12 - PART 5 Infectious Diseases

01 - SECTION 1 Basic Considerations in Infectious Diseases

SECTION 1 Basic Considerations in Infectious Diseases

Section 1 Basic Considerations in Infectious Diseases Neeraj K. Surana, Dennis L. Kasper

Approach to the Patient

with an Infectious Disease ■ ■HISTORIC PERSPECTIVE The origins of the field of infectious diseases are humble. The notion that communicable diseases were due to a miasma (“bad air”) can be traced back to at least the mid-sixteenth century. Not until the work of Louis Pasteur and Robert Koch in the late nineteenth century was there credible evidence supporting the germ theory of disease—i.e., that microorganisms are the direct cause of infections. In contrast to this relatively slow start, the twentieth century saw remarkable advances in the field of infectious diseases, and the etiologic agents of numerous infectious diseases were soon identified. Furthermore, the discovery of antibiotics and the advent of vaccines against some of the most deadly and debilitating infections greatly altered the landscape of human health. Indeed, the twentieth century saw the elimination of smallpox, one of the great scourges in the history of humanity. These remarkable successes prompted Sir Frank MacFarlane Burnet, a noted immunolo­ gist and Nobel laureate, to write in a 1962 publication entitled Natural History of Infectious Diseases: “In many ways one can think of the middle of the twentieth century as the end of one of the most impor­ tant social revolutions in history, the virtual elimination of infectious disease.” Professor Burnet was not alone in this view. Robert Peters­ dorf, a renowned infectious disease expert and former editor of this textbook, wrote in 1978 that “even with my great personal loyalties to infectious diseases, I cannot conceive a need for 309 more [graduating trainees in infectious diseases] unless they spend their time culturing each other.” Given the enormous growth of interest in the microbiome in the past 20 years, Dr. Petersdorf’s statement might have been ironi­ cally clairvoyant, although he could have had no idea what was in store for humanity, with an onslaught of new, emerging, and reemerging infectious diseases. Clearly, even with all the advances of the twentieth century, infectious diseases continue to represent a formidable challenge for patients and physicians alike. Furthermore, during the latter half of the century, several chronic diseases were demonstrated to be directly or indirectly caused by infectious microbes; perhaps the most notable examples are the associations of Helicobacter pylori with peptic ulcer disease and gastric carcinoma, human papillomavirus with cervi­ cal cancer, and hepatitis B and C viruses with liver cancer. In fact, ~16% of all malignancies are now known to be associated with an infectious cause. In addition, numerous emerging and reemerging infectious diseases continue to have a dire impact on global health: HIV/AIDS, SARS-CoV-2, Ebola, and mpox are but a few examples. The fear of weaponizing pathogens for bioterrorism is ever present and poses a potentially enormous threat to public health. Moreover, escalating antimicrobial resistance in clinically relevant microbes (e.g., carbapenem-resistant Enterobacteriaceae and Acinetobacter spp., Candida auris, drug-resistant Mycobacterium tuberculosis, and vancomycin-resistant enterococci) signifies that the administration of antimicrobial agents—once thought to be a panacea—requires appropriate stewardship. For all these reasons, infectious diseases continue to exert grim effects on individual patients as well as on international public health. Even with all the successes of the past century, physicians must be as thoughtful about infectious diseases now as they were at the beginning of the twentieth century.

Infectious Diseases PART 5 ■ ■GLOBAL CONSIDERATIONS Infectious diseases remain the second leading cause of death world­ wide. Although the rate of infectious disease–related deaths has decreased dramatically over the past 25 years, there were still 9.6 mil­ lion such deaths in 2019 (Fig. 124-1A). These deaths disproportion­ ately affect children <1 year of age, adults older than 70 years, and persons living in low- and middle-income countries (Fig. 124-1B and 124-1C; Chap. 487); in 2019, ~17% of all deaths worldwide were related to infectious diseases, with a rate as high as ~69% in sub-Saharan Africa. Given that infectious diseases are still a major cause of global mortal­ ity, understanding the local epidemiology of disease is critically impor­ tant in evaluating patients. Diseases such as HIV/AIDS have decimated southern Africa, with HIV-infected adults representing 16–20% of the total population in countries like South Africa, Botswana, and Lesotho, and more than 25% in Eswatini. Moreover, drug-resistant tuberculosis is rampant throughout the former Soviet-bloc countries, India, China, and South Africa. The ready availability of this type of information allows physicians to develop appropriate differential diagnoses and treatment plans for individual patients. Programs such as the Global Burden of Disease seek to quantify human losses (e.g., deaths, disability-adjusted life-years) due to diseases by age, sex, and country over time; these data not only help inform local, national, and international health policy but can also help guide local medical decision-making. Even though some diseases (e.g., pandemic influenza, mpox) are seemingly geographically restricted, the increasing ease of rapid worldwide travel has raised concern about their swift spread around the globe. Indeed, human migration has historically been the source of epidemics: Yersinia pestis spread along trade routes in the fourteenth century, Native American populations were devastated by diseases such as smallpox and Salmonella that were imported by European explorers in the fifteenth and sixteenth centuries, military maneu­ vers helped facilitate the spread of the 1918 influenza pandemic, and religious pilgrimages (e.g., the Hajj) provide the means for worldwide dissemination of diseases. The continued effects of global travel on the spread of infectious diseases are perhaps best highlighted by the SARS-CoV-2 pandemic (Chap. 204). Although this virus was first identified in Wuhan, China, it quickly spread across the globe and brought an abrupt end to virtually all travel and commerce throughout the world, plunging economies into a deep recession, resulting at one point in more than half the world’s population living under stay-athome orders, and causing the death of ~7 million people worldwide. Not only can travelers carry person-to-person transmitted infections (e.g., SARS-CoV-2, HIV) anywhere in the world, but they can also introduce vector-borne infections to new geographic areas (e.g., chi­ kungunya and Zika viruses) and contribute to the worldwide spread of multidrug-resistant organisms. The world’s increasing interconnected­ ness has profound implications not only for the global economy but also for medicine and the spread of infectious diseases. ■ ■UNDERSTANDING THE MICROBIOTA Normal, healthy humans are colonized with ~40 trillion bacteria as well as countless viruses, fungi, and archaea; taken together, these microor­ ganisms outnumber human cells by ~10 times in the human body (Chap. 484). The major reservoir of these microbes is the gastrointesti­ nal tract, but substantial numbers of microbes live in the female genital tract, the oral cavity, and the nasopharynx. There is increasing interest in the skin and lungs as sites where microbial colonization might be highly relevant to the biology and disease susceptibility of the host. These com­ mensal organisms provide the host with myriad benefits, from aiding in metabolism to shaping the immune system. With regard to infectious diseases, the vast majority of infections are caused by organisms that are part of the normal microbiota (e.g., Staphylococcus aureus, Streptococcus pneumoniae, Pseudomonas aeruginosa), with relatively few infections due to organisms that are strictly pathogens (e.g., Neisseria gonorrhoeae, rabies virus). Perhaps it is not surprising that a general understanding

02 - 124 Approach to the Patient with an Infectious Disease

124 Approach to the Patient with an Infectious Disease

Section 1 Basic Considerations in Infectious Diseases Neeraj K. Surana, Dennis L. Kasper

Approach to the Patient

with an Infectious Disease ■ ■HISTORIC PERSPECTIVE The origins of the field of infectious diseases are humble. The notion that communicable diseases were due to a miasma (“bad air”) can be traced back to at least the mid-sixteenth century. Not until the work of Louis Pasteur and Robert Koch in the late nineteenth century was there credible evidence supporting the germ theory of disease—i.e., that microorganisms are the direct cause of infections. In contrast to this relatively slow start, the twentieth century saw remarkable advances in the field of infectious diseases, and the etiologic agents of numerous infectious diseases were soon identified. Furthermore, the discovery of antibiotics and the advent of vaccines against some of the most deadly and debilitating infections greatly altered the landscape of human health. Indeed, the twentieth century saw the elimination of smallpox, one of the great scourges in the history of humanity. These remarkable successes prompted Sir Frank MacFarlane Burnet, a noted immunolo­ gist and Nobel laureate, to write in a 1962 publication entitled Natural History of Infectious Diseases: “In many ways one can think of the middle of the twentieth century as the end of one of the most impor­ tant social revolutions in history, the virtual elimination of infectious disease.” Professor Burnet was not alone in this view. Robert Peters­ dorf, a renowned infectious disease expert and former editor of this textbook, wrote in 1978 that “even with my great personal loyalties to infectious diseases, I cannot conceive a need for 309 more [graduating trainees in infectious diseases] unless they spend their time culturing each other.” Given the enormous growth of interest in the microbiome in the past 20 years, Dr. Petersdorf’s statement might have been ironi­ cally clairvoyant, although he could have had no idea what was in store for humanity, with an onslaught of new, emerging, and reemerging infectious diseases. Clearly, even with all the advances of the twentieth century, infectious diseases continue to represent a formidable challenge for patients and physicians alike. Furthermore, during the latter half of the century, several chronic diseases were demonstrated to be directly or indirectly caused by infectious microbes; perhaps the most notable examples are the associations of Helicobacter pylori with peptic ulcer disease and gastric carcinoma, human papillomavirus with cervi­ cal cancer, and hepatitis B and C viruses with liver cancer. In fact, ~16% of all malignancies are now known to be associated with an infectious cause. In addition, numerous emerging and reemerging infectious diseases continue to have a dire impact on global health: HIV/AIDS, SARS-CoV-2, Ebola, and mpox are but a few examples. The fear of weaponizing pathogens for bioterrorism is ever present and poses a potentially enormous threat to public health. Moreover, escalating antimicrobial resistance in clinically relevant microbes (e.g., carbapenem-resistant Enterobacteriaceae and Acinetobacter spp., Candida auris, drug-resistant Mycobacterium tuberculosis, and vancomycin-resistant enterococci) signifies that the administration of antimicrobial agents—once thought to be a panacea—requires appropriate stewardship. For all these reasons, infectious diseases continue to exert grim effects on individual patients as well as on international public health. Even with all the successes of the past century, physicians must be as thoughtful about infectious diseases now as they were at the beginning of the twentieth century.

Infectious Diseases PART 5 ■ ■GLOBAL CONSIDERATIONS Infectious diseases remain the second leading cause of death world­ wide. Although the rate of infectious disease–related deaths has decreased dramatically over the past 25 years, there were still 9.6 mil­ lion such deaths in 2019 (Fig. 124-1A). These deaths disproportion­ ately affect children <1 year of age, adults older than 70 years, and persons living in low- and middle-income countries (Fig. 124-1B and 124-1C; Chap. 487); in 2019, ~17% of all deaths worldwide were related to infectious diseases, with a rate as high as ~69% in sub-Saharan Africa. Given that infectious diseases are still a major cause of global mortal­ ity, understanding the local epidemiology of disease is critically impor­ tant in evaluating patients. Diseases such as HIV/AIDS have decimated southern Africa, with HIV-infected adults representing 16–20% of the total population in countries like South Africa, Botswana, and Lesotho, and more than 25% in Eswatini. Moreover, drug-resistant tuberculosis is rampant throughout the former Soviet-bloc countries, India, China, and South Africa. The ready availability of this type of information allows physicians to develop appropriate differential diagnoses and treatment plans for individual patients. Programs such as the Global Burden of Disease seek to quantify human losses (e.g., deaths, disability-adjusted life-years) due to diseases by age, sex, and country over time; these data not only help inform local, national, and international health policy but can also help guide local medical decision-making. Even though some diseases (e.g., pandemic influenza, mpox) are seemingly geographically restricted, the increasing ease of rapid worldwide travel has raised concern about their swift spread around the globe. Indeed, human migration has historically been the source of epidemics: Yersinia pestis spread along trade routes in the fourteenth century, Native American populations were devastated by diseases such as smallpox and Salmonella that were imported by European explorers in the fifteenth and sixteenth centuries, military maneu­ vers helped facilitate the spread of the 1918 influenza pandemic, and religious pilgrimages (e.g., the Hajj) provide the means for worldwide dissemination of diseases. The continued effects of global travel on the spread of infectious diseases are perhaps best highlighted by the SARS-CoV-2 pandemic (Chap. 204). Although this virus was first identified in Wuhan, China, it quickly spread across the globe and brought an abrupt end to virtually all travel and commerce throughout the world, plunging economies into a deep recession, resulting at one point in more than half the world’s population living under stay-athome orders, and causing the death of ~7 million people worldwide. Not only can travelers carry person-to-person transmitted infections (e.g., SARS-CoV-2, HIV) anywhere in the world, but they can also introduce vector-borne infections to new geographic areas (e.g., chi­ kungunya and Zika viruses) and contribute to the worldwide spread of multidrug-resistant organisms. The world’s increasing interconnected­ ness has profound implications not only for the global economy but also for medicine and the spread of infectious diseases. ■ ■UNDERSTANDING THE MICROBIOTA Normal, healthy humans are colonized with ~40 trillion bacteria as well as countless viruses, fungi, and archaea; taken together, these microor­ ganisms outnumber human cells by ~10 times in the human body (Chap. 484). The major reservoir of these microbes is the gastrointesti­ nal tract, but substantial numbers of microbes live in the female genital tract, the oral cavity, and the nasopharynx. There is increasing interest in the skin and lungs as sites where microbial colonization might be highly relevant to the biology and disease susceptibility of the host. These com­ mensal organisms provide the host with myriad benefits, from aiding in metabolism to shaping the immune system. With regard to infectious diseases, the vast majority of infections are caused by organisms that are part of the normal microbiota (e.g., Staphylococcus aureus, Streptococcus pneumoniae, Pseudomonas aeruginosa), with relatively few infections due to organisms that are strictly pathogens (e.g., Neisseria gonorrhoeae, rabies virus). Perhaps it is not surprising that a general understanding

Number of deaths (in millions)

A

PART 5 Infectious Diseases 0.1 C 0.2 0.3 0.4 0.5 0.6 FIGURE 124-1  Magnitude of infectious disease–related deaths globally. A. The absolute number (blue line; left axis) and rate (red line; right axis) of infectious disease– related deaths throughout the world since 1990. B. Age-specific rates of infectious disease–related deaths in 2019. In both A and B, the charts depict the mean estimate and 95% uncertainty intervals. C. A map depicting country-specific data for the percent of total deaths that were attributable to communicable, maternal, neonatal, and nutritional disorders in 2019. (Source: Institute for Health Metrics and Evaluation. Used with permission. All rights reserved.) of the microbiota is essential in the evaluation of infectious diseases. Individuals’ microbiotas have a major impact on their susceptibility to infectious diseases and even their responses to vaccines. Site-specific knowledge of the indigenous microbiota may facilitate appropriate interpretation of culture results, aid in selection of empirical antimicro­ bial therapy based on the likely causative agents, and provide additional impetus for rational antibiotic use to minimize the untoward effects of these drugs on the “beneficial” microbes that inhabit the body. ■ ■WHEN TO CONSIDER AN INFECTIOUS ETIOLOGY The title of this chapter may appear to presuppose that the physician knows when a patient has an infectious disease. In reality, this chapter can serve only as a guide to the evaluation of a patient in whom an infectious disease is a possibility. Once a specific diagnosis is made, the reader should consult the subsequent chapters that deal with specific microorganisms in detail. The challenge for the physician is to recog­ nize which patients may have an infectious disease as opposed to some other underlying disorder. This task is greatly complicated by the fact that infections have an infinite range of presentations, from acute lifethreatening conditions (e.g., meningococcemia) to chronic diseases of varying severity (e.g., H. pylori–associated peptic ulcer disease) to

Rate of death (per 100,000)

Rate of death (per 100,000)

0–27 days 28–364 days 1–4 years 5–9 years 10–19 years 20–29 years 30–39 years 40–49 years 50–59 years 60–69 years 70–79 years 80–89 years 90+ years Age B no symptoms at all (e.g., latent M. tuberculosis infection). While it is impossible to generalize about a presentation that encompasses all infections, common findings in the history, physical examination, and basic laboratory testing often suggest that the patient either has an infectious disease or should be more closely evaluated for one. This chapter focuses on these common findings and how they may direct the ongoing evaluation of the patient. APPROACH TO THE PATIENT Infectious Disease See also Chap. 127. HISTORY As in all of medicine, a complete and thorough history is paramount in the evaluation of a patient with a possible infectious disease. The history is critical for developing a focused differential diagnosis and for guiding the physical exam and initial diagnostic testing. Although a detailing of all the elements of a history is beyond the scope of this chapter, specific components relevant to infectious diseases require

particular attention. In general, these aspects focus on two areas: (1) an exposure history that may identify microorganisms with which the patient may have come into contact and (2) host-specific factors that may predispose to the development of an infection. Exposure History  •  History of infections or exposure to drug-resistant microbes  Information about a patient’s previous infections, with the associated microbial susceptibility profiles, is very helpful in determining possible etiologic agents. Specifically, know­ ing whether a patient has a history of infection with drug-resistant organisms (e.g., methicillin-resistant S. aureus, vancomycin-resistant Enterococcus species, enteric organisms that produce an extendedspectrum β-lactamase or carbapenemase) or may have been exposed to drug-resistant microbes (e.g., during a recent stay in a hospital, nursing home, or long-term acute-care facility) may alter the choice of empirical antibiotics. For example, a patient presenting with sepsis who is known to have a history of invasive infection with a multi­ drug-resistant isolate of P. aeruginosa should be treated empirically with an antimicrobial regimen that will cover this strain. Social history  Although the social history taken by physicians is often limited to inquiries about a patient’s alcohol and tobacco use, a complete social history can offer a number of clues to the underlying diagnosis. Knowing whether the patient has any highrisk behaviors (e.g., unsafe sexual behaviors, intravenous [IV] drug use), potential hobby-associated exposures (e.g., avid gardening, with possible Sporothrix schenckii exposure), or occupational expo­ sures (e.g., increased risk for M. tuberculosis exposure in funeral service workers) can facilitate diagnosis. The importance of the social history is exemplified by a case in 2009 in which a laboratory researcher died of a Y. pestis infection acquired during his work; although this patient had visited both an outpatient clinic and an emergency department, his records at both sites failed to include his occupation—information that potentially could have led quickly to appropriate treatment and infection control measures. Dietary habits  Because certain pathogens are associated with specific dietary habits, inquiring about a patient’s diet can provide insight into possible exposures. For example, Shiga toxin–producing strains of Escherichia coli, and Toxoplasma gondii are associated with the consumption of raw or undercooked meat; Salmonella typhimurium, Listeria monocytogenes, and Mycobacterium bovis with unpasteurized milk; Leptospira species, parasites, and enteric bacteria with unpurified water; and Vibrio species, norovirus, hel­ minths, and protozoa with raw seafood. Animal exposures  Because animals are often important vectors of infectious diseases, patients should be asked about exposures to any animals, including contact with their own pets, visits to petting zoos, or random encounters (e.g., home rodent infestation). For example, dogs can carry ticks that serve as agents for the transmis­ sion of several infectious diseases, including Lyme disease, Rocky Mountain spotted fever, and ehrlichiosis. Cats are associated with Bartonella henselae infection, reptiles with Salmonella infection, rodents with leptospirosis, and rabbits with tularemia (Chap. 146). Travel history  Attention should be paid to both international and domestic travel. Fever in a patient who has recently returned from abroad significantly broadens the differential diagnosis (Chap. 130) and, as exemplified by the COVID-19 pandemic, can help identify the beginnings of international outbreaks. Even a remote history of international travel may reflect patients’ exposure to infections with pathogens such as M. tuberculosis or Strongyloides stercoralis. Similarly, domestic travel may have exposed patients to pathogens that are not normally found in their local environment and therefore may not routinely be considered in the differential diagnosis. For example, a patient who has recently visited California or Martha’s Vineyard may have been exposed to Coccidioides immi­ tis or Francisella tularensis, respectively. Beyond simply identifying locations that a patient may have visited, the physician needs to delve deeper to learn what kinds of activities and behaviors the

patient engaged in during travel (e.g., the types of food and sources of water consumed, freshwater swimming, animal exposures) and whether the patient had the necessary immunizations and/or took the necessary prophylactic medications prior to travel; these addi­ tional exposures, which the patient may not think to report without specific prompting, are as important as exposures during a patient’s routine daily living. Host-Specific Factors  Because many opportunistic infections (e.g., with Pneumocystis jirovecii, Aspergillus species, or JC virus) affect primarily immunocompromised patients, it is of vital importance to determine the immune status of the patient. Defects in the immune system may be due to an underlying disease (e.g., malignancy, HIV infection, malnutrition), a medication (e.g., chemotherapy, gluco­ corticoids, monoclonal antibodies to components of the immune system), a treatment modality (e.g., total body irradiation, splenec­ tomy), or a primary immunodeficiency. The type of infection for which the patient is at increased risk varies with the specific type of immune defect. In concert with determining whether a patient is immunocompromised for any reason, the physician should review the immunization record to ensure that the patient is adequately pro­ tected against vaccine-preventable diseases (Chap. 129). PHYSICAL EXAMINATION Like the history, a thorough physical examination is crucial in evaluating patients with an infectious disease. Some elements of the physical exam (e.g., skin, lymphatics) that are often performed in a cursory manner as a result of the ever-increasing pace of medi­ cal practice may help identify the underlying diagnosis. Moreover, serial exams are critical since new findings may appear as the illness progresses. A description of all the elements of a physical exam is beyond the scope of this chapter, but the following components have particular relevance to infectious diseases. CHAPTER 124 Vital Signs  Given that elevations in temperature are often a hall­ mark of infection, paying close attention to the temperature may be of value in diagnosing an infectious disease (Chap. 20). The idea that 37°C (98.6°F) is the normal human body temperature dates to the nineteenth century and was initially based on axillary measure­ ments. Rectal temperatures more accurately reflect the core body temperature and are 0.4°C (0.7°F) and 0.8°C (1.4°F) higher than oral and axillary temperatures, respectively. This idea of a “normal” body temperature does not consider the fact that temperatures tend to be higher later in the day, in women, and in younger people. Moreover, the average body temperature seems to have dropped ~0.03°C every decade since the early 1800s to a new normal of ~36.7°C. Although the definition of fever varies greatly throughout the medical literature, the most common definition, which is based on studies defining fever of unknown origin (Chap. 22), uses a core temperature ≥38.3°C (≥101°F). Although fever is very commonly associated with infection, it is also documented in many other diseases (Chap. 20). For every 1°C (1.8°F) increase in core tempera­ ture, the heart rate typically rises by ~10 beats/min. Table 124-1 lists infections that are associated with relative bradycardia (Faget’s sign), where patients have a lower heart rate than might be expected for a given body temperature. Although this pulse–temperature dissociation is not highly sensitive or specific for establishing a diagnosis, it is potentially useful in low-resource settings given its ready availability and simplicity. Approach to the Patient with an Infectious Disease Lymphatics  There are ~600 lymph nodes throughout the body, and infections are an important cause of lymphadenopathy. A physical examination should include evaluation of lymph nodes in multiple regions (e.g., popliteal, inguinal, epitrochlear, axillary, multiple cervical regions), with notation of the location, size (nor­ mal, <1 cm), presence or absence of tenderness, and consistency (soft, firm, or rubbery) and of whether the nodes are matted (i.e., connected and moving together). Nodes that are small and firm can also be described as “shotty,” referring to the size and consistency of buckshot pellets. Of note, palpable epitrochlear nodes are always

TABLE 124-1  Causes of Relative Bradycardia Infectious Causes Intracellular organisms     Gram-negative bacteria Salmonella typhi Francisella tularensis Brucella spp. Coxiella burnetii (Q fever) Leptospira interrogans Legionella pneumophila Mycoplasma pneumoniae   Tick-borne organisms Rickettsia spp. Orientia tsutsugamushi (scrub typhus) Babesia spp.   Other Corynebacterium diphtheriae Plasmodium spp. (malaria) Viruses/viral infections Yellow fever virus Dengue virus Viral hemorrhagic feversa Viral myocarditis Noninfectious Causes   Drug fever Beta blocker use Central nervous system lesions Malignant lymphoma Factitious fever aPrimarily early in the course of infection with Marburg or Ebola virus. PART 5 Infectious Diseases pathologic. Of patients presenting with lymphadenopathy, 75% have localized findings, and the remaining 25% have generalized lymphadenopathy (i.e., that involving more than one anatomic region). Localized lymphadenopathy in the head and neck region is found in 55% of patients, inguinal lymphadenopathy in 14%, and axillary lymphadenopathy in 5%. Determining whether the patient has generalized versus localized lymphadenopathy can help narrow the differential diagnosis, as various infections present differently. Skin  The fact that many infections have cutaneous manifestations gives the skin examination particular importance in the evaluation of patients (Chaps. 21, 61, 134, and A1). It is important to perform a complete skin exam, with attention to both front and back. Spe­ cific rashes are often extremely helpful in narrowing the differential diagnosis of an infection (Chaps. 21 and A1). In numerous anec­ dotal instances, patients in the intensive care unit have had “fever of unknown origin” that was actually due to unrecognized pressure ulcers. Moreover, close examination of the distal extremities for splinter hemorrhages, Janeway lesions, or Osler’s nodes may yield evidence of endocarditis or other causes of septic emboli. Foreign Bodies  As previously mentioned, many infections are caused by members of the indigenous microbiota. These infections typically occur when these microbes escape their normal habitat and enter a new one. Thus, maintenance of epithelial barriers is one of the most important mechanisms in protection against infection. However, hospitalization of patients is often associated with breaches of these barriers—e.g., due to placement of IV lines, surgical drains, or tubes (e.g., endotracheal tubes and Foley cath­ eters) that allow microorganisms to localize in sites to which they normally would not have access (Chap. 147). Accordingly, knowing what lines, tubes, and drains are in place is helpful in ascertaining what body sites might be infected. DIAGNOSTIC TESTING Laboratory and radiologic testing has advanced greatly over the past few decades and has become an important component in the evaluation of patients. The dramatic increase in the number

of serologic diagnostics, antigen tests, and molecular diagnostics available to the physician has, in fact, revolutionized medical care. However, all of these tests should be viewed as adjuncts to the his­ tory and physical examination—not a replacement for them. The selection of initial tests should be based directly on the patient’s history and physical exam findings. Moreover, diagnostic testing should generally be limited to those conditions that are reasonably likely and treatable, important in terms of public health consider­ ations, and/or capable of providing a definitive diagnosis that will consequently limit other testing. White Blood Cell (WBC) Count  Elevations in the WBC count are often associated with infection, although many viral infections are associated with leukopenia. It is important to assess the WBC dif­ ferential, given that different classes of microbes are associated with various leukocyte types. For example, bacteria are associated with an increase in polymorphonuclear neutrophils, often with elevated levels of earlier developmental forms such as bands; viruses are associated with an increase in lymphocytes; and certain parasites are associated with an increase in eosinophils. Table 124-2 lists the major infectious causes of eosinophilia. Inflammatory Markers  The erythrocyte sedimentation rate (ESR) and the C-reactive protein (CRP) level are indirect and direct measures of the acute-phase response, respectively, that can be used to assess a patient’s general level of inflammation. Moreover, these markers can be followed serially over time to monitor disease progress/resolution. It is noteworthy that the ESR changes relatively slowly, and its measurement more often than weekly usually is not useful; in contrast, CRP concentrations change rapidly, and daily measurements can be useful in the appropriate context. Although these markers are sensitive indicators of inflammation, neither is very specific. An extremely elevated ESR (>100 mm/h) has a 90% predictive value for a serious underlying disease (Table 124-3). Work is ongoing to identify other potentially useful inflammatory markers (e.g., procalcitonin, serum amyloid A protein); their clini­ cal utility requires further validation. Analysis of Cerebrospinal Fluid (CSF)  Assessment of CSF is criti­ cal for patients with suspected meningitis or encephalitis. An open­ ing pressure should always be recorded, and fluid should routinely be sent for cell counts, Gram’s stain and culture, and determination of glucose and protein levels. A CSF Gram’s stain typically requires

105 bacteria/mL for reliable positivity; its specificity approaches 100%. Table 124-4 lists the typical CSF profiles for various infec­ tions. In general, CSF with lymphocytic pleocytosis and a low glucose concentration suggests either infection (e.g., with Listeria, M. tuberculosis, or a fungus) or a noninfectious disorder (e.g., neo­ plastic meningitis, sarcoidosis). Bacterial antigen tests of CSF (e.g., latex agglutination tests for Haemophilus influenzae type b, group B Streptococcus, S. pneumoniae, and Neisseria meningitidis) are not recommended for screening, given that these tests are no more sensitive than Gram’s stain; however, these assays can be helpful in presumptively identifying organisms seen on Gram’s stain. In con­ trast, other antigen tests (e.g., for Cryptococcus) and some CSF sero­ logic testing (e.g., for Treponema pallidum, Coccidioides) are highly sensitive and are useful for select patients. In addition, polymerase chain reaction (PCR) analysis of CSF is increasingly being used for the diagnosis of bacterial (e.g., N. meningitidis, S. pneumoniae, mycobacteria) and viral (e.g., herpes simplex virus, enterovirus) infections; while these molecular tests permit rapid diagnosis with a high degree of sensitivity and specificity, they often do not allow determination of antimicrobial resistance profiles. Cultures  The mainstays of infectious disease diagnosis include the culture of infected tissue (e.g., surgical specimens) or fluid (e.g., blood, urine, sputum, pus from a wound). Samples can be sent for culture of bacteria (aerobic or anaerobic), fungi, or viruses. Ideally, specimens are collected before the administration of antimicrobial therapy; in instances where this order of events is not clinically

TABLE 124-2  Major Infectious Causes of Eosinophiliaa ORGAN INVOLVED ORGANISM EXPOSURE GEOGRAPHIC DISTRIBUTION DEGREE OF EOSINOPHILIAb Central nervous system Angiostrongylus Raw seafood Asia Mild Gnathostoma Raw poultry and seafood Asia Moderate to extreme Eye Loa loa Insect bite Africa Moderate (expatriates), mild (patients living in endemic areas) Onchocerca Insect bite Africa Mild (expatriates), moderate (patients living in endemic areas) Lung Chlamydia trachomatis Sexual transmission Worldwide Mild Strongyloides Soil Tropical Moderate (acute), mild (chronic) Toxocara canis/Toxocara catic Dogs, soil Worldwide Moderate to extreme Paragonimus Crabs and crayfish Asia Moderate (acute), mild (chronic) Coccidioides immitis Soil Southwestern United States Mild (acute), extreme (disseminated) Brugia malayi Insect bite Asia Mild to moderate Pneumocystis jirovecii Air Worldwide Mild Liver Schistosoma japonicum Freshwater swimming Asia Moderate (acute), mild (chronic) Schistosoma mansoni Freshwater swimming Africa, Middle East, Latin America Fasciola Watercress Worldwide Moderate Clonorchis Raw seafood Asia Mild to moderate Opisthorchis Raw seafood Asia Mild to moderate Intestines Ascarisd Raw fruits and vegetables, contaminated water Hookworm Soil Worldwide Mild to moderate Trichuris Raw fruits and vegetables, contaminated water Cystoisospora belli Contaminated water and food Worldwide Mild Dientamoeba fragilis Unclear; spread via fecal–oral route Capillaria Raw seafood Asia Extreme Heterophyes Raw seafood Asia, Middle East Mild Anisakis Raw seafood Worldwide Mild Baylisascaris procyonise Soil North America Moderate to extreme Hymenolepis nana Contaminated water, soil Worldwide Mild Bladder Schistosoma haematobium Freshwater swimming Africa, Middle East Moderate (acute), mild (chronic) Muscle Trichinella Pork Worldwide Moderate to extreme Lymphatics Wuchereria bancroftid Insect bite Tropical Moderate to extremef Bartonella henselae Cats Worldwide Mild Other Recovery from bacterial or viral infections — — Mild HIV Contaminated bodily fluid Worldwide Mild Cryptococcus neoformans Soil Worldwide Moderate to extreme (disseminated) aThere are numerous noninfectious causes of eosinophilia, such as atopic disease, DRESS (drug reaction with eosinophilia and systemic symptoms) syndrome, and pernicious anemia, which can cause mild eosinophilia; drug hypersensitivity and serum sickness, which can cause mild to moderate eosinophilia; collagen vascular disease, which can cause moderate eosinophilia; and malignancy, Churg-Strauss syndrome, and hyper-IgE syndromes, which can cause moderate to extreme eosinophilia. bMild: 500–1500 cells/μL; moderate: 1500–5000 cells/μL; extreme: >5000 cells/μL. cCan also affect the liver and the eyes. dCan also affect the lungs. eCan also affect the eyes and the central nervous system. fLevels are typically higher with pulmonary infections. feasible, microscopic examination of the specimen (e.g., Gram-stained or potassium hydroxide [KOH]–treated preparations) is particularly important. Culture of the organism(s) allows identification of the etiologic agent(s), determination of the antimicrobial susceptibil­ ity profile, and—when there is concern about an outbreak—isolate typing. While cultures are extremely useful in the evaluation of patients, determining whether culture results are clinically meaning­ ful or represent contamination (e.g., a non-aureus, non-lugdunensis staphylococcal species growing in a blood culture) can sometimes be challenging and requires an understanding of the patient’s immune status, exposure history, and microbiota. In some cases, serial cultures to demonstrate clearance of the organism may be helpful. Pathogen-Specific Testing  Numerous pathogen-specific tests (e.g., serology, antigen testing, PCR testing) are commercially

Moderate (acute), mild (chronic) Worldwide Mild to extreme Tropical Mild CHAPTER 124 Worldwide Mild Approach to the Patient with an Infectious Disease available, and many hospitals now offer some of these tests in-house to facilitate rapid turnaround that ultimately enhances patient care. The reader is directed to relevant chapters on the pathogens of interest for specific details. Some of these tests (e.g., universal PCRs, shotgun metagenomic sequencing) identify organisms that currently are not easily cultivable and have unclear relationships to disease, thereby complicating diagnosis. As these tests become more com­ monplace, the relevance of some of these previously unrecognized bacteria to human health will likely become more apparent. Radiology  Imaging provides an important adjunct to the physical examination, allowing evaluation for lymphadenopathy in regions that are not externally accessible (e.g., mediastinum, intraabdomi­ nal sites), assessment of internal organs for evidence of infection, and facilitation of image-guided percutaneous sampling of deep spaces. The choice of imaging modality (e.g., CT, MRI, ultrasound,

TABLE 124-3  Causes of an Extremely Elevated Erythrocyte Sedimentation Rate (>100 mm/h) ETIOLOGIC CATEGORY (% OF CASES) SPECIFIC CAUSES Infectious diseases (35–40) Subacute bacterial endocarditis Abscesses Osteomyelitis Tuberculosis Urinary tract infection Inflammatory diseases (15–20) Giant cell arteritis Rheumatoid arthritis Systemic lupus erythematosus Malignancies (15–20) Multiple myeloma Leukemias Lymphomas Carcinomas Other (20–35) Drug hypersensitivity reactions (drug fever) Ischemic tissue injury/trauma Renal diseases nuclear medicine, use of contrast) is best made in consultation with a radiologist to ensure that the results will address the physician’s specific concerns. TREATMENT Physicians often must balance the need for empirical antibiotic treat­ ment with the patient’s clinical condition. When clinically feasible, it is best to obtain relevant samples (e.g., blood, CSF, tissue, purulent exudate) for culture prior to the administration of antibiotics, as antibiotic treatment often makes subsequent diagnosis more difficult. Although a general maxim for antibiotic treatment is to use a regimen with as narrow a spectrum as possible (Chap. 149), empirical regi­ mens are necessarily somewhat broad, given that a specific diagnosis has not yet been made. Table 124-5 lists empirical antibiotic treatment regimens for commonly encountered infectious presentations. These regimens should be narrowed as appropriate once a specific diagno­ sis is made. In addition to antibiotics, there is sometimes a role for adjunctive therapies, such as intravenous immunoglobulin G (IVIG) pooled from healthy adults or hyperimmune globulin prepared from PART 5 Infectious Diseases TABLE 124-4  Typical Cerebrospinal Fluid Profiles for Meningitis and Encephalitisa BACTERIAL MENINGITIS VIRAL MENINGITIS   NORMAL WBC count (per μL) <5

1000 25–500 40–600 150–2000 25–100 50–500 Differential of WBC 60–70% lymphocytes, ≤30% monocytes/ macrophages ↑↑PMNs (≥80%) Predominantly lymphocytesc Lymphocytes or PMNs, depending on specific organism Gram’s stain Negative Positive (in >60% of cases) Negative Rarely positive Negative Occasionally positivee Negative Glucose (mg/dL) 40–85 <40 Normal ↓ to normal Normal <50 in 75% of cases Protein (mg/dL) 15–45 100 20–80 150–300 50–200 100–200 50–100 Opening pressure (mmH2O) 50–180 300 100–350 160–340 Normal 150–280 Normal to ↑ Common causes — Streptococcus pneumoniae, Neisseria meningitidis Enteroviruses Candida, Cryptococcus, and Aspergillus spp. aNumbers indicate typical results, but actual results may vary. bCerebrospinal fluid characteristics depend greatly on the specific organism. cNeutrophils may predominate early in the disease course. dPatients typically have striking eosinophilia as well. eSensitivity can be increased by examination of a smear of protein coagulum (pellicle) and the use of acid-fast stains. Abbreviations: PMNs, polymorphonuclear neutrophils; WBC, white blood cell.

the blood of individuals with high titers of specific antibodies to select pathogens (e.g., cytomegalovirus, hepatitis B virus, rabies virus, vaccinia virus, Clostridium tetani, varicella-zoster virus, Clostridium botulinum toxin). Although the data suggesting efficacy are limited, IVIG is sometimes used for patients with suspected staphylococcal or streptococcal toxic shock syndrome. INFECTION CONTROL When evaluating a patient with a suspected infectious disease, the physician must consider what infection control methods are nec­ essary to prevent transmission of any possible infection to other people. In 2007, the U.S. Centers for Disease Control and Prevention published guidelines for isolation precautions that are available for download at www.cdc.gov/infectioncontrol/guidelines/isolation/. Per­ sons exposed to certain pathogens (e.g., N. meningitidis, HIV, Bacillus anthracis) should receive postexposure prophylaxis to prevent disease acquisition. (See relevant chapters for details on specific pathogens.) WHEN TO OBTAIN AN INFECTIOUS DISEASE CONSULT At times, primary physicians need assistance with patient manage­ ment from a diagnostic and/or therapeutic perspective. Multiple studies have demonstrated that an infectious disease consult is asso­ ciated with improved outcomes, shorter length of hospital stay, and decreased costs for patients with various diseases. For example, in a prospective cohort study of patients with S. aureus bacteremia, infec­ tious disease consultation was independently associated with a 56% reduction in 28-day mortality. While artificial intelligence–based chatbots are beginning to be utilized in healthcare settings, they are not yet sophisticated enough to supplant an actual infectious dis­ ease consultation. In addition, infectious disease specialists provide other services (e.g., infection control, antimicrobial stewardship, management of outpatient antibiotic therapy, occupational exposure programs) that have been shown to benefit patients. Whenever such assistance would be advantageous to a patient with a possible infection, the primary physician should opt for an infectious disease consult. Specific situations that might prompt a consult include (1) difficult-to-diagnose patients with presumed infections, (2) patients who are not responding to treatment as expected, (3) patients with a complicated medical history (e.g., organ transplant recipients, patients immunosuppressed due to autoimmune or inflammatory conditions), and (4) patients with “exotic” diseases (i.e., diseases that are not typically seen within the region). FUNGAL MENINGITISb PARASITIC MENINGITIS TUBERCULOUS MENINGITIS ENCEPHALITIS Predominantly lymphocytesc Predominantly lymphocytesc ↑↑ Eosinophils (≥50%)d Normal Angiostrongylus cantonensis, Gnathostoma spinigerum, Baylisascaris procyonis Mycobacterium tuberculosis Herpesviruses, enteroviruses, influenza virus, rabies virus

TABLE 124-5  Initial Empirical Antibiotic Therapy for Common Infectious Disease Presentationsa CLINICAL SYNDROME COMMON ETIOLOGIES ANTIBIOTIC(S) COMMENTS SEE CHAPTER(S) Septic shock Staphylococcus aureus, Streptococcus pneumoniae, enteric gram-negative bacilli Vancomycin, 15 mg/kg q12hb plus A broad-spectrum antipseudomonal β-lactam (piperacillin-tazobactam, 4.5 g q6h; imipenem,

1 g q8h; meropenem, 1 g q8h; or cefepime,

1–2 g q8–12h) Meningitis S. pneumoniae, Neisseria meningitidis Vancomycin, 15 mg/kg q12hb plus Ceftriaxone, 2 g q12h CNS abscess Streptococcus spp., Staphylococcus spp., anaerobes, gram-negative bacilli Vancomycin, 15 mg/kg q12hb plus Ceftriaxone, 2 g q12h plus Metronidazole, 500 mg q8h Acute endocarditis (native valve) S. aureus, Streptococcus spp., coagulase-negative staphylococci Vancomycin, 15 mg/kg q12hb plus Cefepime, 2 g q8h Pneumonia   CommunityS. pneumoniae, Mycoplasma pneumoniae, Haemophilus influenzae, Chlamydia pneumoniae No comorbiditiesh: Azithromycin, 500 mg PO? 1, then 250 mg PO qd 4 days With comorbiditiesh: Levofloxacin, 750 mg PO qd acquired, outpatient   Inpatient, non-ICU Above plus Legionella spp. A respiratory fluoroquinolone (moxifloxacin,

400 mg IV/PO qd; gemifloxacin, 320 mg PO qd; or levofloxacin, 750 mg IV/PO qd) or A β-lactam (cefotaxime, ceftriaxone, or ampicillin-sulbactam) plus azithromycin   Inpatient, ICU Above plus S. aureus A β-lactam plus Azithromycin or a respiratory fluoroquinolone   Hospital-acquired pneumoniad S. pneumoniae, H. influenzae, S. aureus, gram-negative bacilli (e.g., Pseudomonas aeruginosa, Klebsiella pneumoniae, Acinetobacter spp.) An antipseudomonal β-lactam (cefepime, 2 g q8h; ceftazidime, 2 g q8h; imipenem, 500 mg q6h; meropenem, 1 g q8h; or piperacillin-tazobactam, 4.5 g q6h) plus An antipseudomonal fluoroquinolone (levofloxacin, 700 mg qd, or ciprofloxacin,

400 mg q8h) or an aminoglycoside (amikacin, 15–20 mg/kg q24hc; gentamicin, 5–7 mg/kg q24he; or tobramycin, 5–7 mg/kg q24he) Complicated intraabdominal infection   Mild to moderate   Anaerobes (Bacteroides spp., Clostridium spp.),

gram-negative bacilli (Escherichia coli), Streptococcus spp. Cefoxitin, 2 g q6h or A combination of metronidazole (500 mg q8–12h) plus one of the following: cefazolin (1–2 g q8h), cefuroxime (1.5 g q8h), ceftriaxone (1–2 g

q12–24h), cefotaxime (1–2 g q6–8h), ciprofloxacin (400 mg q12h), levofloxacin

(750 mg qd) severity High-risk patient or high degree of severity Same as above A carbapenem (imipenem, 500 mg q6h; meropenem, 1 g q8h; doripenem, 500 mg q8h) or Piperacillin-tazobactam, 3.375 g q6hf or A combination of metronidazole (500 mg q8h) plus an antipseudomonal cephalosporin (cefepime, 2 g q8h; ceftazidime, 2 g q8h)

If a pseudomonal species is likely, a second antipseudomonal agent should be added.

Dexamethasone (0.15 mg/kg IV q6h for 2–4 d) should be added for patients with suspected or proven pneumococcal meningitis, with the first dose administered 10–20 min before the first dose of antibiotics. 143 and pathogenspecific chapters —

—

If MRSA is a consideration, add vancomycin (15 mg/kg q8–12hb) or linezolid (600 mg q12h); daptomycin should not be used in patients with pneumonia. 131 and pathogenspecific chapters CHAPTER 124 Approach to the Patient with an Infectious Disease If MRSA is a consideration, add vancomycin (15 mg/kg q8–12hb) or linezolid (600 mg q12h); daptomycin should not be used in patients with pneumonia. If MRSA is a consideration, add vancomycin (15 mg/kg q12hb) 137, 182, and

pathogen-specific chapters (Continued)

03 - 125 Molecular Mechanisms of Microbial Pathogenesis

125 Molecular Mechanisms of Microbial Pathogenesis

TABLE 124-5  Initial Empirical Antibiotic Therapy for Common Infectious Disease Presentationsa CLINICAL SYNDROME COMMON ETIOLOGIES ANTIBIOTIC(S) COMMENTS SEE CHAPTER(S) Skin and soft tissue infection S. aureus, Streptococcus pyogenes Dicloxacillin, 250–500 mg PO qid or Cephalexin, 250–500 mg PO qid or Clindamycin, 300–450 mg PO tid or Nafcillin/oxacillin, 1–2 g q4h aThis table refers to immunocompetent adults with normal renal and hepatic function. All doses listed are for parenteral administration unless indicated otherwise. Local antimicrobial susceptibility profiles may influence the choice of antibiotic. Therapy should be tailored once a specific etiologic agent and its susceptibilities are identified. bTrough levels for vancomycin should be 15–20 μg/mL. cTrough levels for amikacin should be <4 μg/mL. dIn patients with late onset (i.e., after ≥5 days of hospitalization) or risk factors for multidrug-resistant organisms. eTrough levels for gentamicin and tobramycin should be <1 μg/mL. fIf P. aeruginosa is a concern, the dosage may be increased to 3.375 g IV q4h or 4.5 g IV q6h. gData on the efficacy of TMP-SMX in skin and soft tissue infections are limited. hComorbidities include chronic heart, lung, liver, or renal disease; diabetes mellitus; alcoholism; malignancy; or asplenia. Abbreviations: CNS, central nervous system; ICU, intensive care unit; MRSA, methicillin-resistant S. aureus; TMP-SMX, trimethoprim-sulfamethoxazole. ■ ■PERSPECTIVE The study of infectious diseases is really a study of host–microbial inter­ actions and represents evolution by both the host and the microbe—an endless struggle in which microbes have generally been more creative and adaptive. Given that nearly one-sixth of deaths worldwide are still related to infectious diseases, it is clear that the war against infectious diseases has not been won. For example, a cure for HIV infection is still lacking, there have been only marginal improvements in the methods for detection and treatment of tuberculosis after more than a half century of research, new infectious disease outbreaks (e.g., viral hemorrhagic fevers, SARS-CoV-2, mpox) continue to emerge, and the threat of microbial bioterrorism remains high. The subsequent chapters in Part 5 detail—on both a syndrome and a microbe-by-microbe basis—the current state of medical knowledge about infectious diseases. At their core, all these chapters carry a similar message: Despite numerous advances in the diagnosis, treatment, and prevention of infectious diseases, much work and research are required before anyone can confidently claim we have achieved “the virtual elimination of infectious disease.” In reality, this goal will never be attained, given the rapid adaptability of microbes. PART 5 Infectious Diseases ■ ■FURTHER READING Baker RE et al: Infectious disease in an era of global change. Nat Rev Microbiol 20:193, 2022. Bartlett JG: Why infectious diseases. Clin Infect Dis 59:S85, 2014. Khabbaz RF et al: Challenges of infectious diseases in the USA. Lancet 384:53, 2014. Maillard A et al: Can chatbot artificial intelligence replace infectious diseases physicians in the management of bloodstream infections? A prospective cohort study. Clin Infect Dis 78:825, 2024. McQuillen DP, MacIntyre AT: The value that infectious disease physicians bring to the healthcare system. J Infect Dis 216:S588, 2017. Rubin EJ et al: Audio interview: Dr. Fauci on infectious disease chal­ lenges. N Engl J Med 388:e82, 2023. Verghese A et al: Inadequacies of physical examination as a cause of medical errors and adverse events: A collection of vignettes. Am J Med 128:1322, 2015. Jordan B. Jastrab, Marcia B. Goldberg

Molecular Mechanisms of Microbial Pathogenesis Infectious diseases involve intricate interactions among the infecting microbe, human tissue and immune system, and the host microbiome. The co-evolution of humans and microbes has led to the emergence of microbial factors that promote infection and corresponding human

(Continued) If MRSA is a consideration, clindamycin, vancomycin (15 mg/kg q12hb), linezolid (600 mg IV/PO q12h), or TMP-SMX

(1–2 double-strength tablets PO bidg) can be used. 134 and pathogenspecific chapters cellular responses to microbes. Among the microbial factors that promote disease are those that alter human cells, inhibit host immune responses, and respond to the microbiota. The process of infection can be divided into several stages: microbial entry into the human body (colonization), microbial attachment in its favored niche and avoid­ ance of host defenses (infection), deployment of microbial factors and host inflammatory responses that damage human tissue (disease), and microbial release into the environment, where the microbe can infect others (transmission). It is notable that for most pathogens, the host inflammatory response contributes substantially to symptoms and tissue damage. Moreover, the microbiota (the collection of microbes that reside in and on the human body) modulates, directly or indi­ rectly, every stage of infection (Chap. 484). This chapter describes the best-understood molecular and cellular mechanisms that contribute to human disease caused by bacterial pathogens. ENTRY INTO THE HUMAN HOST Infectious diseases occur when a live pathogen enters or a toxic patho­ gen product is ingested by the host. Entry of a live microbe into a host can occur by crossing a tissue barrier, through entry into or penetration of the skin or a mucous membrane of the respiratory, gastrointestinal, or genitourinary tract. If live bacteria spread to the connective tissue surrounding a barrier, an infectious collection may form (e.g., skin or intraabdominal abscess, empyema). Microbial penetration of barrier surfaces may also result in bloodstream infection, which can in turn lead to infection of other organ systems. Infection of the skin and underlying soft tissue (cellulitis) typically occurs when bacteria reach the dermis via a physical disruption of the epidermal barrier. This disruption can result from trauma, fungal infection, pressure ulcers, or venous stasis ulcers. Bacteria that are already associated with the skin are most likely to traverse this barrier defect; therefore, Staphylococcus aureus and group A streptococci are the most common causes of cellulitis. In patients with diabetic foot ulcers, infections with Pseudomonas aeruginosa and anaerobic bacteria are also common. Entry into the respiratory tract occurs via respiratory droplet nuclei (airborne particles 1–5 μm in diameter) or via fomites introduced on a contaminated hand. Infectious droplet nuclei are generated when an individual with a communicable respiratory infection (e.g., tuberculo­ sis, influenza, COVID-19 infection) sneezes, coughs, or talks, or when environmental fluid contaminated with a noncommunicable infec­ tious agent is aerosolized (e.g., Legionnaires’ disease, aspergillosis). A cough may generate 3000 particles, whereas a sneeze may generate up to 40,000 particles; large particles may evaporate down to 0.5 μm, and hygroscopic particles can increase in size as they pass through the moist respiratory tree. Entry into the gastrointestinal tract occurs via ingestion of con­ taminated food or water or via person-to-person contact. Patho­ gens for which the minimum infectious inoculum is large (e.g., 108 organisms for epidemic spread of Vibrio cholerae, 105 organisms for Salmonella enterica serovar Typhimurium) are generally acquired via

contaminated food or water, whereas pathogens for which the mini­ mum infectious inoculum is small (e.g., 101 to 102 organisms for Shigella spp.) are typically acquired by person-to-person spread. Entry into the genitourinary tract occurs via colonization of the urethral meatus or vaginal introitus with fecal organisms followed by ascension of the organisms into the bladder or kidneys or via instru­ mentation. Pyelonephritis can also result from hematogenous seeding of the kidney. ESTABLISHMENT OF INFECTION ■ ■NICHE Live Pathogens  Many bacterial pathogens display tissue tropism; sites of infection are often pathogen-specific and restricted, such that adjacent tissues may be uninvolved. For example, group A streptococci cause pharyngitis and soft tissue infection, but rarely pneumonia. Cholera is an infection of the small intestine, whereas Shigella spp. cause disease only in the rectosigmoid colon. To establish infection, the pathogen must access and then survive within its niche (Table 125-1). In the respiratory tree, the site at which pathogens settle is determined by mode of spread: droplet nuclei reach the bronchial tree or alveoli, whereas fomites reach the pharynx or nasal passages. Pathogens move through the gastrointestinal tract via normal intestinal motility. Tissue association and invasion are dictated by bacterial interac­ tion with host factors, such as glycan-decorated receptors and the extracellular matrix. The environmental conditions of the niche trigger expression of virulence factors required for establishing infection; for example, bile salts in the gut stimulate the expression of V. cholerae adhesins and the germination of Clostridioides difficile spores. Bacteria commonly manipulate their niche environment in ways that facilitate TABLE 125-1  Bacterial Pathogens, Diseases, and Niches MOST COMMON TROPISM BACTERIUM DISEASE Skin, respiratory tract, small intestine Bacillus anthracis Anthrax Respiratory tract Bordetella pertussis Pertussis Systemic Borrelia burgdorferi Lyme disease Systemic Brucella abortus Brucellosis Systemic Burkholderia pseudomallei Melioidosis Eyes, venereal Chlamydia trachomatis Various chlamydioses, including trachoma Colon Clostridioides difficile Colitis Pharynx Corynebacterium diphtheriae Diphtheria Systemic Coxiella burnetii Q fever Colon Enterohemorrhagic Escherichia coli   Stomach Helicobacter pylori Gastritis, gastric ulcers Respiratory tract Legionella pneumophila Legionnaires’ disease Systemic, central nervous system Listeria monocytogenes Listeriosis Respiratory tract Mycobacterium tuberculosis Tuberculosis Urogenital tract Neisseria gonorrhoeae Gonorrhea Respiratory tract Pseudomonas aeruginosa   Systemic Salmonella enterica serovar Typhi Typhoid fever Gastrointestinal tract Salmonella enterica serovar Typhimurium   Colon, rectum Shigella spp. Dysentery, shigellosis Multiple sites Staphylococcus aureus   Soft tissue Group A Streptococcus   Small intestine Vibrio cholerae Cholera Systemic Yersinia pestis Plague

infection. For example, the gastroduodenal pathogen Helicobacter pylori converts urea into ammonia, thereby increasing the pH of the acidic stomach environment, which creates a more hospitable environment.

Preformed Toxins  A small number of diseases are caused by ingestion of preformed bacterial toxins. The most common among these are S. aureus enterotoxins and the Bacillus cereus emetic entero­ toxin. S. aureus enteroxins are strongly emetic and can be present in prepared foods such as dairy, meat, eggs, salads, and produce. B. cereus produces the peptide toxin cereulide, which acts as a potassium ionophore and induces emesis; this toxin is most often found in rice or other starchy food that has been improperly refrigerated. Although these pathogens may be killed when food is cooked, their toxins are heat stable. Less common but important is botulinum toxin, produced by Clostridium botulinum and the cause of botulism. Botulinum toxin blocks neurotransmitter release in motor neurons, inhibiting the cen­ tral nervous system and resulting in a flaccid paralysis that may be complicated by fatal respiratory failure. Although botulinum toxin is heat-labile, C. botulinum spores are heat-stable. ■ ■ATTACHMENT Attachment of bacteria to host tissue surfaces is a prerequisite for the pathogen’s establishment of an infection and is mediated by specific receptor–ligand interactions. The tissue specificity of the host cell surface receptor repertoire is a critical factor in delimiting a patho­ gen’s niche(s). These physical associations additionally facilitate the pathogen’s avoidance of host clearance mechanisms (see “Avoidance of Innate Immune Responses,” below) and may contribute to formation of biofilms by the pathogen (see “Biofilms,” below). Because adhesion to cellular receptors often triggers cellular signal transduction and innate immune signaling, therapeutic blockade of this interaction may in some circumstances exacerbate infection. CHAPTER 125 Adhesins  Bacterial pathogens have evolved a wide range of strate­ gies by which to attach to the diverse host cell structures they encoun­ ter. For many bacterial pathogens, ligands or adhesins for specific host receptors are known. Adhesins comprise a wide variety of surface structures, including single proteins, carbohydrates, glycoproteins, lip­ ids, lipoproteins, and multiprotein filamentous complexes that extend several micrometers from the bacterial surface, each anchored in the outer-surface cell envelope. Most bacteria produce multiple adhesins with varying specificity, enabling the pathogen to interact with mul­ tiple receptors, including those on several distinct cell and tissue types encountered during the process of infection. These interactions are often partially redundant, are serologically variable, and contribute additively or synergistically with other binding interactions. Molecular Mechanisms of Microbial Pathogenesis Common classes of adhesins are pili (also known as fimbriae), flagella, and autotransporter proteins (Table 125-2). Pili are hairlike extensions consisting of a polymer of the major pilin subunit capped with minor pilins that provide the adherence function of the structure. Pili are classified by type and are produced by many gram-negative bac­ teria and a smaller number of gram-positive bacteria. To date, efforts to prevent infection with pilus-based vaccines have been unsuccessful. Types of pili include type I, type P, and type IV. Type I pili frequently function at mucosal surfaces. For example, they mediate the close association of uropathogenic Escherichia coli (UPEC) with bladder epi­ thelial cells and the ability of this pathogen to persist, causing relapsing urinary tract infections. UPEC also produces type P pili and afimbrial adhesins. These adhesins bind sugar moieties on host surface glycopro­ teins, with varied specificity depending on the adhesin. The minor pilin lectins at the tip of the pili generally bind D-mannose glycans, albeit with strain specificity. For example, the type I pilus adhesin FimH of intes­ tinal E. coli strains often preferentially binds oligomannose, whereas the FimH of UPEC strains commonly binds monomannose. Thus, the same pilus structures in different bacterial strains can dictate adher­ ence to distinct tissues. Type IV pili (Tfp) are widespread among gramnegative bacteria, and similar structures exist among gram-positive bacteria. These are evolutionarily related to the type II secretion system (T2SS) and, in addition to mediating adherence, allow the uptake of

TABLE 125-2  Classes of Bacterial Adhesion Proteins and Their Host Receptors ADHESIN EXAMPLE RECEPTOR Type I pili Fim protein, uropathogenic Escherichia coli Terminal mannose of uroplakin N-glycan in urinary epithelial cells Type P pili Pap protein, uropathogenic E. coli Galactose disaccharides Type IV pili Tfp protein, Neisseria gonorrhoeae CD64, CR3, I domain– containing integrins MSCRAMM SdrC protein, Staphylococcus aureus β-Neurexin Opa Opa protein, Neisseria meningitidis CEACAMsa Flagellum FliC protein, Pseudomonas aeruginosa Asialo-GM1 ganglioside Autotransporter Invasin, Yersinia pseudotuberculosis β1-Integrins Autotransporter Ag85, Mycobacterium tuberculosis Fibronectin aCarcinoembryonic antigen–related cell adhesion molecules. DNA into bacteria and the motility of bacteria on surfaces. The Tfp of Neisseria spp. and V. cholerae mediate aggregation of individual bacte­ ria into microcolonies, which promotes colonization. Flagella are polymeric helical filaments that propel bacteria through liquid environments by rotating about their long axis. Because flagella confer the ability to swim toward a target surface, often following a chemotactic gradient where chemical sensing influences the direction of motility, they are vital virulence factors of many pathogenic bacteria. Flagella can be localized to one end of the bacterial cell (polar) or distributed around the bacterial surface (peritrichous). Flagella are evo­ lutionarily related to type III secretion systems (T3SSs) (see “Replicative Niche” and “Survival in the Vacuole,” below) and have been shown in some instances to be responsible for the secretion of bacterial toxins. Flagella may also act as adhesins, binding to mucins of mucosal sur­ faces in the case of the gastrointestinal pathogens enteropathogenic E. coli (EPEC) and enterohemorrhagic E. coli (EHEC). PART 5 Infectious Diseases Autotransporter proteins comprise a subdivision of type V secretion systems (T5SSs), which are prevalent among gram-negative bacteria Effector proteins Host cytosol Host Plasma membrane Outer membrane Peptidoglycan Bacterium Inner membrane Bacterial cytoplasm Type III secretion system Type IV secretion system Type VI secretion system Type V secretion system FIGURE 125-1  Major bacterial secretion systems involved in pathogenesis. Schematic of the types III, IV, V, and VI secretion systems (T3SS, T4SS, T5SS, and T6SS, respectively) of gram-negative bacteria. The T3SS, T4SS, and T6SS deliver bacterial effector proteins into host cells, whereas the T5SS participates in adhesion to the surface of cells. The architecture of the extracellular portion of the T4SS and how it translocates effector proteins remain poorly defined. Colored shapes with hooks in the host plasma membrane represent host membrane proteins that, it is thought, may participate in these processes.

(Fig. 125-1). Extended adhesive projections anchored in the bacte­ rial outer membrane, autotransporter proteins are pivotal virulence determinants for several human pathogens, including the filamentous hemagglutinin of Bordetella pertussis (the etiologic agent of whooping cough) and the IcsA adhesin and intracellular motility determinant of Shigella spp. (responsible for dysenteric diseases). The autotransporter proteins intimin and invasin are required for the intimate associa­ tion of adhering and effacing pathogens, such as EPEC, and invasion by Yersinia spp., respectively. Besides these main classes of adhesins, other bacterial surface proteins involved in adhesion include the Opa family of membrane proteins of Neisseria spp. and the gram-positive cell wall–anchored microbial surface component recognizing adhesive matrix molecules (MSCRAMMs) of S. aureus and various enterococci. Receptors  Carbohydrates (glycans) on the surface of and secreted by human cells play major roles in the adherence of bacterial patho­ gens. The surfaces of human cells are coated with glycoproteins and glycolipids, whereas the extracellular matrix (ECM) scaffold of tissues is mainly composed of secreted proteoglycans. Most mucosal surfaces are covered with a layer of mucus, which consists primarily of mucins, a family of proteins that are heavily glycosylated. Among the human enzymes involved in glycan decoration of secreted proteins is FUT2, for which ~20% of the human population harbors two nonfunctional alleles; the importance of human glycans in infection is highlighted by the observation that individuals who lack functional FUT2 dis­ play increased susceptibility to certain bacterial infections, decreased susceptibility to certain viral infections, and altered susceptibility to chronic noninfectious inflammatory diseases. These data are con­ founded by a relative increase in secretion of sialylated glycans in indi­ viduals lacking functional FUT2. Glycans participate in many adhesive interactions and serve as receptors for certain bacterial toxins. Certain bacteria enzymatically alter host glycans in a manner that enables improved access to the epithelial surface, as in the case of V. cholerae and the oral pathogen Tannerella forsythia. The GM1 ganglioside, a glycolipid, is the receptor for cholera toxin (CTX), Pseudomonas aeruginosa flagella, and the Clostridium per­ fringens α toxin, and contributes to tropism of the gastric pathogen H. pylori through interactions with the bacterial membrane proteins BabA and SabA. Heparan sulfate proteoglycans and other glycos­ aminoglycan-conjugated proteins are commonly associated with the basolateral membranes of epithelial layers and act as ligands for the

chlamydial OmcB protein and the E. coli cytotoxic necrotizing factor toxin; these interactions promote initial bacterial adherence in prox­ imity to the plasma membrane and lead to engagement of additional surface molecules. In mammals, the ECM and integrin proteins are ubiquitous. The ECM consists of laminin, vimentin, and type IV collagen, which inter­ act via fibronectin with integrin receptors in the plasma membrane. Because of direct interaction with plasma membrane–embedded inte­ grins, ECM alterations can result in signal transduction that directly influences immune cell behavior. Many bacterial pathogens engage integrins and the functionally related carcinoembryonic antigen– related cell adhesion molecules (CEACAMs) as host cell receptors and as triggers for their internalization into human cells (see “Mechanisms of Microbial Entry into Cells,” below). The receptor for adherent-invasive E. coli, a pathogenic type of E. coli, is CEACAM6, whose levels are increased on epithelial cells in inflammatory bowel disease; adherentinvasive E. coli adhere at increased levels in Crohn disease, and sites of excessive bacterial adherence display increased inflammation. Among the many other host cell surface proteins that serve as receptors for bacterial adherence is the cystic fibrosis transmembrane conductance regulator (CFTR), a chloride channel involved in the maintenance of adequate hydration of mucosal surfaces. Mutation of the CFTR gene gives rise to the hereditary disease cystic fibrosis. These patients are hypersusceptible to respiratory infection because the cilia are unable to clear viscous mucus from the bronchial epithelial surface. The remarkably high frequency of cystic fibrosis (>2.5%) in Caucasian populations has been attributed to relative resistance to Salmonella enterica serovar Typhi infection, since S. Typhi adheres to CFTR via type IV pili. Epithelial cells utilize CFTR during internalization-mediated clearance of the extracellular pathogen P. aeruginosa, potentially con­ tributing to the decreased ability of cystic fibrosis patients to clear infections by this pathogen. ■ ■REPLICATIVE NICHE Once bacterial pathogens find their niche and associate with target host cells, they must replicate to persist. Some pathogens remain predominantly associated with the surface of epithelial cells (e.g., Bordetella, Pseudomonas, Vibrio, and Clostridium spp.); others pre­ dominantly enter into cells, where they may survive within the cytosol (e.g., Shigella, Francisella, and Listeria spp.) or within a membranebound compartment (vacuole) comprised of host-derived lipids (e.g., Salmonella, Legionella pneumophila). Phagocytic cells actively engulf pathogens for destruction (phagocytosis); to remain associated with the cell surface, extracellular bacteria must avoid or inhibit phagocytosis. Many pathogens deliver cytoskeleton-disrupting proteins that perturb the phagocytic process. Others create structures, such as P. aeruginosa biofilms and streptococcal cell chains, that impede phagocytosis. A subpopulation of any infecting pathogen will nevertheless be phago­ cytosed by macrophages, neutrophils, and/or dendritic cells (phago­ cytes). Many pathogens have evolved mechanisms to survive within phagocytes; for example, during colonization of the nasopharynx, Streptococcus pneumoniae survives within vacuoles in dendritic cells and macrophages. Nonphagocytic cells, such as epithelial cells, may also internalize bacteria in processes triggered by either the host cell (as a clearance mechanism) or the pathogen (for tissue invasion). Extracellular Pathogens  Extracellular pathogens replicate at the surface of host cells. Many secrete enzymes that liberate nutrients and other factors that fashion a hospitable niche. For example, V. cholerae secretes the mucinase vibriolysin that degrades the mucus barrier overlying intestinal epithelial cells, providing bacterial access to the cell surface. When phagocytosed, V. cholerae inhibits further phagocytosis of extracellular bacteria. Yersinia spp., which associates with the surface of leukocytes, delivers into these cells bacterial proteins that inhibit phagocytosis. EPEC and EHEC efface the microvilli of the brush border of intes­ tinal epithelial cells and adhere to the surface of these cells. These bac­ teria induce formation of actin-rich pedestals at the plasma membrane that inhibit bacterial internalization. Tight junctions mediate intimate

associations between neighboring epithelial cells, thereby maintaining tissue barrier function; the dissolution of tight junctions by pathogens facilitates bacterial penetration into the tissue.

BIOFILMS  Some extracellular pathogens, including S. aureus and P. aeruginosa, establish chronic infections by producing extracellular polymeric matrices called biofilms, which encase the bacteria. Biofilms commonly develop where tissue integrity has been compromised, such as in burn wounds and on prosthetic material. The biofilm matrix is composed of extracellular polysaccharides and DNA, to which the bac­ teria adhere. Biofilms protect bacteria from phagocytosis while impair­ ing the diffusion of antibodies and antibiotics, enabling the bacteria to avoid elimination and persist. Mechanisms of Microbial Entry into Cells  Bacterial patho­ gens that are predominantly intracellular during infection can induce internalization into host cells. Even pathogens that replicate within normally phagocytic cells such as macrophages (e.g., Salmonella spp., L. monocytogenes) possess mechanisms for inducing internalization into these cells. TRIGGER MECHANISM  Bacterial systems that deliver proteins into human cells are critical to many aspects of pathogenesis (Fig. 125-1). An important example is the type III secretion system (T3SS), which delivers proteins from the bacterial cytoplasm directly into the cytosol of eukaryotic cells. Evolutionarily conserved T3SSs are found in many gram-negative bacterial pathogens. Delivery of bacterial proteins (effector proteins) into the host cell can have dramatic effects on host cell function; for many pathogens, these effects include induction of bacterial uptake into the host cell and alteration of cellular processes in ways that promote infection. CHAPTER 125 The T3SS forms an apparatus on the bacterial surface that resembles a needle and syringe, with a channel down its long axis (Fig. 125-1). The base of the organelle spans the two bacterial membranes. Anchored to the organelle base and protruding from the bacterial surface is a long needle-like structure. Upon contact of the needle tip with the eukary­ otic plasma membrane, two proteins secreted through the channel form a pore in the cell’s plasma membrane. The tip of the needle docks onto the extracellular face of the pore, forming a continuous conduit between the bacterial cytoplasm, where proteins are synthesized, and the host cell cytosol. Bacterial effector proteins are then delivered into the cell through this conduit. Molecular Mechanisms of Microbial Pathogenesis For those pathogens that use a T3SS to induce entry into human cells (Salmonella, Shigella, and Chlamydia spp.), several of the first effector proteins to be translocated activate actin polymerization immediately beneath the point where the bacterium is docked on the plasma membrane. Polymerized actin pushes outwardly against the plasma membrane, generating large membrane ruffles that engulf the bacterium into a membrane-bound vacuole (Fig. 125-2). Shigella spp. escape the vacuole and reside in the host cell cytosol, whereas Salmonella and Chlamydia spp. predominantly reside within the vacuole. ZIPPER MECHANISM  Several invasive pathogens enter host cells using a zipper-like mechanism that does not require a T3SS. In these instances, bacterial surface molecules engage with and stimulate the clustering of host cell receptors, which form close associations with the bacterium in a processive fashion until the plasma membrane is tightly opposed to and surrounding the bacterium (Fig. 125-2). For example, Y. pseudotuberculosis binds β1-integrins; this binding stimulates phos­ phoinositide 3-kinase (PI3K) and protein kinase B (Akt) signaling pathways, leading to zipper-mediated entry. L. monocytogenes engages E-cadherin and the Met receptor, activating β-catenin signaling and zipper-mediated entry. Subpopulations of bacteria classically consid­ ered extracellular, such as P. aeruginosa and S. aureus, may also pro­ mote bacterial uptake in a similar fashion. ENTRY OF CYTOSOLIC BACTERIA INTO NEIGHBORING CELLS  Bacteria are initially taken up into a vacuole (nonphagocytic cells) or a phago­ some (phagocytic cells). Yet for some pathogens, residence in the vacu­ ole or phagosome is transient; the lifestyles of these pathogens include residence in the cytosol, actin-based motility, and cell-to-cell spread. L. monocytogenes, Shigella spp., spotted fever group Rickettsia spp. (the

Bacterial effector proteins Pathogencontaining vacuoles Trigger Zipper FIGURE 125-2  Common mechanisms of bacterial invasion. The mechanisms for internalization of bacteria into nonphagocytic cells are typically classified as trigger or zipper mechanisms. As examples of the trigger mechanism, Shigella and Salmonella spp. use their T3SSs to deliver into host cells effector proteins that manipulate the cytoskeleton in ways leading to the formation of cytoskeleton-supported membrane ruffles. These membrane ruffles extend and surround the pathogen, with consequent endocytosis of the bacterium. As examples of the zipper mechanism, bacterial membrane proteins of Yersinia and Listeria spp. induce clustering of host receptors. Clustering and subsequent intracellular signaling result in the uptake of the bacterium in a tightly opposed vacuole. etiologic agents of Rocky Mountain spotted fever, rickettsialpox, and other spotted fever diseases), and Burkholderia pseudomallei (the etio­ logic agent of melioidosis) rapidly lyse the vacuole. L. monocytogenes produces a pore-forming toxin, listeriolysin O, and phospholipases that mediate escape into the cytosol, whereas the mechanism of vacu­ olar escape of Shigella spp. is still uncertain except for a requirement of its T3SS. PART 5 Infectious Diseases Once they have escaped into the cytosol, L. monocytogenes, Shigella spp., B. pseudomallei, and spotted fever group Rickettsia spp. spread into adjacent cells via actin-based motility. These organisms recruit actin-polymerizing host factors that drive actin polymerization at one end of the bacterium, propelling the microbe through the cell cytosol. Actin-based motility pushes bacteria into host cell membranes to form protrusions that are taken up by the neighboring cell, resolving into a double-membrane vacuole containing the bacterium. This vacuole is lysed once more, releasing bacteria into the cytosol to begin another round of replication. Subversion of the host cytoskeleton aids bacterial cytosolic survival and promotes dissemination through epithelial tis­ sues. Burkholderia spp. instead trigger the infected host cell to fuse its membranes with adjacent cells in a T6SS-dependent manner, forming a multinucleated giant cell with contiguous cytoplasm. SURVIVAL IN THE VACUOLE After uptake into cells, the majority of invasive bacterial pathogens remain in a vacuole. Uptake vacuoles normally enter the endosomal pathway, acquiring host proteins that promote vesicle maturation, acidification, and degradation of contents. Many bacterial pathogens contained in these endosomes subvert this intracellular trafficking in ways that block endosomal maturation and lysosomal degradation of the pathogen. A distinct subset of bacteria actively damage vacuolar membrane integrity, thereby escaping into the cytosol, where they replicate. For bacteria that remain in the vacuole, various bacterial effector proteins are delivered by secretion systems either into the vacuolar space or across the vacuolar membrane into the cytosol

(Fig. 125-1), where these proteins manipulate host processes to the benefit of the pathogen. ■ ■TYPE III SECRETION SYSTEMS T3SSs (Fig. 125-1) are versatile virulence systems, helping bacteria such as P. aeruginosa remain extracellular, but promoting both the uptake and the intracellular survival of the intracellular pathogens S. enterica serovar Typhimurium, Chlamydia spp., and Shigella spp.

S. enterica serovar Typhimurium uses distinct T3SSs for the invasion and subsequent maintenance of the Salmonella-containing vacuole (SCV), whereas Chlamydia and Shigella each encode only one such system. The effector proteins delivered are the major determinants of the lifestyles

of these pathogens; Shigella quickly escapes the vacuole in a T3SS-dependent manner, whereas Chlamydia remains in the vacu­ olar compartment (the inclusion) and uses effector proteins to hijack cellular traffick­ ing and perturb innate immune responses. The SCV and the Chlamydia inclusion exhibit distinct traits: the SCV is associated with thin membranous tubules that aid nutrient acquisition, whereas the inclusion is localized at the microtubule-organiz­ ing center, which is thought to facilitate recruitment of vesicles. ■ ■TYPE IV SECRETION SYSTEMS For other pathogens, type IV secretion systems (T4SSs; Fig. 125-1)—conceptually similar, evolutionarily distinct effector protein delivery systems—are key to the ability to survive in cellular phagocytic vacuoles. These T4SSs are similar to the T3SSs described above in that they form a multiprotein apparatus that contains a continuous channel between the bacterial cytoplasm and the eukaryotic cell cytosol. However, T4SSs are functionally more diverse than T3SSs, in that (1) they are present among both gram-negative and gram-positive bacteria as well as some archaea, (2) a large subset of T4SSs transport DNA in a process called conjugation, and (3) some T4SSs deliver proteins, typically toxins, into bacterial cells rather than eukaryotic cells. Not surprisingly, T4SSs also display great structural diversity. For the human pathogens L. pneumophila and Coxiella burnetii, bacterial effector proteins delivered across the vacuolar membrane into the cell cytosol by a T4SS alter maturation of the vacuole in a manner that makes it hospitable for bacterial survival; the resulting vacuole is known as a Legionella-containing or Coxiella-containing vacuole (LCV or CCV, respectively). The delivered effector proteins block fusion of lysosomes with the vacuole, thereby preventing bacterial degradation by lysosomal enzymes; manipulate host vesicular trafficking pathways; and remodel intracellular membranes to alter the lipid and protein content of the vacuolar membrane. For example, the L. pneumophila phospholipase VipD, a T4SS substrate, reduces the levels of phosphati­ dylinositol 3-phosphate on the LCV, thereby preventing recruitment of the host protein Rab5 GTPase, which is involved in endosomal matura­ tion. The mature LCV membrane contains many features that resemble cellular endoplasmic reticulum. In the case of C. burnetii, the vacuole displays more lysosome-like characteristics, including a relatively low pH, which induces the activation of T4SS effector delivery. The deliv­ ered bacterial effectors stimulate efficient vesicle recruitment from endosomal and autophagosomal (see “Autophagy,” below) networks, massively increasing the membrane of the CCV until it occupies much of the cytosol. For both L. pneumophila and C. burnetii, the mature bacterium-containing vacuoles are hospitable to bacterial replication and long-term survival. ■ ■OTHER SECRETION SYSTEMS Francisella tularensis, the etiologic agent of the zoonotic infection tularemia, is a gram-negative facultative intracellular bacterium that displays a tropism for macrophages. F. tularensis resides in a phago­ some, the maturation of which it delays by remodeling the lipid content of the vacuolar membrane through the action of its type VI secretion system (T6SS; Fig. 125-1). F. tularensis then avoids destruction by lysosomal enzymes by escaping into the cytosol, where it replicates. Another pathogen that escapes into the cytosol of macrophages is the respiratory pathogen Mycobacterium tuberculosis. Here, the ESX-1 type VII secretion system (T7SS) is required for lysis of the phago­ somal membrane. For both of these pathogens, few effectors of their cognizant secretion system have been characterized, and the process of phagosomal membrane lysis remains poorly understood.

■ ■NUTRIENT ACQUISITION Regardless of the site of bacterial infection, microbes must acquire nutrients to replicate. Both extracellular and vacuolar pathogens must circumvent the nutrient-poor conditions of these sites. Also impacting extracellular bacteria is the microbiota, which contributes to nutritional immunity through competition for resources (Chap. 484). Bacterial pathogens possess an arsenal of nutrient acquisition mechanisms to overcome these challenges. For example, upon oxidative stress, to maximize nutrient uptake, several pathogens secrete metal ion-binding proteins through T4SSs (Fig. 125-1) and upregulate expression of cell surface receptors. Many bacteria manipulate host restriction of nutrients, with intracellular bacteria commonly delivering factors that inhibit host mRNA translation to increase the available pool of amino acids. Pathogens within vacuoles have evolved methods of redirecting host nutrient transport to these vacuoles (see “Cellular Trafficking,” below). ■ ■CELLULAR TRAFFICKING Because the vacuolar environment is poor in nutrients, bacteria that survive in vacuoles have evolved intricate strategies for the manipu­ lation of host processes to meet their metabolic needs. Vacuolar pathogens modulate endosomal trafficking to prevent degradation by lysosomal compartments. Whereas cytoskeletal rearrangements are regulated by small GTPases of the Rho family, the transport of vesicles through the cell cytosol is controlled by Rab and Arf GTPases. The coevolution of vacuole-residing bacteria with their eukaryotic hosts has promoted the acquisition of eukaryotic-like domains in many secreted bacterial effector proteins. Many of these effector proteins mimic host cellular trafficking proteins such as guanine nucleotide exchange fac­ tors (GEFs), which regulate GTPase activation, and soluble N-ethylma­ leimide-sensitive factor attachment protein receptors (SNAREs), which control vesicle fusion with vacuolar and other membranes. These bac­ terial effectors cause the accumulation of host protein and lipid mark­ ers on vacuolar membranes, thereby causing the vacuoles to resemble other host compartments. Consequently, host vesicles are redirected from the endoplasmic reticulum, Golgi apparatus, and secretory path­ ways to the pathogen-containing vacuole, thus delivering nutrients to the pathogen. Chlamydia spp., which are obligate intracellular bacteria, acquire host-derived lipids in the bacterial membranes. Dissection of the modes of action of bacterial effectors has revealed novel biochemi­ cal modifications of host trafficking regulators. For example, the L. pneumophila T4SS secretes two effector proteins that inactivate the cellular GTPase Rab1: AnkX inactivates Rab1 by transferring to it the phospholipid phosphatidylcholine, and SidM modifies Rab1 by adding an adenosine monophosphate molecule to it. This bacterium delivers

300 effector proteins through its T4SS; some of these proteins (metaeffectors) modulate the activity of other effectors rather than host factors. The coordinated activity of the numerous effector proteins provides precise spatiotemporal control of host processes. AVOIDANCE OF INNATE IMMUNE RESPONSES For the host to respond to and clear an infection, it must first be capa­ ble of sensing it. The route of infection and properties of the pathogen contribute to determining the nature of the immune response. The human immune system is generally considered to consist of two arms: the innate immune system, initiated by germline-encoded sensors, and the adaptive immune system, where lymphocytes are clonally selected in response to specific antigens. Initial interactions of pathogens are with specific host cell types, influenced by bacterial tropism; the result­ ing release of danger signals and cytokines from the innate immune response shapes the later adaptive immune response. Epithelia (e.g., in the intestinal and respiratory tracts) are commonly the first tissues that a pathogen encounters. The mucosal surfaces of these epithelia are covered in a gel-like mucus layer that contains secretory immunoglobulins, antimicrobial peptides, and commensal bacteria, which offer the first line of defense that infecting pathogens must overcome (Chap. 484). In the gastrointestinal tract, this barrier can be overcome through hijacking of microfold cells (M cells) in the

follicle-associated epithelia. M cells sample antigens, including intact pathogenic bacteria, from the gut lumen and deliver them to the underlying gut-associated lymphoid tissue in a process known as trans­ cytosis. Within the gut-associated lymphoid tissue, dendritic cells, mac­ rophages, and neutrophils engulf transcytosed material for destruction. Lysosomal degradation of pathogenic bacteria in macrophages and dendritic cells can then lead to antigen presentation, where the hydro­ lytic products are processed for display by the major histocompatibility complexes (MHCs). These antigen-presenting cells migrate through the lymphatic system to lymph nodes, where the adaptive immune response is stimulated through activation of B-cell and T-cell clones.

■ ■COMPLEMENT Once bacteria have crossed epithelial barriers into the lamina propria, they encounter components of humoral immunity. The complement system is a complex of plasma and tissue-resident proteins that can undergo activation from their pre-protein or zymogen forms to either label pathogens for phagocytosis (opsonization) or lyse them directly (Fig. 125-3). Complement activation commences with detection of the pathogen through the binding of circulating antibodies, which are recognized by the C1 complex (classical pathway), or of lectins (lectin pathway) to surface carbohydrates. These pathways converge at the deposition of the C3 convertase complex on the bacterium. The alter­ native pathway is the third route of complement activation, whereby an alternative C3 convertase complex is formed either spontaneously or by the action of plasma factors, promoting amplification of the cascade. Activation of further complement components results in chemokine production, bacterial opsonization, or assembly of the membrane attack complex pore on the bacterial surface, which lyses the pathogen. Bacterial virulence factors have been described that inhibit each step of the complement pathway, highlighting the importance of this anti­ microbial system to host immunity. The various strategies of bacterial interference include prevention of initial recognition by assembly of a polysaccharide capsule, modification of bacterial lipopolysaccha­ ride (LPS), degradation of complement components, and masking of surfaces with host proteins. Streptococci employ numerous mecha­ nisms for complement evasion. The streptococcal proteases ScpA and SpeB degrade many complement proteins, whereas the bacterial surface-exposed M proteins bind the complement inhibitor C4BP, thereby preventing C3b deposition. The thick peptidoglycan layer of gram-positive bacteria provides modest resistance to membrane attack complex–mediated lysis. Furthermore, streptococci produce a hyaluronic capsule that shields the bacterial surface from complement recognition. In comparison, Borrelia burgdorferi, the etiologic agent of Lyme disease, encodes a CD59-like protein that inhibits the comple­ tion of membrane attack complex assembly. Whereas many pathogens inhibit opsonization, thereby avoiding phagocytosis, other bacteria, including Francisella and Yersinia spp., promote this process, leading to uptake into the phagosomal compartment, wherein they replicate and prevent lysosomal fusion. CHAPTER 125 Molecular Mechanisms of Microbial Pathogenesis ■ ■LYSOSOMES Lysosomes are vesicular organelles of the endosomal system that con­ tain hydrolytic and antimicrobial enzymes within an acidic environ­ ment. These enzymes are crucial for recycling of cellular material but also participate in direct killing of microbes and initiation of antibac­ terial immunity. Lysozyme is a lysosomal enzyme that hydrolyzes the polymeric glycan chains in peptidoglycan, resulting in bacterial lysis. Liberated peptidoglycan fragments may then stimulate innate immune receptors to drive antibacterial immunity. Lysozyme is highly effec­ tive against most gram-positive bacteria; however, the peptidoglycan layer of gram-negative bacteria lies between the bacterial outer and inner membranes, making it inaccessible to lysozyme. Gram-positive bacteria can decrease their susceptibility to lysozyme with peptidogly­ can modifications that block access of lysozyme to the glycan chain, such as acetylation of sugars in the glycan strand. Metabolites present within the lysosome may also contribute to immunity; for example, the mitochondria-derived metabolite itaconate accumulates within lysosomes during infection of macrophages with S. enterica serovar Tymphimurium, where it limits bacterial growth.

C1 complex Classical pathway Lectin pathway C3 convertase Streptococcus pyogenes C5 convertase C3 cleavage Neisseria gonorrhoeae Membrane attack complex PART 5 Infectious Diseases Borrelia burgdorferi FIGURE 125-3  Overview of the complement system in bacterial infection. The classical and lectin pathways of complement activation are initiated by the binding of the C1 complex to antibodies bound to bacteria or to lectins binding carbohydrate moieties on the bacterial surface, respectively. A cascade of proteolytic cleavage generates the C3 convertase complex on the bacterial surface, which, upon activation, leads to C5 convertase formation. In the alternative pathway, spontaneous C3 cleavage forms alternative C3 products that lead to bacterial opsonization and to amplification of C3 convertase activation. The three complement pathways converge at the point of C3 convertase activation. In a manner similar to that seen for bacteria opsonized with circulating immunoglobulin, opsonized bacteria bind to surface receptors of phagocytes, triggering their engulfment for destruction and stimulation of the adaptive immune system. The C5 convertase, generated by conventional C3 convertase activation, recruits further complement proteins to form the membrane attack complex that directly lyses bacteria. Many bacterial pathogens inhibit specific points in the complement cascade, a selection of which are shown with blunt-ended arrows; in contrast, others, such as Francisella spp., promote their own opsonization and subsequent phagocytosis, enabling them to reach their intracellular replicative niche. ■ ■ANTIMICROBIAL PEPTIDES Antimicrobial peptides, such as the positively charged defensins and cathelicidins, are concentrated in degradative compartments. These molecules bind the anionic surface of bacteria and perturb the integ­ rity of LPS of the gram-negative outer membrane and wall teichoic acids in the gram-positive cell wall. Many bacterial pathogens modify their surfaces to reduce the net negative charge, thereby decreasing the binding of positively charged antimicrobial peptides. S. aureus and the respiratory pathogen Burkholderia cenocepacia secrete proteases that cleave antimicrobial peptides, facilitating survival at mucosal surfaces and within phagosomes. ■ ■OXIDATIVE BURST Pathogens are exposed to reactive oxygen species and reactive nitrogen species produced by cells responding to infection. A well-characterized example is the neutrophil oxidative burst, in which the membrane com­ plex nicotinamide adenine dinucleotide phosphate oxidase (NADPH oxidase) produces superoxide radicals through the transfer of electrons to molecular oxygen. NADPH oxidase activation at the site of infec­ tion generates reactive oxygen species in the phagosome of neutrophils and/or in the extracellular space. Bacterial detoxification enzymes,

Pseudomonas aeruginosa Alternative pathway Immunoglobulins Opsonization Francisella tularensis Phagocytosis Lysis including superoxide dismutases, peroxiredoxins, and catalases, enable bacteria to survive this oxidative burst. Many bacteria can also block the activity of NADPH oxidase; several pathogens prevent assembly of the NADPH oxidase complex by T3SS effector protein–mediated posttranslational modification of small GTPases. The importance of NADPH oxidase in immune defense is highlighted clinically by the marked susceptibility to certain bacterial and fungal infections in patients with chronic granulomatous disease, which results from non­ functional alleles for essential subunits of this complex. ■ ■NEUTROPHIL EXTRACELLULAR TRAPS The oxidative burst also stimulates the release of the proinflammatory tumor necrosis factor α, azurophilic granules, and neutrophil extracel­ lular traps (NETs). NETs are an extruded mesh of decondensed chro­ matin that are thought to ensnare pathogens, thereby restricting their spread and destroying them. Streptococci encode deoxyribonucleases (DNases) and S. aureus nuclease degrades NETs. S. aureus then con­ verts the nucleotide products of this degradation into deoxyadenosine, which functions as a potent macrophage toxin. This process showcases how bacterial pathogens may utilize antimicrobial responses to their own benefit.

■ ■PATTERN RECOGNITION RECEPTORS, GUARD PROTEINS, AND THEIR EVASION The first line of detection of pathogens by the innate immune system is via recognition of molecular patterns that are indicative of the pres­ ence of pathogens, termed pathogen-associated molecular patterns (PAMPs). Host proteins referred to as pattern-recognition receptors (PRRs) recognize PAMPs, enabling cells to establish that an infection is occurring. Membrane-associated PRRs surveil the extracellular or endosomal milieu, whereas cytosolic PRRs monitor for threats within the cytosol. Cytosolic ligands tend to stimulate stronger proinflamma­ tory responses than extracellular ligands, and detection of bacteriaderived PAMPs tends to elicit a more powerful inflammatory response than detection of virus-derived PAMPs. Thus, both the subcellular localization and ligand specificity of PRRs play important roles in dic­ tating the character and amplitude of an immune response. Another class of innate immune proteins, termed guards, can also initiate innate immune signaling in a process referred to as effector-triggered immunity. In contrast to the direct binding of PRRs to PAMPs, guards become activated upon sensing activities or cellular changes associated with infection. TOLL-LIKE RECEPTOR DETECTION OF EXTRACELLULAR AND PHAGOSOMAL LIGANDS The most extensively studied PRRs for bacterial PAMPs are Toll-like receptors (TLRs), which are membrane-associated. On phagocytic cells, TLRs may employ scavenger receptors as co-stimulatory surface recep­ tors. Different TLRs recognize distinct conserved molecules that include LPS, peptidoglycan, flagellin, and nucleic acids (Table 125-3). Upon binding their cognate ligands, TLRs trigger assembly of supramolecular organizing centers, large protein complexes that are signaling platforms for signal transduction. Ultimately, TLR signaling results in nuclear translocation of transcription factors, including nuclear factor-κB (NF-κB), AP-1, and interferon-regulatory factors (IRFs), which induce expression of cytokines, chemokines, and other immunity-related genes (Fig. 125-4). ALTERATION OF PAMPS TO PREVENT IMMUNE RECOGNITION  Patho­ gens have evolved a plethora of strategies to avoid immunity; one com­ mon strategy is to mask PAMPs. For example, TLR4 binds efficiently to LPS that contains hexa-acylated lipid A. Y. pestis, the etiologic agent of plague, produces hexa-acylated LPS in the flea vector, but upon transmission into a human host switches to production of tetraacylated LPS, which does not activate TLR4. Tetra-acylated LPS is also synthesized by F. tularensis, H. pylori, and the periodontal pathogen Porphyromonas gingivalis. Bacteria can also mask immune recognition of flagella. The flagel­ lum of H. pylori facilitates bacterial access to the gastric epithelium and is essential for virulence. Flagellin, the monomeric protein subunit of the flagellum, is a ligand for TLR5, but H. pylori bypasses immune recognition by producing a flagellin epitope that does not bind TLR5. CpG motifs in mammalian DNA are usually methylated, but CpG TABLE 125-3  Pattern Recognition Receptors of the Innate Immune System and Their Ligands PATTERN RECOGNITION RECEPTOR LIGAND OR MODE OF ACTIVATION TLR2 (with TLR1 or TLR6) Lipoproteins TLR4 LPS TLR5 Flagellin TLR9 CpG DNA NLRP1 Enzymatic cleavage NLRP3 Ionic flux, mitochondrial damage NLRP6 Lipoteichoic acid, RNA NAIPs/NLRC4 Flagellin/T3SS STING Cyclic dinucleotides NOD1 and NOD2 Peptidoglycan

motifs in bacterial DNA are not; thus, methylation state provides a method of discriminating self from nonself. TLR9 recognizes CpGcontaining unmethylated DNA in intracellular membrane compart­ ments of the endosomal system. Group A streptococci secrete the Sda1 DNase that degrades CpG DNA, thereby reducing the immunogenicity of lysed bacteria.

INHIBITION OF NF-kB AND MAPK SIGNALING  NF-κB signaling plays a central role in controlling both antibacterial and steady-state immu­ nity. A plethora of bacterial effector proteins target components of the NF-κB pathway in ways that maintain repression of NF-κB-dependent proinflammatory genes, thereby counteracting activation of innate immune responses triggered by their conserved PAMPs. Many bacteria produce virulence factors with eukaryotic-like domains that enable pathogens to hijack host signal transduction, including the NF-κB pathway. Ubiquitination, a posttranslational modification in which a chain of the small protein ubiquitin is cova­ lently linked to a target protein, is a common bacterial mechanism of host signaling inhibition. Ubiquitin is added to a target protein by an E3 ubiquitin ligase, which transfers ubiquitin from an E2 ubiquitinconjugating enzyme to the target protein. Depending on the precise structure of the ubiquitin chain, the modified protein is subsequently degraded by the proteasome, targeted to particular vesicular compart­ ments, or recruits adaptor proteins that initiate signal transduction. The translocation of NF-κB into the nucleus requires the succes­ sive phosphorylation, ubiquitination, and degradation of IκBα by the IκB kinase (IKK) complex, which in resting cells inhibits NF-κB; the dissociation of the IKK complex from NF-κB enables the latter to translocate into the nucleus, where it functions as a transcription factor (Fig. 125-4C). The T3SS of Shigella spp. delivers into cells sev­ eral E3 ubiquitin ligases that target components of the NF-κB pathway for degradation. The intracellular pathogens C. trachomatis and B. pseudomallei secrete bacterial deubiquitinases through T3SS and T2SS, respectively, which cleave the ubiquitin moiety from IκBα, thereby pre­ venting its degradation, maintaining inhibited NF-κB in the cytosol, and repressing proinflammatory gene expression. CHAPTER 125 Molecular Mechanisms of Microbial Pathogenesis Other enzymatic activities of bacterial effectors can also inhibit NF-κB activation by ubiquitin-dependent or -independent processes. Shigella spp. and L. pneumophila deliver effectors via their T3SS and T4SS, respectively, that inactivate E2 enzymes to prevent modification and degradation of the IKK complex. EPEC and Shigella spp. release cysteine methyltransferases that inhibit IKK modification, whereas S. enterica serovar Typhimurium targets NF-κB directly for degrada­ tion via T3SS effectors with metalloprotease activity. Another cellular pathway of pro-inflammatory signaling stimu­ lated by PRRs is the mitogen-activated protein kinase (MAPK) phos­ phorylation cascade, which activates transcription factor AP-1. The B. anthracis metalloprotease exotoxin lethal factor ablates this kinase cascade by cleaving the N-terminal region of MAPKs. Homologous T3SS effectors of Yersinia spp., S. enterica serovar Typhimurium, and V. parahaemolyticus acetylate MAPKs, preventing their phosphorylation. Finally, Shigella spp. and S. enterica serovar Typhimurium produce phosphothreonine lyases that cleave phosphorylated threonine resi­ dues from MAPKs, preventing their activities. The broad range of effector protein mechanisms that bacteria employ to subvert TLR and MAPK signaling underscores the criti­ cal roles of these pathways in preventing pathogens from establishing infection (Fig. 125-4). PRR SENSING OF CYTOSOLIC INFECTION  Whereas the extracellular environment is surveilled for signs of infection by TLRs, the cytosol is monitored by other PRRs. Among the signaling pathways activated upon recognition of intracellular bacterial pathogens are the cytosolic PRRs NOD1 and NOD2, which activate NF-κB signaling upon binding modified peptidoglycan fragments. These PRRs are critical for defense against numerous bacterial pathogens, yet polymorphisms in the NOD1 and NOD2 genes confer susceptibility to inflammatory bowel disease. Another important cytosolic (and occasionally nuclear) innate immune surveillance system is the cyclic guanosine monophosphate–adenosine monophosphate (cGAMP) synthase-stimulator of interferon genes

Flagellin GBPs Innate immunity mechanisms Degradation Ubiquitination SMOC assembly A Immune GTPases K+ Nutrient depletion Vacuole rupture Inflammasome activation Cytosolic PAMPs / DAMPs Phagophore formation Autophagosome formation Degradation Lysosome PART 5 Infectious Diseases Autophagy Inflammasomes B C FIGURE 125-4  Overview of innate immune recognition and response to bacterial pathogens. A. Immune GTPases. Proteins of the GBP family associate with the bacterial surface, LPS shed from the bacterial surface, or pathogen-containing vacuoles, disrupting the integrity of the pathogen and leading to bacterial lysis. Alternatively, labeling of intracellular pathogens with immune GTPases may stimulate ubiquitination of the bacterium and its destruction by cell-autonomous immune mechanisms such as autophagy. B. Autophagy. Recognition of bacterial invasion leads to host-mediated sequestration of the pathogen in an autophagosome and destruction of the pathogen upon fusion of the autophagosome with lysosomes. Bacteria may be directly labeled by ubiquitination, damaged vacuoles are recognized by the binding of galectins to exposed glycans, and/or the cellular autophagy may be induced in response to sensing starvation. C. Pattern recognition receptors and innate immune signaling. Conserved PAMPs, DAMPs, and danger signals are recognized by all cells, triggering innate immune signaling pathways and proinflammatory gene expression. TLRs surveil the cell surface and endosomal interior for PAMPs that include LPS, nucleic acids, and flagellin. Cytosolic PRRs and guards, including NLRs and ALRs, recognize similar ligands as well as distinct stimuli, including dysregulation of ion homeostasis. TLR activation promotes supramolecular organizing center assembly and NF-κB signaling. Under resting conditions, NF-κB is sequestered in the cytosol by IκBα, but TLR activation leads to IκBα phosphorylation, ubiquitination, and degradation, with the consequent release of NF-κB. Resultant translocation of NF-κB into the nucleus leads to pro-inflammatory gene expression and the production and release of cytokines. Stimulation of cytosolic PRRs and guards may lead to assembly of supramolecular organizing centers known as inflammasomes, resulting in activation of inflammatory caspases. A well-characterized example is the activation of caspase-1 by canonical and noncanonical inflammasomes, subsequent activation of the pore-forming protein GSDMD, maturation and release of proinflammatory cytokines IL-1β and IL-18, and pyroptotic death of the cell. (cGAS-STING) pathway. In response to recognition of cytosolic DNA, cGAS synthesizes the second messenger cGAMP. Upon bind­ ing cGAMP, STING activates interferon regulatory factor 3 (IRF3) and NF-κB, which subsequently promote transcription of interferonregulated proinflammatory genes. The products of these genes include cytokines that alert bystander cells and recruit immune cells from the circulation, creating a heightened immune state in the local area. INFLAMMASOMES  Some cytosolic PRRs, including members of the NOD- and AIM2-like receptor (NLR and ALR, respectively) families, can seed formation of supramolecular organizing centers known as inflammasomes. Upon binding to a PAMP, these PRRs oligomerize and recruit the zymogen procaspase-1; some PRRs recruit procaspase-1 directly, whereas others bind the adaptor protein ASC, which then recruits procaspase-1. Upon its recruitment, procaspase-1 undergoes autocatalytic cleavage, thereby converting into active caspase-1 and completing formation of an active inflammasome. PRRs capable of triggering inflammasome assembly include NAIP-NLRC4, AIM2, and NLRP6. Select guard proteins can also trigger inflammasome assem­ bly, including NLRP3, Pyrin, and NLRP1. In addition, caspase-4 and -5 (and their murine homolog caspase-11) serve as PRRs that drive assembly of the “non-canonical” NLRP3 inflammasome (see below). The importance of inflammasome signaling is underscored by a pleth­ ora of polymorphisms within inflammasome-associated genes that predispose to autoinflammatory disorders or susceptibility to infection.

LPS Nucleic acids Peptidoglycan Extracellular PAMPs TLRs IκBα degradation NF-κB Nucleus Pro-inflammatory gene expression Cytokine release Gasdermin D pore formation IL-1β IL-1β Caspase-1 activation Pyroptosis Upon their activation within an inflammasome, caspases drive immunity by cleaving inflammasome effector proteins. The bestdescribed inflammasome effectors are gasdermin D (GSDMD) and IL-1 family cytokines, including interleukin-1β (IL-1β) and IL-18. Cleavage of GSDMD liberates its N-terminal domain, which forms pores in the plasma membrane; cleavage of the zymogen forms of IL-1 family cytokines converts them into mature, active cytokines, which are released through GSDMD pores and bind the IL-1 receptor, in turn driving robust inflammation (Fig. 125-4). The formation of GSDMD pores in the plasma membrane is often associated with cell death, termed pyroptosis (inflammatory cell death). Inflammasome-seeding PRRs are activated by binding to PAMPs. NAIPs sense the needle protein of the T3SS or the related flagel­ lin protein and subsequently recruit NLRC4 to seed inflammasome formation. Many pathogens have evolved mechanisms to overcome activation of the NAIP-NLRC4 inflammasome. In L. pneumophila, L. monocytogenes, P. aeruginosa, and S. enterica serovar Typhimurium, flagellin expression is repressed during infection; similarly, in S. enterica serovar Typhimurium, expression of an immunogenic T3SS is repressed and that of a less immunogenic T3SS is upregulated.

S. enterica serovar Typhimurium also produces a T3SS effector that blunts NLRC4 expression. AIM2 is activated upon binding cytosolic double-stranded DNA, which is released during lysis of cytosolic bac­ teria. Bacterial cell wall and membrane modifications that reduce bac­ teriolysis dampen AIM2 activation during cytosolic infection. Overall,

the inhibition of inflammasomes by bacterial pathogens enables patho­ gens to avoid elimination by the innate immune system. The NLRP3 inflammasome appears to be activated upon sens­ ing cell stress and homeostatic dysregulation, including via ion flux, mitochondrial damage, and release of oxidized mitochondrial DNA. Because these processes are triggered by membrane damage, NLRP3 can be activated by many bacterial pore-forming toxins; bacteria may avoid stimulating NLRP3 by downregulating the production of toxins. The pyrin inflammasome becomes activated upon the inactivation of Rho GTPases, thereby serving as a sensor of cytoskeletal perturbations. C. difficile, C. botulinum, and B. cenocepacia all produce effectors that inactivate RhoA via distinct covalent modifications, which induces cell death by alteration of the actin cytoskeleton yet ultimately drives pyrin activation. Meanwhile, NLRP1 acts as bait for cleavage by microbial proteases or E3 ubiquitin ligases; partial degradation of NLRP1 stimu­ lates its activity and subsequent pyroptosis. Caspases-4 and -5 trigger formation of the noncanonical NLRP3 inflammasome in response to LPS. In human cells, LPS is bound by the PRR NLRP11, which facilitates activation of caspase-4; alterna­ tively, caspases-4 and -5 can themselves serve as PRRs for LPS. Upon activation, these caspases cleave GSDMD but not IL-1β; the forma­ tion of GSDMD membrane pores indirectly drives activation of the NLRP3 inflammasome, which subsequently cleaves IL-1β. S. flexneri produces an effector that inactivates caspase-4 via ADP-riboxanation, which blocks host cell mechanisms of cytosolic bacteriolysis, thereby preventing release of LPS into the cytosol. Although caspases-4 and -5 play important roles in host defense, they also contribute to pathologic inflammation during gram-negative sepsis, as the murine homolog caspase-11 drives mortality in murine models of sepsis. Immune GTPases  During many infections, the initial pathogen interaction triggers expression of host immunity-related genes that prime host cells for a more robust response. For example, detection of bacteria can drive interferon signaling, which induces production of the guanylate binding protein (GBP) family of immune GTPases. GBPs are recruited to pathogen-containing vacuoles or bacterial membranes and promote the release of PAMPs, making them available as ligands for PRRs. Some GBPs extract LPS from the surface of cytosolic gramnegative bacterial pathogens, facilitating noncanonical inflammasome activation. GBPs contribute to lysis of F. novicida (a subspecies of F. tularensis) and Neisseria meningitidis, causing release of bacterial DNA into the cytosol. Recruitment of GBPs to Shigella spp. prevents bacte­ rial actin-based motility and promotes bacterial ubiquitination, which targets the bacteria for proteasomal degradation; simultaneously, Shigella spp. secrete an effector that ubiquitinates GBPs, targeting them for degradation. Inhibition of Nonpyroptotic Cell Death  Several types of regu­ lated (programmed) cell death besides pyroptosis can occur, of which inflammatory necroptosis and noninflammatory apoptosis are best characterized. In necroptosis, activation of any of a variety of innate immune signaling pathways (interferon-γ, TLRs) leads to kinasedependent assembly of a plasma membrane pore complex that induces cell lysis and release of DAMPs into the extracellular space, events that induce an intense inflammatory response. Necroptosis can drive both pathologic inflammation and antibacterial immunity. In apoptosis, cells condense and form membranous blebs without releasing cytosolic contents and thus without inducing inflammation in adjacent tissues. Apoptotic blebs and dead cells are disposed of by macrophages in a process known as efferocytosis. In addition to its role during infection, apoptosis plays a key role in development and is required for normal cell turnover in many epithelial tissues, includ­ ing the intestine. Apoptosis can be initiated by intrinsic or extrinsic cellular stimuli, resulting in activation of apoptosis-specific caspases that demolish cellular contents. The intrinsic pathway occurs upon dis­ solution of the mitochondrial membrane potential, which commonly occurs during infection due to cellular stress or the action of bacterial toxins. The extrinsic pathway can be stimulated by interactions of cell-surface death receptors with proinflammatory ligands like TNF-α.

The gastrointestinal pathogens S. enterica serovar Typhimurium, Yersinia enterocolitica, and pathogenic E. coli inhibit signal transduc­ tion from cell-surface death receptors by modifying or cleaving the components of death receptor–associated supramolecular organizing centers (Fig. 125-4), thereby blocking cell death. Inhibition of extrinsic apoptotic signaling can stimulate alternative inflammatory cell death pathways, including necroptosis, highlighting the functional redun­ dancy of cell death pathways in some contexts.

Vacuolar Chlamydia spp. activate survival pathways through phos­ phoinositide 3-kinase (PI3K) and Wnt/β-catenin signaling, while simultaneously inhibiting apoptosis. T4SS effectors of vacuolar L. pneumophila and C. burnetii inhibit apoptosis by sequestering proapoptotic host factors. Cyclomodulins, a group of bacterial effector proteins that arrest the cell cycle, are delivered into cells by EPEC and EHEC. The cyclomodulin Cif deamidates ubiquitin and related proteins, inactivating host E3 ligases involved in cell cycle progression and thereby reducing epithelial turnover; this disruption can be sensed by host cells and triggers other cell death pathways. The diversity of mechanisms that bacteria have accumulated to modulate host cell death pathways highlights the importance of these pathways during infection. Autophagy  Host cells deploy mechanisms of cell-autonomous immunity to neutralize microbial threats. One such mechanism is autophagy, which recycles organelles and protein complexes by gener­ ating double-membrane compartments (autophagosomes) around the cargo and delivering their contents to the lysosome; the cargo is subse­ quently degraded to amino acids and other molecules. Autophagy can isolate and dispose of cytosolic pathogens; host polysaccharide-binding molecules termed galectins can bind microbes and induce ubiquitina­ tion of the bacterial surface, which drives recruitment of LC3, a host protein that triggers engulfment into autophagosomes. CHAPTER 125 Intracellular bacterial pathogens must inhibit autophagy to survive. Common mechanisms used by bacteria to interfere with autophagy include masking bacterial ubiquitination sites, inhibiting autophago­ some maturation, and altering host pathways that regulate autophagy. L. monocytogenes recruits the host factor Arp2/3 to mask the bacterial surface, F. tularensis produces an LPS O-antigen that prevents surface access of E3 ligases, and S. flexneri produces an effector protein that blocks ubiquitination of the bacterial surface protein IcsA. To inhibit autophagosomal flux, L. pneumophila secretes an effector protein that cleaves LC3, S. enterica serovar Typhimurium secretes effector proteins that block activation of a kinase required for autophagosome genera­ tion, and M. tuberculosis and C. trachomatis prevent autophagosomelysosome fusion. An important metabolic trigger of autophagy is intracellular depletion of amino acids, which occurs upon replication of metabolically active intracellular pathogens and triggers inactivation of a master regulator of cell metabolism, mammalian target of rapamy­ cin (mTOR). mTOR inactivation derepresses autophagy; invasive pathogens including M. tuberculosis, S. enterica serovar Typhimurium, and Shigella spp. can reactivate mTOR signaling during infection, thereby blunting autophagy. Molecular Mechanisms of Microbial Pathogenesis Epigenetic Control of Innate Immune Responses  Dysregu­ lated immune responses (e.g., sepsis, severe COVID-19 infection) can cause morbidity or death, whereas weak immune responses may lead to pathogen-induced morbidity and mortality. A cellular mecha­ nism that promotes appropriate amplitude of the immune response is innate immune training, in which cells are epigenetically primed for immunity via alterations in chromatin structure. Posttranslational modification of histones influences chromatin structure and access of transcription factors to regulatory elements in the DNA. Mounting evidence indicates that the microbiota modifies the methylation state of promoters of genes involved in immunity, facilitating appropriately tuned responses to infection. Pathogens also regulate host responses at the epigenetic level. B. anthracis and L. pneumophila deliver into cells histone methyltransferases that control inflammation and ribosome activity. L. monocytogenes listeriolysin O, the P. aeruginosa T3SS pore, and other pore-forming toxins exert epigenetic modulation through

alterations in ion homeostasis, thereby manipulating the gene expres­ sion profiles of infected host cells.

■ ■INHIBITION OF ADAPTIVE IMMUNE RESPONSES Early interactions of pathogens with host cells trigger the innate immune response, releasing cytokines that recruit additional antigenpresenting immune cells to the site of infection. To establish chronic infection, bacterial pathogens need not only to suppress the innate immune system but also to avoid elimination by the adaptive immune response. The adaptive immune system comprises clonally expanded lineages of B and T lymphocytes that have been activated by antigenpresenting cells, the best characterized of which are dendritic cells and macrophages. B cells are required for humoral immunity; genetic dis­ eases that affect B-cell function often manifest as the inability to pro­ duce adequate antibody titers to clear extracellular bacterial infections. T cells generally mediate cell-mediated immunity, helping the host clear infected cells. The activation of T cells occurs by the presentation of processed antigen on MHC molecules of antigen-presenting cells. The activation of T cells is controlled by the specificity of MHC mol­ ecules and of their receptors. The large variety of receptors possessed by T cells enables collective recognition of a wide variety of antigens. Mutations that ablate the development of the adaptive immune system result in severe combined immunodeficiency, rendering the individual extremely vulnerable to infection. M. tuberculosis is notorious for its ability to establish latent infec­ tions, avoiding elimination by the immune system for decades. Once phagocytosed, the pathogen induces secretion of immunosuppressive cytokines, including IL-6, IL-10, and transforming growth factor β. The result is inhibition of interferon-γ-dependent gene expression, with downregulation of MHC class II and other immune-stimulatory molecules, which inhibits induction of helper CD4+ T lymphocytes. Similarly, B. pertussis induces dendritic cells to produce IL-10, which skews T-cell maturation into regulatory T cells, dampening the immune response. The S. enterica serovar Typhimurium T3SS effec­ tor protein SteD depletes MHC class II molecules from the surface of dendritic cells by the activation of the E3 ubiquitin ligase MARCH8. MARCH8 ubiquitinates MHC class II, interfering with its trafficking to the cell surface and thereby decreasing interaction with and subsequent activation of T lymphocytes. PART 5 Infectious Diseases In addition to preventing antigen-presenting cells from stimulating lymphocytes, pathogens may also directly alter B-cell and T-cell activ­ ity. The Yersinia T3SS effector YopH, a protein tyrosine phosphatase, dephosphorylates B-cell and T-cell receptors and thus prevents both signal transduction upon stimulation by antigen-presenting cells and lymphocyte activation. Staphylococcal toxic shock syndrome is induced by the production of superantigens by S. aureus. These extremely inflammatory exotoxins are potent activators of T cells, stimulating exuberant and at times fatal cytokine production. Rather than binding the specificity-determining variable regions of MHC molecules and the T-cell receptor, superantigens bind invariable regions and are therefore able to nonspecifically activate vast numbers of T cells, with a consequent cytokine storm. ■ ■BACTERIAL CYTOTOXINS Many bacteria produce cytotoxins—toxins that trigger host cell death. The best-characterized group of bacterial cytotoxins are the AB tox­ ins, which are composed of an enzymatically active (A) subunit and a binding (B) subunit, which interacts with the cellular receptor. This family of toxins includes Shiga toxins of Shigella spp. and some strains of pathogenic E. coli, diphtheria toxin of Corynebacterium diphtheriae, pertussis toxin of B. pertussis, and CTX of V. cholerae. In general, upon binding to cell surface receptors, the toxin is endocytosed, whereupon the A-subunit translocates across the endosome membrane into the cytosol where its toxic enzymatic activity is stimulated. CTX and per­ tussis toxin ADP-ribosylate G-protein regulators of adenylate cyclases, thereby increasing cellular cyclic AMP (cAMP) concentrations. Within the cell, increased cAMP perturbs ion homeostasis and apoptosis; on a tissue level, increased intracellular cAMP induces chloride secretion via CFTR and inhibits sodium chloride absorption, driving massive

fluid secretion into the lumen of the small intestine and the diarrheal symptoms of cholera. C. perfringens and P. aeruginosa produce non-AB family toxins that ADP-ribosylate elongation factor-2, thereby inhibit­ ing host translation. S. enterica serovar Typhimurium and C. difficile produce toxins that target actin, inhibiting normal cellular cytoskeletal rearrangements. As dictated by the host targets modified, the effects of bacterial toxins on the host cell generally fall into the few broad categories of cytoskeletal manipulation, inhibition of innate immune signaling, and hijacking of cellular trafficking. Pore-forming toxins are secreted virulence factors produced by many extracellular pathogens, including S. pneumoniae, C. perfringens, B. anthracis, and L. monocytogenes, among many others. S. aureus produces several types of pore-forming toxins, including leukocidins, hemolysins, and phenol-soluble modulins. Leukocidins and α- and γ-hemolysins oligomerize within host plasma membranes, leading to host cell lysis. The cell-type specificities of leukocidins and hemolysins are driven by toxin binding to specific cell surface receptors. Conversely, phenolsoluble modulins are small amphipathic peptides that directly insert into cell membranes. Lytic toxins protect S. aureus from phagocytosis and prevent an infected host from developing a protective adaptive immune response. Host cells can counter cytolysins by releasing exosomes, vesicles that act as decoys by removing cytolysins from the local environment. ■ ■TISSUE DAMAGE AND PATHOGEN DISSEMINATION Much of the pathology associated with bacterial infection results from proinflammatory immune responses. Infected cells may continually signal in ways that alert the immune system, even though, as described above, many bacteria avoid elimination by cell-autonomous and immune cell–mediated mechanisms. In the intestine, tight junctions mediate inti­ mate associations between epithelial cells that maintain tissue barrier function, linking the cytoskeletal networks of adjacent cells through intimate association of protein complexes across the cell membranes. Many intestinal pathogens perturb the integrity of the gut epithelium either by manipulating cell polarity or by disrupting intercellular junc­ tions. C. perfringens, V. cholerae, pathogenic E. coli, Shigella spp., and S. enterica serovar Typhi all produce toxins in the gastrointestinal tract that disrupt tight junctions, consequently disrupting the barrier function of the tissue and facilitating access of the pathogen to deeper tissue. The RtxA multifunctional repeats-in-toxin (MARTX) toxin of V. cholerae causes cell rounding and barrier failure through actin-crosslinking activ­ ity, yet avoids eliciting substantial immune responses by simultaneously inactivating phospholipases and Rho GTPases. The serine protease autotransporters (SPATEs) of Shigella spp. and some pathogenic E. coli are typically secreted into the gut lumen or mucus layer, whereupon they cleave components of epithelial junctions and mucins in ways that facili­ tate tissue penetration. Tissue damage permits access to the underlying mucosal layers, the lymphatics, and the bloodstream; for some patho­ gens, this access enables seeding of other organs. Transmission to New Hosts  The host represents a replicative niche for bacterial pathogens, in which they multiply and are transmitted to new hosts. The mode of transmission is typically aligned with the mode of entry. For example, for respiratory pathogens, coughing induced by tissue damage in the lung aerosolizes the pathogen, enabling inhalation by and colonization of a new host. Similarly, gastrointestinal pathogens elicit diarrhea and are transmitted via the direct fecal–oral route or via contamination of crops or food with waste from an infected individual. The understanding of the spread of infectious diseases permits the institution of basic hygiene procedures that greatly diminish transmis­ sion rates—for example, hand washing, decontamination of communal surfaces, and adoption of social distancing measures. Bacteriophages and Pathogen Lifestyle  The reservoir of V. cholerae is aquatic environments. Disease is acquired through the inges­ tion of contaminated seawater or seafood. In regions where cholera is endemic, the disease displays seasonal peaks. This seasonality is associ­ ated with blooms of bacteria-targeting viruses (bacteriophages), which infect V. cholerae organisms, replicate, and lyse and kill the bacterial host. Bacterial lysis releases viral particles into the aquatic environment,

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126 Microbial Genomics and Infectious Disease

whereupon they can infect other V. cholerae. Consequently, the bacte­ riophages regulate the abundance of pathogens, with ramifications for the epidemiology of the disease. Whether bacteriophages contribute to cyclical control of other pathogens is currently unknown. ■ ■SUMMARY Bacterial pathogens display myriad mechanisms of colonization, adhe­ sion, invasion, dissemination, and manipulation of host pathways. Infectious diseases result when pathogens successfully establish them­ selves within the host. Symptoms are usually the result of ensuing fights between the pathogen and the immune system. The incredible diversity of virulence determinants highlights the success of the host in combat­ ing infection. Further elucidating how bacteria cause infection will better our understanding of both human and microbial biology and will provide new opportunities for successful therapeutic intervention against infectious and inflammatory diseases. ■ ■FURTHER READING Costa TRD et al: Secretion systems in gram-negative bacteria: Struc­ tural and mechanistic insights. Nat Rev Microbiol 13:343, 2015. Fitzgerald KA, Kagan JC: Toll-like receptors and the control of immunity. Cell 180:1044, 2020. Galluzzi L et al: Molecular mechanisms of cell death: Recommenda­ tions of the Nomenclature Committee on Cell Death 2018. Cell Death Differ 25:486, 2018. Jastrab JB, Kagan JC: Strategies of bacterial detection by inflamma­ somes. Cell Chem Biol 31:835, 2024. Lamason RL, Welch MD: Actin-based motility and cell-to-cell spread of bacterial pathogens. Curr Opin Microbiol 35:48, 2017. Remick BC et al: Effector-triggered immunity. Annu Rev Immunol 41:453, 2023. Ribet D, Cossart P: How bacterial pathogens colonize their hosts and invade deeper tissues. Microbes Infect 17:173, 2015. Rivera-Cuevas R et al: Human guanylate-binding proteins in intra­ cellular pathogen detection, destruction, and host cell death induc­ tion. Curr Opin Immunol 84:102373, 2023. Stones DH, Krachler AM: Against the tide: The role of bacterial adhesion in host colonization. Biochem Soc Trans 44:1571, 2016. Tsolis RM, Bäumler AJ: Gastrointestinal host–pathogen interaction in the age of microbiome research. Curr Opin Microbiol 53:78, 2020. Wu YW, Li F: Bacterial interaction with host autophagy. Virulence 10:352, 2019. Roby P. Bhattacharyya, Yonatan H. Grad,

Deborah T. Hung

Microbial Genomics and Infectious Disease Just as microscopy opened the worlds of microbiology by providing a tool with which to visualize microorganisms, technological advances in genomics provide microbiologists with powerful methods to character­ ize the genetic map that underlies all microbes with unprecedented resolution, thereby illuminating their complex and dynamic interac­ tions with each other, the environment, and human health. The field of infectious disease genomics encompasses a vast frontier of active research that is transforming public health and the clinical practice of infectious diseases. While genetics has long played a key role in elucidating the process of infection and impacting clinical infectious diseases, the ability to extend our thinking and approaches beyond the study of single genes to an examination of the sequence, structure,

and function of entire genomes allows us to identify new possibilities for research and opportunities to change clinical practice and disease surveillance. From the development of diagnostics with unprecedented sensitivity, specificity, and speed to the design of novel public health surveillance tools and interventions, technical and statistical genomic innovations are reshaping our understanding of the influence of the microbial world on human health and providing us with new tools to diagnose, track, and combat infection. In this chapter, we explore the application of genomics methods to microbial pathogens and the infections they cause. We discuss innovations that are driving the development of diagnostic approaches as well as the discovery of new pathogens, providing insight into novel therapeutic approaches and paradigms, and advancing methods in infectious disease epidemiol­ ogy and the study of pathogen evolution that can inform infection control measures, public health responses to outbreaks, and vaccine development. We draw on examples in current practice and from the recent scientific literature as signposts that point toward ways in which pathogen genomics may influence infectious diseases in the short and long terms, and we highlight applications to SARS-CoV-2 and the COVID-19 pandemic. Table 126-1 provides definitions for a selection of important terms used in genomics.

MICROBIAL DIAGNOSTICS The basic goals of a clinical microbiology laboratory are to establish the presence of a pathogen in a clinical sample, to identify the pathogen, and, when possible, to provide other information that can help guide clinical management and affect prognosis, such as antibiotic suscep­ tibility profiles or the presence of virulence factors. To date, clinical microbiology laboratories have largely approached these goals phe­ notypically by growth-based assays and biochemical testing. Bacteria, for instance, were historically grouped algorithmically into species by their characteristic microscopic appearance, nutrient requirements for growth, and ability to catalyze certain reactions. Antibiotic susceptibil­ ity is still determined in most cases by assessing bacterial growth in the presence of antibiotic. CHAPTER 126 Microbial Genomics and Infectious Disease With the sequencing revolution paving the way to easy access of complete pathogen genomes, we can more systematically define the genetic basis for these observable phenotypes. Compared with tradi­ tional growth-based methods for bacterial diagnostics that dominate the clinical microbiology laboratory, nucleic acid–based diagnostics that build on this genomic information promise improved speed, sen­ sitivity, specificity, and breadth of information. Bridging clinical and research laboratories, adaptations of genomic technologies have begun to deliver on this promise (Table 126-2). ■ ■HISTORICAL LIMITATIONS AND PROGRESS THROUGH GENETIC APPROACHES The molecular diagnostics revolution in the clinical microbiology labo­ ratory is well under way, born of necessity in the effort to identify and characterize microbes that are refractory to traditional culture meth­ ods. Historically, diagnosis of many so-called unculturable pathogens has relied largely on serology and antigen detection. However, these methods provide only limited clinical information because of their suboptimal sensitivity and specificity, and, for serology, the long delays that diminish their utility for real-time patient management and the inability to characterize pathogens beyond identifying past exposure. Newer tests to detect pathogens based on nucleic acid content have already offered improvements in the select cases in which they have been applied. Unlike direct pathogen detection, serologic diagnosis—measurement of the host’s response to pathogen exposure—can typically be made only in retrospect, requiring both acute- and convalescent-phase serum samples. For chronic infections, distinguishing active from latent infection or identifying repeat exposure from serology alone can be difficult or impossible, depending on the syndrome. In addition, serologic diagnosis is variably sensitive, depending on the organism and the patient’s immune status. For instance, tuberculosis is notori­ ously difficult to identify by serologic methods; tuberculin skin testing using purified protein derivative (PPD) is especially insensitive in

TABLE 126-1  Glossary of Selected Terms in Genomics TERM DEFINITION Contig A DNA sequence representing a continuous fragment of a genome, assembled from overlapping sequences; relevant for de novo assembly of sequence data that do not align to previously sequenced genomes Genome The entire set of heritable genetic material within an organism Horizontal gene transfer The transfer of genes between organisms through mechanisms other than by clonal descent, such as through transformation, conjugation, or transduction Genomic epidemiology An approach to inferring and reconstructing transmission events, population dynamics, and epidemiological patterns using comparative analysis of microbial genome sequences Metagenomics Analysis of genetic material from multiple species directly from primary samples without requiring prior culture steps Microarray A collection of DNA oligonucleotides (“oligos”) spatially arranged on a solid surface and used to detect or quantify sequences in a sample of interest that are complementary (and therefore bind) to one or more of the arrayed oligos Microbial genomewide association study (GWAS) An analytic framework to test statistical associations between microbial genotypes and phenotypes of interest, such as antibiotic resistance and virulence Mobile genetic elements DNA elements that can move within a genome and can be transferred between genomes through horizontal gene transfer (e.g., plasmids, bacteriophages, and transposons) Multilocus sequence typing A method for typing organisms based on DNA sequence fragments from a prespecified set of genes Next-generation sequencing High-throughput sequencing using a parallelized sequencing process that produces millions of sequences concurrently, far beyond the capacity of prior dyeterminator methods PART 5 Infectious Diseases Nucleic acid amplification test (NAAT) A biochemical assay that evaluates for the presence of a particular string of nucleic acids through amplification by one of several methods, including polymerase and ligase chain reactions Polymerase chain reaction (PCR) A type of NAAT used to amplify a specific region of DNA by means of specific oligonucleotide primers and a DNA polymerase Single-nucleotide polymorphism (SNP) Point mutations in DNA, the number of which in different microbial isolates is a measure of their genetic distance from one another Transcriptome The catalog of the full set of messenger RNA (mRNA) transcripts from a cell or organism, which are typically measured by microarray or by next-generation sequencing of complementary DNA (cDNA) via a process called RNA-Seq Whole genome sequencing (WGS) A process that determines the full DNA sequence of an organism’s genome; has been greatly facilitated by nextgeneration sequencing technology active disease and possibly cross-reactive with vaccines or other myco­ bacteria. Even the newer interferon γ release assays (IGRAs), which measure cytokine release from T lymphocytes in response to Mycobac­ terium tuberculosis–specific antigens in vitro, have limited sensitivity in immunodeficient hosts. Neither PPD testing nor IGRAs can distin­ guish latent from active infection. Serologic Lyme disease diagnostics suffer similar limitations: in patients from endemic regions, the pres­ ence of IgG antibodies to Borrelia burgdorferi may reflect prior expo­ sure rather than active disease, while IgM antibodies are imperfectly sensitive and specific (50% and 80%, respectively, in early disease). The complicated nature of these tests, particularly in view of the nonspe­ cific symptoms that may accompany Lyme disease, has had substantial implications for public perception of Lyme disease and antibiotic mis­ use in endemic areas. Similarly, syphilis, a chronic infection caused by Treponema pallidum, is notoriously difficult to stage by serology alone, requiring multiple different nontreponemal and treponemal tests (e.g., rapid protein reagin and fluorescent treponemal antibody, respectively)

in conjunction with clinical suspicion. Complementing serology, anti­ gen detection can improve sensitivity and specificity in select cases but has been validated only for a limited set of infections. Typically, structural elements of pathogens are detected, including components of viral envelopes (e.g., hepatitis B surface antigen, HIV p24 antigen, SARS-CoV-2 spike protein), cell surface markers in certain bacteria (e.g., Streptococcus pneumoniae, Legionella pneumophila serotype 1) or fungi (e.g., Cryptococcus, Histoplasma), and less specific fungal cellwall components such as galactomannan and β-glucan (e.g., Aspergillus and other fungi). Given the impracticality of culture and the lack of sensitivity or sufficient clinical information afforded by serologic and antigenic methods, the push toward nucleic acid–based diagnostics originated in pursuit of viruses and fastidious bacteria, becoming part of the standard of care for select organisms in U.S. hospitals. Such tests, including polymerase chain reaction (PCR) and other nucleic acid amplification tests (NAATs), are now widely used for many viral infec­ tions, both chronic (e.g., HIV infection, hepatitis C) and acute (e.g., influenza, SARS-CoV-2, respiratory syncytial virus). NAATs provide essential information about both the initial diagnosis and the response to therapy and, in some cases, genotypically predict drug resistance. Indeed, progression from antigen detection to PCR transformed our understanding of the natural course of HIV infection, with profound implications for treatment (Fig. 126-1A). In the early years of the AIDS pandemic, p24 antigenemia was detected in acute HIV infection but then disappeared for years before emerging again with progression to AIDS (Fig. 126-1B). Without a marker demonstrating viremia, the role of treatment during HIV infection prior to the development of clinical AIDS was uncertain, and assessing treatment efficacy was challeng­ ing. With the emergence of PCR as a progressively more sensitive test (now able to detect as few as 20 copies of virus per milliliter of blood), viremia was recognized as a near-universal feature of HIV infection. Given the challenges of phenotypic assays, genotypic antiviral resis­ tance testing was also adopted early for HIV and is now the standard of care before the initiation of therapy in developed countries. These developments have been transformative in guiding therapy in early disease and, together with the development of less toxic therapies, have helped to shape policy that is moving toward ever-earlier introduction of antiretroviral therapy in HIV infection. Reverse transcriptase PCR (RT-PCR) assays are the core method for detecting SARS-CoV-2 virus in the acute phase, forming a critical component of the clinical and public health response to COVID-19, just as they were on a smaller scale for the related coronaviruses SARS-CoV and Middle East respi­ ratory syndrome (MERS)-CoV. Tests for SARS-CoV-2 represent the largest implementation of a molecular infectious disease assay to date and play a critical role in both clinical diagnostics and public health measures to contain the COVID-19 pandemic. As they are for viral testing, nucleic acid–based tests have become the diagnostic tests of choice for fastidious bacteria, including the common sexually transmitted bacterial pathogens Neisseria gonor­ rhoeae and Chlamydia trachomatis as well as the tick-borne Ehrlichia chaffeensis and Anaplasma phagocytophilum. More recently, nucleic acid amplification–based detection has offered improved sensitivity for diagnosis of the important nosocomial pathogen Clostridioides difficile, and NAATs have provided clinically relevant information on the pres­ ence of cytotoxins A and B as well as molecular markers of hyperviru­ lence, such as the North American pulsotype 1 (NAP1) strain that is enriched in severe illness. The importance of genomics in selecting loci for diagnostic assays and in monitoring test sensitivity was highlighted by the emergence in Sweden of a newly recognized variant of C. tracho­ matis with a deletion that includes the gene targeted by a set of com­ mercial NAATs. By evading detection through this deletion (and thus avoiding treatment), this strain came to be highly prevalent in some areas of Sweden. While nucleic acid–based tests remain the diagnostic approach of choice for fastidious bacteria, this example of “diagnostic escape” serves as a reminder of the need for careful development and ongoing monitoring of molecular diagnostics. In contrast, for typical bacterial pathogens for which culture meth­ ods are well established, growth-based assays still dominate in the

TABLE 126-2  Selected Clinical Applications of Infectious Disease Genomics APPLICATION TECHNOLOGY NOTES/EXAMPLES Organism Identification Viral detection PCR, RT-PCR Identification of HIV, HBV, HCV, respiratory viruses including SARS-CoV-2 and influenza, and others for diagnosis and response to therapy TB detection PCR Amplification of the rpoB gene for species-specific identification of Mycobacterium tuberculosis Pathogen detection PCR, RT-PCR, NAAT Multiplexed identification of dozens of viruses, bacteria, yeasts, and parasites from a variety of clinical specimens Bacterial detection 16S ribosomal gene sequencing Targeted amplification and sequencing of regions of the 16S rRNA gene for identification of suspected bacterial infections undiagnosed by conventional methods Pathogen detection Cell-free DNA sequencing Unbiased amplification and sequencing of cell-free nucleic acid from blood, with analytical comparison of resulting nonhuman sequences to genomes of known pathogens and contaminants, in order to identify circulating pathogen DNA; anecdotal clinical use to establish etiology of systemic or focal infection, though clinical utility and optimal use cases still evolving Pathogen Discovery Bacterial pathogens Sequencing, metagenomic assembly Unbiased “shotgun” sequencing of isolated nucleic acid from patient samples to identify associated pathogens; proofs-of-concept: new Bradyrhizobium species associated with cord colitis; Escherichia coli O104:H4 from 2011 diarrheal outbreak in Germany; Leptospira species from one patient’s cerebrospinal fluid; research use only at this time Viral pathogens Microarray, sequencing Hybridization of clinical samples to microarrays from phylogenetically diverse known viruses identified the first SARS coronavirus and others. Direct sequencing has identified SARS-CoV-2, West Nile virus, and MERS-CoV, among others. Use is primarily in research. Antibiotic Resistance MRSA detection PCR Detection of the mecA gene, the genotypic cause of methicillin resistance in Staphylococcus aureus VRE detection PCR Detection of the vanA or vanB gene, the main genotypic causes of vancomycin resistance in Enterococcus MDR-TB detection PCR, NAAT Detection of polymorphisms in the rpoB gene from M. tuberculosis, which account for 95% of rifampin resistance. Other probes available for inhA and katG genes can detect up to 85% of isoniazid resistance. Carbapenemase detection PCR Detection of genes encoding one of several types of enzymes (KPC, NDM, OXA-48, IMP, VIM) that hydrolyze carbapenems, accounting for much but not all carbapenemase resistance in Enterobacteriaceae HIV resistance detection Targeted sequencing Targeted sequencing of specific genes with known resistance-conferring mutations; now the standard of care prior to initial therapy in the United States and Europe Epidemiology Outbreak and epidemic tracking Sequencing Application to tracking outbreaks and epidemics on local and international scales, including spread of carbapenemase-producing Klebsiella, S. aureus, M. tuberculosis, E. coli, Vibrio cholerae, Ebola virus, Zika virus, and influenza virus Evolution and spread of pathogens Sequencing Sequencing collections of pathogens from individual patients or environmental reservoirs such as wastewater to shed light on pathogen dissemination, virulence factors, and antibiotic resistance determinants; innumerable examples, including V. cholerae, influenza virus, Ebola virus, Zika virus, and SARS-CoV-2 Abbreviations: HBV, hepatitis B virus; HCV, hepatitis C virus; MDR, multidrug-resistant; MERS, Middle East respiratory syndrome; MRSA, methicillin-resistant S. aureus; NAAT, nucleic acid amplification test; PCR, polymerase chain reaction; RT, reverse transcriptase; SARS, severe acute respiratory syndrome; TB, tuberculosis; VRE, vancomycin-resistant enterococci. clinical laboratory. Informed by decades of clinical microbiology, these tests have served clinicians well, yet the limitations of growth-based tests—in particular, the delays associated with waiting for growth— have left opportunities for improvements. Driven by this need, mass spectrometry–based assays that offer highly accurate organism iden­ tification within a few hours of a positive blood culture are widely adopted in clinical microbiology laboratories, largely supplanting biochemical tests for organism identification in well-resourced areas. Looking ahead, molecular diagnostics, greatly informed by the vast quantity of microbial genome sequences generated in recent years, offers a way forward. First, sequencing studies can readily identify key genes (or noncoding nucleic acids) that can be developed into targets for clinical assays using PCR or hybridization assay platforms. Second, sequencing itself is becoming cheap and rapid enough to be performed on clinical specimens in certain cases, with consequent unbiased detec­ tion or characterization of pathogens. One of the biggest drivers for the implementation of novel molecu­ lar technologies in the diagnosis of infectious diseases is the desire for more rapid—or even real-time—pathogen identification, ideally with antibiotic susceptibility information on those microbes for which resistance to the current anti-infective armamentarium is of concern. Such real-time tests have the potential to transform infectious disease management, impacting antibiotic stewardship in the outpatient set­ ting, mortality risk in the critically ill (i.e., patients in whom early

CHAPTER 126 Microbial Genomics and Infectious Disease administration of effective antibiotics is the most significant factor in decreasing mortality risk), hospital admission, and length of hospital stay; the extent of this impact will depend on the economic forces that will help define the breadth of their deployment. On the public health level, such tests will likely play a role in improving antibiotic stewardship, thereby influencing the rise of antibiotic resistance and enabling surveillance of outbreaks by local, national, and international networks. In the United States and the United Kingdom, for example, public health agencies use genome sequencing to track food-borne pathogens and identify outbreaks and have rapidly expanded the rou­ tine use of genomics in identifying and characterizing other pathogens, from mycobacteria (both M. tuberculosis and nontuberculous myco­ bacteria) to N. gonorrhoeae. Further, international efforts to track the spread of viral diseases, particularly the vast efforts to sequence SARSCoV-2 and monitor the emergence and spread of its variants, as well as recent work on mpox, Ebola, and Zika outbreaks and ongoing work on seasonal influenza, offer opportunities for improving interven­ tions, surveillance, and prevention efforts, ranging from more accurate selection of the strains to include in vaccine development to improved design of trials to evaluate novel vaccines and therapies. Technological innovations are lowering several critical barriers to the widespread adoption of genomics and other molecular methods. Specifically, for NAAT, the need for rapid thermal cycling and coldchain storage for reagents has significantly impeded implementation

GENERAL MILESTONES HIV genome sequenced First AIDS case series published First clinical cases worldwide HIV first isolated AZT (NRTI) approved Saquinavir (PI) approved

Phenotypic resistance testing available HIV antibody test approved p24 antigen test approved A DIAGNOSTIC MILESTONES AIDS Acute HIV Chronic HIV PART 5 Infectious Diseases Relative level years weeks B FIGURE 126-1  A. Timeline of select milestones in HIV management. Genomic advances are shown in bold type. The approvals and recommendations indicated apply to the United States. ARV, antiretroviral; AZT, zidovudine; NRTI, nucleoside reverse transcriptase (RT) inhibitor; NNRTI, nonnucleoside RT inhibitor; PI, protease inhibitor. B. Viral dynamics in the natural history of HIV infection. Three diagnostic markers are shown: HIV antibody (Ab), p24 antigen (p24), and viral load (VL). Dashed gray line represents limit of detection. (Adapted from data in HH Fiebig et al: Dynamics of HIV viremia and antibody seroconversion in plasma donors: Implications for diagnosis and staging of primary HIV infection. AIDS 17:1871, 2003.) in resource-limited settings. Recent efforts aim to overcome these chal­ lenges by developing isothermal amplification protocols and lyophilized reagents that do not require refrigeration or sophisticated instrumenta­ tion. For clinical sequencing, (1) the cost and speed of sequencing and analysis methods continue to fall precipitously; (2) automation and miniaturization of the preparation of a sample for sequencing prom­ ise to reduce cost and minimize the expertise needed; and (3) direct sequencing technologies that eliminate the complex molecular biology required to prepare clinical samples for sequencing are improving in accuracy and robustness. Further barriers exist, including the need for careful processing of clinical samples to minimize contamination, and standardized pipelines to process data and present clinicians with easily interpretable and readily actionable results. However, as these advances give rise to rapid, accurate diagnostic tests, the ultimate goal is to inform a clinician in real time whether antibiotics are indicated and, if so, which will be effective. Real-time diagnostics will allow more efficient deployment of our precious antibiotic arsenal, thus improving both societal and patient-specific outcomes in much the same way that a rapid, sensitive troponin assay has transformed bedside management of chest pain.

Nevirapine (NNRTI) approved First once-a-day combination ARV approved Frontier: clinical impact of rare sequence variants HIV genotype recommended before ARV start HIV viral load test approved HIV genotypic resistance testing approved VL Ab p24 Time ■ ■ORGANISM IDENTIFICATION To adapt nucleic acid detection to diagnostic tests and thus to identify pathogens on a wide scale, sequences must be found that are conserved enough within a species to identify the diversity of strains that may be encountered in various clinical settings, but divergent enough to distinguish one species from another. Until recently, this problem has been solved for bacteria by targeting the element of a bacterial genome that is most highly conserved within a species, the 16S ribosomal RNA (rRNA) subunit. Among many examples, this method has now been used to confirm Mycobacterium chimaera infections in several patients after cardiothoracic surgery, leading ultimately to recognition of a widespread outbreak. At present, 16S PCR amplification from tis­ sue specimens can be performed by specialty laboratories, though its sensitivity and clinical utility to date have remained somewhat limited, in part because of the scarcity and relative fragility of pathogen nucleic acid in the sampled tissue, which necessitates reliable, sensitive nucleic acid amplification. As such barriers are reduced through technological advances and as the causes of culture-negative infection are clarified (perhaps in part through sequencing efforts), these tests may become both more accessible and more helpful.

With the wealth of sequencing data now available, other regions beyond 16S rRNA can be targeted for bacterial species identification. These other genomic loci can provide additional information about a clinical isolate that is relevant to patient management. For instance, detection of the presence, or potentially even the expression, of toxin genes such as C. difficile toxins A and B or Shiga toxin can provide clinicians with additional information that will help distinguish com­ mensals or colonizing bacteria from pathogens and thus aid in prog­ nostication and management as well as in diagnosis. Beyond bacteria, one commonly used approach to PCR-based pathogen detection is so-called “syndromic panels” of multiplexed PCR to identify common causes of clinical infection syndromes, including upper respiratory infection, gastroenteritis, and meningoencephalitis. The most frequently deployed syndromic panel is the respiratory viral panel, which typically includes primer sets targeting a combination of influenza, parainfluenza, respiratory syncytial virus, adenovirus, rhinovirus, enterovirus, metapneumovirus, and common-cold corona­ viruses, sometimes in conjunction with unculturable bacteria such as Mycoplasma, Chlamydophila, and Bordetella species. With the advent of the COVID-19 pandemic, SARS-CoV-2 was quickly added to this mul­ tiplex PCR panel within months of its development as a stand-alone PCR test. The goal of such panels is to capture common infectious causes of these syndromes in a single, standardized diagnostic test, ideally streamlining the diagnostic evaluation. The ready identification of a plausible etiologic agent may offer diagnostic clarity if judiciously used and carefully considered in the clinical context of each patient. The widespread adoption of SARS-CoV-2 PCR testing in the earli­ est days of the COVID-19 pandemic underscored the crucial role of precise etiologic pathogen diagnosis in patient care, triage, infection control, and epidemiology. One challenge with PCR-based assays is the relative complexity of the molecular biology and consequent need for advanced technol­ ogy for implementation, including instruments and reagents. Several recent approaches have advanced the molecular biology of nucleic acid detection with the aim of increasing deployability of NAATs for use in resource-limited or even field settings. These methods couple nucleic acid detection to an enzymatic readout, enabling catalytic signal amplification. Several such approaches build on the intrinsic sensitivity, specificity, and amplification of the CRISPR (clustered, regularly interspaced short palindromic repeats) effectors Cas12a and Cas13a as nucleic acid sensors. Distinct from the famous gene editing CRISPR effector Cas9, these robust and versatile enzymes recognize short nucleic acid targets with high specificity and transduce this bind­ ing event into “collateral cleavage” of nearby nucleic acids that can be engineered to create a signal using fluorescent reporter constructs. Crucially, this biotechnology can be made to work in conjunction with isothermal enzymatic preamplification steps to achieve remarkable sensitivity, all robustly enough to withstand lyophilization that enables long-term storage before being reconstituted in the field. Such assays are still in the early stages of development, but they have shown prom­ ise and could play a critical role in global diagnostics and surveillance. While amplification tests such as PCR and other NAATs exemplify one approach to nucleic acid detection, other approaches exist, includ­ ing detection by hybridization. Although not currently used in the clinical realm, techniques for multiplexed detection and identifica­ tion of pathogens by hybridization to microarrays or in solution are being developed for other purposes. Of note, these different detection techniques require different degrees of conservation. Highly sensi­ tive amplification methods require a high degree of sequence identity between PCR primer pairs and their short, specific target sequences; even a single base-pair mismatch (particularly near the 3′ end of the primer) may interfere with detection. In contrast, hybridizationbased tests are more tolerant of mismatch and thus can be used to detect important regions that may be less precisely conserved within a species, thus potentially allowing detection of clinical isolates from a given species with greater diversity between isolates. Such assays take advantage of the predictable binding interactions of nucleic acids and do not require enzymology, broadening the range of conditions under which such assays are feasible, including directly on primary clinical

specimens. The applicability of hybridization-based methods toward either DNA or RNA opens the possibility of expression profiling, which can uncover phenotypic information from nucleic acid content.

Both PCR and hybridization methods target specific, known organ­ isms. At the other extreme, as sequencing costs decline, metagenomic sequencing from patient samples is increasingly feasible. This shot­ gun sequencing approach is unbiased—i.e., can detect any microbial sequence, however divergent or unexpected. In one example, a clinical sample of cerebrospinal fluid from an immunocompromised patient with signs and symptoms of chronic meningitis was found through metagenomic sequencing and analysis to contain small amounts of Leptospira DNA. In light of this information, retrospective PCR testing confirmed the diagnosis of neuroleptospirosis, which had been missed prior to the sequencing result. The patient was treated with penicillin G and clinically recovered. Increasingly, efforts are under way to bring whole genome sequencing to other clinical samples, including sputum and blood, to more readily identify pathogens. One such assay certified for clinical use in the United States—a shotgun metagenomic sequenc­ ing approach applied to cell-free DNA circulating in the bloodstream that aims to identify pathogens both in blood and other body sites—is gaining adoption in cases where traditional culture-based methods fail to identify a pathogen in patients with symptoms consistent with infection. This new approach brings its own set of challenges, however, including the need to recognize pathogenic sequences against a back­ ground of expected host and commensal sequences and to distinguish true pathogens from either colonizers or laboratory contaminants. The burgeoning field of microbiome research is driving technology devel­ opment for sequencing and analyzing complex microbial communities. Lessons from this field will inform diagnostic efforts. CHAPTER 126 ■ ■PATHOGEN DISCOVERY In addition to clinical diagnostic applications, novel genomic technolo­ gies, including whole genome sequencing, are being applied to clinical research specimens with a goal of identifying new pathogens in a vari­ ety of circumstances. The tremendous sensitivity and unbiased nature of sequencing is also ideal in searching clinical samples for unknown or unsuspected pathogens. Microbial Genomics and Infectious Disease Causal inference in infectious diseases has progressed since the time of Koch, whose historical postulates provided a rigorous framework for attributing a disease to a microorganism. To modernize Koch’s postu­ lates, an organism, whether it can be cultured or not, should induce disease upon introduction into a healthy host if it is to be implicated as a causative pathogen. Current sequencing technologies are ideal for advancing this modern version of Koch’s postulates because they can identify candidate causal pathogens with unprecedented sensitivity and in an unbiased way, unencumbered by limitations such as cultur­ ability. Yet, as direct sequencing on primary patient samples greatly expands our ability to recognize associations between microbes and disease states, critical thinking and experimentation will remain vital in establishing causality. Virus discovery in particular has been greatly facilitated by new nucleic acid technology. These frontiers were first notably explored with high-density microarrays containing spatially arrayed sequences from a phylogenetically diverse collection of viruses. Despite bias toward those with homology to known viruses, novel viruses in clini­ cal samples were successfully identified on the basis of their ability to hybridize to these prespecified sequences. This methodology famously contributed to identification of the coronavirus causing severe acute respiratory syndrome (SARS). Once discovered, the SARS coronavirus was rapidly sequenced: the full genome was assembled in April 2003, <6 months after recognition of the first case. With the advent of next-generation sequencing, unbiased pathogen discovery is now addressed through a process known as metagenomic assembly (Fig. 126-2), largely supplanting other methods. Sequences of random nucleotide fragments can be generated from clinical speci­ mens with no a priori knowledge of pathogen identity through a pro­ cess called shotgun sequencing. This collection of sequences can then be computationally aligned to host (i.e., human) sequences, with aligned sequences removed and remaining sequences compared with other

DNA extraction host +/– microbial DNA high-throughput sequencing clinical specimen + + de novo assembly of unmapped reads “novel” microbe phylogenetic comparison to known genomes aligned reads genome fragments (“contigs”) FIGURE 126-2  Workflow of metagenomic assembly for pathogen discovery. DNA is isolated from a specimen of interest (e.g., tissue, body fluid) containing a mixture of host DNA and nucleic acids from coexisting microbes, either commensal or pathogenic. All DNA (and RNA, if a reverse transcription step is added) is then sequenced, yielding a mixture of DNA sequence fragments (“reads”) from the organisms present. Except for reads that do not align (“map”) to any known sequence, these reads are aligned to existing reference genomes for the host or any known microbes. The unmapped reads are computationally assembled de novo into the largest contiguous stretches of DNA possible (“contigs”), representing fragments of previously unsequenced genomes. These genome fragments (contigs) are then mapped onto a phylogenetic tree based on their sequence. Some may represent known but as-yet-unsequenced organisms, while others will represent novel species. (Figure prepared with valuable input from Dr. Ami S. Bhatt, personal communication.) known genomes to detect the presence of known microorganisms. Sequence fragments that remain unaligned suggest the presence of an additional organism that cannot be matched to a known, characterized genome; these reads can be assembled into contiguous nucleic acid stretches that can be compared with known sequences to construct the genome of a potentially novel organism. Assembled genomes (or parts of genomes) can then be compared with known genomes to infer the phylogeny of new organisms and identify related classes or traits. Thus, not only can this process identify unanticipated pathogens, but it can even identify undiscovered organisms. PART 5 Infectious Diseases The emergence of COVID-19 provides a dramatic example that illustrates advances in pathogen discovery technology in the interven­ ing 16 years since SARS-CoV was discovered: the causal coronavirus, SARS-CoV-2, was identified through metagenomic sequencing within about 1 month of the first known case and just weeks after the outbreak was first recognized. Sequencing and assembly were completed within 5 days of the discovery of the new virus, and a NAAT was released 1 day later. Given the ensuing ravages of COVID-19 and the cost of delays of even a few weeks in implementing this new diagnostic test in some locations, it is sobering to contemplate the added harm had this outbreak occurred even a decade earlier. This timeline illustrates the advancing power and speed of new diagnostic technologies but also underscores the pressing need for continued progress. As metagenomic sequencing and assembly techniques become more robust, this technology holds great promise for identifying micro­ organisms that are associated with clinical conditions of unknown etiology. Conventional methods already have unexpectedly linked numerous conditions with specific agents of infection—e.g., cervi­ cal and oropharyngeal cancers with human papillomavirus (HPV), Kaposi’s sarcoma with human herpesvirus 8, and certain lymphomas and, more recently, multiple sclerosis with Epstein-Barr virus. Recently, Zika virus, first described in the 1940s, was found to be increasing in incidence as a cause of febrile syndromes, particularly in Central and South America. A concurrent increase in the incidence of micro­ cephaly was noted that temporally and geographically matched the Zika epidemics. Zika was suspected to be neurotropic because of a previously recognized association with Guillain-Barré syndrome, but the strongest link between Zika virus and microcephaly came when the virus itself was detected by both quantitative reverse transcription PCR (RT-qPCR) and whole genome sequencing in postmortem fetal brain tissue from microcephalic infants. An argument for causality was built

taxonomic assignment of reads mixed reads microbe 1 microbe 2 host unmapped on the foundation of epidemiologic evidence and direct viral detec­ tion, both of which were built on nucleic acid detection and genome sequencing. Sequencing techniques offer unprecedented sensitivity and specificity for identifying foreign nucleic acid sequences that may suggest other such pathogen-associated conditions—from malignan­ cies to inflammatory conditions to unexplained fevers or other clini­ cal syndromes—associated with organisms from viruses to bacteria to parasites. Caution is needed, though: in the absence of the ability to ful­ fill Koch’s postulates, sequence-based identification of a microbe from patient specimens is not, on its own, sufficient to ascribe pathogenicity. The increasing sensitivity of these methods warrants greater rigor and care in defining what is “noise” and what represents a pathogen. As sequencing-based discovery expands, microbes may be found to be associated with conditions not classically thought of as infectious, such as the link between maternal Zika virus infection and fetal micro­ cephaly. Studies of bowel flora in laboratory animals and even humans already suggest correlations between microbe composition and various aspects of metabolic and cardiovascular health. Improved methods for pathogen detection will continue to uncover unexpected correla­ tions between microbes and disease states, but the mere presence of a microbe does not establish causality. Fortunately, once the relatively laborious and computationally intensive metagenomic sequencing and assembly efforts have identified a pathogen, further detection can more easily be undertaken with targeted methods such as PCR or hybridiza­ tion, which may be more scalable and amenable to in situ confirma­ tion. This capacity should facilitate the additional careful investigation that will be required to progress beyond correlation and to draw causal inference. ■ ■ANTIBIOTIC RESISTANCE At present, antibiotic resistance in bacteria and fungi is conventionally determined by isolating a single colony from a cultured clinical speci­ men and testing its growth in the presence of drug. The requirement for multiple growth steps in these conventional assays has several con­ sequences. First, only culturable pathogens can be readily processed. Second, this process requires considerable infrastructure to support the sterile environment needed for culture-based testing of diverse organisms. Finally, and perhaps most significantly, even the fastestgrowing organisms require 1–2 days of processing for identification and 2–3 days for determination of susceptibilities. Some slow-growing organisms take even longer; for instance, weeks must pass before

drug-resistant M. tuberculosis can be identified by growth phenotype. Given the clinical imperative in serious illness to begin effective therapy early, this inherent delay in susceptibility determination has obvious implications for empirical antibiotic use: broad-spectrum antibiotics often must be chosen up front in situations where it is later shown that preferred narrower-spectrum drugs would have been effective or even that no antibiotics were appropriate (i.e., in viral infections). Even with this strategy, as resistant organisms become more common, the empiri­ cal choice can be incorrect, often with devastating consequences. Realtime identification of the infecting organism and information on its susceptibility profile would guide initial therapy and support judicious antibiotic use, ideally improving patient outcomes while aiding in the ever-escalating fight against antibiotic resistance by reserving the use of broad-spectrum agents for cases in which they are truly needed. Molecular diagnostics and sequencing offer a way to accelerate detection of a pathogen’s antibiotic susceptibility profile. If a genotype that confers resistance can be identified, this genotype can be targeted for molecular detection. In infectious disease, this approach has most comprehensively come to fruition for HIV (Fig. 126-1A). (In a concep­ tually parallel application of genomic analysis, molecular detection of certain resistance determinants in cancers informs selection of targeted chemotherapy.) Extensive sequencing of HIV strains and correla­ tions drawn between viral genotypes and phenotypic resistance have delineated the majority of mutations in key HIV genes, such as reverse transcriptase, protease, and integrase, that confer resistance to the antiret­ roviral agents that target these proteins. For instance, the single amino acid substitution K103N in the HIV reverse transcriptase gene predicts resistance to the first-line nonnucleoside reverse transcriptase inhibi­ tor efavirenz, and its detection informs a clinician to choose a different agent. The effects of these common mutations on HIV susceptibility to various drugs—as well as on viral fitness—are curated in publicly avail­ able databases. Thus, genotypes are now routinely used to predict drug resistance in HIV, as phenotypic resistance assays are far more cumber­ some than targeted sequencing. Though it was not implemented at the level of individual patients, sequencing-based detection of circulating SARS-CoV-2 variants predicted susceptibility to monoclonal antibod­ ies due to changes in the spike protein and, thus, informed decisions made by the U.S. Food and Drug Administration (FDA) on granting and rescinding approval for such COVID-19 therapies as the pandemic progressed. As new therapies are introduced, this targeted sequenc­ ing–based approach to drug resistance will likely prove important in other viral infections. The challenge of predicting drug susceptibility from genotype is more daunting for bacteria than for HIV, yet considerable progress has been made toward sequencing-based determination of bacterial antibiotic susceptibility. Bacteria have far more complex genomes than viruses, with thousands of genes on their chromosomes (many of which can functionally interact in ways that escape a priori predic­ tion) and the capacity to acquire many more through horizontal gene transfer of plasmids and mobile genetic elements within and between species. Thus, the task of comprehensively defining all possible genetic resistance mechanisms is orders of magnitude more complex in bac­ teria than in viruses, which typically have far more limited genomes. Despite these challenges, considerable progress has been made in recent years. In select cases where biological factors appear to have con­ strained the genotypic basis for resistance to a small, well-defined set of mutations, genotypic assays for antibiotic resistance are already being integrated into clinical practice. One important example is the detec­ tion of methicillin-resistant Staphylococcus aureus (MRSA). S. aureus is one of the most common and serious bacterial pathogens of humans, particularly in health care settings. Resistance to methicillin—the most effective class of antistaphylococcal antibiotics—is common, even in community-acquired strains. Vancomycin—an alternative drug to methicillin—is effective against MRSA but is measurably inferior to methicillin against methicillin-susceptible S. aureus (MSSA). Analysis of clinical MRSA isolates has demonstrated that the molecular basis for resistance to methicillin in essentially all cases stems from the expres­ sion of an alternative penicillin-binding protein (PBP2A) encoded by the gene mecA, which is found within a transferable genetic element

called mec. This mobile cassette has spread rapidly through the S. aureus population via horizontal gene transfer and selection from widespread antibiotic use. Because methicillin resistance is essentially always due to the presence of the mecA gene, MRSA is particularly amenable to molecular detection. A PCR test for the mecA gene, which saves hours to days compared with standard culture-based methods, has been approved by the FDA to augment (but not replace) traditional culturebased susceptibility testing. Similar to MRSA, vancomycin-resistant enterococci (VRE) harbor one of a limited number of van genes found to be responsible for resistance to this important antibiotic, which occurs through alteration of the mechanism for cell wall cross-linking that vancomycin inhibits. Detection of one of these genes by PCR indicates resistance. More recently, multiplexed PCR assays targeting carbapenemase genes (those encoding the KPC, NDM, OXA-48, IMP1, and VIM carbapenemases), which are responsible for a significant fraction of carbapenem resistance (though not all instances), can predict some resistance to this crucial antibiotic class, though they are not comprehensive enough to confirm susceptibility if absent. Despite these caveats, such assays to detect even a limited set of high-value resistance genes are gaining use in high-resourced settings. Finally, a PCR assay targeting the highly conserved RNA polymerase gene serves not only to detect M. tuberculosis directly in sputum samples but also to detect resistance to rifampin, since the determinants of resistance to this RNA polymerase inhibitor map almost exclusively to a short region of this gene. Since rifampin resistance is epidemiologically associated with, though not causal for, multidrug resistance, this assay identifies strains at high risk for multidrug resistance, enhancing its value. This test has transformed tuberculosis testing where available, improving sensitivity and providing a limited measure of susceptibility testing to guide therapy.

CHAPTER 126 Although identification and rapid detection of monogenic resis­ tance determinants have improved, bacteria typically evolve multiple, diverse resistance mechanisms to most antibiotics; thus, outside of these edge cases, resistance prediction often requires probing for and integration of multiple genetic lesions, targets, or mechanisms. For instance, at least five distinct modes of resistance to fluoroquinolones are known: reduced import, increased efflux, target site mutation, drug modification, and shielding of the target sites by expression of another protein. These mechanisms are typically present in combi­ nation in clinically resistant isolates; thus, the problem of detecting genetic resistance is often a combinatorial one. In another clinically important example, while carbapenem resistance in Enterobacteriaceae is often explained by the presence of carbapenemases, resistance may also develop when other, less broad-spectrum β-lactamases are found in combination with porin mutations or upregulated efflux pumps. These more complex mechanisms prevent PCR-based carbapenemase detection assays from identifying other mechanisms of carbapenem resistance. Additionally, plasmids and transposable elements, which often are enriched for antibiotic resistance determinants, may be more technically and analytically challenging to sequence, although newer long-read sequencing technologies are beginning to address these challenges. To further complicate genetic prediction, changes in gene expression (which may be detectable through mutations in promoter regions or regulatory genes without coding mutations in known resis­ tance determinants) and even gene copy number (which may occur without changes in primary sequence) of resistance determinants play critical roles in some cases of genetic resistance. Thus, while predicting resistance when determinants are found is rapidly becoming feasible, the more clinically relevant task of predicting susceptibility when no known resistance determinants are found remains more difficult. Microbial Genomics and Infectious Disease To build on early successes with the goal of advancing beyond binary detection of monogenic resistance determinants, the ultimate frontier for genetic prediction of bacterial antibiotic resistance lies in more comprehensive prediction of a resistance phenotype from sequence information—a task similar to HIV resistance prediction. Yet there is no comprehensive compendium of genetic elements conferring resistance and their pairwise and higher-order interactions with each other and with the genetic background of bacterial pathogens. Non­ viral genomes are much larger than viral ones, and their abundance

and diversity are such that thousands of genetic differences often exist between clinical isolates of the same species, of which perhaps only one or a few may contribute to resistance. In addition, new mechanisms may emerge in the face of antibiotic deployment or with the release of new drugs, and genetic prediction of resistance will inevitably lag behind the emergence of unforeseen mechanisms. While confident prediction of bacterial antibiotic resistance from sequencing determinants may therefore seem daunting, the vast expansion of microbial sequencing capacity, combined with analytic methods such as microbial genomewide association studies and machine learning algorithms, offers powerful analytical approaches to this “needle in a haystack” problem and has permitted remarkable advances in the predictive power of sequence determinants to date. Particularly in M. tuberculosis, where horizontal gene transfer is minimal and the pathogen is essentially restricted to human hosts to facilitate more representative sampling, a remarkable fraction of phenotypic resistance can be explained by known genetic determinants. Because of these biologic advantages, as well as the slow and laborious growth process that impedes traditional phenotypic assessment, whole genome sequencing has proven quite effective at predicting susceptibility profiles in this organism, to the point that the United Kingdom now routinely performs whole genome sequencing in parallel with phenotypic antibiotic susceptibility testing for M. tuberculosis in what some hope will be a precursor to fully whole genome sequencing–based antibiotic susceptibility testing. Even in more highly variable pathogens, with sequencing of sufficient numbers of susceptible and resistant pathogens, and more sophisticated analyti­ cal algorithms, sequence-based prediction methods are improving in predictive accuracy, at least within the geographic region from which the test samples have been sequenced.

PART 5 Infectious Diseases It is important to note that genotype-based analytical methods largely identify correlates, not necessarily surrogates or determinants, of resistance. In HIV diagnostics, surrogates (i.e., causal determinants of resistance) were found to be more reliable predictors than mere correlates in expanding sequencing-based resistance prediction to the general population. Without a mechanistic understanding of genetic resistance, a correlative relationship may be lineage-specific and less generalizable. Especially with multiple possible mechanisms of resis­ tance to a given antibiotic and ongoing evolutionary pressure resulting in the development and acquisition of new modes of resistance, a geno­ typic approach to diagnosing antibiotic resistance is likely to remain challenging and to require ongoing vigilance in constantly correlating genotypic with more traditional phenotypic methods. An important corollary benefit of a genomic approach to resistance prediction, anchored in phenotypic validation, could be the systematic identifica­ tion of outliers with unexplained resistance. These strains can form the basis for understanding newly emerging resistance mechanisms, which can in turn inform new drug development endeavors. Under­ standing resistance mechanisms may also help direct infection control efforts. For instance, the first identification of the mcr-1 (mobilized colistin resistance) gene on a plasmid, together with other antibiotic resistance determinants, heightened concern about colistin-resistant Enterobacterales identified first in China and later elsewhere because it implied transferrable multidrug resistance. Early recognition of these potentially dangerous strains elucidated the immediate need for strict containment protocols. Furthermore, metagenomic studies have high­ lighted the silent carriage of resistance genes in the microbiomes of returning travelers and their subsequent spread to household contacts. In parallel with advancing sequencing technologies, progress in computational techniques, bioinformatics and statistics, and data stor­ age, as well as experimental confirmatory testing of hypotheses, will be needed to advance toward the ambitious goal of a comprehensive compendium of global antibiotic resistance determinants. Open shar­ ing and careful curation of new sequence information will be of para­ mount importance, as will iterative or even continuous comparison of predictions with ongoing phenotypic testing to assess performance and allow prediction algorithms to keep up with newly evolving or emerg­ ing resistance mechanisms. We continuously observe the accumulation of new or unanticipated modes of resistance from ongoing evolutionary pressure caused by the

widespread clinical use of antibiotics. Even with MRSA, perhaps the best-studied case of antibiotic resistance and a model of relative sim­ plicity with a single known monogenic resistance determinant (mecA), a genotype-based approach to resistance detection proved imperfect. One limitation was a recall of the initial commercial genotypic resis­ tance assay that was deployed for the identification of MRSA. A clinical isolate of S. aureus that emerged in Belgium expressed a variant of the mec cassette not detected by the assay’s PCR primers. New primers were added to detect this new variant, and the assay was reapproved for use. This example illustrates the need for ongoing monitoring of any genotypic resistance assay. A second limitation is that a contradiction can occur between genotypic and phenotypic evidence for resistance. Up to 5% of MSSA strains have been reported to carry a copy of the mecA gene that is either nonfunctional or not expressed. Thus, the erroneous identification of these strains as MRSA by genotypic detec­ tion would lead to administration of the inferior antibiotic vancomycin rather than the preferred β-lactam therapy. These examples illustrate one of the prime challenges of mov­ ing beyond growth-based assays: genotype is merely a proxy for the resistance phenotype that directly informs patient care. Alternative approaches currently under development attempt to circumvent the limitations of genotypic resistance testing by returning to phenotypic assays, albeit more rapid ones. One such approach is informed by genomic methods: transcriptional profiles serve as a rapid phenotypic signature for antibiotic response. Conceptually, since dying cells are transcriptionally distinct from cells fated to survive, susceptible bac­ teria enact different transcriptional profiles after antibiotic exposure than resistant ones, independent of the mechanism of resistance. These differences can be measured and, since transcription is one of the most rapid responses to cell stress (minutes to hours), can be used to determine whether cells are resistant or susceptible much more rap­ idly than is possible if growth in the presence of antibiotics is awaited (days). Like DNA, RNA can be readily detected through predictable rules governing base pairing via either amplification or hybridizationbased methods. Changes in a carefully selected set of transcripts form an expression signature that can represent the total cellular response to antibiotic without requiring full characterization of the entire transcriptome. Preliminary proof-of-concept studies suggest that this approach may identify antibiotic susceptibility based on transcriptional phenotype much more quickly than is possible with growth-based assays. Other rapid phenotype-based approaches to antibiotic suscepti­ bility testing, including automated microscopy, ultrafine measurements of mass fluctuations, and others, are under development as well, with the former approved for clinical use. Because of its sensitivity in detecting even very rare nucleic acid fragments, sequencing provides an unprecedented depth of study into complex populations of cells and tissues. The strength of this depth and sensitivity applies not only to the detection of rare, novel pathogens in a sea of host signal but also to the identification of heterogeneous pathogen subpopulations in a single host that may differ, for example, in drug resistance profiles or pathogenesis determinants. For instance, recent studies have highlighted the diversification of pathogens in chronic bacterial infections, such as Pseudomonas in the lungs of patients with cystic fibrosis or M. tuberculosis in disseminated infec­ tion, perhaps allowing for niche specialization within the host. Such diversification has long been recognized in chronic viral populations, as exemplified by HIV. Future studies will be needed to elucidate the clinical significance of these variable subpopulations, even as deep sequencing is now providing unprecedented levels of detail about majority and minority members of this population. ■ ■HOST-BASED DIAGNOSTICS While pathogen-based diagnostics continue to be the mainstay for con­ firming infection, serologic testing and nonspecific biomarkers—such as erythrocyte sedimentation rate, C-reactive protein level, and even total white blood cell and neutrophil counts—have long been the basis of a strategy for measuring host responses to aid in the diagnosis of infection. Even recently identified host biomarkers of bacterial infec­ tion, such as procalcitonin, have fallen short in their versatility, with

positive and negative predictive values that are thus far adequate for only a few narrow applications but inadequate for generalized clinical use. Here, too, the application of genomics is now being explored to improve upon this approach, given the previously described limitations of serologic testing and the lack of specificity of protein biomarkers identified to date. Rather than using antibody responses as a retrospec­ tive biomarker for infection, recent efforts have focused on transcrip­ tomic analysis of the host response as a new direction with diagnostic implications for human disease. For instance, while pathogen-based diagnostic tests to distinguish active from latent tuberculosis infection have proven elusive, the tran­ scriptional profile of circulating white blood cells exhibits a differential pattern of expression of nearly 400 transcripts that distinguish active from latent tuberculosis; this expression pattern is driven in part by changes in interferon-inducible genes in the myeloid lineage. In a validation cohort, this transcriptional signature was able to distinguish patients with active versus latent disease, to distinguish tuberculosis infection from other pulmonary inflammatory states or infections, and to track responses to treatment in as little as 2 weeks, with normaliza­ tion of expression toward that of patients without active disease over 6 months of effective therapy. Such a test could play an important role not only in the management of patients but also as a marker of efficacy in clinical trials of new therapeutic agents. More recently, a distilled three-transcript signature has shown promise for distinguishing active from latent tuberculosis, raising hopes of a deployable assay in the near term. Similarly, considerable progress has been made toward identify­ ing host transcriptional signatures in circulating blood cells that distinguish viral from bacterial causes of upper respiratory infec­ tion, with better performance characteristics than current clinical parameters or available protein biomarkers. Additional host signa­ tures have been reported that distinguish among bacterial infection, viral infection, and inflammatory states; identify Lyme disease; identify influenza; and even distinguish between gram-positive and gram-negative bacterial infections. In some cases, results have been extended to different host populations—including adults and chil­ dren, and those with varying immune function—which obviously will be critical for generalizing such an approach. Thus, profiling of host transcriptional dynamics could augment the information obtained from studies of pathogens, both enhancing diagnosis and monitoring the progression of illness and the response to therapy. The frontier of genomic applications to understand host response to infection, with the potential of identifying biomarkers or even underlying disease biology, continues to rapidly advance, incor­ porating novel technological and computational approaches such as single-cell host transcriptional profiling of infected patients, to understand complex processes such as sepsis. In this era of genome-wide association studies and attempts to move toward personalized medicine, genomic approaches are also being applied to the identification of host genetic loci and factors that con­ tribute to infection susceptibility. Such loci will have undergone strong selection among populations in which the disease is endemic. Through identification of the beneficial genetic alleles among individuals who survive in such settings, markers for susceptibility or resistance are being discovered; these markers can be translated to diagnostic tests to identify susceptible individuals to implement preventive or pro­ phylactic interventions. Further, such studies may offer mechanistic insight into the pathogenesis of infection and inform new methods of therapeutic intervention. Such beneficial genetic associations were recognized long before the advent of genomics, as in the protective effects of the negative Duffy blood group or heterozygous hemoglobin abnormalities against Plasmodium infection. Genomic approaches allow more systematic and widespread application of this principle to identify not only people with increased susceptibility to prevalent diseases (e.g., HIV infection, tuberculosis, and cholera) but also host factors that contribute to and thus might predict the severity of dis­ ease. In one recent example, polymorphisms in certain genetic loci, or specific circulating autoantibodies, were found to be associated with severe COVID-19.

THERAPEUTICS Genomics has the potential to impact infectious disease therapeutics in two ways. By transforming the speed or type of diagnostic informa­ tion that can be attained, it can influence therapeutic decision-making. Alternatively, by opening new avenues to a better understanding of pathogenesis, providing new ways to disrupt infection, and delineating new approaches to antibiotic discovery or characterization of vaccine targets, it has the potential to facilitate the development of new thera­ peutic agents.

■ ■GENOMIC DIAGNOSTICS INFORMING THERAPEUTICS Efforts at antibiotic discovery are declining, with few new agents in the pipeline and even fewer new drugs (in particular, few agents with new mechanisms of action) entering the market. This phenomenon is due in part to the lack of economic incentives for the private sec­ tor; however, it is also attributable in part to the enormous challenges involved in the discovery and development of antibiotics. Most recent efforts have focused on broad-spectrum antibiotics; the development of a chemical entity that works across an extremely diverse set of organisms (i.e., species more divergent from each other than a human is from an amoeba) is far more challenging than the development of an agent that is designed to target a single bacterial species. Neverthe­ less, the concept of narrow-spectrum antibiotics has heretofore been rejected because of the lack of early diagnostic information that would guide the selection of such agents. Thus, rapid diagnostics providing antibiotic susceptibility information that can guide antibiotic selection in real time has the potential to alter and simplify antibiotic strategies by allowing a paradigm shift away from broad-spectrum drugs and toward narrow-spectrum agents. Such a paradigm shift clearly would have additional implications for antibiotic resistance, helping to limit selective pressure applied to pathogens and commensal bacteria dur­ ing therapy. CHAPTER 126 In yet another diagnostic paradigm with the potential to impact therapeutic interventions, genomics is opening new avenues to a better understanding not only of different host susceptibilities to infection but also of different host responses to therapy. For example, the role of glucocorticoids in tuberculous meningitis has long been debated. Recently, polymorphisms in the human genetic locus LTA4H, which encodes a leukotriene-modifying enzyme, were found to modulate the inflammatory response to tuberculosis. Patients with tuberculous men­ ingitis who were homozygous for the proinflammatory LTA4H allele were most helped by adjunctive glucocorticoid treatment, while those who were homozygous for the anti-inflammatory allele were nega­ tively affected by steroid treatment. Steroids have become part of the standard of care in tuberculous meningitis, but this study suggests that perhaps only a subset of patients benefit from this anti-inflammatory adjunct (while others may be harmed) and further suggests a genetic means of prospectively identifying this subset. Thus, genomic diagnos­ tic tests may eventually approach the goal of personalized medicine, informing diagnosis, prognosis, and treatment decisions by revealing the pathogenic potential of the microbe and by detecting individual­ ized host responses to both infection and therapy. Microbial Genomics and Infectious Disease ■ ■GENOMICS IN DRUG AND VACCINE DEVELOPMENT Genomic technologies are dramatically changing research on host– pathogen interactions, with a goal of increasingly influencing the process of therapeutic discovery and development. Sequencing offers several possible avenues into antimicrobial therapeutic discovery. First, genome-scale molecular methods have paved the way for comprehen­ sive identification of all essential genes encoded by a pathogen, thereby systematically identifying critical vulnerabilities within a pathogen that could be targeted therapeutically. Second, genome-scale methodologies offer rapid ways to address the mechanism of action of newly identified hits from compound screens. Whole genome sequencing offers a rapid, unbiased way to detect mutations arising in resistant mutants dur­ ing selection. Similarly, transcriptional profiling can provide insights into mechanisms of action of new candidate drugs. For instance, the

transcriptional signature of cell wall disruptors (e.g., β-lactams) is distinct from that of DNA-damaging agents (e.g., fluoroquinolones) or protein synthesis inhibitors (e.g., aminoglycosides). Either approach can thus suggest a mechanism of action or flag compounds for pri­ oritization because of a potentially novel activity. In an alternative strategy for determining mechanisms of action, genome-wide RNA interference or CRISPR screens can be used to identify genes required for antimicrobial efficacy. This approach provided new insights into the mechanism of action of drugs that have been in use for decades for human African trypanosomiasis. Third, sequencing can readily identify the most conserved regions of a pathogen’s genomes and cor­ responding gene products; this information is invaluable in narrowing antigen candidates in vaccine development. These surface proteins can be expressed recombinantly and tested for the ability to elicit a sero­ logic response and protective immunity. This process, termed reverse vaccinology, has proved particularly useful for pathogens that are diffi­ cult to culture or poorly immunogenic. After decades of development, the utility of this approach became dramatically apparent with the rapid development of mRNA vaccines targeting conserved regions of the SARS-CoV-2 genome, fueling the most rapid development of a vac­ cine in history. Comparative genomics informed by prior coronavirus sequences enabled the first mRNA vaccine design to begin within days of the first SARS-CoV-2 sequence being made publicly available, and now annual updates are guided by ongoing sequencing of circulating variants, as discussed in detail below.

Genomics has been employed in both developing vaccines and defining their impact on microbial epidemiology and ecology. Exam­ ples include recent studies of influenza, malaria, S. pneumoniae, and HPV following vaccine introduction. Extensive sequencing of influ­ enza viruses has been valuable in understanding the modest efficacy of seasonal influenza vaccination, and the combination of genomics and antigenic cartography is used to select strains to include in subsequent influenza vaccines. Beyond this, sequence conservation informs efforts to design more robust pan-influenza or pan-coronavirus vaccines. The RTS,S/AS01 malaria vaccine was analyzed by targeted sequencing of parasites from vaccinated and control populations during a phase 3 trial conducted at 11 sites in Africa; these analyses revealed reduced vaccine efficacy against parasites with amino acid mutations in the circumsporozoite protein targeted by the vaccine. Similarly, studies of the more established pneumococcal vaccines (the 7- and 13-valent polysaccharide conjugate vaccines, PCV-7 and PCV-13) documented serotype replacement: strains targeted by the vaccine have dramatically decreased in prevalence following widespread vaccination campaigns. Given that specific serotypes of HPV (e.g., types 16 and 18) clearly are more strongly associated than others with carcinogenesis, HPV vaccines have capitalized on serotype replacement, targeting vaccine strains to specifically prevent infection with the more dangerous sero­ types. Such a strategy, informed by pathogen genomics, aims to protect individuals and ideally to decrease the circulating burden of more virulent strains within society. PART 5 Infectious Diseases Large-scale gene content analysis from sequencing or expression profiling enables new research directions that provide novel insights into the interplay of pathogen and host during infection or coloniza­ tion. One important goal of such research is to suggest new therapeutic approaches to disrupt this interaction in favor of the host. Indeed, one of the most immediate applications of next-generation sequencing technology has come from simply characterizing human pathogens and related commensal or environmental strains and then finding genomic correlates for pathogenicity. For instance, as Escherichia coli varies from a simple nonpathogenic, lab-adapted strain (K-12) to a Shiga toxin–producing enterohemorrhagic gastrointestinal pathogen (O157:H7), it displays up to a 25% difference in gene content, though it is classified as the same species. Similarly, some isolates of Entero­ coccus—a genus notorious for its increasing incidence of resistance to common antibiotics such as ampicillin, vancomycin, and aminoglyco­ sides—also contain recently acquired genetic material comprising up to 25% of the genome on mobile genetic elements. This fact suggests that horizontal gene transfer plays an important role in the organisms’ adaptation as nosocomial pathogens. On closer study, this genome

expansion is associated with loss of CRISPR elements, which protect the bacterial genome from invasion by certain foreign genetic mate­ rial, and may thus facilitate the acquisition of antibiotic resistance– conferring genetic elements. While loss of this regulation appears to impose a competitive disadvantage in antibiotic-free environments, these drug-resistant strains thrive in the presence of even some of the best antienterococcal therapies. In addition to insights gained from genome sequencing, transcriptomic and proteomic profiling of patho­ gens under various conditions that mimic colonization or infection, including existence as biofilms or in polymicrobial communities, intra­ cellular infection models, antibiotic exposure, and nutrient starvation, has begun to reveal novel biologic features that may be targeted by the next generation of therapies. At the cutting edge of the host–pathogen interface, single-cell transcriptomic methodologies are rapidly increas­ ing in feasibility and extent, revealing previously unknown heterogene­ ity in the potential outcomes of intracellular infection. Thus, genomic studies are transforming our understanding of infec­ tion, offering evidence of virulence factors or toxins and providing insight into ongoing evolution of pathogenicity and drug resistance. One goal of such studies is to identify therapeutic agents that can disrupt the pathogenic process. There is currently much interest in the theoretical concept of antivirulence drugs that inhibit virulence factors rather than killing the pathogen outright as a means to intervene in infection. Further, with sequencing ever more accessible and efficient, ongoing large-scale studies have unprecedented statistical power to associate clinical outcomes with pathogen and host genotypes and thus to further reveal vulnerabilities in the infection process that can be targeted for disruption. Although this is just the beginning, such stud­ ies point to a tantalizing future in which the clinician is armed with genomic predictors of infection outcome and therapeutic response to guide clinical decision-making. EPIDEMIOLOGY OF INFECTIOUS DISEASES Epidemiologic studies of infectious diseases have several main goals: to identify and characterize outbreaks, to describe the pattern and dynamics of an infectious disease as it spreads through populations, and to identify interventions that can limit or reduce the burden of disease. One classic, paradigmatic example is John Snow’s elucidation of the origin of the 1854 London cholera outbreak. Snow used careful geographic mapping of cases to determine that the likely source of the outbreak was contaminated water from the Broad Street pump, and by removing the pump handle, he aborted the outbreak. Whereas that effort was undertaken without knowledge of the causative agent of cholera, advances in microbiology and genomics have expanded the purview of epidemiology to consider not just the disease but also the pathogen and its genetic variants, its virulence factors, and the complex relationships between microbial and host populations. Through use of genomic tools such as high-throughput sequenc­ ing, the diversity of a microbial population can be rapidly described with unprecedented resolution, with discrimination between isolates that have single-nucleotide differences across the entire genome and advancement beyond prior approaches that relied on phenotypes (such as antibiotic susceptibility profiles) or genetic markers (such as multilocus sequence typing). The development of statistical methods grounded in molecular genetics and evolutionary theory has estab­ lished analytical approaches that translate descriptions of microbial population diversity and structure into descriptions of the origin and history of pathogen spread. By linking phylogenetic reconstruction with epidemiologic and demographic data, genomic epidemiology presents the opportunity to track transmission from person to person and across demographic and geographic boundaries, to infer transmis­ sion patterns of both pathogens and sequence elements that confer phenotypes of interest, and to estimate the transmission dynamics of outbreaks. ■ ■TRANSMISSION NETWORKS Whole genome sequencing of pathogen genomes can be used to infer transmission and identify point-source outbreaks. As reported in a seminal paper in 2010, a study of MRSA in a Thai hospital demonstrated

the use of whole genome sequencing in reconstructing the transmis­ sion of a pathogen from patient to patient by integrating the analysis of accumulation of mutations over time with the dates and hospital locations of the infected individuals. Since then, multiple instances of the use of whole genome sequencing to define and motivate interven­ tions aimed at interrupting transmission chains have been reported. In another MRSA outbreak in a special-care baby unit in Cambridge, United Kingdom, whole genome sequencing extended the traditional infection control analysis, which relies on typing organisms by their antibiotic susceptibilities, to sequencing of isolates from clinical sam­ ples. This approach identified an otherwise unrecognized outbreak of a specific MRSA strain that was occurring against a background of the usual pattern of infection caused by a diverse circulating population of MRSA strains. The analysis showed evidence of transmission among mothers within the special-care baby unit and in the community and demonstrated the key role of MRSA carriage in a single health care provider in the persistence of the outbreak. In yet another example, in response to the observation of 18 cases of infection by carbapenemaseproducing Klebsiella pneumoniae over 6 months at the National Insti­ tutes of Health Clinical Research Center, genome sequencing of the isolates was used to discriminate between the possibilities that these cases represented multiple, independent introductions into the health care system or a single introduction with subsequent transmission. On the basis of network and phylogenetic analysis of genomic and epide­ miologic data, the authors reconstructed the likely relationships among the isolates from patient to patient, demonstrating the nosocomial spread of a single resistant Klebsiella strain. Similar approaches have elucidated the extent to which presumed nosocomial C. difficile, VRE, and carbapenem-resistant Enterobacterales represent within-hospital transmission—of bacterial strains or of plasmids and mobile genetic elements—versus independent acquisitions of unrelated pathogens. With these demonstrations of the potential contribution of genomics to hospital infection-control efforts, an important avenue of research seeks to develop statistical methods with which to ascertain when such tools are useful and their cost-effectiveness when compared with that of current nongenomic approaches. Genome sequencing of clinical specimens of viruses has been used to understand their patterns of spread and the clinical and epidemio­ logic implications of genetic variants. As RNA viruses use an errorprone RNA-dependent RNA polymerase, they accumulate mutations at a rapid rate, facilitating inferences about the dynamics and patterns of spread. These tools have been applied to the study of outbreaks of well-known viruses, such as recent outbreaks of yellow fever in South America and mumps in the United States, as well as recent zoonotic pathogens, such as the coronaviruses MERS-CoV and SARS-CoV. The sequencing of SARS-CoV-2 in the context of the pandemic has offered a powerful example of the contributions that genomic epidemiology can make to, and its increasingly central role in, tracking the spread of a pathogen both locally and globally and informing policy and public health decision-making. Moreover, tracking variants and combining the genome sequences with epidemiologic line-list data enable inves­ tigation of the extent to which new variants cause differing symptom profiles and levels of severe disease. The uncovering of unexpected transmission events by genomic epi­ demiology studies is motivating investigations into pathogen ecology and modes of transmission. Whole genome sequencing established the clonality of several high-profile outbreaks, enabling the discovery of dangerous contaminants such as Burkholderia pseudomallei in aro­ matherapy spray, Pseudomonas in eyedrops, Exserohilum in injectable corticosteroids, Fusarium in epidural anesthetic preparations, E. coli O157:H7 in beef, and Mycobacterium chimaera in the temperaturecontrol systems used during cardiac bypass. Each of these outbreaks caused considerable morbidity and in some cases mortality before being localized and prevented on the basis of investigations informed and accelerated by genomic epidemiology. As more studies aim to carefully define the origins and spread of infectious agents using the high-resolution lens of whole genome sequencing, fundamental questions arise about the diversity of infect­ ing and colonizing microbial populations. Traditional microbiologic

methods include taking a single colony from a growth plate as repre­ sentative of the population. However, the more diverse the colonizing or infecting pathogen population, the less representative these individ­ ual isolates are and the greater the possibility for introducing error into whole genome sequencing–based methods while reconstructing trans­ mission. Sequencing studies of multiple colonies of an S. aureus strain colonizing a single individual showed a “cloud” of diversity. What is the clinical significance of this diversity? What are the processes that generate and limit it? What amount of diversity is transmitted under different conditions and routes of transmission? How do the answers to these questions vary by infectious organism, type of infection, host, and response to treatment? More comprehensive descriptions of diver­ sity, population dynamics, transmission bottlenecks, and the forces that shape and influence the growth and spread of microbial populations will be a critically important focus of future investigations.

■ ■ORIGINS AND DYNAMICS OF PATHOGEN SPREAD In addition to reconstructing the transmission chains of local out­ breaks, genomics-based epidemiologic methods reveal broad-scale geographic and temporal spread of pathogens. Four examples include the origins of cholera in Haiti, the history of HIV-1 group M, the spread of Ebola in West Africa, and the timing and nature of spread of the zoonotic COVID-19 pandemic, which brought genomic tracking of pathogen variants to the forefront of public consciousness for a time. Cholera, a dehydrating diarrheal illness caused by infection with Vibrio cholerae, first spread worldwide from the Indian subcontinent in the 1800s and has since caused seven pandemics; the seventh pandemic has been ongoing since the 1960s. An investigation into the geographic patterns of cholera spread in the seventh pandemic used genome sequences from a global collection of 154 V. cholerae strains repre­ senting isolates from 1957 to 2010. This investigation revealed that the seventh pandemic has comprised at least three overlapping waves spreading out from the Indian subcontinent (Fig. 126-3A). Further, analysis of the genome of an isolate of V. cholerae from the 2010 out­ break of cholera in Haiti showed it to be more closely related to isolates from South Asia than to isolates from neighboring Latin America, sup­ porting the hypothesis that the outbreak was derived from V. cholerae introduced into Haiti by human travel (likely from Nepal) rather than by environmental or more geographically proximal sources. A subse­ quent study that dated the time to the most recent common ancestor of a population of V. cholerae isolates from Haiti provided further sup­ port for a single point-source introduction from Nepal. Application of similar methods that integrate pathogen genome sequences, mutation rates, geographic locations, and phylogenetic inference to HIV-1 group M dated the origin of the virus to the 1920s and the city of Kinshasa (then called Leopoldville), the capital of the Democratic Republic of the Congo (then called the Belgian Congo). This work established an understanding of how a boom in industry and a city with extensive railroad connections provide a scaffolding along which a virus can rapidly spread geographically. CHAPTER 126 Microbial Genomics and Infectious Disease Genome sequencing has proven invaluable in understanding the geographic, demographic, climatic, and administrative factors that drove, sustained, and limited the 2013–2016 Ebola outbreak that rav­ aged West Africa (Fig. 126-3B) as well as the factors and patterns of transmission of Zika virus in the Americas and most recently the tim­ ing and origins of SARS-CoV-2 transmission in human populations. With the rapid availability of the SARS-CoV-2 viral genome sequence, data from a set of cases from early in the pandemic enabled inference of the time to the most recent common ancestor, supporting that SARSCoV-2 entered circulation in human populations in Wuhan, China, sometime in late November to early December of 2019. Subsequently, large, coordinated sequencing networks have been able to recreate its patterns of global spread, discover new variants, and monitor as these variants disseminate. For the first time, scientists and even the public were able to watch viral evolution through a population in almost real time. First, a single-nucleotide polymorphism (D614G in the viral spike protein) displaced the original strain in early 2020, followed in subsequent years by other variants such as Alpha, Delta, Omicron, and then various Omicron sublineages becoming dominant either

A 1.00 0.75 PART 5 Infectious Diseases Variant Fraction Omicron (and subvariants) Founder strain 0.50 0.25 0.00 Oct 2020 Apr 2021 Oct 2021 Apr 2022 Oct 2022 Apr 2023 Oct 2023 C B FIGURE 126-3  A. Transmission events inferred from phylogenetic reconstruction of 154 Vibrio cholerae isolates from the seventh cholera pandemic. Date ranges represent estimated time to the most recent common ancestor for strains transmitted from source to destination locations, based on a Bayesian model of the phylogeny. (Reprinted by permission from the Nature Publishing Group, Nature 477:462. Evidence for several waves of global transmission in the seventh cholera pandemic, A Mutreja et al © 2011.) B. Inferred Ebola virus spread in West Africa (Liberia, red; Guinea, green; and Sierra Leone, blue) by phylogeographic methods using virus genome sequences, dates, and an evolutionary model. The lines reflect spread between population centroids of each administrative region, going from the thin end to the thick end and colored by a time scale. (Reprinted by permission from Nature Publishing Group, Nature 544:309. Virus genomes reveal factors that spread and sustained the Ebola epidemic, G Dudas et al © 2017.) C. SARS-CoV-2 variant proportions in the United States from the start of the COVID-19 pandemic through March 6, 2024. Each major variant family that rose to dominance in the United States is labeled. (Variant proportions are taken from https://covariants.org, accessed April 21, 2024, which derives underlying variant data from the Global Initiative on Sharing All Influenza Data [GISAID].) regionally or globally (Fig. 126-3C) as their mutation profile conferred advantages in increasingly immune populations. One compelling area of innovation in molecular epidemiology is the development of wastewater-based surveillance. Wastewater sur­ veillance has been used to monitor for the appearance of poliovirus, among others, but its widespread and widely publicized use for SARSCoV-2 in “nowcasting” quantitative estimates of epidemic trends has galvanized interest in using wastewater to monitor disease trends and pathogens and their characteristics. The U.S. Centers for Disease Con­ trol and Prevention has sought to establish a national wastewater sur­ veillance system, building on the SARS-CoV-2 work. These tools can help provide a population-level view of the first appearance of a patho­ gen, variant, or antibiotic resistance determinant and can also help monitor the population burden and hence the epidemic curve. Work remains to develop robust, repeatable, and accurate methods across many pathogens, particularly those that can replicate or exchange DNA

Delta Alpha in wastewater environments. Nonetheless, this data source has great promise for providing early signals to decision-makers to help inform public health actions. These efforts illustrate the remarkable promise of genome sequenc­ ing in improving outbreak response strategies by elucidating previously hidden origins and paths of disease spread and details of the forces that shape epidemics. The combination of in-the-field sequencing with portable sequencing platforms, rapid data sharing, and rapid open analysis through sites such as nextstrain.org offers a paradigm by which real-time genomic epidemiology, including wastewater-based epidemiology, may contribute to “weather maps,” enabling prediction of epidemic patterns in space and time and thus providing guidance for public health interventions to slow or control their spread. Increasing numbers of investigations into the spread of many patho­ gens are contributing to a growing atlas of maps describing routes, patterns, and tempos of microbial diversification and dissemination,

not just for agents of emerging infectious diseases but for common pathogens as well. Such studies will create a vast amount of data that can be used to investigate the diversity and microbiologic links within distinct niches and the patterns of spread from one niche to another. The increasingly broad adoption of genome sequencing by health care and public health institutions ensures that the available catalog of genome sequences and associated epidemiologic data will grow very rapidly. For example, updating from the pulsed-field gel electropho­ resis techniques that have been used to define strains of food-borne pathogens since the late 1980s, PulseNet—the U.S. Centers for Disease Control and Prevention network for monitoring these pathogens—has instituted routine genome sequencing. The COVID-19 pandemic fur­ ther underscores the importance of building a new global public health infrastructure in which sequencing plays a central role to facilitate early disease discovery, rapid and close tracking of spread, and develop­ ment of diagnostics and targeted effective interventions. With higherresolution description of microbial diversity and of the dynamics of that diversity over time and across epidemiologic and demographic boundaries and evolutionary niches, we will gain even greater insights into the relationships of transmission routes and patterns of historical spread. ■ ■EPIDEMIC POTENTIAL Defining pathogen transmissibility is a critical step in the development of public health surveillance and intervention strategies because this information can help to predict the epidemic potential of an outbreak. Transmissibility can be estimated by a variety of methods, including inference from the growth rate of an epidemic and the generation time of an infection (the mean interval between infection of an index case and infection of the people infected by that index case). Genome sequencing and analysis of a well-sampled population provide another method by which to derive similar fundamental epidemiologic param­ eters. One key measure of transmissibility is the basic reproduction number, defined as the number of secondary infections generated from a single primary infectious case. When the basic reproduction number is >1, an outbreak has epidemic potential; when it is <1, the outbreak will become extinct. On the basis of sequences from influenza virus samples obtained from infected patients very early in the 2009 H1N1 influenza pandemic, the basic reproduction number was estimated through a population genomic analysis at 1.2; this result provided greater confidence to estimates derived by traditional epidemiologic data, which ranged from 1.4 to 1.6. In addition, with the assumption of a molecular clock model, sequences of H1N1 samples together with information about when and where the samples were obtained have been used to estimate the date and location of the pandemic’s origin, providing insight into disease origins and dynamics. Integrating viral genomics with other types of data—such as the timing and nature of mitigation efforts and the impact of those efforts on mobility—will expand the toolkit with which to assess the impact of public health interventions on slowing and controlling disease spread. These tools may also be applied to institutional infection control: with the devel­ opment of return-to-work protocols, sequencing offers one option to help learn the extent to which infections arose from within-institution spread. Because the magnitude and intensity of the public health response are guided by the predicted size of an outbreak, the ability of genomic methods to cast light on a pathogen’s origin and epidemic potential adds an important dimension to the contributions of these methods to infectious disease epidemiology. ■ ■PATHOGEN EVOLUTION Beyond describing transmission and dynamics, pathogen genomics can provide insight into the evolution of pathogens and the interactions of selective pressures, the host, and pathogen populations, which can have implications for clinical decision-making and the development of vaccines and therapeutics. From a clinical perspective, this process is central to the acquisition of antibiotic resistance, the generation of increasing pathogenicity or new virulence traits, the evasion of host immunity and clearance (leading to chronic infection), and vaccine efficacy.

Microbial genomes evolve through a variety of mechanisms, includ­ ing mutation, duplication, insertion, deletion, recombination, and horizontal gene transfer. Segmented viruses (e.g., influenza virus) can reassort gene segments within multiply infected cells. The pandemic 2009 H1N1 influenza A virus, for example, appears to have been generated through reassortment of several avian, swine, and human influenza strains. Such potential for the evolution of novel pandemic strains has precipitated concern about the possible evolution to trans­ missibility of virulent strains that have been associated with high mortality rates but have not yet exhibited efficient human infectivity. Experiments with H5N1 avian influenza, for example, have defined five mutations that render it transmissible, at least in ferrets—the animal model system for human influenza. Studies that examine the genomes of pathogens collected longitudinally from individual infec­ tions have similarly demonstrated their evolution as they adapt to host environments and new immune and therapeutic pressures.

The continuous antigenic evolution of seasonal influenza offers an example of how studies of pathogen evolution can impact surveillance and vaccine development. Frequent updates to the annual influenza vaccine are needed to ensure protection against the dominant strains. These updates are based on anticipating which viral populations from a pool of substantial locally and globally diverse circulating viruses will predominate in the upcoming season. Toward that end, sequencingbased studies of influenza virus dynamics have shed light on the global spread of influenza, providing concrete data on patterns of spread and helping to elucidate the origins, emergence, and circulation of novel strains. Through analysis of >1000 influenza A H3N2 virus isolates over the 2002–2007 influenza seasons, Southeast Asia was identified as the usual site from which diversity originates and spreads worldwide. Further studies of global isolate collections have shed further light on the diversity of circulating virus, showing that some strains persist and circulate outside of Asia for multiple seasons. Similar studies with SARS-CoV-2, including genome sequences from orders of magnitude more clinical specimens than the aforementioned influenza study, have helped identify where new SARS-CoV-2 variants have emerged and informed vaccine composition. CHAPTER 126 Microbial Genomics and Infectious Disease Not only do genomic epidemiology studies have the potential to help guide vaccine selection and development, but they also help to track what happens to pathogens circulating in the population in response to vaccination. By describing pathogen evolution under the selective pressure of a vaccinated population, such studies can play a key role in surveillance and identification of virulence determinants and perhaps may even help to predict the future evolution of escape from vaccine protection. The seven-valent pneumococcal conjugate vaccine (PCV-7) targeted the seven serotypes of S. pneumoniae respon­ sible for the majority of invasive disease at the time of its introduction in 2000; since then, PCV-7 has dramatically reduced the incidence of pneumococcal disease and mortality. However, sequencing of >600 Massachusetts pneumococcal isolates from 2001 to 2007 has shown that, in the pneumococcal population, previously rare nonvaccine serotypes are replacing vaccine serotypes and that some vaccine strains have persisted despite vaccination by recombining the vaccine-targeted capsule locus with a cassette of capsule genes from non-vaccine-targeted serotypes. Studying the virulence of these persistently circulating strains can help to rationally update vaccine composition. The large collections of pathogen genome sequences are driving development of tools to decipher the genetic basis for antibiotic resis­ tance, virulence, and infection risk. Some pathogens have distinct types of clinical manifestations, the basis for which we are just beginning to unravel with the aid of genomics. For example, Listeria is a food-borne pathogen that can cause both central nervous system infections and maternal/neonatal infections. Although all Listeria isolates are treated the same from a public health perspective, variation in outcomes exists and appears to be linked to the strains’ genomic background. Molecular analysis of a national reference laboratory’s collections of well-characterized specimens, based on the fraction of immunocom­ petent people in which they caused disease, revealed that some clonal complexes of Listeria appear to be more virulent than others. Linking epidemiology and comparative genomics then enabled enumeration

of putative virulence factors that contribute to the clinical phenotypes as well as identification and confirmation of a novel gene cluster that mediates central nervous system tropism. This approach illustrates progress toward a future in which we can link pathogen identification with risk, thereby informing resource use and allocation.

GLOBAL CONSIDERATIONS While cutting-edge genomic technologies are largely implemented in the developed world, their application to infectious diseases perhaps offers the biggest potential impact in less developed regions where the burden of these infections is greatest. This globalization of genomic technology and its extensions has already begun in each of the areas of focus highlighted in this chapter; it has occurred both through the application of advanced technologies to samples collected in the devel­ oping world and through the adaptation and importation of technolo­ gies directly to the developing world for on-site implementation as they become more globally accessible. Genomic characterization of the pathogens responsible for such important global illnesses, such as tuberculosis, malaria, trypanoso­ miasis, cholera, and COVID-19, has led to insights in diagnosis, treat­ ment, and infection control. For instance, with the increasing burden of drug-resistant tuberculosis in the developing world, a molecular diag­ nostic test has been developed to detect rifampin-resistant tuberculo­ sis. The genetic basis for rifampin resistance has been well defined by targeted sequencing: characteristic mutations in the molecular target of rifampin, RNA polymerase, account for the vast majority of instances of rifampin resistance. At least in areas that can afford to implement it, a rapid, automated PCR assay that can detect both M. tuberculosis and a rifampin-resistant allele of RNA polymerase directly in clinical samples has been implemented in parts of Africa and Asia, transforming the recognition and management of incident tuberculosis and multidrug resistance where they are most prevalent. Since rifampin resistance frequently accompanies resistance to other antibiotics, this test can suggest the presence of multidrug-resistant M. tuberculosis within hours instead of weeks, without the infrastructure required for culture. PART 5 Infectious Diseases High-resolution genomic tracking of the spread of epidemics—from cholera to Ebola to Zika to COVID-19—has yielded insights into which public health measures may prove most effective in controlling local epidemics. Many genomic tracking efforts have involved close collaborations with local scientists and public health officials, and considerable investment in sequencing infrastructure in sub-Saharan Africa has made on-location epidemic tracking in the event of another such outbreak feasible. Such investment can not only enable real-time outbreak recognition and tracking but also provide the infrastructure needed to capitalize on the many other benefits of high-throughput sequencing as they are developed. The early returns of such invest­ ments are exemplified by the rapid reporting of genome sequences for SARS-CoV-2, with substantial insights from sequencing efforts across the world; perhaps most notably, the Omicron variant was first identified in South Africa, enabling preparations for what became a global sweep in the subsequent weeks. Overall, sequencing efforts have become cheaper and have moved closer to point-of-care with each passing year. As these technologies synergize with efforts to globalize information-technology resources, global implementation of genomic methods promises to spread state-of-the-art methods for diagnosis, treatment, and epidemic tracking of infections to areas that need these capabilities the most. GENOMICS AND THE COVID-19 PANDEMIC The COVID-19 pandemic, which began in 2019 and spread worldwide in 2020, resulted in hundreds of millions of documented infections and millions of deaths and serves as a prime example of the pandemic potential of infectious pathogens. It also demonstrated the central role that genomic tools now play in response to infectious outbreaks, ranging from enabling diagnostics and vaccines to tracking evolution, virulence, and transmissibility of the pathogen. The rapid discovery of SARS-CoV-2 and sequencing of its genome was complete within weeks of the recognition of the clinical syndrome. The rapid public sharing of this genome sequence led directly to two key interventions: diagnostic

assay development via RT-qPCR and vaccine design. Crucially, vaccine development was informed by homology of the SARS-CoV-2 sequence to SARS and MERS coronaviruses. The dominant antigen of those viruses, the surface protein Spike, was well characterized, enabling the design of the first SARS-CoV-2 vaccines to begin the day after the genome sequence was shared. The progress of the most rapidly developed and validated vaccine in human history was unquestionably accelerated by genomic technology. Whole genome sequencing has also played a large role in outbreak tracking and confirmation of case clusters in institutional settings such as hospitals or congregate living facilities, in helping to distinguish reinfections from recrudescence or prolonged viral shedding, in monitoring spread through societ­ ies, and in tracking pathogen evolution, including the emergence of new variants of concern with altered transmissibility, severity, and/ or partial evasion of the immune response generated to prior versions of the virus, vaccines, or monoclonal antibody therapeutics. Finally, cutting edge genomic methods including single-cell transcriptional profiling and genome-wide association studies are contributing to our understanding of the wide variability in outcomes of SARS-CoV-2 infection, ranging from asymptomatic carriage to death. Overall, just as the global response to the COVID-19 pandemic underscores the indispensable role that genomics methods have come to play in the clinical and public health management of infectious diseases, the dev­ astating impact of this pandemic reveals the urgent need for further development and implementation of tools for disease surveillance and response. SUMMARY By illuminating the genetic information that encodes the most fun­ damental processes of life, genomic technologies are transforming many aspects of medicine. In infectious diseases, methods such as next-generation sequencing and genome-scale expression analysis offer information of unprecedented depth about individual microbes as well as microbial communities. This information is expanding our understanding of the interactions of microorganisms with each other, their human hosts, and the environment. Despite technological and financial barriers that slow the widespread adoption of large-scale pathogen sequencing in clinical and public health settings, genomic methodologies have utterly transformed the research landscape in infectious disease and are beginning to make meaningful inroads into clinical settings. As even vaster amounts of data are generated, innovations in data storage, development of bioinformatics tools to manipulate the data, standardization of methods, and training of end-users in both the research and clinical realms will be required. The cost-effectiveness and applicability of whole genome sequenc­ ing, particularly in the clinic, remain to be studied, and studies of the impact of genome sequencing on patient outcomes will be needed to clarify the contexts in which these new methodologies can make the greatest contributions to patient well-being. The ongoing efforts to overcome limitations through collaboration, teaching, and reduction of financial obstacles should be applauded and expanded. With advances in genomic technologies and computational analysis, our ability to detect, characterize, treat, monitor, prevent, and control infections has advanced rapidly in recent years and will continue to do so, with the hope of heralding a new era where the clinician is better armed to combat infection and promote human health. ■ ■FURTHER READING Bullman S et al: Emerging concepts and technologies for the discov­ ery of microorganisms involved in human disease. Annu Rev Pathol 12:217, 2017. Burnham CD et al: Diagnosing antimicrobial resistance. Nat Rev Microbiol 15:697, 2017. Levy JI et al: Wastewater surveillance for public health. Science 379:26, 2023. CRyPTIC Consortium et al: Prediction of susceptibility to first-line tuberculosis drugs by DNA sequencing. N Engl J Med 379:1403, 2018. Dudas G et al: Virus genomes reveal factors that spread and sustained the Ebola epidemic. Nature 544:309, 2017.

05 - 127 Approach to the Acutely Ill Infected Febrile Patient

127 Approach to the Acutely Ill Infected Febrile Patient

Gardy JL, Loman NJ: Towards a genomics-informed, real-time, global pathogen surveillance system. Nat Rev Genet 19:9, 2018. Grubaugh ND et al: Tracking virus outbreaks in the twenty-first century. Nat Microbiol 4:10, 2019. Loman NJ, Pallen MJ: Twenty years of bacterial genome sequencing. Nat Rev Microbiol 13:787, 2015. Mutreja A et al: Evidence for several waves of global transmission in the seventh cholera pandemic. Nature 477:462, 2011. Viana R et al: Rapid epidemic expansion of the SARS-CoV-2 Omicron variant in southern Africa. Nature 603:679, 2022. Tamar F. Barlam

Approach to the Acutely

Ill Infected Febrile Patient The physician treating the acutely ill febrile patient must be able to recognize infections that require emergent attention. If such infec­ tions are not adequately evaluated and treated at initial presenta­ tion, the opportunity to alter an adverse outcome may be lost. In this chapter, the clinical presentations of and approach to patients with infectious disease emergencies are discussed. These infec­ tious processes and their treatments are discussed in detail in other chapters. APPROACH TO THE PATIENT Acute Febrile Illness Before the history is elicited and a physical examination is per­ formed, an immediate assessment of the patient’s general appear­ ance can yield valuable information. The perceptive physician’s subjective sense that a patient is septic or toxic often proves accu­ rate. Visible agitation or anxiety in a febrile patient can be a harbin­ ger of critical illness. HISTORY Presenting symptoms are frequently nonspecific. Detailed ques­ tions should be asked about the onset and duration of symptoms and about changes in severity or rate of progression over time. Host factors, such as extremes of age and comorbid conditions, may increase the risk of infection with certain organisms or of a more fulminant course than is usually seen. Lack of splenic func­ tion, alcoholism with significant liver disease, IV drug use, HIV infection, diabetes, malignancy, morbid obesity, organ transplan­ tation, and chemotherapy all predispose to specific infections and frequently to increased severity. The patient should be questioned about factors that might help identify a nidus for invasive infec­ tion, such as recent upper respiratory tract infections, influenza, or varicella; prior trauma; disruption of cutaneous barriers due to lacerations, burns, surgery, body piercing, or decubiti; and the presence of foreign bodies or prosthetic devices. Travel, presence during a natural disaster such as a hurricane or tsunami, contact with pets or other animals, or activities that might result in tick or mosquito exposure can lead to diagnoses that would not other­ wise be considered. Recent dietary intake, medication use, social or occupational contact with ill individuals, vaccination history, recent sexual contacts, and menstrual history may be relevant. Pregnancy might increase the risk and severity of some illnesses, such as influenza or COVID-19, or increase the risk of significant morbidity for the fetus, as in Listeria or Zika virus infection. A

detailed review of systems should include any neurologic signs or sensorium alterations, rashes or skin lesions, and focal pain or tenderness. PHYSICAL EXAMINATION A complete physical examination should be performed, with spe­ cial attention to several areas that are sometimes given short shrift in routine examinations such as assessment of the patient’s general appearance and a detailed skin, soft tissue, and neurologic evaluation. The patient may appear either anxious and agitated or lethargic and apathetic. Fever is usually present, although elderly patients and compromised hosts (e.g., patients who are uremic or cir­ rhotic and those who are taking glucocorticoids or nonsteroidal anti-inflammatory drugs) may be afebrile despite serious under­ lying infection. Critically ill patients may be hypothermic, with a high risk of organ failure and mortality. Mortality at 30 days decreases with increasing temperature at presentation. Measure­ ment of blood pressure, heart rate, and respiratory rate and oxy­ gen saturation helps determine the degree of hemodynamic and metabolic compromise. The etiologic diagnosis may become evident in the context of a thorough skin examination (Chap. 21). Petechial rashes are typically seen with meningococcemia or Rocky Mountain spotted fever (RMSF; see Fig. A1-16); erythroderma is associated with toxic shock syndrome (TSS). On soft tissue and muscle examina­ tion, areas of erythema or duskiness, edema, and tenderness may indicate underlying necrotizing fasciitis, myositis, or myonecrosis. The neurologic examination must include a careful assessment of mental status for signs of early encephalopathy. Evidence of nuchal rigidity or focal neurologic findings should be sought. CHAPTER 127 DIAGNOSTIC WORKUP After a quick clinical assessment, diagnostic material should be obtained rapidly and antibiotic and supportive treatment begun. Blood (for cultures; baseline complete blood count with differential; measurement of serum electrolytes, blood urea nitrogen, serum creatinine, and serum glucose; liver function tests and serum lac­ tate; and d-dimer) can be obtained at the time an IV line is placed and before antibiotics are administered. Three sets of blood cultures should be performed for patients with possible acute endocarditis. Blood smears from patients at risk for severe parasitic disease, such as malaria or babesiosis (Chaps. 231, 232, and A2), must be examined for the diagnosis and quantitation of parasitemia. Blood smears may also be diagnostic in ehrlichiosis and anaplasmosis. Testing of a nasopharyngeal sample for COVID-19 or influenza may be indicated. Approach to the Acutely Ill Infected Febrile Patient Patients with possible meningitis should have cerebrospinal fluid (CSF) obtained before the initiation of antibiotic therapy. Focal findings, depressed mental status, or papilledema should be evaluated by brain imaging prior to lumbar puncture, which, in this setting, could initiate herniation. Antibiotics should be administered before imaging but after blood for cultures has been drawn. If CSF cultures are negative, blood cultures will provide the diagnosis in 50–70% of cases. Molecular diagnostic techniques (e.g., broadrange 16S rRNA gene polymerase chain reaction testing for bacte­ rial meningitis pathogens) are of increasing importance in the rapid diagnosis of life-threatening infections. CT or MRI may be necessary to evaluate focal processes that require urgent surgical intervention. Other diagnostic procedures, such as wound cultures, should not delay the initiation of treatment for more than minutes. Once emergent evaluation, diagnostic pro­ cedures, and (if appropriate) surgical consultation (see below) have been completed, other laboratory tests can be conducted. Appropri­ ate radiography; CT and/or MRI imaging; urinalysis; measurement of the erythrocyte sedimentation rate, C-reactive protein, and/or procalcitonin; and transthoracic or transesophageal echocardiogra­ phy all may prove important.

TREATMENT The Acutely Ill Patient In the acutely ill patient, empirical antibiotic therapy for presumed bacterial or fungal infection is critical and should be administered without undue delay in addition to fluid resuscitation and vasopres­ sor support as needed. Increased prevalence of antibiotic resistance in community-acquired bacteria must be considered when antibiot­ ics are selected. Table 127-1 lists first-line empirical regimens for infections considered in this chapter. In addition to the rapid initia­ tion of antibiotic therapy, several of these infections require urgent surgical attention. Neurosurgical evaluation for subdural empyema, otolaryngologic surgery for possible mucormycosis, and cardiotho­ racic surgery for critically ill patients with acute endocarditis are as important as antibiotic therapy. For infections such as necrotizing fasciitis and clostridial myonecrosis, rapid surgical intervention supersedes other diagnostic or therapeutic maneuvers. Adjunctive treatments may reduce morbidity and mortality rates and include dexamethasone for bacterial meningitis or IV immu­ noglobulin for TSS. Adjunctive therapies should usually be initiated within the first hours of treatment; however, dexamethasone for bacterial meningitis must be given before or at the time of the first dose of antibiotic. Glucocorticoids may also be harmful—e.g., when given in the setting of cerebral malaria or viral hepatitis. SPECIFIC PRESENTATIONS The infections considered below according to common clinical pre­ sentation can have rapidly catastrophic outcomes, and their immediate recognition and treatment can be life-saving. PART 5 Infectious Diseases ■ ■SEPSIS WITHOUT AN OBVIOUS FOCUS OF PRIMARY INFECTION Patients initially have a brief prodrome of nonspecific symptoms and signs that progresses quickly to hemodynamic instability with hypoten­ sion, tachycardia, tachypnea, respiratory distress, and altered mental status. Disseminated intravascular coagulation (DIC) with clinical evidence of a hemorrhagic diathesis is a poor prognostic sign. Septic Shock (See also Chap. 315)  Patients with bacteremia leading to septic shock may have a primary site of infection (e.g., pneumonia, pyelonephritis, or cholangitis) that is not evident initially. Elderly patients who may have atypical presentations and often have comorbid conditions, hosts compromised by malignancy and neutro­ penia, and patients who have recently undergone a surgical procedure or hospitalization are at increased risk for an adverse outcome. Gramnegative bacteremia with organisms such as Pseudomonas aeruginosa or Escherichia coli and gram-positive infection with organisms such as Staphylococcus aureus (including methicillin-resistant S. aureus [MRSA]) or group A streptococci can present as intractable hypo­ tension and multiorgan failure. Treatment can usually be initiated empirically on the basis of the presentation, host factors (Chap. 315), and local patterns of bacterial resistance. Outcomes are worse when antimicrobial treatment is delayed or when the responsible pathogen ultimately proves not to be susceptible to the initial regimen. The increasing prevalence of multidrug-resistant organisms makes this especially relevant. Broad-spectrum antimicrobial agents are therefore recommended and should be instituted rapidly, preferably within the first hours after presentation with septic shock. Pharmacodynamics are altered in sepsis due to increased volume of distribution and renal clearance, so it is important to adequately dose antimicrobials. Risk fac­ tors for fungal infection should be assessed, as the incidence of fungal septic shock is increasing. Nonbacterial causes of shock, such as den­ gue virus infection, should be considered in endemic areas. Glucocor­ ticoids are often considered for patients with severe sepsis who do not respond to fluid resuscitation and vasopressor therapy, but conclusive evidence for efficacy in this setting is lacking. Overwhelming Infection in Asplenic Patients (See also Chap. 315)  Patients without splenic function are at risk for

overwhelming bacterial sepsis compared with the general population. The median interval between splenectomy and sepsis is 4–6 years, with a range of 1–19 years. Almost 50% or more of these infections occur within the first l or 2 years, but the increased risk persists throughout life. Encapsulated bacteria cause the majority of infections. Strepto­ coccus pneumoniae is the most common isolate, causing 40–70% of cases. Children less than 5 years of age are at 15 times higher risk of invasive pneumococcal disease than adults, who are more likely to have antibody to these organisms. The risk of infection with Haemophilus influenzae or Neisseria meningitidis also is greater in patients without splenic function, but reported cases are declining. Severe clinical mani­ festations of infections due to other organisms, such as E. coli, S. aureus, Bordetella holmesii, Capnocytophaga, Babesia, and Plasmodium species, have been described. Babesiosis (See also Chap. 232)  A history of recent travel to endemic areas raises the possibility of infection with Babesia. Cases are increasing in the United States, particularly in the Northeast. Between 1 and 4 weeks after a tick bite, the patient experiences chills, fatigue, anorexia, myalgia, arthralgia, shortness of breath, nausea, and neuro­ logic symptoms such as headache, confusion, delirium or impaired consciousness; ecchymosis and/or petechiae are occasionally seen. The tick that most commonly transmits Babesia, Ixodes scapularis, also transmits Borrelia burgdorferi (the agent of Lyme disease) and Anaplasma; co-infection can occur and may result in more severe disease. Infection with the European species Babesia divergens is more frequently fulminant than that due to the U.S. species Babesia microti. B. divergens causes a febrile syndrome with hemolysis, jaundice, hemo­ globinemia, and renal failure and is associated with a mortality rate as high as 40%. Severe babesiosis is especially common in asplenic hosts but does occur in hosts with normal splenic function, particularly those >60 years of age and those with underlying immunosuppres­ sive conditions such as HIV infection or malignancy. Complications include renal failure, acute respiratory failure, heart failure, DIC, and splenic rupture. Other Sepsis Syndromes  Tularemia (Chap. 175) has been reported in every U.S. state except Hawaii. This disease is associated with wild rabbit, tick, horse-fly, and tabanid fly contact. It can be transmitted by arthropod bite, handling of infected animal carcasses, consumption of contaminated food and water, or inhalation. The typhoidal form can be associated with gram-negative septic shock and a mortality rate of >30%, especially in patients with underlying comor­ bid or immunosuppressive conditions. Plague occurs infrequently in the United States (Chap. 176), primarily after contact with ground squirrels, prairie dogs, or chipmunks, but is endemic in other parts of the world; >90% of all cases occur in Africa with Madagascar especially affected. The septic form is particularly rare and is associated with shock, multiorgan failure, and a 30% mortality rate. Pneumonic plague is rapidly progressive and fatal without treatment. These infections should be considered in the appropriate epidemiologic setting. The Centers for Disease Control and Prevention (CDC) lists Francisella tularensis and Yersinia pestis (the agents of tularemia and plague, respectively) along with Bacillus anthracis (the agent of anthrax) as important organisms that might be used for bioterrorism (Chap. S4). ■ ■SEPSIS WITH SKIN MANIFESTATIONS

(SEE ALSO CHAP. 21) Sepsis can be associated with diverse skin findings. In one study, almost 18% of patients with severe sepsis had secondary skin findings, such as purpura, petechiae, or ecchymoses. Maculopapular rashes may reflect early meningococcal or rickettsial disease but are usually associated with nonemergent infections. Exanthems are usually viral. Primary HIV infection commonly presents with a rash that is typically maculo­ papular and involves the upper part of the body but can spread to the palms and soles. The patient is usually febrile and can have lymphade­ nopathy, severe headache, dysphagia, diarrhea, myalgias, and arthral­ gias. Recognition of this syndrome provides an opportunity to prevent transmission and to institute early treatment.

TABLE 127-1  Empirical Treatment for Common Infectious Disease Emergenciesa CLINICAL SYNDROME POSSIBLE ETIOLOGIES TREATMENT COMMENTS SEE CHAP(S) Sepsis without a Clear Focus Septic shock Pseudomonas spp., gramnegative enteric bacilli, Staphylococcus spp., Streptococcus spp. Vancomycin (15 mg/kg q12h)b plus either Piperacillin/tazobactam (4.5 g) q8h via extended infusion (EI)c or cefepime (2 g) q8h via EI Overwhelming postsplenectomy sepsis Streptococcus pneumoniae, Haemophilus influenzae, Neisseria meningitidis Ceftriaxone (2 g q12h) plus vancomycin (15 mg/kg q12h)b If a β-lactam–sensitive strain is identified, vancomycin can be discontinued and a narrower-spectrum agent, such as penicillin, considered based on susceptibility testing. Babesiosis Babesia microti (U.S.),

B. divergens (Europe) Atovaquone (750 mg q12h) plus azithromycin (500 mg q24h) Sepsis with Skin Findings Meningococcemia N. meningitidis Ceftriaxone (2 g q12h) or penicillin (4 mU q4h) Rocky Mountain spotted fever (RMSF) Rickettsia rickettsii Doxycycline (100 mg bid) If both meningococcemia and RMSF are being considered, use ceftriaxone (2 g q12h) plus doxycycline (100 mg bid). If RMSF is diagnosed, ceftriaxone can be discontinued. Purpura fulminans S. pneumoniae, H. influenzae, N. meningitidis Ceftriaxone (2 g q12h) plus vancomycin (15 mg/kg q12h)b If a β-lactam-sensitive strain is identified, vancomycin can be discontinued. Erythroderma: toxic shock syndrome Group A Streptococcus, Staphylococcus aureus Vancomycin (15 mg/kg q12h)b plus clindamycin (600 mg q8h) Sepsis with Soft Tissue Findings Necrotizing fasciitis Group A Streptococcus, mixed aerobic/anaerobic flora Vancomycin (15 mg/kg q12h)b plus piperacillin/tazobactam (4.5 q q8h via EI)c plus clindamycin (600 mg q8h) Clostridial myonecrosis Clostridium perfringens Penicillin (2 mU q4h) plus clindamycin (600 mg q8h) Neurologic Infections Bacterial meningitis S. pneumoniae,

N. meningitidis Ceftriaxone (2 g q12h) plus vancomycin (15 mg/kg q12h)b If a β-lactam–sensitive strain is identified, vancomycin can be discontinued. If the patient is >50 years old or has comorbid disease, add ampicillin (2 g q4h) for Listeria coverage. Dexamethasone (10 mg q6h for 4 days) started before, or at the time of, the first dose of antibiotic improves outcome in adults with meningitis (especially pneumococcal). Brain abscess, suppurative intracranial infections Streptococcus spp., Staphylococcus spp., anaerobes, gram-negative bacilli Vancomycin (15 mg/kg q12h)b plus metronidazole (500 mg q8h) plus ceftriaxone (2 g q12h) Cerebral malaria Plasmodium falciparum Artesunate (2.4 mg/kg IV at 0, 12, and 24 h; then once daily) Spinal epidural abscess Staphylococcus spp., gram-negative bacilli Vancomycin (15 mg/kg q12h)b plus either Piperacillin/tazobactam (4.5 g q8h via EI) or cefepime (2 g q8h via EI)c Focal Infections Acute bacterial endocarditis Ceftriaxone (2 g q12h) or cefepime (2 g q8h via EI)c plus vancomycin (15 mg/kg q12h)b S. aureus, β-hemolytic streptococci, HACEK group,e Neisseria spp.,

S. pneumoniae aThese empirical regimens include coverage for gram-positive pathogens that are resistant to β-lactam antibiotics. Local resistance patterns should be considered and may alter the need for empirical vancomycin or for expanded coverage for antibiotic-resistant gram-negative pathogens. bA vancomycin loading dose of 20–25 mg/kg can be considered in critically ill patients. Dosing must be adjusted based on pharmacokinetic/pharmacodynamic monitoring. Daptomycin (10 mg/kg once daily) can be considered in place of vancomycin as alternate coverage for β-lactam resistant gram-positive organisms. Data for use in central nervous system infections are limited but emerging. cEI, extended infusion. β-Lactam antibiotics may exhibit unpredictable pharmacodynamics in sepsis. Prolonged or continuous infusions are often used. dThe optimal dose of IV immunoglobulin has not been determined, but the median dose in observational studies is 2 g/kg (total dose administered for 1–5 days). eHaemophilus spp., Aggregatibacter spp., Cardiobacterium hominis, Eikenella corrodens, and Kingella kingae.

Empirical therapy should be tailored to local resistance patterns. Carbapenem or aminoglycoside antibiotics should be considered for empirical therapy when rates of multidrugresistant gram-negative organisms are high or for patients with risk factors for resistant organisms. Adjust treatment when culture data become available. 152, 153, 166, 170, 315

Clindamycin (600 mg q8h) plus quinine (650 mg q8h) can be used in severe disease not responding to atovaquone and azithromycin. Treatment with doxycycline (100 mg bid) for potential co-infection with Borrelia burgdorferi or Anaplasma spp. may be prudent. 229, 232 Ceftriaxone eradicates nasopharyngeal carriage of the organism. Close contacts require chemoprophylaxis with rifampin (600 mg q12h for 2 days) or ciprofloxacin (a single dose, 500 mg).

151, 160, 162,

CHAPTER 127 If a penicillin- or oxacillin-sensitive strain is isolated, these agents are superior to vancomycin (penicillin, 2 mU q4h; or oxacillin, 2 g IV q4h). The site of toxigenic bacteria should be debrided; IV immunoglobulin can be used in severe cases.d 152, 153 Urgent surgical evaluation is critical. Empirical therapy should be tailored to local resistance patterns. For mixed aerobic/ anaerobic infections, clindamycin can be discontinued. Adjust treatment when culture data become available. 134, 152, 153 Approach to the Acutely Ill Infected Febrile Patient Urgent surgical evaluation is critical.

Urgent surgical evaluation is critical. If a penicillin- or oxacillin-sensitive strain is isolated, these agents are superior to vancomycin (penicillin, 4 mU q4h; or oxacillin, 2 g q4h).

Avoid glucocorticoids. Until IV artesunate is available, treatment can be initiated with oral artemether-lumefantrine. Atovaquone-proguanil, quinine, and mefloquine are other options. 229, 231

Surgical evaluation is essential. If a β-lactam–sensitive strain is isolated, oxacillin, 2 g q4h or cefazolin 2 gm IV q8h is superior to vancomycin. Adjust treatment when culture data become available. Evaluation by cardiology and cardiothoracic surgery is essential.

Petechial rashes caused by viruses are seldom associated with hypotension or a toxic appearance, although there can be exceptions (e.g., severe measles or arboviral infection). Petechial rashes limited to the distribution of the superior vena cava are rarely associated with severe disease. In other settings, petechial rashes require more urgent attention.

Meningococcemia (See also Chap. 160)  Almost three-quarters of patients with N. meningitidis bacteremia have a rash. Meningococ­ cemia most often affects young children (i.e., those 6 months to 5 years old). In sub-Saharan Africa, the high prevalence of serogroup A meningococcal disease has been a threat to public health for more than a century. Thousands of deaths occur annually in this area, which is known as the “meningitis belt,” and large epidemic waves occur approximately every 8–12 years. In the setting of aggressive vaccination programs against serogroup A, disease due to other serogroups, such as serogroups C, W135, and X, are increasing. Outside Africa, out­ breaks for the past 50 years reported in the United States and Europe are caused primarily by serogroup C (approximately 60%) followed by serogroup B (29%). In the United States, sporadic cases and outbreaks occur in day-care centers, schools (grade school through college, particularly among college freshmen living in residential halls), and army barracks. Outbreaks have also been described in people expe­ riencing homelessness. People with underlying comorbidities, such as cancer, renal or liver disease, or solid organ transplant recipients, are at increased risk. Household contacts of index cases are at 400–800 times greater risk of disease than the general population. Patients may have fever, headache, nausea, vomiting, myalgias, changes in mental status, and meningismus. However, the rapidly progressive form of disease is not usually associated with meningitis. The rash is initially pink, blanching, and maculopapular, appearing on the trunk and extremi­ ties, but then becomes hemorrhagic, forming petechiae. Petechiae are first seen at the ankles, wrists, axillae, mucosal surfaces, and palpebral and bulbar conjunctiva, with subsequent spread on the lower extremi­ ties and to the trunk. A cluster of petechiae may be seen at pressure points—e.g., where a blood pressure cuff has been inflated. In rapidly progressive meningococcemia (10–20% of cases), the petechial rash quickly becomes purpuric (see Fig. A1-41), and patients develop DIC, multiorgan failure, and shock; 50–60% of these patients die, and survi­ vors often require extensive debridement or amputation of gangrenous extremities. Hypotension with petechiae for <12 h is associated with significant mortality. Cyanosis, coma, oliguria, metabolic acidosis, and elevated partial thromboplastin time also are associated with a fatal outcome. Antibiotics given in the office by the primary care provider before hospital evaluation and admission may improve prognosis; this observation suggests that early initiation of treatment may be lifesaving. Members of the patient’s household and other persons with close contact should receive antibiotic prophylaxis with ciprofloxacin, rifampin, or ceftriaxone. Meningococcal conjugate vaccines are protec­ tive against serogroups A, C, Y, and W135 and are recommended for children 11–12 years of age with a booster dose at 16 years of age, and for other high-risk patients. Vaccines active against serogroup B are recommended for high-risk individuals ≥10 years of age and may be appropriate for teens and young adults (16 through 23 years of age). PART 5 Infectious Diseases Rocky Mountain Spotted Fever and Other Rickettsial Dis­ eases (See also Chap. 192)  RMSF is a tickborne disease that occurs throughout North and South America. It is caused primarily by Rickettsia rickettsii but can be caused by other rickettsiae (e.g., R. parkeri, R. akari). Up to 40% of patients do not report a history of a tick bite, but a history of travel or outdoor activity (e.g., camping in tick-infested areas) can often be ascertained. For the first 3 days, headache, fever, malaise, myalgias, nausea, vomiting, and anorexia are documented. By day 3, half of patients have skin findings. Blanching macules develop initially on the wrists and ankles and then spread over the legs and trunk. The lesions become hemorrhagic and are frequently petechial. The rash spreads to palms and soles later in the course. The centripetal spread is a classic feature of RMSF but occurs in a minority of patients. Moreover, 10–15% of patients with RMSF never develop

a rash. The patient can be hypotensive and develop noncardiogenic pulmonary edema, confusion, lethargy, and encephalitis progressing to coma. The CSF contains 10–100 cells/μL, usually with a predominance of mononuclear cells. The CSF glucose level is often normal; the pro­ tein concentration may be slightly elevated. Renal and hepatic injury as well as bleeding secondary to vascular damage are noted. Delayed recognition and treatment are associated with a greater risk of death; mortality rates are 20–35% if treatment is delayed or not prescribed compared with ~4% when treated with doxycycline within 5 days of onset. Native Americans, Alaskan natives, Pacific Islanders, children 5–9 years of age, adults >70 years old, and persons with underlying immunosuppression are at increased risk of death as well. Other rickettsial diseases cause significant morbidity and mortality worldwide. Mediterranean spotted fever caused by Rickettsia conorii is found in Africa, southwestern and south-central Asia, and southern Europe. Patients have fever, flu-like symptoms, and an inoculation eschar at the site of the tick bite. A maculopapular rash develops within 1–7 days, involving the palms and soles but sparing the face. Elderly patients or those with diabetes, alcoholism, uremia, or congestive heart failure are at risk for severe disease characterized by neurologic involvement, respiratory distress, and gangrene of the digits or purpura fulminans. Mortality rates associated with this severe form of disease approach 50% without treatment, but a single day of doxycycline is associated with much improved outcomes. Epidemic typhus, caused by Rickettsia prowazekii, is transmitted in louse-infested environments and emerges in conditions of extreme poverty, war, refugee camps, and natural disaster. Patients experience a sudden onset of high fevers, severe headache, cough, myalgias, and abdominal pain. A maculo­ papular rash develops (primarily on the trunk) in more than half of patients and can progress to petechiae and purpura. Serious signs include delirium, coma, seizures, noncardiogenic pulmonary edema, skin necrosis, and peripheral gangrene. Mortality rates approached 60% in the preantibiotic era and continue to exceed 10–15% in con­ temporary outbreaks. Scrub typhus, caused by Orientia tsutsugamushi (a separate genus in the family Rickettsiaceae), is transmitted by larval mites or chiggers and is one of the most common infections in south­ eastern Asia and the western Pacific. The organism is found in areas of heavy scrub vegetation (e.g., along riverbanks) and most common in rural areas among farming communities. Patients may have an inocula­ tion eschar and may develop a maculopapular rash, lymphadenopathy, and dyspnea. Severe cases progress to pneumonia, meningoencepha­ litis, myocarditis, DIC, and renal failure. Mortality rates range from 1% to 70% and vary by location, increasing age, myocarditis, delirium, pneumonitis, or signs of hemorrhage. If recognized in a timely fashion, rickettsial disease is very respon­ sive to tetracycline-based treatment. Doxycycline (100 mg twice daily for 1–14 days) is the treatment of choice for both adults and children. Combination therapy with doxycycline and azithromycin reduces complications and death in patients with severe scrub typhus. Purpura Fulminans (See also Chaps. 160 and 315)  Purpura fulminans is the cutaneous manifestation of DIC and presents as large ecchymotic areas and hemorrhagic bullae. Progression of petechiae to purpura, ecchymoses, and gangrene is associated with congestive heart failure, septic shock, acute renal failure, acidosis, hypoxia, hypotension, and death. Purpura fulminans has been associated primarily with N. meningitidis but, in splenectomized patients, may be associated with S. pneumoniae, H. influenzae, and S. aureus. Ecthyma Gangrenosum  Septic shock caused by P. aeruginosa or, less often, Aeromonas hydrophila or other gram-negative organisms, can be associated with ecthyma gangrenosum (see Figs. 170-1 and A1-34): hemorrhagic vesicles surrounded by a rim of erythema with central necrosis and ulceration, most frequently located on the legs and trunk. Echthyma gangrenosum is most common among patients with neutropenia, extensive burns, and hypogammaglobulinemia. Other Infections Associated with Rash  Vibrio vulnificus and other noncholera Vibrio bacteremic infections (Chap. 173) can cause focal skin lesions and overwhelming sepsis in hosts with chronic liver

disease, heavy alcohol consumption, iron storage disorders, diabe­ tes, renal insufficiency, hematologic disease, or malignancy or other immunocompromising conditions. More than 95% of the cases are in the subtropical Pacific and Atlantic Oceans coastal regions in the Northern Hemisphere. After ingestion of contaminated raw shellfish (typically oysters from the Gulf Coast in U.S. cases), there is a sudden onset of malaise, chills, fever, and hypotension. The patient develops bullous or hemorrhagic skin lesions, usually on the lower extremities, and 75% of patients have leg pain. The mortality rate can be as high as 35–60%, particularly when the patient presents with hypotension and septicemia. Outcomes are improved when patients are treated with fluoroquinolones with or without cephalosporins or with tetracyclinecontaining regimens. Other infections, caused by agents such as Aeromonas, Klebsiella, and E. coli, can cause hemorrhagic bullae and death due to overwhelming sepsis in cirrhotic patients. Capnocytoph­ aga canimorsus can cause septic shock in asplenic or cirrhotic patients. Infection typically follows a dog bite. Serovars A–C appear more virulent, constituting 92% of human infections but only 7.6% of canine isolates. Patients present with fever, chills, myalgia, vomiting, diarrhea, dyspnea, confusion, and headache. Findings can include an exan­ them or erythema multiforme (see Figs. 59-9 and A1-24A), cyanotic mottling or peripheral cyanosis, petechiae, and ecchymosis. About one-third of patients with sepsis develop septic shock, and 11–30% of patients with this fulminant form die of overwhelming sepsis and DIC. Survivors may require amputation because of gangrene. Erythroderma  TSS (Chaps. 152 and 153) is usually associated with erythroderma. The patient presents with fever, malaise, myalgias, nausea, vomiting, diarrhea, and confusion. There is a sunburn-type rash that may be subtle and patchy but is usually diffuse and is found on the face, trunk, and extremities. Erythroderma, which desquamates after 1–2 weeks, is more common in Staphylococcus-associated than in Streptococcus-associated TSS. Hypotension develops rapidly—often within hours—after the onset of symptoms. Early renal failure may precede hypotension and distinguishes this syndrome from other sep­ tic shock syndromes. There may be no indication of a primary focal infection, although possible cutaneous or mucosal portals of entry for the organism can be ascertained through a careful history. Coloniza­ tion rather than overt infection of the vagina or a postoperative wound, for example, is typical with staphylococcal TSS, and the mucosal areas appear hyperemic but not infected. Streptococcal TSS is more often associated with skin or soft tissue infection (including necrotizing fasciitis), and patients are more likely to be bacteremic. TSS caused by Clostridium sordellii is associated with childbirth or with skin injec­ tion of black-tar heroin. The diagnosis of TSS is defined by the clini­ cal criteria of fever, rash, hypotension, and multiorgan involvement, although fever is typically absent when TSS is caused by C. sordellii. The mortality rate is 5% for menstruation-associated TSS, 10–15% for nonmenstrual TSS, 30–70% for streptococcal TSS, and up to 90% for obstetric C. sordellii TSS. Clindamycin improves outcomes when included in the treatment regimen. The use of IV immunoglobulin is associated with improved survival in some studies, in conjunction with clindamycin therapy. Viral Hemorrhagic Fevers  Viral hemorrhagic fevers (Chaps. 215 and 216) are zoonotic illnesses caused by viruses that reside in either animal reservoirs or arthropod vectors. These diseases occur world­ wide and are restricted to areas where the host species live. They are caused by four major groups of viruses: Arenaviridae (e.g., Lassa fever in Africa), Bunyaviridae (e.g., Rift Valley fever in Africa; hantavirus hemorrhagic fever with renal syndrome in Asia; and Crimean-Congo hemorrhagic fever, which has an extensive geographic distribution), Filoviridae (e.g., Ebola and Marburg virus infections in Africa), and Flaviviridae (e.g., yellow fever in Africa and South America and dengue in Asia, Africa, and the Americas). Lassa fever and Ebola and Marburg virus infections are also transmitted from person to person. The vec­ tors for most viral fevers are found in rural areas; dengue and yellow fever are important exceptions. After a prodrome of fever, myalgias, and malaise, patients develop evidence of vascular damage, petechiae,

and local hemorrhage. Shock, multifocal hemorrhaging, and neuro­ logic signs (e.g., seizures or coma) predict a poor prognosis. Dengue (Chap. 215) is the most common arboviral disease worldwide. More than 100–400 million cases occur each year, ~25% of which are symptomatic with at least 12,000–20,000 deaths. Patients have a triad of symptoms: hemorrhagic manifestations, evidence of plasma leak­ age, and platelet counts of <100,000/μL. Mortality rates are 10–20%. If dengue shock syndrome develops, which is associated with severe hepatitis and neurologic involvement, mortality rates can reach 40%. Ebola infection has been associated with outbreaks with high mortality rates. From 1976 to 2022, 35 outbreaks were reported with an average case fatality rate of approximately 50%. Symptoms can appear 2–21 days after exposure, but most patients become ill within 9 days. The patient first presents with fatigue, fever, headache, and muscle pains, and the illness can progress to multiorgan failure and hemorrhaging. Careful volume-replacement therapy to maintain blood pressure and intravascular volume is key to survival in these infections.

Other viral illnesses with rash, such as measles, can be associated with significant mortality rates, especially in patients who develop organ complications such as pneumonia, pancreatitis or encephalitis. Measles continues to be responsible for more than 100,000 deaths per year worldwide, and to cause outbreaks in populations with low vac­ cination rates. ■ ■SEPSIS WITH A SOFT TISSUE/MUSCLE

PRIMARY FOCUS See also Chap. 134. Necrotizing Fasciitis  This infection is characterized by extensive necrosis of the subcutaneous tissue and fascia. It may arise at a site of minimal trauma or surgical incision and may also be associated with recent varicella, childbirth, or muscle strain. Diabetes mellitus, IV drug use, chronic liver or renal disease, and malignancy are associated risk factors. The most common causes of necrotizing fasciitis are group A streptococci alone (Chap. 153) and a mixed facultative and anaerobic flora (Chap. 134). The incidence of group A streptococcal necrotizing fasciitis has been increasing for the past quarter-century; surveillance in the United States links 13% of the cases to active IV drug use or homelessness. Physical findings are initially minimal except for soft tissue edema and mild erythema compared with the severity of pain and the degree of fever. The infected area is red, hot, shiny, swollen, and exquisitely tender. In untreated infection, the overlying skin develops blue-gray patches after 36 h, and cutaneous bullae and necrosis develop after 3–5 days. Necrotizing fasciitis due to a mixed flora, but not that due to group A streptococci, can be associated with gas production. Without treatment, pain decreases because of thrombosis of the small blood vessels and destruction of the peripheral nerves—an ominous sign. The mortality rate is 11–34% overall and increases when in association with TSS. With surgery, outcomes are significantly better. Necrotizing fasciitis may also be due to Clostridium perfringens (Chap. 159); in this condition, the patient is extremely toxic and the mortality rate is high. Within 48 h, rapid tissue invasion and systemic tox­ icity associated with hemolysis and death ensue. The distinction between this entity and clostridial myonecrosis is made by muscle biopsy. CHAPTER 127 Approach to the Acutely Ill Infected Febrile Patient Clostridial Myonecrosis (See also Chap. 159)  Myonecrosis is often associated with trauma or surgery but can develop spontane­ ously. The incubation period is usually 12–24 h long, and massive necrotizing gangrene develops within hours of onset. Systemic toxicity, shock, and death can occur within 12 h. The patient’s pain and toxic appearance are out of proportion to physical findings. On examina­ tion, the patient is febrile, apathetic, tachycardic, and tachypneic and may express a feeling of impending doom. Hypotension and renal failure develop later, and hyperalertness is evident preterminally. The skin over the affected area is bronze-brown, mottled, and edematous. Bullous lesions with serosanguineous drainage and a mousy or sweet odor can develop. Crepitus can occur secondary to gas production in muscle tissue. The mortality rate is >65% for spontaneous myonecrosis, which is often associated with Clostridium septicum or C. tertium and underlying malignancy. The mortality rates associated with trunk and

limb infection are 63% and 12%, respectively, and any delay in surgical treatment increases the risk of death.

■ ■NEUROLOGIC INFECTIONS WITH OR

WITHOUT SEPTIC SHOCK Bacterial Meningitis (See also Chap. 143)  Bacterial meningi­ tis is one of the most common infectious disease emergencies involv­ ing the central nervous system. Although hosts with cell-mediated immune deficiency (including transplant recipients, diabetic patients, elderly patients, and cancer patients receiving certain chemotherapeu­ tic agents) are at particular risk for Listeria monocytogenes meningitis, most cases in adults are due to S. pneumoniae (30–60%) and N. men­ ingitidis (10–35%). The classic presentation of fever, meningismus, and altered mental status is seen in only one-third to one-half of patients and is more common in adults over 60 years of age, although the elderly can also present without fever or meningeal signs. Cerebral dysfunction is evidenced by confusion, delirium, and lethargy that can progress to coma. In some cases, the presentation is fulminant, with sepsis and brain edema; papilledema at presentation is unusual and suggests another diagnosis (e.g., an intracranial lesion). Focal signs, including cranial nerve palsies (IV, VI, VII), can be seen in 10–20% of cases; 50–70% of patients have bacteremia. A poor outcome is associ­ ated with coma, seizures, hypotension, a purpuric rash, a pneumococ­ cal etiology, respiratory distress, a CSF glucose level of <0.6 mmol/L (<10 mg/dL), a CSF protein level of >2.5 g/L, and peripheral leukope­ nia or thrombocytopenia. Rapid initiation of treatment is essential; the odds of an unfavorable outcome may increase by 30% for each hour that treatment is delayed. Dexamethasone is an adjunctive treatment for meningitis in adults, especially for infections caused by S. pneu­ moniae, but is not recommended for Listeria meningitis. It must be given before or with the first dose of antibiotics; otherwise, it is unlikely to improve outcomes. PART 5 Infectious Diseases Suppurative Intracranial Infections (See also Chap. 145)  Sup­ purative intracranial infections present along with sepsis and hemo­ dynamic instability. Rapid recognition of the toxic patient with central neurologic signs is crucial to improvement of the prognosis of these entities. Patients with diabetes or hematologic disease may be at increased risk for these infections. Subdural empyema arises from the paranasal sinus in 60–70% of cases. Microaerophilic streptococci and staphylococci are the predominant etiologic organisms. The patient is toxic, with fever, headache, and nuchal rigidity. Of all patients, 75% have focal signs and 6–20% die. Despite improved survival rates, 15–44% of patients are left with permanent neurologic deficits. Septic cavernous sinus thrombosis follows a facial or sphenoid sinus infec­ tion; 70% of cases are due to staphylococci (including MRSA), and the remainder are due primarily to aerobic or anaerobic streptococci. Fungi have been common in some series. A unilateral or retro-orbital headache progresses to a toxic appearance and fever within days. Three-quarters of patients have unilateral periorbital edema that becomes bilateral and then progresses to ptosis, proptosis, ophthalmo­ plegia, and papilledema. The mortality rate is as high as 30% in older studies with frequent neurologic sequelae but recent reports indicate improved survival as high as 90%. Septic thrombosis of the superior sagittal sinus spreads from the ethmoid or maxillary sinuses and is caused by S. pneumoniae, other streptococci, and staphylococci. The fulminant course is characterized by headache, nausea, vomiting, rapid progression to confusion and coma, nuchal rigidity, and brainstem signs. Broad-spectrum antibiotics and early surgical intervention at the primary site of infection may improve outcomes. Anticoagulation or steroids are of uncertain benefit. Brain Abscess (See also Chap. 145)  Brain abscess often occurs without systemic signs. Almost half of patients are afebrile, and pre­ sentations are more consistent with a space-occupying lesion in the brain; 70% of patients have headache and/or altered mental status, 50% have focal neurologic signs, and 25% have papilledema. Abscesses can present as single or multiple lesions resulting from contiguous foci or hematogenous infection, such as endocarditis, or after surgery or

trauma. The infection progresses over several days from cerebritis to an abscess with a mature capsule. More than one-fourth to one-half of infections are polymicrobial, with an etiology consisting of aerobic bac­ teria (primarily streptococcal species) and anaerobes. Abscesses arising hematogenously are especially apt to rupture into the ventricular space, causing a sudden and severe deterioration in clinical status and a high mortality rate. Otherwise, mortality is low (<20%) but morbidity is high (30–55%). Patients presenting with stroke and a parameningeal infectious focus, such as sinusitis or otitis, may have a brain abscess; physicians must maintain a high level of suspicion. Prognosis worsens in patients with a fulminant course, delayed diagnosis, abscess rupture into the ventricles, multiple abscesses, or abnormal neurologic status at presentation. Patients who receive medical therapy alone, compared with those receiving combined medical and surgical interventions, have higher mortality and a greater chance of neurologic sequelae. Cerebral Malaria (See also Chap. 231)  This entity should be urgently considered for inhabitants of, or recent travelers to, areas endemic for malaria. Any patient with a change in mental status or repeated seizure in the setting of fulminant malaria has cerebral malaria. Fulminant malaria is caused by Plasmodium falciparum and is associated with temperatures of >40°C (>104°F), hypotension, jaundice, acute respiratory distress syndrome, and bleeding. Patients present with a febrile illness and lethargy or other neurologic signs, In adults, this nonspecific febrile illness progresses to coma over several days; occasionally, coma occurs within hours and death within 24 h. Nuchal rigidity and photophobia are rare. On physical examina­ tion, symmetric encephalopathy is typical, and upper motor neuron dysfunction with decorticate and decerebrate posturing can be seen in advanced disease. Unrecognized infection results in a 20–30% mortal­ ity rate. Intravenous artesunate is associated with better outcomes. Children with neurologic deficit at hospital discharge, seizure recur­ rence during treatment, and/or ischemic neural injury on MRI are at particular risk for neurologic and mental health sequelae. Intracranial and Spinal Epidural Abscesses (See also

Chap. 453)  Spinal and intracranial epidural abscesses (SEAs and ICEAs) can result in permanent neurologic deficits, sepsis, and death. At-risk patients include those with diabetes mellitus; IV drug use; chronic alcohol abuse; recent spinal trauma, surgery, or epidural anesthesia; and other comorbid conditions, such as HIV infection. Fungal epidural abscess and meningitis have been linked to epidural or paraspinal glu­ cocorticoid injections. In the United States and Canada, where early treatment of otitis and sinusitis is typical, ICEA is rare but the number of cases of SEA is on the rise. ICEAs typically present as fever, mental status changes, and neck pain, while SEAs often present as fever, local­ ized spinal tenderness, and back pain. ICEAs are typically polymicro­ bial, whereas SEAs are most often due to hematogenous seeding, with staphylococci the most common etiologic agent. Early diagnosis and treatment, which may include surgical drainage, minimize rates of mortality and permanent neurologic sequelae. Outcomes are worse for SEA due to MRSA, for infection at a higher vertebral-body level, for impaired neurologic status on presentation, and for dorsal rather than ventral abscess location. Elderly patients and persons with renal failure, malignancy, and other comorbidities also have less favorable outcomes. ■ ■OTHER FOCAL SYNDROMES WITH A FULMINANT COURSE Infection at virtually any primary focus (e.g., osteomyelitis, pneu­ monia, pyelonephritis, or cholangitis) can result in bacteremia and sepsis. Rapid clinical deterioration and death can be associated with destruction of the primary site of infection, as is seen in endocarditis and in infections of the oropharynx (e.g., Ludwig’s angina or epiglot­ titis, in which edema suddenly compromises the airway). TSS has been associated with focal infections such as septic arthritis, peritonitis, sinusitis, and wound infection. Lemierre’s syndrome is jugular septic thrombophlebitis primarily caused by Fusobacterium necrophorum, but it can be caused by other Fusobacterium spp., anaerobic streptococci and other bacteria. Lemierre’s syndrome is associated with metastatic infectious emboli (primarily to the lung but sometimes to the liver or

other organs) and sepsis, with mortality rates of >15%. Fusobacterium bloodstream infections have been associated with occult gastrointesti­ nal or genitourinary malignancy. Rhinocerebral Mucormycosis (See also Chap. 224)  Individ­ uals with diabetes or immunocompromising conditions such as solid organ transplants or hematologic malignancies are at risk for invasive rhinocerebral mucormycosis. Patients present with low-grade fever, dull sinus pain, diplopia, decreased mental status, decreased ocular motion, chemosis, proptosis, dusky or necrotic nasal turbinates, and necrotic hard-palate lesions that respect the midline. Without rapid recognition, surgical intervention, and antifungal therapy, the process continues on an inexorable invasive course, with mortality rates of 50–85% or greater. Uncontrolled diabetes and increasing age are nega­ tive prognostic factors. Acute Bacterial Endocarditis (See also Chap. 133)  This entity presents with a much more aggressive course than subacute endocarditis. Bacteria such as S. aureus, S. pneumoniae, L. monocy­ togenes, Haemophilus species, and streptococci of groups A, B, and G attack native valves. Native-valve endocarditis caused by S. aureus (including MRSA strains) is increasing. Mortality rates range from 10% to 40%. The host may have comorbid conditions such as underlying malignancy, diabetes mellitus, IV drug use, or alcoholism. Rheumatic valvular disease remains the most prevalent risk factor in Africa but is much less common in the United States. The patient presents with fever, fatigue, and malaise <2 weeks after onset of infection. On physi­ cal examination, a changing murmur and congestive heart failure may be noted. Hemorrhagic macules on palms or soles (Janeway lesions) sometimes develop. Petechiae, Roth’s spots, splinter hemorrhages, and splenomegaly are unusual. Rapid valvular destruction, particularly of the aortic valve, results in pulmonary edema and hypotension. Myo­ cardial abscesses can form, eroding through the septum or into the conduction system and causing life-threatening arrhythmias or highdegree conduction block. Large friable vegetations can result in major arterial emboli, metastatic infection, or tissue infarction including stroke. Older patients with S. aureus endocarditis are especially likely to present with nonspecific symptoms—a circumstance that delays diagnosis and worsens prognosis. Rapid intervention is crucial for a successful outcome. Inhalational Anthrax (See also Chap. S4)  Inhalational anthrax, the most severe form of disease caused by B. anthracis, had not been reported in the United States for more than 25 years until the use of this organism as an agent of bioterrorism in 2001. Patients presented with malaise, fever, cough, nausea, drenching sweats, short­ ness of breath, and headache. Rhinorrhea was unusual. All patients had abnormal chest roentgenograms at presentation. Pulmonary infiltrates, mediastinal widening, and pleural effusions were the most common findings. Hemorrhagic meningitis was documented in 38% of these patients. Survival was more likely when antibiotics were given during the prodromal period and when multidrug regimens with at least two bacteriocidal antibiotic agents were used. In the absence of urgent intervention with antimicrobial agents and supportive care, inhala­ tional anthrax progresses rapidly to hypotension, cyanosis, and death. Viral Respiratory Tract Illness  Viral respiratory tract illnesses can cause severe disease; several new syndromes have been described in the past decade. For patients who present with a respiratory illness and a relevant exposure and travel history, these viral illnesses must be considered and appropriate infection control measures instituted in addition to supportive care. Avian and Swine Influenza (See also Chap. 206)  Human cases of avian influenza have occurred primarily in Southeast Asia, particularly Vietnam (H5N1) and China (H7N9). Avian influenza should be considered in patients with severe respiratory tract illness, particularly if they have been exposed to poultry. Patients present with high fever, an influenza-like illness, and lower respiratory tract symp­ toms; this illness can progress rapidly to bilateral pneumonia, acute respiratory distress syndrome, multiorgan failure, and death. Younger

age appears to be associated with a lower risk of complications. Early antiviral treatment with neuraminidase inhibitors should be initiated along with aggressive supportive measures. Unlike avian influenza, whose human-to-human transmission has so far been rare and has not been sustained, influenza caused by a novel swine-associated A(H1N1) virus emerged in 2009 and caused more than 60 million cases and 12,000 deaths in 1 year. Patients most at risk of severe disease were children <5 years of age, elderly persons, patients with underlying chronic conditions, and pregnant women. Obesity also has been identi­ fied as a risk factor for severe illness. The virus continues to circulate as a seasonal influenza virus, along with A(H1N1) strains. Although influenza cases dropped during the peak of the COVID-19 pandemic, they are again increasing and cause 3–5 million cases of severe illness and 300,000–650,000 deaths each year.

SARS, COVID-19, and MERS (See Chaps. 204 and 205)  Novel betacoronaviruses have now been identified as the cause of severe acute respiratory syndrome (SARS, caused by SARS-CoV-1), Middle East respiratory syndrome (MERS, caused by MERS-CoV), and COVID-19 (caused by SARS-CoV-2). Although the three viruses have many simi­ larities, they differ in degree of contagion and case fatality rates. SARS and MERS “super-spreaders” are very symptomatic and typically criti­ cally ill, hospitalized patients. This is in contrast to COVID-19 where patients with highly transmissible virus are often asymptomatic or have mild disease. SARS emerged in 2002 in China and was diagnosed in several countries, primarily in Asia. Possible animal reservoirs include bats and civets. SARS-CoV-1 is characterized by efficient human trans­ mission but relatively low mortality. The potential SARS-CoV-1 pan­ demic was controlled through identification and isolation of infected patients. There have been no reported cases since 2004. CHAPTER 127 MERS likely has a bat reservoir and was first recognized in 2012 in Saudi Arabia. Human cases have been associated with direct and indi­ rect contact with dromedary camels. Unlike SARS and COVID-19, MERS exhibits inefficient human transmission but carries a high mortality rate. As of 2023, >2600 cases had been confirmed, most in the Arabian Peninsula, with 35% mortality. MERS ranges from asymptomatic infec­ tion to acute respiratory distress syndrome, multiorgan failure, and death. Elderly men with comorbidities appear to be at highest risk for poor outcomes. Despite little documented human-to-human transmis­ sion in the community, nosocomial infection must be prevented by adherence to strict infection control practices. MERS is currently a low-level public health threat and is likely to remain so unless the virus mutates and its transmissibility increases. Approach to the Acutely Ill Infected Febrile Patient COVID-19 caused by SARS-CoV-2 resulted in a pandemic of his­ toric proportion. Originally linked to cases in China, SARS-CoV-2 spread internationally at a rapid pace. As of December 2023, cases approached 773 million worldwide with almost 7 million confirmed deaths; the United States has reported more than 103 million cases and over 1.1 million deaths. Actual infection may exceed reported cases by 8−10 fold. The primary mode of transmission of COVID-19 is through direct person-to-person contact via respiratory droplets; transmission by the airborne route or by contact with contaminated surfaces is much less common. Greatest risk of infection is associated with close and prolonged contact such as in household or congregate settings. Rates of secondary infection among household contacts are high: >50% in studies early in the pandemic. Although ~80% of patients with symptomatic COVID-19 infection have mild disease such as cough, sore throat, fever, gastrointestinal symptoms, and taste and smell alterations, ~5% of patients develop respiratory failure, shock, and multiorgan system failure. Patients typi­ cally have been symptomatic for 5−7 days before progression to severe pneumonia and hypoxemia. Shock, often in the setting of cardiac injury/myocarditis, arrhythmias or cardiomyopathy, and thromboem­ bolic complications such as pulmonary emboli or stroke, can occur. Other manifestations include acute kidney or liver injury. Laboratory findings include lymphopenia, elevated lactate dehydrogenase, and often evidence of a cytokine-release type syndrome with elevated C-reactive protein, d-dimer, ferritin, and proinflammatory cytokines such as interleukin 6 (IL-6). Chest radiographs and CT imaging reveal

06 - 128 Principles of Immunization

128 Principles of Immunization

diffuse ground-glass opacities. Consolidation, predominantly in the lower lobes and peripherally, is a frequent finding. Mortality rates for patients requiring intensive care and mechanical ventilation have improved significantly over the course of the pandemic but remain ≥20% for patients who progress to that disease severity. Risk factors for severe disease and poor outcomes include age ≥65 years, morbid obe­ sity, diabetes, cardiovascular or cerebrovascular disease, hypertension, chronic obstructive lung disease, and chronic kidney disease. Severe disease and mortality is also increased in males and pregnant women.

Treatment currently includes supportive care, antiviral therapy, anti-inflammatory therapy, and anticoagulation prophylaxis or treat­ ment. Patients with respiratory distress are hypoxemic but maintain pulmonary mechanical functioning; thus, invasive mechanical ventila­ tion should be delayed until other interventions have been exhausted. In a meta-analysis, dexamethasone or other glucocorticoids reduced 28-day mortality in patients with severe COVID-19 disease compared with standard care: 32% and 40%, respectively (odds ratio, 0.66; 95% confidence interval, 0.53–0.82). Thus, dexamethasone (6 mg IV/PO daily) is indicated for patients with hypoxemia. IL-6 inhibitors such as tocilizumab may be indicated for patients with rapidly escalating oxy­ gen requirements. Secondary bacterial pneumonia in COVID-19 is not common, and antibacterial agents should not be routinely prescribed. With the availability of highly effective vaccines, prevention is the most important strategy to control COVID-19. Vaccination has reduced progression to severe disease and hospitalization. In addition, treat­ ment with antiviral agents such as nirmatrelvir/ritonavir or remdesivir can reduce risk of progression to severe disease and hospitalization for patients with mild symptoms. Hantavirus Pulmonary Syndrome (See also Chap. 215) 

Hantavirus pulmonary syndrome has been documented in the United States since 1993 (primarily the southwestern states, west of the Mis­ sissippi River), Canada, and South America. Most cases occur in rural areas and are associated with exposure to rodents. Patients present with a nonspecific viral prodrome of fever, malaise, myalgias, nausea, vomit­ ing, and dizziness that may progress to pulmonary edema, respiratory failure, and death. Hantavirus pulmonary syndrome causes myocardial depression and increased pulmonary vascular permeability; there­ fore, careful fluid resuscitation and use of pressor agents are crucial. Use of venoarterial extracorporeal membrane oxygenation (ECMO) for severe Hantavirus pulmonary syndrome has been successful and should be considered. Aggressive cardiopulmonary support during the first few hours of illness can be life-saving in this high-mortality syn­ drome. The early onset of thrombocytopenia may help distinguish this syndrome from other febrile illnesses in an appropriate epidemiologic setting. PART 5 Infectious Diseases SUMMARY Acutely ill febrile patients with the syndromes discussed in this chapter require close observation, aggressive supportive measures, and—in most cases—admission to intensive care units. The most important task of the physician is to distinguish these patients from other infected febrile patients whose illness will not progress to fulminant disease. The alert physician must recognize the acute infectious disease emer­ gency and proceed with appropriate urgency. ■ ■FURTHER READING Hasburn R: Progress and challenges in bacterial meningitis: A review. JAMA 328:2147, 2022. Hua C et al: Necrotising soft-tissue infections. Lancet Infect Dis 23:e81, 2023. Kollef MH et al: Timing of antibiotic therapy in the ICU. Crit Care 25:360, 2021. Theilacker C et al: Overwhelming postsplenectomy infection: A prospective multicenter cohort study. Clin Infect Dis 62:871, 2016. Wilder-Smith A et al: Dengue. Lancet 393:350, 2019.

Kathleen M. Neuzil, Kathryn M. Edwards

Principles of

Immunization The birth of vaccinology is often ascribed to Edward Jenner’s obser­ vation in the 18th century that infection of milkmaids with cowpox, termed vaccinia and hence the term vaccination, conferred protection against smallpox. This discovery catalyzed a sequence of events that ultimately led to the eradication of smallpox, one of the most disfigur­ ing and lethal infections of humans. Vaccination programs are among the most impactful and cost-effective health interventions. Consid­ ered to be one of the greatest achievements in public health, vaccines are estimated to have averted more than 37 million deaths globally between 2000 and 2019. This represents a 45% reduction in deaths compared with a scenario of no vaccination. Vaccines are defined as inactivated or attenuated pathogens or com­ ponents of a pathogen (nucleic acid, protein) that, when administered to the host, stimulate a protective response by the cells of the immune system (Table 128-1). Adjuvants may be added to vaccines to nonspe­ cifically boost the immune response. Vaccination is the act of introduc­ ing a vaccine into the body to induce protection from a specific disease. Immunization is a process by which a person becomes resistant to a particular infectious disease or pathogen, usually by vaccination. How­ ever, immunity may also be conferred passively. ■ ■IMMUNE RESPONSES TO VACCINES While Chapter 360 provides an overview of immune responses, a few additional concepts are relevant to vaccines. Vaccines targeted for new infectious pathogens, such as SARS-CoV-2 at the onset of the pandemic, or vaccines given to infants and young children without prior expo­ sure to a pathogen, will trigger a primary immune response with peak antibody titers achieved several weeks after administration. It may take several doses of a vaccine to induce a priming response, particularly if the vaccine is inactivated or a toxoid. Live, attenuated vaccines gener­ ate more robust responses and generally only require one or two doses. Subsequent booster doses of vaccines trigger a secondary response with rapid induction of antibody within several days to a week. Vaccines administered against common pathogens, such as influenza, will trigger secondary booster responses since all older children and adults would have encountered influenza virus and/or will have received past influ­ enza vaccines. Immune responses to vaccines are generally measured in terms of serum antibody levels, but mucosal antibodies, cellular responses, and memory cells also are stimulated after vaccination and function to prevent disease and sustain immunity. Passive Immunity  Vaccines stimulate active immunity—inducing an immune response by the host to the vaccine antigen—and will be the focus of most of this section. Passive immunity also serves an impor­ tant role in prophylaxis. Passive immunity results from the exogenous introduction of antibodies. Originally, such antibodies came from another person or animal and were relatively short-lived. Examples included hepatitis A, tetanus, and rabies immunoglobulins. The more recent capability to generate monoclonal antibodies to infectious pathogens, along with methods to extend the half-life of antibodies, has expanded their prophylactic use. Monoclonal antibodies targeted to SARS-CoV-2 strains were beneficial during the pandemic, par­ ticularly in populations who did not mount robust immune responses to vaccines. Currently, monoclonal antibodies are routinely used in infants and young children for the prevention of respiratory syncytial virus (RSV) infections, and their use in adults will likely continue or increase in the years ahead. These agents are evaluated for their safety and efficacy using similar methodology as in vaccine studies but will not be discussed extensively in this chapter. Another type of passive antibody protection is maternal immu­ nization, whereby vaccines are given to the pregnant person and the

TABLE 128-1  Categories and Characteristics of Approved Vaccines and Adjuvants EXAMPLES OF APPROVED VACCINES ADVANTAGES DISADVANTAGES TECHNOLOGY DESCRIPTION Live attenuated Weakened or attenuated form of the pathogen that causes disease Measles, mumps, rubella, varicella, oral poliomyelitis, nasal influenza vaccine, oral rotavirus Inactivated Pathogens or toxins rendered nonreplicating through heat or chemical processes (may be entire pathogen or parts of it) Inactivated poliomyelitis, hepatitis A, whole-cell pertussis, tetanus and diphtheria toxoids Purified protein-based (split or subunit) Specific parts of the pathogen are produced in culture Influenza, acellular pertussis, recombinant shingles Polysaccharide– protein conjugates Type of subunit vaccine that combines a polysaccharide antigen with a protein carrier to improve immune responses Pneumococcal, meningococcal, typhoid, Hib Virus-like particles One or more proteins arranged to closely resemble viruses Hepatitis B, HPV Broad and robust immunity Noninfectious Replicating viral vector Replicating, nonpathogenic viruses deliver nucleic acid to host for in vivo production of antigen Vesicular stomatitis virus– based Ebola vaccine Nonreplicating viral vector Replication-deficient nonpathogenic viruses deliver nucleic acid to host for in vivo production of antigen Chimp adenovirus–based COVID-19 vaccine Nucleic acid (DNA, RNA) Lipid nanoparticles deliver nucleic acid to host for in vivo production of antigen COVID-19 mRNA vaccines Rapid development and manufacturing timelines Effective and safe for majority of population TECHNOLOGY EXAMPLES CURRENT USE MECHANISM OF ACTION Alum Aluminum hydroxide or aluminum phosphate Multiple vaccines—e.g., HPV, hepatitis A, DTaP Oil-in-water emulsion AS03, MF59 Influenza Increases recruitment of innate immune cells and enhanced antigen uptake Toll-like receptor (TLR) agonists Monophosphoryl lipid A (MPL) Cytosine-phosphate-guanine (CpG) HPV (AS04—MPL and alum) Shingles (MPL and QS21) Hepatitis B (CPG) Saponins QS-21, Matrix-M Malaria vaccines (MPL, QS21, and Matrix-M) COVID-19 (Matrix M) Delivery platforms Lipid nanoparticle (LNP) COVID-19 mRNA vaccines Improved lymphatic transport enhances antigen uptake and presentation antibody generated by their immune system is transferred to the fetus through the placenta. Examples of vaccines given to the pregnant person, largely for the protection of the baby, include tetanus, acellular pertussis, and RSV vaccines. Other vaccines given to pregnant persons for protection of both the mother and the baby include influenza and COVID-19. ■ ■VACCINE FORMULATIONS Vaccine formulations have evolved over time (Fig. 128-1). In the late 19th to mid-20th century, methods to inactivate whole bacteria and detoxify bacterial toxins led to early whole-cell typhoid and cholera vaccines, as well as diphtheria and tetanus toxoids. The discovery that repeated passage of organisms in unnatural hosts could select for avirulent strains provided the foundation for Bacille Calmette-Guérin (BCG) vaccine for tuberculosis and yellow fever vaccines. The abil­ ity to grow viruses in cell culture informed the development of live

VACCINES Mimics natural infection Effective priming with durable immunity Large-scale manufacturing capabilities Single or two doses often sufficient Difficult to reach desired level of attenuation Safety concerns for certain populations (e.g., pregnant women, immunocompromised patients) Stability Induces broad immune response to multiple antigens Large-scale manufacturing capabilities Reactogenic Multiple doses needed May require adjuvant Highly specific immune response Noninfectious Low reactogenicity Ease of production Multiple doses may be needed May require adjuvant Limited cross-protective immunity Conjugate protein may also provide immunity (e.g., tetanus) Strong immune responses in all ages, including infants Technically difficult and costly to produce Individual conjugate required for each vaccine serotype Technically difficult to produce Lower stability Induces broad immune response High manufacturing scalability Immunity to vector may dampen immune response Safety of vector—may not be suitable for certain immunocompromised populations CHAPTER 128 Induces broad immune response High manufacturing scalability Immunity to vector may dampen immune response Principles of Immunization May require several doses/boosters to maintain immunity Reactogenic ADJUVANTS Possible depot effect; increases antibody production Activates TLR to enhance antigen presentation and enhance adaptive immune responses Prolongs antigen bioavailability and enhances antigen signals attenuated vaccines against poliomyelitis, measles, mumps, rubella, and varicella viruses between 1950 and 1980. During the next three decades, the fields of biotechnology and molecular genetics burgeoned. New vaccines that circumvented poor immune responses to bacterial polysaccharides by their conjugation to proteins included pneumococ­ cal, Haemophilus influenzae type b, and meningococcal vaccines. The development of recombinant DNA technology allowed antigens to be expressed in cell lines, leading to hepatitis B and human papillomavirus (HPV) vaccines. With the COVID-19 pandemic, a new generation of vaccines and platform technologies emerged with unprecedented speed. While many of these new technologies had been in development for years, the public health need and the influx of government resources pro­ pelled these technologies through the approval and deployment phases at an accelerated pace. The gene-based (e.g., mRNA) and vectorbased technological advancements have been most visible. For years,

Major Conceptual and Technological Advances Vaccinology Era: Concept of immunity Immunology Virology Production: Eggs Animals Chikungunya

SARS-CoV-2 RSV

Ebola (VSV) HPV Rotavirus Varicella Jpn. Enceph. Hepatitis A Hepatitis B Rubella Mumps Adenovirus Measles Poliovirus Influenza Yellow fever Rabies Smallpox

Viral Vaccines

1820 1840 1860 1880 1900 1920 1940 1960 1980 2000 2020 2040 2060 2080 2100 FIGURE 128-1  Major conceptual and technological advances for viral vaccines in the vaccinology era. (Reproduced with permission from Barney Graham.) mRNA technology was limited by its instability and inefficiency of in vivo transfection, until the lipid nanoparticle (LNP) delivery system improved the RNA-based vaccine delivery. Further, new adjuvants and structure-guided antigen design of the SARS-CoV-2 spike protein transformed the field. These technological advances were accompanied by innovations in trial design, real-time safety surveillance and effec­ tiveness evaluations at a massive scale, transmission modeling, and sophisticated analysis of immunologic correlates of protection. PART 5 Infectious Diseases Adjuvants  Adjuvants are substances that are added to vaccines to enhance the immune responses to the antigen. Replicating viral vaccines generally do not need adjuvants, but they are often needed in inactivated or protein-based vaccines. Early emulsions, such as Freund’s adjuvant, stimulated humoral and cellular responses but were very reactogenic. Alum, the oldest adjuvant formulated as aluminum hydroxide or phosphate, has been in use for more than 90 years. It pro­ motes humoral immune responses, has an excellent safety record, and is included in many of the vaccines administered to infants and young children. As the understanding of molecular mechanisms involved in immune responses progressed, new adjuvants were developed. Oilin-water emulsions, including MF59 and AS03, contain squalene as a vehicle and enhance innate and adaptive immune responses. They are approved in combination with certain influenza vaccines. As Toll-like receptors were discovered, adjuvants were developed to combine with the newer, more purified protein and subunit vaccines. The Toll-like receptor agonists TLR4 and TLR9 have been included in licensed vac­ cines. These include monophosphoryl lipid A (MPL) with alum (AS04) for HPV and cytosine-phosphate-guanine (CpG) for hepatitis B. More recently, MPL has been combined with a natural saponin component, QS-21, for use in the recombinant shingles vaccine and the RTS,S malaria vaccine. Likewise, a saponin-based adjuvant, Matrix-M, is approved for use in the recombinant COVID-19 vaccine and the R21 malaria vaccine. In addition to enhancing delivery of mRNA vaccines, LNPs induce inflammation and have an adjuvanting effect. Additional adjuvants are being studied in experimental vaccines, and others will likely be included in licensed products in the future. ■ ■VACCINE DEVELOPMENT The pathway from vaccine discovery to population impact encom­ passes a broad set of components summarized in Fig. 128-2. These steps include understanding of the burden of the pathogen to be prevented and the immune factors involved in protection; antigen dis­ covery; preclinical animal studies advancing to human trials assessing immune responses, safety, and efficacy; vaccine licensure and policymaking; implementation; and finally monitoring of vaccine uptake, impact, and safety after implementation. Well-established systems and processes, both pre- and postlicensure, ensure that vaccines are safe and effective.

Structure-based design Mol Bio Cells Chemical synthesis Bioreactors Structural biology Gene-based delivery High-throughput sequencing Rapid manufacturing Rapid gene synthesis Single cell analysis including B cell lineages Human mAb isolation Nanoparticle display Custom animal models Multi-omics glycobiology Cell biology Imaging Establishing the Need for an Immunization  Generally, before extensive resources are expended to formulate and develop vaccines, it is important to establish the burden of disease, the population at risk for the disease, and the mechanisms involved in protection. Much of this can be done through existing surveillance systems, such as those at the Centers for Disease Control and Prevention (CDC), that measure the burden of disease and the population at risk. In certain situations, vaccines are developed before their potential benefit is fully known, based on the theoretical risk to the population. Examples of these vac­ cines are those directed against potentially pandemic influenza strains or other pandemic threats. Phases of Vaccine Development  Like the development of phar­ maceutical agents, vaccine development progresses through preclinical and three distinct clinical stages: phases 1, 2, and 3. Under specific circumstances where phase 3 trials may be delayed or challenging, controlled human infection models (CHIM) may be part of the devel­ opment pathway. Initially, research focuses on the identification of an antigen or antigens with the potential to stimulate a protective immune response that will prevent disease upon encounter with the pathogen. For most vaccines the generation of an antibody directed to an important com­ ponent of the pathogen is associated with protection. Examples are vaccines directed against the attachment proteins such as the spike in SARS-CoV-2 vaccines or against toxins such as the tetanus or diph­ theria vaccines. These proposed vaccines are administered in preclini­ cal studies to small animals, often mice; their immune responses are measured, and safety established. Generation of immunity against the pathogen must be demonstrated, or the vaccine will not undergo fur­ ther testing. Toxicity studies are conducted in animals to detect safety signals, and some vaccinated animals are challenged with wild-type pathogens after vaccination to establish protection. Phase 1, 2, and 3 Clinical Trials  When preclinical studies in animals demonstrate that the vaccine stimulates an immune response and there are no toxicity concerns, vaccines then undergo phase 1 clini­ cal trials. In the United States, prior to the onset of phase 1 studies, the vaccine manufacturer must submit an Investigational New Drug (IND) application to the U.S Food and Drug Administration (FDA) that outlines the vaccine approach prior to the start of the study. When the IND is approved, phase 1 trials can begin. These trials enroll limited numbers of healthy participants, usually fewer than 100 individuals and generally between the ages of 18 to approximately 55 years, pri­ marily to test the safety of the new experimental vaccine, although immunogenicity also is measured. Subjects enrolled in phase 1 studies are well informed about the risks and the potential benefits of the vac­ cines and are screened for their ability to be monitored closely and to adhere to rigorous safety assessments. These assessments include daily

BURDEN OF ILLNESS STUDIES TO INFORM PUBLIC HEALTH NEED/MARKET NEED ASSAY and PROCESS DEVELOPMENT, FORMULATION AND MANUFACTURING PRECLINICAL STAGE PHASE I PHASE II PHASE III/HUMAN CHALLENGE STUDIES DISCOVERY & EXPLORATORY STAGE • PATHOGENESIS • IMMUNE RESPONSES • ANTIGEN DESIGN • ADJUVANTS • ANIMAL MODELS • ORGANOID SYSTEMS • TRIAL DESIGN, EXECUTION, ANALYSIS • IMMUNOGENICITY AND CO- ADMINISTRATION • SAFETY AND EFFICACY • CORRELATES OF PROTECTION AND SYSTEMS BIOLOGY • MICROBIOME AND -OMICS BIOINFORMATICS, STATISTICS, COMPUTATIONAL BIOLOGY FIGURE 128-2  Vaccine development: a continuum from antigen discovery to population impact. (Figure created by Kathleen M. Neuzil, MD, MPH.) monitoring of local and systemic adverse events with measurement of temperature and size of redness and swelling at the injection site, and with detailed assessments of systemic reactions that may result in limitations of normal activities after vaccine administration. Phase 1 studies often involve a dose-ranging component such that the first enrolled subjects are administered the lowest doses of vaccine, and, if tolerated, the doses are increased. Many phase 1 studies have indepen­ dent Data Safety and Monitoring Committees (DSMCs) composed of vaccine- and pathogen-specific experts, independent of the study site investigators and sponsors of the study, who assess the reactions and approve the advancing of the dose level based on the safety profile of the administered vaccines. Phase 1 studies have halting rules such that if severe reactions are seen, the study will be stopped. These halting rules might include hospitalization for a vaccine-related adverse event, or a severe reaction attributed to the vaccine in several participants. All participants in the phase 1 trials will have immunologic studies per­ formed as well to determine the magnitude of the antibody responses and, in some situations, the function of the antibody generated and/or T lymphocyte responses to the vaccines. At the conclusion of the phase 1 trial, the safety and immunogenicity data are reviewed and presented to the FDA for approval to advance to phase 2 studies. Phase 2 studies involve several hundred subjects and often have a larger age range than that evaluated in phase 1. They expand the safety profile and assess immune responses in larger numbers of subjects. Again, meticulous attention to safety assessment is included in these studies, often with independent DSMCs to assess the reaction profile. Finally, if phase 2 studies confirm safety and immunogenicity in the expanded populations and with approval of the FDA, phase 3 trials can begin. Phase 3 trials are designed to determine whether the vaccines will prevent a predefined endpoint, generally the prevention of laboratory-

confirmed disease with the vaccine-directed pathogen. Subjects enrolled in phase 3 studies are randomized and blind to the receipt of either another irrelevant vaccine or a control agent, usually consisting of a placebo. Subjects and investigators are blind to group assignment and remain so throughout the phase 3 trial. After administration of the vaccine, study participants are followed closely, and if they develop predefined symptoms of disease they are tested for the presence of the pathogen. The determination of the primary endpoint of the study and the level of vaccine efficacy is the study goal. When feasible, one of the additional outcomes of the phase 3 trial is the determination of a serologic correlate of protection. For example, a correlate could be an antibody level associated with protection from disease. This

PHASE IV SAFETY & IMPACT DELIVERY REGULATORY APPROVAL POLICY & FINANCING • POLITICAL WILL • COST-EFFECTIVENESS • TRANSMISSION MODELING • LOGISTICS & PLATFORMS • ALTERNATE SCHEDULES • VACCINE HESITANCY • COMMUNITY ACCEPTANCE • COMMUNICATIONS would facilitate evaluation of additional vaccines in population groups not involved in the original trial and would not require assessment of actual protection from disease. While this is a highly sought-after end­ point, it is often not possible to determine. Prior to submission of the vaccine for licensure there will need to be lot-consistency assessments that determine that several lots of the vaccines can be made that are consistent in their safety and immunogenicity endpoints. Controlled Human Infection Models (CHIMs)  CHIMs may be part of the development pathway of a vaccine. Such models involve intentionally infecting healthy participants with a pathogen and fol­ lowing them carefully for development of disease. The evaluation of vaccines in these models may provide unique immunologic insights into correlates of protection, may establish a rapid pathway for downse­ lection of candidates to move to later-phase trials or to endemic areas, and may inform vaccine approval. For example, an oral cholera vaccine for travelers in the United States was approved by the FDA based on a controlled human challenge study that established efficacy against clinical cholera. The determination to use CHIM is made in consulta­ tion with the FDA and based on the premise that the size, complexity, and expense of a study—for example, a study of a cholera vaccine with a disease endpoint in travelers—would render a phase 3 trial infeasible and significantly delay the approval of the vaccine. Other well-established CHIMs that are used to assess vaccines include those for influenza, typhoid fever, shigellosis, and malaria. CHAPTER 128 Principles of Immunization Vaccine Licensure  At the conclusion of the phase 3 efficacy trials and consistency lot testing, data are presented to the national regula­ tory authorities who ultimately will decide whether the vaccine will be licensed. In the United States, the FDA will require that the product be labeled for how it will be used, based on the data generated in the phase 3 clinical trial. They will seek the guidance of a standing advisory group of experienced clinicians, vaccine experts, epidemiologists, and other subject matter experts, called the Vaccine and Related Biological Prod­ ucts Advisory Committee (VRBPAC), to advise whether the vaccine should be licensed. The FDA poses specific questions to the VRBPAC that relate to both vaccine efficacy and safety. All the adverse events reported in clinical trials are comprehensively presented and discussed with the committee. Any significant safety concerns raised may delay licensure, and additional safety studies may be proposed. Ultimately, although the FDA will seek the guidance of the VRBPAC, it will decide independently whether to license the vaccine and what requirements will be placed on that license. The inclusion of children and pregnant women in vaccine licensure will generally not occur unless the vaccines

Vaccine development and testing Submission to FDA for a Biologics License Application FDA licensure Vaccines and Related Biological Products Advisory Committee Advises Advisory Committee on Immunization Practices Advises CDC consideration ACP’s Board of Regents consideration Recommendations for use published in MMWR Insurance or Medicare coverage Uptake and financing PART 5 Infectious Diseases Public sector FIGURE 128-3  Development and dissemination of vaccine recommendations and policies for adults. ACP, American College of Physicians. (From Annals of Internal Medicine, JC Smith et al: Immunization Policy Development in the United States: The Role of the Advisory Committee on Immunization Practices, Vol. 150, No. 1. Copyright © 2009 American College of Physicians. All Rights Reserved. Reprinted with the permission of American College of Physicians, Inc.) have been specifically studied in those populations for their safety and immunogenicity. This process is outlined in Fig. 128-3. The Advisory Committee on Immunization Practices (ACIP) Process  Following licensure of vaccines by the FDA, the group that has played the major role in determining recommendations for vaccina­ tion in the United States is the Advisory Committee on Immunization Practices (ACIP) of the CDC. The ACIP provides recommendations for the use of vaccines in children, adolescents, and adults and issues a yearly vaccination schedule for all age groups and clinical conditions. Since 1995, the ACIP, American Academy of Pediatrics, American Academy of Family Physicians, American College of Physicians, American College of Obstetricians and Gynecologists, and other professional organizations have worked together to coordinate vaccine schedules and policies. Members of the ACIP have expertise in a variety of disciplines related to the charge to the committee, including clinical medicine, epidemiology, vaccinology, public health, implementation science, and economic evaluations. The ACIP charter states, “Committee delib­ erations on use of vaccines to control disease in the U.S. shall include consideration of disease epidemiology and burden of disease, vac­ cine efficacy and effectiveness, vaccine safety, the quality of evidence reviewed, analyses and implementation issues.” The committee may revise or withdraw their recommendation(s) regarding a particular vaccine as new information on disease epidemiology, effectiveness or safety, economic considerations, or other issues becomes available. Once approved by the CDC Director, the ACIP recommendations are published in the Morbidity and Mortality Weekly Report (MMWR) and become official CDC immunization use policy. Among the responsi­ bilities of the ACIP is prioritizing use in settings where there are short­ ages of vaccines so that the highest-priority groups for whom vaccine might be recommended can receive them. The immunization delivery system in the United States consists of both public and private providers. Since 1993 the Vaccines for Children

Advises ACP’s Adult Immunization Initiative Physician Advisory Board Recommendations for use published in Annals Private sector (VFC) Program provides vaccines to children whose parents or guard­ ians may not be able to afford them. Serving as one of the nation’s most important contributors to health equity, the program helps ensure that all children have a better chance of getting their recommended vaccinations on schedule and staying healthy. In contrast, most vaccines for adults are delivered by private healthcare providers, although many of the vac­ cines are purchased using federal or other government funds. This may contribute to the relatively low coverage of adult vaccines compared with childhood vaccines. The Affordable Care Act requires private insurers to cover ACIP-recommended vaccines at in-network providers. Vaccine Policy and Postlicensure Surveillance  Once a vac­ cine is licensed for use in the United States and recommendations are made by the ACIP, systems need to be put in place to provide access to vaccines and to remove barriers to access, such as cost. Ongoing monitoring of vaccine effectiveness and safety are critical to evaluate issues such as waning immunity, to identify groups at heightened risk for vaccine failure, and to determine adverse events following vaccina­ tion that are causally related, especially rare events that could not be detected in prelicensure trials. Estimations of the burden of causally vaccine-related adverse events can then be weighed against the benefits of the vaccine to determine if any changes in recommendations are warranted. Disease surveillance is crucial to determine who continues to acquire disease, risk factors for disease, and the role of vaccine fail­ ure versus failure to vaccinate in disease prevention. After Vaccine Implementation  Prelicensure clinical trials usually enroll thousands of participants. However, there are questions that can­ not be answered in these studies. These questions include (1) What is the duration of protection from disease, and is there a need for boosters? (2) What is the effectiveness of the vaccine in population groups not evalu­ ated in the clinical trials? Effectiveness can be measured in observational studies post-licensure by comparing disease incidence in vaccinees

07 - 129 Immunization Principles and Vaccine Use

129 Immunization Principles and Vaccine Use

versus nonvaccinees; (3) Are there rare causally related adverse events? If so, what is the incidence rate, and are there risk factors for developing the adverse event that could lead to vaccine contraindications? and (4) What is the impact of vaccination in protecting the community (i.e., herd immunity) by preventing or reducing transmission? Answering these questions requires a comprehensive surveillance system to detect and determine characteristics of disease in the postvaccine era and whether such disease is the result of failure to vaccinate or vaccine failure. If the former, what measures can be taken to enhance vaccine uptake, or should recommendations for vaccination be broadened if substantial numbers of cases are occurring in groups for whom vaccine is not rec­ ommended? If there is evidence of vaccine failure, is it the result of vac­ cine mishandling (e.g., improper storage) or is the rate of failure within expected levels (e.g., the measured vaccine effectiveness is within levels expected based on the prelicensure trials)? If effectiveness is low, are there groups at higher risk for vaccine failure, and if so, would additional doses of vaccines or alternative schedules reduce that risk? Assessing Vaccine Safety  Adequately assessing vaccine safety is critical to the success of immunization programs and requires an existing comprehensive system to monitor safety. In the United States, there are several systems in place to assess safety in the postlicensure setting. The Vaccine Adverse Event Reporting System (VAERS) allows providers, parents, and patients to report adverse events. The VAERS program functions more to raise hypotheses about whether receipt of a vaccine or vaccines causes an adverse event than it does to evaluate causation. The Vaccine Safety Datalink (VSD) is a collaborative project between CDC’s Immunization Safety Office (ISO) and eight health care organizations. The VSD was initiated in 1990 and continues to moni­ tor safety of vaccines over large populations and to conduct studies to assess rare and serious adverse events following immunization. The Clinical Immunization Safety Assessment (CISA) Project is a national network of vaccine safety experts from the CDC’s ISO, seven medical research centers, and other partners, which provides a comprehensive vaccine safety public health service to the nation. Vaccine Injury Compensation  Most vaccine-preventable dis­ eases are transmitted person-to-person. Thus, when individuals are vaccinated, they are protected from disease and indirectly are protect­ ing others in society who either cannot be vaccinated (e.g., have medi­ cal contraindications to vaccine) or fail to make an adequate immune response. Therefore, if someone is injured by vaccine, society should provide that person compensation. This is the basis of the National Vaccine Injury Compensation Program (NVICP). This program offers compensation for the injured vaccine recipient and reduces the risk of liability for the vaccine provider and the manufacturer, as persons who receive vaccines covered by the NVICP must first go through this com­ pensation process before suing the provider or manufacturer. Vaccine Hesitancy  Over the past decade there has been an increase in vaccine hesitancy. Reasons for vaccine hesitancy are multifactorial and include concerns over vaccine safety, questioning whether specific vaccines are needed to prevent disease, and mistrust of the public health sector and medical professionals recommending the vaccines. This hesitancy has increased since the COVID-19 pandemic coinci­ dent with the proliferation of antivaccine tropes and with concern over loss of autonomy of individuals to make decision for themselves. These concerns can often be alleviated or lessened by addressing the spe­ cific questions raised by the hesitant individual and by a presumptive approach where vaccines are presented as the standard of care. A strong recommendation for a vaccine by a trusted health care provider is one proven mechanism to reduce vaccine hesitancy. Websites maintained by the CDC, NIH, and various specialty and subspeciality organiza­ tions have extensive information about vaccines, and providers should refer their patients to these credible sources. ■ ■SUMMARY Few programs have had the impact of vaccines in reducing health burdens. This is the result of a rigorous system to ensure that recom­ mended vaccines are safe and effective, and of an equally rigorous

system to ensure that persons for whom vaccines are recommended receive them. With the development and introduction of new vaccines, public health gains are expected to increase in coming years as long as high vaccine coverage levels can be maintained.

■ ■FURTHER READING Cunningham AL et al: Vaccine development: From concept to early clinical testing. Vaccine 34:6655, 2016. Li X et al: Estimating the health impact of vaccination against ten pathogens in 98 low-income and middle-income countries from 2000 to 2030: A modelling study. Lancet 397:398, 2021. McCarthy NL et al: Monitoring vaccine safety using the Vaccine Safety Datalink: Utilizing immunization registries for pandemic influenza. Vaccine 29:4891, 2011. National Academies of Sciences, Engineering and Medicine: The critical public health value of vaccines: Tackling issues of access and hesitancy: Proceedings of a workshop. Washington DC, National Academies Press, 2021. National Vaccine Advisory Committee: Protecting the public’s health: Critical functions of the Section 317 Immunization Pro­ gram—a report of the National Vaccine Advisory Committee. Public Health Rep 128:78, 2013. O’leary ST et al: Strategies for improving vaccine communication and uptake. Pediatrics 153:e2023065483, 2024. Pickering LK et al: Principles of vaccine licensure, approval and rec­ ommendations for use. Mayo Clin Proc 95:600, 2020. Plotkin SA: Correlates of protection induced by vaccination. Clin Vaccine Immunol 17:1055, 2010. Shattock AJ et al: Contribution of vaccination to improved survival CHAPTER 129 and health: Modelling 50 years of the Expanded Programme on Immunization. Lancet 403:2307, 2024. Walton LR et al: The history of the United States Advisory Committee on Immunization Practices (ACIP). Vaccine 33:405, 2015. Immunization Principles and Vaccine Use Sarah Meyer, Amanda Cohn,

Georgina Peacock

Immunization Principles

and Vaccine Use Few medical interventions of the past century can rival the effect that immunization has had on longevity, economic savings, and quality of life. Twenty-one diseases are now preventable through vaccines routinely administered to children and adults in the United States (Table 129-1), and most vaccine-preventable diseases of childhood are at historically low levels (Table 129-2). In the past few years, the adult immunization landscape has changed substantially with an increased number of recommended routine vaccines. The COVID-19 pandemic identified a need for improved infra­ structure to ensure adult vaccination, especially for those who are uninsured and who have limited access to health systems. Health care providers in a variety of settings deliver the vast majority of vaccines in the United States and therefore play an integral role in the nation’s public health system. ■ ■VACCINE IMPACT Direct and Indirect Effects  Many immunizations against specific infectious diseases protect individuals against infection and thereby prevent symptomatic illnesses. In addition, specific vaccines may blunt the severity of clinical illness (e.g., rotavirus vaccines and severe gastroenteritis) or reduce complications (e.g., zoster vaccines and

TABLE 129-1  Diseases Preventable with Vaccines Routinely Administered in the United States to Children and/or Adults CONDITION TARGET POPULATION(S) FOR ROUTINE USE Pertussis Children, adolescents, adults Diphtheria Children, adolescents, adults Tetanus Children, adolescents, adults Poliomyelitis Children Measles Children Mumps Children Rubella, congenital rubella syndrome Children Hepatitis B Children and high-risk adults Haemophilus influenzae type b infection Children and high-risk adults Hepatitis A Children and high-risk adults Influenza Children, adolescents, adults Varicella Children Pneumococcal disease Children, older adults, and high-risk adultsa Serogroups A, C, W, Y meningococcal disease Adolescents and high-risk children

and adults Serogroup B meningococcal disease High-risk children and adultsa Rotavirus infection Infants Human papillomavirus infection, cervical and anogenital cancers Adolescents and young adultsa Zoster Older adults and high-risk adults Respiratory syncytial virus Infants, high-risk children, pregnant personsa PART 5 Infectious Diseases COVID-19 Children, adolescents, adults Dengue High-risk children (i.e., those living in endemic areas and with laboratory confirmation of prior infection) Mpox High-risk adults aOthers in certain age groups may be vaccinated based on shared clinical decision-making. postherpetic neuralgia). Some immunizations also reduce transmis­ sion of infectious disease agents from immunized persons to others, thereby reducing the impact of infection spread. This indirect impact is known as herd immunity. The level of immunization in a population that is required to achieve indirect protection of unimmunized persons varies substantially with the specific vaccine and disease. For example, because of how transmissible measles is, an estimated 95% of the popu­ lation needs to be vaccinated to achieve herd immunity, whereas with polio, an estimated 80% coverage is needed. Over the past 30 years, due to the implementation of the Vaccines for Children Program, children have had broad access to routine childhood vaccines, regardless of insurance status. For routinely rec­ ommended vaccines in the United States, major declines in rates of vaccine-preventable diseases among both children and adults have become evident (Table 129-2). For example, vaccination of children <5 years of age against Streptococcus pneumoniae has led to not only a 96% reduction in invasive pneumococcal disease, but also substantial reductions in incidence among adults through herd immunity. Among children born during 1994–2023, the childhood vaccination series will prevent 508 million illnesses and 1,129,000 deaths over the course of their lifetime and save nearly $2.7 trillion in societal costs (U.S.). Control, Elimination, and Eradication of Vaccine-Preventable Diseases  Immunization programs are associated with the goals of controlling, eliminating, or eradicating a disease. Control of a vaccinepreventable disease reduces poor illness outcomes and often limits the disruptive impacts associated with outbreaks of disease in com­ munities, schools, and institutions. Control programs can also reduce absences from work for ill persons and for parents caring for sick

TABLE 129-2  Decline in Vaccine-Preventable Diseases in the

United States following Widespread Implementation of National

Vaccine Recommendations REDUCTION (%) IN CASES AFTER WIDESPREAD VACCINATION ANNUAL NO. OF PREVACCINE CASES (AVERAGE) NO. OF CASES REPORTED IN 2023a CONDITION Smallpox 29,005

Diphtheria 21,053

99 Measles 530,217

99 Mumps 162,344

99 Pertussis 200,752

Polio (paralytic) 16,316

Rubella 47,745

99 Congenital rubella syndrome

Tetanus

Haemophilus influenzae type b infection age <5 years 20,000 27b

99 Hepatitis A 117,333 11,500c

Hepatitis B (acute) 66,232 13,300c

Invasive pneumococcal infection: all ages 63,067 17,700d

Rotavirus hospitalizations

(<3 years old) 62,500 16,250e

Varicella 4,085,120 26,919f

a2023 reported cases unless otherwise specified (provisional as of February 2024). bAn additional 12 type b infections are estimated to have occurred among 257 reports of H. influenzae infection caused by unknown types among children <5 years of age. cData from the CDC’s Viral Hepatitis Surveillance, 2021. dUnpublished data from the CDC’s Active Bacterial Core Surveillance, 2020. eData from the CDC’s New Vaccine Surveillance Network, 2021; U.S. rotavirus disease now has a biennial pattern. fData from CDC’s varicella program, 2021. Source: Adapted from SW Roush et al: JAMA 298:2155, 2007 and Morb Mortal Wkly Rep 65:924, 2017. children, decrease absences from school, and limit health care utiliza­ tion associated with treatment visits. Elimination of a disease is a more demanding goal than control, usually requiring the reduction to zero of cases in a defined geographic area but sometimes defined as reduction in the indigenous sustained transmission of an infection in a geographic area. As of 2024, the United States had eliminated indigenous transmission of measles, rubella, poliomyelitis, and diphtheria. Importation of pathogens from other parts of the world continues to be important, and public health efforts are intended to respond promptly to such cases in order to limit forward spread of the infectious agent. Eradication of a disease is achieved when its elimination can be sus­ tained without the need to continue interventions. The only vaccinepreventable disease of humans that has been globally eradicated thus far is smallpox. Although smallpox vaccine is no longer given routinely, the disease has not reemerged naturally because all chains of human transmission were interrupted through earlier vaccination efforts and humans were the only natural reservoir of the virus. Currently, a major health initiative is targeting the global eradication of polio. Two of the three wild poliovirus types (types 2 and 3) have been eradicated globally. However, endemic transmission of wild poliovirus type 1 con­ tinues in Afghanistan and Pakistan, and circulating vaccine-derived polioviruses have been detected in areas of the world where poliovirus had been eliminated, including a case of vaccine-derived poliovirus type 2 in New York in 2022. Detection of a case of disease that has been targeted for eradication or elimination is considered a sentinel event that could permit the infectious agent to become reestablished in the community or region. Therefore, such episodes must be promptly reported to public health authorities.

Epidemic and Pandemic Preparedness and Response  Clusters of cases of a vaccine-preventable disease detected in an institution, a medical practice, or a community may signal important changes in the pathogen, vaccine, or environment. Several factors can give rise to increases in vaccine-preventable disease, including (1) low rates of immunization that result in an accumulation of susceptible persons (e.g., measles resurgence among vaccination abstainers); (2) changes in the infectious agent that permit it to escape vaccine-induced protection (e.g., non-vaccine-type pneumococci); (3) waning of vaccine-induced immunity (e.g., pertussis among adolescents and adults vaccinated in early childhood); and (4) point-source introductions of large inocula (e.g., food-borne exposure to hepatitis A virus). Reporting episodes of outbreak-prone diseases to public health authorities can facilitate recognition of clusters that require further interventions. The COVID-19 pandemic highlighted the importance of a strong immunization program: robust surveillance systems to detect emerging infectious disease threats; public–private partnerships to accelerate the development of novel vaccines; and systems in place to rapidly imple­ ment a vaccination program and monitor vaccine safety and effective­ ness. On a global scale, the “100 Days Mission” is being explored by the Coalition for Epidemic Preparedness Innovations (CEPI) and partners as a response to the next “Disease X” to make safe, effective vaccines, therapeutics, and diagnostics within 100 days of identification. PUBLIC HEALTH REPORTING  Recognition of suspected cases of dis­ eases targeted for elimination or eradication—along with other dis­ eases that require urgent public health interventions, such as contact tracing, administration of chemo- or immunoprophylaxis, or epi­ demiologic investigation for common-source exposure—is typically associated with special reporting requirements. Many diseases against which vaccines are routinely used, including measles, pertussis, and Haemophilus influenzae type b invasive disease, are nationally notifi­ able. Clinicians and laboratory staff have a responsibility to report some vaccine-preventable disease occurrences to local or state public health authorities according to specific case-definition criteria. All providers should be aware of state or city disease-reporting require­ ments and the best ways to contact public health authorities. A prompt response to vaccine-preventable disease outbreaks can greatly enhance the effectiveness of control measures. GLOBAL CONSIDERATIONS  Vaccinations are estimated to prevent 3.5–5 million deaths every year on a global scale. The COVID-19 pandemic led to disruptions of routine vaccination programs, resulting in declines in vaccine coverage in more than 100 countries. Although there have been promising signs of recovery, with diphtheria-pertussistetanus (DPT) coverage in 2022 nearly recovered to 2019 levels, gaps remain for other vaccines. For example, 22 million children missed their first dose of measles vaccine in 2022 compared with 19 million in 2019. As a result, the World Health Organization, the United Nations Children’s Fund, Gavi, the Vaccine Alliance, and the Bill & Melinda Gates Foundation launched “The Big Catch-Up” to support recovery of childhood vaccination rates to at least prepandemic levels. Enhancing Immunization in Adults  Although immunization has become a centerpiece of routine pediatric medical visits, it has not been as well integrated into routine health care for adults. This chapter focuses on immunization principles and vaccine use in adults. Accumulating evidence suggests that immunization coverage can be increased through efforts directed at consumer-, provider-, institu­ tion-, and system-level factors. The literature suggests that the applica­ tion of multiple strategies is more effective at raising coverage rates than is the use of any single strategy. RECOMMENDATIONS FOR ADULT IMMUNIZATIONS  The CDC’s Advi­ sory Committee on Immunization Practices (ACIP) makes recom­ mendations for administration of vaccines approved or authorized by the U.S. Food and Drug Administration (FDA) for use in children and adults in the U.S. civilian population. The ACIP is a federal advisory committee that consists of up to 20 voting members (experts in fields associated with immunization) appointed by the Secretary of the U.S. Department of Health and Human Services, as well as ex officio

members representing federal agencies and nonvoting representatives of various liaison organizations, including major medical societies and managed-care organizations. ACIP recommendations are available at www.cdc.gov/acip-recs/hcp/vaccine-specific/.

ACIP makes several types of recommendations. Routine, catch-up, and risk-based recommendations are those in which everyone in a particular age or risk group is recommended to receive vaccination. Examples include recombinant zoster vaccination for adults age ≥50 years and hepatitis B vaccination for adults age 19–59 years. Shared clinical decision-making recommendations are individually based and informed by a decision process between the health care provider and patient. With shared clinical decision-making recommendations, the decision to vaccinate is informed by the best available evidence on who may benefit from vaccination; the individual’s characteristics, values, and preferences; the health care provider’s clinical discretion; and the characteristics of the vaccine being considered. Examples of shared clinical decision-making recommendations include human papilloma­ virus vaccination of adults age 27-45 years and serogroup B meningococ­ cal vaccination of adolescents and young adults age 16-23 years. ADULT IMMUNIZATION SCHEDULES  Immunization schedules for adults in the United States are updated regularly, through an adden­ dum after ACIP votes on a new recommendation as well as an annual update, and can be found online (www.cdc.gov/vaccines/hcp/imzschedules/adult-age.html). The adult immunization schedule is also approved by seven provider organizations and published annually in Annals of Internal Medicine and Morbidity and Mortality Weekly Report (www.cdc.gov/mmwr). The adult immunization schedules for 2024 are summarized in Fig. 129-1. Additional information and specifications are contained in the footnotes to these schedules. CHAPTER 129 ■ ■IMMUNIZATION PRACTICE STANDARDS Administering immunizations to adults involves a number of pro­ cesses, such as deciding whom to vaccinate, assessing vaccine contra­ indications and precautions, providing vaccine information statements (VISs), ensuring appropriate storage and handling of vaccines, admin­ istering vaccines, and maintaining vaccine records. In addition, pro­ vider reporting of adverse events that follow vaccination is an essential component of the vaccine safety monitoring system. In 2014, the Stan­ dards for Adult Immunization Practice were revised to help providers take steps to ensure that their patients are fully immunized, including assessing the immunization status of patients at every clinical encoun­ ter, strongly recommending vaccines that patients need, administering vaccines or referring the patient to a vaccination provider, and docu­ menting vaccines received by the patient. Immunization Principles and Vaccine Use Deciding Whom to Vaccinate  Every effort should be made to ensure that adults receive all indicated vaccines as expeditiously as pos­ sible. When adults present for care, their immunization history should be assessed and recorded, and this information should be used to identify needed vaccinations according to the most current version of the adult immunization schedule. Decision-support tools incorporated into electronic health records can provide prompts for needed vaccina­ tions. Standing orders, which are often used for routinely indicated vaccines (e.g., influenza and zoster vaccines), permit a nurse or another approved licensed practitioner to administer vaccines without a spe­ cific physician order, thus lowering barriers to adult immunization. Assessing Contraindications and Precautions  Before vac­ cination, all patients should be screened for contraindications and precautions. A contraindication is a condition that increases the risk of a serious adverse reaction to vaccination. A vaccine should not be administered when a contraindication is documented. For example, a history of an anaphylactic reaction to a dose of vaccine or to a vac­ cine component is a contraindication for further doses. A precaution is a condition that may increase the risk of an adverse event or that may compromise the ability of the vaccine to evoke immunity (e.g., administering measles vaccine to a person who has recently received immune globulins or other blood products and may consequently have transient passive immunity to measles virus). Normally, a vaccine is

Vaccine 19–26 years 27–49 years 50–64 years ≥65 years COVID-19 1 or more doses of updated (2023–2024 Formula) vaccine (see notes) Influenza inactivated (IIV4) or Influenza recombinant (RIV4) 1 dose annually or or Influenza live, attenuated (LAIV4) 1 dose annually Respiratory syncytial virus (RSV)

Seasonal administration during pregnancy. See notes. ≥60 years Tetanus, diphtheria, pertussis (Tdap or Td) 1 dose Tdap each pregnancy; 1 dose Td/Tdap for wound management (see notes) 1 dose Tdap, then Td or Tdap booster every 10 years Measles, mumps, rubella (MMR)

1 or 2 doses depending on indication (if born in 1957 or later) For healthcare personnel, see notes Varicella (VAR)

2 doses (if born in 1980 or later) 2 doses Zoster recombinant (RZV)

2 doses for immunocompromising conditions (see notes) 2 doses Human papillomavirus (HPV)

2 or 3 doses depending on age at initial vaccination or condition 27 through 45 years Pneumococcal (PCV15, PCV20, PPSV23) See notes Hepatitis A (HepA)

2, 3, or 4 doses depending on vaccine Hepatitis B (HepB)

Meningococcal A, C, W, Y (MenACWY) 1 or 2 doses depending on indication, see notes for booster recommendations Meningococcal B (MenB)

2 or 3 doses depending on vaccine and indication, see notes for booster recommendations 19 through 23 years Haemophilus influenzae type b (Hib) 1 or 3 doses depending on indication Mpox PART 5 Infectious Diseases Recommended vaccination for adults who meet age requirement, lack documentation of vaccination, or lack evidence of immunity Recommended vaccination for adults with an additional risk factor or another indication HIV infection CD4 percentage and count Immunocompromised (excluding HIV infection) Men who have sex with men VACCINE Pregnancy COVID-19 See notes IIV4 or RIV4 1 dose annually LAIV4 1 dose annually if age 19–49 years RSV Seasonal administration. See notes See notes See notes Tdap or Td Tdap: 1 dose each pregnancy 1 dose Tdap, then Td or Tdap booster every 10 years MMR * VAR * See notes RZV See notes HPV * 3 dose series if indicated Pneumococcal HepA HepB See notes Age ≥60 years MenACWY MenB Hib Asplenia: 1 dose Mpox See notes See notes See notes Not recommended for all adults, but recommended for some adults based on either age OR increased risk for or severe outcomes from disease Recommended based on shared clinical decision-making Recommended for all adults who lack documentation of vaccination, OR lack evidence of immunity a. Precaution for LAIV4 does not apply to alcoholism. b. See notes for influenza; hepatitis B; measles, mumps, and rubella; and varicella vaccinations. c. Hematopoietic stem cell transplant. FIGURE 129-1  Recommended adult immunization schedules, United States, 2024. Additional information, including footnotes for each vaccine, contraindications, and precautions, can be found at www.cdc.gov/vaccines/hcp/imz-schedules/adult-age.html. The recommendations in this schedule were approved by the Centers for Disease Control and Prevention (CDC) Advisory Committee on Immunization Practices (ACIP), the American Academy of Family Physicians (AAFP), the American College of Physicians (ACP), the American College of Obstetricians and Gynecologists (ACOG), and the American College of Nurse-Midwives (ACNM). For complete statements by the ACIP, visit www.cdc.gov/acip-recs/hcp/vaccine-specific/.

See notes 2, 3, or 4 doses depending on vaccine or condition Recommended vaccination based on shared clinical decision-making No recommendation/ Not applicable Kidney failure, End-stage renal disease or on dialysis Asplenia, complement deficiency Heart or lung disease Chronic liver disease; alcoholisma Diabetes 1 dose annually if age 19–49 years Precaution: Might be indicated if benefit of protection outweighs risk of adverse reaction Contraindicated or not recommended *Vaccinate after pregnancy, if indicated Recommended for all adults, and additional doses may be necessary based on medical condition or other indications. See notes. No Guidance/ Not Applicable

not administered when a precaution is noted. However, situations may arise when the benefits of vaccination outweigh the estimated risk of an adverse event, and the provider may decide to vaccinate the patient despite the precaution. In some cases, contraindications and precautions are temporary and may lead to mere deferral of vaccination until a later time. For example, moderate or severe acute illness with or without fever is generally considered a transient precaution to vaccination and results in postponement of vaccine administration until the acute phase has resolved; thus, the superimposition of adverse effects of vaccination on the underlying illness and the mistaken attribution of a manifestation of the underlying illness to the vaccine are avoided. Contraindications and precautions to vaccines licensed in the United States for use in adults are summarized in Table 129-3. It is important to recognize con­ ditions that are not contraindications in order not to miss opportunities for vaccination. For example, in most cases, mild acute illness (with or without fever) or a history of a mild to moderate local reaction to a previous dose of the vaccine are not contraindications to vaccination. History of Immediate Hypersensitivity to a Vaccine Compo­ nent  A severe allergic reaction (e.g., anaphylaxis) to a previous dose of a vaccine or to one of its components is a contraindication to vac­ cination. While most vaccines have many components, substances to which individuals are most likely to have had a severe allergic reaction include egg protein, gelatin, and yeast. In addition, although natural rubber (latex) is not a vaccine component, some vaccines are supplied in vials or syringes that contain natural rubber latex. These vaccines can be identified by the product insert and should not be administered to persons who report a severe (anaphylactic) allergy to latex unless the benefit of vaccination clearly outweighs the risk for a potential allergic reaction. The much more common local or contact hypersensitivity to latex, such as to medical gloves (which contain synthetic latex that is not linked to allergic reactions), is not a contraindication to adminis­ tration of a vaccine supplied in a vial or syringe that contains natural rubber latex. ■ ■SPECIFIC ADULT POPULATIONS Pregnant Persons  Several vaccines are recommended for preg­ nant persons in the United States: tetanus and diphtheria toxoids and acellular pertussis (Tdap), inactivated influenza, COVID-19, and respiratory syncytial virus (RSV) vaccine, as well as hepatitis B vaccine (if not already vaccinated). Tdap and RSV vaccine are administered to pregnant persons specifically to prevent severe pertussis and RSV disease, respectively, in their infants, while influenza, COVID-19, and hepatitis B (HepB) vaccines are given to protect both the pregnant person and infant. Tdap vaccine is recommended during each pregnancy at the 27th through 36th week of gestation (preferably during the earlier part of this time period), regardless of prior vaccination status, in order to prevent pertussis in young infants. Annual influenza vaccination is rec­ ommended for everyone age ≥6 months, including pregnant persons in any trimester, ideally during September or October. Vaccination during July or August can be considered for pregnant persons in their third trimester if vaccine is available. Staying up to date with COVID-19 vaccination is recommended for everyone age ≥6 months, including pregnant persons in any trimester. It is recommended that every infant receive protection against severe RSV disease, either through maternal RSV vaccination during the 32nd through 36th week of gestation (dur­ ing September to January) or through infant administration of nirse­ vimab, a long-acting monoclonal antibody. Pregnant persons may need additional vaccines as well, such as HepB vaccine if not already vaccinated, since all adults age 19 through 59 years are recommended to receive HepB vaccination. Pregnancy is not a contraindication to administration of most other inactivated vaccines when otherwise indicated or when the benefits of vaccination are judged to outweigh potential risks (e.g., serogroup B meningo­ coccal vaccination); it is recommended that human papillomavirus (HPV) and recombinant zoster vaccination are delayed until after pregnancy. Live-virus vaccines (e.g., measles, mumps, and rubella

[MMR], varicella) are contraindicated during pregnancy because of the hypothetical risk that vaccine virus replication will cause congenital infection or have other adverse effects on the fetus. Patients who are breast-feeding or trying to get pregnant should stay up to date on all recommended vaccines, especially as certain congenital infections (e.g., rubella) are preventable through vaccination.

Immunocompromised Persons  Immunocompromised persons are at increased risk for severe outcomes from infectious diseases. Therefore, staying up to date on recommended vaccines is impor­ tant for this population. Immunocompromised patients may need to receive vaccines at an earlier age than the general population (e.g., pneumococcal and zoster at age ≥19 years instead of age ≥50 years), receive additional doses of a recommended vaccine (e.g., three-dose primary series and additional doses of COVID-19), receive vaccines based on a particular type of immunocompromising condition (e.g., meningococcal vaccination for those with complement deficiency), or have different vaccine recommendations based on degree of immune suppression, particularly for live-virus vaccines. Live-virus vaccines elicit an immune response due to replication of the attenuated (weak­ ened) vaccine virus that is contained by the recipient’s immune system. In persons with compromised immune function, enhanced replication of vaccine viruses is possible and could lead to disseminated infection with the vaccine virus. For this reason, live-virus vaccines are con­ traindicated for persons with severe immunosuppression, the defini­ tion of which may vary with the vaccine. Severe immunosuppression may be caused by many disease conditions, including hematologic or other malignancy. In some of these conditions, all affected persons are severely immunocompromised. In others (e.g., HIV infection), the degree to which the immune system is compromised depends on the severity of the condition, which in turn depends on the stage of disease or treatment. For example, MMR vaccine may be given to HIVinfected persons with CD4 percentage of ≥15 and count of ≥200/µL. CHAPTER 129 Older Adults  As age increases, the ability to mount adaptive and innate immune responses declines, resulting in increased susceptibility to infectious diseases and reduced responses after vaccination. This immunosenescence, along with increased rates of underlying condi­ tions, increases the risk of severe outcomes of infectious diseases in older adults. Long-term care facility residents among this age group are potentially at even greater risk due to increased frailty as well as risk of transmission associated with congregate settings. In addition to staying up to date on all recommended vaccines (e.g., COVID-19, Td/Tdap), there are specific vaccine recommendations for older adults: zoster vaccine in adults age ≥50 years, pneumococcal conjugate vaccine for adults age ≥50 years, and RSV vaccine in all adults aged ≥75 years and in those age 60–74 years at increased risk for severe RSV. Further­ more, adults age ≥65 years are recommended to receive a higher-dose influenza vaccine (i.e., high-dose, adjuvanted, or recombinant vaccine) rather than the standard-dose vaccine. Immunization Principles and Vaccine Use Health Care Workers  Health care workers are recommended to stay up to date with all vaccines recommended for them based on age or underlying condition (including influenza and COVID-19 vac­ cines), and they may be recommended to receive additional vaccines due to their occupational exposure (e.g., meningococcal vaccine for laboratory personnel who handle Neisseria meningitidis isolates). As part of their participation in the Centers for Medicare and Medicaid Services’ Hospital Inpatient Quality Reporting program, acute-care hospitals and select other facilities are required to report the propor­ tion of their health care personnel who have received seasonal influ­ enza and COVID-19 vaccination. Some institutions and jurisdictions have added mandates on influenza vaccination of health care workers and have expanded on earlier requirements related to vaccination or proof of immunity for hepatitis B, measles, mumps, rubella, and varicella. ■ ■VACCINE INFORMATION STATEMENTS A Vaccine Information Statement (VIS) is a one-page (twosided) information sheet produced by the CDC that informs vaccine

PART 5 Infectious Diseases TABLE 129-3  Contraindications and Precautions for Commonly Used Vaccines in Adults VACCINE FORMULATION CONTRAINDICATIONS AND PRECAUTIONS All vaccines Contraindication Severe allergic reaction (e.g., anaphylaxis) after a previous vaccine dose or to a vaccine component Precaution Moderate or severe acute illness with or without fever. Defer vaccination until illness resolves. Td Precautions GBS within 6 weeks after a previous dose of TT-containing vaccine History of Arthus-type hypersensitivity reactions after a previous dose of TD- or DT-containing vaccines (including MenACWY). Defer vaccination until at least 10 years have elapsed since the last dose. Tdap Contraindication History of encephalopathy (e.g., coma or prolonged seizures) not attributable to another identifiable cause within 7 days of administration of a vaccine with pertussis components, such as DTaP or Tdap Precautions GBS within 6 weeks after a previous dose of TT-containing vaccine Progressive or unstable neurologic disorder, uncontrolled seizures, or progressive encephalopathy. Defer vaccination until a treatment regimen has been established and the condition has stabilized. History of Arthus-type hypersensitivity reactions after a previous dose of TT- or DT-containing vaccines (including MenACWY). Defer vaccination until at least 10 years have elapsed since the last dose. HPV Contraindications History of immediate hypersensitivity to yeast Pregnancy (vaccinate after pregnancy if indicated) MMR Contraindications History of immediate hypersensitivity reaction to gelatina or neomycin Pregnancy (vaccinate after pregnancy if indicated) Known severe immunodeficiency (e.g., hematologic and solid tumors; chemotherapy; congenital immunodeficiency; long-term immunosuppressive therapy; severe immunocompromise due to HIV infection) Family history of altered immunocompetence (i.e., congenital or hereditary immunodeficiency in a first-degree relative), unless the immune competence of the potential vaccine recipient has been substantiated clinically or verified by a laboratory Precautions Recent receipt (within 11 months) of antibody-containing blood product History of thrombocytopenia or thrombocytopenic purpura Need for tuberculin skin testing or interferon γ release assay (IGRA) testing Varicella Contraindications Pregnancy (vaccinate after pregnancy if indicated) Known severe immunodeficiency History of immediate hypersensitivity reaction to gelatina or neomycin Family history of altered immunocompetence (i.e., congenital or hereditary immunodeficiency in a first-degree relative), unless the immune competence of the potential vaccine recipient has been substantiated clinically or verified by a laboratory Precaution Recent receipt (within 11 months) of antibody-containing blood product Receipt of specific antiviral drugs (acyclovir, famciclovir, or valacyclovir) 24 h before vaccination Use of aspirin or aspirin-containing products Influenza, inactivated, injectable Precautionb History of GBS within 6 weeks after a previous influenza vaccine dose Influenza, live attenuated nasal spray Contraindicationsb Pregnancy Immunosuppression, including that caused by medications or by HIV infection; known severe immunodeficiency (e.g., hematologic and solid tumors; chemotherapy; congenital immunodeficiency; long-term immunosuppressive therapy; severe immunocompromise due to HIV infection) Close contact with severely immunosuppressed persons who require a protected environment, such as isolation in a bone marrow transplantation unit Receipt of oseltamivir or zanamivir within 48 h before vaccination, peramivir within the previous 5 days, or baloxavir within the previous 17 days. Active cerebrospinal fluid/oropharyngeal communications/leaks Cochlear implants Anatomic or functional asplenia (e.g., sickle cell disease) Precautions History of GBS within 6 weeks of a previous influenza vaccine dose Medical conditions that might predispose to higher risks of complications from influenza, such as diabetes mellitus; chronic pulmonary disease (including asthma); chronic cardiovascular disease (except hypertension); renal, hepatic, neurologic/neuromuscular, hematologic, or metabolic disorders (Continued)

TABLE 129-3  Contraindications and Precautions for Commonly Used Vaccines in Adults VACCINE FORMULATION CONTRAINDICATIONS AND PRECAUTIONS Pneumococcal polysaccharide None, other than those listed for all vaccines Pneumococcal conjugate None, other than those listed for all vaccines Hepatitis A Contraindication History of immediate hypersensitivity to neomycin Hepatitis B Contraindications History of immediate hypersensitivity to yeast (for Engerix-B and Recombivax-HB) Pregnancy: Heplisav-B and PreHevbrio are not recommended; use other hepatitis B vaccines if indicated Meningococcal conjugate None, other than those listed for all vaccines Serogroup B meningococcal Precaution Pregnancy (vaccination may be indicated if benefits of protection outweigh risks of adverse reaction) Zoster Precaution Current herpes zoster infection COVID-19 Precaution Diagnosed nonsevere allergy (e.g., urticaria beyond the injection site) or nonsevere, immediate (onset <4 h) allergic reaction after administration of a previous dose Myocarditis or pericarditis within 3 weeks after a dose Multisystem inflammatory syndrome in children (MIS-C) or multisystem inflammatory syndrome in adults (MIS-A) Respiratory syncytial virus None, other than those listed for all vaccines Mpox None, other than those listed for all vaccines aExtreme caution must be exercised in administering MMR, varicella, or live zoster vaccine to persons with a history of anaphylactic reaction to gelatin or gelatin-containing products. Before administration, skin testing for sensitivity to gelatin can be considered. However, no specific protocols for this purpose have been published. bHistory of severe allergic reaction (e.g., anaphylaxis) to egg is a labeled contraindication to the use of inactivated influenza vaccine and live attenuated influenza vaccine. However, CDC’s Advisory Committee on Immunization Practices recommends that any licensed, recommended, and appropriate inactivated influenza vaccine or recombinant influenza vaccine may be administered to persons with egg allergy of any severity (www.cdc.gov/acip-recs/hcp/vaccine-specific/flu.html). Abbreviations: DT, diphtheria toxoid; DTaP, diphtheria, tetanus, and pertussis; GBS, Guillain-Barré syndrome; HPV, human papillomavirus; MenACWY, quadrivalent meningococcal conjugate vaccine; MMR, measles, mumps, and rubella; Td, tetanus and diphtheria toxoids; Tdap, tetanus and diphtheria toxoids and acellular pertussis;

TT, tetanus toxoid. recipients (or their parents or legal representatives) about the benefits and risks of a vaccine. VISs are mandated by the National Childhood Vaccine Injury Act (NCVIA) of 1986 and—whether the vaccine recipi­ ent is a child or an adult—must be provided for any vaccine covered by the Vaccine Injury Compensation Program (VICP). As of February 2024, vaccines that are covered by the NCVIA and that are licensed for use in adults include tetanus, diphtheria, hepatitis A, hepatitis B, HPV, influenza, MMR, pneumococcal conjugate, meningococcal, polio, and varicella vaccines. When combination vaccines for which no separate VIS exists are administered (e.g., hepatitis A and B combination vac­ cine), all relevant VISs should be provided. In addition, although provi­ sion of a VIS is not specifically mandated under the Public Readiness and Emergency Preparedness (PREP) Act, which authorizes the Coun­ termeasures Injury Compensation Program (CICP), CDC has pub­ lished VISs for the approved or authorized vaccines currently covered under the CICP, including COVID-19 and mpox vaccines. The CICP provides compensation for injuries that occur after the administration of certain countermeasures. VISs also exist for some vaccines not covered by the VICP or CICP, such as pneumococcal polysaccharide, Japanese encephalitis, rabies, herpes zoster, typhoid, and yellow fever vaccines. The use of these VISs is encouraged but is not mandated. All current VISs are available at two websites: the CDC’s Vaccine Information Statements site (www.cdc.gov/vaccines/hcp/vis/) and the Immunization Action Coalition’s site (www.immunize.org/vaccines/vis/ about-vis/). (The latter site also includes translations of the VISs.) VISs from these sites can be downloaded and printed. ■ ■STORAGE AND HANDLING Injectable vaccines are packaged in multidose vials, single-dose vials, or manufacturer-filled single-dose syringes. The live attenuated nasal-spray influenza vaccine is packaged in single-dose sprayers. Oral typhoid

(Continued) CHAPTER 129 Immunization Principles and Vaccine Use vaccine is packaged in capsules. Some vaccines, such as MMR and varicella, come as lyophilized (freeze-dried) powders that must be recon­ stituted (i.e., mixed with a liquid diluent) before use. The lyophilized powder and the diluent come in separate vials. Diluents are not inter­ changeable but rather are specifically formulated for each type of vaccine; only the specific diluent provided by the manufacturer for each type of vaccine should be used. Once lyophilized vaccines have been reconsti­ tuted, their shelf-life is limited and they must be stored under appropri­ ate temperature and light conditions. For example, varicella must be protected from light and administered within 30 min of reconstitution; recombinant zoster and MMR vaccines likewise must be protected from light but can be used up to 6 and 8 h after reconstitution, respectively. Vaccines are stored either at refrigerator temperature (2–8°C) or at freezer temperature (–15°C or colder). In general, inactivated vaccines (e.g., inactivated influenza, pneumococcal polysaccharide, and menin­ gococcal conjugate vaccines) are stored at refrigerator temperature, while vials of lyophilized-powder live-virus vaccines (e.g., varicella, MMR vaccines) are stored at freezer temperature. Diluents for lyophi­ lized vaccines may be stored at refrigerator or room temperature. Live attenuated influenza vaccine—a live-virus liquid formulation adminis­ tered by nasal spray—is stored at refrigerator temperature. Vaccine storage and handling errors can result in the loss of vaccines worth millions of dollars, and administration of improperly stored vaccines may elicit inadequate immune responses or adverse events in patients. To improve the standard of vaccine storage and handling practices, the CDC has published detailed guidance (available at www. cdc.gov/vaccines/hcp/admin/storage/toolkit/storage-handling-toolkit. pdf). For vaccine storage, the CDC recommends stand-alone units— i.e., self-contained units that either refrigerate or freeze but do not do both—as these units maintain the required temperatures better than combination refrigerator/freezer units. Dormitory-style combined refrigerator/freezer units should never be used for vaccine storage.

The temperature of refrigerators and freezers used for vaccine stor­ age must be monitored and recorded at least twice each workday. Ide­ ally, continuous thermometers that measure and record temperature all day and all night are used, and minimal and maximal temperatures are read and documented each workday. The CDC recommends the use of calibrated digital thermometers with a probe in thermal-buffered material; more detailed information on specifications of storage units and temperature-monitoring devices is provided at the link given above.

■ ■ADMINISTRATION OF VACCINES Most parenteral vaccines recommended for routine administration to adults in the United States are given by either the intramuscular (IM) or the subcutaneous (SC) route; one influenza vaccine formulation approved for use in persons 2–49 years of age is given intranasally. Some vaccines can be given by multiple routes; for example, MMRV vaccine, one of the MMR vaccines (M-M-R II), and 23-valent pneumo­ coccal polysaccharide vaccine can be given by either IM or SC route, and the mpox vaccine can be given intradermally or subcutaneously. Vaccines given to adults by the SC route are administered with a 5/8-inch needle into the upper outer-triceps area. Vaccines adminis­ tered to adults by the IM route are injected into the deltoid muscle (Fig. 129-2) with a needle whose length should be selected on the basis of the recipient’s sex and weight to ensure adequate penetra­ tion into the muscle. Current guidelines indicate that, for men and women weighing <152 lb (<70 kg), a 1-inch needle is sufficient; for women weighing 152–200 lb (70–90 kg) and men weighing 152–260 lb (70–118 kg), a 1- to 1.5-inch needle is needed; and for women weigh­ ing >200 lb (>90 kg) and men weighing >260 lb (>118 kg), a 1.5-inch needle is required. Additional illustrations of vaccine injection loca­ tions and techniques may be found at www.immunize.org/wp-content/ uploads/catg.d/p2020.pdf. PART 5 Infectious Diseases Aspiration, the process of pulling back on the plunger of the syringe after skin penetration but prior to injection, is not necessary because no large blood vessels are present at the recommended vaccine injec­ tion sites. Multiple vaccines can be administered at the same visit; indeed, administration of all needed vaccines at one visit is encouraged. Studies have shown that in general, vaccines are as effective when administered simultaneously as they are individually, and simultaneous administra­ tion of multiple vaccines is not associated with an increased risk of Site of intramuscular injection: deltoid Dermis Fatty tissue (subcutaneous) Muscle tissue FIGURE 129-2  Technique for IM administration of vaccine.

adverse effects and may increase uptake due to not needing to return for subsequent vaccinations. If more than one vaccine must be admin­ istered in the same limb, the injection sites should be separated by 1–2 inches so that any local reactions can be differentiated. If a vaccine and an immune globulin preparation are administered simultaneously (e.g., Td vaccine and tetanus immune globulin), a separate anatomic site should be used for each injection. For certain vaccines (e.g., hepatitis B vaccine), multiple doses are required for an adequate and persistent antibody response. The rec­ ommended vaccination schedule specifies the interval between doses. Many adults who receive the first dose in a multiple-dose vaccine series do not complete the series or do not receive subsequent doses within the recommended interval; this lack of adherence to protocol com­ promises vaccine efficacy and/or the duration of protection. Providers should implement recall systems that will prompt patients to return for subsequent doses in a vaccination series at the appropriate intervals. With some exceptions (e.g., oral typhoid vaccine), an interruption in the schedule does not require restarting of the entire series or the addi­ tion of extra doses. Syncope may follow vaccination, especially in adolescents and young adults. Serious injuries, including skull fracture and cerebral hemorrhage, have occurred. Adolescents and adults should be seated or lying down during vaccination. The majority of reported syncope episodes after vaccination occur within 15 min. The CDC recommends that vaccine providers consider observing vaccine recipients, particu­ larly adolescents, with patients seated or lying down for 15 min after vaccination to decrease the risk of injury should they develop syncope. If syncope develops, patients should be observed until the symptoms resolve. Anaphylaxis is a rare complication of vaccination. All facilities providing immunizations should have an emergency kit containing aqueous epinephrine for administration in the event of a systemic anaphylactic reaction. ■ ■MAINTENANCE OF VACCINE RECORDS All vaccines administered should be fully documented in the patient’s permanent medical record. Documentation should include the date of administration, the name or common abbreviation of the vaccine, the vaccine lot number and manufacturer, the administration site, the VIS edition, the date the VIS was provided, and the name, address, and title of the person who administered the vaccine. Increasing use Intramuscular needle insertion 90°

of two-dimensional bar codes on vaccine vials and syringes that can be scanned for data entry into compatible electronic medical records and immunization information systems may facilitate more complete and accurate recording of required information. ■ ■VACCINE SAFETY MONITORING AND ADVERSE EVENT REPORTING Prelicensure Evaluations of Vaccine Safety  Before vaccines are licensed by the FDA, they are evaluated in clinical trials with vol­ unteers. These trials are conducted in three progressive phases. Phase 1 trials are small, usually involving <100 volunteers. Their purposes are to provide a basic evaluation of safety and to identify common adverse events. Phase 2 trials, which are larger and may involve several hundred participants, collect additional information on safety and are usually designed to evaluate immunogenicity as well. Data gained from phase 2 trials can be used to determine the composition of the vaccine, the number of doses required, and a profile of common adverse events. Vaccines that appear promising are evaluated in phase 3 trials, which typically involve several hundred to several thousand volunteers and are generally designed to demonstrate vaccine efficacy and provide additional information on vaccine safety. Postlicensure Monitoring of Vaccine Safety  After licensure, a vaccine’s safety is assessed by several mechanisms. The NCVIA of 1986 requires health care providers to report certain adverse events that follow vaccination. As a mechanism for that reporting, the Vaccine Adverse Event Reporting System (VAERS) was established in 1990 and is jointly managed by the CDC and the FDA. This safety surveillance system collects reports of adverse events associated with vaccines cur­ rently approved or authorized in the United States. Adverse events are defined as untoward events that occur after immunization and that might be caused by the vaccine product or vaccination process. While the VAERS was established in response to the NCVIA, reporting of any adverse event following vaccination—whether in a child or an adult, and whether or not it is believed to have actually been caused by vaccination—is encouraged through the VAERS. The adverse events that health care providers are required to report are listed in the reportable-events table at vaers.hhs.gov/docs/VAERS_Table_of_Report­ able_Events_Following_Vaccination.pdf. The number of VAERS reports submitted varies each year. In 2019, VAERS received more than 48,000 reports. Approximately 85–90% of the reports described mild side effects such as fever, arm soreness, or mild irritability; the remaining reports are classified as serious. However, a report to VAERS does not mean that a vaccine caused an adverse event. Anyone can file a VAERS report, including health care providers, manufacturers, and vaccine recipients or their parents or guardians. VAERS reports may be submitted online or in paper form (vaers.hhs. gov/reportevent.html); additional information can be obtained by email (info@vaers.org) or phone (800-822-7967). The VAERS form asks for the following information: the type of vaccine received; the timing of vaccination; the time of onset of the adverse event; and the recipient’s current illnesses or medications, history of adverse events following vaccination, and demographic characteristics (e.g., age and sex). This information is entered into a database. The individual who reported the adverse event then receives a confirmation letter by mail with a VAERS identification number that can be used if additional informa­ tion is submitted later. In selected cases of serious adverse reaction, the patient’s recovery status may be followed up at 60 days and 1 year after vaccination. The FDA and the CDC have access to VAERS data and use this information to monitor vaccine safety and conduct research studies. VAERS data (minus personal information) are also available to the public. While the VAERS provides useful information on vaccine safety, this passive reporting system has important limitations. One is that events following vaccination are merely reported; the system cannot assess whether a given type of event occurs more often than expected after vaccination. A second is that event reporting is incomplete and is biased toward events that are believed to be more likely to be due to vaccination and that occur relatively soon after vaccination. To obtain

more systematic information on adverse events occurring in both vac­ cinated and unvaccinated persons, the Vaccine Safety Datalink project was initiated in 1991. Directed by the CDC, this project includes 11 managed-care organizations in the United States; member databases include information on immunizations, medical conditions, demo­ graphics, laboratory results, and medication prescriptions. In addition to these systems, CDC, FDA, and other federal partners use multiple other systems and data sources to conduct comprehensive vaccine safety monitoring, including CDC’s V-safe system, FDA’s Biologics Effectiveness and Safety (BEST) system, CMS’s Medicare claims data­ base, and the Department of Defense’s Vaccine Adverse Event Clinical System. In addition, postlicensure evaluations of vaccine safety may be conducted by the vaccine manufacturer. In fact, such evaluations are often required by the FDA as a condition of vaccine licensure.

■ ■CONSUMER ACCESS TO AND DEMAND FOR IMMUNIZATION By removing barriers to the consumer or patient, providers and health care institutions can improve vaccine use. Financial barriers have tra­ ditionally been important constraints. Fortunately, some progress has been made to mitigate out-of-pocket costs for vaccination of adults. The Affordable Care Act enacted in 2010 requires that most private insurance plans provide coverage for immunizations that appear on the approved immunization schedules without deductibles or copays when administered by an in-network provider. In addition, as of 2023, the Inflation Reduction Act eliminated out-of-pocket costs for ACIPrecommended vaccines for patients with Medicare prescription drug coverage and requires state Medicaid programs to cover and pay for ACIP-recommended adult vaccines without cost-sharing. However, barriers to vaccination remain for uninsured adults. A comprehensive Vaccines for Adults program to provide vaccines to uninsured adults has been proposed but has not yet been authorized for funding. To help ensure access to COVID-19 vaccines for uninsured and underinsured adults, CDC launched the Bridge Access Program for COVID-19 vaccines in 2023; this program ended in 2024. CHAPTER 129 Immunization Principles and Vaccine Use In addition to removing financial barriers, other strategies that enhance patients’ access to vaccination include extended office hours (e.g., evening and weekend hours) and scheduled vaccination-only clinics where waiting times are reduced. In recent years, pharmacies have become an increasingly important venue for adult vaccination and have helped improve equitable access to vaccines, given that the majority of the adult population lives within 5 miles of a pharmacy. Other locations outside the “medical home” (e.g., through occupa­ tional clinics, universities, and retail settings) also can expand access for adults who do not make medical visits frequently. Health promotion efforts aimed at increasing the demand for immunization are common. Direct-to-consumer advertising by phar­ maceutical companies has been used for some newer adolescent and adult vaccines. Efforts to raise consumer demand for vaccines have not increased immunization rates unless implemented in conjunction with other strategies that target strengthening of provider practices or reduction of consumer barriers. Attitudes and beliefs related to vacci­ nation can be considerable impediments to consumer demand. Many adults view vaccines as important for children but are less familiar with vaccinations targeting disease prevention in adults. Several vac­ cines are recommended for adults with certain medical risk factors, but self-identification as a high-risk individual is relatively rare. Com­ munication research suggests that adults are motivated to get vaccines to protect their own health and many would get vaccinated to protect loved ones. Adults with chronic conditions are more likely to be aware that they need to protect their own health. ■ ■STRATEGIES FOR PROVIDERS AND HEALTH CARE FACILITIES Recommendation from the Provider  Health care providers can have great influence on patients with regard to immunization. Studies repeatedly show that a health care provider is the most trusted source of vaccine information, and patients who receive a strong vaccine rec­ ommendation from a provider are more likely to get vaccinated than

08 - 130 Health Recommendations for International Travel

130 Health Recommendations for International Travel

those who do not. Strong provider recommendations using a presump­ tive approach (e.g., “You’re due for the flu shot today” vs “What do you want to do about the flu shot?”) have been shown to improve vaccine acceptance (Chap. 3). Providers should be well informed about vaccine risks and benefits so that they can address patients’ common concerns. The CDC and seven provider organizations review and update the schedule for adult immunization on an annual basis and have devel­ oped educational materials to facilitate provider–patient discussions about vaccination (www.cdc.gov/vaccines/hcp/).

System Supports  Medical offices can incorporate a variety of meth­ ods to ensure that providers consistently offer specific immunizations to patients with indications for specific vaccines. Decision-support tools have been incorporated into some electronic health records to alert the provider when specific vaccines are indicated. Manual or automated reminders and standing orders have been discussed (see “Deciding Whom to Vaccinate,” above) and have consistently improved vaccina­ tion coverage in both office and hospital settings. Most clinicians’ esti­ mates of their own performance diverge from objective measurements of their patients’ immunization coverage; quantitative assessment and feedback have been shown in pediatric and adolescent practices to increase immunization performance significantly. Some health plans have instituted incentives for providers with high rates of immunization coverage. Specialty providers, including obstetrician–gynecologists, may be the only providers serving some high-risk patients with indica­ tions for selected vaccines (e.g., pneumococcal polysaccharide vaccine, zoster vaccine for immunocompromised adults) as well as for other routine vaccinations (e.g., influenza, COVID-19). Immunization Requirements  Vaccination against selected com­ municable diseases is required for attendance at many universities and colleges as well as for service in the U.S. military and in some occupa­ tional settings (e.g., child care, laboratory, veterinary, and health care). Immunizations are recommended and sometimes required for travel to certain countries (Chap. 130). PART 5 Infectious Diseases ■ ■VACCINATION SETTINGS Vaccination of adults occurs in a variety of settings. While the “medical home” remains a critical setting for vaccination of adults, other settings such as pharmacies have become an increasingly important venue for adult vaccination and help to expand equitable access and convenience to vaccination. Other vaccination settings outside the medical home include health department venues, workplaces, and schools or colleges. Regardless of setting, it remains important for standards of immu­ nization practice to be followed. Consumers should be provided with information on the vaccine and how to report adverse events (e.g., via provision of a VIS), and procedures should ensure that documentation of vaccine administration is forwarded to the primary care provider and the state or city public health immunization registry. Detailed documentation may be required for employment, school attendance, and travel. Personalized health records can help consumers keep track of their immunizations, and some occupational health clinics have incorporated automated immunization reports that help employees stay up to date with recommended vaccinations. Some pharmacy chain establishments are using automated systems to report immunization information to the state or local immunization information system. ■ ■PERFORMANCE MONITORING Tracking of immunization coverage at national, state, institution, and practice levels can yield feedback to practitioners and programs and facilitate quality improvement. Healthcare Effectiveness Data and Information Set (HEDIS) measures related to adult immunization facilitate comparison of health plans. CDC utilizes a number of sur­ veys to monitor vaccination coverage among adults and track progress toward achievement of Healthy People 2030 targets for immuniza­ tion coverage. These sources include the National Health Interview Survey, the Behavioral Risk Factor Surveillance System, the National Immunization Surveys, Internet panel surveys, and other sources. Vaccination coverage among adults remains suboptimal, and statespecific immunization coverage for certain vaccines reveals substantial

geographic variation in coverage. There are persistent disparities in adult immunization coverage rates by race and ethnicity, as well as other sociodemographic factors. In contrast, racial and economic disparities in immunization of young children have been dramatically reduced during the past 30 years. Much of this progress is attributed to the Vaccines for Children Program, which since 1994 has entitled eligible children, including those who are uninsured or underinsured, to receive free vaccines. ■ ■FUTURE TRENDS Although most vaccines developed in the twentieth century targeted common acute infectious diseases of childhood, more recently devel­ oped vaccines prevent chronic conditions prevalent among adults. Hep­ atitis B vaccine prevents hepatitis B–related cirrhosis and hepatocellular carcinoma, HPV vaccine prevents some types of cervical cancer, genital warts, and anogenital cancers and may also prevent some oropharyn­ geal cancers, and the herpes zoster subunit vaccine protects against zoster and postherpetic neuralgia. New targets of vaccine development and research may further broaden the definition of vaccine-preventable disease. Research is ongoing on vaccines to prevent insulin-dependent diabetes mellitus, nicotine addiction, and Alzheimer’s disease. Expand­ ing strategies for vaccine development are incorporating molecular approaches such as RNA, DNA, vector, and peptide vaccines. New tech­ nologies, such as the use of transdermal and other needle-less routes of administration, are being applied to vaccine delivery. Acknowledgment The authors thank Anne Schuchat, MD, Lisa A. Jackson, MD, and Nancy Messonnier, MD, for their significant contributions to this chapter in the previous editions. ■ ■FURTHER READING Centers for Disease Control and Prevention: Epidemiology and Prevention of Vaccine-Preventable Diseases, 13th ed, Hamborsky J et al (eds). Washington DC, Public Health Foundation, 2015. Kroger A et al: General best practice guidelines for immunization. Best practices guidance of the Advisory Committee on Immunization Practices (ACIP). Available at www.cdc.gov/vaccines/hcp/acip-recs/ general-recs/downloads/general-recs.pdf.  Accessed May 11, 2024. Mcneil MM et al: The vaccine safety datalink: Successes and chal­ lenges monitoring vaccine safety. Vaccine 32:5390, 2014. National Vaccine Advisory Committee: Recommendations from the National Vaccine Advisory Committee: Standards for adult immunization practice. Public Health Rep 129:115, 2014. Plotkin SA et al (eds): Plotkin’s Vaccines, 7th ed. Philadelphia, Elsevier, 2017. Whitney CW et al: Benefits from immunization during the Vaccines for Children Program era—United States, 1994–2013. MMWR Morb Mortal Wkly Rep 63:352, 2014. Jesse Waggoner, Henry M. Wu

Health Recommendations

for International Travel In recent decades, international travel has increased dramatically with globalization and greater access to international flights. According to the United Nations World Tourism Organization, international tourist arrivals increased 50.3% from 2010 to 2019; arrivals exceeded 1.4 billion in 2019, with the highest rate of growth in arrivals to destinations in Asia and the Pacific. In 2018, according to the United Nations Conference on Trade and Development, total global merchandise exports reached

a record 19.5 trillion U.S. dollars, a nearly threefold increase over the previous two decades. Although travel in 2020 dropped drastically during the COVID-19 pandemic, there has been recovery since 2021, resuming prepandemic growth trends. International travel has brought social, economic, and cultural ben­ efits to the world; however, travel also widens the range of infections to which an individual may be exposed. The speed of air travel has been a major factor in the ease with which emerging infectious diseases have quickly spread worldwide in recent years. In the nineteenth century, intercontinental travel took long enough that travelers often recovered or perished from acute infections before arrival at their destinations. However, in the jet age, the time required to circumnavigate the globe has decreased to <24 h. This duration is shorter than the incubation periods for almost all infections, increasing the likelihood that infected travelers can arrive at their destinations prior to symptom onset. Epi­ demics can result; examples include severe acute respiratory syndrome (SARS) in 2003, Ebola virus disease in 2014, and the COVID-19 pan­ demic in 2020. Furthermore, introduction of pathogens into vulnerable regions can subsequently lead to infections becoming endemic, as was observed with the reintroduction of dengue throughout much of the Americas beginning in the 1970s and the global spread of HIV infec­ tions in the 1980s. Additional challenges include the increasing diversity of travel­ ers. While tourism, business travel, and mission work continue to be popular, recent decades have seen increasing numbers of other types of travelers, including students, migrants, medical tourists, and per­ sons visiting their countries of origin (“visiting friends and relatives” [VFR] travelers). Furthermore, an increasing range of individuals with risk factors for illness or injury are traveling internationally, including elderly persons, infants, pregnant women, and persons with chronic medical conditions (e.g., immunocompromising conditions). Whether practicing travel medicine, primary care, or other specialties, providers will encounter patients who travel internationally. This chapter outlines key considerations and preventive measures for international travelers, particularly those traveling to low- and middle-income countries. EPIDEMIOLOGY OF TRAVEL-RELATED CONDITIONS Unanticipated medical problems during travel are common. Although reported rates of travel-related morbidity and mortality vary widely by destination, traveler type, and study methodology, as many as 43–79% of travelers report developing a travel-related illness. Most illnesses are minor, with diarrhea often the most commonly reported and fewer than 1–3% of travelers reporting hospitalization. Among vaccinepreventable infections in travelers to lower-income countries, influenza historically has been the most commonly reported, although COVID-19 has also become common among travelers since 2020. Typhoid and hepatitis A are reported much less often, but typically are still reported more frequently than other infections commonly discussed in travel medicine but not frequently diagnosed in travelers, including cholera, Japanese encephalitis, meningococcal disease, rabies, poliomyelitis, and yellow fever. However, outbreaks such as the emergence of yellow fever in coastal southeastern Brazil in 2017–2018 or cholera associated with the 2010 earthquake in Haiti can result in an increased incidence of travel-associated cases. Among causes of death in travelers, studies suggest that cardiovascular events (likely associated with preexisting cardiac conditions) and injury are much more common than infec­ tions. Among U.S. citizens, the injuries most commonly causing deaths during international travel in 2016–2017 were motor vehicle accidents, homicide, drowning and maritime injuries, and suicide. Stressors encountered during travel can also exacerbate or uncover psychiatric disorders, and psychological conditions such as depression and anxiety are a common reason for medical evacuation. GENERAL APPROACH TO ADVISING INTERNATIONAL TRAVELERS Whether advising travelers in a travel clinic or in a primary care set­ ting, providers must cover a few key elements in a pretravel consulta­ tion (Table 130-1). These include (1) a trip risk assessment based on

detailed review of the itinerary and the traveler’s medical profile; (2) immunizations; (3) prevention of arthropod-borne infections, includ­ ing malaria chemoprophylaxis (when indicated); (4) food and water precautions and travelers’ diarrhea management; and (5) prevention of injuries and other conditions associated with travel.

A detailed itinerary, including cities and areas in a country to be vis­ ited, activities, and type of accommodations, is critical for assessment of the risks of the trip and determination of the indications for specific vaccinations, malaria prophylaxis, and other preventive measures. Trip duration, sequence of countries visited, and transit stops are important considerations, especially in the assessment of immunization require­ ments such as those for yellow fever. Numerous online resources offer recommendations for immunizations and malaria prophylaxis, which vary significantly among countries or even within certain countries (Table 130-2). The U.S. Centers for Disease Control and Prevention (CDC) travelers’ health website (www.cdc.gov/travel) is a source of comprehensive, up-to-date, country-specific recommendations on numerous topics, including immunizations, malaria chemoprophy­ laxis, and travel health notices for outbreaks and emerging infections. Because recommendations and requirements can change unexpectedly, providers are advised to routinely review guidance for each country prior to making recommendations. Consideration of the traveler’s medical profile is critical to recom­ mendations for appropriate preventive measures. Key considerations include the patient’s age, medical and vaccination histories, current medications, allergies (drug, food, and environmental), and pregnancy status. Although any chronic medical problem can be relevant to travel, common issues that can require particular attention include immuno­ compromising conditions (including HIV infection and treatment with immunosuppressive and immunomodulatory medications), cardiac and pulmonary conditions, pregnancy status, and severe allergies. Sig­ nificant hepatic and renal impairment due to any etiology can affect the choice of malaria prophylaxis. CHAPTER 130 Although travel clinics with providers specializing in travel medicine— e.g., those with a Certificate in Travelers’ Health issued by the Inter­ national Society of Travel Medicine—are now common in many cities worldwide, many travelers do not seek pretravel consultations, often because of an underappreciation of travel risks or a lack of awareness of the resource. Primary care providers are encouraged to routinely ask their patients about upcoming travel. Although some travel-specific vaccinations are often available only at specialized clinics, many recom­ mended vaccines are available from general practitioners or pharma­ cies. Reasons to refer a patient to a travel specialist include the need for vaccines used exclusively for travel, complex itineraries or traveler medical histories, or unfamiliarity with recommended immunizations or malaria chemoprophylaxis. Because several vaccines are given as a series and all vaccines theoretically take a week or more to induce protective immunity, referral to a travel clinic at least 4–6 weeks before travel is ideal. However, when this time frame is not possible, consulta­ tions can still provide much benefit. Health Recommendations for International Travel While the decision of whether to travel is ultimately that of the traveler, travel medicine providers play a key role in helping travelers identify the risks of a trip so that they can make an informed decision based on their personal risk tolerance. Occasionally, some situations can warrant advice against travel, including trips to areas with danger­ ous outbreaks or security situations or trips by a traveler who is unable to undertake critical preventive measures (e.g., travel to highly malari­ ous areas without chemoprophylaxis). Unfortunately, travel-related vaccinations and chemoprophylactic medications can be expensive and often are not covered by health insurance. To assist travelers on a limited budget, providers can prioritize preventive measures according to degree of risk so that decisions to decline a recommendation are not based on cost alone. IMMUNIZATIONS FOR TRAVELERS Historically, the field of travel medicine considered immunizations as routine, recommended, or required. Because infections prevented by routine immunizations for children and adults are encountered worldwide, travelers should be up to date with these immunizations.

TABLE 130-1  Overview of the Pretravel Consultation CONSULTATION ELEMENT ITEMS TO BE COVERED INTERVENTIONS, ADVICE Risk Assessment Itinerary • Destination countries and regions • Timing • Duration of trip • Mode of travel • Accommodations • Reason for travel and anticipated activities • Altitude Traveler • Medical history, medications • Allergies • Pregnancy status and planning • General risk tolerance Immunizations Itinerary Recommended and required vaccinations for itinerary • Administration of vaccines to meet recommendations and Traveler • Immunization history • Precautions and contraindications for specific vaccines Malaria and Arthropod-borne Infection Prevention Itinerary • Malaria and other arthropod-borne infection risk at destination • Accommodations and activities • Local malaria resistance to chemoprophylaxis drugs PART 5 Infectious Diseases Traveler • Precautions and contraindications to specific malaria chemoprophylaxis agents • Drug–drug interactions of regular medications with malaria chemoprophylaxis Gastrointestinal Illness Itinerary • Destination hygienic standards and water quality • Source of meals (e.g., restaurants, street vendors, home-cooking) Traveler • Travel style • Adventurous eating habits • Drug–drug interactions of regular medications with self-treatment antibiotics Other Possible Topics to Address   • Waterborne infection (schistosomiasis, leptospirosis) prevention • Injury and crime avoidance • Animal bite and rabies prevention • Sexually transmitted infections • Altitude illness • Venous thromboembolism • Jet lag • Motion sickness • Severe food, insect sting, and environmental allergies • Travel health and medical evacuation insurance • Traveler health kits and travel with medications • Mental health and cultural adaptation aICVP, International Certificate of Vaccine Prophylaxis (“yellow card”). Recommended travel vaccines in adults are those that are not included in routine schedules but that should be considered because of antici­ pated risks during travel. Required immunizations are those that are mandated by international regulations or specific countries for entry or exit. These three categories are not mutually exclusive or fixed, as many vaccinations that originally were used exclusively for travel (e.g., hepatitis A vaccine) are now given routinely in the United States. For required travel immunizations, proof of vaccination is pro­ vided on the International Certificate of Vaccination Prophylaxis (ICVP, commonly called the “yellow card”) issued by travel medicine

• Risk assessment that considers the itinerary, traveler, and ability to implement recommended preventive measures • Shared decision-making regarding whether to travel requirements • Provision of official documentation (ICVPa) for required immunizations • Prescription of malaria chemoprophylaxis when indicated • Administration of vaccines for arthropod-borne infections when indicated and available • Arthropod-bite avoidance advice • Advice on early recognition of malaria symptoms See Fig. 130-2   providers. When appropriate, medical waivers for required vaccina­ tions can be granted and documented on the ICVP. While enforcement of vaccine requirements can be unpredictable, travelers without proof or medical waiver for a required immunization can be subject to entry barriers, vaccination upon arrival, quarantine, or other penalties. In some situations, a vaccine may not be routinely recommended for a specific country but is still required for entry. The most common of these situations involves yellow fever vaccination, when a traveler who has recently been in an endemic country enters certain nonendemic countries (see “Yellow Fever,” below).

TABLE 130-2  Online Resources for Travelers and Travel Medicine Providers SUBJECT RESOURCESa General and country-specific recommendations, clinic directories CDC Travelers’ Health, www.cdc.gov/travel Country-specific immunization and malaria prevention advice, travel health notices, travel and yellow fever vaccine clinic listings CDC Health Information for International Travel (Yellow Book), available at www.cdc.gov/travel Comprehensive travel medicine reference covering general topics and specific infections, immunizations, special traveler populations, and common itineraries Heading Home Healthy, www.headinghomehealthy.org Traveler and provider tools for trip-specific CDC recommendations U.S. State Department, www.travel.state.gov Country profiles, travel advisories, Smart Traveler Enrollment Program (STEP), traveler advice Government of Canada Travel and Tourism, www.travel.gc.ca Canadian guidelines and advice for international travel National Travel Health Network and Centre (NaTHNaC), www.nathnac.net British resource for international travel and travel medicine providers International Society of Travel Medicine, www.istm.org Global travel clinic directory, resources for travel medicine providers International Association for Medical Assistance to Travellers, www.iamat.org International clinic directory, advice on travel health insurance World Health Organization, www.who.int/travel-advice Travel health updates, traveler advice, technical guidance American Society of Tropical Medicine and Hygiene, www.astmh.org Directory for clinical specialists in tropical medicine, travel medicine, and medical parasitology Jet lag prevention Jet Lag Rooster Jet Lag Calculator, https://sleepopolis.com/calculators/jet-lag/ Online tool to create jet lag prevention plan for a specific itinerary For travelers with specific conditions American College of Obstetrics and Gynecology, acog.org/search#q=travel&sort=relevancy Advice for pregnant travelers The Global Database on HIV-Specific Travel & Residence Restrictions, www.hivtravel.org General information for HIV-infected travelers and preexposure prophylaxis users and database on HIV-related entry restrictions Asthma and Allergy Foundation of America, www.aafa.org/traveling-with-asthma-allergies Advice for traveling with asthma and allergies FARE, www.foodallergy.org Resources for persons with severe food allergies, including chef card templates in several languages aAll websites last accessed March 21, 2024. Abbreviation: CDC, Centers for Disease Control and Prevention. All vaccines that are commonly administered for travel can gener­ ally be given on the same day; however, oral typhoid vaccine should be administered at least 8 h after oral cholera vaccine. Limited evidence on immunogenicity suggests that the response to live virus vaccines may be impaired if they are given on different days <28 days apart. For this reason, live virus vaccines (i.e., yellow fever, measles–mumps–rubella [MMR], live attenuated influenza, and varicella) should be given on the same day or spaced at least 28 days apart. If neither of these schedules is possible, the recommendation is to repeat the second vaccination after at least 28 days. Table 130-3 outlines common immunizations for travel in adults. ■ ■IMMUNIZATIONS FOR TRAVELERS TO MOST DESTINATIONS Hepatitis A  Hepatitis A is one of the more commonly reported vac­ cine-preventable infections in travelers. Transmission occurs primarily through direct person-to-person contact (fecal–oral transmission) or contaminated food and water, and travel is among the most common risk factors for infection among cases reported in the United States. Travelers are at highest risk in countries with inadequate sanitation and hygienic practices; levels of hepatitis A endemicity are highest in South Asia and sub-Saharan Africa. Although hepatitis A immuniza­ tion is now routinely recommended for persons with certain medical conditions and for all children in the United States, many adults have not been vaccinated. A single dose of monovalent hepatitis A vaccine is considered protective for younger, healthy adults when given prior to travel, and a booster vaccine dose given 6–18 months after the

CHAPTER 130 Health Recommendations for International Travel primary dose confers lifelong immunity. For persons >40 years old, immunocompromised persons, and other individuals with chronic medical conditions that might impair immune response, administra­ tion of hepatitis A immune globulin (0.1 mL/kg) at a separate site at the time of primary vaccination can be considered. No efficacy data are available to support single-dose use of hepatitis A/B combined vaccine (Twinrix) before travel. Hepatitis B  Hepatitis B is transmitted through contact with con­ taminated blood, blood products, or other bodily fluids. Travelers are strongly advised against high-risk activities, including tattooing, body piercing, and unprotected sexual intercourse. Even when avoiding activities that pose a high risk of exposure to hepatitis B, travelers seek­ ing health care can be exposed through inadequate infection-control measures or blood-product screening. While all travelers may benefit from hepatitis B vaccination, ensuring immunity to hepatitis B is par­ ticularly important for long-term travelers, health care workers, and persons who have sexual encounters. Nonimmune travelers who are departing too soon to complete the standard schedule for recombinant hepatitis B vaccine can consider the rapid hepatitis A/B combined vac­ cine (Twinrix) series or the recombinant hepatitis B vaccine with novel adjuvant (Heplisav-B). Influenza and COVID-19  Since respiratory infections such as seasonal influenza and COVID-19 are the most common vaccine-

preventable infections acquired during travel, travelers should be advised to be up to date with routine seasonal influenza and COVID-19 vaccinations. Because of variations in influenza seasons worldwide and

TABLE 130-3  Common Travel Immunizations PRIMARY SERIES IN

UNVACCINATED ADULTS BOOSTER INTERVAL PREGNANCY CONSIDERATIONS VACCINE Consider for Most Destinations Hepatitis A, inactivated (Havrix, Vaqta) 2 doses 6–12 months apart (Havrix); 2 doses 6–18 months apart (Vaqta) Hepatitis A/B combined (Twinrix) 3 doses at 0, 1, and 6 months; accelerated series: 3 doses on days 0, 7, and 21–30 Hepatitis B, recombinant and recombinant with novel adjuvant (Heplisav-B) Recombinant, 3 doses at 0, 1, and 6 months; recombinant with novel adjuvant, 2 doses at 0 and 1 month Measles, mumps, and rubella (MMR) 2 doses (≥28 days apart) None recommended Contraindicated Specific Destinations or Activities Chikungunya vaccine, live 1 dose Undetermined Inadequate data, precaution advised Cholera, live attenuated (Vaxchora), inactivated oral (Dukoral) 1 dose (Vaxchora); 2 doses 1–6 weeks apart (Dukoral) Japanese encephalitis, inactivated Vero cell culture–derived (IXIARO) 2 doses on days 0 and 7–28 ≥1 year after primary series Inadequate data, precaution advised against use unless risk of infection outweighs theoretical vaccine risk Meningococcal meningitis, quadrivalent conjugate 1 dose 5 years May be used if indicated Poliomyelitis, inactivated 3 doses if previously unvaccinated Single lifetime adult booster for persons who received primary series as children Rabies, human diploid cell (HDCV), purified chick embryo cell (PCECV) 2 doses on days 0 and 7 A third dose (21 days to 3 years after initial series) or titer check (1–3 years after primary series) recommended for travelers with ongoing (>3 year) exposure riska PART 5 Infectious Diseases Tick-borne encephalitis vaccine, inactivated 3 doses; second dose 14 days to 3 months after the first, and third dose 5–12 months after the secondb Typhoid, Vi capsular polysaccharide and oral live attenuated 1 dose 2 years for Vi capsular polysaccharide; 5 years for oral live attenuated Yellow fever 1 dose None routine; 10-year boosters recommended for certain groups aReaders are advised to refer to the Centers for Disease Control and Prevention Yellow Book or Advisory Committee on Immunization Practices (ACIP) 2022 guidance on rabies vaccination for detailed rabies preexposure prophylaxis recommendations based on exposure risk categories. bTravelers unable to receive the third dose before travel should receive two doses before travel. year-round risk in tropical regions, influenza vaccine should be offered to unvaccinated travelers even when the influenza season has already peaked locally. Travel medicine providers can reinforce the importance of these immunizations for all travelers by emphasizing that even uncomplicated infection can disrupt travel and put their contacts at risk. Preventing infections among those at higher risk for severe illness (e.g., the elderly and persons with immunocompromising conditions) is particularly important, since the availability of appropriate treat­ ments and supportive care can be limited in many destinations. For this reason, other vaccines for respiratory infections not typically associated with travel, including pneumococcus and respiratory syncytial virus, are potentially important for certain travelers at higher risk of severe illness. Other preventative advice for respiratory infections (wearing masks in high-risk situations or avoiding crowded, poorly ventilated areas) can be important for persons at high risk for infection. For travelers at high risk for complicated influenza or COVID-19 infections, providing a course of antivirals for self-treatment might be considered for travel to areas with limited access to treatment. However, travelers must be counseled on the proper use of antivirals, i.e., reserving their use for confirmed or strongly suspected infections. In addition to available SARS-CoV-2 home test kits, rapid multiplex influenza and SARS-CoV-2 tests are now available for home use in the United States. These kits are useful for travelers; however, at-home tests are not sufficiently sensitive to rule out an infection and travelers should seek formal medical evaluation whenever possible. Measles, Mumps, and Rubella  According to the World Health Organization (WHO), numbers of measles cases have increased

vaccinations (Chap. 129). While the risk of tetanus and pertussis exists worldwide, diphtheria is endemic or epidemic primarily in countries without adequate levels of vaccination. Because a tetanus booster is recommended for tetanus-prone injuries if the preceding booster dose was received >5 years earlier, some experts consider an early booster (before 10 years) for travelers engaging in activities with a high risk of injury in destinations with limited access to health care. A diphtheria booster within 5 years of travel is also recommended for travelers going to countries experiencing a diphtheria outbreak. Typhoid Fever  Typhoid fever, caused by Salmonella enterica sero­ type Typhi, is transmitted through ingestion of contaminated food and water. Most cases of typhoid fever reported in the United States are diagnosed in travelers after being acquired in South Asia. Africa and Southeast Asia also are considered high risk. East Asia, South America, and the Caribbean are considered lower risk. As with other food- and waterborne infections, travelers to endemic areas are at increased risk when consuming food or drink under unhygienic conditions. The risk of typhoid fever is usually lower than the risk of travelers’ diarrhea and hepatitis A; however, rising levels of antimicrobial resistance in endemic regions (particularly in South Asia) have increased the impor­ tance of prevention. Two vaccines are approved for travelers: injectable Vi capsular polysaccharide vaccine and oral live attenuated vaccine. Oral vaccine is contraindicated for immunocompromised persons, and completion of the vaccination course requires 1 week (four doses sepa­ rated by 48 h). Neither vaccine provides protection against paratyphoid fever (caused by S. enterica subtype Paratyphi A, B, or C), which is less commonly reported in U.S. travelers. Varicella  Travelers should have evidence of varicella immunity. For most U.S. travelers, this immunity can consist of documented receipt of two doses of varicella vaccine, laboratory evidence of immunity, confir­ mation of prior varicella or herpes zoster by a health care provider, or birth in the United States before 1980. ■ ■IMMUNIZATIONS FOR CERTAIN

REGIONS OR SITUATIONS Yellow Fever  Yellow fever is endemic in much of sub-Saharan Africa and South America (Fig. 130-1). Requirements for yellow fever immunization are among the most common entry rules encountered by travelers. Some endemic countries require proof of immunization for all international arrivals. Other countries, including many nonen­ demic countries that are prone to epidemics, require proof of immuni­ zation for arriving travelers who have recently (i.e., within 10 days prior to arrival) traveled to endemic countries. Transit stops for ≥12 h in an endemic country also can result in a requirement for proof of immu­ nization. The United States has no requirement regarding yellow fever immunization for travelers entering the country. Country-specific recommendations and requirements for yellow fever immunization are available from the CDC Travelers’ Health website (Table 130-2). Yellow fever vaccine is available only through state-authorized offi­ cial yellow fever vaccination clinics, and its administration is recorded on the ICVP. Yellow fever immunization is considered valid for entry purposes beginning 10 days after administration and extending for the lifetime of the vaccinee. Evidence indicates that a single dose of yel­ low fever vaccine provides most recipients with long-term protection; therefore, the previous requirement for boosting every 10 years was removed by the WHO from the International Health Regulations in 2016. Booster doses of yellow fever vaccine are still recommended after 10 years for certain individuals, including women who were pregnant during primary immunization, persons who were infected with HIV at the time of vaccination, and persons who received a hematopoietic stem cell transplant after immunization (provided they are sufficiently immunocompetent). Booster doses are also recommended for travel­ ers who will be at particularly high risk of yellow fever during travel, including travel to areas experiencing epidemics, prolonged stays in highly endemic areas, or travel during peak transmission seasons. All licensed yellow fever vaccines are live attenuated products. Contraindications include severe immunosuppression (e.g., during

immunosuppressive or immunomodulatory therapy, in primary immu­ nodeficiencies, or with symptomatic HIV infection or a CD4+ T lymphocyte count of <200/μL), malignant neoplasms, thymus gland disorders, and severe egg allergies. Precautions in adults include an age of ≥60 years, pregnancy, breastfeeding, and asymptomatic HIV infection with a CD4+ T lymphocyte count of 200–499/μL. Although medical waivers can be issued by yellow fever clinics, travelers must also consider the risks of traveling to endemic areas without vaccina­ tion. Common mild adverse reactions to vaccination include fevers, body aches, lymphadenopathy, localized swelling, and rash. Rare severe adverse events include anaphylaxis, neurologic complications (e.g., meningitis, encephalitis, Guillain-Barré syndrome), and yellow fever vaccine–associated viscerotropic disease (YEL-AVD). YEL-AVD is similar to yellow fever and can result in death. The risk of YEL-AVD is estimated to be ~0.3 case per 100,000 doses administered, with increased risk among immunosuppressed and elderly persons.

Poliomyelitis  Although wild-type poliovirus has been eradicated from most of the world, poliomyelitis caused by circulating vaccinederived poliovirus has been sporadically reported in numerous coun­ tries in Africa, the Middle East, and Asia where immunization rates are inadequate. For adults who have had the primary childhood polio vac­ cination series and are traveling to countries with reported wild-type or circulating vaccine-derived poliovirus transmission in the previous 12 months, a single booster of inactivated poliovirus vaccine before travel is recommended. This recommendation is sometimes extended to countries bordering those with poliovirus transmission when the risk of imported cases is high, especially for health care or humanitarian aid workers. Travelers who stay >4 weeks in certain countries considered high risk for exporting polio can also be subject to exit requirements for proof of recent vaccination (4 weeks to 12 months before depar­ ture). Because the list of countries with recommendations for polio booster doses is continually updated, providers should routinely review current polio booster guidance in online resources (Table 130-2). CHAPTER 130 Health Recommendations for International Travel Cholera  Most countries at risk for cholera are in Africa and Asia, with the exception of the island of Hispaniola in the Americas. In 2022, an upsurge in cases was observed worldwide, and this trend contin­ ued into 2023. Transmission occurs mostly through consumption of contaminated water, although contaminated food or person-to-person contact also can be responsible. The risk to travelers is extremely low when safe food and water precautions are followed. Cholera vaccina­ tion can be considered for travelers to endemic regions, particularly those visiting areas experiencing outbreaks, health care workers, or travelers who cannot adhere to strict hygienic practices. Individuals at higher risk for severe disease (e.g., those with type O blood or comor­ bid conditions) and those who will be in situations where access to health care will be difficult also might consider vaccination. A singledose live attenuated oral cholera vaccine (Vaxchora) is approved for travelers in the United States and the European Union. This vaccine had an efficacy of 90% at 10 days and 80% at 3 months after admin­ istration; the duration of protection and the need for booster doses remain to be determined. Oral killed cholera vaccines are available outside the United States. Meningococcal Disease  Endemic and epidemic meningococcal disease can occur worldwide; however, immunization is primarily rec­ ommended for high-risk travelers, including those going to countries in the African “meningitis belt” during the dry season (December to June), when large-scale epidemics can take place. Historically, menin­ gococcal disease in the meningitis belt was caused by serogroup A; however, more recently, serogroups C, W-135, and X have caused outbreaks following the successful serogroup A vaccination program (Meningitis Vaccine Project, https://www.path.org/our-impact/articles/ about-meningitis-vaccine-project/). Travelers should receive quadri­ valent meningococcal vaccine, which protects against serogroups A, C, Y, and W-135. Conjugated meningococcal vaccines are generally preferred over polysaccharide vaccines because of their increased immunogenicity and the reduced carriage of meningococci by vaccin­ ees; in fact, the polysaccharide vaccines are no longer available in the

PART 5 Infectious Diseases A B FIGURE 130-1  Yellow fever recommendations in (A) Africa and (B) the Americas. Vaccination of travelers to areas with low exposure risk (designated in green) is not routinely recommended but can be considered for travelers at increased risk due to high exposure to mosquitoes or prolonged travel. Recommendations current as of November 2022. See Centers for Disease Control and Prevention Travelers’ Health website for current recommendations. (Reproduced with permission from J Nemhauser et al [eds]: Yellow Book 2024 Health Information for International Travel. New York, Oxford University Press; 2023.)

United States. Pilgrims traveling to the Kingdom of Saudi Arabia for the Hajj and Umrah pilgrimages are required to demonstrate proof of vaccination with quadrivalent meningococcal vaccine within the preced­ ing 3 years. Vaccination against serogroup B meningococcal disease is not recommended for travelers except in specific outbreak situations. Japanese Encephalitis  Japanese encephalitis is a potentially severe viral infection passed to humans by evening-biting mosquitoes in much of Asia and parts of the western Pacific (Chap. 215). Although the WHO estimates that as many as 68,000 cases occur each year in Asia, the risk to travelers from nonendemic areas is estimated to be <1 case per 1 million travelers. However, certain travelers are at increased risk, such as long-term expatriates, persons traveling in rural areas during peak transmission seasons, and those with increased outdoor exposure (particularly during the evening). Short-term travelers to urban areas appear to be at lowest risk. In addition to mosquito avoid­ ance measures, vaccination can be considered for travelers at risk for infection. Multiple Japanese encephalitis vaccines are available world­ wide. An inactivated Vero cell culture–derived vaccine (IXIARO) is available in the United States and Europe and can be given to adults as a two-dose primary series, with doses administered 7–28 days apart. Rabies  Rabies (Chap. 214) is endemic to all continents except Antarctica and to numerous islands worldwide. Although many mam­ malian species can be infected with rabies virus, terrestrial carnivores and bats are the main reservoirs. In countries without animal control or routine pet vaccination, bites from infected dogs are often the most common source of rabies infection. Management of animal bites car­ rying a rabies risk is a common reason travelers seek urgent health care during or after travel. Individuals at higher risk for exposure may include children, long-term travelers, travelers whose activities will put them at higher risk of animal contact (e.g., field biologists, cavers), and travelers to remote areas. All travelers should be advised about animal bite avoidance and management of bite and scratch injuries (even minor), including thorough washing of the wound with soap and water (or povidone iodine) and immediate medical evaluation to determine whether rabies postexposure prophylaxis is indicated. The CDC-recommended postexposure prophylaxis regimen requires administration of rabies immune globulin and a rabies vaccination series. Rabies preexposure immunization series can be considered for travelers at higher risk of exposure, particularly those to destinations where access to rabies immune globulin can be difficult. Individuals receiving the rabies preexposure vaccination series should be advised that they will require urgent postexposure booster doses of vaccine fol­ lowing potential exposures. In the United States, the rabies preexposure vaccination series was revised in 2022, and a two-dose series (7 days apart) is now approved for travelers with risk of exposure. A third dose (between 21 days and 3 years after the initial series) or confirmation of protective titer (1–3 years after the initial series) is recommended for travelers who will be at risk for >3 years (see detailed discussion in the CDC Yellow Book, Table 130-2). Tick-Borne Encephalitis  Tick-borne encephalitis (Chap. 215) is endemic to parts of Europe and Asia, ranging from France to northern Japan. The infection is transmitted primarily through the bite of an infected Ixodes tick. Transmission is also possible through ingestion of unpasteurized dairy products from infected goats, sheep, or cows. Travelers to endemic regions who engage in outdoor activities in for­ ested areas should consider vaccination prior to travel. An inactivated tick-borne encephalitis vaccine is approved in the United States, given with a three-dose primary series (second dose 14 days to 3 months after the first, and the third dose 5–12 months after the second). For short-term travelers unable to receive the third dose before travel, evi­ dence suggests that vaccine efficacy following the initial two doses is sufficient enough to warrant vaccination with two doses. All travelers should use personal protective measures against tick bites (see “Preven­ tion of Arthropod-Borne Infections,” below). Dengue  Dengue (Chap. 215) is the most common arthropod-borne virus (arbovirus) worldwide, transmitted by Aedes mosquitoes. It is the

leading cause of fever in travelers returning from most tropical and subtropical destinations outside of sub-Saharan Africa, where malaria is most common. While most travel-related cases are uncomplicated, severe dengue can occur in travelers, including dengue hemorrhagic fever and dengue shock syndrome. There is currently no U.S. Food and Drug Administration (FDA)-approved dengue vaccine in the United States for travelers, since the only FDA-approved vaccine (Dengvaxia; tetravalent live-attenuated dengue vaccine) has indications limited to individuals residing in regions of endemicity with evidence of a past dengue virus infection. In the European Union (and some other nonU.S. countries), a newer tetravalent live attenuated dengue vaccine (Qdenga) is approved for persons aged 4 and older. It can be considered for use in travelers, although questions remain regarding its ability to elicit protection against all dengue virus serotypes, specifically in seronegative recipients. All travelers should use personal protective measures against mosquito bites (see “Prevention of Arthropod-Borne Infections,” below).

Chikungunya  Chikungunya virus (Chap. 215) is an arbovirus transmitted by Aedes mosquitoes that has spread worldwide in recent decades. Approximately 100–200 chikungunya cases are reported annually in U.S. travelers. The highest risk of infection is among travelers to countries experiencing outbreaks. Chikungunya causes an acute febrile illness, and although life-threatening illness is rare, many patients develop severe arthralgias that might become chronic. A live chikungunya vaccine was approved in the United States in 2023 with an indication for individuals 18 years of age and older at increased risk of exposure to chikungunya virus. The vaccine is given as a single dose and is contraindicated in immunocompromised persons. In these tri­ als, ~12% of individuals developed chikungunya-like symptoms. While most reactions were mild or moderate in severity and short-lived, 1.6% of vaccine recipients developed more significant symptoms or fever ≥39°C. New guidance on its use in travelers has been recently released (https://www.cdc.gov/chikungunya/hcp/vaccine/index.html). CHAPTER 130 Health Recommendations for International Travel PREVENTION OF ARTHROPOD-BORNE INFECTIONS ■ ■MALARIA PREVENTION Malaria (Chap. 231) results from infection with one or more of five species of protozoan parasites: Plasmodium falciparum, P. vivax, P. ovale, P. malariae, and P. knowlesi. The disease remains a significant cause of morbidity and mortality worldwide, and it is the leading cause of life-threatening infections in travelers. The majority of the disease burden, both in endemic populations and in travelers, occurs in regions of sub-Saharan Africa where P. falciparum is endemic. In 2018, nearly all of the 1823 malaria cases reported in the United States were acquired during travel to endemic countries. In most cases, the patient had not taken chemoprophylaxis, had adhered to the regimen inconsistently, or had taken an incorrect regimen. Among cases in U.S. residents, visiting friends or relatives was the most common reason for travel. Although there have been significant recent developments in malaria vaccines for use in endemic countries, these vaccines are not approved for travelers. Key elements of malaria prevention in travelers include mosquito avoidance, chemoprophylaxis, and early recognition of infection to prevent the development of severe disease. Plasmodium species are transmitted by Anopheles mosquitoes, which typically bite from dusk until dawn. Limiting activities to enclosed or well-screened areas during these hours is recommended. Travelers should wear longsleeved shirts and long pants to limit the amount of exposed skin and should apply a recommended insect repellent to any skin that remains exposed. Both the CDC and the WHO provide lists of reliable insect repellents, with products that contain DEET (20–50%) as the active ingredient being preferred by most experts. Travelers may also wear permethrin-treated clothing to increase protection. If sleeping areas are not well screened, it is recommended that individuals sleep under insecticide-treated bed nets. Indoor insecticide sprays or spatial repel­ lents should be used with caution; the efficacy of these measures in

malaria prevention has not been proven, and there may be risks from direct inhalation.

In addition to mosquito avoidance, chemoprophylaxis to prevent symptomatic disease is recommended for travelers to higher-risk regions. Current drugs used for malaria chemoprophylaxis include atovaquone-proguanil (Malarone), chloroquine (and hydroxychloro­ quine), doxycycline, mefloquine, primaquine, and tafenoquine (see Chap. 231 for detailed regimens). Chemoprophylaxis recommenda­ tions should be based on careful review of the traveler’s itinerary to determine malaria risk, predominant malaria species, and drug resistance patterns. The CDC and other travel medicine resources (Table 130-2) provide current country-specific risk assessments and chemoprophylaxis recommendations. Other important considerations include the traveler’s medical history and routine medications; this information is essential in assessing for contraindications to specific drugs or drug–drug interactions. Primaquine and tafenoquine are active against the dormant liver stages of P. vivax and P. ovale, and both drugs are useful as prophylaxis in persons without glucose-6-phos­ phate dehydrogenase (G6PD) deficiency traveling to areas where these species are more common. Primaquine can also be used for presump­ tive antirelapse therapy in returned travelers without G6PD deficiency who had a prolonged risk of exposure to P. vivax or P. ovale and did not use primaquine or tafenoquine for chemoprophylaxis. Currently approved regimens are well tolerated, and concerns that severe side effects are common are not supported by clinical evidence (e.g., hepa­ titis with atovaquone-proguanil or psychosis with mefloquine). Provid­ ers should emphasize that malaria chemoprophylaxis can be life-saving and that, when prophylaxis is indicated, the benefits outweigh the low risk of serious adverse reactions. PART 5 Infectious Diseases After the onset of malaria, progression to severe disease can be rapid; therefore, early recognition of symptoms is crucial. Febrile trav­ elers should seek evaluation as soon as possible during or after travel to malarious areas. Although the CDC does not recommend that travelers carry malaria treatment medications for standby emergency therapy as an alternative to chemoprophylaxis, this approach is accepted by some authorities for some travelers to lower-risk areas instead of chemoprophylaxis. However, because chemoprophylaxis failures can occur and locally acquired antimalarial agents can be substandard, prescription of a “reliable-supply” treatment course can be considered (in addition to chemoprophylaxis), to be used as needed under the advice of a medical professional. In the United States, atovaquoneproguanil and artemether-lumefantrine (Coartem) are available oral treatments for malaria, although atovaquone-proguanil should not be prescribed as a reliable-supply treatment course when it is being used for chemoprophylaxis. ■ ■PREVENTION OF OTHER ARTHROPOD-BORNE INFECTIONS Arthropod-borne infections are present worldwide, and arboviral infections are particularly numerous. Risk of arbovirus acquisition may vary from year to year and by season (rainy vs dry). In addition, risk can increase dramatically during outbreak periods, as witnessed during recent outbreaks of infection with chikungunya virus and Zika virus in the Americas. While dengue, chikungunya, and Zika infections result from the bite of infected Aedes mosquitoes, arboviruses may also be transmitted by non-Aedes mosquitoes (Japanese encephalitis virus, Mayaro virus, o’nyong-nyong virus), ticks (tick-borne encepha­ litis virus, Powassan virus), and biting midges (Oropouche virus). In addition to arboviral illness, travelers are at risk for numerous other arthropod-borne infections, including—but not limited to—leish­ maniasis (sand flies), filariasis (mosquitoes), African trypanosomiasis (tsetse flies), Chagas disease (triatomine bugs), and many tick-borne infections such as rickettsial diseases (e.g., African tick bite fever, scrub typhus) and Lyme disease. Travel immunizations for arthropod-borne infections are limited to yellow fever, Japanese encephalitis, tick-borne encephalitis, dengue, and chikungunya vaccines, although availability of these vaccines can vary worldwide. Many arboviral infections have no approved vaccines, and furthermore, some travelers might be unable to receive certain

vaccines due to cost or medical barriers (precautions or contraindica­ tions). For these reasons, arthropod bite avoidance remains a corner­ stone of arthropod-borne infection prevention. Recommendations for avoiding arthropod bites in general are similar to those provided for Anopheles mosquito avoidance to prevent malaria. However, in contrast to malaria, many disease-transmitting arthropods bite during the day and may be encountered across a wider range of environments. Travel­ ers at risk for tick bites can tuck pants into socks and perform daily self-inspections for attached ticks. Avoidance of sleeping in mud or thatch housing in areas endemic for Chagas disease is recommended. GASTROINTESTINAL ILLNESS Depending on the itinerary and season, as many as 30–70% of travelers report travelers’ diarrhea (TD). Symptoms can include urgently passed loose stools, abdominal pain, fever, and vomiting, and more severe cases can result in volume depletion or bloody diarrhea (dysentery). The majority (~80%) of TD cases result from bacterial infections, with the most common pathogens being Escherichia coli and Campylobacter, Shigella, and Salmonella species. A minority of cases are caused by viruses, preformed bacterial toxins, and protozoa (most commonly Giardia). Although travelers may develop gastrointestinal illness dur­ ing travel to any destination, TD is most likely to occur in low- and middle-income countries, where unhygienic food preparation prac­ tices constitute the greatest risk for development of disease. Precautions  Recommendations for the prevention of TD center on appropriate food and beverage selection as well as hand hygiene. Food is safest when cooked and served hot. Uncooked fruits and vegetables, unless they can be washed and peeled by the traveler, are considered risky. Dairy products should be pasteurized. Travelers are advised to drink bottled or purified water during travel and to avoid drinking bev­ erages with ice, which may be made with water from unsafe sources. Before drinking any bottled or canned beverage, travelers should con­ firm that the factory seal is intact, as refilling of bottles with untreated water or questionable beverages is common. Street-side vendors can be particularly risky. Individuals traveling to more remote areas without access to bottled water can use one of a number of methods to purify water, including boiling, chemical treatments, filtration, or ultraviolet irradiation devices. Prophylaxis  For prophylaxis of TD, bismuth subsalicylate can be considered for short-term use. This medication, taken as two tablets (or 2 oz of liquid) four times daily, has been shown to decrease TD incidence by 50% in Mexico. The safety of bismuth subsalicylate pro­ phylaxis when used for >21 days has not been established. Further­ more, the high dosing frequency of this regimen, common side effects (constipation, black tongue), and potential drug interactions (e.g., with acetazolamide or warfarin) limit its utility. No recommendation can be made for probiotics for TD prevention given the limited number of studies and varied results, which may vary by specific strain of probiotic used. Likewise, data supporting prebiotics for TD prevention are limited and inconclusive. Prophylactic antibiotics generally are not recom­ mended for TD prevention given increasing concerns about adverse reactions, colonization or infection with multidrug-resistant pathogens, and development of Clostridioides difficile infection. However, shortterm antibiotic prophylaxis (with rifaximin favored over fluoroquino­ lones) can be considered in rare situations for travelers at high risk for complications from TD. Oral killed cholera vaccine (Dukoral, available in Europe and Canada) shows some cross-protection against entero­ toxigenic E. coli; however, given the wide range of TD pathogens, the protection conferred by this vaccine against TD is likely to be minimal. Self-Treatment  In general, TD is a self-limited illness, with symp­ toms resolving in 3–7 days for bacterial infections. Recovery times are typically shorter for infections with viral pathogens and may be prolonged for parasitic infections. For travelers who develop TD, initiation of self-treatment should be based on the patient-assessed functional impact of illness (Fig. 130-2). TD can be considered mild (not distressing and has no impact on activities), moderate (distressing and may interfere with planned activities), or severe (incapacitating

• Food and water precautions • Definition of travelers’ diarrhea and severity classification • Importance of oral rehydration and salt intake for all types of travelers’ diarrhea • Different travelers’ diarrhea treatments and possible provision of antibiotics for self-treatment • Antibiotic prophylaxis (considered only for travelers at high risk for complications from travelers’ diarrhea) Pre-travel During travel Self-determination of travelers’ diarrhea severity Moderate Distressing or interferes with activities Mild Tolerable, not distressing, and does not interfere with planned activities May use: Loperamide or antibiotics* or loperamide and antibiotics* May use: Loperamide or bismuth subsalicylates *Azithromycin is preferred as first line for severe diarrhea or diarrhea acquired in areas with widespread quinolone resistance including Southeast and South Asia. Quinolones and rifaximin may be considered second-line treatment in moderate diarrhea or severe diarrhea without dysentery or high fever. FIGURE 130-2  Management of travelers’ diarrhea. (Adapted from MS Riddle et al: Guidelines for the prevention and treatment of travelers’ diarrhea: A graded expert panel report. J Travel Med 24:S63, 2017.) or prevents participation in planned activities). All dysenteric TD is considered severe. For all levels of severity, replacement of fluid and electrolyte losses resulting from diarrhea and/or vomiting is a mainstay of treatment. In severe cases, replacement with oral rehydration solu­ tion (available over the counter) is ideal; however, milder cases can be managed with any potable liquid. In addition, for patients with mild or moderate TD, self-treatment with antimotility agents alone (e.g., loperamide) can be considered. Antibiotic treatment can decrease the duration of TD to 1–2 days, with potential further benefits from adjunctive loperamide. However, the risks of adverse effects, drug–drug interactions, and alterations in the traveler’s microbiota are increasingly recognized in patients treated with antibiotics. Consequences of an altered microbiota can include C. difficile colitis and acquisition of multidrug-resistant organisms. Studies have shown that international travelers, particularly those who take antibiotics during travel, are at risk for becoming colonized with multidrug-resistant organisms, including extended-spectrum β-lactamase–producing Enterobacteriaceae. Travelers colonized with multidrug-resistant organisms may be at elevated risk for drug-resistant infections (e.g., urinary tract infection). The role of travelers in the global spread of multidrug-resistant organisms is uncertain. Given these concerns, routine self-treatment with antibiotics is recommended only in severe TD. Self-treatment with antibiotics, with or without anti­ motility agents, can be considered for moderate TD cases. Increasing quinolone resistance, most clearly documented in Campylobacter in Southeast and South Asia, has limited the utility of this antibiotic class for TD. The authors’ preferred antibiotic regimen for TD self-treatment is azithromycin at a dose of 500 mg daily for 3 days, although a single 1000-mg dose also is effective. Rifaximin or rifamycin SV can also be considered, especially for persons unable to take other antibiotics because of drug–drug interactions. However, Campylobacter species are resistant to rifamycins, and their efficacy against dysentery has not been established. Treatment regimens for TD are covered in detail in Chap. 138. Empirical treatment of acute TD with antiprotozoal agents such as metronidazole is not recommended. A small proportion of individuals develop prolonged symptoms (≥14 days), which may result from persistent infection (most often secondary to protozoa), secondary infection (C. difficile), or postin­ fectious irritable bowel syndrome. Antibiotic treatment for acute TD has not been proven to reduce the incidence of postinfectious

Counsel travelers on Severe Incapacitating or prevents planned activities Non-dysentery Dysentery Should use: Antibiotics* Should use: Antibiotics* with or without loperamide CHAPTER 130 irritable bowel syndrome, and patients with protracted symptoms should undergo a thorough evaluation. PREVENTION OF OTHER TRAVEL-RELATED PROBLEMS Health Recommendations for International Travel ■ ■ACTIVITY-SPECIFIC INFECTION RISKS Travelers should avoid direct contact with freshwater bodies (lakes, ponds, rivers) because of possible risks of leptospirosis and schistoso­ miasis. Schistosomiasis is endemic in Africa, Asia, and South America. Diving in African Rift Valley lakes (especially Lake Malawi) and raft­ ing on the Nile River are popular activities that put travelers at risk for schistosomiasis. Appropriate footwear is important in tropical coun­ tries to prevent infection with Strongyloides stercoralis and hookworm as well as snakebites. Animals of all types (wild, stray, or even pets) are best avoided to minimize bite risk. Travelers who engage in casual sex, including that with commercial sex workers, should be aware that the risk of sexually transmitted infections (Chap. 141) can be high, especially when barrier protections are not used. The multinational outbreak of mpox in 2022 was spread through close (skin-to-skin) contact, largely among networks of men who have sex with men (MSM). Given the continued transmission of mpox worldwide, vac­ cination against mpox should be considered for travelers who may engage in high-risk activity. Injection drug use, tattooing, and even acupuncture in unhygienic settings can pose a high risk for blood-borne infections such as HIV infection and hepatitis B and C. ■ ■VENOUS THROMBOEMBOLISM Travelers are at risk for venous thromboembolism (Chap. 122), par­ ticularly after long-haul flights or other extended periods of limited mobility. General precautions for prevention include ambulation during travel, calf exercises, and aisle seating. Travelers at increased risk for venous thromboembolism may benefit from graduated com­ pression stockings. Anticoagulation may be considered for high-risk individuals. ■ ■ALTITUDE ILLNESS Travelers to high-altitude destinations (>2500 m) should be counseled on altitude illness, and the prescription of medications for prophylaxis, such as acetazolamide, may be indicated (Chap. 475). Popular highaltitude destinations include Cusco, Peru (the usual gateway to Machu

Picchu), mountains that attract climbers (e.g., Mt. Kilimanjaro), and Nepal (trekking).

■ ■MOTION SICKNESS Motion sickness triggered by various modes of transportation can be significant for many travelers. Nonpharmacologic interventions can be helpful, such as avoiding known triggers, staying hydrated, getting adequate rest, and optimizing positions in vehicles near windows. For travelers with significant motion sickness history, over-the-counter and prescription antihistamines (e.g., dimenhydrinate, cyclizine, pro­ methazine) can be effective in preventing and treating motion sickness; however, sedation and anticholinergic effects can limit their use. Non­ sedating antihistamines appear to be less effective. A prescription for transdermal scopolamine patches can be helpful for susceptible travel­ ers who anticipate prolonged risk periods (e.g., on cruises or extended bus trips), although like the antihistamines, anticholinergic and central nervous system effects are a concern with scopolamine, particularly in elderly travelers. While the evidence supporting complementary and integrative health approaches to prevent and treat motion sickness is limited or anecdotal, approaches such as acupressure bands or ginger are relatively simple to use and might have some benefit for individual travelers. Travelers should be counseled about potential drug–drug interactions between supplements and their routine medications. ■ ■JET LAG Jet lag (Chap. 33) occurs when travel across time zones causes the traveler’s circadian rhythm to become asynchronous with the local time zone. Symptoms are most significant with travel across more than three time zones and can result in poor sleep, daytime sleepiness (with poor physical and mental performance), gastrointestinal symptoms, and altered mood. Strategies to help circadian rhythms adjust to new time zones include shifting of sleep schedules prior to travel, timed light exposures after arrival, and melatonin use. Online resources to assist travelers in timing interventions to minimize jet lag are available (Table 130-2), although none has been validated in clinical trials. While caffeine use can reduce daytime drowsiness, it can also disrupt sleep. Prescription of hypnotics (e.g., zolpidem) for travel-related insomnia should generally be avoided, since adverse effects, including excessive fatigue and impaired cognition upon awakening, can be problematic during travel. When used, the lowest effective dose of a hypnotic medication should be used, and travelers should be cautioned about use during flights (when extended immobilization is problematic) or any situation when a full course of sleep is not possible. Sedativehypnotic and anxiolytic medications are among the classes of drugs that are potentially restricted by certain countries (see “Traveling with Prescription Medications,” below). PART 5 Infectious Diseases ■ ■INJURIES International travel presents numerous factors that contribute a higher risk of injuries and death. Travelers may face unfamiliar environments and language barriers and rely heavily on other people (e.g., drivers and tour operators) for protection. Furthermore, in low- and middleincome countries, safety protections that are typical in high-income countries are often less stringent, unenforced, or nonexistent. Travelers often exhibit increased risk-taking behaviors during travel, frequently in association with the use of alcohol. When injuries do occur, access to adequate trauma care can be limited. Motor vehicle accidents are a common cause of injury deaths in travelers. In addition to poor road conditions, traffic rules are often less strictly enforced overseas. Riding on motorbikes (especially without a helmet), on overcrowded public transit, in improperly maintained vehicles, and in vehicles without seatbelts should be avoided. Drowning prevention and crime avoidance are important safety topics. The U.S. State Department provides coun­ try-specific safety and security advice for U.S. travelers (Table 130-2). TRAVELERS WITH PREEXISTING

MEDICAL CONDITIONS Travel is increasingly common for persons with chronic medical conditions. Risks vary depending on the condition, destination, and activities. Travelers with chronic medical conditions are encouraged to

plan their trips carefully and to consult with their physicians to assess fitness for travel. Notably, cardiovascular events are a frequent cause of in-flight emergencies and death during travel. Travel across time zones and changes in diet can create challenges in conditions—e.g., diabetes mellitus—that require regulation of diet and consistent medication timing. Providers can assist travelers by providing copies of prescrip­ tions (or medication lists), a medical problems list, and a baseline electrocardiogram. Adverse events caused by drug–drug interactions can be diffi­ cult to manage during travel, especially in destinations with limited emergency care. Therefore, providers should review the traveler’s medication list for potential drug interactions when prescribing pro­ phylactic or self-treatment medications. Azithromycin and quinolones prescribed for travelers’ diarrhea self-treatment can cause additive QT interval prolongation when used with some antidepressant and antiar­ rhythmic medications. Malaria prophylaxis medications can affect the international normalized ratio in patients who are taking warfarin. ■ ■IMMUNOCOMPROMISED TRAVELERS An increasing number of immunocompromised persons are traveling, including organ transplant recipients, HIV-infected persons, cancer patients, persons with asplenia, and persons receiving immunosup­ pressive therapies (e.g., biologic agents, antimetabolites, or chronic high-dose glucocorticoids). Although each of these conditions has unique risks, general concerns include increased susceptibility to infec­ tion, decreased vaccine efficacy, and—for patients with severe immu­ nosuppression—contraindications to live virus vaccines. Routinely recommended precautions (e.g., food and water hygiene, insect bite avoidance) are particularly important for travelers with immunocom­ promising conditions. Conditions associated with severe immunocompromise that pre­ clude use of live virus vaccines include active leukemia or lymphoma, generalized malignancy, graft-versus-host disease, HIV/AIDS (with a CD4+ count of <200 cells/μL), and congenital immunodeficiencies. Immunosuppressive therapies that preclude live virus vaccines include high-dose glucocorticoid treatment (defined as ≥20 mg of prednisone or the equivalent daily for ≥2 weeks), alkylating agent administration, antimetabolite therapy (e.g., azathioprine, methotrexate), transplantrelated immunosuppression, cancer chemotherapy, radiation therapy, and treatment with biologic agents, including tumor necrosis factor blockers, checkpoint inhibitors, and lymphocyte-depleting agents. If possible, travel immunizations should be administered prior to iatro­ genic immunosuppression (≥2 weeks for inactivated vaccines and ≥4 weeks for live vaccines). The duration of immunosuppression after discontinuation of immunosuppressive therapies can be prolonged, particularly for biologic agents. Travelers unable to receive a required yellow fever vaccine because of immunosuppression should be given a medical waiver if travel cannot be avoided. Providers are advised to review the detailed guidance for immunizing immunocompromised travelers provided in the CDC Yellow Book (Table 130-2) and other resources. ■ ■HIV-INFECTED TRAVELERS Infection risk in HIV-infected individuals generally correlates with the level of immunosuppression (i.e., the CD4+ T-cell count). Adults with HIV infection and CD4+ counts of >500 cells/μL are generally considered to have levels of risk similar to those faced by travelers without immunocompromising conditions. Live MMR and varicella vaccines can generally be administered to HIV-infected travelers with a CD4+ count of >200 cells/μL for ≥6 months. Guidance for yellow fever immunization of HIV-infected persons is reviewed above. Oral live attenuated typhoid and live attenuated influenza vaccines should not be administered to HIV-infected persons, given the availability of polysaccharide and inactivated versions of these vaccines, respectively. The number of countries that restrict the entry of HIV-infected per­ sons has decreased in recent years, particularly for short-term travelers and tourists. However, HIV-infected travelers should review the poli­ cies of their destination, especially when they plan to work abroad or stay for longer terms. Unnecessary disclosures of HIV status to customs

or immigration officials should be avoided. Resources include embas­ sies in destination countries, the U.S. State Department, and online resources for travelers with HIV infection (Table 130-2). ■ ■PREGNANT TRAVELERS Travel medicine providers should assess pregnancy status and the pos­ sibility of conception during travel. The pregnant traveler faces numer­ ous unique risks. These include limited availability of emergency care for pregnancy complications, increased risk of certain infections, and exposure to specific infections that can result in pregnancy complica­ tions. Although most airlines will allow pregnant women to travel up to 36 weeks of pregnancy, the American College of Obstetrics and Gyne­ cology recommends travel during the middle period of pregnancy (weeks 14–28), when morning sickness has improved, before mobil­ ity becomes impaired, and when the risk of spontaneous abortion or premature labor is minimal. Pregnancy-related contraindications and relative contraindications to travel are numerous and are reviewed in the CDC Yellow Book (Table 130-2). Pregnant travelers are at increased risk of various infections (e.g., malaria, influenza, hepatitis E, listeriosis) and/or severity of illness when traveling. Some infections, notably Zika virus disease, toxoplas­ mosis, and rubella, can result in birth defects or fetal death. Pregnant travelers should contemplate the infectious risks of their destination and consider delaying travel to areas where particularly dangerous infections, such as malaria or Zika, are present. Currently, only meflo­ quine and chloroquine are approved for malaria chemoprophylaxis in pregnancy, and plasmodial resistance to these drugs can further limit options. Travel immunizations considered safe during pregnancy also are limited (Table 130-3). When used as directed, insect repellents registered by the Environmental Protection Agency, such as DEET, are considered safe during pregnancy. ■ ■TRAVELERS WITH SEVERE ALLERGIES Travelers with severe allergies to food, insect stings, and environ­ mental allergens can be at increased risk during travel, particularly in destinations without adequate emergency care. Avoidance of food allergens can be challenging, particularly when eating in restaurants or catered settings. Language barriers can present difficulties in avoiding exposures to allergens in food or medications. Regional variations in culinary practices and ingredients can lead to unexpected food allergen exposure. Outdoor activities can increase the risk of stings by hyme­ nopterous insects (bees, wasps, ants). Solo travelers can face particular challenges when experiencing severe allergies. Providers should ensure that travelers have an emergency care plan for severe allergies and an adequate supply of emergency selftreatment, including epinephrine auto-injectors and antihistamines. For travelers who will have no immediate access to health care, it can be prudent to bring other medications that might be indicated, includ­ ing rescue bronchodilator inhalers (for individuals with asthma) and short courses of glucocorticoids. Written documentation of the allergic disorder and self-treatment medications should be carried, especially when injectable medications are involved. Travelers with severe food allergies should alert restaurants and hosts. Printable food allergy alert cards in various languages are available online (Table 130-2). OTHER PRETRAVEL PREPARATIONS ■ ■TRAVEL HEALTH KITS A carefully planned travel health kit can minimize the need to seek care for self-treatable conditions. The ideal contents depend on the destination, duration, and activities during travel as well as on indi­ vidual health issues. Routine and trip-specific prescription medications (e.g., malaria prophylaxis, travelers’ diarrhea self-treatment antibiot­ ics) should be carried in original labeled prescription bottles to aid in identification. A digital thermometer and typically used over-thecounter medications, such as analgesics and antipyretics, antidiarrheal medications, medications for motion sickness, antacids, laxatives, oral rehydration salts, antihistamines, and topical steroid creams, can be important. Basic first-aid items, such as gloves, bandages, tape,

antibiotic ointment, and tweezers, are helpful. Critical medications should always be carried and not packed in checked luggage; however, travelers must consider any restrictions about flying with sharp objects or liquids, particularly in their carry-on baggage.

■ ■TRAVELING WITH PRESCRIPTION MEDICATIONS Carrying copies of prescriptions (or a signed medication list from a physician’s office) is recommended. Many countries, particularly in Asia, the Middle East, and Africa, have stringent restrictions on certain drugs that are less restricted in the United States. These regulations can include controlled substances such as opioid analgesics, anxiolytics and sedatives, and medications for attention deficit hyperactivity disorder. Even some over-the-counter medications such as pseudoephedrine and diphenhydramine are restricted in certain countries. Requirements for traveling with restricted medications can include carrying copies of prescriptions or even obtaining advance approval from the destination country’s health ministry. Levels of enforcement and penalties for vio­ lations vary widely. Travelers who plan to carry potentially restricted medications should contact the embassy of their destination to review policies. The International Narcotics Control Board also maintains a listing of regulations for travelers carrying controlled medications to specific countries (Table 130-2). Travelers taking antiviral medications for HIV preexposure prophylaxis going to countries that restrict entry to persons living with HIV/AIDS might take specific precautions to avoid challenges when entering (see online resource for HIV-infected travelers, Table 130-2). Travelers should carry amounts appropriate for the duration of their itinerary, including a limited number of extra doses for use in cases of a delay in return. CHAPTER 130 Although many medications that require prescription in the United States are available over the counter overseas, the quality of locally acquired pharmaceuticals can vary. Counterfeit medications, par­ ticularly antimalarials, are common in much of the world. Whenever possible, travelers are cautioned to avoid obtaining critical medications such as malaria chemoprophylaxis during travel. Health Recommendations for International Travel ■ ■HEALTH CARE OVERSEAS AND TRAVEL

HEALTH INSURANCE Travelers should consider where they would seek urgent or emergency health care, particularly if they have chronic health conditions, are pregnant, or will participate in activities with a high risk of injury or illness (e.g., altitude sickness). International travelers should be aware that most countries do not accept routine health insurance from other countries and that such insurance is unlikely to cover out-of-pocket health care costs or to provide assistance in identifying providers overseas. Travelers are advised to review their health insurance policies before travel to assess the scope of international coverage (including emergency care, hospitalization, psychiatric care, and obstetric care, if applicable) and the availability of 24-h physician-backed support. For many travelers, supplemental insurance coverage of some type is prudent. Travel insurance usually consists of coverage for financial losses due to trip cancellation (e.g., due to unexpected illness) or lost baggage. Supplemental travel health insurance policies cover health care costs overseas and typically provide 24-h support centers. Medical evac­ uation (medevac) insurance can be a part of a travel health insurance policy or a stand-alone policy and covers medical evacuation when it is determined that the local level of care is inadequate. Further informa­ tion on travel health insurance is available from online resources listed in Table 130-2. SPECIAL TRAVEL POPULATIONS Travelers are increasingly diverse in their reasons for and types of travel, each of which poses unique risks and challenges (Table 130-4). A major challenge in travel medicine is presented by VFR travelers who are visiting their countries of origin. VFR travelers face increased risks for travel-related infections, as they often travel to areas not frequented by tourists, stay in local homes, and adopt local food and transportation habits. Immunity resulting from malaria infection is not long-term, but immigrants from malaria-endemic countries often incorrectly assume that they are immune. Barriers to appropriate

TABLE 130-4  Risks and Prevention Strategies in Special Travel Populations GROUP RISKS AND CHALLENGES PREVENTION STRATEGIES Travelers visiting friends and relatives (VFR) • Greater likelihood of visiting areas outside usual travel destinations • Frequent adoption of food, accommodation, and transportation habits similar to those of locals • May underestimate importance of travel immunizations or malaria prophylaxis • Financial or cultural barriers to seeking pretravel advice or immunization Budget travelers • Financial barriers to seeking pretravel advice or immunization • Lower quality of accommodations, transportation, and food establishments Last-minute travelers • Minimal advance notice for pretravel consultations or immunizations Long-term travelers • Increased risk of infection and injury due to longer duration • Increased likelihood of adopting local food, accommodation, and transportation standards • Potential need for extended supply of malaria chemoprophylaxis Health care workers on medical missions • Risk of infections acquired through patient care because of inadequate infection-control standards • Potentially high prevalence of untreated transmissible infections in patients • Limited or no access to urgent postexposure PART 5 Infectious Diseases prophylaxis for HIV infection and hepatitis B • Exposure to emerging infections and outbreaks Medical tourists • Nosocomial infections and other complications of medical procedures overseas • Substandard accreditation, infection control, safety guidelines, drugs, and blood-product screening • Increased risk of thromboembolism following surgery Abbreviation: CDC, Centers for Disease Control and Prevention. pretravel advice can include financial and language issues or a lack of trust in the medical system. POSTTRAVEL MEDICAL CARE Acute febrile illness in returning travelers can represent a potentially life-threatening illness, such as malaria, typhoid fever, or leptospirosis. Early diagnosis and treatment can be critical and potentially lifesaving for many travel-related infections. Although most acute febrile illnesses have incubation periods of <14 days, infections including typhoid, malaria, leptospirosis, and acute schistosomiasis can have prolonged incubation periods. Travelers should be advised to always inform their health care providers of their travel history, even when their travel does not immediately precede illness onset. Exposure risk occurring as much as a year prior to illness onset should be considered for malaria. Providers unfamiliar with infections common to a region recently visited by an acutely ill traveler should consult with infectious disease or travel medicine specialists and/or their local public health departments. OUTBREAKS AND EMERGING

INFECTIOUS DISEASES Emerging and reemerging infectious diseases create challenges for international travel. During outbreaks of novel or emerging infec­ tions (e.g., the 2020 COVID-19 pandemic, recent Ebola virus disease epidemics in western and central Africa, or the emergence of Zika in the Americas in 2015–2016), information can be limited or can rapidly change. Significant travel disruption can also occur when outbreaks of

• Questions about planned travel during routine care visits • Prioritization of vaccines and prophylaxis for highest-risk infections when resources are limited • Prioritization of vaccines and prophylaxis for highest-risk infections when resources are limited • Education about high-risk activities (e.g., motorbike taxis) • Several vaccines are effective with a single dose. • Some vaccination series can be accelerated. • Some malaria chemoprophylaxis can be started 1 day prior to entering risk areas. • Consider broad immunization coverage and standing malaria chemoprophylaxis supply for aircrews or other travelers with unpredictable, last-minute trips. • Increased emphasis on importance of certain vaccines, such as hepatitis B, rabies, typhoid, or Japanese encephalitis (in endemic areas) • Long-term malaria chemoprophylaxis • Ensure that traveler has received recommended immunizations for health care workers. • Advise traveler to assess availability of adequate personal protective equipment and medications for HIV postexposure prophylaxis and to consider potential need to bring own supplies. • Advise traveler against working with organizations inexperienced in delivering care in the destination area. • Advise traveler on potential risks. • Direct tourist to internationally accredited facilities and providers. • Tourist should acquire a copy of medical records for providers who will provide follow-up care. • See CDC Yellow Book (Table 130-2) for specific resources. well-known infections take place in previously unaffected areas, such as the 2017–2018 yellow fever outbreak in southeastern Brazil, which included the metropolitan areas of Rio de Janeiro and São Paolo, two major travel destinations for which vaccination had not previously been recommended. Providers counseling travelers can help them make informed decisions by carefully reviewing available travel health notices, surveillance reports, and potential risks to travelers. Indi­ viduals may be advised against travel to certain areas when risks are significant. ■ ■FURTHER READING Angelo KM et al: What proportion of international travellers acquire a travel-related illness? A review of the literature. J Travel Med 24:1, 2017. Keystone JS et al (eds): Travel Medicine, 4th ed. Edinburgh, Elsevier, 2019. Mace KE: Malaria Surveillance—United States, 2018. MMWR Surveill Summ 71:1, 2022. Nemhauser J et al (eds): Yellow Book 2024 Health Information for International Travel. New York, Oxford University Press, 2023. Rao AK et al: Use of a modified preexposure prophylaxis vaccination schedule to prevent human rabies: Recommendations of the Advisory Committee on Immunization Practices–United States, 2022. MMWR Morb Mortal Wkly Rep 71:619, 2022. Riddle MS et al: Guidelines for the prevention and treatment of trav­ elers’ diarrhea: A graded expert panel report. J Travel Med 24:S63, 2017.

09 - SECTION 2 Clinical Syndromes- Community-Acquired Infections

SECTION 2 Clinical Syndromes: Community-Acquired Infections

Section 2 Clinical Syndromes: Community-Acquired Infections Lionel A. Mandell,

Michael S. Niederman

Pneumonia DEFINITION Pneumonia is an infection of the pulmonary parenchyma. Despite significant morbidity and mortality, it is often misdiagnosed, mis­ treated, and underestimated. Pneumonia has usually been classified as community-acquired (CAP), hospital-acquired (HAP), or ventilatorassociated (VAP). A fourth category, healthcare-associated pneumonia, should be discontinued because it did not reliably predict infection with resistant pathogens and was associated with increased use of broad-spectrum antibiotics. Rather than relying on a predefined subset of pneumonia cases, it is better to assess patients individually based on risk factors for infection with a resistant organism, such as certain comorbid illnesses, recent hospitalization, or recent antibiotic therapy. Pneumonia caused by macroaspiration of oropharyngeal or gastric contents, usually referred to as aspiration pneumonia, is best thought of as a point on the continuum that includes CAP and HAP. Estimates suggest that aspiration pneumonia accounts for 5–15% of CAP cases, but reliable figures for HAP are unavailable. The airways or pulmonary parenchyma may be involved, and patients usually represent a clinical phenotype with risk factors for macroaspiration and involvement of characteristic anatomic pulmonary locations. In this chapter, we will not be dealing with pneumonia in immuno­ compromised hosts. PATHOPHYSIOLOGY Pneumonia is the result of the proliferation of microbial pathogens in the alveoli and the host’s response to them. Until recently, it was thought that the lungs were sterile and that pneumonia resulted from the introduction of potential pathogens into this sterile environment. Typically, this introduction occurred through microaspiration of oro­ pharyngeal organisms into the lower respiratory tract. The overcoming of innate and adaptive immunity by such microorganisms could result in the clinical syndrome of pneumonia. A complex and diverse community of bacteria in the lungs consti­ tutes the lung microbiota. Awareness of this microbiota has prompted a rethinking of how pneumonia develops. Mechanical factors, such as the hairs and turbinates of the nares, the branching tracheobronchial tree, mucociliary clearance, and gag and cough reflexes, play roles in host defense but are insufficient to effectively block bacterial access to the lower airways. In the absence of a sufficient barrier, microorgan­ isms may reach the lower respiratory tract by a variety of pathways, including inhalation, microaspiration, and direct mucosal dispersion. The constitution of the lung microbiota is determined by three fac­ tors: microbial entry into the lungs, microbial elimination, and regional growth conditions for bacteria, such as pH, oxygen tension, and tem­ perature. The key question, however, is how a dynamic homeostasis among bacterial communities results in acute infection. Pneumonia therefore does not appear to be the result of the invasion of a sterile space by a particular microorganism but is more likely an emergent phenomenon dependent upon a number of mechanisms, including self-accelerating positive feedback loops. A possible model for pneumonia is as follows. An inflamma­ tory event resulting in epithelial and/or endothelial injury results in the release of cytokines, chemokines, and catecholamines, some of which may selectively promote the growth of certain bacteria, such as Streptococcus pneumoniae and Pseudomonas aeruginosa. This cycle of inflammation, enhanced nutrient availability, and release of potential

bacterial growth factors may result in a positive feedback loop that further accelerates inflammation and the growth of particular bacteria, which may then become dominant. In cases of CAP and HAP, the trigger may be a viral infection compounded by microaspiration of oropharyngeal organisms. In cases of true aspiration pneumonia, the trigger may possibly be the macroaspiration event itself.

Once triggered, innate and adaptive immune responses can help contain potential pathogens and prevent the development of pneumo­ nia. However, in the face of continuing inflammation (and especially if a positive feedback loop becomes sustainable), the process may proceed to a full-fledged pneumonia syndrome. Inflammatory mediators such as interleukin 6 and tumor necrosis factor result in fever, and chemo­ kines such as interleukin 8 and granulocyte colony-stimulating factor increase local neutrophil numbers. Mediators released by macrophages and neutrophils may create an alveolar capillary leak, resulting in impaired oxygenation, hypoxemia, and radiographic infiltrates. Bacte­ ria themselves may produce toxins that further amplify the inflamma­ tory response. Moreover, some bacterial pathogens appear to interfere with the hypoxic vasoconstriction that would normally occur with fluid-filled alveoli, possibly resulting in severe hypoxemia. Decreased compliance due to capillary leak, hypoxemia, increased respiratory drive, increased secretions, and occasionally infection-related bron­ chospasm all lead to worsening dyspnea. If severe enough, changes in lung mechanics secondary to reductions in lung volume, compliance, and intrapulmonary shunting of blood may cause respiratory failure. PATHOLOGY Classic pneumonia evolves through a series of stages. The initial stage is edema with a proteinaceous exudate and often bacteria in the alveoli. Next is a rapid transition to the red hepatization phase. Erythrocytes in this intraalveolar exudate give this stage its name. In the third phase, gray hepatization, erythrocytes have been lysed and degraded. The neutrophil is the predominant cell, fibrin deposition is abundant, and bacteria have disappeared. This phase corresponds with the successful containment of the infection and improvement in gas exchange. In the final phase, resolution, the macrophage reappears as the dominant cell in the alveolar space and the debris of neutrophils, bacteria and fibrin has been cleared, as has the inflammatory response. CHAPTER 131 Pneumonia This pattern has been described best for lobar pneumococcal pneu­ monia but may not apply to pneumonia of all etiologies. In VAP, respi­ ratory bronchiolitis may precede the development of a radiologically apparent infiltrate. A bronchopneumonia pattern is most common in nosocomial pneumonias, whereas a lobar pattern is more common in bacterial CAP. Despite the radiographic appearance, viral and Pneumo­ cystis pneumonias represent alveolar rather than interstitial processes. COMMUNITY-ACQUIRED PNEUMONIA ■ ■ETIOLOGY Numerous microbes may cause CAP, including a variety of bacteria, viruses, fungi, and protozoa. Newer viral pathogens include metapneumoviruses, the coronavi­ ruses responsible for severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS), and the SARS-CoV-2 coronavirus. First described in December 2019, SARS-CoV-2 and its associated clinical disease, COVID-19, reached pandemic proportions and are a cause of significant morbidity and mortality. The COVID-19 pandemic has changed the etiologic profile of CAP, and the ultimate role that the SARS-CoV-2 virus will play as a cause of CAP remains to be seen. The virus and the disease are discussed in detail in Chap. 205. Although most CAP cases are caused by relatively few pathogens, an accurate determination of their prevalence is difficult because labora­ tory testing methods are often insensitive and indirect (Table 131-1). Separation of potential agents into “typical” bacterial pathogens and “atypical” organisms may be helpful, although both types of pathogens can lead to similar clinical syndromes. The former group includes S. pneumoniae, Haemophilus influenzae, and, in selected patients, Staphylo­ coccus aureus and gram-negative bacilli such as Klebsiella pneumoniae and P. aeruginosa. The “atypical” organisms include Mycoplasma pneumoniae,

10 - 131 Pneumonia

131 Pneumonia

Section 2 Clinical Syndromes: Community-Acquired Infections Lionel A. Mandell,

Michael S. Niederman

Pneumonia DEFINITION Pneumonia is an infection of the pulmonary parenchyma. Despite significant morbidity and mortality, it is often misdiagnosed, mis­ treated, and underestimated. Pneumonia has usually been classified as community-acquired (CAP), hospital-acquired (HAP), or ventilatorassociated (VAP). A fourth category, healthcare-associated pneumonia, should be discontinued because it did not reliably predict infection with resistant pathogens and was associated with increased use of broad-spectrum antibiotics. Rather than relying on a predefined subset of pneumonia cases, it is better to assess patients individually based on risk factors for infection with a resistant organism, such as certain comorbid illnesses, recent hospitalization, or recent antibiotic therapy. Pneumonia caused by macroaspiration of oropharyngeal or gastric contents, usually referred to as aspiration pneumonia, is best thought of as a point on the continuum that includes CAP and HAP. Estimates suggest that aspiration pneumonia accounts for 5–15% of CAP cases, but reliable figures for HAP are unavailable. The airways or pulmonary parenchyma may be involved, and patients usually represent a clinical phenotype with risk factors for macroaspiration and involvement of characteristic anatomic pulmonary locations. In this chapter, we will not be dealing with pneumonia in immuno­ compromised hosts. PATHOPHYSIOLOGY Pneumonia is the result of the proliferation of microbial pathogens in the alveoli and the host’s response to them. Until recently, it was thought that the lungs were sterile and that pneumonia resulted from the introduction of potential pathogens into this sterile environment. Typically, this introduction occurred through microaspiration of oro­ pharyngeal organisms into the lower respiratory tract. The overcoming of innate and adaptive immunity by such microorganisms could result in the clinical syndrome of pneumonia. A complex and diverse community of bacteria in the lungs consti­ tutes the lung microbiota. Awareness of this microbiota has prompted a rethinking of how pneumonia develops. Mechanical factors, such as the hairs and turbinates of the nares, the branching tracheobronchial tree, mucociliary clearance, and gag and cough reflexes, play roles in host defense but are insufficient to effectively block bacterial access to the lower airways. In the absence of a sufficient barrier, microorgan­ isms may reach the lower respiratory tract by a variety of pathways, including inhalation, microaspiration, and direct mucosal dispersion. The constitution of the lung microbiota is determined by three fac­ tors: microbial entry into the lungs, microbial elimination, and regional growth conditions for bacteria, such as pH, oxygen tension, and tem­ perature. The key question, however, is how a dynamic homeostasis among bacterial communities results in acute infection. Pneumonia therefore does not appear to be the result of the invasion of a sterile space by a particular microorganism but is more likely an emergent phenomenon dependent upon a number of mechanisms, including self-accelerating positive feedback loops. A possible model for pneumonia is as follows. An inflamma­ tory event resulting in epithelial and/or endothelial injury results in the release of cytokines, chemokines, and catecholamines, some of which may selectively promote the growth of certain bacteria, such as Streptococcus pneumoniae and Pseudomonas aeruginosa. This cycle of inflammation, enhanced nutrient availability, and release of potential

bacterial growth factors may result in a positive feedback loop that further accelerates inflammation and the growth of particular bacteria, which may then become dominant. In cases of CAP and HAP, the trigger may be a viral infection compounded by microaspiration of oropharyngeal organisms. In cases of true aspiration pneumonia, the trigger may possibly be the macroaspiration event itself.

Once triggered, innate and adaptive immune responses can help contain potential pathogens and prevent the development of pneumo­ nia. However, in the face of continuing inflammation (and especially if a positive feedback loop becomes sustainable), the process may proceed to a full-fledged pneumonia syndrome. Inflammatory mediators such as interleukin 6 and tumor necrosis factor result in fever, and chemo­ kines such as interleukin 8 and granulocyte colony-stimulating factor increase local neutrophil numbers. Mediators released by macrophages and neutrophils may create an alveolar capillary leak, resulting in impaired oxygenation, hypoxemia, and radiographic infiltrates. Bacte­ ria themselves may produce toxins that further amplify the inflamma­ tory response. Moreover, some bacterial pathogens appear to interfere with the hypoxic vasoconstriction that would normally occur with fluid-filled alveoli, possibly resulting in severe hypoxemia. Decreased compliance due to capillary leak, hypoxemia, increased respiratory drive, increased secretions, and occasionally infection-related bron­ chospasm all lead to worsening dyspnea. If severe enough, changes in lung mechanics secondary to reductions in lung volume, compliance, and intrapulmonary shunting of blood may cause respiratory failure. PATHOLOGY Classic pneumonia evolves through a series of stages. The initial stage is edema with a proteinaceous exudate and often bacteria in the alveoli. Next is a rapid transition to the red hepatization phase. Erythrocytes in this intraalveolar exudate give this stage its name. In the third phase, gray hepatization, erythrocytes have been lysed and degraded. The neutrophil is the predominant cell, fibrin deposition is abundant, and bacteria have disappeared. This phase corresponds with the successful containment of the infection and improvement in gas exchange. In the final phase, resolution, the macrophage reappears as the dominant cell in the alveolar space and the debris of neutrophils, bacteria and fibrin has been cleared, as has the inflammatory response. CHAPTER 131 Pneumonia This pattern has been described best for lobar pneumococcal pneu­ monia but may not apply to pneumonia of all etiologies. In VAP, respi­ ratory bronchiolitis may precede the development of a radiologically apparent infiltrate. A bronchopneumonia pattern is most common in nosocomial pneumonias, whereas a lobar pattern is more common in bacterial CAP. Despite the radiographic appearance, viral and Pneumo­ cystis pneumonias represent alveolar rather than interstitial processes. COMMUNITY-ACQUIRED PNEUMONIA ■ ■ETIOLOGY Numerous microbes may cause CAP, including a variety of bacteria, viruses, fungi, and protozoa. Newer viral pathogens include metapneumoviruses, the coronavi­ ruses responsible for severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS), and the SARS-CoV-2 coronavirus. First described in December 2019, SARS-CoV-2 and its associated clinical disease, COVID-19, reached pandemic proportions and are a cause of significant morbidity and mortality. The COVID-19 pandemic has changed the etiologic profile of CAP, and the ultimate role that the SARS-CoV-2 virus will play as a cause of CAP remains to be seen. The virus and the disease are discussed in detail in Chap. 205. Although most CAP cases are caused by relatively few pathogens, an accurate determination of their prevalence is difficult because labora­ tory testing methods are often insensitive and indirect (Table 131-1). Separation of potential agents into “typical” bacterial pathogens and “atypical” organisms may be helpful, although both types of pathogens can lead to similar clinical syndromes. The former group includes S. pneumoniae, Haemophilus influenzae, and, in selected patients, Staphylo­ coccus aureus and gram-negative bacilli such as Klebsiella pneumoniae and P. aeruginosa. The “atypical” organisms include Mycoplasma pneumoniae,

TABLE 131-1  Microbial Causes of Community-Acquired Pneumonia, by Site of Care HOSPITALIZED PATIENTS OUTPATIENTS NON-ICU ICU Streptococcus pneumoniae Mycoplasma pneumoniae Haemophilus influenzae Chlamydia pneumoniae Respiratory virusesa S. pneumoniae M. pneumoniae C. pneumoniae H. influenzae Legionella spp. Respiratory virusesa S. pneumoniae Staphylococcus aureus Legionella spp. Gram-negative bacilli H. influenzae Respiratory virusesa aInfluenza A and B viruses, SARS-CoV-2 and other coronaviruses, human metapneumovirus, adenoviruses, respiratory syncytial viruses, parainfluenza viruses. Abbreviation: ICU, intensive care unit. Chlamydia pneumoniae, and Legionella species as well as respiratory viruses such as influenza, adenoviruses, human metapneumoviruses, respiratory syncytial virus, and coronaviruses. With the increasing use of pneumococcal vaccine, the incidence of pneumococcal pneumonia is decreasing. M. pneumoniae plays more of a role in ambulatory cases, whereas Legionella tends to be associated with more serious cases and can be found in outbreaks, as well. C. pneumoniae now appears to account for <1% of CAP cases. Viruses are recognized as increasingly important in pneumonia, and polymerase chain reaction (PCR)–based testing indi­ cates their presence in the respiratory tract of 20–30% of healthy adults and in the same percentage of pneumonia patients, including those who are severely ill. The most common are influenza, parainfluenza, and respiratory syncytial viruses. Whether they are true etiologic pathogens, co-pathogens, or simply colonizers cannot always be determined. Atypi­ cal organisms cannot be cultured on standard media or seen on Gram stain, but their frequency and importance have significant implications for therapy. They are intrinsically resistant to all β-lactam antibiotics and require treatment with a macrolide, fluoroquinolone, or a tetracycline. In the 10–15% of CAP cases that are polymicrobial, the etiology usually includes a combination of typical and atypical pathogens. PART 5 Infectious Diseases Earlier literature suggested that aspiration pneumonia was caused primarily by anaerobes, with or without aerobic pathogens. A shift, however, has been noted recently: if aspiration pneumonia is acquired in a community or hospital setting, the likely pathogens are those usually associated with CAP or HAP. Anaerobes may still play a role, especially in patients with poor dentition, lung abscess, necrotizing pneumonia, or empyema. S. aureus pneumonia is known to complicate influenza virus infection. However, methicillin-resistant S. aureus (MRSA) has been reported as a primary etiologic agent of CAP. Although cases caused by MRSA are relatively uncommon, clinicians must be aware of its potentially serious consequences, such as necrotizing pneumonia. Two factors have led to this problem: the spread of MRSA from the hospital setting to the community and the emergence of genetically distinct MRSA strains in the community associated with bacterial toxin pro­ duction. Community-associated MRSA (CA-MRSA) strains may infect healthy individuals who have had no association with health care. Despite a careful history, physical examination, and radiographic studies, the causative pathogen is often difficult to predict with certainty, and in more than half of cases, a specific etiology is not determined. Nevertheless, epidemiologic and risk factors may suggest certain pathogens (Table 131-2). ■ ■EPIDEMIOLOGY It is estimated that 7 million or more CAP cases occur annually in the United States. The annual incidence in adults ranges from 16 to 23 per 1000 population. The incidence of hospitalization is 650/100,000 but rises dramatically to 2000/100,000 yearly in the elderly. Overall, approximately 30% of patients are hospitalized, resulting in 1.5 million admissions. Along with influenza, CAP is the eighth leading cause of death in the United States, resulting in >60,000 deaths annually. The mortality rate among outpatients is usually <5% but ranges from ~12 to 40% among hospitalized patients. The exact rate depends on whether

TABLE 131-2  Epidemiologic Factors Suggesting Possible Causes of Community-Acquired Pneumonia FACTOR POSSIBLE PATHOGEN(S) Alcoholism Streptococcus pneumoniae, oral anaerobes, Klebsiella pneumoniae, Acinetobacter spp., Mycobacterium tuberculosis COPD and/or smoking Haemophilus influenzae, Pseudomonas aeruginosa, Legionella spp., S. pneumoniae, Moraxella catarrhalis, Chlamydia pneumoniae Structural lung disease

(e.g., bronchiectasis) P. aeruginosa, Burkholderia cepacia, Staphylococcus aureus Dementia, stroke, decreased level of consciousness Oral anaerobes, gram-negative enteric bacteria Lung abscess CA-MRSA, oral anaerobes, endemic fungi, M. tuberculosis, nontuberculous mycobacteria Travel to Ohio or St. Lawrence river valley Histoplasma capsulatum Travel to southwestern United States Hantavirus, Coccidioides spp. Travel to Southeast Asia Burkholderia pseudomallei, avian influenza virus Stay in hotel or on cruise ship in previous 2 weeks Legionella spp. Local influenza activity Influenza virus, S. pneumoniae, S. aureus Exposure to infected humans SARS-CoV-2 Exposure to birds H. capsulatum, Chlamydia psittaci Exposure to rabbits Francisella tularensis Exposure to sheep, goats, parturient cats Coxiella burnetii Abbreviations: CA-MRSA, community-acquired methicillin-resistant Staphylococcus aureus; COPD, chronic obstructive pulmonary disease; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2. treatment takes place in or outside the intensive care unit (ICU). In the United States, CAP is the leading cause of death from infection among patients >65 years of age. Moreover, 18% of hospitalized CAP patients are readmitted within 1 month of discharge. The overall yearly CAP cost is estimated at $17 billion. Risk factors for CAP in general and pneumococcal pneumonia in particular have implications for treatment. They include alcoholism, asthma, immunosuppression, institutionalization, and age >70 years. In the elderly, decreased cough and gag reflexes and reduced antibody and Toll-like receptor responses increase the likelihood of pneumonia. Risk factors for pneumococcal pneumonia include dementia, seizure disor­ ders, heart failure, cerebrovascular disease, alcoholism, tobacco smoking, chronic obstructive pulmonary disease (COPD), and HIV infection. CA-MRSA pneumonia is more likely in patients with skin coloniza­ tion or infection with CA-MRSA at other sites, and after viral infection. Enterobacteriaceae tend to infect patients recently hospitalized, given antibiotics, or who have comorbidities such as alcoholism, heart failure, or renal failure. P. aeruginosa is a particular problem in patients with severe structural lung disease (e.g., bronchiectasis, cystic fibrosis, or severe COPD). Risk factors for Legionella infection include diabetes, hematologic malignancy, cancer, severe renal disease, HIV infection, smoking, male gender, and a recent hotel stay or cruise ship trip. ■ ■CLINICAL MANIFESTATIONS The clinical presentation of pneumonia can vary from indolent to fulminant and from mild to fatal. Manifestations of worsening severity include both constitutional findings and those limited to the lung and associated structures. The patient is frequently febrile and/or tachy­ cardic and may experience chills and/or sweats. Cough may be non­ productive or productive of mucoid, purulent, or blood-tinged sputum. Gross hemoptysis is suggestive of necrotizing pneumonia (e.g., that due to CA-MRSA). Depending on severity, shortness of breath may be present, and pleural involvement may result in chest pain. Up to 20% of

patients may have gastrointestinal symptoms such as nausea, vomiting, or diarrhea. Other symptoms may include fatigue, headache, myalgias, and arthralgias. Findings on physical examination vary with the degree of pulmo­ nary consolidation and the presence or absence of a significant pleural effusion. An increased respiratory rate and use of accessory muscles of respiration are common. Palpation may reveal increased or decreased tactile fremitus, and the percussion note can vary from dull to flat, reflecting underlying consolidated lung and pleural fluid, respectively. Crackles, bronchial breath sounds, and possibly a pleural friction rub may be heard. The clinical presentation may be less obvious in the elderly, who may initially display new-onset or worsening confusion, or worsening of a chronic illness, but few other manifestations. Severely ill patients may have septic shock and organ failure. In cases of CAP, symptoms can range from almost nonexistent to severe, and in those with aspiration pneumonia, chest radiographic findings are often in gravity-dependent parts of the lung. ■ ■DIAGNOSIS When confronted with possible CAP, the physician must ask two ques­ tions: Is this pneumonia, and, if so, what is the likely pathogen? The former question is answered by clinical and radiographic methods, whereas the latter requires laboratory techniques. Clinical Diagnosis  The differential diagnosis includes infectious and noninfectious entities, including acute bronchitis, exacerbations of chronic bronchitis, heart failure, and pulmonary embolism. The importance of a careful history cannot be overemphasized. The diag­ nosis of CAP requires a compatible history, such as cough, sputum pro­ duction, fever and dyspnea, and a new infiltrate on chest radiography. Unfortunately, the sensitivity and specificity of physical examina­ tion findings are only 58% and 67%, respectively. Chest radiography is often unable to differentiate CAP from other conditions. Radiographic findings may suggest increased severity (e.g., cavitation or multilobar involvement) and occasionally suggest an etiologic diagnosis, such as pneumatoceles in S. aureus infection or an upper-lobe cavitating lesion in tuberculosis. Computed tomography (CT) may be of value in sus­ pected loculated effusion or cavitary cases or in postobstructive pneu­ monia caused by a tumor or foreign body. For outpatients, clinical and radiologic assessments are usually all that is required before treatment is started since most laboratory results are not available soon enough to influence initial management. In certain cases, the availability of rapid point-of-care outpatient tests can be important, such as for rapid diagnosis of influenza infection, and can prompt specific anti-influenza treatment and secondary prevention measures. Etiologic Diagnosis  The etiology of pneumonia usually cannot be determined solely on the basis of clinical or radiographic presenta­ tion. Data from >17,000 emergency department CAP cases showed an etiologic determination in only 7.6%. Except for CAP patients admitted to the ICU, no data exist showing that treatment directed at a specific pathogen is statistically superior to empirical therapy. The benefit of establishing a microbial etiology may be questioned, particularly in light of the cost of diagnostic testing. However, a number of reasons exist for attempting an etiologic diagnosis. Identification of a specific or unexpected pathogen allows focusing of the initial empirical regi­ men, with a consequent decrease in antibiotic selection pressure and the risk of resistance. Pathogens with important public safety implica­ tions, such as Mycobacterium tuberculosis, influenza, and SARS-CoV-2 viruses, may be found. Finally, without susceptibility data, trends in resistance cannot be followed accurately, and appropriate empirical therapeutic regimens are harder to devise. GRAM STAIN AND CULTURE OF SPUTUM  The main purpose of the sputum Gram stain is to ensure suitability of a specimen for culture. To be suitable, a sputum sample must have >25 neutrophils and <10 squamous epithelial cells per low-power field. However, staining may also identify certain pathogens (e.g., S. pneumoniae, S. aureus, and gram-negative bacteria). The sensitivity and specificity of the sputum Gram stain and culture are highly variable. Even in cases of proven

bacteremic pneumococcal pneumonia, the positive sputum cultures are ≤50%.

Many patients, particularly elderly individuals, may be unable to produce an appropriate sputum sample. Others may be taking antibiot­ ics that interfere with culture results. Inability to produce sputum can be caused by dehydration, whose correction may result in increased sputum production and a more obvious infiltrate on chest radiography. For patients admitted to the ICU and intubated, a deep-suction aspirate or bronchoalveolar lavage sample has a high yield on culture when sent to the laboratory as soon as possible. Since pathogens in severe and mild CAP may differ (Table 131-1), the greatest benefit of staining and culturing respiratory secretions is to alert the physician to unexpected and/or resistant pathogens and to permit appropriate modification of therapy. Other stains and cultures (e.g., for M. tuberculosis or fungi) may be useful as well. The sputum Gram stain and culture are recom­ mended only for hospitalized CAP patients, particularly those with severe cases or those with risks of MRSA or P. aeruginosa infection. BLOOD CULTURES  The yield from blood cultures, even when samples are collected before antibiotic therapy, is disappointingly low. Only 5–14% of cultures from hospitalized CAP patients are positive, and the most common pathogen is S. pneumoniae followed by S. aureus and P. aeruginosa. Since recommended empirical regimens all provide pneumococcal coverage, a blood culture positive for this pathogen has little, if any, effect on clinical outcome. However, susceptibility data may allow narrowing of antibiotic therapy in appropriate cases, and such data help to track microbial resistance patterns on a national basis. Because of the low yield and lack of significant impact on outcome, blood cultures are not considered de rigueur for all hospitalized CAP patients. Certain high-risk patients should have blood cultured, includ­ ing those with neutropenia secondary to pneumonia, asplenia, comple­ ment deficiencies, chronic liver disease, or severe CAP and those at risk of MRSA or P. aeruginosa infection. CHAPTER 131 URINARY ANTIGEN TESTS  Two commercially available tests detect pneumococcal and Legionella urinary antigens. The Legionella pneu­ mophila test detects only serogroup 1, which accounts for most community-acquired cases of Legionnaires’ disease in the United States. Its sensitivity and specificity are 70% and 99%, respectively. The pneu­ mococcal urine antigen test also is quite sensitive and specific (70% and

90%, respectively). Although false-positive results can be obtained for pneumococcus-colonized children, the test is generally reliable. Both tests can detect antigen even after initiation of appropriate antibiotic therapy. Testing for urinary pneumococcal antigen can be reserved for severe cases; Legionella antigen can be sought in severe cases and when relevant epidemiologic factors are present. Pneumonia POLYMERASE CHAIN REACTION  PCR tests amplify a microorganism’s DNA or RNA, and multiplex PCR panels test for a number of viral and bacterial pathogens. These tests dramatically improve response times, compared to standard cultures, but the contamination of respiratory specimens by upper-airway flora may make semiquantitative or quan­ titative assays necessary for best results. PCR of nasopharyngeal swabs has become the standard for diagnosis of respiratory viral infection, including influenza and coronaviruses. PCR can also detect the nucleic acid of Legionella species, M. pneumoniae, C. pneumoniae, and myco­ bacteria. The cost-effectiveness of PCR testing, however, has not been definitively established. SEROLOGY  A fourfold rise in specific IgM antibody titer between acute- and convalescent-phase serum samples is generally considered diagnostic of infection with a particular pathogen. Until recently, serologic tests were used to help identify atypical pathogens as well as selected unusual organisms such as Coxiella burnetii. However, these tests have fallen out of favor because of the delays in obtaining conva­ lescent phase results and difficulties with interpretation. BIOMARKERS  Two of the most commonly used markers are C-reactive protein (CRP) and procalcitonin (PCT). Levels of these acute-phase reactants increase in the presence of an inflammatory response, par­ ticularly to bacterial pathogens. Nevertheless, PCT is insufficiently accurate for use in the diagnosis of bacterial CAP, and initial serum

PCT levels should not be used as a basis for withholding initial anti­ biotic treatment. CRP is considered even less sensitive than PCT for detecting bacterial pathogens. These tests should not be used alone but, in conjunction with findings from the history, physical examination, radiography, and laboratory tests, may facilitate antibiotic stewardship and appropriate management of seriously ill CAP patients.

TREATMENT Community-Acquired Pneumonia SITE OF CARE The decision to hospitalize a patient with CAP has considerable implications. The cost of inpatient management exceeds outpa­ tient treatment by a factor of 20, and hospitalization accounts for most CAP-related expenditures. However, late admission to the ICU is associated with increased mortality rates. The choice can be difficult: some patients can be managed at home, while others require hospitalization. Tools that objectively assess the risk of adverse outcomes, including severe illness and death, can help to minimize unnecessary hospitalizations. The two most frequently used are the Pneumonia Severity Index (PSI), a prognostic model that identifies patients at low risk of dying, and the CURB-65 criteria, which yield a severity-of-illness score. To determine the PSI, points are given for 20 variables, including age, coexisting illness, and abnormal physical and laboratory find­ ings. Based on the score, patients are assigned to one of five classes with these mortality rates: class 1, 0.1%; class 2, 0.6%; class 3, 2.8%; class 4, 8.2%; and class 5, 29.2%. Using the PSI results in lower admission rates for class 1 and 2 patients. Class 3 patients could ide­ ally be admitted to an observation unit pending further decisions. PART 5 Infectious Diseases The CURB-65 criteria include five variables: confusion (C); urea

7 mmol/L (U); respiratory rate ≥30/min (R); blood pressure— systolic ≤90 mmHg or diastolic ≤60 mmHg (B); and age ≥65 years. Patients with a score of 0 (a 30-day mortality rate of 1.5%) can be treated as outpatients. With a score of 1 or 2, the patient should be hospitalized unless the score is entirely or in part attributable to an age of ≥65 years; in such cases, hospitalization may not be neces­ sary. Among patients with scores of ≥3, mortality rates are 22% overall; these patients may require ICU admission. The PSI score has greater efficacy and has been more robustly validated than the CURB-65 criteria but is more difficult to calculate. In general, if a patient is unable to maintain oral intake, if compliance may be an issue when assessed on the basis of mental condition or living situation (e.g., cognitive impairment or home­ lessness), or if the patient’s O2 saturation on room air is <92%, hospitalization is necessary. If these considerations do not apply, clinical judgment in conjunction with a prediction rule should be used to determine the site of care. Neither PSI nor CURB-65 is accurate in determining the need for ICU admission. Patients with septic shock requiring vasopressors or with acute respiratory failure requiring intubation and mechanical ventilation should be admitted directly to an ICU (Table 131-3), and those with three of the nine minor criteria listed in the latter table should be admitted to an ICU or a high-level monitoring unit. Mortality rates were higher among less ill patients who were admit­ ted to a medical floor but then deteriorated than among equally ill patients initially monitored in the ICU. ANTIBIOTIC RESISTANCE Antimicrobial resistance is a significant problem that threatens to diminish our therapeutic armamentarium. Antibiotic misuse results in increased antibiotic selection pressure that can affect resistance locally and globally by clonal dissemination. For CAP, the main resistance issues currently involve S. pneumoniae and CA-MRSA. S. pneumoniae  In general, pneumococcal resistance to β-lactams is acquired by (1) direct DNA incorporation and remodeling of penicillin-binding proteins through contact with closely related

TABLE 131-3  Criteria for Severe Community-Acquired Pneumonia Minor criteria   Respiratory rate ≥30 breaths/min   PaO2/FiO2 ratio ≤250   Multilobar infiltrates   Confusion/disorientation   Uremia (BUN level ≥20 mg/dL)   Leukopenia (WBC count <4000 cells/μL)   Thrombocytopenia (platelet count <100,000 cells/μL)   Hypothermia (core temperature <36°C)   Hypotension requiring aggressive fluid resuscitation Major criteria   Respiratory failure requiring invasive mechanical ventilation   Septic shock requiring vasopressors Abbreviations: BUN, blood urea nitrogen; PaO2/FiO2, arterial oxygen pressure/ fraction of inspired oxygen; WBC, white blood cell. oral commensal bacteria (e.g., viridans group streptococci), (2) the process of natural transformation, or (3) mutation of certain genes. Susceptibility to penicillins depends upon treatment with intra­ venous (IV) or oral agents. For IV: Susceptible: minimum inhibitory concentration (MIC) ≤2 µg/mL   Intermediate: MIC >2 and ≤4 µg/mL   Resistant: MIC ≥4 µg/mL For oral: Susceptible: MIC ≤0.06 µg/mL   Intermediate: MIC >0.06 and ≤1 µg/mL   Resistant: MIC >1 µg/mL For non–central nervous system pneumococcal infections, decreased susceptibility to penicillin is mitigated by usual doses. In the United States, only 0.6% of pneumococci are resistant to cef­ triaxone and cefotaxime. Risk factors for penicillin-resistant pneu­ mococcal infection include recent antimicrobial therapy, age of <2 or >65 years, attendance at a day-care center, recent hospitalization, and HIV infection. In contrast to penicillin resistance, macrolide resistance is increas­ ing in S. pneumoniae through several mechanisms. Target-site modi­ fication caused by ribosomal methylation in 23S rRNA encoded by the ermB gene results in high-level resistance (MIC, ≥64 μg/mL) to macrolides, lincosamides, and streptogramin B–type antibiotics. The efflux mechanism encoded by the mef gene (M phenotype) is usually associated with low-level resistance (MIC, usually <16 µg/mL). These two mechanisms account for ~40% and ~60%, respectively, of resis­ tant pneumococcal isolates in the United States. High-level resistance to macrolides is more common in Europe, whereas lower-level resis­ tance predominates in North America. The prevalence of macrolideresistant S. pneumoniae exceeds 25% in some countries; in Canada, it is ~22%, and in the United States approximately 40%. Much of this resistance is high-level, and treatment failures may result. In these situations, a macrolide should not be used as empirical monotherapy. In the United States and Canada, 87.5% of pneumococci are suscep­ tible to doxycycline. The rate of pneumococcal resistance to fluoroquinolones (e.g., levofloxacin, moxifloxacin, and gemifloxacin) is usually <2%. Changes can occur in one or both target sites (topoisomerases II and IV) and are attributable to mutations in the gyrA and parC genes, respectively. An efflux pump may also play a role in pneu­ mococcal resistance to fluoroquinolones. Isolates resistant to drugs from three or more antimicrobial classes with different mechanisms of action are considered multi­ drug-resistant (MDR) strains. The propensity for an association of pneumococcal resistance to penicillin with reduced susceptibility to other drugs, e.g. macrolides, tetracyclines, and trimethoprimsulfamethoxazole, is of concern. In the United States, 58.9% of

penicillin-resistant pneumococcal blood isolates are also resistant to macrolides. The most important risk factor for antibiotic-resistant pneumo­ coccal infection is use of a specific antibiotic within the previous 3 months. A history of prior antibiotic treatment is a critical factor in avoiding the use of an inappropriate antibiotic. CA-MRSA  CAP due to MRSA may be caused by the classic hospi­ tal-acquired strains or by genotypically and phenotypically distinct community-acquired strains. Most infections with the former are acquired either directly or indirectly during contact with the health care environment. However, in some hospitals, CA-MRSA strains are displacing the classic hospital-acquired strains, suggesting that the newer community-acquired strains may be more robust. Methicillin resistance in S. aureus is determined by the mecA gene, which encodes for resistance to all β-lactam drugs At least five staphylococcal chromosomal cassette mec (SCCmec) types have been described. The typical hospital-acquired strain usu­ ally has a type II or III SCCmec element, whereas CA-MRSA has type IV. CA-MRSA isolates tend to be less resistant than the older hospital-acquired strains and are often susceptible to trimethoprim-sulfamethoxazole, clindamycin, and tetracycline in addition to vancomycin and linezolid. CA-MRSA strains also carry genes for superantigens such as enterotoxins B and C and PantonValentine leukocidin; the latter is a membrane-tropic toxin that can create cytolytic pores in neutrophils, monocytes, and macrophages. Risk factors for MRSA include colonization or prior infection and MRSA as suggested by gram-positive cocci in clusters on sputum Gram stain. Other factors that may raise suspicion of MRSA infec­ tion include recent antibiotics, hospitalization, influenza, cavitary or necrotizing pneumonia, or empyema. M. pneumoniae  Macrolide-resistant M. pneumoniae has been reported in a number of countries, including Germany (3%), Japan (30%), China (95%), and France and the United States (5–13%). Mycoplasma resistance to macrolides is increasing as a result of binding-site mutation in domain V of 23S rRNA. Gram-Negative Bacilli  A detailed discussion of resistance among gram-negative bacilli is beyond the scope of this chapter (see Chap. 166). Fluoroquinolone resistance among community isolates of Escherichia coli is increasing. Enterobacter species are typically resistant to cephalosporins, and the drugs of choice to treat these organisms are usually fluoroquinolones or carbapenems. Similarly, when infections due to bacteria producing extended-spectrum β-lactamases (ESBLs) are documented or suspected, a carbapenem should be considered. INITIAL ANTIBIOTIC MANAGEMENT Since the etiology of CAP is rarely known at the outset of treatment, initial therapy is usually empirical and designed to cover the likeli­ est pathogens. In all cases, treatment should be initiated as expedi­ tiously as possible. CAP treatment guidelines in the United States from the American Thoracic Society (ATS) and the Infectious Dis­ eases Society of America (IDSA) consider the likely pathogens, risk of antimicrobial resistance, severity of illness, site of care, and risk of infection with specific bacteria such as MRSA and P. aeruginosa (Fig. 131-1, Tables 131-4 and 131-5). In the figure and the tables, the antibiotics are not listed in order of preference. The approach to treatment of aspiration pneumonia is based on a number of factors, including site of acquisition (community vs hospital), normal or abnormal chest radiograph, and additional variables such as illness severity, state of dentition, and risk of infection with an MDR pathogen. Routine coverage of anaerobes is unnecessary unless dentition is poor or there is a lung abscess or necrotizing pneumonia. Our approach to CAP treatment (Tables 131-4 and 131-5) is very similar to the CAP guidelines with the exceptions listed below. Outpatients  The exceptions to the CAP guidelines that we follow in treating patients are as follows:

Nonsevere Severe No risk Risk No risk Risk Recent hospitalization and antibiotics (PO or IV) ± local validation* Recent hospitalization and antibiotics (PO or IV) ± local validation* Prior respiratory isolation Prior respiratory isolation Add treatment Add treatment Obtain cultures† Add treatment FIGURE 131-1  Algorithm for assessment of inpatient risk of infection with methicillin-resistant Staphylococcus aureus (MRSA) or Pseudomonas aeruginosa. Underlying lung disease (e.g., bronchiectasis or very severe chronic obstructive pulmonary disease) are also risks for P. aeruginosa infection. *Local validation consists of information on local prevalence, resistance, and risk factors. †Can also use MRSA rapid nasal polymerase chain reaction (PCR) if available. IV, intravenous; PO, oral. • We usually initiate coverage that includes atypical organisms as well as S. pneumoniae. • Generally, we do not consider the risk of infection with P. aeruginosa CHAPTER 131 or MRSA particularly significant in outpatients. • Prior antibiotic use should include both oral and parenteral agents. Patients are stratified into two groups: those without comorbid­ ity or risk factors for antibiotic resistance and those with comor­ bidities (e.g., chronic heart, lung, liver, or kidney disease; diabetes; alcoholism; malignancy; or asplenia) with or without risk factors for resistance (Table 131-4). As a general rule, if patients have been treated with a drug from a particular class of antibiotics within the previous 3 months, drugs from a different class should be used to minimize resistance issues. Pneumonia For those without comorbidity or resistance risk factors, amox­ icillin alone or doxycycline is recommended in the guidelines. Monotherapy with amoxicillin is based on evidence of its efficacy in the treatment of hospitalized CAP patients. This recommenda­ tion is a change from the 2007 IDSA/ATS CAP guidelines. As a rule, however, we usually tend to initiate treatment that includes coverage for S. pneumoniae as well as the atypical pathogens (Table 131-4). TABLE 131-4  Initial Treatment Strategies for Outpatients with Community-Acquired Pneumonia STATUS STANDARD REGIMEN No comorbidities or risk factors for antibiotic resistancea Combination therapy with amoxicillin (1 g tid) + either a macrolideb or doxycycline (100 mg bid) or Monotherapy with doxycycline (100 mg bid) or Monotherapy with a macrolideb,c With comorbiditiesd ± risk factors for antibiotic resistancea Combination therapy with amoxicillin/clavulanatee or a cephalosporinf + either a macrolideb or doxycycline (100 mg bid) or Monotherapy with a respiratory fluoroquinoloneg aAntibiotic treatment within the past 3 months or contact with the health care system. bAzithromycin (500 mg on day 1, then 250 mg/d for 4 days), clarithromycin (500 mg bid), or clarithromycin ER (1000 mg/d). cIf local prevalence of pneumococcal resistance is <25%. dIncluding chronic heart, lung, liver, or kidney disease; diabetes mellitus; alcoholism; malignancy; or asplenia. e500/125 mg tid or 875/125 mg bid. fCefpodoxime (200 mg bid) or cefuroxime (500 mg bid). gLevofloxacin (750 mg/d), moxifloxacin (400 mg/d), or gemifloxacin (320 mg/d).

TABLE 131-5  Initial Treatment for Inpatients with or without Risk Factors for Infection with MRSA or Pseudomonas aeruginosa DISEASE SEVERITY, RISK STATUS REGIMEN Nonsevere No risk factors A β-lactama + a macrolideb or A respiratory fluoroquinolonec Prior respiratory isolation Add coverage for MRSAd or Pseudomonas aeruginosae Recent hospitalization, antibiotic treatment, ± LVf Add coverage for MRSAd or P. aeruginosae only if cultures are positive Severe No risk factors A β-lactama + a macrolideb or A β-lactama + respiratory fluoroquinolonec Prior respiratory isolation Add coverage for MRSAd or P. aeruginosae Recent hospitalization, antibiotic treatment ± LVf Add coverage for MRSAd or P. aeruginosae aAmpicillin-sulbactam (1.5–3 g q6h), ceftriaxone (1–2 g/d), cefotaxime (1–2 g q8h), ceftaroline (600 mg q12h), or ertapenem (1 g/d). bAzithromycin (500 mg/d) or clarithromycin (500 mg bid). cLevofloxacin (750 mg/d), moxifloxacin (400 mg/d), or gemifloxacin (320 mg/d). dVancomycin (15 mg/kg q12h, with adjustment based on serum levels) or linezolid (600 mg q12h). ePiperacillin-tazobactam (4.5 g q6h), cefepime (2 g q8h), ceftazidime (2 g q8h), imipenem (500 mg q6h), meropenem (1 g q8h), or aztreonam (2 g q8h). fObtain cultures. MRSA rapid nasal polymerase chain reaction can also be used if available. Abbreviations: LV, local validation (local prevalence, resistance, risk factors); MRSA, methicillin-resistant Staphylococcus aureus. PART 5 Infectious Diseases Monotherapy with a macrolide is recommended in the guide­ lines only if there are contraindications to amoxicillin or doxy­ cycline and there is documented low risk of macrolide resistance (<25%). Otherwise, outpatient treatment is quite similar to the regimens recommended in the 2007 IDSA/ATS guidelines. Two relatively newer agents, lefamulin (a pleuromutilin) and omadacy­ cline (a tetracycline) are possible options for CAP patients unable to take β-lactams and/or wanting to avoid the fluoroquinolones. They are available in the United States but not in Canada. Treatment of influenza (Chap. 206) and COVID-19 (Chap. 205) is discussed in their own chapters. Inpatients  Our exceptions to the recommendations in the CAP guidelines are as follows: • As a general rule, when initiating treatment for infection with P. aeruginosa, we use double coverage. • The presence of all three risk factors is not required for drug resistance (recent hospitalization, recent oral or IV antibiotic treatment, ± local validation) (Fig. 131-1, Table 131-5). The main considerations for determining initial empirical treat­ ment of hospitalized CAP patients are clinical severity and risk of infection with drug-resistant pathogens such as MRSA or

P. aeruginosa. Hospitalization alone is not now considered a sig­ nificant risk factor for these pathogens. Hospitals should collect local data on MRSA and P. aeruginosa regarding prevalence, risk factors for infection, and antibiotic susceptibilities. Patients can be categorized as having nonsevere or severe CAP (Table 131-3), and those in each category may or may not have risk factors for MRSA or P. aeruginosa (Fig. 131-1). In scenarios involving these variables in hospitalized CAP patients, empirical treatment for either of these pathogens should be added to standard therapy in those previously colonized or infected with these pathogens, but not in the patient who is considered nonsevere and whose only risk factors are recent hospitalization and antibiotic treatment ± local validation data (Fig. 131-1). In this setting, if we begin treatment, we try to de-escalate if appropriate. In most patients, cultures should be performed but treatment usually withheld unless culture results or rapid nasal PCR results for MRSA are positive.

Nonsevere, No Risk Factors  For patients with nonsevere infection and no risk factors, treatment should consist of either a combination of a β-lactam and a macrolide or monotherapy with a respiratory fluoroquinolone (Table 131-5). In the event of con­ traindications to macrolides and fluoroquinolones, a β-lactam plus doxycycline may be used. Treatment with a β-lactam plus macrolide combination or a fluoroquinolone alone results in lower mortality than monotherapy with a β-lactam. Severe, No Risk Factors  Patients with severe infection but no risk factors should receive combination therapy with either a β-lactam plus macrolide or a β-lactam and a respiratory fluoroqui­ nolone (Table 131-5). Observational studies suggest that combina­ tion therapy with a β-lactam plus macrolide may be preferable to a β-lactam plus fluoroquinolone. Nonsevere and Severe, with Risk Factors  To date, there are no prediction rules reliably identifying patients who should be started empirically on treatment for MRSA or P. aeruginosa. Current risk fac­ tors for infection with these pathogens are hierarchical. Prior isolation of these organisms, especially from the respiratory tract within the previous year, is a more robust risk factor than recent hospitalization and expo­ sure to parenteral antibiotics. For P. aeruginosa, underlying lung disease (e.g., bronchiectasis or very severe COPD) also is an important risk fac­ tor. If MRSA or P. aeruginosa has been isolated previously, appropriate empirical therapy should be started in both severe and nonsevere cases

(Table 131-5). We prefer linezolid over vancomycin as first-line treat­ ment for MRSA because of its inhibition of bacterial exotoxin and better lung penetration. If the organism is not isolated from respiratory secretions or blood and/or the nasal or bronchoalveolar lavage PCR test for MRSA is negative and the patient is improving at 48 h, treat­ ment may be de-escalated to a standard regimen. If, however, the risk factors are recent hospitalization and anti­ biotic use within the previous 3 months, appropriate samples should be obtained for culture, and, in severe cases only, extendedspectrum treatment for MRSA or P. aeruginosa should be initiated. Depending on the severity of infection, local data on P. aeruginosa resistance, and antibiotic use within the previous 90 days, single- or double-drug coverage should be used such as antipseudomonal β-lactam plus ciprofloxacin, levofloxacin, or aminoglycoside. If two antipseudomonal agents are started, the drugs should be from different classes. Whenever possible, assessment for possible de-escalation of therapy is urged. If the patient’s illness is not severe, empirical extended treatment should be withheld until culture results are available. Regardless of the site of care, CAP patients with proven influenza should be given anti-influenza treatment (e.g., oseltamivir) as well as appropriate antibacterial therapy. Physicians should be vigilant about possible superinfection with MRSA. If a viral pathogen such as influenza or SARS-CoV-2 is found and no bacterial pathogen is obvious, antibacterial treatment can be discontinued. However, in those with severe illness, the possibility of bacterial-viral coinfection should be considered. Although hospitalized patients have traditionally received initial therapy by the IV route, some drugs, particularly the fluoroqui­ nolones, are very well absorbed and may be given orally from the outset to select patients. For those initially treated with IV agents, a switch to oral treatment is appropriate when the patient can ingest and absorb the drugs, is hemodynamically stable, and is showing clinical improvement. A 5-day course of treatment is usually suffi­ cient for uncomplicated CAP, but longer treatment may be required for patients who have not stabilized clinically and for those with bacteremia, metastatic infection, or infection with a more virulent pathogen such as P. aeruginosa or MRSA. There are some data to suggest that in select patients who are doing well and are clinically stable, treatment may be discontinued after 3 days. ADJUNCTIVE MEASURES In addition to appropriate antimicrobial therapy, certain adjunctive measures should be used. Adequate hydration, oxygen therapy for

Pneumonia

CHAPTER 131 TABLE 131-6  Microbiologic Causes of Ventilator-Associated Pneumonia NON-MDR PATHOGENS (CORE PATHOGENS) MDR PATHOGENS Streptococcus pneumoniae Other Streptococcus spp. Haemophilus influenzae Methicillin-sensitive Staphylococcus aureus Antibiotic-sensitive Enterobacteriaceae   Escherichia coli   Klebsiella pneumoniae   Proteus spp.   Enterobacter spp.   Serratia marcescens Pseudomonas aeruginosa Methicillin-resistant S. aureus Acinetobacter spp. Antibiotic-resistant Enterobacteriaceae   ESBL-positive strains   Carbapenem-resistant strains Legionella pneumophila Burkholderia cepacia Aspergillus spp. Abbreviations: ESBL, extended-spectrum β-lactamase; MDR, multidrug-resistant. hypoxemia, vasopressor treatment, and assisted ventilation when necessary are critical to successful treatment. Glucocorticoids may be beneficial in cases of severe CAP requiring invasive or noninva­ sive mechanical ventilation or with shock. Recent data show a mor­ tality benefit for corticosteroid therapy in those with severe CAP (ventilated and nonventilated), especially if there is a high level of systemic inflammation (CRP >15 mg/dL); therapy is usually con­ tinued for 8–14 days and given either by intermittent or continu­ ous infusion. Data support the use of dexamethasone plus a Janus kinase inhibitor or an interleukin 6 inhibitor in COVID-19 patients with rapidly increasing oxygen needs and systemic inflammation. FAILURE TO IMPROVE Patients slow to respond to therapy should be reevaluated at about day 3 (or sooner if their condition is worsening), with several sce­ narios considered. A number of noninfectious conditions mimic pneumonia, including pulmonary edema, pulmonary embolism, lung carcinoma, radiation and hypersensitivity pneumonitis, and connective tissue disease involving the lungs. If the patient truly has CAP and empirical treatment is aimed at the likely expected pathogens, lack of response may be explained in a number of ways. The pathogen may be resistant to the drug, or a sequestered focus (e.g., lung abscess or empyema) may prevent antibiotic access to the pathogen. The patient may be getting the wrong drug or the correct drug at the wrong dose or frequency of administration. Another possibility is that CAP has been diagnosed correctly but an unexpected pathogen (e.g., CA-MRSA, M. tuberculosis, or a fungus) is the cause. Nosocomial superinfections—both pulmonary and extrapulmonary—are other possible explanations for a hospitalized patient’s failure to improve. In all cases of delayed response or wors­ ening condition, the patient must be carefully reassessed and appro­ priate studies initiated, possibly including CT or bronchoscopy. COMPLICATIONS Complications of severe CAP include respiratory failure, shock and multiorgan failure, and exacerbation of comorbid illnesses. Three particularly noteworthy conditions are metastatic infection, lung abscess, and complicated pleural effusion. Metastatic infection (e.g., brain abscess or endocarditis) is unusual and requires a high degree of suspicion and a detailed workup for proper treatment. Lung abscess may occur in association with aspiration pneumonia or with infection caused by pathogens such as CA-MRSA, P. aeru­ ginosa, or (rarely) S. pneumoniae. A significant pleural effusion should be tapped for diagnostic and therapeutic purposes. If the fluid has a pH <7.2, a glucose level of <2.2 mmol/L, and a lactate dehydrogenase concentration of >1000 U/L or if bacteria are seen or cultured, drainage is needed. Cardiovascular events with pneumonia, particularly in the elderly and usually in association with pneumococcal pneumonia and influenza, are increasingly recognized. These events, which may be acute or whose occurrence may extend to at least 1 year, include congestive heart failure, arrhythmia, myocardial infarction, or stroke and may be caused by a variety of mechanisms, including increased myocardial load and/or destabilization of atherosclerotic plaques by inflammation. FOLLOW-UP Fever and leukocytosis usually resolve within 2–4 days in otherwise healthy CAP patients, but physical findings may persist longer. Chest radiographic abnormalities are slowest to resolve (4–12 weeks), with the speed of clearance depending on the patient’s age and underlying lung disease, and the etiologic pathogen. Patients may be discharged from hospital once their clinical condition, includ­ ing any comorbidity, is stable. The site of residence after discharge (nursing home, home with family, home alone) is an important consideration, particularly for elderly patients. For a hospitalized patient, we generally recommend a follow-up radiograph ~4–6 weeks later. If relapse or recurrence occurs, particularly in the same lung segment, the possibility of an underlying neoplasm or other local abnormalities (e.g., focal bronchiectasis) must be considered. For individuals managed as outpatients, routine follow-up chest radiog­ raphy is not necessary if they are nonsmokers, if they are otherwise well, and if symptoms resolved within 5–7 days. ■ ■PROGNOSIS The prognosis depends on the patient’s age, comorbidities, and site of treatment (inpatient or outpatient). Young patients without comorbid­ ity do well and usually recover fully after ~2 weeks. Older patients and those with comorbid conditions may take several weeks longer to recover fully. The overall mortality rate for the outpatient group is <5%. For patients requiring hospitalization, overall mortality ranges from 12% to 40%, depending on the patient category and the processes of care, particularly the timely administration of appropriate antibiotics. Recent data, especially in older patients, show that the 1-year mortality following CAP exceeds the 30-day mortality. ■ ■PREVENTION The main preventive measure is vaccination. Recommendations of National Advisory Committees on Immunization Practices should be followed (Chap. 129). VENTILATOR-ASSOCIATED PNEUMONIA Research on hospital-acquired pneumonia has focused on VAP (onset ≥48 h after mechanical ventilation). However, the same information and principles can also be applied to ventilated HAP and to nonICU HAP. Approximately 70% of HAP cases are acquired outside the ICU and 30% in the ICU; the fact that 35% of all HAP patients need mechanical ventilation defines ventilated HAP as a distinct entity. In nonintubated patients with HAP, an expectorated sputum sample is used for microbiologic diagnosis, but results are confounded by fre­ quent colonization by oral pathogens. Microbiologic information in VAP and ventilated HAP is obtained from direct access to deep lower respiratory tract samples, which provide reliable microbiologic data; however, these samples can also contain colonizing pathogens. ■ ■ETIOLOGY Potential etiologic agents of VAP include both MDR and non-MDR bacterial pathogens (Table 131-6). The non-MDR group of “core pathogens” is nearly identical to the pathogens found in severe CAP (Table 131-1); it is not surprising that such pathogens predominate if VAP develops in the first 5–7 days of the hospital stay. However, if patients have other risk factors (particularly prior antibiotic treat­ ment), MDR pathogens are a consideration, even early in the hospital course. The relative frequency of individual MDR pathogens can vary significantly from hospital to hospital and even between different criti­ cal care units within the same institution. Most hospitals have problems with P. aeruginosa and MRSA, but other MDR pathogens are often institution-specific. Less commonly, fungal and viral pathogens cause VAP, usually affecting severely immunocompromised patients. Rarely,

community-associated viruses cause mini-epidemics, usually when introduced by ill health care workers.

■ ■EPIDEMIOLOGY Pneumonia is a common complication among patients requiring mechanical ventilation. Prevalence estimates vary between 6 and 52 cases per 100 patients, depending on the population studied. On any given day in the ICU, an average of 10% of patients will have pneumonia— VAP in the overwhelming majority of cases. Although in recent years the frequency of this infection was declining as a result of effective prevention strategies, with the advent of COVID-19, there has been an increase in its frequency. The frequency of VAP changes with the dura­ tion of mechanical ventilation, with the highest hazard ratio in the first 5 days and a plateau in additional cases (1% per day) after ~2 weeks. However, the cumulative rate among patients who remain ventilated for as long as 30 days is as high as 70%. These rates often do not reflect the recurrence of VAP in the same patient. Once a ventilated patient is transferred to a chronic-care facility or to home, the incidence of pneumonia drops significantly, especially in the absence of other risk factors for pneumonia. However, in chronic ventilator units, purulent tracheobronchitis becomes a significant issue, often interfering with efforts to wean patients off mechanical ventilation (Chap. 313). Three factors are critical in the pathogenesis of VAP: colonization of the oropharynx with pathogenic microorganisms, aspiration of these organisms from the oropharynx into the lower respiratory tract, and compromise of normal host defense mechanisms. Most risk factors and their corresponding prevention strategies pertain to one of these three factors (Table 131-7). The most important risk factor is the endotracheal tube, which bypasses the normal mechanical factors preventing aspiration. While the presence of an endotracheal tube may prevent large-volume aspi­ ration, microaspiration is actually exacerbated by secretions pooling above the cuff. The endotracheal tube and the concomitant need for suctioning can damage the tracheal mucosa, thereby facilitating tracheal colonization. In addition, pathogenic bacteria can form a glycocalyx biofilm on the tube’s surface that protects them from both antibiotics and host defenses. The bacteria can also be dislodged dur­ ing suctioning (done preferably with a closed catheter system) and can reinoculate the trachea, or tiny fragments of a glycocalyx can embolize to distal airways, carrying bacteria with them. The ventilator circuit tubing can harbor pathogenic organisms that can wash back to the patient if manipulated too often; thus, circuits are changed only when soiled and with each new patient. Heat moisture exchangers are changed every 5–7 days or if visibly soiled or malfunctioning. PART 5 Infectious Diseases In a high percentage of critically ill patients, the normal oropharyn­ geal flora is replaced by pathogenic microorganisms. The most impor­ tant risk factors are antibiotic selection pressure, cross-infection from other infected/colonized patients or contaminated equipment, severe systemic illness, and malnutrition. Of these factors, antibiotic exposure poses the greatest risk by far. Pathogens such as P. aeruginosa almost never cause infection in patients without prior exposure to antibiotics. The recent emphasis on hand hygiene has lowered the cross-infection rate. Almost all intubated patients experience microaspiration and are at least transiently colonized with pathogenic bacteria. However, only around one-third of colonized patients develop VAP. Colony counts increase to high levels, sometimes days before the development of clini­ cal pneumonia; these increases suggest that the final step in VAP devel­ opment, independent of aspiration and oropharyngeal colonization, is the overwhelming of host defenses by a large bacterial inoculum. Severely ill patients with sepsis and trauma appear to enter a state of immunoparalysis several days after admission to the ICU—a time that corresponds to the greatest risk of developing VAP. The mechanism of this immunosuppression is not clear, although hyperglycemia and frequent transfusions adversely affect the immune response. ■ ■CLINICAL MANIFESTATIONS The clinical manifestations of HAP and VAP are nonspecific: fever, leukocytosis, increased respiratory secretions, and pulmonary

TABLE 131-7  Pathogenic Mechanisms and Corresponding Prevention Strategies for Ventilator-Associated Pneumonia PATHOGENIC MECHANISM PREVENTION STRATEGY Oropharyngeal colonization with pathogenic bacteria     Elimination of normal flora, Avoidance of prolonged antibiotic courses; consider oral chlorhexidinea overgrowth by pathogenic bacteria   Large-volume oropharyngeal Short course of prophylactic antibiotics for comatose patients; short course of prophylactic inhaled aminoglycoside antibioticsb aspiration around time of intubation   Gastroesophageal reflux Postpyloric enteral feeding with orally placed feeding tubea; avoidance of high gastric residuals, prokinetic agents   Bacterial overgrowth of stomach Avoidance of prophylactic agents that raise gastric pHa; selective decontamination of digestive tract with nonabsorbable antibioticsa Cross-infection from other colonized patients Hand washing, especially with alcoholbased hand rub; intensive infection control educationb; isolation; proper cleaning of reusable equipment Large-volume aspiration Ventilator circuit humidification Endotracheal intubation; rapid-sequence intubation technique; avoidance of sedation; decompression of small-bowel obstruction Change ventilator circuits only when soiled and with new patient; drain ventilator circuit condensate away from patient; replace heat moisture exchanger every 5–7 days or if soiled or malfunctioninga Microaspiration around endotracheal tube     Endotracheal intubation Noninvasive ventilationb   Prolonged duration of ventilation Daily awakening from sedation,b weaning protocolsb   Abnormal swallowing function Early percutaneous tracheostomyb   Secretions pooled above Head of bed elevatedb; continuous aspiration of subglottic secretions with specialized endotracheal tubeb; avoidance of reintubation; minimization of sedation and patient transport; prophylactic PEEPc of 5–8 cm endotracheal tube Altered lower respiratory host defenses Tight glycemic controla; lowering of hemoglobin transfusion threshold aStrategies with negative randomized trials or conflicting results. bStrategies demonstrated to be effective in at least one randomized controlled trial. cPositive end-expiratory pressure. consolidation on physical examination, along with a new or changing radiographic infiltrate. The frequency of abnormal chest radiographs before the onset of pneumonia in intubated patients and the limitations of portable radiographic technique make interpretation of radiographs more difficult than in patients who are not intubated. Other clinical features may include tachypnea, tachycardia, worsening oxygenation, and increased minute ventilation. Serial changes in oxygenation may identify pneumonia earlier than other findings and may also be a means to monitor improvement with therapy. ■ ■DIAGNOSIS No single set of criteria is reliably diagnostic of pneumonia in a ven­ tilated patient. The inability to accurately identify such patients com­ promises efforts to prevent and treat VAP and even calls into question estimates of the impact of VAP on mortality rates. Application of clinical criteria typical for CAP consistently results in overdiagnosis of VAP, largely because of (1) frequent tracheal colo­ nization with pathogenic bacteria in patients with endotracheal tubes, (2) multiple alternative causes of radiographic infiltrates in mechani­ cally ventilated patients, and (3) the high frequency of other sources

of fever in critically ill patients. The differential diagnosis of VAP includes atypical pulmonary edema, pulmonary contusion, alveolar hemorrhage, hypersensitivity pneumonitis, acute respiratory distress syndrome, and pulmonary infarction. Findings of fever and/or leu­ kocytosis may have alternative causes, including antibiotic-associated diarrhea, central line–associated infection, sinusitis, urinary tract infection, pancreatitis, and drug fever. Conditions mimicking pneumo­ nia are often documented in patients in whom VAP has been ruled out by accurate diagnostic techniques. Most of these alternative diagnoses do not require antibiotic treatment; require antibiotics different from those used to treat VAP (fungal or viral pneumonia); or require some additional intervention, such as surgical drainage or catheter removal, for optimal management. This diagnostic dilemma has led to debate and controversy about whether a quantitative-culture approach as a means of eliminating false-positive clinical diagnoses is superior to a clinical approach enhanced by principles learned from quantitative-culture studies. The most recent IDSA/ATS guidelines for HAP/VAP give a weak recom­ mendation for a clinical approach based on semiquantitative cultures, with consideration of the availability of resources, cost, and the avail­ ability of expertise. The guidelines acknowledge that the use of a quan­ titative approach may result in less antibiotic use, which may be critical for antibiotic stewardship in the ICU. Therefore, the approach at each institution—or potentially for each patient—should be individualized and based on local colonization rates, local diagnostic expertise, and recent history of antibiotic therapy. Quantitative-Culture Approach  This method uses quantitative cultures of deep respiratory tract samples to distinguish colonization from true infection. The more distal in the respiratory tree the diagnos­ tic sampling, the more specific are the results and therefore the lower the threshold of growth necessary to diagnose pneumonia and exclude colonization. For example, an endotracheal aspirate yields proximal samples, and the diagnostic threshold is 106 cfu/mL. The protected specimen brush method, in contrast, collects distal samples and has a threshold of 103 cfu/mL. Conversely, sensitivity declines as more distal secretions are obtained, especially when they are collected blindly (i.e., by a technique other than bronchoscopy). Additional tests that may increase the diagnostic yield include Gram staining, differential cell counts, staining for intracellular organisms, and detection of local protein levels elevated in response to infection. If the quantitative approach is used, therapy decisions should be linked to culture results (no antibiotics if below the diagnostic thresh­ old), with antibiotics withheld until results are available unless the patient is critically ill. Studies have documented less antibiotic use with this approach than with the clinical approach, but the results are less clear if antibiotic decisions are not directly linked to culture data. One common limitation of the quantitative approach is that the use of a new and effective antibiotic agent in the 24–48 h prior to sampling can lead to false-negative results. With antimicrobial-sensitive microorganisms, a single antibiotic dose can reduce colony counts below the diagnos­ tic threshold. After 3 days, the operating characteristics of the tests improve to the point at which they are equivalent to results obtained when no prior antibiotic therapy has been given. Conversely, colony counts above the diagnostic threshold during antibiotic therapy sug­ gest that the current antibiotics are ineffective. In addition, quantitative cultures may give results below the diagnostic threshold if samples are collected early in the course of infection or if sampling is delayed until after an effective host response has reduced bacterial counts. Ideally, a specimen should be obtained as soon as pneumonia is suspected and before antibiotic therapy is initiated or changed. Clinical Approach  The lack of specificity of a clinical diagnosis of VAP has hampered its utility, but this approach has been improved by the addition of microbiologic and other laboratory data. Tracheal aspirates generally yield at least twice as many potential pathogens as quantitative cultures, but the causative pathogen is almost always pres­ ent. The absence of bacteria in Gram-stained endotracheal aspirates makes pneumonia an unlikely cause of fever or pulmonary infiltrates. These findings, coupled with a heightened awareness of the alternative

diagnoses possible in patients with suspected VAP, can prevent inap­ propriate antibiotic overtreatment. Furthermore, the absence of an MDR pathogen in tracheal aspirate cultures eliminates the need for MDR coverage, allowing de-escalation of empirical antibiotic therapy. Similarly, with newer and more sensitive molecular diagnostic meth­ ods, a suspected MDR pathogen can be eliminated as a therapy target if test results are negative. A clinical approach that focuses on careful antimicrobial use and de-escalation of therapy after culture results become available may have an impact on the avoidance of antimi­ crobial overuse and the consideration of alternative sites of infection similar to that of a quantitative-culture approach.

TREATMENT Ventilator-Associated Pneumonia Many studies have demonstrated higher mortality rates with the delay of initially appropriate empirical antibiotic therapy. The key to appropriate antibiotic management of VAP is an appreciation of the resistance patterns of the most likely pathogens in a given patient and consideration of local microbiology. ANTIBIOTIC RESISTANCE Because of a higher risk of infection with MDR pathogens (Table 131-6), VAP is treated with antibiotics different from those used for severe CAP. Antibiotic selection pressure leads to the frequent involvement of MDR pathogens by selecting either for drug-resistant isolates of common pathogens (e.g., MRSA and Enterobacteriaceae producing ESBLs or carbapenemases) or for intrinsically resistant pathogens (e.g., P. aeruginosa and Acinetobacter species). Frequent use of β-lactam drugs, especially cephalosporins, appears to be the major risk factor for infection with MRSA and ESBL-positive strains. CHAPTER 131 P. aeruginosa can develop resistance to all routinely used anti­ biotics, and, even if initially sensitive, P. aeruginosa isolates may develop resistance during treatment. Either derepression of resis­ tance genes or selection of resistant clones within the large bacterial inoculum associated with most pneumonias may be the cause. Acinetobacter species, Stenotrophomonas maltophilia, and Burkholderia cepacia are intrinsically resistant to many of the empirical antibiotic regimens employed (see below). VAP caused by these pathogens typically emerges during treatment of other infections, and resis­ tance is always evident at initial diagnosis. EMPIRICAL THERAPY Recommended options for empirical therapy are listed in Table 131-8. Treatment should be started once diagnostic specimens have been obtained. The major factors in the selection of agents are the pres­ ence of risk factors for MDR pathogens and the predicted risk of death (≤15% is considered low risk). Choices among the various options listed depend on local patterns of resistance and—a very important factor—the patient’s prior antibiotic exposure. Knowl­ edge of the local hospital’s—and even the specific ICU’s—anti­ biogram and the local incidence of specific MDR pathogens (e.g., MRSA) is critical in selecting appropriate empirical therapy. Pneumonia The majority of patients without risk factors for MDR infec­ tion can be treated with a single agent. In fact, mortality is lower with a single agent than with combination therapy for those with a low mortality risk. For these patients, monotherapy options listed in Table 131-8 are active against the core pathogens, as well as nonresistant P. aeruginosa. However, in selected settings, it may be possible to use a nonpseudomonal agent such as cefotaxime or moxifloxacin. Unfortunately, the proportion of patients with no MDR risk factors is <10% in some ICUs and is unknown for HAP patients. The major difference from CAP is the markedly lower incidence of atypical pathogens in VAP; the exception is Legionella, which can be a nosocomial pathogen, especially with local epidem­ ics due to breakdowns in the treatment of potable water in the hos­ pital. The standard recommendation for patients with risk factors for MDR infection and a high mortality risk is for three antibiotics:

TABLE 131-8  Empirical Antibiotic Treatment of Hospital-Acquired and Ventilator-Associated Pneumonia NO RISK FACTORS FOR RESISTANT GRAM-NEGATIVE PATHOGEN RISK FACTORS FOR RESISTANT GRAM-NEGATIVE PATHOGENa (CHOOSE ONE FROM EACH COLUMN) Piperacillintazobactam (4.5 g IV q6h) Cefepime (2 g IV q8h) Levofloxacin (750 mg IV q24h) Piperacillin-tazobactam (4.5 g IV q6h) Cefepime (2 g IV q8h) Ceftazidime (2 g IV q8h) Imipenem (500 mg IV q6h) Meropenem (1 g IV q8h) Consider newer agentsc Amikacin (15–20 mg/kg IV q24h) Gentamicin (5–7 mg/kg IV q24h) Tobramycin (5–7 mg/kg IV q24h) Ciprofloxacin (400 mg IV q8h) Levofloxacin (750 mg IV q24h) Colistin (loading dose of 5 mg/kg IV followed by maintenance doses of 2.5 mg × [1.5 × CrCl + 30] IV q12h) Polymyxin B (2.5–3.0 mg/kg per day IV in 2 divided doses) Risk Factors for MRSAb (Add to above) Linezolid (600 mg IV q12h) or Adjusted-dose vancomycin (trough level, 15–20 mg/dL) aPrior antibiotic therapy, prior hospitalization, local antibiogram. bPrior antibiotic therapy, prior hospitalization, known MRSA colonization, chronic hemodialysis, local documented MRSA pneumonia rate >10% (or local rate unknown). cNewer agents can have activity against resistant P. aeruginosa (ceftazidime-avibactam, ceftolozane-tazobactam, imipenem-relebactam, plazomicin), carbapenem-resistant Enterobacteriaceae (ceftazidime-avibactam, imipenem-relebactam, meropenemvaborbactam), metallo-β-lactamase–producing Enterobacteriaceae (ceftazidimeavibactam, cefiderocol), Stenotrophomonas (cefiderocol), and Acinetobacter spp. (cefiderocol, sulbactam-durlobactam). Abbreviations: CrCl, creatinine clearance rate; MRSA, methicillin-resistant Staphylococcus aureus. PART 5 Infectious Diseases two directed at P. aeruginosa and other resistant gram-negative organisms and one directed at MRSA. However, in the absence of septic shock, a single agent may be effective for these patients, provided there is a single agent that is likely to be effective against at least 90% of the gram-negative pathogens in that ICU. Empirical combination therapy enhances the likelihood of initially appropri­ ate therapy over that with monotherapy. A β-lactam agent provides the greatest coverage, yet even the broadest-spectrum agent—a carbapenem—still constitutes inappropriate initial therapy in up to 10–15% of cases at some centers. The emergence of carbapenem resistance at some institutions requires the addition of polymyx­ ins to the combination-therapy options. A number of emerging agents may modify our approach to therapy. New antipseudomonal agents include ceftazidime–avibactam, ceftolozane–tazobactam, imipenem–relebactam, and plazomicin. Therapy for carbapenemresistant Enterobacteriaceae can consist of ceftazidime–avibactam, imipenem–relebactam, or meropenem–vaborbactam, while Entero­ bacteriaceae that produce metallo-β-lactamases can be treated with ceftazidime–avibactam or cefiderocol. Acinetobacter spp. can be treated with cefiderocol (as part of a combination regimen) or with sulbactam-durlobactam, and Stenotrophomonas can be treated with cefiderocol. SPECIFIC TREATMENT Once an etiologic diagnosis is made, broad-spectrum empirical therapy can be modified (de-escalated) to specifically address the known pathogen. For patients with MDR risk factors, antibiotic regimens can be reduced to a single agent in most cases. Only a minority of cases require a complete course with two or three drugs. A negative tracheal-aspirate culture or growth below the threshold for quantitative cultures of samples obtained before any antibiotic change strongly suggests that antibiotics should be discontinued or that an alternative diagnosis should be pursued. Identification of other confirmed or suspected sites of infection may require ongoing antibiotic therapy, but the spectrum of pathogens (and the

corresponding antibiotic choices) may be different from those for VAP. A 7- or 8-day course of therapy is just as effective as a 2-week course and is associated with less frequent emergence of antibioticresistant strains. Exceptions include cases in which initial therapy is inappropriate or consists of second-line antibiotics and cases caused by some more resistant organisms, such as carbapenemase-producing Acinetobacter species. In these situations, serial measurements of procalcitonin may help guide duration of therapy. A major controversy regarding specific therapy for VAP con­ cerns the need for ongoing combination treatment of Pseudomonas pneumonia. No randomized controlled trials have demonstrated a benefit of combination therapy with a β-lactam and an aminogly­ coside, nor have subgroup analyses in other trials found a survival benefit with such a regimen. Combination therapy may increase the likelihood of initially appropriate therapy and may have value in bacteremic infection with septic shock, but the benefit may last for only a few days. The unacceptably high rates of clinical failure and death despite combination therapy among patients with VAP caused by P. aeruginosa (see “Failure to Improve,” below) indicate that better regimens are needed, perhaps including aerosolized anti­ biotics. In most cases of Pseudomonas pneumonia, current guide­ lines recommend against continuing combination therapy after the isolate’s antimicrobial susceptibility is known. FAILURE TO IMPROVE Treatment failure is not uncommon in VAP, especially that caused by MDR pathogens. VAP caused by MRSA is associated with a 40% clinical failure rate when treated with standard-dose vancomycin. One proposed but unproven solution is the use of high-dose indi­ vidualized treatment, although the risk of renal toxicity increases with this strategy. In addition, the MIC of MRSA to vancomycin has been increasing, and a high percentage of clinical failures occur when the MIC is in the upper range of sensitivity (i.e., 1.5–2 μg/mL). Linezolid appears to be 15% more efficacious than even adjusted-dose vancomycin and is preferred in patients with renal insufficiency and those infected with high-MIC isolates of MRSA. VAP due to Pseudomonas has a 40–50% failure rate, no matter what the regimen. Therapy-related causes of clinical failure include not using the recommended combination regimen (Table 131-8) and inadequate antibiotic dosing. However, the emergence of β-lactam resistance during therapy is an important problem, especially in infection with Pseudomonas and Enterobacter species. Recurrent VAP caused by the same pathogen is possible because the biofilm on endotracheal tubes allows persistence and reintroduction of the microorganism. Studies of VAP caused by Pseudomonas show that approximately half of recurrent cases are caused by a new strain. Some studies have suggested that treatment failure may be less com­ mon with optimized β-lactam dosing and use of either prolonged or continuous infusion therapy. Possible causes of treatment failure can be difficult to determine early in the therapeutic course and can include superinfection, the presence of extrapulmonary infection, as well as patient factors such as severe comorbid illness and immunosuppression. Serial quantitative cultures may clarify the microbiologic response, and recent data in ICU patients have shown a role for biomarkers, such as procalcitonin, to guide duration of therapy in conjunction with the patient’s initial response to treatment. COMPLICATIONS Apart from death, the major complication of VAP is prolonga­ tion of mechanical ventilation, with corresponding increases in the duration of ICU and hospital stay. In most studies, the need for additional mechanical ventilation resulting from VAP justifies aggressive efforts at prevention. In rare cases, necrotizing pneumonia (e.g., due to P. aeruginosa or S. aureus) can cause significant pulmonary hemorrhage or empy­ ema. More commonly, necrotizing infections result in the longterm complications of bronchiectasis and parenchymal scarring leading to recurrent pneumonia. Other long-term complications of

pneumonia can include need for prolonged oxygen therapy, a cata­ bolic state in a patient already nutritionally at risk, the necessity for ongoing rehabilitation, and—in the elderly—an inability to return to independent function and the need for nursing home placement. FOLLOW-UP Clinical improvement, if it occurs, is usually evident within 48–72 h of the initiation of antimicrobial treatment, usually with an improve­ ment in oxygenation. Because findings on chest radiography often worsen initially during treatment, they are less helpful than clinical criteria as an indicator of response to therapy. ■ ■PROGNOSIS VAP is associated with crude mortality rates as high as 50–70%, but the real issue is attributable mortality. Many patients with VAP have underlying diseases that would result in death even if VAP did not occur. Attributable mortality exceeded 25% in one matched-cohort study, while more recent studies have suggested much lower rates (5–10%), although patients with VAP complicating COVID-19 have a higher attributable mortality than those with other forms of VAP. Some variability in VAP mortality rates is clearly related to the type of patient and ICU studied. VAP in trauma patients is not associated with attributable mortality, possibly because many of the patients were otherwise healthy before being injured. The causative pathogen also plays a major role. Generally, MDR pathogens are associated with significantly greater attributable mortality than non-MDR pathogens. Pneumonia caused by some pathogens (e.g., S. maltophilia) is simply a marker for a patient whose immune system is highly compromised and is therefore at high risk. ■ ■PREVENTION (TABLE 131-7) Because endotracheal intubation is a risk factor for VAP, the most impor­ tant preventive intervention is to avoid intubation or minimize its dura­ tion. Successful noninvasive ventilation avoids many of the problems associated with endotracheal tubes. Strategies that minimize the dura­ tion of ventilation through daily holding of sedation and formal weaning protocols have also been highly effective in preventing VAP. Unfortunately, a tradeoff in risks is sometimes necessary. Aggressive attempts to extubate early may result in reintubation(s) and increase aspiration, posing a risk of VAP. Heavy continuous sedation increases VAP risk, but self-extubation because of insufficient sedation is also a risk. The tradeoffs also apply to antibiotic therapy. Short-course antibi­ otic prophylaxis can decrease the risk of early-onset VAP in comatose patients requiring intubation, and data suggest that antibiotics decrease VAP rates overall. Conversely, prolonged courses of antibiotics consis­ tently increase the risk of MDR VAP; pseudomonal VAP is rare among patients who have not recently received antibiotics. In one recent randomized trial, 3 days of daily inhaled aminoglycoside prophylaxis reduced the occurrence of VAP for the next 28 days, with no impact on mortality or antibiotic use. Minimizing microaspiration around the endotracheal tube cuff also can prevent VAP. Simply elevating the head of the bed (at least 30° above horizontal, but preferably 45°) and using specially modified endotracheal tubes that allow removal of the secretions pooled above the cuff can prevent microaspiration. The risk-to-benefit ratio of trans­ porting the patient outside the ICU for diagnostic tests or procedures should be carefully considered since VAP rates are increased among transported patients. The role played by overgrowth of the normal bowel flora in the stomach—in the presence of elevated gastric pH—in the pathogen­ esis of VAP is questionable. Therefore, avoidance of agents that raise gastric pH may be relevant only in certain populations, such as liver transplant recipients and patients who have undergone other major intraabdominal procedures or who have bowel obstruction. MRSA and nonfermenters such as P. aeruginosa and Acinetobacter species are not normally part of the bowel flora but reside primarily in the nose and on the skin, respectively. In outbreaks of VAP due to specific pathogens, the possibility of a breakdown in infection control measures (particularly contamination

of reusable equipment) should be investigated. Even high rates of pathogens that are already common in a particular ICU may result from cross-infection. Education and reminders of the need for consis­ tent hand washing and other infection-control practices can minimize this risk.

HOSPITAL-ACQUIRED PNEUMONIA While less well studied than VAP, HAP in nonintubated patients—both inside and outside the ICU—is similar to VAP. The main differences are the higher frequency of non-MDR pathogens and the generally better underlying host immunity in nonintubated patients. The lower frequency of MDR pathogens allows monotherapy in a larger propor­ tion of cases of HAP than of VAP. However, the bacteriology and outcome of ventilated HAP patients may be very similar to those of patients with VAP. The only pathogens that may be more common in the non-VAP population are anaerobes because of a greater risk of macroaspira­ tion and the lower oxygen tensions in the lower respiratory tract of these patients. Anaerobes usually contribute only to polymicrobial pneumonias, and specific therapy targeting anaerobes probably is not needed since many of the recommended antibiotics are active against anaerobes. Diagnosis is even more difficult for HAP in the nonintubated patient than for VAP. Lower respiratory tract samples appropriate for culture are considerably more difficult to obtain from nonintubated patients. Many of the underlying diseases that predispose a patient to HAP are also associated with an inability to cough adequately. Since blood cultures are infrequently positive (<15% of cases), the major­ ity of patients with HAP do not have culture data on which antibiotic modifications can be based, and de-escalation is less likely. Despite these difficulties, the better host defenses in non-ICU patients result in lower mortality rates than are documented for VAP and for ventilated HAP. In addition, the risk of antibiotic failure is lower in HAP. CHAPTER 131 Pneumonia GLOBAL IMPACT From the available data, it is virtually impossible to accurately assess the impact of pneumonia from a global perspective. Any differences in incidence, disease burden, and costs across different age, ethnic, and racial groups are compounded by differences among countries in terms of etiologic pathogens, resistance rates, access to health-care and diagnostic facilities, and vaccine availability and use. A standard approach with clearly defined outcome measures is needed before the impact of pneumonia can be accurately evaluated. However, simple extrapolation from U.S. data for CAP and HAP/ VAP shows that pneumonia has a significant impact on quality of life, morbidity, health costs, and mortality rates and that this impact has implications for patients and for society as a whole. Acknowledgment The authors gratefully acknowledge the contributions of Richard Wun­ derink, MD, to this chapter in a prior edition. ■ ■FURTHER READING Dequin PF et al: Hydrocortisone in severe community–acquired pneu­ monia. N Engl J Med 388:1931, 2023. Dickson RP et al: Towards an ecology of the lung: New conceptual models of pulmonary microbiology and pneumonia pathogenesis. Lancet Respir Med 2:238, 2014. File TM Jr: Community-acquired pneumonia. N Engl J Med 389:632, 2023. Jain S et al: Community-acquired pneumonia requiring hospitaliza­ tion among U.S. adults. N Engl J Med 373:415, 2015. Kalil AC et al: Management of adults with hospital-acquired and ventilator-associated pneumonia: 2016 clinical practice guidelines by the Infectious Diseases Society of America and the American Tho­ racic Society. Clin Infect Dis 63:e61, 2016. Mandell LA, Niederman MS: Aspiration pneumonia. N Engl J Med 380:651, 2019. Mandell LA et al: Infectious Diseases Society of America/Ameri­ can Thoracic Society consensus guidelines on the management of

100 - SECTION 15 Infections Due to RNA Viruses

SECTION 15 Infections Due to RNA Viruses

virus; (2) the fact that the infection can be transmitted by cell-free or cell-associated virus; (3) the fact that the HIV provirus integrates itself into the genome of the target cell and may remain in a latent form unex­ posed to the immune system; (4) the likely need for the development of effective mucosal immunity; and, importantly, (5) the difficulty that the immune system has in readily mounting broadly neutralizing antibodies in response to natural infection with HIV (see below). Early attempts to develop a vaccine with the envelope protein gp120 aimed at inducing neutralizing antibodies in humans were unsuccess­ ful; the elicited antisera failed to neutralize primary isolates of HIV. In this regard, two phase 3 trials were undertaken in the United States and Thailand using soluble gp120, and the vaccines failed to protect human volunteers from HIV infection. In addition, two separate vaccine trials aimed at eliciting CD8+ T cell responses to prevent infection and, if unsuccessful in preventing infection, to control postinfection viremia, also failed at both goals. In 2009, a vaccine using a poxvirus vector prime expressing various viral proteins followed by an envelope protein boost was assessed in a 16,000-person clinical trial (RV144) conducted in Thailand among predominantly low-HIV-prevalence heterosexu­ als. The vaccine provided the first positive, albeit very modest, signal ever reported in an HIV vaccine trial, showing 31% protection against acquisition of infection. Such a result is certainly not sufficient justifi­ cation for clinical use of the vaccine; however, it served as an important first step in the direction of the development of a safe and effective vaccine against HIV infection. Follow-up studies of RV144 indicate that nonneutralizing or weakly neutralizing antibody responses against certain constant epitopes in the otherwise highly variable V1–V2 region of the HIV envelope may be associated with the modest degree of protection observed in that clinical trial. Three additional similar studies were undertaken in high-HIV-prevalence countries in sub-Saharan Africa as well as in the Americas and certain European countries in attempts to improve on the results of RV144 by a variety of approaches, including increasing the number of vaccine boosts with envelope protein, the use of mosaic antigens, and the addition of adjuvant. Unfortunately, all three of these phase 3 studies of candidate vaccines failed to show efficacy. Another study was terminated early due to lack of efficacy. An area of HIV vaccine research that is currently being actively pursued is the attempt to induce broadly neutralizing antibodies by developing as immunogens for vaccination certain epitopes on the HIV envelope that are the targets of naturally occurring broadly neutralizing antibodies during HIV infection (Fig. 208-30). It is curious that only about 20% of people with HIV develop broadly neutralizing antibod­ ies in response to natural infection and they do so only after 2–3 years of ongoing infection. By the time these antibodies appear, they can neutralize a broad range of primary HIV isolates, but they appear to be ineffective against the autologous virus in the infected subject. Upon close examination, these broadly neutralizing antibodies manifest a high degree of somatic mutations that accumulated over time and are responsible for their affinity maturation and broadly neutralizing capacity. The goal of current efforts is to develop the conformationally correct HIV envelope epitopes that, when used as immunogens, would direct the immune response of an uninfected individual to the produc­ tion of broadly neutralizing antibodies over a reasonable time frame by sequential immunizations. It remains to be seen whether this approach will be feasible. ■ ■FURTHER READING Bekker LG et al: HIV infection. Nat Rev Dis Primers 9:42, 2023. Beyrer C et al: Is HIV epidemic control by 2030 realistic? Lancet HIV 7:e489, 2024. Centers for Disease Control and Prevention (CDC): Clinical Guidance for PrEP. Available at www.cdc.gov/hivnexus/hcp/prep/. Centers for Disease Control and Prevention (CDC): Clinical Guidance for PEP. Available at www.cdc.gov/hivnexus/hcp/pep/. Centers for Disease Control and Prevention (CDC): Clinical Care of HIV. Available at www.cdc.gov/hivnexus/hcp/clinical-care/. Cohn LB et al: Biology of the HIV-1 latent reservoir and implications for cure strategies. Cell Host Microbe 27:519, 2020.

Collins DR et al: CD8+ T cells in HIV control, cure and prevention.

Nat Rev Immunol 20:471, 2020. Eisinger RW et al: HIV viral load and transmissibility of HIV infec­ tion: Undetectable equals untransmittable. JAMA 321:451, 2019. Fauci AS, Lane HC: Four decades of HIV/AIDS—much accom­ plished, much to do. N Engl J Med 383:1, 2020. Grosso TM et al: HIV and aging, biological mechanisms, and thera­ pies: What do we know? AIDS Rev 25:79, 2022. Haynes BF et al: Strategies for HIV-1 vaccines that induce broadly neutralizing antibodies. Nat Rev Immunol 23:142, 2023. Horberg M et al: Primary care guidance for persons with human immunodeficiency virus: 2024 update by the HIV Medicine Associa­ tion of the Infectious Diseases Society of America. Clin Infect Dis, 2024. [Epub ahead of print.] Kazer SW: Evolution and diversity of immune responses during acute HIV Infection. Immunity 53:908, 2020. Landovitz RJ et al: Prevention, treatment and cure of HIV infection. Nat Rev Microbiol 21:657, 2023. Lisco A et al: Reappraisal of idiopathic CD4 lymphocytopenia at 30 years. N Engl J Med 388:1681, 2023. Moir S, Fauci AS: B-cell responses to HIV infection. Immunol Rev 275:33, 2017. Nkolola JP, Barouch DH: Prophylactic HIV-1 vaccine trials: Past, present, and future. Lancet HIV 11:e117, 2024. Panel on Opportunistic Infections in Adults and Ado­ lescents with HIV: Guidelines for the Prevention and Treat­ ment of Opportunistic Infections in Adults and Adolescents with HIV. Available at clinicalinfo.hiv.gov/en/guidelines/hiv-clin­ ical-guidelines-adult-and-adolescent-opportunistic-infections/ whats-new. Saez-Cirion A, Sereti I: Immunometabolism and HIV-1 pathogen­ CHAPTER 209 esis: Food for thought. Nat Rev Immunol 21:5, 2021. UN Joint Programme On HIV/AIDS (UNAIDS): 2024 global AIDS report — The Urgency of Now: AIDS at a Crossroads. Available at www

.unaids.org/en/resources/documents/2024/global-aids-update-2024. U.S. Department of Health and Human Services Panel on Viral Gastroenteritis Antiretroviral Guidelines for Adults and Adolescents: Guidelines for the use of antiretroviral agents in adults and adoles­ cents living with HIV. Available at clinicalinfo.hiv.gov/en/guidelines/ hiv-clinical-guidelines-adult-and-adolescent-arv/whats-new. Venter WDF et al: The long wait for long-acting HIV prevention and treatment formulations. Lancet HIV 11:e711, 2024. Section 15 Infections Due to RNA Viruses Umesh D. Parashar, Roger I. Glass

Viral Gastroenteritis Acute infectious gastroenteritis is a common illness that affects persons of all ages worldwide. It is a leading cause of death among children in developing countries, accounting for an estimated 0.5 million deaths each year, and is responsible for up to 6–8% of all hospitalizations among children in industrialized countries, including the United States. Elderly persons, especially those with debilitating health conditions, also are at risk of severe complications and death from acute gastroen­ teritis. Among healthy young adults, acute gastroenteritis is rarely fatal but incurs substantial medical and social costs, including those of time lost from work. Several enteric viruses have been recognized as important etiologic agents of acute infectious gastroenteritis (Table 209-1, Fig. 209-1).

101 - 209 Viral Gastroenteritis

209 Viral Gastroenteritis

virus; (2) the fact that the infection can be transmitted by cell-free or cell-associated virus; (3) the fact that the HIV provirus integrates itself into the genome of the target cell and may remain in a latent form unex­ posed to the immune system; (4) the likely need for the development of effective mucosal immunity; and, importantly, (5) the difficulty that the immune system has in readily mounting broadly neutralizing antibodies in response to natural infection with HIV (see below). Early attempts to develop a vaccine with the envelope protein gp120 aimed at inducing neutralizing antibodies in humans were unsuccess­ ful; the elicited antisera failed to neutralize primary isolates of HIV. In this regard, two phase 3 trials were undertaken in the United States and Thailand using soluble gp120, and the vaccines failed to protect human volunteers from HIV infection. In addition, two separate vaccine trials aimed at eliciting CD8+ T cell responses to prevent infection and, if unsuccessful in preventing infection, to control postinfection viremia, also failed at both goals. In 2009, a vaccine using a poxvirus vector prime expressing various viral proteins followed by an envelope protein boost was assessed in a 16,000-person clinical trial (RV144) conducted in Thailand among predominantly low-HIV-prevalence heterosexu­ als. The vaccine provided the first positive, albeit very modest, signal ever reported in an HIV vaccine trial, showing 31% protection against acquisition of infection. Such a result is certainly not sufficient justifi­ cation for clinical use of the vaccine; however, it served as an important first step in the direction of the development of a safe and effective vaccine against HIV infection. Follow-up studies of RV144 indicate that nonneutralizing or weakly neutralizing antibody responses against certain constant epitopes in the otherwise highly variable V1–V2 region of the HIV envelope may be associated with the modest degree of protection observed in that clinical trial. Three additional similar studies were undertaken in high-HIV-prevalence countries in sub-Saharan Africa as well as in the Americas and certain European countries in attempts to improve on the results of RV144 by a variety of approaches, including increasing the number of vaccine boosts with envelope protein, the use of mosaic antigens, and the addition of adjuvant. Unfortunately, all three of these phase 3 studies of candidate vaccines failed to show efficacy. Another study was terminated early due to lack of efficacy. An area of HIV vaccine research that is currently being actively pursued is the attempt to induce broadly neutralizing antibodies by developing as immunogens for vaccination certain epitopes on the HIV envelope that are the targets of naturally occurring broadly neutralizing antibodies during HIV infection (Fig. 208-30). It is curious that only about 20% of people with HIV develop broadly neutralizing antibod­ ies in response to natural infection and they do so only after 2–3 years of ongoing infection. By the time these antibodies appear, they can neutralize a broad range of primary HIV isolates, but they appear to be ineffective against the autologous virus in the infected subject. Upon close examination, these broadly neutralizing antibodies manifest a high degree of somatic mutations that accumulated over time and are responsible for their affinity maturation and broadly neutralizing capacity. The goal of current efforts is to develop the conformationally correct HIV envelope epitopes that, when used as immunogens, would direct the immune response of an uninfected individual to the produc­ tion of broadly neutralizing antibodies over a reasonable time frame by sequential immunizations. It remains to be seen whether this approach will be feasible. ■ ■FURTHER READING Bekker LG et al: HIV infection. Nat Rev Dis Primers 9:42, 2023. Beyrer C et al: Is HIV epidemic control by 2030 realistic? Lancet HIV 7:e489, 2024. Centers for Disease Control and Prevention (CDC): Clinical Guidance for PrEP. Available at www.cdc.gov/hivnexus/hcp/prep/. Centers for Disease Control and Prevention (CDC): Clinical Guidance for PEP. Available at www.cdc.gov/hivnexus/hcp/pep/. Centers for Disease Control and Prevention (CDC): Clinical Care of HIV. Available at www.cdc.gov/hivnexus/hcp/clinical-care/. Cohn LB et al: Biology of the HIV-1 latent reservoir and implications for cure strategies. Cell Host Microbe 27:519, 2020.

Collins DR et al: CD8+ T cells in HIV control, cure and prevention.

Nat Rev Immunol 20:471, 2020. Eisinger RW et al: HIV viral load and transmissibility of HIV infec­ tion: Undetectable equals untransmittable. JAMA 321:451, 2019. Fauci AS, Lane HC: Four decades of HIV/AIDS—much accom­ plished, much to do. N Engl J Med 383:1, 2020. Grosso TM et al: HIV and aging, biological mechanisms, and thera­ pies: What do we know? AIDS Rev 25:79, 2022. Haynes BF et al: Strategies for HIV-1 vaccines that induce broadly neutralizing antibodies. Nat Rev Immunol 23:142, 2023. Horberg M et al: Primary care guidance for persons with human immunodeficiency virus: 2024 update by the HIV Medicine Associa­ tion of the Infectious Diseases Society of America. Clin Infect Dis, 2024. [Epub ahead of print.] Kazer SW: Evolution and diversity of immune responses during acute HIV Infection. Immunity 53:908, 2020. Landovitz RJ et al: Prevention, treatment and cure of HIV infection. Nat Rev Microbiol 21:657, 2023. Lisco A et al: Reappraisal of idiopathic CD4 lymphocytopenia at 30 years. N Engl J Med 388:1681, 2023. Moir S, Fauci AS: B-cell responses to HIV infection. Immunol Rev 275:33, 2017. Nkolola JP, Barouch DH: Prophylactic HIV-1 vaccine trials: Past, present, and future. Lancet HIV 11:e117, 2024. Panel on Opportunistic Infections in Adults and Ado­ lescents with HIV: Guidelines for the Prevention and Treat­ ment of Opportunistic Infections in Adults and Adolescents with HIV. Available at clinicalinfo.hiv.gov/en/guidelines/hiv-clin­ ical-guidelines-adult-and-adolescent-opportunistic-infections/ whats-new. Saez-Cirion A, Sereti I: Immunometabolism and HIV-1 pathogen­ CHAPTER 209 esis: Food for thought. Nat Rev Immunol 21:5, 2021. UN Joint Programme On HIV/AIDS (UNAIDS): 2024 global AIDS report — The Urgency of Now: AIDS at a Crossroads. Available at www

.unaids.org/en/resources/documents/2024/global-aids-update-2024. U.S. Department of Health and Human Services Panel on Viral Gastroenteritis Antiretroviral Guidelines for Adults and Adolescents: Guidelines for the use of antiretroviral agents in adults and adoles­ cents living with HIV. Available at clinicalinfo.hiv.gov/en/guidelines/ hiv-clinical-guidelines-adult-and-adolescent-arv/whats-new. Venter WDF et al: The long wait for long-acting HIV prevention and treatment formulations. Lancet HIV 11:e711, 2024. Section 15 Infections Due to RNA Viruses Umesh D. Parashar, Roger I. Glass

Viral Gastroenteritis Acute infectious gastroenteritis is a common illness that affects persons of all ages worldwide. It is a leading cause of death among children in developing countries, accounting for an estimated 0.5 million deaths each year, and is responsible for up to 6–8% of all hospitalizations among children in industrialized countries, including the United States. Elderly persons, especially those with debilitating health conditions, also are at risk of severe complications and death from acute gastroen­ teritis. Among healthy young adults, acute gastroenteritis is rarely fatal but incurs substantial medical and social costs, including those of time lost from work. Several enteric viruses have been recognized as important etiologic agents of acute infectious gastroenteritis (Table 209-1, Fig. 209-1).

TABLE 209-1  Viral Causes of Gastroenteritis among Humans VIRUS FAMILY GENOME PRIMARY AGE GROUP AT RISK Group A rotavirus Reoviridae Double-strand segmented RNA Children <5 years

EIA (commercial), RT-PCR, EM, PAGE Norovirus Caliciviridae Positive-sense single-strand RNA All ages

RT-PCR, EM, EIA (commercial) Sapovirus Caliciviridae Positive-sense single-strand RNA Children <5 years + RT-PCR, EM Astrovirus Astroviridae Positive-sense single-strand RNA Children <5 years + EIA, RT-PCR, EM Adenovirus (mainly types 40 and 41) Adenoviridae Double-strand DNA Children <5 years +/+ + EIA (commercial), PCR, EM Abbreviations: EIA, enzyme immunoassay; EM, electron microscopy; PAGE, polyacrylamide gel electrophoresis; PCR, polymerase chain reaction; RT-PCR, reversetranscription PCR. Although most viral gastroenteritis is caused by RNA viruses, the DNA viruses that are occasionally involved (e.g., adenovirus types 40 and 41) are included in this chapter. Illness caused by these viruses is characterized by the acute onset of vomiting and/or diarrhea, which may be accompanied by fever, nausea, abdominal cramps, anorexia, and malaise. As shown in Table 209-2, several features can help dis­ tinguish gastroenteritis caused by viruses from that caused by bacterial agents. However, the distinction based on clinical and epidemiologic parameters alone is often difficult, and laboratory tests are required to confirm the diagnosis. ■ ■HUMAN CALICIVIRUSES Etiologic Agent  The Norwalk virus is the prototype strain of a group of small (27–40 nm), nonenveloped, round, icosahedral viruses with relatively amorphous surface features on visualization by electron microscopy. Molecular cloning and characterization have demonstrated that the viruses have a single, positive-strand RNA genome ~7.5 kb in length and possess a single virion-associated protein—similar to that of typical caliciviruses—with a molecular mass of 60 kDa. On the basis of these molecular characteristics, these viruses are presently classified into two genera belonging to the fam­ ily Caliciviridae, the noroviruses and the sapoviruses, which are fur­ ther classified into genogroups and genotypes. Of the 10 recognized norovirus genogroups in humans and animals, 35 different genotypes belonging to 5 genogroups (GI, GII, GIV, GVIII, and GIX) are known to infect humans. PART 5 Infectious Diseases Epidemiology  Infections with human caliciviruses are common worldwide, and most adults have antibodies to these viruses. Antibody is acquired at an earlier age in developing countries—a pattern con­ sistent with the presumed fecal–oral mode of transmission. Infections occur year-round, although, in temperate climates, a distinct increase has been noted in cold-weather months. Noroviruses may be the most common infectious agents of mild gastroenteritis in the community Rotavirus Adenovirus Astrovirus Calicivirus—NV Calicivirus—SV Torovirus Picobirnavirus Enterovirus 22 FIGURE 209-1  Viral agents of gastroenteritis. NV, norovirus; SV, sapovirus.

CLINICAL SEVERITY DETECTION ASSAYS and affect all age groups, whereas sapoviruses primarily cause gastro­ enteritis in children. Noroviruses also cause traveler’s diarrhea, and outbreaks have occurred among military personnel deployed to various parts of the world. The limited data available indicate that norovirus may be the second most common viral agent (after rotavirus) among young children and the most common agent among older children and adults. In the United States and some other developed countries, with the decline in severe rotavirus disease following implementation of a rotavirus vaccination program, norovirus has become the leading cause of medically attended gastroenteritis in young children. Noroviruses are also recognized as the major cause of epidemics of gastroenteritis worldwide. In the United States, ~50% of all reported outbreaks of gastroenteritis are caused by noroviruses. Norovirus is transmitted predominantly by the fecal–oral route but is also present in vomitus. Because an inoculum with very few viruses can be infectious, transmission can occur by aerosolization, by contact with contaminated fomites, and by person-to-person contact. Viral shedding and infectivity are greatest during the acute illness, but challenge studies with Norwalk virus in volunteers indicate that viral antigen may be shed by asymptomatically infected persons and also by symptomatic persons before the onset of symptoms and for several weeks after the resolution of illness. Viral shedding can be prolonged in immunocompromised individuals. Pathogenesis  The exact sites and cellular receptors for attachment of viral particles have not been determined. Data suggest that carbohy­ drates that are similar to human histo-blood group antigens (HBGA) and are present on the gastroduodenal epithelium of individuals with the secretor phenotype may serve as ligands for the attachment of norovirus. Additional studies must more fully elucidate norovirus– carbohydrate interactions, including strain-specific variations. After the infection of volunteers, reversible lesions are noted in the upper jejunum, with broadening and blunting of the villi, shortening of the microvilli, vacuolization of the lining epithelium, crypt hyperplasia,

TABLE 209-2  Characteristics of Gastroenteritis Caused by Viral and Bacterial Agents FEATURE VIRAL GASTROENTERITIS BACTERIAL GASTROENTERITIS Setting Incidence similar in developing and developed countries More common in settings with poor hygiene and sanitation Infectious dose Low (10–100 viral particles) for most agents High (>105 bacteria) for Escherichia coli, Salmonella, Vibrio; medium (102–105 bacteria) for Campylobacter jejuni; low (10–100 bacteria) for Shigella Seasonality In temperate climates, winter seasonality for most agents; year-round occurrence in tropical areas Incubation period 1–3 days for most agents; can be shorter for norovirus 1–7 days for common agents (e.g., Campylobacter, E. coli, Shigella, Salmonella); a few hours for bacteria producing preformed toxins (e.g., Staphylococcus aureus, Bacillus cereus) Reservoir Primarily humans Depending on bacterial species, human (e.g., Shigella, Salmonella), animal (e.g., Campylobacter, Salmonella, E. coli), and water (e.g., Vibrio) reservoirs exist Fever Common with rotavirus and norovirus; uncommon with other agents Vomiting Prominent and can be the only presenting feature, especially in children Diarrhea Common; nonbloody in almost all cases Prominent and occasionally bloody with agents causing inflammatory diarrhea Duration 1–3 days for norovirus and sapovirus; 2–8 days for other viruses Diagnosis This is often a diagnosis of exclusion in clinical practice. Commercial enzyme immunoassays are available for detection of rotavirus and adenovirus, but identification of other agents is limited to research and public health laboratories. Treatment Supportive therapy to maintain adequate hydration and nutrition should be given. Antibiotics and antimotility agents are contraindicated. and infiltration of the lamina propria by polymorphonuclear neu­ trophils and lymphocytes. The lesions persist for at least 4 days after the resolution of symptoms and are associated with malabsorption of carbohydrates and fats and a decreased level of brush-border enzymes. Adenylate cyclase activity is not altered. No histopathologic changes are seen in the stomach or colon, but gastric motor function is delayed, and this alteration is believed to contribute to the nausea and vomiting that are typical of this illness. Clinical Manifestations  Gastroenteritis caused by human cali­ civiruses has a sudden onset following an average incubation period of 24 h (range, 12–72 h). The illness generally lasts 12–60 h and is characterized by one or more of the following symptoms: nausea, vomiting, abdominal cramps, and diarrhea. Vomiting is more preva­ lent among children, whereas a greater proportion of adults develop diarrhea. Constitutional symptoms are common, including headache, fever, chills, and myalgias. The stools are characteristically loose and watery, without blood, mucus, or leukocytes. White cell counts are generally normal; rarely, leukocytosis with relative lymphopenia may be observed. Death is a rare outcome and usually results from severe dehydration in vulnerable persons (e.g., elderly patients with debilitat­ ing health conditions). Immunity  Approximately 50% of persons challenged with Norwalk virus become ill and acquire short-term immunity against the infecting strain. In early human volunteer studies, immunity to Norwalk virus appeared to correlate inversely with level of antibody; i.e., persons with higher levels of preexisting antibody to Norwalk virus were more sus­ ceptible to illness on rechallenge. This paradoxical observation was later explained by data indicating that some individuals have a genetic predis­ position to illness, with specific HBGA phenotypes influencing suscep­ tibility to norovirus infection. Contemporary data show that functional antibodies that block norovirus binding to HBGAs correlate with protec­ tive immunity in human volunteer challenge and vaccination studies. Furthermore, initial studies have demonstrated that norovirus grown in vitro in the newly developed human intestinal enteroid (HIE) cell-based system can be neutralized by sera containing blocking antibodies. Diagnosis  Cloning and sequencing of the genomes of Norwalk and several other human caliciviruses have allowed the development

More common in summer or rainy months, particularly in developing countries with a high disease burden Common with agents causing inflammatory diarrhea (e.g., Salmonella, Shigella) Common with bacteria producing preformed toxins; less prominent in diarrhea due to other agents 1–2 days for bacteria producing preformed toxins; 2–8 days for most other bacteria Fecal examination for leukocytes and blood is helpful in differential diagnosis. Culture of stool specimens, sometimes on special media, can identify several pathogens. Molecular techniques are useful epidemiologic tools but are not routinely used in most laboratories. Supportive hydration therapy is adequate for most patients. Antibiotics are recommended for patients with dysentery caused by Shigella or diarrhea caused by Vibrio cholerae and for some patients with Clostridium difficile colitis. CHAPTER 209 of assays based on polymerase chain reaction (PCR) for detection of virus in stool and vomitus. PCR-based detection assays for norovirus combined with those for multiple enteric pathogens are commercially available and are increasingly used in clinical settings. Virus-like parti­ cles (VLPs) produced by expression of capsid proteins in a recombinant baculovirus vector have been used to develop enzyme immunoassays (EIAs) for detection of virus in stool or a serologic response to a spe­ cific viral antigen. Commercial EIA kits for detection in stool have limited sensitivity and usefulness in clinical practice and are of greatest utility in outbreaks, in which many specimens are tested and only a few need be positive to identify norovirus as the cause. Viral Gastroenteritis TREATMENT Infections with Norwalk and Related Human Caliciviruses The disease is self-limited, and oral rehydration therapy is generally adequate. If severe dehydration develops, IV fluid therapy is indi­ cated. No specific antiviral therapy is available. Prevention  Epidemic prevention relies on situation-specific mea­ sures, such as control of contamination of food and water, exclusion of ill food handlers, and reduction of person-to-person spread through good personal hygiene and disinfection of contaminated fomites. The role of immunoprophylaxis is not clear, given the lack of long-term immunity from natural disease, but efforts to develop norovirus vaccines are ongo­ ing. Vaccines based on VLPs are being tested in human volunteers. In a proof-of-concept trial, the efficacy of a monovalent GI.1 VLP vaccine was 47% among volunteers who received the vaccine intranasally and were then challenged with a homologous strain. In a second trial, noro­ virus disease severity was reduced in volunteers who received a bivalent G1.1/GII.4 VLP vaccine intramuscularly (with the GII.4 component including a consensus sequence from three different GII.4 strains) and were subsequently challenged with a GII.4 norovirus strain. Data from the first field efficacy study of this bivalent vaccine conducted in ~4700 healthy U.S. Navy recruits given 1 intramuscular injection of the bivalent vaccine were recently reported. While the primary endpoint of

protection against homotypic infection could not be evaluated because only 6 total moderate/severe cases due to GI.1 or GII.4 norovirus strains occurred during the trial, the vaccine efficacy was 61.8% (95.01% con­ fidence interval, 20.8–81.6%) for moderate/severe norovirus acute gas­ troenteritis due to any type. These initial data are encouraging; however, key issues to be further studied include the duration of protection and the level of heterotypic protection against antigenically distinct strains, particularly given the continuing and rapid natural evolution leading to the emergence of novel norovirus strains.

■ ■ROTAVIRUS Etiologic Agent  Rotaviruses are members of the family Reoviri­ dae. The viral genome consists of 11 segments of double-strand RNA enclosed in a triple-layered, nonenveloped, icosahedral capsid 75 nm in diameter. Viral protein 6 (VP6), the major structural protein, is the target of commercial immunoassays and determines the group specific­ ity of rotaviruses. Seven major groups of rotavirus (A through G) exist; human illness is caused primarily by group A and, to a much lesser extent, by groups B and C. Two outer-capsid proteins, VP7 (G-protein) and VP4 (P-protein), determine serotype specificity, induce neutral­ izing antibodies, and form the basis for binary classification of rota­ viruses (G and P types). The segmented genome of rotavirus allows genetic reassortment (i.e., exchange of genome segments between viruses) during co-infection—a property that plays a role in viral evolution and that has been utilized in the development of reassortant animal/human rotavirus–based vaccines. Epidemiology  Worldwide, nearly all children are infected with rotavirus by 3–5 years of age. Neonatal infections are common but are often asymptomatic or mild, presumably because of protection by maternal antibody or breast milk. Compared with rotavirus disease in industrialized countries, disease in developing countries occurs at a younger age, is less seasonal, is more frequently caused by uncommon or multiple rotavirus strains, and is more often fatal. Moreover, because of suboptimal access to hydration therapy, rotavirus is a leading cause of diarrheal death among children in the developing world, with the PART 5 Infectious Diseases Rates per 100,000 PY: 0 to <10 10 to <50 FIGURE 209-2  Rotavirus mortality rates by country, per 100,000 children <5 years of age. (From JE Tate et al: Global, regional, and national estimates of rotavirus mortality in children <5 years of age, 2000–2013. Clin Infect Dis 62:S96, 2016.)

highest mortality rates among children in sub-Saharan Africa and southern Asia (Fig. 209-2). First infections after 3 months of age are likely to be symptomatic, and the incidence of disease peaks among children 4–23 months of age. Reinfections are common, but the severity of disease decreases with each repeat infection. Therefore, severe rotavirus infections are less common among older children and adults than among younger individuals. Nevertheless, rotavirus can cause illness in parents and caretakers of children with rotavirus diarrhea, immunocompromised persons, travelers, and elderly individuals and should be considered in the differential diagnosis of gastroenteritis among adults. In tropical settings, rotavirus disease occurs year-round, with less pronounced seasonal peaks than in temperate settings, where rotavirus disease occurs predominantly during the cooler fall and winter months. Before the introduction of rotavirus vaccine in the United States, the rotavirus season each year began in the Southwest during the autumn and early winter (October through December) and migrated across the continent, peaking in the Northeast during late winter and spring (March through May). The reasons for this characteristic pattern are not clear but may be correlated with statespecific differences in birth rates, which could influence the rate of accumulation of susceptible infants after each rotavirus season. After the implementation of routine vaccination of U.S. infants against rotavirus in 2006, the characteristic prevaccine geotemporal pattern of U.S. rotavirus was dramatically altered, and these changes were accompanied by substantial declines in rotavirus detections by a national network of sentinel laboratories. During episodes of rotavirus-associated diarrhea, virus is shed in large quantities in stool (107–1012/g). Viral shedding detectable by EIA usually subsides within 1 week but may persist for >30 days in immu­ nocompromised individuals; it may be detected for longer periods by sensitive molecular assays, such as PCR. The virus is transmitted pre­ dominantly through the fecal–oral route. Spread through respiratory secretions, person-to-person contact, or contaminated environmental surfaces has been postulated to explain the rapid acquisition of anti­ body in the first 3 years of life, regardless of sanitary conditions. 50 to <100 ≥100

At least 10 different G serotypes of group A rotavirus have been identified in humans, but only 5 types (G1 through G4 and G9) are common. While human rotavirus strains that possess a high degree of genetic homology with animal strains have been identified, animal-tohuman transmission appears to be uncommon. Group B rotaviruses have been associated with several large epidem­ ics of severe gastroenteritis among adults in China since 1982 and have also been identified in India. Group C rotaviruses have been associated with a small proportion of pediatric gastroenteritis cases in several countries worldwide. Pathogenesis  Rotaviruses infect and ultimately destroy mature enterocytes in the villous epithelium of the proximal small intestine. The loss of absorptive villous epithelium, coupled with the prolifera­ tion of secretory crypt cells, results in secretory diarrhea. Brush-border enzymes characteristic of differentiated cells are reduced, and this change leads to the accumulation of unmetabolized disaccharides and consequent osmotic diarrhea. Studies in mice indicate that a non­ structural rotavirus protein, NSP4, functions as an enterotoxin and contributes to secretory diarrhea by altering epithelial cell function and permeability. In addition, rotavirus may evoke fluid secretion through activation of the enteric nervous system in the intestinal wall. Rotavirus antigenemia and viremia are common among children with acute rotavirus infection, although the antigen and RNA levels in serum are substantially lower than those in stool. Clinical Manifestations  The clinical spectrum of rotavirus infec­ tion ranges from subclinical infection to severe gastroenteritis leading to life-threatening dehydration. After an incubation period of 1–3 days, the illness has an abrupt onset, with vomiting frequently preceding the onset of diarrhea. Up to one-third of patients may have a temperature of >39°C. The stools are characteristically loose and watery and only infrequently contain red or white cells. Gastrointestinal symptoms generally resolve in 3–7 days. Respiratory and neurologic features in children with rotavirus infection have been reported, but causal associations have not been proven. Moreover, rotavirus infection has been associated with a vari­ ety of other clinical conditions (e.g., sudden infant death syndrome, necrotizing enterocolitis, intussusception, Kawasaki disease, and type 1 diabetes), but no causal relationship has been confirmed with any of these syndromes. Rotavirus does not appear to be a major opportunistic pathogen in children with HIV infection. In severely immunodeficient children, rotavirus can cause protracted diarrhea with prolonged viral excretion and, in rare instances, can disseminate systemically. Persons who are immunosuppressed for bone marrow transplantation also are at risk for severe or even fatal rotavirus disease. Immunity  Protection against rotavirus disease is correlated with the presence of virus-specific secretory IgA antibodies in the intestine and, to some extent, the serum. Because virus-specific IgA production at the intestinal surface is short-lived, complete protection against disease is only temporary. However, each infection and subsequent reinfection confers progressively greater immunity; thus, severe disease is most common among young children with first or second infections. Immunologic memory is believed to be important in the attenuation of disease severity upon reinfection. Diagnosis  Illness caused by rotavirus is difficult to distinguish clini­ cally from that caused by other enteric viruses. Because large quantities of virus are shed in feces, the diagnosis can usually be confirmed by a wide variety of commercially available EIAs or by techniques for detect­ ing viral RNA, such as gel electrophoresis, probe hybridization, or PCR. TREATMENT Rotavirus Infections Rotavirus gastroenteritis can lead to severe dehydration; appropri­ ate treatment should be instituted early. Standard oral rehydra­ tion therapy is successful for most children who can take fluids

by mouth, but IV fluid replacement may be required for patients who are severely dehydrated or are unable to tolerate oral therapy because of frequent vomiting. The therapeutic roles of probiotics, bismuth subsalicylate, enkephalinase inhibitors, and nitazoxanide have been evaluated in clinical studies but are not clearly defined. Antibiotics and antimotility agents should be avoided. In immuno­ compromised children with chronic symptomatic rotavirus disease, orally administered immunoglobulins or colostrum may result in the resolution of symptoms, but the best choices regarding agents and their doses have not been well studied, and treatment decisions are often empirical.

Prevention  Efforts to develop rotavirus vaccines were pursued because it was apparent—given the similar rates in less developed and industrialized nations—that improvements in hygiene and sanitation were unlikely to reduce disease incidence. The first rotavirus vaccine licensed in the United States in 1998 was withdrawn from the market within 1 year because it was linked with a low incidence of intussuscep­ tion, a form of bowel obstruction. In 2006, promising safety and efficacy (85−98% against severe rota­ virus disease) data for two new rotavirus vaccines—RotaTeq (Merck, United States) and Rotarix (GlaxoSmithKline, Belgium)—were reported from large clinical trials conducted in North America, Europe, and Latin America. Both vaccines are now recommended for routine immuniza­ tion of all U.S. infants, and their use has led to a >70–80% decline in rotavirus hospitalizations and emergency department visits at hospitals across the United States. Somewhat unexpectedly, rotavirus vaccination of young infants has also resulted in the added benefit of declines in rotavirus disease among children who miss vaccination and even among older children and adults who are not eligible for vaccination in some settings. The reason is likely to be a reduction in community transmis­ sion of rotavirus because of vaccination—i.e., herd protection. In April 2009, the World Health Organization (WHO) recommended the use of rotavirus vaccines in all countries worldwide. As of December 2003, over 120 countries, including several low-income countries in Africa and Asia, have incorporated rotavirus vaccine into their national childhood immunization programs (Fig. 209-3). Large declines in severe morbid­ ity and mortality from childhood diarrhea have been documented in many countries. Postmarketing surveillance has identified a low risk of intussusception in some high- and middle-income countries; however, the benefits of vaccination exceed the risks, and no changes in vaccine administration policy have been implemented. An intussusception risk has not been identified in several postmarketing evaluations in develop­ ing countries to date. CHAPTER 209 Viral Gastroenteritis The different epidemiology of rotavirus disease and the greater prevalence of co-infection with other enteric pathogens, of comorbidi­ ties, and of malnutrition in developing countries may adversely affect the performance of oral rotavirus vaccines, as is the case with oral vaccines against poliomyelitis, cholera, and typhoid in these regions. Therefore, evaluation of the efficacy of rotavirus vaccines in resourcepoor settings of Africa and Asia was specifically recommended, and these trials have now been completed. As anticipated, the efficacy of rotavirus vaccines was moderate (50–65%) in these settings when com­ pared with that in industrialized countries. Despite modest efficacy, routine use of rotavirus vaccines in low-income African countries with a heavy disease burden has yielded substantial public health benefits. Several manufacturers in emerging markets, including India, China, Vietnam, Indonesia, and Brazil, are developing candidate rotavirus

vaccines. Beginning in 2016, two Indian-made rotavirus vaccines— Rotavac (Bharat Biotech, India) and Rotasiil (Serum Institute, India)— were implemented in India’s routine childhood immunization program, which has since expanded to all Indian states with a birth cohort of

25 million. In trials conducted in low-income countries, the efficacy of Rotavac and Rotasiil ranged from 36 to 66%, similar to the efficacy of multinational vaccines in these settings. In 2018, these two vaccines were prequalified by WHO, allowing their procurement with funding support from Gavi, the Vaccine Alliance, in low-income countries outside India.

102 - 210 Enterovirus, Parechovirus, and Reovirus Infections

210 Enterovirus, Parechovirus, and Reovirus Infections

RV Vaccine Introduction Current Vaccine Intro Status Current Vaccine Intro Status Introduced Not Introduced Planning Introduced Subnationally Program Suspended January 8, 2024 © The International Vaccine Access Center (IVAC) FIGURE 209-3  Countries that have implemented national rotavirus vaccination programs, January 8, 2024. (Source: View-Hub, http://www.view-hub.org/viz/. © The International Vaccine Access Center [IVAC].) PART 5 Infectious Diseases ■ ■OTHER VIRAL AGENTS OF GASTROENTERITIS Enteric adenoviruses of serotypes 40 and 41 belonging to subgroup F are 70- to 80-nm viruses with double-strand DNA that cause ~2–12% of all diarrhea episodes in young children. Unlike adenoviruses that cause respiratory illness, enteric adenoviruses are difficult to cultivate in cell lines, but they can be detected with commercially available EIAs. Adenovirus types 31 and 42–49 have been linked to diarrhea in HIVinfected and other immunocompromised persons. Astroviruses are 28- to 30-nm viruses with a characteristic icosahe­ dral structure and a positive-sense, single-strand RNA. At least seven serotypes have been identified, of which serotype 1 is most common. Astroviruses are primarily pediatric pathogens, causing ~2–10% of cases of mild to moderate gastroenteritis in children. The availability of simple immunoassays to detect virus in fecal specimens and of molecular methods to confirm and characterize strains will permit more comprehensive assessment of the etiologic role of these agents. Toroviruses are 100- to 140-nm, enveloped, positive-strand RNA viruses that are recognized as causes of gastroenteritis in horses (Berne virus) and cattle (Breda virus). Their role as a cause of diarrhea in humans is still unclear, but studies from Canada have demonstrated associations between torovirus excretion and both nosocomial gastroenteritis and necrotizing enterocolitis in neonates. These associations require further evaluation. Picobirnaviruses are small, bisegmented, double-strand RNA viruses that cause gastroenteritis in a variety of animals. Their role as primary causes of gastroenteritis in humans remains unclear, but several studies have found an association between picobirnaviruses and gastroenteritis in HIV-infected adults. Several other viruses (e.g., enteroviruses, reoviruses, pestiviruses, Aichivirus, and parvovirus B) have been identified in the feces of patients with diarrhea, but their etiologic role in gastroenteritis has not been proven. Diarrhea has also been noted as a manifestation of infection with recently recognized viruses that primarily cause severe respiratory illness: the SARS-CoV viruses, the influenza A/H5N1 virus, and the current pandemic strain of influenza A/H1N1 virus. ■ ■FURTHER READING Armah G et al: Vaccine value profile for norovirus. Vaccine 41 Suppl 2:S134, 2023. Clark A et al: Estimating the global impact of rotavirus vaccines on child mortality. Int J Infect Dis 137:90, 2023.

Cohen A et al: Aetiology and incidence of diarrhoea requiring hos­ pitalisation in children under 5 years of age in 28 low-income and middle-income countries: findings from the Global Pediatric Diar­ rhea Surveillance. BMJ Global Health 7:e009548, 2022. Flynn T et al: Viral gastroenteritis. Lancet 403:862, 2024. Tate J et al: Global, regional, and national estimates of rotavirus mortality in children <5 years of age, 2000–2013. Clin Infect Dis 62:S96, 2016. Jeffrey I. Cohen

Enterovirus, Parechovirus,

and Reovirus Infections ENTEROVIRUSES ■ ■CLASSIFICATION AND CHARACTERIZATION Enteroviruses, members of the family Picornaviridae, are so designated because of their ability to multiply in the gastrointestinal tract. Despite their name, these viruses are not a prominent cause of gastroenteritis. Enteroviruses encompass more than 115 human serotypes: 3 sero­ types of poliovirus, 23 serotypes of coxsackievirus A, 6 serotypes of coxsackievirus B, 29 serotypes of echovirus, enteroviruses 68–71, and multiple new enteroviruses (beginning with enterovirus 73) that have been identified by molecular techniques. Human enteroviruses have been reclassified into four species designated A–D. Echoviruses 22 and 23 have been reclassified as parechoviruses 1 and 2 on the basis of low nucleotide homology and differences in viral proteins. Enterovirus and parechovirus surveillance conducted in the United States by the Cen­ ters for Disease Control and Prevention (CDC) in 2022 showed that the most common enteroviruses and parechoviruses were enterovirus D68 (41.1% of cases) and human parechovirus 2 (20.7%), followed by

coxsackievirus A6, A9, B3, and B5, each of which accounted for 4–5% of all isolates. Human enteroviruses contain a single-stranded RNA genome sur­ rounded by an icosahedral capsid comprising four viral proteins. These viruses have no lipid envelope and are stable in acidic environments, including the stomach. They are susceptible to chlorine-containing cleansers but resistant to inactivation by standard disinfectants (e.g., alcohol, detergents) and can persist for days at room temperature. ■ ■PATHOGENESIS AND IMMUNITY Much of what is known about the pathogenesis of enteroviruses has been derived from studies of poliovirus infection. After ingestion, poliovirus is thought to infect epithelial cells in the mucosa of the gastrointestinal tract and then to spread to and replicate in the submu­ cosal lymphoid tissue of the tonsils and Peyer’s patches. The virus next spreads to the regional lymph nodes, a viremic phase ensues, and the virus replicates in organs of the reticuloendothelial system. In some cases, a second episode of viremia occurs and the virus replicates fur­ ther in various tissues, sometimes causing symptomatic disease. It is uncertain whether poliovirus reaches the central nervous system (CNS) during viremia or whether it also spreads via peripheral nerves. Since viremia precedes the onset of neurologic disease in humans, it has been assumed that the virus enters the CNS via the bloodstream. The poliovirus receptor is a member of the immunoglobulin super­ family. Poliovirus infection is limited to primates, largely because their cells express the viral receptor. Studies demonstrating the poliovirus receptor in the end-plate region of muscle at the neuromuscular junc­ tion suggest that, if the virus enters the muscle during viremia, it could travel across the neuromuscular junction up the axon to the anterior horn cells. Studies of monkeys and of transgenic mice expressing the poliovirus receptor show that, after IM injection, poliovirus does not reach the spinal cord if the sciatic nerve is cut. Taken together, these findings suggest that poliovirus can spread directly from muscle to the CNS by neural pathways. Poliovirus can usually be cultured from the blood 3–5 days after infection, before the development of neutralizing antibodies. While viral replication at secondary sites begins to slow 1 week after infection, it continues in the gastrointestinal tract. Poliovirus is shed from the oropharynx for up to 3 weeks after infection and from the gastrointesti­ nal tract for as long as 12 weeks; hypogammaglobulinemic patients can shed poliovirus for >20 years. During replication in the gastrointestinal tract, attenuated oral poliovirus can mutate, reverting to a more neu­ rovirulent phenotype within a few days; however, additional mutations are probably required for full neurovirulence. One patient with hypo­ gammaglobulinemia who had been infected 12 years earlier and was receiving IV immune globulin suddenly developed quadriplegia and respiratory muscle paralysis and died; analysis showed that the virus had reverted to a more wild-type sequence. Humoral and secretory immunity in the gastrointestinal tract is important for the control of enterovirus infections. Enteroviruses induce specific IgM, which usually persists for <6 months, and specific IgG, which persists for life. Capsid protein VP1 is the predominant target of neutralizing antibody, which generally confers lifelong protection against subsequent disease caused by the same serotype but does not prevent infection or virus shedding. Enteroviruses also induce cellular immu­ nity of uncertain significance. Patients with impaired cellular immunity are not known to develop unusually severe disease when infected with enteroviruses. In contrast, the severe infections in patients with agam­ maglobulinemia emphasize the importance of humoral immunity in controlling enterovirus infections. Disseminated enterovirus infections have occurred in hematopoietic cell transplant recipients. IgA antibodies are instrumental in reducing poliovirus replication in and shedding from the gastrointestinal tract. Breast milk contains IgA specific for enterovi­ ruses and can protect humans from infection. ■ ■EPIDEMIOLOGY Enteroviruses have a worldwide distribution. More than 50% of nonpoliovirus enterovirus infections and >90% of poliovirus infec­ tions are subclinical. When symptoms do develop, they are usually

nonspecific and occur in conjunction with fever; only a minority of infections are associated with specific clinical syndromes. The incu­ bation period for most enterovirus infections ranges from 2 to 14 days but usually is <1 week.

Enterovirus infection is more common in socioeconomically dis­ advantaged areas, especially in those where conditions are crowded and in tropical areas where hygiene is poor. Infection is most com­ mon among infants and young children; serious illness develops most often during the first few days of life and in older children and adults. In developing countries, where children are infected at an early age, poliovirus infection has less often been associated with paralysis; in countries with better hygiene, older children and adults are more likely to be seronegative, become infected, and develop paralysis. Passively acquired maternal antibody reduces the risk of symptomatic infection in neonates. Young children are the most frequent shedders of entero­ viruses and are usually the index cases in family outbreaks. In temper­ ate climates, enterovirus infections occur most often in the summer and fall; no seasonal pattern is apparent in the tropics. Most enteroviruses are transmitted primarily by the fecal–oral or oral–oral route. Patients are most infectious shortly before and after the onset of symptomatic disease, when virus is present in the stool and throat. The ingestion of virus-contaminated food or water also can cause disease. Certain enteroviruses (such as enterovirus 70, which causes acute hemorrhagic conjunctivitis) can be transmitted by direct inoculation from the fingers to the eye. Airborne transmission is important for some viruses that cause respiratory tract disease, such as coxsackievirus A21. Enteroviruses can be transmitted across the placenta from mother to fetus, causing severe disease in the newborn. The transmission of enteroviruses through blood transfusions or insect bites has not been documented. Nosocomial spread of coxsackievirus and echovirus has taken place in hospital nurseries. Outbreaks of enteroviruses correlate with levels of preexisting immunity to specific serotypes and birth rates. CHAPTER 210 Enterovirus, Parechovirus,and Reovirus Infections ■ ■CLINICAL FEATURES Poliovirus Infection  Most infections with poliovirus are asymp­ tomatic. After an incubation period of 3–6 days, ~5% of patients pres­ ent with a minor illness (abortive poliomyelitis) manifested by fever, malaise, sore throat, anorexia, myalgias, and headache. This condition usually resolves in 3 days. About 1% of patients present with aseptic meningitis (nonparalytic poliomyelitis). Examination of cerebrospinal fluid (CSF) reveals lymphocytic pleocytosis, a normal glucose level, and a normal or slightly elevated protein level; CSF polymorphonuclear leukocytes may be present early. In some patients, especially children, malaise and fever precede the onset of aseptic meningitis. PARALYTIC POLIOMYELITIS  The least common presentation is that of paralytic disease. After one or several days, signs of aseptic meningitis are followed by severe back, neck, and muscle pain and by the rapid or gradual development of motor weakness. In some cases, the disease appears to be biphasic, with aseptic meningitis followed first by appar­ ent recovery but then (1–2 days later) by the return of fever and the development of paralysis; this form is more common among children than among adults. Weakness is generally asymmetric, is proximal more than distal, and may involve the legs (most commonly); the arms; or the abdominal, thoracic, or bulbar muscles. Paralysis develops dur­ ing the febrile phase of the illness and usually does not progress after defervescence. Urinary retention also may occur. Examination reveals weakness, fasciculations, decreased muscle tone, and reduced or absent reflexes in affected areas. Transient hyperreflexia sometimes precedes the loss of reflexes. Patients frequently report sensory symptoms, but objective sensory testing usually yields normal results. Bulbar paralysis may lead to dysphagia, difficulty in handling secretions, or dysphonia. Respiratory insufficiency due to aspiration, involvement of the respira­ tory center in the medulla, or paralysis of the phrenic or intercostal nerves may develop, and severe medullary involvement may lead to circulatory collapse. Most patients with paralysis recover some func­ tion weeks to months after infection. About two-thirds of patients have residual neurologic sequelae.

Paralytic disease is more common among older individuals, preg­ nant women, and persons exercising strenuously or undergoing trauma at the time of CNS symptoms. Tonsillectomy predisposes to bulbar poliomyelitis, and IM injections increase the risk of paralysis in the involved limb(s).

VACCINE-ASSOCIATED POLIOMYELITIS  The risk of developing polio­ myelitis after oral vaccination is estimated at 1 case per 2.5 million doses. The risk is ~2000 times higher among immunodeficient per­ sons, especially persons with hypo- or agammaglobulinemia. Before 1997, an average of eight cases of vaccine-associated poliomyelitis occurred—in both vaccinees and their contacts—in the United States each year. With the change in recommendations first to a sequential regimen of inactivated poliovirus vaccine (IPV) and oral poliovirus vaccine (OPV) in 1997 and then to an all-IPV regimen in 2000, the number of cases of vaccine-associated polio declined. From 1997 to 1999, six such cases were reported in the United States followed by one case in 2022. POSTPOLIO SYNDROME  The postpolio syndrome presents as new onset of weakness, fatigue, fasciculations, and pain with additional atrophy of the muscle group involved during the initial paralytic disease 20–40 years earlier. The syndrome is more common among women and with increasing time after acute disease. The onset is usually insidious, and weakness occasionally extends to muscles that were not involved dur­ ing the initial illness. The prognosis is generally good; progression to further weakness is usually slow, with plateau periods of 1–10 years. The postpolio syndrome is thought to be due to progressive dysfunc­ tion and loss of motor neurons that compensated for the neurons lost during the original infection and not to persistent or reactivated poliovirus infection. PART 5 Infectious Diseases Other Enteroviruses  An estimated 5–10 million cases of symp­ tomatic disease due to enteroviruses other than poliovirus occur in the United States each year. Among neonates, enteroviruses are the most common cause of aseptic meningitis and nonspecific febrile illnesses. Certain clinical syndromes are more likely to be caused by certain serotypes (Table 210-1). NONSPECIFIC FEBRILE ILLNESS (SUMMER GRIPPE)  The most com­ mon clinical manifestation of enterovirus infection is a nonspecific febrile illness. After an incubation period of 3–6 days, patients present TABLE 210-1  Manifestations Commonly Associated with Enterovirus Serotypes SEROTYPE(S) OF INDICATED VIRUS ECHOVIRUS (E) AND ENTEROVIRUS (Ent) MANIFESTATION COXSACKIEVIRUS Acute hemorrhagic conjunctivitis A24 E70 Aseptic meningitis A2, 4, 7, 9, 10; B1–5 E4, 6, 7, 9, 11, 13, 16, 18, 19, 30, 33; Ent70, 71 Encephalitis A9; B1–5 E3, 4, 6, 7, 9, 11, 18, 25, 30; Ent71 Exanthem A4, 5, 6, 9, 10, 16; B1, 3–5 E4–7, 9, 11, 16–19, 25, 30; Ent71 Generalized disease of the newborn B1–5 E4–7, 9, 11, 14, 16, 18, 19 Hand-foot-and-mouth disease A5–7, 9, 10, 16; B1, 2, 5 Ent71 Herpangina A1–10, 16, 22; B1–5 E6, 9, 11, 16, 17, 25, 30; Ent71 Myocarditis, pericarditis A4, 9, 16; B1–5 E6, 9, 11, 22 Paralysis A4, 7, 9; B1–5 E2–4, 6, 7, 9, 11, 18, 30; EntD68, 70, 71 Pleurodynia A1, 2, 4, 6, 9, 10, 16; B1–6 E1–3, 6, 7, 9, 11, 12, 14, 16, 19, 24, 25, 30 Pneumonia A9, 16; B1–5 E6, 7, 9, 11, 12, 19, 20, 30; EntD68, 71

with an acute onset of fever, malaise, and headache. Occasional cases are associated with upper respiratory symptoms, and some cases include nausea and vomiting. Symptoms often last for 3–4 days, and most cases resolve in a week. While infections with other respiratory viruses occur more often from late fall to early spring, febrile illness due to enteroviruses frequently occurs in the summer and early fall. GENERALIZED DISEASE OF THE NEWBORN  Most serious enterovirus infections in infants develop during the first week of life, although severe disease can occur up to 3 months of age. Neonates often pres­ ent with an illness resembling bacterial sepsis, with fever, irritability, and lethargy. Laboratory abnormalities include leukocytosis with a left shift, thrombocytopenia, elevated values in liver function tests, and CSF pleocytosis. The illness can be complicated by myocarditis and hypotension, fulminant hepatitis and disseminated intravascular coagulation, meningitis or meningoencephalitis, or pneumonia. It may be difficult to distinguish neonatal enterovirus infection from bacterial sepsis, although a history of a recent virus-like illness in the mother provides a clue. ASEPTIC MENINGITIS AND ENCEPHALITIS  In children and young adults, enteroviruses are the cause of up to 90% of cases of aseptic meningitis in which an etiologic agent can be identified. Patients with aseptic meningitis typically present with an acute onset of fever, chills, headache, photophobia, and pain on eye movement. Nausea and vomiting also are common. Examination reveals meningismus without localizing neurologic signs; drowsiness or irritability also may be apparent. In some cases, a febrile illness may remit and return sev­ eral days later in conjunction with signs of meningitis. Other systemic manifestations may provide clues to an enteroviral cause, including diarrhea, myalgias, rash, pleurodynia, myocarditis, and herpangina. Examination of the CSF invariably reveals pleocytosis; the CSF cell count shows a shift from neutrophil to lymphocyte predominance within 1 day of presentation, and the total cell count does not exceed 1000/μL. The CSF glucose level is usually normal (in contrast to the low CSF glucose level in mumps), with a normal or slightly elevated protein concentration. Partially treated bacterial meningitis may be particu­ larly difficult to exclude in some instances. Enteroviral meningitis is more common in summer and fall in temperate climates, while viral meningitis of other etiologies is more common in winter and spring. Symptoms ordinarily resolve within a week, although CSF abnormali­ ties can persist for several weeks. Enteroviral meningitis is often more severe in adults than in children. Neurologic sequelae are rare, and most patients have an excellent prognosis. Enteroviral encephalitis is much less common than enteroviral aseptic meningitis. Occasional highly inflammatory cases of enteroviral men­ ingitis may be complicated by a mild form of encephalitis that is recog­ nized on the basis of progressive lethargy, disorientation, and sometimes seizures. Less commonly, severe primary encephalitis may develop. An estimated 10–35% of cases of viral encephalitis are due to enteroviruses. Immunocompetent patients generally have a good prognosis. Patients with hypogammaglobulinemia, agammaglobulinemia, or severe combined immunodeficiency may develop chronic meningitis or encephalitis; about half of these patients have a dermatomyositis-like syndrome, with peripheral edema, rash, and myositis. They may also have chronic hepatitis. Patients may develop neurologic disease while receiving immunoglobulin replacement therapy. Echoviruses (espe­ cially echovirus 11) are the most common pathogens in this situation. Paralytic disease due to enteroviruses other than poliovirus occurs sporadically and is usually less severe than poliomyelitis. Most cases are due to enterovirus 70 or 71 or to coxsackievirus A7 or A9. GuillainBarré syndrome is also associated with enterovirus infection. While earlier studies suggested a link between enteroviruses and chronic fatigue syndrome, most recent studies have not demonstrated such an association. ACUTE FLACCID MYELITIS  Patients with acute flaccid myelitis pres­ ent with fever or respiratory symptoms and progress within hours to a few days to flaccid paralysis in one or more limbs. The disease is much more frequent in children. Less commonly, the disease can affect

cranial nerves and respiratory or bulbar muscles. Like polio and some other enteroviruses, the disease affects the anterior horn cells in the spinal cord; gray matter changes can be seen on MRI of the spinal cord. The CSF shows a lymphocytic pleocytosis and often a mildly elevated protein. Cases of acute flaccid myelitis have occurred in late summer or early fall since 2012. Several studies have shown antibodies to entero­ viruses in the CSF; antibodies to enterovirus D68 are most frequently detected. While enterovirus D68 has been detected in respiratory, stool, and nasopharyngeal samples from patients with acute flaccid myelitis, the virus has been rarely detected in the CSF. Treatment is supportive, and most patients have persistent neurologic deficits. PLEURODYNIA (BORNHOLM DISEASE)  Patients with pleurodynia present with an acute onset of fever and spasms of pleuritic chest or upper abdominal pain. Chest pain is more common in adults, and abdominal pain is more common in children. Paroxysms of severe, knifelike pain usually last 15–30 min and are associated with diaphore­ sis and tachypnea. Fever peaks within an hour after the onset of parox­ ysms and subsides when pain resolves. The involved muscles are tender to palpation, and a pleural rub may be detected. The white blood cell count and chest x-ray results are usually normal. Most cases are due to coxsackievirus B and occur during epidemics. Symptoms resolve in a few days, and recurrences are rare. Treatment includes the administra­ tion of nonsteroidal anti-inflammatory agents or the application of heat to the affected muscles. MYOCARDITIS AND PERICARDITIS  Enteroviruses are estimated to cause up to one-third of cases of acute myocarditis. Coxsackievirus B and its RNA have been detected in pericardial fluid and myocardial tissue in some cases of acute myocarditis and pericarditis. Most cases of enteroviral myocarditis or pericarditis occur in newborns, ado­ lescents, or young adults. More than two-thirds of patients are male. Patients often present with an upper respiratory tract infection that is followed by fever, chest pain, dyspnea, arrhythmias, and occasionally heart failure. A pericardial friction rub is documented in half of cases, and the electrocardiogram shows ST-segment elevations or ST- and T-wave abnormalities. Serum levels of myocardial enzymes are often elevated. Neonates commonly have severe disease, while older children and adults recover completely. Up to 10% of cases progress to chronic dilated cardiomyopathy. Chronic constrictive pericarditis also may be a sequela. EXANTHEMS  Enterovirus infection is the leading cause of exanthems in chil­ dren in the summer and fall. While exanthems are associated with many enteroviruses, certain types have been linked to specific syndromes. Echovi­ ruses 9 and 16 have frequently been associated with exanthem and fever. Rashes may be discrete or confluent, beginning on the face and spreading to the trunk and extremities. Echovirus 9 is the most common cause of a rubel­ liform (discrete) rash. Unlike the rash of rubella, the enteroviral rash occurs in the summer and is not associated with lymphadenopathy. Roseola-like rashes develop after defervescence, with mac­ ules and papules on the face and trunk. The Boston exanthem, caused by echo­ virus 16, is a roseola-like rash. A variety of other rashes have been associated with enteroviruses, including erythema multiforme (see Fig. A1-24A) and vesicular, urticarial, petechial, bullous, or purpuric lesions. Enanthems also occur, including lesions that resemble the Koplik’s spots seen with measles (see Fig. A1-2). A B C D FIGURE 210-1  Vesicular eruptions of the hand (A), knee (B), and mouth (C) of a 6-year-old boy with coxsackievirus A6 infection. Several of his fingernails were shed 2 months later (D). (Images reprinted courtesy of Centers for Disease Control and Prevention/Emerging Infectious Diseases.)

HAND-FOOT-AND-MOUTH DISEASE (FIG. 210-1)  After an incubation period of 4–6 days, patients with hand-foot-and-mouth disease pres­ ent with fever, anorexia, and malaise; these manifestations are followed by the development of sore throat and vesicles (see Fig. A1-22) on the buccal mucosa and often on the tongue and then by the appearance of tender vesicular lesions on the dorsum of the hands, sometimes with involvement of the palms. The vesicles may form bullae and quickly ulcerate. About one-third of patients also have lesions on the palate, uvula, or tonsillar pillars, and one-third have a rash on the feet (includ­ ing the soles) or on the buttocks. Generalized rashes also have been reported. The disease is highly infectious, with attack rates of close to 100% among young children. The lesions usually resolve in 1 week. Most cases are due to coxsackievirus A16 or enterovirus 71.

An epidemic of enterovirus 71 infection in Taiwan in 1998 resulted in thousands of cases of hand-foot-and-mouth disease or herpangina (see below). Severe complications included CNS disease, myocarditis, and pulmonary hemorrhage. About 90% of those who died were children ≤5 years old, and death was associated with pulmonary edema or pulmonary hemorrhage. CNS disease included aseptic meningitis, flaccid paralysis (similar to that seen in poliomyelitis), and rhomb­ encephalitis with myoclonus and tremor or ataxia. The mean age of patients with CNS complications was 2.5 years, and MRI in cases with encephalitis usually showed brainstem lesions. Follow-up of children at 6 months showed persistent dysphagia, cranial nerve palsies, hypoven­ tilation, limb weakness, and atrophy; at 3 years, persistent neurologic sequelae were documented, with delayed development and impaired cognitive function. Yearly epidemics of enterovirus 71 infection have occurred in China since 2008, with thousands of cases and hundreds of deaths each year. Infections have been associated with fever, rash, brainstem encephalitis with myoclonic jerks, and limb trembling; some cases have progressed to seizures and coma. Lung findings include pulmonary edema and hemorrhage. While the level of creatine kinase MB is sometimes elevated, myocardial necrosis generally is not found. CHAPTER 210 Enterovirus, Parechovirus,and Reovirus Infections Cyclic epidemics occur every 2–3 years in other Asian countries. However, the virus circulates at lower rates in the United States, Europe, and Africa. In the United States, hand-foot-and-mouth disease is most commonly associated with coxsackievirus A16. Between November 2011 and February 2012, outbreaks of hand-foot-and-mouth disease

due to coxsackievirus A6 occurred in several U.S. states, and 19% of the affected persons were hospitalized.

HERPANGINA  Herpangina is usually caused by coxsackievirus A and presents as acute-onset fever, sore throat, odynophagia, and grayishwhite papulovesicular lesions on an erythematous base that ulcerate. The lesions can persist for weeks; are present on the soft palate, anterior pillars of the tonsils, and uvula; and are concentrated in the posterior portion of the mouth. In contrast to herpes stomatitis, enteroviral herpangina is not associated with gingivitis. Acute lymphonodular pharyngitis associated with coxsackievirus A10 presents as white or yellow nodules surrounded by erythema in the posterior oropharynx. The lesions do not ulcerate. ACUTE HEMORRHAGIC CONJUNCTIVITIS  Patients with acute hemor­ rhagic conjunctivitis present with an acute onset of severe eye pain, blurred vision, photophobia, and watery discharge from the eye. Examination reveals edema, chemosis, and subconjunctival hemor­ rhage and often shows punctate keratitis and conjunctival follicles as well (Fig. 210-2). Preauricular adenopathy is often found. Epidemics and nosocomial spread have been associated with enterovirus 70 and coxsackievirus A24. Outbreaks have been due to coxsackievirus A24 in China and India (2010), Japan (2011), and Thailand (2014). Systemic symptoms, including headache and fever, develop in 20% of cases, and recovery is usually complete in 10 days. The sudden onset and short duration of the illness help to distinguish acute hemorrhagic conjunc­ tivitis from other ocular infections, such as those due to adenovirus and Chlamydia trachomatis. Paralysis has been associated with some cases of acute hemorrhagic conjunctivitis due to enterovirus 70 during epidemics. PART 5 Infectious Diseases OTHER MANIFESTATIONS  Enteroviruses are an infrequent cause of childhood pneumonia and the common cold. From mid-August 2014 to January 2015, enterovirus D68 infection was confirmed in more than 1000 persons with mild to severe respiratory illnesses in 49 U.S. states. Nearly all reported cases were in children, many of whom had asthma. Severe respiratory illness due to enterovirus D68 continues to occur; case numbers were lower in 2020, likely due to mitigation efforts for COVID-19, but rose in 2022. A prospective study of >300 children showed that prolonged shedding of enteroviruses in the stool was asso­ ciated with development of islet cell autoantibodies and type 1 diabetes. Coxsackievirus B has been isolated at autopsy from the pancreas of a few children presenting with type 1 diabetes mellitus; however, most attempts to isolate the virus have been unsuccessful. Other diseases that have been associated with enterovirus infection include parotitis, bronchitis, bronchiolitis, croup, infectious lymphocytosis, polymyosi­ tis, acute arthritis, and acute nephritis. ■ ■DIAGNOSIS Isolation of enterovirus in cell culture had been the traditional diag­ nostic procedure; PCR is used now more often. Cultures of stool, FIGURE 210-2  Acute hemorrhagic conjunctivitis due to enterovirus 70. (Image reprinted courtesy of Jerri Ann Jenista, MD.)

nasopharyngeal, or throat samples from patients with enterovirus diseases do not prove that the virus is directly associated with disease because these sites are frequently colonized for weeks in patients with subclinical infections. Isolation of virus from the throat is more likely to be associated with disease than is isolation from the stool since virus is shed for shorter periods from the throat. Cultures of CSF, serum, fluid from body cavities, or tissues are positive less frequently, but a positive result is indicative of disease caused by enterovirus. Cultures are more likely to be positive earlier than later in the course of infec­ tion. Cultures may be negative because of the presence of neutralizing antibody, lack of susceptibility of the cells used, or inappropriate han­ dling of the specimen. Coxsackievirus A may require inoculation into special cell-culture lines or into suckling mice. Identification of the enterovirus serotype is useful primarily for epi­ demiologic studies and, with a few exceptions, has little clinical utility. It is important to identify serious infections with enterovirus during epidemics and to distinguish the vaccine strain of poliovirus from the other enteroviruses in the throat or in the feces. Stool and throat samples for culture as well as acute- and convalescent-phase serum specimens should be obtained from all patients with suspected polio­ myelitis. In the absence of a positive CSF culture, a positive culture of stool obtained within the first 2 weeks after the onset of symptoms is most often used to confirm the diagnosis of poliomyelitis. If poliovi­ rus infection is suspected, two or more fecal and throat swab samples should be obtained at least 1 day apart and cultured for enterovirus as soon as possible. If poliovirus is isolated, it should be sent to the CDC for identification as either wild-type or vaccine virus. Reverse-transcription polymerase chain reaction (PCR) has been used to amplify viral nucleic acid from CSF, serum, urine, stool, conjunctiva, throat swabs, and tissues. A pan-enterovirus PCR assay can detect all human enteroviruses. With the proper controls, PCR of the CSF is highly sensitive (70–100%) and specific (>80%) and is more rapid than culture. PCR of the CSF is less likely to be positive when patients present ≥3 days after the onset of meningitis or with enterovirus 71 infection; in these cases, PCR of throat or rectal swabs— although less specific than PCR of CSF—should be considered. PCR of serum is also highly sensitive and specific in the diagnosis of disseminated disease. PCR may be particularly helpful for the diag­ nosis and follow-up of enterovirus disease in immunodeficient patients receiving immunoglobulin therapy, whose viral cultures may be nega­ tive. Antigen detection is less sensitive than PCR. Serologic diagnosis of enterovirus infection is limited by the large number of serotypes and the lack of a common antigen. Demonstra­ tion of seroconversion may be useful in rare cases for confirmation of culture results, but serologic testing is usually limited to epidemiologic studies. Serum should be collected and frozen soon after the onset of disease and again ~4 weeks later. Measurement of neutralizing titers is the most accurate method for antibody determination; measurement of complement-fixation titers is usually less sensitive. Titers of virusspecific IgM are elevated in both acute and chronic infection. TREATMENT Enterovirus Infections Most enterovirus infections are mild and resolve spontaneously; however, intensive supportive care may be needed for cardiac, hepatic, or CNS disease. IV, intrathecal, or intraventricular immu­ noglobulin has been used with apparent success in some cases for the treatment of chronic enterovirus meningoencephalitis and dermatomyositis in patients with hypogammaglobulinemia or agammaglobulinemia. The disease may stabilize or resolve during therapy; however, some patients decline inexorably despite therapy. IV immunoglobulin often prevents severe enterovirus disease in these patients. IV administration of immunoglobulin with high titers of antibody to the infecting virus has been used in some cases of life-threatening infection in neonates, who may not have maternally acquired antibody. In one trial involving neonates with enterovirus infections, immunoglobulin containing very high titers

of antibody to the infecting virus reduced rates of viremia; how­ ever, the study was too small to show a substantial clinical benefit. The level of enteroviral antibodies varies with the immunoglobu­ lin preparation. While a phase 2 trial of pleconaril for neonatal enterovirus sepsis showed that the time to serum PCR negativity was reduced and the survival rate increased in newborns who had confirmed enterovirus infections and were treated with the drug, the differences did not reach significance and the drug is not avail­ able on a compassionate-use basis. Pocapavir and vapendavir also are being tested for enterovirus infections; resistance developed rapidly to OPV in a clinical trial of pocapavir. Glucocorticoids are contraindicated. Good hand-washing practices and the use of gowns and gloves are important in limiting nosocomial transmission of enteroviruses during epidemics. Enteric precautions are indicated for 7 days after the onset of enterovirus infections. Inactivated enterovirus 71 vac­ cines have been licensed in China. ■ ■PREVENTION AND ERADICATION OF POLIOVIRUS (See also Chap. 129) After a peak of 57,879 cases of poliomyelitis in the United States in 1952, the introduction of IPV in 1955 and of OPV in 1961 ultimately eradicated disease due to wild-type poliovirus in the Western Hemisphere. Such disease has not been documented in the United States since 1979, when cases occurred among religious groups who had declined immunization. In the Western Hemisphere, paralysis due to wild-type poliovirus was last documented in 1991. Paralysis due to vaccine-derived poliovirus (VDPV) was reported in 2022 in New York (see below). In 1988, when ~350,000 cases of polio occurred in 125 countries, the World Health Organization adopted a resolution to eradicate poliomyelitis by the year 2000. Wild-type poliovirus type 2 and wildtype poliovirus type 3 were declared eradicated in 2015 and 2019, respectively. The Americas were certified free of indigenous wild-type poliovirus transmission in 1994, the Western Pacific Region in 2000, the European Region in 2002, and Southeast Asia in 2014. After a nadir of 496 cases in 2001, 21 countries that had previously been free of polio reported cases imported from 6 polio-endemic countries in 2002–2005. By 2006, polio transmission had been reduced in most of these 21 countries. In 2017, there were 22 cases of wild-type polio, the lowest ever reported for 1 year—all of these cases were from Pakistan and Afghanistan. In 2021 wild-type polio remerged in Africa. After another peak of 176 cases in 2019, the number of cases of wild-type polio had fallen to 12 in 2023, all from 2 countries and all due to polio type 1 (Table 210-2). Polio is a source of concern for unimmunized or partially immunized travelers. Clearly, global eradication of polio is necessary to eliminate the risk of importation of wild-type virus. TABLE 210-2  Laboratory-Confirmed Cases of Poliomyelitis in 2023 COUNTRY WILD-TYPE POLIO VACCINE-DERIVED POLIO Pakistan

Afghanistan

Democratic Republic of the Congo

Nigeria

Chad

Guinea

Madagascar

Mali

Central African Republic

Others

61a Total

aOthers with <10 cases; Kenya, Somalia, Yemen 8 cases; Cote d’Ivoire, Indonesia 6 cases; Mozambique 5 cases; Benin, Burkina Fossa, Niger, South Sudan 3 cases; United Republic of Tanzania 2 cases; Burundi, Ethiopia, Israel, Mauritania, Zambia, Zimbabwe 1 case each.

Outbreaks are thought to have been facilitated by suboptimal rates of vaccination, isolated pockets of unvaccinated children, poor sanitation and crowding, improper vaccine-storage conditions, and a reduced level of response to one of the serotypes in the vaccine. While the global eradication campaign has markedly reduced the number of cases of endemic polio, doubts have been raised as to whether eradication is a realistic goal, given the large number of asymptomatic infections and the political instability in developing countries.

Use of OPV, especially in areas with low vaccination rates, has been associated with vaccine-derived polio due to mutations that result in restoration of viral fitness and neurovirulence during prolonged replication in individuals or person-to-person transmission. Vaccinederived polio was recognized in Egypt in 1983–1993, and hundreds of cases have been reported in many countries, including 385 cases in Nigeria in 2005–2012. Epidemics have been rapidly terminated after intensive vaccination with OPV. In 2005, a case of vaccine-derived polio occurred in an unvaccinated U.S. woman returning from a visit to Central and South America. In the same year, an unvaccinated immunocompromised infant in Minnesota was found to be shedding VDPV; further investigation identified 4 of 22 infants in the same com­ munity who were shedding the virus. All 5 infants were asymptomatic. These outbreaks emphasize the need for maintaining high levels of vaccine coverage and continued surveillance for circulating virus. In 2016, only 5 cases were reported; however, this number increased with a peak of 1113 cases in 2020 and subsequently declined to 526 cases of vaccine-derived polio in 2023 from 24 countries; 96% of these cases were from Africa and 3% were from the Eastern Mediterranean Region (Table 210-2). This decline in VDPV type 2 is associated with the use of a safer, novel type 2 OPV (see below). A case of vaccine-derived polio type 2 occurred in an unvaccinated adult in New York in 2022 and was genetically linked to wastewater collected in the area about 1 month before and after the case; the patient had not traveled internationally during the incubation period. VDPV due to OPV2 has been detected in wastewater from other countries in 2022, including Canada, Israel, and the United Kingdom, where virus transmission has been elimi­ nated. From 2018 to March 2020, 92% of cases of vaccine-derived polio were due to type 2 virus. Cessation of vaccination with type 2 OPV is believed to be responsible for this increase in polio type 2. IPV is used in most industrialized countries and OPV in most developing coun­ tries, including those in which polio still is or recently was endemic. While IM injections of other vaccines (live or attenuated) can be given concurrently with OPV, unnecessary IM injections should be avoided during the first month after OPV vaccination because they increase the risk of vaccine-associated paralysis. Since 1988, an enhanced-potency inactivated poliovirus vaccine has been available in the United States. CHAPTER 210 Enterovirus, Parechovirus,and Reovirus Infections After several doses of OPV alone, the seropositivity rate for indi­ vidual poliovirus serotypes may still be suboptimal for children in developing countries; one or more supplemental doses of IPV can increase the rate of seropositivity for these serotypes. Against a given serotype, monovalent OPV containing only that serotype is more immunogenic than trivalent vaccine because of a lack of interference from other serotypes. Given the eradication of wild-type poliovirus type 2 and the establishment of OPV type 2 as the primary cause of vaccine-derived polio, bivalent OPV (types 1 and 3), which had been shown to be superior to trivalent OPV in inducing antibodies to types 1 and 3, replaced trivalent OPV vaccine in April 2016. However, outbreaks of vaccine-derived polio due to polio type 2 have required vaccination with monovalent OPV type 2. A novel type 2 oral polio­ virus vaccine (nOPV2) was engineered to be impaired in reversion to neurovirulence. After ~700 million doses of nOPV2 were administered in response to circulating VDPV type 2, from 2021 to 2023, 61 cases of paralysis were associated with nOPV2, with all the cases in Africa. The rate of emergence of paralysis with nOPV2 is about 10-fold lower than with monovalent OPV2. Another approach to reduce VDPV is to vac­ cinate with IPV followed by OPV. Addition of at least one dose of tri­ valent IPV after immunization with bivalent OPV will also reduce the risk of vaccine-derived polio associated with type 2 virus and enhance immunity to poliovirus types 1 and 3. Accordingly, in 2016, ~90% of countries included trivalent IPV in their immunization schedules.

103 - 211 Measles (Rubeola)

211 Measles (Rubeola)

TABLE 210-3  Recommendations for Poliovirus Vaccination of Adults

  1. Most adults in the United States have little risk for exposure to polioviruses, and most are immune as a result of vaccination during childhood. Vaccination with IPV is recommended for those at greater risk for exposure to polioviruses than the general population: a. travelers to areas or countries where polio is epidemic or endemic; b. laboratory workers who handle specimens that might contain polioviruses; c. health care workers or other caregivers who have close contact with patients who might be excreting wild-type polioviruses; and d. adults who are identified by public health authorities as being part of a group or population at increased risk because of an outbreak.
  2. Adults who are unvaccinated or whose vaccination status is unknown and who are at increased risk should receive three doses of IPV. Two doses of IPV should be administered at intervals of 4–8 weeks; a third dose should be administered 6–12 months after the second.
  3. Adults who have had a primary series of polio vaccine and who are at increased risk should receive another dose of IPV. Currently, data do not indicate a need for more than a single lifetime booster dose with IPV for adults. Abbreviation: IPV, inactivated poliovirus vaccine. Source: From https://www.cdc.gov/vaccines/vpd/polio/hcp/recommendations.html. As the frequency of wild-type polio declines and reports of polio asso­ ciated with circulating VDPV increase, the World Health Organization is investigating whether IPV can be produced from OPV strains that require less biocontainment, ultimately replacing OPV. OPV and IPV induce antibodies that persist for at least 5 years. Both vaccines induce IgG and IgA antibodies. Compared with recipients of IPV, recipients of OPV shed less virus and less frequently develop reinfection with wild-type virus after exposure to poliovirus. Although IPV is safe and efficacious, OPV offers the advantages of ease of admin­ istration, lower cost, and induction of intestinal immunity resulting in a reduction in the risk of community transmission of wild-type virus. Because of progress toward global eradication of polio and the contin­ ued occurrence of cases of vaccine-associated polio, an all-IPV regi­ men was recommended in 2000 for childhood poliovirus vaccination in the United States, with vaccine administration at 2, 4, and 6–18 months and 4–6 years of age. The risk of vaccine-associated polio should be discussed before OPV is administered. Recommendations for vaccina­ tion of adults are listed in Table 210-3. PART 5 Infectious Diseases There are concerns about discontinuing vaccination in the event that endemic spread of poliovirus is eliminated. Among the reasons for these concerns are that poliovirus is shed from some immunocom­ promised persons for >25 years, that vaccine-derived poliovirus can circulate and cause disease, and that wild-type poliovirus is present in research laboratories and vaccine manufacturing facilities. Antivirals and monoclonal antibodies are in development to reduce or terminate shedding of poliovirus by long-term virus excretors. Pocapavir was shown to reduce shedding of OPV type 1 in a clinical trial, but rapid development of resistance with virus transmission, despite reduced shedding, indicates that combination therapy with other antivirals and/ or monoclonal antibodies will be needed. PARECHOVIRUSES Human parechoviruses (HPeVs), like enteroviruses, are members of the family Picornaviridae. The 19 serotypes of HPeV commonly cause infections in early childhood. Infections with HPeV type 1 (HPeV-1) occur throughout the year, while other parechovirus infections occur more commonly in summer and fall. Infections with HPeVs present similarly to those due to enteroviruses and may cause generalized disease of the newborn, aseptic meningitis, encephalitis, seizures, paralysis, exanthems, respiratory tract disease, rash, hepatitis, and gas­ troenteritis. While HPeV-1 is the most common serotype and generally causes mild disease, deaths of infants in the United States have been associated with HPeV-1, HPeV-3, and HPeV-6. HPeVs can be isolated from the same sites as enteroviruses, including the nasopharynx, stool, and respiratory tract secretions. PCR using pan-enterovirus primers does not detect HPeVs, and while PCR assays are performed by the

CDC and research laboratories, many commercial laboratories do not perform the test. REOVIRUSES Reoviruses are double-stranded RNA viruses encompassing three sero­ types. Serologic studies indicate that most humans are infected with reoviruses during childhood. Most infections either are asymptomatic or cause mild upper respiratory tract symptoms. Reovirus is consid­ ered a rare cause of mild gastroenteritis or meningitis in infants and children. Speculation regarding an association of reovirus type 3 with idiopathic neonatal hepatitis and extrahepatic biliary atresia is based on an elevated prevalence of antibody to reovirus in some affected patients and the detection of viral RNA by PCR in hepatobiliary tissues in some studies. Orthoreoviruses have been associated with human disease—e.g., Melaka and Kampar viruses with fever and acute respi­ ratory disease in Malaysia and Nelson Bay virus with acute respiratory disease in a traveler from Bali. ■ ■FURTHER READING Lee SE et al: Progress toward poliomyelitis eradication – Worldwide, January 2021–March 2023. MMWR Morb Mortal Wkly Rep 72:517, 2023. Link-Gelles R et al: Public health response to a case of paralytic poliomyelitis in an unvaccinated person and detection of poliovirus in wastewater - New York, June–August 2022. Morb Mortal Wkly Rep 19;1065, 2022. Ma KC et al: Increase in acute respiratory illnesses among children and adolescents associated with rhinoviruses and enteroviruses, including enteroviris D68–United States, July–September 2022. MMWR Morb Mortal Wkly Rep 71:1265, 2022. Pallansch MA: Circulating poliovirus in New York—new instance of an old problem. N Engl J Med 387:1725, 2022. Tomatis Souverbielle C et al: Update on nonpolio enterovirus and parechovirus infections in neonates and young infants. Curr Opin Pediatr 35:380, 2023. Alex C. Kong, William J. Moss

Measles (Rubeola) ■ ■DEFINITION Measles is a highly contagious viral disease characterized by a pro­ dromal illness of fever, cough, coryza, and conjunctivitis followed by the appearance of a generalized maculopapular rash. Before the wide­ spread use of measles vaccines, it was estimated that measles caused

2 million deaths worldwide each year. ■ ■GLOBAL CONSIDERATIONS Remarkable progress has been made in reducing global measles incidence and mortality rates through measles vaccination. In the Americas, intensive vaccination and surveillance efforts—based in part on the successful Pan American Health Organization strategy of periodic nationwide measles vaccination campaigns (supplementary immunization activities [SIAs])—and high levels of routine measles vaccine coverage interrupted endemic transmission of measles virus. The World Health Organization’s (WHO’s) Region of the Americas was declared to have eliminated measles in September 2016—the first region in the world to do so. However, endemic measles virus trans­ mission was reestablished, and the region lost its measles elimination status. As such, no WHO region has achieved and sustained measles elimination status, highlighting the importance of maintaining high measles vaccination coverage.

In the United States, high-level coverage with two doses of measles vaccine eliminated endemic measles virus transmission in 2000. How­ ever, imported cases and low measles vaccine coverage in some com­ munities threaten this goal. The 1274 measles cases reported in the United States in 2019 represent the highest count since 1992. Histori­ cally low levels of measles cases were reported early after the COVID-19 pandemic began but were not sustained, with resurgence of measles globally and in the United States. Progress also has been made in reducing measles incidence and mortality rates in sub-Saharan Africa and Asia because of increasing routine measles vaccine coverage and provision of a second dose of measles vaccine through mass measles vaccination campaigns and routine childhood immunization programs. From 2000 to 2022, the estimated annual number of global measles deaths per year decreased 82%, from 772,854 (95% confidence interval [CI]: 580,969−1,064,580) to 136,216 (95% CI: 97,058−190,234). Measles vaccination prevented an estimated 57.2 million deaths over this period. Despite this progress, the fact that >100,000 children die each year from a preventable disease such as measles attests to the need for greater resources and efforts to identify and reach unvaccinated children. The COVID-19 pandemic caused severe disruptions to immuniza­ tion activities, further threatening progress toward measles control and elimination. Almost 40 million children were estimated to have missed a dose of measles vaccine in 2021, including 25 million children who missed their first dose. Large-scale measles outbreaks occurred in 22 countries in 2021 and 37 countries in 2022. Over the same period, the estimated number of measles cases and deaths increased by 18% and 43%, respectively. The Measles and Rubella Partnership (MRP)—formerly the Measles and Rubella Initiative (MRI)—is working to improve immunization coverage and address setbacks caused by the pandemic. Since its incep­ tion in 2001, MRI has played an important role in reducing global measles incidence and mortality rates, providing governments and communities in 88 countries with technical and financial support for routine immunization activities, mass vaccination campaigns, and dis­ ease surveillance systems. In 2023, MRI was rebranded as MRP, and the partnership that had historically been led by the American Red Cross, the United Nations Foundation, UNICEF, and the U.S. Centers for Disease Control and Prevention (CDC) expanded to formally include longtime partners Gavi, the Vaccine Alliance and the Bill and Melinda Gates Foundation as core partners. ■ ■ETIOLOGY Measles virus is a spherical, nonsegmented, single-stranded, negativesense RNA virus and a member of the Morbillivirus genus in the family Paramyxoviridae. Measles was originally a zoonotic infection, arising from animal-to-human transmission of an ancestral morbillivirus thousands of years ago, when human populations attained sufficient size to sustain virus transmission. Although RNA viruses typically have high mutation rates, measles virus is an antigenically monotypic virus, i.e., the surface proteins responsible for inducing protective immunity retained their antigenic structure across time and distance because of their key role in binding cellular receptors. The public health signifi­ cance of this stability is that measles vaccines developed decades ago from a single strain of measles virus remain protective worldwide. Both wild-type and attenuated measles viruses are inactivated by ultraviolet light and heat, necessitating a cold chain for vaccine transport and storage. ■ ■EPIDEMIOLOGY Measles virus is one of the most highly contagious directly transmit­ ted pathogens. Outbreaks can occur in populations in which <10% of persons are susceptible. Chains of transmission are common among household contacts, school-age children, and health care workers. There are no latent or persistent measles virus infections that result in a prolonged infectious period, nor are there animal reservoirs for the virus. Thus, measles virus can be maintained in human populations only by an unbroken chain of acute infections, which requires a contin­ uous supply of susceptible individuals. Newborns become susceptible

to measles virus infection when passively acquired maternal antibodies are lost, generally before 6–9 months of age. When not immunized, these infants account for the bulk of new susceptible individuals that sustain measles virus transmission.

Endemic measles has a typical temporal pattern characterized by yearly seasonal epidemics superimposed on longer epidemic cycles of 2–5 years or more. In temperate climates, annual measles outbreaks typically occur in the late winter and early spring. These annual out­ breaks are probably attributable to social networks facilitating trans­ mission (e.g., congregation of children at school) and environmental factors favoring the viability and transmission of measles virus. Measles cases continue to occur during interepidemic periods in large popula­ tions but at low incidence. The longer epidemic cycles occurring every several years result from the accumulation of susceptible persons over successive birth cohorts and the subsequent decline in the number of susceptibles following an outbreak. Secondary attack rates among susceptible household and institu­ tional contacts generally exceed 90%. The average age at which measles occurs depends on rates of contact with infected persons, protective maternal antibody decline, and vaccine coverage. In densely popu­ lated urban settings with low-level vaccination coverage, measles is a disease of infants and young children. The cumulative incidence can reach 50% by 1 year of age, with a significant proportion of children acquiring measles before 9 months—the age at which the first of two routine vaccine doses are administered in many countries, in line with the schedule recommended by the WHO’s Expanded Programme on Immunization. As measles vaccine coverage increases or population density decreases, the age distribution shifts toward older children. In such situations, measles cases predominate in school-age children. Infants and young children, although susceptible if not protected by maternal antibodies or vaccination, are not exposed to measles virus at a rate sufficient to cause a heavy disease burden in this age group. As vaccination coverage increases further, the age distribution of cases may be shifted into adolescence and adulthood. This distribution is seen in measles outbreaks in the United States and necessitates targeted measles vaccination programs for these older age groups. Some coun­ tries have a bimodal distribution, with measles cases predominantly in young infants and adults. CHAPTER 211 Measles (Rubeola) Persons with measles are infectious for several days before and after the onset of rash, when levels of measles virus in blood and body fluids are highest and when cough, coryza, and sneezing that facilitate virus spread are most severe. The contagiousness of measles before the onset of recognizable disease hinders the effectiveness of isolation measures. Medical settings are well-recognized sites of measles virus trans­ mission. Children may present to health care facilities during the prodrome, when the diagnosis is not obvious, although the child is infectious and is likely to infect susceptible contacts. Susceptible health care workers can acquire measles from infected children and transmit measles virus to others. Nosocomial transmission can be reduced by maintenance of a high index of clinical suspicion particularly during outbreaks, use of appropriate isolation precautions when measles is suspected, administration of measles vaccine to susceptible children and health care workers, and documentation of health care workers’ immunity to measles (i.e., proof of receipt of two doses of measles vaccine or detection of IgG antibodies to measles virus). As efforts at measles control are increasingly successful, public per­ ceptions of the risk of measles diminish and may be replaced by con­ cerns about possible adverse events associated with measles vaccine. Consequently, measles outbreaks have occurred because of opposition to vaccination on religious or philosophical grounds or unfounded fears of serious adverse events (see “Active Immunization,” below, and Chap. 3). ■ ■PATHOGENESIS Measles virus is transmitted primarily by respiratory droplets over short distances and, less commonly, by small-particle aerosols that remain suspended in the air for long periods. Airborne transmission appears to be important in certain settings, including schools, physicians’ offices, hospitals, and enclosed public places. The virus can

be transmitted by direct contact with infected secretions but does not survive for long on fomites.

The incubation period for measles is ~10 days to fever onset and 14 days to rash onset. This period may be shorter in infants and longer (up to 3 weeks) in adults. Infection is initiated when measles virus is deposited in the respiratory tract, oropharynx, or conjunctivae (Fig. 211-1A). During the first 2–4 days after infection, measles virus proliferates locally in the respiratory mucosa, primarily in dendritic cells and lymphocytes, and spreads to draining lymph nodes. Virus then enters the bloodstream by budding from infected lymphocytes, producing the viremia that disseminates infection throughout the body. Replication of measles virus in the target organs, together with the host’s immune response, are responsible for the signs and symp­ toms of measles that occur 8–12 days after infection and mark the end of the incubation period (Fig. 211-1B). Thymus Liver Skin Virus titer (pfu) Severity of clinical symptoms Lung Respiratory epithelium Local lymph nodes Blood Spleen Lymphatic tissue PART 5 Infectious Diseases

A Days after infection Rash Conjunctivitis Cough Fever Koplik’s spots Koplik’s spots

B Days after infection CD4+ T cells Immune suppression CD8+ T cells IgM IgG Immune response

Days after infection C FIGURE 211-1  Measles virus infection: pathogenesis, clinical features, and immune responses. A. Spread of measles virus, from initial infection of the respiratory tract through dissemination to the skin. B. Appearance of clinical signs and symptoms, including Koplik’s spots and rash. C. Antibody and T-cell responses to measles virus. The signs and symptoms of measles arise coincident with the host immune response. (Reproduced with permission from WJ Moss and DE Griffin: Global measles elimination. Nat Rev Microbiology 4:900, 2006.)

■ ■IMMUNE RESPONSES Host immune responses to measles virus are essential for viral clearance, clinical recovery, and the establishment of long-term protective immunity (Fig. 211-1C). Early nonspecific (innate) immune responses during the prodromal phase include activation of natural killer cells and increased production of antiviral proteins. The adaptive immune responses consist of measles virus–specific antibody and cellular responses. The protective efficacy of antibodies to measles virus is illustrated by the immunity conferred to infants from passively acquired maternal antibodies and the protection of exposed, susceptible individuals after administration of anti–measles virus immunoglobulin. The first measles virus–specific antibodies produced after infection are of the IgM subtype, with a sub­ sequent switch to predominantly IgG1 and IgG3 isotypes. The IgM anti­ body response is typically absent following reexposure or revaccination and serves as a marker of primary infection. The importance of cellular immunity to measles virus is demon­ strated by the ability of children with agammaglobulinemia (congenital inability to produce antibodies) to recover fully from measles and the contrasting picture for children with severe defects in T lymphocyte function who often develop severe or fatal disease (Chap. 362). The initial predominant TH1 response (characterized by interferon-γ) is essential for viral clearance, and the later TH2 response (characterized by interleukin-4) promotes the development of measles virus–specific antibodies that are critical for protection against reinfection. The duration of protective immunity following wild-type measles virus infection is generally thought to be lifelong. Immunologic mem­ ory to measles virus includes both continued production of measles virus–specific antibodies by long-lived plasma cells and memory B cells as well as circulation of measles virus–specific CD4+ and CD8+ T lymphocytes. However, the intense immune responses induced by measles virus infection are paradoxically associated with depressed responses to unrelated (non–measles virus) antigens. This state of immunosuppres­ sion persists for at least several weeks to months beyond resolution of the acute illness, enhances susceptibility to secondary infections with bacteria and viruses that cause pneumonia and diarrhea, and is thus responsible for a substantial proportion of measles-related morbidity and deaths. Delayed-type hypersensitivity responses to recall antigens, such as tuberculin, are suppressed, and cellular and humoral responses to new antigens are impaired. Reactivation of latent tuberculosis and remission of autoimmune diseases after measles have been described and are attributed to this period of immune suppression. Importantly, measles results in reductions in the magnitude and diversity of anti­ bodies against previously encountered viral and bacterial antigens, impairing immunologic memory. This mechanism may explain why child morbidity and mortality due to other infectious diseases may be increased for >2 years after measles. APPROACH TO THE PATIENT Measles Clinicians should consider measles in persons presenting with fever and generalized erythematous rash, particularly when measles virus is known to be circulating or the patient has a history of travel to endemic areas. Appropriate precautions must be taken to prevent nosocomial transmission. The diagnosis requires laboratory confirmation except during large outbreaks in which an epidemiologic link to a confirmed case can be established. Care is largely supportive and consists of the administration of vitamin A and antibiotics for secondary bacterial infections (see “Treatment,” below). Complications of measles, including bacterial infections and encephalitis, may occur after acute illness and require careful monitoring, particularly in immunocompromised persons. ■ ■CLINICAL MANIFESTATIONS In most persons, the signs and symptoms of measles are highly char­ acteristic (Fig. 211-1B). Fever and malaise beginning ~10 days after exposure are followed by cough, coryza, and conjunctivitis. These

signs and symptoms increase in severity over 4 days. Koplik’s spots (see Fig. A1-2) develop on the buccal mucosa ~2 days before the rash appears. Koplik’s spots are pathognomonic of measles and consist of bluish white dots ~1 mm in diameter surrounded by erythema. The lesions appear first on the buccal mucosa opposite the lower molars but rapidly increase in number and may involve the entire buccal mucosa. They fade with the onset of rash. The characteristic rash of measles (see Fig. A1-3) begins 2 weeks after infection, when the clinical manifestations are most severe, and signal the host’s immune response to the replicating virus. Headache, abdominal pain, vomiting, diarrhea, and myalgia may be present. The rash of measles begins as erythematous macules behind the ears and on the neck and hairline. The rash progresses to involve the face, trunk, and arms, with involvement of the legs and feet by the end of the second day. Areas of confluent rash appear on the trunk and extremi­ ties, and petechiae may be present. The rash fades slowly in the same order of progression as it appeared, usually beginning on the third or fourth day after onset. Resolution of the rash may be followed by des­ quamation, particularly in undernourished children. Because the characteristic rash of measles is a consequence of the cellular immune response, it may not develop in persons with impaired cellular immunity. These persons have a high case–fatality rate and fre­ quently develop giant cell pneumonitis caused by measles virus. ■ ■DIFFERENTIAL DIAGNOSIS The differential diagnosis of measles includes other causes of fever, morbilliform rash, and conjunctivitis, including rubella, Kawasaki dis­ ease, infectious mononucleosis, roseola, scarlet fever, Rocky Mountain spotted fever, enterovirus or adenovirus infection, and drug sensitivity. Rubella is a milder illness without cough and with distinctive posterior auricular or suboccipital lymphadenopathy. The rash of roseola (exan­ them subitum) (see Fig. A1-5) appears after fever has subsided. The atypical lymphocytosis in infectious mononucleosis contrasts with the leukopenia commonly observed in children with measles. ■ ■DIAGNOSIS Measles is readily diagnosed on clinical grounds by clinicians famil­ iar with the disease, particularly during outbreaks. Koplik’s spots are especially helpful because they appear early and are pathognomonic. Clinical diagnosis is more difficult (1) during the prodromal illness;

(2) when the rash is attenuated by passively acquired antibodies or prior immunization; (3) when the rash is absent or delayed in immu­ nocompromised children or severely undernourished children with impaired cellular immunity; and (4) in regions where the incidence of measles is low and other pathogens are responsible for most illnesses with fever and rash. The CDC case definition for measles requires (1) a generalized maculopapular rash of at least 3 days’ duration; (2) fever of at least 38.3°C (101°F); and (3) cough, coryza, or conjunctivitis. Serology is the most common method of laboratory diagnosis. The detection of measles virus–specific IgM in a single specimen of serum or oral fluid is considered diagnostic of acute infection, as is a four­ fold or greater increase in measles virus–specific IgG antibody levels between acute- and convalescent-phase serum specimens. Primary infection in the immunocompetent host results in antibodies that are often detectable within 1–3 days of rash onset and reach peak levels in 2–4 weeks. However, measles virus–specific IgM antibodies may not be detectable until 4–5 days or more after rash onset, resulting in falsenegative test results if the specimen is obtained too early, and usually fall to undetectable levels within 4–8 weeks of rash onset. Several methods for measurement of antibodies to measles virus are available. Neutralization tests are sensitive and specific, and the results are highly correlated with protective immunity. However, these tests require propagation of measles virus in cell culture and thus are expensive and laborious. Commercially available measles IgM enzyme immunoassays are most frequently used. Measles can also be diagnosed by isolation of the virus in cell culture from respiratory secretions, nasopharyngeal or conjunctival swabs, blood, or urine. Direct detection of giant cells in respiratory secretions, urine, or tissue obtained by biopsy provides another method of diagnosis.

For detection of measles virus RNA by reverse-transcription poly­ merase chain reaction, primers targeted to highly conserved regions of measles virus genes are used. Extremely sensitive and specific, this assay may also permit identification and characterization of measles virus genotypes for molecular epidemiologic studies and can distin­ guish wild-type from vaccine virus strains.

TREATMENT Measles There is no specific antiviral therapy for measles. Treatment con­ sists of general supportive measures such as hydration and adminis­ tration of antipyretic agents. Because secondary bacterial infections are a major cause of morbidity and death attributable to measles, effective case management involves prompt antibiotic treatment for patients who have clinical evidence of bacterial infection, including pneumonia and otitis media. Vitamin A (available in oral and parenteral formulations) is effective for the treatment of measles and can markedly reduce rates of morbidity and mortality. The WHO recommends administration of once-daily oral doses of 200,000 IU of vitamin A for 2 consecu­ tive days to all children with measles who are ≥12 months of age. Lower doses are recommended for younger children: 100,000 IU per day for children 6–11 months of age and 50,000 IU per day for children <6 months old. A third dose is recommended 2–6 weeks later for children with evidence of vitamin A deficiency. While such deficiency is not a widely recognized problem in the United States, many American children with measles do, in fact, have low serum levels of vitamin A, and these children experience increased measles-associated morbidity. CHAPTER 211 Anecdotal reports have described the recovery of previously healthy pregnant and immunocompromised patients with measles pneumonia and of immunocompromised patients with measles encephalitis after treatment with aerosolized and IV ribavirin. However, the clinical benefits of ribavirin in measles have not been conclusively demonstrated in clinical trials. Measles (Rubeola) ■ ■COMPLICATIONS Most complications of measles involve the respiratory tract and include the effects of measles virus itself and secondary bacterial infections. Giant cell pneumonitis due to replication of measles virus in the lungs can develop in immunocompromised persons. Acute laryngotracheo­ bronchitis (croup) can occur during measles and may result in airway obstruction, particularly in young children. Many children with mea­ sles develop diarrhea, which contributes to and can exacerbate existing undernutrition. Most complications of measles result from secondary bacterial infections of the respiratory tract that are attributable to a state of immune suppression after acute measles. Otitis media and broncho­ pneumonia are most common. Recurrence of fever or failure of fever to subside with the rash suggests secondary bacterial infection. Severe complications of measles involve the central nervous system (CNS). Post-measles encephalomyelitis complicates ~1 in 1000 cases, affecting mainly older children and adults. Encephalomyelitis occurs within 2 weeks of rash onset and is characterized by fever, seizures, and a variety of neurologic abnormalities. The finding of periventricular demyelination, the induction of immune responses to myelin basic protein, and the absence of measles virus in the brain suggest that postmeasles encephalomyelitis is an autoimmune disorder triggered by measles virus infection. Rarer CNS complications that occur months to years after acute infection are measles inclusion body encephalitis (MIBE) and subacute sclerosing panencephalitis (SSPE). In contrast to post-measles encephalomyelitis, MIBE and SSPE are caused by per­ sistent measles virus infection. MIBE is a rare but fatal complication that affects individuals with defective cellular immunity and typically occurs months after infection. SSPE is a slowly progressive disease characterized by seizures and progressive deterioration of cognitive and motor functions, with death occurring 5–15 years after measles

virus infection. SSPE most often develops in persons infected with measles virus at <2 years of age.

■ ■PROGNOSIS Most persons with measles recover and develop long-term protective immunity to reinfection. Measles case–fatality proportions vary with the average age of infection, the nutritional and immunologic status of the population, measles vaccine coverage, and access to health care. Among previously vaccinated persons who do become infected, disease is less severe and mortality rates are significantly lower. In most developed countries, the case–fatality rate is 0.01–0.1%, but in endemic areas of sub-Saharan Africa, the measles case–fatality rate may be 5–10% or even higher. Measles is a major cause of childhood deaths in refugee camps and in internally displaced populations, where case–fatality rates have been as high as 20–30%. ■ ■PREVENTION Passive Immunization  Human immunoglobulin given shortly after exposure can attenuate the clinical course of measles. In immu­ nocompetent persons, administration of immunoglobulin within 72 h of exposure usually prevents measles virus infection and almost always prevents clinical measles. Administered up to 6 days after exposure, immunoglobulin will still prevent or modify the disease. Prophylaxis with immunoglobulin is recommended for susceptible household and nosocomial contacts who are at risk of developing severe measles, particularly children <1 year of age, immunocompromised persons (including immunocompromised persons living with HIV who were previously immunized with live attenuated measles vaccine), and pregnant women. Except for premature infants, children <6 months of age usually will be partially or completely protected by passively acquired maternal antibody. Infants born to women with vaccineinduced measles immunity become susceptible to measles at a younger age than infants born to women with acquired immunity from natural infection. If measles is diagnosed in a household member, all unimmu­ nized children in the household should receive immunoglobulin. The recommended dose is 0.5 mL/kg given intramuscularly with a maxi­ mum total dose of 15 mL. Immunocompromised and pregnant persons should receive 400 mg/kg intravenously. IV immunoglobulin contains antibodies to measles virus, and the usual dose of 100–400 mg/kg gen­ erally provides adequate prophylaxis for measles exposures occurring as long as 3 weeks or more after IV immunoglobulin administration. PART 5 Infectious Diseases Active Immunization  The first live attenuated measles vaccine was developed by passage of the Edmonston strain in chick embryo fibroblasts to produce the Edmonston B virus, which was licensed in 1963 in the United States but was reactogenic. Further passage of Edmonston B virus produced the more attenuated and less reactogenic Schwarz vaccine. The Moraten (“more attenuated Enders”) strain, which was licensed in 1968 and is used in the United States, is geneti­ cally identical to the Schwarz strain. The Edmonston-Zagreb vaccine, also derived from the Edmonston B strain, is widely used in many countries and was passaged in human diploid cells. Lyophilized measles vaccines are relatively stable, but reconstituted vaccine rapidly loses potency. Live attenuated measles vaccines are inactivated by light and heat and lose about half their potency at 20°C and almost all their potency at 37°C within 1 h after reconstitution. Therefore, a cold chain must be maintained before and after reconsti­ tution. Antibodies first appear 12–15 days after vaccination, and titers peak at 1–3 months. Measles vaccines are often combined with other live attenuated virus vaccines, such as those for mumps and rubella (MMR) and for mumps, rubella, and varicella (MMRV). The recommended age of first vaccination varies from 6 to 15 months and represents a balance between the optimal age for seroconversion and the probability of acquiring measles before that age. The propor­ tions of children who develop protective levels of antibody after the first measles vaccination approximate 85% at 9 months of age and 95% at 12 months. Common childhood illnesses concomitant with vac­ cination may reduce the level of immune response, but such illnesses are not valid reasons to withhold vaccination. Measles vaccines have

been well tolerated and immunogenic in children and adults living with HIV, although antibody levels may wane more rapidly. Because of the potential severity of wild-type measles virus infection in children living with HIV, routine measles vaccination is recommended except for those who are severely immunocompromised. Measles vaccination is contraindicated in individuals with other severe deficiencies of cel­ lular immunity because of the possibility of disease due to progressive pulmonary or CNS infection with the vaccine virus. The duration of vaccine-induced immunity is at least several decades, if not longer. Rates of secondary vaccine failure 10–15 years after immunization have been estimated at ~5% but are likely lower when vaccination takes place after 12 months of age. Decreasing anti­ body concentrations do not necessarily imply a complete loss of protec­ tive immunity as a secondary immune response usually develops after reexposure to measles virus, with a rapid rise in antibody titers in the absence of overt clinical disease. Standard doses of currently licensed measles vaccines are safe for immunocompetent children and adults. Fever up to 39.4°C (103°F) occurs in ~5–15% of seronegative vaccine recipients, and ~5% of vaccine recipients develop a transient rash. Mild transient thrombocytopenia has been reported, with an incidence of 1 case per ~40,000 MMR recipients. Since the publication of a report in 1998 falsely hypothesizing that MMR vaccine may cause a syndrome of autism and intestinal inflamma­ tion, much public attention has focused on this purported association. The events that followed publication of this report led to diminished vac­ cine coverage in the United Kingdom and provide important lessons in the misinterpretation of epidemiologic evidence and the communication of scientific results to the public. The publication that incited the concern was a case series describing 12 children with a regressive developmental disorder and chronic enterocolitis; 9 of these children had autism. In 8 of the 12 cases, the parents associated onset of the developmental delay with MMR vaccination. This simple temporal association was misinterpreted and misrepresented as a possible causal relationship, first by the lead author of the study and then by elements of the media and the public. Subsequently, many comprehensive reviews and additional epidemiologic studies refuted evidence of a causal relationship between MMR vaccina­ tion and autism, and the offending publication was retracted. ■ ■PROSPECTS FOR MEASLES ERADICATION Progress in global measles control has renewed discussion of measles eradication. In contrast to poliovirus eradication, the eradication of measles virus will not entail challenges posed by prolonged shedding of potentially virulent vaccine viruses and asymptomatic reservoirs. However, in comparison with smallpox eradication, higher levels of population immunity will be necessary to interrupt measles virus transmission, more highly skilled health care workers will be required to administer measles vaccines, and containment through case detec­ tion and ring vaccination will be more difficult for measles virus because of infectivity before rash onset. New tools, such as microarray patches (MAPs) to deliver measles vaccine, could facilitate mass vaccination campaigns and vaccination of hard-to-reach children such as those residing in remote rural areas. In May 2023, Micron Biomedical reported positive results in a phase 1/2 clinical trial in The Gambia comparing the results of immuniza­ tion by measles- and rubella-containing (MR) vaccine administered by MAP versus subcutaneous injection. This trial was the first of its kind to use microarray technology in children and found similar rates of seroconversion in MR vaccine–naïve children (92.9–100% for MAP and 89.7–100% for subcutaneous) and seroprotection in all age groups (93.2–100% for MAP and 89.8–100% for subcutaneous). Further stud­ ies and the overcoming of manufacturing and regulatory hurdles will be needed before MAPs become available. Despite enormous progress, measles remains a leading vaccinepreventable cause of childhood mortality worldwide and continues to cause outbreaks in communities with low vaccination coverage rates. As the world looks to rebuild immunization services disrupted by the COVID-19 pandemic and improve these services through the ambi­ tious MRP agenda, measles outbreaks will continue to remind us of the challenges to be overcome.

104 - 212 Rubella (German Measles)

212 Rubella (German Measles)

Acknowledgment The authors acknowledge the substantial contributions of Kaitlin Rainwater-Lovett, who coauthored prior editions of this chapter. ■ ■FURTHER READING De Swart RL, Moss WJ: The immunological basis for immunization series: Module 7: Measles. Update 2020. Geneva: World Health Organization, 2020. Griffin DE: Measles immunity and immunosuppression. Curr Opin Virol 46:9, 2020. Hübschen JM et al: Measles. Lancet 399:678, 2022. Mina MJ et al: Measles virus infection diminishes preexisting antibod­ ies that offer protection from other pathogens. Science 366:599, 2019. Minta AA et al: Progress toward measles elimination—Worldwide, 2000–2022. MMWR Morb Mortal Wkly Rep 72:1262, 2023. Moss WJ: Measles. Lancet 380:2490, 2017. Moss WJ et al: Feasibility assessment of measles and rubella eradica­ tion. Vaccine 39:3544, 2021. Phadke VK et al: Vaccine refusal and measles outbreaks in the US. JAMA 324:1344, 2020. Strebel PM, Orenstein WA: Measles. N Engl J Med 381:349, 2019. World Health Organization: Measles vaccines: WHO position paper—April 2017. Wkly Epidemiol Rec 92:205, 2017. Alan C. Ou, Ludmila M. Perelygina,

Laura A. Zimmerman, Susan E. Reef

Rubella (German Measles) Rubella was historically viewed as a variant of measles or scarlet fever. After an epidemic of rubella in Australia in the early 1940s, the ophthalmologist Norman Gregg noticed the occurrence of congenital cataracts among infants whose mothers had reported rubella during early pregnancy, and congenital rubella syndrome (CRS; see “Clinical Manifestations,” below) was first described. Not until 1962 was a sepa­ rate viral agent for rubella isolated. ■ ■ETIOLOGY Rubella virus is a member of the Matonaviridae family the genus Rubivirus. This single-strand RNA enveloped virus measures 40–80 nm in diameter. Its nucleocapsid consists of ~10-kb positive-sense RNA genome surrounded by a protein shell composed of a core protein and a single-layer lipoprotein envelope with spike-like projections contain­ ing two glycoproteins, E1 and E2. There is only one antigenic type of rubella virus, and humans are its only known reservoir. ■ ■PATHOGENESIS AND PATHOLOGY Although the pathogenesis of postnatal (acquired) rubella has been well documented, data on pathology are limited because of the mild­ ness of the disease. Rubella virus is spread from person to person via respiratory droplets. Primary implantation and replication in the nasopharynx are followed by spread to the lymph nodes. Subsequent viremia occurs, which in pregnant women often results in infection of the placenta. Placental virus replication may lead to infection of fetal organs. The pathology of CRS in the infected fetus is well defined, with almost all organs found to be infected; however, the pathogenesis of CRS is only poorly delineated. In tissue, infections with rubella virus have diverse effects, ranging from no obvious impact to cell destruction. The hallmark of fetal infection is a chronic infection with persistence throughout fetal development in utero and for up to 1 year after birth. Individuals with acquired rubella may shed virus from 7 days before rash onset to ~5–7 days thereafter. Both clinical and subclinical

infections are considered contagious. Infants with CRS may shed large quantities of virus from bodily secretions, particularly from the throat and in the urine, up to 1 year of age. Outbreaks of rubella, including some in nosocomial settings, have originated with index cases of CRS. Thus, only individuals immune to rubella virus should have contact with infants who have CRS or who are congenitally infected with rubella virus but are not showing signs of CRS.

■ ■EPIDEMIOLOGY The largest recent rubella epidemic in the United States took place in 1964–1965, when an estimated 12.5 million cases occurred, resulting in ~20,000 cases of CRS. Since the introduction of the routine rubella vac­ cination program in the United States in 1969, the number of rubella cases reported each year has dropped by >99%; the rate of vaccination coverage with rubella-containing vaccine (RCV) has been >90% among children 19–35 months old since 1996. In the United States, a goal for the elimination of rubella and CRS by 2000 was set in 1989. Interrup­ tion of endemic transmission of rubella virus was achieved by 2001. In 2004, a panel of experts agreed unanimously that rubella was no longer an endemic disease in the United States. The criteria used to document lack of endemic transmission included low disease incidence, high nationwide rubella antibody seroprevalence, outbreaks that were few and contained (i.e., small numbers of cases), and lack of endemic virus transmission (as assessed by genetic sequencing). Although interrup­ tion of endemic transmission has been sustained since 2001, rubella virus importations continue to occur, and cases continue to develop among susceptible persons. During 2010−2022, 66 cases of rubella were reported; 71% of these cases were in persons 20−49 years old— an age group that includes women of childbearing age. During this period, 13 cases of CRS were reported, all from foreign-born mothers. Therefore, health care providers should remain vigilant, considering the possibility of rubella virus infection in adults (especially those emi­ grating or returning from countries without rubella control programs) and recognizing the potential for CRS among their infants. CHAPTER 212 Rubella (German Measles) The Global Measles and Rubella Strategic Framework 2021–2030 envisions a “world free from measles and rubella” with the goal to “achieve and sustain the regional measles and rubella elimination goals.” By 2023, five of the six World Health Organization (WHO) regions had rubella elimination goals (see “Prevention” below). Although rubella and CRS are no longer endemic in the WHO Region of the Americas, they remain important public health problems globally. The number of rubella cases reported worldwide in 2000 was ~700,000; this figure declined to 17,407 in 2022. However, the number of rubella cases may be underestimated because cases are often mild, patients may not seek care, cases may not be recognized or may not be reported, and, in some countries, cases are identified through measles surveillance systems that are not specific for rubella. Despite a continued increase in the number of countries with rubella vaccination programs, 25% of the world’s children remained unvaccinated against rubella in 2022. In 2010, it was estimated that 105,000 cases of CRS occurred annually globally. ■ ■CLINICAL FEATURES Acquired Rubella  Acquired rubella commonly presents with a generalized maculopapular rash that usually lasts up to 3 days (Fig. 212-1), although as many as 50% of cases may be subclinical or without rash. When the rash occurs, it is usually mild and may be dif­ ficult to detect in persons with darker skin. In younger children, rash is usually the first sign of illness. However, in older children and adults, a 1- to 5-day prodrome often precedes the rash and may include lowgrade fever, malaise, and upper respiratory symptoms. The incubation period is 14 days (range, 12–23 days). Lymphadenopathy, particularly occipital and postauricular, may be noted during the second week after exposure. Although acquired rubella is usually thought of as a benign disease, arthralgia and arthritis are common in infected adults, particularly women. Thrombocytope­ nia and encephalitis are less common complications. Congenital Rubella Syndrome  The most serious consequence of rubella virus infection can develop when a woman becomes infected

FIGURE 212-1  Mild maculopapular rash of rubella in a child. during pregnancy, particularly during the first trimester. The resulting complications may include miscarriage, fetal death, premature deliv­ ery, or live birth with congenital defects. Infants infected with rubella virus in utero may have myriad physical defects (Table 212-1), which most commonly relate to the eyes, ears, and heart. This constellation of severe birth defects is known as CRS. In addition to permanent manifestations, there are a host of transient physical manifestations, including thrombocytopenia with purpura/petechiae (e.g., dermal erythropoiesis, “blueberry muffin syndrome”). Some infants may be born with congenital rubella virus infection but have no apparent signs or symptoms of CRS and are referred to as “infants with congenital rubella virus infection only.” PART 5 Infectious Diseases ■ ■DIAGNOSIS Acquired Rubella  Clinical diagnosis of acquired rubella is difficult because of the mimicry of many illnesses with rashes, the varied clini­ cal presentations, and the high rates of subclinical and mild disease. Illnesses that may be similar to rubella in presentation include scarlet fever, roseola, toxoplasmosis, fifth disease, measles, Zika, and illnesses with suboccipital and postauricular lymphadenopathy. Thus, labora­ tory documentation of rubella virus infection is considered the only reliable way to confirm acute disease. Laboratory assessment of rubella virus infection is conducted by serologic and virus detection methods. For acquired rubella, serologic diagnosis is most common and depends on the demonstration of IgM antibodies in an acute-phase serum specimen or a fourfold rise in IgG antibody titer between acute- and convalescent-phase specimens. To TABLE 212-1  Common Transient and Permanent Manifestations in Infants with Congenital Rubella Syndrome TRANSIENT MANIFESTATIONS PERMANENT MANIFESTATIONS Hepatosplenomegaly Interstitial pneumonitis Thrombocytopenia with purpura/ petechiae (e.g., dermal erythropoiesis or “blueberry muffin syndrome”) Hemolytic anemia Bony radiolucencies Intrauterine growth retardation Adenopathy Meningoencephalitis Hearing impairment/deafness Congenital heart defects (patent ductus arteriosus, pulmonary arterial stenosis) Eye defects (cataracts, cloudy cornea, microphthalmos, pigmentary retinopathy, congenital glaucoma) Microcephaly Central nervous system sequelae (mental and motor delay, autism)

detect a rise in IgG antibody titer indicative of acute disease, the acutephase serum specimen should be collected within 7–10 days after onset of illness and the convalescent-phase specimen ~14–21 days after the first specimen. The enzyme-linked immunosorbent assay IgM capture technique is considered most accurate for serologic diagnosis, but the indirect IgM assays also are acceptable. After rubella virus infection, IgM antibody may be detectable for up to 6 weeks. In case of a negative result for IgM in specimens taken earlier than day 5 after rash onset, serologic testing should be repeated. Although uncommon, reinfection with rubella virus is possible, and IgM antibodies may be present. In this instance, IgG avidity testing is used in conjunction with IgG testing to distinguish primary rubella infection from reinfection. The detection of low-avidity antibodies in a patient’s serum indicates recent infection. The presence of mature (high-avidity) IgG antibodies most likely indicates an infection occur­ ring at least 2 months previously. Avidity testing may be particularly useful in diagnosing rubella in pregnant women and assessing the risk of CRS. Rubella virus is typically detected in the nasopharynx during the prodromal period and for as long as 2 weeks after rash onset. However, viral specimens (nasopharyngeal swabs are preferred but throat swabs or urine are also acceptable) should be collected as soon after symptom onset as possible, preferably 1–3 days after onset, but no later than 7 days after onset. Rubella is usually diagnosed by viral RNA detection in a reverse-transcriptase polymerase chain reaction (RT-PCR) assay; rubella virus isolation can also be used to diagnose rubella. Congenital Rubella Syndrome  The classic triad of CRS—clinical manifestations of cataracts, hearing impairment, and heart defects—is seen in ~10% of infants with CRS. Infants may present with differ­ ent combinations of defects depending on when infection occurs during gestation. Hearing impairment is the most common single defect of CRS. However, as with acquired rubella, laboratory diag­ nosis of congenital infection is highly recommended, particularly because most features of the clinical presentation are nonspecific and may be associated with other intrauterine infections. Early diagnosis of CRS allows the prompt implementation of infection control measures and facilitates appropriate medical intervention for specific disabilities. Diagnostic tests used to confirm CRS include serologic assays and virus detection. In an infant with congenital infection, serum IgM anti­ bodies are normally present for up to 6 months but may be detectable for up to 1 year after birth. In some instances, IgM may not be detect­ able until 1 month of age; thus, infants who have symptoms consistent with CRS but who test negative shortly after birth should be retested at 1 month. A rubella serum IgG titer persisting beyond the time expected after passive transfer of maternal IgG antibody (i.e., a rubella titer that does not decline at the expected rate of a twofold dilution per month) is another serologic criterion used to confirm CRS. In congenital infection, rubella virus is detected most commonly from nasopharyngeal and throat swabs and urine. Infants with con­ genital rubella may excrete virus for up to 1 year, but specimens for RT-PCR are most likely to be positive if obtained within the first 6 months after birth. Rubella virus in infants with CRS can also be detected by virus culture. Rubella Diagnosis in Pregnant Women  In the United States, screening for rubella IgG antibodies is recommended as part of routine prenatal care. Pregnant women with a positive IgG antibody serologic test are considered immune. Susceptible pregnant women should be vaccinated postpartum. A susceptible pregnant woman exposed to rubella virus should be tested for IgM antibodies and, if positive, confirmed by testing for low-avidity IgG antibodies to determine whether she was infected during pregnancy. Pregnant women with evidence of acute infec­ tion must be clinically monitored, and gestational age at the time of maternal infection must be determined to assess the possibility of risk to the fetus. Among women infected with rubella virus during the first 10 weeks of gestation, the risk of delivering an infant with CRS is 90%. The risk of birth defects declines with infection later in gestation, and fetal defects are rarely associated with maternal

rubella after the 16th week of gestation, although sensorineural hear­ ing deficit may occur with infection as late as 20 weeks. Because of the potential for false-positive results, rubella IgM antibody testing is not recommended for pregnant women with no history of illness or contact with a rubella-like illness. TREATMENT Rubella No specific therapy is available for rubella virus infection. Symptombased treatment for various manifestations, such as fever and arthral­ gia, is appropriate. Immunoglobulin does not prevent rubella virus infection after exposure and therefore is not recommended as routine postexposure prophylaxis. Although immunoglobulin may modify or suppress symptoms, it can create an unwarranted sense of secu­ rity: infants with congenital rubella have been born to women who received immunoglobulin shortly after exposure. Administration of immunoglobulin should be considered only if a pregnant woman who has been exposed to a person with rubella will not consider termina­ tion of the pregnancy under any circumstances. In such cases, IM administration of 20 mL of immunoglobulin within 72 h of rubella exposure may reduce—but does not eliminate—the risk of rubella. ■ ■PREVENTION After the isolation of rubella virus in the early 1960s and the occur­ rence of a devastating rubella pandemic in 1964–1965, a vaccine for rubella was developed and licensed in 1969. The majority of rubellacontaining vaccines (RCVs) used worldwide are combined measles and rubella (MR) or measles, mumps, and rubella (MMR) formulations. A tetravalent measles, mumps, rubella, and varicella (MMRV) vaccine is available but is not widely used. Available rubella-containing vaccines are live attenuated vaccine virus. The public health burden of rubella virus infection is measured primarily through the occurrence of CRS cases among women who were infected during pregnancy. The 1964–1965 rubella epidemic in Vaccine intro - Rubella vaccine by Country - 2022 Yes Yes (Partial) No Not applicable 4000 km FIGURE 212-2  Countries using rubella vaccine in national childhood immunization schedules, 2022. Disclaimer—The boundaries and names shown and the designations used on this map do not imply the expression or any opinion whatsoever on the part of the World Health Organization concerning the legal status of any country, territory, city, or area nor of its authorities, or concerning the delimitation of its frontiers or boundaries. Dotted or dashed lines on maps represent approximate border lines for which there may not be full agreement. (Reproduced with permission from the World Health Organization; 2024.)

the United States resulted in >30,000 infections during pregnancy. CRS occurred in ~20,000 infants born alive, including >11,000 infants who were deaf, >3500 infants who were blind, and almost 2000 infants with intellectual disability. The medical cost of this epidemic exceeded $1.5 billion. It has been estimated that the lifetime medical costs for children with CRS range from $11,255 in low-income countries to $934,000 in high-income countries.

In most countries, there are few data to document the epidemiology of CRS, but clusters of CRS cases have been reported in developing coun­ tries. Before the introduction of routine immunization against rubella in the United States, the incidence of CRS was 0.1–0.2 case per 1000 live births during endemic periods and 1–4 cases per 1000 live births dur­ ing epidemic periods. Where rubella virus is circulating and women of childbearing age are susceptible, CRS cases will continue to occur. The most effective method of preventing acquired rubella and CRS is through vaccination with an RCV. One dose induces seroconversion in ≥95% of persons ≥1 year of age. Immunity is considered long-term and is probably lifelong. The most commonly used vaccine globally is the RA27/3 virus strain. The recommendation for routine rubella vac­ cination schedules in the United States is a first dose of MMR vaccine at 12–15 months of age and a second dose at 4–6 years. Other persons recommended to receive a dose of a rubella-containing vaccine include adolescents and adults without documented evidence of immunity, individuals in congregate settings (e.g., college students, military per­ sonnel, childcare and health care workers), international travelers, and susceptible women before and after pregnancy. Because of the theoretical risk of transmission of live attenuated rubella vaccine virus to the developing fetus, women known to be pregnant should not receive RCV. In addition, pregnancy should be avoided for 28 days after receipt of RCV. In follow-up studies of ~3000 unknowingly pregnant women who received rubella vaccine, no infant was born with CRS. Receipt of RCV during pregnancy is not ordinarily a reason to consider termination of the pregnancy. CHAPTER 212 In 2022, 175 (90%) of the 194 member countries of the WHO rec­ ommended inclusion of RCV in the routine childhood vaccination schedule (Fig. 212-2). Goals for the elimination of rubella and CRS Rubella (German Measles)

105 - 213 Mumps

213 Mumps

have been established in the WHO American, European, Southeast Asian, African, and Western Pacific regions. The Eastern Mediterra­ nean region has not yet set such goals. Unfortunately, the COVID-19 pandemic, which began in late 2019, led to extensive disruptions of routine vaccination services in many countries. Thus, it is essential that all children are up to date with rubella vaccination, especially those who missed vaccination during the pandemic. To protect against rubella vaccine throughout the life course, additional strategies to immunize adolescents and adults will be needed to ensure adults of childbearing age are protected from the risk of having an infant with CRS.

■ ■FURTHER READING Centers for Disease Control and Prevention: Control and prevention of rubella: Evaluation and management of suspected outbreaks, rubella in pregnant women, and surveillance for con­ genital rubella syndrome. MMWR Morb Mortal Wkly Rep 50:1,

Centers for Disease Control and Prevention: Notice to readers: Revised ACIP recommendation for avoiding pregnancy after receiv­ ing a rubella-containing vaccine. MMWR Morb Mortal Wkly Rep 50:1117, 2001. Centers for Disease Control and Prevention: Manual for the Sur­ veillance of Vaccine-Preventable Diseases, SW Roush et al (eds). Atlanta, Centers for Disease Control and Prevention, Last Reviewed: March 6, 2020, Chapters 14–15 (Rubella & Congenital Rubella Syndrome). Available at https://www.cdc.gov/vaccines/pubs/surv-manual/chapters

.html.  Accessed March 5, 2024. Centers for Disease Control and Prevention: Rubella, in Epidemiology and Prevention of Vaccine Preventable Diseases, 14th ed.

E Hall et al (eds). Washington, DC, Public Health Foundation, August 2021, Chapter 20. Available at https://www.cdc.gov/vaccines/ pubs/pinkbook/front-matter.html.  Accessed March 5, 2024. Ou AC et al: Progress toward rubella and congenital rubella syndrome PART 5 Infectious Diseases control and elimination–Worldwide, 2012–2022. MMWR Morb Mortal Wkly Rep 73:162, 2024. Reef SE, Plotkin SA: Rubella vaccine, in Vaccines, SA Plotkin, WA Orenstein (eds). Philadelphia, Saunders, 2024, pp 1025–1056. Thompson K, Odahowski C: The costs and valuation of health impacts of measles and rubella risk management policies. Risk Anal 36:1357, 2016. Vynnycky E et al: Estimates of the global burden of congenital rubella syndrome, 1996–2019. Int J Infect Dis 137:149, 2023. World Health Organization: Rubella, module 11, in The Immuno­ logical Basis for Immunization Series. Geneva, WHO, 2008. Available at https://www.who.int/publications/i/item/9789241596848.  Accessed August 10, 2023. World Health Organization: Rubella vaccines: WHO position paper. Wkly Epidemiol Rec 95:301, 2020. Available at https://www.who

.int/publications/i/item/WHO-WER9527.  Accessed August 10, 2023. World Health Organization: Global Vaccine Action Plan 2011–2020. Geneva, WHO, 2013. Available at http://www.who.int/ immunization/global_vaccine_action_plan/GVAP_doc_2011_2020/ en/.  Accessed August 10, 2023. World Health Organization: Measles and rubella strategic frame­ work 2021–2030. Geneva: WHO; 2020. License: CC BY-NC-SA 3.0 IGO. World Health Organization: Immunization Agenda 2030: A global strategy to leave no one behind. Available at https://www

.who.int/teams/immunization-vaccines-and-biologicals/strategies/ ia2030.  Accessed March 5, 2024.

Jessica Leung, Nina B. Masters

Mumps Mumps is an acute, self-limited, systemic viral illness typically charac­ terized by parotitis or other salivary gland swelling. Although mumps was once considered a universal childhood disease in the United States, routine mumps vaccination—with a one-dose policy implemented in 1977 and a two-dose policy in 1989—led to a >99% reduction in cases by the early 2000s. However, since 2006, there has been an increase in mumps cases in the United States, the majority among fully vaccinated persons. Mumps should be suspected in all patients with parotitis or mumps complications (see “Clinical Manifestations”), regardless of age, vaccination status, or travel history. ■ ■ETIOLOGIC AGENT Mumps is an acute viral illness caused by a paramyxovirus from the Rubulavirus genus in the Paramyxoviridae family. This single-stranded, negative-sense, enveloped RNA virus is ~15.3 kb in size and encodes several minor proteins and seven major proteins. There is only one mumps virus serotype. One of the seven major encoded proteins, the small hydrophobic (SH) protein exhibits hypervariability among strains; thus, the SH gene nucleotide sequence (316 nucleotides) is used to genotype the virus for molecular epidemiologic purposes. Mumps virus is rapidly inactivated by formalin, ether, chloroform, heat, and ultraviolet light. The 12 known genotypes of mumps virus are designated by the let­ ters A to N (except E and M). In the United States, >98% of mumps virus specimens genotyped from 2015 through 2017 were genotype G. Most mumps vaccines licensed globally are composed of virus strains from genotype A, B, or N. The mumps virus strain (Jeryl Lynn) used in vaccines in the United States is genotype A. ■ ■EPIDEMIOLOGY Mumps occurs worldwide and is endemic in many countries. In the absence of routine vaccination, the annual incidence of mumps is 100–1000 cases per 100,000 population, with epidemic peaks every 2–5 years. From 1999 to 2018, on average, >500,000 mumps cases were reported to the World Health Organization annually, with fewer reported cases from 2019 to 2021 (between 160,000 and 270,000). However, the global mumps incidence is challenging to estimate, as few countries routinely collect the pertinent data. As of 2021, mumps vaccine was introduced in 123 WHO member states. Mumps incidence has been reduced by 97–99% in countries with a routine two-dose mea­ sles, mumps, and rubella (MMR) vaccination schedule and by 87–88% in those with a one-dose vaccination program. However, since the mid2000s, large mumps outbreaks have been reported among populations with high two-dose MMR coverage in countries with routine mumps immunization programs. Most outbreaks have occurred in settings with intense or frequent close contact, such as universities, close-knit communities, and correctional facilities, and most of these cases have occurred in fully vaccinated persons. Despite these outbreaks, mumps incidence is still much higher in countries that do not have routine mumps vaccination. In the United States, prior to licensure of a vaccine for mumps in 1967, >100,000 mumps cases occurred annually. After the implementa­ tion of a one-dose mumps vaccination policy in 1977 and a subsequent two-dose policy in 1989, reported mumps cases declined to an annual average of ~300 by the early 2000s. However, since 2006, there has been an increase in mumps cases reported in the United States, with several peak years (Fig. 213-1). During the highest peak in recent cases, from January 2016 through June 2017, 150 mumps outbreaks and 9200 outbreak-associated cases were reported in a range of settings and groups, including schools, universities, athletic teams and facili­ ties, church groups, workplaces, and large parties and events. While a majority of cases occurred in fully vaccinated young adults in asso­ ciation with large university outbreaks, about one-third of cases have

Number of Mumps Cases

FIGURE 213-1  Reported mumps cases: United States, 2000–2022. (Source: National Notifiable Diseases Surveillance System (NNDSS), Notifiable Infectious Disease Data Tables. Atlanta, GA, CDC Division of Health Informatics and Surveillance, 2022. Available at https://www.cdc.gov/nndss/data-statistics/infectious-tables/index.html.) affected children or adolescents, most of whom were vaccinated. Since 2020, there has been a large reduction in reported mumps cases likely due to social distancing and other COVID-19 prevention measures, with 150–700 reported annually. As of 2023, mumps is endemic in the United States, and there are no elimination goals for the disease. Multiple factors are likely involved in being at risk for mumps infec­ tion among vaccinated persons. Following vaccination, these factors include (1) failure to develop an immune response, (2) the develop­ ment of a low-level immune response that is insufficient for protection, (3) a decrease in immunity over time (waning immunity) after initial development of a vaccine-induced immune response, (4) lower levels of vaccine-induced antibodies to the circulating wild-type virus strains than to the vaccine virus strain, and (5) a lower frequency of subclini­ cal immunologic boosting due to lack of exposure to wild-type virus during periods of low disease incidence. ■ ■PATHOGENESIS Humans are the only known natural reservoir for mumps virus, which is transmitted through direct contact with respiratory droplets or saliva of an infected person. The average incubation period is 16–18 days, with a range of 12–25 days. A person is most infectious from 2 days before until 5 days after onset of parotitis or other salivary gland swelling. However, mumps virus has been detected in saliva as early as 7 days before onset and as late as 9 days after onset of these manifestations. Mumps virus has been isolated from urine and seminal fluid up to 14 days after onset of parotitis, although no studies have assessed transmissibil­ ity of the virus through these fluids. Primary mumps virus replication likely occurs in the nasal mucosa or upper respiratory mucosal epithelium. Given the range of symp­ toms, it is assumed that, after infection of the respiratory mucosa, the virus spreads to regional lymph nodes. Mononuclear cells and cells within regional lymph nodes can become infected; such infection facilitates the development of viremia, which usually lasts 3–5 days. Viremia can result in a range of acute inflammatory reactions, most commonly in the salivary glands (resulting in parotitis) and the testes (resulting in orchitis). Other sites of virus dissemination include the kidneys (reflected in the frequency of viruria), the central nervous sys­ tem (CNS), the pancreas, the heart, the ovaries, the mammary glands, the perilymphatic fluid within the cochlea, and (during pregnancy) the fetus. Little is known about the pathology of mumps since the disease is rarely fatal. Affected salivary glands contain perivascular and intersti­ tial mononuclear-cell infiltrates and exhibit hemorrhage with promi­ nent edema. Serum and urine amylase levels may be elevated as a result

of inflammation and tissue damage in the parotid gland. Necrosis of acinar and epithelial duct cells is evident in the salivary glands and in the germinal epithelium of the seminiferous tubules of the testes. The virus probably enters cerebrospinal fluid (CSF) through the choroid plexus or via transiting mononuclear cells during plasma viremia. Although relevant data are limited, in many cases, mumps encephalitis appears to be a para- or postinfectious process (as suggested by perive­ nous demyelination and perivascular mononuclear-cell inflammation) rather than the result of a direct cytotoxic effect caused by viral inva­ sion of the CNS. However, although rare, primary mumps encephalitis does occur, as shown by mumps virus isolation from brain tissue. Infection of the perilymphatic fluid likely develops via retrograde pen­ etration by the virus from the cervical lymph nodes following viremia, but infection could also occur via the CSF in cases of mumps CNS infection, given that the perilymph communicates with the CSF. Virus in the perilymph can result in infection of the cochlea and damage to the organ of Corti and the tectorial membrane, leading to transient or permanent deafness. Evidence of placental and intrauterine spread has been found in both early and late gestation. Virus frequently dis­ seminates to the kidneys, but kidney involvement in mumps is almost always benign. CHAPTER 213 Mumps ■ ■CLINICAL MANIFESTATIONS While typically presenting with parotitis or other salivary gland swell­ ing, mumps infection can be asymptomatic or present as nonspecific respiratory symptoms, though serious complications such as sensori­ neural hearing loss can occur. Fully vaccinated persons can contract mumps, but vaccinated persons are at a lower risk for mumps and mumps complications. Mumps infection is asymptomatic in ~20% of unvaccinated patients; the proportion asymptomatic among vaccinated persons is unknown. Parotitis can be preceded by several days by a prodrome of low-grade fever, malaise, myalgia, headache, and anorexia. Parotitis typically lasts for 5 days (range, 3–7 days); most cases resolve within 10 days. Parotitis is generally bilateral and may not occur synchronously on both sides; unilateral involvement occurs in about one-third of cases. Swelling of the parotid gland is accompanied by tenderness and obliteration of the space between the earlobe and the angle of the mandible (Figs. 213-2 and 213-3). The patient frequently reports an earache and jaw pain and finds it difficult to eat, swallow, or talk. The orifice of the parotid duct is commonly red and swollen. The submaxillary and sublingual glands are involved less often than the parotid gland and are rarely involved alone. In ~6% of mumps cases, obstruction of lymphatic drainage secondary to bilateral salivary gland swelling may lead to presternal

A B FIGURE 213-2  The same person before mumps acquisition (A) and on day 3 of acute bilateral parotitis (B). (Courtesy of patient C.M. From JD Shanley: The resurgence of mumps in young adults and adolescents. Cleve Clin J Med 74:42, 2007. Reprinted with permission. Copyright © 2007 Cleveland Clinic Foundation. All rights reserved.) pitting edema, associated often with submandibular adenitis and rarely with the more life-threatening supraglottic edema. The most frequent complications of mumps include orchitis, oopho­ ritis, mastitis, pancreatitis, hearing loss, meningitis, and encephalitis. Complications can occur in the absence of parotitis and are more com­ mon among adults than among children and among males than among females, likely due to rates of orchitis. Orchitis (testicular inflammation), usually accompanied by fever, is the most common complication, developing in up to 30% of unvac­ cinated and 6% of vaccinated postpubertal males. This complication is rare in children. Orchitis typically occurs during the first week of parotitis but can develop up to 6 weeks after parotitis. Both testes are involved in ~10–30% of cases. The testis is painful and tender and can be enlarged to several times its normal size. Pain and swelling may last for 1 week, while tenderness may last for several weeks. Testicular atrophy develops in ~30–50% of affected testicles. The development of anti-sperm antibodies, reduced testosterone production, and impaired sperm mobility through oligospermia, azoospermia, or asthenosper­ mia may lead to temporary sterility or subfertility. However, no studies have assessed the risk of permanent infertility in men with mumps orchitis. PART 5 Infectious Diseases Approximately 7% of unvaccinated and ≤1% of vaccinated postpu­ bertal women develop oophoritis, which may be associated with lower abdominal pain and vomiting. The rate of mastitis in mumps has been estimated to be as high as 30% among unvaccinated postpubertal women and as low as ≤1% among vaccinated postpubertal women. Pancreatitis occurs in ~4% of unvaccinated and <1% of vaccinated mumps patients. Mumps pancreatitis, which may present as abdominal Parotid gland (enlarged) Parotid gland Sternocleidomastoid muscle Ear-gland axis Ear-gland axis FIGURE 213-3  Schematic drawings of a normal parotid gland (left) and a parotid gland infected with mumps virus (right). An enlarged cervical lymph node is usually posterior to the imaginary line. (Reproduced with permission from A Gershon et al: Krugman’s Infectious Diseases of Children, 11th ed. Philadelphia, Elsevier, 2004.)

pain, is difficult to diagnose because an elevated serum amylase level can be associated with either parotitis or pancreatitis. However, serum lipase is elevated in pancreatitis and the presence of both elevated serum amylase and lipase can help determine if pancreatitis is present in addition to parotitis. Hearing loss associated with mumps infection can occur in up to 4% of unvaccinated and <1% of vaccinated mumps patients. Mumps-related hearing loss is usually sudden in onset, uni­ lateral, and transient and may be associated with vestibular symptoms. Bilateral and permanent hearing loss are rare. Mumps virus is highly neurotropic, with subclinical CNS involve­ ment occurring in up to 55% of patients as manifested by CSF pleo­ cytosis. However, symptomatic CNS infection is less common. Aseptic meningitis occurs in ≤1% of vaccinated patients and up to 10% of unvaccinated patients and is a self-limited manifestation without sig­ nificant risk of death or long-term sequelae. Symptoms of aseptic men­ ingitis, including stiff neck, headache, and drowsiness, typically appear ~5 days after parotitis. Encephalitis develops in ≤1% of patients, who present with high fever, marked changes in the level of consciousness, seizures, and focal neurologic symptoms. Electroencephalographic abnormalities may be seen. Permanent sequelae are sometimes iden­ tified in survivors, and adult infections more commonly have poor outcomes than pediatric infections. The mortality rate associated with mumps encephalitis is ~1.5%. Other CNS problems occasionally asso­ ciated with mumps include cerebellar ataxia, facial palsy, transverse myelitis, hydrocephalus, Guillain-Barré syndrome, flaccid paralysis, and behavioral changes. Although rare and self-limited, myocarditis and endocardial fibro­ elastosis may represent severe complications of mumps infection; however, mumps-associated electrocardiographic abnormalities have been reported in up to 15% of cases. Other unusual complications include thyroiditis, nephritis, arthritis, hepatic disease, keratouveitis, and thrombocytopenic purpura. Abnormal renal function is common, but severe, life-threatening nephritis is rare. Mumps infection in pregnant women is generally benign and is not more severe than in women who are not pregnant. Evidence suggesting an association between maternal mumps infection and an increased rate of spontaneous abortion or intrauterine fetal death is inconclusive. Both mumps reinfection after natural infection and recurrent infection (in which parotid gland swelling resolves and then, weeks to months later, develops on the same or the other side) can occur. In the past, mumps reinfection was thought to be rare, but more recent data have suggested that it may be more common than previously thought. Death due to mumps is exceedingly rare. ■ ■DIFFERENTIAL DIAGNOSIS Mumps is the only cause of epidemic parotitis, although an increase in parotitis cases may also result from increased influenza activity— specifically, infection with influenza A virus subtype H3N2. Other infectious causes of parotitis include parainfluenza virus types 1–3, Epstein-Barr virus, human herpesviruses 6A and 6B, herpes simplex viruses types 1 and 2, coxsackievirus A, adenovirus, parvovirus B19, echovirus, lymphocytic choriomeningitis virus, and HIV. Laboratory testing for sporadic parotitis cases caused by these infectious pathogens can help rule out mumps. Parotitis can also develop in the setting of sarcoidosis, Sjögren’s syndrome, Mikulicz’s syndrome, Parinaud’s oculoglandular syndrome, uremia, diabetes mellitus, laundry starch ingestion, malnutrition, cir­ rhosis, and some drug treatments. Unilateral parotitis can be caused by ductal obstruction, cysts, and tumors. In the absence of parotitis or other salivary gland enlargement, symptoms of other visceral-organ and/or CNS involvement may predominate, and a laboratory diagnosis is required. Other entities should be considered when manifestations consistent with mumps appear in organs other than the parotid. For example, testicular torsion may produce a painful scrotal mass resem­ bling that seen in mumps orchitis. Orchitis can also be caused by bac­ terial infections in the prostate and urinary tract, sexually transmitted diseases such as chlamydia and gonorrhea, and other viral infections such as those with coxsackievirus, varicella, echovirus, and cytomega­ lovirus. Oophoritis can also be caused by sexually transmitted diseases

such as chlamydia and gonorrhea. A number of viruses (e.g., enterovi­ ruses) can cause aseptic meningitis that is clinically indistinguishable from that due to mumps virus. ■ ■LABORATORY DIAGNOSIS If mumps is suspected, infection is confirmed by virologic methods, but serologic testing can aid in diagnosis. Especially in vaccinated patients, a negative virologic or serologic result in a person with clinical signs of mumps does not rule out mumps infection. Virologic methods for confirming mumps include reverse transcrip­ tion polymerase chain reaction (RT-PCR) and viral culture. RT-PCR is preferred because of its sensitivity, specificity, and timeliness. Mumps virus and viral RNA can be detected in blood, saliva, urine, and CSF. Buccal swabs provide the best specimens for virus detection. The parotid gland should be massaged for 30 s prior to collection of the buccal swab sample. As maximal viral shedding occurs within 5 days after symptom onset, specimens for mumps virologic testing ideally should be collected as close to parotitis onset as possible. The diagnos­ tic yield of urine specimens increases over time up to 10 days after par­ otitis onset, but buccal specimens are more likely than urine specimens to result in virus detection at any time point. Serologic methods that can aid in the diagnosis of mumps include detection of mumps-specific IgM antibodies or a fourfold rise between acute- and convalescent-phase IgG antibodies. In unvaccinated per­ sons, IgM antibody is usually detectable within 5 days after onset, reaches a maximal level a week after onset, and remains elevated for weeks or months. Failure to detect mumps IgM in vaccinated patients is very common, as the IgM response is often undetectable, transient, or delayed in these individuals. Collection of specimens >3 days after onset may improve IgM detection. Additionally, IgM can yield falsepositive results due to serologic cross-reactions. Use of IgG testing is generally not recommended, as IgG titers in vaccinated or previously infected patients may already be elevated at the time of acute-phase specimen collection, such that a fourfold rise is not detected in the convalescent-phase specimen. TREATMENT Mumps Mumps is generally a benign, self-resolving illness. Therapy for parotitis and other clinical manifestations is symptom based and supportive. The administration of analgesics and the application of warm or cold compresses to the parotid area may be helpful. Testicular pain may be minimized by the local application of cold compresses and gentle support for the scrotum. Anesthetic blocks also may be used. Neither the administration of glucocorticoids nor incision of the tunica albuginea is of proven value in severe orchitis. Mumps immune globulin is not recommended for postexposure prophylaxis or treatment. ■ ■PREVENTION Vaccination is the best prevention measure against mumps. Mumps vaccine is commonly included as part of the combination measles– mumps–rubella (MMR) vaccine or the combination measles–mumps– rubella–varicella (MMRV) vaccine. All mumps vaccines currently on the market are live attenuated virus vaccines. Strains used in mumps vaccines have included Jeryl Lynn, RIT 4385, Urabe Am9, Rubini, Leningrad-3 and Leningrad-Zagreb. Urabe- and Rubini-containing vaccines are no longer available. The Jeryl Lynn and RIT 4385 strains are the only strains used in mumps vaccines in the United States since 1967. In the United States, children are recommended to receive the first MMR dose at 12–15 months of age and the second dose at 4–6 years. MMR vaccine is licensed for use in persons age ≥12 months; MMRV vaccine is licensed for use in persons age 12 months through 12 years. Due to the potential increased risk of febrile seizures, it is recom­ mended that MMR vaccine and varicella vaccine be administered for the first dose in children 12–15 months of age. Adequate vaccination

against mumps is defined as two doses of MMR for school-aged chil­ dren (i.e., grades K–12) and for adults at high risk (i.e., health care workers, international travelers, and students at post–high school educational institutions) and one dose for preschool-aged children and adults not at high risk. During an outbreak, a second dose should be considered for children age 1–4 years and adults who have received one dose. In 2017, after an increase in cases among persons with two MMR doses and a study demonstrating added benefit of a third MMR vaccine dose for individual protection, a third dose was recommended for use during outbreaks, specifically for groups whom public health authorities identify as at increased risk of acquiring mumps; public health authorities will inform providers of these groups at increased risk. As the duration of protection provided by a third dose of MMR vaccine is unknown and may be short term (<1 year), there is no cur­ rent recommendation for a routine third dose.

The effectiveness of Jeryl Lynn containing MMR vaccine in pre­ venting mumps was 72% after one dose (RR 0.24, 95% CI 0.08 to 0.76; 6 cohort studies; 9915 children; moderate certainty evidence), 86% after two doses (RR 0.12, 95% CI 0.04 to 0.35; 5 cohort studies; 7792 children; moderate certainty evidence). The effectiveness of the mumps component is lower than that of the measles component (twodose effectiveness of 97%) and the rubella component (one-dose effec­ tiveness of 97%). Incremental vaccine effectiveness of a third MMR dose—compared with two doses—during outbreaks is estimated at 78% (range, 61–88%). In general, most recipients of mumps vaccine will seroconvert after vaccination and will have detectable antibodies to mumps virus; how­ ever, antibody levels start to decline soon after vaccination. Vaccineinduced neutralizing antibodies to wild-type strains may be lower in titer and may decline more rapidly than antibodies to the vaccine strain (Jeryl Lynn). However, most young adults given two vaccine doses in childhood appear to retain memory B cells. CHAPTER 213 Mumps vaccines are generally very safe. Urabe and LeningradZagreb mumps strain vaccines have been associated with a slightly increased risk of aseptic meningitis, but there is no evidence of this risk for Jeryl Lynn and RIT 4385 mumps strain vaccines. There is a twofold greater risk of febrile seizures among children 12–23 months of age after receipt of the first dose of MMRV vaccine than after the first dose of MMR vaccine, with or without simultaneous varicella vaccination; this risk has not been found among vaccinated children 4–6 years of age. Mumps There is no known immune correlate of protection for mumps; a positive IgG titer indicates only that a person has been exposed to mumps virus through either vaccination or natural infection and does not predict protection against infection. Therefore, all close contacts of a mumps patient should be advised to self-monitor for mumps symp­ toms for 25 days after their last exposure. Further, IgG titers should not be used to infer immunity in close contacts as it may indicate acute infection rather than immunity. MMR vaccine has not been shown to prevent illness or alter clinical severity in persons already infected with mumps virus and is not recommended as postexposure prophylaxis for immediate close contacts of mumps patients. Acknowledgment The authors acknowledge and thank Drs. Mariel Marlow and Stephen Rubin, authors of prior editions of this chapter. ■ ■FURTHER READING Di Pietrantonj C et al: Vaccines for measles, mumps, rubella, and varicella in children. Cochrane Database Syst Rev 4:CD004407, 2020. Krow-Lucal E et al: Measles, mumps, rubella vaccine (PRIORIX): Recommendations of the Advisory Committee on Immunization Practices—United States, 2022. MMWR Morb Mortal Wkly Rep 71: 1465, 2022. Marin M et al: Recommendation of the Advisory Committee on Immunization Practices for use of a third dose of mumps virus–con­ taining vaccine in persons at increased risk for mumps during an outbreak. MMWR Morb Mortal Wkly Rep 67:33, 2018. Masarani M et al: Mumps orchitis. J R Soc Med 99:573, 2006.

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214 Rabies and Other Rhabdovirus Infections

Mcclean HQ et al: Prevention of measles, rubella, congenital rubella

syndrome, and mumps, 2013: Summary recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR Recomm Rep 62:1, 2013. Rasheed MAU et al: Decreased humoral immunity to mumps in young adults immunized with MMR vaccine in childhood. Proc Natl Acad Sci USA 116:19071, 2019. Rota JS et al: Comparison of the sensitivity of laboratory diagnostic methods from a well-characterized outbreak of mumps in New York City in 2009. Clin Vaccine Immunol 20:391, 2013. Rubin S et al: Molecular biology, pathogenesis and pathology of mumps virus. J Pathol 235:242, 2015. Shepersky L et al: Mumps in vaccinated children and adolescents: 2007–2019. Pediatrics 148:e2021051873, 2021. Tappe J et al: Characteristics of reported mumps cases in the United States: Vaccine 2018-2023. Vaccine 2024. Available at https://pubmed

.ncbi.nlm.nih.gov/39019661/. World Health Organization: WHO immunological basis for immu­ nization series. Module 16: Mumps update 2020. Available at https:// apps.who.int/iris/bitstream/handle/10665/338004/9789240017504eng.pdf. Alan C. Jackson

Rabies and Other

Rhabdovirus Infections PART 5 Infectious Diseases RABIES Rabies is a rapidly progressive, acute infectious disease of the central nervous system (CNS) in humans and animals that is caused by infection with rabies virus. The infection is normally transmitted from animal vectors via a bite exposure. Rabies has encephalitic and paralytic forms that progress to death. ■ ■ETIOLOGIC AGENT Rabies virus is a member of the family Rhabdoviridae. Two genera in this family, Lyssavirus and Vesiculovirus, contain species that cause FIGURE 214-1  Distribution of global rabies vectors. (Courtesy of the Centers for Disease Control and Prevention.)

human disease. Rabies virus is a lyssavirus that infects a broad range of mammals and causes serious neurologic disease when transmit­ ted to humans. This single-strand RNA virus has a nonsegmented, negative-sense (antisense) genome that consists of 11,932 nucleotides and encodes five proteins: nucleocapsid protein, phosphoprotein, matrix protein, glycoprotein, and a large polymerase protein. Rabies virus variants, which can be characterized by distinctive nucleotide sequences, are associated with specific animal reservoirs. Six other non–rabies virus species in the Lyssavirus genus have been reported to cause a clinical picture similar to rabies. Vesicular stomatitis virus, a vesiculovirus, causes vesiculation and ulceration in cattle, horses, and other animals and causes a self-limited, mild, systemic illness in humans (see “Other Rhabdoviruses,” below). ■ ■EPIDEMIOLOGY Rabies is a zoonotic infection that occurs in a variety of mammals throughout the world except in Antarctica and on some islands. Rabies virus is usually transmitted to humans by the bite of an infected animal. Worldwide, endemic canine rabies is estimated to cause 59,000 human deaths annually. Most of these deaths occur in Asia and Africa, with rural populations and children disproportionally affected. Thus, in many resource-poor and resource-limited countries, canine rabies continues to be a threat to humans. However, in Latin America, rabies control efforts in dogs have been quite successful in recent years. Endemic canine rabies has been eliminated from the United States and most other resource-rich countries. Rabies is endemic in wildlife species, and a variety of animal reservoirs have been identified in different countries of the world (Fig. 214-1). Surveillance data from 2021 identified 3579 confirmed animal cases of rabies in the United States and Puerto Rico. Only 9.6% of these cases were in domestic animals, including 222 cases in cats,

50 in dogs, and 42 in cattle. In North American wildlife reservoirs, includ­ ing bats, raccoons, skunks, and foxes, the infection is endemic, with involvement of one or more rabies virus variants in each reservoir species (Fig. 214-2). “Spillover” of rabies to other wildlife species and to domestic animals occurs. Bat rabies virus variants are present in every state except Hawaii and are responsible for most indigenously acquired human rabies cases in the United States. Raccoon rabies is endemic along the entire eastern coast of the United States. Skunk rabies is present in the midwestern states, with another focus in California. Rabies in foxes occurs in New Mexico, Arizona, and Alaska. In the United States, there were five human rabies deaths in 2021 and none in 2019, 2020, and 2022. In Canada and Europe, epizootics of rabies in red foxes have been well controlled with the use of baits containing rabies vaccine. A similar approach, along with additional measures, is used in Canada to control incursions of raccoon rabies from the United States.

FIGURE 214-2  Distribution of the major rabies virus variants among wild terrestrial reservoirs in the United States and Puerto Rico, 2017–2021. Darker shading indicates counties with confirmed animal rabies cases in the past 5 years; lighter shading represents counties bordering enzootic counties without animal rabies cases that did not satisfy criteria for adequate surveillance. Small nonenzootic areas with no rabies cases reported in the past 15 years are shaded if they are in the vicinity of known enzootic counties and do not satisfy criteria for adequate surveillance. ARC FX rabies virus variant, Arctic fox RVV; AZ FX, Arizona fox RVV; CA SK, California skunk RVV; ERC, Eastern raccoon RVV; MG, Dog-mongoose RVV; NC SK, North central skunk RVV; SC SK, South central skunk RVV. (Reproduced with permission from X Ma et al: Rabies surveillance in the United States during 2021. J Am Vet Med Assoc 261:1045, 2023.) Rabies virus variants isolated from humans or other mammalian species can be identified by reverse-transcription polymerase chain reaction (RT-PCR) amplification and sequencing or by characteriza­ tion with monoclonal antibodies. These techniques are helpful in human cases with no known history of exposure. Worldwide, most human rabies (~99%) is transmitted from dogs in countries with endemic canine rabies and dog-to-dog transmission, and human cases can be imported by travelers returning from these regions. In North America, indigenously acquired human disease is usually associated with transmission from bats; there may be no known history of bat bite or other bat exposure in these cases. Most human cases are due to a bat rabies virus variant associated with silver-haired and tricolored bats and, less commonly, from Brazilian (Mexican) free-tail bats. These are small bats whose bite may not be recognized, and the virus has adapted for replication at skin temperature and in cell types that are present in the skin. Transmission from nonbite exposures is relatively uncommon. Aerosols generated in the laboratory or in caves containing millions of Brazilian free-tail bats have rarely caused human rabies. Transmission has resulted from corneal transplantation and also from solid-organ transplantation and a vascular conduit (for a liver transplant) from undiagnosed donors with rabies in Texas, Florida, Germany, Kuwait, Syria, and China. Human-to-human transmission is extremely rare, although hypothetical concern about transmission to health care work­ ers has prompted the implementation of barrier techniques to prevent exposures from patients with rabies. ■ ■PATHOGENESIS The incubation period of rabies (defined as the interval between expo­ sure and the onset of clinical disease) is usually 20–90 days, but in rare

CHAPTER 214 Rabies and Other Rhabdovirus Infections cases is either as short as a few days or >1 year. During most of the incubation period, rabies virus is thought to be present at or close to the site of inoculation (Fig. 214-3). In muscles, the virus is known to bind to nicotinic acetylcholine receptors on postsynaptic membranes at neuromuscular junctions, but the exact details of viral entry into the skin and subcutaneous tissues have not yet been clarified. Rabies virus spreads centripetally along peripheral nerves toward the spinal cord or brainstem via retrograde fast axonal transport (rate, up to ~250 mm/d), with delays at intervals of ~12 h at each synapse. Once the virus enters the CNS, it rapidly disseminates to other regions of the CNS via fast axonal transport along neuroanatomic connections. Neurons are prominently infected in rabies; infection of astrocytes is unusual. After CNS infection becomes established, there is centrifugal spread along sensory and autonomic nerves to other tissues, includ­ ing the salivary glands, heart, adrenal glands, and skin. Rabies virus replicates in acinar cells of the salivary glands and is secreted in the saliva of rabid animals that serve as vectors of the disease. There is no well-documented evidence for hematogenous spread of rabies virus. Pathologic studies show mild inflammatory changes in the CNS in rabies, with mononuclear inflammatory infiltration in the leptomen­ inges, perivascular regions, and parenchyma, including microglial nodules called Babes nodules. Degenerative neuronal changes usually are not prominent, and there is little evidence of neuronal death; neuronophagia is observed occasionally. The pathologic changes are surprisingly mild in light of the clinical severity and fatal outcome of the disease. The most characteristic pathologic finding in rabies is the Negri body (Fig. 214-4). Negri bodies are eosinophilic cytoplasmic inclusions in brain neurons that are composed of rabies virus pro­ teins and viral RNA. These inclusions occur in a minority of infected neurons, are commonly observed in Purkinje cells of the cerebellum

Brain

Centrifugal spread along nerves to salivary glands, skin, cornea, and other organs Eye Salivary glands

Virus binds to nicotinic acetylcholine receptors at neuromuscular junction

Replication in motor neurons of the spinal cord and local dorsal root ganglia and rapid ascent to brain Dorsal root ganglion Sensory nerves to skin Skeletal muscle

Virus travels within axons in peripheral nerves via retrograde fast axonal transport PART 5 Infectious Diseases Spinal cord Viral replication in muscle

Virus inoculated

FIGURE 214-3  Schematic representation of events in rabies pathogenesis following peripheral inoculation of rabies virus by an animal bite. (Reproduced with permission from AC Jackson: Pathogenesis, in Rabies: Scientific Basis of the Disease and Its Management, 3rd ed. Oxford, UK, Elsevier Academic Press, 2013.) and in pyramidal neurons of the hippocampus, and are less frequently seen in cortical and brainstem neurons. Negri bodies are not observed in all cases of rabies. The lack of prominent degenerative neuronal changes has led to the concept that neuronal dysfunction—rather than neuronal death—is responsible for clinical disease in rabies. Experimental studies indicate that oxidative stress due to mitochon­ drial dysfunction plays an important role. The basis for behavioral changes, including the aggressive behavior of rabid animals, is not well understood but may be related to infection of serotonergic neu­ rons in the brainstem. ■ ■CLINICAL MANIFESTATIONS For rabies prevention, the emphasis must be on postexposure prophy­ laxis (PEP) initiated after a recognized exposure and before any symp­ toms or signs develop. Rabies should usually be suspected on the basis of the clinical presentation with or without a history of an exposure. The disease generally presents as atypical encephalitis with relative preservation of consciousness. Rabies may be difficult to recognize late in the clinical course when progression to coma has occurred. A minority of patients (~20%) present with acute flaccid paralysis. There are prodromal, acute neurologic, and comatose phases that usually progress to death despite aggressive therapy (Table 214-1).

Prodromal Features  The clinical features of rabies begin with nonspecific prodromal manifestations, including fever, malaise, head­ ache, nausea, and vomiting. Anxiety or agitation also may occur. The earliest specific neurologic symptoms of rabies include paresthesias, pain, or pruritus near the site of the exposure, one or more of which occur in 50–80% of patients and strongly suggest rabies. The wound has usually healed by this point, and these symptoms prob­ ably reflect infection with associated inflam­ matory changes in local dorsal root or cranial sensory ganglia.

Infection of brain neurons with neuronal dysfunction Encephalitic Rabies  Two acute neuro­ logic forms of rabies are seen in humans: the encephalitic (furious) form in 80% and the paralytic form in 20%. Some of the manifesta­ tions of encephalitic rabies, including fever, confusion, hallucinations, combativeness, and seizures, may be seen in other viral encephaliti­ des as well. Autonomic dysfunction is common in rabies and may result in hypersalivation, gooseflesh, cardiac arrhythmia, and priapism. In encephalitic rabies, episodes of hyperexcitability are typically followed by periods of complete lucidity that become shorter as the disease progresses. Rabies encephalitis is distinguished by early brainstem involvement, which results in the classic features of hydrophobia (involun­ tary, painful contraction of the diaphragm and accessory respiratory, laryngeal, and pharyn­ geal muscles in response to swallowing liquids)

(Fig. 214-5) and aerophobia (the same features caused by stimulation from a draft of air). These symptoms are probably due to dysfunction of infected brainstem neurons that normally inhibit inspiratory neurons near the nucleus ambiguus, resulting in exaggerated defense reflexes that protect the respiratory tract. The combination of hypersalivation and pharyngeal dysfunction is responsible for the classic appearance of “foaming at the mouth.” Brainstem dysfunction progresses rapidly, and coma—followed within days by death—is the rule unless the course is prolonged by supportive measures. With such measures, late complications can include cardiac and/or respiratory failure, disturbances of water balance (syndrome of inappropriate antidiuretic hormone secretion or diabetes insipidus), noncardiogenic pulmonary edema, and gastrointestinal hemorrhage. Cardiac arrhythmias may be due to dysfunction affecting vital centers in the brainstem or autonomic pathways or to myocarditis. Multipleorgan failure is common in patients treated aggressively in critical care units. Paralytic Rabies  About 20% of patients have paralytic rabies in which muscle weakness predominates and cardinal features of encephalitic rabies (hyperexcitability, hydrophobia, and aerophobia) are lacking. There is early and prominent flaccid muscle weakness, often beginning in the bitten extremity and spreading to produce quadriparesis and facial weakness. Sphincter involvement is common, sensory involvement is usually mild, and these cases are commonly misdiagnosed as Guillain-Barré syndrome. Patients with paralytic rabies generally survive a few days longer than those with encephalitic rabies, but multiple-organ failure nevertheless ensues. ■ ■LABORATORY INVESTIGATIONS Most routine laboratory tests in rabies yield normal results or show nonspecific abnormalities. Complete blood counts are usually nor­ mal. Examination of cerebrospinal fluid (CSF) often reveals mild

FIGURE 214-4  Three large Negri bodies in the cytoplasm of a cerebellar Purkinje cell from an 8-year-old boy who died of rabies after being bitten by a rabid dog in Mexico. (Reproduced with permission from AC Jackson, E Lopez-Corella. N Engl J Med 335:568, 1996; © Massachusetts Medical Society.) mononuclear-cell pleocytosis with a mildly elevated protein level. Severe pleocytosis (>1000 white cells/μL) is unusual and should prompt a search for an alternative diagnosis. Imaging is usually per­ formed to exclude other diagnostic possibilities. Computed tomogra­ phy (CT) head scans are usually normal in rabies. Magnetic resonance imaging (MRI) brain scans may show signal abnormalities in the brain­ stem or other gray-matter areas, but these findings are variable and nonspecific. Electroencephalograms typically show only nonspecific abnormalities. Of course, important tests in suspected cases of rabies include those that may identify an alternative, potentially treatable diagnosis (see “Differential Diagnosis,” below). ■ ■DIAGNOSIS In North America, a diagnosis of rabies often is not considered until relatively late in the clinical course, even with a typical clinical pre­ sentation. This diagnosis should be considered in patients presenting with acute atypical encephalitis or acute flaccid paralysis, including those in whom Guillain-Barré syndrome is suspected. The absence of an animal-bite history is common in North America, particularly due to unrecognized bat exposures. The lack of hydrophobia is not unusual in rabies. Once rabies is suspected, rabies-specific laboratory tests should be performed to confirm the diagnosis. Diagnostically TABLE 214-1  Clinical Stages of Rabies STAGE TYPICAL DURATION SYMPTOMS AND SIGNS Incubation period 20–90 days None Prodrome 2–10 days Fever, malaise, anorexia, nausea, vomiting; paresthesias, pain, or pruritus at the wound site Acute Neurologic Disease   Encephalitic (80%) 2–7 days Anxiety, agitation, hyperactivity, bizarre behavior, hallucinations, autonomic dysfunction, hydrophobia   Paralytic (20%) 2–10 days Flaccid paralysis in limb(s) progressing to quadriparesis with facial paralysis Coma, deatha 0–14 days   aRecovery is rare. Source: Adapted from MAW Hattwick: Rabies virus, in Principles and Practice of Infectious Diseases, GL Mandell et al (eds). New York, Wiley, 1979.

FIGURE 214-5  Hydrophobic spasm of inspiratory muscles associated with terror in a patient with encephalitic (furious) rabies who is attempting to swallow water. (Copyright DA Warrell, Oxford, UK; with permission.) CHAPTER 214 useful specimens include serum, CSF, fresh saliva, skin biopsy samples from the neck, and brain tissue (rarely obtained before death). Because skin biopsy relies on the demonstration of rabies virus antigen and/or RNA in cutaneous nerves at the base of hair follicles, samples are usu­ ally taken from hairy skin at the nape of the neck. Corneal impression smears are of low diagnostic yield and are generally not performed. Negative antemortem rabies-specific laboratory tests never exclude a diagnosis of rabies, and tests may need to be repeated after an interval for diagnostic confirmation. Rabies Virus–Specific Antibodies  In a previously unimmu­ nized patient, serum neutralizing antibodies to rabies virus are diagnostic. However, because rabies virus infects immunologically privileged neuronal tissues, serum antibodies may not develop until late in the disease. Antibodies may be detected within a few days after the onset of symptoms, but some patients die without detectable anti­ bodies. The presence of rabies virus–specific neutralizing antibodies in the CSF suggests rabies encephalitis, regardless of immunization status. A diagnosis of rabies is questionable in patients who recover from their illness without developing serum neutralizing antibodies to rabies virus. RT-PCR Amplification  Detection of rabies virus RNA by RTPCR is highly sensitive and specific. This technique can detect virus in fresh saliva samples, skin biopsy specimens, CSF (less sensitive), and brain tissues. In addition, RT-PCR with genetic sequencing can distinguish among rabies virus variants, permitting identification of the probable source of an infection. Direct Fluorescent Antibody Testing  Direct fluorescent anti­ body (DFA) testing with rabies virus antibodies conjugated to fluo­ rescent dyes is highly sensitive and specific for the detection of rabies virus antigen in tissues; the test can be performed quickly and applied to skin biopsy and brain tissue samples. In skin biopsy samples, rabies virus antigen may be detected in cutaneous nerves at the base of hair follicles. Rabies and Other Rhabdovirus Infections ■ ■DIFFERENTIAL DIAGNOSIS The diagnosis of rabies may be difficult without a history of animal exposure, and no exposure to an animal (e.g., a bat) may be recalled. The presentation of rabies is usually quite different from that of acute

viral encephalitis due to most other causes, including herpes simplex encephalitis and arboviral (e.g., West Nile) encephalitis. Early neuro­ logic symptoms may occur at the site of the bite, and there may be early features of brainstem involvement with preservation of consciousness. Anti–N-methyl-d-aspartate receptor (anti-NMDA) encephalitis occurs in young patients (especially females) and is characterized by behav­ ioral changes, autonomic instability, hypoventilation, and seizures. Many other antibodies are also associated with autoimmune encepha­ litis. Postinfectious (immune-mediated) encephalomyelitis may follow influenza, measles, mumps, and other infections; it may also occur as a sequela of immunization with rabies vaccines derived from neural tissues, which are now infrequently used and only in resource-limited and resource-poor countries. Rabies may present with unusual neuro­ psychiatric symptoms and may be misdiagnosed as a psychiatric dis­ order. Rabies hysteria (now classified as a somatic symptom disorder) may occur as a psychological response to the fear of rabies and is often characterized by a shorter incubation period than rabies, aggressive behavior, inability to communicate, and a long course with recovery.

As previously mentioned, paralytic rabies may mimic GuillainBarré syndrome. In these cases, fever, bladder dysfunction, a normal sensory examination, and CSF pleocytosis favor a diagnosis of rabies. Conversely, Guillain-Barré syndrome may occur as a complication of rabies vaccination with a neural tissue–derived product (e.g., suckling mouse brain vaccine) and may be mistaken for paralytic rabies (i.e., vaccine failure). TREATMENT Rabies PART 5 Infectious Diseases There is no established treatment for rabies. Aggressive manage­ ment with supportive care in critical care units has resulted in the survival of at least 33 patients with rabies. Many of these survivors have recently been reported from India. There have been many recent treatment failures (>60) with the combination of therapeutic (induced) coma, antiviral drugs, and sometimes ketamine— measures that were used in a healthy survivor in whom neutralizing antibodies to rabies virus were detected at presentation. No antiviral therapy has demonstrated any efficacy in the treatment of rabies. Expert opinion is recommended before a course of experimental therapy is embarked upon. A palliative approach may be appropri­ ate for many patients who are not considered candidates for aggres­ sive management. ■ ■PROGNOSIS Rabies is an almost uniformly fatal disease but is nearly always prevent­ able after recognized exposures with appropriate postexposure therapy during the early incubation period (see below). All but 2 of 33 docu­ mented survivors of rabies received one or more doses of rabies vaccine before disease onset. The survivors who had not received vaccine had neutralizing antibodies to rabies virus in CSF at clinical presentation. Most patients with rabies die within several days of the onset of illness, despite aggressive care in a critical care unit. ■ ■PREVENTION Postexposure Prophylaxis  Since there is no effective therapy for rabies, it is extremely important to prevent the disease after an animal exposure. Figure 214-6 shows the steps involved in making decisions about PEP. Based on the exposure history and local epidemiologic information, the physician must decide whether initiation of PEP is warranted. Healthy dogs, cats, or ferrets may be confined and observed for 10 days. PEP is not necessary if the animal remains healthy. If the animal develops signs of rabies during the observation period, it should be euthanized immediately; the head should be transported to the laboratory under refrigeration, rabies virus should be sought by DFA testing, and viral isolation should be attempted by cell culture and/or mouse inoculation. Any animal other than a dog, cat, or ferret should be euthanized immediately and the head submitted for laboratory

Rabies prophylaxis Did the animal bite the patient or did saliva contaminate a scratch, abrasion, open wound, or mucous membrane? No None Yes Is rabies known or suspected to be present in the species and the geographic area? None No Yes No Was the animal captured? RIG and vaccine Yes Yes Was the animal a normally behaving dog, cat, or ferret? Does the animal become ill under observation over the next 10 days? Yes No No Does laboratory examination of the brain by fluorescent antibody staining confirm rabies? No None Yes RIG and vaccine FIGURE 214-6  Algorithm for rabies postexposure prophylaxis. RIG, rabies immune globulin. (From L Corey, in Harrison’s Principles of Internal Medicine, 15th ed. E Braunwald et al [eds]: New York, McGraw-Hill, 2001.) examination. In high-risk exposures and in areas where canine rabies is endemic, rabies prophylaxis should be initiated without waiting for laboratory results. If the laboratory results prove to be negative, it may safely be concluded that the animal’s saliva did not contain rabies virus, and immunization should be discontinued. If an animal escapes after an exposure, it must be considered rabid, and PEP must be initiated unless information from public health officials indicates otherwise (i.e., there is no endemic rabies in the area). Although controversial, the use of PEP may be warranted when a person (e.g., a small child or a sleeping adult) has been present in the same space as a bat and an unrecognized bite cannot be reliably excluded. PEP includes local wound care and both active and passive immuni­ zation. It is important that current recommendations are followed very closely because minor deviations can lead to failure of prophylactic measures. Local wound care is essential and may greatly decrease the risk of rabies virus infection. Wound care should not be delayed, even if the initiation of immunization is postponed pending the results of the 10-day observation period. All bite wounds and scratches should be washed thoroughly with soap and water. Devitalized tissues should be debrided, tetanus prophylaxis given, and antibiotic treatment initiated whenever indicated. Previously unvaccinated persons (but not those who have previ­ ously been immunized) should be passively immunized with rabies immune globulin (RIG). If RIG is not immediately available, it should be administered no later than 7 days after the first vaccine dose. After day 7, endogenous antibodies are being produced, and passive immu­ nization may actually be counterproductive. If anatomically feasible, the entire dose of RIG (20 IU/kg) should be infiltrated at the site of the bite, and any RIG remaining after infiltration of the bite site should be administered IM at a distant site. Recent recommendations by the World Health Organization indicate that under certain circumstances the remainder of the dose does not need to be administered after local

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215 Arthropod-Borne and Rodent-Borne Virus Infections

infiltration of the wound(s). With multiple or large wounds, the RIG preparation may need to be diluted in order to obtain a sufficient volume for adequate infiltration of all wound sites. If the exposure involves a mucous membrane, the entire dose should be administered IM. Rabies vaccine and RIG should never be administered at the same site or with the same syringe. Commercially available RIG in the United States is purified from the serum of hyperimmunized human donors. These human RIG preparations are much better tolerated than are the equine-derived preparations still in use in some countries (see below). Serious adverse effects of human RIG are uncommon. Local pain and low-grade fever may occur. Two purified inactivated rabies vaccines are available for rabies PEP in the United States. They are highly immunogenic and remarkably safe compared with earlier vaccines. Four 1-mL doses of rabies vaccine should be given IM in the deltoid area. (The anterolateral aspect of the thigh also is acceptable in children.) Gluteal injections, which may not always reach muscle, should not be given and have been associated with rare vaccine failures. Ideally, the first dose should be given as soon as possible after exposure; failing that, it should be given without further delay. The three additional doses should be given on days 3, 7, and 14; a fifth dose on day 28 is no longer recommended except in immunocompromised patients. Pregnancy is not a contraindication for immunization. Glucocorticoids and other immunosuppressive medi­ cations may interfere with the development of active immunity and should not be administered during PEP unless they are essential. Rou­ tine measurement of serum neutralizing antibody titers is not required, but titers should be measured 2–4 weeks after immunization in immu­ nocompromised persons. Local reactions (pain, erythema, edema, and pruritus) and mild systemic reactions (fever, myalgias, headache, and nausea) are common; anti-inflammatory and antipyretic medications may be used, but immunization should not be discontinued. Systemic allergic reactions are uncommon, but anaphylaxis does occur rarely and can be treated with epinephrine and antihistamines. The risk of rabies development should be carefully considered before the decision is made to discontinue vaccination because of an adverse reaction. Most of the burden of rabies PEP is borne by persons with the fewest resources. In addition to the rabies vaccines discussed above, vaccines grown in either primary cell lines (hamster or dog kidney) or con­ tinuous cell lines (Vero cells) are satisfactory and are available in many countries outside the United States. Less expensive vaccines derived from neural tissues are still used in a diminishing number of develop­ ing countries; however, these vaccines are associated with serious neu­ roparalytic complications, including postinfectious encephalomyelitis and Guillain-Barré syndrome. The use of these vaccines should be discontinued as soon as possible, and progress has been made in this regard. Worldwide, more than 10 million individuals receive postexpo­ sure rabies vaccine each year. If human RIG is unavailable, purified equine RIG can be used in the same manner at a dose of 40 IU/kg. The incidence of anaphylactic reac­ tions and serum sickness has been low with recent equine RIG products. Preexposure Rabies Vaccination  Preexposure rabies prophy­ laxis should be considered for people with an occupational or rec­ reational risk of rabies exposures and also for certain travelers to rabies-endemic areas. The primary schedule consists of two doses of rabies vaccine given on days 0 and 7. Serum neutralizing antibody tests help determine the need for subsequent booster doses. When a previ­ ously immunized individual is exposed to rabies, two booster doses of vaccine should be administered on days 0 and 3. Wound care remains essential. As stated above, RIG should not be administered to previ­ ously vaccinated persons. OTHER RHABDOVIRUSES ■ ■OTHER LYSSAVIRUSES A growing number of lyssaviruses other than rabies virus have been discovered to infect bat populations in Europe, Africa, Asia, and Australia. Six of these viruses have produced a very small number of cases of a human disease indistinguishable from rabies, including European bat lyssaviruses 1 and 2, Australian bat lyssavirus, Irkut virus,

and Duvenhage virus. Mokola virus, a lyssavirus that has been isolated from shrews with an unknown reservoir species in Africa, may also produce human disease indistinguishable from rabies.

■ ■VESICULAR STOMATITIS VIRUS Vesicular stomatitis is a viral disease of cattle, horses, pigs, and some wild mammals. Vesicular stomatitis virus is a member of the genus Vesiculovirus in the family Rhabdoviridae. Outbreaks of vesicular stoma­ titis in horses and cattle occur sporadically in the southwestern United States. The animal infection is associated with severe vesiculation and ulceration of oral tissues, teats, and feet and may be clinically indistin­ guishable from the more dangerous foot-and-mouth disease. Epidemics are usually seasonal, typically beginning in the late spring, and are prob­ ably due to arthropod vectors. Direct animal-to-animal spread can also occur, although the virus cannot penetrate intact skin. Transmission to humans usually results from direct contact with infected animals (particularly cattle) and occasionally follows laboratory exposure. In human disease, early conjunctivitis is followed by an acute influenza-like illness with fever, chills, nausea, vomiting, headache, retrobulbar pain, myalgias, substernal pain, malaise, pharyngitis, and lymphadenitis. Small vesicular lesions may be present on the buccal mucosa or on the fingers. Encephalitis is very rare. The illness usually lasts 3–6 days, with complete recovery. Subclinical infections are common. A serologic diagnosis can be made on the basis of a rise in titer of complement-fixing or neutral­ izing antibodies. Therapy is symptom-based. ■ ■FURTHER READING Fooks AR et al: Current status of rabies and prospects for elimination. Lancet 384:1389, 2014. Fooks AR, Jackson AC (eds): Rabies: Scientific Basis of the Disease CHAPTER 215 and Its Management, 4th ed. London, Elsevier Academic Press, 2020. Jackson AC: Treatment of rabies. In: Post TW, ed. UpToDate. Waltham, Massachusetts: Wolters Kluwer, 2023. www.uptodate.com. Letchworth GJ et al: Vesicular stomatitis. Vet J 157:239, 1999. Manning SE et al: Human rabies prevention—United States, 2008: Arthropod-Borne and Rodent-Borne Virus Infections
Recommendations of the Advisory Committee on Immunization Practices. MMWR Recomm Rep 57:1, 2008. World Health Organization: WHO Expert Consultation on Rabies: Third Report (WHO Technical Report Series No. 1012). Geneva, World Health Organization, 2018. Available at https://iris.who.int/ bitstream/handle/10665/272364/9789241210218-eng.pdf. Accessed September 20, 2024. Jens H. Kuhn, Ian Crozier

Arthropod-Borne and

Rodent-Borne Virus

Infections This chapter summarizes the major features of selected arthropodborne and rodent-borne viruses and associated infections and/or disease. Numerous viruses of this category are spread in nature among animals without ever infecting humans. Other viruses incidentally infect humans, with few causing disease. Some viruses are regularly introduced into the human population by arthropods (specifically, insects and ticks) or by chronically infected rodents. These zoonotic viruses are taxonomically diverse and therefore dif­ fer fundamentally in virion morphology, replication strategy, genomic organization, and genome sequence. Although a virus’s classification in a taxon is enlightening regarding natural maintenance strategies, sensitivity to antiviral agents, and aspects of pathogenesis, it does not

necessarily predict which clinical symptoms and signs (if any) the viral infection will cause in humans. Zoonotic viruses are evolving, and “new” zoonotic viruses are regularly discovered. The epizootiol­ ogy and epidemiology of zoonotic viruses continue to change because of environmental alterations affecting vectors, reservoirs, and hosts, including wildlife, livestock, and humans. Zoonotic viruses are most numerous in the tropics but are also found in temperate and frigid climates. The distribution and seasonal activity of zoonotic viruses may vary in a manner likely to depend largely on ecologic conditions (e.g., rainfall and temperature), which can affect the density of virus vectors and reservoirs, likelihood of exposure, and development of infection.

Arthropod-borne viruses (arboviruses) infect their vectors after ingestion of blood meals from viremic, usually nonhuman vertebrates; some infections may be the result of saliva-activated transmission. Arthropod vectors develop chronic systemic infection as the viruses penetrate their guts and spread throughout their bodies to their sali­ vary glands; such virus dissemination, referred to as extrinsic incuba­ tion, typically lasts 1–3 weeks in mosquitoes. If the salivary glands become involved, the arthropod vectors are competent to continue the chain of transmission by infecting vertebrates during subsequent blood meals. Alternatively, virus maintenance in arthropod vectors may be achieved through transovarial transmission to progeny. Generally, the arthropods are unharmed by the infections and natural vertebrate part­ ners have only transient viremia without overt disease. Rodent-borne viruses are maintained in nature by transmission among rodents, which become chronically infected. A high degree of rodent–virus specificity may be observed, and overt disease in the reservoir hosts is rare. ETIOLOGY Arthropod-borne and rodent-borne zoonotic viruses belong mostly to the classes Alsuviricetes (family Togaviridae), Bunyaviricetes (fami­ lies Arenaviridae, Hantaviridae, Nairoviridae, Peribunyaviridae, and

Phenuiviridae), Flasuviricetes (family Flaviviridae), Insthoviricetes (family Orthomyxoviridae), Monjiviricetes (families Bornaviridae and Rhabdoviridae), and Resentoviricetes (families Sedoreoviridae and

Spinareoviridae) (Table 215-1). An exception is Syr-Darya Valley fever virus, an ixodid tick-borne cardiovirus (Pisoniviricetes: Picornaviridae) that causes febrile disease and has been found in Central Asia. PART 5 Infectious Diseases ■ ■ALSUVIRICETES: TOGAVIRIDAE Members of the family Togaviridae have linear, positive-sense RNA genomes (≈9.7–11.8 kb) and form enveloped icosahedral virions

(≈60–70 nm in diameter) that bud from the plasma membrane of the infected cell. The togavirids discussed in this chapter are all members of the genus Alphavirus and are transmitted to vertebrates by mosquitoes. ■ ■BUNYAVIRICETES: ARENAVIRIDAE Members of the family Arenaviridae that infect humans are all assigned to the genus Mammarenavirus. Mammarenaviruses form spherical, oval, or pleomorphic enveloped and spiked virions (≈50–300 nm in diameter) that bud from the plasma membrane of the infected cell. The particles contain two genomic single-stranded RNAs (small [S], ≈3.5 kb, and large [L], ≈7.5 kb), encoding structural proteins in an ambisense orientation. Most mammarenaviruses persist by chroni­ cally infecting rodents. Human mammarenaviruses are maintained by muroid rodents that are often continuously viremic and commonly transmit viruses vertically and horizontally. One mammarenavirus associated with human infections is maintained by shrews. Strikingly, each mammarenavirus is predominantly adapted to one particular type of rodent. Humans usually become infected through inhalation of or direct contact with infected rodent excreta or secreta (e.g., from aerosolized whole rodents in harvesting machines or aerosolized dried urine or feces when sweeping floors in barns or houses) and direct contact with rodents in traps. Person-to-person transmission of mam­ marenaviruses occurs but is uncommon. ■ ■BUNYAVIRICETES: HANTAVIRIDAE, NAIROVIRIDAE, PERIBUNYAVIRIDAE, AND PHENUIVIRIDAE Members of these families that infect humans form spherical to pleomorphic enveloped virions containing three genomic RNAs

(S, ≈1–2 kb; medium [M], 3.6–5.3 kb; and L, 6.4–12.3 kb) of negativesense (hantavirids, nairovirids, and peribunyavirids) or negative-sense or ambisense polarities (phenuivirids). These bunyaviricetes mature into particles ≈80–120 nm in diameter in the Golgi complexes of infected cells and exit these cells by exocytosis. Hantavirids that infect humans are classified in the genus Orthohan­ tavirus and are maintained by muroid rodents that chronically shed virions. As with mammarenaviruses, individual orthohantaviruses are usually specifically adapted to a particular type of rodent. However, orthohantaviruses do not cause chronic viremia in their rodent hosts and are transmitted only horizontally from rodent to rodent. Also like mammarenaviruses, orthohantaviruses infect humans primarily through inhalation of or direct contact with rodent excreta or secreta, and person-to-person transmission occurs but is not common (with the notable exception of Andes virus). Nairovirids that infect humans are classified in the genus Orthonai­ rovirus. Orthonairoviruses are predominantly maintained by ixodid ticks, which transmit these viruses vertically (transovarially and trans­ stadially) to progeny and horizontally through contact with viremic vertebrate hosts and subsequent transmission to healthy vertebrates. Humans are usually infected via a tick bite or during handling of infected vertebrates. Peribunyavirids of one genus (Orthobunyavirus) infect humans. Orthobunyaviruses are primarily mosquito-borne (and, in rare cir­ cumstances, transmitted by midges or sandflies) and have viremic vertebrate intermediate hosts. Many orthobunyaviruses are transmitted vertically (transovarially) to their mosquito hosts. Numerous orthobu­ nyaviruses have been associated with human infection and disease. Phenuivirids are transmitted vertically (transovarially) in their arthropod vectors and horizontally through viremic vertebrate hosts. Human phenuivirids are found in four genera: Bandavirus, Phlebo­ virus, Tanzavirus, and Uukuvirus. Bandaviruses and uukuviruses are transmitted by ticks, whereas human phleboviruses are transmitted by sandflies (the ecology of tanzaviruses is unclear). ■ ■FLASUVIRICETES: FLAVIVIRIDAE The family Flaviviridae currently includes only one genus (Orthoflavi­ virus) that comprises arthropod-borne human viruses. Orthoflavivi­ ruses have single-stranded positive-sense RNA genomes (≈11 kb) and form spherical enveloped particles or virions (40–60 nm in diameter). The orthoflaviviruses discussed in this chapter belong to two phyloge­ netically and antigenically distinct groups that are transmitted among vertebrates by mosquitoes (generally, Aedes and Culex spp.) and, typi­ cally, ixodid ticks, respectively. Vectors are usually infected when they feed on viremic hosts. As in the case of most other viruses discussed in this chapter, humans are accidental hosts, typically infected by arthro­ pod bites. Arthropods maintain orthoflavivirus infections horizontally, although vertical (transovarial) transmission has been documented. Under some circumstances, orthoflaviviruses can be transmitted by aerosol or via contaminated food products; in particular, raw milk can transmit tick-borne encephalitis virus. ■ ■INSTHOVIRICETES: ORTHOMYXOVIRIDAE The family Orthomyxoviridae includes two genera of medically rele­ vant arthropod-borne viruses: Quaranjavirus and Thogotovirus. Quar­ anjaviruses are transmitted among birds by argasid ticks, whereas thogotoviruses have a predilection for mammalian host reservoirs and can be transmitted by argasid and ixodid ticks and mosquitoes. ■ ■MONJIVIRICETES: BORNAVIRIDAE

AND RHABDOVIRIDAE Bornavirids and rhabdovirids that infect humans have linear non­ segmented, negative-sense RNA genomes (bornavirids, ≈9 kb; rhab­ dovirids, ≈11–15 kb) and form spherical (bornavirids, 90–130 nm in diameter) and bullet-shaped to pleomorphic enveloped particles (rhab­ dovirids, 100–430 nm long and 45–100 nm wide). The only known rodent-borne bornavirid that infects humans is a member of genus Orthobornavirus and is transmitted by squirrels. The rhabdovirid genus Tibrovirus contains several uncharacterized human pathogens

TABLE 215-1  Zoonotic Arthropod- and Rodent-Borne Viruses That Infect Humans VIRUS TAXON VIRUS (ABBREVIATION) MAJOR NONHUMAN HOST(S)a VECTOR(S) SYNDROMEb Alphavirus (family Togaviridae) Barmah Forest virus (BFV) Cattle, horses, marsupials? Biting midges (Culicoides marksi), mosquitoes (Aedes camptorhynchus, Ae. normanensis, Ae. notoscriptus, Ae. vigilax, Culex annulirostris)   Chikungunya virus (CHIKV) Bats, nonhuman primates Mosquitoes (predominantly Aedes aegypti and Ae. Albopictus)   Eastern equine encephalitis virus (EEEV) Freshwater swamp passeriform birds, but also opportunistic amphibians, other birds (emu, gallinaceous poultry, pheasants), reptiles, and mammals (goats, horses, pigs, rodents)   Everglades virus (EVEV) Cotton deermice (Peromyscus gossypinus), hispid cotton rats (Sigmodon hispidus)   Madariaga virus (MADV) Likely bats, marsupials, reptiles, rodents Mosquitoes (Culex, Culiseta spp.) F&M, E   Mayaro virus (MAYV) Nonhuman primates, possums, rodents; possibly caimans, horses, sheep   Mucambo virus (MUCV) Nonhuman primates, rodents Mosquitoes (Culex, Ochlerotatus spp.) F&M, E   O’nyong-nyong virusd (ONNV) Unknown Mosquitoes (in particular Anopheles gambiae, A. funestus, Mansonia spp.)   Ross River virus (RRV) Marsupials, rodents Mosquitoes (Aedes normanensis, Ae. vigilax, Culex annulirostris)   Semliki Forest virus (SFV) Birds, rodents Mosquitoes (Aedes, Culex spp.) A&R   Sindbis viruse (SINV) Typically birds, but also frogs and rats Typically mosquitoes (Culex, Culiseta spp.), but tick isolation has been reported   Tonate virus (TONV) Birds, Suriname crested oropendolas (Psarocolius decumanus)   Una virus (UNAV) Birds, horses, nonhuman primates, rodents   Venezuelan equine encephalitis virus (VEEV) Equids, rodents Mosquitoes (Aedes, Culex spp., Psorophora confinnis)   Western equine encephalitis virus (WEEV) Equids, lagomorphs, passeriform birds, pheasants Bandavirus (family Phenuiviridae) Bhanja virusf (BHAV) Cattle, four-toed hedgehog (Atelerix albiventris), goats, sheep, striped ground squirrels (Xerus erythropus)   Heartland virus (HRTV) Cattle, deer, elk, goats, raccoons, sheep? Ixodid ticks (Amblyomma americanum) F&M   Severe fever with thrombocytopenia syndrome virusg (SFTSV) Camels, cats, cattle, chickens, dogs, goats, hedgehogs, pigs, rodents, sheep? Coltivirus (family Spinareoviridae) Colorado tick fever virus (CTFV) Predominantly golden-mantled ground squirrels (Spermophilus lateralis), but also bushy-tailed woodrats (Neotoma cinerea), Columbian ground squirrels (Spermophilus columbianus), eastern deermice (Peromyscus maniculatus), least chipmunks (Tamias minimus), North American porcupines (Erethizon dorsata), Richardson ground squirrel (Spermophilus richardsonii), Uinta chipmunks (Tamias umbrinus), yellowpine chipmunks (Tamias amoenus) among lesser important mammal hosts   Eyach virus (EYAV) Lagomorphs, rodents Ixodid ticks (Ixodes ricinus, I. ventalloi) F&M, E   Salmon River virus (SRV) Unknown Ixodid ticks (Ixodes spp.) F&M, E Mammarenavirus (family Arenaviridae) Chapare virus (CHAPV) Unidentified cricetid rodents None VHF   Flexal virus (FLEV) Unidentified cricetid rodents None (F&M)   Guanarito virus (GTOV) Predominantly short-tailed zygodonts (Zygodontomys brevicauda) but also Alston’s cotton rats (Sigmodon alstoni)

A&R A&Rc Mosquitoes (Aedes, Coquillettidia, Culex spp., Anopheles quadrimaculatus, Culiseta melanura, Mansonia perturbans, Psorophora spp.) E Mosquitoes (Culex cedecei) F&M, E Mosquitoes (predominantly Haemagogus spp., but also Aedes, Culex, Mansonia, Psorophora, Sabethes spp.) A&R A&R A&R A&R CHAPTER 215 Mosquitoes (Anopheles, Coquillettidia, Culex, Mansonia, Uranotaenia, Wyeomyia spp.), sandflies (Lutzomyia spp.) F&M, E Mosquitoes (Aedes, Anopheles, Coquillettidia, Culex, Ochlerotatus, Psorophora spp.) F&M Arthropod-Borne and Rodent-Borne Virus Infections
F&M, E Mosquitoes (Aedes spp., Culex tarsalis, Culiseta spp.) E Ixodid ticks (Amblyomma, Dermacentor, Haemaphysalis, Hyalomma, Rhipicephalus spp.) F&M, E Ixodid ticks (predominantly Haemaphysalis longicornis, but also Amblyomma testudinarium, H. concinna, H. flava, Ixodes nipponensis, Rhipicephalus microplus) F&M, VHF Ixodid ticks (Dermacentor andersoni, possibly D. occidentalis) F&M, E None VHF (Continued)

TABLE 215-1  Zoonotic Arthropod- and Rodent-Borne Viruses That Infect Humans VIRUS TAXON VIRUS (ABBREVIATION) MAJOR NONHUMAN HOST(S)a VECTOR(S) SYNDROMEb   Junín virus (JUNV) Predominantly drylands lauchas (Calomys musculinus) but also Azara’s akodonts (Akodon azarae) and little lauchas (Calomys laucha)   Lassa virus (LASV) Predominantly Natal mastomys (Mastomys natalensis), but also reddishwhite mastomys (M. erythroleucus), African hylomuscus (Hylomyscus pamfi), and Baoule’s mice (Mus baoulei)   Lymphocytic choriomeningitis virus (LCMV) Predominantly house mice (Mus musculus), but also murid long-tailed field mice (Apodemus sylvaticus), softfurred mice (Praomys spp.), and golden hamsters (Mesocricetus auratus)   Lujo virus (LUJV) Unknown None VHF   Machupo virus (MACV) Big lauchas (Calomys callosus) None VHF   Sabiá virus (SBAV) Unidentified cricetid rodents None VHF   Whitewater Arroyo virus (WWAV)l White-throated woodrats (Neotoma albigula) Orbivirus (family Sedoreoviridae) Kemerovo virus (KEMV) Birds, rodents Ixodid ticks (Ixodes persulcatus) F&M, E   Lebombo virus (LEBV) Unknown Mosquitoes (Aedes, Mansonia spp.) F&M   Orungo virus (ORUV) Camels, cattle, goats, nonhuman primates, sheep   Tribeˇc virus (TRBV)m Bank voles (Myodes glareolus), birds, common pine voles (Microtus subterraneus), goats, hares PART 5 Infectious Diseases Orthobornavirus (family Bornaviridae) Variegated squirrel bornavirus 1 (VSBV1) Finlayson’s squirrel (Callosciurus finlaysonii), Prevost’s squirrels (Callosciurus prevostii), Swinhoe’s striped squirrel (Tamiops swinhoei), variegated squirrels (Sciurus variegatoides) Orthobunyavirus (family Peribunyaviridae) Apeú virus (APEUV) Bare-tailed woolly opossums (Caluromys philander) and other opossums; rodents; black howlers (Alouatta caraya), tufted capuchins (Cebus apella)   Bangui virus (BGIV) Unknown Unknown F&M   Batai virus (BATV)n Birds, camels, cattle, goats, rodents, sheep   Bunyamwera virus (BUNV) Birds, cows, goats, horses, sheep Mosquitoes (Aedes spp.) F&M   Bwamba virus (BWAV) Unknown Mosquitoes (Aedes, Anopheles, Mansonia spp.)   Cache Valley virus (CVV) Cattle, deer, dogs, foxes, groundhogs, horses, nonhuman primates, pigs, rabbits, raccoons, rodents   California encephalitis virus (CEV) Lagomorphs, rodents Mosquitoes (Aedes, Culex, Culiseta, Psorophora spp.)   Caraparú virus (CARV) Rodents, tufted capuchins (C. apella) Mosquitoes (Culex spp.) F&M   Catú virus (CATUV) Bats, capuchins (Cebus spp.), opossums, rodents   Cristoli virus (/) Unknown Mosquitoes? E   Fort Sherman virus (FSV) Cattle, goats, horses, sheep? Mosquitoes? F&M   Gan Gan virus (GGV) Unknown Mosquitoes (Aedes, Culex spp.) A&R   Germiston virus (GERV) Rodents Mosquitoes (Culex spp.) F&M   Guamá virus (GMAV) Bats, capuchins (Cebus spp.), howlers (Alouatta spp.), marsupials, rodents   Guaroa virus (GROV) Unknown Mosquitoes (Anopheles spp.) F&M   Ilesha virus (ILEV) Unknown Mosquitoes (Anopheles gambiae) F&M, (VHF)   Inkoo virus (INKV) Cattle, foxes, hares, moose, reindeer, rodents

(Continued) None VHF None F&M, VHF None F&M, E, (VHF) None (E) Mosquitoes (Aedes, Anopheles, Culex spp.) F&M, E Ixodid ticks (Ixodes persulcatus, I. ricinus) F&M None E Mosquitoes (Aedes, Culex spp.) F&M Mosquitoes (Aedes abnormalis, A. curtipes, Anopheles barbirostris, A. maculipennis, A. phillippinensis, Culex gelidus, other spp.) F&M F&M Mosquitoes (Aedes, Anopheles, Coquillettidia, Culiseta, Psorophora spp.) F&M, E F&M, E Mosquitoes (Culex spp.) F&M Mosquitoes (Aedes, Culex, Limatus, Mansonia, Psorophora, Trichoprosopon spp.) F&M Mosquitoes (Aedes spp.) F&M, E (Continued)

TABLE 215-1  Zoonotic Arthropod- and Rodent-Borne Viruses That Infect Humans VIRUS TAXON VIRUS (ABBREVIATION) MAJOR NONHUMAN HOST(S)a VECTOR(S) SYNDROMEb   Iquitos virus (IQTV) Unknown Unknown F&M   Itaquí virus (ITQV) Capuchins (Cebus spp.), opossums, rodents   Jamestown Canyon virus (JCV) Predominantly white-tailed deer (Odocoileus virginianus), but also bison, elk, and moose   Keystone virus (KEYV) Rabbits, squirrels Mosquitoes (Ochlerotatus atlanticus) E   La Crosse virus (LACV) Chipmunks, squirrels Mosquitoes (Ochlerotatus triseriatus) F&M, E   Lumbo virus (LUMV) Unknown Mosquitoes (Aedes pembaensis) F&M, E   Madrid virus (MADV) Capuchins (Cebus spp.), opossums, rodents   Maguari virus (MAGV) Birds, cattle, horses, sheep, water buffalo Mosquitoes (Aedes, Anopheles, Culex, Psorophora, Wyeomyia spp.)   Marituba virus (MTBV) Capuchins (Cebus spp.), opossums, rodents   Murutucú virus (MURV) Capuchins (Cebus spp.), opossums, palethroated sloths (Bradypus tridactylus), rodents   Nepuyo virus (NEPV) Bats (Artibeus spp.), rodents Mosquitoes (Culex spp.) F&M   Ngari virus (NRIV) Cattle, goats, sheep Mosquitoes (Aedes, Anopheles, Culex spp.) F&M, VHF   Nyando virus (NDV) Unknown Mosquitoes (Aedes, Anopheles spp.), sandflies (Lutzomyia spp.)   Oriboca virus (ORIV) Capuchins (Cebus spp.), opossums, rodents   Oropouche virus (OROV) Marmosets (Callithrix spp.), pale-throated sloths (B. tridactylus)   Ossa virus (OSSAV) Rodents Mosquitoes (Culex spp.) F&M   Pongola virus (PGAV) Cattle, donkeys, goats, sheep Mosquitoes (Aedes, Anopheles, Mansonia spp.)   Restan virus (RESV) Unknown Mosquitoes (Culex spp.) F&M   Shokwe virus (SHOV) Rodents Mosquitoes (Aedes, Anopheles, Mansonia spp.)   Shuni virus (SHUV) Horses, livestock Mosquitoes (Culex theileri, Culicoides spp.) E   Snowshoe hare virus (SSHV) Collared lemmings, snowshoe hares, squirrels, voles   Tacaiuma virus (TCMV) Nonhuman primates Mosquitoes (Anopheles, Haemagogus spp.)   Tˇ ahynˇ a virus (TAHV) Boars, cattle, deer, dogs, eulipotyphla, foxes, hares, horses, pigs, rodents   Tataguine virus (TATV) Unknown Mosquitoes (Anopheles spp.) F&M   Trubanaman virus (TRUV) Unknown Mosquitoes (Anopheles, Culex spp.) (A&R)   Wyeomyia virus (WYOV) Unknown Mosquitoes (Wyeomyia spp.) F&M   Xingu virus (XINV) Unknown Unknown F&M   Zungarococha virus (ZUNV) Unknown Unknown F&M Orthoflavivirus (family Flaviviridae) Alkhurma hemorrhagic fever virus (AHFV)j Livestock? Argasid ticks (Ornithodoros savignyi), ixodid ticks (Hyalomma dromedarii, H. rufipes)   Apoi virus (APOIV) Field mice, voles Unknown E   Banzi virus (BANV) Rodents? Mosquitoes (Culex rubinotus) F&M   Bussuquara virus (BSQV) Cotton rats, monkeys, spiny-rats Mosquitoes (Culex spp.) F&M   Cacipacoré virus (CPCV) Birds, horses, nonhuman primates, water buffalo   Dakar bat virus (DBV) Bats Unknown F&M   Dengue viruses 1–4 (DENV 1–4) Nonhuman primates Mosquitoes (predominantly Aedes aegypti, A. albopictus)   Edge Hill virus (EHV) Bandicoots, dogs, wallabies Mosquitoes (Aedes vigilax, Culex annulirostris)   Ilhéus virus (ILHV) Birds, coatis, nonhuman primates, rodents, reptiles, sloths, water buffalo

(Continued) Mosquitoes (Culex spp.) F&M Mosquitoes (Aedes, Anopheles, Coquillettidia, Culiseta, Ochlerotatus spp.) F&M, E Mosquitoes (Culex spp.) F&M F&M Mosquitoes (Culex spp.) F&M Mosquitoes (Coquillettidia, Culex spp.) F&M F&M Mosquitoes (Aedes, Culex, Mansonia, Psorophora spp.) F&M CHAPTER 215 Biting midges (Culicoides paraensis), mosquitoes (Coquillettidia venezuelensis, Culex quinquefasciatus, Mansonia spp., Ochlerotatus serratus) F&M, E F&M Arthropod-Borne and Rodent-Borne Virus Infections
F&M Mosquitoes (Aedes, Culiseta, Ochlerotatus, Simulium spp.) F&M, E F&M Mosquitoes (Aedes, Anopheles, Culex, Culiseta spp.) F&M, E VHF Mosquitoes (Aedes, Anopheles, Culex spp.) F&M F&M, VHF F&M Mosquitoes (Aedes, Culex, Coquillettidia, Haemagogus, Ochlerotatus, Psorophora, Sabethes, Trichoprosopon spp.) E (Continued)

TABLE 215-1  Zoonotic Arthropod- and Rodent-Borne Viruses That Infect Humans VIRUS TAXON VIRUS (ABBREVIATION) MAJOR NONHUMAN HOST(S)a VECTOR(S) SYNDROMEb   Japanese encephalitis virus (JEV) Ardeid wading birds (in particular herons), horses, pigs   Karshi virus (KSIV) Great gerbils (Rhombomys opimus) Argasid ticks (Ornithodoros capensis), ixodid ticks (Hyalomma asiaticum)   Kédougou virus (KEDV) Unknown Mosquitoes (Culex spp.) F&M   Kokobera virus (KOKV) Macropods, horses Mosquitoes (Culex spp.) A&R   Koutango virus (KOUV) Gerbils Mosquitoes (Aedes spp.) F&M   Kyasanur Forest disease virus (KFDV)k Indomalayan vandeleurias (Vandeleuria oleracea), roof rats (Rattus rattus)   Modoc virus (MODV) North American deermice (Peromyscus maniculatus)   Murray Valley encephalitis virush (MVEV) Birds (egrets, herons) Mosquitoes (predominantly C. annulirostris)   Ntaya virus (NTAV) Birds, domestic animals Mosquitoes (Culex spp.) F&M   Omsk hemorrhagic fever virus (OHFV) Migratory birds, rodents Ixodid ticks (predominantly Dermacentor spp.)   Powassan virus (POWV) Red squirrels (Tamiasciurus hudsonicus), white-footed deermice (Peromyscus leucopus), woodchucks (Marmota monax), other small mammals   Rio Bravo virus (RBV) Bats Unknown F&M   Rocio virus (ROCV) Rufous-collared sparrows (Zonotrichia capensis), various migratory birds, horses, water buffalo (Bubalus bubalis)   Sepik virus (SEPV) Unknown Mosquitoes (Mansonia septempunctata) F&M PART 5 Infectious Diseases   Spondweni virus (SPOV) Cattle, sheep Mosquitoes (Aedes, Culex, Mansonia spp.) F&M   St. Louis encephalitis virus (SLEV) Columbiform and passeriform birds (finches, sparrows)   Tick-borne encephalitis virus (TBEV) Passeriform birds, deer, eulipotyphla, goats, grouse, small mammals, rodents, sheep   Usutu virus (USUV) Accipitriform, columbiform, passeriform, and strigiform birds   Wesselsbron virus (WESSV) Cattle, goats, rodents, sheep Mosquitoes (Aedes spp.) F&M   West Nile virus (WNV)i Passeriform birds (blackbirds, crows, finches, sparrows), small mammals, horses   Yellow fever virus (YFV) Nonhuman primates (Alouatta, Ateles, Cebus, Cercopithecus, Colobus spp.)   Zika virus (ZIKV) Nonhuman primates (Macaca, Pongo spp.) Orthohantavirus (family Hantaviridae) Amur virus (AMRV) Korean field mice (Apodemus peninsulae) None VHF   Anajatuba virus (ANJV) Fornes’ colilargos (Oligoryzomys fornesi) None P   Andes virus (ANDV) Long-tailed colilargos (Oligoryzomys longicaudatus)   Araraquara virus (ARAV) Hairy-tailed akodonts (Necromys lasiurus) None P   Araucária virus (ARAUV) Black-footed colilargos (Oligoryzomys nigripes)   Bayou virus (BAYV) Marsh rice rats (Oryzomys palustris) None P   Bermejo virus (BMJV) Chacoan colilargos (Oligoryzomys chacoensis)   Black Creek Canal virus (BCCV) Hispid cotton rats (Sigmodon hispidus) None P   Blue River virus (BRV) White-footed deermice (Peromyscus leucopus)   Caño Delgadito virus (CADV) Alston’s cotton rats (Sigmodon alstoni) None P

(Continued) Mosquitoes (Culex spp., in particular C. tritaeniorhynchus) E F&M, E Ixodid ticks (predominantly Haemaphysalis spinigera) VHF Unknown E E VHF Ixodid ticks (in particular Ixodes cookei, other Ixodes spp., Dermacentor spp.) E Mosquitoes (Aedes, Culex, Psorophora spp.) E Mosquitoes (predominantly Culex spp., in particular C. nigripalpus, C. pipiens, C. quinquefasciatus, C. tarsalis) E Ixodid ticks (Ixodes gibbosus, I. persulcatus, I. ricinus; sporadically Dermacentor, Haemaphysalis, Hyalomma spp.) F&M, E, (VHF) Mosquitoes (Culex spp., in particular C. pipiens, but also Aedes spp, Anopheles spp., Ochlerotatus spp.) (E) Mosquitoes (Culex spp., in particular C. pipiens, C. quinquefasciatus, C. restuans, C. tarsalis) E Mosquitoes (Aedes spp., in particular Ae. aegypti) VHF Mosquitoes (Aedes spp.) F&M, A&R None P None P None P None P (Continued)

TABLE 215-1  Zoonotic Arthropod- and Rodent-Borne Viruses That Infect Humans VIRUS TAXON VIRUS (ABBREVIATION) MAJOR NONHUMAN HOST(S)a VECTOR(S) SYNDROMEb   Castelo dos Sonhos virus (CASV) Brazilian colilargos (Oligoryzomys eliurus) None P   Catacamas virus (CATV) Coues’ oryzomys (Oryzomys couesi) None P   Choclo virus (CHOV) Fulvous colilargos (Oligoryzomys fulvescens)   Dobrava virus (DOBV) Caucasus field mice (Apodemus ponticus), striped field mice (A. agrarius), yellow-necked field mice (A. flavicollis)   El Moro Canyon virus (ELMCV) Western harvest mice (Reithrodontomys megalotis)   Go–u virus (GOUV) Brown rats (Rattus norvegicus), roof rats (R. rattus), Oriental house rats (R. tanezumi)   Hantaan virus (HTNV) Striped field mice (Apodemus agrarius) None VHF   Juquitiba virus (JUQV) Black-footed colilargos (Oligoryzomys nigripes)   Kurkino virus (KURV) Striped field mice (Apodemus agrarius) None VHF   Laguna Negra virus (LANV) Little lauchas (Calomys laucha) None P   Lechiguanas virus (LECV) Flavescent colilargos (Oligoryzomys flavescens)   Maciel virus (MCLV) Dark-furred akodonts (Necromys obscurus)   Maripa virus (MARV) Unknown None P   Monongahela virus (MGLV) Eastern deermice (Peromyscus maniculatus)   Muju virus (MUJV) Korean red-backed voles (Myodes regulus)   New York virus (NYV) White-footed deermice (Peromyscus leucopus)   Orán virus (ORNV) Long-tailed colilargos (Oligoryzomys longicaudatus)   Paranoá virus (PARV) Unknown None P   Pergamino virus (PRGV) Azara’s akodonts (Akodon azarae) None P   Puumala virus (PUUV) Bank voles (Myodes glareolus) None P, VHF   Rio Mamoré virus (RIOMV) Common bristly mice (Neacomys spinosus)   Saaremaa virus (SAAV) Striped field mice (Apodemus agrarius) None VHF   Seoul virus (SEOV) Brown rats (Rattus norvegicus), roof rats (R. rattus)   Sin Nombre virus (SNV) Western deermice (Peromyscus sonoriensis)   Sochi virus (SOCV) Caucasus field mice (Apodemus ponticus) None VHF   Tula virus (TULV) Common voles (Microtus arvalis), East European voles (M. levis), field voles (M. agrestis)   Tunari virus (TUNV) Unknown None P Orthonairovirus (family Nairoviridae) Aigai virus (AIGV) Cattle, goats, tortoises Ixodid ticks (Hyalomma spp., Rhipicephalus spp.)   Avalon virus (AVAV) European herring gulls (Larus argentatus) Ixodid ticks (Ixodes uriae) (Polyradiculoneuritis?)   Crimean-Congo hemorrhagic fever virus (CCHFV) Cattle, dogs, goats, hares, hedgehogs, mice, ostriches, sheep   Dugbe virus (DUGV) Northern giant pouched rats (Cricetomys gambianus), Zébu cattle (Bos primigenius)   Erve virus (ERVEV) Greater white-toothed shrews (Crocidura russula)   Issyk-kul virus (ISKV) Bats, birds Biting midges (Culicoides schultzei), horseflies (Tabanus agrestis), mosquitoes (Aedes caspius, Anopheles hyrcanus), argasid ticks (Argas vespertilionis, A. pusillus), ixodid ticks (Ixodes vespertilionis)

(Continued) None F&M, P None VHF None VHF None VHF None P None P None P None P CHAPTER 215 None VHF None P None P Arthropod-Borne and Rodent-Borne Virus Infections
None P None VHF None P None (P), VHF VHF Predominantly ixodid ticks (Hyalomma spp.) VHF Biting midges (Culicoides spp.), ixodid ticks (Amblyomma, Hyalomma, Rhipicephalus spp.) F&M Unknown (Thunderclap headache?) F&M (Continued)

TABLE 215-1  Zoonotic Arthropod- and Rodent-Borne Viruses That Infect Humans VIRUS TAXON VIRUS (ABBREVIATION) MAJOR NONHUMAN HOST(S)a VECTOR(S) SYNDROMEb   Nairobi sheep disease viruso (NSDV) Sheep Ixodid ticks (Amblyomma, Haemaphysalis, Rhipicephalus spp.), mosquitoes (Culex spp.)   Sönglïng virus (SGLV) Great gerbils (Rhombomys opimus) Ixodid ticks (Ixodes crenulatus, Ixodes persulcatus, Haemaphysalis concinna, and Haemaphysalis longicornis)   Taˇchéng tick virus 1 (TcTV1) Sheep, tortoises Ixodid ticks (Dermacentor marginatus, Hyalomma aegyptium)   Tamdy virus (TAMV) Gerbils, other mammals (including Bactrian camels), birds   Wetland virus (WELV) Horses, pigs, sheep, North China zokors Ixodid ticks (Dermacentor, Ixodes, Haemaphysalis spp.)   Yezo virus (YEZV) Unknown Ixodid ticks (Ixodes persulcatus) F&M Phlebovirus (family Phenuiviridae) Adria virus (ADRV) Unknown Sandflies E   Alenquer virus (ALEV) Unknown Unknown F&M   Candirú virus (CDUV) Unknown Unknown F&M   Chagres virus (CHGV) Unknown Sandflies (Lutzomyia spp.) F&M   Chios virus (/) Unknown Unknown E   Coclé virus (CCLV) Unknown Sandflies F&M   Echarate virus (ECHV) Unknown Unknown F&M   Granada virus (GRV = GRAV) Unknown Sandflies F&M   Maldonado virus (MLOV) Unknown Unknown F&M   Morumbi virus (MRBV = MRMBV) Unknown Unknown F&M PART 5 Infectious Diseases   Punta Toro virus (PTV) Unknown Sandflies (Lutzomyia spp.) F&M   Rift Valley fever virus (RVFV) Cattle, sheep Mosquitoes (Aedes, Anopheles, Coquillettidia, Culex, Eretmapodites, Mansonia spp.)   Sandfly fever Cyprus virus (SFCV) Unknown Unknown F&M   Sandfly fever Ethiopia virus (SFEV) Unknown Sandflies F&M   Sandfly fever Naples virus (SFNV) Unknown Sandflies (Phlebotomus papatasi, P. perfiliewi, P. perniciosus)   Sandfly fever Sicilian virus (SFSV) Eulipotyphla, least weasels (Mustela nivalis), rodents   Sandfly fever Turkey virus (SFTV) Unknown Sandflies (Phlebotomus spp.) F&M   Serra Norte virus (SRNV) Unknown Unknown F&M   Toscana virus (TOSV) Migratory birds? Sandflies (Phlebotomus papatasi, P. perfiliewi, Sergentomyia spp.) Quaranjavirus (family Orthomyxoviridae) Quaranfil virus (QRFV) Birds Argasid ticks (Argas arboreus) F&M Seadornavirus (family Sedoreoviridae) Banna virus (BAV) Cattle, pigs Biting midges (Culicoides spp.), mosquitoes (Aedes, Anopheles, Culiseta spp.), ticks Tanzavirus (family Phenuviridae) Dar es Salaam virus (DeSV) Unknown Unknown F&M Tibrovirus (family Rhabdoviridae) Bas-Congo virus (BASV) Artiodactyls? Biting midges? (VHF)   Ekpoma virus 1 (EKV1) Artiodactyls? Biting midges? F&M   Ekpoma virus 2 (EKV2) Artiodactyls? Biting midges? F&M   Mundri virus (MUNV) Artiodactyls? Biting midges? E (New onset nodding syndrome) Thogotovirus (family Orthomyxoviridae) Bourbon virus (BRBV) Raccoons (Procyon lotor), white-tailed deer (Odocoileus virginianus)?   Dhori virus (DHOV)p Bats, camels, horses Mosquitoes (Aedes, Anopheles, Culex spp.), argasid ticks (Ornithodoros spp.), ixodid ticks (Dermacentor, Hyalomma spp.)   Thogoto virus (THOV) Camels, cattle Ixodid ticks (Amblyomma, Hyalomma, Rhipicephalus spp.)

(Continued) F&M F&M F&M Ixodid ticks (Dermacentor, Hyalomma spp.) F&M F&M F&M, E, VHF F&M Sandflies (predominantly Phlebotomus papatasi) F&M F&M, E E Ixodid ticks (Amblyomma americanum) F&M F&M, E F&M, E (Continued)

TABLE 215-1  Zoonotic Arthropod- and Rodent-Borne Viruses That Infect Humans VIRUS TAXON VIRUS (ABBREVIATION) MAJOR NONHUMAN HOST(S)a VECTOR(S) SYNDROMEb Uukuvirus (family Phenuiviridae) Taˇchéng tick virus 2 (TcTV2) Unknown Ixodid ticks (Dermacentor marginatus, Dermacentor nuttalli, Dermacentor silvarum, Hyalomma asiaticum)   Uukuniemi virus (UUKV) Birds, cattle, rodents Ixodid ticks (Ixodes spp.) F&M Vesiculovirus (family Rhabdoviridae) Chandipura virus (CHPV) Hedgehogs Mosquitoes (Aedes aegypti), sandflies (Phlebotomus, Sergentomyia spp.)   Isfahan virus (ISFV) Great gerbils (Rhombomys opimus) Sandflies (Phlebotomus papatasi) F&M   Piry virus (PIRYV) Gray four-eyed opossums (Philander opossum)   Vesicular stomatitis Indiana virus (VSIV) Cattle, horses, pigs Sandflies (Lutzomyia spp.) F&M   Vesicular stomatitis New Jersey virus (VSNJV) Cattle, horses, pigs Biting midges (Culicoides spp.), chloropid flies, mosquitoes (Culex, Mansonia spp.), muscoid flies (Musca spp.), simuliid flies aMammalian names as listed in Wilson & Reeder’s Mammal Species of the World, 3rd edition (https://www.departments.bucknell.edu/biology/resources/msw3/). bAbbreviations refer to the syndromes most associated with the viruses: A&R, arthritis and rash; E, encephalitis; F&M, fever and myalgia; P, pulmonary; VHF, viral hemorrhagic fever. Abbreviations are placed in parentheses when cases are controversial. cIn the older literature, chikungunya virus often is also listed as a causative agent of VHF. However, later studies revealed that, in most cases, people with “chikungunya hemorrhagic fever” were co-infected with one or more dengue viruses, an observation suggesting that the VHF was severe dengue. dAlso known as Igbo-Ora virus. eAlso known as Ockelbo virus (OCKV), Pogosta virus, and Karelian fever virus (KFV). fAlso known as Palma virus (PALV). gAlternatives used in the literature are Huáiyángsha – n virus (HYSV) and Hénán fever virus (HNFV). hAlso known as Alfuy virus (ALFV). iAlso includes Kunjin virus (KUNV). jAlso spelled Alkhumra hemorrhagic fever virus (AHFV) and known as Alkhurma/Alkhumra virus (ALKV). kAlso known as Nánjiànyí(n) virus. lWhitewater Arroyo virus is often listed as a causative agent of VHF in the literature, but convincing data associating this virus with VHF have not been published. mAlso known as Brezová virus, Cvilín virus, Kharagysh virus, Koliba virus, or Lipovník virus. nAlso known as Cˇalovo virus (CVOV) or Chittoor virus (CHITV). oAlso known as Ganjam virus (GV). pAlso known as Astra virus and Batken virus (BKNV). that cause fever and myalgia (F&M) syndrome and possibly viral hem­ orrhagic fever, whereas the genus Vesiculovirus includes confirmed human arthropod-borne viruses, all of which are transmitted by insects (biting midges, mosquitoes, and sandflies). The general properties of rhabdovirids are discussed in more detail in Chap. 214. ■ ■RESENTOVIRICETES: SEDOREOVIRIDAE

AND SPINAREOVIRIDAE The families Sedoreoviridae and Spinarevoviridae are established for viruses with linear, multisegmented, double-stranded RNA genomes (≈16–29 kb in total). These viruses produce particles that have icosa­ hedral symmetry and are 60–80 nm in diameter. In contrast to all other virions discussed in this chapter, resentovirions are not enveloped and thus are insensitive to detergent inactivation. Human arthropod-borne viruses are found within the genera Coltivirus (family Spinareoviri­ dae), Orbivirus, and Seadornavirus (both in family Sedoreoviridae). Arthropod-borne coltiviruses possess 12 genome segments and are transmitted by numerous tick types transstadially but not transovari­ ally. Therefore, maintenance of the transmission cycle involves viremic mammalian hosts infected by tick bites. Arthropod-borne orbiviruses have 10 genome segments and are transmitted by mosquitoes or ixodid ticks, whereas relevant seadornaviruses have 12 genome segments and are transmitted exclusively by mosquitoes. EPIDEMIOLOGY Arthropod-borne and rodent-borne viruses are generally confined to the areas inhabited by their reservoir hosts and/or vectors. Con­ sequently, a patient’s geographic origin or travel history can provide important clues in the differential diagnosis. Table 215-2 lists the approximate geographic distribution of most arthropod-borne and rodent-borne infections. Many of these diseases can be acquired in rural or urban settings. However, diseases commonly associated with urban outbreaks include yellow fever (YF), dengue without/with warning signs (previously called dengue fever), severe dengue (previ­ ously called dengue hemorrhagic fever and dengue shock syndrome), chikungunya virus disease, hemorrhagic fever with renal syndrome (HFRS) caused by Seoul virus, sandfly fever caused by sandfly fever Naples and Sicilian viruses, and Oropouche virus disease. DIAGNOSIS In patients with suspected viral infection, a history of mosquito bite(s) has little diagnostic significance, but a history of tick bite(s) is more useful. Patients infected with mammarenaviruses or orthohantaviruses

(Continued) (E) F&M, E Mosquitoes (Aedes, Culex, Toxorhynchites spp.) F&M F&M sometimes report exposure to rodents. Although clinical signs and epidemiologic clues may enable presumptive etiologic diagnosis in epi­ demic settings, laboratory diagnosis is required in all cases. For most arthropod-borne and rodent-borne virus diseases, acute-phase serum samples (collected within 3 or 4 days of onset) have yielded isolates. Paired serum samples have been used to show rising antibody titers. Intensive efforts to develop rapid tests for viral hemorrhagic fevers (VHFs) have resulted in reliable antigen-detection enzyme-linked immunosorbent assays (ELISAs), IgM-capture ELISAs, and multiplex polymerase chain reaction (PCR) assays. These tests can provide a diagnosis based on a single serum sample within a few hours and are particularly useful for assessment of patients with severe disease. More sensitive reverse transcriptase PCR (RT-PCR) assays may yield diagno­ ses based on samples without detectable antigen and may also provide useful truly quantifiable genetic information about the etiologic agent. CHAPTER 215 Arthropod-Borne and Rodent-Borne Virus Infections
At diagnosis, patients with encephalitides generally are no longer viremic or antigenemic and usually do not have virions in cerebrospi­ nal fluid (CSF). In this situation, either serologic detection of IgM or RT-PCR detection of viral nucleic acid may enable diagnosis. Increas­ ingly, IgM-capture ELISAs are used for the simultaneous testing of serum and CSF. IgG ELISA or classic serology is useful in the evalua­ tion of past exposure to viruses, many of which circulate in areas with minimal medical infrastructures and sometimes cause only mild or subclinical infections (and are not recognized). CLINICAL DISEASE SYNDROMES There is a wide spectrum of possible human responses to infection with arthropod-borne or rodent-borne viruses, and knowledge of the outcome of most of these infections is limited. People infected with these viruses may not develop or recognize symptoms or signs of ill­ ness. If viral disease is recognized, it can usually be grouped into one of five broad syndromic categories: F&M, arthritis and rash (A&R), encephalitis, pulmonary disease, or VHF (Table 215-3). Although a useful clinical heuristic, it should be acknowledged that these catego­ ries often overlap in the complex spectra of disease caused by arthro­ pod-borne and rodent-borne viruses. Furthermore, illness caused by many of these viruses is often best known by the most severe disease phenotypes, which typically do not result in the most common disease manifestations. For example, infections with West Nile virus (WNV) and Venezuelan equine encephalitis virus (VEEV) are discussed in this chapter as encephalitides, but during epidemics, many patients pres­ ent with much milder F&M. Similarly, Rift Valley fever virus (RVFV)

TABLE 215-2  Geographic Distribution of Zoonotic Arthropod-Borne or Rodent-Borne Viral Diseases BUNYAVIRICETES (Arenaviridae, Hantaviridae, Nairoviridae, Peribunyaviridae, Phenuiviridae) FLASUVIRICETES (Flaviviridae) AREAa Africa Aigai, Bangui, Batai, Bhanja, Bunyamwera, and Bwamba virus infections, Crimean-Congo hemorrhagic fever, Dar es Salaam, Dugbe, Germiston, Ilesha virus infections, Lassa fever, Lujo virus, Nairobi sheep disease virus infection, Ngari, Nyando, and Pongola virus infections, Rift Valley fever, sandfly fever, Shokwe, Shuni, Tataguine virus infections Alkhurma hemorrhagic fever, Banzi virus infection, Dakar bat, dengue without/with warning signs/severe dengue, Kédougou, Koutango, Ntaya, Spondweni, Usutu, Wesselsbron, West Nile virus infections, yellow fever, Zika virus disease Central Asia Bhanja, Issyk-kul virus infections, Far Eastern tick-borne encephalitis, Karshi, Powassan, West Nile virus infections Crimean-Congo hemorrhagic fever, sandfly fever, Tˇahynˇ a, Tamdy virus infections Eastern Asia Crimean-Congo hemorrhagic fever, hemorrhagic fever with renal syndrome, sandfly fever, severe fever with thrombocytopenia syndrome, Taˇchéng tick virus 1 (and 2) and Tˇahynˇ a, Tamdy, So Apoi virus infection, dengue without/ with warning signs/ severe dengue, Far Eastern tickborne encephalitis, Japanese encephalitis, Karshi virus infection, Kyasanur Forest disease –nglıˇng, wetland, and Yezo virus infections PART 5 Infectious Diseases Southern Asia Batai, Bhanja virus infections, Crimean-Congo hemorrhagic fever, hemorrhagic fever with renal syndrome, Nairobi sheep disease virus infection, sandfly fever Dengue without/with warning signs/severe dengue, Japanese encephalitis, Kyasanur Forest disease, West Nile virus infection, Zika virus disease SouthEastern Asia Batai virus infection, hemorrhagic fever with renal syndrome Dengue without/with warning signs/severe dengue, Japanese encephalitis, West Nile virus infection, Zika virus disease Western Asia Aigai, Batai, Bhanja virus infections, Crimean-Congo hemorrhagic fever, hemorrhagic fever with renal syndrome, sandfly fever, Taˇchéng tick virus 1, Tamdy virus infections Alkhurma hemorrhagic fever, Central European tickborne encephalitis, dengue without/with warning signs/severe dengue, West Nile virus infection Latin/ Central America and the Caribbean Alenquer, Apeú virus infections, Argentinian hemorrhagic fever, Bolivian hemorrhagic fever, “Brazilian hemorrhagic fever”, Bunyamwera, Candirú, Caraparú, Catú, Chagres, Chapare, Coclé, Echarate, (Flexal,) Fort Sherman, Guamá, Guaroa virus infections, hantavirus pulmonary syndrome, Iquitos, Itaquí, Juquitiba virus infections, lymphocytic choriomeningitis, Madrid, Maguari, Maldonado, Marituba, Mayaro, Morumbi, Murutucú, Nepuyo, Oriboca virus infections, Oropouche virus disease, Ossa, Punta Toro, Restan, Serra Norte, Tacaiuma, Trinidad virus infections, Venezuelan hemorrhagic fever, Wyeomyia, Xingu, Zungarococha virus infections Bussuquara, Cacipacoré virus infections, dengue without/with warning signs/severe dengue, Ilhéus, Rio Bravo virus infections, Rocio viral encephalitis, Spondweni, St Louis encephalitis, yellow fever, Zika virus disease

TYPE OF DISEASEb RESENTOVIRICETES (Sedoreoviridae, Spinareoviridae) MONJIVIRICETES (Bornaviridae, Rhabdoviridae) INSTHOVIRICETES (Orthomyxoviridae) ALSUVIRICETES (Togaviridae) Dhori, Quaranfil, Thogoto virus infections Lebombo, Orungo, Tribecˇ virus infections (Bas-Congo virus infection,) Ekpoma virus 1, Ekpoma virus 2, Mundri virus infections Chikungunya virus disease, o’nyongnyong fever, Semliki Forest, Sindbis virus infections Dhori virus infections — Isfahan virus infection Sindbis virus infection — Banna virus infection — — Dhori, Quaranfil, Thogoto virus infections — Chandipura, Isfahan virus infections, variegated squirrel bornavirus 1? Chikungunya virus disease — — — Chikungunya virus disease Dhori, Quaranfil virus infections — — Chikungunya virus disease — — Piry fever, vesicular stomatitis fever Chikungunya virus disease, Madariaga, Mayaro, Mucambo, Tonate, Una virus infections, Venezuelan equine encephalitis (Continued)

TABLE 215-2  Geographic Distribution of Zoonotic Arthropod-Borne or Rodent-Borne Viral Diseases BUNYAVIRICETES (Arenaviridae, Hantaviridae, Nairoviridae, Peribunyaviridae, Phenuiviridae) FLASUVIRICETES (Flaviviridae) AREAa Northern America (Avalon,) Cache Valley virus infections, California encephalitis, hantavirus pulmonary syndrome, Heartland and Keystone virus infections, lymphocytic choriomeningitis, Nepuyo, snowshoe hare, (Whitewater Arroyo virus) infections Dengue without/with warning signs/severe dengue, Modoc virus infection, Powassan virus disease, Rio Bravo virus infection, St Louis encephalitis, West Nile virus infection, Zika virus disease Europe Adria, Aigai, (Avalon,) Batai, Bhanja, Cristoli virus infections, California encephalitis, CrimeanCongo hemorrhagic fever, (Erve virus infection), hemorrhagic fever with renal syndrome, Inkoo, Issykkul virus infections, lymphocytic choriomeningitis, sandfly fever, snowshoe hare, Tˇahynˇ a, Uukuniemi virus infections Central European tick-borne encephalitis, dengue without/with warning signs/severe dengue, Ntaya virus infection, Omsk hemorrhagic fever, Powassan, Usutu, West Nile virus infections Oceania Batai, Gan Gan, (Trubanaman) virus infections Dengue without/ with warning signs/ severe dengue, Edge Hill virus infection, Japanese encephalitis, Kokobera virus infection, Murray Valley encephalitis, Sepik virus, West Nile virus infections, Zika virus disease aGeographic names here and throughout the chapter are as recommended by the UN geoscheme (https://unstats.un.org/unsd/methodology/m49/). bDisease names according to the World Health Organization’s International Classification of Diseases 11th revision (ICD-11; https://icd.who.int/browse11/l-m/en). Quotation marks indicate common usage in the absence of ICD-11 recognition. Diseases not acknowledged by the ICD-11 are designated as “virus infection(s).” is best known as a cause of VHF, but the prevalence of milder F&M syndrome is far higher after infection, with encephalitis and ocular disease also occurring occasionally. Lymphocytic choriomeningitis virus (LCMV) is classified in this chapter as a cause of F&M syndrome because it is the most common disease manifestation. Even when cen­ tral nervous system (CNS) disease evolves during infection with this virus, neurologic manifestations are usually mild and preceded by F&M. However, this virus may also cause fetal microcephaly. Over­ lap between syndromic categories is further complicated by evolving nomenclature around their classification. For example, infection with any dengue virus (DENV-1–4) is considered as a cause of F&M syn­ drome because this disease presentation, historically called “dengue fever,” is by far the most common manifestation. However, severe manifestations of DENV infection have a complicated pathogenesis: the historical classification of disease as “dengue hemorrhagic fever” included a subset of patients with “dengue shock syndrome,” which is of tremendous consequence for pediatric populations in particular areas of the world. Further complicating this overlap, a relatively recent World Health Organization (WHO) revision of disease classification recommended a less descriptive but more pragmatic use of “dengue without warning signs,” “dengue with warning signs,” and “severe den­ gue” to describe the same spectrum and enhance clinical management and case reporting. Unfortunately, most of the known arthropod-borne and rodentborne viral diseases have not been studied in detail with modern medi­ cal approaches. Thus, available data may be incomplete, at relatively low resolution, or biased toward severe disease. Data on geographic distribution are often difficult to interpret. Frequently, the literature is not clear as to whether the data pertain to the distribution of a particu­ lar virus or to the areas where human disease has been observed. In

(Continued) TYPE OF DISEASEb RESENTOVIRICETES (Sedoreoviridae, Spinareoviridae) MONJIVIRICETES (Bornaviridae, Rhabdoviridae) INSTHOVIRICETES (Orthomyxoviridae) ALSUVIRICETES (Togaviridae) Bourbon virus infection Colorado tick fever, Salmon River virus infection Vesicular stomatitis fever Eastern equine encephalitis, Everglades virus infection, western equine encephalitis Dhori, Thogoto virus infections Eyach, Kemerovo, Tribecˇ virus infections — Chikungunya virus disease, Sindbis virus infection — — — Barmah Forest virus infection, Ross River disease, Sindbis virus infection CHAPTER 215 Arthropod-Borne and Rodent-Borne Virus Infections
addition, the designations for viruses and viral diseases have changed multiple times over decades. In this chapter, virus and taxon names are in line with the latest reports of the International Committee on Taxonomy of Viruses, and disease names are in accordance with the WHO’s International Classification of Diseases 11th revision (ICD-11). When needed for clarity or historical reference, alternative nomencla­ ture is used. Considering this syndromic approach, it should be noted that the variable clinical manifestations of particular viruses may be captured over a number of sections. ■ ■FEVER AND MYALGIA (F&M) F&M is by far the most common clinical syndrome and has the most favorable outcomes after arthropod-borne and rodent-borne infec­ tions. However, it is also a common prodrome en route to more inva­ sive syndromes associated with these infections. In logical sequence, therefore, F&M will be discussed initially but should not be considered in isolation. Indeed, clinicians evaluating patients with F&M should pay careful attention for symptoms and signs that suggest the A&R, neuroinvasive, pulmonary, or VHF syndromes that are subsequently discussed in this chapter. Although many of the viruses listed in Table 215-1 probably cause F&M, only some of these viruses have prominent associations with the syndrome that are considered bio­ medically important. F&M syndrome typically begins with the abrupt onset of fever, chills, intense myalgia, and malaise. Patients may also report joint pain, but true arthritis is not found. Anorexia is characteristic and may be accompanied by nausea or vomiting. Headache is common and may be severe, with photophobia and retroorbital pain. Physical findings are minimal and usually confined to conjunctival injection, pharyn­ geal erythema/exudate, muscle tenderness, abdominal tenderness,

PART 5 Infectious Diseases TABLE 215-3  Clinical Syndromes Caused by Zoonotic Arthropod-Borne or Rodent-Borne Viruses SYNDROME VIRUS Fever and myalgia (F&M) Arenaviridae: (Flexal,) Lassa and lymphocytic choriomeningitis viruses Flaviviridae: Bussuquara, Banzi, Cacipacoré, Dakar bat, dengue 1–4, Edge Hill, Karshi, Kédougou, Koutango, Ntaya virus, Rio Bravo, Sepik, Spondweni, tick-borne encephalitis, Wesselsbron, and Zika viruses Hantaviridae: Choclo virus Nairoviridae: Dugbe, Issyk-kul, Nairobi sheep disease, So –nglıˇng viruses, Taˇchéng tick virus 1, Tamdy, wetland, Yezo virus Orthomyxoviridae: Bourbon, Dhori, and Thogoto viruses Peribunyaviridae: Apeú, Bangui, Batai, Bunyamwera, Bwamba, Cache Valley, California encephalitis, Caraparú, Catú, Fort Sherman, Germiston, Guamá, Guaroa, Ilesha, Inkoo, Iquitos, Itaquí, Jamestown Canyon, La Crosse, Lumbo, Madrid, Maguari, Marituba, Nepuyo, Ngari, Nyando, Oriboca, Oropouche, Ossa, Pongola, Restan, Shokwe, snowshoe hare, Tacaiuma, Tˇahynˇ a, Tataguine, Wyeomyia, Xingu, and Zungarococha viruses Phenuiviridae: Alenquer, Bhanja, Candirú, Chagres, Dar es Salaam, Echarate, Heartland, Maldonado, Morumbi, Punta Toro, Rift Valley fever, sandfly fever Cyprus, sandfly fever Ethiopia, sandfly fever Naples, sandfly fever Sicilian, sandfly fever Turkey, Serra Norte, severe fever with thrombocytopenia syndrome, Toscana, and Uukuniemi viruses Sedoreoviridae: Kemerovo, Lebombo, Orungo, and Tribecˇ viruses Spinareoviridae: Colorado tick fever, Eyach, and Salmon River viruses Rhabdoviridae: Ekpoma virus 1, Ekpoma virus 2, Chandipura, Isfahan, Piry, vesicular stomatitis Indiana, and vesicular stomatitis New Jersey viruses Togaviridae: Everglades, Madariaga, Mucambo, Tonate, Una, and Venezuelan equine encephalitis viruses Arthritis and rash (A&R) Flaviviridae: Kokobera and Zika viruses Peribunyaviridae: Gan Gan virus, (Trubanaman virus) Togaviridae: Barmah Forest, chikungunya, Mayaro, o’nyong-nyong, Ross River, Semliki Forest, and Sindbis viruses Encephalitis Arenaviridae: lymphocytic choriomeningitis virus, (Whitewater Arroyo virus) Flaviviridae: Apoi, Ilhéus, Japanese encephalitis, Karshi, Modoc, Murray Valley encephalitis, Powassan, Rocio, St. Louis encephalitis, tick-borne encephalitis, Usutu, and West Nile viruses Orthomyxoviridae: Dhori and Thogoto viruses Bornaviridae: variegated squirrel bornavirus 1 Peribunyaviridae: Cache Valley, California encephalitis, Cristoli, Inkoo, Jamestown Canyon, Keystone, La Crosse, Lumbo, Oropouche, snowshoe hare, Shuni, and Tˇahynˇ a viruses Phenuiviridae: Adria, Bhanja, Chios, Rift Valley fever, (Taˇchéng tick virus 2), and Toscana viruses Sedoreoviridae: Banna, Kemerovo, and Orungo viruses Spinareoviridae: Colorado tick fever, Eyach, and Salmon River viruses Rhabdoviridae: Chandipura, Mundri viruses Togaviridae: eastern equine encephalitis, Everglades, Madariaga, Mucambo, Tonate, Venezuelan equine encephalitis, and western equine encephalitis viruses Pulmonary disease Hantaviridae: Anajatuba, Andes, Araucária, bayou, Bermejo, Black Creek Canal, Blue River, Caño Delgadito, Castelo dos Sonhos, Catacamas, Choclo, Juquitiba, Laguna Negra, Lechiguanas, Maciel, Monongahela, New York, Orán, Paranoá, Pergamino, Puumala, Rio Mamoré, Sin Nombre, (Tula,) and Tunari viruses Viral hemorrhagic fever (VHF) Arenaviridae: Chapare, Guanarito, Junín, Lassa, Lujo, (lymphocytic choriomeningitis,) Machupo, and Sabiá viruses Hantaviridae: Amur, Dobrava, El Moro Canyon, go – u, Hantaan, Kurkino, Muju, Puumala, Saaremaa, Seoul, Sochi, and Tula viruses Nairoviridae: Crimean-Congo hemorrhagic fever virus Peribunyaviridae: (Ilesha virus,) Ngari viruses Phenuiviridae: Rift Valley fever and severe fever with thrombocytopenia syndrome viruses Flaviviridae: Alkhurma hemorrhagic fever, dengue 1–4, Kyasanur Forest disease, Omsk hemorrhagic fever, (tick-borne encephalitis,) and yellow fever viruses Rhabdoviridae: (Bas-Congo virus) Viruses are placed in parentheses if cases were controversial. and possibly the presence of a nonpruritic maculopapular rash that may have a petechial component. The spectrum of disease varies from subclinical to temporarily incapacitating. When present, the evolution and duration of symptoms/signs are quite variable (generally 2–5 days) and may be biphasic after some viral infections. Less-common findings include epistaxis (not necessarily indicating a bleeding diathesis) and signs of aseptic meningitis. Even in the presence of headache, photo­ phobia, and meningismus, challenges in obtaining and examining the CSF in remote areas makes diagnosis difficult. Although upper and lower respiratory symptoms/signs and radiographic evidence of pul­ monary infiltrates are noted in some patients, the agents causing this syndrome are not primary respiratory pathogens. F&M syndrome is the most nonspecific of the disease syndromes caused by arboviral infections. The early stages of other syndromes discussed in this chapter begin similarly and are encompassed in a broad differential diagnosis that also includes community-acquired parasitic infections (e.g., malaria), bacterial infections (e.g., anicteric leptospirosis and rickettsial diseases), and other viral infections. F&M syndrome is often described as “influenza-like,” but the usual absence of cough and coryza makes influenza an unlikely confounder except at the earliest stages. Treatment is supportive, but acetylsalicylic acid is generally avoided because of the potential for exacerbated bleeding or Reye syndrome. Complete recovery is the expected outcome for symptomatic patients, although prolonged asthenia and nonspecific symptoms have been described, particularly after infection with LCMV or DENV-1–4. Nonetheless, it must be reemphasized that this non­ specific syndrome may be the prodrome for arthritic, neuroinvasive, pulmonary, or VHF syndromes with less favorable outcomes. Efforts to prevent viral infections causing this syndrome are best targeted to vector control, which, however, may be expensive or impossible. Destruction of mosquito breeding sites is generally the most economically and environmentally sound approach. Emerging containment technologies include the release of genetically modified mosquitoes and the spread of Wolbachia bacteria to limit mosquito

multiplication rates. Depending on the vector and its habits, other possible strategies include the use of window screens on dwellings or other barriers (e.g., permethrin-impregnated bed nets), judi­ cious application of arthropod repellents (e.g., N,N,-diethyltoluamide [DEET]) to the skin, use of long-sleeved (ideally permethrin-impregnated) clothing, and avoidance of vector habitats, particularly at peak feeding times. Bunyaviricetes  F&M syndrome is caused by numerous bunyavi­ ricetes, many of which cause isolated individual infections and usually do not cause epidemics. These viruses include arenavirids (mammare­ naviruses), hantavirids (orthohantaviruses), nairovirids (orthonai­ roviruses), peribunyavirids (orthobunyaviruses), and phenuivirids (bandaviruses and phleboviruses). ARENAVIRIDS  Infection with LCMV is the only human mammare­ navirus infection resulting predominantly in F&M syndrome. LCMV is transmitted to humans from the common house mouse (Mus mus­ culus) by aerosols of excreta or secreta. The virus is maintained in the mouse mainly by vertical transmission from infected dams. Infected mice remain viremic and shed virus for life, with high concentrations of virus in all tissues. Mouse-acquired infections of colonies of pet hamsters also can serve as a link for human acquisition. Patients may have a history of residence in rodent-infested housing or other expo­ sure to rodents. An antibody prevalence of ≈5–10% has been reported among urban adults from Argentina, Germany, and the United States. In addition, laboratory infections among scientists and animal care­ takers can occur because the virus is widely used in immunology laboratories to study T-lymphocyte function and can silently infect cell cultures and passaged tumor lines. Transmission has occurred via organ transplantation. Lymphocytic choriomeningitis (LCM) differs from the general F&M syndrome in a characteristic gradual onset of the otherwise typical illness. Orchitis, transient alopecia, arthritis, pharyngitis, cough, and maculopapular rash are associated with LCM. An estimated one fourth (or less) of patients experience an initial febrile phase of 3–6 days. After a brief remission, many develop recurrent fever accompanied by meningeal (severe headache, photophobia, nausea and vomiting, and neck stiffness) and/or less common encephalitic signs (altered mental status or level of consciousness, sensorimotor deficits) as part of a ≈1-week CNS phase. Patients with neuroinvasive disease, includ­ ing patients with clear-cut signs of encephalitis, almost always recover fully. Rarely, disease is complicated by transient hydrocephalus (that may require shunting) and myelitis. During the initial febrile phase, leukopenia and thrombocytopenia are common, and virus can usually be isolated from blood. During the CNS phase, the virus may be found in the CSF, and antibodies are detected in the blood. The pathogenesis of LCM is thought to resemble manifestations resulting from direct intracranial inoculation of the virus into adult mice. The onset of the immune response leads to T-cell-mediated immunopathologic menin­ gitis. During the meningeal phase, CSF monocyte counts range from the hundreds to the low thousands per microliter, and, unusually for a viral meningitis, hypoglycorrhachia is found in one-third of patients. IgM-capture ELISA, immunochemistry, and RT-PCR are used in the diagnosis of lymphocytic choriomeningitis. IgM-capture ELISA of serum and CSF usually yields positive results; RT-PCR assays have been developed for CSF detection. Particular diagnostic chal­ lenges arise in immunosuppressed patients with fulminant infections transmitted by recent organ transplantation: in the absence of typical immune responses, molecular diagnosis via RT-PCR or immunohisto­ chemistry may be required. Infection should be suspected in acutely ill febrile patients with marked leukopenia and thrombocytopenia and meningitis syndromes. In patients with aseptic meningitis, a diagnosis of LCM is suggested by the following: a well-marked febrile prodrome, adult age, occurrence in the autumn, low CSF glucose levels, or CSF monocyte counts of >1,000/μL. Throughout a pregnancy, maternalto-fetal transmission may occur, leading to fetal death (in the first trimester) or to consequent congenital hydrocephalus, microcephaly, and/or chorioretinitis (in the second or third trimesters). Because a mild maternal infection may be unrecognized or unrecalled, specific

antibodies to the virus should be sought in both mother and fetus under suspicious circumstances, particularly in neonatal hydrocephalus testing negative for toxoplasmosis, rubella, cytomegalovirus, herpes simplex, and HIV-1/HIV-2 (TORCH) pathogens.

PERIBUNYAVIRIDS  Apeú, Caraparú, Itaquí, Madrid, Marituba, Muru­ tucú, Nepuyo, Oriboca, Ossa, Restan, and Zungarococha viruses are among the most common causes of arboviral infection in humans entering South American jungles. These viruses cause acute febrile disease and are transmitted by mosquitoes in neotropical forests. Oropouche virus is transmitted in urban settings in Central and South America by biting midges (Culicoides paraensis), which often breed to high density in cacao husks and other vegetable detritus found in towns and cities. Explosive epidemics involving thousands of patients with Oropouche virus disease have been reported in Brazil and Peru, with smaller outbreaks in Panama and French Guiana and most recent outbreaks in Cuba and Haiti. Direct human-to-human transmission does not occur. Sloths and nonhuman primates may play a role as ver­ tebrate hosts during maintenance sylvatic cycles. After a 3- to 10-day incubation period, most patients develop typical F&M syndrome with a biphasic evolution (recurrence of symptoms in the second week of illness) and, uncommonly, hemorrhagic manifestations. However, a small percentage of patients develop severe illness with neurologic (meningoencephalitis, Guillain-Barré) or hemorrhagic syndromes that have been associated with fatal outcome. Recently, infection of pregnant women has been associated with spontaneous miscarriage, intrauterine fetal death, and congenital anomalies. Oropouche virus has been detected in umbilical cord blood and fetal tissues in these clinical settings. Serologic detection (IgM or a rise in antibody titer on paired samples) or PCR confirms the diagnosis of acute F&M syndrome; IgM and viral nucleic acid have been detected in the CSF in rare cases of neuroinvasive disease. Postacute sequelae have not been described, though some patients have prolonged asthenia after the acute syndrome. Specific therapeutics or vaccines are not available. CHAPTER 215 Iquitos virus, a recently discovered reassortant and close relative of Oropouche virus, causes disease that is easily mistaken for Oropouche virus disease; its overall epidemiologic significance remains to be determined. Arthropod-Borne and Rodent-Borne Virus Infections
PHENUIVIRIDS  The genus Phlebovirus includes numerous viruses that may cause human infection. Sandfly fever Cyprus virus, sandfly fever Ethiopia virus, sandfly fever Sicilian virus, and sandfly fever Turkey virus (and the encephalitis-causing Chios virus) are genetically and antigenically very closely related. In contrast, sandfly fever Naples virus (SFNV) is only genetically and antigenically distantly related to these viruses. SFNV has not been detected in sandflies, humans, or nonhuman vertebrates since the 1980s and therefore may be extinct, but closely related viruses, such as Granada virus and Toscana virus, continue to circulate. Toscana virus is thus far the only Phlebovirus transmitted by sandflies that is known to cause central and peripheral nervous system disease, such as encephalitis, meningitis, or polymyelo­ radiculopathy. Phlebotomus sandflies transmit the virus, probably by biting small mammals and humans. Female sandflies may be infected orally during blood meals and may transmit the virus to progeny when they lay their eggs. This prominent transovarial transmission confounds virus control. Sandfly fever is found in the circum-Mediterranean area, extend­ ing to the east through the Balkans into parts of China as well as into Western Asia. Sandflies are found in both rural and urban settings and are known for their short flight ranges and small sizes; the latter enables them to penetrate standard mosquito screens and netting. Epidemics have been described in the wake of natural disasters and wars. After World War II, extensive spraying in parts of Europe to control malaria greatly reduced sandfly populations and transmission of SFNV; the incidence of sandfly fever continues to be low. A common pattern of disease in endemic areas consists of high attack rates among visitors (including military personnel) but little or no disease in the local population, having developed protective immu­ nity after childhood infection. Toscana virus infection is common dur­ ing the summer among rural residents and vacationers, particularly in

Italy, Spain, and Portugal; several cases have been identified in travelers returning to Germany and Scandinavia. Clinical presentation is typical for F&M syndrome, with onset 3–6 days after a sandfly bite (a papule at the bite site may be observed). Notably, outbreaks of aseptic meningitis (virus can be isolated from the CSF) or meningoencephalitis have been associated with Toscana virus infection.

Coclé and Punta Toro viruses are not directly related to sandfly fever viruses but, like them, are transmitted by sandflies and cause a sandflyfever-like disease. These two viruses are maintained in Latin American and Caribbean tropical forests, respectively, where the vectors rest on tree buttresses. Epidemics have not been reported, but antibody preva­ lence among inhabitants of villages in endemic areas indicates a cumu­ lative lifetime exposure rate of >50% in the case of Punta Toro virus. Heartland virus is notable as an emerging virus in the eastern, mid­ western, and southern United States (>60 cases since its first descrip­ tion in 2012), where it causes often-fatal infection among males >50 years of age with comorbidities. Flavivirids  The most clinically significant orthoflaviviruses that cause F&M syndrome are DENV-1–4 and Zika virus (ZIKV). In fact, dengue without or with warning signs (“dengue,” historically called “dengue fever”—to be distinguished from severe dengue) is probably the most prevalent arthropod-borne viral disease worldwide, with ≈400 million infections occurring per year, of which ≈100 million (25%) cause clinical illness. Dengue is endemic in >100 countries worldwide, including those in Africa, the Americas, the eastern Medi­ terranean, Southeastern Asia, and the western Pacific. More than half of the world’s population is considered at risk, although Asia bears 70% of the global burden, with alarming increases over the past decade including, for example, >400,000 cases in 2019 in the Philippines. Year-round transmission of DENV-1–4 occurs between latitudes 25°N and 25°S, but seasonal forays of the viruses into the United States and Europe have been documented. The principal vectors for all four viruses are YF mosquitoes (Aedes aegypti). Through increasing spread of mosquitoes throughout the tropics and subtropics and international travel by infected humans, large areas of the world have become vul­ nerable to the introduction of DENV-1–4. Thus, dengue and severe dengue (see “Viral Hemorrhagic Fever,”) are becoming increasingly common. For instance, conditions favorable to DENV-1–4 transmis­ sion via YF mosquitoes exist in Hawaii and the southern United States. The range of a lesser vector of DENV-1–4, the Asian tiger mosquito (Aedes albopictus), now extends from Asia to the continental United States, the Indian Ocean, parts of Europe, and Hawaii. Also anthropophilic, YF mosquitoes typically breed near human habitation, using relatively fresh water in locations such as jars, vases, discarded containers, coconut husks, and old tires. These mosquitoes usually bite during the day. Bursts of dengue and severe dengue cases are to be expected in the southern United States, particularly along the Mexican border, where containers of water may be infested with YF mosquitoes. Airconditioned buildings with screened vents may inhibit transmission of many arboviruses, including DENV-1–4. PART 5 Infectious Diseases Most primary DENV infections are subclinical. After an incuba­ tion period of ≈4–7 days, symptomatic patients present with three evolving phases: febrile, critical, and recovery. Although most patients presenting with F&M syndrome do not go through a critical phase, early recognition of the critical phase consistent with severe dengue must be considered in all patients. In most patients, dengue begins with the typical sudden onset of high-grade fever, frontal headache, retroorbital pain, back pain, and severe myalgia. These symptoms and signs gave rise to the colloquial designation of dengue as “break-bone fever.” A transient macular rash is often present at illness onset, and conjunctival redness, pharyngeal erythema, lymphadenopathy, and hepatomegaly may be noted on physical examination. The illness may last a week and include additional symptoms and signs (anorexia, nausea or vomiting, and marked cutaneous hypersensitivity). Near the time of defervescence (days 3–5), a maculopapular rash begins on the trunk and spreads to the extremities and the face. Epistaxis and scat­ tered petechiae are often noted in uncomplicated dengue (without/ with warning signs), and preexisting gastrointestinal lesions may bleed

during the acute illness. A positive tourniquet test—i.e., the detection of 10 or more new petechiae in one square inch of the upper arm after a 5-min blood pressure cuff inflation to midway between systolic and diastolic pressure—may demonstrate microvascular damage, but this finding is more likely to be associated with severe dengue. After defervescence and a brief remission, a subset of patients has recurrence of fever and other signs in a “saddle-back” pattern. Regardless, most patients recover from acute illness by 7–10 days, though convalescence can be prolonged. Laboratory findings of dengue (without/with warning signs) include leukopenia, thrombocytopenia, and, in many cases, modest elevations of serum aspartate aminotransferase (AST) activity without hepatic synthetic dysfunction. The diagnosis is made by antigen-detection ELISA or RT-PCR during the acute phase or by IgM ELISA or paired serology during recovery. Virus is readily isolated from blood in the acute phase if mosquito inoculation or mosquito cell culture is used. Further clinical management and vaccine prevention are discussed in reference to severe dengue. ZIKV is an emerging pathogen that is transmitted to nonhuman primates and humans by Aedes mosquitoes. The virus was discov­ ered 1947 in a sentinel rhesus monkey (Macaca mulatta) and Aedes africanus mosquitoes in the Zika Forest in what was then the British Protectorate of Uganda. Human ZIKV infection was first documented during a YF outbreak in 1954 in Nigeria. Later, ZIKV infections were recognized in Southeastern Asia and Southern Asia. Prior to 2007, only 14 clinically identified cases of Zika virus disease had been reported. In recent years, the number of reported ZIKV infections has increased steadily and rapidly, with large but generally mild disease outbreaks on Yap Island and Micronesia (2007) and in Cambodia (2010), the Philippines (2012), and French Polynesia (2013–2014). ZIKV disease in the Americas was first reported on Easter Island, Chile (2014), and in Brazil (2015). By the end of 2015, an estimated 440,000 to 1.3 million cases had occurred in Brazil. At the end of May 2017, ZIKV infections had been recorded on five continents in 85 countries, including Mexico and the United States. Beginning in 2018, the global activity of ZIKV declined rather rapidly for unknown reasons. Phylogenetic analysis of all available African ZIKV isolates revealed two geographically overlapping clades (Eastern and Western Africa). A descendant Asian lineage, represented by viruses collected from mosquitoes trapped in homes in Malaysia, was first reported in 1969. All ZIKV isolates causing human cases outside of Africa trace back to this Asian lineage. Human infections are usually asymptomatic or benign and selfresolving and are most commonly misdiagnosed as dengue without/ with warning signs or influenza. Typically, ZIKV disease is charac­ terized by low-grade fever, an itchy maculopapular rash, arthralgia/ myalgia, nonpurulent conjunctivitis, headache, and malaise. Other clinical manifestations include involvement of the gastrointestinal (nausea, vomiting, abdominal pain, and/or diarrhea), genitourinary (hematospermia), and, less commonly, cardiac (myocarditis and/or pericarditis), ocular (uveitis), and auditory (hearing loss) systems. The most concerning complications of ZIKV infection are neurologic syndromes, such as, Guillain-Barré syndrome and congenital fetal microcephaly, which may be associated either with fetal death or serious developmental delay in newborn infants. Other neurologic complications include encephalitis, meningoencephalitis, transverse myelitis, demyelinating polyneuropathies, cerebrovascular ischemia, retinopathies, and neurologic birth defects. Although most human ZIKV infections are acquired after bites by infected female mosqui­ toes, transmission may also occur perinatally or via breast-feeding, sexual contact with an infected person, transfusion of blood products, or organ transplantation. Infectious ZIKV or ZIKV RNA has been documented in the semen of male patients for up to 69 and 188 days after illness onset, respectively; sexual transmission associated with persistence has been documented up to 41 days after illness onset. Antiviral treatments (curative or preventive) and licensed vaccines against ZIKV are not yet available. Of note, some evidence suggests that prior exposure to ZIKV infection may increase the risk of severe disease syndrome and poor outcomes from DENV infection.

Orthomyxovirids  Bourbon virus is emerging as a rare cause of F&M in humans in at least 14 states of the United States (≈60 cases since discovery in 2015). Onset is typical, but severe disease progres­ sion occurs among older individuals with comorbidities, including multisystem organ (kidney, respiratory, and/or hemodynamic) failure that has been fatal. Sedoreovirids  Several orbiviruses (Lebombo, Kemerovo, Orungo, and Tribeč viruses) can cause F&M in humans. Lebombo and Orungo viruses are transmitted by mosquitoes, whereas Kemerovo and Tribeč viruses are transmitted by ticks. Spinareovirids  Several coltiviruses (Colorado tick fever, Eyach, and Salmon River viruses) can cause F&M in humans. All are trans­ mitted by ticks. The most significant spinareovirid arthropod-borne disease is Colorado tick fever. Several hundred patients with this disease are reported annually in the United States and Canada. The infection is acquired between March and November through the bite of Rocky Mountain wood ticks (Dermacentor andersoni) in mountain­ ous western regions at altitudes of 1200–3000 m. Small mammals serve as amplifying hosts. After a tick exposure (reported by almost all patients) and a mean incubation period of 1–14 days, the most common presentation is F&M, often with headache; rash develops in a minority of patients. Meningoencephalitis is not uncommon, espe­ cially in children, and disseminated intravascular coagulation (DIC) with hemorrhagic manifestations, pericarditis, myocarditis, orchitis, and pulmonary disease have also been reported. Leukopenia and thrombocytopenia are noted. The disease usually lasts 7–10 days and is often biphasic. Historically, the most important differential diagnos­ tic considerations have been Rocky Mountain spotted fever (although Colorado tick fever is much more common in Colorado) and tulare­ mia. Notably, seropositivity may be delayed for 10–14 days, limiting the utility of typical serologic testing early in the disease course; molecular RT-PCR testing may be used in this setting. Also, Colorado tick fever virus replicates for several weeks in erythropoietic cells and can be found in erythrocytes. This feature, detected in erythroid smears stained by immunofluorescence, can be diagnostically helpful and is important during screening of blood donors. ■ ■ARTHRITIS AND RASH (A&R) Arthritides are common clinical presentations (or manifestations) of several viral diseases, such as hepatitis B, hepatitis C, parvovirus B19 infection, and rubella, and occasionally accompany infection due to adenovirids, enteroviruses, herpesvirids, mumps virus, or HIV-1/ HIV-2. Arthropod-borne alphaviruses are also common causes of arthritides—usually acute febrile syndromes with joint involvement often accompanied by a maculopapular rash. Rheumatic involvement includes arthralgia with or without typical inflammatory signs, includ­ ing periarticular redness, swelling (less commonly, joint effusions), and immobility. Most alphavirus infections are less severe and have fewer articular manifestations in children than in adults. In temperate climates, these ailments are summer diseases. The most significant alphavirus causes of arthritides are chikungunya virus disease, Ross River disease, Barmah Forest virus infection, and Sindbis virus infec­ tion. ZIKV infections may also be associated with joint manifestations. Less significant but historically notable are viruses that caused isolated cases or epidemics. A large (>2 million cases), albeit isolated, epidemic was caused by o’nyong-nyong virus from 1959 to 1962 (o’nyong-nyong fever). Mayaro virus, Semliki Forest virus, and Una virus caused iso­ lated cases or limited and infrequent outbreaks (30 to several hundred cases per year). Symptoms and signs of infections with these viruses often are similar to those observed with chikungunya virus disease. Two orthobunyaviruses—Gan Gan virus and Trubanaman virus—and the orthoflavivirus Kokobera virus have been associated with single cases of polyarthritic disease. Except for a vaccine to prevent chikun­ gunya virus disease, no specific therapies or licensed vaccines exist. Chikungunya Virus Disease  Historically, chikungunya virus (CHIKV) was considered endemic in rural areas of Africa, with inter­ mittent outbreaks occurring in towns and cities of both Africa and Asia.

In 2004, a large epidemic began in the Indian Ocean region (specifically on the islands of Réunion and Mauritius) and was most likely spread by travelers. YF mosquitoes and Asian tiger mosquitoes are the major CHIKV vectors. All regions with established populations of these two vectors have now documented local mosquito-borne transmission, and the virus has now been identified across Africa, Asia, Europe, and the Americas, including in the continental United States, where suitable vector mosquitoes are present in southern states and local transmission has been documented. From December 2013 to June 2023, >3.6 million cases were reported from 50 countries or territories in the Americas.

The disease is more common in adults. After an incubation period of 2–10 days, the abrupt onset of fever (often severe, with a saddle­ back pattern) and severe arthralgia are accompanied by constitutional symptoms and signs, including chills, abdominal pain, anorexia, conjunctival injection, headache, nausea, and photophobia. Migratory polyarthritis mainly affects the small joints of the ankles, feet, hands, and wrists, but the larger joints may be involved. Rash may appear at the outset or several days into the illness; its development often coin­ cides with defervescence around day 2 or 3 of clinical illness. The rash is most intense on the trunk and limbs and may desquamate. In hos­ pitalized patients with severe acute disease, uncommon but reported manifestations include cardiorespiratory, neurologic, renal, neurologic, and ocular complications; deaths have been reported, typically in the context of large outbreaks. Young children develop less prominent symptoms and signs and are therefore less frequently hospitalized. Children also often develop a bullous rather than a maculopapular/ petechial rash. Although pregnant women are not at risk of more severe disease, there is a high risk of maternal–fetal transmission, particularly during the intrapartum period, that, in some cases, has led to fetal death. Recovery may require weeks, and a significant portion of middle-aged to older patients develop chronic arthritis or arthralgia syndromes (typically involving the same joints) that may be disabling. The risk of chronic musculoskeletal syndromes is increased in patients with preexisting osteoarthritis or severe acute disease who test positive for the human leukocyte antigen B27 subtype (HLA-B27) and may be associated with cryoglobulins. Although petechiae and epistaxis are occasionally seen, CHIKV should not be considered as a cause of VHF. Laboratory abnormalities may include transient lymphopenia and mild thrombocytopenia, as well as elevated activities of AST and concentra­ tions of C-reactive protein (CRP). Treatment of chikungunya virus disease relies on acetaminophen and nonsteroidal anti-inflammatory drugs (NSAIDs). For patients with refractory arthritis, glucocorti­ coids or disease-modifying antirheumatic drugs (DMARDs) may be considered. A live-attenuated vaccine to prevent chikungunya virus disease has been approved by the U.S. Food and Drug Administration (FDA) and is undergoing postmarketing evaluation. Other vaccine and therapeutic candidates in preclinical development may benefit from the designation of CHIKV as the representative pathogen for arthritic alphaviruses. CHAPTER 215 Arthropod-Borne and Rodent-Borne Virus Infections
Ross River Disease and Barmah Forest Virus Infection  Ross River virus (RRV) and Barmah Forest virus (BFV) cause diseases that are clinically indistinguishable (hence the previously common disease designation of “epidemic polyarthritis” for both infections). RRV has caused epidemics in Australia, Papua New Guinea, and the South Pacific since the beginning of the 20th century. In 1979 and 1980, the virus swept through the Pacific Islands, causing >500,000 infections. From 1991 to 2011, the virus caused 92,559 infections in rural and suburban areas of Australia. From 2014 to 2015, >10,000 cases were recorded in Australia. RRV is predominantly transmitted by Aedes nor­ manensis, Aedes vigilax, and Culex annulirostris mosquitoes. Marsupials (e.g., kangaroos, wallabies, and koalas) and rodents are probably the main vertebrate hosts, but infection also occurs in horses and other livestock. BFV infections have been increasingly documented since the early 1990s. For instance, from 1991 to 2011, 21,815 cases were recorded in Australia, and new data indicate that the disease also occurs in Papua New Guinea. BFV is transmitted by both Aedes and Culex mosquitoes and has been isolated from biting midges. The vertebrate hosts remain to be determined, but serologic studies implicate horses and possums.

Of the human RRV and BFV infections surveyed, 55–75% were asymptomatic; however, clinical illness can be debilitating when it occurs. After a typical incubation period of 3–9 days, patients develop the sudden onset of symmetric joint pain predominantly involving the peripheral extremities. Generally, a nonitchy, diffuse, maculopapular rash (more common in BFV infection) develops coincidentally or follows shortly; but, in some patients, rash can precede joint pain by several days. Constitutional symptoms (e.g., low-grade fever, asthenia, headache, myalgia, and nausea) are not prominent or are absent in many cases. Most patients are incapacitated for considerable periods (6 months or more) by joint involvement that interferes with grasping, sleeping, and walking. Ankle, interphalangeal, knee, metacarpopha­ langeal, and wrist joints are most often involved, although elbows, shoulders, and toes may also be affected. Periarticular swelling and tenosynovitis are common, and one third of patients have true arthritis (more common in Ross River disease). Myalgia and nuchal stiffness may accompany joint pain. Only half of all patients with arthritis can resume normal activities within 4 weeks, and 10% continue to limit their activities after 3 months. Occasionally, patients are symptomatic for >1 year but without progressive arthropathy.

In the diagnosis of either infection, clinical laboratory values are normal or variable. Tests for rheumatoid factor and antinuclear anti­ bodies are negative, and the erythrocyte sedimentation rate is acutely elevated. Joint fluid contains 1000–60,000 monocytes per microliter, and viral antigen can usually be detected in macrophages. Serologic demonstration of rising antibody titers is the cornerstone of diagnosis, though it should be noted that virus-specific IgM antibodies may per­ sist for month to years. Isolation of the virus from blood after mosquito inoculation or growth of the virus in cell culture is possible early in the illness. Because of the great economic impact of annual epidemics in Australia, an inactivated RRV vaccine has been under advanced devel­ opment; phase 3 trials were completed in 2015 with promising results, but the candidate vaccine has not yet been developed for the market. Analgesics or NSAIDS are used for symptomatic treatment. PART 5 Infectious Diseases Sindbis Virus Infection  Sindbis virus is typically transmitted to birds primarily by infected Culex and Culiseta mosquitoes that also vector human transmission over a range that includes Africa, Asia, Europe, and Australia. Infections with northern European or southern African variants are particularly likely in rural environments. Although infections may be subclinical, rash and arthralgia typically develop after an incubation period of <1 week. Constitutional clinical signs are not marked, and fever is modest or absent. The rash, which lasts ≈1 week, begins on the trunk, spreads to the extremities, and evolves from macules to papules that often vesiculate. The arthritis is polyartic­ ular, migratory, and incapacitating, with resolution of the acute phase in a few days. The ankles, elbows, knees, phalangeal joints, wrists, and—to a much lesser extent—proximal and axial joints are involved. Persistence of joint pain and occasionally arthritis may continue for months or even years despite lack of deformities. ■ ■ENCEPHALITIS The major encephalitis viruses are found in the families Flaviviridae, Peribunyaviridae, Rhabdoviridae, and Togaviridae. However, individual agents of other families, including Dhori virus and Thogoto virus (Orthomyxoviridae) and Banna virus (Sedoreoviridae), have caused isolated cases of encephalitis. Arboviral encephalitides are seasonal diseases, commonly occurring in the warmer months. Their incidence varies markedly with time and place, depending on ecologic factors that determine vector activity and human exposure. The causative viruses differ substantially in terms of the ratio of clinical to subclinical infections, case-fatality rate (CFR), and postacute sequelae. Humans are not considered important amplifiers of these viruses. All the viral encephalitides discussed in this section share a similar pathogenesis. An infected arthropod ingests blood from a human and, through this contact, initiates infection. Initial viremia, thought to originate from the lymphoid system, leads to multifocal entry into the CNS, presumably via infection of olfactory neuroepithelium and sub­ sequent passage through the cribriform plate via infected macrophages

or infection of brain capillary endothelial cells. During the viremic phase, there may be little or no recognizable disease except in tickborne orthoflavivirus encephalitides, which have clearly delineated phases of fever and systemic illness. CNS dysfunction or damage and associated clinical symptoms/signs arise partly from direct neuronal infection and subsequent damage and partly from edema, inflammation, and other indirect effects of the host response. The usual pathologic features of arboviral encephalitides are focal necroses of neurons, inflammatory glial nodules, and perivas­ cular lymphoid cuffing. Involved areas display the “luxury perfusion” phenomenon, with normal or increased total blood flow and low oxygen extraction. The typical patient presents with a prodrome of nonspe­ cific constitutional symptoms and signs, including fever, abdominal pain, sore throat, and respiratory signs. Headache, meningeal signs, photophobia, and vomiting quickly follow. The severity of human infection widely varies from an absence of symptoms/signs to febrile headache, aseptic meningitis, and full-blown encephalitis. The propor­ tions and severity of these manifestations vary with the infecting virus and with known (e.g., age) and mostly unknown host immunogenetic determinants. Encephalitic involvement of deeper brain structures may be signaled by lethargy, somnolence, and cognitive deficit detected by mental status examination. More severely affected patients are obvi­ ously disoriented and may become comatose. Tremors, loss of abdomi­ nal reflexes, cranial nerve palsies, hemiparesis, monoparesis, difficulty swallowing, limb-girdle syndrome, and frontal lobe signs are common. Spinal and motor neuron diseases have been documented after WNV and Japanese encephalitis virus infections. Seizures and focal signs may be evident early or may appear during the disease. Some patients pres­ ent with an abrupt onset of fever, convulsions, and other signs of CNS involvement. Encephalitis usually lasts a few days to several weeks and may be fatal, or recovery may be slow (with weeks or months before the return of maximal recoverable function) or incomplete (with persisting long-term deficits). Difficulty concentrating, fatigability, tremors, and personality changes are common during recovery. The diagnosis of arboviral encephalitides depends on careful history-taking (travel/exposures), physical examination (in particular, of a febrile patient with signs of CNS dysfunction), and virus-specific or serology-specific laboratory testing. Clinicians should (1) consider empirical acyclovir treatment for herpesvirus meningoencephalitis and antibiotic treatment for bacterial meningitis until test results are received; (2) exclude intoxination and metabolic or oncologic causes, including paraneoplastic syndromes, hyperammonemia, liver failure, and anti-N-methyl-d-aspartate (NMDA) receptor encephalitis; and

(3) rule out a brain abscess or a cerebrovascular event. Leptospirosis, neurosyphilis, Lyme disease, cat-scratch disease, opportunistic infec­ tions in immunocompromised patients, and more recently described viral encephalitides (e.g., Nipah virus infection), among others, should be considered in the differential diagnosis if epidemiologically relevant. CSF examination usually shows a modest increase in white blood cell (WBC) counts—in the tens or hundreds or perhaps a few thousand. In early phases of disease, a significant proportion of CSF WBCs may be polymorphonuclear leukocytes (PMNs), but monocytes later pre­ dominate. CSF glucose concentrations are generally normal. There are exceptions to this pattern of findings. In eastern equine encephalitis, for example, PMNs may predominate during the first 72 h of disease, and hypoglycorrhachia may be detected. In lymphocytic choriomen­ ingitis, lymphocyte counts may be in the thousands, and glucose concentrations may be diminished. A humoral immune response is usually detectable at or near the onset of disease. Both serum (sampled in the acute or convalescent phase) and CSF should be examined for IgM antibodies, and plaque-reduction neutralization testing and/or RT-PCR should be conducted to detect viruses. Viruses generally can­ not be isolated from blood or CSF, although Japanese encephalitis virus has been recovered from CSF of patients with severe disease. RT-PCR analysis of CSF may yield positive results but cannot exclude diagno­ ses. Metagenomic/high-throughput sequencing of CSF may support diagnosis of arboviral encephalitides. Viral antigen is present in brain tissue, although its distribution may be focal. Electroencephalography usually shows diffuse abnormalities and thus is not directly helpful.

Experience with medical imaging is still evolving but is generally nonspecific, as most patients do not present with pathognomonic abnormalities; however, imaging may be useful to rule out other sus­ pected causes of disease. Both computed tomography (CT) and mag­ netic resonance imaging (MRI) scans may show normal results except for evidence of preexisting conditions or occasional diffuse edema. Notably, images acquired early may be normal and imaging abnor­ malities may only be detected later as the disease evolves. Although not pathognomonic, characteristic imaging abnormalities for some arbo­ viral encephalitides have been reported, such as with eastern equine encephalitis (focal abnormalities) and severe Japanese encephalitis (hemorrhagic bilateral thalamic abnormalities). Supportive care for severely ill patients may require management of elevated intracranial pressure, the syndrome of inappropriate secretion of antidiuretic hormone, respiratory failure, or seizures. Specific thera­ pies for these viral encephalitides are not available. The only practical preventive measures are vector management and personal protective measures against the arthropod transmitting the virus. Direct humanto-human transmission has not been confirmed. For Japanese or Central European/Far Eastern tick-borne encephalitides, vaccination should be considered in specific circumstances (see relevant sections that follow). Flavivirids  The most significant orthoflavivirus encephalitides are Central European/Far Eastern tick-borne encephalitides, Japanese encephalitis, St. Louis encephalitis, and WNV infection. Murray Valley encephalitis and Rocio virus infection resemble Japanese encepha­ litis but are documented only occasionally in Australia and Brazil. Powassan virus has caused ≈400 cases of often-severe disease (CFR ≈8%), frequently occurring among children in eastern Canada and the United States. Usutu virus causes sporadic human infections (e.g., 105 cases from 2012 to 2021 in Europe), but such infections may be underdiagnosed. central european/far eastern tick-borne encepha­ litides  Tick-borne encephalitis virus (TBEV) is currently sub­ divided into four groups: the western/European subtype (previously called Central European encephalitis virus), the (Ural-)Siberian sub­ type (previously called Russian spring–summer encephalitis virus), the Far Eastern subtype, and the louping ill subtype (previously called louping ill virus, or, in Japan, Negishi virus). Small mammals, grouse, deer, and sheep are the vertebrate amplifiers for these viruses, which are transmitted by ticks. The risk of infection varies by geographic area and can be highly localized. Human infections usually follow outdoor activities during which tick bites occur or consumption of raw (unpasteurized) milk or cheese from infected goats or, less commonly, infected cows or sheep. Milk seems to represent the main transmission route for louping ill subtype viruses, which cause disease very rarely. Several thousand infections with TBEV are recorded each year among people of all ages. Tick-borne encephalitis occurs between April and October, with a peak in June and July. Western/European viruses classically caused bimodal disease. After an incubation period of 7–14 days, the illness begins with an influenzalike febrile prodrome (fever, arthralgia/myalgia, headaches, and nausea and/or vomiting) that lasts for 2–4 days and is thought to correlate with viremia. A subsequent remission for several days is followed by the recurrence of fever and the onset of neurologic signs. The neuro­ invasive phase (7–10 days before onset of improvement) varies from mild aseptic meningitis (more common among younger patients) to severe (meningo)encephalitis with coma, seizures, tremors, and motor signs. Spinal and medullary involvement can lead to typical limbgirdle paralysis and respiratory paralysis. Most patients with western/ European virus infections recover (CFR ≈1–2%), and only a minority of patients have significant residual deficits. However, patients with (Ural-)Siberian virus infections have worse outcomes (CFR ≈7–8%) and are more likely to experience prolonged infections and develop chronic disability. Infections with Far Eastern viruses generally run a more abrupt and severe course. The encephalitic syndrome caused by these viruses

sometimes begins without a remission from the fever-myalgia phase and has more severe manifestations than the western/European syn­ drome. CFR is high (≈20–40%), and major sequelae—most notably, lower motor neuron paralyses of the proximal muscles of the extremi­ ties, trunk, and neck—are common, developing in approximately half of patients. Thrombocytopenia may occur during the initial febrile ill­ ness, resembling the early hemorrhagic phase of some other tick-borne orthoflavivirus infections, such as Kyasanur Forest disease. In the early stage of the illness, virus may be detected by PCR or isolated from the blood; however, after the onset of CNS manifestations, virus cannot typically be detected in or isolated from the CSF, and diagnosis requires detection of IgM antibodies in serum and/or CSF.

Diagnosis of Central European/Far Eastern tick-borne encepha­ litides primarily relies on serology and detection of viral genomes by RT-PCR. There is no specific therapy for infection. However, effec­ tive alum-adjuvanted, formalin-inactivated virus vaccines (FSMEIMMUN and Encepur) are produced in Austria, Germany, and Russia in chicken embryo cells. Two doses of the Austrian vaccine separated by an interval of 1–3 months appear to be effective in the field, though antibody responses are similar when vaccine doses are given 2 weeks apart. Because rare cases of postvaccination Guillain-Barré syndrome have been reported, vaccination should be reserved for people likely to experience rural exposure in an endemic area during the season of transmission. Cross-neutralization for the western/European and Far Eastern variants has been established, but there are no published field studies on cross-protection among formalin-inactivated vaccines. Because up to 4% of ticks in endemic areas may be infected, the use of immunoglobulin prophylaxis of Central European/Far Eastern tickborne encephalitides has increased in some regions. Prompt adminis­ tration of high-titer-specific immunoglobulin is routine in some areas (e.g., Russia) but has been discontinued in many European countries because of concerns for antibody-mediated enhancement of infections and/or disease. CHAPTER 215 JAPANESE ENCEPHALITIS  Japanese encephalitis is the most significant viral encephalitis in Asia. Each year, ≈68,000 cases and ≈13,600–20,400 deaths are reported. Japanese encephalitis virus is found throughout Asia—including in the Russian Far East, Japan, China, India, Pakistan, and Southeastern Asia—and causes occasional epidemics on western Pacific Islands. The virus has previously been considered endemic in Australia’s Torres Strait Islands; from 2021 to 2022, 45 cases caus­ ing seven deaths were identified across a broad region of eastern and southern Australia. The virus is particularly common in areas where irrigated rice fields attract the natural avian vertebrate hosts and pro­ vide abundant breeding sites for Culex tritaeniorhynchus mosquitoes, which transmit the virus to humans. Amplification by pigs, which subsequently abort pregnancies, and horses, which develop encepha­ litis, may be significant, as well. Vaccination of domestic pigs and horses may reduce the transmission of the virus. After an incubation period of 5–15 days, clinical signs of Japanese encephalitis range from nonspecific febrile illness (nausea, vomiting, diarrhea, and/or cough) to aseptic meningitis, meningoencephalitis, acute flaccid paralysis, and severe encephalitis. Common findings are cerebellar signs, cranial nerve palsies, and cognitive and speech impairments. A Parkinsonian presentation and seizures are typical in severe cases. Patients may pres­ ent with neuropsychiatric manifestations, including abnormal behavior and acute psychosis. MRI detection of thalamic abnormalities is spe­ cific but insensitive. CFR in hospitalized patients is high (≈20–30%) and long-term neurologic dysfunction and disability are common in survivors. However, most long-term residents of endemic areas even­ tually seroconvert after natural environmental exposures and develop protective immunity. Effective vaccines are available and indicated for naïve individuals who are relocating or traveling frequently or over extended periods to endemic rural areas. Usually, two intramuscular doses of the vaccine are given 28 days apart, with the second dose administered at least 1 week prior to travel. Arthropod-Borne and Rodent-Borne Virus Infections
ST. LOUIS ENCEPHALITIS  St. Louis encephalitis virus is transmitted among birds by mosquitoes. This virus causes a low-level endemic infection among rural residents of the central and western United States,

where Culex tarsalis mosquitoes serve as vectors. Urban mosquitoes (Culex pipiens and Culex quinquefasciatus) have been responsible for epidemics, resulting in hundreds or even thousands of cases in cit­ ies of the central and eastern United States. Most cases occur in June through October in the United States, but sporadic cases of the disease have been noted throughout the year in Latin/Central America and the Caribbean. The urban mosquitoes breed in accumulations of stagnant water and sewage with high organic content. The elimination of open sewers and trash-filled drainage systems is expensive and may not be possible. However, installation of screens on windows and vents of houses and implementation of personal protective measures may be effective in preventing infection. Mosquitoes are most active out­ doors at dusk; bites can be avoided by modifying activities and using repellents.

Most St. Louis encephalitis virus infections are subclinical; sus­ ceptibility to severe disease increases with age. Infections that result in aseptic meningitis or mild encephalitis are concentrated among children and young adults, whereas severe and/or fatal cases primar­ ily affect the elderly. Infection rates are similar in all age groups; a pathophysiologic explanation for higher risk of severe disease in older individuals is unclear. After an incubation period of 4–21 days, patients typically present with a nonspecific prodrome (fever, malaise, myalgia, and/or headache), followed by rapid-onset CNS manifestations with neurologic abnormalities that commonly include nuchal rigidity, hypo­ tonia, hyperreflexia, myoclonus, and tremors. Severe cases can include cranial nerve palsies, hemiparesis, and seizures. Of interest, during and after the prodrome, patients often report dysuria and may have viral antigenuria and pyuria. Overall CFR is ≈7% but may be increased up to ≈20% among patients >60 years of age. Recovery is slow, and emotional lability, difficulty concentrating, memory issues, asthenia, and tremors are commonly prolonged in older convalescent patients. The CSF of patients with St. Louis encephalitis usually contains tens to hundreds of WBCs, with a lymphocytic predominance and a left shift. The CSF glucose concentration is normal in these patients. PART 5 Infectious Diseases WEST NILE VIRUS INFECTION  WNV is now the primary cause of arboviral encephalitis in the United States. From 1999 to 2022, 28,684 cases of neuroinvasive disease (e.g., meningitis, encephalitis, and acute flaccid paralysis), with 2641 deaths, and 26,891 cases of nonneuroin­ vasive infection, with 135 deaths, were reported. WNV was initially described as being transmitted among wild birds by Culex mosquitoes in Africa, Asia, and southern Europe. In addition, the virus has been implicated in severe and fatal hepatic necrosis in Africa. WNV was introduced into New York City via diseased birds in 1999 and subse­ quently spread to other areas of the northeastern United States, causing die-offs among crows, exotic zoo birds, and other birds. The virus has continued to spread and is now found in all states (of the United States) as well as in Canada, Mexico, South America, and the Caribbean islands. C. pipiens mosquitoes remain the major vectors in the northeastern United States, but mosquitoes of several other Culex species and Asian tiger mosquitoes are also involved. Jays compete with crows and other corvids as amplifiers elsewhere in the United States. The majority of WNV infections are subclinical. After an incuba­ tion period of 3–14 days, 20% of those infected develop typical F&M syndrome (without CNS involvement); <1% develop aseptic meningitis and severe encephalitis, particularly among the elderly. The febrile syn­ drome associated with WNV infection is notable for the frequent—rather than occasional—appearance of a maculopapular rash concentrated on the trunk (especially in children) and the development of lymph­ adenopathy. Back pain, fatigue, headache, myalgia, retroorbital pain, sore throat, nausea and vomiting, and arthralgia (but not arthritis) are common symptoms. Most patients fully recover, though asthenia may persist for several weeks. Patients who develop neuroinvasive disease typically have meningitis, encephalitis, or acute flaccid paraly­ sis syndromes, often with some overlap. The risk of encephalitis, neurologic sequelae, and death is increased in male, elderly, diabetic, and hypertensive patients and in patients with previous CNS disease, but the pathophysiologic determinants of such a wide spectrum of disease phenotype and severity have otherwise been unexplained.

Host genetic risk factors for neuroinvasive disease and death may include chemokine receptor CCR5 deficiency. Recent identification of a high prevalence (≈35%) of preexisting autoantibodies against type I interferons in patients with encephalitis (vs patients with subclinical WNV infection, in whom the prevalence approximated the general population) suggests this predisposition may underlie 40% of cases of WNV encephalitis. In addition to the more severe motor and cognitive sequelae, milder neurologic findings may include cranial nerve palsies, tremor, slight abnormalities in motor skills, and loss of executive func­ tions. Given the burden of disease, intense clinical interest, and widely available diagnostics, more unusual features are increasingly described (including chorioretinitis, myocarditis, myositis and rhabdomyolysis, orchitis, flaccid paralysis with histologic lesions resembling poliomy­ elitis, and initial presentation with fever and focal neurologic deficits in the absence of diffuse encephalitis). Immunosuppressed patients may have fulminant courses or may develop persistent CNS infection. Postacute neurologic sequelae are common, especially in patients with severe neuroinvasive disease syndromes, and thus far have persisted in some patients for >5 years. Virus transmission through both trans­ plantation and blood transfusion has necessitated screening of blood and organ donors by nucleic acid–based tests. A low but non-zero risk of maternal-to-fetal/neonatal transmission has been reported. Diagnosis rests upon detection of IgM antibodies in serum or CSF. In patients with potential prior exposures to sero-cross-reactive flavivi­ ruses, plaque-reduction neutralization testing for specific neutralizing antibodies or molecular testing (PCR) may aid in specific diagnosis. Treatment is supportive only, and ventilatory support may be required for severe neuroinvasive disease. Although an equine vaccine is avail­ able, prevention of WNV infection in humans relies on avoidance of mosquito bites, vector control, and safe handling of potentially infected carcasses. Peribunyavirids  •  CALIFORNIA ENCEPHALITIS  California encephalitis virus has been implicated in only a very few cases of encephalitis (California encephalitis sensu stricto), whereas its close relative, La Crosse virus (LACV), is the major cause of this disease (historically ≈80–100 cases per year in the United States, although prevalence has been lower in recent years). La Crosse encephalitis is most reported in the upper midwestern United States but is also found in other areas of the central and southeastern parts of the country, such as West Virginia, Tennessee, North Carolina, and Georgia. Inkoo, Jamestown Canyon, Lumbo, snowshoe hare, and Ťahyňa viruses are close relatives of LACV that also cause human disease (California encephalitis sensu lato, including La Crosse encephalitis). Transovarial infection is a strong component of transmission of these viruses in Aedes and Ochlerotatus mosquitoes. The vector of LACV is the Och­ lerotatus triseriatus mosquito. These mosquitoes are infected by trans­ ovarial transmission, venereal transmission, and feeding on viremic chipmunks and other mammals. O. triseriatus mosquitoes breed in standing water in locations such as tree holes and abandoned tires; they bite during daylight hours. Risk factors for human cases include recre­ ation in forested areas, residence at a forest’s edge, and nearby standing water. Intensive environmental modification based on these findings has reduced the incidence of disease in a highly endemic area in the midwestern United States. Most humans are infected from July through September. Asian tiger mosquitoes efficiently transmit LACV to mice and have transmit­ ted it transovarially in the laboratory. This aggressive anthropophilic mosquito has the capacity to urbanize, and its possible transmission of virus to humans is of concern. The prevalence of antibody to LACV in humans is 20% or higher in endemic areas, indicating that infection is common but most often asymptomatic or subclinical. Neuroinvasive disease after LACV infection varies from aseptic meningitis accompanied by confusion to severe and occasionally fatal encephalitis (CFR <0.5%). CNS disease has been recognized primar­ ily in children <15 years of age. After an incubation period of ≈3–7 days, a usual nonspecific febrile prodrome is followed by the sudden onset of CNS manifestations, including headache and lethargy, often with nausea, vomiting, convulsions (in half of patients), and coma (in

one-third of patients). Focal seizures, hemiparesis, tremor, aphasia, chorea, Babinski signs, and other evidence of significant neurologic dysfunction are common during acute neuroinvasive disease. Most patients recover completely, but ≈10% develop postacute sequelae, including recurrent seizures, focal neurologic deficits, and cognitive and behavioral problems that may affect learning abilities. The WBC count is commonly elevated and left-shifted in patients with LACV infection, sometimes reaching 20,000/µL. CSF leukocyte counts are typically 30–500/µL, usually with a monocyte predominance (although 25–90% of cells are PMNs in some patients). The blood protein concentration is normal or slightly increased, and the glucose concentration is normal. Specific virologic diagnosis based on IgMcapture assays of serum and CSF is efficient. The only human anatomic site from which virus has been isolated is the brain. Treatment is supportive over a 1- to 2-week acute phase and focused on the prevention and management of status epilepticus, cerebral edema, and the syndrome of inappropriate secretion of antidiuretic hormone. A phase 2B clinical trial of IV ribavirin in children with LACV infection was discontinued during dose escalation because of adverse effects. Jamestown Canyon virus has been implicated in several cases of encephalitis in adults (≈10–40 cases per year since 2011), usually with a significant respiratory illness at onset. Human infection with this virus has been documented in Northern America (in the United States in at least 10 states and in Canada), where the vector mosquito (Aedes stimulans) feeds on its main host, the white-tailed deer (Odocoileus vir­ ginianus). Ťahyňa virus can be found in Africa, China, Central Europe, and Russia. The virus is a prominent cause of undifferentiated febrile disease but can also cause pharyngitis, pulmonary syndromes, aseptic meningitis, or meningoencephalitis. Rhabdovirids  •  CHANDIPURA VIRUS INFECTION  Chandipura virus is an emerging and increasingly significant human virus in India, where it is transmitted among hedgehogs by mosquitoes and sandflies. In humans, the disease begins as a rapid-onset influenza-like illness, with fever, headache, abdominal pain, nausea, and vomiting. These manifestations are followed by infection-related or autoimmune-mediated encephalitis that may cause altered mental status and seizures. Notably, neither the virus nor abnormal cells are detected in CSF; the brain has not been examined histopathologically, but limited cerebral imaging data query whether CNS disease is more likely related to cerebrovas­ cular events (vasospasm/vasculitis) than to infectious encephalitis. Several hundred cases of Chandipura virus infection are recorded in India every year in children, with a CFR ≈45–80%. Infections with other arthropod-borne rhabdovirids (Isfahan, Piry, vesicular stomatitis Indiana, and vesicular stomatitis New Jersey viruses) may imitate the early febrile stage of Chandipura virus infection. Togavirids  •  EASTERN EQUINE ENCEPHALITIS  This disease is encountered primarily in swampy foci along the east coast and, more recently and specifically, the southeastern coast of the United States, with a few inland foci as far removed from the coastlines as Michigan, Wisconsin, Arkansas, and Montana (≈4–40 cases/year). Humans are mostly infected from June through October. During this period, Culi­ seta mosquitoes infect birds and virus spills over into other vectors, such as Aedes sollicitans or Aedes vexans mosquitoes, which are more likely to feed on mammals. Especially with spread to the southeastern coast, there is concern over the amplifying role of introduced Asian tiger mosquitoes, which have been found to be infected with eastern equine encephalitis virus and are an effective experimental vector in the laboratory. Horses are a common target for the virus. Contact with unvaccinated horses may be associated with human disease, but horses probably do not play a significant role in amplification of the virus. Most infections are subclinical. After an incubation period of ≈5–10 days, symptomatic individuals develop a nonspecific febrile prodrome, after which a small proportion (2% of adults, 6% of chil­ dren) develop sudden and rapidly progressive neuroinvasive disease with (meningo)encephalitis syndromes associated with focal neuro­ logic deficits, seizures, profoundly altered mental status, and coma.

Neuroinvasive disease is highly fatal (CFR ≥30–50%) and >50% survivors have residual postacute sequelae. Acute PMN CSF pleocy­ tosis, often occurring during the first 1–3 days of disease, is another indication of severity, and it parallels peripheral leukocytosis with a left shift. Extensive necrotic lesions and PMN infiltrates are found at post­ mortem examination of the brain. Specific treatment is not available, although the therapeutic role of IV immunoglobulin (IVIG) remains under query. A formalin-inactivated vaccine has been used to protect laboratory workers but is not generally available or applicable.

VENEZUELAN EQUINE ENCEPHALITIS  VEEV is separated into epi­ zootic viruses (subtypes IA/B and IC) and enzootic viruses (subtypes ID, IE, and IF). Closely related enzootic viruses are Everglades virus, Mucambo virus, and Tonate virus. Enzootic viruses are found pri­ marily in humid tropical-forest habitats and are maintained between culicoid mosquitoes and rodents. These viruses cause acute febrile syn­ dromes in humans but are not pathogenic for horses and do not cause epizootics. Everglades virus has caused sporadic cases of encephalitis in humans in Florida. Extrapolation from the rate of genetic change suggests that Everglades virus may have been introduced into Florida <200 years ago. Everglades virus is most closely related to the ID-subtype viruses that appear to have given evolutionary rise to the epizootic variants active in South America. Epizootic viruses have an unknown natural cycle but periodically cause extensive epizootics/epidemics in equids and humans in the Americas. These epizootics/epidemics are the result of high-level vire­ mia in horses and mules, which transmit the infection to several types of mosquitoes, which in turn infect humans. Humans also have highlevel viremia, but their role in virus transmission is unclear. Relatively restricted epizootics of Venezuelan equine encephalitis (VEE) occurred repeatedly in South America in <10-year intervals from the 1930s until 1969, when a massive epizootic, including tens of thousands of equine and human infections, spread throughout Central America and Mexico, reaching southern Texas in 1971. Genetic sequencing suggested that the virus from that outbreak originated from residual “un-inactivated” IA/B-subtype virus in veterinary vaccines. The out­ break was terminated in Texas with a live-attenuated vaccine (TC-83), originally developed for human use by the U.S. Army; the epizootic virus was then used for further production of inactivated veterinary vaccines. No further major outbreaks occurred until 1995 and 1996, when large epizootics of VEE occurred in Colombia/Venezuela and Mexico, respectively. Of the >85,000 clinical cases, 4% (more children than adults) had neurologic symptoms/signs, resulting in 300 deaths. The viruses involved in these epizootics, as well as previously epizo­ otic IC viruses, are close phylogenetic relatives of known enzootic ID viruses. This finding suggests that active virus evolution and selection of epizootic viruses are underway in South America. CHAPTER 215 Arthropod-Borne and Rodent-Borne Virus Infections
During epizootics, extensive human infection is typical, with clini­ cal disease occurring in 10–60% of infected individuals. Most infec­ tions result in notable acute febrile syndromes, whereas relatively few (5–15%) result in neurologic disease. A low rate of CNS invasion is supported by the absence of encephalitis after the many infections (from exposure to aerosols) that have occurred in the laboratory set­ ting. Small-molecule and monoclonal antibody-based antiviral thera­ peutics are in preclinical development but not yet approved. These (and vaccine; see below) efforts may benefit from the designation of VEEV as a representative pathogen for the encephalitic alphaviruses. The prevention of epizootic VEE depends on vaccination of horses with the attenuated TC-83 vaccine or with an inactivated vaccine pre­ pared from that variant. Enzootic viruses are genetically and antigeni­ cally different from epizootic viruses, and protection against the former with vaccines prepared from the latter is relatively ineffective. Humans can be protected by immunization with similar vaccines prepared from Everglades virus, Mucambo virus, and VEEV, but the use of the vac­ cines is restricted to laboratory personnel because of reactogenicity, possible fetal pathogenicity, and limited availability. WESTERN EQUINE ENCEPHALITIS  The primary maintenance cycle of western equine encephalitis virus in the western United States and Canada involves Aedes, C. tarsalis, and Culiseta mosquitoes and

birds (principally sparrows and finches). Equids and humans become infected, and both suffer encephalitis without amplifying the virus. St. Louis encephalitis virus is transmitted via a similar cycle in the same regions harboring western equine encephalitis virus; disease caused by the former occurs about a month earlier than that caused by the latter (July through October). Large epidemics of western equine encephalitis occurred in the western and central United States and Canada from the 1930s through the 1950s but have been uncommon since then. From 1964 through 2010, only 640 cases were reported in the United States. This decline in incidence may reflect, in part, the integrated approach to mosquito management employed in irrigation projects and, in part, the increasing use of agricultural pesticides. The decreased incidence of western equine encephalitis almost certainly reflects the increased tendency for humans to be indoors behind closed or screened windows at dusk—the peak biting period of the major vector.

Most infections are subclinical or mild. After an incubation period of ≈5–10 days, most symptomatic patients develop a nonspecific febrile prodrome with spontaneous recovery. The small minority of patients who develop neuroinvasive disease present with a typical viral menin­ goencephalitis syndrome (headache, vomiting, neck and back stiffness, and/or altered mental status). The frequency and morbidity of severe neurologic disease are increased among infants and very young chil­ dren; in addition, CFR is high in this and in very elderly populations (3–7% overall). One-third of individuals who have convulsions dur­ ing the acute illness have subsequent seizure activity. Infants <1 year of age—particularly those in the first months of life—are at serious risk of persistent motor and cognitive deficits. Of those 5–9 years of age, twice as many males as females develop clinical encephalitis; the contribution of biological versus behavioral factors (increased outdoor exposure to the vector) to incidence differences is uncertain. Specific treatment is not available. A formalin-inactivated vaccine has been used to protect laboratory workers but is not generally available. PART 5 Infectious Diseases ■ ■PULMONARY DISEASE Hantavirus pulmonary syndrome (HPS) was first described in 1993, but retrospective identification of cases by immunohistochemistry (1978) and serology (1959) supports the idea that HPS is a recently discovered disease rather than a truly new one. The causative agents are orthohantaviruses of a distinct phylogenetic lineage that is associated with the cricetid rodent subfamily Sigmodontinae. Sin Nombre virus, which chronically infects western deermice (Peromyscus sonoriensis), is the most important agent of HPS in the United States. Several other related viruses (Anajatuba, Andes, Araraquara, Araucária, bayou, Bermejo, Black Creek Canal, Blue River, Caño Delgadito, Castelo dos Sonhos, Catacamas, Choclo, Juquitiba, Laguna Negra, Lechiguanas, Maciel, Maripa, Monongahela, New York, Orán, Paranoá, Pergamino, Rio Mamoré, and Tunari viruses) cause the disease in Northern and South America. Andes virus is unusual in that it has been implicated in human-to-human transmission. HPS particularly affects rural resi­ dents living in dwellings permeable to rodent entry or workers in occu­ pations that risk rodent exposure. Each type of rodent has particular habits; in the case of deermice, these behaviors include living in and around human habitation. After a typical incubation period of 2–3 weeks, HPS begins with a nonspecific prodrome of ≈3–4 days (range 1–11 days) comprising fever, malaise, myalgia, and—in many cases—gastrointestinal distur­ bances (e.g., abdominal pain, nausea, and vomiting). Dizziness is com­ mon, and vertigo is occasional. Severe prodromal symptoms/signs may bring some patients to medical attention, but most cases are recognized as the pulmonary phase begins, with typical signs of slightly lowered blood pressure, tachycardia, tachypnea, mild hypoxemia, thrombocy­ topenia, and early radiographic signs of pulmonary edema. Physical findings in the chest are often surprisingly scant. The conjunctival and cutaneous signs of vascular involvement seen in orthohantavirus VHFs (see “Viral Hemorrhagic Fever,” below) are uncommon but may be more frequent with HPS outside Northern America. Early recogni­ tion and triage of patients at this stage are critical because, shortly after onset, hypotension and noncardiogenic edema progress rapidly to hypoxemic respiratory failure.

The differential diagnosis of HPS includes undifferentiated sep­ sis syndromes and severe acute respiratory infections and should be informed by local epidemiology. Considerations include severe atypical pneumonia (Legionella, Mycoplasma, and Q fever), influenza, rickettsial disease, meningococcemia, septicemic plague, tularemia, leptospirosis, dengue, and YF. Fever with severe abdominal pain and tenderness merits consideration of the causes of a “surgical” abdomen and pyelonephritis. A specific diagnosis is best made when antigenspecific IgM, present in most symptomatic patients, is detected in acute-phase serum. Sin Nombre virus antigen testing detects antibod­ ies to the related HPS-causing orthohantaviruses. Occasionally, het­ erotypic viruses will cross-react (only to the IgG ELISA), but the very low seroprevalence of these viruses in normal populations queries this laboratory finding. Orthohantaviral RNA is cleared quickly, although RT-PCR may be useful to detect virus in tissue biopsies, thrombus, or at autopsy. RT-PCR and DNA sequencing may be needed in specifi­ cally identifying the infecting virus in areas outside the home range of deermice, in atypical cases, and for molecular epidemiology. During the prodrome, differentiating HPS from other causes is difficult, but diagnostically helpful features emerge at or soon after initial presentation. A new dry cough, usually absent at disease onset, may signal the pulmonary phase. Early in disease, interstitial edema may be evident radiographically; with progression, bilateral alveolar infiltrates in a central pattern and with a normal-sized heart signal the development of noncardiogenic edema. Pleural effusions are often seen. Coincident thrombocytopenia, circulating atypical lymphocytes (lymphoblasts), and a left shift (often with marked leukocytosis) are considered almost diagnostic in experienced centers. Hemoconcen­ tration, hypoalbuminemia, and proteinuria also aid in diagnosis. Although thrombocytopenia and partial thromboplastin time (PTT) prolongation are usually found, clinical evidence of coagulopathy or laboratory indications of DIC are found in only a minority of severely ill patients. Patients with severe illness also have acidosis and elevated serum lactate concentrations. Oliguria and mildly increased values in serum creatinine concentrations are common, but patients with severe HPS often have markedly decreased glomerular filtration. Some American orthohantaviruses other than Sin Nombre virus (e.g., Andes virus) have been associated with more severe renal dysfunction, but few such cases have been studied. After recognition, early triage-based monitoring and management of hemodynamic, respiratory, and renal dysfunction during the first few hours are critical to improved outcomes. During this period, hypo­ tension and hemoconcentration suggest intravascular volume deple­ tion; however, aggressive IV fluid therapy is not advised in the setting of widespread capillary leak, frequently depressed cardiac output, and oliguria. Mild hypoxemia may be managed by oxygen administra­ tion alone, but mechanical ventilatory support of respiratory failure and shock is often needed. Pulmonary artery catheterization enables pulmonary capillary wedge pressure–guided inotropic and vasopres­ sor support and cautious intravascular volume replacement. The most critically ill patients with a low cardiac index or refractory hypoxemia despite this support may benefit from extracorporeal membrane oxy­ genation, ideally before the onset of shock. CFR is high (≈30–40%), even with appropriate management, but most patients surviving the first 48 h of hospitalization are extubated and discharged within a few days with no apparent long-term sequelae. Although ribavirin inhibits orthohantaviruses in vitro and has been effectively used in patients with HFRS due to Hantaan virus, an underpowered randomized clini­ cal trial of ribavirin for HPS failed to show efficacy. Preclinical devel­ opment of novel small-molecule– and monoclonal antibody-based therapeutics for HPS is ongoing. ■ ■VIRAL HEMORRHAGIC FEVER VHF syndrome encompasses a small number of acute viral infections that cause severe disease and multisystem organ dysfunction sharing a mutual pathophysiologic disruption to vascular function, stability, and integrity. In practice, viral infections classified as VHFs vary widely across a spectrum of clinical phenotypes and severity; indeed, hemor­ rhagic manifestations are typically not the most prominent nor severe

features of disease. Nonetheless, the VHF designation largely captures a useful common theme: hemorrhagic manifestations are a sign of wide­ spread vascular dysfunction or damage that plays a central role in local, organ-specific, and systemic expression of disease. Direct or indirect damage to the microvasculature leads to increased permeability and (particularly when platelet function is decreased) to actual disruption and local hemorrhage most evident in the skin and mucous mem­ branes. Cutaneous flushing and conjunctival suffusion are examples of common observable abnormalities due to dysfunction of local cir­ culation. Similarly, affected microvasculature may be fragile to trauma and external forces, such as the application of a blood pressure cuff causing a positive tourniquet sign. Although overt hemorrhage occurs infrequently, subtle local hemorrhagic signs (e.g., bleeding of the gums or at IV catheter sites) are often present on careful examination. Cir­ culatory dysfunction may also contribute to specific organ dysfunction or damage that may be particularly prominent in some VHF cases; for instance, the kidneys are primary targets in HFRS, and the liver is a pri­ mary target in YF. Regarding severe systemic manifestations, hypoten­ sion and shock are not usually a direct result of life-threatening blood loss but rather an indication of generalized circulatory dysfunction contributing to hemodynamic instability. Despite this common theme, the pathogenesis of VHF is poorly understood and varies among the viruses regularly implicated in the syndrome. In some viral infections, direct damage to the vascular system or even to parenchymal cells of target organs is an important factor; in other viral infections, soluble mediators are thought to play a major role in the development of hem­ orrhage or hemodynamic derangement. Symptom initiation in the acute phase in most VHF syndromes is associated with ongoing virus replication and viremia, typically with nonspecific fever and arthralgia/myalgia, usually of abrupt onset (mammarenavirus infections are exceptions, as they often develop gradually). Within a few days, patients present for medical atten­ tion with severe fatigue and prostration that is often accompanied by anorexia, severe headache, sore throat, chest pain, gastrointestinal symptoms (abdominal pain, nausea, vomiting, and/or diarrhea), diz­ ziness, hyperesthesia, and photophobia. Initial examination often reveals only an acutely ill febrile patient with asthenia, conjunctival suffusion, tenderness to palpation of muscles or abdomen, and border­ line hypotension or postural hypotension (perhaps with tachycardia). Petechiae (often best visualized in the axillae), flushing of the head and thorax, periorbital edema, and proteinuria are common. AST activities are usually elevated at presentation or within a day or two thereafter. Hemoconcentration from vascular leakage is usually evident and is most marked in HFRS and in severe dengue. Seriously ill patients develop more severe clinical signs, including initial organ dysfunction typical of the causative virus; continued high viral loads coupled with ineffective and dysregulated immune responses lead to progressive multiorgan dysfunction characterized by shock, acute kidney injury, CNS injury (encephalopathy, coma, seizures), severe hemorrhagic manifestations, and death. One of the major diagnostic clues is travel to an endemic or active outbreak area within the incubation period for a given syndrome. Except in infections with Seoul virus, DENV-1–4, and yellow fever virus (YFV), which have urban hosts/vectors, rural travel exposures are more commonly associated with a VHF. In addition, several travelassociated diseases—falciparum malaria, shigellosis, typhoid fever, leptospirosis, relapsing fever, and rickettsial diseases—should also be considered in the differential diagnosis, as they are treatable despite being potentially fatal. Early recognition of VHF is crucial for timely initiation of appropri­ ate supportive clinical care and virus-specific therapy (if available). Key components of clinical care include prompt isolation and hospitaliza­ tion even in absence of clinical signs; rapid triage and ongoing moni­ toring of vital signs; the prevention and management of dehydration, hypovolemia, and hemodynamic instability (with closely monitored IV fluids and vasopressors); the prevention and management of acute kid­ ney injury and electrolyte abnormalities; empiric or specific treatment of common concurrent or secondary bacterial or parasitic infections; replacement of blood products (packed red blood cells, clotting factors,

and platelets as indicated); and the usual precautionary measures used in the treatment of patients with hemorrhagic diatheses. DIC should be treated only with clear laboratory evidence and if laboratory monitor­ ing of therapy is feasible; there is no proven benefit of such therapy. Of note, VHF patients are commonly hypovolemic from insensible fluid and gastrointestinal losses and require intravascular fluids to maintain blood pressure and glomerular filtration; however, VHF patients may also have decreased cardiac output, capillary leak syndromes, and oli­ guria that require judicious fluid management and inotropic/vasopres­ sor support. Specific therapy is available for several of the VHFs. Strict barrier nursing and other precautions against infection of medical staff and visitors are indicated when VHFs are encountered, except when the illness is due to DENV-1–4, orthohantaviruses, RVFV, or YFV.

Novel VHF-causing agents are still being discovered. Some of these viruses have recently emerged and continue to cause disease (e.g., severe fever with thrombocytopenia syndrome virus [SFTSV]). Others have caused isolated VHF-like cases without recurrence, including viruses for whom Koch’s postulates are yet to be fulfilled (e.g., Bas-Congo virus). Bunyaviricetes  The most significant VHF-causing bunyaviricetes are arenavirids (Junín, Lassa, and Machupo viruses), hantavirids, nai­ rovirids (Crimean-Congo hemorrhagic fever virus), and phenuivirids (RVFV and SFTSV). Other bunyaviricetes—e.g., the Garissa variant of Ngari virus and Ilesha virus (both orthobunyaviruses) or Chapare, Guanarito, Lujo, and Sabiá viruses (all mammarenaviruses)—have caused sporadic VHF outbreaks. ARGENTINIAN AND BOLIVIAN HEMORRHAGIC FEVERS  These severe diseases (with CFR reaching >50% in individual outbreaks) are caused by Junín virus and Machupo virus, respectively. Clinical presentations of both diseases are similar, but epidemiology differs because of the distribution and behavior of the distinct rodent reservoirs. Argentinian hemorrhagic fever has been recorded only in rural areas of Argentina, whereas Bolivian hemorrhagic fever seems to be confined to rural Bolivia. Unusually for arboviruses, infection with the causative agents almost always results in disease, and all ages and both sexes are affected. Person-to-person or nosocomial transmission is rare but has occurred. The transmission of Junín virus from male survivors to their partners suggests the need to avoid intimate contact in early convalescence (and counseling in this regard for all patients with mammarenavirus hemorrhagic fevers). In contrast to LF (see below), thrombocytopenia—often marked—is the rule, mucosal hemorrhage is common, and CNS dysfunction (e.g., marked confusion, tremors of the upper extremities and tongue, and cerebellar signs) is much more common in disease caused by Junín virus and Machupo virus (20–30%). Some cases follow a predominantly neurologic course, with a poor prognosis. Convalescence is protracted and frequently associ­ ated with sequelae such as hearing loss. CHAPTER 215 Arthropod-Borne and Rodent-Borne Virus Infections
Typical clinical laboratory findings (thrombocytopenia, leukope­ nia, and proteinuria) support the diagnosis. Argentinian and Bolivian hemorrhagic fevers are readily treated with convalescent plasma given within the first 8 days of illness; however, the use of convalescent plasma may be associated with neurologic sequelae. In the absence of passive antibody therapy, (off-label) IV ribavirin in the dose recommended for LF is likely to be effective in all South American VHFs caused by mammarenaviruses. A safe, effective, live-attenuated vaccine exists for Argentinian hemorrhagic fever. After vaccination of >250,000 highrisk people in the endemic area, the incidence of this VHF decreased markedly. In experimental animals, this vaccine is cross-protective against Bolivian hemorrhagic fever. Other mammarenaviruses have caused isolated or sporadic VHF in South America, including Guanarito virus (Venezuelan hemorrhagic fever) and Chapare and Sabiá viruses. Multivalent vaccine approaches against all five VHF-causing mam­ marenaviruses in this region are in preclinical development. LASSA FEVER  Lassa virus (LASV) is known to cause endemic and epidemic disease in Nigeria, Sierra Leone, Guinea, and Liberia, although it is probably more widely distributed in Western Africa. In Western Africa alone, probably tens to hundreds of thousands of LASV

infections occur annually, and ≈20% of these develop LF, which can be a prominent cause of febrile disease. For example, in one hospital in Sierra Leone, laboratory-confirmed LF is consistently responsible for one-fifth of admissions to the medical wards. LASV can be trans­ mitted by close person-to-person contact. Nosocomial spread has occurred but is uncommon if appropriate infection prevention and control practices are followed. All ages and both sexes are affected; the incidence of disease is highest in the dry season, but transmission occurs year-round.

The majority of LASV infections are subclinical or mild. After a highly variable incubation period of 1–3 weeks, symptomatic patients have the gradual onset of a nonspecific febrile prodrome (among the VHF agents, only mammarenaviruses are typically associated with a gradual onset of illness). Progression in the second week of illness includes sore throat, cough, retrosternal chest and back pain, and gastrointestinal symptoms (abdominal pain, vomiting, and/or diar­ rhea). A maculopapular rash may be noted in light-skinned patients. Further disease progression includes conjunctival infection, head and neck edema, and respiratory (pneumonia and/or pleural effu­ sions) and cardiac (pericarditis and/or pericardial effusions) mani­ festations. In late stages, multisystem organ dysfunction occurs with acute kidney injury, CNS manifestations (encephalopathy, coma, and/ or seizures), shock, and hemorrhage (seen in only ≈15–30% of LF patients overall). Laboratory abnormalities may include leukocytosis, thrombocytopenia, elevated serum creatinine levels, hyperkalemia, hypoalbuminemia, elevated AST and ALT activities, and proteinuria. Viral load, age, pregnancy status, clinical signs (CNS features, face and neck swelling, hypotension, hemorrhage, and/or jaundice), and laboratory abnormalities (high serum creatinine levels, AST activity, leukocytosis, and/or thrombocytopenia) have been associated with fatal outcome. Pregnancy is associated with higher case fatality, espe­ cially during the last trimester, when fetal death is 90%. Interruption of pregnancy via medical abortion may increase survival rates, but data on LF and pregnancy outcomes remain sparse. In survivors, a high rate (≈30%) of sensorineural deafness (onset during the acute illness or in convalescence, usually bilateral) occurs; other postacute sequelae of LF are poorly defined but include neurologic and mental health issues. The virus is detected in the urine during convalescence and has been detected in seminal fluid early in recovery. Reinfection may occur but has not been associated with severe disease. PART 5 Infectious Diseases Aggressive supportive and critical care, including renal replacement therapy when available, is required to improve outcomes. Regarding LASV-specific therapy, observational studies in the 1980s informed the current practice of treating patients with (off-label) ribavirin (IV route preferred). Historically, ribavirin appeared to be partially effective in reducing fatality from that documented among retrospective controls. However, possible side effects, such as reversible anemia (which usually does not require transfusion), dependent hemolytic anemia, and bone marrow suppression, need to be kept in mind. When used, ribavirin should be given by slow IV infusion in a dose of 32 mg/kg; this dose should be followed by 16 mg/kg every 6 h for 4 days and then by 8 mg/kg every 8 h for 6 days. Small-molecule and monoclonal antibody therapeutics are in advanced preclinical development, and research networks in Western Africa are preparing for randomized clinical tri­ als that will be critical to inform best specific therapeutic approaches. Inactivated LASV vaccines failed in preclinical studies, but several promising vaccine platforms are under experimental evaluation. HEMORRHAGIC FEVER WITH RENAL SYNDROME  HFRS is the most significant VHF today, with >100,000 cases of severe disease in Asia annually and thousands of mild infections in Europe. The disease is widely distributed in Eurasia. The major causative viruses are Puumala virus (Europe), Dobrava virus (the Balkans), and Hantaan virus (Eastern Asia). Amur, gōu, Kurkino, Muju, Saaremaa, Sochi, and Tula viruses also cause HFRS but much less frequently and in more geo­ graphically confined areas that are determined by the distribution of reservoir hosts. Seoul virus is an exception; because it is associated with brown rats (Rattus norvegicus), which migrate on ships, the virus has a worldwide distribution. Despite the wide distribution of Seoul virus,

only mild or moderate HFRS occurs in Asia; human disease has been difficult to identify in many areas of the world, although an outbreak in North America has been described. Most cases of HFRS occur in rural residents or vacationers; again, the exception is Seoul virus infection, which may be acquired in an urban, rural, or laboratory setting. Classic Hantaan virus infection in Korea and in rural China is most common in the spring and fall and is related to rodent density and agricultural practices. Human infection is acquired primarily through aerosols of rodent urine, although virus is also present in rodent saliva and feces. Patients with HFRS are not infectious. Most patients with hantavirus infections do not develop severe dis­ ease. Classic features of HFRS include fever, hypotension, hemorrhage, and acute kidney injury, with severe disease expressed in four identifi­ able stages after a typical 1- to 2-week incubation period:

  1. The febrile stage lasts 3 or 4 days and is identified by the abrupt onset of fever, headache, severe myalgia, thirst, anorexia, and often nausea and vomiting. Photophobia, retroorbital pain, and pain on ocular movement are common, and the vision may become blurred with ciliary body inflammation. Flushing over the face, the anterior neck, and back is characteristic, as are pharyngeal injection, perior­ bital edema, and conjunctival suffusion. Petechiae often develop in areas of pressure, the conjunctivae, and the axillae. Back pain and tenderness to percussion at the costovertebral angle reflect massive retroperitoneal edema. Thrombocytopenia and mild to moderate DIC is present, and hemorrhagic manifestations may occur. Other early laboratory findings include proteinuria and an active urinary sediment.
  2. During the hypotensive stage (lasting from a few hours to 48 h), the blood pressure falls (sometimes with shock) with a compensatory tachycardia (vs the relative bradycardia typical of the febrile phase). Kinin activation and capillary leak are marked, leading to hemocon­ centration. Leukocytosis with a left shift develops, and thrombocy­ topenia persists. Atypical lymphocytes—which, in fact, are activated CD8+ and, to a lesser extent, CD4+ T cells—circulate. Renal perfu­ sion is compromised from local and systemic circulatory changes resulting in necrosis of tubules, particularly at the corticomedullary junction, and oliguria. Consequent inability to concentrate the urine develops, and the urine’s specific gravity falls to 1.010. Proteinuria is marked. Diffuse hemorrhage may appear, including hematemesis, hemoptysis, and melena.
  3. During the oliguric stage, hemorrhagic tendencies continue, prob­ ably in large part because of uremic bleeding defects. Oliguria persists for 3–10 days before the return of renal function marks the onset of the polyuric stage.
  4. During the polyuric stage (diuresis with hyposthenuria), the inability to concentrate the urine may lead to volume depletion and electro­ lyte abnormalities. Mild cases of HFRS may be much less stereotypical, presenting only with fever, gastrointestinal abnormalities, and transient oliguria followed by hyposthenuria. Infections with Puumala virus, the most common cause of HFRS in Europe (“nephropathia epidemica”), result in a similar general but much-attenuated presentation. Bleeding mani­ festations are found in only 10% of patients, hypotension rather than shock is usually documented, and oliguria is present in only about half of patients. The dominant features may be fever, abdominal pain, proteinuria, mild oliguria, and sometimes blurred vision or glaucoma, followed by polyuria and hyposthenuria in recovery. CFR is <1%. HFRS should be suspected in patients with exposure in an endemic (typically rural) area. Prompt recognition of the disease permits rapid hospitalization and expectant management of shock and renal failure. Clinical laboratory parameters include leukocytosis (which may be leukemoid and with a left shift), thrombocytopenia, elevated serum creatinine level, proteinuria, and hematuria. HFRS is readily diagnosed by an IgM-capture ELISA that is positive at admission or within 24–48 h thereafter. The isolation of orthohantaviruses is difficult, but RT-PCR of renal biopsy, thrombus (collected early in the clinical course), or postmortem tissues may yield molecular diagnosis and should be used if definitive identification of the infecting virus is required.

Mainstays of therapy are management of shock with vasopressor support and modest crystalloid infusion, IV human serum albumin administration, timely renal replacement therapy to prevent volume overload that may result in pulmonary edema, and control of hyper­ tension to prevent intracranial hemorrhage. Use of IV ribavirin has reduced CFR and morbidity in severe cases, provided treatment is begun within the first 4 days of illness. CFR approaches 15% but should be lower than 5% with adequate supportive and critical care. Generally, surviving patients make a full recovery, even of glomerular function, and postacute sequelae have not been identified. CRIMEAN-CONGO HEMORRHAGIC FEVER (CCHF)  CCHFV causes severe VHF over a wide endemic area (Africa, Asia, and Europe), determined primarily by the distribution of ixodid Hyalomma ticks. Incidence has been increasing in known endemic areas (particularly in Western Asia), and recent autochthonous infections in nonen­ demic areas, such as Spain, suggest that the at-risk geography is expanding. Because of the propensity of CCHFV-transmitting ticks to feed on domestic livestock and some wild mammals, veterinary serosurveys are the most effective mechanism for the monitoring of virus circulation. Human infections are acquired via tick bites, during the crushing of infected ticks, and contact with blood or body fluids of infected animals or humans (including nosocomial and vertical transmission). Domestic animals do not become ill but do develop viremia. Thus, risk of acquiring CCHFV occurs during sheep shear­ ing, animal slaughter, or contact with infected hides or carcasses from recently slaughtered infected animals. Nosocomial outbreaks are common and are usually related to extensive blood exposure or needlesticks. The majority of infections are subclinical. After an incubation period that varies by exposure to a tick bite (1–3 days) or blood or body fluids (3–7 days, perhaps shorter after nosocomial exposures to highly viremic patients), patients develop a sudden-onset nonspecific febrile prodrome typical to VHF. Severe illness is also typical, with the possible exceptions of a profound and dysregulated inflamma­ tory component, more severe hemorrhagic manifestations (including extended ecchymoses and/or retinal and pulmonary hemorrhage), and more severe liver injury, resulting in jaundice in some patients. Recovery (for most patients; sequelae are not identified) or rapid progression to fatal disease occurs in the second week of illness with multiorgan (renal and respiratory) dysfunction, shock, and profound coagulopathy leading to massive bleeding. Clinical laboratory findings indicate coagulopathy as well as markers of liver (elevations in activi­ ties of AST/ALT and bilirubin concentrations) and muscle (elevations in creatine phosphokinase levels) injury. Predictors of fatal outcome include clinical features (hematemesis, melena, and/or altered mental status), laboratory biomarkers (viral load, thrombocytopenia, elevated AST/ALT levels, and/or obvious DIC), and delayed CCHFV-specific IgM and IgG responses. The mainstay of treatment is supportive and critical care, including extracorporeal support of organ dysfunction. The pathophysiology argues for combined virus-specific and diseasemodifying immunomodulatory approaches. The benefit of IV ribavi­ rin, suggested in observational studies that were likely confounded, has not been borne out in randomized trials. Small-molecule antivirals (favipiravir and molnupiravir), monoclonal antibodies, and the use of convalescent plasma are in preclinical evaluation, but clinical studies are needed. The routine use of steroids and/or IVIG is not currently recommended. No human or veterinary vaccines are recommended. RIFT VALLEY FEVER  The natural range of RVFV was previously con­ fined to sub-Saharan Africa, with circulation of the virus markedly enhanced by substantial rainfall. The El Niño Southern Oscillation phenomenon of 1997 facilitated subsequent spread of Rift Valley fever (RVF) to the Arabian Peninsula, with epidemic disease in 2000. The virus has also been found in Madagascar and Egypt, where it caused major epidemics in 1977–1979, 1993, and thereafter. RVFV is maintained by transovarial transmission in floodwater Aedes mosquitoes and presumably also via a vertebrate amplifier. Increased transmission during particularly heavy rains leads to epizootics char­ acterized by high-level viremia in cattle, goats, or sheep. Numerous

types of mosquitoes feed on these animals and become infected, thereby increasing the possibility of human infections. Remote sens­ ing via satellite can detect the ecologic changes associated with high rainfall that predict the likelihood of RVFV transmission. High-resolution satellites can also detect the special depressions in floodwaters from which the mosquitoes emerge. The virus can be transmitted to humans by mosquitoes but most commonly by contact with blood or aerosols from domestic animals. Therefore, transmission risk is high during birthing, and both abortuses and placentas need to be handled with caution. Risk is also high during animal slaughter but decreases thereafter as anaerobic glycolysis in postmortem tissues results in an acidic environment that rapidly inactivates bunyaviricetes in carcasses. Neither person-to-person nor nosocomial transmission of RVFV has been documented.

RVFV is unusual in that it causes several clinical syndromes. Most infections are either subclinical or produce a typical F&M syndrome after an incubation period of 2–6 days. A much smaller proportion (<1%) of patients develop typical VHF notable for prominent liver involvement (jaundice) and an extremely rapid course with high case fatality (50%, within 3–6 days), often with renal failure and DIC/

hemorrhagic manifestations. Further, <1% of patients develop a late onset (1–4 weeks) meningoencephalitis syndrome that is not usually fatal but does lead to severe neurologic sequelae. Perhaps 10% of oth­ erwise mild infections develop a late-onset (1–3 weeks) retinal vascu­ litis causing blurred or loss of vision. Funduscopic examination reveals edema, hemorrhages, and infarction of the retina as well as optic nerve degeneration. In a small proportion of patients (<1 in 200), retinal vas­ culitis is followed by viral encephalitis. Of interest, the late onset of both retinal disease and meningoencephalitis—after serum neutralizing anti­ body has developed—suggests (so far, undefined) immunopathology. CHAPTER 215 No proven therapy exists for RVF. Epidemic disease is best pre­ vented by vaccination of livestock. The ability of this virus to propagate after introduction into Egypt suggests that other potentially recep­ tive areas, including the United States, should develop response plans. RVF, like VEE, is likely to be controlled only with adequate stocks of an effective live-attenuated vaccine, but global stocks are unavailable. A formalin-inactivated vaccine confers immunity in humans; however, quantities are limited, and three injections are required. This vaccine is recommended for potentially exposed laboratory workers and for veterinarians working in sub-Saharan Africa. A new live-attenuated vaccine, MP-12, is being tested in humans (phase 2 trials have been completed). The vaccine is safe and licensed for use in sheep and cattle. A modified replication-incompetent chimpanzee adenovirus (ChAdOx1) vectored vaccine has recently been shown to be safe and immunogenic in phase 1 trials. In addition, several vaccines are being developed specifically for use in animals. Arthropod-Borne and Rodent-Borne Virus Infections
SEVERE FEVER WITH THROMBOCYTOPENIA SYNDROME  This tickborne disease is caused by SFTSV. The primary tick vector, the Asian long-horned tick (Haemaphysalis longicornis), is endemic in areas of southeast Asia and Oceania, and geographic distribution has been increasing, likely via migratory birds (in Asia) or by importation of pets or livestock (in the United States, where the vector is now widespread and considered invasive). Numerous human infections have been reported during the past few years from China, and cases have also been detected in Japan, the Republic of Korea, Vietnam, and Thailand. Seroprevalence in endemic areas of China approaches 5%. The clinical presentation ranges from mild nonspecific febrile syndromes to severe VHF with high CFR (>12%) particularly in older individuals. After a 1- to 2-week incubation period, patients develop a nonspecific febrile prodrome that may include abdominal pain, vomiting, and diarrhea. Common laboratory abnormalities include prominent thrombocy­ topenia, leukopenia (often with atypical lymphocytes), markers of hepatocyte injury, and elevated ferritin levels. Most patients recover spontaneously; however, in the second week of illness, some patients develop severe disease with multiorgan dysfunction that can include acute renal injury, myocarditis, meningoencephalitis, DIC with hem­ orrhage, and hemophagocytic lymphohistiocytosis. Diagnosis requires RT-PCR, as serologic tests are not available. No vaccines or approved

therapeutics are available (though ribavirin and favipiravir have been used in patients) and treatment relies on supportive care.

Flasuviricetes  The most significant orthoflaviviruses that cause VHF are the mosquito-borne DENV-1–4 and YFV. These viruses are widely distributed and cause tens to hundreds of thousands of infec­ tions each year. Alkhurma hemorrhagic fever virus (>600 cases since discovery in 1995), Kyasanur Forest disease virus (≈10,000 cases over 60 years), and Omsk hemorrhagic fever virus (isolated infections every year with intermittent larger outbreaks) are geographically very restricted but prevalent tick-borne orthoflaviviruses that cause VHF, sometimes with subsequent viral encephalitis. Tick-borne encephalitis virus has caused VHF in a few patients. There is currently no therapy for infection with these VHFs, but an inactivated vaccine has been used in India to prevent Kyasanur Forest disease (and there is a vaccine to prevent tick-borne encephalitis; see above). SEVERE DENGUE  Although most individuals infected with DENV1–4 have either subclinical infection or F&M syndrome (the febrile phase), some of these patients enter a critical (or plasma leakage) phase—often as fever declines—and develop the criteria for severe dengue: a definite or presumptive diagnosis of dengue infection plus either (1) plasma leakage severe enough to cause shock or respiratory distress, or (2) severe bleeding, or (3) severe organ dysfunction. Early recognition and action long before this critical period are crucial to ini­ tiate appropriate supportive care. Indeed, all patients with a presump­ tive diagnosis of dengue (of any severity) should be initially assessed for designated warning signs (dengue with warning signs, directing inpatient management) as well as for the criteria for severe dengue (i.e., a provisional diagnosis of severe dengue should be made clinically). PART 5 Infectious Diseases The complex determinants of risk for this progression to severe dengue include host and viral factors but center most notably around the potential for immune-mediated enhancement of disease. Several weeks after convalescence from infection with DENV-1–4, the tran­ sient protection conferred by that infection against reinfection with a heterotypic DENV usually wanes. Heterotypic reinfection may result in classic dengue without/with warning signs or, less commonly, in severe dengue. In the past 20 years, YF mosquitoes have progressively rein­ vaded Latin America and other areas, and frequent travel by infected individuals has introduced multiple variants of DENV-1–4 from many geographic areas. Thus, the pattern of hyperendemic transmission of multiple DENV serotypes established in the Americas and the Carib­ bean has led to the emergence of severe dengue as a major problem. Among the millions of DENV-1–4 infections, ≈500,000 cases of severe dengue occur annually, with a CFR of ≈2.5%. The induction of vascular permeability and shock depends on multiple factors, such as the pres­ ence or absence of enhancing and nonneutralizing antibodies, age (sus­ ceptibility to severe dengue drops considerably after 12 years of age), sex (females are more often affected than males), race (white people are more often affected than black people), nutritional status, and the timing and sequence of infections (e.g., DENV-1 infection followed by DENV-2 infection seems to be more severe than DENV-4 infection fol­ lowed by DENV-2 infection). The presence of neutralizing antibodies is associated with decreased viremia on subsequent infection. In addi­ tion, considerable heterogeneity exists among variants in each DENV population. For instance, Southeastern Asian DENV-2 variants have more potential to cause severe dengue than do other variants. Recent evidence points to a key role for the DENV NS1 protein in the vascular leak phenomenon associated with severe dengue. In milder cases of severe dengue, restlessness, lethargy, thrombocy­ topenia (<100,000/μL), and hemoconcentration are detected 2–5 days after the onset of typical dengue, usually at the time of defervescence. The maculopapular rash that often develops in dengue (without/ with warning signs) may also appear in severe dengue. However, severe dengue is most notoriously identified as the consequence of a vascular leak syndrome leading to intravascular volume depletion, hypoalbuminemia, serosal effusions (pleural, ascitic), and, in severe cases, circulatory collapse (i.e., shock, typically lasting 2–3 days), often with an accompanying narrowed pulse pressure, hepatomegaly, and cyanosis. Bleeding tendencies (evidenced by a positive tourniquet test

and petechiae) or overt bleeding in the absence of underlying causes (e.g., preexisting gastrointestinal lesions) may be detected but are less common in children. Organ involvement may include mild hepatic injury, CNS abnormalities (e.g., altered mental status, seizures), cardiac abnormalities (e.g., arrhythmias), renal disturbances (e.g., acute kidney injury), and ocular dysfunction. A virologic diagnosis of severe dengue can be made by the usual means (nucleic acid amplification or antigen detection) in the first 5 days of infection, after which diagnosis relies on serologic testing. Combination testing—point-of-care rapid tests for NS1 antigen and IgM antibody assays—is increasingly used in the clinical setting. The early detection of IgG (starting at 4 days after illness onset, rapidly ris­ ing) suggests a secondary rather than primary infection, in which IgG is detected later and at much lower titers. However, serologic crossreactivity after infection with (or vaccination for) antigenically similar orthoflaviviruses (e.g., YFV, ZIKV, and Japanese encephalitis virus) confounds diagnosis. As above, patients with severe dengue should be hospitalized and rapidly triaged (and then closely monitored) for hemodynamic insta­ bility and bleeding. In general, appropriate IV fluids should be admin­ istered as early as possible, preferably before shock develops. Based on clinical parameters (vital signs, urine output, hematocrit, bleeding, and signs of volume overload), algorithmic approaches have been devel­ oped to standardize appropriate fluid management (crystalloid vs col­ loid vs blood rate) and monitoring of patients with and without shock and/or bleeding. CFR varies greatly depending on early recognition and quality of treatment. However, most patients with severe dengue respond well to supportive therapy, and the overall CFR at experienced clinical centers in the tropics is probably as low as 1%. When patients enter the convalescent phase, significant fluid reabsorption (of extra­ vascular fluid and effusions) occurs as the plasma leak resolves, blood pressure improves, and diuresis occurs. Notably, a patchy erythematous pruritic rash may develop and desquamate during recovery. Significant postacute sequelae include prolonged fatigue and depression. The key to control of both dengue (without/with warning signs) and severe dengue is the control of YF mosquitoes, which also reduces the risk of urban YF and CHIKV circulation. Control efforts have been handicapped by the presence of nondegradable tires and long-lived plastic containers in trash repositories—creating perfect mosquito breeding grounds upon filling with water during rainfall—and by insecticide resistance. Urban poverty and an inability of the public health community to mobilize the human population to respond to the need to eliminate mosquito breeding sites are also factors in lack of mosquito control. New approaches that may be considered in the future of vector control include the release of Aedes mosquitoes infected with Wolbachia or carrying dominant lethal genetic mutations that will be passed on to offspring. A tetravalent live-attenuated dengue vaccine based on the attenuated YFV 17D platform (CYD-TDV, or Dengvaxia) was licensed in 2015 and registered in 20 countries for individuals 9–45 years of age. However, retrospective analysis of phase 3 trials in Latin America and Asia suggested protection from severe dengue only in previously seropositive individuals; indeed, the risk of severe dengue was actually increased in seronegative vaccine recipients over that in nonvaccinated seronegative individuals, suggesting that a “first serologic hit” from the vaccine predisposes naïve recipients to more severe primary dengue infection. Strategic revision to avoid vaccineenhanced disease pivoted to include prevaccination serologic screening aimed to avoid vaccination of seropositive individuals; however, the requirement for prevaccine testing and decision-making is challeng­ ing. Recently, live-attenuated candidate vaccines based on modified recombinant DENVs have been or are being evaluated, with these learnings in mind. Qdenga (TAK-3), given in two doses 3 month apart, has shown overall efficacy against infection and hospitalization in sero­ positive and seronegative children, although not against DENV-3 (in fact, a very small signal of potentially increased risk of hospitalization after DENV-3 infection was observed), and the trial did not include DENV-4 infections. This vaccine has been approved for use in several European countries. Recommendations globally are currently for introduction in settings with high transmission intensity for maximal

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216 The Filovirids: Orthoebolavirus and Orthomarburgvirus Infections

public health impact while reducing potential risk in seronegative individuals. Further postmarket evaluation is ongoing. More recently, a phase 3 trial of Butantan-DV, a single-dose tetravalent vaccine, was shown to prevent symptomatic DENV-1 and DENV-2 infection; in the absence of DENV-3 and DENV-4 infections during the trial, efficacy against these serotypes is unknown. TVOO3, a single-dose vaccine designed to balance DENV serotype-specific components, is currently undergoing phase 3 clinical evaluation; similar concerns about safety are being addressed. YELLOW FEVER  YFV had caused major epidemics in Africa and Europe before its transmission by YF mosquitoes was discovered in 1900. Urban YF became established in the Americas because of colonization with YF mosquitoes—originally an African mosquito. Subsequently, different types of mosquitoes and nonhuman primates were found to maintain YFV in Africa and in Central and South American rain forests. Trans­ mission to humans is incidental, occurring via bites from mosquitoes that have fed on viremic monkeys. After the identification of Ae. aegypti as the vector of YF, containment strategies were aimed at increased mosquito control. Today, urban YFV transmission occurs only in some African cities, but the threat exists in the cities of South America, where reinfestation by YF mosquitoes has taken place, and DENV-1–4 transmission by these mosquitoes is common. Despite the existence of a highly effective and safe vaccine, several hundred jungle cases of YF occur annually in South America, and 84,000–170,000 severe jungle and urban cases (resulting in 29,000–60,000 fatal outcomes) occurred in Africa in 2013 alone. In 2016, a large urban outbreak (Luanda, Angola) spilled over to generate local transmission in large cities in neighboring countries (e.g., Kinshasa, Democratic Republic of the Congo) as well as travel-related cases in China; the signal of a global threat that included exportation to Asia stimulated ongoing efforts to identify and vaccinate highest-risk populations in 40 targeted countries in Africa and South America, to reactively vaccinate people in outbreak settings, and to increase measures to prevent exportation. YF is a typical VHF notable for prominent hepatic necrosis. After an incubation period of 3–6 days, patients present with a nonspecific febrile illness (fatigue, myalgia, backache, headaches, photophobia, anorexia, nausea or vomiting) associated with viremia typically lasting 3–4 days. After defervescence, 10–15% of patients develop recrudes­ cent fever and “intoxication” characterized by severe dysfunction of the liver and other organs. Hepatic failure leads to the characteristic jaundice, bleeding (gastrointestinal tract, nasopharyngeal mucosa), abdominal pain with nausea and vomiting, and hyperammonemic encephalopathy; acute kidney injury leads to oliguria, azotemia, and marked albuminuria; and myocardial injury and encephalitis have been described. Abnormalities in liver function tests range from mod­ est elevations of hepatic aminotransferase activities in mild cases to severe liver injury, hyperbilirubinemia, and the synthetic dysfunction of acute hepatic failure. After early leukopenia, leukocytosis occurs as disease progresses, and coagulation abnormalities are common. Treat­ ment is supportive only. Although most infections are subclinical, 50% of patients who enter the toxic phase die in the next 7–10 days. Urban YF can be prevented by the control of YF mosquitoes. The continuing sylvatic cycles require vaccination of all visitors to areas of potential transmission with live-attenuated variant 17D vaccine virus, which cannot be transmitted by mosquitoes. With few excep­ tions, reactions to the vaccine are minimal; immunity is provided within 10 days and lasts for at least 25–35 years. Recent meta-analytic review of 39 clinical studies suggests seroprotection is high (>90%) and perhaps lifelong after one vaccine dose in adults from nonen­ demic areas, raising questions about the utility of further boosting in this population. An egg allergy mandates caution in vaccine admin­ istration. Although fetal harm has not been documented, pregnant women should be immunized only if they are at risk of exposure to YFV. Because vaccination has been associated with several cases of encephalitis in children <6 months of age, it is contraindicated in this age group and not recommended for infants 6–8 months of age unless the risk of exposure is very high. Rare, serious, multisystemic adverse reactions (occasionally fatal), including vaccine-associated

“viscerotropic” YF, have been reported, particularly affecting the elderly and those with congenital (mutations affecting interferon α [IFNα] signaling) or acquired (autoantibodies to IFNα, thymoma) impairment affecting viral control. The risk-to-benefit ratio should be weighed before vaccine administration to individuals ≥60 years of age. Nevertheless, the number of deaths of unvaccinated travelers with YF exceeds the number of deaths from vaccination, and a liberal vaccina­ tion policy for travelers to involved areas should be pursued. Timely information on changes in YF distribution and YF vaccine require­ ments can be obtained from the U.S. Centers for Disease Control and Prevention (https://wwwnc.cdc.gov/travel/yellowbook/2024/preparing/ yellow-fever-vaccine-malaria-prevention-by-country).

Acknowledgment The authors gratefully acknowledge the major contributions of Clarence J. Peters and additional contributions by Rémie N. Charrel to this chapter in previous editions and thank Anya Crane (IRF-Frederick) for editing the manuscript. ■ ■FURTHER READING Centers for Disease Control and Prevention: Arbovirus catalog. Available at https://wwwn.cdc.gov/arbocat/. Accessed February 4, 2024. Howley PM, Knipe DM (eds): Fields Virology. Volume 1: Emerging Viruses, 7th ed. Philadelphia, Wolters Kluwer/Lippincott Williams & Wilkins, 2020. International Committee on Taxonomy of Viruses (ICTV): Virus taxonomy: The ICTV report on virus classification and taxon nomen­ clature. Available at https://ictv.global/report. Accessed February 4, 2024. Lvov DK et al: Zoonotic Viruses of Northern Eurasia: Taxonomy and CHAPTER 216 Ecology. London, Elsevier/Academic Press, 2015. Singh SK, Ruzek D (eds): Viral Hemorrhagic Fevers. Boca Raton, FL, CRC Press, 2013. Vasilakis N, Gubler DJ (eds): Arboviruses: Molecular Biology, Evolution and Control. Haverhill, UK, Caister Academic Press, 2016. Filovirids: Orthoebolavirus and Orthomarburgvirus Infections ■ ■WEBSITE International Committee on Taxonomy of Viruses (ICTV). https://ictv.global/. Accessed February 4, 2024. Jens H. Kuhn, Ian Crozier

Filovirids: Orthoebolavirus

and Orthomarburgvirus Infections Several viruses in the family Filoviridae cause severe infections in humans that are often fatal. Introduction of filovirids into human populations is an extremely rare event that most likely occurs by direct or indirect contact with reservoir hosts (known and unknown) or by contact with sick or deceased filovirid-infected mammals. Filovirids are highly infectious but not exceptionally contagious. Human-tohuman transmission occurs through direct person-to-person contact or exposure to infected bodily fluids or tissues; there is no evidence of aerosol or respiratory droplet transmission in natural outbreak settings. Infections manifest initially with a nonspecific influenza-like febrile illness that rapidly progresses to commonly include gastrointestinal manifestations and, in severe illness, coagulopathy, multiple-organ dysfunction syndrome, shock, and death. Although the prevalence and source remain controversial, serologic footprints of subclinical

acute filovirid infections have been identified since the first descrip­ tions of filovirid disease outbreaks. Filovirid disease survivors may be persistently infected in immune-privileged tissue compartments, com­ monly in the male reproductive tract, but also in the central nervous system (CNS), and in intraocular tissues and fluids; notably, filoviral persistence has been rarely associated with the re-ignition of outbreaks. Historically, the prevention of filovirid infections has consisted primar­ ily of tried-and-true epidemiologic approaches (e.g., isolation of cases, contact tracing, effective infection prevention and control, and safe burial practices). Treatment of disease traditionally consisted only of limited supportive clinical care (often constrained by in-field capacity); indeed, filovirid-specific vaccines or therapeutic agents had not been rigorously evaluated in humans prior to the 2013–2016/2021 outbreaks of Ebola virus disease (EVD) that occurred in Western Africa. Building on the knowledge gained in Western Africa and during the 2018–2020 EVD outbreak in the Democratic Republic of the Congo, prevention and treatment strategies now include the widespread deployment of an effective vaccine specific to Ebola virus (EBOV); the use of effective therapeutics based on virus-specific monoclonal antibodies (mAbs), which were identified in a first-of-its-kind randomized controlled trial; and the delivery of improved supportive care. Although these advances have essentially become new standards for prevention and treatment of EVD, the same cannot yet be said for other filovirid diseases.

Filovirids are categorized as World Health Organization (WHO) Risk Group 4 pathogens. Consequently, all work with material potentially containing replicating filovirids should be conducted in maximum containment (biosafety level 4) laboratories, or the viruses should be inactivated prior to activities in biosafety level 2/3 laborato­ ries. These viruses must be handled by experienced personnel wearing appropriate personal protective equipment (PPE; see “Control and Prevention,” below) and following rigorous standard operating proce­ dures. In addition, when filovirid disease outbreaks are suspected or confirmed, the appropriate national authorities and WHO reference laboratories should be contacted immediately. PART 5 Infectious Diseases ■ ■ETIOLOGY The family Filoviridae includes nine official genera (Figs. 216-1 and 216-2). Human pathogens are found in two of these genera, Orthoebolavirus and Orthomarburgvirus. Collectively, these patho­ gens cause “filovirid” (“filovirus”) disease (FVD; International Clas­ sification of Diseases, Eleventh Revision [ICD-11], code 1D60). FVD is subdivided into “Ebola disease” (EBOD; ICD-11, code 1D60.0), caused by four of six classified orthoebolaviruses (Bundibugyo virus [BDBV], EBOV, Sudan virus [SUDV], and Taï Forest virus [TAFV]), and “Marburg disease” (MARD; ICD-11, code 1D60.1), caused by the two orthomar­ burgviruses, Marburg virus and Ravn virus. Accordingly, specific diseases are optimally named with the virus-specific cause such that, for example, “EVD” is distinguished from “Sudan virus disease (SVD).” Mammalian filovirids have linear, nonsegmented, negative-sense RNA genomes that are ≈19 kb in length. These genomes contain seven genes that encode seven structural proteins: a nucleopro­ tein (NP), a polymerase cofactor (VP35), a matrix protein (VP40), a glycoprotein (GP1,2), a transcriptional activator (VP30), a ribonucleo­ protein complex-associated protein (VP24), and a large (L) protein that contains an RNA-directed RNA polymerase domain. Orthoebolaviruses, but not orthomarburgviruses, additionally encode three nonstruc­ tural proteins of unknown functions (sGP, ssGP, and Δ-peptide). Filovirions are unique among human virus particles in that they are predominantly pleomorphic filaments but also assume torus-like or 6-like shapes (width ≈91–98 nm; average length <1 μm). These envel­ oped virions contain helical ribonucleoprotein capsids and are covered with GP1,2 spikes (Fig. 216-3). ■ ■EPIDEMIOLOGY The majority of recorded FVD outbreaks, including the 2013–2016 EVD outbreak, can be traced back to single index patients who trans­ mitted the infection to others. Although small outbreaks may not have been recorded historically, the epidemiology of EVD transmission chains suggests that only ≈50 natural host-to-human spillover events

Realm Riboviria Kingdom Orthornavirae Phylum Negarnaviricota Subphylyum Haploviricotina Class Monjiviricetes Order Mononegavirales Family Filoviridae Genus Cuevavirus Species Cuevavirus lloviuense Virus: Lloviu virus (LLOV) Genus Dianlovirus Species “Dianlovirus dehongense” (proposed) Virus: Déhóng virus (DEHV) Species Dianlovirus menglaense Virus: Měnglà virus (MLAV) Genus Loebevirus Species Loebevirus percae Virus: Lötschberg virus (LTBV) Genus Oblavirus Species Oblavirus percae Virus: Oberland virus (OBLV) Genus Orthoebolavirus Species Orthoebolavirus bombaliense Virus: Bombali virus (BOMV) Species Orthoebolavirus bundibugyoense Virus: Bundibugyo virus (BDBV) Species Orthoebolavirus restonense Virus: Reston virus (RESTV) Species Orthoebolavirus sudanense Virus: Sudan virus (SUDV) Species Orthoebolavirus taiense Virus: Taï Forest virus (TAFV) Species Orthoebolavirus zairense Virus: Ebola virus (EBOV) Genus Orthomarburgvirus Species Orthomarburgvirus marburgense Virus 1: Marburg virus (MARV) Virus 2: Ravn virus (RAVV) Genus Striavirus Species Striavirus antennarii Virus: Xīlaˇng virus (XILV) Genus Tapjovirus Species Tapjovirus bothropis Virus: Tapajós virus (TAPV) Genus Thamnovirus Species Thamnovirus kanderense Virus: Kander virus (KNDV) Species Thamnovirus percae Virus: Fiwi virus (FIWIV) Species Thamnovirus thamnaconi Virus: Huángjiāo virus (HUJV) Filovirids that are known to infect humans are depicted in the same nonblack colors as in figures. FIGURE 216-1  Filovirid taxonomy (2024).   have occurred since the discovery of filovirids in 1967. However, outbreaks associated with reignition of human-to-human transmis­ sion from a persistently infected survivor of a previous outbreak have more recently been described. Outbreak frequency, size, and overall

FJ217162 TAFV/H.sap/CIV/94/Pau-CI

FJ217161 BDBV/H.sap/UGA/07/But-811250

AF086833 EBOV/H.sap/COD/76/Yam-May 0.96 MF319185 BOMV/M.con/SLE/16/Nor-PREDICT_SLAB000156

AY729654 SUDV/H.sap/UGA/00/Gul-808892 0.96 AF522874 RESTV/M.fas/USA/89/Phi89-Pen KX371873 “Bat2162”

KP233864 “BtFV/WD04” MW775011 LLOV/M.sch/HUN/19/378

DQ217792 MARV/H.sap/KEN/80/MtE-Mus

DQ447649 RAVV/H.sap/KEN/87/KiC-810040 0.77

OP924273 DEHV/R.les/CHI/16/Rl133-16 KX371887 MLAV/Rousettus/CHN/15/Sha-Bat9447-1

BR001752 TAPJ/B.atr/15/BRA MW093492 KNDV/P.flu/CHE/17/CH17 MG599980 XILV/A.str/CHN/17/Wen-XYHYS28627

MN510772 FIWIV/P.flu/CHE/17/CH17

MG599981 HUJV/T.sep/CHN/17/Wen-LQMMTII17328

OQ186623 LTBV/P.flu/CHE/17/CH17 MN510773 OBLV/P.flu/CHE/17/CH17 0.5 FIGURE 216-2  Filovirid phylogeny/evolution. Midpoint-rooted maximum-likelihood tree inferred by using filovirid large gene (L) sequences. Bootstrap values are shown at each node. The scale bar indicates nucleotide substitutions per site. Tips of branches are labeled with GenBank accession numbers followed by filovirid isolate designation. Filovirids that are known to infect humans are depicted in the same nonblack colors as in other figures. BDBV, Bundibugyo virus; BOMV, Bombali virus; DEHV, Déhóng virus; EBOV, Ebola virus; FIWIV, Fiwi virus; HUJV, Huángjiao virus; KNDV, Kander virus; LLOV, Lloviu virus; LTBV, Lötschberg virus; MARV, Marburg virus; MLAV, Meˇnglà virus; OBLV, Oberland virus; RAVV, Ravn virus; RESTV, Reston virus; SUDV, Sudan virus; TAFV, Taï Forest virus; TAPV, Tapajós virus; XILV, Xi-laˇng virus. (Adapted and expanded from JH Kuhn et al: Filoviridae, in Fields Virology, Vol 1, 7th ed, PM Howley et al (eds). Philadelphia, Wolters Kluwer/Lippincott Williams & Wilkins, 2020, pp 449–503. Analysis courtesy of Nicholas Di Paola, PhD, USAMRIID, Fort Detrick, MD, USA. Figure courtesy of Jiro Wada, NIH/NIAID/DCR/IRF-Frederick, Fort Detrick, MD, USA.) case-fatality rate are likely the result of complex interactions of the specific filovirid, the reservoir hosts (known and unknown), the sus­ ceptible human population (e.g., varying with age, unknown genetic determinants of susceptibility and disease severity, and risk behavior), and the geographic setting (e.g., local public health capacity, socioeco­ nomic conditions, and cultural practices). As of November 13, 2024, 35,608 cases of human filovirid disease and 15,886 deaths had been recorded (Fig. 216-4). These numbers emphasize both the high case-fatality rate (number of deaths per num­ ber of sick people; 44.6%) and the overall low mortality (reflecting the impact on the healthy population) related to filovirid diseases. Of these totals, 28,652 cases and 11,325 deaths occurred during the 2013–2016 EVD outbreak in Western Africa; this was the largest of all recorded FVD outbreaks. Natural FVD outbreaks had not been considered a global threat until regional and then global spread during this outbreak challenged that tenet. Filovirids that are pathogenic in humans appear to be exclusively endemic to equatorial (Western, Middle, and Eastern) FIGURE 216-3  Ultrastructure of filovirions. Left: Colorized scanning electron micrograph of Ebola virus particles (green) attached to the surface of an infected grivet (Chlorocebus aethiops (Linnaeus, 1758)) Vero E6 producer cell (blue). Right: Colorized transmission electron micrograph of a Marburg virus particle collected from purified Vero E6 producer cell supernatant. (Figure courtesy of John G. Bernbaum and Jiro Wada, NIH/NIAID/DCR/IRF-Frederick, Fort Detrick, MD, USA.)

Orthoebolavirus New genus? Cuevavirus Orthomarburgvirus Dianlovirus ˉ Tapjovirus ˉ Striavirus Thamnovirus ˉ Oblavirus Loebevirus CHAPTER 216 Africa (Fig. 216-5), although this distribution may change if natural or artificial environmental alterations lead to filovirid host migration and increased contacts between nonhuman hosts and humans. Outbreaks have been contained when high-risk activities (e.g., ritual washing as part of burial practices) have been limited or been made safer with appropriate infection prevention and control. Of particular importance is accessibility to health care centers with staff trained and equipped (e.g., with PPE) for adequate infection prevention and control, to limit both community spread and nosocomial amplification. The incidence of FVD may have increased over the past two decades (Fig. 216-4 and Fig. 216-5), but debate continues as to whether this increase is due to increased filovirid activity, more frequent human interaction with filovirid hosts, or improvement in surveillance capabilities. Filovirids: Orthoebolavirus and Orthomarburgvirus Infections FVD outbreaks are associated with distinct meteorologic and geo­ graphic conditions and are probably associated with distinct hosts or reservoirs. The four orthoebolaviruses that cause disease in humans appear to be endemic in humid rainfor­ ests. EVD outbreaks in particular have often been associated with hunting in forests or contact with bushmeat (i.e., meat from apes, other nonhuman pri­ mates, duikers, or bush pigs). Ecologic studies suggest that EBOV may play a role in extensive and often-fatal epizoot­ ics among wild ape populations. How­ ever, only one orthoebolavirus, TAFV, has been isolated from nonhuman pri­ mates in the wild. Orthomarburgviruses, on the other hand, seem to infect hosts inhabiting arid woodlands, and associ­ ated outbreaks have almost always been epidemiologically linked to individuals visiting or working in natural or engi­ neered caves or mines. The cave-dwelling Egyptian rousette (Rousettus aegyptiacus (Geoffroy, 1810)), a pteropodid (fruit) bat, serves as a natural and subclini­ cally infected reservoir for both Marburg virus and Ravn virus. Although bats are suspected hosts for orthoebolaviruses as

Country (Year) BDBV COD (2012) Uganda (2007−2009) Zaire (1976) Zaire (1977) Gabon (1994−1995) Zaire (1995) Russia (1996) Gabon (1996) Gabon → South Africa (1996−1997) Gabon; COG (2001−2002) COG → Gabon (2002) COG (2002−2003) COG (2003−2004) Russia (2004) COG (2005) EBOV COD (2007) COD (2008−2009) Guinea → France, Germany, Italy, Liberia,

Mali, Netherlands, Nigeria,

Norway, Senegal, Sierra Leone,

Spain, Switzerland, UK, USA

(2013−2016, '21*) COD (2014) COD (2017) COD (2018) COD → Uganda (2018−2020, '21*,

'21*, '22*) COD (2020) COD (2022) RESTV USA (1989) Sudan (1976) PART 5 Infectious Diseases UK (1976) Sudan (1979) Uganda (2000−2001) SUDV Sudan (2004) Uganda (2011) Uganda (2012) Uganda (2012) Uganda (2022−2023) TAFV Côte d’Ivoire → Switzerland (1994) Orthoebolaviruses total: Uganda → West Germany,

Yugoslavia (1967) Rhodesia → )

( a cirf A h t u o S Kenya (1980) USSR (1988) USSR (1990) COD (1998−2000) Angola (2004−2005) Uganda (2007) MARV Uganda → Netherlands (2008) Uganda → USA (2008) Uganda (2012) Uganda (2014) Uganda (2017) Guinea (2021) Tanzania (2023) Ghana (2022) Equatorial Guinea (2023) Rwanda (2024) Kenya (1987) COD (1998−2000) RAVV Uganda (2007) Orthomarburgviruses total: Filovirids total: FIGURE 216-4  Characteristics of outbreaks of human filovirid disease. Seven of 17 classified filovirids have caused disease in humans. Left column: Outbreaks considered to be caused by independent zoonotic spillover are listed by virus in chronological order in the left column. Outbreaks considered to be related to reignition from a persistently infected survivor of a prior outbreak are included with the initial outbreak (denoted by asterisk). Laboratory infections are in gray italicized text. International case exportations are indicated with arrows. Right column: Numbers of fatal cases and total cases are summarized. Middle column: The case-fatality rate (colored dots) for each outbreak is plotted on a 0–100% scale along with 99% confidence intervals (gray horizontal bars). The overall case-fatality rate for disease caused by a particular virus is delineated by vertical colored lines, with vertical colored dashed lines indicating the corresponding 99% confidence intervals. The overall case-fatality rates for all Orthoebolavirus, all Orthomarburgvirus, and all filovirid disease outbreaks are shown by (underlaid) vertical gray bars. BDBV, Bundibugyo virus; COD, Democratic Republic of the Congo (formerly Zaire); COG, Republic of the Congo; EBOV, Ebola virus; MARV, Marburg virus; RAVV, Ravn virus; RESTV, Reston virus; SUDV, Sudan virus; TAFV, Taï Forest virus; UK, United Kingdom; USSR, Union of Soviet Socialist Republics (today Russia). (Adapted and expanded from JH Kuhn et al: Evaluation of perceived threat differences posed by filovirus variants. Biosecur Bioterror 9:361, 2011. Figure courtesy of Jiro Wada, NIH/NIAID/DCR/IRF-Frederick, Fort Detrick, MD, USA.)

Total cases Deceased Lethality (%)

11,337 28,675

2,303 3,494

14,887 33,838

15,439 35,008

15,886 35,608

Switzerland Senegal Mali USA Guinea '13−'16,'21* Nigeria Sierra Leone '21 '22 '94 Liberia Ghana Côte d'Ivoire USA and three European countries Equatorial Guinea Kenya USA and five European countries Gabon BDBV EBOV SUDV TAFV MARV RAVV '94−'95 '01−'02 '96 Gabon '02−'03 '03−'04 FIGURE 216-5  Geographic distribution of human filovirid disease outbreaks and years of occurrence. Arrows indicate international case exportations. BDBV, Bundibugyo virus; COD, Democratic Republic of the Congo (formerly Zaire); COG, Republic of the Congo; EBOV, Ebola virus; MARV, Marburg virus; RAVV, Ravn virus; SUDV, Sudan virus; TAFV, Taï Forest virus. (Figure courtesy of Jiro Wada, NIH/NIAID/DCR/IRF-Frederick, Fort Detrick, MD, USA.) well, definitive proof is lacking. To date, the nonpathogenic Bombali virus is the only orthoebolavirus for which coding-complete genome nucleic acid has been detected in bats. EBOV and Reston virus have been loosely connected to frugivorous and insectivorous bats by means of antibody or genome fragment detection, whereas the hosts of BDBV, SUDV, and TAFV are enigmatic. ■ ■PATHOGENESIS Human infections typically occur through direct exposure of skin lesions or mucosal surfaces to contaminated body fluids or material or by parenteral inoculation (e.g., via accidental needlesticks or reuse of needles in poorly equipped hospitals). Numerous studies, both in vitro and in vivo (in several animal models of human disease), have

South Sudan (Sudan) '04 '76,'79 '76 '98−'00 '77 '17 Uganda '12 '76 '23 '80 '98−'00 '14 '20 '87 '96−'97 COD (Zaire) '22 '18 '23 '24 COG '07 Tanzania Rwanda '95 '08−'09 '04−'05 Angola CHAPTER 216 '75 Zimbabwe (Rhodesia) Filovirids: Orthoebolavirus and Orthomarburgvirus Infections South Africa South Africa '00−'01 COD (Zaire) Uganda COG '00−'01 '17 Netherlands West Germany and Yugoslavia '12 '11 '07−'09 '11,'12 '18−'20, '21*,'21*, '22* '14 '02 '22−'23 '07 '67 '07 '08 '00−'01 '05 '12 illuminated aspects of FVD pathogenesis (Fig. 216-6). The GP1,2 spikes on the surface of filovirions determine their cell and tissue tropism by engaging cell-surface molecules and the intracellular filovirid receptor NPC intracellular cholesterol transporter 1 (NPC1). One of the hallmarks of filovirid pathogenesis is a pronounced mod­ ulation and dysregulation of immune responses. The first targets of filovirions are local macrophages, monocytes, and dendritic cells. Sev­ eral structural proteins of filovirions (i.e., VP35, VP40, and/or VP24) then suppress intrinsic and innate immune responses by, for example, inhibiting the type I interferon antiviral pathways. This immuno­ modulation ultimately enables a productive filovirid infection, result­ ing in very high viral titers (>106 PFU/mL of serum in humans) with dissemination to most tissues. In tissues, filovirions infect additional

Onset of signs and symptoms Incubation Early phase Peak phase Recovery Nonspecific prodrome: fever, fatigue, anorexia, myalgia, and headache Gastrointestinal symptoms: nausea, vomiting, diarrhea, and abdominal pain Rash Hemorrhagic manifestations Hemodynamic instability, Shock Renal dysfunction/failure Respiratory dysfunction/failure Neurologic manifestations Cardiac dysfunction (myocarditis and pericarditis) ↓WBCs, ↓PLTs ↑WBCs (↑PMNs), ↓Hb, ↓HCT ↑PLTs Hepatic injury: ↑AST, ↑ALT Hypoglycemia Renal dysfunction: ↑BUN, ↑creatinine Abnormal electrolytes: ↓Na+, ↑or↓K+, ↓Ca2+, ↓Mg2+ Metabolic acidosis: ↑lactate, ↓HCO3 – ↑CPK, myoglobinuria PART 5 Infectious Diseases Hypoalbuminemia Coagulopathy: ↑PT, ↑PTT, ↑D-dimer Inflammation: ↑CRP Magnitude Days since disease onset

−10 to −7 FIGURE 216-6  Ebola virus disease course. ALT, alanine aminotransferase; AST, aspartate aminotransferase; BUN, blood urea nitrogen; CPK, creatine phosphokinase; CRP, C-reactive protein; Hb, hemoglobin; HCT, hematocrit; PLTs, platelets; PMNs, polymorphonuclear leukocytes; PT, prothrombin time; PTT, partial thromboplastin time; WBCs, white blood cells. (Adapted and expanded from JH Kuhn et al: Filoviridae, in Fields Virology, 7th ed, Vol. 1. Howley PM et al (eds). Philadelphia, Wolters Kluwer/Lippincott Williams & Wilkins, 2020, pp 449–503. Figure courtesy of Jiro Wada, NIH/NIAID/DCR/IRF-Frederick, Fort Detrick, MD, USA.) phagocytic cells, including other macrophages (alveolar, peritoneal, and pleural macrophages; Kupffer cells in the liver; and microglia in the CNS), epithelial cells (e.g., adrenal cortical cells and hepatocytes), stromal cells (fibroblasts), and endothelial cells. Infection is cytolytic in some—but not all—infected cells; e.g., hepatocyte necrosis likely con­ tributes to elevated aminotransferase activities, and hepatic synthetic dysfunction contributes to coagulopathy. Infection leads to the secre­ tion of soluble signaling molecules (varying with the cell type) that likely contribute to forward dysregulation of immune responses and ultimately to multiorgan dysfunction syndromes. For instance, infected macrophages secrete proinflammatory cytokines, leading to further recruitment of monocyte-derived macrophages to the site of infection. In contrast, infected dendritic cells are not activated to secrete cyto­ kines, and the expression of major histocompatibility class II antigens is partially suppressed, with consequently deficient antigen presenta­ tion. Immunosuppression also occurs in part by massive lymphoid depletion in lymph nodes, spleen, and thymus in the absence of effec­ tive humoral and cell-mediated immune responses, especially in severe or fatal disease. Results from animal studies suggest that depletion is

Uveitis Clinical presentation Innate immune response Laboratory findings Viremia (nonsurvivor) Viremia (survivor) IgM Cellular immune response Humoral immune response (IgG) a direct consequence of considerable lymphocyte death; this explana­ tion would also account for the severe lymphopenia that develops in patients. In addition to potential florid filovirid dissemination, another consequence of lymphocyte depletion may be susceptibility to second­ ary bacterial and fungal infections. Other pathogenic hallmarks of filovirid infections include coagu­ lopathy and endothelial dysfunction. Along with hepatic synthetic dysfunction, disseminated intravascular coagulation may contribute though this remains controversial. Thrombocytopenia, increased con­ centrations of tissue factor, consumption of clotting factors, increased concentrations of fibrin degradation products (D-dimers), and declin­ ing concentrations of protein C are typical features of severe disease. Consequently, fibrin deposition and microthrombotic small-vessel occlusion and necrotic/hypoxic infarction may occur in some tissues, particularly in the gonads and, less often, in the kidneys and spleen. In addition, petechiae, ecchymoses, extensive visceral effusions, and other hemorrhagic signs are observed in internal organs, mucous membranes, and skin. However, actual severe blood loss is a rare event (although it frequently occurs during or after childbirth). Most likely,

aberrance in cytokines or other factors, such as nitric oxide, and direct infection and activation of endothelial cells are responsible for upregu­ lated permeability of blood-vessel endothelia. This upregulation leads to fluid redistribution and interstitial tissue edema and hypovolemic or septic shock are common features of severe disease. Despite this long list of pathogenetic hallmarks, increasing evidence from clinical settings suggests that effective filovirid-specific adaptive immune responses do develop, coinciding with control and clearance of viremia and subsequent clinical improvement in surviving patients. However, depending on the severity of illness (including organ dys­ function and late complications), clinical illness may be protracted and recovery incomplete. ■ ■CLINICAL MANIFESTATIONS Diseases caused by orthoebolaviruses and orthomarburgviruses present very similar clinical phenotypes that cannot be distinguished at the bedside and, for all practical purposes, may be considered the same disease; this approach may change as higher-resolution characteriza­ tion of human FVD accrues. The prevalence and character of clinical signs do not differ significantly in disease caused by disparate filovirids (except for the possibly apathogenic Reston virus), although, with the exception of the 2013–2016 EVD outbreak, the numbers of deeply characterized clinical observations are very low. The incubation period is 2–25 days (most commonly 7–10 days), after which infected people develop a nonspecific influenza-like syndrome characterized by sud­ den onset of fever and chills, severe headaches, cough, myalgia, phar­ yngitis, arthralgia of the larger joints, development of a maculopapular rash, and other symptoms/signs. A subsequent second phase (≈5–7 days after disease onset and thereafter) involves the gastrointestinal (nausea and vomiting and/or diarrhea, sometimes with abdominal pain), respiratory (chest pain, cough, and/or dyspnea), cardiovascular (hypotension and/or edema), and central nervous (confusion, head­ ache, and/or coma) systems. Common hemorrhagic manifestations include subconjunctival injection, petechial rash, gingival bleeding, and bleeding at injection sites; epistaxis, hematemesis, hematuria, and melena occur but are less common. Patients usually succumb to acute disease 4–14 days after infection, often with severe multiorgan dysfunction that commonly includes shock and acute renal or respira­ tory failure. Typical laboratory findings are leukopenia (with cell counts as low as 1000/μL) with a left shift prior to leukocytosis, thrombocyto­ penia (with counts as low as 50,000/μL), increased activities of liver enzymes (aspartate aminotransferase > alanine aminotransferase, γ-glutamyltransferase), increased creatinine and urea concentrations with proteinuria, electrolyte derangement (hypokalemia or hyperkale­ mia, hyponatremia, and/or hypocalcemia), hypoglycemia, hypoalbu­ minemia, prolonged prothrombin and partial thromboplastin times, and elevated creatine phosphokinase activities. Nonspecific markers of systemic inflammation (e.g., C-reactive protein concentrations) may be markedly elevated in severely ill patients. ■ ■DIAGNOSIS Filovirid disease cannot be diagnosed based on clinical presentation alone. Numerous diseases common in equatorial Africa need to be considered in the differential diagnosis of a febrile patient. Almost all of these occur at a much higher incidence than FVD and are much more likely diagnoses in non-outbreak settings; however, during outbreaks and in peri-outbreak periods, timely and accurate labora­ tory diagnosis to rule in or rule out filovirid infection is critical. The most important infectious disease mimics of FVD are falciparum malaria and typhoid fever; also important are enterohemorrhagic Escherichia coli enteritis, gram-negative septicemia (including shig­ ellosis), meningococcal septicemia, rickettsial infections, fulminant viral hepatitis, leptospirosis, measles, and other high-consequence viral infections (in particular, Lassa fever and yellow fever). Rarer noninfectious possibilities, including venomous snakebites, war­ farin intoxication, and the many causes of acquired or inherited

coagulopathy, also must be considered in the bleeding patient. An exposure history—including exposure to caves or mines; direct con­ tact with bats, nonhuman primates, or “bushmeat”; direct contact with severely ill local residents; or admission to rural hospitals with patient-to-patient or patient-to-health-care-worker clusters of ill­ ness—should raise the index of suspicion.

If FVD is suspected based on epidemiologic and/or clinical mani­ festations, infectious disease specialists and the proper public health authorities should be notified immediately. Laboratory diagnosis of FVD is relatively straightforward but ideally requires the capacity for maximum containment, which usually is not available in filoviridendemic countries. Increasingly, laboratory diagnosis is performed with patient samples inactivated in mobile field “glove boxes” by on-site personnel trained in the safe use of diagnostic assays adapted for field use in lower-containment settings. Consequently, diagnostic samples should be collected and processed with great caution and with the use of appropriate PPE and strict barrier techniques. With adherence to established biosafety precautionary measures, samples should be sent in suitable transport media to national or international reference labo­ ratories. Acute-phase blood/serum is the preferred diagnostic speci­ men because it usually contains high titers of filovirions. The current assay of choice for the diagnosis of filovirid infection is reverse-transcription polymerase chain reaction (RT-PCR) target­ ing one or more filovirid genes; a typical detection limit is 1000–5000 PFU/mL of serum, depending on the assay. When available, safe, rapid-turnaround, and standardized in-field PCR-based approaches (e.g., the Cepheid GeneXpert platform) are now standardly deployed during outbreaks. Antigen-capture enzyme-linked immunosorbent assays (ELISAs) or immunochromatographic assays for the detec­ tion of filovirion proteins are in development and may be useful as point-of-care rapid diagnostic tests (RDTs) in the future; sensitivity limitations of EBOV-specific antigen-based RDTs restrict current use to diagnosis of postmortem samples only. Direct immunoglobulin M (IgM) capture, direct IgG capture, or IgM-capture ELISAs (including multiplex approaches) are used for the detection of filovirion-targeting antibodies from patients in the later stages of disease (i.e., those who have been able to mount a detectable antibody response), including survivors. All of these assays can be conducted on samples treated with guanidinium isothiocyanate (for RT-PCR), cobalt-60 irradiation (for ELISA), or other effective measures (heat or irradiation treatment) that render filovirids noninfectious. Virus isolation in cell culture and plaque assays for quantification or diagnostic confirmation are rela­ tively simple but must be performed in maximum-containment labo­ ratories. If available, electron microscopic examination of inactivated samples or cultures for the unique filamentous structures of filovirions can further support diagnosis (Fig. 216-3). Formalin-fixed skin biopsy samples and possibly skin swabs can be useful for safe postmortem diagnoses. In-field (or near in-field) rapid genome sequencing was first deployed to inform classic epidemiology in Western Africa during the 2013–2016 EVD outbreak and has become a mainstay of outbreak control and response, even in endemic settings. CHAPTER 216 Filovirids: Orthoebolavirus and Orthomarburgvirus Infections TREATMENT Filovirid Disease Clinical management of patients with suspected or confirmed filo­ virid disease should be conducted by health care workers who have been well-trained in the complex care of FVD patients and who are using appropriate PPE in care environments with appropri­ ate infection prevention and control measures (see “Control and Prevention,” below). Historically, management of FVD had been entirely supportive (and even that limited by resource constraints) because the efficacy and safety of specific antiviral countermea­ sures had not been rigorously studied outside of animal mod­ els of disease. The 2013–2016 EVD outbreak in Western Africa highlighted the need to conduct rigorous, feasible, and ethical clinical research during outbreaks. Building on challenges and les­ sons learned in that setting, the first-of-its-kind Pamoja Tulinde

Maisha (PALM) randomized clinical trial was conducted during the 2018–2020/2021 EVD outbreak in the Democratic Republic of the Congo and identified two therapeutics based on mAbs specific to EBOV to improve survival rates. mAb114 (ansuvimab-zykl) and REG-EB3 (atoltivimab, maftivimab, and odesivimab-ebgn) were subsequently approved for the treatment of EVD in PCR-confirmed adults (including pregnant women) and children. Both are adminis­ tered via carefully monitored single-dose infusions initiated as soon as possible after diagnosis. In addition, efforts during this outbreak demonstrated the will and capacity to deliver more advanced sup­ portive and critical care accompanying specific therapeutics in the African outbreak setting. Although evidence is lacking, con­ sensus treatment strategies include those generally recommended for severe septicemia/sepsis/shock (Chap. 315) and should be applied with an emphasis on standard approaches—i.e., monitoring and response to respiratory dysfunction (e.g., oxygen), circulatory dysfunction (e.g., intravascular fluid repletion and vasopressor sup­ port), and CNS dysfunction (e.g., ruling out of reversible causes, notably hypoglycemia)—as well as the detection and management of acute kidney injury, hemorrhage, electrolyte derangements, and nutritional status and the prevention and treatment of secondary or co-infections. Pain management and administration of anti­ pyretics, antiemetics, and antidiarrheal agents may be considered. Crucial strategies to improve outcomes in the most severely ill FVD patients include preventing organ dysfunction and providing safe and effective temporary organ support (e.g., mechanical ventilation and renal replacement) to expand the window for administration of medical countermeasures and development of effective endogenous immune responses.

PART 5 Infectious Diseases ■ ■COMPLICATIONS Even in patients who have initial virologic or clinical improvement, complications in the second or third week of illness may include secondary infections; persistent renal dysfunction and electrolyte abnormalities; respiratory insufficiency; neurologic compromise (e.g., EBOV-related meningoencephalitis, cerebrovascular events, and sei­ zures); cardiac dysfunction (e.g., myocarditis and pericarditis); and venous thrombosis. Pregnancy and labor are historically associated with severe complications and poor outcomes in filovirid disease due to clotting factor consumption, fetal loss, and/or severe blood loss during birth; in the era of EBOV-specific therapeutics and optimized supportive care, maternal and fetal outcomes are improved. A number of sequelae have been self-reported or historically described in survivors of FVD, including prolonged and sometimes incapacitating arthralgia and myalgia, asthenia, alopecia, visual prob­ lems (including uveitis), hearing loss, memory loss and neurocognitive dysfunction, mental health conditions (anxiety, depression, and post­ traumatic stress disorder), and reproductive problems. Well-controlled observational studies of EVD survivors were first conducted in the aftermath of the 2013–2016 Western Africa EVD outbreak. Com­ pared with their close-contact controls, Liberian EVD survivors had an increased incidence of headache, arthralgia and myalgia, memory loss, fatigue, and urinary frequency as well as abnormal results in abdominal, chest, neurologic, musculoskeletal, and ocular exams. Of individual and public health significance is the potential for persistence of filovirids in immune-privileged tissue compartments (and their associated fluids) in FVD survivors, most commonly in the semen of male survivors (with the rare but documented potential for sexual transmission) and rarely in the CNS (causing recrudescent meningo­ encephalitis), the eye (causing recrudescent uveitis), and the placenta (causing transmission or placental insufficiency). True relapses that resemble the course of primary FVD are extremely rare but have been described. The risk of the remote reignition of outbreaks from persistently infected FVD survivors is apparently rare but obviously consequential. Notably, these events have only been detected after the two largest EVD outbreaks, suggesting a likely limitation, with the caveat that available samples and genomic tools to determine outbreak relatedness have historically not been available. EVD outbreaks in the

Democratic Republic of the Congo in 2021 were genomically linked to the previous 2018–2020 outbreak. After a 5- to 7-year interim, an EVD outbreak in Guinea in 2021 was genomically linked to the 2013–2016 Western African outbreak; this event remains enigmatic as corroborat­ ing epidemiologic links have not been found. ■ ■PROGNOSIS Among the most severe of acute viral diseases in humans, FVD gener­ ally has a poor prognosis, although with much greater heterogene­ ity than was historically assumed; i.e., the 70–90% case-fatality rate ascribed for many decades to EVD has required revision. With an incomplete evidence base, the outcome probably depends on factors that include the particular filovirid causing the infection (Fig. 216-4), host factors (age, immune status, coinfections, and unknown host genetic factors), virus exposure route and dose, viral load, the presence and severity of organ dysfunction, and—critically—the availability of filovirid-specific countermeasures and requisite supportive care. Despite the receipt of effective EBOV-specific therapeutics, outcomes in severe EVD patients with high viral loads and complex organ dysfunction remain suboptimal and will require future research to evaluate and advance virus-targeted (e.g., combination therapeutic), disease-targeted (e.g., immunomodulatory), and optimized supportive care strategies. After resolution of acute disease, the long-term health and survival outcomes for FVD survivors are unknown. It is uncertain how increased access to filovirid-specific therapeutics and life-saving support will impact filovirid persistence in survivors; short- and longterm surveillance will be necessary to avoid individual consequences (e.g., relapse and/or recrudescent inflammatory syndromes) and public health consequences (e.g., reignition of outbreak transmission chains in the peri-outbreak or post-outbreak period). Although remarkable progress in understanding and improving outcomes in EVD has occurred in recent years, the same cannot yet be said for disease caused by filovirids other than EBOV, either in acute disease or in convalescence. ■ ■CONTROL AND PREVENTION Prevention of viral exposure in nature is challenged by an incomplete understanding of filovirid ecology. To prevent orthomarburgvirus infection, people entering or living in areas where Egyptian rousettes can be found should avoid direct or indirect contact with these ani­ mals. Prevention in nature is more difficult in the case of pathogenic orthoebolaviruses, largely because ecologic reservoirs have not yet definitively been identified. EVD outbreaks have been associated with hunting or consumption of nonhuman primates more than exposure to bats. Indeed, the mechanism of introduction of orthoebolaviruses into nonhuman primate populations, if it occurs at all, is unclear. (Only one pathogenic orthoebolavirus, TAFV, has unequivocally been detected in wild nonhuman primates.) Therefore, to prevent orthoebolavirus infection, the only evidenced advice that can be offered to travelers and locals is to avoid contact with “bushmeat,” nonhuman primates, and bats. In any setting, the local involvement of medical anthro­ pologists and careful community engagement is strongly advised to ensure preventive risk communication is not received as threatening or patronizing. Biomedical prevention strategies have historically been limited to tried-and-true pillars of outbreak control, centering on the identifica­ tion and isolation of cases, contact tracing, ensuring that health care workers and other response personnel have appropriate training and capacity in infection prevention and control, and preventing high-risk transmission events. Measures aimed at preventing and controlling infection, including relatively simple barrier nursing techniques, vigi­ lant use of appropriate PPE, quarantine, and contact tracing, usually effectively terminate or at least contain FVD outbreaks. Isolation of infected people and their contacts and avoidance of direct personto-person contact without appropriate PPE usually prevent further spread, as the virions are not transmitted through droplets or aerosols under natural conditions. Typical protective gear sufficient to prevent filovirid infections consists of disposable gloves, gowns, and shoe

109 - SECTION 16 Fungal Infections

SECTION 16 Fungal Infections

covers and a face shield and/or goggles. If available, N-95 or N-100 respi­ rators may be used to further limit infection risk. Positive-air-pressure respirators should be considered for high-risk medical procedures, such as intubation or suctioning. Medical equipment used in the care of an infected patient, such as gloves or syringes, should never be reused. Because filovirions are enveloped, disinfecting with detergents (e.g., 1% sodium deoxycholate, diethyl ether, or phenolic compounds) is relatively straightforward. Bleach solutions are recommended at 1:100 for surface disinfection and 1:10 for application to excreta or corpses. Whenever possible, potentially contaminated materials should be auto­ claved, irradiated, or destroyed. Emerging from research conducted during the 2013–2016 EVD outbreak in Western Africa, a vaccine based on a recombinant vesicu­ lar stomatitis Indiana virus expressing EBOV GP1,2 (rVSV-ZEBOV/ Ervebo) was the first filovirid vaccine approved for use in the United States and the European Union (EU). It is now widely deployed in both a reactive-ring vaccination strategy, targeting close contacts and their contacts in EVD outbreak settings, and for the preexposure vaccination of health care workers in at-risk regions. More recently, a heterologous dose vaccine candidate incorporating EBOV GP1,2 into an adenovirus vector (Ad26.ZEBOV-GP/Zabdeno) followed by a vaccinia virus vec­ tor incorporating multiple filovirid antigens (MVA-BN-filo/Mvabea) has been shown to be safe and immunogenic in humans. Though evaluation of efficacy in a clinical trial has not been possible, immu­ nobridging data gained during nonhuman primate experimentation led to regulatory authorization in the EU under “exceptional circum­ stances”; the two-dose requirement likely limits current use to proac­ tive preexposure prevention in “peri-outbreak” settings rather than reactive “in-outbreak” reactive strategies. Development and evaluation of this and other vaccine candidates continue toward complementary preventive approaches for non-outbreak or peri-outbreak settings, with emphasis on the durability of immune responses and increases in preventive breadth toward other filovirids. Even in the absence of high-level evidence, expert consensus informs the targeted use of EBOV-specific vaccine or postexposure prophylaxis (PEP) to prevent infection or disease in health care workers considered to have had a high-risk EBOV exposure (e.g., after needlestick injury). Evidence is needed to inform the use of PEP in high-risk contacts in the field outbreak setting. For male survivors, abstinence from sexual activity with a partner for at least 12 months after disappearance of clinical signs is recommended, unless testing proves semen to be free of filovirid RNA. (The use of condoms is generally recommended for all sexual activities.) Reproductive tract and CNS tissues, including ocular tissues and fluids from survivors, should be handled with appropriate precautions until demonstrated to be filovirid-RNA free. The role of filovirid-specific therapeutics in the prevention or treatment of filoviral persistence is unclear. ■ ■FURTHER READING Cnops L et al: Essentials of filoviral load quantification. Lancet Infect Dis 16:e134, 2016. Crozier I et al: The evolution of medical countermeasures for Ebola virus disease: Lessons learned and next steps. Vaccines (Basel) 10:1213, 2022. Dudas G et al: Virus genomes reveal factors that spread and sustained the Ebola epidemic. Nature 544:309, 2017. Hoenen T et al: Therapeutic strategies to target the Ebola virus life cycle. Nat Rev Microbiol 17:593, 2019. Jacob ST et al: Ebola virus disease. Nat Rev Dis Primers 6:13, 2020. Kuhn JH et al: Filoviridae, in Fields Virology, Vol 1, 7th ed, PM Howley et al (eds). Philadelphia, Wolters Kluwer/Lippincott Williams & Wilkins, 2020, pp 449–503. Matz KM et al: Ebola vaccine trials: Progress in vaccine safety and immunogenicity. Expert Rev Vaccines 18:1229, 2019. Mulangu S et al: A randomized, controlled trial of Ebola virus disease therapeutics. N Engl J Med 381:2293, 2019. Regules JA et al: A recombinant vesicular stomatitis virus Ebola vaccine. N Engl J Med 376:330, 2017.

Section 16 Fungal Infections Michail S. Lionakis, John E. Edwards, Jr.

Pathogenesis, Diagnosis,

and Treatment of Fungal Infections DEFINITION AND ETIOLOGY In recent decades, human fungal infections have dramatically increased worldwide as a result of the AIDS pandemic, the widespread use of antibacterial agents, and the introduction of cytotoxic agents and precision medicine biologics for the treatment of autoimmune and neoplastic diseases and for use in patients undergoing solid organ transplantation or hematopoietic stem cell transplantation. Moreover, of great concern has been the recent rise in fungal infections caused by drug-resistant species, such as azole- and/or echinocandin-resistant Candida glabrata and Candida auris and azole-resistant Aspergillus fumigatus. Among the ~5 million fungal species, only a few cause human infections (Table 217-1). Fungal infections are classified as mucocutaneous and deep organ infections on the basis of anatomic location and as endemic and opportu­ nistic infections on the basis of epidemiology. Mucocutaneous infections can cause serious morbidity but are rarely fatal. Deep organ infections cause severe illness and often carry a high mortality rate. The endemic mycoses are caused by fungi that are not part of the normal human microbiota but are environmentally acquired. The opportunistic myco­ ses are caused by fungi (Candida, Aspergillus) that often are components of the human microbiota and whose ubiquity in nature renders them easily acquired by immunosuppressed hosts (Table 217-1). Opportunis­ tic fungi cause serious infections when impaired host immune responses allow the organisms to transition from commensals to invasive patho­ gens. Endemic fungi typically cause self-limited disease in immunocom­ petent hosts but severe illness in immunosuppressed patients. CHAPTER 217 Pathogenesis, Diagnosis, and Treatment of Fungal Infections Fungi are morphologically classified as yeast, mold, and dimorphic. Yeasts are seen as round single cells or budding organisms. Molds grow as filamentous forms called hyphae both at room temperature and in tissue. Aspergillus, Mucorales, and dermatophytes that infect skin and nails are mold fungi. Variations exist within this classification system. For instance, when Candida infects tissue, both yeasts and filamen­ tous forms (pseudohyphae) may be present (except in the cases of C. glabrata and C. auris, which form only yeasts in tissue); in contrast, Cryptococcus exists only in yeast form. Dimorphic is the term used to describe fungi that have two forms; they grow as yeasts or large spheri­ cal structures in tissue but as filamentous forms at room temperature in the environment (Table 217-1). Patients acquire deep organ infection by molds and endemic dimorphic fungi via inhalation. Skin dermatophytes are primarily environmentally acquired, but human-to-human transmission may also occur. The commensal Candida invades deep tissues from sites of mucocutaneous colonization, usually in the gastrointestinal tract or the skin in the case of C. auris. In this chapter, we outline general principles of immunology, diagno­ sis, and treatment related to the most common human fungal infections. ■ ■PATHOGENESIS In the past decade, our understanding of fungal recognition pathways and of tissue-specific innate and adaptive antifungal host defense mechanisms has markedly expanded. A major breakthrough has been the discovery and functional characterization of the C-type lectin recep­ tor/spleen tyrosine kinase/caspase recruitment domain–containing

protein 9 (CLR/SYK/CARD9) signaling pathway, which mediates fungal polysaccharide recognition and orchestrates proinflamma­ tory mediator production, leukocyte recruitment, inflammasome

11 - 132 Lung Abscess

132 Lung Abscess

community-acquired pneumonia in adults. Clin Infect Dis 44:S27, 2007. Martin-Loeches I et al: ERS/ESICM/ESCMID/ALAT guidelines for

the management of severe community-acquired pneumonia. Eur Respir J 61:2200735, 2023. Metlay JP et al: Diagnosis and treatment of adults with communityacquired pneumonia. An Official Clinical Practice Guideline of the American Thoracic Society and Infectious Diseases Society of America. Am J Respir Crit Care Med 200:e45, 2019. Vacheron CH et al: Attributable mortality of ventilator-associated pneumonia among patients with COVID-19. Am J Respir Crit Care Med 206:161, 2022. Rebecca M. Baron, Beverly W. Baron,

Miriam Baron Barshak

Lung Abscess Lung abscess represents necrosis and cavitation of the lung following microbial infection. Lung abscesses can be single or multiple but usu­ ally are marked by a single dominant cavity >2 cm in diameter. PART 5 Infectious Diseases ■ ■ETIOLOGY The low prevalence of lung abscesses makes them difficult to study in randomized controlled trials. Although the incidence of lung abscesses has decreased in the antibiotic era, they are still a source of significant morbidity and mortality. Lung abscesses usually are characterized as either primary (~80% of cases) or secondary. Primary lung abscesses generally arise from aspi­ ration, often are caused principally by anaerobic bacteria, and occur in the absence of an underlying pulmonary or systemic condition. Secondary lung abscesses arise in the setting of an underlying condi­ tion, such as a postobstructive process (e.g., bronchial foreign body, tumor) or a systemic process (e.g., HIV infection, another immuno­ compromising condition). Lung abscesses can also be characterized as acute (<4–6 weeks in duration) or chronic (~40% of cases). ■ ■EPIDEMIOLOGY The majority of the existing epidemiologic information involves pri­ mary lung abscesses. In general, middle-aged men are more commonly affected than middle-aged women. The major risk factor for primary lung abscesses is aspiration. Patients at particular risk for aspiration, such as those with altered mental status, alcoholism, drug overdose, seizures, bulbar dysfunction, prior cerebrovascular or cardiovascular events, or neuromuscular disease, are most commonly affected. At additional risk are patients with esophageal dysmotility or esophageal lesions (strictures or tumors) and those with gastric distention and/or gastroesophageal reflux, especially those who spend substantial time in the recumbent position. It is widely thought that colonization of the gingival crevices by anaerobic bacteria or microaerophilic streptococci (especially in patients with gingivitis and periodontal disease), combined with a risk of aspiration, is important in the development of lung abscesses. In fact, many physicians consider it extremely rare for lung abscesses to develop in the absence of teeth as a nidus for bacterial colonization. The importance of these risk factors in the development of lung abscesses is highlighted by a significant reduction in abscess inci­ dence in the late 1940s that coincided with a change in oral surgical technique: beginning at that time, these operations were no longer performed with the patient in the seated position without a cuffed endotracheal tube, and the frequency of perioperative aspiration events was thus decreased. In addition, the introduction of penicillin around

TABLE 132-1  Examples of Microbial Pathogens That Can Cause Lung Abscesses CLINICAL CONDITION PATHOGENS Primary lung abscess (often with risk factors for aspiration) Anaerobes (e.g., Peptostreptococcus spp., Prevotella spp., Bacteroides spp., milleri group streptococci), microaerophilic streptococci Secondary lung abscess (often with underlying immunocompromise) Staphylococcus aureus, gram-negative rods (e.g., Pseudomonas aeruginosa, Enterobacteriaceae), Nocardia spp., Aspergillus spp., Mucorales, Cryptococcus spp., Legionella spp., Rhodococcus equi, Pneumocystis jirovecii Embolic lesions Staphylococcus aureus (often from endocarditis), Fusobacterium necrophorum (Lemierre’s syndrome; see text for details) Endemic infections (with or without underlying immunocompromise) Mycobacterium tuberculosis (as well as Mycobacterium avium and Mycobacterium kansasii), Coccidioides spp., Histoplasma capsulatum, Blastomyces spp., parasites (e.g., Entamoeba histolytica, Paragonimus westermani, Strongyloides stercoralis) Miscellaneous conditions Bacterial pathogen (often S. aureus) after influenza or another viral infection, Actinomyces spp. the same time significantly reduced the incidence of and mortality rate from lung abscess. ■ ■PATHOGENESIS Primary Lung Abscesses  The development of primary lung abscesses is thought to originate when chiefly anaerobic bacteria (as well as microaerophilic streptococci) in the gingival crevices are aspirated into the lung parenchyma in a susceptible host (Table 132-1). Patients who develop primary lung abscesses usually carry an overwhelming burden of aspirated material or are unable to clear the bacterial load. Pneumonitis develops initially (exacerbated in part by tissue damage caused by gastric acid); then, over a period of 7–14 days, the bacteria produce parenchymal necrosis and cavitation whose extent depends on host–pathogen interaction (Fig. 132-1). Anaerobes are thought to pro­ duce more extensive tissue necrosis in polymicrobial infections in which virulence factors of the various bacteria can act synergistically to cause more significant tissue destruction. Secondary Lung Abscesses  The pathogenesis of secondary abscesses depends on the predisposing factor. For example, in cases of bronchial obstruction from malignancy or a foreign body, the A B FIGURE 132-1  Representative chest CT scans demonstrating development of lung abscesses. This patient was immunocompromised from underlying lymphoma and developed severe Pseudomonas aeruginosa pneumonia, as represented by a left lung infiltrate with concern for central regions of necrosis (panel A, black arrow). Two weeks later, areas of cavitation with air-fluid levels were visible in this region and were consistent with the development of lung abscesses (panel B, white arrow). (Images provided by Dr. Ritu Gill, formerly of the Division of Chest Radiology, Brigham and Women’s Hospital, Boston; with permission.)

obstructing lesion prevents clearance of oropharyngeal secretions, leading to abscess development. With underlying systemic conditions (e.g., immunosuppression after bone marrow or solid organ transplan­ tation), impaired host defense mechanisms lead to increased suscepti­ bility to the development of lung abscesses caused by a broad range of pathogens, including opportunistic organisms (Table 132-1). Lung abscesses also arise from septic emboli, either in tricuspid valve endocarditis (often involving Staphylococcus aureus) or in Lemierre’s syndrome, in which an infection begins in the pharynx (classically involving Fusobacterium necrophorum) and then spreads to the neck and the carotid sheath (which contains the jugular vein) to cause septic thrombophlebitis. ■ ■PATHOLOGY AND MICROBIOLOGY Primary Lung Abscesses  The dependent segments (posterior upper lobes and superior lower lobes) are the most common locations of primary lung abscesses, given the predisposition of aspirated materi­ als to be deposited in these areas. Generally, the right lung is affected more commonly than the left because the right mainstem bronchus is less angulated. Primary lung abscesses often are polymicrobial, primarily including anaerobic organisms as well as microaerophilic streptococci (Table 132-1). The retrieval and culture of anaerobes can be complicated by the con­ tamination of samples with microbes from the oral cavity, the need for expeditious transport of the cultures to the laboratory, the need for early plating with special culture techniques, the prolonged time required for culture growth, and the need for collection of specimens prior to admin­ istration of antibiotics. When attention is paid to these factors, rates of recovery of specific isolates are reportedly as high as 78%. Because it is not clear that knowing the identity of the causative anaerobic isolate alters the approach to treatment of a primary lung abscess, practice has shifted away from the use of specialized tech­ niques to obtain material for culture, such as transtracheal aspiration and bronchoalveolar lavage with protected brush specimens that allow recovery of culture material while avoiding contamination from the oral cavity. When no pathogen is isolated from a primary lung abscess (which occurs as often as 40% of the time), the abscess is termed a nonspecific lung abscess, and the presence of anaerobes is often pre­ sumed. A putrid lung abscess refers to cases with foul-smelling breath, sputum, or empyema; these manifestations are essentially diagnostic of an anaerobic lung abscess. Secondary Lung Abscesses  The location of secondary abscesses may vary with the underlying cause. The microbiology of second­ ary lung abscesses can encompass a broad bacterial spectrum, with infection by Pseudomonas aeruginosa and other gram-negative rods the most common. In addition, a broad array of pathogens can be identified in patients from certain endemic areas and in specific clini­ cal scenarios (e.g., a significant incidence of fungal infections among immunosuppressed patients following bone marrow or solid organ transplantation). Because immunocompromised hosts and patients without the classic presentation of a primary lung abscess can be infected with a wide array of unusual organisms (Table 132-1), it is of special importance to obtain culture material to target therapy. ■ ■CLINICAL MANIFESTATIONS Clinical manifestations initially may be similar to those of pneumo­ nia, with fevers, cough, sputum production, and chest pain; a more chronic and indolent presentation that includes night sweats, fatigue, and anemia is often observed with anaerobic lung abscesses. A subset of patients with putrid lung abscesses may report discolored phlegm and foul-tasting or foul-smelling sputum. Patients with lung abscesses due to non-anaerobic organisms, such as S. aureus, may present with a more fulminant course characterized by high fevers and rapid progression. Findings on physical examination may include fevers, poor denti­ tion, and/or gingival disease as well as amphoric and/or cavernous breath sounds on lung auscultation. Additional findings may include digital clubbing and the absence of a gag reflex.

■ ■DIFFERENTIAL DIAGNOSIS The differential diagnosis of lung abscesses is broad and includes other noninfectious processes that result in cavitary lung lesions, including lung infarction, malignancy, sequestration, cryptogenic organizing pneumonia, sarcoidosis, vasculitides and autoimmune diseases (e.g., granulomatosis with polyangiitis), lung cysts or bullae containing fluid, and septic emboli (e.g., from tricuspid valve endocarditis). Other less common entities can include pulmonary manifestations of diseases that usually present at locations other than the chest (e.g., inflamma­ tory bowel disease, pyoderma gangrenosum).

■ ■DIAGNOSIS Lung abscesses are documented by chest imaging. Although a chest radiograph usually detects a thick-walled cavity with an air-fluid level, CT permits better definition and may provide earlier evidence of cavitation. CT may also yield additional information regarding a pos­ sible underlying cause of lung abscess, such as malignancy, and may help distinguish a peripheral lung abscess from a pleural infection. This distinction has important implications for treatment because a pleural space infection, such as an empyema, may require urgent drainage. As described earlier (see “Pathology and Microbiology,” above), more invasive diagnostics (such as transtracheal aspiration) were tra­ ditionally undertaken for primary lung abscesses, whereas empirical therapy that includes drugs targeting anaerobic organisms currently is used more often. While sputum can be collected noninvasively for Gram’s stain and culture, which may yield a pathogen, the infection is likely to be polymicrobial, and culture results may not reflect the pres­ ence of anaerobic organisms. Increasing use of molecular techniques for bacterial detection (e.g., 16S RNA gene amplification) may eventu­ ally yield more specific pathogen identification. As stated above, many physicians consider putrid-smelling sputum to be virtually diagnostic of an anaerobic infection. CHAPTER 132 When a secondary lung abscess is present or empirical therapy fails to elicit a response, sputum and blood cultures are advised in addi­ tion to serologic studies for opportunistic pathogens (e.g., viruses and fungi causing infections in immunocompromised hosts). Additional diagnostics, such as bronchoscopy with bronchoalveolar lavage or protected brush specimen collection and CT-guided percutaneous needle aspiration, can be undertaken. Risks posed by these more invasive diagnostics include spillage of abscess contents into the other lung (with bronchoscopy) and pneumothorax and bronchopleural fistula development (especially with CT-guided needle aspiration). However, early diagnostics in secondary abscesses, especially in immu­ nocompromised hosts, are particularly important because the patients involved may be especially fragile, at risk for infection with a broad array of pathogens, and therefore less likely than other patients to respond to empirical therapy. Lung Abscess TREATMENT Lung Abscess The availability of antibiotics in the 1940s and 1950s established therapy with this drug class as the primary approach to the treat­ ment of lung abscess. Previously, surgery had been relied upon much more frequently. For many decades, penicillin was the antibiotic of choice for primary lung abscesses in light of its anaerobic cover­ age; however, because oral anaerobes can produce β-lactamases, clindamycin has proved superior to penicillin in clinical trials. For primary lung abscesses, the recommended regimens are (1) clindamy­ cin (600 mg IV three times daily; then, with the disappearance of fever and clinical improvement, 300 mg PO four times daily) or (2) an IV-administered β-lactam/β-lactamase combination, followed— once the patient’s condition is stable—by orally administered amoxicillin-clavulanate. This therapy should be continued until imaging demonstrates that the lung abscess has cleared or regressed to a small scar. Treatment duration may range from 3–4 weeks to as long as 14 weeks, with some literature suggesting that a course of at least 6 weeks may be associated with better outcomes. One small

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217 Pathogenesis, Diagnosis, and Treatment of Fungal Infections

covers and a face shield and/or goggles. If available, N-95 or N-100 respi­ rators may be used to further limit infection risk. Positive-air-pressure respirators should be considered for high-risk medical procedures, such as intubation or suctioning. Medical equipment used in the care of an infected patient, such as gloves or syringes, should never be reused. Because filovirions are enveloped, disinfecting with detergents (e.g., 1% sodium deoxycholate, diethyl ether, or phenolic compounds) is relatively straightforward. Bleach solutions are recommended at 1:100 for surface disinfection and 1:10 for application to excreta or corpses. Whenever possible, potentially contaminated materials should be auto­ claved, irradiated, or destroyed. Emerging from research conducted during the 2013–2016 EVD outbreak in Western Africa, a vaccine based on a recombinant vesicu­ lar stomatitis Indiana virus expressing EBOV GP1,2 (rVSV-ZEBOV/ Ervebo) was the first filovirid vaccine approved for use in the United States and the European Union (EU). It is now widely deployed in both a reactive-ring vaccination strategy, targeting close contacts and their contacts in EVD outbreak settings, and for the preexposure vaccination of health care workers in at-risk regions. More recently, a heterologous dose vaccine candidate incorporating EBOV GP1,2 into an adenovirus vector (Ad26.ZEBOV-GP/Zabdeno) followed by a vaccinia virus vec­ tor incorporating multiple filovirid antigens (MVA-BN-filo/Mvabea) has been shown to be safe and immunogenic in humans. Though evaluation of efficacy in a clinical trial has not been possible, immu­ nobridging data gained during nonhuman primate experimentation led to regulatory authorization in the EU under “exceptional circum­ stances”; the two-dose requirement likely limits current use to proac­ tive preexposure prevention in “peri-outbreak” settings rather than reactive “in-outbreak” reactive strategies. Development and evaluation of this and other vaccine candidates continue toward complementary preventive approaches for non-outbreak or peri-outbreak settings, with emphasis on the durability of immune responses and increases in preventive breadth toward other filovirids. Even in the absence of high-level evidence, expert consensus informs the targeted use of EBOV-specific vaccine or postexposure prophylaxis (PEP) to prevent infection or disease in health care workers considered to have had a high-risk EBOV exposure (e.g., after needlestick injury). Evidence is needed to inform the use of PEP in high-risk contacts in the field outbreak setting. For male survivors, abstinence from sexual activity with a partner for at least 12 months after disappearance of clinical signs is recommended, unless testing proves semen to be free of filovirid RNA. (The use of condoms is generally recommended for all sexual activities.) Reproductive tract and CNS tissues, including ocular tissues and fluids from survivors, should be handled with appropriate precautions until demonstrated to be filovirid-RNA free. The role of filovirid-specific therapeutics in the prevention or treatment of filoviral persistence is unclear. ■ ■FURTHER READING Cnops L et al: Essentials of filoviral load quantification. Lancet Infect Dis 16:e134, 2016. Crozier I et al: The evolution of medical countermeasures for Ebola virus disease: Lessons learned and next steps. Vaccines (Basel) 10:1213, 2022. Dudas G et al: Virus genomes reveal factors that spread and sustained the Ebola epidemic. Nature 544:309, 2017. Hoenen T et al: Therapeutic strategies to target the Ebola virus life cycle. Nat Rev Microbiol 17:593, 2019. Jacob ST et al: Ebola virus disease. Nat Rev Dis Primers 6:13, 2020. Kuhn JH et al: Filoviridae, in Fields Virology, Vol 1, 7th ed, PM Howley et al (eds). Philadelphia, Wolters Kluwer/Lippincott Williams & Wilkins, 2020, pp 449–503. Matz KM et al: Ebola vaccine trials: Progress in vaccine safety and immunogenicity. Expert Rev Vaccines 18:1229, 2019. Mulangu S et al: A randomized, controlled trial of Ebola virus disease therapeutics. N Engl J Med 381:2293, 2019. Regules JA et al: A recombinant vesicular stomatitis virus Ebola vaccine. N Engl J Med 376:330, 2017.

Section 16 Fungal Infections Michail S. Lionakis, John E. Edwards, Jr.

Pathogenesis, Diagnosis,

and Treatment of Fungal Infections DEFINITION AND ETIOLOGY In recent decades, human fungal infections have dramatically increased worldwide as a result of the AIDS pandemic, the widespread use of antibacterial agents, and the introduction of cytotoxic agents and precision medicine biologics for the treatment of autoimmune and neoplastic diseases and for use in patients undergoing solid organ transplantation or hematopoietic stem cell transplantation. Moreover, of great concern has been the recent rise in fungal infections caused by drug-resistant species, such as azole- and/or echinocandin-resistant Candida glabrata and Candida auris and azole-resistant Aspergillus fumigatus. Among the ~5 million fungal species, only a few cause human infections (Table 217-1). Fungal infections are classified as mucocutaneous and deep organ infections on the basis of anatomic location and as endemic and opportu­ nistic infections on the basis of epidemiology. Mucocutaneous infections can cause serious morbidity but are rarely fatal. Deep organ infections cause severe illness and often carry a high mortality rate. The endemic mycoses are caused by fungi that are not part of the normal human microbiota but are environmentally acquired. The opportunistic myco­ ses are caused by fungi (Candida, Aspergillus) that often are components of the human microbiota and whose ubiquity in nature renders them easily acquired by immunosuppressed hosts (Table 217-1). Opportunis­ tic fungi cause serious infections when impaired host immune responses allow the organisms to transition from commensals to invasive patho­ gens. Endemic fungi typically cause self-limited disease in immunocom­ petent hosts but severe illness in immunosuppressed patients. CHAPTER 217 Pathogenesis, Diagnosis, and Treatment of Fungal Infections Fungi are morphologically classified as yeast, mold, and dimorphic. Yeasts are seen as round single cells or budding organisms. Molds grow as filamentous forms called hyphae both at room temperature and in tissue. Aspergillus, Mucorales, and dermatophytes that infect skin and nails are mold fungi. Variations exist within this classification system. For instance, when Candida infects tissue, both yeasts and filamen­ tous forms (pseudohyphae) may be present (except in the cases of C. glabrata and C. auris, which form only yeasts in tissue); in contrast, Cryptococcus exists only in yeast form. Dimorphic is the term used to describe fungi that have two forms; they grow as yeasts or large spheri­ cal structures in tissue but as filamentous forms at room temperature in the environment (Table 217-1). Patients acquire deep organ infection by molds and endemic dimorphic fungi via inhalation. Skin dermatophytes are primarily environmentally acquired, but human-to-human transmission may also occur. The commensal Candida invades deep tissues from sites of mucocutaneous colonization, usually in the gastrointestinal tract or the skin in the case of C. auris. In this chapter, we outline general principles of immunology, diagno­ sis, and treatment related to the most common human fungal infections. ■ ■PATHOGENESIS In the past decade, our understanding of fungal recognition pathways and of tissue-specific innate and adaptive antifungal host defense mechanisms has markedly expanded. A major breakthrough has been the discovery and functional characterization of the C-type lectin recep­ tor/spleen tyrosine kinase/caspase recruitment domain–containing

protein 9 (CLR/SYK/CARD9) signaling pathway, which mediates fungal polysaccharide recognition and orchestrates proinflamma­ tory mediator production, leukocyte recruitment, inflammasome

TABLE 217-1  Major Fungal Infections, Associated At-Risk Patient Populations, and Diagnostic Tests INFECTION (MOST COMMON FUNGAL GENERA AND SPECIES) CLINICAL SYNDROME(S) RISK FACTOR(S) DIAGNOSTIC TEST(S) Mold (Filamentous) Fungi Aspergillosis (Aspergillus fumigatus,

A. terreus, A. flavus, A. niger,

A. nidulansa) Pneumonia or disseminated infection ABPA Keratitis Neutropenia, glucocorticoids, HSCT, post-influenza or COVID-19, BTK inhibition Atopic individuals Direct inoculation Mucormycosis (Rhizopus, Rhizomucor,

Mucor, Cunninghamella, and Lichtheimia spp.) Sinopulmonary infection Rhinocerebral infection Necrotizing skin infection Neutropenia, HSCT Diabetic ketoacidosis Direct inoculation (e.g., tornado victims) Fusariosis (Fusarium solani, F. oxysporum) Pneumonia or disseminated infection Keratitis Neutropenia Direct inoculation Scedosporiosis (Scedosporium apiospermum) Pneumonia or disseminated infection Neutropenia, glucocorticoids, HSCT Phaeohyphomycosis (Cladophialophora, Alternaria, Phialophora, Rhinocladiella, Exophiala, and Exserohilum spp.) Sinopulmonary, CNS, or disseminated infection Skin infection Allergic sinusitis HSCT, neutropenia, glucocorticoids, healthy individuals (for CNS), TNF-α inhibition Direct inoculation Atopic individuals Dermatophytosis (Trichophyton, Microsporum, and Epidermophyton spp.) Skin and nail infections Healthy individuals Culture or microscopic examination of scrapings or clippings: chains of arthrospores (diagnostic) PART 5 Infectious Diseases Eumycetoma (Madurella mycetomatis) Skin and subcutaneous infections Healthy individuals Culture and macroscopic and histologic examination of grains harvested from biopsy or aspiration Yeast Fungi Mucosal candidiasisc Oropharyngeal or esophageal candidiasis Vulvovaginal candidiasis AIDS, glucocorticoids Antibiotic use (Candida albicans, C. glabrata) Invasive candidiasisc Candidemia Disseminated infection (spleen, liver, kidney, eye, heart, CNS) Critical illness (ICU) Neutropenia, glucocorticoids (C. albicans, C. glabrata,

C. parapsilosis, C. tropicalis,

C. auris) Cryptococcosis (Cryptococcus neoformans, C. gattii) Pneumonia Osteomyelitis Meningoencephalitis AIDS, glucocorticoids Sarcoidosis AIDS, AAbs to IFN-γ or GM-CSF, BTK or JAK inhibition Trichosporonosisd Superficial skin infection (white piedra) Disseminated infection (skin, eye) Healthy individuals Neutropenia, glucocorticoids, HSCT, SOT (Trichosporon asahii, T. mucoides, T. asteroides) Endemic Dimorphic Fungi Histoplasmosis (Histoplasma capsulatum, H. duboisii [in Africa]) Self-limited pneumonia Disseminated infection (liver, bone, bone marrow) Fibrosing mediastinitis Healthy individuals AIDS, SOT, glucocorticoids, AAbs to IFN-γ, JAK or TNF-α inhibition Blastomycosis (Blastomyces dermatitidis, B. gilchristii) Pneumonia Disseminated infection (skin, bone, mucosal surfaces, genitourinary tract) Healthy individuals AIDS, glucocorticoids, TNF-α inhibition Coccidioidomycosis (Coccidioides immitis, C. posadasii) Self-limited pneumonia Disseminated infection (CNS, bone) Healthy individuals AIDS, glucocorticoids, TNF-α inhibition Paracoccidioidomycosis (Paracoccidioides brasiliensis, P. lutzii) Pneumonia Disseminated infection (skin, bone, mucosal surfaces) Healthy individuals AIDS, glucocorticoids

Culture of BAL fluid: low sensitivity, nonspecific (colonization, contamination) Histologic examination of tissueb: acute-angle septate hyphae Biomarkers: GM (BAL > serum); serum BDG (nonspecific) Culture of BAL fluid or sinus tissue: very low sensitivity Histologic examination of tissue: ribbon-like aseptate hyphae Biomarkers: Negative Culture of tissue or blood: one of the few molds recovered from blood Histologic examination of tissue: acute-angle septate hyphae Biomarkers: GM can be positive; BDG (nonspecific) Culture of BAL: low sensitivity, nonspecific (colonization, contamination) Histologic examination of tissue: acute-angle septate hyphae Biomarkers: BDG can be positive Culture of ordinarily sterile site Histologic examination of tissue: cell walls may appear dark brown or golden on H&E; Fontana-Masson may stain fungal melanin Culture of mucosal surfaces Histologic examination of esophageal tissue or wet preparation (10% KOH) of vaginal discharge: yeast and/or pseudohyphae Culture of blood: low sensitivity Histologic examination of tissue: yeast and/or pseudohyphae Biomarkers/other tests: BDG (nonspecific); T2 magnetic resonance in whole blood Culture of CSF, BAL fluid, blood Microscopic examination of tissue or CSF: encapsulated yeast (GMS, India ink, mucicarmine stain) Biomarkers: Cryptococcus Ag (serum, CSF) is sensitive and specific Culture of tissue or blood Histologic examination of tissue: yeasts, hyphae, and arthroconidia Biomarkers: BDG can be positive Culture of blood or tissue: low sensitivity; weeks needed for growth Histologic examination of tissue: yeast with narrow-based budding Other tests: Histoplasma Ag (urine > serum > BAL); BDG can be positive; serology (CF) can be useful in non-AIDS patients Culture of BAL or tissue: low sensitivity; weeks needed for growth Histologic examination of tissue: yeast with broad-based budding Other tests: serology (CF, ID) has low sensitivity; Blastomyces Ag test cross-reacts with other endemic fungi; GM can be positive Culture is diagnostice Histologic examination: spherules Other tests: serology (CF, ID); Coccidioides Ag test can be useful in CNS infection; BDG can be positive Culture of tissue: active disease; several weeks needed for growth Histologic examination of KOH preparations or tissue: yeast with budding in steering-wheel pattern Other tests: serology (ID, CF); Paracoccidioides Ag test (Continued)

TABLE 217-1  Major Fungal Infections, Associated At-Risk Patient Populations, and Diagnostic Tests INFECTION (MOST COMMON FUNGAL GENERA AND SPECIES) CLINICAL SYNDROME(S) RISK FACTOR(S) DIAGNOSTIC TEST(S) Sporotrichosis (Sporothrix schenckii) Lymphocutaneous infection (ascending lymphangitis) Disseminated infection Direct inoculation AIDS, glucocorticoids Talaromycosis (Talaromyces marneffei) Pneumonia Disseminated infection (skin, bone, mucosal surfaces) Healthy individuals AIDS, glucocorticoids, AAbs to IFN-γ Adiaspiromycosis (Emmonsia crescens, E. parva) Pneumonia Occupational dust exposure Culture: nonculturable Histologic examination: thick-walled adiaspore within granuloma Emergomycosis (Emergomyces africanus, E. pasteurianus) Disseminated infection (lungs, skin) AIDS, SOT Culture of infected tissue Histologic examination of tissue: yeast with narrow-based budding Biomarkers: Histoplasma Ag can be positive Chromoblastomycosis (Fonsecaea pedrosoi, F. monophora) Skin and subcutaneous tissue infections Healthy individuals Culture of infected tissue Histologic examination of scrapings (KOH) or tissue (GMS): sclerotic bodies (pathognomonic) Other Fungi    Pneumocystosisf Pneumonia Disseminated infection (eye, CNS, skin, gastrointestinal tract) AIDS, glucocorticoids, BTK inhibition AIDS (Pneumocystis jirovecii) aA. nidulans is seen almost exclusively in chronic granulomatous disease. bGMS or PAS stains. cSome Candida species form pseudohyphae. dTrichosporon species are yeast-like fungi that also generate septate hyphae and arthroconidia. eCoccidioides is a laboratory hazard. It is important to notify the microbiology laboratory if this infection is suspected. fPneumocystis is present in cyst and trophozoite forms. Abbreviations: AAbs, autoantibodies; ABPA, allergic bronchopulmonary aspergillosis; Ag, antigen; BAL, bronchoalveolar lavage; BDG, β-D-glucan; BTK, Bruton’s tyrosine kinase; CF, complement fixation; CNS, central nervous system; CSF, cerebrospinal fluid; GM, galactomannan; GM-CSF, granulocyte-macrophage colony-stimulating factor; GMS, Gomori methenamine silver; H&E, hematoxylin and eosin; HSCT, hematopoietic stem cell transplantation; ICU, intensive care unit; ID, immunodiffusion; IFN-γ, interferon γ; JAK, Janus kinase; KOH, potassium hydroxide; PAS, periodic acid–Schiff; PCR, polymerase chain reaction; SOT, solid organ transplantation; TNF-α, tumor necrosis factor α. activation, and Th17 cell differentiation upon fungal invasion. Human inherited CARD9 deficiency causes severe mucocutaneous and inva­ sive fungal disease and is the only known primary immunodeficiency to feature fungus-specific infection susceptibility without a predispo­ sition to other infections, autoimmunity, allergy, or cancer. Notably, CARD9-deficient patients develop infections by certain fungi in cer­ tain tissues, including (1) chronic mucocutaneous candidiasis linked to defective interleukin (IL) 17 responses; (2) infections of the central nervous system (CNS) caused by Candida (but also by Aspergillus and phaeohyphomycetes) linked to impaired microglial-neutrophilic responses; and (3) deep dermatophytosis. Thus, the clinical use of SYK inhibitors for autoimmunity and cancer may cause opportunistic fun­ gal disease. Human inherited deficiency of Toll-like receptor (TLR) sig­ naling does not lead to spontaneous fungal disease, yet polymorphisms in TLR pathway molecules may increase the risk of fungal disease in critically ill or immunosuppressed persons, and TLR stimulation may boost protective CLR immunity, as has been shown with the TLR7 agonist imiquimod in chromoblastomycosis. The development of clinically relevant animal models of mycoses and the phenotypic characterization of fungal infections that develop in patients with primary immunodeficiencies and in recipients of immune pathway-targeting biologics have led to the delineation of fungus-, cell-, and tissue-specific requirements for antifungal host defense (Fig. 217-1). At the mucosal interface, IL-17-producing lymphoid cells play a critical role in protection by driving epithelial cell production of anti­ microbial peptides that restrict mucosal Candida invasion. Indeed, AIDS patients are at risk for mucosal—but not invasive—candidiasis. Concordantly, inherited deficiency of IL-17 signaling caused by muta­ tions in IL17F, IL17RA, IL17RC, or TRAF3IP2 (encoding the IL-17 receptor adaptor ACT1) or pharmacologic inhibition of IL-17 signal­ ing by biologics that target IL-12p40, IL-23p19, IL-17A, IL-17A/IL17F, or IL-17RA cause mucosal—but not invasive—candidiasis. Other conditions that underlie a predisposition to chronic mucocutaneous

(Continued) Culture of tissue (diagnostic) Histologic examination: cigar-shaped yeast, often with surrounding asteroid body Culture of tissue (diagnostic) Histologic examination of tissue: yeasts with transverse septa Biomarkers: GM is often positive Culture: nonculturable Histologic examination (gold standard): special (GMS, Diff-Quik) or immunofluorescence stains Biomarkers/other tests: BDG (nonspecific); BAL fluid PCR (sensitive; can be positive in colonized individuals) CHAPTER 217 Pathogenesis, Diagnosis, and Treatment of Fungal Infections candidiasis include primary immunodeficiencies due to mutations in STAT3, STAT1, DOCK8, JNK1, IRF8, RORC, and CARD9, all of which impair Th17 cells, as well as thymoma and autoimmune polyendocri­ nopathy–candidiasis–ectodermal dystrophy (APECED), which feature autoantibodies to IL-17A, IL-17F, and IL-22. In APECED, exacerbated

T cell–derived interferon γ (IFN-γ)/STAT1 responses disrupt the integrity of the oral epithelial barrier, thereby promoting mucosal fungal inva­ sion and infection; remission of candidiasis can be achieved with JAK inhibition in APECED and STAT1 gain of function (GOF). Of note, vaginal candidiasis (unlike oropharyngeal and esophageal candidiasis) develops in the setting of antibiotic treatment, not AIDS or IL-17–targeted biologics; this observation underscores the role of the microbiota in fungal control at the vaginal—but not the oral—mucosa. On the other hand, neutrophils—but not lymphocytes—are criti­ cal for control of invasive infections caused by Aspergillus (and other inhaled molds) and Candida (Fig. 217-1). Indeed, patients with che­ motherapy-induced neutropenia and patients undergoing allogeneic hematopoietic stem cell transplantation are at risk for invasive aspergil­ losis and candidiasis. Both oxidative and nonoxidative burst–dependent effector mechanisms are operational within neutrophils for fungal killing. Inherited deficiency in neutrophil superoxide generation due to mutations in the six subunits of the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase complex causes chronic granulomatous disease, a prototypic primary immunodeficiency that carries a lifetime risk for invasive aspergillosis of ~40%; infrequently (i.e., in <5% of cases, mostly infants), chronic granulomatous disease predisposes to invasive candidiasis. The unexpected development of invasive mold infections in recipients of Bruton’s tyrosine kinase (BTK) inhibitors has recently uncovered the critical role of BTK in promoting myeloid phagocyte-dependent antifungal effector functions. Host defenses against fungi that reside within macrophages, such as Cryptococcus, Pneumocystis, and endemic dimorphic fungi, depend on the interplay of IFN-γ–producing lymphoid cells and IL-12–producing macrophages that enable intramacrophagic fungal killing (Fig. 217-1).

IL6R IL23R Th17 cell Th1 cell Neutrophil JAK2 TYK2 STAT3 JAK2 TYK2 RORγT CXCL1 Nucleus fc T cell CXCR2 AAbs NADPH p22phox gp91phox IL-17A/IL-17F AAbs p67phox p47phox p40phox IL-17F IL-17A IL-22 IL-22R1 IL-10Rβ IL-17RA IL-17RC NADP+ PART 5 Infectious Diseases Candida yeast and pseudohyphae
Multilobed nucleus STAT3 ACT1 Epithelial cells Neutrophil Lung FIGURE 217-1  Host defense against fungi. Left: Production of IL-17A, IL-17F, and IL-22 by Th17 cells, Tc17 cells, γδ T cells, and innate lymphoid cells confers protection from mucosal Candida invasion. STAT3 promotes Th17 differentiation via RORγt induction. IL-17A and IL-17F bind to IL-17RA and IL-17RC on epithelial cells and signal via ACT1 to produce antimicrobial peptides that inhibit fungal growth. IL-22 binds to its receptor on epithelial cells and activates STAT3 to mediate epithelial proliferation and repair. Middle: Activation of CXCR2+ neutrophils recruited from blood in the Aspergillus-infected lung enables assembly of the six subunits of NADPH oxidase and superoxide generation that promotes fungal killing. Production of reactive oxygen species by neutrophils is facilitated by recruited monocyte-derived and plasmacytoid dendritic cells via type I and type III IFNs and GM-CSF. Right: The interaction of Th1 cells with macrophages is protective against intramacrophagic endemic dimorphic fungi, Pneumocystis, and Cryptococcus. Upon fungal uptake, macrophages produce IL-12 that binds to its receptor on T cells and activates STAT4, with consequent release of IFNγ. IFN-γ binds to its receptor on macrophages and activates STAT1, thereby enabling fungal killing. TNF-α and GM-CSF are also critical for macrophage activation. AAbs, autoantibodies; IL, interleukin; IFN, interferon; JAK, Janus kinase; GM-CSF, granulocyte-macrophage colony-stimulating factor; NADPH, nicotinamide adenine dinucleotide phosphate; RORγt; RAR-related orphan receptor γ; SOD, superoxide dismutase; STAT, signal transducer and activator of transcription; TNF, tumor necrosis factor; TYK2, tyrosine kinase 2. Indeed, AIDS patients and those receiving glucocorticoids, which affect lymphocytes and macrophages both quantitatively and qualitatively, are at risk for severe infections by these fungi. Accordingly, inherited impair­ ment of the IL-12/IFN-γ signaling axis caused by mutations in IL12RB1, IFNGR1, IFNGR2, STAT1, IRF8, or GATA2 underlies susceptibility to severe infection by intramacrophagic fungi (and other intramacrophagic pathogens, such as mycobacteria and salmonellae). In addition, the IFN-γ–targeting monoclonal antibody emapalumab, JAK inhibitors that block IFN-γ–dependent cellular responses, and autoantibodies to IFN-γ predispose to infection with intramacrophagic fungi, as do biologics targeting tumor necrosis factor α (TNF-α) and autoantibodies to gran­ ulocyte-macrophage colony-stimulating factor (GM-CSF). The latter predisposing factors reveal the central role of these two Th1-associated cytokines—TNF-α and GM-CSF—in macrophage activation. Taken together, these observations show that the cellular and molec­ ular factors that drive protective antifungal immune responses vary greatly with the anatomic site of the infection, the offending fungus, and the patient population (Table 217-1). The growing body of data on human immunologic responses to fungi promises to inform precision medicine strategies for risk assessment, prophylaxis, immunotherapy, and vaccination of vulnerable patients.

Blood IFNγ CXCR2 Nucleus STAT4 STAT4 CXCL2 TYK2 JAK2 GM-CSF IL12Rβ1 IL12Rβ2 IFNγ IFN-λ Aspergillus conidia AAbs IL-12 IFNγ IFN-γR2 TNFα IFN-γR1 e– O2 – JAK2 STAT1 SOD JAK1 Phagosome STAT1 Phagosome H2O2 Yeast (Histoplasma, Cryptococcus) Macrophage IL-12 Nucleus GM-CSF ■ ■DIAGNOSIS The diagnostic modalities used for various fungal infections are out­ lined in Table 217-1 and are detailed in the chapters on specific myco­ ses that follow in this section. Definitive diagnosis of a fungal infection requires histopathologic identification of the fungus invading tissue with parallel culture of the fungus from the specimen. Certain fungi have dis­ tinctive morphologic features that facilitate diagnosis (Table 217-1). The stains most often used to identify fungi are periodic acid–Schiff (PAS) and Gomori methenamine silver (GMS). Candida, unlike other fungi, is visible on Gram-stained tissue smears. Hematoxylin and eosin stains define accompanying histologic features of fungal disease (granuloma formation, angioinvasion, necrosis) but are insufficient to reliably identify fungi in tissue. A positive India ink stain of cerebrospinal fluid (CSF) is diagnostic for cryptococcosis. Most laboratories use calcofluor white staining coupled with fluorescence microscopy to identify fungi in fluid specimens. A positive fungal culture of blood or tissue may signify either patient colonization or lab contamination instead of true infection, with the most likely scenario depending on the fungus and the anatomic site. In blood, Candida can be detected with any of the widely used automated blood culture systems, but the lysiscentrifugation technique increases the sensitivity of blood cultures for

both Candida and other less common fungi (e.g., Histoplasma). Matrixassisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) is now used extensively for detection and specia­ tion of fungi recovered from culture. The several available fungal-antigen and serologic tests vary in sen­ sitivity and specificity. The most reliable of these tests are the antibody to Coccidioides, Histoplasma antigen, and cryptococcal polysaccharide antigen. Serologic tests are also available for other endemic dimorphic fungi (Table 217-1). The galactomannan test—especially in the bron­ choalveolar lavage fluid—is useful for the diagnosis of aspergillosis; however, false-negative results are common, particularly in patients receiving antifungal prophylaxis, and false-positive results may occur with other fungal infections. The β-glucan test has a high negative pre­ dictive value for invasive candidiasis but lacks specificity. T2 magnetic resonance is now approved by the U.S. Food and Drug Administration (FDA) for detection of Candida in blood. Several polymerase chain reaction and nucleic acid hybridization assays exist for fungal detection but are not standardized and are not widely used in the clinic. ■ ■ANTIFUNGAL DRUGS This section provides a brief overview of available agents for the treat­ ment of fungal infections. Drug regimens and schedules are detailed in the chapters on specific mycoses that follow in this section. Since fungal organisms, like human cells, are eukaryotic, the identification of drugs that selectively kill or inhibit fungi but that are not toxic to human cells poses challenges. Indeed, far fewer antifungal than anti­ bacterial agents have been introduced into clinical medicine. Early initiation of appropriate antifungal therapy is a critical determi­ nant of favorable outcome, as has been shown for candidemia, aspergil­ losis, and mucormycosis. In addition, source control of the infection is important—e.g., with removal of the central venous catheter in candi­ demia, drainage of abdominal abscesses in intraabdominal candidiasis, and surgical debridement of sinus tissue in mucormycosis. Moreover, an essential factor in a favorable prognosis in patients with opportunistic mycoses is the achievement of immune reconstitution—e.g., with neu­ trophil recovery, tapering of glucocorticoids or other immunosuppres­ sive drugs, or initiation of combination antiretroviral therapy in AIDS. ■ ■AMPHOTERICIN B The advent of amphotericin B (AmB) in the 1950s revolutionized the treatment of deep-seated mycoses. Before the availability of AmB, cryptococcal meningitis and other disseminated fungal infections were nearly always fatal. AmB remains the broadest-spectrum antifungal agent. Its fungicidal mechanism of action involves forming extramem­ branous sponge-like aggregates that extract fungal ergosterol from lipid bilayers. AmB remains the preferred antifungal agent for the treatment of mucormycosis and fusariosis and for induction therapy for cryptococ­ cal meningitis and disseminated infections caused by endemic dimor­ phic fungi. However, AmB has several limitations, including lack of a licensed oral formulation and significant toxicity from the intravenous preparations, primarily renal and infusion-related (fever, chills, throm­ bosis). The introduction of lipid AmB formulations has ameliorated these toxicities, and the lipid formulations have largely replaced the original deoxycholate formulation in resource-rich settings. In develop­ ing countries, AmB deoxycholate is still widely used because of the high cost of the lipid formulations. The two lipid formulations commonly used in the clinic are liposomal AmB and AmB lipid complex, which exhibit comparable efficacy, toxicity, and tissue penetration profiles. ■ ■AZOLES Azoles offer important advantages over AmB, such as the availability of oral and IV formulations and a lack of renal toxicity. The mechanism of action of azoles involves inhibition of lanosterol 14α-demethylase and ergosterol synthesis in the fungal cell membrane, with a consequent accumulation of toxic sterol intermediates and growth arrest. Unlike AmB, azoles are considered fungistatic. Fluconazole  Fluconazole plays an important role in the treatment of several fungal infections. Its major advantages are the availability of oral and IV formulations, a long half-life, penetration into most body

fluids (ocular fluid, CSF, urine), and minimal toxicity. This drug rarely causes liver toxicity; high doses may result in alopecia, dry mouth, and a metallic taste. Notably, the administration of even low doses of fluco­ nazole to pregnant women for the treatment of vaginal candidiasis was recently linked to miscarriage and stillbirth. Fluconazole has no activ­ ity against molds and most endemic dimorphic fungi and is less active than the newer azoles against C. glabrata and C. krusei.

Fluconazole is the preferred agent for the treatment of coccidioidal meningitis, although relapses may occur despite therapy. Fluconazole is also used as consolidation and maintenance therapy for cryptococcal meningitis and for the treatment of mucosal candidiasis. It is used for treating candidemia in patients who are not critically ill or immuno­ suppressed; in these patients, fluconazole was found to be as effica­ cious as AmB. Because of increasing rates of azole-resistant Candida strains, echinocandin treatment is preferred, which is then replaced by fluconazole once a susceptible Candida species is recovered. Flu­ conazole is effective as prophylaxis in recipients of high-risk liver and allogeneic bone marrow transplants, although many centers now use posaconazole in neutropenic patients, given its added activity against molds. Fluconazole prophylaxis in leukemic patients, in AIDS patients with low CD4+ T-cell counts, and in patients on surgical intensive care units is controversial. Itraconazole  Itraconazole is available in oral (capsule, suspension) and IV formulations and has broader antifungal activity—i.e., against molds and endemic dimorphic fungi—than fluconazole. Itraconazole is the drug of choice for mild to moderate histoplasmosis and blasto­ mycosis and has also been used to treat chronic coccidioidomycosis, phaeohyphomycosis, sporotrichosis, and mucocutaneous mycoses such as oropharyngeal candidiasis, tinea versicolor, tinea capitis, and onychomycosis. Although it is approved by the FDA for use in febrile neutropenic patients, most centers now use newer azoles in such patients. Disadvantages of itraconazole include its poor CSF penetra­ tion, the use of cyclodextrin in its oral suspension and IV formulation, and its variable level of absorption in the capsule form, which requires monitoring of blood levels in patients receiving capsules for dis­ seminated mycoses. A new formulation of itraconazole, called SUBAitraconazole (for “super-bioavailability”), which results in improved absorption and less variable plasma levels, has recently been approved by the FDA. Itraconazole is a potent CYP3A4 inhibitor; this character­ istic leads to significant drug interactions. The drug causes hepatotox­ icity and cardiac toxicity that may manifest as congestive heart failure. CHAPTER 217 Pathogenesis, Diagnosis, and Treatment of Fungal Infections Voriconazole  Voriconazole is also available in oral and IV formula­ tions, has far broader antifungal activity than fluconazole (including

C. glabrata, C. krusei, Aspergillus, Scedosporium, and endemic dimorphic fungi—but not Mucorales), and penetrates into most body fluids (ocular fluid, CSF). It is the preferred agent for the treatment of aspergillosis and also has been used to treat scedosporiosis and as step-down (but not pri­ mary) therapy for coccidioidomycosis, blastomycosis, and histoplasmo­ sis. Voriconazole is considerably more expensive than fluconazole, and as with itraconazole, its use is associated with numerous interactions with drugs typically used in patients at risk for fungal infections. Hepatotoxic­ ity, visual disturbances, and skin rashes (including photosensitivity) are relatively common, and long-term use requires skin cancer surveillance. A unique toxicity of voriconazole among azoles is fluorosis-associated periostitis. It is crucial to monitor drug levels because (1) voriconazole is metabolized in the liver by CYP2C9, CYP3A4, and CYP2C19; and (2) human genetic variation in CYP2C19 activity exists and can lead to sig­ nificant interpatient variability in drug levels. Dosages should be reduced in patients with hepatic, but not renal, failure; however, because the IV formulation is prepared in cyclodextrin, it should be given with caution to patients with severe renal failure. Posaconazole  Posaconazole has broader activity than voricon­ azole, including activity against Mucorales. Both oral (suspension, tablet) and IV formulations are available. Posaconazole is approved by the FDA for antifungal prophylaxis in neutropenic leukemic patients and allogeneic hematopoietic stem cell transplant recipients as well as for treatment of oropharyngeal candidiasis, including infections refractory

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218 Histoplasmosis

to fluconazole or itraconazole. Posaconazole has been reported to be effective salvage therapy for aspergillosis, mucormycosis, fusariosis, cryptococcosis, histoplasmosis, and coccidioidomycosis, although controlled clinical trials are lacking. The tablet formulation is not ham­ pered by the suboptimal absorption that occurs with the suspension; the tablet also results in higher and more reliable blood levels of the drug. Posaconazole is less hepatotoxic than voriconazole and does not cause the skin, visual, or bone toxicity that occurs with voriconazole. However, the use of posaconazole is linked to significant P450-related drug interactions and can lead to pseudohyperaldosteronism, which may manifest with hypokalemia, edema, and/or hypertension.

Isavuconazole  Isavuconazole is available in oral and IV formula­ tions and has broad antifungal activity similar to that of posaconazole. Isavuconazole is approved by the FDA for treatment of aspergillosis (on the basis of a randomized controlled trial that found it noninferior to voriconazole) and mucormycosis (on the basis of an open-label, non­ comparative trial of 37 patients). Isavuconazole appears to be less hepa­ totoxic than voriconazole; it does not cause skin, bone, or visual toxicity, and it causes fewer P450-associated drug interactions than voriconazole. Oteseconazole  The newest azole—a tetrazole—oteseconazole, is approved by the FDA for the treatment of recurrent vulvovaginal candidiasis. It has potent activity against Candida (including azoleresistant strains) species and endemic dimorphic fungi. Its long halflife allows for once-weekly dosing following induction. Oteseconazole does not appear to cause hepatotoxicity or P450-associated drug inter­ actions, but it is contraindicated in women of reproductive potential as it may cause fetal ocular toxicity. PART 5 Infectious Diseases ■ ■ECHINOCANDINS The echinocandins include the FDA-approved drugs caspofungin, anidulafungin, micafungin, and rezafungin, which are available solely as an IV formulation and inhibit β-1,3-glucan synthase, an enzyme that is crucial for fungal cell-wall synthesis but is not a constituent of human cells. Rezafungin, a new generation echinocandin, has a prolonged half-life that allows for weekly dosing with front-loading. The different echinocandins have comparable efficacy, toxicity, and tissue penetration profiles; are fungicidal for Candida and fungistatic for Aspergillus; and have no activity against other molds, Cryptococcus, or endemic dimor­ phic fungi. Their most common use to date is in candidal infections. These drugs offer three major advantages: minimal toxicity, minimal drug interactions, and activity against all Candida species. The minimal inhibitory concentrations (MICs) of echinocandins are higher against Candida parapsilosis than against other Candida species, but the higher MICs do not translate into less clinical efficacy against this species. In controlled trials, caspofungin was as efficacious as AmB against candidemia and invasive candidiasis and as efficacious as fluconazole against candidal esophagitis. Caspofungin has also been efficacious as salvage therapy for aspergillosis. Anidulafungin is approved by the FDA as therapy for candidemia in nonneutropenic patients and for Candida esophagitis, abdominal infection, and peritonitis. In con­ trolled trials, anidulafungin was noninferior and possibly superior to fluconazole against candidemia and invasive candidiasis and was as efficacious as fluconazole against candidal esophagitis. Micafungin is approved by the FDA for the treatment of candidal esophagitis and candidemia and for antifungal prophylaxis in hematopoietic stem cell transplantation. Moreover, micafungin yielded favorable results when used for the treatment of invasive aspergillosis and candidiasis in open-label trials. Rezafungin was recently approved by the FDA for the treatment of candidemia and invasive candidiasis. ■ ■IBREXAFUNGERP Ibrexafungerp is an oral antifungal agent that belongs to a new class of glucan synthase inhibitors called triterpenoids and has potent activity against Candida species. Ibrexafungerp has been approved by the FDA for the treatment of vulvovaginal candidiasis and recurrent vulvovaginal candidiasis as it maintains its antifungal efficacy at low pH that is often observed in the vaginal mucosa. Its use is contraindicated in pregnant and lactating women.

■ ■FLUCYTOSINE (5-FLUOROCYTOSINE) Flucytosine use has diminished as newer antifungal drugs have been developed. Its mechanism of action involves intrafungal conversion to 5-fluorouracil, which inhibits fungal DNA synthesis. The use of flucytosine in combination with AmB as induction therapy for cryp­ tococcal meningitis is based on the drugs’ synergistic interaction and favorable flucytosine CSF penetration that promotes a rapid decline of the cryptococcal burden in the CSF. Flucytosine is also used in combination with AmB for the treatment of candidal meningitis and endocarditis, although comparative trials with AmB monotherapy are lacking. Flucytosine monotherapy is not recommended as it is associ­ ated with the development of resistance. Flucytosine can cause bone marrow suppression and liver toxicity, which are intensified when the drug is used with AmB. ■ ■TERBINAFINE Terbinafine inhibits squalene epoxidase and ergosterol synthesis, is used for onychomycosis and ringworm infection, and is as effective as itraconazole in both conditions. Although active against other fungi, terbinafine penetrates poorly into tissues beyond the skin and nails and therefore is not preferred for systemic mycoses. Terbinafine carries a risk for hepatotoxicity. ■ ■TOPICAL ANTIFUNGAL AGENTS A detailed discussion of topical agents for mucocutaneous mycoses is beyond the scope of this chapter; the reader is referred to Chap. 225 and the dermatology literature. Azoles such as clotrimazole, micon­ azole, and ketoconazole are often used topically to treat common cutaneous mycoses as well as oropharyngeal and vaginal candidiasis. In vaginal candidiasis, oral fluconazole given once has the advantage of not requiring repeated intravaginal application. The polyenes nystatin and AmB have also been used topically for oropharyngeal and vaginal candidiasis. Agents from other classes that are used to treat these con­ ditions include ciclopirox, haloprogin, terbinafine, naftifine, tolnaftate, and undecylenic acid. ■ ■FURTHER READING Bennett JE: Introduction to mycoses, in Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases, 9th ed, JE Bennett et al (eds). Philadelphia, Elsevier Saunders, 2020, pp 3082–3086. Lionakis MS et al: Immune responses to fungal pathogens and thera­ peutic prospects. Nat Rev Immunol 23:433, 2023. Pappas PG et al: Clinical mycology today: A synopsis of the mycoses study group education and research consortium (MSGERC) second biennial meeting, September 27–30, 2018, Big Sky, Montana, a pro­ posed global research agenda. Med Mycol 58:569, 2020. Chadi A. Hage, L. Joseph Wheat

Histoplasmosis ■ ■ETIOLOGY Histoplasma capsulatum, a thermal dimorphic fungus, is the etiologic agent of histoplasmosis. In most endemic areas in North America, H. capsulatum var. capsulatum is the causative agent. In Central and South America, histoplasmosis is common and is caused by genetically different clades of H. capsulatum var. capsulatum. In Africa, H. capsulatum var. duboisii is also found. Yeasts of var. duboisii are larger than those of var. capsulatum. Mycelia—the naturally infectious form of Histoplasma—have a characteristic appearance, with microconidial and macroconidial forms (Fig. 218-1). Microconidia are oval and are small enough

FIGURE 218-1  Spiked spherical conidia of H. capsulatum (lacto-phenol cotton blue stain) grown in the laboratory at room temperature. (2–4 μm) to reach the terminal bronchioles and alveoli. Shortly after infecting the host, mycelia transform into the yeasts that are found inside macrophages and other phagocytes. The yeast forms are characteristically small (2–5 μm), with occasional narrow budding (Fig. 218-2). In the laboratory, mycelia are best grown at room tem­ perature, whereas yeasts are grown at 37°C on enriched media. A B FIGURE 218-2  A. Small (2–5 μm) narrow budding yeasts of H. capsulatum from bronchoalveolar lavage fluid (Grocott’s methenamine silver stain). B. Intracellular yeasts of H. capsulatum within an alveolar macrophage from a patient with AIDS and disseminated histoplasmosis (Giemsa stain).

■ ■EPIDEMIOLOGY Histoplasmosis is the most prevalent endemic mycosis in North America. Although this fungal disease has been reported throughout the world, its endemicity is particularly notable in the Ohio and Mississippi river valleys of North America and in certain parts of Mexico, Central and South America (Brazil), Africa, and Asia. Histo­ plasmosis is increasingly reported outside of the traditionally known endemic areas. The geographic distribution of histoplasmosis is related to the humid and acidic nature of the soil in the endemic areas. Soil enriched with bird or bat droppings promotes the growth and sporula­ tion of Histoplasma. Disruption of soil containing the organism leads to aerosolization of the microconidia and exposure of humans nearby. Activities associated with high-level exposure and high infection attack rate include spelunking, excavation, cleaning of chicken coops, demoli­ tion and remodeling of old buildings, and cutting of dead trees. Most cases seen outside of highly endemic areas represent imported disease, e.g., in Europe, histoplasmosis is diagnosed fairly often, mostly in emigrants from or travelers to endemic areas on other continents. The epidemiology of histoplasmosis is changing as a result of global climate changes and with the continued expansion of at-risk populations and the acceleration of intercontinental and international travel that brings this infection to areas of the world that are not known to be endemic. The population at risk for histoplasmosis continues to grow as a result of increasing numbers of patients receiving immunosuppressive thera­ pies for autoimmune disorders, cancers, and organ transplants.

■ ■PATHOGENESIS AND PATHOLOGY Infection follows inhalation of microconidia (Fig. 218-1). Once they reach the alveolar spaces, microconidia are rapidly recognized and engulfed by alveolar macrophages, where they transform into yeasts (Fig. 218-2), a process that is integral to the pathogenesis of histo­ plasmosis and is dependent on the availability of calcium and iron inside the phagocytes. The yeasts are capable of evading the immune system and multiplying inside resting macrophages. Neutrophils and then lymphocytes are attracted to the site of infection. Before the development of cellular immunity, yeasts use the phagosomes as a vehicle for translocation to local draining lymph nodes, whence they spread hematogenously throughout the reticuloendothelial system. Effective cellular immunity develops ~2 weeks after infection. T cells produce interferon-γ to assist the macrophages in killing the organism and controlling the progression of disease. Interleukin 12 and tumor necrosis factor α (TNF-α) play an essential role in cellular immunity to H. capsulatum. In the immunocompetent host, macrophages, lym­ phocytes, and epithelial cells eventually organize and form granulomas that contain the organisms. These granulomas typically fibrose and calcify; calcified lung nodules, mediastinal lymph nodes, and hepatosplenic calcifications are frequently found in healthy individuals from endemic areas. In immunocompetent hosts, infection with H. capsulatum confers protective immunity to reinfection. In patients with impaired cellular immunity, the infection is not properly contained and can disseminate throughout the reticuloendothelial system. Progressive disseminated histoplasmosis (PDH) can involve multiple organs, most commonly the lungs, bone marrow, spleen, liver (Fig. 218-3), adrenal glands, and mucocutaneous membranes. Unlike latent tuberculosis, inactive histoplasmosis does not reactivate. In patients with mildly impaired immune systems, active infection may smolder and eventu­ ally worsen with further decline in immunity. CHAPTER 218 Histoplasmosis 3 µm Structural lung disease (e.g., emphysema) impairs the clearance of pulmonary histoplasmosis leading to the development of chronic pulmonary disease. This chronic process is characterized by progres­ sive inflammation, tissue necrosis, and fibrosis mimicking cavitary tuberculosis. 5 µm ■ ■CLINICAL MANIFESTATIONS The clinical spectrum of histoplasmosis ranges from asymptomatic infection to life-threatening illness. The attack rate and the extent and severity of the disease depend on the intensity of exposure, the immune status of the exposed individual, and the underlying lung architecture of the host.

FIGURE 218-3  Intracellular yeasts (arrows) of H. capsulatum in a liver biopsy specimen (hematoxylin and eosin stain) from a patient who developed progressive disseminated histoplasmosis while receiving anti–tumor necrosis factor therapy for rheumatoid arthritis. PART 5 Infectious Diseases In immunocompetent individuals with low-level exposure, most Histoplasma infections are either asymptomatic or mild and self-limited. Of adults residing in endemic areas, up to 75% have immunologic and/or radiographic evidence of previous infection without clinical manifestations. Asymptomatic lung nodules representing controlled histoplasmosis are frequently found on chest computed tomography (CT) scans obtained during screening for lung cancer in smokers from endemic areas. When symptoms of acute histoplasmosis develop, they usually appear 1–4 weeks after exposure. Heavy exposure leads to a flulike illness with fever, chills, sweats, headache, myalgia, anorexia, dry cough, dyspnea, and chest pain. Chest radiographs usually show signs of pneumonitis with prominent hilar or mediastinal adenopathy. Pulmonary infiltrates may be focal with light exposure or diffuse with heavy exposure. Rheumatologic symptoms of arthralgia or arthritis, often associated with erythema nodosum, occur in 5–10% of patients with acute histoplasmosis. Pericarditis may also develop. These mani­ festations represent inflammatory responses to the acute pulmonary infection rather than extrapulmonary spread. Affected hilar or medi­ astinal lymph nodes may undergo necrosis and coalesce to form large mediastinal masses that can cause compression of great vessels, proximal airways, and the esophagus. These necrotic lymph nodes may also rupture and create fistulas between mediastinal structures (e.g., bronchoesophageal fistulae). PDH is typically seen in immunocompromised individuals, who account for ~70% of cases. Common risk factors include AIDS (CD4+ T-cell count, <200/μL), extremes of age, the administration of immunosuppressive medications to prevent or treat rejection fol­ lowing transplantation (e.g., prednisone, mycophenolate, calcineurin inhibitors), and the use of methotrexate, anti-TNF-α agents, and other biologic response modifiers for autoimmune disorders. PDH may also occur in healthy individuals, some of whom may have rare undiag­ nosed genetic immunodeficiencies of the relevant immune pathways (IFN-γ and TNF-α). Workup for these conditions should be considered in healthy subjects with PDH. The clinical spectrum of PDH ranges from an acute, rapidly fatal course—with diffuse interstitial or reticulonodular lung infiltrates

causing respiratory failure, shock, coagulopathy, and multiorgan failure—to a subacute or chronic course with a focal organ distribution. Common manifestations include fever, weight loss, hepatospleno­ megaly, and thrombocytopenia. Other findings may include meningitis or focal brain lesions, ulcerations of the oral mucosa, gastrointestinal ulcerations and bleeding, and adrenal insufficiency. Prompt recogni­ tion of this devastating illness is of paramount importance in patients with severe manifestations or with underlying immunosuppression, especially those due to AIDS (Chap. 208) or anti-TNF therapy. Chronic cavitary histoplasmosis is seen in smokers who have structural lung disease (e.g., bullous emphysema). This chronic ill­ ness is characterized by productive cough, dyspnea, low-grade fever, night sweats, and weight loss. Chest radiographs usually show upperlobe infiltrates, cavitation, and pleural thickening—findings resem­ bling those of tuberculosis. Without treatment, the course is slowly progressive. Fibrosing mediastinitis is an uncommon but serious complication of histoplasmosis. In certain patients, acute infection is followed for unknown reasons by progressive fibrosis around the hilar and medi­ astinal lymph nodes, encasing mediastinal structures with potentially devastating consequences. Major manifestations include superior vena cava syndrome, obstruction of pulmonary vessels, and airway obstruc­ tion. Patients may experience recurrent pneumonia, hemoptysis, or respiratory failure. Fibrosing mediastinitis is fatal in up to one-third of cases. In healed histoplasmosis, calcified mediastinal nodes or lung paren­ chymal nodules may erode through the walls of the airways and cause hemoptysis and expectoration of calcified material. This condition is called broncholithiasis. The clinical features and management of histoplasmosis caused by the genetically different clades in Central and South America are similar to those of the disease in North America. African histoplasmo­ sis caused by var. duboisii is clinically distinct and is characterized by frequent skin and bone involvement. ■ ■DIAGNOSIS Recommendations for the diagnosis and treatment of histoplasmosis are summarized in Table 218-1. Once suspected, the diagnosis of histoplasmosis is usually straightforward as many diagnostic tools are now available in the United States. This is not the case in resourcelimited endemic regions of Central America, South America, and Africa, where the diagnosis is often delayed, with consequently poor outcomes. Fungal culture remains the gold standard diagnostic test for his­ toplasmosis. However, culture results may not be known for up to 1 month, and cultures are often negative in less severe cases. Cultures are positive in ~75% of patients with PDH and chronic pulmonary histo­ plasmosis. Cultures of bronchoalveolar lavage (BAL) fluid are positive in about half of patients with acute pulmonary histoplasmosis causing diffuse infiltrates and hypoxemia. In PDH, the culture yield is highest for BAL fluid, bone marrow aspirate, and blood. Cultures of sputum or bronchial washings are usually positive in chronic pulmonary his­ toplasmosis. Cultures are typically negative, however, in other forms of histoplasmosis. Fungal stains of cytopathology or biopsy materials showing struc­ tures resembling Histoplasma yeasts are helpful in the diagnosis of PDH, yielding positive results in about half of cases. Yeasts can be seen in BAL fluid (Fig. 218-2) from patients with diffuse pulmonary infiltrates, in bone marrow biopsy samples, and in biopsy specimens of other involved organs (e.g., liver, adrenal glands). Occasionally, yeasts are seen within circulating phagocytes on blood smears from patients with severe PDH. However, staining artifacts and other fungal elements sometimes stain positively and may be misidentified as Histo­ plasma yeasts. Culture and pathology are no longer performed in most patients because diagnosis is more often established by antigen detec­ tion and/or serology, more rapidly and without subjecting the patient to invasive procedures. The detection of Histoplasma antigen in body fluids is extremely use­ ful in the diagnosis of PDH and acute diffuse pulmonary histoplasmosis.

TABLE 218-1  Recommendations for the Diagnosis and Treatment of Histoplasmosis TYPE OF HISTOPLASMOSIS DIAGNOSTIC TESTS TREATMENT RECOMMENDATIONS COMMENTS Acute pulmonary, mild to moderate with no improvement by the time of diagnosis Histoplasma antigen (BAL fluid, serum, urine) Cytopathology and fungal culture of BAL fluid Histoplasma serology (ID and CF), (EIA): IgG and IgM Acute pulmonary, severe illness, respiratory failure (ARDS) Histoplasma antigen (BAL fluid, serum, urine) Cytopathology and fungal culture of BAL fluid Histoplasma serology (ID and CF), (EIA): IgG and IgM Chronic/cavitary pulmonary Histoplasma serology (ID and CF), (EIA): IgG and IgM Fungal culture of sputum or BAL fluid Progressive disseminated Histoplasma antigen (BAL fluid, serum, urine) Histoplasma serology (ID and CF), (EIA): IgG and IgM Fungal culture of blood or bone marrow aspirate Cytopathology on biopsy of affected organ Central nervous system Histoplasma antigen CSF Histoplasma serology (ID and CF), (EIA): IgG and IgM Fungal culture of CSF Abbreviations: AmB, amphotericin B; BAL, bronchoalveolar lavage; CF, complement fixation; CSF, cerebrospinal fluid; EIA, enzyme immunoassay; ID, immunodiffusion; MRI, magnetic resonance imaging. The sensitivity of this method is >95% in patients with PDH and >80% in patients with severe acute pulmonary histoplasmosis resulting from heavy exposure, if both urine and serum are tested. Antigen levels correlate with severity of illness in PDH and can be used to follow dis­ ease progression, as levels predictably decrease with effective therapy. Increasing antigen levels also predict relapse. Histoplasma antigen can be detected in cerebrospinal fluid from patients with Histoplasma men­ ingitis and in BAL fluid from those with pulmonary histoplasmosis. Lateral flow antigen detection allows the diagnosis of histoplasmosis at the bedside as a point-of-care testing method, which might offer access to rapid testing in resource-limited areas of the world where traditional antigen testing might not be available. Cross-reactivity occurs with African histoplasmosis, blastomycosis, coccidioidomycosis, paracoc­ cidioidomycosis, talaromycosis, and rarely aspergillosis. Serologic tests, including immunodiffusion (ID), complement fixa­ tion (CF), and IgG and IgM enzyme immunoassay (EIA), are useful for the diagnosis of histoplasmosis, especially in immunocompetent patients. One month may be required for the detection of antibodies after the onset of infection by ID or CF, but antibodies may be detected earlier by more sensitive methods (EIA). IgM appears first then declines, and IgG appears later and increases during the infection. EIA for IgG and IgM antibodies provides a more accurate method for moni­ toring changes and antibody levels. Serologic tests are especially useful for the diagnosis of chronic pulmonary histoplasmosis. Limitations of ID and CF, however, include insensitivity early in the course of infec­ tion and reduced sensitivity in immunosuppressed patients, especially those receiving immunosuppression for organ transplantation. Also, antibodies may persist for several years after infection. Positive results from past infection may lead to a misdiagnosis of active histoplasmosis in a patient with another disease process. TREATMENT Histoplasmosis Treatment is indicated for all patients with PDH or chronic pulmo­ nary histoplasmosis as well as for most symptomatic patients with acute pulmonary histoplasmosis who have not improved by the time the diagnosis is established, especially in those with diffuse

Itraconazole (200 mg bid) for 6–12 weeks. Monitor renal and hepatic function. Patients with mild cases usually recover without therapy, but itraconazole should be considered if the patient’s condition is not already improving by the time the diagnosis is established. Lipid AmB (3–5 mg/kg per day) ±

glucocorticoids for 1–2 weeks; then itraconazole (200 mg bid) for 6–12 weeks. Monitor renal and hepatic function. Adjust itraconazole dose to achieve blood levels of 2–5 μg/mL. Itraconazole (200 mg bid) Adjust dose to achieve blood levels of 2–5 μg/mL for at least 12 months. Monitor hepatic function. Continue treatment until radiographic findings show no further improvement. Monitor for relapse after treatment is stopped. Lipid AmB (3–5 mg/kg per day) for 1–2 weeks; then itraconazole (200 mg bid); adjust dose to achieve blood levels of 2–5 μg/mL for at least 12 months. Monitor renal and hepatic function. Liposomal AmB is preferred, but the AmB lipid complex may be used because of cost. Chronic antifungal maintenance therapy may be necessary if the degree of immunosuppression cannot be substantially reduced. Monitor antigen levels in urine and serum during treatment to ensure proper response. Liposomal AmB (5 mg/kg per day) for 4–6 weeks; then itraconazole (200 mg bid) Adjust dose to achieve blood levels of 2–5 μg/mL for at least 12 months. Monitor renal and hepatic function. A longer course of lipid AmB is recommended because of the high risk of relapse. Itraconazole should be continued until CSF or MRI abnormalities clear. CHAPTER 218 infiltrates and difficulty breathing. In most other cases of pulmo­ nary histoplasmosis, treatment is not recommended, especially if the immune system of the host is intact and the degree of exposure is not heavy. The symptoms usually are mild, subacute, and not pro­ gressive, and the illness resolves without therapy. Treatment should be considered if the symptoms are not improving within a month. Histoplasmosis The preferred treatments for histoplasmosis (Table 218-1) include the lipid formulations of amphotericin B in severe cases and itraconazole in others. Liposomal amphotericin B is more effec­ tive and better tolerated than the deoxycholate formulation and is more effective in patients with AIDS and PDH. The deoxycholate formulation of amphotericin B is an alternative to a lipid formula­ tion for patients at low risk for nephrotoxicity and if liposomal amphotericin B is not available. Posaconazole and isavuconazole are alternatives for patients who cannot take itraconazole. Histoplasma may develop resistance to fluconazole and voriconazole, and they are not the preferred alternative to itraconazole, especially in immu­ nocompromised patients. In severe cases requiring hospitalization, a lipid formulation of amphotericin B is used first for 2 weeks, followed by itraconazole. In patients with meningitis, a lipid formulation of amphotericin B should be given for 4–6 weeks before switching to itraconazole. In immunosuppressed patients, the degree of immunosuppression should be reduced if possible, although immune reconstitution inflammatory syndrome (IRIS) may ensue. Antiretroviral treat­ ment improves the outcome of PDH in patients with AIDS and is recommended; however, whether antiretroviral treatment should be delayed to avoid IRIS is unknown. Blood levels of itraconazole should be monitored to ensure ade­ quate drug exposure, with target concentrations of the parent drug and its hydroxy metabolites measuring 2–5 μg/mL. Drug interac­ tions should be carefully assessed; itraconazole not only is cleared by cytochrome P450 metabolism but also inhibits cytochrome P450. This profile causes interactions with many other medications used routinely in organ transplant recipients. The duration of treatment for acute pulmonary histoplasmosis is 6–12 weeks, while that for PDH and chronic pulmonary his­ toplasmosis is at least 1 year. Antigen levels in urine and serum should be monitored during and for at least 1 year after therapy

112 - 219 Coccidioidomycosis

219 Coccidioidomycosis

for PDH. Stable or rising antigen levels suggest treatment failure or relapse and should raise concerns regarding proper intake of itraconazole (capsule formulation with food), adherence to treat­ ment, drug absorption, itraconazole serum concentrations, and drug interactions.

Lifelong itraconazole maintenance therapy is recommended for patients with persistently suppressed immunity but not for those with immune recovery—e.g., patients with AIDS who complete at least 1 year of itraconazole and show no signs of active infection including Histoplasma antigen levels <2 ng/mL, CD4+ T-cell count recovery to at least 150/μL (preferably >250/μL), and HIV sup­ pression with viral load <50 copies/mL on antiretroviral therapy. Similarly, maintenance therapy may not be necessary in other immunocompromised patients if the clinical findings have cleared, antigen levels are <2 ng/mL, and immunosuppression is substan­ tially reduced. Fibrosing mediastinitis, which represents a chronic fibrotic reac­ tion to past mediastinal histoplasmosis rather than an active infec­ tion, does not respond to antifungal therapy. Often patients with mediastinal granuloma have chronic or progressive courses and receive treatment with itraconazole and corticosteroids to reduce disease progression. ■ ■FURTHER READING Abdallah W et al: Diagnosis of histoplasmosis using the MVista Histo­ plasma galactomannan antigen qualitative lateral flow-based immu­ noassay: A multicenter study. Open Forum Infect Dis 8:ofab454, 2021. Azar MM et al: Clinical perspectives in the diagnosis and manage­ PART 5 Infectious Diseases ment of histoplasmosis. Clin Chest Med 38:403, 2017. Azar MM et al: Current concepts in the epidemiology, diagnosis, and management of histoplasmosis. Semin Respir Crit Care Med 41:13, 2020. Bahr NC et al: Histoplasmosis infections worldwide: Thinking outside of the Ohio River valley. Curr Trop Med Rep 2:70, 2015. Hage CA et al: A multicenter evaluation of tests for diagnosis of histo­ plasmosis. Clin Infect Dis 53:448, 2011. Neil M. Ampel

Coccidioidomycosis ■ ■DEFINITION AND ETIOLOGY Coccidioidomycosis, commonly known as Valley fever (see “Epidemi­ ology,” below), is caused by dimorphic soil-dwelling fungi of the genus Coccidioides. Genetic analysis has demonstrated the existence of at least two species, C. immitis and C. posadasii. These species are indis­ tinguishable with regard to the clinical disease they cause and their appearance on routine laboratory media, although C. posadasii may grow more quickly at higher temperatures. Thus, the organisms will be referred to simply as Coccidioides for the remainder of this chapter. ■ ■EPIDEMIOLOGY Coccidioidomycosis is confined to the Western Hemisphere between the latitudes of 40°N and 40°S. In the United States, areas of high ende­ micity include the southern San Joaquin Valley of California (hence the sobriquet “Valley fever”) and the south-central region of Arizona. However, infection may be acquired in other areas of the southwestern United States, including the other southern counties in California, southern Nevada, southwestern Utah, southern New Mexico, and western Texas. Cases acquired well outside the recognized endemic

areas, including in eastern Washington state and in northeastern Utah, have also been described, and a recent climate model suggests that the endemic range may expand further east and north over time. Outside the United States, coccidioidomycosis is endemic to northern Mexico as well as to localized regions of Central America. In South America, there are endemic foci in Colombia, Venezuela, northeastern Brazil, Paraguay, Bolivia, and north-central Argentina. The fungus is known to inhabit the native soils within the endemic regions. Because of the difficulty in isolating Coccidioides from envi­ ronmental sites, the precise characteristics of potentially infectious soil are not known, but alluvial alkaline deposits in regions of relative aridity with moderate temperature ranges are most closely associated with endemicity. In the United States, several outbreaks of coccidioi­ domycosis have been associated with soil from archeologic excava­ tions of Amerindian sites both within and outside of the recognized endemic region. When isolated from the soil, Coccidioides is found no more than 2–20 cm below the surface, nor is it usually isolated from cultivated soil. In addition to its saphrophytic soil phase, Coccidioides appears to have an endozoan life cycle, likely infecting burrowing rodents. This would suggest that the organisms are concentrated in these rodents and their burrows rather than extant in the soil. In endemic areas, most cases of human coccidioidomycosis occur without obvious soil or dust exposure. Climatic factors may increase the risk of infection. Periods of aridity following rainy seasons are associated with increases in the number of symptomatic infections. The number of reported cases of coccidioidomycosis has been steadily increasing over the past two decades. In 2019, a total of 20,003 cases were reported. The majority of these were either from Arizona or Cali­ fornia, in approximately equal numbers, with Nevada, New Mexico, and Utah reporting a small fraction of cases. The factors associated with these increases have not been elucidated but likely include an influx of older individuals without prior coccidioidal infection into endemic areas, construction activity, increased reporting, and chang­ ing climate. ■ ■PATHOGENESIS, PATHOLOGY, AND

IMMUNE RESPONSE The life cycle of Coccidioides is depicted in Fig. 219-1. On agar media and in the environment, Coccidioides organisms exist as filamentous molds or mycelia. Individual filaments (hyphae) elongate and branch, some growing upward. Portions of these aerial hyphae thicken and sep­ tate, and alternating cells then degenerate by autolysis, leaving barrelshaped viable spores called arthroconidia. Measuring approximately 2 μm by 5 μm, individual arthroconidia can easily dislodge from the hypha and become airborne. These may persist in the air for prolonged periods. When arthroconidia are inhaled by a susceptible host, their small size allows them to evade initial mechanical mucosal defenses and reach deep into the bronchial tree, where infection is initiated. Once within a susceptible host, the individual arthroconidia enlarge, become rounded, and develop internal septations. The resulting struc­ tures, called spherules, may attain sizes up to 200 μm and are unique to Coccidioides. The septations encompass uninuclear elements called endospores. Spherules may rupture and release packets of endospores that can themselves develop into new spherules, thus propagating infection locally. If returned to artificial media or the soil, the fungus reverts to its mycelial stage. Clinical observations and data from animal studies strongly sup­ port the critical role of a robust cellular immune response in the host’s control of coccidioidomycosis. Necrotizing granulomas containing spherules are typically identified in patients with resolved pulmonary infection. In disseminated disease in which infection is not controlled, granulomas are generally poorly formed or do not develop at all, and a polymorphonuclear leukocyte response is frequently seen. In patients who are asymptomatic or in whom the initial pulmonary infection has resolved, delayed-type hypersensitivity to coccidioidal skin anti­ gens has been routinely documented and T-cell activation has been observed in peripheral blood stimulated with coccidioidal antigens. In persons who have developed protective cellular immunity, spherules exist in the lungs for prolonged periods, possibly lifelong, in a latent

Host Maturing spherule Rupturing spherule Early spherule Endospore Spherule stage ~ 5 µm Arthroconidium Environment Mycelial stage FIGURE 219-1  Life cycle of Coccidioides, including the mycelial phase in the environment and the spherule phase in the host. state. This results in the persistence of protective immunity but also places those infected at risk of recurrent active disease if that immunity should wane. ■ ■CLINICAL AND LABORATORY MANIFESTATIONS After infection, 60% of individuals are completely asymptomatic. The other 40% have symptoms that are related primarily to pulmonary infection, including fever, cough, and pleuritic chest pain. Symptoms generally occur from several days to up to 3 weeks after inhalation of arthroconidia. The risk of symptomatic illness increases with age. In addition to these local symptoms indicative of infection, there are several manifestations of primary pulmonary coccidioidomycosis that are due to an immunologic response. Most prominent among these are cutaneous reactions. A diffuse, erythematous maculopapular rash, known as toxic erythema, has been noted in some cases. In addi­ tion, erythema nodosum (see Fig. A1-39)—typically over the lower extremities—and erythema multiforme (see Fig. A1-24)—usually in a necklace distribution—may occur. Lesions consistent with Sweet syn­ drome have also been reported (Chap. 21). Cutaneous manifestations are especially common in women. Symmetrical arthralgias (“desert rheumatism”), often involving the ankles, knees, and hips, may also occur with or without cutaneous manifestations. Primary pulmonary coccidioidomycosis is often misdiagnosed as a community-acquired bacterial pneumonia. However, the diagnosis of primary pulmonary coccidioidomycosis is strongly suggested by the findings of rash or symmetrical arthralgias in a patient with an appropriate exposure history. The finding of any of the following is also strongly suggestive of coccidioidomycosis: a history of night sweats, marked fatigue, peripheral-blood eosinophilia, failure to improve with antibacterial therapy, and an upper lobe infiltrate or hilar or mediasti­ nal lymphadenopathy on chest imaging. In most patients, primary pulmonary coccidioidomycosis resolves without sequelae over several weeks. However, several pneumonic complications may arise. Pulmonary nodules are residua of the pri­ mary pneumonia. Generally single, frequently located in the upper lobes, and usually ≤4 cm in diameter, nodules are often discovered on routine chest radiograph in asymptomatic patients. Calcifica­ tion is uncommon. Coccidioidal pulmonary nodules can be difficult

to distinguish radiographically from pulmonary malignancies. Like malignancies, coccidioidal nodules often enhance on positron emis­ sion tomography. However, unlike malignancies, routine computed tomography (CT) imaging often demonstrates multiple nodules and there may be microsatellite lesions, scattered smaller nodules sur­ rounding the larger one. These findings are not specific, and biopsy may be required to distinguish between these two entities.

Pulmonary cavities occur when a nodule extrudes its contents into the bronchial tree, resulting in a thin-walled shell. Frequently asymp­ tomatic, these cavities may on occasion be associated with persistent cough, hemoptysis, and pleuritic chest pain. They may also become secondarily infected with bacterial oral flora, environmental fungi such as Aspergillus species, or even with Coccidioides growing from the cavity wall. Rarely, a cavity may rupture into the pleural space, causing pyopneumothorax. In such cases, patients present with acute dyspnea, and the chest radiograph reveals a collapsed lung with a pleu­ ral air-fluid level. Chronic or persistent pulmonary coccidioidomycosis manifests with prolonged fever, cough, and weight loss and is radio­ graphically associated with pulmonary scarring, fibrosis, and cavities. It is most common in patients who already have chronic lung disease due to other etiologies. In some cases, primary pneumonia presents as a diffuse reticulo­ nodular pulmonary process in association with dyspnea and fever. This manifestation of pulmonary coccidioidomycosis may occur in settings of intense environmental exposure or in those with profoundly sup­ pressed cellular immunity, such as persons with untreated HIV-1 infec­ tion and markedly depressed peripheral blood CD4 cell count. In the latter case, this picture is associated with unrestrained fungal growth and is frequently associated with fungemia. CHAPTER 219 Clinical dissemination occurs in approximately 1% of infected indi­ viduals and is defined as finding the fungus outside the thoracic cavity. Dissemination is more likely to occur in male patients, particularly those of African and perhaps Filipino ancestry, and in persons with depressed cellular immunity, including patients with HIV-1 infection and peripheral blood CD4 counts of <250/μL, those receiving chronic glucocorticoid therapy, those with allogeneic solid-organ transplants, and those being treated with tumor necrosis factor-α antagonists or other biological response modifiers. Women who acquire new coc­ cidioidal infection during the second or third trimester of pregnancy or postpartum also are at significant risk for disseminated disease. Com­ mon sites for dissemination include the skin, bones, joints, soft tissues, and meninges. Dissemination may follow symptomatic or asymptom­ atic pulmonary infection and may involve only one site or multiple anatomic foci. When it occurs, clinical dissemination is usually evident within the first 6 months after primary pulmonary infection and is generally associated with a lack of cellular immunity to Coccidioides. Coccidioidomycosis Of the disseminated syndromes, coccidioidal meningitis is the most dire and is uniformly fatal if untreated. Patients usually present with a persistent dull headache, often accompanied by lethargy, and confusion. Nuchal rigidity, if present, is not severe. Examination of cerebrospinal fluid (CSF) demonstrates lymphocytic pleocytosis with profound hypoglycorrhachia and elevated protein levels. CSF eosino­ philia is occasionally observed. The diagnosis is usually established by finding coccidioidal complement-fixing antibody (see below) in the CSF in association with the CSF inflammatory pattern described above. The fungus is isolated from the CSF in fewer than one-third of cases. Magnetic resonance imaging with gadolinium frequently demonstrates enhancement in the basilar meninges. With or without appropriate therapy, patients may develop hydrocephalus, usually com­ municating, which presents clinically as a marked decline in mental status, often with gait disturbances. An elevated opening CSF pressure on lumbar puncture and dilated ventricles on brain imaging are hall­ marks of this condition. ■ ■DIAGNOSIS Serology is the most common method to establish the diagnosis of coccidioidomycosis. Several techniques are available, including the traditional tube-precipitin (TP) and complement-fixation (CF) assays, immunodiffusion TP and CF (IDTP and IDCF), and enzyme

immunoassay (EIA) to detect IgM and IgG antibodies. TP and IgM antibodies are found in serum soon after infection and persist for weeks to months. They are not useful for gauging severity of disease. The CF and IgG antibodies occur later in the course of the disease and persist longer than TP and IgM antibodies. Rising CF titers are a reflec­ tion of fungal growth and are associated with clinical progression, and the presence of CF antibody in CSF is indicative of coccidioidal men­ ingitis. Antibodies disappear over time in persons whose clinical illness resolves and the presence of coccidioidal antibodies in the serum does not indicate immunity or control of coccidioidomycosis.

Because of its commercial availability, relatively rapid turnaround time, and higher sensitivity than other tests, EIAs for IgM and IgG are recommended for the initial diagnosis of coccidioidomycosis. Both tests, but particularly the IgM EIA, are occasionally falsely positive, and results should be interpreted cautiously when used for screening pur­ poses in asymptomatic individuals. If either the IgM or IgG EIA serol­ ogy is positive, immunodiffusion tests (IDTP and IDCF) or traditional TP and CF assays should be performed for confirmation. If the IDCF or traditional CF assays are qualitatively positive, a CF titer should be requested. The optical density obtained from the EIA IgG does not correlate with the serologic titer of either the IDCF or traditional CF assay and should not be used as a surrogate for a titer. The CF titer is an important prognostic tool. High titers, for example, those ≥1:32, sug­ gest unrestrained fungal growth, predict disease activity, and suggest the possibility of extrathoracic dissemination. Coccidioides grows within 3–7 days at 37°C on a variety of artificial media, including blood agar. Therefore, it is always useful to obtain samples of sputum or other respiratory fluids and tissue for culture in suspected cases of coccidioidomycosis. The clinical laboratory should always be alerted to the possibility of this diagnosis, since Coccidioides poses a significant laboratory hazard if it is inadvertently inhaled. The organism can also be identified directly. While treatment of samples with potassium hydroxide is rarely fruitful, examination of sputum or other respiratory fluids after Papanicolaou, Gomori methenamine silver, or calcofluor white staining may reveal spherules in a significant proportion of patients with pulmonary coccidioidomycosis. For fixed tissues (e.g., those obtained from biopsy specimens), spherules with surrounding inflammation can be demonstrated with hematoxylineosin or Gomori methenamine silver staining. PART 5 Infectious Diseases A commercially available test for coccidioidal antigenuria and anti­ genemia has been developed and appears to be useful in immunosup­ pressed patients with severe or disseminated disease. It is also useful when the CSF is assayed in cases of suspected coccidioidal meningitis. False-positive results may occur in cases of histoplasmosis or blastomy­ cosis. Some laboratories offer genomic detection by polymerase chain reaction; this assay does not appear to be more sensitive than culture but can be more rapid. TREATMENT Coccidioidomycosis Currently, two classes of antifungal agents are useful for the treat­ ment of coccidioidomycosis (Table 219-1). While once prescribed routinely, amphotericin B in all its formulations is now reserved for only the most severe cases of dissemination and for intrathecal or intraventricular administration in patients with coccidioidal men­ ingitis in whom other antifungal therapy has failed. The original formulation of amphotericin B, which is dispersed with deoxycholate, is usually administered intravenously in doses of 0.7–1.0 mg/kg

either daily or three times per week. The newer lipid-based for­ mulations are associated with less renal toxicity, but there are no studies indicating whether they lead to better improvement than the deoxycholate formulation in coccidioidomycosis. The lipid disper­ sions are administered intravenously at doses of 3–5 mg/kg daily or three times per week. Triazole antifungals are currently the principal drugs used to treat most cases of coccidioidomycosis. Clinical trials have demon­ strated the usefulness of both fluconazole and itraconazole as initial

TABLE 219-1  Clinical Presentations of Coccidioidomycosis, Their Frequency, and Recommended Initial Therapy for the Immunocompetent Host CLINICAL PRESENTATION FREQUENCY, % RECOMMENDED THERAPY Asymptomatic infection

None Primary pneumonia (focal)

In most cases, nonea Diffuse pneumonia <1 Amphotericin B followed by prolonged oral triazole therapy Pulmonary sequelae

     Nodule   None      Cavity   In most cases, noneb      Chronic pneumonia   Prolonged triazole therapy Disseminated disease ≤1        Skin, bone, joint, soft   Prolonged triazole therapyc tissue disease      Meningitis Lifelong triazole therapyd aTreatment is indicated for hosts with depressed cellular immunity as well as for those with prolonged symptoms and signs of increased severity, including night sweats for >3 weeks, weight loss of >10%, a complement-fixation titer of >1:16, and extensive pulmonary involvement on chest radiography. bTreatment (usually with the oral triazoles fluconazole and itraconazole) is recommended for persistent symptoms. cIn severe cases, some clinicians would use amphotericin B as initial therapy. dIntraventricular or intrathecal amphotericin B is recommended in cases of triazole failure. Hydrocephalus may occur, requiring a cerebrospinal fluid shunt. Note: See text for dosages and durations. agents. Fluconazole has been the triazole of choice for the treat­ ment of coccidioidomycosis in the past because of availability, cost, predictable oral absorption, and perceived lack of severe adverse events. However, there are no studies demonstrating its superiority over other triazole antifungals, and evidence indicates that itra­ conazole is more effective against bone and joint disease and may be more effective for other forms of coccidioidomycosis. For both drugs, a minimal oral adult dosage of 400 mg/d should be used. The maximum dose of itraconazole is 200 mg three times daily, but higher doses of fluconazole may be given and 800 mg is frequently prescribed for coccidioidal meningitis. The newer triazole anti­ fungals, voriconazole and posaconazole, are useful for all types of clinical disease, including meningitis, and should be considered in cases where fluconazole or itraconazole therapy has failed. To date, isavuconazole has been used in limited circumstances in coccidioi­ domycosis but also appears to be effective, including for meningitis. High-dose triazole therapy may be teratogenic during the first trimester of pregnancy, and triazole therapy should be avoided in pregnant women during this period. There are several new antifungal agents that are promising for the treatment of coccidioidomycosis in the future. These include olorofim, oteseconazole, ibrexafungerp, fosmanogepix, and oral liquid nanocrystal amphotericin B. At this time, only olorofim has been studied in human coccidioidomycosis, and none are currently approved for use in coccidioidomycosis. However, olorofim has been studied in human coccidioidomycosis and has received U.S. Food and Drug Administration breakthrough therapy designation for difficult-to-treat Coccidioides meningitis. Most patients with focal primary pulmonary coccidioidomycosis do not require antifungal therapy. Patients for whom antifungal therapy should be considered include those with underlying cellular immunodeficiencies and those with prolonged symptoms and signs of extensive disease. Specific criteria include symptoms persisting for ≥2 months, night sweats occurring for >3 weeks, weight loss of

10%, a serum CF antibody titer of >1:16, and extensive pulmo­ nary involvement apparent on chest radiography. When antifungal therapy is used, either fluconazole or itraconazole at 400 mg daily anywhere from 6 weeks to 6 months is considered appropriate. Diffuse pulmonary coccidioidomycosis represents a special situation. Because most patients with this form of disease are

113 - 220 Blastomycosis

220 Blastomycosis

profoundly hypoxemic and critically ill, many clinicians favor beginning therapy with an amphotericin B formulation combined with an oral triazole antifungal. The triazole antifungal therapy is continued alone once clinical improvement occurs and should be continued for 6 months to 1 year. The nodules that may occur after primary pulmonary coccidi­ oidomycosis do not require treatment. As noted above, these nod­ ules are not easily distinguished from pulmonary malignancies by means of radiographic imaging. Close clinical follow-up and biopsy may be required to separate these two entities. Most pulmonary cavities do not require therapy. Antifungal treatment should be con­ sidered in patients with persistent cough, pleuritic chest pain, and hemoptysis. Occasionally, pulmonary coccidioidal cavities become secondarily infected (see above). This development is often mani­ fested by an air-fluid level within the cavity. Bacterial oral flora or Aspergillus species are commonly involved, and therapy directed at these organisms should be considered. Surgical removal of the cavity may be required in cases of persistent productive cough and hemoptysis or in those cases of a persistently growing cavity. In addition, cavities >4 cm in diameter are unlikely to resolve sponta­ neously, and surgical extirpation should be considered. Surgery is always required in cases of pyopneumothorax. For chronic pulmo­ nary coccidioidomycosis, prolonged antifungal therapy—lasting for at least 1 year—is usually required, with monitoring of symptoms, radiographic changes, sputum cultures, and serologic titers. Most cases of disseminated coccidioidomycosis require pro­ longed antifungal therapy. The duration of treatment is based on clinical improvement in conjunction with a significant decline in serum CF antibody titer. Such therapy routinely is continued for at least several years. Relapse occurs in 15–30% of individuals once therapy is discontinued and it is important to continue to monitor such patients on a regular basis (e.g., every 3–4 months) after anti­ fungal therapy is discontinued. Coccidioidal meningitis poses a special challenge. While most patients with this form of disease respond to treatment with oral tri­ azoles, 80% experience relapse when therapy is stopped. Therefore, lifelong therapy is recommended. In cases of triazole failure, intra­ thecal or intraventricular amphotericin B may be used. Installation requires considerable expertise and should be undertaken only by an experienced health care provider. Shunting of CSF in addition to appropriate antifungal therapy is required in cases of meningitis complicated by hydrocephalus. It is prudent to obtain expert con­ sultation in all cases of coccidioidal meningitis. PREVENTION There are no proven methods to reduce the risk of acquiring coccid­ ioidomycosis among residents of an endemic region, but avoidance of inhalation of uncultivated soil or dust is a reasonable measure. For individuals with suppressed cellular immunity, the risk of developing symptomatic coccidioidomycosis is greater than that in the general population. Among those about to undergo allogeneic solid-organ transplantation, antifungal therapy is appropriate prior to transplantation when there is evidence of active or recent coc­ cidioidomycosis. Several transplant centers in the endemic region provide universal antifungal prophylaxis for 6 months to 1 year after solid-organ and allogeneic stem cell transplantation, and lifelong universal prophylaxis has been advocated after lung trans­ plantation to prevent the development of coccidioidomycosis after transplantation. Cases of donor-transmitted coccidioidomycosis have been reported. Donors who are living or have lived in a coccidioidal endemic region should be screened serologically for coccidioido­ mycosis before transplantation and organ donation deferred if there is evidence of active infection. Data on the use of antifungal agents for prophylaxis in other situations are limited. The administration of prophylactic anti­ fungals is not recommended for HIV-1-infected persons who live in an endemic region. Most experts would administer a triazole antifungal to patients with a history of active coccidioidomycosis

or a positive coccidioidal serology in whom therapy with tumor necrosis factor-α antagonists or other biological response modifiers is being considered.

There are recent efforts to develop a vaccine for coccidioido­ mycosis, and a live avirulent product has demonstrated promising results in a canine model. Future studies will determine if this is a viable strategy for preventing or ameliorating coccidioidal infection in humans. ■ ■FURTHER READING Galgiani JN et al: 2016 Infectious Diseases Society of America (IDSA) clinical practice guideline for the treatment of coccidioidomycosis. Clin Infect Dis 63:e112, 2016. Gorris ME et al: Expansion of coccidioidomycosis endemic regions in the United States in response to climate change. Geohealth 3:308, 2019. Shubitz LF et al: Δcps1 vaccine protects dogs against experimentally induced coccidioidomycosis. Vaccine 39:6894, 2021. Taylor JW, Barker BM: The endozoan, small-mammal reservoir hypothesis and the life cycle of Coccidioides species. Med Mycol 57:S16, 2019. Troung CN et al: Universal lifelong fungal prophylaxis and risk of coccidioidomycosis in lung transplant recipients living in an endemic area. Clin Infect Dis 74:1966, 2021. CHAPTER 220 Gregory M. Gauthier, Bruce S. Klein

Blastomycosis Blastomycosis ■ ■DEFINITION Blastomycosis is a pyogranulomatous disease that follows the inhalation of Blastomyces conidia or hyphal fragments. Typically, Blastomyces causes pulmonary infection; however, a subset of patients will have dissemi­ nated disease that involves organs such as the skin, bone, brain, or geni­ tourinary system. Blastomycosis is considered a primary fungal infection because it affects persons with either intact or impaired immune systems. A delay in diagnosis is common because blastomycosis mimics other diseases such as bacterial pneumonia, tuberculosis, and malignancy. Diagnosis involves culture- and nonculture-based tests. Amphotericin B formulations and triazoles are the drugs of choice for treatment. ■ ■ETIOLOGY Blastomyces is a species complex comprising B. dermatitidis, B. gil­ christii, B. helicus, B. percursus, B. emzantsi, B. silverae, and B. parvus. B. silverae and B. parvus are not known to commonly infect humans. Blastomyces species exhibit thermal dimorphism, which involves the ability to convert between hyphal and yeast morphologies in response to temperature. In the soil (22–25°C), Blastomyces grows as septate hyphae that produce infectious conidia. Among the Blastomyces spe­ cies, B. helicus is unique because its hyphae grow in a coiled pattern and it does not sporulate under in vitro growth conditions. In organs and tissues (37°C), Blastomyces grows as a pathogenic yeast (Fig. 220-1) that elicits pyogranulomatous inflammation. The yeast form of all Blas­ tomyces species grows as broad-based budding yeast cells, with subtle differences in size among the different species (4–29 μm). ■ ■EPIDEMIOLOGY Although the majority of Blastomyces infections occur in North America, blastomycosis is a systemic fungal infection of global importance, with infections also occurring in Africa and Asia. In the United States, the traditional geographic range for Blastomyces includes the Mississippi

FIGURE 220-1  Blastomyces yeast at 37°C, with broad-based budding between mother and daughter cells (arrow). Bar = 10 μm. (Gregory M. Gauthier, MD, MS.) and Ohio River basins, the St. Lawrence River basin, states bordering the Great Lakes, and southeastern states. In Canada, the traditional geographic range includes the provinces of Saskatchewan, Manitoba, Ontario, and Quebec. In North America, B. dermatitidis is located throughout the traditional geographic range. B. gilchristii is geographi­ cally restricted to Minnesota, Wisconsin, Canada, and areas along the St. Lawrence River. B. dermatitidis and B. gilchristii are thought to have diverged 1.9 million years ago during the Pleistocene epoch, with

B. gilchristii restricted to formerly glaciated areas. B. dermatitidis is found in glaciated and nonglaciated areas. In the environment, B. dermatitidis and B. gilchristii are not uniformly distributed; rather, they grow in eco­ logic niches often referred to as microfoci, which are characterized by acidic, sandy soils that are found near water and that contain decaying organic matter such as vegetation or wood. In upstate New York State, blastomycosis is an emerging pathogen in the Capitol District and upper Susquehanna River Subbasin with B. dermatitidis (88.4%) more com­ mon than B. gilchristii (11.6%). In Canada, blastomycosis is an emerging pathogen in the province of Saskatchewan. B. helicus infections have been reported in the western United States (California, Montana, Idaho, Colorado, Nebraska, Texas) and Canada (Saskatchewan, Alberta); their ecologic niche has yet to be defined. The geographic range and ecologic niche for B. parvus and B. silverae are unknown. PART 5 Infectious Diseases Outside of North America, blastomycosis has been reported in Africa (>100 cases), India (<10 cases), and Israel. On the basis of mor­ phologic analysis, nearly all clinical isolates of Blastomyces in Africa were originally thought to be B. dermatitidis. However, molecular phylogenetic analysis of human clinical isolates has demonstrated that multiple Blastomyces species exist in Africa, including B. derma­ titidis, B. gilchristii, B. percursus, and B. emzantsi. A combination of internal transcribed spacer (ITS) sequencing, multilocus sequence typing (MLST), and whole genome sequencing was used to identify a new species, B. emzantsi, and to differentiate B. percursus from other Blastomyces species. MLST has identified a B. dermatitidis isolate from Rwanda and B. gilchristii from Zimbabwe and South Africa. Analysis of 20 isolates from South Africa collected over a 40-year period identified them as either B. emzantsi or B. percursus. The geographic distribu­ tion and ecologic niche of the four Blastomyces species in Africa are unknown. In India, there have been fewer than 10 autochthonous cases of blastomycosis, with the majority identified by morphologic analysis. One autochthonous case (caused by B. percursus) with molecular con­ firmation has been reported from Israel. Epidemiologic information about blastomycosis derives primar­ ily from passive laboratory surveillance, retrospective studies, and

outbreak investigations. The lack of sensitive skin testing and serologic testing, along with difficulty in isolating Blastomyces from the envi­ ronment by culture or molecular methods, has limited an in-depth epidemiologic understanding of blastomycosis. In North America, blastomycosis is reportable in five U.S. states (Minnesota, Wisconsin, Michigan, Arkansas, and Louisiana) and two Canadian provinces (Manitoba, Ontario). The annual incidence of blastomycosis in the tra­ ditional endemic area ranges from 0.11 to 2.17 cases/100,000 persons. In older persons (Medicare beneficiaries, 1999–2008), the nationwide annual incidence of blastomycosis was 0.7/100,000, with the highest rates in the midwestern and southern regions of the United States. Analysis of Healthcare Cost and Utilization Project (HCUP) data esti­ mated that 11,776 persons were hospitalized for blastomycosis in the United States from 2010 through 2020, with the majority of patients from midwestern (58.8%) and southern (31.4%) states. In certain places, such as Vilas County, Wisconsin, and Kenora, Ontario, blas­ tomycosis is hyperendemic, with annual incidence rates ranging from 40 to 117 cases/100,000 persons. Incidence data likely underestimate the true burden of infection because they are limited to persons with clinically apparent infection. Patients with asymptomatic or subclinical infections are undercounted. Most Blastomyces infections are sporadic and can occur in either rural or urban areas. There have been at least 20 outbreaks of blastomy­ cosis in the United States since the mid-1950s. Wisconsin, Minnesota, and North Carolina have had multiple outbreaks. The majority of outbreaks have been in rural areas, but several have occurred in urban settings. Activities associated with outbreaks include construction (of homes, cabins, factories, and roads), excavation of dirt, participation in water sports (canoeing, tubing on a river, and fishing), and exposure to a community compost pile or to beaver dams. Blastomyces infection is typically acquired from disturbed soil, which liberates infectious particles that are then inhaled into the lungs. An investigation of a blastomycosis outbreak in Marathon County, Wisconsin (2009–2010), found that 45% of patients were of Hmong ethnicity. A retrospective study from the Marshfield Clinic in Wiscon­ sin (1999–2014) found that 14.4% of patients with blastomycosis were of Asian ethnicity—a figure higher than was anticipated given that <2.5% of the population within the catchment area is Asian, includ­ ing a large Hmong population. These findings suggest that persons of Hmong ethnicity have an increased risk of acquiring blastomycosis. A combination of whole genome sequencing and immunologic analyses indicated that polymorphisms in the interleukin 6 (IL-6) gene in the Hmong population result in decreased IL-6 production, which in turn impairs development of IL-17-producing CD4+ T lymphocytes. IL-17 is a critical cytokine for recruitment and activation of innate immune cells such as neutrophils and macrophages active against Blastomy­ ces. Thus, alterations in IL-6 production may be responsible for the increased risk of blastomycosis in the Hmong population. Although data are limited, persons of Hmong ethnicity do not appear to be at increased risk for disseminated blastomycosis. Increased incidence rates of blastomycosis have also been reported in indigenous people of Canada and the United States. Compared with Caucasians, Asian and indigenous persons with blastomycosis tend to have fewer underlying medical conditions and to be younger. ■ ■PATHOGENESIS A defining feature of the Blastomyces species complex is the ability to respond to shifts in temperature by switching between hyphal and yeast forms. In the soil, Blastomyces grows as mold cells with hyphae that produce conidia. Hyphal growth promotes environmental sur­ vival, genetic diversity through mating, and production of infectious conidia that facilitate transmission of Blastomyces from the environ­ ment to mammals, including humans. At 37°C (the core temperature of mammals), Blastomyces hyphae and conidia convert into patho­ genic yeast that upregulate yeast phase–specific virulence factors and downregulate host immune defenses, thereby facilitating infection. Virulence traits that Blastomyces shares with Histoplasma, Coccidioides, Sporothrix, and Paracoccidioides are thermotolerance at 37°C, intracel­ lular survival, and capacity to cause infection in persons with either

healthy or impaired immune defenses. Although Emergomyces and Talaromyces marneffei (formerly Penicillium marneffei) exhibit thermal dimorphism, growth as yeast at 37°C, and intracellular survival, these dimorphic fungi tend to cause infection primarily in immunocompro­ mised persons. The morphologic switch from hyphae to yeast at 37°C is driven chiefly by temperature and is coupled with the uptake of exogenous cysteine. Cysteine uptake is required to complete the transition to the yeast form because it helps restart mitochondrial respiration, which ceases during the morphologic switch. Over the past two decades, knowledge about the genetic mechanisms that promote the tempera­ ture-dependent transition between hyphae and yeast has substantially increased. The discovery of dimorphism-regulating kinase 1 (DRK1), which encodes a group III hybrid histidine kinase that is part of the high-osmolarity glycerol (HOG) signaling pathway, provided genetic proof that that the transition to yeast is essential for virulence of the thermally dimorphic fungi. Disruption of DRK1 by gene deletion or RNA interference resulted in Blastomyces cells that grew as hyphae at 37°C instead of yeast. Although viable at 37°C, these cells had altered cell-wall composition, failed to upregulate the Blastomyces adhesin 1 (BAD1, formerly WI-1) virulence factor, and were avirulent in a mouse model of lethal pulmonary infection. Subsequent studies of Histoplasma and Talaromyces demonstrated that the function of DRK1 is conserved with regard to thermal dimorphism and virulence. The temperature-dependent transition in the other direction—from yeast to hyphae—is regulated in part by a GATA-transcription factor encoded by siderophore biosynthesis repressor in Blastomyces (SREB), which influences neutral lipid metabolism. In addition, sensing of chitin by NGT1 and NGT2 N-acetylglucosamine transporters accelerates the conversion to hyphae following a drop in temperature from 37°C to 22°C. These two mechanisms are conserved in Histoplasma capsulatum. As a primary fungal pathogen, Blastomyces is one of the few fungi that can infect immunocompetent persons. In its yeast form, Blastomy­ ces evades and modulates immune defenses. Following disruption of soil, conidia that are aerosolized and inhaled into the lungs are phago­ cytosed by pulmonary macrophages, in which a subset of the conidia germinate as yeast and replicate during the early phases of infection. Blastomyces is also capable of replicating outside of macrophages. Upon conversion to the yeast phase, an essential virulence factor encoded by BAD1 is upregulated. BAD1 encodes a multifunctional 120-KDa cellsurface protein that facilitates yeast adherence to lung epithelial cells via interaction with heparin sulfate, attachment to host immune cells by binding to CR3 and CD14 complement receptors, and downregulation of tumor necrosis factor alpha (TNF-α) in macrophages and neutro­ phils. In addition, the BAD1 protein impairs activation of CD4+ T lym­ phocytes, thereby decreasing the production of IL-17 and interferon gamma (IFN-γ). In vivo transcriptional profiling of B. dermatitidis yeast during pulmonary infection demonstrated that BAD1 is the most highly upregulated gene. Deletion of BAD1 renders B. dermatitidis avirulent in a murine model of pulmonary infection. Thus, BAD-1 is essential for virulence in B. dermatitidis and likely in B. gilchristii as well. In contrast, BAD1 is absent from the sequenced genomes of

B. helicus, B. parvus, B. silverae, B. percursus, and B. emzantsi. Additional factors that contribute to the virulence of Blastomyces yeast include relative resistance to oxidative stress, upregulation of catalase and superoxide dismutase during infection, active uptake of zinc by a PRA1-encoded zincophore and transmembrane transporter (ZRT1), and cleavage of granulocyte-macrophage colony-stimulating factor by dipeptidyl peptidase IVA, which blocks activation of innate immune cells (macrophages, neutrophils) and their recruitment to the lung. APPROACH TO THE PATIENT Blastomycosis On the basis of outbreak investigations, it is estimated that 50% of persons exposed to Blastomyces develop symptomatic infec­ tion after a 3-week to 3-month incubation period. The relatively long incubation period means that patients can be diagnosed with

blastomycosis throughout the year. Blastomycosis has been referred to as the “the great pretender” because it can mimic infectious and noninfectious diseases. Blastomycotic pneumonia clinically and radiographically resembles community-acquired bacterial pneu­ monia, viral pneumonia, tuberculosis, and lung cancer. Patients often receive two or three courses of antibiotics before pulmonary blastomycosis is diagnosed. Without fungal stain and culture, cuta­ neous lesions can mimic skin cancer, sarcoidosis, and pyoderma gangrenosum. Rarely, blastomycosis can mimic laryngeal cancer. The most important aspect of the approach to a patient with a compatible illness is the consideration of Blastomyces as an etiologic agent in the differential diagnosis. This awareness facilitates early diagnosis and treatment, enhancing the potential for improved clinical outcomes. Clinical clues to blastomycosis, especially in persons who reside in or visit endemic regions, include pneumonia that does not improve with antibiotic treatment, pneumonia with extrapulmonary manifestations (e.g., skin lesions, osteomyelitis, central nervous system [CNS] involvement), and skin ulcers that do not respond to standard therapy. Blastomycosis should also be considered in persons from (or visited) an endemic area who have unexplained respiratory failure or acute respiratory distress syn­ drome (ARDS). In addition, a detailed exposure history can elevate blastomycosis in the differential diagnosis; approximately 50–60% of patients will have environmental risk factors for blastomycosis. This history should also include inquiries about a pet or family member with blastomycosis; these factors have been reported in 7.7–10% and 4–9% of patients, respectively. CHAPTER 220 ■ ■CLINICAL MANIFESTATIONS Pulmonary Blastomycosis  Pulmonary manifestations occur in 69–93% of patients with symptomatic blastomycosis and are the most common clinical feature of infection. Signs and symptoms can include fever, chills, productive or nonproductive cough, shortness of breath, hemoptysis, malaise, and decreased appetite. Pulmonary blastomycosis also can manifest as asymptomatic infection, a brief influenza-like ill­ ness, acute pneumonia, chronic pneumonia, or ARDS. Radiographic findings in the lungs include lobar consolidation, a mass lesion, inter­ stitial infiltrates, nodule(s), a miliary pattern, cavitary disease, and dif­ fuse involvement of multiple lobes. Hilar adenopathy, pleural effusion, and empyema are uncommon. No distinctive features differentiate blastomycosis from other pulmonary diseases. Diabetes, receipt of a solid organ transplant, immunosuppression, and multilobar pneumo­ nia are risk factors for severe pulmonary blastomycosis. Approximately 4–15% of patients with pulmonary blastomycosis develop ARDS, which is characterized by a fulminant course and high mortality rates ranging from 40 to 89% in most studies. The mortality rate in ARDS is increased when the diagnosis is delayed. Blastomycosis Disseminated Blastomycosis  Disseminated blastomycosis occurs in 15–48% of patients and has the potential to involve nearly any organ in the body. The most common site of dissemination is the skin, in which the infection can manifest as papules, ulcers, verrucous lesions, or abscesses. The second most common site is bone, with consequent osteomyelitis characterized by bone pain, soft tissue swelling, soft tissue abscess, and sinus tract formation. Typically, a single bone is involved; however, multifocal osteomyelitis can occur. The most common sites for osteomyelitis include the spine, long bones, and ribs. Dissemination to the CNS (e.g., manifesting as meningitis, an abscess, or a mass lesion), the larynx, or the genitourinary system (e.g., to the prostate or epididy­ mis) occurs in fewer than 10%; the majority of the affected patients have concomitant involvement of other organs, such as the lung or the skin. Factors that influence dissemination include the infecting Blasto­ myces species, the duration of pulmonary symptoms, and concomitant AIDS. Multiple studies from Wisconsin, a state in which B. derma­ titidis and B. gilchristii are endemic, have demonstrated that B. der­ matitidis is more likely to cause disseminated infection (31.4–47.8% of cases), whereas B. gilchristii tends to remain localized to the lung (90.7–92.2%). Surprisingly, immunosuppression has only a minimal

influence on dissemination, an observation suggesting that Blastomyces virulence factors have a greater impact than host immune defenses. The frequency of disseminated blastomycosis among solid organ transplant recipients, persons receiving cancer chemotherapy, and patients under­ going pharmacologic immunosuppression is similar to that among patients with intact immune systems. Although patients treated with TNF-α antagonists are considered at risk for blastomycosis, the clinical manifestations and frequency of disseminated disease are unknown in this group because of a paucity of published data. Persons with AIDS and CD4+ T lymphocyte counts of <100/μL are an exception: they are at increased risk for CNS dissemination. Blastomycosis in pregnancy is uncommon, is typically diagnosed in the second or third trimester (91%), and most frequently manifests as pneumonia (74%) or dis­ seminated infection (48%). Transmission to the neonate by either the transplacental route or aspiration of infected vaginal secretions is rare. Persons infected with B. helicus can have localized pulmonary infection or disseminated disease; they are typically immunosuppressed (e.g., as a result of solid organ transplantation, chemotherapy, HIV infection, or lupus) and have a high mortality rate (71.4% in seven patients). In contrast to B. dermatitidis and B. gilchristii, B. helicus commonly causes fungemia. Infections with B. percursus and B. emzantsi are often of long duration (persisting for 4 weeks to 5 years) and can involve the lungs or become disseminated (skin, bone, brain).

■ ■DIAGNOSIS Timely diagnosis of blastomycosis requires a high degree of clinical suspicion because its clinical and radiographic presentations mimic more common etiologies, such as community-acquired pneumonia, malignancy, and tuberculosis. Laboratory findings such as leukocy­ tosis, mild anemia, increased C-reactive protein level, and elevated erythrocyte sedimentation rate are nonspecific. Once suspected, the diagnosis of blastomycosis is straightforward and involves microscopic examination of stained specimens, fungal culture, and antigen testing. The poor sensitivity of complement fixation (9%) and immunodiffu­ sion (28%) renders serologic testing diagnostically dispensable. How­ ever, a recently developed serologic test designed to detect antibodies to BAD1 has a sensitivity of 87% and a specificity of 94–99%. PART 5 Infectious Diseases A presumptive diagnosis of blastomycosis can be made by staining of clinical specimens and looking for broad-based budding yeast with a doubly refractile cell wall. Along with the broad-based budding pat­ tern, yeast size (4–29 μm) allows Blastomyces to be distinguished from other fungi. An exception is B. helicus, which has the potential to be confused with Histoplasma because of its small-sized yeast. Respiratory tract specimens such as sputum, tracheal aspirate, and bronchoalveolar (BAL) fluid can be stained with calcofluor, 10% potassium hydroxide, or Papanicolaou stain. Purulent drainage can be stained in a similar manner. The sensitivity of staining of respiratory samples ranges from 50 to 90%. Tissue samples for histopathology should be stained with Gomori methenamine silver or periodic acid–Schiff stain and assessed for pyogranulomatous inflammation and broad-based budding yeast. Traditional stains, such as Gram’s stain or hematoxylin and eosin, do not permit optimal visualization of Blastomyces yeast. Growth of Blastomyces in cultures of respiratory tissue or body fluid samples provides a definitive diagnosis of blastomycosis but typically requires 5–28 days of incubation. Special media such as Sabouraud dextrose, potato dextrose, and brain–heart infusion are required because Blastomyces does not grow well on standard bacterio­ logic media. Most clinical microbiology laboratories incubate fungal cultures at 25–30°C, a temperature that results in hyphal growth of Blastomyces. Unfortunately, Blastomyces hyphae are not morphologi­ cally distinct enough to confirm diagnosis. Thus, fungal identification and diagnosis are commonly confirmed via chemiluminescent DNA probe or, less commonly, via conversion to yeast upon incubation at 37°C. Diagnosis can also be confirmed by polymerase chain reaction. Neither the chemiluminescent DNA probe nor morphologic analysis of yeast by light microscopy differentiates among the different species of Blastomyces. Moreover, some species, such as B. emzantsi, are difficult to convert to yeast at 37°C. The species of Blastomyces is not typically determined in clinical labs because DNA sequencing is required.

An antigen test that detects a conserved galactomannan component in the Blastomyces cell wall has supplanted serologic testing. This test can be performed on urine, blood, BAL fluid, and cerebrospinal fluid. The sensitivity of the antigen test is 85–93% for urine and 57–82% for serum. Infection burden appears to influence test sensitivity, with a lower burden of infection resulting in reduced sensitivity. The antigen test can detect B. dermatitidis, B. gilchristii, and B. helicus; however, its utility for detection of other Blastomyces species is unknown. Crossreactions in the antigen test occur during infection with other dimor­ phic fungi, including H. capsulatum (96%), Paracoccidioides species (100%), and T. marneffei (70%). Among these, only H. capsulatum is found in the same endemic region as Blastomyces. Rare cross-reactions can occur with Aspergillus and Cryptococcus infections. Antigen levels in urine and blood decline with successful treatment, and their mea­ surement can be used to monitor the response to antifungal therapy. TREATMENT Blastomycosis Guidelines for the treatment of blastomycosis have been published by the Infectious Diseases Society of America (2008), the American Thoracic Society (2011), the American Society of Transplanta­ tion (2019), and European Confederation of Medical Mycology (2021). Although there are isolated reports of self-limited pul­ monary blastomycosis, there are no criteria to determine which patients will experience a resolution of infection. Thus, treatment is recommended for all patients with blastomycosis in order to prevent progressive infection, respiratory failure, and disseminated disease. Antifungal selection is influenced by immune status, CNS involvement, pregnancy, medical comorbidities (e.g., congestive heart failure, prolonged QT interval), and drug–drug interactions. Antifungal drugs active against Blastomyces include amphotericin B

(AmB) formulations and triazoles. The minimal amount of beta- (1,3)-glucan in the Blastomyces yeast cell wall renders echinocan­ dins ineffective, and they should not be used to treat blastomycosis. Hematologic, hepatic, and renal function should be assessed prior to initiation of antifungal therapy, and possible drug–drug interac­ tions should be evaluated. In addition, patients should be educated about proper administration of triazole antifungals. For example, itraconazole capsules require an acidic gastric environment for optimal absorption and should be taken with food and an acidic beverage to improve bioavailability; they cannot be used by persons taking antacids, H2 antagonists, or proton pump inhibitors. In con­ trast, itraconazole solution can be given to patients receiving gastric acid–lowering therapies and should be taken without food. Treatment for blastomycosis is summarized in Table 220-1. For immunocompetent patients with pulmonary or disseminated blasto­ mycosis of mild or moderate severity (e.g., treatable in the outpatient setting), itraconazole therapy for 6 months is recommended. For severe blastomycosis (e.g., that requiring hospitalization), induction therapy with lipid AmB for 7–14 days (or until clinical improve­ ment), followed by itraconazole treatment for 6–12 months, is recommended. Although not well studied, combination antifungal therapy with lipid AmB and itraconazole (or another azole) can be considered for patients with severe pulmonary blastomycosis. In patients with ARDS, prednisone can be considered; however, the benefits of steroids are unclear. Osteomyelitis due to blastomycosis requires at least 12 months of antifungal therapy, and some patients may require surgical debridement. For blastomycosis involving the CNS, lipid AmB is administered for 4–6 weeks and is followed by treatment with voriconazole, itraconazole, or fluconazole for at least 12 months. Although fluconazole has excellent CNS penetration, its minimum inhibitory concentration (MIC) against B. dermatitidis and B. gilchristii is higher than that of either itraconazole or vori­ conazole. Emerging data suggest that isavuconazonium sulfate can be used for treatment of CNS blastomycosis. Immunosuppressed patients should be treated with 7–14 days of lipid AmB followed by 12 months of itraconazole. For patients

114 - 221 Cryptococcosis

221 Cryptococcosis

TABLE 220-1  Treatment of Blastomycosis SEVERITY OF INFECTION SITE OF INFECTION THERAPY PATIENT POPULATION Immunocompetent Mild to moderatea Lung Itraconazole for 6–12 monthsb     Disseminated Itraconazole for 6–12 monthsb (≥12 months for osteomyelitis)   Severec CNS Lipid AmB (5 mg/kg dailyd,e for 4–6 weeks) followed by voriconazole (200–400 mg bid), itraconazoleb or fluconazole (800 mg daily) for at least 12 months of treatment     Lung Lipid AmB (3–5 mg/kg dailye,f for 7–14 days) followed by itraconazoleb for 6–12 months     Disseminated Lipid AmB (3–5 mg/kg dailye,f for 7–14 days) followed by itraconazoleb for 12 months of treatment

(≥12 months for osteomyelitis) Immunocompromised Any severity CNS Lipid AmB (5 mg/kg dailyd,e for 4–6 weeks) followed by voriconazole (200–400 mg bid), itraconazoleb or fluconazole (800 mg daily) for at least 1 year of treatmentg     Lung or disseminated Lipid AmB (3–5 mg/kg dailye,f for 7–14 days) followed by itraconazoleb for 12 monthsg Pregnanth Any severity Any site Lipid AmB (3–5 mg/kg dailye,f for 6–8 weeks), with avoidance of azole antifungals aMild to moderate infections can typically be managed in the outpatient setting. bA loading dose of 200 mg PO tid for 3 days followed by 200 mg PO daily or bid, with dosing based on serum itraconazole levels. The goal for levels of total itraconazole (i.e., itraconazole plus hydroxyitraconazole) is 1–5 μg/mL. Liquid itraconazole has greater bioavailability than the capsule formulation. Liquid itraconazole and oral capsules are administered differently (see text for details). Serum itraconazole levels should be measured after steady state has been reached (2 weeks). Because of the drug’s long half-life, blood for serum itraconazole determinations can be drawn regardless of the time of administration. In contrast, serum drug levels for voriconazole, posaconazole, and isavuconazole should be measured before a dose is administered when steady state has been reached (~1 week). cSevere blastomycosis requires hospitalization on a medical ward, an intermediate care unit, or an intensive care unit. dLipid amphotericin B (AmB) is the preferred formulation because it has the best central nervous system (CNS) penetration among AmB formulations. For patients with CNS blastomycosis that results in neurologic dysfunction, surgical intervention should be considered. eFor patients with CNS blastomycosis, severe pulmonary blastomycosis, or severe disseminated blastomycosis, combination therapy with lipid AmB plus a triazole antifungal can be considered; however, this combination has not been formally studied. For patients with acute respiratory distress syndrome, adjunctive steroid therapy with prednisone (40–60 mg daily for 1–2 weeks) can be considered; however, the benefit of steroid administration is not well defined. fIf lipid AmB is not available, then AmB deoxycholate (0.7–1. 0 mg/kg daily) can be substituted; however, this formulation is associated with higher rates of nephrotoxicity and infusion reactions than lipid AmB. gConsider lifelong suppression with itraconazole (200 mg daily) if immunosuppression cannot be reversed. This decision should be made on a case-by-case basis; not all immunosuppressed patients require lifelong suppressive therapy. In addition, lifelong antifungal suppression can be considered in patients who experience relapse after appropriate therapy. hAll women of childbearing age should undergo pregnancy testing before initiation of therapy. requiring irreversible immunosuppression, indefinite suppressive azole therapy may be needed; however, in light of the heterogene­ ity of this patient population, a decision about suppressive therapy should be made on a case-by-case basis. The majority of solid organ transplant recipients do not require lifelong suppression because rates of relapse are low when treatment guidelines are followed. For pregnant women, lipid AmB treatment for 6–8 weeks is recommended because, unlike the triazole antifungals, lipid AmB is not teratogenic. Fluconazole can cause craniofacial, skeletal, and cardiac defects in the developing fetus (Antley-Bixler-like syn­ drome); voriconazole and posaconazole also can result in skeletal abnormalities. Itraconazole increases the risk of spontaneous abor­ tion. Before starting antifungal therapy, women of childbearing age with blastomycosis should have a pregnancy test. Voriconazole, posaconazole, and isavuconazonium sulfate have potent activity against B. dermatitidis and B. gilchristii and can be considered as alternatives for persons who cannot tolerate itra­ conazole. These agents, along with itraconazole and AmB, also exhibit good activity against newly identified species of Blastomy­ ces, such as B. helicus, B. percursus, and B. emzantsi. Fluconazole MICs against B. percursus and B. emzantsi are higher than those of other triazoles. Moreover, fluconazole appears to have poor activity against B. helicus, B. parvus, and B. silverae. ■ ■PROGNOSIS Mortality rates for blastomycosis range from 5 to 13%; most deaths are associated with respiratory failure due to ARDS. Analysis of 11,776 hospital admissions for blastomycosis from 2010 through 2020 (HCUP data) demonstrated a 7.9% mortality rate, with increased mortality in patients who were older or who had underlying chronic obstructive pulmonary disease or concomitant malignancy (solid or hematologic). The vast majority of patients who recover from pulmonary blastomy­ cosis do not experience long-term loss of pulmonary function. Cutane­ ous blastomycosis typically results in scarring. ■ ■PREVENTION Prevention of blastomycosis is challenging because most infections are sporadic and unpredictably acquired from the environment. How­ ever, substantial progress has been made in understanding vaccinemediated immunity conferred by a live, attenuated vaccine strain that

CHAPTER 221 is deficient in BAD1. When injected subcutaneously into mice, the

B. dermatitidis BAD1-null vaccine strain induces sterilizing immunity by activating TH17 lymphocytes to protect against lethal pulmonary challenge. Major antigenic components of the vaccine identified to date include calnexin and Blastomyces endoglucanase-2, which is con­ served in other pathogenic fungi, including Histoplasma capsulatum, Coccidioides species, Aspergillus species, Fonsecaea pedrosoi, and Pseu­ dogymnoascus destructans. Neither the BAD1-null attenuated vaccine nor recombinant antigen-based vaccines are commercially available. Cryptococcosis ■ ■FURTHER READING Benedict K et al: Blastomycosis-associated hospitalizations, United States 2010-2020. J Fungi 9:867, 2023. Limper AH et al: An official American Thoracic Society statement: Treatment of fungal infections in adult pulmonary and critical care patients. Am J Respir Crit Care Med 183:96, 2011. Smith D et al: Clinical testing guidance for coccidioidomycosis, histo­ plasmosis, and blastomycosis in patients with community-acquired pneumonia for primary and urgent care providers. Clin Infect Dis 78:1559, 2024. Arturo Casadevall, Shmuel Shoham

Cryptococcosis ■ ■DEFINITION AND ETIOLOGY Cryptococcus, a genus of yeast-like fungi, is the etiologic agent of cryp­ tococcosis. In 2022, the World Health Organization (WHO) declared Cryptococcus neoformans as a critical priority pathogen. Until recently, cryptococcal strains were separated into two species. However, genome sequencing studies have now revealed tremendous diversity among isolates previously assigned to each species, leading to the proposal that each of the prior species classifications includes numerous new species. Hence, C. neoformans and C. gattii are now considered as

species complexes. However, for clinical purposes, these species com­ plexes cause indistinguishable disease referred to as cryptococcosis. Consequently, this chapter will continue to use the nomenclature C. neoformans and C. gattii with the understanding that these terms refer to species complexes.

■ ■EPIDEMIOLOGY Cryptococcosis was first described in the 1890s but remained rare until the mid-twentieth century, when advances in diagnosis and increases in the number of immunosuppressed individuals markedly raised its reported prevalence. Although serologic evidence of cryptococcal infection is common among immunocompetent individuals, cryp­ tococcal disease (cryptococcosis) is rare in the absence of impaired immunity. Conditions associated with high risk for C. neoformans infection include hematologic malignancies, receipt of solid-organ transplants, advanced liver disease, diabetes mellitus, illnesses neces­ sitating glucocorticoid therapy, and advanced HIV infection with CD4+ T lymphocyte counts of <200/μL. In contrast, C. gattii–related disease is not generally associated with specific immune deficits and often occurs in immunocompetent individuals but is associated with autoantibodies to granulocyte-macrophage colony-stimulating factor. Cryptococcal infection is acquired from the environment.

C. neoformans and C. gattii species complexes inhabit different eco­ logic niches. C. neoformans is frequently found in soils contaminated with avian excreta and can be recovered from shaded and humid soils contaminated with pigeon droppings. In contrast, C. gattii is not found in bird feces. Instead, it inhabits a variety of arboreal species, includ­ ing eucalyptus trees. C. neoformans strains are found throughout the world; however, var. grubii (serotype A) strains are far more common than var. neoformans (serotype D) strains among both clinical and environmental isolates. C. gattii was thought to be largely limited to tropical regions until an outbreak of cryptococcosis caused by a new serotype B strain began in Vancouver in 1999. This outbreak has extended into the United States, and C. gattii infections are being encountered increasingly in several states in the Pacific Northwest. PART 5 Infectious Diseases The global burden of cryptococcosis was estimated in 2012 at

~1 million cases, with >600,000 deaths annually, although the prevalence of this disease has declined since then with the greater availability of antiretroviral therapy (ART) for HIV. Since the onset of the HIV pan­ demic in the early 1980s, the overwhelming majority of cryptococcosis cases have occurred in patients with AIDS (Chap. 208). To compre­ hend the impact of HIV infection on the epidemiology of cryptococ­ cosis, it is instructive to note that in the early 1990s there were >1000 cases of cryptococcal meningitis each year in New York City—a figure far exceeding that for all cases of bacterial meningitis. With the advent of effective ART, the incidence of AIDS-related cryptococcosis has been sharply reduced among treated individuals. Therefore, most cases of cryptococcosis now occur in resource-limited regions of the world. The disease remains distressingly common in regions where ART is not readily available (e.g., parts of Africa and Asia); in these regions, up to one-third of patients with AIDS have cryptococcosis. Among HIVinfected persons, those with a decreased percentage of memory B cells expressing IgM may be at greater risk for cryptococcosis. In wealthier countries, most cryptococcosis cases are in people without HIV. ■ ■PATHOGENESIS Cryptococcal infection is almost always acquired by inhalation of aerosolized particles. The exact nature of the infectious particles is not known; the two leading candidate forms are small desiccated yeast cells and basidiospores. Occasionally, infection is acquired via direct inoculation of the skin. Little is known about the pathogenesis of initial infection. Serologic studies have shown that cryptococcal infection is acquired in childhood, but it is not known whether the initial infection is symptomatic. Given that cryptococcal infection is common while disease is rare, the consensus is that pulmonary defense mechanisms in immunologically intact individuals are highly effective at containing this fungus. It is not clear whether initial infection leads to a state of immunity or whether most individuals are subject throughout life to frequent and recurrent infections that resolve without clinical disease.

FIGURE 221-1  Cryptococcal antigen in human brain tissue, as revealed by immunohistochemical staining. Brown areas show polysaccharide deposits in the midbrain of a patient who died of cryptococcal meningitis. (Reproduced with permission from SC Lee et al: Immunohistochemical localization of capsular polysaccharide antigen in the central nervous system cells in cryptococcal meningoencephalitis. Am J Pathol 148:1267, 1996.) However, some human cryptococcal infections lead to a state of latency in which viable organisms are harbored for prolonged periods, possibly in granulomas. Thus, the inhalation of cryptococcal cells and/or spores can be followed by either clearance or establishment of the latent state. The consequences of prolonged harboring of cryptococcal cells in the lung are not known, but evidence from animal studies indicates that the organisms’ prolonged presence could alter the immunologic milieu in the lung and predispose to allergic airway disease. Cryptococcosis usually presents clinically as pulmonary disease and/or chronic meningoencephalitis. The mechanisms by which the fungus undergoes extrapulmonary dissemination and enters the central nervous system (CNS) remain poorly understood. Current evidence suggests that both direct fungal-cell migration across the endothelium and fungal-cell carriage inside macrophages as “Trojan horse” invaders can occur. Cryptococcus species have well-defined virulence factors that include the expression of the polysaccharide cap­ sule, the ability to make melanin, and the elaboration of enzymes (e.g., phospholipase and urease) that enhance the survival of fungal cells in tissue. Among these virulence factors, the capsule and melanin produc­ tion have been most extensively studied. The cryptococcal capsule is antiphagocytic, and the capsular polysaccharide has been associated with numerous deleterious effects on host immune function. Crypto­ coccal infections can elicit little or no tissue inflammatory response. The immune dysfunction seen in cryptococcosis has been attributed to the release of copious amounts of capsular polysaccharide into tissues, where it probably interferes with local immune responses (Fig. 221-1). In clinical practice, the capsular polysaccharide is the antigen that is measured as a diagnostic marker of cryptococcal infection. APPROACH TO THE PATIENT Cryptococcosis Cryptococcosis should be included in the differential diagnosis when any patient presents with findings suggestive of chronic pul­ monary or CNS infection. Concern about cryptococcosis is height­ ened by a history of headache and neurologic symptoms in a patient with an underlying immunosuppressive disorder such as advanced HIV infection, malignancy, immunosuppressive use, or solid-organ transplantation. Evaluation of cerebrospinal fluid (CSF) is critical for diagnosis of CNS disease and should include measurements of CSF pressure, protein, glucose, cell count, Gram stain, cultures, and a cryptococcal antigen assay.

■ ■CLINICAL MANIFESTATIONS The clinical manifestations reflect the site of infection. The spectrum of disease consists predominantly of meningoencephalitis and pneumonia, but skin and soft tissue infections also occur and cryptococcosis can affect any tissue or organ. Symptoms may arise from fungal tissue inva­ sion and from an overactive immune response. In CNS disease, symp­ toms may reflect elevated intracranial pressure (ICP). CNS involvement usually presents as signs and symptoms of chronic meningitis, such as headache, fever, lethargy, sensory deficits, memory deficits, cranial nerve paresis, vision deficits, and meningismus. Cryptococcal meningi­ tis differs from bacterial meningitis in that many patients present with symptoms of several weeks in duration. In addition, classic characteris­ tics of meningeal irritation, such as meningismus, may be absent. Indo­ lent cases can present as subacute dementia or depression. Meningeal cryptococcosis can lead to sudden catastrophic vision loss. Pulmonary cryptococcosis usually presents as cough, increased sputum production, and chest pain. Radiographic findings include nodules, infiltrates, and masses. C. gattii pneumonia can present with granulomatous masses known as cryptococcomas. Fever develops in a minority of cases. Pulmonary cryptococcosis can follow an indolent course, and many cases probably do not come to clinical attention. In fact, many cases are discovered incidentally during the workup of an abnormal chest radiograph obtained for other purposes. Pulmonary cryptococcosis can be associated with antecedent diseases such as malignancy, diabetes, and tuberculosis. Skin lesions are common in patients with disseminated cryptococ­ cosis and can be highly variable, including papules, plaques, purpura, vesicles, tumor-like lesions, and rashes. The spectrum of cryptococ­ cosis in HIV-infected patients is so varied and has changed so much since the advent of ART that a distinction between HIV-related and HIV-unrelated cryptococcosis is no longer pertinent. In highly immu­ nosuppressed patients, the lesions of cutaneous cryptococcosis often resemble those of molluscum contagiosum (Fig. 221-2; Chap. 201). ■ ■DIAGNOSIS A diagnosis of cryptococcosis requires the demonstration of cryp­ tococcal cells or antigen in normally sterile tissues. Visualization of encapsulated fungal cells in CSF mixed with India ink remains a useful rapid diagnostic technique. Cryptococcal cells can also be visualized on Gram stain. However, these staining techniques may yield negative results in patients with a low fungal burden or be misread as positive by inexperienced operators. Cultures of CSF and blood that are positive for cryptococcal cells are diagnostic for cryptococcosis. CSF examina­ tion usually reveals evidence of chronic meningitis with mononuclear FIGURE 221-2  Disseminated fungal infection. A liver transplant recipient developed six cutaneous lesions similar to the one shown. Biopsy and serum antigen testing demonstrated Cryptococcus. Important features of the lesion include a benign-appearing fleshy papule with central umbilication resembling molluscum contagiosum. (Photo courtesy of Dr. Lindsey Baden; with permission.)

cell pleocytosis and increased protein levels. A particularly useful test is cryptococcal antigen (CRAg) detection in CSF and blood. The assay is based on serologic detection of cryptococcal polysaccharide and is both sensitive and specific. A major advance in recent years was the intro­ duction of rapid point-of-care CRAgs that provide results in minutes. A positive CRAg test provides strong presumptive evidence for cryp­ tococcosis; however, because the result is often negative in pulmonary cryptococcosis, the test is less useful for diagnosing pulmonary disease and is of only limited usefulness in monitoring the response to therapy. Polymerase chain reaction is available and effective for the diagnosis of cryptococcosis, but physicians should seek confirmation of positive or negative tests with independent methods such as CRAg and culture.

In areas of Africa where there is a high prevalence of HIV infection, routine screening of blood for CRAg in HIV-infected patients with low CD4+ T lymphocyte counts may identify individuals at high risk of cryptococcal disease who are candidates for antifungal therapy. CRAg screening has shown that a significant proportion of HIV-infected patients hospitalized with pneumonia in Thailand harbor cryptococcal infection. Inexpensive point-of-care CRAg tests in the form of lateral flow assays provide rapid and accurate information and are of great diagnostic benefit in resource-limited regions. TREATMENT Cryptococcosis When the infection is nonsevere and limited to the lungs or other non-CNS sites, treatment is typically with fluconazole. While pul­ monary cryptococcosis in an immunocompetent host sometimes resolves without therapy, given the propensity of Cryptococcus spe­ cies to disseminate from the lung, the inability to gauge the host’s immune status precisely, and the availability of low-toxicity therapy in the form of fluconazole, the current recommendation for nonse­ vere pulmonary cryptococcosis in an immunocompetent individual is fluconazole 400 mg daily for 6–12 months. Fluconazole can cause drug interactions, QT interval prolongation, and liver dysfunction (especially at higher doses), and the dose should be adjusted for renal function. In general, extrapulmonary cryptococcosis without CNS involvement requires less intensive therapy, with the caveat that morbidity and death are associated with meningeal involve­ ment. Thus, the decision to categorize cryptococcosis as “extra­ pulmonary without CNS involvement” should be made only after evaluation of the CSF reveals no evidence of cryptococcal infection. CHAPTER 221 Cryptococcosis When cryptococcosis is severe or involves the CNS, treatment involves induction, consolidation, and maintenance phases. The induction phase ideally includes a combination of intravenous amphotericin B (AmB) and oral flucytosine (5-FC). If feasible, lipid formulations of AmB should be used because those have less toxicity than deoxycholate AmB. The recommended daily doses are liposo­ mal AmB 3–5 mg/kg, AmB lipid complex 5 mg/kg, and deoxycholate AmB 0.7–1 mg/kg. The dose of 5-FC is 25 mg/kg four times daily. 5-FC can cause bone marrow suppression, and the dose should be adjusted for renal function. The duration of induction therapy is a minimum of 2 weeks but may require extension to 4–6 weeks when there is poor initial response to therapy, neurologic complications, or brain cryptococcomas and in people without HIV. Cryptococcal cells with negative cultures may be seen on CSF staining, and elevated cryptococcal antigen may be present in CSF and blood, long after clearance of cultures. These do not indicate treatment failure. For people with HIV, WHO recommends several alternative approaches to induction. These include a single dose of liposomal AmB 10 mg/kg given along with 14 days of 5-FC at 25 mg/kg four times daily and fluconazole 1200 mg daily. If liposomal AmB is not available, a 7-day course of AmB deoxycholate 1 mg/kg per day can be used in its place. If AmB is not available, an all-oral 14-day induction regimen of fluconazole 1200 mg daily and 5-FC 25 mg/kg

four times daily can be used. The consolidation phase is typically with fluconazole 400–800 mg daily for 8 weeks. This is followed by fluconazole maintenance at 200–400 mg daily for a year or longer.

115 - 222 Candidiasis

222 Candidiasis

The goal of maintenance is to prevent recurrent disease, and its duration depends on the immune status of the patient. Individu­ als who remain deeply immunocompromised, such as those with HIV whose CD4+ T lymphocyte counts remain <200/μL or have not attained control of HIV with ART may need to remain on flu­ conazole indefinitely. Newer triazoles like voriconazole, posaconazole, and isavuconazole are highly active against cryptococcal strains and appear to be clinically effective, but clinical experience with these agents is limited. An oral formulation (cochleated) of AmB is in development. The echinocandins are not effective against Cryptococcus species. Addition of a short course of interferon γ to antifungal therapy in patients with HIV infection increases clear­ ance of cryptococci from the CSF. Antifungal drug resistance has not been a major problem with cryptococcal strains, but there are increasing reports of drug-resistant strains, including some emerging during the prolonged therapy needed for cryptococco­ sis. Hence, cryptococcosis that is refractory to antifungal therapy should prompt an investigation into the susceptibility of the clinical isolates in question.

Cryptococcal meningoencephalitis may be associated with increased ICP, which can damage the brain and cranial nerves. CSF pressure should be measured and increased ICP diligently man­ aged in such patients. The mechanism leading to increased ICP is often excess CSF fluid. Neurologic symptoms, including headache, blurred vision, cranial nerve abnormalities, and altered mental sta­ tus, are typical clues to increased ICP. Management requires reduc­ tion of pressure by repeated therapeutic lumbar punctures and the placement of shunts. Neither mannitol nor acetazolamide is effec­ tive. Glucocorticoids are not helpful unless the elevated ICP is from a brain lesion with associated mass effect and edema. PART 5 Infectious Diseases Several clinical syndromes in cryptococcosis are driven by an overactive immune response. In people with HIV disease who are treated with ART, an immune reconstitution inflammatory syndrome (IRIS) occurs when immunity rebounds in the setting of treated cryptococcosis (or an undiagnosed asymptomatic infec­ tion). Similar syndromes can occur in transplant recipients whose immunosuppressive regimens have been reduced to help control the infection and even in non-immunocompromised people with infection that had appeared to be resolving. The immune response triggers an inflammatory reaction that can be difficult to distin­ guish from a relapsing infection. Symptoms can include fevers, headache, lymphadenopathy, and pulmonary, CNS, and cutaneous manifestations. Administration of prophylactic dexamethasone in HIV-associated cryptococcosis is not recommended because it has been associated with reduced fungal clearance and increased mor­ tality. The approach to patients that have already developed IRIS and the related immune-driven syndromes must attempt to balance resurgent immunity against immune-mediated damage. Manage­ ment is individualized and can involve the use of glucocorticoids to reduce inflammation. Apart from the difficulties in distinguishing these inflammatory syndromes from cryptococcal relapse, their management is complex because the cause is often triggered by the desirable outcome of improving immunity, which is important in controlling cryptococcal infection and preventing relapses. A major consideration for clinicians treating symptomatic AIDSrelated cryptococcosis is when to begin ART, which can trigger rebounding immunity. Current recommendations are to start ART 4−6 weeks after initiating antifungal therapy. Screening with serum CRAg is recommended for asymptomatic people with HIV and CD4+ T lymphocyte counts of <100–200/μL. Positive tests should be followed up with careful evaluation for active disease including CSF analysis. For those with antigenemia but no evidence of active disease, a preemptive fluconazole regimen is recommended with 14 days of induction dosing at 800 mg daily and up to 1200 mg daily when fungal burden is high (e.g., CRAg titers exceed 1:80) followed by consolidation and maintenance dosing. Initiation of ART should start 2 weeks after initiation of antifungal therapy in such circumstances.

■ ■PROGNOSIS AND COMPLICATIONS Even with antifungal therapy, cryptococcosis is associated with high rates of morbidity and death. For most patients with cryptococcosis, the most important prognostic factors are the extent and the dura­ tion of the underlying immunologic deficits that predisposed them to develop the disease. Cryptococcosis is often curable with antifungal therapy in individuals with no apparent immunologic dysfunction, but in patients with irreversible immunosuppression, the best that can be hoped for is that antifungal therapy will induce remission, which can then be maintained with lifelong suppressive therapy. Before the advent of ART, the median overall survival period for AIDS patients with cryptococcosis was <1 year. Cryptococcosis in patients with underlying neoplastic disease has a particularly poor prognosis. For CNS crypto­ coccosis, poor prognostic markers are a CSF assay positive for yeast cells on initial India ink examination (evidence of a heavy fungal bur­ den), high CSF pressure, low CSF glucose levels, low CSF pleocytosis (<2/μL), recovery of yeast cells from extraneural sites, absence of anti­ body to capsular polysaccharide, a CSF or serum cryptococcal antigen level of ≥1:32, and concomitant glucocorticoid therapy or hematologic malignancy. A response to treatment does not guarantee cure since relapse of cryptococcosis is common even among patients with rela­ tively intact immune systems and immune reconstitution syndromes can occur in patients who had been improving with antifungal therapy. Complications of CNS cryptococcosis include cranial nerve deficits, vision and hearing loss, and cognitive impairment. ■ ■PREVENTION No vaccine is available for cryptococcosis. Primary prophylaxis with fluconazole 100–200 mg daily is an approach in high-risk HIV patients (e.g., CD4+ T lymphocyte count <100/μL) and can be used in places where cryptococcal antigen testing is not readily available. Since ART raises the CD4+ T lymphocyte count, it constitutes an immunologic form of prophylaxis. ■ ■FURTHER READING Alanio A: Dormancy in Cryptococcus neoformans: 60 years of accu­ mulating evidence. J Clin Invest 130:3353, 2020. Boyer-Chammard T et al: Recent advances in managing HIV-associated cryptococcal meningitis. F1000Res 8:F1000 Faculty Rev-743, 2019. Kwon-Chung KJ et al: The case for adopting the “species complex” nomenclature for the etiologic agents of cryptococcosis. mSphere 2:e00357, 2017. Robertson EJ et al: Cryptococcus neoformans ex vivo capsule size is associated with intracranial pressure and host immune response in HIV-associated cryptococcal meningitis. J Infect Dis 209:74, 2014. Ssebambulidde K et al: Treatment recommendations for non-HIV associated cryptococcal meningoencephalitis including management of post-infectious inflammatory response syndrome. Front Neurol 13:994396, 2022. Tugume L et al: Cryptococcal meningitis. Nat Rev Dis Primers 9:62, 2023. Michail S. Lionakis, Shakti Singh,

Ashraf S. Ibrahim, John E. Edwards, Jr.

Candidiasis The genus Candida encompasses >150 species, only a few of which cause disease in humans. With rare exceptions (although the excep­ tions are increasing in number), the human pathogens are C. albicans, C. guilliermondii (recently revised to Meyerozyma guilliermondii), C. krusei (recently revised to Pichia kudriavzevii), C. parapsilosis,

C. tropicalis, C. lusitaniae (recently revised to Clavispora lusitaniae),

C. dubliniensis, C. glabrata (recently revised to Nakaseomyces glabratus), and the emerging, multidrug-resistant C. auris, which has been respon­ sible for several outbreaks in health care facilities in recent years. Ubiq­ uitous in nature, they inhabit the gastrointestinal tract (including the mouth and oropharynx), the female genital tract, and the skin in the majority of healthy persons. Although cases of candidiasis have been described since antiquity in debilitated patients, the advent of Candida species as common human pathogens dates to the introduction of modern therapeutic approaches that suppress normal host defense mechanisms. Of those advances, the most important are the use of antibacterial agents that alter the normal human microbiota and allow nonbacterial species to become more prevalent in the commensal flora, the use of indwelling intravenous catheters, and the use of cytotoxic, immunosuppressive treatments for malignant and autoimmune disor­ ders. With the introduction of antifungal agents, the causes of Candida infections shifted from an almost complete dominance of C. albicans to the common involvement of C. glabrata and the other species listed above. The non-albicans species now account for approximately half of all cases of candidemia and hematogenously disseminated candidiasis. Recognition of this change is clinically important since the various spe­ cies differ in susceptibility and are increasingly resistant to the newer antifungal agents. Candida is a small, thin-walled, ovoid yeast that measures 4–6 μm in diameter and reproduces by budding. Organisms of this genus occur in three forms in tissue: blastospores, pseudohyphae, and hyphae. Candida grows readily on simple media; lysis centrifugation enhances its recovery from blood. Species are identified by biochemi­ cal testing (currently with automated devices) or on special agar (e.g., CHROMagar). ■ ■EPIDEMIOLOGY Candida are present in humans as commensals, in animals, in foods, and on inanimate objects. In developed countries, where contemporary medical therapeutics are commonly used, Candida species are now among the most common nosocomial pathogens. In the United States, these species are among the four most common pathogens isolated from the blood of hospitalized patients. In fact, in a recent pointprevalence study in the United States, Candida species were the most common organisms infecting the bloodstream of hospitalized patients. In regions where advanced medical care is not readily available, muco­ cutaneous Candida infections, such as thrush, are more common than deep-organ infections, which rarely occur. However, the incidence of deep-organ candidiasis has been increasing steadily as advances in health care—such as therapy with broad-spectrum antibiotics, more aggressive treatment of cancer, and the use of immunosuppression for sustaining organ transplants—have been implemented. In aggregate, the global incidence of infections due to Candida species has risen over the past few decades. C. auris is an emerging species of Candida that has spread rapidly in recent years to >50 countries and is a major public health concern. It was designated an urgent threat in the 2019 Centers for Disease Con­ trol and Prevention (CDC) Antimicrobial Resistance Threat Report, and it was included within the critical priority group in the 2022 World Health Organization (WHO) fungal priority pathogens list. This concern stems from its occurrence in health care facilities, its ability to adhere to and persist long term on inanimate objects (in hospitals) and the human skin despite decolonization efforts, its association with substantial mortality, its propensity for misidentification as other Candida species, the incomplete understanding of its environmental reservoirs, and its multidrug resistance to the current antifungal thera­ peutic armamentarium, with some C. auris strains being resistant to all antifungal drug classes currently available for treatment. C. auris (auris meaning ear in Latin) was first identified in 2009 from the ear drain­ age of a patient with an ear infection in Japan. However, subsequent retrospective analysis of Candida strain collections identified the earli­ est known C. auris strain to date back to 1996 in South Korea. Notably, whole genome sequencing analysis of C. auris strains from South Asia, East Asia, South America, South Africa, and Iran found that although

strains within each geographic region are closely related to each other, they are distinct compared to strains from other geographic regions. These findings indicate that C. auris emerged independently in mul­ tiple geographic locations around the same time; the epidemiologic reasons for this emergence remain poorly understood but may relate to the increasing use of antifungal drugs and climate change.

The presence of a central venous catheter and/or other invasive medical devices and recent residence in nursing homes are major risk factors for C. auris colonization and infection. Screening of selected patients who are in a hospital or nursing home where C. auris has been cultured and are at risk for dissemination from a colonization site may help in implementing effective infection control measures. Hand hygiene with an alcohol-based hand sanitizer is recommended when hands are not visibly soiled, in which case washing with soap and water is preferred. Identifying the source of contamination, if possible, and using an Environmental Protection Agency (EPA)-registered hospitalgrade disinfectant effective against Clostridioides difficile spores are desirable. If a patient develops an invasive or bloodstream infection, it is recommended that the health care facility informs the CDC, or a similar agency in other countries, and adheres to recommendations for infection control, including isolation of patients (contact or enhanced barrier precautions), use of proper personal protective coverings, enforcement of hospital environment hygiene, and communicating with other health care facilities if the patient is being transferred. ■ ■PATHOGENESIS In the most severe form of Candida infection, the organisms dis­ seminate hematogenously and form microabscesses and small mac­ roabscesses in major organs. Although the exact mechanism is not known, Candida probably enters the bloodstream from mucosal surfaces after growing to large numbers as a consequence of bacterial suppression by antibacterial drugs and breaches in the integrity of the mucosal barrier; alternatively, in some instances, the organism may enter the bloodstream from the skin via central venous catheters. A change from the blastospore stage to the pseudohyphal and hyphal stages is generally considered integral to Candida’s penetration into tissue. However, C. glabrata and C. auris can cause life-threatening infection, even though they do not transform into pseudohyphae or hyphae. Adherence to both epithelial and endothelial cells is thought to be the first step in invasion and infection; several adhesins have been identified as well as a mucosal toxin, candidalysin. Biofilm formation also is considered important in pathogenesis. Numerous reviews of cases of hematogenously disseminated candidiasis have identified the predisposing factors or conditions associated with disease (Table 222-1). CHAPTER 222 Candidiasis Several genes that are involved in the pathogenesis of other Candida species—such as those responsible for biofilm formation, proteinases, lipases, phospholipases, hydrolases, adhesins, secreted aspartyl prote­ ases, and transporters involved in azole resistance—are also present in C. auris. Unlike other Candida species, several C. auris strains exhibit aggregate-forming properties in vivo, which may enable immune eva­ sion. In addition, C. auris shows a unique tolerance to high temperature and saline concentrations and can grow optimally at up to 42°C and in a 10% saline concentration, making it possible to exist and persist in harsh environments. Furthermore, C. auris has significant affinity for TABLE 222-1  Well-Recognized Factors and Conditions Predisposing to Hematogenously Disseminated Candidiasis Antibacterial agents Indwelling intravenous catheters Hyperalimentation fluids Indwelling urinary catheters Parenteral glucocorticoids Severe burns CARD9 deficiency (central nervous system) Abdominal and thoracic surgery Cytotoxic chemotherapy Immunosuppressive agents for organ transplantation Respirators Myeloperoxidase deficiency Neutropenia Low birth weight (neonates) Diabetes

abiotic surfaces such as plastic materials and medical devices, as well as human skin and nasal and ear cavities, which may account for its persistent colonization capabilities. The C. auris–specific adhesin sur­ face colonization factor 1 (SCF1) was recently identified as a key fungal factor governing biofilm formation, colonization of skin and medical devices, and virulence during invasive infection.

Innate immunity is the most important defense mechanism against hematogenously disseminated candidiasis, and the neutrophil is the most potent component of this defense. Macrophages also play an important host defense role. On the other hand, interleukin (IL) 17– producing innate and adaptive lymphoid cells contribute significantly to defense against mucocutaneous candidiasis as evidenced by several monogenic disorders of IL-17 receptor signaling that manifest with chronic mucocutaneous candidiasis (CMC) (see “Clinical Manifestations,” below). Although many immunocompetent individuals have antibodies to Candida, the role of these antibodies in defense against the organism is not clear. Multiple genetic polymorphisms in host immune-related genes that predispose to both disseminated and focal candidiasis have been identified and may contribute to patient susceptibility. ■ ■CLINICAL MANIFESTATIONS Mucocutaneous Candidiasis  Thrush is characterized by white, adherent, painless, discrete or confluent patches in the mouth, on the tongue, or in the esophagus, occasionally with fissuring at the corners of the mouth. This form of disease caused by Candida can also occur at points of contact with dentures (called “denture sore mouth”). Organisms are identifiable in gram-stained scrapings from lesions. The occurrence of thrush in a young, otherwise healthy-appearing person should prompt an investigation for underlying HIV infec­ tion. More commonly, thrush is seen as a nonspecific manifestation of severe debilitating illness. Vulvovaginal candidiasis is accompanied by pruritus, pain, and vaginal discharge, which is usually thin but may contain whitish “curds” in severe cases. In contrast to oral thrush, HIV is not considered a major risk factor for vulvovaginal candidiasis. Instead, many women who receive antibiotics, particularly β-lactams, may develop vulvovaginal candidiasis. A subset of patients with recur­ rent vulvovaginitis may have a deficiency in the surface expression of Dectin-1, encoded by CLEC7A, a major recognition factor for β-glucan on the surface of Candida and/or in the downstream adaptor molecule CARD9, which ultimately increases the propensity for recurrent muco­ cutaneous (including vaginal) infections. PART 5 Infectious Diseases Other Candida skin infections include paronychia, a painful swelling at the nail–skin interface; onychomycosis, a fungal nail infection rarely caused by this genus; intertrigo, an erythematous irritation with red­ ness and pustules in the skin folds; balanitis, an erythematous-pustular infection of the glans penis; erosio interdigitalis blastomycetica, an infec­ tion between the digits of the hands or toes; folliculitis, with pustules developing most frequently in the area of the beard; perianal candidia­ sis, a pruritic, erythematous, pustular infection surrounding the anus; mastitis; and diaper rash, a common erythematous, pustular perineal infection in infants. Generalized disseminated cutaneous candidiasis, another form of infection that occurs primarily in infants, is character­ ized by widespread eruptions over the trunk, thorax, and extremities. The diagnostic macronodular lesions of hematogenously disseminated candidiasis (Fig. 222-1) indicate a high probability of dissemination to multiple organs as well as the skin. While the lesions are seen predomi­ nantly in immunocompromised patients treated with cytotoxic drugs, they may also develop in patients without neutropenia. CMC is a heterogeneous infection of the hair, nails, skin, and mucous membranes that persists despite intermittent antifungal therapy. The onset of disease usually comes in infancy or within the first two decades of life, but in rare cases, it occurs in later life. The condition may be mild and limited to a specific area of the skin or nails, or it may take a severely disfiguring form (Candida granuloma) characterized by exophytic outgrowths on the skin. CMC is usually associated with specific immunologic dysfunction; most frequently reported is a failure of lymphocytes to secrete or respond to type-17 cytokines following stimulation by Candida antigens in vitro. A subset of the affected

FIGURE 222-1  Macronodular skin lesions associated with hematogenously disseminated candidiasis. Candida organisms are usually but not always visible on histopathologic examination. The fungi grow when a portion of the biopsied specimen is cultured. Therefore, for optimal identification, both histopathology and culture should be performed. (Image courtesy of Dr. Noah Craft and the Victor Newcomer collection at UCLA, archived by Logical Images, Inc.; with permission.) patients has mutations in the IL-17 receptors IL-17RA or IL-17RC, its adaptor molecule ACT1 (TRAF3IP2), or, more often, in STAT1. Approximately half of patients with CMC have associated endocrine abnormalities either in the setting of gain-of-function mutations in STAT1 or in the context of autoimmune polyendocrinopathy–candidiasis– ectodermal dystrophy (APECED) syndrome. This syndrome is due to mutations in the autoimmune regulator (AIRE) gene and is most prevalent among Finns, Iranian Jews, and Sardinians. Conditions that usually follow the onset of the disease include hypoparathyroidism, adrenal insufficiency, autoimmune thyroiditis, autoimmune hepatitis, autoimmune pneumonitis, alopecia, pernicious anemia, intestinal mal­ absorption, and primary hypogonadism. In addition, dental enamel dysplasia, vitiligo, nail dystrophy, asplenia, and calcification of the brain and tympanic membranes may occur. Patients with CMC rarely develop hematogenously disseminated candidiasis, reflecting their intact neutrophil function. Deeply Invasive Candidiasis  Deeply invasive Candida infections may or may not be due to hematogenous seeding. Deep esophageal infection may result from penetration by organisms from superficial esophageal erosions; joint or deep-wound infection from contiguous spread of organisms from the skin; kidney infection from catheterinitiated ascending spread of organisms through the urinary tract; infection of intraabdominal organs and the peritoneum from perfora­ tion of the gastrointestinal tract; and gallbladder infection from retro­ grade migration of organisms from the gastrointestinal tract into the biliary drainage system. However, more commonly, deeply invasive candidiasis results from hematogenous seeding of various organs as a complication of candi­ demia. Once the organism gains access to the intravascular compart­ ment (either from the gastrointestinal tract or, less often, from the skin through the site of an indwelling intravascular catheter), it may spread hematogenously to a variety of deep organs. The brain, chorioretina (Fig. 222-2), heart, and kidneys are most commonly infected and the liver and spleen are less commonly affected in nonneutropenic hosts (but most often involved in neutropenic patients). In fact, nearly any organ can become involved, including the endocrine glands, pancreas, heart valves (native or prosthetic), skeletal muscle, joints (native or prosthetic), bones, and meninges. Candida organisms can also spread hematogenously to the skin and cause classic macronodular lesions (Fig. 222-1). Frequently, painful muscular involvement is evident beneath the area of affected skin. Chorioretinal involvement and skin involvement are highly significant since both findings are associ­ ated with a very high probability of abscess formation in multiple

FIGURE 222-2  Hematogenous Candida endophthalmitis. A classic off-white lesion projecting from the chorioretina into the vitreous causes the surrounding haze. The lesion is composed primarily of inflammatory cells rather than organisms. Lesions of this type may progress to cause extensive vitreal inflammation and eventual loss of the eye. Partial vitrectomy, combined with IV and possibly intravitreal antifungal therapy, may be helpful in controlling the lesions. (Image courtesy of Dr. Gary Holland; with permission.) deep organs as a result of generalized hematogenous seeding. Ocular involvement (Fig. 222-2) may require specific treatment (e.g., partial vitrectomy or intraocular injection of antifungal agents) to prevent per­ manent blindness. An ocular examination is indicated for patients with candidemia, whether or not they have ocular manifestations. C. auris invasive infections are similar to those of other Candida species and are most frequently associated with recent surgical procedures, immu­ nosuppression, invasive devices such as catheters or various support or drainage tubes, and extended hospital stays. In the majority of invasive infections, C. auris has been isolated from the blood, but invasion of the kidney or spleen, and its recovery from cerebrospinal, bile, perito­ neal, and pleural fluids demonstrate its invasiveness and dissemination potential. C. auris–associated candidemia can be life-threatening, with a crude mortality rate of 30–60%. ■ ■DIAGNOSIS The diagnosis of Candida infection is established by visualization of pseudohyphae or hyphae on wet mount (saline and 10% KOH), tis­ sue Gram stain, periodic acid–Schiff stain, or methenamine silver stain in the presence of inflammation. Absence of organisms on hematoxylin-eosin staining does not reliably exclude Candida infec­ tion. The most challenging aspect of diagnosis is determining which patients with Candida isolates have hematogenously disseminated A B C FIGURE 222-3  C. auris colony morphology and color on CHROMagar plates. A. Candida mixed culture: culture of C. glabrata (purple), C. tropicalis (navy blue), and C. auris (white, circled in red). B. C. auris showing multiple colony morphologies. C. C. auris after Salt SAB Dulcitol Broth enrichment. (From CDC: Identification of Candida auris. Available at: https://www.cdc.gov/fungal/candida-auris/identification.html.)

candidiasis. For instance, recovery of Candida from sputum, urine, or peritoneal catheters may indicate mere colonization rather than deepseated infection, and Candida isolation from the blood of patients with indwelling intravascular catheters may reflect inconsequential seeding of the blood from or growth of the organisms on the catheter. Despite extensive research into both antigen and antibody detection systems, there is currently no widely available and validated diagnostic test to distinguish patients with inconsequential seeding of the blood from those whose positive blood cultures represent hematogenous dissemi­ nation to multiple organs. Many studies have examined the utility of the β-glucan test; at present, its greatest utility is its negative predictive value (~90%). Meanwhile, the presence of ocular or macronodular skin lesions is highly suggestive of widespread infection of multiple deep organs. Despite extensive diagnostic tests for hematogenous dissemi­ nation, such as polymerase chain reaction and T2 technology, no test is fully validated or widely available at present. Matrix-assisted laser desorption–ionization–time-of-flight mass spectrometry (MALDITOF MS) is now being used extensively for detection and speciation and is useful for the correct diagnosis of C. auris.

C. auris can be misdiagnosed in the microbiology laboratory, often leading to inappropriate treatment and delay in the implementation of appropriate infection control measures. Preliminary testing by culturing the fungus and examination of colony morphology may help in the initial identification, but this must be confirmed with more advanced diagnos­ tic methods. For example, features such as budding yeast morphology, absence of hyphal growth or germ tubes, and growth at 40–42°C (unlike other Candida species) on CHROMagar that may appear white, pink, red, or purple should raise suspicion for C. auris (Fig. 222-3). CHAPTER 222 Several advanced molecular techniques accurately identify C. auris strains and therefore are being used for the follow-up testing and confirmation of the specimens that failed to be identified by tradi­ tional methods. MALDI-TOF equipment with upgraded libraries, such as Bruker Biotyper MALDI-TOF (CA System library version claim 4 or research use only [RUO] libraries versions 2014 [5627] and more recent), and using the bioMérieux VITEK MALDI-TOF MS (IVD v3.2 or RUO libraries with Saramis Ver 4.14 database and Sac­ charomycetaceae update), are the most common methods of C. auris identification. Other supplemental MALDI-TOF databases, such as MicrobeNet, which include additional C. auris strains from the four phylogenetic clades (i.e., South Asian, East Asian, South American, and South African) also can be used for the identification of C. auris strains. Sequencing of the D1–D2 region of the 28s rDNA or the internal transcribed region (ITS) of rDNA can also correctly identify C. auris. Recently, an automated, qualitative nucleic acid multiplex in vitro diagnostic test by GenMark called ePlex Blood Culture Identifica­ tion Fungal Pathogen (BCID-FP) Panel was approved by the U.S. Food and Drug Administration for C. auris testing. Also, several polymerase chain reaction–based detection methods have been reported to identify

Candidiasis

TABLE 222-2  Typical Decision-Making Steps in the Diagnosis of C. auris NO. METHOD DATABASE/SOFTWARE INITIAL FINDING CONFIRMATION 1. Bruker Biotyper MALDI-TOF RUO libraries C. auris C. auris CA System library C. auris C. auris 2. bioMérieux VITEK MS MALDI-TOF RUO library C. auris C. auris IVD library (v3.2) C. auris C. auris Older IVD libraries C. haemulonii C. auris possible: Needs further workup C. lusitaniae C. auris possible: Needs further workup No identification C. auris possible: Needs further workup 3. VITEK 2 YST Software version 8.01 C. auris C. auris confirmed C. haemulonii C. auris possible: Needs further workup C. duobushaemulonii C. auris possible: Needs further workup Candida spp. not identified C. auris possible: Needs further workup Older versions C. haemulonii C. auris possible: Needs further workup C. duobushaemulonii C. auris possible: Needs further workup Candida spp. not identified C. auris possible: Needs further workup 4. API 20C   Rhodotorula glutinis, if characteristic red color absent C. auris possible: Needs further workup C. sake C. auris possible: Needs further workup Candida spp. not identified C. auris possible: Needs further workup 5. API ID 32C   C. intermedia C. auris possible: Needs further workup C. sake C. auris possible: Needs further workup Saccharomyces kluyveri C. auris possible: Needs further workup 6. BD Phoenix   C. catenulata C. auris possible: Needs further workup C. haemulonii C. auris possible: Needs further workup Candida spp. not identified C. auris possible: Needs further workup PART 5 Infectious Diseases 7. MicroScan   C. lusitaniae No hyphal growth present Can rule out C. lusitaniae, C. guilliermondii, and C. parapsilosis. C. auris possible: Needs further workup C. guilliermondii C. parapsilosis C. lusitaniae Hyphal growth present Possibly C. lusitaniae, C. guilliermondii, C. parapsilosis, or C. auris: Needs further workup C. guilliermondii C. parapsilosis C. famata C. auris possible: Needs further workup Candida spp. not identified C. auris possible: Needs further workup 8. RapID Yeast Plus   C. parapsilosis → Test on corneal agar Candida spp. not identified C. auris possible: Needs further workup 9. GenMark ePlex BCID-FP Panel   C. auris C. auris confirmed Abbreviations: IVD, in vitro diagnostic; RUO, research use only. Source: Adapted from CDC: Identification of Candida auris. Available at: https://www.cdc.gov/fungal/candida-auris/pdf/Testing-algorithm_by-Method_508.pdf. C. auris in various specimens. Table 222-2 outlines the typical decision-making steps in the diagnosis of C. auris by using different methods. A suspicious C. auris specimen is usually sent to a regional reference laboratory for further testing and confirmation of C. auris. TREATMENT Candida Infections MUCOCUTANEOUS CANDIDA INFECTION The treatment of mucocutaneous candidiasis is summarized in Table 222-3. CANDIDEMIA AND SUSPECTED HEMATOGENOUSLY DISSEMINATED CANDIDIASIS All patients with candidemia are treated with a systemic anti­ fungal agent. A certain percentage of patients, including many of those who have candidemia associated with an indwelling intra­ vascular catheter, probably have “benign” candidemia rather than

No hyphal growth present Can rule out C. parapsilosis. C. auris possible: Needs further workup Hyphal growth present Possibly C. parapsilosis or C. auris: Needs further workup deep-organ seeding. However, because there is no reliable way to distinguish benign candidemia from deep-organ infection, and because antifungal drugs less toxic than amphotericin B are avail­ able, antifungal treatment for candidemia—with or without clinical evidence of deep-organ involvement—has become the standard of practice. In addition, if an indwelling intravascular catheter is TABLE 222-3  Treatment of Mucocutaneous Candidal Infections DISEASE PREFERRED TREATMENT ALTERNATIVES Cutaneous Topical azole Topical nystatin Vulvovaginal Oral fluconazole (150 mg) or ibrexafungerp (300 mg twice daily for 1 day) or azole cream or suppository Nystatin suppository Oral (thrush) Fluconazole tablets (100–200 mg/d) Clotrimazole trashes, nystatin Esophageal Fluconazole tablets (100–200 mg/d) or itraconazole solution (200 mg/d) Caspofungin, micafungin, or amphotericin B

TABLE 222-4  Agents for the Treatment of Disseminated Candidiasis ROUTE OF ADMINISTRATION DOSEa COMMENT AGENT Amphotericin B deoxycholate IV only 0.5–1.0 mg/kg daily Mostly replaced by lipid formulations Amphotericin B lipid formulations     Not approved as primary therapy by the U.S. Food and Drug Administration, but used commonly because they are less toxic than amphotericin B deoxycholate   Liposomal (AmBiSome, Abelcet) IV only 3.0–5.0 mg/kg daily     Lipid complex (ABLC) IV only 3.0–5.0 mg/kg daily     Colloidal dispersion (ABCD) IV only 3.0–5.0 mg/kg daily Associated with frequent infusion reactions Azolesb         Posaconazole IV and oral 300 mg/d (IV) 200 mg tid (oral)   Fluconazole IV and oral 400 mg/d Most commonly used   Voriconazole IV and oral 400 mg/d Multiple drug interactions, visual hallucinations, fluorosis, phototoxicity Approved for candidemia in nonneutropenic patients Echinocandins     Broad spectrum against Candida species; approved for disseminated candidiasis; less toxic than amphotericin B formulations   Caspofungin IV only 50 mg/d     Anidulafungin IV only 100 mg/d     Micafungin   Rezafungin IV only IV only 100 mg/d 400 mg loading dose, 200 mg once weekly thereafter aFor loading doses and adjustments in renal failure, see Pappas PG et al: Clinical practice guidelines for the management of candidiasis: 2016 update by the Infectious Diseases Society of America. Clin Infect Dis 62:e1, 2016. The recommended duration of therapy is 2 weeks beyond the last positive blood culture and the resolution of signs and symptoms of infection. bAlthough ketoconazole is approved for the treatment of disseminated candidiasis, it has been replaced by the newer agents listed in this table. Posaconazole has been approved for prophylaxis in neutropenic patients and for oropharyngeal candidiasis. present, it is best to remove or replace the device whenever feasible. Moreover, an infectious disease consultation is recommended as it has been associated with improved patient outcomes. The drugs used for the treatment of candidemia and suspected disseminated candidiasis are listed in Table 222-4. Various lipid formulations of amphotericin B, four echinocandins, the azoles fluconazole and voriconazole, and in some instances, the newer triazole posaconazole are used; no agent within a given class has been clearly identified as superior to the others. Most institutions choose an agent from each class on the basis of their own specific microbial epidemiology, strategies to minimize toxicities, and cost considerations. An echinocandin is the first choice of treatment. The U.S. Food and Drug Administration (FDA) recently approved the new-generation echinocandin rezafungin for the treatment of candidemia and invasive candidiasis in adult patients; rezafungin has a prolonged half-life, which allows for once-weekly 200-mg dos­ ing following a 400-mg front-loading dose. Echinocandin treatment continues until sensitivities or speciation is determined. In stable patients, many centers then switch to fluconazole if a sensitive strain is identified and there is no evidence of hematogenous dissemina­ tion. For hemodynamically unstable or neutropenic patients, initial treatment with echinocandins is warranted, and once the clinical response has been assessed and the pathogen specifically identified, the regimen can be altered according to the sensitivities. At present, the vast majority of C. albicans isolates are sensitive to fluconazole. Isolates of C. glabrata and C. krusei are less sensitive to fluconazole and more sensitive to polyenes and echinocandins. C. parapsilosis is less sensitive to echinocandins in vitro; however, this lesser sen­ sitivity is considered clinically insignificant. Posaconazole has been approved for prophylaxis, including against Candida, in neutropenic patients. Itraconazole is rarely used for Candida nowadays, and isa­ vuconazole is not recommended for this indication. Antifungal drug resistance is one of the hallmarks of C. auris infections. Some C. auris strains have multidrug resistance with elevated minimal inhibitory concentrations (MICs) to all three major antifungal classes—azoles, echinocandins, and polyenes— resulting in limited treatment options. A CDC study reported antifungal resistance in C. auris strains obtained from 54 patients

Approved for prophylaxis   CHAPTER 222 in India, Pakistan, South Africa, and Venezuela: 93% were resistant to fluconazole, 35% to amphotericin B, and 7% to echinocandins; 41% of the tested strains were resistant to two antifungal classes, and, alarmingly, 4% of the tested strains were resistant to all three classes of antifungal drugs. A 2023 CDC report indicated that rates of C. auris echinocandin resistance tripled in 2021 compared to the prior 2 years. Almost all C. auris strains that have been identified have elevated MICs for fluconazole with variable susceptibilities to other triazoles (Table 222-5), associated with mutations in ERG11encoded lanosterol demethylase and/or overexpression of drug transporters/efflux pumps. Candidiasis Due to the high rates of azole resistance among C. auris strains, the use of echinocandins is recommended as first-line therapy for

C. auris infection. By contrast, the CDC discourages the use of antifungal drugs for the treatment of colonization of C. auris in the absence of invasive or bloodstream infection. A history of patient travel or residence in a health care or nursing facility with a known TABLE 222-5  Typical MICs of Available Antifungal Drugs for C. auris TENTATIVE RESISTANCE BREAKPOINTSa MIC RANGE, μg/mL DRUG MIC MIC50 MIC90 Amphotericin B ≥2 0.06–8 0.5–1 2–4 Fluconazole ≥32 0.12–≥64 ≥64 ≥64 Itraconazole N/A 0.032–2 0.06–0.5 0.25–1 Voriconazole N/A 0.032–16 0.5–2 2–8 Posaconazole N/A 0.015–16 0.016–0.5 0.125–2 Isavuconazole N/A 0.015–4 0.125–0.25 0.5–2 Caspofungin ≥2 0.03–16 0.25–1 1–2 Anidulafungin ≥4 0.015–16 0.125–0.5 0.5–1 Micafungin ≥4 0.015–8 0.125–0.25 0.25–2 aTentative resistance breakpoints per Centers for Disease Control and Prevention (CDC) (www.cdc.gov/fungal/candida-auris/c-auris-antifungal.html). Abbreviations: MIC, minimum inhibitory concentration; N/A, not available. Source: Adapted from CDC: Antifungal susceptibility testing and interpretation. Available at: www.cdc.gov/fungal/candida-auris/c-auris-antifungal.html.

TABLE 222-6  List of CDC-Recommended Echinocandin Doses for the Treatment of C. auris Infections CHILDREN

(>2 MONTHS) INFANTS

(<2 MONTHS) DRUG ADULTS Caspofungin Loading dose 70 mg IV, then 50 mg IV daily Loading dose 70 mg/m2 per day IV, then 50 mg/m2 per day IV 25 mg/m2 per day IV Anidulafungin Loading dose 200 mg IV, then 100 mg IV daily Not approved for use in children Not approved for use in children Micafungin 100 mg IV daily 2 mg/kg per day IV with option to increase to 4 mg/ kg per day IV in children at least 40 kg 10 mg/kg per day IV Abbreviation: CDC, Centers for Disease Control and Prevention. Source: Adapted from CDC: Treatment and management of infections and colonization. Available at: www.cdc.gov/fungal/candida-auris/c-auris-treatment.html. outbreak of C. auris infection, as well as drug susceptibility data of identified strains, act as a guide for the effective choice of treatment of invasive and bloodstream infections. C. auris is known to develop antibiotic resistance during treatment. Therefore, the emergence of antifungal resistance should be closely monitored with follow-up cul­ tures and repeat susceptibility testing. Antibiotic stewardship should be implemented to ameliorate the risk of development of drug resis­ tance. Patients may remain colonized with C. auris during or after the successful treatment of invasive C. auris infection. Therefore, infec­ tion control measures should be implemented throughout patient care. Table 222-6 outlines CDC-recommended echinocandin doses for the initial antifungal treatment for C. auris infections. PART 5 Infectious Diseases In cases of echinocandin resistance, liposomal amphotericin B (5 mg/kg per day) can be considered. For neonates and infants (<2 months old), amphotericin B deoxycholate (1 mg/kg per day) treatment can be initiated. If this fails, liposomal amphotericin B (5 mg/kg per day) can be given. In very severe cases, if all treat­ ment options fail, echinocandins per CDC recommendations can be given (Table 222-6). Other considerations for C. auris infection management can be referenced from the 2016 Infectious Diseases Society of America (IDSA) Clinical Practice Guideline for the Man­ agement of Candidiasis. Some generalizations exist regarding the management of specific Candida infections. Recovery of Candida from sputum is almost never indicative of underlying pulmonary candidiasis and does not by itself warrant antifungal treatment. Similarly, Candida in the urine of a patient with an indwelling bladder catheter may represent colonization only, rather than bladder or kidney infection. However, the threshold for systemic treatment is lower in general in severely ill patients in this category since it is impossible to distinguish colo­ nization from lower or upper urinary tract infection. If the isolate is

C. albicans, most clinicians use oral fluconazole rather than a bladder washout with amphotericin B, which was more commonly used in the past. Although echinocandins are poorly excreted into the urine, they may be an option, especially for non-albicans isolates. The doses and duration are the same as for disseminated candidiasis. The sig­ nificance of the recovery of Candida from abdominal drains in post­ operative patients is unclear, but again, the threshold for treatment is generally low because most of the affected patients have been sub­ jected to risk factors predisposing them to disseminated candidiasis. In addition, there has been a considerable increase in the recognition and diagnosis of intraabdominal candidiasis. Removal of the infected valve and long-term antifungal admin­ istration constitute appropriate treatment for Candida endocardi­ tis. Although definitive studies are not available, patients usually are treated for weeks with a systemic antifungal agent (Table 222-4) and then given chronic suppressive therapy for months or years (sometimes indefinitely) with an oral azole (usually fluconazole at 400–800 mg/d). Hematogenous Candida endophthalmitis is a special problem requiring ophthalmologic consultation. When lesions are expanding

or are threatening the macula, an IV polyene combined with flucy­ tosine (25 mg/kg four times daily) has been the regimen of choice, although comparative studies with other regimens have not yet been reported. As more data on the newer triazoles (e.g., voriconazole) and the echinocandins become available, new strategies involving these agents are developing, although it is important to note that echinocandins exhibit low penetration in ocular tissue. Of para­ mount importance is the decision to perform a partial vitrectomy. This procedure debulks the infection and can preserve sight, which may otherwise be lost due to vitreal scarring. All patients with can­ didemia should undergo ophthalmologic examination because of the relatively high frequency of this ocular complication (up to 15–20% in some case series). This examination can detect a developing eye lesion early in its course; in addition, identification of a lesion signi­ fies a probability of ~90% of deep-organ abscesses and may prompt prolongation of therapy for candidemia beyond the recommended 2 weeks after the last positive blood culture. Although the basis for the consensus is a very small data set, the recommended treatment for Candida meningitis is a polyene (Table 222-4) plus flucytosine (25 mg/kg four times daily). Development of Candida meningo­ encephalitis in an otherwise immunocompetent individual should raise suspicion for deficiency in the C-type lectin receptor adaptor molecule CARD9 and should prompt genetic testing to rule out this monogenic disorder. Successful treatment of Candida-infected pros­ thetic material (e.g., an artificial joint) nearly always requires removal of the infected material followed by long-term administration of an antifungal agent selected on the basis of the isolate’s sensitivity and the logistics of administration. ■ ■PROPHYLAXIS The use of antifungal agents to prevent Candida infections has been controversial, but some general principles have emerged. Most centers administer prophylactic fluconazole (400 mg/d) to recipients of alloge­ neic hematopoietic stem cell transplantation. High-risk liver transplant recipients are also given fluconazole prophylaxis in most centers. The use of prophylaxis for neutropenic patients has varied considerably from center to center; many centers that elect to give prophylaxis to this population use either fluconazole (200–400 mg/d) or a lipid formula­ tion of amphotericin B (AmBisome, 1–2 mg/d). Caspofungin (50 mg/d) also has been used. Posaconazole (200 mg three times daily) has been approved by the FDA for prophylaxis in neutropenic patients; it is gain­ ing in popularity and may replace fluconazole in settings when mold activity is desired. Prophylaxis is sometimes given to surgical patients at very high risk for candidiasis. The widespread use of prophylaxis for nearly all patients in general surgical or medical intensive care units is not—and should not be—a common practice for three reasons: (1) the incidence of disseminated candidiasis is relatively low, (2) the cost–benefit ratio is suboptimal, and (3) increased resistance with widespread prophylaxis is a valid concern. Prophylaxis for oropharyngeal or esophageal candidiasis in HIVinfected patients is not recommended unless there are frequent recurrences. ■ ■FURTHER READING Lionakis MS, Edwards JE jr: Candida species, in Mandell, Douglas, and Bennett’s Principles of Infectious Diseases, 10th ed. Blaser MJ et al (eds). Philadelphia, Elsevier, 2025. Pappas PG et al: Invasive candidiasis. Nat Rev Dis Primers 62:e1, 2018. Pechacek J, Lionakis MS: Host defense mechanisms against Candida auris. Expert Rev Anti Infect Ther 21:1087, 2023. Proctor DM et al: Integrated genomic, epidemiologic investigation of Candida auris skin colonization in a skilled nursing facility. Nat Med 27:1401, 2021. Santana DJ et al: A Candida auris-specific adhesin, Scf1, governs sur­ face association, colonization, and virulence. Science 381:1461, 2023. Tsai SV et al: Burden of candidemia in the United States, 2017. Clin Infect Dis 71:e449, 2020.

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223 Aspergillosis

David W. Denning

Aspergillosis Aspergillosis is the collective term used to describe all disease enti­ ties caused by any one of ~50 pathogenic and allergenic species of Aspergillus. Only those species that grow at 37°C can cause invasive infection, although some species without this ability can cause allergic syndromes. Each common pathogenic species is actually a complex of many species (many of them cryptic) but is referred to as a single species here for simplicity. A. fumigatus is responsible for most cases of invasive aspergillosis, >95% of cases of chronic aspergillosis, and most allergic syndromes. A. flavus is more prevalent in some hospitals and causes a higher proportion of cases of sinus infections, cutaneous infections, and keratitis than A. fumigatus. A. niger can cause invasive infection but more commonly colonizes the respiratory tract and causes external otitis. A. terreus causes only invasive disease, usually with a poor prognosis. A. nidulans occasionally causes invasive infec­ tion, primarily in patients with chronic granulomatous disease (CGD). ■ ■EPIDEMIOLOGY AND ECOLOGY Aspergillus has a worldwide distribution, most commonly growing in decomposing plant materials (i.e., compost) and in bedding. This hya­ line (nonpigmented), septate, branching mold produces vast numbers of conidia (spores) on stalks above the surface of mycelial growth. Aspergilli are found in indoor and outdoor air, on surfaces, and in water from surface reservoirs. Daily exposures vary from a few to many millions of conidia; high numbers of conidia are encountered in hay barns and other very dusty environments. The required size of the infecting inoculum is uncertain; however, only intense exposures (e.g., during construction work, handling of moldy bark or hay, or composting) are demonstrated to cause disease—acute community-acquired pulmonary aspergillosis— in healthy immunocompetent individuals or exacerbations of COPD. Allergic syndromes are exacerbated by continuous antigenic exposure arising from sinus or airway colonization or from nail infection. Highefficiency particulate air (HEPA) filtration is often protective against infection; thus, HEPA filters should be installed and monitored for effi­ ciency in operating rooms, burn units and in areas of the hospital that house high-risk patients. The incubation period of invasive aspergillosis after exposure is highly variable, extending in documented cases from 2 to 90 days. Thus, community acquisition of an infecting strain frequently mani­ fests as invasive infection during hospitalization, although nosocomial acquisition is also common. Outbreaks usually are directly related to a contaminated air source or construction in the hospital. Global aspergillosis incidence and prevalence have been estimated (Table 223-1). The frequency of different manifestations of asper­ gillosis varies considerably with geographic location; most notably, chronic granulomatous sinusitis is rare outside the Middle East and India. Fungal (mycotic) keratitis is particularly common in Southeast Asia but occurs globally. Chronic pulmonary aspergillosis follows pulmonary tuberculosis in ~6–13% of treated cases and also mimics pulmonary tuberculosis as smear-negative or “clinically diagnosed” tuberculosis. Aspergillus onychomycosis, usually of the toenail, has been reported in as low as <1% and as high as 35% of cases of onycho­ mycosis and is more common in diabetes. ■ ■RISK FACTORS AND PATHOGENESIS The primary risk factors for invasive aspergillosis are profound neu­ tropenia, glucocorticoid use, and underlying respiratory disease; risk increases with longer duration of these conditions. Higher doses of glucocorticoids increase the risk of both acquisition of invasive asper­ gillosis and death from the infection. Neutrophil and/or phagocyte dysfunction also is an important risk factor, as evidenced by aspergil­ losis in CGD, advanced HIV infection, and relapsed leukemia. Invasive aspergillosis is increasingly recognized (if actively sought) in medical intensive care units (2–5%), those with severe influenza (8–25%) and

TABLE 223-1  Disease Frequency and Diagnostic Sensitivity for Different Manifestations of Aspergillosis TYPE OF DISEASE INVASIVE CHRONIC ALLERGIC PARAMETER Incidence/100,000a 27.6 23.6 ?b Prevalence/100,000a — 55.4 286c Global burdena ~2,116,000 ~6,141,000 ~11,690,000 Mortality rate without treatmenta ~100% ~50% <1% Respiratory Diagnostic Sensitivityd Culturee √ √-√√e √-√√e Microscopy √ √ √ Antigen √√√ √√ ? Real-time PCR √√ √√ √√ Blood Diagnostic Sensitivityd Culture x x x Antigen √√ √ √ β-D-Glucan √√ √ ? Real-time PCR √√ x x IgG antibodyf √√f √√√ √√√ IgE antibody x √√ √√√√ aDenning DW: Global incidence and mortality of severe fungal disease. Lancet Infect Dis 24:e428, 2024. bAllergic fungal disease can develop at any age, usually in adulthood; the annual frequency with which it occurs is not known. cAllergic bronchopulmonary aspergillosis and severe asthma with fungal sensitization. dKey for sensitivity: 1 check = limited (as the text indicates, 10–30% for culture); 2 checks = higher; 3 checks = >80%; and 4 checks = ~95%. eMultiple specimens, fungal media, and high-volume fungal culture increase sensitivity. fHigh cross-reactivity between A. fumigatus and other Aspergillus spp. gIgG is a useful assay in subacute invasive and nonneutropenic patients but may not be positive on initial testing. Abbreviation: PCR, polymerase chain reaction. CHAPTER 223 Aspergillosis severe COVID-19 (~10%), and patients admitted to hospital with chronic obstructive pulmonary disease (COPD; 1.3–3.9%). Tempo­ rary abrogation of protective responses from glucocorticoid use or compensatory anti-inflammatory response syndrome is a significant risk factor. Many patients have some evidence of prior pulmonary disease—typically, a history of pneumonia or COPD. Many new immu­ nomodulating agents, such as infliximab and ibrutinib, increase the risk of invasive aspergillosis, as does severe liver disease. Patients with chronic pulmonary aspergillosis have a wide spectrum of underlying pulmonary disease, including current and prior tuber­ culosis, sarcoidosis, prior pneumothorax, or COPD. These patients are apparently immunocompetent, but natural killer and/or interleukin 12 or gamma interferon production defects are common. Their inflamma­ tory immune (TH1-like) response is suboptimal, and persistent inflam­ mation is typical. Glucocorticoids accelerate disease progression. Allergic bronchopulmonary aspergillosis (ABPA) usually compli­ cates asthma and cystic fibrosis. Many genetic associations indicate a strong basis for the development of a TH2-like and “allergic” response to A. fumigatus. Remarkably, high-dose glucocorticoid treatment for exacerbations of ABPA almost never leads to invasive aspergillosis. Fungal, and especially Aspergillus, sensitization is especially common in those with poorly controlled asthma. Environmental exposure and Aspergillus sensitization drive some COPD exacerbations. Most patients with Aspergillus bronchitis have bronchiectasis, with or with­ out cystic fibrosis, including heterozygous CFTR mutations. Different genetic traits are associated with invasive, chronic, and allergic aspergillosis; the majority of people probably are not at risk for aspergillosis. Multiple gene variants appear to be necessary for susceptibility to each form of aspergillosis. ■ ■CLINICAL FEATURES AND APPROACH

TO THE PATIENT (Table 223-2) Invasive Pulmonary Aspergillosis  Both the frequency of inva­ sive disease and the pace of its progression increase with greater

TABLE 223-2  Major Manifestations of Aspergillosis ORGAN INVASIVE (ACUTE AND SUBACUTE) CHRONIC SAPROPHYTIC ALLERGIC Lung Angioinvasive (in neutropenia), nonangioinvasive, granulomatous Chronic cavitary, chronic fibrosing, bronchitis, Aspergillus nodule Sinus Acute invasive Chronic invasive, chronic granulomatous Brain Abscess, hemorrhagic infarction, meningitis Granulomatous, meningitis None None Skin Acute disseminated, locally invasive (trauma, burns, IV access) External otitis, onychomycosis None None Heart Endocarditis (native or prosthetic), pericarditis None None None Eye Keratitis, endophthalmitis None None None described degrees of immunocompromise. Invasive aspergillosis is arbitrarily classified as acute and subacute, with courses of ≤1 month and 1–3 months, respectively. More than 80% of cases of invasive aspergil­ losis involve the lungs, and most are community acquired. The most common clinical features are no symptoms at all, fever, cough (some­ times productive), nondescript chest discomfort, trivial hemoptysis, and shortness of breath. Although the fever often responds to gluco­ corticoids, the disease progresses. In ventilated patients, screening for Aspergillus antigen on tracheobronchial lavage fluid is necessary for diagnosis as radiology is not distinctive. The keys to early diagnosis in at-risk patients are a high index of suspicion, screening for circulating antigen (in leukemia), and urgent computed tomography (CT) of the thorax. Invasive aspergillosis is one of the most common diagnostic errors revealed at autopsy. Invasive Sinusitis  The sinuses are involved in 5–10% of cases of invasive aspergillosis, especially affecting patients with leukemia and recipients of hematopoietic stem cell transplants. In addition to fever, the most common features are nasal or facial discomfort, blocked nose, and nasal discharge (sometimes bloody). Endoscopic examination of the nose reveals pale, dusky or necrotic-looking tissue in any location. CT or magnetic resonance imaging (MRI) of the sinuses is essential but does not distinguish invasive Aspergillus sinusitis from preexisting allergic or bacterial sinusitis early in the disease process. Tracheobronchitis  Occasionally, only the airways are infected by Aspergillus. The resulting manifestations seen on bronchoscopy range from acute or chronic bronchitis to ulcerative or pseudomembranous tracheobronchitis. These entities are particularly common among lung transplant recipients and patients on artificial ventilation. Obstruc­ tion with mucous plugs may occur and is called obstructing bronchial aspergillosis in immunocompromised patients and mucous impaction in other patients, such as those with ABPA. Aspergillus Bronchitis  Recurrent chest infections that only par­ tially improve with antibiotic treatment and are associated with signifi­ cant breathlessness or coughing up of thick sputum plugs are typical features of Aspergillus bronchitis. Patients are not significantly immu­ nocompromised and usually have bronchiectasis or cystic fibrosis. Occasional patients present with respiratory failure because of airway obstruction with mucus. Concurrent bacterial bronchitis is common. The diagnosis rests on recurrent detection of Aspergillus in the airway by microscopy, culture, or polymerase chain reaction (PCR). Aspergillus IgG is usually detectable. Disseminated Aspergillosis  In the most severely immunocom­ promised patients, Aspergillus disseminates from the lungs to multiple organs—most often to the brain but also to the skin, thyroid, bone, kidney, liver, gastrointestinal tract, eye (endophthalmitis), and heart valve. Aside from cutaneous lesions, the most common features are gradual clinical deterioration over 1–3 days, with low-grade fever and features of mild sepsis, and nonspecific abnormalities in laboratory tests. In most cases, at least one localization becomes apparent before death. Blood cultures are almost always negative. PART 5 Infectious Diseases

TYPE OF DISEASE Aspergilloma (single), airway colonization Allergic bronchopulmonary, severe asthma with fungal sensitization, hypersensitivity pneumonitis Maxillary fungal ball Allergic fungal sinusitis, eosinophilic fungal rhinosinusitis Cerebral Aspergillosis  Hematogenous dissemination to the brain is a devastating complication of invasive aspergillosis. Single or mul­ tiple lesions may develop. In acute disease, hemorrhagic infarction is most typical, and cerebral abscess is common. Rarer manifestations include meningitis, mycotic aneurysm, and cerebral granuloma (mim­ icking a brain tumor). Local spread from cranial sinuses also occurs. Postoperative infection develops rarely and is exacerbated by glucocor­ ticoids, which are often given after neurosurgery. The presentation can be either acute or subacute, with mood changes, focal signs, seizures, and decline in mental status. MRI is the most useful immediate investi­ gation; unenhanced CT of the brain is usually nonspecific, and contrast is often contraindicated because of poor renal function. Cerebral asper­ gillosis is disproportionately common in those on ibrutinib. Endocarditis  Most cases of Aspergillus endocarditis are pros­ thetic-valve infections resulting from contamination during surgery. Native-valve disease is reported, especially as a feature of dissemi­ nated infection and in persons using illicit IV drugs. Culture-negative endocarditis with large vegetations is the most common presentation; embolectomy occasionally reveals the diagnosis. Cutaneous Aspergillosis  Dissemination of Aspergillus occasion­ ally results in cutaneous features, usually an erythematous or purplish nontender area that progresses to a necrotic eschar. Direct invasion of the skin occurs in neutropenic patients at the site of IV catheter inser­ tion and in burn patients. Surgical, burn, and trauma wounds may become infected with Aspergillus (especially A. flavus). Chronic Pulmonary Aspergillosis  The hallmark of chronic cavitary pulmonary aspergillosis (Fig. 223-1) is one or more pulmo­ nary cavities expanding over a period of months or years in associa­ tion with pulmonary symptoms and systemic manifestations such as fatigue and weight loss. Often mistaken initially for tuberculosis, >90% of chronic cavitary pulmonary aspergillosis cases occur in patients with prior pulmonary disease (e.g., current or past tuberculosis or nontuberculous mycobacterial infection, sarcoidosis, rheumatoid lung disease, pneumothorax, bullae) or lung surgery. The onset is insidious, and systemic features (weight loss, fatigue) may be more prominent than pulmonary symptoms. On CT scan, an irregular internal cavity surface and thickened cavity walls are typical and indicative of dis­ ease activity. Irregular material, fluid level, and a well-formed fungal ball are seen in some cavities. Multiple cavities are more common than a single cavity, and most cavities are in the upper lobes. Pleural thickening and pericavitary infiltrates are typical and most obvious if a positron emission tomography scan has been done as part of the workup. Chronic cavitary pulmonary aspergillosis is usually caused by A. fumigatus, but A. niger has been implicated, particularly in diabetic patients, as have other species. IgG antibodies to Aspergillus are detect­ able in ~90% of patients with chronic cavitary pulmonary aspergillosis. Some patients have concurrent infections with mycobacteria and/ or other bacterial pathogens. The most significant complication is life-threatening hemoptysis, which may be the presenting manifesta­ tion. If untreated, chronic cavitary pulmonary aspergillosis typically

C C C C C FIGURE 223-1  Computed tomography (CT) scan image of the chest in a patient with long-standing bilateral chronic cavitary pulmonary aspergillosis. This patient had a history of several bilateral pneumothoraces and had required bilateral pleurodesis in 1990. CT then demonstrated multiple bullae, and sputum cultures grew A. fumigatus. The patient had initially weakly and later strongly positive serum IgG Aspergillus antibody tests. This scan (2003) shows a mixture of thick- and thinwalled cavities in both lungs (each marked with C), with a probable fungal ball (black arrow) protruding into the large cavity on the patient’s right side (R). There is also considerable pleural thickening bilaterally. progresses (sometimes relatively rapidly) to unilateral or upper-lobe fibrosis. This end-stage entity is termed chronic fibrosing pulmonary aspergillosis (destroyed lung). Aspergilloma  Aspergilloma (fungal ball) is a late manifesta­ tion of chronic cavitary pulmonary aspergillosis, but some patients are asymptomatic. The inside of a pulmonary cavity allows fungal growth that peels off, forming the layers of the fungal ball. Signs and symptoms associated with single (simple) aspergillomas are minor, including cough (sometimes productive), hemoptysis, wheezing, and mild fatigue. More significant signs and symptoms are associated with chronic cavitary pulmonary aspergillosis and should be treated as such. About 10% of fungal balls resolve spontaneously (by being coughed up), but the cavity may still be infected and the patient symptomatic. Aspergillus Nodule  A recently recognized form of chronic pulmo­ nary aspergillosis is the Aspergillus nodule, which may resemble early lung carcinoma and may cavitate. Nodules may be single or multiple and 5–50 mm in diameter. Larger mass lesions are rarely seen. Nodules are usually avid on positron emission tomography. IgG antibodies to Aspergillus are detectable in ~65% of patients with an Aspergillus nodule. Chronic Aspergillus Sinusitis  Three entities are subsumed under this broad designation: fungal ball of the sinus, chronic invasive sinus­ itis, and chronic granulomatous sinusitis. Fungal ball of the sinus is lim­ ited to the maxillary sinus (except in rare cases involving the sphenoid sinus) in which the sinus cavity is filled with a fungal ball. Maxillary disease is associated with prior upper-jaw root canal work and chronic (bacterial) sinusitis. About 90% of CT scans show focal hyperattenu­ ation related to concretions; on MRI scans, the T2-weighted signal is decreased, whereas it is increased in bacterial sinusitis. Removal of the fungal ball is curative. No tissue invasion is demonstrable histologically or radiologically. In contrast, chronic invasive sinusitis is a slowly destructive pro­ cess that most commonly affects the ethmoid and sphenoid sinuses. Patients are usually but not always immunocompromised to some degree (e.g., as a result of diabetes or HIV infection). Imaging of the cranial sinuses shows opacification of one or more sinuses, local bone destruction, and invasion of local structures. The differential diagnosis is wide, including other infections. Apart from a history of chronic nasal discharge and blockage, loss of the sense of smell, and persistent headache, the usual presenting features are related to local involve­ ment of critical structures. The orbital apex syndrome (blindness and proptosis) is characteristic. Facial swelling, cavernous sinus thrombosis,

carotid artery occlusion, and pituitary fossa and brain and skull-base invasion are complications.

Chronic granulomatous sinusitis due to Aspergillus is most com­ monly seen in the Middle East and India and is often caused by A. flavus. It typically presents late, with facial swelling and unilateral proptosis. The prominent granulomatous reaction histologically dis­ tinguishes this disease from chronic invasive sinusitis, in which tissue necrosis with a low-grade mixed-cell infiltrate is typical. IgG antibodies to A. flavus are usually detectable. Allergic Bronchopulmonary Aspergillosis  In almost all cases, ABPA represents a hypersensitivity reaction to A. fumigatus; other causes are other aspergilli and other fungi. ABPA occurs in ~2.5% of patients with asthma who are referred to secondary care, although it may be less common in the United States and more common in those from the Indian subcontinent. In cystic fibrosis, up to 15% of teenag­ ers are affected. Episodes of bronchial obstruction with mucous plugs leading to coughing fits, “pneumonia,” consolidation, and breathless­ ness are typical. Many patients report coughing up thick sputum casts, often brown in color. Eosinophilia commonly develops before systemic glucocorticoids are given. The cardinal diagnostic test is detection of Aspergillus-specific IgE (or a positive skin-prick test in response to A. fumigatus extract) together with an elevated serum level of total IgE (usually >1000 IU/mL). The presence of hyperattenuated mucus in airways is highly specific. Bronchiectasis is characteristic, and a few patients develop chronic cavitary pulmonary aspergillosis. Severe Asthma with Fungal Sensitization (SAFS)  Many adults with severe asthma do not fulfill the criteria for ABPA and yet are allergic to fungi. Although A. fumigatus is a common allergen, numerous other fungi (e.g., Cladosporium and Alternaria species) are implicated by skin-prick testing and/or specific IgE testing. Serum total IgE concentrations are <1000 IU/mL, and bronchial-wall thickening is common. ABPA and SAFS are collectively referred to as fungal asthma. CHAPTER 223 Aspergillosis Allergic Fungal Rhinosinusitis  Like the lungs, the sinuses manifest allergic responses to Aspergillus and other fungi. The affected patients present with chronic (i.e., perennial) sinusitis that is relatively unresponsive to antibiotics. Many of these patients have nasal polyps, and all have congested nasal mucosae and sinuses full of mucoid mate­ rial. The histologic hallmarks of allergic fungal sinusitis are local eosin­ ophilia and Charcot-Leyden crystals. Removal of abnormal mucus and polyps, with local and occasionally systemic administration of gluco­ corticoids, usually leads to resolution. Persistent or recurrent signs and symptoms may require more extensive surgery (ethmoidectomy) and sometimes oral antifungal therapy. Recurrence is common, often after another bacterial or viral infection. Superficial Aspergillosis  Aspergillus can cause keratitis, onycho­ mycosis, and otitis externa. The first can be difficult to diagnose early enough to save the patient’s sight. Natamycin (5%) eye drops are the opti­ mal therapy for fungal keratitis, often with surgery. Otitis externa usually resolves with debridement and local application of antifungal agents. ■ ■DIAGNOSIS Several techniques are required to establish the diagnosis of any form of aspergillosis with confidence (Table 223-1). Acute Invasive Aspergillosis  Patients with acute invasive asper­ gillosis have a relatively heavy load of fungus in the affected organ; thus, antigen detection, PCR, microscopy, culture, and/or histopathol­ ogy usually confirm the diagnosis. However, the pace of progression leaves only a narrow window for making the diagnosis without losing the patient, and some invasive procedures are not possible because of coagulopathy, respiratory compromise, and other factors. Many cases of invasive aspergillosis are missed clinically and are diagnosed only at autopsy. Histologic examination of affected tissue reveals either infarction, with invasion of blood vessels by many fungal hyphae, or acute necrosis, with limited inflammation and fewer hyphae. Asper­ gillus hyphae are hyaline, narrow, and septate, with branching at 45° in infected tissue. Hyphae can be seen in cytology or microscopy

preparations, which therefore provide a rapid means of presumptive diagnosis.

One Aspergillus antigen test relies on detection of galactomannan release from Aspergillus organisms during growth, the other a novel protein antigen. Respiratory sample antigen detection is more sensitive than serum and is critical in the intensive care unit patient in whom radiology is nonspecific. Positive serum antigen results usually precede clinical or radiologic features by several days. The sensitivity of antigen detection is reduced by antifungal therapy. A positive culture supports the diagnosis, given that multiple other (rarer) fungi can mimic Aspergillus species histologically, but only 10–30% of patients with invasive aspergillosis have a positive culture. Bacterial agar is less sensitive than fungal media for culture; thus, if physicians do not request fungal culture, the diagnosis may be missed. High-volume fungal cultures enhance yield. A positive culture may represent noninvasive forms of aspergillosis or airway colonization. Both antigen detection and real-time PCR are faster and much more sensitive than culture of respiratory samples and blood. Definitive confirmation of a diagnosis of invasive aspergillosis requires (1) a positive culture of a sample taken directly from an ordinarily sterile site (e.g., a brain abscess) or (2) positive results of both histologic testing and culture (or molecular confirmation of Aspergillus spp.) of a sample taken from an affected organ (e.g., sinuses or skin). Most diagnoses of invasive aspergillosis are inferred from fewer data, including the presence of the halo sign on a thoracic CT scan—a localized ground-glass appear­ ance representing hemorrhagic infarction surrounds a nodule or consoli­ dation. Halo signs are present for ~7 days early in the course of infection in neutropenic patients and are a good prognostic feature, reflecting an early diagnosis. Nodules with halo signs are a feature of COVID-19 itself; invasive aspergillosis diagnosis requires supportive evidence. Other char­ acteristic radiologic features of invasive pulmonary aspergillosis include nodules and pleural-based infarction or cavitation, but nonspecific con­ solidation is common (Fig. 223-2). PART 5 Infectious Diseases Chronic Aspergillosis  For chronic aspergillosis, Aspergillus anti­ body testing combined with characteristic imaging is sufficient for the diagnosis. Biopsy of Aspergillus nodules reveals hyphae surrounded by cells of chronic inflammation and sometimes granulomas. Antibody titers fall slowly with successful therapy. Cultures are infrequently positive but are important in checking for azole resistance. PCR of spu­ tum is often strongly positive. Some patients with chronic pulmonary aspergillosis often have elevated titers of total and Aspergillus-specific IgE. ABPA, SAFS, and Allergic Aspergillus Sinusitis  ABPA and SAFS are diagnosed serologically with elevated specific and total serum IgE levels or with skin-prick tests. Allergic Aspergillus sinusitis is usually diagnosed histologically, accompanied by Aspergillus IgE antibody. TREATMENT Aspergillosis Antifungal drugs active against Aspergillus include voriconazole, itraconazole, posaconazole, isavuconazole, caspofungin, micafun­ gin, and amphotericin B (AmB). Possible interactions with other drugs must be considered before azoles are prescribed. In addi­ tion, plasma itraconazole and voriconazole concentrations vary substantially from one patient to another, and many authorities recommend monitoring levels to ensure that drug concentrations are adequate but not excessive. Initial IV administration is preferred for acute invasive aspergillosis and oral administration for all other diseases that require antifungal therapy. Current recommendations are shown in Table 223-3. Voriconazole, isavuconazole, and posaconazole are the preferred agents for invasive aspergillosis; caspofungin, micafungin, and lipidassociated AmB are second-line agents. AmB is not active against A. terreus or A. nidulans; multi-azole resistance in A. fumigatus is present in 3–20% of isolates but is increasing, especially in Southeast Asia; and A. niger is resistant to isavuconazole. An infectious disease consultation is advised for patients with invasive disease, given the

A B FIGURE 223-2  Markedly different appearances of invasive aspergillosis on computed tomography scan of the thorax. A. Patient with myelodysplasia and moderate neutropenia showing small right-sided nodules with minimal surrounding ground glass and a separate area of ground glass only on the left laterally. B. Patient with multiple myeloma undergoing intensive chemotherapy with corticosteroids showing bilateral areas of consolidation and some nonspecific atelectasis with probable ground glass surrounding the right-sided lesion. The anterior component of the left-sided lesion is demarcated by the fissure. complexity of management. Immune reconstitution can complicate recovery. The duration of therapy for invasive aspergillosis varies from ~3 months to several years, depending on the patient’s immune status and response to therapy. Relapse occurs if the response is sub­ optimal and immune reconstitution is not complete. Voriconazole is currently the preferred oral agent for chronic aspergillosis with itraconazole or posaconazole as substitutes when failure, emergence of resistance, or adverse events occur. Because chronic cavitary pulmonary aspergillosis responds slowly, therapy for at least 12 months is necessary, and disease control may require years of treatment, whereas the duration of treatment for other forms of chronic and allergic aspergillosis requires case-by-case evaluation. Glucocorticoids should be avoided in chronic cavitary pulmonary aspergillosis unless covered by adequate antifungal therapy; mortality is increased by 240% with corticosteroids. Acute exacerbations of ABPA respond well to voriconazole, itraconazole, or a short course of glucocorticoids—long-term azole therapy usu­ ally helps minimize corticosteroid exposure and maintain remis­ sion. Antifungal response in Aspergillus bronchitis is gratifying, but relapse after 4 months of therapy is common. Resistance in A. fumigatus to one or more azoles, although uncommon, is increasingly found globally. Resistance may be derived from azole fungicide use for crops. Resistance arising from multiple mechanisms may develop during long-term treatment, and a positive culture during antifungal therapy is an indication for susceptibility testing. Surgical treatment is important in several forms of aspergillosis, including fungal ball of the sinus and single aspergillomas, in which

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224 Mucormycosis

TABLE 223-3  Treatment of Aspergillosisa INDICATION PRIMARY TREATMENT PRECAUTIONS SECONDARY TREATMENT COMMENTS Invasive diseaseb Voriconazole, isavuconazole, posaconazole Drug interactions (especially with voriconazole)c AmB, caspofungin, micafungin Prophylaxis Posaconazole tablet, itraconazole solution SUBA-itraconazole Vincristine, cyclophosphamide interaction Single aspergilloma Surgical resection Multicavity disease: poor outcome of surgery, medical therapy preferable Chronic pulmonary diseaseb Voriconazole, itraconazole Poor absorption of itraconazole capsules with proton pump inhibitors or H2 blockers ABPA/SAFS (“fungal asthma”) Itraconazole Some glucocorticoid interactions, including with inhaled formulations aFor information on duration of therapy and drug resistance in certain Aspergillus species, see text. bAn infectious disease consultation is appropriate for these patients. cOnline drug-interaction resource: www.aspergillus.org.uk/content/antifungal-drug-interactions. Note: After loading doses, the oral dose is usually 200 mg bid for voriconazole and itraconazole, 65 mg bid for SUBA-itraconazole, 300 mg qd for posaconazole tablets, and 200 mg qd for isavuconazole. The IV dose of voriconazole for adults is 6 mg/kg twice at 12-h intervals (loading doses) followed by 4 mg/kg q12h; a larger dose is required for children and teenagers; a lower dose may be safer for persons >70 years of age. Plasma monitoring is helpful in optimizing the dosage. The IV dose of isavuconazole is 200 mg tid for 2 days (loading dose) followed by 200 mg qd. Caspofungin is given as a single loading dose of 70 mg and then at 50 mg/d; some authorities use 70 mg/d for patients weighing >80 kg, and lower doses are required with hepatic dysfunction. Micafungin is given as 50 mg/d for prophylaxis and as at least 150 mg/d for treatment; this drug has not yet been approved by the U.S. Food and Drug Administration (FDA) for this indication. AmB deoxycholate is given at a daily dose of 1 mg/kg if tolerated. Several strategies are available for minimizing renal dysfunction. Lipid-associated AmB is given at 3 mg/kg (AmBisome) or 5 mg/kg (Abelcet). Different regimens are available for aerosolized AmB, but none is FDA approved. Other considerations that may alter dose selection or route include age; concomitant medications; renal, hepatic, or intestinal dysfunction; and drug tolerability. Abbreviations: ABPA, allergic bronchopulmonary aspergillosis; AmB, amphotericin B; SAFS, severe asthma with fungal sensitization ICU, intensive care unit. surgery is curative; invasive aspergillosis involving bone, heart valve, sinuses, and proximal areas of the lung (to avoid catastrophic hemop­ tysis); brain abscess; keratitis; and endophthalmitis. In allergic fungal sinusitis, removal of abnormal mucus and polyps, with local and occasionally systemic glucocorticoid treatment, usually leads to reso­ lution. Persistent or recurrent signs and symptoms may require more extensive surgery (ethmoidectomy) and possibly antifungal therapy. Surgery is problematic in chronic cavitary pulmonary aspergillosis, often resulting in serious complications. Bronchial artery emboliza­ tion is preferred for problematic hemoptysis. ■ ■PROPHYLAXIS In situations in which moderate or high risk is predicted (e.g., after induction therapy for acute myeloid leukemia), the need for antifungal prophylaxis for superficial and systemic candidiasis and for invasive aspergillosis is generally accepted. Fluconazole is commonly used in these situations but has no activity against Aspergillus species. Itracon­ azole solution or super bioavailability (SUBA)-itraconazole capsules provide enough bioavailability for modest efficacy, the latter with fewer adverse events. Posaconazole tablets are more effective in reducing infection rates and the need for empirical antifungal therapy. Some data support the use of IV micafungin in those with azole contraindica­ tions. No prophylactic regimen is completely successful. ■ ■OUTCOME Invasive aspergillosis is curable if immune reconstitution occurs, whereas allergic and chronic forms are not. The mortality rate for invasive aspergillosis is 30–70% if the infection is treated but is 100% if the diagnosis is missed. Cerebral aspergillosis, Aspergillus endocarditis, and bilateral extensive invasive pulmonary aspergillosis have very poor outcomes, as does invasive infection in persons with late-stage AIDS, relapsed uncontrolled leukemia or liver failure. For those with chronic cavitary pulmonary aspergillosis, ~70% deteriorate clinically without antifngal therapy. The mortality rate for chronic cavitary pulmonary aspergillosis is 15–20% in the first year, ~40% over 5 years, and 50–60% over 10 years if the patient is actively treated with antifungal agents. Antifungal therapy fails in ~30% of recipients and still more often if azole resistance is present. Both ABPA and SAFS patients respond to glucocorticoids and antifungal therapy; ~60% respond to itraconazole and ~80% to

As primary therapy, azoles have a 20% higher response rate than AmB. Therapeutic drug monitoring is recommended for voriconazole and isavuconazole in ICU patients. Micafungin, aerosolized AmB Some centers monitor plasma levels of itraconazole and posaconazole. Itraconazole, voriconazole, intracavity AmB Single large cavities with an aspergilloma are best resected. Relapse reduced by pre- and perioperative antifungal therapy. Posaconazole, IV AmB, IV micafungin Resistance may emerge during treatment, especially if plasma drug levels are low. Therapeutic azole drug monitoring recommended. Voriconazole, posaconazole, inhaled AmB Long-term therapy is helpful in most cases. No evidence indicates whether therapy modifies progression to bronchiectasis/fibrosis. CHAPTER 224 voriconazole and posaconazole (if tolerated). Inhaled amB is effective in and tolerated by ~15% of patients. Long-term antifungal therapy often allows oral glucocorticoids to be stopped and inhaled glucocor­ ticoid dose. Relapse after discontinuation is common. Mucormycosis ■ ■FURTHER READING Denning DW, Pfavayi LT: Poorly controlled asthma: Easy wins and future prospects for addressing fungal allergy. Allergol Int 72:493, 2023. Kang N et al: Clinical characteristics and treatment outcomes of patho­ logically confirmed Aspergillus nodules. J Clin Med 9:2185, 2020. Li Z, Denning DW: The impact of corticosteroids on the outcome of fungal disease: A systematic review and meta-analysis. Curr Fungal Infect Rep 17:54, 2023. Sehgal IS et al: Efficacy of 12-months oral itraconazole versus 6-months oral itraconazole to prevent relapses of chronic pulmonary aspergillosis: An open-label, randomised controlled trial in India. Lancet Infect Dis 22:1052, 2022. Ullman AJ et al: Diagnosis and management of Aspergillus diseases: Executive summary of the 2017 ESCMID-ECMM-ERS guideline. Clin Microbiol Infect 24:e1ee38, 2018. Brad Spellberg, Ashraf S. Ibrahim

Mucormycosis Mucormycosis represents a group of life-threatening infections caused by fungi of the order Mucorales of the subphylum Mucoromycotina. Mucormycosis is highly invasive and relentlessly progressive, resulting in higher rates of morbidity and mortality than many other infections. The mortality rates from mucormycosis have declined in recent years as a result of early initiation of more effective antifungal therapies. However, mortality remains high overall, often driven by progression of the underlying predisposing condition.

TABLE 224-1  Taxonomy of Fungi Causing Mucormycosis (Subphylum Mucoromycotina, Order Mucorales) FAMILY GENUS (SPECIES LISTED FOR SOME) Mucoraceae Rhizopus oryzae Rhizopus delemar Rhizopus microsporus Rhizomucor Mucor Actinomucor Lichtheimiaceae Lichtheimia (formerly Mycocladus, formerly Absidia) Cunninghamellaceae Cunninghamella Thamnidiaceae Cokeromyces Mortierellaceae Mortierella Saksenaceae Saksenaea Apophysomyces Syncephalastraceae Syncephalastrum ■ ■ETIOLOGY The fungal order Mucorales consists of seven families that are known to cause mucormycosis (Table 224-1). Rhizopus oryzae and R. delemar (both in the family Mucoraceae) are by far the most common causes of mucormycosis in the Western Hemisphere. Less frequently isolated species of the Mucoraceae that cause a similar spectrum of infections include R. microsporus, Rhizomucor pusillus, Lichtheimia corymbifera (formerly Absidia corymbifera), Apophysomyces elegans, and Mucor species. Increasing numbers of cases of mucormycosis due to infection by mold in the family Cunninghamellaceae have also been reported, particularly in highly immunocompromised patients. Other Mucorales can be the major cause of disease in certain geographic areas (e.g., A. elegans in India and M. irregularis in China), or in outbreaks follow­ ing natural disasters (e.g., A. trapeziformis outbreak following Joplin tornado). Only rare case reports have demonstrated the ability of fungi in the remaining families of the Mucorales to cause mucormycosis. PART 5 Infectious Diseases ■ ■PATHOGENESIS The Mucorales are ubiquitous environmental fungi to which humans are constantly exposed. These fungi cause infection primarily in patients with uncontrolled diabetes, defects in phagocytic function (e.g., neutropenia or exposure to high doses of corticosteroids), and/ or elevated levels of free iron, which supports fungal growth in serum and tissues. In the past, iron-overloaded patients with end-stage renal failure who were treated with deferoxamine had a high risk of develop­ ing rapidly fatal disseminated mucormycosis; deferoxamine is an iron chelator for the human host, but it serves as a fungal xenosiderophore, directly delivering iron to the Mucorales. Furthermore, patients with diabetic ketoacidosis (DKA) are at high risk of developing rhinocer­ ebral mucormycosis. The acidosis causes dissociation of iron from sequestering proteins, resulting in enhanced fungal survival and virulence. The ketoacid β-hydroxybutyrate also increases expression of host and fungal receptors that result in fungal adherence and penetra­ tion into tissues. Nevertheless, the majority of diabetic patients who present with mucormycosis are not acidotic, and, even absent acidosis, hypergly­ cemia directly contributes to the risk of mucormycosis by at least four likely mechanisms: (1) hyperglycation of iron-sequestering proteins, disrupting normal iron sequestration; (2) upregulation of a mam­ malian cell receptor (GRP78) that binds to Mucorales, enabling tissue penetration (due to both a direct effect of hyperglycemia and increas­ ing levels of free iron); (3) induction of poorly characterized defects in phagocytic function; and (4) enhanced expression of CotH, a Mucorales-specific protein that mediates host cell invasion by binding to GRP78 (due to hyperglycemia and the resulting free iron). ■ ■EPIDEMIOLOGY Mucormycosis typically occurs in patients with diabetes mellitus, solidorgan or hematopoietic stem cell transplantation (HSCT), prolonged

neutropenia or corticosteroid use, or malignancy. During the pan­ demic, mucormycosis occurred commonly as a co-infection in patients with COVID-19 severe enough to warrant hospitalization. Such infec­ tions were heavily described in low- and middle-income countries (LMICs) and in India in particular. However, the incidence of mucor­ mycosis was higher in India than most other countries even before the pandemic. Hence, it is unclear if the virus itself somehow predisposed to fungal superinfection or if the routine use of high-dose cortico­ steroid therapy for COVID-19, possibly combined with high rates of diabetes mellitus in affected populations, was the primary driver of such infections. As mentioned, the majority of diabetic patients are not acidotic on presentation with mucormycosis. Furthermore, patients often have no previously recognized history of diabetes mellitus when they present with mucormycosis. In these instances, presentation with mucormyco­ sis may result in the first clinical recognition of hyperglycemia, which often has been unmasked by recent glucocorticoid use. Thus, a high index of suspicion of mucormycosis must be maintained, even in the absence of a known history of diabetes, if hyperglycemia is present and patients present with sinusitis, particularly with possible orbital extension. In patients undergoing HSCT, mucormycosis develops at least as commonly during nonneutropenic as during neutropenic periods, probably because of glucocorticoid treatment of graft-versus-host disease. Mucormycosis can occur as isolated cutaneous or subcutane­ ous infection in immunologically normal individuals after traumatic implantation of soil or vegetation (e.g., due to natural disasters, motor vehicle accidents, or severe injuries in war zones) or in nosocomial settings via direct access through intravenous catheters, subcutaneous injections, or maceration of the skin by a moist dressing. Patients receiving antifungal prophylaxis with either itraconazole or voriconazole may be at increased risk of mucormycosis. These patients typically present with disseminated mucormycosis, the most lethal form of disease. Breakthrough mucormycosis also has been described in patients receiving posaconazole, isavuconazole, or echinocandin prophylaxis. ■ ■CLINICAL MANIFESTATIONS Mucormycosis presentations are often as one of five well-defined clini­ cal syndromes: rhino-orbital-cerebral, pulmonary, cutaneous, gastroin­ testinal, and disseminated disease. However, infection of any body site can occur. Patients with specific defects in host defense tend to develop specific syndromes. For example, patients with diabetes mellitus and/ or DKA typically develop the rhino-orbital-cerebral form and much more rarely develop pulmonary or disseminated disease. In contrast, pulmonary mucormycosis occurs most commonly in leukemic patients who are receiving chemotherapy and in patients undergoing HSCT. Rhino-Orbital-Cerebral Disease  Rhino-orbital-cerebral mucormycosis continues to be the most common form of the disease worldwide. Most cases occur in patients with diabetes, although such cases are also described in the transplantation setting, often along with glucocorticoid-induced diabetes mellitus. The initial symptoms of rhino-orbital-cerebral mucormycosis are nonspecific and include eye or facial pain and facial numbness followed by the onset of conjuncti­ val suffusion and swelling, and blurry vision. In contrast to the acute, bright red, periocular skin manifestations typical of acute bacterial orbital cellulitis, periorbital skin in patients with rhino-orbital-cerebral mucormycosis may take on a more dusky, subacute appearance. Fever may be absent in up to half of cases. White blood cell counts are typi­ cally elevated as long as the patient has functioning bone marrow. If untreated, infection usually spreads from the ethmoid sinus to the orbit, resulting in compromise of extraocular muscle function and proptosis, typically with chemosis. From the orbit, the fungus can spread contiguously or hematogenously to the frontal lobe of the brain and/or via venous drainage to the cavernous sinus. Onset of signs and symptoms in the contralateral eye, with resulting bilateral proptosis, chemosis, vision loss, and ophthalmoplegia, is ominous, suggesting the development of cavernous sinus thrombosis.

Upon visual inspection, infected tissue often has a normal appear­ ance during the earliest stages of fungal spread, which can make diagnosis difficult; blind biopsies of normal-appearing sinus tissue are warranted when suspicion for mucormycosis is high. Tissue then pro­ gresses through an erythematous phase, with or without edema, before the onset of a violaceous appearance and finally the development of a black necrotic eschar. Infection can sometimes extend from the sinuses into the mouth and produce painful necrotic ulcerations of the hard palate, but this is a late finding that suggests extensive, well-established infection. One common misperception about mucormycosis is that it is always rapidly progressive. In fact, the rate of progression is extremely vari­ able and is possibly dependent on the immune status of the patient, the infectious inoculum, and the causative Mucorales species, some of which are more virulent and/or have faster growth rates than others. Patients may go from initial symptoms to death in days; alternatively, it can take months or even a year or more for lethal progression to occur. Pulmonary Disease  Pulmonary mucormycosis is the second most common manifestation. Symptoms include dyspnea, cough, and chest pain; fever is often but not invariably present. Angioinvasion results in necrosis, cavitation, and/or hemoptysis. Lobar consolidation, isolated masses, nodular disease, cavities, or wedge-shaped infarcts may be seen on chest radiography. Chest computed tomography (CT) is the best method for determining the extent of pulmonary mucormycosis and may demonstrate evidence of infection before it is seen on chest x-ray. In the setting of cancer, where mucormycosis may be difficult to differentiate from aspergillosis, the presence of ≥10 pulmonary nodules, pleural effusion, or concomitant sinusitis make mucormy­ cosis more likely. It is important to distinguish mucormycosis from aspergillosis because treatments for these infections may differ. Indeed, voriconazole—the first-line treatment for aspergillosis—exacerbates mucormycosis in mouse and fly models of infection. Isavuconazole was noninferior to voriconazole for the treatment of aspergillosis in a randomized controlled trial and also has activity against Mucorales. Hence if there is doubt about whether infection is caused by a septated mold (e.g., Aspergillus) or a Mucorales, inclusion of isavuconazole in a treatment regimen is a reasonable consideration. Consideration must also be given to the possibility of dual infection with both a septated mold and Mucorales; dual infection is not infrequently encountered in highly compromised patients. Cutaneous Disease  Cutaneous mucormycosis may result from external implantation of the fungus or from hematogenous dissemina­ tion. External implantation–related infection has been described in the setting of soil exposure from trauma (e.g., in a motor vehicle accident, a natural disaster, or combat-related injuries), penetrating injury with plant material (e.g., a thorn), injections of medications (e.g., insulin), catheter insertion, contamination of surgical dressings, and use of tape to secure endotracheal tubes. Cutaneous disease can be highly invasive, penetrating into muscle, fascia, and even bone. Necrotizing fasciitis caused by mucormycosis carries a mortality rate approaching 80%. Necrotic cutaneous lesions in the setting of hematogenous dissemination also are associated with an extremely high mortality rate. However, with prompt, aggressive surgical debridement, isolated, nonnecrotizing cutaneous mucormycosis has a favorable prognosis and a low mortality rate. Gastrointestinal Disease  In the past, gas­ trointestinal mucormycosis occurred primarily in premature neonates in association with dis­ seminated disease and necrotizing enterocolitis. However, there has been a marked increase in case reports describing adults with neutropenia, glu­ cocorticoid use, or other immunocompromising conditions. In addition, gastrointestinal disease has been reported as a nosocomial process fol­ lowing administration of medications mixed with A B FIGURE 224-1  Histopathology sections of Rhizopus delemar in infected brain. A. Broad, ribbon-like, nonseptate hyphae in the parenchyma (arrows) and a thrombosed blood vessel with extensive intravascular hyphae (arrowhead) (hematoxylin and eosin). B. Extensive, broad, ribbon-like hyphae invading the parenchyma (Gomori methenamine silver).

contaminated wooden applicator sticks. Nonspecific abdominal pain and distention associated with nausea and vomiting are the most com­ mon symptoms. Gastrointestinal bleeding is common, and fungating masses may be seen in the stomach at endoscopy. The disease may progress to visceral perforation, with extremely high mortality rates.

Disseminated and Miscellaneous Forms of Disease  Hema­ togenously disseminated mucormycosis may originate from any pri­ mary site of infection. The most common site of dissemination is the brain, but metastatic lesions may also be found in any other organ. Mortality rates for widely disseminated mucormycosis exceed 90%; however, these high rates are likely to be due in part to the underlying predisposing condition leading to the infection and the inability to surgically remove the infected foci. Mucormycosis may affect any body site, including bones, mediasti­ num, trachea, kidneys, peritoneum (in association with dialysis), scalp (causing a kerion), and even isolated infection of teeth. ■ ■DIAGNOSIS A high index of suspicion is required for diagnosis of mucormycosis. Unfortunately, autopsy series have shown that up to half of cases are diagnosed only postmortem. Because the Mucorales are environmental isolates, definitive diagnosis requires a positive culture from a sterile site (e.g., a needle aspirate, a tissue biopsy specimen, or pleural fluid) or histopathologic evidence of invasive mucormycosis. A probable diagnosis of mucormycosis can be established by culture from a non­ sterile site (e.g., sputum or bronchoalveolar lavage) or the detection of Mucorales on the surface of histopathology samples (without visualiza­ tion of evidence of invasion) when a patient has appropriate risk factors as well as clinical and radiographic evidence of disease. In such cases, given the urgency of administering therapy early, the patient should be treated while confirmation of the diagnosis is awaited. CHAPTER 224 Biopsy with histopathologic examination remains the most sensi­ tive and specific modality for definitive diagnosis (Fig. 224-1). Biopsy reveals characteristic wide (≥6- to 30-μm), thick-walled, ribbon-like, aseptate hyphal elements that branch at right angles. Other fungi, including Aspergillus, Fusarium, and Scedosporium species, have septa, are thinner, and branch at acute angles. However, artificial septa may result from folding of tissue during processing (which may also alter the appearance of the angle of branching), which can make Mucorales appear to have septa. Thus, the width and the ribbon-like form of the fungus are likely the most reliable features distinguishing mucor­ mycosis from other pathogenic molds. The Mucorales are visualized most effectively with periodic acid–Schiff or hematoxylin and eosin; in contrast to many other fungi, methenamine silver may not result in optimal staining. While histopathology can identify the Mucorales, species can be identified only by culture. Several studies showed that polymerase chain reaction (PCR) of Mucorales-specific targets is use­ ful in diagnosing mucormycosis. However, the U.S. Food and Drug Administration (FDA) has not approved any of these PCR-based assays for this purpose. Mucormycosis

Unfortunately, cultures are positive in fewer than half of cases of mucormycosis. Nevertheless, the Mucorales are not fastidious organ­ isms and tend to grow quickly (i.e., within 48–96 h) on culture media. The likely explanation for the low sensitivity of culture is that the Mucorales form long filamentous structures that are killed by tissue homogenization—the standard method for preparing tissue cultures in the clinical microbiology laboratory. Therefore, the laboratory should be advised when a diagnosis of mucormycosis is suspected, and the tissue should be cut into sections and placed in the center of culture dishes rather than homogenized. Because there is also substantial vari­ ability among isolates in optimal growth temperature, growth at both room temperature and 37°C is advisable.

Imaging techniques often yield subtle findings that underestimate the extent of disease. For example, the most common finding on CT or magnetic resonance imaging (MRI) of the head or sinuses of a patient with rhino-orbital mucormycosis is sinusitis that is indistinguish­ able from bacterial sinusitis. While sinusitis is almost always seen on CT scans in patients with the rhino-orbital-cerebral disease, erosion through the sinus bones and into the orbit is rarely seen on CT even when it is clinically present. MRI is more sensitive (~80%) for detecting orbital and central nervous system (CNS) disease than is CT. High-risk patients should always undergo endoscopy and/or surgical exploration, with biopsy of the areas of suspected infection. If mucormycosis is sus­ pected, initial empirical therapy with a polyene antifungal agent should be initiated while the diagnosis is being confirmed. ■ ■DIFFERENTIAL DIAGNOSIS Other mold infections, including aspergillosis, scedosporiosis, fusa­ riosis, and infections caused by the dematiaceous fungi (brownpigmented soil organisms), can cause clinical syndromes identical to mucormycosis. Histopathologic examination usually allows distinc­ tion of the Mucorales from these other organisms, and a positive culture permits definitive species identification. As stated above, it is important to distinguish the Mucorales from these other fungi, as the preferred antifungal treatments differ (i.e., polyenes for the Muco­ rales vs expanded-spectrum triazoles for most septate molds). The entomophthoromycoses caused by Basidiobolus and Conidiobolus also can cause identical clinical syndromes. These fungi cannot be readily distinguished from the Mucorales by histopathology but can be reli­ ably distinguished by culture. Fortunately, entomophthoromycoses are uncommon in developed countries and can be treated with polyenes; it is not urgent to distinguish them from mucormycosis. PART 5 Infectious Diseases In a patient with sinusitis and proptosis, orbital cellulitis and cavern­ ous sinus thrombosis caused by bacterial pathogens (most commonly Staphylococcus aureus, but also streptococcal and gram-negative spe­ cies) must be excluded. Klebsiella rhinoscleromatis is a rare cause of an indolent facial rhinoscleroma syndrome that may appear similar to mucormycosis. Finally, the Tolosa-Hunt syndrome is characterized by painful ophthalmoplegia, ptosis, headache, and cavernous sinus inflammation; biopsies and clinical follow-up may be needed to distin­ guish the Tolosa-Hunt syndrome from mucormycosis; there is a lack of progression of the former entity. TREATMENT Mucormycosis GENERAL PRINCIPLES Optimizing the chances for successful treatment of mucormyco­ sis requires four steps: (1) early initiation of therapy; (2) surgi­ cal debridement, when possible; (3) rapid reversal of underlying predisposing risk factors, if possible; and (4) proceeding to treat underlying malignancy, if present, without waiting to complete antifungal therapy first. Early initiation of antifungal therapy requires maintaining a high index of suspicion for at-risk patients. Multiple studies have found that earlier initiation of polyene-based therapy improves survival of patients with mucormycosis. Because the disease can present subtly at first and because confirmation of the diagnosis can take days,

therapy often must be started empirically before the diagnosis is established. When there is a reasonable suspicion of mucormycosis, clinicians should not hesitate to initiate therapy with a lipid polyene as soon as possible, since the toxicity of lipid polyenes (unlike that of amphotericin B [AmB] deoxycholate) is rarely substantial after one or two doses. Blood vessel thrombosis and resulting tissue necrosis during mucormycosis can result in poor penetration of antifungal agents to the site of infection. Therefore, debridement of all necrotic tis­ sues can help eradicate the disease. Surgery has been found (by logistic regression and in multiple case series) to be an independent variable for favorable outcome in patients with mucormycosis. However, these data are confounded by the fact that sicker patients are often unable to tolerate surgical procedures. Thus, a moderated approach in which tissue is debrided when and to the extent it is safe to do so is advisable. Limited data from a retrospective study support the use of intraoperative frozen sections to delineate the margins of infected tissues, with sparing of tissues lacking evidence of infection. Rapidly reversing hyperglycemia, acidosis, or iron overload, and lowering corticosteroid doses are important to achieving cure. Indeed, a recent study confirmed that resolution of acidosis in mice with DKA via the administration of sodium bicarbonate (used in the mice in lieu of insulin) improved survival. Administration of glucocorticoids predisposes animals to death from mucormycosis in experimental models. Similarly, iron administration to patients with active mucormycosis should be avoided, as iron exacer­ bates infection in experimental models. Blood transfusion typically results in some liberation of free iron due to hemolysis, so a conser­ vative approach to red blood cell transfusions is advisable. One of the most common errors made in management of mucor­ mycosis is the belief that mucormycosis must be eradicated before an underlying malignancy can be treated. This belief can result in halting or delaying treatment for the underlying disease (e.g., chemotherapy or transplantation) until the mucormycosis is cured. Three fallacies belie this concern. First, mucormycosis will not be definitively eradicated until near-normal immunity is restored; the antifungals provide a holding action and are unlikely to be curative until the underlying disease is treated. Second, modern antifungals can halt progression of mucormycosis temporarily, enabling aggres­ sive chemotherapy or transplantation to be administered to cure the underlying disease. Finally, the primary driver of death in such patients is typically progression of the underlying disease due to failure to treat it appropriately. Initially, some consideration can be given to moderating the level of aggressiveness of the chemotherapy and resulting duration and depth of neutropenia. The aggressiveness of immune suppression and antifungal therapy can then be adjusted during the course of treatment in response to changes in clinical status. Nevertheless, chemotherapy should be given sufficiently aggressively to attempt cure of the underlying disease. These patients are extremely com­ plex, and multidisciplinary, team-based care is advisable. ANTIFUNGAL THERAPY Primary therapy for mucormycosis should be based on a polyene antifungal agent (Table 224-2), except perhaps in mild localized infection (e.g., isolated suprafascial cutaneous infection) that has been mostly excised surgically in an immunocompetent patient. Lipid formulations of AmB are significantly less nephrotoxic than AmB deoxycholate, can be administered at higher doses, and are probably more effective for this purpose. Liposomal amphotericin B (LAmB) may be preferred to amphotericin B lipid complex (ABLC) for management of brain infection based on retrospective survival data and superior brain penetration; there is no clear efficacy advantage of either agent for infections outside the brain, although LAmB may be less nephrotoxic than ABLC (but is also considerably more expensive in many settings). Starting dosages of 1 mg/kg per day for AmB deoxycholate and

5 mg/kg per day for LAmB and ABLC are commonly given to adults

TABLE 224-2  Antifungal Options for the Treatment of Mucormycosisa DRUG RECOMMENDED DOSAGE ADVANTAGES AND SUPPORTING STUDIES DISADVANTAGES First-Line Antifungal Therapy AmB deoxycholate 1.0–1.5 mg/kg once per day • >5 decades of clinical experience • Inexpensive • FDA approved for treatment of mucormycosis LAmB 5–10 mg/kg once per day • Less nephrotoxic than AmB deoxycholate • Better CNS penetration than AmB deoxycholate or ABLC • Better outcomes than with AmB deoxycholate in murine models and a retrospective clinical review ABLC 5 mg/kg once per day • Less nephrotoxic than AmB deoxycholate • Murine and retrospective clinical data suggest benefit of combination therapy with echinocandins Second-Line/Salvage Option Isavuconazole 200 mg of isavuconazole

(372 mg of isavuconazonium sulfate), load q8h × 6 followed by once-daily dosing • Efficacy similar to that of LAmB in mouse models • FDA approved for treatment of mucormycosis • May be a rational empirical option when septate mold vs mucormycosis is not yet established Posaconazole 200 mg four times per day • In vitro activity against the Mucorales, with lower MICs than isavuconazole • Retrospective data for salvage therapy in mucormycosis Combination Therapyb Echinocandin plus lipid polyene Standard echinocandin doses • Favorable toxicity profile • Synergistic in murine disseminated mucormycosis • Retrospective clinical data suggest superior outcomes for rhino-orbital-cerebral mucormycosis. Lipid polyene plus azole (posaconazole or isavuconazole) Standard doses • Favorable toxicity profile • Limited efficacy data, with no available Triple therapy (lipid polyene plus echinocandin plus azole) Standard doses • Maximal aggressiveness • Limited efficacy data, with no available aPrimary therapy should generally include a polyene. Non-polyene-based regimens may be appropriate for patients who refuse polyene therapy or for relatively immunocompetent patients with mild disease (e.g., isolated suprafascial cutaneous infection) that can be surgically eradicated. bProspective randomized trials are necessary to confirm the suggested benefit (from animal and small retrospective human studies) of combination therapy for mucormycosis. Dose escalation of any echinocandin is not recommended because of a paradoxical loss of benefit of combination therapy at echinocandin doses of ≥3 mg/kg per day. Abbreviations: ABLC, AmB lipid complex; AmB, amphotericin B; CNS, central nervous system; FDA, U.S. Food and Drug Administration; LAmB, liposomal AmB; MIC, minimal inhibitory concentration. Source: Reproduced with permission from B Spellberg et al: Recent advances in the management of mucormycosis: From bench to bedside. Clin Infect Dis 48:1743, 2008. and children to treat mucormycosis. Dose escalation of LAmB to 7.5 or 10 mg/kg per day for CNS mucormycosis may be consid­ ered in light of the limited penetration of polyenes into the brain. A recent observational study failed to find superior outcomes of LAmB dosed at 10 mg/kg per day. However, very few patients with brain involvement were included, and the analysis was not stratified by site of infection. Therefore, 10 mg/kg per day may be reasonable to consider for treating aggressive brain infection but is unlikely of benefit, and will increase toxicity, for other patients. Because of autoinduction of metabolism, which results in paradoxically lower drug levels, there is no advantage to escalating the LAmB dose above 10 mg/kg per day, and doses of 5 mg/kg per day are probably adequate for nonbrain infections. ABLC dose escalation above 5 mg/kg

per day is not advisable given the lack of relevant data and the drug’s potential toxicity. In multiple studies, various combinations of lipid polyenes (both ABLC and LAmB) plus echinocandins (e.g., caspofungin, micafun­ gin, and anidulafungin) improved survival rates among mice with disseminated mucormycosis (including CNS disease). Furthermore, combination lipid polyene–echinocandin therapy was associated with significantly better outcomes than polyene monotherapy in a

• Highly toxic • Poor CNS penetration • Expensive • Expensive • Possibly less efficacious than LAmB for CNS infection • Much less clinical experience • Clinical study supporting approval was small and historically controlled • Substantially lower blood levels than isavuconazole • No data on initial therapy for mucormycosis, and no evidence for posaconazole in combination with other therapy • Experience limited, potential use for CHAPTER 224 salvage therapy • Limited clinical data on combination therapy Mucormycosis evidence of superiority vs monotherapy evidence for superiority vs monotherapy or dual therapy retrospective clinical study involving patients with rhino-orbital-cerebral mucormycosis (including CNS disease). The effect of echinocan­ dins appears to be to downmodulate the virulence of the fungus and reduce tissue necrosis and destruction from fungal invasion. On the basis of such data, some experts prefer combination lipid polyene– echinocandin therapy as a first-line option. However, at least one retrospective study did not find an advantage of any combina­ tion regimens (including polyene-azole, polyene-echinocandin, or others) in patients who primarily had malignancy as the underlying disease. Ultimately, definitive randomized controlled trials are needed to establish whether the combination is superior in efficacy to monotherapy for mucormycosis. When used, echino­ candins should be administered at standard, FDA-approved doses, since dose escalation has resulted in paradoxical loss of efficacy in preclinical models. In contrast to deferoxamine, the iron chelator deferasirox is fun­ gicidal against clinical isolates of the Mucorales. In mice with DKA and disseminated mucormycosis, combination deferasirox–LAmB therapy resulted in synergistic improvement of survival rates and reduced the fungal burden in the brain. Unfortunately, a small, randomized, double-blind, phase 2 safety clinical trial of adjunctive

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225 Superficial Fungal Infections

therapy with deferasirox (plus LAmB) documented excess mortality among patients treated with deferasirox. Of note, the study popula­ tion included primarily patients with active malignancy, and few patients in the study had diabetes mellitus as their only risk factor. Deferasirox is therefore contraindicated as therapy in patients with active malignancy, but its role in patients who have diabetes melli­ tus without malignancy (the setting in which its preclinical efficacy was optimal) remains uncertain.

Posaconazole and isavuconazole are the only FDA-approved azoles with reliable in vitro activity against the Mucorales. However, there are limited data regarding the efficacy of posaconazole mono­ therapy for mucormycosis, and in contrast to polyene-echinocandin therapy, no data support the use of combination posaconazolepolyene regimens. Although the minimal inhibitory concentra­ tions (MICs) of isavuconazole against the Mucorales are four- to eightfold higher than those of posaconazole, blood levels may be higher with standard isavuconazole dosing than with posaconazole. Isavuconazole is FDA-approved for the treatment of mucormycosis based on a small, historically controlled study. Given this limited dataset, many experts continue to think that lipid polyenes are first-line options and that isavuconazole, like posaconazole, is best reserved for oral transitional therapy in patients whose condition has substantially improved on polyene-based therapy, or for salvage therapy in patients who are intolerant of polyene-based regimens or whose infection is refractory to these regimens. No studies in mice or humans have ever shown benefit of add­ ing posaconazole to another antifungal agent for combination therapy of mucormycosis. In contrast, in mice, a combination of isavuconazole plus polyene regimens did prolong survival time and lowered tissue fungal burden compared to monotherapy. However, this combination has not been studied compared to monotherapy in people. Some experts use triple therapy with a polyene, echino­ candin, and either posaconazole or isavuconazole for patients who have extensive disease or whose disease has progressed on prior therapy. Empirical, dual lipid polyene–azole therapy is a rational choice in a patient with likely invasive mold infections when septate molds and mucormycosis are both in the differential diagnosis and the etiologic agent has not yet been confirmed. Alternatively, initial therapy with isavuconazole monotherapy may be reasonable for a brief period of time in a stable patient if mucormycosis is felt to be possible but less likely than a septated mold infection. PART 5 Infectious Diseases The roles of recombinant cytokines and neutrophil transfusions in the primary treatment of mucormycosis are not clear, although it is intuitive that earlier recovery of neutrophil counts should improve survival rates. Limited data from uncontrolled case series have described the use of hyperbaric oxygen in centers with the appropriate technical expertise and facilities; its efficacy remains undefined. As mentioned previously, one study in mice with DKA found that administration of sodium bicarbonate improved survival from mucormycosis; however, because insulin was not adminis­ tered to the mice, it is unclear whether the therapeutic effect is clinically relevant. In general, antifungal therapy for mucormycosis should be con­ tinued until resolution of clinical signs and symptoms of infec­ tion and resolution of underlying immunosuppression. However, after a week or two of daily therapy in a patient who is clinically improving, it is reasonable to consider switching to thrice-weekly lipid polyene doses—with ultimate weaning down to twice-weekly doses—for maintenance therapy. For patients with mucormyco­ sis who are receiving immunosuppressive medications, second­ ary antifungal prophylaxis is typically continued for as long as the immunosuppressive regimen is administered. Transitioning to azoles for chronic suppression is a reasonable alternative to continuing polyene therapy in this setting, with re-initiation of polyenes during periods of deep neutropenia. One common source of error in the long-term management of mucormycosis is follow-up radiology. Analysis of data from the DEFEAT Mucor study indicated that early radiographic progres­ sion (within the first 2 weeks) did not predict long-term mortality.

Changing the therapeutic plan based on early radiographic changes can result in therapeutic errors. For example, it is common for CNS Mucorales to cavitate in the brain parenchyma over time. This does not necessarily reflect therapeutic failure, but rather may reflect increased immune reactivity to the fungus, particularly in patients recovering from neutropenia or with removal of immune suppres­ sion. Thus, it may not be advisable to obtain serial radiographic studies in the short-term, and if such studies are obtained, caution should be used in reacting to their results. Greater emphasis should be placed on clinical response, particularly within the first 2–4 weeks after initiation of therapy. ■ ■PROGNOSIS Over the past two decades, the prognosis of mucormycosis has sub­ stantially improved with aggressive antifungal therapy. Even CNS infection is often successfully treated. As mentioned, the key driver of outcome may be control of the patient’s predisposing condition. ■ ■FURTHER READING Alqarihi A et al: Mucormycosis in 2023: An update on pathogenesis and management. Front Cell Infect Microbiol 13:1254919, 2023. Baldin C, Ibrahim AS: Molecular mechanisms of mucormycosis: The bitter and the sweet. PLoS Pathog 13:e1006408, 2017. Cornely O et al: Global guideline for the diagnosis and management of mucormycosis: An initiative of the ECMM in cooperation with ESCMID/EFISG. Lancet Infect Dis 19:e405, 2019. Danion F et al: What is new in pulmonary mucormycosis? J Fungi (Basel) 9:307, 2023. Pettrikos G et al: Epidemiology of mucormycosis in Europe. Clin Microbiol Infect 20:67, 2014. Joseph Pechacek, Carol A. Kauffman,

Michail S. Lionakis

Superficial Fungal

Infections Fungal infections of the skin and skin structures are caused by molds and yeasts that do not invade deeper tissues but rather cause disease merely by inhabiting the superficial layers of skin, hair follicles, and nails. These agents cause the most common human fungal infections but only rarely cause serious infections. ■ ■MALASSEZIA INFECTIONS Etiologic Agents, Epidemiology, and Pathogenesis  Malassezia species, primarily M. furfur and M. pachydermatis, are lipophilic yeasts that generally cause only minor skin infections but, on occasion, can cause invasive infection. Malassezia species are part of the indigenous human microbiota found in the stratum corneum of the back, chest, scalp, and face—areas rich in sebaceous glands. The organisms do not invade below the stratum corneum and generally elicit little inflam­ mation. Cutaneous interleukin (IL)-17 signaling controls Malassezia species in the skin, but excessive, Malassezia-induced IL-17 responses have been associated with exacerbating atopic inflammation. Clinical Manifestations  Malassezia species cause tinea versicolor (also called pityriasis versicolor), folliculitis, and seborrheic dermati­ tis. Tinea versicolor presents as flat round scaly patches of hypo- or hyperpigmented skin on the neck, chest, or upper arms. The lesions are usually asymptomatic but can be pruritic. They can be mistaken

for vitiligo, but the latter is not scaly. Folliculitis occurs on the back and chest and mimics bacterial folliculitis. Seborrheic dermatitis manifests as erythematous pruritic scaly lesions in the eyebrows, moustache, nasolabial folds, and scalp (dandruff). Seborrheic dermatitis can be severe in patients with AIDS. Fungemia and disseminated infection occur rarely with Malassezia species, and they almost always occur in premature neonates receiving parenteral lipid nutrition preparations. Diagnosis  Malassezia infections are diagnosed clinically in most cases. If scrapings are collected on a microscope slide on which a drop of potassium hydroxide has been placed, a mixture of budding, bottleshaped yeasts and short septate hyphae is seen. To culture Malassezia from patients with suspected disseminated infection, sterile olive oil must be added to the medium. Treatment and Prognosis  Topical creams and lotions, includ­ ing selenium sulfide shampoo, ketoconazole shampoo or cream, and terbinafine cream, are effective in treating Malassezia infections and are usually given for 2 weeks. Mild topical steroid creams are some­ times used to treat seborrheic dermatitis. For extensive disease, oral itraconazole or fluconazole (200 mg/day) can be used for 5–7 days. The rare cases of fungemia caused by Malassezia species are treated with amphotericin B (AmB) or an azole, such as voriconazole, prompt removal of central catheters, and discontinuation of parenteral lipid infusions. Malassezia skin infections are benign and self-limited, although recurrences are common. The outcome of systemic infec­ tion depends on the host’s underlying conditions, but most infected neonates do well. ■ ■MUCOCUTANEOUS CANDIDA INFECTIONS Candidiasis is discussed in Chap. 222, and this section briefly describes mucocutaneous manifestations of candidiasis. Etiologic Agents, Epidemiology, and Pathogenesis  The most common species responsible for mucocutaneous candidiasis—e.g., oral thrush, esophageal candidiasis, vulvovaginal candidiasis (VVC)—is Candida albicans. Candida species are commensal organisms residing primarily in the oral mucosa, gastrointestinal tract, and genitourinary tract with the skin being colonized in certain settings, such as after antibiotic use in the intensive care unit or by certain species such as C. parapsilosis or C. auris. C. auris is an emerging multidrug-resistant spe­ cies with a propensity for long-term colonization of the human skin, health care environments, and medical devices and for causing noso­ comial outbreaks. C. auris grows avidly in human sweat and colonizes deep skin layers including hair follicles. IL-17–producing lymphoid cells at mucocutaneous barrier sites such as TH17 cells, γδ T cells, and innate lymphoid cells mediate control against Candida. Risk factors for oral thrush include inherited and pharmacologic defects of IL-17 signaling, AIDS, and conditions with excessive mucosal interferon γ responses such as autoimmune polyendocrine syndrome type 1. Antibiotic use, diabetes, and pregnancy are major risk factors for VVC. Skin candidiasis is uncommon and most often seen in inborn errors of immunity that cause chronic mucocutaneous candidiasis, primarily STAT1 gain-of-function. Diagnosis  Diagnosis of oral thrush is generally clinical with white mucosal plaques that are removed with gentle scraping. Culture reveals the responsible Candida species. Biopsy of affected tissue may be needed to differentiate thrush from mimics such as leukoplakia. Biopsy of affected oral or esophageal tissue shows yeasts, pseudohyphae, and hyphae invading into the epithelium; observing only yeast forms super­ ficial to epithelial tissue usually represents solely colonization. VVC is diagnosed with culture and microscopy of vaginal secretions collected via swabs. Skin and nail candidiasis is diagnosed by skin biopsy or nail clippings with culture and microscopy. Treatment and Prognosis  Mild oral thrush may be treated with topical agents such as nystatin suspension or clotrimazole troches, but moderate and severe disease or esophageal involvement should be treated with systemic, primarily azole therapy. VVC may be treated with topical or oral azoles. Newer options for VVC include the

triterpenoid antifungal ibrexafungerp, which inhibits β-glucan, and oteseconazole, a long-acting tetrazole that cannot be used in patients of reproductive potential owing to potential fetal ocular toxicity.

■ ■DERMATOPHYTE INFECTIONS Etiologic Agents, Epidemiology, and Pathogenesis  The molds that cause skin infections in humans include the genera Tricho­ phyton, Microsporum, and Epidermophyton. These organisms, which are not components of the normal skin microbiota, can live within the keratinized structures of the skin—hence the term dermatophytes. Dermatophytes occur worldwide, and infections with these organisms are extremely common with an estimated ~1 billion people affected globally. Some organisms cause disease only in humans and can be transmitted by person-to-person contact and by fomites, such as hairbrushes or wet floors, that have been contaminated by infected individuals. Several species cause infections in cats and dogs and can readily be transmitted from these animals to humans, and others are spread from contact with soil. The characteristic ring shape of cutane­ ous lesions is the result of the organisms’ outward growth in a cen­ trifugal pattern in the stratum corneum. Fungal nail invasion usually occurs through the lateral or superficial nail plates and then spreads throughout the nail; when hair shafts are invaded, the organisms can be found either within the shaft or surrounding it. Symptoms are caused by the inflammatory reaction elicited by fungal antigens and not by tissue invasion. Dermatophyte infections are restricted by IL-17 responses and occur more often in males; progesterone can inhibit dermatophyte growth. CHAPTER 225 Clinical Manifestations  Dermatophyte infection of the skin is often called ringworm. This term is confusing because worms are not involved. Tinea, the Latin word for worm, describes the serpentine nature of the skin lesions. Tinea is a less confusing term and can be used with the name of the body part affected—e.g., tinea capitis (head), tinea pedis (feet), tinea corporis (body), tinea cruris (crotch), and tinea unguium (nails, more often termed onychomycosis). Superficial Fungal Infections Tinea capitis occurs most commonly in 3- to 7-year-old children. Children with tinea capitis usually present with well-demarcated scaly patches in which hair shafts are broken off right above the skin; alope­ cia can result. Tinea corporis manifests as well-demarcated, annular, pruritic, scaly lesions that undergo central clearing. Usually, one or sev­ eral small lesions are present; however, in some patients, tinea corporis can involve much of the trunk. The rash should be differentiated from contact dermatitis, eczema, and psoriasis. Tinea cruris is seen almost exclusively in men. The perineal rash is erythematous and pustular, has a discrete scaly border, is without satellite lesions, and is usually pru­ ritic. The rash must be differentiated from intertriginous candidiasis, erythrasma, and psoriasis. Tinea pedis also is also more common among men. It usually starts in the web spaces of the toes; peeling, maceration, and pruritus are followed by development of a scaly pruritic rash along the lateral and plantar surfaces of the feet. Hyperkeratosis of the soles of the feet often ensues. Tinea pedis has been implicated in lower-extremity cellulitis, as strepto­ cocci and staphylococci can gain entrance to the tissues through fissures between the toes. Onychomycosis affects toenails more often than fin­ gernails and is most common among persons who have tinea pedis. The nail becomes thickened and discolored and may crumble; onycholysis almost always occurs. Onychomycosis is more common in older adults and in persons with vascular disease, diabetes mellitus, and nail trauma. Fungal infection must be differentiated from psoriasis, which can mimic onychomycosis but usually has associated skin lesions. Diagnosis  Many dermatophyte infections are diagnosed by their clinical appearance. If the diagnosis is in doubt, scrapings should be taken from the edge of a lesion with a scalpel blade, transferred to a slide to which a drop of potassium hydroxide is added, and examined under a microscope for the presence of hyphae. Cultures are indicated if an outbreak is suspected or the patient does not respond to therapy. Treatment and Prognosis  Dermatophyte infections usually respond to topical therapy with azoles or terbinafine. Lotions or sprays

TABLE 225-1  Suggested Oral Treatment for Extensive Tinea Infections and Onychomycosis ANTIFUNGAL AGENT SUGGESTED DOSAGE COMMENTS Extensive Tinea Infection Terbinafine 250 mg/day for

1–2 weeks Adverse reactions minimal with short treatment period Itraconazolea 200 mg/day for

1–2 weeks Adverse reactions minimal with short treatment period except for drug interactions Onychomycosis Terbinafine 250 mg/day for

3 months Slightly superior to itraconazole; monitor for hepatotoxicity Itraconazolea 200 mg/day for

3 months or 200 twice daily for

1 week each month for 3 months Drug interactions frequent; monitor for hepatotoxicity; rarely causes hypokalemia, hypertension, edema; use with caution in patients with congestive heart failure Other triazoles such as posaconazole may be considered as alternative therapy aItraconazole capsules require food and gastric acid for absorption, whereas itraconazole solution is taken on an empty stomach. The newer SUBA-itraconazole formulation exhibits improved oral bioavailability and reduced interpatient variability. are easier than creams to apply to large or hairy areas. Particularly for tinea cruris, the affected area should be kept as dry as possible. When patients have extensive skin lesions, oral itraconazole or terbinafine can hasten resolution (Table 225-1). Terbinafine interacts with fewer drugs than itraconazole and is generally the first-line systemic agent. PART 5 Infectious Diseases Onychomycosis generally does not respond to topical therapy, although efinaconazole topical solution applied to the affected nail for as long as a year has been beneficial in several trials. Itraconazole and terbinafine both accumulate in the nail plate and can treat onychomy­ cosis (Table 225-1). The major decision to be made regarding therapy is whether the extent of nail involvement justifies the use of systemic antifungal agents that may have adverse effects, may interact with other drugs, and are costly. Relapses of tinea cruris and tinea pedis are com­ mon and should be treated as early as possible with topical creams to avoid development of more extensive disease. Relapses of onychomy­ cosis follow treatment in 25–30% of cases. ■ ■SPOROTRICHOSIS Etiologic Agent, Epidemiology, and Pathogenesis  Sporothrix schenckii complex comprises six closely related organisms; S. schenckii and S. brasiliensis are the species that cause most human infections.

S. globosa has grown in importance in Asia. Sporothrix species are rela­ tively intolerant to heat and are found worldwide in sphagnum moss, decaying vegetation, and soil. Sporotrichosis most commonly affects persons who participate in outdoor activities such as landscaping, gar­ dening, and tree farming. Infected animals—such as dogs, armadillos, and most commonly cats—can transmit S. schenckii to humans. A large ongoing outbreak of sporotrichosis in Brazil caused by S. brasiliensis has been traced to cats, which are highly susceptible to this infection. Sporotrichosis is primarily a localized infection of skin and subcutane­ ous tissues that follows traumatic conidial inoculation. Osteoarticular sporotrichosis is uncommon, occurring most often in middle-aged men who abuse alcohol, and pulmonary sporotrichosis occurs almost exclusively in persons with chronic obstructive pulmonary disease following fungal inhalation. Dissemination occurs almost entirely in markedly immunocompromised patients, especially those with AIDS. Clinical Manifestations and Differential Diagnosis  Days or weeks after inoculation, a papule develops at the site and then usually ulcerates but is not very painful. Similar lesions develop sequentially along the lymphatic channels proximal to the original lesion (Fig. 225-1). Some patients develop a fixed cutaneous lesion that can be verrucous

FIGURE 225-1  Several nodular lesions in a lymphangitic spread pattern that developed on the patient’s arm after traumatic inoculation. Cultures from the biopsied lesion (circle) yielded growth of Sporothrix schenckii. or ulcerative and that remains localized without lymphatic exten­ sion. The differential diagnosis of lymphocutaneous sporotrichosis includes nocardiosis (especially by Nocardia brasiliensis), tularemia, nontuberculous mycobacterial infection (especially by Mycobacterium marinum), and leishmaniasis. Osteoarticular sporotrichosis is usually monoarticular and can present as chronic synovitis or septic arthritis. Pulmonary sporotrichosis presents with cavitary or occasionally mul­ tifocal noncavitary pneumonia and must be differentiated from tuber­ culosis and other fungal pneumonias. Numerous ulcerated skin lesions, with or without spread to visceral organs (including the central ner­ vous system [CNS]), are characteristic of disseminated sporotrichosis. Diagnosis  S. schenckii usually grows readily as a mold on Sab­ ouraud’s agar when material from a cutaneous lesion biopsy is incu­ bated at room temperature. Histopathologic examination of biopsy material shows a mixed granulomatous and pyogenic reaction, and tiny oval or cigar-shaped yeasts sometimes can be seen with special stains. Treatment and Prognosis  Guidelines for the management of the various forms of sporotrichosis have been published by the Infectious Diseases Society of America (Table 225-2). Itraconazole is the drug of choice for cutaneous and lymphocutaneous sporotrichosis. Fluco­ nazole is less effective, voriconazole is ineffective, and posaconazole has been used successfully in a small number of patients. Saturated solution of potassium iodide (SSKI) continues to be used for lympho­ cutaneous infection because of its low cost; however, SSKI is poorly TABLE 225-2  Suggested Treatment for Sporotrichosis FIRST-LINE THERAPY ALTERNATIVES/COMMENTS DISEASE Cutaneous, lymphocutaneous Itraconazole, 200 mg/ day until 2–4 weeks after lesions resolve SSKI, increasing dosesa Terbinafine, 500 mg twice daily Pulmonary, osteoarticular Itraconazole,

200 mg twice daily for 12 months Lipid AmBb for severe pulmonary disease until stable; then itraconazole Disseminated, central nervous system Lipid AmBb for

4–6 weeks Itraconazole, 200 mg twice daily after AmB for 12 months Patients with AIDS: itraconazole maintenance, 200 mg/day until CD4+ T cell count is >200/μL

for ≥12 months aThe starting dosage is 5–10 drops three times daily in water or juice. The dosage is increased weekly by 10 drops per dose, as tolerated, up to 40–50 drops three times daily. bThe dose of lipid AmB is 3–5 mg/kg daily; the higher dose should be used when the central nervous system is involved. Abbreviations: AmB, amphotericin B; SSKI, saturated solution of potassium iodide.

119 - 226 Uncommon Disseminated Fungal Infections

226 Uncommon Disseminated Fungal Infections

tolerated because of adverse reactions, including metallic taste, salivary gland swelling, rash, and fever. High-dose terbinafine may be effective for lymphocutaneous infection. Treatment for lymphocutaneous spo­ rotrichosis is continued for 2–4 weeks after all lesions have resolved, usually for a total of 3–6 months. The success rate for treatment of lymphocutaneous sporotrichosis is 90–100%. Pulmonary and osteoarticular forms of sporotrichosis are treated with itraconazole for at least 1 year. Severe pulmonary infection and disseminated sporotrichosis, including that involving the CNS, should be treated initially with amphotericin B (AmB), with a switch to itraconazole after improvement has been noted. Lifelong suppressive therapy with itraconazole is often required for patients with AIDS. These forms of sporotrichosis respond poorly to antifungal therapy. ■ ■EUMYCETOMA AND CHROMOBLASTOMYCOSIS Etiologic Agents, Epidemiology, and Pathogenesis  Dema­ tiaceous or brown-black fungi, the common soil organisms that cause phaeohyphomycoses, contain melanin, which causes the hyphae and conidia to be darkly pigmented. The term phaeohyphomycosis is used to describe any infection with a pigmented mold. This definition encom­ passes two cutaneous syndromes discussed herein—eumycetoma and chromoblastomycosis—and all other types of invasive infections caused by these organisms, which are discussed in Chap. 226. It is important to note that eumycetomas can be caused by hyaline molds and by brown-black molds and that only ~50% of all mycetomas are due to fungi. Actinomycetes cause the remainder (Chap. 180). The most common cause of eumycetoma is Madurella species. Fonsecaea, Phialophora, and Cladophialophora species are responsible for most cases of chromoblastomycosis and exhibit a pathogenic muri­ form phase. These infections are most often seen in resource-limited rural settings and affect mostly adult men. Inoculation is via minor, often unnoticed, trauma with soil or plant material contamination. Although exposure to these pathogens is common, the development of clinical disease is infrequent. Eumycetoma is found predominantly in tropical and subtropical areas with most cases reported from Sudan, Mexico, and India. Neutrophils mediate early inflammation around the hyphae-containing grains, and macrophages form granulomas at the site of infection. Chromoblastomycosis is also prevalent in tropical and subtropical regions, with most cases reported from Latin America, Madagascar, and China. A broader geographic distribution is seen with certain species such as Cladophialophora carrionii, which thrives in semiarid climates and is a chief cause of chromoblastomycosis in Australia. Fol­ lowing traumatic inoculation, transformation of the fungus into thickwalled, multiseptated, muriform cells ensues. These cells can evade phagocytosis, resulting in chronic disease development. Clinical Manifestations and Differential Diagnosis 

Eumycetoma—commonly called Madura foot—is a chronic cutane­ ous and subcutaneous infection that usually occurs on the lower extremities and manifests with swelling, sinus tract development, and appearance of grains that are actually fungal colonies discharged from the sinus tract. As the infection progresses, adjacent fascia and bony structures may become involved. The disease is indolent and disfigur­ ing, progressing slowly over years. Complications include fractures of infected bone and bacterial superinfection. Chromoblastomycosis is an indolent subcutaneous infection char­ acterized by nodular, verrucous, or plaque-like painless lesions that occur predominantly on the lower extremities and grow slowly over months to years. There is hardly ever extension to adjacent structures, as is seen with eumycetoma. Long-term consequences include bacterial superinfection, chronic lymphedema, and (rarely) the development of squamous cell carcinoma. Diagnosis  A tentative clinical diagnosis of mycetoma can be made when a patient presents with a lesion characterized by swelling, sinus tracts, and grains. Histopathologic examination and culture are neces­ sary to confirm that the etiologic agent is a mold and not an actino­ mycete. In chromoblastomycosis, the diagnosis rests on the histologic

demonstration of muriform cells or “sclerotic bodies” in tissues; culture establishes the causative pigmented mold. In culture-negative cases, PCR-based detection of the 28S ribosomal subunit or the rRNA inter­ nal transcribed spacer (ITS) may assist with identification.

Treatment and Prognosis  Treatment of eumycetoma and chro­ moblastomycosis involves both surgical excision of lesions and antifun­ gal therapy. Surgical removal of lesions is most effective if performed before extensive spread has occurred and is strongly recommended as a mainstay of treatment for both syndromes. In chromoblastomycosis, cryosurgery, heat therapy, and laser therapy have been used with vari­ able success. Optimal antifungal therapy may vary depending on the causative fungal agent. Eumycetoma has been treated with itracon­ azole, voriconazole, posaconazole, and less commonly terbinafine with variable success rates. Long-term antifungal therapy is required, and cost or tolerability of these agents may limit their use. Itraconazole, ter­ binafine, and flucytosine have been used to treat chromoblastomycosis, again with variable success and with cost or tolerability limiting longterm treatment. Muriform cells may respond to antifungal therapy dif­ ferently from hyphal forms, so caution is needed in interpreting in vitro susceptibility results. Species identification may be useful in determin­ ing optimal therapy, as different dematiaceous molds can differ in their susceptibilities to azoles, AmB, and flucytosine. ■ ■FURTHER READING Bonifaz A, Tirado-Sanchez A: Cutaneous disseminated and extra­ cutaneous sporotrichosis: Current status of a complex disease. J Fungi 3:6, 2017. Hay R: Dermatophytosis (ringworm) and other superficial mycoses, in CHAPTER 226 Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases, 10th ed. Blaser MJ et al (eds). Philadelphia, Elsevier, 2025. Hospenthal D: Uncommon fungi and related species, in Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases, 10th ed. Blaser MJ et al (eds). Philadelphia, Elsevier, 2025. Uncommon Disseminated Fungal Infections Joseph Pechacek, Carol A. Kauffman,

Michail S. Lionakis

Uncommon Disseminated

Fungal Infections ENDEMIC MYCOSES (DIMORPHIC FUNGI) Dimorphic fungi—so named for their ability to convert between mold and yeast forms (or spherules or adiaspores in the case of Coccidioides and Emmonsia crescens, respectively) in a temperature-dependent manner—are found in discrete environmental niches as molds that produce conidia, which are the infectious form. In infected tissues and at temperatures of >35°C, they are found in the yeast form. The most common invasive endemic mycoses in the United States—

histoplasmosis, coccidioidomycosis, and blastomycosis—are discussed in Chaps. 218–220, respectively, and sporotrichosis is discussed in Chap. 225. ■ ■PARACOCCIDIOIDOMYCOSIS Etiologic Agent, Epidemiology, and Pathogenesis  The spe­ cies constituting the Paracoccidioides brasiliensis complex and the less frequently reported Paracoccidioides lutzii are thermally dimorphic fungi found in humid areas of Central and South America. The high­ est incidence of Paracoccidioides infections is seen in Brazil, Colombia, Venezuela, and Ecuador although infections have been identified as far north as Mexico. Most cases are found in an ecological niche

characterized by high rainfall and humidity and the presence of tropi­ cal or subtropical forests. A striking male-to-female ratio ranges from 14:1 to as high as 70:1 in various reports; this observation is largely accounted for by chronic disease, as the acute form of the infection lacks this stark male predominance. Most patients are middle-aged or elderly men from rural areas. Paracoccidioidomycosis develops after the inhalation of aerosolized conidia encountered in the environment into the lungs. In this early stage, neutrophils and macrophages play an important role in controlling the infection and preventing dissemina­ tion. Granulomas may be found in the lung and at sites of extrapulmo­ nary dissemination in infected patients. Disease rarely develops at the time of the initial infection but appears years later, presumably after reactivation of a latent infection. Inborn errors of immunity affecting the interleukin (IL)-12/interferon (IFN)-γ signaling axis predispose to severe, disseminated paracoccidioidomycosis.

Clinical Manifestations  The majority of patients exposed to Paracoccidioides develop asymptomatic pulmonary disease, which is contained and does not become clinically apparent. When clinical disease develops, two major syndromes are associated with paracoc­ cidioidomycosis: the acute or juvenile form and the chronic or adult form. The acute form is uncommon, develops typically in persons <30 years old, and manifests primarily as disseminated infection of the reticuloendothelial system. The interval from initial fungal exposure to developing the acute form of paracoccidioidomycosis varies from as early as 45 days to as late as several months. Immu­ nocompromised individuals also can develop this type of rapidly progressive disease. Peripheral eosinophilia is often seen in this form of the disease. The chronic form of paracoccidioidomycosis accounts for ~90% of cases and predominantly affects older men. The primary PART 5 Infectious Diseases TABLE 226-1  Suggested Treatment for Uncommon Disseminated Fungal Infections DISEASE FIRST-LINE THERAPY ALTERNATIVES/COMMENTS Paracoccidioidomycosis Chronic (adult form) Itraconazolea, 100–200 mg/day for

6–12 months Voriconazolea, 200 mg twice daily for 6–12 months Posaconazolea, 300 mg/day for 6–12 months TMP-SMX, 160/800 mg twice daily for 12–36 months Acute (juvenile form) Lipid AmBb until improvement Itraconazolea, 200 mg twice daily after AmB for 12 months Voriconazole or posaconazole at doses noted above may be used Talaromycosis Mild or moderate Itraconazolea, 200 mg twice daily for

12 weeks Voriconazolea, 200 mg twice daily Posaconazolea 300 mg/day Severe Lipid AmBb until improvement Itraconazolea, 200 mg twice daily after AmB for 12 weeks Voriconazole or posaconazole may be considered as an alternative Maintenance therapy (AIDS) Itraconazole, 200 mg/day until CD4+

T cell count is >100/μL for ≥6 months   Emergomycosis (Emmonsiosis) Mild or moderate Itraconazolea 200 mg twice daily for

12 weeks Voriconazolea, 200 mg twice daily Posaconazolea 300 mg/day Severe Lipid AmBb until improvement Itraconazolea, 200 mg twice daily after AmB for 12 weeks Voriconazole or posaconazole may be considered as an alternative Phaeohyphomycosis Voriconazolea, 200 mg twice daily Itraconazolea, 200 mg twice daily Posaconazolea, 300 mg/day Lipid AmB may be effective against some mold species Fusariosis Voriconazolea, 200–300 mg twice daily Lipid AmB, 5 mg/kg/day Posaconazolea, 300 mg/day Lipid AmB plus voriconazole or posaconazole is used by some physicians for initial therapy. Investigational olorofim or fosmanogepix may be considered if available. Voriconazolea, 200–300 mg twice daily Posaconazolea, 300 mg/day Not susceptible to AmB. Lomentospora prolificans is resistant to almost all commercially available antifungal drugs. Investigational olorofim or fosmanogepix may be considered if available. Scedosporiosis and lomentosporiosis Trichosporonosis Voriconazolea, 200–300 mg twice daily Posaconazolea, 300 mg/day aDosing should be adjusted for appropriate drug trough levels. bThe dose of lipid AmB is 3–5 mg/kg daily; the higher dose should be used when the central nervous system is involved. Abbreviations: AmB, amphotericin B; TMP-SMX, trimethoprim-sulfamethoxazole.

manifestations are progressive pulmonary disease, primarily in the lower lobes, with fibrosis and ulcerative and nodular mucocutane­ ous lesions that occur primarily in mucous membranes of the upper respiratory tract and that must be differentiated from leishmaniasis (Chap. 233) and squamous cell carcinoma (Chap. 82). Metastatic foci of paracoccidioidomycosis may also occur via hematogenous or lymphatic spread, most often involving the central nervous system (CNS), skin, or adrenal glands, which can lead to the development of adrenal insufficiency. Diagnosis  The diagnosis is established by growth of the mold form of Paracoccidioides in culture at room temperature. A presumptive diagnosis can be made by detection of the distinctive thick-walled yeast in tissue biopsy specimens, which appears as a central yeast with mul­ tiple daughter cells attached circumferentially via narrow-necked buds and is classically described as a mariner’s wheel. Serologic testing also can aid in diagnosis, although it should be noted that certain available tests may not detect P. lutzii. Treatment and Prognosis  Itraconazole is the treatment of choice for paracoccidioidomycosis (Table 226-1) owing to its longer-standing experience in this disease, although voriconazole and posaconazole also are active in vitro and have been effective in small patient cohorts. Sulfonamides have been used for years and are the least costly agents; however, the response is slower and the relapse rate higher. Seriously ill patients, such as those with respiratory failure, should be initially treated with amphotericin B (AmB) with subsequent azole therapy following clinical improvement. Patients with paracoccidioidomycosis have an excellent response to therapy, regardless of the causative species, but pulmonary fibrosis can be progressive in those with chronic disease.

■ ■TALAROMYCOSIS Etiologic Agent, Epidemiology, and Pathogenesis  Talaromyces marneffei (formerly Penicillium marneffei) is a thermally dimorphic fungus that is endemic in the soil in certain areas of Vietnam, Thailand, and other southeastern Asian countries. The epidemiology of talaromycosis (formerly penicilliosis) is linked to bamboo rats that are infected with the fungus but rarely manifest disease. The disease occurs most often among persons living in rural areas in which the rats are found, but there is no evidence for transmission of the infec­ tion directly from rats to humans. Accidental laboratory inoculation has also been reported. Infection is rare in immunocompetent hosts, and most cases are reported in persons who have advanced AIDS. Other immune deficiencies associated with a higher risk of developing talaromycosis include corticosteroid use, solid organ transplantation, idiopathic CD4 lymphocytopenia, inborn errors of immunity affect­ ing the IL-12/IFN-γ signaling axis, and neutralizing autoantibodies against IFN-γ. Infection is presumed to result from the inhalation of conidia from the environment though a definitive environmental res­ ervoir that has yet to be identified. The organism converts to the yeast phase in the lungs and then spreads hematogenously throughout the reticuloendothelial system. IFN-γ–primed monocytes/macrophages and neutrophils both play a role in control of Talaromyces via fungal phagocytosis and killing. Clinical Manifestations  The clinical manifestations of talaromy­ cosis mimic those of disseminated histoplasmosis and include fever, fatigue, weight loss, dyspnea, lymphadenopathy, hepatosplenomegaly, and mucocutaneous lesions, which appear as papules that often umbili­ cate and resemble molluscum contagiosum (Chap. 201). Patients with mild talaromycosis may present with isolated skin lesions. Diagnosis  Talaromycosis is diagnosed by culture of T. marneffei from blood or from biopsy samples of skin, bone marrow, or lymph node. The organism usually grows within 1 week as a mold producing a distinctive red pigment that diffuses into the agar. Histopathologic examination of tissues, blood smears, or material from skin lesions shows oval or elliptical yeast-like organisms that reproduce via bipolar fission (i.e., appearing as two elliptical yeasts separated by a central sep­ tation) and can quickly establish a presumptive diagnosis. Intramacro­ phage yeast-like organisms can also be seen on tissue or lymph node sections or bone marrow aspirates. T. marneffei–specific antigen and polymerase chain reaction (PCR)-based assays are under investigation. Treatment and Prognosis  Disseminated talaromycosis is usually fatal if not treated. With treatment, the mortality rate is ~10%. For mild or moderate infection, itraconazole is the drug of choice. Voriconazole also can be used, and posaconazole has good in vitro activity and has been successfully used in few case reports. Severe infection should be treated with AmB until clinical improvement occurs; then therapy can be changed to an effective triazole (Table 226-1). For patients with AIDS, suppressive therapy with itraconazole is recommended until the CD4+ T cell count has been >100 cells/μL for at least 6 months. Primary prophylaxis may be considered in certain patients with AIDS in endemic areas who are unable to receive effective antiretroviral therapy. ■ ■EMERGOMYCOSIS (DISSEMINATED EMMONSIOSIS) Etiologic Agents, Epidemiology, and Pathogenesis  As the name implies, Emergomyces (several members of which are formerly of the genus Emmonsia) is a genus of emerging thermally dimorphic fungi consisting of seven species, six of which have been shown to cause human disease. Among them, E. pasteurianus is the most geo­ graphically widespread species, and E. africanus has been the most studied given its relatively high incidence of infection in South Africa. Infection by Emergomyces species have now been reported in Europe, Asia (primarily in China and India), Africa (Uganda and South Africa), and North America (United States and Canada). Although Emergo­ myces has yet to be cultured from the environment and no animal

reservoir has been identified thus far, infection is thought to begin, as with other dimorphic fungi, through inhalation of conidia released by the mold form of the pathogen in the environment. The organism then undergoes temperature-dependent transformation into the invasive yeast form within the lungs and disseminates hematogenously to cause systemic disease in immunocompromised patients. Clinically appar­ ent disease appears largely restricted to patients with impaired cellular immunity, including those with AIDS, hematologic malignancy, or solid organ transplantation, and recipients of immunosuppressive therapy such as corticosteroids.

Clinical Manifestations  Pulmonary disease can manifest with parenchymal lesions and/or endobronchial lesions. Although isolated lung involvement has been described, the vast majority of patients with pulmonary disease have concomitant cutaneous involvement, which most frequently manifests with disseminated ulcers, papules, or nod­ ules. Dissemination to the liver, bone marrow, and lymph nodes may also occur, whereas dissemination to the CNS is rare. Diagnosis  Diagnosis is made by culturing Emergomyces species from blood or tissue or by showing tissue invasion upon histologic examination. Emergomyces may grow from aerobic blood culture bot­ tles within 7 days; the use of fungal isolator culture bottles may improve sensitivity. The yeast form appears on histopathology as intracellular and extracellular round or ovoid yeasts with narrow-based budding that may be misidentified as Histoplasma capsulatum; therefore, defini­ tive identification is made on mold phase in culture. It is worth noting that the commercially available Histoplasma antigen test has a high degree of cross-reactivity with Emergomyces species. CHAPTER 226 Treatment and Prognosis  Given the lack of clinical trials to guide optimal treatment of emergomycosis, treatment recommendations are currently based on expert opinion and largely mirror treatment rec­ ommendations for disseminated histoplasmosis. Immunosuppressed patients with emergomycosis should initiate treatment with AmB until clinical improvement occurs followed by therapy with a mold-active azole. Itraconazole is the azole most often used, although in vitro sus­ ceptibility testing results of voriconazole and posaconazole have been favorable. In case reports, itraconazole monotherapy has been used successfully for mild disease. Prolonged secondary prophylaxis may be considered in patients without a reversible cause of immunodeficiency. Uncommon Disseminated Fungal Infections ■ ■ADIASPIROMYCOSIS Adiaspiromycosis is a rare infection caused predominantly by the mold Emmonsia crescens, which is found in soil. Occupational dust exposure is a major risk factor. Human infections have been broadly reported including from the Americas, Europe, and Asia. The lungs are by far the most frequently affected organ, although cases of ocular and appendiceal disease have been described. The disease is defined by the presence of adiaspores in affected tissue, which results in a marked inflammatory response. After exposure to conidia via inhalation, ingestion, or mucosal contact with dust, the host mounts an immune response leading to the development of noncaseating granulomas. Once within host tissue and driven by thermal pressure, the conidia enlarge to thick-walled adiaspores measuring up to 500 μm in diam­ eter that are incapable of budding or replication. Clinical sequelae, including respiratory failure, are caused by the host’s granulomatous inflammatory response in infected tissue. As E. crescens has not been cultured from human tissue with adiaspiromycosis, diagnosis depends upon histopathologic identification of adiaspores with surrounding granulomas, often with the assistance of molecular methods. The dis­ ease is often self-limiting. However, in severe disease, corticosteroids, with or without antifungal therapy, have been used successfully in several reports. MOLD INFECTIONS Molds produce conidia or spores, which cause infection via inha­ lation or traumatic skin inoculation. In infected tissues, filamen­ tous structures termed hyphae form. The most common invasive mold infections—aspergillosis and mucormycosis—are discussed

in Chaps. 223 and 224, respectively. Dematiaceous or brown-black molds cause phaeohyphomycoses that can be either localized or disseminated infections, the latter mostly in immunocompromised patients. Three genera of hyaline (nonpigmented) molds, Fusarium, Scedosporium, and Lomentospora, have become prominent pathogens among immunocompromised patients. Infections with these fungi mimic aspergillosis in their clinical manifestations and their histo­ pathologic appearance in tissues.

■ ■PHAEOHYPHOMYCOSES Dematiaceous or brown-black fungi cause phaeohyphomycoses. The term phaeohyphomycosis is used to describe any infection with a melaninproducing fungus with filamentous growth. Melanin is a virulence fac­ tor for all pigmented molds. Most dematiaceous fungi cause localized subcutaneous infections after direct inoculation. Disseminated infec­ tions and serious focal visceral infections can occur in immunocompe­ tent individuals but are much more often seen in immunocompromised patients. Two specific cutaneous phaeohyphomycoses—eumycetoma and chromoblastomycosis—are discussed in Chap. 225. Etiologic Agents, Epidemiology, and Pathogenesis  A large number of genera and species of pigmented molds can cause human infection. Most are found in the soil or on plants, and some cause economically important plant diseases. Alternaria, Exophiala, Curvu­ laria, Cladophialophora, Bipolaris, and Wangiella species are among the more common pigmented molds reported to cause human infec­ tion. Infections with dematiaceous molds are acquired by traumatic inoculation into the eye or through the skin, by inhalation, or by injection of contaminated medication. In 2012, Exserohilum species caused a large outbreak in the United States of severe—and in some patients fatal—infections of the CNS following the injection of methyl­ prednisolone contaminated with this fungus. Subcutaneous infections follow traumatic inoculation and are frequently caused by Curvularia, Exophiala, and Phialophora, among other genera. Several organisms, specifically Cladophialophora bantiana and Rhinocladiella mackenziei, are neurotropic and likely to cause CNS infection. When a patient is immunocompromised or when a pigmented mold is injected directly into a deep structure, these organisms become opportunistic patho­ gens, invading blood vessels and mimicking better-known opportunis­ tic infections, such as aspergillosis. PART 5 Infectious Diseases Disseminated phaeohyphomycosis may occur in patients with solid organ or hematopoietic stem cell transplantation or after corti­ costeroid use. It has also been described in patients with autosomal recessive deficiency in CARD9, an intracellular adaptor protein that relays fungus-sensing signals from multiple cell-surface C-type lectin receptors (CLRs). A recent report indicated that ~70% of putatively immunocompetent patients who developed disseminated and brain phaeohyphomycosis carried deleterious variants in CLEC7A, the gene that encodes for the CARD9-coupled CLR Dectin-1, which recognizes fungal cell wall β-glucan. Dectin-1 and CARD9 protect against this infection by mediating the production of proinflammatory cytokines in response to fungal infection, promoting macrophage fungal killing. Clinical Manifestations  Dematiaceous molds are the most com­ mon cause of allergic fungal sinusitis and a less common cause of invasive fungal sinusitis. Keratitis occurs with traumatic corneal inoculation. Even in many immunocompromised patients, inocula­ tion through the skin generally produces only localized nodular lesions at the entry site. Other immunocompromised patients develop pneumonia, brain abscess, or disseminated infection. Patients with brain abscess often have no other signs of dissemination at the time of diagnosis and typically present with an indolent course. Certain species, such as Cladophialophora bantiana, exhibit a propensity for intraventricular spread within the CNS. In the Exserohilum outbreak mentioned above, epidural injections of fungus-contaminated gluco­ corticoids led to meningitis, basilar stroke, epidural abscess and phleg­ mon, vertebral osteomyelitis, and arachnoiditis. Diagnosis  The specific diagnosis of infection with a pigmented mold is established by growth of the organism in culture. Histologically,

FIGURE 226-1  Histologic stains revealing pigmented mold from brain biopsy of a man with multifocal brain lesions. Cultures grew Cladophialophora bantiana. Images show tissue stained with hematoxylin and eosin (left panel) showing goldenbrown pigmented hyphae (arrow) within multinucleated giant cells and associated neutrophilic infiltrate and Fontana-Masson stain (right panel) highlighting the melanin found in the cell wall of this organism. Scale bars, 20 μm. (Images taken by Dr. Stefania Pittaluga, NCI.) observation of melanized hyphal forms appearing golden-brown on biopsy specimens stained with hematoxylin and eosin supports the diagnosis of phaeohyphomycosis. Specialized melanin stains, such as the Fontana-Masson stain, highlight melanized fungal structures in infected tissues and provide important diagnostic support (Fig. 226-1). PCR assays are increasingly used in the diagnosis of infection due to dematiaceous molds but are available only through fungal reference laboratories. Treatment and Prognosis  Surgical debridement is vital for the treatment of phaeohyphomycosis in addition to antifungal treatment, which may need to be prolonged, especially in patients with CNS involvement. The choice of antifungal agent to treat disseminated and focal visceral infections with brown-black molds is based on the loca­ tion and extent of the infection, in vitro test results, and clinical experi­ ence with the specific infecting organism. AmB is not effective against many of these organisms but has been used successfully against some species (Table 226-1). Itraconazole, voriconazole, or posaconazole can be used in the treatment of localized infections. Voriconazole has been the preferred azole when infections involve the CNS because it reaches adequate concentrations at that anatomic site; recent reports support posaconazole as an alternative azole. For certain pigmented molds, such as C. bantiana, flucytosine may have a role in addition to AmB or an azole. Disseminated and focal visceral infections, especially those involving the CNS, are associated with high mortality rates. ■ ■FUSARIOSIS Etiologic Agent, Epidemiology, and Pathogenesis  Fusarium species, which are found worldwide in soil and on plants, have emerged as major opportunistic pathogens in markedly immunocompromised patients. Strains of the F. solani complex are most commonly respon­ sible for human infections, followed by F. oxysporum, F. proliferatum, and other species. Fusarium are molds with distinctive banana-shaped macroconidia. While Fusarium keratitis and onychomycosis can occur in immunocompetent patients, disseminated disease develops in immunodeficient patients. An outbreak of severe Fusarium keratitis among soft contact lens wearers was traced back to a particular brand of contact lens solution and individual contact lens cases that had been contaminated with this mold. In 2022 and 2023, two separate outbreaks of F. solani meningitis were reported in previously healthy young women who had received epidural anesthesia for plastic surgery proce­ dures at two private clinics in Mexico. The case-fatality rate was ~50%. Disseminated infection is reported most often in patients who have a hematologic malignancy, are neutropenic, have received hemato­ poietic stem cell or solid organ transplantation or corticosteroids, or have severe burn wounds. Neutrophils are the most critical cells for protection against fusariosis; hence the correlation with profound and prolonged neutropenia and susceptibility to disseminated infec­ tion. Neutrophil-derived reactive oxygen species are partly respon­ sible for effective host defense against Fusarium. Patients with chronic granulomatous disease (CGD), who carry deleterious mutations in the NADPH oxidase complex that impair the phagocyte oxidative burst,

are at risk for fusariosis, albeit with far lower frequency compared with aspergillosis. Several studies have shown that the prognosis of fusariosis in neutropenic patients is significantly improved in those who recovered their neutrophil counts relative to those who remained persistently neutropenic. Exposure to Fusarium can be through inhalation of conidia or direct inoculation via trauma, but disseminated disease can also occur secondary to pre-existing local disease, such as sinusitis or ony­ chomycosis. For example, the presence of Fusarium onychomycosis preceding allogeneic hematopoietic stem cell transplantation has been associated with a higher risk of developing disseminated fusariosis post-transplantation. Clinical Manifestations  In immunocompetent persons, Fusar­ ium species cause localized infections of various organs. These organ­ isms are a common cause of fungal keratitis, which can extend into the anterior chamber of the eye, cause loss of vision, and require corneal transplantation. Onychomycosis due to Fusarium species, while a potential source of discomfort in immunocompetent patients, is a source of subsequent hematogenous dissemination and should be aggressively sought and treated in neutropenic patients. In profoundly immunocompromised patients, fusariosis is angioinvasive, and clinical manifestations mimic those of aspergillosis. Pulmonary infection is characterized by multiple nodular lesions that are best identified on CT scan. Fusariosis typically presents with persistent fever in neutropenic patients despite the administration of broad-spectrum antibiotics. Disseminated fusariosis differs from disseminated aspergillosis in that skin lesions are extremely common with fusariosis and may develop shortly after or even concurrently with the development of fever; the lesions are nodular or necrotic, are usually painful, and may appear over time in different locations (Fig. 226-2). Diagnosis  The diagnostic approach usually includes both docu­ mentation of the growth of Fusarium species from involved tissue and demonstration of invasion by histopathologic techniques that show acute angle septate hyphae in tissues. The organism is difficult to differ­ entiate from Aspergillus species in tissues; therefore, identification with culture is imperative. An extremely helpful diagnostic clue is growth in blood cultures, which are positive in as many as 50% of patients with disseminated fusariosis. A positive blood culture may lead to rapid identification; subsequent rapid growth on agar reveals the character­ istic banana-shaped macroconidia of Fusarium. Further identification of the species and antimicrobial susceptibility testing, generally in a reference laboratory, may aid in choosing the most appropriate therapy. Treatment and Prognosis  Fusarium species are resistant to many antifungal agents. A lipid formulation of AmB, voriconazole, or posaconazole is recommended. Many physicians use both a lipid FIGURE 226-2  Painful necrotic foot lesion that developed over a week in a woman with acute leukemia who had been neutropenic for 2 months. Fusarium species grew from a punch biopsy. (Courtesy of Dr. Nessrine Ktaich.)

formulation of AmB and either voriconazole or posaconazole because susceptibility information is not standardized and is not always predic­ tive of clinical response, bearing in mind that the prognosis is largely dictated by the degree of immune restoration. F. solani complex is frequently resistant to all azoles, while other species may have lower minimum inhibitory concentrations (MICs) to voriconazole and posaconazole. AmB is the most active antifungal drug against Fusar­ ium; however, the MICs are relatively high for these organisms com­ pared with other molds (Table 226-1). Mortality rates for disseminated fusariosis can be as high as 85%, with survival dependent on reversal of neutropenia. With the improved antifungal therapy now available, mortality rates have fallen to ~50%. However, if neutropenia persists, the mortality rate approaches 100%.

Two newer investigational antifungal agents show promise for the treatment of fusariosis. Fosmanogepix, which inhibits fungal Gwt1 and glycosylphosphatidylinositol–mannoprotein synthesis and anchoring, exhibits in vitro activity against F. solani, including against strains that are resistant to all available antifungals; it has been reported to success­ fully treat a few patients with meningeal and disseminated fusariosis. Olorofim, which inhibits fungal dihydroorotate dehydrogenase and pyrimidine synthesis, has shown favorable, but strain-specific, activity against F. solani complex in vitro. ■ ■SCEDOSPORIOSIS AND LOMENTOSPORIOSIS Etiologic Agent, Epidemiology, and Pathogenesis  Scedo­ sporium apiospermum complex, which is composed of several related species, is reported more often as a cause of human infection than Lomentospora prolificans, formerly Scedosporium prolificans, but both are major emerging opportunistic pathogens in immunocompro­ mised hosts and can cause pneumonia, disseminated infection, and brain abscess. Organisms of the S. apiospermum complex are found worldwide in temperate climates in tidal flats, swamps, ponds, manure, and soil. L. prolificans is also found in soil but is more geographically restricted with a greater representation of reported infections from Australia and Spain. Infection occurs predominantly through inhala­ tion of conidia, but direct traumatic inoculation through the skin or the eye can also occur. S. apiospermum infections of the lungs and CNS have been described in putatively immunocompetent patients fol­ lowing near-drowning accidents, likely owing to the associated heavy burden of fungal exposure from contaminated water. CHAPTER 226 Uncommon Disseminated Fungal Infections Clinical Manifestations  Among immunocompetent persons, Scedosporium and Lomentospora species are a prominent cause of eumycetoma (Chap. 225). Keratitis resulting from accidental corneal inoculation is a sight-threatening infection. Traumatic inoculation during accidents may also lead to deep-seated infections, such as osteo­ articular infections including septic arthritis, in immunocompetent patients. In patients who have hematologic malignancies, especially those with acute leukemia and prolonged neutropenia, recipients of solid organ or hematopoietic stem cell transplants, CGD patients, and patients receiving corticosteroids, these organisms are angioinvasive and can cause pneumonia and extrapulmonary dissemination, with a propensity for CNS spread. Pulmonary infection mimics that of asper­ gillosis both clinically and in appearance on CT scan: nodules, cavities, and lobar infiltrates are common radiographic findings. Contiguous spread from the lung to adjacent anatomic sites, such as the vertebrae, has rarely been described. Disseminated infection often involves the heart, brain, and many other organs. Skin lesions are not as common or as painful as those in patients with fusariosis. Diagnosis  Diagnosis depends on the growth of Scedosporium or Lomentospora species from infected tissue and the histologic demon­ stration of acute-angle septate hyphae invading tissues. Culture evi­ dence is essential because these molds are difficult to morphologically differentiate from Aspergillus in tissues. Demonstration of tissue inva­ sion is essential because these ubiquitous environmental molds can be mere contaminants or colonizers in the respiratory tract, especially in persons with bronchiectasis. As with Fusarium, L. prolificans can grow in blood cultures, but S. apiospermum usually does not.

12 - 133 Infective Endocarditis

133 Infective Endocarditis

study suggested that moxifloxacin (400 mg/d PO) is as effective and well tolerated as ampicillin-sulbactam. Notably, metronidazole is not effective as a single agent: it covers anaerobic organisms but not the microaerophilic streptococci that are often components of the mixed flora of primary lung abscesses.

In secondary lung abscesses, antibiotic coverage should be directed at the identified pathogen, and a prolonged course (until resolution of the abscess is documented) is often required. Treat­ ment regimens and courses vary widely, depending on the immune state of the host and the identified pathogen. Other interventions may be necessary as well, such as relief of an obstructing lesion or treatment directed at the underlying condition predisposing the patient to lung abscess. Similarly, if the condition of patients with presumed primary lung abscess fails to improve, additional studies to rule out an underlying predisposing cause for a secondary lung abscess are indicated. Although it can take as long as 7 days for patients receiving appropriate therapy to defervesce, as many as 10–20% of patients may not respond at all, with continued fevers and progression of the abscess cavity on imaging. An abscess >6–8 cm in diameter is less likely to respond to antibiotic therapy without additional interven­ tions. Options for patients who do not respond to antibiotics and whose additional diagnostic studies fail to identify a pathogen that can be treated include surgical resection and percutaneous drainage of the abscess, especially when the patient is a poor surgical candi­ date. Timing of surgical intervention can be challenging; the goal is to balance the morbidity/mortality risk of a procedure with the need for definitively clearing the abscess in the setting of persistent infection that is not responsive to nonsurgical approaches. Possible complications of percutaneous drainage include bacterial contami­ nation of the pleural space as well as pneumothorax and hemothorax. Traversing normal lung parenchyma might represent a risk factor for major complications from percutaneous abscess drainage. PART 5 Infectious Diseases ■ ■COMPLICATIONS Larger cavity size on presentation may correlate with the development of persistent cystic changes (pneumatoceles) or bronchiectasis. Addi­ tional possible complications include recurrence of abscesses despite appropriate therapy, extension to the pleural space with development of empyema, life-threatening hemoptysis, and massive aspiration of lung abscess contents. ■ ■PROGNOSIS AND PREVENTION Reported mortality rates for primary abscesses have been as low as 2%, while rates for secondary abscesses are generally higher—as high as 75% in some case series. Other poor prognostic factors include age

60, malignancy-related abscesses, the presence of aerobic bacteria, sepsis at presentation, symptom duration of >8 weeks, and abscess size 6 cm. Mitigation of underlying risk factors may be the best approach to prevention of lung abscesses, with attention directed toward airway protection, oral hygiene, and minimized sedation with elevation of the head of the bed for patients at risk for aspiration. Prophylaxis against certain pathogens in at-risk patients (e.g., recipients of bone marrow or solid organ transplants or patients whose immune systems are signifi­ cantly compromised by HIV infection) may be undertaken. APPROACH TO THE PATIENT Lung Abscess For patients with a lung abscess and a low likelihood of malignancy (e.g., smokers <45 years old) and with risk factors for aspiration, it is reasonable to administer empirical treatment and then to pursue further evaluation if therapy does not elicit a response. However, some clinicians may opt for up-front cultures, even in primary lung abscesses. In patients with risk factors for malignancy or other underlying conditions (especially immunocompromised hosts) or with an atypical presentation, earlier diagnostics should be

considered, such as bronchoscopy with biopsy or CT-guided needle aspiration. Bronchoscopy should be performed early in patients whose history, symptoms, or imaging findings are consistent with possible bronchial obstruction. In patients from areas endemic for tuberculosis or patients with other risk factors for tuberculosis (e.g., underlying HIV infection), induced sputum samples should be examined early in the workup to rule out this disease. ■ ■FURTHER READING Bartlett JG: How important are anaerobic bacteria in aspiration pneumonia: When should they be treated and what is optimal therapy. Infect Dis Clin North Am 27:149, 2013. Desai H, Agrawal A: Pulmonary emergencies: Pneumonia, acute respiratory distress syndrome, lung abscess, and empyema. Med Clin North Am 96:1127, 2012. Lee JH et al: Percutaneous transthoracic catheter drainage for lung abscess: A systematic review and meta-analysis. Eur Radiol 32:1184, 2022. Maitre T et al: Pyogenic lung abscess in an infectious disease unit: A 20-year retrospective study. Ther Adv Respir Dis 17534666211003012, 2021. Ott SR et al: Moxifloxacin vs ampicillin/sulbactam in aspiration pneu­ monia and primary lung abscess. Infection 36:23, 2008. Raymond D: Surgical intervention for thoracic infections. Surg Clin North Am 94:1283, 2014. Vaarst JK et al: Lung abscess: Clinical characteristics of 222 Danish patients diagnosed from 2016 to 2021. Respir Med 216: 107305, 2023. Sara E. Cosgrove, Michael T. Melia

Infective Endocarditis The prototypic lesion of infective endocarditis (IE), the vegetation (Fig. 133-1), is a mass of platelets, fibrin, microorganisms, and scant inflammatory cells. Infection most commonly involves heart valves but may also occur on the low-pressure side of a ventricular septal defect, on mural endocardium damaged by aberrant jets of blood or foreign bodies, or on intracardiac devices. The analogous process involving arteriovenous shunts, arterio-arterial shunts (patent ductus arteriosus), or a coarctation of the aorta is called infective endarteritis. FIGURE 133-1  Vegetations (arrows) due to viridans streptococci endocarditis involving the mitral valve.

IE can be classified according to the temporal evolution of disease, the site of infection, the cause of infection, or the predisposing risk factor (e.g., injection drug use, health care–associated). Acute IE is a hectically febrile illness that rapidly damages cardiac structures, seeds extracardiac sites, and, if untreated, progresses to death within weeks. Subacute IE follows an indolent course; causes structural cardiac dam­ age only slowly, if at all; rarely metastasizes; and is gradually progressive unless complicated by a major embolic event or a ruptured mycotic aneurysm. In the United States and likely in other developed countries, the incidence of IE is estimated to be 15 cases per 100,000 population per year, with progressive increases during recent decades. While con­ genital heart diseases remain a constant predisposition, predisposing conditions in developed countries have shifted from chronic rheumatic heart disease (still common in developing countries) to injection drug use, degenerative valve disease, and intracardiac devices. Although the incidence of IE is increased among the elderly, recent data indicate age-adjusted mortality rates in people ≥55 years old have declined in the United States. Recently, however, there has been acceleration in mortality in people aged 25–44 years, likely associated with an increase in opioid use disorder (OUD) and injection drug use in this age group. In developed countries, 25–35% of cases of native-valve endocarditis (NVE) are health care–associated, and 16–30% of all cases are pros­ thetic-valve infections (PVE). The risk of PVE is greatest during the initial year after valve replacement; gradually declines to a low, stable rate thereafter; and is greater for bioprosthetic valves than mechanical valves. The incidence and rate of decline of transcatheter aortic valve replacement (TAVR)-PVE are similar to those for surgically implanted bioprosthetic aortic valves. IE involving cardiovascular implantable electronic devices (CIED-IE)—greater on implanted defibrillators and resynchronization devices than on permanent pacemakers—occurs in 0.5–1.14 cases per 1000 recipients. ■ ■ETIOLOGY Although many species of bacteria and fungi cause sporadic episodes of IE, a few bacterial species cause the majority of cases (Table 133-1). Recent large studies from developed areas identify Staphylococcus aureus as the most common bacterial species causing IE. The oral cavity, skin, and upper respiratory tract are the respective primary portals for viridans streptococci, staphylococci, and HACEK organ­ isms (Haemophilus species, Aggregatibacter species, Cardiobacterium TABLE 133-1  Organisms Causing Major Clinical Forms of Infective Endocarditis (IE)   NATIVE-VALVE IE   COMMUNITY-

ACQUIRED (N = 1718) HEALTH CARE– ASSOCIATED (N = 1110) <2 (N = 144)

2-12 (N = 31) 12 (N = 194) (N = 295) (N = 337) ORGANISM(S) Streptococcib

Pneumococci

— — — —

— Enterococcic

Staphylococcus aureus 28a 52d

Coagulase-negative staphylococci

Fastidious gram-negative coccobacilli (HACEK group)e

— — —

— — Gram-negative bacilli

Candida spp. <1

Polymicrobial/miscellaneous

Diphtheroids — <1

—

Culture-negative

aIncludes methicillin-susceptible and -resistant isolates. bIncludes viridans streptococci; Streptococcus gallolyticus; other non–group A, groupable streptococci; and Abiotrophia and Granulicatella spp. (nutritionally variant, pyridoxal-requiring streptococci). cPrimarily E. faecalis or nonspeciated isolates; occasionally E. faecium or other less likely species. dMethicillin resistance is common among these S. aureus strains. eIncludes Haemophilus spp., Aggregatibacter spp., Cardiobacterium hominis, Eikenella corrodens, and Kingella kingae. Abbreviations: CIED, cardiac implantable electronic device; TAVR, transcatheter aortic valve replacement. Note: Data are compiled from multiple studies.

hominis, Eikenella corrodens, and Kingella kingae). Streptococcus gal­ lolyticus subspecies gallolyticus (formerly S. bovis biotype 1) originates from the gastrointestinal tract and is associated with colonic polyps and tumors. Enterococci enter the bloodstream primarily from the genitourinary tract. Health care–associated IE, most commonly caused by S. aureus, coagulase-negative staphylococci (CoNS), and entero­ cocci, may have either a nosocomial onset (55%) or a community onset (45%). IE complicates 8–25% of episodes of catheter-associated S. aureus bacteremia; the higher rates are detected in high-risk patients studied by transesophageal echocardiography (TEE) (see “Cardiac Imaging,” below).

PVE arising within 2 months of valve surgery—i.e., early PVE—is generally nosocomial and is the result of intraoperative contamination of the prosthesis or a postoperative infection. This nosocomial origin is reflected in the microbial causes: S. aureus, CoNS, facultative gram-neg­ ative bacilli, diphtheroids, and fungi. The portals of entry and organisms causing PVE beginning >12 months after surgery—i.e., late PVE—are similar to those in community-acquired NVE. Regardless of the time of onset after surgery, the majority of CoNS strains that cause PVE are resistant to methicillin. The microbiology of TAVR-PVE, while generally similar to that of PVE, is notable for an increased frequency of entero­ cocci. Risk factors associated with TAVR-PVE include male sex, diabetes, renal failure, and moderate postimplantation aortic valve regurgitation. CIED-IE involves the device or the endothelium at points of device contact. Occasionally, there is concurrent valvular infection. One-third of cases of CIED-IE present within 3 months after device implantation or manipulation, one-third between 3 and 12 months, and one-third

1 year. S. aureus and CoNS cause the majority of cases. CHAPTER 133 IE in people who inject drugs (PWID), especially that involving the tricuspid valve, is commonly caused by S. aureus, which is often resis­ tant to methicillin. Left-sided valve infections in PWID have a more varied etiology. In addition to the usual causes of IE, infection due to Enterobacterales, Pseudomonas aeruginosa, Candida species, and spo­ radically by unusual organisms (Bacillus, Lactobacillus, Corynebacterium species) is encountered. Infective Endocarditis About 5–15% of patients with IE have negative blood cultures; in one-third to one-half of these cases, cultures are negative because of prior antibiotic exposure. The remainder are infected by fastidious organisms, such as some streptococci; nutritionally variant bacteria now designated Granulicatella, Gemella, and Abiotrophia species; Coxiella burnetii; and Bartonella species. Some fastidious organisms PROPORTION OF CASES PROSTHETIC-VALVE IE AT INDICATED TIME OF ONSET (MONTHS) AFTER VALVE SURGERY TAVR PVE CIED-IE

occur in characteristic geographic settings (e.g., C. burnetii and Barton­ ella species in Europe, Brucella species in the Middle East). Tropheryma whipplei causes an indolent, culture-negative form of IE. C. burnetii has a predilection for prosthetic valves. Corynebacterium species and Cuti­ bacterium acnes may involve intracardiac devices and be slow to grow in blood cultures. Mycobacterium chimaera, which may be difficult to recover from blood cultures unless special media is used, has caused a global outbreak of PVE and disseminated infection as a result of aero­ sols from contaminated heater-cooler machines used during cardiopul­ monary bypass. Lastly, atrial myxoma, marantic endocarditis, and the antiphospholipid antibody syndrome may mimic culture-negative IE.

■ ■PATHOGENESIS The undamaged endothelium is resistant to infection by most bacteria. Endothelial injury (e.g., at the site of impact of high-velocity blood jets or on the low-pressure side of a cardiac structural lesion) allows either direct infection by virulent organisms or the development of a platelet–fibrin thrombus—a condition called nonbacterial throm­ botic endocarditis (NBTE). This thrombus serves as a site of bacterial attachment during transient bacteremia. The cardiac conditions most commonly resulting in NBTE are mitral regurgitation, aortic stenosis, aortic regurgitation, ventricular septal defects, and complex congenital heart disease. NBTE also arises as a result of a hypercoagulable state; this phenomenon gives rise to marantic endocarditis (uninfected veg­ etations seen in patients with malignancy and chronic diseases) and to bland vegetations complicating systemic lupus erythematosus and antiphospholipid antibody syndrome. Organisms that cause IE enter the bloodstream from colonized body surfaces or sites of infection. S. aureus adherence to intact endothelium may be mediated by local inflammation inducing von Willebrand factor on endothelial cell surfaces with resulting adherence of both platelets and S. aureus. Alternatively, S. aureus adherence to injured endothelium may be mediated by local deposition of fibrin and cir­ culating von Willebrand factor on exposed subendothelial tissue to which in turn S. aureus adhere directly. Other microorganisms in the blood adhere to NBTE. The organisms that commonly cause IE have surface adhesin molecules, collectively called microbial surface com­ ponents recognizing adhesin matrix molecules (MSCRAMMs) that mediate adherence to NBTE sites or injured endothelium. Adherence is facilitated by fibronectin-binding proteins present on many grampositive bacteria; by clumping factor (a fibrinogen- and fibrin-binding surface protein) on S. aureus; by fibrinogen-binding surface proteins (Fss2), collagen-binding surface protein (Ace), and Ebp pili (the latter mediating platelet adherence) on Enterococcus faecalis; and by glucans or FimA (a member of the family of oral mucosal adhesins) on strep­ tococci. Fibronectin-binding proteins are required for S. aureus inva­ sion of intact endothelium; thus, these surface proteins may facilitate infection of previously normal valves. If resistant to the bactericidal activity of serum and the microbicidal peptides released locally by platelets, adherent organisms proliferate to form dense microcolonies. Microorganisms also induce platelet deposition and a localized pro­ coagulant state by eliciting tissue factor from the endothelium and, in the case of S. aureus, from monocytes as well. Fibrin deposition combines with platelet aggregation and microorganism proliferation to generate an infected vegetation. Organisms deep in vegetations are metabolically inactive (nongrowing) and relatively resistant to killing by antimicrobial agents. Proliferating surface organisms are shed into the bloodstream continuously. PART 5 Infectious Diseases The clinical manifestations of IE—other than constitutional symp­ toms, which probably result from cytokine production—arise from damage to intracardiac structures; embolization of vegetation frag­ ments leading to infection or infarction of remote tissues; hematog­ enous infection of sites during bacteremia; and tissue injury due to the deposition of circulating immune complexes or immune responses to deposited bacterial antigens. ■ ■CLINICAL MANIFESTATIONS The highly variable clinical IE syndrome spans a continuum between acute and subacute presentations. Most forms of IE share clinical and

TABLE 133-2  Clinical and Laboratory Features of Infective Endocarditis FEATURE FREQUENCY, % Fever 80–90 Chills and sweats 40–75 Anorexia, weight loss, malaise 25–50 Myalgias, arthralgias 15–30 Back pain 7–15 Heart murmur 80–85 New/worsened regurgitant murmur 20–50 Arterial emboli 20–50 Splenomegaly 15–50 Clubbing 10–20 Neurologic manifestations 20–40 Peripheral manifestations (Osler’s nodes, subungual hemorrhages, Janeway lesions, Roth’s spots) 2–15 Petechiae 10–40 Laboratory manifestations     Anemia 70–90   Leukocytosis 20–30   Microscopic hematuria 30–50   Elevated erythrocyte sedimentation rate 60–90   Elevated C-reactive protein level

90   Rheumatoid factor

  Circulating immune complexes 65–100   Decreased serum complement 5–40 laboratory manifestations (Table 133-2). The causative microorganism is primarily responsible for the temporal course of IE. β-Hemolytic streptococci, S. aureus, and pneumococci typically result in an acute course, and IE caused by Staphylococcus lugdunensis (a coagulasenegative species) or by enterococci may present acutely. Subacute IE is typically caused by viridans streptococci, enterococci, CoNS, and the HACEK group. IE caused by Bartonella species, T. whipplei, C. burnetii, or M. chimaera is exceptionally indolent. In patients with subacute presentations, fever is typically low-grade, rarely exceeding 39.4°C (103°F); in contrast, temperatures of 39.4°– 40°C (103°–104°F) are often noted in acute IE. Fever may be blunted in patients who are elderly, are severely debilitated, or have renal failure. Cardiac Manifestations  Although heart murmurs are usually indicative of the predisposing cardiac pathology rather than of IE, valvular damage and ruptured chordae may result in new regurgitant murmurs. In acute IE involving a normal valve, murmurs may be absent initially but ultimately are detected in 85% of cases. Congestive heart failure (CHF) resulting from valve dysfunction or, occasion­ ally, intracardiac fistulae develop in 30–40% of patients. Extension of leaflet infection into adjacent annular or myocardial tissue results in paravalvular abscesses, which in turn may cause intracardiac fistulae with new murmurs. Aortic paravalvular infection may burrow into the upper ventricular septum and interrupt the conduction system, lead­ ing to varying degrees of heart block. Mitral paravalvular abscesses are more distant from the conduction system and rarely cause conduction abnormalities. Coronary artery emboli occur in 2% of patients and may result in myocardial infarction. Noncardiac Manifestations  The classic nonsuppurative periph­ eral manifestations of subacute IE (e.g., Janeway lesions; Fig. 133-2A) are related to prolonged infection; with early diagnosis and treat­ ment, these have become infrequent. In contrast, septic embolization mimicking some of these lesions (subungual hemorrhage, Osler’s nodes) is common in patients with acute S. aureus IE (Fig. 133-2B). Musculoskeletal pain usually remits promptly with treatment but

A B FIGURE 133-2  A. Janeway lesions on the toe (left) and plantar surface (right) of the foot in subacute Neisseria mucosa infective endocarditis (IE). (Images courtesy of Rachel Baden, MD.) B. Septic emboli with hemorrhage and infarction due to acute Staphylococcus aureus IE. must be distinguished from focal metastatic infections (e.g., spondy­ lodiscitis), which may complicate 10–15% of cases. Hematogenously seeded focal infection occurs most often in the skin, spleen, kidneys, skeletal system, and meninges. Arterial emboli, one-half of which precede the diagnosis of IE, are clinically apparent in up to 50% of patients. S. aureus IE, mobile vegetations >10 mm in diameter, and infection involving the mitral valve anterior leaflet are independently associated with an increased risk of embolization. Embolic arterial occlusion causes regional pain or ischemia-induced organ dysfunc­ tion (e.g., of the kidney, spleen, bowel, extremity). Cerebrovascular emboli presenting as strokes or occasionally as encephalopathy com­ plicate 15–35% of cases; however, evidence of clinically asymptomatic emboli is found on magnetic resonance imaging (MRI) in 30–65% of patients with left-sided IE. The frequency of stroke is 8 per 1000 patient-days during the week prior to diagnosis and decreases to 4.8 and 1.7 per 1000 patient-days during the first and second weeks of effective antimicrobial therapy, respectively. Only 3% of strokes occur after 1 week of effective therapy. Emboli occurring late during or after effective therapy do not in themselves constitute evidence of failed antimicrobial treatment. Other neurologic complications include aseptic or purulent menin­ gitis, intracranial hemorrhage due to hemorrhagic infarcts or ruptured mycotic aneurysms, and seizures. Mycotic aneurysms are focal dila­ tions of arteries occurring at points in the artery wall that have been weakened by infection in the vasa vasorum or where septic emboli have lodged. Microabscesses in the brain and meninges occur commonly in S. aureus IE; intracerebral abscesses requiring surgical drainage are infrequent. Immune complex deposition on the glomerular basement mem­ brane causes diffuse hypocomplementemic glomerulonephritis and renal dysfunction, which typically improve with effective antimicrobial therapy. Embolic renal infarcts cause flank pain and hematuria but rarely renal dysfunction. Splenic infarcts or abscess can manifest as left upper abdominal, pleuritic chest, or left shoulder pain. Manifestations with Specific Predisposing Conditions  Among PWID, 35–60% of IE is limited to the tricuspid valve and presents with fever with faint or no murmur and without peripheral manifesta­ tions. Septic pulmonary emboli, which are common with tricuspid IE, cause cough, pleuritic chest pain, nodular pulmonary infiltrates, and occasionally empyema or pyopneumothorax. Infection of the aortic or mitral valve presents with the typical clinical features of IE, including peripheral manifestations. Health care–associated IE has typical manifestations unless associ­ ated with an intracardiac device or masked by the symptoms of concur­ rent illness. CIED-IE may be associated with obvious (especially within 6 months of device manipulation) or cryptic generator pocket infection or arise through bacteremic seeding without pocket infection. Fever, sepsis, minimal murmur, and occasionally pulmonary symptoms due to septic emboli are seen. Late-onset PVE and TAVR-PVE present with typical clinical features. In early PVE, symptoms may be masked by recent surgery. In both early and late PVE, paravalvular infection is common and often results in partial valve dehiscence, regurgitant murmurs, CHF, or disruption of the conduction system.

■ ■DIAGNOSIS Careful clinical, microbiologic, and echocardiographic evaluations should be pursued when febrile patients have IE predispositions, cardiac or noncardiac (e.g., stroke or splenic infarct) features of IE, or blood cultures yielding an IE-associated organism. Duke Criteria  The diagnosis of IE is established with certainty only when vegetations are examined histologically and microbiologi­ cally. Nevertheless, a common clinical approach utilizes a diagnostic schema based on clinical, laboratory, and echocardiographic findings commonly encountered in patients with IE (Table 133-3). Now known as the Duke–International Society for Cardiovascular Infectious Dis­ eases (Duke-ISCVID) Criteria for IE, the criteria were updated in 2023 to encompass new epidemiologic data and microbiologic testing and imaging methodologies; these modifications have been validated in at least four bacteremia cohorts and show sensitivity in the 80–93% range. Clinical judgment must be exercised to use the criteria effectively. A clinical diagnosis of definite IE requires two major criteria, one major and three minor criteria, or five minor criteria. IE is rejected if an alternative diagnosis is established, if there is no recurrence despite therapy for <4 days, or if surgery or autopsy after <4 days of antimicro­ bial therapy yields no histologic evidence of IE. Cases not classified as definite or rejected are considered possible IE when either one major and one minor criterion or three minor criteria are fulfilled. Absent extenuating circumstances, patients with definite and possible IE are treated as having IE. CHAPTER 133 Infective Endocarditis Blood Cultures  The Duke-ISCVID Criteria reiterate that multiple blood cultures are the gold standard for diagnosing IE. Collection of three cultures from separate venipuncture sites is still recommended but is no longer required. Further, the bacterial species considered “typical” for causing IE are defined as those whose recovery from blood has been strongly associated with IE; the pathogen list has been expanded to include all streptococcal species except S. pneumoniae and S. pyogenes, S. lugdunensis, E. faecalis regardless of the primary source, and “streptococcus-like bacteria” (e.g., Granulicatella spp., Abiotrophia spp., and Gemella spp.). To fulfill a major criterion, a typical organism that causes IE (e.g., those listed previously plus S. aureus and HACEK organisms) must be recovered in two or more blood culture sets; other organisms, considered “nontypical” must grow in three or more blood culture sets and the clinical presentation must be unexplained by an extracardiac focus of infection. In patients with intracardiac prosthetic material, CoNS, Corynebacterium striatum, Corynebacterium jeikeium, C. acnes, Serratia marcescens, Pseudomonas aeruginosa, nontubercu­ lous mycobacteria (e.g., M. chimaerae), and Candida species should be considered “typical.” Otherwise these organisms must be found in three or more blood culture sets to satisfy a major criterion.   In patients with suspected NVE, PVE, TAVR-PVE, or CIED-IE who have not received antibiotics during the prior 2 weeks, three twobottle blood culture sets containing the appropriate volume of blood (10 mL per bottle) should be obtained, ideally from different venipunc­ ture sites. If the cultures remain negative after 48–72 h, two or three additional blood culture sets should be obtained, and the laboratory should be consulted for advice regarding optimal culture techniques.

TABLE 133-3  The Modified Duke Criteria for the Clinical Diagnosis of Infective Endocarditis (IE)a Major Criteria A. Microbiologic Criteria 1.  Positive blood culture Microorganism that commonly cause IE in 2 or more separate blood culture sets (Typical—i.e., S. aureus, S. lugdunensis, E. faecalis, all streptococcal species except S. pneumoniae and S. pyogenes, Granulicatella spp., Abiotrophia spp., Gemella spp., HACEK group organisms and in the setting of intracardiac prosthetic material, CoNS, C. striatum, C. jeikeium, S. marcescens, P. aeruginosa, C. acnes, nontuberculosis mycobacteria, Candida spp.) or Microorganisms that occasionally or rarely cause IE isolated from 3 or more separate blood culture sets (Nontypical) 2.  Positive laboratory tests Positive polymerase chain reaction (PCR) for Coxiella burnetti, Bartonella spp., or Tropheryma whipplei from blood or Single blood culture growing C. burnetti or phase I IgG Ab titer ≥ 1:800 or Indirect immunofluorescence assays (IFA) for IgM Ab and IgG Ab to

B. henselae or B. quintana with IgG Ab titer ≥ 1:800 B. Imaging Criteria 1.  Echocardiography or cardiac CT showing vegetation, valvular/leaflet perforation/aneurysm, abscess, pseudoaneurysm or intracardiac fistula or New valvular regurgitation (new/worsening murmur is NOT sufficient; requires echocardiographic evidence) or New prosthetic valve dehiscence/insufficiency 2.  FDG-PET/CT with abnormal metabolic activity involving a native or PART 5 Infectious Diseases prosthetic valve, ascending aortic graft, intracardiac device leads, or other prosthetic material C. Surgical Criteria Evidence of IE documented by direct inspection during heart surgery Minor Criteria A. Predisposition: previous history of IE, injection drug use, prosthetic valve, previous valve repair, congenital heart disease (e.g., bicuspid AV), CIED, more than mild regurgitation or stenosis, hypertrophic cardiomyopathy B. Fever: T > 38.0°C (100.4°F) C. Vascular phenomenon: arterial emboli, septic pulmonary infarcts, mycotic aneurysm, intracranial hemorrhage, conjunctival hemorrhage, Janeway lesions, cerebral/splenic abscesses, purulent purpura D. Immunologic phenomena: glomerulonephritis, Osler nodes, Roth spots, positive rheumatoid factor E. Microbiologic evidence: positive blood cultures (not meeting above criteria) or Positive culture, PCR, or other nucleic acid-based test for an organism consistent with IE from a non-endovascular site or single finding of a skin bacterium by PCR on a valve or wire without additional clinical or microbiological supporting evidence F. Imaging criteria: abnormal metabolic activity on FDG-PET/CT within 3 months of implantation of a prosthetic valve, ascending aortic graft, intracardiac device lead G. Physical exam criteria: New valvular regurgitation on auscultation DIAGNOSIS Definite IE: 1. Pathologic criteria (microorganisms or active endocarditis identified in a vegetation/intra-cardiac abscess from cardiac tissue, prosthetic material, arterial embolus) 2. Clinical criteria (2 major or 1 major + 3 minor or 5 minor) Possible IE: 1 major + 1 minor or 3 minor Rejected IE: does not meet above criteria or firm alternative dx or lack of recurrence with <4 days of antibiotics or no evidence on autopsy  Abbreviations: Ab, antibody; AV, aortic valve; CIED, cardiac implantable electronic device; CoNS, coagulase-negative staphylococci; CT, computed tomography; FDG, 18F-fluorodeoxyglucose; HACEK, Haemophilus species, Aggregatibacter species, Cardiobacterium hominis, Eikenella corrodens, and Kingella kingae; PET, positron emission tomography. Source: Reproduced with permission from VG Fowler Jr et al: The 2023 DukeInternational Society for Cardiovascular Infectious Diseases criteria for infective endocarditis: Updating the modified Duke criteria. Clin Infect Dis. 77:518, 2023, Table 2.

Pending culture results, empirical antimicrobial therapy should be withheld initially from hemodynamically and clinically stable patients with suspected subacute IE, especially those who have received antibiotics within the preceding 2 weeks. The delay allows blood for additional cultures to be obtained without the confounding effect of empirical treatment. Patients with sepsis or deteriorating hemodynam­ ics who may require urgent surgery should receive empirical treatment immediately after the initial three sets of blood cultures are obtained. Non–Blood Culture Tests  Non–blood culture laboratory criteria have been added to microbiologic major criteria in the Duke-ISCVID Criteria to implicate organisms that are difficult to recover by blood culture. These include polymerase chain reaction (PCR) or other nucleic acid–based techniques identifying C. burnetii, Bartonella spp., or T. whipplei from the blood and indirect immunofluorescence assays for IgM and IgG antibodies to Bartonella henselae or B. quintana with IgG titer ≥1:800. Next-generation (shotgun metagenomic) sequenc­ ing of pathogen DNA from serum has emerged as a novel nonculture technology capable of identifying a wide array of organisms in blood culture–negative IE. However, Duke-ISCVID Criteria recommend results from such testing that yield organisms other than C. burnetii, Bartonella spp., or T. whipplei be considered minor criteria at this time. In vegetations recovered at surgery or by embolectomy, pathogens can be identified by culture and histopathologic examination with spe­ cial stains. A sample of the vegetation should be collected using sterile technique and saved for molecular testing using PCR with organismspecific primers (e.g., C. burnetii, Bartonella, T. whipplei, C. acnes, Mycoplasma hominis) or broad-range PCR targeting 16S ribosomal RNA (or 28S rRNA, if fungi are suspected) followed by sequencing for organism identification. Histopathology may inform the selection of specific molecular tests. Molecular testing is a useful diagnostic tech­ nology when the histopathology of a vegetation is consistent with IE; however, it cannot be used to establish the viability of residual bacteria in vegetations. Additionally, molecular testing is only moderately sensi­ tive, and thus, a negative test cannot exclude IE. When tissue is limited, molecular testing should be prioritized over culture. Cardiac Imaging  Echocardiography anatomically confirms and measures vegetations, detects intracardiac complications, and assesses cardiac function. Transthoracic echocardiography (TTE) is exception­ ally specific; however, in 20% of patients, the images are inadequate. TTE fails to detect vegetations in 20–35% of patients with definite clinical IE, missing vegetations <2 mm in diameter. It is not optimal for evaluating prosthetic valves, especially TAVR with large stents, or detecting intracardiac complications. TEE detects vegetations in >90% of patients with definite IE; nevertheless, initial studies may yield falsenegative results in 6–18% of IE patients, especially in TAVR-PVE. A negative TEE, when IE is likely, does not exclude the diagnosis but rather warrants repeating the study in 7–10 days. TEE is sometimes augmented by three-dimensional TEE, which can better visualize veg­ etations and perivalvular extension of infection. Other imaging should be pursued when anatomic confirmation of IE is unclear, when TEE is not confirmatory or is contraindicated, and in suspected PVE. Electrocardiographic-gated multislice cardiac CT angiogram (CTA), which is less sensitive than TEE in detection of vegetations, valvular perforation, and paravalvular leakage but superior in defining pseudoaneurysm or abscess, may be definitive. Further, it can be used in lieu of preoperative cardiac catheterization to assess coronary artery patency in patients at low to intermediate risk of coronary disease. 18F-Fluorodeoxyglucose positron emission tomog­ raphy (FDG-PET)/CT is less sensitive than TEE or CTA in detecting intracardiac pathology in NVE or CIED-IE but provides increased sensitivity in assessing suspected PVE, including TAVR-PVE, infection of ascending aorta grafts, extracardiac complications, left ventricular assist device (LVAD) infection, and CIED pocket and lead infection. As a whole-body image, findings may modify therapy in 25% of NVE and PVE patients. However, FDG-PET/CT is costly, requires preprocedure patient preparation, can have false-positive results in patients with recent valve surgery (<3 months), and requires experienced radiogra­ phers for interpretation. Of note, findings indicative of IE on CTA and

FDG-PET/CT are major criteria and considered equivalent to echocar­ diography in the Duke-ISCVID schema. In population-based studies and large series (using various diag­ nostic criteria), IE occurs frequently among patients who have mono­ microbial bacteremia due to those gram-positive organisms that are commonly associated with IE. For example, 12–17% of patients with blood cultures growing E. faecalis have IE; 7% of patients with blood cultures growing non-β-hemolytic streptococci have IE; and 8–14% of patients with blood cultures growing S. aureus have IE. Among patients with one or more positive monomicrobial blood culture, IE risk-prediction scoring systems have been developed to identify patients who are at sufficient risk of IE to justify echocardiographic assessment (Table 133-4). Because S. aureus bacteremia is associated with a high prevalence of IE and a resultant high risk for mortality, echocardiographic evaluation (high-quality TTE or preferably TEE) is recommended routinely. Prediction scores suggest that with S. aureus bacteremia, a patient with any of the features listed in Table 133-4 incurs at least a 6% risk of IE, with risk increasing when multiple features are present. Thus, when present, these findings are a strong indication for early TEE. In their absence, TTE should suffice unless other findings suggest IE. Among patients with either monomicrobial E. faecalis or non-β-hemolytic streptococcal bacteremia, any three of the respective listed features (Table 133-4) are associated with a significant frequency of IE. For these patients, the estimated number needed to test with TEE to detect IE is 2.4 and 3.6, respectively. While these predictive scoring systems need further evaluation and should be used with clinical judgment, they appear to have a high sensitivity and therefore a high negative predictive value, which allows identifica­ tion of patients at low risk of IE where echocardiography, particularly TEE, can be omitted. An approach to echocardiographic evaluation of patients with suspected IE is illustrated in Fig. 133-3. Other Studies  Many studies that are not diagnostic—i.e., complete blood count, creatinine determination, liver function tests, chest radi­ ography, and electrocardiography—are important in the management of patients with IE. The erythrocyte sedimentation rate, C-reactive protein level, rheumatoid factor, and circulating immune complex titer are commonly increased in IE (Table 133-2). Brain MRI/magnetic TABLE 133-4  Features Guiding the Need for Echocardiographic Assessment in Patients with Selected Monomicrobial Bacteremia BLOOD CULTURE ISOLATE S. AUREUSa E. FAECALISb NON-a-HEMOLYTIC STREPTOCOCCIc Intracardiac device Symptoms ≥7 days Symptoms ≥7 days Preexisting valve disease (including prior endocarditis or valve prosthesis) Emboli

2 positive cultures Injection drug use ≥2 positive cultures One species: S. gallolyticus, S. sanguinis, S. mutans (not S. anginosus) Cerebral/peripheral emboli Unknown origin (no focus) Preexisting valve disease (including prior endocarditis or valve prosthesis) Meningitis Heart murmur Heart murmur Persistent bacteremia (≥72 h) Preexisting valve disease (including prior endocarditis or valve prosthesis) Community acquisition Vertebral osteomyelitis     Community acquisition     Nonnosocomial health care associated (including hemodialysis)     Source: aS Tubiana et al: J Infect 72:544, 2016 and A Showler et al: JACC Cardiovasc Imaging 8:924, 2015. bA Berge et al: Infection 47:45, 2019. cT Sunnerhagen et al: Clin Infect Dis 66:693, 2018.

resonance angiography (MRA) should be obtained in patients with neurologic signs or symptoms, including unusual headache, to assess for emboli, hemorrhage, or mycotic aneurysms. The findings can sup­ port the IE diagnosis as well as provide evidence requiring changes in planned surgical treatment. Patients with recalcitrant back pain or focal spine tenderness should undergo spine MRI targeted to the appropri­ ate level based on symptoms to assess for osteomyelitis and epidural abscess. Contrast-enhanced whole-body tomography to detect silent emboli in patients without localizing symptoms is not likely to enhance diagnostic accuracy and is associated with significant risk of kidney injury due to contrast media exposure; thus, it should not be per­ formed routinely.

TREATMENT Infective Endocarditis ANTIMICROBIAL THERAPY To cure IE, all bacteria in the vegetation must be killed. This is difficult because local host defenses are deficient and because the bacteria are largely nongrowing and metabolically inactive and thus are less easily killed by antibiotics. Consequently, therapy must be prolonged. Antibiotics are generally given parenterally to achieve serum concentrations that, through passive diffusion, result in effective concentrations in the depths of the vegetation. The deci­ sion to initiate treatment empirically must balance the need to establish a microbiologic diagnosis against the potential disease progression or the need to control infection prior to urgent surgery (see “Blood Cultures,” above). Infection at other sites (such as the meninges), allergies, end-organ dysfunction, interactions with con­ comitantly administered medications, and risks of adverse events must be considered in the selection of therapy. CHAPTER 133 The regimens recommended for the treatment of PVE (except that caused by staphylococci), although given for several weeks longer, are similar to those used to treat NVE (Table 133-5). Rec­ ommended antibiotic dosing and duration of therapy, which is measured from the time blood cultures become negative, should be followed unless alterations are required by end-organ dysfunction or adverse events. Infective Endocarditis Organism-Specific Therapies  •  Streptococci  The recom­ mended therapies for streptococcal IE are based on the minimal inhibitory concentration (MIC) of penicillin for the causative isolate (Table 133-5). The 2-week penicillin/gentamicin and ceftri­ axone/gentamicin regimens should not be used to treat NVE com­ plicated by cardiac or extracardiac abscess or PVE. Caution should be exercised in considering aminoglycoside-containing regimens in patients at increased risk for aminoglycoside toxicity (renal or eighth cranial nerve). The regimens recommended for relatively penicillin-resistant streptococci are advocated for treatment of group B, C, or G streptococcal IE. Granulicatella, Abiotrophia, and Gemella species are treated with the regimens for moderately peni­ cillin-resistant streptococci, as is PVE caused by these organisms or by streptococci with a penicillin MIC of >0.1 μg/mL (Table 133-5). Enterococci  Enterococci are resistant to oxacillin, nafcillin, and the cephalosporins and are only inhibited—not killed—by the cell wall–active agents penicillin, ampicillin, teicoplanin (not avail­ able in the United States), and vancomycin, with ampicillin and penicillin being preferred when susceptible. Enterococci are killed by the synergistic interaction of these cell wall–active antibiotics combined with gentamicin, unless the isolate exhibits high-level resistance to gentamicin, defined as growth of the isolate in the presence of gentamicin at ≥500 μg/mL. Bactericidal synergy with other aminoglycosides—tobramycin, netilmicin, kanamycin, and amikacin—is unpredictable even in the absence of high-level resis­ tance; thus, they are not used to treat enterococcal IE. Although the dose of gentamicin used to achieve bactericidal synergy in treating enterococcal IE is smaller than that used in stan­ dard therapy, nephrotoxicity is not uncommon during treatment

Low initial patient risk and low clinical suspicion Initial TTE Initial TEE – + + Increased suspicion during clinical course Low suspicion persists Rx High-risk echo features* No high-risk echo features TEE No TEE unless clinical status deteriorates TEE for detection of complications – Clinical judgment regarding treatment + Rx Look for other source FIGURE 133-3  The diagnostic use of transesophageal and transthoracic echocardiography (TEE and TTE, respectively). †High initial patient risk for infective endocarditis (IE) or evidence of intracardiac complications (new regurgitant murmur, new electrocardiographic conduction changes, or congestive heart failure). *High-risk echocardiographic features include large vegetations, valve insufficiency, paravalvular infection, or ventricular dysfunction. Rx indicates initiation of antibiotic therapy. CTA, electrocardiogram-gated cardiac computed tomography (CT) angiogram; FDG-PET/CT, fluorodeoxyglucose-positron emission tomography CT. ^See text for discussion of these modalities. (Reproduced with permission from AS Bayer: Diagnosis and management of infective endocarditis and its complications. Circulation 98:2936, 1998, Figure 1.) PART 5 Infectious Diseases lasting 4–6 weeks. High concentrations of ampicillin plus ceftri­ axone or cefotaxime, by expanded binding of penicillin-binding proteins, also kill E. faecalis in vitro and in animal models of IE. Nonrandomized comparative studies suggest that high-dose regimens using ampicillin-ceftriaxone appear comparable and less nephrotoxic than penicillin or ampicillin plus gentamicin for treat­ ment of E. faecalis (but not E. faecium) IE and may also provide effective treatment when strains possess high-level gentamicin resistance. This regimen has been used increasingly to address not only high-level gentamicin-resistant strains but also to minimize nephrotoxicity. Alternatively, if there is a contraindication to an ampicillin-ceftriaxone regimen, shorter 2- to 3-week courses of synergistic gentamicin can be considered. The combinations of vancomycin (or teicoplanin) or gentamicin with ceftriaxone are not bactericidal for E. faecalis and are not recommended for treatment of enterococcal IE. If a combination regimen cannot be used due to resistance or toxicity, an 8- to 12-week course of a single cell wall–active agent can be considered, although the patient should be followed carefully for evidence of failure. Treatment of IE caused by E. faecium, which is generally more antibiotic resistant than E. faecalis and may be vancomycin resis­ tant, is not well established. Successful treatment of IE caused by vancomycin-resistant enterococci with high-dose daptomycin (10–12 mg/kg IV once daily), often in combination with ampicillin or other β-lactams, has been reported. If the isolate susceptibility allows treatment with penicillin or ampicillin plus gentamicin, this is preferred. These cases should be managed in conjunction with an infectious disease consultant. Staphylococci  Management of S. aureus bacteremia and IE in conjunction with infectious disease consultants has been associ­ ated with improved outcomes and is recommended. Treatment of staphylococcal IE (Table 133-5) is based on the presence of a

High initial patient risk†; moderate to high clinical suspicion or difficult imaging candidate IE suspected – Look for other source of symptoms High suspicion persists Rx + – Repeat TEE Alternative diagnosis established – + Consider CTA or FDG-PET/CT^ Rx – + Follow-up TEE or TTE to reassess vegetations, complications, or Rx response as clinically indicated prosthetic valve or foreign device, the native valve(s) involved (right vs left side), and the antibiotic susceptibility of the isolate. Penicillin resistance and, except in specific countries, methicillin resistance are widespread among staphylococci. Thus, empirical therapy for possible staphylococcal IE should use a regimen effective against methicillin-resistant organisms. Therapy should be revised to an antistaphylococcal penicillin if the isolate is susceptible to methicil­ lin. Cefazolin is generally considered an alternative β-lactam agent for the treatment of methicillin-susceptible S. aureus (MSSA) IE. Ease of administration and reduced adverse events compared to treatment with an antistaphylococcal penicillin have prompted use of cefazolin as a primary agent in this setting. Concerns, however, have been raised about inactivation of cefazolin by type A and C staphylococcal β-lactamases (these do not hydrolyze antistaphylo­ coccal penicillins), resulting in treatment failure in high-inoculum infections. Initiating treatment with an antistaphylococcal penicil­ lin until there is source control and a reduced inoculum and then transitioning to cefazolin should be considered. The addition of gentamicin to a β-lactam antibiotic or vancomycin to enhance therapy for left-sided NVE has not improved survival rates and is associated with nephrotoxicity. Guidelines do not recommend the routine addition of gentamicin, fusidic acid, rifampin, or daptomy­ cin to regimens for MSSA NVE. For treatment of NVE due to methicillin-resistant S. aureus (MRSA), vancomycin, dosed to achieve trough concentrations of 15 μg/mL (or an area under the time-concentration curve/broth microdilution MIC ratio [AUC:MIC] >400 achieved with the assis­ tance of a pharmacist), is recommended, with the caveat that high vancomycin trough concentrations may be associated with nephro­ toxicity. Although resistance to vancomycin among staphylococci is rare, reduced vancomycin susceptibility among MRSA strains is increasingly encountered. Isolates with a vancomycin MIC of 4–8 μg/mL have intermediate susceptibility and are referred to as

TABLE 133-5  Antibiotic Treatment for Infective Endocarditis Caused by Common Organismsa ORGANISM(S) DRUG (DOSE, DURATION) COMMENTS Streptococci   For PVE 6-week regimens are preferred. Penicillin-susceptible streptococci, S. gallolyticus (MIC ≤0.12 μg/mL) • Penicillin G (2–3 mU IV q4h for 4 weeks) Can use ampicillin or amoxicillin (2 g IV q4h) if penicillin is unavailable. • Ceftriaxone (2 g once daily for 4 weeks) Can use ceftriaxone in patients with nonimmediate penicillin allergy.   • Vancomycinb (15 mg/kg IV q12h for 4 weeks) Use vancomycin for patients with immediate (urticarial) or severe penicillin allergy. Obtain allergy consultation for further evaluation including role of β-lactam desensitization.   • Penicillin G (2–3 mU IV q4h) or ceftriaxone

(2 g IV once daily) for 2 weeks plus Gentamicinc (3 mg/kg daily IV or IM, as a single dosed for 2 weeks) Relatively penicillin-resistant streptococci, S. gallolyticus (MIC >0.12 μg/mL and

<0.5 μg/mLe) • Penicillin G (4 mU IV q4h) or ceftriaxone

(2 g IV daily) for 4 weeks plus Gentamicinc (3 mg/kg daily IV or IM, as a single dosed for 2 weeks)   • Vancomycinb as noted above for 4 weeks Use vancomycin for patients with immediate (urticarial) or severe penicillin allergy. Obtain allergy consultation for further evaluation including role of β-lactam desensitization. Ceftriaxone alone or with gentamicin can be used in patients with nonimmediate β-lactam allergy. Moderately penicillin-resistant streptococci (MIC, ≥0.5 μg/mL and <8 μg/mL); Granulicatella, Abiotrophia, or Gemella spp. • Penicillin G (4–5 mU IV q4h) or ceftriaxone (2 g IV daily) for 6 weeks plus Gentamicinc (3 mg/kg daily IV or IM in 2–3 equally divided doses for 6 weeks)   • Vancomycinb as noted above for 6 weeks Regimen is preferred by some. Enterococcie   For PVE, 6-week regimens are preferred.   • Ampicillin (2 g IV q4h) plus ceftriaxone (2 g IV q12h), both for 6 weeks   • Penicillin G (4–5 mU IV q4h) for 4–6 weeks plus gentamicinc (3 mg/kg daily IV or 1 mg/kg IV q8h) for 2–6 weeks   • Ampicillin (2 g IV q4h) for 4–6 weeks plus gentamicinc (3 mg/kg daily IV or 1 mg/kg IV q8h) for 2–6 weeks   • Vancomycinb (15 mg/kg IV q12h) for 6 weeks plus gentamicinc (3 mg/kg daily IV or 1 mg/kg IV q8h) for 2–6 weeks Staphylococci (S. aureus and coagulase-negative) MSSA infecting native valves (no foreign devices) including complicated right-sided and left-sided endocarditis. • Nafcillin, oxacillin, or flucloxacillin (2 g IV q4h for 4–6 weeks) • Cefazolin (2 g IV q8h for 4–6 weeks) Can use cefazolin regimen for patients with nonimmediate penicillin allergy; see text regarding cefazolin vs antistaphylococcal penicillin as primary therapy. Addition of gentamicin not recommended.   • Vancomycinb (15 mg/kg IV q12h for 4–6 weeks) MRSA infecting native valves (no foreign devices) • Vancomycinb (15 mg/kg IV q8–12h) or daptomycin (8–10 mg/kg daily) for 4–6 weeks MSSA infecting prosthetic valves • Nafcillin, oxacillin, or flucloxacillin (2 g IV q4h for 6–8 weeks) plus Gentamicinc (1 mg/kg IM or IV q8h for 2 weeks) plus • Rifampinf (300 mg PO q8h for 6–8 weeks) MRSA infecting prosthetic valves • Vancomycinb (15 mg/kg IV q12h for 6–8 weeks) plus Gentamicinc (1 mg/kg IM or IV q8h for 2 weeks) plus • Rifampinf (300 mg PO q8h for 6–8 weeks)

Can use ampicillin or amoxicillin (2 g IV q4h) if penicillin is unavailable. Avoid 2-week regimen when risk of aminoglycoside toxicity is increased and in prosthetic-valve or complicated endocarditis. Penicillin G at a dose of 4 mU IV q4h or ceftriaxone 2 g once daily for 6 weeks both with or without gentamicin during the initial 2 weeks preferred for PVE caused by streptococci with penicillin MICs ≤0.12 μg/mL. Can use ampicillin or amoxicillin (2 g IV q4h) if penicillin is unavailable. Penicillin G at a dose of 4 mU IV q4h or ceftriaxone 2 g once daily for 6 weeks both with gentamicin during the initial 2 weeks preferred for PVE caused by streptococci with penicillin MICs >0.12 μg/mL. Can use ampicillin or amoxicillin (2 g IV q4h) if penicillin is unavailable. CHAPTER 133 Use for E. faecalis isolates with or without high-level resistance to gentamicin or for patients at risk for aminoglycoside nephrotoxicity. Infective Endocarditis Can treat NVE for 4 weeks if symptoms last <3 months. Treat NVE for 6 weeks if

3 months of symptoms. Can abbreviate gentamicin course in some patients (see text). Can use IV amoxicillin in lieu of ampicillin (same dose). Can abbreviate gentamicin course in some patients (see text). Use vancomycin plus gentamicin only for penicillin-allergic patients (preferable to desensitize to penicillin if immediate [urticarial] allergy; consult allergy) and for isolates resistant to penicillin/ampicillin. Addition of gentamicin is not recommended. For uncomplicated right-sided endocarditis, a 2-week course may be effective (see text). Only use vancomycin for patients with immediate (urticarial) or severe penicillin allergy until allergy consultation can be obtained for β-lactam desensitization evaluation; addition of gentamicin not recommended. No role for routine use of rifampin (see text). For daptomycin treatment, see text. Use gentamicin during initial 2 weeks; determine gentamicin susceptibility and await blood culture clearance before initiating rifampin (see text); if patient is highly allergic to penicillin, use regimen for MRSA and obtain allergy consultation; if β-lactam allergy is of the minor nonimmediate type, cefazolin can be substituted for oxacillin, nafcillin, or flucloxacillin. Use gentamicin during initial 2 weeks; determine gentamicin susceptibility and await blood culture clearance before initiating rifampin (see text). Daptomycin (8–10 mg/kg daily) is an alternative to vancomycin. (Continued)

TABLE 133-5  Antibiotic Treatment for Infective Endocarditis Caused by Common Organismsa ORGANISM(S) DRUG (DOSE, DURATION) COMMENTS HACEK organisms   For PVE, 6-week regimens are preferred.   • Ceftriaxone (2 g once daily IV for 4 weeks)     • Ampicillin/sulbactam (3 g IV q6h for 4 weeks) Use ampicillin alone (2 g IV q4h) only if β-lactamase production can be excluded. If the isolate is susceptible, ciprofloxacin (500 mg by mouth q12h or 400 mg IV q12h) can be used. Coxiella burnetii   • Doxycycline (100 mg PO q12h) plus hydroxychloroquine (200 mg PO q8h), both for at least 18 (native valve) or 24 (prosthetic valve) months Bartonella spp.   • Doxycycline (100 mg q12h PO) for 6 weeks plus Gentamicin (1 mg/kg IV q8h for 2 weeks) aRegimens adapted from the guidelines of the American Heart Association and the European Society of Cardiology (ESC). Doses of gentamicin, vancomycin, and daptomycin must be adjusted for reduced renal function. Ideal body weight is used to calculate doses of gentamicin and daptomycin per kilogram (men = 50 kg + 2.3 kg per inch over 5 feet; women = 45.5 kg + 2.3 kg per inch over 5 feet). bVancomycin dose is based on actual body weight. Adjust for trough level of 10–15 μg/mL for streptococcal and enterococcal infections and 15–20 μg/mL for staphylococcal infections (see text). cTarget peak and trough serum concentrations of divided-dose gentamicin 1 h after a 20- to 30-min infusion or IM injection are ~3.5 μg/mL and ≤1 μg/mL, respectively. dNetilmicin (4 mg/kg qd, as a single dose) can be used in lieu of gentamicin for streptococcal infection only. eAntimicrobial susceptibility must be evaluated; see text. fRifampin increases warfarin and dicumarol requirements for anticoagulation. Abbreviations: HACEK, Haemophilus species, Aggregatibacter species, Cardiobacterium hominis, Eikenella corrodens, and Kingella kingae; MIC, minimal inhibitory concentration; MRSA, methicillin-resistant S. aureus; MSSA, methicillin-susceptible S. aureus; NVE, native-valve endocarditis; PVE, prosthetic-valve endocarditis. vancomycin-intermediate S. aureus (VISA). Isolates with a vanco­ mycin MIC of 2 μg/mL may harbor subpopulations with higher MICs. These isolates, called heteroresistant VISA (hVISA), are not detectable by routine susceptibility testing and yet may impair van­ comycin effectiveness. PART 5 Infectious Diseases Daptomycin has provided effective alternative treatment for left-sided NVE caused by documented daptomycin-susceptible MRSA and is associated with less nephrotoxicity than vancomycin (although it is associated with myositis and, rarely, eosinophilic pneumonitis). Although it is U.S. Food and Drug Administration (FDA) approved only for right-sided IE at a dose of 6 mg/kg daily, most recommend doses of 8–10 mg/kg daily for treatment of leftsided IE. Because receipt of vancomycin has been associated with emergence of daptomycin nonsusceptibility, caution should be exercised when switching from vancomycin to daptomycin mono­ therapy in a patient who has received vancomycin without clearing blood cultures. Daptomycin activity against MRSA—even against some daptomycin-nonsusceptible isolates—is enhanced when given in combination with nafcillin or ceftaroline. Case series suggest that high-dose daptomycin combined with nafcillin or ceftaroline, ceftaroline combined with trimethoprim/sulfamethoxazole, or cef­ taroline alone (600 mg IV q8h) may be effective salvage treatment for vancomycin-unresponsive MRSA IE and should be considered in patients with sepsis, in those with multifocal sites of infection, or when bacteremia persists for 4 or more days. Eradicable sources of bacteremia should always be addressed; failure of source control is a very common reason for persistent MRSA bacteremia. MSSA IE that is uncomplicated and limited to the tricuspid or pulmonic valve can often be treated with 2 weeks of oxacillin or nafcillin (but not vancomycin). Prolonged fever (≥5 days) during therapy or multiple septic pulmonary emboli mandate standardduration therapy. Right-sided MRSA IE is treated for at least 4 weeks with vancomycin or daptomycin (6 mg/kg daily); 2-week courses of therapy are suboptimal. Left-sided staphylococcal IE is treated with a minimum of 6 weeks of therapy. Staphylococcal PVE is treated with a multidrug regimen for 6–8 weeks (Table 133-5). To achieve long-term bacterial eradica­ tion, rifampin, which kills staphylococci embedded in biofilm adherent to foreign material, is considered an essential component of this regimen. Rifampin resistance can emerge during therapy. To prevent emergence of resistance, administration of rifampin should be delayed until initial therapy with two agents (gentamicin plus an

(Continued) Follow serology to monitor response during treatment (antiphase I IgG and IgA decreased fourfold and IgM antiphase II negative) and thereafter for relapse. If doxycycline is not tolerated, use azithromycin (500 mg PO daily). Some experts recommend that doxycycline be continued for 3–6 months unless all infection is resected surgically. antistaphylococcal penicillin or vancomycin selected on the basis of susceptibility testing) has eradicated bacteremia (reduced the inoc­ ulum). The isolate’s susceptibility to gentamicin or an alternative agent as well as to rifampin should be established before rifampin treatment is begun. The impact of adding rifampin and gentamicin on survival of patients with staphylococcal PVE has not been dem­ onstrated convincingly; thus, it is reasonable to discontinue these agents if patients are experiencing toxicity or drug interactions. Other Organisms  In the absence of meningitis, IE caused by Streptococcus pneumoniae isolates with a penicillin MIC of ≤4 μg/ mL can be treated with IV penicillin (4 million units IV every 4 h), ceftriaxone (2 g once daily), or vancomycin. Ceftriaxone or vanco­ mycin is preferred for pneumococcal strains with a penicillin MIC of ≥2 μg/mL. If meningitis is suspected, treatment with vancomycin plus ceftriaxone—at the doses advised for meningitis—should be initiated until susceptibility results are known. Definitive therapy should then be selected on the basis of meningitis breakpoints (penicillin MIC, 0.06 μg/mL; ceftriaxone MIC, 0.5 μg/mL). Pneu­ mococcal NVE is treated for 4 weeks and pneumococcal PVE for 6 weeks. P. aeruginosa IE is treated with an antipseudomonal cephalosporin and traditionally high doses of tobramycin, and IE caused by Enterobacterales is treated with a β-lactam antibiotic, also traditionally, plus an aminoglycoside, although data substantiating a need for combination therapy for these organisms are limited. Therapy for Candida IE consists of a lipid formulation of ampho­ tericin B (5 mg/kg IV daily) with or without flucytosine (25 mg/kg PO q6h) (if using flucytosine, monitor renal function, flucytosine levels, and bone marrow function). Alternatively, a high-dose echi­ nocandin regimen can be used. If there is valve dysfunction or PVE, early surgery is advised, as is long-term (if not indefinite) oral azole suppression. Absent valve dysfunction, medical treatment with long-term oral azole suppression may achieve results comparable to surgical treatment. Empirical Therapy and Treatment for Culture-Negative IE  In designing therapy to be administered before culture results are known or when cultures are truly negative, clinical clues to etiol­ ogy (e.g., acute vs subacute presentation, NVE, early or late PVE, the patient’s predispositions) as well as epidemiologic clues (e.g., region of residence, animal exposure) must be considered. Thus, empirical therapy for acute IE should cover MRSA and in a PWID or for health care–associated NVE potentially antibiotic-resistant

gram-negative bacilli. Treatment with vancomycin plus gentamicin or cefepime, initiated immediately after blood cultures are obtained, covers these organisms as well as many others. For empirical treat­ ment of NVE with a subacute presentation, vancomycin plus ceftri­ axone is reasonable. For blood culture–pending PVE, vancomycin, gentamicin, and cefepime should be used if the prosthetic valve has been in place for ≤1 year. Empirical therapy for late PVE (valve in place for >1 year) is similar to that for culture-negative NVE. Therapy is revised once a pathogen has been identified. In the treatment of blood culture–negative episodes, marantic endocarditis and the antiphospholipid antibody syndrome must be considered. In the absence of prior antibiotic therapy, it is unlikely that infection due to S. aureus, CoNS, enterococci, or Enterobacterales will present with negative blood cultures; thus, recommended empirical therapy targets fastidious streptococci, Abiotrophia, Gemella, Granulicatella, the HACEK group, and Bar­ tonella species. Pending the availability of diagnostic data, blood culture–negative subacute NVE is treated with vancomycin plus ampicillin-sulbactam (12 g every 24 h) or ceftriaxone; doxycycline (100 mg twice daily) is added for enhanced Bartonella coverage. If cultures are negative because of prior antibiotic administration, pathogens likely to be inhibited by the specific prior therapy should be considered. TAVR-PVE  The vast majority of these patients are treated medi­ cally with classic PVE antibiotic regimens for the given pathogen. Selection of empirical therapy pending blood culture results simi­ larly parallels that for classic PVE but with recognition that entero­ coccal infection occurs with increased frequency. CIED-IE  Antimicrobial therapy for CIED-IE (as well as for gen­ erator pocket and lead infection) is adjunctive to complete removal of the device. The antimicrobial selected is based on the causative organism and should be used as recommended for NVE (Table 1335). Bacteremic CIED infection may be complicated by coincident left-sided NVE, PVE, or remote-site infection (e.g., osteomyelitis), which may require modification of antimicrobial therapy. A 4- to 6-week course of IE-targeted therapy is recommended for patients with CIED-IE and for those with bacteremia that continues after device removal. Generator pocket infection without bacteremia is treated with a 10- to 14-day course, some of which can be given orally. In the absence of another source, S. aureus bacteremia (and persistent CoNS bacteremia) is likely indicative of CIED-IE or valvular IE and should be investigated and managed accordingly. However, not all bloodstream infections in these patients indicate IE. If evidence suggesting CIED-IE is lacking, bloodstream infec­ tion due to gram-negative bacilli, streptococci, and enterococci species may not indicate CIED-IE and can be treated with anti­ microbial therapy for the alternative diagnosis. However, in these patients, relapse after antimicrobial therapy increases the likelihood of CIED-IE and warrants treatment as such. Attempted salvage of an infected CIED with antibiotics alone and long-term suppressive therapy is usually unsuccessful and should be reserved for patients whose devices cannot be removed or who decline removal. Careful follow-up is required. Partial Oral Antibiotic Treatment of IE  Recent studies have examined the use of oral antibiotics to complete therapy in patients who have received an initial course of intravenous treatment (with or without cardiac surgery). A noninferiority, multicenter, randomized study found mortality among patients with left-sided IE caused by streptococci, enterococci, and staphy­ lococci who received partial oral treatment comparable to that of patients who were treated intravenously for the full course of therapy (6.5% and 7.5%, respectively). Four hundred clini­ cally stable patients (20% of the population screened) who had received at least 10 days of parenteral therapy (or at least 7 days after surgery) were enrolled. IE was caused by streptococci (49%), enterococci (24%), and MSSA (22%); no patients had MRSA. Of note, the median duration of intravenous treatment before the

switch to oral agents was 16 days (interquartile range, 13–23 days); thus, some patients may have been effectively cured by intrave­ nous therapy with or without surgery prior to transition to oral therapy. The results in this highly selected and monitored cohort with relatively small numbers of patients with enterococcal and S. aureus IE may not be generalizable to most patients with IE. If oral therapy is considered in patients who meet the criteria in the studied population, the decision on agents and duration should be made in consultation with infectious diseases.

Use of Long-Acting Lipoglycopeptide Agents  Dalbavancin, a lipoglycopeptide with a long half-life allowing for infrequent dos­ ing, is FDA approved for treatment of skin and soft tissue infections. Only limited data are available supporting its use in the treatment of IE, particularly due to S. aureus, with most being case series of patients who received dalbavancin upon discharge from the hospi­ tal after having received several weeks of active intravenous therapy. Consideration can be given to using this agent in patients who are clinically stable with negative blood cultures and in patients with complicated IE who are not candidates to continue hospitalization or to transition to outpatient parenteral therapy, in consultation with infectious diseases. Outpatient Parenteral Antimicrobial Therapy  Fully compliant, clinically stable patients who are no longer bacteremic, are not febrile, and have no clinical or echocardiographic findings that suggest an impending complication may complete IV therapy as outpatients. Careful follow-up and a stable residential setting are necessary, as are predictable IV access and use of antimicrobial agents that are stable in solution and less frequently associated with severe adverse effects. Recommended regimens should not be com­ promised to accommodate outpatient therapy. CHAPTER 133 Monitoring Antimicrobial Therapy  Antibiotic-related adverse events occur in 25–40% of IE patients and commonly arise after several weeks of therapy. Blood tests to detect renal, hepatic, and hematologic toxicity should be performed periodically. Serum concentrations of aminoglycosides and vancomycin should be monitored and doses adjusted to optimize treatment and minimize toxicity. Infective Endocarditis Control of peripheral sites of infection—source control—should be addressed promptly. Blood cultures should be repeated daily until sterile in patients with IE due to S. aureus or difficult-totreat organisms and rechecked if there is recrudescent fever. Blood cultures become sterile after 2 days of appropriate therapy when infection is caused by viridans streptococci, E. faecalis, or HACEK organisms. In MSSA IE, β-lactam therapy results in sterile cultures in 3–5 days, whereas in MRSA IE, the duration of bacteremia is often longer with vancomycin or daptomycin treatment. MRSA bacteremia persisting despite an appropriate dosage of vancomycin or daptomycin may indicate emergence of reduced susceptibility in the infecting strain and point to a need for alternative therapy. When fever persists for 7 days despite appropriate antibiotic ther­ apy, patients should be evaluated further for paravalvular abscess, extracardiac abscesses (spleen, kidney), or complications (embolic events). Recrudescent fever raises the possibility of these complica­ tions but also of drug reactions or complications of hospitaliza­ tion. It is advisable to obtain follow-up echocardiography after the completion of therapy to assess valvular function. Of note, veg­ etations become smaller with effective therapy; however, 3 months after cure, 50% are unchanged, and 25% each are slightly larger or smaller. Antithrombotic Therapy  Because patients with IE are at risk for hemorrhagic transformation of embolic strokes and for intracere­ bral hemorrhage from septic arteritis or ruptured mycotic aneu­ rysms, initiation of antithrombotic (anticoagulant or antiplatelet) therapy requires careful consideration of the risks and benefits. Antithrombotic therapy can render such bleeding catastrophic. Neither anticoagulant nor antiplatelet therapy reduces the risk of emboli in patients with NVE, and thus, such treatment is not

TABLE 133-6  Indications for Cardiac Surgical Treatment in Patients with Endocarditis Surgery Required for Optimal Outcome Native-valve or prosthetic-valve endocarditis   Moderate or severe congestive heart failure or shock due to valve dysfunction   Paravalvular extension of infection with abscess, fistula, or heart block   Persistent bacteremia without an extracardiac cause despite 7–10 days of optimal antimicrobial therapy   Lack of effective antimicrobial therapy (e.g., fungal [see text regarding Candida spp.], Brucella, multidrug-resistant gram-negative bacillary endocarditis)   Prosthetic-valve endocarditis   Partially dehisced unstable prosthetic valve  Surgery to Be Strongly Considered for Improved Outcomea Prosthetic-valve endocarditis   S. aureus infection with intracardiac complications   Relapse after optimal antimicrobial therapy Native-valve endocarditis   Large (>10-mm) hypermobile vegetation, particularly with prior systemic embolus and significant valve dysfunctionb   Very large (>30-mm) vegetation   Persistent unexplained fever (≥10 days) in blood culture–negative endocarditis   Poorly responsive or relapsed endocarditis due to highly antibiotic-resistant enterococci or gram-negative bacilli aCarefully consider surgery. Multiple findings are often combined to justify surgery. bIn the group with an estimated low cardiac-surgery mortality risk (see text). PART 5 Infectious Diseases indicated for that purpose. However, patients with IE may have coexisting conditions wherein anticoagulation is indicated. Thus, in the absence of a contraindication (i.e., no clinical or imaging evidence of a recent large embolic stroke, intracerebral hemor­ rhage, or mycotic aneurysm), anticoagulant therapy is given to patients who have a mechanical prosthetic valve, atrial fibrilla­ tion with either mitral stenosis or a CHA2DS2-VASc score ≥2, or deep-vein thrombophlebitis. Most experts use unfractionated or low-molecular-weight heparin for ease of reversal. Anticoagulant TABLE 133-7  Timing of Cardiac Surgical Intervention in Patients with Endocarditis INDICATION FOR SURGICAL INTERVENTION   TIMING STRONG SUPPORTING EVIDENCE Emergent (same day) Valve dysfunction with pulmonary edema or cardiogenic shock Acute aortic regurgitation plus preclosure of mitral valve   Sinus of Valsalva abscess ruptured into right heart     Rupture into pericardial sac   Urgent (within 1–2 days) Valve obstruction by vegetation Unstable (dehisced) prosthesis Acute aortic or mitral regurgitation with heart failure (New York Heart Association class III or IV)   Septal perforation Mobile vegetation >30 mm   Paravalvular extension of infection with or without new electrocardiographic conduction system changes   Lack of effective antibiotic therapy   Elective (earlier usually preferred) Progressive paravalvular prosthetic regurgitation Valve dysfunction plus persisting infection after ≥7–10 days of antimicrobial therapy Fungal (mold) endocarditis     Antibiotic-resistant organisms aSupported by a single-institution randomized trial showing benefit from early surgery. Implementation requires clinical judgment. If surgery is elected, it must be done early (see text). Source: Reproduced with permission from L Olaison, G Pettersson: Current best practices and guidelines: Indications for surgical intervention in infective endocarditis. Infect Dis Clin North Am 16:453, 2002.

therapy should be reversed, at least temporarily, in most patients who have had an acute ischemic stroke or an intracerebral hemorrhage. SURGICAL TREATMENT The indications for cardiac surgical treatment of IE (Table 133-6) have been derived from observational studies and expert opinion. The strength of specific indications varies; thus, the risks and benefits as well as the timing of surgery must be individualized (Table 133-7). These are best weighed by a team that includes cardiologists, cardiac surgeons, infectious disease physicians, and neurologists if there have been neurologic complications. Between 25% and 40% of patients with left-sided IE undergo cardiac surgery during active infection, with slightly higher surgery rates for PVE than NVE. The benefit of surgery has been assessed primarily in retrospective studies comparing populations of medically and sur­ gically treated patients matched for the necessity of surgery, with adjustments for predictors of death (comorbidities) and the timing of surgical intervention (a correction for survival bias). Although study results vary, surgery for NVE based on current indications appears to convey a significant survival benefit (27–55%), which is greatest among those with the most pressing indications. The survival benefit becomes more apparent after ≥6 months. The effect of surgery for PVE is more nuanced, with survival ben­ efits accruing largely to those with intracardiac complications. Of note, surgery itself carries mortality risks that may offset survival benefits in patients with lesser indications. Among patients with TAVR-PVE, 50–80% are reported to have an indication for surgical intervention—yet because of high pre-TAVR estimated operative mortality, <15% undergo surgery. Some patients with significant aortic regurgitation after medical cure of infection have undergone valve-in-valve redo-TAVR. Indications  •  Congestive Heart Failure  Moderate to severe refractory CHF caused by new or worsening valve dysfunction or intracardiac fistulae is the major indication for cardiac surgery. Surgery can relieve functional stenosis due to large vegetations or restore competence to damaged regurgitant valves by repair or replacement. At 6–12 months of follow-up of patients with left-sided NVE or PVE and moderate to severe CHF due to valve CONFLICTING EVIDENCE, BUT MAJORITY OF OPINIONS FAVOR SURGERY   Vegetation diameter >10 mm plus severe but not urgent aortic or mitral valve dysfunctiona Major embolus plus persisting large vegetation (>10 mm)   Staphylococcal prosthetic-valve endocarditis with intracardiac complications Early prosthetic-valve endocarditis (≤2 months after valve surgery) Candida spp. endocarditis (see text)

dysfunction, survival is significantly improved among those treated surgically compared with those treated medically. The survival ben­ efit with surgery is inversely related to the severity of preoperative CHF; thus, surgery should not be delayed in the face of deteriorat­ ing hemodynamics. Paravalvular Infection  This complication, which is most com­ mon with aortic valve infection, occurs in 10–15% of patients with NVE and in 45–60% of those with PVE. It is suggested clinically by persistent unexplained fever during appropriate therapy, new electrocardiographic conduction disturbances, or pericarditis. TEE with color Doppler is the test of choice to detect paravalvular abscesses (sensitivity, ≥85%). Occasionally, three-dimensional TEE, ECG-gated CTA, or FDG-PET/CT demonstrates paravalvular infection not detected by TEE. For optimal outcome, paravalvular infection requires surgery, especially when fever persists, fistulae develop, prostheses are dehisced and unstable, or infection relapses after appropriate treatment. Cardiac rhythm must be monitored since high-grade heart block may require insertion of a pacemaker. Uncontrolled Infection  Continued positive blood cultures or otherwise unexplained persistent fevers despite optimal antibiotic therapy may reflect uncontrolled infection that warrants surgery. Surgical treatment is also advised for IE caused by organisms against which effective antimicrobial therapy is lacking (e.g., yeasts, molds, P. aeruginosa, other highly antibiotic-resistant bacteria, Brucella species). S. aureus IE  The mortality rate for S. aureus PVE exceeds 50% with medical treatment and may be reduced with surgical treat­ ment. Nevertheless, surgery is not routinely advised for uncom­ plicated S. aureus PVE. Rather, survival benefits are most likely in those with paravalvular infection, dysfunctional valves, and CHF. Surgical treatment of S. aureus NVE should be guided by the stan­ dard indications. Isolated tricuspid-valve S. aureus IE, even with persistent fever, rarely requires surgery. Prevention of Systemic Emboli  Persisting morbidity and/or death may result from cerebral or coronary artery emboli. Anti­ thrombotic therapy does not prevent systemic emboli in NVE. The frequency of embolization decreases rapidly with effective antimi­ crobial therapy. Thus, if emboli are to be prevented through surgical intervention, surgery must occur very early. Vegetation characteris­ tics defined echocardiographically can identify patients at high risk of embolization but do not identify those patients in whom surgery to prevent emboli will increase survival. In a small randomized trial in patients who were at low risk of surgery-related mortality and had large vegetations (>10 mm) and significant valve dysfunction, emboli were prevented by early surgery (≤48 h after diagnosis), but there was no survival benefit. Rarely is prevention of emboli the sole indication for surgery; more often, this may be an additional benefit of early surgery for other indications. Valve repair, with the consequent avoidance of a prosthesis, improves the benefit-to-risk ratio of surgery performed to eliminate vegetations. CIED-IE  Removal of all hardware is recommended for patients with established CIED-IE as well as for pocket or intracardiac lead infection. Percutaneous lead extraction is preferred; if hard­ ware remains after attempted percutaneous extraction, surgical removal should be considered. With lead vegetations >2 cm, there is a risk of a pulmonary embolism; nevertheless, the need for CIED removal surgically is unclear. Removal of the infected CIED during the initial hospitalization is associated with increased 30-day and 1-year survival rates over those attained with anti­ biotic therapy and device retention. The CIED, if needed, can be reimplanted at a new site after at least 10–14 days of effective antimicrobial therapy. CIEDs should be replaced when patients undergo surgery for IE. Timing of Cardiac Surgery  With life-threatening indications for surgery (valve dysfunction and severe CHF, perivalvular abscess, major prosthesis dehiscence), surgery during the initial days of

therapy is associated with greater survival than later surgery. With less compelling indications, surgery may reasonably be delayed to allow further treatment as well as improvement in overall health (Table 133-7). Recrudescent IE on a newly implanted prosthetic valve follows surgery for active NVE and PVE in 2% and 6–15% of patients, respectively. These frequencies do not justify the increased mortality risk associated with delaying surgery in patients with severe heart failure, valve dysfunction, and uncontrolled infections. Delay is justified when infection and CHF are controlled with medi­ cal therapy.

Neurologic complications of IE may be exacerbated during car­ diac surgery. The risk of neurologic deterioration is related to the type and severity of the preoperative neurologic complication and the interval between the complication and surgery. In a nonob­ tunded patient with an ischemic stroke and hemorrhage excluded by imaging, cardiac surgery, if urgent, should be performed early because risk of hemorrhagic conversion or other neurologic side effects is low. In patients with intracranial hemorrhage and need for urgent surgery, early surgery should be pursued as long as the patient is expected to have meaningful recovery. Similar rates of postoperative intracranial hemorrhage and no difference in mortal­ ity have been observed in patients with and without preoperative intracranial hemorrhage. Nonurgent cardiac surgery should be delayed for 2–3 weeks after a large nonhemorrhagic embolic infarc­ tion and for 4 weeks after a significant cerebral hemorrhage. A rup­ tured mycotic aneurysm should be treated before cardiac surgery. Antibiotic Therapy after Cardiac Surgery  Organisms have been detected on Gram stain—or their DNA has been detected by PCR— in excised valves from 45% of patients who have completed the recommended therapy for IE. However, organisms, most of which are unusual or antibiotic resistant, are rarely cultured from these valves. Detection of organisms or their DNA does not necessarily indicate antibiotic failure; in fact, relapse after surgery for active IE is uncommon. Thus, in uncomplicated NVE caused by susceptible organisms, the duration of preoperative plus postoperative treat­ ment should equal the total duration of recommended therapy. For IE complicated by perivalvular abscess, partially treated PVE, or valves culture-positive for the original organism, a full course of therapy should be given postoperatively. CHAPTER 133 Infective Endocarditis Treatment of IE in PWID  PWID should be treated according to the standard guidelines for antibiotic selection and surgical inter­ vention. Additionally, OUD must be recognized as an ongoing predisposition for IE and treated; this includes medication-assisted therapy that is initiated during hospitalization and continued with­ out delay upon discharge. Addressing OUD significantly increases completion of antibiotic therapy, decreases resumption of injection drug use, and decreases recurrent IE and requirement for cardiac surgery. Extracardiac Complications  Splenic abscess develops in 3–5% of patients with IE. Effective therapy requires either image-guided percutaneous drainage or splenectomy. Mycotic aneurysms occur in 2–15% of IE patients; one-half of these cases involve the cere­ bral arteries and present as headaches, focal neurologic symp­ toms, or hemorrhage. Cerebral aneurysms should be monitored by angiography. Some will resolve with effective antimicrobial therapy, but those that have leaked or persist or enlarge should be treated surgically, if possible. Extracerebral aneurysms present as local pain, a mass, local ischemia, or bleeding; these are treated surgically. ■ ■OUTCOME IE is a heterogeneous disease that occurs in extremely heterogeneous patient populations. Adverse outcomes are associated with older age, severe comorbid conditions and diabetes, delayed diagnosis, involve­ ment of prosthetic valves or the aortic valve, an invasive (S. aureus) or antibiotic-resistant (P. aeruginosa, yeast) pathogen, intracardiac and major neurologic complications, and health care–associated infection.

TABLE 133-8  Antibiotic Regimens for Prophylaxis of Endocarditis in Adults with High-Risk Cardiac Lesionsa,b A. Standard oral regimen   Amoxicillin: 2 g PO 1 h before procedure B. Inability to take oral medication   Ampicillin: 2 g IV or IM within 1 h before procedure C. Penicillin allergy   1. Cephalexinc: 2 g PO 1 h before procedure   2. Clarithromycin or azithromycin: 500 mg PO 1 h before procedure   3. Doxycycline: 100 mg PO 1 h before procedure D. Penicillin allergy, inability to take oral medication   Cefazolinc or ceftriaxonec: 1 g IV or IM 30 min before procedure aDosing for children: for amoxicillin, ampicillin, cephalexin, or cefadroxil, use 50 mg/kg PO; cefazolin, 25 mg/kg IV; clindamycin, 20 mg/kg PO or 25 mg/kg IV; clarithromycin, 15 mg/kg PO; and vancomycin, 20 mg/kg IV. bFor high-risk lesions, see Table 133-9. Prophylaxis is not advised for other lesions. cDo not use cephalosporins in patients with immediate hypersensitivity (urticaria, angioedema, anaphylaxis) to penicillin. Source: Table created using the guidelines published by the American Heart Association and the European Society of Cardiology (W Wilson et al: Circulation 116:1736, 2007; W Wilson et al: Circulation 143:e963, 2021; and G Habib et al: Eur Heart J 30:2369, 2009). Death or poor outcome often is related not to failure of antibiotic therapy but rather to the interactions of comorbidities and IE-related end-organ complications. In developed countries, overall survival rates are 80–85%; however, rates vary considerably among subpopula­ tions of IE patients. The outcome for a given patient depends on that individual’s infection, the complexity of required therapy, and preex­ isting comorbidities. About 85–90% of patients with NVE caused by viridans streptococci, HACEK organisms, or enterococci (susceptible to synergistic therapy) survive. For S. aureus NVE in patients who do not inject drugs, survival rates are 55–70%; rates are 85–90% among PWID. However, 1-year mortality rises to 20–30% among PWID if substance use disorder is not successfully addressed. PVE beginning within 2 months after valve replacement results in mortality rates of 40–50%, whereas rates are only 10–20% in late-onset cases. In the elderly population with TAVR-PVE the in-hospital mortality is 35–50% and increases to 60–75% at 1 year. Crude survival rates after successful treatment of IE generally are 80–90% and 70–80% at 1 and 2 years, respectively. PART 5 Infectious Diseases ■ ■PREVENTION Prevention of IE has been a goal of clinical practice; however, the evi­ dence establishing benefit from antibiotic prophylaxis for IE is insuffi­ cient to recommend it as a widespread standard of care. The American Heart Association and the European Society of Cardiology recommend limiting prophylactic antibiotics (Table 133-8) to only patients at high­ est risk for severe morbidity or death from IE (Table 133-9). TABLE 133-9  High-Risk Cardiac Lesions for Which Endocarditis Prophylaxis Is Advised Before Dental Procedures Prosthetic heart valves or material Left ventricular assist devices or implantable heart Prior endocarditis Unrepaired cyanotic congenital heart disease, including palliative shunts or conduits Completely repaired congenital heart defects during the 6 months after repair Repaired congenital heart disease with residual defects adjacent to prosthetic material Surgical or transcatheter pulmonary artery valve or conduit placement Valvulopathy developing after cardiac transplantation Source: Table created using the guidelines published by the American Heart Association and the European Society of Cardiology (W Wilson et al: Circulation 116:1736, 2007; W Wilson et al: Circulation 143:e963, 2021; and G Habib et al: Eur Heart J 30:2369, 2009).

In at-risk patients, maintaining good dental hygiene is recom­ mended, and antibiotic prophylaxis is recommended only when there is manipulation of gingival tissue or the periapical region of the teeth or perforation of the oral mucosa (including with respiratory tract surgery). Recent studies suggest that severe adverse events related to amoxicillin prophylaxis are exceedingly rare; however, clindamycin prophylaxis has been associated with low but significant rates of fatal and nonfatal adverse reactions with Clostridioides difficile infection. Consequently, the American Heart Association now recommends against the use of clindamycin for prophylaxis. Although prophylaxis is not advised for patients undergoing gastrointestinal or genitourinary tract procedures, genitourinary tract infections (or skin infection) should be treated before or when these sites undergo procedures. In patients with aortic or mitral valve regurgitation or a prosthetic valve, treatment of acute Q fever for 12 months with doxycycline plus hydroxychloroquine (see Table 133-4) is highly effective in preventing C. burnetii IE. Acknowledgment The authors would like to acknowledge the important contributions of Adolf W. Karchmer, MD, to this chapter in prior editions. ■ ■FURTHER READING Baddour LM et al: Infective endocarditis in adults: Diagnosis, anti­ microbial therapy, and management of complications: A scientific statement for healthcare professionals from the American Heart Association. Circulation 132:1435, 2015. Baddour LM et al: Update on cardiovascular implantable electronic device infections and their prevention, diagnosis, and management: A scientific statement from the American Heart Association. Circula­ tion 149:e201, 2024. Baddour LM et al: Management of infective endocarditis in people who inject drugs: A scientific statement from the American Heart Association. Circulation 146:e187, 2022. Bourque JM et al: 18F-FDG PET/CT and radiolabeled leukocyte SPECT/CT imaging for the evaluation of cardiovascular infection in the multimodality context: ASNC Imaging Indications (ASNC I2) Series Expert Consensus Recommendations from ASNC, AATS, ACC, AHA, ASE, EANM, HRS, IDSA, SCCT, SNMMI, and STS. Clin Infect Dis 2024. Corrected and republished in Heart Rhythm 21:e1, 2024. Chirouze C et al: Impact of early valve surgery on outcome of Staphy­ lococcus aureus prosthetic valve infective endocarditis: Analysis in the International Collaboration of Endocarditis–Prospective Cohort Study. Clin Infect Dis 60:741, 2015. Chobufo MD et al: Trends in infective endocarditis mortality in the United States: 1999 to 2020: A cause for alarm. J Am Heart Assoc 12:e031589, 2023. Delgado V et al: 2023 ESC guidelines for the management of endo­ carditis. Eur Heart J 44:3948, 2023. Duval X et al: Impact of systematic whole-body 18F-fluorodeoxyglu­ cose PET/CT on the management of patients suspected of infective endocarditis: The prospective multicenter TEPvENDO study. Clin Infect Dis 73:393, 2021. Fowler VG Jr et al: The 2023 Duke-International Society for Car­ diovascular Infectious Diseases criteria for infective endocarditis: Updating the modified Duke criteria. Clin Infect Dis 77:518, 2023. Liesman RM et al: Laboratory diagnosis of infective endocarditis. J Clin Microbiol 55:2599, 2017. Regueiro A et al: Association between transcatheter aortic valve replacement and subsequent infective endocarditis and in-hospital death. JAMA 316:1083, 2016. Wilson W et al: Prevention of viridans group streptococcal infective endocarditis: A scientific statement from the American Heart Asso­ ciation. Circulation 143:e963, 2021.

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227 Pneumocystis Infections

Treatment and Prognosis  Scedosporium and Lomentospora species are intrinsically resistant to AmB, echinocandins, and some azoles. Voriconazole has been the agent of choice for S. apiospermum, and posaconazole has been increasingly used for this infection (Table 226-1). L. prolificans is resistant in vitro to almost every commercially avail­ able antifungal agent; the addition of agents such as terbinafine to a voriconazole regimen has been attempted because in vitro data sug­ gest possible synergy between these two agents against some strains of

L. prolificans. Mortality rates for invasive S. apiospermum infection are ~50%, but those for invasive L. prolificans infection remain as high as 85–100%. The novel antifungal agents fosmanogepix and olorofim have shown in vitro activity against both S. apiospermum and L. prolificans and have demonstrated efficacy in preclinical mouse models of these infections. Ongoing clinical trials for these agents will help determine their clinical efficacy in infected patients.

YEAST INFECTIONS In addition to the far more common human pathogenic yeasts Candida and Cryptococcus, which are discussed in Chaps. 221 and 222 the yeast-like fungus Trichosporon has emerged as a significant opportu­ nistic pathogen among immunocompromised patients. ■ ■TRICHOSPORONOSIS Etiologic Agent, Epidemiology, and Pathogenesis  The genus Trichosporon encompasses many species, some of which cause localized infection of hair and nails; white piedra is a superficial infection of the hair and scalp caused by T. ovoides that is characterized by the appear­ ance of white nodules along the hair shaft. The most common species responsible for invasive infection is Trichosporon asahii, although other species also can cause disseminated disease. Trichosporon species grow as yeast-like colonies in vitro; in vivo, however, hyphae, pseudohyphae, and arthroconidia can be seen in addition to yeast forms. These yeasts are commonly found in soil, sewage, and water and in rare instances can colonize the human skin and the gastrointestinal and respiratory tracts. Most infections follow inhalation or entry via central venous catheters. The presence of foreign material is a risk factor for trichospo­ ronosis, as both catheter-associated infections and prosthetic cardiac valve infections have been described. Systemic infection occurs almost exclusively in immunocompromised hosts, especially those who have hematologic malignancies, are neutropenic, have received a solid organ or hematopoietic stem cell transplantation, or are receiving cortico­ steroids. Severe neutropenia is a major risk factor for disseminated disease. Myeloid phagocytes and their oxidative burst are critical for protection against Trichosporon, as illustrated by the development of invasive trichosporonosis in patients with CARD9 deficiency or CGD. Clinical Manifestations  Disseminated trichosporonosis resem­ bles invasive candidiasis, and fungemia is often the initial manifesta­ tion of infection typically presenting with fever. Pneumonia, skin lesions, and sepsis are common, although many patients can present without clinically apparent lung infection. The skin lesions in dissemi­ nated disease begin as papules or nodules surrounded by erythema and progress to central necrosis. Renal involvement is common in dissemi­ nated disease and may cause funguria and hematuria. A rare chronic form of infection mimics hepatosplenic candidiasis, now known as chronic disseminated candidiasis. PART 5 Infectious Diseases Diagnosis  The diagnosis of systemic Trichosporon infection is established by growth of the organism from infected tissues or from blood. Histopathologic examination of a skin lesion showing a mixture of yeast forms, arthroconidia, pseudohyphae, and hyphae can lead to an early presumptive diagnosis of trichosporonosis. The presence of arthroconidia in particular can help differentiate trichosporonosis from candidemia. The serum cryptococcal antigen latex agglutination test may be positive in patients with disseminated trichosporonosis because T. asahii and Cryptococcus neoformans share polysaccharide antigens. Treatment and Prognosis  Rates of response to AmB have been disappointing, and many Trichosporon isolates are resistant to AmB in

vitro. Voriconazole has been the antifungal agent of choice (Table 226-1). The mortality rates for disseminated Trichosporon infection have been as high as 70% but are decreasing with the use of voricon­ azole; however, neutrophil recovery is vital in overcoming this infec­ tion, and mortality rates with persistent neutropenia remain very high despite antifungal therapy. ■ ■FURTHER READING De Almeida Junior JN, Hennequin C: Invasive Trichosporon infec­ tions: A systematic review on a re-emerging fungal pathogen. Front Microbiol 7:1629, 2016. Hospenthal D: Uncommon fungi and related species in Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases, 10th ed, Blaser MJ et al (eds). Philadelphia, Elsevier, 2025. Neoh CF et al: Scedosporiosis and lomentosporiosis: Modern perspec­ tives on these difficult-to-treat rare mold infections. Clin Microbiol Rev 37:e0000423, 2024. Nucci M et al: Fusariosis. Semin Respir Crit Care Med 36:706, 2015. Revankar SG et al: A Mycoses Study Group international prospec­ tive study of phaeohyphomycosis: An analysis of 99 proven/probable cases. Open Forum Infect Dis 4:ofx200, 2017. Alison Morris, Henry Masur

Pneumocystis Infections ■ ■DEFINITION AND DESCRIPTION Pneumocystis is an opportunistic pathogen that is an important cause of pneumonia in immunocompromised hosts, particularly those with HIV infection (Chap. 208), organ transplants, or hema­ tologic malignancies and those receiving high-dose glucocorticoids or certain immunosuppressive monoclonal antibodies. Pneumocystis was discovered in rodents in 1909 and was initially believed to be a protozoan. Because Pneumocystis cannot be cultured, understanding its biology has been limited, but molecular techniques have demon­ strated that the organism is actually a fungus. Formerly known as Pneumocystis carinii, the species specific to humans has been renamed Pneumocystis jirovecii; the species that is specific to rats is Pneumocys­ tis carinii; there are multiple variants of Pneumocystis that are specific to other animals. ■ ■EPIDEMIOLOGY Pneumocystis jirovecii pneumonia (PCP) came to medical atten­ tion in the early 1950s when pathologists in Czechoslovakia rec­ ognized Pneumocystis in the alveolar exudates of infants involved in nursery outbreaks of interstitial pneumonia, outbreaks that had been described in Europe since the 1920s. Among adults, PCP was rarely recognized until the populations of immunosuppressed adults increased due to the development of immunosuppressive thera­ pies for solid-organ transplantation, bone marrow transplantation, cancer, and autoimmune disorders, and until the development of better pulmonary diagnostic techniques such as bronchoscopy. In 1981, PCP was first reported in men who had sex with men and in intravenous (IV) drug users who had no obvious cause of immuno­ suppression. These cases were subsequently recognized as the first cases of what came to be known as the acquired immunodeficiency syndrome (AIDS) (Chap. 208). The incidence of PCP increased dramatically as the AIDS epidemic grew: without PCP chemoprophylaxis or antiretroviral therapy (ART), 80–90% of patients with HIV/AIDS in North America and Western Europe ultimately developed one or more episodes of PCP. While its incidence declined with the introduction of anti-Pneumocystis

prophylaxis and durably effective ART, PCP has continued to be an important cause of AIDS-associated morbidity in the United States and Western Europe, particularly in individuals who do not know they are infected with HIV until they are profoundly immunosuppressed and in people living with HIV (PLWH) with CD4+ T lymphocyte counts of <200/μL who are not receiving ART or PCP prophylaxis. PCP also develops in HIV-uninfected patients who are immuno­ compromised secondary to congenital immunodeficiencies, hemato­ logic or malignant neoplasms, stem cell or solid-organ transplantation, and treatment with immunosuppressive medications. The incidence of PCP depends on the degree and duration of immunosuppression. PCP is increasingly reported among individuals receiving tumor necro­ sis factor α inhibitors and certain (but not all) immunosuppressive monoclonal antibodies for autoimmune, rheumatologic, or neoplastic diseases. In many health care systems, PCP occurs more often due to non-HIV-related immunosuppression than due to HIV infection. While clinical disease due to Pneumocystis in immunocompetent hosts has not been clearly documented, studies have shown that Pneu­ mocystis organisms can cause subclinical infection among children and adults who are not immunocompromised and can be associated with pulmonary pathology. The relevance of these organisms to acute or chronic clinical syndromes, such as chronic obstructive pulmonary disease (COPD), in immunocompetent patients is being investigated. In some developing countries, the incidence of PCP among PLWH has been reported to be lower than that in more industrialized coun­ tries. This lower incidence may be due to competing mortality from infectious diseases such as tuberculosis and bacterial pneumonia, which typically occur before patients become immunosuppressed enough to develop PCP. Geographic variations in Pneumocystis expo­ sure and underdiagnosis attributable to lack of diagnostic resources also may explain the apparent lower frequency of PCP in some countries. ■ ■PATHOGENESIS AND PATHOLOGY Life Cycle and Transmission  The life cycle of Pneumocystis likely involves both sexual and asexual reproduction. The organism exists as a trophic form, a cyst, and a precyst. Studies in rodents show that immunocompetent animals can serve as reservoirs for respiratory transmission of P. carinii (the infecting species in rats) to immunocom­ petent and immunosuppressed rats. Human Pneumocystis is thought to be transmitted by a respiratory route as well. P. jirovecii, like all pneu­ mocystis species, is host-specific. Thus, humans are not infected, for example, by P. carinii (rodents) or P. oryctolagi (rabbits), but are only infected by P. jirovecii, which is from other humans. Serologic and molecular studies have demonstrated that most humans are exposed to P. jirovecii and infected early in life. It was historically thought that Pneumocystis pneumonia usually developed from reactivation of latent infection. However, molecular evidence makes it clear that children and adults can develop PCP from primary infection or reinfection. The source of infection is thought to be either healthy or immunosuppressed individuals who themselves experi­ enced recent infection or reinfection, or immunosuppressed persons with clinical PCP. Nosocomial outbreaks among immunosuppressed persons occur in inpatient and outpatient settings. The utility of drop­ let or airborne isolation for preventing transmission from patients with PCP to other immunosuppressed individuals has been debated; no clear evidence exists, although it seems prudent to isolate patients with active PCP from other immunosuppressed patients using at least droplet precautions. Role of Immunity  Defects in cellular and/or humoral immunity predispose to development of PCP. Such defects may be congenital, or they may be acquired as a result of HIV infection or treatment with certain immunosuppressive drugs such as glucocorticoids, fludarabine, temozolomide, temsirolimus, rituximab, or alemtuzumab. CD4+ T cells are critical in host defense against Pneumocystis. Among PLWH, the incidence of PCP is inversely related to the CD4+ T-cell count: at least 80% of cases occur at counts of <200/μL, and most cases actually develop at counts of <100/μL. HIV viral load is another factor that

predisposes patients to PCP. Clinicians must recognize that PCP can occur at CD4+ T-cell counts >200/μL in persons with HIV infection, but such occurrences are uncommon, especially with CD4+ T-cell counts substantially higher than 200/μL.

CD4+ T-cell counts are less useful in predicting the risk of PCP in patients who are immunosuppressed for reasons other than HIV infec­ tion. In these populations, a CD4+ T-cell count <200/μL is a sensitive indicator of susceptibility to PCP. However, a CD4+ T-cell count >200/μL in many populations does not imply protection. Lung Pathology  Pneumocystis has a unique tropism for the lung. Organisms are presumably inhaled into the alveolar space after being exhaled by another human. Clinically apparent pneumonia occurs only if an individual is immunocompromised. Pneumocystis prolifer­ ates in the lung, provoking a mononuclear cell response. The alveoli become filled with proteinaceous material, and alveolar damage results in increased alveolar-capillary injury and surfactant abnormalities. Stained lung sections typically show foamy, vacuolated alveolar exudates composed largely of viable and nonviable organisms (Fig. 227-1A). Interstitial edema and fibrosis may develop, and organisms can be seen in the alveolar space with silver or other stains. The organisms can also be seen when tissue is subjected to colorimetric or immunofluorescent staining (Fig. 227-1B–D). ■ ■CLINICAL FEATURES Clinical Presentation  PCP presents as acute or subacute pneu­ monia that may initially be characterized by a vague sense of dyspnea alone, but that subsequently manifests as fever and nonproductive cough with progressive shortness of breath. Patients may ultimately progress to respiratory failure and death. Extrapulmonary manifesta­ tions of PCP are rare, but can include involvement of almost any organ, most notably the lymph nodes, spleen, and liver. CHAPTER 227 Physical Examination, Oxygen Saturation, and Imaging  The physical examination findings in PCP are nonspecific. Patients have decreased oxygen saturation—at rest or with exertion—that, without treatment, progresses to severe hypoxemia. Patients may initially have a normal chest examination and no adventitious sounds, but later develop diffuse rales and signs of consolidation. Pneumocystis Infections Laboratory Findings  The results of routine laboratory tests are nonspecific in PCP. Serum levels of lactate dehydrogenase (LDH) are often elevated as a result of pulmonary damage; however, a normal LDH level does not rule out PCP, nor is an elevated LDH value specific for PCP. The peripheral white blood cell count may be elevated in rela­ tion to the patient’s baseline values, but the increase is usually modest and the baseline may have been below usual normal limits due to HIV infection. Hepatic and renal function are typically normal. Radiographic Findings  Although the initial chest radiograph may be normal when patients have mild symptoms, the classic radiographic appearance of symptomatic PCP consists of diffuse bilateral interstitial infiltrates that are perihilar and symmetric (Fig. 227-2A)—another finding that is not specific for PCP. The interstitial infiltrates can prog­ ress to alveolar filling (Fig. 227-2B). High-resolution chest computed tomography (CT) shows diffuse ground-glass opacities in virtually all patients with PCP, often before a routine chest radiograph becomes abnormal (Fig. 227-2C). A normal chest CT essentially rules out the diagnosis of PCP. Pneumatoceles and pneumothoraces are characteris­ tic chest radiographic findings, especially in patients with HIV infection (Fig. 227-2D). A wide variety of atypical radiographic findings have been described, including asymmetric patterns, upper-lobe infiltrates, mediastinal adenopathy, nodules, cavities, and effusions. ■ ■DIAGNOSIS The optimal sample for a specific microbiologic diagnostic examina­ tion depends on how ill the patient is and what resources are avail­ able. Before the 1990s, diagnoses of PCP were usually established by open lung biopsy; later, transbronchial lung biopsy was employed. Hematoxylin and eosin (H and E) staining of pulmonary tissue

A B PART 5 Infectious Diseases C D FIGURE 227-1  Direct microscopy of Pneumocystis pneumonia. A. Transbronchial lung biopsy stained with hematoxylin and eosin shows eosinophilic alveolar filling. B. Methenamine silver–stained bronchoalveolar lavage (BAL) fluid. C. Giemsa-stained BAL fluid. D. Immunofluorescent stain of BAL fluid. demonstrates a foamy alveolar filling and a mononuclear interstitial infiltrate (Fig. 227-1A). This appearance is pathognomonic for PCP even though the organisms cannot be specifically identified with this H and E stain. Since human pneumocystis has never been success­ fully cultured, the diagnosis is typically established in lung tissue or pulmonary secretions by staining of the cyst and or trophozoite—e.g., with methenamine silver (Fig. 227-1B), toluidine blue O, or Giemsa (Fig. 227-1C)—or by staining with a specific immunofluorescent antibody (Fig. 227-1D). Since the 1990s, bronchoalveolar lavage (BAL) has become the most common method for obtaining a sample of respiratory secretions in which to detect Pneumocystis organisms. The demonstration of organ­ isms in BAL fluid is almost 100% sensitive and specific for PCP. The organisms are identified in pulmonary secretions with the specific stains indicated above for lung biopsy. While expectorated sputum or throat swabs have very low sensitivity, an induced sputum sample obtained and interpreted by an experienced provider can be highly sensitive and specific; however, the sensitivity is dependent on both the characteristics of the patient, the quality of the sputum sample, and the experience of the center conducting the test and is widely variable (55–90%). Many laboratories offer polymerase chain reaction (PCR) test­ ing of respiratory specimens for Pneumocystis in preference to direct microscopy of stained respiratory secretions. However, PCR tests are so sensitive that it is difficult to distinguish patients with colonization (i.e., those whose acute lung disease is due to some other process but who have low levels of Pneumocystis DNA in their lungs) from those with acute pneumonia due to Pneumocystis. The PCR cycle number is probably helpful, i.e., the lower the cycle number, the more DNA was

detected. However, PCR tests for PCP are not standardized and speci­ men collection can vary considerably resulting in variable quality of the specimens assessed. Therefore, there is no specific PCR threshold that can be deemed to be especially convincing. Such PCR tests on appropriate samples may be more useful for ruling out a diagnosis of PCP if they are negative than for definitively attributing the disease to Pneumocystis if they are positive. There has been considerable interest in serologic tests, such as assays for (1→3)-β-d-glucan, a component of the fungal cell wall. Serum (1→3)-β-d-glucan levels are frequently elevated in patients with PCP. However, serum or BAL (1→3)-β-d-glucan levels are not perfectly sensitive or highly specific for PCP. There are increasing numbers of studies of serum PCR tests for Pneumocystis, but such serum tests are not yet useful for establishing the presence or absence of PCP. ■ ■COURSE AND PROGNOSIS Untreated, PCP is invariably fatal. Patients with HIV infection often have an indolent course that may present early as mild exercise intoler­ ance or chest tightness without fever or cough and a normal or nearly normal posterior–anterior chest radiograph. However, this process progresses over days, weeks, or even a few months to fever, cough, dif­ fuse alveolar infiltrates, and profound hypoxemia. Some patients with HIV infection and most patients with other types of immunosuppres­ sion have more acute disease that progresses over a few days to respira­ tory failure. Rare patients also develop distributive shock due to PCP. A few unusual patients present with extrapulmonary manifestations in the skin or soft tissue, retina, brain, liver, kidney, or spleen. Extrapul­ monary disease is nonspecific in presentation and can be diagnosed only by histology. When there is extrapulmonary clinical disease in a

A B D C FIGURE 227-2  Radiographs in Pneumocystis pneumonia. A. Posterior–anterior chest radiograph showing symmetric interstitial infiltrates. B. Posterior–anterior chest radiograph showing symmetric alveolar infiltrates (courtesy of Alison Morris). C. Computed tomography (CT) image demonstrating symmetric interstitial infiltrates and ground-glass opacities. D. CT image showing symmetric interstitial infiltrates, ground-glass opacities, and pneumatoceles. patient with PCP, the priority is to determine what other concurrent infectious or neoplastic process might be present, given the rarity of extrapulmonary pneumocystosis. Factors that influence mortality risk of PCP include the patient’s age and degree of immunosuppression as well as the presence of preexisting lung disease, the need for mechanical ventilation, and the development of a pneumothorax. With advances in supportive criti­ cal care, the prognosis for patients with PCP who require intubation and respiratory support has improved and now depends to a large extent on comorbidities and the prognosis of the underlying disease. Some patients do not respond to therapy for 4–8 days, and thus sup­ portive care for a minimum of 10 days is a reasonable consideration if such support is compatible with the patient’s wishes and the prog­ nosis of comorbidities. In fact, if patients of any level of severity are treated with specific therapy but without corticosteroids, they often deteriorate during the first few days, presumably due to enhanced inflammation induced by dying organisms. Patients whose condi­ tion continues to deteriorate after 3 or 4 days or have not improved after 7–10 days should be reevaluated to determine whether other infectious processes are present (either having been missed on initial evaluation or having developed during treatment), whether initial anti-Pneumocystis treatment has failed, or whether noninfectious processes (e.g., congestive heart failure, pulmonary emboli, pulmo­ nary hypertension, drug toxicity, or a neoplastic process) are causing pulmonary dysfunction.

CHAPTER 227 Pneumocystis Infections TREATMENT P. jirovecii Pneumonia The treatment of choice for PCP is trimethoprim-sulfamethoxazole (TMP-SMX), given either IV or PO for 14 days to non-HIVinfected patients with mild disease and for 21 days to all other patients (Table 227-1). TMP-SMX, which interferes with the organ­ ism’s folate metabolism, is at least as effective as alternative agents and is better tolerated. TMP-SMX can cause leukopenia, hepatitis, rash, fever, elevation of potassium and creatinine, and anaphylactic and anaphylactoid reactions. Patients with HIV infection have an unusually high incidence of hypersensitivity to TMP-SMX. Moni­ toring of serum drug levels is useful if renal function or toxicities are issues in order to enhance the likelihood that therapy will be effective and toxicity will be avoided. Maintenance of a 2-h post­ dose serum sulfamethoxazole level of 100–150 μg/mL has been associated with a successful outcome. Resistance to TMP-SMX cannot be measured by organism growth inhibition in the labora­ tory because human Pneumocystis cannot be successfully cultured. However, mutations in the target gene for sulfamethoxazole that confer in vitro sulfa resistance when found in other organisms have been recognized in Pneumocystis. The clinical relevance of these mutations for the response to therapy is uncertain. Sulfadiazine plus pyrimethamine, an oral regimen more often used for treatment of toxoplasmosis, also is highly effective.

TABLE 227-1  Treatment of Pneumocystis Pneumoniaa DRUG(S) DOSE, ROUTE ADVERSE EFFECTS First-Choice Agent TMP-SMX TMP (5 mg/kg) plus SMX (25 mg/kg) q6–8h PO or IV (i.e., 2 double-strength tablets tid or qid) Fever, rash, cytopenias, hepatitis, hyperkalemia Alternative Agents Atovaquone 750 mg bid PO Rash, fever, hepatitis Clindamycin plus Primaquine 300–450 mg q6h PO or 600 mg q6–8h IV 15–30 mg qd PO Hemolysis (G6PD deficiency), methemoglobinemia, neutropenia, rash Pentamidine 3–4 mg/kg qd IV Hypotension, azotemia, cardiac arrhythmias (torsades des pointes), pancreatitis, dysglycemias, hypocalcemia, neutropenia, hepatitis Adjunctive Agent Prednisone or methylprednisolone 40 mg bid × 5 d, 40 mg qd × 5 d, 20 mg qd × 11 d; PO or IV Peptic ulcer disease, hyperglycemia, mood alteration, hypertension aTreatment can be administered for 14 days to non-HIV-infected patients with mild disease and for 21 days to all other patients. Abbreviations: G6PD, glucose-6-phosphate dehydrogenase; TMP-SMX, trimethoprim-sulfamethoxazole. Either intravenous pentamidine or the combination of clindamy­ cin plus primaquine is are options for patients who cannot tolerate TMP-SMX and for patients in whom treatment with TMP-SMX appears to be failing. Pentamidine must be administered IV over at least 60 min to avoid potentially lethal hypotension. Adverse effects can be severe and irreversible and include renal dysfunction, dysgly­ cemia (life-threatening hypoglycemia that can occur days or weeks after initial infusion and be followed by hyperglycemia), neutropenia, and torsades de pointes. Clindamycin plus primaquine is effective, but primaquine can be given only by the oral route—a disadvantage for patients who cannot ingest or absorb oral drugs and primaquine can cause hemolysis in patients with glucose-6-phosphate dehydro­ genase deficiency and can cause methemoglobinemia. Oral atova­ quone is also a reasonable option for patients with mild disease who have no impediments to absorbing an oral drug that requires a highfat meal for optimal absorption. There is some evidence for activity of echinocandins against the cyst form (but not the trophozoite form) of Pneumocystis, but the role for echinocandins as part of combination therapy is currently uncertain, and these drugs should never be used as single drug therapy for PCP. PART 5 Infectious Diseases A major advance in therapy for PCP was the recognition that glucocorticoids could improve survival rates among PLWH with moderate to severe disease (initial room air Po2 <70 mmHg or alveolar–arterial oxygen gradient ≥35 mmHg). Glucocorticoids appear to reduce the pulmonary inflammation that occurs after specific therapy is started and organisms begin to die, eliciting inflammation. Therapy with glucocorticoids should be the stan­ dard of care for patients with HIV infection and moderate to severe PCP. Therapy with glucocorticosteroids is also probably effective for patients with other immunodeficiencies who have moderate to severe PCP. This treatment should be started for moderate or severe disease when therapy for PCP is initiated, even if the diagnosis is suspected but has not yet been confirmed. If PLWH or HIVuninfected patients are receiving high-dose glucocorticoids when they develop PCP, there are theoretical advantages to decreasing the steroid dose to improve immune function, but there is no convinc­ ing evidence on which to base any specific strategy. No definitive trials have identified the best therapeutic algorithm for patients in whom TMP-SMX treatment for PCP is failing. If no other treatable infectious or noninfectious processes are detected and pulmonary dysfunction appears to be due to PCP alone, many

authorities would switch from TMP-SMX to either IV pentamidine or IV clindamycin plus oral primaquine. Clindamycin-primaquine is certainly less toxic than IV pentamidine. Some authorities would add the second drug or drug combination to TMP-SMX rather than switching regimens. If patients are not already receiving them, glucocorticoids should be added to the regimen; the dosage and regimen are usually chosen empirically and depend on what glu­ cocorticoid regimen (if any) the patient was receiving when PCP therapy was begun. For PLWH who present with PCP before the initiation of ART, ART should be started within the first 2 weeks of therapy for PCP in most situations. Immune reconstitution inflammatory syndrome (IRIS) can occur, and the decision to initiate ART thus requires considerable clinical judgement that factors in the severity of pneu­ monia, the response of PCP to therapy, and concurrent medical conditions. ■ ■PREVENTION The most effective method for preventing PCP is to eliminate the cause of immunosuppression by withdrawing immunosuppressive therapy or treating the underlying cause (e.g., HIV infection). Patients who are susceptible to PCP benefit from chemoprophylaxis during the period of susceptibility. For patients with HIV infection, CD4+ T-cell counts are a reliable marker of susceptibility, and counts <200/μL are an indi­ cation to start or continue prophylaxis (Table 227-2). For patients who are immunosuppressed as a result of factors other than HIV infection, CD4+ T-cell counts <200/μL are a plausible but not absolute marker of susceptibility. However, counts >200/μL are not a reliable marker of protection from PCP. For these patients who are immunosuppressed due to causes other than HIV infection, the period of susceptibility is usually estimated on the basis of experi­ ence with the underlying disease and the specific immunosuppressive regimen. Cessation of prophylaxis has been associated with clusters of cases in certain patient populations, such as solid-organ transplant recipients, where the period of susceptibility is not well-defined. Patients receiving a prolonged course of high-dose glucocorticoids TABLE 227-2  Prophylaxis of Pneumocystis Pneumonia DRUG(S) DOSE, ROUTE COMMENTS First-Choice Agent TMP-SMX 1 tablet (double- or single-strength) qd PO Incidence of hypersensitivity is high. Rechallenge for non-lifethreatening hypersensitivity; consider dose-escalation protocol. Alternative Agents Dapsone 50 mg bid or 100 mg qd PO Hemolysis is associated with G6PD deficiency. Dapsone plus Pyrimethamine plus Leucovorin 50 mg qd PO Leucovorin ameliorates cytopenias due to pyrimethamine. 50 mg weekly PO 25 mg weekly PO Dapsone plus Pyrimethamine plus Leucovorin 200 mg weekly PO Leucovorin ameliorates cytopenias due to pyrimethamine. 75 mg weekly PO 25 mg weekly PO Pentamidine 300 mg monthly via Respirgard II nebulizer Aerosol may cause bronchospasm. Pentamidine is probably less effective than TMP-SMX or dapsone regimens. Atovaquone 1500 mg qd PO Requires fatty meal for optimal absorption. Abbreviations: G6PD, glucose-6-phosphate dehydrogenase; TMP-SMX, trimethoprim-sulfamethoxazole.

121 - SECTION 17 Protozoal and Helminthic Infections- General Considerations

SECTION 17 Protozoal and Helminthic Infections: General Considerations

appear to be particularly susceptible to PCP. The glucocorticoid exposure threshold that warrants chemoprophylaxis is ill-defined, but such preventive therapy should be strongly considered for any patient who is receiving more than the equivalent of 20 mg of pred­ nisone daily for 30 days or who is receiving glucocorticoids in con­ junction with other immunosuppressive agents. Clinical experience also suggests that chemoprophylaxis is useful for patients receiving certain immunosuppressive agents (e.g., tumor necrosis factor inhibi­ tors, antithymocyte globulin, rituximab, and alemtuzumab). The duration of such chemoprophylaxis is empirically estimated based on prior clinical experience and immunologic factors that would plausi­ bly relate to immunity, such as CD4+ T-cell counts, recognizing that such estimates are not precise. TMP-SMX is the most effective prophylactic drug; few patients experience a PCP breakthrough when they are reliably taking a recom­ mended TMP-SMX chemoprophylactic regimen. Several TMP-SMX regimens have been used successfully. Regimens of one single-strength or double-strength tablet daily are the regimens with which there is the most experience, but one double-strength tablet two or three times weekly also has been recommended for various PLWH and non-HIVinfected populations of patients. For patients who cannot tolerate TMP-SMX (usually because of hypersensitivity or bone marrow suppression), alternative drugs include daily dapsone, weekly dapsone-pyrimethamine, atovaquone, and monthly aerosol pentamidine. Patients who develop hypersensi­ tivity to TMP-SMX can sometimes tolerate the drug if a gradual doseescalation protocol is used. Atovaquone is effective and well tolerated; however, this drug is available only as an oral preparation, and gastro­ intestinal absorption is unpredictable in patients with abnormal gas­ trointestinal motility or function. Aerosolized pentamidine is effective, but it is not as effective as TMP-SMX and may not provide protection in areas of the lung that are not well-ventilated. Dapsone cross-reacts with sulfonamides in a substantial fraction of patients and is rarely use­ ful in patients with a history of life-threatening reactions to TMP-SMX. ■ ■FURTHER READING Buchacz K et al: Incidence of AIDS-defining opportunistic infections in a multicohort analysis of HIV-infected persons in the United States and Canada, 2000–2010. J Infect Dis 214:862, 2016. Del Corpo O et al: Diagnostic accuracy of serum (1-3)-β-D-glucan for Pneumocystis jirovecii pneumonia: A systematic review and metaanalysis. Clin Microbiol Infect 26:1137, 2020. Lécuyer R et al: Characteristics and prognosis factors of Pneumocystis jirovecii pneumonia according to underlying disease. Chest 165:1319, 2024. Le Gal S et al: Pneumocystis infection outbreaks in organ trans­ plantation units in France: A nation-wide survey. Clin Infect Dis 70:2216, 2020. Ma L et al: Genome analysis of three Pneumocystis species reveals adaptation mechanisms to life exclusively in mammalian hosts. Nat Commun 7:10740, 2016. Panel on Opportunistic Infections in HIV-Infected Adults and Adolescents: Guidelines for the prevention and treatment of opportunistic infections in HIV-infected adults and adoles­ cents: Recommendations from the Centers for Disease Control and Prevention, the National Institutes of Health, and the HIV Medi­ cine Association of the Infectious Diseases Society of America. Available at https://clinicalinfo.hiv.gov/en/guidelines/hiv-clinicalguidelines-adult-and-adolescent-opportunistic-infections/pneumocystis0?view=full.  Accessed December 21, 2024. Wills NK et al: The prevalence of laboratory-confirmed Pneumocystis jirovecii in HIV-infected adults in Africa: A systematic review and meta-analysis. Med Mycol 59:802, 2021. Zolopa A et al: Early antiretroviral therapy reduces AIDS progression/ death in individuals with acute opportunistic infections: A multicenter randomized strategy trial. PLoS One 4:e5575, 2009.

Section 17 Protozoal and Helminthic Infections: General Considerations Sharon L. Reed, Charles E. Davis

Introduction to

Parasitic Infections The word parasite comes originally from the Greek parasitos (para, alongside of; and sitos, food), meaning someone who eats at another’s table or lives at another’s expense. Although the same is true of many bacteria and viruses, the designation parasite is reserved, by conven­ tion, for helminths and protozoa. These organisms are larger and more complex than bacteria, with a eukaryotic cell structure similar to that of human host cells. Historically, this similarity has made it difficult to find effective antiparasitic agents that do not cause unacceptable toxicity to human cells. Fortunately, intensive research and modern techniques have now provided suitable agents for safe and effective treatment of most parasitic infections. See Chap. S12 for details on diagnostic procedures and Chap. 229 for details on treatment. Internal parasites of human beings are divided into two types: hel­ minths (worms) and protozoa. Helminths are multicellular organisms that can often be seen with the naked eye (Chap. 237). There are two phyla: Platyhelminthes (flat worms) and Nemathelminthes (round­ worms). Both phyla include some genera that mature in the gastroin­ testinal tract and others that migrate through the tissue after ingestion or skin penetration. Tables S12-1 and S12-2 present the helminthic genera, their definitive and intermediate hosts, their geographic distri­ butions, and the parasitic stages in the human body. CHAPTER 228 Introduction to Parasitic Infections The key to understanding which helminths use humans as defini­ tive hosts is to remember that helminth ova develop into larvae, and larval stages develop into adults. Humans serve as the definitive host when they ingest helminth larvae, which develop into adults in the intestine and usually cause mild disease, often without any symptoms. (The exception is ingestion of the late-stage larvae of the somatic or tissue flukes, as shown in Table S12-1.) In contrast, if humans ingest helminth ova and serve as the intermediate host, the ova develop into larvae, which penetrate the intestine, migrate through the tissue, and invade organs where they mature into adults. Intermediate hosts with parasitic invasion of organs may experience severe disease. Protozoa are microscopic single-celled organisms. Among the many differences between helminths and protozoans, the most important is the ability of protozoa (like bacteria) to multiply within the human body and cause overwhelming infections. A major mechanism pro­ moting unrestrained growth is evasion of the host immune response either by antigenic variation (Trypanosoma brucei) or by survival inside host cells (e.g., Plasmodium, Babesia, Cryptosporidium, Leishmania, and Toxoplasma). In contrast, almost all helminths require stages in other hosts to complete their life cycles and multiply. As a result, except for Strongyloides and Capillaria, which can complete their life cycle in humans, increases in the burden of infection with helminths require repeated exogenous reinfections. Thus, permanent residents of endemic countries, who are exposed repeatedly, may have heavy severe infections, while most travelers with one or two exposures are unlikely to experience the full spectrum of chronic helminthic infections. In contrast to helminthic infections, naïve patients with their first protozoal infection usually are the most severely affected because partial immunity often limits the number of parasites during recurrent infections. Protozoan replication to large numbers in the host also pro­ motes the development of drug-resistant forms, especially in malaria (Chap. 229). Because protozoa belong to many different phyla, it is easier to understand the pathogenesis and management of protozoal infections when they are classified by the site of infection (intestinal protozoans, free-living amebae, and blood and tissue protozoans)

122 - 228 Introduction to Parasitic Infections

228 Introduction to Parasitic Infections

appear to be particularly susceptible to PCP. The glucocorticoid exposure threshold that warrants chemoprophylaxis is ill-defined, but such preventive therapy should be strongly considered for any patient who is receiving more than the equivalent of 20 mg of pred­ nisone daily for 30 days or who is receiving glucocorticoids in con­ junction with other immunosuppressive agents. Clinical experience also suggests that chemoprophylaxis is useful for patients receiving certain immunosuppressive agents (e.g., tumor necrosis factor inhibi­ tors, antithymocyte globulin, rituximab, and alemtuzumab). The duration of such chemoprophylaxis is empirically estimated based on prior clinical experience and immunologic factors that would plausi­ bly relate to immunity, such as CD4+ T-cell counts, recognizing that such estimates are not precise. TMP-SMX is the most effective prophylactic drug; few patients experience a PCP breakthrough when they are reliably taking a recom­ mended TMP-SMX chemoprophylactic regimen. Several TMP-SMX regimens have been used successfully. Regimens of one single-strength or double-strength tablet daily are the regimens with which there is the most experience, but one double-strength tablet two or three times weekly also has been recommended for various PLWH and non-HIVinfected populations of patients. For patients who cannot tolerate TMP-SMX (usually because of hypersensitivity or bone marrow suppression), alternative drugs include daily dapsone, weekly dapsone-pyrimethamine, atovaquone, and monthly aerosol pentamidine. Patients who develop hypersensi­ tivity to TMP-SMX can sometimes tolerate the drug if a gradual doseescalation protocol is used. Atovaquone is effective and well tolerated; however, this drug is available only as an oral preparation, and gastro­ intestinal absorption is unpredictable in patients with abnormal gas­ trointestinal motility or function. Aerosolized pentamidine is effective, but it is not as effective as TMP-SMX and may not provide protection in areas of the lung that are not well-ventilated. Dapsone cross-reacts with sulfonamides in a substantial fraction of patients and is rarely use­ ful in patients with a history of life-threatening reactions to TMP-SMX. ■ ■FURTHER READING Buchacz K et al: Incidence of AIDS-defining opportunistic infections in a multicohort analysis of HIV-infected persons in the United States and Canada, 2000–2010. J Infect Dis 214:862, 2016. Del Corpo O et al: Diagnostic accuracy of serum (1-3)-β-D-glucan for Pneumocystis jirovecii pneumonia: A systematic review and metaanalysis. Clin Microbiol Infect 26:1137, 2020. Lécuyer R et al: Characteristics and prognosis factors of Pneumocystis jirovecii pneumonia according to underlying disease. Chest 165:1319, 2024. Le Gal S et al: Pneumocystis infection outbreaks in organ trans­ plantation units in France: A nation-wide survey. Clin Infect Dis 70:2216, 2020. Ma L et al: Genome analysis of three Pneumocystis species reveals adaptation mechanisms to life exclusively in mammalian hosts. Nat Commun 7:10740, 2016. Panel on Opportunistic Infections in HIV-Infected Adults and Adolescents: Guidelines for the prevention and treatment of opportunistic infections in HIV-infected adults and adoles­ cents: Recommendations from the Centers for Disease Control and Prevention, the National Institutes of Health, and the HIV Medi­ cine Association of the Infectious Diseases Society of America. Available at https://clinicalinfo.hiv.gov/en/guidelines/hiv-clinicalguidelines-adult-and-adolescent-opportunistic-infections/pneumocystis0?view=full.  Accessed December 21, 2024. Wills NK et al: The prevalence of laboratory-confirmed Pneumocystis jirovecii in HIV-infected adults in Africa: A systematic review and meta-analysis. Med Mycol 59:802, 2021. Zolopa A et al: Early antiretroviral therapy reduces AIDS progression/ death in individuals with acute opportunistic infections: A multicenter randomized strategy trial. PLoS One 4:e5575, 2009.

Section 17 Protozoal and Helminthic Infections: General Considerations Sharon L. Reed, Charles E. Davis

Introduction to

Parasitic Infections The word parasite comes originally from the Greek parasitos (para, alongside of; and sitos, food), meaning someone who eats at another’s table or lives at another’s expense. Although the same is true of many bacteria and viruses, the designation parasite is reserved, by conven­ tion, for helminths and protozoa. These organisms are larger and more complex than bacteria, with a eukaryotic cell structure similar to that of human host cells. Historically, this similarity has made it difficult to find effective antiparasitic agents that do not cause unacceptable toxicity to human cells. Fortunately, intensive research and modern techniques have now provided suitable agents for safe and effective treatment of most parasitic infections. See Chap. S12 for details on diagnostic procedures and Chap. 229 for details on treatment. Internal parasites of human beings are divided into two types: hel­ minths (worms) and protozoa. Helminths are multicellular organisms that can often be seen with the naked eye (Chap. 237). There are two phyla: Platyhelminthes (flat worms) and Nemathelminthes (round­ worms). Both phyla include some genera that mature in the gastroin­ testinal tract and others that migrate through the tissue after ingestion or skin penetration. Tables S12-1 and S12-2 present the helminthic genera, their definitive and intermediate hosts, their geographic distri­ butions, and the parasitic stages in the human body. CHAPTER 228 Introduction to Parasitic Infections The key to understanding which helminths use humans as defini­ tive hosts is to remember that helminth ova develop into larvae, and larval stages develop into adults. Humans serve as the definitive host when they ingest helminth larvae, which develop into adults in the intestine and usually cause mild disease, often without any symptoms. (The exception is ingestion of the late-stage larvae of the somatic or tissue flukes, as shown in Table S12-1.) In contrast, if humans ingest helminth ova and serve as the intermediate host, the ova develop into larvae, which penetrate the intestine, migrate through the tissue, and invade organs where they mature into adults. Intermediate hosts with parasitic invasion of organs may experience severe disease. Protozoa are microscopic single-celled organisms. Among the many differences between helminths and protozoans, the most important is the ability of protozoa (like bacteria) to multiply within the human body and cause overwhelming infections. A major mechanism pro­ moting unrestrained growth is evasion of the host immune response either by antigenic variation (Trypanosoma brucei) or by survival inside host cells (e.g., Plasmodium, Babesia, Cryptosporidium, Leishmania, and Toxoplasma). In contrast, almost all helminths require stages in other hosts to complete their life cycles and multiply. As a result, except for Strongyloides and Capillaria, which can complete their life cycle in humans, increases in the burden of infection with helminths require repeated exogenous reinfections. Thus, permanent residents of endemic countries, who are exposed repeatedly, may have heavy severe infections, while most travelers with one or two exposures are unlikely to experience the full spectrum of chronic helminthic infections. In contrast to helminthic infections, naïve patients with their first protozoal infection usually are the most severely affected because partial immunity often limits the number of parasites during recurrent infections. Protozoan replication to large numbers in the host also pro­ motes the development of drug-resistant forms, especially in malaria (Chap. 229). Because protozoa belong to many different phyla, it is easier to understand the pathogenesis and management of protozoal infections when they are classified by the site of infection (intestinal protozoans, free-living amebae, and blood and tissue protozoans)

(Table S12-3). Immunocompromised hosts are at risk of disseminated infection with several protozoa, including Leishmania, Toxoplasma, Cryptosporidium, and Trypanosoma cruzi, which are AIDS-defining illnesses. Among the helminths, Strongyloides can disseminate in immunocompromised individuals.

HELMINTHIC INFECTIONS The Platyhelminthes (flatworms) are categorized as tapeworms (ces­ todes) and flukes (trematodes). Tapeworms are composed of a head or scolex bearing the holdfast organs and segments, which become gravid as they mature. Some tapeworms can reach lengths of many yards; the longest tapeworms develop in the intestine, where they rarely cause serious disease. In contrast, flukes are small leaf-shaped organisms whose size is not a measure of disease severity. ■ ■FLATWORMS Cestodes  Tapeworms cause either intestinal or somatic infection, depending on the species. Intestinal infections occur when the human host ingests larvae in the tissue of the intermediate host, whereas somatic infections occur when humans accidentally ingest ova excreted from the wild or domesticated definitive animal host. INTESTINAL TAPEWORMS  As shown in Table S12-1, humans acquire most intestinal tapeworms by eating the insufficiently cooked flesh of the intermediate host. Thus, Taenia saginata is commonly called the beef tapeworm, Taenia solium the pork tapeworm, and Diphylloboth­ rium latum the fish tapeworm. Hymenolepis nana is capable of com­ pleting its life cycle in the human intestine and is acquired by ingestion of infected grain beetles or of ova from infected humans or mice. None of these parasites causes significant damage, and infection is usually asymptomatic. There are two occasional exceptions. When people ingest T. solium ova from their own intestine or from another infected individual, it can cause somatic infection. D. latum avidly absorbs vitamin B12 in the intestine and can cause pernicious anemia in 1–2% of infected Scandinavians with a genetic predisposition. PART 5 Infectious Diseases SOMATIC TAPEWORMS  There are three major causes of somatic tape­ worm infections. Two species of Echinococcus cause echinococcosis.

E. granulosus is acquired by accidental ingestion of ova from dogs infected when fed the infected tissues of sheep or other animals by sheepherders or hunters. E. multilocularis is transmitted primarily in sub-Arctic areas when humans ingest ova from foxes, dogs, or cats that have been infected through consumption of the tissues of infected rodents. Both species cause hydatid cysts when the eggs hatch into lar­ vae, penetrate the intestine, and migrate into the liver or lung. Ingested T. solium ova cause somatic disease (cysticercosis) when the larvae penetrate the intestine, migrate into tissue, and form cysts (cysterci), usually in the muscles or central nervous system (CNS). Trematodes  Flukes also cause both intestinal and somatic infections (Chap. 241 and Table S12-1). Most fluke infections are localized to Asia, Africa, Southeast Asia, or the Pacific islands. Infection with intestinal flukes is usually asymptomatic, although heavy infections sometimes cause abdominal discomfort and mucous diarrhea. Liver flukes and lung flukes cause somatic infections when humans ingest a larval form from an intermediate host. Adults develop in the intestine, migrate into adjacent tissues, and cause disease. The major liver flukes (Clonorchis sinensis, Opisthorchis spp., and Fasciola hepatica) are causes of recurrent bacterial cholangitis (due to obstruction) or portal hypertension and cir­ rhosis. Only F. hepatica can be acquired worldwide; it is especially com­ mon in sheep-raising areas, where the animals ingest water plants (e.g., watercress). The lung flukes (Paragonimus spp.) occur globally except in Europe; most lesions occur as pulmonary cysts, although occasional lesions develop in the CNS or the abdominal cavity. The blood flukes cause schistosomiasis, one of the most common and serious parasitic infections (Chap. 241 and Table S12-1). The major species are Schistosoma mansoni, S. haematobium, and S. japonicum. All are transmitted to humans when free-swimming larvae exit an infected snail in freshwater and penetrate the skin. Swimmer’s itch sometimes follows skin penetration but is usually of short duration.

The larvae then wander in the skin until they find a blood vessel and migrate to the target organ. S. mansoni and S. japonicum migrate to the mesentery vessels and eventually make their way to the liver, while S. haematobium targets the veins around the ureter and blad­ der. Extensive egg deposition by S. mansoni and S. japonicum and the immune reactions to the ova cause granuloma formation and, with many repeated exposures, portal vein obstruction and cirrhosis. The same process in the ureters and bladders during infection with S. hae­ matobium eventually interferes with urine flow and leads to repeated urinary tract infections and kidney damage. ■ ■ROUNDWORMS Nematodes  Roundworms are nonsegmented bisexual organisms. The species that infect humans include intestinal and tissue groups. Humans may also acquire certain nonhuman mammalian round­ worms that either can be limited to the skin or can migrate to tissues and cause serious disease (the larva migrans syndromes). INTESTINAL ROUNDWORMS  The major intestinal roundworms are Ascaris lumbricoides, Necator americanus (New World hookworms), Ancylostoma duodenale (Old World hookworms), Trichuris trichiura (whipworms), Enterobius vermicularis (pinworms), and Strongyloides stercoralis. Taken together, infections caused by intestinal roundworms are the most common infections in the world. Ascaris, hookworms, and Trichuris infect about 900 million individuals, and at least 100–370 million have strongyloidiasis. These infections are most common in resource-poor developing countries, especially where people defecate outside and/or human feces is used as fertilizer (“night soil”). Infection is transmitted either by ingestion of ova (A. lumbricoides, T. trichiura, and E. vermicularis) or by active penetration of the skin by larvae (hookworms and S. stercoralis) (Table S12-2). Intestinal roundworms cause serious health problems in residents of endemic regions with poor sanitation, but travelers are at low risk of developing significant disease from most of these parasites. Intestinal blockage and malnutrition from heavy Ascaris infections and anemia from heavy hookworm infections are now restricted to areas of heavy endemicity. Except in the case of Strongyloides and Capillaria, which can reproduce in the body, multiple exposures over time are neces­ sary for the development of severe disease. Strongyloides infection persists over decades and can disseminate when the immune system is compromised. Although Capillaria remains localized to the intestine, infections can become so heavy that protein-losing enteropathy and malnutrition cause serious disease. The life cycles of Ascaris and the hookworms involve migration through the heart and lungs before development into adults in the intestine. In particular, Ascaris occasionally causes eosinophilic pneu­ monia (Loeffler’s syndrome) during heavy infections. Pinworms are the most common causes of intestinal roundworm infection persisting in the United States and other developed countries. The anal and peri­ neal itching caused by pinworm migration out of the anus and subse­ quent egg deposition is well known to families throughout the world. TISSUE ROUNDWORMS  The major diseases caused by tissue round­ worms are filariasis, angiostrongyliasis, gnathostomiasis, and trichinel­ losis. By far, the most important globally is filariasis; the thread-like filarial worms infect an estimated 80 million individuals in tropical and subtropical areas of the world. Four filarial species cause three distinct diseases: lymphatic filariasis (Wuchereria bancrofti and Brugia malayi), river blindness (Onchocercus volvulus), and loiasis (Loa loa, the African eye worm). Humans, the major reservoir, acquire these infections from bites of infected arthropods (Table S12-2). The larvae develop into adults, which remain static in tissue: the lymphatics for lymphatic filariasis and subcutaneous tissue for O. volvulus and L. loa. After adults mate, next-stage larvae are produced, and their migration causes additional damage. Repeated bouts of migrating larvae and blocking of the lymphatics by adults are necessary to establish the syndrome of lymphatic filaria­ sis; thus, it is unusual for the short-term traveler (<3 months’ residence in an endemic region) to develop significant disease. In river blindness, the larvae produced by adult O. volvulus migrate through the skin and

eye, causing skin damage and eventual blindness. Loiasis is a milder disease restricted to central and western Africa. Although both the adults and the larvae of L. loa migrate through the skin and eye, many infected individuals are asymptomatic, and the infection is often diag­ nosed only when an adult worm migrates across the subconjunctival tissue and is visible to the patient and the physician. Red lumps in the skin from heavy cutaneous migration are called Calabar swellings. The other four major roundworm tissue infections are acquired by ingestion of larvae in undercooked food. The sources for trichinellosis are swine and other large mammals; for gnathostomiasis, freshwater fish and chicken; for ancylostomiasis, snails, fish, prawns, and crabs; and for Guinea worm, infected water fleas. Guinea worm infection (dracunculiasis, caused by Dracunculus medinensis) has been almost eradicated. Trichinella spiralis larvae penetrate the intestine and migrate widely, with a preference for skeletal tissue; the release of eosinophils and IgE causes muscle soreness and may cause palpebral swelling and other manifestations of generalized allergic reactions. Angiostrongylus cantonensis is the most common parasitic cause of eosinophilic men­ ingitis. Ingested larvae penetrate the intestine and migrate to the brain and meninges, where they quickly die and attract massive numbers of eosinophils. Although complications can occur, most individuals recover spontaneously. Gnathostoma spinigerum larvae also penetrate the intestine and migrate, showing a preference for the skin, eyes, and meninges. Mechanical damage from the migration and inflammation produced by the resultant immune reaction can cause boil-like lesions on the skin, painful eye damage, and eosinophilic meningitis. Although eosinophilic meningitis caused by G. spinigerum is less common than that caused by A. cantonensis, it is often more severe and can result in paralysis or brain hemorrhage. PROTOZOAL INFECTIONS ■ ■INTESTINAL PROTOZOA Entamoeba histolytica is the one intestinal protozoan that causes inva­ sive disease. This disease consists of dysentery or bloody diarrhea that must be differentiated from that due to bacteria such as Salmonella, Campylobacter, and Shigella. Although amebiasis usually has a slower onset with lower fever than these bacterial infections, E. histolytica can disseminate from the bloodstream to cause distant abscesses, particu­ larly of the liver. The diagnosis cannot be made by identification of the characteristic cyst or trophozoites (Chap. 230), as they are identical to those of the noninvasive E. dispar, which is more common globally. Cryptosporidium and Giardia are the most common water-borne pro­ tozoal infections. Cryptosporidium can cause major outbreaks because it is highly infectious and resistant to high levels of chlorine (Chap. 236). Without immune reconstitution, immunosuppressed patients, particu­ larly those with AIDS, can develop severe, even fatal watery diarrhea. Infections caused by the remaining intestinal protozoans—Giardia, Isos­ pora, Cyclospora, and microsporidia (Chap. 236)—have a much more indolent course, with intermittent diarrhea. Microsporidia, unique intracellular protozoa that form infectious spores, may cause limited gastrointestinal infection in immunocompetent hosts, but patients with AIDS can develop chronic diarrhea and wasting or disseminated infec­ tion to the biliary or respiratory tract. ■ ■FREE-LIVING AMOEBAS The free-living amoebas Acanthamoeba and Naegleria are found world­ wide in freshwater and brackish water (Chap. 230 and Table S12-3). Organisms of these two genera cause very different syndromes. In immunocompromised individuals, Acanthamoeba may cause invasive infection, with brain masses and skin lesions. However, all humans are susceptible to Acanthamoeba keratitis after trauma to the eye and expo­ sure to contaminated water. In contrast, naeglerial meningitis, acquired in warm lakes or hot springs, causes sudden pyogenic and usually fatal meningitis. Balamuthia, reported only from the Americas, causes indo­ lent meningoencephalitis, with both cerebrospinal fluid pleocytosis and a space-occupying lesion, in immunocompetent patients. Despite the availability of miltefosine, which is active in vitro against Naegleria, infection of the CNS is almost universally fatal.

■ ■BLOOD AND TISSUE PROTOZOANS

Plasmodium and Babesia  Malaria, caused by six species of Plasmo­ dium, carries higher mortality rates than any other parasitic infection (Chap. 231). All species are transmitted in tropical and subtropical areas by female Anopheles mosquitoes. Plasmodium falciparum is most common in sub-Saharan Africa, where it causes more than 80% of malaria infections and 90% of malarial deaths. Infection with

P. falciparum may be particularly severe because the organism can invade any erythrocyte, reaches very high parasite loads, damages organs by adhering to vascular epithelium, and is the most likely Plasmodium species to be resistant to antimalarial drugs. Plasmodium vivax, the dominant cause of malaria outside sub-Saharan Africa, reaches lower levels of parasitemia and exhibits less drug resistance because it invades only reticulocytes with Duffy antigen. Many Africans, especially in the western part of the continent, lack the Duffy blood group; consequently, Plasmodium ovale, another cause of milder malaria, can compete successfully with P. vivax. Both P. vivax and P. ovale pro­ duce persistent liver forms, which must be treated with primaquine (Chap. 229). Because malaria can cause a variety of symptoms ranging from acute fever to coma, this diagnosis must be considered in any traveler or immigrant from a malarial area. Recently, locally acquired P. vivax malaria has been detected in Florida and Texas and P. falci­ parum in Maryland; thus, malaria should be included in the differential diagnoses of unexplained fever even in patients who have never left the United States. Babesia also infects erythrocytes and may cause a nonspecific febrile illness or, in asplenic patients, severe infection. This parasite is carried by ixodid ticks and is geographically limited to the northeastern and midwestern United States, with only sporadic cases in Europe and other temperate areas. CHAPTER 228 Trypanosomes  The three species of trypanosomes all have flagel­ lated bloodstream forms, but they cause very different diseases. T. cruzi, the cause of Chagas disease, is transmitted in South and Central America in the feces of blood-sucking reduviid bugs (Chap. 234). After initial parasitemia, patients are often asymptomatic for years while the parasite multiplies intracellularly in muscle and ganglion cells. Although only a minority of patients go on to develop organ damage (megaesophagus and cardiomyopathy), all infected patients can spread the disease through transfusions, mother-to-child transmission, and organ transplants. Introduction to Parasitic Infections African trypanosomiasis is limited to sub-Saharan Africa, where it is transmitted by the bite of a tsetse fly. A history of a tsetse bite and the presence of a painful chancre are strong diagnostic clues (Chap. 234). Although the parasites causing this disease in western Africa (Trypano­ soma brucei gambiense) and eastern Africa (T. brucei rhodesiense) look identical, they are genetically and clinically distinct. T. b. gambiense causes low-level parasitemia with cyclical fevers over months or years before CNS invasion, whereas T. b. rhodesiense causes high-level para­ sitemia, invades the CNS early on, and can lead to death within weeks of onset. Leishmania  Leishmaniasis is caused by more than 20 species of obligate intracellular protozoa transmitted by sandflies, which are present in almost 100 countries in tropical and temperate zones (Chap. 233). A wide spectrum of clinical symptoms result, ranging from self-healing, painless skin ulcers to mucocutaneous disease with destruction of the nose and palate to disseminated visceral leishmaniasis with hepatic and splenic involvement. The resulting disease depends on the infect­ ing strain and the host immune response. Visceral leishmaniasis can present as an acute febrile illness, with the later development of hepatosplenomegaly, and is an AIDS-defining illness in HIV-infected patients. More than 90% of cases of visceral leishmaniasis occur in India, Bangladesh, Ethiopia, Sudan, and Brazil. Toxoplasma  Toxoplasma gondii is an obligate intracellular parasite that is found worldwide. Infection follows ingestion of oocysts in food or water contaminated by cat feces, ingestion of tissue cysts in under­ cooked meat, or transplacental transmission. After gastrointestinal invasion, tachyzoites can invade any nucleated cell and cause lifelong infection in most patients (Chap. 235). Clinical manifestations depend

on the host’s age and immune status at the time of infection. Congeni­ tal toxoplasmosis results from primary maternal infection; outcomes are most severe early in pregnancy and include visual, hearing, and cognitive impairments. Babies infected later in pregnancy may appear normal but can develop chorioretinitis decades later. Primary infection in immunocompetent hosts may be asymptomatic, may present as an infectious mononucleosis–like syndrome, or may manifest as chorio­ retinitis during outbreaks. During immunosuppression by AIDS or organ transplantation, reactivation of latent cerebral infection can be fatal unless diagnosed and treated early.

APPROACH TO THE PATIENT Parasitic Infection A thorough history and physical examination are the keys to diag­ nosis of any disease and particularly of parasitic infections. Because many of the more serious parasitic infections are uncommon in the United States, a travel history, particularly to developing nations, is a critical component. The longer the stay in an area endemic for significant parasitic infections, the greater the risk, even for healthy travelers. In addition, other factors increase the chance of acquiring these infections. Notably, immunocompromise greatly increases the likelihood of developing some of the more serious parasitic infec­ tions. Even healthy travelers with adventure itineraries, extensive travel to rural areas, or involvement in war zones or refugee camps are at increased risk. Immigrants from developing countries may seek care for symptoms or signs associated with parasitic infections. Information on the patient’s immunization history and adher­ ence to appropriate malarial chemoprophylaxis is critical. The recent approval of the first parasitic vaccines against P. falciparum is very exciting, but it will be targeted initially only for children in high prevalence areas. Typhoid fever is much less likely to be the cause of prolonged fever in an immunized individual. Similarly, PART 5 Infectious Diseases TABLE 228-1  Parasitic Infections, by Organ System and Signs/Symptomsa ORGAN SYSTEM, MAJOR SIGN(S)/SYMPTOM(S) PARASITE(S) GEOGRAPHIC DISTRIBUTION COMMENTS Skin Serpentine rash Hookworm Worldwide Can cause anemia in heavy infections   Strongyloides Moist tropics and subtropics Disseminated infection in immunocompromise   Toxocara (animal roundworm) Tropical and temperate zones Cutaneous or visceral larva migrans Itchy skin rash Onchocerca Mexico, Central/South America, Africa Painless ulcers Leishmania Tropics and subtropics Amastigotes detectable in biopsies; may cause destructive mucocutaneous infection; AIDS-defining infection Skin nodules Onchocerca Mexico, South America, Africa Large nodules of adult worms   Loa loa (African eye worm) Western and central Africa Migratory nodules   Gnathostoma Southeast Asia and China Migratory nodules with eosinophilia Painful nodules, especially involving feet Dracunculus (Guinea worm) Africa Nearly eradicated Central Nervous System Somnolence, seizures, coma Plasmodium falciparum Subtropics and tropics Cerebral malaria, especially in children   Trypanosoma brucei rhodesiense Sub-Saharan eastern Africa Painful chancre from tsetse fly bite; death in weeks to months Space-occupying lesions, seizures Acanthamoeba Worldwide Immunocompromised individuals   Balamuthia Americas Indolent meningoencephalitis with brain mass   Toxoplasma Worldwide Reactivation disease in immunocompromise; ring-enhancing lesions; AIDS-defining infection   Taenia solium Mexico, Central/South America, Africa   Schistosoma japonicum Far East Aberrant eggs can form brain or spinal cord masses.   Schistosoma mansoni Africa, Central/South America Aberrant eggs can form brain or spinal cord masses.

hepatitis A or B is unlikely to be the cause of jaundice and fever in fully immunized patients. In this era of increasing drug resistance, even adherence to appropriate malarial chemoprophylaxis does not guarantee that fever is not malarial. Nevertheless, most travel­ ers who acquire malaria have taken inadequate or no prophylaxis. Although these considerations do not prove that the symptoms are caused by parasites, they narrow the differential diagnosis. There are many other important aspects of the history, includ­ ing when symptoms began. Was the individual still in the endemic area at the time, or did the symptoms commence after return to the United States? If they started during travel, was any treatment received? Malaria must be the first consideration in a febrile patient returning from an endemic area. If the patient was well upon return from travel, the timing of symptom onset is a critical point. For example, if the chief manifestation is fever that began >10–14 days after departure from the endemic region, many tropical diseases can be ruled out, including dengue fever, chikungunya fever, and Zika virus infection. On the other hand, fever beginning several months or later after return makes malaria a likely diagnosis. Trav­ elers’ diarrhea, the most common complaint of travelers, is usually caused by bacteria or viruses and resolves in a short time with or without treatment. Travelers’ diarrhea that persists for weeks is much more likely to be parasitic in origin. Most patients who consult physicians after international travel either have troublesome symptoms or have been referred for symp­ toms or signs whose source was unclear to a referring caregiver. After a careful travel history including the individual’s symptoms and the exact geographic zones visited, a thorough physical exami­ nation must be conducted. The symptoms, signs, and physical findings should help to establish possible diagnoses. Table 228-1 breaks down the symptoms of major parasitic infections by organ system and geographic distribution, with comments on clinical and epidemiologic associations. Larvae detectable in skin snips and nodules Cysticercosis; variable sized or calcified larval cysts on CT (Continued)

TABLE 228-1  Parasitic Infections, by Organ System and Signs/Symptomsa ORGAN SYSTEM, MAJOR SIGN(S)/SYMPTOM(S) PARASITE(S) GEOGRAPHIC DISTRIBUTION COMMENTS Pyogenic meningitis Naegleria Worldwide Motile trophozoites in fresh cerebrospinal fluid; pyogenic; rapid death Eosinophilic meningitis Angiostrongylus (rat lung worm) Southeast Asia, Pacific, Caribbean   Gnathostoma Southeast Asia and China Migratory nodules Eyes Painful corneal ulcers Acanthamoeba Worldwide Freshwater and brackish water; corneal trauma; long-wear contact lenses Corneal opacification Onchocerca Mexico, Central/South America, Africa Congenital or adult visual loss Toxoplasma Worldwide Primary infection in pregnancy and normal hosts; reactivation infection in immunocompromised Retinal mass Toxocara Worldwide Ocular larva migrans Visible roundworm in eye Onchocerca Mexico, Central/South America, Africa   L. loa Western and central Africa Worms may cross eye during migration. Pain, possible vision loss Gnathostoma Southeast Asia and China Migratory skin nodules, eosinophilia Lungs Pulmonary nodule/abscess Paragonimus Far East, Africa, Americas Ectopic migration to abdomen or central nervous system Cough, transient infiltrates, eosinophilia Migrating helminths Worldwide Loeffler’s syndrome from migrating Ascaris, hookworm, Strongyloides Heart Pulmonary edema P. falciparum (complication) Tropics and subtropics End-organ damage from severe malaria Cardiomegaly, arrhythmias Trypanosoma cruzi Mexico, Central/South America Late amastigote infection of myocardium; AIDS-defining infection Gastrointestinal Tract Hepatosplenomegaly Malaria (multiple episodes) Tropics and subtropics Splenomegaly with anemia and recurrent fever are hallmarks of malaria.   S. mansoni Africa, Central/South America Portal obstruction with cirrhosis and late varices   Leishmania donovani complex Tropics and subtropics Visceral leishmaniasis; AIDS-defining infection Hepatomegaly Entamoeba histolytica Tropics Acute with fever, right-upper-quadrant pain; or chronic with enlarged liver; hypoechoic abscess(es) on ultrasound or CT   Echinococcus Sheep-raising areas Characteristic cysts of liver > lung   Fasciola Sheep-raising areas Eosinophilia Cholangitis Clonorchis China, Southeast Asia Recurrent cholangitis and late cholangiocarcinoma   Microsporidia Worldwide AIDS   Cryptosporidium Worldwide AIDS-defining infection Bloody diarrhea E. histolytica Tropics Less fever than in diarrhea of bacterial etiology   S. mansoni Africa, Central/South America Only in heavy, acute infection with fever and eosinophilia   S. japonicum Far East Only in heavy, acute infection Watery diarrhea Cryptosporidium Worldwide Severe in immunocompromised patients   Giardia Worldwide Foul-smelling stool with steatorrhea   Isospora belli Worldwide Fever, abdominal pain, chronic diarrhea   Microsporidia Worldwide Chronic diarrhea with AIDS   Capillaria Southeast Asia, Egypt Malabsorption, wasting Passage of large roundworm (>6 cm) Ascaris Worldwide Patients may confuse the roundworm with an earthworm. Small roundworms visible around anus Pinworm Worldwide Anal itching; eggs rarely detected by ova and parasite (O&P) exam   Trichuris Worldwide Rectal prolapse with heavy infection in children Passage of tapeworm segments T. solium or Taenia saginata Worldwide Usual reason for seeking medical care   Diphyllobothrium latum Worldwide Pernicious anemia in genetically predisposed Scandinavians Genitourinary System Itchy discharge Trichomonas vaginalis Worldwide Common sexually transmitted disease of both sexes Hematuria Schistosoma haematobium Africa Hematuria with negative cultures, urinary tract infections, and late bladder cancer

(Continued) Most common cause globally of eosinophilic meningitis; spontaneous resolution Immune response to microfilaria in cornea Worms may cross eye during migration. CHAPTER 228 Introduction to Parasitic Infections (Continued)

123 - 229 Agents Used to Treat Parasitic Infections

229 Agents Used to Treat Parasitic Infections

TABLE 228-1  Parasitic Infections, by Organ System and Signs/Symptomsa ORGAN SYSTEM, MAJOR SIGN(S)/SYMPTOM(S) PARASITE(S) GEOGRAPHIC DISTRIBUTION COMMENTS Muscular System Myalgias, myositis Trichinella Worldwide Palpebral swelling; high-level eosinophilia Bloodstream Fever without localizing symptoms Plasmodium Tropics and subtropics Consider in any patient from a malarious area.   Babesia New England, United States Geographically limited; worse with splenectomy   T. brucei rhodesiense, T. brucei gambiense Sub-Saharan Africa Limited to tsetse fly range; painful chancre; adenopathy and cyclical fevers; early (rhodesiense) or late (gambiense) central nervous system involvement   Filariae Asia, India Periodic fever with eosinophilia, adenolymphangitis, chronic lymphangitis   L. donovani complex Tropics and subtropics Hepatosplenomegaly, fever, wasting; AIDS-defining infection aSee also text and Tables S12-1, S12-2, and S12-3 for vectors and routes of transmission. ■ ■FURTHER READING Blackburn D et al: Outbreak of locally acquired mosquito-transmitted malaria—Florida and Texas, May–July 2023. MMWR 72:973, 2023. Conrad MD et al: Evolution of partial resistance to artemisinins in malaria parasites in Uganda. N Engl J Med 389:722, 2023. Diaz AV et al: Reaching the World Health Organization elimination targets for schistosomiasis: The importance of a One Health perspective. Philos Trans R Soc Lond B Biol Sci 378:20220274, 2023. Loukas A et al: The yin and yang of human soil-transmitted helminths. PART 5 Infectious Diseases Int J Parasitol 51:1243, 2021. Rubin EJ: Making the worm turn. N Engl J Med 388:1908, 2023. Thomas A. Moore

Agents Used to Treat Parasitic Infections Parasitic infections continue to afflict more than half of the world’s population and impose a substantial health burden, particularly in underdeveloped nations, where they are most prevalent. The reach of some parasitic diseases, including malaria, has expanded over the past few decades due to factors such as deforestation, population shifts, global warming, and other climatic events. Although there have been significant advances in vaccine development and vector control, chemotherapy remains the single most effective means of controlling parasitic infections. Efforts to combat the spread of some diseases are hindered by the development and spread of drug resistance, the limited introduction of new antiparasitic agents, the proliferation of counterfeit medications, profiteering, and, most recently, the widespread and unsupported use of antiparasitic agents to treat COVID-19, all of which have dramatically increased the cost of these once-affordable agents. However, there are good reasons to be optimistic. Ambitious global initiatives aimed at controlling or eliminating threats such as AIDS, tuberculosis, and malaria continue to demonstrate success. The ongoing efforts of multinational partnerships to address the substantial burden imposed by neglected tropical diseases have generated new effective antiparasitic agents. In addition, agents approved for other uses are being re-evaluated for antiparasitic efficacy, and some have been subsequently repurposed. This chapter deals exclusively with the agents used to treat infections due to parasites. Specific treatment recommendations for the

(Continued) parasitic diseases of humans are listed in subsequent chapters. Many of the agents discussed herein are approved by the U.S. Food and Drug Administration (FDA) but are considered investigational for the treatment of certain infections. Drugs marked in the text with an asterisk (*) are available through the Centers for Disease Control and Prevention (CDC) Drug Service (telephone: 404-639-3670; email: drugservice@ cdc.gov). Drugs marked with a dagger (†) are available only through their manufacturers; contact information for these manufacturers may be available from the CDC. Table 229-1 presents a brief overview of each agent (including some drugs that are covered in other chapters), along with major toxicities, spectrum of activity, and safety for use during pregnancy and lactation. Albendazole  Like all benzimidazoles, albendazole acts by selectively binding to free β-tubulin in nematodes, inhibiting the polymerization of tubulin and the microtubule-dependent uptake of glucose. Irreversible damage occurs in gastrointestinal (GI) cells of the nematodes, resulting in starvation, death, and expulsion by the host. This fundamental disruption of cellular metabolism offers treatment for a wide range of parasitic diseases. Albendazole is poorly absorbed from the GI tract, a feature that is advantageous for the treatment of intestinal helminths but not for that of tissue helminth infections (e.g., hydatid disease and neurocysticercosis), which requires a sufficient amount of active drug to reach the site of infection. Administration with a high-fat meal (~40 g) increases the drug’s absorption by up to fivefold. The metabolite albendazole sulfoxide is responsible for the drug’s therapeutic effect outside the gut lumen. Albendazole sulfoxide crosses the blood-brain barrier, reaching a level significantly higher than that achieved in plasma. The high concentrations of albendazole sulfoxide attained in cerebrospinal fluid (CSF) may explain the efficacy of albendazole in the treatment of neurocysticercosis. Albendazole is extensively metabolized in the liver, but there are few data regarding the drug’s use in patients with hepatic disease. Singledose albendazole therapy in humans is largely without side effects (overall frequency, ≤1%). More prolonged courses (e.g., as administered for cystic and alveolar echinococcal disease) have been associated with liver function abnormalities and bone marrow toxicity. Thus, when prolonged use is anticipated, the drug should be administered in treatment cycles of 28 days interrupted by 14-day intervals off therapy. Prolonged therapy with full-dose albendazole (800 mg/d) should be approached cautiously in patients also receiving drugs with known effects on the cytochrome P450 system. Amodiaquine  Amodiaquine has been widely used in the treatment of malaria for >60 years. Like chloroquine (the other major 4-aminoquinoline), amodiaquine is now of limited use because of the spread of resistance. Amodiaquine interferes with hemozoin formation through complexation with heme. It is rapidly absorbed and acts as a prodrug after oral administration; the principal plasma metabolite, monodesethylamodiaquine, is the predominant antimalarial agent. Amodiaquine

TABLE 229-1  Overview of Agents Used for the Treatment of Parasitic Infections DRUGS BY CLASS PARASITIC INFECTION(S) ADVERSE EFFECTS 4-Aminoquinolines             Amodiaquine Malariab Agranulocytosis, hepatotoxicity No information Not assigned Yesc   Chloroquine Malariab Occasional: pruritus, nausea, vomiting, headache, hair depigmentation, exfoliative dermatitis, reversible corneal opacity Rare: irreversible retinal injury, nail discoloration, blood dyscrasias   Piperaquine Malariab Occasional: GI disturbances None reported Not assigned Yes 8-Aminoquinolines             Primaquine Malariab Frequent: hemolysis in patients with G6PD deficiency Occasional: methemoglobinemia, GI disturbances Rare: CNS symptoms   Tafenoquine Malariab Frequent: hemolysis in patients with G6PD deficiency, mild GI upset Occasional: methemoglobinemia, headache Aminoalcohols             Halofantrine Malariab Frequent: abdominal pain, diarrhea Occasional: ECG disturbances (dose-related prolongation of QTc and PR interval), nausea, pruritus; contraindicated in persons who have cardiac disease or who have taken mefloquine in the preceding 3 weeks   Lumefantrine Malariab Occasional: nausea, vomiting, diarrhea, abdominal pain, anorexia, headache, dizziness Aminoglycosides             Paromomycin Amebiasis,b infection with Dientamoeba fragilis, giardiasis, cryptosporidiosis, leishmaniasis Frequent: GI disturbances (oral dosing only) Occasional: nephrotoxicity, ototoxicity, vestibular toxicity (parenteral dosing only) Amphotericin B   Amphotericin B Leishmaniasis,e amebic meningoencephalitis Frequent: fever, chills, hypokalemia, hypomagnesemia, nephrotoxicity Occasional: vomiting, dyspnea, hypotension deoxycholate   Amphotec (InterMune)   Amphotericin B lipid complex, ABLC (Abelcet)   Amphotericin B, liposomal (AmBisome) Antimonials   Meglumine Leishmaniasis Frequent: arthralgias/myalgias, pancreatitis, ECG changes (QT prolongation, T-wave flattening or inversion) antimoniateh Artemisinin and derivatives Malariag Occasional: neurotoxicity (ataxia, convulsions), nausea, vomiting, anorexia, contact dermatitis   Arteether     No information Not assigned Yesc   Artemether     Artemether levels decreased by darunavir, etravirine, and nevirapine   Artesunateh     Mefloquine: levels decreased and clearance accelerated by artesunate   Dihydroartemisinin     Mefloquine: increased absorption Not assigned Yesc

MAJOR DRUG–DRUG INTERACTIONS PREGNANCY CLASSa BREAST MILK Antacids and kaolin: reduced absorption of chloroquine Ampicillin: bioavailability reduced by chloroquine Cimetidine: increased serum levels of chloroquine Cyclosporine: serum levels increased by chloroquine Not assignedd Yesc Quinacrine: potentiated toxicity of primaquine Contraindicated Yes No information Not assigned Yes Concomitant use of agents that prolong QTc interval contraindicated C No information CHAPTER 229 Plasma levels increased by darunavir and nevirapine, decreased by etravirine Not assigned No information Agents Used to Treat Parasitic Infections No major interactions Oral: B Parenteral: not assignedd No information Antineoplastic agents: renal toxicity, bronchospasm, hypotension Glucocorticoids, ACTH, digitalis: hypokalemia Zidovudine: increased myelo- and nephrotoxicity B No information   Antiarrhythmics and tricyclic antidepressants: increased risk of cardiotoxicity   Not assigned   No information       C Yesc C Yesc (Continued)

TABLE 229-1  Overview of Agents Used for the Treatment of Parasitic Infections DRUGS BY CLASS PARASITIC INFECTION(S) ADVERSE EFFECTS Atovaquone Malaria,b babesiosis Frequent: nausea, vomiting Occasional: abdominal pain, headache Azoles   Fluconazole   Itraconazole   Ketoconazole Leishmaniasis Serious: hepatotoxicity Rare: exfoliative skin disorders, anaphylaxis Benzimidazoles           PART 5 Infectious Diseases   Albendazole Ascariasis, capillariasis, clonorchiasis, cutaneous larva migrans, cysticercosis,b echinococcosis,b enterobiasis, eosinophilic enterocolitis, gnathostomiasis, hookworm, lymphatic filariasis, microsporidiosis, strongyloidiasis, trichinellosis, trichostrongyliasis, trichuriasis, visceral larva migrans Occasional: nausea, vomiting, abdominal pain, headache, reversible alopecia, elevated aminotransferases Rare: leukopenia, rash   Mebendazole Ascariasis,b capillariasis, eosinophilic enterocolitis, enterobiasis,b hookworm,b trichinellosis, trichostrongyliasis, trichuriasis,b visceral larva migrans Occasional: diarrhea, abdominal pain, elevated aminotransferases Rare: agranulocytosis, thrombocytopenia, alopecia   Thiabendazole Strongyloidiasis,b cutaneous larva migrans,b visceral larva migransb Frequent: anorexia, nausea, vomiting, diarrhea, headache, dizziness, asparagus-like urine odor Occasional: drowsiness, giddiness, crystalluria, elevated aminotransferases, psychosis Rare: hepatitis, seizures, angioneurotic edema, StevensJohnson syndrome, tinnitus   Triclabendazole Fascioliasis, paragonimiasis Occasional: abdominal cramps, diarrhea, biliary colic, transient headache   Benznidazole Chagas disease Frequent: rash, pruritus, nausea, leukopenia, paresthesias Clindamycin Babesiosis, malaria, toxoplasmosis Occasional: pseudomembranous colitis, abdominal pain, diarrhea, nausea/vomiting Rare: pruritus, skin rashes Diloxanide furoate Amebiasis Frequent: flatulence Occasional: nausea, vomiting, diarrhea Rare: pruritus

(Continued) MAJOR DRUG–DRUG INTERACTIONS PREGNANCY CLASSa BREAST MILK Plasma levels decreased by rifampin, tetracycline, atazanavir, efavirenz, lopinavir/ritonavir; bioavailability decreased by metoclopramide C No information Warfarin, oral hypoglycemics, phenytoin, cyclosporine, theophylline, digoxin, dofetilide, quinidine, carbamazepine, rifabutin, busulfan, docetaxel, vinca alkaloids, pimozide, alprazolam, diazepam, midazolam, triazolam, verapamil, atorvastatin, cerivastatin, lovastatin, simvastatin, tacrolimus, sirolimus, indinavir, ritonavir, saquinavir, alfentanil, buspirone, methylprednisolone, trimetrexate: plasma levels increased by azoles Carbamazepine, phenobarbital, phenytoin, isoniazid, rifabutin, rifampin, antacids, H2-receptor antagonists, proton pump inhibitors, nevirapine: decreased plasma levels of azoles Clarithromycin, erythromycin, indinavir, ritonavir: increased plasma levels of azoles C Yes Dexamethasone, praziquantel: plasma level of albendazole sulfoxide increased by ~50% C Yesc Cimetidine: inhibited mebendazole metabolism C No information Theophylline: serum levels increased by thiabendazole C No information No information Not assigned Yes No major interactions Not assigned No information No major interactions B Yesc None reported Contraindicated No information (Continued)

TABLE 229-1  Overview of Agents Used for the Treatment of Parasitic Infections DRUGS BY CLASS PARASITIC INFECTION(S) ADVERSE EFFECTS Eflornithinef (difluoromethylornithine, DFMO) Trypanosomiasis Frequent: pancytopenia Occasional: diarrhea, seizures Rare: transient hearing loss Emetine and dehydroemetine Amebiasis, fascioliasis Severe: cardiotoxicity Frequent: pain at injection site Occasional: dizziness, headache, GI symptoms Folate antagonists             Dihydrofolate           reductase inhibitors     Pyrimethamine Malaria,b isosporiasis, toxoplasmosisb Occasional: folate deficiency Rare: rash, seizures, severe skin reactions (toxic epidermal necrolysis, erythema multiforme, StevensJohnson syndrome)     Proguanil and Malaria Occasional: urticaria Rare: hematuria, GI disturbances chlorproguanil     Trimethoprim Cyclosporiasis, isosporiasis Hyperkalemia, GI upset, mild stomatitis   Dihydropteroate Malaria,b toxoplasmosisb Frequent: GI disturbances, allergic skin reactions, crystalluria Rare: severe skin reactions (toxic epidermal necrolysis, erythema multiforme, Stevens-Johnson syndrome), agranulocytosis, aplastic anemia, hypersensitivity of the respiratory tract, hepatitis, interstitial nephritis, hypoglycemia, aseptic meningitis synthetase inhibitors: sulfonamides     Sulfadiazine     Sulfamethoxazole     Sulfadoxine   Dihydropteroate           synthetase inhibitors: sulfones     Dapsone Leishmaniasis, malaria, toxoplasmosis Frequent: rash, anorexia Occasional: hemolysis, methemoglobinemia, neuropathy, allergic dermatitis, anorexia, nausea, vomiting, tachycardia, headache, insomnia, psychosis, hepatitis Rare: agranulocytosis Fumagillin Microsporidiosis Rare: neutropenia, thrombocytopenia None reported No information No information Furazolidone Giardiasis Frequent: nausea/vomiting, brown urine Occasional: rectal itching, headache Rare: hemolytic anemia, disulfiramlike reactions, MAO inhibitor interactions Iodoquinol Amebiasis,b balantidiasis, D. fragilis infection Occasional: headache, rash, pruritus, thyrotoxicosis, nausea, vomiting, abdominal pain, diarrhea Rare: optic neuritis, peripheral neuropathy, seizures, encephalopathy Lactones             Ivermectin Ascariasis, cutaneous larva migrans, gnathostomiasis, loiasis, lymphatic filariasis, onchocerciasis,b scabies, strongyloidiasis,b trichuriasis Occasional: fever, pruritus, headache, myalgias Rare: hypotension   Moxidectin Onchocerciasis Occasional: fever, pruritus, headache, myalgias Rare: orthostatic hypotension, elevated transaminases

(Continued) MAJOR DRUG–DRUG INTERACTIONS PREGNANCY CLASSa BREAST MILK No major interactions Contraindicated No information None reported X No information Sulfonamides, proguanil, zidovudine: increased risk of bone marrow suppression when used concomitantly C Yes Atazanavir, efavirenz, lopinavir/ ritonavir: plasma levels of proguanil decreased C Yes Methotrexate: reduced clearance Warfarin: effect prolonged Phenytoin: hepatic metabolism increased C Yes Thiazide diuretics: increased risk of thrombocytopenia in elderly patients Warfarin: effect prolonged by sulfonamides Methotrexate: levels increased by sulfonamides Phenytoin: metabolism impaired by sulfonamides Sulfonylureas: effect prolonged by sulfonamides B Yes CHAPTER 229 Agents Used to Treat Parasitic Infections Rifampin: lowered plasma levels of dapsone C Yes Risk of hypertensive crisis when administered for >5 days with MAO inhibitors C No information No major interactions C No information No major interactions C Yesc No major interactions C Yesc (Continued)

TABLE 229-1  Overview of Agents Used for the Treatment of Parasitic Infections DRUGS BY CLASS PARASITIC INFECTION(S) ADVERSE EFFECTS Macrolides             Azithromycin Babesiosis Occasional: nausea, vomiting, diarrhea, abdominal pain Rare: angioedema, cholestatic jaundice   Spiramycinh Toxoplasmosis Occasional: GI disturbances, transient skin eruptions Rare: thrombocytopenia, QT prolongation in an infant, cholestatic hepatitis Mefloquine Malariab Frequent: lightheadedness, nausea, headache Occasional: confusion; nightmares; insomnia; visual disturbance; transient and clinically silent ECG abnormalities, including sinus bradycardia, sinus arrhythmia, first-degree AV block, prolongation of QTc interval, and abnormal T waves Rare: psychosis, convulsions, hypotension Melarsoprolf Trypanosomiasis Frequent: myocardial injury, encephalopathy, peripheral neuropathy, hypertension Occasional: G6PD-induced hemolysis, erythema nodosum leprosum Rare: hypotension PART 5 Infectious Diseases Metrifonate Schistosomiasis Frequent: abdominal pain, nausea, vomiting, diarrhea, headache, vertigo, bronchospasm Rare: cholinergic symptoms Miltefosine Leishmaniasis,b primary amebic meningoencephalitis Frequent: mild and transient (1–2 days) GI disturbances within first 2 weeks of therapy (resolve after treatment completion); motion sickness Occasional: reversible elevations of creatinine and aminotransferases Niclosamide Intestinal cestode infectionsb Occasional: nausea, vomiting, dizziness, pruritus Nifurtimox Chagas disease Frequent: nausea, vomiting, abdominal pain, insomnia, paresthesias, weakness, tremors Rare: seizures (all reversible and dose-related) Nitazoxanide Cryptosporidiosis,b giardiasisb Occasional: abdominal pain, diarrhea Rare: vomiting, headache Nitroimidazoles   Metronidazole   Amebiasis,b balantidiasis, dracunculiasis, giardiasis b, trichomoniasis,b D. fragilis infection   Frequent: nausea, headache, anorexia, metallic aftertaste Occasional: vomiting, insomnia, vertigo, paresthesias, disulfiram-like effects Rare: seizures, peripheral neuropathy   Tinidazole Amebiasis,b giardiasis, trichomoniasis Occasional: nausea, vomiting, metallic taste

(Continued) MAJOR DRUG–DRUG INTERACTIONS PREGNANCY CLASSa BREAST MILK Cyclosporine and digoxin: levels increased by azithromycin Nelfinavir: increased levels of azithromycin B Yes No major interactions Not assignedd Yesc Administration of halofantrine <3 weeks after mefloquine use may produce fatal QTc prolongation. Mefloquine may lower plasma levels of anticonvulsants. Levels are decreased and clearance is accelerated by artesunate. Mefloquine decreases plasma levels of ritonavir and possibly other protease inhibitors. C Yes No major interactions Not assigned No information No major interactions B No No major interactions Not assigned No information No major interactions B No information No major interactions Not assigned No information Increases plasma levels of highly protein-bound drugs (e.g., phenytoin, warfarin) B No information   Warfarin: effect enhanced by metronidazole Disulfiram: psychotic reaction Phenobarbital, phenytoin: accelerate elimination of metronidazole Lithium: serum levels elevated by metronidazole Cimetidine: prolonged half-life of metronidazole Oral solutions of antiretrovirals containing alcohol: disulfiram effect due to alcohol   B   Yes See metronidazole C Yes (Continued)

TABLE 229-1  Overview of Agents Used for the Treatment of Parasitic Infections DRUGS BY CLASS PARASITIC INFECTION(S) ADVERSE EFFECTS Oxamniquine Schistosomiasis Occasional: dizziness, drowsiness, headache, orange urine, elevated aminotransferases Rare: seizures Pentamidine isethionate Leishmaniasis, trypanosomiasis Frequent: hypotension, hypoglycemia, pancreatitis, sterile abscesses at IM injection sites, GI disturbances, reversible renal failure Occasional: hepatotoxicity, cardiotoxicity, delirium Rare: anaphylaxis Piperazine and derivatives             Piperazine Ascariasis, enterobiasis Occasional: nausea, vomiting, diarrhea, abdominal pain, headache Rare: neurotoxicity, seizures   Diethylcarbamazinef Lymphatic filariasis, loiasis, tropical pulmonary eosinophilia Frequent: dose-related nausea, vomiting Rare: fever, chills, arthralgias, headache Praziquantel Clonorchiasis,b cysticercosis, diphyllobothriasis, hymenolepiasis, taeniasis, opisthorchiasis, intestinal trematodes, paragonimiasis, schistosomiasisb Frequent: abdominal pain, diarrhea, dizziness, headache, malaise Occasional: fever, nausea Rare: pruritus, singultus Pyrantel pamoate Ascariasis, eosinophilic enterocolitis, enterobiasis,b hookworm, trichostrongyliasis Occasional: GI disturbances, headache, dizziness, elevated aminotransferases Pyronaridine Malaria Occasional: headache, nausea None reported to date B Yes Quinacrineh Giardiasisb Frequent: headache, nausea, vomiting, bitter taste Occasional: yellow-orange discoloration of skin, sclerae, urine; begins after 1 week of treatment and lasts up to 4 months after drug discontinuation Rare: psychosis, exfoliative dermatitis, retinopathy, G6PD-induced hemolysis, exacerbation of psoriasis, disulfiramlike effects Quinine and quinidine Malaria, babesiosis Frequent: cinchonism (tinnitus, hightone deafness, headache, dysphoria, nausea, vomiting, abdominal pain, visual disturbances, postural hypotension), hyperinsulinemia resulting in life-threatening hypoglycemia Occasional: deafness, hemolytic anemia, arrhythmias, hypotension due to rapid IV infusion Quinolones             Ciprofloxacin Cyclosporiasis, isosporiasis Occasional: nausea, diarrhea, vomiting, abdominal pain/discomfort, headache, restlessness, rash Rare: myalgias/arthralgias, tendon rupture, CNS symptoms (nervousness, agitation, insomnia, anxiety, nightmares, or paranoia); convulsions

(Continued) MAJOR DRUG–DRUG INTERACTIONS PREGNANCY CLASSa BREAST MILK No major interactions C No information No major interactions C No information None reported C No information None reported Not assignedd No information No major interactions B Yes CHAPTER 229 No major interactions C No information Primaquine: toxicity potentiated by quinacrine C No information Agents Used to Treat Parasitic Infections Carbonic anhydrase inhibitors, thiazide diuretics: reduced renal elimination of quinidine Amiodarone, cimetidine: increased quinidine levels Nifedipine: decreased quinidine levels; quinidine slows metabolism of nifedipine Phenobarbital, phenytoin, rifampin: accelerated hepatic elimination of quinidine Verapamil: reduced hepatic clearance of quinidine Diltiazem: decreased clearance of quinidine X Yesc Probenecid: increased serum levels of ciprofloxacin Theophylline, warfarin: serum levels increased by ciprofloxacin C Yes (Continued)

TABLE 229-1  Overview of Agents Used for the Treatment of Parasitic Infections DRUGS BY CLASS PARASITIC INFECTION(S) ADVERSE EFFECTS Suraminf Trypanosomiasis Frequent: immediate: fever, urticaria, nausea, vomiting, hypotension; delayed (up to 24 h): exfoliative dermatitis, stomatitis, paresthesias, photophobia, renal dysfunction Occasional: nephrotoxicity, adrenal toxicity, optic atrophy, anaphylaxis Tetracyclines Balantidiasis, D. fragilis infection, malaria; lymphatic filariasis (doxycycline) Frequent: GI disturbances Occasional: photosensitivity dermatitis Rare: exfoliative dermatitis, esophagitis, hepatotoxicity aBased on U.S. Food and Drug Administration (FDA) pregnancy categories of A–D, X. bApproved by the FDA for this indication. cNot believed to be harmful. dUse in pregnancy is recommended by international organizations outside the United States. eOnly AmBisome has been approved by the FDA for this indication. fAvailable through the CDC. gOnly artemether (in combination with lumefantrine) and artesunate have been approved by the FDA for this indication. hAvailable through the manufacturer. Abbreviations: ACTH, adrenocorticotropic hormone; AV, atrioventricular; CNS, central nervous system; ECG, electrocardiogram; G6PD, glucose 6-phosphate dehydrogenase; GI, gastrointestinal; MAO, monoamine oxidase. and its metabolites are all excreted in urine, but there are no recommen­ dations concerning dosage adjustment in patients with impaired renal function. Agranulocytosis and hepatotoxicity can develop with repeated use; therefore, this drug should not be used for prophylaxis. Indeed, because of its adverse effects and widespread resistance, amodiaquine is no longer in use in Europe or the United States, and it was dropped from malaria control programs as a single agent by the WHO in 1990; however, it remains effective in some areas when combined with other antimalarial drugs (e.g., artesunate, sulfadoxine-pyrimethamine). PART 5 Infectious Diseases Amphotericin B  See Table 229-1 and Chap. 217. Antimonials†  Despite associated adverse reactions and the need for prolonged parenteral treatment, the pentavalent antimonial com­ pounds (designated Sbv) have remained the first-line therapy for all forms of leishmaniasis throughout the world, primarily because they are affordable and effective and have survived the test of time. Pentavalent antimonials are active only after bioreduction to the tri­ valent Sb(III) form, which inhibits trypanothione reductase, a critical enzyme involved in the oxidative stress management of Leishmania species. The fact that Leishmania species use trypanothione rather than glutathione (which is used by mammalian cells) may explain the parasite-specific activity of antimonials. The drugs are taken up by the reticuloendothelial system, and their activity against Leishmania species may be enhanced by this localization. Resistance is a major problem in some areas. Although low-level unresponsiveness to Sbv was identified in India in the 1970s, incre­ mental increases in both the recommended daily dosage (to 20 mg/kg) and the duration of treatment (to 28 days) satisfactorily compensated for the growing resistance until around 1990. Since then, the capacity of Sbv to induce long-term cure in patients with kala-azar who live in eastern India has steadily eroded. Co-infection with HIV impairs the treatment response. Pentavalent antimonials are available in aqueous solution and are administered parenterally. Antimony appears to have two elimination phases. When the drug is administered IV, the mean half-life of the first phase is <2 h; the mean half-life of the terminal elimination phase is nearly 36 h. This slower phase may be due to conversion of pentavalent antimony to a trivalent form that is the likely cause of the side effects often seen with prolonged therapy. In 2020, the global manufacturer of sodium stibogluconate notified the CDC that the product would be discontinued due to the inability to source the necessary raw materials. As a result, CDC ended its dis­ tribution program following expiration of the last lot in use. However, meglumine antimoniate is available from the manufacturer through the FDA (301-796-1400). Artemisinin Derivatives*  Artesunate, artemether, artemotil, and the parent compound artemisinin are sesquiterpene lactones derived from the wormwood plant Artemisia annua. These agents are at least

(Continued) MAJOR DRUG–DRUG INTERACTIONS PREGNANCY CLASSa BREAST MILK No major interactions Not assigned No information Warfarin: effect prolonged by tetracyclines D Yes 10-fold more potent in vivo than other antimalarial drugs and presently show no cross-resistance with known antimalarial drugs; thus, they have become first-line agents for the treatment of severe falciparum malaria. The artemisinin compounds are rapidly effective against the asexual blood forms of Plasmodium species but are not active against intrahepatic forms. With the exception of artesunate, artemisinin and its derivatives are highly lipid soluble and readily cross both host and parasite cell membranes. One factor that explains the drugs’ highly selective toxicity against malaria is that parasitized erythrocytes con­ centrate artemisinin and its derivatives to concentrations 100-fold higher than those in uninfected erythrocytes. The antimalarial effect of these agents results primarily from the active metabolite dihydroar­ temisinin; in the presence of heme or molecular iron, the endoperoxide moiety of dihydroartemisinin decomposes, generating free radicals and other metabolites that damage parasite proteins. The compounds are available for oral, rectal, IV, or IM administration, depending on the derivative. Following FDA approval in May 2020, artesunate became commercially available in the United States. Artemisinin and its derivatives are cleared rapidly from the cir­ culation. Their short half-lives limit their value for prophylaxis and monotherapy. Side effects appear to be minor, although sinus bra­ dycardia and transient first-degree heart block have been reported. Although seen in animal models, embryotoxicity and neurotoxicity have not been identified in humans despite active investigation. These agents should be used only in combination with another, longer-acting agent (e.g., artesunate-mefloquine, dihydroartemisinin-piperaquine). A combined formulation of artemether and lumefantrine is widely available for the treatment of acute uncomplicated falciparum malaria acquired in areas where Plasmodium falciparum is resistant to chloroquine and antifolates. Atovaquone  Atovaquone is a hydroxynaphthoquinone that exerts broad-spectrum antiprotozoal activity via selective inhibition of para­ site mitochondrial electron transport. This agent exhibits potent activity against toxoplasmosis and babesiosis when used with pyri­ methamine and azithromycin, respectively. Atovaquone possesses a novel mode of action against Plasmodium species, inhibiting the elec­ tron transport system at the level of the cytochrome bc1 complex. The drug is active against both the erythrocytic and the exoerythrocytic stages of Plasmodium species; however, because it does not eradicate hypnozoites from the liver, patients with P. vivax or P. ovale infections must be given radical prophylaxis. Malarone is a fixed-dose combination of atovaquone and progua­ nil used for malaria prophylaxis as well as for the treatment of acute, uncomplicated P. falciparum malaria. Malarone has been shown to be effective in regions with multidrug-resistant P. falciparum. Resistance to atovaquone develops rapidly via mutations in the parasite’s mito­ chondrial cytochrome b complex. However, the mutations result in sterility of female parasites; thus, atovaquone-resistant parasites cannot

be transmitted to another person. This situation may explain why clini­ cal resistance has yet to be reported. The bioavailability of atovaquone varies considerably. Absorption after a single oral dose is slow, increases two- to threefold with a fatty meal, and is dose-limited above 750 mg. The elimination half-life is increased in patients with moderate hepatic impairment. Because of the potential for drug accumulation, the use of atovaquone is generally contraindicated in persons with a creatinine clearance rate <30 mL/min. No dosage adjustments are needed in patients with mild to moderate renal impairment. Azithromycin  See Table 229-1 and Chap. 149. Azoles  See Table 229-1 and Chap. 217. Benznidazole  Introduced in 1971, this oral nitroimidazole deriva­ tive is used to treat Chagas disease, with cure rates of 80–90% recorded in acute infections. Benznidazole is believed to exert its trypanocidal effects by generating oxygen radicals to which the parasites are more sensitive than mammalian cells because of a relative deficiency in antioxidant enzymes. Benznidazole also appears to alter the balance between pro- and anti-inflammatory mediators by downregulating the synthesis of nitrite, interleukin (IL) 6, and IL-10 in macrophages. Benznidazole is highly lipophilic and readily absorbed. The drug is extensively metabolized; only 5% of the dose is excreted unchanged in the urine. Benznidazole is well tolerated; adverse effects are rare and usually manifest as GI upset or pruritic rash. Following FDA approval in 2018, this drug is now commercially available in the United States. Chloroquine  This 4-aminoquinoline has marked, rapid schizon­ ticidal and gametocidal activity against blood forms of P. ovale and

P. malariae and against susceptible strains of P. vivax and P. falciparum. It is not active against intrahepatic forms (P. vivax and P. ovale). Para­ sitized erythrocytes accumulate chloroquine in significantly greater concentrations than do normal erythrocytes. Chloroquine, a weak base, concentrates in the food vacuoles of intraerythrocytic parasites because of a relative pH gradient between the extracellular space and the acidic food vacuole. Once it enters the acidic food vacuole, chlo­ roquine is rapidly converted to a membrane-impermeable protonated form and is trapped. Continued accumulation of chloroquine in the parasite’s acidic food vacuoles results in drug levels that are 600-fold higher at this site than in plasma. The high accumulation of chloro­ quine results in an increase in pH within the food vacuole to a level above that required for the acid proteases’ optimal activity, inhibiting parasite heme polymerase; as a result, the parasite is effectively killed with its own metabolic waste. Compared with susceptible strains, chloroquine-resistant plasmodia transport chloroquine out of intra­ parasitic compartments more rapidly and maintain lower chloroquine concentrations in their acid vesicles. Hydroxychloroquine, a congener of chloroquine, is equivalent to chloroquine in its antimalarial efficacy but is preferred to chloroquine for the treatment of autoimmune disor­ ders because it produces less ocular toxicity when used in high doses. Chloroquine is well absorbed. However, because it exhibits exten­ sive tissue binding, a loading dose is required to yield effective plasma concentrations. A therapeutic drug level in plasma is reached 2–3 h after oral administration (the preferred route). Chloroquine can be administered IV, but excessively rapid parenteral administration can result in seizures and death from cardiovascular collapse. The mean half-life of chloroquine is 4 days, but the rate of excretion decreases as plasma levels decline, making once-weekly administration possible for prophylaxis in areas with sensitive strains. About one-half of the par­ ent drug is excreted in urine, but the dose should not be reduced for persons with acute malaria and renal insufficiency. Ciprofloxacin  See Table 229-1 and Chap. 149. Clindamycin  See Table 229-1 and Chap. 149. Dapsone  See Table 229-1 and Chap. 188. Dehydroemetine  Emetine is an alkaloid derived from ipecac; dehy­ droemetine is synthetically derived from emetine and is considered less

toxic. Both agents are active against Entamoeba histolytica and appear to work by blocking peptide elongation and thus inhibiting protein synthesis. Emetine is rapidly absorbed after parenteral administration, rapidly distributed throughout the body, and slowly excreted in the urine in unchanged form. Both agents are contraindicated in patients with renal disease.

Diethylcarbamazine*  A derivative of the antihelminthic agent piperazine with a long history of successful use, diethylcarbamazine (DEC) remains the treatment of choice for lymphatic filariasis and loiasis and has also been used for visceral larva migrans. Although piperazine itself has no antifilarial activity, the piperazine ring of DEC is essential for the drug’s activity. DEC’s mechanism of action remains to be fully defined. Proposed mechanisms include immobilization due to inhibition of parasite cholinergic muscle receptors, disrup­ tion of microtubule formation, and alteration of helminthic surface membranes resulting in enhanced killing by the host’s immune system. DEC enhances adherence properties of eosinophils. The develop­ ment of resistance under drug pressure (i.e., a progressive decrease in efficacy when the drug is used widely in human populations) has not been observed, although DEC has variable effects when administered to persons with filariasis. Monthly administration provides effective prophylaxis against both bancroftian filariasis and loiasis. DEC is well absorbed after oral administration, with peak plasma concentrations reached within 1–2 h. No parenteral form is available. The drug is eliminated largely by renal excretion, with <5% found in feces. If more than one dose is to be administered to an individual with renal dysfunction, the dose should be reduced commensurate with the reduction in creatinine clearance rate. Alkalinization of the urine prevents renal excretion and increases the half-life of DEC. Use in patients with onchocerciasis can precipitate a Mazzotti reaction, with pruritus, fever, and arthralgias. Like other piperazines, DEC is active against Ascaris species. Patients co-infected with this nematode may expel live worms after treatment. CHAPTER 229 Agents Used to Treat Parasitic Infections Diloxanide Furoate  Diloxanide furoate, a substituted acetanilide, is a luminally active agent used to eradicate the cysts of E. histolytica. After ingestion, diloxanide furoate is hydrolyzed by enzymes in the lumen or mucosa of the intestine, releasing furoic acid and the ester diloxanide; the latter acts directly as an amebicide. Diloxanide furoate is given alone to asymptomatic cyst passers. For patients with active amebic infections, diloxanide is generally admin­ istered in combination with a 5-nitroimidazole such as metronidazole or tinidazole. Diloxanide furoate is rapidly absorbed after oral admin­ istration. When coadministered with a 5-nitroimidazole, diloxanide levels peak within 1 h and disappear within 6 h. About 90% of an oral dose is excreted in the urine within 48 h, chiefly as the glucuronide metabolite. Diloxanide furoate is contraindicated in pregnant and breast-feeding women and in children <2 years of age. Eflornithine*  Eflornithine (difluoromethylornithine, or DFMO) is a fluorinated analogue of the amino acid ornithine. Although originally designed as an antineoplastic agent, eflornithine has proven effective against some trypanosomatids. Eflornithine has specific activity against all stages of infection with Trypanosoma brucei gambiense; however, it is inactive against T. b. rhodesiense. The drug acts as an irreversible suicide inhibitor of orni­ thine decarboxylase, the first enzyme in the biosynthesis of the poly­ amines putrescine and spermidine. Polyamines are essential for the synthesis of trypanothione, an enzyme required for the maintenance of intracellular thiols in the correct redox state and for the removal of reactive oxygen metabolites. However, polyamines are also essential for cell division in eukaryotes, and ornithine decarboxylase is similar in trypanosomes and mammals. The selective antiparasitic activity of eflornithine is partly explained by the structure of the trypanosomal enzyme, which lacks a 36-amino-acid C-terminal sequence found on mammalian ornithine decarboxylase. This difference results in a lower turnover of ornithine decarboxylase and a more rapid decrease of poly­ amines in trypanosomes than in the mammalian host. The diminished effectiveness of eflornithine against T. b. rhodesiense appears to be due

to the parasite’s ability to replace the inhibited enzyme more rapidly than T. b. gambiense.

Eflornithine is less toxic but more costly than conventional therapy. It can be administered IV or PO. The dose should be reduced in renal failure. Eflornithine readily crosses the blood-brain barrier; CSF levels are highest in persons with the most severe central nervous system (CNS) involvement. Flubendazole  This agent, a methylcarbamate benzimidazole, is highly active against a broad spectrum of gut nematodes and filaria. Its antihelminthic effect is similar to other benzimidazoles; however, it is also an effective inducer of reactive oxygen species and disrupts glucose transport and metabolism. It has limited solubility in water, so bioavailability is very low, but absorption increases if taken after meals, and is further enhanced when reconstituted as a cyclodextrin-based formulation. It is approved for human use in Europe but not in the Unites States. Fosmidomycin  Originally developed in the 1970s as an antibiotic, this Streptomyces-derived agent is an aminopropylphosphonic acid that exhibits antimalarial activity through inhibition of 1-deoxy-Dxylulose 5-phosphate reductoisomerase, an essential enzyme on the non-mevalonate pathway of isoprenoid biosynthesis, a pathway that is common in most eukaryotes but absent in humans. For >15 years, fosmidomycin has been evaluated alone and in combination with other antimalarials, and it has demonstrated both safety and efficacy. However, the drug is highly charged—resulting in a short plasma halflife (3.5 h)—causing high rates of recrudescent malaria, particularly in children. It is not currently available, but clinical studies are ongoing. Fumagillin†  Originally discovered as an antiangiogenic compound derived from the fungus Aspergillus fumigatus, fumagillin is a waterinsoluble antibiotic that is active against microsporidia and is used topically to treat ocular infections due to Encephalitozoon species. When given systemically, fumagillin was effective but caused thrombocytope­ nia in all recipients in the second week of treatment; this side effect was readily reversed when administration of the drug was stopped. Fuma­ gillin acts by binding to methionine aminopeptidase 2, thus inhibiting microsporidial replication by irreversibly blocking the active site. PART 5 Infectious Diseases Furazolidone  This nitrofuran derivative is an effective alternative agent for the treatment of giardiasis and also exhibits activity against Isospora belli. Because it is the only agent active against Giardia that is available in liquid form, it is most often used to treat young chil­ dren. Furazolidone undergoes reductive activation in Giardia lamblia

trophozoites—an event that, unlike the reductive activation of met­ ronidazole, involves an NADH oxidase. The killing effect correlates with the toxicity of reduced products, which damage important cellular components, including DNA. Although furazolidone had been thought to be largely unabsorbed when administered orally, the occurrence of systemic adverse reactions indicates that this is not the case. More than 65% of the drug dose can be recovered from the urine as colored metab­ olites. Omeprazole reduces the oral bioavailability of furazolidone. Furazolidone is a monoamine oxidase (MAO) inhibitor; thus, caution should be used in its concomitant administration with other drugs (especially indirectly acting sympathomimetic amines) and in the consumption of food and drink containing tyramine during treat­ ment. However, hypertensive crises have not been reported in patients receiving furazolidone, and it has been suggested that—because furazolidone inhibits MAOs gradually over several days—the risks are small if treatment is limited to a 5-day course. Because hemolytic anemia can occur in patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency and glutathione instability, furazolidone treatment is contraindicated in mothers who are breast-feeding and in neonates. Halofantrine  This 9-phenanthrenemethanol is one of three classes of arylaminoalcohols first identified as potential antimalarial agents by the World War II Malaria Chemotherapy Program. Its activity is believed to be similar to that of chloroquine, although it is an oral alternative for the treatment of malaria due to chloroquine-resistant P. falciparum.

Halofantrine is thought to share one or more mechanisms with the 4-aminoquinolines, forming a complex with ferriprotoporphyrin IX and interfering with the degradation of hemoglobin. It has been shown to bind to plasmepsin, a hemoglobin-degrading enzyme unique to plasmodia. Halofantrine exhibits erratic bioavailability, but its absorption is significantly enhanced when it is taken with a fatty meal. The elimi­ nation half-life of halofantrine is 1–2 days; it is excreted mainly in feces. Halofantrine is metabolized into N-debutyl-halofantrine by the cytochrome P450 enzyme CYP3A4. Grapefruit juice should be avoided during treatment because it increases both halofantrine’s bioavailability and halofantrine-induced QT interval prolongation by inhibiting CYP3A4 at the enterocyte level. Halofantrine should not be given simultaneously with or <3 weeks after mefloquine because of the potential occurrence of a fatal prolongation of the QTc interval on electrocardiography. Iodoquinol  Iodoquinol (diiodohydroxyquin), a hydroxyquinoline, is an effective luminal agent for the treatment of amebiasis, balantidia­ sis, and infection with Dientamoeba fragilis. Its mechanism of action is unknown. It is poorly absorbed. Because the drug contains 64% organi­ cally bound iodine, it should be used with caution in patients with thy­ roid disease. Iodine dermatitis occurs occasionally during iodoquinol treatment. Protein-bound serum iodine levels may be increased dur­ ing treatment and can interfere with certain tests of thyroid function. These effects may persist for as long as 6 months after discontinuation of therapy. Iodoquinol is contraindicated in patients with liver disease. Most serious are the reactions related to prolonged high-dose therapy (optic neuritis, peripheral neuropathy), which should not occur if the recommended dosage regimens are followed. Ivermectin  Ivermectin (22,23-dihydroavermectin) is a derivative of the macrocyclic lactone avermectin produced by the soil-dwelling actinomycete Streptomyces avermitilis. Ivermectin is active at low doses against a wide range of helminths and ectoparasites. It is the drug of choice for the treatment of onchocerciasis, strongyloidiasis, cutaneous larva migrans, and scabies. Ivermectin is highly active against micro­ filariae of the lymphatic filariases but has no macrofilaricidal activity. When ivermectin is used in combination with other agents such as DEC or albendazole for treatment of lymphatic filariasis, synergistic activity is seen. Although active against the intestinal helminths Ascaris lumbricoides and Enterobius vermicularis, ivermectin is only variably effective in trichuriasis and is ineffective against hookworms. Wide­ spread use of ivermectin for treatment of intestinal nematode infec­ tions in sheep and goats has led to the emergence of drug resistance in veterinary practice; this development may portend problems in human medical use. Data suggest that ivermectin acts by opening the neuromuscular membrane-associated, glutamate-dependent chloride channels. The influx of chloride ions results in hyperpolarization and muscle paralysis— particularly of the nematode pharynx, with consequent blockage of the oral ingestion of nutrients. As these chloride channels are present only in invertebrates, paralysis is seen only in the parasite. Ivermectin is available for administration to humans only as an oral formulation. The drug is highly protein bound; it is almost completely excreted in feces. Both food and beer increase the bioavailability of ivermectin significantly. Ivermectin is distributed widely throughout the body; animal studies indicate that it accumulates at the highest concentration in adipose tissue and liver, with little accumulation in the brain. Few data exist to guide therapy in hosts with conditions that may influence drug pharmacokinetics. Ivermectin is generally administered as a single dose of 150–200 μg/kg. In the absence of parasitic infection, the adverse effects of ivermectin in therapeutic doses are minimal. Adverse effects in patients with filarial infections include fever, myalgia, malaise, lightheadedness, and (occasionally) postural hypotension. The severity of such side effects is related to the intensity of parasite infection, with more symp­ toms in individuals with a heavy parasite burden. In onchocerciasis, skin edema, pruritus, and mild eye irritation may also occur. The adverse effects are generally self-limiting and only occasionally require

symptom-based treatment with antipyretics or antihistamines. More severe complications of ivermectin therapy for onchocerciasis include encephalopathy in patients heavily infected with Loa loa. Lumefantrine  Lumefantrine (benflumetol), a fluorene arylamino­ alcohol derivative synthesized in the 1970s by the Chinese Academy of Military Medical Sciences (Beijing), has marked blood schizonticidal activity against a wide range of plasmodia. This agent conforms struc­ turally and in mode of action to other arylaminoalcohols (quinine, mefloquine, and halofantrine). Lumefantrine exerts its antimalarial effect as a consequence of its interaction with heme, a degradation product of hemoglobin metabolism. Although its antimalarial activity is slower than that of the artemisinin-based drugs, the recrudescence rate with the recommended lumefantrine regimen is lower. The phar­ macokinetic properties of lumefantrine are reminiscent of those of halofantrine, with variable oral bioavailability, considerable augmenta­ tion of oral bioavailability by concomitant fat intake, and a terminal elimination half-life of ~4–5 days in patients with malaria. Artemether and lumefantrine have synergistic activity, and the combined formulation of artemether and lumefantrine is effective for the treatment of falciparum malaria in areas where P. falciparum is resistant to chloroquine and antifolates. Mebendazole  This benzimidazole is a broad-spectrum antipara­ sitic agent widely used to treat intestinal helminthiases. Its mechanism of action is similar to that of albendazole; however, it is a more potent inhibitor of parasite malic dehydrogenase and exhibits a more spe­ cific and selective effect against intestinal nematodes than the other benzimidazoles. Mebendazole is available only in oral form but is poorly absorbed from the GI tract; only 5–10% of a standard dose is measurable in plasma. The proportion absorbed from the GI tract is extensively metabolized in the liver. Metabolites appear in the urine and bile; impaired liver or biliary function results in higher plasma mebendazole levels in treated patients. No dose reduction is warranted in patients with renal function impair­ ment. Because mebendazole is poorly absorbed, its incidence of side effects is low. Transient abdominal pain and diarrhea sometimes occur, usually in persons with massive parasite burdens. Mefloquine  Mefloquine is used for prophylaxis of chloroquineresistant malaria; high doses can be used for treatment. Despite the development of drug-resistant strains of P. falciparum in parts of Africa and Southeast Asia, mefloquine remains an effective drug throughout most of the world. Cross-resistance of mefloquine with halofantrine and with quinine has been documented in limited areas. Like quinine and chloroquine, this quinoline is active only against the asexual eryth­ rocytic stages of malarial parasites. Unlike quinine, however, meflo­ quine has a relatively poor affinity for DNA and, as a result, does not inhibit the synthesis of parasitic nucleic acids and proteins. Although both mefloquine and chloroquine inhibit hemozoin formation and heme degradation, mefloquine differs in that it forms a complex with heme that may be toxic to the parasite. Mefloquine HCl is poorly water soluble and intensely irritating when given parenterally; thus, it is available only in tablet form. Its absorption is adversely affected by vomiting and diarrhea but is sig­ nificantly enhanced when the drug is administered with or after food. About 98% of the drug binds to protein. Mefloquine is excreted mainly in the bile and feces; therefore, no dose adjustment is needed in per­ sons with renal insufficiency. The drug and its main metabolite are not appreciably removed by hemodialysis. No dosage adjustments are indi­ cated for the achievement of plasma concentrations in dialysis patients. Pharmacokinetic differences have been detected among various ethnic populations; however, these distinctions are of minor importance com­ pared with host immune status and parasite susceptibility. In patients with impaired liver function, the elimination of mefloquine may be prolonged, leading to higher plasma levels. Mefloquine should be used with caution by individuals participating in activities requiring alertness and fine-motor coordination because dizziness, vertigo, or tinnitus can develop and persist. If the drug is to be administered for a prolonged period, periodic evaluations are

recommended, including liver function tests and ophthalmic examina­ tions. Sleep abnormalities (insomnia, abnormal dreams) have occa­ sionally been reported. Psychosis and seizures occur rarely; mefloquine should not be prescribed to patients with neuropsychiatric conditions. The development of acute anxiety, depression, restlessness, or confu­ sion may be considered prodromal to a more serious event, and the drug should be discontinued.

Concomitant use of quinine, quinidine, or drugs producing β-adrenergic blockade may cause significant electrocardiographic abnormalities or cardiac arrest. Halofantrine must not be given simul­ taneously with or <3 weeks after mefloquine because a potentially fatal prolongation of the QTc interval on electrocardiography may occur. No data exist on mefloquine use after halofantrine use. Administration of mefloquine with quinine or chloroquine may increase the risk of convulsions. Mefloquine may lower plasma levels of anticonvulsants. Caution should be exercised with concomitant antiretroviral therapy, because mefloquine has been shown to exert variable effects on rito­ navir pharmacokinetics that are not explained by hepatic CYP3A4 activity or ritonavir protein binding. Vaccinations with attenuated live bacteria should be completed at least 3 days before the first dose of mefloquine. Women of childbearing age who are traveling to areas where malaria is endemic should be warned against becoming pregnant and encouraged to practice contraception during malaria prophylaxis with mefloquine and for up to 3 months thereafter. However, in the case of unplanned pregnancy, use of mefloquine is not considered an indica­ tion for pregnancy termination. Analysis of prospectively monitored cases demonstrates a prevalence of birth defects and fetal loss compa­ rable to background rates. Melarsoprol*  Melarsoprol has been used since 1949 for the treat­ ment of human African trypanosomiasis. This trivalent arsenical compound is indicated for the treatment of African trypanosomiasis with neurologic involvement and for the treatment of early disease that is resistant to suramin or pentamidine. Melarsoprol, like other drugs containing heavy metals, interacts with thiol groups of several different proteins; however, its antiparasitic effects appear to be more specific. Trypanothione reductase is a key enzyme involved in the oxidative stress management of both Trypanosoma and Leishmania species, help­ ing to maintain an intracellular reducing environment by reduction of disulfide trypanothione to its dithiol derivative dihydrotrypano­ thione. Melarsoprol sequesters dihydrotrypanothione, depriving the parasite of its main sulfhydryl antioxidant, and inhibits trypanothione reductase, depriving the parasite of the essential enzyme system that is responsible for keeping trypanothione reduced. These effects are synergistic. The selectivity of arsenical action against trypanosomes is due at least in part to the greater melarsoprol affinity of reduced trypanothione than of other monothiols (e.g., cysteine) on which the mammalian host depends for maintenance of high thiol levels. Melar­ soprol enters the parasite via an adenosine transporter; drug-resistant strains lack this transport system. CHAPTER 229 Agents Used to Treat Parasitic Infections Melarsoprol is always administered IV. A small but therapeuti­ cally significant amount of the drug enters the CSF. The compound is excreted rapidly, with ~80% of the arsenic found in feces. Melarsoprol is highly toxic. The most serious adverse reaction is reactive encephalopathy, which affects 6% of treated individuals and usually develops within 4 days of the start of therapy, with an average case–fatality rate of 50%. Glucocorticoids are administered with melar­ soprol to prevent this development. Because melarsoprol is intensely irritating, care must be taken to avoid infiltration of the drug. Metrifonate  Metrifonate has selective activity against Schistosoma haematobium. This organophosphorous compound is a prodrug that is converted nonenzymatically to dichlorvos (2,2-dichlorovinyl dimeth­ ylphosphate, DDVP), a highly active chemical that irreversibly inhibits the acetylcholinesterase enzyme. Schistosomal cholinesterase is more susceptible to dichlorvos than is the corresponding human enzyme. The exact mechanism of action of metrifonate is uncertain, but the drug is believed to inhibit tegumental acetylcholine receptors that mediate glucose transport.

Metrifonate is administered in a series of three doses at 2-week intervals. After a single oral dose, metrifonate produces a 95% decrease in plasma cholinesterase activity within 6 h, with a fairly rapid return to normal. However, 2.5 months are required for erythrocyte cholinester­ ase levels to return to normal. Treated persons should not be exposed to neuromuscular blocking agents or organophosphate insecticides for at least 48 h after treatment.

Metronidazole and Other Nitroimidazoles  See Table 229-1 and Chap. 149. Miltefosine  In the early 1990s, miltefosine (hexadecylphosphocho­ line), originally developed as an antineoplastic agent, was discovered to have significant antiproliferative activity against Leishmania species, T. cruzi, and T. brucei parasites in vitro and in experimental animal models. Miltefosine is the first oral drug that has proved to be highly effective and comparable to amphotericin B against visceral leishmani­ asis in India, where antimonial-resistant cases are prevalent. Miltefo­ sine is also effective in previously untreated visceral infections. Cure rates in cutaneous leishmaniasis are comparable to those obtained with antimony. Miltefosine is also effective against the free-living ameba Naegleria fowleri. The activity of miltefosine is attributed to interaction with cell signal transduction pathways and inhibition of phospholipid and sterol bio­ synthesis. Resistance to miltefosine has not been observed clinically. The drug is readily absorbed from the GI tract, is widely distributed, and accumulates in several tissues. The efficacy of a 28-day treatment course in Indian visceral leishmaniasis is equivalent to that of ampho­ tericin B therapy; however, it appears that a shortened course of 21 days may be equally efficacious. PART 5 Infectious Diseases General recommendations for the use of miltefosine are limited by the exclusion of specific groups from the published clinical trials: per­ sons <12 or >65 years of age, persons with the most advanced disease, breast-feeding women, HIV-infected patients, and individuals with significant renal or hepatic insufficiency. Moxidectin  Like ivermectin, moxidectin is a macrocyclic lactone that is an effective antihelminthic. In 2018, the FDA approved its use for the treatment of onchocerciasis. The primary mode of action of moxidectin is believed to be like that of ivermectin; however, there are likely different binding sites, as suggested by the identification of ivermectin-resistant helminths that are susceptible to moxidectin. The drug is well tolerated, with most adverse effects attributed to death of microfilariae. Some adverse effects occurred more commonly com­ pared with ivermectin, including orthostatic hypotension (5 vs 2%) and elevated transaminases (1 vs 0.6%). In clinical trials, no clinically significant differences in the pharmacokinetics were observed with age, gender, weight, or renal impairment. The effect of hepatic dysfunction is unknown. Niclosamide†  Niclosamide is active against a wide variety of adult tapeworms but not against tissue cestodes. The drug uncouples oxida­ tive phosphorylation in parasite mitochondria, thereby blocking the uptake of glucose by the intestinal tapeworm and resulting in the para­ site’s death. Niclosamide rapidly causes spastic paralysis of intestinal cestodes in vitro. Its use is limited by its side effects, the necessarily long duration of therapy, the recommended use of purgatives, and— most important—limited availability (i.e., on a named-patient basis from the manufacturer). Niclosamide is poorly absorbed. Tablets are given on an empty stomach in the morning after a liquid meal the night before, and this dose is followed by another 1 h later. For treatment of hymenolepiasis, the drug is administered for 7 days. A second course is often pre­ scribed. The scolex and proximal segments of the tapeworms are killed on contact with niclosamide and may be digested in the gut. However, disintegration of the adult tapeworm results in the release of viable ova, which theoretically can result in autoinfection. Although fears of the development of cysticercosis in patients with Taenia solium infections have proved unfounded, it is still recommended that a brisk purgative be given 2 h after the first dose.

Nifurtimox  This nitrofuran compound is an inexpensive and effective oral agent for the treatment of acute Chagas disease. Try­ panosomes lack catalase and have very low levels of peroxidase; as a result, they are very vulnerable to by-products of oxygen reduc­ tion. When nifurtimox is reduced in the trypanosome, a nitro anion radical is formed and undergoes autooxidation, resulting in the generation of the superoxide anion O2 –, hydrogen peroxide (H2O2), hydroperoxyl radical (HO2), and other highly reactive and cytotoxic molecules. Despite the abundance of catalases, peroxidases, and superoxide dismutases that neutralize these destructive radicals in mammalian cells, nifurtimox has a poor therapeutic index. Prolonged use is required, but the course may have to be interrupted because of drug toxicity, which develops in 40–70% of recipients. Nifurtimox is well absorbed and undergoes rapid and extensive biotransformation; <0.5% of the original drug is excreted in urine. In 2020, the FDA approved this agent for the treatment of Chagas disease in children; it is now commercially available. Nitazoxanide  Nitazoxanide is a 5-nitrothiazole compound used for the treatment of cryptosporidiosis and giardiasis; it is active against other intestinal protozoa as well. The drug is approved for use in children 1–11 years of age. The antiprotozoal activity of nitazoxanide is believed to be due to interference with the pyruvate-ferredoxin oxidoreductase (PFOR) enzyme–dependent electron transfer reaction that is essential to anaerobic energy metabolism. Studies have shown that the PFOR enzyme from G. lamblia directly reduces nitazoxanide by transfer of electrons in the absence of ferredoxin. The DNA-derived PFOR protein sequence of Cryptosporidium parvum appears to be similar to that of G. lamblia. Interference with the PFOR enzyme–dependent electron transfer reaction may not be the only pathway by which nitazoxanide exerts antiprotozoal activity. After oral administration, nitazoxanide is rapidly hydrolyzed to an active metabolite, tizoxanide (desacetyl-nitazoxanide). Tizoxanide then undergoes conjugation, primarily by glucuronidation. It is rec­ ommended that nitazoxanide be taken with food; however, no studies have been conducted to determine whether the pharmacokinetics of tizoxanide and tizoxanide glucuronide differ in fasted versus fed sub­ jects. Tizoxanide is excreted in urine, bile, and feces, and tizoxanide glucuronide is excreted in urine and bile. The pharmacokinetics of nitazoxanide in patients with impaired hepatic and/or renal function have not been studied. Tizoxanide is highly bound to plasma protein (>99.9%). Therefore, caution should be used when administering this agent concurrently with other highly plasma protein–bound drugs that have narrow therapeutic indices, as competition for binding sites may occur. Oxamniquine  This tetrahydroquinoline derivative is an effective alternative agent for the treatment of S. mansoni, although susceptibil­ ity to this drug exhibits regional variation. Oxamniquine exhibits anti­ cholinergic properties, but its primary mode of action seems to rely on ATP-dependent enzymatic drug activation generating an intermediate that alkylates essential macromolecules, including DNA. In treated adult schistosomes, oxamniquine produces marked tegumental altera­ tions like those seen with praziquantel but that develop less rapidly, becoming evident 4–8 days after treatment. Oxamniquine is administered orally as a single dose and is well absorbed. Food retards absorption and reduces bioavailability. About 70% of an administered dose is excreted in urine as a mixture of pharmacologically inactive metabolites. Patients should be warned that their urine might have an intense orange-red color. Side effects are uncommon and usually mild, although hallucinations and seizures have been reported. Paromomycin (Aminosidine)  First isolated in 1956, this amino­ glycoside is an effective oral agent for the treatment of infections due to intestinal protozoa. Parenteral paromomycin appears to be effective against visceral leishmaniasis in India. Paromomycin inhibits protozoan protein synthesis by binding to the 30S ribosomal RNA in the aminoacyl-tRNA site, causing misreading

of mRNA codons. Paromomycin is less active against G. lamblia than standard agents; however, like other aminoglycosides, paromomycin is poorly absorbed from the intestinal lumen, and the high levels of drug in the gut compensate for this relatively weak activity. If absorbed or administered systemically, paromomycin can cause ototoxicity and nephrotoxicity. However, systemic absorption is very limited, and tox­ icity should not be a concern in persons with normal kidneys. Topical formulations are not generally available. Pentamidine Isethionate  This diamidine is an effective alterna­ tive agent for some forms of leishmaniasis and trypanosomiasis. It is available for parenteral and aerosolized administration. Although its mechanism of action remains undefined, it is known to exert a wide range of effects, including interaction with trypanosomal kinetoplast DNA; interference with polyamine synthesis by a decrease in the activ­ ity of ornithine decarboxylase; and inhibition of RNA polymerase, topoisomerase, ribosomal function, and the synthesis of nucleic acids and proteins. Pentamidine isethionate is well absorbed, highly tissue bound, and excreted slowly over several weeks, with an elimination half-life of 12 days. No steady-state plasma concentration is attained in persons given daily injections; the result is extensive accumulation of pentami­ dine in tissues, primarily the liver, kidney, adrenal gland, and spleen. Pentamidine does not penetrate well into the CNS. Pulmonary con­ centrations of pentamidine are increased when the drug is delivered in aerosolized form, but not when it is delivered systemically. Rapid (<1-h) infusion of intravenous pentamidine often results in hypotension. Because electrolyte disturbances and mild to moderate nephrotoxicity occur commonly, pentamidine should be used with caution with other nephrotoxic agents. Pancreatitis and QT prolonga­ tion may also occur; cumulative damage to pancreatic islet cells may result in drug-induced diabetes mellitus. Similarly, hypoglycemia can develop, although much less commonly when pentamidine is given by the inhaled route. Piperaquine  This bisquinoline was synthesized in the 1960s and used widely for malaria control in China until resistance emerged. The development of artemisinin-based combination therapy led to its evaluation as a partner drug, and it is now combined with dihydroarte­ misinin. Piperaquine is highly lipophilic and has a prolonged half-life (~20 days), thus providing a period of posttreatment prophylaxis. The drug’s mechanisms of action and resistance have not been well studied but are presumed to be similar to the other 4-aminoquinolines. Piperazine  The antihelminthic activity of piperazine is confined to ascariasis and enterobiasis. Piperazine acts as an agonist at extra­ synaptic γ-aminobutyric acid (GABA) receptors, causing an influx of chloride ions in the nematode somatic musculature. Although the ini­ tial result is hyperpolarization of the muscle fibers, the ultimate effect is flaccid paralysis, leading to the expulsion of live worms. Patients should be warned, as this occurrence can be unsettling. Praziquantel  This heterocyclic pyrazinoisoquinoline derivative is highly active against a broad spectrum of trematodes and cestodes. It is the mainstay of treatment for schistosomiasis and is a critical part of community-based control programs. All of the effects of praziquantel can be attributed either directly or indirectly to an alteration of intracellular calcium concentrations. Although the exact mechanism of action remains unclear, the major mechanism is disruption of the parasite tegument, causing tetanic contractures with loss of adherence to host tissues and, ultimately, disintegration or expulsion. Praziquantel induces changes in the anti­ genicity of the parasite by causing the exposure of concealed antigens. Praziquantel also produces alterations in schistosomal glucose metabo­ lism, including decreases in glucose uptake, lactate release, glycogen content, and ATP levels. Praziquantel exerts its parasitic effects directly and does not need to be metabolized to be effective. It is well absorbed but undergoes exten­ sive first-pass hepatic clearance. Levels of the drug are increased when it is taken with food, particularly carbohydrates, or with cimetidine.

Serum levels are reduced by glucocorticoids, chloroquine, carbam­ azepine, and phenytoin. Praziquantel is completely metabolized in humans, with 80% of the dose recovered as metabolites in urine within 4 days. It is not known to what extent praziquantel crosses the placenta, but retrospective studies suggest that it is safe in pregnancy.

Patients with schistosomiasis who have heavy parasite burdens may develop abdominal discomfort, nausea, headache, dizziness, and drows­ iness. Symptoms begin 30 min after ingestion, may require spasmolytics for relief, and usually disappear spontaneously after a few hours. Primaquine Phosphate  Primaquine, an 8-aminoquinoline, has a broad spectrum of activity against all stages of plasmodial development in humans but has been used most effectively for eradication of the hepatic stage of these parasites. Primaquine must be metabolized by the host to be effective. It is, in fact, rapidly metabolized; only a small fraction of the dose of the parent drug is excreted unchanged. Although the parasiticidal activity of the three oxidative metabolites remains unclear, they are believed to affect both pyrimidine synthesis and the mitochondrial electron transport chain. The metabolites appear to have significantly less antimalarial activity than primaquine; however, their hemolytic activity is greater than that of the parent drug. Primaquine causes marked hypotension after parenteral admin­ istration and therefore is given only by the oral route. It is rapidly and almost completely absorbed from the GI tract. Patients should be tested for G6PD deficiency before they receive primaquine. The drug may induce the oxidation of hemoglobin into methemoglobin, regardless of the G6PD status of the patient. Primaquine is otherwise well tolerated. CHAPTER 229 Proguanil (Chloroguanide)  Proguanil inhibits plasmodial dihy­ drofolate reductase and is used with atovaquone for oral treatment of uncomplicated malaria or with chloroquine for malaria prophylaxis in parts of Africa without widespread chloroquine-resistant P. falciparum. Proguanil exerts its effect primarily by means of the metabolite cycloguanil, whose inhibition of dihydrofolate reductase in the para­ site disrupts deoxythymidylate synthesis, thus interfering with a key pathway involved in the biosynthesis of pyrimidines required for nucleic acid replication. There are no clinical data indicating that folate supplementation diminishes drug efficacy; women of childbearing age for whom atovaquone/proguanil is prescribed should continue taking folate supplements to prevent neural tube birth defects. Agents Used to Treat Parasitic Infections Proguanil is extensively absorbed regardless of food intake. The drug is 75% protein bound. The main routes of elimination are hepatic biotransformation and renal excretion; 40–60% of the proguanil dose is excreted by the kidneys. Drug levels are increased and elimination is impaired in patients with hepatic insufficiency. Pyrantel Pamoate  Pyrantel is a tetrahydropyrimidine formulated as pamoate. This safe, well-tolerated, inexpensive drug is used to treat a variety of intestinal nematode infections but is ineffective in trichu­ riasis. Pyrantel pamoate is usually effective in a single dose. Its target is the nicotinic acetylcholine receptor on the surface of nematode somatic muscle. Pyrantel depolarizes the neuromuscular junction of the nematode, resulting in its irreversible paralysis and allowing the natural expulsion of the worm. Pyrantel pamoate is poorly absorbed from the intestine; >85% of the dose is passed unaltered in feces. The absorbed portion is metabolized and excreted in urine. Piperazine is antagonistic to pyrantel pamoate and should not be used concomitantly. Pyrantel pamoate has minimal toxicity at the oral doses used to treat intestinal helminthic infection. It is not recommended for pregnant women or for children <12 months old. Pyrimethamine  When combined with short-acting sulfon­ amides, this diaminopyrimidine is effective in malaria, toxoplas­ mosis, and isosporiasis. Unlike mammalian cells, the parasites that cause these infections cannot use preformed pyrimidines obtained through salvage pathways but rather rely completely on de novo pyrimidine synthesis, for which folate derivatives are essential cofac­ tors. The efficacy of pyrimethamine is increasingly limited by the

development of resistant strains of P. falciparum and P. vivax. Single amino acid substitutions to parasite dihydrofolate reductase confer resistance to pyrimethamine by decreasing the enzyme’s binding affinity for the drug.

Pyrimethamine is well absorbed; the drug is 87% bound to human plasma proteins. In healthy volunteers, drug concentrations remain at therapeutic levels for up to 2 weeks; drug levels are lower in patients with malaria. At the usual dosage, pyrimethamine alone causes little toxicity except for occasional skin rashes and, more rarely, blood dyscrasias. Bone marrow suppression sometimes occurs at the higher doses used for toxoplasmosis; at these doses, the drug should be administered with folinic acid. Pyronaridine  This potent antimalarial is a benzonaphthyridine derivative first synthesized by Chinese researchers in 1970. Like chloroquine, pyronaridine targets hematin formation, inhibiting the production of β-hematin by forming complexes with it, with conse­ quent enhancement of hematin-induced hemolysis. However, this drug is more potent than chloroquine: for complete lysis, pyronaridine is required at only 1/100th of the concentration needed with chloroquine. It also inhibits glutathione-dependent heme degradation. Despite its similar mode of action, pyronaridine remains effective against chloroquine-resistant strains. When combined with artesunate, it is effective for the treatment of acute, uncomplicated infection caused by P. falciparum or P. vivax in areas of low transmission with evidence of artemisinin resistance. Pyronaridine is readily absorbed, widely distributed throughout the body, metabolized by the liver, and excreted in urine and stool. Its use is contraindicated in patients with severe liver or kidney impairment. Pyronaridine inhibits both CYP2D6 and P-glycoprotein in vitro, and these effects may have clinical relevance for patients taking medica­ tions for cardiac disease (e.g., metoprolol and digoxin). PART 5 Infectious Diseases Quinacrine†  Despite being one of the first antimalarials, the anti­ protozoal mechanism of quinacrine has not been fully elucidated. It is generally considered to intercalate into DNA and thereby inhibit repli­ cation and transcription. The drug also inhibits adenosine uptake, ATP incorporation into RNA, and NADH oxidase—the same enzyme that activates furazolidone. The differing relative quinacrine uptake rate between human cells and G. lamblia may explain the selective toxicity of the drug. Resistance correlates with decreased drug uptake. Quinacrine is rapidly absorbed from the intestinal tract and is widely distributed in body tissues. Alcohol is best avoided because of a disulfiram-like effect. Although its production was discontinued in 1992, quinacrine can be obtained commercially from compounding pharmacies. Quinine and Quinidine  When combined with another agent, the cinchona alkaloid quinine is effective for the oral treatment of both uncomplicated, chloroquine-resistant malaria and babesiosis. Quinine acts rapidly against the asexual blood stages of all forms of the human malaria parasites. For severe malaria, only quinidine (the dextroisomer of quinine) is available in the United States. Quinine concentrates in the acidic food vacuoles of Plasmodium species. The drug inhibits the nonenzymatic polymerization of the highly reactive, toxic heme mol­ ecule into the nontoxic polymer pigment hemozoin. Quinine is readily absorbed when given orally. In patients with malaria, the elimination half-life of quinine increases according to the severity of the infection. However, toxicity is avoided by an increase in the concentration of plasma glycoproteins. The cinchona alkaloids are extensively metabolized, particularly by CYP3A4; only 20% of the dose is excreted unchanged in urine. The drug’s metabolites are also excreted in urine and may be responsible for toxicity in patients with renal failure. Renal excretion of quinine is decreased when cimetidine is taken and increased when the urine is acidic. The drug readily crosses the placenta. Quinidine is both more potent as an antimalarial and more toxic than quinine. Its use requires cardiac monitoring. Dose reduction is necessary in persons with severe renal impairment.

Spiramycin†  This macrolide is used to treat acute toxoplasmosis in pregnancy and congenital toxoplasmosis. While the mechanism of action is similar to that of other macrolides, the efficacy of spiramycin in toxoplasmosis appears to stem from its rapid and extensive intracel­ lular penetration, which results in macrophage drug concentrations 10–20 times greater than serum concentrations. Spiramycin is rapidly and widely distributed throughout the body and reaches concentrations in the placenta up to five times those in serum. This agent is excreted mainly in bile. Indeed, in humans, the urinary excretion of active compounds represents only 20% of the administered dose. Serious reactions to spiramycin are rare. Of the available macrolides, spiramycin appears to have the lowest risk of drug interactions. Compli­ cations of treatment are rare but, in neonates, can include life-threaten­ ing ventricular arrhythmias that disappear with drug discontinuation. Spiramycin is not formally approved for use in the United States, but it is accessible through a compassionate use program for toxoplasmosis in pregnancy through the FDA (301-796-1400). Sulfonamides  See Table 229-1 and Chap. 149. Suramin*  This derivative of urea is the drug of choice for the early stage of African trypanosomiasis. The drug is polyanionic and acts by forming stable complexes with proteins, thus inhibiting multiple enzymes essential to parasite energy metabolism. Suramin appears to inhibit all trypanosome glycolytic enzymes more effectively than it inhibits the corresponding host enzymes. Suramin is parenterally administered. It binds to plasma proteins and persists at low levels for several weeks after infusion. Its metabo­ lism is negligible. This drug does not penetrate the CNS. Tafenoquine  Tafenoquine is an 8-aminoquinoline with causal prophylactic activity. Its prolonged half-life (2–3 weeks) allows longer dosing intervals when the drug is used for prophylaxis. Tafenoquine has been well tolerated in clinical trials. When tafenoquine is taken with food, its absorption is increased by 50% and the most commonly reported adverse event—mild GI upset—is diminished. Like prima­ quine, tafenoquine is a potent oxidizing agent, causing hemolysis in patients with G6PD deficiency as well as methemoglobinemia. It has been commercially available since FDA approval in 2018. Tetracyclines  See Table 229-1 and Chap. 149. Thiabendazole  Discovered in 1961, thiabendazole remains one of the most potent of the numerous benzimidazole derivatives. However, its use has declined significantly because of a higher frequency of adverse effects than is seen with other, equally effective agents. Thiabendazole is active against most intestinal nematodes that infect humans. Although the exact mechanism of its antihelminthic activity has not been fully elucidated, it is likely to be similar to that of other benzimidazole drugs: namely, inhibition of polymerization of parasite β-tubulin. The drug also inhibits the helminth-specific enzyme fumarate reductase. In animals, thiabendazole has antiinflammatory, antipyretic, and analgesic effects, which may explain its usefulness in dracunculiasis and trichinellosis. Thiabendazole also suppresses egg and/or larval production by some nematodes and may inhibit the subsequent development of eggs or larvae passed in feces. Despite the emergence and global spread of thiabendazole-resistant trichostrongyliasis among sheep, there have been no reports of drug resistance in humans. Thiabendazole is available in tablet form and as an oral suspension. The drug is rapidly absorbed from the GI tract but can also be absorbed through the skin. Thiabendazole should be taken after meals. This agent is extensively metabolized in the liver before ultimately being excreted; most of the dose is excreted within the first 24 h. The usual dose of thiabendazole is determined by the patient’s weight, but some treatment regimens are parasite specific. No adjustments are recom­ mended in patients with renal or hepatic failure; only cautious use is advised. Coadministration of thiabendazole to patients taking theophylline can result in an increase in theophylline levels by >50%. Therefore,

124 - SECTION 18 Protozoal Infections

SECTION 18 Protozoal Infections

serum levels of theophylline should be monitored closely in this situation. Tinidazole  This nitroimidazole is effective for the treatment of amebiasis, giardiasis, and trichomoniasis. Like metronidazole, tini­ dazole must undergo reductive activation by the parasite’s metabolic system before it can act on protozoal targets. Tinidazole inhibits the synthesis of new DNA in the parasite and causes degradation of exist­ ing DNA. The reduced free-radical derivatives alkylate DNA, with consequent cytotoxic damage to the parasite. This damage appears to be produced by short-lived reduction intermediates, resulting in helix destabilization and strain breakage of DNA. The mechanism of action and side effects of tinidazole are similar to those of metronidazole, but adverse events appear to be less frequent and severe with tinidazole. In addition, the significantly longer half-life of tinidazole (>12 h) offers potential cure with a single dose. Tribendimidine  Tribendimidine, a diamidine derivative of ami­ nophenylamidine amidantel, is a cholinergic agonist that is selective for the nicotinic acetylcholine receptors of nematode muscle. This novel agent has a broad spectrum of activity against a wide variety of helminths and is highly effective against food-borne trematodes, with a similar cure rate to praziquantel. Clinical trials have demonstrated efficacy of a single dose alone or in combination with other helminthics against soil-transmitted helminth infections. The drug is an L-type nicotinic acetylcholine receptor agonist and exhibits the same method of action as levamisole and pyrantel; therefore, it may not be effective in regions where resistance to these agents is widespread. It is not avail­ able in the United States. Triclabendazole  In contrast to other benzimidazoles, the antihel­ minthic activity of triclabendazole is highly specific for Fasciola and Paragonimus species, with little activity against nematodes, cestodes, and other trematodes. Triclabendazole is effective against all stages of Fasciola species. The active sulfoxide metabolite of triclabendazole binds to fluke tubulin by assuming a unique nonplanar configuration and disrupts microtubule-based processes. Resistance to triclaben­ dazole in veterinary use has been reported in Australia and Europe; however, no resistance has been documented in humans. Triclabendazole is rapidly absorbed after oral ingestion; administra­ tion with food enhances its absorption and shortens the elimination half-life of the active metabolite. Both the sulfoxide and the sulfone metabolites are highly protein bound (>99%). Treatment with tricla­ bendazole is typically given in one or two doses. No clinical data are available regarding dose adjustment in renal or hepatic insufficiency; however, given the short course of therapy and extensive hepatic metabolism of triclabendazole, dose adjustment is unlikely to be neces­ sary. No information exists on drug interactions. Trimethoprim-Sulfamethoxazole  See Table 229-1 and Chap. 149. ■ ■FURTHER READING Dziduch K et al: The current directions of searching for antiparasitic drugs. Molecules 27:1534, 2022. Fehintola FA et al: Drug interactions in the treatment and chemo­ prophylaxis of malaria in HIV infected individuals in sub-Saharan Africa. Curr Drug Metab 12:51, 2011. Keiser J, Häberli C: Evaluation of commercially available anthelmin­ thics in laboratory models of human intestinal nematode infections. ACS Infect Dis 7:1177, 2021. Keiser J et al: Antiparasitic drugs for paediatrics: Systematic review, formulations, pharmacokinetics, safety, efficacy, and implications for control. Parasitology 138:1620, 2011. Kelesidis T, Falagas ME: Substandard/counterfeit antimicrobial drugs. Clin Microbiol Rev 28:443, 2015. Pink R et al: Opportunities and challenges in antiparasitic drug discovery. Nat Rev Drug Discov 4:727, 2005.

Section 18 Protozoal Infections Rosa M. Andrade, Sharon L. Reed

Amebiasis and Infection

with Free-Living

Amebae AMEBIASIS ■ ■DEFINITION Amebiasis is an infection caused by Entamoeba histolytica, an intestinal protozoan. Its spectrum of clinical syndromes ranges from asymptom­ atic colonization (90% of cases) to invasive amebiasis, which accounts for 10% of affected individuals. Invasive amebiasis frequently presents as intestinal colitis (dysentery or diarrhea) or as extraintestinal ame­ biasis, in which abscesses of the liver are more commonly found than involvement of the lungs or brain. ■ ■LIFE CYCLE AND TRANSMISSION E. histolytica is acquired by ingestion of viable cysts from fecally con­ taminated water, food, or hands (Fig. 230-1). Food-borne exposure is the most prevalent form of transmission. It occurs when food handlers are shedding cysts or food is being grown with feces-contaminated soil, fertilizer, or water. Less common means of transmission include oral and anal sexual practices and, in rare instances, direct rectal inoculation through colonic irrigation devices. Motile trophozoites are released from cysts in the small intestine and, in most patients, remain as harmless commensals in the large bowel. After encystation, infec­ tious cysts are shed in the stool and can survive for several weeks in a moist environment. In some patients, the trophozoites invade either the bowel mucosa, causing symptomatic colitis, or the bloodstream, causing distant abscesses of the liver, lungs, or brain. The trophozoites may not encyst in patients with active dysentery, and motile hematoph­ agous trophozoites are frequently present in fresh stools. Trophozoites are rapidly killed by exposure to air or stomach acid and therefore can­ not transmit infection. CHAPTER 230 Amebiasis and Infection with Free-Living Amebae
■ ■EPIDEMIOLOGY E. histolytica infection typically affects underdeveloped tropical regions with poor sanitation systems and hygiene, occurring often in children <5 years of age. This infection is widespread in the Indian subconti­ nent and Africa, parts of East Asia (Thailand), and Central and South America (Mexico and Colombia). According to the Global Burden of Disease 2016 study, amebiasis accounts for 26,748 all-age deaths, including 4567 children <5 years old. In contrast, returning travelers, recent immigrants, men who have sex with men (MSM), military personnel, and inmates of institutions are the main groups at risk for amebiasis in developed countries. The GeoSentinel Surveillance Network, which encompasses information from tropical medicine clinics on six continents, showed that, among long-term travelers (trip duration, >6 months), diarrhea due to E. histo­ lytica was among the most common diagnoses. In countries like Japan and Spain, amebiasis is emerging as a sexually transmitted disease, first reported in HIV-positive MSM. However, in Japan, E. histolytica infec­ tions, symptomatic or asymptomatic, seem to be steadily spreading among non-HIV men and women. Worldwide, E. histolytica is the second most common cause of death related to parasitic infection (after malaria). Invasive colitis and liver abscesses are tenfold more common among men than among women; this difference has been attributed to a disparity in complementmediated killing and effects of testosterone on the secretion of inter­ feron γ. The wide spectrum of clinical disease caused by Entamoeba is due in part to the differences between the two major infecting species,

125 - 230 Amebiasis and Infection with Free-Living Amebae

230 Amebiasis and Infection with Free-Living Amebae

serum levels of theophylline should be monitored closely in this situation. Tinidazole  This nitroimidazole is effective for the treatment of amebiasis, giardiasis, and trichomoniasis. Like metronidazole, tini­ dazole must undergo reductive activation by the parasite’s metabolic system before it can act on protozoal targets. Tinidazole inhibits the synthesis of new DNA in the parasite and causes degradation of exist­ ing DNA. The reduced free-radical derivatives alkylate DNA, with consequent cytotoxic damage to the parasite. This damage appears to be produced by short-lived reduction intermediates, resulting in helix destabilization and strain breakage of DNA. The mechanism of action and side effects of tinidazole are similar to those of metronidazole, but adverse events appear to be less frequent and severe with tinidazole. In addition, the significantly longer half-life of tinidazole (>12 h) offers potential cure with a single dose. Tribendimidine  Tribendimidine, a diamidine derivative of ami­ nophenylamidine amidantel, is a cholinergic agonist that is selective for the nicotinic acetylcholine receptors of nematode muscle. This novel agent has a broad spectrum of activity against a wide variety of helminths and is highly effective against food-borne trematodes, with a similar cure rate to praziquantel. Clinical trials have demonstrated efficacy of a single dose alone or in combination with other helminthics against soil-transmitted helminth infections. The drug is an L-type nicotinic acetylcholine receptor agonist and exhibits the same method of action as levamisole and pyrantel; therefore, it may not be effective in regions where resistance to these agents is widespread. It is not avail­ able in the United States. Triclabendazole  In contrast to other benzimidazoles, the antihel­ minthic activity of triclabendazole is highly specific for Fasciola and Paragonimus species, with little activity against nematodes, cestodes, and other trematodes. Triclabendazole is effective against all stages of Fasciola species. The active sulfoxide metabolite of triclabendazole binds to fluke tubulin by assuming a unique nonplanar configuration and disrupts microtubule-based processes. Resistance to triclaben­ dazole in veterinary use has been reported in Australia and Europe; however, no resistance has been documented in humans. Triclabendazole is rapidly absorbed after oral ingestion; administra­ tion with food enhances its absorption and shortens the elimination half-life of the active metabolite. Both the sulfoxide and the sulfone metabolites are highly protein bound (>99%). Treatment with tricla­ bendazole is typically given in one or two doses. No clinical data are available regarding dose adjustment in renal or hepatic insufficiency; however, given the short course of therapy and extensive hepatic metabolism of triclabendazole, dose adjustment is unlikely to be neces­ sary. No information exists on drug interactions. Trimethoprim-Sulfamethoxazole  See Table 229-1 and Chap. 149. ■ ■FURTHER READING Dziduch K et al: The current directions of searching for antiparasitic drugs. Molecules 27:1534, 2022. Fehintola FA et al: Drug interactions in the treatment and chemo­ prophylaxis of malaria in HIV infected individuals in sub-Saharan Africa. Curr Drug Metab 12:51, 2011. Keiser J, Häberli C: Evaluation of commercially available anthelmin­ thics in laboratory models of human intestinal nematode infections. ACS Infect Dis 7:1177, 2021. Keiser J et al: Antiparasitic drugs for paediatrics: Systematic review, formulations, pharmacokinetics, safety, efficacy, and implications for control. Parasitology 138:1620, 2011. Kelesidis T, Falagas ME: Substandard/counterfeit antimicrobial drugs. Clin Microbiol Rev 28:443, 2015. Pink R et al: Opportunities and challenges in antiparasitic drug discovery. Nat Rev Drug Discov 4:727, 2005.

Section 18 Protozoal Infections Rosa M. Andrade, Sharon L. Reed

Amebiasis and Infection

with Free-Living

Amebae AMEBIASIS ■ ■DEFINITION Amebiasis is an infection caused by Entamoeba histolytica, an intestinal protozoan. Its spectrum of clinical syndromes ranges from asymptom­ atic colonization (90% of cases) to invasive amebiasis, which accounts for 10% of affected individuals. Invasive amebiasis frequently presents as intestinal colitis (dysentery or diarrhea) or as extraintestinal ame­ biasis, in which abscesses of the liver are more commonly found than involvement of the lungs or brain. ■ ■LIFE CYCLE AND TRANSMISSION E. histolytica is acquired by ingestion of viable cysts from fecally con­ taminated water, food, or hands (Fig. 230-1). Food-borne exposure is the most prevalent form of transmission. It occurs when food handlers are shedding cysts or food is being grown with feces-contaminated soil, fertilizer, or water. Less common means of transmission include oral and anal sexual practices and, in rare instances, direct rectal inoculation through colonic irrigation devices. Motile trophozoites are released from cysts in the small intestine and, in most patients, remain as harmless commensals in the large bowel. After encystation, infec­ tious cysts are shed in the stool and can survive for several weeks in a moist environment. In some patients, the trophozoites invade either the bowel mucosa, causing symptomatic colitis, or the bloodstream, causing distant abscesses of the liver, lungs, or brain. The trophozoites may not encyst in patients with active dysentery, and motile hematoph­ agous trophozoites are frequently present in fresh stools. Trophozoites are rapidly killed by exposure to air or stomach acid and therefore can­ not transmit infection. CHAPTER 230 Amebiasis and Infection with Free-Living Amebae
■ ■EPIDEMIOLOGY E. histolytica infection typically affects underdeveloped tropical regions with poor sanitation systems and hygiene, occurring often in children <5 years of age. This infection is widespread in the Indian subconti­ nent and Africa, parts of East Asia (Thailand), and Central and South America (Mexico and Colombia). According to the Global Burden of Disease 2016 study, amebiasis accounts for 26,748 all-age deaths, including 4567 children <5 years old. In contrast, returning travelers, recent immigrants, men who have sex with men (MSM), military personnel, and inmates of institutions are the main groups at risk for amebiasis in developed countries. The GeoSentinel Surveillance Network, which encompasses information from tropical medicine clinics on six continents, showed that, among long-term travelers (trip duration, >6 months), diarrhea due to E. histo­ lytica was among the most common diagnoses. In countries like Japan and Spain, amebiasis is emerging as a sexually transmitted disease, first reported in HIV-positive MSM. However, in Japan, E. histolytica infec­ tions, symptomatic or asymptomatic, seem to be steadily spreading among non-HIV men and women. Worldwide, E. histolytica is the second most common cause of death related to parasitic infection (after malaria). Invasive colitis and liver abscesses are tenfold more common among men than among women; this difference has been attributed to a disparity in complementmediated killing and effects of testosterone on the secretion of inter­ feron γ. The wide spectrum of clinical disease caused by Entamoeba is due in part to the differences between the two major infecting species,

Necrotic abscess 5 Following GI infection (usually asymptomatic), trophozoites may invade through the blood stream, causing necrotic abscesses, particularly of the liver. Encystation occurs in the large intestines Trophozoites can invade the large bowel, causing flask-shaped ulcers and bloody diarrhea.

90% of patients are asymptomatically colonized but can still pass infectious cysts. PART 5 Infectious Diseases Trophozoite FIGURE 230-1  Life cycle of Entamoeba histolytica. GI, gastrointestinal; RBCs, red blood cells. E. histolytica and E. dispar. E. histolytica has unique surface antigens, is genetically distinct, and possesses virulence properties that distinguish it from the morphologically identical E. dispar. Most asymptomatic carriers, including MSM and patients with AIDS, harbor E. dispar and have self-limited infections. In this respect, E. dispar is dissimilar to other enteric pathogens such as Cryptospo­ ridium and Cystoisospora belli, which can cause self-limited illnesses in immunocompetent hosts but devastating diarrhea in patients with AIDS. These observations indicate that E. dispar is incapable of causing invasive disease. Through genomic sequencing, new species of Ent­ amoeba have been identified: E. moshkovskii and E. bangladeshi. These new species are microscopically indistinguishable from E. histolytica. Although E. moshkovskii causes diarrhea, weight loss, and colitis in mice, a prospective evaluation of children from the Mirpur community of Dhaka, Bangladesh, found that most children who had diarrheal dis­ eases associated with E. moshkovskii were simultaneously infected with at least one other enteric pathogen. E. bangladeshi nov. sp., Bangladesh was first reported in 2012 in this Bangladeshi community; however, it has been isolated in South African subjects of all ages in recent years. Additional clinical and epidemiologic studies are needed to discern the true role of E. bangladeshi in the human host. ■ ■PATHOGENESIS AND PATHOLOGY Both trophozoites and cysts are found in the intestinal lumen, but only trophozoites of E. histolytica invade tissue. The trophozoite is 20–60 μm in diameter and contains vacuoles and a nucleus with a characteristic central nucleolus. Trophozoites attach to colonic mucus and epithelial cells by Gal/GalNAc adherence lectin and release glycosidases and

Cysts and trophozoites are passed into soil or water. Cyst

Cyst are ingested in contaminated food or water. Stool Small intestines Large intestines

Excystation occurs in the small intestines, releasing a single motile trophozoite that colonizes the large bowel. RBCs proteases that cause degradation of mucus polymers. Extracellular cys­ teine proteinases degrade collagen, elastin, IgA, IgG, and the anaphyla­ toxins C3a and C5a. After disruption of the mucous layer, trophozoites damage the mucosa by contact-dependent and contact-independent cytotoxicity. The contact-dependent cytotoxicity is attributable to induction of apoptotic cell death; trogocytosis-mediated cell death (ingestion of fragments of living cells); and lysis of inflammatory cells (neutrophils, monocytes, and lymphocytes), colonic cells, and hepatic cells through release of phospholipase A and pore-forming peptides. Contact-independent cytotoxicity follows production of inflammatory mediators, such as prostaglandin E2, by trophozoites, ultimately lead­ ing to increased ion permeability of intercellular tight junctions. E. histolytica trophozoites are constantly exposed to reactive oxygen and nitrogen species arising from their own metabolism and from the host during tissue invasion. The ability to resist reactive oxygen species or reactive nitrogen species such as nitric oxide or S-nitrosothiols (e.g., S-nitrosoglutathione [GSNO] and S-nitrosocysteine [CySNO]) is also a virulence factor. Since E. histolytica lacks glutathione and glutathione reductase, it relies on its thioredoxin–thioredoxin reductase system to prevent, regulate, and repair the damage caused by oxidative stress. This antioxidant system is versatile: it has the ability to reduce reactive nitrogen species and use an alternative electron donor, such as nico­ tinamide adenine dinucleotide. In addition, trophozoites preincubated with E. coli 055 are more resistant to hydrogen peroxide–mediated killing by using bacterially produced malate dehydrogenase and oxalo­ acetate for protection from oxidative stress. Metronidazole, the current standard of therapy for amebiasis, seems to exert its antiparasitic effect through inhibition of this antioxidant system.

Phagocytosis is a virulence factor that leads to a defective prolifera­ tion of E. histolytica if inhibited. Trophozoites use membrane-associated carbohydrate-binding proteins to phagocytose intestinal bacteria, especially gram-negative Enterobacteriaceae, for their nutrients. Inter­ actions with commensal bacteria, such as Escherichia coli, can attenuate the virulence of E. histolytica by decreasing the expression of Gal/GalNAc lectin. In contrast, ingestion of enteropathogenic bacteria, such as enteropathogenic E. coli and Shigella dysenteriae, increases expression of the Gal/GalNAc lectin and enhances E. histolytica cyste­ ine protease activity. The interactions between E. histolytica and the gut microbiome can determine the severity of disease. During the first 2 years of life, the gut immune system and the microbiome mature rapidly. In one study, ~80% of children from the Bangladeshi community of Dhaka were found to be infected with E. histolytica by 2 years of age. Fecal anti–Gal/ GalNAc lectin IgA was associated with protection from reinfection, while a high parasite burden in the first year of life was associated with the expansion of Prevotella copri in their gut microbiome and pres­ ence of diarrhea. In adults, the diversity of the gut microbiome can determine the pathogenesis of disease. As shown in a Japanese cohort through next-generation sequencing analysis, the gut microbiome in asymptomatic E. histolytica carriers was more homogeneous, with high abundance of Ruminococcaceae, Coriobacteriaceae, and Clostridiaceae, while these bacteria were absent or in low abundance in patients with invasive disease (symptomatic). These observations suggest the protec­ tive role of these bacteria. Antimicrobial peptides, such as cathelicidins, are an important component of innate immunity and are induced by E. histolytica upon intestinal invasion in a mouse model. In this model, cecal cathelicidinrelated antimicrobial peptide mRNA increased by >4-fold at 3 days and >100-fold at 7 days. However, E. histolytica remained resistant to cathelicidin-mediated killing, probably because the antimicrobial peptide was digested by amebic cysteine proteinases. Other in vitro studies have shown that α- and β-defensins may damage E. histolytica membrane integrity. IgA plays a critical role in acquired immunity to E. histolytica. A study in Bangladeshi schoolchildren revealed that an intestinal IgA response to Gal/GalNAc reduced the risk of new E. histolytica infec­ tion by 64%. Serum IgG antibody is not protective; titers correlate with the duration of illness rather than with the severity of disease. Indeed, Bangladeshi children with a serum IgG response were more likely than those without such a response to develop new E. histolytica infection. In infants from this same Bangladeshi community, passive immunity conferred by maternal parasite-specific IgA via breastfeeding resulted in a 39% reduced risk of infection and a 64% reduced risk of diarrheal disease from E. histolytica during the first year of life. However, this protection appeared to be species-specific, with little or no protection conferred from infections with other species such as E. dispar or E. bangladeshi. This Bangladeshi cohort has furthered our understanding of the genetic susceptibility factors associated with E. histolytica disease. Heterozygosity of the major histocompatibility complex (MHC) class II allele DQB1∗0601 was found to protect against amebic intesti­ nal disease, which supports the role of antigen processing and CD4+ T cells in resistance to amebiasis. Adipocyte leptin receptors (LEPRs) are expressed on intestinal epithelial cells, prevent apoptosis, promote tissue repair, and may decrease neutrophil infiltration. In this cohort, a single amino acid substitution (Q223R) in LEPRs nearly quadrupled the risk for amebic intestinal disease in children and increased the risk for amebic liver abscesses in adults. Similarly, variations in the locus of cAMP-responsive element modulator/cullin 2 (CREM/CUL2) may increase the risk for diarrhea in children who acquired E. histolytica within their first year of life. Interestingly, both genetic variations, Q223R and CREM, are overrepresented in this geographical region. The earliest intestinal lesions are microulcerations of the mucosa of the cecum, sigmoid colon, or rectum that release erythrocytes, inflam­ matory cells, and epithelial cells. A colonoscopy reveals small ulcers with heaped-up margins and normal intervening mucosa (Fig. 230-2A). Submucosal extension of ulcerations under viable-appearing surface

A CHAPTER 230 Amebiasis and Infection with Free-Living Amebae
B FIGURE 230-2  Endoscopic and histopathologic features of intestinal amebiasis. A. Appearance of ulcers on colonoscopy (arrows). B. Inflammatory infiltrate and Entamoeba histolytica trophozoites (arrows) in invasive amebic colitis (hematoxylin and eosin). (Courtesy of the Department of Pathology and Gastroenterology, San Diego VA Medical Center.) mucosa causes the classic “flask-shaped” ulcer containing trophozoites at the margins of dead and viable tissues. Although neutrophilic infiltrates may accompany early lesions in animals, human intestinal infection is marked by a paucity of inflammatory cells, probably in part because of the killing of neutrophils by trophozoites (Fig. 230-2B). Treated ulcers characteristically heal with little or no scarring. Occasionally, however, full-thickness necrosis and perforation occur. Rarely, intestinal infection results in the formation of a mass lesion, or ameboma, in the bowel lumen. The overlying mucosa is usually thin and ulcerated, while other layers of the wall are thickened, edematous, and hemorrhagic; this condition results in exuberant formation of granulation tissue with little fibrous-tissue response. Amebic liver abscesses are age- and gender-dependent. Men 30–60 years of age are most commonly infected at a rate 10–12 times higher than women in the same age group. Studies in animal models have demonstrated that testosterone may increase susceptibility to amebic liver abscess by modulating the secretion of interferon γ by natural killer T cells, which are activated through E. histolytica lipopeptido­ phosphoglycan present on the surface of ameba trophozoites. Liver

abscesses are always preceded by intestinal colonization, which may be asymptomatic. Blood vessels may be compromised early by wall lysis and thrombus formation. Trophozoites invade veins to reach the liver through the portal venous system. E. histolytica is resistant to complement-mediated lysis—a property critical to survival in the bloodstream. Inoculation of amebae into the portal system of hamsters results in an acute cellular infiltrate consisting predominantly of neu­ trophils. Later, the neutrophils are lysed by contact with amebae, and the release of neutrophil toxins may contribute to necrosis of hepato­ cytes. The liver parenchyma is replaced by necrotic material that is sur­ rounded by a thin rim of congested liver tissue. Although the necrotic contents of a liver abscess are classically described as “anchovy paste,” the fluid is variable in color; it is composed of bacteriologically sterile granular debris with few or no cells. Amebae, if seen, tend to be found near the capsule of the abscess.

■ ■CLINICAL SYNDROMES Intestinal Amebiasis  The most common type of amebic infection is asymptomatic cyst passage (90% of patients). Even in highly endemic areas, most patients harbor E. dispar. Symptomatic amebic colitis develops 2–6 weeks after the ingestion of infectious cysts. A gradual onset of lower abdominal pain and mild diarrhea is followed by malaise, weight loss, and diffuse lower abdomi­ nal or back pain. Cecal involvement may mimic acute appendicitis. Patients with full-blown dysentery may pass 10–12 stools per day. The stools contain little fecal material and consist mainly of blood and mucus. In contrast to those with bacterial diarrhea, fewer than 40% of patients with amebic dysentery are febrile. Virtually all patients have heme-positive stools. PART 5 Infectious Diseases More fulminant intestinal infection, with severe abdominal pain, high fever, and profuse diarrhea, is rare and occurs predominantly in children. Patients may develop toxic megacolon, in which there is severe bowel dilation with intramural air. Patients receiving glucocor­ ticoids are at risk for severe amebiasis. The association between severe amebiasis complications and glucocorticoid therapy emphasizes the importance of excluding amebiasis when inflammatory bowel disease is suspected. An occasional patient presents with only an asymptomatic or tender abdominal mass caused by an ameboma, which is easily con­ fused with cancer on barium studies. A positive serologic test or biopsy can prevent unnecessary surgery in this setting. Environmental enteropathy (“impoverished gut”; blunted smallintestinal villi with lamina propria inflammation) is observed in tropi­ cal developing areas with endemic enteric infections, such as amebiasis. It is associated with functional gastrointestinal impairment causing malnutrition and stunted growth in children within the first 2 years of life. Bangladeshi children with symptomatic E. histolytica infections were 2.9 times more likely to be malnourished and 4.7 times more likely to be short for their age than were children without symptomatic infections. These factors affect their cognitive development and may be linked to loss of productivity in adulthood. Amebic Liver Abscess  Extraintestinal infection by E. histolytica most often involves the liver. Of travelers who develop an amebic liver abscess after leaving an endemic area, 95% do so within 5 months. Young patients with an amebic liver abscess are more likely than older patients to present in the acute phase with prominent symptoms of <10 days’ duration. Most patients are febrile and have right-upperquadrant pain, which may be dull or pleuritic in nature and may radiate to the shoulder. Point tenderness over the liver and right-sided pleural effusion are common. Jaundice is rare. Although the initial site of infection is the colon, fewer than one-third of patients with an amebic abscess have active diarrhea. Older patients from endemic areas are more likely to have a subacute course lasting 6 months, with weight loss and hepatomegaly. About one-third of patients with chronic presenta­ tions are febrile. Thus, the clinical diagnosis of an amebic liver abscess may be difficult to establish because the symptoms and signs are often nonspecific. Since 10–15% of patients present only with fever, amebic liver abscess must be considered in the differential diagnosis of fever of unknown origin (Chap. 22).

Complications of Amebic Liver Abscess  Pleuropulmonary involvement, which is reported in 20–30% of patients, is the most frequent complication of amebic liver abscess. Manifestations include sterile effusions, contiguous spread from the liver, and rupture into the pleural space. Sterile effusions and contiguous spread usually resolve with medical therapy, but frank rupture into the pleural space requires drainage. A hepatobronchial fistula may cause cough productive of large amounts of necrotic material that may contain amebae. This dra­ matic complication carries a good prognosis. Abscesses that rupture into the peritoneum may present as an indolent leak or an acute abdo­ men and require both percutaneous catheter drainage and medical therapy. Rupture into the pericardium, usually from abscesses of the left lobe of the liver, carries the gravest prognosis; it can occur during medical therapy and requires surgical drainage. Involvement of Other Extraintestinal Sites  The genitourinary tract may become involved by direct extension of amebiasis from the colon or by hematogenous spread of the infection. Painful genital ulcers, characterized by a punched-out appearance and profuse dis­ charge, may develop secondary to extension from either the intestine or the liver. Both of these conditions respond well to medical therapy. Cerebral involvement has been reported in fewer than 0.1% of patients in large clinical series. Symptoms and prognosis depend on the size and location of the lesion. ■ ■DIAGNOSTIC TESTS Laboratory Diagnosis  Stool examinations, serologic tests, and noninvasive imaging of the liver remain the most important pro­ cedures in the diagnosis of amebiasis in most parts of the world. Fecal findings suggestive of amebic colitis include a positive test for heme, a paucity of neutrophils, and amebic cysts or trophozoites. The definitive diagnosis of amebic colitis is made by the demonstration of hematophagous trophozoites of E. histolytica. Because trophozoites are killed rapidly by water, drying, or barium, it is important to examine at least three fresh stool specimens. Examination of a combination of wet mounts, iodine-stained concentrates, and trichrome-stained prepara­ tions of fresh stool and concentrates for cysts or trophozoites confirms the diagnosis in 75–95% of cases. Cultures of amebae are more sensi­ tive but are not routinely available. If stool examinations are negative, sigmoidoscopy with biopsy of the edge of ulcers may increase the yield, but this procedure is dangerous during fulminant colitis because of the risk of perforation. Trophozoites in a biopsy specimen from a colonic mass confirm the diagnosis of ameboma, but trophozoites are rare in liver aspirates because they are found in the abscess capsule and not in the readily aspirated necrotic center. Accurate diagnosis requires experience, since the trophozoites may be confused with neutrophils and the cysts must be differentiated morphologically from those of Entamoeba hartmanni, Entamoeba coli, and Endolimax nana, which do not cause clinical disease and do not warrant therapy. Unfortunately, the cysts of E. histolytica cannot be distinguished microscopically from those of E. dispar, E. moshkovskii, or E. bangladeshi. Therefore, the microscopic diagnosis of E. histolytica can be made only by the detec­ tion of Entamoeba trophozoites that have ingested erythrocytes. Serology is an important addition to the methods used for parasito­ logic diagnosis of invasive amebiasis. Enzyme-linked immunosorbent assays are positive in >90% of cases with colitis, ameboma, or liver abscess. Positive results in conjunction with the appropriate clinical syndrome suggest active disease because serologic findings usually revert to negative within 6–12 months. Even in highly endemic areas such as South Africa, fewer than 10% of asymptomatic individuals have a positive amebic serology. The interpretation of the indirect hemag­ glutination test is difficult because titers may remain positive for as long as 10 years. Early in infection, up to 10% of patients with acute amebic liver abscess may have negative serologic findings, which usually become positive within a week. In contrast to carriers of E. dispar, most asymp­ tomatic carriers of E. histolytica develop antibodies. Thus, serologic tests are helpful in assessing the risk of invasive amebiasis in asymp­ tomatic, cyst-passing individuals in nonendemic areas. Serologic tests

should also be performed in patients with ulcerative colitis before the institution of glucocorticoid therapy to prevent the development of severe colitis or toxic megacolon owing to unsuspected amebiasis. More sensitive, specific, and rapid tests in stool are now in use in most developed countries. Enzyme immunoassays using monoclo­ nal antibodies for the Gal/GalNAc lectin allow specific detection of

E. histolytica in 96-well plates or a single cassette for rapid detection. Sensitivity is 87% and specificity >90% in stool and can also be used in liver abscess aspirates. The number of commercially available PCR tests has significantly expanded. Most are multiplex for GI protozoa alone or include major bacterial and viral pathogens, as well. The sensitivity and specificity approaches 100%, and results can be available in as little as an hour. Their cost precludes use in most developing countries. Other amplification tests such as a loop-mediated isothermal amplification (LAMP) assay may be a potential alternative for direct detection of E. histolytica DNA in pus samples from amebic liver abscesses. LAMP is a relatively simple, rapid, and low-cost method of DNA amplification that could be a better alternative for diagnosis in developing countries. Routine hematology and chemistry tests usually are not very helpful in the diagnosis of invasive amebiasis. About three-fourths of patients with an amebic liver abscess have leukocytosis (>10,000 cells/μL); this condition is particularly likely if symptoms are acute or complica­ tions have developed. Invasive amebiasis does not elicit eosinophilia. Anemia, if present, is usually multifactorial. Even with large liver abscesses, liver enzyme levels are normal or minimally elevated. The alkaline phosphatase level is most often elevated and may remain so for months. Aminotransferase elevations suggest acute disease or a complication. Radiographic Studies  Radiographic barium studies are poten­ tially dangerous in acute amebic colitis. Amebomas are usually identi­ fied first by a barium enema, but biopsy is necessary for differentiation from carcinoma. Radiographic techniques such as ultrasonography, CT, and MRI are all useful for detection of the round or oval hypoechoic cyst. More than 80% of patients who have had symptoms for >10 days have a single abscess of the right lobe of the liver (Fig. 230-3). Approximately 50% of patients who have had symptoms for <10 days have multiple abscesses. Findings associated with complications include large abscesses (>10 cm)

in the superior part of the right lobe, which may rupture into the pleural space; multiple lesions, which must be differentiated from pyogenic abscesses; and lesions of the left lobe, which may rupture into the pericardium. Because abscesses resolve slowly and may increase in size despite a clinical response to therapy, frequent follow-up ultraso­ nography may prove confusing. Complete resolution of a liver abscess FIGURE 230-3  Abdominal CT scan of a large amebic abscess of the right lobe of the liver. (Courtesy of the Department of Radiology, UCSD Medical Center, San Diego; with permission.)

within 6 months can be anticipated in two-thirds of patients, but 10% may have persistent abnormalities for a year.

Differential Diagnosis  The differential diagnosis of intestinal amebiasis includes bacterial diarrheas (Chap. 138) caused by Campy­ lobacter (Chap. 169); enteroinvasive Escherichia coli (Chap. 166); and species of Shigella (Chap. 172), Salmonella (Chap. 171), and Vibrio (Chap. 173). Because the typical patient with amebic colitis has less prominent fever than in these other conditions as well as heme-positive stools with few neutrophils, correct diagnosis requires bacterial cul­ tures, microscopic examination of stools, amebic serologic testing, and stool specific antigens or PCR. As has been mentioned, amebiasis must be ruled out in any patient thought to have inflammatory bowel disease. Because of the variety of presenting signs and symptoms, amebic liver abscess can easily be confused with pulmonary or gallbladder dis­ ease or with any febrile illness with few localizing signs, such as malaria (Chap. 231) or typhoid fever (Chap. 171). The diagnosis should be considered in members of high-risk groups who have recently traveled outside the United States (Chap. 130) and in inmates of institutions. Once radiographic studies have identified an abscess in the liver, the most important differential diagnosis is between amebic and pyogenic abscess. Patients with pyogenic abscess typically are older and have a history of underlying bowel disease or recent surgery. Amebic serol­ ogy is helpful, but aspiration of the abscess, with Gram’s staining and culture of the material, may be required for differentiation of the two diseases. CHAPTER 230 TREATMENT Amebiasis INTESTINAL DISEASE (TABLE 230-1) The drugs used to treat amebiasis can be classified according to their primary site of action. Luminal amebicides are poorly absorbed and reach high concentrations in the bowel, but their activity is limited to cysts and trophozoites close to the mucosa. Only two luminal drugs are available in the United States: iodoquinol and paromo­ mycin. Indications for the use of luminal agents include eradication of cysts in patients with colitis or a liver abscess and treatment of asymptomatic carriers. The majority of asymptomatic individuals who pass cysts are colonized with E. dispar, which does not war­ rant specific therapy. However, it is prudent to treat asymptomatic individuals who pass cysts unless E. dispar colonization can be definitively demonstrated by specific antigen-detection tests. Amebiasis and Infection with Free-Living Amebae
Tissue amebicides reach high concentrations in the blood and tissue after oral or parenteral administration. The development of nitroimidazole compounds, especially metronidazole, was a major advance in the treatment of invasive amebiasis. Patients with ame­ bic colitis should be treated with IV or oral metronidazole. Side effects include nausea, vomiting, abdominal discomfort, and a disulfiram-like reaction. Another, longer-acting imidazole com­ pound, tinidazole, is likewise effective and is available in the United States. All patients should also receive a full course of therapy with TABLE 230-1  Drug Therapy for Amebiasis INDICATION THERAPY Asymptomatic carriage Luminal agent: iodoquinol (650-mg tablets), 650 mg tid for 20 days; or paromomycin (250-mg tablets), 500 mg tid for 10 days Acute colitis Metronidazole (250- or 500-mg tablets), 750 mg PO or IV tid for 5–10 days; or tinidazole, 2 g/d PO for 3 days plus Luminal agent as above Amebic liver abscess Metronidazole, 750 mg PO or IV for 5–10 days; or tinidazole, 2 g PO once; or ornidazole,a 2 g PO once plus Luminal agent as above aNot available in the United States.

a luminal agent, since metronidazole does not eradicate cysts. Resis­ tance to metronidazole has been selected in the laboratory but has not been found in clinical isolates. Relapses are not uncommon and probably represent reinfection or failure to eradicate amebae from the bowel because of an inadequate dosage or duration of therapy. AMEBIC LIVER ABSCESS Metronidazole is the drug of choice for amebic liver abscess. Longer-acting nitroimidazoles (tinidazole and ornidazole) have been effective as single-dose therapy in developing countries. With early diagnosis and therapy, mortality rates from uncomplicated amebic liver abscess are <1%. There is no evidence that combined therapy with two drugs is more effective than the single-drug regi­ men. Studies of South Africans with liver abscesses demonstrated that 72% of patients without intestinal symptoms had bowel infec­ tion with E. histolytica; thus, all treatment regimens should include a luminal agent to eradicate cysts and prevent further transmission. Amebic liver abscess recurs rarely.

More than 90% of patients respond dramatically to metronida­ zole therapy with decreases in both pain and fever within 72 h. Indica­ tions for aspiration of liver abscesses are (1) the need to rule out a pyogenic abscess, particularly in patients with multiple lesions; (2) the lack of a clinical response in 3–5 days; (3) the threat of imminent rupture; and (4) the need to prevent rupture of left-lobe abscesses into the pericardium. There is no evidence that aspira­ tion, even of large abscesses (up to 10 cm), accelerates healing. Percutaneous drainage may be successful even if the liver abscess has already ruptured. Surgery should be reserved for instances of bowel perforation and rupture into the pericardium. PART 5 Infectious Diseases ■ ■PREVENTION Amebic infection is spread by ingestion of food or water contaminated with cysts. Since an asymptomatic carrier may excrete up to 15 million cysts per day, prevention of infection requires adequate sanitation and eradication of cyst carriage. In high-risk areas, infection can be mini­ mized by the avoidance of unpeeled fruits and vegetables and the use of bottled water. Because cysts are resistant to readily attainable levels of chlorine, disinfection by iodination (tetraglycine hydroperiodide) is recommended. There is no effective prophylaxis. INFECTION WITH FREE-LIVING AMEBAE ■ ■EPIDEMIOLOGY There are multiple genera of free-living amebae, but the major human pathogens are Acanthamoeba, Naegleria, and Balamuthia. All of these parasites can cause serious central nervous system (CNS) infections, which are almost always fatal. Acanthamoeba and Naegleria are dis­ tributed throughout the world and have been isolated from a wide variety of fresh and brackish water, including water from taps, lakes, hot springs, swimming pools, heating and air-conditioning units, and hospital water networks, and even from the nasal passages of healthy children. Encystation may protect these protozoa from desiccation and food deprivation. The persistence of Legionella pneumophila in water supplies is attributable in part to chronic infection of free-living amebae, particularly Acanthamoeba. In vitro studies have suggested that several pathogens that can resist phagosome-mediated killing may be able to survive within water systems in free-living amebae. These pathogens include Pseudomonas aeruginosa, nontuberculous Myco­ bacteria (both slow-growing species—e.g., those in the Mycobacterium avium complex, M. kansasii, and M. gordonae—and rapid-growing species—e.g., M. chelonae and M. abscessus), and viruses such as adenoviruses and echoviruses. In contrast, the environmental niche of free-living amebae of the genus Balamuthia appears to be soil. A soil sample from a flowerpot was linked to a fatal infection in a child. Cases have been reported from all continents except Africa, but the majority of cases are from warm, dry areas of the southwestern United States and Latin America. With better recognition of these pathogens, additional risk fac­ tors have been identified. Since 2010, five cases of Naegleria fowleri

infection have been reported in northern U.S. states and have been associated with exposure to piped water, which represents a new eco­ logic niche. Since 2009, three clusters of Balamuthia mandrillaris infec­ tions have been associated with organ transplantation. Acanthamoeba species have caused large outbreaks of microbial keratitis associated with contact lens wear. ■ ■NAEGLERIA INFECTIONS Primary amebic meningoencephalitis (PAM) is a fulminant CNS infec­ tion caused by the free-living ameba Naegleria fowleri, which thrives in warm freshwater of lakes and rivers. In the United States, 157 cases of PAM were reported from 1962 through 2022 for a total of 381 cases worldwide. Historically in southern states and more frequently during the summer, the number of infections reported annually has remained stable (0–8). However, cases in northern states have been on the rise. In 2010, a PAM case was reported for the first time from Minnesota; this case was followed by additional cases from Minnesota, Indiana, and Kansas in 2011 and 2012. With climate change, other areas may be at risk because of higher temperatures. It commonly affects healthy, male individuals (75%) with a median age of 14 years (range, 1 month–

85 years). Most patients (91%) were exposed to recreational freshwa­ ter from lakes, reservoirs, rivers, streams, or ditches. The remaining cases were due to tap-water exposure through nasal irrigation with a neti pot, playing on a backyard waterslide, and swimming in a poorly maintained pool. Despite evidence of extra-CNS dissemination of N. fowleri in PAM patients post-mortem, transplantation of their solid organs have not been associated with PAM in transplant recipients. The risk of PAM in transplant recipients needs to be carefully con­ sidered in individual cases. PAM occurs after N. fowleri trophozoites enter the CNS through the olfactory neuroepithelium following inhalation of water or dust contaminated with trophozoites or cysts. Trophozoites then migrate to the brain, avoiding the protective blood-brain barrier and invoking a neutrophilic response. After an incubation period of 2–15 days, severe headache, high fever, nausea, vomiting, and meningismus develop. Photophobia and palsies of the third, fourth, and sixth cranial nerves are common. Rapid progression to seizures and coma may follow. The prognosis is uniformly poor: most patients die within a week. The diagnosis of Naegleria infection should be considered in any patient who has purulent meningitis without evidence of bacteria on Gram’s staining, antigen detection assay, and culture. Other labora­ tory findings resemble those for fulminant bacterial meningitis, with elevated intracranial pressure, high white blood cell counts (up to 20,000/μL), and elevated protein concentrations and low glucose levels in cerebrospinal fluid (CSF). Diagnosis depends on the detection of motile trophozoites in wet mounts of fresh spinal fluid. Antibodies to Naegleria species have been detected in healthy adults; thus, serologic testing is not useful in the diagnosis of acute infection. Diagnostic PCR and histochemical staining of biopsies are available through the CDC. A number of antimicrobial agents have in vitro activity against N. fowleri, but the prognosis remains poor. The few survivors have been treated with different combinations of amphotericin B, fluconazole, azithromycin, rifampin, and dexamethasone. The new antiparasitic agent miltefosine—an alkylphosphocholine compound used to treat breast cancer and visceral leishmaniasis—is active in vitro against Nae­ gleria, Acanthamoeba, and Balamuthia and is now commercially avail­ able (impavido.com). All seven survivors among reported worldwide cases (381) included miltefosine as well as the five drugs above. Early diagnosis, prompt combination therapy including miltefosine, and aggressive management of neurologic complications, including thera­ peutic hypothermia, are important factors in better outcomes. A clini­ cian whose patient may have PAM should contact the CDC Emergency Operations Center at (770) 488-7100 for assistance in diagnosis by PCR and treatment recommendations (which should include miltefosine). ■ ■ACANTHAMOEBA INFECTIONS Granulomatous Amebic Encephalitis  Infection with at least eight different Acanthamoeba species follows a more indolent course

than Naegleria infection and typically occurs in chronically ill or debilitated patients. Risk factors include lymphoproliferative disor­ ders, chemotherapy, glucocorticoid therapy, lupus erythematosus, post-transplant, and AIDS. Infection usually reaches the CNS hema­ togenously from a primary focus in the sinuses, skin, or lungs. In the CNS, the onset is insidious, and the syndrome often mimics a spaceoccupying lesion. Altered mental status, headache, and stiff neck may be accompanied by focal findings such as cranial nerve palsies, ataxia, and hemiparesis. Cutaneous ulcers or hard nodules containing amebae are frequently detected in AIDS patients with disseminated Acantham­ oeba infection. Examination of the CSF for trophozoites may be diagnostically help­ ful, but lumbar puncture may be contraindicated because of increased intracerebral pressure. CT frequently reveals cortical and subcorti­ cal lesions of decreased density consistent with embolic infarcts. In other patients, multiple enhancing lesions with edema may mimic the CT appearance of toxoplasmosis (Chap. 235). Demonstration of the trophozoites and cysts of Acanthamoeba on wet mounts or in biopsy specimens establishes the diagnosis. Culture on nonnutrient agar plates seeded with Escherichia coli also may be helpful. A real-time PCR assay to detect Acanthamoeba, Naegleria, and Balamuthia as well as a fluo­ rescein-labeled antiserum for the detection of protozoa in biopsy speci­ mens are available from the CDC. Granulomatous amebic encephalitis in patients with AIDS or transplant recipients may have an accelerated course (with survival for only 3–40 days) because of the difficulty these individuals have in forming granulomas. Various antimicrobial agents have been used to treat Acanthamoeba infection, but miltefosine should be included in combination therapy. Keratitis  The incidence of keratitis caused by Acanthamoeba has increased in the past 20 years, in part as a result of improved diag­ nosis. Earlier infections were associated with trauma to the eye and exposure to contaminated water. At present, most infections are linked to extended-wear contact lenses, and rare cases are associated with laser-assisted in situ keratomileusis (LASIK). Risk factors include the use of homemade saline, the wearing of lenses while swimming, and inadequate disinfection. Since contact lenses presumably cause micro­ scopic trauma, early corneal findings may be nonspecific. The first symptoms usually include tearing and the painful sensation of a foreign body. Once infection is established, progression is rapid. The character­ istic clinical sign is an annular, paracentral corneal ring representing a corneal abscess. Deeper corneal invasion and loss of vision may follow. The differential diagnosis includes bacterial, mycobacterial, and herpetic infection. The irregular polygonal cysts of Acanthamoeba (Fig. 230-4) may be identified in corneal scrapings or biopsy mate­ rial, and trophozoites can be grown on special media. Cysts are resistant to available drugs, and the results of medical therapy have been disappointing. Some reports have suggested partial responses FIGURE 230-4  Double-walled cyst of Acanthamoeba castellani, as seen by phasecontrast microscopy. (From DJ Krogstad et al, in A Balows et al [eds]: Manual of Clinical Microbiology, 5th ed. Washington, DC, American Society for Microbiology, 1991.)

to polyhexamethylene biguanide (0.2%) or propamidine isethionate (0.1%) eyedrops. Severe infections usually require keratoplasty.

■ ■BALAMUTHIA INFECTIONS Balamuthia mandrillaris is a free-living ameba that was first identified in 1986 as the cause of a fatal infection in a mandrill baboon at the Wild Animal Park in San Diego, California. Since then more than 100 cases have been reported in the United States and 200 worldwide. The para­ site has been isolated from soil and dust and is probably widespread in the environment. It is an important etiologic agent of granulomatous amebic encephalitis, cutaneous lesions, and sinus infections in humans. The potential risk factors for granulomatous amebic encephalitis iden­ tified by the California Encephalitis Project include young age, immu­ nocompromising conditions, and Hispanic ethnicity. The infection likely starts with percutaneous or mucous membrane exposure and then spreads hematogenously to the brain and other organs—a pattern that explains the risk for transmission through organ transplantation. In 2009–2012, three clusters of organ transplant–transmitted B. man­ drillaris infections were detected by recognition of severe unexpected illness in multiple recipients from the same donor after an incubation period of 17–24 days. Frequently, Balamuthia affects immunocompetent individuals, in whom the course is typically subacute, with focal neurologic signs, fever, seizures, and headaches leading to death within 1 week to sev­ eral months after onset. Skin lesions may occur on the face, trunk, or extremities. In addition to dust inhalation, inoculation of trophozoites or cysts from stagnant water may occur through open wounds or mucous membranes. Diagnosis relies on examination of CSF, which reveals mononuclear or neutrophilic pleocytosis, elevated protein levels, and normal to low glucose concentrations. Amebae are rarely isolated from CSF and require culture in the presence of cell monolay­ ers. Multiple hypodense lesions are usually detected with imaging stud­ ies (Fig. 230-5). The differential diagnosis includes tuberculomas (Chap. 183) and neurocysticercosis (Chap. 242). Testing for Bala­ muthia is part of the multiplex PCR test from the CDC. Fluorescent antibodies for serology and immunohistochemistry also are available. CHAPTER 230 Amebiasis and Infection with Free-Living Amebae
The nine surviving patients in the United States have been treated with a variety of drugs, including pentamidine, flucytosine, FIGURE 230-5  Brain MRI of amebic meningoencephalitis due to Balamuthia mandrillaris. A large lesion in the parieto-occipital lobe and other smaller lesions are seen. (Courtesy of the Department of Radiology, UCSD Medical Center, San Diego.)

126 - 231 Malaria

231 Malaria

sulfadiazine, and macrolides. The CDC recommends that miltefosine now be included, as for treatment of other free-living amebae. Clini­ cians should contact the CDC Emergency Operations Center at (770)-488-7100 for assistance in diagnosis and treatment.

■ ■FURTHER READING Amebiasis Shirley DAT et al: A review of the global burden, new diagnostics, and current therapeutics for amebiasis. Open Forum Infect Dis 5:ofy161, 2018. Uddin MJ et al: Host protective mechanisms to intestinal amebiasis. Trends Parasitol 37:165, 2021. Yanagawa Y et al: Clinical features and gut microbiome of asymptom­ atic Entamoeba histolytica infection. Clin Infect Dis 73:e3163, 2021. Yanagawa Y et al: Diversity of plasticity of virulent characteristics of Entamoeba histolytica. Trop Med and Infect Dis 8:255, 2023. Free-Living Amebae Cope JR et al: The epidemiology and clinical features of Balamuthia mandrillaris disease in the United States, 1974–2016. Clin Infect Dis 68:1815, 2019. Gharpure R et al: Epidemiology and clinical characteristics of pri­ mary amebic meningoencephalitis caused by Naegleria fowleri: A global review. Clin Infect Dis 73:e19, 2021. Haston JC et al: The epidemiology and clinical features of non-keratitis Acanthamoeba infections in the United States, 1956–2020. Open Forum Infect Dis 10:ofac682, 2023. PART 5 Infectious Diseases Elizabeth A. Ashley, Nicholas J. White

Malaria Humanity has but three great enemies: Fever, famine, and war; of these by far the greatest, by far the most terrible, is fever. —William Osler, 1896 Malaria is a protozoan disease transmitted by the bite of infected female Anopheles mosquitoes and is the most important of the para­ sitic diseases of humans. In 2022, there were an estimated 249 million cases in 85 malaria-endemic countries and 608,000 deaths (i.e., ~1660 deaths each day). Two countries had an estimated 43% of these deaths: TABLE 231-1  Characteristics of Plasmodium Species Infecting Humans CHARACTERISTIC P. FALCIPARUM P. VIVAX P. OVALEa P. MALARIAE P. KNOWLESI Duration of intrahepatic phase (days) 5.5

5.5 Number of merozoites released per infected hepatocyte 30,000 10,000 15,000 15,000 20,000 Approximate duration of erythrocytic cycle (h)

Red cell preference Younger cells (but can invade cells of all ages) Reticulocytes and cells up to 2 weeks old Morphology Usually only ring forms; banana-shaped gametocytes Irregularly shaped large rings and trophozoites; enlarged erythrocytes; Schüffner’s dots Pigment color Black Yellow-brown Dark brown Brown-black Dark brown Ability to cause relapses No Yes Yes No No aGenomic studies have revealed P. ovale to be two sympatric species: P. ovale curtisi and P. ovale wallikeri, which are morphologically very similar but may have different incubation periods and latencies. bAlso known as James’s dots.

Nigeria (31%) and Democratic Republic of the Congo (12%). Malaria was eliminated from the United States, Canada, Europe, and Russia

50 years ago, but its prevalence rose in many parts of the tropics between 1970 and 2000. In response to this rise, there was substantial investment aimed at increasing access to accurate diagnosis, effective treatments, and insecticide-treated bed nets. Between 2000 and 2015, malaria mortality rates decreased dramatically as a result of highly effective control programs in several countries; since then, however, progress has reversed and estimated global case numbers, mainly in sub-Saharan Africa, have risen steadily. Meanwhile an increasing num­ ber of countries that had low malaria transmission are now targeting malaria elimination. This ambitious goal is threatened by increasing resistance to antimalarial drugs and insecticides. Malaria remains today, as it has been for centuries, a heavy burden on tropical communities, a threat to nonendemic countries, and a danger to travelers. ETIOLOGY AND PATHOGENESIS Six species of the genus Plasmodium cause nearly all malarial infections in humans. These are P. falciparum, P. vivax, two morphologically iden­ tical sympatric species of P. ovale (curtisi and wallikeri), P. malariae, and—in Southeast Asia—the monkey malaria parasite P. knowlesi (Table 231-1). Occasionally humans are also infected with the monkey parasites P. simium (South America) and P. cynomolgi (Southeast Asia). While almost all deaths are caused by falciparum malaria, P. knowlesi and occasionally P. vivax can also cause severe illness. Human infec­ tion begins when a female anopheline mosquito inoculates plasmodial sporozoites from its salivary glands during a blood meal (Fig. 231-1). These microscopic motile forms of the malaria parasite are carried rapidly via the bloodstream to the liver, where they invade hepatic parenchymal cells and begin a period of asexual reproduction. By this amplification process (known as intrahepatic or preerythrocytic schizogony), a single sporozoite may produce from 10,000 to >30,000 daughter merozoites. These few swollen infected liver cells containing the hepatic schizonts eventually burst, discharging motile merozoites into the bloodstream. The merozoites then invade red blood cells (RBCs) to become trophozoites and, in nonimmune subjects, multiply 6- to 20-fold every 48 h (P. knowlesi, 24 h; P. malariae, 72 h). When the parasites reach densities of ~50/μL of blood (~100 million parasites in total in the blood of an adult), the symptomatic stage of the infec­ tion begins. In P. vivax and P. ovale infections, a proportion of the intrahepatic forms do not divide immediately but remain inert for a period ranging from 2 weeks to ≥1 year. These dormant forms, or hypnozoites, are the cause of the relapses that characterize infection with these species. Attachment of merozoites to erythrocytes is mediated via a com­ plex interaction with several different binding ligands and specific erythrocyte surface receptors. P. falciparum merozoites bind via FINDING FOR INDICATED SPECIES Reticulocytes Older cells Younger cells Infected erythrocytes, enlarged and oval with tufted ends; Schüffner’sb dots Band or rectangular forms of trophozoites common Resembles P. falciparum (early trophozoites) or P. malariae (later trophozoites, including band forms)

Sporozoites Liver Ookinete Merozoites Zygote RBC Gamete In mosquito gut Schizont Gametocytes FIGURE 231-1  The malaria transmission cycle from mosquito to human and targets of immunity. In Plasmodium vivax and P. ovale infections some liver stage parasites remain dormant (“hypnozoites”) and awake weeks or months later to cause relapses. RBC, red blood cell. erythrocyte- binding antigen 175 to glycophorin A and via EBA-140 to glycophorin C. The other glycophorins (B and D) also contribute. The merozoite reticulocyte-binding protein homologue 5 (PfRh5) plays a critical role binding to red cell basigin (CD147, EMMPRIN). P. vivax binds to receptors on developing erythrocytes. The Duffy blood-group antigen Fya or Fyb plays an important role in invasion of P. vivax and P. knowlesi. Most West Africans and people with origins in that region have the Duffy-negative FyFy phenotype and are generally resistant to P. vivax malaria. During the first few hours of intraerythrocytic development, the small “ring forms” of the different malaria species appear similar under light microscopy. As the trophozoites enlarge, species-specific characteristics become evident, malaria pigment (hemozoin) becomes visible, and the parasite assumes an irregular or ameboid shape. By the Predominant species circulating P. faIciparum P. knowlesi P. vivax P. falciparum + P. vivax FIGURE 231-2  Malaria-endemic countries showing predominant Plasmodium species. Plasmodium vivax is common in the Horn of Africa and in Mauritania but relatively unusual elsewhere in the continent.

end of the intraerythrocytic life cycle, the parasite has consumed two-thirds of the RBC’s hemoglobin and has grown to occupy most of the cell. It is now called a schizont. Multiple nuclear divisions have taken place (schizogony or merog­ ony). The infected RBC then ruptures to release 6–30 daughter merozoites, each potentially capable of invading a new RBC and repeating the cycle. The disease in human beings is caused by the direct effects of the asexual parasite— RBC invasion and destruction—and by the host’s reaction. Some of the bloodstage parasites develop into morpho­ logically distinct, longer-lived sexual forms (gametocytes) that can transmit malaria. In falciparum malaria, a delay of several asexual cycles precedes this switch to gametocytogenesis. Female gametocytes typically outnumber males by 4:1. Immature (stage I to IV) game­ tocytes are sequestered preferentially in the bone marrow and are also found in the spleen, brain, and intestine.

Pre-erythrocytic Antibodies to sporozoites block invasion of hepatocytes CD4+ and CD8+ T cells kill intrahepatic parasites Antibodies to merozoites block invasion of RBCs Antibodies to malaria “toxins” Asexual erythrocytic Transmission Antibodies to parasite antigens on infected RBCs block cytoadherence to endothelium and augment splenic clearance Cell-mediated immunity and antibody-dependent cytotoxicity kill intraerythocytic parasites Antibodies block fertilization, development, and invasion After being ingested in the blood meal of a biting female anopheline mos­ quito, the male gametocyte exflagellates and divides rapidly into eight motile male gametes. These fuse with female gametocytes, undergoing two rounds of sexual division (meiosis) to form a zygote in the insect’s midgut. This zygote matures into an ookinete, which penetrates and encysts in the mosquito’s gut wall. The resulting oocyst expands by asexual division until it bursts to liberate myriad motile sporozoites, which then migrate in the hemolymph to the salivary gland of the mosquito to await inoculation into another human at the next feed, thus completing the parasite’s life cycle. CHAPTER 231 Malaria EPIDEMIOLOGY Malaria occurs throughout most of the tropical regions of the world (Fig. 231-2). P. falciparum predominates in Africa, New Guinea, and Hispaniola (i.e., the Dominican Republic and Haiti); P. vivax is more

common in Central and South America and most of Asia. The preva­ lence of these two species is approximately equal in Oceania. P. malariae is found in most endemic areas, especially throughout sub-Saharan Africa, but is much less common. P. ovale is relatively unusual outside of Africa. In endemic areas, submicroscopic asymptomatic infections (detectable by polymerase chain reaction [PCR]) with all human malaria parasites are much more common than microscopy-detected infections. P. knowlesi causes human infections commonly on the island of Borneo and, to a lesser extent, elsewhere in Southeast Asia, where the main hosts, long-tailed and pig-tailed macaques, are found.

The epidemiology of malaria is complex and may vary considerably even within relatively small geographic areas. Endemicity traditionally has been defined in terms of rates of microscopy-detected parasitemia or palpable spleens in children 2–9 years of age and has been classi­ fied as hypoendemic (<10%), mesoendemic (11–50%), hyperendemic (51–75%), and holoendemic (>75%). In holo- and hyperendemic areas (e.g., certain regions of tropical Africa or coastal New Guinea) where there is intense P. falciparum transmission, people may sustain one or more infectious mosquito bites per week and are infected repeatedly throughout their lives. In such settings, malaria morbidity and mortal­ ity are substantial during early childhood. Immunity against disease is hard won in these areas following repeated symptomatic infections in childhood, but, if the child survives, infections become increasingly asymptomatic. These asymptomatic older children and adults are a major source of malaria transmission. As control measures progress and urbanization expands, environmental conditions become less conducive to malaria transmission, and all age groups may lose protec­ tive immunity and become susceptible to illness. Constant, frequent, year-round infection is termed stable transmission. In areas where transmission is low, erratic, or focal, full protective immunity is not acquired, and symptomatic disease may occur at all ages. This situation usually exists in hypoendemic areas and is termed unstable transmis­ sion. Even in stable transmission areas, there is often an increased incidence of symptomatic malaria during the rainy season coincid­ ing with increased mosquito breeding and transmission. Malaria can behave like an epidemic disease in some areas, particularly those with unstable malaria, such as northern India (the Punjab region), the Horn of Africa, Rwanda, Burundi, southern Africa, and Madagascar. Epi­ demics may occur when changes in environmental, economic, or social conditions (e.g., heavy rains following drought or migration—usually of refugees or workers—from a nonmalarious region to an area of high transmission) are compounded by relaxation or breakdown in malaria control and prevention services caused by underinvestment, war, or civil disorder. Epidemics often result in high mortality rates among all age groups. The principal determinants of the epidemiology of malaria are the number (density), the human-biting habits, and the longevity of the anopheline mosquito vectors. More than 100 of the >400 anoph­ eline species can transmit malaria, but the ~40 species that do so com­ monly vary considerably in their efficiency as malaria vectors. More specifically, the transmission of malaria is directly proportional to the density of the vector, the square of the number of human bites per day per mosquito, and the tenth power of the probability of the mosquito’s surviving for 1 day. Mosquito longevity is particularly important as a determinant of malaria transmissibility because the parasite’s develop­ ment within the mosquito—from gametocyte ingestion to subsequent inoculation (sporogony)—lasts 8–30 days, depending on ambient temperature. In order to transmit malaria, the vector mosquito must therefore survive for >7 days. Sporogony is not completed at cooler temperatures—i.e., <16°C (<60.8°F) for P. vivax and <21°C (<69.8°F) for P. falciparum; thus, transmission does not occur below these tem­ peratures or at high altitudes. Global warming is resulting in malaria transmission at higher altitudes than previously. The most effective mosquito vectors of malaria are those, such as the Anopheles gambiae species complex in Africa, that are long-lived, occur in high densities in tropical climates, breed readily, and bite humans in preference to other animals. The entomologic inoculation rate (i.e., the number of sporo­ zoite-positive mosquito bites per person per year) is the most common measure of malaria transmission and varies from <1 in some parts of Latin America and Southeast Asia to >300 in parts of tropical Africa. PART 5 Infectious Diseases

PATHOPHYSIOLOGY ■ ■ERYTHROCYTE CHANGES After invading an erythrocyte, the growing malarial parasite pro­ gressively consumes and degrades intracellular proteins, principally hemoglobin. The potentially toxic heme is detoxified by lipid-mediated crystallization to biologically inert hemozoin (malaria pigment). The parasite also alters the RBC membrane by changing its transport prop­ erties, exposing cryptic surface antigens, and inserting new parasitederived proteins. The RBC becomes more irregular in shape, more antigenic, and less deformable. In P. falciparum infections, membrane protuberances appear on the erythrocyte’s surface 12–15 h after cell invasion. These “knobs” extrude a high-molecular-weight, antigenically variant, strain-specific erythro­ cyte membrane adhesive protein (PfEMP1) that mediates attachment to receptors on venular and capillary endothelium (cytoadherence). Several vascular receptors have been identified; intercellular adhesion molecule 1 and endothelial protein C receptor are important in the brain, VAR2CSA (which binds chondroitin sulfate A) predominates in the placenta, and CD36 binds parasitized RBCs in most other organs. Erythrocytes containing more mature parasites stick inside and even­ tually block capillaries and venules. Infected RBCs may also adhere to uninfected RBCs (to form rosettes) and to other parasitized erythro­ cytes (agglutination). The processes of cytoadherence, rosetting, and agglutination are central to the pathogenesis of falciparum malaria. They result in the sequestration of infected RBCs in vital organs (particularly the brain), where they interfere with microcirculatory flow and metabolism. Sequestered P. falciparum parasites continue to develop out of reach of the principal host defense mechanism: splenic processing and filtration. As a consequence, only the younger ring forms of the asexual P. falciparum parasites circulate in the peripheral blood, and the level of peripheral parasitemia variably underestimates the true number of parasites within the body. In severe malaria, unin­ fected erythrocytes also become less deformable, which compromises their passage through the partially obstructed capillaries and venules and shortens their survival. In the other human malarias, significant sequestration does not occur, and all stages of the parasite’s development are evident on peripheral-blood smears. P. vivax and P. ovale show a marked predilec­ tion for young RBCs and P. malariae for old cells; these species produce a level of parasitemia that seldom exceeds 2%. In contrast, P. falciparum can invade erythrocytes of all ages and may be associated with very high parasite densities. Dangerously high parasite densities may also occur in P. knowlesi infections, with rapid increases as a result of the shorter (24-h) asexual life cycle. ■ ■HOST RESPONSE Initially, the host responds to malaria infection by activating nonspe­ cific defense mechanisms. Splenic immunologic and filtrative clearance functions are augmented, and the removal of both parasitized and uninfected erythrocytes is accelerated. The spleen also removes dam­ aged ring-form parasites (a process known as “pitting”) from within the red cell and returns the once-infected cells back to the circula­ tion, where their survival is shortened. The parasitized cells escaping splenic removal are destroyed when the schizont ruptures. The mate­ rial released induces monocyte/macrophage activation and the release of proinflammatory cytokines, which cause fever and other pathologic effects. Temperatures of ≥40°C (≥104°F) damage mature parasites; in untreated infections, the effect of such temperatures is to further synchronize the parasitic cycle, with eventual production of the regu­ lar fever spikes and rigors that originally characterized the different malarias. These regular fever patterns (quotidian, daily; tertian, every 2 days; quartan, every 3 days) are seldom seen today as patients receive prompt and effective antimalarial treatment. The geographic distributions of the thalassemias, sickle cell dis­ ease, hemoglobins C and E, hereditary ovalocytosis, and glucose6-phosphate dehydrogenase (G6PD) deficiency closely resemble that of falciparum malaria before the introduction of control measures. This similarity suggests that these genetic disorders confer protection

against death from falciparum malaria. HbA/S heterozygotes (sickle cell trait) have a sixfold reduction in the risk of dying from severe fal­ ciparum malaria and are correspondingly protected from the bacterial infections that complicate malaria. Hemoglobin S–containing RBCs impair parasite growth at low oxygen tensions, and P. falciparum–infected RBCs containing hemoglobin S or C exhibit reduced cytoadherence because of reduced surface presentation of the adhesin PfEMP1. Para­ site multiplication in HbA/E heterozygotes is reduced at high parasite densities. In Melanesia, children with α-thalassemia have more fre­ quent malaria (both vivax and falciparum) in the early years of life, which appears to protect them against severe disease. In Melanesian ovalocytosis and in Africans with the Dantu blood group, rigid eryth­ rocytes resist merozoite invasion. G6PD deficiency provides some pro­ tection against severe P. falciparum infections but has a much stronger protective effect against P. vivax infections. Nonspecific host defense mechanisms stop the infection’s expan­ sion, and the subsequent strain-specific immune response then con­ trols the infection. Eventually, exposure to sufficient strains confers protection from high-level parasitemia and disease but not from infection (premunition). Asymptomatic parasitemia is very common among adults and older children living in regions with stable and intense malaria transmission (i.e., holo- or hyperendemic areas) and also in parts of low-transmission areas. Parasitemia in asymptomatic infections fluctuates in density but often averages ~5000/mL—which is just below the level of microscopy detection but sufficient to generate transmissible densities of gametocytes. Immunity is mainly specific for both the species and the strain of infecting malarial parasite. Both humoral immunity and cellular immunity are necessary for protection, but the mechanisms of each are incompletely understood (Fig. 231-1). Immune individuals have a polyclonal increase in serum levels of IgM, IgG, and IgA, although much of this antibody is unrelated to protec­ tion. Antibodies to a variety of parasite antigens presumably act in concert to limit in vivo replication of the parasite. In P. falciparum infections, the variant surface adhesin PfEMP1 is the most important of these antigens. Passive transfer of maternal antibody contributes to the partial protection of infants from severe malaria in the first months of life. This complex immunity to disease declines when a person lives outside an endemic area for several months or longer. Several factors retard the development of cellular immunity to malaria. These factors include the absence of major histocompatibility antigens on the surface of infected RBCs, which precludes direct T-cell recognition; malaria antigen–specific immune unresponsiveness; and the enormous strain diversity of malarial parasites, along with the ability of the parasites to express variant immunodominant antigens on the erythrocyte surface that change during the course of infection. Parasites may persist in the blood for months or years (or, in the case of P. malariae, for decades) if treatment is not given. The complexity of the immune response in malaria, the sophistication of the parasites’ evasion mechanisms, and the lack of a good in vitro correlate with clinical immunity have all slowed progress toward effective vaccines. CLINICAL FEATURES Malaria is a common cause of fever in tropical countries. Clinical diag­ nosis is notoriously unreliable. The first symptoms of malaria are non­ specific; the lack of a sense of well-being, headache, fatigue, abdominal discomfort, and muscle aches followed by fever are all similar to the symptoms of a minor viral illness. In some instances, a prominence of headache, chest pain, abdominal pain, cough, arthralgia, myalgia, or diarrhea may suggest another diagnosis. Although headache may be severe in malaria, the neck stiffness and photophobia of meningitis do not occur. While myalgia may be prominent, it is not usually as severe as in dengue fever, and the muscles are not tender as in leptospirosis or typhus. Nausea, vomiting, and orthostatic hypotension are common. The classic malarial paroxysms, in which fever spikes, chills, and rig­ ors occur at regular intervals, are unusual and at presentation suggest infection (often relapse) with P. vivax or P. ovale. The temperature of nonimmune individuals and children often rises above 40°C (104°F), with tachycardia and sometimes delirium. Childhood febrile convul­ sions may occur with any of the malarias, but generalized seizures

are associated specifically with falciparum malaria and may herald the development of encephalopathy (cerebral malaria). Many clinical abnormalities have been described in acute malaria, but most patients with uncomplicated infections have few abnormal physical findings other than fever, malaise, mild anemia, and (in some cases) a palpable spleen. Anemia is common among young children living in areas with stable transmission (e.g., much of West Africa) and increases in preva­ lence where resistance has compromised the efficacy of antimalarial drugs. Frequent vivax malaria relapse is an important cause of anemia in young children in some areas (e.g., on the island of New Guinea). In nonimmune individuals with acute malaria, the spleen takes several days to become palpable, but splenic enlargement is found in a high proportion of otherwise healthy individuals in malaria-endemic areas and reflects repeated infections. Slight enlargement of the liver is also common, particularly among young children. Jaundice may develop in patients with otherwise uncomplicated malaria and usually resolves over 1–3 weeks. Malaria is not associated with a rash. Petechial hem­ orrhages in the skin or mucous membranes—features of viral hemor­ rhagic fevers and leptospirosis—develop only very rarely in severe falciparum malaria, although thrombocytopenia is usual.

■ ■SEVERE FALCIPARUM MALARIA Appropriately and promptly treated, uncomplicated falciparum malaria (i.e., where the patient can sit or stand unaided and can swallow medicines and food) carries a mortality rate of <0.1%. However, once vital-organ dysfunction occurs or the total proportion of erythrocytes infected increases to >2% (a level corresponding to >1012 parasites in an adult), mortality risk rises steeply, depending on the immunity of the host. The major manifestations of severe falciparum malaria are shown in Table 231-2, and features indicating a poor prognosis are listed in Table 231-3. CHAPTER 231 Cerebral Malaria  Coma is a characteristic and ominous feature of falciparum malaria and, even with treatment, has been associated with death rates of ~20% among adults and 15% among children. Any obtundation, delirium, or abnormal behavior in falciparum malaria should be taken very seriously. The onset of coma may be gradual or sudden following a convulsion. Malaria Cerebral malaria manifests as a diffuse symmetric encephalopathy; focal neurologic signs are unusual. Although some passive resistance to head flexion may be detected, signs of meningeal irritation are absent. The eyes may be divergent, and bruxism and a pout reflex are common, but other primitive reflexes are usually absent. The corneal reflexes are preserved, except in deep coma. Muscle tone may be either increased or decreased. The tendon reflexes are variable, and the plantar reflexes may be flexor or extensor; the abdominal and cremasteric reflexes are absent. Flexor or extensor posturing may be seen. On routine fun­ doscopy, ~15% of patients have retinal hemorrhages; with pupillary dilation and indirect ophthalmoscopy, this figure increases to 30–40%. Papilledema (8% among children, rare among adults) and cotton wool spots (<5%) also occur. More specific funduscopic abnormalities

(Fig. 231-3) include discrete spots of retinal opacification (30–60%) and decolorization of a retinal vessel or segment of vessel (occasional cases). Convulsions, which are usually generalized and often repeated, occur in ~10% of adults and up to 50% of children with cerebral malaria. More covert seizure activity is common, particularly among children, and may manifest as repetitive tonic–clonic eye movements or even hypersalivation. Whereas adults rarely (<3% of cases) suffer obvious neurologic sequelae, ~10% of children surviving cerebral malaria— especially those with hypoglycemia, severe anemia, repeated seizures, and deep coma—have residual neurologic deficits when they regain consciousness; hemiplegia, cerebral palsy, cortical blindness, deafness, and impaired cognition may all occur. The majority of these deficits improve markedly or resolve completely within 6 months. However, the prevalence of some other deficits increases over time; ~10% of children surviving cerebral malaria have a persistent language deficit. There may also be deficits in learning, planning and executive functions, atten­ tion, memory, and nonverbal functioning. The incidence of epilepsy is increased and life expectancy is decreased among these children.

TABLE 231-2  Manifestations of Severe Falciparum Malaria SIGNS MANIFESTATIONS Major Unarousable coma/ cerebral malaria Failure to localize or respond appropriately to noxious stimuli; coma persisting for >30 min after generalized convulsion; a Glasgow Coma Score <11, or in young children a Blantyre Coma Score of <3 Acidemia/acidosis Arterial pH of <7.25, base deficit >8 meq/L, or plasma bicarbonate level of <15 mmol/L; venous lactate level of ≥5 mmol/L; manifests as labored deep breathing, often termed “respiratory distress” Severe normochromic, normocytic anemia Hematocrit of <15% or hemoglobin level of <50 g/L (<5 g/dL) with parasite density of >10,000/μLa Renal failure Serum or plasma creatinine level of >265 μmol/L (>3 mg/dL) or blood urea level of >20 μmol/Lb Pulmonary edema/ adult respiratory distress syndrome Noncardiogenic pulmonary edema, often aggravated by overhydration; radiologically confirmed or oxygen saturation <92% on room air with a respiratory rate >30/ min, often with chest wall indrawing and crepitations on auscultation Hypoglycemia Plasma glucose level of <2.2 mmol/L (<40 mg/dL) Hypotension/shock Systolic blood pressure of <70 mmHg in children 1–5 years or <80 mmHg in adults; with evidence of impaired perfusion or capillary refill >2 s Bleeding/ disseminated intravascular coagulation Significant bleeding and hemorrhage from the gums, nose, and gastrointestinal tract and/or evidence of disseminated intravascular coagulation Convulsions More than two generalized seizures in 24 h; signs of continued seizure activity, sometimes subtle (e.g., tonicclonic eye movements without limb or face movement) PART 5 Infectious Diseases Other Hemoglobinuriac Macroscopic black, brown, or red urine; not associated with effects of oxidant drugs and red blood cell enzyme defects (such as G6PD deficiency) Extreme weakness Prostration; inability to sit unaidedd Hyperparasitemia Parasitemia level of >5% in nonimmune patients (>10% in any patient) Jaundice Serum bilirubin level of >50 mmol/L (>3 mg/dL) if combined with a parasite density of >100,000/μL or other evidence of vital-organ dysfunction aThis is nonspecific and may include patients with chronic anemia; a parasitemia threshold of 100,000/μL is more specific for acute malarial anemia. bThese are criteria for adults. Lower values reflect severe malaria in children. cHemoglobinuria may also occur in uncomplicated malaria and in patients with G6PD deficiency, particularly if they take oxidant drugs such as primaquine. dIn children who are normally able to sit. Abbreviation: G6PD, glucose-6-phosphate dehydrogenase. Hypoglycemia  Hypoglycemia, an important and common com­ plication of severe malaria, is associated with a poor prognosis and is particularly problematic in children and pregnant women. Hypoglyce­ mia in malaria results from both a failure of hepatic gluconeogenesis and an increase in the consumption of glucose by the host and, to a much lesser extent, the malaria parasites. Hypoglycemia may be compounded by quinine, a powerful stimulant of pancreatic insulin secretion, which is still sometimes used in endemic areas for the treat­ ment of both severe and uncomplicated falciparum malaria. In severe disease, the clinical diagnosis of hypoglycemia is difficult: the usual physical signs (sweating, gooseflesh, tachycardia) are absent, and the neurologic impairment caused by hypoglycemia cannot be distin­ guished from that caused by malaria. Acidosis  Acidosis, resulting from accumulation of organic acids, is an important cause of death from severe malaria, which in adults is often compounded by renal impairment. Hypovolemia is not a major contributor to acidosis. Lactic acidosis is caused by the combination of anaerobic glycolysis in tissues where sequestered parasites inter­ fere with microcirculatory flow, lactate production by the parasites,

TABLE 231-3  Features Indicating a Poor Prognosis in Severe Falciparum Malaria Clinical Marked agitation Hyperventilation (respiratory distress) Low core temperature (<36.5°C; <97.7°F) Bleeding Deep coma Repeated convulsions Anuria Shock Laboratory Biochemistry   Hypoglycemia (<2.2 mmol/L)   Hyperlactatemia (≥5 mmol/L)   Acidemia (arterial pH <7.25, base deficit >8 meq/L, or serum HCO3 <15 mmol/L)   Elevated serum creatinine (>265 μmol/L)   Elevated total bilirubin (>50 μmol/L)   Elevated liver enzymes (AST/ALT 3 times upper limit of normal)   Elevated muscle enzymes (CPK ↑, myoglobin ↑)   Elevated urate (>600 μmol/L) Hematology   Leukocytosis (>12,000/μL)   Severe anemia (PCV <10%)   Coagulopathy   Low platelet count (<50,000/μL)   Prolonged prothrombin time (>3 s)   Prolonged partial thromboplastin time   Decreased fibrinogen (<200 mg/dL) Parasitology   Hyperparasitemia   Increased mortality at >100,000/μL   High mortality at >500,000/μL   >20% of parasites identified as pigment-containing trophozoites and schizonts   >5% of neutrophils contain visible malaria pigment Note: Increased risk of concomitant bacteremia in adults if >20% parasitemia. Abbreviations: ALT, alanine aminotransferase; AST, aspartate aminotransferase; CPK, creatine phosphokinase; PCV, packed cell volume. and a failure of hepatic and renal lactate clearance. Hyperlactate­ mia commonly coexists with hypoglycemia. Acids of gut origin are important contributors to acidosis. In children, ketoacidosis may also contribute. Hydroxyphenyllactic acid, α-hydroxybutyric acid, and β-hydroxybutyric acid concentrations are elevated. Acidotic breathing, sometimes called “respiratory distress,” is a sign of poor prognosis. It is followed often by circulatory failure refractory to volume expansion or inotropic drug treatment and ultimately by respiratory arrest. Plasma concentrations of bicarbonate or lactate are the best biochemical prog­ nosticators in severe malaria. Noncardiogenic Pulmonary Edema  Adults with severe falci­ parum malaria may develop noncardiogenic pulmonary edema (adult respiratory distress syndrome) even after several days of antimalarial therapy. The mortality rate is >80%. The pathogenesis is unclear. Pul­ monary edema can be readily precipitated by overly vigorous adminis­ tration of IV fluid. Noncardiogenic pulmonary edema can also develop in otherwise uncomplicated vivax malaria, where recovery is usual. Renal Impairment  Acute kidney injury is common in severe falciparum malaria. Oliguric renal failure requiring temporary renal replacement therapy is an important manifestation in adults but is unusual among children. The pathogenesis of renal failure is unclear but may be related to erythrocyte sequestration and agglutination interfering with renal microcirculatory flow and metabolism. Clini­ cally and pathologically, this syndrome manifests as acute tubular

FIGURE 231-3  The eye in cerebral malaria: perimacular whitening and palecentered retinal hemorrhages. (Courtesy of N. Beare, T. Taylor, S. Harding, S. Lewallen, and M. Molyneux; with permission.) necrosis. Acute renal failure may occur simultaneously with other vitalorgan dysfunction (in which case the mortality risk is high) or may progress as other disease manifestations resolve. In survivors, urine flow resumes in a median of 4 days, and serum creatinine levels return to normal in a mean of 17 days (Chap. 321). Early dialysis or hemo­ filtration considerably improves the chances of survival, particularly in acute hypercatabolic renal failure. Hematologic Abnormalities  Anemia results from accelerated RBC removal by the spleen, obligatory RBC destruction at parasite schi­ zogony, and ineffective erythropoiesis. In severe malaria, the deform­ ability of both infected and uninfected RBCs is reduced. The degree of reduced deformability correlates with prognosis and with the develop­ ment of anemia. Splenic clearance of all RBCs is increased. In nonim­ mune individuals and in areas with unstable transmission, anemia can develop rapidly and transfusion is often required. A hemoglobin of ≤3 g/dL on presentation is associated with increased mortality. Acute hemolytic anemia with massive hemoglobinuria (“blackwater fever”) may occur. Hemoglobinuria may contribute to renal injury. Some patients with blackwater fever have G6PD deficiency, but in the majority of cases, it is unclear why massive hemolysis has occurred. In nonimmune patients, sudden hemolysis may follow many days after artesunate treatment of hyperparasitemia, usually as a result of rela­ tively synchronous loss of once-parasitized “pitted” RBCs. As a conse­ quence of repeated malarial infections, children in high-transmission areas are usually anemic and often develop severe anemia. This results from both shortened survival of uninfected RBCs and marked dys­ erythropoiesis. Anemia is a common consequence of antimalarial drug resistance, which results in repeated or continued infection. Slight coagulation abnormalities are common in falciparum malaria, and mild thrombocytopenia is usual (a normal platelet count should question the diagnosis of malaria). Fewer than 5% of patients with severe malaria have significant bleeding with evidence of disseminated intravascular coagulation. Hematemesis from stress ulceration or acute gastric erosions may occur rarely. Liver Dysfunction  Mild hemolytic jaundice is common in malaria. Severe jaundice is associated with P. falciparum infections; is more common among adults than among children; and results from hemolysis, hepatocyte injury, and cholestasis. Liver failure does not occur. When accompanied by other vital-organ dysfunction (often

TABLE 231-4  Relative Incidence of Severe Complications of Falciparum Malaria NONPREGNANT ADULTS PREGNANT WOMEN CHILDREN COMPLICATION Anemia + ++ +++ Convulsions + + +++ Hypoglycemia + +++ +++ Jaundice +++ +++ + Renal failure +++ +++ + Pulmonary edema ++ +++ + Note: –, rare; +, infrequent; ++, frequent; +++, very frequent. renal impairment), liver dysfunction carries a poor prognosis. Hepatic dysfunction contributes to hypoglycemia, lactic acidosis, and impaired drug metabolism. Occasional patients with falciparum malaria may develop deep jaundice (with hemolytic, hepatic, and cholestatic com­ ponents) without evidence of other vital-organ dysfunction, in which case the prognosis is good. Other Complications  HIV/AIDS and malnutrition predispose to more severe malaria in nonimmune individuals. Malaria anemia is worsened by concurrent infections with intestinal helminths, hook­ worm in particular. Approximately 6% of children diagnosed with severe malaria have concomitant bacteremia. In adults, the proportion is lower (<1%), except in those with very high parasite counts (>20% parasitemia). In areas of moderate and high malaria transmission, differentiating severe malaria from sepsis with incidental parasitemia in childhood is very difficult, and severe malaria is overdiagnosed. In endemic areas, Salmonella spp. bacteremia has been associated specifically with P. falciparum infections. Chest infections and cath­ eter-associated urinary tract infections are common among patients who are unconscious for >3 days. Aspiration pneumonia may follow generalized convulsions. The frequencies of complications of severe falciparum malaria are summarized in Table 231-4. CHAPTER 231 Malaria ■ ■MALARIA IN PREGNANCY Malaria in early pregnancy causes fetal loss. In areas of high malaria transmission, falciparum malaria in primi- and secundigravid women is associated with low birth weight (average reduction, ~170 g) and con­ sequently increased infant mortality rates. In general, infected mothers in areas of stable transmission remain asymptomatic despite intense accumulation of parasitized erythrocytes in the placental microcircu­ lation. Maternal HIV infection predisposes pregnant women to more frequent and higher-density malaria infections, predisposes their new­ borns to congenital malarial infection, and exacerbates the reduction in birth weight associated with malaria. In areas with unstable transmission of malaria, pregnant women are prone to severe infections and are particularly likely to develop high P. falciparum parasitemias complicated by anemia, hypoglycemia, and acute pulmonary edema. Fetal distress, premature labor, and stillbirth or low birth weight are common results. Fetal death is common in severe malaria. Congenital malaria occurs in <5% of newborns of infected mothers; its frequency and the level of parasitemia are related directly to the timing of maternal infection and the parasite density in maternal blood and in the placenta. P. vivax malaria in pregnancy is also associated with a reduction in birth weight (average, 110 g), but in contrast to fal­ ciparum malaria, this effect is more pronounced in multigravid than in primigravid women. About 300,000 women die in childbirth yearly, with most deaths occurring in low-income countries; maternal death from hemorrhage at childbirth is correlated with malaria-induced anemia. ■ ■MALARIA IN CHILDREN Most of the estimated >600,000 deaths from falciparum malaria each year are in young African children. Convulsions, coma, hypoglycemia, metabolic acidosis, and severe anemia are relatively common among children with severe malaria, whereas deep jaundice, oliguric acute kid­ ney injury, and acute pulmonary edema are unusual. Severely anemic children may present with labored deep breathing, which previously

has been attributed incorrectly to “anemic congestive cardiac failure” but is usually caused by metabolic acidosis. In general, children toler­ ate antimalarial drugs well and respond rapidly to treatment. Younger children, and in particular malnourished children, require higher body weight–adjusted antimalarial drug doses than older children or adults.

■ ■TRANSFUSION MALARIA Malaria can be transmitted by blood transfusion, needlestick injury, or organ transplantation. The incubation period is often short because there is no preerythrocytic stage of development, and thus there are no relapses of P. vivax and P. ovale infections. The clinical features and management of these cases are the same as for naturally acquired infec­ tions, although primaquine is not needed for radical cure of vivax or ovale malaria as there are no liver stages. CHRONIC COMPLICATIONS OF MALARIA ■ ■HYPERREACTIVE MALARIAL SPLENOMEGALY Chronic or repeated malarial infections produce hypergammaglobu­ linemia; normochromic, normocytic anemia; and, in certain situations, splenomegaly. Some residents of malaria-endemic areas in tropical countries exhibit an abnormal immunologic response to repeated infections that is characterized by massive splenomegaly, hepato­ megaly, marked elevations in serum IgM and malarial antibody titers, hepatic sinusoidal lymphocytosis, and (in Africa) peripheral B-cell lymphocytosis. This syndrome has been associated with the produc­ tion of cytotoxic IgM antibodies to CD8+ T lymphocytes, antibodies to CD5+ T lymphocytes, and an increase in the ratio of CD4+ to CD8+ T cells. These events may lead to uninhibited B-cell production of IgM and the formation of cryoglobulins (IgM aggregates and immune com­ plexes). This immunologic process stimulates lymphoid hyperplasia and clearance activity and eventually produces splenomegaly. Patients with hyperreactive malarial splenomegaly present with an abdominal mass or a dragging sensation in the abdomen and occasional sharp abdominal pains suggesting perisplenitis. There is usually anemia and some degree of pancytopenia (hypersplenism). In some cases, malaria parasites cannot be found in peripheral-blood smears by microscopy. Respiratory and skin infections are common and many patients die of overwhelming sepsis. Persons with hyperreactive malarial splenomegaly living in endemic areas should receive antimalarial chemoprophylaxis; the results are usually good. In nonendemic areas, anti­ malarial treatment is advised. Some cases have been mistaken for hematologic malignancy. However, in other cases refractory to therapy, clonal lymphop­ roliferation may develop, and this can evolve into a malignant lymphoproliferative disorder. PART 5 Infectious Diseases ■ ■QUARTAN MALARIAL NEPHROPATHY Chronic or repeated infections with P. malariae (and possibly with other malarial species) may cause soluble immune complex injury to the renal glomeruli, resulting in the nephrotic syndrome. Other unidentified factors must contribute to this process since only a very small proportion of infected patients develop renal disease. The his­ tologic appearance is that of focal or segmental glomerulonephritis with splitting of the capillary basement membrane. Subendothelial dense depos­ its are seen on electron microscopy, and immu­ nofluorescence reveals deposits of complement and immunoglobulins and P. malariae antigens are often visible. A coarse-granular pattern of basement membrane immunofluorescent deposits (predominantly IgG3) with selective proteinuria carries a better prognosis than a fine-granular, predominantly IgG2 pattern with nonselective pro­ teinuria. Quartan nephropathy is mainly a disease A B C D E F FIGURE 231-4  Thin blood films of Plasmodium falciparum. A. Young trophozoite. B. Old trophozoite. C. Trophozoites in erythrocytes and pigment in polymorphonuclear cells. D. Mature schizont. E. Female gametocyte. F. Male gametocyte. (Reproduced from Bench Aids for the Diagnosis of Malaria Infections, 2nd ed, with the permission of the World Health Organization.)

of children and is rarely reported nowadays. It usually responds poorly to treatment with either antimalarial agents or glucocorticoids and cytotoxic drugs. ■ ■BURKITT’S LYMPHOMA AND EPSTEIN-BARR VIRUS INFECTION It is possible that malaria-related immune dysregulation provokes infection with lymphoma viruses. Childhood Burkitt’s lymphoma is strongly associated with Epstein-Barr virus (EBV) and with high transmission of P. falciparum. Chronic P. falciparum malaria drives large numbers of EBV-infected cells through the lymph node germinal centers and deregulates activation-induced cytidine deaminase, result­ ing in DNA damage, c-myc translocations, and sometimes lymphoma. DIAGNOSIS OF MALARIA When a patient in or from a malarious area presents with fever, thick and thin blood smears should be prepared and examined immediately to confirm the diagnosis and identify the species of infecting parasite (Figs. 231-4 through 231-10). In general, if the blood smear is negative when examined by an experienced microscopist, the patient does not have malaria. If reliable microscopy is not available, a rapid test should be performed. Malaria is not a clinical diagnosis. ■ ■DEMONSTRATION OF THE PARASITE The definitive diagnosis of malaria rests on the demonstration of asexual forms of the parasite in stained peripheral-blood smears. Of the Romanowsky stains, Giemsa at pH 7.2 is preferred; Field’s, Wright’s, or Leishman’s stain can also be used. Staining of parasites with the fluo­ rescent dye acridine orange allows more rapid diagnosis of malaria (but not speciation of the infection) in patients with low-level parasitemia. Both thin (Figs. 231-4, 231-5, 231-10) and thick (Figs. 231-6, 231-7, 231-8, and 231-9) blood smears should be examined. The thin blood smear should be air-dried, fixed in anhydrous methanol, then stained; the RBCs in the tail of the film should then be examined under oil immersion (×1000 magnification). The density of parasitemia is expressed as the number of parasitized erythrocytes per 1000 RBCs. The thick blood film should be of uneven thickness. The smear should be dried thoroughly and stained without fixing. As many layers of

A B C D E FIGURE 231-5  Thin blood films of Plasmodium vivax. A. Young trophozoite. B. Old trophozoite. C. Mature schizont. D. Female gametocyte. E. Male gametocyte. (Reproduced from Bench Aids for the Diagnosis of Malaria Infections, 2nd ed, with the permission of the World Health Organization.) erythrocytes overlie one another and are lysed during the staining pro­ cedure, the thick film has the advantage of concentrating the parasites (by 40- to 100-fold compared with a thin blood film) and thus increas­ ing diagnostic sensitivity. Both parasites and white blood cells (WBCs) are counted, and the number of parasites per unit volume is calculated from the total leukocyte count. Alternatively, a WBC count of 8000/μL is assumed. This figure is converted to the number of parasitized eryth­ rocytes per microliter. A minimum of 200 WBCs should be counted under oil immersion. Interpretation of blood smears, particularly thick films, requires some experience because artifacts are common. Before a thick smear is judged to be negative, 100–200 fields should be examined. In high-transmission areas, the presence of up to 10,000 parasites/μL of blood may be tolerated without symptoms or signs in partially immune individuals. Thus, in these areas, the detection of low-density malaria parasitemia is sensitive but has low specificity in identifying malaria as the cause of illness. Because the prevalence of asymptomatic parasitemia is often high, low-density parasitemia is a common incidental finding in other conditions causing fever. ■ ■RAPID DIAGNOSTIC TESTS Rapid, simple, sensitive, and specific antibody-based diagnostic stick or card tests that detect P. falciparum–specific histidine-rich protein 2 (PfHRP2), lactate dehydrogenase, or occasionally aldolase antigens in finger-prick blood samples have become the main method of A B FIGURE 231-6  Thick blood films of Plasmodium falciparum. A. Trophozoites. B. Gametocytes. (Reproduced from Bench Aids for the Diagnosis of Malaria Infections, 2nd ed, with the permission of the World Health Organization.)

CHAPTER 231 malaria diagnosis in endemic areas (Table 231-5). Some of these rapid diagnostic tests (RDTs) carry a second antibody (either pan-malaria or P. vivax–specific) and so distinguish falciparum malaria from the less dangerous malarias. PfHRP2-based RDTs may remain positive for several weeks after acute infection. This prolonged positivity is a disadvantage in high-transmission areas where infections are frequent but helps in the diagnosis of severe malaria in patients who have taken antimalarial drugs and cleared peripheral parasitemia but who still have a strongly positive PfHRP2 test. A major disadvantage of RDTs is that they do not quantify parasitemia. Widespread use of PfHRP2 RDTs has put strong selection pressure on P. falciparum populations in some areas, leading to an increased prevalence of mutant parasites with deletion of PfHRP2/3 genes that are not detected by the current generation of PfHRP2-based tests. This is a particular problem in the horn of Africa but increasingly reported elsewhere. Malaria The relationship between parasite density and prognosis is complex and variable; in general, patients with >105 parasites/μL are at increased risk of dying, but nonimmune patients may die with much lower counts, and partially immune persons may tolerate parasite densities many times higher with only minor symptoms. In severe malaria, a poor prognosis is indicated by a predominance of more mature P. falciparum parasites (i.e., >20% of parasites with visible pigment) in the peripheral-blood film or by the presence of phagocytosed malarial pigment in >5% of neutrophils (an indicator of recent schizogony). In P. falciparum infections, gametocytemia peaks 1 week after the peak of asexual parasite densities. Because the mature gametocytes of P. falci­ parum (unlike those of other plasmodia) are not affected by most anti­ malarial drugs, their persistence does not mean there is drug resistance or a need to re-treat if a full course of appropriate treatment has been given. Phagocytosed malarial pigment seen inside peripheral-blood monocytes may provide a clue to recent infection if malaria parasites are not detectable. After parasite clearance, this intraphagocytic malar­ ial pigment is often evident for several days in peripheral-blood films, and for longer in bone marrow aspirates or smears of fluid expressed after intradermal puncture. Molecular diagnosis by PCR amplification of parasite nucleic acid is more sensitive than microscopy or rapid diagnostic tests for detecting malaria parasites and defining malarial species. PCR is used in refer­ ence centers but should not be used for primary diagnosis in endemic

A B C FIGURE 231-7  Thick blood films of Plasmodium vivax. A. Trophozoites. B. Schizonts. C. Gametocytes. (Reproduced from Bench Aids for the Diagnosis of Malaria Infections, 2nd ed, with the permission of the World Health Organization.) areas as it is too sensitive. In epidemiologic surveys, ultrasensitive PCR detection (1000 times more sensitive than microscopy) has proved very useful in identifying asymptomatic infections as control and eradica­ tion programs drive parasite prevalences down to very low levels and in identifying residual “hot spots” of transmission. Serologic diagnosis with either indirect fluorescent antibody or enzyme-linked immuno­ sorbent assays is useful for screening of prospective blood donors and may prove useful as a measure of transmission intensity in future epide­ miologic studies. Serology has no place in the diagnosis of acute illness. ■ ■LABORATORY FINDINGS IN ACUTE MALARIA Normochromic, normocytic anemia is usual. The leukocyte count is generally normal, although it may be raised in very severe infections. There is slight monocytosis, lymphopenia, and eosinopenia, with reac­ tive lymphocytosis and eosinophilia in the weeks after acute infection. The platelet count is usually reduced to ~105/μL (a normal platelet count may point to another diagnosis). The erythrocyte sedimentation rate, plasma viscosity, and levels of C-reactive protein and other acutephase proteins are elevated. Severe infections may be accompanied by prolonged prothrombin and partial thromboplastin times and by more severe thrombocytopenia. Antithrombin III levels are reduced even in mild infection. In uncomplicated malaria, plasma concentra­ tions of electrolytes, blood urea nitrogen (BUN), and creatinine are usually normal. Findings in severe malaria may include metabolic acidosis, with low plasma concentrations of glucose, sodium, bicar­ bonate, phosphate, and albumin, together with elevations in lactate, BUN, creatinine (adjusted for age and body mass), urate, muscle and liver enzymes, and conjugated and unconjugated bilirubin. Hypergam­ maglobulinemia is usual in immune and semi-immune subjects living in malaria-endemic areas. Urinalysis generally gives normal results. In adults and children with cerebral malaria, the mean cerebrospinal fluid (CSF) opening pressure at lumbar puncture is ~160 mm H2O; usually the CSF content is normal or there is a slight elevation of total protein level (<1.0 g/L [<100 mg/dL]) and cell count (<20/μL). PART 5 Infectious Diseases TREATMENT Malaria Patients with severe malaria and those unable to take oral drugs should receive parenteral antimalarial therapy immediately (Table 231-6). Antimalarial drug susceptibility testing can be A B C FIGURE 231-8  Thick blood films of Plasmodium ovale. A. Trophozoites. B. Schizonts. C. Gametocytes. (Reproduced from Bench Aids for the Diagnosis of Malaria Infections, 2nd ed, with the permission of the World Health Organization.)

performed but yields results too slowly to influence the choice of treatment. Molecular markers of resistance to various antimalarial drugs have been identified such as dhfr mutations (antifols), Pfcrt mutations (chloroquine), Pfkelch13 mutations (artemisinin), Pfpm1 or Pfpm2 gene amplification (piperaquine) and Pfmdr1gene ampli­ fication (mefloquine), which are used in population surveillance studies. The World Health Organization (WHO) recommends

artemisinin-based combination therapy (ACT) as first-line treat­ ment for uncomplicated P. falciparum malaria in malaria-endemic areas. An ACT comprises an artemisinin derivative (e.g., artesunate, artemether, or dihydroartemisinin), combined with a single partner drug from another antimalarial drug class in a fixed-dose cofor­ mulation. ACTs are also the recommended first-line treatment for P. knowlesi infections, and either chloroquine or an ACT is recom­ mended for the other malarias. The choice of ACT depends on the likely sensitivity of the infecting parasites to the partner drug. ACTs may be unavailable in temperate countries, where treatment recom­ mendations are limited to the registered available drugs. Despite increasing evidence of chloroquine resistance in P. vivax (from parts of Indonesia, Oceania, eastern and southern Asia, and Central and South America), chloroquine remains an effective treatment for P. vivax malaria in many areas and for P. ovale and P. malariae infections everywhere. Artemisinin resistance in P. falciparum emerged in Southeast Asia in the late 2000s, where it was followed by piperaquine and mefloquine resistance in some areas. Significant artemisinin resis­ tance is now prevalent throughout the Greater Mekong Subregion and has now emerged and spread in East Africa. As new antima­ larials are still years away, it has been suggested that current ACTs should now combine two slowly eliminated partner drugs to pro­ vide mutual protection against resistance (triple ACTs). Falsified or substandard antimalarial drugs are sold in many Asian and African countries and may be the cause of treatment failures. Characteris­ tics of antimalarial drugs are shown in Table 231-7. SEVERE MALARIA In large randomized controlled clinical trials, parenteral artesunate, a water-soluble artemisinin derivative, reduced severe falciparum malaria mortality rates by 35% in Asian adults and children and by 22.5% in African children compared with quinine treatment. Artesunate therefore is now the drug of choice for all patients with

A B C FIGURE 231-9  Thick blood films of Plasmodium malariae. A. Trophozoites. B. Schizonts. C. Gametocytes. (Reproduced from Bench Aids for the Diagnosis of Malaria Infections, 2nd ed, with the permission of the World Health Organization.) A B C D FIGURE 231-10  Thin blood films of Plasmodium knowlesi. A. Young ring stages B. Older ring stages C. Mature trophozoites D. Schizonts (B and C may look very similar to the “band forms” of P. malariae). (From Kesinee Chotivanich, Cell and Tissue Culture Resources Unit laboratory and MORU, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand.) TABLE 231-5  Standard Methods for the Diagnosis of Malariaa METHOD PROCEDURE ADVANTAGES DISADVANTAGES Thick blood filmb Blood should be uneven in thickness but thin enough that the hands of a watch can be read through part of the spot. Stain dried, unfixed blood spot with Giemsa, Field’s, or another Romanowsky stain. Count number of asexual parasites per 200 WBCs (or per 500 WBCs at low densities). Count and report gametocytes separately.c Thin blood filmd Stain fixed smear with Giemsa, Field’s, or another Romanowsky stain. Count number of RBCs containing asexual parasites per 1000 RBCs. In severe malaria, assess stage of parasite development and count neutrophils containing malaria pigment.e Count and report gametocytes separately.c PfHRP2 dipstick or card test A drop of blood is placed on the stick or card, which is then immersed in washing solutions. Monoclonal antibody capture of parasitic antigens reads out as a colored band. Plasmodium LDH dipstick or card test A drop of blood is placed on the stick or card, which is then immersed in washing solutions. Monoclonal antibody capture of parasitic antigens reads out as two colored bands. One band is genus specific (all malarias) or P. vivax specific, and the other band is specific for P. falciparum. Microtube concentration methods with acridine orange staining Blood is collected in a specialized tube containing acridine orange, anticoagulant, and a float. After centrifugation, which concentrates the parasitized cells around the float, fluorescence microscopy is performed. aMalaria cannot be diagnosed clinically with accuracy, but treatment should be started on clinical grounds if laboratory confirmation is likely to be delayed. In areas of the world where malaria is endemic and transmission rates are high, low-level asymptomatic parasitemia is common in otherwise healthy people. Thus, malaria may not be the cause of a fever in a parasitemic patient, although in this context, the presence of >10,000 parasites/μL (~0.2% parasitemia) does indicate that malaria is likely to be the cause. Antibody and polymerase chain reaction (PCR) tests have no role in the diagnosis of malaria except that PCR is increasingly used for genotyping and speciation in mixed infections and for detection of low-level parasitemia in asymptomatic residents of endemic areas. bAsexual parasites/200 WBCs × 40 = parasite count/μL (assumes a WBC count of 8000/μL). See Figs. 231-6 through 231-10. cP. falciparum gametocytemia may persist for days or weeks after clearance of asexual parasites. Gametocytemia without asexual parasitemia does not indicate active infection. dParasitized RBCs (/1000) × hematocrit × 125.6 = parasite count/μL. See Figs. 231-4, 231-5 and 231-10. eThe presence of >100,000 parasites/μL (~2% parasitemia) is associated with an increased risk of severe malaria, but some patients have severe malaria with lower counts. At any level of parasitemia, the finding that >50% of parasites are tiny rings (cytoplasm thickness less than half of nucleus width) carries a relatively good prognosis. In a severely ill patient, the presence of visible pigment in >20% of parasites or of phagocytosed pigment in >5% of polymorphonuclear leukocytes (indicating massive recent schizogony) carries a worse prognosis. fPersistence of PfHRP2 is a disadvantage in high-transmission settings, where many asymptomatic people have positive tests, but can be used to diagnostic advantage in low-transmission settings when a sick patient has previously received unknown treatment (which, in endemic areas, often consists of antimalarial drugs). In this situation, a positive PfHRP2 test indicates that the illness is falciparum malaria, even if the blood smear is negative. Abbreviations: LDH, lactate dehydrogenase; PfHRP2, P. falciparum histidine-rich protein 2; RBCs, red blood cells; WBCs, white blood cells.

CHAPTER 231 Sensitive (0.001% parasitemia); species specific; inexpensive Requires experience (artifacts may be misinterpreted as low-level parasitemia); underestimates true count Malaria Rapid; species specific; inexpensive; in severe malaria, provides prognostic informatione Insensitive (<0.05% parasitemia); uneven distribution of P. vivax, as enlarged infected red cells concentrate at leading edge Robust and relatively inexpensive; rapid; sensitivity similar to or slightly lower than that of thick films (~0.001% parasitemia) Detects only Plasmodium falciparum; remains positive for weeks after high-density infectionsf; does not quantitate P. falciparum parasitemia; evasion of detection by certain strains due to polymorphisms/deletions in HRP2/3 genes Rapid; sensitivity similar to or slightly lower than that of thick films for P. falciparum (~0.001% parasitemia) May miss low-level parasitemia with P. vivax, P. ovale, and P. malariae and may not speciate these organisms; does not quantitate P. falciparum parasitemia; lower sensitivity for detection of P. knowlesi, which may be misidentified as P. falciparum Sensitivity similar or superior to that of thick films (~0.001% parasitemia); ideal for processing large numbers of samples rapidly Does not speciate or quantitate; requires fluorescence microscopy

TABLE 231-6  Regimens for the Treatment of Malariaa TYPE OF DISEASE OR TREATMENT REGIMEN(S) Uncomplicated Malaria Known chloroquine-sensitive strains of Plasmodium vivax, P. malariae, P. ovale, P. falciparumb Chloroquine (10 mg of base/kg stat followed by 5 mg/kg at 12, 24, and 36 h or by 10 mg/kg at 24 h and 5 mg/kg at 48 h) or Amodiaquine (10–12 mg of base/kg qd for 3 days) Radical treatment for P. vivax or P. ovale infection (prevention of relapse) In addition to chloroquine or amodiaquine or ACT, primaquine (0.5 mg of base/kg qd in Southeast Asia and Oceania [total dose 7 mg/kg] and 0.25 mg/kg elsewhere [total dose 3.5 mg/kg]) should be given for 14 days to prevent relapse.c In mild G6PD deficiency, 0.75 mg of base/kg should be given once weekly for 8 weeks. Primaquine should not be given in severe G6PD deficiency. Single dose tafenoquine (adult dose 300 mg) is being introduced for radical cure in some areas alongside quantitative G6PD testing. Can be given only If G6PD levels are normal. P. falciparum malaria Artesunated,e (4 mg/kg qd for 3 days) plus sulfadoxine (25 mg/kg)/pyrimethamine (1.25 mg/kg) as a single dose or Artesunated (4 mg/kg qd for 3 days) plus amodiaquine (10 mg of base/kg qd for 3 days)d,e or Artemether-lumefantrined (1.5/9 mg/kg bid for 3 days with food) or Artesunated (4 mg/kg qd for 3 days) plus mefloquine (24–25 mg of base/kg—either 8 mg/kg qd for 3 days or 15 mg/kg on day 2 and then 10 mg/kg on day 3)f or DHA-piperaquined (target dose: 4/24 mg/kg qd for 3 days in children weighing <25 kg and 4/18 mg/kg qd for 3 days in persons weighing ≥25 kg) or Artesunate-pyronaridined (4/12 mg/kg qd for 3 days) Second-line treatment/treatment of imported malaria (if other ACT not available) Artesunatee (2 mg/kg qd for 7 days) or quinine (10 mg of salt/kg tid for 7 days) plus 1 of the following 3: Tetracyclinef (4 mg/kg qid for 7 days) Doxycyclinef (3 mg/kg qd for 7 days) Clindamycin (10 mg/kg bid for 7 days) or Atovaquone-proguanil (20/8 mg/kg qd for 3 days with food) PART 5 Infectious Diseases Severe Falciparum Malariag,h,i   Artesunatee (2.4 mg/kg stat IV followed by 2.4 mg/kg at 12 and 24 h and then daily if necessary; for children weighing <20 kg, give 3 mg/kg per dose) or, if unavailable, Artemethere (3.2 mg/kg stat IM followed by 1.6 mg/kg qd) or, if unavailable, Quinine dihydrochloride (20 mg of salt/kgj infused over 4 h, followed by 10 mg of salt/kg infused over 2–8 h q8hk) aIn endemic areas where malaria transmission is low, except in pregnant women and infants, a single dose of primaquine (0.25 mg of base/kg) should be added as a gametocytocide to all falciparum malaria treatments to prevent transmission. This is safe, even in G6PD deficiency. bVery few areas now have chloroquine-sensitive P. falciparum malaria. cRecent large studies indicate that these total doses can be condensed into 7-day primaquine regimens. dIn areas where the partner drug to artesunate is known to be effective. Fixed-dose co-formulated combinations are available. The World Health Organization recommends artemisinin combination regimens as first-line therapy for falciparum malaria in all tropical countries and advocates use of fixed-dose combinations. eArtemisinin derivatives are not readily available in some temperate countries. f Tetracycline and doxycycline should not be given to pregnant women or to children <8 years of age. gOral treatment should be substituted as soon as the patient recovers sufficiently to take fluids by mouth. hArtesunate is the drug of choice when available. The data from large studies in Southeast Asia showed a 35% lower mortality rate than with quinine, and very large studies in Africa showed a 22.5% reduction in mortality rate compared with quinine. The doses of artesunate in children weighing <20 kg should be 3 mg/kg. iPatients with severe malaria acquired in an area where artemisinin resistance has been confirmed should be given parenteral artesunate and quinine together at standard treatment doses. jA loading dose should not be given if therapeutic doses of quinine have definitely been administered in the previous 24 h. kInfusions can be given in 0.9% saline and 5–10% dextrose in water. Infusion rates for quinine should be carefully controlled. Abbreviations: ACT, artemisinin combination therapy; DHA, dihydroartemisinin; G6PD, glucose-6-phosphate dehydrogenase. severe malaria everywhere. Artesunate is given by IV injection but is also absorbed rapidly following IM injection. Artemether and the closely related drug artemotil (arteether) are oil-based formulations given by IM injection; they are erratically absorbed and do not confer the same survival benefit as artesunate. A rectal formulation of artesunate has been developed as a community-based prerefer­ ral treatment for patients in the rural tropics who cannot take oral medications. Prereferral administration of rectal artesunate has been shown to decrease mortality rates among severely ill children without access to immediate parenteral treatment. IV artesunate is approved by the U.S. Food and Drug Administration for the treatment of severe malaria and can be obtained from major drug distributors in the United States. Although parenteral quinine is steadily being replaced by parenteral artesunate in endemic areas, it may still have a small role in the treatment of presumed artemis­ inin-resistant severe falciparum malaria, where a precautionary

approach is adopted and both artesunate and quinine are given together in full doses. Severe falciparum malaria constitutes a medical emergency requiring intensive nursing care and careful management. Frequent evaluation of the patient’s condition is essential. Adjunctive treat­ ments such as high-dose glucocorticoids, urea, heparin, dextran, desferrioxamine, antibody to tumor necrosis factor α, high-dose phenobarbital (20 mg/kg), mannitol, or large-volume fluid or albu­ min boluses have proved either ineffective or harmful in clinical trials and should not be used. In acute renal failure or severe meta­ bolic acidosis, hemofiltration or hemodialysis should be started as early as possible. In severe malaria, parenteral antimalarial treatment should be started immediately, and in endemic areas, because of the diffi­ culty in distinguishing sepsis from severe malaria, broad-spectrum antibiotics should also be started in children. Artesunate, given

TABLE 231-7  Properties of Antimalarial Drugs DRUG(S)a PHARMACOKINETIC PROPERTIES ANTIMALARIAL ACTIVITY MINOR TOXICITY MAJOR TOXICITY Quinine Good oral and IM absorption (quinine); Cl and Vd reduced, but plasma protein binding (principally to α1 acid glycoprotein) increased (90%) in malaria; quinine t1/2: 16 h in malaria, 11 h in healthy persons Acts mainly on trophozoite blood stage; kills gametocytes of P. vivax, P. ovale, and P. malariae (but not P. falciparum); no action on liver stages Chloroquine Good oral absorption, very rapid IM and SC absorption; complex pharmacokinetics; enormous Cl and Vd (unaffected by malaria); blood concentration profile determined by distribution processes in malaria; t1/2: 1–2 months. Active desethyl metabolite about 25% of parent drug concentrations As for quinine, but acts slightly earlier in asexual cycle Piperaquine Adequate oral absorption, may be enhanced by fats; similar pharmacokinetics to chloroquine; t1/2: 21–28 days As for chloroquine; resistance has emerged in Southeast Asia Amodiaquine Good oral absorption; largely converted to active metabolite desethylamodiaquine; t1/2: 4–5 days As for chloroquine, but more active against chloroquine-resistant P. falciparum Primaquine Complete oral absorption; active metabolite produced mainly via CYP2D6; t1/2: 5–7 h Radical cure; eradicates hepatic forms of P. vivax and P. ovale; kills P. falciparum gametocytes; kills developing liver stages of all species Mefloquine Adequate oral absorption; no parenteral preparation; t1/2: 14–20 days (shorter in malaria) As for quinine; resistance has emerged in Southeast Asia Lumefantrine Highly variable absorption related to fat intake; t1/2: 3–4 days As for quinine None identified None identified Artemisinin and derivatives (artemether, artesunate) Good oral absorption; good absorption of IM artesunate but slow and variable absorption of IM artemether; artesunate and artemether biotransformed to active metabolite dihydroartemisinin; all drugs eliminated very rapidly; t1/2: <1 h Broader stage specificity and more rapid than other drugs; no action on liver stages; kills all but fully mature gametocytes of P. falciparum Pyrimethamine Good oral absorption, variable IM absorption; t1/2: 4 days For blood stages, acts mainly on mature forms; causal prophylactic Proguanilb (chloroguanide) Good oral absorption; biotransformed to active metabolite cycloguanil; t1/2: 16 h; biotransformation reduced by oral contraceptive use and in pregnancy Causal prophylactic; not used alone for treatment Atovaquoneb Highly variable absorption related to fat intake; t1/2: 30–70 h Acts mainly on trophozoite blood stage None identified None identified Tetracycline, doxycyclinec Excellent absorption; t1/2: 8 h for tetracycline, 18 h for doxycycline Weak antimalarial activity; should not be used alone for treatment Pyronaridine Rapid variable absorption, large Vd; t1/2: 12–14 days Acts mainly on trophozoite blood stage; kills gametocytes of P. vivax, P. ovale, and P. malariae (but not P. falciparum); no action on liver stages Arterolane t1/2: 3 h Broad stage specificity; no action on liver stages; kills all but fully mature gametocytes of P. falciparum aSeveral antimalarial drugs are formulated as different salts (e.g., phosphate, sulfate, hydrochloride) and are therefore prescribed as base equivalents. For example, chloroquine phosphate 250 salt contains 155 mg base equivalent. It is very important to check when prescribing that the correct dose is being given. bAtovaquone and proguanil are prescribed as a fixed-dose combination. This and proguanil alone should not be given if the estimated glomerular filtration rate is <30 mL/min. cTetracycline and doxycycline should not be given to pregnant women or to children <8 years of age. Abbreviations: Cl, systemic clearance; ECG, electrocardiogram; G6PD, glucose-6-phosphate dehydrogenase; Vd, total apparent volume of distribution.

Common: cinchonism (tinnitus, high-tone hearing loss, nausea, vomiting, dysphoria, postural hypotension); ECG QT interval prolongation (usually by <10%). Rare: diarrhea, visual disturbance, rashes. Note: very bitter taste Common: hypoglycemia. Rare: hypotension, blindness, deafness, cardiac arrhythmias, thrombocytopenia, hemolysis, hemolytic-uremic syndrome, vasculitis, cholestatic hepatitis, neuromuscular paralysis. Common: nausea, dysphoria, pruritus in dark-skinned patients, postural hypotension, ECG QT prolongation. Rare: accommodation difficulties, keratopathy, hypoglycemia, rash. Note: bitter taste but usually well tolerated Acute: hypotensive shock (parenteral), cardiac arrhythmias, neuropsychiatric reactions. Chronic: retinopathy (cumulative dose, >100 g), skeletal and cardiac myopathy Occasional epigastric pain, diarrhea, ECG QT prolongation None identified Nausea (tastes better than chloroquine), dysphoria, headache, bradycardia, ECG QT prolongation Agranulocytosis; hepatitis, mainly with prophylactic use; should not be used with efavirenz Nausea, vomiting, diarrhea, abdominal pain, hemolysis, methemoglobinemia Serious hemolytic anemia in severe G6PD deficiency; hemoglobinuria CHAPTER 231 Nausea, giddiness, dysphoria, fuzzy thinking, sleeplessness, nightmares, sense of dissociation Neuropsychiatric reactions, convulsions, encephalopathy Malaria Reduction in reticulocyte count (but not anemia); neutropenia at high doses; in some cases, delayed anemia after treatment of severe malaria with hyperparasitemia Anaphylaxis, urticaria, fever Well tolerated Megaloblastic anemia, pancytopenia, pulmonary infiltration Well tolerated; mouth ulcers and rare alopecia Megaloblastic anemia in renal failure Gastrointestinal intolerance, deposition in growing bones and teeth (tetracycline), photosensitivity, moniliasis, benign intracranial hypertension Renal failure in patients with impaired renal function (tetracycline) Gastrointestinal intolerance, anemia, transient elevation of aminotransferases, hypoglycemia, headache None identified Gastrointestinal intolerance, transient elevation of aminotransferases None identified

by either IV or IM injection, is simple to administer, very safe, and rapidly effective. It does not require dose adjustments in liver dysfunction or renal failure. It should be used in pregnant women with severe malaria. If artesunate is unavailable and arte­ mether or quinine is used, an initial loading dose must be given so that therapeutic concentrations are reached as soon as possible. Quinine causes dangerous hypotension if injected rapidly and so must be administered carefully by rate-controlled infusion only. If this approach is not possible, quinine may be given by deep IM injections into the anterior thigh. The optimal therapeutic range for quinine in severe malaria is not known with certainty, but total plasma concentrations of 8–15 mg/L for quinine are effective and do not cause serious toxicity. If the patient remains seriously ill or in acute renal failure for >2 days, maintenance doses of quinine should be reduced by 30–50% to prevent toxic accumulation of the drug. The initial dose should never be reduced. Convulsions should be treated promptly with IV (or rectal) benzodiazepines. The role of prophylactic anticonvulsants in children is uncertain. If respira­ tory support is not available, a full loading dose of phenobarbital (20 mg/kg) to prevent convulsions should not be given as it may cause respiratory arrest. Levetiracetam is the preferred anticonvul­ sant to control seizures.

When the patient is unconscious, the blood glucose level should be measured every 6 h for at least 24 h. All patients should receive a continuous infusion of dextrose, and blood concentra­ tions ideally should be maintained above 4 mmol/L. Hypoglyce­ mia (<2.2 mmol/L or 40 mg/dL) usually occurs within the first 24 h and should be treated immediately with bolus glucose. The parasite count and hematocrit should be measured every 6–12 h. Anemia develops rapidly. There is uncertainty as to the optimal thresholds for transfusion as there is some evidence that moderate anemia may be beneficial in a patient with severe malaria and vital organ dysfunction. It is recommended that if the hematocrit falls to <20%, whole blood (preferably fresh) or packed cells should be transfused slowly, with careful attention to circulatory status. In areas with higher malaria transmission, where blood for transfu­ sion is in short supply, a threshold of 15% is widely used. Renal function should be checked at least daily. Children presenting with very severe anemia (hemoglobin <4 g/dL) and acidotic breathing require immediate blood transfusion. Management of fluid balance is difficult in severe malaria, particularly in adults, because of the thin dividing line between overhydration (leading to pulmonary edema) and underhydration (contributing to renal impairment). Fluid balance management is different from that in sepsis: fluid boluses are potentially dangerous in severe malaria. Nasogastric feeding should be delayed in nonintubated patients (for 60 h in adults and 36 h in children) to reduce the risk of aspiration pneu­ monia. As soon as the patient can take fluids, oral therapy should be substituted for parenteral treatment and a full 3-day course of ACT given. Mefloquine should be avoided as follow-on treatment for severe malaria because of the increased risk of post-malaria neurologic syndrome. PART 5 Infectious Diseases In areas of high transmission of both P. falciparum and P. vivax (the island of New Guinea), severe and potentially life-threatening anemia is common among children, and both species contribute. Elsewhere, severe vivax malaria may occur but is uncommon. Many patients have had comorbidities contributing to vital-organ dysfunction. Noncardiogenic pulmonary edema may occur. P. knowlesi can cause severe disease associated with high parasite densities. Acute kidney injury, respiratory distress, and shock have all been described, but cerebral malaria does not occur. Treatment for severe vivax and knowlesi malaria should follow the recommen­ dations given for falciparum malaria. UNCOMPLICATED MALARIA P. falciparum and P. knowlesi infections should be treated with an artemisinin-based combination because of their propensity for high parasite densities and severe disease. Infections with sensitive strains of P. vivax, P. malariae, and P. ovale should be treated either with an

ACT or oral chloroquine (total dose, 25 mg of base/kg). The ACT regi­ mens now recommended are safe and effective in adults, children, and pregnant women (all trimesters). The WHO recommends that artemether-lumefantrine should be given preferentially in the first trimester of pregnancy, if available, since there is the most experi­ ence with this drug. The rapidly eliminated artemisinin component is an artemisinin derivative (artesunate, artemether, or dihydroar­ temisinin) given for 3 days, and the partner drug is usually a more slowly eliminated antimalarial to which P. falciparum in the area is sensitive. Six ACT regimens are currently recommended by the WHO: artemether-lumefantrine, artesunate-mefloquine, dihydro­ artemisinin-piperaquine, artesunate-sulfadoxine-pyrimethamine, artesunate-amodiaquine, and artesunate-pyronaridine. In areas of low malaria transmission or to contain resistance spread, a single dose of primaquine (0.25 mg/kg) should be added to ACT as a P. falciparum gametocytocide to reduce the transmissibility of the infection. This low dose of primaquine is safe even in G6PD deficiency. Pregnant women should not be given primaquine. Atovaquone-proguanil is highly effective everywhere, although it is seldom used in endemic areas because of its high cost and the propensity for rapid emergence of resistance. Clinical recovery is slower after atovaquone-proguanil treatment than after ACT. Of great concern is the spread of artemisinin-resistant P. falciparum in Southeast Asia and East Africa. Infections with these resistant parasites are cleared slowly from the blood, with parasite clearance half-lives >5 h and clearance times typically exceeding 3 days. Cure rates with ACT have fallen to unacceptably low levels in some areas. Triple antimalarial combinations are under evaluation with promising results to date. The 3-day ACT regimens are all well tolerated, although meflo­ quine is associated with increased rates of vomiting and dizziness. As second-line treatment for recrudescence following first-line therapy, a different ACT regimen may be given. Patients should be monitored for vomiting for 1 h after the administration of any oral antimalarial drug. If there is vomit­ ing, the dose should be repeated. Symptom-based treatment, with acetaminophen (paracetamol) administration, lowers fever and thereby reduces the patient’s propensity to vomit these drugs. Minor central nervous system (CNS) reactions (nausea, dizziness, sleep disturbances) are common. The incidence of serious adverse neuropsychiatric reactions to mefloquine treatment is ~1 in 1000 in Asia but may be as high as 1 in 200 among African and white ethnic groups. Amodiaquine may also cause adverse CNS effects. All the antimalarial quinolines (chloroquine, piperaquine amodiaquine, mefloquine, and quinine) exacerbate the orthostatic hypotension associated with malaria, and all are tolerated better by children than by adults. Chloroquine, amodiaquine, and piperaquine all prolong ventricular repolarization (QT prolongation) but, at currently rec­ ommended doses, are not proarrhythmic. Pregnant women, young children, patients unable to tolerate oral therapy, and nonimmune individuals (e.g., travelers) with suspected malaria should be evalu­ ated carefully and hospitalization considered. If there is any doubt as to the identity of the infecting malarial species, treatment for falciparum malaria should be given. A negative blood smear read by an experienced microscopist makes malaria very unlikely but does not rule it out completely; thick blood films should be checked again 1 and 2 days later to exclude the diagnosis. Nonimmune patients receiving treatment for malaria should have daily parasite counts performed to confirm reduction until the thick films are negative for asexual parasite stages. If the asexual parasitemia has not cleared by 7 days (and adherence is assured), second-line treat­ ment should be administered. To eradicate persistent liver stages and prevent relapse (radi­ cal treatment), primaquine (0.5 mg of base/kg in East Asia and Oceania and 0.25 mg/kg elsewhere) should be given once daily for 14 days to patients with P. vivax or P. ovale infection after laboratory tests for G6PD deficiency have proved negative. The same total dose may be given over 7 days. If the patient has a mild variant of G6PD deficiency, primaquine can be given in a

dose of 0.75 mg of base/kg (maximum, 45 mg) once weekly for 8 weeks. Pregnant women with vivax or ovale malaria should not be given primaquine but should receive suppressive prophylaxis with chloroquine (5 mg of base/kg per week) until 1 month after delivery, after which radical treatment can be given. The slowly eliminated 8-aminoquinoline tafenoquine has been registered in some countries (currently recommended adult dose 300 mg, which may be too low; dose-optimization studies are ongo­ ing). This allows radical cure to be given in a single dose. The consequent risk of protracted hemolysis in G6PD deficiency, including in female heterozygotes who may test as normal with current G6PD screens (which detect <30–40% of normal enzyme activity), requires that all patients should have a quantitative test of G6PD activity before receiving tafenoquine. Only those with

70% of normal activity should receive the drug. MANAGEMENT OF COMPLICATIONS OF MALARIA Acute Renal Failure  If plasma levels of BUN or age- and weightadjusted creatinine rise despite adequate rehydration, fluid admin­ istration should be restricted to prevent volume overload. As in other forms of hypercatabolic acute kidney injury, renal replace­ ment therapy is best performed early (Chap. 321). Hemofiltration and hemodialysis are more effective than peritoneal dialysis and are associated with lower mortality risk. Some patients with renal impairment pass small volumes of urine sufficient to allow con­ trol of fluid balance; these cases can be managed conservatively if other indications for dialysis do not arise. Renal function usually improves within days, but full recovery may take weeks. Acute Pulmonary Edema (Acute Respiratory Distress Syndrome)  This syndrome is caused by increased pulmonary capillary permeability. Patients should be positioned with the head of the bed at a 45° elevation and should be given oxygen and IV diuretics. Positive-pressure ventila­ tion should be started early if the immediate measures fail (Chap. 316). Rarely, patients may require extracorporeal membrane oxygenation. Hypoglycemia  An initial slow injection of 20% dextrose (2 mL/kg over 10 min) should be followed by an infusion of 10% dextrose (0.10 g/kg per hour). The blood glucose level should be checked regularly thereafter as recurrent hypoglycemia is common, par­ ticularly among patients receiving quinine. In severely ill patients, hypoglycemia commonly occurs together with metabolic (lactic) acidosis and carries a poor prognosis. Sepsis  In malaria-endemic areas where a high proportion of children are parasitemic, it is usually impossible to distinguish severe malaria from bacterial sepsis with confidence. These chil­ dren should be treated with both antimalarials and broad-spectrum antibiotics with activity against nontyphoidal Salmonella species from the outset. Ideally all patients with severe malaria should have blood cultures taken, and empirical antibiotics should also be given to adults with >20% parasitemia. Antibiotics should be considered for severely ill patients of any age who are not responding to anti­ malarial treatment or who deteriorate unexpectedly. Other Complications  Patients who develop spontaneous bleeding should be given fresh blood and IV vitamin K. Convulsions should be treated with IV or rectal benzodiazepines and, if necessary, respi­ ratory support. Aspiration pneumonia should be suspected in any unconscious patient with convulsions, particularly with persistent hyperventilation; IV antimicrobial agents and oxygen should be administered, and pulmonary hygiene should be undertaken. GLOBAL CONSIDERATIONS The goal of global eradication of malaria remains a challenge. After substantial progress from 2000 to 2015, the global burden of malaria has been steadily increasing. Malaria eradication will require strong leadership, increased national commitment, and substantial interna­ tional support. The two main tools used to control malaria are insec­ ticide-treated bed nets (ITNs), previously shown to reduce all-cause

mortality in African children by 20%, and the ACTs. Pyrethroid resis­ tance and artemisinin resistance in Africa pose major threats to global malaria control. Newer ITNs combine pyrethroids with chlorfenapyr (a pyrrole insecticide) or pyriproxyfen (an insect growth regulator). New drugs to treat malaria are in development but still years away. Two very similar pre-erythrocytic vaccines (RTS,S/AS01 and RTS/ matrix M) have been recommended by WHO for deployment. These vaccines provide relatively short-duration protection and are given to young children. Other challenges to malaria eradication include the widespread distribution of Anopheles breeding sites, the enormous number of infected persons, behavioral changes (to avoid ITN contact) in anopheline mosquito vectors, and inadequacies in human and mate­ rial resources, infrastructure, and control programs. Eliminating vivax malaria is further hindered by the lack of a simple, safe, radical curative regimen. The animal reservoir of P. knowlesi presents another challenge to eradication of this species.

MALARIA PREVENTION Malaria may be contained by judicious use of insecticides to kill the mosquito vector, rapid diagnosis, patient management, and—where effective and feasible—administration of intermittent preventive treat­ ments, seasonal malaria chemoprevention, or chemoprophylaxis to high-risk groups such as pregnant women and young children. Indoor residual spraying with insecticides is practiced as part of an integrated vector control program in endemic countries, and a combination neo­ nicotinoid-pyrethroid has been developed in an attempt to delay the emergence of insecticide resistance. Focal elimination of P. falciparum can be accelerated safely by mass treatment with slowly eliminated antimalarials such as dihydroartemisinin-piperaquine. In addition to the two malaria vaccines now available, an irradiated live sporozoite vaccine is in late-stage development, and research on many other vac­ cine candidates is ongoing. CHAPTER 231 ■ ■PERSONAL PROTECTION AGAINST MALARIA Simple measures to reduce the frequency of bites by infected mosqui­ toes in malarious areas are very important. These measures include the avoidance of exposure to mosquitoes at their peak feeding times (usu­ ally dusk to dawn) and the use of insect repellents containing 10–35% DEET (or, if DEET is unacceptable, 7% picaridin), suitable clothing, and ITNs or other insecticide-impregnated materials. Widespread use of bed nets treated with residual pyrethroids reduces the incidence of malaria in areas where vectors bite indoors at night. Malaria ■ ■CHEMOPROPHYLAXIS (Table 231-8; https://wwwnc.cdc.gov/travel/yellowbook/2020/travelrelated-infectious-diseases/malaria) Recommendations for malaria pro­ phylaxis depend on knowledge of local patterns of drug sensitivity in Plasmodium species and the likelihood of acquiring malarial infection. Drugs effective against resistant P. falciparum should be used (atovaquoneproguanil, doxycycline, or mefloquine). Chemoprophylaxis is never entirely reliable, and malaria should always be considered in the differ­ ential diagnosis of fever in patients who have traveled to endemic areas, even if they are taking prophylactic antimalarial drugs. Chemoprevention in Pregnancy  Pregnant women planning to visit malarious areas should be warned about the potential risks and advised to avoid all nonessential travel. All pregnant women who live in endemic areas should be encouraged to attend regular antenatal clin­ ics. Mefloquine is the only drug advised for pregnant women traveling to areas with drug-resistant malaria; this drug is generally considered safe in the second and third trimesters of pregnancy; the data on firsttrimester exposure, although limited, are reassuring. Chloroquine and proguanil are regarded as safe, but there are now very few regions where these drugs can be recommended for protection. The safety of other prophylactic antimalarial agents in pregnancy has not been established. In endemic areas, intermittent preventive treatment in pregnancy (IPTp) involves giving treatment doses of sulfadoxine-pyrimethamine (SP) at each antenatal visit (maximum, once monthly) in the second and third trimesters of pregnancy. Women with HIV infection who are taking trimethoprim-sulfamethoxazole as prophylaxis should not

TABLE 231-8  Drugs Used in the Prophylaxis of Malariaa DRUG USAGE ADULT DOSE PEDIATRIC DOSE COMMENTS Atovaquoneproguanil (Malarone) Prophylaxis in areas with chloroquine- or mefloquine-resistant Plasmodium falciparum 1 adult tablet POb 5–8 kg: ½ pediatric tabletc daily ≥8–10 kg: ¾ pediatric tablet daily ≥10–20 kg: 1 pediatric tablet daily ≥20–30 kg: 2 pediatric tablets daily ≥30–40 kg: 3 pediatric tablets daily ≥40 kg: 1 adult tablet daily Chloroquine phosphate (Aralen and generic) Prophylaxis only in the very few areas with chloroquine-sensitive P. falciparumc or areas with P. vivax only 300 mg of base (500 mg of salt) PO once weekly Doxycycline (many brand names and generic) Prophylaxis in areas with chloroquine- or mefloquine-resistant P. falciparumd 100 mg PO qd (except in pregnant women; see Comments) Hydroxychloroquine sulfate (Plaquenil) An alternative to chloroquine for primary prophylaxis only in the very few areas with chloroquine-sensitive P. falciparumd or areas with P. vivax only 310 mg of base (400 mg of salt) PO once weekly PART 5 Infectious Diseases Mefloquine (Lariam and generic) Prophylaxis in areas with chloroquine-resistant P. falciparumd 228 mg of base (250 mg of salt) PO once weekly Primaquine For prevention of malaria in areas with mainly P. vivax 30 mg of base (52.6 mg of salt) PO qd Primaquine Terminal prophylaxis to decrease risk of relapses of P. vivax and P. ovale 30 mg of base (52.6 mg of salt) PO qd for 14 days after departure from the malarious area aSeveral antimalarial drugs are formulated as different salts (e.g., phosphate, sulfate, hydrochloride) and are therefore prescribed as base equivalents. For example, chloroquine phosphate 250 salt contains 155 mg base equivalent. It is very important to check when prescribing that the correct dose is being given. bAn adult tablet contains 250 mg of atovaquone and 100 mg of proguanil hydrochloride. cA pediatric tablet contains 62.5 mg of atovaquone and 25 mg of proguanil hydrochloride. dVery few areas now have chloroquine-sensitive falciparum malaria. eOne tablet contains 228 mg of base (250 mg of salt). Abbreviation: G6PD, glucose-6-phosphate dehydrogenase. Source: Centers for Disease Control and Prevention, https://www.cdc.gov/malaria/travelers/index.html. be given concomitant SP. Dihydroartemisinin-piperaquine is being evaluated as an alternative. Children born to nonimmune mothers in malaria-endemic areas (usually expatriates moving to these areas) should receive prophylaxis from birth. Chemoprevention for Children in Endemic Areas  Seasonal malaria chemoprevention (SMC) involves giving monthly treatment doses of amodiaquine and SP to children (between 3 months and 5 years of age) in endemic areas of Africa. Originally designed for the intense 3- to 4-month rainy season malaria transmission period across the Sahel region of Africa, it is now introduced in other parts of Africa

Begin 1–2 days before travel to malarious areas. Take daily at the same time each day while in the malarious areas and for 7 days after leaving such areas. Atovaquone-proguanil is contraindicated in persons with severe renal impairment (creatinine clearance rate, <30 mL/min). In the absence of data, it is not recommended for children weighing <5 kg, pregnant women, or women breast-feeding infants weighing <5 kg. Atovaquone-proguanil should be taken with food or a milky drink. 5 mg of base/kg (8.3 mg of salt/ kg) PO once weekly, up to maximum adult dose of 300 mg of base Begin 1–2 weeks before travel to malarious areas. Take weekly on the same day of the week while in the malarious areas and for 4 weeks after leaving such areas. Chloroquine may exacerbate psoriasis. ≥8 years of age: 2 mg/kg PO qd, up to adult dose Begin 1–2 days before travel to malarious areas. Take daily at the same time each day while in the malarious areas and for 4 weeks after leaving such areas. Doxycycline is contraindicated in children aged <8 years and in pregnant women. 5 mg of base/kg (6.5 mg of salt/ kg) PO once weekly, up to maximum adult dose of 310 mg of base Begin 1–2 weeks before travel to malarious areas. Take weekly on the same day of the week while in the malarious areas and for 4 weeks after leaving such areas. Hydroxychloroquine may exacerbate psoriasis. ≤9 kg: 4.6 mg of base/kg (5 mg of salt/kg) PO once weekly

9–19 kg: ¼ tablete once weekly 19–30 kg: ½ tablet once weekly 30–45 kg: ¾ tablet once weekly 45 kg: 1 tablet once weekly Begin 1–2 weeks before travel to malarious areas. Take weekly on the same day of the week while in the malarious areas and for 4 weeks after leaving such areas. Mefloquine is contraindicated in persons allergic to this drug or related compounds (e.g., quinine and quinidine) and in persons with active or recent depression, generalized anxiety disorder, psychosis, schizophrenia, other major psychiatric disorders, or seizures. Use with caution in persons with psychiatric disturbances or a history of depression. Mefloquine is not recommended for persons with cardiac conduction abnormalities. 0.5 mg of base/kg (0.8 mg of salt/kg) PO qd, up to adult dose; should be taken with food Begin 1–2 days before travel to malarious areas. Take daily at the same time each day while in the malarious areas and for 7 days after leaving such areas. Primaquine is contraindicated in persons with G6PD deficiency. It is also contraindicated during pregnancy. 0.5 mg of base/kg (0.8 mg of salt/kg), up to adult dose, PO qd for 14 days after departure from the malarious area This therapy is indicated for persons who have had prolonged exposure to P. vivax and/or P. ovale. It is contraindicated in persons with G6PD deficiency as well as during pregnancy. where malaria is less seasonal and more drug resistant, although its effectiveness there is not known. Hundreds of millions of doses are now dispensed annually. Perennial malaria chemoprevention (PMC) is a loosely described intervention involving giving treatment doses, usually of SP, to young children when they attend health services for immunizations or whenever convenient for control programs. It has replaced intermittent preventive treatment for infants. Children should not receive both SMC and PMC. Chemoprevention in Travelers  Travelers to a malaria-endemic region should start taking antimalarial drugs 2 days to 2 weeks before

127 - 232 Babesiosis

232 Babesiosis

departure so that any untoward reactions can be detected before travel and therapeutic antimalarial blood concentrations will be present if and when any infections develop (Table 231-8). Antimalarial prophy­ laxis should continue for 4 weeks after the traveler has left the endemic area, except if atovaquone-proguanil or primaquine has been taken; these drugs have significant activities against the liver stage of the infection (causal prophylaxis) and can be discontinued 1 week after departure from the endemic area. If suspected malaria develops while a traveler is abroad, obtaining a reliable diagnosis and antimalarial treat­ ment locally is a top priority. Presumptive self-treatment for malaria with atovaquone-proguanil (for 3 consecutive days) or one of the artemisinin-based combinations can be considered under special cir­ cumstances; medical advice on self-treatment should be sought before departure for malaria-endemic areas and as soon as possible after illness begins. Every effort should be made to confirm the diagnosis. Atovaquone-proguanil (3.75/1.5 mg/kg or 250/100 mg, daily adult dose) is a fixed-combination, once-daily prophylactic agent that is very well tolerated by adults and children. This combination is effective against all types of malaria, including multidrug-resistant falciparum malaria. Atovaquone-proguanil is best taken with food or a milky drink to optimize absorption. It is not recommended if the estimated glomerular filtration rate is <30 mL/min. There are insufficient data on safety for prophylaxis in pregnancy. Mefloquine (250 mg of salt weekly, adult dose) has been widely used for malarial prophylaxis because it is usually effective against multidrug-resistant falciparum malaria and is reasonably well toler­ ated. Mefloquine has been associated with rare episodes of psychosis and seizures at prophylactic doses; these reactions are more frequent at the higher doses used for treatment. More common side effects with prophylactic mefloquine include mild nausea, dizziness, fuzzy think­ ing, disturbed sleep patterns, vivid dreams, dysphoria, and malaise. Mefloquine is contraindicated for use by travelers with known hyper­ sensitivity and by persons with active or recent depression, anxiety disorder, psychosis, schizophrenia, another major psychiatric disorder, or seizures; it is not recommended for persons with cardiac conduc­ tion abnormalities, although the evidence that it is cardiotoxic is very weak. Daily administration of doxycycline (100 mg daily, adult dose) is an effective alternative to atovaquone-proguanil or mefloquine. Doxy­ cycline is generally well tolerated but may cause vulvovaginal thrush, diarrhea, and photosensitivity and is not recommended for prophylaxis in children <8 years old or pregnant women, although evidence that it is harmful in children is lacking. Chloroquine can no longer be relied upon to prevent P. falciparum infections but is still used to prevent and treat malaria due to the other human Plasmodium species and for P. falciparum malaria in Central American countries west and north of the Panama Canal and in Caribbean countries. Chloroquine-resistant P. vivax has been reported from parts of eastern Asia, Oceania, and Central and South America. High-level resistance in P. vivax is prevalent in Oceania and Indonesia. Chloroquine is generally well tolerated, although some patients cannot take it because of malaise, headache, visual symptoms (due to revers­ ible keratopathy), gastrointestinal intolerance, alopecia, or pruritus. Chloroquine is considered safe in pregnancy. With chronic administra­ tion for >5 years, a characteristic dose-related retinopathy may develop, but this condition is rare at the doses used for antimalarial prophylaxis. Idiosyncratic or allergic reactions are also rare. Skeletal and/or cardiac myopathy is a potential problem with protracted prophylactic use, although it is more likely to occur at the high doses used in the treat­ ment of rheumatoid arthritis. Neuropsychiatric reactions and skin rashes are unusual. Amodiaquine should not be used for weekly pro­ phylaxis because continuous weekly use is associated with a high risk of agranulocytosis (~1 person in 2000) and hepatotoxicity (~1 person in 16,000). Chloroquine, amodiaquine, and piperaquine all cause moder­ ate electrocardiograph QT prolongation but have not been associated with ventricular arrhythmias at therapeutic doses. Primaquine (0.5 mg of base/kg or a daily adult dose of 30 mg taken with food), an 8-aminoquinoline compound, has proved safe and effec­ tive in the prevention of drug-resistant falciparum and vivax malaria in adults. Primaquine can be considered for adults (with the exception

of pregnant women) who are intolerant to other recommended drugs. Abdominal pain can be prevented by taking primaquine with food. G6PD deficiency must be excluded before primaquine is prescribed. In the past, the dihydrofolate reductase inhibitors pyrimethamine and proguanil (chloroguanide) were administered widely, but the rapid selection of resistance in both P. falciparum and P. vivax has limited their use. Whereas antimalarial quinolines such as chloroquine (a 4-aminoquinoline) act only on the erythrocyte stage of parasitic development, the dihydrofolate reductase inhibitors (as well as ato­ vaquone and primaquine) also inhibit preerythrocytic growth in the liver (causal prophylaxis) and development in the mosquito (sporon­ tocidal activity). Proguanil is safe and well tolerated, although mouth ulceration occurs in ~8% of persons using this drug; it is considered safe for antimalarial prophylaxis in pregnancy. Prophylactic use of the combination of SP or amodiaquine is not recommended for weekly administration because of an unacceptable incidence of severe toxicity, principally exfoliative dermatitis and other skin rashes (SP) (incidence, 1 in 7000; fatal reactions, 1 in 18,000), agranulocytosis (amodiaquine), hepatitis, and pulmonary eosinophilia (both drugs).

Because of the increasing spread and intensity of antimalarial drug resistance, the Centers for Disease Control and Prevention (CDC) rec­ ommends that travelers and their providers consider their destination, type of travel, and current medications and health risks when choosing antimalarial chemoprophylaxis. There is an increasingly appreciated problem of falsified and substandard antimalarial drugs (and other medicines) on the shelves of pharmacies in Southeast Asia and subSaharan Africa; hence, travelers should purchase their preventive drugs from a reputable source before going to a malarious country. Consulta­ tion for the evaluation of prophylaxis failures or treatment of malaria can be obtained from state and local health departments and the CDC Malaria Hotline (770) 488-7788 or (855) 856-4713 [toll free]) or the CDC Emergency Operations Center (770-488-7100 [after hours]). CHAPTER 232 Acknowledgment The authors gratefully acknowledge the substantial contributions of Joel G. Breman, MD, to this chapter in the previous editions. Babesiosis ■ ■FURTHER READING World Health Organization: WHO guidelines for malaria. October 16, 2023. Available at: https://www.who.int/teams/globalmalaria-programme/guidelines-for-malaria.  Accessed March 1, 2024. Edouard Vannier, Jeffrey A. Gelfand

Babesiosis Babesiosis is an emerging infectious disease caused by protozoan para­ sites of the genus Babesia that invade and eventually lyse red blood cells (RBCs). Most cases occur in the United States during the summer months and are caused by Babesia microti, a species typically found in small rodents and transmitted by the deer tick, Ixodes scapularis. Symptoms are those of a flu-like illness. For most patients, a single standard course of atovaquone plus azithromycin is sufficient to achieve cure. Highly immu­ nocompromised patients and asplenic individuals are at risk of persistent infection and should be treated for a longer duration. Adjunct red cell exchange can be useful for severe cases. In the absence of vaccine and chemoprophylaxis, persons at risk of severe babesiosis should minimize their exposure to ticks and, if possible, avoid endemic areas. ■ ■ETIOLOGY, EPIDEMIOLOGY, AND

MODES OF TRANSMISSION A few Babesia species have been implicated as etiologic agents of human babesiosis. These species use wild or domesticated mammals

as reservoir hosts and are maintained in their enzootic cycle by ticks. Humans are incidental, dead-end hosts. Most cases are reported from across the Northern Hemisphere; the predominant etiologic agent varies by continent.

United States  •  GEOGRAPHIC DISTRIBUTION  Most cases (>95%) are caused by B. microti and are reported from the Northeast (Massachusetts, Rhode Island, Connecticut, Vermont, New Hampshire, Maine, New York, New Jersey) and the upper Midwest (Minnesota, Wisconsin) (Fig. 232-1). Other Babesia species seldom cause disease. Symptomatic infection with Babesia duncani and B. duncani–type organisms has been reported from Washington State, Oregon, and California. Symptomatic infection with Babesia divergens–like organisms has been documented in the central states of Arkansas, Missouri, Michigan, and Kentucky and also in Washington State and Pennsylvania. INCIDENCE  Cases are reported weekly by state health departments to the Centers for Disease Control and Prevention (CDC) via the National Notifiable Diseases Surveillance System. In 2023, more than 3200 cases were reported from 33 of the 40 states in which babesiosis is notifiable. In 2011, when babesiosis became a nationally notifiable disease, 1126 cases were reported to the CDC. The increase in incidence over time is best explained by a greater density of ticks in highly endemic areas and by the northward expansion of these ticks. Although B. microti and Borrelia burgdorferi (the agent of Lyme disease; Chap. 191) are transmitted by the same tick species, the deer tick I. scapularis, the geographic expansion of babesiosis has lagged behind that of Lyme disease. This delay likely reflects the poor ecologic fitness of B. microti compared with that of B. burgdorferi and is consistent with the observation that B. burgdorferi helps maintain B. microti in their shared enzootic cycle. Other factors contributing to the rise in the incidence of babesiosis include greater exposure to ticks due to forest fragmentation in suburban areas and increased leisure activities in grassy or wooded areas. PART 5 Infectious Diseases I. ricinus B. divergens B. microti I. scapularis B. duncani FIGURE 232-1  Geographic distribution of human babesiosis and associated tick vectors. Dark colors indicate areas where human babesiosis is endemic or sporadic (defined by ≥5 cases). Light colors indicate areas where tick vectors are present but human babesiosis is rare (<5 cases), undocumented, or absent. Circles depict single cases except in six locations (Colombia, Mexico, Montenegro, Poland, and the provinces of Gansu and Shandong in China) where all patients were diagnosed at one hospital or identified via survey in one location. Colors distinguish the etiologic agents: red for Babesia microti, orange for B. duncani, blue for B. divergens and B. divergens–like, green for B. venatorum, pink for B. crassa–like, brown for B. bovis and B. bigemina, and black for B. motasi–like. White circles depict cases caused by uncharacterized Babesia isolates or by isolates for which the full sequence of the 18S rRNA gene was not available or ruled out all of the Babesia species listed above. Asymptomatic infections and cases of travel-associated babesiosis are omitted.

MODES OF TRANSMISSION  •  Tick Bite  B. microti is acquired primarily during the blood meal of an I. scapularis tick. Less than one-half (~45%) of patients recall a tick bite within the 8 weeks prior to symptom onset. Both nymphs and adult ticks can transmit B. microti; tick larvae are not infected because B. microti is not transmitted transovarially. Most cases (~90%) present from June through August because nymphs—the primary vector—are active from late spring to early summer. Patients who present in the fall may have acquired B. microti from an adult female tick. B. duncani and B. divergens–like organisms are thought to be transmitted by Dermacentor albipictus and Ixodes dentatus ticks, respectively. Blood Transfusion  More than 300 cases of transfusion-transmitted babesiosis (TTB) caused by B. microti have been reported. Most cases involve packed RBCs; a few have been attributed to frozen-deglycerolized RBCs and whole blood–derived platelets contaminated with RBCs but none to apheresis platelets. At the time of transfusion, the age of the refrigerated RBC units has ranged from 4 to 42 days, indicating that B. microti remains viable throughout the RBC unit’s shelf life. Acellular blood components (plasma, cryoprecipitate) have not been implicated. TTB has occurred year-round because B. microti can persist for more than a year in untreated asymptomatic carriers. Given the seasonality of tick-borne babesiosis, two-thirds of the donations implicated in TTB cases have been obtained from June through October. Like that of tickborne babesiosis, the incidence of TTB sharply increased during the first two decades of the millennium. In May 2020, the American Red Cross (ARC) began to test all blood donations collected in the 15 jurisdictions that account for 99% of tick-borne babesiosis cases and 95% of TTB cases. During the following 13 months, no suspected TTB cases were reported to the ARC. In contrast, from 2010 to 2017, B. microti– positive donations were linked to 81 TTB cases (~10 cases per year). Screening of the blood supply relies on the amplification of parasite 18S rRNA gene transcripts. In addition to B. microti, the Procleix Babesia B. venatorum & B. crassa I. persulcatus I. ovatus B. microti

Assay detects B. duncani and B. divergens, which have been implicated in a handful of TTB cases. The assay is exquisitely sensitive; the 95% limit of detection is 3 parasites per mL of blood for B. microti and B. duncani, and 2 parasites per mL of blood for B. divergens. Vertical Transmission  Passage of B. microti across the placenta has been documented but is rare. Symptoms typically develop during the third to the sixth week of life and often consist of fever accompanied by pallor and lethargy. Parasitemia at presentation has ranged from 2% to 5%. Both the neonate and the mother typically are seropositive for

B. microti IgG antibody. Most cases of neonatal babesiosis, however, are acquired through blood transfusion or tick bite. Infants (<1 year of age) account for <1% of the annual number of cases of babesiosis reported to the CDC. Solid Organ Transplantation  This unusual mode of transmission has been highlighted in a single case report. Two patients received a diagnosis of babesiosis 8 weeks after transplantation of a kidney allograft obtained from a single donor who had received multiple transfusions shortly before his death. Corneas from the deceased donor were transplanted, but neither recipient was infected with B. microti; this outcome sug­ gests that RBCs that had remained in the vasculature or fluids of the donated kidneys were the source of B. microti. Risk Factors  Most patients (~80%) who present with symptoms of babesiosis are ≥50 years. Those who are admitted to a hospital are a decade older (median age, 68 years) than those who are not (median age, 59 years). Aside from tick exposure and advanced age, major risk factors for severe babesiosis include asplenia and immunosuppression. Asplenia can be congenital, functional (e.g., due to celiac disease or hemoglobinopathies such as sickle cell disease and thalassemia), or acquired (due to splenectomy). Immunosuppression often is iatro­ genic and associated with conditions such as autoimmune disorders, chronic inflammatory disorders, malignancies, or transplantation. Immunosuppression can be inherent in comorbidities such as X-linked agammaglobulinemia, common variable hypogammaglobulinemia, and HIV/AIDS. Risk factors for TTB include conditions that require transfusion of blood components, particularly RBCs. Outside the United States  Travel-associated babesiosis may become more common if, as anticipated, Babesia species continue to emerge worldwide (Fig. 232-1). EUROPE  More than 40 cases have been attributed to B. divergens since the index case was reported from Croatia in 1957. The infection is rarely diagnosed in immunocompetent individuals; most patients lack a spleen or have functional hyposplenism. Most cases occur in France and Ireland, particularly in regions with cattle farms. B. divergens, a parasite of cattle, is transmitted by the castor bean tick Ixodes ricinus. Other Babesia species rarely cause disease. B. divergens–like parasites have been implicated in 1 case on the Canary Islands and in another in western France. Five cases caused by Babesia venatorum have been reported from Italy, Austria, Germany, and Sweden. Two cases of infection with Babesia crassa–like organisms have been reported from Slovenia and western France. All 9 patients had been splenectomized. In Germany, a normosplenic patient with acute myeloid leukemia presumably acquired B. microti during the transfusion of whole blood–derived platelets. In eastern Poland, mild babesiosis caused by B. microti has occurred in normosplenic patients. I. ricinus is the vector for B. microti and B. venatorum and the presumed vector for B. divergens–like parasites. Haemaphysalis concinna is the likely vec­ tor for B. crassa–like organisms. B. microti is found in small rodents, B. venatorum in roe deer, and B. crassa in sheep. Rabbits are suspected reservoirs for B. divergens–like parasites. ASIA  B. microti was recognized as a human pathogen in Taiwan in the late 1990s. In the past decade, babesiosis has gained the status of emerging infectious disease in mainland China. In the northeastern province of Heilongjiang, B. venatorum and B. crassa–like organisms have caused mild disease in immunocompetent individuals, whereas antibodies specific for B. microti have been detected in blood donors. The taiga tick Ixodes persulcatus is a competent vector for B. microti

and the presumed vector for B. venatorum. In this province, B. crassa– like organisms are found in I. persulcatus and H. concinna ticks. A case of B. venatorum infection was documented in a child residing in the northwestern Xinjiang Autonomous Region. Several cases of B. microti infection have been reported from southern China, mostly from Yunnan; two of the patients were co-infected with Plasmodium species, one with P. falciparum and the other with P. vivax.

The single case of babesiosis reported from Japan was acquired through blood transfusion and was caused by a B. microti organism that defines the Kobe lineage. Two cases have been reported from South Korea; both occurred in splenectomized individuals and were caused by Babesia motasi–like organisms, which are parasites of sheep and goats. In one case, Haemaphysalis longicornis was the presumed vector. REMAINDER OF THE WORLD  Three cases of babesiosis caused by B. microti have been reported from Canada: two from southern Mani­ toba and one from southern Nova Scotia. Asymptomatic B. microti infection has been detected in a few blood donors from southwestern Ontario. Babesiosis is a reportable disease in Manitoba and Quebec. In Yucatan, Mexico, four cases of febrile illness have been attributed to B. microti. Evidence that Babesia bovis and Babesia bigemina may cause human illness has come from Uraba, a region of Colombia where cattle ranching is important and malaria is endemic. In west­ ern India, the death of a young, splenectomized farmer was caused by a novel Babesia species. A fatal case of B. microti infection has been reported from New South Wales, Australia; the causative agent was likely imported. ■ ■CLINICAL MANIFESTATIONS CHAPTER 232 United States  •  B. MICROTI INFECTION  Symptoms typically appear 1–4 weeks after the bite of an infected tick but 3–7 weeks (median, 37 days; range, 11–176 days) after transfusion of contami­ nated blood components. Patients experience a gradual onset of fatigue with or without malaise that is followed within days by fever and one or more of the following: chills, night sweats, headache, myalgia, and anorexia. Fever is persistent or intermittent and has reached 40.9°C (105.6°F). Less common symptoms include arthralgia, nausea, dry cough, neck stiffness, emotional lability, and sore throat. Diarrhea, vomiting, abdominal pain, and joint swelling are rare. Dark urine and jaundice raise the suspicion of severe hemolytic anemia and may be accompanied by shortness of breath. Babesiosis On physical examination, fever is the salient feature. The skin may be pale or yellowish. A focal red rash, if present, denotes the site of the tick bite; an erythema migrans rash (Fig. A1-8) signifies concur­ rent Lyme disease (Chap. 191) or southern tick-associated rash illness (STARI) (Chap. 190). Ecchymoses and petechiae are rare. Examination of the eyes may be remarkable for scleral icterus, which is consistent with severe hemolysis. Retinopathy with splinter hemorrhages and reti­ nal infarcts are rare. Tenderness of the abdominal upper left quadrant suggests splenomegaly, which may be accompanied by hepatomegaly. Left-sided abdominal pain raises suspicion of splenic infarction or splenic rupture. Unexplained hypotension accompanied by tachycardia reinforces suspicion of splenic rupture. Splenic infarction and subcap­ sular hematoma can occur in the absence of splenic rupture. Splenic infarction and splenic rupture are confirmed by CT. Splenic infarcts appear as wedge-shaped hypodense lesions. A hyperdense fluid sur­ rounding the spleen is consistent with hemoperitoneum caused by splenic rupture. Severe babesiosis requires hospital admission. The median length of hospital stay is 4 days (interquartile range, 3–7). Severe babesiosis can be accompanied by one or several complications. The leading compli­ cation is acute renal failure (20%). The second most common compli­ cation is acute respiratory failure (7%). Less common complications include supraventricular arrhythmia, heart failure, disseminated intra­ vascular coagulation, and shock. At least one transfusion of red blood cells is given to one of five patients hospitalized for babesiosis. Patients diagnosed >7 days after symptom onset are predisposed to severe complicated babesiosis, which has been defined as an illness requir­ ing admission to an intensive care unit (ICU); an illness complicated

by acute respiratory distress syndrome, heart failure or shock; or an illness requiring dialysis, intubation or RBC exchange. Among clini­ cal features, diarrhea and nausea or vomiting are strong predictors of severe babesiosis as just defined. Asplenia and autoimmune disorders predispose to severe disease, but underlying cardiac conditions do not. Concurrent Lyme disease does not decrease the risk of severe disease but has been associated with a lower risk for hospitalization; the impli­ cation is that a diagnosis of babesiosis should be considered in any patient diagnosed with Lyme disease.

Despite therapy, babesiosis can be fatal. Prior to the turn of the mil­ lennium, when clindamycin plus quinine was the regimen of choice for treatment of babesiosis, fatality rates ranging from 5% to 9% were reported. Since atovaquone plus azithromycin has become the first-line therapy, a fatal outcome is rare. Of the 7612 cases of babesiosis reported to the CDC in 2011–2015, 46 (0.6%) ended in death. Death is more frequent among those ≥65 years of age. A review of claims for 10,305 Medicare recipients who were diagnosed with babesiosis between 2006 and 2013 revealed that 1% had died within 30 days. Among those admitted to a hospital, the fatality rate was 3%. Fatality is high in immunocompromised patients (~20%) and those with splenic rupture or splenic infarction (6%). OTHER BABESIA INFECTIONS  The eight documented cases of B. duncani infection reported in the United States were moderate to severe; one patient died. Symptoms were similar to those evoked by B. microti. All seven patients infected with B. divergens–like organisms had been splenectomized and experienced a severe illness that required hospi­ talization; three died. Global Considerations  Most cases of B. divergens infection in Europe have occurred in individuals who lack a spleen or have func­ tional hyposplenism. Symptoms develop suddenly and consist of fever (>41°C [>105.8°F]), headache, lumbar or abdominal pain, and dark urine. Jaundice and hemoglobinuria are common. Without immediate therapy, including immediate red cell exchange, patients often develop renal failure with or without acute respiratory distress or failure. In spleen-intact individuals, B. divergens infection typically evokes a mild illness. All 5 patients infected with B. venatorum in Europe had been splenectomized; their illness ranged from mild to severe, and none died. The 32 cases of B. venatorum infection reported from northeastern China occurred in spleen-intact residents. Symptoms were similar to those evoked by B. microti, although chills were rare. Seven patients were hospitalized for intermittent fever as high as 40°C. Only 4 patients were treated with clindamycin (without quinine); all 32 patients recovered. Cases of B. crassa–like infection reported from northeastern China also occurred in spleen-intact residents. Fever, fatigue, and myalgia were less common than among patients infected with B. microti, but headache was as common and nausea or vomiting more common. No patient was admitted to a hospital. Only 3 of the 31 patients were given clindamycin (without quinine); none died. In Europe, 2 cases of B. crassa–like infection occurred in splenectomized individuals and were severe. Neither ended in death, although 1 patient required intensive care. Cases of B. motasi–like infection in South Korea were severe; 1 patient died but the other recovered following clindamycin monotherapy. PART 5 Infectious Diseases ■ ■PATHOGENESIS Anemia  RBC debris generated by hemolysis may accumulate in the kidney vasculature and cause renal failure. Free hemoglobin is rapidly complexed by haptoglobin. Once haptoglobin is depleted, the heme group is oxidized, released from hemoglobin, and complexed by hemopexin. Excessive hemolysis results in excess free hemoglobin, which scavenges and consumes nitric oxide, leading to thrombus formation and vascular inflammation. RBC lysis in small capillaries of the spleen may trigger localized necrosis, leading to splenic infarc­ tion. Exposed to oxidative stress, RBCs become poorly deformable and are filtered out by splenic macrophages as they attempt to pass through the red pulp. Erythrophagocytosis, along with the mounting of an immune response, contributes to splenomegaly and splenic

rupture. Massive phagocytosis of RBCs, platelets, and white blood cells has led to hemophagocytic lymphohistiocytosis, a fatal condi­ tion. Persistent anemia, despite resolution of infection, has been attributed to autoantibodies that tag RBCs for clearance and may activate the complement system. Inflammation  Fever, chills, and sweats likely result from the systemic inflammatory response triggered by RBC lysis. Excessive inflammation promotes end-organ pathology, leading to renal and pulmonary compromise. The spleen is the immunodominant organ in babesiosis. This feature, along with the clearance of parasitized RBCs, explains why asplenia is a major risk factor for severe disease. Protective immunity involves CD4+ T cells, particularly Th1 cells, as revealed by high-grade parasitemia in mice depleted of CD4+ T cells or treated with an interferon γ–neutralizing antibody. The importance of CD4+ T cells is corroborated by the severity of babesiosis in patients with AIDS and in allograft recipients. In immunocompetent mice, Tfh cell expansion is accompanied with germinal center formation and antibody secretion. Although nearly every immunocompetent patient tests positive for B. microti antibody at diagnosis, a role for antibodies is uncertain. Persistent, relapsing babesiosis often occurs in patients treated with rituximab for a cancer or an autoimmune disorder; this observation suggests that B cells, and presumably antibodies, are criti­ cal for parasite clearance in some individuals. ■ ■DIAGNOSIS A diagnosis of babesiosis should be considered for patients who experi­ ence symptoms compatible with babesiosis and may have been exposed to ticks in an endemic area, particularly from late spring to early fall, or were transfused with blood components, particularly packed RBCs, in the past 6 months. Given that I. scapularis ticks can be coinfected with B. microti and B. burgdorferi, babesiosis should be considered in any Lyme disease patient for whom symptoms worsen or do not abate within days or weeks of initiation of appropriate antibiotic therapy. Conversely, because one-half of patients diagnosed with babesiosis are infected with B. burgdorferi, a diagnosis of babesiosis should prompt a diagnostic evaluation for Lyme disease. Other tick-borne pathogens, although rarely implicated in cases of coinfection with B. microti, may be considered; these include Anaplasma phagocytophilum (Chap. 192) and Borrelia miyamotoi (Chap. 190). Routine Laboratory Testing  The complete blood count often is remarkable for anemia. An elevated reticulocyte count signifies stress-induced erythropoiesis. Low levels of haptoglobin or elevated levels of lactate dehydrogenase are consistent with hemolysis. Severe anemia often is preceded by severe thrombocytopenia. The white blood cell (WBC) count is reduced, unchanged, or elevated. Elevated levels of alkaline phosphatase, aspartate aminotransferase, and alanine aminotransferase signify hepatocyte injury. Elevated total bilirubin levels result from hemolysis but may also denote hepatic compromise. Elevated levels of blood urea nitrogen and serum creatinine indicate renal compromise. Urinalysis may reveal excess urobilinogen, hemo­ globinuria, and/or proteinuria. Given that babesiosis is an imitator of HELLP (hemolysis, elevated liver enzymes, and low platelet count) syndrome, a diagnosis of babesiosis should be considered for pregnant women who are at risk of tick exposure and have laboratory abnormali­ ties that define this syndrome. There is no consensus on the use of a particular laboratory param­ eter as predictor of severe babesiosis. In a study of severe disease as defined above (see “Clinical Manifestations”), a total bilirubin level of >1.9 mg/dL was highly predictive of severe disease, whereas WBC counts of <5 × 103/μL were associated with a better prognosis. Param­ eters associated with severe disease also included WBC counts of

10 × 103/μL and creatinine levels of >1.2 mg/dL. An earlier study identified alkaline phosphatase levels of >125 IU/L and WBC counts of 5 × 103/μL as strong predictors of severe disease, in this case defined as a hospital stay of >2 weeks, an ICU stay of >2 days, or death. Specific Testing  A definitive diagnosis of babesiosis is made by microscopic examination of Giemsa-stained thin blood smears

A B C D FIGURE 232-2  Giemsa-stained thin blood films showing Babesia microti parasites. B. microti is an obligate parasite of erythrocytes. Trophozoites may appear as ring forms (A) or as ameboid forms (B). Merozoites can be arranged in tetrads that are pathognomonic (C). Extracellular parasites can be noted (D), particularly when parasitemia is high. (Reproduced with permission from E Vannier, PJ Krause: Human babesiosis. N Engl J Med 366:2397, 2012.) (Fig. 232-2) or amplification of Babesia DNA in blood. Babesia trophozoites appear round, oval, or ameboid. The ring form is most common and lacks the central brownish (hemozoin) deposit typical of Plasmodium falciparum late-stage trophozoites (see Fig. A2-1). For travelers who have returned from P. falciparum–endemic areas and reside in a Babesia-endemic area, a negative result in the BinaxNOW malaria test readily rules out falciparum malaria when microscopy cannot. The presence of extracellular merozoites, particularly when parasitemia is high, and the absence of gametocytes and schizonts also distinguish babesiosis from malaria. Merozoites are arranged in pairs and occasionally in tetrads (the “Maltese cross”). Tetrads are pathognomonic of babesiosis and can be seen in human erythrocytes infected with B. microti, B. duncani, B. venatorum, or B. divergens–like organisms. Parasitemia typically ranges from 0.1 to 10% in immuno­ competent patients but has reached 30–40% in immunocompromised patients. Parasitemia of >4% is a risk factor for severe, complicated disease. Splenic complications, however, often occur in patients with low-grade parasitemia (median, 1%; range, 0.1–30%). If parasites cannot be identified by microscopy and babesiosis is still suspected, amplification of Babesia DNA is recommended. Real-time polymerase chain reaction (PCR) assays, which amplify the parasite 18S rRNA gene, detect as few as 1–10 parasites/μL of blood. Use of a fluorescent probe allows for speciation of the causative agent. Realtime PCR is recommended for the monitoring of low-grade infection in patients at risk of relapsing babesiosis. A single positive serologic result is not sufficient to establish a diagnosis of babesiosis because antibodies can persist for >1 year after the illness has resolved and the parasite has been cleared. An indirect fluorescent antibody test is most commonly used. For B. microti, IgM titers of ≥1:20 and IgG titers of ≥1:64 are considered positive. IgG titers of ≥1:1024 suggest active or recent infection. Antibodies to B. microti do not react with B. duncani or B. divergens antigen. Sera from patients infected with B. venatorum or B. crassa–like organisms react with B. divergens antigen.

TREATMENT Babesiosis MILD TO MODERATE B. MICROTI ILLNESS Mild to moderate babesiosis caused by B. microti is treated with atovaquone plus azithromycin administered orally for 7–10 days. Dosages for adults and children are provided in Table 232-1. Symptoms usually abate within 48 h after initiation of therapy and resolve within 1–2 weeks. If symptoms persist despite therapy for babesiosis, initiating doxycycline while testing for B. burgdorferi or other tick-borne pathogens such as A. phagocytophilum and

B. miyamotoi is essential. Fatigue may persist for weeks to months but does not warrant, on its own, that treatment be extended or resumed. Parasite DNA can be detected for as long as 3 months, but follow-up PCR testing is not recommended because relapse of infection in immunocompetent individuals is unlikely. SEVERE B. MICROTI ILLNESS First-Line Antimicrobial Therapy  The preferred regimen for the treatment of severe babesiosis caused by B. microti is oral atova­ quone plus IV azithromycin (Table 232-1). Use of this combina­ tion is supported by a retrospective study of 40 patients who were admitted for severe babesiosis, including 11 who were admitted to the ICU. All but 1 of the 40 patients improved following ini­ tiation of treatment with atovaquone plus azithromycin, and were discharged. Clindamycin plus quinine, the first regimen to ever bring cure to a babesiosis patient, is an alternative choice. Use of quinine is impeded by a risk for QTc prolongation and cinchonism, including tinnitus; these adverse events often require premature discontinuation. A prospective, nonblind, randomized clinical trial established that atovaquone plus azithromycin is as effective as clindamycin plus quinine in clearing B. microti parasites and resolving symptoms of non-life-threatening babesiosis. Adverse events were reported by 15% of patients treated with atovaquone plus azithromycin but 72% of patients treated with clindamycin plus quinine. No trial has compared the 2 regimens for treatment of severe babesiosis. CHAPTER 232 Babesiosis Intravenous azithromycin should be initiated at 500 mg/d, along with atovaquone. Laboratory parameters should be monitored daily until symptoms abate and parasitemia is <4%. Thereafter, azithro­ mycin can be administered orally. If the patient has an intact spleen and is not immunocompromised, the dosage can be reduced to 250 mg/d. The regimen is administered for 7–10 days, but the duration should be extended if symptoms persist. If the patient is asplenic or immunocompromised, azithromycin should be main­ tained at 500 mg/d. Given the risk for prolonged or relapsing babe­ siosis in such patients, the regimen should be administered until symptoms have resolved and parasites are no longer seen on blood smear for at least 2 weeks. Adjunct Exchange Transfusion  RBC exchange (RCE) is rec­ ommended when parasitemia is high and the clinical status of the patient deteriorates in the context of complications such as severe hemolysis or critical organ dysfunction, particularly renal or pulmonary compromise. The primary purpose of RCE is to rapidly reduce parasite burden; RCE also corrects anemia. Plasma exchange seldom is used in severe, complicated babesiosis although it removes circulating inflammatory mediators and byproducts of hemolysis such as free hemoglobin, free heme, and unconjugated bilirubin. Therapeutic apheresis is performed in close consultation with transfusion medicine services. The criteria for RCE are not strictly defined. It is common prac­ tice to initiate RCE when parasitemia is >10%, although no study has been conducted to identify the threshold above which RCE pro­ vides greatest benefit. A recent case series illustrates the limitation of using parasitemia as the sole criterion to initiate RCE in the hope of preventing death. In this study of 19 patients, pre-RCE parasit­ emia (mean, 12.9%; 95% confidence interval [CI], 9.4–16.4) was a predictor of post-RCE length of hospital stay but not of mortality;

TABLE 232-1  Treatment of Human Babesiosis ADULTS CHILDREN Mild to Moderate B. microti Infectiona Atovaquone (750 mg q12h PO) plus Azithromycin (500 mg/d PO on day 1 followed by 250 mg/d PO on subsequent days) Atovaquone (20 mg/kg q12h PO; maximum, 750 mg/dose) plus Azithromycin (10 mg/kg qd PO on day 1 [maximum, 500 mg], 5 mg/kg qd PO thereafter [maximum, 250 mg]) Severe B. microti Infectionb,c Preferredd Preferred Atovaquone (20 mg/kg q12h PO; maximum, 750 mg/dose) plus Azithromycin (10 mg/kg qd IV followed by 10 mg/kg qd PO [maximum, 500 mg]) Alternative Clindamycin (7–10 mg/kg q6–8h IV followed by 7–10 mg/kg q6–8h PO [maximum, 600 mg/dose]) plus Quinine (8 mg/kg q8h PO; maximum, 650 mg/dose) Consider exchange transfusion Atovaquone (750 mg q12h PO) plus Azithromycin (500 mg qd IV followed by 250–500 mg qd PO) Alternativee,f Clindamycin (600 mg q6h IV followed by 600 mg q8h PO) plus Quinine (650 mg q6–8h PO) Consider exchange transfusion B. divergens Infectiong Immediate complete exchange transfusion plus Clindamycin (600 mg q6–8h IV) plus Quinine (650 mg q8h PO) Immediate complete exchange transfusion plus Clindamycin (7–10 mg/kg q6–8h IV; maximum, 600 mg/dose) plus Quinine (8 mg/kg q8h PO; maximum, 650 mg/dose) PART 5 Infectious Diseases aTreat for 7–10 days. bTreat for 7–10 days, but extend duration if symptoms persist. cFor severely immunocompromised patients, antimicrobial therapy should be given for at least 6 consecutive weeks, including 2 final weeks during which parasites are no longer detected on blood smear. dIf the risk of QTc prolongation or allergy associated with use of azithromycin is a concern, clindamycin can be substituted for azithromycin. For severely immunocompromised patients, IV clindamycin can be added to atovaquone plus azithromycin at initiation of treatment. eClindamycin plus quinine is no longer the preferred regimen because quinine often is discontinued due to QTc prolongation or other side effects, including tinnitus. This regimen can be considered for cases that respond poorly to atovaquone plus azithromycin. fOther alternative regimens have been used successfully, as documented in a limited number of case reports. If quinine toxicity is a concern, atovaquone can be substituted for quinine. For cases that respond poorly to atovaquone plus azithromycin, atovaquone-proguanil can be added to the two-drug regimen or can be substituted for atovaquone. gA few cases of B. divergens infection in Europe have been treated successfully with atovaquone plus azithromycin or atovaquoneproguanil plus azithromycin. this finding is consistent with the concept that end-organ dysfunc­ tion is a correlate of host inflammation rather than parasite burden. Post-RCE parasitemia (mean, 3.4%; 95% CI, 1.9–4.9) was associated neither with post-RCE length of hospital stay nor with mortality; this finding advocates against the use of repeat RCE. There was a trend toward an association between creatinine levels at admission and mortality, pointing to the greater benefit of RCE in patients with renal compromise. RCE has been withheld when the patient is stable and parasitemia has begun to resolve following initiation of antimicrobial therapy. In a study that compared 9 such patients with 19 patients who underwent RCE and received antimicrobial therapy, the two groups differed neither by the parasitemia at

admission (mean, 11.0% vs 11.6%) nor by the time to parasitemia <1% (day 7 vs day 8). Highly Immunocompromised Patients  Patients are at risk for per­ sistent, relapsing babesiosis when the immune response to Babesia is severely impaired. Major risk factors include asplenia, HIV/ AIDS, and immunosuppressive regimens given for transplantation, an autoimmune disorder, or malignancy. Many of these regimens include rituximab or another B cell–depleting antibody. Antimi­ crobial therapy should be administered for at least 6 consecutive weeks, including 2 weeks beyond the first definitive negative PCR

test for B. microti. Given the duration of treatment, atovaquone plus azithromycin is the preferred regimen. Azithromycin should be administered intravenously and initiated at 500 mg/d. Laboratory parameters should be monitored daily until symptoms abate and parasitemia is <4%. Thereafter, azithromycin can be administered orally, but the dosage should be maintained at 500 mg/d because lower dosages may promote antimicrobial resistance. Once the patient is no longer critically ill, routine laboratory testing can be performed every 2 to 3 days. When parasites are no longer observed on blood smear, real-time PCR should be used to monitor the infec­ tion. After discontinuation of therapy, close follow-up is recom­ mended. If symptoms recur, blood smears and/or real-time PCR should be ordered. Antimicrobial Resistance  Parasitologic or clinical relapse has been documented in highly immunocompromised patients, par­ ticularly when antimicrobial therapy is interrupted or administered at a reduced dosage. Some patients who relapse while or after being treated with atovaquone plus azithromycin have been managed with clindamycin plus quinine (Table 232-1, footnote e). When quinine toxicity is a concern, clindamycin has been added to atovaquone plus azithromycin. An informed approach is to identify mutations that have arisen in the parasite genome and may explain antimicrobial resistance. Atovaquone targets the parasite cytochrome b (Cytb). When resistance to atovaquone is predicted to be partial, atovaquone-proguanil

can be substituted for atovaquone (Table 232-1, footnote f). The rationale is that, as with P. falciparum, proguanil lowers the concentration of atovaquone required to collapse the mitochondrial membrane potential. Resistance to azithromycin has been attrib­ uted to missense mutations in the ribosomal protein subunit L4 gene (RLP4) and to a mutation in domain V of the 23S rRNA gene, which carries the peptidyl transferase activity of the parasite ribo­ some. Domain V is also the target of clindamycin. When resistance to azithromycin is explained by a mutation in RPL4, clindamycin can be substituted for azithromycin. A mutation in domain V that prevents binding of azithromycin and clindamycin precludes sub­ stitution of one for the other. Such mutation was recently identified in a patient who relapsed while being treated with a three-drug regimen that included azithromycin and clindamycin. Following initiation of tafenoquine at the loading dose of 600 mg (given over 3 consecutive days), parasitemia rapidly declined. Tafenoquine was administered along with atovaquone and proguanil. Maintenance therapy, which consisted of a 300-mg weekly dose of tafenoquine, was discontinued 5 weeks after the last positive B. microti PCR test. Tafenoquine, which acts by inducing reactive oxygen species, is well suited to substitute for drugs that target a particular gene product. Tafenoquine should not be prescribed to patients with glucose6-phosphate dehydrogenase deficiency because their RBCs have reduced antioxidant capacity and would not sustain the oxidative stress caused by tafenoquine. Splenic Rupture  Splenic rupture typically occurs in young, healthy patients with low-grade parasitemia. If the patient is hemo­ dynamically unstable, emergent splenectomy should be performed. If the patient is hemodynamically stable but bleeding persists, splenic arterial embolization should be considered. In the absence of hemoperitoneum, splenic rupture should be managed without surgery but with careful hemodynamic monitoring. Removal of the spleen leaves patients at risk for relapsing babesiosis or severe disease caused by other microorganisms. OTHER BABESIA INFECTIONS B. duncani and B. divergens–like infections typically have been treated with IV clindamycin (600 mg three or four times daily or 1200 mg twice daily) plus oral quinine (600–650 mg three times daily) for 7–10 days. RBC exchange is initiated for patients with severe hemolytic anemia and/or end-organ failure. In a recent case of severe B. divergens-like infection, a RBC exchange was performed and cure was achieved following a 2-week course of atovaquone plus azithromycin.

128 - 233 Leishmaniasis

233 Leishmaniasis

GLOBAL CONSIDERATIONS In Europe, B. divergens infection is considered a medical emer­ gency. The recommended approach is immediate, complete RCE combined with administration of clindamycin plus oral quinine (Table 232-1). Some cases have been cured with RCE and clindamy­ cin monotherapy. Anemia may persist for >1 month and require blood transfusion. A severe case of B. divergens infection resolved during therapy with atovaquone plus azithromycin. A relapse in a spleen-intact individual was treated with atovaquone-proguanil plus azithromycin. The first-line therapy for B. venatorum infec­ tion in Europe has been IV or oral clindamycin plus quinine. In a patient intolerant to quinine, infection was cured after administra­ tion of atovaquone plus azithromycin. A pediatric case of mild

B. venatorum infection in China was successfully treated by a stan­ dard course of atovaquone plus azithromycin. ■ ■PREVENTION Given the lack of vaccine and chemoprophylaxis, individuals who reside in endemic areas, especially those at risk of severe babesiosis, should wear protective clothing, apply tick repellents to the skin and permethrin to clothing, and limit outdoor activities where ticks abound from May through October. The skin should be thoroughly examined after outdoor activities and ticks carefully removed with tweezers. As babesiosis continues to expand into new areas and because climate change has begun to reshape this expansion, physicians should be increasingly aware of this once neglected disease. ■ ■FURTHER READING Krause PJ et al: Clinical practice guidelines by the Infectious Diseases Society of America (IDSA): 2020 guideline on diagnosis and manage­ ment of babesiosis. Clin Infect Dis 72:185, 2021. Rogers R et al: Broad antimicrobial resistance in a case of relapsing babesiosis successfully treated with tafenoquine. Clin Infect Dis 76:741, 2023. Swanson M et al: Trends in reported babesiosis cases — United States, 2011–2019. MMWR Morb Mortal Wkly Rep 72:273, 2023. Tannous T et al: Red cell exchange as adjunctive therapy for babesio­ sis: Is it really effective? Transfus Med Rev 35:16, 2021. Tonnetti L et al: Babesia blood testing: The first-year experience. Transfusion 62:135, 2022. Shyam Sundar

Leishmaniasis Encompassing a complex group of disorders, leishmaniasis is caused by unicellular eukaryotic obligatory intracellular protozoa of the genus Leishmania and primarily affects the host’s reticuloendothelial system. Leishmania species produce widely varying clinical syndromes rang­ ing from self-healing cutaneous ulcers to fatal visceral disease. These syndromes fall into three broad categories: visceral leishmaniasis (VL), cutaneous leishmaniasis (CL), and mucosal leishmaniasis (ML). ■ ■ETIOLOGY AND LIFE CYCLE Leishmaniasis is caused by ~20 species of the genus Leishmania in the order Kinetoplastida and the family Trypanosomatidae (Table 233-1). Several clinically important species are of the subspecies Viannia. The organisms are transmitted by phlebotomine sandflies of the genus Phle­ botomus in the “Old World” (Asia, Africa, and Europe) and the genus Lutzomyia in the “New World” (the Americas). Transmission may be anthroponotic (i.e., the vector transmits the infection from infected humans to healthy humans) or zoonotic (i.e., the vector transmits the

infection from an animal reservoir to humans). Human-to-human transmission via shared infected needles has been documented in IV drug users in the Mediterranean region. In utero transmission to the fetus occurs rarely.

Leishmania organisms occur in two forms: extracellular, flagellate promastigotes (length, 10–20 μm) in the sandfly vector and intracellu­ lar, nonflagellate amastigotes (length, 2–4 μm; Fig. 233-1 in vertebrate hosts, including humans. Promastigotes are introduced through the proboscis of the female sandfly into the skin of the vertebrate host. Neutrophils predominate among the host cells that first encounter and take up promastigotes at the site of parasite delivery. The infected neutrophils may undergo apoptosis and release viable parasites that are taken up by macrophages, or the apoptotic cells may themselves be taken up by macrophages and dendritic cells. The parasites multiply as amastigotes inside macrophages, causing cell rupture with subsequent invasion of other macrophages. While feeding on infected hosts, sand­ flies pick up amastigotes, which transform into the flagellate form in the flies’ posterior midgut and multiply by binary fission; the promas­ tigotes then migrate to the anterior midgut and can infect a new host when flies take another blood meal. ■ ■EPIDEMIOLOGY Leishmaniasis occurs in 99 countries—most of them developing—in tropical and temperate regions (Fig. 233-2). More than 1 billion people live in areas endemic for leishmaniasis and are at risk of infection. An estimated 30,000 new cases of VL and more than 1 million new cases of CL occur annually. CL is common in South America, Africa, and Asia, whereas VL commonly occurs in East Africa, Brazil, and the Indian subcontinent. ML is limited to South America. The distribution of Leishmania is limited by the distribution of sandfly vectors. CHAPTER 233 ■ ■VISCERAL LEISHMANIASIS VL (also known as kala-azar, a Hindi term meaning “black fever”) is caused by the Leishmania donovani complex, which includes

L. donovani and Leishmania infantum; these species are responsible for anthroponotic and zoonotic transmission, respectively. East Africa now has the highest incidence of VL, followed by Brazil and the Indian subcontinent. In the Indian subcontinent (India, Nepal, and Bangladesh), where a VL elimination program has been implemented, VL incidence has markedly declined. In these three countries, the World Health Organization (WHO) recently announced the elimina­ tion of VL as a public health problem in Bangladesh and a steep decline in the incidence in Nepal, and India reported a 98.7% decline. Zoonotic VL is reported from all countries in the Middle East, Pakistan, and other countries from western Asia to China. Endemic foci also exist in the independent states of the former Soviet Union, mainly Georgia and Azerbaijan. In the Horn of Africa, Sudan, South Sudan, Ethiopia, Kenya, Uganda, and Somalia report VL. In Sudan and South Sudan, large outbreaks are thought to be anthroponotic, although zoonotic transmission also occurs. VL is rare in West and sub-Saharan Africa. Leishmaniasis Mediterranean VL, long an established endemic disease due to

L. infantum, has a large canine reservoir and was seen primarily in infants before the advent of HIV infection. In Mediterranean Europe, 70% of adult VL cases are associated with HIV co-infection. The combination is deadly because of the combined impact of the two infections on the immune system. IV drug users are at particular risk. Other forms of immunosuppression (e.g., that associated with organ transplantation) also predispose to VL. In the Americas, disease caused by L. infantum is endemic from Mexico to Argentina, but 90% of cases in the New World are reported from northeastern Brazil. After the introduction of highly active antiretroviral therapy, the incidence of HIV–VL co-infection declined significantly in Europe; however, ~30 and 5% of VL patients are co-infected with HIV in Ethiopia and India, respectively. Immunopathogenesis  The majority of individuals infected by L. donovani or L. infantum mount a successful immune response and control the infection, never developing symptomatic disease. Fortyeight hours after intradermal injection of killed promastigotes, these individuals exhibit delayed-type hypersensitivity (DTH) to leishman­ ial antigens in the leishmanin skin test (also called the Montenegro

TABLE 233-1  Geographic Distribution and Characteristic Epidemiology of Leishmaniases CLINICAL SYNDROME SPECIES VECTOR RESERVOIR TRANSMISSION SETTING ORGANISM, ENDEMIC REGION Leishmania donovani Complex South Asia VL, PKDL L. donovani Phlebotomus argentipes Sudan, South Sudan, Somalia, Ethiopia, Kenya, Uganda VL, PKDL L. donovani P. orientalis,

P. martini Mediterranean basin, Middle East, Central Asia, China VL, CL L. infantum P. perniciosus,

P. ariasi Middle East, Saudi Arabia, Yemen VL L. donovani P. perniciosus,

P. ariasi Central and South America VL, CL L. infantuma Lutzomyia longipalpis Azerbaijan, Armenia, Georgia, Kazakhstan, Kyrgyzstan, Tajikistan, Turkmenistan, Uzbekistan VL L. infantum P. turanicus Humans, dogs, foxes Anthroponotic, zoonotic L. tropica Western India to Turkey, parts of North and East Africa CL, leishmaniasis recidivans L. tropica P. sergenti Humans Anthroponotic Urban domestic, peridomestic L. major Western and Central Asia, North and sub-Saharan Africa CL L. major P. papatasi,

P. duboscqi Kazakhstan, Turkmenistan, Uzbekistan CL L. major P. papatasi,

P. duboscqi L. aethiopica PART 5 Infectious Diseases Ethiopia, Uganda, Kenya CL, DCL L. aethiopica P. longipes,

P. pedifer Subspecies Viannia Peru, Ecuador CL, ML L. (V.) peruviana Lutzomyia verrucarum,

L. peruensis Guyana, Surinam, French Guyana, Ecuador, Brazil, Colombia, Bolivia CL, ML L. (V.) guyanensis L. umbratilis Sloths, arboreal anteaters, opossums Central America, Ecuador, Colombia CL, ML L. (V.) panamensis L. trapidoi Sloths Zoonotic Tropical forest and deforested areas South and Central America CL, ML L. (V.) braziliensis Lutzomyia spp., L. umbratilis, Psychodopygus wellcomei L. mexicana Complex Central America and northern parts of South America CL, ML, DCL L. amazonensis L. flaviscutellata Forest rodents Zoonotic Tropical forest and deforested areas   CL, ML, DCL L. mexicana L. olmeca Variety of forest rodents and marsupials   CL, DCL L. pifanoi L. olmeca Variety of forest rodents and marsupials aL. infantum is designated L. chagasi in the New World. Abbreviations: CL, cutaneous leishmaniasis; DCL, diffuse cutaneous leishmaniasis; ML, mucosal leishmaniasis; PKDL, post–kala-azar dermal leishmaniasis; VL, visceral leishmaniasis. skin test). Results in mouse models indicate that the development of acquired resistance to leishmanial infection is controlled by the pro­ duction of interleukin (IL) 12 by antigen-presenting cells and the sub­ sequent secretion of interferon (IFN) γ, tumor necrosis factor (TNF) α, and other proinflammatory cytokines by the T helper 1 (TH1) subset of T lymphocytes. The immune response in patients developing active VL is complex; in addition to increased production of multiple proinflam­ matory cytokines and chemokines, patients with active disease have markedly elevated levels of IL-10 in serum as well as enhanced IL-10 mRNA expression in lesional tissues. A direct role for IL-10 in the pathology of VL in humans is supported by studies demonstrating that

Humans Anthroponotic Rural, domestic Humans, rodents in Sudan, canines Anthroponotic, occasionally zoonotic Majority peridomestic, occasionally sylvatic Dogs, foxes, jackals Zoonotic Domestic, peridomestic Dogs, foxes, jackals Zoonotic Domestic, peridomestic Foxes, dogs, opossums Zoonotic Domestic, peridomestic, periurban Domestic Nile rats, rodents Zoonotic Sylvatic, peridomestic Gerbils Zoonotic Rural Hyraxes Zoonotic Sylvatic, peridomestic Wild rodents Zoonotic Andean Valleys Zoonotic Tropical forest Forest rodents, peridomestic animals Zoonotic Tropical forest and deforested areas Zoonotic Tropical forest and deforested areas Zoonotic Tropical forest and deforested areas IL-10 blockade can enhance antigen-specific IFN-γ responses in whole blood from VL patients and IL-10 neutralization promotes parasitic killing. The main disease-promoting activity of IL-10 in VL may be to condition host macrophages for enhanced survival and growth of the parasite. IL-10 can render macrophages unresponsive to activa­ tion signals and inhibit killing of amastigotes by downregulating the production of TNF-α and nitric oxide. Multiple antigen-presentation functions of dendritic cells and macrophages are also suppressed by IL-10. Patients with such suppression do not have positive leishmanin skin tests, nor do their peripheral-blood mononuclear cells respond to leishmanial antigens in vitro. Organs of the reticuloendothelial system

FIGURE 233-1  A macrophage with numerous intracellular amastigotes (2–4 μm) in a Giemsa-stained splenic smear from a patient with visceral leishmaniasis. Each amastigote contains a nucleus and a characteristic kinetoplast consisting of multiple copies of mitochondrial DNA. A few extracellular parasites are also visible. are predominantly affected, with remarkable enlargement of the spleen, liver, and lymph nodes in some regions. The tonsils and intestinal submucosa are also heavily infiltrated with parasites. Bone marrow dysfunction results in pancytopenia. Clinical Features  On the Indian subcontinent and in the Horn of Africa, persons of all ages are affected by VL. In endemic areas of the Americas and the Mediterranean basin, immunocompetent infants and small children as well as immunodeficient adults are affected especially often. The incubation period is 2–3 months but may be up to 1 year or more. The most common presentation of VL is an abrupt onset of moderate- to high-grade fever associated with rigor and chills. Fever may continue for several weeks with decreasing intensity, and the patient may become afebrile for a short period before experiencing another bout of fever. The spleen may be palpable by the second week of illness and, depending on the duration of illness, may become hugely enlarged (Fig. 233-3). Hepatomegaly (usually moderate) soon follows. Visceral leishmaniasis Cutaneous leishmaniasis Visceral and cutaneous leishmaniasis Low endemic region Non endemic region FIGURE 233-2  Worldwide distribution of human leishmaniasis.

Lymphadenopathy is common in most endemic regions of the world except the Indian subcontinent, where it is rare. Patients lose weight and feel weak, and the skin gradually develops dark discoloration due to hyperpigmentation that is most easily seen in brown-skinned individuals. In advanced illness, hypoalbuminemia may manifest as pedal edema and ascites. Anemia appears early and may become severe enough to cause congestive heart failure. Epistaxis, retinal hemor­ rhages, and gastrointestinal bleeding are associated with thrombocyto­ penia. Secondary infections such as pneumonia, tuberculosis, bacillary or amebic dysentery, and gastroenteritis are common. Herpes zoster, chickenpox, boils in the skin, and scabies may also occur. Untreated, the disease is fatal in most patients, including 100% of those with HIV co-infection.

Leukopenia and anemia occur early and are followed by throm­ bocytopenia. There is a marked polyclonal increase in serum immu­ noglobulins. Serum levels of hepatic aminotransferases are raised in a significant proportion of patients, and serum bilirubin levels are elevated occasionally. Renal dysfunction is uncommon. Laboratory Diagnosis  Demonstration of amastigotes in smears of tissue aspirates remains the gold standard for the diagnosis of VL (Fig. 233-1). The sensitivity of splenic smears is >95%, whereas smears of bone marrow (60–85%) and lymph node aspirates (50%) are less sensitive. Culture of tissue aspirates increases sensitivity. Splenic aspiration is invasive and may be dangerous in untrained hands. To circumvent these invasive procedures, several serologic techniques are currently used to detect antibodies to Leishmania. An enzyme-linked immunosorbent assay (ELISA) and the indirect immunofluorescent antibody test (IFAT) are used in sophisticated laboratories. CHAPTER 233 In the field, however, a rapid immunochromatographic test based on the detection of antibodies to a recombinant antigen (rK39) consisting of 39 amino acids conserved in the kinesin region of L. infantum is used worldwide. The test requires only a drop of fingerprick blood or serum, and the result can be read within 15 min. Except in East Africa (where both its sensitivity and specificity are lower), the sensitivity of the rK39 rapid diagnostic test (RDT) in immunocompetent individuals is ~98% and its specificity is ~90%. RDTs— based on a new synthetic polyprotein, rK28—performed better in Sudan, but are not available commercially. Since these antibody detection tests remain positive for years after cure, they cannot be used for measurement of cure or detection of relapse. Qualitative detection of leishmanial nucleic acid by polymerase chain reaction (PCR) and quantitative detection by Leishmaniasis

PART 5 Infectious Diseases FIGURE 233-3  A patient with visceral leishmaniasis has a hugely enlarged spleen visible through the surface of the abdomen. Splenomegaly is the most important feature of visceral leishmaniasis. real-time PCR are highly sensitive and have the added advantage of species identification. However, because the capacity to perform these tests is confined to specialized laboratories, they are yet to be used for routine diagnosis of VL in endemic areas. Using blood samples, loopmediated isothermal amplification (LAMP) of nucleic acids performs better compared with microscopy. A simple visual readout with minimum laboratory equipment requirements makes it attractive for pointof-care diagnostics for VL and post–kala-azar dermal leishmaniasis (PKDL), and it is likely to be used increasingly in the future. Differential Diagnosis  VL is easily mistaken for malaria. Other febrile illnesses that may mimic VL include typhoid fever, tuberculosis, brucellosis, schistosomiasis, and histoplasmosis. Splenomegaly due to portal hypertension, chronic myeloid leukemia, tropical splenomegaly syndrome, and (in Africa) schistosomiasis may also be confused with VL. Fever with neutropenia or pancytopenia in patients from an endemic region strongly suggests a diagnosis of VL; hypergammaglobulinemia in patients with long-standing illness strengthens the diagnosis. In nonendemic countries, a careful travel history is essential when any patient presents with fever. TREATMENT Visceral Leishmaniasis GENERAL CONSIDERATIONS Severe anemia should be corrected by blood transfusion, and other comorbid conditions should be managed promptly. Treatment of VL is complex because the optimal drug, dosage, and duration vary with the endemic region. Despite completing recommended treatment,

some patients experience relapse (most often within 6−12 months), and prolonged follow-up is recommended. A pentavalent antimonial is the drug of choice in most endemic regions of the world, but there is widespread resistance to antimony in the Indian state of Bihar, where either amphotericin B (AmB)—deoxycholate or liposomal—or miltefosine is preferred. Dose requirements for AmB are lower in India than in the Americas, Africa, or the Mediterranean region. In Mediterranean countries, where cost is seldom an issue, liposomal AmB (LAmB) is the drug of choice. In immunocompetent patients, relapses are uncommon with AmB in its deoxycholate and lipid formulations. Antileishmanial therapy has recently evolved as new drugs and delivery systems have become available and resistance to antimonial compounds has emerged. Except for AmB (deoxycholate and lipid formulations), antileishmanial drugs are available in the United States only from the Centers for Disease Control and Prevention Drug Service (telephone: 404-639-3670; email: drugservice@cdc.gov). PENTAVALENT ANTIMONIAL COMPOUNDS Two pentavalent antimonial (SbV) preparations are available: sodium stibogluconate (100 mg of SbV/mL) and meglumine antimoniate (85 mg of SbV/mL). The daily dose is 20 mg/kg by IV infusion or IM injection, and therapy continues for 28–30 days. Cure rates exceed 90% in Africa, the Americas, and most of the Old World but are <50% in Bihar, India, as a result of resistance. Adverse reactions to SbV treatment are common and include arthralgia, myalgia, and elevated serum levels of aminotransferases. Electrocardiographic changes are common. Concave ST-segment elevation is not significant, but prolongation of QTc to >0.5 s may herald ventricular arrhythmia and sudden death. Chemical pancreatitis is common but usually does not require discontinuation of treatment; severe clinical pancreatitis occurs in immunosuppressed patients. AMPHOTERICIN B AmB (and its liposomal formulations) is currently used as a firstline drug in the Indian Subcontinent. Because of a better safety profile, LAmB is now the preferred drug of choice in other parts of the world, as well. Conventional AmB deoxycholate is administered in doses of 0.75–1.0 mg/kg on alternate days for a total of 15 infusions. Fever with chills is an almost universal adverse reaction to AmB infusions. Nausea and vomiting are also common, as is thrombophlebitis in the infused veins. Acute toxicities can be minimized by administration of antihistamines like chlorpheniramine and antipyretic agents like acetaminophen before each infusion. AmB can cause renal dysfunction and hypokalemia and, in rare instances, elicits hypersensitivity reactions, bone marrow suppression, and myocarditis, all of which can be fatal. Several lipid formulations of AmB, developed to replace the deoxycholate formulation, are preferentially taken up by reticuloendothelial tissues. Because very little free drug is available to cause toxicity, a large amount of drug can be delivered over a short period. LAmB has been used extensively to treat VL in all parts of the world. With a terminal half-life of ~150 h, LAmB can be detected in the liver and spleen of animals for several weeks after a single dose. In addition to oral miltefosine (see below), this is the only drug approved by the U.S. Food and Drug Administration (FDA) for the treatment of VL; the regimen is 3 mg/kg daily on days 1–5, 14, and 21 (total dose, 21 mg/kg). However, the total-dose requirement for different regions of the world varies widely. In Asia, it is 10–15 mg/kg;
in Africa, ~18 mg/kg; and in Mediterranean/American regions, ≥20 mg/kg. The daily dose is flexible (1–10 mg/kg). In a study in India, a single dose of 10 mg/kg cured infection in 96% of patients. This single-dose regimen is used in the elimination program in the Indian subcontinent. Adverse effects of LAmB are usually mild and include infusion reactions, backache, and occasional reversible nephrotoxicity. PAROMOMYCIN Paromomycin (aminosidine) is an aminocyclitol-aminoglycoside antibiotic with antileishmanial activity. Its mechanism of action

against Leishmania is yet to be established. Paromomycin is approved in India for the treatment of VL at an IM dose of 11 mg of base/kg daily for 21 days; this regimen produces a cure rate of 94.6%. However, the optimal dose has not been established in other endemic regions. Paromomycin is a relatively safe drug, but some patients develop hepatotoxicity, reversible ototoxicity, and (in rare instances) nephrotoxicity and tetany. Paromomycin, in combina­ tion with SbV, is used in sub-Saharan Africa. MILTEFOSINE Miltefosine, an alkylphosphocholine, is the first oral compound approved for the treatment of leishmaniasis in several endemic countries including the United States. This drug has a long half-life (150–200 h); its mechanism of action is not clearly understood. The recommended therapeutic regimens consist of 28 consecutive days of treatment in doses of 50 mg for patients weighing <25 kg, a twicedaily dose of 50 mg for patients weighing ≥25 kg, and 2.5 mg/kg

for children 2–11 years of age. In children, allometric dosing has been suggested for adequate exposure to the drug. These regimens have resulted in a cure rate of ~94% in India. However, recent stud­ ies from the Indian subcontinent indicate a decline in the cure rate. Doses in other regions remain to be established. Because of its long half-life, miltefosine is prone to induce resistance in Leishmania. Its adverse effects include mild to moderate vomiting and diarrhea in 40 and 20% of patients, respectively; these reactions usually subside spontaneously after a few days. Rare instances of severe allergic dermatitis, hepatotoxicity, and nephrotoxicity have been reported. Because miltefosine is expensive and is associated with significant adverse events, it is best administered as directly observed therapy after meals to minimize the gastrointestinal adverse events, and to ensure completion of treatment and to minimize the risk of resis­ tance induction. Because miltefosine is teratogenic in rats, its use is contraindicated during pregnancy and (unless contraceptive mea­ sures are strictly adhered to for at least 5 months after treatment) in women of childbearing age. MULTIDRUG THERAPY Multidrug therapy for leishmaniasis is likely to be preferred in the future. Its potential advantages in VL include (1) better compli­ ance and lower costs associated with shorter treatment courses and decreased hospitalization, (2) less toxicity due to lower drug doses and/or shorter duration of treatment, and (3) a reduced likelihood that resistance to either agent will develop. In a study from India, one dose of LAmB (5 mg/kg) followed by miltefosine for 7 days, or paromomycin for 10 days, or both miltefosine and paromomycin simultaneously for 10 days (in their usual daily doses) produced a cure rate of >97% (all three combinations). In Africa, a combination of SbV and paromomycin given for 17 days was as effective and safe as SbV given for 30 days and is the preferred first-line treatment of VL. Recently, in a phase 3 trial from East Africa, this prevailing 17-day regimen was compared with combinations of paromomycin and miltefosine for 14 days as doses described above. The cure rates were similar, and this new combination has been proposed as an alternative to the standard 17-day regimen. Prognosis of Treated VL Patients  Recovery from VL is quick. Within a week after the start of treatment, defervescence, regression of splenomegaly, weight gain, and recovery of hematologic param­ eters are evident. With effective treatment, no parasites are recovered from tissue aspirates at the posttreatment evaluation. Continued clinical improvement over 12 months is suggestive of cure. A small percentage of patients (with the exact figure depending on the regimen used) relapse but respond well to retreatment with AmB or its lipid formulations. VL in the Immunocompromised Host  HIV/VL co-infection has been reported from 45 countries. Where both infections are endemic, VL behaves as an opportunistic infection in HIV-1-infected patients. HIV infection can increase the risk of VL development by several-fold in endemic areas. Co-infected patients usually show the

classic signs of VL, but they may present with atypical features due to loss of immunity and involvement of unusual anatomic locations, e.g., infiltration of the skin, oral mucosa, gastrointestinal tract, lungs, and other organs. Serodiagnostic tests may be negative in up to 50% of patients. Parasites can be recovered from unusual sites such as bron­ choalveolar lavage fluid and buffy coat. LAmB is the drug of choice for HIV/VL co-infection—both for primary treatment and for treatment of relapses. A total dose of 40 mg/kg, administered as 4 mg/kg on days 1–5, 10, 17, 24, 31, and 38, is considered optimal for the Americas and Mediterranean countries and is approved by the FDA, but most patients experience a relapse within 1 year. Recently, there has been a significant change in the WHO recommendations for treatment of HIV–VL co-infection in the Indian subcontinent and East Africa. LAmB infusions (total dose 30 mg/kg; 5 mg/kg on days 1, 3, 5, 7, 9, and 11) co-administered with oral miltefosine (100 mg/d for 14 days for the Indian subcontinent and 28 days for East Africa) should now be used to treat HIV–VL co-infection. AmB deoxycholate can also be used where LAmB is not accessible. Reconstitution of patients’ immunity by antiretroviral therapy has led to a dramatic decline in the incidence of co-infection in the Mediterranean basin. In contrast, HIV–VL coinfection is on the rise in African and Asian countries. Ethiopia is worst affected: up to 30% of VL patients are also infected with HIV. Because restoration of the CD4+ T-cell count to >200/μL does decrease the fre­ quency of relapse, antiretroviral therapy (in addition to antileishmanial therapy) is a cornerstone of the management of HIV–VL co-infection. Secondary prophylaxis with pentamidine or lipid AmB has been shown to delay relapses, but no regimen has been established as optimal.

CHAPTER 233 Post–Kala-Azar Dermal Leishmaniasis  On the Indian sub­ continent and in Sudan and other East African countries, 2–50% of patients develop skin lesions concurrent with or after the cure of VL. Most common are hypopigmented macules, papules, and/or nodules or diffuse infiltration of the skin and sometimes of the oral mucosa. The African and Indian diseases differ in several respects; important features of PKDL in these two regions are listed in Table 233-2, and disease in an Indian patient is depicted in Fig. 233-4. Leishmaniasis In PKDL, parasites are scanty in hypopigmented macules but may be seen and cultured more easily from nodular lesions. Cellular infil­ trates are heavier in nodules than in macules. Lymphocytes are the dominant cells; next most common are histiocytes and plasma cells. In about half of cases, epithelioid cells—scattered individually or forming compact granulomas—are seen. The diagnosis is based on history, clin­ ical findings supported by demonstration of parasites in the slit skin smears, or by demonstrating parasitic DNA in PCR. Sensitivity of skin smears is quite low, and positive serology does not help as it remains positive for several years after the cure of VL. Indian PKDL was treated with prolonged courses (up to 120 days) of pentavalent antimonials. This prolonged course was toxic and frequently led to noncompliance. TABLE 233-2  Clinical, Epidemiologic, and Therapeutic Features of Post–Kala-Azar Dermal Leishmaniasis: East Africa and the Indian Subcontinent FEATURE EAST AFRICA INDIAN SUBCONTINENT Most affected country Sudan and South Sudan Bangladesh Incidence among patients with VL ~50% 5–15% Interval between VL and PKDL During VL to 6 months 6 months to 3 years Age distribution Mainly children Any age History of prior VL Yes Not necessarily Rashes of PKDL in presence of active VL Yes No Treatment Sodium stibogluconate for 2–3 months Miltefosine for 12 weeks Natural course Spontaneous cure in majority of patients Spontaneous cure rarely Abbreviations: PKDL, post–kala-azar dermal leishmaniasis; VL, visceral leishmaniasis.

PART 5 Infectious Diseases FIGURE 233-4  Post–kala-azar dermal leishmaniasis in an Indian patient. Note nodules of varying size involving the entire face. The face is erythematous, and the surface of some of the large nodules is discolored. The alternative—three to four 20-day courses of AmB spread over several months—is expensive and unacceptable for most patients. Except for cosmetic reasons, these patients do not have any physical limitation, and thus motivation for such long and arduous treatment is very low. This leads to either no or incomplete treatment. In the Indian subcontinent, the currently recommended regimen is oral miltefosine for 12 weeks, in the usual daily doses. Though initially cure rates were high, a decline in its efficacy is now being reported in some studies. Furthermore, there are reports of ocular toxicity with this regimen in 3.7% of patients. The efficacy of either LAmB alone (20 mg/kg)

or in combination with miltefosine (for 3 weeks) in PKDL has been recently tested in the Indian subcontinent; the efficacy was greater than 80% with both regimens. In East Africa, a majority of patients experi­ ence spontaneous healing. In those with persistent lesions, the response to 60 days of treatment with a pentavalent antimonial is good. ■ ■CUTANEOUS LEISHMANIASIS CL can be broadly divided into Old World and New World forms. Old World CL caused by Leishmania tropica is anthroponotic and is con­ fined to urban or suburban areas throughout its range. Zoonotic CL is most commonly due to Leishmania major, which naturally parasitizes several species of desert rodents that act as reservoirs over wide areas of the Middle East, Africa, and central Asia. Local outbreaks of human disease are common. Major outbreaks currently affect Afghanistan, Syria, Iraq, Lebanon, and Turkey in association with refugees and population movement. CL is increasingly seen in tourists and military personnel on mission in CL-endemic regions of countries and as a coinfection in HIV-infected patients. Leishmania aethiopica is restricted to the highlands of Ethiopia, Kenya, and Uganda, where it is a natural parasite of hyraxes. New World CL is mainly zoonotic and is most often caused by Leishmania mexicana, Leishmania (Viannia) panamensis, and Leishmania amazonensis. A wide range of forest animals act as

reservoirs, and human infections with these species are predominantly rural. As a result of extensive urbanization and deforestation, Leishmania (Viannia) braziliensis has adapted to peridomestic and urban animals, and CL due to this organism is increasingly becoming an urban dis­ ease. In the United States, a few cases of CL have been acquired indig­ enously in Texas. Immunopathogenesis  As in VL, the proinflammatory (TH1) response in CL may result in either asymptomatic or subclinical infec­ tion. However, in some individuals, the immune response causes ulcer­ ative skin lesions, the majority of which heal spontaneously, leaving a scar. Healing is usually followed by immunity to reinfection with that species of parasite. Clinical Features  A few days or weeks after the bite of a sandfly, a papule develops and grows into a nodule that ulcerates over weeks or months. The base of the ulcer, which is usually painless, consists of necrotic tissue and crusted serum, but secondary bacterial infection sometime occurs. The margins of the ulcer are raised and indurated. Lesions may be single or multiple and vary in size from 0.5 to >3 cm (Fig. 233-5). Lymphatic spread and lymph gland involvement may be palpable and may precede the appearance of the skin lesion. There may be satellite lesions, especially in L. major and L. tropica infections. The lesions usually heal spontaneously after 2–15 months. Lesions due to L. major and L. mexicana tend to heal rapidly, whereas those due to L. tropica and parasites of subspecies Viannia heal more slowly. In CL caused by L. tropica, new lesions—usually scaly, erythematous papules and nodules—develop in the center or periphery of a healed sore, a condition known as leishmaniasis recidivans. Lesions of L. mexicana and Leishmania (Viannia) peruviana closely resemble those seen in the Old World; however, lesions on the pinna of the ear are com­ mon, chronic, and destructive in the former infections. L. mexicana is responsible for chiclero’s ulcer, the so-called self-healing sore of Mexico. CL lesions on exposed body parts (e.g., the face and hands), permanent scar formation, and social stigmatization may cause anxiety and depression and may affect the quality of life of CL patients. Differential Diagnosis  A typical history (an insect bite followed by the events leading to ulceration) in a resident of or a returning traveler from an endemic zone, strongly suggests CL. Cutaneous tuber­ culosis, fungal infections, leprosy, sarcoidosis, and malignant ulcers are sometime mistaken for CL. Laboratory Diagnosis  Demonstration of amastigotes in material obtained from a lesion remains the diagnostic gold standard. Micro­ scopic examination of slit skin smears, aspirates, or biopsies of the lesion is used for detection of parasites. Culture of smear or biopsy material may yield Leishmania. PCR or LAMP is more sensitive than FIGURE 233-5  Cutaneous leishmaniasis in a Bolivian child. There are multiple ulcers resulting from several sandfly bites. The edges of the ulcers are raised. (Courtesy of P. Desjeux, Retired Medical Officer, World Health Organization, Geneva, Switzerland.)

microscopy and culture and allows identification of Leishmania to the species level. Quantitative PCR may be used to monitor treatment responses. This information is important in decisions about therapy because responses to treatment can vary with the species. TREATMENT Cutaneous Leishmaniasis Although lesions heal spontaneously in the majority of cases, their spread or persistence indicates that treatment may be needed. One or a few small lesions due to “self-healing species” can be treated with topical agents. Systemic treatment is required for lesions over the face, hands, or joints; multiple lesions; large ulcers; lymphatic spread; New World CL with the potential for development of ML; and CL in HIV-co-infected patients. A pentavalent antimonial is the first-line drug for all forms of CL and is used in a dose of 20 mg/kg for 20 days. The exceptions to this rule are CL caused by Leishmania (Viannia) guyanensis, for which pentamidine isethionate is the drug of choice (two injections of

4 mg of salt/kg separated by a 48-h interval), and CL due to L. aethi­ opica, which responds to paromomycin (16 mg/kg daily) but not to antimonials. Relapses usually respond to a second course of treat­ ment. In Peru, topical imiquimod (5–7.5%) plus parenteral antimo­ nials have been shown to cure CL more rapidly than antimonials alone. Azoles and triazoles have been used with mixed responses in both Old and New World CL but have not been adequately assessed for this indication in clinical trials. In L. major infection, oral flu­ conazole (200 mg/d for 6 weeks) resulted in a higher rate of cure than placebo (79% vs 34%) and also cured infection faster. Adverse effects include gastrointestinal symptoms and hepatotoxicity. Keto­ conazole (600 mg/d for 28 days) is 76–90% effective in CL due to

L. (V.) panamensis and L. mexicana in Panama and Guatemala. Orally administered miltefosine makes it very attractive and is a major advancement in the treatment of CL. Miltefosine has been used in CL in doses of 2.5 mg/kg for 28 days. This agent is effective against L. major infections. Recent Pan American Health Organiza­ tion guidelines recommend intralesional pentavalent antimonials in patients with localized CL caused by L. braziliensis and L. ama­ zonensis. There is a strong recommendation for oral miltefosine treatment for New World CL where L. panamensis, L. mexicana, L. guyanensis, and L. braziliensis are the species involved. Overall, there has been a tilt toward the use of miltefosine in CL and ML instead of antimonials, both in the New and Old Worlds. In Brazil, miltefosine cured 71% of patients with L. (V.) guyanensis infection. Other drugs, such as dapsone, allopurinol, rifampin, azithromycin, and pentoxifylline, have been used either alone or in combinations, but most of the relevant studies have had design limitations that preclude meaningful conclusions. Small lesions (≤3 cm in diameter) may conveniently be treated weekly until cure with an intralesional injection of a pentavalent antimonial at a dose adequate to blanch the lesion (0.2–2.0 mL). An ointment containing 15% paromomycin sulfate, either alone or with 0.5% gentamicin or 12% methylbenzonium chloride, cured 70–82% of lesions due to L. major in 20 days and may be suitable for lesions caused by other species. Heat therapy with an FDA-approved radio­ frequency generator and cryotherapy with liquid nitrogen have also been used successfully. Diffuse Cutaneous Leishmaniasis (DCL)  DCL is a rare form of leishmaniasis caused by L. amazonensis and L. mexicana in South and Central America and by L. aethiopica in Ethiopia and Kenya. DCL is characterized by the lack of a cell-mediated immune response to the parasite, the uncontrolled multiplication of which thus continues unabated. The DTH response does not develop, and lymphocytes do not respond to leishmanial antigens in vitro. DCL patients have a polarized immune response with high levels of immunosuppressive cytokines, including IL-10, transforming growth factor (TGF) β, and IL-4, and low concentrations of IFN-γ. Profound immunosuppression

leads to widespread cutaneous disease. Lesions may initially be con­ fined to the face or a limb but spread over months or years to other areas of the skin. They may be symmetrically or asymmetrically dis­ tributed and include papules, nodules, plaques, and areas of diffuse infiltration. These lesions do not ulcerate. The overlying skin is usu­ ally erythematous in pale-skinned patients. The lesions are teeming with parasites, which are therefore easy to recover. DCL does not heal spontaneously and is difficult to treat. If relapse and drug resistance are to be prevented, treatment should be continued for some time after lesions have healed and parasites can no longer be isolated. In the New World, repeated 20-day courses of pentavalent antimonials are given, with an intervening drug-free period of 10 days. Miltefosine has been used for several months with a good initial response. Combinations should be tried. In Ethiopia, a combination of paromomycin (14 mg/kg

per day) and sodium stibogluconate (10 mg/kg per day) is effective.

■ ■MUCOSAL LEISHMANIASIS The subgenus Viannia is widespread from the Amazon basin to Paraguay and Costa Rica and is responsible for deep sores and for ML (Table 233-1). In L. (V.) braziliensis infections, cutaneous lesions may be simultaneously accompanied by mucosal spread of the disease or followed by spread years later. ML is typically caused by L. (V.) bra­ ziliensis and rarely by L. amazonensis, L. (V.) guyanensis, and L. (V.) panamensis. Young men with chronic lesions of CL are at particular risk. Overall, ~3% of infected persons develop ML. Not every patient with ML has a history of prior CL. ML is almost entirely confined to the Americas. In rare cases, ML may also be caused by Old World species like L. major, L. infantum (L. chagasi), or L. donovani. CHAPTER 233 Immunopathogenesis and Clinical Features  The immune response is polarized toward a TH1 response, with marked increases of IFN-γ and TNF-α and varying levels of TH2 cytokines (IL-10 and TGF-β). Patients have a stronger DTH response with ML than with CL, and their peripheral-blood mononuclear cells respond strongly to leishmanial antigens. The parasite spreads via the lymphatics or the bloodstream to mucosal tissues of the upper respiratory tract. Intense inflammation leads to destruction, and severe disability ensues. Lesions in or around the nose or mouth (espundia; Fig. 233-6) are the typical presentation of ML. Patients usually provide a history of self-healed CL preceding ML by 1–5 years. Typically, ML presents as nasal stuffiness and bleeding followed by destruction of nasal cartilage, perforation of the nasal septum, and collapse of the nasal bridge. Subsequent involve­ ment of the pharynx and larynx leads to difficulty in swallowing and phonation. The lips, cheeks, and soft palate may also be affected. Sec­ ondary bacterial infection is common, and aspiration pneumonia may be fatal. Despite the high degree of TH1 immunity and the strong DTH response, ML does not heal spontaneously. Leishmaniasis Laboratory Diagnosis  Tissue biopsy is essential for identification of parasites, but the rate of detection is poor unless PCR techniques are used. The strongly positive DTH response fails to distinguish between past and present infection. TREATMENT Mucosal Leishmaniasis The regimen of choice is a pentavalent antimonial agent admin­ istered at a dose of 20 mg of SbV/kg for 30 days. Patients with ML require long-term follow-up with repeated oropharyngeal and nasal examination. With failure of therapy or relapse, patients may receive another course of an antimonial but then become unre­ sponsive, presumably because of resistance in the parasite. In this situation, AmB should be used. An AmB deoxycholate dose total­ ing 25–45 mg/kg is appropriate. There are no controlled trials of LAmB, but administration of 2–3 mg/kg for 20 days is considered adequate. Miltefosine (2.5 mg/kg for 28 days) cured 71% of ML patients in Bolivia. The more extensive the disease, the worse is the prognosis; thus, prompt, effective treatment and regular follow-up are essential.

129 - 234 Chagas Disease and African Trypanosomiasis

234 Chagas Disease and African Trypanosomiasis

PART 5 Infectious Diseases FIGURE 233-6  Mucosal leishmaniasis in a Brazilian patient. There is extensive inflammation around the nose and mouth, destruction of the nasal mucosa, ulceration of the upper lip and nose, and destruction of the nasal septum. (Courtesy of R. Dietz, Universidade Federal do Espírito Santo, Vitória, Brazil.) ■ ■PREVENTION OF LEISHMANIASIS No vaccine is available for humans for any form of leishmaniasis, although several candidates are in early phases of development. Inocu­ lation with live L. major (“leishmanization”) is practiced in Iran; 80% of recipients were protected, according to one report. Anthroponotic leishmaniasis is controlled by case finding, treatment, and vector control with insecticide-impregnated bed nets and curtains and residual insecti­ cide spraying. Control of zoonotic leishmaniasis is more difficult. Use of insecticide-impregnated collars for dogs, treatment of infected domestic dogs, and culling of street dogs are measures that have been used with uncertain efficacy to prevent transmission of L. infantum. In Brazil, canine vaccines have been found to promote a decrease in the human and canine incidence of zoonotic VL. Two vaccines, Leishmune and LeishTec, are licensed in Brazil; Leishmune provides significant protection to vaccinated dogs. CaniLeish and LetiFend are the two licensed canine vaccines approved for use in Europe. Personal prophylaxis with bed nets and repellants may reduce the risk of CL infections in the New World. ■ ■FURTHER READING Dorlo TP et al: Optimal dosing of miltefosine in children and adults with visceral leishmaniasis. Antimicrob Agents Chemother 56:3864, 2012. Mann S et al: A review of leishmaniasis: Current knowledge and future directions. Curr Trop Med Rep 8:121, 2021. Pan American Health Organization: Synthesis of evidence and recommendations: Guideline for the treatment of leishmaniasis in the region of the Americas 47:e43, 2023. Singh OP, Sundar S: Visceral leishmaniasis elimination in India: Progress and the road ahead. Expert Rev Anti Infect Ther 20:1381, 2022. World Health Organization: WHO guideline for the treatment of visceral leishmaniasis in HIV co-infected patients in East Africa and South-East Asia. Geneva: World Health Organization, 2022. Available at https://iris.who.int/bitstream/handle/10665/354703/9789240048294-eng. pdf?sequence=1. Accessed September 15, 2023.

François Chappuis, Yves Jackson

Chagas Disease and

African Trypanosomiasis Myriads of protozoan parasites of the genus Trypanosoma infect plants and animals worldwide. Among these, three are of clinical significance for humans: T. cruzi causes Chagas disease, and T. brucei gambiense and T. brucei rhodesiense cause human African trypanosomiasis (HAT), which is also known as “sleeping sickness.” Despite obvious differences in their geographic distribution, parasitic life cycle, clinical presentation, treatment, and outcome, these vector-borne diseases are archetypal examples of neglected tropical diseases. More broadly, these infectious diseases affect neglected populations of the lowest socio­ economic class who have limited access to care and who live either in remote rural areas of low- or middle-income tropical/subtropical countries or in urban areas of both endemic and nonendemic coun­ tries. Most drugs to treat these conditions are several decades old, their availability is limited, and their efficacy and/or safety is suboptimal. Other trypanosome species (e.g., T. congolense and T. evansi) pre­ dominantly cause nonhuman zoonoses and only occasionally cause illness in humans. CHAGAS DISEASE (AMERICAN TRYPANOSOMIASIS) ■ ■DEFINITION First described in 1909 by Carlos Chagas, Chagas disease (American trypanosomiasis) is a zoonosis caused by the flagellated protozoan T. cruzi. After a frequently asymptomatic acute phase, 30–40% of patients develop potentially life-threatening chronic cardiomyopathy and/or digestive tract dysfunction over decades. Acute reactivation may occur in immunocompromised patients. Chagas disease imposes an important human and social burden in Latin America and has recently spread outside its natural boundaries to become a global public health problem. The vast majority of affected individuals are unaware of being infected and do not have access to appropriate clini­ cal management and counseling. ■ ■TRANSMISSION Vectorial Transmission  T. cruzi infection is primarily a zoonosis transmitted to a range of wild and domestic mammals by blood-sucking triatomine bugs. Sylvatic, peridomiciliary, and intradomiciliary vec­ torial cycles sometimes overlap. Over a large geographic area in the Americas (from northern Argentina to the southern United States), most human infections are intradomiciliary, arising from a triatomine bite during nighttime sleep. Feces released by triatomines during a blood meal contain the infective metacyclic form of T. cruzi that enters the human body through cutaneous breaks, mucosae, or conjunctivae. Despite some laboratory research showing the potential for transmis­ sion by bedbugs, there is no evidence that bedbugs actually transmit T. cruzi to humans. Nonvectorial Transmission  Other modes of transmission can cause infection in both endemic and nonendemic regions. T. cruzi can be transmitted congenitally from mother to newborn, by transfusion of blood products, by tissue or organ transplantation, or by ingestion of contaminated food or drink. Congenital infection occurs in 1–10% of newborns of infected mothers. The risk of infection from contami­ nated blood products is low (1.7% overall, 13% for platelet recipients, and close to 0 for recipients of red blood cells and plasma). Transmis­ sion by infected organ and tissue transplants mostly affects heart, liver, and kidney recipients. Oral transmission is increasingly reported after ingestion of contaminated food (berries) or drinks (fruit or sugar cane juice) and occasionally causes outbreaks.

■ ■EPIDEMIOLOGY An estimated 6–7 million people are infected by T. cruzi, including

1 million individuals with chronic cardiomyopathy. However, the true global burden of Chagas disease is in fact uncertain. The highest num­ bers of infected individuals reside in Argentina, Brazil, and Mexico; the prevalence is highest in Bolivia (6.1%), Argentina (3.6%), and Paraguay (2.1%). In highly endemic regions of these countries, the prevalence may exceed 40%. Formerly restricted to poor rural populations, the distribution of cases—and, to some extent, T. cruzi transmission—has progressively extended to cities in the context of rapid urbanization and rural migration. A recent history of migration from a rural area is the main risk factor in urban settings. Overall, the prevalence and incidence of Chagas disease have sharply declined in recent decades because of improved housing and socio­ economic conditions as well as public health interventions, includ­ ing regional vector-control initiatives, implementation of systematic screening of blood products, and improved detection of congenital transmission. Several countries have been declared free of domicili­ ary transmission as a result of sustained residual insecticide-spraying campaigns. This progress is threatened by adaptation of the vector to the periurban environment, its resurgence in areas where spraying has been discontinued, the development of resistance to pyrethroid insec­ ticides, and the persistence of peridomiciliary transmission. A growing number of localized outbreaks are being reported in previously stable areas, with the Amazon basin particularly at risk. Chagas disease distribution has expanded to nonendemic countries in the context of increased global travel, with cases reported more fre­ quently in North America, Western Europe, Australia, and Japan. The United States harbors up to 300,000 cases, mostly among immigrants from Central America. In addition, sporadic vector-borne infec­ tions occur in the southern states. Western Europe has an estimated 68,000–123,000 cases, and Japan and Australia a few thousand cases. Despite the implementation of blood bank screening and of some dedicated medical programs, only a small proportion of cases have been identified and properly managed to date. A low level of awareness among health care professionals and difficulties experienced by some groups in accessing care appear to be major drivers. At-risk migrant communities are frequently subject to factors that render them socially, legally, or economically vulnerable. Moreover, the cultural perception of Chagas as a disease embedded in poverty can create a social stigma that complicates its management at the community level. In contrast to immigrants, international tourists visiting endemic countries are at very low risk of being infected, whether by reduviid bug bites or by other routes, and reports of Chagas disease in travelers are rare. ■ ■PATHOLOGY Several T. cruzi strains have been identified. These strains have par­ tially overlapping transmission cycles and geographic distributions, but no definitive evidence supports an association of certain strains with specific clinical manifestations or with variation in disease severity. The rarity of digestive tract involvement north of the Amazon basin suggests that specific parasitic and host genetic factors may influence the disease course. The pathogenesis of Chagas disease results from the complex interactions between the pathogen and the host immune response. Many questions about the relative importance of these inter­ actions, including the role of autoimmune mechanisms, remain unan­ swered. After local penetration of trypomastigotes, parasites rapidly enter the bloodstream and disseminate through the body, infecting a wide range of nucleated cells in which they differentiate into amasti­ gotes (Fig. 234-1). The innate immune response triggered by parasite mucins and DNA leads to a predominantly T helper 1 response. The production of various proinflammatory cytokines and the activation of CD8+ T lymphocytes reduce parasitemia to a subpatent level within 4–8 weeks, a point marking the end of the acute phase. Immune evasion mechanisms allow persistent low-intensity prolif­ eration of amastigotes and their release into the bloodstream, with sub­ sequent infection of potentially all types of nucleated cells—notably cardiac, skeletal, and smooth-muscle cells. Mechanisms that have been postu­ lated to determine the pathogenic evolution toward cardiomyopathy

FIGURE 234-1  A cluster of Trypanosoma cruzi amastigotes with an inflammatory infiltrate in the placenta of a congenitally infected newborn infant. include the parasites’ persistence and the host’s inability to downregu­ late the initial immune response, resulting in cell-mediated damage and an imbalance of T helper 1 and 2 responses with excessive produc­ tion of proinflammatory cytokines. Secondary mechanisms, such as microcirculation abnormalities and dysautonomia, also may influence the progression of tissue damage. Genome-wide association stud­ ies suggest that genetic variation may contribute to cardiomyopathy development. CHAPTER 234 In the myocardium, chronic inflammation results in cellular destruction and the development of fibrosis leading to a segmental loss of contractility and dilation of the chambers, with the associated risk of left ventricle apical aneurism. Focal hypoperfusion and tissue damage are sources of ventricular arrhythmias, while scarring lesions mostly affect the conduction system. Autonomic cell destruction leads to vagal and sympathetic denervation whose exact clinical significance remains to be clarified. Chagas Disease and African Trypanosomiasis T. cruzi appears to have a direct toxic effect on digestive tract intra­ mural autonomic ganglion cells. Over time, the loss of neural cells affects muscular tone, leading to motility disorders and ultimately to organ dilation (megaviscera syndrome). The esophagus and colon are primarily affected, but lesions may occur along the whole digestive tract. Inadequate relaxation of the lower esophageal sphincter causes symptoms of achalasia, whereas damage to the colon ultimately mimics Hirschsprung’s disease, with severe constipation and the risk of volvu­ lus and toxic dilation. Factors reducing the cellular immune response, such as HIV infection, posttransplantation immunosuppressive therapies, or hematologic malignancies, may increase intracellular replication of amastigotes, with increased parasitemia (reactivation). Lesions develop predominantly in the central nervous system (CNS), the heart, and the skin. Among HIV-positive patients, the risk of reactivation is ~20% in the absence of antiretroviral therapies and occurs when the CD4+ T cell count falls to <100/μL. Clinically manifest T. cruzi reactivation is an AIDS-defining opportunistic infection. ■ ■CLINICAL MANIFESTATIONS The clinical manifestations of T. cruzi infection vary greatly among individuals. The infection course is divided into two phases that are associated with different clinical features, duration, and prognosis (Table 234-1). The acute phase remains undetected and undiagnosed in most individuals. While 5–10% of these early infections spontaneously resolve without treatment, T. cruzi persists for life in most individuals (the chronic phase); 60–70% of these individuals never develop appar­ ent tissue damage (the indeterminate form), but the remaining 30–40% progress toward detectable organ damages of variable severity over decades (the determinate form). These chronic complications include cardiac (20–30%), digestive (5–20%), or mixed (5–10%) disorders. There

TABLE 234-1  Characteristics of the Stages of Trypanosoma cruzi Infection PHASE OR SETTING CONTEXT ONSET OF FIRST SYMPTOMS CLINICAL MANIFESTATIONS DURATION PROGNOSIS Acute (congenital) ~5% risk of maternal transmission to newborn At birth or weeks after delivery

90% asymptomatic; rare lymphadenopathy, hepatosplenomegaly, jaundice, respiratory distress, growth retardation Acute Vector-borne transmission; oral transmission (ingestion of contaminated food/ drinks); blood product transfusion; tissue/organ transplantation 1–2 weeks after vectorial transmission; may be sooner (days) after oral transmission or later (months) after transfusion/ transplantation 90% asymptomatic or mild febrile illness; local swelling at inoculation site (eyelid [Romaña sign] or skin [chagoma]); polyadenopathy; splenomegaly; myocarditis, hepatitis, and encephalitis more frequent with oral transmission Chronic (indeterminate form) Balanced immune response after acute phase subsides No symptoms Normal clinical examination and ECG result Chronic (determinate form) Predominant inflammatory response (in cardiomyopathy only) Years to decades after initial infection Dyspnea, chest pain, palpitation, syncope, sudden death, stroke, dysphagia, regurgitation, constipation, fecaloma, volvulus, peripheral neuropathy Acute (reactivation) Severe immunosuppression Variable Myocarditis, erythema nodosum, panniculitis, Toxoplasma-like focal brain lesion, meningoencephalitis Abbreviation: ECG, electrocardiography. is no predictor of evolution toward clinical manifestations during the chronic phase. In patients with cardiomyopathy, bundle branch blocks are usually the first signs and may cause no symptoms for years until more severe conduction system disease, arrhythmias, and left ventricular dysfunction occur. Advanced cardiac damage entails a worse prognosis than other cardiomyopathies—notably, ischemic heart disease. PART 5 Infectious Diseases APPROACH TO THE PATIENT Chagas Disease (American Trypanosomiasis) More than 90% of infections go undiagnosed, and cases are fre­ quently identified at a late stage once chronic complications develop. The vast majority of T. cruzi–infected individuals are asymptomatic (i.e., in the indeterminate form of the chronic phase). An awareness of potential Chagas disease is important for general practitioners as well as for physicians from various specialties, including gastroen­ terologists, cardiologists, neurologists, obstetricians, pediatricians, and infectious disease specialists. Outside endemic areas, screening for Chagas disease should be proposed when any Latin American individual has evocative symptoms and signs, including abnormali­ ties on electrocardiography (ECG) or increased risk of (1) T. cruzi infection (Chagas disease in the mother or other family members; origins in a highly endemic country or area; history of unscreened blood transfusion in Latin America); (2) transmission to others (e.g., via pregnancy or blood or organ donation); or (3) reactivation (current or pending immunosuppression). Screening of the rela­ tives of an index case may identify additional cases. ■ ■DIAGNOSIS AND STAGING Diagnostic Confirmation  Diagnostic strategies depend on the clinical phase (Table 234-2). Detection of circulating parasites by microscopy of the blood after concentration (e.g., by the Strout method, microhematocrit) or by nucleic acid–based assay (polymerase chain reaction [PCR]) is the best diagnostic approach when the parasit­ emia level is high—i.e., during the acute phases, including reactivation. Once parasitemia becomes undetectable by microscopy (a point mark­ ing the end of the acute phase), diagnosis relies on immunologic tests that detect anti–T. cruzi IgG. The most common techniques include a conventional or recombinant enzyme-linked immunosorbent assay (ELISA) and immunofluorescence assays. Two positive serologic tests using different techniques and targeting different antigens confirm the

2–8 weeks Favorable when infant is born alive; unknown rate of in utero or neonatal death 4–8 weeks Mortality: 0.1–5% with oral transmission or myocarditis/ encephalitis Lifelong or until determinate phase No attributable mortality Chronic 5-year mortality: 2–63%, depending on extent of cardiac damage; most important causes of death: cardiac failure and sudden death, followed by stroke Variable Mortality depends on rapidity of diagnosis and treatment and on underlying conditions diagnosis of Chagas disease during the chronic phase. In the presence of discordant serologic results, a third serologic test is warranted. Some of the immunochromatographic rapid diagnostic tests on the market have sufficient sensitivity and specificity to be used as first-line screen­ ing tests where laboratory facilities are not easily accessible. If the rapid diagnostic test result is positive, at least one conventional serologic assay is necessary to confirm infection. Diagnosis of congenital infection relies on examination of cord and/ or peripheral blood by microscopy or PCR during the first days or weeks of life. A test conducted after 4 weeks of age is most accurate: PCR earlier in life may be falsely positive, likely because of the passage of T. cruzi DNA fragments from the mother to the child. If results are negative, serologic tests should be performed at 9 months of age, once maternal antibodies have been cleared. During the chronic phase, the limited sensitivity (50–80%) of con­ ventional PCR restricts its usefulness for primary diagnosis; however, PCR can document therapeutic failure if it yields positive results after the completion of treatment. In the United States, the Centers for Disease Control and Prevention (CDC) provides reference laboratory testing (see contact information in the treatment section). TABLE 234-2  Diagnostic Procedures of Choice for Clinical Stages of T. cruzi Infection TECHNIQUE OF CHOICE SAMPLE DIAGNOSTIC CRITERIA STAGE Acute Microscopy after concentration, PCR Peripheral blood, cerebrospinal or other body fluids Positivity in one test Acute (early congenital during first

9 months of life) Microscopy after concentration, PCR Cord or peripheral blood Positivity in one test Chronic (indeterminate and determinate forms) Serology Peripheral blood Positivity in two tests with different techniques and antigens Reactivation Microscopy after concentration, PCR Peripheral blood, cerebrospinal or other body fluids Positivity with evidence of increasing parasitemia on serial samples or extremely high parasite load Abbreviation: PCR, polymerase chain reaction.

D1 V1 D2 D3 AVR AVL AVF FIGURE 234-2  Electrocardiogram of a 43-year-old patient shows bradycardia with high-grade atrioventricular blocks. Disease Staging  Once T. cruzi infection is confirmed, clinicians should assess the presence of complications and concomitant factors that may influence the course of the disease. The initial evaluation includes a thorough cardiac, neurologic, and digestive history and a clinical examination. Twelve-lead ECG with a 30-s strip is a good screening test for Chagas-associated cardiomyopathy. The most fre­ quently found abnormalities are right bundle branch block, left anterior fascicular block, ventricular premature beats, repolarization disorders, Q waves, and low QRS voltage (Fig. 234-2). An abnormal ECG result or the presence of suggestive cardiac symptoms warrants further investiga­ tion. Echocardiography and the 24-h Holter test are the preferred meth­ ods for assessment of chamber dilation, apical aneurysm, ventricular dysfunction, and arrhythmias. Depending on the findings, the workup can be supplemented by MRI or electrophysiologic studies. Gastro­ enterologic investigations are performed in patients with suggestive symptoms, such as dysphagia and severe constipation. Barium esopha­ gography and enema are first-line diagnostic procedures, which can be supplemented by esophageal manometry. Megacolon is diagnosed when the sigmoid or descending colon diameter is ≥6.5 cm. Comorbidities, including other cardiovascular risk factors, immu­ nosuppressive conditions, and other chronic infections (e.g., with Strongyloides stercoralis or HIV) should be investigated. TABLE 234-3  Chagas Treatment Regimens and Adverse Reactions to Benznidazole and Nifurtimox DRUG REGIMEN DURATION ADVERSE EVENTS IN ADULTS (FREQUENCY) Benznidazole Age <12 years: 5–7.5 mg/kg per day in 2 doses Age >12 years: 5 mg/kg per day in 2 doses 30–60 days Allergic dermatitis (29–50%), anorexia and weight loss (5–40%), Nifurtimox Age <10 years: 15–20 mg/kg per day in 3 or 4 doses Age 11–16 years: 12.5–15 mg/kg per day in 3 or 4 doses Age >16 years: 8–10 mg/kg per day in 3 or 4 doses 60–90 days Anorexia and weight loss (50–81%), nausea and vomiting Source: From C Bern: Chagas’ Disease. N Engl J Med 373:456, 2015. Copyright © 2015 Massachusetts Medical Society. Reprinted with permission from Massachusetts Medical Society.

V2 V3 V4 V5 V6 CHAPTER 234 TREATMENT Chagas Disease (American Trypanosomiasis) Chagas Disease and African Trypanosomiasis ETIOLOGIC TREATMENT Only two drugs, benznidazole and nifurtimox (Table 234-3), have shown persistent efficacy against T. cruzi infection when adminis­ tered for ≥30 days. While these drugs have been used since the early 1970s, many questions remain about their mode of action and effi­ cacy at the different stages of infection. The treatment goal depends on the clinical stage; the overall objectives are to cure patients who have recent infection or reactivation, to reduce morbidity, and to prevent transmission at later stages. Treatment is most effective during the acute (including congenital) phase and the early chronic phase (i.e., in patients <18 years of age), with a 60–100% cure rate. The efficacy of treatment during the indeterminate form of the chronic phase in patients >18 years old is not known; however, treatment may protect against the development of cardiac damage later in life and eliminate the risk of vertical transmission when given before conception. In adults with chronic cardiomyopathy, benznidazole has no impact on disease progression and mortal­ ity risk. Neither benznidazole nor nifurtimox is effective against PREMATURE DISCONTINUATION (RATE) 7–20% paresthesia (0–30%), peripheral neuropathy (0–30%), nausea and vomiting (0–5%), leukopenia and thrombocytopenia (<1%) 6–44% (15–50%), abdominal discomfort (12–40%), headaches (13–70%), dizziness and vertigo (12–33%), anxiety and depression (10–49%), insomnia (10–54%), myalgia (13–30%), peripheral neuropathy (2–5%), memory loss (6–14%), leukopenia (<1%)

digestive complications. Treatment is contraindicated during preg­ nancy and in advanced renal or hepatic failure. Preferred regimens and drug tolerance vary with age. Adverse events are more frequent among adults, who are therefore at increased risk of premature treatment discontinuation (Table 234-3). As benznidazole seems better tolerated than nifurtimox in adults, it is the recommended first-line drug in this age range. Close (e.g., weekly) clinical and biologic monitoring is necessary during treatment. While treatment is usually prescribed for 60 days, the optimal duration remains a matter of debate, with a growing interest in shorter courses.

Treatment should be undertaken for all children, women of child-bearing age, patients in the acute phase, and patients with reactivation. Given the uncertainties about the impact of treatment, the decision to treat patients >18 years old who have the indeter­ minate form of the chronic phase should be made on an individual basis after discussing the pros and cons with the patient. A nega­ tive pregnancy test is mandatory before initiating treatment, as the recommended drugs have not been proven to be safe in pregnancy. The efficacy of second-line treatment (e.g., nifurtimox after failure with benznidazole) has not been evaluated to date. The limited efficacy of current regimens and the understanding that living parasites are a driver of immunopathologic processes have fueled interest in novel therapeutic approaches. These include the addition of immunomodulatory interventions to antiparasitic treatment and the use of combinations of antiparasitic drugs. Information on drugs can be obtained through the CDC (Parasitic Diseases Public Inquiries line [404-718-4745] or chagas@cdc.gov), or the CDC Emergency Operations Center (770-488-7100). The U.S. Food and Drug Administration has approved the use of benznidazole for treatment of children 2–12 years and nifurti­ mox for those 0–17 years of age, and in older patients based on clinical decision. PART 5 Infectious Diseases NONETIOLOGIC TREATMENT The management of Chagas cardiomyopathy generally follows the management guidelines for heart failure, conduction disturbances, or ventricular arrhythmia of other etiologies. Given the high risk of sudden death, early initiation of treatment with amiodarone or implantation of a cardioverter defibrillator should be considered in the presence of pathologic electrophysiologic abnormalities. Anti­ coagulation is recommended for primary and secondary prevention of cardioembolic events in the presence of an intramural thrombus or apical aneurysm. Strict control of other cardiovascular risk fac­ tors is warranted. Chagas cardiomyopathy is a prominent indication for heart transplantation in Latin America; some evidence indicates that the results are better than in cardiomyopathy of other etiolo­ gies. Posttransplantation immunosuppression requires close moni­ toring, given the high risk of reactivation. Treatment of digestive dysmotility includes dietary counseling and meals rich in fiber and hydration, with smaller portions eaten more frequently. Drugs releasing the lower esophageal sphincter (e.g., nifedipine or isosorbide dinitrate before meals), pneumatic balloon dilatation, or laparoscopic myotomy improve upper gas­ trointestinal symptoms in the early stage. Use of botulinum toxin is effective but requires repeated injections. Laxatives and enemas alleviate chronic constipation in most patients. Surgery is indicated in patients with distressing symptoms that are refractory to medical treatment. CLINICAL FOLLOW-UP Defining the optimal cure after treatment remains very challenging and is a crucial topic of research. While the search for biomarkers (including through proteomics) to identify early indicators of treat­ ment response holds some promise, serologic follow-up remains the cornerstone of posttreatment monitoring in the acute phase. In the chronic phase, there is no assay of proven value for documentation of response. The time needed for negative seroconversion after treatment indeed depends on the duration of infection. The interval is short (usually months, sometimes up to 2 years) when infection

is treated during the acute (including congenital) phase. In contrast, decades are required in adults infected during childhood. A posi­ tive result in a posttreatment PCR indicates treatment failure, but a negative result cannot be interpreted because of the low sensitivity of PCR during the chronic phase. The status of patients with nega­ tive PCR results but persistent positive serology is therefore uncer­ tain, but these patients should be considered potentially infective as long as serologic tests continue to yield positive results. All patients, treated or not, should be regularly monitored. The basic yearly assessment includes history-taking for detection of new symptoms, clinical examination, and 12-lead ECG. ■ ■PREVENTION In the absence of a vaccine, preventive measures—primary (prevention of T. cruzi transmission), secondary (avoidance of complications), and tertiary (reduction of morbidity and mortality)—are necessary. Screen­ ing of blood donations is being progressively implemented in endemic areas and in countries to which high-risk groups are immigrating, and screening should be extended to organ donation. When sustained over prolonged periods, vector control is an effective and cost-effective strat­ egy to curb intradomiciliary transmission. Insecticide-impregnated bed nets (as used for malaria) provide individual protection against reduviid bug bites. Screening of child-bearing-age and pregnant Latin American migrant women has been highly cost-effective in Spain, although the cost per case detected varies with the prevalence of infec­ tion in the targeted population. Early identification of cases through passive and active screening of the population at risk, along with provi­ sion of treatment, may reduce the risk of complications and secondary transmission, particularly congenital transmission. Finally, identifica­ tion and treatment of cardiac complications and prevention of cardio­ embolic events at an early stage positively influence the disease course. ■ ■GLOBAL CONSIDERATIONS With its geographic expansion, Chagas disease has become a global health issue, predominantly affecting vulnerable people on four con­ tinents. Yet, as with other neglected tropical diseases, progress against Chagas is limited by a lack of research and development and a lack of financial and political commitment. For example, the production and registration of existing drugs, and access to them, are still prob­ lematic in many countries, including the United States. Difficulties in research on and development of new drugs are compounded by the lack of financial incentives. The future of Chagas disease is likely to be influenced by global phenomena. Climatic changes, population aging, increasing prevalence of noncommunicable comorbidities (e.g., diabetes, hypertension) in low- and middle-income countries, and increasing use of immunosuppressive drugs are likely to impact the epidemiology, clinical course, and burden of Chagas disease. To tackle these challenges, clinical, public health, and policy interventions need to be scaled up and improved in areas of high or hidden prevalence (e.g., in the Chaco Region of Argentina, Bolivia, and Paraguay and in Mexico, Western Europe, and the United States, respectively). HUMAN AFRICAN TRYPANOSOMIASIS (SLEEPING SICKNESS) ■ ■DEFINITION HAT is a life-threatening illness caused by infection with extracellular protozoan parasites that are transmitted by tsetse flies in sub-Saharan Africa. T. b. gambiense and T. b. rhodesiense are the two pathogenic subspecies affecting humans; their epidemiologic and clinical features largely differ. ■ ■EPIDEMIOLOGY The geographic range of HAT is restricted to sub-Saharan Africa in line with the distribution of its vector, the tsetse fly (Glossina species; Fig. 234-3). HAT due to T. b. gambiense is endemic in 24 countries of western and central Africa. Between 1999 and 2020, the number of reported cases fell by 98% (from 27,862 to 565) as a result of success­ ful control measures based on systematic screening of populations at

FIGURE 234-3  Areas at risk for human African trypanosomiasis, 2016–2020. (Reproduced from Franco JR et al: The elimination of human African trypanosomiasis: Achievements in relation to WHO road map targets for 2020. PLoS Negl Trop Dis 2022; 16(1): e0010047, Figure 4.) risk, diagnostic confirmation, and treatment of infected individuals. During the same period, the number of reported cases of HAT due to T. b. rhodesiense fell by 84% (from 619 to 98) in the 10 diseaseendemic countries of eastern and southeastern Africa. However, the ratio of reported to unreported cases remains uncertain for disease caused by both species. In 2020, most cases of T. b. gambiense HAT were reported by the Democratic Republic of the Congo (DRC; 70%), whereas Malawi reported most of the cases caused by T. b. rhod­ esiense (91%). The geographic distributions of T. b. gambiense and T. b. rhodesiense do not overlap, but the two species are present in distinct regions of Uganda. A roadmap for HAT elimination as a public health problem by 2020 was mapped out by the World Health Organization (WHO) with two primary indicators: the number of cases reported annually (target: <2000; reached since 2018) and the area at risk reporting ≥1 case/10,000 people/year (target: reduction of 90% by 2016−2020 compared with the 2000−2004 baseline; 83% reduction reached in 2020). The next goal set by WHO is the global elimination of transmission by 2030. Humans are the predominant reservoir of T. b. gambiense. Rare cases of vertical (in utero) or transfusional transmission have been reported, but almost all patients are infected by the bite of tsetse flies during their daily activities along or near rivers, where the flies live and reproduce. In contrast, T. b. rhodesiense causes zoonosis in a variety of wild and domesticated animals (e.g., antelopes and cattle, respectively), which act as reservoirs. Humans are infected by T. b. rhodesiense via tsetse bites in woodland savannah. Honey gatherers, game park rangers, poachers, and firewood collectors are particularly at risk. Imported cases of HAT are occasionally diagnosed among African immigrants and other travelers. While long-term travelers (>30 days) are at increased risk of T. b. gambiense HAT, most imported cases of T. b. rhodesiense HAT are seen in short-term travelers, typically following visits to game parks.

CHAPTER 234 Chagas Disease and African Trypanosomiasis ■ ■PATHOLOGY AND PATHOGENESIS T. b. rhodesiense and T. b. gambiense, unlike other trypanosome spe­ cies, can infect humans because they resist lytic factors in human serum—namely, apolipoprotein L-1 (APOL1). Human APOL1 variants are prevalent in individuals of African ancestry, conferring protection against livestock trypanosome species, but at the cost of increasing the likelihood of chronic kidney disease. The serum resistance–associated protein is responsible for resistance in T. b. rhodesiense, whereas other mechanisms, notably involving the T. b. gambiense–specific glycopro­ tein (TgsGP) gene, are used by T. b. gambiense. Trypanosomes are transmitted to humans by the tsetse bite, pro­ liferate, and induce a local inflammatory reaction that is sometimes clinically apparent as a chancre. Trypanosomes then disseminate into the hematolymphatic system, with lymph nodes becoming enlarged after infiltration by mononuclear cells and lymphocytes. The degree of enlargement of the liver and spleen is usually mild to moderate, with infiltration by mononuclear cells as a prominent feature. Trypanosomes multiply in the blood, but their presence and density vary. This variation is mainly due to a cyclic immuneevasion process, whereby the parasite population can be decimated by the host’s immune response until the reemergence of offspring parasites that express a different variant surface glycoprotein to which the immune system is temporarily blind. Each trypanosome genome encodes a repertoire of ~1000 variant surface glycoproteins between which the parasites can switch genetically. Trypanosomes also multiply in extravascular tissues during the first stage of illness. The skin, skeletal muscles, serous membranes (peritoneum, pleurae, and pericardium), and heart can be involved, with interstitial infil­ tration of mononuclear cells and vasculitis evident on microscopic examination. Myocarditis and pericarditis with myocardial degeneration and interstitial hemorrhage are common features of T. b. rhodesiense infection.

The CNS is invaded weeks to months (T. b. rhodesiense) or months to years (T. b. gambiense) after initial infection. This invasion cor­ responds to the second stage of HAT, which is defined by the presence of trypanosomes or mononuclear cells in the cerebrospinal fluid (CSF). The white matter is predominantly affected, with perivascular prolifera­ tion of astrocytes, microglial cells, and Mott’s (morular) cells that contain IgM in intracellular vacuoles. The location of white-matter lesions in the brain correlates with the main neurologic clinical features. The cere­ bral cortex and neurons are spared until the terminal stages of illness. Because reversible inflammatory lesions predominate over the irrevers­ ible destruction of tissue, neuropsychiatric symptoms and signs resolve partially or completely during or after treatment of second-stage HAT.

APPROACH TO THE PATIENT Human African Trypanosomiasis HAT is usually lethal in the absence of treatment. Therefore, early diagnosis is crucial; physicians should include HAT in the dif­ ferential diagnosis of several clinical syndromes when a patient has traveled or lived in at-risk sub-Saharan African countries, and obtaining a thorough recent and remote travel history from the patient is a prerequisite for diagnosis. In particular, HAT due to

T. b. gambiense should be suspected in patients with persistent and intermittent fever or headaches, progressive neuropsychiatric dis­ orders, and biologic signs of systemic inflammation, even if the last exposure occurred several years previously. HAT due to T. b. rhod­ esiense should be suspected in patients with an acute febrile illness and a recent exposure to tsetse flies in an eastern African country, especially if diagnostic tests for malaria are negative. PART 5 Infectious Diseases ■ ■CLINICAL MANIFESTATIONS The clinical presentations of T. b. gambiense and T. b. rhodesiense HAT usually differ. T. b. gambiense HAT is a slowly evolving illness with a long incubation period (months to years) and a prolonged disease course. In contrast, T. b. rhodesiense HAT is an acute febrile illness with a short (<3-week) incubation period and a shorter (weeks to months) disease course. There are exceptions to this classic pattern. Acute forms of T. b. gambiense HAT have been reported, especially among travelers, and chronic forms of T. b. rhodesiense HAT occur in the southern range of its geographic distribution (e.g., Zambia and Malawi). Trypano-resistance (i.e., self-resolving first-stage infections) and trypano-tolerance (i.e., the long-term persistence of parasites [e.g., in the skin] without clinical features of disease) have been reported for T. b. gambiense. Concomitant HIV co-infection does not seem to pre­ dispose individuals to an increased risk of HAT, and the impact of the virus on the clinical presentation of HAT is not known. T. b. gambiense  The occurrence of trypanosomal chancre is reported in a sizeable proportion of travelers, but very rarely in patients living in endemic areas, where the nonpurulent, painful, and itchy nodule can easily be confused with a lesion caused by the bite of another arthro­ pod. The chancre spontaneously disappears in 1–3 weeks. SYSTEMIC FEATURES  After an asymptomatic incubation period that usually lasts for weeks or months but occasionally lasts for years, patients may present with irregular and remittent fever, sometimes accompanied by fatigue, malaise, and myalgia. Fever is more frequent among travelers than among natives, but the absence of fever in no way rules out the disease. Circinate or serpiginous rashes, commonly called trypanids, can occur on the trunk and on proximal parts of the extremities. Trypanids are almost impossible to detect on dark skin and have been reported only in Caucasians. Pruritus is a common but non­ specific symptom that affects up to half of patients during the second stage. Painless edema of the face and extremities occasionally occurs during the first phase. Enlarged lymph nodes—a classic sign of HAT—are detected in 38–85% of patients at both disease stages. Cervical palpation is essen­ tial in patients with suspected HAT. The lateroposterior cervical group (Winterbottom sign) and the supraclavicular group are most commonly

affected. Lymph nodes are movable, soft initially, harder later, and pain­ less. A variable proportion of patients present with mild to moderate hepatomegaly and splenomegaly. Signs of myocarditis and pericarditis are occasionally detected by ECG and echocardiography but are usu­ ally clinically silent. Symptoms of HAT may mimic hypothyroidism or adrenal insufficiency, but thyroid and adrenal function tests yield normal results. Loss of libido, impotence, and amenorrhea, with decreased levels of testosterone and estradiol, are common in second-stage patients and are most likely caused by dysfunction of the hypothalamic–pituitary axis. NEUROPSYCHIATRIC FEATURES  Most patients with second-stage ill­ ness have no or only mild specific neuropsychiatric symptoms and signs, which, when they develop, tend to do so late in the disease course. In contrast, some nonspecific features, such as headaches and mood and behavioral changes, are present in both disease stages but become more permanent and severe during the second stage. As mentioned earlier, HAT is commonly called “sleeping sickness” because of various sleep disturbances (daytime somnolence, nocturnal insomnia) that are more pronounced late in the second stage. Dysregulation of the daily sleep/ wake cycle and fragmentation of sleeping patterns are characteristic. Depending on the area of the brain affected, various neurologic syn­ dromes also can develop, including disorders that are pyramidal-related (e.g., motor weakness, rare instances of hemiplegia), extrapyramidalrelated (e.g., rigidity, paratonia), and cerebellar-related (e.g., ataxia, abnormal gait). Fine tremor, resting myoclonus, and abnormal (athetoid or choreic) movements also have been reported. Mental disorder is a key feature of HAT and can easily be misdiagnosed as primary psychiatric ill­ ness. Common presentations are antisocial or aggressive behavior, mood disorders (e.g., irritability, indifference), apathy or hyperactivity, and depression or psychosis (e.g., delirium, hallucinations). In the final stage of illness, decreased consciousness, dementia, and sometimes epilepsy are present, leading to coma, bed sores, aspiration pneumonia, or other bacterial infections and ultimately to death. T. b. rhodesiense  The clinical presentation of T. b. rhodesiense HAT can be similar to that of T. b. gambiense HAT in areas (e.g., Zambia, Malawi) that characteristically harbor specific parasite genotypes and host factors. The typical acute form with an incubation period of <3 weeks generally occurs in the northern range of the disease’s distribution (e.g., Tanzania, Uganda) and in travelers. The initial try­ panosomal chancre is clinically similar to that seen in T. b. gambiense HAT but is more common, especially among travelers. SYSTEMIC FEATURES  Fever can be high and occurs in both first- and second-stage patients, often in association with headaches and with diffuse myalgia and arthralgia. Pruritus and edema of the face and legs can be present. Lymphadenopathies have been reported in variable proportions in both disease stages and predominately affect the submandibular, axillary, and inguinal regions. Mild to moderate hepatomegaly and splenomegaly are documented in a minority of patients. Myocarditis and pericarditis appear to influence clinical course and outcome, even though clinical features of cardiac fail­ ure or arrhythmia have not been prominent findings in large case series. In contrast, conduction abnormalities, with various degrees of atrioventricular block, have been reported in travelers. Sepsis-like features, with disseminated intravascular coagulation and multipleorgan failure, can occur in the terminal stage. NEUROPSYCHIATRIC FEATURES  Neuropsychiatric symptoms and signs in T. b. rhodesiense HAT are reported with varying frequency but overall are similar to those described above for T. b. gambiense HAT. The notable exception in T. b. rhodesiense disease is a more rapid evolu­ tion toward coma and death. ■ ■DIAGNOSIS The clinical and biologic features of T. b. gambiense and T. b. rhodesiense HAT—anemia, thrombocytopenia, elevated levels of C-reactive protein and IgM—are not sufficiently specific, and current drug regimens are not sufficiently practical to allow the initiation of treatment solely on the basis of suspicion. Diagnostic confirmation is therefore required in all patients.

T. b. gambiense  The diagnosis of T. b. gambiense HAT is based on a three-step approach: screening, diagnostic confirmation, and staging. SCREENING  Immunologic (serologic) methods constitute the pre­ ferred screening tool. The card agglutination test for trypanosomiasis (CATT) has been used in most endemic areas for several decades. The test reagent contains stained, freeze-dried trypanosomes of selected variable-antigen types. If specific antibodies are present in the patient’s blood or serum, agglutination can be seen with the naked eye. The sensitivity of the CATT on undiluted blood or serum is 69–100% (>90% in most studies), with some regional variation; its specificity is 84–99%. The CATT and associated equipment (e.g., a rotator) are manufactured and distributed by the Institute of Tropical Medicine in Antwerp, Belgium, but are not widely available outside endemic areas. In recent years, lateral flow tests have been developed and commercial­ ized, first based on whole parasites and later on recombinant antigens. Their diagnostic performance is comparable to that of the CATT. Other serologic test formats (ELISA, immunofluorescence, indirect hemagglutination) are available in some reference laboratories in both endemic and nonendemic countries. DIAGNOSTIC CONFIRMATION  The microscopic observation of try­ panosomes in the lymph, blood, skin, or CSF confirms the diagnosis. Direct observation of motile trypanosomes on a wet preparation of lymph obtained by cervical lymph node puncture is simple and cheap but has limited sensitivity (50–65% in most studies). Trypanosomes can be found in the blood but often occur at low densities. Therefore, stained thin and thick blood smears have very low sensitivity. Sensitiv­ ity is improved (to 40–60% in most studies) with the microhematocrit centrifugation technique, which is based on microscopic examination of the buffy coat after centrifugation of four to six microhematocrit tubes. The most sensitive method (~90%) is the miniature anionexchange centrifugation technique, which is based on the visualization of trypanosomes in eluate after the passage of a large volume (500 μL) of blood through an anion-exchange column and subsequent centrifu­ gation. Trypanosomes can also be visualized on microscopic examina­ tion of skin biopsies, even when parasites have not been visualized in the blood or lymph node. STAGING  Staging is based on the examination of CSF obtained by lum­ bar puncture. Second-stage HAT is defined by the presence in CSF of a raised leukocyte count (>5/μL) and/or of trypanosomes. The latter can be detected in the cell-counting chamber or, preferably, after centrifuga­ tion of the CSF. Staging is no longer an obligatory step in settings where fexinidazole is used as first-line treatment for both first- and secondstage HAT patients, except for young children (<6 years or weighing <20 kg) and for patients with neuropsychiatric symptoms and signs consistent with severe HAT, i.e., mental confusion, abnormal behavior, logorrhea, anxiety, ataxia, tremor, motor weakness, speech impairment, abnormal gait or movements, or seizures (see “Treatment,” below). Several molecular methods based on PCR or loop-mediated isother­ mal amplification have been developed, mostly based on the detection of multiple-copy DNA targets of the Trypanozoon group (to which T. brucei belongs) or the single-copy TgsGP gene of T. b. gambiense. None of these methods has been fully validated for diagnostic pur­ poses, and a positive result of their application to blood should be interpreted as suspected rather than confirmed HAT. Molecular meth­ ods applied to CSF (to detect biomarkers) have not proved more accu­ rate than classic methods for staging and have yielded false-positive results in a substantial proportion of cases. T. b. rhodesiense  The diagnosis of T. b. rhodesiense HAT is usually simpler because parasites are more numerous in body fluids. They can occasionally be visualized in a chancre aspirate. In light of the lack of available serologic tests and the high sensitivity of parasite detection methods in blood, wet mounts, thin/thick smears (Fig. 234-4), and the microhematocrit or other concentration techniques are used for both screening and confirmation. Because the modalities of treatment of T. b. rhodesiense are stage dependent, staging remains an obligatory step, and the definition and methods used are the same as for T. b. gambiense HAT.

FIGURE 234-4  Trypanosoma brucei rhodesiense in blood (thin smear, Giemsa stain). (Credit to the DPDx team, U.S. Centers for Disease Control and Prevention, Atlanta.) TREATMENT Human African Trypanosomiasis The management of HAT is based on general supportive therapy (e.g., rehydration, pain management), treatment of concomitant infections (e.g., malaria, pneumonia), and antiparasitic treatment. The modalities of antitrypanosomal treatment depend on the Trypanosoma species, the stage of illness, and the presence of

contraindications (Table 234-4). T. B. GAMBIENSE Fexinidazole, a nitroimidazole compound, is the first effective oral treatment against HAT. It is administered with food for 10 days, divided into a 4-day loading phase and a 6-day maintenance phase. It is highly effective (>95% cure rate) in patients with firststage and nonsevere second-stage HAT, the latter being defined as

<100 leukocytes/μL in the CSF. Fexinidazole is associated with a lower cure rate (87%) in patients with severe second-stage (≥100 leukocytes/μL in the CSF) HAT. The most relevant adverse reactions reported in clinical trials are vomiting, headache, and neuropsychiatric disorders (e.g., insomnia, anxiety, agitation). Fexi­ nidazole is contraindicated in patients with hepatic insufficiency or at increased risk of QT interval prolongation. In the absence of safety and efficacy data, it remains contraindicated in small children (<6 years and/or weighing <20 kg). CHAPTER 234 Chagas Disease and African Trypanosomiasis Pentamidine isethionate is highly effective (>95%) against firststage T. b. gambiense HAT and is an excellent alternative to fexi­ nidazole when the latter is contraindicated or not available. It is generally well tolerated and can therefore be administered in peripheral health care centers in endemic countries (Fig. 234-5). Hypotension after injection is common but generally mild. Hypo­ glycemia or hyperglycemia occasionally occurs, but permanent dia­ betes is very rare. Severe adverse events, such as acute pancreatitis and anaphylaxis, occur extremely rarely. Nifurtimox–eflornithine combination therapy is very effective (>95% cure rate) and safe in patients with second-stage HAT, including patients with severe (≥100 leukocytes/μL in the CSF) illness. Common adverse reactions include gastrointestinal distur­ bances (nausea, vomiting, abdominal pain), headache, anorexia, and reversible bone marrow toxicity (anemia, leukopenia). Convul­ sions and psychosis are reported in <5% of patients. Acoziborole, administered orally as a single-dose treatment (three tablets), cured >95% of 208 patients age >15 years with first- and second-stage HAT. Provided that its efficacy and safety is confirmed in a higher number of individuals, acoziborole could become the preferred treatment for T. b. gambiense HAT in the future.

TABLE 234-4  Treatment of Human African Trypanosomiasis (HAT) FIRST-LINE TREATMENT DISEASE AND STAGE ALTERNATIVE TREATMENT DRUG(S) AND ROUTE DOSE AND DURATION T. b. gambiense HAT First stage Fexinidazole PO ≥35 kg: 1800 mg for 4 days, followed by 1200 mg for 6 days 20–34 kg: 1200 mg for 4 days, followed by 600 mg for 6 daysa Nonsevere second stage (6–99 leukocytes/ μL in the cerebrospinal fluid [CSF]) Fexinidazole PO ≥35 kg: 1800 mg for 4 days, followed by 1200 mg for 6 days 20–34 kg: 1200 mg for 4 days, followed by 600 mg for 6 daysa Severe second stage (≥100 leukocytes/μL in the CSF) Eflornithine IV + nifurtimox PO Eflornithine: 200 mg/kg bid for 7 days Nifurtimox: 5 mg/kg tid for 10 days T. b. rhodesiense HAT First stage Suramin IV 4–5 mg/kg on day 1 followed by 5 weekly injections of 20 mg/kg (e.g., days 3, 10, 17, 24, 31)c Pentamidine isethionate IM or IVb: 4 mg/kg per day for 7 days Second stage Melarsoprol IV 2.2 mg/kg per day for 10 days — aFexinidazole should not be administered in children <6 years and weighing <20 kg. bFor IV administration, slow infusion (60–120 min) should be used. cThe maximal dose is 1 g per injection; the drug should be diluted in distilled water. Sources: Control and surveillance of human African trypanosomiasis: Report of a WHO Expert Committee. WHO Technical Report Series 984, 2013; WHO interim guidelines for the treatment of gambiense human African trypanosomiasis. August 2019; https://www.who.int/publications/i/item/9789241550567. T. B. RHODESIENSE Suramin has been used for >90 years and remains the first-line treatment for first-stage T. b. rhodesiense HAT. Common adverse events are pyrexia and nephrotoxicity, which is usually mild and reversible but necessitates surveillance of albuminuria and renal function before each dose. PART 5 Infectious Diseases Because eflornithine is ineffective against T. b. rhodesiense, melar­ soprol, an arsenic-based derivative, remains in use for second-stage T. b. rhodesiense HAT. Reactive encephalopathy is a life-threatening adverse event that occurs in 5–18% of patients, with an associated mortality rate of 10–70%. The efficacy of concomitant high-dose prednisolone to prevent reactive encephalopathy in patients with T. b. rhodesiense HAT is not known. Other severe but less frequent adverse reactions to melarsoprol include exfoliative dermatitis, FIGURE 234-5  Intramuscular injection of pentamidine by a nurse in a village health center, Province Orientale, Democratic Republic of the Congo.

Pentamidine isethionate IM or IVb: 4 mg/kg per day for 7 days Eflornithine: 200 mg/kg bid for 7 days plus Nifurtimox: 5 mg/kg tid for 10 days Fexinidazole: ≥35 kg: 1800 mg for 4 days, followed by 1200 mg for 6 days 20–34 kg: 1200 mg for 4 days, followed by 600 mg for 6 daysa bloody diarrhea, peripheral neuropathy, renal dysfunction, and liver toxicity. Phlebitis is common, as is soft tissue necrosis if the drug is accidentally given paravenously. The 10-day fexinidazole oral treatment regimen (see above for T. b. gambiense HAT) was recently studied as an alternative to suramin and melarsoprol in Malawi and Uganda. Fexinidazole was administered in 45 patients with first-stage (n = 10) and secondstage (n = 35) patients and cured 43 (96%). ■ ■PROGNOSIS Provided that treatment guidelines are properly followed, >95% of patients with first-stage and second-stage T. b. gambiense HAT are definitively cured with fexinidazole, pentamidine, and nifurtimox– eflornithine combination therapy. The overall case–fatality rate is <1% except in very advanced cases. Because relapses can occur long after completion of treatment, follow-up visits are advised every 6 months for at least 2 years. If clinical features of HAT are present, both blood and CSF examinations are indicated. Patients with second-stage T. b. rhodesiense HAT are at a 5–10% risk of dying during or after melarsoprol treatment, but relapses are very rare. ■ ■GLOBAL CONSIDERATIONS The elimination of sleeping sickness as a public health problem has been achieved, thanks to increased control activities run by national control programs and nongovernmental medical organiza­ tions, improved funding, and the end of several civil wars (e.g., in Angola) in the past 20 years. Funding for research, development, and implementation of improved diagnostic (e.g., rapid diagnostic tests), therapeutic (e.g., oral drugs), and vector control tools remains crucial to sustain recent achievements and to move on to the next objective, i.e., the global elimination of transmission by 2030. ■ ■FURTHER READING Bern C et al: Chagas disease in the United States: A public health approach. Clin Microbiol Rev 33:e00023-19, 2019. Büscher P et al: Human African trypanosomiasis. Lancet 390:2397, 2017. de Sousa AS et al: Chagas disease. Lancet 403:203, 2024. Lindner AK et al: New WHO guidelines for treatment of gambiense human African trypanosomiasis including fexinidazole: Substantial changes for clinical practice. Lancet Infect Dis 20:e38, 2020. Urech K et al: Sleeping sickness in travelers—Do they really sleep? PLoS Negl Trop Dis 5:e1358, 2011.

13 - 134 Infections of the Skin, Muscles, and Soft Tissues

134 Infections of the Skin, Muscles, and Soft Tissues

Dennis L. Stevens, Amy E. Bryant

Infections of the Skin,

Muscles, and Soft Tissues Skin and soft tissue infections occur in all races, all ethnic groups, and all geographic locations, although some have unique geographic niches. In modern times, the frequency and severity of some skin and soft tissue infections have increased for several reasons. First, microbes are rapidly disseminated throughout the world via efficient air travel, acquiring genes for virulence factors and antibiotic resistance. Second, natural disasters, such as earthquakes, tsunamis, tornadoes, and hur­ ricanes, appear to be increasing in frequency, and the injuries sustained during these events commonly cause major skin and soft tissue damage that predisposes to infection. Third, trauma and casualties resulting from combat and terrorist activities can markedly damage or destroy tissues and provide both endogenous and exogenous pathogens with ready access to deeper structures. Unfortunately, because the marvels of modern medicine may not be available during human-instigated and natural disasters, primary treatment may be delayed and the likelihood of severe infection and death increased. ANATOMIC RELATIONSHIPS: CLUES TO THE DIAGNOSIS OF SOFT TISSUE INFECTIONS Skin and soft tissue infections have been common human afflictions for centuries. However, between 2000 and 2004, hospital admissions for these infections rose by 27%, a remarkable increase that was attributable largely to the emergence of the USA300 clone of methicillinresistant Staphylococcus aureus (MRSA). This chapter provides an anatomic approach to understanding the types of soft tissue infections and the diverse microbes responsible. Protection against infection of the epidermis depends on the mechanical barrier afforded by the stratum corneum since the epi­ dermis itself is devoid of blood vessels (Fig. 134-1). Disruption of this layer by burns or bites, abrasions, foreign bodies, primary dermatologic disorders (e.g., herpes simplex, varicella, ecthyma gangrenosum), sur­ gery, or vascular or pressure ulcers allows penetration of bacteria to the deeper structures. Similarly, the hair follicle can serve as a portal either for components of the normal flora (e.g., Staphylococcus) or for extrinsic bacteria (e.g., Pseudomonas in hot-tub folliculitis). Intracel­ lular infection of the squamous epithelium with vesicle formation may arise from cutaneous inoculation, as in infection with herpes simplex virus (HSV) type 1; from the dermal capillary plexus, as in varicella and Crust Bulla Eschar Vesicle Hair follicle Stratum corneum Stratum germinativum Erysipelas Dermal papillae Sebaceous gland Post-capillary venule Cellulitis Subcutaneous fat Necrotizing fasciitis Deep fascia Lymphatic channel Vein Artery Myositis Muscle Bone FIGURE 134-1  Structural components of the skin and soft tissues, superficial infections, and infections of the deeper structures. The rich capillary network beneath the dermal papillae plays a key role in the localization of infection and in the development of the acute inflammatory reaction.

infections due to other viruses associated with viremia; or from cutane­ ous nerve roots, as in herpes zoster. Bacteria infecting the epidermis, such as Streptococcus pyogenes, may be translocated laterally to deeper structures via lymphatics, an event that results in the rapid superficial spread of erysipelas. Later, engorgement or obstruction of lymphat­ ics causes flaccid edema of the epidermis, another characteristic of erysipelas.

The rich plexus of capillaries beneath the dermal papillae provides nutrition to the stratum germinativum, and physiologic responses of this plexus produce important clinical signs and symptoms. For exam­ ple, infective vasculitis of the plexus results in petechiae, Osler nodes, Janeway lesions, and palpable purpura, which, if present, are impor­ tant clues to the existence of endocarditis (Chap. 133). In addition, metastatic infection within this plexus can result in cutaneous mani­ festations of disseminated fungal infection (Chap. 222), gonococcal infection (Chap. 161), Salmonella infection (Chap. 171), Pseudomonas infection (i.e., ecthyma gangrenosum; Chap. 170), meningococcemia (Chap. 160), and staphylococcal infection (Chap. 152). The plexus also provides bacteria with access to the circulation, thereby facilitat­ ing local spread or bacteremia. The postcapillary venules of this plexus are a prominent site of polymorphonuclear leukocyte sequestration, diapedesis, and chemotaxis to the site of cutaneous infection. Amplification of these physiologic mechanisms by excessive levels of cytokines or bacterial toxins causes leukostasis, venous occlusion, and pitting edema. Edema with purple bullae, ecchymosis, and cutaneous anesthesia suggests loss of vascular integrity and necessitates explora­ tion of the deeper structures for evidence of necrotizing fasciitis or myonecrosis. An early diagnosis requires a high level of suspicion in instances of unexplained fever and of pain and tenderness in the soft tissue, even in the absence of acute cutaneous inflammation. CHAPTER 134 Table 134-1 indicates the chapters in which the infections described below are discussed in greater detail. Many of these infections are illustrated in the chapters cited or in Chap. A1. Infections of the Skin, Muscles, and Soft Tissues INFECTIONS ASSOCIATED WITH VESICLES (Table 134-1) Vesicle formation due to infection is caused by viral proliferation within the epidermis. In varicella and variola, viremia precedes the onset of a diffuse centripetal rash that progresses from macules to vesicles, then to pustules, and finally to scabs over the course of 1–2 weeks. Vesicles of varicella have a “dewdrop” appearance and develop in crops randomly about the trunk, extremities, and face over 3–4 days. Herpes zoster occurs in a single dermatome; the appear­ ance of vesicles is preceded by pain for several days. Zoster may occur in persons of any age but is most common among immunosuppressed individuals and elderly patients, whereas most cases of varicella occur in young children. Vesicles due to HSV are found on or around the lips (HSV-1) or genitals (HSV-2) but also may appear on the head and neck of young wrestlers (herpes gladiatorum) or on the digits of health care workers (herpetic whitlow). Recurrent herpes labialis (HSV-1) and herpes genitalis (HSV-2) commonly follow primary infection. Coxsackievirus A16 characteristically causes vesicles on the hands, feet, and mouth of children. Orf is caused by a DNA virus related to small­ pox virus and infects the fingers of individuals who work around goats and sheep. Molluscum contagiosum virus induces flaccid vesicles on the skin of healthy and immunocompromised individuals. Although variola (smallpox) in nature was eradicated as of 1977, postmillennial terrorist events have renewed interest in this devastating infection (Chap. S4). Viremia beginning after an incubation period of 12 days is followed by a diffuse maculopapular rash, with rapid evolution to vesicles, pustules, and then scabs. Secondary cases can occur among close contacts. Rickettsialpox begins after mite-bite inoculation of Rickettsia akari into the skin. A papule with a central vesicle evolves to form a 1- to 2.5-cm painless crusted black eschar with an erythematous halo and proximal adenopathy. While more common in the northeastern United States and Ukraine in 1940–1950, rickettsialpox has recently been described in Ohio, Arizona, and Utah. Blistering dactylitis is a painful, vesicular, localized S. aureus or group A streptococcal infec­ tion of the pulps of the distal digits of the hands.

PART 5 Infectious Diseases TABLE 134-1  Skin and Soft Tissue Infections LESION, CLINICAL SYNDROME INFECTIOUS AGENT(S) SEE ALSO CHAP(S). Vesicles       Smallpox Variola virus S4   Chickenpox Varicella-zoster virus

  Shingles (herpes zoster) Varicella-zoster virus

  Cold sores, herpetic whitlow, herpes gladiatorum Herpes simplex virus

  Hand-foot-and-mouth disease Coxsackievirus A16

  Orf Parapoxvirus

  Molluscum contagiosum Molluscum contagiosum poxvirus

  Rickettsialpox Rickettsia akari

  Blistering distal dactylitis Staphylococcus aureus or Streptococcus pyogenes 152, 153 Bullae   Staphylococcal scalded-skin syndrome S. aureus

  Necrotizing fasciitis S. pyogenes, Clostridium spp., mixed aerobes and anaerobes 153, 159, 182   Gas gangrene Clostridium spp.

  Halophilic Vibrio Vibrio vulnificus

Crusted lesions       Bullous impetigo/ecthyma S. aureus

  Impetigo contagiosa S. pyogenes

  Ringworm Superficial dermatophyte fungi

  Sporotrichosis Sporothrix schenckii

  Histoplasmosis Histoplasma capsulatum

  Coccidioidomycosis Coccidioides immitis

  Blastomycosis Blastomyces dermatitidis

  Cutaneous leishmaniasis Leishmania spp.

  Cutaneous tuberculosis Mycobacterium tuberculosis

  Nocardiosis Nocardia asteroides

Folliculitis       Furunculosis S. aureus

  Hot-tub folliculitis Pseudomonas aeruginosa

  Swimmer’s itch Schistosoma spp.

  Acne vulgaris Propionibacterium acnes

Papular and nodular lesions       Fish-tank or swimming-pool granuloma Mycobacterium marinum

  Creeping eruption (cutaneous larva migrans) Ancylostoma braziliense

  Dracunculiasis Dracunculus medinensis

  Cercarial dermatitis Schistosoma mansoni

  Verruca vulgaris Human papillomaviruses 1, 2, 4

  Condylomata acuminata (anogenital warts) Human papillomaviruses 6, 11, 16, 18

  Onchocerciasis nodule Onchocerca volvulus

  Cutaneous myiasis Dermatobia hominis

  Verruca peruana Bartonella bacilliformis

  Cat-scratch disease Bartonella henselae

  Lepromatous leprosy Mycobacterium leprae

  Secondary syphilis (papulosquamous and nodular lesions, condylomata lata) Treponema pallidum

  Tertiary syphilis (nodular gummatous lesions) T. pallidum

Ulcers with or without eschars       Anthrax Bacillus anthracis S4   Ulceroglandular tularemia Francisella tularensis 175, S4   Bubonic plague Yersinia pestis 176, S4   Buruli ulcer Mycobacterium ulcerans

  Leprosy M. leprae

  Cutaneous tuberculosis M. tuberculosis

  Chancroid Haemophilus ducreyi

  Primary syphilis T. pallidum

Erysipelas S. pyogenes

Cellulitis Staphylococcus spp., Streptococcus spp., various other bacteria Various Necrotizing fasciitis       Streptococcal gangrene S. pyogenes

  Fournier gangrene Mixed aerobic and anaerobic bacteria

  Staphylococcal necrotizing fasciitis Methicillin-resistant S. aureus

Myositis and myonecrosis       Pyomyositis S. aureus

  Streptococcal necrotizing myositis S. pyogenes

  Gas gangrene Clostridium spp.

  Nonclostridial (crepitant) myositis Mixed aerobic and anaerobic bacteria

  Synergistic nonclostridial anaerobic myonecrosis Mixed aerobic and anaerobic bacteria

The recent spike in mpox (formerly monkeypox virus; MPXV) (Chap. 201) cases has raised concerns due to its clinical resemblance to smallpox in terms of symptom onset, timing of rash occurrence, and rash distribution. MPXV infection is generally less severe than smallpox in terms of complication rate and levels of scarification. The case fatality rate (1–10%) is also less than that of smallpox. Though largely endemic in West and Central Africa, cases have recently emerged in at least 10 other African countries. In addition, cases have been reported in Israel, the United Kingdom, and in six midwestern states of the United States. Most non-African cases are associated with tourism to endemic African countries or with exposure to small mam­ mals imported from such areas. However, human-to-human disease transmission was clearly documented in a British health care worker who attended a patient that had acquired the infection during a trip to Nigeria. Most patients are in lower age groups (<40 years) due to a lack cross-protective immunity, having been born after discontinuation of the smallpox eradication campaign in the 1980s. This finding height­ ens the concern for potential global disease spread. INFECTIONS ASSOCIATED WITH BULLAE (Table 134-1) Staphylococcal scalded-skin syndrome (SSSS) in neonates is caused by a toxin (exfoliatin) from phage group II S. aureus. SSSS must be distinguished from toxic epidermal necrolysis (TEN), which occurs primarily in adults, is drug-induced, and is associated with a higher mortality rate. Punch biopsy with frozen section is useful in making this distinction since the cleavage plane is the stratum corneum in SSSS and the stratum germinativum in TEN (Fig. 134-1). Treatment with intravenous immune globulin plus corticosteroids may reduce recovery time and improve prognosis in patients with TEN. Necrotizing fasciitis and gas gangrene also induce bulla formation (see “Necrotizing Fasciitis,” below). Halophilic Vibrio infection can be as aggressive and fulminant as necrotizing fasciitis; a helpful clue in its diagnosis is a his­ tory of exposure to waters of the Gulf of Mexico or the Atlantic seaboard or (in a patient with cirrhosis) the ingestion of raw seafood. The etio­ logic organism (Vibrio vulnificus) is highly susceptible to tetracycline. INFECTIONS ASSOCIATED WITH

CRUSTED LESIONS (Table 134-1) Impetigo contagiosa is caused by S. pyogenes, and bul­ lous impetigo is due to S. aureus. Both skin lesions may have an early bullous stage but then appear as thick crusts with a golden-brown color. Epidemics of impetigo caused by MRSA have been reported. Streptococcal lesions are most common among children 2–5 years of age, and epidemics may occur in settings of poor hygiene, par­ ticularly among children in lower socioeconomic settings in tropical climates. It is important to recognize impetigo contagiosa because of its relationship to poststreptococcal glomerulonephritis. Rheumatic fever is not a complication of skin infection caused by S. pyogenes. Superficial dermatophyte infection (ringworm) can occur on any skin surface, and skin scrapings with KOH staining are diagnostic. Primary infections with dimorphic fungi such as Blastomyces dermatitidis and Sporothrix schenckii can initially present as crusted skin lesions resem­ bling ringworm. Disseminated infection with Coccidioides immitis can also involve the skin, and biopsy and culture should be performed on crusted lesions when the patient is from an endemic area. Crusted nodular lesions caused by Mycobacterium chelonae have been described in HIV-seropositive patients. FOLLICULITIS (Table 134-1) Hair follicles serve as portals for a number of bacteria, although S. aureus is the most common cause of localized folliculitis. Sebaceous glands empty into hair follicles and ducts and, if these portals are blocked, form sebaceous cysts that may resemble staphy­ lococcal abscesses or may become secondarily infected. Inflammation of sweat glands (hidradenitis suppurativa) also can mimic infection of hair follicles, particularly in the axillae, but new treatments with potent anti-inflammatory agents hold promise. Chronic folliculitis is uncom­ mon except in acne vulgaris, where constituents of the normal flora (e.g., Propionibacterium acnes) may play a role.

Diffuse folliculitis occurs in two settings. Hot-tub folliculitis is caused by Pseudomonas aeruginosa in waters that are insufficiently chlorinated and maintained at temperatures of 37–40°C. Infection is usually selflimited, although bacteremia and shock have been reported. Swimmer’s itch occurs when a skin surface is exposed to water infested with fresh­ water avian schistosomes. Warm water temperatures and alkaline pH are suitable for mollusks that serve as intermediate hosts between birds and humans. Free-swimming schistosomal cercariae readily penetrate human hair follicles or pores but quickly die and elicit a brisk allergic reaction, causing intense itching and erythema.

PAPULAR AND NODULAR LESIONS (Table 134-1) Raised lesions of the skin occur in many different forms. Mycobacterium marinum infections of the skin may present as cel­ lulitis or as raised erythematous nodules. Similar lesions caused by Mycobacterium abscessus and M. chelonae have been described among patients undergoing cosmetic laser surgery and tattooing, respectively. Erythematous papules are early manifestations of cat-scratch disease (with lesions developing at the primary site of inoculation of Bartonella henselae) and bacillary angiomatosis (also caused by B. henselae). Raised serpiginous or linear eruptions are characteristic of cutaneous larva migrans, which is caused by burrowing larvae of dog or cat hook­ worms (Ancylostoma braziliense) and which humans acquire through contact with soil that has been contaminated with dog or cat feces. Similar burrowing raised lesions are present in dracunculiasis caused by migration of the adult female nematode Dracunculus medinensis. Nodules caused by Onchocerca volvulus measure 1–10 cm in diameter and occur mostly in persons bitten by Simulium flies in Africa. The nodules contain the adult worm encased in fibrous tissue. Migration of microfilariae into the eyes may result in blindness. Verruga peruana is caused by Bartonella bacilliformis, which is transmitted to humans by the sandfly Phlebotomus. This condition can take the form of single gigantic lesions (several centimeters in diameter) or multiple small lesions (several millimeters in diameter). Numerous subcutaneous nodules may also be present in cysticercosis caused by larvae of Taenia solium. Multiple erythematous papules develop in schistosomiasis; each represents a cercarial invasion site. Skin nodules as well as thick­ ened subcutaneous tissue are prominent features of lepromatous lep­ rosy. Large nodules or gummas are features of tertiary syphilis, whereas flat papulosquamous lesions are characteristic of secondary syphilis. Human papillomavirus may cause singular warts (verruca vulgaris) or multiple warts in the anogenital area (condylomata acuminata). The latter are major problems in HIV-infected individuals and others with defects in cell-mediated immunity. CHAPTER 134 Infections of the Skin, Muscles, and Soft Tissues ULCERS WITH OR WITHOUT ESCHARS (Table 134-1) Cutaneous anthrax begins as a pruritic papule, which develops within days into an ulcer with surrounding vesicles and edema and then into an enlarging ulcer with a black eschar. Cutaneous anthrax may cause chronic nonhealing ulcers with an overlying dirtygray membrane, although lesions may also mimic psoriasis, eczema, or impetigo. Ulceroglandular tularemia may have associated ulcerated skin lesions with painful regional adenopathy. Although buboes are the major cutaneous manifestation of plague, ulcers with eschars, papules, or pustules also are present in 25% of cases. Mycobacterium ulcerans typically causes chronic skin ulcers on the extremities of individuals living in the tropics. Mycobacterium leprae may be associated with cutaneous ulcerations in patients with lepro­ matous leprosy related to Lucio’s phenomenon, in which immunemediated destruction of tissue bearing high concentrations of M. leprae bacilli occurs, usually several months after initiation of effective therapy. Mycobacterium tuberculosis also may cause ulcerations, pap­ ules, or erythematous macular lesions of the skin in both immunocom­ petent and immunocompromised patients. Decubitus ulcers are due to tissue hypoxemia secondary to pressureinduced vascular insufficiency and may become secondarily infected with components of the skin and gastrointestinal flora, including anaerobes. Ulcerative lesions on the anterior shins may be due to pyo­ derma gangrenosum, which must be distinguished from similar lesions

of infectious etiology by histologic evaluation of biopsy sites. Ulcerated lesions on the genitals may be either painful (chancroid) or painless (primary syphilis).

ERYSIPELAS (Table 134-1) Erysipelas is due to S. pyogenes and is characterized by an abrupt onset of fiery-red swelling of the face or extremities. The distinctive features of erysipelas are well-defined indurated margins, particularly along the nasolabial fold; rapid progression; and intense pain. Flaccid bullae may develop during the second or third day of illness, but extension to deeper soft tissues is rare. Treatment with peni­ cillin is effective; swelling may progress despite appropriate treatment, although fever, pain, and the intense red color diminish. Desquama­ tion of the involved skin occurs 5–10 days into the illness. Infants and elderly adults are most commonly afflicted, and the severity of systemic toxicity varies. CELLULITIS (Table 134-1) Cellulitis is an acute inflammatory condition of the skin that is characterized by localized pain, erythema, swelling, and heat. It may be caused by indigenous flora colonizing the skin and append­ ages (e.g., S. aureus and S. pyogenes) or by a wide variety of exogenous bacteria. Because the exogenous bacteria involved in cellulitis occupy unique niches in nature, a thorough history (including epidemiologic data) offers important clues to etiology. When there is drainage, an open wound, or an obvious portal of entry, Gram stain and culture provide a definitive diagnosis. In the absence of these findings, the bacterial etiology of cellulitis is difficult to establish, and in some cases, staphylococcal and streptococcal cellulitis may have similar features. Even with needle aspiration of the leading edge or a punch biopsy of the cellulitis tissue itself, cultures are positive in only 20% of cases. This observation suggests that relatively low numbers of bacteria may cause cellulitis and that the expanding area of erythema within the skin may be a direct effect of extracellular toxins or of the soluble mediators of inflammation elicited by the host. PART 5 Infectious Diseases Bacteria may gain access to the epidermis through cracks in the skin, abrasions, cuts, burns, insect bites, surgical incisions, and IV catheters. Cellulitis caused by S. aureus spreads from a central local­ ized infection, such as an abscess, folliculitis, or an infected foreign body (e.g., a splinter, a prosthetic device, an IV catheter). MRSA is rapidly replacing methicillin-sensitive S. aureus (MSSA) as a cause of cellulitis in both inpatient and outpatient settings. Cellulitis caused by MSSA or MRSA is usually associated with a focal infection, such as a furuncle, a carbuncle, a surgical wound, or an abscess; the U.S. Food and Drug Administration preferentially refers to these types of infec­ tion as purulent cellulitis. In contrast, cellulitis due to S. pyogenes is a more rapidly spreading, diffuse process that is frequently associated with lymphangitis and fever and should be referred to as nonpurulent cellulitis. Recurrent streptococcal cellulitis of the lower extremities may be caused by organisms of group A, C, or G in association with chronic venous stasis or with saphenous venectomy for coronary artery bypass surgery. Streptococci also cause recurrent cellulitis among patients with chronic lymphedema resulting from elephantiasis, lymph node dissection, or congenital disorders. Recurrent staphylococcal cutane­ ous infections are more common among individuals who have Job syndrome, the autosomal dominant hyper-IgE recurrent infection syndrome, and among nasal carriers of staphylococci. Cellulitis caused by Streptococcus agalactiae (group B Streptococcus) occurs primarily in elderly patients and those with diabetes mellitus or peripheral vascular disease. Haemophilus influenzae typically causes periorbital cellulitis in children in association with sinusitis, otitis media, or epiglottitis. It is unclear whether this form of cellulitis will (like meningitis) become less common as a result of the impressive efficacy of the H. influenzae type b vaccine. Many other bacteria also cause cellulitis. It is fortunate that these organisms occur in such characteristic settings that a good history pro­ vides useful clues to the diagnosis. Cellulitis associated with cat bites and, to a lesser degree, with dog bites is commonly caused by Pasteu­ rella multocida, although in the latter case Staphylococcus intermedius

and Capnocytophaga canimorsus also must be considered. Sites of cellulitis and abscesses associated with dog bites and human bites also contain a variety of anaerobic organisms, including Fusobacterium, Bacteroides, aerobic and anaerobic streptococci, and Eikenella cor­ rodens. Pasteurella is notoriously resistant to dicloxacillin and nafcillin but is sensitive to other β-lactam antimicrobial agents as well as to quinolones, tetracycline, and erythromycin. Amoxicillin-clavulanate, ampicillin-sulbactam, and cefoxitin are good choices for the treatment of animal or human bite infections. Aeromonas hydrophila causes aggressive cellulitis and occasionally necrotizing fasciitis in tissues surrounding lacerations sustained in freshwater (lakes, rivers, and streams). This organism remains sensitive to aminoglycosides, fluo­ roquinolones, chloramphenicol, trimethoprim-sulfamethoxazole, and third-generation cephalosporins; it is resistant to ampicillin, however. P. aeruginosa causes three types of soft tissue infection: ecthyma gan­ grenosum in neutropenic patients, hot-tub folliculitis, and cellulitis fol­ lowing penetrating injury. Most commonly, P. aeruginosa is introduced into the deep tissues following a significant penetrating injury, such as occurs when a person steps on a nail. Treatment includes surgical inspection and drainage, particularly if the injury also involves bone or joint capsule. Choices for empirical treatment (while antimicrobial susceptibility data are awaited) include an aminoglycoside, a thirdgeneration cephalosporin (ceftazidime, cefoperazone), a semisynthetic penicillin (ticarcillin or piperacillin), or a fluoroquinolone (Chap. 170). Gram-negative bacillary cellulitis, including that due to P. aerugi­ nosa, is most common among hospitalized, immunocompromised hosts. Cultures and sensitivity tests are critically important in this set­ ting because of multidrug resistance (Chap. 170). The gram-positive aerobic rod Erysipelothrix rhusiopathiae is most often associated with fish and domestic swine and causes cellulitis primarily in bone renderers and fishmongers. E. rhusiopathiae remains susceptible to most β-lactam antibiotics (including penicillin), eryth­ romycin, clindamycin, tetracycline, and cephalosporins but is resistant to sulfonamides, chloramphenicol, and vancomycin. Its resistance to vancomycin, which is unusual among gram-positive bacteria, is of potential clinical significance since this agent is sometimes used in empirical therapy for skin infection. Fish food containing the water flea Daphnia is sometimes contaminated with M. marinum, which can cause cellulitis or granulomas on skin surfaces exposed to the water in aquariums or injured in swimming pools (Chap. 185). Ethambutol and either clarithromycin or azithromycin are the best therapies. NECROTIZING FASCIITIS (Table 134-1) Necrotizing fasciitis, formerly called streptococcal gan­ grene, may be associated with group A Streptococcus or mixed aerobic– anaerobic bacteria or may occur as a component of gas gangrene caused by Clostridium perfringens. Strains of MRSA that produce the Panton-Valentine leukocidin (PVL) toxin have been reported to cause necrotizing fasciitis. Early diagnosis may be difficult when pain or unexplained fever is the only presenting manifestation. Swelling then develops and is followed by brawny edema and tenderness. With pro­ gression, dark-red induration of the epidermis appears, along with bul­ lae filled with blue or purple fluid. Later the skin becomes friable and takes on a bluish, maroon, or black color. By this stage, thrombosis of blood vessels in the dermal papillae (Fig. 134-1) is extensive. Extension of infection to the level of the deep fascia causes this tissue to take on a brownish-gray appearance. Rapid spread occurs along fascial planes, through venous channels and lymphatics. Patients in the later stages are toxic and frequently manifest shock and multiorgan failure. Necrotizing fasciitis caused by mixed aerobic–anaerobic bacteria begins with a breach in the integrity of a mucous membrane barrier, such as the mucosa of the gastrointestinal or genitourinary tract. The portal can be a malignancy, a diverticulum, a hemorrhoid, an anal fis­ sure, or a urethral tear. Other predisposing factors include peripheral vascular disease, diabetes mellitus, surgery, and penetrating injury to the abdomen. Leakage into the perineal area results in a syndrome called Fournier gangrene, characterized by massive swelling of the scrotum and penis with extension into the perineum or the abdominal wall and the legs.

Necrotizing fasciitis caused by S. pyogenes has increased in frequency and severity since 1985. There are two distinct clinical presentations: patients without, versus those with, a defined portal of bacterial entry. Infections in the first category often begin deep at the site of a nonpen­ etrating, relatively minor trauma, such as a bruise or a muscle strain. Seeding of the site via transient bacteremia is likely, although most patients deny antecedent streptococcal infection. Affected patients present with only severe pain and fever and are frequently misdiag­ nosed (e.g., thrombophlebitis), given pain-relieving drugs, and sent home. Later in the course, the classic signs of necrotizing fasciitis, such as purple (violaceous) bullae, skin sloughing, and progressive toxicity, develop. Mortality in this setting is high, and survivors often undergo repeated surgeries including amputations. In infections of the second type, S. pyogenes may reach the deep fascia from a site of cutaneous infection or penetrating trauma. These patients have early signs of superficial skin infection with progression to necrotizing fasciitis. In either setting, toxicity is severe, and renal impairment may precede the development of shock. In 20–40% of cases, myositis occurs con­ comitantly, and, as in gas gangrene (see below), serum creatine phos­ phokinase levels may be markedly elevated. Necrotizing fasciitis due to mixed aerobic–anaerobic bacteria may be associated with gas in deep tissue, but gas usually is not present when the cause is S. pyogenes or MRSA. Prompt surgical exploration down to the deep fascia and mus­ cle is essential. Necrotic tissue must be surgically removed, and Gram staining and culture of excised tissue are useful in establishing whether group A streptococci, mixed aerobic–anaerobic bacteria, MRSA, or Clostridium species are present (see “Treatment,” below). MYOSITIS AND MYONECROSIS (Table 134-1) Muscle involvement can occur with viral infection (e.g., influenza, dengue, or coxsackievirus B infection) or parasitic invasion (e.g., trichinellosis, cysticercosis, or toxoplasmosis). Although myalgia develops in most of these infections, severe muscle pain is the hall­ mark of pleurodynia (coxsackievirus B), trichinellosis, and bacterial infection. Acute rhabdomyolysis predictably occurs with clostridial and streptococcal myositis but may also be associated with influenza virus, echovirus, coxsackievirus, Epstein-Barr virus, and Legionella infections. Pyomyositis is usually due to S. aureus, is common in tropical areas, and generally has no known portal of entry. Cases of pyomyositis caused by MRSA producing the PVL toxin have been described among children in the United States. Muscle infection begins at the exact site of blunt trauma or muscle strain. Infection remains localized, and shock does not develop unless organisms produce toxic shock syndrome toxin 1 or certain enterotoxins and the patient lacks antibodies to the toxin produced by the infecting organisms. In contrast, S. pyogenes may induce primary myositis (referred to as streptococcal necrotizing

myositis) in association with severe systemic toxicity. Myonecrosis occurs concomitantly with necrotizing fasciitis in ~50% of cases. Both are part of the streptococcal toxic shock syndrome. Gas gangrene usually follows severe penetrating injuries that result in interruption of the blood supply and introduction of soil into wounds. Such cases of traumatic gangrene are usually caused by the clostridial species C. perfringens, C. septicum, and C. histolyticum. Rarely, latent or recurrent gangrene can occur years after penetrating trauma; dormant spores that reside at the site of previous injury are most likely responsible. Spontaneous nontraumatic gangrene among patients with neutropenia, gastrointestinal malignancy, diverticulosis, or recent radiation therapy to the abdomen is caused by several clos­ tridial species, of which C. septicum is the most commonly involved. The tolerance of this anaerobe to oxygen probably explains why it can initiate infection spontaneously in normal tissue anywhere in the body. Gas gangrene of the uterus, especially that due to Clostridium

sordellii, historically occurred as a consequence of illegal or selfinduced abortion and nowadays also follows spontaneous abortion, vaginal delivery, and Caesarean section. C. sordellii has also been impli­ cated in medically induced abortion. Postpartum C. sordellii infections in young, previously healthy women present with little or no fever, lack of a purulent discharge, refractory hypotension, extensive peripheral

FIGURE 134-2  CT showing edema and inflammation of the left chest wall in a patient with necrotizing fasciitis and myonecrosis caused by group A Streptococcus. edema and effusions, hemoconcentration, and a markedly elevated white blood cell count. The infection is often fatal, with death ensuing rapidly. C. sordellii and C. novyi have also been associated with cutane­ ous injection of black tar heroin; mortality rates are lower among these individuals, probably because their injection-site infections are readily apparent and diagnosis is therefore prompt. Synergistic nonclostridial anaerobic myonecrosis, also known as necrotizing cutaneous myositis and synergistic necrotizing cellulitis, is a variant of necrotizing fasciitis caused by mixed aerobic and anaerobic bacteria with the exclusion of clostridial organisms (see “Necrotizing Fasciitis,” above). CHAPTER 134 DIAGNOSIS This chapter emphasizes the physical appearance and location of lesions within the soft tissues as important diagnostic clues. Other crucial considerations in narrowing the differential diagnosis are the temporal progression of the lesions as well as the patient’s travel history, animal exposure or bite history, age, underlying disease status, and lifestyle. However, even the astute clinician may find it challenging to diagnose all infections of the soft tissues by history and inspection alone. Soft tissue radiography, computed tomography (CT) (Fig. 134-2), and magnetic resonance imaging (MRI) may be useful in determining the depth of infection and should be performed when the patient has rapidly progressing lesions or evidence of a sys­ temic inflammatory response syndrome. These tests are particularly valuable for defining a localized abscess or detecting gas in tissue. Unfortunately, they may reveal only soft tissue swelling and thus are not specific for fulminant infections such as necrotizing fasciitis or myonecrosis caused by group A Streptococcus (Fig. 134-2), where gas is not found in lesions. Infections of the Skin, Muscles, and Soft Tissues Aspiration of the leading edge or punch biopsy with frozen section may be helpful if the results of imaging tests are positive, but negative results occur in ~80% of cases. There is some evidence that aspiration alone may be superior to injection and aspiration with normal saline. Frozen sections are especially useful in distinguishing SSSS from TEN and are quite valuable in cases of necrotizing fasciitis. Open surgical inspection, with debridement as indicated, is the best way to determine the extent and severity of infection and to obtain material for Gram staining and culture. Such an aggressive approach is important and may be lifesaving if undertaken early in the course of fulminant infec­ tions when there is evidence of systemic toxicity. TREATMENT Infections of the Skin, Muscles, and Soft Tissues A full description of the treatment of all the clinical entities described herein is beyond the scope of this chapter. As a guide to the clinician in selecting appropriate treatment, the antimicrobial agents useful in the most common and the most fulminant cutane­ ous infections are listed in Table 134-2. Newer antibiotics approved

TABLE 134-2  Treatment of Common Infections of the Skin DIAGNOSIS/CONDITION PRIMARY TREATMENT ALTERNATIVE TREATMENT SEE ALSO CHAP(S). Animal bite (prophylaxis or early infection)a Amoxicillin–clavulanate (875/125 mg

PO bid) Animal bitea (established infection) Ampicillin–sulbactam (1.5–3 g IV q6h) Clindamycin (600–900 mg IV q8h) plus Ciprofloxacin (400 mg IV q12h) or cefoxitin (2 g IV q6h) Bacillary angiomatosis Erythromycin (500 mg PO qid) Doxycycline (100 mg PO bid)

Herpes simplex (primary genital) Acyclovir (400 mg PO tid for 10 days) Famciclovir (250 mg PO tid for 5–10 days) or valacyclovir

(1000 mg PO bid for 10 days) Herpes zoster (immunocompetent host >50 years of age) Acyclovir (800 mg PO 5 times daily for 7–10 days) Cellulitis (staphylococcal or streptococcalb,c) Nafcillin or oxacillin (2 g IV q4–6h) Cefazolin (1–2 g q8h) or ampicillin/sulbactam (1.5–3 g IV q6h) or erythromycin (0.5–1 g IV q6h) or clindamycin (600–900 mg IV q8h) MRSA skin infectiond Vancomycin (1 g IV q12h) Linezolid (600 mg IV q12h)

Necrotizing fasciitis

(group A streptococcalb) Clindamycin (600–900 mg IV q6–8h) plus penicillin G (4 million units IV q4h) Necrotizing fasciitis (mixed aerobes and anaerobes) Ampicillin (2 g IV q4h) plus clindamycin (600–900 mg IV q6–8h) plus ciprofloxacin (400 mg IV q6–8h) Gas gangrene Clindamycin (600–900 mg IV q6–8h) plus penicillin G (4 million units IV q4–6h) aPasteurella multocida, a species commonly associated with both dog and cat bites, is resistant to cephalexin, dicloxacillin, clindamycin, and erythromycin. Eikenella corrodens, a bacterium commonly associated with human bites, is resistant to clindamycin, penicillinase-resistant penicillins, and metronidazole but is sensitive to trimethoprim-sulfamethoxazole and fluoroquinolones. bThe frequency of erythromycin resistance in group A Streptococcus is currently ~5% in the United States but has reached 70–100% in some other countries. Most, but not all, erythromycin-resistant group A streptococci are susceptible to clindamycin; clindamycin-resistant group A streptococci are susceptible to linezolid and to tedizolid. Approximately 90% of Staphylococcus aureus strains are sensitive to clindamycin, but resistance—both intrinsic and inducible—is increasing. cSevere hospital-acquired S. aureus infections or community-acquired S. aureus infections that are not responding to the β-lactam antibiotics recommended in this table may be caused by methicillin-resistant strains, requiring vancomycin, daptomycin, or linezolid or tedizolid. dSome strains of methicillin-resistant S. aureus (MRSA) remain sensitive to tetracycline and trimethoprim-sulfamethoxazole. Daptomycin (4 mg/kg IV q24h) or tigecycline (100-mg loading dose followed by 50 mg IV q12h) is an alternative treatment for MRSA. PART 5 Infectious Diseases by the U.S. Food and Drug Administration for uncomplicated skin and soft tissue infections including ceftaroline, dalbavancin, orita­ vancin, tedizolid, delafloxacin, and omadacycline. Furuncles, carbuncles, and abscesses caused by MRSA and MSSA are common, and their treatment depends upon the size of the lesion. Furuncles <2.5 cm in diameter are usually treated with moist heat. Those that are larger (4.5 cm of erythema and indura­ tion) require surgical drainage, and the occurrence of these larger lesions when associated with fever, chills, or leukocytosis requires both drainage and antibiotic treatment. Previous studies in children demonstrated that surgical drainage of abscesses (mean diameter, 3.8 cm) was as effective when used alone as when combined with trimethoprim-sulfamethoxazole treatment. However, the rate of recurrence of new lesions was lower in the group undergoing both drainage and antibiotic treatment. A more recent study in patients with predominantly MRSA localized abscesses suggested that a 7- to 10-day course of treatment with trimethoprim-sulfamethoxazole or clindamycin was associated with higher cure rates and fewer recurrences. Early and aggressive surgical exploration is essential in cases of suspected necrotizing fasciitis, myositis, or gangrene to (1) visu­ alize the deep structures, (2) remove necrotic tissue, (3) reduce compartment pressure, and (4) obtain suitable material for Gram staining and for aerobic and anaerobic cultures and antimicrobial sensitivity testing. Appropriate empirical antibiotic treatment for mixed aerobic–anaerobic infections is reviewed in Chap. 159. Group A streptococcal and clostridial infection of the fascia and/or muscle carries a mortality rate of 20–50% with penicillin treatment. In experimental models of streptococcal and clostridial necrotiz­ ing fasciitis/myositis, clindamycin has exhibited markedly superior efficacy, but no comparative clinical trials have been performed. A retrospective study of children with invasive group A streptococ­ cal infection demonstrated higher survival rates with clindamycin treatment than with β-lactam antibiotic therapy. Current guide­ lines recommend treatment with clindamycin (or linezolid) plus a β-lactam antibiotic. The emergence of macrolide resistance among

Doxycycline (100 mg PO bid)

Famciclovir (500 mg PO tid for 7–10 days) or valacyclovir

(1000 mg PO tid for 7 days)

152, 153 Clindamycin (600–900 mg IV q6–8h) plus a cephalosporin (first- or second-generation)

Vancomycin (1 g IV q6h) plus metronidazole (500 mg IV q6h) plus ciprofloxacin (400 mg IV q6–8h)

Clindamycin (600–900 mg IV q6–8h) plus cefoxitin (2 g IV q6h)

group A streptococci has, however, created therapeutic dilemmas for patients allergic to penicillin and for those with life-threatening infections, such as necrotizing fasciitis/myonecrosis and strep­ tococcal toxic shock syndrome, where clindamycin has been the treatment of choice. In such instances, linezolid or tedizolid could be used. The use of intravenous immune globulin (IVIg) in patients with invasive group A streptococcal infections has been controversial, and early reports failed to demonstrate efficacy or were terminated early due to low enrollment. More recent prospective studies and one five-study meta-analysis demonstrated that administration of IVIg (often in combination with clindamycin) significantly reduced 30- and 90-day mortality. The combination of clindamycin and IVIg may help reduce production and activities of circulating tox­ ins produced by group A Streptococcus. IVIg could be given early, and more than one dose should be used because different batches of IVIg have variable neutralizing activity against streptococcal exotoxins. Hyperbaric oxygen (HBO) treatment has been suggested to be useful in gas gangrene due to clostridial species. However, no definitive comparative trials of HBO have been done for the treat­ ment for invasive group A streptococcal infections, although some reports state that such treatment reduces mortality and the need for further debridements. The use of HBO should not delay, or be used in preference to, surgical debridement when the latter is indicated; such delays significantly contribute to mortality. Antibiotic treat­ ment should be continued until all signs of systemic toxicity have resolved, all devitalized tissue has been removed, and granulation tissue has developed (Chaps. 153, 159, and 182). In summary, infections of the skin and soft tissues are diverse in presentation and severity and offer a great challenge to the clinician. This chapter provides an approach to diagnosis and understand­ ing of the pathophysiologic mechanisms involved in these infec­ tions. More in-depth information is found in chapters on specific infections.

130 - 235 Toxoplasma Infections

235 Toxoplasma Infections

Kami Kim

Toxoplasma Infections ■ ■DEFINITION Toxoplasmosis is caused by infection with the obligate intracellular parasite Toxoplasma gondii. Acute infection acquired after birth is typi­ cally asymptomatic, but some immunocompetent individuals can pres­ ent with systemic or ocular disease. Acute infection is thought to result in the lifelong chronic persistence of cysts in the host’s tissues. The classic presentation of toxoplasmosis is encephalitis in immunocom­ promised individuals (especially HIV-positive individuals or transplant recipients) in whom latent infection has reactivated. Among the clini­ cal manifestations of the disease are lymphadenopathy, encephalitis, myocarditis, pneumonitis, and retinitis. Congenital toxoplasmosis is an infection of newborns that results from the transplacental passage of parasites from an infected mother to the fetus. These infants may be asymptomatic at birth, but many children later manifest signs and symptoms, including chorioretinitis, strabismus, epilepsy, and psycho­ motor retardation. Toxoplasmosis can also present as acute disease (typically chorioretinitis) associated with food- or waterborne sources. ■ ■ETIOLOGY T. gondii is an intracellular coccidian that infects both birds and mam­ mals. Up to a third of the world’s population is thought to be infected latently with this organism. There are two distinct stages in the life cycle that are transmissible to humans (Fig. 235-1). Tissue cysts that contain bradyzoites are transmitted in undercooked meat. After an intermediate host (e.g., a human, mouse, sheep, pig) ingests the cyst, it is rapidly digested by the acidic-pH gastric secretions. Sporulated oocysts that contain sporozoites are products of the sexual cycle in feline intestines and acquired by ingestion of food or water contami­ nated with infected cat feces. Bradyzoites or sporozoites are released, enter the intestinal epithelium, and transform into rapidly dividing Intermediate host: birds, mammals, humans Bradyzoites encyst within the CNS and muscle of the infected host. Oocysts are excreted in cat feces. Contaminated soil is ingested by birds, mammals, and humans. Definitive host FIGURE 235-1    Life cycle of Toxoplasma gondii. The cat is the definitive host in which the sexual phase of the cycle is completed. Oocysts shed in cat feces can infect a wide range of animals, including birds, rodents, grazing domestic animals, and humans. The bradyzoites found in the muscle of food animals may infect humans who eat insufficiently cooked meat products, particularly lamb and pork. Although human disease can take many forms, congenital infection and encephalitis from reactivation of latent infection in the brains of immunosuppressed persons are the most important manifestations. CNS, central nervous system. (Courtesy of Dominique Buzoni-Gatel, Institut Pasteur, Paris.)

tachyzoites. The tachyzoites can infect and replicate in all mammalian cells except red blood cells. The parasite actively penetrates the cell and forms a parasitophorous vacuole. Parasite replication continues within the vacuole. After the parasites reach a critical mass, intracellular sig­ naling within the host and the parasite results in parasite egress from the vacuole. The host cell is destroyed, and the released tachyzoites infect adjoining cells. Parasites can disseminate throughout the body as free tachyzoites or within phagocytic cells in the bloodstream or via lymphatics. Tachyzoites actively invade host cells and can cross epithelial and endothelial barriers.

The tachyzoite replication cycle within an infected organ causes cytopathology and clinical symptoms. Most tachyzoites are eliminated by the host’s humoral and cell-mediated immune responses. Tissue cysts containing bradyzoites develop 7–10 days after systemic tachyzo­ ite infection. These tissue cysts occur in various host organs but persist principally within the central nervous system (CNS) and muscle. The development of this chronic stage completes the asexual portion of the life cycle. Active infection in the immunocompromised host is usu­ ally due to the spontaneous release of encysted parasites that undergo rapid transformation into tachyzoites within the CNS that cannot be contained by the immune system. The sexual stage in the life cycle takes place in the cat (the defini­ tive host) and is defined by the formation of oocysts within the feline host intestine. This enteroepithelial cycle begins with the ingestion of the bradyzoite tissue cysts and, after several intermediate stages, culminates in the production of gametes. Gamete fusion produces a zygote, which envelops itself in a rigid wall and is secreted in the feces as an unsporulated oocyst. After 2–3 days of exposure to air at ambient temperature, the noninfectious oocyst sporulates to produce eight sporozoite progeny. The sporulated oocyst can be ingested by an intermediate host. It is in the intermediate host that T. gondii completes its life cycle. CHAPTER 235 Sporulated oocysts are environmentally hardy and very infectious; they are thought to be sources of waterborne outbreaks such as those reported in Victoria (British Columbia, Canada) and in South Amer­ ica. In the Northern Hemisphere, T. gondii strains are predominantly of three genotypes. Strains found in South and Central America are more Toxoplasma Infections virulent than those from the Northern

Hemisphere, are frequently of the type I virulent genotype or atypical geno­ types, and are more likely to be associ­ ated with symptomatic disease, usually ocular posterior uveitis. Ocular toxo­ plasmosis should be considered in a person from Central or South America

with ocular symptoms and retinal abnormalities. Severe disease, includ­ ing sepsis, fever of unknown origin, and pneumonia, has been reported and should be considered in a patient with travel history to South or Central America. There have been reports of outbreaks in North America among individuals who have ingested under­ cooked game, especially venison, and these strains appear more virulent as the attack rate is often high. Prevalence of T. gondii in Africa suggests T. gondii infection is common and that some strains may also be quite virulent. Tachyzoites infect all nucleated cells in the host, replicate, and cause tissue damage. Toxoplasmic encephalitis ■ ■EPIDEMIOLOGY T. gondii infects a wide range of mam­ mals and birds. Its seroprevalence depends on the locale and the age of the population. Generally, hot arid climatic conditions are associated with a low prevalence of infection. In the United States and most European

countries, the seroprevalence increases with age and exposure. In the United States, seroprevalence has steadily decreased, with 11% of individuals >6 years old having serologic evidence of Toxoplasma exposure in a 2011–2014 survey, with foreign-born Americans having a higher rate of seroprevalence. In most other regions of the world, the seroprevalence is higher, with a seroprevalence as high as 78% reported in Brazil. Because of increased awareness of foodborne infections, the prevalence of seropositivity has decreased worldwide over the past two decades, although it remains very high in Central and South America.

■ ■TRANSMISSION Oral Transmission  Most cases of human Toxoplasma infection are thought to be acquired by the oral route. Transmission can be attribut­ able to ingestion of sporulated oocysts from contaminated soil, food, or water. During acute feline infection, a cat may excrete as many as 100 million oocysts per day. These sporozoite-containing oocysts are highly infectious and may remain viable for many years in soil or water. Humans infected during an oocyst-transmitted infection develop stage-specific antibodies to the oocyst/sporozoite. Children and adults also acquire infection from tissue cysts contain­ ing bradyzoites. Undercooking or insufficient freezing of meat is an important source of infection in the developed world. Toxoplasmosis has been associated with eating raw or undercooked food including ground beef, lamb, or venison or drinking unpasteurized goat milk. More recent epidemiologic studies have associated acute infections with ingestion of untreated water or shellfish (oysters, mussels, and clams). Transmission via Blood or Organs  In addition to being trans­ mitted orally, T. gondii can be transmitted directly from a seropositive donor to a seronegative recipient in a transplanted heart, heart–lung, kidney, liver, or pancreas. Viable parasites can be cultured from refrig­ erated anticoagulated blood, which may be a source of infection in individuals receiving blood transfusions. T. gondii reactivation has been reported in bone marrow, hematopoietic stem cell, and liver transplant recipients as well as in individuals with AIDS. Finally, laboratory personnel can be infected after contact with contaminated needles or glassware or with infected tissue. PART 5 Infectious Diseases Transplacental Transmission  On average, about one-third of all women who acquire infection with T. gondii during pregnancy transmit the parasite to the fetus; the remainder give birth to normal, uninfected babies. Of the various factors that influence fetal outcome, gestational age at the time of infection is the most critical (see below). Recrudescent maternal infection is rarely the source of congenital disease, although rare cases of transmission by immunocompromised women (e.g., those infected with HIV or those receiving high-dose glu­ cocorticoids) have been reported. Thus, women who are seropositive before pregnancy usually are protected against acute infection and do not give birth to congenitally infected neonates. There is essentially no risk for congenital infection if the mother becomes infected ≥6 months before conception. If infection is acquired <6 months before conception, the likelihood of transplacental infection increases as the interval between infection and conception decreases. Women with documented acute toxoplasmosis should be counseled to use appropriate measures to prevent pregnancy for 6 months after infection. In pregnancy, if the mother becomes infected during the first trimester, the incidence of transplacental infection is lowest (~15%), but the disease in the neonate is most severe. If maternal infection occurs during the third trimester, the incidence of transplacental infec­ tion is greatest (65%), but the infant is usually asymptomatic at birth. Infected infants who are normal at birth may have a higher incidence of learning disabilities and chronic neurologic sequelae than uninfected children. Only a small proportion (20%) of women infected with

T. gondii develop clinical signs of infection. Often the diagnosis is first appreciated when routine postconception serologic tests show evidence of specific antibody. Chronic toxoplasmosis has not been thought to affect pregnancy, but there have been recent studies suggesting a greater incidence of adverse pregnancy outcomes without evidence of

reactivated toxoplasmosis. The lingering effects of chronic infection are controversial (see below) and an area of ongoing investigation. ■ ■PATHOGENESIS Upon the host’s ingestion of either tissue cysts containing bradyzoites or oocysts containing sporozoites, the parasites are released from the cysts by the digestive process. Bradyzoites are resistant to the effect of pepsin and invade the host’s gastrointestinal tract. Within enterocytes (or other gut-associated cells), the parasites undergo morphologic transformation, giving rise to invasive tachyzoites. From the gastroin­ testinal tract, parasites disseminate to a variety of organs, particularly lymphatic tissue, skeletal muscle, myocardium, retina, placenta, and the CNS. At these sites, the parasite infects host cells, replicates, and invades the adjoining cells. In this fashion, the hallmarks of the infec­ tion develop: cell death and focal necrosis surrounded by an acute inflammatory response. In the immunocompetent host, both the humoral and the cellular immune responses control infection; parasite virulence and tissue tropism may be strain specific. Tachyzoites are sequestered by a variety of immune mechanisms, including induction of parasiticidal antibody, activation of macrophages with radical intermediates, production of interferon γ (IFN-γ), and stimulation of CD8+ cytotoxic T lym­ phocytes. These antigen-specific lymphocytes are capable of killing both extracellular parasites and target cells infected with parasites. As tachyzoites are cleared from the acutely infected host, tissue cysts con­ taining bradyzoites begin to appear, usually within the CNS, skeletal muscle, and the retina. Toxoplasma secretes signaling molecules into infected host cells, and these molecules modulate host gene expression, host metabolism, and host immune response. It was initially thought that cysts with bradyzoites are not eliminated by the immune system, but more recent studies in the murine model indicate that both CD8+ T cells and alternatively activated macrophages are able to kill cysts in vivo. The ability to eliminate cysts may depend on the genetic back­ ground of the infected host as well as the parasite genotype that initially infected the host. Immunocompromised or fetal hosts lack the immune factors neces­ sary to control the spread of tachyzoite infection. This altered immune state allows the persistence of tachyzoites and gives rise to progressive focal destruction in affected organs (i.e., necrotizing encephalitis, pneumonia, and myocarditis). It is thought that all infected individuals have persistent infection with cysts containing bradyzoites with lifelong infection remaining sub­ clinical. Although bradyzoites are in a slow metabolic phase, bradyzoites can replicate, and cysts do rupture within the CNS. These subclinical cycles of cyst ruptures followed by development of new bradyzoitecontaining cysts are the probable source of recrudescent infection in immunocompromised individuals and the most likely stimulus for the persistence of antibody titers in the immunocompetent host. The persistence of toxoplasmosis has been hypothesized to be a contributing factor to a variety of neuropsychiatric conditions, includ­ ing schizophrenia and bipolar disease, but the contribution of chronic toxoplasmosis to human disease remains controversial. In rodents, chronic T. gondii infection has significant effects on behavior, increas­ ing predation. Epidemiologic studies such as the National Health and Nutrition Examination Survey (NHANES) study show a correlation between T. gondii seropositivity and a number of chronic diseases, including diabetes and cognitive dysfunction. ■ ■PATHOLOGY Cell death and focal necrosis due to replicating tachyzoites induce an intense mononuclear inflammatory response in any tissue or cell type infected. Tachyzoites rarely can be visualized by routine histo­ pathologic staining of these inflammatory lesions. However, immu­ nofluorescent staining with parasitic antigen–specific antibodies can reveal the organism. In contrast to the inflammatory process caused by tachyzoites, bradyzoite-containing cysts cause inflammation only at the early stages of development. Once the cysts reach maturity, the inflam­ matory process is blunted, and the cysts remain relatively immunologi­ cally quiescent within the brain matrix until they rupture.

Lymph Nodes  During acute infection, lymph node biopsy dem­ onstrates characteristic findings, including follicular hyperplasia and irregular clusters of tissue macrophages with eosinophilic cytoplasm. Granulomas rarely are evident in these specimens. Although tachyzo­ ites are not usually visible, parasites can be demonstrated by subinocu­ lation of infected tissue into mice, with resultant disease, or by PCR. PCR amplification of DNA fragments of Toxoplasma genes is effective and sensitive in establishing lymph node infection by tachyzoites. Eyes  In the eye, infiltrates of monocytes, lymphocytes, and plasma cells may produce uni- or multifocal lesions. Granulomatous lesions and retinochoroiditis can be observed in the posterior chamber after acute necrotizing retinitis. Other ocular complications include irido­ cyclitis, cataracts, and glaucoma. T. gondii is the most common cause of posterior uveitis in immunocompetent individuals and ocular toxo­ plasmosis is a common clinical presentation in outbreaks. Central Nervous System  During CNS involvement, both focal and diffuse meningoencephalitis can be documented, with evidence of necrosis and microglial nodules. Necrotizing encephalitis in patients without AIDS is characterized by small diffuse lesions with perivas­ cular cuffing in contiguous areas. In the AIDS population, polymor­ phonuclear leukocytes may be present in addition to monocytes, lymphocytes, and plasma cells. Cysts containing bradyzoites frequently are found contiguous with the necrotic tissue border. As a conse­ quence of antiretroviral therapy (ART) for AIDS, the incidence of toxoplasmosis has decreased in the developed world. The incidence of toxoplasmosis in underresourced settings is not known due to lack of diagnostic infrastructure but is likely to be higher than in the United States, particularly in regions of the world with populations with large numbers of untreated patients living with HIV. Lungs and Heart  Among patients with AIDS who die of toxoplas­ mosis, 40–70% have involvement of the lungs and heart. Interstitial pneumonitis can develop in neonates and immunocompromised patients and rarely in immunocompetent individuals. Thickened and edematous alveolar septa infiltrated with mononuclear and plasma cells are apparent. This inflammation may extend to the endothelial walls. Tachyzoites and bradyzoite-containing cysts have been observed within the alveolar membrane. Superimposed bronchopneumonia can be caused by other microbial agents. Cysts and aggregates of parasites in cardiac muscle tissue are evident in patients with AIDS who die of toxoplasmosis. Focal necrosis surrounded by inflammatory cells is associated with hyaline necrosis and disrupted myocardial cells. Pericardi­ tis is associated with toxoplasmosis in some patients. Gastrointestinal Tract  Rare cases of human gastrointestinal tract infection with T. gondii have presented as ulcerations in the mucosa. Acute infection in certain strains of inbred mice (C57BL/6) results in lethal ileitis within 7–9 days. This inflammatory bowel disease has been recognized in several other mammalian species, including pigs and nonhuman primates. Other Sites  Pathologic changes during disseminated infection are similar to those described for the lymph nodes, eyes, and CNS. In patients with AIDS, the skeletal muscle, pancreas, stomach, and kidneys can be involved, with necrosis, invasion by inflammatory cells, and (rarely) tachyzoites detectable by routine staining. Large necrotic lesions may cause direct tissue destruction. In addition, secondary effects from acute infection of these various organs, including pancreatitis, myositis, and glomerulonephritis, have been reported. ■ ■HOST IMMUNE RESPONSE Acute Toxoplasma infection evokes a cascade of protective immune responses in the immunocompetent host. Toxoplasma enters the host at the gut mucosal level and evokes a mucosal immune response that includes the production of antigen-specific secretory IgA. Titers of serum IgA antibody directed at the tachyzoite surface antigen p30/ SAG1 are a useful marker for congenital and acute toxoplasmosis. Within the host, T. gondii rapidly induces detectable levels of both IgM and IgG serum antibodies. Monoclonal gammopathy of the IgG

class can occur in congenitally infected infants. IgM levels may be increased in newborns with congenital infection. The polyclonal IgG antibodies evoked by infection are parasiticidal in vitro in the pres­ ence of serum complement and are the basis for the Sabin-Feldman dye test. However, cell-mediated immunity is the major protective response evoked by the parasite during host infection. Macrophages are activated after phagocytosis of antibody-opsonized parasites. If the parasite is not phagocytosed and enters the macrophage, monocytes, or dendritic cells by active penetration, these “Trojan horses” repre­ sent a mechanism for transport and dissemination to distant organs. Toxoplasma stimulates a robust interleukin (IL) 12 response by human dendritic cells.

The CD4+ and CD8+ T-cell responses are antigen-specific and fur­ ther stimulate the production of a variety of important lymphokines that expand the T-cell and natural killer cell repertoire. T. gondii is a potent inducer of a TH1 phenotype, with IL-12 and IFN-γ playing an essential role in the control of the parasites’ growth in the host. Regula­ tion of the inflammatory response is at least partially under the control of a TH2 response that includes the production of IL-4 and IL-10 in seropositive individuals. Human T-cell clones of both the CD4+ and the CD8+ phenotypes are cytolytic against parasite-infected macro­ phages. These T-cell clones produce cytokines that are “microbistatic.” IL-18, IL-7, and IL-15 upregulate the production of IFN-γ and may be important during acute and chronic infection. The effect of IFN-γ may be paradoxical, with stimulation of a host downregulatory response as well. Although T. gondii infection is believed to be recrudescent in patients with AIDS or other immunocompromised states, antibody titers are not useful in establishing reactivation or in following the activity of infection. An absence of positive serologies suggests an alter­ native diagnosis, although AIDS patients may have borderline positive or low serologies and transplant patients treated with immunosuppres­ sive agents including those specific for B cells may become seronega­ tive. T cells from AIDS patients with reactivation of toxoplasmosis fail to secrete both IFN-γ and IL-2. This alteration in the production of these critical immune cytokines contributes to the persistence of infection. Toxoplasma infection develops late in the course of AIDS (CD4+ count <100/μL), when the loss of T cell–dependent protective mechanisms, particularly CD8+ T cells, becomes most pronounced. CHAPTER 235 Toxoplasma Infections ■ ■CLINICAL MANIFESTATIONS In persons whose immune systems are intact, acute toxoplasmosis is usually asymptomatic and self-limited. This condition can go unrec­ ognized in 80–90% of adults and children with acquired infection. The asymptomatic nature of this infection makes diagnosis difficult in mothers infected during pregnancy. In contrast, the wide range of clinical manifestations in congenitally infected children includes severe neurologic complications such as hydrocephalus, microcephaly, intellectual disability, and chorioretinitis. If prenatal infection is severe, multiorgan failure and subsequent intrauterine fetal death can occur. In children and adults, chronic infection can persist throughout life, with little consequence to the immunocompetent host. Toxoplasmosis in Immunocompetent Patients  The most common manifestation of acute toxoplasmosis is cervical lymphade­ nopathy. The nodes may be single or multiple, are usually nontender, are discrete, and vary in firmness. Lymphadenopathy also may be found in suboccipital, supraclavicular, inguinal, and mediastinal areas. Generalized lymphadenopathy occurs in 20–30% of symptomatic patients. Between 20 and 40% of patients with lymphadenopathy also have headache, malaise, fatigue, and fever (usually with a temperature of <40°C [<104°F]). A smaller proportion of symptomatic individu­ als have myalgia, sore throat, abdominal pain, maculopapular rash, meningoencephalitis, and confusion. Rare complications associated with infection in the normal immune host include pneumonia, myocarditis, encephalopathy, pericarditis, and polymyositis. These manifestations are often associated with more virulent parasitic genotypes/strains. Signs and symptoms associated with acute infection usually resolve within several weeks, although

the lymphadenopathy may persist for some months. In one epidemic, toxoplasmosis was diagnosed correctly in only 3 of the 25 patients who consulted physicians. If toxoplasmosis is considered in the differential diagnosis, routine laboratory and serologic screening should precede node biopsy.

In North America and Europe, there are three predominant geno­ types, but strains are more genetically diverse in Central and South America. Genotypes of T. gondii prevalent in South America are more virulent than those typically seen in North America or Europe. These genotypes may be associated with acute or recurrent ocular disease in immunocompetent individuals and have also been associated with pneumonitis and a fulminant sepsis picture in immunologically nor­ mal individuals. Thus, a detailed history, particularly regarding travel and countries of residence, is critical for establishing a diagnosis. Indi­ viduals from South or Central America may have frequent recurrences of ocular toxoplasmosis that may require treatment and suppressive therapy. The results of routine laboratory studies are usually unremarkable except for minimal lymphocytosis, an elevated erythrocyte sedimenta­ tion rate, and a nominal increase in serum aminotransferase levels. Evaluation of cerebrospinal fluid (CSF) in cases with evidence of encephalopathy or meningoencephalitis shows an elevation of intra­ cranial pressure, mononuclear pleocytosis (10–50 cells/mL), a slight increase in protein concentration, and (occasionally) an increase in the gamma globulin level. PCR amplification of the Toxoplasma DNA target sequence in CSF is specific for active toxoplasmosis, but not sensitive. PCR of ocular fluid or bronchoalveolar lavage may also be positive in those with ocular or pulmonary toxoplasmosis, respectively. The CSF of chronically infected individuals is normal. PART 5 Infectious Diseases Infection of Immunocompromised Patients  Patients with AIDS, transplant patients, and those receiving immunosuppressive therapy for lymphoproliferative disorders are at greatest risk for devel­ oping acute toxoplasmosis. Toxoplasmosis has also been reported after treatment with antibodies to tumor necrosis factor. The infection may be due either to reactivation of latent infection or to acquisition of parasites from exogenous sources such as blood or transplanted organs. In individuals with AIDS, >95% of cases of Toxoplasma encephalitis (TE) are believed to be due to recrudescent infection. In most of these cases, encephalitis develops when the CD4+ T-cell count falls below 100/μL. In immunocompromised hosts, the disease may be rapidly fatal if untreated. Thus, accurate diagnosis and initiation of appropriate therapy are necessary to prevent fulminant infection. Toxoplasmosis is a principal opportunistic infection of the CNS in persons with AIDS. Individuals with AIDS who are seropositive for T. gondii are at high risk for encephalitis. Before the advent of highly effective ART, about one-third of the 15–40% of adult AIDS patients in the United States who were latently infected with T. gondii devel­ oped TE. TE may still be a presenting infection in individuals who are unaware of their positive HIV status. Individuals may be at relatively high risk for reactivation after allo­ geneic hematopoietic stem cell transplantation (HSCT), particularly if complicated by graft-versus-host reaction. Weekly PCR screening of blood from patients seroposi­ tive prior to HSCT is recommended, although not all centers routinely monitor HSCT patients for toxoplasmosis. Screening Toxoplasma serolo­ gies (donor and recipient) before transplantation identifies patients potentially at risk for reactivated toxoplasmosis. Serologies should be performed prior to initiation of immunosuppressive agents. The signs and symptoms of acute toxoplas­ mosis in immunocompromised patients princi­ pally involve the CNS (Fig. 235-2). More than 50% of patients with clinical manifestations have intracerebral involvement. Clinical findings at presentation range from nonfocal to focal dys­ function. CNS findings include encephalopathy, meningoencephalitis, and mass lesions. Patients FIGURE 235-2  Toxoplasmic encephalitis in a 36-year-old patient with AIDS. The multiple lesions are demonstrated by magnetic resonance imaging scanning (T1-weighted with gadolinium enhancement). (Courtesy of Clifford Eskey, Dartmouth Hitchcock Medical Center, Hanover, NH; with permission.)

may present with altered mental status (75%), fever (10–72%), seizures (33%), headaches (56%), and focal neurologic findings (60%), includ­ ing motor deficits, cranial nerve palsies, movement disorders, dysmet­ ria, visual-field loss, and aphasia. Patients who present with evidence of diffuse cortical dysfunction develop evidence of focal neurologic disease as infection progresses. This altered condition is due not only to the necrotizing encephalitis caused by direct invasion by the parasite but also to secondary effects, including vasculitis, edema, and hemor­ rhage. The onset of infection can range from an insidious process over several weeks to an acute presentation with fulminant focal deficits, including hemiparesis, hemiplegia, visual-field defects, localized head­ ache, and focal seizures. Although lesions can occur anywhere in the CNS, the areas most often involved appear to be the brainstem, basal ganglia, pituitary gland, and corticomedullary junction. Brainstem involvement gives rise to a variety of neurologic dysfunctions, including cranial nerve palsy, dysmetria, and ataxia. With basal ganglion infection, patients may develop hydrocephalus, choreiform movements, and choreo­ athetosis. Toxoplasma usually causes encephalitis, and meningeal involvement is uncommon. CSF findings may be unremarkable or may include a modest increase in cell count and in protein—but not glucose—concentration. Cerebral toxoplasmosis must be differentiated from other oppor­ tunistic infections or tumors in the CNS of AIDS patients. The dif­ ferential diagnosis includes herpes simplex encephalitis, cryptococcal meningitis, progressive multifocal leukoencephalopathy, and primary CNS lymphoma. Involvement of the pituitary gland can give rise to panhypopituitarism and hyponatremia from inappropriate secretion of vasopressin (antidiuretic hormone). HIV-associated neurocognitive disorder (HAND) may present as cognitive impairment, attention loss, and altered memory. Brain biopsy in patients who have been treated for TE but who continue to exhibit neurologic dysfunction often fails to identify organisms. Autopsies of Toxoplasma-infected patients have demonstrated the involvement of multiple organs, including the lungs, gastrointestinal tract, pancreas, skin, eyes, heart, and liver. Toxoplasma pneumonia can be confused with Pneumocystis pneumonia. Respiratory involvement usually presents as dyspnea, fever, and a nonproductive cough and may rapidly progress to acute respiratory failure with hemoptysis, metabolic acidosis, hypotension, and (occasionally) disseminated intravascular coagulation. Histopathologic studies demonstrate necrosis and a mixed cellular infiltrate. The presence of organisms is a helpful diagnostic indicator, but organisms can also be found in healthy tissue. Infection of the heart is usually asymptomatic but can be associated with cardiac tamponade or biventricular failure. Infections of the gastrointestinal tract and the liver have been documented. Congenital Toxoplasmosis  Between 400 and 4000 infants born each year in the United States are affected by congenital toxoplasmo­ sis. Acute infection in mothers acquiring T. gondii during pregnancy is usually asymptomatic; most women are diagnosed via prenatal serologic screening. Infection of the placenta leads to hematogenous

infection of the fetus. As gestation proceeds, the proportion of fetuses that become infected increases, but the clinical severity of the infection declines. Although infected children may initially be asymptomatic, the persistence of T. gondii can result in reactivation and clinical disease—most frequently chorioretinitis—decades later. Factors asso­ ciated with relatively severe disabilities include delays in diagnosis and in initiation of therapy, neonatal hypoxia and hypoglycemia, profound visual impairment (see “Ocular Infection,” below), uncorrected hydro­ cephalus, and increased intracranial pressure. If treated appropriately, upward of 70% of children have normal developmental, neurologic, and ophthalmologic findings at follow-up evaluations. Treatment for 1 year with pyrimethamine, a sulfonamide, and folinic acid is tolerated with minimal toxicity (see “Treatment,” below). Ocular Infection  Infection with T. gondii is estimated to cause 35% of all cases of chorioretinitis in the United States and Europe. It was formerly thought that the majority of cases of ocular disease were due to congenital infection. Ocular toxoplasmosis in immunocompetent individuals occurs more commonly than was previously appreciated and has been associated with outbreaks traced to oocyst contamination of soil or water in Victoria (British Columbia) and in South America. Outbreaks have also been reported in North America after ingestion of undercooked venison. A variety of ocular manifestations are documented, including blurred vision, scotoma, photophobia, and eye pain. Macular involve­ ment occurs, with loss of central vision, and nystagmus is secondary to poor fixation. Involvement of the extraocular muscles may lead to dis­ orders of convergence and to strabismus. Ophthalmologic examination should be undertaken in newborns with suspected congenital infec­ tion. As the inflammation resolves, vision improves, but episodic flareups of chorioretinitis, which progressively destroy retinal tissue and lead to glaucoma, are common. The ophthalmologic examination reveals yellow-white, cotton-like patches with indistinct margins of hyperemia. As the lesions age, white plaques with distinct borders and black spots within the retinal pigment become more apparent. Lesions usually are located near the posterior pole of the retina; they may be single but are more commonly multiple. Congenital lesions may be unilateral or bilateral and show evidence of massive chorioretinal degeneration with extensive fibrosis. Surrounding these areas of involvement are a normal retina and vasculature. In patients with AIDS, retinal lesions are often large, with diffuse retinal necrosis, and include both free tachyzoites and cysts containing bradyzoites. Toxoplasmic chorioretinitis may be a prodrome to the development of encephalitis. ■ ■DIAGNOSIS Tissue and Body Fluids  The differential diagnosis of acute toxo­ plasmosis can be made by appropriate culture, serologic testing, and PCR (Table 235-1). PCR is the mainstay for detection of organisms in tissue or biological fluids, but a negative PCR does not rule out toxoplasmosis. Isolation or PCR of T. gondii from the patient’s body fluids (blood, CSF, or bronchoalveolar lavage) reflects acute infection, whereas isolation from biopsied tissue is an indication only of the presence of tissue cysts and should not be misinterpreted as evidence of acute toxoplasmosis. Persistent parasitemia in patients with latent, asymptomatic infection is rare. Histologic examination of lymph nodes may suggest the characteristic changes described above. Demonstra­ tion of tachyzoites in lymph nodes establishes the diagnosis of acute toxoplasmosis. Histologic demonstration of cysts containing brady­ zoites confirms prior infection with T. gondii but may represent latent rather than acute infection. Serology  Serologic testing has become the routine method of diag­ nosis. Diagnosis of acute infection with T. gondii can be established by detection of the simultaneous presence of IgG and IgM antibodies to Toxoplasma in serum. The presence of circulating IgA favors the diagnosis of an acute infection. The Sabin-Feldman dye test, the indi­ rect fluorescent antibody test, and the enzyme-linked immunosorbent assay (ELISA) all measure circulating IgG antibody to Toxoplasma. Positive IgG titers (>1:10) can be detected as early as 2–3 weeks after

TABLE 235-1  Differential Laboratory Diagnosis of Toxoplasmosis DISTINGUISHING CHARACTERISTICS CLINICAL SETTING ALTERNATIVE DIAGNOSIS Mononucleosis syndrome Epstein-Barr virus infection Serology/PCR Cytomegalovirus infection PCR/viral load/serology HIV infection Serology/antigen/viral load Bartonella infection (catscratch disease) Biopsy (PCR or culture)/ serology Lymphoma Biopsy Congenital infection Cytomegalovirus infection PCR Herpes simplex virus infection PCR Rubella virus infection Serology Syphilis Serology Listeriosis Bacterial culture Chorioretinitis in immunocompetent individual Tuberculosis Bacterial culture/PCR Syphilis Serology Histoplasmosis Serology/culture/antigen Chorioretinitis in AIDS patient Cytomegalovirus infection Characteristic exam Syphilis Serology Herpes simplex virus infection PCR CHAPTER 235 Varicella-zoster virus infection PCR Fungal infection PCR/culture CNS lesions in AIDS patient Lymphoma or metastatic tumor Tissue biopsy Brain abscess Culture/biopsy Toxoplasma Infections Progressive multifocal leukoencephalopathy PCR for JC virus Fungal infection Antigen/PCR/biopsy/ culture Mycobacterial infection PCR/biopsy/culture Abbreviations: CNS, central nervous system; PCR, polymerase chain reaction. Source: Reproduced with permission from JD Schwartzman: Toxoplasmosis, in Principles and Practice of Clinical Parasitology. Hoboken, Wiley; 2001. infection. These titers usually peak at 6–8 weeks and decline slowly to a new baseline level that persists for life. Antibody avidity increases with time and can be useful in difficult cases during pregnancy for establishing when infection may have occurred. The serum IgM titer should be measured in concert with the IgG titer to better establish the time of infection; either the double-sandwich IgM-ELISA or the IgM-immunosorbent assay (IgM-ISAGA) should be used. Both assays are specific and sensitive, with fewer false-positive results than other commercial tests. The double-sandwich IgA-ELISA is more sensitive than the IgM-ELISA for detecting congenital infection in the fetus and newborn. Although a negative IgM result with a positive IgG titer indicates distant infection, IgM can persist for >1 year and should not necessarily be considered a reflection of acute disease. If acute toxo­ plasmosis is suspected, a more extensive panel of serologic tests can be performed. In the United States, testing is available at the Remington Laboratory for Specialty Diagnostics (https://www.sutterhealth.org/ services/lab-pathology/toxoplasma-serology-laboratory). Molecular Diagnostics  Molecular approaches can directly detect T. gondii in biologic samples independent of the serologic response. Results obtained with PCR have high specificity and clini­ cal utility in the diagnosis of toxoplasmosis but may only be available in specialty laboratories. While very specific, depending on the body fluid type tested, the sensitivity of PCR of body fluids may be low, so diagnostic algorithms typically incorporate serologic testing of blood

or body fluids. Real-time PCR, if available, can provide quantitative results. Molecular epidemiologic studies with polymorphic mark­ ers have been useful in correlating clinical signs and symptoms of disease with different T. gondii genotypes that may vary in virulence. Next-generation sequencing approaches (metagenomics) of blood or body fluids are often useful, particularly in immunocompromised individuals.

The Immunocompetent Adult or Child  For the patient who presents with lymphadenopathy only, a positive IgM titer is an indi­ cation of acute infection—and an indication for therapy, if clinically warranted (see “Treatment,” below). The serum IgM titer should be determined again in 3 weeks. An elevation in the IgG titer without an increase in the IgM titer suggests that infection is present but is not acute. If there is a borderline increase in either IgG or IgM, the titers should be reassessed in 3–4 weeks. The Immunocompromised Host  A presumptive clinical diag­ nosis of TE in patients with AIDS is based on clinical presentation, history of exposure (as evidenced by positive serology), and radiologic evaluation. To detect latent infection with T. gondii, HIV-infected persons should be tested for IgG antibody to Toxoplasma soon after HIV infection is diagnosed. When these criteria are used, the predic­ tive value is as high as 80%. More than 97% of patients with AIDS and toxoplasmosis have IgG antibody to T. gondii in serum. IgM serum antibody usually is not detectable. Although IgG titers do not correlate with active infection, serologic evidence of infection almost always precedes the development of TE. It is therefore important to determine the Toxoplasma antibody status of all patients infected with HIV. Anti­ body titers may range from negative to 1:1024 in patients with AIDS and TE. Fewer than 3% of patients have no demonstrable antibody to Toxoplasma at diagnosis of TE. PART 5 Infectious Diseases Patients with TE have focal or multifocal abnormalities demon­ strable by computed tomography (CT) or magnetic resonance imag­ ing (MRI). Neuroradiologic evaluation should include double-dose contrast CT of the head. By this test, single and frequently multiple contrast-enhancing lesions (<2 cm) may be identified. MRI usually demonstrates multiple lesions located in both hemispheres, with the basal ganglia and corticomedullary junction most commonly involved; MRI provides a more sensitive evaluation of the efficacy of therapy than does CT (Fig. 235-2). These findings are not pathog­ nomonic of Toxoplasma infection, because 40% of CNS lymphomas are multifocal and 50% are ring-enhancing. For both MRI and CT scans, the rate of false-negative results is ~10%. The finding of a single lesion on an MRI scan increases the likelihood of primary CNS lymphoma (in which solitary lesions are four times more likely than in TE) and strengthens the argument for the performance of a brain biopsy. A therapeutic trial of anti-Toxoplasma medications is frequently used to assess the diagnosis. Treatment of presumptive TE with pyrimethamine plus sulfadiazine or clindamycin results in quantifiable clinical improvement in >50% of patients by day 3. Leucovorin is administered to prevent bone marrow toxicity. By day 7, >90% of treated patients show evidence of improvement. In contrast, if patients fail to respond or have lymphoma, clinical signs and symptoms worsen by day 7. Patients in this category require brain biopsy with or without a change in therapy. This procedure can now be performed by a stereotactic CT-guided method that reduces the potential for complications. Brain biopsy for T. gondii identifies organisms in 50–75% of cases. PCR amplification of CSF may also confirm toxoplasmosis or suggest alternative diagnoses (Table 235-1), such as progressive multifocal leukoencephalopathy (JC virus posi­ tive) or primary CNS lymphoma (Epstein-Barr virus positive). CT and MRI with contrast are currently the standard diagnostic imaging tests for TE. As in other conditions, the radiologic response may lag behind the clinical response. Resolution of lesions may take from 3 weeks to 6 months. Some patients show clinical improvement despite worsening radiographic findings. Congenital Infection  The issue of concern when a pregnant woman has evidence of recent T. gondii infection is whether the fetus

is infected. PCR analysis of the amniotic fluid has replaced fetal blood sampling. Serologic diagnosis is based on the persistence of IgG anti­ body or a positive IgM titer after the first week of life (a time frame that excludes placental leak). The IgG determination should be repeated every 2 months. An increase in IgM beyond the first week of life is indicative of acute infection. Up to 25% of infected newborns may be seronegative and have normal routine physical examinations. Thus, assessment of the eye and the brain, with ophthalmologic testing, CSF evaluation, and radiologic studies, is important in establishing the diagnosis. Ocular Toxoplasmosis  The serum antibody titer may not cor­ relate with the presence of active lesions in the fundus, particularly in cases of congenital toxoplasmosis. In general, a positive IgG titer (measured in undiluted serum if necessary) in conjunction with typical lesions establishes the diagnosis. If lesions are atypical and the serum antibody titer is in the low-positive range, the diagnosis is presump­ tive. The parasitic antigen–specific polyclonal IgG assay as well as parasite–specific PCR may facilitate the diagnosis. PCR of ocular samples has better yield than PCR of blood, but negative PCR does not rule out the diagnosis. Diagnosis may also be established by ocular fluid Western blot or comparison of ocular fluid antibody with blood antibody (Goldmann-Witmer coefficient). The clinical diagnosis of ocular toxoplasmosis can be supported in 60–90% of cases by labora­ tory tests, depending on the time of anterior chamber puncture and the panel of antibody analyses used. TREATMENT Toxoplasmosis CONGENITAL INFECTION Congenitally infected neonates are treated with daily oral pyri­ methamine (1 mg/kg) and sulfadiazine (100 mg/kg) with folinic acid for 1 year. Depending on the signs and symptoms, prednisone (1 mg/kg per day) may be used for congenital infection. Some U.S. states and some countries routinely screen pregnant women (France, Austria) and/or newborns (Denmark, Massachusetts). Management and treatment regimens vary with the country and the treatment center. Most experts use spiramycin to treat preg­ nant women who have acute toxoplasmosis early in pregnancy and use pyrimethamine/sulfadiazine/folinic acid to treat women who seroconvert after 18 weeks of pregnancy or in cases of documented fetal infection. Studies suggest that treatment during pregnancy decreases the severity of infection. Many women who are infected in the first trimester elect termination of pregnancy. Those who do not terminate pregnancy are offered prenatal anti­ biotic therapy to reduce the frequency and severity of Toxoplasma infection in the infant. The optimal duration of treatment for a child with asymptomatic congenital toxoplasmosis is not clear, although most clinicians in the United States would treat the child for 1 year in light of cohort investigations conducted by the National Collaborative Chicago-Based, Congenital Toxoplasmosis Study. INFECTION IN IMMUNOCOMPETENT PATIENTS Immunologically competent adults and older children who have only lymphadenopathy do not require specific therapy unless they have persistent, severe symptoms. Patients with ocular toxoplas­ mosis are usually treated for 6 weeks with pyrimethamine plus either sulfadiazine or clindamycin and sometimes with prednisone. Trimethoprim-sulfamethoxazole (TMP-SMX) can also be given if pyrimethamine cannot be obtained (5 mg/kg bid based on TMP). Treatment should be supervised by an ophthalmologist familiar with Toxoplasma disease. Ocular disease can be self-limited without treatment, but therapy is typically considered for lesions that are severe or close to the fovea or optic disc. Prolonged treatment with TMP-SMX prevents recurrences of ocular toxoplasmosis while on treatment and is often considered in individuals with frequent flares

in a 1- to 2-year period. Flares are more common in individuals who have acquired infection in South America. Whether treatment improves long-term visual outcomes is unclear. Other clinical pre­ sentations of toxoplasmosis in immunocompetent individuals are treated 2–4 weeks with duration and decision to treat based upon response and severity of clinical symptoms. INFECTION IN IMMUNOCOMPROMISED PATIENTS Clinical Treatment   Immunocompromised patients, such as patients with AIDS and/or transplant recipients, should be treated for acute toxoplasmosis as toxoplasmosis is rapidly fatal if untreated. The recommended treatment is pyrimethamine (200 mg load, 50 mg/d if <60 kg, 75 mg/d if >60 kg) plus sulfadiazine (1000 mg qid for <60 kg; 1500 mg qid >60 kg) plus leucovorin (10–25 mg/d) to reduce hematologic toxicity for a minimum of 6 weeks. In cases of sulfa intolerance, clindamycin (600 mg qid) can be substituted. TMP-SMX (5 mg/kg of TMP, 25 mg/kg SMX bid) appears to be an effective alternative for treatment of TE in resource-poor settings where the preferred combination of pyrimethamine plus sulfadia­ zine is not available. Pyrimethamine is very expensive in the United States, so many clinicians prescribe TMP-SMX if pyrimethamine cannot be obtained. Most experts continue to prefer pyrimethamine plus sulfadiazine because of better synergy between the drugs and fewer clinical failures relative to TMP-SMX. Primary Prophylaxis in AIDS    The incidence of TE has declined as the survival of patients with HIV infection has increased through the use of ART. The incidence of TE in underresourced settings is unknown because serologic testing and imaging are not available. AIDS patients who are seropositive for T. gondii and who have a CD4+ T lymphocyte count of <100/μL should receive prophylaxis against TE. The daily dose of TMP-SMX (one double-strength tablet qd) recommended for prophylaxis of Pneumocystis jirovecii pneumonia (PJP; formerly Pneumocystis carinii or PCP) is effective against TE. If patients cannot tolerate TMP-SMX, the recommended alterna­ tive is dapsone-pyrimethamine, which likewise is effective against PJP. Atovaquone with or without pyrimethamine also can be con­ sidered. Prophylactic monotherapy with dapsone, pyrimethamine, azithromycin, clarithromycin, or aerosolized pentamidine is prob­ ably insufficient. Discontinuing Primary Prophylaxis  Prophylaxis against TE can be discontinued in HIV-positive patients who have responded to ART and whose CD4+ T lymphocyte count has been >200/μL for 3 months. Although patients with CD4+ T lymphocyte counts of <100/μL are at greatest risk for developing TE, the risk that this condition will develop when the count has increased to 100–200/μL has not been established. Thus, prophylaxis should be discontinued when the count has increased to >200/μL. Prophylaxis should be recommenced if the CD4+ T lymphocyte count again decreases to <100–200/μL. Individuals who have completed initial therapy for TE should receive treatment indefinitely until immune reconstitution, with a CD4+ T-cell count of >200/μL, is achieved with ART. Combination therapy with pyrimethamine plus sulfadiazine plus leucovorin is effective. An alternative to sulfadiazine in this regimen is clindamy­ cin or TMP-SMX. Discontinuing Secondary Prophylaxis (Long-Term Maintenance Therapy)  Patients receiving secondary prophylaxis for TE are at low risk for recurrence when they have completed initial therapy for TE, remain asymptomatic, and have evidence of restored immune function. Individuals with HIV infection should have a CD4+

T lymphocyte count of >200/μL for at least 6 months after combined ART (cART). A repeat MRI brain scan is recommended. Secondary

prophylaxis should be reintroduced if the CD4+ T lymphocyte count decreases to <200/μL.

Prophylaxis in Allogeneic HSCT or Solid Organ Transplant  The incidence of toxoplasmosis is lower in seropositive allogeneic HSCT given TMP-SMX prophylaxis, so prophylaxis should be given after engraftment for at least 6 months with weekly PCR monitoring performed after transplant. TMP-SMX regimens will prevent both toxoplasmosis and P. jirovecii. ■ ■PREVENTION Seronegative immunocompromised or pregnant persons should be counseled regarding sources of Toxoplasma infection. The chances of primary infection with Toxoplasma can be reduced by not eating undercooked meat and by avoiding oocyst-contaminated material (i.e., a cat’s litter box). Specifically, lamb, beef, pork, and venison should be cooked to an internal temperature of 63°C (145°F) measured in the thickest portion of the cut and rested for 3 min. Ground meat should be cooked to 71°C (145°F), whereas poultry should be cooked to 74°C (165°F). Hands should be washed thoroughly after work in the garden, and all fruits and vegetables should be washed. Freezing meat to –20°C (–4°F) also kills cysts. Ingestion of raw shellfish is a risk factor for toxoplasmosis, given that the filter-feeding mechanism of clams and mussels concentrates oocysts. If the patient owns a cat, the litter box should be cleaned or changed daily, preferably by an HIV-negative, nonpregnant person; alterna­ tively, patients should wash their hands thoroughly after changing the litter box. Litter boxes should be changed daily if possible, as freshly excreted oocysts will not have sporulated and will not be infectious. Patients should be encouraged to keep their cats inside and not to adopt or handle stray cats. Cats should be fed only canned or dried commercial food or well-cooked table food, not raw or undercooked meats. Patients need not be advised to part with their cats or to have their cats tested for toxoplasmosis. Ideally, blood intended for transfu­ sion into Toxoplasma-seronegative immunocompromised individuals should be screened for antibody to T. gondii. Although such serologic screening is not routinely performed, seronegative women should be screened for evidence of infection several times during pregnancy if they are exposed to environmental conditions that put them at risk for infection with T. gondii. HIV-positive individuals should adhere closely to these preventive measures. CHAPTER 235 Toxoplasma Infections Acknowledgment The author would like to acknowledge Dr. Lloyd Kasper for his numerous contributions to our understanding of the pathogenesis of toxoplasmosis and his essential role in preparation of this chapter for prior editions. ■ ■FURTHER READING Aerts R et al: Guidelines for the management of Toxoplasma gondii infection and disease in patients with haematological malignancies and after haematopoietic stem-cell transplantation: Guidelines from the 9th European Conference on Infections in Leukaemia, 2022. Lancet Infect Dis 24:e291, 2024. Cifuentes-Gonzalez C et al: Risk factors for recurrences and visual impairment in patients with ocular toxoplasmosis: A systematic review and meta-analysis. PLoS One A18:e0283845, 2023. Jones JL et al: Toxoplasma gondii infection in the United States, 2011–2014. Am J Trop Med Hyg 98:551 2018. Schumacher AC et al: Toxoplasmosis outbreak associated with Toxo­ plasma gondii-contaminated venison−high attack rate, unusual clini­ cal presentation, and atypical genotype. Clin Infect Dis 72:1557, 2021. Song G et al: Toxoplasma gondii seropositivity and cognitive function­ ing in older adults: An analysis of cross-sectional data of the National Health and Nutrition Examination Survey 2011–2014. BMJ Open 14:e071513, 2024.

131 - 236 Protozoal Intestinal Infections and Trichomoniasis

236 Protozoal Intestinal Infections and Trichomoniasis

Peter F. Weller, Edward T. Ryan

Protozoal Intestinal

Infections and

Trichomoniasis PROTOZOAL INFECTIONS ■ ■GIARDIASIS Giardia duodenalis (also known as G. lamblia or G. intestinalis) is a cosmopolitan protozoal parasite that inhabits the small intestines of humans and other mammals. Giardiasis is one of the most com­ mon parasitic diseases in both developed and developing countries worldwide, causing both endemic and epidemic intestinal disease and diarrhea. In resource-constrained areas with limited access to adequate sanitation and safer water, prevalence rates of giardiasis may be 20–40%, especially among young children. Life Cycle and Epidemiology (Fig. 236-1)  Infection follows the ingestion of environmentally hardy cysts, which excyst in the small PART 5 Infectious Diseases Excystation follows exposure to stomach acid and intestinal proteases, releasing trophozoite forms that multiply by binary fission and reside in the upper small bowel adherent to enterocytes. Causes: Asymptomatic infection, acute diarrhea, or chronic diarrhea and malabsorption. Small bowel may demonstrate villous blunting, crypt hypertrophy, and mucosal inflammation. Encystation occurs under conditions of bile salt concentration changes and alkaline pH. Smooth-walled cysts can contain two trophozoites. Cysts are ingested (10–25 cysts) in contaminated water or food or by direct fecal-oral transmission (as in day-care centers). Cysts can survive in the environment (up to several weeks in cold water). They may also infect nonhuman mammalian species. Cysts and trophozoites are passed in the stool into the environment. FIGURE 236-1  Life cycle of Giardia. (Reproduced with permission from RL Guerrant et al [eds]: Tropical Infectious Diseases: Principles, Pathogens and Practice, 2nd ed. Elsevier, 2006.)

FIGURE 236-2  Flagellated, binucleate Giardia trophozoites. intestine, releasing flagellated trophozoites (Fig. 236-2) that multiply by binary fission. Giardia remains a pathogen of the proximal small bowel and does not disseminate hematogenously. Trophozoites remain free in the lumen or attach to the mucosal epithelium by means of a ventral sucking disk. As a trophozoite encounters altered conditions, it forms a morphologically distinct cyst, which is the stage of the para­ site usually found in the feces. Trophozoites may be present and even predominate in loose or watery stools, but it is the resistant cyst that survives outside the body and is responsible for transmission. Cysts do not tolerate heating or desiccation, but they do remain viable for months in cold fresh water. The number of cysts excreted varies widely but can approach 107 per gram of stool. Ingestion of as few as 10 cysts is sufficient to cause infection in humans. Because cysts are infectious when excreted, person-to-person transmission occurs where fecal hygiene is poor. Giardiasis is especially prevalent in day-care centers; person-to-person spread also takes place in other institutional settings with poor fecal hygiene and during anal– oral contact. If food is contaminated with Giardia cysts after cooking or preparation, foodborne transmission can occur. Waterborne transmis­ sion accounts for episodic infections (e.g., in campers and travelers) and for major epidemics in metropolitan areas. Surface water, ranging from mountain streams to large municipal reservoirs, can become contaminated with fecally derived Giardia cysts. The efficacy of water as a means of transmission is enhanced by the small infectious inocu­ lum of Giardia, the prolonged survival of cysts in cold water, and the resistance of cysts to killing by routine chlorination methods that are adequate for controlling bacteria. Viable cysts can be eradicated from water by either boiling or filtration. In the United States, Giardia (like Cryptosporidium; see below) can cause waterborne epidemics of gastroenteritis. Giardia is common in developing countries, and infections may be acquired by travelers. There are several recognized genotypes or assemblages of G. duode­ nalis. Human infections are due to assemblages A and B, whereas other assemblages are more common in other animals, including cats and dogs. Like beavers from reservoirs implicated in epidemics, dogs and cats have been found to be infected with assemblages A and B; this finding suggests both that these animals may have been infected from human sources and that they might be sources of further human infections. Giardiasis, like cryptosporidiosis, creates a significant economic burden because of the costs incurred in the installation of water filtration systems required to prevent waterborne epidemics, in the management of epidemics that involve large communities, and in the evaluation and treatment of endemic infections. Pathophysiology  The reasons that some, but not all, infected patients develop clinical manifestations and the mechanisms by which Giardia causes alterations in small-bowel function are largely unknown. Although trophozoites adhere to the epithelium, they are not invasive but may elicit apoptosis of enterocytes, epithelial bar­ rier dysfunction, and epithelial cell malabsorption and secretion.

TABLE 236-1  Diagnosis of Intestinal Protozoal Infections PARASITE STOOL O+P FECAL ACID-FAST STAIN FECAL ANTIGEN IMMUNOASSAYS FECAL NAATS OTHER Giardia +   + + DFA Cryptosporidium ± + + + DFA Cystoisospora ± + +   Cyclospora ± + +   Dientamoeba ±   + +   Balantidium +         Microsporidia –     + Special fecal stains, tissue biopsies Abbreviations: DFA, direct immunofluorescence assay; NAATs, nucleic acid amplification tests; O+P, conventional ova and parasites. Consequent lactose intolerance and, in a minority of infected adults and children, significant malabsorption are clinical signs of the loss of epithelial brush-border enzyme activities. In most infections, the morphology of the bowel is unaltered; however, in chronically infected, symptomatic patients, the histopathologic findings (including flattened villi) and the clinical manifestations at times resemble those of tropical sprue and gluten-sensitive enteropathy. The pathogenesis of diarrhea in giardiasis is not known. The natural history of Giardia infection varies markedly. Infections may be asymptomatic, transient, recurrent, or chronic. G. duodenalis parasites vary genotypically, and such variations might contribute to different courses of infection. Parasite as well as host factors may be important in determining the course of infection and disease. Both cellular and humoral responses develop in human infections, but their precise roles in disease pathogenesis and/or control of infection are unknown. Because patients with hypogammaglobulinemia suffer from prolonged, severe infections that are poorly responsive to treatment, humoral immune responses appear to be important. The greater sus­ ceptibilities of the young than of the old and of newly exposed persons than of chronically exposed populations suggest that at least partial protective immunity may develop. Clinical Manifestations  Disease manifestations of giardiasis range from asymptomatic carriage to fulminant diarrhea and malabsorption. Most infected persons are asymptomatic, but in epidemics, the propor­ tion of symptomatic cases may be higher. Symptoms may develop sud­ denly or gradually. In persons with acute giardiasis, symptoms develop after an incubation period that lasts at least 5–6 days and usually 1–3 weeks. Prominent early symptoms include diarrhea, abdominal pain, bloating, belching, flatus, nausea, and vomiting. Although diarrhea is common, upper intestinal manifestations such as nausea, vomiting, bloating, and abdominal pain may predominate. The duration of acute giardiasis is usually >1 week, although diarrhea often subsides. Individuals with chronic giardiasis may present with or without having experienced an antecedent acute symptomatic episode. Diarrhea is not necessarily prominent, but increased flatus, loose stools, sulfurous belching, and (in some instances) weight loss occur. Symptoms may be continual or epi­ sodic and may persist for years. Some persons who have relatively mild symptoms for long periods recognize the extent of their discomfort only in retrospect. Fever, the presence of blood and/or mucus in the stools, and other signs and symptoms of colitis are uncommon and suggest a different diagnosis or a concomitant illness. Symptoms tend to be inter­ mittent yet recurring and gradually debilitating, in contrast with the acute disabling symptoms associated with many enteric bacterial infec­ tions. Because of the less severe illness early on and the propensity for chronic infections, patients may seek medical advice late in the course of the illness; however, disease can be severe, resulting in malabsorption, weight loss, growth retardation in children, and dehydration. A number of extraintestinal manifestations have been described, such as urticaria, anterior uveitis, and arthritis; whether these are caused by giardiasis or concomitant processes is unclear. Giardiasis can be severe in patients with hypogammaglobulinemia and can complicate other preexisting intestinal diseases, such as that occurring in cystic fibrosis. In patients with AIDS, Giardia can cause enteric illness that is refractory to treatment.

Diagnosis (Table 236-1)  Giardiasis is diagnosed by detection of parasite antigens in the feces, by identification of cysts in the feces or of trophozoites in the feces or small intestines, or by nucleic acid amplifi­ cation tests (NAATs). Cysts are oval and measure 8–12 μm × 7–10 μm. Mature cysts characteristically contain four nuclei. Trophozoites are pear-shaped, dorsally convex, flattened parasites with two nuclei and four pairs of flagella (Fig. 236-2). The diagnosis is sometimes difficult to establish. Direct examination of fresh or properly preserved stools as well as concentration methods can be used. Because cyst excretion is variable and may be undetectable at times, repeated examination of stool, sampling of duodenal fluid, and biopsy of the small intestine may be required to detect the parasite. Tests for parasitic antigens in stool are often more sensitive than or comparable to microscopic examina­ tion, are easier to perform, have a more rapid turn-around time, are often cost-comparable, and are often the first-line test in many institu­ tions. NAATs are also highly sensitive. CHAPTER 236 TREATMENT Giardiasis Protozoal Intestinal Infections and Trichomoniasis
Cure rates with metronidazole (250–500 mg thrice daily for 5 days) are usually >90%. Tinidazole (2 g once by mouth) may be more effective than metronidazole. Nitazoxanide (500 mg twice daily for 3 days) is an alternative agent for treatment of giardiasis. Paro­ momycin, an oral aminoglycoside that is not well absorbed, can be given to symptomatic pregnant patients, although information is limited on how effectively this agent eradicates infection. Alben­ dazole and mebendazole also can be used as alternative agents, although their efficacy is lower than that of single-dose tinidazole. Quinacrine, secnidazole, ornidazole, and furazolidone are other alternative agents that have demonstrated some efficacy. Almost all patients respond to therapy and are cured, although some with chronic giardiasis experience delayed resolution of symptoms after eradication of Giardia. For many of the latter patients, residual symptoms probably reflect delayed regeneration of intestinal brush-border enzymes. Continued infection should be documented by stool examinations before treatment is repeated. Patients who remain infected after repeated treatments should be evaluated for reinfection through family members, close personal contacts, and environmental sources as well as for hypogamma­ globulinemia. In cases refractory to multiple treatment courses, prolonged therapy with metronidazole (750 mg thrice daily for 21 days) or therapy with varied combinations of multiple agents has been successful. Prevention  Giardiasis can be prevented by consumption of uncon­ taminated food and water and by personal hygiene during the pro­ vision of care for infected children. Boiling or filtering potentially contaminated water prevents infection. ■ ■CRYPTOSPORIDIOSIS The coccidian parasite Cryptosporidium causes diarrheal disease that is self-limited in immunocompetent human hosts but can be severe in persons with AIDS or other forms of immunodeficiency. Two species

of Cryptosporidium, C. hominis and C. parvum, cause most human infections.

Life Cycle and Epidemiology  Cryptosporidium species are widely distributed in the world. In resource-constrained areas, cryptosporidiosis is the second most common cause of moderate to severe diarrhea during the first two years of life (following rotavirus). Cryptosporidiosis is acquired by the consumption of oocysts (50% infectious dose: ~10–100 C. hominis/parvum oocysts in nonimmune individuals), which excyst to liberate sporozoites that in turn enter and infect intestinal epithelial cells. The parasite’s further develop­ ment involves both asexual and sexual cycles, which produce forms capable of infecting other epithelial cells and of generating oocysts that are passed in the feces. Cryptosporidium species infect a num­ ber of animals, and C. parvum can spread from infected animals to humans. Since oocysts are immediately infectious when passed in feces, person-to-person transmission takes place in day-care centers and among household contacts and medical providers. Waterborne transmission (especially that of C. hominis) accounts for infections in travelers and for common-source epidemics. Oocysts are quite hardy and resist killing by routine chlorination. Both drinking water and recreational water (e.g., pools, waterslides) have been increasingly recognized as sources of infection. Pathophysiology  Although intestinal epithelial cells harbor cryp­ tosporidia in an intracellular vacuole, the means by which secretory diarrhea is elicited remain uncertain. No characteristic pathologic changes are found by biopsy. The distribution of infection can be spotty within the principal site of infection, the small bowel. Cryptosporidia are found in the pharynx, stomach, and large bowel of some patients and at times in the respiratory tract. Especially in patients with AIDS, involvement of the biliary tract can cause papillary stenosis, sclerosing cholangitis, or cholecystitis. PART 5 Infectious Diseases Clinical Manifestations  Asymptomatic infections can occur in both immunocompetent and immunocompromised hosts. In immu­ nocompetent persons, symptoms develop after an incubation period of ~1 week and consist principally of watery nonbloody diarrhea, sometimes in conjunction with abdominal pain, nausea, anorexia, fever, and/or weight loss. In these hosts, the illness usually subsides after 1–2 weeks. In contrast, in immunocompromised hosts (especially those with AIDS and CD4+ T-cell counts <100/μL), diarrhea can be chronic, persistent, and remarkably profuse, causing clinically signifi­ cant fluid and electrolyte depletion. Stool volumes may range from 1 to 25 L/d. Weight loss, wasting, and abdominal pain may be severe. Bili­ ary tract involvement can manifest as mid-epigastric or right-upperquadrant pain. Diagnosis  (Table 236-1) Evaluation starts with fecal examination for small oocysts, which are smaller (4–5 μm in diameter) than the fecal stages of most other parasites. Because conventional stool exami­ nation for ova and parasites (O+P) does not detect Cryptosporidium, specific testing must be requested. Detection is enhanced by evaluation of stools (obtained on multiple days) by several techniques, including modified acid-fast and direct immunofluorescent stains and enzyme immunoassays. NAATs also are useful. Cryptosporidia can also be identified by light and electron microscopy at the apical surfaces of intestinal epithelium from biopsy specimens of the small bowel and, less frequently, the large bowel. TREATMENT Cryptosporidiosis Nitazoxanide, approved by the U.S. Food and Drug Administration (FDA) for the treatment of cryptosporidiosis, is available in tablet form for adults (500 mg twice daily for 3 days) and as an elixir for children. This agent has not been effective for the treatment of immunosuppressed patients or HIV-infected patients, in whom improved immune status due to antiretroviral therapy can lead to amelioration of cryptosporidiosis. Otherwise, treatment includes

supportive care with replacement of fluids and electrolytes and administration of antidiarrheal agents. Biliary tract obstruction may require papillotomy or T-tube placement. Prevention requires minimizing exposure to infectious oocysts in human or animal feces. Use of submicron water filters may minimize acquisition of infection from drinking water. ■ ■CYSTOISOSPORIASIS The coccidian parasite Cystoisospora belli (formerly referred to as Isospora belli) causes human intestinal disease. Infection is acquired by the consumption of oocysts, after which the parasite invades intestinal epithelial cells and undergoes both sexual and asexual cycles of devel­ opment. Oocysts excreted in stool are not immediately infectious but must undergo further maturation. Although C. belli infects many animals, little is known about the epidemiology or prevalence of this parasite in humans. It is most com­ mon in tropical and subtropical countries. Acute infections can begin abruptly with fever, abdominal pain, and watery nonbloody diarrhea and can last for weeks or months. In patients who have AIDS or are immunocompromised for other reasons, infections often are not selflimited but rather resemble cryptosporidiosis, with chronic, profuse watery diarrhea. Eosinophilia, which is not found in other enteric protozoan infections, may be detectable in immunocompetent hosts. The diagnosis (Table 236-1) is usually made by detection of the large (~25 μm) oocysts in stool by modified acid-fast staining. Oocyst excre­ tion may be low-level and intermittent; if repeated stool examinations are unrevealing, sampling of duodenal contents by aspiration or smallbowel biopsy (often with electron microscopic examination) may be necessary. NAATs are effective newer diagnostic tools. TREATMENT Cystoisosporiasis Trimethoprim-sulfamethoxazole (TMP-SMX, 160/800 mg two times daily for 7–10 days; and, for HIV-infected patients, then continuing three times daily for 3–4 weeks) is effective. For patients intolerant of sulfonamides, pyrimethamine (50–75 mg/d) or cipro­ floxacin 500 mg two times a day can be used. Relapses can occur in persons with AIDS and necessitate maintenance therapy with TMP-SMX (160/800 mg three times per week). ■ ■CYCLOSPORIASIS Cyclospora cayetanensis, a cause of diarrheal illness, is globally distrib­ uted: illness due to C. cayetanensis has been reported in the United States, Asia, Africa, Latin America, and Europe. The epidemiology of this parasite has not yet been fully defined, but waterborne transmission and foodborne transmission (e.g., by basil, sweet peas, and imported raspberries) have been recognized. The full spectrum of illness attribut­ able to Cyclospora has not been delineated. Some infected patients may be without symptoms, but many have diarrhea, flulike symptoms, and flatulence and belching. The illness can be self-limited, can wax and wane, or, in many cases, can involve prolonged diarrhea, anorexia, and upper gastrointestinal symptoms, with sustained fatigue and weight loss in some instances. Diarrheal illness may persist for >1 month. Cyclospora can cause enteric illness in patients infected with HIV. The parasite is detectable in epithelial cells of small-bowel biopsy samples and elicits secretory diarrhea by unknown means. The absence of fecal blood and leukocytes indicates that disease due to Cyclospora is not caused by destruction of the small-bowel mucosa. The diagno­ sis (Table 236-1) can be made by detection of spherical 8- to 10-μm oocysts in the stool, although routine stool O+P examinations are not sufficient. Specific fecal examinations must be requested to detect the oocysts, which are variably acid-fast and are fluorescent when viewed with ultraviolet light microscopy. NAATs are sensitive. Cyclospo­ riasis should be considered in the differential diagnosis of prolonged diarrhea, with or without a history of travel by the patient to other countries.

TREATMENT Cyclosporiasis Cyclosporiasis is treated with TMP-SMX (160/800 mg twice daily for 7–10 days). HIV-infected patients may require longer treatment courses, may experience relapses after treatment, and thus may require longer-term suppressive maintenance therapy. Nitazoxanide and ciprofloxacin may be alternative agents in patients unable to receive TMP-SMX. ■ ■MICROSPORIDIOSIS Microsporidia are obligate intracellular spore-forming protozoa that infect many animals and cause disease in humans, especially as opportunistic pathogens in AIDS. Microsporidia are members of a distinct phylum, Microspora, which contains hundreds of genera and thousands of species. The various microsporidia are differentiated by their developmental life cycles, ultrastructural features, and molecular Microsporidia Enterocytozoon bieneusi, Encephalitozoon spp., etc. Intracellular multiplication via merogony and sporogony E. bieneusi in epithelial cell Polar tubule pierces host epithelial cell, injects sporoplasm While E. bieneusi is primarily in the gastrointestinal tract, other species may invade the lung or eye or disseminate to cause: Presumed ingestion or respiratory acquisition of spores Person-to-person, zoonotic, waterborne, or food-borne transmission? Diagnostic spores present in stool, urine, respiratory fluids, cerebrospinal fluid, or various tissue specimens FIGURE 236-3  Life cycle of microsporidia. (Reproduced with permission from RL Guerrant et al [eds]: Tropical Infectious Diseases: Principles, Pathogens and Practice, 2nd ed. Elsevier 2006.)

taxonomy based on ribosomal RNA. The complex life cycles of the organisms result in the production of infectious spores (Fig. 236-3). Currently, at least 17 species of microsporidia are recognized as causes of human disease, most commonly of the genera Enterocytozoon and Encephalitozoon. Although some microsporidia are probably prevalent causes of self-limited or asymptomatic infections in immunocompe­ tent patients, little is known about how microsporidiosis is acquired.

Microsporidiosis is most common among patients with AIDS, less common among patients with other types of immunocompromise, and rare among immunocompetent hosts. In patients with AIDS, intestinal infections with Enterocytozoon bieneusi and Encephalitozoon intestinalis are recognized to contribute to chronic diarrhea and wasting; these infections have been found in 10–40% of patients with chronic diar­ rhea. Both organisms have been found in the biliary tracts of patients with cholecystitis. E. intestinalis may also disseminate to cause fever, diarrhea, sinusitis, cholangitis, and bronchiolitis. In patients with AIDS, Encephalitozoon hellem has caused superficial keratoconjunc­ tivitis as well as sinusitis, respiratory tract disease, and disseminated Encephalitozoon intestinalis in epithelial cells, endothelial cells, or macrophages CHAPTER 236 Protozoal Intestinal Infections and Trichomoniasis
Spore-laden host epithelial cells sloughed into lumina of gastrointestinal, respiratory, or genitourinary tract Chronic diarrhea Cholangitis Sinusitis Bronchitis Nephritis Cystitis/prostatitis Keratoconjunctivitis Encephalitis Sloughed cells degenerate; spores shed in bodily fluids

infection. Myositis due to Pleistophora has been documented. Nosema, Vittaforma, and Microsporidium have caused stromal keratitis associ­ ated with trauma in immunocompetent patients.

Microsporidia are small gram-positive organisms with mature spores measuring 0.5–2 μm × 1–4 μm. Diagnosis of microsporidial infections in tissue often requires electron microscopy, although intra­ cellular spores can be visualized by light microscopy with hematoxylin and eosin, Giemsa, or tissue Gram’s stain. Light microscopy with a modified trichrome stain or fluorescent assays can be used on stool, tis­ sue, or urine. NAATs are useful for diagnosis and speciation. Definitive therapies for microsporidial infections remain to be established. For superficial keratoconjunctivitis due to E. hellem, E. cuniculi, E. intesti­ nalis, and E. bieneusi, topical therapy with fumagillin suspension has shown promise (Chap. 229). For enteric infections with E. intestinalis in HIV-infected patients, therapy with albendazole may be efficacious (Chap. 229). ■ ■OTHER INTESTINAL PROTOZOA Balantidiasis  Balantidium coli is a large ciliated protozoal parasite that can produce a spectrum of large-intestinal disease analogous to amebiasis. The parasite is widely distributed in the world. Since it infects pigs, cases in humans are more common where pigs are raised. Infective cysts can be transmitted from person to person and through water, but many cases are due to the ingestion of cysts derived from porcine feces in association with slaughtering, with use of pig feces for fertilizer, or with contamination of water supplies by pig feces. Ingested cysts liberate trophozoites, which reside and replicate in the large bowel. Many patients remain asymptomatic, but some have persisting intermittent diarrhea, and a few develop more fulminant dysentery. In symptomatic individuals, the pathology in the bowel— both gross and microscopic—is similar to that seen in amebiasis, with varying degrees of mucosal invasion, focal necrosis, and ulceration. Balantidiasis, unlike amebiasis, only rarely spreads hematogenously to other organs. The diagnosis is made by detection of the parasite in stool or sampled colonic tissue. Tetracycline (500 mg four times daily for

10 days) is an effective therapeutic agent. Metronidazole is an alterna­ tive agent. PART 5 Infectious Diseases Blastocystosis  Blastocystis hominis is an organism of uncertain pathogenicity. Some patients who pass B. hominis in their stools are asymptomatic, whereas others have diarrhea and associated intesti­ nal symptoms. Diligent evaluation reveals other potential bacterial, viral, or protozoal causes of diarrhea in some but not all patients with symptoms. Because the pathogenicity of B. hominis is uncertain and because therapy for Blastocystis infection is neither specific nor uni­ formly effective, patients with prominent intestinal symptoms should be fully evaluated for other infectious causes of diarrhea. If diarrheal symptoms associated with Blastocystis are prominent, metronidazole (500–750 mg thrice daily for 5–10 days) or tinidazole (2 g once) can be tried. Alternative agents include paromomycin, TMP-SMX, and nitazoxanide. Dientamoebiasis  Dientamoeba fragilis is unique among intestinal protozoa in that it has a trophozoite stage but not a cyst stage. How trophozoites survive to transmit infection is not known. When symp­ toms develop in patients with D. fragilis infection, they are generally mild and include intermittent diarrhea, abdominal pain, and anorexia. The diagnosis is made by the detection of trophozoites in stool; the lability of these forms accounts for the greater yield when fecal samples are preserved immediately after collection. NAATs are more sensitive than fecal microscopy. Paromomycin (25–35 mg/kg per day in three doses for 7 days) or metronidazole (500–750 mg three times daily for 10 days) is appropriate for treatment. Tetracycline, doxycycline, tinida­ zole, secnidazole, and ornidazole are alternative agents. Sarcocystosis  Humans can be the definitive or dead-end inter­ mediate host for Sarcocystis spp. sarcocystoses are zoonotic infections. Humans can act as definitive hosts (the host in which a parasite com­ pletes its sexual life cycle) following consumption of undercooked beef

or pork that contain the protozoa Sarcocystis hominis or S. suihominis, respectively. In this setting, humans can develop intestinal symptoms including nausea, vomiting, and diarrhea that is often self-resolving within a few days. Human intestinal infection can result in transient shedding of infectious oocysts that are consumed by cattle or pigs in the environment, and in these hosts the protozoa encyst in muscle tissue, allowing completion of the life cycle when these tissues are con­ sumed by humans. Separately, humans can develop a transient myositis with peripheral eosinophilia following ingestion of oocysts of S. nesbitti shed by snakes. Intestinal sarcocystosis can be detected by stool exami­ nation, although oocysts are not usually detectable during the diarrheal phase. Intestinal sarcocystosis is self-limited and treated supportively. TRICHOMONIASIS Various species of trichomonads can be found in the mouth (in associ­ ation with periodontitis) and occasionally in the gastrointestinal tract. Trichomonas vaginalis—one of the most prevalent protozoal parasites in the United States—is a pathogen of the genitourinary tract and a major cause of symptomatic vaginitis (Chap. 141). Life Cycle and Epidemiology  T. vaginalis is a pear-shaped, actively motile organism that measures about 10 × 7 μm, replicates by binary fission, and inhabits the lower genital tract of females and the urethra and prostate of males. There are 2–3 million infections each year in the United States. While the organism can survive for a few hours in moist environments and could be acquired by direct contact, person-to-person venereal transmission accounts for virtually all cases of trichomoniasis. Its prevalence is greatest among persons with mul­ tiple sexual partners and among those with other sexually transmitted diseases (Chap. 141). Clinical Manifestations  Many men infected with T. vaginalis are asymptomatic, although some develop urethritis and a few have epididymitis or prostatitis. In contrast, infection in women, which has an incubation period of 5–28 days, is more often symptomatic and manifests with malodorous vaginal discharge (often yellow), vulvar erythema and itching, dysuria or urinary frequency (in 30–50% of patients), and dyspareunia. These manifestations, however, do not clearly distinguish trichomoniasis from other types of infectious vaginitis. Diagnosis  Detection of motile trichomonads by microscopic exam­ ination of wet mounts of vaginal or prostatic secretions has been the conventional means of diagnosis. Although this approach provides an immediate diagnosis, its sensitivity for the detection of T. vaginalis is only ~50–60% in routine evaluations of vaginal secretions. Direct immunofluorescent antibody staining is more sensitive (70–90%) than wet-mount examinations. T. vaginalis can be recovered from the ure­ thra of both males and females and is detectable in males after prostatic massage. NAATs are highly sensitive and specific for urine and for endocervical and vaginal swabs from women and often considered first-line diagnostics. TREATMENT Trichomoniasis Metronidazole (either a single 2-g dose or 500-mg doses twice daily for 7 days), tinidazole (a single 2-g dose) or secnidazole (a single 2-g dose) is effective. All sexual partners must be treated concur­ rently to prevent reinfection, especially from asymptomatic males. In males with persistent symptomatic urethritis after therapy for nongonococcal urethritis, metronidazole therapy should be consid­ ered for possible trichomoniasis. Alternatives to metronidazole for treatment during pregnancy are not readily available. Reinfection often accounts for apparent treatment failures, but strains of T. vagi­ nalis exhibiting high-level resistance to metronidazole have been encountered. Treatment of these resistant infections with higher oral doses, parenteral doses, or concurrent oral and vaginal doses of metronidazole or with tinidazole has been successful.

132 - SECTION 19 Helminthic Infections

SECTION 19 Helminthic Infections

■ ■FURTHER READING Bourli P et al: Waterborne transmission of protozoan parasites: A review of worldwide outbreaks–an update 2017–2022. J Water Health 21:1421, 2023. Buret AG et al: Update on Giardia: Highlights from the Seventh Inter­ national Giardia and Cryptosporidim Conference. Parasite 27:49, 2020. Carter BL et al: Health sequelae of human cryptosporidiosis in indus­ trialized countries: A systematic review. Parasit Vectors 13:443, 2020. Coffey CM et al: Evolving epidemiology of reported giardiasis cases in the United States, 1995–2016. Clin Infect Dis 72:764, 2021. Fitri LE et al: Diagnostic methods of common intestinal protozoa: Current and future immunological and molecular methods. Trop Med Infect Dis 7:253, 2022. Hemphill A et al: Comparative pathobiology of the intestinal protozoan parasites Giardia lamblia, Entamoeba histolytica and Cryptosporidium parvum. Pathogens 8:116, 2019. Mathison BA, Pritt BS: Cyclosporiasis—updates on clinical presen­ tation, pathology, clinical diagnosis, and treatment. Microorganisms 9:1863, 2021. Ramanan P, Pritt BS: Extraintestinal microsporidiosis. J Clin Microbiol 52:3839, 2014. Van Den Broucke S et al: Microscopic detection of intestinal Sarcocystis infection diagnosed in international travelers at the Institute of Tropical Medicine, Antwerp, Belgium, from 2001 to 2020. Am J Trop Med Hyg 109:327, 2023. Van German TO, Muzny CA: Recent advances in the epidemiology, diagnosis, and management of Trichomonas vaginalis infection. F1000Res 8:1666, 2019. Van Gerwen OT et al: Trichomoniasis. Infect Dis Clin North Am 37: 245, 2023. Van Gestel RSFE et al: A clinical guideline on Dientamoeba fragilis infections. Parasitology 146:1131, 2018. Widmer G et al: Update on Cryptosporidium spp: Highlights from the Seventh International Giardia and Cryptosporidim Conference. Parasite 27:14, 2020. Section 19 Helminthic Infections Peter F. Weller, Edward T. Ryan

Introduction to

Helminthic Infections The word helminth is derived from the Greek helmins (“parasitic worm”). Helminthic worms are highly prevalent and, depending on the species, may exist as free-living organisms or as parasites of plant or animal hosts. The parasitic helminths have co-evolved with specific mammalian and other host species. Accordingly, most helminthic infections are restricted to nonhuman hosts, and only rarely do these zoonotic helminths accidentally cause human infections. Helminthic parasites of humans belong to two phyla: Nemathel­ minthes, which includes nematodes (roundworms), and Platyhelmin­ thes, which includes cestodes (tapeworms) and trematodes (flukes). Helminthic parasites of humans reside within the human body and hence are the cause of true infections. In contrast, parasites of other genera that reside only on mucocutaneous surfaces of humans (e.g., the parasites causing myiasis and scabies) are considered to represent infestations rather than infections. Helminthic parasites differ substantially from protozoan parasites in several respects. First, protozoan parasites are unicellular organ­ isms, whereas helminthic parasites are multicellular worms that possess

differentiated organ systems. Second, helminthic parasites have complex life cycles that require sequential stages of development outside the human host. Thus, most helminths do not complete their replication within the human host; rather, they develop to a certain stage within the mammalian host and, as part of their obligatory life cycle, must mature further outside that host. During the “extra-human” stages of their life cycle, helminths exist either as free-living organisms or as parasites within another host species and thereafter mature into new developmen­ tal stages capable of infecting humans. Thus, with only two exceptions (Strongyloides stercoralis and Capillaria philippinensis, which are capable of internal human reinfections), increases in the number of adult hel­ minths (i.e., the “worm burden”) within the human host require repeated exogenous reinfections. In the case of protozoan parasites, a brief, even singular exposure (e.g., a single mosquito bite transmitting malaria) may lead rapidly to intense parasite loads and overwhelming infections; in contrast, for all but the two helminths noted above, increases in worm burden require multiple and usually ongoing exposures to infectious forms, such as ingestion of eggs of intestinal helminths or waterborne exposures to infectious cercariae of Schistosoma mansoni. This require­ ment is germane both to the consideration of helminthic infections in individuals and to ongoing global efforts to interrupt and/or minimize the acquisition of helminthic infections by humans.

Third, helminthic infections have a predilection toward stimulation of host immune responses that elicit eosinophilia within human tissues and blood. The many protozoan infections characteristically do not elicit eosinophilia in infected humans, with only three exceptions (two intestinal protozoan parasites, Cystoisospora belli and Dientamoeba fragilis, and tissue-borne Sarcocystis species). The magnitude of helminthelicited eosinophilia tends to correlate with the extent of tissue inva­ sion by larvae or adult helminths. For example, in several helminthic infections, including acute schistosomiasis (Katayama syndrome), paragonimiasis, and hookworm and Ascaris infections, eosinophilia is most pronounced during the early phases of infection, when migra­ tions of infecting larvae and progression of subsequent developmental stages through the tissues are greatest. In established infections, local eosinophilia is often present around helminths in tissues, but blood eosinophilia may be intermittent, mild, or absent. In helminthic infec­ tions in which parasites are well contained within tissues (e.g., echino­ coccal cysts) or confined within the lumen of the intestinal tract (e.g., adult Ascaris or tapeworms), eosinophilia is usually absent. CHAPTER 237 Introduction to Helminthic Infections ■ ■NEMATODES Nematodes are nonsegmented roundworms. Species of nematodes are remarkably diverse and abundant in nature. Among the many thou­ sands of nematode species, few are parasites of humans. Most nema­ todes are free-living, and these species have variably evolved to survive in diverse ecologic niches, including saltwater, freshwater, or soil. The well-studied organism Caenorhabditis elegans is a free-living nema­ tode. Nematodes can be either beneficial or deleterious parasites of plants. Parasitic nematodes have co-evolved with specific mammalian hosts and have no capacity to live their full life cycles in other hosts. Uncommonly, humans are exposed to infectious stages of nonhuman nematode parasites, and the resultant zoonotic nematode infections can elicit inflammatory and immune responses as larval forms migrate and die in the unsuitable human host. Examples include pulmonary coin lesions due to mosquito-transmitted infections with the dog heartworm Dirofilaria immitis; eosinophilic meningoencephalitis due to ingested eggs of the raccoon ascarid Baylisascaris procyonis; and eosinophilic meningitis due to ingestion of larvae of the rat lungworm Angiostrongylus cantonensis. Nematode parasites of humans include worms that reside in the intestinal tract or localize in extraintestinal vascular or tissue sites. Roundworms are sexually dimorphic, with separate male and female forms that copulate to produce offspring (except for S. stercoralis, whose adult females are hermaphroditic in the human intestinal tract). Depending on the species, fertilized females release either larvae or eggs containing larvae. Nematodes have five developmental stages: an adult stage and four sequential larval stages. These parasites character­ istically are surrounded by a durable outer cuticular layer. Nematodes

133 - 237 Introduction to Helminthic Infections

237 Introduction to Helminthic Infections

■ ■FURTHER READING Bourli P et al: Waterborne transmission of protozoan parasites: A review of worldwide outbreaks–an update 2017–2022. J Water Health 21:1421, 2023. Buret AG et al: Update on Giardia: Highlights from the Seventh Inter­ national Giardia and Cryptosporidim Conference. Parasite 27:49, 2020. Carter BL et al: Health sequelae of human cryptosporidiosis in indus­ trialized countries: A systematic review. Parasit Vectors 13:443, 2020. Coffey CM et al: Evolving epidemiology of reported giardiasis cases in the United States, 1995–2016. Clin Infect Dis 72:764, 2021. Fitri LE et al: Diagnostic methods of common intestinal protozoa: Current and future immunological and molecular methods. Trop Med Infect Dis 7:253, 2022. Hemphill A et al: Comparative pathobiology of the intestinal protozoan parasites Giardia lamblia, Entamoeba histolytica and Cryptosporidium parvum. Pathogens 8:116, 2019. Mathison BA, Pritt BS: Cyclosporiasis—updates on clinical presen­ tation, pathology, clinical diagnosis, and treatment. Microorganisms 9:1863, 2021. Ramanan P, Pritt BS: Extraintestinal microsporidiosis. J Clin Microbiol 52:3839, 2014. Van Den Broucke S et al: Microscopic detection of intestinal Sarcocystis infection diagnosed in international travelers at the Institute of Tropical Medicine, Antwerp, Belgium, from 2001 to 2020. Am J Trop Med Hyg 109:327, 2023. Van German TO, Muzny CA: Recent advances in the epidemiology, diagnosis, and management of Trichomonas vaginalis infection. F1000Res 8:1666, 2019. Van Gerwen OT et al: Trichomoniasis. Infect Dis Clin North Am 37: 245, 2023. Van Gestel RSFE et al: A clinical guideline on Dientamoeba fragilis infections. Parasitology 146:1131, 2018. Widmer G et al: Update on Cryptosporidium spp: Highlights from the Seventh International Giardia and Cryptosporidim Conference. Parasite 27:14, 2020. Section 19 Helminthic Infections Peter F. Weller, Edward T. Ryan

Introduction to

Helminthic Infections The word helminth is derived from the Greek helmins (“parasitic worm”). Helminthic worms are highly prevalent and, depending on the species, may exist as free-living organisms or as parasites of plant or animal hosts. The parasitic helminths have co-evolved with specific mammalian and other host species. Accordingly, most helminthic infections are restricted to nonhuman hosts, and only rarely do these zoonotic helminths accidentally cause human infections. Helminthic parasites of humans belong to two phyla: Nemathel­ minthes, which includes nematodes (roundworms), and Platyhelmin­ thes, which includes cestodes (tapeworms) and trematodes (flukes). Helminthic parasites of humans reside within the human body and hence are the cause of true infections. In contrast, parasites of other genera that reside only on mucocutaneous surfaces of humans (e.g., the parasites causing myiasis and scabies) are considered to represent infestations rather than infections. Helminthic parasites differ substantially from protozoan parasites in several respects. First, protozoan parasites are unicellular organ­ isms, whereas helminthic parasites are multicellular worms that possess

differentiated organ systems. Second, helminthic parasites have complex life cycles that require sequential stages of development outside the human host. Thus, most helminths do not complete their replication within the human host; rather, they develop to a certain stage within the mammalian host and, as part of their obligatory life cycle, must mature further outside that host. During the “extra-human” stages of their life cycle, helminths exist either as free-living organisms or as parasites within another host species and thereafter mature into new developmen­ tal stages capable of infecting humans. Thus, with only two exceptions (Strongyloides stercoralis and Capillaria philippinensis, which are capable of internal human reinfections), increases in the number of adult hel­ minths (i.e., the “worm burden”) within the human host require repeated exogenous reinfections. In the case of protozoan parasites, a brief, even singular exposure (e.g., a single mosquito bite transmitting malaria) may lead rapidly to intense parasite loads and overwhelming infections; in contrast, for all but the two helminths noted above, increases in worm burden require multiple and usually ongoing exposures to infectious forms, such as ingestion of eggs of intestinal helminths or waterborne exposures to infectious cercariae of Schistosoma mansoni. This require­ ment is germane both to the consideration of helminthic infections in individuals and to ongoing global efforts to interrupt and/or minimize the acquisition of helminthic infections by humans.

Third, helminthic infections have a predilection toward stimulation of host immune responses that elicit eosinophilia within human tissues and blood. The many protozoan infections characteristically do not elicit eosinophilia in infected humans, with only three exceptions (two intestinal protozoan parasites, Cystoisospora belli and Dientamoeba fragilis, and tissue-borne Sarcocystis species). The magnitude of helminthelicited eosinophilia tends to correlate with the extent of tissue inva­ sion by larvae or adult helminths. For example, in several helminthic infections, including acute schistosomiasis (Katayama syndrome), paragonimiasis, and hookworm and Ascaris infections, eosinophilia is most pronounced during the early phases of infection, when migra­ tions of infecting larvae and progression of subsequent developmental stages through the tissues are greatest. In established infections, local eosinophilia is often present around helminths in tissues, but blood eosinophilia may be intermittent, mild, or absent. In helminthic infec­ tions in which parasites are well contained within tissues (e.g., echino­ coccal cysts) or confined within the lumen of the intestinal tract (e.g., adult Ascaris or tapeworms), eosinophilia is usually absent. CHAPTER 237 Introduction to Helminthic Infections ■ ■NEMATODES Nematodes are nonsegmented roundworms. Species of nematodes are remarkably diverse and abundant in nature. Among the many thou­ sands of nematode species, few are parasites of humans. Most nema­ todes are free-living, and these species have variably evolved to survive in diverse ecologic niches, including saltwater, freshwater, or soil. The well-studied organism Caenorhabditis elegans is a free-living nema­ tode. Nematodes can be either beneficial or deleterious parasites of plants. Parasitic nematodes have co-evolved with specific mammalian hosts and have no capacity to live their full life cycles in other hosts. Uncommonly, humans are exposed to infectious stages of nonhuman nematode parasites, and the resultant zoonotic nematode infections can elicit inflammatory and immune responses as larval forms migrate and die in the unsuitable human host. Examples include pulmonary coin lesions due to mosquito-transmitted infections with the dog heartworm Dirofilaria immitis; eosinophilic meningoencephalitis due to ingested eggs of the raccoon ascarid Baylisascaris procyonis; and eosinophilic meningitis due to ingestion of larvae of the rat lungworm Angiostrongylus cantonensis. Nematode parasites of humans include worms that reside in the intestinal tract or localize in extraintestinal vascular or tissue sites. Roundworms are sexually dimorphic, with separate male and female forms that copulate to produce offspring (except for S. stercoralis, whose adult females are hermaphroditic in the human intestinal tract). Depending on the species, fertilized females release either larvae or eggs containing larvae. Nematodes have five developmental stages: an adult stage and four sequential larval stages. These parasites character­ istically are surrounded by a durable outer cuticular layer. Nematodes

134 - 238 Trichinellosis and Other Tissue Nematode Infections

238 Trichinellosis and Other Tissue Nematode Infections

have a nervous system; a muscular system, including muscle cells under the cuticle; and a developed intestinal tract, including an oral cavity and an elongated gut that ends in an anal pore. Adults may range in size from minute to >1 meter in length (with Dracunculus medinen­ sis, for example, at the long end of this spectrum).

Humans acquire infections with nematode parasites by various routes, depending on the parasitic species. Ingestion of eggs passed in human feces is a major global health problem with many of the intes­ tinal helminths (e.g., Ascaris lumbricoides). In other species, infecting larvae penetrate skin exposed to fecally contaminated soil (e.g., S. ster­ coralis, hookworms) or traverse the skin after the bite of infected insect vectors (e.g., filariae). Some nematode infections are acquired by con­ sumption of specific animal-derived foods (e.g., trichinellosis from raw or undercooked pork or wild carnivorous mammals). As noted above, only two nematodes, S. stercoralis and C. philippinensis, can internally reinfect humans; thus, for all other nematodes, any increases in worm burden must be due to continued exogenous reinfections. ■ ■CESTODES Tapeworms are the cestode parasites of humans. Adult tapeworms are elongated, segmented, hermaphroditic flatworms that reside in the intestinal lumen or, in their larval forms, may live in extraintestinal tissues. Tapeworms include a head (scolex) and a number of attached segments (proglottids). The worms attach to the intestinal tract via their scolices, which may possess suckers, hooks, or grooves. The scolex is the site of formation of new proglottids. Tapeworms do not have a functional gut tract; rather, each tapeworm segment passively and actively obtains nutrients through its specialized surface tegu­ ment. Mature proglottids possess both male and female sex organs, but insemination usually occurs between adjacent proglottids. Fertilized proglottids release eggs that are passed in the feces. When ingested by an intermediate host, an egg releases an oncosphere that penetrates the gut and develops further in tissues as a cysticercus. Humans acquire infection by ingesting animal tissues that contain cysticerci, and the resultant tapeworms develop and reside in the proximal small bowel (e.g., Taenia solium, T. saginata). Alternatively, if humans ingest eggs of these cestodes that have been passed in human or animal feces, onco­ spheres develop and can cause space-occupying extraintestinal cystic lesions in tissues; examples include cysticercosis due to T. solium and hydatid disease due to species of Echinococcus. PART 5 Infectious Diseases ■ ■TREMATODES Trematodes of medical importance include blood flukes, intestinal flukes, and tissue flukes. Adult flukes are often leaf-shaped flatworms. Oral and/ or ventral suckers help adult flukes maintain their positions in situ. Flukes have an oral cavity but no distal anal pore. Nutrients are obtained both through their integument and by ingestion into the blind intestinal tract. Flukes are hermaphroditic except for blood flukes (schistosomes), which are sexually dimorphic. Eggs are passed in human feces (Fasciola, Fascio­ lopsis, Clonorchis, Schistosoma japonicum, S. mansoni), urine (Schistosoma haematobium), or sputum and feces (Paragonimus). Expelled eggs release miracidia—usually in water—that infect specific snail species. Within snails, parasites multiply and cercariae are released. Depending on the species, cercariae can penetrate the skin (schistosomes) or can develop into metacercariae that can be ingested with plants (e.g., watercress for Fasciola) or with fish (Clonorchis) or crabs (Paragonimus). ■ ■CONCLUSION Many of the so-called neglected tropical diseases are due to helmin­ thic infections. The health impacts of many helminthic infections are varied and are based on the frequent need for repeated exposures to increase the worm burdens in infected humans. In global regions where exposures to specific helminths occur even in childhood (e.g., fecally derived intestinal nematodes, mosquito-transmitted filariae, or waterborne snail-transmitted schistosomes), the morbidities in infected individuals can include nutritional, developmental, cognitive, and functional impairments. Ongoing global mass-treatment programs are currently aimed at diminishing the local intensity of infection and prevalences of specific helminths and their consequent impacts on the health of local populations.

Peter F. Weller, Edward T. Ryan

Trichinellosis and

Other Tissue Nematode

Infections Nematodes are elongated, symmetric roundworms. Parasitic nema­ todes of medical significance may be broadly classified as either pre­ dominantly intestinal or tissue nematodes. The intestinal nematodes are covered in Chap. 239; filarial tissue-dwelling nematodes and Dracunculus medinensis infection are both covered in Chap. 240. This chapter covers the tissue nematodes that cause trichinellosis, visceral and ocular larva migrans, cutaneous larva migrans, cerebral angio­ strongyliasis, and gnathostomiasis. All of these zoonotic infections result from incidental exposure to infectious nematodes. The clinical symptoms of these infections are due largely to invasive larval stages that (except in the case of Trichinella) do not reach maturity in humans. ■ ■TRICHINELLOSIS Trichinellosis develops after the ingestion of meat containing cysts of Trichinella (e.g., pork or other meat from a carnivore/omnivore). Although most infections are mild and asymptomatic, heavy infections can cause severe enteritis, periorbital edema, myositis, and (infrequently) death. Life Cycle and Epidemiology  At least nine species of Trichinella and 13 genotypes are recognized as causes of infection in humans. Two species are distributed worldwide: T. spiralis, which is found in a great variety of carnivorous and omnivorous animals, and T. pseudospiralis, which is found in mammals and birds. T. nativa is present in Arctic and subarctic regions and infects bears, foxes, and walruses; T. nelsoni is found in equatorial eastern Africa, where it is common among felid predators and scavengers such as hyenas and bush pigs; and T. britovi is found in Europe, western Africa, and western Asia among carnivores but not among domestic swine. T. murrelli is present in wild animals in North American and Japan. T. papuae is found in Papua New Guinea, Thailand, Taiwan, and Cambodia in domestic and feral pigs and in salt­ water crocodiles and turtles. T. zimbabwensis is present in crocodiles in Tanzania. T. patagoniensis is found in cougars in South America. After human consumption of trichinous meat, encysted larvae are liberated by digestive acid and proteases (Fig. 238-1). The larvae invade the small-bowel mucosa and mature into adult worms. After ~1 week, female worms release newborn larvae that migrate via the circulation to striated muscle. The larvae of all species except T. pseu­ dospiralis, T. papuae, and T. zimbabwensis then encyst by inducing a radical transformation in the muscle cell architecture. Host immune responses may help to expel intestinal adult worms but have few del­ eterious effects on muscle-dwelling larvae. Human trichinellosis classically has been caused by the ingestion of infected pork products and thus can occur in almost any location where the meat of domestic or wild swine is eaten. Increasingly, human trichi­ nellosis has also been acquired from the meat of other animals, including dogs (in parts of Asia and Africa), horses (in Italy and France), and bears and walruses (in northern regions). Although cattle and horses (being herbivores) are not natural hosts of Trichinella, beef and horse meat have been implicated in outbreaks when contaminated or adulterated with trichinous pork or perhaps following ingestion of fodder contaminated with meat parts from naturally infected animals. Approximately 10–20 cases of trichinellosis are reported annually in the United States, down from 400–500 cases per year in the 1940s, largely reflecting the impact of control programs targeting education, pig-raising, and the processing and freezing of pork. Consumption of wild game (especially bear and walrus meat) now accounts for the majority of cases reported each year in North America. Most mild cases probably remain undiagnosed. Pathogenesis and Clinical Features  Clinical symptoms of trich­ inellosis arise from the successive phases of parasite enteric invasion,

Larvae are released in the stomach and mature into adults over 1–2 wks in the small bowel, causing: Irritation and mild abdominal cramping or even diarrhea Encysted larvae ingested in undercooked pork, boar, horse, or bear *T. papuae, T. zimbabwensis, and T. pseudospiralis do not encyst. FIGURE 238-1  Life cycle of Trichinella spiralis (cosmopolitan); nelsoni (equatorial Africa); britovi (Europe, western Africa, western Asia); nativa (Arctic); murrelli (North America); papuae (Papua New Guinea); zimbabwensis (Tanzania); and pseudospiralis (cosmopolitan). CNS, central nervous system. (Reproduced with permission from RL Guerrant et al [eds]: Tropical Infectious Diseases: Principles, Pathogens and Practice, 2nd ed. Elsevier, 2006.) larval migration, and muscle encystment (Fig. 238-1). Most light infec­ tions (those with <10 larvae per gram of muscle) are asymptomatic, whereas heavy infections (which can involve >50 larvae per gram of muscle) can be life-threatening. An initial enteric phase due to release of ingested muscle larvae may elicit diarrhea, abdominal pain, consti­ pation, and nausea during the first weeks after infection. Symptoms due to larval migration and muscle invasion begin to appear in the second week after infection. The migrating Trichinella larvae provoke a marked local and systemic hypersensitivity reaction, with fever and eosinophilia. Periorbital and facial edema is common, as are hemorrhages in the subconjunctivae, retina, and nail beds (“splinter” hemorrhages). A maculopapular rash, headache, cough, dyspnea, or dys­ phagia sometimes develops. Myocarditis with tachyarrhythmias or heart failure—and, less commonly, encephalitis or pneumonitis—may develop and accounts for most deaths of patients with trichinellosis. Upon onset of larval encystment in muscle 2–3 weeks after infec­ tion, symptoms of myositis with myalgias, muscle edema, and weak­ ness develop, usually overlapping with the inflammatory reactions to migrating larvae. The most commonly involved muscle groups include the extraocular muscles; the biceps; and the muscles of the jaw, neck, lower back, and diaphragm. Peaking ~3 weeks after infection, symptoms subside only gradually during a prolonged convalescence. Uncommon infections with T. pseudospiralis, whose larvae do not encapsulate in muscles, elicit a prolonged polymyositis-like illness. Laboratory Findings and Diagnosis  Blood eosinophilia devel­ ops in >90% of patients with symptomatic trichinellosis and may peak at a level of >50% 2–4 weeks after infection. Serum levels of muscle enzymes, including creatine phosphokinase, are elevated in most symptomatic patients. Patients should be questioned thoroughly about their consumption of pork or wild animal meat and about illness in other individuals who ate the same meat. A presumptive clinical diagnosis can be based on fevers, eosinophilia, periorbital edema, and myalgias after a suspect meal. A rise in the titer of parasite-specific antibody, which usually does not occur until after the third week of infection, confirms the diagnosis. Alternatively, a definitive diagnosis requires surgical biopsy of at least 1 g of involved muscle; the yields

Larvae migrate, penetrate striated muscle, reside in “nurse-cells,” and encyst,* causing: Muscle pain, fever, periorbital edema, eosinophilia, occasional CNS or cardiac damage Similar cycle (as humans) in swine or other carnivores (rats, bears, foxes, dogs, or horses) CHAPTER 238 are highest near tendon insertions in symptomatic muscles. The fresh muscle tissue should be compressed between glass slides and examined microscopically (Fig. 238-2) because larvae may be missed by exami­ nation of routine histopathologic sections alone. Trichinellosis and Other Tissue Nematode Infections
TREATMENT Trichinellosis Most lightly infected patients recover uneventfully with bed rest, anti­ pyretics, and analgesics. Glucocorticoids like prednisone (Table 238-1) are beneficial for severe myositis and myocarditis. Mebendazole and albendazole are active against enteric stages of the parasite, but their efficacy against encysting/encysted larvae has not been conclusively demonstrated. FIGURE 238-2  Trichinella larva encysted in a characteristic hyalinized capsule in striated muscle tissue. (CDC/Wadsworth Center, New York State Department of Health.)

TABLE 238-1  Therapy for Tissue Nematode Infections INFECTION SEVERITY TREATMENT Trichinellosis Mild Supportive   Moderate Albendazole (400 mg bid × 10–14 days) or     Mebendazole (200–400 mg tid × 3 days, then 500 mg tid × 10 days)   Severe Add glucocorticoids (e.g., prednisone, 1 mg/kg qd × 5 days) Visceral larva migrans Mild to moderate Supportive Severe Glucocorticoids (as above)   Ocular Not fully defined; albendazole (800 mg bid for adults, 400 mg bid for children) with glucocorticoids × 5–20 days has been effective Cutaneous larva migrans   Ivermectin (single dose, 200 μg/kg) or   Albendazole (200 mg bid × 3 days) Angiostrongyliasis Mild to moderate Supportive Not fully defined: albendazole (15 mg/kg per day in two divided doses × 14 days) with glucocorticoids (as above)   Severe Not fully defined: albendazole (15 mg/kg per day in two divided doses × 14 days) with glucocorticoids (as above) Gnathostomiasis   Ivermectin (200 μg/kg per day × 2 days)a or     Albendazole (400 mg bid × 21 days)a aThese agents may have efficacy for cutaneous gnathostomiasis; their use in neurologic and ocular gnathostomiasis is less certain and may be detrimental. Steroids are often given during neurologic and ocular gnathostomiasis. PART 5 Infectious Diseases Prevention  Larvae are usually killed by cooking pork until it is no longer pink or by freezing it at −15°C for 3 weeks. However, Arctic T. nativa larvae in walrus or bear meat are relatively resistant and may remain viable despite freezing. ■ ■VISCERAL AND OCULAR LARVA MIGRANS Visceral larva migrans is a syndrome caused by nematodes that are normally parasitic for nonhuman host species. In humans, these nematode larvae do not develop into adult worms but instead migrate through host tissues and elicit eosinophilic inflammation. The most common form of visceral larva migrans is toxocariasis due to larvae of the canine ascarid Toxocara canis; the syndrome is due less commonly to the feline ascarid T. cati and even less commonly to the pig ascarid Ascaris suum. Rare cases with eosinophilic meningoencephalitis have been caused by the raccoon ascarid Baylisascaris procyonis. Life Cycle and Epidemiology  The canine roundworm T. canis is distributed among dogs worldwide. Ingestion of infective eggs by dogs is followed by liberation of Toxocara larvae, which penetrate the gut wall and migrate intravascularly into canine tissues, where most remain in a developmentally arrested state. During pregnancy, some larvae resume migration in bitches and infect puppies prena­ tally (through transplacental transmission) or after birth (through suckling). Thus, in lactating bitches and puppies, larvae return to the intestinal tract and develop into adult worms, which produce eggs that are released in the feces. Eggs must undergo embryonation over several weeks to become infectious. Humans acquire toxocariasis mainly by ingesting soil/sand—such as found in public parks and playgrounds— contaminated by dog or cat feces that contains infective T. canis eggs. Pathogenesis and Clinical Features  Clinical disease most com­ monly afflicts preschool children. After humans ingest Toxocara eggs, the larvae hatch and penetrate the intestinal mucosa, from which they are carried by the circulation to a wide variety of organs and tissues. The larvae invade the liver, lungs, central nervous system (CNS), and other sites, provoking intense local eosinophilic granulomatous responses. The degree of clinical illness depends on larval number

and tissue distribution, reinfection, and host immune responses. Most light infections are asymptomatic and may be evidenced only by blood eosinophilia. Characteristic symptoms of visceral larva migrans include fever, malaise, anorexia and weight loss, cough, wheezing, and rashes. Hepatosplenomegaly is common. These features may be accom­ panied by extraordinary peripheral eosinophilia at levels that may approach 90%. Uncommonly, seizures or behavioral disorders develop. Rare deaths are due to severe neurologic, pneumonic, or myocardial involvement. The ocular form of the larva migrans syndrome occurs when Toxocara larvae invade the eye. An eosinophilic granulomatous mass, most commonly in the posterior pole of the retina, develops around the entrapped larva. The retinal lesion can mimic retinoblastoma in appearance, and mistaken diagnosis of the latter condition can lead to unnecessary enucleation. The spectrum of eye involvement also includes endophthalmitis, uveitis, and chorioretinitis. Unilateral visual disturbances, strabismus, and eye pain are the most common presenting symptoms. In contrast to visceral larva migrans, ocular toxocariasis usually develops in older children or young adults with no history of pica; these patients seldom have eosinophilia or visceral manifestations. Diagnosis  In addition to eosinophilia, leukocytosis and hypergam­ maglobulinemia may be evident in visceral larva migrans; these attri­ butes are often absent in ocular larva migrans. Transient pulmonary infiltrates are apparent on chest x-rays of about one-half of patients with symptoms of pneumonitis. The clinical diagnosis can be con­ firmed by an enzyme-linked immunosorbent assay for toxocaral anti­ bodies, although this assay may be negative in ocular larva migrans. Stool examination for parasite eggs is worthless in toxocariasis, since the larvae do not develop into egg-producing adults in humans. TREATMENT Visceral and Ocular Larva Migrans The vast majority of Toxocara infections are self-limited and resolve without specific therapy. In patients with severe myocardial, CNS, or pulmonary involvement, glucocorticoids may be employed to reduce inflammatory complications. Available anthelmintic drugs, including mebendazole and albendazole, have not been shown conclusively to alter the course of larva migrans, although most individuals with moderate to severe visceral larva migrans receive albendazole with concomitant steroids (Table 238-1). Treatment of ocular disease is not fully defined, but the administration of steroids (to decrease ocular inflammation) and albendazole has been effec­ tive. Control measures include prohibiting dog excreta in public parks and playgrounds, deworming dogs, and preventing pica in children. ■ ■CUTANEOUS LARVA MIGRANS Cutaneous larva migrans (“creeping eruption”) is a serpiginous skin eruption caused by burrowing larvae of animal hookworms, usually the dog and cat hookworm Ancylostoma braziliense. The larvae hatch from eggs passed in dog and cat feces and mature in the soil. Humans become infected after skin contact with soil in areas frequented by dogs and cats. Cutaneous larva migrans is prevalent among children and travelers in regions with warm humid climates. After larvae penetrate the skin, erythematous lesions form along the tortuous tracks of their migration along the dermal-epidermal junction; the larvae advance several centimeters in a day. The intensely pruritic lesions may occur anywhere on the body and can be numerous if the patient has lain on the ground. Vesicles and bullae may form. The animal hookworm larvae do not mature in humans and, without treatment, will die after an interval ranging from weeks to a couple of months, with resolution of skin lesions. The diagnosis is made on clinical grounds. Skin biopsies only rarely detect diagnos­ tic larvae. Symptoms can be alleviated by ivermectin or albendazole (Table 238-1).

2 weeks 3rd-stage larvae (consumed in snail or slime) penetrate gut, go to CNS (then lung in rat) Larvae consumed by land snail/slug (Achatina fulica) FIGURE 238-3  Life cycle of Angiostrongylus cantonensis (rat lung worm) found in Southeast Asia and the Pacific Basin as well as on Caribbean islands, in countries of Central and South America, and in the southern United States. CNS, central nervous system. (Reproduced with permission from RL Guerrant et al [eds]: Tropical Infectious Diseases: Principles, Pathogens and Practice, 2nd ed. Elsevier, 2006.) ■ ■ANGIOSTRONGYLIASIS Angiostrongylus cantonensis, the rat lungworm, is the most common cause of human eosinophilic meningitis (Fig. 238-3). Life Cycle and Epidemiology  This infection occurs principally in Southeast Asia and the Pacific Basin (including Hawaii) but has spread to other areas of the world, including the Caribbean islands, countries in Central and South America, and the southern United States. A. cantonensis larvae produced by adult worms in the rat lung migrate to the gastrointestinal tract and are expelled with the feces. They develop into infective larvae in land snails and slugs. Humans acquire the infection by ingesting raw infected mollusks; vegetables con­ taminated by mollusk slime; or crabs, freshwater shrimp, and certain marine fish that have themselves eaten infected mollusks. The larvae then migrate to the brain. Pathogenesis and Clinical Features  The parasites eventually die in the CNS, but not before initiating pathologic consequences that, in heavy infections, can result in permanent neurologic sequelae or death. Migrating larvae cause marked local eosinophilic inflammation and hemorrhage, with subsequent necrosis and granuloma formation around dying worms. Clinical symptoms develop 2–35 days after the ingestion of larvae, typically at about 2 weeks. Patients usually present with an insidious or abrupt excruciating frontal, occipital, or bitempo­ ral headache. Neck stiffness, nausea and vomiting, and paresthesias are also common. Fever, cranial and extraocular nerve palsies, seizures, paralysis, and lethargy are uncommon. Laboratory Findings  Examination of cerebrospinal fluid (CSF) is mandatory in suspected cases and usually reveals an elevated opening pressure, a white blood cell count of 150–2000/μL, and an eosinophilic pleocytosis of >20%. The protein concentration is usually elevated and the glucose level normal. The larvae of A. cantonensis are only rarely seen in CSF. Peripheral-blood eosinophilia may be mild. The diagnosis is generally based on the clinical presentation of eosinophilic menin­ gitis together with a compatible epidemiologic history. Serologic tests are sometimes available in endemic regions, although they have not been standardized and may be negative at onset of clinical symptoms. A polymerase chain reaction assay that detects A. cantonensis DNA in CSF or other tissues is available, including through the U.S. Centers for Disease Control and Prevention (CDC) and the Hawaii Department of Public Health in the United States.

Eosinophilic meningitis Adult in pulmonary artery produces fertile eggs; larvae hatch, penetrate arterioles, migrate up bronchi, and are coughed up, swallowed, and passed in feces viable in fresh water CHAPTER 238 TREATMENT Angiostrongyliasis Many individuals with eosinophilic meningitis from angiostron­ gyliasis improve with supportive therapy alone. Steroids are often given to decrease inflammation. Specific chemotherapy is of uncertain benefit and should never be given without steroids to prevent exacerbation of inflammatory brain lesions; when given, albendazole is usually employed. (Table 238-1). Repeated lumbar punctures with removal of CSF can assist with symptom relief. In most patients, cerebral angiostrongyliasis has a self-limited course, and recovery is complete. The infection may be prevented by adequately cooking snails, crabs, and prawns and inspecting vegetables for mollusk infestation. Other parasitic or fungal causes of eosinophilic meningitis in endemic areas may include gnathos­ tomiasis (see below), paragonimiasis (Chap. 241), schistosomiasis (Chap. 241), neurocysticercosis (Chap. 242), and coccidioidomy­ cosis (Chap. 219). Trichinellosis and Other Tissue Nematode Infections
A separate species of Angiostrongylus, A. costaricensis, can inhabit the mesenteric arteries of humans, causing an eosinophilic inflam­ matory mass in abdominal viscera. A. costaricensis is a zoonosis (rat–mollusk/slug life cycle) found in the New World, especially Latin America, and occurs following ingestion of slugs or raw produce contaminated by slug slime. Diagnosis is most commonly made at the time of surgical excision, which is often curative. ■ ■GNATHOSTOMIASIS Infection of human tissues with larvae of Gnathostoma spinigerum can cause eosinophilic meningoencephalitis, migratory cutaneous swell­ ings, or invasive masses of the eye and visceral organs. Life Cycle and Epidemiology  Human gnathostomiasis occurs in many countries and is notably endemic in Southeast Asia and parts of China and Japan. In nature, the mature adult worms parasitize the gastrointestinal tract of dogs and cats. First-stage larvae hatch from eggs passed into water and are ingested by Cyclops species (water fleas). Infective third-stage larvae develop in the flesh of many animal species (including fish, frogs, eels, snakes, chickens, and ducks) that have eaten either infected Cyclops or another infected second intermediate host. Humans typically acquire the infection by eating raw or undercooked

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239 Intestinal Nematode Infections

fish or poultry. Raw fish dishes, such as som fak in Thailand and sashimi in Japan, account for many cases of human gnathostomiasis. Some cases in Thailand result from the local practice of applying frog or snake flesh as a poultice.

Pathogenesis and Clinical Features  Clinical symptoms are due to the aberrant migration of a single larva into cutaneous, visceral, neural, or ocular tissues. After invasion, larval migration may cause local inflammation, with pain, cough, or hematuria accompanied by fever and eosinophilia. Painful, itchy, migratory swellings may develop in the skin, particularly in the distal extremities or periorbital area. Cutaneous swellings usually last ~1 week but often recur intermit­ tently over many years. Larval invasion of the eye can provoke a sightthreatening inflammatory response. Invasion of the CNS results in eosinophilic meningitis with myeloencephalitis, a serious complication due to ascending larval migration along a large nerve tract. Patients characteristically present with agonizing radicular pain and paresthe­ sias in the trunk or a limb, which are followed shortly by paraplegia. Cerebral involvement, with focal hemorrhages and tissue destruction, is often fatal. Diagnosis and Treatment  Cutaneous migratory swellings with marked peripheral eosinophilia, supported by an appropriate geo­ graphic and dietary history, generally constitute an adequate basis for a clinical diagnosis of gnathostomiasis. However, patients may present with ocular or cerebrospinal involvement without anteced­ ent cutaneous swellings. In the latter case, eosinophilic pleocytosis is demonstrable (usually along with hemorrhagic or xanthochromic CSF), but worms are almost never recovered from CSF. Computed tomography or magnetic resonance imaging of the brain during neu­ ronal gnathostomiasis often demonstrates cerebral hemorrhage from the destructive migration of the parasite. Surgical removal of the parasite from subcutaneous or ocular tissue, though rarely feasible, is both diagnostic and therapeutic. Albendazole or ivermectin may be helpful, especially for cutaneous gnathostomiasis (Table 238-1). At present, cerebrospinal involvement is managed with supportive measures and generally with a course of glucocorticoids; the role of albendazole or ivermectin is uncertain and could be detrimental. Gnathostomiasis can be prevented by adequate cooking of fish and poultry in endemic areas. PART 5 Infectious Diseases ■ ■FURTHER READING Auer H, Walochnik J: Toxocariasis and the clinical spectrum. Adv Parasitol 109:111, 2020. Centers for Disease Control and Prevention: Surveillance for trichinellosis—United States, 2015, Annual Summary. Atlanta, GA: U.S. Department of Health and Human Services, CDC, 2017. Clinical Subcommittee of the Hawaii Governor’s Joint Task Force on Rat Lungworm Disease. Preliminary guidelines for the diagnosis and treatment of human neuroangiostrongyli­ asis (rat lungworm disease) in Hawaii. https://health.hawaii.gov/docd/ files/2018/08/RLWD_Preliminary_Clinical_Guidelines_FINAL_082918. pdf.  Accessed December 1, 2023. Diaz JH et al: The disease ecology, epidemiology, clinical manifestations, and management of trichinellosis linked to consumption of wild animal meat. Wilderness Environ Med 31:235, 2020. Lupi O et al: Mucocutaneous manifestations of helminth infections. Nematodes. J Am Acad Dermatol 73:929, 2015. Martins YC et al: Central nervous system manifestations of Angiostrongylus cantonensis infection. Acta Trop 141:46, 2015. Rostami A et al: Meat sources of infection for outbreaks of human trichinellosis. Food Microbiol 64:65, 2017. Sitcar AD et al: Raccoon roundworm infection associated with cen­ tral nervous system disease and ocular disease—six states, 2013–2015. Morbid Mortal Wkly Rep 65:930, 2016.

David J. Diemert, Thomas B. Nutman

Intestinal Nematode

Infections More than one billion individuals worldwide are infected with one or more species of intestinal nematode. Table 239-1 summarizes biologic and common clinical features of infections due to the major intestinal parasitic nematodes. These parasites are most common in regions with inadequate sanitation and disposal of fecal waste, particularly in resource-limited countries in the tropics and subtropics, although they have also been seen with increasing frequency among immigrants and refugees to developed countries. Although intestinal nematode infec­ tions are not usually fatal, they contribute to malnutrition, impaired physical and cognitive development, and diminished work capacity. It is interesting that these helminth infections may protect some indi­ viduals from allergic diseases. Humans may on occasion be infected with nematode parasites that ordinarily infect animals; these zoonotic infections produce diseases such as trichostrongyliasis, anisakiasis, capillariasis, and abdominal angiostrongyliasis. Intestinal nematodes are roundworms that range in length from 1 mm to many centimeters when mature (Table 239-1). Their life cycles are complex and highly varied; some species, including Strongyloides stercoralis and Enterobius vermicularis, can be transmitted directly from person to person, while others, such as Ascaris lumbricoides, Trichuris trichiura, and the hookworms, require a soil phase for devel­ opment. Because most helminth parasites cannot fully complete their life cycle within the same human host, heavy burdens of adult worms require repeated exposure to the parasite in its infectious stage, whether larva or egg. Hence, clinical disease, as opposed to asymptomatic (or subclinical) infection, generally develops only with repeated exposure and is typically related to infection intensity. In children with marginal nutritional status, intestinal helminth infections may impair growth and development. Eosinophilia and elevated serum IgE antibody levels are features of many helminth infections, the latter particularly when the helminth life cycle involves tissue migration such as A. lumbricoides, S. stercoralis, or the hookworms. Significant protective immunity to intestinal nematodes appears not to develop in humans. ■ ■ASCARIASIS A. lumbricoides is the largest intestinal nematode parasite of humans, reaching up to 40 cm in length. Most infected individuals have low worm burdens and are asymptomatic. Clinical disease arises from larval migra­ tion in the lungs or the effects of adult worms in the intestines. Life Cycle  Adult worms live in the lumen of the small intestine. Mature female Ascaris worms are extraordinarily fecund, each pro­ ducing over 200,000 eggs a day that are expelled within feces. Ascarid eggs, which are remarkably resistant to environmental stresses, become infective after several weeks of maturation in warm, moist soil and can remain infective for years. After infective eggs are swallowed, gastric acid dissolves their protective outer layer, releasing larvae into the intestine that invade the mucosa, migrate through the circulation to the lungs, penetrate into the alveoli, ascend the bronchial tree, and return— through swallowing—to the small intestine, where they develop into adult worms. The time between initial infection and egg detection in the feces is typically between 2 and 3 months. Adult worms live for 1–2 years. Epidemiology  Ascaris is widely distributed in tropical and sub­ tropical regions as well as in other humid areas in more temperate regions of the world. Transmission typically occurs through fecally contaminated soil and is due either to inadequate treatment of sewage or to the use of human feces as fertilizer. With their propensity for poorer oral hygiene, younger children are most often affected. Infec­ tion outside endemic areas, though uncommon, can occur when eggs on transported produce are ingested.

TABLE 239-1  Major Human Intestinal Parasitic Nematodes NECATOR AMERICANUS, ANCYLOSTOMA DUODENALE, ANCYLOSTOMA CEYLANICUM (HOOKWORM) ASCARIS LUMBRICOIDES (ROUNDWORM) FEATURE Global prevalence in humans (millions)

Endemic areas Hot, humid regions Hot, humid regions Hot and warm, humid regions Hot, humid regions Worldwide Infective stage Egg Filariform larva Filariform larva Egg Egg Route of infection Oral Percutaneous Percutaneous or autoinfective Gastrointestinal location of worms Small intestine Jejunal mucosa Small intestinal, mucosa Cecum, colonic mucosa Adult worm size 15–40 cm 7–13 mm 1–2 mm 30–50 mm 2–13 mm Pulmonary passage of larvae Yes Yes Yes No No Incubation perioda (days) 60–75 40–100 25–30 70–90 35–45 Longevity 1 year N. americanus: 2–5 years A. duodenale: 6–8 years A. ceylanicum: 6–8 yearsb Fecundity (eggs/day/ worm) 240,000 N. americanus: 9000–10,000 A. duodenale: 10,000–28,000 A. ceylanicum: 5,000–15,000 Principal symptoms Gastrointestinal symptoms; rarely, biliary obstruction or, in heavy infections, gastrointestinal obstruction Iron-deficiency anemia in moderate and heavy infections Diagnosis Eggs in stool Eggs in fresh stool, larvae in old stool Treatment Mebendazole Albendazole Ivermectin Moxidectin Pyrantel pamoate Mebendazole Albendazole aTime from infection to egg production by mature female worm. bAssumed but no evidence base in humans. Clinical Features  During the lung phase of larval migration, ~9–12 days after egg ingestion, patients may develop an irritating nonproductive cough and burning substernal discomfort that is aggravated by coughing or deep inspiration. Dyspnea and bloodtinged sputum are less common. Fever can occur. Eosinophilia develops during this symptomatic phase and subsides slowly over weeks. Chest imaging may reveal rounded infiltrates a few millime­ ters to several centimeters in size that are hallmarks of eosinophilic pneumonitis (Löffler’s syndrome). These infiltrates may be transient and intermittent, clearing after several weeks. Where there is sea­ sonal transmission of the parasite such as on the Arabian peninsula, seasonal pneumonitis with eosinophilia may develop in previously infected and sensitized hosts. In light infections, adult worms in the small intestine usually cause no symptoms. In heavier infections, a large bolus of entangled worms can cause pain and small-bowel obstruction, sometimes complicated by perforation, intussusception, or volvulus. Obstruction is more com­ mon in children due to their having intestinal lumens of narrow diam­ eter. Single worms may cause disease when they migrate into aberrant sites. A large worm can enter and occlude the biliary tree, causing bili­ ary colic, cholecystitis, cholangitis, pancreatitis, or (rarely) intrahepatic abscesses. Migration of an adult worm up the esophagus can provoke coughing or vomiting up the worm. In highly endemic areas, intestinal and biliary ascariasis can rival acute appendicitis as a cause of surgical acute abdomen. Laboratory Findings  Most cases of ascariasis can be diagnosed by microscopic detection of characteristic Ascaris eggs (65 × 45 μm) in

PARASITIC NEMATODE TRICHURIS TRICHIURA (WHIPWORM) ENTEROBIUS VERMICULARIS (PINWORM) STRONGYLOIDES STERCORALIS Oral Oral Cecum, appendix Decades (due to autoinfection) 5 years 2 months 5000–10,000 3000–7000 2000–10,000 Gastrointestinal symptoms; malabsorption or sepsis in hyperinfection Gastrointestinal symptoms, rectal prolapse, or anemia in heavy infection Perianal pruritus CHAPTER 239 Larvae in stool or duodenal aspirate; sputum in hyperinfection; serology Eggs in stool Eggs from perianal skin on cellulose acetate tape Ivermectin Albendazole Mebendazole Albendazole Ivermectin  Mebendazole Albendazole Pyrantel pamoate Intestinal Nematode Infections fecal samples, although increasingly, polymerase chain reaction (PCR) of worm DNA extracted from stool is being used in research and some clinical settings. Occasionally, patients present after passing an adult worm—identifiable by its large size and smooth cream-colored surface— in the stool or, much less commonly, through the mouth or nose. During the early transpulmonary migratory phase, when eosinophilic pneumonitis occurs, larvae can be found in sputum or gastric aspirates before diagnostic eggs appear in the stool. The eosinophilia that is prominent during this early stage usually decreases to minimal levels in established infection. Adult worms may be visualized, occasionally serendipitously, on contrast radiographic studies of the gastrointestinal tract. A plain abdominal film may reveal masses of worms in gas-filled loops of bowel in patients with intestinal obstruction. Pancreaticobili­ ary worms can be detected by ultrasound and endoscopic retrograde cholangiopancreatography; the latter method has been used to extract biliary Ascaris worms. TREATMENT Ascariasis Ascariasis should always be treated to prevent potentially serious complications. Albendazole (400 mg once), mebendazole (100 mg twice daily for 3 days or 500 mg once), pyrantel pamoate (11 mg/kg once with a maximum dose not to exceed 1 g), ivermectin (150–200 μg/kg once), and moxidectin (8 mg once) are effective. These medi­ cations are contraindicated in pregnancy, however. Mild diarrhea and abdominal pain are uncommon side effects of these agents.

Partial intestinal obstruction should be managed with nasogastric suction, IV fluid administration, and instillation of piperazine through the nasogastric tube, although complete obstruction and its severe complications require immediate surgical intervention.

■ ■HOOKWORM Three species (Necator americanus, Ancylostoma duodenale, and Ancylostoma ceylanicum) are responsible for most human hookworm infections. Most infected individuals are asymptomatic. Hookworm disease develops from a combination of factors—heavy worm burden, prolonged duration of infection, and inadequate iron intake—and results in iron-deficiency anemia and, on occasion, hypoproteinemia. Life Cycle  Adult hookworms, which are ~1 cm long, use buccal teeth (Ancylostoma) or cutting plates (Necator) to attach to the smallbowel mucosa where they secrete enzymes that enable them to invade submucosal tissues and ingest blood and villous tissue. Female adult hookworms produce thousands of eggs daily. The eggs are deposited with feces in soil, where rhabditiform larvae hatch and develop over a 1-week period into infectious filariform larvae. Infective larvae pen­ etrate the skin and reach the lungs by way of the bloodstream. There they invade alveoli and ascend the airways to the larynx before being swallowed and reaching the small intestine. The prepatent period from skin penetration to appearance of eggs in the feces is ~6–8 weeks, but it may be longer with Ancylostoma spp. Less commonly, larvae of Ancy­ lostoma spp., if swallowed orally, can survive and develop directly into adult worms that parasitize the intestinal mucosa. Adult hookworms may survive over a decade but usually live ~6–8 years for A. duodenale and 2–5 years for N. americanus. PART 5 Infectious Diseases Epidemiology  Over 400 million people are infected with hook­ worms worldwide. Whereas N. americanus is the predominant spe­ cies throughout the tropics and subtropics, A. duodenale is more geographically restricted to areas of North Africa and northern Asia. A. ceylanicum is much less common than the other two species and is mainly found in Southeast Asia. Age prevalence studies have shown a constant increase in hookworm prevalence throughout childhood and into adulthood. Older children have the greatest intensity of hookworm infection; however, in rural areas where fields are fertil­ ized with human feces, older working adults also may be heavily infected. Clinical Features  Most hookworm infections are of light intensity and therefore are clinically asymptomatic. Infective larvae may pro­ voke a pruritic maculopapular rash (“ground itch”) at the site of skin penetration as well as serpiginous tracks of subcutaneous migration (similar to those of cutaneous larva migrans; Chap. 238) in previously sensitized hosts. Larvae migrating through the lungs occasionally cause mild transient pneumonitis, but this condition develops less frequently in hookworm infection than in ascariasis. In the early intes­ tinal phase, infected persons may develop epigastric pain, abdominal bloating, and other abdominal symptoms accompanied by eosino­ philia. The major consequence of chronic hookworm infection is iron deficiency. Symptoms are minimal if iron stores are adequate, but undernourished individuals develop symptoms of progressive irondeficiency anemia and hypoproteinemia, including fatigue, weakness, and shortness of breath. Laboratory Findings  The diagnosis is established by the finding of characteristic 40 × 60 μm oval hookworm eggs in the feces. Stool concentration techniques may be required to detect light infections. Eggs of the three species are indistinguishable by light microscopy, whereas PCR provides significant improvement in species-specific diagnosis, although this remains a research tool that is not available in commercial laboratories. In a stool sample that is not fresh, hookworm eggs may have hatched to release rhabditiform larvae, which must be differentiated from those of S. stercoralis. Hypochromic microcytic anemia, occasionally with eosinophilia or hypoalbuminemia, is char­ acteristic of hookworm disease.

TREATMENT Hookworm Infection Hookworm infection can be treated with several safe and effective anthelmintic drugs, including albendazole (400 mg once daily for

3 days) and mebendazole (500 mg once daily or 100 mg twice daily for 3 days). Mild iron-deficiency anemia can often be treated with oral iron supplementation alone. Severe hookworm disease with protein loss and malabsorption necessitates nutritional support and oral iron replacement along with deworming. There is significant concern that the benzimidazoles (mebendazole and albendazole) are becoming much less effective against human hookworms. Ancylostoma caninum and Ancylostoma braziliense  A. caninum, the canine hookworm, has been identified as a cause of human eosino­ philic enteritis, especially in northeastern Australia. In this rare zoo­ notic infection, adult hookworms attach to the small intestine (where they may be visualized by endoscopy) and elicit abdominal pain and intense local eosinophilia. Treatment with mebendazole (100 mg twice daily for 3 days) or albendazole (400 mg once) or endoscopic removal is effective. Both of these animal hookworm species can cause cutane­ ous larva migrans (“creeping eruption”; Chap. 238). ■ ■STRONGYLOIDIASIS S. stercoralis is distinguished by its ability—unique among helminths (except for Capillaria; see below)—to complete its life cycle and repli­ cate within a human host. This capacity permits ongoing cycles of auto­ infection as infective larvae are internally produced that can develop into egg-producing adult worms without exiting the host. Infection with S. stercoralis can thus persist for decades without further exposure to exogenous infective larvae. In immunocompromised hosts, large numbers of invasive Strongyloides larvae can disseminate widely and can be fatal. Life Cycle  In addition to a parasitic life cycle in the human host, Strongyloides can undergo a free-living cycle of development in the soil (Fig. 239-1). This adaptability facilitates the parasite’s survival in the absence of mammalian hosts. Rhabditiform larvae passed in feces can transform into infectious filariform larvae either directly or after a free-living phase of development. Humans acquire S. stercoralis when filariform larvae in fecally contaminated soil penetrate the skin or mucous membranes. The larvae then travel through the bloodstream to the lungs, where they break into the alveolar spaces, ascend the bronchial tree, are swallowed, and thereby reach the small intestine. There the larvae mature into adult worms that embed in the mucosa of the proximal small bowel. The minute (1- to 2-mm long) parasitic adult female worms reproduce by parthenogenesis; adult males do not exist in the human host. Eggs hatch in the intestinal mucosa, releasing rhabditiform larvae that migrate to the lumen and pass with the feces into soil. Alternatively, rhabditiform larvae in the bowel can develop directly into filariform larvae that penetrate the colonic wall or perianal skin and enter the circulation to repeat the migration that establishes ongoing internal reinfection. Epidemiology  S. stercoralis is spottily distributed in tropical areas and other warm, humid regions and is common in Southeast Asia, subSaharan Africa, and Brazil. In the United States, the parasite is endemic in parts of the Southeast and is found in immigrants, refugees, travel­ ers, and military personnel who have lived in endemic areas. Clinical Features  In uncomplicated strongyloidiasis, many patients are asymptomatic or have mild cutaneous and/or abdominal symptoms. Recurrent urticaria, often involving the buttocks, is the most common cutaneous manifestation. Larvae migrating in the skin can elicit a pathognomonic serpiginous eruption known as larva cur­ rens. This pruritic, raised, erythematous lesion advances as rapidly as 10 cm/h along the course of larval migration. Adult parasites burrow into the duodenojejunal mucosa and can cause abdominal (usually midepigastric) pain, which resembles peptic ulcer disease except that

Larvae migrate via bloodstream or lymphatics to lungs, ascend airway to trachea and pharynx, and are swallowed. Hyperinfection: With immunosuppression, larger numbers of filariform larvae develop, penetrate bowel, and disseminate, causing: Filariform larvae (450 µm) Rhabditiform larvae in soil FIGURE 239-1  Life cycle of Strongyloides stercoralis. (Reproduced with permission from RL Guerrant et al [eds]: Tropical Infectious Diseases: Principles, Pathogens and Practice, 2nd ed. Elsevier, 2006.) it is aggravated by food ingestion. Nausea, watery diarrhea, abdominal bloating, occult gastrointestinal bleeding, and weight loss can occur. Small-bowel obstruction may develop with early, heavy infection. Pul­ monary symptoms are rare in uncomplicated strongyloidiasis. Eosino­ philia is common, with levels fluctuating over time. The ongoing autoinfection cycle of S. stercoralis is normally con­ strained by the host’s immune system. Impairment of host cellmediated immunity, especially with high-dose glucocorticoid therapy and less commonly with other immunosuppressive medications, or associated with infection with human T-cell lymphotropic virus type 1 (HTLV-1), may lead to hyperinfection and the generation of large numbers of filariform larvae. Colitis, enteritis, or malabsorption may develop. Hyperinfection can also lead to disseminated strongyloidiasis, characterized by larvae invading not only gastrointestinal tissues and the lungs but also the central nervous system, skin, peritoneum, liver, and kidneys. Moreover, bacteremia may develop because of the spread of enteric flora through disrupted mucosal barriers. Gram-negative sepsis, pneumonia, or meningitis may complicate or dominate the clinical course. Eosinophilia is often absent in severely infected patients. Dis­ seminated strongyloidiasis, particularly in patients with unsuspected infection who are taking glucocorticoids, has a high mortality rate. Diagnosis  In uncomplicated strongyloidiasis, visualizing rhab­ ditiform larvae by light microscopic examination of a fecal sample is diagnostic. Rhabditiform larvae are ~250 μm long, with a short buccal cavity that distinguishes them from hookworm larvae. In uncom­ plicated infections, few larvae are passed and single stool examina­ tions detect only about one-third of cases. Serial examinations and the use of the agar plate detection method improve the sensitivity of stool diagnosis. Again, PCR has begun to be used more widely and provides increased diagnostic sensitivity. In hyperinfection, a single

2-mm hermaphroditic adult s penetrate small-bowel mucosa and release eggs, which hatch to rhabditiform larvae. Lung or intestinal stage may cause: Eosinophilia and intermittent epigastric pain Autoinfection: Transform within the intestine into filariform larvae, which penetrate perianal skin or bowel mucosa, causing: Rhabditiform larvae (250 µm) Pruritic larva currens Eosinophilia Larvae shed in stool Colitis, polymicrobial sepsis, pneumonitis, or meningitis Free-living 1-mm adults in soil CHAPTER 239 Direct development Eggs in soil Indirect development (heterogonic) (can multiply outside host for several generations) in soil Intestinal Nematode Infections microscopy-based stool examination is usually positive given the large number of larvae being produced. Strongyloides larvae may also be found by sampling of the duodenojejunal contents by aspiration or biopsy. An enzyme-linked immunosorbent assay for serum antibod­ ies to antigens of Strongyloides is a sensitive method for diagnosing uncomplicated infections. Such serologic testing should be performed for patients with prior potential exposure, especially those with eosino­ philia and/or those who may be starting glucocorticoid therapy or undergoing solid organ transplantation. In disseminated strongyloi­ diasis, filariform larvae are easily detected in stool as well as in samples obtained from sites of potential larval migration, including sputum, bronchoalveolar lavage fluid, or surgical drainage fluid. TREATMENT Strongyloidiasis Even in the asymptomatic state, strongyloidiasis must be treated because of the potential for subsequent hyperinfection and fatal dissemination. Ivermectin (200 μg/kg daily for 2 days) is more effective than albendazole (400 mg daily for 3 days). Decreases in eosinophil count and antibody titer indicate a response to treat­ ment. For disseminated strongyloidiasis, treatment with ivermectin should be extended for at least 14 days after fecal examinations have become negative. In potentially immunocompromised hosts, the course of ivermectin should be repeated 2 weeks after initial treatment. Ivermectin has been successfully given per rectum in those unable to take ivermectin orally and can be given parenterally (using veterinary preparations) through single-patient expanded access protocols.

■ ■TRICHURIASIS Most infections with Trichuris trichiura are asymptomatic, but heavy infections may cause gastrointestinal symptoms. Like the other soiltransmitted helminths, whipworm is distributed globally in the trop­ ics and subtropics and is most common among poor children from resource-limited regions of the world.

Life Cycle  Adult Trichuris worms reside in the colon and cecum, their thin anterior portions embedded in the superficial mucosa. Thousands of eggs laid daily by adult female worms pass with the feces and mature in the soil. After ingestion, infective eggs hatch in the duodenum, releasing larvae that mature before migrating to the large bowel. The entire cycle takes ~3 months, and adult worms may live for several years. Clinical Features  Most individuals infected with T. trichiura have no symptoms or eosinophilia. Heavy infections may result in anemia, abdominal pain, anorexia, and bloody or mucoid diarrhea resembling inflammatory bowel disease. Rectal prolapse can result from massive infections in children due to protracted tenesmus. Moderately heavy T. trichiura burdens also contribute to impaired physical growth. Diagnosis and Treatment  The characteristic 50 × 20 μm lemonshaped T. trichiura eggs are readily detected on microscopic examina­ tion of stool. Adult worms, which are 3–5 cm long, are occasionally seen on proctoscopy. PCR is being used increasingly in settings where it is available. Albendazole (400 mg daily for 3 days) or mebendazole (100 mg twice daily for 3 days) is safe and moderately effective for treatment. The addition of ivermectin (200 μg/kg daily for 3 doses) to either benzimidazole drug increases cure rates significantly. PART 5 Infectious Diseases ■ ■ENTEROBIASIS (PINWORM) E. vermicularis is more common in temperate climates than in the tropics. In the United States, ~40 million persons are infected with pinworms, with most cases occurring in children. Life Cycle and Epidemiology  Adult Enterobius worms are ~5–10 mm long and dwell in the cecum. Gravid female worms migrate nocturnally out of the anus to the perianal region where they release up to 10,000 immature eggs each. The eggs become infective within hours and are transmitted by hand-to-mouth passage. From ingested eggs, larvae hatch and mature into adults. This life cycle takes ~1 month, and adult worms survive for ~2 months. Autoinfection results from perianal scratching and transport of infective eggs on the fingers or under the nails to the mouth. Retroinfection can also occur by larvae released from embryonated eggs on the perianal skin migrating back into the rectum. Because of the ease of person-to-person spread, pin­ worm infections are common among family members. Clinical Features  Most pinworm infections are asymptomatic. Perianal pruritus is the cardinal symptom. The itching, which is often worse at night as a result of the nocturnal emergence of female worms, may lead to excoriation and bacterial superinfection. Heavy infections have been associated with abdominal pain, weight loss, and rarely acute appendicitis. Uncommonly, pinworms may migrate into the female genital tract, causing vulvovaginitis and pelvic or peritoneal granulo­ mas. Eosinophilia is uncommon. Diagnosis  Since pinworm eggs are not released in feces, the diagno­ sis cannot be made by conventional fecal ova and parasite microscopy. Instead, eggs are detected by the application of clear cellulose acetate tape to the perianal region in the morning. After the tape is transferred to a slide, microscopic examination can detect pinworm eggs, which are oval, measure 55 × 25 μm, and are flattened along one side. TREATMENT Enterobiasis Infected children and adults should be treated with mebendazole (100 mg once), albendazole (400 mg once), or pyrantel pamoate

(11 mg/kg once), with the same treatment repeated after 2 weeks because the drugs do not kill eggs or early-stage larvae. Treatment of household members is recommended to eliminate asymptomatic reservoirs of potential reinfection. Linens and clothing should be washed in hot water followed by a hot dryer to kill deposited eggs. ■ ■TRICHOSTRONGYLIASIS Trichostrongylus species, which are normally parasites of herbivorous animals, occasionally infect humans, particularly in Asia, Africa, and Australia. Humans acquire the infection by accidentally ingesting Trichostrongylus larvae on contaminated leafy vegetables. The larvae do not migrate in humans but mature directly into adult worms in the small bowel. The pathology caused by these worms is similar to hookworm, although they ingest far less blood. Most infected persons are asymptomatic, but heavy infections may give rise to mild anemia and eosinophilia. In stool examinations, Trichostrongylus eggs resemble hookworm eggs but are larger (85 × 115 μm). Treatment consists of mebendazole, albendazole, or pyrantel pamoate (Chap. 229). ■ ■ANISAKIASIS Anisakiasis is a gastrointestinal infection caused by the accidental ingestion in uncooked saltwater fish of nematode larvae belonging to the family Anisakidae. The incidence of anisakiasis in the United States has increased as a result of the growing popularity of raw fish dishes. Most cases occur in Japan, the Netherlands, and Chile, where raw fish—sashimi, pickled green herring, and ceviche, respectively— are national culinary staples. Anisakid nematodes parasitize large sea mammals such as whales, dolphins, and seals. As part of a complex parasitic life cycle involving marine food chains, infectious larvae migrate to the musculature of a variety of fish species that serve as intermediate hosts. Both Anisakis simplex and Pseudoterranova decipiens have been implicated in human anisakiasis, but an identi­ cal gastric syndrome may be caused by the red larvae of eustrongylid parasites of fish-eating birds. When humans consume infected raw or undercooked fish, live larvae may be coughed up within 48 h. Alternatively, larvae may immediately penetrate the mucosa of the stomach. Within hours, vio­ lent upper abdominal pain accompanied by nausea and occasionally vomiting ensues, mimicking an acute abdomen. The diagnosis can be established by direct visualization on upper endoscopy, outlining of the worm by contrast radiographic studies, or histopathologic exami­ nation of extracted tissue. Extraction of the burrowing larvae during endoscopy is curative. In addition, larvae may pass to the small bowel, where they penetrate the mucosa and provoke a vigorous eosinophilic granulomatous response. Symptoms may appear 1–2 weeks after the infective meal, with intermittent abdominal pain, diarrhea, nausea, and fever resembling the manifestations of Crohn’s disease. Ingestion of Anisakis-derived proteins through consumption of fish meat con­ taining Anisakis parasites can elicit allergic gastrointestinal and even anaphylactic responses. Anisakid eggs are not found in the stool since the larvae do not mature in humans. Serologic tests have been developed but are not widely available. Anisakid larvae in saltwater fish are killed by cooking to 60°C, freez­ ing at –20°C for 3 days, or commercial blast freezing, but usually not by salting, marinating, or cold smoking. No medical treatment is available; surgical or endoscopic removal should be undertaken. ■ ■CAPILLARIASIS Intestinal capillariasis is caused by ingestion of raw fish infected with Capillaria philippinensis. Subsequent autoinfection can lead to a severe wasting syndrome. The disease occurs in the Philippines and Thailand and, on occasion, elsewhere in Asia. The natural life cycle of C. philippinensis involves fish from fresh and brackish water. When humans eat infected raw fish, the larvae mature in the intestine into adult worms, which produce invasive larvae that cause intestinal inflammation and villus loss and can develop into new adult worms without leaving the human host. Capillariasis has an insidious onset

136 - 240 Filarial and Related Infections

240 Filarial and Related Infections

with nonspecific abdominal pain and watery diarrhea. If untreated, repeated rounds of autoinfection can lead to protein-losing enter­ opathy, severe malabsorption, and ultimately death from cachexia, cardiac failure, or superinfection. The diagnosis is made by identify­ ing characteristic peanut-shaped (20 × 40 μm) eggs on microscopic examination of stool. Severely ill patients require hospitalization and supportive therapy in addition to prolonged anthelmintic treatment with albendazole (400 mg twice daily for 10 days) or mebendazole (500 mg daily for 20 days; Chap. 229). ■ ■ABDOMINAL ANGIOSTRONGYLIASIS Abdominal angiostrongyliasis is primarily a disease of children living in Latin America. The zoonotic parasite Angiostrongylus costaricensis causes eosinophilic ileocolitis after the ingestion of contaminated vegetation. A. costaricensis normally parasitizes the cotton rat and other rodents, with slugs and snails serving as intermediate hosts. Humans become infected by accidentally ingesting infective larvae in mollusk slime deposited on fruits and vegetables; children are at highest risk. The larvae penetrate the gut wall and migrate to mesen­ teric arterioles, where they develop into adult worms. Eggs deposited in the gut wall provoke an intense eosinophilic granulomatous reac­ tion, and adult worms may cause mesenteric arteritis, thrombosis, or frank bowel infarction. Symptoms may mimic those of appendicitis, including abdominal pain and tenderness, fever, vomiting, and a pal­ pable mass in the right iliac fossa. Leukocytosis and eosinophilia are prominent. CT with contrast typically shows inflamed bowel, often with concomitant obstruction, but a definitive diagnosis is usually made histologically after partial bowel resection. Pathologic exami­ nation reveals a thickened bowel wall with eosinophilic granulomas surrounding the Angiostrongylus eggs. In nonsurgical cases, the diag­ nosis rests solely on clinical grounds because larvae and eggs cannot be detected in the stool and serologic tests are not available. Medical therapy for abdominal angiostrongyliasis is of uncertain efficacy. Careful observation and surgical resection for severe symptoms are the mainstays of treatment. Acknowledgment The authors wish to acknowledge and thank Peter F. Weller, MD, author of prior editions of this chapter. ■ ■FURTHER READING Costa IN et al: Diagnosis of human strongyloidiasis: Application in clinical practice. Acta Trop 223:106081, 2021. Else KJ et al: Whipworm and roundworm infections. Nat Rev Dis Primers 6:44, 2020. Holland C et al: Global prevalence of Ascaris infection in humans (2010–2021): A systematic review and meta-analysis. Infect Dis Pov­ erty 11:113, 2022. Krolewiecki A et al: Strongyloidiasis: A neglected tropical disease. Infect Dis Clin North Am 33:135, 2019. Loukas A et al: Hookworm infection. Nat Rev Dis Primers 2:16088, 2016. Montressor A et al: The global progress of soil-transmitted helmin­ thiases control in 2020 and World Health Organization targets for 2030. PLoS Negl Trop Dis 14:e0008505, 2020. Moser W et al: Efficacy of recommended drugs against soil transmit­ ted helminths: Systematic review and network meta-analysis. BMJ 358:j4307, 2017. O’Connell EM et al: Ancylostoma ceylanicum hookworm in Myanmar refugees, Thailand, 2012-2015. Emerg Infect Dis 24:1472, 2018. Shamsi S et al: A critical review of anisakidosis cases occurring globally. Parasitol Res 122:1733, 2023.

Philip J. Budge, Thomas B. Nutman

Infections Filarial worms are nematodes that dwell in the subcutaneous tissues and the lymphatics. Eight filarial species infect humans (Table 240-1); of these, four—Wuchereria bancrofti, Brugia malayi, Onchocerca volvulus, and Loa loa—are responsible for most symptomatic filarial infec­ tions. Filarial parasites, which infect an estimated 170 million persons worldwide, are transmitted by specific species of mosquitoes or other arthropods and have a complex life cycle, including insect-borne infective larval stages and adult worms that reside in either lymphatic or subcutaneous tissues of humans. The offspring of adults are micro­ filariae, which, depending on their species, are 200–250 μm long and 5–7 μm wide, may or may not be enveloped in a loose sheath, and either circulate in the blood or migrate through the skin (Table 240-1). To complete the life cycle, microfilariae are ingested by the arthropod vector and develop over 1–2 weeks into new infective larvae. Adult worms live for many years, whereas microfilariae survive for 3–36 months. The intracellular bacterial endosymbiont Wolbachia has been found in all stages of Brugia, Wuchereria, Mansonella, and Onchocerca species and has become a target for antifilarial chemotherapy. Usually, infection is established only with repeated, prolonged exposures to infective larvae. Since the clinical manifestations of filarial diseases develop relatively slowly, these infections should be considered chronic, with possible long-term debilitating effects. The nature, severity, and timing of clinical manifestations in patients with filarial infections who are native to endemic areas and have lifelong exposure may differ significantly from those who are travelers or who have recently moved to these areas. Characteristically, filarial disease is more acute and intense in newly exposed individuals than in natives of endemic areas. CHAPTER 240 Filarial and Related Infections LYMPHATIC FILARIASIS Lymphatic filariasis is caused by W. bancrofti, B. malayi, or Brugia timori. The threadlike adult parasites reside in afferent lymphatics or lymph nodes, where they may remain viable for more than two decades. ■ ■EPIDEMIOLOGY W. bancrofti, the most widely distributed filarial parasite of humans, affects an estimated 51 million people and is found throughout the tropics and subtropics, including Asia and the Pacific Islands, Africa, areas of South America, and the Caribbean basin. Humans are the only definitive host for the parasite. Generally, the subperiodic form is found only in the Pacific Islands; elsewhere, W. bancrofti is noctur­ nally periodic. Nocturnally periodic forms of microfilariae are scarce in peripheral blood by day and increase at night, whereas subperiodic forms are present in peripheral blood at all times and reach maximal levels in the afternoon. Natural vectors for W. bancrofti are Culex mosquitoes in urban settings and Anopheles or Aedes mosquitoes in rural areas. Brugian filariasis due to B. malayi occurs primarily in eastern India, Indonesia, Malaysia, and the Philippines. B. malayi also has two forms distinguished by the periodicity of microfilaremia. The more common nocturnal form is transmitted in areas of coastal rice fields, while the subperiodic form is found in forests. B. malayi naturally infects cats as well as humans. The distribution of B. timori is limited to the islands of southeastern Indonesia. ■ ■PATHOLOGY The principal pathologic changes of lymphatic filariasis result from inflammatory damage to the lymphatics, which is typically caused by adult worms and not by microfilariae. Adult worms live in afferent lymphatics or sinuses of lymph nodes and cause lymphatic dilation and thickening of the vessel walls. The infiltration of plasma cells,

TABLE 240-1  Characteristics of the Filariae ORGANISM PERIODICITY DISTRIBUTION VECTOR LOCATION OF ADULT Wuchereria bancrofti Nocturnal Cosmopolitan areas worldwide, including South America, Africa, southern Asia, Papua New Guinea, China, Indonesia   Subperiodic Eastern Pacific Aedes (mosquitoes) Lymphatic tissue Blood + Brugia malayi Nocturnal Southeast Asia, Indonesia, India Mansonia, Anopheles (mosquitoes) Lymphatic tissue Blood +   Subperiodic Indonesia, Southeast Asia Coquillettidia, Mansonia (mosquitoes) Lymphatic tissue Blood + Brugia timori Nocturnal Indonesia Anopheles (mosquitoes) Lymphatic tissue Blood + Loa loa Diurnal West and Central Africa Chrysops (deerflies) Subcutaneous tissue Blood + Onchocerca volvulus None South and Central America, Africa Simulium (blackflies) Subcutaneous tissue Skin, eye – Mansonella ozzardi None South and Central America Culicoides (midges) Undetermined site Blood –   None Caribbean Simulium (blackflies) Undetermined site Blood – Mansonella perstans None South and Central America, Africa Culicoides (midges) Body cavities, mesentery, perirenal tissue Mansonella streptocerca None West and Central Africa Culicoides (midges) Subcutaneous tissue Skin – eosinophils, and macrophages in and around the infected vessels, along with endothelial and connective tissue proliferation, leads to tortuosity of the lymphatics and damaged or incompetent lymph valves. Lymph­ edema and chronic stasis changes with hard or brawny edema develop in the overlying skin. These consequences of filarial infection are due both to the direct effects of the worms and to the host’s inflammatory response to the parasite. Inflammatory responses are believed to cause the granulomatous and proliferative processes that precede total lymphatic obstruction. It is thought that the lymphatic vessel remains patent as long as the worm remains viable, and that the death of the worm leads to enhanced granulomatous reactions and fibrosis. Lymphatic obstruction results, and despite collateralization, lymphatic function is compromised. PART 5 Infectious Diseases ■ ■CLINICAL FEATURES The most common presentations of the lymphatic filariases are asymp­ tomatic (or subclinical) microfilaremia, hydrocele (Fig. 240-1), acute adenolymphangitis (ADL), and chronic lymphatic disease. In areas FIGURE 240-1  Hydrocele associated with Wuchereria bancrofti infection.

MICROFILARIAL LOCATION SHEATH Culex, Anopheles (mosquitoes) Lymphatic tissue Blood + Blood – where W. bancrofti or B. malayi is endemic, most infected individuals have few overt clinical manifestations of filarial infection despite the presence of circulating microfilariae in the peripheral blood. Although they may be clinically asymptomatic, virtually all persons with

W. bancrofti or B. malayi microfilaremia have some degree of subclini­ cal disease that includes microscopic hematuria and/or proteinuria, dilated (and tortuous) lymphatics (visualized by imaging), and—in men with W. bancrofti infection—scrotal lymphangiectasia (detect­ able by ultrasound). Despite these findings, most individuals appear to remain clinically asymptomatic for years; in relatively few does the infection progress to either acute or chronic disease. ADL is characterized by high fever, lymphatic inflammation (lym­ phangitis and lymphadenitis), and transient local edema. The lym­ phangitis is retrograde, extending peripherally from the lymph node draining the area where the adult parasites reside. Regional lymph nodes are often enlarged, and the entire lymphatic channel can become indurated and inflamed. Concomitant local thrombophlebitis can occur as well. In brugian filariasis, a single local abscess may form along the involved lymphatic tract and subsequently rupture to the sur­ face. The lymphadenitis and lymphangitis can involve both the upper and lower extremities in both bancroftian and brugian filariasis, but involvement of the genital lymphatics occurs almost exclusively with W. bancrofti infection. This genital involvement can be manifested by funiculitis, epididymitis, and scrotal pain and tenderness. In endemic areas, another type of acute disease—dermatolymphangioadenitis (DLA)—is recognized as a syndrome that includes high fever, chills, myalgias, and headache. Edematous inflammatory plaques clearly demarcated from normal skin are seen. Vesicles, ulcers, and hyperpig­ mentation also may be noted. There is often a history of trauma, burns, irradiation, insect bites, punctiform lesions, or chemical injury. Entry lesions, especially in the interdigital area, are common. DLA is often diagnosed as cellulitis. If lymphatic damage progresses, transient lymphedema can develop into lymphatic obstruction and the permanent changes associated with elephantiasis (Fig. 240-2). Brawny edema follows early pitting edema, the subcutaneous tissues thicken, and hyperkeratosis occurs. Fissur­ ing of the skin develops, as do hyperplastic changes. Superinfection of these poorly vascularized tissues becomes a problem. In bancroftian filariasis, in which genital involvement is common, hydroceles may develop (Fig. 240-1); in advanced stages, this condition may evolve into scrotal lymphedema and scrotal elephantiasis. Furthermore, if there is obstruction of the retroperitoneal lymphatics, increased renal lym­ phatic pressure leads to rupture of the renal lymphatics and the devel­ opment of chyluria, which is usually intermittent and most prominent in the morning.

FIGURE 240-2  Elephantiasis of the lower extremity associated with Wuchereria bancrofti infection. The clinical manifestations of filarial infections in travelers or trans­ migrants who have recently entered an endemic region are distinctive. Given a sufficient number of bites by infected vectors, usually over a 3- to 6-month period, recently exposed patients can develop acute lym­ phatic or scrotal inflammation with or without urticaria and localized angioedema. Lymphadenitis of epitrochlear, axillary, femoral, or ingui­ nal lymph nodes is often followed by evolving retrograde lymphangi­ tis. Acute attacks are short-lived and are not usually accompanied by fever. With prolonged exposure to infected mosquitoes, these attacks, if untreated, become more severe and lead to permanent lymphatic inflammation and obstruction. ■ ■DIAGNOSIS A definitive diagnosis can be made only by detection of the para­ sites and hence can be difficult. Adult worms localized in lymphatic vessels or nodes are largely inaccessible. Microfilariae can be found in blood, in hydrocele fluid, or (occasionally) in other body fluids. Such fluids can be examined microscopically, either directly or—for greater sensitivity—after concentration of the parasites by the pas­ sage of fluid through a polycarbonate cylindrical-pore filter (pore size, 3 μm) or by the centrifugation of fluid fixed in 2% formalin (Knott’s concentration technique). The timing of blood collection is critical and should be based on the periodicity of the microfilariae in the endemic region involved. Many infected individuals do not have microfilaremia, and definitive diagnosis in such cases can be difficult. Assays for circulating antigens of W. bancrofti permit the diagnosis of microfilaremic and cryptic (amicrofilaremic) infection. Two tests are commercially available: an enzyme-linked immunosorbent assay (ELISA) and a rapid-format lateral flow assay. Both assays have sen­ sitivities of 93–100% and specificities approaching 100%, although false positives can occur in individuals infected with Loa loa (see “Loiasis” below). There are currently no tests for circulating antigens in brugian filariasis. Polymerase chain reaction (PCR)–based assays for DNA of W. ban­ crofti and B. malayi in blood have been developed. A number of studies indicate that the sensitivity of this diagnostic method is equivalent to or greater than that of parasitologic methods.

In cases of suspected lymphatic filariasis, examination of the scro­ tum, the lymph nodes, or (in female patients) the breast by means of high-frequency ultrasound in conjunction with Doppler techniques may result in the identification of motile adult worms within dilated lymphatics (Video 240-1). Worms may be visualized in the lymphatics of the spermatic cord in up to 80% of men infected with W. bancrofti. Live adult worms have a distinctive pattern of movement within the lymphatic vessels (termed the filarial dance sign). Radionuclide lymphoscintigraphic imaging of the limbs reliably demonstrates wide­ spread lymphatic abnormalities in both subclinical microfilaremic persons and those with clinical manifestations of lymphatic pathology. Although of potential utility in the delineation of anatomic changes associated with infection, lymphoscintigraphy is unlikely to assume primacy in the diagnostic evaluation of individuals with suspected infection; it is principally a research tool, although it has been used more widely for assessment of lymphedema of any cause. Eosinophilia and elevated serum concentrations of IgE and antifilarial antibody support the diagnosis of lymphatic filariasis. There is, however, exten­ sive cross-reactivity between filarial antigens and antigens of other helminths. Of note, W. bancrofti– and B. malayi–specific antigens have been identified and are now available for use in rapid diagnostic tests with specificities of >98%. However, seropositivity cannot be equated with active infection: residents of endemic areas can become sensitized to filarial antigens through exposure to infective mosquitoes without having patent filarial infections.

The ADL associated with lymphatic filariasis must be distinguished from thrombophlebitis, infection, and trauma. Retrograde evolution is a characteristic feature that helps distinguish filarial lymphangitis from ascending bacterial lymphangitis. Chronic filarial lymphedema must also be distinguished from the lymphedema of malignancy, post­ operative scarring, trauma, chronic edematous states, and congenital lymphatic system abnormalities. CHAPTER 240 Filarial and Related Infections TREATMENT Lymphatic Filariasis With newer definitions of clinical syndromes in lymphatic filariasis and new tools to assess clinical status (e.g., ultrasound, lymphos­ cintigraphy, circulating filarial antigen assays, PCR), approaches to treatment based on infection status can be considered. Orally administered diethylcarbamazine (DEC; 6 mg/kg daily for 12 days), which has both macro- and microfilaricidal properties, remains the drug of choice for the treatment of active lymphatic fila­ riasis (defined by microfilaremia, antigen positivity, or adult worms on ultrasound), although albendazole (400 mg twice daily by mouth for 21 days) also has demonstrated macrofilaricidal efficacy. A 4- to 6-week course of oral doxycycline (targeting the intracellular Wol­ bachia) also has significant macrofilaricidal activity, as does DEC/ albendazole used daily for 7 days. The addition of DEC to a 3-week course of doxycycline is also efficacious in lymphatic filariasis. Regimens that combine single doses of albendazole (400 mg) with either DEC (6 mg/kg) or ivermectin (200 μg/kg) all have a sustained microfilaricidal effect and are the mainstay of programs for the eradication of lymphatic filariasis in Africa (albendazole/ ivermectin) and elsewhere (albendazole/DEC) (see “Prevention and Control,” below). More recently, a regimen using single doses of the three major antifilarial drugs (albendazole/DEC/ivermectin)

has been shown to sustain microfilarial clearance out to at least 2 years. As already mentioned, a growing body of evidence indicates that, although they may be asymptomatic, virtually all persons with W. bancrofti or B. malayi microfilaremia have some degree of sub­ clinical disease (hematuria, proteinuria, abnormalities on lympho­ scintigraphy). Thus, early treatment of asymptomatic persons who have microfilaremia is recommended to prevent further lymphatic damage. For ADL, supportive treatment (including the administra­ tion of antipyretics and analgesics) is recommended, as is antibiotic therapy if secondary bacterial infection is likely. Similarly, because

lymphatic disease is associated with the presence of adult worms, treatment with DEC is recommended for microfilaria-negative car­ riers of adult worms.

In persons with chronic manifestations of lymphatic filariasis, treatment regimens that emphasize hygiene, prevention of sec­ ondary bacterial infections, and physiotherapy have gained wide acceptance for morbidity control. These regimens are similar to those recommended for lymphedema of most nonfilarial causes and are known by a variety of names, including complex deconges­ tive physiotherapy and complex lymphedema therapy. Hydroceles (Fig. 240-1) can be managed surgically. With chronic manifesta­ tions of lymphatic filariasis, drug treatment should be reserved for individuals who have evidence of active infection; however, in some settings, a 6-week course of doxycycline has been shown to provide improvement in filarial lymphedema irrespective of disease activity. Side effects of DEC treatment in infected individuals may include fever, chills, arthralgias, headaches, nausea, and vomit­ ing. Both the development and the severity of these reactions are directly related to the number of microfilariae circulating in the bloodstream. The adverse reactions may represent either an acute hypersensitivity reaction to the antigens being released by dead and dying parasites or an inflammatory reaction induced by the intracellular Wolbachia endosymbionts freed from their intracel­ lular niche. Ivermectin has a side effect profile similar to that of DEC when used in lymphatic filariasis. In patients infected with L. loa who have high levels of microfilaremia, DEC—like ivermectin (see “Loiasis,” below)—can elicit severe encephalopathic complications. When used in single-dose regimens for the treatment of lymphatic filariasis, albendazole is associated with relatively few side effects. PART 5 Infectious Diseases ■ ■PREVENTION AND CONTROL To protect themselves against filarial infection, individuals must avoid contact with infected mosquitoes by using personal protective measures, including bed nets, particularly those impregnated with insecticides such as permethrin. Mass drug administration (MDA) is the current approach to elimination of lymphatic filariasis as a public health problem. The underlying tenet of this approach is that mass annual distribution of antifilarial chemotherapy—albendazole with either DEC (for all areas except those where onchocerciasis is co-endemic; see section on onchocerciasis treatment, below) or iver­ mectin or with both ivermectin and DEC (triple-drug therapy)—will profoundly suppress microfilaremia. If the suppression is sustained, then transmission can be interrupted. Created by the World Health Organization in 1997, the Global Programme to Eliminate Lymphatic Filariasis is based on mass administration of single annual doses of DEC plus albendazole in nonAfrican regions and of albendazole plus ivermectin in Africa. Available information from late 2022 indicated that >935 million persons in 68 countries had thus far participated. Not only has lymphatic filariasis been eliminated in some defined areas, but collateral benefits—avoidance of disability and treatment of intestinal helminths and other condi­ tions (e.g., scabies and louse infestation)—also have been noted. The strategy of the global program is being refined, and attempts are being made to integrate this effort with other mass-treatment strategies (e.g., deworming programs, malaria control, and trachoma control) in an integrated control strategy. TROPICAL PULMONARY EOSINOPHILIA Tropical pulmonary eosinophilia (TPE) is a distinct syndrome that develops in some individuals infected with the lymphatic-dwelling filarial species. Most cases have been reported from India, Pakistan, Sri Lanka, Brazil, Guyana, and Southeast Asia; the decreasing inci­ dence of TPE in the past decade probably reflects global MDA efforts. ■ ■CLINICAL FEATURES The main features include a history of residence in filaria-endemic regions, paroxysmal cough and wheezing (usually nocturnal and

probably related to the nocturnal periodicity of microfilariae), weight loss, low-grade fever, lymphadenopathy, and pronounced blood eosin­ ophilia (>3000 eosinophils/μL). Chest x-rays or computed tomography (CT) scans may be normal but generally show increased bronchovascu­ lar markings. Diffuse miliary lesions or mottled opacities may be pres­ ent in the middle and lower lung fields. Tests of pulmonary function show restrictive abnormalities in most cases and obstructive defects in half. Characteristically, total serum IgE levels (4–40 KIU/mL) and antifilarial antibody levels are markedly elevated. ■ ■PATHOLOGY In TPE, microfilariae and parasite antigens are rapidly cleared from the bloodstream by the lungs. The clinical symptoms result from allergic and inflammatory reactions elicited by the cleared parasites. In some patients, trapping of microfilariae in other reticuloendothelial organs can cause hepatomegaly, splenomegaly, or lymphadenopathy. A prominent, eosinophil-enriched, intra-alveolar infiltrate is common, and with it comes the release of cytotoxic proinflammatory eosinophil granule proteins that may mediate some of the pathology seen in TPE. In the absence of successful treatment, interstitial fibrosis can lead to progressive pulmonary damage. ■ ■DIFFERENTIAL DIAGNOSIS TPE must be distinguished from asthma, Löffler’s syndrome, allergic bronchopulmonary aspergillosis, allergic granulomatosis with polyan­ giitis (eosinophilic granulomatosis with polyangiitis or Churg-Strauss syndrome), other systemic vasculitides (most notably, periarteritis nodosa), chronic eosinophilic pneumonia, and the hypereosinophilic syndromes (HESs). TREATMENT Tropical Pulmonary Eosinophilia DEC is used at a daily dosage of 4–6 mg/kg for 14 days. Symptoms usually resolve within 3–7 days after the initiation of therapy. Relapse, which occurs in ~12–25% of cases (sometimes after an interval of several years), requires re-treatment. ONCHOCERCIASIS ■ ■EPIDEMIOLOGY Onchocerciasis (“river blindness”) is caused by the filarial nematode O. volvulus, which infects an estimated 37 million individuals in 31 countries worldwide. The majority of individuals infected with O. volvulus live in the equatorial region of Africa extending from the Atlantic coast to the Red Sea. In the Americas, the only remaining countries with isolated foci are Venezuela and Brazil. The infection is also found in Yemen. ■ ■ETIOLOGY Infection in humans begins with the deposition of infective larvae on the skin by the bite of an infected blackfly. The larvae develop into adults, which are typically found in subcutaneous nodules. About 7 months to 3 years after infection, the gravid female releases microfilariae that migrate out of the nodule and throughout the tissues, concentrating in the dermis. Infection is transmitted to other persons when a female fly ingests microfilariae from the host’s skin and these microfilariae then develop into infective larvae. Adult O. volvulus females and males are ~40–60 cm and ~3–6 cm in length, respectively. The life span of adults can be as long as 18 years, with an average of ~9 years. Because the blackfly vector breeds along free-flowing rivers and streams (particu­ larly in rapids) and generally restricts its flight to an area within several kilometers of these breeding sites, both biting and disease transmission are most intense in these locations. ■ ■PATHOLOGY Onchocerciasis primarily affects the skin, eyes, and lymph nodes. In contrast to the pathology in lymphatic filariasis, the damage in

onchocerciasis is elicited by microfilariae and not by adult parasites. In the skin, there are mild but chronic inflammatory changes that can result in loss of elastic fibers, atrophy, and fibrosis. The subcutaneous nodules (onchocercomata) consist primarily of fibrous tissues sur­ rounding the adult worm, often with a peripheral ring of inflammatory cells surrounded by an endothelial layer (characterized as lymphatic in origin). In the eye, neovascularization and corneal scarring lead to cor­ neal opacities and blindness. Inflammation in the anterior and poste­ rior chambers frequently results in anterior uveitis, chorioretinitis, and optic atrophy. Although punctate opacities are due to an inflammatory reaction surrounding dead or dying microfilariae, the pathogenesis of most manifestations of onchocerciasis is still unclear. ■ ■CLINICAL FEATURES Skin  Pruritus and rash are the most common manifestations of onchocerciasis. The pruritus can be incapacitating; the rash is typically a papular eruption (Fig. 240-3) that is generalized rather than localized to a particular region of the body. Long-term infection results in exag­ gerated and premature wrinkling of the skin, loss of elastic fibers, and epidermal atrophy that can lead to loose, redundant skin and hypo- or hyperpigmentation. Localized eczematoid dermatitis can cause hyper­ keratosis, scaling, and pigmentary changes. In an immunologically hyperreactive form of onchodermatitis (commonly termed sowdah or localized onchodermatitis), the affected skin darkens as a consequence of the profound inflammation that occurs as microfilariae in the skin are cleared. Onchocercomata  These subcutaneous nodules, which can be pal­ pable and/or visible, contain the adult worm. They are most common over the coccyx and sacrum, the trochanter of the femur, the lateral anterior crest, and other bony prominences. Nodules vary in size and characteristically are firm and not tender. It has been estimated that, for every palpable nodule, there are four deeper nonpalpable ones. Ocular Tissue  Visual impairment is the most serious complication of onchocerciasis and usually affects only those persons with moder­ ate or heavy infections. Lesions may develop in all parts of the eye. The most common early finding is conjunctivitis with photophobia. Punctate keratitis—acute inflammatory reactions surrounding dying microfilariae and manifested as “snowflake” opacities—is common among younger patients and resolves without apparent complications. Sclerosing keratitis occurs in 1–5% of infected persons and is the lead­ ing cause of onchocercal blindness. Anterior uveitis and iridocyclitis FIGURE 240-3  Papular eruption as a consequence of onchocerciasis.

develop in ~5% of infected persons. Characteristic chorioretinal lesions develop as a result of atrophy and hyperpigmentation of the retinal pigment epithelium. Constriction of the visual fields and overt optic atrophy may occur.

Lymph Nodes  Mild to moderate lymphadenopathy is common, particularly in the inguinal and femoral areas, where the enlarged nodes may hang down in response to gravity (“hanging groin”), some­ times predisposing to inguinal and femoral hernias. Other Manifestations  Some heavily infected individuals develop cachexia with loss of adipose tissue and muscle mass. A form of dwarf­ ism, Nakalanga dwarfism, has been attributed to pituitary involvement in this infection. An association between onchocerciasis and epilepsy (including an epidemic form termed nodding syndrome) has gained attention recently. Among adults who become blind, there is a three- to fourfold increase in mortality rate. ■ ■DIAGNOSIS Definitive diagnosis depends on the detection of an adult worm in an excised nodule or, more commonly, of microfilariae in a skin snip. Skin snips are obtained with a corneal-scleral punch or by lifting of the skin with the tip of a needle and excision of a small (1- to 3-mm) piece with a sterile scalpel blade. Both methods collect a blood-free skin biopsy sample extending to just below the epidermis. The biopsy tissue can be incubated in tissue culture medium or in saline on a glass slide or flat-bottomed microtiter plate. After incubation for 2–4 h (or occasionally overnight in light infections), microfilariae emer­ gent from the skin can be seen by low-power microscopy or can be detected by PCR. CHAPTER 240 Eosinophilia and elevated serum IgE levels are common, but, because these features are seen in many parasitic infections, are not diagnostic in themselves. Immunoassays to detect antibodies to Onchocercaspecific antigens are being used both in specialized laboratories and at the point of contact in rapid-diagnostic formats. Filarial and Related Infections TREATMENT Onchocerciasis The main goals of therapy are to prevent the development of irre­ versible lesions and to alleviate symptoms. Chemotherapy is the mainstay of management. Ivermectin, a semisynthetic macrocyclic lactone active against microfilariae, is the first-line agent for the treatment of onchocerciasis. It is given orally in a single dose of 150 μg/kg, either yearly or semiannually. More frequent ivermectin administration (every 3 months) has been suggested to ameliorate pruritus and skin disease. After treatment, most individuals have few or no reactions. Pruri­ tus, cutaneous edema, and/or maculopapular rash occur in ~1–10% of treated individuals. In areas of Africa co-endemic for O. volvulus and L. loa, however, ivermectin is contraindicated (as it is for preg­ nant or breast-feeding women) because of severe posttreatment encephalopathy, especially in patients who are heavily microfilaremic for L. loa (>30,000 microfilariae/mL). Although ivermectin treatment results in a marked drop in microfilarial density, its effect can be short-lived (<3 months in some cases). Thus, it is occasionally nec­ essary to give ivermectin more frequently for persistent symptoms. Another macrocyclic lactone, moxidectin, has a more prolonged microfilaricidal effect but is not yet commercially available. A 6-week course of doxycycline is macrofilaristatic, rendering female adult worms sterile for long periods. ■ ■PREVENTION Vector control has been beneficial in highly endemic areas in which breeding sites are vulnerable to insecticide spraying, but most areas endemic for onchocerciasis are not suited to this type of control. Community-based administration of ivermectin every 6–12 months is being used to interrupt transmission in endemic areas. This measure, in conjunction with vector control, has already helped eliminate the

FIGURE 240-4  Adult Loa loa worm being surgically removed after its subconjunctival migration. infection in most of Latin America and has reduced the prevalence of disease in many endemic foci in Africa. No drug has proved useful for prophylaxis of O. volvulus infection. LOIASIS ■ ■ETIOLOGY AND EPIDEMIOLOGY Loiasis is caused by L. loa (the African eye worm), which is present in the rainforests of West and Central Africa. Adult parasites (females, 50–70 mm long and 0.5 mm wide; males, 25–35 mm long and 0.25 mm wide) live in subcutaneous tissues. Microfilariae circulate in the blood with a diurnal periodicity that peaks between 10:00 a.m. and 2:00 p.m. PART 5 Infectious Diseases ■ ■CLINICAL FEATURES Manifestations of loiasis in natives of endemic areas may differ from those in temporary residents or visitors. Among the indigenous popu­ lation, loiasis is often an asymptomatic infection with microfilaremia. Infection may be recognized only after subconjunctival migration of an adult worm (Fig. 240-4) or may be manifested by episodic Calabar swellings—evanescent localized areas of angioedema and erythema developing on the extremities and less frequently at other sites. Nephropathy, encephalopathy, and cardiomyopathy can occur but are infrequently diagnosed. Although historically considered relatively benign in endemic populations, recent studies show loiasis is associated with chronic morbidity and increased mortality in endemic populations. In patients who are not residents of endemic areas, aller­ gic symptoms predominate, episodes of Calabar swelling tend to be more frequent, microfilaremia is less common, and eosinophilia and increased levels of antifilarial antibodies are characteristic. ■ ■PATHOLOGY The pathogenesis of the manifestations of loiasis is poorly understood. Calabar swellings are thought to result from a hypersensitivity reaction to adult worm antigens. ■ ■DIAGNOSIS Definitive diagnosis of loiasis requires the detection of microfilariae in the peripheral blood or the isolation of the adult worm from the eye (Fig. 240-4) or from a subcutaneous biopsy specimen collected from a site of swelling developing after treatment. PCR-based assays for the detection of L. loa DNA in blood are available in specialized laboratories and are highly sensitive and specific, as are some newer recombinant antigen–based serologic techniques. In practice, the diagnosis must often be based on a characteristic history and clinical presentation, blood eosinophilia, and elevated levels of antifilarial antibodies, particu­ larly in travelers to an endemic region, who are often amicrofilaremic.

TREATMENT Loiasis DEC (8–10 mg/kg per day administered orally for 21 days) is effec­ tive against both the adult and the microfilarial forms of L. loa, but multiple courses are frequently necessary before loiasis resolves completely. In cases of heavy microfilaremia, allergic or other inflammatory reactions can take place during treatment, including central nervous system involvement with coma and encephalitis. Heavy infections can be treated initially with apheresis to remove the microfilariae and with glucocorticoids (40–60 mg of predni­ sone per day) followed by low doses of DEC (0.5 mg/kg per day). If antifilarial treatment has no adverse effects, the prednisone dose can be tapered rapidly and the dose of DEC gradually increased to 8–10 mg/kg per day. Albendazole or ivermectin is effective in reducing microfilarial loads, although neither is approved for this purpose by the U.S. Food and Drug Administration. Moreover, ivermectin is contra­ indicated in patients with >30,000 microfilariae/mL because this drug has been associated with severe adverse events (including encephalopathy and death) in heavily infected patients with loiasis in West and Central Africa. DEC (300 mg weekly) is an effective prophylactic regimen for loiasis. STREPTOCERCIASIS Mansonella streptocerca, found mainly in the tropical forest belt of Africa from Ghana to the Democratic Republic of the Congo, is trans­ mitted by biting midges. The major clinical manifestations involve the skin and include pruritus, papular rashes, and pigmentation changes. Many infected individuals have inguinal adenopathy, although most are asymptomatic. The diagnosis is made by detection of the charac­ teristic microfilariae in skin snips. Ivermectin at a single dose of 150 μg/kg leads to sustained suppression of microfilariae in the skin and is probably the treatment of choice for streptocerciasis. MANSONELLA PERSTANS INFECTION M. perstans, distributed across the center of Africa and in northeastern South America, is transmitted by midges. Adult worms reside in serous cavities—pericardial, pleural, and peritoneal—as well as in the mesentery and the perirenal and retroperitoneal tissues. Microfilariae circulate in the blood without periodicity. The clinical and pathologic features of the infection are poorly defined. Most patients appear to be asymptomatic, but manifestations may include transient angioedema and pruritus of the arms, face, or other parts of the body (analogous to the Calabar swellings of loiasis); fever; headache; arthralgias; and right-upper-quadrant pain. Occasionally, pericarditis and hepatitis occur. The diagnosis is based on the demonstration of microfilariae in blood or serosal effusions. Perstans filariasis is often associated with peripheral-blood eosinophilia and antifilarial antibody elevations. With the identification of a Wolbachia endosymbiont in M. perstans, doxycycline (200 mg twice a day) for 6 weeks has been established as the first effective treatment for this infection. MANSONELLA OZZARDI INFECTION The distribution of M. ozzardi is restricted to Central and South America and certain Caribbean islands. Adult worms are rarely recovered from humans. Microfilariae circulate in the blood without periodicity. Although this organism has often been considered nonpathogenic, headache, articular pain, fever, pulmonary symptoms, adenopathy, hepatomegaly, pruritus, and eosinophilia have been ascribed to M. ozzardi infection. The diagnosis is made by detection of microfilariae in peripheral blood. Ivermectin is effective in treating this infection. ZOONOTIC FILARIAL INFECTIONS Dirofilariae that affect primarily dogs, cats, and raccoons occasionally infect humans incidentally, as do Brugia and Onchocerca parasites that affect small mammals. Because humans are an abnormal host, the

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241 Schistosomiasis and Other Trematode Infections

parasites never develop fully. Pulmonary dirofilarial infection caused by the canine heartworm Dirofilaria immitis generally presents in humans as a solitary pulmonary nodule. Chest pain, hemoptysis, and cough are uncommon. Infections with Dirofilaria repens (from dogs) or Dirofilaria tenuis (from raccoons) can cause local subcutaneous nodules in humans. Zoonotic Brugia infection can produce isolated lymph node enlargement, whereas zoonotic Onchocerca species (par­ ticularly O. lupi) can cause subconjunctival masses. Eosinophilia levels and antifilarial antibody titers are not commonly elevated. Excisional biopsy is both diagnostic and curative. These infections usually do not respond to antifilarial chemotherapy. DRACUNCULIASIS (GUINEA WORM INFECTION) ■ ■ETIOLOGY AND EPIDEMIOLOGY The incidence of dracunculiasis, caused by Dracunculus medinensis, has declined dramatically because of global eradication efforts. How­ ever, between 2017 and 2020, there were increases in the number of human cases, followed by a decline in 2021 and 2022. At the end of 2022, there were a total of 13 human cases of Guinea worm disease across four African countries, with 6 cases in Chad, 5 cases in South Sudan, and 1 each in Ethiopia and Central African Republic. Low-level persistence of infection in animal reservoirs (dogs and cats) is a major challenge to ongoing eradication efforts. Humans acquire D. medinensis when they ingest water containing infective larvae derived from Cyclops, a crustacean that is the inter­ mediate host. Larvae penetrate the stomach or intestinal wall, mate, and mature. The adult male probably dies; the female worm develops over a year and migrates to subcutaneous tissues, usually in the lower extremity. As the thin female worm, ranging in length from 30 cm to 1 m, approaches the skin, a blister forms that, over days, breaks down and forms an ulcer. When the blister opens, large numbers of motile, rhabditiform larvae can be released into stagnant water; ingestion by Cyclops completes the life cycle. ■ ■CLINICAL FEATURES Few or no clinical manifestations of dracunculiasis are evident until just before the blister forms, when there is an onset of fever and gen­ eralized allergic symptoms, including periorbital edema, wheezing, and urticaria. The emergence of the worm is associated with local pain and swelling. When the blister ruptures (usually as a result of immer­ sion in water) and the adult worm releases larva-rich fluid, symptoms are relieved. The shallow ulcer surrounding the emerging adult worm heals over weeks to months. Such ulcers, however, can become second­ arily infected, the result being cellulitis, local inflammation, abscess formation, or (uncommonly) tetanus. Occasionally, the adult worm does not emerge but becomes encapsulated and calcified. ■ ■DIAGNOSIS The diagnosis is based on the findings developing with the emergence of the adult worm, as described above. TREATMENT Dracunculiasis Gradual extraction of the worm by winding of a few centimeters on a stick each day remains the common and effective practice. Worms may be excised surgically. No drug is effective in treating dracunculiasis. ■ ■PREVENTION Prevention, which remains the only real control measure, depends on the provision of safe drinking water. Acknowledgment The authors wish to acknowledge and thank Peter F. Weller, MD, author of prior editions of this chapter.

■ ■FURTHER READING Hopkins DR et al: Progress toward global eradication of dracunculiasis—

January 2022–June 2023. Morb Mortal Wkly Rep 72:1230, 2023. King CL et al: Single-dose triple-drug therapy for Wuchereria bancrofti— 5-year follow-up. N Engl J Med 382:1956, 2020. Opoku NO et al: Single dose moxidectin versus ivermectin for Onchocerca volvulus infection in Ghana, Liberia, and the Democratic Republic of the Congo: A randomised, controlled, double-blind phase 3 trial. Lancet 392:1207, 2018. Ramharter M et al: The African eye worm: Current understanding of the epidemiology, clinical disease, and treatment of loiasis. Lancet Infect Dis 24:e165, 2024. Taylor MJ et al: Lymphatic filariasis and onchocerciasis. Lancet 376:1175, 2010. VIDEO 240-1  Filarial dance sign. Ultrasound showing dilated lymphatic vessel (upper right) containing live, motile, adult worms.

Schistosomiasis and

Other Trematode

Infections CHAPTER 241 Birgitte Jyding Vennervald,

Anna-Sofie Stensgaard Schistosomiasis and Other Trematode Infections
Trematodes, or flatworms, are a group of helminths that belong to the phylum Platyhelminthes. The adult flatworms share some common characteristics, such as macroscopic size (from one to several centi­ meters); dorsoventrally flattened, bilaterally symmetric bodies; and two suckers—oral and ventral. Except for schistosomes, which have separate sexes, all human parasitic trematodes are hermaphroditic. Their life cycles involve a mammalian/human definitive host, in which sexual reproduction by adult worms takes place, and an intermediate host (snails), in which asexual multiplication occurs. Some species of trematodes have more than one intermediate host. Humans are infected either by direct penetration of intact skin (schistosomiasis) or by ingestion of raw freshwater fish, crustaceans, or aquatic plants with metacercariae—the infective larval stage. Significant trematode infections of humans may be divided according to the location of the adult worms: blood, liver (biliary tree), intestines, or lungs (Table 241-1). Adult worms do not multiply within the mammalian host but can live for up to 30 years. Infections are often chronic. Although it is relatively rare to encounter patients with trematode infections in the United States, many millions of people are infected worldwide. Both schistosomiasis and food-borne trematode infections are poverty-related chronic diseases with high morbidity and a signifi­ cant public health impact. Various factors may increase the spread of the infections globally. Increasing temperatures may render new areas suitable for the intermediate host snails, and an increase in travel and migration may increase the number of patients with trematode infections—for example, in the United States. APPROACH TO THE PATIENT Trematode Infection In the evaluation of a patient in whom trematode infection is sus­ pected, certain questions are highly relevant and can assist in estab­ lishing a diagnosis: Where have you been? If you have traveled, when

TABLE 241-1  Major Human Trematode Infections GEOGRAPHIC DISTRIBUTION TREMATODE TRANSMISSION ROUTE Blood Flukes Intestinal schistosomiasis Schistosoma mansoni Skin penetration by cercariae released from snails (Biomphalaria spp.) Africa, Brazil, Venezuela, Surinam, the Caribbean

(low risk) Shistosoma japonicum Skin penetration by cercariae released from snails (Oncomelania spp.) China, Indonesia, Philippines Schistosoma guineensis and Schistosoma intercalatum Skin penetration by cercariae released from snails (Bulinus spp.) Rainforest areas of Central Africa Schistosoma mekongi Skin penetration by cercariae released from snails (Neotricula aperta) Several districts of Cambodia and Lao People’s Democratic Republic (PDR) Urogenital schistosomiasis Schistosoma haematobium Skin penetration by cercariae released from snails (Bulinus spp.) Africa, Middle East, Corsica (France) Liver Flukes Clonorchis sinensis Ingestion of metacercariae in freshwater fish Asia, including Republic of Korea, China, Taiwan, Vietnam Opisthorchis viverrini Ingestion of metacercariae in freshwater fish Northeast Thailand, Lao PDR, Cambodia, Vietnam Opisthorchis felineus Ingestion of metacercariae in freshwater fish Former Soviet Union, Kazakhstan, Ukraine,

Turkey PART 5 Infectious Diseases Fasciola hepatica Ingestion of metacercariae on aquatic plants or in

water Worldwide Fasciola gigantica Ingestion of metacercariae on aquatic plants or in

water Africa, Asia Intestinal Flukes Fasciolopsis buski Ingestion of metacercariae on aquatic plants Bangladesh, China, India, Indonesia, Lao PDR, Malaysia, Taiwan, Thailand, Vietnam Echinostoma spp. Ingestion of freshwater fish, frogs, mussels, snails China, India, Indonesia, Japan, Malaysia, Russia, Republic of Korea, Philippines, Thailand Heterophyes heterophyes, several other species Ingestion of metacercariae in freshwater or brackishwater fish Egypt, Greece, Islamic Republic of Iran, Italy, Japan, Republic of Korea, Sudan, Tunisia, Turkey Lung Flukes Paragonimus westermani Ingestion of metacercariae in crayfish or crabs Tropical and subtropical areas of eastern and southern Asia and

sub-Saharan Africa Paragonimus kellicotti Ingestion of metacercariae in crayfish or crabs North America did you return? What activities have you been involved in (trekking, swimming, whitewater rafting)? What have you been eating (local dishes while traveling; raw, poorly cooked, or pickled freshwater fish or crustaceans)? Definitive diagnosis is based on detection of parasite eggs in stool, urine, sputum, and sometimes tissue samples or on serologic tests. The presence of eosinophilia and a history of travel to endemic areas should raise suspicion of trematode infection. The U.S. Centers for Disease Control and Prevention (CDC) can provide guidance with respect to diagnosis and treatment.

SCHISTOSOMIASIS Human schistosomiasis is caused by six species of the parasitic genus Schistosoma: S. mansoni, S. japonicum, S. mekongi, S. intercalatum, and the recently described S. guineensis cause intestinal disease, and S. haematobium causes urogenital disease (Table 241-1). The infection may cause considerable intestinal, hepatic, and genitourinary morbid­ ity. Avian schistosomes may penetrate human skin but die in subcuta­ neous tissue, producing only cutaneous manifestations. ■ ■ETIOLOGY Schistosoma infection is contracted through contact with freshwater bodies harboring infected intermediate-host snails (Fig. 241-1). Cer­ cariae, the infective larval stage released from the snail, penetrate intact human skin within a few minutes after attaching to the skin. After pen­ etration, the cercariae transform to schistosomula, which then enter a small vein or lymphatic vessel, circulate in the bloodstream through the lung capillaries, and are pumped via the heart to all parts of the body to reach the portal vein. There, the worms mature into adult males or females, pair, and migrate to their final location in the mesenteric or pelvic venous plexus. The interval from cercarial penetration to sexual maturation and egg production, termed the prepatent period, lasts 5–7 weeks (up to 12 weeks for S. haematobium). The female worm then begins to produce eggs, which are excreted via feces or, for S. haematobium, urine. Approxi­ mately 50% of eggs are retained in tissue, where they are responsible for organ-specific morbidity (see “Pathogenesis,” below). When excreted eggs reach water, they hatch and release a free-swimming larval stage (miracidium), which, after penetrating a host snail, undergoes sev­ eral rounds of asexual multiplication. After ~4–6 weeks, infective cercariae are shed from the infected snails into the water. One snail, infected by one miracidium, can shed thousands of cercariae per day for several months; thus, the transmission potential of schistosomes is enormous. Environmental temperature is an important determinant of the development rate and the number of cercariae produced and released by the snails. The schistosome egg (Fig. 241-2) is the only stage of the parasites’ life cycle that can be detected in humans, either in excreta or in tissue biopsies. The eggs are large and can easily be distinguished morpho­ logically from other helminth eggs. S. haematobium eggs are ~140 µm long, with a terminal spine; S. mansoni eggs are ~150 µm long, with a lateral spine; and S. japonicum eggs are smaller, rounder, and ~90 mm long, with a small lateral spine or knob. Adult schistosomes are ~1–2 cm long. The male worm is flat, and the body forms a groove or gynecophoric canal in which the mature adult female is held like a sausage in a hotdog roll. Females are longer, thinner, and rounded. The females produce hundreds (African species) to thousands (Asian species) of eggs per day. Each ovum contains a ciliated miracidium larva, which secretes proteolytic enzymes that help the eggs to migrate into the lumen of the bladder (S. haematobium) or the intestine (other species). The lifespan of an adult schistosome aver­ ages 3–5 years but can be as long as 30 years. Schistosome worms feed on red blood cells; the debris is regurgitated in the host’s blood, where it can be detected as circulating antigens (see “Diagnosis,” below). Adult schistosomes persist in the bloodstream for years and have evolved strategies of evading attack using immune effector mecha­ nisms. This immune evasion is a result of several processes, such as binding of host proteins to the schistosome surface, which renders the parasite invisible to the host immune system. The genome of schistosomes is relatively large (~300 Mb). Wholegenome sequences are available for S. mansoni, S. japonicum,

and S. haematobium. ■ ■EPIDEMIOLOGY Because of the complex life cycle of schistosomes, with snails as an intermediate host and humans as the final host, transmission is depen­ dent on freshwater habitats that are suitable for the snails, are areas of human activity, and have climatic conditions favoring the survival of the snails and the development of the parasites inside the snail host. These requirements are reflected in the global distribution of

Infected definitive host Definitive host Cercaria Snail (intermediate host) FIGURE 241-1  Life cycle of schistosomiasis. schistosomiasis as well as in its microgeographic distribution within an endemic area. For S. mansoni, S. haematobium, S. intercalatum, and S. guineensis, humans are the most important definitive host.

S. japonicum and S. mekongi are zoonotic parasites, with a wide range of definitive hosts such as pigs, water buffaloes, and various rodents. It is estimated that at least 251.4 million people required preven­ tive treatment in 2021, of whom more than 75.3 million people were reported to have been treated. Schistosomiasis transmission has been reported from 78 countries, of which 51 endemic countries have mod­ erate to high transmission (Fig. 241-3). More than 70% of infected people live in sub-Saharan Africa. Schistosomiasis is the most impor­ tant of the neglected tropical diseases and is second only to malaria in public health impact. It is a poverty-related disease, and infection is prevalent in areas where adequate water supplies and sanitary facilities are lacking. In these areas, people come into contact with infested water through a variety of activities, including bathing, washing clothes, and collecting water for drinking or cooking. In some areas, adults have a high occupational risk of exposure; fishermen, canal cleaners, and workers in rice fields fall into this category. Among children, playing in FIGURE 241-2  Schistosoma haematobium eggs.

water and swimming pose a risk. Large-scale irri­ gation and hydroelectric power operations can create suitable habitats for host snails and thus increase the risk of schistosomiasis transmission.

In general, children living in endemic areas initially acquire infection at ~3–4 years of age— i.e., when they are old enough to walk and come into contact with infested water. However, infection does occur in much younger chil­ dren. As children grow older, the prevalence and intensity of infection increase, peaking around puberty. A characteristic feature of schistosomia­ sis infection in human populations is a convex age–prevalence curve, with low prevalence in very young children, higher prevalence in older children with a peak at 10–15 years of age, and declining prevalence in adults. The same pat­ tern is observed between age and intensity of infection and is attributable to various factors. Generally, children have more frequent, pro­ longed, and extensive water contact than adults through activities like playing and swimming. Furthermore, several studies have indicated that acquired immunity to schistosomiasis develops slowly over several years, so that adults are reinfected to a much lesser extent than children. These factors, combined with progressive spon­ taneous death of adult worms from infections acquired during childhood, lead to lower levels of infection in the adult population. Eggs shed in urine and stool Eggs CHAPTER 241 It is important to note that ongoing climate change, resulting in ris­ ing water temperatures and altered precipitation patterns, affects the distribution, reproduction, survival, and dispersal of the intermediate host snails, as well as the rate of parasite development within the snail. Therefore, climate change could considerably alter the distribution and abundance of the intermediate host snail and its schistosome parasites, resulting in notable shifts in the global distribution, disease dynamics, and transmission of schistosomiasis in the future. Schistosomiasis and Other Trematode Infections
■ ■PATHOGENESIS Cercarial invasion may be associated with dermatitis arising from dermal and subdermal inflammatory reactions in response to dying cercariae that trigger innate immune responses. However, most mani­ festations of schistosomiasis—in the acute, established, and chronic phases of infection—are due to immunologic reactions to eggs retained in host tissues. Around the time when oviposition commences, acute schistoso­ miasis (Katayama fever) may occur (see “Clinical Features,” below). Antigen excess from eggs results in the formation of soluble immune complexes, which may be deposited in several tissues and initiate a serum sickness–like illness. All evidence suggests that schistosome eggs, and not adult worms, induce the organ-specific morbidity caused by schistosome infections. Approximately half of the eggs are not excreted via feces or urine but are trapped in intestinal or hepatic tissue (S. mansoni, S. japonicum, and S. mekongi) or in the bladder and urogenital system (S. haematobium). The eggs induce a granulo­ matous host immune response composed primarily of lymphocytes, eosinophils, and alternatively activated macrophages. The lymphocytes produce various TH2 cytokines such as interleukins 4, 5, and 13. Later, in the chronic phase of infection, regulatory cytokines are respon­ sible for immunomodulation or downregulation of host responses to schistosome eggs and play an important role in reducing the size of granulomas. When S. mansoni or S. japonicum eggs are swept into the small portal branches of the liver via the portal vein, they lodge in the pre­ sinusoidal periportal tissues. The formation of granulomas around the eggs can cause significant enlargement of the spleen and liver. High-intensity infections in children are often accompanied by hepa­ tosplenomegaly that generally decreases over time, partly because the

B A FIGURE 241-3  Global distribution of human schistosomiasis. A. Schistosoma mansoni infection (dark blue) is endemic in Africa, the Middle East, South America, and a few Caribbean countries. S. intercalatum infection (green) is endemic in sporadic foci in West and Central Africa. B. Schistosoma haematobium infection (purple) is endemic in Africa and the Middle East. The major endemic countries for S. japonicum infection (green) are China, the Philippines, and Indonesia. Schistosoma mekongi infection (red) is endemic in sporadic foci in Southeast Asia. (Reprinted from CH King, AAF Mahmoud: Schistosomiasis and other trematode infections, in DL Kasper et al [eds], Harrison’s Principles of Internal Medicine, 19th ed. New York, McGraw-Hill Education, 2015, pp 1423–1429.) PART 5 Infectious Diseases number of eggs being deposited in the tissue gradually declines after the early teenage years as partial immunity to new infections develops and partly because of immunologic downregulation of the granulo­ matous response. However, in some infected individuals, egg-induced granulomatous responses lead to severe periportal fibrosis (Symmers clay pipestem fibrosis), with deposition of collagen around the portal vein, occlusion of the smaller portal branches, and severe, often irre­ versible, pathology. Occlusion of the portal branches may result in marked portal hypertension (Fig. 241-4). The signs and symptoms of S. haematobium infection relate to the worms’ predilection for the veins of the urogenital plexus and result from deposition of eggs in the bladder, ureters, and genital organs. During established active infection, clusters of living eggs in the urogenital tissues can be found surrounded by intense inflammatory reactions and intense tissue eosinophilia. Movement of egg clusters into the lumen of the bladder is often followed by sloughing off of the FIGURE 241-4  Portal hypertension with ascites. (Photo courtesy of Birgitte J. Vennervald.)

epithelial surface, ulceration, and bleeding. Intense egg-induced tissue inflammation can result in bladder wall thickening and development of masses and pseudopolyps. Inflammation and granuloma formation around the ureteral ostia can lead to hydronephrosis. Generally, late chronic-stage infections are characterized by accu­ mulation of dead calcified eggs in tissue. Characteristic cervical lesions are found in S. haematobium infections, including active-stage lesions with intense tissue inflammation around live eggs and chronic-stage sandy patches with clusters of calcified eggs. ■ ■CLINICAL FEATURES In general, disease manifestations of schistosomiasis occur in three stages—acute, active, and chronic—according to the duration and intensity of infection. Cercarial Dermatitis (“Swimmer’s Itch”)  Cercarial penetra­ tion of the skin may result in a maculopapular rash called cercarial dermatitis or “swimmer’s itch.” Cercarial dermatitis can develop in people who have not previously been exposed to schistosomiasis (e.g., travelers), whereas it is rare among people living in endemic areas. A particularly severe form of cercarial dermatitis is commonly seen after exposure to cercariae from avian schistosomes. These cercariae cannot complete their development in humans and die in the skin, causing an inflammatory allergic reaction. This form of cercarial dermatitis can occur in people who have been in contact with water from lakes (e.g., in Europe or the United States) where various species of water birds, such as ducks, geese, and swans, are found. The rash may last for 1–2 weeks. This condition normally requires no treatment, but sys­ temic antihistamines, topical antihistamines, or glucocorticoids can be used to reduce symptoms. Acute Schistosomiasis (Katayama Fever)  Symptomatic acute schistosomiasis, also known as Katayama fever or Katayama syndrome, is usually seen in travelers who have contracted the infection for the first time. The onset occurs between 2 weeks and 3 months after expo­ sure to the parasite. The symptoms may appear suddenly and include fever, myalgia, general malaise and fatigue, headache, nonproductive cough, and intestinal symptoms such as abdominal tenderness or pain. Various combinations of these symptoms are often accompanied by eosinophilia and transient pulmonary infiltrates. Many patients

recover spontaneously from acute schistosomiasis after 2–10 weeks, but the illness follows a more severe clinical course in some individuals, with weight loss, dyspnea, diarrhea, and hepatomegaly. Severe cerebral or spinal cord manifestations may occur, and even light infections may cause severe illness. The syndrome can, in rare cases, be fatal. Differential diagnosis includes many other febrile infectious dis­ eases with acute onset, including malaria, salmonellosis, and acute hep­ atitis. Fever and eosinophilia occur in trichinosis, tropical eosinophilia, invasive ankylostomiasis, strongyloidiasis, visceral larva migrans, and infections with Opisthorchis and Clonorchis species. Katayama fever is rare in people chronically exposed to infection in areas endemic for S. mansoni or S. haematobium. Intestinal Schistosomiasis (S. mansoni, S. japonicum)  In intes­ tinal schistosomiasis, adult worms are located in the mesenteric veins, and disease manifestations are associated with parasite eggs passing through or becoming trapped in intestinal tissue. This event induces mucosal granulomatous inflammation with microulcerations, super­ ficial bleeding, and sometimes pseudopolyposis. The symptoms tend to be more pronounced with a high intensity of infection and include intermittent abdominal pain, loss of appetite, and sometimes bloody diarrhea. The clinical manifestations of S. intercalatum, S. guineensis, and S. mekongi infection are generally milder. Hepatosplenic Schistosomiasis  Hepatosplenic schistosomiasis is caused by schistosome eggs trapped in liver tissue and occurs in S. mansoni and S. japonicum infections. There are two distinct clinical entities: early inflammatory hepatosplenomegaly and late hepatosplenic disease with periportal fibrosis. Early inflammatory hepatosplenic schistosomiasis is the main entity seen in children and adolescents. The liver is enlarged, especially the left lobe, and is smooth and firm. The spleen is enlarged, often extending below the umbilicus, and is firm or hard. Generally, ultraso­ nography shows no hepatic fibrosis. This form of hepatosplenic schis­ tosomiasis may be found in up to 80% of infected children. Its severity is closely associated with the intensity of infection and may also be associated with concomitant chronic exposure to malaria. Late hepatosplenic schistosomiasis with periportal or Symmers fibrosis may develop in young and middle-aged adults with long-standing, high-level exposure to infection. Patients with periportal fibrosis may excrete very few or no eggs in feces. During the early stage, the liver is enlarged, especially the left lobe; it is smooth and firm or hard. The spleen is enlarged, often massively, and is firm or hard. The patient may report a left hypochondrial mass with discomfort and anorexia. Ultrasonography reveals typical periportal fibrosis and dilation of the portal vein. Other complications include delayed growth and puberty, especially in S. japonicum infections, and severe anemia. Severe hepa­ tosplenic schistosomiasis may lead to portal hypertension, but hepatic function usually remains normal, even in cases with marked periportal fibrosis and portal hypertension. Ascites, attributable both to portal hypertension and to hypoalbu­ minemia, may be seen, especially in S. japonicum infection. Patients with severe hepatosplenic disease and portal hypertension may develop esophageal varices detectable by endoscopy or ultrasound. These patients may experience repeated bouts of hematemesis, melena, or both. Hematemesis is the most severe complication of hepatosplenic schistosomiasis, and death may result from massive loss of blood. Urogenital Schistosomiasis (S. haematobium)  The signs and symptoms of S. haematobium infection relate to the worms’ predilec­ tion for the veins of the urogenital tract. Two stages of infection are recognized. An active stage occurring mainly in children, adolescents, and younger adults is characterized by egg excretion in the urine, with proteinuria and macroscopic or microscopic hematuria and deposi­ tion of eggs in the urinary tract. A chronic stage in older individuals is characterized by sparse or no urinary egg excretion despite urogenital tract pathology. A characteristic sign in the active stage is painless, terminal hema­ turia. Dysuria and suprapubic discomfort or pain are associated with active urogenital schistosomiasis and may persist throughout the

course of active infection. Eggs deposited in the bladder mucosa may give rise to an intense inflammatory response of the bladder wall, which may cause ureteric obstruction and lead to hydroureter and hydronephrosis. These early inflammatory lesions, including obstruc­ tive uropathy, can be visualized by ultrasonography.

As the infection progresses, the inflammatory component decreases and fibrosis becomes more prominent. The symptoms at this stage are nocturia, urine retention, dribbling, and incontinence. Cystoscopy reveals “sandy patches” composed of large numbers of calcified eggs surrounded by fibrous tissue and an atrophic mucosal surface. The ureters are less commonly involved, but ureteral fibrosis can cause irreversible obstructive uropathy that can progress to uremia. Egg deposition may cause granulomas and lesions in the genital organs, most commonly in the cervix and vagina in women and the seminal vessels in men. The results may include dyspareunia, abnormal vaginal discharge, contact bleeding, and lower back pain in women and perineal pain, painful ejaculation, and hematospermia in men. Genital symptoms like bloody discharge and genital itch are associated with S. haematobium infection in school-aged girls living in schistosomiasis-endemic areas. Symptoms such as hematospermia and perineal discomfort have been described in travelers, and eggs have been demonstrated in seminal fluid. An association between female genital schistosomiasis and HIV infection has been demonstrated, but the impact of genital schistosomiasis on HIV transmission needs further elucidation. S. haematobium has been classified by the International Agency for Research on Cancer (IARC) as definitely carcinogenic to humans (i.e., a group 1 carcinogen). Chronic S. haematobium infection is associated with squamous cell carcinoma of the urinary bladder. CHAPTER 241 Other Manifestations  Worms and eggs can sometimes be located in ectopic sites, causing site-specific manifestations and symptoms. Neuroschistosomiasis is one of the most severe clinical forms of schis­ tosomiasis and is caused by the inflammatory response around eggs in the cerebral or spinal venous plexus. S. mansoni and S. haematobium worms can end up in the spinal venous plexus, where they may cause transverse myelitis—an acute complication sometimes seen in travelers returning home with schistosomiasis. S. japonicum is mainly associated with granulomatous lesions in the brain, causing epileptic seizures, encephalopathy with headache, visual impairment, motor deficit, and ataxia. Pulmonary schistosomiasis is caused by portacaval shunting of eggs into the lung capillaries, where they induce granulomas in the perialveolar area. The consequences may be fibrosis, pulmonary hyper­ tension, and cor pulmonale. Schistosomiasis and Other Trematode Infections
■ ■DIAGNOSIS Anamnestic information on recent travels to endemic areas and expo­ sure to freshwater bodies through recreational or other activities is important in the diagnosis of schistosomiasis in travelers. Information about exact geographic locations can facilitate identification of the relevant species of Schistosoma. Eosinophilia is a common finding and is often associated with helminthic infections such as schistosomiasis. Detection of schistosome eggs in stool or urine is indicative of active infection and is the standard diagnostic method. The diagnosis is often based on the detection of eggs in a fixed small amount of excreta—e.g., 50 mg of stool or filtration of 10 mL of urine. This method is widely used among populations in endemic areas and allows quantitation of the level of infection (eggs per gram of feces or per 10 mL of urine). However, levels of egg excretion in people from nonendemic areas may be very low, in which case a larger sample and concentration methods (e.g., formol-ether concentration) may be needed. Eggs can also be detected in rectal biopsies (both S. mansoni and S. haematobium) and occasionally in Pap smears and semen samples (S. haematobium). Polymerase chain reaction (PCR)–based detection of parasite DNA in stool or urine is more sensitive than parasitologic methods and is increasingly used. Schistosoma DNA can be detected in cerebrospinal fluid samples for diagnosis of neuroschistosomiasis. Serology, with detection of specific antibodies to schistosomes, is useful in travelers but less so in people from endemic areas where

transmission is ongoing. The serologic assays employed at the CDC are a Falcon assay screening test/enzyme-linked immunosorbent assay (FAST-ELISA) using S. mansoni adult microsomal antigen and a con­ firmatory species-specific immunoblot assay performed in light of the patient’s travel history.

Schistosome proteoglycans—circulating anodic and cathodic anti­ gens (CAAs and CCAs)—regurgitated into the bloodstream by the feeding worms can be detected in serum and urine by ELISA or monoclonal antibody–based lateral flow assays. The presence of CAA or CCA is an indication of active infection, and levels of these antigens correlate well with the intensity of infection. However, detection of CAAs and CCAs is not currently suitable for diagnosis in travelers, who are likely to have low levels of infection and very few worms, but promising results have been obtained using an ultrasensitive lateral flow assay. A commercially available point-of-care assay (Rapid Medical Diagnostics, Pretoria, South Africa) that detects CCA in urine is now widely used for screening of infected communities in relation to mass drug administration programs. TREATMENT Schistosomiasis The drug of choice for treatment of schistosomiasis is praziquantel. It is administered orally, is available as 600-mg tablets, and is effec­ tive against all schistosome species infecting humans. The drug is safe and well tolerated. Standard regimens are shown in Table 241-2. In patients who are not cured by initial treatment, the same dose can be repeated at weekly intervals for 2 weeks. Since praziquantel does not affect the young migrating stages of the schistosomes, it may be necessary to repeat the dose 6–12 weeks later, especially if eosinophilia or symptoms persist despite treatment. PART 5 Infectious Diseases As a general principle, all patients with acute schistosomiasis should be treated with praziquantel. Glucocorticoids can be added in Katayama fever to suppress the hypersensitivity reaction. How­ ever, treatment for acute schistosomiasis or Katayama fever must be adjusted appropriately for each case, and in the most severe cases, management in an acute-care setting is necessary. Praziquantel is effective in cerebral S. japonicum infections, resulting in rapid dissipation of cerebral edema and resolution of TABLE 241-2  Treatment of Schistosomiasis and Food-Borne Trematode Infections INFECTION DRUG OF CHOICE ADULT DOSEa Schistosoma mansoni,

S. haematobium,

S. intercalatum, S. guineensis Praziquantelb 40 mg/kg PO in 2 divided doses for 1 day S. japonicum, S. mekongi Praziquantel 60 mg/kg PO in 3 divided doses for 1 day Clonorchis sinensis, Opisthorchis viverrini, Opisthorchis felineus Praziquantel 25 mg/kg PO tid for

2 consecutive days Fasciola hepatica, Fasciola gigantica Triclabendazolec 2 doses of 10 mg/kg PO given 12 h apart Fasciolopsis buski Praziquantel 75 mg/kg PO in 3 divided doses for 1 day Echinostoma spp., Heterophyes heterophyes, several other species Praziquantel 25 mg/kg PO tid Paragonimus westermani, Paragonimus kellicotti Praziquantel Triclabendazolec 25 mg/kg PO tid for

2 consecutive days 10 mg/kg PO once (or twice, 12–24 h apart) aThe pediatric dose is the same as the adult dose in all instances. bThe safety of praziquantel in children <4 years old has not been established, although many children in this age group have been treated with praziquantel during mass drugadministration programs. cIn February 2019, the U.S. Food and Drug Administration (FDA) approved triclabendazole for treatment of fascioliasis in patients at least

6 years of age.

cerebral masses. However, glucocorticoids and anticonvulsants are sometimes needed in neuroschistosomiasis. The effect of antischistosomal treatment on disease manifesta­ tions depends on the stage and severity of the lesions. Early hepa­ tosplenomegaly, mild or moderate fibrosis, and urinary bladder lesions seen during active infection resolve after chemotherapy. However, for late-stage manifestations (e.g., severe fibrosis with portal hypertension), praziquantel treatment is only one com­ ponent of management, since the main complications are due to obstructive pathology. Management of portal hypertension and prevention of bleeding from esophageal varices should follow clini­ cal guidelines for treatment of these conditions. ■ ■PREVENTION AND CONTROL Schistosomiasis is contracted through direct contact with infested freshwater. Travelers should be made aware of the risk of infection if they come into contact with freshwater sources in schistosomiasisendemic areas. For people living in rural areas where schistosomiasis is endemic, it may be very difficult, if not impossible, to avoid water contact—for example, during occupational activities such as fishing and working in rice fields. Schistosomiasis is a poverty-related disease, and access to safe water and good sanitary facilities may rarely be avail­ able. Because S. japonicum is a zoonotic parasite, preventive measures should target not only the human population but also animals such as water buffalo, which act as reservoirs for infection. Praziquantel treatment of infected people, often during mass drugadministration programs, is a cornerstone of the management and control of schistosomiasis. Regular treatment will reduce the level of schistosomiasis morbidity in affected populations. However, treatment should be combined with other relevant strategies, such as control of the intermediate host snails, improved water-quality and sanitation facilities, and health education. Schistosomiasis control measures should be integrated into local health programs. There have been intensive efforts to develop vaccines, but none is yet available. Three vaccine candidates have been tested in clinical phase 1 or 2 trials. Only one candidate, S. haematobium 28GST, has been tested in a clinical phase 3 trial in populations living in an endemic area. The vaccine candidate was immunogenic and well tolerated by infected children, but a sufficient efficacy was not reached. FOOD-BORNE TREMATODE INFECTIONS Food-borne trematode infections are a group of zoonotic diseases caused by hepatic, intestinal, and pulmonary parasitic flukes. These infections are contracted by ingestion of infective parasites in under­ cooked aquatic food or water plants. Infections can cause severe liver and lung disease, and together, this may result in 2 million life-years lost to disability and death worldwide every year. ■ ■LIVER FLUKES The most important liver flukes causing human infections are the related species Opisthorchis viverrini and Opisthorchis felineus, which cause opisthorchiasis; Clonorchis sinensis, which causes clonorchiasis; and Fasciola hepatica and Fasciola gigantica, which cause fascioliasis (Table 241-1). Opisthorchiasis and Clonorchiasis  O. viverrini is found mainly in northeastern Thailand, Laos, and Cambodia; O. felineus mainly in Europe and Asia, including the former Soviet Union; and C. sinensis in Asia, including Korea, China, Taiwan, Vietnam, Japan, and Asian regions of Russia. Parasite eggs excreted from infected humans or animals are ingested by a host snail (the first intermediate host), where they undergo several developmental stages. Cercariae are then released from the snail and penetrate freshwater fish (the second intermediate host), encysting as metacercariae in the muscles or under the scales. Humans become infected by eating raw or undercooked fish from endemic countries. After ingestion, the metacercariae excyst in gastric juices and migrate via the duodenum, the ampulla of Vater, and the extrahepatic biliary system to the intrahepatic bile ducts.

TABLE 241-3  Clinical Features of Food-Borne Trematode Infections   SYMPTOMS OR SIGNS   INFECTION COMPLICATIONS EARLY OR ACUTE STAGE ESTABLISHED OR CHRONIC STAGE Liver Flukes Clonorchis sinensis, Opisthorchis viverrini, Opisthorchis felineus Often asymptomatic; sometimes hepatitis-like symptoms and high fever (especially with O. felineus) Fasciola hepatica, Fasciola gigantica Acute onset (1–4 weeks after infection) with high fever, weight loss, sometimes urticaria and liver tenderness Intestinal Flukes Fasciolopsis buski, Echinostoma spp., Heterophyes heterophyes, several other species Often asymptomatic; sometimes nonspecific gastrointestinal symptoms Lung Flukes Paragonimus westermani, Paragonimus kellicotti Often asymptomatic; sometimes insidious onset with anorexia and weight loss aCarcinogenesis has not yet been established for O. felineus. The clinical manifestations of infection with Opisthorchis species and C. sinensis are similar. Pathologic changes are typically seen in the bile ducts, liver, and gallbladder (Table 241-3). Tissue damage and intense inflammation are caused by mechanical and chemical irrita­ tion and immune responses to worms or worm products, and chronic inflammation may result in the development of cholangiocarcinoma. Both O. viverrini and C. sinensis are classified by the IARC as definitely carcinogenic (class 1). Acute and light infections are mostly asymptom­ atic, but hepatitis-like signs and symptoms, with high fever and chills, have been reported, especially in O. felineus infections. In general, only heavily infected people have symptoms and severe complications (Table 241-3). The diagnosis of these infections is based on microscopic identifica­ tion of parasite eggs in stool specimens. The eggs of Opisthorchis are indistinguishable from those of Clonorchis. Fascioliasis  Fascioliasis occurs in many areas of the world and usu­ ally is caused by Fasciola hepatica, a common liver fluke of sheep and cattle. F. hepatica is found in more than 50 countries on all continents except Antarctica; F. gigantica is less widespread. The areas with the highest known rates of human Fasciola infection are in the Andean highlands of Bolivia and Peru. In other areas where fascioliasis is found, human cases are sporadic. Unlike the other liver flukes, Fasciola species have no second intermediate host, as their infectious metacercariae adhere directly to aquatic plants. Humans usually acquire infection by ingesting aquatic plants, such as watercress, that contain viable metacercariae or by drinking water with free metacercariae. After metacercariae have excysted in the duodenum, Fasciola spe­ cies migrate through the intestinal wall into the body cavity, penetrate the liver capsule, and move through the liver into the bile ducts. This migration route is different from that of other liver flukes and gives rise to symptoms during the acute migratory phase; the parasites may cause tissue destruction, focal bleeding, and inflammation. Some migrating flukes may deviate from their usual route to cause ectopic infections. In the established latent stage of infection, the parasites may cause bile duct inflammation, resulting in thickening and expansion of the ducts, fibrosis, and ultimately biliary obstruction (Table 241-3). Although some infected people are asymptomatic in the latent phase, others may experience repeated relapses of acute manifestations. The most widely used diagnostic approach is direct detection of Fasciola eggs by microscopic examination of stool or of duodenal or

Biliary colic, cholestatic jaundice, recurrent cholangitis and cholelithiasis; hepatomegaly, gallbladder enlargement, periductal fibrosis. Light infections are often asymptomatic and remain so for years. Pancreatitis, cholangiocarcinomaa Biliary colic, cholestatic jaundice, recurrent cholangitis and cholelithiasis; thickening, enlargement, and fibrosis of biliary ducts; sometimes repeated relapses of acute symptoms Pancreatitis. In rare cases: ectopic infections in the central nervous system, orbital area, gastrointestinal tract, lungs, and other organs. Rarely, fascioliasis can be fatal. Heavy infection may lead to ulceration of intestinal mucosa and malabsorption. Mild infections are often asymptomatic. Malnutrition, anemia; rarely, ectopic infection in the central nervous system Bronchitis-, asthma-, and tuberculosis-like symptoms and signs such as chronic cough, dyspnea, bloody (“rusty”) sputum Pulmonary cyst formation; ectopic infection in the central nervous system, eyes, skin, heart, abdominal and reproductive organs biliary aspirates. Eggs generally cannot be detected until 3–4 months after exposure, whereas antibodies to the parasite may become detect­ able 2–4 weeks after exposure. More than one stool specimen may be needed for diagnosis, especially in light infections. CHAPTER 241 ■ ■INTESTINAL FLUKES More than 70 species of intestinal flukes can cause human infection. These parasites are found in different geographic areas, with a relatively high prevalence in Southeast Asia. Humans are infected by ingestion of infective metacercariae attached to aquatic plants (Fasciolopsis buski) or encysted in freshwater fish. Flukes mature in the human intestines, and eggs are passed with feces. Mechanical irritation of the intestinal wall and inflammation may lead to nonspecific gastrointestinal symptoms such as diarrhea, constipation, and abdominal pain. Most individuals infected with intestinal flukes are asymptomatic, but heavy infections can be severe, with intestinal mucosal ulcerations and malabsorption (Table 241-3). The diagnosis is established by detection of eggs in stool samples. However, eggs from various intestinal trematodes are often morphologically similar, and it is very difficult to distinguish among species. A cautionary note: Fasciola eggs can be difficult to distinguish on the basis of morphologic criteria from the eggs of the intestinal fluke F. buski. The distinction has implications for therapy: infection with F. buski is treated with praziquantel, which is not effective against fascioliasis (Table 241-2). Schistosomiasis and Other Trematode Infections
■ ■LUNG FLUKES Paragonimiasis is a parasitic lung infection caused by lung flukes of the genus Paragonimus. It is a food-borne parasitic zoonosis, with most cases reported from Asia and attributable to consumption of raw or undercooked freshwater crustaceans. Paragonimus westermani and related species (e.g., Paragonimus africanus) are endemic in West Africa, Central and South America, and Asia. The United States has one indigenous species of lung fluke, Paragonimus kellicotti. Paragonimus species require two intermediate hosts: first, a freshwa­ ter snail; and second, a freshwater crustacean, such as a freshwater crab. Humans are infected by consuming raw or undercooked infected crus­ taceans containing Paragonimus metacercariae. Paragonimus infects other carnivores such as cats, dogs, foxes, rodents, and pigs in addi­ tion to humans. After ingestion, metacercariae quickly penetrate the duodenum and traverse the peritoneal cavity, diaphragm, and parietal pleura to mature into hermaphroditic worm pairs in the pleural spaces or lungs within 6–10 weeks. Adults cross-fertilize in cystic cavities

138 - 242 Cestode Infections

242 Cestode Infections

in the pleural spaces or lungs within another 4–16 weeks and release unembryonated eggs into bronchioles. The eggs are then coughed up in bloody (“rusty”) sputum and either discharged in sputum or swallowed and later excreted in feces. Unembryonated eggs are passed from the mammalian host into freshwater ecosystems, where they infect inter­ mediate host snails.

The symptoms and signs of paragonimiasis are fever, cough, hemop­ tysis, and peripheral eosinophilia. Some patients with paragonimiasis and low parasite burdens may remain relatively asymptomatic for prolonged periods or may have recurrent attacks of cough, sputum production, fever, and night sweats that mimic tuberculosis. Infective metacercariae may migrate to extrapulmonary sites such as the brain (cerebral paragonimiasis). Pulmonary paragonimiasis is diagnosed by detection of parasite ova in sputum and/or feces. Serology can be helpful in egg-negative cases and in cerebral paragonimiasis. Anamnestic information about the consumption of raw or undercooked freshwater crabs by immigrants, expatriates, and returning travelers—and, in the United States, the con­ sumption of raw or undercooked crayfish from freshwater river systems where P. kellicotti is endemic—is important in patients presenting with fever, cough, hemoptysis, pleural effusions, and peripheral eosinophilia. TREATMENT Food-Borne Trematode Infections Praziquantel and triclabendazole are the two drugs of choice; Table 241-2 summarizes the dosages recommended for the various trematode infections. All confirmed cases of human paragonimiasis should be treated with praziquantel (Table 241-2) to avoid the com­ plications of extrapulmonary disease. Surgical management may be needed for pulmonary or cerebral lesions. PART 5 Infectious Diseases ■ ■CONTROL AND PREVENTION Drugs are currently the main method of controlling the morbidity associated with food-borne trematode infections, but integrated pro­ grams (including improved sanitation; food inspections; and informa­ tion, education, and communication campaigns) are important for sustainable disease control. Collaboration with other sectors (e.g., agricultural, environmental, and educational) is necessary to tackle highly complex situations in which human behavior, biological factors, and agricultural practices all play a role. ■ ■FURTHER READING Andrade G et al: Decline in infection-related morbidities following drug-mediated reductions in the intensity of Schistosoma infec­ tion: A systematic review and meta-analysis. PLoS Negl Trop Dis 11:e0005372, 2017. Cucchetto G et al: High-dose or multi-day praziquantel for imported schistosomiasis? A systematic review. J Travel Med 26:taz050, 2019. De Leo GA et al: Schistosomiasis and climate change. BMJ 371:m4324, 2020. Fried B, Abruzzi A: Food-borne trematode infections of humans in the United States of America. Parasitol Res 106:1263, 2010. Fürst T et al: Global burden of human food-borne trematodiasis: A systematic review and meta-analysis. Lancet Infect Dis 12:210, 2012. Jordan P et al (eds): Human Schistosomiasis. CAB International, Wall­ ingford, 1993. Keiser J, Utzinger J: Food-borne trematodiases. Clin Microbiol Rev 22:466, 2009. Mcmanus DP et al: Schistosomiasis. Nat Rev Dis Primers 4:13, 2018. Ross AG et al: Katayama syndrome. Lancet Infect Dis 7:218, 2007. Sripa B et al: Update on pathogenesis of opisthorchiasis and cholangio­ carcinoma. Adv Parasitol 102:97, 2018. World Health Organization: Female Genital Schistosomiasis: A Pocket Atlas for Clinical Health-Care Professionals. Geneva, World Health Organization, 2015. Available at http://brightresearch.org/ wp-content/uploads/2016/05/FGS-pocket-atlas_eng.pdf. WHO/HTM/ NTD/2015.4, 2015. Accessed March 16, 2020.

A. Clinton White, Jr., Miguel M. Cabada

Cestode Infections Cestodes, or tapeworms, are members of the flatworm phylum (Platyhelminthes) and comprise the subphylum (Cestoda). The adult worms are segmented worms found in the gastrointestinal tract of the definitive host. The larval forms are typically cystic and found in the tissues of the intermediate host. Human cestode infections include both tapeworm and larval-form infections. For tapeworm infections, humans are the definitive hosts, with adult tapeworms living in the gastrointestinal tract (Taenia saginata, Dibothriocephalus latus, and Dipylidium caninum). Humans are an intermediate or dead-end host with larval-stage parasites living in the tissues for Echinococcus spp., Spirometra spp., and Taenia multiceps. Humans may be either the intermediate hosts or the definitive hosts for Taenia solium and are the hosts of both stages of Rodentolepis nana (formerly Hymenolepis nana). The tapeworm forms are typically elongated and ribbon-shaped and attach to the intestinal mucosa by means of sucking cups or hooks located on the scolex. A short, narrow neck is found at the base of the scolex from which proglottids (segments) form. As proglottids mature, they are displaced from the neck by the formation of new, less mature segments. The elongating chain of attached proglottids, called the stro­ bila, constitutes the bulk of the tapeworm. The length of the strobila varies between species. Some tapeworms consist of more than 1000 proglottids and may be several meters long. Mature proglottids are hermaphroditic and produce eggs, which are intermittently released. Most human tapeworms require at least one intermediate host to complete the life cycle. After ingestion of the eggs or proglottids by an intermediate host, the eggs are activated to release the invasive larvae (oncospheres). The oncosphere penetrates the intestinal mucosa and migrates to tissues and develops into an encysted form known as a cysticercus (single scolex), a coenurus (multiple scolices), or a hydatid (cyst with daughter cysts, each containing multiple protoscolices). After ingestion of the cystic forms by the definitive host, the scolex evaginates and develops into a tapeworm. ■ ■TAENIASIS SAGINATA AND TAENIASIS ASIATICA The beef tapeworm T. saginata is found in all countries where raw or undercooked beef is eaten. It is most prevalent in sub­Saharan African and Middle Eastern countries. Taenia asiatica is closely related to T. saginata and is found in Asia, with pigs as intermediate hosts. Since the clinical manifestations and morphology of the two species are very similar, they are discussed together. Etiology and Pathogenesis  Humans serve as the definitive host for the adult stage of T. saginata and T. asiatica. The scolex attaches to the small intestines and the strobila with 1000–2000 proglottids can reach 8 m in length. The scolex of T. asiatica has an unarmed (with no hooks) rostellum and four suckers, whereas T. saginata has no rostel­ lum but attaches via four prominent suckers. Each gravid segment has 15–30 uterine branches (in contrast to 8–12 for T. solium). The eggs of the three human-infecting Taenia species are indistinguishable morphologically and are 30–40 μm in diameter, contain the oncosphere with six hooklets, and have a thick brown striated shell. Eggs can live for months to years on vegetation until they are ingested by cattle or other herbivores (T. saginata) or pigs (T. asiatica). After ingestion, the onco­ sphere egresses the egg, invades into the intestinal wall, and is distributed throughout the body via the bloodstream. The invasive larva transforms into the cysticercus (cystic forms) in muscle or viscera of the intermediate host. When raw or undercooked meat containing the larvae is ingested, the cysticercus evaginates and forms a tapeworm in the human intestines. The adult worm matures over several months to produce eggs. The pro­ glottids or eggs are then shed intermittently with stool. Clinical Manifestations  Patients may be asymptomatic or may note passing proglottids in their feces. Patients may note the sensation

of something moving while the motile proglottids of T. saginata are passed. Patients occasionally note abdominal pain or discomfort, nau­ sea, change in appetite, or weakness. Weight loss is unusual. Diagnosis  Diagnosis depends on detection of eggs or proglottids in the stool. Eggs may not be found by stool examination but are sometimes only present in the perianal area; thus, when suspected, the perianal region should be examined with use of a cellophanetape swab (as in pinworm infection; Chap. 239). Antigen-detection assays are more sensitive than microscopic examination but are not commercially available. Distinguishing T. saginata or T. asiatica from

T. solium requires examination of mature proglottids or the scolex or via molecular tests. Eosinophilia and elevated levels of serum IgE are usually absent. TREATMENT Taeniasis saginata and Taeniasis asiatica A single dose of praziquantel (10 mg/kg) is highly effective. Niclosamide (adult dose, 2 g; 1 g for children weighing 11−34 kg) is also effective but is less available. Nitazoxanide can also be used. Prevention  Adequate cooking of beef and pork viscera or exposure to temperatures as low as 56°C for 5 min will destroy cysticerci. Refrig­ eration, salting for long periods, or freezing at –10°C for 9 days also kills the parasites in beef. General preventive measures include inspec­ tion of beef and pork viscera and proper disposal of human feces. ■ ■TAENIASIS SOLIUM AND CYSTICERCOSIS T. solium, also known as the pork tapeworm, causes two forms of infec­ tion in humans: adult tapeworms in the intestine (taeniasis) or larval forms in the tissues (cysticercosis). Humans are the only hosts for the

T. solium tapeworm; pigs are the usual intermediate hosts, although other animals may rarely harbor the cysticercus forms. T. solium has a wide global distribution including all areas where pigs are raised with access to human feces. Cysticercosis is highly prevalent in Latin America, sub­Saharan Africa, China, India, and Southeast Asia. Cysticercosis also occurs in nonendemic nations due to the immigration of tapeworm carriers or cysticercosis-infected persons from endemic areas. Etiology and Pathogenesis  The tapeworms of T. solium reside primarily in the small intestines. The scolex attaches to the mucosa via sucking disks and the armed rostellum with two rows of hooklets. The mature tapeworm can reach a length of 3 m and have up to 1000 proglottids. The adult tapeworms are thought to live for just few years. Each mature proglottid may produce approximately 50,000 eggs. Pro­ glottids are released from the terminal end of the tapeworm intermit­ tently and excreted into the feces. The eggs are immediately infective for both humans and pigs. After ingestion of eggs by the intermediate host, the oncospheres are released, penetrate the intestinal wall, and are carried via the bloodstream to tissues. In the pig, larvae usually mature in striated muscle of the neck, tongue, and trunk. By 60–90 days, the invasive larvae transform into the encysted larval stage. In the pigs, cys­ ticerci typically survive for months to years, until the pig is slaughtered. Humans develop intestinal tapeworm infections after ingestion of contaminated pork. Human cysticercosis follows ingestion of T. solium eggs. Transmission is associated with close contact with a tapeworm carrier. The eggs are sticky and may be found under the fingernails of tapeworm carriers. The tapeworm carrier can also infect themselves by ingestion of ova, likely via the fecal-oral route. Clinical Manifestations  Intestinal infections (taeniasis) with

T. solium are often asymptomatic. Some patients note passage of pro­ glottids in stool. The proglottids are typically off-white in color and 1–3 cm in length, 0.5–1 cm wide, and about 1 mm thick. The clinical manifestations of cysticercosis are variable. Cysticerci can be found anywhere in the body but are commonly detected in the central ner­ vous system (CNS), skeletal muscle, subcutaneous tissue, or the eye.

Involvement of the brain, spine, or cerebrospinal fluid (CSF) is termed neurocysticercosis. The clinical manifestations of human cysticercosis vary with the location of the cysticerci as well as with the extent of asso­ ciated inflammatory responses or scarring. The most common clinical manifestations are neurologic symptoms. Headache is common with all CNS forms of disease. Seizures are the most frequent clinical manifes­ tation and are associated with inflammation of the brain parenchyma surrounding the cysticercus. Seizures may be focal, focal with second­ ary generalization, or generalized. Another common clinical manifes­ tation is with symptoms of hydrocephalus, which may result from CSF flow obstruction by cysticerci and/or accompanying inflammation or communicating hydrocephalus from arachnoiditis. The symptoms of increased intracranial pressure may include headache, nausea and vomiting, dizziness, and ataxia. Patients with hydrocephalus may pres­ ent with altered mental status or papilledema with altered visual acuity. Some patients present with intermittent acute hydrocephalus (termed Bruns’ syndrome) associated with change in position due to the cysti­ cercus working as a ball valve. Cysticerci at the base of the brain or in the subarachnoid space may cause chronic meningitis or arachnoiditis, communicating hydrocephalus, mass lesions, hemorrhages, or strokes.

Diagnosis  The diagnosis of tapeworm infection with T. solium is made by the demonstration of eggs or proglottids, as described for

T. saginata. However, eggs and proglottids are only shed intermittently, limiting the sensitivity of direct testing. Antigen-capture enzymelinked immunosorbent assay (ELISA), polymerase chain reaction (PCR), and serology for tapeworm stage-specific antigens are more sensitive but are only available as research techniques. The diagnosis of neurocysticercosis can be difficult since the symptoms are nonspe­ cific and there is no readily available material for demonstration of the parasite. A group of international experts proposed revised diagnostic criteria (Table 242-1). The diagnosis is only certain with definite dem­ onstration of the parasite (absolute criteria). Definitive diagnosis is possible with histologic observation of the parasite in excised tissue, by funduscopic visualization of the parasite in the subretinal or vitreous spaces of the eye, or by neuroimaging demonstrating a cystic lesion containing a characteristic scolex (Fig. 242-1). With high-resolution neuroimaging, the scolex can often be identified. In other cases, a clinical diagnosis is based on a combination of clinical presentation, radiographic studies, exposure or evidence demonstrating presence of the parasites by antigen-detection, quantitative PCR, or even nextgeneration sequencing in spinal fluid. CHAPTER 242 Cestode Infections Neuroimaging is the primary major diagnostic method (Fig. 242-1). Demonstration of a cystic lesion with a mural nodule consistent with a scolex (“dot-in-hole”) is diagnostic. Major findings include cystic lesions with or without enhancement (e.g., ring enhancement), one or more nodular calcifications (which may also have associated edema or enhancement), focal enhancing lesions, or cystic lesions in the subarachnoid space. Cysticerci in the brain parenchyma are usually 5–20 mm in diameter and round. Cystic lesions in the subarachnoid space or fissures may enlarge up to 6 cm in diameter and may be lobulated. The cyst wall for cysticerci in the subarachnoid space or ventricles is usually very thin, and the cyst fluid is often isodense with CSF. Thus, obstructive hydrocephalus or enhancement of the basilar meninges may be the only finding on computed tomography (CT) in extraparenchymal neurocysticercosis. However, since these findings are less specific, they are considered only minor criteria. Cysticerci in the ventricles or subarachnoid space are more readily identified by magnetic resonance imaging (MRI), especially fast imaging employing steady-state acquisition (FIESTA) or three-dimensional constructive interference in steady state (3D CISS). CT is more sensitive than MRI in identifying calcified lesions, whereas MRI is more sensitive than CT for identifying small cystic lesions, scolexes, and enhancement. Sponta­ neous resolution, resolution after albendazole therapy, or mobile cystic lesions within the ventricles are findings that can support the diagnosis of neurocysticercosis. Exposure history significantly modifies the interpretation of neu­ roimaging studies. Detection of specific antibodies to or antigens of

T. solium are major exposure criteria. Antibody tests using unfractionated

TABLE 242-1  Revised Diagnostic Criteria for Neurocysticercosisa 1. Absolute criteria

a. Histologic demonstration of the parasite from biopsy of a brain or spinal cord lesion

b. Visualization of subretinal cysticercus

c. Conclusive demonstration of a scolex with a cystic lesion on neuroimaging studies 2. Neuroimaging criteria

a. Major neuroimaging criteria Cystic lesions without a discernible scolex, typical small enhancing lesions, multilobulated cystic lesions in the subarachnoid space, typical parenchymal brain calcifications

b. Confirmative neuroimaging criteria Resolution of cystic lesions spontaneously or after cysticidal drug therapy Migration of ventricular cysts documented on sequential neuroimaging studies

c. Minor neuroimaging criteria Obstructive hydrocephalus or abnormal enhancement of basal leptomeninges 3. Clinical/exposure criteria

a. Major clinical/exposure criteria Detection of specific anticysticercal antibodies (e.g., by enzyme-linked immunoelectrotransfer blot [EITB]) or cysticercal antigens by wellstandardized immunodiagnostic tests Cysticercosis outside the central nervous system Evidence of a household contact with T. solium infection

b. Minor clinical/exposure criteria Clinical manifestations suggestive of neurocysticercosis Individuals coming from or living in an area where cysticercosis is endemic PART 5 Infectious Diseases aDiagnosis is confirmed by one absolute criterion, by two major criteria or one major and one confirmatory neuroimaging criteria plus any clinical/exposure criterion, or by one major neuroimaging criterion plus two clinical/exposure criteria (including at least one major clinical/exposure criterion), together with the exclusion of other pathologies producing similar neuroimaging findings. A probable diagnosis is supported by one major neuroimaging criterion plus any two clinical/ exposure criteria or by one minor neuroimaging criterion plus at least one major clinical/exposure criterion. Source: Reproduced with permission from OH Del Brutto et al: Revised diagnostic criteria for neurocysticercosis. J Neurol Sci 372:202, 2017. antigens (e.g., ELISAs using crude parasite antigen) have high rates of false-positive and false-negative results and should be avoided. An immunoblot assay (enzyme-linked immunoelectrotransfer blot [EITB]) using lentil lectin–purified glycoproteins is >99% specific and sensitive in patients with multiple cysts. However, patients with single intracranial lesions or with calcifications may be seronegative. Serum samples are more sensitive than CSF using EITB. Each of the diagnostic antigens has been cloned, and assays using recombinant or synthetic antigens are in development. Assays using monoclonal antibodies to detect parasite antigen in the blood, CSF, or urine may also facilitate diagnosis and patient follow-up. Antigen-detection assays are currently available commercially in Europe but not in the United States. More recently, real-time PCR has been employed for diagnosis and follow-up of extraparenchymal disease. Other major clinical/exposure criteria for neurocysticercosis include the presence of cysticerci outside the CNS (e.g., typical cigar-shaped calcifications in muscle) or exposure to a tapeworm carrier or a house­ hold member infected with T. solium. Minor clinical/exposure criteria include residence in an endemic village or clinical symptoms sugges­ tive of neurocysticercosis (e.g., seizures or obstructive hydrocephalus). Studies from India validated clinical criteria for diagnosis in selected cases. In patients from endemic areas who had single enhancing lesions presenting with seizures, a normal physical examination, and no evi­ dence of systemic disease (e.g., no fever, adenopathy, or chest radio­ graphic abnormalities), the presence on CT of round lesions 5–20 mm

in diameter with no midline shift was almost always caused by neu­ rocysticercosis. Definite or probable diagnosis can be made using the criteria and combinations of criteria listed in the footnote of

Table 242-1. Patients often have CSF pleocytosis with a predominance of lymphocytes, neutrophils, or eosinophils. The protein level in CSF may be elevated; the glucose concentration is usually normal but can be markedly reduced. TREATMENT Taenia solium and Cysticercosis Tapeworm infection by T. solium infection is treated with a single dose of praziquantel (10 mg/kg). However, praziquantel may occa­ sionally trigger an inflammatory response in the CNS if con­ comitant cryptic cysticercosis is present. Niclosamide (2 g) is also effective but is not as widely available. INITIAL MANAGEMENT OF NEUROCYSTICERCOSIS Initial management of neurocysticercosis should focus on treat­ ment of seizures or hydrocephalus. Seizures can be controlled with antiseizure medications. Seizure medications can usually be tapered after 6 months in patients with single enhancing lesions in whom imaging normalizes and in whom there are no breakthrough seizures. Subjects with multiple parenchymal lesions require more prolonged therapy. However, antiseizure medications can often be tapered off after 2 years if lesions resolve without development of calcifications and patients remain free of seizures. Patients with calcified lesions are at higher risk of recurrent seizures, especially if the lesions are associated with perilesional edema or enhancement. MANAGING HYDROCEPHALUS For patients with hydrocephalus, the reduction of intracranial pressure should be the priority of the initial therapy. Patients with cysticerci in the cerebral ventricles typically present with obstruc­ tive hydrocephalus, and the preferred approach is removal of the cysticercus via neurosurgery. Cysticerci in the lateral or third ven­ tricles should be removed via neuroendoscopy. Antiparasitic drugs make the cysticerci more friable and should be avoided prior to surgery. The cysticerci in the fourth ventricle can be approached by microdissection using a posterior approach or, in some cases, via neuroendoscopy. When complete removal of the cysticercus is not possible, a diverting procedure, such as ventriculoperitoneal shunting, can be used to manage hydrocephalus. Historically, shunt failure was a major problem. The risk of shunt failure may be lim­ ited by administration of antiparasitic drugs and glucocorticoids. ANTIPARASITIC DRUGS AND ANTI-INFLAMMATORY THERAPY Antiparasitic drug treatment is never an emergency in neuro­ cysticercosis and should wait until patients are stabilized with antiseizure and anti-inflammatory medications and exclusion of intraocular disease. Antiparasitic drugs should never be started in patients with elevated intracranial pressure. Antiparasitics do hasten resolution of neuroradiologic abnormalities in parenchymal neurocysticercosis. Clinical benefits consist mainly of decreasing the number of recurrent generalized seizures. In viable parenchy­ mal cysticercosis, most authorities recommend antiparasitic drugs, especially albendazole (15 mg/kg per day for 8–28 days). A combi­ nation of albendazole and praziquantel (50 mg/kg per day) is more effective in patients with more than two cystic lesions. A longer course or combination therapy is needed in patients with multiple subarachnoid cysticerci. Both antiparasitic agents may exacerbate the inflammatory response around the dying parasite, thereby exac­ erbating seizures or hydrocephalus. Patients receiving these drugs should be carefully monitored. High-dose glucocorticoids should always be used during treatment (e.g., dexamethasone 0.1–0.4 mg/kg per day or prednisone 60 mg/d). For patients with subarachnoid cysts or giant cysticerci, antiinflammatory medications such as glucocorticoids are needed to reduce arachnoiditis and accompanying vasculitis. Most authorities recommend prolonged courses of antiparasitic drugs as well as shunting when hydrocephalus is present. Patients typically require prolonged anti-inflammatory treatment along with antiparasitics.

FIGURE 242-1  Neurocysticercosis is caused by Taenia solium. Neurologic infection can be classified based on the location and viability of the parasites. Upper left: Parenchymal viable cysts (FLAIR MRI sequence). Upper center: Parenchymal viable cysts (postcontrast T1 MRI sequence). Upper right: Single enhancing lesion (postcontrast T1 MRI sequence). Bottom left: Extensive basal subarachnoid neurocysticercosis in the anterior fossa (FLAIR MRI sequence). Bottom center: Viable cyst in the fourth ventricle (FLAIR MRI sequence). Bottom right: Intraparenchymal brain calcifications (noncontrasted CT scan). Lesions are marked with arrowheads. FLAIR, fluid-attenuated inversion recovery. (Modified with permission from White AC Jr, Garcia HH. Updates on the management of neurocysticercosis. Curr Opin Infect Dis. 2018;31(5):377-382. Lippincott Williams & Wilkins.) Methotrexate and, in some cases, tumor necrosis factor inhibitors have been used as steroid-sparing agents in patients requiring pro­ longed therapy. In patients with diffuse cerebral edema and elevated intracranial pressure due to multiple inflamed parenchymal lesions, glucocorticoids are the mainstay of therapy, and antiparasitic drugs should be avoided. For ocular and spinal lesions, drug-induced inflammation may cause irreversible damage. Intraocular disease should be managed surgically. Recent data suggest that spinal dis­ ease is best managed using both medical and surgical therapy. Prevention  Prevention of T. solium tapeworm infection consists of precautions in handling pork, as described for T. asiatica, and thoroughly cooking or freezing pork to destroy the cysticerci. Pork inspections and condemnation of infected meat prevent transmission. The prevention of cysticercosis involves good personal hygiene including handwash­ ing, effective disposal of feces, and treatment and prevention of human intestinal infections. Optimal eradication programs to eradicate T. solium in endemic areas include mass chemotherapy administered to human and porcine populations and vaccinations of pigs. A vaccine for porcine infection is licensed in India and a few other countries. ■ ■ECHINOCOCCOSIS Echinococcosis (also known as hydatid disease) refers to infection by the larval stage of Echinococcus species (E. granulosus sensu lato,

E. multilocularis, or E. vogeli). E. granulosus sensu lato parasites

CHAPTER 242 Cestode Infections produce cystic hydatid disease or cystic echinococcosis, which is preva­ lent worldwide in most areas where livestock is raised in association with dogs. E. granulosus sensu lato is a complex of several distinct spe­ cies with important genotypic and phenotypic differences. Human cys­ tic echinococcosis is caused by E. granulosus sensu stricto (genotypes 1–3), E. canadensis (genotypes 6–8 and 10), and E. ortleppi (genotype 5). Some classify genotypes 6 and 7 as a separate species—E. intermedius. E. granulosus sensu lato parasites are found on all continents, with areas of high prevalence in western China, central Asia, the Middle East, the Mediterranean region, eastern Africa, and parts of South America.

E. multilocularis causes multilocular or alveolar echinococcosis charac­ terized by locally invasive lesions. Alveolar echinococcosis is prevalent in Alpine, sub-Arctic, and Arctic regions of the northern hemisphere, including western China, central Asia, central and northern Europe, and in isolated areas of North America with an expanding range of endemic areas. Neotropical echinococcosis (formerly termed polycys­ tic hydatid disease) is caused by E. vogeli and E. oligarthrus, which are only found in limited foci in South America. Echinococcal species require both intermediate and definitive hosts. The definitive hosts are usually canines (dogs, foxes, wolves) that har­ bor the small tapeworms in the intestine and shed eggs in stool. After the ingestion of eggs, oncospheres invade through the intestines into the circulatory system and form cysts in the intermediate hosts including sheep, cattle, goats, camels, pig, and horses for E. granulosus sensu lato and mice and other rodents for E. multilocularis. When a dog

(E. granulosus) or fox/wolf (E. multilocularis) ingests viscera containing cysts, the protoscolices in the cyst fluid develop into tapeworms in the intestine, completing the life cycle. Humans are a dead-end intermedi­ ate host and not part of the parasite’s life cycle.

Etiology  The adult tapeworms of E. granulosus sensu lato are small (5­ mm long) and live for 5–20 months in the small intestine of dogs. Each tapeworm has only three proglottids: one immature, one mature, and one gravid. The latter are shed and release eggs that are morpho­ logically similar to Taenia eggs. Heavy infections of dogs with many tapeworms are common in endemic areas. When humans ingest the eggs, the invasive oncospheres are released from eggs, penetrate the intestinal mucosa, enter the portal circulation, and are carried mostly to the liver and lungs. However, virtually any organ can be infected including kidneys, spleen, heart, bone, and brain. Larvae of E. granu­ losus sensu lato develop into fluid-filled unilocular hydatid cysts. The wall of the cystic lesion consists of an external membrane and an inner germinal layer, which are surrounded by the host’s adventitial layer that may contain different patterns of inflammation and fibrosis. Daughter cysts and germinating cystic structures called brood capsules develop from the inner aspect of the germinal layer. New organisms, called protoscolices, develop in large numbers within the brood capsule. A protoscolex is an invaginated scolex with the capacity to form an adult tapeworm if ingested by a definitive host or form a new cystic lesion if released in the intermediate host’s tissues. The cysts expand slowly over a period of years and may contain thousands of protoscolices. E. multilocularis has a life cycle involving wild canines such as foxes or wolves, which are the main definitive hosts. Domestic dogs can also serve as definitive hosts of these tapeworms. Small rodents are the main intermediate hosts. Humans are dead-end intermediate hosts that develop alveolar echinococcosis. The larval stage of this parasite forms multilocular, small irregular cysts with proliferating and invasive capacity. The parasite larvae invade the host tissue by peripheral exten­ sion of processes from the germinal layer. These lesions do not contain brood capsules or protoscolices. PART 5 Infectious Diseases Clinical Manifestations  The slowly enlarging echinococcal cysts generally remain asymptomatic for years until their expanding size elicits organ-specific symptoms. Spontaneous or traumatic rupture may lead to type I allergic reactions including anaphylaxis. The liver is involved in two-thirds of E. granulosus sensu lato infections and nearly all E. multi­ locularis infections. The lungs are involved in about 20% of E. granulosus sensu lato infections. The parasites are often discovered incidentally on a routine x-ray or ultrasound study prior to onset of symptoms. Symptoms of hepatic cystic echinococcosis may include abdomi­ nal fullness or pain or a palpable mass in the right upper quadrant. A B FIGURE 242-2.  Imaging studies of cystic echinococcosis. A. Chest x-ray film of a patient with bilateral cysts. The hollow arrows show well-defined cyst walls with a complicated right chest cyst and an intact left chest cyst. B. Liver ultrasound of a patient with CE1 cysts. The hollow arrow shows a well-defined bilayer cyst wall.

Compression of a bile duct or leakage of cyst fluid into the biliary tree may present with symptoms mimicking cholelithiasis. Jaundice can result from biliary obstruction. Rupture of or leakage from a hydatid cyst may present more acutely with symptoms including fever, pruri­ tus, urticaria, eosinophilia, or anaphylaxis. Cystic echinococcosis in the lungs may present with chronic cough, shortness of breath, chest pain, or hemoptysis. Rupture into the bronchial tree leads to sudden expectoration of the cyst fluid and membranes and rupture into the pleural cavity may produce pleuritic chest pain and hydatid empyema. Rupture of hydatid cysts, which can occur spontaneously, after trauma, or during surgery, may lead to release of protoscolices into the patient’s tissues, each of which can form new cysts. Hydatid disease may also involve bone (invasion of the medullary cavity with bony erosion pro­ ducing pathologic fractures), the CNS (space-occupying lesions), the heart (conduction defects, pericarditis), and the pelvis (pelvic mass). E. multilocularis characteristically presents as a slow-growing hepatic mass, which typically presents decades after the initial infec­ tion. The lesions resemble tumors causing progressive destruction of the liver and extension into adjoining structures. Frequent symptoms include upper-quadrant and epigastric discomfort. Physical examina­ tion may reveal liver enlargement and obstructive jaundice. The lesions may infiltrate adjoining organs (e.g., diaphragm, kidneys, or lungs) or may metastasize to the spleen, lungs, or brain. Diagnosis  Imaging studies are the main diagnostic methods to detect and evaluate echinococcal cysts. Chest x-ray or CT can iden­ tify pulmonary cysts due to E. granulosus sensu lato, which appear as rounded masses of uniform density (Fig. 242-2). Ultrasound, CT, or MRI can be used to identify cystic echinococ­ cosis lesions in solid organs of the abdomen, particularly in the liver. The ultrasound classification proposed by the World Health Organiza­ tion Informal Working Group on Echinococcosis has diagnostic and management applications for liver cystic echinococcosis (Figs. 242-2 and 242-3). MRI can also be used to classify lesions. MRI and CT may be more useful than ultrasound to evaluate the presence of cyst com­ plications, such as communication with the biliary tree, that preclude some management options. Imaging of cystic echinococcosis shows well-defined cysts walls and, in some cases, internal trabeculation, dense cyst material, and/or calcifications (Fig. 242-3). In some cases, the protoscolices and brood capsules of E. granulosus complex may be visible within the cysts as fine particles termed hydatid sand. Identi­ fication of daughter cysts within the larger cyst is diagnostic of cystic echinococcosis disease. Eggshell or mural calcification on CT is also diagnostic of E. granulosus infection. In contrast, ultrasound or CT of alveolar hydatid cysts often reveals an indistinct solid mass. Some cases will display central necrosis or plaque-like calcifications.

Imaging of cystic echinococcosis Ultrasound CT scan MRI CE 1 CE 2 CE 3a CE 3b CE 4 CE 5 FIGURE 242-3  Management of cystic hydatid disease caused by Echinococcus granulosus should be based on viability of the parasite, which can be estimated from radiographic appearance. Staging is done by imaging studies including ultrasound, CT, or MRI and includes lesions classified as active, transitional, and inactive. Active cysts include types CL (with a cystic lesion and no visible cyst wall), CE1 (with a visible cyst wall and internal echoes [snowflake sign]), and CE2 (with a visible cyst wall and internal septation). Transitional cysts may have detached laminar membranes (CE3a) or may be partially collapsed (CE3b). Inactive cysts include types CE4

(a nonhomogeneous mass) and CE5 (a cyst with a thick calcified wall). For cystic echinococcosis disease, a definitive diagnosis can also be made by the examination of aspirated fluids for protoscolices and/or hooklets. However, due to the potential risk of fluid leakage resulting in either dissemination of infection or, more rarely, anaphylactic reac­ tions, aspiration should only be performed by experienced interven­ tionists and following the precautions used for percutaneous treatment (see below). Serodiagnostic assays can be useful, but current serologic tests are insensitive and cannot be used to exclude the diagnosis of echinococcosis. ELISA and immunoblot assays for specific antibody are positive in ~90% of cases of hydatid liver disease. By contrast, the sensitivity is only ~50% for patients with cysts in the lungs. TREATMENT Echinococcosis CYSTIC HYDATID DISEASE Optimal treatment of cystic echinococcosis varies depending on the size, stage, location, and clinical manifestations of cysts. In the past, surgery was the main treatment method, but numerous studies

CHAPTER 242 Cestode Infections have demonstrated that other treatment modalities may be just as effective and lead to less morbidity. Staging is recommended for cystic echinococcosis of the liver, which allows assessment of the cyst size and viability (Fig. 242-3). CL, CE1, and CE3a lesions <5 cm in diameter may respond well to chemotherapy with albendazole alone. Other small liver cysts (<5 cm) such as CE2 or CE3b are less responsive to medical treatment alone. Larger CE1 lesions and uncomplicated CE3a lesions in the liver can often be managed by PAIR (percutaneous aspiration, instillation of scolicidal agents, and reaspiration). Contraindications to PAIR include cysts communi­ cating with the biliary tree, large cysts (>10 cm), or superficial cysts, which are more likely to rupture and spill protoscolices. Albenda­ zole (15 mg/kg daily in two divided doses) should be initiated at least 2 days before the procedure and continued for at least 4 weeks afterward. Fine-needle ultrasound or CT-guided aspiration should enter the cyst through solid tissues to limit spillage of cyst fluid. Aspiration can confirm the diagnosis by microscopic demonstra­ tion of protoscolices or hooks. After aspiration, bilirubin should be measured in the cyst fluid using a dipstick, or contrast material should be injected to detect occult communications with the biliary

tract. If no bile is found and no communication is visualized, instil­ lation of scolicidal agents (usually hypertonic saline) and reaspira­ tion are performed. PAIR, when performed by a skilled practitioner, results in cure and relapse rates equivalent to surgery, with less perioperative morbidity and shorter hospitalization. In some cen­ ters, CE2 lesions have been treated by modified catheter drainage, including puncture of each daughter cyst within the primary cyst.

Patients with treatment failure can often be treated again suc­ cessfully with PAIR or additional courses of medical therapy. Response to treatment is best assessed by serial imaging studies, with attention to cyst size and consistency. Cysts may not dem­ onstrate complete radiologic resolution even though no viable protoscolices are present. Those cysts classified as CE4 or CE5 are considered nonviable and require periodic reevaluations to assess for reactivation. Surgery remains the treatment of choice for complicated cystic echinococcosis (e.g., cysts communicating with the biliary tract), for most thoracic and intracranial cysts, and when PAIR is not pos­ sible. Liver cysts should be removed via a pericystectomy, in which the entire cyst and the surrounding fibrous tissue are removed to prevent spillage and recurrence. Recent reports demonstrate that, in experienced hands, cysts can often be safely removed by laparoscopic or robotic surgery. The risks posed by leakage of fluid during surgery or PAIR include anaphylaxis and dissemination of protoscolices. Spillage can be minimized by careful dissection and using surgical draping soaked in hypertonic saline. Infusion of scolicidal agents is no longer recommended because of problems with hypernatremia, intoxication, or sclerosing cholangitis. Alben­ dazole should be administered adjunctively, beginning several days to weeks before resection of the liver cyst and continuing for several weeks afterwards. Albendazole at 10-15 mg/kg divided in two daily doses (400 mg BID in adults) is recommended for treatment in association with clinical and laboratory monitoring. ALVEOLAR HYDATID DISEASE Surgical resection is required to attempt cure of E. multilocularis infection. Complete removal of the parasite continues to offer the best chance for cure. Patients who have undergone presumed cura­ tive resection should be treated with albendazole for at least 2 years after presumptively curative surgery. Positron emission tomography can be used to follow disease activity. Unfortunately, most cases are only diagnosed at a stage in which complete resection is impos­ sible; in these cases, albendazole treatment should be continued indefinitely, with careful monitoring. In some cases of larger lesions, complete removal of the liver followed by liver transplantation has been used. Unfortunately, the immunosuppression required to pre­ vent rejection of the transplanted liver also promotes proliferation of E. multilocularis and reinfection of the transplanted liver. Thus, indefinite treatment with albendazole is required. PART 5 Infectious Diseases Prevention  Cystic echinococcosis can be prevented by adminis­ tering praziquantel to infected dogs, by preventing dogs from having access to viscera from infected animals, or by vaccinating sheep. Limit­ ing the number of stray dogs that are more likely to evade prevention measures may help decrease transmission. In Europe, E. multilocu­ laris infection is associated with human settlement encroaching into forested areas, gardening in those areas, and collecting activities in forested areas. Thus, gloves and hand hygiene should be used in these situations. Praziquantel-impregnated bait can be used to treat tape­ worms in wild canines. ■ ■RODENTOLEPIS NANA (PREVIOUSLY HYMENOLEPIS NANA) The dwarf tapeworm, previously known as H. nana, is the most com­ mon human cestode infection. Recent molecular data have led to the reclassification of this organism to a different genus and new name Rodentolepis nana. R. nana is endemic worldwide, including tem­ perate and tropical regions. Transmission is mostly person-to-person by the fecal-oral route.

Etiology and Pathogenesis  R. nana is the only human cestode that does not require an intermediate host. Both the larval and adult stages coexist in the intestine of infected persons. The adult tapeworm is only ~2 cm long and lives in the human proximal ileum. The tiny proglottids are rarely seen in the stool. They release spherical eggs, 30–44 μm in diameter, containing the invasive larvae termed onco­ sphere, which has six hooklets. The eggs are immediately infective after leaving the host and survive for ≤10 days in the environment. After the eggs are ingested, the oncosphere is released, penetrates the intestinal villi, and develops into the cysticercoid larval form within the epithe­ lium. After a few days, these larvae reenter the intestinal lumen, attach to the mucosa, and mature into adult tapeworms over 10–12 days. The life span of adult R. nana worms is typically ~4–10 weeks. However, infection is perpetuated by cycles of reinfection and autoinfection in which some eggs hatch in the intestinal lumen and form the cysticer­ coid larva without leaving the host. Clinical Manifestations  R. nana infection is most often asymp­ tomatic. However, infection may be associated with diarrhea, abdomi­ nal pain, and weight loss, particularly in children with the highest burden of infection. Diagnosis  Infection is diagnosed by finding the characteristic R. nana eggs in microscopy of the stool. TREATMENT Rodentolepis nana Infection The treatment of choice for R. nana is praziquantel (25 mg/kg once), which is active against both the adult worms and the cysti­ cercoids in the intestinal wall. Nitazoxanide (500 mg twice a day for 3 days) has been used as an alternative treatment. Prevention  Since R. nana is acquired by the fecal-oral route, improved sanitation and personal hygiene can be used to eliminate disease. Hand washing in the household and school is important. Mass chemotherapy and improved hygiene have been used to control epidemics. ■ ■HYMENOLEPIASIS DIMINUTA Hymenolepis diminuta is a cestode of rodents that occasionally causes infection in small children. Infection is acquired by ingesting uncooked cereal and other foods contaminated by fleas and other insects that serve as intermediate hosts for H. diminuta. Infection is diagnosed by detection of eggs in the stool. The treatment of choice is praziquantel (25 mg/kg once). ■ ■DIPHYLLOBOTHRIASIS (DIBOTHRIOCEPHALUS/ ADENOCEPHALUS) The broad fish tapeworms comprised several species that were for­ merly classified under the genus Diphyllobothrium. Molecular and phylogenetic studies have now demonstrated several important dif­ ferences between these parasites, which led to modification of the taxonomic classification of cestodes of the Diphyllobothriidae family infecting humans. Dibothriocephalus latus (formerly Diphyllobothrium latum), Adenocephalus pacificus (formerly Diphyllobothrium pacifi­ cum), and Dibothriocephalus nihonkaiensis (formerly Diphyllobothrium nihonkaiensis) are the most common species infecting humans. These parasites were initially identified in freshwater lakes, rivers, and deltas of the Northern Hemisphere and central Africa. However, they are also found in marine environments in the northern Pacific Ocean

(D. nihonkaisensis) and the Pacific coast of South America (A. pacificus). Etiology and Pathogenesis  The adult tapeworm can reach a length of up to 25 m, making them the longest tapeworms of humans. The scolex attaches to the small intestinal mucosa by a modified sucker called bothria, which is located on the elongated scolex. The mature tapeworms have 3000–4000 proglottids, producing ~1 million eggs per day. D. latus eggs hatch in fresh water and release the free-swimming coracidium. The coracidia are ingested by small freshwater crustaceans

(Cyclops or Diaptomus species). Within the infected crustaceans, the procercoid larvae develop. When the infected crustaceans are ingested by fish, the procercoid larva migrate to the fish’s flesh and transform into a sparganum or plerocercoid larva. Humans acquire the infection by ingesting infected raw or smoked fish. Within 3–5 weeks, the tape­ worm matures into an adult in the human intestine. For A. pacificus, the definite hosts include seals, dogs, and humans, and the second intermediate hosts are marine fish. Clinical Manifestations  Most infections by these tapeworms are asymptomatic. Some patients note abdominal discomfort, diarrhea, vomiting, weakness, or weight loss. The proglottids may be passed in stool or found incidentally during endoscopy. Diphyllobothriidae can cause acute abdominal pain or intestinal obstruction in rare cases.

D. latus tapeworms have avid receptors for vitamin B12 that can inter­ fere with absorption and, in patients with other risk factors, produce vitamin B12 deficiency. In patients with B12 deficiency, pernicious anemia and neurologic sequelae may develop. Diagnosis  The diagnosis is made by the detection of the character­ istic eggs in the stool. The eggs possess a single shell with an opercu­ lum at one end and a knob at the other, which may be confused with trematode eggs by the inexperienced technician. Mild to moderate eosinophilia may be detected. Examination of the tapeworms passed also provides a diagnosis, as proglottids have a characteristic uterus with a rosette-like shape. TREATMENT Diphyllobothriasis Praziquantel (5–10 mg/kg once) is highly effective against all Diphyllobothriid species. Parenteral vitamin B12 may be given for B12 deficiency. Prevention  Heating fish to 54°C for 5 min or freezing at –18°C for 24 h kills the larval forms. Placing fish in brine with a high salt concen­ tration for long periods can also kill the plerocercoid larvae. ■ ■DIPYLIDIASIS Dipylidium caninum is a common tapeworm of dogs and cats. Dogs, cats, and occasionally humans become infected by swallowing fleas harboring the intermediate forms (cysticercoid larvae). Children are more often infected than adults. Infections are usually asymptomatic except for the passage of motile proglottids in stool and, less often, vague abdominal symptoms. The diagnosis is made by the detection of proglottids or the characteristic egg packets in the stool. Eggs within the packet resemble other Taenia eggs. The motile proglottids resemble flattened grains of rice and, in small children, may migrate out of the anus. Praziquantel is the treatment of choice. Prevention should mainly focus on antiparasitic and flea treatment of dogs and cats.

■ ■SPARGANOSIS The plerocercoid larvae of Diphyllobothriid tapeworms of the genus Spirometra cause human sparganosis. Dogs, cats, and pigs are hosts of the adult tapeworm; crustaceans of the Cyclops species in fresh water are the first intermediate host; and snakes, frogs, or birds are the sec­ ond intermediate hosts. Human infection commonly follows topical application of poultices with infected flesh from snakes, frogs, or birds used in traditional medicine. Infection can also be acquired by the ingestion of water containing infected Cyclops and raw or undercooked meat from infected snakes, birds, or some mammals. Infection com­ monly presents as a subcutaneous swelling that contains the parasite. Periorbital infections can present with swelling and intraocular infec­ tions may lead to blindness. Infections of the brain can present as a mass or slowly migrating lesions. Proliferative lesions may cause tissue infiltration and are poorly responsive to medical treatment. Surgical excision is used to treat localized sparganosis.

■ ■COENUROSIS Coenurosis is a rare infection of humans by the larval stage (coenurus) of the dog tapeworms Taenia multiceps or Taenia serialis. The main clinical manifestations are space-occupying cystic lesions in various tissues. The commonly involved tissues include the CNS or subcuta­ neous tissue. Surgical excision is usually required for both definitive diagnosis and treatment. There are limited data on response to anti­ parasitic treatment. Acknowledgment The authors acknowledge and thank Peter F. Weller, MD, author of prior editions of this chapter. CHAPTER 242 ■ ■FURTHER READING Brunetti E et al: Expert consensus for the diagnosis and treatment of cystic and alveolar echinococcosis in humans. Acta Trop 114:1, 2010. Del Brutto OH et al: Revised diagnostic criteria for neurocysticercosis. Cestode Infections J Neurol Sci 372:202, 2017. Kern P et al: The echinococcoses: Diagnosis, clinical management and burden of disease. Adv Parasitol 96:259, 2017. Nash TE et al: Natural history of treated subarachnoid neurocysticer­ cosis. Am J Trop Med Hyg 102:78, 2020. Nguyen DC et al: The brief case: The boy who cried worm. J Clin Microbiol 61:e00553, 2013. Wen H et al: Echinococcosis: Advances in the 21st century. Clin Microbiol Rev 32:e00075, 2019. White AC Jr et al: Diagnosis and treatment of neurocysticercosis: 2017 clinical practice guidelines by the Infectious Diseases Society of America (IDSA) and the American Society of Tropical Medicine and Hygiene (ASTMH). Clin Infect Dis 66:1159, 2018.

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135 Infectious Arthritis

■ ■FURTHER READING Aronoff DM, Marrazzo JM: Infections caused by Clostridium per­ fringens and Paeniclostridium sordellii after unsafe abortion. Lancet Infect Dis 23:e48, 2023. Bruun T et al: Risk factors and predictors of mortality in streptococcal necrotizing soft-tissue infections: A multicenter prospective study. Clin Infect Dis 72:293, 2021. Bryant AE et al: Emerging erythromycin and clindamycin resistance in group A Streptococcus: Efficacy of linezolid and tedizolid in experi­ mental necrotizing infection. J Glob Antimicrob Resist 22:601, 2020. Daum RS et al: A placebo-controlled trial of antibiotics for smaller skin abscesses. N Engl J Med 376:2545, 2017. Davies MR et al: Emergence of scarlet fever Streptococcus pyogenes emm12 clones in Hong Kong is associated with toxin acquisition and multidrug resistance. Nat Genet 47:84, 2015. Gessain A et al: Monkeypox. N Engl J Med 387:1783, 2022. Linner A et al: Clinical efficacy of polyspecific intravenous immuno­ globulin therapy in patients with streptococcal toxic shock syndrome: A comparative observational study. Clin Infect Dis 59:851, 2014. Rafei R et al: A global snapshot on the prevalent macrolide-resistant emm types of group A Streptococcus worldwide, their phenotypes and their resistance marker genotypes during the last two decades: A systematic review. Infect Genet Evol 99:105258, 2022. Stevens DL et al: Necrotizing soft tissue infections. Infect Dis Clin North Am 35:135, 2021. Talan DA et al: Bacteriologic analysis of infected dog and cat bites. Emergency Medicine Animal Bite Infection Study Group. N Engl J Med 340:85, 1999. Nongnooch Poowanawittayakom,

Lawrence C. Madoff

Infectious Arthritis Although Staphylococcus aureus, streptococci, and Neisseria gonorrhoeae are the most common causes of infectious arthritis, various mycobac­ teria, spirochetes, fungi, and viruses also infect joints (Table 135-1). Since acute bacterial infection can destroy articular cartilage rapidly, all inflamed joints must be evaluated without delay to exclude non­ infectious processes and determine appropriate antimicrobial therapy and drainage procedures. For more detailed information on infectious arthritis caused by specific organisms, the reader is referred to the chapters on those organisms. Acute bacterial infection typically involves a single joint or a few joints. Subacute or chronic monoarthritis or oligoarthritis suggests mycobacterial or fungal infection; episodic inflammation is seen in syphilis, Lyme disease, and the reactive arthritis that follows enteric infections and chlamydial urethritis. Acute polyarticular inflammation occurs as an immunologic reaction during the course of endocarditis, rheumatic fever, disseminated neisserial infection, and acute viral hepatitis. Viruses often infect multiple joints; however, bacterial infec­ tions generally cause mono- or oligoarthritis except in persons with underlying diseases such as rheumatoid arthritis. APPROACH TO THE PATIENT Infectious Arthritis Aspiration of synovial fluid or arthrocentesis—an essential element in the evaluation of potentially infected joints—can be performed without difficulty in most cases by the insertion of a large-bore

TABLE 135-1  Differential Diagnosis of Arthritis Syndromes CHRONIC MONARTICULAR ARTHRITIS ACUTE MONARTICULAR ARTHRITIS POLYARTICULAR ARTHRITIS Staphylococcus aureus Streptococcus pneumoniae β-Hemolytic streptococci Gram-negative bacilli Neisseria gonorrhoeae Candida spp. Crystal-induced arthritis Fracture Hemarthrosis Foreign body Osteoarthritis Ischemic necrosis Monoarticular rheumatoid arthritis Mycobacterium tuberculosis Nontuberculous mycobacteria Borrelia burgdorferi Treponema pallidum Candida spp. Sporothrix schenckii Coccidioides immitis Blastomyces dermatitidis Aspergillus spp. Cryptococcus neoformans Nocardia spp. Brucella spp. Legg-Calvé-Perthes disease Osteoarthritis Neisseria meningitidis N. gonorrhoeae Nongonococcal bacterial arthritis Bacterial endocarditis Candida spp. Poncet’s disease (tuberculous rheumatism) Hepatitis B virus Parvovirus B19 HIV Human T-lymphotropic virus type 1 Rubella virus Arthropod-borne viruses Sickle cell disease flare Reactive arthritis Serum sickness Acute rheumatic fever Inflammatory bowel disease Systemic lupus erythematosus Rheumatoid arthritis/ Still’s disease Other vasculitides Sarcoidosis CHAPTER 135 Infectious Arthritis needle into the site of maximal fluctuance or tenderness or by the route of easiest access. Ultrasonography or computed tomography (CT) may be used to guide aspiration of difficult-to-localize effu­ sions of the hip and, occasionally, the shoulder and other joints. Normal synovial fluid contains <180 cells (predominantly mononu­ clear cells) per microliter. Synovial cell counts averaging 100,000/μL (range, 25,000–250,000/μL), with >90% neutrophils, are character­ istic of acute bacterial infections. Crystal-induced, rheumatoid, and other noninfectious inflammatory arthritides usually are associated with <30,000–50,000 cells/μL; cell counts of 10,000–30,000/μL, with 50–70% neutrophils and the remainder lymphocytes, are common in mycobacterial and fungal infections. Definitive diagnosis of an infectious process relies on identification of the pathogen in stained smears of synovial fluid, isolation of the pathogen from cultures of synovial fluid and blood, or detection of microbial nucleic acids and proteins by nucleic acid amplification tests (NAATs) and immuno­ logic techniques. Gram stain is positive in about 30−50% of cases, and synovial fluid culture is positive in >60% of nongonococcal bacte­ rial arthritis cases. Matrix-assisted laser desorption/ionization–time of flight (MALDI-TOF) mass spectrometry is helpful in patients who have negative culture and high suspicion of infectious arthritis. Sonication of explanted prosthetic joints (placement of the material into liquid and then immersion in an ultrasound bath) increases the yield of organism detection, especially in the case of prior antibiotic use within 14 days. ACUTE BACTERIAL ARTHRITIS ■ ■PATHOGENESIS Bacteria enter the joint from the bloodstream; from a contiguous site of infection in bone or soft tissue; or by direct inoculation during surgery, injection, animal or human bite, or trauma. In hematogenous infection, bacteria escape from synovial capillaries, which have no limiting base­ ment membrane, and within hours provoke neutrophilic infiltration

of the synovium. Neutrophils and bacteria enter the joint space; later, bacteria adhere to articular cartilage. Degradation of cartilage begins within 48 h as a result of increased intraarticular pressure, release of proteases and cytokines from chondrocytes and synovial macrophages, and invasion of the cartilage by bacteria and inflammatory cells. Histologic studies reveal bacteria lining the synovium and cartilage as well as abscesses extending into the synovium, cartilage, and—in severe cases—subchondral bone. Synovial proliferation results in the formation of a pannus over the cartilage, and thrombosis of inflamed synovial vessels develops. Bacterial factors that appear important in the pathogenesis of infective arthritis include various surface-associated adhesins in S. aureus that permit adherence to cartilage and endotoxins that promote chondrocyte-mediated breakdown of cartilage.

■ ■MICROBIOLOGY The hematogenous route of infection is the most common route in all age groups, and nearly every bacterial pathogen is capable of causing septic arthritis. In infants, group B streptococci, gram-negative enteric bacilli, and S. aureus are the most common pathogens. Since the advent of the Haemophilus influenzae vaccine, the predominant causes among children <5 years of age have been S. aureus, Streptococcus pyogenes (group A Streptococcus), and (in some centers) Kingella kingae. Among young adults and adolescents, N. gonorrhoeae is the most commonly implicated organism. S. aureus (including methicillin-resistant S. aureus [MRSA]) accounts for most nongonococcal isolates in adults of all ages; gram-negative bacilli, pneumococci, and β-hemolytic streptococci—

particularly groups A and B but also groups C, G, and F—are involved in up to one-third of cases in older adults, especially those with under­ lying comorbid illnesses. Gram-negative bacilli such as Pseudomonas may occur in immunocompromised patients or intravenous drug users. PART 5 Infectious Diseases Infections after surgical procedures or penetrating injuries are due most often to S. aureus and occasionally to other gram-positive bacte­ ria or gram-negative bacilli. Infections with coagulase-negative staphy­ lococci are unusual except after the implantation of prosthetic joints or arthroscopy. Anaerobic organisms, often in association with aerobic or facultative bacteria, are found after human bites and when decu­ bitus ulcers or intraabdominal abscesses spread into adjacent joints. Polymicrobial infections complicate traumatic injuries with extensive contamination. Bites and scratches from cats and other animals may introduce Pasteurella multocida or Bartonella henselae into joints either directly or hematogenously, and bites from humans may introduce Eikenella corrodens or other components of the oral flora. Penetration of a sharp object through a shoe is associated with Pseudomonas aeruginosa arthritis in the foot. ■ ■NONGONOCOCCAL BACTERIAL ARTHRITIS Epidemiology  Although hematogenous infections with virulent organisms such as S. aureus, H. influenzae, and pyogenic streptococci occur in healthy persons, there is an underlying host predisposition in many cases of septic arthritis. Patients with rheumatoid arthritis have the highest incidence of infective arthritis (most often secondary to S. aureus) because of chronically inflamed joints; glucocorticoid therapy; and frequent breakdown of rheumatoid nodules, vasculitic ulcers, and skin overlying deformed joints. Diabetes mellitus, glucocor­ ticoid therapy, hemodialysis, intravenous drug use, and malignancy all carry an increased risk of infection with S. aureus and gram-negative bacilli. Tumor necrosis factor inhibitors (e.g., etanercept, infliximab), which increasingly are used for the treatment of rheumatoid arthritis, predispose to mycobacterial infections and possibly to other pyogenic bacterial infections and could be associated with septic arthritis in this population. Pneumococcal infections complicate alcoholism, deficien­ cies of humoral immunity, and hemoglobinopathies. Pneumococci, Salmonella species, and H. influenzae cause septic arthritis in persons infected with HIV. Persons with primary immunoglobulin deficiency are at risk for mycoplasmal arthritis, which, while rare, results in per­ manent joint damage if tetracycline and replacement therapy with IV immunoglobulin are not administered promptly. IV drug users acquire staphylococcal and streptococcal infections from their own flora and

acquire pseudomonal and other gram-negative infections from drugs and injection paraphernalia. Clinical Manifestations  Patients with acute septic arthritis usu­ ally present with joint pain often with limitation of passive and active joint movement, joint swelling, and/or erythema. Approximately 90% of patients present with involvement of a single joint—most com­ monly the knee; less frequently the hip; and still less often the shoulder, wrist, or elbow. Small joints of the hands and feet are more likely to be affected after direct inoculation or a bite. Among IV drug users, infections of the spine, sacroiliac joints, and sternoclavicular joints (Fig. 135-1) are more common than infections of the appendicular skeleton. Poly­ articular infection is most common among patients with rheumatoid arthritis and may resemble a flare of the underlying disease. The usual presentation consists of moderate to severe pain that is uniform around the joint, effusion, muscle spasm, and decreased range of motion. Fever in the range of 38.3–38.9°C (101–102°F) and some­ times higher is common but may not be present, especially in persons with rheumatoid arthritis, renal or hepatic insufficiency, or conditions requiring immunosuppressive therapy. The inflamed, swollen joint is usually evident on examination except in the case of a deeply situated joint such as the hip, shoulder, or sacroiliac joint. Cellulitis, bursitis, and acute osteomyelitis, which may produce a similar clinical picture, should be distinguished from septic arthritis by preservation of pas­ sive range of motion and less-than-circumferential swelling. A focus of extraarticular infection, such as a boil, pneumonia, or endocarditis, should be sought. Peripheral-blood leukocytosis with a left shift and elevation of the erythrocyte sedimentation rate or C-reactive protein level are common. Plain radiographs show evidence of soft tissue swelling, joint space widening, and displacement of tissue planes by the distended capsule. Narrowing of the joint space and bony erosions indicate advanced infection and a poor prognosis. Ultrasound is useful for detecting effusions in the hip, and CT or MRI can demonstrate infections of the sacroiliac joint, the sternoclavicular joint, and the spine very well. Laboratory Findings  Specimens of peripheral blood and synovial fluid should be obtained before antibiotics are administered. Blood cultures are positive in up to 50–70% of S. aureus infections but are less frequently positive in infections due to other organisms. The synovial fluid is turbid, serosanguineous, or frankly purulent. Gram-stained smears confirm the presence of large numbers of neutrophils. Levels of total protein and lactate dehydrogenase in synovial fluid are elevated, FIGURE 135-1  Acute septic arthritis of the sternoclavicular joint. A man in his forties with a history of cirrhosis presented with a new onset of fever and lower neck pain. He had no history of IV drug use or previous catheter placement. Jaundice and a painful swollen area over his left sternoclavicular joint were evident on physical examination. Cultures of blood drawn at admission grew group B Streptococcus. The patient recovered after treatment with IV penicillin. (Courtesy of the late Francisco M. Marty, MD, Brigham and Women’s Hospital, Boston; with permission.)

and the glucose level is depressed; however, these findings are not specific for infection, and measurement of these levels is not neces­ sary for diagnosis. The synovial fluid should be examined for crystals because gout and pseudogout can resemble septic arthritis clinically, and infection and crystal-induced disease occasionally occur together. Organisms are seen on synovial fluid smears in nearly three-quarters of infections with S. aureus and streptococci and in 30–50% of infec­ tions due to gram-negative and other bacteria. Cultures of synovial fluid are positive in >90% of cases. Inoculation of synovial fluid into bottles containing liquid media for blood cultures increases the yield of a culture, especially if the pathogen is a fastidious organism or the patient is taking an antibiotic, but should be interpreted in the context of the Gram’s stain result. Pathogen nucleic acid amplification based assays (NAAT) or MALDI-TOF mass spectrometry, when available, can be useful for the diagnosis of partially treated or culture-negative arthritis. Inflammatory markers such as erythrocyte sedimentation rate and C-reactive protein tend to be elevated in septic arthritis but are nonspecific. Serum procalcitonin elevation is only ~50% sensitive and should not be used to rule out infectious arthritis. Synovial fluid procalcitonin might be useful, although data are limited. TREATMENT Nongonococcal Bacterial Arthritis Prompt administration of systemic antibiotics and drainage of the involved joint can prevent destruction of cartilage, postinfectious degenerative arthritis, joint instability, or deformity. Once samples of blood and synovial fluid have been obtained for culture, empiri­ cal antibiotics should be directed against the bacteria visualized on smears or the pathogens that are likely in light of the patient’s age and risk factors. Initial therapy should consist of IV-admin­ istered bactericidal agents; direct instillation of antibiotics into the joint is not necessary to achieve adequate levels in synovial fluid and tissue. If there are gram-positive cocci on the smear, IV vancomycin (15−20 mg/kg/dose) every 8–12 h should be started empirically. If methicillin-resistant S. aureus is an unlikely patho­ gen (e.g., when it is not widespread in the community), cefazolin (2 g every 8 h), oxacillin (2 g every 4 h), or nafcillin (2 g every 4 h) should be given. If initial Gram’s stain shows gram-negative bacilli, the risk for P. aeruginosa infection should be evaluated. If the patient does not have risk factors for P. aeruginosa infection, an IV third-generation cephalosporin such as cefotaxime (2 g every 8 h) or ceftriaxone (2 g every 24 h) provides adequate empirical coverage for most community-acquired infections. In addition, if the patient has a higher risk for P. aeruginosa infection, then anti-pseudomonal coverage such as cefepime (2 g every 8−12 h) or ceftazidime (2 g every 8 h) should be given. Double coverage of Pseudomonas with cephalosporin and ciprofloxacin or aminoglycoside can be consid­ ered empirically in severely ill patients or in the setting of highly resistant Pseudomonas infection incidence. Definitive therapy is based on the identity and antibiotic sus­ ceptibility of the bacteria isolated in culture. Infections due to staphylococci are treated with cefazolin, oxacillin, nafcillin, or van­ comycin for 4 weeks. In patients without evidence of endocarditis, IV antibiotics can be used for 7–14 days of treatment followed by oral antibiotics to complete the treatment course. Pneumococcal and streptococcal infections due to penicillin-susceptible organisms respond to 2 weeks of therapy with penicillin G (2 million units IV every 4 h); infections caused by H. influenzae and by strains of Streptococcus pneumoniae that are resistant to penicillin are treated with cefotaxime or ceftriaxone for 2 weeks. Most enteric gramnegative infections can be cured in 3–4 weeks by a second- or thirdgeneration cephalosporin given IV or by a fluoroquinolone such as levofloxacin (500 mg IV or PO every 24 h). P. aeruginosa infec­ tion should be treated for at least 2 weeks with antipseudomonal cephalosporin such as ceftazidime (2 g IV every 8 h), cefepime (2 g every 8–12 h). If tolerated, this regimen is continued for an additional

2 weeks; alternatively, a fluoroquinolone such as ciprofloxacin (750 mg PO twice daily) may be given.

Timely drainage of pus and necrotic debris from the infected joint is required for a favorable outcome. Needle aspiration of read­ ily accessible joints such as the knee may be adequate if loculations or particulate matter in the joint does not prevent its thorough decompression. Arthroscopic drainage and lavage may be employed initially or within several days if repeated needle aspiration fails to relieve symptoms, decrease the volume of the effusion and the synovial white cell count, and clear bacteria from smears and cul­ tures. In some cases, arthrotomy is necessary to remove loculations and debride infected synovium, cartilage, or bone. Septic arthritis of the hip is best managed with arthrotomy, particularly in young children, in whom infection threatens the viability of the femoral head. Septic joints do not require immobilization except for pain control before symptoms are alleviated by treatment. Weight bear­ ing should be avoided until signs of inflammation have subsided, but frequent passive motion of the joint is indicated to maintain full mobility. Although some clinical studies suggest that adjunctive use of glucocorticoids showed some benefit in children with septic arthritis, they are not widely used in clinical practice. ■ ■GONOCOCCAL ARTHRITIS Epidemiology  In the past, gonococcal arthritis (Chap. 161) accounted for up to 70% of episodes of infectious arthritis in persons <40 years of age in the United States. As the rates of mucosal gonor­ rhea have fallen in the United States, it is likely that the proportion of septic arthritis caused by N. gonorrhoeae also has fallen considerably. Arthritis due to N. gonorrhoeae is a consequence of bacteremia arising from gonococcal infection or, more frequently, from asymptomatic gonococcal mucosal colonization of the urethra, cervix, or pharynx. Women are at greatest risk during menses and during pregnancy and overall are two to three times more likely than men to develop disseminated gonococcal infection (DGI) and arthritis. Persons with complement deficiencies, especially of the terminal components, are prone to recurrent episodes of gonococcemia. Eculizumab, which is a long-acting monoclonal antibody targeting the C5 complement com­ ponent and used mainly for the treatment of paroxysmal nocturnal hemoglobinuria, also has been reported to be associated with dissemi­ nated gonococcal infection. CHAPTER 135 Infectious Arthritis Clinical Manifestations and Laboratory Findings  The most common manifestation of DGI is a syndrome of arthritis-dermatitis. Patients present with fever, chills, rash, tenosynovitis, and articular symptoms. Small numbers of papules that progress to hemorrhagic pustules develop on the trunk and the extensor surfaces of the distal extremities. Migratory arthritis and tenosynovitis of the knees, hands, wrists, feet, and ankles are prominent. The cutaneous lesions and articular findings are believed to be the consequence of an immune reaction to circulating gonococci and immune-complex deposition in tissues. Thus, cultures of synovial fluid are consistently negative, and blood cultures are positive in <45% of patients. Synovial fluid may be difficult to obtain from inflamed joints and usually contains only 10,000–20,000 leukocytes/μL. True gonococcal septic arthritis is less common than the DGI syn­ drome and always follows DGI, which is unrecognized in one-third of patients. A single joint such as the hip, knee, ankle, or wrist is usually involved. Synovial fluid, which contains >50,000 leukocytes/μL, can be obtained with ease; the gonococcus is evident only occasionally in Gram-stained smears, and cultures of synovial fluid are positive in <40% of cases. Blood cultures are almost always negative. Because it is difficult to isolate gonococci from synovial fluid and blood, specimens for culture should be obtained from potentially infected mucosal sites. NAAT-based urine tests also may be positive. Culture requires endocervical (in female patients) or urethral (in male patients) swab specimens. Culture is available for detection of rectal, oropharyngeal, and conjunctival gonococcal infection. Cultures and Gram-stained smears of skin lesions are occasionally positive.

All specimens for culture should be plated onto Thayer-Martin agar directly or in special transport media at the bedside and transferred promptly to the microbiology laboratory in an atmosphere of 5% CO2. NAAT assays are extremely sensitive in detecting gonococcal DNA in synovial fluid, but they are not FDA approved for this purpose. A dramatic alleviation of symptoms within 12–24 h after the initiation of appropriate antibiotic therapy supports a clinical diagnosis of the DGI syndrome if cultures are negative.

TREATMENT Gonococcal Arthritis Initial treatment consists of ceftriaxone (1 g IV or IM every 24 h) to cover possible penicillin-resistant organisms. Once local and sys­ temic signs are clearly resolving, a 7-day course of antibiotics may be completed with daily IM ceftriaxone given at 500 mg daily (or 1g for those weighing over 150 kg). An oral fluoroquinolone such as ciprofloxacin (500 mg twice daily) may be used if the organism is known to be susceptible. If penicillin-susceptible organisms are isolated, amoxicillin (500 mg four times daily) may be used. Sup­ purative arthritis usually responds to needle aspiration of involved joints and 7–14 days of antibiotic treatment. Arthroscopic lavage or arthrotomy is rarely required. The Centers for Disease Control rec­ ommends that when chlamydial infection has not been excluded, treatment with PO doxycycline 100 mg twice daily for 7 days should be given. Sexual partners should be offered testing and presumptive treatment for gonorrhea and chlamydial infection. It is notewor­ thy that arthritis symptoms similar to those seen in DGI occur in meningococcemia. A dermatitis–arthritis syndrome, purulent monoarthritis, and reactive polyarthritis have been described. All respond to treatment with appropriate antibiotics. PART 5 Infectious Diseases SPIROCHETAL ARTHRITIS ■ ■LYME DISEASE Lyme disease (Chap. 191) due to infection with the spirochete Borrelia burgdorferi causes arthritis in up to 60% of persons who are not treated. Intermittent arthralgias and myalgias—but not arthritis—occur within days or weeks of inoculation of the spirochete by the Ixodes tick. Later, there are three patterns of joint disease: (1) Fifty percent of untreated persons experience intermittent episodes of monoarthritis or oligoar­ thritis involving the knee and/or other large joints. The symptoms wax and wane without treatment over months, and each year, 10–20% of patients report loss of joint symptoms. (2) Twenty percent of untreated persons develop a pattern of waxing and waning arthralgias. (3) Ten percent of untreated patients develop chronic inflammatory synovitis that results in erosive lesions and destruction of the joint. Serologic tests for IgG antibodies to B. burgdorferi are positive in >90% of per­ sons with Lyme arthritis, and a NAAT detects Borrelia DNA in synovial fluid in 85% of patients. TREATMENT Lyme Arthritis Lyme arthritis generally responds well to therapy. A regimen of oral doxycycline (100 mg twice daily for 28 days), oral amoxicillin (500 mg three times daily for 28 days), or parenteral ceftriax­ one (2 g/d for 2–4 weeks) is recommended. Patients who do not respond to a total of 2 months of oral therapy or 1 month of par­ enteral therapy are unlikely to benefit from additional antibiotic therapy and are treated with anti-inflammatory agents or synovec­ tomy. Failure of therapy is associated with host features such as the human leukocyte antigen DR4 (HLA-DR4) genotype, persistent reactivity to OspA (outer-surface protein A), and the presence of hLFA-1 (human leukocyte function–associated antigen 1), which cross-reacts with OspA.

■ ■SYPHILITIC ARTHRITIS Articular manifestations occur in different stages of syphilis (Chap. 187). In early congenital syphilis, periarticular swelling and immobilization of the involved limbs (Parrot’s pseudoparalysis) compli­ cate osteochondritis of long bones. Clutton’s joint, a late manifestation of congenital syphilis that typically develops between ages 8 and 15 years, is caused by chronic painless synovitis with effusions of large joints, particularly the knees and elbows. Secondary syphilis may be associated with arthralgias, with symmetric arthritis of the knees and ankles and occasionally of the shoulders and wrists, and with sacroi­ liitis. The arthritis follows a subacute to chronic course with a mixed mononuclear and neutrophilic synovial-fluid pleocytosis (typical cell counts, 5000–15,000/μL). Immunologic mechanisms may contribute to the arthritis, and symptoms usually improve rapidly with penicillin therapy. In tertiary syphilis, Charcot joint results from sensory loss due to tabes dorsalis. Penicillin is not helpful in this setting. MYCOBACTERIAL ARTHRITIS Tuberculous arthritis (Chap. 183) accounts for ~1% of all cases of tuberculosis and 10% of extrapulmonary cases. The most common presentation is chronic granulomatous monoarthritis. An unusual syndrome, Poncet’s disease, is a reactive symmetric form of polyar­ thritis that affects persons with visceral or disseminated tuberculosis. No mycobacteria are found in the joints, and symptoms resolve with antituberculous therapy. Unlike tuberculous osteomyelitis (Pott’s disease) (Chap. 136), which typically involves the thoracic and lumbar spine (50% of cases), tuber­ culous arthritis primarily involves the large weight-bearing joints, in particular the hips, knees, and ankles, and only occasionally involves smaller non-weight-bearing joints. Progressive monoarticular swelling and pain develop over months or years, and systemic symptoms are seen in only half of all cases. Tuberculous arthritis occurs as part of a disseminated primary infection or through late reactivation, often in persons with HIV infection or other immunocompromised hosts. Coexistent active pulmonary tuberculosis is unusual. Aspiration of the involved joint yields fluid with an average cell count of 20,000/μL, with ~50% neutrophils. Acid-fast staining of the fluid yields positive results in fewer than one-third of cases, and cul­ tures are positive in 80%. Culture of synovial tissue taken at biopsy is positive in ~90% of cases and shows granulomatous inflammation in most. NAAT can shorten the time to diagnosis to 1 or 2 days. Radio­ graphs reveal peripheral erosions at the points of synovial attachment, periarticular osteopenia, and eventually joint-space narrowing. Therapy for tuberculous arthritis is the same as that for tuberculous pulmonary dis­ ease, requiring the administration of multiple agents for 6–9 months. Therapy is more prolonged in immunosuppressed individuals, such as those infected with HIV. Various atypical mycobacteria (Chap. 185) found in water and soil may cause chronic indolent arthritis. Such disease results from trauma and direct inoculation associated with farming, gardening, or aquatic activities. Smaller joints, such as the digits, wrists, and knees, are usu­ ally involved. Involvement of tendon sheaths and bursae is typical. The mycobacterial species involved include Mycobacterium marinum, M. avium complex, M. terrae, M. kansasii, M. fortuitum, and M. chelonae. In persons who have HIV infection or are receiving immunosuppressive therapy, hematogenous spread to the joints has been reported for M. kansasii, M. avium complex, and M. haemophilum. Diagnosis usu­ ally requires biopsy and culture, and therapy is based on antimicrobial susceptibility patterns. FUNGAL ARTHRITIS Fungi are an unusual cause of chronic monoarticular arthritis. Granu­ lomatous articular infection with the endemic dimorphic fungi Coc­ cidioides immitis, Blastomyces dermatitidis, and (less commonly) Histoplasma capsulatum (Fig. 135-2) results from hematogenous seed­ ing or direct extension from bony lesions in persons with disseminated disease. Joint involvement is an unusual complication of sporotrichosis (infection with Sporothrix schenckii) among gardeners and other per­ sons who work with soil or sphagnum moss. Articular sporotrichosis

A B C FIGURE 135-2  Chronic arthritis caused by Histoplasma capsulatum in the left knee. A. A man in his sixties from El Salvador presented with a history of progressive knee pain and difficulty walking for several years. He had undergone arthroscopy for a meniscal tear 7 years before presentation (without relief) and had received several intraarticular glucocorticoid injections. The patient developed significant deformity of the knee over time, including a large effusion in the lateral aspect. B. An x-ray of the knee showed multiple abnormalities, including severe medial femorotibial joint-space narrowing, several large subchondral cysts within the tibia and the patellofemoral compartment, a large suprapatellar joint effusion, and a large soft tissue mass projecting laterally over the knee. C. MRI further defined these abnormalities and demonstrated the cystic nature of the lateral knee abnormality. Synovial biopsies demonstrated chronic inflammation with giant cells, and cultures grew H. capsulatum after 3 weeks of incubation. All clinical cystic lesions and the effusion resolved after 1 year of treatment with itraconazole. The patient underwent a left total-knee replacement for definitive treatment. (Courtesy of the late Francisco M. Marty, MD, Brigham and Women’s Hospital, Boston; with permission.) is six times more common among men than among women, and alco­ holics and other debilitated hosts are at risk for polyarticular infection. Candida infection involving a single joint—usually the knee, hip, or shoulder—results from surgical procedures, intraarticular injec­ tions, or (among critically ill patients with debilitating illnesses such as diabetes mellitus or hepatic or renal insufficiency and patients receiving immunosuppressive therapy) hematogenous spread. Candida infections in IV drug users typically involve the spine, sacroiliac joints, or other fibrocartilaginous joints. Unusual cases of arthritis due to Aspergillus species, Cryptococcus neoformans, Pseudallescheria boydii, and the dematiaceous fungi also have resulted from direct inoculation or disseminated hematogenous infection in immunocompromised persons. In the United States, a 2012 national outbreak of fungal arthri­ tis (and meningitis) caused by Exserohilum rostratum was linked to intraspinal and intraarticular injection of a contaminated preparation of methylprednisolone acetate. The synovial fluid in fungal arthritis usually contains 10,000–40,000 cells/μL, with ~70% neutrophils. Stained specimens and cultures of synovial tissue often confirm the diagnosis of fungal arthritis when studies of synovial fluid give negative results. Treatment consists of drainage and lavage of the joint and systemic administration of an anti­ fungal agent directed at a specific pathogen. The doses and duration of therapy are the same as for disseminated disease (see Part 5, Section 16). In fungal prosthetic joint infection, the removal of all prosthetic joint material is highly recommended. VIRAL ARTHRITIS Viruses produce arthritis by infecting synovial tissue during systemic infection or by provoking an immunologic reaction that involves joints. As many as 50% of women report persistent arthralgias and 10% report frank arthritis within 3 days of the rash that follows natural infection with rubella virus and within 2–6 weeks after receipt of live-virus vac­ cine. Episodes of symmetric inflammation of fingers, wrists, and knees uncommonly recur for >1 year, but a syndrome of chronic fatigue, low-grade fever, headaches, and myalgias can persist for months or years. IV immunoglobulin has been helpful in selected cases. Self-limited monoarticular or migratory polyarthritis may develop within 2 weeks of the parotitis of mumps; this sequela is more common among men than women. Approximately 10% of children and 60% of women develop polyarthritis or polyarthralgia in small joints after infection with parvovirus B19. In adults, arthropathy sometimes occurs without fever or rash. Pain and stiffness, with less prominent swelling (pri­ marily of the hands but also of the knees, wrists, and ankles), usually resolve within weeks, although a small proportion of patients develop chronic arthropathy.

About 2 weeks before the onset of jaundice, up to 10% of persons with acute hepatitis B develop an immune complex–mediated, serum sickness–like reaction with maculopapular rash, urticaria, fever, and arthralgias. Less common developments include symmetric arthritis involving the hands, wrists, elbows, or ankles and morning stiffness that resembles a flare of rheumatoid arthritis. Symptoms resolve at the time jaundice develops. Many persons with chronic hepatitis C infec­ tion report persistent arthralgia or arthritis, both in the presence and in the absence of cryoglobulinemia. CHAPTER 135 Painful arthritis often accompanies the fever and rash of several arthropod-borne viral infections, including those caused by Zika, chikungunya, O’nyong-nyong, Ross River, Mayaro, and Barmah For­ est viruses (Chap. 215). Symmetric arthritis involving the hands and wrists may occur during the convalescent phase of infection with lym­ phocytic choriomeningitis virus. Patients infected with an enterovirus frequently report arthralgias, and echovirus has been isolated from patients with acute polyarthritis. Infectious Arthritis Several arthritis syndromes are associated with HIV infection. An incomplete form of reactive arthritis with painful lower-extremity oligoarthritis may follow an episode of urethritis in HIV-infected per­ sons. HIV-associated reactive arthritis appears to be extremely com­ mon among persons with the HLA-B27 haplotype, but sacroiliac joint disease is unusual and is seen mostly in the absence of HLA-B27. Up to one-third of HIV-infected persons with psoriasis develop psoriatic arthritis. Painless monoarthropathy and persistent symmetric polyar­ thropathy occasionally complicate HIV infection. Chronic persistent oligoarthritis of the shoulders, wrists, hands, and knees occurs in women infected with human T-lymphotropic virus type 1. Synovial thickening, destruction of articular cartilage, and leukemic-appearing atypical lymphocytes in synovial fluid are characteristic, but progression to T cell leukemia is unusual. PARASITIC ARTHRITIS Arthritis due to parasitic infection is rare. The guinea worm Dra­ cunculus medinensis may cause destructive joint lesions in the lower extremities as migrating gravid female worms invade joints or cause ulcers in adjacent soft tissues that become secondarily infected. Hydatid cysts infect bones in 1–2% of cases of infection with Echi­ nococcus granulosus. The expanding destructive cystic lesions may spread to and destroy adjacent joints, particularly the hip and pelvis. In rare cases, chronic synovitis has been associated with the presence of schistosomal eggs in synovial biopsies. Monoarticular arthritis in children with lymphatic filariasis appears to respond to therapy with diethylcarbamazine even in the absence of microfilariae in synovial fluid. Reactive arthritis has been attributed to hookworm,

Strongyloides, Cryptosporidium, and Giardia infection in case reports, but confirmation is required.

POSTINFECTIOUS OR REACTIVE ARTHRITIS Reactive polyarthritis develops several weeks after ~1% of cases of non­ gonococcal urethritis and 2% of enteric infections, particularly those due to Yersinia enterocolitica, Shigella flexneri, Campylobacter jejuni, Clostridioides difficile, and Salmonella species. Only a minority of these patients have the other findings of classic reactive arthritis, including urethritis, conjunctivitis, uveitis, oral ulcers, and rash. Studies have identified microbial DNA or antigen in synovial fluid or blood, but the pathogenesis of this condition is poorly understood. The arthritis may occur several days or weeks after the infection and can be associ­ ated with dactylitis, enthesitis, or extraarticular involvement such as conjunctivitis. Reactive arthritis is most common among young men (except after Yersinia infection) and has been linked to the HLA-B27 locus as a potential genetic predisposing factor. Patients report painful, asym­ metric oligoarthritis that affects mainly the knees, ankles, and feet. Low back pain is common, and radiographic evidence of sacroiliitis is found in patients with long-standing disease. Most patients recover within 6 months, but prolonged recurrent disease is more common in cases that follow chlamydial urethritis. Anti-inflammatory agents help relieve symptoms, but the role of prolonged antibiotic therapy in elimi­ nating microbial antigen from the synovium is controversial. Migratory polyarthritis and fever constitute the usual presentation of acute rheumatic fever in adults (Chap. 371). This presentation is distinct from that of poststreptococcal reactive arthritis, which also follows infections with group A Streptococcus but is not migratory, lasts beyond the typical 3-week maximum of acute rheumatic fever, and responds poorly to aspirin. PART 5 Infectious Diseases INFECTIONS IN PROSTHETIC JOINTS Infection complicates 0.5–2% of total joint replacements. Prosthetic joint infection occurs more often in knee arthroplasty compared with hip arthroplasty. The majority of infections are acquired intraopera­ tively or immediately postoperatively as a result of wound breakdown or infection; less commonly, these joint infections develop later after joint replacement and are the result of hematogenous spread or direct inoculation. The presentation may be acute, with fever, pain, and local signs of inflammation, especially in infections due to S. aureus, pyogenic streptococci, and gram-negative bacilli. Alternatively, infec­ tion may persist for months or years without causing constitutional symptoms when less virulent organisms—such as coagulase-negative staphylococci, Cutibacterium (formerly Propionibacterium) species, enterococci, or diphtheroids—are involved. Such indolent infections usually are acquired during joint implantation and are discovered dur­ ing evaluation of chronic unexplained pain or after a radiograph shows loosening of the prosthesis; the erythrocyte sedimentation rate and C-reactive protein level are usually elevated in such cases. The diagnosis is best made by needle aspiration of the joint; acci­ dental introduction of organisms during aspiration must be avoided meticulously. Synovial fluid pleocytosis with a predominance of poly­ morphonuclear leukocytes is highly suggestive of infection, since other inflammatory processes uncommonly affect prosthetic joints. Culture and Gram’s stain usually yield the responsible pathogen. Sonication of explanted prosthetic material can improve the yield of culture, presum­ ably by breaking up bacterial biofilms on the surfaces of prostheses. Semiquantitative culture should be obtained since a low number of organisms can represent contaminants. Molecular diagnostic testing of synovial fluid, sonicate fluid, or periprosthetic tissue also may improve the yield when routine culture is negative; however, it is not always available in microbiology laboratories. Use of special media for unusual pathogens such as fungi, atypical mycobacteria, and Mycoplasma may be necessary if routine and anaerobic cultures are negative. Molecular diagnostic results should be interpreted cautiously since they may represent colonization rather than a true pathogen. Testing of synovial fluid for alpha-defensin, C-reactive protein, leukocyte esterase, and

calprotectin can be considered in challenging cases. These tests must be interpreted in the context of the synovial fluid leukocyte count and neutrophil percentage. TREATMENT Prosthetic Joint Infections Treatment includes surgery and high doses of parenteral antibiotics, which are given for 4–6 weeks because bone is usually involved. In most cases, the prosthesis must be removed and replaced to cure the infection. Implantation of a new prosthesis is best delayed for several weeks or months because relapses of infection occur most commonly within this time frame. In some cases, reimplantation is not possible, and the patient must manage without a joint, with a fused joint, or even with amputation. Cure of infection without removal of the prosthesis is occasionally possible in cases that are due to streptococci or pneumococci and that lack radiologic evidence of loosening of the prosthesis. In these cases, antibiotic therapy must be initiated within several days of the onset of infec­ tion, and the joint should be drained vigorously by open arthrotomy or arthroscopically, preferably with polyethylene liner exchange, to have a more successful outcome. In selected patients who prefer to avoid the high morbidity rate associated with joint removal and reimplantation, lifelong suppression of the infection with antibiot­ ics may be a reasonable goal. A high cure rate with retention of the prosthesis has been reported when the combination of oral rifampin and another antibiotic (e.g., a quinolone, an antistaphylococcal penicillin, or vancomycin) is given for 3–6 months to persons with staphylococcal prosthetic joint infection of short duration. This approach is based on the ability of rifampin to kill organisms adher­ ent to foreign material and in the stationary growth phase. ■ ■PREVENTION To avoid the disastrous consequences of infection, candidates for joint replacement should be selected with care. All modifiable risk factors should be identified and minimized preoperatively to improve the surgical outcome and prevent surgical site infection. Preoperative screening for S. aureus with decolonization should be considered. Rates of infection are particularly high among patients with rheumatoid arthritis, persons who have undergone previous surgery on the joint, and persons with medical conditions requiring immunosuppressive therapy. Perioperative antibiotic prophylaxis, usually with cefazolin, and measures to decrease intraoperative contamination, such as lami­ nar flow, have lowered the rates of perioperative infection to <1% in many centers. After implantation, measures should be taken to prevent or rapidly treat extraarticular infections that might give rise to hema­ togenous spread to the prosthesis. The effectiveness of prophylactic antibiotics for the prevention of hematogenous infection after dental procedures has not been demonstrated; in fact, viridans streptococci and other components of the oral flora are extremely unusual causes of prosthetic joint infection. Accordingly, the American Dental Associa­ tion and the American Academy of Orthopaedic Surgeons do not rec­ ommend antibiotic prophylaxis for most dental patients with total joint replacements and have stated that there is no convincing evidence to support its use. Similarly, guidelines issued by the American Urological Association and the American Academy of Orthopaedic Surgeons do not recommend the use of prophylactic antibiotics for most patients with prosthetic joints who are undergoing urologic procedures but state that prophylaxis should be considered in certain situations—e.g., for patients (especially immunocompromised patients) who are under­ going a procedure posing a relatively high risk of bacteremia (e.g., lithotripsy or surgery involving bowel segments). ■ ■FURTHER READING Bardin T: Gonococcal arthritis. Best Pract Res Clin Rheumatol 17:201, 2003. Beam E, Osmon D: Prosthetic joint infection update. Infect Dis Clin North Am 32:843, 2018.

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136 Osteomyelitis

Borzio R et al: Predictors of septic arthritis in the adult population. Orthopedics 39:e657, 2016. Franssila R, Hedman K: Infection and musculoskeletal conditions: Viral causes of arthritis. Best Pract Res Clin Rheumatol 20:1139, 2006. Harrington JT: Mycobacterial and fungal arthritis. Curr Opin Rheumatol 10:335, 1998. Huotari K et al: High cure rate for acute streptococcal prosthetic joint infections treated with debridement, antimicrobials, and implant retention in a specialized tertiary care center. Clin Infect Dis 67:1288, 2018. Meehan AM et al: Outcome of penicillin-susceptible streptococcal prosthetic joint infection treated with debridement and retention of the prosthesis. Clin Infect Dis 36:845, 2003. Mohammad M et al: The role of Staphylococcus aureus lipoproteins in hematogenous septic arthritis. Sci Rep 10:7936, 2020. Osmon DR et al: Diagnosis and management of prosthetic joint infection: Clinical practice guidelines by the Infectious Diseases Society of America. Clin Infect Dis 56:e1, 2013. Patel R: Periprosthetic joint infection. N Engl J Med 388:251, 2023. Ross J et al: Septic arthritis and the opioid epidemic: 1465 cases of culture-positive native joint septic arthritis from 1990−2018. Open Forum Infect Dis 7:ofaa089, 2020. Zeller V et al: One-stage exchange arthroplasty for chronic peripros­ thetic hip infection: Results of a large prospective cohort study. J Bone Joint Surg Am 96:e1, 2014. Zimmerli W et al: Prosthetic-joint infections. N Engl J Med 351:145, 2004. Werner Zimmerli

Osteomyelitis Osteomyelitis, an infection of bone, can be caused by various microorganisms that arrive at bone through different routes. Spon­ taneous hematogenous osteomyelitis may occur in otherwise healthy individuals, whereas local microbial spread mainly affects either indi­ viduals who have underlying disease (e.g., vascular insufficiency) or patients who have compromised skin or other tissue barriers, with con­ sequent exposure of bone. The latter situation typically follows surgery involving bone, such as sternotomy or orthopedic repair. The manifestations of osteomyelitis are different in children and adults. In children, circulating microorganisms seed mainly long bones, whereas in adults, the vertebral column is the most commonly affected site. Management of osteomyelitis differs greatly depending on whether an implant is involved. The most important aim of the management of either type of osteomyelitis is to prevent progression to chronic osteomyelitis by rapid diagnosis and prompt treatment. Therefore, unambiguous case definitions are required. Device-related bone and joint infection necessitates a multidisciplinary approach requiring antibiotic therapy and, in many cases, surgical removal of the device. For most types of osteomyelitis, the optimal duration and route of anti­ biotic treatment have not been established in clinical trials. Therefore, the recommendations for therapy in this chapter reflect mainly expert opinions. CLASSIFICATION There is no generally accepted, comprehensive system for classification of osteomyelitis, primarily because of the multifaceted presentation of this infection. Different specialists are confronted with different facets of bone disease. Most often, however, general practitioners or

internists are the first to encounter patients with the initial signs and symptoms of osteomyelitis. These primary care physicians should be able to recognize this disease in any of its forms. Osteomyelitis cases can be classified by various criteria, including pathogenesis, duration of infection, location of infection, and presence or absence of foreign material. The widely used Cierny-Mader staging system is useful mainly for trauma surgeons. It classifies osteomyelitis according to anatomic site, comorbidity, and radiographic findings, with stratifica­ tion of long-bone osteomyelitis to optimize surgical management; this system encompasses both systemic and local factors affecting immune status, metabolism, and local vascularity.

Any of three mechanisms can underlie osteomyelitis: (1) hematog­ enous spread; (2) spread from a contiguous site following surgery; and (3) secondary infection in the setting of vascular insufficiency or con­ comitant neuropathy. Hematogenous osteomyelitis in adults typically involves the vertebral column. In only about half of patients a primary focus can be detected. The most common primary foci of infection are the urinary tract, skin/soft tissue, intravascular catheterization sites, and the endocardium. Spread from a contiguous source follows either bone trauma or surgical intervention. Wound infection leading to osteomyelitis typically occurs after cardiovascular intervention involv­ ing the sternum, orthopedic repair after open fracture, or prosthetic joint insertion. Osteomyelitis secondary to vascular insufficiency or peripheral neuropathy most often follows chronic, progressively deep skin and soft tissue infection of the foot. The most common underlying condition is diabetes. In diabetes that is poorly controlled, the diabetic foot syndrome is caused by skin, soft tissue, and bone ischemia com­ bined with motor, sensory, and autonomic neuropathy. CHAPTER 136 Classification of osteomyelitis according to the duration of infec­ tion, although ill defined, is useful because the management of acute and chronic osteomyelitis differs. However, not a defined duration of infection, but the presence or absence of bone necrosis (sequesters) is crucial. Acute osteomyelitis without bone necrosis can generally be treated with antibiotics alone. In contrast, for chronic osteomyelitis antibiotic treatment should be combined with debridement surgery. Acute hematogenous or contiguous osteomyelitis evolves over a short period—i.e., a few days or weeks. In contrast, subacute or chronic osteomyelitis lasts for weeks or months before treatment is started. Typical examples of a subacute course are vertebral osteomyelitis due to tuberculosis or brucellosis and delayed implant-associated infections caused mainly by low-virulence microorganisms (coagulase-negative staphylococci, Cutibacterium acnes). A special form of osteomyelitis is Brodie’s abscess, mainly occurring in young males. Chronic osteomy­ elitis develops when insufficient therapy leads to persistence or recur­ rence, most often after sternal, mandibular, or foot infection. Osteomyelitis Classification by location distinguishes among cases in the long bones, the vertebral column, and the periarticular bones. Long bones are generally involved after hematogenous seeding in children or contiguous spread following trauma or surgery. The risk of vertebral osteomyelitis in adults increases with age. Periarticular osteomyelitis, which complicates septic arthritis that has not been adequately treated, is especially common in periprosthetic joint infection. Osteomyelitis involving a foreign device requires surgical manage­ ment for cure. Even acute implant-associated infection calls for pro­ longed antimicrobial therapy. Therefore, identification of this type of disease is of practical importance. VERTEBRAL OSTEOMYELITIS ■ ■PATHOGENESIS Vertebral osteomyelitis, also referred to as disk-space infection, septic diskitis, spondylodiskitis, or spinal osteomyelitis, is the most common manifestation of hematogenous bone infection in adults. This designa­ tion reflects a pathogenic process leading to involvement of the adja­ cent vertebrae and the corresponding intervertebral disk. In adults, the disk is avascular. Microorganisms invade via the segmental arterial cir­ culation in adjacent endplates and then spread into the disk. Alterna­ tive routes of infection are retrograde seeding through the prevertebral venous plexus and direct inoculation during spinal surgery, epidural

infiltration, or trauma. In the setting of implant surgery, microorgan­ isms are inoculated either during the procedure or, if wound healing is impaired, in the early postoperative period.

■ ■EPIDEMIOLOGY Vertebral osteomyelitis occurs more often in male than in female patients (ratio, 1.5:1). Between 1995 and 2008, the incidence rate increased from 2.2 to 5.8 cases/100,000 person-years. There is a clear age-dependent increase. Men age ≥70 years have a sixfold higher inci­ dence rate than those <70 years. The observed increase in reported cases over time may reflect improvements in diagnosis resulting from the broad availability of MRI technology. In addition, the fraction of cases of vertebral osteomyelitis acquired in association with health care is increasing as a consequence of comorbidity and the rising number of invasive interventions. ■ ■MICROBIOLOGY Vertebral osteomyelitis is typically classified as pyogenic or nonpyo­ genic. However, this distinction is arbitrary: in “nonpyogenic” cases (tuberculous, brucellar), macroscopic pus formation (caseous necrosis, abscess) is quite common. A more accurate scheme is to classify cases as acute or subacute/chronic. Whereas the microbiologic spectrum of acute cases is similar in different parts of the world, the spectrum of subacute/chronic cases varies according to the geographic region. The great majority of cases are monomicrobial in etiology. Of episodes of acute vertebral osteomyelitis, 40–50% are caused by Staphylococcus aureus, 12% by streptococci, and 20% by gram-negative bacilli—mainly Escherichia coli (9%) and Pseudomonas aeruginosa (6%). Subacute vertebral osteomyelitis is typically caused by Mycobacterium tubercu­ losis or Brucella species in regions where these microorganisms are endemic. Osteomyelitis due to viridans streptococci also has a subacute presentation; these infections most often occur as secondary foci in patients with endocarditis. In vertebral osteomyelitis due to Candida species, the diagnosis is often delayed by several weeks; this etiology should be suspected in IV drug users who do not use sterile parapher­ nalia. In implant-associated spinal osteomyelitis, coagulase-negative staphylococci and C. acnes—which, in the absence of an implant, are generally considered contaminants—typically cause low-grade (chronic) infections. As an exception, coagulase-negative staphylococci can cause native spinal osteomyelitis in cases of prolonged bacteremia (e.g., in patients with infected pacemaker electrodes or implanted vas­ cular catheters that are not promptly removed). PART 5 Infectious Diseases ■ ■CLINICAL MANIFESTATIONS The signs and symptoms of vertebral osteomyelitis are nonspecific. Only about half of patients develop fever >38°C (>100.4°F), perhaps because patients frequently use analgesic drugs. Back pain is the lead­ ing initial symptom (>85% of cases). The location of the pain corre­ sponds to the site of infection: the cervical spine in ~10% of cases, the thoracic spine in 30%, and the lumbar spine in 60%. One exception is involvement at the thoracic level in two-thirds of cases of tuberculous osteomyelitis and at the lumbar level in only one-third. This difference is due to direct mycobacterial spread via pleural or mediastinal lymph nodes in pulmonary tuberculosis. Neurologic deficits, such as radiculopathy, weakness, or sensory loss, are observed in about one-third of cases of vertebral osteomyelitis. Neurologic signs and symptoms are caused mostly by spinal epidural abscess. This complication starts with severe localized back pain and progresses to radicular pain, reflex changes, sensory abnormalities, motor weakness, bowel and bladder dysfunction, and paralysis. A primary focus should always be sought but is found in only half of cases. Overall, endocarditis is identified in ~10% of patients. In osteo­ myelitis caused by viridans streptococci, endocarditis is the source in about half of patients. Implant-associated spinal osteomyelitis can present as either early- or late-onset infection. Early-onset infection is diagnosed within 30 days after implant placement. S. aureus is the most common patho­ gen. Wound healing impairment and fever are the leading findings. Late-onset infection is diagnosed beyond 30 days after surgery, with

low-virulence organisms such as coagulase-negative staphylococci or C. acnes as typical infecting agents. Fever is rare. One-quarter of patients have a sinus tract. Because of the delayed course and the lack of classic signs of infection, rapid diagnosis requires a high degree of suspicion. ■ ■DIAGNOSIS Leukocytosis and neutrophilia have low levels of diagnostic sensitivity (only 65% and 40%, respectively). In contrast, an increased erythrocyte sedimentation rate or C-reactive protein (CRP) level has been reported in 98% and 100% of cases, respectively; thus, these tests are helpful in excluding vertebral osteomyelitis. The fraction of blood cultures that yield positive results depends heavily on whether the patient has been pretreated with antibiotics; across studies, the range is 30–78%. In view of this low rate of positive blood culture after antibiotic treatment, such therapy should be withheld until microbial growth is proven unless the patient has sepsis syndrome. In patients with negative blood cultures, CT-guided or open biopsy is needed. Whether a CT-guided biopsy with a negative result is repeated or followed by open biopsy depends on the experience of personnel at the specific center. Bone samples should be cultured for aerobic, anaerobic, and fungal agents, with a portion of the sample sent for histopathologic study. In cases with a subacute/chronic presentation, a suggestive history, or a granuloma detected during histopathologic analysis, mycobacteria and brucellae also should be sought. When blood and tissue cultures are negative despite suggestive histopathology, nonculture techniques (eubacterial or multiplex polymerase chain reaction analysis, metagenomics) of biopsy specimens or aspirated pus should be considered. These tech­ niques allow detection of unusual pathogens such as Helicobacter spp. or Tropheryma whipplei. Given that signs and symptoms of osteomyelitis are nonspecific, the clinical differential diagnosis of febrile back pain is broad, includ­ ing pyelonephritis, pancreatitis, and viral syndromes. In addition, multiple noninfectious pathologies of the vertebral column, such as osteoporotic fracture, seronegative spondylitis (ankylosing spondylitis, psoriasis, reactive arthritis, enteropathic arthritis), and spinal stenosis must be considered. Imaging procedures are the most important tools not only for the diagnosis of vertebral osteomyelitis but also for the detection of pyo­ genic complications and alternative conditions (e.g., bone metastases or osteoporotic fractures). Plain radiography is a reasonable first step in evaluating patients without neurologic symptoms and may reveal an alternative diagnosis. Because of its low sensitivity, plain radiography generally is not helpful in acute osteomyelitis, but it can be useful in subacute or chronic cases. The gold standard is MRI, which should be performed expeditiously in patients with neurologic impairment in order to rule out a herniated disk or to detect pyogenic complications in a timely manner (Fig. 136-1, left). Even if the pathologic findings on MRI suggest vertebral osteomyelitis, alternative diagnoses should be considered, especially when blood cultures are negative. The most common alternative diagnosis is erosive osteochondrosis. Septic bone necrosis, gouty spondylodiskitis, and erosive diskovertebral lesions (Andersson lesions) in ankylosing spondylitis may likewise mimic ver­ tebral osteomyelitis. CT is less sensitive than MRI but may be helpful in guiding a percutaneous biopsy. Positron emission tomography (PET) with 18F-fluorodeoxyglucose, which has a high degree of diagnostic accuracy, is an alternative imaging procedure when MRI is contra­ indicated (Fig. 136-1, right). 18F-fluorodeoxyglucose PET should be considered for patients with implants and patients in whom several foci are suspected. TREATMENT Vertebral Osteomyelitis The aims of therapy for vertebral osteomyelitis are (1) elimination of the pathogen(s), (2) protection from further bone loss, (3) relief of back pain, (4) prevention of complications, and (5) stabilization, if needed.

A FIGURE 136-1  Left: MRI from a 53-year-old man suffering from prosthetic aortic valve endocarditis (Aggregatibacter actinomycetemcomitans). In addition, he experienced lumbar pain for 7 weeks. MRI sagittal sequence shows on T1 fat-saturated post-gadolinium image enhancement in the intervertebral disk space (ventral arrow) and a small epidural abscess (dorsal arrow). Right: PET/CT from the same patient 4 weeks earlier. PET/CT fusion shows fluorodeoxyglucose uptake at L5 ventral (small arrow) and dorsal of S1 (large arrow: epidural abscess). (Figures courtesy of Damien Toia, MD, Kantonsspital Baselland; with permission.) Table 136-1 summarizes suggested antimicrobial regimens for infections attributable to the most common etiologic agents. For optimal antimicrobial therapy, identification of the infecting agent is required. Therefore, in patients without sepsis syndrome, antibiotics should not be administered until the pathogen is identified in a blood culture, a bone biopsy, or an aspirated pus collection. Traditionally, bone infections are at least initially treated by the IV route. However, the preference for the IV route is not evidence based. There are no good arguments for the assumption that IV therapy is superior to oral administration if the following requirements are met: (1) opti­ mal antibiotic spectrum, (2) excellent bioavailability of the oral drug, (3) clinical studies confirming efficacy of the oral drug, (4) normal intestinal function, and (5) no vomiting. Indeed, in a controlled trial in patients with bone and joint infections, including vertebral osteomyelitis, oral antibiotic therapy was noninferior to intravenous therapy when used during the first 6 weeks. Nevertheless, a short initial course of parenteral therapy with a β-lactam antibiotic may lower the risk of emergence of fluoroquinolone resistance, especially if P. aeruginosa infection is treated with ciprofloxacin or staphylo­ coccal infection with the combination of a fluoroquinolone plus rifampin. These suggestions are based on observational studies and expert opinion. A randomized, controlled trial showed that 6 weeks of antibiotic treatment is not inferior to a 12-week course in patients with pyogenic vertebral osteomyelitis. The cure rate was 90.9% in both groups 1 year after therapy. Thus, prolonged antibiotic therapy is required only for patients with undrained abscesses and for patients with spinal implants. Treatment efficacy should be regularly monitored through inquiries about signs and symptoms (fever, pain) and assessment for signs of inflammation (elevated CRP concentra­ tions). Follow-up MRI is appropriate only for patients with pyogenic complications since the correlation between clinical healing and improvement on MRI is very poor. Surgical treatment generally is not needed in acute hematog­ enous vertebral osteomyelitis. However, it is always necessary in implant-associated spinal infection. Early infections (those occur­ ring up to 30 days after internal stabilization) can be cured with debridement, implant retention, and a 3-month course of antibi­ otics (Table 136-2). In contrast, in late infection with a duration

B CHAPTER 136 of >30 days, implant removal and a 6-week course of antibiotics (Table 136-1) are required for complete elimination of the infection. If implants cannot be removed, oral suppressive long-term treat­ ment should follow the initial course of IV antibiotics. The optimal duration of suppressive therapy is unknown. However, if antibiotic therapy is discontinued after, for example, 1 year, close clinical and laboratory (CRP) follow-up is needed. Osteomyelitis ■ ■COMPLICATIONS Complications should be suspected when there is persistent pain, a persistently increased CRP level, and new-onset or persistent neuro­ logic impairment. In cases of persistent pain with or without signs of inflammation, paravertebral, epidural, or psoas abscesses must be sought. Epidural abscesses occur in 15–20% of cases. This complica­ tion is more common in the cervical column (30%) than in the lumbar spine (12%). Risk factors for severe neurologic deficit were epidural abscess, cervical and/or thoracic involvement, and S. aureus vertebral osteomyelitis. Persistent pain despite normalization of CRP values indicates mechanical complications such as severe osteonecrosis or spinal instability. These patients require a consult with an experienced orthopedic surgeon. ■ ■GLOBAL CONSIDERATIONS The incidence rate of acute vertebral osteomyelitis is similar in dif­ ferent regions of the world. In contrast, subacute/chronic vertebral osteomyelitis predominates in defined regions. Cases attributable to brucellosis predominate in endemic areas such as the Middle East, Africa, Central and South America, and the Indian subcontinent. Tuberculosis is an especially frequent cause in Africa and Asia (India, Indonesia, China), where more than two-thirds of the global tubercu­ losis burden is reported. Thus, specific diagnostic tests are needed in patients either living in or having traveled to these regions. OSTEOMYELITIS IN LONG BONES ■ ■PATHOGENESIS Osteomyelitis in long bones is a consequence of hematogenous seed­ ing, exogenous contamination during trauma (open fracture), or

TABLE 136-1  Antibiotic Therapy for Osteomyelitis in Adults without Implantsa MICROORGANISM ANTIMICROBIAL AGENT (DOSE,b ROUTE) Staphylococcus spp.     Methicillin-susceptible Nafcillin or oxacillinc (2 g IV q6h)   followed by   Rifampin (300–450 mg PO q12h) plus levofloxacin (750 mg PO q24h or 500 mg PO q12h)   Methicillin-resistant Vancomycind (15 mg/kg IV q12h) or daptomycin (8–10 mg/kg IV q24h)   followed by   Rifampin (300–450 mg PO q12h)   plus   Levofloxacin (750 mg PO q24h or 500 mg PO q12h) or TMP-SMXe (1 double-strength tablet PO q8h)

or fusidic acid (500 mg PO q8h) Streptococcus spp. Penicillin Gc (5 million units IV q6h) or ceftriaxone (2 g IV q24h) Enterobacteriaceae   Quinolone-susceptible   Quinolone-resistantf Ciprofloxacin (750 mg PO q24h) Imipenem (500 mg IV q6h) or meropenem (1–2g IV q8h) Pseudomonas aeruginosa Cefepime or ceftazidime (2 g IV q8h) plus an aminoglycosideg   or   Piperacillin-tazobactam (4.5 g IV q8h) plus an aminoglycosideg for 2–4 weeks   followed by PART 5 Infectious Diseases   Ciprofloxacinh (750 mg PO q12h) Anaerobes Clindamycin (600 mg IV q6–8h) for 2–4 weeks   followed by   Clindamycini (300 mg PO q6h) aUnless otherwise indicated, the total duration of antimicrobial treatment is generally 6 weeks. bAll dosages are for adults with normal renal function. cWhen the patient has delayed-type penicillin hypersensitivity, cefuroxime (1.5 g IV q6–8h) can be administered. When the patient has immediate-type penicillin hypersensitivity, the penicillin should be replaced by vancomycin (1 g IV q12h). dTarget vancomycin trough level: 15–20 μg/mL. eTrimethoprim-sulfamethoxazole. A double-strength tablet contains 160 mg of trimethoprim and 800 mg of sulfamethoxazole. fIncluding isolates producing extended-spectrum β-lactamase. gThe need for addition of an aminoglycoside has not yet been proven. However, this addition may decrease the risk of emergence of resistance to the β-lactam. hThe rationale for starting ciprofloxacin treatment only after pretreatment with a β-lactam is the increased risk of emergence of quinolone resistance in the presence of a heavy bacterial load. iAlternatively, penicillin G (5 million units IV q6h) or ceftriaxone (2 g IV q24h) can be used against gram-positive anaerobes (e.g., Cutibacterium acnes), and metronidazole (500 mg IV/PO q8h) can be used against gram-negative anaerobes (e.g., Bacteroides spp.). Source: From W Zimmerli: Vertebral osteomyelitis. N Engl J Med 362:1022, 2010. Copyright © 2010 Massachusetts Medical Society. Reprinted with permission from Massachusetts Medical Society. perioperative contamination during surgery involving bone. Hematog­ enous infection in long bones typically occurs in children. In adults, the leading pathogenic source is exogenous fracture-related infection, mainly associated with internal fixation devices. For classification, the presence of a sequestrum and the status of the surrounding soft tissue are crucial for the decision as to whether a surgical intervention is required. Chronic osteomyelitis can be reactivated after a symptomfree interval of >70 years. Such recurrences are most common among elderly patients who developed S. aureus osteomyelitis in the preanti­ biotic era. ■ ■EPIDEMIOLOGY In adults, most cases of long-bone osteomyelitis are posttraumatic or postsurgical; less frequently, late recurrence arises from hematog­ enous infections during childhood. For postoperative or postsurgical osteomyelitis, the term “facture-related infection” was generated. The risk of infection depends on the type of fracture. After closed fracture, implant-associated infection occurs in fewer than 1% of patients. In contrast, after open fracture, the risk of osteomyelitis ranges from ~2%

up to 30%, with the precise figure depending on the degree of tissue damage during trauma and the time between injury and admission to a specialized center. ■ ■MICROBIOLOGY The spectrum of microorganisms causing hematogenous long-bone osteomyelitis does not differ from that in vertebral osteomyelitis. S. aureus is most commonly isolated in each type of osteomyelitis. In rare cases, mycobacteria or fungal agents such as Cryptococcus species, Sporothrix schenckii, Blastomyces dermatitidis, or Coccidioides species are found in patients who live or have traveled in endemic regions. Impaired cellular immunity (e.g., in HIV infection or after transplantation) predisposes to these etiologies. Coagulase-negative staphylococci are the second most common etiologic agents (after S. aureus) in implant-associated osteomyelitis. After open fracture, contiguous long-bone osteomyelitis is typically caused by gram-negative bacilli or a polymicrobial mixture of organisms. ■ ■CLINICAL MANIFESTATIONS The leading symptoms in adults with primary or recurrent hematog­ enous long-bone osteomyelitis are pain and low-grade fever. Infection occasionally manifests as clinical sepsis and local signs of inflamma­ tion (erythema and swelling). After internal fixation, osteomyelitis can be classified as early (acute; <3 weeks), delayed (3–10 weeks), or late (chronic) infection. Early/acute long-bone osteomyelitis manifests as signs of surgical site infection, such as erythema and impaired wound healing. Acute implant-associated infection may also follow hematog­ enous seeding at any time after implantation of a device. Typical symp­ toms are new-onset pain and signs of sepsis. Delayed or late (chronic) infections are usually caused by low-virulence microorganisms or occur after ineffective treatment of early-onset infection. Patients may present with persisting pain, subtle local signs of inflammation, intermittent discharge of pus, or fluctuating erythema over the scar (Fig. 136-2). A special form of subacute osteomyelitis is Brodie’s abscess, which is characterized by pain (98%) and swelling (53%), mainly in the tibia or the femur. Fever and inflammatory markers are typical. The median delay from symptoms to diagnosis is 3 months. Thus, a young patient with unclear localized pain in the tibia or femur should be worked-up with an imaging modality (plain x-ray, MRI, or CT). ■ ■DIAGNOSIS The diagnostic workup for acute hematogenous long-bone osteomy­ elitis is similar to that for vertebral osteomyelitis. Bone remodeling and thus marker uptake are increased for at least 1 year after surgery. Therefore, the three-phase bone scan is not useful during this interval. However, in late recurrences it allows rapid diagnosis at low cost. If the results are positive, CT is required to estimate the extent of inflamed tissue and detect bone necrosis (sequesters). Implant-associated infec­ tion should be suspected if CRP values do not return to the normal range or rise after an initial decrease. Clinical and laboratory suspicion should prompt surgical exploration and sampling. In osteomyelitis of >1 year’s duration, single-photon emission CT plus conventional CT (SPECT/CT) is a good option, either with 99mTc methylene diphosphonate (99mTc-MDP)–labeled leukocytes or with labeled monoclonal antibodies to granulocytes. Surgical debridement is needed for diagnostic (biopsy culture, histology) and therapeutic reasons. TREATMENT Osteomyelitis in Long Bones Treatment for acute hematogenous infection in long bones is iden­ tical to that for acute vertebral osteomyelitis (Table 136-1). The suggested duration of antibiotic therapy is 4–6 weeks. In patients with good soft tissue condition and no sequestra or implants, generally no surgical intervention is required. According to a controlled trial, oral treatment can be given, provided that a regi­ men with excellent oral biocompatibility is available. An initial IV course can be as short as a few days, if the microorganism and

TABLE 136-2  Antibiotic Therapy for Osteomyelitis Associated with Orthopedic Devices MICROORGANISM ANTIMICROBIAL AGENTa (DOSE, ROUTE) Staphylococcus spp. Recommendation for initial treatment phase (2 weeks with implant)   Methicillin-susceptible Rifampin (450 mg PO/IV q12hb)   plus   Nafcillin or oxacillinc (2 g IV q6h)   Methicillin-resistant Rifampin (450 mg PO/IV q12hb)   plus   Vancomycin (15 mg/kg IV q12h) or daptomycin (8–10 mg/kg IV q24h) Staphylococcus spp. Recommendation after completion of initial treatment phase   Rifampin (450 mg PO q12hb)   plus   Levofloxacin (750 mg PO q24h or 500 mg PO q12h) or ciprofloxacin (750 mg PO q12h) or fusidic acid (500 mg PO q8h) or TMP-SMXd (1 double-strength tablet PO q8h) or minocycline (100 mg PO q12h) or linezolid (600 mg PO q12h) or clindamycin (1200–1350 mg/d PO in 3 or 4 divided doses) Streptococcus spp.e Penicillin Gc (18–24 million units/d IV in 6 divided doses) or ceftriaxone (2 g IV q24h) for 4 weeks   followed by   Amoxicillin (750–1000 mg PO q6–8h) or clindamycin (1200–1350 mg/d PO in 3 or 4 divided doses) Enterococcus spp.f     Penicillin-susceptible Penicillin Gc (24 million units/d IV in 6 divided doses) or ampicillin or amoxicilling (2 g IV q4–6h)   Penicillin-resistant Vancomycin (15 mg/kg IV q12h) or daptomycin (6–10 mg/kg IV q24h) or linezolid (600 mg IV/PO q12h) Enterobacteriaceae A β-lactam selected in light of in vitro susceptibility profile for 2 weeksh   followed by   Ciprofloxacin (750 mg PO q12h) Enterobacter spp.i and nonfermentersj (e.g., Pseudomonas aeruginosa) Cefepime or ceftazidime (2 g IV q8h) or meropenem (1–2 g IV q8hk) for 2–4 weeks followed by Ciprofloxacin (750 mg PO q12h) Cutibacterium spp. Penicillin Gc (18–24 million units/d IV in 6 divided doses) or clindamycin (600–900 mg IV q8h) for 2–4 weeks   followed by   Amoxicillin (750–1000 mg PO q6–8h) or clindamycin (1200–1350 mg/d PO in 3 or 4 divided doses) Gram-negative anaerobes

(e.g., Bacteroides spp.) Metronidazole (500 mg IV/PO q8h) Mixed bacteria (without

methicillin-resistant staphylococci) Ampicillin-sulbactam (3 g IV q6h) or amoxicillin-clavulanatel (2.2 g IV q6h) or piperacillin-tazobactam (4.5 g IV q8h)

or imipenem (500 mg IV q6h) or meropenem (1–2 g IV q8hk) for 2–4 weeks followed by Individualized oral regimens chosen in light of antimicrobial susceptibility aAntimicrobial agents should be chosen in light of the isolate’s in vitro susceptibility, the patient’s drug allergies and intolerances, potential drug interactions, and contraindications to specific drugs. All dosages recommended are for adults with normal renal and hepatic function. See text for total durations of antibiotic treatment. bOther dosages and intervals of administration with equivalent success rates have been reported. cWhen the patient has delayed-type penicillin hypersensitivity, cefazolin (2 g IV q8h) can be administered. When the patient has immediate-type penicillin hypersensitivity, the penicillin should be replaced by vancomycin (1 g IV q12h). dTrimethoprim-sulfamethoxazole. A double-strength tablet contains 160 mg of trimethoprim and 800 mg of sulfamethoxazole. eDetermination of the minimal inhibitory concentration (MIC) of penicillin is advisable. fCombination therapy with an aminoglycoside is optional since its superiority to monotherapy for prosthetic joint infection is unproved. When using combination therapy, monitor for signs of aminoglycoside ototoxicity and nephrotoxicity; the latter is potentiated by other nephrotoxic agents (e.g., vancomycin). gFor patients with hypersensitivity to penicillin, see treatment options for penicillin-resistant enterococci. hCiprofloxacin (PO or IV) can be administered to patients with hypersensitivity to β-lactams. iCeftriaxone and ceftazidime should not be administered for treatment targeting Enterobacter species, even strains that test susceptible in the laboratory, but can be used against nonfermenters. Strains producing extended-spectrum β-lactamases should not be treated with any cephalosporin, including cefepime. Enterobacter infections can also be treated with ertapenem (1 g IV q24h); however, ertapenem is not effective against Pseudomonas spp. and other nonfermenters. jAddition of an aminoglycoside is optional. Use of two active drugs can be considered in light of the patient’s clinical condition. kThe recommended dosage is in line with the guidelines of the Infectious Diseases Society of America. In Europe, 2 g IV q8h is suggested for P. aeruginosa infections. lNot available as an IV formulation in the United States. Source: Modified from W Zimmerli et al: N Engl J Med 351:1645, 2004. Massachusetts Medical Society. its antibiotic susceptibility is known. In recurrences of chronic osteomyelitis as well as in each type of exogenous osteomyelitis (acute, chronic, with or without an implant), a combination of surgical debridement, obliteration of dead space, and long-term antibiotic therapy is required. The length of therapy depends on the completeness of the surgical intervention (removal of seques­ tra, implants, and necrotic tissue). The therapeutic aims in patients whose infections are associ­ ated with internal fixation devices are consolidation of the fracture and prevention of chronic osteomyelitis. Stable implants can be maintained except in patients with uncontrolled sepsis. In a sys­ tematic review reporting the outcome of 276 patients, the success rate with retention of the implant was 86–100% in early, 82–89% in delayed, and only 67% in late fracture-related infection. Appropri­ ate antimicrobial therapies are listed in Table 136-2. The cure rate

CHAPTER 136 Osteomyelitis for early staphylococcal implant-associated infections treated with a fluoroquinolone plus rifampin is >90%. Rifampin is efficacious against staphylococcal biofilms of ≤3 weeks’ duration. Similarly, fluoroquinolones are active against biofilms formed by gram-negative bacilli. In these cases, a short initial course of IV therapy with a β-lactam antibiotic is suggested to minimize the risk of emergence of resistance to the oral drugs. The total duration of treatment is 3 months, and the device can be retained even after antibiotics have been discontinued. In contrast, in cases caused by rifampin-

resistant staphylococci or fluoroquinolone-resistant gram-negative bacilli, all hardware should be removed after consolidation of the fracture and before discontinuation of antibiotics. These patients are treated with an oral antibiotic (suppressive therapy) as long as the hardware is retained.

FIGURE 136-2  A 42-year-old man who had sustained a malleolar fracture 6 weeks previously had persistent pain and slight inflammation after orthopedic repair. His infection was treated with oral antibiotics without debridement surgery. This insufficient management of an implant-associated Staphylococcus aureus infection was complicated by a sinus tract. ■ ■COMPLICATIONS The main complication of long-bone osteomyelitis is the persistence of infection with progression to chronic osteomyelitis. This risk is especially high after internal fixation of an open fracture and among patients with implant-associated osteomyelitis that is treated without surgical debridement. In longstanding osteomyelitis, recurrent sinus tracts result in severe damage to skin and soft tissue (Fig. 136-2). Patients who have chronic open wounds need a therapeutic approach combining orthopedic repair and plastic reconstructive surgery. PART 5 Infectious Diseases ■ ■GLOBAL CONSIDERATIONS In North American and Western European countries, tuberculous osteomyelitis is extremely rare, occurring mainly in very old people, HIV-infected patients, and immigrants from endemic countries. In contrast, in countries where the prevalence of tuberculosis is high (India, Indonesia, China), tuberculous osteomyelitis must routinely be considered. PERIPROSTHETIC JOINT INFECTION ■ ■PATHOGENESIS Implanted foreign material is highly susceptible to local infection due to local immunodeficiency around the device. Infection occurs by either the exogenous or the hematogenous route. More rarely, con­ tiguous spread from adjacent sites of osteomyelitis or deep soft-tissue infection may cause periprosthetic joint infection (PJI). The fact that foreign devices are covered with host proteins such as fibronectin favors the adherence of staphylococci and the formation of a biofilm that resists phagocytosis. ■ ■EPIDEMIOLOGY The risk of infection manifesting during the first 2 postoperative years varies according to the joint. It is lowest after hip and knee arthroplasty (0.3–1.5%) and highest after ankle and elbow replacement (4–10%). The risk of hematogenous PJI is highest in the early postoperative period. However, hematogenous seeding occurs throughout life, and most cases therefore develop >2 years after implantation. The rate of risk for secondary PJI during S. aureus bacteremia is 30–40%. ■ ■MICROBIOLOGY About 50–70% of cases of PJI are caused by staphylococci (S. aureus and coagulase-negative staphylococci), 6–10% by streptococci, 4–10% by gram-negative bacilli, and the rest by various other microorganisms. In patients with hematogenous PJI, the fraction of streptococci and

FIGURE 136-3  Acute postoperative periprosthetic joint infection of the left hip caused by group B streptococci in a 68-year-old woman. gram-negative bacilli is higher and the fraction of coagulase-negative staphylococci is much lower. All microorganisms can cause PJI, includ­ ing fungi and mycobacteria. C. acnes causes up to one-third of episodes of periprosthetic shoulder infection. ■ ■CLASSIFICATION AND CLINICAL MANIFESTATIONS PJI is traditionally classified as early (<3 months after implantation), delayed (3–24 months after surgery), or late (>2 years after implanta­ tion). For therapeutic decision-making (see below), it is more useful to classify PJI as (1) acute hematogenous PJI with <3 weeks of symptoms, (2) early postinterventional PJI manifesting within 1 month after sur­ gery, or (3) chronic PJI with symptom duration of >3 weeks. Acute exogenous PJI typically presents with local signs of infection (Fig. 136-3). In contrast, acute hematogenous PJI is most often caused by S. aureus and is characterized by new-onset pain. Local inflamma­ tory signs are rare in hip PJI but frequent in knee PJI. Fever is rare after the initial phase of bacteremia. Key findings in chronic PJI are joint effusion, local pain, implant loosening, and occasionally a sinus tract. Chronic PJI is most commonly caused by low-virulence microorgan­ isms such as coagulase-negative staphylococci or C. acnes. These infec­ tions are characterized by nonspecific symptoms, such as chronic pain caused by low-grade inflammation or early loosening. ■ ■DIAGNOSIS Blood tests such as the measurement of CRP (elevated levels, ≥10 mg/L) and erythrocyte sedimentation rate (elevated rates, ≥30 mm/h) are sensitive (91–97%) but not specific (70–78%). Synovial fluid cell counts are ~90% sensitive and specific, with threshold values of 1700 leukocytes/μL in periprosthetic knee infection and 4200 leukocytes/ μL in periprosthetic hip infection. A biomarker, α-defensin, can be tested in synovial fluid; this biomarker is highly specific and therefore useful in confirming PJI. However, this test is expensive and its sensi­ tivity is limited; therefore, it should not be used for screening. During debridement surgery, at least three but optimally six tissue samples should be obtained for culture and histopathology. If implant mate­ rial (modular parts, screws, or the prosthesis) is removed, sonication of this material followed by culture and/or use of molecular methods to examine the sonicate fluid allows the detection of microorganisms in biofilms. The three-phase bone scan is very sensitive for detecting PJI but is not specific. As mentioned above, this test does not differentiate bone remodeling from infection and therefore is not useful during at least the first year after implantation. CT and MRI detect soft tissue infec­ tion, prosthetic loosening, and bone erosion, but imaging artifacts caused by metal implants limit their use. 18F-fluorodeoxyglucose PET (18F-FD-PET) is an alternative method with good sensitivity but low specificity for the detection of PJI. Therefore, 18F-FDG-PET/CT is use­ ful only in excluding but not confirming PJI.

TREATMENT Periprosthetic Joint Infection The outcome following treatment of PJI is better when managed using a multidisciplinary approach involving an experienced ortho­ pedic surgeon, an infectious disease specialist, a plastic reconstruc­ tive surgeon, and a microbiologist. Therefore, most patients are referred to a specialized center. In general, the goal of treatment is cure—i.e., a pain-free functional joint with complete eradication of the infecting pathogen(s). However, for patients with severe comor­ bidity, lifelong suppressive antimicrobial therapy may be preferred. As a rule, antimicrobial therapy without surgical intervention is not curative but merely suppressive. There are four curative surgi­ cal options: debridement and implant retention, one-stage implant exchange, two-stage implant exchange, and implant removal with­ out replacement. The least invasive treatment option should be selected for each patient without compromising the cure rate. The choice can be guided following a treatment algorithm. However, the different surgical options have not been tested in a randomized controlled trial. Implant retention offers a good chance of infectionfree survival (>80%) only if the following conditions are fulfilled: (1) acute infection, (2) stable implant, (3) pathogen susceptible to a biofilm-active antimicrobial agent (see below), and (4) skin and soft tissue in good condition. Table 136-2 summarizes pathogen-specific antimicrobial therapy for PJI. Initial IV therapy is followed by long-term oral antibiotics. Efficacious treatment is best defined in staphylococcal implantassociated infections. Rifampin exhibits excellent activity against biofilms composed of susceptible staphylococci. Because of the risk of rapid emergence of resistance, rifampin must always be com­ bined with another effective antibiotic. If gram-negative infections are treated with implant retention, fluoroquinolones should be used because of their activity against gram-negative biofilms. ■ ■PREVENTION OF HEMATOGENOUS INFECTION As mentioned above, hematogenous seeding may occur throughout life. This risk is highest during S. aureus bacteremia from a distant focus. Therefore, documented bacterial infections should be promptly treated in patients with prosthetic joints. However, according to a prospective case-control study, the risk of prosthetic hip or knee infec­ tion is not increased following dental procedures. Therefore, antibiotic prophylaxis is not needed during dental work. ■ ■GLOBAL CONSIDERATIONS Rifampin and fluoroquinolones are still the only antimicrobial agents with good activity against staphylococcal and gram-negative biofilms, respectively. Thus, in countries with high rates of rifampin resistance in staphylococci and/or high rates of fluoroquinolone resistance in gramnegative bacilli, debridement with implant retention generally does not yield a good cure rate. STERNAL OSTEOMYELITIS ■ ■PATHOGENESIS Sternal osteomyelitis occurs primarily after sternal surgery (with the entry of exogenous organisms) and more rarely by hematogenous seeding or contiguous extension from adjacent sites of sternocostal arthritis. Exogenous sternal osteomyelitis after open sternal surgery is also called deep sternal-wound infection. Exogenous infection may fol­ low minor sternal trauma, sternal fracture, and manubriosternal septic arthritis. Tuberculous sternal osteomyelitis typically manifests during hematogenous seeding in children or as reactivated infection in adults. Reactivation is sometimes preceded by blunt trauma. In rare cases, tuberculous sternal osteomyelitis is caused by continuous infection from an infected internal mammary lymph node. ■ ■EPIDEMIOLOGY The incidence of poststernotomy wound infection varies from 0.5% to 2%, but figures are even higher among patients with risk factors such

as diabetes, obesity, chronic renal failure, emergency surgery, use of bilateral internal mammary artery grafts, and re-exploration for bleed­ ing. Rapid diagnosis and correct management of superficial sternal wound infection prevent its progression to sternal osteomyelitis. Pri­ mary (hematogenous) sternal osteomyelitis accounts for only 0.3% of all cases of osteomyelitis. Risk factors are IV drug use, HIV infection, radiotherapy, blunt trauma, cardiopulmonary resuscitation, alcohol abuse, liver cirrhosis, and hemoglobinopathy.

■ ■MICROBIOLOGY Poststernotomy osteomyelitis is generally caused by S. aureus (10–20% of cases), coagulase-negative staphylococci (40–60%), gram-negative bacilli (5-15%), or C. acnes (2–10%). The spectrum of microorganisms greatly varies in different populations (as represented in different pub­ lications); this is perhaps due to local epidemiologic conditions, such as antimicrobial stewardship and the incidence of chronic infections. Polymicrobial cases are rare in acute infection and indicate exogenous superinfection during therapy. Hematogenous sternal osteomyelitis is caused most commonly by S. aureus. Other microorganisms play a role in special populations—e.g., P. aeruginosa in IV drug users, Salmonella species in individuals with sickle cell anemia, and M. tuberculosis in patients from endemic areas who have previously had tuberculosis. ■ ■CLINICAL MANIFESTATIONS Exogenous sternal osteomyelitis manifests as fever, increased local pain, erythema, wound discharge, and sternal instability (Fig. 136-4). Contiguous mediastinitis is a feared complication, occurring in ~10–30% of patients with sternal osteomyelitis. Hematogenous sternal osteomyelitis is characterized by sternal pain, swelling, and erythema. In addition, most patients have systemic signs and symptoms of sepsis. CHAPTER 136 The differential diagnosis of hematogenous sternal osteomyelitis includes immunologic processes typically presenting as systemic or multifocal inflammation of the sternum or of the sternoclavicular or sternocostal joints (e.g., SAPHO [synovitis, acne, pustulosis, hyperos­ tosis, osteitis], vasculitis, chronic multifocal relapsing osteomyelitis). Osteomyelitis ■ ■DIAGNOSIS In primary sternal osteomyelitis, the diagnostic workup does not differ from that in other types of hematogenous osteomyelitis (see above). When a patient has grown up in regions where tuberculosis is endemic, a specific workup for mycobacterial infection should be performed, especially if osteomyelitis had its onset after a blunt sternal trauma. In secondary sternal osteomyelitis, leukocyte counts may be normal, but the CRP level is >100 mg/L in most cases. Tissue sampling for microbiologic studies is crucial. In osteomyelitis associated with ster­ nal wires, low-virulence microorganisms, such as coagulase-negative staphylococci, play an important role. In order to differentiate between colonization and infection, samples from at least three deep biopsies should be subjected to microbiologic examination. Superficial swab cultures are not diagnostic and may be misleading. No studies have FIGURE 136-4  Sternal osteomyelitis caused by Staphylococcus epidermidis 5 weeks after sternotomy for aortocoronary bypass in a 72-year-old man.

compared the value of the various imaging modalities in suspected pri­ mary sternal osteomyelitis. However, MRI is the current gold standard for detection of each type of osteomyelitis.

TREATMENT Sternal Osteomyelitis In cases of deep sternal-wound infection, a combined approach using both surgery and antibiotic treatment is required. Antimi­ crobial therapy should be started immediately after samples have been obtained for microbiologic analyses in order to control clini­ cal sepsis. To protect a newly inserted heart valve, initial treatment should be directed against staphylococci, with consideration of the local susceptibility pattern. In centers with a high prevalence of methicillin-resistant S. aureus, vancomycin or daptomycin should be added to a broad-spectrum β-lactam drug. As soon as cultures of blood and/or deep wound biopsies have confirmed the pathogen’s identity and susceptibility pattern, treatment should be optimized and narrowed accordingly. Tables 136-1 and 136-2 show appropri­ ate therapeutic choices for the most frequently identified microor­ ganisms causing sternal osteomyelitis in the absence and presence, respectively, of an implanted device. In a recent observational study of patients with staphylococcal deep sternal-wound infection, the use of a rifampin-containing regimen was predictive of success. The optimal duration of antibiotic therapy has not been established. In acute sternal osteomyelitis without hardware, a 6-week course is the rule. In patients with remaining sternal wires, treatment duration is generally prolonged to 3 months (Table 136-2). Like other types of tuberculous bone infection, tuberculous sternal osteomyelitis is treated for 6–12 months. PART 5 Infectious Diseases Primary sternal osteomyelitis can generally be treated without surgery. In contrast, in secondary sternal osteomyelitis, debride­ ment is always required. This procedure should be performed by a team of experienced surgeons, since mediastinitis, bone infection, and skin and soft tissue damage may need to be treated during the same intervention. ■ ■PROGNOSIS Primary sternal osteomyelitis poses a minimal mortality risk. In con­ trast, the mortality rates from secondary sternal osteomyelitis during the first year after diagnosis are 6–20%. ■ ■GLOBAL CONSIDERATIONS In endemic areas, microorganisms such as M. tuberculosis, Salmonella species, and Brucella species should be considered during sampling for microbiologic diagnosis. FOOT OSTEOMYELITIS ■ ■PATHOGENESIS Osteomyelitis of the foot is a feared complication in patients with diabetic foot ulcers. It also usually occurs in patients with peripheral arterial insufficiency, or peripheral neuropathy and after foot surgery. These entities are often linked to each other, especially in diabetic patients with late complications. However, foot osteomyelitis is also seen in patients with isolated peripheral neuropathy and can manifest as implant-associated osteomyelitis in patients without comorbidity due to a deep wound infection after foot surgery (hallux valgus surgery, arthrodesis, total ankle arthroplasty). Foot osteomyelitis is acquired almost exclusively by the exogenous route. It is a complication of deep pressure ulcers and of impaired wound healing after surgery. ■ ■EPIDEMIOLOGY About 34% of people with diabetes develop a foot ulcer during their lifetime. Among patients with foot ulcer, the annual incidence of pro­ gression to foot osteomyelitis is 5%. The condition starts with skin and soft tissue lesions and progresses to osteomyelitis, especially in patients with risk factors. Diabetic foot osteomyelitis increases the risk of

FIGURE 136-5  Neuropathic joint disease (Charcot foot) complicated by chronic foot osteomyelitis in a 78-year-old woman with diabetes mellitus complicated by severe neuropathy. amputation. With adequate management of the early stage of diabetic foot infections, the rate of amputation can be lowered. ■ ■RISK FACTORS Risk factors for diabetic foot infection are (1) peripheral motor, sensory, and autonomic neuropathy; (2) neuro-osteoarthropathic deformities (Charcot foot; Fig. 136-5); (3) arterial insufficiency; (4) uncontrolled hyperglycemia; (5) disabilities such as reduced vision; and (6) maladaptive behavior. ■ ■MICROBIOLOGY The correlation between cultures from bone biopsy and those from wound swabs or even deep soft-tissue punctures is poor. In a study of 31 patients with simultaneous sampling, the correlation between needle biopsy and bone biopsy cultures was only 24%. The correlation is better when S. aureus is isolated (40–50%) than when anaerobes (20–35%), gram-negative bacilli (20–30%), or coagulase-negative staphylococci (0–20%) are identified. When only bone-biopsy samples are considered, the leading pathogens are S. aureus (25–40%), anaer­ obes (5–20%), and various gram-negative bacilli (18–40%). The precise distribution depends on whether the patient has already been treated with antibiotics. Anaerobes are especially prevalent in chronic wounds. Pretreatment typically selects for P. aeruginosa, methicillin-resistant S. aureus, or enterococci. ■ ■DIAGNOSIS In many cases, foot osteomyelitis can be diagnosed clinically, without imaging procedures. Most clinicians rely on the “probe-to-bone” test, which has a positive predictive value of ~90% in populations with a high pretest probability. Thus, in a patient with diabetes who is hospitalized for a chronic deep foot ulcer, the diagnosis of foot osteomyelitis is highly probable if bone can be directly touched with a metal instrument. In a patient with a lower pretest probability, MRI should be performed because of its high sensitivity (80–100%) and specificity (80–90%). Plain x-rays have typical features character­ izing diabetic foot osteomyelitis. However, their sensitivity is only 30–90% and the specificity varies between 50% and 90%; thus, it is mainly useful for follow-up of patients with confirmed diabetic foot osteomyelitis. TREATMENT Foot Osteomyelitis As mentioned above, correlation between cultures of bone and those of wound swabs or wound punctures is poor. Antibiotic treatment should be based on bone culture. If no bone biopsy is

16 - 137 Intraabdominal Infections and Abscesses

137 Intraabdominal Infections and Abscesses

performed, empirical therapy chosen in light of the most common infecting agents and the type of clinical syndrome should be given. In a controlled therapeutic trial of diabetic patients in whom no infected bone needed to be resected, the outcome of a 6-week course of antibiotics was not different from a 12-week course. In patients undergoing surgical debridement, a 3-week course of sys­ temic antibiotics was noninferior to a 6-week course in a prospec­ tive, randomized pilot trial. Wound debridement combined with antibiotics renders amputation unnecessary in about two-thirds of patients. For the diagnosis and treatment of diabetic foot infec­ tions, the following management strategies should be considered. If a foot ulcer is clinically infected, prompt empirical antimicrobial therapy may prevent progression to osteomyelitis. When the risk of methicillin-resistant S. aureus is considered high, an agent active against these strains (e.g., vancomycin) should be chosen. If the patient has not recently received antibiotics, the spectrum of the selected antibiotic must include gram-positive cocci (e.g., clindamycin, ampicillin-sulbactam). If the patient has received antibiotics within the past month, the spectrum of empirical anti­ biotics should include gram-negative bacilli (e.g., clindamycin plus a fluoroquinolone). If the patient has risk factors for Pseudomonas infection (previous colonization, residence in a warm climate, frequent exposure of the foot to water), an empirical antipseudo­ monal agent (e.g., piperacillin-tazobactam, cefepime) is indicated. If osteomyelitis is suspected either on clinical grounds (probe to bone) or on the basis of imaging procedures (MRI), bone biopsy should be performed. If infected bone is not entirely removed by surgery, the patient should be treated for 4–6 weeks in line with the identified pathogen(s) and their susceptibility. Treatment should initially be given by the IV route. Whether therapy can later be administered by the oral route depends on the bioavailability of oral drugs that cover the infecting agents. In the future, phage therapy may be a valuable additive therapeutic option. ■ ■GLOBAL CONSIDERATIONS The number of multiresistant microorganisms causing diabetic foot infection is increasing. The prevalence of methicillin-resistant S. aureus is 5–43% in various countries. Gram-negative pathogens are more common than gram-positive bacteria in Asia. In a study of 102 patients with diabetic foot infection from India, 69% of aerobic gram-negative bacilli produced extended-spectrum β-lactamase and 43% of S. aureus isolates were methicillin resistant. Risk factors for multidrug-resistant microorganisms are poor glycemic control, prolonged duration of infection, and large ulcer size. ■ ■FURTHER READING Depypere M et al: Pathogenesis and management of fracture-related infection. Clin Microbiol Infect 26:572, 2020. Li H-K et al: Oral versus intravenous antibiotics for bone and joint infection. N Engl J Med 380:425, 2019. Osmon Dr et al: Diagnosis and management of prosthetic joint infec­ tion: Clinical practice guidelines by the Infectious Diseases Society of America. Clin Infect Dis 56:e1, 2013. Senneville E et al: IWGDF/IDSA guidelines on the diagnosis and treatment of diabetes-related foot infections (IWGDF/IDSA 2023). 40:e3687, 2024. Zimmerli W (ed): Bone and Joint Infections. From Microbiology to Diagnostics and Treatment, 2nd ed. West Sussex, UK, John Wiley & Sons, Ltd, 2021. Zimmerli W: Clinical practice. Vertebral osteomyelitis. N Engl J Med 362:1022, 2010.

Miriam Baron Barshak

Intraabdominal Infections

and Abscesses Intraperitoneal infections generally arise because a normal anatomic barrier is disrupted. This disruption may result from a variety of causes—e.g., when the appendix, a diverticulum, or an ulcer ruptures; when the bowel wall is weakened by ischemia, tumor, or inflammation (e.g., in inflammatory bowel disease); or with adjacent inflammatory processes, such as pancreatitis or pelvic inflammatory disease, in which enzymes (in the former case) or organisms (in the latter) may leak into the peritoneal cavity. Whatever the inciting event, once inflammation develops and organisms usually contained within the bowel or another organ enter the normally sterile peritoneal space, a knowable series of events takes place. Intraabdominal infections occur in two stages: peritonitis and—if the patient survives this stage and goes untreated— abscess formation. The types of microorganisms predominating in each stage of infection are responsible for the pathogenesis of disease. PERITONITIS Peritonitis is a life-threatening event that is often accompanied by bacteremia and sepsis syndrome (Chap. 315). The peritoneal cavity is large but is divided into compartments. The upper and lower peri­ toneal cavities are divided by the transverse mesocolon; the greater omentum extends from the transverse mesocolon and from the lower pole of the stomach to line the lower peritoneal cavity. The pancreas, duodenum, and ascending and descending colon are located in the anterior retroperitoneal space; the kidneys, ureters, and adrenals are found in the posterior retroperitoneal space. The other organs, includ­ ing the liver, stomach, gallbladder, spleen, jejunum, ileum, transverse and sigmoid colon, cecum, and appendix, are within the peritoneal cavity. The cavity is lined with a serous membrane that can serve as a conduit for fluids—a property exploited in peritoneal dialysis (Fig. 137-1). A small amount of serous fluid is normally present in the CHAPTER 137 Intraabdominal Infections and Abscesses Left subphrenic Right subphrenic Lesser sac Subhepatic Left paracolic Right paracolic Pelvic FIGURE 137-1  Diagram of the intraperitoneal spaces, showing the circulation of fluid and potential areas for abscess formation. Some compartments collect fluid or pus more often than others. These compartments include the pelvis (the lowest portion), the subphrenic spaces on the right and left sides, and Morrison’s pouch, which is a posterosuperior extension of the subhepatic spaces and is the lowest part of the paravertebral groove when a patient is recumbent. The falciform ligament separating the right and left subphrenic spaces appears to act as a barrier to the spread of infection; consequently, it is unusual to find bilateral subphrenic collections. (Reproduced with permission from B Lorber [ed]: Atlas of Infectious Diseases, vol VII: Intra-abdominal infections, hepatitis, and gastroenteritis. Springer; 1996.)

peritoneal space, with a protein content (consisting mainly of albumin) of <30 g/L and <300 white blood cells (WBCs, generally mononuclear cells) per microliter. In bacterial infections, leukocyte recruitment into the infected peritoneal cavity consists of an early influx of polymor­ phonuclear leukocytes (PMNs) and a prolonged subsequent phase of mononuclear cell migration. The phenotype of the infiltrating leu­ kocytes during the course of inflammation is regulated primarily by resident-cell chemokine synthesis.

■ ■PRIMARY (SPONTANEOUS) BACTERIAL PERITONITIS Peritonitis is either primary (without an apparent source of contami­ nation) or secondary. The types of organisms found and the clinical presentations of these two processes are different. In adults, primary bacterial peritonitis (PBP) occurs most commonly in conjunction with cirrhosis of the liver (frequently the result of alcoholism). However, the disease has been reported in adults with metastatic malignant disease, postnecrotic cirrhosis, chronic active hepatitis, acute viral hepatitis, congestive heart failure, systemic lupus erythematosus, and lymph­ edema as well as in patients with no underlying disease. Although PBP virtually always develops in patients with preexisting ascites, it is, in general, an uncommon event, occurring in ≤10% of cirrhotic patients. The cause of PBP has not been established definitively but is believed to involve hematogenous spread of organisms in a patient in whom a diseased liver and altered portal circulation result in a defect in the usual filtration function. Organisms multiply in ascites, a good medium for growth. Proteins of the complement cascade are found in peritoneal fluid, with lower levels in cirrhotic patients than in patients with ascites of other etiologies. The opsonic and phagocytic properties of PMNs are diminished in patients with advanced liver disease. Cir­ rhosis is associated with alterations in the gut microbiota, including an increased prevalence of potentially pathogenic bacteria such as Enterobacteriaceae. Small-intestinal bacterial overgrowth is frequently present in advanced stages of liver cirrhosis and has been linked with pathologic bacterial translocation and PBP. Factors promoting these changes in cirrhosis may include deficiencies in Paneth cell defensins, reduced intestinal motility, decreased pancreatobiliary secretions, and portal-hypertensive enteropathy. PART 5 Infectious Diseases The presentation of PBP differs from that of secondary peritonitis. The most common manifestation is fever, which is reported in up to 80% of patients. Ascites is found but virtually always predates infec­ tion. Abdominal pain, an acute onset of symptoms, and peritoneal irritation during physical examination can be helpful diagnostically, but the absence of any of these findings does not exclude this oftensubtle diagnosis. Nonlocalizing symptoms (such as malaise, fatigue, or encephalopathy) or jaundice or acute kidney injury without another clear etiology also should prompt consideration of PBP in a susceptible patient. It is vital to sample the peritoneal fluid of any cirrhotic patient with ascites and fever. The finding of >250 PMNs/μL is diagnostic for PBP. This criterion does not apply to secondary peritonitis (see below). The microbiology of PBP also is distinctive. While enteric gram-negative bacilli such as Escherichia coli are commonly encountered, gram-posi­ tive organisms such as streptococci, enterococci, or even pneumococci are sometimes found. In an important development, widespread use of quinolones to prevent PBP in high-risk subgroups of patients, frequent hospitalizations, and exposure to broad-spectrum antibiotics have led to a change in the etiology of infections in patients with cirrhosis, with more gram-positive bacteria and extended-spectrum β-lactamase (ESBL)–producing Enterobacteriaceae in recent years. Risk factors for multidrug-resistant infections include nosocomial origin of infection, long-term norfloxacin prophylaxis, recent infection with multiresistant bacteria, and recent use of β-lactam antibiotics. In PBP, a single organ­ ism is typically isolated; anaerobes are found less frequently in PBP than in secondary peritonitis, in which a mixed flora including anaerobes is the rule. In fact, if PBP is suspected and multiple organisms including anaerobes are recovered from the peritoneal fluid, the diagnosis must be reconsidered and a source of secondary peritonitis sought. The diagnosis of PBP is not easy. It depends on the exclusion of a primary intraabdominal source of infection. Contrast-enhanced CT

FIGURE 137-2  Pneumoperitoneum. Free air under the diaphragm on an upright chest film suggests the presence of a bowel perforation and associated peritonitis. (Image courtesy of Dr. John Braver; with permission.) is useful in identifying an intraabdominal source for infection. It may be difficult to recover organisms from cultures of peritoneal fluid, presumably because the burden of organisms is low. However, the yield can be improved if 10 mL of peritoneal fluid is placed directly into a blood culture bottle. Because bacteremia frequently accompanies PBP, blood should be cultured simultaneously. To maximize the yield, culture samples should be collected prior to administration of antibi­ otics. There is interest in identifying biomarkers in ascites that may be associated with PBP. No specific radiographic studies are helpful in the diagnosis of PBP. A plain film of the abdomen would be expected to show ascites. Chest and abdominal radiography should be performed when patients have abdominal pain to exclude free air, which signals a perforation (Fig. 137-2). TREATMENT Primary Bacterial Peritonitis Treatment for PBP is directed at the isolate from blood or peritoneal fluid. Gram’s staining of peritoneal fluid often gives negative results in PBP. Delays in antibiotic treatment of PBP are associated with increased mortality. Therefore, until culture results become avail­ able, prompt therapy should cover gram-negative aerobic bacilli and gram-positive cocci. Third-generation cephalosporins such as cefotaxime (2 g q8h, administered IV) provide reasonable initial coverage in moderately ill patients if the local prevalence of multi­ drug resistant organisms is low. Broad-spectrum antibiotics, such as β-lactam/β-lactamase inhibitor combinations (e.g., piperacillin/ tazobactam, 3.375 g q6h IV for adults with normal renal function) or ceftriaxone (2 g q24h IV), also are options, with vancomycin for patients who have had prior infection or colonization with methicillin-resistant Staphylococcus aureus. Broader empirical cov­ erage aimed at resistant hospital-acquired gram-negative bacteria (e.g., carbapenem or newer agents, such as ceftolozane-tazobactam or ceftazidime-avibactam) and vancomycin-resistant Enterococcus (e.g., daptomycin) may be appropriate for nosocomially acquired PBP or in patients with prior antibiotic-resistant infections or criti­ cal illness until culture results become available. Empirical coverage for anaerobes is not necessary. A mortality benefit from albumin (1.5 g/kg of body weight within 6 h of detection and 1.0 g/kg on day 3) has been demonstrated for patients with PBP, with greatest

benefit among those who present with serum creatinine levels ≥1 mg/dL, blood urea nitrogen levels ≥30 mg/dL, or total bilirubin levels ≥5 mg/dL. After the infecting organism is identified, therapy should be narrowed to target the specific pathogen. Patients with PBP usually respond within 72 h to appropriate antibiotic therapy. Antimicrobial treatment can be administered for as little as 5 days if rapid improvement occurs and blood cultures are negative, but a course of up to 2 weeks may be required for patients with bac­ teremia and for those whose improvement is slow. Persistence of WBCs in the ascitic fluid after therapy should prompt a search for additional diagnoses. Prognosis: PBP is associated with significant morbidity and mortality, perhaps reflecting the fact that PBP risk is highest among patients with advanced liver disease. In a 2018 study of hospitalized patients with PBP in the United States, in-hospital mortality was 17.6%. Another study in 2019 found that among patients at a U.S. tertiary academic center, mortality following PBP was 23% at 30 days and 37% at 90 days. The morbidity and mortality associated with PBP has led to interest in strategies for PBP prevention. Prevention  •  PRIMARY PREVENTION  Several observational studies and a meta-analysis raise the concern that gastric acid sup­ pression may increase the risk of PBP. No prospective studies have yet addressed whether avoidance of such therapy may prevent PBP. A 2021 guideline from the American Association for the Study of Liver Diseases recommends chronic antibiotic prophylaxis be considered, after discussing risks and benefits, with a regimen described in the next section for patients who are at highest risk for PBP—that is, those with an ascitic-fluid total protein level <1.5 g/dL along with impaired renal function (creatinine, ≥1.2 mg/dL; blood urea nitrogen, ≥25 mg/dL; or serum sodium, ≤130 mEq/L) and/or liver failure (Child-Turcotte-Pugh score, ≥9; and bilirubin, ≥3 mg/dL). A course of antibiotic prophylaxis is recommended for patients with cirrhosis and gastrointestinal bleed­ ing until bleeding has resolved and vasoactive drugs are stopped. SECONDARY PREVENTION  PBP has a high rate of recurrence. Up to 70% of patients experience a recurrence within 1 year. Antibiotic prophylaxis has been recommended for patients with a history of PBP to reduce this rate and improve short-term survival rates. Prophylactic regimens for adults with normal renal function include fluoroqui­ nolones (ciprofloxacin, 500 mg weekly; or norfloxacin [not available in the United States], 400 mg/d) or trimethoprim-sulfamethoxazole (one double-strength tablet daily). However, these recommendations are being reconsidered in recent years as the risks and benefits evolve. Long-term administration of broad-spectrum antibiotics in this setting has been shown to increase the risk of severe staphylococcal infec­ tions and of infections with multidrug-resistant organisms (MDRO). Additionally, quinolone prophylaxis is less effective in patients who are colonized with MDRO. There is increased interest in using rifaximin, a broad-spectrum antibiotic that is used already for hepatic encepha­ lopathy and is not absorbed, for PBP prophylaxis (1200 mg daily). ■ ■SECONDARY PERITONITIS Secondary peritonitis develops when bacteria contaminate the perito­ neum as a result of spillage from an intraabdominal viscus. The organ­ isms found almost always constitute a mixed flora in which facultative gram-negative bacilli and anaerobes predominate, especially when the contaminating source is colonic. Early in the course of infection, when the host response is directed toward containment, exudate containing fibrin and PMNs is found. Early death in this setting is attributable to gram-negative bacillary sepsis and to potent endotoxins circulating in the bloodstream (Chap. 315). Gram-negative bacilli, particularly E. coli, are common bloodstream isolates, but Bacteroides fragilis bac­ teremia also occurs. The severity of abdominal pain and the clinical course depend on the inciting process. The organisms isolated from the peritoneum also vary with the source of the initial process and the normal flora at that site. Secondary peritonitis can result primarily from chemical irritation and/or bacterial contamination. For example, as long as the patient is not achlorhydric, a ruptured gastric ulcer will

release low-pH gastric contents that will serve as a chemical irritant. The normal flora of the stomach comprises the same organisms found in the oropharynx but in lower numbers. Thus, the bacterial burden in a ruptured ulcer is negligible compared with that in a ruptured appen­ dix. The normal flora of the colon below the ligament of Treitz contains ~1011 anaerobic organisms/g of feces but only 108 aerobes/g; therefore, anaerobic species account for 99.9% of the bacteria (Chap. 484). Leak­ age of colonic contents (pH 7–8) does not cause significant chemical peritonitis, but infection is intense because of the heavy bacterial load.

Depending on the inciting event, local symptoms may occur in sec­ ondary peritonitis—for example, epigastric pain from a ruptured gas­ tric ulcer. In appendicitis (Chap. 342), the initial presenting symptoms are often vague, with periumbilical discomfort and nausea followed in a number of hours by pain more localized to the right lower quadrant. Unusual locations of the appendix (including a retrocecal position) can complicate this presentation further. Once infection has spread to the peritoneal cavity, pain increases, particularly with infection involving the parietal peritoneum, which is innervated extensively. Patients usu­ ally lie motionless, often with knees drawn up to avoid stretching the nerve fibers of the peritoneal cavity. Coughing and sneezing, which increase pressure within the peritoneal cavity, are associated with sharp pain. There may or may not be pain localized to the infected or dis­ eased organ from which secondary peritonitis has arisen. Patients with secondary peritonitis generally have abnormal findings on abdominal examination, with marked voluntary and involuntary guarding of the anterior abdominal musculature. Later findings include tenderness, especially rebound tenderness. In addition, there may be localized findings in the area of the inciting event. In general, patients are febrile, with marked leukocytosis and a left shift of the WBCs to band forms. CHAPTER 137 While recovery of organisms from peritoneal fluid is easier in secondary than in primary peritonitis, a tap of the abdomen is rarely the procedure of choice in secondary peritonitis. An exception is in cases involving trauma, where the possibility of a hemoperitoneum may need to be excluded early. Emergent studies (such as abdominal CT) to find the source of peritoneal contamination should be under­ taken if the patient is hemodynamically stable; unstable patients may require surgical intervention without prior imaging. Results of cultures from chronic drain sites are not reliable for defining the etiology of infections. Intraabdominal Infections and Abscesses TREATMENT Secondary Peritonitis Treatment for secondary peritonitis includes early administration of antibiotics aimed particularly at aerobic gram-negative bacilli and anaerobes (see below). The most appropriate regimen depends on the anticipated flora and the degree of illness. Community-

acquired infections associated with mild to moderate disease can be treated with many drugs covering these organisms, including broad-spectrum β-lactam/β-lactamase inhibitor combinations (e.g., ticarcillin/clavulanate, 3.1 g q4–6h IV; or piperacillin/tazobactam, 3.375 g q6h IV) or a combination of either a fluoroquinolone (e.g., levofloxacin, 750 mg q24h IV) or a third-generation cephalosporin (e.g., ceftriaxone, 2 g q24h IV) plus metronidazole (500 mg q8h IV). Eravacycline is a newer antibiotic in the tetracycline class that has been approved by the U.S. Food and Drug Administration for treatment of complicated intraabdominal infections (1 mg/ kg q12h IV). Patients in intensive care units and/or those with health care–associated infections should receive antibiotics target­ ing more resistant gram-negative organisms such as Pseudomonas aeruginosa—e.g., imipenem (500 mg q6h IV), meropenem (1 g q8h IV), higher-dose piperacillin/tazobactam (4.5 g IV q6h), or drug combinations such as cefepime (2 g IV q8h) or ceftazidime (2 g IV q8h) plus metronidazole. The role of enterococci and Can­ dida species in mixed infections is controversial; however, because cephalosporin-based regimens lack activity against enterococci, ampicillin or vancomycin can be added to these regimens for enterococcal coverage in very ill patients until culture results are

available. For patients known to be colonized with ampicillin-resistant, vancomycin-resistant enterococci (VRE), a VRE-active agent, such as linezolid or daptomycin, should be included. Antifungal cover­ age is warranted if there is growth of Candida species from a sterile site. Patients who are known to be colonized with highly resistant gram-negative organisms may require treatment with a newer agent such as ceftazidime/avibactam or ceftolozane/tazobactam. Second­ ary peritonitis usually requires both surgical intervention to address the inciting process and antibiotics to treat early bacteremia, to decrease the incidence of abscess formation and wound infection, and to prevent distant spread of infection. Although surgery is rarely indicated in PBP in adults, it may be life-saving in second­ ary peritonitis. In a recent study, the urgency and success of source control impacted the odds of survival, while the choice of empiric antibiotics did not. Recombinant human activated protein C (APC) was considered at one time for treatment of severe sepsis from causes including secondary peritonitis but was withdrawn from the market in 2011 after it was determined that the drug was associated with an increased risk of bleeding and that evidence for its benefi­ cial effects was inadequate. Thus APC should not be used for sepsis or septic shock outside randomized clinical trials.

Peritonitis may develop as a complication of abdominal surger­ ies. These infections may be accompanied by localizing pain and/ or nonlocalizing signs or symptoms such as fever, malaise, anorexia, and toxicity. As a nosocomial infection, postoperative peritonitis may be associated with organisms such as staphylococci, compo­ nents of the gram-negative hospital microflora, and the microbes that cause PBP and secondary peritonitis, as described above. PART 5 Infectious Diseases ■ ■PERITONITIS IN PATIENTS UNDERGOING CONTINUOUS AMBULATORY PERITONEAL DIALYSIS A third type of peritonitis arises in patients who are undergoing continuous ambulatory peritoneal dialysis (CAPD). Unlike PBP and secondary peritonitis, which are caused by endogenous bacteria, CAPD-associated peritonitis usually involves skin organisms. The pathogenesis of infection is similar to that of intravascular device– related infection, in which skin organisms migrate along the catheter, which both serves as an entry point and exerts the effects of a foreign body. Exit-site or tunnel infection may or may not accompany CAPD-

associated peritonitis. Like PBP, CAPD-associated peritonitis is usually caused by a single organism. Peritonitis is, in fact, the most common reason for discontinuation of CAPD. Improvements in equipment design, especially the Y-set connector, have resulted in a decrease from one case of peritonitis per 9 months of CAPD to one case per 24 months. Diabetes was reported to be a risk factor for CAPD-associated perito­ nitis in a study from Taiwan. The clinical presentation of CAPD peritonitis resembles that of sec­ ondary peritonitis in that diffuse pain and peritoneal signs are common. The dialysate is usually cloudy and contains >100 WBCs/μL, >50% of which are neutrophils. However, the number of cells depends in part on dwell time. According to a guideline from the International Society for Peritoneal Dialysis (2016), for patients undergoing automated peri­ toneal dialysis who present during their nighttime treatment and whose dwell time is much shorter than with CAPD, the clinician should use the percentage of PMNs rather than the absolute number of WBCs to diagnose peritonitis. As the normal peritoneum has very few PMNs, a proportion above 50% is strong evidence of peritonitis even if the abso­ lute WBC count does not reach 100/μL. Meanwhile, patients undergoing automated peritoneal dialysis without a daytime exchange who present with abdominal pain may have no fluid to withdraw, in which case 1 L of dialysate should be infused and permitted to dwell a minimum of 1–2 h, then drained, examined for turbidity, and sent for cell count with differential and culture. The differential (with a shortened dwell time) may be more useful than the absolute WBC count. In equivocal cases or in patients with systemic or abdominal symptoms in whom the effluent appears clear, a second exchange is performed, with a dwell time of at least 2 h. Clinical judgment should guide initiation of therapy.

The most common organisms are Staphylococcus species, which accounted for ~45% of cases in one series. Historically, coagulase-

negative staphylococcal species were identified most commonly in these infections, but these isolates have more recently been decreas­ ing in frequency. Staphylococcus aureus is more often involved among patients who are nasal carriers of the organism than among those who are not, and this organism is the most common pathogen in overt exitsite infections. Gram-negative bacilli and fungi such as Candida species also are found. Vancomycin-resistant enterococci and vancomycin-

intermediate S. aureus have been reported to produce peritonitis in CAPD patients. The finding of more than one organism in dialysate culture should prompt evaluation for secondary peritonitis. As with PBP, culture of dialysate fluid in blood culture bottles improves the yield. To facilitate diagnosis, several hundred milliliters of removed dialysis fluid should be concentrated by centrifugation before culture. TREATMENT CAPD Peritonitis Empirical therapy for CAPD peritonitis should be directed at S. aureus, coagulase-negative Staphylococcus, and gram-negative bacilli until the results of cultures become available. Guidelines sug­ gest that agents should be chosen on the basis of local experience with resistant organisms. In some centers, a first-generation cepha­ losporin such as cefazolin (for gram-positive bacteria) and a fluoro­ quinolone or a third-generation cephalosporin such as ceftazidime (for gram-negative bacteria) may be reasonable; in areas with high rates of infection with methicillin-resistant S. aureus, van­ comycin should be used instead of cefazolin, and gram-negative coverage may need to be broadened—e.g., with an aminoglycoside, ceftazidime, cefepime, or a carbapenem. Broad coverage including vancomycin should be particularly considered for patients with septic physiology or exit-site infections. Vancomycin should also be included in the regimen if the patient has a history of colonization or infection with methicillin-resistant S. aureus or has a history of severe allergy to penicillins and cephalosporins. Loading doses are administered intraperitoneally; doses depend on the dialysis method and the patient’s renal function. Intraperitoneal antibiotics are given either continuously (i.e., with each exchange) or intermit­ tently (i.e., once daily, with the dose allowed to remain in the perito­ neal cavity for at least 6 h). If the patient is severely ill, IV antibiotics should be added at doses appropriate for the patient’s degree of renal failure. The clinical response to an empirical treatment regi­ men should be rapid; if the patient has not responded after 48–96 h of treatment, new samples should be collected for cell counts and cul­ tures, and catheter removal should be considered. For patients who lack exit-site or tunnel infection, the typical duration of antibiotic treatment is 14 days. For patients with exit-site or tunnel infection, catheter removal should be considered, and a longer duration of antibiotic therapy (up to 21 days) may be appropriate. In fungal infections, the catheter should be removed immediately. ■ ■TUBERCULOUS PERITONITIS See Chap. 183. INTRAABDOMINAL ABSCESSES ■ ■INTRAPERITONEAL ABSCESSES Abscess formation is common in untreated peritonitis if overt gramnegative sepsis either does not develop or develops but is not fatal. In experimental models of abscess formation, mixed aerobic and anaero­ bic organisms have been implanted intraperitoneally. Without therapy directed at anaerobes, animals develop intraabdominal abscesses. As in humans, these experimental abscesses may stud the peritoneal cavity, lie within the omentum or mesentery, or even develop on the surface of or within viscera such as the liver. Pathogenesis and Immunity  There is often disagreement about whether an abscess represents a disease state or a host response. In a

sense, it represents both: while an abscess is an infection in which via­ ble infecting organisms and PMNs are contained in a fibrous capsule, it is also a process by which the host confines microbes to a limited space, thereby preventing further spread of infection. In any event, abscesses do cause significant symptoms, and patients with abscesses can be quite ill. Experimental work has helped to define both the host cells and the bacterial virulence factors responsible—most notably in the case of B. fragilis. This organism, although accounting for only 0.5% of the normal colonic flora, is the anaerobe most frequently isolated from intraabdominal infections, is especially prominent in abscesses, and is the most common anaerobic bloodstream isolate. On clinical grounds, therefore, B. fragilis appears to be uniquely virulent. Moreover, B. fragilis acts alone to cause abscesses in animal models of intraabdominal infec­ tion, whereas most other Bacteroides species must act synergistically with a facultative organism to induce abscess formation. Of the several virulence factors identified in B. fragilis, one is critical: the capsular polysaccharide complex found on the bacterial surface. This complex comprises at least eight distinct surface polysaccharides. Structural analysis of these polysaccharides has shown an unusual motif of oppositely charged sugars. Polysaccharides having these zwitterionic characteristics, such as polysaccharide A, evoke a host response in the peritoneal cavity that localizes bacteria into abscesses. B. fragilis and polysaccharide A have been found to adhere to primary mesothelial cells in vitro; this adherence, in turn, stimulates the pro­ duction of tumor necrosis factor α and intercellular adhesion molecule 1 by peritoneal macrophages. Although abscesses characteristically contain PMNs, the process of abscess induction depends on the stimu­ lation of T lymphocytes by these unique zwitterionic polysaccharides. The stimulated CD4+ T lymphocytes secrete leukoattractant cytokines and chemokines. The alternative pathways of complement and fibrino­ gen also participate in abscess formation. While antibodies to the capsular polysaccharide complex enhance bloodstream clearance of B. fragilis, CD4+ T cells are critical in immu­ nity to abscesses. When administered experimentally, B. fragilis poly­ saccharide A has immunomodulatory characteristics and stimulates CD4+ T regulatory cells via an interleukin 2–dependent mechanism to produce interleukin 10. Interleukin 10 downregulates the inflamma­ tory response, thereby preventing abscess formation. Clinical Presentation  Of all intraabdominal abscesses, 74% are intraperitoneal or retroperitoneal and are not visceral. Most intraperi­ toneal abscesses result from fecal spillage from a colonic source, such as an inflamed appendix. Abscesses can also arise from other processes. They usually form within weeks of the development of peritonitis and may be found in a variety of locations from omentum to mesentery, pelvis to psoas muscles, and subphrenic space to a visceral organ such as the liver, where they may develop either on the surface of the organ or within it. Periappendiceal and diverticular abscesses occur com­ monly. Diverticular abscesses are least likely to rupture. Infections of the female genital tract and pancreatitis also are among the more common causative events. When abscesses occur in the female genital tract—either as a primary infection (e.g., tuboovarian abscess) or as an infection extending into the pelvic cavity or peritoneum—B. fragilis figures prominently among the organisms isolated. B. fragilis is not found in large numbers in the normal vaginal flora. For example, it is encountered less commonly in pelvic inflammatory disease and endo­ metritis without an associated abscess. In pancreatitis with leakage of damaging pancreatic enzymes, inflammation is prominent. Therefore, clinical findings such as fever, leukocytosis, and even abdominal pain do not distinguish pancreatitis itself from complications such as pan­ creatic pseudocyst, pancreatic abscess (Chap. 359), or intraabdominal collections of pus. Especially in cases of necrotizing pancreatitis, in which the incidence of local pancreatic infection may be as high as 30%, needle aspiration under CT guidance is performed to sample fluid for culture. Traditionally, many centers have prescribed preemp­ tive antibiotics for patients with necrotizing pancreatitis. Imipenem is frequently used for this purpose because it reaches high tissue levels in the pancreas (although it is not unique in this regard). Random­ ized controlled studies have not demonstrated a benefit from this

practice, and guidelines no longer recommend preemptive antibiotics for patients with acute pancreatitis. If needle aspiration yields infected fluid in the setting of acute necrotizing pancreatitis, antibiotic treat­ ment is appropriate in conjunction with surgical and/or percutaneous drainage of infected material. Infected pseudocysts that occur remotely from acute pancreatitis are unlikely to be associated with significant amounts of necrotic tissue and may be treated with either surgical or percutaneous catheter drainage in conjunction with appropriate anti­ biotic therapy.

Diagnosis  Scanning procedures have considerably facilitated the diagnosis of intraabdominal abscesses. Abdominal CT probably has the highest yield, although ultrasonography is particularly useful for the right upper quadrant, kidneys, and pelvis. Both indium-labeled WBCs and gallium tend to localize in abscesses and may be useful in finding a collection. Because gallium is taken up in the bowel, indiumlabeled WBCs may have a slightly greater yield for abscesses near the bowel. Neither indium-labeled WBC scans nor gallium scans serve as a basis for a definitive diagnosis, however; both need to be followed by other, more specific studies, such as CT, if an area of possible abnormality is identified. PET scanning should also be considered due to its ready availability and because it provides more resolution. Abscesses contiguous with or contained within diverticula are particu­ larly difficult to diagnose with scanning procedures. Although barium should not be injected if a perforation is suspected, a barium enema occasionally may detect a diverticular abscess not diagnosed by other procedures. If one study is negative, a second study sometimes reveals a collection. Although exploratory laparotomy has been less commonly used since the advent of CT, this procedure still must be undertaken on occasion if an abscess is strongly suspected on clinical grounds. CHAPTER 137 TREATMENT Intraperitoneal Abscesses Intraabdominal Infections and Abscesses An algorithm for the management of patients with intraabdominal (including intraperitoneal) abscesses by percutaneous drainage is presented in Fig. 137-3. Treatment of intraabdominal infections involves determination of the initial focus of infection, admin­ istration of broad-spectrum antibiotics targeting the organisms involved, and performance of a drainage procedure if one or more definitive abscesses have formed. Antimicrobial therapy, in general, is adjunctive to drainage and/or surgical correction of an underlying lesion or process in intraabdominal abscesses. Results of cultures from drain sites are not reliable for defining the etiology of infections. Unlike the intraabdominal abscesses resulting from most causes, for which drainage of some kind is generally required, Percutaneous drainage No improvement by 48 h Defervescence by 24–48 h Repeat CT scan with dilute Hypaque injection into cavity and attempt further drainage Successful drainage and defervescence No drainage or no improvement Drain out when criteria for catheter removal satisfied Surgery FIGURE 137-3  Algorithm for the management of patients with intraabdominal abscesses by percutaneous drainage. Antimicrobial therapy should be administered concomitantly. (Reproduced with permission from B Lorber [ed]: Atlas of Infectious Diseases, vol VII: Intra-abdominal infections, hepatitis, and gastroenteritis. Philadelphia, Current Medicine, 1996, p 1.30, as adapted from OD Rotstein, RL Simmons, in SL Gorbach et al [eds]: Infectious Diseases. Philadelphia, Saunders; 1992.)

abscesses associated with diverticulitis usually wall off locally after rupture of a diverticulum, so that surgical intervention is not rou­ tinely required.

A number of agents exhibit excellent activity against aerobic gram-negative bacilli. Because death in intraabdominal sepsis is linked to gram-negative bacteremia, empirical therapy for intra-

abdominal infection always needs to include adequate coverage of gram-negative aerobic, facultative, and anaerobic organisms. Even if anaerobes are not cultured from clinical specimens, they still must be covered by the therapeutic regimen. Empirical antibiotic therapy should be the same as that discussed above for secondary peritonitis. Most clinical treatment failures are due to failure to drain the abscess and thereby achieve source control. The appro­ priate duration of antibiotic treatment for abdominal abscesses depends on whether the presumptive source of the intraabdominal infection has been controlled. With adequate source control, anti­ biotic treatment may be limited to 4 or 5 days. ■ ■VISCERAL ABSCESSES Liver Abscesses  The liver is the organ most subject to the develop­ ment of abscesses. In one study of 540 intraabdominal abscesses, 26% were visceral. Liver abscesses made up 13% of the total number, or 48% of all visceral abscesses. Liver abscesses may be solitary or mul­ tiple; they may arise from hematogenous spread of bacteria or from local spread from contiguous sites of infection within the peritoneal cavity. In the past, appendicitis with rupture and subsequent spread of infection was the most common source for a liver abscess. Currently, associated disease of the biliary tract is most common. Pylephlebitis (suppurative thrombosis of the portal vein), usually arising from infec­ tion in the pelvis but sometimes from infection elsewhere in the perito­ neal cavity, is another common source for bacterial seeding of the liver. PART 5 Infectious Diseases Fever is the most common presenting sign of liver abscess. Some patients, particularly those with associated disease of the biliary tract, have symptoms and signs localized to the right upper quadrant, includ­ ing pain, guarding, punch tenderness, and even rebound tenderness. Nonspecific symptoms, such as chills, anorexia, weight loss, nausea, and vomiting, also may develop. Only 50% of patients with liver abscesses, however, have hepatomegaly, right-upper-quadrant tender­ ness, or jaundice; thus, one-half of patients have no symptoms or signs to direct attention to the liver. Fever of unknown origin may be the only manifestation of liver abscess, especially in the elderly. Diagnostic studies of the abdomen, especially the right upper quadrant, should be a part of any workup for fever of unknown origin. The single most reliable laboratory finding is an elevated serum concentration of alka­ line phosphatase, which is documented in 70% of patients with liver abscesses. Other tests of liver function may yield normal results, but 50% of patients have elevated serum levels of bilirubin, and 48% have elevated concentrations of aspartate aminotransferase. Other labora­ tory findings include leukocytosis in 77% of patients, anemia (usually normochromic, normocytic) in 50%, and hypoalbuminemia in 33%. Concomitant bacteremia is found in one-third to one-half of patients. A liver abscess is sometimes suggested by chest radiography, especially if a new elevation of the right hemidiaphragm is seen; other suggestive findings include a right basilar infiltrate and a right pleural effusion. Imaging studies are the most reliable methods for diagnosing liver abscesses. These studies include ultrasonography, CT (Fig. 137-4), indium-labeled WBC or gallium scan, and MRI. More than one such study may be required. Organisms recovered from liver abscesses vary with the source. In liver infection arising from the biliary tree, enteric gram-negative aero­ bic bacilli and enterococci are common isolates. Klebsiella pneumoniae liver abscess has been well described in Southeast Asia for more than 20 years and has become an emerging syndrome in North America and elsewhere. These community-acquired infections have been linked to a virulent hypermucoviscous K. pneumoniae phenotype and to a specific genotype. The typical syndrome includes liver abscess, bacteremia, and metastatic infection in the eye(s) and lung(s). Ampicillin/amoxicillin therapy started within the previous 30 days has been associated with

FIGURE 137-4  Multilocular liver abscess on CT scan. Multiple or multilocular abscesses are more common than solitary abscesses. (Reprinted with permission from B Lorber [ed]: Atlas of Infectious Diseases, vol VII: Intra-abdominal Infections, Hepatitis, and Gastroenteritis. Philadelphia, Current Medicine, 1996, Fig. 1.22.) increased risk for this syndrome, presumably because of selection for the causative strain. Unless previous surgery has been performed, anaerobes are not generally involved in liver abscesses arising from biliary infections. In contrast, in liver abscesses arising from pelvic and other intraperitoneal sources, a mixed flora including both aerobic and anaerobic species is common; B. fragilis is the species most frequently isolated. With hematogenous spread of infection, usually only a single organism is encountered; this species may be S. aureus or a strepto­ coccal species such as one in the Streptococcus milleri group. Liver abscesses may also be caused by Candida species; such abscesses usu­ ally follow fungemia in patients receiving chemotherapy for cancer and often present when PMNs return after a period of neutropenia. Amebic liver abscesses are not an uncommon problem (Chap. 230). Amebic serologic testing gives positive results in >95% of cases. In addition, polymerase chain reaction (PCR) testing has been used in recent years. Negative results from these studies help to exclude this diagnosis. TREATMENT Liver Abscesses (Fig. 137-3) Drainage is the mainstay of therapy for intraabdomi­ nal abscesses, including liver abscesses; the approach can be either percutaneous (with a pigtail catheter kept in place or possibly with a device that can perform pulse lavage to fragment and evacuate the semisolid contents of a liver abscess), transluminal (with endo­ scopic ultrasound guidance), or surgical. However, there is growing interest in medical management alone for pyogenic liver abscesses. The drugs used for empirical therapy include the same ones used in intraabdominal sepsis and secondary bacterial peritonitis. Usu­ ally, blood cultures and a diagnostic aspirate of abscess contents should be obtained before the initiation of empirical therapy, with antibiotic choices adjusted when the results of Gram’s staining and culture become available. Cases treated without definitive drainage generally require longer courses of antibiotic therapy. When percu­ taneous drainage was compared with open surgical drainage, the average length of hospital stay for the former was almost twice that for the latter, although both the time required for fever to resolve and the mortality rate were the same for the two procedures. The mortality rate was appreciable despite treatment, averaging 15%. Several factors predict the failure of percutaneous drainage and therefore may favor primary surgical intervention. These factors include the presence of multiple, sizable abscesses; viscous abscess contents that tend to plug the catheter; associated disease (e.g., disease of the biliary tract) requiring surgery; the presence of yeast; communication with an untreated obstructed biliary tree; or the lack of a clinical response to percutaneous drainage in 4–7 days.

Treatment of candidal liver abscesses often entails initial admin­ istration of liposomal amphotericin B (3–5 mg/kg IV daily) or an echinocandin, with subsequent fluconazole therapy (Chap. 222). In some cases, therapy with fluconazole alone (6 mg/kg daily) may be used—e.g., in clinically stable patients whose infecting isolate is susceptible to this drug. Splenic Abscesses  Splenic abscesses are much less common than liver abscesses. The incidence of splenic abscesses has ranged from 0.14 to 0.7% in various autopsy series. The clinical setting and the organ­ isms isolated usually differ from those for liver abscesses. The degree of clinical suspicion for splenic abscess needs to be high because this condition is frequently fatal if left untreated. Even in the most recently published series, diagnosis was made only at autopsy in 37% of cases. Although splenic abscesses may arise occasionally from contiguous spread of infection or from direct trauma to the spleen, hematogenous spread of infection is more common. Bacterial endocarditis is the most common associated infection (Chap. 133). Splenic abscesses can develop in patients who have received extensive immunosuppressive therapy (particularly those with malignancy involving the spleen) and in patients with hemoglobinopathies or other hematologic disorders (especially sickle cell anemia). Although ~50% of patients with splenic abscesses have abdominal pain, the pain is localized to the left upper quadrant in only one-half of these cases. Splenomegaly is found in ~50% of cases. Fever and leukocytosis are generally present; the development of fever preceded diagnosis by an average of 20 days in one series. Left-sided chest find­ ings may include abnormalities to auscultation, and chest radiographic findings may include an infiltrate or a left-sided pleural effusion. CT scan of the abdomen has been the most sensitive diagnostic tool. Ultra­ sonography can yield the diagnosis but is less sensitive. Liver–spleen scan or gallium scan also may be useful. Streptococcal species are the most common bacterial isolates from splenic abscesses, followed by S. aureus—presumably reflecting the associated endocarditis. An increase in the prevalence of gram-negative aerobic isolates from splenic abscesses has been reported; these organisms often derive from a urinary tract focus, with associated bacteremia, or from another intraabdominal source. Salmonella species are seen fairly commonly, especially in patients with sickle cell hemoglobinopathy. Anaerobic species accounted for only 5% of isolates in the largest collected series, but the reporting of a number of “sterile abscesses” may indicate that optimal techniques for the isolation of anaerobes were not used. TREATMENT Splenic Abscesses Because of the high mortality figures reported for splenic abscesses, splenectomy with adjunctive antibiotics has traditionally been con­ sidered standard treatment and remains the best approach for complex, multilocular abscesses or multiple abscesses. However, percutaneous drainage has worked well for single, small (<3-cm) abscesses in some studies and may also be useful for patients with high surgical risk. Patients undergoing splenectomy should be vaccinated against encapsulated organisms (Streptococcus pneu­ moniae, Haemophilus influenzae, Neisseria meningitidis). The most important factor in successful treatment of splenic abscesses is early diagnosis. Perinephric and Renal Abscesses  Perinephric and renal abscesses are not common. The former accounted for only ~0.02% of hospital admissions and the latter for ~0.2% in Altemeier’s series of 540 intraabdominal abscesses. Before antibiotics became available, most renal and perinephric abscesses were hematogenous in origin, usually complicating prolonged bacteremia, with S. aureus most commonly recovered. Now, in contrast, >75% of perinephric and renal abscesses arise from a urinary tract infection. Infection ascends from the blad­ der to the kidney, with pyelonephritis preceding abscess development. Bacteria may directly invade the renal parenchyma from medulla

to cortex. Local vascular channels within the kidney may facilitate the transport of organisms. Areas of abscess developing within the parenchyma may rupture into the perinephric space. The kidneys and adrenal glands are surrounded by a layer of perirenal fat that, in turn, is surrounded by Gerota’s fascia, which extends superiorly to the diaphragm and inferiorly to the pelvic fat. Abscesses extending into the perinephric space may track through Gerota’s fascia into the psoas or transversalis muscles, into the anterior peritoneal cavity, superiorly to the subdiaphragmatic space, or inferiorly to the pelvis. Of the risk factors that have been associated with the development of perinephric abscesses, the most important is concomitant nephrolithiasis obstruct­ ing urinary flow. Of patients with perinephric abscess, 20–60% have renal stones. Other structural abnormalities of the urinary tract, prior urologic surgery, trauma, and diabetes mellitus also have been identi­ fied as risk factors.

The organisms most frequently encountered in perinephric and renal abscesses are E. coli, Proteus species, and Klebsiella species. E. coli, the aerobic species most commonly found in the colonic flora, seems to have unique virulence properties in the urinary tract, including factors promoting adherence to uroepithelial cells. The urease of Proteus species splits urea, thereby creating a more alkaline and more hospitable environment for bacterial proliferation. Proteus species are frequently found in association with large struvite stones caused by the precipitation of magnesium ammonium sulfate in an alkaline environment. These stones serve as a nidus for recurrent urinary tract infection. Although a single bacterial species is usually recovered from a perinephric or renal abscess, multiple species may be found. If a urine culture is not contaminated with periurethral flora and is found to contain more than one organism, a perinephric or renal abscess should be considered in the differential diagnosis. Urine cultures may also be polymicrobial in cases of bladder diverticulum. CHAPTER 137 Candida species can cause renal abscesses. Fungi of this genus may spread to the kidney hematogenously or by ascension from the bladder. The hallmark of the latter route of infection is ureteral obstruction with large fungal balls. Intraabdominal Infections and Abscesses The presentation of perinephric and renal abscesses is quite non­ specific. Flank pain and abdominal pain are common. At least 50% of patients are febrile. Pain may be referred to the groin or leg, particularly with extension of infection. The diagnosis of perinephric abscess, like that of splenic abscess, is frequently delayed, and the mortality rate in some series is appreciable, although lower than in the past. Perinephric or renal abscess should be most seriously considered when a patient presents with symptoms and signs of pyelonephritis and remains febrile after 4 or 5 days of treatment. Moreover, when a urine culture yields a polymicrobial flora, when a patient is known to have renal stones, or when fever and pyuria coexist with a sterile urine culture, these diagnoses should be entertained. Renal ultrasonography and abdominal CT are the most useful diagnostic modalities. If a renal or perinephric abscess is diagnosed, nephrolithiasis should be excluded, especially when a high urinary pH suggests the presence of a urea-splitting organism. TREATMENT Perinephric and Renal Abscesses Treatment for perinephric and renal abscesses, like that for other intraabdominal abscesses, includes drainage of pus and antibiotic therapy directed at the organism(s) recovered. For perinephric abscesses, percutaneous drainage is usually successful. Psoas Abscesses  The psoas muscle is another location in which abscesses are encountered. Psoas abscesses may arise from a hema­ togenous source, by contiguous spread from an intraabdominal or pelvic process, or by contiguous spread from nearby bony structures (e.g., vertebral bodies). Associated osteomyelitis due to spread from bone to muscle or from muscle to bone is common in psoas abscesses. When Pott’s disease was common, Mycobacterium tuberculosis was a frequent cause of psoas abscess. Currently, either S. aureus or a mixture

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138 Acute Infectious Diarrheal Diseases and Bacterial Food Poisoning

of enteric organisms including aerobic and anaerobic gram-negative bacilli is usually isolated from psoas abscesses in the United States. S. aureus is most likely to be isolated when a psoas abscess arises from hematogenous spread or a contiguous focus of osteomyelitis; a mixed enteric flora is the most likely etiology when the abscess has an intraab­ dominal or pelvic source. Patients with psoas abscesses frequently pres­ ent with fever, lower abdominal or back pain, or pain referred to the hip or knee. CT is the most useful diagnostic technique.

TREATMENT Psoas Abscesses Treatment includes surgical drainage and the administration of an antibiotic regimen directed at the inciting organism(s). Pancreatic Abscesses  See Chap. 359. ■ ■FURTHER READING Biggins SW et al: Diagnosis, evaluation, and management of ascites, spontaneous bacterial peritonitis, and hepatorenal syndrome: 2021 practice guidance by the American Association for the Study of Liver Diseases. Hepatology 74;1014.2021. De Pascale G et al: Poor timing and failure of source control are risk factors for mortality in critically ill patients with secondary peritonitis. Intensive Care Med 48:1593, 2022. Li PK et al: ISPD peritonitis recommendations: 2016 update on prevention and treatment. Perit Dial Int 36:481, 2016. Oliver A et al: Role of rifaximin in spontaneous bacterial peritonitis PART 5 Infectious Diseases prevention. South Med J 111:660 2018. Roediger R, Lisker-Melman M: Pyogenic and amebic infections of the liver. Gastroenterol Clin North Am 49:361, 2020. Ross JT et al: Secondary peritonitis: Principles of diagnosis and intervention. BMJ 361:k1407, 2018. Van Hooste W et al: Infections caused by hypervirulent Klebsiella pneumoniae in non-endemic countries: Three case reports and review of the literature. Acta Clin Belg 78:229, 2023. Ana A. Weil, Regina C. LaRocque

Acute Infectious

Diarrheal Diseases and

Bacterial Food Poisoning Diarrheal disease mortality has decreased substantially in the past three decades. Nevertheless, acute diarrheal disease is still a leading cause of illness globally and is associated with an estimated 1.57 million deaths per year. Across low-income and middle-income countries, one in ten deaths among children <5 years of age is attributable to diar­ rhea, with substantial variation in incidence and mortality between countries. The morbidity from diarrhea also is significant. Recurrent intestinal infections are associated with physical and mental stunting, wasting, micronutrient deficiencies, and malnutrition. In short, diar­ rheal disease is a driving factor in global morbidity and mortality. The wide range of clinical manifestations of acute gastrointestinal illnesses is matched by the wide variety of infectious agents involved, including viruses, bacteria, and parasites (Table 138-1). This chapter discusses factors that enable gastrointestinal pathogens to cause dis­ ease, reviews host defense mechanisms, and delineates an approach to the evaluation and treatment of patients presenting with acute

diarrhea. Individual organisms causing acute gastrointestinal illnesses are discussed in detail in subsequent chapters. PATHOGENIC MECHANISMS Enteric pathogens have developed a variety of tactics to overcome host defenses. Understanding the virulence factors employed by these organisms is important in the diagnosis and treatment of clinical disease. ■ ■INOCULUM SIZE The number of microorganisms that must be ingested to cause disease varies considerably from species to species. For Shigella, enterohemor­ rhagic Escherichia coli, Giardia lamblia, or Entamoeba, as few as 10–100 bacteria or cysts can produce infection, while 105−108 Vibrio cholerae organisms must be ingested to cause disease. The infective dose of Sal­ monella varies widely, depending on the species, host, and food vehicle. The ability of organisms to overcome host defenses has important implications for transmission; Shigella, enterohemorrhagic E. coli, Ent­ amoeba, and Giardia can spread by person-to-person contact, whereas under some circumstances, Salmonella may need to grow in food for several hours before reaching an effective infectious dose. ■ ■ADHERENCE Many organisms must adhere to the gastrointestinal mucosa as an initial step in the pathogenic process; thus, organisms that can com­ pete with the normal bowel flora and colonize the mucosa have an important advantage in causing disease. Specific cell-surface proteins involved in attachment of bacteria to intestinal cells are important virulence determinants. V. cholerae, for example, adheres to the brush border of small-intestinal enterocytes via specific surface adhesins, including the toxin-coregulated pilus and other accessory coloniza­ tion factors. Enterotoxigenic E. coli, which causes watery diarrhea, produces an adherence protein called colonization factor antigen that is necessary for colonization of the upper small intestine by the organism prior to the production of enterotoxin. Enteropathogenic E. coli, an agent of diarrhea in young children, and enterohemor­ rhagic E. coli, which causes hemorrhagic colitis and the hemolyticuremic syndrome, produce virulence determinants that allow these organisms to attach to and efface the brush border of the intestinal epithelium. ■ ■TOXIN PRODUCTION The production of one or more exotoxins is important in the pathogen­ esis of numerous enteric organisms. Such toxins include enterotoxins, which cause watery diarrhea by acting directly on secretory mecha­ nisms in the intestinal mucosa; cytotoxins, which cause destruction of mucosal cells and associated inflammatory diarrhea; and neurotoxins, which act directly on the central or peripheral nervous system. The prototypical enterotoxin is cholera toxin, a heterodimeric pro­ tein composed of one A and five B subunits. The A subunit contains the enzymatic activity of the toxin, while the B subunit pentamer binds holotoxin to the enterocyte surface receptor, the ganglioside GM1. After the binding of holotoxin, a fragment of the A subunit is translocated across the eukaryotic cell membrane into the cytoplasm, where it catalyzes the adenosine diphosphate ribosylation of a guano­ sine triphosphate (GTP)–binding protein and causes persistent activa­ tion of adenylate cyclase. The result is an increase of cyclic adenosine monophosphate in the intestinal cell, which increases Cl– secretion and decreases Na+ absorption, leading to a loss of fluid and the production of diarrhea. Enterotoxigenic strains of E. coli may produce a protein called heatlabile enterotoxin (LT) that is like cholera toxin and causes secretory diarrhea by the same mechanism. Alternatively, enterotoxigenic strains of E. coli may produce heat-stable enterotoxin (ST), one form of which causes diarrhea by activation of guanylate cyclase and elevation of intracellular cyclic guanosine monophosphate. Some enterotoxigenic strains of E. coli produce both LT and ST. Bacterial cytotoxins, in contrast, destroy intestinal mucosal cells and produce the syndrome of dysentery, with bloody stools containing

TABLE 138-1  Gastrointestinal Pathogens Causing Acute Diarrhea MECHANISM LOCATION ILLNESS STOOL FINDINGS EXAMPLES OF PATHOGENS INVOLVED Noninflammatory (enterotoxin) Proximal small bowel Watery diarrhea No fecal leukocytes; mild or no increase in fecal lactoferrin Inflammatory (invasion or cytotoxin) Colon or distal small bowel Dysentery or inflammatory diarrhea Fecal polymorphonuclear leukocytes; substantial increase in fecal lactoferrin Penetrating Distal small bowel Enteric fever Fecal mononuclear leukocytes Abbreviations: LT, heat-labile enterotoxin; ST, heat-stable enterotoxin. inflammatory cells. Enteric pathogens that produce such cytotoxins include Shigella dysenteriae type 1, Vibrio parahaemolyticus, and Clos­ tridioides difficile. S. dysenteriae type 1 and Shiga toxin–producing strains of E. coli produce potent cytotoxins and have been associated with outbreaks of hemorrhagic colitis and hemolytic-uremic syndrome. Neurotoxins are usually produced by bacteria outside the host and therefore cause symptoms soon after ingestion. Included are the staph­ ylococcal and Bacillus cereus toxins, which act on the central nervous system to produce vomiting. ■ ■INVASION Dysentery may result not only from the production of cytotoxins but also from bacterial invasion and destruction of intestinal mucosal cells. Infections due to Shigella and enteroinvasive E. coli are characterized by the organisms’ invasion of mucosal epithelial cells, intraepithelial mul­ tiplication, and subsequent spread to adjacent cells. Salmonella causes inflammatory diarrhea by invading the bowel mucosa but generally is not associated with the destruction of enterocytes or the full clinical syndrome of dysentery. Salmonella enterica serovar Typhi and Yersinia enterocolitica can penetrate intact intestinal mucosa, multiply intra­ cellularly in Peyer patches and intestinal lymph nodes, and then dis­ seminate through the bloodstream to cause enteric fever—a syndrome characterized by fever, headache, relative bradycardia, abdominal pain, splenomegaly, and leukopenia. HOST DEFENSES Given the enormous number of microorganisms ingested with every meal, the normal host must combat a constant influx of potential enteric pathogens. Studies of infections in patients with alterations in defense mechanisms have led to a greater understanding of the variety of ways in which the normal host can protect itself against disease. ■ ■INTESTINAL MICROBIOTA The large numbers of bacteria that normally inhabit the intestine (the intestinal microbiota) act as an important host defense mechanism, preventing colonization by potential enteric pathogens. Mechanisms of colonization resistance are not fully understood but include geographic and nutritional exclusion at the mucosal surface. Persons with fewer intestinal bacteria, such as infants who have not yet developed normal enteric colonization or patients receiving antibiotics, are at greater risk of developing infections with enteric pathogens. The bacterial com­ munity composition of the intestinal microbiota is as important as abundance of organisms present. More than 99% of the normal colonic microbiota is made up of anaerobic bacteria, and the acidic pH and volatile fatty acids produced by these organisms appear to be critical elements in resistance to colonization. ■ ■GASTRIC ACID The acidic pH of the stomach is an important barrier to enteric patho­ gens, and an increased frequency of infections due to Salmonella,

G. lamblia, and a variety of helminths has been reported among patients who have undergone gastric surgery or are achlorhydric for some other reason. Neutralization of gastric acid with antacids, proton pump

Vibrio cholerae, enterotoxigenic Escherichia coli (LT and/or ST), enteroaggregative E. coli, Clostridium perfringens, Bacillus cereus, Staphylococcus aureus, Aeromonas hydrophila, Plesiomonas shigelloides, rotavirus, norovirus, enteric adenoviruses, Giardia lamblia, Cryptosporidium spp., Cyclospora spp., microsporidia Shigella spp., Salmonella spp., Campylobacter jejuni, enterohemorrhagic E. coli, enteroinvasive E. coli, Yersinia enterocolitica, Listeria monocytogenes, Vibrio parahaemolyticus, Clostridium difficile, A. hydrophila, P. shigelloides, Entamoeba histolytica, Klebsiella oxytoca Salmonella Typhi, Y. enterocolitica inhibitors, or H2 blockers—a common practice in the management of hospitalized patients—similarly increases the risk of enteric coloniza­ tion. In addition, some microorganisms can survive the extreme acid­ ity of the gastric environment; rotavirus and Shigella, for example, are highly stable to acidity. ■ ■INTESTINAL MOTILITY Peristalsis is the major mechanism for clearance of bacteria from the proximal small intestine. When intestinal motility is impaired (e.g., by treatment with opioids or other antimotility drugs, anatomic abnor­ malities, or hypomotility states), the frequency of bacterial overgrowth and infection of the small bowel with enteric pathogens is increased. Some patients whose treatment for Shigella infection consists of diphenoxylate hydrochloride with atropine (Lomotil) experience pro­ longed fever and shedding of organisms, while patients treated with opioids for mild Salmonella gastroenteritis have a higher frequency of bacteremia than those not treated with opioids. CHAPTER 138 ■ ■INTESTINAL MUCIN A complex layer of mucus, produced by specialized secretory cells, covers the stomach, small intestine, and large intestine, and separates the lumenal commensal microbiota from the epithelium. The thickness and constituents of this mucus barrier vary throughout the gastro­ intestinal tract. The mucus barrier turns over rapidly and comprises glycoproteins and a range of antimicrobial molecules and secreted immunoglobulins directed against specific microbial antigens. Enteric pathogens have evolved a wide range of strategies to overcome this barrier and thus to reach the underlying epithelium and cause disease. For example, pathogens can penetrate the mucus layer by secreting enzymes to degrade the mucus or through flagella-mediated motil­ ity. Some organisms, such as Shigella, secrete toxins that can diffuse through the mucus layer and disrupt the underlying epithelium. The resulting reduction of mucus production allows the pathogen to reach the cell surface. Acute Infectious Diarrheal Diseases and Bacterial Food Poisoning
■ ■IMMUNITY Both cellular immune responses and antibody production play impor­ tant roles in protection from enteric infections. Humoral immunity to enteric pathogens consists of systemic IgG and IgM as well as secre­ tory IgA. The mucosal immune system may be the first line of defense against many gastrointestinal pathogens. The binding of bacterial antigens to the lumenal surface of M cells in the distal small bowel and the subsequent presentation of antigens to subepithelial lymphoid tis­ sue leads to the proliferation of sensitized and specialized lymphocytes. These lymphocytes circulate and populate all of the mucosal tissues of the body as IgA-secreting plasma cells. ■ ■GENETIC DETERMINANTS Host genetic variation influences susceptibility to diarrheal dis­ eases. People with blood group O show increased susceptibility to disease due to V. cholerae, Shigella, E. coli O157, and norovirus. Polymorphisms in genes encoding inflammatory mediators have been associated with the outcome of infection with enteroaggregative E. coli, enterotoxin-producing E. coli, Salmonella, C. difficile, and V. cholerae.

APPROACH TO THE PATIENT Infectious Diarrhea or Bacterial Food Poisoning The approach to the patient with possible infectious diarrhea or bacterial food poisoning is shown in Fig. 138-1. HISTORY Diarrhea is defined as the passage of loose or watery stools that conform to the shape of a container three or more times in a 24-h period. The answers to questions with high discriminating value can quickly narrow the range of potential causes of diarrhea and help determine whether treatment is needed. Important elements of the narrative history are detailed in Fig. 138-1. Diarrhea, Nausea, or Vomiting Symptomatic therapy Oral rehydration therapy (see Table 138-5) Assess: Duration (>1 day) Severity (see text) Yes PART 5 Infectious Diseases Obtain history: Duration1 Fever2 Appearance of stool3 Frequency of bowel movements4 Abdominal pain5 Tenesmus6 Vomiting7 Common source8 Antibiotic use9 Travel10 and

Obtain stool to be examined for WBCs (and, if >10 days, for parasites) Inflammatory (WBCs; see Table 138-1) Examine stool for parasites Noninflammatory (no WBCs; see Table 138-1) Continue symptomatic therapy (Table 138-5); further evaluation if no resolution Culture for: Shigella, Salmonella, Campylobacter jejuni Consider: Clostridioides difficile cytotoxin Consider: Empirical antimicrobial therapy (Table 138-5) FIGURE 138-1  Clinical algorithm for the approach to patients with community-acquired infectious diarrhea or bacterial food poisoning. Key to superscripts: 1. Diarrhea lasting >2 weeks is generally defined as chronic; in such cases, many of the causes of acute diarrhea are much less likely, and a new spectrum of causes needs to be considered. 2. Fever often implies invasive disease, although fever and diarrhea may also result from infection outside the gastrointestinal tract, as in malaria. 3. Stools that contain blood or mucus indicate ulceration of the large bowel. Bloody stools without fecal leukocytes should alert the laboratory to the possibility of infection with Shiga toxin–producing enterohemorrhagic Escherichia coli. Bulky white/gray stools suggest a small-intestinal process that is causing malabsorption. An acute illness with profuse “rice-water” stools, with an appearance of water after rice has been cooked, suggests cholera or a similar toxigenic process. 4. Frequent stools over a given period can provide the first warning of impending dehydration. 5. Abdominal pain may be most severe in inflammatory processes like those due to Shigella, Campylobacter, and necrotizing toxins. Painful abdominal muscle cramps, caused by electrolyte loss, can develop in severe cases of cholera. Bloating is common in giardiasis. An appendicitislike syndrome should prompt a culture for Yersinia enterocolitica with cold enrichment. 6. Tenesmus (painful rectal spasms with a strong urge to defecate but little passage of stool) may be a feature of cases with proctitis, as in shigellosis or amebiasis. 7. Vomiting implies an acute infection (e.g., a toxin-mediated illness or food poisoning) but can also be prominent in a variety of systemic illnesses (e.g., malaria) and in intestinal obstruction. 8. Asking patients whether anyone they know is sick is a more efficient means of identifying a common source than is constructing a list of recently eaten foods. If a common source seems likely, specific foods can be investigated. See text for a discussion of bacterial food poisoning. 9. Current antibiotic therapy or a recent history of treatment suggests Clostridioides difficile diarrhea (Chap. 139). Stop antibiotic treatment if possible and consider tests for C. difficile toxins. Antibiotic use may increase the risk of chronic intestinal carriage following salmonellosis. 10. See text (and Chap. 130) for a discussion of traveler’s diarrhea. (From The New England Journal of Medicine. Bacterial and Protozoal Gastroenteritis. RL Guerrant, DA Bobak 325:327-340. Copyright @1991 Massachusetts Medical Society. Reprinted with permission from Massachusetts Medical Society.)

PHYSICAL EXAMINATION The examination of patients for signs of dehydration provides essential information about the severity of the diarrheal illness and the need for rapid therapy. Mild dehydration is indicated by thirst, dry mouth, decreased axillary sweat, decreased urine output, and slight weight loss. Signs of moderate dehydration include an ortho­ static fall in blood pressure, skin tenting, and sunken eyes (or, in infants, a sunken fontanelle). Signs of severe dehydration include lethargy, obtundation, feeble pulse, hypotension, and shock. DIAGNOSTIC APPROACH After assessing severity of illness, the clinician must distinguish between inflammatory and noninflammatory disease. Using the Resolution No Continued illness Specific antiparasitic therapy

history and epidemiologic features of the case, the clinician can then rapidly evaluate the need for further efforts to define a specific etiology and for therapeutic intervention. Examination of a stool sample may supplement the narrative history. Grossly bloody or mucoid stool suggests an inflammatory process. A test for fecal leukocytes (preparation of a thin smear of stool on a glass slide, addition of a drop of methylene blue, and examination of the wet mount) can suggest inflammatory disease in patients with diarrhea, although the predictive value of this test is still debated. A test for fecal lactoferrin, which is a marker of fecal leukocytes, is more sensitive and is available in latex agglutination and enzyme-linked immunosorbent assay formats. Stool culture or molecular testing is useful for diagnosing a causative organism, particularly in patients with severe illness or high-risk comorbidities. Causes of acute infec­ tious diarrhea, categorized as inflammatory and noninflammatory, are listed in Table 138-1. POST DIARRHEA COMPLICATIONS Short- or long-term complications may follow the resolution of an acute diarrheal episode. The clinician should inquire about prior diarrheal illness if the conditions listed in Table 138-2 are observed. EPIDEMIOLOGY ■ ■TRAVEL HISTORY Of the several million people who travel from temperate industrialized countries to tropical regions of Asia, Africa, and Central and South America each year, 20–50% experience a sudden onset of abdominal cramps, anorexia, and watery diarrhea; thus, traveler’s diarrhea is the most common travel-related infectious illness (Chap. 130). The time of onset is usually 3 days to 2 weeks after the traveler’s arrival in a resource-poor area; most cases begin within the first 3–5 days. The illness is generally self-limited, lasting 1–5 days. The high rate of diar­ rhea among travelers is related to the ingestion of contaminated food or water. The pathogens that cause traveler’s diarrhea vary considerably with location (Table 138-3), as does the pattern of antimicrobial resistance. In all areas, enterotoxigenic and enteroaggregative E. coli are the most common pathogens among persons with the classic secretory traveler’s diarrhea syndrome. Infection with Campylobacter jejuni is especially common in areas of Asia. ■ ■LOCATION Closed and semi-closed communities, including day-care centers, schools, residential facilities, and cruise ships, are important settings TABLE 138-2  Postdiarrhea Complications of Acute Infectious Diarrheal Illness COMPLICATION COMMENTS Chronic diarrhea (diarrhea lasting

4 weeks) • Lactase deficiency • Small-bowel bacterial overgrowth • Malabsorption syndromes (tropical Occurs in ~1% of travelers with acute diarrhea Protozoa account for approximately one-third of cases and celiac sprue) Initial presentation or exacerbation of inflammatory bowel disease May be precipitated by traveler’s diarrhea Irritable bowel syndrome Occurs in ~10% of travelers with traveler’s diarrhea Reactive arthritis Particularly likely after infection with invasive organisms (Shigella, Salmonella, Campylobacter, Yersinia) Hemolytic-uremic syndrome (hemolytic anemia, thrombocytopenia, and renal failure) Follows infection with Shiga toxin–producing bacteria (Shigella dysenteriae type 1 and enterohemorrhagic Escherichia coli) Guillain-Barré syndrome Particularly likely after Campylobacter infection

TABLE 138-3  Causes of Traveler’s Diarrhea APPROXIMATE PERCENTAGE OF CASES COMMENTS ETIOLOGIC AGENT Bacteria 50–75 Enterotoxigenic Escherichia coli 10–45 Single most important agent Enteroaggregative E. coli 5–35 Emerging enteric pathogen with worldwide distribution Campylobacter jejuni 5–25 More common in Asia Shigella 0–15 Major cause of dysentery Salmonella 0–15 — Others 0–5 Including Aeromonas, Plesiomonas, and Vibrio cholerae Viruses 0–20 Norovirus 0–10 Associated with cruise ships Rotavirus 0–5 Particularly common among children Parasites 0–10 Giardia lamblia 0–5 Affects hikers and campers who drink from freshwater streams Cryptosporidium 0–5 Resistant to chlorine treatment of water sources Entamoeba histolytica <1 — Cyclospora <1 — CHAPTER 138 Other 0–10 Acute food poisoninga 0–5 — No pathogen identified 10–50 — aFor etiologic agents, see Table 138-4. Source: After DR Hill et al: The practice of travel medicine: Guidelines by the Infectious Diseases Society of America. Clin Infect Dis 43:1499, 2006. Acute Infectious Diarrheal Diseases and Bacterial Food Poisoning
for outbreaks of enteric infections. Norovirus, which is highly con­ tagious and robust in surviving on surfaces, is the most common etiologic agent associated with outbreaks of acute gastroenteritis. Other common organisms, often spread by fecal–oral contact in such communities, are Shigella, C. jejuni, and Cryptosporidium. Rotavirus is rarely a cause of pediatric diarrheal outbreaks in the United States since rotavirus vaccination was broadly recommended in 2006. Simi­ larly, hospitals are sites in which enteric infections are concentrated. Diarrhea is one of the most common manifestations of nosocomial infections. C. difficile is the predominant cause of nosocomial diar­ rhea among adults in the United States, and outbreaks of norovirus infection are common in health care settings. Klebsiella oxytoca has been identified as a cause of antibiotic-associated hemorrhagic colitis. Enteropathogenic E. coli has been associated with outbreaks of diarrhea in nurseries for newborns. One-third of elderly patients in chronic-care institutions develop a significant diarrheal illness each year; more than one-half of these cases are caused by cytotoxin-producing C. difficile. Antimicrobial therapy can predispose to pseudomembranous colitis by altering the normal colonic microbiota, which then permits the multi­ plication of C. difficile (Chap. 139). ■ ■AGE Globally, most morbidity and mortality from enteric pathogens involve children <5 years of age. Breast-fed infants are protected from patho­ gens in contaminated food and water and derive some protection from maternal antibodies, but their risk of infection rises dramatically when they begin to eat solid foods. Exposure to rotavirus is universal, with most children experiencing their first infection in the first or second year of life if not vaccinated. Older children and adults are more com­ monly infected with norovirus. Other organisms with higher attack rates among children than among adults include enterotoxigenic, enteropathogenic, and enterohemorrhagic E. coli; Shigella; C. jejuni; and G. lamblia.

■ ■HOST IMMUNE STATUS Immunocompromised hosts are at elevated risk of acute and chronic infectious diarrhea. Individuals with defects in cell-mediated immu­ nity (including those with AIDS) are at particularly high risk of invasive enteropathies, including salmonellosis, listeriosis, and cryp­ tosporidiosis. Individuals with hypogammaglobulinemia are at par­ ticular risk of C. difficile colitis and giardiasis and can develop chronic infections with viral pathogens such as norovirus. Patients with cancer are more likely to develop C. difficile infection due to chemo­ therapy-associated antibiotic exposure and frequent hospitalizations. Infectious diarrhea can be life-threatening in immunocompromised hosts, with complications including persistent infection, bacteremia, and metastatic seeding of infection. Furthermore, dehydration may compromise renal function and increase the toxicity of immunosup­ pressive drugs.

■ ■BACTERIAL FOOD POISONING If the history and the stool examination indicate a noninflammatory etiology of diarrhea and there is evidence of a common-source out­ break, questions concerning the ingestion of specific foods and the time of onset of diarrhea after a meal can provide clues to the bacterial cause of the illness. Potential causes of bacterial food poisoning are shown in Table 138-4. Bacterial disease caused by an enterotoxin elaborated outside the host, such as that due to Staphylococcus aureus or B. cereus, has the shortest incubation period (1–6 h) and generally lasts <12 h. Most cases of staphylococcal food poisoning are caused by contamination from Staphylococcus-infected human carriers. Staphylococci can multiply at a wide range of temperatures; thus, if food is left to cool slowly and remains at room temperature after cooking, the organisms will have the opportunity to form enterotoxin. Outbreaks following picnics where potato salad, mayonnaise, and cream pastries have been served offer classic examples of staphylococcal food poisoning. Diarrhea, nausea, vomiting, and abdominal cramping are common, while fever is rare. PART 5 Infectious Diseases B. cereus can produce either a syndrome with a short incuba­ tion period—the emetic form, mediated by a staphylococcal type of enterotoxin—or one with a longer incubation period (8–16 h)—the diarrheal form, caused by an enterotoxin resembling E. coli LT, in which TABLE 138-4  Bacterial Food Poisoning INCUBATION PERIOD, ORGANISM SYMPTOMS COMMON FOOD SOURCES 1–6 h Staphylococcus aureus Nausea, vomiting, diarrhea Ham, poultry, potato or egg salad, mayonnaise, cream pastries Bacillus cereus Nausea, vomiting, diarrhea Fried rice 8–16 h Clostridium perfringens Abdominal cramps, diarrhea (vomiting rare) Beef, poultry, legumes, gravies B. cereus Abdominal cramps, diarrhea (vomiting rare) Meats, vegetables, dried beans, cereals

16 h Vibrio cholerae Watery diarrhea Shellfish, water Enterotoxigenic Escherichia coli Watery diarrhea Salads, cheese, meats, water Enterohemorrhagic

E. coli Bloody diarrhea Ground beef, roast beef, salami, raw milk, raw vegetables, apple juice Salmonella spp. Inflammatory diarrhea Beef, poultry, eggs, dairy products Campylobacter jejuni Inflammatory diarrhea Poultry, raw milk Shigella spp. Dysentery Potato or egg salad, lettuce, raw vegetables Vibrio parahaemolyticus Dysentery Mollusks, crustaceans

diarrhea and abdominal cramps are characteristic but vomiting is uncommon. The emetic form of B. cereus food poisoning is associated with contaminated fried rice; the organism is common in uncooked rice, and its heat-resistant spores survive boiling. If cooked rice is not refrigerated, the spores can germinate and produce toxin. Frying before serving may not destroy the preformed, heat-stable toxin. Food poisoning due to Clostridium perfringens also has a slightly longer incubation period (8–14 h) and results from the survival of heat-resistant spores in inadequately cooked meat, poultry, or legumes. After ingestion, toxin is produced in the intestinal tract, causing mod­ erately severe abdominal cramps and diarrhea; vomiting is rare, as is fever. The illness is self-limited, rarely lasting >24 h. Not all food poisoning has a bacterial cause. Nonbacterial agents of short-incubation food poisoning include capsaicin, which is found in hot peppers, and a variety of toxins found in fish and shellfish (Chap. 471). ■ ■LABORATORY EVALUATION Many cases of noninflammatory diarrhea are self-limited or can be treated empirically, and in these instances, the clinician may not need to determine a specific etiology. Potentially pathogenic E. coli cannot be distinguished from normal fecal flora by routine culture, and tests to detect enterotoxins are not available in most clinical laboratories. In situations in which cholera is a concern, stool should be cultured on selective media such as thiosulfate–citrate–bile salts–sucrose (TCBS) or tellurite–taurocholate–gelatin (TTG) agar; rapid diagnostic tests are also available. A latex agglutination test has made the rapid detection of rotavirus in stool practical for many laboratories, while reverse-transcriptase polymerase chain reaction (PCR) and specific antigen enzyme immunoassays have been devel­ oped for the identification of norovirus. Stool specimens should be examined by immunofluorescence-based rapid assays, PCR, or (less sensitive) standard microscopy for Giardia cysts or Cryptosporidium if the level of clinical suspicion regarding the involvement of these organisms is high. All patients with fever and evidence of inflammatory diarrhea acquired outside the hospital should be evaluated for Salmonella, Shigella, and Campylobacter. Salmonella and Shigella can be selected on MacConkey agar as non-lactose-fermenting (colorless) colonies or can be grown on Salmonella–Shigella agar or in selenite enrichment broth, both of which inhibit most organisms except these pathogens. Evalua­ tion of nosocomial diarrhea should initially focus on C. difficile; stool culture for other pathogens in the hospital setting is extremely low yield and is not cost-effective. Toxins A and B produced by pathogenic strains of C. difficile can be detected by rapid enzyme immunoassays, latex agglutination tests, or PCR (Chap. 139). Isolation of C. jejuni requires inoculation of fresh stool onto selective growth medium and incubation at 42°C in a microaerophilic atmosphere. In many labora­ tories in the United States, E. coli O157:H7 is among the most common pathogens isolated from visibly bloody stools. Shiga-toxin containing strains of this enterohemorrhagic serotype can be identified by PCR or in specialized laboratories by serotyping, but also can be identified presumptively as lactose-fermenting, indole-positive colonies of sor­ bitol nonfermenters (white colonies) on sorbitol MacConkey plates. If the clinical presentation suggests the possibility of intestinal amebiasis, stool should be examined by a rapid antigen detection assay or by (less sensitive and less specific) microscopy. Multiplex nucleic acid amplifi­ cation methods for detection of many stool pathogens (viral, bacterial, and parasitic) are increasingly being used in clinical microbiology laboratories to decrease the time to detection of a pathogen. These tests are more sensitive and rapid than standard culture methods and may detect multiple pathogens, which may lead to challenges in interpreta­ tion. For bacterial enteric infections, the lack of a microbial isolate prevents determination of antimicrobial susceptibility and typing of strains by public health authorities in order to detect and respond to common-source outbreaks. For this reason, the Centers for Disease Control and Prevention suggests that diagnosis of an enteric bacterial infection by a nucleic acid amplification method should be followed by attempted isolation of the pathogen by culture.

TREATMENT Infectious Diarrhea or Bacterial Food Poisoning In many cases, a specific diagnosis is not necessary or not available to guide treatment. The clinician can proceed with the informa­ tion obtained from the history, stool examination, and evaluation of dehydration severity. Empirical regimens for the treatment of traveler’s diarrhea are listed in Table 138-5. The mainstay of treatment is adequate rehydration. The treat­ ment of cholera and other dehydrating diarrheal diseases was revolutionized by the development and promotion of oral rehydra­ tion solution (ORS), the efficacy of which depends on the fact that glucose-facilitated absorption of sodium and water in the small intestine remains intact in the presence of cholera toxin. The use of ORS has reduced cholera mortality rates from >50% (in untreated cases) to <1%. Several ORS formulas have been developed and tested. Initial preparations were based on the treatment of patients with cholera and included a solution containing 3.5 g of sodium chloride, 2.5 g of sodium bicarbonate (or 2.9 g of sodium citrate), 1.5 g of potassium chloride, and 20 g of glucose (or 40 g of sucrose) per liter of water. Such a preparation can still be used for the treat­ ment of severe cholera. Many causes of secretory diarrhea, however, are associated with less electrolyte loss than occurs in cholera. Beginning in 2002, the World Health Organization recommended a reduced-osmolarity/reduced-salt ORS that is better tolerated and TABLE 138-5  Treatment of Traveler’s Diarrhea on the Basis of Clinical Featuresa CLINICAL SYNDROME SUGGESTED THERAPY Watery diarrhea (no blood in stool, no fever), 1 or 2 unformed stools per day without distressing enteric symptoms Oral fluids (oral rehydration solution, Pedialyte, Lytren, or flavored mineral water) and saltine crackers Watery diarrhea (no blood in stool, no fever), 1 or 2 unformed stools per day with distressing enteric symptoms Bismuth subsalicylate (for adults): 30 mL or 2 tablets (262 mg/tablet) every 30 min for 8 doses; or loperamideb: 4 mg initially followed by 2 mg after passage of each unformed stool, not to exceed 8 tablets (16 mg) per day (prescription dose) or 4 caplets (8 mg) per day (over-the-counter dose); drugs can be taken for 2 days. Antibacterial drugc can be considered in selected circumstances. Dysentery (passage of bloody stools) or fever (>37.8°C) Antibacterial drugc Vomiting, minimal diarrhea Bismuth subsalicylate (for adults; see dose above) Diarrhea in infants (<2 years old) Fluids and electrolytes (oral rehydration solution, Pedialyte, Lytren); continue feeding, especially with breast milk; seek medical attention for moderate dehydration, fever lasting >24 h, bloody stools, or diarrhea lasting more than several days aAll patients should take oral fluids (Pedialyte, Lytren, or flavored mineral water) plus saltine crackers. If diarrhea becomes moderate or severe, if fever persists, or if bloody stools or dehydration develops, the patient should seek medical attention. bLoperamide should not be used by patients with fever or dysentery; its use may prolong diarrhea in patients with infection due to Shigella or other invasive organisms. cThe recommended antibacterial drugs are as follows: If the level of suspicion is low for fluoroquinolone-resistant Campylobacter: Adults: (1) A fluoroquinolone such as ciprofloxacin, 750 mg as a single dose or 500 mg bid for 3 days; levofloxacin, 500 mg as a single dose or 500 mg qd for 3 days; or norfloxacin, 800 mg as a single dose or 400 mg bid for 3 days. (2) Azithromycin, 1000 mg as a single dose or 500 mg qd for 3 days. (3) Rifaximin, 200 mg tid or 400 mg bid for 3 days (not recommended for use in dysentery). Children: Azithromycin, 10 mg/kg on day 1, 5 mg/kg on days 2 and 3 if diarrhea persists. If fluoroquinolone-resistant Campylobacter is suspected (for example, following travel to Southeast Asia): Adults: Azithromycin (at above dose for adults). Children: Same as for children traveling to other areas (see above). Source: After DR Hill et al: The practice of travel medicine: Guidelines by the Infectious Diseases Society of America. Clin Infect Dis 43:1499, 2006.

more effective than classic ORS. This preparation contains 2.6 g of sodium chloride, 2.9 g of trisodium citrate, 1.5 g of potassium chloride, and 13.5 g of glucose (or 27 g of sucrose) per liter of water. ORS formulations containing rice or cereal as the carbohydrate source may be even more effective than glucose-based solutions. Patients who are severely dehydrated, are unable to drink, or in whom vomiting precludes oral therapy should receive IV solutions such as Ringer’s lactate in addition to ORS when the patient can ingest it safely.

Most cases of traveler’s diarrhea (usually due to enterotoxigenic or enteroaggregative E. coli or to Campylobacter) can be treated effectively with rehydration, bismuth subsalicylate, or antiperistal­ tic agents. Antimicrobial agents can shorten the duration of illness from 3–4 days to 24–36 h but may be associated with the acquisi­ tion of multidrug-resistant organisms; their use should therefore be reserved for severe cases. Changes in diet have not been shown to have an impact on the duration of illness, and the efficacy of probi­ otics to hasten recovery continues to be debated. Most individuals who present with dysentery (bloody diarrhea and fever) should be treated empirically with an antimicrobial agent (e.g., a fluoroquino­ lone or a macrolide, depending on local antimicrobial susceptibility patterns) pending diagnostic testing. Individuals with shigellosis should receive a 3- to 7-day course. Patients with severe or pro­ longed Campylobacter infection often benefit from antimicrobial treatment. Because of widespread resistance of Campylobacter to fluoroquinolones, especially in parts of Asia, a macrolide antibiotic such as erythromycin or azithromycin is preferred for this infection. Treatment of salmonellosis must be tailored to the individual patient. Since administration of antimicrobial agents often prolongs intestinal colonization with Salmonella, these drugs are usually reserved for individuals at high risk of complications from dis­ seminated salmonellosis, such as infants, patients with prosthetic devices, patients over age 50, and immunocompromised persons. Antimicrobial agents should not be administered to individuals (especially children) in whom enterohemorrhagic E. coli infection is suspected. Laboratory studies of enterohemorrhagic E. coli strains have demonstrated that many antibiotics induce replication of Shiga toxin–producing lambdoid bacteriophages, thereby significantly increasing toxin production by these strains. Clinical studies have supported these laboratory results, and antibiotics may increase by 20-fold the risk of hemolytic-uremic syndrome and renal failure during enterohemorrhagic E. coli infection. A clinical clue in the diagnosis of the latter infection is bloody diarrhea in a patient with a low-grade fever or who is afebrile. CHAPTER 138 Acute Infectious Diarrheal Diseases and Bacterial Food Poisoning
PROPHYLAXIS Improvements in safe water infrastructure to limit fecal–oral spread of enteric pathogens will be necessary if the prevalence of diarrheal dis­ eases is to be significantly reduced in resource-poor countries. Travelers can reduce their risk of diarrhea by eating only hot, freshly cooked food; by avoiding raw vegetables, salads, and unpeeled fruit; and by drinking only boiled or treated water and avoiding ice. Historically, few travelers to tourist destinations adhere to these dietary restrictions, even after pretravel counseling. Bismuth subsalicylate is an inexpensive agent for the prophylaxis of traveler’s diarrhea; it is taken at a dosage of 2 tablets (525 mg) four times a day. Treatment appears to be effective and safe for up to 3 weeks, but adverse events such as temporary darkening of the tongue, constipation, and tinnitus can occur. A meta-analysis sug­ gests that probiotics may lessen the likelihood of traveler’s diarrhea by ~15%, but further studies are needed. Prophylactic antimicrobial agents, although effective, are not generally recommended for the prevention of traveler’s diarrhea except when travelers are immunosuppressed or have underlying illness that places them at high risk for morbidity from gastrointestinal infection. If prophylaxis is indicated, the nonabsorbed antibiotic rifaximin can be considered. The possibility of exerting a major impact on the worldwide mor­ bidity and mortality associated with diarrheal diseases has led to inten­ sive efforts to develop effective vaccines against common bacterial and

18 - 139 Clostridioides difficile Infection, Including Pseudomembranous Colitis

139 Clostridioides difficile Infection, Including Pseudomembranous Colitis

viral enteric pathogens. An effective rotavirus vaccine is available. Vac­ cines against V. cholerae are available and recommended in areas where active transmission is ongoing, although the protection they offer is incomplete and short lived. A typhoid conjugate vaccine is now recom­ mended by the World Health Organization for use in countries where typhoid is endemic. At present, there are no effective commercially available vaccines against pathogenic E. coli, Shigella, Campylobacter, nontyphoidal Salmonella, norovirus, or intestinal parasites.

Acknowledgment The authors thank Stephen B. Calderwood, MD, Edward T. Ryan, MD, and Richelle C. Charles, MD, for their significant contributions to this chapter in the previous editions. ■ ■FURTHER READING Brown AB et al: Travel-related diagnoses among U.S. nonmigrant trav­ elers or migrants presenting to U.S. GeoSentinel Sites – GeoSentinel Network, 2012-2021. MMWR Surveill Summ 72:1, 2023. Global Burden of Disease Cause of Death Collaborators: Global, regional, and national age-sex-specific mortality for 282 causes of death in 195 countries and territories, 1980-2017: A systematic analysis for the Global Burden of Disease Study 2017. Lancet 392:1736, 2018. Goldenberg JZ et al: Probiotics for the prevention of pediatric antibioticassociated diarrhea. Cochrane Database Syst Rev 12:CD004827, 2015. Guttman JA, Finlay BB: Subcellular alterations that lead to diarrhea during bacterial pathogenesis. Trends Microbiol 16:535, 2008. Hyesuk S et al: Vaccines against gastroenteritis, current progress and challenges. Gut Microbes 11:1486, 2020. Levine MM et al: Diarrhoeal disease and subsequent risk of death in infants and children residing in low-income and middle-income countries: Analysis of the GEMS case-controlled study and 12-month GEMS-1A follow-on study. Lancet Glob Health 8:e202, 2020. Local Burden of Disease Diarrhoea Collaborators: Mapping geo­ PART 5 Infectious Diseases graphical inequalities in childhood diarrhoeal morbidity and mortal­ ity in low-income and middle-income countries, 2000-17: Analysis for the Global Burden of Disease Study 2017. Lancet 395:1779, 2020. Rodgers AP et al: Impact of enteric bacterial infections at and beyond the epithelial barrier. Nat Rev Microbiol 21:260, 2023. Shane AL et al: Infectious Diseases Society of America. Clinical practice guidelines for the diagnosis and management of infectious diarrhea. Clin Infect Dis 65:e45, 2017. Teh R et al: Review of the role of gastrointestinal multiplex polymerase chain reaction in the management of diarrheal illness. J Gastroenterol Hepatol 36:3286, 2021. Tsolis RM, Baumler AJ: Gastrointestinal host-pathogen interaction in the age of microbiome research. Curr Opin Microbiol 53:78, 2020. Dale N. Gerding, Stuart Johnson

Clostridioides difficile

Infection, Including Pseudomembranous Colitis ■ ■DEFINITION Clostridioides difficile infection (CDI) is a unique colonic disease that is acquired most commonly in association with antimicrobial use and the consequent disruption of the normal colonic microbiota. The most commonly diagnosed diarrheal illness acquired in the hospital, CDI results from the ingestion of spores of C. difficile that vegetate, multi­ ply, and secrete toxins, causing diarrhea and, in the most severe cases, pseudomembranous colitis (PMC).

■ ■ETIOLOGY AND EPIDEMIOLOGY C. difficile is an obligately anaerobic, gram-positive, spore-forming bacillus whose spores are found widely in nature, particularly in the environment of hospitals and chronic-care facilities. CDI occurs fre­ quently in hospitals and nursing homes (or shortly after discharge from these facilities) where the level of antimicrobial use is high and the environment is contaminated by C. difficile spores. Clindamycin, ampicillin, and cephalosporins were the first antibiot­ ics associated with CDI. The second- and third-generation cephalospo­ rins, particularly cefotaxime, ceftriaxone, cefuroxime, and ceftazidime, are agents frequently responsible for this condition, and the fluoroqui­ nolones (ciprofloxacin, levofloxacin, and moxifloxacin) are the most recent drug class to be implicated in hospital outbreaks. Penicillin/

β-lactamase-inhibitor combinations such as ticarcillin/clavulanate and piperacillin/tazobactam pose significantly less risk. However, all anti­ biotics, including vancomycin (the agent most commonly used to treat CDI), have been found to carry a risk of subsequent CDI. A minority of cases, especially in the community, are reported in patients without documentation of prior antibiotic exposure. C. difficile is acquired exogenously—most often in the hospital or nursing home, but also in the outpatient setting—and is carried in the stool of both symptomatic and asymptomatic patients. The rate of fecal colonization increases in proportion to length of hospital stay and is often ≥20% among adult patients hospitalized for >2 weeks; in contrast, the rate is 1–3% among community residents. CDI is the most com­ mon health care–associated infection in the United States, with an esti­ mated 462,100 cases in 2017. Between 2011 and 2017, the total burden of CDI in the United States decreased by 24%, which was due primarily to decreases in health care–associated CDI. The estimated burden of community-associated CDI was unchanged, but it now approximates the health care–associated rate in the United States. Asymptomatic fecal carriage of C. difficile in healthy neonates is very common, with repeated colonization by multiple strains in infants <1–2 years of age, but associated disease in these infants is extremely rare if it occurs at all. Spores of C. difficile are found on environmental surfaces (where the organism can persist for months) and on the hands of hospital personnel who fail to practice good hand hygiene. Hospital epidemics of CDI have been attributed to a single C. difficile strain and to multiple different strains, introduced by patients on admission, that are present simultaneously. Other identified risk factors for CDI include older age, greater severity of underlying illness, gastrointestinal surgery, use of electronic rectal thermometers, enteral tube feeding, and antacid treatment. Use of proton pump inhibitors may be a risk factor, but this risk is probably modest, and no firm data have impli­ cated these agents in patients who are not already receiving antibiotics. ■ ■PATHOLOGY AND PATHOGENESIS Spores of toxigenic C. difficile are ingested, survive gastric acidity, ger­ minate in the small bowel, and, in the presence of a disrupted micro­ biota, colonize the lower intestinal tract, where they elaborate two large toxins: toxin A (an enterotoxin) and toxin B (a cytotoxin). These toxins initiate processes resulting in the disruption of epithelial-cell barrier function, diarrhea, and pseudomembrane formation. Toxin A is a potent neutrophil chemoattractant, and both toxins glucosylate the GTP-binding proteins of the Rho subfamily that regulate the actin cell cytoskeleton. Data from studies using molecular disruption of toxin genes in isogenic mutants suggest that toxin B may be the more impor­ tant virulence factor, which is consistent with the well-documented occurrence of clinical disease caused by toxin A–negative strains but not by toxin B–negative strains. Disruption of the cytoskeleton results in loss of cell shape, adherence, and tight junctions, with consequent fluid leakage. A third toxin, binary toxin CDT, was previously found in only ~6% of strains but is present in all isolates of the widely recognized epidemic NAP1/BI/027 strain (see “Global Considerations,” below); this toxin is related to C. perfringens iota toxin. Its role in the pathogen­ esis of CDI has not yet been defined. The pseudomembranes of PMC are confined to the colonic mucosa and initially appear as 1- to 2-mm whitish-yellow plaques. The inter­ vening mucosa appears unremarkable, but, as the disease progresses,

FIGURE 139-1  Autopsy specimen showing confluent pseudomembranes covering the cecum of a patient with pseudomembranous colitis. Note the sparing of the terminal ileum (arrow). the pseudomembranes coalesce to form larger plaques and become confluent over the entire colon wall (Fig. 139-1). The whole colon is usually involved, but 10% of patients have rectal sparing. Viewed microscopically, the pseudomembranes have a mucosal attachment point and contain necrotic leukocytes, fibrin, mucus, and cellular debris. The epithelium is eroded and necrotic in focal areas, with neu­ trophil infiltration of the mucosa. Patients colonized with C. difficile were initially thought to be at high risk for CDI. However, four prospective studies have shown that colonized patients who have not previously had CDI actually have a decreased risk of CDI, possibly because many of these patients are colonized by nontoxigenic strains. At least three events are proposed as essential for the development of CDI (Fig. 139-2). Exposure to anti­ microbial agents is the first event and establishes susceptibility to CDI, most likely through disruption of the normal gastrointestinal micro­ biota. The second event is exposure to toxigenic C. difficile. Given that the majority of patients do not develop CDI after the first two events, a third event is clearly essential for its occurrence. Candidate third C. difficile exposure C. difficile exposure Antimicrobial(s) Healthcare Exposure (Increased chance of receiving an antibiotic and Increased chance of being exposed to spores of C. difficile) FIGURE 139-2  Pathogenesis model for Clostridioides difficile infection (CDI). At least three events are integral to C. difficile pathogenesis. Exposure to antibiotics establishes susceptibility to C. difficile infection. If susceptible, the patient may ingest nontoxigenic (nonpathogenic) or toxigenic strains of C. difficile as a second event. Acquisition of toxigenic C. difficile may be followed by asymptomatic colonization or CDI, depending on one or more additional factors, including the strain of C. difficile and an inadequate host anamnestic antibody response to C. difficile toxins.

events include exposure to a C. difficile strain of particular virulence, exposure to antimicrobial agents especially likely to cause CDI, and an inadequate host immune response. The host anamnestic immune response as has been shown for serum IgG antibody response to toxin A of C. difficile is likely one factor in the third event that determines which patients develop diarrhea and which patients remain asymp­ tomatic. The majority of humans probably first develop antibody to C. difficile toxins when colonized asymptomatically during the first year of life or after CDI in childhood. Infants are thought not to develop symptomatic CDI because they lack suitable mucosal toxin receptors that develop later in life. In adulthood, there is evidence that serum antibodies to both toxin A and B protect against recurrent CDI. Two large clinical trials in which intravenous monoclonal antibodies to toxin A and toxin B were used together and as single agents in addi­ tion to standard antibiotic therapy showed that rates of recurrent CDI were significantly lower with the combination of antibodies and with the toxin B antibody alone than with placebo plus standard therapy. Antibody to toxin A alone was ineffective.

■ ■GLOBAL CONSIDERATIONS Rates and severity of CDI in the United States, Canada, and Europe increased markedly in the early 2000s. Rates in U.S. hospitals tripled between 2000 and 2005. Hospitals in Montreal, Quebec, reported rates in 2005 that were four times higher than the 1997 baseline, with directly attributable mortality of 6.9% (increased from 1.5%). An epi­ demic strain, variously known as toxinotype III, REA type BI, PCR ribotype 027, and pulsed-field type NAP1 (collectively designated NAP1/BI/027), likely accounted for much of the increase in incidence. Two clones of NAP1/BI/027 originated in the United States and Canada and spread to the United Kingdom, Europe, and Asia. This epi­ demic strain was characterized by (1) an ability to produce 16–23 times as much toxin A and toxin B as control strains in vitro, (2) the presence of binary toxin CDT, and (3) high-level resistance to all fluoroquino­ lones. National control policies instituted in England in 2006 resulted in a marked decline in CDI cases, and restriction of fluoroquinolones, in particular, was correlated with near elimination of fluoroquinoloneresistant strains of C. difficile (i.e., NAP1/BI/027) there by 2013. This epidemic strain has likewise decreased in the United States, with data from the Centers for Disease Control and Prevention showing a decrease among health care–associated isolates from 31% to 15% (and from 19% to 6% in community-associated isolates) between 2011 and 2017. New strains have been and will probably continue to be impli­ cated in outbreaks, including a strain commonly found in food animals that also carries binary toxin and has been associated with high mortal­ ity rates in human infections (toxinotype V, ribotype 078). Currently, the most frequently isolated community-associated strain in the United States is ribotype 106 (REA group DH), which was previously found to be epidemic in the United Kingdom. CHAPTER 139 Clostridioides difficile Infection, Including Pseudomembranous Colitis Asymptomatic C. difficile colonization CDI Acquisition of a toxigenic strain of  C. difficile and presence of additional factor(s) result in CDI.

■ ■CLINICAL MANIFESTATIONS Diarrhea is the most common manifestation caused by C. difficile. Stools are almost never grossly bloody and range from soft and unformed to watery or mucoid in consistency, with a characteristic odor. Clinical and laboratory findings include fever in 28% of cases, abdominal pain in 22%, and leukocytosis in 50%. When adynamic ileus (which is seen on x-ray in ~20% of cases) results in cessation of stool passage, the diagnosis of CDI is frequently overlooked. A clue to the presence of unsuspected CDI in these patients is unexplained leukocytosis, with ≥15,000 white blood cells (WBCs)/μL. Such patients are at high risk for complications of CDI, particularly toxic megacolon and sepsis.

C. difficile diarrhea recurs after treatment in ~15–30% of cases and remains one of the most challenging treatment dilemmas. Recurrences may represent either relapses due to the same strain or reinfections with a new strain. Susceptibility to recurrence of clinical CDI is likely a result of continued disruption of the normal fecal microbiota caused by the antibiotic used to treat CDI. ■ ■DIAGNOSIS The diagnosis of CDI is based on a combination of clinical criteria: (1) diarrhea (≥3 unformed stools per 24 h for ≥2 days) with no other rec­ ognized cause plus (2) detection of toxin A or B in the stool, detection of toxin-producing C. difficile in the stool by nucleic acid amplification testing (NAAT; e.g., polymerase chain reaction [PCR]) or by culture, or visualization of pseudomembranes in the colon. PMC is a more advanced form of CDI and is visualized at endoscopy in only ~50% of patients with diarrhea who have a positive stool culture and toxin assay for C. difficile (Table 139-1). Endoscopy is a rapid diagnostic tool in seriously ill patients with suspected PMC and an acute abdomen, but a negative result in this examination does not rule out CDI. PART 5 Infectious Diseases Despite the array of tests available for C. difficile and its toxins (Table 139-1), no single test has high sensitivity, high specificity, and rapid turnaround. Most laboratory tests for toxins, including enzyme immunoassays (EIAs), lack sensitivity. NAATs (including PCR) are widely used diagnostically and are both rapid and sensitive; however, concern has been raised that PCR may detect colonization with toxigenic C. difficile in patients who have diarrhea for a reason other than CDI. Confirmation of the presence of toxin in the stool in addition to NAAT or glutamate dehydrogenase (GDH) positivity is recommended in the European CDI guidelines for diagnosis of CDI, and inclusion of a stool toxin test is recommended in the U.S. guidelines when there are no prior criteria for stool submission. Test algorithms that include NAAT followed by toxin EIA for NAAT+ results, and GDH plus toxin EIA arbitrated by NAAT when the two initial test results do not agree, have become widely used; however, when results of individual algorithm tests are discrepant (NAAT+ or GDH+/toxin EIA–), most patients are nonetheless treated for CDI. TABLE 139-1  Relative Sensitivity and Specificity of Diagnostic Tests for Clostridioides difficile Infection (CDI) TYPE OF TEST RELATIVE SENSITIVITYa RELATIVE SPECIFICITYa COMMENT Stool culture for C. difficile ++++ +++ Most sensitive test; specificity of ++++ if the C. difficile isolate tests positive for toxin; turnaround time too slow for practical use Cell culture cytotoxin test on stool +++ ++++ With clinical data, is diagnostic of CDI; highly specific but not as sensitive as stool culture; slow turnaround time Enzyme immunoassay for toxins A and B in stool ++ to +++ +++ With clinical data, is diagnostic of CDI; rapid results, but not as sensitive as stool culture or cell culture cytotoxin test Enzyme immunoassay for C. difficile common antigen in stool +++ to ++++ +++ Detects glutamate dehydrogenase found in toxigenic and nontoxigenic strains of C. difficile and other stool organisms; more sensitive and less specific than enzyme immunoassay for toxins; requires confirmation with a toxin test; rapid results Nucleic acid amplification tests for C. difficile toxin A or B gene in stool ++++ +++ Detects toxigenic C. difficile in stool; widely used in United States for clinical testing; more sensitive than enzyme immunoassay toxin testing; marked increase in CDI diagnoses when implemented Colonoscopy or sigmoidoscopy + ++++ Highly specific if pseudomembranes are seen; insensitive compared with other tests aAccording to both clinical and test-based criteria. Note: ++++, >90%; +++, 71–90%; ++, 51–70%; +, ~50%.

Empirical treatment is appropriate if stool testing is delayed. Testing of asymptomatic patients is not recommended except for epidemio­ logic study purposes. In particular, so-called tests of cure following treatment are not recommended because >50% of patients continue to harbor the organism and its toxin after diarrhea has ceased and test results are not predictive of recurrence of CDI. The results of such tests should not be used to restrict placement of patients in long-term care or nursing home facilities. TREATMENT Clostridioides difficile Infection PRIMARY CDI When possible, discontinuation of any ongoing antimicrobial administration is recommended as the first step in treatment of CDI. Earlier studies indicated that 15–23% of patients respond to this simple measure. However, with the advent of the NAP1/BI/027 epidemic strain and the associated rapid clinical deterioration of some patients, prompt initiation of specific CDI treatment has become the standard. General treatment guidelines include hydra­ tion and the avoidance of antiperistaltic agents and opiates, which may mask symptoms and possibly worsen disease. Nevertheless, antiperistaltic agents have been used safely with vancomycin or metronidazole treatment for mild to moderate CDI. Oral administration of fidaxomicin was suggested as firstline treatment for CDI in the 2021 Infectious Diseases Society of America (IDSA) and Society for Healthcare Epidemiology of America (SHEA) focused update guidelines on management of CDI in adults. However, because of resource availability issues, oral vancomycin remains an acceptable alternative. Oral metro­ nidazole is recommended only for mild or moderate CDI when fidaxomicin or vancomycin is not available. IV vancomycin is ineffective for CDI. Fidaxomicin is available only for oral admin­ istration. Two large clinical trials comparing vancomycin and fidaxomicin indicated comparable clinical resolution of diar­ rhea in ~90% of patients, and the rate of recurrent CDI was significantly lower with fidaxomicin. The largest randomized controlled trial of vancomycin versus metronidazole showed that the vancomycin cure rate was superior to the metronidazole cure rate (81% vs 73%; p = .034) for all patients with CDI, regardless of severity. Although the mean time to resolution of diarrhea is 2–4 days, the response to metronidazole may be much slower. Treatment should not be deemed a failure until a drug has been given for at least 6 days. On the basis of data for shorter courses of vancomycin and the results of four large clinical trials, it is recommended that vancomycin or fidaxomicin be given for at least 10 days. Metronidazole was never approved for CDI by the

TABLE 139-2  Recommendations for the Treatment of Clostridioides difficile Infection (CDI) CLINICAL SETTING TREATMENT(S) COMMENTS Initial episode, mild to moderate Fidaxomicin (200 mg bid × 10 d) or alternatively Oral vancomycin (125 mg qid × 10 d) Initial episode, severe Oral vancomycin (125 mg qid × 10 d) or alternatively Fidaxomicin (200 mg bid × 10 d) Initial episode, fulminant Vancomycin (500 mg PO or via nasogastric tube) plus metronidazole (500 mg IV q8h) plus consider Rectal instillation of vancomycin (500 mg in 100 mL of normal saline as a retention enema q6–8h) First recurrence Fidaxomicin (200 mg bid × 10 d) or Oral vancomycin (125 mg qid × 10 d) or Oral vancomycin followed by a taper-and-pulse regimena Multiple recurrences Oral vancomycin treatment followed by a taper-and-pulse regimen or Fidaxomicin (200 mg bid × 10 d or 200 mg bid × 5 d followed by every other day × 20 d) or Vancomycin (125 mg qid × 10 d), then stop vancomycin and start rifaximin (400 mg bid × 2 weeks) or Fecal microbiota replacement therapy (FMRT) Patients at high risk of recurrent CDI who are receiving vancomycin, fidaxomicin, or metronidazole Bezlotoxumab 10 mg/kg given IV Bezlotoxumab is adjuvant therapy (in addition to and during antibiotic treatment) for patients at high risk for recurrent CDI. Risk factors include age >65 years, immunocompromised host, severe CDI on presentation, and prior episode of CDI in the past 6 months. aA typical taper-and-pulse vancomycin regimen following a 10-day treatment course includes: 125 mg bid × 1 week, then daily × 1 week, then q2–3d for 2–8 weeks. U.S. Food and Drug Administration (FDA), and its use for CDI treatment declined markedly after publication of the 2017 IDSA/ SHEA CDI guidelines. It is important to initiate treatment with oral vancomycin or fidaxomicin for patients who appear seriously ill, particularly if they have a high WBC count (>15,000/μL) or creatinine level (≥1.5 mg/dL) (Table 139-2). Small randomized trials of nitazoxanide, bacitracin, rifaximin, and fusidic acid for treatment of CDI have been conducted. These drugs have not been extensively studied, shown to be superior, or approved by the FDA for CDI, but they provide potential alternatives to vancomy­ cin and fidaxomicin. RECURRENT CDI Overall, ~15–30% of successfully treated patients experience recurrences of CDI following treatment. CDI recurrence is sig­ nificantly lower in patients treated with fidaxomicin than in those treated with vancomycin. Vancomycin and metronidazole have comparable recurrence rates, and metronidazole is not recom­ mended for treatment of recurrent CDI. Patients who have a first recurrence of CDI have an even higher rate of second recur­ rence. Fidaxomicin is superior to vancomycin in reducing further recurrences in patients who have had one CDI recurrence (Table 139-2). Recurrent disease, once thought to be relatively mild, has now been documented to pose a significant (11%) risk of seri­ ous complications (shock, megacolon, perforation, colectomy, or death within 30 days). There is no standard treatment for multiple recurrences, but the use of vancomycin in a tapering and pulsed dosing regimen every other day for 2–8 weeks has been used for years as a practical approach to treating these patients, and recent data suggest it is still effective. Other recommended treat­ ment options for patients with multiple CDI recurrences include fidaxomicin in standard or extended/pulsed dosing regimens, vancomycin followed by rifaximin, or fecal microbiota transplan­ tation (FMT) via nasoduodenal tube, colonoscope, enema, or oral capsules (Table 139-2). FMT has been widely used over the past decade, and the availability of stool banks and oral capsule formu­ lations made this approach more practical. Recently, the FDA has approved two microbiota replacement therapies and discontinued enforcement discretion for centralized donor stool banks. A fecal

Oral metronidazole is less effective than the other options and may necessitate a longer treatment course for response. Metronidazole (500 mg tid × 10–14 d) is recommended only if vancomycin or fidaxomicin is not readily accessible and for mild to moderate disease only. Indicators of severe disease may include leukocytosis (≥15,000 white blood cells/μL) and a creatinine level ≥1.5 mg/dL. Fulminant CDI is defined as severe CDI with the addition of hypotension, shock, ileus, or toxic megacolon. The duration of treatment may need to be >2 weeks and is dictated by response. Treatment for the initial episode may be considered when choosing treatment for the first recurrence. It is recommended that FMRT by fecal microbiota (Rebyota, live-jslm) given by enema or fecal microbiota spores (Vowst, live-brpk) given orally be considered only after appropriate antibiotic treatment for recurrent CDI. CHAPTER 139 microbiota suspension (Rebyota, live-jslm) and a suspension of live fecal microbiota spores for oral delivery (Vowst, live-brpk) are now approved for patients with recurrent CDI to prevent further recurrence. Both of these products are adjunctive treatments that are indicated for patients who have completed antibiotic treat­ ment for CDI. Clostridioides difficile Infection, Including Pseudomembranous Colitis In addition to antibacterial therapies, another adjunctive treat­ ment is now available for patients who are receiving standard-ofcare antibacterial agents and who are at high risk for recurrent CDI (rCDI). Bezlotoxumab, a monoclonal antibody directed against C. difficile toxin B, has been shown to reduce the risk of rCDI by an absolute rate of ~10% when administered to patients currently receiving vancomycin, fidaxomicin, or metronidazole. Risk factors for rCDI in the clinical trials included age >65 years, immunocom­ promise, severe CDI on presentation, and prior episode of CDI in the past 6 months. SEVERE COMPLICATED OR FULMINANT CDI Fulminant (rapidly progressive and severe) CDI presents the most difficult treatment challenge. Patients with fulminant disease often do not have diarrhea, and their illness mimics an acute surgical abdomen. Sepsis (hypotension, fever, tachycardia, leukocytosis) may result from fulminant CDI. An acute abdomen (with or with­ out toxic megacolon) may include signs of obstruction, ileus, colonwall thickening and ascites on abdominal CT, and peripheral-blood leukocytosis (≥20,000 WBCs/μL). With or without diarrhea, the differential diagnosis of an acute abdomen, sepsis, or toxic megaco­ lon should include CDI if the patient has received antibiotics in the past 2 months. Cautious sigmoidoscopy or colonoscopy to visualize PMC and an abdominal CT examination are the best diagnostic tests in patients without diarrhea. Medical management of fulminant CDI is suboptimal because of the difficulty of delivering oral fidaxomicin, metronidazole, or vancomycin to the colon in the presence of ileus (Table 139-2). The combination of vancomycin (given orally or via nasogastric tube and by retention enema) plus IV metronidazole has been used with some success in uncontrolled studies, as has IV tigecycline in small-scale uncontrolled studies. Surgical colectomy may be life­ saving if there is no response to medical management. If possible,

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140 Urinary Tract Infections: Cystitis, Prostatitis, and Pyelonephritis

colectomy should be performed before the serum lactate level reaches 5 mmol/L. However, mortality and morbidity associated with colectomy may be reduced by performing instead a laparo­ scopic ileostomy followed by colon lavage with polyethylene glycol and vancomycin infusion into the colon via the ileostomy.

■ ■PROGNOSIS The mortality rate attributed to CDI, previously found to be 0.6–3.5%, has reached 6.9% in recent outbreaks and is progressively higher with increasing age. Most patients recover, but recurrences are common. ■ ■PREVENTION AND CONTROL Strategies for the prevention of CDI are of two types: those aimed at preventing transmission of the organism to the patient and those aimed at reducing the risk of CDI if the organism is transmitted. Transmission of C. difficile in clinical practice has been prevented by gloving of personnel, elimination of the use of contaminated electronic thermometers, and use of hypochlorite (bleach) solution for environ­ mental decontamination of patients’ rooms. Hand hygiene is critical; hand washing is recommended in CDI outbreaks because alcohol hand gels are not sporicidal. CDI outbreaks have been best controlled by restricting the use of specific antibiotics, such as clindamycin, second- and third-generation cephalosporins, and fluoroquinolones. Outbreaks of CDI due to antibiotic-resistant strains have resolved promptly when specific antibiotic use is restricted. Future primary prevention strate­ gies include use of monoclonal antibodies, as well as biotherapeutics with live organisms to prevent colonization. Vaccines have to date been unsuccessful. PART 5 Infectious Diseases ■ ■FURTHER READING Dingle KE et al: Effects of control interventions on Clostridium difficile infection in England: An observational study. Lancet Infect Dis 17:411, 2017. Feuerstadt P et al: SER-109, an oral microbiome therapy for recur­ rent Clostridioides difficile infection. N Engl J Med 386:220, 2022. Guh AY et al: Trends in U.S. burden of Clostridioides difficile infection and outcomes. N Engl J Med 382:1320, 2020. Guh AY et al: Potential underreporting of treated patients using a Clostridioides difficile testing algorithm that screens with a nucleic acid amplification test. Infect Cont Hosp Epidemiol 2024. Online ahead of print. He M et al: Emergence and global spread of epidemic healthcareassociated Clostridium difficile. Nat Genet 45:109, 2013. Johnson S et al: Vancomycin, metronidazole, or tolevamer for Clostridium difficile infection: Results from two multinational, randomized, controlled trials. Clin Infect Dis 59:345, 2014. Johnson S et al: Clinical practice guideline by the Infectious Diseases Society of America (IDSA) and Society for Healthcare Epidemiology of America (SHEA): 2021 Focused update guidelines on management of Clostridioides difficile infection in adults. Clin Infect Dis 73:755, 2021. Khanna S et al: Efficacy and safety of RBX2660 in PUNCH CD3, a phase III, randomized, double‑blind, placebo‑controlled trial with a Bayesian primary analysis for the prevention of recurrent Clostridioi­ des difficile infection. Drugs 82:1527, 2022. Kociolek LK et al: Natural Clostridioides difficile toxin immunization in colonized infants. Clin Infect Dis 70:2095, 2020. Louie TJ et al: Fidaxomicin versus vancomycin for Clostridium difficile infection. N Engl J Med 364:422, 2011. McDonald LC et al: Clinical practice guidelines for Clostridium difficile infection in adults and children: 2017 update by the Infec­ tious Diseases Society of America (IDSA) and the Society for Health­ care Epidemiology of America (SHEA). Clin Infect Dis 66:987, 2018. Polage CR et al: Overdiagnosis of Clostridium difficile infection in the molecular test era. JAMA Intern Med 175:1792, 2015. See I et al: NAP1 strain type predicts outcomes from Clostridium dif­ ficile infection. Clin Infect Dis 58:1394, 2014. Wilcox MH et al: Bezlotoxumab for prevention of Clostridium difficile infection recurrence. N Engl J Med 376:305, 2017.

Urinary Tract Infections:

Cystitis, Prostatitis, and Pyelonephritis Barbara W. Trautner, Prathit A. Kulkarni,

Nicolás W. Cortés-Penfield, Kalpana Gupta In the preantibiotic era, urinary tract infection (UTI) caused significant morbidity. Hippocrates described acute cystitis as sometimes lasting for a year before either resolving or worsening to involve the kidneys. Early twentieth-century chemotherapeutics for UTI were ineffective, and persistent infection was common. Nitrofurantoin, which became available during the 1950s, was the first tolerable and effective agent for UTI treatment. Today, UTI remains common, being one of the leading reasons for which antibiotics are used within and outside the hospital. Patients typically favorably and quickly to appropriately chosen mod­ ern antimicrobials. Since the most common manifestation of UTI is acute cystitis, which is far more prevalent among women than among men, most clinical research on UTI has involved healthy young adult women recruited from college campuses or large U.S. health maintenance organizations. Therefore, clinicians must understand that UTI remains relatively under­ studied in other patient populations, and they must use expert judgment to decide when and to what extent UTI clinical trial data and treatment guidelines can be generalized to other groups of patients. ■ ■DEFINITIONS The term urinary tract infection refers to an infection somewhere along the urinary tract that produces clinical symptoms. Infection can be caused either by bladder invasion by a new urinary pathogen or by a shift in the existing urinary bacteria’s dynamic equilibrium with the host; both causes result in local tissue injury and inflammation. Thus, while UTI encompasses a broad range of clinical presentations, from acute simple cystitis to bacteremic emphysematous pyelonephritis, importantly, the presence alone of bacteria in the urinary tract in a patient without related symptoms does not constitute UTI. The pres­ ence of bacteria in the urine without associated symptoms is instead termed asymptomatic bacteriuria (ASB). This distinction between UTI and ASB has major clinical impli­ cations. Both UTI and ASB connote the presence of bacteria in the urinary tract and are usually accompanied by white blood cells in the urine (termed pyuria). However, while antibiotic treatment of UTI is almost always indicated to relieve the symptoms attributable to urinary bacterial infection and to prevent progression of the infection, ASB typically poses no threat for most patients and usually does not require treatment. Limited data suggest inappropriate treatment of ASB might in fact increase risk for future UTI, perhaps by facilitating bladder colo­ nization with more virulent and potentially more resistant pathogens. Two exceptions to the general rule that ASB should not be treated are patients who are pregnant or who are about to undergo certain urologic procedures. In pregnant patients, untreated ASB is associated with pyelonephritis, preterm delivery, and low birth weight, which justifies use of routine ASB screening and treatment. In patients undergoing procedures that cause urologic mucosal injury (e.g., lithotripsy or transurethral resection of the prostate), bacterial translocation from urine to blood can occur; therefore, screening for, and periprocedural treatment of, ASB is recommended in this setting. In this chapter, the term urinary tract infection denotes symptom­ atic disease; cystitis, symptomatic infection of the bladder; prostatitis, symptomatic infection of the prostate; and pyelonephritis, symptomatic infection of the kidneys. Uncomplicated urinary tract infection refers to an infection confined to the bladder in a woman or man without a urinary catheter, whereas complicated urinary tract infection refers to infection that extends beyond the bladder (e.g., prostatitis, pyelone­ phritis, bacteremia, or UTI in the setting of intermittent or indwelling

urinary catheterization, termed catheter-associated UTI [CAUTI]). Recurrent urinary tract infection is not necessarily complicated; indi­ vidual episodes can be uncomplicated and treated as such. This approach to UTI categorization differs from the classic approach, in which UTI in men and other populations perceived to be at higher risk for poor outcome (e.g., patients with diabetes, anatomic abnormali­ ties of the urinary tract, or immunocompromise) was automatically considered complicated. The new updated approach more closely reflects actual clinical practice. For the frontline clinician, the key con­ siderations in diagnostic workup and therapy for UTI include whether the patient is stable for outpatient management, whether a source of recalcitrant and/or recurrent infection (e.g., obstructing renal calculi) needs to be identified, and whether the prescribed antimicrobial agents must achieve adequate levels in the blood and renal tissue in addition to the urine. Prostatitis, usually categorized as either acute bacterial prostatitis (ABP) or chronic bacterial prostatitis (CBP), can complicate cystitis or arise hematogenously (e.g., as a metastatic focus of Staphylococcus aureus bacteremia) in men and transgender women. While prostatitis is certainly an infection of the genitourinary tract, it is largely unrepre­ sented in UTI clinical trial data, and earlier ABP literature suggests that prolonged therapy with antimicrobials penetrating the prostate might be required. Therefore, clinical practice guidelines for complicated UTI cannot be assumed to be generalizable to patients with prostatitis. Urethritis is occasionally caused by Escherichia coli and other urinary pathogens but is predominantly due to Neisseria gonorrhea, Chlamydia trachomatis, and Mycoplasma genitalium. Hence, symptoms such as urethral pain, pruritus, or discharge should prompt evaluation for sexually transmitted infections (STIs), which are covered in Chap. 141. ■ ■EPIDEMIOLOGY AND RISK FACTORS During the neonatal period, the incidence of UTI is slightly higher among males than among females because male infants more com­ monly have congenital urinary tract anomalies. After 50 years of age, obstruction from prostatic hypertrophy becomes common in men, and incidence of UTI is almost as high among men as among women. Between 1 year and ~50 years of age, however, UTI and recurrent UTI are predominantly conditions of females, with most true male UTI occurring in the context of urinary catheterization or anatomic abnormalities. The prevalence of ASB is ~5% among women between ages 20 and 40 and might be as high as 40–50% among elderly women and men. Up to 80% of women develop at least one UTI during their lifetime, predominantly acute simple cystitis. The lifetime prevalence of UTI in men in the United States is estimated to be 14%. About 20–30% of women who have had one episode of UTI will have recurrent episodes. Early recurrence (within 2 weeks) is usually regarded as relapse rather than reinfection and might indicate the need to evaluate the patient for a sequestered focus of ongoing infection. The rate of UTI recurrence ranges from 0.3 to 7.6 infections per patient per year, with an average of 2.6 reinfections per year. It is not uncom­ mon for multiple recurrences to follow an initial infection, resulting in clustering of episodes. Clustering may be related to the presence of a new risk factor, sloughing of the protective outer bladder epithelial layer in response to bacterial attachment during acute cystitis, or possibly

bacteria into the genitourinary tract or serve as a nidus of infection, and (3) factors that disrupt or impair local mucosal defenses.

The flushing action of normal urination is a key mechanism by which the host controls urinary bacterial populations and mitigates bacterial ascent from the urethra to the bladder and kidneys. Ana­ tomic factors that result in urinary obstruction or retention have been associated with cystitis, pyelonephritis, and recurrent UTI. These risk factors include congenital urinary tract abnormalities, cystoceles, pelvic-organ prolapse, bladder dysfunction leading to overflow urinary incontinence, and benign prostatic hypertrophy. Importantly, not all men with UTI have detectable urinary tract abnormalities; this point is particularly relevant for men ≤45 years of age. Factors that promote bacterial entry into the urinary tract predis­ pose to UTI. Sexual activity is likely the most common among these and is temporally associated with cystitis, with an increased relative risk in one study ranging from 1.4 with weekly intercourse to 4.8 with intercourse five times weekly. For men, insertive rectal intercourse and lack of circumcision are associated with increased UTI risk, the latter likely because E. coli is tends to colonize the glans and prepuce and subsequently migrate into the urinary tract. For men and women, urinary instrumentation (e.g., intermittent or chronic catheterization or existence of a draining nephrostomy tube) dramatically increases bacterial entry into the urinary tract and risk of UTI. In addition, for­ eign bodies in the urinary tract (e.g., stents and kidney stones) also can become colonized with bacteria and serve as niduses for recurrent UTI. Factors that disrupt or impair local mucosal defenses, usually either by irritating the urogenital mucosa or by disrupting the normal female urogenital microbiome (i.e., symbiotic lactobacilli), predispose to UTI. Such factors include recent use of diaphragms or spermicide and menopause. Women with diabetes have two- to threefold higher rates of ASB and UTI than women without diabetes, and this risk increases further with a longer duration of having had diabetes and with the need for insulin rather than oral medications. The specific mechanisms for this risk are not clear, although impaired cytokine secretion or diabetes complications leading to bladder dysfunction likely contribute. Whether glycosuria is major contributor to UTI risk remains controversial. The U.S. Food and Drug Administration has issued a drug-safety warning for the observed association between use sodium–glucose cotransporter 2 (SGLT-2) inhibitors for treatment of diabetes and UTI. The postulated mechanism is that these drugs increase excretion of glucose in the urine. CHAPTER 140 Urinary Tract Infections: Cystitis, Prostatitis, and Pyelonephritis ■ ■ETIOLOGY E. coli is the predominant pathogen across the spectra of UTI clinical syndromes and patient populations. In cases of acute cystitis in the United States, E. coli accounts for 75–90% of isolates, Staphylococcus saprophyticus for 5–15% (with more frequent isolation from younger women), and other Enterobacterales species (i.e., Klebsiella, Proteus, Enterobacter, and Citrobacter) for the majority of the remainder. The microbiology of pyelonephritis is similar to that of acute cystitis, as would be expected since pyelonephritis usually develops as ascend­ ing infection from the bladder. The microbiology of CAUTI is more diverse, but E. coli remains the predominant organism. Other impor­ tant pathogens in CAUTI are Pseudomonas aeruginosa, enterococci, Staphylococcus aureus, and Candida. Genetic sequencing of the bladder microbiome has consistently demonstrated that more bacterial spe­ cies are present than can be identified by routine culture methods, in both symptomatic and asymptomatic states. The clinical significance of these noncultivable organisms is unknown, but such work demon­ strates that the healthy bladder is often not a sterile site. Antimicrobial resistance among Enterobacterales species that cause UTI is increasing. One recent surveillance study of isolates from the United States demonstrated resistance rates of >20% to trimethoprimsulfamethoxazole (TMP-SMX), fluroquinolones, and nitrofurantoin among ambulatory patients in many regions of the country. In addi­ tion, approximately 6% of isolates were resistant to three or more antibiotic classes, and approximately 9% of isolates had production of extended-spectrum β-lactamase. Surveillance studies conducted in South America and Europe have yielded similar findings. The increased

prevalence of multidrug-resistant uropathogens has left few oral options for therapy in some cases and no single agent or agents that can confidently be recommended for empirical ambulatory treatment of UTI without regard to geographic region. Since resistance rates vary in different areas, local antibiogram data can help inform which drugs should be used as preferred empirical therapy in UTI, with the caveat that organisms are identified only in cases in which urine is sent for culture—typically, when complicated or recurrent UTI is suspected— and so may overestimate antimicrobial resistance in healthy ambula­ tory patients with a primary episode of UTI.

■ ■PATHOGENESIS The urinary tract can be viewed as an anatomic unit linked by a con­ tinuous column of urine extending from the kidneys to the urethra. In the majority of UTIs, bacteria establish infection by ascending from the urethra to the bladder. Continuing ascent up the ureter to the kidney is the pathway for most renal parenchymal infections. The other main route of entry into the genitourinary tract is hematogenous in cases where bacteremia precedes UTI. Importantly, introduction of bacteria into the bladder does not inevitably lead to sustained and symptomatic infection. The interplay of host, pathogen, and environmental factors determines whether tissue invasion and symptomatic infection will ensue (Fig. 140-1). For example, bacteria often enter the bladder after sexual intercourse, but normal voiding and innate host defense mecha­ nisms in the bladder eliminate these organisms. Any foreign body in the urinary tract, such as a urinary catheter or stone, provides an inert surface for bacterial colonization. Additionally, abnormal micturition and/or significant residual urine volume promotes infection. In the simplest terms, anything that increases the likelihood of bacteria enter­ ing the bladder and staying there increases risk of developing UTI. Intracellular bacterial communities of infecting organisms within the bladder epithelium have been demonstrated in animal models of UTI and in exfoliated human urothelial cells, but the clinical impact of this phenomenon in humans is not yet clear. PART 5 Infectious Diseases Bacteria can also gain access to the urinary tract through the blood­ stream. However, hematogenous spread only accounts for less than 2% of documented UTIs and usually results from bacteremia caused by relatively virulent organisms, such as S. aureus and Salmonella. The isolation of either of these pathogens from a urine culture warrants consideration of concomitant bacteremia. Hematogenous infections can also produce focal renal abscesses or pyelonephritis. The pathogenesis of candiduria in particular is distinct because the hematogenous route is more common in that situation. The presence of Candida in the urine of a non-instrumented immunocompetent patient implies either genital contamination or potentially widespread visceral dissemination. By contrast, candiduria is common in catheter­ ized patients, particularly following antimicrobial therapy for CAUTI or inappropriate treatment of ASB. Host

Genetic background

Behavioral factors

Underlying disease

Tissue-specific receptors Organism

Type of organism

Presence of virulence factors

Expression of virulence factors Organism Host Infection, colonization, or elimination Environment Environment

Vaginal ecology

Anatomy/urinary retention

Medical devices FIGURE 140-1  Pathogenesis of urinary tract infection. The relationship among specific host, pathogen, and environmental factors determines the clinical outcome.

Environmental Factors  •  VAGINAL ECOLOGY  Vaginal ecology is an important environmental factor affecting risk of UTI in women. Colonization of the vaginal introitus and periurethral area with organ­ isms from the intestinal flora (usually E. coli) is the critical initial step in the pathogenesis of UTI. Sexual intercourse is associated with an increased risk of vaginal colonization with E. coli and thereby increases risk of UTI. Nonoxynol-9 in spermicide is toxic to normal vaginal lactobacilli and thus is likewise associated with an increased risk of vaginal colonization and bacteriuria due to E. coli. In postmenopausal women, the previously predominant vaginal lactobacilli are replaced with colonizing Gram-negative bacteria because of vaginal atrophy. Topical estrogens have been demonstrated to reduce frequency of recurrent UTIs in postmenopausal women without altering systemic hormone levels. Given the side effects of systemic hormone replace­ ment, oral estrogens should not be used to prevent UTI. ANATOMIC AND FUNCTIONAL ABNORMALITIES  Any condition that permits urinary stasis or obstruction predisposes the patient to devel­ opment of UTI. Foreign bodies such as stones or urinary catheters provide an inert surface for bacterial colonization and formation of a persistent biofilm. Thus, vesicoureteral reflux, ureteral obstruction secondary to prostatic hypertrophy, neurogenic bladder, and urinarydiversion surgery all create an environment that can lead to UTI. Inhibition of ureteral peristalsis and decreased ureteral tone leading to vesicoureteral reflux are important in the pathogenesis of pyelonephri­ tis in pregnant patients. Anatomic factors—in particular, the length of the urethra—are considered to be the primary reason why UTI is predominantly an illness of young women rather than of young men. Host Factors  The genetic background of the host influences the individual’s susceptibility to recurrent UTI. In women, famil­ ial disposition to UTI is well documented. Additionally, women with recurrent UTI are more likely to have had their first UTI before the age of 15 years and to have a maternal history of UTI. A component of the underlying pathogenesis of this familial predisposition to recur­ rent UTI might be persistent vaginal colonization with E. coli, even during asymptomatic periods. Vaginal and periurethral mucosal cells from women with recurrent UTI bind uropathogenic bacteria three­ fold more than do mucosal cells from women without recurrent infec­ tion. This difference appears to be at least partly mediated by not expressing certain blood group antigens, facilitating adherence of E. coli to the urothelium. Mutations in host innate immune response genes (e.g., those coding for Toll-like receptors and the interleukin 8 receptor) also have been linked to recurrent UTI and pyelonephritis. Microbial Factors  Strains of E. coli that cause invasive symp­ tomatic infection of the urinary tract in otherwise normal hosts often possess and express genetic virulence factors, including bacterial-surface adhesins that mediate microbial binding to specific receptors on the surface of uroepithelial cells. The best-studied adhes­ ins are the P fimbriae, hairlike protein structures that interact with a specific receptor on renal epithelial cells. (The letter P denotes the abil­ ity of these fimbriae to bind to blood-group antigen P, which contains a d-galactose-d-galactose residue.) P fimbriae are important in the pathogenesis of pyelonephritis and subsequent bloodstream invasion from the kidney. Another adhesin is the type 1 pilus (fimbria), which all E. coli strains possess but which is not always expressed. Type 1 pili are thought to play a key role in initiating E. coli bladder infection; they mediate binding to mannose on the luminal surface of bladder uroepi­ thelial cells. Toxins, metal (iron)-acquisition systems, biofilm forma­ tion, and bacterial capsules also can contribute to the ability of pathogenic E. coli to thrive in the bladder. APPROACH TO THE PATIENT Clinical Syndromes and Diagnostic Approaches The key questions to be addressed when UTI is suspected are: •  Does the patient actually have UTI (rather than ASB or uri­ nary symptoms not related to infection)?

•  If the patient does have UTI, is it confined to the bladder (cystitis), or is there evidence that infection might be present beyond the bladder? •  If the diagnosis is uncertain, would it be safe to delay empiri­ cal antimicrobial therapy while attempting to achieve more diagnostic certainty? The answers to these immediate clinical questions will shape the diagnostic and therapeutic approach. ASYMPTOMATIC BACTERIURIA A diagnosis of ASB should be strongly suspected when a patient with a positive urine culture does not have either localizing urinary symptoms or systemic symptoms that are unexplained. The clinical presentation is typically bacteriuria that is detected incidentally when a patient undergoes a screening urine culture. Accordingly, diagnostic stewardship (i.e., not sending urine cultures in patients who lack clinical signs and symptoms that suggest UTI except when screening patients who are pregnant or about to undergo urologic procedures) is the key intervention in reducing misdiagnosis of UTI and inappropriate treatment of ASB. In the setting of pyuria (presence of white blood cells in the urine) and bacteriuria in a patient without symptoms localizing to the urinary tract, systemic symptoms and signs such as fever, altered mental status, and leukocytosis could be features of UTI but do not on their own merit a diagnosis of UTI unless other potential etiologic causes for such findings have been adequately explored and ruled out. Critically, the degree of pyuria in an asymptomatic patient should not influence the diagnosis of ASB versus true UTI. Very high levels of pyuria are common in urinary stasis, such as in patients with end-stage renal disease or obstructed urinary catheters. In practice, one of the most challenging clinical scenarios to dif­ ferentiate ASB from true UTI occurs when an older patient presents with isolated altered mentation, often superimposed on chronic cognitive deficits and potentially with little prior documentation. Accurately diagnosing the cause of such a patient’s acute encepha­ lopathy is challenging both because the patient might not be able to reliably report localizing urinary symptoms and because true UTI is an important diagnostic consideration on a long list of possible causes of the patient’s symptoms. In such a patient, the treating clinician should first establish whether there is evidence of sepsis. If not, the next consideration is whether empirical therapy for poten­ tially serious bacterial infection can be safely withheld while the diagnostic process is ongoing and symptomatic treatment for other noninfectious causes of delirium is given. The clinician should seek out collateral history, such as by calling the patient’s caregivers if none are present at the bedside, to elicit information about what occurred prior to the patient’s development of acute encephalopa­ thy. A history of prior recurrent UTIs is might not be accurate and instead reflects a history of persistent ASB. The clinician must be vigilant to avoid premature diagnostic closure. Specifically, the clinician should rule out other potential causes of delirium, particu­ larly those whose treatment is time-sensitive or that may worsen with if the diagnosis is not considered and addressed. CYSTITIS The typical symptoms of cystitis are dysuria, urinary frequency, and urinary urgency. Other common symptoms include supra­ pubic discomfort or tenderness and new nocturia, hesitancy, or gross hematuria. Fever, rigors, and unilateral back or flank pain are all inconsistent with uncomplicated cystitis and should provoke investigation for infection beyond the bladder (i.e., involving the kidneys, prostate, or bloodstream). Cystitis in women can be treated on the basis of history alone. However, if the symptoms are not specific or if a reliable history cannot be obtained, then a urine dipstick (or, if rapidly available, urine microscopy) should be performed. In the scenario of a woman with symptoms potentially suggestive of cystitis, a negative dipstick result does not fully rule out UTI, and appropriate next

steps would include formal urine microscopy with reflex to culture, possibly a pelvic examination or STI testing, and close clinical follow-up. In pregnant patients, in patients suspected to have a resistant organism, or in cases of recurrent UTI, a urine culture is specifically warranted to guide appropriate therapy. Urine dipstick and/or urine microscopy alone are not sufficient. The signs and symptoms of cystitis in men are similar to those in women, but this diagnosis should be approached cautiously, as cys­ titis in men is less common, except in the setting of urinary cathe­ terization/instrumentation or obstructive uropathy due to prostatic hypertrophy. Urine cultures should be obtained for all cases of male UTI, as the documentation of bacteriuria can differentiate the less common syndromes of acute and chronic bacterial prostatitis from the more common entity of chronic pelvic pain syndrome, which is not associated with bacteriuria and not improved by antibiotics. PROSTATITIS Prostatitis includes both infectious and noninfectious abnormali­ ties of the prostate gland. Infections can be acute or chronic, are almost always bacterial in nature, and are far less common than the noninfectious entity chronic pelvic pain syndrome (formerly known as chronic prostatitis). ABP presents as dysuria, frequency, and pain in the prostatic pelvic or perineal area. It is typically a severe illness that can be accompanied by fever, rigors, and/or bladder outlet obstruction. CBP presents more insidiously as recurrent episodes of cystitis, sometimes with associated pelvic and perineal pain. Systemic signs of infection are typically absent, and many patients are evaluated for CBP in the ambulatory setting. Males with recur­ rent cystitis should be evaluated for a prostatic focus as well as for urinary retention. CHAPTER 140 Men with febrile UTI can have an elevated serum level of prostate-specific antigen and an enlarged prostate and enlarged seminal vesicles on ultrasound—findings suggestive of prostate involvement. In a study of 85 men with febrile UTI, symptoms of urinary retention, early recurrence of UTI, hematuria at follow-up, and voiding difficulties were predictive of surgically correctable disorders. Men with none of these symptoms had normal upper and lower urinary tracts on urologic workup. In general, men with febrile UTI, including at the first episode, should have imaging performed (CT or ultrasound) if it has not been performed previ­ ously. The purpose of such investigation is specifically to evaluate for urinary tract abnormalities or obstruction. If the diagnosis of UTI is unclear or if UTI is recurrent, referral for urologic consulta­ tion is appropriate. Urinary Tract Infections: Cystitis, Prostatitis, and Pyelonephritis PYELONEPHRITIS Fever is one of the main features distinguishing cystitis from pyelo­ nephritis, and while “febrile UTI” is sometimes used as a diagnos­ tic category in UTI research, practically speaking, many of these patients have pyelonephritis or other infection that has extended beyond the bladder (hence the inclusion of fever as a criterion for complicated UTI). Mild pyelonephritis can present with low-grade fever with or without lower back or costovertebral angle pain, whereas severe pyelonephritis often manifests with high fever, rigors, nausea, vom­ iting, and flank pain. Symptoms are generally acute in onset and may or may not be preceded by symptomatic cystitis. Obstructive uropathy related to acute papillary necrosis (i.e., when the sloughed papillae obstruct the ureter) can complicate pyelonephritis. Acute papillary necrosis can also occur in sickle cell disease and analge­ sic nephropathy. In the rare cases of bilateral papillary necrosis, a rapid rise in the serum creatinine level might be the first indication of the condition. Emphysematous pyelonephritis is a particularly severe form of pyelonephritis associated with production of gas in the renal and perinephric tissues. It occurs almost exclusively in patients with poorly controlled diabetes (Fig. 140-2). Xanthogranu­ lomatous pyelonephritis occurs when chronic urinary obstruction (often by staghorn calculi), together with chronic infection, leads to suppurative destruction of renal tissue (Fig. 140-3). On pathologic

FIGURE 140-2  Emphysematous pyelonephritis. Infection of the right kidney of a diabetic man by Escherichia coli, a gas-forming, facultative anaerobic uropathogen, has led to destruction of the renal parenchyma (arrow) and tracking of gas through the retroperitoneal space (arrowhead). PART 5 Infectious Diseases examination, the residual renal tissue frequently has a yellow color­ ation, with infiltration by lipid-laden macrophages (hence the term xanthogranulomatous). Pyelonephritis can also be complicated by intraparenchymal abscess formation; this development should be suspected when a patient has continued fever and/or bacteremia despite antibacterial therapy. CATHETER-ASSOCIATED URINARY TRACT INFECTION (CAUTI) Clinically, CAUTI can be defined by the presence of symptoms along with presence of bacteriuria and pyuria. CAUTI may cause localizing urinary symptoms or otherwise unexplained systemic manifestations, such as fever. The accepted threshold for bacteriuria to meet the definition of CAUTI is ≥103 colony-forming units per milliliter of urine. The central diagnostic difficulty of CAUTI is that, because cathe­ ters provide a conduit for bacteria to enter the bladder, bacteriuria is inevitable with urinary catheterization. The typical signs and symp­ toms of UTI, including pain, urgency, dysuria, fever, and peripheral leukocytosis, also have less predictive value for diagnosis of UTI in catheterized patients. Since bacteriuria and pyuria are expected in this population, the diagnostic evaluation of a febrile patient with a urinary catheter should not end with an abnormal urinalysis. To make a diagnosis of CAUTI, first, signs and symptoms of UTI (localizing and/or systemic) should be present along with pyuria and bacteriuria. Additionally, other infectious and noninfectious causes of the patient’s symptoms should be systematically ruled out before settling on the diagnosis of CAUTI. ■ ■DIAGNOSTIC TOOLS History  The diagnosis of any of the UTI syndromes or ASB begins with a detailed history (Fig. 140-4). Patient-reported symptoms have high positive predictive value in uncomplicated cystitis, at least among adult women; data are lacking for other populations. Self-diagnoses among women with recurrent UTI are particularly accurate and account for the success of patient-initiated treatment for recurrent cystitis. A meta-analysis of the diagnostic utility of history and physical findings for UTI concluded that, in women presenting with at least one localizing urinary symptom (dysuria, frequency, hematuria, or back

A B FIGURE 140-3  Xanthogranulomatous pyelonephritis. A. This photograph shows extensive destruction of renal parenchyma due to long-standing suppurative inflammation. The precipitating factor was obstruction by a staghorn calculus, which has been removed, leaving a depression (arrow). The mass effect of xanthogranulomatous pyelonephritis can mimic renal malignancy. B. A large staghorn calculus (arrow) is seen obstructing the renal pelvis and calyceal system. The lower pole of the kidney shows areas of hemorrhage and necrosis with collapse of cortical areas. (Images courtesy of Dharam M. Ramnani, MD, Virginia Urology Pathology Laboratory, Richmond, VA.) pain) and no complicating factors, the probability of acute cystitis or pyelonephritis is 50%. If vaginal discharge and complicating factors are absent and risk factors for UTI are present, then the probability of UTI is close to 90%. Further laboratory evaluation with dipstick testing or urine culture prior to antimicrobial treatment is not necessary in such patients unless there is concern for resistant pathogens (e.g., in patients presenting with recurrent UTI, prior treatment failure, known coloni­ zation with extensively resistant pathogens, or high local prevalence of resistance to the empirical antibiotic being given). In applying the patient’s history as a diagnostic tool, one significant con­ cern is that STIs—caused by C. trachomatis in particular—might be inap­ propriately treated as UTI. Importantly, women with multiple sexual partners and inconsistent use of condoms are at increased risk for both UTI and STIs, and symptoms alone do not always distinguish between these conditions. Dysuria might instead be due to cervicitis (C. trachomatis, N. gonorrhoeae), vaginitis (Candida albicans, Trichomonas vaginalis), herpetic urethritis, interstitial cystitis, or noninfectious vaginal or vul­ var irritation. Vaginal discharge in particular should prompt consider­ ation of STI. In this particular patient population, consideration should

Clinical Presentation Patient Characteristics Diagnostic and Management Considerations Otherwise healthy woman who is not pregnant, low risk for multidrug resistance Woman with a history of or risk factors for STI Acute onset of urinary symptoms Dysuria Frequency Urgency Male with perineal, pelvic, or prostatic pain Patient with indwelling urinary catheter All other patients Otherwise healthy woman who is not pregnant Acute onset of back pain, nausea/vomiting, or fever with or without cystitis symptoms All other patients Systemic symptoms Elderly patients; patients with spinal cord injury, immunocompromise, no alternate diagnosis Fever Altered mental status Leukocytosis Consider complicated UTI Consider other etiologies Urine culture Blood cultures Positive urine culture in patient who is pregnant, or patient undergoing invasive urologic procedure No urinary symptoms Positive urine culture in all other patients Positive urine culture in patient with indwelling catheter Otherwise healthy woman who is not pregnant Recurrent acute urinary symptoms Male patient FIGURE 140-4  Diagnostic approach to urinary tract infection (UTI). NTF, nitrofurantoin; TMP-SMX, trimethoprim-sulfamethoxazole; UTI, urinary tract infection. be given to STI screening either at the time of empirical treatment for UTI or if symptoms do not resolve with treatment. Urine Dipstick Test, Urinalysis, and Urine Culture  Useful tools in the diagnosis of UTI include the urine dipstick test (for nitrite and leukocyte esterase) and microscopic urinalysis (for counts of red and white blood cells), both of which provide point-of-care informa­ tion that can primarily be used for ruling out UTI in patients who do not have a clear clinical diagnosis. Urine culture, another diagnostic tool, can retrospectively provide proof that bacteria were present in the urinary tract and can provide information on susceptibilities of the

Consider uncomplicated cystitis No urine culture needed Consider telephone management Consider uncomplicated cystitis or STI Urinalysis, culture STI evaluation, pelvic exam Consider acute prostatitis Urinalysis and culture Consider urology evaluation Consider CAUTI Exchange or remove catheter Urinalysis and culture Blood cultures if fever Consider complicated UTI Urinalysis and culture Address any modifiable anatomic or functional abnormalities Consider pyelonephritis Urinalysis and culture Consider outpatient management Consider pyelonephritis or acute prostatitis (male) CHAPTER 140 Urine culture Blood cultures Urinary Tract Infections: Cystitis, Prostatitis, and Pyelonephritis Consider ASB Screening and treatment warranted Consider ASB No additional workup or treatment needed Consider CA-ASB No additional workup or treatment needed Remove unnecessary catheters Consider recurrent cystitis Urine culture to establish diagnosis Consider prophylaxis or patient-initiated management (see text) Consider chronic bacterial prostatitis Consider urology consult isolated organism(s). A key point is that patients with urinary catheter­ ization will almost always have both pyuria and bacteriuria. Therefore, presence of pyuria and bacteriuria alone in the absence of symptoms do not indicate a diagnosis of UTI. Understanding the parameters of the dipstick test is important in interpreting its results. Only members of the Enterobacterales family convert nitrate to nitrite, and enough nitrite must accumulate in the urine to reach the threshold of detection. This poses three problems limiting nitrites’ sensitivity and specificity. First, nitrites could be posi­ tive in a patient with Enterobacterales ASB. Second, nitrites would be negative in a patient with UTI due to non-Enterobacterales organisms.

Third, nitrites might also be falsely negative in a woman with acute cys­ titis who is voiding frequently because of significant oral fluid intake.

The leukocyte esterase test detects this enzyme in neutrophils present in the host’s urine, whether the cells are intact or lysed. Practi­ cally speaking, it is a less sensitive and specific surrogate for pyuria as measured by urine microscopy. Many reviews have attempted to describe the diagnostic accuracy of dipstick testing for UTI. The most important point for practicing clinicians is that a dipstick test negative for both nitrite and leukocyte esterase should prompt consideration of diagnoses other than UTI to explain the patient’s symptoms, keeping in mind that the dipstick results can be falsely negative in a small per­ centage of cases. Importantly, a negative urine dipstick test is not suf­ ficiently sensitive to rule out bacteriuria in pregnant women, for whom ASB screening should be performed via urine culture. Urine microscopy reveals pyuria in nearly all cases of cystitis and hematuria in approximately 30% of cases. Pyuria has historically been defined as >10 leukocytes per high-powered microscopy field (HPF). Modern data indicate that, at least for older women, median urine white blood cells (WBCs) per HPF are far higher in UTI. As such, the cutoff of >10 WBCs/HPF has poor specificity (36%) for UTI, and a cutoff closer to 250 WBCs/HPF might better correlate with presence of urinary symptoms. However, at present, >10 WBCs/HPF remains the generally accepted standard for what defines presence of pyuria. Detection of bacteria in a urine culture from a patient with symptoms of cystitis can confirm the diagnosis of UTI. However, culture results often do not become available until at least 24 h after a patient presents for care, with identification of individual organisms and their suscepti­ bilities usually requiring an additional 24–48 h. Furthermore, identify­ ing the presence of bacteria in the urine does not necessarily imply the presence of symptoms; therefore, a positive urine culture is consistent with both UTI and ASB. This issue is a perennial source of inaccuracy for retrospective studies of UTI, which are often flawed by defining UTI as the combination of pyuria and bacteriuria without requiring clinical documentation of symptoms. Studies of women with symptoms of cysti­ tis have found that a colony count threshold of >102 bacteria/mL is more sensitive (95%) than a threshold of 105/mL for the diagnosis of acute cystitis in women. Fewer data are available in men about the threshold of bacteriuria to establish cystitis. When interpreting urine culture results, the clinician should consider that contamination with the normal microbial flora of the distal urethra, vagina, or skin is common and that these contaminants can grow to high numbers when the collected urine is allowed to stand at room temperature. In most instances, a culture that yields mixed bacterial species is contaminated except in settings of long-term catheterization, chronic urinary retention, or the presence of a fistula between the urinary tract and the gastrointestinal or genital tract. PART 5 Infectious Diseases TREATMENT Urinary Tract Infections The approach to diagnosis and treatment is influenced by which of the UTI clinical syndromes is suspected and presence of risk factors for resistant pathogens (Fig. 140-4). GENERAL CONSIDERATIONS FOR ANTIMICROBIAL THERAPY Studies indicate that treatment of UTI is usually the first or second most common indication for antimicrobials in ambulatory, inpa­ tient, and long-term-care settings. Responsible use of antimicrobi­ als for this common infection has broad implications for preserving antibiotic effectiveness into the future. Importantly, antimicrobial therapy is warranted for any true (symptomatic) UTI. Studies of nonantibiotic treatment for UTI (e.g., with nonsteroidal anti-inflammatory drugs alone) suggest that this approach delays clinical response and predisposes patients to more invasive infections. Delayed therapy, in which a patient receives a prescription for antibiotics but fills it only if symptoms fail to resolve in a day or two, has the potential advantage of avoiding antibiotic use in those patients who do not have cysti­ tis to begin with; however, this approach has the downsides of

prolonging discomfort and a small increased risk of progression to pyelonephritis in patients who do have true UTI. Potentially higheryield antimicrobial stewardship interventions in UTI management include encouraging clinicians to avoid unnecessary treatment of ASB, to avoid ordering urine cultures in patients who lack urinary or systemic symptoms suggestive of UTI, and to avoid prescribing overly long antibiotic durations for UTI. In 1999, trimethoprim-sulfamethoxazole (TMP-SMX) was rec­ ommended as the treatment of choice for uncomplicated cystitis in the first published UTI guidelines of the Infectious Diseases Society of America. However, local uropathogen resistance patterns and national differences in the availability of certain antimicrobial agents are such that providing a global or even national recom­ mendation for a single preferred empirical antibiotic for UTI is challenging. Instead, clinicians must use their knowledge of local resistance patterns and individual patient factors (e.g., allergies and potential for drug–drug interactions) to select from groups of antimicrobials based on their efficacy, toxicity risks, and potential for collateral damage (defined below). The choice of antimicrobial agent, the dose, and the duration of therapy also depend on the clinical syndrome (that is, which type of UTI specifically is being treated) and the rapidity of clinical improvement. Collateral damage refers to the adverse ecologic effects of anti­ microbial therapy, including killing of the patient’s normal flora (predisposing to Clostridioides difficile infection) and selection of drug-resistant organisms. The implication of collateral damage for UTI management is that a drug that is highly efficacious for treat­ ment of UTI is not necessarily the optimal first-line agent if it also has pronounced secondary effects on the normal flora or is likely to significantly adversely affect resistance patterns. Drugs used for UTI that have a minimal effect on fecal flora include pivmecil­ linam, fosfomycin, and nitrofurantoin. In contrast, TMP-SMX, quinolones, and β-lactams (particularly the late-generation cepha­ losporins) affect the fecal flora more significantly and are notably the agents for which rising resistance has been documented. Choosing judiciously whether to initiate antibiotic therapy and then selecting the agent with the least potential for collateral dam­ age to be given for the shortest effective duration are important factors in global efforts to stem the rise of antimicrobial-resistant organisms. Common risk factors for resistance to specific anti­ microbials in UTI include prior urine or blood cultures with an organism resistant to that antimicrobial, recent exposure to that antimicrobial, or travel to an area where resistance to that antimi­ crobial is especially prevalent. For each of these factors, the magni­ tude of risk is greater with more recent exposures in the preceding year; the greatest increase in risk occurs with fluoroquinolones. More data is available documenting this risk of resistance with prior exposure to the antibiotic for fluoroquinolones than for other antibiotics. Other general risk factors for antimicrobial resistance in UTI include urinary catheterization, prior exposure to antibiotics of any class, and residence in a healthcare facility. ASYMPTOMATIC BACTERIURIA Critically, treatment of ASB does not decrease frequency of subse­ quent true UTIs. Clinical trial data from renal transplant recipients suggest that ASB treatment might even increase future risk of infec­ tion. Importantly, though, treatment of ASB in pregnant patients and patients undergoing high-risk urologic procedures is recom­ mended to reduce risk of complications (see discussion above) and should be guided by urine culture results. In all other populations, screening for and treating ASB are discouraged. Specifically, a urine culture does not need to be obtained at the time of placement or removal of a urinary catheter if the patient is asymptomatic. The majority of cases of catheter-associated bacteriuria are asymptom­ atic and do not warrant antimicrobial therapy. CYSTITIS Many episodes of uncomplicated cystitis in women (i.e., cystitis without systemic symptoms, symptoms of pyelonephritis, or urinary

TABLE 140-1  Treatment Strategies for Acute Uncomplicated Cystitis DRUG DOSE DURATION COMMON SIDE EFFECTS Nitrofurantoin 100 mg bid Women or men: 5–7 days Nausea, headache TMP-SMX 1 DS tablet bid Women: 3 days Men: 7 days Rash, urticaria, nausea, vomiting, hematologic abnormalities Fosfomycin 3-g sachet Women: 1 day Men: qod × 3 doses Pivmecillinam 400 mg bid Women: 3–7 days Nausea, vomiting, diarrhea Fluoroquinolones Dose varies by agent Women: 3 days Men: 7 days β-Lactams Dose varies by agent Women or men: 5–7 days Diarrhea, nausea, vomiting, rash, urticaria Abbreviations: DS, double-strength; TMP-SMX, trimethoprim-sulfamethoxazole; bid, twice a day; qod every other day. catheterization) can be managed remotely (Fig. 140-4). Studies of telephone-management algorithms for UTI indicate low risk of serious complications, with the important caveat that such algo­ rithms have generally involved otherwise healthy women who are at low risk of complications of UTI; thus, patients with other UTI syndromes or more complicated prior histories require further diag­ nostic evaluation and might not be as adequately managed remotely. As described above, cystitis in men is unusual except in the setting of urinary catheterization or obstructive uropathy. Before settling on this diagnosis in men, however, the clinician should be confident that ABP, CBP, and pyelonephritis with or without uri­ nary obstruction have been excluded. True uncomplicated cystitis in afebrile men can be treated with only 7 days of antimicrobial therapy; the durations shorter than 7 days used for uncomplicated cystitis in women have not been adequately studied in men. Several effective therapeutic regimens are available for acute uncomplicated cystitis in women; data are very limited on therapy for cystitis in men (Table 140-1). Preferred agents for treatment of acute uncomplicated cystitis include TMP-SMX, nitrofuran­ toin, fosfomycin, and pivmecillinam. Fosfomycin is somewhat less studied than TMP-SMX and nitrofurantoin, but it is also effective and is preferred in cystitis because of its selective concentration in urine, low risk for collateral damage, and potential to retain efficacy against MDR E. coli. Pivmecillinam was approved for treatment of cystitis in the United States in 2024. It also often retains activity against MDR E. coli, including some expressing extended-spectrum β-lactamases. Alternative agents for treatment of cystitis include fluoroquino­ lones and β-lactams. β-lactams require longer durations of therapy (5–7 days) even in uncomplicated cystitis and are associated with higher clinical failure rates (compared with either fluoroquinolones or TMP-SMX) in some randomized controlled trials. Despite these limitations, β-lactams are favored over fluoroquinolones as an alternative agent for treatment of uncomplicated cystitis because of the bacterial resistance and side effects associated with fluoroqui­ nolones. Fluoroquinolones are highly effective for treating UTI but cause high collateral damage as well as numerous rare but serious drug toxicities (e.g., collagen vascular pathology such as tendon rupture, retinal detachment, and aortic aneurysm), leading the U.S. Food and Drug Administration (FDA) to advise against using fluoroquinolones for uncomplicated cystitis in women unless no alternatives are available. Importantly, in men, if there is concern for prostatic involvement, fluoroquinolones or TMP-SMX are pre­ ferred over β-lactams because fluoroquinolones have superior pen­ etration into the prostate. The pros and cons of each specific agent are discussed in more detail below. TMP-SMX remains an excellent option for treatment of cys­ titis in areas where local uropathogen antibiogram data do not clearly favor other preferred agents or when the patient has recent microbiology demonstrating a TMP-SMX–susceptible uropatho­ gen. Otherwise, the optimal setting for empirical use of TMP-SMX is uncomplicated cystitis in a patient who has an established rela­ tionship with the practitioner and has no barriers to seeking further care if their symptoms do not respond promptly.

Diarrhea, nausea, headache Nausea, vomiting, diarrhea, headache, drowsiness, insomnia Resistance to nitrofurantoin remains low despite >70 years of use, probably because several mutational steps are required for bacterial resistance to this drug. Nitrofurantoin remains highly active against E. coli. However, importantly, most Proteus, Provi­ dencia, Morganella, Pseudomonas, and Serratia and some Klebsiella and Enterobacter strains are intrinsically resistant to this drug. A 5-day course of nitrofurantoin is as effective as a 3-day course of TMP-SMX for treatment of acute uncomplicated cystitis in women. Nitrofurantoin should not be used in patients with a creatinine clearance <30 mL/min. It does not reach significant levels in tissue and so cannot be used to treat pyelonephritis or prostatitis. Addi­ tionally, we advise against long-term (i.e., multiple years) prescrip­ tion of nitrofurantoin as prophylaxis for recurrent UTIs because of the risk of sometimes irreversible pulmonary toxicities. CHAPTER 140 Guidelines also recommend fosfomycin, which interferes with bacterial cell-wall formation, as an option for uncomplicated cysti­ tis, given its easy dosing and low collateral damage. While fosfomy­ cin is active against E. coli, its activity against other Enterobacterales species is less reliable. Moreover, fosfomycin susceptibility testing is difficult to perform, and the results of such testing are not typi­ cally included in standard automated microbiologic susceptibility reports. Oral fosfomycin is given as a single 3-g dose sachet (pow­ der) that is dissolved in a glass of water and swallowed. Similar to nitrofurantoin, renal tissue levels of fosfomycin are low, and a dif­ ferent antibiotic should be used if treating pyelonephritis. While not extensively studied for this indication, fosfomycin has been used in the treatment of prostatitis to manage pathogens resistant to other oral agents. Urinary Tract Infections: Cystitis, Prostatitis, and Pyelonephritis Despite their often high rates of urinary secretion, β-lactams as a class have not performed as well as TMP-SMX or fluoroquinolones for treatment of acute cystitis. Rates of pathogen eradication are lower, and relapse rates are higher with β-lactam drugs compared with other agents. The generally accepted explanation for this observation is that β-lactams fail to eradicate uropathogens from the vaginal reservoir, which is specific to female patients with UTI. Supporting this hypothesis are limited indirect data indicating that treatment success with cephalexin is as good as or better than success with other classes in male UTI. Amoxicillin resistance has become widespread in E. coli, and many strains resistant to TMPSMX are also resistant to some or all of the other oral β-lactams. Thus, as described for TMP-SMX above, β-lactams such as cef­ podoxime or cefixime are optimally used for patients known to have susceptible uropathogens or in areas where local antibiograms demonstrate that resistance remains uncommon. The fluoroquinolones ciprofloxacin and levofloxacin are highly effective as short-course therapy for cystitis due to susceptible flora. By contrast, moxifloxacin in particular might not reach adequate urinary levels. The two main concerns about using fluoroquino­ lones for acute cystitis are the propagation of fluoroquinolone resistance, not only among uropathogens but also among other organisms causing more serious and difficult-to-treat infections at other sites, and fluoroquinolones’ rare but potentially serious adverse effects (increased risk of Achilles’ tendon rupture, neuropa­ thy, and aortic dissection). In light of these potential detrimental

effects, the FDA issued an advisory against using fluoroquinolones to treat acute cystitis in patients who have other effective therapeu­ tic options.

Urinary analgesics are appropriate in certain situations to help the patient with resolution of bladder discomfort in cystitis. The urinary tract analgesic phenazopyridine is widely used but can cause significant nausea. Combination analgesics containing uri­ nary antiseptics (such as methenamine and methylene blue), a urine-acidifying agent (sodium phosphate), and an antispasmodic agent (hyoscyamine) also are available. There are no well-established criteria for when to use these agents, although as mentioned above, multiple clinical trials demonstrate that symptomatic treat­ ments alone are not an adequate substitute for antimicrobials when patients have true UTI. PROSTATITIS The prostate is involved in the majority of cases of febrile UTI in men and can also be a source of recurrent cystitis. ABP should specifically be suspected in men presenting with UTI with systemic symptoms without another clear anatomic location of UTI, such as pyelonephritis. CBP should be suspected when patients have recurrent UTI, especially if not in the context of urinary catheter­ ization). Both ABP and CBP should be suspected when patients have prostatic symptoms. If ABP is suspected, antimicrobial therapy should be promptly initiated after urine and blood cultures are obtained. For ABP, at least 2 weeks and potentially up to 4 weeks of a fluoroquinolone or TMP-SMX is recommended for susceptible uropathogens. Identification of a prostatic abscess should prompt consultation to urologic specialists. For CBP, a 4- to 6-week course of antibiotics is typically given, and recurrences (not uncommon) may warrant up to a 12-week course of therapy. These durations are based on older observational studies and historic norms of practice and need be to clarified with modern randomized controlled trials. PART 5 Infectious Diseases PYELONEPHRITIS Patients with pyelonephritis have tissue-invasive disease and are often systemically ill. Therefore, empirical therapy should be reli­ able and rapidly reach therapeutic levels in both renal parenchyma and blood. In years past, increasing TMP-SMX resistance made fluoroquinolones the empirical treatment of choice for pyelone­ phritis, and quinolones remain among the best-studied agents for acute pyelonephritis. However, quinolone resistance is now also increasingly prevalent. For hospitalized patients, we recommend empirical therapy with an IV β-lactam for pyelonephritis, with attention to choosing an agent to which the patient has not recently had a resistant uro­ pathogen in urine or blood and appears highly reliable based on the local antibiogram. For patients with pyelonephritis who are not and do not need to be hospitalized, we recommend empirical therapy with TMP-SMX, ciprofloxacin, or levofloxacin based on local anti­ biogram data, along with urine culture collection and short-term follow-up. Optimally, an initial dose of IV or IM β-lactam or an aminoglycoside to ensure effective empirical therapy can be pre­ scribed, where this is practical and feasible. Ambulatory patients with pyelonephritis should be hospitalized if they are unable to tolerate oral medications (e.g., due to nausea and vomiting) or are unlikely to adhere to short-term follow-up. For susceptible isolates, clinical trial data indicate that 5–7 days of an oral fluroquinolone or 7–14 days of oral TMP-SMX (depend­ ing on rapidity of clinical improvement) are highly effective options for pyelonephritis. Oral β-lactam agents are less effective and should be used with caution and close follow-up. Nitrofurantoin, pivmecillinam, and fosfomycin should not be used for pyelonephri­ tis. Options for parenteral therapy for uncomplicated pyelonephri­ tis include late-generation cephalosporins (such as ceftriaxone or cefepime), piperacillin-tazobactam, a carbapenem, or fluoroqui­ nolones (although the high bioavailability of this class makes IV administration superfluous for patients with who can tolerate and absorb oral medications). Alternative parenteral antibiotics include

novel β-lactam/β-lactamase inhibitors (e.g., meropenem-vaborbactam) and aminoglycosides, which are considered alternative due to cost and toxicity profiles, respectively. In general, the treatment of pyelonephritis should be guided by urine culture results and clinical resolution of symptoms. Pyelone­ phritis due to drug-resistant uropathogens requiring nonpreferred antibiotics should be treated in consultation with infectious disease specialists. Once the patient has responded clinically, oral therapy can be substituted for parenteral therapy. Treatment can be con­ cluded at 7 days for patients who have clinically responded (i.e., res­ olution of fever and return to hemodynamic stability with adequate source control) within that timeframe and who can be treated with a fluoroquinolone or TMP-SMX. Oral β-lactams may necessitate a longer course of treatment for pyelonephritis. Patients with pyelonephritis who have not had a clinical response within 72 h should undergo further evaluation for an uncontrolled source of infection. Pyelonephritis can be complicated by acute papillary necrosis causing urinary obstruction, which requires intervention to relieve the obstruction, overcome the infection, and preserve renal function. Perinephric and renal cortical abscesses may require drainage percutaneous drainage. Percutaneous drain­ age can also be used as the initial therapy in emphysematous pyelo­ nephritis and can be followed by elective nephrectomy as needed depending upon the patient’s clinical status in consultation with urologic specialists. Xanthogranulomatous pyelonephritis (XGP) is treated with close collaboration with urologic colleagues and often requires nephrectomy; antibiotic treatment duration should be individualized in cases of XGP. CATHETER-ASSOCIATED UTI The bacteriology of CAUTI is diverse, and urine culture results are essential to guide treatment. The catheter should be exchanged (or, if possible, removed entirely) during treatment for CAUTI, the purpose of which is to remove biofilm-associated organisms that could serve as a nidus for reinfection. When possible, this should be done prior to sending urine cultures to ensure that the culture results reflect the microbiology of the bladder rather than the cath­ eter walls. Most clinicians treat CAUTI for 7–14 days based on clinical response; the need for longer therapy than is used in uncomplicated cystitis is supported by a randomized controlled trial that identified more relapses after 3 versus 14 days of antimicrobials. Further stud­ ies on the optimal duration of therapy in CAUTI are needed. The best strategy for prevention of CAUTI is to avoid insertion of unnecessary catheters and to remove catheters once they are no longer necessary. Quality-improvement collaboratives that have addressed technical aspects of CAUTI prevention (such as avoid­ ance of inappropriate catheterization) and team-communication strategies have shown the benefit of such approaches in decreasing rates of CAUTI in both acute- and long-term-care settings. Anti­ microbial catheters impregnated with silver or nitrofurazone have not been shown to provide significant clinical benefit with regard to reducing CAUTI rates and should not be used routinely. Evidence is insufficient to systematically recommend suprapubic catheters and condom urinary catheters as alternatives to indwelling urinary catheters as a way to prevent bacteriuria. Intermittent catheteriza­ tion may be preferable to long-term indwelling urethral catheteriza­ tion in certain populations (e.g., patients with spinal cord injury) to prevent both infectious and anatomic complications. UTI IN PREGNANT PATIENTS Aminopenicillins (ampicillin and amoxicillin) and cephalosporins have been used extensively in pregnancy and are the drugs of choice for treatment of ASB or UTI in pregnant patients. Importantly, though, widespread resistance to aminopenicillins precludes their use for pregnant patients with UTI if urine culture results are not already available. One retrospective case-control study suggested an association between nitrofurantoin use in the first trimester and birth defects, but this association has not been confirmed, and the

American College of Obstetrics and Gynecology (ACOG) suggests that nitrofurantoin can be used during all trimesters if β-lactam agents are not an option. Sulfonamides (including TMP-SMX) should be avoided both in the first trimester (because of possible teratogenic effects) and near term (because of a possible role in the development of neonatal kernicterus), although ACOG suggests TMP-SMX can be used as a potential treatment option at both times if no alternatives are available. Fluoroquinolones are avoided dur­ ing pregnancy because of possible adverse effects on fetal cartilage development. Pregnant patients with ASB or cystitis are generally treated for 5–7 days because of the absence of evidence to support shorter durations of therapy in this patient population. For preg­ nant patients with pyelonephritis, guidelines recommend parenteral β-lactam therapy with or without aminoglycosides based on limited observational data and historic norms of practice. CANDIDURIA The appearance of Candida in the urine is an increasingly common complication of indwelling catheterization, particularly for patients in the intensive-care unit, those taking broad-spectrum antimicro­ bial drugs, and those with underlying diabetes mellitus. In many studies, >50% of urinary Candida isolates have been found to be non–C. albicans species. The clinical presentation of candiduria varies widely—from asymptomatic colonization to pyelonephritis and sepsis—thereby making management challenging in some instances, particularly for patients with multiple other ongoing medical conditions. Removal of the urinary catheter results in reso­ lution of candiduria in more than one-third of asymptomatic cases. As with ASB, treatment of asymptomatic patients with candiduria has not been found to reduce recurrence of candiduria or subse­ quent infection. Therefore, treatment of asymptomatic patients is not routinely recommended for most patients. By contrast, candiduria should be treated in patients who have symptomatic cystitis or pyelonephritis or those who are asymptom­ atic but are at high risk for disseminated disease (e.g., patients with neutropenia, patients who will undergo urologic manipulation, and low-birth-weight infants). Fluconazole (200–400 mg/d for 7–14 days) reaches high levels in the urine and is the first-line regi­ men for Candida infections of the urinary tract. Although instances of successful eradication of candiduria by some of the newer azoles and echinocandins have been reported, these agents are relatively unstudied and are not recommended as first-line agents but can be considered in certain cases. For Candida isolates with high levels of resistance to fluconazole, oral flucytosine and/or parenteral ampho­ tericin B are additional options. Bladder irrigation with amphotericin B is generally not recommended. ■ ■PREVENTION OF RECURRENT UTI IN WOMEN Recurrence of uncomplicated cystitis in women is common, and a preventive strategy is indicated if recurrent UTIs are interfering with a patient’s lifestyle. The threshold of three or more symptomatic episodes per year to intervene or develop a specific preventive strategy is not absolute; clinicians are recommended to engage in shared decisionmaking with each individual patient to make determinations about a UTI preventive strategy. Initial evaluation of a patient with recurrent cystitis should include confirmation of the diagnosis of recurrent UTI, imaging to identify renal calculi or another potential nidus of infection, and assessment for urinary retention. Referrals to urology, urogynecology, and/or pelvic physical therapy might be helpful when the diagnosis is unclear or an anatomic abnormality is suspected. Nonantimicrobial prophylactic approaches to recurrent UTI are increasingly being studied. Observational data indicate that increasing fluid intake might reduce recurrent UTIs, and such an approach is gen­ erally well tolerated by healthy adults. Two recent clinical trials found that methenamine hippurate (converted to the antiseptic formaldehyde in the bladder) is well tolerated and might reduce recurrent UTIs as much as antibiotic prophylaxis. A recent Cochrane review of random­ ized clinical trials of cranberry products also found a significant benefit

for adult women and children with recurrent UTI. Less is known about which products and what precise dosing are optimal. Lactobacillus probiotics are under investigation for UTI prevention, with one recent factorial trial suggesting that intravaginal but not oral lactobacilli products reduce recurrent UTIs. We advise caution, however, in rec­ ommending probiotics to immunocompromised patients, in whom probiotic-associated invasive infections have been described. Studies of mannose products for UTI prevention have produced mixed results.

Three antibiotic prophylactic strategies are available: continu­ ous (using daily or thrice-weekly dosing), postcoital (one dose after intercourse), and patient-initiated therapy. For the continuous and postcoital strategies, low doses of TMP-SMX or nitrofurantoin are typi­ cally utilized. Such approaches are usually highly effective during the period of active antibiotic intake. A prophylactic regimen is typically prescribed for 6 months and then discontinued. If bothersome infec­ tions recur, the prophylactic program can be reinstituted for the same duration or for a longer period. Selection of resistant strains in the fecal flora has been documented in studies of patients taking prophylactic antibiotics. An approach of alternating antibiotic prophylaxis with periods of nonantimicrobial prevention could hypothetically allow for intermittent recovery of the gut microbiome. The choice between methenamine and cranberry products should be based on shared decision-making and individual patient response. Some studies suggest that although the rate of UTI will be decreased when using nonantimicrobial prophylaxis compared with no prophylaxis, the rate of UTI might be slightly higher than dur­ ing the antibiotic-suppression period. Patient-initiated therapy involves supplying the patient with materi­ als for urine culture and with a course of antibiotics for self-medication at the first symptoms of infection. The urine culture is refrigerated and delivered to the physician’s office for confirmation of the diagnosis. When an established and reliable patient–provider relationship exists, the urine culture can be omitted as long as the symptomatic episodes respond completely to short-course therapy and are not followed by relapse. CHAPTER 140 Urinary Tract Infections: Cystitis, Prostatitis, and Pyelonephritis ■ ■PROGNOSIS Cystitis is a risk factor for recurrent cystitis and pyelonephritis. ASB is common among elderly and catheterized patients but does not in itself increase risk of infection or death. The relationships among recurrent UTI, chronic pyelonephritis, and renal insufficiency have been widely studied. In the absence of anatomic abnormalities such as reflux, recur­ rent infections in children and adults do not lead to chronic pyelone­ phritis or renal failure. Additionally, infection does not play a primary role in chronic interstitial nephritis; the primary etiologic factors in this condition are analgesic overuse, obstruction, reflux, and toxin exposure. By contrast, in the presence of underlying renal abnormali­ ties (particularly obstructing stones), infection as a secondary factor can accelerate renal parenchymal damage. ■ ■FURTHER READING Bilsen M et al: Current pyuria cutoffs promote inappropriate urinary tract infection diagnosis in older women. Clin Infect Dis 76:2070, 2023. Brehm TJ et al: Acute and chronic infectious prostatitis in older adults. Infect Dis Clin North Am 37:175, 2023. Christmas MM et al: Menopause hormone therapy and urinary symptoms: A systematic review Menopause 30:672, 2023. Drekonja DM et al: Effect of 7 vs 14 days of antibiotic therapy on reso­ lution of symptoms among afebrile men with urinary tract infection: A randomized clinical trial. JAMA 326:324, 2021. Gupta V et al: Effectiveness of prophylactic oral and/or vaginal pro­ biotic supplementation in the prevention of recurrent urinary tract infections: A randomized, double-blind, placebo-controlled trial. Clin Infect Dis 78:1154, 2024. Hooton TM et al: Asymptomatic bacteriuria and pyuria in premeno­ pausal women. Clin Infect Dis 72:1332, 2021. Lafaurie M et al: Antimicrobial for 7 or 14 days for febrile urinary tract Infection in men: A multicenter noninferiority double-blind,

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141 Sexually Transmitted Infections: Overview and Clinical Approach

placebo-controlled, randomized clinical trial. Clin Infect Dis 76:2154, 2023. Patel PK et al: Strategies to prevent catheter-associated urinary tract

infections in acute-care hospitals: 2022 update. Infect Control Hosp Epidemiol 44:1209, 2023. Roth R et al: The urobiome in men and women: A clinical review. Clin Microbiol Infect 29:1242, 2023. Uitrakul S et al: The incidence and risk factors of urinary tract infec­ tion in patients with type 2 diabetes mellitus using SGLT2 inhibitors: A real-world observational study. Medicines (Basel) 9:59, 2022. Jeanne M. Marrazzo

Sexually Transmitted

Infections: Overview

and Clinical Approach CLASSIFICATION AND EPIDEMIOLOGY Worldwide, most adults acquire at least one sexually transmitted infec­ tion (STI), and many remain at risk for complications. Each day, for example, more than 1 million STIs are acquired worldwide, placing many affected persons at risk for adverse reproductive health out­ comes and neoplasia. Certain STIs, such as syphilis, gonorrhea, HIV infection, hepatitis B, and chancroid, often occur in highly intercon­ nected sexual networks characterized by high rates of partner change or multiple concurrent partners. Such networks, for example, often include persons who engage in transactional sex, some men who have sex with men (MSM), and persons involved in the use of illicit drugs. Other STIs are distributed more evenly throughout populations. For example, chlamydial infections, genital human papillomavirus (HPV) infections, and genital herpes can spread efficiently even in relatively low-risk populations. Finally, modern technologies based on detection of nucleic acid have accelerated elucidation of the role of sexual trans­ mission in the spread of some viruses, including Ebola virus and Zika virus, and have provided new evidence of apparent sexual transmission of several bacteria, including group C Neisseria meningitidis and anaer­ obes associated with bacterial vaginosis (BV). PART 5 Infectious Diseases In general, the product of three factors determines the initial rate of spread of any STI within a population: rate of sexual exposure of sus­ ceptible to infectious people, efficiency of transmission per exposure, and duration of infectivity of those infected. Accordingly, efforts to prevent and control STIs aim to decrease the rate of sexual exposure of susceptible to infected persons (e.g., through education and efforts to change sexual behavior norms and through control efforts aimed at reducing the proportion of the population infected, including post­ exposure prophylaxis [PEP]); to decrease the duration of infectivity (through early diagnosis and curative or suppressive treatment); and to decrease the efficiency of transmission (through promotion of condom use and safer sexual practices, use of effective vaccines, and male medi­ cal circumcision). In all societies, STIs rank among the most common of all infec­ tious diseases, with at least 40 microorganisms now classified as pre­ dominantly sexually transmitted or as frequently sexually transmissible (Table 141-1). In developing countries, with three-quarters of the world’s population and 90% of the world’s STIs, factors such as popu­ lation growth (especially in adolescent and young-adult age groups), rural-to-urban migration, wars, limited or no provision of reproductive health services for women, and poverty create exceptional vulnerability to disease resulting from unprotected sex. During the 1990s in China, Russia, the other states of the former Soviet Union, and South Africa,

TABLE 141-1  Sexually Transmitted and Sexually Transmissible Microorganisms BACTERIA VIRUSES OTHERa Transmitted in Adults Predominantly by Sexual Intercourse Neisseria gonorrhoeae Chlamydia trachomatis Treponema pallidum Haemophilus ducreyi Klebsiella (Calymmatobacterium) granulomatis Ureaplasma urealyticum Mycoplasma genitalium HIV (types 1 and 2) Human T-cell lymphotropic virus type 1 Herpes simplex virus type 2 Human papillomavirus (multiple genital genotypes) Hepatitis B virusb Trichomonas vaginalis Pthirus pubis Molluscum contagiosum virus Sexual Transmission Repeatedly Described but Not Well Defined or Not the Predominant Mode Mycoplasma hominis Gardnerella vaginalis and other vaginal bacteria Group B Streptococcus Mobiluncus spp. Helicobacter cinaedi Helicobacter fennelliae Anaerobes associated with bacterial vaginosis Leptotrichia/Sneathia Group C Neisseria meningitidis Cytomegalovirus Human T-cell lymphotropic virus type 2 Hepatitis C virus (?) Hepatitis D virus Herpes simplex virus type 1 Zika virus Ebola virus (?) Epstein-Barr virus Human herpesvirus type 8 Candida albicans Sarcoptes scabiei Transmitted by Sexual Contact Involving Oral–Fecal Exposure; of Declining Importance in Men Who Have Sex with Men Shigella spp. Campylobacter spp. Hepatitis A virus Giardia lamblia Entamoeba histolytica aIncludes protozoa, ectoparasites, and fungi. bAmong U.S. patients for whom a risk factor can be ascertained, most hepatitis B virus infections are transmitted sexually. internal social structures changed rapidly as borders opened to the West, unleashing enormous new epidemics of HIV infection and other STIs; such patterns persist in the face of ongoing conflicts. Despite advances in the provision of highly effective antiretroviral therapy worldwide, HIV remains the leading cause of death in some develop­ ing countries, and HPV and hepatitis B virus (HBV) remain important causes of cervical and hepatocellular carcinoma, respectively—two of the most common (and preventable) malignancies in the developing world. Sexually transmitted herpes simplex virus (HSV) infection causes most genital ulcer disease throughout the world, and an increas­ ing proportion of cases of genital herpes occur in developing countries with generalized HIV epidemics, where the positive feedback loop between HSV and HIV transmission remains intractable. Despite this consistent link, randomized trials evaluating the efficacy of antiviral therapy in suppressing HSV in both HIV-uninfected and HIV-infected persons have demonstrated no protective effect against acquisition or transmission of HIV. The World Health Organization estimated that 357 million new cases of four curable STIs—gonorrhea, chlamydial infection, syphilis, and trichomoniasis—occurred annually in recent years. Up to 50% of women of reproductive age in developing countries have BV (arguably acquired sexually). All of these curable STIs have been associated with increased risk of HIV transmission or acquisition. In the United States, the prevalence of antibody to HSV-2 began to fall in the late 1990s, especially among adolescents and young adults; the decline was presumably due to delayed sexual debut, increased condom use, and lower rates of multiple (four or more) sex partners— all well documented by the U.S. Youth Risk Behavior Surveillance System. The estimated annual incidence of HBV infection has also declined dramatically since the mid-1980s; this decrease is probably attributable to now-widespread administration of hepatitis B vaccine in infancy. Genital HPV remains the most common sexually transmit­ ted pathogen in the United States, infecting 60% of a cohort of initially

HPV-negative, sexually active Washington state college women within 5 years in a study conducted from 1990 to 2000—i.e., during the pre–HPV immunization era. Global expansion of HPV vaccine cover­ age among young women has already shown promise in reducing the incidence of infection with the HPV types included in the vaccines and of conditions associated with these viruses, including invasive cervical cancer; however, gaps in immunization coverage remain. In industrialized countries, fear of HIV infection in the mid-1980s and through the mid-2000s, coupled with widespread behavioral interventions and better-organized systems of care for the curable STIs, initially helped curb the transmission of several STDs. However, with current antiretroviral therapy, HIV has become for many a chronic disease associated with a normal life span and high quality of life, and HIV pre-exposure prophylaxis (PrEP) has proven highly effective in preventing HIV acquisition, further reducing motivation to use barrier protection such as condoms. Rates of gonorrhea and syphilis remain higher in the United States than in any other Western industrialized country. In the United States, the Centers for Disease Control and Prevention (CDC) has compiled reported rates of STIs since 1941. The incidence of reported gonorrhea peaked at 468 cases per 100,000 population in the mid-1970s and fell to a low of 98 cases per 100,000 in 2012; in 2018, the case rate was 179.1 per 100,000 persons, which is more than an 80% increase since 2009 when the number of new cases reached an all-time low. With increased testing and more sensitive tests, the incidence of reported Chlamydia trachomatis infection has been increasing steadily since reporting began in 1984, reaching an all-time peak of 457.6 cases per 100,000 in 2011. The incidence of primary and secondary syphilis per 100,000 peaked at 71 cases in 1946, fell rapidly to 3.9 cases in 1956, ranged from ~10 to 15 cases through 1987 (with markedly increased rates among MSM and African Americans), and then fell to a nadir of 2.1 cases in 2000–2001 (with rates falling most rapidly among hetero­ sexual African Americans). However, since 1996, with the introduc­ tion of highly active antiretroviral therapy, gonorrhea, syphilis, and chlamydial infection have had a remarkable resurgence among MSM in North America and Europe, where outbreaks of a rare type of chla­ mydial infection (lymphogranuloma venereum [LGV]) that had virtu­ ally disappeared during the AIDS era have occurred. In 2022, 207,255 cases of syphilis (all stages and congenital syphilis) were reported in the United States, which is the greatest number of cases reported since 1950 and an increase of 17.3% since 2021. In 2022, 45% of primary and secondary syphilis cases among males reported to the CDC were in MSM, but incidence has also increased in women, with a concomitant increase in congenital syphilis. In 2022, 3755 cases of congenital syphi­ lis were reported, including 282 congenital syphilis-related stillbirths and infant deaths. The national congenital syphilis rate of 102.5 cases per 100,000 live births in 2022 represents a 30.6% increase relative to 2021 and is the highest reported rate since 1991. Moreover, the uptake of oral PrEP for HIV-1 acquisition has increased among MSM since its initial approval for this purpose in 2012 and has been associated with reports of reduced condom-use frequency and concomitantly increased STI acquisition. These devel­ opments have resulted in a soaring incidence of STIs, with increas­ ing co-infection with HIV and other sexually transmitted pathogens (particularly Treponema pallidum, the cause of syphilis; and Neisseria gonorrhoeae, the cause of gonorrhea), primarily among MSM. MANAGEMENT OF COMMON SEXUALLY TRANSMITTED DISEASE SYNDROMES Although other chapters discuss management of specific STIs, most patients are managed (at least initially) on the basis of presenting symptoms and signs and associated risk factors, even in industrialized countries. Table 141-2 lists some of the most common clinical sexually transmitted disease (STD) syndromes and their microbial etiologies. Strategies for their management are outlined below. Chapters 207 and 208 address the management of infections with human retroviruses. STD care and management begin with risk assessment and proceed to clinical assessment, diagnostic testing or screening, treatment, and prevention. Risk assessment guides detection and interpretation of

TABLE 141-2  Major Sexually Transmitted Disease Syndromes and Sexually Transmitted Microbial Etiologies SEXUALLY TRANSMITTED MICROBIAL ETIOLOGIES SYNDROME AIDS HIV types 1 and 2 Urethritis: males Neisseria gonorrhoeae, Chlamydia trachomatis, Mycoplasma genitalium, Ureaplasma urealyticum (subspecies urealyticum), Trichomonas vaginalis, HSV, some anaerobic bacteria, Leptotrichia/Sneathia Epididymitis C. trachomatis, N. gonorrhoeae, and (in older men or men who have sex with men) coliform bacteria Lower genital tract infections: females   Cystitis/urethritis C. trachomatis, N. gonorrhoeae, HSV   Mucopurulent cervicitis C. trachomatis, N. gonorrhoeae, M. genitalium   Vulvitis Candida albicans, HSV   Bartholinitis C. albicans, T. vaginalis   Vulvovaginitis C. albicans, T. vaginalis   BV BV-associated bacteria (see text) Acute pelvic inflammatory disease N. gonorrhoeae, C. trachomatis, BV-associated bacteria, M. genitalium, group B streptococci Infertility N. gonorrhoeae, C. trachomatis, BV-associated bacteria Ulcerative lesions of the genitalia HSV-1, HSV-2, Treponema pallidum, Haemophilus ducreyi, C. trachomatis (LGV strains), Klebsiella (Calymmatobacterium) granulomatis CHAPTER 141 Complications of pregnancy/ puerperium Several pathogens implicated Intestinal infections   Proctitis C. trachomatis, N. gonorrhoeae, HSV,

T. pallidum Sexually Transmitted Infections: Overview and Clinical Approach
  Proctocolitis or enterocolitis Campylobacter spp., Shigella spp., Entamoeba histolytica, Helicobacter spp., other enteric pathogens   Enteritis Giardia lamblia Acute arthritis with urogenital infection or viremia N. gonorrhoeae (e.g., DGI), C. trachomatis

(e.g., reactive arthritis), HBV Genital and anal warts HPV (30 genital types) Mononucleosis syndrome CMV, HIV, EBV Hepatitis Hepatitis viruses, T. pallidum, CMV, EBV Neoplasias   Squamous cell dysplasias HPV (especially types 16, 18, 31, 45) and cancers of the cervix, anus, vulva, vagina, or penis   Kaposi’s sarcoma, bodyHHV-8 cavity lymphomas   T-cell leukemia HTLV-1   Hepatocellular carcinoma HBV Tropical spastic paraparesis HTLV-1 Scabies Sarcoptes scabiei Pubic lice Pthirus pubis Abbreviations: BV, bacterial vaginosis; CMV, cytomegalovirus; DGI, disseminated gonococcal infection; EBV, Epstein-Barr virus; HBV, hepatitis B virus; HHV-8, human herpesvirus type 8; HPV, human papillomavirus; HSV, herpes simplex virus; HTLV, human T-cell lymphotropic virus; LGV, lymphogranuloma venereum. symptoms that could denote an STD; decisions on screening or pro­ phylactic/preventive treatment; risk reduction counseling and inter­ vention (e.g., hepatitis B vaccination); treatment of partners of patients with known infections; and behavioral risk reduction by the patient. Consideration of routine demographic data (e.g., identified gender, age, area of residence) is a simple first step in this risk assessment. For example, national guidelines strongly recommend routine screening of

TABLE 141-3  Eleven-Question Sexually Transmitted Disease (STD)/ HIV Risk Assessment Framing Statement   In order to provide the best care for you today and to understand whether we should consider certain infections, I’d like to talk about your sexual behavior. Screening Questions   (1)  Do you have any reason to think you might have a sexually transmitted infection? If so, what reason? (2)  For all adolescents <18 years old: Have you begun having any kind of sex yet? STD History   (3)  Have you ever had any sexually transmitted infections or any genital infections? If so, which ones? Sexual Preference   (4)  Have you had sex with men, women, or both? Injection Drug Use   (5)  Have you ever injected yourself with drugs? (If yes, have you ever shared needles or injection equipment?) (6)  Have you ever had sex with anyone who had ever injected drugs? Characteristics of Partner(s)   (7)  Have any of your sex partners had any sexually transmitted infections? If so, which ones? (8)  Have any of your sex partners had other sex partners during the time you’ve been together? STD Symptoms Checklist   (9)  Have you recently developed any of these symptoms? For Men For Women PART 5 Infectious Diseases (a)  Discharge of pus (drip) from the (a)  Abnormal vaginal discharge penis (b)  Genital sores (ulcers) or rash (increased amount, abnormal odor, abnormal yellow color) (b)  Genital sores (ulcers), rash, or itching Sexual Practices, Past 2 Months (for patients answering yes to any of the above questions, to guide examination and testing) (10)  Now I’d like to ask what parts of your body may have been sexually exposed to an STD (e.g., your penis, mouth, vagina, anus). Query About Interest in STD Screening Tests (for patients answering no to all of the above questions) (11)  Would you like to be tested for HIV or any other STDs today? (If yes, clinician can explore which STD and why.) Source: Adapted from JR Curtis, KK Holmes, in KK Holmes et al (eds): Sexually Transmitted Diseases, 4th ed. New York, McGraw-Hill, 2008. sexually active females ≤25 years of age for C. trachomatis infection. Table 141-3 provides a set of 11 STD/HIV risk-assessment questions that clinicians can pose verbally or that health care systems can adapt (with yes/no responses) into a routine self-administered questionnaire. The initial framing statement gives permission to discuss topics that may be difficult for the patient to disclose. Risk assessment is followed by clinical assessment (elicitation of information on specific current symptoms and signs of STDs). Con­ firmatory diagnostic tests (for persons with symptoms or signs) or screening tests (for those without symptoms or signs) may involve microscopic examination, culture, nucleic acid amplification tests (NAATs), or serology. Initial syndrome-based treatment should cover the most likely causes. For certain syndromes, results of rapid tests can narrow the spectrum of this initial therapy (e.g., pH of vaginal fluid for women with vaginal discharge, Gram’s stain of urethral discharge for men with urethral discharge, rapid plasma reagin test for genital ulcer to assess the probability of syphilis). After the institution of treatment, STD management proceeds to the “4 Cs” of prevention and control: contact tracing (see “Prevention and Control of STIs,” below), ensuring compliance with therapy, and counseling on risk reduction, including condom promotion and provision as well as motivational interviewing for risk reduction.

Consistent with current guidelines, all adults should be screened for infection with HIV-1 at least once, and more frequently if they are at elevated risk for acquisition of this infection. ■ ■URETHRITIS IN MEN Urethritis in men produces urethral discharge, dysuria, or both, usually without frequency of urination. Causes include N. gonorrhoeae, C. trachomatis, Mycoplasma genitalium, Ureaplasma urealyticum, Tricho­ monas vaginalis, HSV, and (rarely) adenovirus. Until recently, C. trachomatis caused ~30–40% of cases of nongono­ coccal urethritis (NGU), particularly in heterosexual men; however, the proportion of cases due to this organism has probably declined in some populations served by effective chlamydial control programs, and older men with urethritis appear less likely to have chlamydial infec­ tion. HSV and T. vaginalis each cause a small proportion of NGU cases in the United States. Recently, multiple studies have consistently impli­ cated M. genitalium as a probable cause of many Chlamydia-negative cases. Fewer studies than in the past have implicated Ureaplasma; the ureaplasmas have been differentiated into U. urealyticum and Urea­ plasma parvum, and a few studies suggest that U. urealyticum—but not U. parvum—is associated with NGU; for this reason, neither testing nor presumptive treatment for ureaplasmas in the setting of urethritis is recommended. Coliform bacteria can cause urethritis in men who engage in insertive anal intercourse. More recently, anaerobic bacte­ ria that are characteristically involved in BV, especially Leptotrichia/ Sneathia species, have occasionally been associated with urethritis in heterosexual men. Recommendations for the initial diagnosis of ure­ thritis in men currently include specific tests only for N. gonorrhoeae and C. trachomatis; they do not yet include testing for M. genitalium, although a NAAT is now commercially available for the latter. APPROACH TO THE PATIENT Urethritis in Men The following summarizes the approach to the male patient with suspected urethritis:

  1. Establish the presence of urethritis. If proximal-to-distal “milk­ ing” of the urethra does not express a purulent or mucopurulent discharge, even after the patient has not voided for several hours (or preferably overnight), a Gram’s-stained smear of an anterior urethral specimen obtained by passage of a small urethrogenital swab 2–3 cm into the urethra usually reveals ≥2 neutrophils per 1000× field when urethritis is present; in gonococcal infec­ tion, such a smear usually reveals gram-negative intracellular diplococci as well. Patients with symptoms who lack objective evidence of urethritis generally do not benefit from repeated courses of antibiotics, and other etiologies of such symptoms may be considered.
  2. Evaluate for complications or alternative diagnoses. A brief history and examination can exclude epididymitis and systemic compli­ cations, such as disseminated gonococcal infection (DGI) and reactive arthritis. Although digital examination of the prostate gland seldom contributes to the evaluation of sexually active young men with urethritis, men with dysuria who lack evidence of urethritis as well as sexually inactive men with urethritis should undergo prostate palpation, urinalysis, and urine culture to exclude bacterial prostatitis and cystitis.
  3. Evaluate for gonococcal and chlamydial infection. An absence of typical gram-negative diplococci on Gram’s-stained smear of urethral exudate containing inflammatory cells warrants a preliminary diagnosis of NGU, as this test is 98% sensitive for the diagnosis of gonococcal urethral infection. However, most men with symptoms and/or signs of urethritis are simultaneously assessed for infection with N. gonorrhoeae and C. trachomatis by NAATs of first-catch urine. The urine specimen tested should consist of the first 10–15 mL of the stream, and if possible, patients should not have voided for the prior 2 h. Culture or NAAT for N. gonorrhoeae may yield positive results even when

Gram’s staining is negative; certain strains of N. gonorrhoeae can result in negative urethral Gram’s stains in up to 30% of cases of urethral infection. Results of tests for gonococcal and chlamydial infection predict the patient’s prognosis (with greater risk for recurrent NGU if neither chlamydiae nor gonococci are found than if either is detected) and can guide both the counseling given to the patient and the management of the patient’s sexual partner(s). 4. Treat urethritis promptly while test results are pending. TREATMENT Urethritis in Men Table 141-4 summarizes the steps in management of urethral dis­ charge and/or dysuria in sexually active men. In practice, if Gram’s stain does not reveal gonococci, urethritis is treated with a regimen effective for NGU, such as azithromycin or doxycycline. Both are generally effective. Although azithromycin has been more effective than doxycycline for M. genitalium infection, the efficacy of azithromycin for treatment of M. genitalium is rapidly declining. Alternatives include moxifloxacin and pristinamycin, a streptogramin antibiotic available in some countries. If gonococci are demonstrated by Gram’s stain or if no diagnostic tests are per­ formed to exclude gonorrhea definitively, treatment should include parenteral cephalosporin therapy for gonorrhea (Chap. 161). Doxy­ cycline is the preferred antibiotic for treating C. trachomatis infec­ tion, which can cause urethral co-infection in men with gonococcal urethritis. Sexual partners with contact to the index patient in the past 60 days should also be tested for gonorrhea and chlamydial infection. Regardless of whether they are tested for these infections, however, they should receive the same regimen given to the index case. Patients with confirmed persistence or recurrence of urethritis after treatment should be re-treated with the initial regimen if they did not comply with the original treatment or were reexposed to an untreated partner. Most persistent urethritis is due to M. genitalium, and prompt diagnostic testing and/or treatment for M. genitalium is recommended. TABLE 141-4  Management of Urethral Discharge in Men USUAL CAUSES USUAL INITIAL EVALUATION Chlamydia trachomatis Neisseria gonorrhoeae Mycoplasma genitalium Ureaplasma urealyticum Trichomonas vaginalis Herpes simplex virus Demonstration of urethral discharge or pyuria Exclusion of local or systemic complications Urethral Gram’s stain to confirm urethritis, detect gram-negative diplococci Test for N. gonorrhoeae, C. trachomatis,

M. genitalium (if indicated and available) Initial Treatment for Patient and Partners Treat gonorrhea (unless excluded): Ceftriaxone (500 mg IMa) For persons weighing ≥150 kg, 1 gram of ceftriaxone IM should be administered Management of Recurrence Confirm objective evidence of urethritis. If patient was reexposed to untreated or new partner, repeat treatment of patient and partner. If patient was not reexposed, consider infection with T. vaginalisb or antibioticresistant M. genitaliumc, and treat accordingly (metronidazole for trichomoniasis; azithromycin for M. genitalium followed by moxifloxacin if needed). aNeither oral cephalosporins nor fluoroquinolones are recommended for treatment of gonorrhea in the United States because of the emergence of increasing fluoroquinolone resistance in N. gonorrhoeae, especially (but not only) among men who have sex with men, and the decreasing susceptibility of a small proportion of gonococci to ceftriaxone (Fig. 136-1). Updates on the emergence of antimicrobial resistance in N. gonorrhoeae can be obtained from the Centers for Disease Control and Prevention at http://www.cdc.gov/std. bIn men, the diagnosis of T. vaginalis infection requires nucleic acid amplification testing of a urethral swab specimen obtained before voiding. cM. genitalium is often resistant to doxycycline and azithromycin but is usually susceptible to the fluoroquinolone moxifloxacin. Moxifloxacin can be considered for treatment of refractory nongonococcal, nonchlamydial urethritis.

National and international guidelines exist for treatment of gonococcal urethritis, typically with ceftriaxone. However, con­ sensus is still lacking on treatment of urethritis that persists after treatment and cure of gonorrhea. Ideally, the approach would involve testing for potential causes of persistent urethritis (e.g., M. genitalium) and antimicrobial susceptibility testing in settings and populations where antimicrobial resistance is emerging. Currently, assays are available that can detect M. genitalium, and some experts believe it is time to integrate such testing into STD care. If M. genitalium is detected, the persistent urethritis can be treated with azithromycin or moxifloxacin in light of local patterns of antimi­ crobial susceptibility.

In heterosexual men with a high likelihood of exposure to tricho­ moniasis, an intraurethral swab specimen and a first-voided urine sample should be tested for T. vaginalis using NAAT. Presumptive treatment with metronidazole or tinidazole (2 g by mouth in a single dose) should be given. For MSM, trichomoniasis is unlikely, and consideration of a course of moxifloxacin is warranted. Because MSM also have the highest prevalence rates of antimicrobialresistant N. gonorrhoeae, this possibility, even if apparently ruled out at the initial presentation, should be kept in mind. ■ ■EPIDIDYMITIS Acute epididymitis, almost always unilateral, produces pain, swelling, and tenderness of the epididymis, with or without symptoms or signs of urethritis. This condition must be differentiated from testicular tor­ sion, tumor, and trauma. Torsion, a surgical emergency, usually occurs in the second or third decade of life and produces a sudden onset of pain, elevation of the testicle within the scrotal sac, rotation of the epi­ didymis from a posterior to an anterior position, and absence of blood flow on Doppler ultrasound. Persistence of symptoms after a course of therapy for epididymitis suggests the possibility of testicular tumor or of a chronic granulomatous disease, such as tuberculosis. In sexually active men under age 35, acute epididymitis is caused most frequently by C. trachomatis and less commonly by N. gonorrhoeae and is usu­ ally associated with overt or subclinical urethritis. Acute epididymitis occurring in older men or following urinary tract instrumentation is usually caused by urinary pathogens. These older men usually have no urethritis but do have bacteriuria. Similarly, epididymitis in MSM who have practiced insertive rectal intercourse is often caused by Enterobacteriaceae. CHAPTER 141 Sexually Transmitted Infections: Overview and Clinical Approach
TREATMENT Epididymitis Ceftriaxone (500 mg as a single dose IM) followed by doxycycline (100 mg by mouth twice daily for 10 days) constitutes effective treat­ ment for epididymitis caused by N. gonorrhoeae or C. trachomatis. Nei­ ther oral cephalosporins nor fluoroquinolones are recommended for treatment of gonorrhea in the United States because of resis­ tance in N. gonorrhoeae, especially (but not only) among MSM (Fig. 141-1). Given rapidly escalating rates of resistance in N. gonor­ rhoeae to azithromycin, this antibiotic is no longer recommended as co-therapy with a parenteral ceftriaxone for gonorrhea. When infection with Enterobacteriaceae is suspected, oral levofloxacin (500 mg once daily for 10 days) added to parenteral ceftriaxone (500 mg IM once) is effective for syndrome-based initial treatment. ■ ■URETHRITIS AND THE URETHRAL

SYNDROME IN WOMEN C. trachomatis, N. gonorrhoeae, and occasionally HSV cause symp­ tomatic urethritis—known as the urethral syndrome in women—that is characterized by “internal” dysuria (usually without urinary urgency or frequency), pyuria, and an absence of Escherichia coli and other uropathogens at counts of ≥102/mL in urine. In contrast, the dysuria associated with vulvar herpes or vulvovaginal candidiasis (and perhaps with trichomoniasis) is often described as “external,” being caused

40% 30% Percentage 20% 10% 0%

Antimicrobials

n (%)

n (%)

n (%)

n (%)

n (%)

n (%)

n (%)

n (%)

n (%)

n (%)

n (%) Azithromycin

(0.3)

(0.3)

(0.6)

(0.4)

(0.9)

(0.6)

(0.2)

(0.4)

(0.2)

(0.2)

(0.5) Cefixime

(0.2)

(0.2)

(0.2)

(0.1)

(0.1)

(0.1)

(0.1) N/A N/A

(0.8)

(1.4) Ceftriaxone

(0.1)

(0.3)

(0.1)

(0.0)

(0.1)

(0.1)

(0.0)

(0.1)

(0.1)

(0.3)

(0.3) Ciprofloxacin

(0.3)

(0.7)

(2.2)

(4.1)

(6.8)

(9.4)

(13.8)

(14.8)

(13.5)

(9.6)

(12.5) Penicillin

(14.2)

(11.4)

(8.2)

(6.6)

(6.5)

(9.4)

(11.5)

(12.9)

(11.2)

(12.5)

(12.9) Tetracycline

(19.9)

(17.0)

(15.2)

(14.4)

(14.4)

(17.3)

(20.6)

(20.5)

(18.2)

(16.7)

(20.2) PART 5 Infectious Diseases Antimicrobials

n (%)

n (%)

n (%)

n (%)

n (%)

n (%)

n (%)

n (%)

n (%)

n (%)

n (%) Azithromycin

(0.3)

(0.3)

(0.6)

(2.5)

(2.6)

(3.6)

(4.4)

(4.6)

(5.1)

(5.8)

(4.6) Cefixime

(1.4)

(0.9)

(0.4)

(0.7)

(0.5)

(0.3)

(0.4)

(0.3)

(0.3)

(0.3)

(0.2) Ceftriaxone

(0.4)

(0.3)

(0.1)

(0.1)

(0.3)

(0.3)

(0.2)

(0.2)

(0.1)

(0.1)

(0.1) Ciprofloxacin

(13.3)

(14.7)

(16.1)

(19.2)

(22.3)

(26.8)

(30.1)

(31.2)

(35.4)

(34.8)

(32.8) Penicillin

(11.8)

(13.2)

(12.2)

(16.2)

(15.7)

(17.8)

(15.8)

(13.7)

(12.8)

(12.3)

(12.0) Tetracycline

(22.8)

(23.4)

(23.7)

(25.3)

(24.2)

(22.6)

(23.1)

(25.6)

(27.8)

(19.7)

(20.6) FIGURE 141-1  Percentage of Tetracycline, Penicillin, or Ciprofloxacin Resistance* or Elevated Cefixime, Ceftriaxone, or Azithromycin Minimum Inhibitory Concentrations (MICs)† by Year, Gonococcal Isolate Surveillance Project (GISP), 2001-2022. *Resistance: ciprofloxacin MIC ≥1.0 μg/mL; penicillin MIC ≥2.0 μg/mL or b-lactamase positive; tetracycline MIC ≥2.0 μg/mL. †Elevated MICs: azithromycin MIC ≥1.0 μg/mL (2000–2004), MIC ≥2.0 μg/mL (2005–2022); ceftriaxone MIC ≥0.125 μg/mL; cefixime MIC ≥0.25 μg/mL. Cefixime susceptibility was not tested in 2007 and 2008. by painful contact of urine with the inflamed or ulcerated labia or introitus. Acute onset, association with urinary urgency or frequency, hematuria, or suprapubic bladder tenderness suggests bacterial cystitis. Among women with symptoms of acute bacterial cystitis, costoverte­ bral pain and tenderness or fever suggest acute pyelonephritis. The management of bacterial urinary tract infection (UTI) is discussed in Chap. 140. Signs of vulvovaginitis, coupled with symptoms of external dysuria, suggest vulvar infection (e.g., with HSV or Candida albicans). Among dysuric women without signs of vulvovaginitis, bacterial UTI must be differentiated from the urethral syndrome by assessment of risk, evalu­ ation of the pattern of symptoms and signs, and specific microbiologic

Ciprofloxacin Tetracycline Penicillin Azithromycin Cefixime Ceftriaxone

Year testing. An STI etiology of the urethral syndrome is suggested by young age, more than one current sexual partner, a new partner within the past month, a partner with urethritis, or coexisting mucopurulent cer­ vicitis (see below). The finding of a single urinary pathogen, such as E. coli or Staphylococcus saprophyticus, at a concentration of ≥102/mL in a properly collected specimen of midstream urine from a dysuric woman with pyuria indicates probable bacterial UTI, whereas pyuria with <102 conventional uropathogens per milliliter of urine (“sterile” pyuria) sug­ gests acute urethral syndrome due to C. trachomatis or N. gonorrhoeae. Gonorrhea and chlamydial infection should be sought by specific tests (e.g., NAATs of vaginal secretions collected with a swab). Among dys­ uric women with sterile pyuria caused by infection with N. gonorrhoeae

or C. trachomatis, appropriate treatment alleviates dysuria. The role of M. genitalium in the urethral syndrome in women remains undefined. ■ ■VULVOVAGINAL INFECTIONS Abnormal Vaginal Discharge  If directly questioned about vagi­ nal discharge during routine health checkups, many women acknowl­ edge having nonspecific symptoms of vaginal discharge that do not correlate with objective signs of inflammation or with actual infection. However, unsolicited reporting of abnormal vaginal discharge often denotes BV or trichomoniasis. Specifically, an abnormally increased amount or an abnormal odor of the discharge is associated with one or both of these conditions. Cervical infection with N. gonorrhoeae or C. trachomatis does not often cause an increased amount or abnormal odor of discharge; however, when these pathogens cause cervicitis, they—like T. vaginalis—often result in an increased number of neu­ trophils in vaginal fluid, which thus takes on a yellow color. Vulvar conditions such as genital herpes or vulvovaginal candidiasis can cause vulvar pruritus, burning, irritation, or lesions as well as external dysuria (as urine passes over the inflamed vulva or areas of epithelial disruption) or vulvar dyspareunia. Certain vulvovaginal infections may have serious sequelae. Tricho­ moniasis, BV, and vulvovaginal candidiasis have all been associated with increased risk of acquisition of HIV infection; BV promotes HIV transmission from HIV-infected women to their male sex partners. Vaginal trichomoniasis and BV early in pregnancy independently TABLE 141-5  Diagnostic Features and Management of Vaginal Infection NORMAL VAGINAL EXAMINATION VULVOVAGINAL CANDIDIASIS TRICHOMONAL VAGINITIS BACTERIAL VAGINOSIS (BV) FEATURE Etiology Uninfected; lactobacilli predominant Candida albicans Trichomonas vaginalis Associated with Gardnerella vaginalis, various anaerobic bacteria, and mycoplasmas Typical symptoms None Vulvar itching and/or irritation Profuse discharge; vulvar itching Discharge           Amount Variable; usually scant Scant Often profuse Moderate   Colora Clear or translucent White White or yellow White or gray   Consistency Nonhomogeneous, flocculent Clumped; adherent plaques Homogeneous Homogeneous, low viscosity; uniformly coats vaginal walls Inflammation of vulvar or vaginal epithelium None Erythema of vaginal epithelium, introitus; vulvar dermatitis, fissures common pH of vaginal fluidb Usually ≤4.5 Usually ≤4.5 Usually ≥5 Usually >4.5 Amine (“fishy”) odor with 10% KOH None None May be present Present Microscopyc Normal epithelial cells; lactobacilli predominant Leukocytes, epithelial cells; mycelia or pseudomycelia in up to 80% of

C. albicans culture–positive persons with typical symptoms Other laboratory findings Isolation of Candida spp. Isolation of T. vaginalis or positive NAATd Diagnosis of BV by NAATd Usual treatment None Azole cream, tablet, or suppository—e.g., miconazole (100-mg vaginal suppository) or clotrimazole (100-mg vaginal tablet) once daily for 7 days Fluconazole, 150 mg orally (single dose) Usual management of sexual partner None None; topical treatment if candidal dermatitis of penis is detected aColor of discharge is best determined by examination against the white background of a swab. bA pH determination is not useful if blood is present or if the test is performed on endocervical secretions. cTo detect fungal elements, vaginal fluid is digested with 10% KOH prior to microscopic examination; to examine for other features, fluid is mixed (1:1) with physiologic saline. Gram’s stain is also excellent for detecting yeasts (less predictive of vulvovaginitis) and pseudomycelia or mycelia (strongly predictive of vulvovaginitis) and for distinguishing normal flora from the mixed flora seen in bacterial vaginosis, but it is less sensitive than the saline preparation for detection of T. vaginalis. dNAAT, nucleic acid amplification test (where available). NAAT for diagnosis of BV typically tests for combinations of BV-associated bacteria and absence of Lactobacillus species.

predict premature onset of labor. BV can also lead to anaerobic bacte­ rial infection of the endometrium and salpinges. Vaginitis may be an early and prominent feature of toxic shock syndrome, and recurrent or chronic vulvovaginal candidiasis develops with increased frequency among women who have systemic illnesses, such as diabetes mellitus or HIV-related immunosuppression (although only a very small propor­ tion of women with recurrent vulvovaginal candidiasis in industrial­ ized countries actually have a serious predisposing illness).

Thus, vulvovaginal symptoms or signs warrant careful evaluation, including speculum and pelvic examination, diagnostic testing, and appropriate therapy specific for the infection identified. Unfortunately, clinicians do not always perform the tests required to establish the cause of such symptoms. Further, self-diagnosis of a specific type of infection—including vulvovaginal candidiasis—is often incorrect. The diagnosis and treatment of the three most common types of vaginal infection are summarized in Table 141-5. Inspection of the vulva and perineum may reveal tender genital ulcerations or fissures (typically due to HSV infection or vulvovaginal candidiasis) or discharge visible at the introitus before insertion of a speculum (suggestive of BV or trichomoniasis). Speculum examina­ tion permits the clinician to discern whether the discharge appears abnormal and whether it emanates from the cervical os (mucoid and, if abnormal, yellow) or from the vagina (not mucoid, since the vaginal epithelium does not produce mucus). Symptoms or signs of abnormal vaginal discharge should prompt testing of vaginal fluid for pH, for a fishy odor when mixed with 10% KOH, and for certain microscopic CHAPTER 141 Sexually Transmitted Infections: Overview and Clinical Approach
Malodorous, slightly increased discharge Erythema of vaginal and vulvar epithelium; colpitis macularis None Leukocytes; motile trichomonads seen in 80–90% of symptomatic patients, less often in the absence of symptoms Clue cells; few leukocytes; no lactobacilli or only a few outnumbered by profuse mixed microbiota, nearly always including

G. vaginalis plus anaerobic species on Gram’s stain (Nugent’s score ≥7) Metronidazole or tinidazole,

2 g orally (single dose) Metronidazole, 500 mg PO bid for 7 days Metronidazole, 500 mg PO bid for 7 days Metronidazole gel, 0.75%, one applicator

(5 g) intravaginally once daily for 5 days Clindamycin, 2% cream, one full applicator vaginally each night for 7 days Examination for sexually transmitted infection; treatment with metronidazole, 2 g PO (single dose) None

features when mixed with saline (motile trichomonads and/or “clue cells”) and with 10% KOH (pseudohyphae or hyphae indicative of vul­ vovaginal candidiasis). Additional objective laboratory tests, described below, are useful for establishing the cause of abnormal vaginal dis­ charge. Gram’s staining of vaginal fluid can be used to characterize the vaginal bacteria using the Nugent score but is used primarily for research purposes and requires familiarity with the morphotypes and scale involved. Of note, NAATs that characterize relative concentra­ tions of BV-associated bacteria and certain Lactobacillus species are now available and offer comparable performance to clinical diagnostic criteria.

TREATMENT Vaginal Discharge Patterns of treatment for abnormal vaginal discharge vary widely. In developing countries, where clinics or pharmacies often dispense treatment based on symptoms alone without examination or test­ ing, oral treatment with metronidazole—particularly with a 7-day regimen—provides reasonable coverage against both trichomonia­ sis and BV, the usual causes of symptoms of vaginal discharge. Met­ ronidazole treatment of sex partners prevents reinfection of women with T. vaginalis, although it does not help prevent the recurrence of BV. Guidelines for syndromic management promulgated by the World Health Organization suggest consideration of treatment for cervical infection and for trichomoniasis, BV, and vulvovaginal can­ didiasis in women with symptoms of abnormal vaginal discharge. However, it is important to note that the majority of chlamydial and gonococcal cervical infections produce no symptoms. PART 5 Infectious Diseases In industrialized countries, clinicians treating symptoms and signs of abnormal vaginal discharge should, at a minimum, differ­ entiate between BV and trichomoniasis because optimal manage­ ment of patients and partners differs for these two conditions. Vaginal Trichomoniasis (See also Chap. 236)  Symptomatic trichomoniasis characteristically produces a profuse, yellow, purulent, homogeneous vaginal discharge and vulvar irritation, sometimes with visible inflammation of the vaginal and vulvar epithelium and petechial lesions on the cervix (the so-called strawberry cervix, best visualized by colposcopy). The pH of vaginal fluid—normally <4.7—usually rises to ≥5. Microscopic examination of vaginal discharge mixed with saline reveals motile trichomonads in most culture-positive cases. However, saline microscopy detects fewer than one-half of all cases, and, espe­ cially in the absence of symptoms or signs, culture or NAAT is usually required for detection of the organism. NAAT for T. vaginalis is more sensitive than culture. Treatment of asymptomatic as well as symptom­ atic cases reduces rates of transmission and prevents later development of symptoms. TREATMENT Vaginal Trichomoniasis Only nitroimidazoles (e.g., metronidazole and tinidazole) consis­ tently cure trichomoniasis. A single 2-g oral dose of metronidazole has been the standard treatment for decades, but it is less effective than a weeklong course; the latter is preferred. Tinidazole has a longer half-life than metronidazole, causes fewer gastrointestinal symptoms, and may be useful in treating trichomoniasis that fails to respond to metronidazole. Treatment of sexual partners, facilitated by dispensing metronidazole to the female patient to give to her partner(s), significantly reduces both the risk of reinfection and the reservoir of infection; treating partners is the standard of care. Intravaginal treatment with 0.75% metronidazole gel is not reliable for vaginal trichomoniasis. Thus, systemic use of metronidazole is still recommended throughout pregnancy for treatment of tricho­ moniasis. In a large randomized trial, metronidazole treatment of trichomoniasis during pregnancy was associated with an increased

frequency of perinatal morbidity. However, most studies, including randomized controlled trials, have shown no adverse effects of met­ ronidazole use during pregnancy on preterm birth or birth defects. Bacterial Vaginosis  BV is a syndrome characterized by symptoms of vaginal malodor and increased white-gray discharge, which appears homogeneous, is low in viscosity, and uniformly covers the vaginal mucosa. BV has been associated with an increased risk of acquiring several other genital infections, including those caused by HIV, C. trachomatis, and N. gonorrhoeae. Other possible risk factors include recent unprotected vaginal intercourse, having a female sex partner, and vaginal douching. Although bacteria associated with BV have been detected under the foreskin of uncircumcised men and have been associated with urethritis, metronidazole treatment of male partners has not reduced the rate of recurrence of BV among affected women. Among women with BV, culture of vaginal fluid has shown mark­ edly increased prevalences and concentrations of Gardnerella vaginalis, Mycoplasma hominis, and several anaerobic bacteria (e.g., Mobiluncus, Prevotella [formerly Bacteroides], and some Peptostreptococcus species) as well as an absence of hydrogen peroxide–producing Lactobacillus species that constitute most of the normal vaginal microbiota and help protect against cervical and vaginal infections. Broad-range polymerase chain reaction (PCR) amplification of 16S rDNA in vaginal fluid, with subsequent identification of specific bacterial species by various meth­ ods, has documented even greater bacterial diversity, including several unique species not previously identified in culture (Fig. 141-2) and Atopobium vaginae, an organism that is strongly associated with BV and is resistant to metronidazole. Other genera newly implicated in BV include Megasphaera, Leptotrichia, Eggerthella, and Dialister. BV is conventionally diagnosed clinically with the Amsel criteria, which include any three of the following four clinical abnormalities: (1) objective signs of increased white homogeneous vaginal discharge; (2) a vaginal discharge pH of >4.5; (3) liberation of a distinct fishy odor (attributable to volatile amines such as trimethylamine) immediately after vaginal secretions are mixed with a 10% solution of KOH; and (4) microscopic demonstration of “clue cells” (vaginal epithelial cells coated with coccobacillary organisms, which have a granular appear­ ance and indistinct borders; Fig. 141-3) on a wet mount prepared by mixing vaginal secretions with normal saline in a ratio of ~1:1. More recently, NAAT targeting the absence of Lactobacillus crispatus and presence of BV-associated anaerobes have offered accurate options for diagnosing BV. BV6: BVAB-1 (green) + BVAB-2 (red) + DAPI (blue) FIGURE 141-2  Broad-range polymerase chain reaction amplification of 16S rDNA in vaginal fluid from a woman with bacterial vaginosis (BV) shows a field of bacteria hybridizing with probes for BV-associated bacterium 1 (BVAB-1, visible as a thin, curved green rod) and for BVAB-2 (red). The inset shows that BVAB-1 has a morphology similar to that of Mobiluncus (curved rod). (Adapted from DN Fredricks et al: Molecular identification of bacteria associated with bacterial vaginosis. N Engl J Med 353:1899, 2005.)

FIGURE 141-3  Wet mount of vaginal fluid showing typical clue cells from a woman with bacterial vaginosis. Note the obscured epithelial cell margins and the granular appearance attributable to many adherent bacteria (×400). (Photograph provided by Lorna K. Rabe, reprinted with permission from S Hillier et al, in KK Holmes et al [eds]: Sexually Transmitted Diseases, 4th ed. New York, McGraw-Hill, 2008.) TREATMENT Bacterial Vaginosis The standard dosage of oral metronidazole for the treatment of BV is 500 mg twice daily for 7 days. Intravaginal treatment with 2% clindamycin cream (one full applicator [5 g containing 100 mg of clindamycin phosphate] each night for 7 nights) or with 0.75% metronidazole gel (one full applicator [5 g containing 37.5 mg of metronidazole] twice daily for 5 days) is also approved for use in the United States and does not elicit systemic adverse reactions; the response to both of these treatments is similar to the response to oral metronidazole. Another nitroimidazole given orally, secnidazole, is also effective (single 2-g dose). Other alternatives include oral clindamycin (300 mg twice daily for 7 days), clindamycin ovules (100 g
intravaginally once at bedtime for 3 days), and oral tinidazole (1 g daily for 5 days or 2 g daily for 3 days). Unfortunately, recurrence over the long term (i.e., several months later) is distressingly common after either oral or intravaginal treatment. A randomized trial comparing intravaginal gel containing 37.5 mg of metronidazole with a suppository containing 500 mg of metronidazole plus nystatin (the latter not marketed in the United States) showed significantly higher rates of recurrence with the 37.5-mg regimen; this result suggests that higher metronidazole dosages may be important in topical intravaginal therapy. Recurrences can be significantly lessened with the twiceweekly use of suppressive intravaginal metronidazole gel. The goal of replenishing the vaginal lactobacilli that sustain vaginal health has recently been bolstered by a randomized trial that demonstrated that weekly vaginal administration of L. crispatus CTV-05 (LACTIN-V) reduced rates of recurrent BV by approximately one-third. A meta-analysis of 18 studies concluded that BV during pregnancy substantially increased the risk of preterm delivery and of spontaneous abortion. However, in most studies, topical intravaginal treatment of BV with clindamycin during pregnancy has not reduced adverse pregnancy outcomes. Numerous trials of oral metronidazole treatment during pregnancy have given inconsistent results, and recent reviews have concluded that antenatal treatment of women with BV—including those with previous preterm delivery— did not reduce the risk of preterm delivery. The U.S. Preventive Services Task Force thus recommends against routine screening of pregnant women for BV. Vulvovaginal Pruritus, Burning, or Irritation  Vulvovaginal candidiasis produces vulvar pruritus, burning, or irritation, generally without symptoms of increased vaginal discharge or malodor. Genital

herpes can produce similar symptoms, with lesions sometimes difficult to distinguish from the fissures and inflammation caused by candidiasis. Signs of vulvovaginal candidiasis include vulvar erythema, edema, fissures, and tenderness. With candidiasis, a white scanty vaginal discharge sometimes takes the form of white thrush-like plaques or cottage cheese–like curds adhering loosely to the vaginal epithelium. C. albicans accounts for nearly all cases of symptomatic vulvovaginal candidiasis, which probably arise from endogenous strains of C. albicans that have colonized the vagina or the intestinal tract. Complicated vulvovaginal candidiasis includes cases that recur four or more times per year; are unusually severe; are caused by non-albicans Candida species; or occur in women with uncontrolled diabetes, debilitation, immunosuppression, or pregnancy.

In addition to compatible clinical symptoms, the diagnosis of vulvovaginal candidiasis involves the demonstration of pseudohyphae or hyphae by microscopic examination of vaginal fluid mixed with saline or 10% KOH or subjected to Gram’s staining. Microscopic examination is less sensitive than culture but correlates better with symptoms. Culture is typically reserved for cases that do not respond to standard first-line antimycotic agents and is undertaken to rule out imidazole or azole resistance (often associated with Candida glabrata) or before the initiation of suppressive antifungal therapy for recurrent disease. TREATMENT Vulvovaginal Pruritus, Burning, or Irritation Symptoms and signs of vulvovaginal candidiasis warrant treatment, usually intravaginal administration of any of several imidazole antibiotics (e.g., miconazole or clotrimazole) for 3–7 days or of a single dose of oral fluconazole (Table 141-5). Over-the-counter marketing of such preparations has reduced the cost of care and made treatment more convenient for many women with recurrent yeast vulvovaginitis. However, most women who purchase these preparations do not have vulvovaginal candidiasis, whereas many have other vaginal infections that require different treatment. Therefore, only women with classic symptoms of vulvar pruritus and a history of previous episodes of yeast vulvovaginitis documented by an experienced clinician should self-treat. Short-course topical intravaginal azole drugs are effective for the treatment of uncomplicated vulvovaginal candidiasis (e.g., clotrimazole, two 100-mg vaginal tablets daily for 3 days; or miconazole, a 1200-mg vaginal suppository as a single dose). Single-dose oral treatment with fluconazole (150 mg) is also effective and is preferred by many patients. Management of complicated cases (see above) and those that do not respond to the usual intravaginal or single-dose oral therapy often involves prolonged or periodic oral therapy; this situation is discussed extensively in the 2015 CDC STD treatment guidelines (https://www.cdc.gov/ sti/hcp/clinical-guidance/?CDC_AAref_Val=https://www.cdc.gov/std/ treatment/). Treatment of sexual partners is not routinely indicated. CHAPTER 141 Sexually Transmitted Infections: Overview and Clinical Approach
Other Causes of Vaginal Discharge or Vaginitis  In the ulcerative vaginitis associated with staphylococcal toxic shock syndrome, Staphylococcus aureus should be promptly identified in vaginal fluid by Gram’s stain and by culture. In desquamative inflammatory vaginitis, smears of vaginal fluid reveal neutrophils, massive vaginal epithelial cell exfoliation with increased numbers of parabasal cells, and grampositive cocci; this syndrome may respond to treatment with 2% clindamycin cream, often given in combination with topical steroid preparations for several weeks. Additional causes of vaginitis and vulvovaginal symptoms include retained foreign bodies (e.g., tampons), cervical caps, vaginal spermicides, vaginal antiseptic preparations or douches, vaginal epithelial atrophy (in postmenopausal women or during prolonged breast-feeding in the postpartum period), allergic reactions to latex condoms, vaginal aphthae associated with HIV infection or Behçet’s syndrome, and vestibulitis. ■ ■MUCOPURULENT CERVICITIS Mucopurulent cervicitis (MPC) refers to inflammation of the columnar epithelium and subepithelium of the endocervix and of any contiguous

columnar epithelium that lies exposed in an ectopic position on the ectocervix. MPC in women represents the “silent partner” of urethritis in men, being equally common and often caused by the same agents (N. gonorrhoeae, C. trachomatis, M. genitalium); however, MPC is more difficult than urethritis to recognize, given the nonspecific nature of symptoms (e.g., abnormal vaginal discharge) and the need for visual­ ization by pelvic examination. As the most common manifestation of these serious bacterial infections in women, MPC can be a harbinger or sign of upper genital tract infection, also known as pelvic inflam­ matory disease (PID; see below). In pregnant women, MPC can lead to obstetric complications. In the pre-NAAT era, more than one-third of cervicovaginal specimens tested for C. trachomatis, N. gonorrhoeae, M. genitalium, HSV, and T. vaginalis revealed no identifiable etiology for MPC (Fig. 141-4). More recent studies employing NAATs for these pathogens have still failed to identify a microbiologic etiology in nearly one-half of women with MPC. Individual bacteria associated with BV may also elicit an inflammatory reaction at the cervix; thus, BV may be a cause of MPC.

The diagnosis of MPC rests on the detection of cardinal signs at the cervix, including yellow mucopurulent discharge from the cervical os, endocervical bleeding upon gentle swabbing, and edematous cervical ectopy (see below); the latter two findings are somewhat more common with MPC due to chlamydial infection, but signs alone do not allow a distinction among the causative pathogens. Unlike the endocervicitis produced by gonococcal or chlamydial infection, cervicitis caused by HSV produces ulcerative lesions on the stratified squamous epithelium of the ectocervix as well as on the columnar epithelium. Yellow cervi­ cal mucus on a white swab removed from the endocervix indicates the presence of polymorphonuclear leukocytes (PMNs). Gram’s staining may confirm their presence, although it adds relatively little to the diagnostic value of assessment for cervical signs. The presence of ≥20 PMNs per 1000× microscopic field within strands of cervical mucus not contaminated by vaginal squamous epithelial cells or vaginal bac­ teria indicates endocervicitis. Detection of intracellular gram-negative diplococci in carefully collected endocervical mucus is quite specific but ≤50% sensitive for gonorrhea. Therefore, NAATs for N. gonor­ rhoeae and C. trachomatis are always indicated in the evaluation of MPC, as is a careful evaluation of vaginal discharge for the causes of vaginitis discussed above. PART 5 Infectious Diseases MG/GC 2% MG/GC/CT 1% MG/CT 2% HSV 5% GC/CT 7% No organism 35% MG 8% TV 10% CT 17% GC 13% FIGURE 141-4  Organisms detected among female sexually transmitted disease clinic patients with mucopurulent cervicitis (n = 167). CT, Chlamydia trachomatis; GC, gonococcus; HSV, herpes simplex virus; MG, Mycoplasma genitalium; TV, Trichomonas vaginalis. (Courtesy of Dr. Lisa Manhart.)

TREATMENT Mucopurulent Cervicitis Although the above criteria for MPC are neither highly specific nor highly predictive of gonococcal or chlamydial infection in some settings, the 2015 CDC STD guidelines call for consideration of empirical treatment for MPC, pending test results, in most cases. Presumptive treatment with antibiotics active against C. trachomatis should be provided for women at increased risk for this common STI (risk factors: age <25 years, new or multiple sex partners, and unprotected sex), especially if follow-up cannot be ensured. Concurrent therapy for gonorrhea is indicated if the prevalence of this infection is substantial in the relevant patient population (e.g., young adults, a clinic with documented high prevalence). In this situation, therapy should include a single-dose regimen effective for gonorrhea plus treatment for chlamydial infection, as outlined in Table 141-4 for the treatment of urethritis. In settings where gonor­ rhea is much less common than chlamydial infection, initial therapy for chlamydial infection alone suffices, pending test results for gon­ orrhea. The etiology and potential benefit of treatment for endocer­ vicitis not associated with gonorrhea or chlamydial infection have not been established. Although the antimicrobial susceptibility of M. genitalium is not yet well defined, the organism frequently per­ sists after doxycycline therapy, and it currently seems reasonable to use azithromycin to treat possible M. genitalium infection in such cases. With resistance of M. genitalium to azithromycin now rec­ ognized, moxifloxacin may be a reasonable alternative. The sexual partner(s) of a woman with MPC should be examined and given a regimen similar to that chosen for the woman unless results of tests for gonorrhea or chlamydial infection in either partner warrant dif­ ferent therapy or no therapy. ■ ■CERVICAL ECTOPY Cervical ectopy, often mislabeled “cervical erosion,” is easily confused with infectious endocervicitis. Ectopy represents the presence of the one-cell-thick columnar epithelium extending from the endocervix out onto the visible ectocervix. In ectopy, the cervical os may contain clear or slightly cloudy mucus but usually not yellow mucopus. Col­ poscopy shows intact epithelium. Normally found during adolescence and early adulthood, ectopy gradually recedes through the second and third decades of life, as squamous metaplasia replaces the ectopic columnar epithelium. Oral contraceptive use favors the persistence or reappearance of ectopy, while smoking apparently accelerates squa­ mous metaplasia. Cauterization of ectopy is not warranted. Ectopy may render the cervix more susceptible to infection with N. gonorrhoeae, C. trachomatis, or HIV. ■ ■PELVIC INFLAMMATORY DISEASE The term pelvic inflammatory disease (PID) usually refers to infection that ascends from the cervix or vagina to involve the endometrium and/or fallopian tubes. Infection can extend beyond the reproductive tract to cause pelvic peritonitis, generalized peritonitis, perihepatitis, perisplenitis, or pelvic abscess. Rarely, infection not related to spe­ cific sexually transmitted pathogens extends secondarily to the pelvic organs (1) from adjacent foci of inflammation (e.g., appendicitis, regional ileitis, or diverticulitis) or BV, (2) as a result of hematogenous dissemination (e.g., of tuberculosis or staphylococcal bacteremia), or (3) as a complication of certain tropical diseases (e.g., schistosomiasis). Intrauterine infection can be primary (spontaneously occurring and usually sexually transmitted) or secondary to invasive intrauterine sur­ gical procedures (e.g., dilation and curettage, termination of pregnancy, insertion of an intrauterine device [IUD], or hysterosalpingography) or to parturition. Etiology  The agents most often implicated in acute PID include the primary causes of endocervicitis (N. gonorrhoeae, C. trachomatis, and M. genitalium) and anaerobes associated with BV. In general, PID is most often caused by N. gonorrhoeae in settings where there is a

high incidence of gonorrhea. M. genitalium has also been significantly associated with histopathologic diagnoses of endometritis and with salpingitis. Anaerobic and facultative organisms (especially Prevotella species, peptostreptococci, E. coli, Haemophilus influenzae, and group B strep­ tococci) as well as genital mycoplasmas have been isolated from the peritoneal fluid or fallopian tubes in a varying proportion (typically one-fourth to one-third) of women with PID studied in the United States. The difficulty of determining the exact microbial etiology of an individual case of PID—short of using invasive procedures for specimen collection—has implications for the approach to empirical antimicrobial treatment of this infection. Epidemiology  In the United States, the estimated annual number of initial visits to physicians’ offices for PID by women 15–44 years of age fell from an average of 400,000 during the 1980s to 250,000 in 1999 and then to 51,000 in 2014. Hospitalizations for acute PID in the United States also declined steadily throughout the 1980s and early 1990s but have remained fairly constant at 70,000–100,000 per year since 1995. Important risk factors for acute PID include the presence of endocervical infection or BV, a history of salpingitis or of recent vaginal douching, and recent insertion of an IUD. Certain other iatrogenic fac­ tors, such as dilation and curettage or cesarean section, can increase the risk of PID, especially among women with endocervical gonococcal or chlamydial infection or BV. Symptoms of N. gonorrhoeae–associated and C. trachomatis–associated PID often begin during or soon after the menstrual period; this timing suggests that menstruation is a risk fac­ tor for ascending infection from the cervix and vagina. Experimental inoculation of the fallopian tubes of nonhuman primates has shown that repeated exposure to C. trachomatis leads to the greatest degree of tissue inflammation and damage; thus, immunopathology probably contributes to the pathogenesis of chlamydial salpingitis. Women using oral contraceptives appear to be at decreased risk of symptomatic PID, and tubal sterilization reduces the risk of salpingitis by preventing intraluminal spread of infection into the tubes. Clinical Manifestations  •  ENDOMETRITIS: A CLINICAL PATHO­ LOGIC SYNDROME  A study of women with clinically suspected PID who were undergoing both endometrial biopsy and laparoscopy showed that those with endometritis alone differed from those who also had salpingitis in significantly less often having lower quadrant, adnexal, or cervical motion or abdominal rebound tenderness; fever; or elevated C-reactive protein levels. In addition, women with endometri­ tis alone differed from those with neither endometritis nor salpingitis in more often having gonorrhea, chlamydial infection, and risk factors such as douching or IUD use. Thus, women with endometritis alone were intermediate between those with neither endometritis nor sal­ pingitis and those with salpingitis with respect to risk factors, clinical manifestations, cervical infection prevalence, and elevated C-reactive protein level. Women with endometritis alone are at lower risk of subsequent tubal occlusion and resulting infertility than are those with salpingitis. SALPINGITIS  Symptoms of nontuberculous salpingitis classically evolve from a yellow or malodorous vaginal discharge caused by MPC and/or BV to midline abdominal pain and abnormal vaginal bleeding caused by endometritis and then to bilateral lower abdominal and pelvic pain caused by salpingitis, with nausea, vomiting, and increased abdominal tenderness if peritonitis develops. The abdominal pain in nontuberculous salpingitis is usually described as dull or aching. In some cases, pain is lacking or atypi­ cal, but active inflammatory changes are found in the course of an unrelated evaluation or procedure, such as a laparoscopic evaluation for infertility. Abnormal uterine bleeding precedes or coincides with the onset of pain in ~40% of women with PID, symptoms of urethritis (dysuria) occur in 20%, and symptoms of proctitis (anorectal pain, tenesmus, and rectal discharge or bleeding) are occasionally seen in women with gonococcal or chlamydial infection. Speculum examination shows evidence of MPC (yellow endocervi­ cal discharge, easily induced endocervical bleeding) in the majority of

women with gonococcal or chlamydial PID. Cervical motion tender­ ness is produced by stretching of the adnexal attachments on the side toward which the cervix is pushed. Bimanual examination reveals uter­ ine fundal tenderness due to endometritis and abnormal adnexal ten­ derness due to salpingitis that is usually, but not necessarily, bilateral. Adnexal swelling is palpable in about one-half of women with acute salpingitis, but evaluation of the adnexae in a patient with marked tenderness is not reliable. The initial temperature is >38°C in only about one-third of patients with acute salpingitis. Laboratory findings include elevation of the erythrocyte sedimentation rate (ESR) in 75% of patients with acute salpingitis and elevation of the peripheral white blood cell count in up to 60%.

Unlike nontuberculous salpingitis, genital tuberculosis often occurs in older women, many of whom are postmenopausal. Presenting symp­ toms include abnormal vaginal bleeding, pain (including dysmenor­ rhea), and infertility. About one-quarter of these women have had adnexal masses. Endometrial biopsy shows tuberculous granulomas and provides optimal specimens for culture. PERIHEPATITIS AND PERIAPPENDICITIS  Pleuritic upper abdominal pain and tenderness, usually localized to the right upper quadrant (RUQ), develop in 3–10% of women with acute PID. Symptoms of perihepatitis arise during or after the onset of symptoms of PID and may overshadow lower abdominal symptoms, thereby leading to a mistaken diagnosis of cholecystitis. In perhaps 5% of cases of acute salpingitis, early laparoscopy reveals perihepatic inflammation ranging from edema and erythema of the liver capsule to exudate with fibrin­ ous adhesions between the visceral and parietal peritoneum. When treatment is delayed and laparoscopy is performed late, dense “violinstring” adhesions can be seen over the liver; chronic exertional or positional RUQ pain ensues when traction is placed on the adhesions. Although perihepatitis, also known as the Fitz-Hugh–Curtis syndrome, was for many years specifically attributed to gonococcal salpingitis, most cases are now attributed to chlamydial salpingitis. In patients with chlamydial salpingitis, serum titers of microimmunofluorescent anti­ body to C. trachomatis are typically much higher when perihepatitis is present than when it is absent. CHAPTER 141 Sexually Transmitted Infections: Overview and Clinical Approach
Physical findings include RUQ tenderness and usually include adnexal tenderness and cervicitis, even in patients whose symptoms do not suggest salpingitis. Results of liver function tests and RUQ ultrasonography are nearly always normal. The presence of MPC and pelvic tenderness in a young woman with subacute pleuritic RUQ pain and normal ultrasonography of the gallbladder points to a diagnosis of perihepatitis. Periappendicitis (appendiceal serositis without involvement of the intestinal mucosa) has been found in ~5% of patients undergoing appendectomy for suspected appendicitis and can occur as a complica­ tion of gonococcal or chlamydial salpingitis. Among women with salpingitis, HIV infection is associated with increased severity of salpingitis and with tuboovarian abscess requiring hospitalization and surgical drainage. Nonetheless, among women with HIV infection and salpingitis, the clinical response to conventional antimicrobial therapy (coupled with drainage of tuboovarian abscess, when found) has usually been satisfactory. Diagnosis  Treatment appropriate for PID must not be withheld from patients who have an equivocal diagnosis; it is better to err on the side of overdiagnosis and overtreatment. On the other hand, it is essential to differentiate between salpingitis and other pelvic pathol­ ogy, particularly surgical emergencies such as appendicitis and ectopic pregnancy or the chronic syndrome of endometriosis. Nothing short of laparoscopy definitively identifies salpingitis, but routine laparoscopy to confirm suspected salpingitis is gener­ ally impractical. Most patients with acute PID have lower abdominal pain of <3 weeks’ duration, pelvic tenderness on bimanual pelvic examination, and evidence of lower genital tract infection (e.g., MPC). Approximately 60% of such patients have salpingitis at laparoscopy, and perhaps 10–20% have endometritis alone. Among the patients with these findings, a rectal temperature >38°C, a palpable adnexal mass, and elevation of the ESR to >15 mm/h also raise the probability

of salpingitis, which has been found at laparoscopy in 68% of patients with one of these additional findings, 90% of patients with two, and 96% of patients with three. However, only 17% of all patients with lap­ aroscopy-confirmed salpingitis have had all three additional findings.

In a woman with pelvic pain and tenderness, increased numbers of PMNs (30 per 1000× microscopic field in strands of cervical mucus) or leukocytes outnumbering epithelial cells in vaginal fluid (in the absence of trichomonal vaginitis, which also produces PMNs in vaginal discharge) increase the predictive value of a clinical diagnosis of acute PID, as do onset with menses, history of recent abnormal menstrual bleeding, presence of an IUD, history of salpingitis, and sexual expo­ sure to a male with urethritis. Appendicitis or another disorder of the gut is favored by the early onset of anorexia, nausea, or vomiting; the onset of pain later than day 14 of the menstrual cycle; or unilateral pain limited to the right or left lower quadrant. Whenever the diag­ nosis of PID is being considered, serum assays for human β-chorionic gonadotropin should be performed; these tests are usually positive with ectopic pregnancy. Ultrasonography and magnetic resonance imaging (MRI) can be useful for the identification of tuboovarian or pelvic abscess. MRI of the tubes can also show increased tubal diameter, intra­ tubal fluid, or tubal wall thickening in cases of salpingitis. The primary value of laparoscopy in women with lower abdominal pain is for the exclusion of other surgical problems that cannot be resolved with noninvasive imaging. Some of the most common or seri­ ous problems that may be confused with salpingitis (e.g., acute appen­ dicitis, ectopic pregnancy, corpus luteum bleeding, ovarian tumor) are unilateral. Unilateral pain or pelvic mass, although not incompatible with PID, is a strong indication for laparoscopy unless the clinical picture warrants laparotomy instead. Atypical clinical findings such as the absence of lower genital tract infection, a missed menstrual period, a positive pregnancy test, or failure to respond to appropriate therapy are other common indications for laparoscopy. Endometrial biopsy is relatively sensitive and specific for the diagnosis of endometritis, which correlates well with the presence of salpingitis. PART 5 Infectious Diseases Vaginal or endocervical swab specimens should be obtained for NAATs for N. gonorrhoeae and C. trachomatis. At a minimum, vaginal fluid should be evaluated for the presence of PMNs, and endocervical secretions ideally should be assessed by Gram’s staining for PMNs and gram-negative diplococci, which indicate gonococcal infection. The clinical diagnosis of PID made by expert gynecologists is confirmed by laparoscopy or endometrial biopsy in ~90% of women who also have cultures positive for N. gonorrhoeae or C. trachomatis. Even among women with no symptoms suggestive of acute PID who were attending an STD clinic or a gynecology clinic in Pittsburgh, endometritis was significantly associated with endocervical gonorrhea or chlamydial infection or with BV, being detected in 26%, 27%, and 15% of women with these conditions, respectively. TREATMENT Pelvic Inflammatory Disease Recommended combination regimens for ambulatory or parenteral management of PID are presented in Table 141-6. Women man­ aged as outpatients should receive a combined regimen with broad activity, such as ceftriaxone (to cover possible gonococcal infec­ tion) followed by doxycycline (to cover possible chlamydial infec­ tion). Metronidazole should be added to enhance activity against anaerobes; in a randomized trial, the addition of metronidazole to ceftriaxone and doxycycline effected reduction in endometrial anaerobes, M. genitalium, and pelvic tenderness. The CDC STD treatment guidelines recommend initiation of empirical treatment for PID in sexually active young women and other women at risk for PID if they are experiencing pelvic or lower abdominal pain, if no other cause for the pain can be identified, and if pelvic examination reveals one or more of the following criteria for PID: cervical motion tenderness, uterine tenderness, or adnexal tenderness. Women with suspected PID can be treated as either outpatients or inpatients. In the multicenter Pelvic Inflammatory

TABLE 141-6  Combination Antimicrobial Regimens Recommended for Outpatient Treatment or for Parenteral Treatment of Pelvic Inflammatory Disease OUTPATIENT REGIMENSa PARENTERAL REGIMENS Ceftriaxone (500 mg IM once) plus Doxycycline (100 mg PO bid for 14 days) plusb Initiate parenteral therapy with either of the following regimens; continue parenteral therapy until 48 h after clinical improvement; then change to outpatient therapy, as described in the text Regimen A Cefotetan (2 g IV q12h) or cefoxitin (2 g IV q6h) plus Doxycycline (100 mg IV or PO q12h) Regimen B Clindamycin (900 mg IV q8h) plus Gentamicin (loading dose of 2 mg/kg IV or IM, then maintenance dose of 1.5 mg/kg q8h) Metronidazole (500 mg PO bid for 14 days) aSee text for discussion of options in the patient who is intolerant of cephalosporins. bThe addition of metronidazole is recommended particularly if bacterial vaginosis or trichomoniasis is present. Source: Adapted from Centers for Disease Control and Prevention: MMWR Recomm Rep 70(RR-04):1, 2021. Disease Evaluation and Clinical Health (PEACH) trial, 831 women with mild to moderately severe symptoms and signs of PID were randomized to receive either inpatient treatment with IV cefoxitin and doxycycline or outpatient treatment with a single IM dose of cefoxitin plus oral doxycycline. Short-term clinical and microbio­ logic outcomes and long-term outcomes were equivalent in the two groups. Nonetheless, hospitalization should be considered when (1) the diagnosis is uncertain and surgical emergencies such as appen­ dicitis and ectopic pregnancy cannot be excluded, (2) the patient is pregnant, (3) pelvic abscess is suspected, (4) severe illness or nausea and vomiting preclude outpatient management, (5) the patient has HIV infection, (6) the patient is assessed as unable to follow or tol­ erate an outpatient regimen, or (7) the patient has failed to respond to outpatient therapy. Some experts also prefer to hospitalize ado­ lescents with PID for initial therapy, although younger women do as well as older women on outpatient therapy. Currently, no agents other than parenteral cephalosporins pro­ vide reliable coverage for gonococcal infection. Thus, adequate oral treatment of women with serious intolerance to cephalosporins is a challenge. If penicillins are an option, amoxicillin/clavulanic acid combined with doxycycline has elicited a short-term clini­ cal response in one trial. Clinical trials performed outside the United States support the effectiveness of oral moxifloxacin. For women whose PID involves quinolone-resistant N. gonorrhoeae, treatment is uncertain but could include parenteral gentamicin or ertapenem. For hospitalized patients, the following two parenteral regimens (Table 141-6) have given nearly identical results in a multicenter randomized trial:

  1. Doxycycline plus either cefotetan or cefoxitin: Administration of these drugs should be continued by the IV route for at least 48 h after the patient’s condition improves and then followed with oral doxycycline (100 mg twice daily) to complete 14 days of therapy.
  2. Clindamycin plus gentamicin in patients with normal renal function: Once-daily administration of gentamicin (with com­ bination of the total daily dose into a single daily dose) has not been evaluated in PID but has been efficacious in other serious infections and could be substituted. Treatment with these drugs should be continued for at least 48 h after the patient’s condition improves and then followed with oral doxycycline (100 mg twice daily) or clindamycin (450 mg four times daily) to complete 14 days of therapy. In cases with tuboovarian abscess, clindamycin rather than doxycycline for continued therapy provides better coverage for anaerobic infection.

FOLLOW-UP Hospitalized patients should show substantial clinical improvement within 3–5 days. Women treated as outpatients should be clinically reevaluated within 72 h. A follow-up telephone survey of women seen in an emergency department and given a prescription for 10 days of oral doxycycline for PID found that 28% never filled the prescription and 41% stopped taking the medication early (after an average of 4.1 days), often because of persistent symptoms, lack of symptoms, or side effects. Women not responding favorably to ambulatory therapy should be hospitalized for parenteral therapy and further diagnostic evaluations, including a consideration of laparoscopy. Sex partners should be evaluated and treated empiri­ cally for gonorrhea and chlamydial infection. After completion of treatment, tests for persistent or recurrent infection with N. gonor­ rhoeae or C. trachomatis should be performed if symptoms persist or recur or if the patient has not complied with therapy or has been reexposed to an untreated sex partner. SURGERY Surgery is necessary for the treatment of salpingitis only in the face of life-threatening infection (such as rupture or threatened rupture of a tuboovarian abscess) or for drainage of an abscess. Conserva­ tive surgical procedures are usually sufficient. Pelvic abscesses can often be drained by posterior colpotomy, and peritoneal lavage can be used for generalized peritonitis. Prognosis  Late sequelae include infertility due to bilateral tubal occlusion, ectopic pregnancy due to tubal scarring without occlusion, chronic pelvic pain, and recurrent salpingitis. The overall post-salpingitis risk of infertility due to tubal occlusion in a large study in Sweden was 11% after one episode of salpingitis, 23% after two episodes, and 54% after three or more episodes. A University of Washington study found a sevenfold increase in the risk of ectopic pregnancy and an eightfold increase in the rate of hysterectomy after PID. Prevention  A randomized controlled trial designed to determine whether selective screening for chlamydial infection reduces the risk of subsequent PID showed that women randomized to undergo screening had a 56% lower rate of PID over the following year than did women receiving the usual care without screening. This report helped prompt U.S. national guidelines for risk-based chlamydial screening of young women to reduce the incidence of PID and the prevalence of post-PID sequelae, while also reducing sexual transmission of C. trachomatis. The CDC and the U.S. Preventive Services Task Force recommend that sexually active women ≤25 years of age be screened annually for genital chlamydial infection. ■ ■ULCERATIVE GENITAL OR PERIANAL LESIONS Genital ulceration reflects a set of important STIs, most of which sharply increase the risk of sexual acquisition and shedding of HIV. In a 1996 study of genital ulcers in 10 of the U.S. cities with the highest rates of primary syphilis, PCR testing of ulcer specimens demonstrated HSV in 62% of patients, T. pallidum in 13%, and Haemophilus ducreyi (the cause of chancroid) in 12–20%. Today, genital herpes represents an even higher proportion of genital ulcers in the United States and other industrialized countries, even with a marked increase in early syphilis. In Asia and Africa, chancroid (Fig. 141-5) was once considered the most common type of genital ulcer, followed in frequency by primary syphilis and then genital herpes (Fig. 141-6). With increased efforts to control chancroid and syphilis and widespread use of broad-spectrum antibiotics to treat STI-related syndromes, together with more frequent recurrences or persistence of genital herpes attributable to HIV infec­ tion, PCR testing of genital ulcers now clearly implicates genital herpes as by far the most common cause of genital ulceration in most develop­ ing countries. LGV due to C. trachomatis (Fig. 141-7) and donovanosis (granuloma inguinale, due to Klebsiella granulomatis; see Fig. 178-1) continue to cause genital ulceration in some developing countries. LGV virtually disappeared in industrialized countries during the first 20 years of the HIV pandemic, but outbreaks are again occurring in

FIGURE 141-5  Chancroid: multiple, painful, punched-out ulcers with undermined borders on the labia occurring after autoinoculation. Europe (including the United Kingdom), in North America, and in Australia. In these outbreaks, LGV typically presents as proctitis, with or without anal lesions, in men who report unprotected receptive anal intercourse, very often in association with HIV and/or hepatitis C virus infection; the latter may be an acute infection acquired through the same exposure. Other causes of genital ulcers include (1) candidiasis and traumatized genital warts; (2) lesions due to genital involvement by more widespread dermatoses; (3) cutaneous manifestations of sys­ temic diseases such as genital mucosal ulceration in Stevens-Johnson syndrome or Behçet’s disease; (4) superinfections of lesions that may originally have been sexually acquired (for example, methicillinresistant S. aureus complicating a genital ulcer due to HSV-2); and (5) localized drug reactions, such as the ulcers occasionally seen with topi­ cal paromomycin cream or boric acid preparations. CHAPTER 141 Sexually Transmitted Infections: Overview and Clinical Approach
Diagnosis  Although most genital ulcerations cannot be diagnosed confidently on clinical grounds alone, clinical findings (Table 141-7) and epidemiologic considerations can guide initial management (Table 141-8) pending results of specific tests. Clinicians should order a rapid serologic test for syphilis in all cases of genital ulcer and treat presumptively while awaiting serology in a patient at increased FIGURE 141-6  Genital herpes. A relatively mild, superficial ulcer is typically seen in episodic outbreaks. (Courtesy of Michael Remington, University of Washington Virology Research Clinic.)

FIGURE 141-7  Lymphogranuloma venereum (LGV): striking tender lymphadenopathy occurring at the femoral and inguinal lymph nodes, separated by a groove made by Poupart’s ligament. This “sign-of-the-groove” is not considered specific for LGV; for example, lymphomas may present with this sign. epidemiologic risk (for example, MSM) or in pregnancy. To evaluate lesions except those highly characteristic of infection with HSV (i.e., those with herpetic vesicles), dark-field microscopy, direct immuno­ fluorescence, and a NAAT for T. pallidum can be useful but are rarely available. It is important to note that 30% of syphilitic chancres—the primary ulcer of syphilis—are associated with an initially nonreactive syphilis serology. All patients presenting with genital ulceration should be counseled and tested for HIV infection. PART 5 Infectious Diseases Typical vesicles or pustules or a cluster of painful ulcers preceded by vesiculopustular lesions suggest genital herpes. These typical clinical manifestations make detection of the virus optional; however, many patients want confirmation of the diagnosis, and differentiation of HSV-1 from HSV-2 has prognostic implications, because the latter causes more frequent genital recurrences and is more infectious to vulnerable sex partners. Painless, nontender, indurated ulcers with firm, nontender inguinal adenopathy suggest primary syphilis. If results of dark-field examina­ tion and a rapid serologic test for syphilis are initially negative, or if these tests are not available, presumptive therapy should be provided on the basis of the individual’s risk. With historically high rates of syphilis among MSM in the United States, therapy for this infection should not be withheld pending watchful waiting and/or subsequent TABLE 141-7  Clinical Features of Genital Ulcers FEATURE SYPHILIS HERPES CHANCROID Incubation period 9–90 days 2–7 days 1–14 days 3 days–6 weeks 1–4 weeks (up to 6 months) Early primary lesions Papule Vesicle Pustule Papule, pustule, or vesicle Papule Number of lesions Usually one Multiple Usually multiple, may coalesce Diameter 5–15 mm 1–2 mm Variable 2–10 mm Variable Edges Sharply demarcated, elevated, round, or oval Erythematous Undermined, ragged, irregular Depth Superficial or deep Superficial Excavated Superficial or deep Elevated Base Smooth, nonpurulent, relatively nonvascular Serous, erythematous, nonvascular Induration Firm None Soft Occasionally firm Firm Pain Uncommon Frequently tender Usually very tender Variable Uncommon Lymphadenopathy Firm, nontender, bilateral Firm, tender, often bilateral with initial episode Source: Reproduced with permission from RM Ballard, in KK Holmes et al (eds): Sexually Transmitted Diseases, 4th ed. New York, McGraw-Hill, 2008.

detection of seroconversion. Repeated serologic testing for syphilis 1 or 2 weeks after treatment of seronegative primary syphilis usually demonstrates seroconversion. “Atypical” or clinically trivial ulcers may be more common manifes­ tations of genital herpes than classic vesiculopustular lesions. Specific tests for HSV in such lesions are therefore indicated (Chap. 197). Com­ mercially available type-specific serologic tests for serum antibody to HSV-2 may give negative results, especially when patients present early with the initial episode of genital herpes or when HSV-1 is the cause of genital herpes. Furthermore, a positive test for antibody to HSV-2 does not prove that the current lesions are herpetic because nearly one-fifth of the general population of the United States becomes seropositive for HSV-2 during early adulthood. Although even “type-specific” tests for HSV-2 that are commercially available in the United States are not 100% specific, a positive HSV-2 serology does enable the clinician to tell the patient that they have probably had genital herpes, should learn to recognize symptoms, and should avoid sex during recurrences. In addition, because genital shedding and sexual transmission of HSV-2 often occur in the absence of symptoms and signs of recurrent herpetic lesions, persons who have a history of genital herpes or who are sero­ positive for HSV-2 should consider disclosure of serostatus to partners and the use of condoms or suppressive antiviral therapy, both of which can reduce the risk of HSV-2 transmission to a sexual partner. Demonstration of H. ducreyi by culture (or by PCR, where avail­ able) is most useful when ulcers are painful and purulent, especially if inguinal lymphadenopathy with fluctuance or overlying erythema is noted; if chancroid is prevalent in the community; or if the patient has recently had a sexual exposure elsewhere in a chancroid-endemic area (e.g., a developing country). Enlarged, fluctuant lymph nodes should be aspirated for culture or PCR to detect H. ducreyi as well as for Gram’s staining and culture to rule out the presence of other pyogenic bacteria. When genital ulcers persist beyond the natural history of initial episodes of herpes (2–3 weeks) or of chancroid or syphilis (up to 6 weeks) and do not resolve with syndrome-based antimicrobial therapy, then— in addition to the usual tests for herpes, syphilis, and chancroid— biopsy is indicated to exclude donovanosis as well as carcinoma and other nonvenereal dermatoses. TREATMENT Ulcerative Genital or Perianal Lesions Immediate syndrome-based treatment for acute genital ulcer (after collection of all necessary diagnostic specimens at the first visit) is often appropriate before all test results become available because patients with typical initial or recurrent episodes of genital or LYMPHOGRANULOMA VENEREUM DONOVANOSIS Usually one; often not detected, despite lymphadenopathy Variable Elevated, round, or oval Elevated, irregular Purulent, bleeds easily Variable, nonvascular Red and velvety, bleeds readily Tender, may suppurate, loculated, usually unilateral Tender, may suppurate, loculated, usually unilateral None; pseudobuboes

TABLE 141-8  Initial Management of Genital or Perianal Ulcer Causative Pathogens HSV Treponema pallidum (primary syphilis) Haemophilus ducreyi (chancroid) Usual Initial Laboratory Evaluation Dark-field examination (if available), direct FA, or PCR for T. pallidum RPR, VDRL, or EIA serologic test for syphilisa Culture, direct FA, ELISA, or PCR for HSV HSV-2-specific serology (consider) In chancroid-endemic area: PCR or culture for H. ducreyi Initial Treatment Herpes confirmed or suspected (history or sign of vesicles): Treat for genital herpes with acyclovir, valacyclovir, or famciclovir. Syphilis confirmed (dark-field, FA, or PCR showing T. pallidum, or RPR reactive): Benzathine penicillin (2.4 million units IM once to patient, to recent [e.g., within

3 months] seronegative partner[s], and to all recent partners)b Chancroid confirmed or suspected (diagnostic test positive, or HSV and syphilis excluded, and persistent lesion):   Ciprofloxacin (500 mg PO as single dose) or   Ceftriaxone (250 mg IM as single dose) or   Azithromycin (1 g PO as single dose) aIf results are negative but primary syphilis is suspected, treat presumptively when indicated by epidemiologic and sexual risk assessment; repeat in 1 week. bThe same treatment regimen is also effective in HIV-infected persons with early syphilis. Abbreviations: EIA, enzyme immunoassay; ELISA, enzyme-linked immunosorbent assay; FA, fluorescent antibody; HSV, herpes simplex virus; PCR, polymerase chain reaction; RPR, rapid plasma reagin; VDRL, Venereal Disease Research Laboratory. anorectal herpes can benefit from prompt oral antiviral therapy (Chap. 197); because early treatment of sexually transmitted causes of genital ulcers decreases further transmission; and because some patients do not return for test results and treatment. A thorough assessment of the patient’s sexual-risk profile and medical history is critical in determining the course of initial management. The patient who has risk factors consistent with exposure to syphilis (e.g., a male patient who reports sex with other men or who has HIV infection) should generally receive initial treatment for syphi­ lis. Empirical therapy for chancroid should be considered if there has been an exposure in an area of the world where chancroid occurs or if regional lymph node suppuration is evident. Finally, empirical antimicrobial therapy may be indicated if ulcers persist and the diagnosis remains unclear after a week of observation despite attempts to diagnose herpes, syphilis, and chancroid. ■ ■PROCTITIS, PROCTOCOLITIS, ENTEROCOLITIS, AND ENTERITIS Sexually acquired proctitis, with inflammation limited to the rectal mucosa (the distal 10–12 cm), results from direct rectal inoculation of typical STD pathogens. In contrast, inflammation extending from the rectum to the colon (proctocolitis), involving both the small and the large bowel (enterocolitis), or involving the small bowel alone (enteritis) can result from ingestion of typical intestinal pathogens through oral– anal exposure during sexual contact. Anorectal pain and mucopuru­ lent, bloody rectal discharge suggest proctitis or proctocolitis. Proctitis commonly produces tenesmus (causing frequent attempts to defecate, but not true diarrhea) and constipation, whereas proctocolitis and enterocolitis more often cause true diarrhea. In all three conditions, anoscopy usually shows mucosal exudate and easily induced mucosal bleeding (i.e., a positive “wipe test”), sometimes with petechiae or mucosal ulcers. Exudate should be sampled for Gram’s staining and other microbiologic studies. Sigmoidoscopy or colonoscopy shows inflammation limited to the rectum in proctitis or disease extending at least up into the sigmoid colon in proctocolitis. Acquisition of HSV, N. gonorrhoeae, or C. trachomatis (including LGV strains of C. trachomatis) during receptive anorectal intercourse

causes most cases of infectious proctitis in women and MSM. Primary and secondary syphilis can also produce anal or anorectal lesions, with or without symptoms. Gonococcal or chlamydial proctitis typically involves the most distal rectal mucosa and the anal crypts and is clini­ cally mild, without systemic manifestations. In contrast, primary proc­ titis due to HSV and proctocolitis due to the strains of C. trachomatis that cause LGV usually produce severe anorectal pain and often cause fever. Perianal ulcers and inguinal lymphadenopathy, most commonly due to HSV, can also occur with LGV or syphilis. Sacral nerve root radiculopathies, usually presenting as urinary retention, laxity of the anal sphincter, or constipation, may complicate primary herpetic proc­ titis. In LGV, rectal biopsy typically shows crypt abscesses, granulomas, and giant cells—findings resembling those in Crohn’s disease; such findings should always prompt rectal culture and serology for LGV, which is a curable infection. Syphilis can also produce rectal granu­ lomas, usually in association with infiltration by plasma cells or other mononuclear cells. Syphilis, LGV, and HSV infection involving the rectum can produce perirectal adenopathy that is sometimes mistaken for malignancy; syphilis, LGV, HSV infection, and chancroid involving the anus can produce inguinal adenopathy because anal lymphatics drain to inguinal lymph nodes.

Diarrhea and abdominal bloating or cramping pain without ano­ rectal symptoms and with normal findings on anoscopy and sigmoid­ oscopy occur with inflammation of the small intestine (enteritis) or with proximal colitis. In MSM without HIV infection, enteritis is often attributable to Giardia lamblia. Sexually acquired proctocolitis is most often due to Campylobacter or Shigella species. CHAPTER 141 TREATMENT Proctitis, Proctocolitis, Enterocolitis, and Enteritis Acute proctitis in persons who have practiced receptive anorec­ tal intercourse is usually sexually acquired. Such patients should undergo anoscopy to detect rectal ulcers or vesicles and petechiae after swabbing of the rectal mucosa; to examine rectal exudates for PMNs and gram-negative diplococci; and to obtain rectal swab specimens for testing for rectal gonorrhea, chlamydial infection, herpes, and syphilis. Pending test results, patients with proctitis should receive empirical syndromic treatment—e.g., with ceftriax­ one (a single IM dose of 500 mg for gonorrhea) plus doxycycline (100 mg by mouth twice daily for 7 days for possible chlamydial infection) plus treatment for herpes or syphilis if indicated. If LGV proctitis is proven or suspected, the recommended treatment is doxycycline (100 mg by mouth twice daily for 21 days); alterna­ tively, 1 g of azithromycin once a week for 3 weeks is likely to be effective but is little studied. Sexually Transmitted Infections: Overview and Clinical Approach
PREVENTION AND CONTROL OF STIs Prevention and control of STIs require the following:

  1. Reduction of the average rate of sexual exposure to STIs through alteration of sexual risk behaviors and behavioral norms among both susceptible and infected persons in all population groups. The necessary changes include reduction in the total number of sexual partners and the number of concurrent sexual partners. The U.S. Preventive Services Task Force recommends intensive behavioral counseling for all sexually active adolescents and adults who are at increased risk for STIs (grade B recommendation). Motivational interviewing is one approach that has elicited behavioral changes, including safer sex practices and more consistent contraception, that contribute to these goals.
  2. Reduction of the efficiency of transmission through the promo­ tion of safer sexual practices, the use of condoms during casual or commercial sex, vaccination against HBV and HPV infection, male circumcision (which reduces risk of acquisition of HIV infec­ tion, chancroid, and perhaps other STIs), and a growing number of other approaches (e.g., early detection and treatment of other STIs to reduce the efficiency of sexual transmission of HIV; provision of

Number whose behaviors and ecologic settings result in exposure to STDs Number who acquire STDs Number who develop symptoms of STDs Number who perceive the symptoms of STDs Number who promptly seek medical care when symptomatic Number seeking care who have ready access to care Number perceived by clinicians as possibly having STDs Number perceived as possibly having STDs who can be tested for STDs Number with objective evidence of STDs who get proper treatment for STDs Number who comply with treatment Number whose partners are treated and who are not reinfected FIGURE 141-8  Critical control points for preventive and clinical interventions against sexually transmitted diseases (STDs). (Adapted from HT Waller and MA Piot: Bull World Health Organ 41:75, 1969 and 43:1, 1970; and from “Resource allocation model for public health planning—a case study of tuberculosis control,” Bull World Health Organ 48[Suppl], 1973.) doxycycline for PEP). Among MSM and transgender women, doxy­ cycline PEP (200 mg taken once orally within 72 h of condomless sex, with a maximum dose of 200 mg each day) can reduce the risk of chlamydia, syphilis, and, in some studies, gonorrhea. However, studies have failed to demonstrate efficacy among cisgender women. 3. Shortening of the duration of infectivity of STIs through early detec­ PART 5 Infectious Diseases tion and curative or suppressive treatment of patients and their sexual partners. Financial and time constraints imposed by many clinical practices, along with the reluctance of some clinicians to ask questions about stigmatized sexual behaviors, often curtail screening and prevention services. As outlined in Fig. 141-8, the success of clinicians’ efforts to detect and treat STIs depends in part on societal efforts to teach young people how to recognize symptoms of STIs; to motivate individuals with symptoms to seek care promptly; to educate persons who are at risk but have no symptoms about what tests they should undergo routinely; and to make high-quality, appropriate care accessible, afford­ able, and acceptable, especially to the young indigent patients most likely to acquire an STI. STI RISK ASSESSMENT Because many infected individuals develop no symptoms or fail to recognize and report symptoms, clinicians should routinely perform an STI risk assessment for teenagers and young adults as a guide to selec­ tive screening. As stated earlier, the U.S. Preventive Services Task Force recommends screening sexually active female patients ≤25 years of age for C. trachomatis whenever they present for health care (at least once a year); older women should be tested if they have more than one sexual partner, have begun a new sexual relationship since the previous test, or have another STI diagnosed. In women 25–29 years of age, chlamydial infection is uncommon but still may reach a prevalence of 3–5% in some settings; information provided by women in this age group on a sex partner’s concurrency (whether a male partner has had another sex partner during the time they have been together) is helpful in iden­ tifying women at increased risk. In some regions of the United States, widespread selective screening and treatment of young women for cervical C. trachomatis infection have been associated with a 50–60% drop in prevalence. Such screening and treatment also protect the individual woman from PID. Sensitive urine-based genetic amplifica­ tion tests permit expansion of screening to men, teenage boys, and girls in settings where examination is not planned or is impractical (e.g.,

during preparticipation sports examinations or during initial medical evaluation of adolescent girls). Vaginal swabs—collected either by the health care provider at a pelvic examination or by the woman herself— are highly sensitive and specific for the diagnosis of chlamydial and gonococcal infection; they are now the preferred type of specimen for screening and diagnosis of these infections. Although gonorrhea is now substantially less common than chla­ mydial infection in women in industrialized countries, screening tests for N. gonorrhoeae are still appropriate for women and teenage girls attending STD clinics and for sexually active teens and young women from areas of high gonorrhea prevalence. Multiplex NAATs that com­ bine screening for N. gonorrhoeae and C. trachomatis—and, more recently, for T. vaginalis—in a single low-cost assay now facilitate the prevention and control of these infections for populations at high risk. All patients who have newly detected STIs or are at high risk for STIs according to routine risk assessment as well as all pregnant women should be encouraged to undergo serologic testing for syphilis and HIV infection, with appropriate counseling. Randomized trials have shown that risk-reduction counseling of patients with STIs significantly lowers subsequent risk of acquiring an STI; such counseling should now be considered a standard component of STI management. Preimmuniza­ tion serologic testing for antibody to HBV is indicated for unvaccinated persons who are known to be at high risk, such as MSM and people who use injection drugs. In most young persons, however, it is more cost-effective to vaccinate against HBV without serologic screening. It is important to recognize that, while immunization against HBV has contributed to marked reductions in the incidence of infection with this virus, the majority of new cases that occur are acquired through sex. In 2006, the CDC’s Advisory Committee on Immunization Prac­ tices (ACIP) recommended the following: (1) Universal hepatitis B vaccination should be implemented for all unvaccinated adults in set­ tings in which a high proportion of adults have risk factors for HBV infection (e.g., STD clinics, HIV testing and treatment facilities, drugabuse treatment and prevention settings, health care settings targeting services to injection drug users or MSM, and correctional facilities). (2) In other primary care and specialty medical settings that provide care to adults at risk for HBV infection, health care providers should inform all patients about the health benefits of vaccination, the risk factors for HBV infection, and the persons for whom vaccination is recommended; they should vaccinate adults who report risk factors for HBV infection as well as any adult who requests protection from HBV infection. To promote vaccination in all settings, health care providers should implement standing orders to identify adults recommended for hepatitis B vaccination, should administer hepatitis B vaccine as part of routine clinical services, should not require acknowledgment of an HBV infection risk factor for adult vaccination, and should use available reimbursement mechanisms to remove financial barriers to hepatitis B vaccination. In 2007, the ACIP made its first recommendation for routine immu­ nization of 9- to 26-year-old girls and women with the quadrivalent HPV vaccine (against HPV types 6, 11, 16, and 18). In 2011, the ACIP recommended routine administration of quadrivalent HPV vaccine to boys at 11 or 12 years of age and to males 13–21 years of age who have not yet been vaccinated or who have not completed the three-dose vac­ cine series; HBV vaccination of men 22–26 years of age has also been recommended. Since that time, a nonavalent HPV vaccine has become available and has largely replaced the earlier vaccines. Moreover, single-dose HPV vaccine has great promise to simplify implementation without compromising efficacy. The optimal age for recommended vaccination is 11–12 years because of the very high risk of HPV infec­ tion after sexual debut. Partner management is the process of identifying and informing partners of infected patients about possible exposure to an STI and of examining, testing, vaccinating, and treating partners as appropriate. In a series of 22 reports concerning partner notification during the 1990s, index patients with gonorrhea or chlamydial infection named a mean of 0.75–1.6 partners, of whom one-fourth to one-third were infected; those with syphilis named 1.8–6.3 partners, with one-third to one-half infected; and those with HIV infection named 0.76–5.31 partners, with

21 - 142 Encephalitis

142 Encephalitis

up to one-fourth infected. Persons who transmit infection or who have recently been infected and are still in the incubation period usually have no symptoms or only mild symptoms and seek medical attention only when notified of their exposure. Therefore, the clinician must encourage patients to participate in partner notification, must ensure that exposed persons are notified and treated, and must guarantee confidentiality to all involved. In the United States, local health departments often offer assistance in partner notification, treatment, and/ or counseling. It seems both feasible and most useful to notify those partners exposed within the patient’s likely period of infectiousness, which is often considered the preceding 1 month for gonorrhea, 1–2 months for chlamydial infection, and up to 3 months for early syphilis. Persons with a new-onset STI have a source contact who gave them the infection; in addition, they may have a secondary (spread or exposed) contact with whom they had sex after becoming infected. The identification and treatment of these two types of contacts have different objectives. Treatment of the source contact (often a casual contact) benefits the community by preventing further transmission and benefits the source contact; treatment of the recently exposed secondary contact (typically a spouse or another steady sexual partner) prevents the development of serious complications (such as PID) in the partner, reinfection of the index patient, and further spread of infection. A randomized trial compared patients’ delivery of therapy to partners exposed to gonorrhea or chlamydial infection with conventional notification and advice to partners to seek evaluation for STD; patients’ delivery of partners’ therapy, also known as expedited partner therapy (EPT), significantly reduced combined rates of reinfection of the index patient with N. gonorrhoeae or C. trachomatis. EPT, which is now commonly used by many practicing physicians, is currently permissible in 46 states and potentially allowable in the remaining four. (Updated information on the legal status of EPT is available at https://www .cdc.gov/sti/hcp/clinical-guidance/expedited-partner-therapy.html?CDC_ AAref_Val=https://www.cdc.gov/std/ept/.) In summary, clinicians and public health agencies share responsibility for the prevention and control of STIs. In the current health care environment, the role of primary care clinicians has become increasingly important in STI prevention as well as in diagnosis and treatment, and the resurgence of bacterial STIs like congenital syphilis—particularly in the setting of HIV co-infection—emphasizes the need for risk assessment and routine screening. Acknowledgment The author wishes to acknowledge King K. Holmes, MD, PhD, for his valuable contributions to this chapter in prior editions. ■ ■FURTHER READING Cannon CA et al: On the horizon: Novel approaches to sexually transmitted infection prevention. Med Clin North Am 108:403, 2024. Gottlieb SL et al: Advancing vaccine development for gonorrhoea and the Global STI Vaccine Roadmap. Sex Health 16:426, 2019. Johnston C, Corey L: Current concepts for genital herpes simplex virus infection: Diagnostics and pathogenesis of genital tract shedding. Clin Microbiol Rev 29:149, 2016. Kirkcaldy RD et al: Neisseria gonorrhoeae antimicrobial resistance among men who have sex with men and men who have sex exclusively with women: The Gonococcal Isolate Surveillance Project, 2005–2010. Ann Intern Med 158:321, 2013. Mlisana K et al: Symptomatic vaginal discharge is a poor predictor of sexually transmitted infections and genital tract inflammation in high-risk women in South Africa. J Infect Dis 206:6, 2012. Price MJ et al: Risk of pelvic inflammatory disease following Chlamydia trachomatis infection: Analysis of prospective studies with a multistate model. Am J Epidemiol 178:484, 2013. Tuddenham S et al: Diagnosis and treatment of sexually transmitted infections: A review. JAMA 327:161, 2022. Unemo M et al: Sexually transmitted infections: Challenges ahead. Lancet Infect Dis 17:e235, 2017. U.S. Preventive Services Task Force: Screening for chlamydia and gonorrhea: US Preventive Services Task Force Recommendation Statement. JAMA 326:949, 2021.

U.S. Preventive Services Task Force: Behavioral counseling inter-

ventions to prevent sexually transmitted infections. JAMA 324:674, 2020. Wiesenfeld HC et al: A randomized controlled trial of ceftriaxone and doxycycline, with or without metronidazole, for the treatment of acute pelvic inflammatory disease. Clin Infect Dis 13:ciaa101, 2020. Workowski KA, Bachmann L: Sexually transmitted disease treatment guidelines, 2021. MMWR Recomm Rep 70:1, 2021. Karen L. Roos, Michael R. Wilson,
Kenneth L. Tyler

Encephalitis ■ ■DEFINITION Encephalitis is defined as an inflammation of the brain caused either by infection, usually with a virus, or from a primary autoimmune process. This chapter will focus on infectious causes of encephalitis. Many patients with encephalitis also have evidence of associated meningitis (meningoencephalitis) and, in some cases, involvement of the spinal cord or nerve roots (encephalomyelitis, encephalomyeloradiculitis). CHAPTER 142 ■ ■CLINICAL MANIFESTATIONS Similar to meningitis, encephalitis is typically an acute febrile illness. The patient with encephalitis commonly has an altered state of consciousness (confusion, behavioral abnormalities), or a depressed level of consciousness ranging from mild lethargy to coma, and evidence of either focal or diffuse neurologic signs and symptoms. Patients with encephalitis may have hallucinations, agitation, personality change, behavioral disorders, and, at times, a frankly psychotic state. Focal or generalized seizures occur in many patients with encephalitis. Virtually every possible type of focal neurologic disturbance has been reported in viral encephalitis; the signs and symptoms reflect the sites of infection and inflammation. The most commonly encountered focal findings are aphasia, ataxia, upper or lower motor neuron patterns of weakness, involuntary movements (e.g., myoclonic jerks, tremor), and cranial nerve deficits (e.g., ocular palsies, facial weakness). Involvement of the hypothalamic-pituitary axis may result in temperature dysregulation, diabetes insipidus, or the development of the syndrome of inappropriate secretion of antidiuretic hormone (SIADH). Even though neurotropic viruses typically cause injury in distinct regions of the central nervous system (CNS), variations in clinical presentations make it impossible to reliably establish the etiology of a specific case of encephalitis on clinical grounds alone (see “Differential Diagnosis,” below). Encephalitis ■ ■ETIOLOGY In the United States, there are an estimated ~20,000 cases of encephalitis per year, although the actual number of cases is likely to be significantly higher. Despite comprehensive diagnostic efforts, most cases of acute encephalitis with a suspected viral etiology remain of unknown cause. Hundreds of viruses are capable of causing encephalitis, although only a limited subset is responsible for most cases in which a specific cause is identified (Table 142-1). The most commonly identified viruses causing sporadic cases of acute encephalitis in immunocompetent adults are herpesviruses (herpes simplex virus [HSV] [Chap. 197], varicella-zoster virus [VZV] [Chap. 198], and EpsteinBarr virus [EBV] [Chap. 199]). Epidemics of encephalitis are caused by arboviruses (Chap. 215), which belong to several different viral taxonomic groups including Alphaviruses (e.g., eastern equine encephalitis [EEE] virus and chikungunya virus), Flaviviruses (e.g., West Nile virus [WNV], St. Louis encephalitis virus, Japanese encephalitis virus,

TABLE 142-1  Viruses Causing Acute Encephalitis in North America COMMON LESS COMMON Herpesviruses   Cytomegalovirusa Rabies Eastern equine encephalitis virus Powassan virus Cytomegalovirusa   Herpes simplex virus 1b   Herpes simplex virus 2   Human herpesvirus 6   Varicella-zoster virus   Epstein-Barr virus Colorado tick fever virus Mumps Jamestown Canyon virus Arthropod-borne viruses   La Crosse virus   West Nile virusc   St. Louis encephalitis virus   Zika Enteroviruses aImmunocompromised host. bThe most common cause of sporadic encephalitis. cThe most common cause of epidemic encephalitis. Powassan virus, Zika virus, dengue virus, and tick-borne encephalitis virus), and Bunyaviruses (e.g., California encephalitis virus serogroup, La Crosse virus, Jamestown Canyon virus). Historically, the largest number of cases of arbovirus encephalitis in the United States has been due to St. Louis encephalitis virus and the California encephalitis virus serogroup. However, since 2002, WNV has been responsible for the majority of arbovirus meningitis and encephalitis cases in the United States. WNV caused 28,684 confirmed cases of neuroinvasive disease (encephalitis, meningitis, or myelitis) in the years 1999–2022 with 2641 deaths. In 2023, there were 1599 reported cases of neuroinvasive disease (encephalitis, meningitis, acute flaccid paralysis). The majority of cases occur in August and September. It is important to recognize that WNV epidemics are unpredictable and that cases have occurred in every state in the continental United States. Since 2006, there have been increasing numbers of cases of the tick-borne Powassan virus primarily in the northeastern United States and Minnesota and Wis­ consin. New causes of viral CNS infections are constantly appearing, as evidenced by multiple outbreaks of cases of encephalitis in Southeast Asia caused by Nipah virus, a member of the Paramyxoviridae family; meningitis in Europe caused by Toscana virus, an arbovirus belonging to the Bunyavirus family; neurologic disorders associated with Zika virus, a flavivirus, in South America; and neurologic disorders associ­ ated with major epidemics of chikungunya virus, a togavirus, in Africa, India, and Southeast Asia. Dengue virus is common in >100 countries worldwide with cases on the rise in the Caribbean and Puerto Rico and rare cases reported in the United States in Florida and in south­ ern Texas. Parechoviruses including human parechovirus 3 (HPeV3), members of the Picornavirus family, have been reported as causes of fever, sepsis, and meningitis in infants (age <3 months) in the United States and abroad. PART 5 Infectious Diseases ■ ■LABORATORY DIAGNOSIS CSF Examination  Cerebrospinal fluid (CSF) examination should be performed in all patients with suspected viral encephalitis unless contraindicated by the presence of severely increased intracranial pressure (ICP). Ideally, at least 20 mL of the initial CSF sample should be collected, with 5–10 mL stored frozen for later studies, including additional direct detection tests like virus-specific polymerase chain reaction (PCR) or metagenomic next-generation sequencing, since many neuroinvasive viruses are only transiently present in the CSF. The characteristic CSF profile is indistinguishable from that of viral men­ ingitis (Chap. 143) and typically consists of a lymphocytic pleocytosis, a mildly elevated protein concentration, and a normal glucose concen­ tration. A CSF pleocytosis (>5 cells/μL) occurs in >95% of immuno­ competent patients with documented viral encephalitis. In rare cases, a pleocytosis may be absent on the initial lumbar puncture (LP) but present on subsequent LPs. Patients who are severely immunocompro­ mised by HIV infection, glucocorticoid or other immunosuppressant

drugs, chemotherapy, or lymphoreticular malignancies may fail to mount a CSF inflammatory response. CSF cell counts exceed 500/μL in only about 10% of patients with encephalitis. Infections with certain arboviruses (e.g., EEE virus or California encephalitis virus), mumps, and lymphocytic choriomeningitis virus (LCMV) may occasionally result in cell counts >1000/μL, but this degree of pleocytosis should suggest the possibility of nonviral infections or other inflammatory processes. Atypical lymphocytes in the CSF may be seen in EBV infec­ tion and less commonly with other viruses, including cytomegalovirus (CMV), HSV, and enteroviruses. Increased numbers of plasmacy­ toid or Mollaret-like large mononuclear cells have been reported in WNV encephalitis. Polymorphonuclear pleocytosis occurs in ~45% of patients with WNV encephalitis and is also a common feature in CMV myeloradiculitis in immunocompromised patients. Large numbers of CSF polymorphonuclear leukocytes may be present in patients with encephalitis due to EEE virus, echovirus 9, and, more rarely, other enteroviruses. However, persisting CSF neutrophilia should prompt consideration of bacterial infection, leptospirosis, amebic infection, and noninfectious processes such as acute hemorrhagic leukoencepha­ litis (Chap. 456). About 20% of patients with encephalitis will have a significant number of red blood cells (>500/μL) in the CSF in a non­ traumatic tap. The pathologic correlate of this finding may be punctate microhemorrhages of the type seen with HSV; however, CSF red blood cells occur with similar frequency and in similar numbers in patients with nonherpetic focal encephalitides. A decreased CSF glucose con­ centration is distinctly unusual in viral encephalitis and should suggest the possibility of bacterial, fungal, tuberculous, parasitic, leptospiral, syphilitic, sarcoid, or neoplastic meningitis. Rare patients with mumps, LCMV, VZV, or advanced HSV encephalitis and many patients with CMV myeloradiculitis have low CSF glucose concentrations. ■ ■CSF POLYMERASE CHAIN REACTION CSF PCR has become the primary diagnostic test for CNS infections caused by HSV, CMV, EBV, HHV-6, and enteroviruses. In the case of VZV CNS infection, CSF PCR and detection of virus-specific IgM or intrathecal antibody synthesis both provide important aids to diag­ nosis. The sensitivity and specificity of CSF PCRs vary with the virus being tested. The sensitivity (~96%) and specificity (~99%) of HSV CSF PCR are equivalent to or exceed those of brain biopsy. It is impor­ tant to recognize that HSV CSF PCR results need to be interpreted after considering the likelihood of disease in the patient being tested, the timing of the test in relationship to onset of symptoms, and the prior use of antiviral therapy. A negative HSV CSF PCR test performed by a qualified laboratory at the appropriate time during illness in a patient with a high likelihood of HSV encephalitis based on clinical and laboratory abnormalities significantly reduces the likelihood of HSV encephalitis but does not exclude it. For example, in a patient with a pretest probability of 35% of having HSV encephalitis, a nega­ tive HSV CSF PCR reduces the posttest probability to ~2%, and for a patient with a pretest probability of 60%, a negative test reduces the posttest probability to ~6%. In both situations, a positive test makes the diagnosis almost certain (98–99%). There have been reports of initially negative HSV CSF PCR tests that were obtained early (≤72 h) follow­ ing symptom onset and that became positive when repeated 1–3 days later. The frequency of positive HSV CSF PCRs in patients with herpes encephalitis also decreases as a function of the duration of illness, with only ~20% of cases remaining positive after ≥14 days. PCR results are generally not affected by ≤1 week of antiviral therapy. In one study, 98% of CSF specimens remained PCR positive during the first week of antiviral therapy, but the numbers fell to ~50% by 8–14 days and to ~21% by >15 days after initiation of antiviral therapy. The sensitivity and specificity of CSF PCR tests for viruses other than HSV have not been definitively characterized. Enteroviral (EV) CSF RT-PCR appears to have a sensitivity and specificity of >95%. EV RT-PCR sensitivity for EV-A71 may be considerably lower (~30% in some reports). Patients with EV-D68-associated acute flaccid myelitis (AFM) only rarely have a positive CSF RT-PCR (<3%) but may have a positive test on nasopharyngeal swab specimens. Parechoviruses are also not detected by standard EV RT-PCRs. The specificity of EBV

CSF PCR has not been established. Positive EBV CSF PCRs associ­ ated with positive tests for other pathogens have been reported and may reflect reactivation of EBV latent in lymphocytes that enter the CNS as a result of an unrelated infectious or inflammatory process. In patients with CNS infection due to VZV, CSF antibody and PCR stud­ ies should be considered complementary because patients may have evidence of intrathecal synthesis of VZV-specific antibodies and nega­ tive CSF PCRs. In the case of WNV infection, CSF PCR appears to be less sensitive than detection of WNV-specific CSF IgM, although PCR testing remains useful in immunocompromised patients who may not mount an effective anti-WNV antibody response. The recent pandemic due to SARS-CoV-2 (COVID-19) has been associated with cases of encephalopathy due to the indirect effects on the nervous system of multiorgan system failure and/or to a hyperinflammatory syndrome and disseminated intravascular coagulation, but also with rare cases of true encephalitis caused by viral CNS invasion. In both sets of patients, nasopharyngeal reverse transcriptase (RT)-PCR tests for SARS-CoV-2 are positive, but only cases with encephalitis have a positive CSF RTPCR for SARS-CoV-2. Rare cases of neuroinvasion by SARS-CoV-2 has also been detected by RT-PCR of brain tissue. Unbiased metagenomic sequencing technologies capable of identi­ fying infectious genomes in CSF, brain, and other tissues have recently shown great promise for rapid diagnosis of obscure cases of encepha­ litis and other brain infections, especially in immunocompromised patients. CSF Culture  CSF culture is generally of limited utility in the diag­ nosis of acute viral encephalitis. Culture may be insensitive (e.g., >95% of patients with HSV encephalitis have negative CSF cultures, as do virtually all patients with EBV-associated CNS disease) and often takes too long to significantly affect immediate therapy. Serologic Studies and Antigen Detection  For many arbovi­ ruses including WNV, serologic studies remain important diagnostic tools. Serum antibody determination is less useful for viruses with high seroprevalence rates in the general population such as HSV, VZV, CMV, and EBV. For viruses with low seroprevalence rates, diagnosis of acute viral infection can be made by documenting seroconversion between acute-phase and convalescent sera (typically obtained after 2–4 weeks) or by demonstrating the presence of virus-specific IgM antibodies. For viruses with high seroprevalence such as VZV and HSV, demonstration of synthesis of virus-specific antibodies in CSF, as shown by an increased IgG index or the presence of CSF IgM anti­ bodies, may be useful and can provide presumptive evidence of CNS infection. Unfortunately, the delay between onset of infection and the host’s generation of a virus-specific antibody response often means that serologic data are useful mainly for the retrospective establishment of a diagnosis, rather than in aiding acute diagnosis or management. In patients with HSV encephalitis, antibodies to HSV-1 glycopro­ teins and HSV glycoprotein antigens have been detected in the CSF. Optimal detection of both HSV antibodies and antigen typically occurs after the first week of illness, limiting the utility of these tests in acute diagnosis. Nonetheless, HSV CSF antibody testing is of value in selected patients whose illness is >1 week in duration and who are CSF PCR negative for HSV. In the case of VZV infection, CSF IgM antibody tests may be positive when PCR fails to detect viral DNA, and both tests should be considered complementary rather than mutually exclusive. Demonstration of CSF WNV IgM antibodies is diagnostic of WNV encephalitis because the high molecular weight of IgM antibodies restricts their passage from serum to CSF through the blood-brain barrier and their presence in CSF is therefore indicative of intrathecal synthesis. Timing of antibody testing may be important because the rate of CSF WNV IgM seropositivity increases during the first week after illness onset, reaching 80% or higher on day 7 after symptom onset. Although serum and CSF IgM antibodies generally persist for only a few months after acute infection, there are exceptions to this rule, and WNV serum IgM has been shown to persist in some patients for >1 year following acute infection.

MRI, CT, and EEG  Patients with suspected encephalitis almost invariably undergo neuroimaging studies and often electroencephalo­ gram (EEG). These tests help identify or exclude alternative diagnoses and assist in the differentiation between a focal and a diffuse encepha­ litic process. Specific focal findings in a patient with encephalitis should always raise the possibility of HSV encephalitis. Examples of focal findings found in HSV encephalitis include: (1) areas of increased signal intensity in the frontotemporal, cingulate, or insular regions of the brain on T2-weighted, fluid-attenuated inversion recovery (FLAIR), or diffusion-weighted magnetic resonance imaging (MRI) (Fig. 142-1); (2) focal areas of low absorption, mass effect, and contrast enhancement in frontotemporal areas on computed tomography (CT); or (3) periodic focal temporal lobe spikes on a background of slow or low-amplitude (“flattened”) activity on EEG. Approximately 10% of patients with PCR-documented HSV encephalitis may have a normal MRI, although nearly 80% will have asymmetric abnormalities in the temporal lobe, and an additional 10% in extratemporal regions. The addition of FLAIR and diffusion-weighted images to the standard MRI sequences enhances sensitivity. Children with HSV encephalitis may have atypical patterns of MRI lesions and often show involvement of brain regions outside the frontotemporal areas. CT is less sensitive than MRI and is normal in up to 20–35% of patients. EEG abnormalities occur in >75% of PCR-documented cases of HSV encephalitis; they typically involve the temporal lobes but are often nonspecific. Some patients with HSV encephalitis have a distinctive EEG pattern consist­ ing of periodic, stereotyped, sharp-and-slow complexes originating in one or both temporal lobes and repeating at regular intervals of 2–3 s. The periodic complexes are typically noted between days 2 and 15 of the illness and are present in two-thirds of pathologically proven cases of HSV encephalitis.

CHAPTER 142 Significant MRI abnormalities are found in only approximately two-thirds of patients with WNV encephalitis, a frequency less than that found with HSV encephalitis. When present, abnormalities often involve deep brain structures, including the thalamus, basal ganglia, and brainstem, rather than the cortex, and may only be apparent on T2/FLAIR images. Similar MRI patterns can be observed in patients infected with other arboviruses, including other flaviviruses such as Japanese encephalitis virus and St. Louis encephalitis virus, as well the Alphavirus EEE virus. EEGs in patients with WNV encephalitis typi­ cally show generalized slowing that may be more anteriorly prominent rather than the temporally predominant pattern of sharp or periodic discharges more characteristic of HSV encephalitis. Patients with VZV encephalitis may show multifocal areas of hemorrhagic and ischemic infarction, reflecting the tendency of this virus to produce a CNS vasculopathy rather than a true encephalitis. Immunocompromised Encephalitis FIGURE 142-1  Coronal fluid-attenuated inversion recovery (FLAIR) magnetic resonance image from a patient with herpes simplex encephalitis. Note the area of increased signal in the right temporal lobe (left side of image) confined predominantly to the gray matter. This patient had predominantly unilateral disease; bilateral lesions are more common but may be quite asymmetric in their intensity.

TABLE 142-2  Use of Diagnostic Tests in Encephalitis The best test for WNV encephalitis is the CSF IgM antibody test. The prevalence of positive CSF IgM tests increases by about 10% per day after illness onset and reaches 70–80% by the end of the first week. Serum WNV IgM can provide evidence for recent WNV infection, but in the absence of other findings does not establish the diagnosis of neuroinvasive disease (meningitis, encephalitis, acute flaccid paralysis). Approximately 80% of patients with proven HSV encephalitis have MRI abnormalities involving the temporal lobes. This percentage likely increases to >90% when FLAIR and diffusion-weighted MRI sequences are also used. The absence of temporal lobe lesions on MRI reduces the likelihood of HSV encephalitis and should prompt consideration of other diagnostic possibilities. The CSF HSV PCR test may be negative in the first 72 h of symptoms of HSV encephalitis. A repeat study should be considered in patients with an initial early negative PCR in whom diagnostic suspicion of HSV encephalitis remains high and no alternative diagnosis has yet been established. Detection of intrathecal synthesis (increased CSF/serum HSV antibody ratio corrected for breakdown of the blood-brain barrier) of HSV-specific antibody may be useful in diagnosis of HSV encephalitis in patients in whom only late (>1 week after onset) CSF specimens are available and PCR studies are negative. Serum serology alone is of no value in diagnosis of HSV encephalitis due to the high seroprevalence rate in the general population. Negative CSF viral cultures are of no value in excluding the diagnosis of HSV or EBV encephalitis. VZV CSF IgM antibodies may be present in patients with a negative VZV CSF PCR. Both tests should be performed in patients with suspected VZV CNS disease. The specificity of EBV CSF PCR for diagnosis of CNS infection is unknown. Positive tests may occur in patients with a CSF pleocytosis due to other causes. Detection of EBV CSF IgM or intrathecal synthesis of antibody to VCA supports the diagnosis of EBV encephalitis. Serologic studies consistent with acute EBV infection (e.g., IgM VCA, presence of antibodies against EA but not against EBNA) can help support the diagnosis. In addition to broad-based PCR assays for bacterial and fungal infections, metagenomic next-generation sequencing (mNGS) allows for unbiased detection of nucleic acids from the whole range of infectious agents (except prions), which can then be confirmed by independent pathogen-specific techniques. Due to the sensitivity of this technology, there is a risk of false-positive results. As this technology becomes refined and the turnaround time faster, mNGS is likely to become a routine test on CSF for the diagnosis of encephalitis. PART 5 Infectious Diseases Abbreviations: CNS, central nervous system; CSF, cerebrospinal fluid; DWI, diffusion-weighted imaging; EA, early antigen; EBNA, EBV-associated nuclear antigen; EBV, Epstein-Barr virus; FLAIR, fluid-attenuated inversion recovery; HSV, herpes simplex virus; IgM, immunoglobulin M; MRI, magnetic resonance imaging; PCR, polymerase chain reaction; VCA, viral capsid antibody; VZV, varicella-zoster virus; WNV, West Nile virus. adult patients with CMV often have enlarged ventricles with areas of increased T2 signal on MRI outlining the ventricles and subependymal enhancement on T1-weighted postcontrast images. Prominent cerebel­ lar T2/FLAIR abnormalities have been observed with Powassan virus encephalitis and in children with herpesviruses like EBV and VZV. Table 142-2 highlights specific diagnostic test results in encephalitis that can be useful in clinical decision-making. Brain Biopsy  Brain biopsy is now generally reserved for patients in whom CSF PCR studies fail to lead to a specific diagnosis and who have focal abnormalities on MRI, no serologic evidence of autoimmune disease, and continue to show progressive clinical deterioration despite treatment with acyclovir and supportive therapy. ■ ■DIFFERENTIAL DIAGNOSIS Infection by a variety of other organisms can mimic viral encephalitis. In studies of biopsy-proven HSV encephalitis, common infectious mimics of focal viral encephalitis included mycobacteria, fungi, rick­ ettsiae, Listeria, Mycoplasma, and other bacteria (including Bartonella sp.) as well as neurosyphilis. There are an increasing number of anti­ bodies reported that cause autoimmune encephalitis and mimic those caused by viral infection, including those associated with antibodies against N-methyl-d-aspartate (NMDA) receptor, two components of the voltage-gated potassium channels/leucine-rich glioma inactivated protein-1 (LGI-1) and contracting-associated protein-like 2 (CASPR2),

α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), γ-aminobutyric acid (GABA) receptors, and glutamic acid decarboxyl­ ase (GAD 65) (Chap. 99). In most cases, diagnosis is made by detection of the specific autoantibodies in serum and/or CSF. NMDA receptor antibodies have been reported in up to 25% of patients who have recov­ ered from HSV encephalitis, and their presence should not exclude appropriate testing and treatment for HSV encephalitis. The develop­ ment of NMDA receptor antibodies in patients with HSV encephalitis may contribute to new or worsening symptoms in the weeks following recovery from HSV encephalitis. Autoimmune encephalitis may also be associated with specific cancers (paraneoplastic) and onconeuro­ nal antibodies (e.g., anti-Hu, Yo, Ma2, amphiphysin, CRMP5, CV2). Subacute or chronic forms of encephalitis may occur in association with autoantibodies against thyroglobulin and thyroperoxidase (Hashi­ moto’s encephalopathy) and with prion diseases. Infection caused by the ameba Naegleria fowleri can also cause acute meningoencephalitis (primary amebic meningoencephalitis), whereas that caused by Acanthamoeba and Balamuthia more typically produces subacute or chronic granulomatous amebic meningoencephalitis. Naegleria thrive in warm, iron-rich pools of water, including those found in drains, canals, and both natural and human-made outdoor pools (Chap. 230). Infection has typically occurred in immunocompe­ tent children with a history of swimming in potentially infected water. The CSF, in contrast to the typical profile seen in viral encephalitis, often resembles that of bacterial meningitis with a neutrophilic pleocy­ tosis and hypoglycorrhachia. Motile trophozoites can be seen in a wet mount of warm, fresh CSF. There have been an increasing number of cases of Balamuthia mandrillaris amebic encephalitis in children and immunocompetent adults, mimicking acute viral encephalitis. This organism has also been associated with encephalitis in recipients of transplanted organs from a donor with unrecognized infection. No effective treatment has been approved, and mortality approaches 100%. There have been a few case reports of patients who recovered with nitroxoline therapy. Encephalitis can be caused by the raccoon pinworm Baylisascaris procyonis. Clues to the diagnosis include a history of raccoon exposure, especially of playing in or eating dirt potentially contaminated with raccoon feces. Most patients are children, and many have an associated eosinophilia. Once nonviral causes of encephalitis have been excluded, the major diagnostic challenge is to distinguish HSV from other viruses that cause encephalitis. This distinction is particularly important because in virtually every other instance the therapy is supportive, whereas specific and effective antiviral therapy is available for HSV, and its effi­ cacy is enhanced when it is instituted early in the course of infection. HSV encephalitis should be considered when clinical features suggest involvement of the inferomedial frontotemporal regions of the brain, including prominent olfactory or gustatory hallucinations, anosmia, unusual or bizarre behavior or personality alterations, or memory dis­ turbance. HSV encephalitis should always be suspected in patients with signs and symptoms consistent with acute encephalitis who have focal findings on clinical examination, neuroimaging studies, or EEG. The diagnostic procedure of choice in these patients is CSF PCR analysis for HSV. A positive CSF PCR establishes the diagnosis, and a negative test dramatically reduces the likelihood of HSV encephalitis (see above). The anatomic distribution of lesions may provide an additional clue to diagnosis. Patients with rapidly progressive encephalitis and prominent brainstem signs, symptoms, or neuroimaging abnormali­ ties may be infected by flaviviruses (WNV, St. Louis encephalitis virus, Japanese encephalitis virus), HSV, enterovirus A71 (EV-A71), rabies, or Listeria monocytogenes. Significant involvement of deep gray matter structures, including the basal ganglia and thalamus, should also sug­ gest possible flavivirus infection. These patients may present clinically with prominent movement disorders (tremor, myoclonus) or other parkinsonian features. Patients with WNV infection can also present with a poliomyelitis-like AFM, as can patients infected with EV-A71, EV-D68, and less commonly, other enteroviruses. Acute flaccid paraly­ sis is characterized by the acute onset of a lower motor neuron type of weakness with flaccid tone, reduced or absent reflexes, and relatively

preserved sensation. Patients often have multisegmental increased FLAIR and T2 signal in the anterior horns of the spinal cord and a CSF lymphocytic pleocytosis. Epidemiologic factors may provide important clues to the diagnosis of viral encephalitis. Particular attention should be paid to the season of the year; the geographic location and travel history; and possible exposure to animal bites or scratches, rodents, and ticks. Although transmission from the bite of an infected dog remains the most com­ mon cause of rabies (Chap. 214) worldwide, in the United States very few cases of dog rabies occur, and the most common risk factor is exposure to bats—although a clear history of a bite or scratch is often lacking. The classic clinical presentation of encephalitic (furious) rabies is fever, fluctuating consciousness, and autonomic hyperactivity. Pho­ bic spasms of the larynx, pharynx, neck muscles, and diaphragm can be triggered by attempts to swallow water (hydrophobia) or by inspiration (aerophobia). Patients may also present with paralytic (dumb) rabies characterized by acute ascending paralysis. Rabies due to the bite of a bat has a different clinical presentation than classic rabies due to a dog or wolf bite. Patients present with focal neurologic deficits, myoclonus, seizures, and hallucinations; phobic spasms are not a typical feature. Patients with rabies have a CSF lymphocytic pleocytosis and may show areas of increased T2 signal abnormality in the brainstem, hippocam­ pus, and hypothalamus. Diagnosis can be made by finding rabies virus antigen in brain tissue or in the neural innervation of hair follicles at the nape of the neck. PCR amplification of viral nucleic acid from CSF and saliva or tears may also enable diagnosis. Serology is frequently negative in both serum and CSF in the first week after onset of infec­ tion, which limits its acute diagnostic utility. No specific therapy is available, and cases are almost invariably fatal, with isolated survivors having devastating neurologic sequelae. State public health authorities provide a valuable resource concern­ ing isolation of particular agents in individual regions. Regular updates concerning the number, type, and distribution of cases of arboviral encephalitis can be found on the Centers for Disease Control and Pre­ vention and U.S. Geological Survey (USGS) websites (http://www.cdc

.gov and http://diseasemaps.usgs.gov). TREATMENT Viral Encephalitis Specific antiviral therapy should be initiated when appropriate. Vital functions, including respiration and blood pressure, should be monitored continuously and supported as required. In the initial stages of encephalitis, many patients will require care in an intensive care unit. Basic management and supportive therapy should include careful monitoring of ICP, fluid restriction, avoidance of hypotonic intravenous solutions, and suppression of fever. Seizures should be treated with standard anticonvulsant regimens, and prophylactic therapy should be considered in view of the high frequency of seizures in severe cases of encephalitis. As with all seriously ill, immobilized patients with altered levels of consciousness, encepha­ litis patients are at risk for aspiration pneumonia, stasis ulcers and decubiti, contractures, deep venous thrombosis and its complica­ tions, and infections of indwelling lines and catheters. Acyclovir is of benefit in the treatment of HSV and should be started empirically in patients with suspected viral encephalitis, especially if focal features are present, while awaiting viral diagnos­ tic studies. Treatment should be discontinued in patients found not to have HSV encephalitis, with the possible exception of patients with severe encephalitis due to VZV or EBV. HSV, VZV, and EBV all encode an enzyme deoxypyrimidine (thymidine) kinase that phos­ phorylates acyclovir to produce acyclovir-5′-monophosphate. Host cell enzymes then phosphorylate this compound to form a triphos­ phate derivative. It is the triphosphate that acts as an antiviral agent by inhibiting viral DNA polymerase and by causing premature termination of nascent viral DNA chains. The specificity of action depends on the fact that uninfected cells do not phosphorylate significant amounts of acyclovir to acyclovir-5′-monophosphate. A

second level of specificity is provided by the fact that the acyclovir triphosphate is a more potent inhibitor of viral DNA polymerase than of the analogous host cell enzymes.

Adults should receive a dose of 10 mg/kg of acyclovir intrave­ nously every 8 h (30 mg/kg per day total dose) for 21 days. Neonatal HSV CNS infection is less responsive to acyclovir therapy than HSV encephalitis in adults; it is recommended that neonates with HSV encephalitis receive 20 mg/kg of acyclovir every 8 h (60 mg/kg per day total dose) for a minimum of 21 days. Prior to intravenous administration, acyclovir should be diluted to a concentration ≤7 mg/mL. (A 70-kg person would receive a dose of 700 mg, which would be diluted in a volume of 100 mL.) Each dose should be infused slowly over 1 h, rather than by rapid or bolus infusion, to minimize the risk of renal dysfunction. Care should be taken to avoid extravasation or intramuscular or subcutaneous administration. The alkaline pH of acyclovir can cause local inflam­ mation and phlebitis (9%). Dose adjustment is required in patients with impaired renal glomerular filtration. Penetration into CSF is excellent, with average drug levels ~50% of serum levels. Complica­ tions of therapy include elevations in blood urea nitrogen and creat­ inine levels (5%), thrombocytopenia (6%), gastrointestinal toxicity (nausea, vomiting, diarrhea) (7%), and neurotoxicity (lethargy or obtundation, disorientation, confusion, agitation, hallucinations, tremors, seizures) (1%). Acyclovir resistance may be mediated by changes in either the viral deoxypyrimidine kinase or DNA poly­ merase. To date, acyclovir-resistant isolates have not been a signifi­ cant clinical problem in immunocompetent individuals. It is now appreciated that some patients with worsening symptoms in the weeks following recovery from HSV encephalitis have developed NMDA receptor encephalitis requiring immunosuppression rather than having developed an acyclovir-resistant isolate. However, there have been reports of clinically virulent acyclovir-resistant HSV iso­ lates from sites outside the CNS in immunocompromised individu­ als, including those with AIDS. CHAPTER 142 Encephalitis Oral antiviral drugs with efficacy against HSV, VZV, and EBV, including acyclovir, famciclovir, and valacyclovir, have not been evaluated in the treatment of encephalitis as primary therapy. Additional oral valaciclovir following a 14- to 21-day course of intravenous acyclovir does not improve outcomes in adult patients with HSV encephalitis. The role of adjunctive intravenous gluco­ corticoids in treatment of HSV and VZV infection remains unclear. Experimental models and case reports of HSV encephalitis suggest that glucocorticoids may be efficacious, although no data from randomized controlled human trials are available. Ganciclovir and foscarnet, as combination therapy, are used in the treatment of CMV-related CNS infections. Cidofovir (see below) may provide an alternative in patients who fail to respond to ganciclovir and foscarnet, although data concerning its use in CMV CNS infections are extremely limited. Ganciclovir is a synthetic nucleoside analogue of 2′-deoxyguano­ sine. The drug is preferentially phosphorylated by virus-induced cellular kinases. Ganciclovir triphosphate acts as a competitive inhibitor of the CMV DNA polymerase, and its incorporation into nascent viral DNA results in premature chain termination. Follow­ ing intravenous administration, CSF concentrations of ganciclovir are 25–70% of coincident plasma levels. The usual dose for treat­ ment of severe neurologic illnesses is 5 mg/kg every 12 h given intravenously at a constant rate over 1 h. Induction therapy is fol­ lowed by maintenance therapy of 5 mg/kg every day for an indefi­ nite period. Induction therapy should be continued until patients show a decline in CSF pleocytosis and a reduction in CSF CMV DNA copy number on quantitative PCR testing (where available). Doses should be adjusted in patients with renal insufficiency. Treat­ ment is often limited by the development of granulocytopenia and thrombocytopenia (20–25%), which may require reduction in or discontinuation of therapy. Gastrointestinal side effects, including nausea, vomiting, diarrhea, and abdominal pain, occur in ~20% of patients. Some patients treated with ganciclovir for CMV retinitis have developed retinal detachment, but the causal relationship to

ganciclovir treatment is unclear. Valganciclovir is an orally bioavail­ able prodrug that can generate high serum levels of ganciclovir, although studies of its efficacy in treating CMV CNS infections are limited.

Foscarnet is a pyrophosphate analogue that inhibits viral DNA polymerases by binding to the pyrophosphate-binding site. Fol­ lowing intravenous infusion, CSF concentrations range from 15 to 100% of coincident plasma levels. The usual dose for serious CMV-related neurologic illness is 60 mg/kg every 8 h administered by constant infusion over 1 h. Induction therapy for 14–21 days is followed by maintenance therapy (60–120 mg/kg per day). Induc­ tion therapy may need to be extended in patients who fail to show a decline in CSF pleocytosis and a reduction in CSF CMV DNA copy number on quantitative PCR tests (where available). Approximately one-third of patients develop renal impairment during treatment, which is reversible following discontinuation of therapy in most, but not all, cases. This is often associated with elevations in serum creatinine and proteinuria and is less frequent in patients who are adequately hydrated. Many patients experience fatigue and nausea. Reductions in serum calcium, magnesium, and potassium occur in ~15% of patients and may be associated with tetany, cardiac rhythm disturbances, or seizures. Cidofovir is a nucleotide analogue that is effective in treating CMV retinitis and equivalent to or better than ganciclovir in some experimental models of murine CMV encephalitis, although data concerning its efficacy in human CMV CNS disease are limited. The usual dose is 5 mg/kg intravenously once weekly for 2 weeks, then biweekly for two or more additional doses, depending on clini­ cal response. Patients must be prehydrated with normal saline (e.g., 1 L over 1–2 h) prior to each dose and treated with probenecid (e.g., 1 g 3 h before cidofovir and 1 g 2 and 8 h after cidofovir). Nephro­ toxicity is common; the dose should be reduced if renal function deteriorates. PART 5 Infectious Diseases Intravenous ribavirin (15–25 mg/kg per day in divided doses given every 8 h) has been reported to be of benefit in isolated cases of severe encephalitis due to California encephalitis (La Crosse) virus. Ribavirin might be of benefit for the rare patients, typically infants or young children, with severe adenovirus or rotavirus encephalitis and in patients with encephalitis due to LCMV or other arenaviruses. However, clinical trials are lacking. Hemolysis, with resulting anemia, has been the major side effect limiting therapy. No specific antiviral therapy of proven efficacy is currently available for treatment of WNV encephalitis. Patients have been treated with interferon-α, ribavirin, an Israeli IVIg preparation that contains high-titer anti-WNV antibody (Omr-IgG-am), and humanized monoclonal antibodies directed against the viral enve­ lope glycoprotein (www.clinicaltrials.gov, identifiers NCT00927953 and 00515385). Omr-IgG-am did not improve outcomes in patients with WNV neuroinvasive disease, but the study design was poten­ tially flawed as some patients received drug up to a week after symptom onset, when expected benefit may have been minimal. Of the six West Nile virus human vaccines that advanced into phase I clinical trials, only two live attenuated virus vaccines have advanced to phase II clinical trials. There has been success with four equine vaccines, but all require multiple primary doses and annual boost­ ers. The ideal human WNV vaccine needs to provide complete and long-lasting protective immunity after the administration of a single dose. Effective vaccines are already in human use for preven­ tion of other flavivirus infections including Japanese encephalitis and yellow fever. The Centers for Disease Control and Prevention (CDC) Clini­ cal Considerations for COVID-19 treatment in outpatients as of January 2024 state that the preferred treatment for mild to mod­ erate COVID-19 infection in adults is oral ritonavir-boosted nir­ matrelvir. This antiviral is U.S. Food and Drug Administration (FDA) approved in adults with mild-to-moderate COVID-19 with symptoms of <5 days in duration who are at high risk of developing severe COVID-19 due to older age (>50 years) or other risk fac­ tors. It has also been approved under emergency use authorization

(EUA) for 12- to 17-year-olds. A 3-day course of intravenous remdesivir is the second preferred treatment option after ritonavirboosted nirmatrelvir for adults and pediatric patients as young as 28 days. Immunocompromised patients may be treated with longer or additional courses. The FDA has issued an EUA for the use of COVID-19 convalescent plasma with high titers of anti–SARSCoV-2 antibodies for the treatment of COVID-19 in immunocom­ promised patients from disease or immunosuppressive therapy. The antiviral molnupiravir can be used for therapy according to the CDC but has been less effective in clinical trials than ritonavirboosted nirmatrelvir or remdesivir. ■ ■SEQUELAE There is considerable variation in the incidence and severity of sequelae in patients surviving viral encephalitis. In the case of EEE virus infec­ tion, nearly 80% of survivors have severe neurologic sequelae. At the other extreme are infections due to EBV, California encephalitis virus, and Venezuelan equine encephalitis virus, where severe sequelae are unusual. For example, ~5–15% of children infected with La Crosse virus have a residual seizure disorder, and 1% have persistent hemi­ paresis. Detailed information about sequelae in patients with HSV encephalitis treated with acyclovir is available from the NIAID-Collab­ orative Antiviral Study Group (CASG) trials. Of 32 acyclovir-treated patients, 26 survived (81%). Of the 26 survivors, 12 (46%) had no or only minor sequelae, 3 (12%) were moderately impaired (gainfully employed but not functioning at their previous level), and 11 (42%) were severely impaired (requiring continuous supportive care). The incidence and severity of sequelae were directly related to the age of the patient and the level of consciousness at the time of initiation of therapy. Patients with severe neurologic impairment (Glasgow Coma Scale score 6) at initiation of therapy either died or survived with severe sequelae. Young patients (<30 years) with good neurologic function at initiation of therapy did substantially better (100% survival, 62% with no or mild sequelae) compared with their older counterparts (>30 years; 64% survival, 57% no or mild sequelae). Many patients with WNV infection have sequelae, including cognitive impairment; weakness; and hyper- or hypokinetic movement disorders, including tremor, myoclonus, and parkinsonism. In a large longitudinal study of prognosis in 156 patients with WNV infection, the mean time to achieve recovery (defined as 95% of maximal predicted score on specific validated tests) was 112–148 days for fatigue, 121–175 days for physical function, 131–139 days for mood, and 302–455 days for mental function (the longer interval in each case representing patients with invasive CNS disease). CHRONIC ENCEPHALITIS ■ ■PROGRESSIVE MULTIFOCAL LEUKOENCEPHALOPATHY Clinical Features and Pathology  Progressive multifocal leuko­ encephalopathy (PML) is characterized pathologically by multifocal areas of demyelination of varying size distributed throughout the brain but sparing the spinal cord and optic nerves. In addition to demyelin­ ation, there are characteristic cytologic alterations in both astrocytes and oligodendrocytes. Astrocytes are enlarged and contain hyper­ chromatic, deformed, and bizarre nuclei and frequent mitotic figures. Oligodendrocytes have enlarged, densely staining nuclei that contain viral inclusions formed by crystalline arrays of JC virus (JCV) particles. Patients often present with visual deficits (45%), typically a homony­ mous hemianopia; mental impairment (38%) (dementia, confusion, personality change); weakness, including hemi- or monoparesis; and ataxia. Seizures occur in ~20% of patients, predominantly in those with lesions abutting the cortex. Almost all patients have an underlying immunosuppressive disor­ der or are receiving immunomodulatory therapy. The most common immunosuppressive disorder associated with PML is AIDS, followed by hematologic malignancies, solid organ and hematopoietic stem cell transplant, and chronic inflammatory diseases, including sarcoidosis. It

has been estimated that up to 5% of AIDS patients will develop PML. There has been considerable progress in the development of diseasemodifying therapies (DMTs) for multiple sclerosis and inflammatory bowel disease. Of the DMTs, the highest risk of PML is associated with natalizumab, a humanized monoclonal antibody that inhibits lymphocyte trafficking into CNS and bowel mucosa by binding to α4 integrins. Overall risk in these patients has been estimated at ~4 PML cases per 1000 treated patients, but the risk depends on a variety of factors including anti-JCV antibody serostatus and the magnitude of the JCV antibody response, prior immunosuppressive therapy use, and duration of natalizumab therapy. Patients who lack detectable JCV antibody have a risk of developing PML of <0.1 case/1000 patients, whereas those who are JCV seropositive and have been exposed to prior immunosuppressive therapy and have received >24 months of natalizumab therapy have a risk of >1.3 cases/100 treated patients. Some recent studies suggest that extended dosing interval regimens of natalizumab (at 6- to 8-week intervals rather than the conventional 4-week interval) may significantly reduce the risk of PML. Among JCV-seropositive individuals, those with higher JCV antibody index values, presumably due to the “immunizing” effects of more frequent JCV reactivations, appear to be at higher risk than those with low antibody indices. Alternative therapies are preferred in patients who are JCV seropositive. PML cases have also been reported in patients receiving other immunomodulatory agents including rituximab, ocrel­ izumab, fingolimod, and dimethyl fumarate, although the relative risks have not been clearly established, and many individual cases are complicated by previous exposure to other therapies including natali­ zumab. Prolonged lymphopenia, a side effect of dimethyl fumarate, is associated with an increased risk of PML. The basic clinical and diagnostic features appear to be similar in HIV-associated PML and PML associated with immunomodulatory drugs with the exception of an increased likelihood of MRI enhancement of PML lesions in immunomodulatory cases. In natalizumab-associated PML, patients will also almost invariably develop clinical and radiographic worsening of lesions with discontinuation of therapy, attributed to development of immune reconstitution inflammatory syndrome (IRIS). Diagnostic Studies  The diagnosis of PML is frequently suggested by MRI. MRI reveals multifocal asymmetric, coalescing white mat­ ter lesions located periventricularly, in the centrum semiovale, in the parietal-occipital region, and in the cerebellum. These lesions have increased signal on T2 and FLAIR images and decreased signal on T1-weighted images. HIV-PML lesions are classically nonenhancing (90%), but patients with immunomodulatory drug-associated PML may have peripheral ring enhancement. PML lesions are not typically associated with edema or mass effect. CT scans, which are less sensitive than MRI for the diagnosis of PML, often show hypodense nonenhanc­ ing white matter lesions. JCV infection may also induce rare cases of encephalitis and cerebellitis in immunocompromised patients that are distinct from PML and have differing neuroimaging features. The CSF is typically normal, although mild elevation in protein and/ or IgG may be found. Pleocytosis occurs in <25% of cases, is predomi­ nantly mononuclear, and rarely exceeds 25 cells/μL. PCR amplification of JCV DNA from CSF has become an important diagnostic tool. The presence of a positive CSF PCR for JCV DNA in association with typi­ cal MRI lesions in the appropriate clinical setting is diagnostic of PML, reflecting the assay’s relatively high specificity (92–100%); however, sensitivity is variable, and a negative CSF PCR does not exclude the diagnosis. In HIV-negative patients and HIV-positive patients not receiving antiretroviral therapy (ART), sensitivity is likely 70–90%. In ART-treated patients, sensitivity may be closer to 60%, reflecting the lower JCV CSF viral load in this relatively more immunocompetent group. Patients with natalizumab-associated PML have highly variable amounts of JCV DNA in CSF. Some patients may have negative CSF PCRs performed in commercial laboratories, where assay detection thresholds are typically >100 JCV DNA copies/μL, but positive results in reference laboratories using supersensitive techniques (detection of 10 JCV copies/μL or less). CSF studies with quantitative JCV PCR indi­ cate that patients with low JCV loads (<100 copies/μL) have a generally

better prognosis than those with higher viral loads. Patients with nega­ tive CSF PCR studies may require brain biopsy for definitive diagnosis. In biopsy or necropsy specimens of brain, JCV antigen and nucleic acid can be detected by immunocytochemistry, in situ hybridization, or PCR amplification.

Serologic studies of JCV antibody are of modest value in diagno­ sis of PML due to the high basal seroprevalence level, although the absence of detectable JCV antibody may be useful in reducing the likelihood of PML in the differential diagnosis, as PML results from viral reactivation in previously infected individuals and virtually all confirmed cases have been JCV seropositive at diagnosis. Antibody testing may also be useful in risk stratification of patients receiving immunomodulatory therapies. TREATMENT Progressive Multifocal Leukoencephalopathy No consistently effective therapy for PML is available. There are case reports of potential beneficial effects of the 5-HT2a receptor antagonist mirtazapine, which may inhibit binding of JCV to its receptor on oligodendrocytes. Retrospective noncontrolled studies have also suggested a possible beneficial effect of treatment with interferon-α. Neither of these agents has been tested in randomized controlled clinical trials. A prospective multicenter clinical trial to evaluate the efficacy of the antimalarial drug mefloquine failed to show benefit. Intravenous and/or intrathecal cytarabine were not shown to be of benefit in a randomized controlled trial in HIVassociated PML, although some experts suggest that cytarabine may have therapeutic efficacy in situations where breakdown of the blood-brain barrier allows sufficient CSF penetration. A random­ ized controlled trial of cidofovir in HIV-associated PML also failed to show significant benefit. Because PML almost invariably occurs in immunocompromised individuals, any therapeutic interventions designed to enhance or restore immunocompetence should be con­ sidered; a small series of patients treated with the PD-1 inhibitor pembrolizumab demonstrated clinical improvement and stabiliza­ tion. Positive results in small case series have also been reported in patients receiving infusions of BK or JC virus–specific cytotoxic T lymphocytes. Perhaps the most dramatic demonstration of the ben­ efit of restoring immune competence is disease stabilization and, in rare cases, improvement associated with an improved immune status of HIV-positive patients with AIDS following institution of ART. In HIV-positive PML patients treated with ART, 1-year sur­ vival is ~50%, although up to 80% of survivors may have significant neurologic sequelae. HIV-positive PML patients with higher CD4 counts (>300/μL) and low or nondetectable HIV viral loads have a better prognosis than those with lower CD4 counts and higher viral loads. Although institution of ART enhances survival in HIVpositive PML patients, the associated immune reconstitution in patients with an underlying opportunistic infection such as PML may also result in a severe CNS inflammatory syndrome (IRIS) associated with clinical worsening, CSF pleocytosis, and the appear­ ance of new enhancing MRI lesions. Patients receiving natalizumab or other immunomodulatory therapies who are suspected of hav­ ing PML should have therapy immediately halted. Patients should be closely monitored for development of IRIS, which is generally treated with intravenous glucocorticoids, although controlled clini­ cal trials of efficacy remain lacking. CHAPTER 142 Encephalitis ■ ■SUBACUTE SCLEROSING PANENCEPHALITIS Subacute sclerosing panencephalitis (SSPE) is a rare, chronic, progres­ sive demyelinating disease of the CNS associated with a chronic non­ permissive infection of brain tissue with measles virus. The frequency has been estimated at 1 in 100,000–500,000 measles cases. An average of five cases per year is reported in the United States. The incidence has declined dramatically since the introduction of a measles vaccine, but we may expect a rise in cases over the coming decades with increasing vaccine hesitancy and rising measles cases in the United States and

22 - 143 Acute Meningitis

143 Acute Meningitis

Europe in children from countries where routine childhood immu­ nizations are not available. Most patients give a history of primary measles infection at an early age (2 years), which is followed after a latent interval of 6–8 years by the development of a progressive neuro­ logic disorder. Some 85% of patients are between 5 and 15 years old at diagnosis. Initial manifestations include poor school performance and mood and personality changes. Typical signs of a CNS viral infection, including fever and headache, do not occur. As the disease progresses, patients develop progressive intellectual deterioration, focal and/or generalized seizures, myoclonus, ataxia, and visual disturbances. In the late stage of the illness, patients are unresponsive, quadriparetic, and spastic, with hyperactive tendon reflexes and extensor plantar responses.

Diagnostic Studies  MRI is often normal early, although areas of increased T2 signal develop in the white matter of the brain and brain­ stem as disease progresses. The EEG may initially show only nonspecific slowing, but with disease progression, patients develop a characteristic periodic pattern with bursts of high-voltage, sharp, slow waves every 3–8 s, followed by periods of attenuated (“flat”) background. The CSF is acellular with a normal or mildly elevated protein concentration and a markedly elevated gamma globulin level (>20% of total CSF protein). CSF antimeasles antibody levels are invariably elevated, and oligoclonal antimeasles antibodies are often present. Measles virus can be cultured from brain tissue using special cocultivation techniques. Viral antigen can be identified immunocytochemically, and viral genome can be detected by in situ hybridization or PCR amplification. TREATMENT Subacute Sclerosing Panencephalitis PART 5 Infectious Diseases No definitive therapy for SSPE is available. Treatment with isopri­ nosine (Inosiplex, 100 mg/kg per day), alone or in combination with intrathecal or intraventricular interferon-α, has been reported to prolong survival and produce clinical improvement in some patients but has never been subjected to a controlled clinical trial. ■ ■PROGRESSIVE RUBELLA PANENCEPHALITIS This is an extremely rare disorder that primarily affects males with con­ genital rubella syndrome, although isolated cases have been reported following childhood rubella. After a latent period of 8–19 years, patients develop progressive neurologic deterioration. The manifestations are similar to those seen in SSPE. CSF shows a mild lymphocytic pleocyto­ sis, slightly elevated protein concentration, markedly increased gamma globulin, and rubella virus–specific oligoclonal bands. No therapy is available. Universal prevention of both congenital and childhood rubella through the use of the available live attenuated rubella vaccine would be expected to eliminate the disease. ■ ■FURTHER READING Bernard-Valnet R et al: Advances in treatment of progressive multifocal leukoencephalopathy. Ann Neurol 90:865, 2021. Hodzic E et al: Steroids for the treatment of viral encephalitis: A systematic literature review and meta-analysis. Neurology 270:3603, 2023. Kaiser JA et al: Twenty years of progress toward West Nile virus vaccine development. Viruses 11:823, 2019. Ramachandran PS, Wilson MR: Metagenomics for neurological infections: Expanding our imagination. Nat Rev Neurol 16:547, 2020. Tunkel AR et al: The management of encephalitis: Clinical practice guidelines by the Infectious Diseases Society of America. Clin Infect Dis 47:303, 2008. Tyler KL: Acute viral encephalitis. N Engl J Med 379:557, 2018. Venkatesan A et al: International Encephalitis Consortium. Case definitions, diagnostic algorithms, and priorities in encephalitis: Consensus statement of the International Encephalitis Consortium. Clin Infect Dis 57:1114, 2013.

Karen L. Roos, Kenneth L. Tyler

Acute Meningitis BACTERIAL MENINGITIS ■ ■DEFINITION Bacterial meningitis is an acute purulent infection within the subarach­ noid space (SAS). It is associated with a central nervous system (CNS) inflammatory reaction that may result in decreased consciousness, seizures, raised intracranial pressure (ICP), and stroke. The meninges, SAS, and brain parenchyma are all frequently involved in the inflam­ matory reaction (meningoencephalitis). ■ ■EPIDEMIOLOGY Bacterial meningitis is the most common form of suppurative CNS infection, with an annual incidence in the United States of ~1.4 cases/100,000 population. The organisms most often responsible for community-acquired bacterial meningitis are Streptococcus pneumoniae (~50%), Neisseria meningitidis (~25%), group B streptococci (~15%), and Listeria monocytogenes (~10%). Haemophilus influenzae type b accounts for <10% of cases of bacterial meningitis in most series. N. meningitidis is the causative organism of recurring epidemics of men­ ingitis every 8–12 years. ■ ■ETIOLOGY S. pneumoniae (Chap. 151) is the most common cause of meningitis in adults >20 years of age, accounting for nearly half the reported cases (1.1 per 100,000 persons per year). There are a number of predispos­ ing conditions that increase the risk of pneumococcal meningitis, the most important of which is pneumococcal pneumonia. Additional risk factors include coexisting acute or chronic pneumococcal sinusitis or otitis media, alcoholism, diabetes, splenectomy, hypogammaglobu­ linemia, complement deficiency, and head trauma with basilar skull fracture and cerebrospinal fluid (CSF) rhinorrhea. The mortality rate remains ~20% despite antibiotic therapy. The incidence of meningitis due to N. meningitidis (Chap. 160) has decreased with the routine immunization of 11- to 18-year-olds with the quadrivalent (serogroups A, C, W-135, and Y) meningococcal gly­ coconjugate vaccine, and adolescents and young adults (16–23 years old) with the serogroup B meningococcal vaccine. A pentavalent meningococcal vaccine (serogroups A, B, C, W-135, and Y) has recently become available. Individuals being treated with complement inhibi­ tors are at increased risk of meningococcal disease and should receive either the quadrivalent vaccine and a serogroup B meningococcal vaccine, or the pentavalent meningococcal vaccine, prior to beginning therapy. Individuals with complement component deficiencies are at increased risk for meningococcal disease and similarly should receive either the quadrivalent and a serogroup B meningococcal vaccine or the pentavalent meningococcal vaccine. The meningococcal vaccines use outer membrane proteins as the vaccine antigens. The serogroup B polysaccharide capsule is poorly immunogenic. The serogroup B meningococcal vaccines do not reduce the risk of bacterial spread of group B meningococcus from vaccinated persons to unimmunized persons as the vaccines do not significantly reduce nasopharyngeal carriage of meningococci, and this remains the major source of personto-person bacterial transmission. In contrast, nasopharyngeal carriage is reduced in vaccinated individuals who have received the conjugate vaccines that cover groups A, C, W, and Y. The presence of petechial or purpuric skin lesions can provide an important clue to the diagnosis of meningococcal infection. In some patients, the disease is fulminant, progressing to death within hours of symptom onset. Infection may be initiated by nasopharyngeal colonization, which can result in either an asymptomatic carrier state or invasive meningococcal disease. The risk of invasive disease following nasopharyngeal colonization depends on both bacterial virulence factors and host immune defense mechanisms, including the host’s capacity to produce antimeningococcal antibodies

and to lyse meningococci by both classic and alternative complement pathways. Individuals with deficiencies of any of the complement com­ ponents, including properdin, are highly susceptible to meningococcal infections. Gram-negative bacilli cause meningitis in individuals with chronic and debilitating diseases such as diabetes, cirrhosis, or alcoholism and in those with urinary tract infections. Gram-negative meningitis can also complicate neurosurgical procedures, particularly craniotomy, and head trauma associated with CSF rhinorrhea or otorrhea. Otitis, mastoiditis, and sinusitis are predisposing and associated conditions for meningitis due to Streptococcus spp., gram-negative anaerobes, Staphylococcus aureus, Haemophilus spp., and Enterobac­ teriaceae. Meningitis complicating endocarditis may be due to viri­ dans streptococci, S. aureus, Streptococcus bovis, the HACEK group (Haemophilus spp., Actinobacillus actinomycetemcomitans, Cardiobac­ terium hominis, Eikenella corrodens, Kingella kingae), or enterococci. Group B streptococcus., or Streptococcus agalactiae (Chap. 153), was previously responsible for meningitis predominantly in neonates, but it has been reported with increasing frequency in individuals aged

50 years, particularly those with underlying diseases. L. monocytogenes (Chap. 156) is an increasingly important cause of meningitis in neonates (<1 month of age), pregnant women, indi­ viduals >60 years, and immunocompromised individuals of all ages. Infection is acquired by ingesting foods contaminated by Listeria. Foodborne human listerial infection has been reported from contami­ nated coleslaw, milk, soft cheeses, and several types of “ready-to-eat” foods, including delicatessen meat and uncooked hotdogs. The frequency of H. influenzae type b (Hib) meningitis in children has declined dramatically since the introduction of the Hib conjugate vaccine, although rare cases of Hib meningitis in vaccinated children have been reported. More frequently, H. influenzae causes meningitis in unvaccinated children and older adults, and non-b H. influenzae is an emerging pathogen (Chap. 162). S. aureus and coagulase-negative staphylococci (Chap. 152) are important causes of meningitis that occurs following invasive neuro­ surgical procedures, particularly shunting procedures for hydrocepha­ lus, or as a complication of the use of subcutaneous Ommaya reservoirs for administration of intrathecal chemotherapy. ■ ■PATHOPHYSIOLOGY The most common bacteria that cause meningitis, S. pneumoniae and N. meningitidis, initially colonize the nasopharynx by attaching to nasopharyngeal epithelial cells. Bacteria are transported across epi­ thelial cells in membrane-bound vacuoles to the intravascular space or invade the intravascular space by creating separations in the apical tight junctions of columnar epithelial cells. Once in the bloodstream, bacteria are able to avoid phagocytosis by neutrophils and classic complement-mediated bactericidal activity because of the presence of a polysaccharide capsule. Bloodborne bacteria can reach the intraven­ tricular choroid plexus, directly infect choroid plexus epithelial cells, and gain access to the CSF. Some bacteria, such as S. pneumoniae, can adhere to cerebral capillary endothelial cells and subsequently migrate through or between these cells to reach the CSF. Bacteria are able to multiply rapidly within CSF because of the absence of effective host immune defenses. Normal CSF contains few white blood cells (WBCs) and relatively small amounts of complement proteins and immuno­ globulins. The paucity of the latter two prevents effective opsoniza­ tion of bacteria, an essential prerequisite for bacterial phagocytosis by neutrophils. Phagocytosis of bacteria is further impaired by the fluid nature of CSF, which is less conducive to phagocytosis than a solid tissue substrate. A critical event in the pathogenesis of bacterial meningitis is the inflammatory reaction induced by the invading bacteria. Many of the neurologic manifestations and complications of bacterial meningitis result from the immune response to the invading pathogen rather than from direct bacteria-induced tissue injury. As a result, neurologic injury can progress even after the CSF has been sterilized by antibiotic therapy. The lysis of bacteria with the subsequent release of cell-wall components into the SAS is the initial step in the induction of the

inflammatory response and the formation of a purulent exudate in the SAS (Fig. 143-1). Bacterial cell-wall components, such as the lipopoly­ saccharide (LPS) molecules of gram-negative bacteria and teichoic acid and peptidoglycans of S. pneumoniae, induce meningeal inflammation by stimulating the production of inflammatory cytokines and chemo­ kines by microglia, astrocytes, monocytes, microvascular endothelial cells, and CSF leukocytes. In experimental models of meningitis, cyto­ kines including tumor necrosis factor alpha (TNF-α) and interleukin 1β (IL-1β) are present in CSF within 1–2 h of intracisternal inoculation of LPS. This cytokine response is quickly followed by an increase in CSF protein concentration and leukocytosis. Chemokines (cytokines that induce chemotactic migration in leukocytes) and a variety of other proinflammatory cytokines are also produced and secreted by leukocytes and tissue cells that are stimulated by IL-1β and TNF-α. In addition, bacteremia and the inflammatory cytokines induce the production of excitatory amino acids, reactive oxygen and nitrogen species (free oxygen radicals, nitric oxide, and peroxynitrite), and other mediators that can induce death of brain cells, especially in the dentate gyrus of the hippocampus.

Much of the pathophysiology of bacterial meningitis is a direct consequence of elevated levels of CSF cytokines and chemokines. TNF-α and IL-1β act synergistically to increase the permeability of the blood-brain barrier, resulting in induction of vasogenic edema and the leakage of serum proteins into the SAS (Fig. 143-1). The subarachnoid exudate of proteinaceous material and leukocytes obstructs the flow of CSF through the ventricular system and diminishes the resorptive capacity of the arachnoid granulations in the dural sinuses, leading to obstructive and communicating hydrocephalus and concomitant interstitial edema. CHAPTER 143 Inflammatory cytokines upregulate the expression of selectins on cerebral capillary endothelial cells and leukocytes, promoting leuko­ cyte adherence to vascular endothelial cells and subsequent migration into the CSF. The adherence of leukocytes to capillary endothelial cells increases the permeability of blood vessels, allowing for the leakage of plasma proteins into the CSF, which adds to the inflammatory exudate. Neutrophil degranulation results in the release of toxic metabolites that contribute to cytotoxic edema, cell injury, and death. Contrary to previous beliefs, CSF leukocytes probably do little to contribute to the clearance of CSF bacterial infection. Acute Meningitis During the very early stages of meningitis, there is an increase in cerebral blood flow, soon followed by a decrease in cerebral blood flow and a loss of cerebrovascular autoregulation (Chap. 318). Narrowing of the large arteries at the base of the brain due to encroachment by the purulent exudate in the SAS and infiltration of the arterial wall by inflammatory cells with intimal thickening (vasculitis) also occur and may result in ischemia and infarction, obstruction of branches of the middle cerebral artery by thrombosis, thrombosis of the major cerebral venous sinuses, and thrombophlebitis of the cerebral cortical veins. The combination of interstitial, vasogenic, and cytotoxic edema leads to raised ICP and coma. Cerebral herniation usually results from the effects of cerebral edema, either focal or generalized; hydrocephalus and dural sinus or cortical vein thrombosis may also play a role. ■ ■CLINICAL PRESENTATION Meningitis can present as either an acute fulminant illness that pro­ gresses rapidly in a few hours or as a subacute infection that progres­ sively worsens over several days. The classic clinical triad of meningitis is fever, headache, and nuchal rigidity, and these features each occur in

80% of adult cases of acute bacterial meningitis, although the com­ plete classic triad is not always present. A decreased level of conscious­ ness occurs in >75% of patients and can vary from lethargy to coma. Nausea, vomiting, and photophobia are also common complaints. Nuchal rigidity (“stiff neck”) is the pathognomonic sign of men­ ingeal irritation and is present when the neck resists passive flexion. Kernig’s and Brudzinski’s signs are also classic signs of meningeal irri­ tation. Kernig’s sign is elicited with the patient in the supine position. The thigh is flexed on the abdomen, with the knee flexed; attempts to passively extend the knee elicit pain when meningeal irritation is present. Brudzinski’s sign is elicited with the patient in the supine

Invasion of SAS by meningeal pathogens Multiplication of organisms and lysis of organisms by bactericidal antibiotics Release of bacterial cell-wall components (endotoxin, teichoic acid) Production of inflammatory cytokines Altered blood-brain barrier permeability Adherence of leukocytes to cerebral capillary endothelial cells Leukocytes migrate into CSF, degranulate, and release toxic metabolites Permeability of blood vessels with leakage of plasma proteins into CSF Exudate in SAS obstructs outflow and resorption of CSF and surrounds and infiltrates cerebral vasculature Cerebral ischemia Blood flow Blood flow Vasogenic edema Obstructive and communicating hydrocephalus and interstitial edema Cytotoxic edema, stroke, seizures PART 5 Infectious Diseases Intracranial pressure Coma FIGURE 143-1  The pathophysiology of the neurologic complications of bacterial meningitis. CSF, cerebrospinal fluid; SAS, subarachnoid space. position and is positive when passive flexion of the neck results in spontaneous flexion of the hips and knees. Although commonly tested on physical examinations, the sensitivity and specificity of Kernig’s and Brudzinski’s signs are uncertain. Both may be absent or reduced in very young or elderly patients, immunocompromised individuals, or patients with a severely depressed mental status. The high prevalence of cervical spine disease in older individuals may result in false-positive tests for nuchal rigidity. Seizures occur as part of the initial presentation of bacterial men­ ingitis or during the course of the illness in 15–40% of patients. Focal seizures are usually due to focal arterial ischemia or infarction, cortical venous thrombosis with hemorrhage, or focal edema. Generalized sei­ zure activity and status epilepticus may be due to hyponatremia, cere­ bral anoxia, or, less commonly, the toxic effects of antimicrobial agents. Raised ICP is an expected complication of bacterial meningitis and the major cause of obtundation and coma in this disease. More than 90% of patients will have a CSF opening pressure >180 mmH2O, and 20% have opening pressures >400 mmH2O. Signs of increased ICP include a deteriorating or reduced level of consciousness, papilledema, dilated poorly reactive pupils, sixth nerve palsies, decerebrate postur­ ing, and the Cushing reflex (bradycardia, hypertension, and irregular respirations). The most disastrous complication of increased ICP is cerebral herniation. The incidence of herniation in patients with bacte­ rial meningitis has been reported to occur in as few as 1% to as many as 8% of cases. Specific clinical features may provide clues to the diagnosis of indi­ vidual organisms and are discussed in more detail in specific chapters

devoted to individual pathogens. The most important of these clues is the rash of meningococcemia, which begins as a diffuse erythematous maculopapular rash resembling a viral exanthem; how­ ever, the skin lesions of meningococce­ mia rapidly become petechial. Petechiae are found on the trunk and lower extremities, in the mucous membranes and conjunctiva, and occasionally on the palms and soles. ■ ■DIAGNOSIS When bacterial meningitis is suspected, blood cultures should be immediately obtained and empirical antimicrobial and adjunctive dexamethasone therapy initiated without delay (Table 143-1). Therapy with dexamethasone should ideally be started 20 min before, or not later than concurrent with, the first dose of antibiotics. The diagnosis of bacte­ rial meningitis is made by examination of the CSF (Table 143-2). The need to obtain neuroimaging studies (computed tomography [CT] or magnetic reso­ nance imaging [MRI]) prior to lumbar puncture (LP) requires clinical judg­ ment. In an immunocompetent patient with no known history of recent head trauma, a normal level of consciousness, and no evidence of papilledema or focal neurologic deficits, it is considered safe to perform LP without prior neuroimag­ ing studies. If LP is delayed in order to obtain neuroimaging studies, empirical antibiotic therapy should be initiated after blood cultures are obtained. Anti­ biotic therapy initiated a few hours prior to LP will not significantly alter the CSF WBC count or glucose concentration, nor is it likely to prevent visualization of organisms by Gram’s stain or detection of bacterial nucleic acid by polymerase chain reaction (PCR) assay. Alterations in cerebral blood flow Production of excitatory amino acids and reactive oxygen and nitrogen species Cell injury and death The classic CSF abnormalities in bacterial meningitis (Table 143-2) are (1) polymorphonuclear (PMN) leukocytosis (>100 cells/μL in 90%), (2) decreased glucose concentration (<2.2 mmol/L [<40 mg/dL] and/or CSF/serum glucose ratio of <0.4 in ~60%), (3) increased pro­ tein concentration (>0.45 g/L [>45 mg/dL] in 90%), and (4) increased opening pressure (>180 mmH2O in 90%). CSF bacterial cultures are positive in >70% of patients, and CSF Gram’s stain demonstrates organ­ isms in >60%. CSF glucose concentrations <2.2 mmol/L (<40 mg/dL) are abnor­ mal, and a CSF glucose concentration of zero can be seen in bacterial meningitis. Use of the CSF/serum glucose ratio corrects for hypergly­ cemia that may mask a relative decrease in the CSF glucose concen­ tration. The CSF glucose concentration is low when the CSF/serum glucose ratio is <0.6. A CSF/serum glucose ratio <0.4 is highly sugges­ tive of bacterial meningitis but may also be seen in other conditions, including fungal, tuberculous, and carcinomatous meningitis. It takes from 30 min to several hours for the concentration of CSF glucose to reach equilibrium with blood glucose levels; therefore, administration of 50 mL of 50% glucose (D50) prior to LP, as commonly occurs in emergency room settings, is unlikely to alter CSF glucose concentra­ tion significantly unless more than a few hours have elapsed between glucose administration and LP. The presently available CSF multiplex PCR pathogen assays, the most common of which is the FilmArray Meningitis/Encephalitis panel (BioFire Diagnostics), detect the nucleic acid of S. pneumoniae,

TABLE 143-1  Antibiotics Used in Empirical Therapy of Bacterial Meningitis and Focal Central Nervous System Infectionsa INDICATION ANTIBIOTIC Preterm infants to infants <1 month Ampicillin + cefotaxime Infants 1–3 months Ampicillin + cefotaxime or ceftriaxone Immunocompetent children >3 months and adults <55 Cefotaxime, ceftriaxone, or cefepime + vancomycin Adults >55 and adults of any age with alcoholism or other debilitating illnesses Ampicillin + cefotaxime, ceftriaxone, or cefepime + vancomycin Hospital-acquired meningitis, posttraumatic or postneurosurgery meningitis, neutropenic patients, or patients with impaired cell-mediated immunity Ampicillin + ceftazidime or meropenem + vancomycin TOTAL DAILY DOSE AND DOSING INTERVAL CHILD (>1 MONTH) ADULT ANTIMICROBIAL AGENT Ampicillin 300 (mg/kg)/d, q6h 12 g/d, q4h Cefepime 150 (mg/kg)/d, q8h 6 g/d, q8h Cefotaxime 225–300 (mg/kg)/d, q6h 12 g/d, q4h Ceftriaxone 100 (mg/kg)/d, q12h 4 g/d, q12h Ceftazidime 150 (mg/kg)/d, q8h 6 g/d, q8h Gentamicin 7.5 (mg/kg)/d, q8hb 7.5 (mg/kg)/d, q8h Meropenem 120 (mg/kg)/d, q8h 6 g/d, q8h Metronidazole 30 (mg/kg)/d, q6h 1500–2000 mg/d, q6h Nafcillin 200 (mg/kg)/d, q6h 12 g/d, q4h Penicillin G 400,000 (U/kg)/d, q4h 20–24 million U/d, q4h Vancomycin 45–60 (mg/kg)/d, q6h 45–60 (mg/kg)d, q6–12hb aAll antibiotics are administered intravenously; doses indicated assume normal renal and hepatic function. bDoses should be adjusted based on serum peak and trough levels: gentamicin therapeutic level: peak: 5–8 μg/mL; trough: <2 μg/mL; vancomycin therapeutic level: peak: 25–40 μg/mL; trough: 5–15 μg/mL. N. meningitidis, Escherichia coli, L. monocytogenes, H. influenzae, and S. agalactiae (group B streptococci). Although these PCR assays have a rapid turnaround time, the sensitivity and specificity for the bacterial meningeal pathogens they test for are not known. The CSF multiplex PCR pathogen assays do not include S. aureus, coagulase-negative staphylococci, and many gram-negative organisms. The PCR assays cannot replace CSF bacteria culture, as culture is required for antimi­ crobial susceptibility testing. Almost all patients with bacterial men­ ingitis will have neuroimaging studies performed during the course of their illness. MRI is preferred over CT because of its superiority in demonstrating areas of cerebral edema and ischemia. In patients with bacterial meningitis, diffuse meningeal enhancement is often seen TABLE 143-2  Cerebrospinal Fluid (CSF) Abnormalities in Bacterial Meningitis Opening pressure

180 mmH2O White blood cells 10/μL to 10,000/μL; neutrophils predominate Red blood cells Absent in nontraumatic tap Glucose <2.2 mmol/L (<40 mg/dL) CSF/serum glucose <0.4 Protein 0.45 g/L (>45 mg/dL) Gram’s stain Positive in >60% Culture Positive in >80% PCR Detects bacterial DNA Abbreviation: PCR, polymerase chain reaction.

after the administration of gadolinium. Meningeal enhancement is not diagnostic of meningitis but occurs in any CNS disease associated with increased blood-brain barrier permeability.

Petechial skin lesions, if present, should be biopsied. The rash of meningococcemia results from the dermal seeding of organisms with vascular endothelial damage, and biopsy may reveal the organism on Gram’s stain. ■ ■DIFFERENTIAL DIAGNOSIS Viral meningoencephalitis, and particularly herpes simplex virus (HSV) encephalitis (Chap. 142), can mimic the clinical presentation of bacte­ rial meningitis (encephalitis). HSV encephalitis typically presents with headache, fever, altered consciousness, focal neurologic deficits (e.g., dysphasia, hemiparesis), and focal or generalized seizures. The findings on CSF studies, neuroimaging, and electroencephalogram (EEG) dis­ tinguish HSV encephalitis from bacterial meningitis. The typical CSF profile with viral CNS infections is a lymphocytic pleocytosis with a normal glucose concentration, in contrast to the PMN pleocytosis and hypoglycorrhachia characteristic of bacterial meningitis. The CSF HSV PCR has a 96% sensitivity and a 99% specificity when CSF is examined 72 h following symptom onset and in the first week of antiviral therapy. MRI abnormalities (other than meningeal enhancement) are not seen in uncomplicated bacterial meningitis. By contrast, in HSV encephali­ tis, on T2-weighted, fluid-attenuated inversion recovery (FLAIR), and diffusion-weighted MRI images, high-signal-intensity lesions are seen in the orbitofrontal, anterior, and medial temporal lobes in the major­ ity of patients within 48 h of symptom onset. Some patients with HSV encephalitis have a distinctive periodic pattern on EEG. CHAPTER 143 Rickettsial disease (Chap. 192) can resemble bacterial meningitis. Rocky Mountain spotted fever (RMSF) is transmitted by a tick bite and caused by the bacteria Rickettsia rickettsii. The disease may pres­ ent acutely with high fever, prostration, myalgia, headache, nausea, and vomiting. Most patients develop a characteristic rash within 96 h of the onset of symptoms. The rash is initially a diffuse erythematous maculopapular rash that may be difficult to distinguish from that of meningococcemia. It progresses to a petechial rash, then to a purpuric rash, and if untreated, to skin necrosis or gangrene. The color of the lesions changes from bright red to very dark red, then yellowishgreen to black. The rash typically begins in the wrist and ankles and then spreads distally and proximally within a matter of a few hours, involving the palms and soles. Diagnosis is made by immunofluores­ cent staining of skin biopsy specimens. Ehrlichioses are also trans­ mitted by a tick bite. These are small gram-negative coccobacilli of which two species cause human disease. Anaplasma phagocytophilum causes human granulocytic ehrlichiosis (anaplasmosis), and Ehrlichia chaffeensis causes human monocytic ehrlichiosis. The clinical and laboratory manifestations of the infections are similar. Patients present with fever, headache, confusion, nausea, and vomiting. Twenty percent of patients have a maculopapular or petechial rash. There is laboratory evidence of leukopenia, thrombocytopenia, and anemia, and mild to moderate elevations in alanine aminotransferases, alkaline phospha­ tase, and lactate dehydrogenase. Patients with RMSF and those with ehrlichial infections may have an altered level of consciousness ranging from mild lethargy to coma, confusion, focal neurologic signs, cranial nerve palsies, hyperreflexia, and seizures. Acute Meningitis Focal suppurative CNS infections, including subdural and epidural empyema and brain abscess, should also be considered (Chap. 145), especially when focal neurologic findings are present. MRI should be performed promptly in all patients with suspected meningitis who have focal features, both to detect the intracranial infection and to search for associated areas of infection in the sinuses or mastoid bones. A number of noninfectious CNS disorders can mimic bacterial meningitis. Subarachnoid hemorrhage (SAH; Chap. 440) is gener­ ally the major consideration. Other possibilities include medicationinduced hypersensitivity meningitis; chemical meningitis due to rupture of tumor contents into the CSF (e.g., from a cystic glioma, craniopharyngioma, epidermoid or dermoid cyst); carcinomatous or lymphomatous meningitis; meningitis associated with inflammatory disorders such as sarcoid, systemic lupus erythematosus (SLE), and

Behçet’s syndrome; pituitary apoplexy; and uveomeningitic syndromes (Vogt-Koyanagi-Harada syndrome).

On occasion, subacutely evolving meningitis (see below and Chap. 144) may be considered in the differential diagnosis of acute meningitis. The principal causes include Mycobacterium tuberculosis (Chap. 183), Cryptococcus neoformans (Chap. 221), Histoplasma capsulatum (Chap. 218), Coccidioides immitis (Chap. 219), and Treponema pallidum (Chap. 187). TREATMENT Acute Bacterial Meningitis EMPIRICAL ANTIMICROBIAL THERAPY (Table 143-1) Bacterial meningitis is a medical emergency. The goal is to begin antibiotic therapy within 60 min of a patient’s arrival in the emergency room. Empirical antimicrobial therapy is initiated in patients with suspected bacterial meningitis before the results of CSF multiplex PCR assays, Gram’s stain, and culture are known.

S. pneumoniae (Chap. 151) and N. meningitidis (Chap. 160) are the most common etiologic organisms of community-acquired bacterial meningitis. Due to the emergence of penicillin- and cephalosporinresistant S. pneumoniae, empirical therapy of community-acquired suspected bacterial meningitis in children and adults should include a combination of dexamethasone, a third- or fourth-generation cephalosporin (e.g., ceftriaxone, cefotaxime, or cefepime), and van­ comycin, plus acyclovir, as HSV encephalitis is the leading disease in the differential diagnosis, and doxycycline during tick season to treat tick-borne bacterial infections. Ceftriaxone or cefotax­ ime provides good coverage for susceptible S. pneumoniae, group B streptococci, and H. influenzae and adequate coverage for N. meningitidis. Cefepime is a broad-spectrum fourth-generation cephalosporin with in vitro activity similar to that of cefotaxime or ceftriaxone against S. pneumoniae and N. meningitidis and greater activity against Enterobacter species and Pseudomonas aeruginosa. In clinical trials, cefepime has been demonstrated to be equivalent to cefotaxime in the treatment of penicillin-sensitive pneumococcal and meningococcal meningitis, and it has been used successfully in some patients with meningitis due to Enterobacter species and

P. aeruginosa. Cefepime has been associated with seizures, myoclo­ nus, and encephalopathy, any of which may limit its use in critically ill patients. Ampicillin should be added to the empirical regimen for coverage of L. monocytogenes in individuals <3 months of age, those

55, or those with suspected impaired cell-mediated immunity because of chronic illness, organ transplantation, pregnancy, malig­ nancy, or immunosuppressive therapy. Metronidazole is added to the empirical regimen to cover gram-negative anaerobes in patients with otitis, sinusitis, or mastoiditis. In hospital-acquired meningi­ tis, and particularly meningitis following neurosurgical procedures, staphylococci and gram-negative organisms including P. aerugi­ nosa are the most common etiologic organisms. In these patients, empirical therapy should include a combination of vancomycin and ceftazidime or meropenem. Ceftazidime or meropenem should be substituted for ceftriaxone or cefotaxime in neurosurgical patients and in neutropenic patients because ceftriaxone and cefotaxime do not provide adequate activity against CNS infection with P. aerugi­ nosa. Meropenem is a carbapenem antibiotic that is highly active in vitro against L. monocytogenes, has been demonstrated to be effective in cases of meningitis caused by P. aeruginosa, and shows good activity against penicillin-resistant pneumococci. In experi­ mental pneumococcal meningitis, meropenem was comparable to ceftriaxone and inferior to vancomycin in sterilizing CSF cultures. When S. pneumoniae, H. influenzae, L. monocytogenes, or aerobic gram-negative bacilli (including P. aeruginosa and E. coli) are pos­ sible meningeal pathogens, based on predisposing and associated conditions, the combination of vancomycin plus meropenem can be recommended as empiric therapy for bacterial meningitis in chil­ dren and adults. Meropenem should not be used as monotherapy. PART 5 Infectious Diseases

TABLE 143-3  Antimicrobial Therapy of Central Nervous System Bacterial Infections Based on Pathogena ORGANISM ANTIBIOTIC Neisseria meningitides     Penicillin-sensitive Penicillin G or ampicillin   Penicillin-resistant Ceftriaxone or cefotaxime Streptococcus pneumoniae     Penicillin-sensitive Penicillin G   Penicillin-intermediate Ceftriaxone or cefotaxime or cefepime   Penicillin-resistant Ceftriaxone (or cefotaxime or cefepime) + vancomycin Gram-negative bacilli (except Pseudomonas spp.) Ceftriaxone or cefotaxime Pseudomonas aeruginosa Ceftazidime or cefepime or meropenem Staphylococci spp.     Methicillin-sensitive Nafcillin   Methicillin-resistant Vancomycin Listeria monocytogenes Ampicillin + gentamicin Haemophilus influenzae Ceftriaxone or cefotaxime if β-lactamase positive; ampicillin if β-lactamase negative Streptococcus agalactiae Penicillin G or ampicillin Bacteroides fragilis Metronidazole Fusobacterium spp. Metronidazole aDoses are as indicated in Table 143-1.
■ ■SPECIFIC ANTIMICROBIAL THERAPY Meningococcal Meningitis (Table 143-3)  Although ceftriax­ one and cefotaxime provide adequate empirical coverage for N. menin­ gitidis, penicillin G remains the antibiotic of choice for meningococcal meningitis caused by susceptible strains. Isolates of N. meningitidis with moderate resistance to penicillin have been identified and are increasing in incidence worldwide. CSF isolates of N. meningitidis should be tested for penicillin and ampicillin susceptibility, and if resistance is found, cefotaxime or ceftriaxone should be substituted for penicillin. A 7-day course of intravenous antibiotic therapy is adequate for uncomplicated meningococcal meningitis. The index case and all close contacts should receive chemoprophylaxis with a 2-day regimen of rifampin (600 mg every 12 h for 2 days in adults and 10 mg/kg every 12 h for 2 days in children >1 year). Rifampin is not recommended in pregnant women. Alternatively, adults can be treated with one dose of azithromycin (500 mg) or one intramuscular dose of ceftriaxone (250 mg). Close contacts are defined as those individuals who have had contact with oropharyngeal secretions, either through kissing or by sharing toys, beverages, or cigarettes. Pneumococcal Meningitis  Antimicrobial therapy of pneu­ mococcal meningitis is initiated with a cephalosporin (ceftriax­ one, cefotaxime, or cefepime) and vancomycin. All CSF isolates of

S. pneumoniae should be tested for sensitivity to penicillin and the cephalosporins. Once the results of antimicrobial susceptibility tests are known, therapy can be modified accordingly (Table 143-3). For

S. pneumoniae meningitis, an isolate of S. pneumoniae is considered to be susceptible to penicillin with a minimal inhibitory concentration (MIC) <0.06 μg/mL and to be resistant when the MIC is >0.12 μg/mL. Isolates of S. pneumoniae that have cephalosporin MICs ≤0.5 μg/mL are considered sensitive to the cephalosporins (cefotaxime, ceftriax­ one, cefepime). Those with MICs of 1 μg/mL are considered to have intermediate resistance, and those with MICs ≥2 μg/mL are considered resistant. For meningitis due to pneumococci, with cefotaxime or cef­ triaxone MICs ≤0.5 μg/mL, treatment with cefotaxime or ceftriaxone is usually adequate. For MIC >1 μg/mL, vancomycin is the antibiotic of choice. Rifampin can be added to vancomycin for its synergistic effect but is inadequate as monotherapy because resistance develops rapidly when it is used alone.

A 2-week course of intravenous antimicrobial therapy is recom­ mended for pneumococcal meningitis. Patients with S. pneumoniae meningitis should have a repeat LP performed 24–36 h after the initiation of antimicrobial therapy to document sterilization of the CSF. Failure to sterilize the CSF after 24–36 h of antibiotic therapy should be considered presumptive evidence of antibiotic resistance. Patients with penicillin- and ceph­ alosporin-resistant strains of S. pneumoniae who do not respond to intravenous vancomycin alone may benefit from the addition of intra­ ventricular vancomycin. The intraventricular route of administration is preferred over the intrathecal route because adequate concentrations of vancomycin in the cerebral ventricles are not always achieved with intrathecal administration. Listeria Meningitis  Meningitis due to L. monocytogenes is treated with ampicillin for at least 3 weeks (Table 143-3). Gentamicin is added in critically ill patients (2 mg/kg loading dose, then 7.5 mg/kg per day given every 8 h and adjusted for serum levels and renal function). The combination of trimethoprim (10–20 mg/kg per day) and sulfa­ methoxazole (50–100 mg/kg per day) given every 6 h may provide an alternative in penicillin-allergic patients. Staphylococcal Meningitis  Meningitis due to susceptible strains of S. aureus or coagulase-negative staphylococci is treated with nafcil­ lin (Table 143-3). Vancomycin is the drug of choice for methicillinresistant staphylococci and for patients allergic to penicillin. In these patients, the CSF should be monitored during therapy. If the CSF is not sterilized after 48 h of intravenous vancomycin therapy, then either intraventricular or intrathecal vancomycin, 20 mg once daily, can be added. Gram-Negative Bacillary Meningitis  The third-generation cephalosporins—cefotaxime, ceftriaxone, and ceftazidime—are equally efficacious for the treatment of gram-negative bacillary meningitis, with the exception of meningitis due to P. aeruginosa, which should be treated with ceftazidime or meropenem (Table 143-3). A 3-week course of intravenous antibiotic therapy is recommended for meningitis due to gram-negative bacilli. ■ ■ADJUNCTIVE THERAPY The release of bacterial cell-wall components by bactericidal antibiot­ ics leads to the production of the inflammatory cytokines IL-1β and TNF-α in the SAS. Dexamethasone exerts its beneficial effect by inhib­ iting the synthesis of IL-1β and TNF-α at the level of mRNA, decreas­ ing CSF outflow resistance, and stabilizing the blood-brain barrier. The rationale for giving dexamethasone 20 min before antibiotic therapy is that dexamethasone inhibits the production of TNF-α by macrophages and microglia only if it is administered before these cells are activated by endotoxin. Dexamethasone does not alter TNF-α production once it has been induced. The results of clinical trials of dexametha­ sone therapy in meningitis due to H. influenzae, S. pneumoniae, and

N. meningitidis have demonstrated its efficacy in decreasing meningeal inflammation and neurologic sequelae such as the incidence of senso­ rineural hearing loss. A prospective European trial of adjunctive therapy for acute bac­ terial meningitis in adults found that dexamethasone reduced the number of unfavorable outcomes (15 vs 25%, p = .03) including death (7 vs 15%, p = .04). The benefits were most striking in patients with pneumococcal meningitis. Dexamethasone (10 mg intravenously) was administered 15–20 min before the first dose of an antimicrobial agent, and the same dose was repeated every 6 h for 4 days. These results were confirmed in a second trial of dexamethasone in adults with pneu­ mococcal meningitis. Therapy with dexamethasone should ideally be started 20 min before, or not later than concurrent with, the first dose of antibiotics. It is unlikely to be of significant benefit if started >6 h after antimicrobial therapy has been initiated. Dexamethasone may decrease the penetration of vancomycin into CSF, and it delays the sterilization of CSF in experimental models of S. pneumoniae men­ ingitis. As a result, to assure reliable penetration of vancomycin into the CSF, children and adults are treated with vancomycin in a dose of

45–60 mg/kg per day. Alternatively, vancomycin can be administered by the intraventricular route. In clinical trials, dexamethasone has also been shown to reduce rates of death and hearing loss with no adverse effects in patients with meningococcal meningitis.

One of the concerns for using dexamethasone in adults with bac­ terial meningitis is that in experimental models of meningitis, dexa­ methasone therapy increased hippocampal cell injury and reduced learning capacity. This has not been the case in clinical series. The efficacy of dexamethasone therapy in preventing neurologic sequelae is also different between high- and low-income countries. Three large randomized trials in low-income countries (sub-Saharan Africa, Southeast Asia) failed to show benefit in subgroups of patients. The lack of efficacy of dexamethasone in these trials has been attributed to late presentation to the hospital with more advanced disease, antibiotic pretreatment, malnutrition, infection with HIV, and treat­ ment of patients with probable, but not microbiologically proven, bacterial meningitis. The results of these clinical trials suggest that patients in sub-Saharan Africa and those in low-income countries with negative CSF Gram’s stain and culture should not be treated with dexamethasone. ■ ■INCREASED INTRACRANIAL PRESSURE Emergency treatment of increased ICP includes elevation of the patient’s head to 30–45°, intubation, hyperventilation (Paco2 25–30 mmHg), and mannitol. Patients with increased ICP should be managed in an intensive care unit; accurate ICP measure­ ments are best obtained with an ICP monitoring device. Treatment of increased ICP is discussed in detail in Chap. 318. CHAPTER 143 ■ ■PROGNOSIS Mortality rate is 3–7% for meningitis caused by H. influenzae, N. men­ ingitidis, or group B streptococci; 15% for that due to L. monocytogenes; and 20% for S. pneumoniae. In general, the risk of death from bacte­ rial meningitis increases with (1) decreased level of consciousness on admission, (2) onset of seizures within 24 h of admission, (3) signs of increased ICP, (4) young age (infancy) and age >50, (5) the presence of comorbid conditions including shock and/or the need for mechani­ cal ventilation, and (6) delay in the initiation of treatment. Decreased CSF glucose concentration (<2.2 mmol/L [<40 mg/dL]) and markedly increased CSF protein concentration (>3 g/L [> 300 mg/dL]) have been predictive of increased mortality and poorer outcomes in some series. Moderate or severe sequelae occur in ~25% of survivors, although the exact incidence varies with the infecting organism. Common sequelae include decreased intellectual function, memory impairment, seizures, hearing loss and dizziness, and gait disturbances. Acute Meningitis VIRAL MENINGITIS ■ ■CLINICAL MANIFESTATIONS Immunocompetent adult patients with viral meningitis usually present with headache, fever, and signs of meningeal irritation coupled with an inflammatory CSF profile (see below). Headache is almost invari­ ably present and often characterized as frontal or retroorbital and frequently associated with photophobia and pain on moving the eyes. Nuchal rigidity is present in most cases but may be mild and present only near the limit of neck anteflexion. Constitutional signs can include malaise, myalgia, anorexia, nausea and vomiting, abdominal pain, and/ or diarrhea. Patients often have mild lethargy or drowsiness; however, profound alterations in consciousness, such as stupor, coma, or marked confusion, do not occur in viral meningitis and suggest the presence of encephalitis or other alternative diagnoses. Similarly, seizures or focal neurologic signs or symptoms or neuroimaging abnormalities indica­ tive of brain parenchymal involvement are not typical of viral meningi­ tis and suggest the presence of encephalitis or another CNS infectious or inflammatory process. ■ ■ETIOLOGY Using a variety of diagnostic techniques, including CSF PCR, culture, and serology, a specific viral cause can be found in 60–90% of cases of

TABLE 143-4  Viruses Causing Acute Meningitis in North America COMMON LESS COMMON Enteroviruses (coxsackieviruses, echoviruses, and the numbered enteroviruses) Varicella-zoster virus Herpes simplex virus 2 Epstein-Barr virus Arthropod-borne viruses (notably WNV) HIV Herpes simplex virus 1 Human herpesvirus 6 Cytomegalovirus Lymphocytic choriomeningitis virus Mumps Zika and other non-WNV arboviruses Abbreviation: WNV, West Nile virus. viral meningitis. The most important agents are enteroviruses (includ­ ing echoviruses and coxsackieviruses in addition to numbered entero­ viruses), varicella-zoster virus (VZV), HSV (HSV-2 > HSV-1), HIV, and arboviruses (Table 143-4). CSF cultures are positive in 30–70% of patients, with the frequency of isolation depending on the specific viral agent. Approximately two-thirds of culture-negative cases of “aseptic” meningitis have a specific viral etiology identified by CSF PCR testing (see below). ■ ■EPIDEMIOLOGY Viral meningitis is not a nationally reportable disease; however, it has been estimated that the incidence is at least 75,000 cases per year in the United States. In temperate climates, there is a substantial increase in cases during the nonwinter months, reflecting the seasonal predomi­ nance of enterovirus and arthropod-borne virus (arbovirus) infections in the summer and fall, with a peak monthly incidence of about 1 reported case per 100,000 population. PART 5 Infectious Diseases ■ ■LABORATORY DIAGNOSIS CSF Examination  The most important laboratory test in the diagnosis of viral meningitis is examination of the CSF. The typical profile is a pleocytosis, a normal or slightly elevated protein concen­ tration (0.2–0.8 g/L [20–80 mg/dL]), a normal glucose concentra­ tion, and a normal or mildly elevated opening pressure (100–350 mmH2O). Organisms are not seen on Gram’s stain of CSF. The total CSF cell count in viral meningitis is typically 25–500/μL, although cell counts of several thousand/μL are occasionally seen, especially with infections due to lymphocytic choriomeningitis virus (LCMV) and mumps virus. Lymphocytes are typically the predominant cell. Rarely, PMNs may predominate in the first 48 h of illness, especially with infections due to echovirus 9, West Nile virus (WNV), eastern equine encephalitis (EEE) virus, or mumps. A PMN pleocytosis occurs in 45% of patients with WNV meningitis and can persist for a week or longer before shifting to a lymphocytic pleocytosis. PMN pleocytosis with low glucose may also be a feature of cytomegalovi­ rus (CMV) infections in immunocompromised hosts. Despite these exceptions, the presence of a CSF PMN pleocytosis in a patient with suspected viral meningitis in whom a specific diagnosis has not been established should prompt consideration of alternative diagnoses, including bacterial meningitis or parameningeal infections. The CSF glucose concentration is typically normal in viral infections, although it may be decreased in 10–30% of cases due to mumps or LCMV. Rare instances of decreased CSF glucose concentration occur in cases of meningitis due to echoviruses and other enteroviruses, HSV-2, and VZV. As a rule, a lymphocytic pleocytosis with a low glucose con­ centration should suggest fungal or tuberculous meningitis, Listeria meningoencephalitis, or noninfectious disorders (e.g., sarcoid, neo­ plastic meningitis). A number of tests measuring levels of various CSF proteins, enzymes, and mediators—including C-reactive protein, lactic acid, lactate dehydrogenase, neopterin, quinolinate, IL-1β, IL-6, soluble IL-2 receptor, β2-microglobulin, and TNF—have been proposed as potential discriminators between viral and bacterial meningitis or as markers of specific types of viral infection (e.g., infection with HIV), but they

remain of uncertain sensitivity and specificity and are not widely used for diagnostic purposes. Polymerase Chain Reaction Amplification of Viral Nucleic Acid  Amplification of viral-specific DNA or RNA from CSF using PCR amplification has become the single most important method for diagnosing CNS viral infections. In both enteroviral and HSV infec­ tions of the CNS, CSF PCR has become the diagnostic procedure of choice and is substantially more sensitive than viral cultures. HSV CSF PCR is also an important diagnostic test in patients with recur­ rent episodes of “aseptic” meningitis, many of whom have amplifiable HSV DNA in CSF despite negative viral cultures. The FilmArray Meningitis/Encephalitis panel (BioFire Diagnostics) includes HSV-1, HSV-2, enteroviruses, VZV, human herpesvirus 6 (HHV-6), and human Parechovirus. The panel does not include CMV, WNV, LCMV, or Epstein-Barr virus (EBV). CSF PCR tests are available for WNV but are not as sensitive as detection of WNV-specific CSF IgM. PCR is also useful in the diagnosis of CNS infection caused by Mycoplasma pneu­ moniae, which can mimic viral meningitis and encephalitis. PCR of throat washings may assist in diagnosis of enteroviral and mycoplasmal CNS infections. PCR of stool specimens may also assist in diagnosis of enteroviral infections (see below). Viral Culture  The sensitivity of CSF cultures for the diagnosis of viral meningitis and encephalitis, in contrast to its utility in bacterial infections, is generally poor. In addition to CSF, specific viruses may also be isolated from throat swabs, stool, blood, and urine. Entero­ viruses and adenoviruses may be found in feces; arboviruses, some enteroviruses, and LCMV in blood; mumps and CMV in urine; and enteroviruses, mumps, and adenoviruses in throat washings. During enteroviral infections, viral shedding in stool may persist for several weeks. The presence of enterovirus in stool is not diagnostic and may result from residual shedding from a previous enteroviral infection; it also occurs in some asymptomatic individuals during enteroviral epidemics. Serologic Studies  The basic approach to the serodiagnosis of viral meningitis is identical to that for viral encephalitis (see Chap. 142). Serologic studies are important for the diagnosis of arboviruses such as WNV; however, these tests are less useful for viruses such as HSV, VZV, CMV, and EBV that have a high seroprevalence in the general population. CSF oligoclonal γ globulin bands occur in association with a num­ ber of viral infections. The associated antibodies are often directed against viral proteins. Oligoclonal bands also occur commonly in certain noninfectious neurologic diseases (e.g., multiple sclerosis) and may be found in nonviral infections (e.g., neurosyphilis, Lyme neuroborreliosis). Other Laboratory Studies  All patients with suspected viral men­ ingitis should have a complete blood count and differential, liver and renal function tests, erythrocyte sedimentation rate (ESR), C-reactive protein, electrolytes, glucose, creatine kinase, aldolase, amylase, and lipase. Neuroimaging studies (MRI preferable to CT) are not absolutely necessary in patients with uncomplicated viral meningitis but should be performed in patients with altered consciousness, seizures, focal neurologic signs or symptoms (see “Differential Diagnosis” below), atypical CSF profiles, or underlying immunocompromising treatments or conditions. ■ ■DIFFERENTIAL DIAGNOSIS The most important issue in the differential diagnosis of viral menin­ gitis is to consider diseases that can mimic viral meningitis, including (1) untreated or partially treated bacterial meningitis; (2) early stages of meningitis caused by fungi, mycobacteria, or Treponema pallidum (neurosyphilis), in which a lymphocytic pleocytosis is common, cul­ tures may be slow growing or negative, and hypoglycorrhachia may not be present early; (3) meningitis caused by agents such as Myco­ plasma, Listeria spp., Brucella spp., Coxiella spp., Leptospira spp., and Rickettsia spp.; (4) parameningeal infections; (5) neoplastic meningitis;

and (6) meningitis secondary to noninfectious inflammatory diseases, including medication-induced hypersensitivity meningitis, SLE and other rheumatologic diseases, sarcoidosis, Behçet’s syndrome, and the uveomeningitic syndromes. Studies in children >28 days of age suggest that the presence of CSF protein >0.5 g/L (sensitivity 89%, specificity 78%) and elevated serum procalcitonin levels >0.5 ng/mL (sensitivity 89%, specificity 89%) were clues to the presence of bacterial as opposed to “aseptic” meningitis. A variety of clinical algorithms for differentiat­ ing bacterial from aseptic meningitis have been developed. One such prospectively validated system, the bacterial meningitis score, suggests that the probability of bacterial meningitis is 0.3% or less (negative predictive value 99.7%, 95% confidence interval 99.6–100%) in chil­ dren with CSF pleocytosis who have (1) a negative CSF Gram’s stain, (2) CSF neutrophil count <1000 cells/μL, (3) CSF protein <80 mg/dL, (4) peripheral absolute neutrophil count of <10,000 cells/μL, and (5) no prior history or current presence of seizures. ■ ■SPECIFIC VIRAL ETIOLOGIES Enteroviruses (EVs) (Chap. 210) are the most common cause of viral meningitis, accounting for >85% of cases in which a specific etiology can be identified. Cases may either be sporadic or occur in clusters. EV71 has produced large epidemics of neurologic disease outside the United States, especially in Southeast Asia, but most recently reported cases in the United States have been sporadic. Enteroviruses are the most likely cause of viral meningitis in the summer and fall months, especially in children (<15 years), although cases occur at reduced frequency year-round. Although the incidence of enteroviral menin­ gitis declines with increasing age, some outbreaks have preferentially affected older children and adults. Meningitis outside the neonatal period is usually benign. Patients present with sudden onset of fever; headache; nuchal rigidity; and often constitutional signs, including vomiting, anorexia, diarrhea, cough, pharyngitis, and myalgias. The physical examination should include a careful search for stigmata of EV infection, including exanthems, hand-foot-mouth disease, herpangina, pleurodynia, myopericarditis, and hemorrhagic conjunctivitis. The CSF profile is typically a lymphocytic pleocytosis (100–1000 cells/μL) with normal glucose and normal or mildly elevated protein concentra­ tion. However, up to 15% of patients, most commonly young infants rather than older children or adults, have a normal CSF leukocyte count. In rare cases, PMNs may predominate during the first 48 h of illness. CSF reverse transcriptase PCR (RT-PCR) is the diagnostic procedure of choice and is both sensitive (>95%) and specific (>100%). CSF RT-PCR has the highest sensitivity if performed within 48 h of symptom onset, with sensitivity declining rapidly after day 5 of symp­ toms. RT-PCR of throat washings or stool specimens may be positive for several weeks, and positive results can help support the diagnosis of an acute enteroviral infection. The sensitivity of routine enteroviral RTPCRs for detecting EV71 is low, and specific testing may be required. Treatment is supportive, and patients usually recover without sequelae. Chronic and severe infections can occur in neonates and in individuals with hypo- or agammaglobulinemia. Arbovirus infections (Chap. 215) occur predominantly in the sum­ mer and early fall. Arboviral meningitis should be considered when clusters of meningitis and encephalitis cases occur in a restricted geo­ graphic region during the summer or early fall. In the United States, the most important causes of arboviral meningitis and encephalitis are WNV, St. Louis encephalitis virus, and the California encephali­ tis group of viruses. In WNV epidemics, avian deaths may serve as sentinel infections for subsequent human disease. A history of tick exposure or travel or residence in the appropriate geographic area should suggest the possibility of Colorado tick fever virus or Powas­ san virus infection, although nonviral tick-borne diseases, including RMSF and Lyme neuroborreliosis, may present similarly. Arbovirus meningitis is typically associated with a CSF lymphocytic pleocytosis, normal glucose concentration, and normal or mildly elevated protein concentration. However, ~45% of patients with WNV meningitis have CSF neutrophilia, which can persist for a week or more. The rarity of hypoglycorrhachia in WNV infection, the absence of posi­ tive Gram’s stains, and the negative cultures help distinguish these

patients from those with bacterial meningitis. Definitive diagnosis of arboviral meningitis is based on demonstration of viral-specific IgM in CSF or seroconversion. The prevalence of CSF IgM increases progressively during the first week after infection, peaking at >80% in patients with neuroinvasive disease; as a result, repeat studies may be needed when disease suspicion is high and an early study is negative. CSF RT-PCR tests are available for some viruses in selected diagnos­ tic laboratories and at the Centers for Disease Control and Prevention (CDC), but in the case of WNV, sensitivity (~70%) of CSF RT-PCR is less than that of CSF serology. WNV CSF RT-PCR may be useful in immunocompromised patients who may have absent or reduced antibody responses.

HSV meningitis (Chap. 197) has been increasingly recognized as a major cause of viral meningitis in adults, and overall, it is probably second in importance to enteroviruses as a cause of viral meningitis, accounting for 5% of total cases overall and undoubtedly a higher frequency of those cases occurring in adults and/or outside of the summer-fall period when enterovirus infections are increasingly com­ mon. In adults, the majority of cases of uncomplicated meningitis are due to HSV-2, whereas HSV-1 is responsible for 90% of cases of HSV encephalitis. HSV meningitis occurs in ~25–35% of women and ~10–15% of men at the time of an initial (primary) episode of geni­ tal herpes. Of these patients, 20% go on to have recurrent attacks of meningitis. Diagnosis of HSV meningitis is usually by HSV CSF PCR because cultures may be negative, especially in patients with recurrent meningitis. Demonstration of intrathecal synthesis of HSV-specific antibody may also be useful in diagnosis, although antibody tests are less sensitive and less specific than PCR and may not become posi­ tive until after the first week of infection. Although a history of or the presence of HSV genital lesions is an important diagnostic clue, many patients with HSV meningitis give no history and have no evidence of active genital herpes at the time of presentation. Most cases of recur­ rent viral or “aseptic” meningitis, including cases previously diagnosed as Mollaret’s meningitis, are due to HSV. CHAPTER 143 Acute Meningitis VZV meningitis (Chap. 198) should be suspected in the presence of concurrent chickenpox or shingles. However, it is important to recognize that VZV is being increasingly identified as an important cause of both meningitis and encephalitis in patients without rash. The frequency of VZV as a cause of meningitis is extremely variable, rang­ ing from as low as 3% to as high as 20% in different series. Diagnosis is usually based on CSF PCR, although the sensitivity of this test is not as high as for the other herpesviruses. VZV serologic studies complement PCR testing, and the diagnosis of VZV CNS infection can be made by the demonstration of VZV-specific intrathecal antibody synthesis and/or the presence of VZV CSF IgM antibodies, or by positive CSF cultures. EBV infections (Chap. 199) may also produce aseptic meningitis, with or without associated infectious mononucleosis. The presence of atypical lymphocytes in the CSF or peripheral blood is suggestive of EBV infection but may occasionally be seen with other viral infections. EBV is almost never cultured from CSF. Serum and CSF serology help establish the presence of acute infection, which is characterized by IgM viral capsid antibodies (VCAs), antibodies to early antigens (EAs), and the absence of antibodies to EBV-associated nuclear antigen (EBNA). CSF PCR is another important diagnostic test, although false-positive results may reflect viral reactivation associated with other infectious or inflammatory processes or the presence of latent viral DNA in lympho­ cytes recruited due to other inflammatory conditions. HIV meningitis should be suspected in any patient presenting with a viral meningitis with known or suspected risk factors for HIV infec­ tion. Meningitis may occur following primary infection with HIV in 5–10% of cases and less commonly at later stages of illness. Cranial nerve palsies, most commonly involving cranial nerves V, VII, or VIII, are more common in HIV meningitis than in other viral infections. Diagnosis can be confirmed by detection of HIV genome in blood or CSF. Seroconversion may be delayed, and patients with negative HIV serologies who are suspected of having HIV meningitis should be monitored for delayed seroconversion. For further discussion of HIV infection, see Chap. 208.

Mumps (Chap. 213) should be considered when meningitis occurs in the late winter or early spring, especially in males (male-to-female ratio 3:1). With the widespread use of the live attenuated mumps vac­ cine in the United States since 1967, the incidence of mumps menin­ gitis has fallen by >95%; however, mumps remains a potential source of infection in nonimmunized individuals and populations, but also in those who have been vaccinated but may have waning immunity. Rare cases (10–100/100,000 vaccinated individuals) of vaccine-associated mumps meningitis have been described, with onset typically 2–4 weeks after vaccination. The presence of parotitis, orchitis, oophoritis, pancreatitis, or elevations in serum lipase and amylase is suggestive of mumps meningitis; however, their absence does not exclude the diagnosis. Clinical meningitis has been estimated to occur in 10–30% of patients with mumps parotitis. Mumps infection confers lifelong immunity, so a documented history of previous infection excludes this diagnosis. A CSF pleocytosis that can exceed 1000 cells/μL is present in 25% of patients with mumps meningitis. Lymphocytes predominate in 75%, although CSF neutrophilia occurs in 25%. Hypoglycorrhachia occurs in 10–30% of patients and may be a clue to the diagnosis when present. Diagnosis is typically made by culture of virus from CSF or by detecting IgM antibodies or seroconversion. CSF PCR is available in some diagnostic and research laboratories.

LCMV infection (Chap. 215) should be considered when aseptic meningitis occurs in the late fall or winter and in individuals with a history of exposure to house mice (Mus musculus), pet or laboratory rodents (e.g., hamsters, rats, mice), or their excreta. Some patients have an associated rash, pulmonary infiltrates, alopecia, parotitis, orchitis, or myopericarditis. Laboratory clues to the diagnosis of LCMV, in addition to the clinical findings noted above, may include the presence of leukopenia, thrombocytopenia, or abnormal liver function tests. Some cases present with a marked CSF pleocytosis (>1000 cells/μL) and hypoglycorrhachia (<30%). Diagnosis is based on serology and/or culture of virus from CSF. PART 5 Infectious Diseases TREATMENT Acute Viral Meningitis Treatment of almost all cases of viral meningitis is primarily symp­ tomatic and includes use of analgesics, antipyretics, and antiemet­ ics. Fluid and electrolyte status should be monitored. Patients with suspected bacterial meningitis should receive appropriate empirical therapy pending culture results (see above). Hospitalization may not be required in immunocompetent patients with presumed viral meningitis and no focal signs or symptoms, no significant alteration in consciousness, and a classic CSF profile (lymphocytic pleocytosis, normal glucose, negative Gram’s stain) if adequate provision for monitoring at home and medical follow-up can be ensured. Immunocompromised patients; patients with significant alteration in consciousness, seizures, or the presence of focal signs and symptoms suggesting the possibility of encephalitis or paren­ chymal brain involvement; and patients who have an atypical CSF profile should be hospitalized. Oral or intravenous acyclovir may be of benefit in patients with meningitis caused by HSV-1 or -2 and in cases of severe EBV or VZV infection. Data concerning treatment of HSV, EBV, and VZV meningitis are extremely limited. Seriously ill patients should probably receive intravenous acyclovir (15–30 mg/kg per day in three divided doses), which can be fol­ lowed by an oral drug such as acyclovir (800 mg five times daily), famciclovir (500 mg tid), or valacyclovir (1000 mg tid) for a total course of 7–14 days. Patients who are less ill can be treated with oral drugs alone. Patients with HIV meningitis should receive antiretroviral therapy (Chap. 208). There is no specific therapy of proven benefit for patients with arboviral encephalitis, including that caused by WNV. Patients with viral meningitis who are known to have deficient humoral immunity (e.g., X-linked agammaglobulinemia) and who are not already receiving either intramuscular γ globulin or intrave­ nous immunoglobulin (IVIg) should be treated with these agents.

Intraventricular administration of immunoglobulin through an Ommaya reservoir has been tried in some patients with chronic enteroviral meningitis who have not responded to intramuscular or intravenous immunoglobulin. Vaccination is an effective method of preventing the devel­ opment of meningitis and other neurologic complications asso­ ciated with poliovirus, mumps, measles, rubella, and varicella infection. A recombinant zoster vaccine (RSV, Shingrix) contains recombinant VZV glycoprotein E in combination with an adjuvant (ASO1B) and has greater efficacy in preventing zoster in adults aged ≥70 years than the previously recommended live attenuated vaccine (Zostrax). The Advisory Committee on Immunization Practices recommends the use of the recombinant zoster vaccine in immunocompetent adults aged ≥50 years and in immunodeficient or immunosuppressed adults ≥19 years of age. ■ ■PROGNOSIS In adults, the prognosis for full recovery from viral meningitis is excellent. Rare patients complain of persisting headache, mild men­ tal impairment, incoordination, or generalized asthenia for weeks to months. The outcome in infants and neonates (<1 year) is less certain; intellectual impairment, learning disabilities, hearing loss, and other lasting sequelae have been reported in some studies. SUBACUTE MENINGITIS ■ ■CLINICAL MANIFESTATIONS Patients with subacute meningitis typically have an unrelenting head­ ache, stiff neck, low-grade fever, and lethargy for days to several weeks before they present for evaluation. Cranial nerve abnormalities and night sweats may be present. This syndrome overlaps that of chronic meningitis, discussed in detail in Chap. 144 but is included here because the meningeal pathogens of subacute meningitis can also present as an acute meningitis. ■ ■ETIOLOGY Common causative organisms include M. tuberculosis, C. neoformans, H. capsulatum, C. immitis, and T. pallidum. Initial infection with

M. tuberculosis is acquired by inhalation of aerosolized droplet nuclei. Tuberculous meningitis in adults does not develop acutely from hematogenous spread of tubercle bacilli to the meninges. Rather, millet seed–sized (miliary) tubercles form in the parenchyma of the brain during hematogenous dissemination of tubercle bacilli in the course of primary infection. These tubercles enlarge and are usually caseating. The propensity for a caseous lesion to produce meningitis is determined by its proximity to the SAS and the rate at which fibrous encapsulation develops. Subependymal caseous foci cause meningitis via discharge of bacilli and tuberculous antigens into the SAS. Myco­ bacterial antigens produce an intense inflammatory reaction that leads to the production of a thick exudate that fills the basilar cisterns and surrounds the cranial nerves and major blood vessels at the base of the brain. Fungal infections are typically acquired by the inhalation of airborne fungal spores. The initial pulmonary infection may be asymptomatic or present with fever, cough, sputum production, and chest pain. The pulmonary infection is often self-limited. A localized pulmonary fungal infection can then remain dormant in the lungs until there is an abnormality in cell-mediated immunity that allows the fungus to reactivate and disseminate to the CNS. The most common pathogen causing fungal meningitis is C. neoformans. This fungus is found worldwide in soil and bird excreta. H. capsulatum is endemic to the Ohio and Mississippi River valleys of the central United States and to parts of Central and South America. C. immitis is endemic to the desert areas of the southwest United States, northern Mexico, and Argentina. Syphilis is a sexually transmitted disease that is manifest by the appearance of a painless chancre at the site of inoculation. T. pallidum invades the CNS early in the course of syphilis. Cranial nerves VII and VIII are most frequently involved.

■ ■LABORATORY DIAGNOSIS The classic CSF abnormalities in tuberculous meningitis are as follows: (1) elevated opening pressure, (2) lymphocytic pleocytosis (10–500 cells/μL), (3) elevated protein concentration in the range of 1–5 g/L, and (4) decreased glucose concentration in the range of 1.1–2.2 mmol/L (20–40 mg/dL). The combination of unrelenting headache, stiff neck, fatigue, night sweats, and fever with a CSF lymphocytic pleocytosis and a mildly decreased glucose concentration is highly suspicious for tuberculous meningitis. The last tube of fluid collected at LP is the best tube to send for a smear for acid-fast bacilli (AFB). If there is a pellicle in the CSF or a cobweb-like clot on the surface of the fluid, AFB can best be demonstrated in a smear of the clot or pellicle. Positive smears are typically reported in only 10–40% of cases of tuberculous meningi­ tis in adults. Cultures of CSF take 4–8 weeks to identify the organism and are positive in ~50% of adults. Culture remains the gold standard to make the diagnosis of tuberculous meningitis. Nucleic acid amplifi­ cation tests for the detection of M. tuberculosis DNA should be sent on CSF if available. The XpertMTB/RIF Ultra and the Xpert MTB/RIF are used most commonly, but the consensus of opinion is that the sensitiv­ ity, and thus the risk of false-negative results, has not been defined. The characteristic CSF abnormalities in fungal meningitis are a mononuclear or lymphocytic pleocytosis, an increased protein con­ centration, and a decreased glucose concentration. There may be eosinophils in the CSF in C. immitis meningitis. Large volumes of CSF are often required to demonstrate the organism on India ink smear or grow the organism in culture. If spinal fluid examined by LP on two separate occasions fails to yield an organism, CSF should be obtained by high-cervical or cisternal puncture. The cryptococcal polysaccharide antigen test is a highly sensitive and specific test for cryptococcal meningitis. A reactive CSF cryptococcal antigen test establishes the diagnosis. The detection of the Histoplasma polysaccharide antigen in CSF establishes the diagnosis of a fungal meningitis but is not specific for meningitis due to H. capsulatum. It may be falsely positive in coccidioidal meningitis. The CSF complement fixation antibody test is reported to have a specificity of 100% and a sen­ sitivity of 75% for coccidioidal meningitis. The diagnosis of syphilitic meningitis is made when a reactive serum treponemal test (fluorescent treponemal antibody absorption test [FTA-ABS] or microhemagglutination assay–T. pallidum [MHA-TP]) is associated with a CSF lymphocytic or mononuclear pleocytosis and an elevated protein concentration, or when the CSF Venereal Disease Research Laboratory (VDRL) test is positive. A reactive CSF FTA-ABS is not definitive evidence of neurosyphilis. The CSF FTA-ABS can be falsely positive from blood contamination. A negative CSF VDRL does not rule out neurosyphilis. A negative CSF FTA-ABS or MHA-TP rules out neurosyphilis. TREATMENT Subacute Meningitis Empirical therapy of tuberculous meningitis is often initiated based on a high index of suspicion without adequate laboratory support. Initial therapy is a combination of isoniazid (300 mg/d), rifampin (10 mg/kg per day), pyrazinamide (30 mg/kg per day in divided doses), ethambutol (15–25 mg/kg per day in divided doses), and pyridoxine (50 mg/d). When the antimicrobial sensitivity of the M. tuberculosis isolate is known, ethambutol can be discontinued. If the clinical response is good, pyrazinamide can be discontinued after 8 weeks and isoniazid and rifampin continued alone for the next 6–12 months. A 6-month course of therapy is acceptable, but therapy should be prolonged for 9–12 months in patients who have an inadequate resolution of symptoms of meningitis or who have positive mycobacterial cultures of CSF during the course of therapy. Dexamethasone therapy is recommended for HIV-negative patients with tuberculous meningitis. The dose is 12–16 mg/d for 3 weeks, and then tapered over 3 weeks.

Meningitis due to C. neoformans in non-HIV, nontransplant patients is treated with induction therapy with amphotericin B (AmB) (0.7 mg/kg IV per day) plus flucytosine (100 mg/kg per day in four divided doses) for at least 4 weeks if CSF culture results are negative after 2 weeks of treatment. Therapy should be extended for a total of 6 weeks in the patient with neurologic complica­ tions. Induction therapy is followed by consolidation therapy with fluconazole 400 mg/d for 8 weeks. Organ transplant recipients are treated with liposomal AmB (3–4 mg/kg per day) or AmB lipid complex (ABLC; 5 mg/kg per day) plus flucytosine (100 mg/kg per day in four divided doses) for at least 2 weeks or until CSF culture is sterile. Follow CSF yeast cultures for sterilization rather than the cryptococcal antigen titer. This treatment is followed by an 8- to 10-week course of fluconazole (400–800 mg/d [6–12 mg/kg] PO). If the CSF culture is sterile after 10 weeks of acute therapy, the dose of fluconazole is decreased to 200 mg/d for 6 months to a year. Patients with HIV infection are treated with AmB or a lipid formulation plus flucytosine for at least 2 weeks, followed by fluconazole for a minimum of 8 weeks. HIV-infected patients may require indefinite maintenance therapy with fluconazole 200 mg/d. Meningitis due to H. capsulatum is treated with AmB (0.7–1.0 mg/kg per day) for 4–12 weeks. A total dose of 30 mg/kg is recommended. Therapy with AmB is not discontinued until fungal cultures are sterile. After completing a course of AmB, maintenance therapy with itraconazole 200 mg two or three times daily is initiated and continued for at least 9 months to a year. C. immitis meningitis is treated with either high-dose fluconazole (1000 mg daily) as monotherapy or intrave­ nous AmB (0.5–0.7 mg/kg per day) for >4 weeks. Intrathecal AmB (0.25–0.75 mg/d three times weekly) may be required to eradicate the infection. Lifelong therapy with fluconazole (200–400 mg daily) is recommended to prevent relapse. AmBisome (5 mg/kg per day) or amphotericin B lipid complex (ABLC; 5 mg/kg per day) can be substituted for AmB in patients who have or who develop signifi­ cant renal dysfunction. The most common complication of fungal meningitis is hydrocephalus. Patients who develop hydrocephalus should receive a CSF diversion device. A ventriculostomy can be used until CSF fungal cultures are sterile, at which time the ven­ triculostomy is replaced by a ventriculoperitoneal shunt.

CHAPTER 143 Acute Meningitis Syphilitic meningitis is treated with aqueous penicillin G in a dose of 3–4 million units intravenously every 4 h for 10–14 days. An alternative regimen is 2.4 million units of procaine penicillin G intramuscularly daily with 500 mg of oral probenecid four times daily for 10–14 days. Either regimen is followed with 2.4 million units of benzathine penicillin G intramuscularly once a week for 3 weeks. The standard criterion for treatment success is reexamina­ tion of the CSF. The CSF should be reexamined at 6-month intervals for 2 years. The cell count is expected to normalize within 12 months, and the VDRL titer to decrease by two dilutions or revert to non­ reactive within 2 years of completion of therapy. Failure of the CSF pleocytosis to resolve or an increase in the CSF VDRL titer by two or more dilutions requires retreatment. ■ ■FURTHER READING Gundamraj V et al: Viral meningitis and encephalitis: An update. Curr Opin Infect Dis 36:177, 2023. Krett JD et al: Neurology of acute viral infections. Neurohospitalist 12:632, 2022. Pfefferle S et al: Implementation of the FilmArray ME panel in labo­ ratory routine using a simple sample selection strategy for diagnosis of meningitis and encephalitis. BMC Infect Dis 20:170, 2020. Roos KL et al: Acute bacterial meningitis, in Infections of the Cen­ tral Nervous System, 4th ed. Scheld WM, Whitley RJ, Marra (eds). Philadelphia, Wolters Kluwer Health, 2014, pp. 365–419. Van de Beek D et al: Community acquired bacterial meningitis. Lancet 398:1171, 2021.

23 - 144 Chronic and Recurrent Meningitis

144 Chronic and Recurrent Meningitis

Avindra Nath, Walter J. Koroshetz,

Michael R. Wilson

Chronic and Recurrent Meningitis Chronic inflammation of the meninges (pia, arachnoid, and dura) can produce profound neurologic disability and may be fatal if not success­ fully treated. Chronic meningitis is diagnosed when a characteristic neurologic syndrome exists for >4 weeks and is associated with a per­ sistent inflammatory response in the cerebrospinal fluid (CSF) (white cell count >5/μL). The causes are varied, and appropriate treatment depends on identification of the etiology. Five categories of disease account for most cases of chronic meningitis: (1) meningeal infections, (2) malignancy, (3) autoimmune inflammatory disorders, (4) chemi­ cal meningitis, and (5) parameningeal infections. In addition, there is increasing recognition that some patients with recurrent meningitis may have monogenic autoinflammatory disorders. ■ ■CLINICAL PATHOPHYSIOLOGY Neurologic manifestations of chronic meningitis (Table 144-1) are determined by the anatomic location of the inflammation and its con­ sequences. Persistent headache, clinical signs of hydrocephalus, cranial neuropathies, radiculopathies, and cognitive or personality changes are the cardinal features. Meningeal signs are uncommon in chronic meningitis. These manifestations can occur alone or in combination. When they appear in combination, it may be indicative of widespread dissemination of the inflammatory process along CSF pathways. In some cases, the presence of an underlying systemic illness points to the probable cause of the meningitis (Fig. 144-1). The diagnosis of chronic meningitis is usually made when the clinical presentation prompts the physician to examine the CSF for signs of inflammation. CSF is produced by the choroid plexus of the cerebral ventricles, exits through the foramina in the fourth ventricle into the subarachnoid space surrounding the brain and spinal cord, circulates around the base of the brain and over the cerebral hemispheres, and is resorbed by arachnoid villi projecting into the superior sagittal sinus where it mixes with blood in the venous sinuses. Recently, a cerebral lymphatic system has been identified that drains the dura mater (Chap. 435); however, its role in chronic meningitis has not been studied. CSF flow provides a pathway for rapid spread of infectious and other infiltrative processes over the brain, spinal cord, cranial, and spinal nerve roots. Spread from the subarachnoid space into brain parenchyma may occur via the arachnoid cuffs that surround blood vessels that penetrate brain PART 5 Infectious Diseases TABLE 144-1  Symptoms and Signs of Chronic Meningitis SYMPTOM SIGN Chronic headache ± Papilledema Neck or back pain/stiffness Brudzinski’s or Kernig’s sign of meningeal irritation Change in personality Altered mental status—drowsiness, inattention, disorientation, memory loss, frontal release signs (grasp, suck, snout), perseveration Facial weakness Peripheral seventh CN paresis Double vision Paresis of CNs III, IV, and/or VI Diminished vision Papilledema, CN II (optic atrophy/inflammation) Hearing loss Eighth CN paresis Arm or leg weakness Myelopathy or radiculopathy Numbness in arms or legs Myelopathy or radiculopathy Urinary retention/ incontinence Myelopathy or radiculopathy Frontal lobe dysfunction (hydrocephalus) Clumsiness Ataxia Abbreviation: CN, cranial nerve.

tissue (Virchow-Robin spaces). Microorganisms can travel along these perivascular spaces to enter the brain. Intracranial Meningitis  In intracranial meningitis, nociceptive nerve fibers of the meninges are stimulated by the inflammatory pro­ cess, resulting in headache, neck pain, or back pain. Obstruction of CSF pathways at the cerebral aqueduct or arachnoid villi may produce hydrocephalus and signs and symptoms of raised intracranial pressure (ICP), including headache, vomiting, apathy or drowsiness, gait insta­ bility, papilledema, visual loss, impaired upgaze, or palsy of the sixth cranial nerve (CN VI). Cognitive and behavioral changes during the course of chronic meningitis may also result from vascular damage due to inflammation around the blood vessels in the subarachnoid space, causing infarction. Inflammatory deposits seeded via the CSF circula­ tion are often prominent around the brainstem and cranial nerves and along the undersurface of the frontal and temporal lobes. Such cases, termed basal meningitis, often present as multiple cranial neuropathies (Chap. 452), with some combination of decreased vision (CN II), facial weakness (CN VII), decreased hearing (CN VIII), diplopia (CNs III, IV, and VI), sensory or motor abnormalities of the oropharynx (CNs IX, X, and XII), decreased olfaction (CN I), or decreased facial sensa­ tion and masseter weakness (CN V). Involvement of the lower CNs is more common because the inflammatory exudate tends to collect at the base of the brain. Spinal Meningitis  In spinal meningitis, injury may occur to motor and sensory nerve roots as they traverse the subarachnoid space and penetrate the meninges. These cases present as multiple radicu­ lopathies with combinations of radicular pain, sensory loss, motor weakness, and urinary or fecal incontinence. In some cases, chronic inflammation causes arachnoiditis with clumping of the lower nerve roots and thickening of the meninges. Preferential involvement of the lower nerve roots results from inflammatory cells that gravitate to the bottom of the intrathecal space. Meningeal inflammation can encircle and damage the spinal cord, resulting in a myelopathy. Slow progressive involvement of multiple CNs and/or spinal nerve roots is likely due to chronic meningitis. Electrophysiologic testing (electromy­ ography, nerve conduction studies, and evoked response testing) may be helpful in determining whether there is involvement of cranial and spinal nerve roots. Systemic Manifestations  In some patients, evidence of systemic disease provides clues to the underlying cause of chronic meningitis. A complete history of travel, sexual exposure, insect bites, and other modes of exposure to infectious agents should be sought. Infectious causes are often associated with fever, malaise, anorexia, and signs of localized or disseminated infection outside the nervous system. Infec­ tious causes are of major concern in immunosuppressed patients and especially in patients with untreated HIV infection, in whom chronic meningitis is most often caused by Mycobacterium tuberculosis or Cryptococcus neoformans and may present without headache, fever, or meningeal signs. In this population, a high index of clinical suspicion needs to be maintained even when there is only mild confusion or a nonspecific headache syndrome even in the context of a pauicellular CSF profile. Noninfectious inflammatory disorders most often pro­ duce systemic manifestations first, but meningitis may be the initial manifestation. Carcinomatous meningitis, caused by CSF seeding with metastatic cancer cells, may or may not be accompanied by clinical evidence of the primary neoplasm. APPROACH TO THE PATIENT Chronic Meningitis The occurrence of chronic or worsening headache, typically constant and nonfocal, clinical signs of hydrocephalus, cranial neuropathy, radiculopathy, and/or cognitive decline in a patient with or without fever should prompt consideration of a lumbar puncture for evidence of meningeal inflammation. On occasion, the diagnosis is made when a contrast-enhanced imaging study

Skin changes Behçet’s syndrome Systemic lupus erythematosus Cryptococcosis Blastomycosis
Eyes Uveitis VKH syndrome Sarcoidosis Lymphoma Fungal infections Taenia solium Tuberculosis Herpes viruses Keratoconjunctivitis Sjögren’s syndrome Hypopyon Behçet’s syndrome Iridocyclitis Behçet’s syndrome Vasculitis Primary CNS vasculitis Bacterial, mycobacterial, spirochete, parasitic and viral infections Malignancy including lymphoma Paraneoplastic Rheumatoid arthritis ANCA associated vasculitis Sarcoidosis Sjögren’s syndrome Systemic lupus erythematosus Lymph nodes Lymphoma Metastatic adenocarcinoma Sarcoidosis Tuberculosis HIV Secondary syphilis Whipple’s disease Pancreas/GI tract IgG4-related disease Whipple’s disease Syphilis Sarcoidosis Bone/Bone marrow Lymphoma/Leukemia Fungal, bacterial and mycobacterial infections Brucellosis (vertebral osteomyelitis) Histiocytic disorders Metastatic adenocarcinoma Rheumatoid arthritis (joint space) FIGURE 144-1  Systemic manifestations that may provide clues to the etiology of chronic meningitis. (magnetic resonance imaging [MRI] or computed tomography [CT]) shows leakage of contrast agent into the meninges. Menin­ geal enhancement is always concerning with the exception of dural enhancement after lumbar puncture, neurosurgical procedures, concussion, or spontaneous CSF leakage. Once chronic meningitis is confirmed by CSF examination, effort is focused on identifying

Sarcoidosis Vasculitis CAPS (urticaria) Syphilis (diffuse rash including palms and soles) Lyme disease Sporotrichosis NOMID Trypanosomiasis IV drug use

Ear Fungal, mycobacterial, bacterial infections (chronic drainage) Varicella zoster virus Sinus Fungal and bacterial infections ANCA associated vasculitis IgG4-related disease Sarcoidosis Syphilis Mouth Dental abscess Behçet’s syndrome (aphthous ulcer) Herpes simplex virus types 1 and 2 HIV (candidiasis) Syphilis Whipple’s disease (oculomasticatory myorhythmia) CHAPTER 144 Thyroid IgG4-related diseases Chronic and Recurrent Meningitis Heart/Lungs Infectious source (right-to-left shunt, pneumonia) Sarcoidosis Syphilis Systemic lupus erythematosus IV drug use (endocarditis) Liver/Spleen Lymphoma Metastatic adenocarcinoma Sarcoidosis Tuberculosis Parasitic infection Brucellosis Rickettsial infection Genitals Herpes simplex virus Syphilis Behçet’s syndrome (aphthous ulcer) the cause (Tables 144-2 and 144-3) by (1) further analysis of the CSF, (2) diagnosis of an underlying systemic infection or noninfec­ tious inflammatory condition, or (3) pathologic examination of meningeal biopsy specimens. Two clinical forms of chronic meningitis exist. In the first, the symptoms are chronic and persistent, whereas in the second, there

TABLE 144-2  Infectious Causes of Chronic Meningitis CAUSATIVE AGENT CSF FORMULA HELPFUL DIAGNOSTIC TESTS RISK FACTORS AND SYSTEMIC MANIFESTATIONS Common Bacterial Causes Partially treated suppurative meningitis Mononuclear or mixed mononuclearpolymorphonuclear cells CSF culture and Gram’s stain; CSF 16s rRNA PCR Parameningeal infection Mononuclear or mixed mononuclearpolymorphonuclear cells Contrast-enhanced CT or MRI to detect parenchymal, subdural, epidural, or sinus infection Mycobacterium tuberculosis Mononuclear cells except polymorphonuclear cells in early infection (commonly

<500 WBC/μL); low CSF glucose; high protein Tuberculin skin test may be negative; interferon gamma release assay; PCR and AFB culture of CSF (sputum, urine, gastric contents if indicated); identify tubercle bacillus on acid-fast stain of CSF or protein pellicle Lyme disease (Bannwarth’s syndrome) Borrelia burgdorferi Mononuclear cells; elevated protein Serum Lyme antibody titer; western blot confirmation; (patients with syphilis may have false-positive Lyme titer) Syphilis (secondary, tertiary) Treponema pallidum Mononuclear cells; elevated protein CSF VDRL; serum VDRL (or RPR); fluorescent treponemal antibodyabsorbed (FTA) or MHA-TP; serum VDRL and RPR may be negative in tertiary syphilis due to waning antibody levels or earlier in the disease course due to very elevated antibody levels (prozone effect) Uncommon Bacterial Causes Actinomyces Polymorphonuclear cells Anaerobic culture Parameningeal abscess or sinus tract (oral or dental focus); pneumonitis PART 5 Infectious Diseases Nocardia Polymorphonuclear; occasionally mononuclear cells; often low glucose Isolation may require weeks; weakly acid fast Brucella Mononuclear cells (rarely polymorphonuclear); elevated protein; often low glucose CSF antibody detection; serum antibody detection Whipple’s disease Tropheryma whipplei Mononuclear cells Biopsy of small bowel or lymph node; CSF PCR for T. whipplei; brain and meningeal biopsy (with PAS stain and EM examination) Rare Bacterial Causes Leptospirosis (occasionally if left untreated may last 3–4 weeks) Fungal Causes Cryptococcus neoformans and var. gattii Mononuclear cells; count not elevated in some patients with AIDS India ink or fungal wet mount of CSF (budding yeast); blood and urine cultures; antigen detection in CSF (false negatives can occur in the setting of high antigen titers [prozone effect]) Coccidioides immitis Mononuclear cells (sometimes 10–20% eosinophils); often low glucose Antibody detection in CSF and serum, antigen detection in CSF Candida species Polymorphonuclear or mononuclear Fungal stain and culture of CSF IV drug abuse; postsurgery; prolonged IV therapy; disseminated candidiasis, recent epidural injection Histoplasma capsulatum Mononuclear cells; low glucose Fungal stain and culture of large volumes of CSF; antigen detection in CSF, serum, and urine; antibody detection in serum, CSF Blastomyces dermatitidis Mononuclear or polymorphonuclear Fungal stain and culture of CSF; biopsy and culture of skin, lung lesions; antibody detection in serum Aspergillus species Mononuclear or polymorphonuclear CSF culture Sinusitis; granulocytopenia or immunosuppression Sporothrix schenckii Mononuclear cells Antibody detection in CSF and serum; CSF culture Rare Fungal Causes Xylohypha (formerly Cladosporium) trichoides and other dark-walled (dematiaceous) fungi such as Curvularia; Drechslera; Mucor; and, after water aspiration, Pseudallescheria boydii; iatrogenic Exserohilum rostratum infection following spinal blocks and Fusarium solani following epidural anesthesia in two clinics in Durango, Mexico, during 2022–2023

History consistent with acute bacterial meningitis and incomplete treatment Otitis media, pleuropulmonary infection, right-to-left cardiopulmonary shunt for brain abscess; focal neurologic signs; neck, back, ear, or sinus tenderness Exposure history; previous tuberculous illness; immunosuppressed, anti-TNF therapy or AIDS; young children; fever, meningismus, night sweats, miliary TB on x-ray or liver biopsy; stroke due to arteritis History of tick bite or appropriate exposure history; erythema chronicum migrans skin rash; arthritis, radiculopathy, Bell’s palsy and other cranial neuropathies, meningoencephalitis–multiple sclerosis-like syndrome Appropriate exposure history; HIV-seropositive individuals at increased risk of aggressive infection; fever; lymphadenopathy; generalized, nonpruritic, mucocutaneous rash; “dementia”; cerebral infarction due to endarteritis; myelopathy Associated brain abscess may be present Intake of unpasteurized dairy products; exposure to goats, sheep, cows; fever, arthralgia, myalgia, vertebral osteomyelitis Diarrhea, weight loss, arthralgias, fever; dementia, ataxia, paresis, ophthalmoplegia, oculomasticatory myoclonus AIDS and immune suppression; pigeon exposure for C. neoformans, decaying wood exposure for C. var. gattii; skin and other organ involvement due to disseminated infection Exposure history—southwestern United States Exposure history—Ohio and central Mississippi River Valley; AIDS; mucosal lesions Midwestern and southeastern United States; usually systemic infection; abscesses, draining sinus, ulcers Traumatic inoculation; IV drug use; ulcerated skin lesion (Continued)

TABLE 144-2  Infectious Causes of Chronic Meningitis (Continued) CAUSATIVE AGENT CSF FORMULA HELPFUL DIAGNOSTIC TESTS RISK FACTORS AND SYSTEMIC MANIFESTATIONS Protozoal Causes Toxoplasma gondii Mononuclear cells Biopsy or response to empirical therapy in clinically appropriate context (including presence of antibody in serum) Trypanosomiasis Trypanosoma gambiense, T. rhodesiense Mononuclear cells; elevated protein Elevated CSF IgM; identification of trypanosomes in CSF and blood smear Rare Protozoal Causes Acanthamoeba sp. causing granulomatous amebic encephalitis and meningoencephalitis in immunocompromised and debilitated individuals; Balamuthia mandrillaris causing chronic meningoencephalitis in immunocompetent hosts Helminthic Causes Cysticercosis (infection with cysts of Taenia solium) Mononuclear cells; may have eosinophils; glucose level may be low Indirect hemagglutination assay in CSF; Serum serology with enzyme-linked immunoelectrotransfer blot preferred over crude extract ELISA; antigen or PCR testing in CSF Gnathostoma spinigerum Eosinophils, mononuclear cells Peripheral eosinophilia History of eating raw fish; common in Thailand and Japan; ocular involvement; subarachnoid hemorrhage; painful radiculopathy Angiostrongylus cantonensis Eosinophils, mononuclear cells Recovery of worms from CSF History of eating raw shellfish; common in tropical Pacific regions; often benign; ocular involvement (rare) Baylisascaris procyonis (raccoon ascarid) Eosinophils, mononuclear cells Immunoblot in CSF (Centers for Disease Control and Prevention) Rare Helminthic Causes Trichinella spiralis (trichinosis); Fasciola hepatica (liver fluke), Echinococcus cysts; Schistosoma spp. The former may produce a lymphocytic pleocytosis, whereas the latter two may produce an eosinophilic response in CSF associated with cerebral cysts (Echinococcus) or granulomatous lesions of brain or spinal cord. Viral Causes Mumps Mononuclear cells Antibody in serum No prior mumps or immunization; orchitis; may produce meningoencephalitis; may persist for 3–4 weeks Lymphocytic choriomeningitis Mononuclear cells; may have low glucose Antibody in serum; PCR for LCMV in CSF Contact with rodents or their excreta; may persist for

3–4 weeks Enteroviruses Mononuclear cells; may have low glucose Virus isolation and/or PCR for enteroviruses from CSF HIV (acute retroviral syndrome) Mononuclear cells PCR for HIV in blood and CSF HIV risk factors; rash, fever, lymphadenopathy; lymphopenia in peripheral blood; syndrome may persist long enough to be considered as “chronic meningitis”; or chronic meningitis may develop in later stages (AIDS) due to HIV Human herpes viruses Mononuclear cells PCR for HSV, EBV, CMV DNA; CSF antibody for HSV, EBV Abbreviations: AFB, acid-fast bacillus; CMV, cytomegalovirus; CSF, cerebrospinal fluid; CT, computed tomography; EBV, Epstein-Barr virus; ELISA, enzyme-linked immunosorbent assay; EM, electron microscopy; FTA, fluorescent treponemal antibody absorption test; HSV, herpes simplex virus; LCMV, lymphocytic choriomeningitis virus; MHA-TP, microhemagglutination assay–T. pallidum; MRI, magnetic resonance imaging; PAS, periodic acid–Schiff; PCR, polymerase chain reaction; RPR, rapid plasma reagin test; TB, tuberculosis; VDRL, Venereal Disease Research Laboratory test. are recurrent discrete episodes of illness. In the latter group, all symptoms, signs, and CSF parameters of meningeal inflammation resolve completely between episodes either spontaneously or in response to a specific therapy. In such patients, the likely etiologies include Mollaret’s meningitis, which is most often due to herpes simplex virus (HSV) type 2; chemical meningitis due to episodic leakage from an epidermoid tumor, craniopharyngioma, or choles­ teatoma into CSF; primary autoimmune inflammatory conditions, including Vogt-Koyanagi-Harada syndrome, Behçet’s syndrome, systemic lupus erythematosus (SLE), rheumatoid arthritis, neu­ rosarcoidosis, IgG4-related disease, granulomatosis with polyan­ giitis, and primary central nervous system (CNS) vasculitis; and drug hypersensitivity with repeated administration of the offending agent. With the wider availability of whole genome sequencing, there is also a growing recognition that patients with inherited auto­ inflammatory syndromes like tumor necrosis factor (TNF) recep­ tor–associated periodic syndrome (TRAPS), cryoporin-associated periodic fever syndrome (CAPS), complement factor I deficiency,

Usually with intracerebral abscesses; common in

HIV-seropositive patients; fever Endemic in Africa; chancre, lymphadenopathy; prominent sleep disorder Usually with multiple cysts in basal meninges and hydrocephalus; cerebral cysts, ocular involvement; muscle calcification Infection follows accidental ingestion of B. procyonis eggs from raccoon feces; ocular involvement; fatal meningoencephalitis CHAPTER 144 Chronic and Recurrent Meningitis Congenital hypogammaglobulinemia; history of recurrent meningitis Recurrent meningitis due to HSV-2 (rarely HSV-1) often associated with genital recurrences; EBV associated with myeloradiculopathy, CMV with polyradiculopathy and neonatal-onset multisystem inflammatory disorder can have recurrent meningitis. The epidemiologic history is of considerable importance in diagnosis of chronic meningitis and may provide direction for selection of laboratory studies. Pertinent features include a history of tuberculosis or exposure; past travel to areas endemic for fungal infections (the San Joaquin Valley in California and southwestern states for coccidioidomycosis, midwestern states for histoplasmosis, southeastern states for blastomycosis); travel to the Mediterranean region or ingestion of imported unpasteurized dairy products (Brucella); time spent in wooded areas endemic for Lyme dis­ ease; exposure to sexually transmitted disease (syphilis, HSV-2); exposure of an immunocompromised host to pigeons and their droppings (Cryptococcus neoformans); exposure to decaying wood (Cryptococcus gattii); gardening (Sporothrix schenckii); ingestion of poorly cooked meat or contact with a household cat (Toxoplasma gondii); residence in Thailand or Japan (Gnathostoma spinigerum), Latin America (Paracoccidioides brasiliensis), or the South Pacific

TABLE 144-3  Noninfectious Causes of Chronic Meningitis CAUSATIVE AGENTS CSF FORMULA HELPFUL DIAGNOSTIC TESTS RISK FACTORS AND SYSTEMIC MANIFESTATIONS Malignancy Mononuclear cells; elevated protein; low glucose Repeated cytologic examination of large volumes of CSF; CSF exam by polarizing microscopy; clonal lymphocyte markers; deposits on nerve roots or meninges seen on myelogram or contrast-enhanced MRI; meningeal biopsy Chemical compounds (may cause recurrent meningitis) Mononuclear or PMNs; low glucose, elevated protein; xanthochromia from subarachnoid hemorrhage in week prior to presentation with “meningitis” Contrast-enhanced CT scan or MRI; cerebral angiogram to detect aneurysm. Enhancement and clumping of nerve roots of the cauda equina in arachnoiditis/ pachymeningitis Primary Inflammation CNS sarcoidosis Mononuclear cells; elevated protein; often low glucose Serum and CSF angiotensin-converting enzyme levels (insensitive); biopsy of extraneural affected tissues or brain lesion/meningeal biopsy, nodular meningeal and parenchymal enhancement Vogt-Koyanagi-Harada syndrome (recurrent meningitis) Mononuclear cells Recurrent meningoencephalitis with uveitis, retinal detachment, alopecia, lightening of eyebrows and lashes, dysacousia, cataracts, glaucoma Isolated granulomatous angiitis of the nervous system Mononuclear cells; elevated protein Angiography (often normal with small vessel angiitis); meningeal biopsy may be necessary if confined to small vessels. VZV PCR in blood, CSF, and biopsy tissue; microhemorrhages with amyloid beta– related angiitis Systemic lupus erythematosus Mononuclear or PMNs Anti-dsDNA antibody, antinuclear antibodies PART 5 Infectious Diseases Behçet’s syndrome (recurrent meningitis) Mononuclear or PMNs; elevated protein Rhombencephalitis Oral and genital aphthous ulcers; iridocyclitis; retinal hemorrhages; pathergic lesions at site of skin puncture Chronic benign lymphocytic meningitis Mononuclear cells Recovery in 2–6 months, diagnosis by exclusion Mollaret’s meningitis (recurrent meningitis) Large endothelial cells and PMNs in first hours, followed by mononuclear cells PCR for HSV; MRI/CT to rule out epidermoid tumor or dural cyst Drug hypersensitivity PMNs; occasionally mononuclear cells or eosinophils Complete blood count (eosinophilia) Exposure to nonsteroidal anti-inflammatory agents, sulfonamides, isoniazid, tolmetin, ciprofloxacin, penicillin, carbamazepine, lamotrigine, IV immunoglobulin, OKT3 antibodies, phenazopyridine; improvement after discontinuation of drug; recurrence with repeat exposure Granulomatosis with polyangiitis (Wegener’s) Mononuclear cells Chest and sinus radiographs; urinalysis; ANCA antibodies in serum; pachymeningitis on contrast-enhanced MRI Neonatal-onset multisystem inflammatory disorder Mononuclear and PMNs Gain-of-function mutation in NLRP3 gene leading to elevated IL-1β IgG4-related hypertrophic pachymeningitis Mild lymphocytic pleocytosis in some cases; normal to mildly increased protein; normal glucose Serum IgG4 levels frequently elevated; ESR and C-reactive protein; pachymeningitis on contrast-enhanced MRI; meningeal biopsy shows swirling “storiform” fibrosis with lymphocytic infiltrates, obliterative phlebitis and IgG4+ plasma cells TNF receptor–associated periodic fever syndrome (TRAPS) Mononuclear cells Mutation in TNFRSF1A gene leading to elevated TNF Complement factor I deficiency PMNs Mutation in complement factor I gene leading to low serum levels of factor I (or dysfunctional factor I) and C3 Cryoporin-associated periodic fever syndrome (CAPS) Mononuclear cells Heterozygous gain-of-function mutations within the NLRP3 gene Other: multiple sclerosis, Sjögren’s syndrome, and rarer forms of vasculitis (e.g., Cogan’s syndrome) Abbreviations: ANCA, antineutrophil cytoplasmic antibodies; CN, cranial nerve; CNS, central nervous system; CSF, cerebrospinal fluid; CT, computed tomography; HSV, herpes simplex virus; ICP, intracranial pressure; IL, interleukin; MRI, magnetic resonance imaging; PCR, polymerase chain reaction; PMNs, polymorphonuclear cells; TNF, tumor necrosis factor; VZV, varicella-zoster virus.

Metastatic cancer of breast, lung, stomach, or pancreas; melanoma, lymphoma, leukemia; meningeal gliomatosis; sarcoma; cerebral dysgerminoma History of recent injection into the subarachnoid space; history of sudden onset of headache; recent resection of acoustic neuroma or craniopharyngioma; epidermoid tumor of brain or spine, sometimes with dermoid sinus tract; pituitary apoplexy CN palsy, especially CN VII and CN II, including optic chiasm; hypothalamic dysfunction, especially diabetes insipidus; abnormal chest radiograph; peripheral neuropathy or myopathy; longitudinally extensive transverse myelitis Subacute dementia; multiple cerebral infarctions; recent zoster ophthalmicus Encephalopathy; seizures; stroke; transverse myelopathy; rash; arthritis; thromboembolism; renal and/ or pulmonary complications Recurrent meningitis; exclude HSV-2; rare cases due to HSV-1; occasional case associated with dural cyst Associated sinus, pulmonary, or renal lesions; CN palsies; skin lesions; peripheral neuropathy Recurrent fever, urticaria, arthralgia, sensorineural hearing loss, papilledema, increased ICP Headache; seizures; focal symptoms from dural involvement in spinal cord/nerve roots, clivus, periorbital, vestibular, and brainstem structures. Systemic IgG4-related disease can involve many tissues including pancreas, thyroid, lungs, retroperitoneum, lacrimal, parotid and submandibular glands, orbits, kidney, aorta, liver Headache, seizures, tinnitus, skin rash, abdominal pain, lymphadenopathy, periorbital edema, joint pain, myalgia Recurrent, steroid-responsive, aseptic, neutrophilic meningitis with or without encephalitis; increased risk for systemic infections with encapsulated bacteria, glomerulonephritis, systemic lupus erythematosus and leukocytoclastic vasculitis Fever, urticaria, amyloidosis, arthralgia, sensorineural hearing loss, myalgias, papilledema, vision changes

(Angiostrongylus cantonensis); rural residence and raccoon expo­ sure (Baylisascaris procyonis); and residence in Latin America, the Philippines, sub-Saharan Africa, or Southeast Asia (Taenia solium/ cysticercosis, schistosomiasis). CNS meliodosis caused by Burkholderia pseudomallei is endemic in South Asia and Australia. Individu­ als with agammaglobulinemia or those receiving B cell–depleting therapy may be susceptible to chronic enterovirus meningitis. Focal cerebral signs in a patient with chronic meningitis sug­ gest the possibility of a brain abscess, parameningeal infection, or infarct; identification of a potential source of infection (chronic draining ear, sinusitis, dental abscess, right-to-left cardiac or pulmonary shunt, chronic pleuropulmonary infection) supports this diagnosis. In some cases, diagnosis may be established by recognition and biopsy of unusual skin lesions (Behçet’s syn­ drome, SLE, cryptococcosis, blastomycosis, Lyme disease, spo­ rotrichosis, trypanosomiasis, IV drug use) or enlarged lymph nodes (lymphoma, sarcoid, tuberculosis, HIV, secondary syphilis, or Whipple’s disease). Ophthalmologic examination may reveal uveitis (Vogt-Koyanagi-Harada syndrome, sarcoidosis, or CNS lymphoma), keratoconjunctivitis sicca (Sjögren’s syndrome), or iridocyclitis (Behçet’s syndrome) and is essential to assess visual loss from papilledema. M. tuberculosis can cause a wide spectrum of ophthalmologic pathology. If neurocysticercosis is suspected, it is important to rule out an intraocular infection requiring surgi­ cal treatment before initiating antihelminthic therapy. Aphthous oral lesions, genital ulcers, and hypopyon (inflammatory cells in the anterior chamber of the eye) suggest Behçet’s syndrome. Hepatosplenomegaly suggests lymphoma, sarcoid, tuberculosis, or brucellosis. Arthralgias could be indicative of Lyme disease or Whipple’s disease. The latter can also cause gastrointestinal symp­ toms, including diarrhea and abdominal pain. Herpetic lesions in the genital area or on the thighs suggest HSV-2 infection. A breast nodule; a suspicious hyperpigmented skin lesion; focal bone pain; hard, fixed lymph nodes; or an abdominal mass suggests possible carcinomatous meningitis. IMAGING Once the clinical syndrome is recognized as a potential mani­ festation of chronic meningitis, proper analysis of the CSF is essential. However, if the possibility of raised ICP exists, a brain imaging study should be performed before lumbar puncture. If ICP is elevated because of a mass lesion, brain swelling, or a block in ventricular CSF outflow (obstructive hydrocephalus), then lumbar puncture carries the potential risk of brain herniation. Obstruc­ tive hydrocephalus usually requires direct ventricular drainage. In patients with open CSF flow pathways, elevated ICP can still occur due to impaired resorption of CSF by arachnoid villi. In such patients, lumbar puncture is usually safe and may be therapeutic. Indeed, repetitive or continuous lumbar drainage may be necessary to prevent abrupt deterioration and death from raised ICP. In some patients, especially those with cryptococcal meningitis, fatal levels of raised ICP can occur without enlarged ventricles. Contrast-enhanced MRI or CT studies of the brain and spinal cord can identify meningeal enhancement, parameningeal infections (including brain abscess), encasement of the spinal cord (malignancy, inflammation, or infection), or nodular deposits on the meninges or nerve roots (malignancy, sarcoidosis, or schistosomiasis) (Fig. 144-2). Imaging studies are also useful to guide biopsy of affected meninges. Lastly, a cyst that recurrently causes a chemical meningitis due to a leak may be better visualized in between clinical episodes when a recent leak has not shrunken the cyst volume. The patterns of enhancement of the different layers of the menin­ ges can be very informative and can be divided into two types: leptomeningeal (pia and arachnoid), when enhancement of the meninges follows the convolutions of the gyri and/or involves the meninges around the basal cisterns; and pachymeningeal (dura), when the enhancement is thick and linear or nodular along the inner surface of the calvarium, falx, or tentorium without extension

into the cortical gyri or basal cistern involvement. For example, infectious meningitis presents mostly as leptomeningitis, while lymphomatous meningitis can present as pachymeningitis. Angiographic studies can identify evidence of cerebral arteritis in patients with chronic meningitis and stroke. CEREBROSPINAL FLUID ANALYSIS The CSF pressure should be measured and samples sent for bacte­ rial, fungal, and mycobacterial culture; Venereal Disease Research Laboratory (VDRL) test; cell count and differential; Gram’s stain; and measurement of glucose and protein. CSF VDRL is a highly specific, but not particularly sensitive, test for syphilis. If CSF VDRL is negative in an otherwise high-risk patient with positive treponemal antibodies in the serum and an otherwise unexplained CSF pleocytosis, empiric treatment for neurosyphilis may still be appropriate. Wet mount for fungus and parasites, India ink preparation, culture for fastidious bacteria and fungi, assays for cryptococcal antigen and oligoclonal immunoglobulin bands, and cytology should be performed. Other specific CSF tests (Tables 144-2 and 144-3) or blood tests and cultures should be ordered as indicated based on the history, physical examination, or prelimi­ nary CSF results (i.e., eosinophilic, mononuclear, or polymorpho­ nuclear meningitis). Rapid diagnosis may be facilitated by serologic tests and polymerase chain reaction (PCR) testing to identify DNA sequences in the CSF that are specific for the suspected pathogen. 16s ribosomal RNA (rRNA) PCR can be used to detect a broad range of bacterial causes of meningitis and can be particularly use­ ful in partially treated meningitis when the yield of culture is low. 18s and 28s rRNA PCR can similarly be useful for detecting a broad range of fungal species. In patients with suspected fungal infections, when other tests are negative, CSF assays for beta-glucans may be a useful adjunct in establishing the diagnosis. Building on progress in parallel deep sequencing and informatics, unbiased metagenomic next-generation sequencing is becoming generally available, repre­ senting an efficient and powerful method for diagnosis of challeng­ ing infectious cases. CHAPTER 144 Chronic and Recurrent Meningitis In most categories of chronic (not recurrent) meningitis, mononuclear cells predominate in the CSF. When neutrophils predominate after 3 weeks of illness, the principal etiologic considerations are Nocardia asteroides, Actinomyces israelii, Brucella, M. tuberculosis (5–10% of early cases only), various fungi (Blas­ tomyces dermatitidis, Candida spp., Histoplasma capsulatum, Aspergillus spp., Pseudallescheria boydii, Cladophialophora ban­ tiana), and noninfectious causes (SLE, exogenous chemical men­ ingitis). When eosinophils predominate or are present in limited numbers in a primarily mononuclear cell response in the CSF, the differential diagnosis includes parasitic diseases (A. canto­ nensis, G. spinigerum, B. procyonis, or Toxocara canis infection; cysticercosis; schistosomiasis; echinococcal disease; T. gondii infection), fungal infections (6–20% eosinophils along with a predominantly lymphocyte pleocytosis, particularly with coc­ cidioidal meningitis), neoplastic disease (lymphoma, leukemia, metastatic carcinoma), or other inflammatory processes (sar­ coidosis, hypereosinophilic syndrome). It is often necessary to broaden the number of diagnostic tests if the initial workup does not reveal the cause. In addition, repeated samples (three or more) of large volumes of lumbar CSF may be required to diagnose certain infectious and malignant causes of chronic meningitis. Lymphomatous or carcinomatous meningitis may be diagnosed by examination of sections cut from a cell block formed by spinning down the sediment from a large volume of CSF. Flow cytometry for malignant cells may also be useful in patients with suspected carcinomatous meningitis. The diagnosis of fungal meningitis may also require large volumes of CSF for culture of sediment. If standard lumbar puncture is unrewarding, a cervical cisternal tap to sample CSF near to the basal meninges may be fruitful. Ventricular fluid may appear sterile in cases with active infection in the lower lumbar space.

A PART 5 Infectious Diseases C FIGURE 144-2  Chronic meningitis illustrating meningeal enhancement on contrast magnetic resonance imaging scan. A and B are images from a patient with chronic meningitis due to carcinoma. C and D are from a patient with chronic meningitis due to Cryptococcus infection. Arrows point to the most prominent areas of meningeal inflammation around the brainstem and cerebellar folia (A), cerebellum (C), along the dorsal spinal cord (B), and clumping of roots in the cauda equina (D). LABORATORY INVESTIGATION In addition to the CSF examination, an attempt should be made to uncover pertinent underlying illnesses. Tuberculin skin test, chest radiograph, urine analysis and culture, blood count and differential, renal and liver function tests, alkaline phosphatase, sedimentation rate, antinuclear antibody, anti-Ro antibody, anti-La antibody, rheu­ matoid factor, and IgG4 level are often indicated. In some cases, a thorough search for a systemic site of infection is indicated. Pul­ monary foci of infection may be present, particularly with fungal

B D or tuberculous disease. Hence, a CT or MRI of the chest and a sputum examination may be helpful. Abnormalities can be pursued by bronchoscopy or transthoracic needle biopsy. A tuberculin skin test is often placed, although the test has limited specificity and sensitivity for diagnosis of active disease. Where available, gamma interferon release assays may be used to diagnose latent tuber­ culosis. Liver, bone marrow, or lymph node biopsy may be diag­ nostic in some cases of miliary tuberculosis, disseminated fungal infection, sarcoidosis, or metastatic malignancy. Positron emission

24 - 145 Brain Abscess and Empyema

145 Brain Abscess and Empyema

tomography with fluorodeoxyglucose may be useful in identifying a systemic site for biopsy in patients with suspected carcinomatous meningitis or sarcoidosis when other tests are unrevealing. Genetic testing can identify mutations that cause rare monogenic autoin­ flammatory disorders or underlying immunocompromised states. MENINGEAL BIOPSY If CSF is not diagnostic, then a meningeal biopsy should be strongly considered in patients who are severely disabled, who need chronic ventricular decompression, or whose illness is progressing rap­ idly. The activities of the surgeon, pathologist, microbiologist, and cytologist should be coordinated so that a large enough sample is obtained and the appropriate cultures and histologic and molecular studies, including electron-microscopic and PCR studies, are per­ formed. The diagnostic yield of meningeal biopsy can be increased by targeting regions that enhance with contrast on MRI or CT. With current microsurgical techniques, most areas of the basal meninges can be accessed for biopsy via a limited craniotomy. In one series, MRI demonstrated meningeal enhancement in 47% of patients undergo­ ing meningeal biopsy; biopsy of an enhancing region was diagnostic in 80% of cases, biopsy of nonenhancing regions was diagnostic in only 9%, and sarcoidosis (31%) and metastatic adenocarcinoma (25%) were the most common conditions identified. Tuberculosis is the most common condition identified in many reports from outside the United States. APPROACH TO THE ENIGMATIC CASE In approximately one-third of cases, the diagnosis is not known despite careful evaluation of CSF and potential extraneural sites of disease. A number of the organisms that cause chronic meningitis may take weeks to be identified by cultures. In enigmatic cases, several options are available, determined by the extent of the clinical deficits and rate of progression. It is prudent to wait until cultures are finalized if the patient is asymptomatic or symptoms are mild and not progressive. Unfortunately, in many cases, progressive neurologic deterioration occurs, and rapid treatment is required. Ventricular-peritoneal shunts may be placed to relieve hydrocepha­ lus, but the risk of disseminating the undiagnosed inflammatory process into the abdomen must be considered. Empirical Treatment  Diagnosis of the causative agent is essential because effective therapies exist for many etiologies of chronic men­ ingitis, but if the condition is left untreated, progressive damage to the CNS and cranial nerves and roots is likely to occur. Occasion­ ally, empirical therapy must be initiated when all attempts at diag­ nosis fail. In general, empirical therapy in the United States consists of antimycobacterial agents, amphotericin B (often combined with flucytosine) for fungal infection, and/or glucocorticoids for non­ infectious inflammatory causes. It is important to direct empirical therapy of lymphocytic meningitis at tuberculosis, particularly if the condition is associated with low CSF glucose, since untreated disease can be devastating within weeks. Prolonged anti-TNF ther­ apy and anti–programmed cell death 1 (PD-1) inhibitors can cause reactivation of TB, and such patients who develop chronic menin­ gitis should be treated empirically with antituberculous therapy if the etiology is uncertain. Even with treatment, TB meningitis can carry high rates of morbidity. In the Mayo Clinic series, the most useful empirical therapy was administration of glucocorticoids rather than antituberculous therapy. When proceeding with empiric glucocorticoids, caution should be maintained whenever a transient response to treatment is noted, as some infectious (e.g., tuberculosis and cysticercosis) and noninfectious (e.g., lymphoma) etiologies may temporarily respond to glucocorticoid monotherapy. Carcino­ matous or lymphomatous meningitis may be difficult to diagnose initially, but the diagnosis becomes evident with time. ■ ■THE IMMUNOSUPPRESSED PATIENT Chronic meningitis is not uncommon in the course of HIV infec­ tion. Pleocytosis and mild meningeal signs often occur at the onset

of HIV infection, and occasionally, low-grade meningitis persists. In worldwide populations, M. tuberculosis is the most common cause of chronic meningitis, followed by C. neoformans. Toxoplasmosis com­ monly presents as intracranial abscesses and also may be associated with meningitis. Other important causes of chronic meningitis in AIDS include infection with Nocardia, Candida, or other fungi; syphilis; and lymphoma (Fig. 144-2). With HIV infection, primary CNS lymphomas may arise, which are typically positive for EBV infection. Toxoplasmo­ sis, nocardiosis, cryptococcosis and other fungal infections are impor­ tant etiologic considerations in individuals with immunodeficiency states other than AIDS, including those due to immunosuppressive medications. Because of the increased risk of chronic meningitis and the attenuation of clinical signs of meningeal irritation in immunosup­ pressed individuals, CSF examination should be performed for any persistent headache or unexplained change in mental state.

■ ■FURTHER READING Aksamit AJ: Chronic meningitis. N Engl J Med 385:930, 2021. Baldwin K, Avila JD: Diagnostic approach to chronic meningitis. Neurol Clin 36:831, 2018. Chang CC et al: Global guideline for the diagnosis and management of cryptococcosis: An initiative of the ECMM and ISHAM in coopera­ tion with the ASM. Lancet Infect Dis 24:e495, 2024. Johnson TP, Nath A: Biotypes of HIV associated neurocognitive dis­ orders. Curr Opin Inf Dis 335:223, 2022. Kacar M et al: Hereditary systemic autoinflammatory diseases and Schnitzler’s syndrome. Rheumatology 58:vi31, 2019. Lu LX et al: IgG4-related hypertrophic pachymeningitis: Clinical fea­ CHAPTER 145 tures, diagnostic criteria and treatment. JAMA Neurol 71:785, 2014. Saifon W et al: Distinguishing clinical characteristics of central ner­ vous system tuberculosis in immunodeficient and non-immunodefi­ cient individuals: A 12-year retrospective study. Ann Clin Microbiol Antimicrob 22:69, 2023. Wilson MR et al: Chronic meningitis investigated via metagenomic Brain Abscess and Empyema next-generation sequencing. JAMA Neurol 75:947, 2018. Karen L. Roos, Kenneth L. Tyler

Brain Abscess and

Empyema BRAIN ABSCESS ■ ■DEFINITION A brain abscess is a focal, suppurative infection within the brain parenchyma, typically surrounded by a vascularized capsule. The term cerebritis is often employed to describe a nonencapsulated brain abscess. ■ ■EPIDEMIOLOGY A bacterial brain abscess is a relatively uncommon intracranial infec­ tion, with an incidence of ~0.3–1.3:100,000 persons per year. Pre­ disposing conditions include otitis media and mastoiditis, paranasal sinusitis, pyogenic infections in the chest or other body sites, penetrat­ ing head trauma or neurosurgical procedures, and dental infections. In immunocompetent individuals, the most important pathogens are Streptococcus spp. (anaerobic, aerobic, and viridans [40%]), Entero­ bacteriaceae (Proteus spp., Escherichia coli sp., Klebsiella spp. [25%]), anaerobes (e.g., Bacteroides spp., Fusobacterium spp. [30%]), and staphylococci (10%). In immunocompromised hosts with underlying HIV infection, organ transplantation, cancer, or immunosuppressive therapy, most brain abscesses are caused by Nocardia spp., Toxoplasma

gondii, Aspergillus spp., Candida spp., and Cryptococcus neoformans. In Latin America and in immigrants from Latin America, the most common cause of brain abscess is Taenia solium (neurocysticercosis). In India and East Asia, mycobacterial infection (tuberculoma) remains a major cause of focal CNS mass lesions.

■ ■ETIOLOGY A brain abscess may develop (1) by direct spread from a contiguous cranial site of infection, such as paranasal sinusitis, otitis media, mas­ toiditis, or dental infection; (2) following head trauma or a neurosurgi­ cal procedure; or (3) as a result of hematogenous spread from a remote site of infection. In up to 25% of cases, no obvious primary source of infection is apparent (cryptogenic brain abscess). Approximately one-third of brain abscesses are associated with otitis media and mastoiditis, often with an associated cholestea­ toma. Otogenic abscesses occur predominantly in the temporal lobe (55–75%) and cerebellum (20–30%). In some series, up to 90% of cere­ bellar abscesses are otogenic. Common organisms include streptococci, Bacteroides spp., Pseudomonas spp., Haemophilus spp., and Enterobac­ teriaceae. Abscesses that develop as a result of direct spread of infec­ tion from the frontal, ethmoidal, or sphenoidal sinuses and those that occur due to dental infections are usually located in the frontal lobes. Approximately 10% of brain abscesses are associated with paranasal sinusitis, and this association is particularly strong in young males in their second and third decades of life. The most common pathogens in brain abscesses associated with paranasal sinusitis are streptococci (especially Streptococcus milleri), Haemophilus spp., Bacteroides spp., Pseudomonas spp., and Staphylococcus aureus. Dental infections are associated with ~2% of brain abscesses, although it is often suggested that many “cryptogenic” abscesses are in fact due to dental infections. The most common pathogens in this setting are streptococci, staphylo­ cocci, Bacteroides spp., and Fusobacterium spp. PART 5 Infectious Diseases Hematogenous abscesses account for ~25% of brain abscesses. Hematogenous abscesses are often multiple, and multiple abscesses often (50%) have a hematogenous origin. These abscesses show a pre­ dilection for the territory of the middle cerebral artery (i.e., posterior frontal or parietal lobes). Hematogenous abscesses are often located at the junction of the gray and white matter and are often poorly encap­ sulated. The microbiology of hematogenous abscesses is dependent on the primary source of infection. For example, brain abscesses that develop as a complication of infective endocarditis are often due to viridans streptococci or S. aureus. Abscesses associated with pyogenic lung infections such as lung abscess or bronchiectasis are often due to streptococci, staphylococci, Bacteroides spp., Fusobacterium spp., or Enterobacteriaceae. Enterobacteriaceae and Pseudomonas aerugi­ nosa are important causes of abscesses associated with urinary sepsis. Congenital cardiac malformations that produce a right-to-left shunt allow bloodborne bacteria to bypass the pulmonary capillary bed and reach the brain. Similar phenomena can occur with pulmonary arteriovenous malformations. The decreased arterial oxygenation and saturation from the right-to-left shunt and polycythemia may cause focal areas of cerebral ischemia, thus providing a nidus for microorgan­ isms that bypassed the pulmonary circulation to multiply and form an abscess. Streptococci are the most common pathogens in this setting. Abscesses that follow penetrating head trauma or neurosurgical procedures are frequently due to methicillin-resistant S. aureus (MRSA), Staphylococcus epidermidis, Enterobacteriaceae, Pseudomonas spp., and Clostridium spp. ■ ■PATHOGENESIS AND HISTOPATHOLOGY Results of experimental models of brain abscess formation suggest that for bacterial invasion of brain parenchyma to occur, there must be preexisting or concomitant areas of ischemia, necrosis, or hypoxemia in brain tissue. The intact brain parenchyma is relatively resistant to infection. Once bacteria have established infection, brain abscess frequently evolves through a series of stages, influenced by the nature of the infecting organism and by the immunocompetence of the host. The early cerebritis stage (days 1–3) is characterized by a perivascu­ lar infiltration of inflammatory cells, which surround a central core

of coagulative necrosis. Marked edema surrounds the lesion at this stage. In the late cerebritis stage (days 4–9), pus formation leads to enlargement of the necrotic center, which is surrounded at its border by an inflammatory infiltrate of macrophages and fibroblasts. A thin capsule of fibroblasts and reticular fibers gradually develops, and the surrounding area of cerebral edema becomes more distinct than in the previous stage. The third stage, early capsule formation (days 10–13), is characterized by the formation of a capsule that is better developed on the cortical than on the ventricular side of the lesion. This stage correlates with the appearance of a ring-enhancing capsule on neu­ roimaging studies. The final stage, late capsule formation (day 14 and beyond), is defined by a well-formed necrotic center surrounded by a dense collagenous capsule. The surrounding area of cerebral edema has regressed, but marked gliosis with large numbers of reactive astrocytes has developed outside the capsule. This gliotic process may contribute to the development of seizures as a sequela of brain abscess. ■ ■CLINICAL PRESENTATION A brain abscess typically presents as an expanding intracranial mass lesion rather than as an infectious process. Although the evolution of signs and symptoms is extremely variable, ranging from hours to weeks or even months, most patients present to the hospital 11–12 days following onset of symptoms. The classic clinical triad of headache, fever, and a focal neurologic deficit is present in <50% of cases. The most common symptom in patients with a brain abscess is headache, occurring in >75% of patients. The headache is often characterized as a constant, dull, aching sensation, either hemicranial or generalized, and it becomes progressively more severe and refractory to therapy. Fever is present in only 50% of patients at the time of diagnosis, and its absence should not exclude the diagnosis. The new onset of focal or generalized seizure activity is a presenting sign in 15–35% of patients. Focal neurologic deficits including hemiparesis, aphasia, or visual field defects are part of the initial presentation in >60% of patients. The clinical presentation of a brain abscess depends on its location, the nature of the primary infection if present, and the level of the intra­ cranial pressure (ICP). Hemiparesis is the most common localizing sign of a frontal lobe abscess. A temporal lobe abscess may present with a disturbance of language (dysphasia) or an upper homony­ mous quadrantanopia. Nystagmus and ataxia are signs of a cerebellar abscess. Signs of raised ICP—papilledema, nausea and vomiting, and drowsiness or confusion—can be the dominant presentation of some abscesses, particularly those in the cerebellum. Meningismus is not present unless the abscess has ruptured into the ventricle or the infec­ tion has spread to the subarachnoid space. ■ ■DIAGNOSIS Diagnosis is made by neuroimaging studies. Magnetic resonance imag­ ing (MRI) (Fig. 145-1) is better than computed tomography (CT) for demonstrating abscesses in the early (cerebritis) stages and is supe­ rior to CT for identifying abscesses in the posterior fossa. Cerebritis appears on MRI as an area of low signal intensity on T1-weighted images with irregular postgadolinium enhancement and as an area of increased signal intensity on T2-weighted images. Cerebritis is often not visualized by CT scan, but when present, appears as an area of hypodensity. On a contrast-enhanced CT scan, a mature brain abscess appears as a focal area of hypodensity surrounded by ring enhance­ ment with surrounding edema (hypodensity). On contrast-enhanced T1-weighted MRI, a mature brain abscess has a capsule that enhances surrounding a hypodense center and surrounded by a hypodense area of edema. On T2-weighted MRI, there is a hyperintense central area of pus surrounded by a well-defined hypointense capsule and a hyperintense surrounding area of edema. It is important to recognize that the CT and MRI appearance, particularly of the capsule, may be altered by treatment with glucocorticoids. The distinction between a brain abscess and other focal CNS lesions such as primary or metastatic tumors may be facilitated by the use of diffusion-weighted imaging sequences on which a brain abscess typically shows increased signal due to restricted diffusion of the abscess cavity with corresponding low signal on apparent diffusion coefficient images.

A B C FIGURE 145-1  Pyogenic brain abscess. Note that the abscess wall enhances prominently after gadolinium administration on the magnetic resonance axial T1-weighted image (A). The abscess is hyperintense on the diffusion-weighted image (B) and dark on the apparent diffusion coefficient (ADC) image (C). (Courtesy of Aaron Kamer, MD; with permission.) Microbiologic diagnosis of the etiologic agent is most accurately determined by Gram’s stain and culture of abscess material obtained by CT-guided stereotactic needle aspiration. Aerobic and anaerobic bacte­ rial cultures and mycobacterial and fungal cultures should be obtained. Blood cultures are positive in ~10% of cases overall but may be posi­ tive in >85% of patients with abscesses due to Listeria. About 50% of patients have a peripheral leukocytosis, 60% an elevated erythrocyte sedimentation rate, and 80% an elevated C-reactive protein. Lumbar puncture (LP) should not be performed in patients with known or suspected focal intracranial infections such as abscess or empyema; cerebrospinal fluid (CSF) analysis contributes nothing to diagnosis or therapy, and LP increases the risk of herniation. ■ ■DIFFERENTIAL DIAGNOSIS Conditions that can cause headache, fever, focal neurologic signs, and seizure activity include brain abscess, subdural empyema, bacte­ rial meningitis (Chap. 143), viral meningoencephalitis (Chap. 142), superior sagittal sinus thrombosis (Chap. 438), and acute disseminated encephalomyelitis (Chap. 456). When fever is absent, primary and metastatic brain tumors become the major differential diagnosis. Less commonly, cerebral infarction or hematoma can have an MRI or CT appearance resembling brain abscess. TREATMENT Brain Abscess Optimal therapy of brain abscesses involves a combination of high-dose parenteral antibiotics and neurosurgical drainage. Empirical therapy of community-acquired brain abscess in an immunocompetent patient typically includes a third- or fourthgeneration cephalosporin (e.g., cefotaxime, ceftriaxone, or cefepime) and metronidazole (see Table 143-1 for antibiotic dosages). In patients with penetrating head trauma or recent neurosurgical procedures, treatment should include ceftazidime as the thirdgeneration cephalosporin to enhance coverage of Pseudomonas spp. and vancomycin for coverage of staphylococci. Meropenem plus vancomycin also provides good coverage in this setting. Aspiration and drainage of the abscess under stereotactic guidance are beneficial for both diagnosis and therapy. Empiri­ cal antibiotic coverage should be modified based on the results of Gram’s stain and culture of the abscess contents. Complete excision

of a bacterial abscess via craniotomy or craniectomy is generally reserved for multiloculated abscesses or those in which stereotactic aspiration is unsuccessful. CHAPTER 145 Medical therapy alone is not optimal for treatment of brain abscess and should be reserved for patients whose abscesses are neurosurgically inaccessible, for patients with small (<2–3 cm) or nonencapsulated abscesses (cerebritis), and for patients whose condition is too tenuous to allow performance of a neurosurgical procedure. All patients should receive a minimum of 6–8 weeks of parenteral antibiotic therapy. The role, if any, of supplemental oral antibiotic therapy following completion of a standard course of parenteral therapy has never been adequately studied. Brain Abscess and Empyema In addition to surgical drainage and antibiotic therapy, patients should receive prophylactic anticonvulsant therapy because of the high risk (~35%) of focal or generalized seizures. Anticonvulsant therapy is continued for at least 3 months after resolution of the abscess, and decisions regarding withdrawal are then based on the electroencephalogram (EEG). If the EEG is abnormal, anticonvulsant therapy should be continued. If the EEG is normal, anticonvulsant therapy can be slowly withdrawn, with close follow-up and repeat EEG after the medication has been discontinued. Glucocorticoids should not be given routinely to patients with brain abscesses. Intravenous dexamethasone therapy (10 mg every 6 h) is usually reserved for patients with substantial periabscess edema and associated mass effect and increased ICP. Dexametha­ sone should be tapered as rapidly as possible to avoid delaying the natural process of encapsulation of the abscess. Serial MRI or CT scans should be obtained on a monthly or twice-monthly basis to document resolution of the abscess. More frequent studies (e.g., weekly) are probably warranted in the sub­ set of patients who are receiving antibiotic therapy alone. A small amount of enhancement may remain for months after the abscess has been successfully treated. ■ ■PROGNOSIS The mortality rate of brain abscess has declined in parallel with the development of enhanced neuroimaging techniques, improved neuro­ surgical procedures for stereotactic aspiration, and improved antibiot­ ics. In modern series, the mortality rate is typically <15%. Significant sequelae, including seizures, persisting weakness, aphasia, or mental impairment, occur in ≥20% of survivors.

NONBACTERIAL CAUSES OF INFECTIOUS FOCAL CNS LESIONS

■ ■ETIOLOGY Neurocysticercosis is the most common parasitic disease of the CNS worldwide. Humans acquire cysticercosis by the ingestion of food contaminated with the eggs of the parasite Taenia solium (Chap. 242). Toxoplasmosis (Chap. 235) is a parasitic disease caused by T. gondii and acquired from the ingestion of undercooked meat and from handling cat feces. ■ ■CLINICAL PRESENTATION The most common manifestation of neurocysticercosis is new-onset partial seizures with or without secondary generalization. Cysticerci may develop in the brain parenchyma and cause seizures or focal neurologic deficits. When present in the subarachnoid or ventricular spaces, cysticerci can produce increased ICP by interference with CSF flow. Spinal cysticerci can mimic the presentation of intraspinal tumors. When the cysticerci first lodge in the brain, they frequently cause little in the way of an inflammatory response. As the cysticercal cyst degenerates, it elicits an inflammatory response that may present clinically as a seizure. Eventually the cyst dies, a process that may take several years and is typically associated with resolution of the inflam­ matory response and, often, abatement of seizures. Primary Toxoplasma infection is often asymptomatic. However, dur­ ing this phase, parasites may spread to the CNS, where they become latent. Reactivation of CNS infection is almost exclusively associated with immunocompromised hosts, particularly those with HIV infec­ tion. During this phase, patients present with headache, fever, seizures, and focal neurologic deficits. PART 5 Infectious Diseases ■ ■DIAGNOSIS The lesions of neurocysticercosis are readily visualized by MRI or CT scans depending on the stage of the lesion. There are four stages of neurocysticercosis: (1) the vesicular stage, (2) the colloidal stage, (3) the granulonodular stage, and (4) the nodular-calcified stage. Lesions with viable parasites appear as cystic lesions, and the scolex can often be visualized on MRI. Cystic lesions with small nodules (scolex) within the cyst are in the vesicular stage of neurocysticercosis (Fig. 145-2A and B). There is no significant edema surrounding a lesion in the vesicular stage. Lesions in the colloidal stage demonstrate peripheral enhancement on postcontrast imaging (Fig. 145-2C) with substantial surrounding edema on T2 images (Fig. 145-2D). In the granulonodu­ lar stage, on postcontrast imaging, the lesion enhances in a homog­ enous fashion (Fig. 145-2E). On fluid-attenuated inversion recovery (FLAIR) images, there is no surrounding edema (Fig. 145-2F). Paren­ chymal brain calcifications are the most common finding and evidence that the parasite is no longer viable. These chronic lesions are best seen on CT (Fig. 145-2G) and can be difficult to detect on MRI. The most sensitive technique for the detection of these small calcific foci on MRI is susceptibility-weighted imaging (SWI). If a confirmatory test for neurocysticercosis is needed, the enzyme-linked immunotransfer blot is recommended. A funduscopic exam is also recommended for all patients with suspected neurocysticercosis. MRI findings of toxoplasmosis consist of multiple lesions in the deep white matter, the thalamus, and basal ganglia and at the graywhite junction in the cerebral hemispheres. With contrast adminis­ tration, the majority of the lesions enhance in a ringed, nodular, or homogeneous pattern and are surrounded by edema. In the presence of the characteristic neuroimaging abnormalities of T. gondii infection, serum IgG antibody to T. gondii should be obtained and, when positive, the patient should be treated. TREATMENT Infectious Focal CNS Lesions Anticonvulsant therapy is initiated when the patient with neuro­ cysticercosis presents with a seizure. Anthelmintic therapy is given to patients based on the stage of the lesion(s). Cysticerci appearing

as cystic lesions in the brain parenchyma with or without pericystic edema or in the subarachnoid space at the convexity of the cerebral hemispheres should be treated with anticysticidal therapy. Cys­ ticidal drugs accelerate the destruction of the parasites, resulting in a faster resolution of the infection. Albendazole monotherapy is recommended for patients with one to two parenchymal cysts. The dose of albendazole is 15 mg/kg per day in two daily doses for 10–14 days. A combination of albendazole plus praziquantel is recommended for patients with more than two viable cysts. Viable cysts are defined as those in the vesicular or colloidal stages (see above). The recommended dose of praziquantel is 50 mg/kg per day for 10–14 days. Prednisone or dexamethasone is begun prior to anticysticidal therapy to reduce the host inflammatory response to degenerating parasites. Only cysts in the vesicular stage, where the cyst contains living larva (scolex seen on CT or MRI), and cysts in the colloidal stage, as the larva degenerates (edema surrounds the lesion), are treated with anticysticidal therapy. Some, but not all, experts recommend anticysticidal therapy for lesions that are in the granulonodular stage (surrounded by a contrast-enhancing ring). There is universal agreement that calcified lesions do not need to be treated with anticysticidal therapy. Antiepileptic therapy can be stopped once a follow-up CT or MRI scan shows resolution of the lesion and the patient has had no seizures for 24 consecutive months. Long-term antiepileptic therapy is recommended when seizures occur after resolution of edema and resorption or calcifica­ tion of the degenerating cyst. CNS toxoplasmosis is treated with a combination of sulfadiazine, 1.5–2.0 g orally qid, plus pyrimethamine, 100 mg orally to load, then 75–100 mg orally qd, plus folinic acid, 10–15 mg orally qd. Folinic acid is added to the regimen to prevent megaloblastic anemia. Therapy is continued until there is no evidence of active disease on neuroimaging studies, which typically takes at least 6 weeks, and then the dose of sulfadiazine is reduced to 2–4 g/d and pyrimethamine to 50 mg/d. Clindamycin plus pyrimethamine is an alternative therapy for patients who cannot tolerate sulfadiazine, but the combination of pyrimethamine and sulfadiazine is more effective. SUBDURAL EMPYEMA A subdural empyema (SDE) is a collection of pus between the dura and arachnoid membranes (Fig. 145-3). ■ ■EPIDEMIOLOGY SDE is a rare disorder that accounts for 15–25% of focal suppurative CNS infections. Sinusitis is the most common predisposing condition and typically involves the frontal sinuses, either alone or in combi­ nation with the ethmoid and maxillary sinuses. Sinusitis-associated empyema has a striking predilection for young males, possibly reflect­ ing sex-related differences in sinus anatomy and development. It has been suggested that SDE may complicate 1–2% of cases of frontal sinusitis severe enough to require hospitalization. As a consequence of this epidemiology, SDE shows an ~3:1 male/female predominance, with 70% of cases occurring in the second and third decades of life. SDE may also develop as a complication of head trauma or neuro­ surgery. Secondary infection of a subdural effusion may also result in empyema, although secondary infection of hematomas, in the absence of a prior neurosurgical procedure, is rare. ■ ■ETIOLOGY Aerobic and anaerobic streptococci, staphylococci, Enterobacteriaceae, and anaerobic bacteria are the most common causative organisms of sinusitis-associated SDE. Staphylococci and gram-negative bacilli are often the etiologic organisms when SDE follows neurosurgical proce­ dures or head trauma. Up to one-third of cases are culture-negative, possibly reflecting difficulty in obtaining adequate anaerobic cultures. ■ ■PATHOPHYSIOLOGY Sinusitis-associated SDE develops as a result of either retrograde spread of infection from septic thrombophlebitis of the mucosal veins

A B C D E F G FIGURE 145-2  The four stages of neurocysticercosis. A, B. The vesicular stage. A. Postcontrast T1 magnetic resonance image (MRI). Note lesion in right parietal area. Small hypointense nodules within the cyst likely represent scolex. B. T2 MRI. The cyst is now visualized as a uniform hyperintense lesion with the small hypointense nodules likely representing scolex. No significant edema is present around the lesion on T2, typical for this stage of the disease. C, D. The colloidal stage. C. A medial left occipital lesion demonstrates peripheral enhancement on postcontrast imaging. D. On fluid-attenuated inversion recovery (FLAIR) MRI, the lesion has substantial surrounding hyperintense edema. E, F. The granulonodular stage. E. Postcontrast T1-weighted imaging demonstrates enhancing lesions in the left putamen and in the genu of the internal capsule near the foramen of Monro. F. These lesions demonstrate no surrounding edema on FLAIR imaging, typical for this stage of the disease. G. The nodular-calcified stage. Computed tomography scan demonstrates typical parenchymal brain calcifications. (Courtesy of Aaron Kamer, MD; with permission.) draining the sinuses or contiguous spread of infection to the brain from osteomyelitis in the posterior wall of the frontal or other sinuses. SDE may also develop from direct introduction of bacteria into the subdural space as a complication of a neurosurgical procedure. The evolution of SDE can be extremely rapid because the subdural space is a large compartment that offers few mechanical barriers to the spread of infection. In patients with sinusitis-associated SDE, suppuration typi­ cally begins in the upper and anterior portions of one cerebral hemi­ sphere and then extends posteriorly. SDE is often associated with other

CHAPTER 145 Brain Abscess and Empyema intracranial infections, including epidural empyema (40%), cortical thrombophlebitis (35%), and intracranial abscess or cerebritis (>25%). Cortical venous infarction produces necrosis of underlying cerebral cortex and subcortical white matter, with focal neurologic deficits and seizures (see below). ■ ■CLINICAL PRESENTATION A patient with SDE typically presents with fever and a progressively worsening headache. The diagnosis of SDE should always be suspected

Subdural empyema Thrombosed veins Dura mater Arachnoid FIGURE 145-3  Subdural empyema. in a patient with known sinusitis who presents with new CNS signs or symptoms. Patients with underlying sinusitis frequently have symptoms related to this infection. As the infection progresses, focal neurologic deficits, seizures, nuchal rigidity, and signs of increased ICP commonly occur. Headache is the most common complaint at the time of presentation; initially it is localized to the side of the subdural infection, but then it becomes more severe and generalized. Contralat­ eral hemiparesis or hemiplegia is the most common focal neurologic deficit and can occur from the direct effects of the SDE on the cortex or as a consequence of venous infarction. Seizures begin as partial motor seizures that then become secondarily generalized. Seizures may be due to the direct irritative effect of the SDE on the underlying cortex or result from cortical venous infarction. In untreated SDE, the increas­ ing mass effect and increase in ICP cause progressive deterioration in consciousness, leading ultimately to coma. PART 5 Infectious Diseases ■ ■DIAGNOSIS MRI (Fig. 145-4) is superior to CT in identifying SDE and any associ­ ated intracranial infections. The administration of gadolinium greatly improves diagnosis by enhancing the rim of the empyema and allowing A B C FIGURE 145-4  Subdural empyema. The purulent fluid collection along the left falx and left frontal convexity is hypointense on T1-weighted images (A) but markedly hyperintense on the diffusion-weighted (B) and the T2 fat-saturation (C) magnetic resonance images.

the empyema to be clearly delineated from the underlying brain paren­ chyma. Cranial MRI is also extremely valuable in identifying sinusitis, other focal CNS infections, cortical venous infarction, cerebral edema, and cerebritis. CT may show a crescent-shaped hypodense lesion over one or both hemispheres or in the interhemispheric fissure. Frequently the degree of mass effect, exemplified by midline shift, ventricular compression, and sulcal effacement, is far out of proportion to the mass of the SDE. CSF examination should be avoided in patients with known or suspected SDE because it adds no useful information and is associated with the risk of cerebral herniation. ■ ■DIFFERENTIAL DIAGNOSIS The differential diagnosis of the combination of headache, fever, focal neurologic signs, and seizure activity that progresses rapidly to an altered level of consciousness includes subdural hematoma, bacterial meningitis, viral encephalitis, brain abscess, superior sagittal sinus thrombosis, and acute disseminated encephalomyelitis. The presence of nuchal rigidity is unusual with brain abscess or epidural empyema and should suggest the possibility of SDE when associated with signifi­ cant focal neurologic signs and fever. Patients with bacterial meningitis also have nuchal rigidity but do not typically have focal deficits of the severity seen with SDE. TREATMENT Subdural Empyema SDE is a medical emergency. Emergent neurosurgical evacuation of the empyema, either through craniotomy, craniectomy, or burr-hole drainage, is the definitive step in the management of this infec­ tion. Empirical antimicrobial therapy for community-acquired SDE should include a combination of a third-generation cephalosporin (e.g., cefotaxime or ceftriaxone), vancomycin, and metronidazole (see Table 143-1 for dosages). Patients with hospital-acquired SDE may have infections due to Pseudomonas spp. or MRSA and should receive coverage with a carbapenem (e.g., meropenem) and vancomycin. Metronidazole is not necessary for antianaero­ bic therapy when meropenem is being used. Parenteral antibiotic therapy should be continued for a minimum of 3–4 weeks after SDE drainage. Patients with associated cranial osteomyelitis may require longer therapy. Specific diagnosis of the etiologic organisms is made based on Gram’s stain and culture of fluid obtained via either burr holes or craniotomy; the initial empirical antibiotic coverage can be modified accordingly.

■ ■PROGNOSIS Prognosis is influenced by the level of consciousness of the patient at the time of hospital presentation, the size of the empyema, and the speed with which therapy is instituted. Long-term neurologic sequelae, which include seizures and hemiparesis, occur in up to 50% of cases. CRANIAL EPIDURAL ABSCESS Cranial epidural abscess is a suppurative infection occurring in the potential space between the inner skull table and dura (Fig. 145-5). ■ ■ETIOLOGY AND PATHOPHYSIOLOGY Cranial epidural abscess is less common than either brain abscess or SDE and accounts for <2% of focal suppurative CNS infections. A cranial epidural abscess develops as a complication of a craniotomy or compound skull fracture or as a result of spread of infection from the frontal sinuses, middle ear, mastoid, or orbit. An epidural abscess may develop contiguous to an area of osteomyelitis, when craniotomy is complicated by infection of the wound or bone flap, or as a result of direct infection of the epidural space. Infection in the frontal sinus, middle ear, mastoid, or orbit can reach the epidural space through retrograde spread of infection from septic thrombophlebitis in the emissary veins that drain these areas or by way of direct spread of infection through areas of osteomyelitis. Unlike the subdural space, the epidural space is really a potential rather than an actual compart­ ment. The dura is normally tightly adherent to the inner skull table, and infection must dissect the dura away from the skull table as it spreads. As a result, epidural abscesses are often smaller than SDEs. Cranial epidural abscesses, unlike brain abscesses, only rarely result from hematogenous spread of infection from extracranial primary sites. The bacteriology of a cranial epidural abscess is similar to that of SDE (see above). The etiologic organisms of an epidural abscess that arises from frontal sinusitis, middle-ear infections, or mastoiditis are usually streptococci or anaerobic organisms. Staphylococci or gram-negative organisms are the usual cause of an epidural abscess that develops as a complication of craniotomy or compound skull fracture. ■ ■CLINICAL PRESENTATION Patients present with fever (60%), headache (40%), nuchal rigidity (35%), seizures (10%), and focal deficits (5%). Development of symp­ toms may be insidious, as the empyema usually enlarges slowly in the confined anatomic space between the dura and the inner table of the skull. Periorbital edema and Pott’s puffy tumor, reflecting underlying associated frontal bone osteomyelitis, are present in ~40%. In patients with a recent neurosurgical procedure, wound infection is invariably present, but other symptoms may be subtle and can include altered mental status (45%), fever (35%), and headache (20%). The diagnosis should be considered when fever and headache follow recent head trauma or occur in the setting of frontal sinusitis, mastoiditis, or otitis media. Epidural abscess FIGURE 145-5  Cranial epidural abscess is a collection of pus between the dura and the inner table of the skull.

■ ■DIAGNOSIS Cranial MRI with gadolinium enhancement is the procedure of choice to demonstrate a cranial epidural abscess. The sensitivity of CT is limited by the presence of signal artifacts arising from the bone of the inner skull table. The CT appearance of an epidural empyema is that of a lens or crescent-shaped hypodense extraaxial lesion. On MRI, an epidural empyema appears as a lentiform or crescent-shaped fluid col­ lection that is hyperintense compared to CSF on T2-weighted images. On T1-weighted images, the fluid collection may be either isointense or hypointense compared to brain. Following the administration of gadolinium, there is linear enhancement of the dura on T1-weighted images. In distinction to subdural empyema, signs of mass effect or other parenchymal abnormalities are uncommon.

TREATMENT Epidural Abscess Immediate neurosurgical drainage is indicated. Empirical anti­ microbial therapy, pending the results of Gram’s stain and cul­ ture of the purulent material obtained at surgery, should include a combination of a third-generation cephalosporin, vancomycin, and metronidazole (see Table 143-1). Ceftazidime or meropenem should be substituted for ceftriaxone or cefotaxime in neurosur­ gical patients. Metronidazole is not necessary for antianaerobic coverage in patients receiving meropenem. When the organism has been identified, antimicrobial therapy can be modified accordingly. Antibiotics should be continued for 3–6 weeks after surgical drain­ age. Patients with associated osteomyelitis may require additional therapy. CHAPTER 145 ■ ■PROGNOSIS The mortality rate is <5% in modern series, and full recovery is the rule in most survivors. Brain Abscess and Empyema SUPPURATIVE THROMBOPHLEBITIS ■ ■DEFINITION Suppurative intracranial thrombophlebitis is septic venous thrombosis of cortical veins and sinuses. This may occur as a complication of bac­ terial meningitis; SDE; epidural abscess; or infection in the skin of the face, paranasal sinuses, middle ear, or mastoid. ■ ■ANATOMY AND PATHOPHYSIOLOGY The cerebral veins and venous sinuses have no valves; therefore, blood within them can flow in either direction. The superior sagittal sinus is the largest of the venous sinuses (Fig. 145-6). It receives blood from the Superior sagittal sinus Transverse sinus Straight sinus Superior ophthalmic vein Inferior ophthalmic vein Sigmoid sinus Internal jugular vein Cavernous sinus FIGURE 145-6  Anatomy of the cerebral venous sinuses.

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146 Infectious Complications of Bites

frontal, parietal, and occipital superior cerebral veins and the diploic veins, which communicate with the meningeal veins. Bacterial menin­ gitis is a common predisposing condition for septic thrombosis of the superior sagittal sinus. The diploic veins, which drain into the superior sagittal sinus, provide a route for the spread of infection from the meninges, especially in cases where there is purulent exudate near areas of the superior sagittal sinus. Infection can also spread to the superior sagittal sinus from nearby SDE or epidural abscess. Dehydration from vomiting, hypercoagulable states, and immunologic abnormalities, including the presence of circulating antiphospholipid antibodies, also contribute to cerebral venous sinus thrombosis. Thrombosis may extend from one sinus to another, and at autopsy, thrombi of different histologic ages can often be detected in several sinuses. Thrombosis of the superior sagittal sinus is often associated with thrombosis of supe­ rior cortical veins and small parenchymal hemorrhages.

The superior sagittal sinus drains into the transverse sinuses (Fig. 145-6). The transverse sinuses also receive venous drainage from small veins from both the middle ear and mastoid cells. The transverse sinus becomes the sigmoid sinus before draining into the internal jugu­ lar vein. Septic transverse/sigmoid sinus thrombosis can be a complica­ tion of acute and chronic otitis media or mastoiditis. Infection spreads from the mastoid air cells to the transverse sinus via the emissary veins or by direct invasion. The cavernous sinuses are inferior to the superior sagittal sinus at the base of the skull. The cavernous sinuses receive blood from the facial veins via the superior and inferior ophthalmic veins. Bacteria in the facial veins enter the cavernous sinus via these veins. Bacteria in the sphenoid and ethmoid sinuses can spread to the cavernous sinuses via the small emissary veins. The sphenoid and eth­ moid sinuses are the most common sites of primary infection resulting in septic cavernous sinus thrombosis. PART 5 Infectious Diseases ■ ■CLINICAL MANIFESTATIONS Septic thrombosis of the superior sagittal sinus presents with head­ ache, fever, nausea and vomiting, confusion, and focal or generalized seizures. There may be a rapid development of stupor and coma. Weakness of the lower extremities with bilateral Babinski’s signs or hemiparesis is often present. When superior sagittal sinus thrombosis occurs as a complication of bacterial meningitis, nuchal rigidity and Kernig’s and Brudzinski’s signs may be present. The oculomotor nerve, the trochlear nerve, the abducens nerve, the ophthalmic and maxillary branches of the trigeminal nerve, and the internal carotid artery all pass through the cavernous sinus (see Fig. 452-7). The symptoms of septic cavernous sinus thrombosis are fever, headache, frontal and retroorbital pain, and diplopia. The classic signs are ptosis, proptosis, chemosis, and extraocular dysmotil­ ity due to deficits of cranial nerves III, IV, and VI; hyperesthesia of the ophthalmic and maxillary divisions of the fifth cranial nerve and a decreased corneal reflex may be detected. There may be evidence of dilated, tortuous retinal veins and papilledema. Headache and earache are the most frequent symptoms of trans­ verse sinus thrombosis. A transverse sinus thrombosis may also pres­ ent with otitis media, sixth nerve palsy, and retroorbital or facial pain (Gradenigo’s syndrome). Sigmoid sinus and internal jugular vein thrombosis may present with neck pain. ■ ■DIAGNOSIS The diagnosis of septic venous sinus thrombosis is suggested by an absent flow void within the affected venous sinus on MRI and confirmed by contrast-enhanced magnetic resonance venography, CT venography, or the venous phase of cerebral angiography. The diagnosis of thrombophlebitis of intracerebral and meningeal veins is suggested by the presence of intracerebral hemorrhage but requires the venous phase of cerebral angiography for definitive diagnosis. TREATMENT Suppurative Thrombophlebitis Septic venous sinus thrombosis is treated with antibiotics, hydra­ tion, and removal of infected tissue and thrombus in septic lateral

or cavernous sinus thrombosis. The choice of antimicrobial therapy is based on the bacteria responsible for the predisposing or asso­ ciated condition. Optimal duration of therapy is unknown, but antibiotics are usually continued for 6 weeks or until there is radiographic evidence of resolution of thrombosis. Anticoagulation with unfractionated or low-molecular-weight heparin is recom­ mended for aseptic venous sinus thrombosis and in the treatment of septic venous sinus thrombosis complicating bacterial meningitis in patients who have progressive neurologic deterioration despite antimicrobial therapy and intravenous fluids. The presence of a small intracerebral hemorrhage from septic thrombophlebitis is not an absolute contraindication to heparin therapy. Successful management of aseptic venous sinus thrombosis has been reported with surgical thrombectomy, catheter-directed urokinase therapy, and a combination of intrathrombus recombinant tissue plasmino­ gen activator (rtPA) and intravenous heparin, but there are not enough data to recommend these therapies in septic venous sinus thrombosis. ■ ■FURTHER READING Bodilsen J et al: European Society of Clinical Microbiology and Infec­ tious Diseases guidelines on diagnosis and treatment of brain abscess in children and adults. Clinical Microbiol Infect 30:66, 2024. Prosty C et al: Revisiting the evidence base for modern-day practice of the treatment of toxoplasmic encephalitis: A systematic review and meta-analysis. Clin Infect Dis 76:e1302, 2023. Ropper AH, Klein JP: Cerebral venous thrombosis. N Engl J Med 385:59, 2021. White AC et al: Diagnosis of Neurocysticercosis: 2017 Clinical Practice Guidelines by the Infectious Diseases Society of America (IDSA) and the American Society of Tropical Medicine and Hygiene (ASTMH). Clin Infect Dis 66:e49, 2018. Nongnooch Poowanawittayakom,

Lawrence C. Madoff

Infectious Complications

of Bites The skin is an essential component of nonspecific immunity, protect­ ing the host from potential pathogens in the environment. Breaches in this protective barrier thus represent a form of immunocompromise that predisposes the patient to infection. Bites and scratches from animals and humans allow the inoculation of microorganisms past the skin’s protective barrier into deeper, susceptible host tissues. Each year in the United States, millions of animal-bite wounds are sustained. The vast majority are inflicted by pet dogs and cats, which number >100 million; the annual incidence of dog and cat bites has been reported as 300 bites per 100,000 population. Other bite wounds are a consequence of encounters with animals in the wild or in occu­ pational settings. While many of these wounds require minimal or no therapy, a significant number result in infection, which may be life-threatening. The microbiology of bite-wound infections in general reflects the oropharyngeal flora of the biting animal, although organ­ isms from the soil, the skin of the animal and the victim, and the animal’s feces may also be involved. DOG BITES In the United States, dogs bite >4.7 million people each year and are responsible for 80% of all animal-bite wounds, an estimated 15–20% of which become infected. Each year, 800,000 Americans seek medical

attention for dog bites; of those injured, 386,000 require treatment in an emergency department, with >1000 emergency department visits each day and ~43 deaths per year. Most dog bites are provoked and are inflicted by the victim’s pet or by a dog known to the victim. These bites are frequently sustained during efforts to break up a dogfight. Children are more likely than adults to sustain canine bites, with the highest inci­ dence of 6 bites per 1000 population among boys 5–9 years old. Victims are more often male than female, and bites most often involve an upper extremity. Among children <4 years old, two-thirds of all these injuries involve the head or neck. Infection typically manifests 8–24 h after the bite as pain at the site of injury with cellulitis accompanied by purulent, sometimes foul-smelling discharge. Septic arthritis and osteomyelitis may develop if a canine tooth penetrates synovium or bone. Systemic manifestations (e.g., fever, lymphadenopathy, and lymphangitis) also may occur. The microbiology of dog-bite wound infections is usu­ ally mixed and includes Pasteurella species, β-hemolytic streptococci, Staphylococcus species (including methicillin-resistant Staphylococcus aureus [MRSA] and Staphylococcus intermedius), Neisseria species (commonly Neisseria weaveri, formerly known as CDC group M-5), Eikenella corrodens, and Capnocytophaga canimorsus. Many wounds also include anaerobic bacteria such as Bacteroides, Fusobacterium, Prevotella, and Porphyromonas species. While most infections resulting from dog-bite injuries are localized to the area of injury, many of the microorganisms involved are capable of causing systemic infection, including bacteremia, meningitis, brain abscess, endocarditis, and chorioamnionitis. These infections are par­ ticularly likely in hosts with edema or compromised lymphatic drain­ age in the involved extremity (e.g., after a bite on the arm in a woman who has undergone mastectomy) and in patients who are immu­ nocompromised by medication or disease (e.g., glucocorticoid use, systemic lupus erythematosus, acute leukemia, or hepatic cirrhosis). In addition, dog bites and scratches may result in systemic illnesses such as rabies (Chap. 214) and tetanus (Chap. 157). Infection with Capnocytophaga canimorsus (and other Capnocyto­ phaga species) following dog-bite wounds (or licking of preexisting wounds) may result in fulminant sepsis, disseminated intravascular coagulation, and renal failure, particularly in hosts who have impaired hepatic function, who have undergone splenectomy, or who are immu­ nosuppressed. This thin gram-negative rod is difficult to culture on most solid media but grows in a variety of liquid media. It may require up to 14 days of incubation to grow on blood cultures. The bacteria are occasionally seen within polymorphonuclear leukocytes on Wrightstained smears of peripheral blood from septic patients. Tularemia (Chap. 175) also has been reported to follow dog bites. CAT BITES Although less common than dog bites, cat bites and scratches result in infection in more than half of all cases. Because the cat’s narrow, sharp canine teeth penetrate deeply into tissue, cat bites are more likely than dog bites to cause septic arthritis and osteomyelitis; the development of these conditions is particularly likely when punctures are located over or near a joint, especially in the hand. Women sustain cat bites more frequently than do men. These bites most often involve the hands and arms. Both bites and scratches from cats are prone to infec­ tion from organisms in the cat’s oropharynx. Pasteurella multocida, a normal component of the feline oral flora, is a small gram-negative coccobacillus implicated in the majority of cat-bite wound infections. Like that of dog-bite wound infections, however, the microflora of cat-bite wound infections is usually mixed. However, the median time from bite to the appearance of signs and symptoms of wound infection is much shorter when compared to dog bites. Other microorganisms causing infection after cat bites are similar to those causing dog-bite wound infections. The same risk factors for systemic infection following dog-bite wounds apply to cat-bite wounds. Pasteurella infections tend to advance rapidly, often within hours, causing severe inflammation accompanied by purulent drainage with adenitis; Pasteurella may also be spread by respiratory droplets from animals, resulting in pneumonia or bacteremia. Like dog-bite wounds, cat-bite wounds may result in

the transmission of rabies or in the development of tetanus. Infection with Bartonella henselae causes cat-scratch disease (Chap. 177) and is an important late consequence of cat bites and scratches. Tularemia (Chap. 175) also has been reported to follow cat bites. Occasionally, sporotrichosis (Chap. 225) has been associated with scratches or bites by animals, especially domestic cats.

OTHER ANIMAL BITES Infections have been attributed to bites from many animal species. Often these bites are sustained as a consequence of occupational exposure (farmers, laboratory workers, veterinarians) or recreational exposure (hunters and trappers, wilderness campers, owners of exotic pets). Generally, the microflora of bite wounds reflects the oral flora of the biting animal. Most members of the cat family, including feral cats, harbor P. multocida. Bite wounds from aquatic animals such as alliga­ tors or piranhas may contain Aeromonas hydrophila. Shark, moray eel, and barracuda bites, like other injuries sustained in saltwater, are often associated with infections with marine Vibrio species. Venomous snakebites (Chap. 471) result in severe inflammatory responses and tis­ sue necrosis—conditions that render these injuries prone to infection. The snake’s oral flora includes many species of aerobes and anaerobes, such as Pseudomonas aeruginosa, Serratia marcescens, Proteus species, Staphylococcus epidermidis, Salmonella species, Bacteroides fragilis, and Clostridium species. Bites from nonhuman primates are highly suscep­ tible to infection with pathogens similar to those isolated from human bites (see below). Bites from Old World monkeys (Macaca) may also result in the transmission of B virus (Macacine herpesvirus 1, Herpes­ virus simiae, Cercopithecine herpesvirus), a cause of serious infection of the human central nervous system. Actinobacillus lignieresii has often been reported in infected wounds of humans bitten by horses, pigs, and sheep. Bites of seals, walruses, and polar bears may cause a chronic sup­ purative infection known as seal finger, which is probably due to one or more species of Mycoplasma, including Mycoplasma phocacerebrale, colonizing these animals. CHAPTER 146 Infectious Complications of Bites Small rodents, including rats, mice, and gerbils, as well as animals that prey on rodents may transmit Streptobacillus moniliformis (a microaerophilic, pleomorphic gram-negative rod) or Spirillum minor (a spirochete); these organisms cause a clinical illness known as rat-bite fever. The vast majority of cases in the United States are streptobacil­ lary, whereas Spirillum infection occurs mainly in Asia. In the United States, the risk of rodent bites is usually greatest among laboratory workers or inhabitants of rodent-infested dwellings (particularly children). Rat-bite fever is distinguished from acute bitewound infection by its typical manifestation after the initial wound has healed. Streptobacillary disease follows an incubation period of 3–10 days. Fever, chills, myalgias, headache, and severe migratory arthralgias are usually followed by a maculopapular rash, which char­ acteristically involves the palms and soles and may become confluent or purpuric. Complications include endocarditis, myocarditis, menin­ gitis, pneumonia, and abscesses in many organs. Haverhill fever is an S. moniliformis infection acquired from contaminated milk or drinking water and has similar manifestations. Streptobacillary rat-bite fever was frequently fatal in the preantibiotic era. The differential diagnosis includes Rocky Mountain spotted fever, Lyme disease, leptospirosis, and secondary syphilis. The diagnosis is made by direct observation of the causative organisms in tissue or blood, by culture of the organisms on enriched media, or by serologic testing with specific agglutinins. Spirillum infection (referred to in Japan as sodoku) causes pain and purple swelling at the site of the initial bite, with associated lymphan­ gitis and regional lymphadenopathy, after an incubation period of 1–4 weeks. The systemic illness includes fever, chills, and headache. The original lesion may eventually progress to an eschar. The infection is diagnosed by direct visualization of the spirochetes in blood or tissue or by animal inoculation. HUMAN BITES Human bites may be self-inflicted; may be sustained by medical per­ sonnel caring for patients; or may take place during fights, domestic abuse, or sexual activity. The risk of infection in human-bite wounds

depends on the depth of the wound. The risk of wound infection ranges from about 2% in superficial wounds to over 25% in deep bite wounds such as clenched-fist injuries. Human-bite wounds become infected more frequently (~10–15% of the time) than do bites inflicted by other animals. These infections reflect the diverse oral microflora of humans, which includes multiple species of aerobic and anaerobic bac­ teria. Common aerobic isolates include viridans streptococci, S. aureus, E. corrodens (which is particularly common in clenched-fist injury; see below), and Haemophilus influenzae. Anaerobic species, includ­ ing Fusobacterium nucleatum and Prevotella, Porphyromonas, and Peptostreptococcus species, are isolated from 50% of wound infections due to human bites; many of these isolates produce β-lactamases. The oral flora of hospitalized and debilitated patients often includes Entero­ bacteriaceae in addition to the usual organisms. Hepatitis B, hepatitis C, herpes simplex virus infection, syphilis, tuberculosis, actinomycosis, and tetanus have been reported to be transmitted by human bites; it is biologically possible to transmit HIV through human bites, although the risk is quite low. In general, postexposure prophylaxis should be considered for bites involving severe trauma with extensive tissue dam­ age and the presence of blood in saliva. There is essentially no risk of transmission if the skin is intact.

Human bites are categorized as either occlusional injuries, which are inflicted by actual biting, or clenched-fist injuries, which are sustained when the fist of one individual strikes the teeth of another, causing traumatic laceration of the hand. For several reasons, clenched-fist injuries, which are sometimes referred to as “fight bite” and which are more common than occlusional injuries, result in particularly serious infections. The deep spaces of the hand, includ­ ing the bones, joints, and tendons, are frequently inoculated with organisms in the course of such injuries. The clenched position of the fist during injury, followed by extension of the hand, may further promote the introduction of bacteria as contaminated tendons retract beneath the skin’s surface Moreover, medical attention is often sought only after frank infection develops. Patients with clenched-fist injury should undergo careful physical examination of the area, including the extensor tendons. PART 5 Infectious Diseases APPROACH TO THE PATIENT Animal or Human Bites A careful history should be elicited, including the type of biting animal, the type of attack (provoked or unprovoked), and the amount of time elapsed since injury. Local and regional publichealth authorities should be contacted to determine whether an individual species could be rabid and/or to locate and observe the biting animal when rabies prophylaxis may be indicated (Chap. 214). Suspicious human-bite wounds should provoke careful ques­ tioning regarding domestic or child abuse. Details on antibiotic allergies, immunosuppression, splenectomy, liver disease, mastec­ tomy, and immunization history should be obtained. The wound should be inspected carefully for evidence of infection, including redness, exudate, and foul odor. The type of wound (puncture, laceration, or scratch); the depth of penetration; and the pos­ sible involvement of joints, tendons, nerves, and bones should be assessed. It is often useful to include a diagram or photograph of the wound in the medical record. In addition, a general physical examination should be conducted and should include an assess­ ment of vital signs as well as an evaluation for evidence of lym­ phangitis, lymphadenopathy, dermatologic lesions, and functional limitations. Injuries to the hand warrant consultation with a hand surgeon for the assessment of tendon, nerve, and muscular dam­ age. Radiographs should be obtained in penetrating wounds to evaluate the evidence of fracture or retained foreign body such as a tooth fragment. Culture and Gram’s staining of all infected wounds are essential; anaerobic cultures should be undertaken if abscesses, devitalized tissue, or foul-smelling exudate is present. A small-tipped swab may be used to culture deep punctures or small lacerations. It is also reasonable to culture samples from

apparently uninfected wounds due to bites inflicted by animals other than dogs and cats, since the microorganisms causing dis­ ease are less predictable in these cases. The microbiology labora­ tory should be notified if fastidious organisms such as E. corrodens are under consideration in human bites. The white blood cell count should be determined, and the blood cultured if systemic infection is suspected. TREATMENT Bite-Wound Infections WOUND MANAGEMENT Wound closure is controversial in bite injuries. Many authori­ ties prefer not to attempt primary closure of wounds that are or may become infected, choosing instead to irrigate these wounds copiously, debride devitalized tissue, remove foreign bodies, and approximate the wound edges. All abscesses should be drained. Delayed primary closure may be undertaken after the risk of infec­ tion is over. Small uninfected wounds may be allowed to close by secondary intention. Puncture wounds due to cat bites should be left unsutured because of the high rate at which they become infected. Facial wounds are usually sutured after thorough cleaning and irrigation because of the importance of a good cosmetic result in this area and because anatomic factors such as an excellent blood supply and the absence of dependent edema lessen the risk of infec­ tion. In general, wounds >12 h old (for bites to the arm or leg) or

24 h old (for bites to the face) should not be closed primarily and may require prophylactic antibiotics (see below). ANTIBIOTIC THERAPY Established Infection  Antibiotics should be administered for all established bite-wound infections and should be chosen in light of the most likely potential pathogens, as indicated by the biting species and by Gram’s stain and culture results (Table 146-1). For dog and cat bites, antibiotics should be effective against S. aureus, Pasteurella species, C. canimorsus, streptococci, and oral anaerobes. For human bites, agents with activity against S. aureus, H. influenzae, and β-lactamase-positive oral anaerobes should be used. The combination of an extended-spectrum penicillin with a β-lactamase inhibitor (amoxicillin/clavulanic acid, ampicillin/ sulbactam) appears to offer the most reliable coverage for these pathogens. Third-generation cephalosporins (ceftriaxone, cefpo­ doxime) also offer substantial coverage when given in conjunc­ tion with a drug that provides anaerobic coverage (clindamycin or metronidazole). The choice of antibiotics for penicillin-allergic patients (particularly those in whom immediate-type hypersensi­ tivity makes the use of cephalosporins hazardous) is more difficult and is based primarily on in vitro sensitivity since data on clinical efficacy are inadequate. The combination of an antibiotic active against gram-positive cocci and anaerobes (such as clindamy­ cin or metronidazole) with trimethoprim-sulfamethoxazole or a fluoroquinolone, which is active against many of the other potential pathogens, would appear reasonable. Moxifloxacin, a fluoroquinolone with anaerobic coverage, can also be considered as a single agent. In vitro data suggest that azithromycin alone provides coverage against most commonly isolated bite-wound pathogens; however, this agent has variable activity against

P. multocida, E. corrodens, and fusobacteria and thus should be avoided unless no alternative agent is available. Empirical use of agents active against MRSA should be considered in high-risk situations while culture results are awaited. Antibiotics are generally given for about 5–7 days and for no more than 14 days, but the response to therapy must be carefully monitored. Failure to respond should prompt a consideration of diagnostic alternatives and surgical evaluation for possible drain­ age or debridement. Complications such as osteomyelitis or septic arthritis mandate a longer duration of therapy.

TABLE 146-1  Management of Wound Infections Following Animal and Human Bites COMMONLY ISOLATED PATHOGENS PREFERRED ANTIBIOTIC(S)a BITING SPECIES Dog Staphylococcus aureus, Pasteurella spp. (mainly

P. multocida and

P. canis), anaerobes, Capnocytophaga canimorsus Amoxicillin/clavulanate (875/125 mg PO q12h) or ampicillin/sulbactam

(3.0 g IV q6h) or Ceftriaxone 2 g IV once daily plus metronidazole 500 mg q8h Cat P. multocida, S. aureus, anaerobes Amoxicillin/clavulanate, ampicillin/sulbactam, or ceftriaxone plus metronidazole as above Human, occlusional Viridans streptococci, S. aureus, Haemophilus influenzae, anaerobes Eikenella corrodense Amoxicillin/clavulanate plus TMX-SMX if consider including MRSA coverage or

ampicillin/sulbactam

or ceftriaxone plus metronidazole (consider adding vancomycin if MRSA coverage required)   Monkey As for human bite As for human bite As for human bite Always For macaque monkeys, consider B virus prophylaxis with acyclovir. Snake Snake oral flora including Pseudomonas, Morganella spp., E. coli, group D streptococci, Salmonella spp., anaerobic organisms including Bacteroides fragilis, Clostridium spp. Piperacillin/tazobactam

3.375 g IV q6–8h Rodent Rat bite fever; Streptobacillus moniliformis, Spirillum minus, Streptobacillus notomytis, Leptospira spp., P. multocida Penicillin VK (500 mg PO qid) or ceftriaxone IV Aquatic animal (alligator, piranha, shark, moray eel, barracuda) Aeromonas hydrophila, marine Vibrio spp. (Vibrio vulnificus) Third-generation cephalosporin (e.g., ceftriaxone, 1 g IV q24h) plus doxycycline (100 mg PO bid) aAntibiotic choices should be based on culture data when available. These suggestions for empirical therapy need to be tailored to individual circumstances and local conditions. MRSA empirical coverage is based on individual risk factors. IV regimens should be used for hospitalized patients. A single IV dose of antibiotics may be given to patients who will be discharged after initial management. bAvoid monotherapy for aerobic coverage due to poor activity against Pasteurella spp. Risk of Clostridioides difficile infection. cCat bite may leave small external wounds but deep puncture wounds. dProphylactic antibiotics are suggested for severe or extensive wounds, facial wounds, and crush injuries; when bone or joint may be involved; delayed wound care >8 h; and when comorbidity is present (see text). Prophylactic antibiotic duration is generally between 3 and 5 days. eEikenella corrodens is resistant to penicillinase-resistant penicillin, first and second generation cephalosporins, clindamycin, metronidazole, and aminoglycosides in vitro. Abbreviations: DS, double-strength; TMP-SMX, trimethoprim-sulfamethoxazole. Management of C. canimorsus bacteremia requires an initial treatment with a 2-week course of IV antibiotics such as penicillin G (2 million units IV every 4 h) or IV ampicillin/sulbactam (3.0 g every 6 h) along with supportive measures. Once the patient is improved, then a switch to oral antibiotics can be considered. Alter­ native agents for the treatment of C. canimorsus infection include cephalosporins or carbapenems. Serious infection with P. multocida (e.g., pneumonia, sepsis, or meningitis) also should be treated with IV penicillin G. Alternative agents include a second- or thirdgeneration cephalosporin or ciprofloxacin. Penicillin resistance is uncommon. Bites by venomous snakes (Chap. 471) may not require antibi­ otic treatment. Because it is often difficult to distinguish signs of infection from tissue damage caused by the envenomation, many authorities continue to recommend treatment directed against the snake’s oral flora—i.e., the administration of broadly active agents such as ceftriaxone (1–2 g IV every 24 h) or ampicillin/sulbactam (3.0 g IV every 6 h). Seal finger appears to respond to doxycycline (100 mg twice daily for a duration guided by the response to therapy).

ALTERNATIVE IN PENICILLIN-ALLERGIC PATIENT PROPHYLAXIS ADVISED FOR EARLY UNINFECTED WOUNDS OTHER CONSIDERATIONS Clindamycinb or metronidazole plus either TMP-SMX

(1 DS tablet PO bid) or ciprofloxacin (500 mg PO bid) Sometimesc Consider rabies prophylaxis. Clindamycin or metronidazole plus TMP-SMX as above or fluoroquinolone Usuallyd Consider rabies prophylaxis. Carefully evaluate for joint/bone penetration. TMP-SMX plus metronidazole Always   Clindamycin or metronidazole plus a fluoroquinolone Evidence does not support the benefit. Can consider in regions with high rates of infection such as Brazil. Administer antivenin for venomous snakebite. Tetanus prophylaxis. CHAPTER 146 Doxycycline (100 mg PO bid) Sometimes   Infectious Complications of Bites Clindamycin or metronidazole plus levofloxacin (750 mg PO qd) plus doxycycline Always Obtain prompt surgical consultation, as risk for necrotizing infection is high with Aeromonas and Vibrio spp. Presumptive or Prophylactic Therapy  The use of antibiotics for patients presenting early (within 8 h) after bite injury is controver­ sial. Although symptomatic infection frequently will not yet have manifested at this point, many early wounds will harbor pathogens, and many will become infected. Studies of antibiotic prophylaxis for wound infections are limited and have often included only small numbers of cases in which various types of wounds have been man­ aged according to various protocols. A meta-analysis of eight ran­ domized trials of prophylactic antibiotics in patients with dog-bite wounds demonstrated a reduction in the rate of infection by 50% with prophylaxis. However, in the absence of sound clinical trials, many clinicians base the decision to treat bite wounds with empirical antibiotics on the species of the biting animal; the location, severity, and extent of the bite wound; and the existence of comorbid condi­ tions in the host. All human- and monkey-bite wounds should be treated presumptively because of the high rate of infection. Most catbite wounds, particularly those involving the hand, should consider prophylactic antibiotics. Other factors favoring treatment for bite wounds include severe injury, as in crush wounds; potential bone or joint involvement; involvement of the hands or genital region; host

26 - SECTION 3 Clinical Syndromes- Health Care–Associated Infections

SECTION 3 Clinical Syndromes: Health Care–Associated Infections

immunocompromise, including that due to diabetes mellitus, liver disease, or splenectomy; involvement of extremities with underlying venous and/or lymphatic compromise; and prior mastectomy on the side of an involved upper extremity. When prophylactic antibiotics are administered, they are usually given for 3–5 days.

Rabies and Tetanus Prophylaxis  Rabies prophylaxis, consisting of both passive administration of rabies immune globulin (with as much of the dose as possible infiltrated into and around the wound) and active immunization with rabies vaccine, should be given in consultation with local and regional public-health authorities for some animal bites and scratches as well as for certain nonbite expo­ sures (Chap. 214). Rabies is endemic in a variety of animals, includ­ ing dogs and cats, in many areas of the world. In the United States, although the majority (90%) of rabid animals reported each year are wild (including raccoons, skunks, foxes, and bats), most rabies prophylaxis is given because of close contact with domestic animals. More cats than dogs are reported rabid each year. Many local health authorities require the reporting of all animal bites. A tetanus booster immunization should be given if the patient has undergone primary immunization but has not received a booster dose in the past 5 years. Patients who have not previ­ ously completed primary immunization should be immunized and should also receive tetanus immune globulin. Elevation of the site of injury is an important adjunct to antimicrobial therapy. Immo­ bilization of the infected area, especially the hand, also is beneficial. Hepatitis B Prophylaxis  Hepatitis B virus can be transmitted, albeit rarely, by exposure of nonintact skin to blood-free saliva. The mainstay of postexposure prophylaxis is active immunization with hepatitis B vaccine, but, in certain circumstances, hepatitis B immune globulin is recommended in addition to vaccine for added protection (Chap. 350). PART 5 Infectious Diseases Acknowledgment The authors would like to acknowledge Drs. Sandeep S. Jubbal and Florencia Pereyra for their prior contributions to this chapter. ■ ■FURTHER READING Abrahamian FM, Goldstein EJC: Microbiology of animal bite wound infections. Clin Microbiol Rev 24:231, 2011. Brook I: Management of human and animal bite wounds: An over­ view. Adv Skin Wound Care 18:197, 2005. Bystritsky R, Chambers H: Cellulitis and soft tissue infections. Ann Intern Med 168:ITC17, 2018. Ellis R, Ellis C: Dog and cat bites. Am Fam Phys 90:239, 2014. Fallouji MA: Traumatic love bites. Br J Surg 77:100, 1990. Fleisher GR: The management of bite wounds. N Engl J Med 340:138, 1999. Kullberg BJ et al: Purpura fulminans and symmetrical peripheral gangrene caused by Capnocytophaga canimorsus (formerly DF-2) septicemia—a complication of dog bite. Medicine (Baltimore) 70:287, 1991. Lohiya GS et al: Human bites: Bloodborne pathogen risk and postex­ posure follow-up algorithm. J Natl Med Assoc 105:92, 2013. Martino R et al: Bacteremia caused by Capnocytophaga species in patients with neutropenia and cancer: Results of a multicenter study. Clin Infect Dis 33:e20, 2001. Morgan M, Palmer J: Dog bites. BMJ 334:413, 2007. Oehler RL et al: Bite-related and septic syndromes caused by cats and dogs. Lancet Infect Dis 9:439, 2009. Stevens DL et al: Practice guidelines for the diagnosis and manage­ ment of skin and soft tissue infections. 2014 update by the Infectious Diseases Society of America. Clin Infect Dis 59:e10, 2014. Weber DJ et al: Infections resulting from animal bites. Infect Dis Clin North Am 5:663, 1991. World Health Organization, Regional Office for South-East Asia: Guidelines for the management of snakebites, 2nd ed, 2016. Available at https://iris.who.int/handle/10665/249547. Accessed February 13, 2024.

Section 3 Clinical Syndromes: Health Care–Associated Infections Mini Kamboj, Tara N. Palmore

Infections Acquired in

Health Care Facilities Health care–associated infections affect at least 3% of hospitalized patients at any given time. Through concerted efforts, national rates of some nosocomial infections were declining before the onset of the COVID-19 pandemic, but infection control challenges related to the pandemic reversed years of progress. The past few years have also seen a rise in incidence of multidrug-resistant infections, which are challenging to treat and contain. However, newer tools combined with evidence-based methods of infection prevention and control are robust and can succeed. This chapter reviews the epidemiology, prevention, and control of health care–associated infections and recent challenges faced by health care epidemiologists. ORGANIZATION, RESPONSIBILITIES, AND OVERSIGHT OF INFECTION PREVENTION AND CONTROL PROGRAMS Infection prevention and control programs are composed of infection preventionists supervised by an experienced team lead. These typically include a doctoral-level (MD/DO/PhD) health care epidemiologist who may report to the chief medical officer or chief quality officer. The number of staff required in an infection prevention and control program depends on the size and complexity of the health care facility and its patients. Infection prevention and control programs are responsible for a broad range of activities, including surveillance and reporting of nosocomial infections; preventing and thwarting transmission of nosocomial patho­ gens through use of isolation and education; reducing device-associated infections through evidence-based interventions; collaborating with occupational health to manage infectious exposures; preparing for and managing emerging infectious diseases; and investigating and control­ ling outbreaks. The team collects and analyzes infection data and reports those data to institutional stakeholders, such as the multidisciplinary Infection Control Committee. Infection preventionists usually perform the mandatory reporting of select nosocomial infection data to the National Healthcare Safety Network that is managed by the Centers for Disease Control and Prevention (CDC). Such reporting is required by the U.S. Centers for Medicare and Medicaid Services and affects facilities’ reimbursement for the care they have provided, i.e., nonpayment for care related to preventable nosocomial infections. SURVEILLANCE Surveillance to detect and prevent health care–associated infections focuses on outcomes, processes, and other related measures that directly or indirectly influence the risk of contracting them. Examples of outcomes include surgical site infections and hospital-onset Clos­ tridioides difficile infections. Key process measures include compliance with evidence-based practices that reduce the risk of infection, such as hand hygiene, central line insertion care, and maintenance practices for indwelling devices. Finally, health care personnel influenza immuniza­ tion rates are an example of a related measure that is tracked at a local, regional, and national level to gauge efforts toward reducing the risk of nosocomial influenza in acute and long-term care settings. Detecting health care–associated infections using a case-finding strategy is a labor- and resource-intensive process. Most U.S. hospitals rely on laboratory-based surveillance as the fundamental data collec­ tion methodology, supplemented with clinical reviews by infection preventionists.

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147 Infections Acquired in Health Care Facilities

immunocompromise, including that due to diabetes mellitus, liver disease, or splenectomy; involvement of extremities with underlying venous and/or lymphatic compromise; and prior mastectomy on the side of an involved upper extremity. When prophylactic antibiotics are administered, they are usually given for 3–5 days.

Rabies and Tetanus Prophylaxis  Rabies prophylaxis, consisting of both passive administration of rabies immune globulin (with as much of the dose as possible infiltrated into and around the wound) and active immunization with rabies vaccine, should be given in consultation with local and regional public-health authorities for some animal bites and scratches as well as for certain nonbite expo­ sures (Chap. 214). Rabies is endemic in a variety of animals, includ­ ing dogs and cats, in many areas of the world. In the United States, although the majority (90%) of rabid animals reported each year are wild (including raccoons, skunks, foxes, and bats), most rabies prophylaxis is given because of close contact with domestic animals. More cats than dogs are reported rabid each year. Many local health authorities require the reporting of all animal bites. A tetanus booster immunization should be given if the patient has undergone primary immunization but has not received a booster dose in the past 5 years. Patients who have not previ­ ously completed primary immunization should be immunized and should also receive tetanus immune globulin. Elevation of the site of injury is an important adjunct to antimicrobial therapy. Immo­ bilization of the infected area, especially the hand, also is beneficial. Hepatitis B Prophylaxis  Hepatitis B virus can be transmitted, albeit rarely, by exposure of nonintact skin to blood-free saliva. The mainstay of postexposure prophylaxis is active immunization with hepatitis B vaccine, but, in certain circumstances, hepatitis B immune globulin is recommended in addition to vaccine for added protection (Chap. 350). PART 5 Infectious Diseases Acknowledgment The authors would like to acknowledge Drs. Sandeep S. Jubbal and Florencia Pereyra for their prior contributions to this chapter. ■ ■FURTHER READING Abrahamian FM, Goldstein EJC: Microbiology of animal bite wound infections. Clin Microbiol Rev 24:231, 2011. Brook I: Management of human and animal bite wounds: An over­ view. Adv Skin Wound Care 18:197, 2005. Bystritsky R, Chambers H: Cellulitis and soft tissue infections. Ann Intern Med 168:ITC17, 2018. Ellis R, Ellis C: Dog and cat bites. Am Fam Phys 90:239, 2014. Fallouji MA: Traumatic love bites. Br J Surg 77:100, 1990. Fleisher GR: The management of bite wounds. N Engl J Med 340:138, 1999. Kullberg BJ et al: Purpura fulminans and symmetrical peripheral gangrene caused by Capnocytophaga canimorsus (formerly DF-2) septicemia—a complication of dog bite. Medicine (Baltimore) 70:287, 1991. Lohiya GS et al: Human bites: Bloodborne pathogen risk and postex­ posure follow-up algorithm. J Natl Med Assoc 105:92, 2013. Martino R et al: Bacteremia caused by Capnocytophaga species in patients with neutropenia and cancer: Results of a multicenter study. Clin Infect Dis 33:e20, 2001. Morgan M, Palmer J: Dog bites. BMJ 334:413, 2007. Oehler RL et al: Bite-related and septic syndromes caused by cats and dogs. Lancet Infect Dis 9:439, 2009. Stevens DL et al: Practice guidelines for the diagnosis and manage­ ment of skin and soft tissue infections. 2014 update by the Infectious Diseases Society of America. Clin Infect Dis 59:e10, 2014. Weber DJ et al: Infections resulting from animal bites. Infect Dis Clin North Am 5:663, 1991. World Health Organization, Regional Office for South-East Asia: Guidelines for the management of snakebites, 2nd ed, 2016. Available at https://iris.who.int/handle/10665/249547. Accessed February 13, 2024.

Section 3 Clinical Syndromes: Health Care–Associated Infections Mini Kamboj, Tara N. Palmore

Infections Acquired in

Health Care Facilities Health care–associated infections affect at least 3% of hospitalized patients at any given time. Through concerted efforts, national rates of some nosocomial infections were declining before the onset of the COVID-19 pandemic, but infection control challenges related to the pandemic reversed years of progress. The past few years have also seen a rise in incidence of multidrug-resistant infections, which are challenging to treat and contain. However, newer tools combined with evidence-based methods of infection prevention and control are robust and can succeed. This chapter reviews the epidemiology, prevention, and control of health care–associated infections and recent challenges faced by health care epidemiologists. ORGANIZATION, RESPONSIBILITIES, AND OVERSIGHT OF INFECTION PREVENTION AND CONTROL PROGRAMS Infection prevention and control programs are composed of infection preventionists supervised by an experienced team lead. These typically include a doctoral-level (MD/DO/PhD) health care epidemiologist who may report to the chief medical officer or chief quality officer. The number of staff required in an infection prevention and control program depends on the size and complexity of the health care facility and its patients. Infection prevention and control programs are responsible for a broad range of activities, including surveillance and reporting of nosocomial infections; preventing and thwarting transmission of nosocomial patho­ gens through use of isolation and education; reducing device-associated infections through evidence-based interventions; collaborating with occupational health to manage infectious exposures; preparing for and managing emerging infectious diseases; and investigating and control­ ling outbreaks. The team collects and analyzes infection data and reports those data to institutional stakeholders, such as the multidisciplinary Infection Control Committee. Infection preventionists usually perform the mandatory reporting of select nosocomial infection data to the National Healthcare Safety Network that is managed by the Centers for Disease Control and Prevention (CDC). Such reporting is required by the U.S. Centers for Medicare and Medicaid Services and affects facilities’ reimbursement for the care they have provided, i.e., nonpayment for care related to preventable nosocomial infections. SURVEILLANCE Surveillance to detect and prevent health care–associated infections focuses on outcomes, processes, and other related measures that directly or indirectly influence the risk of contracting them. Examples of outcomes include surgical site infections and hospital-onset Clos­ tridioides difficile infections. Key process measures include compliance with evidence-based practices that reduce the risk of infection, such as hand hygiene, central line insertion care, and maintenance practices for indwelling devices. Finally, health care personnel influenza immuniza­ tion rates are an example of a related measure that is tracked at a local, regional, and national level to gauge efforts toward reducing the risk of nosocomial influenza in acute and long-term care settings. Detecting health care–associated infections using a case-finding strategy is a labor- and resource-intensive process. Most U.S. hospitals rely on laboratory-based surveillance as the fundamental data collec­ tion methodology, supplemented with clinical reviews by infection preventionists.

Widespread adoption of automated surveillance systems to collect, analyze, and combine infection and antimicrobial prescription data from diverse sources within electronic records has improved surveil­ lance efficiency and scalability. Challenges with reliability, interfacility standardization, and the need for considerable human input to refine and ensure comprehensive data flow and adjudication of infection definitions continue to leave considerable room for improvement in electronic surveillance. Leveraging natural language processing mod­ els for health care–associated infection surveillance should further enhance existing systems. Surveillance data interpretation can be complex as hospitals apply health care–associated infection metrics for diverse purposes, including monitoring disease trends, detecting out­ breaks, driving quality improvement, and meeting mandated reporting requirements to state and federal agencies, and as a vital part of valuebased care measures. Because of its role as the nation’s mandatory reporting mechanism for health care–associated infections, the CDC’s National Healthcare Safety Network benchmarks and tracks health care–associated infec­ tions and provides the measures and analytical tools that allow com­ parison across facilities. Infection rates are expressed as standardized infection ratios, calculated by dividing the number of observed infec­ tions by the number of predicted infections. EPIDEMIOLOGIC BASIS AND GENERAL MEASURES FOR PREVENTION AND CONTROL Patients in health care facilities are vulnerable to transmission of patho­ gens from other patients, visitors, staff, or the inanimate health care environment. Health care personnel may convey multidrug-resistant organisms between patients and the environment on their hands or on shared patient care equipment (e.g., stethoscopes or portable imag­ ing machines). Spread can also occur from contaminated plumbing, person to person via a respiratory route (e.g., influenza or group A Streptococcus from health care personnel), or via contamination of food, water, or medications. Infections that arise within the first 48–72 h after admission are generally considered community acquired or, if the patient was trans­ ferred from another facility, attributed to the transferring institution. Those occurring henceforth are considered health care associated, or nosocomial. Patient factors that increase vulnerability to nosocomial infection include presence of an invasive device, immune deficiency (congenital or acquired), renal insufficiency, diabetes, and other major comorbidities. When patients’ own microbiota have been altered by exposure to antibiotics or other medications, they become more susceptible to colo­ nization with multidrug-resistant organisms. Colonized patients serve as unintentional reservoirs for transmission of resistant organisms to other patients or to the hospital environment, including shared equip­ ment. Patients can develop nosocomial infections from these transmit­ ted pathogens (cross-transmission) or from their own microbiota. Hand hygiene and environmental cleaning are essential interventions to interrupt transmission. Hand hygiene can be performed either with soap and water or alcohol-based hand gel, with special requirements for care of patients who have diarrhea (see “Health Care–Associated Diarrhea,” below). Environmental cleaning and disinfection are focused on frequently touched surfaces, such as doorknobs, bed rails, and objects in the bathroom, and are similarly adjusted for disinfectant-resistant pathogens when necessary. ■ ■COLONIZATION Nosocomial bacteria or yeast that are transmitted to a patient may be unintentionally ingested, leading to colonization with the organism, a carrier state. Patients whose microbiota have been altered by antibiot­ ics are more susceptible to colonization with nosocomial organisms. Colonization pressure, the proportion of patients in a ward who are colonized, is a risk factor for spread of resistant organisms. Patients who are immunocompromised or whose immune barriers have been breached by surgery or invasive devices, are at increased risk of infec­ tion from the nosocomial organisms with which they are colonized.

■ ■OUTBREAK INVESTIGATION AND RESPONSE Transmission in a health care facility may be detected via careful surveillance or by notification from a clinician or laboratory. Investi­ gation typically involves molecular typing of bacterial or fungal (less commonly viral) isolates to ascertain whether they are, in fact, closely related and represent an outbreak. The gold standard for these com­ parisons is whole genome sequencing, which has high resolution for determining clonality and, in the case of bacteria, conveys valuable information about plasmid-carried resistance genes.

Once transmission has been established, outbreak investigation involves a series of steps including drafting a case definition, finding cases, reviewing medical records, performing descriptive epidemiol­ ogy, and developing a hypothesis regarding the source of the outbreak. Control measures are implemented while surveillance (including environmental cultures in some cases) is conducted. Communicating with patients, staff, facility leaders, and public health is integral to the outbreak response. NOSOCOMIAL AND DEVICE-RELATED INFECTIONS Invasive procedures and indwelling devices deliver supportive care and can save lives, but they provide potential portals of entry for pathogens. Infections in patients who undergo surgery or have indwelling devices are important targets for prevention in health care facilities. Postsurgical infections occur more frequently when surgery is per­ formed in the setting of emergencies, tissue damage from trauma or radiation, uncontrolled infection elsewhere in the body, malnutrition, chemotherapy, and other conditions that impair wound healing. CHAPTER 147 Routine perioperative checklists enumerate activities that reduce the risk of surgical site infections (Table 147-1). After years of decline due to prevention efforts, rates of devicerelated and other nosocomial infections rose during the COVID-19 pandemic. These infections are a substantial cause of morbidity and mortality for hospitalized patients and are often due to antimicrobialresistant infections. Infections Acquired in Health Care Facilities ■ ■CATHETER-ASSOCIATED URINARY

TRACT INFECTIONS Catheter-associated urinary tract infections occur more frequently in patients with critical illness, older age, and female sex. Sterile and atraumatic insertion of catheters and preservation of a closed drainage system help prevent contamination. Infections are preventable largely by concerted efforts to avoid the placement of indwelling urinary cath­ eters and by reevaluating daily the duration of their use. The presence of asymptomatic bacteriuria, or bacteria without symptoms of urinary tract infection, often leads to improper antimi­ crobial treatment in patients with indwelling urinary catheters. Apart from pregnant patients, most others need not undergo urine culture or treatment for asymptomatic bacteriuria. Diagnostic stewardship efforts include guidelines to reduce collection of urinalysis and culture from patients in whom these studies are not indicated to avoid subsequent inappropriate treatment of asymptomatic bacteriuria. In the elderly, this often occurs in the setting of nonspecific symptoms, such as confusion, that are not related to asymptomatic bacteriuria. Improper treatment of asymptomatic bacteriuria is often followed by improper testing for “clearance” of bacteriuria, which results in a vicious cycle of escalating antimicrobial use and resistance. ■ ■HEALTH CARE–ASSOCIATED PNEUMONIA Hospital-acquired pneumonia is often due to aspiration of oral or gastric contents or, less commonly, hematogenous transmission from a remote source. Because of changes in patients’ microbiota in medical facilities, bacterial causes of nosocomial pneumonia are frequently anti­ microbial resistant. The most likely pathogens involved in nosocomial pneumonia and its treatment are discussed in Chaps. 131 and 147. Ventilator-associated pneumonia is a subset of nosocomial pneumo­ nia that occurs in up to 10% of ventilated patients. Mechanical ventila­ tion can be complicated by a variety of infectious and noninfectious problems that are encompassed under the term ventilator-associated

TABLE 147-1  Evidence-Based Bundled Measures to Reduce the Risk of Select Device- and Procedure-Related Infections Prevention of Surgical Site Infections Treat active infections prior to surgery Administer prophylactic antibiotics within 1 h before surgery and discontinue immediately after surgery Wear surgical attire that covers hair, mouth, and nose Perform hand hygiene with an antiseptic agent Perform antisepsis of the surgical site with chlorhexidine/alcohol solutions whenever possible If hair must be removed, clip rather than shave (to avoid skin microtrauma) Implement operating room asepsis, minimizing movement into and out of

the room Maintain operating room air at positive pressure, with moderate humidity

(20–60%) and at least 20 air changes per hour Prevention of Ventilator-Associated Pneumonia Attempt to avoid intubation using high-flow nasal oxygen or noninvasive

positive-pressure ventilation Elevate head of bed to 30–45 degrees to reduce risk from gastric contents Brush teeth daily Use aseptic care of all respiratory care equipment Avoid use of tap water for rinsing any respiratory care equipment Follow strategies to promote earlier extubation: • Minimize sedation • Provide early exercise and mobilization • Provide early enteral feeding • Assess readiness for extubation on a daily basis PART 5 Infectious Diseases Prevention of Catheter-Associated Urinary Tract Infection Place indwelling catheters only when strictly necessary, e.g., to relieve obstruction, and not for convenience Use aseptic equipment and technique for catheter insertion and urinary tract instrumentation Minimize urinary tract instrumentation Minimize manipulation of or entry into urinary catheter systems, and avoid catheter irrigation Reevaluate daily the need for continued use of an indwelling urinary catheter in each patient Use alternative methods to avoid indwelling catheters, e.g., bladder scans, condom catheters, intermittent catheterization Prevention of Catheter-Associated Central Line Infections Optimize nurse-to-patient ratio Consider alternatives to central line (e.g., peripheral IV, midline catheter) Catheter insertion • Use a checklist to ensure adherence to insertion bundle • Use hand hygiene • Use maximum sterile barrier precautions and aseptic technique • Use chlorhexidine-alcohol antisepsis to prepare the site • Use ultrasound guidance • Use an all-inclusive catheter insertion kit • The subclavian vein is least prone and the femoral vein most prone to infection in the intensive care unit (ICU) setting Catheter maintenance • Administer daily chlorhexidine gluconate baths to ICU patients with central lines • Scrub the hub with alcohol • Cleanse the catheter site with chlorhexidine-based antiseptic with every dressing change, at a minimum every 7 days (earlier if site is soiled or dressing disrupted) • Use disinfectant caps • Apply chlorhexidine-impregnated dressings • Reevaluate daily the need for continued use of each central line in each patient

events. Ventilator-associated pneumonia can prolong the duration of ventilation and intensive care unit (ICU) stay or even prove fatal. Like other device-related infections, ventilator-associated pneumonia is preventable by limiting use of the invasive device (Table 147-1). ■ ■CATHETER-ASSOCIATED BLOODSTREAM INFECTIONS Central venous catheters represent another major target for infection prevention. Although they are often necessary for patient care, central lines are foreign objects that lie in direct contact with the bloodstream. Without meticulous care, each health care personnel interaction with the catheter serves as a potential contamination event. Catheter-associated bloodstream infections triple a patient’s risk of in-hospital death. Similar to other device-related infections, the rate of catheter-related bloodstream infections had declined in the years leading up to 2020, when infection control gains were lost with the onset of the pandemic. Preventing catheter-associated bloodstream infections begins with aseptic insertion technique using a bundle, or checklist, of evidencebased steps (Table 147-1). Catheter maintenance practices are impor­ tant for ongoing prevention of infection and include meticulous, aseptic technique in handling the catheter. Daily full-body cleansing with 2% chlorhexidine gluconate is effective for preventing blood­ stream infections, particularly central line infections, in the ICU and are standard of care. Although studies outside the ICU show mixed results, some hospitals utilize the baths for all patients with central lines because of the treatment’s low side effect profile and the potential impact of the intervention. TRANSMISSION-BASED PRECAUTIONS Standard precautions are the basic set of practices that health care personnel should follow to minimize exposure to potentially infec­ tious material and prevent pathogen transmission while caring for all patients in all health care settings. The key elements of standard precautions are applied based on the situational risk and include hand hygiene; respiratory etiquette; personal protective equipment use depending on the potential for exposure to blood, body fluid, or infec­ tious material; and safe injection practices. Additional pathogen-specific prevention measures are called trans­ mission-based precautions. When patients harbor a suspected or confirmed communicable disease, transmission-based precautions are used to prevent spread in the health care setting. The fundamental modes of pathogen transmission inform the recommended prevention methods categorized by the World Health Organization (WHO) and CDC into contact, droplet, and airborne precautions. Contact precautions utilize hand hygiene and barrier methods (gown and gloves) to prevent the spread of ubiquitous pathogens in a colonized or infected person’s immediate environment, for example, C. difficile and multidrug-resistant organisms. Institutions differ in use of contact precautions for ubiquitous resistant organisms such as methicillin-resistant Staphylococcus aureus. Droplet precautions are implemented when a patient has respiratory symptoms or a confirmed respiratory infection with a pathogen that is transmitted from person to person. Protective equipment includes a gown, gloves, eye protection, and a mask or respirator, given recent evidence that respiratory viruses are spread by both droplets (respira­ tory particles ≥ 100 microns in size) and aerosols (sprays of particles <100 microns). The principal mode of respiratory pathogen spread is by inhalation of respiratory particles emitted by an infected person during sneezing, coughing, talking, and breathing. Host factors such as infection stage, symptoms, immune status, and environmental conditions, including ventilation and humidity, influence pathogen transmissibility. Respiratory droplets remain suspended in the air only briefly and travel short distances. Airborne transmission occurs via tiny particles 100 that can travel larger distances and remain suspended in the air for an extended dura­ tion. Measles, tuberculosis, varicella, and COVID-19 are infectious agents that pose a major risk of airborne transmission. Airborne pre­ cautions require respirators and placement of the infected individual in a negative-pressure airborne isolation room with a minimum air flow

rate of 12 air changes per hour with direct exhaust outside the building or recirculation through a high-efficiency particulate air (HEPA) filter and respirator use. Some conditions may merit combinations of these precautions (e.g., patients with varicella and SARS-CoV-2 are placed in contact and air­ borne precautions). EMERGING INFECTIOUS DISEASES ■ ■EPIDEMICS, EMERGING INFECTIOUS DISEASES, AND AGENTS OF BIOTERRORISM Preparedness for newly evolved and reemerging human pathogens is critical to infection prevention and control activities. The 21st century has witnessed the emergence of several infectious threats and two major pandemics: 2009 H1N1 swine influenza and 2019 SARS-CoV-2, with their vast worldwide impact and, in the case of SARS-CoV-2, millions of deaths. The changing of global ecosystems from deforesta­ tion, climate change, and extensive urbanization with a rising world population has increased the proximity between humans and wildlife. The confluence of these factors can generate conditions that favor mutational viral evolution and, ultimately, cross-species spillover of otherwise geographically confined zoonotic illnesses, as was seen with the outbreaks of Ebola in 2014, Zika in 2016, SARS-CoV-2, and the 2022–2023 international monkeypox (mpox) epidemic. Another emerging challenge relates to socio-behavioral perceptions and beliefs linked to vaccines; the perpetuation of misinformation through social media around childhood and adult immunizations has amplified vaccine hesitancy, threatening the maintenance of safe vacci­ nation rates and herd immunity. The most urgent threat is from highly contagious illnesses such as measles, mumps, polio, and varicella. The current global rise in measles cases has disproportionately affected unvaccinated or partially vaccinated individuals. Therefore, addressing vaccine access, misinformation, and declining immunization rates is critical to avert outbreaks of vaccine-preventable illnesses with a high public health impact. Finally, health care personnel awareness of intentional human threats from potential agents of bioterrorism such as Bacillus anthracis and variola virus is essential. Natural smallpox is no longer a hazard; routine vaccination against smallpox ended in the United States in 1971. However, there is a possibility of laboratory-maintained variola outside of the WHO-approved repositories. In the event of an acciden­ tal or intentional release, vaccination is the primary strategy to stop smallpox spread. Two currently licensed vaccines for smallpox preven­ tion are the replication-competent vaccinia virus vaccine, ACAM2000, and the live, nonreplicating modified vaccinia Ankara vaccine. Both vaccines are also recommended for prevention of mpox in high-risk groups. B. anthracis is a category A pathogen with a high case fatality rate and potential for mass casualty if aerosolized. After the U.S. inha­ lational anthrax cases in 2001 from mailed letters containing anthrax spores that resulted in five deaths, protocols for a mass disaster related to B. anthracis exposure have been a biodefense preparedness priority. ■ ■VIRAL RESPIRATORY INFECTIONS WITH PANDEMIC POTENTIAL Safety protocols for patients and health care personnel from new and reemerging contagious respiratory illnesses including novel influenza A viruses, Middle East respiratory syndrome coronavirus (MERSCoV), and other novel coronaviruses are vital. Adopting an integrated all-hazard screening approach to identify and implement infection control measures, initiate diagnostic testing, and safely deliver care is critical. Lessons from the COVID-19 pandemic underscore the importance of maintaining par levels of personal protective equip­ ment, testing supplies, and essential therapeutics. Regular training of frontline health care personnel on appropriate donning and doffing of personal protective equipment ensures readiness to handle patients with contagious illnesses safely. Testing for novel pathogens may only be available through public health laboratories; however, developing the capability for laboratory-developed tests and existing infrastruc­ tures to rapidly adopt testing protocols ensures timely diagnosis,

contact investigations, and postexposure management, which is criti­ cal for effective containment. Health care facilities should leverage a structured information-sharing approach through hospital incident command system activation. From a public health standpoint, robust surveillance mechanisms, clear and concise guidelines for clinicians and the public, and alignment among federal agencies, state and local public health, and commercial laboratories are critical to ensure testing capacity and vaccine and therapeutics access.

■ ■HIGHLY PATHOGENIC AVIAN INFLUENZA Among the potential pandemic pathogens, avian influenza A (H5 and H7 strains) has an exceptionally high impact due to its constant viral evolution and potential for adaptation to transmit effectively in humans. The most immediate pandemic threat is posed by highly pathogenic H5N1 avian influenza, with ~900 known human infections worldwide since 2003 and a case fatality rate of ~50%. Sustained per­ son-to-person transmission of this group of viruses has not occurred thus far. H5N1 began circulating in North American wild birds and poultry in late 2021, with subsequent detections in mammals, includ­ ing U.S. dairy cattle herds and barn cats, in multiple U.S. states and numerous cases of animal-to-human transmission. The recommended prevention measures against novel influenza in hospitals are similar to those against other high-consequence patho­ gens, including contact and airborne precautions and eye protection. Close contacts should receive antiviral prophylaxis with oseltamivir. Regarding diagnostics, influenza targets on commercial multiplex reverse transcriptase polymerase chain reaction (PCR) panels are insufficient for strain identification and additional, dedicated testing in public health laboratories from nasopharyngeal swabs, washes, and conjunctival swabs is required to establish the diagnosis in the appro­ priate clinical and epidemiologic context. CHAPTER 147 ■ ■MPOX Previously a geographically constrained virus endemic to West Africa mpox Clade II virus caused s global outbreak in 2022. On its heels, in central Africa, cases of Clade I, endemic to that region, soared. The virus is zoonotic, and spreads from person to person through close contact of mucosal surfaces or nonintact skin with open lesions or via contaminated inanimate objects such as bed linens. The role of respira­ tory transmission without close physical contact is less well established. Despite initial concerns, health care–associated transmission of mpox in nonendemic countries is rare, with multiple reported occupational transmissions after direct inoculation from needlestick injuries. Post­ exposure prophylaxis with the JYNNEOS or ACAM2000 vaccine series is recommended for high-risk exposures. Infections Acquired in Health Care Facilities ■ ■VIRAL HEMORRHAGIC FEVER PREPAREDNESS During the 2014–2015 West African Ebola epidemic, symptomatic returning travelers presented to U.S. hospitals, resulting in occupa­ tional Ebola infections in two nurses at one facility. These and other events led to formation of a national network of Regional Emerging Special Pathogen Treatment Centers at large hospitals that have built specially engineered containment wards and trained staff to care for patients with suspected or confirmed Ebola, Marburg, or other viral hemorrhagic fever diseases. A series of additional hospitals are pre­ pared to receive and provide temporary care for such patients prior to transferring them to these treatment centers. Finally, all medical facili­ ties and emergency services should have plans for managing patients with highly contagious diseases who may present unexpectedly. HEALTH CARE–ASSOCIATED DIARRHEA ■ ■CLOSTRIDIOIDES DIFFICILE INFECTION Diarrhea that begins in health care facilities or is attributable to recent care in a health care facility is considered health care associated.

C. difficile infection (Chap. 139) is not only the most frequent cause but also the most frequent hospital-acquired infection in the United States. C. difficile colitis has symptoms that can range in severity from mild diarrhea to life-threatening toxic megacolon. Receipt of antibiotics is the most common cause of C. difficile infection, making antimicrobial

stewardship the first-line measure for its prevention. Restricting use of antibiotics that are highly associated with subsequent C. difficile infec­ tion, such as quinolones and clindamycin, has been shown to reduce the incidence of the disease. Longitudinal genomic surveillance shows that patients who enter the hospital already colonized with C. difficile are by far the most likely to develop the infection during their hospital stay.

Patients who have suspected or confirmed C. difficile infection are placed in contact isolation to contain spores that infected patients shed in high numbers and that heavily contaminate their skin and envi­ ronment. These spores are hardy, survive for a prolonged time in the health care environment, and can be transmitted on hands of health care personnel, on surfaces, or on shared patient care equipment. Because alcohol-based hand gel and standard hospital cleaners do not kill C. difficile spores, handwashing with soap and water is preferred to mechanically remove them, and sporicidal disinfectants such as bleach are used to clean the rooms of infected patients. ■ ■NOROVIRUS Norovirus is another important cause of health care–associated diar­ rhea that can be transmitted among patients and staff and cause outbreak in facilities. A nonenveloped virus that is poorly inactivated by alcohol-based hand gel and standard hospital cleaners, norovirus is also addressed with handwashing with soap and water and bleach cleaning. Another potential cause of health care–associated diarrhea is a foodborne outbreak, which can start with symptomatic food handlers or point-source food contamination that originates outside the facility. TUBERCULOSIS Tuberculosis (Chap. 183) requires special infection prevention and occupational health measures. Early identification, isolation, and test­ ing of patients who may have active tuberculosis are critical steps in tuberculosis control in health care facilities. Patients with suspected or confirmed tuberculosis should be managed with airborne precautions (see “Transmission-Based Precautions”). Health care personnel man­ aging patients with infectious tuberculosis use fit-tested particulate respirators, known in the United States and Canada as N95 respirators because they filter 95% of airborne particles, or powered air-purifying respirators that draw air through a HEPA filter into a hood. PART 5 Infectious Diseases Health care personnel should undergo preemployment screening for tuberculosis, preferably with an interferon-γ release assay rather than a tuberculin skin test due to its higher positive predictive value. Staff with positive tests should be evaluated further with symptom screening and chest imaging to assess for active or latent tuberculosis. Staff who have latent tuberculosis should be encouraged to undergo treatment to reduce the risk of reactivation and transmission to patients. In hospitals with high caseloads and local prevalence of tuberculosis, follow-up testing may be performed routinely among staff in certain disciplines. In most facilities, follow-up testing is event driven, such as following exposure to a patient with active tuberculosis. FUNGAL INFECTIONS ■ ■MOLD INFECTIONS Some immunocompromised hospitalized patients are highly suscepti­ ble to nosocomial mold infections, which are usually acquired through inhalation. Patients who will experience prolonged neutropenia dur­ ing treatment of leukemia and stem cell transplant recipients require protective isolation rooms that are specially engineered with positivepressure, HEPA-filtered airflow and sealed seams to repel mold spores, which, at 2–4 microns in size, are ubiquitous in dust and air currents. Vulnerable patients should wear masks when leaving their rooms to go to areas of the hospital that lack such measures. Exposure to hospital construction is a well-described risk factor for invasive fungal infec­ tions among immunosuppressed patients; thus, all construction sites within the hospital must employ negative airflow, HEPA filtration, sealed walls, and sticky doormats to minimize leakage of particles con­ taining mold spores into patient care areas.

■ ■CANDIDA INFECTIONS Candida infections represent the vast majority of health care–associated fungal infections. Many of these arise from endogenous sources in patients who are immunosuppressed, have undergone surgery, or have invasive devices in place. Although Candida albicans represent a majority of Candida infections, antifungal prophylaxis may select out non-albicans species such as Candida glabrata. Candida auris  Some Candida infections are attributed to exogenous transmission. Candida auris was first isolated in Japan in 2009 and subsequently emerged as a global nosocomial pathogen. Since the first U.S. case was identified in 2015, the incidence has increased steadily, with a sharp acceleration during the COVID-19 pandemic. C. auris has become an increasing cause of candidiasis in hospitals and in long-term care facilities where patients are mechanically ventilated. A multidrug-resistant species that is adapted to the health care environ­ ment, C. auris survives for extended periods on surfaces, and rapid recontamination from colonized persons makes sustained disinfection challenging. Shared medical equipment such as glucometers, ultrasound machines, blood pressure cuffs, and axillary temperature probes are potential sources of transmission between patients, and thorough dis­ infection after each use is vital to prevent the spread of C. auris. Hand hygiene and contact precautions should be followed strictly when caring for a colonized or infected patient. The organism is resistant to some hospital disinfectant cleaners and requires use of select disinfec­ tants. C. auris is of substantial public health concern, such that efforts to contain its spread require collaboration among infection control, public health, and laboratory experts on a facility, local, and regional basis. Patients transferred from long-term care facilities are often screened for C. auris by PCR or culture and isolated pending the results of screening (Fig. 147-1). THE HEALTH CARE BUILT ENVIRONMENT Patients, staff, and visitors continually shed microorganisms into the built environment of a health care facility (surfaces and plumbing). Organisms are also introduced via the external environment, includ­ ing potable water. Patients who are immunologically vulnerable may acquire pathogens from the built environment. Health care facilities must reduce their risk through adherence to current guidelines and through surveillance for such infections. ■ ■ENVIRONMENTAL CLEANING Floors and frequently touched surfaces in health care facilities should be cleaned often with disinfectant cleaners in order to reduce the burden of communicable bacteria, viruses, and fungi. Patient care equipment (e.g., stethoscopes, portable x-ray cartridges) should be disinfected between patients to avoid becoming point sources for transmission of multidrug-resistant pathogens. Sporicidal cleaners and ultraviolet C light are often used as adjunctive methods for inactivat­ ing C. difficile spores and other tenacious pathogens. Clinical trial data suggest that standard cleaning plus adjunctive ultraviolet C may reduce nosocomial transmission of pathogens over cleaning alone. ■ ■WATER SAFETY Waterborne infections arising from wastewater (drains and toilets) as well as potable water (sinks, showers, and fountains) pose a potential risk to vulnerable patients in health care facilities. Because many of these infections are preventable, hospitals are required to have a water management plan to anticipate and reduce risk to patients. Tap water is not sterile, and contamination of potable hospital water and plumbing occurs most frequently when waterborne organisms (e.g., Legionella pneumophila, nontuberculous mycobacteria, and many others) colonize the biofilm within the pipes or cooling towers of a health care facility. Low disinfectant levels, warm temperature ranges, and stagnation of water promote organism growth, which can lead to transmission to patients via aerosols or droplets. Immunosuppressed patients are the most susceptible to waterborne infections; pneumonia in this patient population should be thoroughly investigated to include testing for waterborne infection.

Facilities work together to protect patients. The Problem Many patients transfer back and forth for treatment within different regional facilities. If patients are colonized or infected with multidrug-resistant or other infectious pathogens(such as C. difficile and C. auris), those organisms are introduced to the different facilities. After introduction, other patients can acquire organisms, silently spreading them throughout the region’s healthcare network. A collaborative approach and timely communication between public health agencies and healthcare facilities are essential to prevent and mitigate the regional spread of contagious pathogens. Facilities convey infection control data promptly to public health FIGURE 147-1  Regional spread and control of antimicrobial resistance. (Modified from https://archive.cdc.gov/#/details?url=https://www.cdc.gov/vitalsigns/stop-spread/ infographic.html.) In the case of nosocomial Legionnaires disease, detection of a single case should prompt a thorough investigation. In facilities with no his­ tory of an outbreak, routine hospital water testing for L. pneumophila is not strictly necessary if robust clinical surveillance is in place. Insti­ tutions that have had nosocomial cases should conduct regular water testing for the organism as part of their water management plan. Wastewater drains in health care facility sinks, tubs, and showers, as well as toilets, serve as silent reservoirs for multidrug-resistant gramnegative organisms and can cause protracted outbreaks. These bacteria reach patients via transmission in droplets or aerosols of contaminated water from splashback, plumbing fixtures, or toilet flushing. Maneu­ vers to reduce risk include installing lids on toilets and angling sink faucets so water does not land directly on the drain. Contamination of medications and medical devices can also lead to nosocomial outbreaks. In the past decade, outbreaks of nontuberculous mycobacterial infections among cardiac surgery patients have been linked to operative heater-cooler units that aerosolized the organisms from contaminated water tanks. ANTIBIOTIC-RESISTANT BACTERIA Implementing measures to stop the spread of multidrug-resistant organisms is a key priority for public health agencies and health care epidemiologists. Globally, 1.27 million deaths were attributed to antibiotic-resistant infections in 2019, with the highest rates in subSaharan Africa. According to recent estimates, infections due to meth­ icillin-resistant S. aureus (MRSA) and extended-spectrum β-lactamase

Facilities must alert receiving locations about infection control issues prior to transferring patients Public health authorities implement collective actions to reduce interfacility transmission CHAPTER 147 Facilities convey infection control data promptly to public health Infections Acquired in Health Care Facilities (ESBL) infections are the most common multidrug-resistant organism infections in U.S. hospitals. While improved infection prevention measures have led to a substantial reduction in MRSA, vancomycinresistant Enterococcus (VRE), carbapenem-resistant Acinetobacter, and multidrug-resistant Pseudomonas infections, the prevalence of carbapenem-resistant Enterobacterales and ESBL-producing Entero­ bacterales has not declined. Infections due to multidrug-resistant organisms, which frequently colonize the human gastrointestinal and respiratory tracts, are associated with higher morbidity and mortality and other adverse clinical consequences such as extended hospital stays, transmission risk, and added health care costs. Without meticu­ lous environmental disinfection, many of these organisms can establish enduring reservoirs in the hospital environment, effectively transmit­ ting between patients and posing the risk of horizontal gene transfer among co-colonizing strains. Coordination of care during interfacility transfer of known patients with MDRO colonization is paramount to ensure timely implementation of infection prevention and control measures (Fig. 147-1). Regardless of pathogen, frequent health care–related exposure, extended hospital stays, residence in long-term care facilities, multiple comorbid conditions, short- or long-term indwelling devices, and anti­ biotic exposure are among the key risk factors for multidrug-resistant organism acquisition. Additionally, community determinants can contribute substantially to spread of certain organisms. For example, people who use drugs, prison inmates, and those without stable hous­ ing conditions are at a greater risk of MRSA infections.

■ ■METHICILLIN-RESISTANT STAPHYLOCOCCUS AUREUS Among hospitalized patients, up to a third of MRSA-colonized individ­ uals develop MRSA infection, and the risk in the pediatric ICU can be as high as 47%. MRSA is the most common health care–associated infec­ tion in neonatal ICUs. Transmission within facilities occurs through indirect contact with contaminated environments and sometimes from contaminated or colonized health care personnel. Screening, contact precautions, and education can reduce MRSA transmission, but most U.S. hospitals selectively deploy MRSA screening only in high-risk settings, including ICUs; dialysis, burn, and transplant units; and post–acute care facilities. Screening is also the fundamental approach to combat MRSA outbreaks.

Among the preventative practices assessed in ICUs, universal gowns and gloves and decolonization have reduced MRSA acquisition and infections. Universal decolonization is more effective than screening and contact precautions, with or without concurrent targeted decoloni­ zation with chlorhexidine bathing and 5-day nasal application of mupi­ rocin. The advantages of decolonizing MRSA carriers extend beyond the hospital stay, and post–hospital discharge measures can reduce the short-term risk of MRSA infections in carriers by 30%. Still, this approach may not achieve sustained benefits over longer time periods. Preventing postoperative and device-related MRSA infections is an essential surgical quality and safety goal for health care settings. A meta-analysis of 39 studies evaluated nasal decolonization and glycopeptide antibacterial prophylaxis before cardiac and orthopedic surgery, demonstrating a reduced risk of S. aureus infections with universal nasal decolonization and all gram-positive surgical site infec­ tions with a combined approach of decolonization and targeted anti­ bacterial prophylaxis. Similarly, universal decolonization methods may be effective in non-ICU settings where there is high utilization of inva­ sive devices to lower the risk of associated infections, including MRSA. PART 5 Infectious Diseases ■ ■VANCOMYCIN-RESISTANT ENTEROCOCCUS (VRE) Soon after VRE emerged in the U.S. in 1987, identifying carriers by screening for gastrointestinal carriage to institute contact precautions proved to be an influential early approach. This commonly applied method successfully controlled the regional spread of this pathogen while its prevalence in health care settings was still low. Initial ani­ mal studies and clinical observations established the critical role of antibiotic exposure to vancomycin, third-generation cephalosporins, and anti-anaerobic agents as risk drivers of VRE acquisition and over­ growth. Subsequently, microbiome analysis elucidated that a higher VRE colonization burden often precedes mucosal translocation in susceptible hosts, also influencing the number of bacteria shed in the environment that can facilitate person-to-person spread. Immuno­ compromised patients with hematologic malignancy, as well as stem cell and liver transplant recipients, are especially vulnerable to VRE colonization and infection. Over the years, VRE has become ubiquitous in many U.S. health care settings. Active surveillance and contact pre­ cautions offer minimal benefit in hyperendemic settings. However, the judicious use of antibiotics through robust stewardship measures and bundles incorporating chlorhexidine baths remain effective in reduc­ ing the impact of VRE by rendering colonized patients less infectious and decreasing the risk of invasive infection. ■ ■MULTIDRUG-RESISTANT GRAM-NEGATIVE BACTERIA Multidrug-resistant gram-negative bacteria are increasingly common in health care settings. Chromosomally encoded resistance mecha­ nisms and plasmid-carried resistance genes make them particularly challenging therapeutic targets. Multidrug resistance is defined as nonsusceptibility to at least one agent from three distinct antibacterial drug classes. Difficult-to-treat resistance is defined as resistance to all first-line antibacterial agents. Extensively drug-resistant organisms exhibit nonsusceptibility to two or fewer antimicrobials in all catego­ ries. Finally, pan-drug resistance is defined by the noneffectiveness of all antimicrobial agents. The rising incidence of drug-resistant gramnegative bacteria has been noted across all types of health care settings,

carrying decreased survival compared with susceptible infections from the same species. Carbapenem-resistant Enterobacterales and Acineto­ bacter baumanii, multidrug-resistant Pseudomonas aeruginosa, and ESBL-producing Escherichia coli and Klebsiella pneumoniae are among the top threats. Extended hospital stays, long-term care facility exposure, antimicro­ bials, medical devices, mechanical ventilation, and impaired immunity are frequent risk factors for acquisition of multidrug-resistant gramnegative bacteria. Outbreaks mainly occur in long-term care facilities, and hospital-based clusters tend to occur in intensive care, hematologyoncology, and liver transplant units. Inadequate disinfection of duo­ denoscopes due to their complex design has made them vulnerable to sterilization lapses, resulting in numerous outbreaks of multidrugresistant gram-negative bacteria. Aggressive control measures are among the cornerstones of pre­ vention. Given an association between antimicrobial overuse and the emergence of multidrug-resistant gram-negative organisms, antimi­ crobial stewardship is a pivotal component in the strategies to prevent their emergence and spread. Targeted screening and contact precau­ tions for rapid identification and isolation of gastrointestinal carriers have proven essential for outbreak management. Universal gown and glove use failed to reduce acquisition. Local epidemiology and inter­ national travel to hyper endemic areas, especially for medical care, are other important risk factors to consider when deciding on targeted screening practices. Finally, there are no proven decolonization strate­ gies for multidrug-resistant gram-negative bacteria. ■ ■DIAGNOSTIC STEWARDSHIP Diagnostic advances, including molecular technologies and highthroughput automated systems, have vastly enhanced clinical laboratory efficiencies and reduced the time to actionable information. However, rapid, convenient, and unrestricted testing can become problematic when used excessively for medically unnecessary investigations. From a health care–associated infection standpoint, inappropriate testing for C. difficile infection and excessive pursuit of urine cultures are pervasive problems in the current health care environment. Estimates show that as much as 40–60% of testing in hospitalized patients may not be medi­ cally indicated. Consequently, overtesting for these two common health care–associated infections leads to antibiotic overuse, patient distress, TABLE 147-2  Diagnostic Stewardship Examples   KEY FOCUS AREAS Urine culture   • Educate clinicians on asymptomatic bacteriuria and appropriate indications to suspect urinary tract infection. • Require documentation of patient symptoms at the time of urine culture order placement. • Implement alerts to discourage testing when patient is asymptomatic, with exceptions (pregnancy, urologic procedures, etc.). • Consider algorithms that incorporate pyuria threshold

(>10 white blood cells per high-power field) to proceed to culture in nonneutropenic patients. • Educate clinicians on appropriate methods for urine collection, storage, and transport for voided and catheterized urine. Avoid collection from the bag or a catheter that has been in place for an extended time. Clostridioides difficile testing   • Set clinical criteria for testing: three or more unformed stools in 24 h in the absence of alternate etiology for the diarrhea. • Implement alerts that discourage an ordering clinician from testing when a patient is on laxatives. • Empower laboratory to reject specimens based on Bristol stool scale. • Apply auto-cancellation of test if sample is not received within 24 h or repeated within 7 days after a negative test or 14 days after a positive test. • Educate clinicians on the lack of utility of a test of cure. • Implement a two-step algorithm for testing including a toxin detection method.

TABLE 147-3  Health Care Personnel Immunizations VACCINE VACCINE TYPE ELIGIBILITY RECOMMENDATIONS COVID-19 Non-live All HCP Per latest CDC recommendations Influenza Non-live All HCP Once annually Hepatitis B Non-live HCP without serologic evidence of immunity or past infection MMR Live HCP without serologic evidence of immunity or past infection Varicella Live HCP without serologic evidence of immunity, clinicianverified history, or serologic evidence of past infection Tetanus-diphtheriapertussis (TdaP) Non-live HCP without immunization in past 10 years Pregnant HCP Meningococcal Non-live Laboratory workers with potential exposure Men ACWY (booster every 5 years) and MenB (booster at 1 year and every 2–3 years thereafter) Abbreviations: CDC, Centers for Disease Control and Prevention; HCP, healthcare personnel; MMR, measles, mumps, rubella. spurious inflation of publicly reported infection rates, and excess health care costs. Clinicians should use interventions implemented through the computerized clinical decision support systems. Lab measures such as reflex or multistep testing algorithms can address the potential harms of overtesting. Interventions based on the electronic health record yield better results than human interventions. Key strategies to reduce inap­ propriate testing are included in Table 147-2. OCCUPATIONAL HEALTH ■ ■VACCINATION OF HEALTH CARE PERSONNEL Health care personnel are at higher risk for acquiring certain vaccinepreventable illnesses and for transmitting infection to patients. The goals of vaccination are to provide personal safety, reduce the risk of occupational acquisition or transmission to preserve the workforce during periods of high community transmission, and protect vulner­ able patients. Pathogens that pose a substantial risk of occupational transmission for which licensed vaccines are available include respira­ tory viruses such as influenza, COVID-19, measles and mumps, vari­ cella, pertussis, hepatitis B, and meningococcus. Vaccine eligibility is assessed at the time of employment, and the schedules for recommended immunizations are shown in Table 147-3. Additional risk-based vaccines may be recommended (e.g., Ebola, vac­ cinia, hepatitis A). Highly contagious illnesses such as measles and var­ icella have caused outbreaks in health care settings, disproportionately affecting underimmunized or unimmunized patients and health care personnel. Even with vaccines that have only moderate effectiveness against infection, higher vaccination uptake is associated with a lower risk of respiratory illness among personnel and a lower risk of nosoco­ mial acquisition of these infections, especially in vulnerable patients. Numerous medical professional societies endorse mandatory health care personnel influenza immunization, and many facilities require it. Health care personnel who provide direct care to patients should be tested for hepatitis B surface antibody 1–2 months after the last dose of the series and reimmunized if antibody levels are <10 mIU/mL. Non­ responders after repeat series are considered susceptible to hepatitis B and should be treated as such in the event of a bloodborne pathogen exposure. ■ ■BLOODBORNE PATHOGEN EXPOSURE Since 1991, the U.S. Occupational Safety and Health Administra­ tion has promulgated a standard to regulate exposure to bloodborne TABLE 147-4  Estimated Risk of Bloodborne Pathogen Transmission from Percutaneous Injury PATHOGEN RISK Hepatitis Ba 6–30% Hepatitis C 1–3% HIVb 0.3% aHighest for hepatitis B e antigen positive. bIf on effective treatment and with an undetectable viral load, the risk is negligible.

Two-dose series (Heplisav-B) or three-dose series (Engerix-B, PreHevbrio, or Recombivax HB) Two doses at a 4-week interval Two doses at a 4-week interval Single dose (booster every 10 years) With every pregnancy pathogens. Advancements in engineering and workplace controls, such as needleless catheter systems, have made the use of medi­ cal devices safer, prevented sharps-related injuries, and reduced bloodborne pathogen transmission events. Despite this, exposure to potentially infectious agents in blood and other bodily fluids through contact with skin, eyes, and other mucous membranes remains a sig­ nificant problem during health care delivery. HIV and hepatitis B and C acquisition after blood exposure are the most common concerning risks; semen, vaginal and rectal fluid, and breast milk can also pose a transmission risk. The risk of bloodborne pathogen transmission after contact with feces, urine, gastric and respiratory secretions, saliva, and sweat that is not contaminated with blood is exceedingly low. Signifi­ cant exposures involve percutaneous injury or skin puncture with a much lower risk from contamination of nonintact skin and mucous membranes. CHAPTER 147 Among the three pathogens, hepatitis B transmits most effectively, with the estimated risks after percutaneous injury from needlesticks or other sharps shown in Table 147-4. Infections Acquired in Health Care Facilities When managing personnel with a bloodborne pathogen exposure, a thorough history and risk evaluation and baseline serologic testing should be conducted. For significant potential hepatitis B exposures in nonimmune persons, hepatitis B immune globulin should ideally be given within 24 h and can effectively prevent transmission when administered up to 7 days from exposure. HIV postexposure prophy­ laxis must be started within 72 after possible exposure, and likely has diminishing efficacy the longer initiation is delayed. Serial testing for HIV, hepatitis B, and hepatitis C should be performed 6 weeks and

3 months after exposure, followed by testing for hepatitis C and HIV at 6 months if the exposed person is found to be immune to hepatitis B or received immune globulin. With widespread vaccination, transmission of hepatitis B from health care personnel to patients is rare. Cases have occurred in the setting of occult hepatitis B with high-grade viremia and invasive pro­ cedures. Guidelines set out recommendations for management of clini­ cians infected with HIV, hepatitis B, and hepatitis C to minimize risks to patients. Unsafe medical device and medication management can spread infection between patients. Outbreaks and transmission events arise from improper disinfection of blood glucose monitors between patient uses, infection control breaches during dialysis, single-dose medication reuse, improper practices with multidose vials, and drug diversion. Such occurrences are entirely preventable with adherence to basic hygiene and patient safety practices. ■ ■FURTHER READING Harris AD et al: Acquisition of antibiotic-resistant gram-negative bacteria in the benefits of universal glove and gown (BUGG) cluster randomized trial. Clin Infect Dis 72:431, 2021. Huang SS et al: Decolonization to reduce postdischarge infection risk among MRSA carriers. N Engl J Med 380:638, 2019. Lyman M et al: Worsening spread of Candida auris in the United States, 2019 to 2021. Ann Intern Med 176:489, 2023.

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148 Infections in Transplant Recipients

Miles-Jay A et al: Longitudinal genomic surveillance of carriage and

transmission of Clostridioides difficile in an intensive care unit. Nat Med 29:2526, 2023. O’Grady NP: Prevention of central line-associated bloodstream infec­ tions. N Engl J Med 389:1121, 2023. Rankin DA et al: Concurrent transmission of multiple carbapenemases in a long-term acute-care hospital. Infect Control Hosp Epidemiol 45:292, 2024. Singh HK et al: Diagnostic stewardship to improve patient outcomes and healthcare-associated infection (HAI) metrics. Infect Control Hosp Epidemiol 45:405, 2024. Jennifer M. Cuellar-Rodriguez,

Juan C. Gea-Banacloche

Infections in Transplant Recipients GENERAL CONCEPTS This chapter discusses infections in recipients of solid organ transplant (SOT), vascular composite allografts (VCAs), and hematopoietic stem cell transplantation (HCT). Despite the notable differences between these procedures, they share the fact that recipients receive immu­ nosuppression (in the case of SOT and VCAs, for life). They become immunocompromised and, consequently, are at increased risk for infection. PART 5 Infectious Diseases Infections in immunocompromised patients may be caused by the common pathogens that affect everybody but also by opportunistic microorganisms that usually do not cause significant illness in healthy people. The physician taking care of transplant recipients should be aware of the risk of unusual pathogens, uncommon presentation of infections, and multiple concomitant infections. In addition, response to treatment may be suboptimal due to the effect of immunosup­ pressive medication and corticosteroids as well as coexisting medical or surgical complications. Professional societies issue and regularly update guidelines for the diagnosis and management of infections in these patients. For SOT, the reader is referred to the guidelines from the American Society of Transplantation Infectious Diseases Community of Practice and, for HCT, to the guidelines published by the American Society for Transplantation and Cellular Therapy and the European Conference on Infections in Leukemia (ECIL). In this textbook, there are chapters dedicated to SOT (Chaps. 271, 309, 325, and 356), HCT (Chap. 119), and cellular therapy (Chap. 483), as well as chapters about every one of the pathogens that will be men­ tioned here. Our goal is to focus on the particulars of infections as they pertain to transplant, as well as to discuss some relevant topics that are not included elsewhere. These include pretransplant evaluation of transplant candidates, assessment of potential donors, infections acquired through the transplant, and prophylaxis of infections. Understanding the similarities between transplants regarding increased risk of infection (e.g., immunosuppressive agents) is impor­ tant, as is understanding some of the basic differences. An obvious and relevant difference is that SOT and VCA transplants include surgical procedures, and the operations are extremely challenging. Most early infections are related to complications of the surgery, and sometimes even late infections may have their origin in the operating room. In contrast, early infections after HCT are related to the neutropenia induced by the conditioning regimen. Another difference is that both SOT and VCA transplants require lifelong immunosuppression to prevent rejection. This means the risk of opportunistic infection never goes away. Conversely, immunosuppression is not typically

administered after autologous HCT (auto-HCT), and it is only given for a few months after allogeneic HCT (allo-HCT). Finally, both SOT and allo-HCT carry a risk of unfavorable immune reactions: rejec­ tion of the transplanted organ and graft-versus-host disease (GHVD), respectively. Both rejection and GVHD are treated by increasing the immunosuppression, with consequent increased risk of infection. INFECTIONS AFTER HEMATOPOIETIC STEM CELL TRANSPLANTATION HCT is a procedure in which a conditioning regimen that eliminates (partially or completely) the recipient’s myeloid and lymphoid immune systems is followed by the infusion of hematopoietic stem cells, with subsequent recovery of bone marrow and immune function. It includes two distinct categories: auto-HCT and allo-HCT. Of the approximately 20,000 HCTs performed in the United States every year, 12,000 are autologous and 8000 allogeneic. In both, a conditioning regimen (vari­ able combinations of chemotherapy, radiation, and serotherapy) is administered. This conditioning regimen eliminates the myeloid pro­ genitors and the lymphoid immune system of the patient. Then, hema­ topoietic stem cells are administered to replenish both the myeloid and the lymphoid immune systems. In the case of autologous transplanta­ tion, the stem cells are from the recipient, harvested in advance and cryopreserved until the day of the transplant. This procedure does not correct defects present in the stem cells (e.g., hemoglobinopathies, primary immunodeficiencies) and, in the treatment of cancer, is not expected to cure a malignancy that is refractory to chemotherapy because the autologous stem cells have no anticancer potential. For malignancies still susceptible to chemotherapy, however, the intensive myeloablative conditioning regimen could be curative, while the infu­ sion of autologous cells would effectively “rescue” the bone marrow. Conversely, in allo-HCT, the donor is another person, and their stem cells are free of whatever genetic defect the recipient’s stem cells had. This means allo-HCT could, in principle, cure stem cell disorders. The reconstituted myeloid system, for instance, will be free of hemoglobin­ opathy. The reconstituted immune system should ultimately become immunocompetent. If a malignancy is being treated, there is potential for the new immune system to fight it and even eradicate it, through a now well-defined phenomenon called graft-versus-leukemia (GVL) or graft-versus-tumor (GVT) effect. From this standpoint, allo-HCT may be considered a form of immunotherapy, and it has been used to cure chemotherapy-refractory malignancies. There is also, unfortunately, the potential for the regenerated immune system to recognize the recipient as “alien” and to mount an immune response against their organs, which is called graft-versus-host disease (GVHD) and can be severe and even fatal. GVHD is common, affecting between 35 and 50% of allo-HCT recipients with varying severity (from mild to lifethreatening). To prevent GVHD, allo-HCT recipients receive a few months of immunosuppression, which is then slowly tapered. GVHD is not expected to occur after auto-HCT, so no immunosuppression is administered after the stem cell infusion. The biological difference between auto-HCT and allo-HCT dictates the different indications of each one. The vast majority of auto-HCTs performed are for multiple myeloma and lymphoma, malignancies that do not involve the stem cells. In this situation, the therapeutic inter­ vention is the conditioning regimen, and the function of the infusion of stem cells (the transplant) is to replenish the myeloid progenitors eliminated by the conditioning. A minute fraction of auto-HCT is used or is being studied in select nonmalignant conditions that could poten­ tially benefit from intense immunoablation. Auto-HCT was superior to standard immunosuppression for the treatment of severe systemic sclerosis in a randomized controlled trial and is also being used in selected patients with multiple sclerosis. It is also being investigated in systemic lupus erythematosus, rheumatoid arthritis, polymyositis/ dermatomyositis, and Crohn disease. The most common indication for allo-HCT is acute leukemia, fol­ lowed by myelodysplastic and myeloproliferative disorders. It is also increasingly used for nonmalignant conditions including aplastic anemia, hemoglobinopathies (e.g., sickle cell disease, thalassemia), and primary immunodeficiency disorders (PIDs; also called inborn errors

of immunity [IEI]) including severe combined immunodeficiencies (SCID), phagocytic disorders such as chronic granulomatous disease, diseases of immune dysregulation such as familial hemophagocytic lymphohistiocytosis, and others. Different conditioning regimens result in varying degrees of organ toxicity, including mucositis, and variable durations of neutropenia. In the case of allo-HCT, additional factors that impact the risk of infection include the source of stem cells (bone marrow, peripherally collected stem cells, or umbilical cord blood [UCB]) and the degree of human leukocyte antigen (HLA) matching and relatedness between the donor and the recipient (in increasing infection risk: matched sibling donor, matched unrelated donor, haploidentical donor, mismatched donor). Infections are common after transplant and account for between 15 and 25% of the deaths in the first 100 days after auto-HCT and between 20 and 40% after allo-HCT, depending on the type of transplant. The risk factors for infection include preexisting conditions (intimately associated with the indication for transplant), conditioning regimen with its attendant neutropenia, mucositis and immunosuppression, and (in the case of allo-HCT) immunosuppression to prevent GVHD and, if this occurs, GVHD and its treatment. As mentioned earlier, however, and in distinction to SOT, most allo-HCT recipients do not receive lifelong immunosuppression. A few years after allo-HCT, a patient without chronic GVHD has an essentially normal immune system. TIMELINE OF INFECTIONS AFTER HCT Considerable experience over decades of transplantation has resulted in learning what the expected infections are and when they are sup­ posed to occur. Anti-infective prophylaxis derives from this knowl­ edge. Nomenclature varies, but we will consider very early infections (from initiation of the conditioning regimen until resolution of neutro­ penia), early infections (from neutrophil engraftment until day +100), and late infections (after day +100) (Table 148-1). Very Early Infections: Preexisting Infections, Baseline Condition, and Neutropenia  We consider the “very early” phase of HCT to be the period between the initiation of the conditioning regimen and recovery from neutropenia (neutrophil engraftment). The intensity of the conditioning (which is classified as myeloablative, reduced intensity, and nonmyeloablative) results in different degrees and durations of neutropenia, mucositis, and immunosuppression. During this time, bacterial infections related to neutropenia and mucosal barrier injury predominate, but one must also consider pre­ existing infections, both common and frequently known in patients undergoing transplant for PID (e.g., known diarrhea and cholangiopa­ thy caused by cryptosporidiosis in patients with SCID) and potentially unknown and acquired during therapies received prior to transplant (e.g., pulmonary aspergillosis developed during induction therapy of acute myeloid leukemia but asymptomatic pulmonary nodules at the time of transplant). A pretransplant infectious diseases consult TABLE 148-1  Common Sources of Infection After Hematopoietic Stem Cell Transplantation INFECTION SITE VERY EARLY (<1 MONTH) EARLY (BEFORE 100 DAYS) LATE (>100 DAYS) Disseminated Aerobic bacteria (gram-negative, gram-positive) Candida, Aspergillus, CMV, EBV, Toxoplasma Skin and mucous membranes HSV, Candida HSV, VZV VZV, HPV (warts) Lungs Aerobic bacteria (gram-negative,

gram-positive), Aspergillus, other molds CMV, community-acquired respiratory viruses, Pneumocystis, Toxoplasma, molds, adenovirus Gastrointestinal tract Clostridioides difficile CMV, adenovirus, norovirus EBV, CMV, norovirus Genitourinary tract BK virus BK virus, adenovirus Brain   HHV-6, Toxoplasma Toxoplasma, JC virus (rare) Bone marrow   CMV, Toxoplasma CMV Abbreviations: cGVHD, chronic graft-versus-host disease; CMV, cytomegalovirus; EBV, Epstein-Barr virus; HHV-6, human herpesvirus type 6; HPV, human papillomavirus; HSV, herpes simplex virus; NTM, nontuberculous mycobacteria; VZV, varicella-zoster virus.

is recommended, when possible, as a review of previous records and imaging may be extremely useful for prevention, diagnosis, and management of infection. An etiologic diagnosis for abnormal find­ ings should be pursued. A review of past infection or colonization with resistant pathogens may also provide actionable information, for instance, determining the choice of empirical antibiotics during neutropenic fever.

Fever during neutropenia (neutropenic fever) occurs in a large pro­ portion of HCT recipients, depending on the conditioning regimen. Neutropenic fever is discussed in depth in Chap. 79, but a summary follows. Fever during neutropenia is considered infectious in origin even if no infection is identified in most cases. The management cen­ ters on the fact that bacterial infections may progress very quickly in the absence of neutrophils, so expediency is critical. A thorough physi­ cal exam should be performed, with emphasis on possible bacterial portals of entry (e.g., mouth, perianal region, catheter exit site); blood cultures should be obtained; and broad-spectrum antibiotics including activity against Pseudomonas aeruginosa should be initiated. Antibiotic choice varies with prior history (e.g., previous infection with multi­ drug-resistant [MDR] gram-negative bacteria or methicillin-resistant Staphylococcus aureus), the clinical presentation (e.g., localizing symp­ toms or signs, hemodynamic instability), and the local patterns of resistance. Once antibiotics are started, they are modified depending on new microbiologic data or changes in the clinical status. In general, persistent fever alone is not an indication to modify the antibacterial regimen. However, persistent fever while receiving broad-spectrum antibiotics identifies a subset of patients with higher likelihood of har­ boring invasive fungal disease (IFD), and a thorough search for IFD or modification of the antifungal regimen is indicated when neutropenic fever continues for >4 days. CHAPTER 148 Bacterial infections during this time are related to the degree and duration of neutropenia and the extent of the mucositis. Consequently, milder, nonmyeloablative conditioning regimens may result in less very early infections. Antibacterial prophylaxis (typically with a fluoroqui­ nolone such as levofloxacin) decreases the frequency of neutropenic fever and is used by some groups, but there is growing concern about the long-term consequences of this practice (potential for worsening transplant outcome due to microbiome disruption), as well as coloniza­ tion with resistant bacteria and Clostridioides difficile colitis. Currently, antibacterial prophylaxis during HCT is not universal. Infections in Transplant Recipients Neutropenia during HCT is usually shorter than after induction of remission of acute leukemia. The most common early fungal infection after HCT used to be candidiasis, but now antifungal prophylaxis is almost universal. Prophylaxis with fluconazole decreases the likelihood of invasive candidiasis and improves survival in the allo-HCT setting. Fluconazole prophylaxis is most often used, but if the likelihood of asper­ gillosis is considered high, an agent with activity against mold, typically posaconazole, is used instead. Aspergillus and other molds are uncom­ mon during short neutropenia, but there may be other factors that put the patient at risk. It should be noted that antimicrobial prophylaxis is PERIOD AFTER TRANSPLANTATION Encapsulated bacteria (Streptococcus pneumoniae, Haemophilus influenzae, Neisseria meningitidis) Community-acquired respiratory viruses, Pneumocystis, Nocardia, S. pneumoniae, H. influenzae NTM (cGVHD)

never completely effective. Breakthrough infections may occur, such as non-albicans Candida when fluconazole is used or Fusarium, Mucorales, or other azole-resistant molds when posaconazole is used.

Risk of early Pneumocystis jirovecii pneumonia (PJP) varies with the indication for transplant and prior treatments, as well as with the conditioning regimen. For instance, a patient receiving allo-HCT for sickle cell disease is not at risk for PJP before receiving the condition­ ing regimen, whereas patients with immunodeficiency or heavily treated for acute lymphoblastic leukemia carry considerable risk. In most cases, prophylaxis against PJP is used. It is well accepted that trimethoprim-sulfamethoxazole (TMP-SMX) is most effective, and it can be used until the infusion of stem cells. After the stem cells are given, the potential for myelotoxicity and delayed engraftment makes other options (e.g., inhaled or intravenous pentamidine) preferable. Atovaquone may be used, but it is expensive, unpalatable, and requires a fatty meal for optimal absorption, which may be difficult during this phase of significant mucositis. Herpes simplex virus (HSV) reactivation occurs early, associated with the mucositis caused by the conditioning regimen. Acyclovir prophylaxis is standard of care, and it effectively prevents HSV and varicella-zoster virus (VZV). Breakthrough HSV must be differenti­ ated from mucositis caused by chemotherapy, and viral polymerase chain reaction (PCR) should be used. If HSV breaks through acyclovir prophylaxis and absorption is not an issue, acyclovir-resistant HSV should be considered. Decisions must be made clinically, since drug resistance can only be determined definitively by phenotypic testing, which takes weeks. Some instances of low-level resistance may be over­ come by switching to high-dose (10 mg/kg per 8 h) IV acyclovir, but if this does not result in clinical improvement, foscarnet 60 mg/kg per 12 h or cidofovir 5 mg/kg weekly should be used. PART 5 Infectious Diseases Cytomegalovirus (CMV) infection is uncommon before engraft­ ment and will be discussed in detail later, but in many centers, HCT recipients who are CMV seropositive start prophylaxis with the anti­ viral letermovir at the time of transplant. Of note, letermovir has no activity against HSV or VZV, so acyclovir should be administered concomitantly. Early Infections: Between Neutrophil Engraftment and Day 100—Reactivation of Latent Infections and Complications of GVHD and Its Treatment  During this phase, the patient has neutrophils but not a functioning lymphoid immune system. Potential infections include exogenous pathogens (e.g., community-acquired respiratory viruses), acquired when the patient is discharged from the hospital and reenters the community, as well as the reactivation of latent pathogens (e.g., herpesviruses, adenovirus, JC virus, and para­ sites such as Toxoplasma gondii). Despite neutrophil engraftment, the patient’s lymphoid immune system was eliminated by the conditioning regimen, and the infused stem cells have not had time to create a func­ tioning immune system. Other factors may impact the risk of infection. In the case of allo-HCT, there is usually ongoing immunosuppression with a calcineurin inhibitor (cyclosporine A or tacrolimus), siroli­ mus, or mycophenolate mofetil. The conditioning regimen may have included particularly immunosuppressive strategies such as sero­ therapy with antithymocyte globulin or alemtuzumab, which have a long half-life and may result in prolonged T-cell depletion that persists after engraftment. The source of the stem cells administered also plays a role: peripherally collected stem cells include more mature T cells than bone marrow, and these may offer some protection against latent pathogens. Ex vivo T-cell depletion (CD34 selection) is sometimes used and results in a product with even fewer mature T cells than bone marrow. UCB also lacks effective cellular immunity against infections latent in the recipient. Finally, if the patient develops acute GVHD, high-dose corticosteroids may be added to the immunosuppressive regimen, increasing the risk not only of latent infection reactivation but also of newly acquired bacterial and fungal infections. If GVHD does not respond to steroids, second-line agents may be added, compound­ ing the immunocompromise of the patient. BACTERIAL INFECTIONS  Mucositis and neutropenia have resolved during this phase. Some patients still have a central venous catheter,

with the attendant risk of catheter-related bloodstream infection. How­ ever, in the absence of GVHD, bacterial infections are relatively uncom­ mon. Conversely, if acute GVHD of the bowel occurs, the patient is at risk of mucosal barrier injury–related bloodstream infection with bowel flora, particularly when high-dose corticosteroids are initiated. C. difficile infection is also common during this period and should be aggressively treated. FUNGAL INFECTIONS  Because neutropenia is no longer a factor, fun­ gal infection during this time is usually associated with corticosteroid use, although other immunosuppressive agents used to treat acute GVHD may also increase the risk. Aspergillosis is the most common mold infection, and patients with GVHD on high-dose corticosteroids (i.e., >0.5 mg/kg of prednisone every other day) or on a combina­ tion of immunosuppressive agents frequently receive posaconazole prophylaxis, based on the results of a randomized controlled trial. Many authorities recommend monitoring serum posaconazole levels, particularly when absorption may be compromised by severe acute GVHD of the bowel. A level <700 ng/µL is considered subtherapeutic, but experts disagree on what levels are desirable for effective prophy­ laxis and/or treatment. The use of serologic markers of fungal infec­ tion (galactomannan and/or beta-d-glucan) is not recommended in this setting, as the sensitivity of the tests is markedly reduced when on mold-active azoles. HERPESVIRUSES  Herpesviruses may reactivate during this phase, par­ ticularly in the setting of T-cell depletion, UCB transplant, or GVHD and its treatment. HSV and VZV are usually effectively prevented by acyclovir prophy­ laxis, but acyclovir-resistant HSV may occur (see the previous section). CMV disease may be prevented by prophylaxis or preemptive manage­ ment and is discussed separately. Human herpesvirus 6 (HHV-6), the cause of exanthem subitum (sixth disease) in children, is carried latently by most adults and reacti­ vates frequently after HCT. Most of the time, reactivation is asymptom­ atic and goes undetected, since routine monitoring of HHV-6 is not recommended. However, it can sometimes cause encephalitis, which may be severe and result in significant morbidity and even death. It presents as limbic encephalitis, typically without a fever and initially with a clear sensorium but with short-term memory loss accompa­ nied by tremor or other involuntary movements. It may progress to abnormal mental status, seizures, and coma. The diagnosis is made in the presence of signs and symptoms of encephalitis with detectable HHV-6 in the cerebrospinal fluid (CSF) and no other explanation for the disease. Brain magnetic resonance imaging (MRI) may be normal initially but later shows bilateral involvement of the amygdala on the temporal lobes. It is not clear whether treatment modifies the out­ come, as HHV-6 tends to get under control in most patients without any intervention and affected patients frequently suffer significant long-term sequelae despite antiviral treatment. Guidelines recommend treatment with ganciclovir or foscarnet, although some experts recom­ mend combining both antivirals. Other less well-established disease associations with HHV-6 include fever, GVHD, pneumonia, poor graft function, liver enzyme abnormalities, and thrombocytopenia. Human herpesvirus 7 (HHV-7) is, like CMV and HHV-6, a betaherpesvirus. It may be detected by PCR in the blood of HCT recipients, but it is not known whether it causes any disease or is a co-factor in other infections such as CMV and HHV-6. BK VIRUS  BK virus, a polyomavirus that infects 80–90% of people, is associated with hemorrhagic cystitis after allo-HCT and, very sel­ domly, nephropathy. The clinical presentation includes dysuria and bladder spasms and hematuria, usually macroscopic. High-level BK viruria is usually present, but this may also happen without symptoms. The increasing use of posttransplant cyclophosphamide as prophylaxis for GVHD may make diagnosis difficult, as cyclophosphamide can cause hemorrhagic cystitis. There is no known effective treatment for BK virus after HCT, but hemorrhagic cystitis, albeit debilitating, is almost always self-limited. In severe cases, intravesical clots may cause obstructive uropathy and require continuous bladder irrigations.

Rarely, BK virus may cause nephropathy after allo-HCT. This entity is discussed in detail in the section on infections after SOT. RESPIRATORY VIRUSES  Besides the reactivation of latent viral infec­ tions expected during this period, once the patient is discharged after transplant, they are exposed to community-acquired respiratory viruses, which have significant mortality when they progress from upper respiratory tract infection to pneumonia. Many respiratory viruses can cause significant morbidity after HCT. SARS-CoV-2, influenza, respiratory syncytial virus (RSV), metapneu­ movirus, and parainfluenza 3 may cause severe disease more often, but multiple enteroviruses and non–severe acute respiratory syndrome (SARS) coronaviruses may occasionally result in pneumonia. Only a few of these infections are amenable to antiviral treatment. Diagnosis usually relies on molecular methods (PCR of nasopharyngeal swab or nasal wash), but rapid viral culture (shell vial) is still used in some centers. A general principle of viral infections is that controlling them (espe­ cially long term) requires the host’s specific immunity, to a degree that bacterial infections do not. In this regard, decreasing the immunosup­ pression, if possible, is a standard part of the management. Unfortu­ nately, this may not be possible in allo-HCT, particularly if the viral infection occurs in the setting of GVHD. If there is no GVHD, some consideration should be given to decreasing the dose of immunosup­ pressive drugs such as calcineurin inhibitors or sirolimus, which are usually carefully monitored through blood levels. Specific antiviral treatment recommendations follow. Coronavirus Disease 2019 (COVID-19)  U.S. Food and Drug Administra­ tion (FDA)-approved antiviral treatments for SARS-CoV-2 include remdesivir and nirmatrelvir-ritonavir (Paxlovid). In nonvaccinated, never-infected, non-immunocompromised patients with comorbid conditions or older than 60 years with COVID-19, nirmatrelvirritonavir given within 3 days of the onset of symptoms resulted in a 90% decrease on the risk of hospitalization or death. More recent studies including people who have been vaccinated and are not at high risk of progression have not confirmed these results. However, many experts still recommend nirmatrelvir-ritonavir for immunocompro­ mised patients. Unfortunately, transplant recipients are frequently receiving medications such as tacrolimus or sirolimus that may make nirmatrelvir-ritonavir use impractical due to drug–drug interactions. The antiviral molnupiravir has an FDA emergency use authoriza­ tion for the treatment of adults with mild-to-moderate COVID-19 who are at high risk for progression to severe disease and for whom other COVID-19 treatment options are not accessible or clinically appropriate. Most experts would at least consider antiviral treatment of COVID-19 in HCT recipients. Depending on the clinical setting, outpatient administration of IV remdesivir may be a more practical option. Bacterial superinfection is uncommon after COVID-19, but aspergillosis has been described. Risk factors for COVID-19-associated pulmonary aspergillosis include high-dose corticosteroids and hema­ tologic malignancy but not necessarily transplant. Influenza Virus  Influenza may be treated with neuraminidase inhibi­ tors (oseltamivir, zanamivir, or peramivir) or baloxavir, an inhibitor of the polymerase acidic (PA) protein. There are limited data to choose between these two options, but more experience with the former. Amantadine and rimantadine, only active against influenza A, are not currently recommended in the United States because of widespread resistance. Respiratory Syncytial Virus  RSV may be treated off-label with ribavirin, either orally or by inhalation. In RSV, lymphopenia (absolute lympho­ cyte count [ALC]) has been found to be a risk factor for the develop­ ment of pneumonia, and some experts recommend using ALC <1000/µL as a guide to initiate early treatment with ribavirin. Adenovirus  Adenovirus occupies a particular place within this group, as it can represent a newly acquired community-acquired infec­ tion or be reactivation of latent virus from prior infection. Because adenovirus may be more common in children, some pediatric centers monitor high-risk patients by PCR of stool and/or blood and attempt

intervention before established adenovirus disease ensues. Drugs with activity against adenovirus include cidofovir, brincidofovir, and (for some species) ribavirin. The response of established adenovirus disease to antivirals is poor.

As mentioned above, all respiratory viral infections may occasionally cause lower respiratory tract disease. When there is lower respiratory tract with any respiratory virus, a search for co-pathogens (bacterial or fungal) is indicated, as these are frequently present. TOXOPLASMOSIS  Toxoplasma gondii infects >2.5 billion people. The prevalence and routine monitoring after HCT vary significantly between countries. Studies from Europe suggest that allo-HCT recipi­ ents who have positive Toxoplasma serology reactivate frequently (8–21%) and early (95% within the first 6 months). The presentation may be just fever (this, in the presence of a positive Toxoplasma PCR, is often called toxoplasmosis infection), but also may include pneumonia (small pulmonary nodules and interstitial pneumonia predominantly at the lung bases), heart disease, brain abscess (single, multiple, or diffuse encephalitis), and disseminated disease sometimes associated with hemophagocytic lymphohistiocytosis. Diagnosis is by blood PCR and/or immunohistochemistry. The standard treatment is with pyri­ methamine and sulfadiazine, but TMP-SMX may be equally effective. Atovaquone is a second-line agent, and there are anecdotal reports in advanced HIV of successful use of the combination of azithromycin and clindamycin. Late Infections (After Day 100)  Day 100 post-HCT remains an arbitrary boundary, as this is when tapering of immunosuppression usually begins. Conceptually, immune reconstitution takes place unim­ peded by immunosuppressants, so the risk of infection should become progressively less. Most infections at this time are caused by commu­ nity-acquired pathogens, although late CMV may also occur. The main risk factor for infection is the presence of GVHD and its treatment. Chronic GVHD (cGVHD) results in functional asplenia, and Strep­ tococcus pneumoniae is a particularly important pathogen. Late fatal infections are predominantly bacterial, but respiratory viral infections such as influenza are still more common than in age-matched popula­ tions. The group at highest risk are patients with cGVHD receiving immunosuppression. CHAPTER 148 Infections in Transplant Recipients INFECTIONS AFTER SOLID ORGAN TRANSPLANT ■ ■RISK FACTORS FOR INFECTION AFTER SOLID ORGAN TRANSPLANT Infectious complications remain a major cause of morbidity and mor­ tality after SOT. The risk of infection is determined by the interaction of various factors, such as age of the recipient; type of transplant and technical complexities associated with it; other invasive procedures; dose, duration, and temporal sequence of immunosuppressive drugs; epidemiologic exposures of both donor and recipient; use of antimi­ crobial prophylaxis; donor-recipient serostatus to certain infections (e.g., CMV, Epstein-Barr virus [EBV], toxoplasmosis); and ongoing viral replication (so-called indirect effects). Assessment of a recipient’s risk for infection helps tailor specific preventive strategies and workup when an infection is suspected. Induction immunosuppression is usually administered immediately prior to transplantation. This is followed by maintenance immunosup­ pression, which frequently consists of triple therapy with a calcineurin inhibitor (cyclosporine A or tacrolimus) or an mTOR inhibitor (siro­ limus or everolimus), mycophenolate mofetil, and corticosteroids. Maintenance immunosuppression may be intensified during periods of rejection, and it may be decreased with time, but it is usually continued for life. The amount of immunosuppression the patient is receiving is important in terms of risk of infection. Age is also an important determinant of susceptibility to infections; it impacts the likelihood of prior exposures to microbial pathogens, either by primary infection or vaccination. History of exposure can have either positive or negative effects. Older patients are more likely to have encountered pathogens that can remain latent and reactivate at the time

of transplant (e.g., tuberculosis [TB]); younger patients have a higher risk of acquiring primary infections after transplant, sometimes from the transplanted organ, which tend to be more severe than those secondary to reactivation of a latent infection (e.g., CMV). In addition, preexisting immunity can have a protective effect against clinical disease (e.g., EBVassociated posttransplant lymphoproliferative disorder [PTLD]).

The type of allograft affects the specific infectious risk due to techni­ cal factors associated with the transplant procedure but also inherent to the transplanted organ and risk of rejection. For example, urinary tract infections are most common after kidney transplant, either from cath­ eter placement or ureter stenting. BK virus is ubiquitous, but associated nephropathy (BK virus-associated nephropathy) is most common after kidney transplant and is an important cause of allograft loss. Infec­ tion after liver transplant frequently results from leaks of biliary and gastrointestinal anastomoses. Cardiac assist devices in heart transplant recipients are a frequent source of infection. Tracheal anastomotic infections, particularly due to fungi, are a significant complication of lung transplantation. In single-lung transplants, recurrent infections of the native lung, such as gram-negative bacilli or fungi, can extend to the transplanted lung. In small intestine transplant, opportunistic and nonopportunistic viral infections of the gastrointestinal tract are com­ mon; these can be severe and even life-threatening (e.g., norovirus). ■ ■TIME COURSE OF INFECTIONS AFTER SOLID ORGAN TRANSPLANT (TABLE 148-2) Despite all the differences in individual risk of infection, some general patterns of infection in the absence of antimicrobial intervention are similar among SOT recipients. This predictable temporal pattern has enabled the institution of specific prophylactic strategies and is useful when developing an infection differential diagnosis. PART 5 Infectious Diseases Early Period: Infections in the First Month After Transplant 

Infections in this period are generally associated with technical compli­ cations of the transplant surgery and are nosocomial. Bacterial infec­ tions are most common during this time across all organs. Infections with prior colonizers (e.g., MDR bacteria) can occur, and bacterial prophylactic strategies should take these into account. Antimicrobial prophylaxis is usually started promptly after transplant. Hepatitis C occurs during this period when hepatitis C–positive donors are used. Opportunistic infections can happen in recipients with comorbid immunodeficiencies or those who received immunosuppression before transplant, but in the absence of these risk factors, the development of classic opportunistic infections during this early posttransplant period should alert to the possibility of donor allograft-transmitted infections (e.g., invasive coccidioidomycosis). Intermediate Period: Infections 1–6 Months After Transplant 

In the absence of routine antimicrobial prophylaxis, this period is char­ acterized by the presence of classic opportunistic infections, such as P. jirovecii and CMV. The incidence of these infections has been sig­ nificantly reduced or delayed with the use of prophylaxis or preemptive therapy. Reactivation of latent infections such as TB, Chagas’ disease, endemic mycoses, and cryptococcosis can occur, so it is important to ascertain the risk before transplant and implement specific prophylactic strategies. Viral infections such as BK virus, adenovirus, RSV, hepatitis B virus, and EBV are common. Invasive fungal infections, specifically Aspergillus and other mold infections, can be problematic during this period of heightened immune suppression, but risk varies significantly by type of organ transplanted. Donor-derived infections may still occur. Late Period: Infections 6 Months After Transplant  This period is less well defined. Patients with satisfactory allograft func­ tion may develop more severe manifestations of community-acquired infections. Prophylaxes are frequently stopped, since the risk of oppor­ tunistic infections is lower than in the earlier period. Patients with poor allograft function due to rejection receive increased immunosuppres­ sion and are at higher risk of opportunistic infections for a variable period thereafter. Frequently, antimicrobial prophylaxes are “reset” when an episode of acute rejection ensues. The type of rejection (e.g., cellular or humoral) and its specific management may influence the

infection risk. Given that all SOT recipients need lifelong immunosup­ pression, increased risk of infection persists. Late-onset CMV, nocar­ diosis, PJP, listeriosis, invasive fungal infections, and EBV-associated PTLD are some examples of opportunistic infections that can be expected in these patients. Patients with ongoing chronic viral reacti­ vation, particularly CMV, are also at increased risk of other opportu­ nistic infections. In addition, lung transplant recipients with chronic graft dysfunction are at high risk of recurrent bacterial pneumonia. Similarly, liver transplant recipients with chronic graft dysfunction fre­ quently develop biliary strictures and recurrent cholangitis. Over time, MDR bacteria can become a problem. VASCULAR COMPOSITE ALLOGRAFTS VCAs refer to transplantation as a single functional unit of multiple tissues, such as muscle, blood vessel and nerve bundles, and associ­ ated viscera or bone. Face, hand, and uterus are common examples. These procedures are increasing in frequency and complexity, but data regarding infection risk are still scarce and inferred from the known risk of SOT. Due to the highly complex anatomy of the allografts and their natural microbial colonization, bacterial infections are most common. Relevant infection prevention strategies are like those used in SOT, such as updated immunizations pretransplant, donor/recipient serologic screening, and targeted antimicrobial prophylaxis. Planning and in-depth communication between the organ procurement team, the surgical team, and an infectious disease consultant are important so that relevant cultures are obtained and results reported back. Surveil­ lance cultures for targeted microbiologic diagnosis will vary depending on the transplanted tissue. For example, for face transplants, in addi­ tion to cultures of respiratory pathogens, sinus cultures for bacteria and fungi are routinely obtained. Similarly, in penile or uterine transplants, sexually transmitted infections such Chlamydia or human papilloma­ virus need to be explicitly ruled out. Opportunistic infection risk is largely driven by the induction, maintenance, and intensification of immunosuppression due to the frequent rejection episodes. Systemic viral infections, in particular CMV and EBV, have been a significant problem after VCA; similar prevention strategies, as in other high-risk SOT recipients, are recom­ mended. Fungal infections, such as cryptococcus or mucocutaneous candidiasis, may mimic rejection, which underscores the importance of establishing a histopathologic and/or microbiologic diagnosis and reassessing frequently. As the field of VCA continues to evolve, the risk of infectious complications needs to be continually assessed so that in the future we can better define its similarities and differences from other types of transplants. DONOR-DERIVED INFECTIONS Donor-derived infections (DDIs) can be expected or unexpected. Expected transmission to the recipient occurs with pathogens that are known to be present in the transplanted organ. Viruses such as CMV, EBV, hepatitis B virus, hepatitis C virus, and BK polyomavirus are examples. Preventive strategies such as targeted antimicrobial prophylaxis and access to highly specific molecular assays minimize the risk of end-organ disease and the overall impact on the recipient despite transmission. Global and regional health and transplantation societies provide detailed recommendations for adequate screen­ ing for certain blood and tissue donor-derived pathogens, such as HIV and viral hepatitis, and provide specific guidance for continued surveillance in recipients of donors with a history of IV drug use, unprotected sexual activity, multiple sexual partners, or incarceration within 30 days of potential donation. Recommendations often vary between organizations and countries. Transmission of unexpected infections occurs when pathogens not detected in an organ donor prior to organ recovery are transmitted to recipients. Screening donors for transmissible pathogens requires clinical and epidemiologic his­ tory as well as laboratory evaluation. Despite clinical and laboratory screening of potential donors for transmissible diseases, unexpected transmission of infection from donor to recipient remains an inherent risk of transplantation, predominantly of solid organ and composite tissue transplantation. Unexpected donor-derived diseases affect <1%

TABLE 148-2  Common Infections After Solid Organ Transplantation by Site of Infection and Transplant Type INFECTED SITE TYPE OF INFECTION RISK FACTOR MANAGEMENT Transplanted organ Bacterial and fungal infections of the graft, anastomotic site, and surgical wound Prior colonization of the donor with MDR organisms Surgical incision infection Bacterial, yeast infections are most common; NTM (rare, mostly thoracic transplants) Pain, erythema, discharge, or dehiscence of wounds, typically within the first 30 days posttransplant Intra-abdominal infections in liver transplant recipients Bacterial and yeast infections Secondary to biliary anastomoses leaks, or Roux-en-Y hepaticojejunostomy or other anastomoses that increase the risk of intestinal reflux into the biliary system; hepatic necrosis, from hepatic vascular thrombosis Intrahepatic infection in liver transplant Recurrent cholangitis (bacteria and yeast) Biliary strictures: anastomotic (acute) or nonanastomotic (chronic allograft rejection) Urinary tract infections in kidney transplant Recurrent cystitis and pyelonephritis (bacteria, in particular Enterobacteriaceae); Mycobacterium tuberculosis and yeasts are less frequent causes of UTI but should be suspected when there is pyuria and/or hematuria and negative cultures Duration of indwelling urinary tract catheters, female recipients, as well as recurrent UTI prior to the transplant Polyomavirusassociated nephropathy in kidney transplant BK, rarely JC virus Heightened immunosuppression, male recipient, older age, and a high BK viremia (>10,000 copies/mL) Pneumonia and tracheobronchitis in lung transplant Bacterial infections: severe or recalcitrant community-acquired viral pathogens (RSV, influenza, SARS-CoV-2); invasive mold infections; fungal tracheobronchial infection; NTM pulmonary disease Pretransplant colonization with resistant organisms, requiring endotracheal intubation prior to the transplant, and mucociliary dysfunction, are significant risk factors for bacterial infections; CLAD is a risk factor for and a result of recurrent viral, bacterial, fungal, and mycobacterial infections Mediastinitis in thoracic organ transplants Bacterial or Candida mediastinitis, with or without sternal osteomyelitis; less common Nocardia spp., NTM, or molds Diabetes mellitus, surgical reoperation, and acute rejection Myocarditis, pericarditis, and cardiomyopathy in heart transplant Toxoplasma gondii infection Toxoplasma pretransplant serology D+/R–   Trypanosoma cruzi Pretransplant recipient seropositivity (hearts from seropositive donors are not used) Infectious hyperammonemia syndrome in lung transplant (less commonly in other transplants) Ureaplasma spp. systemic infection (less commonly Mycoplasma spp.) that results in encephalopathy due to high ammonia levels >200 μmol/L Donor lung colonization Targeted antimicrobial treatment and direct ammonia removal through renal replacement therapy, lactulose, and reducing enteral protein intake. Prevalence in transplanted organs is roughly 10%, and screening should be considered. Abbreviations: CLAD, chronic lung allograft dysfunction; D+/R–, donor positive, recipient negative (serology); IR, interventional radiology; MDR, multidrug-resistant; NTM, nontuberculous mycobacteria; PCR, polymerase chain reaction; RSV, respiratory syncytial virus; TMP-SMX, trimethoprim-sulfamethoxazole; UTI, urinary tract infection. of all transplant recipients, and most are infections. (Besides infection, donors may also transmit unsuspected malignancies and other condi­ tions [e.g., hemochromatosis], but these are beyond the scope of this chapter and will not be discussed.) Unexpected DDIs occur predomi­ nantly in cadaveric donation, where screening of potentially transmis­ sible infection is hampered by the incomplete assessment of the donor’s

Institute targeted antimicrobial treatment as soon as prior colonization is known (peritransplant or immediately posttransplant). Source control (surgical or interventional radiology) and continued directed antimicrobials. Optimal duration after source control is unknown. Usual correlates of adequate treatment are clinical, laboratory, and radiologic resolution. Collection of microbiologic specimens for culture. Thorough washout and debridement. Imaging to evaluate for the presence of deeper infection. Antimicrobials should be adjusted based on microbiologic results, and duration guided by clinical response. Obtain blood cultures. Deep tissue or fluid cultures obtained intraoperatively or by IR. Source control. Antimicrobials adjusted based on microbiologic results (polymicrobial infections are the norm). Optimal duration of antimicrobials is unknown. Usual correlates of adequate treatment are clinical, laboratory, and radiologic resolution. Biliary drainage and antimicrobials. Prompt catheter and ureteral stent removal. Targeted antimicrobials. If recurrent UTI, evaluate for anatomic abnormalities (ureteral reflux, ureterovesical junction stenosis, neurogenic bladder, bladder diverticulum). CHAPTER 148 The only effective intervention is reduction in immunosuppression. An allograft kidney biopsy is necessary when there is suspicion of rejection or a different infection. Early antimicrobial therapy if possible. For viral respiratory pathogens, prevention with seasonal annual vaccination is recommended, with monoclonal antibodies when available. In patients with CLAD, respiratory rehabilitation and pulmonary toileting. Infections in Transplant Recipients Deep tissue cultures (intraoperatively). Duration of antimicrobials is unknown. Correlates of adequate treatment are clinical, laboratory, and radiologic resolution. Diagnosis is established by the identification of tachyzoites in the endomyocardial biopsy or PCR in tissue and/or pericardial fluid. Preemptive monitoring via blood PCR and TMP-SMX prophylaxis reduce incidence of disease. Clinical manifestations are fever, myocarditis, and painful skin lesions. Preemptive monitoring via blood PCR to detect early reactivation is indicated in all previously known seropositive recipients. Antitrypanosomal treatment (benznidazole or nifurtimox) for all patients with reactivation and tissueinvasive disease. risk. Almost all become apparent within 90 days of the transplant sur­ gery. Overall mortality is around 15%, but it is much higher (30–50%) in some fungal (e.g., aspergillosis, coccidioidomycosis) and parasitic infections (e.g., strongyloidiasis, toxoplasmosis). The risk of DDI needs to be balanced against the growing number of patients on the trans­ plant waitlist and their associated risks of mortality. The early diagnosis

of a DDI requires a high index of suspicion. Clinicians should be on alert to the possibility of an unexpected transmission. DDI should be considered when opportunistic infections happen outside the expected at-risk period, in the case of infections with no clear epidemiologic risk factors in the recipient, with unusual clinical syndromes, and with persistent undiagnosed fever, especially during the first 90 days. Trans­ plant and infectious disease physicians should be alerted to clusters of infections in recipients from the same donor. When an unexpected DDI is suspected, besides prompt diagnosis and treatment in the recipient, immediate communication with public health authorities and the organ procurement organization is essential to decrease the risk of transmission/disease in other recipients.

Donors with a known or suspected active infection at the time of donation, such as pneumonia, urinary tract infection or bloodstream infection, may transmit the infection to the recipient. Living donors with an identified active infection should ideally be treated and transplantation delayed until infection resolves. In deceased donors, targeted antimicrobial prophylaxis for recipients is highly effective in preventing disease transmission when the infection is due to sus­ ceptible bacteria. Bacteremia in deceased donors is not uncommon, and studies have shown that, in adult recipients, donor bacteremia is not associated with adverse outcomes if a feasible treatment option exists. Bacterial meningitis carries a similar risk of transmission. Blood cultures should be obtained to rule out occult bacteremia in deceased donors, but bacteremia is generally not considered a contraindica­ tion to organ procurement and transplantation, unless no effective antimicrobial therapy is available. However, if the organ selected for transplantation is not infected or colonized and the donor is not bac­ teremic, the risk of transmission is low. These scenarios require careful analysis, and decisions should be individualized. The patient should be involved in deciding the acceptance of donors who may entail a higher risk for the recipient. PART 5 Infectious Diseases Meningoencephalitis in SOT recipients can be devastating. Viruses cause most cases, and effective antiviral treatments exist for only a few of these. Parasitic and fungal meningoencephalitis are also associated with significant morbidity and mortality. Development of meningoencepha­ litis or other neurologic syndrome in an SOT recipient, especially early after transplantation, should raise suspicion for DDI. Meningoencepha­ litis of unknown etiology in a donor is one of the few remaining absolute contraindications of transplantation. Even in patients in whom the etiol­ ogy is suspected, caution is advised, and a comprehensive evaluation for infectious causes, with special consideration for opportunistic infection when the donor is from an endemic area, should be undertaken. TB and endemic mycoses have been well-documented donortransmitted infections and are associated with significant risk of mortality or graft loss. Routine screening is recommended in areas of endemicity or in other recognized high-risk scenarios for TB, such as incarceration, displacement, or homelessness. Screening in deceased donors is frequently limited by the extent of the medical history and time constraints. IMPORTANT PATHOGENS IN

TRANSPLANT RECIPIENTS ■ ■CYTOMEGALOVIRUS CMV is the most important virus in both SOT and HCT recipients. CMV is usually acquired in early childhood and establishes latency for life in many different cell types, including endothelial cells and dendritic cells. Seropositivity for CMV (defined as positive anti-CMV IgG) varies by geographic region and ranges from 30 to 97%, increas­ ing with age. CMV infection is defined as detection of the virus in the blood (typically by PCR, which has replaced the old pp65 antigenemia test). CMV DNA may be quantified in whole blood or in plasma, with results expressed in international units per microliter (IU/µL), and only results using the same test should be compared. CMV viremia is usually asymptomatic, but SOT recipients may develop a syndrome of fever, malaise, myalgias, moderate leukopenia with occasional atypical peripheral blood lymphocytes, and mild transaminitis concomitant with the detection of CMV in the peripheral blood. This is called CMV

syndrome. Tissue-invasive CMV disease is diagnosed by histopatho­ logic proof of invasion by the virus and tissue damage. Pneumonitis is the most feared end-organ disease after allo-HCT, and gastrointestinal disease is the most common, both after SOT and allo-HCT, but many other organs may be affected. Typically, CMV infection precedes development of disease by a few days, but sometimes CMV gastroin­ testinal disease occurs before the virus may be detected in the blood, or simultaneously. Clinically significant CMV infection is a term used in some clinical trials of CMV prophylaxis and includes invasive disease and/or a viremia level that triggers treatment with antivirals. Before widespread use of interventions aimed at reducing the incidence of CMV end-organ disease (i.e., pneumonitis, carditis, gastroenteritis, hepatitis), this occurred frequently during the first 1–6 months after transplantation and was associated with high morbidity and mortality. In addition, active CMV replication is associated with several “indirect effects,” including increased rates of bacterial and fungal infection, increased graft dysfunction, transplant vascular disease, acute rejec­ tion, and, after HCT, increased GVHD and increased nonrelapse transplant-related mortality. Avoiding CMV disease is an important part of transplant medicine. The process starts with donor selection, as the serostatuses of both donor and recipient are the main determinants of the risk of disease. In SOT, the highest risk is seen with a seropositive donor going into a seronegative recipient (D+/R–). In this setting, up to 50% of recipients may become viremic after prophylaxis is stopped, with lung, small bowel, pancreas, and combined kidney-pancreas having higher risks. After HCT, the highest risk is for seropositive recipients (who harbor the virus in many of their tissues) receiving stem cells from seronega­ tive donors (D–/R+), 60–70% of whom may develop CMV viremia. When several donors are available, avoiding high-risk CMV serostatus combinations is part of the selection process. Once the transplant takes place, prevention of CMV disease can be achieved by two different strategies: universal prophylaxis or preemptive management. There are two strategies to prevent CMV disease after transplant: universal prophylaxis, in which every recipient at high risk is given antivirals (usually valganciclovir after SOT or letermovir after allo-HCT) for an arbitrary period (usually 3–6 months after SOT and 100 days after allo-HCT), or preemptive management, in which CMV is monitored in the blood weekly and anti-CMV treatment initiated when a particu­ lar level of viremia is detected. Of note, valganciclovir (a prodrug of ganciclovir) is active against HSV and VZV, but letermovir is not, so if letermovir is used, acyclovir or valaciclovir must be added for HSV/ VZV prophylaxis. The tolerable level of viremia differs based on type of transplant, timing of the infection, and other factors that increase the risk of CMV disease. In UCB transplant or T-cell-depleted transplants, lower levels of viremia may trigger treatment, as the kinetics of viral replication and risk of progression to end-organ disease may be sig­ nificantly higher in these patients. Randomized trials comparing both strategies have been performed only after allo-HCT for kidney and liver transplant but are not available for SOT involving other organs. Prophylaxis and preemptive treatment seem similarly effective in terms of overall mortality, and transplant centers may choose one over the other based on considerations such as logistics (more complicated with preemptive therapy) and cost (potentially higher with letermo­ vir). Treatment of significant viremia is with ganciclovir or foscarnet. Cidofovir and maribavir are inferior, and the former is potentially more toxic. Ganciclovir and foscarnet are considered equally effective but differ in toxicity: ganciclovir has significant myelotoxicity, and foscar­ net causes electrolyte abnormalities that usually cannot be managed in the outpatient setting, as well as nephrotoxicity. If viremia increases after 2 weeks of optimal antiviral treatment or if it does not decrease, the infection is considered “refractory” or “possibly refractory” CMV, respectively, and testing for genetic resistance to antivirals is recom­ mended. The antiviral maribavir is FDA approved for resistant/refractory CMV, but its efficacy is only moderate, with <20% of patients with resistant/refractory CMV maintaining control of the virus at 16 weeks. Invasive disease due to CMV varies with the type of transplant. After allo-HCT, CMV pneumonia is the most severe, but it is now uncommon with the use of prophylaxis or preemptive management.

Involvement of the gastrointestinal tract (most commonly CMV coli­ tis, but any segment of the bowel may be affected) is more frequent, in part because the time between asymptomatic viremia and CMV colitis is shorter. After SOT, CMV reactivation may start in the transplanted organ in CMV-seropositive donors (e.g., CMV pneumonia in lung transplant, enteritis in small-bowel transplant). Hepatitis, retinitis, and, rarely, central nervous system (CNS) involvement (myelopathy, encephalopathy) may also occur. Frequency of CMV monitoring after prophylaxis or treatment is completed is not well established. If available, immune monitoring, via commercially available CMV-specific interferon γ (IFN-γ) release assays that detect CMV-specific T cells in whole blood, may allow for more precise targeting of monitoring and prophylactic strategies. ■ ■EPSTEIN-BARR VIRUS Most adults are infected with EBV, which remains latent in B cells. When an EBV-seropositive donor provides an allograft to an EBVseronegative recipient (D+/R–), EBV infection (detection of EBV in blood or plasma) is very common. This primary EBV infection may be asymptomatic, present with end-organ disease such as hepatitis or enteritis, or cause a mononucleosis-like febrile illness with or without lymphadenopathy, atypical lymphocytes, and culture-negative exuda­ tive pharyngitis. EBV-associated PTLD is a heterogeneous group of clinical syndromes associated with uncontrolled lymphoproliferation (typically of B cells, but cases of natural killer– and T-cell EBV-positive PTLD occur), which ranges from polyclonal proliferation to true lymphoma containing clonal chromosomal abnormalities. The diag­ nosis requires tissue examination, which may show a polymorphic or monomorphic proliferation of B cells or even frank lymphoma. Risk of PTLD after SOT is greater in EBV-seronegative transplant recipients, who acquire a primary infection through the allograft. Consequently, this scenario is most common in pediatric transplant recipients. Viral burden within the lymphoid tissue of the transplanted organ appears to be a risk factor for PTLD, with intestinal transplant recipients at the highest risk and kidney lowest. EBV-positive PTLD is most frequent in the first year after SOT, but late PTLD incidence increases after 7–10 years and is related to duration of immunosuppression. Late PTLD can present as primary CNS lymphoma and gastrointestinal tract disease. EBV viral load monitoring and radiologic evaluation can assist in early diagnosis. Positron emission tomography/computed tomography (CT) imaging is useful in establishing lymph node or end-organ disease involvement, thus allowing for targeted tissue sampling to establish the diagnosis. In SOT, reduction of immunosuppression is the preferred initial treatment strategy. Rituximab is recommended next if the PTLD cells express CD20. Cytotoxic chemotherapy may be required. For refractory cases, adoptive immunotherapy with EBV-specific T cells, including chimeric antigen receptor T cells and third-party trained T cells, such as tabelecleucel, have been used. All donors and recipients should undergo EBV serologic testing before transplant. Preemptive monitor­ ing of EBV viral load in high-risk (EBV D+/R–) SOT recipients is sug­ gested, and reduction of immunosuppression is recommended in EBV viremic patients. Withdrawal of immunosuppression may trigger graft rejection. Specific viral load cutoffs that should trigger an intervention have not been established; there is considerable interlaboratory vari­ ability even with commercial EBV PCR assays. Serial monitoring with the same assay (e.g., whole blood or plasma) and the same laboratory is necessary to establish a trend. Rapidly rising EBV viral load is a sig­ nificant risk factor for PTLD. EBV-positive PTLD is an early complication of allo-HCT, occurring in the first 2–4 months. In this case, the virus is usually present in the recipient, and the risk is highest in EBV D–/R+ pair and cord blood stem cell source, although any D/R mismatch (D+/R− or D−/R+) in the setting of T-cell-depleted grafts is recognized as a significant risk. EBV D/R matching is recommended whenever possible. The overall frequency is ~3%, varying between 1% in matched sibling donor trans­ plant to up to 10% in unrelated UCB transplants. The risk depends on the degree of T-cell depletion because it is the T-cell function after transplant (derived from the donor) that will control EBV. The man­ agement is like that described above, with the caveats that decreasing

immunosuppression after allo-HCT may be difficult or impossible if GVHD is present and that, if the stem cell donor is available and is EBV positive, administration of lymphocytes from the donor (donor lymphocyte infusion) may be a successful strategy, as a significant fraction of the T-cell repertoire is directed against EBV. Donor-derived adoptive immunotherapy with EBV-specific T cells or off-the-shelf commercially available EBV-specific T cells, such as tabelecleucel, are increasingly used as second-line therapy.

■ ■POLYOMAVIRUSES Polyomaviruses have been identified as frequent causes of allograft dysfunction in kidney transplant recipients. BK virus (BKV) and JC virus (JCV) are the most common pathogenic poliomaviruses that infect humans. BKV (most commonly) and JCV have been associated with tubulointerstitial nephritis and nephropathy, usually referred to as polyomavirus-associated nephropathy (PVAN). Ureteric stenosis and hemorrhagic cystitis can also occur. BKV infection is most common in kidney transplant recipients. Viruria is common, but some patients develop viremia, which in the absence of an intervention results in PVAN and allograft loss. The presumptive diagnosis of BKV nephropa­ thy can be established with renal dysfunction and viremia, but acute rejection and PVAN can coexist and can be clinically indistinguishable. Definitive histopathologic diagnosis requires identification of viral cytopathic changes and immunohistochemistry for SV40 antigen or BKV-specific antigen detection in the tissue. Reduction of immuno­ suppression is the mainstay of treatment. Preemptive monitoring of BKV viral load can aid in the early detection of BK viremia; increasing viral loads are predictive of PVAN, and careful reduction of immuno­ suppression can prevent renal dysfunction and graft loss. CHAPTER 148 BKV after allo-HCT was discussed earlier, in the section on early infections after HCT, above. After PVAN, PML caused by JCV CNS infection is the most com­ mon disease caused by polyomaviruses in SOT, although it is signifi­ cantly less common than in other immunocompromised patients, such as those with advanced HIV. Diagnosis is established by a compatible neurologic syndrome, molecular detection of JC virus in the CSF, and radiologic evidence of demyelinating disease. As with other polyoma­ virus infections, no effective antiviral therapy exists, and reduction of immunosuppression is commonly employed, with overall poor response rates. Directed T-cell therapies are under development, and immune checkpoint inhibition (e.g., pembrolizumab) has been used successfully in some cases. Infections in Transplant Recipients ■ ■FUNGAL INFECTIONS Invasive fungal infections are a common infectious complication in transplant recipients. Newer antifungals and corticosteroid-sparing immunosuppressive regimens have decreased the overall incidence of invasive fungal infections and improved outcomes. Invasive candidiasis is the most common invasive fungal infection in SOT recipients. Other fungal infections such as invasive aspergillosis, cryptococcosis, and endemic mycoses are also prevalent. In HCT, almost universal antifun­ gal prophylaxis with activity against Candida has made mold infections relatively more frequent. Candidiasis  Invasive candidiasis is most often an early compli­ cation after SOT. Length of hospitalization, frequent use of broadspectrum antimicrobials, intravascular catheters, renal replacement therapy, and critical illness are all risk factors for increased incidence of invasive candidiasis. Additionally, the complex nature of abdominal organ transplant surgical procedures increases the risk of invasive can­ didiasis. In liver transplant recipients, choledochojejunostomy, largevolume transfusion of cellular blood products (including platelets and packed red blood cells), reoperation, and retransplantation are all risk factors for invasive candidiasis. Pancreas transplantation with enteric drainage and small-bowel transplantation are particularly prone to be complicated by candidemia or invasive candidiasis. Targeted antifungal prophylaxis in liver, pancreas, and small-bowel transplant recipients is routine and decreases the incidence of invasive candidiasis. All forms of invasive candidiasis in transplant recipients merit antifungal treatment, and echinocandins are the initial treatment of

choice, except in CNS disease or urinary tract infection. In addition to antifungal treatment, source control, such as catheter removal, and mechanical drainage of abdominal or surgical collections are the mainstay of successful treatment. Lung transplant recipients are at risk of invasive disease at the site of tracheal anastomosis, which can lead to anastomotic leak. Mycotic aneurisms of the main vessel of the allograft are associated with a high risk of rupture and usually represent a DDI or result from contamination of the preservation fluid. Early diagnosis and aggressive medical and surgical treatment are indispensable for overall survival.

Aspergillus and Other Molds  Invasive pulmonary aspergillosis is the most common invasive mold infection in transplant recipients and the overall most common invasive fungal infection in lung trans­ plant recipients. Airway and allograft colonization with Aspergillus spp. is a common but also invasive disease. Lung transplant recipients experience the full spectrum of Aspergillus spp. pathology, including tracheobronchitis, bronchial anastomotic infections, invasive pulmo­ nary aspergillosis, and disseminated disease. Tracheobronchitis and bronchial anastomotic infections are characterized by ulceration and cartilage invasion. Dissemination, including to the brain, can occur and should be ruled out in all transplant recipients. Risk factors are single-lung transplant, lung transplant for cystic fibrosis, airway isch­ emia, rejection with increased immunosuppression, and chronic lung allograft dysfunction (CLAD). Risk factors for invasive aspergillosis (IA) in other SOTs include CMV infection and disease and degree of immunosuppression, such as high-dose corticosteroids for rejection. Retransplantation and posttransplant liver or renal hepatic failure, as well as prolonged neutropenia, increase the overall risk of IA. Targeted antimold prophylaxis in those at high risk of IA is recom­ mended. Monitoring serum galactomannan (GMN) does not perform well in SOT recipients. Targeted diagnostic bronchoalveolar lavage (BAL) GMN and serum GMN are preferred when there is suspicion of invasive disease. CT imaging can assist in the early diagnosis of IA. The CT findings are variable (Fig. 148-1). Whereas dense, wellcircumscribed pulmonary nodules, sometimes with halo sign, are char­ acteristic in patients with hematologic malignancies, SOT recipients more frequently present with nonspecific pulmonary findings such as alveolar and centrilobular infiltrates, and the halo sign is almost never seen. Despite early diagnosis and treatment, the mortality associated with IA is still high. Other mold infections in SOT are less frequent but significantly more common than in nontransplant populations. PART 5 Infectious Diseases Aspergillosis after HCT is a complication of prolonged neutropenia or the use of high-dose corticosteroids for the treatment of GVHD. Prophylaxis with posaconazole is frequently used in patients receiving

0.5 mg/kg of prednisone every day or being treated for GVHD with two or more immunosuppressants. Some of the newer treatments for GVHD, such as ibrutinib and ruxolitinib, seem to significantly increase the risk of IA. Presentation is nonspecific with dyspnea, cough, and chest pain. Fever may be absent. Lack of systemic toxicity is frequent A B C FIGURE 148-1  Radiologic findings are often nonspecific in transplant recipients. A. Nocardia 3 months after kidney transplant in a patient who was not receiving trimethoprim-sulfamethoxazole. A fine-needle aspirate showed only necrosis, but the bronchoalveolar lavage (BAL) grew Nocardia cyriacigeorgica. B. Tuberculosis 3 months after heart transplant in a patient with known latent tuberculosis infection who was receiving isoniazid prophylaxis. The isolate of Mycobacterium tuberculosis was resistant to isoniazid. C. Aspergillosis in a neutropenic patient receiving allogeneic hematopoietic stem cell transplant for acute myelogenous leukemia. The BAL was positive for galactomannan, but the culture was negative. The mold was later identified as a non-fumigatus Aspergillus by next-generation sequencing of a surgical specimen.

in mold infections. Establishing an accurate diagnosis is essential, since most mold infections can have similar presentations, and antifungal susceptibility varies significantly between molds. Cryptococcosis  With prolonged immunosuppression, SOTs are at risk of cryptococcal infection and disease. These tend to occur later after SOT, with most cases occurring between 1 and 3 years after the transplant. Earlier presentation should alert to the possibility of donorderived cryptococcosis. Advanced liver disease is a risk factor for cryptococcosis, and liver transplant recipients may present earlier after transplant, especially if mental status changes peri-transplant were attributed to hepatic encephalopathy but no microbiologic workup was completed. Other presentations include skin papules or cellulitis, pneumonia, and asymptomatic or minimally symptomatic lung nod­ ules. Both Cryptococcus neoformans and Cryptococcus gattii are known pathogens in SOT recipients. Serum cryptococcal antigen assay allows early noninvasive diagnosis but requires a high index of suspicion. In patients with CNS disease, both serum and CSF cryptococcal antigen are extremely useful in the early diagnosis of cryptococcal disease. In the evaluation of patients with mental status changes with or without fever or with cryptococcal disease outside of the CNS, patients should undergo a lumbar puncture with CSF opening pressure, as well as cell count and culture. These are useful for the diagnosis of CSF involve­ ment, but also to evaluate the appropriate treatment response. Moni­ toring of cryptococcal antigen to determine the duration of therapy is not recommended because it may persist despite microbiologic and clinical resolution. Immune reconstitution inflammatory syndrome has been described in SOT recipients when immunosuppression is tapered aggressively in cryptococcal meningitis and may result in worsening of the initial symptoms despite microbiologic cure. Cryptococcosis is less common after allo-HCT, probably due to the prevalence of antifungal prophylaxis. Endemic Mycoses  SOT recipients are second only to people with advanced HIV in terms of risk of disseminated endemic mycoses. Given its worldwide endemicity, histoplasmosis is the most common, but disseminated coccidioidomycosis, in particular CNS disease, is associated with higher mortality, and patients who survive usually require lifelong antifungal therapy. Invasive forms of all endemic myco­ ses have been described in transplant recipients. Pneumocystis jirovecii  PJP is an important complication of SOT and allo-HCT, and prophylaxis early after transplant is used routinely. Breakthroughs are uncommon if patients are receiving TMP-SMX but may be seen with second-line prophylaxis such as inhaled pentamidine or atovaquone. Lung transplant recipients retain a lifelong risk of infec­ tion. Acute or subacute shortness of breath, with significant hypox­ emia, is the most common presentation. Radiologic changes may be subtle and out of proportion to the hypoxemia, with a broad alveolararterial gradient. Many radiologic presentations have been described, but the most common are ground-glass opacities. Nodules and atypical

consolidations have also been described, especially in patients who receive less effective prophylactic agents, such as inhaled pentamidine or atovaquone. Most patients with PJP have elevated β-d-glucan in serum. The definitive diagnosis is established by identification of Pneu­ mocystis trophozoites in a respiratory sample from an induced sputum, a BAL, or a lung biopsy, but microscopy has low sensitivity, and increas­ ingly, PCR in the BAL has become the diagnostic modality of choice. A positive PCR requires clinical interpretation because some patients may be merely colonized. During treatment, an immune reconstitution inflammatory syndrome (IRIS)-like condition can occur, so tapering of immunosuppression is usually not advised in transplant recipients newly diagnosed with PJP. The use of adjuvant corticosteroids in nonHIV patients with severe PJP remains controversial. ■ ■MYCOBACTERIAL INFECTIONS Mycobacterium tuberculosis  TB remains a major challenge and public health threat in transplant recipients. Incidence of active TB among SOT recipients is estimated to be 20–75 times higher than in the general population. Incidence varies significantly among different geographic regions and ethnic groups. Most cases occur between 6 and 12 months after the transplant and are associated with very high mortality; many more cases result in graft loss. Donor-derived TB has been also well documented after SOT, and these cases can present earlier after transplant. In contrast, TB after HCT is relatively uncom­ mon and presents early within the first year after allo-HCT. In areas of medium or high endemicity, cases of active TB develop most com­ monly from reactivation of old foci of infection, but primary infection after transplant can also occur. Disseminated disease is common, and fever is the most common presenting symptom (>90%). In those with only pulmonary involvement, fever is present in ~60%. Involvement of every organ and atypical presentations have been reported (Fig. 148-1). Good-quality data regarding the optimal combinations of drugs and length of therapy are lacking in organ transplant recipients. Decisions are based on case series, recommendations for the general population and other immunocompromised individuals such as people living with HIV, and expert opinion. Most cases require 6–9 months of treatment, but in disseminated disease with CNS involvement, extrapulmonary TB, cavitary TB, or culture positivity after 2 months of treatment, 12–18 months of treatment are recommended. Daily therapy is recom­ mended over intermittent dosing, given signals of higher relapse rates with intermittent dosing and concerns of less reliable immunosup­ pressive drug levels due to drug–drug interactions that may ultimately result in higher rejection rates or graft loss. Rifamycin-containing regimens are preferred for all types of TB due to the potent sterilizing activity of such regimens. Rifampin, the most used rifamycin, is a potent inducer of the cytochrome P450 enzyme, which increases the metabolism of many drugs, including calcineurin inhibitors (cyclo­ sporine and tacrolimus), mTOR inhibitors (sirolimus and everolimus), and corticosteroids; its use has been frequently associated with acute rejection. Rifampin-based treatment is extremely problematic. Even though there is less experience with the use of rifabutin and it is not available worldwide, rifabutin is a weaker inducer of cytochrome p450, and drug interactions are significantly easier to manage than with rifampin. Other rifamycin-sparing treatment protocols can be used in organ transplant recipients to avoid drug–drug interactions; however, when a rifamycin is not used, shorter courses (6–9 months) should not be used. Use of a rifamycin is also highly desirable in patients with severe, disseminated, or cavitary disease. The initial treatment regimen should follow local recommendations based on local resistance pat­ terns and tailored to available susceptibility data. Newer antitubercu­ lous antimicrobials have not been studied in transplant recipients but are promising. Reduction of immunosuppressive therapies, which help control the infection, should be done slowly if at all, as IRIS is a wellknown complication of mycobacterial disease. If symptoms worsen with lowering immunosuppression, IRIS should be considered. Prevention of TB after transplantation relies on adequately iden­ tifying those at risk of reactivation or recipients of an organ from a donor who has latent TB infection (LTBI) and treating LTBI. Screening

relies on known epidemiologic exposures and probing the individual’s immune response to TB antigens. There are two methods available for screening for prior TB exposure (for additional information, see Chap. 183), the tuberculin skin test (TST) and the IFN-γ release assays (IGRAs); both assays are less sensitive in immunocompromised hosts. In transplant candidates or recipients, a TST >5 mm is consistent with LTBI. Patients with prior positive bacillus Calmette-Guérin vaccina­ tion can have a positive TST from prior vaccination. In these patients, IGRA is preferable. End-stage renal disease, end-stage liver disease, and receipt of prior chemotherapies are all well-known risk factors that can lead to a false-negative TST or indeterminate results in the IGRAs. In patients with indeterminate results, the decision to treat LTBI should be made clinically, according to epidemiologic risk factors based on history. In patients with a negative TST, a second TST should be performed at about 2 weeks to look for a booster effect. An IGRA test can be performed after an initial negative TST in those at risk. In patients with evidence of LTBI by TST or IGRA, active TB should be ruled out before proceeding to transplant. Live donors should undergo a similar screening as recipients. TST and IGRAs may not be avail­ able on deceased donors; therefore, risk should be assessed based on a detailed history from the donor’s family regarding previous active or latent TB and any treatment received and chest imaging.

All transplant recipients with LTBI should receive treatment. Con­ sideration should be given to time to transplantation, drug–drug inter­ actions with ongoing treatments, and potential toxicities. In patients in whom the transplant is unlikely to occur until at least 4–6 months, shorter courses are suggested (e.g., rifampin for 4 months or isoniazid plus rifapentine for 12 weeks). If the transplant is likely to occur sooner, isoniazid for 9 months is recommended. When possible, LTBI should be treated prior to transplantation; however, the transplant should not be delayed to complete treatment. For liver transplant candidates or HCT candidates receiving chemotherapy, treating prior to transplanta­ tion is challenging due to potential hepatoxicity or significant drug– drug interactions. In these patients, it may be safer to begin therapy after transplantation once liver function has normalized or the con­ ditioning regimen has been completed. SOT recipients who received an organ from an untreated donor with known LTBI should also be treated. If a living donor has LTBI, treatment of the donor should pri­ oritize their risk of progression to active TB following national guide­ lines for the treatment of LTBI. CHAPTER 148 Infections in Transplant Recipients Nontuberculous Mycobacteria  Nontuberculous mycobacteria (NTM) are ubiquitous in the environment, and some species are important opportunistic human pathogens. Transplant recipients are at increased risk of NTM infection. The lungs are the most common site of infection and the most frequently colonized organ. Disease due to NTM can be separated into pulmonary, disseminated, catheter- and cardiac implantable electronic device–associated infection, surgical site deep-seated infection, and skin and soft tissue (SST) infections. Lung followed by heart transplant recipients are at highest risk of post­ transplant NTM disease. Because structural lung disease increases the risk of colonization, allo-HCT recipients with cGVHD of the lungs are also at risk. Lung transplant candidates with cystic fibrosis or chronic obstructive pulmonary disease are at increased risk of being colonized or infected by NTMs, and pretransplant isolation of NTM is associated with increased risk of NTM disease after transplant. NTM disease is associated with increased mortality and CLAD. Most common infections are due to Mycobacterium abscessus and M. avium complex. Diagnosis of NTM disease from nonsterile sites, especially the respiratory tract, can be a challenge. Validation of criteria suggested for the routine diagnosis of pulmonary NTM in immunocompetent individuals is lacking in transplant recipients; however, these provide a useful framework for the diagnosis of pulmonary NTM in transplant recipients. In lung transplant recipients, the threshold for treatment of infection should be lower, given the high rate of infection, secondary dissemination, or development of CLAD. Treatment of NTM is com­ plex and often compromised due to drug–drug interaction and drug toxicities. Species-level identification is necessary to design a treat­ ment regimen. For detailed treatment strategies, the reader is referred

to Chap. 185 and American Thoracic Society/British Thoracic Society guidelines. The length of treatment should be guided by the specific clinical scenario; localized SST infections require shorter treatment durations, whereas pulmonary or disseminated NTMs require at least 1 year (frequently longer).

All patients with NTM infection pretransplant should start treat­ ment before transplantation. Routine screening for NTM infection in lung transplant candidates is recommended, and those found to be colonized or infected should be treated, ideally for at least 3 months prior to proceeding to transplant. Rapidly growing NTMs, especially M. abscessus, have been associated with a high risk of deep surgical site infection and with frequent bone involvement. Treatment regimens should be based on species identification and susceptibility testing when appropriate, as well as stabilization of disease before transplant and ability to tolerate the designed regimen. Therapeutic drug moni­ toring can help to reduce toxicity. Bilateral lung transplant is preferred in those already colonized or infected with NTMs prior to transplant. Despite these measures, the risk of posttransplant infection is high, and the patient should be counseled about this risk. Some PIDs that result in susceptibility to mycobacterial infection present with disseminated NTM or TB. Without successful replace­ ment of the deficient immune system via allo-HCT, these infections are difficult to treat. Management of these infections is challenging; aggressive antimicrobial treatment is required before, during, and early after transplant. Ideally, infection and associated inflammatory response should be partially controlled prior to transplantation, which usually requires at least 3 months of treatment before the starting con­ ditioning. Drug–drug interactions and drug toxicities in the peri- and posttransplant period are a challenge. In the first few months after engraftment, IRIS-like phenomena are common in patients being treated for active mycobacterial infection at the time of transplantation. PART 5 Infectious Diseases Definitive diagnosis of IRIS is a challenge, as there are no clear diagnostic criteria for this disease. Classically, there is worsening of symptoms or imaging at the site of prior infection that coincides with tapering of immune suppression. Adequate management requires ongoing treatment of the underlying infection, and augmented immu­ nosuppression or anti-inflammatory therapy. Once symptoms are TABLE 148-3  Prophylactic Regimens Commonly Used to Decrease Risk of Infection in Transplant Recipients RISK FACTOR ORGANISM PROPHYLACTIC DRUG EXAMINATION(S)a Neutropenia, mucositis Candida Fluconazole Candida is part of the normal GI flora; everyone with disruption of the mucosal integrity is at risk Prolonged neutropenia, high-dose corticosteroids, enhanced immunosuppression Aspergillus and other molds Posaconazole Blood tests (GMN, β-d-glucan) are less sensitive if triazoles are being administered and are always negative in mucormycosis Travel to or residence in an area with known risk of endemic fungal infection Histoplasma, Blastomyces, Coccidioides, Talaromyces marneffei Triazoles considered in context of clinical and laboratory assessment Chronic hepatitis B HBV Entecavir HBV serology, HBV DNA Chronic hepatitis C HCV Prophylaxis not used; treat to achieve persistent virological response Latent herpesviruses HSV, VZV, significantly lower efficacy for CMV, EBV Acyclovir or valacyclovir Serologic tests for HSV, VZV, CMV, HHV-6, EBV, KSHV (HHV-8); PCR   CMV Letermovir (more experience in HCT) or valganciclovir (active against HSV and VZV) Exposure (unknown reservoir) or colonization Pneumocystis jirovecii Trimethoprim-sulfamethoxazole (TMP-SMX) Second line: dapsone, pentamidine, atovaquone Parasites Toxoplasma gondii Strongyloides stercoralis TMP-SMX or atovaquone for toxoplasma Ivermectin for Strongyloides LTBI Mycobacterium tuberculosis Isoniazid or rifampin in patients with recent seroconversion, positive chest imaging, or recent known exposure and no previous treatment, once active TB is ruled out aSerologic examination, TST, and IGRAs may be less reliable after transplantation. Abbreviations: CMV, cytomegalovirus; EBV, Epstein-Barr virus; GI, gastrointestinal; GMN, galactomannan; HBV, hepatitis B virus; HCT, hematopoietic stem cell transplantation; HCV, hepatitis C virus; HHV, human herpesvirus; HSV, herpes simplex virus; IGRA, interferon γ release assay; KSHV, Kaposi’s sarcoma–associated herpesvirus; LTBI, latent TB infection; PCR, polymerase chain reaction; TB, tuberculosis; TST, tuberculin skin test; VZV, varicella-zoster virus.

controlled, tapering of immunosuppression should be done slowly, taking into consideration other potential toxicities of ongoing immu­ nosuppression and predisposition to other opportunistic infections. ■ ■NOCARDIOSIS Nocardia is an aerobic gram-positive bacillus found in the soil that infects predominantly immunocompromised people. It is more fre­ quently acquired by inhalation, although skin inoculation also occurs (typical with Nocardia braziliensis). The presentation varies from acute with fever, cough, and dyspnea, to subacute or chronic with night sweats and weight loss. Radiologic findings are commonly dense nod­ ules, sometimes with cavitation, but a variety of pulmonary infiltrates may be seen (Fig. 148-1). Dissemination to the CNS occurs in 25% of cases and is frequently asymptomatic. Brain MRI is recommended in all patients with Nocardia to rule out brain abscess. Antibiotic sus­ ceptibility can be predicted based on species identification, but this may take time. All Nocardia species are susceptible to linezolid, so this antibiotic may be administered empirically until identification and susceptibilities are obtained. Most isolates are also susceptible to TMP-SMX, amikacin, and imipenem or meropenem. Combination of two or more antibiotics and long duration of treatment (usually 6–12 months) is recommended by many experts, but comparative tri­ als are not available. PREVENTION OF INFECTIONS IN TRANSPLANT RECIPIENTS There are three different and complementary approaches to prevent infections after transplant: minimizing exposures (lifestyle modifica­ tions), immunizations, and chemoprophylaxis. Hand hygiene and avoiding sick contacts are some of the most impactful ways to avoid infection. Other suggested lifestyle modifications include recommen­ dations for food and water safety, specifically making sure the drinking water is safe, avoiding raw or poorly cooked eggs and meat, and peeling or carefully washing fruits and vegetables. Hobbies and pets are impor­ tant components of healthy rehabilitation after transplant, but some specific recommendations can make these safer and reduce the risks of infection. Examples of these are to avoid landscaping, gardening, and Chest imaging, antigen testing, serology HCV serology, HCV RNA     Serologic test for Toxoplasma and Strongyloides TST and/or IGRA; if indeterminate, clinical assessment of exposure and risk

29 - SECTION 4 Therapy for Bacterial Diseases

SECTION 4 Therapy for Bacterial Diseases

construction sites or to wear protective gear if this is not possible. Using a mask in encounters with high risk of aerosol transmission is recom­ mended. There is a general recommendation against getting new pets for the first 6–12 months after transplant. Avoiding tick and mosquito bites is particularly important. Different professional societies provide advice to transplant recipients. In the United States, the American Society of Transplantation (AST) (myast.org) and the Centers for Dis­ ease Control and Prevention (cdc.gov) are useful resources. Available and required immunizations vary by country, and recommendations are modified as new vaccines and new data become available. Articles published in 2023 are provided in the “Further Reading” section, but updated information may be found in the AST website, and the CDC also presents the guidelines of the Advisory Committee on Immuniza­ tion Practices. Specific prophylaxes against many infections have been presented in the text; a summary is presented in Table 148-3. ■ ■FURTHER READING Amengual JE, Pro B: How I treat posttransplant lymphoproliferative disorder. Blood 142:1426, 2023. Dadwal SS et al: American Society of Transplantation and Cellular Therapy Series, 2: Management and prevention of aspergillosis in hematopoietic cell transplantation recipients. Transplant Cell Ther 27:201, 2021. Fishman JA: Infection in organ transplantation. Am J Transplant 17:856, 2017. Hakki M et al: American Society for Transplantation and Cellular Therapy Series, 3: Prevention of cytomegalovirus infection and dis­ ease after hematopoietic cell transplantation. Transplant Cell Ther 27:707, 2021. Kaul DR et al: Ten years of donor-derived disease: A report of the dis­ ease transmission advisory committee. Am J Transplant 21:689, 2021. Reynolds G et al: Vaccine schedule recommendations and updates for patients with hematologic malignancy post-hematopoietic cell trans­ plant or CAR T-cell therapy. Transpl Infect Dis 25(suppl 1):e14109, 2023. Stewart AG, Kotton CN: What’s new: Updates on cytomegalovirus in solid organ transplantation. Transplantation 108:884, 2024. Timsit JF et al: Diagnostic and therapeutic approach to infectious diseases in solid organ transplant recipients. Intensive Care Med 45:573, 2019. Viganò M et al: Vaccination recommendations in solid organ trans­ plant adult candidates and recipients. Vaccines (Basel) 11:1611, 2023. Wolfe CR et al: Donor-derived guidelines: Guidelines from the American Society of Transplantation Infectious Diseases Community of Practice. Clin Transplant 33:e13547, 2019. Section 4 Therapy for Bacterial Diseases

Treatment and

Prophylaxis of Bacterial Infections David C. Hooper, Erica S. Shenoy,

Alyssa R. Letourneau, Ramy H. Elshaboury Antimicrobial agents have had a major impact on human health. Together with vaccines, they have contributed to reduced mortality, extended life span, and enhanced quality of life. Among drugs used in human medicine, however, they are distinctive in that their use

promotes the occurrence of drug resistance in the pathogens they are designed to treat as well as in other “bystander” organisms. Indeed, the history of antimicrobial development has been driven in large part by the medical need engendered by the emergence of resistance to each generation of agents. Thus, the careful and appropriate use of antimicrobial drugs is particularly important not only for optimizing efficacy and minimizing adverse effects but also for minimizing the risk of resistance and preserving the value of existing agents. Although this chapter focuses on antibacterial agents, the optimal use of all antimicro­ bials depends on an understanding of each drug’s mechanism of action, spectrum of activity, mechanisms of resistance, pharmacology, and adverse effect profile. This information is applied in the context of the patient’s clinical presentation, underlying conditions, and epidemiology to define the site and likely nature of the infection or other condition and thus to choose the best therapy. Gathering of microbiologic infor­ mation is especially important for refining therapeutic choices based on the documented pathogen and susceptibility data whenever possible; this information also makes it possible to choose more targeted therapy, thereby reducing the risk of selection of resistant bacteria that can occur with use of agents with a broader spectrum of activity than needed for the patient. Durations of therapy are chosen according to the nature of the infection and the patient’s response to treatment and are informed by clinical studies when they are available, with the understanding that shorter courses are less likely than longer courses to promote the emergence of resistance. This chapter and the one that follows provide specific information that is necessary for making informed choices among antibacterial agents. The mechanisms of action of antibacterial agents are discussed in detail in the text of this chapter, and mecha­ nisms of resistance are discussed in detail in Chap. 150. Both types of mechanisms, which are related to each other, are summarized for the most commonly used groups of agents in Table 150-1. A schematic of antibacterial targets is provided in Fig. 150-1.

CHAPTER 149 MECHANISMS OF ACTION

(SEE TABLE 150-1) Multiple essential components of bacterial cell structures and metabo­ lism have been the targets of antibacterial agents used in clinical medicine, and the interaction of an agent with its target results in either inhibition of bacterial growth and replication (bacteriostatic effect) or bacterial killing (bactericidal effect). In general, targets have been cho­ sen because they either do not exist in mammalian cells and physiol­ ogy or are sufficiently different from their mammalian counterparts to allow selective bacterial targeting. Treatment with bacteriostatic agents is effective when the patient’s host defenses are sufficient to contribute to eradication or sufficient reduction of the infecting pathogen. In patients with impaired host defenses (e.g., neutropenia) or infections at body sites with impaired or limited host defenses (e.g., meningitis and endocarditis), bactericidal agents are generally preferred. Treatment and Prophylaxis of Bacterial Infections ■ ■INHIBITION OF CELL WALL SYNTHESIS The bacterial cell wall, which is external to the cytoplasmic membrane and has no counterpart in mammalian cells, protects bacterial cells from lysis under low osmotic conditions. The cell wall is a crosslinked peptidoglycan composed of a polymer of alternating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), four-amino-acid stem peptides linked to each NAM, and a peptide cross-bridge that links adjacent stem peptides to form a net-like structure. Several steps in peptidoglycan synthesis are targets of anti­ bacterial agents. Inhibition of cell-wall synthesis generally results in a bactericidal effect that is linked to cell lysis. This effect results not only from the blocking of new cell-wall formation but from the uninhibited action of cell wall–remodeling enzymes called autolysins, which cleave peptidoglycan as part of normal cell-wall growth and cell division. In gram-positive bacteria, the peptidoglycan is the most external cell structure, but in gram-negative bacteria, an asymmetric lipid outer membrane is external to the peptidoglycan and contains diffusion channels called porins. The space between the outer membrane and the peptidoglycan and cytoplasmic membrane is referred to as the periplasmic space. Most antibacterial drugs enter the gram-negative

30 - 149 Treatment and Prophylaxis of Bacterial Infections

149 Treatment and Prophylaxis of Bacterial Infections

construction sites or to wear protective gear if this is not possible. Using a mask in encounters with high risk of aerosol transmission is recom­ mended. There is a general recommendation against getting new pets for the first 6–12 months after transplant. Avoiding tick and mosquito bites is particularly important. Different professional societies provide advice to transplant recipients. In the United States, the American Society of Transplantation (AST) (myast.org) and the Centers for Dis­ ease Control and Prevention (cdc.gov) are useful resources. Available and required immunizations vary by country, and recommendations are modified as new vaccines and new data become available. Articles published in 2023 are provided in the “Further Reading” section, but updated information may be found in the AST website, and the CDC also presents the guidelines of the Advisory Committee on Immuniza­ tion Practices. Specific prophylaxes against many infections have been presented in the text; a summary is presented in Table 148-3. ■ ■FURTHER READING Amengual JE, Pro B: How I treat posttransplant lymphoproliferative disorder. Blood 142:1426, 2023. Dadwal SS et al: American Society of Transplantation and Cellular Therapy Series, 2: Management and prevention of aspergillosis in hematopoietic cell transplantation recipients. Transplant Cell Ther 27:201, 2021. Fishman JA: Infection in organ transplantation. Am J Transplant 17:856, 2017. Hakki M et al: American Society for Transplantation and Cellular Therapy Series, 3: Prevention of cytomegalovirus infection and dis­ ease after hematopoietic cell transplantation. Transplant Cell Ther 27:707, 2021. Kaul DR et al: Ten years of donor-derived disease: A report of the dis­ ease transmission advisory committee. Am J Transplant 21:689, 2021. Reynolds G et al: Vaccine schedule recommendations and updates for patients with hematologic malignancy post-hematopoietic cell trans­ plant or CAR T-cell therapy. Transpl Infect Dis 25(suppl 1):e14109, 2023. Stewart AG, Kotton CN: What’s new: Updates on cytomegalovirus in solid organ transplantation. Transplantation 108:884, 2024. Timsit JF et al: Diagnostic and therapeutic approach to infectious diseases in solid organ transplant recipients. Intensive Care Med 45:573, 2019. Viganò M et al: Vaccination recommendations in solid organ trans­ plant adult candidates and recipients. Vaccines (Basel) 11:1611, 2023. Wolfe CR et al: Donor-derived guidelines: Guidelines from the American Society of Transplantation Infectious Diseases Community of Practice. Clin Transplant 33:e13547, 2019. Section 4 Therapy for Bacterial Diseases

Treatment and

Prophylaxis of Bacterial Infections David C. Hooper, Erica S. Shenoy,

Alyssa R. Letourneau, Ramy H. Elshaboury Antimicrobial agents have had a major impact on human health. Together with vaccines, they have contributed to reduced mortality, extended life span, and enhanced quality of life. Among drugs used in human medicine, however, they are distinctive in that their use

promotes the occurrence of drug resistance in the pathogens they are designed to treat as well as in other “bystander” organisms. Indeed, the history of antimicrobial development has been driven in large part by the medical need engendered by the emergence of resistance to each generation of agents. Thus, the careful and appropriate use of antimicrobial drugs is particularly important not only for optimizing efficacy and minimizing adverse effects but also for minimizing the risk of resistance and preserving the value of existing agents. Although this chapter focuses on antibacterial agents, the optimal use of all antimicro­ bials depends on an understanding of each drug’s mechanism of action, spectrum of activity, mechanisms of resistance, pharmacology, and adverse effect profile. This information is applied in the context of the patient’s clinical presentation, underlying conditions, and epidemiology to define the site and likely nature of the infection or other condition and thus to choose the best therapy. Gathering of microbiologic infor­ mation is especially important for refining therapeutic choices based on the documented pathogen and susceptibility data whenever possible; this information also makes it possible to choose more targeted therapy, thereby reducing the risk of selection of resistant bacteria that can occur with use of agents with a broader spectrum of activity than needed for the patient. Durations of therapy are chosen according to the nature of the infection and the patient’s response to treatment and are informed by clinical studies when they are available, with the understanding that shorter courses are less likely than longer courses to promote the emergence of resistance. This chapter and the one that follows provide specific information that is necessary for making informed choices among antibacterial agents. The mechanisms of action of antibacterial agents are discussed in detail in the text of this chapter, and mecha­ nisms of resistance are discussed in detail in Chap. 150. Both types of mechanisms, which are related to each other, are summarized for the most commonly used groups of agents in Table 150-1. A schematic of antibacterial targets is provided in Fig. 150-1.

CHAPTER 149 MECHANISMS OF ACTION

(SEE TABLE 150-1) Multiple essential components of bacterial cell structures and metabo­ lism have been the targets of antibacterial agents used in clinical medicine, and the interaction of an agent with its target results in either inhibition of bacterial growth and replication (bacteriostatic effect) or bacterial killing (bactericidal effect). In general, targets have been cho­ sen because they either do not exist in mammalian cells and physiol­ ogy or are sufficiently different from their mammalian counterparts to allow selective bacterial targeting. Treatment with bacteriostatic agents is effective when the patient’s host defenses are sufficient to contribute to eradication or sufficient reduction of the infecting pathogen. In patients with impaired host defenses (e.g., neutropenia) or infections at body sites with impaired or limited host defenses (e.g., meningitis and endocarditis), bactericidal agents are generally preferred. Treatment and Prophylaxis of Bacterial Infections ■ ■INHIBITION OF CELL WALL SYNTHESIS The bacterial cell wall, which is external to the cytoplasmic membrane and has no counterpart in mammalian cells, protects bacterial cells from lysis under low osmotic conditions. The cell wall is a crosslinked peptidoglycan composed of a polymer of alternating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), four-amino-acid stem peptides linked to each NAM, and a peptide cross-bridge that links adjacent stem peptides to form a net-like structure. Several steps in peptidoglycan synthesis are targets of anti­ bacterial agents. Inhibition of cell-wall synthesis generally results in a bactericidal effect that is linked to cell lysis. This effect results not only from the blocking of new cell-wall formation but from the uninhibited action of cell wall–remodeling enzymes called autolysins, which cleave peptidoglycan as part of normal cell-wall growth and cell division. In gram-positive bacteria, the peptidoglycan is the most external cell structure, but in gram-negative bacteria, an asymmetric lipid outer membrane is external to the peptidoglycan and contains diffusion channels called porins. The space between the outer membrane and the peptidoglycan and cytoplasmic membrane is referred to as the periplasmic space. Most antibacterial drugs enter the gram-negative

bacterial cell through a porin channel, since the outer membrane is a major diffusion barrier. Although the peptidoglycan layer is thicker in gram-positive (20–80 nm) than in gram-negative (1 nm) bacteria, peptidoglycan itself constitutes only a limited diffusion barrier for antibacterial agents.

β-Lactams  The β-lactam drugs, including penicillins, cepha­ losporins, monobactams, and carbapenems, target transpeptidase enzymes (also called penicillin-binding proteins [PBPs]) involved in the stem-peptide cross-linking step. Inhibitors of β-lactamases—bacterial enzymes that can degrade β-lactams—are used in combination with some β-lactams to expand their spectrum of activity. Glycopeptides and Lipoglycopeptides  The glycopeptides, including vancomycin and teicoplanin, and the lipoglycopeptides, including telavancin, dalbavancin, and oritavancin, bind the two terminal d-alanine residues of the stem peptide, hindering the glyco­ syltransferase involved in polymerizing NAG–NAM units as well as transpeptidases. Vancomycin also binds to the lipid II intermediate that delivers cell-wall precursor subunits. The additional binding of teicoplanin, telavancin, dalbavancin, and oritavancin to the bacterial cytoplasmic membrane contributes to their increased potency. Both β-lactams and glycopeptides interact with their targets external to the cytoplasmic membrane. Bacitracin (Topical) and Fosfomycin  These agents interrupt enzymatic steps in the production of peptidoglycan precursors in the cytoplasm. ■ ■INHIBITION OF PROTEIN SYNTHESIS Most inhibitors of bacterial protein synthesis target bacterial ribo­ somes, whose differences from eukaryotic ribosomes allow selective antibacterial action. Some inhibitors bind to the 30S ribosomal sub­ unit and others to the 50S subunit. Most protein synthesis–inhibiting agents are bacteriostatic; aminoglycosides are an exception and are bactericidal. PART 5 Infectious Diseases Aminoglycosides  Aminoglycosides (amikacin, gentamicin, kana­ mycin, netilmicin, streptomycin, tobramycin, and plazomicin) bind irreversibly to 16S ribosomal RNA (rRNA) of the 30S ribosomal subunit, blocking the translocation of peptidyl transfer RNA (tRNA) from the A (aminoacyl) to the P (peptidyl) site and, at low concentra­ tions, causing misreading of messenger RNA (mRNA) codons, and thus cause the introduction of incorrect amino acids into the peptide chain; at higher concentrations, translocation of the peptide chain is blocked. Cellular uptake of aminoglycosides is dependent on the elec­ trochemical gradient across the bacterial membrane. Under anaerobic conditions, this gradient is reduced, with a consequent reduction in the uptake and activity of the aminoglycosides. Spectinomycin is a related aminocyclitol antibiotic that also binds to 16S rRNA of the 30S ribo­ somal subunit but at a different site. This drug inhibits translocation of the growing peptide chain but does not trigger codon misreading and produces only a bacteriostatic effect. Tetracyclines  Tetracyclines (doxycycline, minocycline, tetracy­ cline) bind reversibly to the 16S rRNA of the 30S ribosomal subunit and block the binding of aminoacyl tRNA to the ribosomal A site, thereby inhibiting peptide elongation. Active transport of tetracyclines into bacterial but not mammalian cells contributes to the selectivity of these agents. Tigecycline, a derivative of minocycline and the only available glycylcycline, acts similarly to the tetracyclines but is distinctive for its ability to circumvent the most common mechanisms of resistance to the tetracyclines. Other new tetracycline derivatives—eravacycline, a fluorocycline, and omadacycline, an aminomethylcycline—like tigecy­ cline are notable for being little affected by prior common tetracycline resistance mechanisms. Macrolides and Ketolides  In contrast to the aminoglycosides and tetracyclines, the macrolides (azithromycin, clarithromycin, and erythromycin) and ketolides (telithromycin) bind to the 23S rRNA of the 50S ribosomal subunit. These agents block translocation of the

growing peptide chain by binding to the tunnel from which the chain exits the ribosome. Lincosamides  Clindamycin is the only lincosamide in clinical use. It binds to the 23S rRNA of the 50S ribosomal subunit, interacting with both the ribosomal A and P sites and blocking peptide bond formation. Streptogramins  The only streptogramin in clinical use is a com­ bination of quinupristin, a group B streptogramin, and dalfopristin, a group A streptogramin. Both components bind to 23S rRNA of the 50S ribosome: dalfopristin binds to both the A and P sites of the peptidyl transferase center, and quinupristin binds to a site that overlaps the macrolide-binding site, blocking the emergence of nascent peptide from the ribosome. The combination is bactericidal, but macrolideresistant bacteria exhibit cross-resistance to quinupristin, and the remaining activity of dalfopristin alone is only bacteriostatic. Chloramphenicol  Chloramphenicol binds reversibly to the 23S rRNA of the 50S subunit in a manner that interferes with the proper positioning of the aminoacyl component of tRNA in the A site. This site of binding is near those of the macrolides and lincosamides. Oxazolidinones  Linezolid and tedizolid are the only oxazolidi­ nones in clinical use. They bind directly to the A site in the 23S rRNA of the 50S ribosomal subunit and block binding of aminoacyl tRNA, inhibiting the initiation of protein synthesis. Pleuromutilins  Lefamulin is the only systemic pleuromutilin in clinical use. It binds to the peptidyl transferase center of the 50S ribo­ somal subunit and prevents the correct positioning of tRNAs, thereby inhibiting peptide bond formation and protein synthesis. Mupirocin  Mupirocin (pseudomonic acid) is used topically. It competes with isoleucine for binding to isoleucyl tRNA synthetase, depleting stores of isoleucyl tRNA and thereby inhibiting protein synthesis. ■ ■INHIBITION OF BACTERIAL METABOLISM Available inhibitors (antimetabolites) target the pathway for synthesis of folate, which is a cofactor in a number of one-carbon transfer reac­ tions involved in the synthesis of some nucleic acids, including the pyrimidine thymidine and all purines (adenine and guanine), as well as some amino acids (methionine and serine) and acetyl coenzyme A. Two sequential steps in folate synthesis are targeted. The selective antibacterial effect stems from the inability of mammalian cells to syn­ thesize folate; they depend instead on exogenous sources. Antibacterial activity, however, may be reduced in the presence of high exogenous concentrations of the end products of the folate pathway (e.g., thymi­ dine and purines) that may occur in some infections, resulting from local breakdown of leukocytes and host tissues. Sulfonamides  Sulfonamides, including sulfadiazine, sulfisoxazole, and sulfamethoxazole, inhibit dihydropteroate synthetase (DHPS), which adds p-aminobenzoic acid (PABA) to pteridine, producing dihy­ dropteroate. Sulfonamides are structural analogues of PABA and act as competing enzyme substrates. Trimethoprim  Subsequent steps in folate synthesis are catalyzed by dihydrofolate synthase, which adds glutamate to dihydropteroate, and dihydrofolate reductase (DHFR), which then generates the final product, tetrahydrofolate. Trimethoprim is a structural analogue of pteridine and inhibits DHFR. Trimethoprim is available alone but is most often used in combination products that also contain sulfa­ methoxazole and thus block two sequential steps in folate synthesis. ■ ■INHIBITION OF DNA AND RNA SYNTHESIS OR ACTIVITY A variety of antibacterial agents act on these processes. Quinolones  The quinolones include nalidixic acid, the first agent in the class, and newer, more widely used fluorinated derivatives (fluoroquinolones), including norfloxacin, ciprofloxacin, levofloxacin, moxifloxacin, gemifloxacin, and delafloxacin. The quinolones are

synthetic compounds that inhibit bacterial DNA synthesis by interact­ ing with the DNA complexes of two essential enzymes, DNA gyrase and DNA topoisomerase IV, which alter DNA topology. Quinolones trap enzyme–DNA complexes in such a way that they block movement of the DNA replication apparatus and can generate lethal double-strand breaks in DNA, resulting in bactericidal activity. Although mammalian cells also have type II DNA topoisomerases related to gyrase and topoi­ somerase IV, the structures of the mammalian enzymes are sufficiently different from those of the bacterial enzymes that quinolones have substantially selective antibacterial activity. Rifamycins  Rifampin, rifabutin, and rifapentine are semisynthetic derivatives of rifamycin B and bind the β subunit of bacterial RNA polymerase, thereby blocking elongation of mRNA. Their action is highly selective for the bacterial enzyme over mammalian RNA polymerases. Nitrofurantoin  The reduction of nitrofurantoin, a nitrofuran com­ pound, by bacterial enzymes produces highly reactive derivatives that are thought to cause DNA strand breakage. Nitrofurantoin is used only for the treatment of lower urinary tract infections. Metronidazole  Metronidazole is a synthetic nitroimidazole with activity limited to anaerobic bacteria and certain anaerobic proto­ zoa. Reduction of its nitro group by the electron-transport system in anaerobic bacteria produces reactive intermediates that damage DNA and result in bactericidal activity. Both nitrofurantoin and metronida­ zole have selective antibacterial activity because the reducing activity needed to produce active derivatives is generated only by bacterial and not mammalian enzymes. ■ ■DISRUPTION OF MEMBRANE INTEGRITY The integrity of the bacterial cytoplasmic membrane—and, in gramnegative bacteria, the outer membrane—is important for bacterial viability. Two bactericidal drugs have membrane targets. Polymyxins  The polymyxins, including polymyxin B and poly­ myxin E (colistin), are cationic cyclic polypeptides that disrupt the cytoplasmic membrane and the outer membrane (the latter by binding lipopolysaccharides, which are negatively charged). Daptomycin  Daptomycin is a lipopeptide that binds the cytoplas­ mic membrane of gram-positive bacteria in the presence of calcium, generating a channel that leads to leakage of cytoplasmic potassium ions and membrane depolarization. PHARMACOKINETICS AND PHARMACODYNAMICS The term pharmacokinetics describes the disposition of a drug in the human body, whereas pharmacodynamics describes the drug action on the pathogen in relation to pharmacokinetic factors. An understand­ ing of the principles governing these two areas is required for effective drug selection, dosing, and prevention of toxicities. ■ ■PHARMACOKINETICS The process of drug disposition consists of four principal phases: absorption, distribution, metabolism, and excretion. These phases determine the time course of drug concentrations in serum, tissues, and body fluids. Absorption  When a drug is administered, absorption is defined as the percentage of the dose that reaches the vasculature. The fraction of a drug, however, that reaches the systemic circulation or the phar­ macologic site of action is termed bioavailability. The bioavailability is more relevant when non-IV routes are used—e.g., the oral, IM, SC, and topical routes. For example, since IV administration provides direct access to the systemic circulation, the bioavailability is therefore 100%. IV and oral dosing for highly bioavailable agents can result in equivalent systemic and tissue concentrations; examples of such agents include metronidazole, fluoroquinolones, tetracyclines, and linezolid. Additionally, many factors can influence oral bioavailability, including

the timing of food consumption relative to drug administration, drugmetabolizing enzymes, efflux transporters, concentration-dependent solubility, and acid degradation. Underlying conditions such as diar­ rhea or ileus can also affect the site of drug absorption and thereby alter its bioavailability. Certain orally administered drugs may have lower bioavailability because of the first-pass effect—the process by which drugs are absorbed in the small intestine through the portal circulation and directly transported to the liver for metabolism before reaching their intended site of action.

Distribution  Distribution describes the process of drug transfer reversibly between the general circulation and body tissues and fluids. After absorption into the systemic circulation and the central compart­ ment (the extensively perfused organs), the drug also distributes into the peripheral compartment (less well-perfused tissues). The volume of distribution (Vd) is a pharmacokinetic parameter that describes the amount of drug in the body at a given time relative to the measured serum concentration. Properties such as the drug’s lipophilicity, parti­ tion coefficient within different body tissues, protein binding, blood flow, penetration of the blood-brain barrier, and pH can affect the Vd and subsequently the concentration in various tissues. Drugs with a small Vd are limited to certain areas within the body (typically extracel­ lular fluid), whereas those with a higher Vd penetrate extensively into tissues and organs. Some antibacterial drugs can bind to serum pro­ teins, resulting in typically lower Vd as only the unbound (free) frac­ tion of the drug distributes into body tissues and fluids. Furthermore, only the unbound fraction of the drug is considered therapeutically active and available to exert antibacterial effects. CHAPTER 149 Metabolism  Metabolism is the chemical transformation of a drug by the body. This modification can occur within several areas; the liver is the organ most commonly involved. Drugs are metabolized by enzymes, but enzyme systems have a finite capacity to metabolize a substrate. If a drug is given in a dose at which the concentration does not exceed the rate of metabolism, the metabolic process is generally linear. If the dose exceeds the amount that can be metabolized, drug accumulation and potential toxicity may occur over time. Drugs are metabolized through phase I or phase II reactions. In phase I reactions, the drug is made more polar through dealkylation, hydroxylation, oxidation, and deamination. Polarity increases water solubility and facilitates removal from the body (e.g., renal elimination). Phase II reactions, which include glucuronidation, sulfation, and acetylation, result in larger and more polar compounds than the parent drug. Both phases usually inactivate the parent drug, although some drugs are rendered more active. The hepatic cytochrome P450 (CYP) enzyme system is mostly responsible for phase I reactions. CYP3A4 is a com­ mon subfamily within this system that is responsible for the majority of phase I metabolism. Antibacterial drugs can be substrates, inhibitors, or inducers of a particular CYP enzyme. Inducers, e.g., rifampin, can increase the production of CYP enzymes and consequently increase the metabolism of other drugs. Inhibitors, such as macrolides, cause a decrease in enzyme activity and therefore an increase in the concentra­ tion of the interacting drug by decreasing the rate of its metabolism. Treatment and Prophylaxis of Bacterial Infections Excretion  Excretion describes the body’s mechanisms of drug elim­ ination. Drugs can be eliminated through more than one mechanism. Renal clearance is the most common route and includes elimination through glomerular filtration, tubular secretion, and/or passive diffu­ sion. Some agents undergo nonrenal clearance and rely on the biliary tract or the intestine for excretion. Rate of excretion affects the half-life of a drug—i.e., defined as the time it takes for the blood concentration of a drug to decrease by one-half. This value can range from minutes to days. Half-life and overall drug clearance time can be extended if the organ responsible for clearance is impaired. For example, patients with renal or hepatic impairment may require dose adjustments that take delayed clearance into account to prevent drug accumulation and toxicity. For example, the majority of β-lactam agents are cleared pre­ dominantly through glomerular filtration, and in the presence of renal impairment, the dosing interval is typically increased to account for the increased half-life.

■ ■PHARMACODYNAMICS The term pharmacodynamics describes the relationship between the drug concentrations that determine its efficacy and those that may produce toxic effects. For an antibacterial agent, the pharmacodynamic focus is the level of drug exposure needed for optimal antibacterial effect in relation to the minimal inhibitory concentration (MIC)—the lowest drug concentration that inhibits the growth of a microorganism under standardized laboratory conditions. Antibacterial effect usually correlates with one or more of the following parameters: (1) concentra­ tion-dependent killing (defined as the ratio of peak drug concentration to the MIC), (2) time-dependent killing (defined as duration of drug concentrations above the MIC), or (3) the area under the concentra­ tion–time curve to the MIC (AUC/MIC), a measure of the overall drug exposure during the dosing interval (Fig. 149-1).

For concentration-dependent killing agents, as the designation implies, the higher the drug peak concentration (Cmax) at the site of action, the higher the rate and extent of bacterial killing. Aminogly­ cosides fit into the Cmax/MIC model of pharmacodynamics activity, and a particular measured peak serum concentration is often tar­ geted to achieve optimal killing. In contrast, time-dependent killing agents reach a ceiling at which higher concentrations do not result in increased effect(s). Rather, these agents are active against bacteria when the drug concentration exceeds the MIC for a desired period of time. The T > MIC predicts clinical efficacy for all β-lactams. The longer the concentration of the β-lactam remains above the MIC for an infect­ ing pathogen during the dosing interval, the greater the killing effect. Fluoroquinolones and vancomycin exemplify agents for which the AUC/MIC is a predictor of efficacy. For example, studies have found that an AUC/MIC ratio of >30 will maximize killing of Streptococcus pneumoniae by fluoroquinolones, whereas AUC/MIC ratios >125 are required to exert their optimal effects against gram-negative patho­ gens. Finally, for some antibacterial drugs such as aminoglycosides, a postantibiotic effect—the delayed regrowth of surviving bacteria after exposure to an antibiotic—supports less frequent dosing, increased drug-free intervals, and likely decreased drug-related toxicities. PART 5 Infectious Diseases APPROACH TO THERAPY The approach to antibiotic therapy is driven by host factors, site of infection, and local resistance profiles of suspected or known patho­ gens. Further, national and local drug shortages and formulary restric­ tions can affect available therapies. Regular monitoring of the patient and collection of laboratory data should be undertaken to streamline antibacterial therapy as appropriate and to investigate the possibility of treatment failure if the patient fails to respond appropriately. ■ ■EMPIRICAL AND DIRECTED THERAPY Therapy is considered empirical when the causative agent has yet to be determined and therapeutic decisions are based on the severity of illness, Peak (Cmax/MIC) Drug concentration AUC/MIC MIC T > MIC Time FIGURE 149-1  Pharmacokinetic and pharmacodynamic model predicting efficacy of antibacterial drugs. AUC, area under the time–concentration curve; Cmax, peak serum concentration of drug; MIC, minimal inhibitory concentration; T > MIC, duration of drug concentrations above the MIC.

the clinician’s assessment of likely pathogens in light of the clinical syn­ drome, the patient’s medical conditions and prior therapy, and relevant epidemiologic factors. For patients with severe illness, empirical therapy often takes the form of an antibacterial combination that provides broad coverage of diverse agents and thus ensures adequate treatment of possible pathogens while additional data are being collected. Directed therapy is predicated on identification of the pathogen, determination of its susceptibility profile, and establishment of the extent of the infection. Directed therapy generally allows the use of more targeted and narrowerspectrum antibacterial agents than does empirical therapy. Information on epidemiology, exposures, and local antibacterial susceptibility patterns can help guide empirical therapy. When empiri­ cal treatment is clinically appropriate, care should be taken to obtain clinical specimens for microbiologic analysis before the initiation of therapy and to adjust therapy as new information is obtained about the patient’s clinical condition and the causal pathogens. Change to directed therapy can limit unnecessary risks of drug side effects as well as selection for antibacterial resistance and the risk of Clostridioides difficile infection and disease. ■ ■SITE OF INFECTION The site of infection is a consideration in antibacterial therapy, largely because of the differing abilities of drugs to penetrate and achieve ade­ quate concentrations at particular body sites. For example, to be effective in the treatment of meningitis, an agent must (1) be able to cross the blood-brain barrier and reach adequate concentrations in the cerebrospi­ nal fluid (CSF) and (2) be active against the relevant pathogen(s). Dexa­ methasone, administered with or 15–20 min before the first dose of an antibacterial drug, has been shown to improve outcomes in patients with some types of acute bacterial meningitis, but its use may reduce penetra­ tion of some antibacterial agents, such as vancomycin, into the CSF. In this case, rifampin is added because its penetration is not reduced by dexamethasone. Infections at sites where pathogens are protected from normal host defenses, penetration of an antibacterial drug is limited, or local conditions (e.g., low pH) limit activity of some agents include, in addition to meningitis, osteomyelitis, prostatitis, intraocular infections, and abscesses. In such cases, consideration must be given to the route of drug delivery (e.g., intravitreal injections) as well as to interventions to drain, debride, or otherwise reduce bacterial load and necrotic material that can reduce antibacterial activity. ■ ■HOST FACTORS Host factors, including immune function, pregnancy, allergies, age, renal and hepatic function, drug–drug interactions, comorbid condi­ tions, and occupational or social exposures, should be considered. Immune Dysfunction  Patients with deficits in immune function that blunt the response to bacterial infection, including neutropenia, deficient humoral immunity, and asplenia (either surgical or functional), are all at increased risk of severe bacterial infection. Such patients should be treated aggressively and often broadly in the early stages of suspected infection pending results of microbiologic tests. For asplenic patients, treatment should include coverage of encapsulated organisms, particu­ larly S. pneumoniae, that may cause rapidly life-threatening infection. For neutropenic patients, initial treatment typically includes antibacterial agents with broad activity against gram-negative bacteria. Pregnancy  Pregnancy affects decisions regarding antibacterial therapy in two respects. First, pregnancy is associated with an increased risk of particular infections (e.g., those caused by Listeria). Second, the potential risks to the fetus that are posed by specific drugs must be considered. As for other drugs, the safety of the vast majority of anti­ bacterial agents in pregnancy has not been established, and such agents are grouped in categories B and C by the U.S. Food and Drug Admin­ istration (FDA). Drugs in categories D and X are contraindicated in pregnancy or lactation due to established risks. Note that in accordance with the Pregnancy and Lactation Labeling Final Rule (PLLR), drugs submitted to the FDA for approval after 2015 do not use the pregnancy risk categories. The risks associated with antibacterial use in pregnancy and during lactation are summarized in Table 149-1.

TABLE 149-1  Risks Associated with Use of Antibacterial Drugs in Pregnancy and Lactation PREGNANCY CATEGORYa ANTIBACTERIAL DRUG FETAL RISK RECOMMENDATIONb BREAST-FEEDING RISK RECOMMENDATIONb B Azithromycin Limited human data. Animal data suggest low risk. Limited human data; probably compatible Cephalosporins (including cephalexin, cefuroxime, cefixime, cefpodoxime, cefotaxime, ceftriaxone) Compatible Compatible Ceftazidime-avibactam No human data; no fetal harm in animal studies Ceftazidime is excreted into human milk in low concentrations. Avibactam is excreted into the milk of lactating rats; no human studies have been conducted. Ceftolozane-tazobactam Compatible Unknown Clindamycin Compatible Compatible Ertapenem No human data; probably compatible Limited human data; probably compatible Erythromycin Compatible (except for estolate salt) Compatible Meropenem and meropenem-vaborbactam No human data. Animal data suggest low risk. No human data; probably compatible Metronidazole Human data suggest low risk. Interrupt breast-feeding for 12–24 h after single 2-g dose. Limited human data; potential toxicity in divided doses Nitrofurantoin Human data suggest risk in third trimester. Limited human data; probably compatible. Higher risk associated with younger infants and those with G6PD deficiency Penicillins (including amoxicillin, ampicillin, cloxacillin) Compatible Compatible Quinupristin-dalfopristin Compatible. Maternal benefit must far outweigh risk to embryo/fetus. Vancomycin Compatible Limited human data; probably compatible C Chloramphenicol Compatible Limited human data; potential toxicity Fluoroquinolones Human data suggest low risk. Limited human data; probably compatible Clarithromycin Limited human data. Animal data suggest high risk. No human data; probably compatible Imipenem-cilastatin Limited human data. Animal data suggest low risk. Limited human data; probably compatible Linezolid Compatible. Maternal benefit must far outweigh risk to embryo/fetus. Telavancin No human data. Animal studies have revealed evidence of teratogenicity.c No human data. Animal studies have revealed evidence of teratogenicityc Tedizolid Limited data. Embryo-fetal studies in mice, rats, and rabbits have demonstrated fetal developmental toxicities. Use only if benefit outweighs risk. Dalbavancin Limited human data. At high doses in animal studies, delayed fetal maturation, increased embryo and offspring death. Use only if benefit outweighs risk. Oritavancin Limited human data. Studies in rats and rabbits demonstrated no harm at 25% of recommended human dose. Use only if benefit outweighs risk. C/D Amikacin Human data suggest low risk. Compatible Gentamicin Human data suggest low risk. Compatible D Kanamycin Human data suggest risk. Limited human data; probably compatible Streptomycin Human data suggest risk. Compatible Sulfonamides Human data suggest risk in third trimester. Limited human data; potential toxicity. Avoid in ill, stressed, premature infants and in infants with hyperbilirubinemia or G6PD deficiency. Tetracyclines Contraindicated in second and third trimesters Compatible Tigecycline Human data suggest risk in second and third trimesters. Not assignedd Cefiderocol No controlled data in human pregnancy; animal studies have not provided evidence of fetal harm. Eravacycline No controlled data in human pregnancy; animal data indicate drug crosses placenta and is associated with risk at higher doses, Imipenem-cilastatin-relebactam No controlled data in human pregnancy; animal studies have not revealed teratogenicity but have shown evidence of increased fetal loss. Lefamuline No controlled data in human pregnancy; animal studies have revealed evidence of fetal harm.

No human data; potential toxicity CHAPTER 149 No human data; potential toxicity Treatment and Prophylaxis of Bacterial Infections Excreted in the breast milk of rats; unknown in humans; caution use Excreted in the breast milk of animals; unknown in humans; caution use Excreted in the breast milk of rats; unknown in humans; caution use No human data; potential toxicity Unknown if excreted in human milk; excreted in animal milk. Unknown if excreted in human milk; excreted in animal milk. Not recommended during and for a period after treatment. Imipenem and cilastatin are excreted into human milk; no data on relebactam in human milk. Relebactam is excreted in animal milk. No human data regarding potential effect on infant. Unknown if excreted in human milk; excreted in animal milk. Breast-feeding is not recommended during use and for 2 days afterward. (Continued)

TABLE 149-1  Risks Associated with Use of Antibacterial Drugs in Pregnancy and Lactation PREGNANCY CATEGORYa ANTIBACTERIAL DRUG FETAL RISK RECOMMENDATIONb BREAST-FEEDING RISK RECOMMENDATIONb Meropenem-vaborbactam No controlled data in human pregnancy; animal studies have revealed evidence of fetal harm (related to vaborbactam component). Omadacycline No controlled data in human pregnancy; however, as this is a tetracycline class antibiotic, may cause deciduous tooth discoloration and bone growth inhibition in second and third trimester of pregnancy; animal data have demonstrated embryofetal lethality, teratogenicity, and embryofetal toxicity. Plazomicin No controlled data in human pregnancy; however, aminoglycoside antibiotics are known to cause fetal harm in pregnancy. aCategory B: Either animal reproduction studies have failed to demonstrate a risk to the fetus, and there are no adequate and well-controlled studies in pregnant women; or animal studies have shown an adverse effect, but adequate and well-controlled studies in pregnant women have failed to demonstrate a risk to the fetus in any trimester. Category C: Animal reproduction studies have shown an adverse effect on the fetus, and there are no adequate and well-controlled studies in humans, but potential benefits may warrant use of the drug in pregnant women despite potential risks. Category D: There is positive evidence of human fetal risk based on adverse-reaction data from investigational or marketing experience or studies in humans, but potential benefits may warrant use of the drug in pregnant women despite potential risks. bFetal risk recommendation and breast-feeding risk recommendation adapted from GG Briggs et al (eds): Drugs in Pregnancy and Lactation, 9th ed. Philadelphia, Lippincott Williams and Wilkins, 2011; and the U.S. Food and Drug Administration (Drugs@FDA). cA registry has been established to monitor pregnancy outcomes of pregnant women exposed to telavancin. Physicians are encouraged to register pregnant patients, or pregnant women may enroll themselves by calling 1-855-633-8479. dThe U.S. Food and Drug Administration is phasing out use of pregnancy categories A, B, C, D, and X. eA pregnancy pharmacovigilance program is available: If this drug is inadvertently administered during pregnancy or if a patient becomes pregnant while receiving this drug, healthcare providers or patients should report drug exposure by calling 1-855-5NABRIVA (1-855-56227482) to enroll. Abbreviation: G6PD, glucose-6-phosphate dehydrogenase. Allergies  Allergies to antibiotics are among the most common allergies reported, and an allergy history should be obtained whenever possible before therapy is chosen. A detailed allergy history can shed light on the type of reaction experienced previously and on whether rechallenge with the same or a related medication is advisable (and, if so, under what circumstances). Allergies to the penicillins are most common. Although as many as 10% of patients may report an allergy to penicillin, studies suggest that >90% of these patients could tolerate a penicillin or cephalosporin. Adverse effects and intolerances (Table 149-2) should be distinguished from true allergies to ensure appropri­ ate selection of antibacterial therapy. PART 5 Infectious Diseases Drug–Drug Interactions  Patients commonly receive other drugs that may interact with antibacterial agents. A summary of the most common drug–drug interactions, by antibacterial class, is provided in Table 149-3. Exposures  Exposures, both occupational and nonoccupational, may provide clues to likely pathogens. When relevant, inquiries about exposure to ill contacts, animals, insects, and water should be included in the history, along with sites of residence and travel within a relevant time frame. Other Host Factors  Age, renal and hepatic function, and comor­ bid conditions are all considerations in the choice of, and schedule for, therapy. Dose adjustments should be made accordingly. In patients with decreased or unreliable oral absorption, intravenous therapy may be preferred to ensure adequate blood levels of drug and delivery of the antibacterial agent to the site of infection. In general, initial treatment for severe and life-threatening infections is given by intravenous injec­ tion to assure prompt and adequate drug delivery. ■ ■DURATION OF THERAPY Whether empirical or directed, the duration of therapy should be established in most clinical situations. Guidelines that synthesize available literature and expert opinion provide recommendations on therapy duration that are based on infecting organism, organ system, and patient factors. For example, the American Heart Association has published guidelines endorsed by the Infectious Diseases Society of America (IDSA) on diagnosis, antibacterial therapy, and manage­ ment of complications of infective endocarditis. Additional guidelines from the IDSA in collaboration with other specialty societies, such as the Society of Healthcare Epidemiology of America, the American Thoracic Society, and the European Society for Clinical Microbiology

(Continued) Meropenem is excreted in human milk; it is unknown if vaborbactam is excreted in human milk. Data on excretion of vaborbactam in animal milk are unknown. Unknown if excreted in human milk; data not available regarding excretion in animal milk. Not recommended during and for a period after treatment. Unknown if excreted in human milk; excreted in animal milk. and Infectious Diseases, exist for a range of infectious syndromes and specific pathogens. In general, where data on adequate durations of therapy exist, shorter courses are preferred to reduce the likelihood of drug adverse effects and selection of resistant bacteria. ■ ■FAILURE OF THERAPY If a patient does not respond to therapy, investigations often should include imaging and the collection of additional specimens for micro­ biologic testing as indicated. Failure to respond can be the result of an antibacterial regimen that does not address the underlying causative organism, the development of resistance during therapy, or the existence of a focus of infection at a site poorly penetrated by systemic therapy. Some infections may also require surgical interventions (e.g., large abscesses, myonecrosis). Fever due to allergic drug reactions may com­ plicate assessment of the patient’s response to antibacterial treatment. ■ ■EXPERT GUIDANCE Selected websites with the most up-to-date information and guidance for the clinician include the following: • Johns Hopkins ABX Guide (www.hopkins-abxguide.org) • IDSA Practice Guidelines (https://www.idsociety.org/ practice-guideline/practice-guidelines) • OneHealth Trust Resistance Map (https://resistancemap.onehealthtrust

.org/index.php) • Centers for Disease Control and Prevention Antibiotic/Antimicro­ bial Resistance (www.cdc.gov/drugresistance/) CLINICAL USE OF ANTIBACTERIAL AGENTS The clinical application of antibacterial therapy is guided by the spectrum of the agent and the suspected or known target pathogen. Infections for which specific antibacterial agents are among the drugs of choice are listed, along with associated pathogens and susceptibility data, in Table 149-4. Resistance rates of specific organisms are dynamic and should be taken into account in the approach to antibacterial therapy. While national resistance rates can serve as a reference, the most useful reference for the clinician is the most recent local labora­ tory antibiogram, which provides details on local resistance patterns, often on an annual or semiannual basis. ■ ■a-LACTAMS The β-lactam class of antibiotics consists of penicillins, cephalospo­ rins, carbapenems, and monobactams. The term β-lactam reflects

TABLE 149-2  Common Adverse Reactions to Antibacterial Agents ANTIBACTERIAL(S) POTENTIAL ADVERSE EFFECTS COMMENTS β-Lactams Hypersensitivity reactions Ranges from rash to anaphylaxis. Cross-reactivity among β-lactams is related to chemical structure and side chain similarity. Neurotoxicity More commonly described with cefepime and imipenem, but likely a class effect. Risk is increased in patients with history of seizures, renal impairment, and advanced age. Neutropenia/hematologic reactions May be related to high doses and prolonged duration Vancomycin Nephrotoxicity Risk increases with vancomycin trough levels >20 μg/mL or concomitant administration with other potentially nephrotoxic agents. The effect is usually reversible. “Red man syndrome” Can be managed with a slower vancomycin infusion and pretreatment with antihistamine Telavancin QT prolongation Interference with coagulation tests May falsely affect INR, PT, aPTT. Perform these tests before the next dose of telavancin (when serum drug levels are at their nadir). Taste disturbances Nephrotoxicity Oritavancin Interference with coagulation tests May falsely affect INR, PT, aPTT. Perform these tests at least 24 h after the dose is administered. Gastrointestinal distress Dalbavancin Gastrointestinal distress Daptomycin Myopathy Monitor CPK levels during therapy. Rhabdomyolysis has been reported but appears to be rare. Eosinophilic pneumonia Aminoglycosides Nephrotoxicity Associated with prolonged use; usually reversible Ototoxicity Can cause both vestibular and cochlear toxicity. Ototoxicity may be irreversible. Fluoroquinolones QTc prolongation Moxifloxacin appears more likely than other quinolones to exert this effect. Risk of arrhythmia increases when these drugs are given concomitantly with other QTc-prolonging agents. Tendinitis Risk is greater among the elderly and patients receiving steroids. Dysglycemia Exacerbation of myasthenia gravis Rifampin Hepatotoxicity Risk is greater when drug is given with other antituberculosis agents. When rifampin is given alone, LFT values may be transiently elevated without symptoms. Orange discoloration of body fluids Tetracyclines, including tigecycline, eravacycline, and omadacycline Photosensitivity Gastrointestinal distress High incidence of diarrhea, nausea, vomiting Macrolides Gastrointestinal distress Erythromycin is occasionally used as a therapeutic agent for some gastric motility disorders. QTc prolongation Azithromycin use is associated with an increased risk of death from cardiovascular causes among patients at high baseline risk. Metronidazole Peripheral neuropathy Associated with prolonged use Clindamycin Diarrhea and pseudomembranous colitis Linezolid, tedizolid Myelosuppression Associated with prolonged use Optic and peripheral neuropathy Associated with prolonged use Lactic acidosis TMP-SMX Hypersensitivity reactions Allergy usually associated with sulfonamide moiety Nephrotoxicity Associated with high doses Hematologic effects Associated with prolonged use Nitrofurantoin Pneumonitis and other pulmonary reactions Associated with prolonged use Peripheral neuropathy Associated with accumulation of nitrofurantoin in renal failure. Avoid use in renal impairment. Fosfomycin Gastrointestinal effects Polymyxins Nephrotoxicity Associated with high dose Neurotoxicity Neuromuscular blockade and muscle weakness are well described and usually reversible. Quinupristindalfopristin Arthralgias and myalgias Chloramphenicol Bone marrow suppression Aplastic anemia or hematopoietic toxicity Pleuromutilin Gastrointestinal Diarrhea QTc prolongation When used in conjunction with CYP3A4 substrates Note: All systemic antibiotics have the potential to alter abdominal flora and induce Clostridioides difficile infection. Abbreviations: aPTT, activated partial thromboplastin time; CPK, creatine phosphokinase; INR, international normalized ratio; LFT, liver function test; PT, prothrombin time; TMP-SMX, trimethoprim-sulfamethoxazole.

CHAPTER 149 Treatment and Prophylaxis of Bacterial Infections

TABLE 149-3  Important Antibacterial Drug Interactions ANTIBACTERIAL(S) INTERACTING AGENT(S) POTENTIAL EFFECT AND MANAGEMENT Nafcillin Warfarin, cyclosporine, tacrolimus Decreased effects of interacting drug via CYP3A4 induction. Monitor levels of affected drug closely if drugs are given concomitantly. Ceftriaxone Calcium-containing IV solutions Concomitant use is contraindicated in neonates (<28 days); the combination can lead to precipitation of ceftriaxone-calcium particulate. Ceftriaxone and calcium-containing solutions can be given to infants >28 days of age provided they are given sequentially and the lines are thoroughly flushed between infusions, or infused via separate lines. Carbapenems Valproic acid Diminished levels of valproic acid. Monitor valproic acid levels closely if drugs are given concomitantly and consider alternative therapies. Linezolid, tedizolid Serotonergic and adrenergic agents (e.g., SSRIs, vasopressors) Quinupristindalfopristin Substrates of CYP3A4 (e.g., warfarin, ritonavir, cyclosporine, diazepam, verapamil) Fluoroquinolones Theophyllinea Can result in theophylline toxicity Sucralfate; antacids containing aluminum, calcium, or magnesium; ferrous sulfate– and zinc-containing multivitamins Tizanidinea Can result in increased levels of tizanidine and hypotensive, sedative effects. Monitor for side effects if drugs are given concomitantly. QTc-prolonging drugs (e.g. azoles, sotalol, amiodarone, dofetilide, fluoxetine) Rifampin Substrates of CYP3A4 (e.g., warfarin, ritonavir, cyclosporine, diazepam, verapamil, protease inhibitors, voriconazole)   Substrates of CYP2C19 (e.g., omeprazole, lansoprazole)   Substrates of CYP2C9 (e.g., warfarin, tolbutamide) PART 5 Infectious Diseases   Substrates of CYP2C8 (e.g., repaglinide, rosiglitazone)   Substrates of CYP2B6 (e.g., efavirenz)   Hormone therapy (e.g., norethindrone) Can result in decreased levels of hormone. If oral contraceptive and rifampin are given concomitantly, use alternative or additional forms of birth control. Tetracyclines Antacids or drugs containing calcium, magnesium, iron, or aluminum Warfarin Increased effect of warfarin. Monitor levels closely if drugs are given concomitantly. Eravacycline: Strong CYP3A4 inducers (e.g., rifampin) Macrolidesb Substrates of CYP3A4 (e.g., warfarin, ritonavir, cyclosporine, diazepam, verapamil, amiodarone)   QTc-prolonging agents (e.g., fluoroquinolones, sotalol)   Protease inhibitors (e.g., ritonavir) Can result in increased levels of both macrolides and protease inhibitors. Avoid concomitant use if possible.   Cimetidine Cimetidine can increase levels of macrolides. Metronidazole Ethanol Can result in disulfiram-like reaction. Ethanol may be present in some formulations of oral drug suspensions (e.g., ritonavir). Warfarin Can increase warfarin effects. Monitor INR closely if drugs are given concomitantly. TMP-SMX Warfarin Increased effect of warfarin. Monitor levels closely if drugs are given concomitantly. Phenytoin Increased levels of phenytoin. Monitor levels closely if drugs are given concomitantly. Methotrexate Increased levels of methotrexate and prolonged exposure. Monitor levels closely if drugs are given concomitantly. Oritavancin Substrates of CYP3A4 (e.g., cyclosporine, warfarin) and CYP2D6 (e.g., aripiprazole) Substrates of CYP2C19 (e.g., omeprazole) and CYP2C9 (e.g., warfarin) Lefamulin QTc-prolonging drugs (e.g., azoles, sotalol, amiodarone, dofetilide, fluoxetine) Strong CYP3A4 inducers (e.g., rifampin) Reduced lefamulin efficacy Strong CYP3A4 strong inhibitors (e.g., ritonavir) Increased lefamulin exposure aDrug reaction described with ciprofloxacin only. bClarithromycin and erythromycin are potent CYP3A4 inhibitors; the probability of a drug interaction with azithromycin is lower. Abbreviations: INR, international normalized ratio; SSRI, selective serotonin-reuptake inhibitor; TMP-SMX, trimethoprim-sulfamethoxazole.

Increased levels of serotonergic and adrenergic agents. Monitor for serotonin syndrome. Tedizolid may have less potential than linezolid to cause this drug interaction. Can result in increased levels of interacting drug Can result in decreased oral absorption of fluoroquinolones. Administer fluoroquinolone

2 h before or 6 h after interacting drug. Increased risk of cardiotoxicity and arrhythmias. Monitor QTc. Can result in decreased levels of interacting drug. Avoid concomitant use if possible. If giving drugs concomitantly, monitor drug levels if possible. Can result in decreased oral absorption of tetracyclines. Administer tetracycline 2 h before or 6 h after interacting drug. Reduced eravacycline efficacy Avoid concomitant administration if possible. Increased risk of cardiotoxicity and arrhythmias. Monitor QTc. Can result in decreased levels of interacting drug. Avoid concomitant use if possible. If giving drugs concomitantly, monitor drug levels if possible. Increased risk of cardiotoxicity and arrhythmias. Monitor QTc.

TABLE 149-4  Drug Indications for Specific Infections, Associated Pathogens, and Sample Susceptibility Rates ANTIMICROBIAL(S) INFECTIONS Penicillin G Syphilis; yaws; leptospirosis; streptococcal infections; pneumococcal infections; actinomycosis; oral and periodontal infections; meningococcal meningitis and meningococcemia; viridans streptococcal endocarditis; clostridial myonecrosis; tetanus; rat-bite fever; Pasteurella multocida infections; erysipeloid (Erysipelothrix rhusiopathiae) Ampicillin, amoxicillin Salmonellosis; acute otitis media; Haemophilus influenzae meningitis and epiglottitis; Listeria monocytogenes meningitis; Enterococcus faecalis UTI Nafcillin, oxacillin MSSA bacteremia and endocarditis Staphylococcus aureus (72%); coagulase-negative staphylococci (49%) Piperacillintazobactam Intraabdominal infections (facultative enteric gram-negative bacilli and obligate anaerobes); infections caused by mixed flora (aspiration pneumonia, diabetic foot ulcers); infections caused by Pseudomonas aeruginosa Cefazolin E. coli UTI; surgical prophylaxis; MSSA bacteremia and endocarditis E. coli (80%) Cefoxitin, cefotetan Intraabdominal infections and pelvic inflammatory disease Bacteroides fragilis (60%)b Ceftriaxone Gonococcal infections; pneumococcal meningitis; viridans streptococcal endocarditis; salmonellosis and typhoid fever; health care–associated infections caused by nonpseudomonal facultative gram-negative enteric bacilli Ceftazidime, cefepime Health care–associated infections caused by facultative gram-negative bacilli and Pseudomonas spp. Ceftaroline CAP caused by S. pneumoniae, MSSA, H. influenzae, K. pneumoniae, Klebsiella oxytoca, and E. coli; acute bacterial skin and skin-structure infections caused by MSSA, MRSA, Streptococcus pyogenes, Streptococcus agalactiae, E. coli, K. pneumoniae, and K. oxytoca Ceftazidimeavibactam, meropenemvaborbactam Complicated UTIs (ceftazidime-avibactam and meropenem-vaborbactam) and complicated intraabdominal infections (ceftazidime-avibactam in combination with metronidazole) caused by resistant gram-negative organisms, including Pseudomonas, and some anaerobes Ceftolozanetazobactam Complicated UTIs and complicated intraabdominal infections (in combination with metronidazole) caused by resistant gram-negative organisms, including Pseudomonas, and some anaerobes Imipenem, meropenem Intraabdominal infections, infections caused by Enterobacter spp. and ESBLproducing gram-negative bacilli Ertapenem CAP; complicated UTIs, including pyelonephritis; acute pelvic infections; complicated intraabdominal infections; complicated skin and skin-structure infections, excluding diabetic foot infections accompanied by osteomyelitis or caused by P. aeruginosa Aztreonam Infections caused by facultative gram-negative bacilli and Pseudomonas in penicillin-allergic patients Vancomycin Bacteremia, endocarditis, and other invasive disease caused by MRSA; pneumococcal meningitis; oral formulation for CDAD Telavancin Health care– and ventilator-associated pneumonia or skin and soft tissue infections caused by MRSA Dalbavancin, oritavancin Complicated skin and soft tissue infections S. aureus: rarely reported for dalbavancin.g Rarely reported for oritavancin.h Daptomycin VRE infections; MRSA bacteremia E. faecalis (99.9%);i E. faecium (99.7%);i S. aureus (99.9%)g Gentamicin, tobramycin, amikacin Combined with penicillin for staphylococcal, enterococcal, or streptococcal endocarditis; combined with β-lactam for gram-negative bacteremia; pyelonephritis Azithromycin, clarithromycin, erythromycin Legionella, Campylobacter, and Mycoplasma infections; CAP; GAS pharyngitis in penicillin-allergic patients; bacillary angiomatosis; gastric infections due to Helicobacter pylori; MAI infections Clindamycin Severe, invasive GAS infections (with a β-lactam); infections caused by obligate anaerobes; infections caused by susceptible staphylococci Doxycycline, minocycline Acute bacterial exacerbations of chronic bronchitis; granuloma inguinale; brucellosis (with streptomycin); tularemia; glanders; melioidosis; spirochetal infections caused by Borrelia (Lyme disease and relapsing fever; doxycycline); infections caused by Vibrio vulnificus; some Aeromonas infections; infections due to Stenotrophomonas (minocycline); plague; ehrlichiosis; chlamydial infections (doxycycline); granulomatous infections due to Mycobacterium marinum (minocycline); rickettsial infections; mild CAP; skin and soft tissue infections caused by gram-positive cocci (e.g., CA-MRSA infections); leptospirosis; syphilis; and actinomycosis in the penicillin-allergic patient

COMMON PATHOGENS (% SUSCEPTIBLE);

RESISTANCE AS NOTEDa Neisseria meningitidis; viridans streptococci (60%); Streptococcus pneumoniae (97% nonmeningitis; 74% meningitis) Escherichia coli (52%); H. influenzae (72%); Salmonella spp. (89%) P. aeruginosa (78%) S. pneumoniae (87% meningitis; 99% nonmeningitis); E. coli (88%); Klebsiella pneumoniae (85%) P. aeruginosa (82% for ceftazidime, 85% for cefepime) Mostly susceptible; four strains of MRSA with ceftaroline MICs >4 μg/mL reported in isolates from a single Greek hospitalc; additional case reports, including in patients without prior exposure to ceftarolined,e P. aeruginosa (84–97%)f MDR Enterobacterales, including carbapenem-resistant Enterobacterales that produce KPCs No activity against metallo-β-lactamases (e.g., NDM) CHAPTER 149 P. aeruginosa (>86% overall; 60–80% of ceftazidime- and meropenem-resistant strains)f MDR Enterobacterales No activity against KPC-producing organisms Treatment and Prophylaxis of Bacterial Infections P. aeruginosa (84%); Acinetobacter calcoaceticus-baumannii complex (84%) (meropenem susceptibilities reported) Enterobacter cloacae (88%); K. pneumoniae (98%)   S. aureus (100%); E. faecalis (93%); E. faecium (41%) S. aureus: none reported E. coli (gentamicin, 90%); P. aeruginosa (amikacin, 93%; gentamicin, 89%); A. calcoaceticus-baumannii complex (gentamicin, 83%) S. pneumoniae (60%); group A streptococci (82%); H. pylori (75%)j S. aureus (73%) S. pneumoniae (54%); S. aureus (97%) (Continued)

TABLE 149-4  Drug Indications for Specific Infections, Associated Pathogens, and Sample Susceptibility Rates ANTIMICROBIAL(S) INFECTIONS Tigecycline CAP caused by S. pneumoniae, H. influenzae, or Legionella pneumophila; complicated skin infections caused by E. coli, MRSA, MSSA, S. pyogenes, Streptococcus anginosus, S. agalactiae, B. fragilis; complicated intraabdominal infections caused by E. coli, vancomycin-susceptible E. faecalis, Citrobacter freundii, E. cloacae, K. pneumoniae, K. oxytoca, Bacteroides spp., Clostridium perfringens, and Peptostreptococcus spp. TMP-SMX Community-acquired UTI; CA-MRSA skin and soft tissue infections E. coli (73%); S. aureus (94%) Sulfonamides Nocardial infections; leprosy (dapsone); toxoplasmosis (sulfadiazine) Unknown Ciprofloxacin, levofloxacin, moxifloxacin, delafloxacin CAP (levofloxacin and moxifloxacin); UTI; bacterial gastroenteritis; health care–associated gram-negative enteric infections; Pseudomonas infections (ciprofloxacin and levofloxacin); skin and skin-structure infections (delafloxacin) Rifampin Staphylococcal foreign body infections (in combination with other antistaphylococcal agents); Legionella pneumonia; Mycobacterium tuberculosis; atypical nontuberculous mycobacterial infection; pneumococcal meningitis when organisms are susceptible or response is delayed Metronidazole Obligate anaerobic gram-negative bacteria (e.g., Bacteroides spp.); abscess in lung, brain, or abdomen; bacterial vaginosis; CDAD Linezolid, tedizolid VRE; uncomplicated and complicated skin and soft tissue infections caused by MSSA and MRSA; CAP with concurrent bacteremia; health care–associated pneumonia Chloramphenicol Infections due to gram-positive and gram-negative organisms resistant to standard alternatives (e.g., Burkholderia) Colistin Infections due to gram-negative bacilli resistant to all other chemotherapy

(e.g., P. aeruginosa, Acinetobacter spp., and Stenotrophomonas maltophilia) Quinupristindalfopristin VRE; complicated skin and skin-structure infections due to MSSA and S. pyogenes PART 5 Infectious Diseases Mupirocin Topical application to nares for S. aureus decolonization S. aureus (74–100%)l Nitrofurantoin UTI caused by most gram-negative bacilli and some gram-positive organisms; prophylaxis in recurrent cystitis Fosfomycin UTI caused by most gram-negative bacilli and some gram-positive organisms; prophylaxis in recurrent cystitis Cefiderocol Complicated UTIs and/or pyelonephritis caused by multidrug-resistant gram-negative bacteria, including extended-spectrum β-lactamase- or carbapenemase-producing organisms and multidrug-resistant P. aeruginosa,

A. baumannii, Stenotrophomonas maltophilia, and Burkholderia cepacia Eravacycline Complicated intraabdominal infections caused by E. coli, K. pneumoniae,

C. freundii, E. cloacae, K. oxytoca, E. faecalis, E. faecium, S. aureus, S. anginosus group, C. perfringens, Bacteroides spp., and Parabacteroides distasonis Imipenem-cilastatinrelebactam Complicated intraabdominal infections, pneumonia, and complicated UTI including pyelonephritis caused by multidrug-resistant organisms including Enterobacterales and against some imipenem-nonsusceptible P. aeruginosa Lefamuline Community acquired pneumonia caused by MRSA, S. pneumoniae, and atypical CAP pathogens Meropenemvaborbactam Complicated UTI caused by KPC-producing Enterobacterales Identified in KPC-producing strains of K. pneumoniaeo Omadacycline Community-acquired bacterial pneumonia caused by S. pneumoniae, S. aureus (methicillin-susceptible isolates), H. influenzae, Haemophilus parainfluenzae,

K. pneumoniae, L. pneumophila, M. pneumoniae, and Chlamydophila pneumoniae Plazomicin Complicated UTIs caused by carbapenemase-producing Enterobacteriaceae Resistance uncommon except in infrequent isolates with plasmid-encoded ribosome modifying methylases aUnless otherwise noted, susceptibility rates are based on isolates from the Massachusetts General Hospital Clinical Microbiology Laboratory collected between January and December 2022. Local rates will vary. bJA Karlowsky et al: Antimicrob Agents Chemother 56:1247, 2012. cRE Mendes et al: J Antimicrob Chemother 67:1321, 2012. dSW Long et al: Antimicrob Agents Chemother 58:6668, 2014. eM Nigo et al: Antimicrob Agents and Chemother 61:e01235, 2017. fD Van Duin, RA Bonomo: Clin Infect Dis 63:234, 2016. gSP McCurdy et al: Antimicrob Agents Chemother 59:5007, 2015. hJA Karlowsky et al: Diag Microbiol Infect Dis 87:349, 2017. iHS Sader et al: J Chemother 23:200, 2011. jJ Torres et al: J Clin Microbiol 39:2677, 2001. kWS Oh et al: Antimicrob Agents Chemother 49:5176, 2005. lAE Simor et al: Antimicrob Agents Chemother 51:3880, 2007. mS Demirci-Duarte et al: Diagn Microbiol Infect Dis 98:115098, 2020. nLJ Scott: Drugs 79:315, 2019. oSun et al Antimicrob Agents Chemother 61:e01694, 2017. Abbreviations: CA-MRSA, community-acquired MRSA; CAP, community-acquired pneumonia; CA-UTI, community-acquired UTI; CDAD, Clostridioides difficile–associated diarrhea; ESBL, extended-spectrum β-lactamase; GAS, group A streptococcal; KPCs, Klebsiella pneumoniae carbapenemases; MAI, Mycobacterium avium-intracellulare; MDR, multidrug-resistant; MIC, minimal inhibitory concentration; MRSA, methicillin-resistant S. aureus; MSSA, methicillin-susceptible S. aureus; NDM, New Delhi metalloβ-lactamase; TMP-SMX, trimethoprim-sulfamethoxazole; UTI, urinary tract infection; VRE, vancomycin-resistant Enterococcus. the drugs’ four-membered lactam ring, which is their core structure. The differing side groups among the agents of this family determine the spectrum of activity. All β-lactams exert a bactericidal effect by inhibiting bacterial cell-wall synthesis. The β-lactams are classified as time-dependent killing agents; therefore, their clinical efficacy is best

(Continued) COMMON PATHOGENS (% SUSCEPTIBLE);

RESISTANCE AS NOTEDa Mostly susceptible, although case reports of resistance

in A. baumannii and K. pneumoniae S. pneumoniae (100% levofloxacin); E. coli (75% for ciprofloxacin and 70% for levofloxacin); P. aeruginosa (78% for ciprofloxacin and 70% for levofloxacin); Salmonella spp. (79% for ciprofloxacin and 77% for levofloxacin) S. aureus (99%), although staphylococci rapidly develop resistance with monotherapy Mostly susceptible; resistance very rare Mostly susceptible; resistance occasionally seen in VRE Unknown P. aeruginosa (case reports, outbreaks) E. faecalis (<20%);k E. faecium (>90%)k E. coli (97%); E. faecalis (99%) Considered low prevalencem Very low resistance rates in initial studies Resistance noted in both gram-negative and gram-positive bacterian Low resistance rates in initial studies Low resistance rates in target pathogens in initial studies Broad spectrum overall, but resistance can occur in

gram-negative isolates; not active against Pseudomonas correlated with the proportion of the dosing interval during which drug levels remain above the MIC for the targeted pathogen. Penicillins and β-Lactamase Inhibitors  Penicillin, the first β-lactam, was discovered in 1928 by Alexander Fleming. Natural

Treatment and Prophylaxis of Bacterial Infections

CHAPTER 149 penicillins, such as penicillin G, are active against non-β-lactamaseproducing gram-positive and gram-negative bacteria, anaerobes, and some gram-negative cocci. Penicillin G is used for penicillin-susceptible streptococcal infections, pneumococcal and meningococcal menin­ gitis, enterococcal endocarditis, and syphilis. The antistaphylococcal penicillins, which have potent activity against methicillin-susceptible Staphylococcus aureus (MSSA), include nafcillin, oxacillin, dicloxa­ cillin, and flucloxacillin. Aminopenicillins, such as ampicillin and amoxicillin, provide added coverage beyond penicillin against gramnegative cocci, such as Haemophilus influenzae, and some Entero­ bacterales, including Escherichia coli, Proteus mirabilis, Salmonella, and Shigella. The aminopenicillins are hydrolyzed by many common β-lactamases. These drugs are commonly used for infections caused by susceptible enterococcal and streptococcal species. IV ampicillin is commonly used in meningitis and endocarditis. Oral amoxicillin may be an option for otitis media, respiratory tract infections, and urinary tract infections. The antipseudomonal penicillins include ticarcillin and piperacillin. These penicillin groups generally offer adequate anaerobic coverage; the exceptions are Bacteroides species (such as Bacteroides fragilis), which produce β-lactamases and are generally resistant. The rising prevalence of β-lactamase-producing bacteria has led to the increased use of β-lactam–β-lactamase inhibitor combina­ tions, such as ampicillin-sulbactam, amoxicillin-clavulanate, ticar­ cillin-clavulanate, piperacillin-tazobactam, ceftolozane-tazobactam, ceftazidime-avibactam, meropenem-vaborbactam, and imipenemrelebactam. The β-lactamase inhibitors themselves do not have antibacterial activity (with the exception of sulbactam, which has activity against Acinetobacter baumannii) but typically inhibit the S. aureus class A β-lactamase, β-lactamases of H. influenzae and Bacte­ roides species, and a number of plasmid-encoded β-lactamases. These combination agents are typically used when broader-spectrum cover­ age is needed—e.g., in pneumonia and intraabdominal infections. Piperacillin-tazobactam is a useful agent for febrile neutropenia, when local P. aeruginosa susceptibility rates are high. Avibactam, vabor­ bactam, and relebactam inhibit a broader spectrum of β-lactamases than the other inhibitors, including extended-spectrum β-lactamases (ESBLs), AmpC β-lactamases, and some carbapenemases (see Chap. 150). Sulbactam combined with the inhibitor durlobactam is limited to treatment for Acinetobacter infections. Cephalosporins  The cephalosporin drug class encompasses sev­ eral generations distinguished by spectrum of antibacterial activity. The first generation (cefazolin, cefadroxil, and cephalexin) largely has activity against gram-positive bacteria, with some additional activity against E. coli, P. mirabilis, and Klebsiella pneumoniae. First-generation cephalosporins are commonly used for infections caused by MSSA and streptococci (e.g., skin and soft tissue infec­ tions). Cefazolin is recommended for surgical prophylaxis against skin organisms. The second generation (cefamandole, cefuroxime, cefaclor, cefprozil, cefuroxime axetil, cefoxitin, and cefotetan) has additional activity against H. influenzae and Moraxella catarrha­ lis. Cefoxitin and cefotetan have potent activity against anaerobes as well, although there is some increased resistance seen for B. fragilis. Second-generation cephalosporins have been used to treat community-acquired pneumonia because of their activity against S. pneumoniae, H. influenzae, and M. catarrhalis. They are also used for other mild or moderate infections, such as acute otitis media and sinusitis. The third-generation cephalosporins are character­ ized by greater potency against gram-negative bacilli and reduced potency against gram-positive cocci. These cephalosporins, which include cefoperazone, cefotaxime, ceftazidime, ceftriaxone, cefdinir, cefixime, and cefpodoxime, are used for infections caused by Entero­ bacterales, although resistance is an increasing concern. Ceftriaxone penetrates the CSF and can be used to treat meningitis caused by H. influenzae, N. meningitidis, and susceptible strains of S. pneumoniae. It is also used for the treatment of later-stage Lyme disease, gono­ coccal infections, and streptococcal endocarditis. It is noteworthy that ceftazidime is the only third-generation cephalosporin with activity against Pseudomonas aeruginosa, but it lacks activity against gram-positive bacteria. This drug is frequently used for pulmonary infections in cystic fibrosis, postneurosurgical meningitis, and febrile neutropenia. The fourth generation of cephalosporins includes cefepime and cefpirome, broad-coverage agents with potent activ­ ity against both gram-negative bacilli, including P. aeruginosa, and gram-positive cocci. The fourth generation has clinical applications similar to those of the third generation and may offer additional activity over the first, second, and third generations in the presence of certain β-lactamases. These agents can be used in bacteremia, febrile neutropenia, and intraabdominal and urinary tract infections. Ceftaroline, a fifth-generation cephalosporin, differs from the other cephalosporins in its added activity against methicillin-resistant S. aureus (MRSA), which is resistant to all other β-lactams. Ceftaro­ line’s gram-negative activity is similar to that of the third-generation cephalosporins but does not include P. aeruginosa. Ceftaroline may be used in community-acquired pneumonia and skin infec­ tions, and emerging data support its use in more severe infections such as bacteremia. Adverse reactions to ceftaroline have included hypersensitivity reactions and neutropenia. Ceftolozane-tazobactam and ceftazidime-avibactam are novel cephalosporin–β-lactamase inhibitor combinations with activity against gram-negative bacteria, including Pseudomonas, and some anaerobes. Both agents have been studied in complicated intraabdominal infections and complicated urinary tract infections. Ceftolozane-tazobactam is thought to be stable against many ESBL-producing organisms because of the tazobactam component. The addition of avibactam to ceftazidime yields a combination agent with activity against AmpC-, ESBL-, and K. pneumoniae carbapenemase (KPC)–producing organisms. These cephalosporin–β-lactamase inhibitor combinations may be of clinical benefit in multidrug-resistant gram-negative infections. Cefiderocol is a novel cephalosporin with enhanced uptake through bacterial iron uptake pathways and stability to a broad range of β-lactamases. It has been studied in complicated urinary tract infections and hospitalacquired and ventilator-associated pneumonia and may provide most benefit for multidrug-resistant gram-negative bacterial infections. Carbapenems  Carbapenems, including doripenem, imipenem, meropenem, and ertapenem, offer the most reliable coverage for strains containing ESBLs. All carbapenems have broad activity against gram-positive cocci, gram-negative bacilli, and anaerobes. None is active against MRSA, but all are active against MSSA, Strep­ tococcus species, and Enterobacterales. Ertapenem is the only car­ bapenem that has poor activity against P. aeruginosa and Acinetobacter. Imipenem is active against penicillin-susceptible Enterococcus faecalis but not Enterococcus faecium. Carbapenems are not active against Enterobacterales containing carbapenemases. Stenotrophomonas maltophilia and some Bacillus species are intrinsically resistant to carbapenems because of a zinc-dependent carbapenemase. Addi­ tion of vaborbactam to meropenem and relebactam to imipenem results in inhibition of AmpC β-lactamases, ESBLs, and KPCs but not metallo-carbapenemases, such as NDM (New Delhi metalloβ-lactamases). Carbapenems may decrease the concentration of valproate products, and caution should be used in patients on this combination of therapy. Monobactams  Aztreonam is the sole monobactam in clinical use. Its activity is limited to gram-negative bacteria and includes P. aerugi­ nosa and most other Enterobacterales. It is inactivated by ESBLs and carbapenemases. The principal use for aztreonam is as an alternative to penicillins, cephalosporins, or carbapenems in patients with a seri­ ous β-lactam allergy. Aztreonam is structurally related to ceftazidime and should be used cautiously in individuals with a serious ceftazidime allergy. It is used in febrile neutropenia and intraabdominal infections when other β-lactams cannot be used. Aztreonam is sometimes used in combination with avibactam for the treatment of NDM carbapenemase gram-negative bacterial infections. Adverse Reactions to β-Lactam Drugs  Agents within the β-lactam class are known for several adverse effects. Gastrointesti­ nal side effects, mainly diarrhea, are common, but hypersensitivity

reactions constitute the most common adverse effect of β-lactams. The reactions’ severity can range from rash to anaphylaxis, but the rate of true anaphylactic reactions is only 0.05%. An individual with an accelerated IgE-mediated reaction to one β-lactam agent may still receive another agent within the class, but caution should be used and a β-lactam that has a dissimilar side chain and a low level of cross-reac­ tivity would be the preferred choice. For example, the second-, third-, and fourth-generation cephalosporins and the carbapenems display very low cross-reactivity in patients with penicillin allergy. Aztreonam is the only β-lactam that has no cross-reactivity with the penicillin group. In cases of severe allergy, desensitization (a graded challenge) to the indicated β-lactam, with close monitoring, may be warranted if other antibacterial options are not suitable.

β-Lactams can rarely cause serum sickness, Stevens-Johnson syn­ drome, nephropathy, hematologic reactions, and neurotoxicity. Neu­ tropenia appears to be related to high doses or prolonged use. Neutropenia and interstitial nephritis caused by β-lactams generally resolve upon discontinuation of the agent. Imipenem and cefepime are associated with an increased risk of seizure, but this risk is likely a class effect and related to high doses or doses that are not adjusted in renal impairment. ■ ■SULBACTAM-DURLOBACTAM Durlobactam is a novel diazabicyclooctane non-β-lactam β-lactamase inhibitor combined with the β-lactamase inhibitor sulbactam for treat­ ment of multidrug-resistant Acinetobacter infections. It was approved in the United States in 2023 for treatment of hospital-acquired and ventilator-associated pneumonia caused by Acinetobacter baumanniicalcoaceticus complex. Adverse reactions include abnormal liver func­ tion tests, diarrhea, anemia, and hypokalemia. PART 5 Infectious Diseases ■ ■GLYCOPEPTIDES AND LIPOGLYCOPEPTIDES Vancomycin is a glycopeptide antibiotic with activity against staphylo­ cocci (including MRSA and coagulase-negative staphylococci), strep­ tococci (including S. pneumoniae), and enterococci. It is not active against gram-negative organisms. Vancomycin also displays activity against Bacillus species, Corynebacterium jeikeium, and gram-positive anaerobes such as Peptostreptococcus, Actinomyces, Clostridium, and Propionibacterium species. Vancomycin has several important clini­ cal uses. It is used for serious infections caused by MRSA, including health care–associated pneumonia, bacteremia, osteomyelitis, and endocarditis. It is also commonly used for skin and soft tissue infec­ tions. Oral vancomycin is not absorbed systemically and is reserved for the treatment of C. difficile infection. Vancomycin is also an alternative for the treatment of infections caused by MSSA in patients who cannot tolerate β-lactams. Resistance to vancomycin is a rising concern. Strains of vancomycin-intermediate S. aureus (VISA) and vancomycin-resistant enterococci (VRE) are not uncommon. Vanco­ mycin appears to be a concentration-dependent killer, with the AUC/ MIC ratio being the best predictor of efficacy (Fig. 149-1). Guidelines recommend targeting a vancomycin trough level of 15–20 μg/mL in MRSA infections in order to maintain an AUC/MIC ratio >400. When using vancomycin, clinicians should monitor for nephrotoxic­ ity. The risk of nephrotoxicity increases when trough levels are >20 μg/mL. Concomitant therapy with other nephrotoxic agents, such as aminoglycosides, also increases the risk. Ototoxicity was reported with early formulations of vancomycin but is currently uncommon because purer formulations are available. Both of these adverse effects are reversible upon discontinuation of vancomycin. Clinicians should be aware of vancomycin infusion reaction (formerly known as “red man syndrome”), a common reaction that presents as a rapid onset of erythematous rash or pruritus on the head, face, neck, and upper trunk. This reaction is caused by histamine release from basophils and mast cells and can be treated with diphenhydramine and slowing of the vancomycin infusion. Telavancin, dalbavancin, and oritavancin are structurally similar to vancomycin and are referred to as lipoglycopeptides. They have anti­ bacterial activity against S. aureus (including MRSA and some strains of VISA and vancomycin-resistant S. aureus [VRSA]), streptococci,

and enterococci. Oritavancin may have activity against some strains of VRE. These lipoglycopeptide agents also provide coverage against anaerobic gram-positive organisms except for Lactobacillus and some Clostridium species. The clinical efficacy of telavancin has been demonstrated in both skin and soft tissue infections and nosocomial pneumonia, and the efficacy of dalbavancin and oritavancin has been shown in skin and soft tissue infections. The vancomycin resistance phenotype may reduce the potency of all three lipoglycopeptides, but the rate of resistance to these drugs among S. aureus and enterococcal isolates has been low. Adverse effects of telavancin include nephrotox­ icity, metallic taste, and gastrointestinal side effects. Clinicians should be aware of the potential for electrocardiographic QTc prolongation that can increase the risk of cardiac arrhythmias when telavancin is used concomitantly with other QTc-prolonging agents. Telavancin may interfere with certain coagulation tests (e.g., causing false eleva­ tions in prothrombin time). Dalbavancin and oritavancin have safety profiles similar to that of vancomycin, with common effects reported as headache and gastrointestinal side effects. These glycolipopeptides should be used cautiously in patients with hypersensitivity reactions to vancomycin, as cross-allergy may be possible. ■ ■LIPOPEPTIDES Daptomycin is a lipopeptide antibiotic with activity against a broad range of gram-positive organisms. It is active against staphylococci (including MRSA and coagulase-negative staphylococci), streptococci, and enterococci. Daptomycin remains active against enterococci that are resistant to vancomycin. In addition, it exhibits activity against Bacillus, Corynebacterium, Peptostreptococcus, and Clostridium species. Daptomycin’s pharmacodynamic parameter for efficacy is concentrationdependent killing. Resistance to daptomycin is rare, but MICs may be higher for some VISA strains. Daptomycin can be used in skin and soft tissue infections, bacteremia, endocarditis, and osteomyelitis. It is an important alternative for MRSA and other gram-positive infections when bactericidal therapy is needed and vancomycin cannot be used. Daptomycin is generally well tolerated, and its main toxicity consists of elevation of creatine phosphokinase (CPK) levels and myopathy. CPK should be monitored during daptomycin treatment, and the drug should be discontinued if muscular toxicities occur. There have also been case reports of reversible eosinophilic pneumonia associated with daptomycin use. ■ ■AMINOGLYCOSIDES The aminoglycosides are a class of antibacterial agents with concen­ tration-dependent activity against most gram-negative organisms. The most commonly used aminoglycosides are gentamicin, tobramycin, and amikacin, although others, such as streptomycin, kanamycin, neomycin, and paromomycin, may be used in special circumstances. Plazomicin is a new aminoglycoside that is less affected by common resistance mechanisms and is approved for treatment of complicated urinary tract infections and acute pyelonephritis. Aminoglycosides have a significant dose-dependent postantibiotic effect; i.e., they have an antibacterial effect even after serum drug levels fall below inhibitory concentrations. The postantibiotic effect and concentration-dependent killing form the rationale behind extended-interval aminoglycoside dosing, in which a larger dose is given once daily rather than smaller doses multiple times daily. Aminoglycosides are active against gramnegative bacilli, such as Enterobacterales, P. aeruginosa, and Acineto­ bacter. They also enhance the activity of cell wall–active agents such as β-lactams or vancomycin against some gram-positive bacteria, including staphylococci and enterococci. This combination therapy is termed synergistic because the effect of both agents provides a killing effect greater than would be predicted from the effects of either agent alone. Amikacin and streptomycin have activity against Mycobacte­ rium tuberculosis, and amikacin has activity against nontuberculous mycobacteria, including Mycobacterium avium complex and Mycobac­ terium abscessus. The aminoglycosides do not have activity against anaerobes, S. maltophilia, or Burkholderia cepacia complex. Amino­ glycosides are used in clinical practice in a variety of infections caused by gram-negative organisms, including bacteremia and urinary tract

infections. They are frequently used in combination for the treatment of P. aeruginosa infection. When used in combination with a cell wall– active agent, gentamicin and streptomycin are also important for the treatment of gram-positive bacterial endocarditis. All aminoglycosides can cause nephrotoxicity and ototoxicity. The risk of nephrotoxicity is not well defined; however, some studies have indicated that the effect may be related to the duration of therapy as well as to the concomitant use of other nephrotoxic agents. Nephrotoxicity is usually reversible, but ototoxicity can be irreversible. ■ ■MACROLIDES AND KETOLIDES The macrolides (azithromycin, clarithromycin, and erythromycin) and ketolides (telithromycin) are classes of antibiotics that inhibit protein synthesis. Compared with erythromycin (the older antibi­ otic), azithromycin and clarithromycin have better oral absorption and tolerability. Azithromycin, clarithromycin, and telithromycin all have broader spectra of activity than erythromycin, which is less frequently used. These agents are commonly used in the treat­ ment of upper and lower respiratory tract infections caused by S. pneumoniae, H. influenzae, M. catarrhalis, and atypical organisms (e.g., Chlamydophila pneumoniae, Legionella pneumophila, and Myco­ plasma pneumoniae); group A streptococcal pharyngitis in penicillinallergic patients; and nontuberculous mycobacterial infections (e.g., caused by Mycobacterium marinum and Mycobacterium chelonae) as well as in the prophylaxis and treatment of M. avium complex infection in patients with HIV/AIDS and in combination therapy for Helicobacter pylori infection and bartonellosis. Enterobacterales, Pseudomonas species, and Acinetobacter species are intrinsically resistant to macrolides as a result of decreased membrane perme­ ability, although azithromycin is active against gram-negative diar­ rheal pathogens. The major adverse effects of this drug class include nausea, vomiting, diarrhea and abdominal pain, prolongation of QTc interval, exacerbation of myasthenia gravis, and tinnitus and reversible deafness, especially in the elderly. Azithromycin specifi­ cally has been associated with an increased risk of death, especially among patients with underlying heart disease, because of the risk of QTc interval prolongation and torsades de pointes arrhythmia. Erythromycin, clarithromycin, and telithromycin inhibit the CYP3A4 hepatic drug-metabolizing enzyme and can result in increased levels of coadministered drugs, including benzodiazepines, statins, war­ farin, cyclosporine, and tacrolimus. Azithromycin does not inhibit CYP3A4 and therefore does not interact with these drugs. ■ ■CLINDAMYCIN Clindamycin is a lincosamide antibiotic and is bacteriostatic against some organisms and bactericidal against others. It is used most often to treat bacterial infections caused by anaerobes (e.g., B. fragilis, Clostridium perfringens, Fusobacterium species, Prevotella melanino­ genicus, and Peptostreptococcus species) and susceptible staphylococci and streptococci. Clindamycin is used for treatment of dental infec­ tions, anaerobic lung abscess, and skin and soft tissue infections. It is used together with bactericidal agents (penicillins or vancomycin) to inhibit new toxin synthesis in the treatment of streptococcal or staphylococcal toxic shock syndrome. Other uses include treatment of infections caused by Capnocytophaga canimorsus, combination therapy for babesiosis and occasionally malaria, and therapy for toxoplas­ mosis. Clindamycin has excellent oral bioavailability. Adverse effects include nausea, vomiting, diarrhea, C. difficile–associated diarrhea and pseudomembranous colitis, maculopapular rash, and rarely StevensJohnson syndrome. ■ ■TETRACYCLINES The older (doxycycline, minocycline, and tetracycline) and newer (tigecycline, eravacycline, and omadacycline) tetracyclines inhibit protein synthesis and are bacteriostatic. These drugs have wide clini­ cal uses. They are used in the treatment of skin and soft tissue infec­ tions caused by gram-positive cocci (including MRSA), spirochetal infections (e.g., Lyme disease, syphilis, leptospirosis, and relapsing fever), rickettsial infections (e.g., Rocky Mountain spotted fever,

scrub typhus), atypical pneumonia, sexually transmitted infections (e.g., Chlamydia trachomatis infection, lymphogranuloma venereum, and granuloma inguinale), infections with Nocardia and Actinomyces, brucellosis, tularemia, Whipple’s disease, and malaria. Tigecycline, eravacycline, and omadacycline are also used in combination with other agents for treatment of M. abscessus. Tigecycline is a glycylcy­ cline derived from minocycline and is available only in IV formula­ tion. It is indicated in the treatment of complicated skin and soft tissue infections, complicated intraabdominal infections, and communityacquired bacterial pneumonia in adults. Tigecycline has activity against MRSA, vancomycin-sensitive enterococci, many Enterobacte­ rales, and Bacteroides species; it has no activity against P. aeruginosa. This drug has been used in combination with colistin for the treat­ ment of serious infections with multidrug-resistant gram-negative organisms. A pooled analysis of 13 clinical trials found an increased risk of death and treatment failure among patients given tigecycline alone; as a result, the FDA mandated a black box warning. Eravacy­ cline is a fluorocycline derivative available in IV formulation with a similar spectrum but more potent than tigecycline in vitro. It has been approved for complicated intraabdominal infections. Omadacycline is an aminomethylcycline derivative available in both IV and oral formulations. It has activity similar to that of tigecycline against grampositive pathogens but is less active against gram-negative pathogens. Omadacycline has been approved for treatment of bacterial skin and skin structure infections and community-acquired bacterial pneumonia. Tetracyclines have reduced absorption when orally coad­ ministered with calcium- and iron-containing compounds, including milk, and doses should be spaced at least 2 h apart. The major adverse reactions to old and new tetracyclines are nausea, vomiting, diarrhea, and photosensitivity. Tetracyclines have been associated with fetal bone-growth abnormalities and should be avoided during pregnancy and in the treatment of children <8 years old.

CHAPTER 149 ■ ■TRIMETHOPRIM-SULFAMETHOXAZOLE Trimethoprim-sulfamethoxazole (TMP-SMX) is an antibiotic with two components that each inhibit a separate step in folate synthesis and produce antibacterial activity. TMP-SMX is active against grampositive bacteria such as staphylococci and streptococci; however, its use against MRSA is usually limited to community-acquired infections, and its activity against Streptococcus pyogenes may not be reliable. This drug is also active against many gram-negative bacteria, including H. influenzae, E. coli, P. mirabilis, Neisseria gonorrhoeae, and S. maltophilia. TMP-SMX is not active against anaerobes or P. aeruginosa. It has many uses because of its wide spectrum of activity and high oral bioavailability. Urinary tract infections, skin and soft tissue infections, and respiratory tract infections are among the com­ mon uses. Another important indication is for both prophylaxis and treatment of Pneumocystis jirovecii infections in immunocompro­ mised patients. Resistance to TMP-SMX has limited its use against many Enterobacterales. Resistance rates among urinary isolates of E. coli are almost 25% in the United States. The most common adverse reactions associated with TMP-SMX are gastrointestinal effects such as nausea, vomiting, and diarrhea. In addition, rash is a common allergic reaction and may preclude the subsequent use of other sulfonamides. With prolonged use, leukopenia, thrombocyto­ penia, and granulocytopenia can develop. TMP-SMX can also cause nephrotoxicity, hyperkalemia, and hyponatremia, which are more common at high doses. TMP-SMX has several important interactions with other drugs (Table 149-3), including warfarin, phenytoin, and methotrexate. Treatment and Prophylaxis of Bacterial Infections ■ ■FLUOROQUINOLONES The fluoroquinolones include norfloxacin, ciprofloxacin, ofloxacin, levofloxacin, moxifloxacin, gemifloxacin, and delafloxacin. Cipro­ floxacin and levofloxacin have the broadest spectrum of activity against gram-negative bacteria, including P. aeruginosa (similar to that of third-generation cephalosporins). Because of the risk of selection of resistance during fluoroquinolone treatment of serious pseudomonal infections, these agents are usually used in combination with an

antipseudomonal β-lactam. Levofloxacin, moxifloxacin, gemifloxacin, and delafloxacin have additional gram-positive activity, including that against S. pneumoniae and some strains of MSSA, and with the excep­ tion of delafloxacin, these agents are used for treatment of communityacquired pneumonia. Strains of MRSA are commonly resistant to all fluoroquinolones except delafloxacin. Moxifloxacin is used as one component of second-line regimens for multidrug-resistant tubercu­ losis. Fluoroquinolones are no longer used for treatment of gonorrhea because of common resistance in N. gonorrhoeae. Fluoroquinolones exhibit concentration-dependent killing, are well absorbed orally, and have elimination half-lives that usually support once- or twicedaily dosing. Oral coadministration with compounds containing high concentrations of aluminum, magnesium, or calcium can reduce fluoroquinolone absorption. The penetration of fluoroquinolones into prostate tissue supports their use for bacterial prostatitis. Fluoroquino­ lones are generally well tolerated but can cause central nervous system (CNS) stimulatory effects, including seizures; peripheral neuropathy; glucose dysregulation; and tendinopathy associated with Achilles ten­ don rupture, particularly in older patients, organ transplant recipients, and patients taking glucocorticoids. Other potential effects on connec­ tive tissues include an association with increased risk of aortic aneu­ rysm. Worsening of myasthenia gravis also has been associated with quinolone use. Moxifloxacin causes modest prolongation of the QTc interval and should be used with caution in patients receiving other QTc-prolonging drugs.

■ ■RIFAMYCINS The rifamycins include rifampin, rifabutin, and rifapentine. Rifampin is the most commonly used rifamycin. For almost all therapeutic indications, it is used in combination with other agents to reduce the likelihood of selection of high-level rifampin resistance. Rifampin is used foremost in the treatment of mycobacterial infections—specifically, as a mainstay of combination therapy for M. tuberculosis infection or as a single agent in the treatment of latent M. tuberculosis infection. In addition, it is often used in the treatment of some nontuberculous mycobacterial infections. Rifampin is used in combination regimens for the treatment of staphylococcal infections, particularly prostheticvalve endocarditis and bone infections with retained hardware. It is a component of combination therapy for brucellosis (with doxycycline) and leprosy (with dapsone for tuberculoid leprosy and with dapsone and clofazimine for lepromatous disease). Rifampin can be used alone for prophylaxis in close contacts of patients with H. influenzae or N. meningitidis meningitis. The drug has high oral bioavailability, which is further enhanced when it is taken on an empty stomach. Rifampin has several adverse effects, including elevated aminotrans­ ferase levels (14%), rash (1–5%), and gastrointestinal events such as nausea, vomiting, and diarrhea (1–2%). Its many clinically relevant interactions with other drugs (Table 149-3) mandate the clinician’s careful review of the patient’s medications before rifampin initiation to assess safety and the need for additional monitoring, including monitoring of drug levels. PART 5 Infectious Diseases ■ ■METRONIDAZOLE Metronidazole is used in the treatment of anaerobic bacterial infections as well as infections caused by protozoa (e.g., amebiasis, giardiasis, trichomoniasis). It is the agent of choice as a component of combina­ tion therapy for polymicrobial abscesses in the lung, brain, or abdo­ men, the etiology of which often includes anaerobic bacteria, and for bacterial vaginosis, pelvic inflammatory disease, and anaerobic infec­ tions, such as those due to Bacteroides, Fusobacterium, and Prevotella species. This drug is an alternative agent for treatment of mild to moderate C. difficile–associated diarrhea. Metronidazole is bactericidal against anaerobic bacteria and exhibits concentration-dependent kill­ ing. It has high oral bioavailability and tissue penetration, including penetration of the blood-brain barrier. The majority of Actinomyces, Propionibacterium, and Lactobacillus species are intrinsically resistant to metronidazole. The major adverse effects include nausea, diarrhea, and a metallic taste. Concomitant ingestion of alcohol may result in a disulfiram-like reaction, and patients are usually instructed to avoid

alcohol during treatment. Long-term treatment carries the risk of leukopenia, neutropenia, peripheral neuropathy, and CNS toxicity manifesting as confusion, dysarthria, ataxia, nystagmus, and oph­ thalmoparesis. Through metronidazole’s effect on the CYP2C9 drugmetabolizing enzyme, its coadministration with warfarin can result in decreased metabolism and enhanced anticoagulant effects that require close monitoring. Concomitant administration of metronidazole with lithium can result in increased serum levels of lithium and associated toxicity; coadministration with phenytoin can result in phenytoin tox­ icity and possibly decreased levels of metronidazole. ■ ■OXAZOLIDINONES Linezolid is a bacteriostatic agent and is indicated for serious infections due to resistant gram-positive bacteria, such as MRSA and VRE. The intrinsic resistance of gram-negative bacteria is mediated primarily by endogenous efflux pumps. Linezolid has excellent oral bioavailability. Adverse effects include myelosuppression and ocular and peripheral neuropathy with prolonged therapy. Peripheral neuropathy may be irreversible. Linezolid is a weak, reversible monoamine oxidase inhibi­ tor, and coadministration with sympathomimetics and foods rich in tyramine should be avoided. Linezolid has been associated with sero­ tonin syndrome when coadministered with selective serotonin reup­ take inhibitors. Tedizolid has properties similar to those of linezolid, but with lower dosing due to greater potency, it may be less likely to cause adverse hematologic and neuropathic effects. ■ ■PLEUROMUTILINS Lefamulin is the only member of the pleuromutilin class approved for systemic use; it is available in IV and oral formulations. Lefamulin has in vitro activity against S. aureus (including MRSA), S. pneumoniae, H. influenzae, and atypical respiratory pathogens, including L. pneu­ mophila, M. pneumoniae, and C. pneumoniae, and has been approved for treatment of community-acquired bacterial pneumonia. Adverse effects are most commonly gastrointestinal, including diarrhea (12%), nausea (5%), and vomiting (3%). Prolongation of QTc interval and hepatic transaminase elevations occur in some patients. There can be interactions with drugs that are either inducers or inhibitors of CYP3A4 or P-glycoprotein transporter. ■ ■NITROFURANTOIN Nitrofurantoin’s antibacterial activity results from the drug’s conver­ sion to highly reactive intermediates that can damage bacterial DNA and other macromolecules. Nitrofurantoin is bactericidal, and its action is concentration dependent. It displays activity against a range of gram-positive bacteria, including S. aureus, Staphylococcus epidermidis, Staphylococcus saprophyticus, E. faecalis, Streptococcus agalactiae, group D streptococci, viridans streptococci, and corynebacteria, as well as gram-negative organisms, including E. coli, Enterobacter, Salmonella, and Shigella species. Nitrofurantoin is used primarily in the treatment of urinary tract infections and is preferred in the treatment of such infections in pregnancy. It may be used for the prevention of recurrent cystitis. Recently, there has been interest in the use of nitrofurantoin for treatment of urinary tract infections caused by ESBL-producing Enterobacterales such as E. coli, although resistance has been growing in Latin America and parts of Europe. Coadministration with magne­ sium should be avoided because of decreased absorption, and patients should be encouraged to take the drug with food to increase its bio­ availability and decrease the risk of adverse effects, which include nau­ sea, vomiting, and diarrhea. Nitrofurantoin may also cause pulmonary fibrosis and drug-induced hepatitis. Because the risk of adverse reac­ tions increases with age, the use of nitrofurantoin in elderly patients is not recommended. Patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency are at elevated risk for nitrofurantoin-associated hemolytic anemia. ■ ■POLYMYXINS Colistin and polymyxin B act by disrupting bacterial cell membrane integrity and are active against the nonenteric pathogens P. aeruginosa and A. baumannii but not against Burkholderia. These drugs also

exhibit activity against many Enterobacterales, with the exceptions of Proteus, Providencia, and Serratia species. They lack activity against gram-positive bacteria. Polymyxins are bactericidal and are available in IV formulations. Colistimethate is converted to the active form (colistin) in plasma. Polymyxins are most often used for infections due to pathogens resistant to multiple other antibacterial agents, including urinary tract infections, hospital-acquired pneumonia, and bloodstream infections. Nebulized formulations have been used for adjunctive treatment of refractory ventilator-associated pneumonia as well as a prevention strategy for patients with bronchiectasis and/ or cystic fibrosis. The most important adverse effect is dose-depen­ dent reversible nephrotoxicity. Neurotoxicity, including paresthesias, muscle weakness, and confusion, is reversible and less common than nephrotoxicity. ■ ■QUINUPRISTIN-DALFOPRISTIN Quinupristin-dalfopristin contains two members of the streptogramin class of antibiotics and kills bacteria by inhibiting protein synthesis. The antibacterial spectrum of quinupristin-dalfopristin includes staph­ ylococci (including MRSA), streptococci, and E. faecium (but not E. faecalis). This drug combination is also active against Corynebacterium species and L. monocytogenes. Quinupristin-dalfopristin is not reliably active against gram-negative organisms. It exhibits concentrationdependent killing, with an AUC/MIC ratio predicting efficacy. The clinical use of quinupristin-dalfopristin is largely for infections due to vancomycin-resistant E. faecium and other gram-positive bacterial infections. The drug has demonstrated efficacy in a variety of infec­ tions, including urinary tract infections, bone and joint infections, and bacteremia. Adverse effects associated with quinupristin-dalfopristin include infusion-related reactions, arthralgias, and myalgias. The arthralgias and myalgias may be severe enough to warrant drug discontinuation. Quinupristin-dalfopristin inhibits the CYP3A4 drugmetabolizing enzyme, with consequent drug interactions (Table 149-3). ■ ■FOSFOMYCIN Fosfomycin is a phosphonic acid antibiotic that has greater activity in acidic environments and is excreted in its active form in the urine. The oral formulation is primarily for prophylaxis and treatment of uncom­ plicated cystitis and should be avoided if there is concern about pyelo­ nephritis. The drug is administered as a single 3-g dose that results in high urine concentrations for up to 48 h. Fosfomycin is active against S. aureus, vancomycin-susceptible enterococci and VRE, and a wide range of gram-negative organisms, including E. coli, Enterobacter spe­ cies, Serratia marcescens, P. aeruginosa, and K. pneumoniae. Notably, the vast majority of ESBL-producing Enterobacterales are susceptible to fosfomycin. A. baumannii and Burkholderia species are resistant. The emergence of resistance to fosfomycin has not been observed during treatment of cystitis but has been documented during IV treat­ ment of respiratory tract infections and osteomyelitis. The few adverse effects that have been reported include nausea and diarrhea. ■ ■CHLORAMPHENICOL The use of chloramphenicol is limited by its potentially serious tox­ icities. When other agents are contraindicated or ineffective, chloram­ phenicol represents an alternative treatment for infections, including meningitis caused by susceptible bacteria such as N. meningitidis, H. influenzae, and S. pneumoniae. It has also been used for the treatment of anthrax, brucellosis, Burkholderia infections, chlamydial infections, clostridial infections, ehrlichiosis, rickettsial infections, and typhoid fever. Adverse reactions include aplastic anemia, myelosuppression, and gray baby syndrome. Chloramphenicol inhibits the CYP2C19 and CYP3A4 drug-metabolizing enzymes and consequently increases levels of many classes of drugs. APPROACH TO PROPHYLAXIS

OF INFECTION Antibacterial prophylaxis is indicated only in selected circumstances (Table 149-5) and should be supported by well-designed studies or expert panel recommendations. In all cases, the risk or severity

of the infection to be prevented should be greater than the adverse consequences of antibacterial therapy, including the potential for selection of resistance. In addition, the timing and duration of antibacterial treatment should be targeted for maximal effect and minimal required exposure. Prophylaxis of surgical infections tar­ gets bacteria that may contaminate the wound during the surgical procedure, including the skin flora of the patient or operating team and the air in the operating room. Delivery of the antibacterial drug within 1 h before the surgical incision is most effective because it maximizes tissue concentrations. For prolonged procedures, redosing may be necessary to maintain effective blood and tissue levels until the wound is closed. Additional dosing is not recommended after the incision is closed, and antimicrobials should be discontinued after incisional closure in the operating room. In patients with nasal car­ riage of S. aureus, preoperative decolonization with nasal mupirocin reduces the rate of S. aureus surgical-site infections and is generally recommended for high-risk procedures such as cardiac surgery and orthopedic implantation of prosthetic devices. For dental procedures, preprocedure antibacterial drugs are recommended for select patient populations to prevent transient bacteremia during the procedure and the seeding of certain high-risk cardiac lesions. Prophylaxis is also used in nonprocedural settings in certain patients who have recurrent infections or who are at risk of serious infection from a spe­ cific exposure (e.g., close contact with a patient with meningococcal meningitis). Extension of prophylaxis beyond the period of infection risk does not add further benefit and may increase the risk of resis­ tance selection or C. difficile disease.

CHAPTER 149 ANTIMICROBIAL STEWARDSHIP In an era of increasing prevalence of multidrug-resistant bacteria and with a substantial amount of inappropriate antimicrobial use, the need for rational antimicrobial prescribing has never been greater (Chap. 150). Antimicrobial stewardship describes the practice of promoting the selection of the appropriate drug, dosage, route, and duration of therapy. Antimicrobial stewardship programs implement a variety of strategies to (1) improve patient care through appropriate antimicrobial use; (2) preserve a vital health care resource by curbing the development of resistance within patient populations; (3) reduce the incidence of adverse effects; and (4) control costs. The Centers for Disease Control and Prevention (CDC) guidelines, The Joint Commission (TJC) Medication Management Standards, and the Centers for Medicare and Medicaid Services (CMS) Conditions of Participation, as well as the 2015 National Action Plan for Combating Antibiotic-Resistant Bacteria, have all supported antimicrobial stew­ ardship in various health care settings. Antimicrobial stewardship programs are typically multidisciplinary and often include infectious disease physicians, clinical pharmacists (usually with special training in infectious disease), clinical microbiologists, information systems specialists, infection prevention and control practitioners, and epi­ demiologists. These teams employ a variety of approaches to achieve the program’s goals. Treatment and Prophylaxis of Bacterial Infections Established strategies of antimicrobial stewardship programs include (1) prospective audit of antimicrobial use, with intervention and feedback; (2) formulary restriction; and (3) preauthorization. Prospective audit and feedback are usually undertaken by an infectious disease physician or a pharmacist. In this process, orders for broadspectrum antimicrobials (e.g., carbapenems) or agents for which more cost-effective alternatives may exist (e.g., daptomycin, ceftazidimeavibactam) are reviewed on a regular basis for appropriateness. In circumstances when antimicrobial use can be further optimized, the stewardship program team can intervene to recommend an alternative. This process has been successful in several quasi-experimental studies, resulting in declines in use of broad-spectrum drugs when unnecessary and decreases in adverse events, such as C. difficile infection. Formulary restriction is the inclusion of a limited set of antimicrobial agents in a hospital formulary for the purpose of limiting indiscriminate use of antimicrobials in the absence of demonstrated benefit. Such restriction also serves to avoid unnecessary drug expenditure. Preauthorization is the practice of requiring clinicians to obtain approval before using

TABLE 149-5  Prophylaxis of Bacterial Infections in Adults CONDITION ANTIBACTERIAL AGENTSa TIMING OR DURATION OF PROPHYLAXIS Surgical Clean (cardiac, thoracic, neurologic, orthopedic, vascular, plastic) Cefazolin (vancomycin,b clindamycin) 1 h before incision; re-dose with long procedures Clean (ophthalmic) Topical neomycin–polymyxin B–gramicidin, topical moxifloxacin Clean-contaminated (head and neck) Cefazolin + metronidazole, ampicillin-sulbactamc (clindamycin) 1 h before incision; re-dose with long procedures Clean-contaminated (hysterectomy, gastroduodenal, biliary, unobstructed small intestine, urologic) Cefazolin, ampicillin-sulbactamc (clindamycin + aminoglycoside, aztreonam, or fluoroquinolone) Clean-contaminated (colorectal, appendectomy) Cefazolin + metronidazole, ampicillin-sulbactam,c ertapenem (clindamycin + aminoglycoside, aztreonam, or fluoroquinolone) Dirty (ruptured viscus) Therapeutic regimen directed at anaerobes and gram-negative bacteria (e.g., ceftriaxone + metronidazole) Dirty (traumatic wound) Therapeutic regimen: cefazolin (clindamycin ± aminoglycoside, aztreonam, or fluoroquinolone) Nonsurgical Dental, oral, or upper respiratory procedures in patients with high-risk cardiac lesions (prosthetic valves, congenital heart defects, prior endocarditis) Amoxicillin PO, ampicillin IM (clindamycin PO, IV) Oral agents 1 h before procedure; injection 30 min before procedure Recurrent S. aureus skin infectionsd Mupirocine Intranasal application for 5 days Recurrent cellulitis associated with lymphatic disruptiond Benzathine penicillin IM monthly, oral penicillin or erythromycin twice daily Recurrent cystitis in womend Nitrofurantoin, TMP-SMX, fluoroquinolone After sexual intercourse or 3 times weekly for up to 1 year Bite wounds Amoxicillin-clavulanate (doxycycline, moxifloxacin) 3–5 days PART 5 Infectious Diseases Recurrent spontaneous bacterial peritonitis in cirrhotic patientsd Fluoroquinolonef Undefined Recurrent pneumococcal meningitis in patient with CSF leak or humoral immune defectd Penicillin Undefined Exposure to patient with meningococcal meningitis Rifampin, ciprofloxacin 2 days (rifampin), single dose (ciprofloxacin) High-risk neutropenia (ANC, ≤100/μL for >7 days)d Levofloxacin or ciprofloxacinf Until neutropenia resolves or fever dictates use of other antibacterials aRegimens in parentheses are alternatives for patients allergic to β-lactams. bVancomycin may be given together with cefazolin to patients known to be colonized with methicillin-resistant Staphylococcus aureus. cCefoxitin or cefotetan may also be considered. dNot considered routine for all patients, but an acceptable consideration among alternative approaches. eUsually coupled with bathing with chlorhexidine-containing skin antiseptic. fChoice of fluoroquinolone prophylaxis must be balanced against the risk of selection of resistance. Abbreviations: ANC, absolute neutrophil count; CSF, cerebrospinal fluid; TMP-SMX, trimethoprim-sulfamethoxazole. selected antimicrobials. Approval may be provided electronically with sophisticated Computerized Provider Order Entry (CPOE) software, after specific criteria for use are met, or after communication with an infectious disease specialist as designated by the stewardship program. These strategies have led to a decrease in C. difficile infections and to improvements in drug susceptibility patterns. Additional strategies used in specific health care settings are guide­ lines and pathways, dose optimization, parenteral-to-oral conversion, antibiotic time-out, and de-escalation of therapy. Documentation of the indication for which each antimicrobial is prescribed is also encouraged. Finally, antimicrobial stewardship teams provide ongoing education of best practices. An evolving and increasingly active area of clinical research to identify best practices, antimicrobial stewardship continues to grow as an essential service in various health care settings. The IDSA, in collaboration with several other professional organiza­ tions, has published guidelines for developing institutional antimicro­ bial stewardship programs (www.idsociety.org/Antimicrobial_Agents/). Acknowledgment The authors thank Christy A. Varughese for her significant contributions to this chapter in the previous editions. ■ ■FURTHER READING Barlam TF et al: Implementing an antibiotic stewardship program: Guidelines by the Infectious Diseases Society of America and the

Every 5–15 min for 5 doses immediately prior to procedure 1 h before incision; re-dose with long procedures 1 h before incision; re-dose with long procedures 1 h before incision; re-dose with long procedures; continue for 3–5 days after procedure 1 h before incision; re-dose with long procedures; continue for 3–5 days after procedure Undefined Society for Healthcare Epidemiology of America. Clin Infect Dis 62:e51, 2016. Bratzler DW et al: Clinical practice guidelines for antimicrobial pro­ phylaxis in surgery. Surg Infect (Larchmt) 14:73, 2013. Calderwood MS et al: Strategies to prevent surgical site infections in acute care hospitals: 2022. Update. Infect Control Hospital Epidemiol 44:695, 2023. Grayson ML et al (eds): Kucers’ The Use of Antibiotics. A Clinical Review of Antibacterial, Antifungal, Antiparasitic and Antiviral Drugs, 7th ed. Boca Raton, CRC Press, 2018. Infectious Diseases Society of America: Practice guidelines by organ system. Available at https://www.idsociety.org/practice-guideline/ practice-guidelines/. Jeffries MN et al: Consequences of avoiding β-lactams in patients with β-lactam allergies. J Allergy Clin Immunol 137:1148, 2016. Labreche MJ et al: Recent updates on the role of pharmacokinetics– pharmacodynamics in antimicrobial susceptibility testing as applied to clinical practice. Clin Infect Dis 61:1446, 2015. Rotschafer J et al (eds): Antibiotic Pharmacodynamics. Methods in Pharmacodynamics and Toxicology. New York, Humana Press, 2016. Shenoy ES et al: Evaluation and management of penicillin allergy: A review. JAMA 321:188, 2019. Tamma PD et al: Infectious Diseases Society of America 2023 guidance on the treatment of antimicrobial resistant gram-negative infections. Clin Infect Dis 18:ciae403, 2024.

31 - 150 Bacterial Resistance to Antimicrobial Agents

150 Bacterial Resistance to Antimicrobial Agents

David C. Hooper

Bacterial Resistance to Antimicrobial Agents ■ ■DEFINITION OF RESISTANCE The action of antimicrobial agents on a range of targets within the bacterial cell can result in inhibition of bacterial growth or in killing of the bacterial cell (Chap. 149). Reduction in or loss of an agent’s antibacterial effect is referred to as resistance, and the properties of or alterations in the bacterium that result in reduced antimicrobial activity are termed resistance mechanisms. Bacteria can be resistant to single or multiple antimicrobials, as detailed in the sections that follow. The occurrence and magnitude of resistance are often assessed in clinical microbiology laboratories by measurement of the lowest drug concentration that inhibits growth of a bacterium (minimal inhibitory concentration, or MIC) with a standardized inoculum and growth conditions. MIC values are generally interpreted as represent­ ing bacterial susceptibility, intermediate susceptibility, or resistance; the interpretation is based on correlations of the MIC values with the pharmacokinetics and delivery of a drug to the site of infection in the body as well as with data from clinical trials. Thus, a clinical laboratory result of “susceptible” for a bacterium predicts a likely clinical response to an appropriately dosed antimicrobial drug by a patient infected with that organism, whereas a result of “resistant” predicts poor or no clinical response to that drug. Breakpoint MIC values for categoriza­ tion of bacteria as susceptible, intermediate, or resistant are generally developed by regulatory and advisory groups and are often based on the distribution of MIC values from a large collection of recent clinical bacterial isolates. Research studies on the mechanisms and epidemiol­ ogy of resistance may in some cases use different and less rigid defini­ tions of resistance based on determination of a reproducible increase in an MIC value relative to a baseline reference MIC, independent of clinical breakpoints. ■ ■MECHANISMS OF RESISTANCE Bacteria use a wide variety of mechanisms to interrupt or circumvent the activity of antibacterial agents (Table 150-1 and Fig. 150-1). Although myriad, these mechanisms can generally be grouped into three categories: (1) alteration or bypassing of targets that exhibit reduced binding of the drug, (2) altered access of the drug to its target by reductions in uptake or increases in active efflux, and (3) a modi­ fication of the drug that reduces its activity. These mechanisms result from either mutations in bacterial chromosomal genes occurring spontaneously during bacterial DNA replication or the acquisition of new genes by DNA transfer from other bacteria or uptake of exogenous DNA. New genes are most often acquired on self-replicating plasmids or other DNA elements transferred from other bacteria. However, some bacteria, such as Streptococcus pneumoniae and Neisseria gonor­ rhoeae, can also take up fragments of environmental DNA from related bacterial species and recombine that DNA directly into their own chro­ mosomes, a process called transformation. Not uncommonly, resistant bacteria have combinations of resistance mechanisms either within one category or among categories; many plasmids contain more than one resistance gene; and bacteria can acquire multiple plasmids. Thus, plasmid acquisition itself can in many cases confer resistance to mul­ tiple antibacterial agents. Resistance to multiple, structurally unrelated antibiotics can also occur by mutations that cause increased expression or, less often, expanded substrate profiles of certain bacterial efflux pumps, some of which are able to transport multiple antibacterial agents as well as other compounds out of the cell. Many antibacterial drugs are derived from natural products of envi­ ronmental microbial species. Some genes encoding resistance to these drugs originate in the drug-producer organism to protect it from its own product and have then been mobilized onto plasmids that spread to other organisms. Surviving nonproducer bacteria in the exposed

natural environment may also have evolved resistance under selection pressure that adds to the reservoir of resistance mechanisms. Exposure to antibacterial agents either in nature or during human, animal, or other use then results in the selection of resistant strains within an otherwise susceptible bacterial population. In some cases, resistance mechanisms may confer disadvantages that render bacterial growth or survival fitness inferior to that of susceptible strains. In a number of examples, however, fitness defects are often mitigated over time by compensatory mutational mechanisms that make the bacteria both resistant and fit and thereby more likely to persist in a reservoir even in the absence of continued antimicrobial selection pressures. Discussed below are the major classes of antimicrobial agents currently in clinical use and the most important mechanisms of resistance encountered in clinical infections.

β-Lactams  β-lactams, the largest class of antibiotics, inhibit bac­ terial cell-wall synthesis by binding to cell-wall transpeptidases, cross-linking enzymes that are also called penicillin-binding proteins (PBPs); PBPs are targets that are unique to bacteria and have no mam­ malian counterpart. The most common mechanism of resistance to β-lactams, particularly in gram-negative bacteria, is their degradation by β-lactamases, enzymes that break down the core β-lactam ring and destroy drug activity. β-Lactamases differ in the spectrum of β-lactams they can degrade. Some β-lactamases are encoded on the bacterial chromosome, and their activity contributes to the intrinsic susceptibility profile of a particular species. Chromosomally encoded β-lactamases can be produced in varying amounts that affect the degree of resistance. In some cases, enzyme expression is physiologi­ cally induced by exposure to certain β-lactams; in other cases, enzyme expression can become constant or constitutive through mutations in genes that encode the regulators of expression of a β-lactamase gene. Other β-lactamases are encoded by genes on acquired plasmids and are usually constitutively expressed. The resistance profiles due to plasmids may be present in some strains of a species but not others, depending on which plasmids the strain has acquired. In gram-positive bacteria, β-lactamases are secreted into the extracellular environment, whereas in gram-negative bacteria these enzymes are secreted into the periplas­ mic space between the cytoplasmic and outer membranes—a limited space that enables the presence of high concentrations of β-lactamase. In gram-negative bacteria, access of β-lactams both to their target PBPs and to β-lactamases requires diffusion across the outer membrane, generally through the porin diffusion channels. Reductions in outermembrane diffusion channels due to mutation can further augment the efficiency of β-lactamase degradation of β-lactams: slow diffusion acts together with the high enzyme concentrations in the periplasmic space to enhance drug degradation and resistance. CHAPTER 150 Bacterial Resistance to Antimicrobial Agents Most strains of Staphylococcus aureus produce a plasmid-encoded β-lactamase that degrades penicillin but not semisynthetic penicil­ lins, such as oxacillin and nafcillin. The greatest diversity among β-lactamases, however, is found in gram-negative bacteria. The most common and earliest identified plasmid-encoded β-lactamases of gram-negative bacteria can inactivate all penicillins and most earlygeneration cephalosporins. Multiple extended-spectrum β-lactamase (ESBL) variants of these early enzymes have emerged and are now widely disseminated. These ESBLs can degrade later-generation cepha­ losporins (ceftriaxone, cefotaxime, ceftazidime) as well as the mono­ bactam aztreonam, and some ESBLs also degrade the fourth-generation cephalosporin cefepime. Carbapenems (imipenem, meropenem, ertapenem, doripenem) generally are not degraded by ESBLs, but additional β-lactamases, called carbapenemases, which degrade car­ bapenems and most if not all other β-lactams, have emerged and are increasing in prevalence. In the United States, Klebsiella pneu­ moniae carbapenemases (KPCs), which are usually found in strains of Escherichia coli and K. pneumoniae, are most widespread, but New Delhi metallo-β-lactamases (NDM carbapenemases), which were found initially on the Indian subcontinent, have now appeared and increased in the United States, as has an OXA group carbapen­ emase, OXA-48. In some cases, high levels of expression of an ESBL or an AmpC chromosomally encoded enzyme (see below), together

TABLE 150-1  The Most Common Mechanisms of Resistance to Antibacterial Agents ANTIBACTERIAL AGENT(S) MAJOR TARGET MECHANISM(S) OF ACTION MECHANISM(S) OF RESISTANCE Cell-wall synthesis Bind cell-wall cross-linking enzymes (PBPs, transpeptidases) β-Lactams (penicillins, cephalosporins, monobactams, carbapenems) Glycopeptides and lipoglycopeptides (vancomycin, teicoplanin, telavancin, dalbavancin, oritavancin) Cell-wall synthesis Block cell wall glycosyltransferases by binding d-Ala-d-Ala stem-peptide terminus Teicoplanin, telavancin, dalbavancin, and oritavancin: affect membrane function Bacitracin Cell-wall synthesis Blocks lipid carrier of cell wall precursors Active drug efflux Fosfomycin Cell-wall synthesis Blocks linkage of stem peptide to NAG by enoyltransferase Aminoglycosides (gentamicin, tobramycin, amikacin, plazomicin) Protein synthesis Bind 30S ribosomal subunit Block translocation of peptide chain Cause misreading of mRNA Tetracyclines (tetracycline, doxycycline, minocycline) Protein synthesis Bind 30S ribosomal subunit Inhibit peptide elongation Tigecycline, eravacycline, omadacycline Protein synthesis Same as tetracyclines Active drug efflux (pumps different from those affecting tetracyclines) Macrolides (erythromycin, clarithromycin, azithromycin) and the ketolide telithromycin Protein synthesis Bind 50S ribosomal subunit Block peptide chain exit Lincosamides (clindamycin) Protein synthesis Bind 50S ribosomal subunit Block peptide bond formation Streptogramins (quinupristin, dalfopristin) Protein synthesis Same as macrolides Same as macrolides Drug-modifying enzymes PART 5 Infectious Diseases Chloramphenicol Protein synthesis Binds 50S ribosomal subunit Blocks aminoacyl tRNA positioning Oxazolidinones (linezolid, tedizolid) Protein synthesis Bind 50S ribosomal subunit Inhibit initiation of peptide synthesis Pleuromutilins (lefamulin) Protein synthesis Bind 50S ribosomal subunit Blocks peptidyl transferase center Mupirocin Protein synthesis Blocks isoleucyl tRNA synthetase Acquired resistant tRNA synthetase (drug bypass) Altered native tRNA synthetase target Sulfonamides (sulfadiazine, sulfisoxazole, and sulfamethoxazole) Folate synthesis Inhibit dihydropteroate synthetase Acquired resistant dihydropteroate synthetase (drug bypass) Trimethoprim Folate synthesis Inhibits dihydrofolate reductase Acquired resistant dihydrofolate reductase (drug bypass) Quinolones (norfloxacin, ciprofloxacin, ofloxacin, levofloxacin, moxifloxacin, gemifloxacin, delafloxacin) DNA synthesis Inhibit DNA gyrase and DNA topoisomerase IV Enzyme–DNA–drug complex: blocks DNA replication apparatus Rifamycins (rifampin, rifabutin, rifapentine) RNA synthesis Inhibit RNA polymerase Altered target Nitrofurantoin Nucleic acid synthesis Reduces reactive drug derivatives that damage DNA Metronidazole Nucleic acid synthesis Reduces reactive drug derivatives that damage DNA Polymyxins (polymyxin B and polymyxin E [colistin]) Cell membrane Bind LPS and disrupt both outer and cytoplasmic membranes Daptomycin Cell membrane Produces membrane channel and membrane leakage Abbreviations: LPS, lipopolysaccharide; NAG, N-acetylglucosamine; PBP, penicillin-binding protein. with reduced porin diffusion channels, can also result in resistance to carbapenems. In Pseudomonas aeruginosa, resistance to carbapenems can occur by mutations that cause reductions in the OprD diffusion channel for imipenem or increased expression of efflux pumps that can remove meropenem from the bacterial cell.

Drug inactivation by β-lactamases Altered PBP targets Reduced diffusion through porin channels Altered iron uptake proteins (cefiderocol) Altered d-Ala-d-Ala target (d-Ala-d-Lac) Increased d-Ala-d-Ala target binding at sites distant from cell wall synthesis enzymes Target enzyme overexpression Drug-modifying enzymes Drug-modifying enzymes Methylation at ribosome binding site Decreased permeation to target due to active efflux Active drug efflux Ribosomal protection proteins Methylation at ribosome binding site Active drug efflux Methylation at ribosome binding site Drug-modifying enzymes Altered rRNA binding site Methylation of ribosome binding site Altered L3 and L4 protein binding site Methylation of ribosome binding site Altered target(s) Active efflux Protection of target from drug Drug-modifying enzyme (ciprofloxacin) Altered drug-activating enzymes Altered drug-activating enzyme Acquired detoxifying enzymes Active efflux Altered cell-membrane charge with reduced drug binding Altered cell-membrane charge with reduced drug binding The chromosomal β-lactamase of K. pneumoniae preferentially degrades penicillins over cephalosporins. In contrast, the chromo­ somal β-lactamase of Enterobacter and related genera, AmpC, can degrade almost all cephalosporins but is normally expressed in only small amounts. Mutations in regulatory genes that cause increased

Gram-Negative Bacterium Antibiotic a-lactamases in periplasmic space b-lactams (including carbapenems for some b-lectamases) Loss of porins carbapenems (imipenem) Porin Altered iron uptake pathways (cefiderocol) Antibiotic Plasmid with antibioticresistant genes Bypass targets trimethoprim (dihydrofolate reductase), sulfonamides (dihydropteroate synthase) Ribosomes Ribosomal mutation or modification tetracyclines, ozazolidinones, lefamulin (TetM or TetO), aminoglycosides (rRNA methylation) Mutations in lipopolysaccharide structure polymyxin antibiotic class FIGURE 150-1  Antibacterial targets and mechanisms of resistance to antibacterial agents, as illustrated in a gram-negative bacterium. Similar mechanisms are found in gram-positive bacteria, but their lack of an outer membrane causes β-lactamases to be excreted outside the cell, rather than into the periplasmic space between the inner and outer membranes, and reduces the efficiency of efflux pumps because exported drugs can re-enter the cell after crossing a single membrane, rather than the two membranes in gram-negative bacteria. Red spheres indicate antibiotics. (From AY Peleg, DC Hooper: Hospital-acquired infections due to gram-negative bacteria. N Engl J Med 362:1084, 2010. Copyright © 2010 Massachusetts Medical Society. Reprinted with permission from Massachusetts Medical Society.) amounts of AmpC to be produced can cause full resistance to penicil­ lins and cephalosporins; the exceptions are cefoxitin and cefepime, which are relatively stable to AmpC. Resistance to cefepime can develop, however, through the combined effects of mutations that cause increased AmpC production and decreased porin diffusion channels. Genes encoding AmpC have also been found on plasmids but are less common than plasmid-encoded ESBLs. A recent novel cephalosporin, cefiderocol, has enhanced stability to β-lactamases and, due to a catechol side group, is actively taken up into the bacterial cell by siderophore iron uptake pathways, rather than diffusing pas­ sively through porin channels. It is active against many gram-negative bacteria that are resistant to other β-lactams, including carbapenems. Reduced susceptibility has been reported to occur in strains with muta­ tions in multiple iron transport genes. Inhibitors of β-lactamases such as clavulanate, sulbactam, tazobac­ tam, avibactam, vaborbactam, relebactam, and durlobactam have been developed and paired with amoxicillin and ticarcillin (clavulanate), ampicillin (sulbactam), piperacillin and ceftolozane (tazobactam), ceftazidime (avibactam), meropenem (vaborbactam), imipenem (rele­ bactam), and sulbactam (durlobactam), which has selective activity against Acinetobacter. These inhibitors, with the exception of sulbac­ tam, have little or no antibacterial activity of their own but inhibit plasmid-mediated β-lactamases, including ESBLs. Only avibactam, vaborbactam, and relebactam inhibit AmpC enzymes and some car­ bapenemases (KPCs but not metallo-carbapenemases, such as NDM). Resistance to β-lactams also occurs through alterations in the drugs’ target transpeptidase enzymes (PBPs) involved in cross-linking of the bacterial cell-wall peptidoglycan structure. In S. pneumoniae,

Overexpression of transmembrane efflux pump b-lactams (meropenem), quinolones, aminoglycosides, tetracycline antibiotics (tigecycline), and chloramphenicol Antibiotic Antibiotic-modifying enzymes aminoglycosides, ciprofloxacin Target mutations quinolones (DNA gyrase and topoisomerase IV) Proteins CHAPTER 150 Protein Lipopolysaccharide Bacterial Resistance to Antimicrobial Agents N. gonorrhoeae, and Neisseria meningitidis, resistance to penicillin occurs by recombination of transformed DNA from related species that results in mosaic PBPs with lower affinity for penicillin. A combination of increased expression of an efflux pump and a porin mutation also causes penicillin resistance in N. gonorrhoeae. In staphylococci, resis­ tance to methicillin and other β-lactams occurs by acquisition of the mec gene, which encodes a PBP2a with reduced drug affinity. PBP2a is a bypass target that can function in cell wall cross-linking in the pres­ ence of β-lactams, bypassing their effect on other PBPs. Ceftaroline is the only β-lactam that has affinity for PBP2a and is thus active against methicillin-resistant staphylococcal strains. Resistance to ceftaroline can occur, however, by mutations in the gene encoding PBP2a that reduce its affinity for the drug. Glycopeptides and Lipoglycopeptides  Glycopeptides and lipoglycopeptides inhibit bacterial cell-wall synthesis by binding to the terminal two d-alanine amino acids on the cell-wall peptidoglycan stem peptides, which are involved in peptidoglycan cross-links. In doing so, these drugs block the transpeptidase cross-linking enzymes and glycosyl transferases necessary for cell-wall synthesis. Resistance to vancomycin in enterococci is due to the acquisition of a set of van genes that result in (1) the production of d-alanine-d-lactate—instead of the normal d-alanine-d-alanine—at the end of the peptidoglycan stem peptide and (2) the reduction of existing d-alanine-d-alanine–termi­ nated peptides. Vancomycin binds d-alanine-d-lactate with a 1000-fold lower affinity than d-alanine-d-alanine. The van genes originated in the organisms that naturally produce vancomycin and have been mobilized and reorganized in transposon mobile genetic elements

and onto plasmids, which can be transferred between enterococci. In rare cases, the van gene cassettes have been transferred from entero­ cocci to S. aureus, with the consequent generation of full vancomycin resistance. In S. aureus, intermediate resistance to vancomycin is more common than full vancomycin resistance and is due to a different mechanism that results from a series of several chromosomal muta­ tions leading to a thickened and poorly cross-linked cell wall. This modified cell wall contains additional d-alanine-d-alanine–terminated stem peptides that bind vancomycin at a site distant from the cell membrane, adjacent to which new peptidoglycan is synthesized and where vancomycin binding blocks transpeptidase and transglycosylase enzymes. Thus, vancomycin’s binding to these distant termini impedes its access to the proximal binding sites that result in inhibition of peptidoglycan synthesis. This intermediate-resistance phenotype was first recognized in patients receiving prolonged courses of vancomycin that created an opportunity for selection of the multiple mutations needed to produce the modified cell wall. Because of the energy costs of a thickened cell wall, this intermediate-resistance phenotype may be unstable, with strains returning to susceptibility in the absence of van­ comycin selection pressure. Susceptibility to telavancin, dalbavancin, and oritavancin is also reduced in strains that exhibit resistance or intermediate susceptibility to vancomycin, although in some cases, the drugs remain sufficiently active that the strains may still be classified as susceptible based on standard clinical laboratory interpretive criteria.

Aminoglycosides  Aminoglycosides are one of several classes of antimicrobials that inhibit protein synthesis by binding to either the 30S or the 50S bacterial ribosomal subunit (both of which differ from eukaryotic ribosomal subunits), with consequent selective antibacterial activity. The aminoglycosides bind to the 30S subunit of the bacterial ribosome. The most common mechanism of resistance to aminogly­ cosides in gram-negative bacteria is due to acquisition of plasmids with genes encoding transferase enzymes that modify aminoglycosides by the addition of acetyl, adenyl, or phosphate groups; these added groups decrease the drugs’ binding affinity to their ribosomal target site. Various transferases differ in which aminoglycosides they modify, and amikacin resistance occurs less often than resistance to gentamicin or tobramycin by these mechanisms. Plazomicin, a recently developed aminoglycoside, is distinctive in that it remains active and is not modi­ fied by most transferases. Another mechanism of plasmid-mediated aminoglycoside resistance is due to methylase enzymes that can methyl­ ate the site of aminoglycoside binding on the 16S ribosomal RNA of the 30S ribosomal subunit and reduce drug binding to its ribosome target, resulting in resistance to all aminoglycosides, including plazomicin. For streptomycin, a single ribosomal protein mutation may also cause resistance. In P. aeruginosa, aminoglycoside resistance can also occur through mutations in regulatory genes causing increased expression of a chromosomally encoded efflux pump, MexXY, which reduces intracel­ lular drug concentrations. PART 5 Infectious Diseases Tetracyclines  These antibiotics bind the 16S ribosomal RNA of the 30S ribosomal subunit at a site distinct from the binding site of the ami­ noglycosides and inhibit bacterial protein synthesis. For tetracyclines, including doxycycline and minocycline, resistance is often plasmidmediated and due either to active efflux pumps, which in some cases are specific for tetracyclines, or to proteins that protect the ribosome from tetracycline action. Some broad-spectrum, chromosomally encoded efflux pumps may also include tetracyclines among their substrates, and regulatory mutations that cause pump overexpression may confer tetracycline resistance together with resistance to other agents that are pump substrates. There have been recent derivatives of the tetracyclines, including the glycylcycline, tigecycline, the fluorocycline, eravacycline, and the aminomethylcycline, omadacycline, which have modifications on the core tetracycline ring structure rendering them less affected or unaffected by the common tetracycline resistance mechanisms. Resis­ tance to the newer tetracyclines can occur, however, through mutations that cause overexpression of some broad-spectrum efflux pumps, partic­ ularly in Proteus species. An uncommon plasmid-encoded tetracycline modification mechanism can also cause resistance to the newer agents.

Macrolides, Ketolides, Lincosamides, and Streptogramins  These antibiotics are also inhibitors of bacterial protein synthesis, in this case through their binding to the 23S RNA of the 50S ribosomal subunit. They are generally active against gram-positive bacteria. Resistance to macrolides, clindamycin, and quinupristin is most often due to acquired Erm methylases that modify the drug-binding site on the ribosome, reducing drug binding. Resistance to quinupristin by this mechanism renders the quinupristin-dalfopristin combination bacteriostatic rather than bactericidal. Telithromycin, a ketolide structurally related to macro­ lides, has an additional binding site on the ribosome and remains active in the presence of some methylases. Methylase gene expression can be induced by exposure to most macrolides but generally not ketolides (e.g., telithromycin); however, bacterial strains constitutively expressing methylase genes can display resistance to both macrolides and ketolides. Acquired genes encoding active efflux pumps can also contribute to resistance to macrolides in streptococci and to resistance to macro­ lides, clindamycin, and dalfopristin in staphylococci. Plasmid-acquired, drug-modifying enzymes in staphylococci can also cause resistance to quinupristin and dalfopristin. Macrolide resistance due to 23S rRNA mutations at the site of drug binding is uncommon in staphylococci and streptococci because of the multiple copies of the rRNA genes on the chromosomes of these species; such resistance mechanisms may occur more frequently, however, in mycobacteria, Helicobacter pylori, and Treponema species, which have only one or two chromosomal cop­ ies of these rRNA genes, thus requiring fewer mutations to generate a resistance phenotype. Among gram-negative bacteria, many of which are not susceptible to current macrolides because of inadequate drug perme­ ation across the outer membrane, some strains with acquired genes for macrolide-modifying enzymes have been described. Chloramphenicol  Chloramphenicol inhibits bacterial protein synthesis by binding to the 23S rRNA of the 50S subunit at a site that overlaps the macrolide-binding site. Chloramphenicol is uncommonly used in human medicine because of infrequent but potentially severe bone marrow toxicity. Resistance to chloramphenicol is most often due to plasmid-encoded, drug-modifying acetyltransferases that have been found in both gram-positive and gram-negative bacteria and whose expression can be induced by drug exposure. Among staphylococci, some resistant strains have been found to have a plasmid-encoded ribosomal methylase that confers resistance to chloramphenicol, clindamycin, and oxazolidinones. As is the case for macrolides, ribo­ somal mutations causing resistance to chloramphenicol are uncommon because of multiple copies of rRNA genes in the common grampositive human pathogens. Plasmid-encoded efflux pumps affecting chloramphenicol specifically have been found in gram-negative bac­ teria, and other pumps affecting chloramphenicol and oxazolidinones have been found in gram-positive bacteria. Oxazolidinones  Linezolid and tedizolid are the only members of the oxazolidinone class of antimicrobials in clinical use, and both are active against gram-positive bacteria only; lack of sufficient activity in gram-negative bacteria results from the ability of native efflux pumps in these bacteria to limit drug access to their cytoplasmic ribosome targets. Oxazolidinones target the bacterial ribosome and inhibit pro­ tein synthesis by binding to 23S rRNA of the 50S subunit at a distinct site that overlaps with the chloramphenicol-binding site. Resistance has been seen in enterococci more often than in staphylococci and, in both organisms, is most often due to mutations in multiple copies of the 23S rRNA genes that reduce drug binding to the ribosome. A plasmidacquired ribosomal methylase gene that enables ribosomal alteration at a site that confers resistance to both linezolid and chloramphenicol has also been found in some strains of both S. aureus and coagulasenegative staphylococci but is not yet widespread. A plasmid-encoded active efflux pump conferring resistance to oxazolidinones (both line­ zolid and tedizolid) and chloramphenicol has been described in animal isolates and a small number of human isolates of Enterococcus faecalis. Pleuromutilins  Lefamulin is the only systemic pleuromutilin in clinical use. Retapamulin has been available for topical use in skin infections. Pleuromutilins inhibit bacterial protein synthesis by

Bacterial Resistance to Antimicrobial Agents

CHAPTER 150 binding to the peptidyl transferase center in the 50S ribosomal sub­ unit, and lefamulin is generally active against gram-positive bacteria, Haemophilus influenzae, Moraxella catarrhalis, and atypical respira­ tory pathogens, such a Mycoplasma pneumoniae and Legionella spp. Although there is partial overlap in the site of binding of lefamulin and those of other antibacterials binding the 50S ribosomal subunit, cross-resistance with macrolides, oxazolidinones, lincosamides, and streptogramins is uncommon. Resistance to lefamulin can occur by mutations in L3 and L4 proteins of the 50S subunit that alter the lefamulin binding site. In addition, the plasmid-encoded Cfr methyl­ ase, which confers resistance to chloramphenicol and oxazolidinones, can also cause resistance to lefamulin by disrupting its binding site. Vga efflux transporters, which cause resistance to lincosamides and streptogramins, also affect pleuromutilins. Mupirocin  Mupirocin is used only in topical formulations, most often for elimination of nasal carriage of S. aureus. It targets bacterial leucyl-tRNA synthetase and inhibits protein synthesis. Resistance to mupirocin occurs by either mutation in the target leucyl-tRNA syn­ thetase (low-level resistance) or the acquisition of a plasmid-encoded resistant tRNA synthetase (high-level resistance), which bypasses drug inhibition of the native, sensitive synthetase. Sulfonamides and Trimethoprim  These agents inhibit the folate biosynthesis pathway at different steps. Sulfonamides are structurally similar to para-aminobenzoic acid (PABA) and competitively inhibit dihydropteroate synthetase, which, in an early step in the pathway, uses PABA to synthesize dihydropteroate, a precursor of dihydrofolate. Trimethoprim inhibits dihydrofolate reductase at a later step in the pathway that generates tetrahydrofolate. Clinical use of folate pathway inhibitors most often consists of the combination of sulfamethoxazole and trimethoprim; on occasion, however, trimethoprim or various sulfonamides are used individually. Resistance to both of these anti­ metabolites can result from mutation in their target enzymes or can be due to plasmid-acquired genes encoding resistant enzymes that bypass the inhibition of the native sensitive enzymes—a resistant dihydropteroate synthetase in the case of sulfonamides and a resistant dihydrofolate reductase in the case of trimethoprim. Resistance to the combination of sulfamethoxazole and trimethoprim requires that the bacterial strain have resistance mechanisms for both agents and yet is relatively common. Resistance due to drug efflux or drug modification has been limited for both sulfonamides and trimethoprim. Quinolones  Quinolones are synthetic inhibitors of bacterial DNA synthesis. They bind to two enzymes required for DNA synthesis: DNA gyrase and DNA topoisomerase IV, which alter DNA conformation and the interlinking of replicated molecules. In addition to inhibiting the enzymes’ catalytic functions of altering DNA topology, they stabilize enzyme–DNA complexes that form a barrier to the DNA replication machinery and are a precursor to lethal double-strand DNA breaks. Although related topoisomerase enzymes are involved in mammalian DNA synthesis, the mammalian and bacterial enzymes are sufficiently different from each other for quinolones to have selective activity against bacteria. Resistance to quinolones is most often due either to chromosomal mutations altering the target enzymes DNA gyrase and DNA topoisomerase IV, with consequent reduction in drug binding, or to mutations that increase the expression of native broad-spectrum efflux pumps for which quinolones (among other compounds) are sub­ strates. In addition, three types of acquired genes can confer reduced susceptibility or low-level resistance by protecting the target enzymes, modifying some quinolones (particularly ciprofloxacin and norfloxacin) to reduce their activity, or generating an efflux of quinolones. These genes are usually located on multidrug-resistance plasmids that have spread worldwide. Their presence can promote higher levels of quino­ lone resistance by enhancing selection of the mutations in chromosomal target genes with exposure to quinolones and can then link quinolone resistance to resistance to other antibacterial drugs that are encoded by the same plasmid. Rifampin, Rifabutin, and Rifapentine  Antimicrobials of the rifamycin class target bacterial RNA polymerase and thereby inhibit transcription of messenger RNA and gene expression. Their activity is generally limited to gram-positive bacteria because native efflux pumps in most gram-negative bacteria reduce drug access to the cytoplasmic enzyme target. Single mutations in the β subunit of RNA polymerase con­ stitute the principal mechanism of acquired rifampin resistance, which is high-level. Thus, rifampin and other rifamycins are used for treatment of infections only in combination with other antibacterial drugs in order to reduce the likelihood of selection of high-level resistance. Metronidazole  Metronidazole is actively taken up by most anaero­ bic bacteria and then converted to reactive drug derivatives that nonspecifically damage cytoplasmic proteins and nucleic acids. Thus, metronidazole lacks a specific cellular target. Acquired resistance to metronidazole in Bacteroides species is rare. Such resistance has been reported in strains that lack the endogenous activating nitroreductase or that have acquired nim genes responsible for further reduction of the DNA-damaging nitroso intermediates to an inactive derivative. Active efflux and enhanced DNA repair mechanisms also have been associ­ ated with resistance. Nitrofurantoin  Nitrofurantoin is used only for treatment of lower urinary tract infections because adequate drug concentrations are found only in urine. Its mechanism of action is not fully understood but is thought to involve generation of reactive derivative molecules (as occurs with metronidazole) that damage DNA and ribosomes. Resis­ tance to nitrofurantoin in E. coli can emerge through a series of muta­ tions that progressively decrease the nitroreductase activity required for generating active nitrofuran metabolites. These mutants are also impaired in growth; this impairment possibly explains the infrequent occurrence of resistance with clinical use of nitrofurantoin. Polymyxins  Because of emerging multidrug resistance in gramnegative bacteria, colistin and polymyxin B have been used for infec­ tions due to resistant Enterobacterales, P. aeruginosa, and Acinetobacter species. Polymyxins are cationic cyclic peptide molecules that bind negatively charged lipopolysaccharides on the gram-negative bacterial outer membrane, with subsequent disruption and permeabilization of both outer-membrane and cytoplasmic-membrane structure. Thus, the polymyxins are bactericidal. Resistance is so far uncommon but can emerge during therapy through mutations that cause reductions in the negative charge of the gram-negative bacterial cell surface, thereby reducing binding of the positively charged colistin. Transferable plasmid-mediated colistin resistance has also been found to be due to mcr-1 and other mcr variants that encode a gene for a phosphoetha­ nolamine transferase that also reduces the negative charge on the cell surface. mcr-containing enteric bacteria have now been identified in Asia, Europe, and the United States. Daptomycin  Daptomycin is active against gram-positive bacteria and interacts with and disrupts the cytoplasmic membrane in a calcium-dependent manner, resulting in bactericidal activity. The mechanisms of resistance to daptomycin are complex and involve mutations in several genes that can alter cell membrane charge and structure and reduce daptomycin binding. Resistance to daptomycin is relatively infrequent but has emerged in some S. aureus strains with intermediate vancomycin susceptibility from patients treated with van­ comycin but not exposed to daptomycin. In some strains of methicillinresistant S. aureus, daptomycin resistance has been linked to acquired susceptibility to β-lactams; combinations of daptomycin with nafcillin or ceftaroline have been successful for treatment of patients infected with resistant strains when daptomycin alone or in combination with other agents has failed. The mechanism of this effect is not yet clear but may involve alteration in surface charge and increased daptomycin binding in the presence of β-lactams. Daptomycin resistance has also been reported in enterococci. ■ ■EPIDEMIOLOGY OF RESISTANCE AND REDUCTION OF ITS OCCURRENCE Multidrug resistance in human bacterial infections has been increasing overall in recent years, substantially limiting the number of antibiotics that can be used to treat some infections. The prevalence of resistance

to various antimicrobials among human pathogens can, however, vary greatly in different geographic areas and even at different institutions in the same area. Thus, specific local data on the occurrence of various types of resistance are an important component of the choice of anti­ microbials for empirical treatment of infection until the responsible pathogen is identified and its specific susceptibilities are determined by the clinical microbiology laboratory. Prompt adjustment of the initially chosen antimicrobial on the basis of species and susceptibility data to best target therapy is equally important. These principles emphasize the importance of obtaining appropriate samples for culture or other diagnostic modalities and susceptibility testing—whenever possible, prior to administration of antimicrobials. They also highlight the importance of rapid and sensitive diagnostic methods and the prompt communication of their results to clinicians to inform best choices of antimicrobials.

The overall prevalence of resistance can be affected by a number of factors, including (1) the extent of resistance reservoirs in the patient population; (2) the selection pressures from use of antimicrobials that favor resistant strains over susceptible ones; and (3) the extent by which resistance is amplified by transmission of resistant strains to patients from their environment or other persons, either directly or indirectly via the contaminated hands of health care workers when hand hygiene and other infection control practices are inadequately followed. The likelihood that an individual patient will be infected with a resistant pathogen is likewise affected by his or her history. Studies have shown that prior antibiotic treatment, prior infection with resistant pathogens, and prior hospitalizations all increase this likelihood. These factors emphasize the importance of the appropriate use of antimicrobials (particularly, the avoidance of their use in clinical conditions in which they are not needed), the use of the shortest courses of therapy sufficient for a successful clinical outcome, and the implementation of antimicrobial stewardship programs (Chap. 149) as well as careful and consistent infection control practices in short-term and long-term-care institutions. Antimicrobial agents are distinct among drug classes in human medicine in that—despite their clear clinical value when used appropriately—the extent of their use can compromise their future utility because of resistance. The remarkable ability of pathogens to acquire resistance is inherent in their biology and emphasizes the necessity for clinicians and institu­ tions to pay careful attention to those factors that can be controlled through judicious antimicrobial use and rigorous infection control and prevention practices. PART 5 Infectious Diseases Efforts to address the problems caused by resistance are now being made worldwide. The U.S. Centers for Disease Control and Preven­ tion (CDC) has recently estimated that >2.8 million resistant bacterial infections occur in the United States each year, with 35,900 deaths, and has identified particular resistant pathogens that are of great­ est concern because of their overall effects on public health (Table 150-2). Enteric bacteria (such as E. coli, K. pneumoniae, and Entero­ bacter spp.) and Acinetobacter spp. that are resistant to carbapenems are included in the “urgent” category because of their increasing occurrence worldwide and because they are often highly resistant to multiple drugs, with few if any active antimicrobials available for treatment. Resistant N. gonorrhoeae is included in this category as well because of the ease with which gonorrhea can be spread from person to person and because limited active agents are now available. Other resistances are common and also affect clinical care, often requiring use of alternatives to first-line agents that can be less effective and less well tolerated. Also affecting clinical care and considered urgent are infections due to Clostridioides difficile. Although not directly due to acquired resistance, C. difficile disease, like resistant bacterial infec­ tions, is linked to antibacterial use (by the disruption of the normal microbiome of the gastrointestinal tract rather than direct resistance selection) and to its ability as a spore-forming bacterium to be spread in health care environments. To address the problems posed by resis­ tance and C. difficile disease, the CDC has emphasized a set of five core actions. (1) Infection prevention and control: These efforts focus on implementation of evidence-based activities to reduce the risks and incidence of device-related infections overall and on improvement of

TABLE 150-2  Antibiotic Resistance Threats in the United States, 2019 THREAT CATEGORY ORGANISMS Urgent Carbapenem-resistant Acinetobacter Candida auris Clostridioides difficile Carbapenem-resistant Enterobacterales Drug-resistant Neisseria gonorrhoeae Serious Drug-resistant Campylobacter Drug-resistant Candida Extended-spectrum β-lactamase–producing Enterobacterales Vancomycin-resistant Enterococcus Multidrug-resistant Pseudomonas aeruginosa Drug-resistant nontyphoidal Salmonella Drug-resistant Salmonella serotype Typhi Drug-resistant Shigella Methicillin-resistant Staphylococcus aureus Drug-resistant Streptococcus pneumoniae Drug-resistant Mycobacterium tuberculosis Concerning Erythromycin-resistant group A Streptococcus Clindamycin-resistant group B Streptococcus Watch List Azole-resistant Aspergillus fumagatis Drug-resistant Mycoplasma genitalium Drug-resistant Bordetella pertussis Source: U.S. Centers for Disease Control and Prevention. compliance with infection control practices that prevent transmis­ sion of resistant pathogens from one person to another, such as hand hygiene and isolation precautions in health care and long-term care settings. (2) Tracking and data: Efforts aim to increase the reporting and sharing of the occurrence of resistance to enhance epidemiologic data and inform targeting of preventive interventions. (3) Antibiotic use and access: Antimicrobial stewardship programs with specific components to track usage and educate clinicians on appropriate use have become required in hospitals, and the CDC has implemented efforts to reduce inappropriate use in outpatient settings, with particu­ lar attention to upper respiratory illnesses that often do not require antimicrobials because of their common self-limited viral causes. (4) Vaccines, therapeutics, and diagnostics: The U.S. Congress and the U.S. Food and Drug Administration, as well as agencies in other countries, have recently developed incentives and enhanced regulatory pathways for drug approval that pharmaceutical companies can use for develop­ ment of antimicrobials that specifically address particular resistant pathogens. Both small and large companies have undertaken efforts in this area. New technologies for rapid detection of resistance and sus­ ceptibility are also being developed by multiple diagnostics companies in order to facilitate the appropriate choice of antimicrobials earlier in the course of illness, providing an important tool for antimicrobial stewardship programs. (5) Environment and sanitation: Reservoirs of resistant bacteria and resistance genes on mobile genetic elements can exist in agriculture and food production and domestic animals and have the potential for introduction into humans. Thus, antibiotic use in these environments can amplify resistance reservoirs and increase the chance of human exposure. Therefore, public health interventions addressing these issues in a One Health approach (https://www.cdc

.gov/onehealth/index.html) are an important component of managing resistance risks. ■ ■FURTHER READING Bush K, Bradford PA: Interplay between β-lactamases and new β-lactamase inhibitors. Nat Rev Microbiol 17:295, 2019. Centers for Disease Control and Prevention: Antibiotic resis­ tance threats in the United States, 2019. Available at https://www.cdc

.gov/antimicrobial-resistance/media/pdfs/2019-ar-threats-report-508. pdf. Accessed June 23, 2020.

32 - SECTION 5 Diseases Caused by Gram-Positive Bacteria

SECTION 5 Diseases Caused by Gram-Positive Bacteria

French GL: Antimicrobial resistance and healthcare-associated infec­ tions, in Hospital Epidemiology and Infection Control, 4th ed. GC Mayhall (ed). Philadelphia, Lippincott Williams & Wilkins, 2012, pp. 1297–1310. Rice LB: Mechanisms of resistance and clinical relevance of resistance to β-lactams, glycopeptides, and fluoroquinolones. Mayo Clin Proc 87:198, 2012. Silver LL, Bush K (eds): Antibiotics and Antibiotic Resistance. Cold Spring Harbor Perspectives in Medicine. New York, Cold Spring Harbor Laboratory Press, 2016. Section 5 Diseases Caused by

Gram-Positive Bacteria David Goldblatt, Katherine L. O’Brien

Pneumococcal

Infections In the late nineteenth century, pairs of micrococci were first recog­ nized in the blood of rabbits injected with human saliva by both Louis Pasteur, working in France, and George Sternberg, an American army physician. The important role of these micrococci in human disease was not appreciated at that time. By 1886, when the organism was designated “pneumokokkus” and Diplococcus pneumoniae, it had been isolated by many independent investigators, and its role in the etiol­ ogy of pneumonia was well known. In the 1930s, pneumonia was the third leading cause of death in the United States (after heart disease and cancer) and was responsible for ~7% of all deaths both in the United States and in Europe. While pneumonia was caused by a host of pathogens, lobar pneumonia—a pattern more likely to be caused by the pneumococcus—accounted for approximately one-half of all pneu­ monia deaths in the United States in 1929. In 1974, the organism was reclassified as Streptococcus pneumoniae. ■ ■MICROBIOLOGY Etiologic Agent  Pneumococci are spherical gram-positive bacteria of the genus Streptococcus. Within this genus, cell division occurs along a single axis, and bacteria grow in chains or pairs—hence the name Streptococcus, from the Greek streptos, meaning “twisted,” and kokkos, meaning “berry.” At least 22 streptococcal species are recognized and are divided further into groups based on their hemolytic properties. S. pneumoniae belongs to the α-hemolytic group that characteristically produces a greenish color on blood agar because of the reduction of iron in hemoglobin (Fig. 151-1). The bacteria are fastidious and grow best in 5% CO2 but require a source of catalase (e.g., blood) for growth on agar plates, where they develop mucoid (smooth/shiny) colonies. Pneumococci without a capsule produce colonies with a rough surface. Unlike that of other α-hemolytic streptococci, their growth is inhibited in the presence of optochin (ethylhydrocupreine hydrochloride), and they are bile soluble. In common with other gram-positive bacteria, pneumococci have a cell membrane beneath a cell wall, which in turn is covered by a polysaccharide capsule. Pneumococci are divided into serogroups or serotypes based on capsular polysaccharide structure, as distinguished with rabbit polyclonal antisera; capsules swell in the presence of spe­ cific antiserum (the Quellung reaction). The most recently discovered serotypes—6C, 6D, 6F, 6G, 6H, 10D, 11E, 20A, 20B, and 35D—have been identified with monoclonal antibodies and by serologic, genetic, and biochemical means. The currently recognized 100 serotypes fall

FIGURE 151-1  Pneumococci growing on blood agar, illustrating α hemolysis and optochin sensitivity (zone around optochin disk). Inset: Gram’s stain, illustrating gram-positive diplococci. (Photographs courtesy of Paul Turner, University of Oxford, United Kingdom.) CHAPTER 151 into 21 serogroups, and each serogroup contains two to eight serotypes with closely related capsules. Detailed genetic analysis of the locus cod­ ing for the polysaccharide capsule, the cps locus, continues to reveal putative novel capsular polysaccharides, variants within existing sero­ groups that are designated with an “X.” In the absence of type-specific antibody, the capsule protects the bacteria from phagocytosis by host cells and is arguably the most important determinant of pneumococcal virulence. Unencapsulated variants are occasionally identified in cases of invasive pneumococcal disease; however, when their genotype is assessed, they often contain capsular genes. Thus it is likely that they were encapsulated in vivo and have stopped producing capsule during the laboratory steps of pathogen isolation. Pneumococcal Infections Virulence Factors  Within the cytoplasm, cell membrane, and cell wall, many molecules that may play a role in pneumococcal pathogenesis and virulence have been identified (Fig. 151-2). These proteins are often involved in direct interactions with host tissues or in concealment of the bacterial surface from host defense mechanisms. Pneumolysin (PLY) is a secreted cytotoxin thought to result in cytolysis of cells and tissues, and LytA enhances pathogenesis. A number of cell wall proteins interfere with the complement pathway, thus inhibiting complement deposition and preventing lysis and/or opsonophagocyto­ sis. The pneumococcal H inhibitor (Hic) impedes the formation of C3 convertase, while pneumococcal surface protein C (PspC), also known as choline-binding protein A (CbpA), binds factor H and is thought to accelerate the breakdown of C3. PspA and CbpA inhibit the deposition of or degrade C3b. To avoid clearance by the mucus, pneumococci utilize the matrix metalloprotease ZmpA, which cleaves mucosal IgA to evade complement activation, preventing agglutination and thus clearance by the mucociliary flow. The numerous pneumococcal proteins thought to be involved in adhesion include pneumococcal surface adhesin A (PsaA) and the exoglycosidases such as neuraminidase (NanA), β-galactosidase (BgaA), and β-N-Acetylglucosaminidase (StrH), which deglycosylate host glycoproteins releasing sugars as a nutrient source and expos­ ing hidden receptors for adhesion. Once through the epithelial bar­ rier, pneumococci utilize PLY and mannose receptor C type lectin 1 (MRC-1/CD206) on the surface of dendritic cells and macrophages to enter cells, where they may survive intracellularly in vacuoles thus

33 - 151 Pneumococcal Infections

151 Pneumococcal Infections

French GL: Antimicrobial resistance and healthcare-associated infec­ tions, in Hospital Epidemiology and Infection Control, 4th ed. GC Mayhall (ed). Philadelphia, Lippincott Williams & Wilkins, 2012, pp. 1297–1310. Rice LB: Mechanisms of resistance and clinical relevance of resistance to β-lactams, glycopeptides, and fluoroquinolones. Mayo Clin Proc 87:198, 2012. Silver LL, Bush K (eds): Antibiotics and Antibiotic Resistance. Cold Spring Harbor Perspectives in Medicine. New York, Cold Spring Harbor Laboratory Press, 2016. Section 5 Diseases Caused by

Gram-Positive Bacteria David Goldblatt, Katherine L. O’Brien

Pneumococcal

Infections In the late nineteenth century, pairs of micrococci were first recog­ nized in the blood of rabbits injected with human saliva by both Louis Pasteur, working in France, and George Sternberg, an American army physician. The important role of these micrococci in human disease was not appreciated at that time. By 1886, when the organism was designated “pneumokokkus” and Diplococcus pneumoniae, it had been isolated by many independent investigators, and its role in the etiol­ ogy of pneumonia was well known. In the 1930s, pneumonia was the third leading cause of death in the United States (after heart disease and cancer) and was responsible for ~7% of all deaths both in the United States and in Europe. While pneumonia was caused by a host of pathogens, lobar pneumonia—a pattern more likely to be caused by the pneumococcus—accounted for approximately one-half of all pneu­ monia deaths in the United States in 1929. In 1974, the organism was reclassified as Streptococcus pneumoniae. ■ ■MICROBIOLOGY Etiologic Agent  Pneumococci are spherical gram-positive bacteria of the genus Streptococcus. Within this genus, cell division occurs along a single axis, and bacteria grow in chains or pairs—hence the name Streptococcus, from the Greek streptos, meaning “twisted,” and kokkos, meaning “berry.” At least 22 streptococcal species are recognized and are divided further into groups based on their hemolytic properties. S. pneumoniae belongs to the α-hemolytic group that characteristically produces a greenish color on blood agar because of the reduction of iron in hemoglobin (Fig. 151-1). The bacteria are fastidious and grow best in 5% CO2 but require a source of catalase (e.g., blood) for growth on agar plates, where they develop mucoid (smooth/shiny) colonies. Pneumococci without a capsule produce colonies with a rough surface. Unlike that of other α-hemolytic streptococci, their growth is inhibited in the presence of optochin (ethylhydrocupreine hydrochloride), and they are bile soluble. In common with other gram-positive bacteria, pneumococci have a cell membrane beneath a cell wall, which in turn is covered by a polysaccharide capsule. Pneumococci are divided into serogroups or serotypes based on capsular polysaccharide structure, as distinguished with rabbit polyclonal antisera; capsules swell in the presence of spe­ cific antiserum (the Quellung reaction). The most recently discovered serotypes—6C, 6D, 6F, 6G, 6H, 10D, 11E, 20A, 20B, and 35D—have been identified with monoclonal antibodies and by serologic, genetic, and biochemical means. The currently recognized 100 serotypes fall

FIGURE 151-1  Pneumococci growing on blood agar, illustrating α hemolysis and optochin sensitivity (zone around optochin disk). Inset: Gram’s stain, illustrating gram-positive diplococci. (Photographs courtesy of Paul Turner, University of Oxford, United Kingdom.) CHAPTER 151 into 21 serogroups, and each serogroup contains two to eight serotypes with closely related capsules. Detailed genetic analysis of the locus cod­ ing for the polysaccharide capsule, the cps locus, continues to reveal putative novel capsular polysaccharides, variants within existing sero­ groups that are designated with an “X.” In the absence of type-specific antibody, the capsule protects the bacteria from phagocytosis by host cells and is arguably the most important determinant of pneumococcal virulence. Unencapsulated variants are occasionally identified in cases of invasive pneumococcal disease; however, when their genotype is assessed, they often contain capsular genes. Thus it is likely that they were encapsulated in vivo and have stopped producing capsule during the laboratory steps of pathogen isolation. Pneumococcal Infections Virulence Factors  Within the cytoplasm, cell membrane, and cell wall, many molecules that may play a role in pneumococcal pathogenesis and virulence have been identified (Fig. 151-2). These proteins are often involved in direct interactions with host tissues or in concealment of the bacterial surface from host defense mechanisms. Pneumolysin (PLY) is a secreted cytotoxin thought to result in cytolysis of cells and tissues, and LytA enhances pathogenesis. A number of cell wall proteins interfere with the complement pathway, thus inhibiting complement deposition and preventing lysis and/or opsonophagocyto­ sis. The pneumococcal H inhibitor (Hic) impedes the formation of C3 convertase, while pneumococcal surface protein C (PspC), also known as choline-binding protein A (CbpA), binds factor H and is thought to accelerate the breakdown of C3. PspA and CbpA inhibit the deposition of or degrade C3b. To avoid clearance by the mucus, pneumococci utilize the matrix metalloprotease ZmpA, which cleaves mucosal IgA to evade complement activation, preventing agglutination and thus clearance by the mucociliary flow. The numerous pneumococcal proteins thought to be involved in adhesion include pneumococcal surface adhesin A (PsaA) and the exoglycosidases such as neuraminidase (NanA), β-galactosidase (BgaA), and β-N-Acetylglucosaminidase (StrH), which deglycosylate host glycoproteins releasing sugars as a nutrient source and expos­ ing hidden receptors for adhesion. Once through the epithelial bar­ rier, pneumococci utilize PLY and mannose receptor C type lectin 1 (MRC-1/CD206) on the surface of dendritic cells and macrophages to enter cells, where they may survive intracellularly in vacuoles thus

Pneumolysin: secreted cytolytic/cytotoxic protein; activates complement and stimulates proinflammatory cytokines Polysaccharide capsule: prevents complement binding; therefore antiphagocytic, target for protective antibody Pneumococcal surface protein A: interferes with complement deposition by blocking alternative complement pathway activation Pneumococcal surface protein C (choline-binding protein A): principal pneumococcal adhesion molecule Pneumococcal iron acquisition A and iron uptake A: lipoprotein components of iron ABC transporters, essential for iron uptake PspA PspC/ CbpA IgA1 protease: degrades human IgA1 PiaA and PiuA CbpG Hyal PsaA Pneumolysin Phosphorylcholine Autolysin PBP Enolase PART 5 Infectious Diseases Histidine triad Neuraminidase (NanA, NanB) Cell membrane Polysaccharide capsule Cell wall FIGURE 151-2  Schematic diagram of the pneumococcal cell surface, with key antigens and their roles highlighted. facilitating spread. To outcompete the other co-colonizing bacteria, the pneumococcus produces bacteriocins called pneumocins that medi­ ate intraspecific competition. Some of the antigens mentioned above are potential vaccine candidates (see “Prevention,” below). Biofilm production by pneumococci is now well recognized and is likely to be an important mechanism aiding survival of pneumococci in the upper respiratory tract and contributing to local disease manifestations such as otitis media. Although the capsule surrounding the cell wall of S. pneumoniae is the basis for categorization by serotype, the disease potential of a sero­ type is also related to the genetic composition of the strain. Molecular genotyping and epidemiology are therefore essential. Conventionally, multilocus sequence typing (MLST) was the gold-standard technique for epidemiologic analyses due to its simplicity and effectiveness. Alleles at seven loci are sequenced, compared with all of the known alleles at that locus and a unique sequence type (ST) assigned using the pneumococcal MLST website (pubmlst.org/organisms/streptococcus-

pneumoniae/). With the advent of high-throughput and relatively inexpensive sequencing techniques, whole-genome sequencing has facilitated even more precise molecular epidemiology: enhanced genomic epidemiology can be performed using ribosomal MLST (rMLST; >50 ribosomal genes) or core genome MLST (cgMLST; >1300 core genes) with assignment via the PubMLST website, while k-mer–based methods fully exploit core and accessory genomic variation (Pop­ PUNK, github.com/bacpop/PopPUNK) to assign Global Pneumococcal Sequence Clusters (GPSCs) via the Global Pneumococcal Sequencing project website (pneumogen.net). Twenty-three years after the first pneumococcal genome was sequenced, >50,000 pneumococcal genomes are now available on pub­ lic nucleotide sequence databases. There are two curated pneumococ­ cal genome databases: the PubMLST Pneumococcal Genome Library contains ~33,000 curated, published, assembled genomes and isolate

Choline-binding protein G: cleaves host extracellular matrix, aiding adhesion Pneumococcal surface antigen A: metal-binding lipoprotein (Zn and Mn); may have a role in adhesion Hyaluronate lyase: degrades hyaluronan and chondroitin sulfate in extracellular matrix Binds to platelet-activating factor receptor on human epithelial cells Releases peptidoglycan, teichoic acid, pneumolysin, and other intracellular contents on autolysis Penicillin-binding proteins: catalyze polymerization of glycan chains and transpeptidation of pentapeptidic moieties within structure of peptidoglycan Neuraminidase: contributes to adherence; removes sialic acids on host glycopeptides and mucin to expose binding sites Binds to fibronectin in host tissues PhtA, B, D, E: cell-surface exposed proteins, unknown function Pili: on cell surface; inhibit phagocytosis, promote invasion Pili provenance data (pubmlst.org/organisms/streptococcus-pneumoniae/pgl/), and the GPS Database Monocle Dataviewer, which contains ~21,000 high-quality genomes with epidemiologic data (www.pneumogen

.net/gps/gps-database-overview/). These databases are publicly avail­ able and free to access. Over the past decade, web applications such as Pathogenwatch (pathogen.watch/) have provided a user-friendly platform to enable fast and reliable analysis of pneumococcal genome data without the requirement for bioinformatic expertise. Users can simply drag and drop genome data into a browser to obtain details such as serotype, genotype (including GPSC and MLST), and antimi­ crobial resistance profiles. In recent years, genome sequence analyses have made major contributions to the understanding of pneumococcal molecular epidemiology, biology, diversity, pathogenicity, and vaccine impact. ■ ■EPIDEMIOLOGY (See also “Global Health,” below.) Pneumococcal infections remain a significant global cause of morbidity and death, particularly among children and the elderly. Rapid and dramatic changes in the epide­ miology of this disease during the past 20 years in several developed countries followed the licensure and routine childhood administration of pneumococcal polysaccharide–protein conjugate vaccine (PCV). With PCV introduction in low- and middle-income countries (LMIC), additional profound changes in pneumococcal ecology and disease epidemiology are occurring. The disease burden and serotype distribu­ tion in the PCV era are influenced not only by the reduction in disease caused by serotypes included in PCV but also by serotype replacement as a result of reductions in vaccine serotypes, concomitant secular trends in pneumococcal strains unrelated to vaccine use, the impact of antibiotic use on pneumococcal strain ecology, and surveillance system attributes that can themselves affect analysis of epidemiologic features of pneumococcal strains and disease.

Serotype Distribution  Not all pneumococcal serotypes are equally likely to cause disease; observed serotype distributions vary by age category, disease syndrome, and geography. Geographic differences may be driven by variations in the relative prevalence of syndromes caus­ ing disease rather than by true serotype distribution differences, as cer­ tain serotypes are more common causes of some syndromes than others (e.g., pneumonia and meningitis). Most data on serotype distribution come from pediatric invasive pneumococcal disease (IPD, defined as infection of a normally sterile site); much less information on global or regional serotype distributions is available for disease in adults. In the era before PCV use, five to seven serotypes caused >60% of IPD cases among children <5 years of age in most parts of the world; seven sero­ types (1, 5, 6A, 6B, 14, 19F, and 23F) accounted for ~60% of such cases in all areas of the world, but in any given region these seven serotypes may not all rank as the most common disease strains. Some serotypes (e.g., types 1 and 5) not only tend to cause disease in areas with a high disease burden but also cause waves of disease in lower-burden areas (e.g., Europe) or outbreaks (e.g., in military barracks; meningitis in sub-Saharan Africa). The widespread use of pneumococcal conjugate vaccines has significantly altered serotype-specific epidemiology, with some of the serotypes identified above now causing little invasive disease in countries with mature vaccine programs and emerging sero­ types, not prominent in the pre–conjugate vaccine era, appearing as important causes of invasive disease. These include serotypes such as 15BC, 22F, 10A, 23B, 12F, 15A, and 8 while some serotypes included in PCVs such as 3 and 19A continue to cause IPD. Nasopharyngeal Carriage  Pneumococci are intermittent inhabit­ ants of the healthy human nasopharynx and are transmitted by respi­ ratory droplets. In children, pneumococcal nasopharyngeal ecology varies by geographic region, socioeconomic status, climate, degree of crowding, and particularly intensity of exposure to other children, with children in day-care settings having higher rates of colonization. In developed-world settings, children serve as the major vectors of pneu­ mococcal transmission. By 1 year of age, ~50% of children have had at least one episode of pneumococcal colonization. Cross-sectional prev­ alence data show rates of pneumococcal carriage ranging from 20% to 50% among children <5 years of age and from 5% to 15% among young and middle-aged adults; Fig. 151-3 shows relevant data from the United Kingdom. Data on colonization rates among healthy elderly individuals are limited. In LMICs, pneumococcal acquisition occurs much earlier, sometimes within the first few days after birth, and nearly all infants have had at least one episode of colonization by 2 months of age. Cross-sectional studies show that up to the age of 5 years, 70–90% of children carry S. pneumoniae in the nasopharynx, and a significant proportion of adults (sometimes >40%) also are colonized. Their high rates of colonization make adults an important source of transmission and may affect community transmission dynamics. 50% 60% 70% 80% 90% 100% Age in years: 0–2 3–4 5–17 18+ Carriage prevalence 0% 10% 20% 30% 40% January February March April May June October November December Swabbing month FIGURE 151-3  Prevalence of pneumococcal carriage in adults and children resident in the United Kingdom who had nasopharyngeal swabs collected monthly for 10 months (no seasonal trend; t test trend, >.05). (Data adapted from D Goldblatt et al: J Infect Dis 192:387, 2005.)

Cases/100,000 population

<1

Age group (years) 5–17 18–34 35–49 50–64

65 FIGURE 151-4  Rates of invasive pneumococcal disease before the introduction of pneumococcal conjugate vaccine, by age group: United States, 1998. (Source: CDC, Active Bacterial Core Surveillance/Emerging Infectious Program Network, 2000. Data adapted from MMWR 49[RR-9], 2000.) Invasive Disease and Pneumonia  IPD develops when S. pneu­ moniae invades the bloodstream and seeds other organs or directly reaches the cerebrospinal fluid (CSF) by local extension. Pneumonia may follow aspiration of pneumococci, although only 10–30% of pneu­ mococcal pneumonia cases are associated with a positive blood culture (and thus contribute to the measured burden of IPD). The substantial variation of IPD rates with age is illustrated by data from the United States for 1998–1999, a period prior to PCV introduction. Rates of IPD were highest among children <2 years of age and among adults ≥65 years of age (188 and 60 cases/100,000, respectively; Fig. 151-4). Since the introduction of PCV, IPD rates among infants and children in the

United States have fallen by >75%, a decrease driven by the near elimina­ tion of vaccine-serotype IPD. A similar impact of PCV on vaccine-sero­ type IPD rates has been consistently observed in countries where PCV has been introduced into the routine pediatric vaccination schedule. However, the magnitudes of change in the non-vaccine-serotype IPD rate in various countries have been heterogeneous; the interpretation of this heterogeneity is a complex issue. In the United States, Canada, and Australia, rates of non-vaccine-serotype IPD have increased but the magnitude of the increase is generally small relative to the substantial reductions in vaccine-serotype IPD. In contrast, in other settings (e.g., Alaska Native communities and adults in the United Kingdom), the reduction in vaccine-serotype IPD has been offset by notable increases in rates of disease caused by non-vaccine serotypes. Explanations for the heterogeneity of findings include replacement disease resulting from vaccine pressure, changes in clinical case investigation, secular trends unrelated to PCV use, antibiotic pressure selecting for resistant organ­ isms, changes in surveillance or reporting systems, rapidity of PCV introduction, and inclusion of a catch-up campaign. Serotype replace­ ment in IPD follows the use of PCV7, PCV10, and PCV13, but the magnitude of this phenomenon may be small relative to the reduction in disease from vaccine serotypes in vaccinated populations. In adults in the UK, however, where rates of IPD due to vaccine serotypes fell following PCV introduction, the increase in IPD secondary to non- vaccine sero­ types is eroding the original impact of PCV. Furthermore, not all vaccine serotypes have declined and persistent disease due to serotypes 3 and 19A in particular has been noted in many settings. CHAPTER 151 Pneumococcal Infections Pneumonia is the most common of the serious pneumococcal dis­ ease syndromes and poses special challenges from a clinical and public health perspective. Most cases of pneumococcal pneumonia are not associated with bacteremia, and in these cases a definitive etiologic diagnosis is difficult or impossible. As a result, estimates of disease bur­ den focus primarily on IPD rates and fail to include the major portion of the burden of serious pneumococcal disease. Among children, PCV trials designed to collect efficacy data on syndrome-based outcomes (e.g., radiographically confirmed pneumonia, clinically diagnosed pneumonia) have revealed the burden of culture-negative pneumococ­ cal pneumonia. These trials have provided the means to infer that only ~5–20% of pneumococcal pneumonia cases result in bacteremia. An important randomized controlled trial of PCV among the elderly in July

the Netherlands (the CAPiTA trial) has revealed the small fraction of adult pneumococcal pneumonia patients who also have bacteremia. Use of high-quality sputum specimens and, in the case of adults with a low likelihood of colonization absent disease, urine antigen detec­ tion both contribute to the diagnosis of nonbacteremic pneumococcal pneumonia. Furthermore, accruing evidence continues to indicate that pneumococcal pneumonia events are often the result of co-infection with viral or other bacterial pathogens. Thus a pneumonia case result­ ing from a pulmonary infection with a single pathogen is probably an uncommon event; rather, most cases of pneumonia likely result from the sequential or contemporaneous co-infection of a host with multiple pathogens, often both viruses and bacteria.

The case–fatality ratios (CFRs) for pneumococcal pneumonia and IPD vary by age, underlying medical condition, and access to care. In addition, the CFR for pneumococcal pneumonia varies with the severity of disease at presentation (rather than according to whether the pneu­ monia episode is associated with bacteremia) and with the patient’s age (from <5% among hospitalized patients 18–44 years old to >12% among those >65 years old, even when appropriate and timely management is available). Notably, the likelihood of death in the first 24 h of hospital­ ization did not change substantially with the introduction of antibiotics; this surprising observation highlights the fact that the pathophysiology of severe pneumococcal pneumonia among adults reflects a rapidly progressive cascade of events that often unfolds irrespective of antibiotic administration. Management in an intensive care unit can provide criti­ cal support for the patient through the acute period, with lower CFRs, while antibiotics address the underlying infection. Rates of pneumococcal disease vary by season, with higher rates in colder than in warmer months in temperate climates; by sex, with males more often affected than females; and by risk group, with risk factors including underlying medical conditions, behavioral issues (e.g., smoking), and ethnic group. In the United States, some Native American populations (including Alaska Natives) and African Americans have higher rates of disease than the general population; the increased risk is probably attributable to socioeconomic conditions and the prev­ alence of underlying risk factors for pneumococcal disease. Medical conditions that increase the risk of pneumococcal infection are listed in Table 151-1. Outbreaks of disease are well recognized in crowded settings with susceptible individuals, such as infant day-care facilities, military barracks, and nursing homes. Furthermore, there is a clear association between preceding viral respiratory disease (especially but not exclusively influenza) and risk of secondary pneumococcal infec­ tions. The significant role of pneumococcal pneumonia in the morbid­ ity and mortality associated with seasonal and pandemic influenza is increasingly well recognized. PART 5 Infectious Diseases Antibiotic Resistance  Reduced pneumococcal susceptibility to penicillin was first noted in 1967, but not until the 1990s did reduced antibiotic susceptibility emerge as a significant clinical and public health issue, with an increasing prevalence of pneumococcal isolates resistant to single or multiple classes of antibiotics and a rising absolute magnitude of minimal inhibitory concentrations (MICs). Strains with reduced susceptibility to penicillin G, cefotaxime, ceftriaxone, mac­ rolides, and other antibiotics are now found worldwide and account for a significant proportion of disease-causing strains in many loca­ tions, especially among children. Vancomycin resistance has not yet been observed in clinical pneumococcal strains. Lack of antimicrobial susceptibility is clearly related to a subset of serotypes, many of which disproportionately cause disease among children. Resistance pheno­ types are based on a diverse array of mutational events and inter- and intraspecies gene-transfer phenomena carried out by several types of mobile genetic elements, with consequent dissemination of success­ ful resistant clones. The vicious cycle of antibiotic exposure, selection of resistant organisms in the nasopharynx, and transmission of these organisms within the community, leading to difficult-to-treat infec­ tions and increased antibiotic exposure, has been interrupted to some extent by the introduction and routine use of PCV. The clinical impli­ cations of pneumococcal antimicrobial nonsusceptibility are addressed in “Treatment,” below.

TABLE 151-1  Clinical Risk Groups for Pneumococcal Infection CLINICAL RISK GROUP EXAMPLES Asplenia or splenic dysfunction Sickle cell disease and other hemoglobinopathies, celiac disease Chronic respiratory disease Chronic obstructive pulmonary disease, bronchiectasis, cystic fibrosis, interstitial lung fibrosis, pneumoconiosis, bronchopulmonary dysplasia, aspiration risk, neuromuscular disease (e.g., cerebral palsy), severe asthma Chronic heart disease Ischemic heart disease, congenital heart disease, hypertension with cardiac complications, chronic heart failure Chronic kidney disease Nephrotic syndrome, chronic renal failure, renal transplantation Chronic liver disease Cirrhosis, biliary atresia, chronic hepatitis Diabetes mellitus Diabetes mellitus requiring insulin or oral hypoglycemic drugs Immunocompromise/ immunosuppression HIV infection, primary immunodeficiency (including B cell, T cell, complement, and some phagocytic disorders), leukemia, lymphoma, Hodgkin’s disease, multiple myeloma, generalized malignancy, chemotherapy, organ or bone marrow transplantation, systemic glucocorticoid treatment for >1 month at a dose equivalent to ≥20 mg/d (children, ≥1 mg/kg per day) Cochlear implants … Cerebrospinal fluid leaks … Miscellaneous Infancy and old age; prior hospitalization; alcoholism; malnutrition; cigarette smoking; day-care center attendance; residence in military training camps, prisons, homeless shelters Note: Groups for whom pneumococcal vaccines are recommended by the Advisory Committee on Immunization Practices can be found at www.cdc.gov/vaccines/ schedules/. ■ ■PATHOGENESIS Pneumococci colonize the human nasopharynx from an early age; colonization acquisition events are generally described as asymptom­ atic, but evidence exists to associate acquisition with mild respira­ tory symptoms, especially in the very young. Bacteria survive in the nasopharynx protected by a variety of factors, including their bacterial capsule and the formation of a biofilm. From the nasopharynx, the bacteria spread either via the bloodstream to distant sites (e.g., brain, joint, bones, peritoneal cavity) or locally to mucosal surfaces where they can cause otitis media or pneumonia. Direct spread from the nasopharynx to the central nervous system (CNS) can occur in rare cases of skull-base fracture, although most cases of pneumococcal meningitis are secondary to hematogenous spread. The pneumococcus is not a static bacterium; rather, it modifies its expression of capsule in adaptation to the external environment. In the nasopharynx, the pneumococcus downregulates capsular expression, averting protective immunologic mechanisms that recognize capsule; rough colonies are the phenotype on culture. Upon invasion by traversal of the epithelium, the pneumococcus upregulates its capsular expression, transforming its appearance on culture to smooth colonies—a change illustrating the dynamic nature of the organism in response to the local environ­ ment. Pneumococci can cause disease in almost any organ or part of the body; however, otitis media, pneumonia, bacteremia, and menin­ gitis are most common. Colonization is a relatively frequent event, yet disease is rare. In the nasopharynx, pneumococci survive in mucus secreted by epithelial cells and in a biofilm they create, where they can avoid local immune factors such as leukocytes and complement. The mucus itself is a component of local defense mechanisms, and the flow of mucus (driven in part by cilia in what is known as the mucociliary escalator) effects mechanical clearance of pneumococci. While many colonization episodes are of short duration, longitudinal studies in adults and children have revealed persistent colonization with a spe­ cific serotype over many months. Colonization eventually results in

the development of capsule- and protein-specific serum IgG antibod­ ies, which are thought to play a role in mediating clearance of bacteria from the nasopharynx. IgG antibodies to surface-exposed cell-wall or secreted proteins also appear in the circulation in an age-dependent fashion or after colonization; these antibodies are likely to have a disease-modifying and/or protective role. Recent acquisition of a new colonizing serotype is more likely to be associated with subsequent invasion, presumably as a result of the absence of type-specific immu­ nity. Intercurrent viral infections make the host more susceptible to pneumococcal colonization, and pneumococcal disease in a colonized individual often follows perturbation of the nasopharyngeal mucosa by such infections. Local cytokine production after a viral infection is thought to upregulate adhesion factors in the respiratory epithelium, allowing pneumococci to adhere via a variety of surface adhesin mol­ ecules, including PsaA, PspA, CbpA, PspC, Hyl, pneumolysin, and the neuraminidases (Fig. 151-2). Adhesion coupled with inflammation induced by pneumococcal factors such as peptidoglycans and teichoic acids results in invasion. It is the inflammation induced by various

bacterium-derived factors that is responsible for the pathology associated with pneumococcal infection. Pneumococcal cell wall–derived teichoic acids and peptidoglycans induce a variety of cytokines, including the proinflammatory cytokines interleukin (IL) 1, IL-6, and tumor necrosis factor, and activate complement via the alternative pathway. Polymor­ phonuclear leukocytes are thus attracted, and an intense inflammatory response is initiated. Pneumolysin also is important in local pathology, inducing proinflammatory cytokine production by local monocytes. The pneumococcal capsule, consisting of polysaccharides with anti­ phagocytic properties (i.e., the capacity to resist complement deposi­ tion in the absence of type-specific antibody), plays an important role in pathogenesis. While most capsular types can cause human disease, certain capsular types are more commonly isolated from sites of infec­ tion. The reason for the dominance of some serotypes over others in IPD is unclear. ■ ■HOST DEFENSE MECHANISMS Innate Immunity  As described above, intact respiratory epithelium and a host of nonspecific or innate immune factors (e.g., mucus, splenic function, complement, neutrophils, and macrophages) constitute the first line of defense against pneumococci. Physical factors such as the cough reflex and the mucociliary escalator are important in clearing bacteria from the lungs. Immunologic factors are critical as well: C-reactive protein (CRP) binds phosphorylcholine in the pneumococcal

cell wall, inducing complement activation and leading to bacterial clear­ ance; Toll-like receptor 2 (TLR2) recognizes pneumococcal-derived lipoproteins. In animal models, the absence of host TLR2 leads to more severe infection and impaired clearance of nasopharyngeal colonization. TLR4 appears to be necessary for the proinflammatory effect of pneumo­ lysin on macrophages. The importance of TLR recognition is underlined by descriptions of an inherited deficiency of human IL-1 receptor– associated kinase 4 (IRAK-4) that manifests as an unusual susceptibility to infection with bacteria, including S. pneumoniae. IRAK-4 is essential for the normal functioning of several TLRs. Other factors that interfere with these nonspecific mechanisms (e.g., viral infections, cystic fibrosis, bronchiectasis, complement deficiency, chronic obstructive pulmonary disease) all predispose to the development of pneumococcal pneumonia. Patients who lack a spleen or have abnormal splenic function (e.g., per­ sons with sickle cell disease) are at high risk of developing overwhelming pneumococcal disease. Acquired Immunity  Acquired immunity induced following colo­ nization or through exposure to cross-reactive antigens rests largely on the development of serum IgG antibody specific for the pneumococcal capsular polysaccharide. Nearly all polysaccharides are T cell–independent antigens; B cells can make antibodies to such antigens without

T cell help. However, in children <1–2 years old, such B cell responses are poorly developed. This delayed ontogeny of capsule-specific IgG in young children is associated with susceptibility to pneumococcal infec­ tion (Fig. 151-4). The extremely high risk of pneumococcal infection in the absence of serum immunoglobulin (i.e., in conditions such as

agammaglobulinemia) highlights the important role of capsular anti­ body in protection against disease. Each serotype’s capsule is chemically distinct, even though for some serotypes the chemical distinction from another type may be a minor one; thus immunity tends to be serotype specific, although some cross-immunity exists. For example, conjugate vaccine–induced antibodies to serotype 6B prevent infection due to serotype 6A but not 6C; cross-protection against serotype 6C requires the administration of vaccines containing 6A. Antibodies to surfaceexposed or secreted pneumococcal proteins (such as pneumolysin, PsaA, and PspA) also appear in the circulation with increasing age of the host and are likely to contribute to protection. Data from murine models suggest that CD4+ T cells may play a role in preventing pneu­ mococcal colonization and disease, and experimental data derived from humans suggest that IL-17-secreting CD4+ T cells may be relevant.

APPROACH TO THE PATIENT Pneumococcal Infections There is no pathognomonic presentation of pneumococcal disease; patients may present with one or more clinical syndromes (e.g., pneumonia, meningitis, sepsis). S. pneumoniae can infect nearly any body tissue, manifesting as disease ranging in severity from mild and self-limited to life-threatening. The differential diagnosis of common clinical syndromes such as pneumonia, otitis media, fever of unknown origin, and meningitis should always include pneumococcal infection. A microbiologically confirmed diagnosis is made in only a minority of pneumococcal cases since, in most circumstances (and especially in pneumonia and otitis media), fluid from the site of infection is not available for etiologic determina­ tion, and infection of body fluids distant from the site of infection (e.g., blood in the case of pneumonia) occurs in only a minority of true pneumococcal cases. Empirical therapy that includes appropri­ ate treatment for S. pneumoniae is often indicated. CHAPTER 151 Algorithms for assessment and management of ill children (Inte­ grated Management of Childhood Illness; IMCI) have been devel­ oped for use in the developing world or in other settings where evaluation by a trained physician may not be feasible. No such algorithms for the management of adults with suspected disease exist. Children who present with signs associated with increased risk of serious disease, such as an inability to drink, convulsions, lethargy, and severe malnutrition, are categorized as having very severe disease without further evaluation by the community health care worker; are given antibiotics; and are immediately referred to a hospital for diagnosis and management. Children who present with cough and tachypnea (the latter defined according to specific age strata) are further stratified into severity categories based on the presence or absence of lower chest wall indrawing and are managed accordingly either with antibiotics alone or with antibiotics and referral to a hospital facility. Children with cough but no tachypnea are categorized as having a nonpneumonia respiratory illness. Pneumococcal Infections ■ ■CLINICAL MANIFESTATIONS The clinical manifestations of pneumococcal disease depend on the site of infection and the duration of illness. Clinical syndromes are classified as noninvasive (e.g., otitis media) or invasive (e.g., bacteremic pneumonia, meningitis) according to whether a normally sterile site is infected. The pathogenesis of noninvasive illness involves contiguous spread from the nasopharynx or skin; invasive disease involves infec­ tion of a normally sterile body fluid or follows bacteremia. Regardless of the mechanism, all pneumococcal infections result from nasopha­ ryngeal acquisition of the organism. Pneumonia  Pneumonia is the most common serious pneumo­ coccal syndrome and is considered invasive when associated with a positive blood culture. Whether to categorize nonbacteremic pneumo­ coccal pneumonia as invasive or noninvasive remains debatable. Pneumococcal pneumonia can present as a mild community-acquired infection at one extreme and as a life-threatening disease requiring intubation and intensive support at the other.

PRESENTING MANIFESTATIONS  The presentation of pneumococcal pneumonia does not reliably distinguish it from pneumonia of other etiologies. In a subset of cases, pneumococcal pneumonia is recognized at the outset as associated with a viral upper respiratory infection and is characterized by the abrupt onset of cough and dyspnea accompanied by fever, shaking chills, and myalgias. The cough evolves from nonpu­ rulent to productive of sputum that is purulent and sometimes tinged with blood. Patients may describe stabbing pleuritic chest pain and sig­ nificant dyspnea indicating involvement of the parietal pleura. Among the elderly, the presenting clinical symptoms may be less specific, with confusion or malaise but without fever or cough. In such cases, a high index of suspicion is required because failure to treat pneumococcal pneumonia promptly in an elderly patient is likely to result in rapid evolution of the infection, with increased severity, morbidity, and risk of death.

FINDINGS ON PHYSICAL EXAMINATION    The clinical signs associ­ ated with pneumococcal pneumonia among adults include tachypnea (defined as >20 breaths/min) and tachycardia, hypotension in severe cases, and fever in most cases (although not in all elderly patients). Respiratory signs are varied, including dullness to percussion in areas of the chest with significant consolidation, crackles on auscultation, reduced expansion of the chest in some cases as a result of splinting to reduce pain, bronchial breathing in a minority of cases, pleural rub in occasional cases, and cyanosis in cases with significant hypoxemia. Among infants with severe pneumonia, chest wall indrawing and nasal flaring are common. Nonrespiratory findings can include upper abdominal pain if the diaphragmatic pleura is involved as well as men­ tal status changes, particularly confusion in elderly patients. PART 5 Infectious Diseases DIFFERENTIAL DIAGNOSIS  The differential diagnosis of pneumococ­ cal pneumonia includes cardiac conditions such as myocardial infarc­ tion and heart failure with atypical pulmonary edema; pulmonary conditions such as atelectasis; and pneumonia caused by viral patho­ gens, mycoplasmas, Haemophilus influenzae, Klebsiella pneumoniae, Staphylococcus aureus, Legionella, or (in HIV-infected and otherwise immunocompromised hosts) Pneumocystis jirovecii. In cases with abdominal symptoms, the differential diagnosis includes cholecystitis, appendicitis, perforated peptic ulcer disease, and subphrenic abscesses. The challenge in cases with abdominal symptoms is to remember to include pneumococcal pneumonia—a nonabdominal process—in the differential diagnosis. DIAGNOSIS    Some authorities advocate treating uncomplicated, nonsevere, community-acquired pneumonia without determining the microbiologic etiology, given that this information is unlikely to alter clinical management. However, efforts to identify the cause of pneu­ monia are important when the disease is more severe and when the diagnosis of pneumonia is not clearly established. The gold standard for etiologic diagnosis of pneumococcal pneumonia is pathologic examination of lung tissue. In lieu of that procedure, evidence of an infiltrate on chest radiography warrants a diagnosis of pneumonia. However, cases of pneumonia without radiographic evidence do occur. An infiltrate can be absent either early in the course of the illness or with dehydration; upon rehydration, an infiltrate usually appears. The radiographic appearance of pneumococcal pneumonia is varied; it classically consists of lobar or segmental consolidation (Fig. 151-5) but in some cases is patchy. More than one lobe is involved in ~30% of cases. Consolidation may be associated with a small pleural effusion or empyema in complicated cases. In children, “round pneumonia,” a distinctly spherical consolidation on chest radiography, is associated with a pneumococcal etiology. Round pneumonia is uncommon in adults. S. pneumoniae is not the only cause of such lesions; other causes, especially cancer, should be considered. Blood drawn from patients with suspected pneumococcal pneu­ monia can be used for supportive or definitive diagnostic tests. Blood cultures are positive for pneumococci in a minority (<30%) of cases of pneumococcal pneumonia, as evidenced especially by vaccine clinical trials, which provide an independent method to reveal the contribu­ tion of the pneumococcus to pneumonia cases. Nonspecific findings

FIGURE 151-5  Chest radiograph depicting classic lobar pneumococcal pneumonia in the right lower lobe of an elderly patient’s lung. include an elevated polymorphonuclear leukocyte count (>15,000/μL in most cases and upward of 40,000/μL in some), leukopenia in <10% of cases (a poor prognostic sign associated with a fatal outcome), and elevated values in liver function tests (e.g., both conjugated and unconjugated hyperbilirubinemia). Anemia, low serum albumin levels, hyponatremia, and elevated serum creatinine levels are all found in ~20–30% of patients. Urinary pneumococcal antigen assays, based on identifying a ubiq­ uitous common cell wall polysaccharide, have facilitated etiologic diag­ nosis, but the application of the results is confounded by the fact that nasopharyngeal colonization with the pneumococcus, in the absence of disease, also results in a positive test. In adults, therefore, a positive pneumococcal urinary antigen test has a predictive value for etiologic attribution of pneumonia because the prevalence of pneumococcal nasopharyngeal colonization is relatively low, although the sensitivity of the assay is modest. In communities, particularly those in low-

income countries, where colonization rates among adults are high, urine antigen assays may be less useful. The same issue holds for chil­ dren, in whom a positive urinary antigen test is usually uninformative for etiologic attribution of their pneumonia illness because coloniza­ tion rates are generally high. A recent advance is the development of quantitative serotype-specific urinary antigen detection assays for up to 24 pneumococcal antigens; their application for adults and children holds promise, especially in detecting serotypes that are rarely identi­ fied in asymptomatic carriage (e.g., serotype 1), even among children. Most cases of pneumococcal pneumonia in adults are diagnosed by Gram’s staining and culture of sputum. The utility of a sputum specimen is directly related to its quality and the patient’s antibiotic treatment status. COMPLICATIONS    Empyema is the most common focal complica­ tion of pneumococcal pneumonia, occurring in <5% of cases. When fluid in the pleural space is accompanied by fever and leukocytosis (even low-grade) after 4–5 days of appropriate antibiotic treatment for pneumococcal pneumonia, empyema should be considered. Para­ pneumonic effusions are more common than empyema, representing a self-limited inflammatory response to pneumonia. Pleural fluid with frank pus, bacteria (detected by microscopic examination), or a pH of ≤7.1 indicates empyema and demands aggressive and complete drain­ age, usually through chest tube insertion.

Meningitis  Pneumococcal meningitis usually presents as a pyo­ genic condition that is clinically indistinguishable from meningitis of other bacterial etiologies. Meningitis can be the primary presenting pneumococcal syndrome or a complication of other conditions such as skull fracture, otitis media, bacteremia, or mastoiditis. Now that

H. influenzae type b vaccine is routinely used in children, S. pneumoniae and Neisseria meningitidis are the most common bacterial causes of meningitis in both adults and children. Pyogenic meningitis, including that due to S. pneumoniae, is associated clinically with findings that include severe, generalized, gradual-onset headache, fever, and nausea as well as specific CNS manifestations such as stiff neck, photophobia, seizures, and confusion. Clinical signs include a toxic appearance, altered consciousness, bradycardia, and hypertension indicative of increased intracranial pressure. A small proportion of adult patients have Kernig’s or Brudzinski’s sign or cranial nerve palsies (particularly of the third and sixth cranial nerves). A definitive diagnosis of pneumococcal meningitis rests on the examination of CSF for (1) evidence of turbidity (visual inspection); (2) elevated protein level, elevated white blood cell count, and reduced glucose concentration (quantitative measurement); and (3) specific identification of the etiologic agent (culture, Gram’s staining, antigen testing, or polymerase chain reaction [PCR]). A blood culture positive for S. pneumoniae in conjunction with clinical manifestations of men­ ingitis also is considered confirmatory. As discussed in “Pneumonia,” above, detection of pneumococcal antigen in urine is considered highly specific among adults because of the low prevalence of nasopharyngeal colonization in this age group. The mortality rate for pneumococcal meningitis is ~20%. In addi­ tion, up to 50% of survivors experience acute or chronic complications, including deafness, hydrocephalus, and mental retardation in children and diffuse brain swelling, subarachnoid bleeding, hydrocephalus, cerebrovascular complications, and hearing loss in adults. Other Invasive Syndromes  S. pneumoniae can cause other inva­ sive syndromes involving virtually any body site. These syndromes include primary bacteremia without other sites of infection (bactere­ mia without a source; occult bacteremia), osteomyelitis, septic arthritis, endocarditis, pericarditis, and peritonitis. The essential diagnostic approach is collection of fluid from the site of infection by sterile technique and examination by Gram’s staining, culture, and—when relevant—capsular antigen assay or PCR. Hemolytic-uremic syndrome can complicate invasive pneumococcal disease. Noninvasive Syndromes  The major noninvasive syndromes caused by S. pneumoniae are sinusitis, bacterial bronchitis, and otitis media; the latter is the most common pneumococcal syndrome and most often affects young children. The manifestations of otitis media include the acute onset of severe pain, fever, deafness, and tinnitus, most frequently in the setting of a recent upper respiratory tract infec­ tion. Clinical signs include a red, swollen, often bulging tympanic membrane with reduced movement on insufflation or tympanography. Redness of the tympanic membrane is not sufficient for the diagnosis of otitis media. Pneumococcal sinusitis is also a complication of upper respiratory tract infections and presents with facial pain, congestion, fever, and— in many cases—persistent nighttime cough. A definitive diagnosis is made by aspiration and culture of sinus material; however, presumptive treatment is most commonly initiated after application of a strict set of clinical diagnostic criteria. Pneumococcal bronchitis is usually seen in the context of pre-existing lung conditions such as bronchiectasis or chronic obstructive pulmonary disease (COPD) and may be caused by nontypeable strains. TREATMENT Pneumococcal Infections Historically, the activity of penicillin against pneumococci made parenteral penicillin G the drug of choice for disease caused by susceptible organisms, including community-acquired pneumonia.

Today, parenteral β-lactam drugs such as ampicillin, cefotaxime, ceftriaxone, and cefuroxime are often used as first-line agents for community-acquired infections. Macrolides and cephalosporins are alternatives for penicillin-allergic patients. While agents such as clindamycin, tetracycline, and trimethoprim-sulfamethoxazole exhibit some activity against pneumococci, resistance to these agents is frequently encountered in different parts of the world.

Penicillin-resistant pneumococci were first described in the mid1960s, at which point tetracycline- and macrolide-resistant strains had already been reported. Multidrug-resistant strains were first described in the 1970s, but it was during the 1990s that pneumo­ coccal drug resistance reached pandemic proportions. The use of antibiotics selects for resistant pneumococci, and strains resistant to β-lactam agents and to multiple drugs are now found all over the world. The emergence of high rates of macrolide and fluoro­ quinolone resistance also has been described. Drug-resistant pneu­ mococci are considered a serious threat by the Centers for Disease Control and Prevention. The molecular basis of penicillin resistance in S. pneumoniae is the alteration of penicillin-binding protein (PBP) genes by transfor­ mation and horizontal transfer of DNA from related streptococcal species. Such alteration of PBPs results in lower affinity for penicil­ lins. Depending on the specific PBP(s) and the number of PBPs altered, the level of resistance ranges from intermediate to high. For many years, penicillin susceptibility breakpoints have been defined by MICs as follows: susceptible, ≤0.06 μg/mL and resistant, ≥2.0 μg/mL. However, in vitro results often were not predictive of the response of a patient to treatment for pneumococcal diseases other than meningitis. Revised recommendations have been based on the penicillin G breakpoints established in 2008 by the Clinical and Laboratory Standards Institute. For IV treatment of meningitis with at least 24 million units per day in 8 divided doses, the suscep­ tibility breakpoint remains ≤0.06 μg/mL, and MICs of ≥0.12 μg/mL indicate resistance. For IV treatment of nonmeningeal infections with 12 million units per day in 6 divided doses, the breakpoints are ≤2 μg/mL for susceptible organisms and ≥8 μg/mL for resistant organisms; a dosage of 18–24 million units per day is recommended for strains with MICs in the intermediate category. CHAPTER 151 Pneumococcal Infections Although guidelines for antibiotic therapy should be driven in part by local patterns of resistance, guidelines from national organizations in many countries (e.g., the Infectious Diseases Society of America/ American Thoracic Society, the British Thoracic Society, the European Respiratory Society) lay out evidence-based approaches. The following guidelines for the treatment of individual sepsis syn­ dromes are based on those advocated by the American Academy of Pediatrics Red Book and updated in 2023. MENINGITIS LIKELY OR PROVEN TO BE DUE

TO S. PNEUMONIAE In areas of the world with an increased prevalence of resistant pneumococci, first-line therapy for persons ≥1 month of age is a combination of vancomycin (adults, 30–60 mg/kg per day; infants and children, 60 mg/kg per day) and cefotaxime (adults, 8–12 g/d in 4–6 divided doses; children, 225–300 mg/kg per day in 1 dose or 2 divided doses) or ceftriaxone (adults, 4 g/d in 1 dose or 2 divided doses; children, 100 mg/kg per day in 1 dose or 2 divided doses). In low-prevalence areas and where the patient has not recently traveled, vancomycin is not included in first-line therapy. If children are hypersensitive to β-lactam agents (penicillins and cephalosporins), rifampin (adults, 600 mg/d; children, 20 mg/d in 1 dose or 2 divided doses) can be substituted for cefotaxime or ceftriaxone and added as a second agent. A repeat lumbar puncture should be considered after 48 h if the organism is not susceptible to penicillin and information on cephalosporin sensitivity is not yet available, if the patient’s clinical condition does not improve or deteriorates, or if dexamethasone has been administered interfering with the ability to interpret clinical responses in the deteriorating patient. When antibiotic sensitivity data become available, treat­ ment should be modified accordingly. If the isolate is sensitive

to penicillin, vancomycin can be discontinued and penicillin can replace the cephalosporin, or cefotaxime or ceftriaxone can be con­ tinued alone. If the isolate displays any resistance to penicillin but is susceptible to the cephalosporins, vancomycin can be discontinued and cefotaxime or ceftriaxone continued. If the isolate exhibits any resistance to penicillin and is not susceptible to cefotaxime and ceftriaxone, vancomycin and high-dose cefotaxime or ceftriaxone can be continued and rifampin may be added. Data support the use of corticosteroids in high-income countries but do not appear to have a beneficial effect in low-income countries. This discrepancy in the efficacy of corticosteroids may be related to differences in availability of appropriate and timely medical care. Glucocorticoids significantly reduce rates of mortality, severe hearing loss, and neurologic sequelae in adults and should be administered to those with community-acquired bacterial meningitis. If dexamethasone is given to either adults or children, it should be administered before or in conjunction with the first antibiotic dose.

SEPSIS (EXCLUDING MENINGITIS) In previously well children with noncritical illness, therapy with a recommended antibiotic should be instigated at the usually rec­ ommended dosages: ampicillin 200 mg/kg/day (doses 6 h apart), cefotaxime, 75–225 mg/kg/day (doses 8 h apart), ceftriaxone, 50–75 mg/kg/day (doses 12–24 h apart) or penicillin G, 250,000–400,000 units/kg per day (in divided doses 4–6 h apart). For critically ill children, including those who have myocarditis or multilobular pneumonia with hypoxia or hypotension, vancomycin may be added if the isolate may possibly be resistant to β-lactam drugs, with its use reviewed once susceptibility data become available. If the organism is resistant to β-lactam agents, therapy should be modi­ fied on the basis of clinical response and susceptibility to other anti­ biotics. Clindamycin or vancomycin can be used as a first-line agent for children with severe β-lactam hypersensitivity, but vancomycin should not be continued if the organism is shown to be sensitive to other non-β-lactam antibiotics. PART 5 Infectious Diseases For outpatient management, oral amoxicillin (45–90 mg/kg/ day, doses 8 h apart) provides effective treatment for virtually all cases of pneumococcal pneumonia. Cephalosporins, which are far more expensive, offer no advantages over amoxicillin. Levofloxacin (500–750 mg/d as a single dose) and moxifloxacin (400 mg/d as a single dose) also are highly likely to be effective in the United States except in patients who come from closed populations where these drugs are used widely or who have themselves been treated recently with a quinolone. Clindamycin (600–1200 mg/d every 6 h) is effec­ tive in 90% of cases and azithromycin (500 mg on day 1 followed by 250–500 mg/d) or clarithromycin (500–750 mg/d as a single dose) in 80% of cases. Treatment failure resulting in bacteremic disease due to macrolide-resistant isolates has been amply documented in patients given azithromycin empirically. As noted above, rates of resistance to all these antibiotics are relatively low in some countries and much higher in others; high-dose amoxicillin remains the best option worldwide. The optimal duration of treatment for pneumococcal pneu­ monia is uncertain, but its continuation for at least 5 days once the patient becomes afebrile appears to be a prudent approach— although in adults, 5 days in total will usually suffice. Cases with a second focus of infection (e.g., empyema or septic arthritis) require longer therapy. ACUTE OTITIS MEDIA Amoxicillin (80–90 mg/kg per day) is recommended for infants <6 months of age and those 6–23 months of age with bilateral dis­ ease. Observation and symptom-based treatment without antibiot­ ics are advocated for nonsevere illness and an uncertain diagnosis in children 6 months to 2 years of age and nonsevere illness (even if the diagnosis seems certain) in children >2 years of age. Although the optimal duration of therapy has not been conclusively estab­ lished, a 10-day course is recommended for younger children and for children with severe disease at any age. For children >6 years

old who have mild or moderate disease, a course of 5–7 days is considered adequate. Patients whose illness fails to respond should be reassessed at 48–72 h. If acute otitis media is confirmed and anti­ biotic treatment has not been started, administration of amoxicillin should be commenced. If antibiotic therapy fails, a change is indi­ cated. Failure to respond to second-line antibiotics (such as highdose amoxicillin-clavulanate) as well indicates that myringotomy or tympanocentesis may need to be undertaken in order to obtain samples for culture. The above recommendations can also be followed for the treat­ ment of sinusitis. Detailed information on the further management of these conditions in children has been published by the American Academy of Pediatrics, the American Academy of Family Physi­ cians, the Pediatric Infectious Diseases Society, and the Infectious Diseases Society of America. ■ ■PREVENTION Measures to prevent pneumococcal disease include vaccination against S. pneumoniae and influenza viruses, reduction of comorbidities that increase the risk of pneumococcal disease, and prevention of antibiotic overuse, which fuels pneumococcal resistance. Capsular Polysaccharide Vaccines  The 23-valent pneumococ­ cal polysaccharide vaccine (PPSV23), containing 25 μg of each capsular polysaccharide, has been licensed for use since 1983. Recommenda­ tions for its use vary by country with an age-based recommendation for those ≥65 years of age most commonly found. However, the utility of PPSV23 is likely to be limited in the future as more countries introduce the newly licensed (2021) extended-valency pneumococcal conjugate vaccines (see “Polysaccharide–Protein Conjugate Vaccines,” below). The effectiveness of PPSV23 against IPD, pneumococcal pneu­ monia, all-cause pneumonia, and death is controversial, with wide variation in observations. The many published meta-analyses of PPSV efficacy have often reached opposing conclusions with regard to a given clinical entity. Generally, observational studies cite greater effec­ tiveness than do controlled clinical trials. The consensus is that PPSV is effective against IPD but is less effective against nonbacteremic pneu­ mococcal pneumonia. However, the results of some published trials, observational studies, and meta-analyses contradict this view. Effec­ tiveness is often lower in the elderly and in immunodeficient patients whose condition is associated with reduced antibody responses to vaccines than in younger, healthier populations. When PPSV is effec­ tive, the duration of protection following a single dose of vaccine is estimated to be ~5 years. Even in the setting of routine pneumococcal conjugate vaccination of infants (which indirectly protects adults from vaccine-serotype strains), disease caused by serotypes not represented in the conjugate vaccine continues to be a significant burden among adults. Polysaccharide–Protein Conjugate Vaccines  Infants and young children respond poorly to PPSV, which contains T cell–independent antigens. Consequently, another class of pneumococcal vaccines, the PCVs, were developed specifically for infants and young children. The first product, a 7-valent PCV, was licensed in 2000 in the United States and two PCV products—containing 10 and 13 serotypes, respectively— were licensed in 2009 and 2010, respectively. The serotypes included in these original PCV formulations were important causes of IPD and antibiotic resistance among young children. Randomized controlled trials and real-world evidence accumulated following their widespread use have demonstrated a high degree of efficacy of PCVs against vaccine-serotype IPD as well as efficacy against pneumonia, otitis media, nasopharyngeal colonization, and all-cause mortality. PCVs are recommended by the World Health Organization for inclusion in routine childhood immunization schedules worldwide, especially in countries with high infant mortality rates. To date 146 countries (75%) have PCV in their National Immunization program, 15 are planning introduction, and 33 have no national decision. In 2021, two new PCVs were licensed for adults: a 15-valent pneumococcal conjugate vaccine, VAXNEUVANCE (Merck), and a

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152 Staphylococcal Infections

20-valent pneumococcal conjugate vaccine, PREVNAR 20 (Pfizer). Both vaccines are now also licensed for infant immunization and are likely to gradually replace PCV10 and PCV13. The additional serotypes in the new vaccines account for a variable fraction of the residual IPD in children (additional 15–45%) and adults (additional 11–33%) for PCV15 and PCV20, respectively, in countries where PCV13 coverage is high. For adults the United States Advisory Committee on Immuni­ zation Practices (ACIP) now recommends PCV20 instead of PPSV23 for all persons ≥65 years of age and for those 2–64 years of age who have underlying medical conditions that put them at increased risk for pneumococcal disease or, if infected, disease of increased severity (Table 151-1; see also www.cdc.gov/vaccines/schedules). United States

recommendations for PPSV23 are now limited to sequential use in the above age groups receiving the PCV15 vaccine. The introduction of PCV in high-income settings has resulted in a

90% reduction in vaccine-serotype IPD among the whole population. This decline has been noted not only in those age groups immunized but also in adults and is attributable to the near elimination of vaccine-

serotype nasopharyngeal colonization in immunized infants, which reduces spread to adults. This protection of unimmunized community members through vaccination of a subset of the community is termed the indirect effect. Increases in colonization with—and concomitantly in disease due to—non-vaccine-serotype strains (i.e., replacement colonization and disease) have been seen. The scale of replacement disease has varied geographically with the impact eroding vaccine impact significantly in the elderly in the United Kingdom while having relatively little impact in the United States (see “Epidemiology,” above). Since vaccine-serotype strains are more commonly resistant to anti­ biotics than are non-vaccine serotypes, use of PCV has also resulted in substantial declines in the proportion and absolute rates of drugresistant pneumococcal disease. The ACIP recommendations for the use of conjugate vaccines can be found at www.cdc.gov/vaccines/hcp/ acip-recs/vacc-specific/pneumo.html. PCV has been shown to prevent pneumococcal infection in HIV-infected adults. Other Prevention Strategies  Pneumococcal disease can be averted through the prevention of illnesses that predispose individu­ als to pneumococcal infections. Relevant measures include smoking cessation and influenza vaccination, as well as improved management and control of diabetes, HIV infection, heart disease, and lung disease. Finally, the reduction of antibiotic misuse is a strategy for the preven­ tion of pneumococcal disease in that antimicrobial resistance directly and indirectly perpetuates organism transmission and disease in the community. ■ ■GLOBAL HEALTH In 2015, pneumococcal infections were estimated to have caused ~317,000 annual deaths worldwide among children 1–59 months of age, accounting for 9.7% of the 3.2 million all-cause deaths and 38% of all pneumonia deaths in this age group. Updated estimates of residual pneumococcal disease burden since the widespread introduction of PCV have not been published. Reliable estimates of adult cases and deaths globally are more difficult to establish because of limited data from parts of the world where most disease occurs. Rates of pneumo­ coccal disease and mortality vary substantially across geographic set­ tings, with the highest rates in selected countries of sub-Saharan Africa and southern Asia, where risk factors for pneumococcal disease— including HIV infection, lack of breast feeding of infants and children, malnutrition, sickle cell disease, and limited access to medical care— are prevalent. Serotypes causing disease exhibit some heterogeneity across geographic settings, but a small number of serotypes universally account for the preponderance of disease in the absence of vaccina­ tion; accordingly, vaccine development and vaccination programs are globally relevant. Reductions in disease from pneumococcal infections are anchored in prevention through the inclusion of pneumococcal vaccines in infant immunization programs, timely assessment and appropriate treatment of persons with pneumococcal infections, and reduction of risk factors for pneumococcal disease. The use of vaccines for the prevention of adult pneumococcal disease, particularly among

the elderly, is currently implemented in high-income countries, with virtually no use in low-income countries where most cases of disease exist.

■ ■FURTHER READING Krone CL et al: Immunosenescence and pneumococcal disease: An imbalance in host–pathogen interactions. Lancet Respir Med 2:141, 2014. Lees JA et al: Fast and flexible bacterial genomic epidemiology with PopPUNK. Genome Res 29:304, 2019. Mackenzie GA et al: The impact of the introduction of pneumococcal conjugate vaccination in invasive pneumococcal disease and pneumonia in The Gambia: 10 years of population-based surveillance. Lancet Infect Dis 21:1293, 2021. Subramanian K et al: Pneumolysin binds to the mannose recep­ tor C type 1 (MRC-1) leading to anti-inflammatory responses and enhanced pneumococcal survival. Nat Microbiol 4:62, 2019. Van Der Poll T, Opal SM: Pathogenesis, treatment, and prevention of pneumococcal pneumonia. Lancet 374:1543, 2009. ■ ■WEBSITES American Academy of Pediatrics: Red Book: The report of the Committee on Infectious Diseases. Available at: aapredbook.aap­ publications.org. Cochrane: Corticosteroids for Bacterial Meningitis. Available at: www. cochrane.org/CD004405/ARI_corticosteroids-bacterial-meningitis. U.S. Department of Health and Human Services: Antibiotic Resistance Threats in the United States 2019. Available at: www.cdc. gov/drugresistance/pdf/threats-report/2019-ar-threats-report-508.pdf. World Health Organization: Summary of WHO Position Paper CHAPTER 152 on Pneumococcal conjugate vaccines in infants and children under 5 years of age, February 2019. Available at: www.who.int/ publications/i/item/10665-310968. Staphylococcal Infections Franklin D. Lowy, Anne-Catrin Uhlemann

Staphylococcal Infections Staphylococcus aureus, the most virulent of the many (≥40) staphylococ­ cal species, has demonstrated its versatility by remaining a major cause of morbidity and mortality worldwide despite the availability of numerous effective antistaphylococcal antibiotics. S. aureus is a pluripotent pathogen, causing disease through both toxin- and non-toxin-mediated mecha­ nisms. It is responsible for numerous nosocomial and community-based infections that range from relatively minor skin and soft tissue infections (SSTIs) to life-threatening systemic infections. The “other” staphylococci, coagulase-negative staphylococci, are less virulent than S. aureus but remain important pathogens in select set­ tings, such as infections involving prosthetic devices. MICROBIOLOGY AND TAXONOMY Staphylococci, gram-positive cocci in the family Micrococcaceae, form grapelike clusters on Gram’s stain (Fig. 152-1). These organisms (~1 μm in diameter) are catalase-positive (unlike streptococcal spe­ cies), nonmotile, aerobic, and facultatively anaerobic. They are capable of prolonged survival on environmental surfaces under varying condi­ tions. Some species have a relatively broad host range, including mam­ mals and birds, whereas the host range for others is quite narrow—i.e., limited to one or two closely related animals. S. aureus is generally distinguished from other staphylococcal spe­ cies by coagulase production, a surface enzyme that converts fibrinogen to fibrin. However, several of the “coagulase-negative staphylococci,”

FIGURE 152-1  Gram’s stain of S. aureus in a sputum sample, illustrating staphylococcal clusters. (From ASM MicrobeLibrary.org. © Pfizer, Inc.) including S. pseudintermedius and S. argenteus, are coagulase-positive. As a result, description of these other staphylococci as non–S. aureus staphylococci (NSaS) is more accurate. S. aureus ferments mannitol, is positive for protein A, and produces DNAse. On blood agar plates, S. aureus forms golden β-hemolytic colo­ nies; in contrast, most NSaS form small, white nonhemolytic colonies. Latex kits that detect both protein A and clumping factor can distin­ guish S. aureus from most other staphylococcal species. Point-of-care tests targeting these two proteins also are used for the rapid detection of staphylococcal colonization. Newer methods such as matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDITOF) are increasingly being used for staphylococcal speciation. PART 5 Infectious Diseases Determining whether multiple staphylococcal isolates from differ­ ent patients are the same or different is often relevant when there is concern that a nosocomial outbreak of staphylococcal infections is due to a common point source (e.g., a contaminated medical instrument). Molecular typing methods, such as pulsed-field gel electrophoresis and sequence-based techniques (e.g., staphylococcal protein A [SpA] typing), have been used for this purpose. More recently, whole-genome sequencing has emerged as the gold standard for discrimination among different isolates. S. AUREUS INFECTIONS ■ ■EPIDEMIOLOGY S. aureus is both a commensal and an opportunistic pathogen. Approx­ imately 20–40% of healthy persons are colonized with S. aureus, with a smaller percentage (~10%) persistently colonized with the same strain. The rate of colonization is elevated among type 1 diabetics, HIV-infected patients, patients undergoing hemodialysis, injection drug users, and individuals with damaged skin. The anterior nares and oropharynx are frequent sites of human colonization, although the skin (especially when damaged), axilla, vagina, and perineum also are often colonized. These colonization sites serve as potential reservoirs for future infections. Most individuals who develop S. aureus infections become infected with a strain that is already a part of their own commensal flora. Breaches of the skin or mucosal membrane allow S. aureus to initiate infection. Person-to-person transmission of S. aureus also occurs, most frequently from direct personal contact with an infected body site. Spread of staphylococci in aerosols of respiratory or nasal secretions from heavily colonized individuals, although rare, has been reported. Some diseases increase the risk of S. aureus infection. Diabetes, for example, combines an increased rate of S. aureus colonization and the use of injectable insulin with the possibility of impaired leuko­ cyte function. Individuals with congenital or acquired qualitative or quantitative defects of polymorphonuclear leukocytes (PMNs) are at increased risk of S. aureus infections; this group includes neutropenic patients (e.g., those receiving chemotherapeutic agents), those with

chronic granulomatous disease, and those with autosomal dominant hyperimmunoglobulin E (Job syndrome) or Chédiak-Higashi syn­ drome. Other groups at risk include individuals with end-stage renal disease, HIV infection, skin abnormalities, or prosthetic devices. S. aureus is a leading cause of health care–associated infections (Chap. 147). It is the most common cause of surgical wound infections and is second only to NSaS as a cause of primary bacteremia. These iso­ lates are often resistant to multiple antibiotics; thus, available therapeutic options may be limited. In the community, S. aureus remains an important cause of SSTIs, respiratory infections, and, especially among injection drug users, infective endocarditis. The increasing use of home infusion therapy also poses a risk of community-acquired staphylococcal infections. In the past three decades, there has been a dramatic change in the epidemiology of infections due to methicillin-resistant S. aureus (MRSA). In addition to its major role as a nosocomial pathogen, MRSA has become an established community-based pathogen. Numerous outbreaks of community-associated MRSA (CA-MRSA) infections have been reported in both rural and urban settings in widely separated regions throughout the world. This trend appears to be due in part to the dramatic increase in MRSA colonization found in the community in different parts of the world. Outbreaks of CA-MRSA infections have occurred among such diverse groups as children, prisoners, athletes, Native Americans, and drug users. Risk factors common to these outbreaks include poor hygienic conditions, close contact, contaminated material, and dam­ aged skin. In some geographic regions of the world, the infections have been caused by a single CA-MRSA strain, while in others, a variety of CA-MRSA strains have been responsible. In the United States, strain sequence type 8 (PFGE type USA300) has been the predominant clone (Fig. 152-2). Although most infections caused by these strains have involved the skin and soft tissue, 5–10% have been invasive and poten­ tially life-threatening. CA-MRSA strains have also been responsible for an increasing number of nosocomial infections. Of concern has been the enhanced capacity of CA-MRSA to cause disease in immunocom­ petent individuals. ■ ■PATHOGENESIS General Concepts  S. aureus is a pyogenic pathogen known for its capacity to induce abscess formation at both local and distant sites (i.e., metastatic infections). This classic pathologic response to S. aureus defines the framework within which infections will progress. The bac­ teria elicit an inflammatory response characterized by an initial intense infiltration of PMNs and a subsequent infiltration of macrophages and fibroblasts. Either the host cellular response (including the deposition of fibrin and collagen) contains the infection with the formation of a fibrinous capsule, or infection spreads to the adjoining tissue or into the bloodstream. In toxin-mediated staphylococcal disease, infection is not invari­ ably present. For example, in staphylococcal food poisoning, once the heat-stable enterotoxin has been released into food, symptoms can develop in the absence of viable bacteria. In staphylococcal toxic shock syndrome (TSS), conditions allowing toxin elaboration at colonization sites (e.g., the presence of a superabsorbent tampon) suffice for initia­ tion of clinical illness. The S. aureus Genome  The complete genomes of S. aureus strains are now readily available. Among the interesting revela­ tions are (1) the high degree of nucleotide sequence similarity of the core genomes of different strains; (2) the acquisition of a relatively large amount of genetic information by horizontal transfer from other bacterial species; and (3) the presence of unique “pathogenicity” or “genomic” islands—mobile genetic elements that contain clusters of enterotoxin and exotoxin genes and/or antimicrobial resistance deter­ minants. Among the genes in these islands is mecA, the gene respon­ sible for methicillin resistance. Methicillin resistance–containing islands have been designated staphylococcal cassette chromosome mec (SCCmec). There are different SCCmec types that range in size from ~20 to 60 kb. Among the more common SCCmec types, types 1–3 are

FIGURE 152-2  Global distribution of community-associated MRSA. Dotted lines indicate possible route of dissemination. Estimates of the areas are shown where infection with the main strains—i.e., ST1 (green), ST8 (red), ST30 (blue), and ST80 (gray hatched)—have been reported. +, Panton-Valentine leukocidin (PVL)-positive strains; –, PVL-negative strains; ±, PVL-positive and -negative strains. (Reproduced with permission from FR DeLeo, M Otto, BN Kreiswirth, HF Chambers: Community-associated methicillin-resistant Staphylococcus aureus. Lancet 375:1557, 2010.) traditionally associated with nosocomial MRSA isolates, whereas types 4–6 have been associated with epidemic CA-MRSA strains. A relatively limited number of MRSA clones have been responsible for most community- and hospital-associated infections worldwide. A comparison of these strains with those from earlier outbreaks (e.g., the phage 80/81 strains from the 1950s) has revealed preservation of much of the nucleotide sequence over time. This observation suggests that these strains possess determinants that facilitate survival and spread. Regulation of Virulence Gene Expression  In both toxinmediated and non-toxin-mediated diseases due to S. aureus, the expression of virulence determinants associated with infection depends on a series of regulatory genes (e.g., accessory gene regulator [agr] and staphylococcal accessory regulator [sar]) that coordinately control the expression of many virulence genes. The regulatory gene agr is part of a quorum-sensing signal transduction pathway that senses and responds to bacterial density. Staphylococcal surface pro­ teins are synthesized during the bacterial exponential growth phase in vitro. In contrast, many secreted proteins, such as α toxin, the entero­ toxins, and assorted enzymes, are released during the post–exponential growth phase in response to transcription of the effector molecule of agr, RNAIII. These regulatory genes appear to serve a similar function in vivo. Successful invasion requires the sequential expression of these different bacterial elements. Bacterial adhesins are needed to initiate coloniza­ tion of host tissue surfaces. The subsequent release of various enzymes enables the colony to obtain nutritional support and permits bacteria to spread to adjacent tissues. Studies with strains in which these regu­ latory genes are inactivated show reduced virulence in several animal models of S. aureus infection. Pathogenesis of Invasive S. aureus Infection  Staphylococci are opportunists. For these organisms to invade the host and cause infection, some or all of the following steps are necessary: contamina­ tion and colonization of host tissue surfaces, breach of cutaneous or mucosal barriers, establishment of a localized infection, invasion, eva­ sion of the host response, and metastatic spread. Colonizing strains or strains transferred from other exposures are introduced into damaged skin, a wound, or the bloodstream. Recurrences of S. aureus infections are common, apparently because of the capacity of these pathogens to

CHAPTER 152 persist in a quiescent state in various tissues, and then to cause recru­ descent infections when suitable conditions arise. S. AUREUS COLONIZATION OF BODY SURFACES  The anterior nares and oropharynx are primary sites of staphylococcal colonization. In the nares, colonization appears to involve the attachment of S. aureus to keratinized epithelial cells. Other factors that contribute to colo­ nization include the influence of other resident nasal flora and their bacterial density, host factors, and nasal mucosal damage (e.g., that resulting from inhalational drug use). Other colonized body sites, such as damaged skin, the groin, and the oropharynx, may be particularly important reservoirs for CA-MRSA strains. Staphylococcal Infections INOCULATION AND COLONIZATION OF TISSUE SURFACES  Staphylo­ cocci may be introduced into tissue as a result of minor abrasions (e.g., mosquito bites), administration of medications such as insulin, or estab­ lishment of IV access with catheters. After introduction into a tissue site, bacteria replicate and colonize the host tissue surface. A family of struc­ turally related S. aureus surface proteins referred to as MSCRAMMs (microbial surface components recognizing adhesive matrix molecules) play an important role as mediators of adherence to these different sites. By adhering to exposed matrix molecules (e.g., fibrinogen, collagen, fibronectin), MSCRAMMs, such as clumping factor and collagen-binding protein, enable the bacteria to colonize different host tissue surfaces; these proteins contribute to the pathogenesis of invasive infections such as endocarditis and septic arthritis by facilitating the adherence of S. aureus to surfaces with exposed fibrinogen or collagen. Although NSaS are classically known for their ability to elaborate biofilms and to colonize prosthetic devices, S. aureus also possesses the genes responsible for biofilm formation, such as the intercellular adhe­ sion (ica) locus. Binding to these devices occurs in a stepwise fashion, involving staphylococcal adherence to serum constituents that have coated the device surface and subsequent biofilm elaboration. S. aureus is thus a frequent cause of biomedical device–related infections. INVASION  After colonization, staphylococci replicate at the initial site of infection, elaborating enzymes that include serine proteases, hyaluronidases, thermonucleases, and lipases. These enzymes facilitate bacterial survival and local spread across tissue surfaces. The lipases may facilitate survival in lipid-rich areas such as the hair follicles, where S. aureus infections are often initiated.

Constitutional findings may result from either localized or systemic infections. The staphylococcal cell wall—consisting of alternating N-acetyl muramic acid and N-acetyl glucosamine units in combination with an additional cell wall component, lipoteichoic acid—can initiate an inflammatory response that includes the sepsis syndrome. Staphy­ lococcal alpha (α) toxin is a critical staphylococcal toxin. It causes pore formation in various eukaryotic cells and can also initiate an inflam­ matory response with findings suggestive of sepsis. The S. aureus toxin Panton-Valentine leukocidin is cytolytic to PMNs, macrophages, and monocytes. Strains elaborating this toxin have been epidemiologically linked with cutaneous and more serious infections (i.e., pneumonia) caused by strains of CA-MRSA.

EVASION OF HOST DEFENSE MECHANISMS  Staphylococci have many host immune evasion strategies that are crucial to their survival. They possess an antiphagocytic polysaccharide microcapsule. Most human S. aureus infections are due to strains with capsular types 5 and 8. The zwitterionic (both negatively and positively charged) S. aureus capsule also plays a critical role in the induction of abscess formation. Protein A, an MSCRAMM unique to S. aureus, acts as an Fc receptor, binding the Fc portion of IgG subclasses 1, 2, and 4 and preventing opsonophagocytosis by PMNs. Both chemotaxis inhibitory protein of staphylococci (CHIPS, a secreted protein) and extracellular adherence protein (EAP, a surface protein) interfere with PMN migration to sites of infection. There are several cytolytic toxins, including α toxin and Panton-Valentine toxin, that are secreted by staphylococci that cause lysis of different host cells and contribute to host tissue damage. An additional potential mechanism of S. aureus evasion is its capac­ ity for intracellular survival. Both professional and nonprofessional phagocytes internalize staphylococci. Internalization by these cells may provide a sanctuary that protects bacteria against the host’s defenses. This phenomenon appears to be especially relevant for hepatic Kupffer cells during staphylococcal bacteremias. The intracellular environment favors the phenotypic expression of S. aureus small-colony variants, which are found in patients receiving antimicrobial therapy (e.g., with aminoglycosides) and in those with cystic fibrosis or osteomyelitis. These variants, whether intra- or extracellular, may facilitate prolonged staphylococcal survival in different tissue sites and enhance the likeli­ hood of recurrences. Finally, S. aureus can survive within PMNs and may use these cells to spread and seed other tissue sites. PART 5 Infectious Diseases PATHOGENESIS OF COMMUNITY-ACQUIRED MRSA INFECTIONS  A number of specific virulence determinants contribute to the patho­ genesis of CA-MRSA infections. A strong epidemiologic association links the presence of the gene for the Panton-Valentine leukocidin with SSTIs and with necrotizing postinfluenza pneumonia. Other determinants that play a role in the pathogenesis of these infections and contribute to the unique virulence of these clones include the arginine catabolic mobile element (ACME), a cluster of unique genes that may facilitate evasion of host defense mechanisms; phenol-soluble modulins, a family of cytolytic peptides; and α toxin. Host Response to S. aureus Infection  The primary host response to S. aureus infection is the recruitment of PMNs. These cells are attracted to infection sites by bacterial components such as formylated peptides or peptidoglycan as well as by the cytokines tumor necrosis factor (TNF) and interleukins (ILs) 1 and 6, which are released by activated macrophages and endothelial cells. Although most individuals have antibodies to staphylococci, it is not clear that antibody levels are qualitatively or quantitatively sufficient to protect against infection. Anticapsular and anti-MSCRAMM antibod­ ies facilitate opsonization in vitro and have been protective against infection in several animal models; however, vaccines with these com­ ponents have not yet successfully prevented staphylococcal infections in clinical trials. Pathogenesis of Toxin-Mediated Disease  S. aureus produces three types of toxins: cytotoxins, pyrogenic toxin superantigens, and exfoliative toxins. Both epidemiologic data and studies in animals suggest that antitoxin antibodies are protective against illness in TSS, staphylococcal food poisoning, and staphylococcal scalded-skin

syndrome (SSSS). Illness develops after toxin synthesis and absorption and the subsequent toxin-initiated host response. ENTEROTOXIN AND TOXIC SHOCK SYNDROME TOXIN 1 (TSST-1)  The pyrogenic toxin superantigens are a family of small-molecular-size, structurally similar proteins that are responsible for two diseases: TSS and food poisoning. TSS results from the ability of TSST-1 and enterotoxins to function as T-cell mitogens. In the normal process of antigen presentation, the antigen is first processed within the cell, and peptides are then presented in the major histocompatibility complex (MHC) class II groove, initiating a measured T-cell response. In con­ trast, TSST-1 and enterotoxins bind directly to the invariant region of MHC—outside the MHC class II groove. TSST-1 and the enterotoxins can then bind T-cell receptors via the vβ chain; this binding results in a dramatic overexpansion of T-cell clones (up to 20% of the total T-cell population). The consequence of this T-cell expansion is a cyto­ kine storm, with the release of inflammatory mediators that include interferon γ, IL-1, IL-6, TNF-α, and TNF-β. The resulting multisystem disease produces a constellation of findings that mimic those found in endotoxin shock; however, the pathogenic mechanisms differ. A different region of the enterotoxin molecule is responsible for the symptoms of food poisoning. The enterotoxins are heat stable and can survive conditions that kill the bacteria. Illness results from the ingestion of preformed toxin; as a result, the incubation period is short (1–6 h). The toxin stimulates the vagus nerve and the vomiting center of the brain. It also appears to stimulate intestinal peristaltic activity. EXFOLIATIVE TOXINS AND SSSS  The exfoliative toxins are responsible for SSSS, most commonly seen in newborns. The toxins that produce disease in humans are of two serotypes: ETA and ETB. These toxins are serine proteases that cleave desmosomal cadherins in the superfi­ cial layer of the skin, triggering exfoliation. The result is a split in the epidermis at the granular level, which is responsible for the superficial desquamation of the skin that typifies this illness. ■ ■DIAGNOSIS Staphylococcal infections are readily diagnosed by Gram’s stain (Fig. 152-1) and microscopic examination of abscess contents or of infected tissue. Routine cultures of infected material usually are positive; blood cultures are sometimes positive even when infections are localized to extravascular sites. S. aureus is rarely a blood culture contaminant. Polymerase chain reaction (PCR)–based assays are now often used for the rapid diagnosis of S. aureus infection. A number of point-of-care tests are available to screen patients for colonization with MRSA. Determining whether patients with documented S. aureus bacteremia also have infective endocarditis or a metastatic focus of infection remains a diagnostic challenge. Uniformly positive cultures of blood collected over time suggest an endovascular infection such as endocarditis (see “Bacteremia, Sepsis, and Infective Endocarditis,” below). ■ ■CLINICAL SYNDROMES (Table 152-1) Skin and Soft Tissue Infections  S. aureus causes a variety of cutaneous infections. Common factors predisposing to S. aureus cuta­ neous infection include chronic skin conditions (e.g., eczema), skin damage (e.g., insect bites, minor trauma), injections (e.g., in diabetes, injection drug use), and poor personal hygiene. These infections are characterized by the formation of pus-containing blisters, which often begin in hair follicles and spread to adjoining tissues. Folliculitis is a superficial infection that involves the hair follicle, with a central area of purulence (pus) surrounded by induration and erythema. Furuncles (boils) are more extensive, painful lesions that tend to occur in hairy, moist regions of the body and extend from the hair follicle to become a true abscess with an area of central purulence. Carbuncles are most often located in the lower neck and are even more severe and painful, resulting from the coalescence of other lesions that extend to a deeper layer of the subcutaneous tissue. In general, furuncles and carbuncles are readily apparent, with pus often expressible or discharging from the abscess. Other cutaneous S. aureus infections include impetigo

TABLE 152-1  Common Illnesses Caused by Staphylococcus aureus Skin and Soft Tissue Infections   Folliculitis   Abscess, furuncle, carbuncle   Cellulitis   Impetigo   Mastitis   Surgical wound infections Musculoskeletal Infections   Septic arthritis   Osteomyelitis (hematogenous or contiguous spread)   Pyomyositis   Psoas abscess Respiratory Tract Infections   Ventilator-associated or nosocomial pneumonia   Septic pulmonary emboli   Postviral pneumonia (e.g., influenza)   Empyema Bacteremia and Its Complications   Sepsis, septic shock   Metastatic foci of infection (kidney, joints, bone, lung)   Infective endocarditis Infective Endocarditis   Injection drug use–associated   Native-valve   Prosthetic-valve   Nosocomial Device-Related Infections (e.g., intravascular catheters, prosthetic joints) Toxin-Mediated Illnesses   Toxic shock syndrome   Food poisoning   Staphylococcal scalded-skin syndrome Invasive Infections Associated with Community-Acquired MethicillinResistant S. aureus   Necrotizing fasciitis   Waterhouse-Friderichsen syndrome   Necrotizing pneumonia   Purpura fulminans and cellulitis. S. aureus is one of the most common causes of surgical wound infections. Mastitis develops in 1–3% of nursing mothers. This infection of the breast, which generally presents within 2–3 weeks after delivery, is characterized by findings that range from cellulitis to abscess forma­ tion. Systemic signs, such as fever and chills, are often present in more severe cases. Musculoskeletal Infections  S. aureus is a common cause of bone infections—both those resulting from hematogenous dissemination and those arising from contiguous spread from a soft tissue site. Hema­ togenous osteomyelitis in children most often involves the long bones. Infections present with fever and bone pain or with a child’s reluctance to bear weight. The white blood cell count and erythrocyte sedimen­ tation rate are often elevated. Blood cultures are positive in ~50% of cases. When necessary, bone biopsies for culture and histopathologic examination are usually diagnostic. In adults, hematogenous osteomyelitis involving the long bones is less common. However, vertebral osteomyelitis is among the more com­ mon clinical presentations. Vertebral bone infections are most often

seen in patients with endocarditis, those undergoing hemodialysis, diabetics, and injection drug users. These infections may present with intense back pain and fever but may also be clinically occult, present­ ing as chronic back pain with low-grade fever. S. aureus is the most common cause of epidural abscess, a complication that can result in neurologic compromise. Patients report difficulty voiding or walking and radicular pain in addition to the symptoms associated with their osteomyelitis. Surgical intervention in this setting often constitutes a medical emergency.

Magnetic resonance imaging (MRI) is the most reliable imaging modality to help establish the diagnosis of osteomyelitis (Fig. 152-3). Routine x-rays are an appropriate first step, but findings may be normal for up to 14 days after the onset of symptoms. If an MRI is not possible, computed tomography (CT) is an acceptable alternative. Bone infections that result from contiguous spread tend to develop from soft tissue infections, such as those associated with diabetic or vascular ulcers, surgery, or trauma. Exposure of bone, a draining fistulous tract, failure to heal, or continued drainage suggests involve­ ment of underlying bone. Bone involvement is established by bone culture and histopathologic examination (revealing evidence of PMN infiltration). Contamination of culture material from adjacent tissue can make the diagnosis of osteomyelitis difficult in the absence of pathologic confirmation. Samples obtained during surgery are the most reliable. An MRI is the most reliable radiologic test to distinguish between osteomyelitis and overlying soft tissue infection with underly­ ing osteitis. In both children and adults, S. aureus is the most common cause of septic arthritis in native joints. If left untreated, this infection is rapidly progressive and may be associated with extensive joint destruction. It presents with intense pain on motion of the affected joint, swelling, and fever. Aspiration of the joint reveals turbid fluid, with >50,000 PMNs/μL and gram-positive cocci in clusters seen on Gram’s stain (Fig. 152-1). In adults, septic arthritis may result from trauma, surgery, or hematogenous dissemination. The most commonly involved joints include the knees, shoulders, hips, and phalanges. Infection frequently develops in joints previously damaged by osteoarthritis or rheumatoid CHAPTER 152 Staphylococcal Infections FIGURE 152-3  S. aureus vertebral osteomyelitis and epidural abscess involving the thoracic disk between T9 and T10. Sagittal postcontrast magnetic resonance imaging of the spine illustrates destruction of the T9–T10 intervertebral space with enhancement (long arrow). There is impingement on the thoracic cord and an epidural collection extending from T9 through T11 (short arrows).

arthritis. Iatrogenic infections resulting from aspiration or injection of agents into the joint also occur. In these settings, the patient experi­ ences increased pain and swelling in the involved joint in association with fever.

Pyomyositis is an unusual infection of skeletal muscles that is seen primarily in tropical climates but also occurs in immunocompromised (e.g., HIV-infected) patients. It is believed to arise from occult bacte­ remia. Pyomyositis presents as fever, swelling, and pain overlying the involved muscle. Aspiration of fluid from the involved tissue yields pus. Although a history of trauma may be associated with the infection, its pathogenesis is poorly understood. Respiratory Tract Infections  Respiratory tract infections caused by S. aureus occur in selected clinical settings. S. aureus is a cause of serious respiratory tract infections in newborns and infants; these infections present with shortness of breath, fever, and respiratory failure. Chest x-ray may reveal pneumatoceles (shaggy, thin-walled cavities). Pneumothorax and empyema are recognized complications. In adults, nosocomial S. aureus pulmonary infections are common among intubated patients in intensive care units. Nasally colonized patients are at increased risk of these infections. The clinical presenta­ tion is no different from pulmonary infections caused by other bacterial pathogens. Patients produce increased volumes of purulent sputum and develop respiratory distress, fever, and new pulmonary infiltrates. Dis­ tinguishing bacterial pneumonia from respiratory failure or other causes of new pulmonary infiltrates in critically ill patients is difficult and relies on a constellation of clinical, radiologic, and laboratory findings. Community-acquired respiratory tract infections due to S. aureus often follow viral infections—most commonly influenza. Patients may present with fever, bloody sputum production, and midlung-field pneumatoceles or multiple, patchy pulmonary infiltrates. Diagnosis is made by sputum Gram’s stain and culture. Blood cultures, although useful, are usually negative. Bacteremia, Sepsis, and Infective Endocarditis  S. aureus bacteremia may be complicated by sepsis, endocarditis, vasculitis, or metastatic seeding (establishment of suppurative collections at other tissue sites). Among the more commonly seeded tissue sites are bones, joints, kidneys, and lungs. The frequency of metastatic seeding during bacteremia has been estimated to be as high as 31%. The incidence of these complications increases with the duration of the bacteremia. PART 5 Infectious Diseases Recognition of these complications by clinical criteria alone is chal­ lenging. Comorbid conditions that are frequently seen in association with S. aureus bacteremia and that increase the risk of complications include diabetes, HIV infection, and renal insufficiency. Other host factors that increase the risk of complications include presentation with community-acquired S. aureus bacteremia, lack of an identifiable primary focus of infection, and the presence of prosthetic devices or material. Clinically, S. aureus sepsis presents in a manner similar to that docu­ mented for sepsis due to other bacteria. The well-described progres­ sion of hemodynamic changes—beginning with respiratory alkalosis and clinical findings of hypotension and fever—is commonly seen. The microbiologic diagnosis is established by positive blood cultures. The overall incidence of S. aureus endocarditis has increased over the past 20 years. S. aureus is now the leading cause of endocarditis worldwide, accounting for 25–35% of cases. This increase is due, at least in part, to the increased use of intravascular devices and, more recently, the upsurge in injection drug use. Studies using transesopha­ geal echocardiography found an endocarditis incidence of ~25% among patients with intravascular catheter–associated S. aureus bacte­ remia. Other factors associated with an increased risk of endocarditis are hemodialysis, the presence of intravascular prosthetic devices at the time of bacteremia, and immunosuppression. Patients with implant­ able cardiac devices (e.g., permanent pacemakers) are at increased risk of endocarditis or device-related infections. Despite the availability of effective antibiotics, mortality rates from these infections continue to range from 20 to 40%, depending on both the host and the nature of the infection. Complications of S. aureus endocarditis include cardiac valvular insufficiency, peripheral emboli, metastatic seeding, vasculitis,

and central nervous system (CNS) involvement (e.g., mycotic aneu­ rysms, embolic strokes). S. aureus endocarditis is encountered in four clinical settings: (1) right-sided endocarditis in association with injection drug use; (2) leftsided native-valve endocarditis; (3) prosthetic-valve endocarditis; and (4) nosocomial endocarditis. In each of these settings, the diagnosis is suspected from the patient’s history and the recognition of physical signs suggestive of endocarditis. These findings include cardiac mani­ festations, such as new or changing cardiac valvular murmurs; cuta­ neous evidence, such as vasculitic lesions, Osler’s nodes, or Janeway lesions; evidence of right- or left-sided embolic disease; and a history suggesting a risk for S. aureus bacteremia. In the absence of anteced­ ent antibiotic therapy, blood cultures are almost uniformly positive. Transthoracic echocardiography, while less sensitive than transesopha­ geal echocardiography, is less invasive and often identifies valvular vegetations. The Duke criteria (Chap. 133) are commonly used to help establish this diagnosis. Acute right-sided tricuspid valvular S. aureus endocarditis is most often seen in patients who inject drugs. The classic presentation includes a high fever, a toxic clinical appearance, pleuritic chest pain, and the production of purulent, sometimes bloody, sputum. Chest x-rays or CT scans reveal evidence of septic pulmonary emboli (small, peripheral, circular lesions that may cavitate with time) (Fig. 152-4). A high percentage of affected patients have no history of antecedent val­ vular damage. At the outset of their illness, patients may present with fever alone, without cardiac or other localizing findings. As a result, a high index of clinical suspicion is essential for diagnosis. Individuals with antecedent cardiac valvular damage more com­ monly present with left-sided native-valve endocarditis involving the damaged valve. These patients tend to be older than those with rightsided endocarditis, their prognosis is worse, and their incidence of complications (including peripheral emboli, cardiac decompensation, cerebrovascular events, and metastatic seeding) is increased. S. aureus is one of the more common causes of prosthetic-valve endocarditis. This infection is especially fulminant in the early post­ operative period and is associated with increased morbidity and mor­ tality. In most instances, medical therapy alone is not sufficient and urgent valve replacement is necessary. Patients are prone to develop valvular insufficiency or myocardial abscesses originating from the region of valve implantation. The increased frequency of nosocomial endocarditis (15–30% of cases, depending on the series) reflects in part the increased use of intravascular devices. This form of endocarditis is most commonly caused by S. aureus. These patients are often critically ill, are receiving antibiotics for various other indications, and have comorbid condi­ tions. As a result, blood cultures may be negative, and the diagnosis missed. Prosthetic Device–Related Infections  S. aureus accounts for a large proportion of prosthetic device–related infections. These infections include intravascular and peritoneal catheters, prosthetic valves, orthopedic devices, pacemakers, left ventricular assist devices, or vascular grafts. In contrast with the more indolent presentation of FIGURE 152-4  Computed tomography scan illustrating septic pulmonary emboli in a patient with methicillin-resistant Staphylococcus aureus bacteremia.

NSaS infections, S. aureus device-related infections are often acute, have both local and systemic manifestations, and tend to progress more rapidly. It is relatively common for a pyogenic collection to be present at the device site. Aspiration of these collections and performance of blood cultures are important components in establishing a diag­ nosis. S. aureus infections tend to occur more commonly soon after implantation unless the device is used for access (e.g., intravascular or hemodialysis catheters). In the latter instance, infections can occur at any time. As in most prosthetic-device infections, successful therapy usually involves removal of the device. Left in place, the device serves as a potential nidus for either persistent or recurrent infections. Urinary Tract Infections  Urinary tract infections (UTIs) are infrequently caused by S. aureus. The presence of S. aureus in the urine often suggests hematogenous dissemination. Ascending S. aureus infections occasionally result from instrumentation of the genitouri­ nary tract. Infections Associated with Community-Acquired MRSA  Although skin and soft tissues are by far the most common sites of infection associated with CA-MRSA, 5–10% of these infections are invasive and can be life-threatening. The latter unique infections, including necrotizing fasciitis, necrotizing pneumonia, and sepsis with Waterhouse-Friderichsen syndrome or purpura fulminans, were rarely associated with S. aureus prior to the emergence of CA-MRSA. These life-threatening infections reflect the increased virulence of CA-MRSA strains. Toxin-Mediated Diseases  •  FOOD POISONING  S. aureus is among the most common causes of foodborne outbreaks in the United States. Staphylococcal food poisoning results from the inoculation of toxin-producing S. aureus into food by colonized food handlers. Toxin is then elaborated in such growth-promoting food as custards, potato salad, or processed meats. Even if the bacteria are killed by warming, the heat-stable toxin is not destroyed. The onset of illness is rapid, occurring within 1–6 h of ingestion; it is characterized by nausea and vomiting, although diarrhea, hypotension, and dehydration may occur. The differential diagnosis includes diarrhea of other etiologies, especially that caused by similar toxins (e.g., the toxins elaborated by Bacillus cereus). The rapidity of onset, the absence of fever, and the epidemic nature of the presentation (without secondary spread) should arouse suspicion of staphylococcal food poisoning. Symptoms gener­ ally resolve within 8–10 h. The diagnosis can be established by the demonstration of bacteria or the documentation of enterotoxin in the implicated food. Treatment is entirely supportive. TOXIC SHOCK SYNDROME  TSS gained attention in the early 1980s, when a nationwide outbreak occurred among young, otherwise healthy, menstruating women. Epidemiologic investigation demonstrated that these cases were associated with the use of a highly absorbent tampon recently introduced to the market. Subsequent studies established the role of TSST-1 in these illnesses. Withdrawal of the tampon from the market resulted in a rapid decline in the incidence of this disease. However, menstrual and nonmenstrual cases continue to be reported. Nonmenstrual cases are seen in patients with surgical or postpartum wound infections, especially when packing of the wound occurs. The clinical presentation is similar in menstrual and nonmenstrual TSS. Evidence of clinical S. aureus infection is not a prerequisite. TSS results from the elaboration of an enterotoxin or the structurally related enterotoxin-like TSST-1. More than 90% of menstrual cases are caused by TSST-1, whereas a high percentage of nonmenstrual cases are caused by enterotoxins (e.g., enterotoxin B). TSS begins with relatively nonspecific flulike symptoms. In menstrual cases, the onset usually comes 2 or 3 days after the start of menstruation. Patients present with fever, hypotension, and erythroderma of variable inten­ sity. Mucosal involvement is common (e.g., conjunctival hyperemia). The illness can rapidly progress to symptoms that include vomiting, diarrhea, confusion, myalgias, and abdominal pain. These symptoms reflect the multisystemic nature of the disease, with involvement of the liver, kidneys, gastrointestinal tract, and/or CNS. Desquamation of the skin occurs during convalescence, usually 1–2 weeks after the onset of

TABLE 152-2  Case Definition of Staphylococcus aureus Toxic Shock Syndrome Clinical Criteria An illness with the following clinical manifestations: • Fever: temperature ≥102.0°F (≥38.9°C) • Rash: diffuse macular erythroderma • Desquamation: 1–2 weeks after rash onset • Hypotension: systolic blood pressure ≤90 mmHg for adults or less than the fifth percentile, by age, for children <16 years old • Multisystem involvement (≥3 of the following organ systems) • Gastrointestinal: vomiting or diarrhea at illness onset • Muscular: severe myalgia or creatine phosphokinase level at least twice ULN • Mucous membrane: vaginal, oropharyngeal, or conjunctival hyperemia • Renal: blood urea nitrogen or creatinine level at least twice ULN for laboratory or urinary sediment with pyuria (≥5 leukocytes per high-power field) in the absence of urinary tract infection • Hepatic: total bilirubin or aminotransferase level at least twice ULN for laboratory • Hematologic: platelet count <105/μL • Central nervous system: disorientation or alterations in consciousness without focal neurologic signs in the absence of fever and hypotension Laboratory Criteria Negative results in the following tests, if obtained: • Blood or cerebrospinal fluid cultures for another pathogena • Serologic tests for Rocky Mountain spotted fever, leptospirosis, or measles CHAPTER 152 Case Classification Probable: a case that meets the laboratory criteria and in which four of the five clinical criteria are fulfilled Confirmed: a case that meets the laboratory criteria and in which all five of the clinical criteria are fulfilled, including desquamation (unless the patient dies before desquamation occurs) Staphylococcal Infections aBlood cultures may be positive for S. aureus. Abbreviation: ULN, upper limit of normal. Source: Centers for Disease Control and Prevention (www.cdc.gov/nndss/ conditions/toxic-shock-syndrome-other-than-streptococcal/case-definition/2011/). illness. Laboratory findings may include azotemia, leukocytosis, hypo­ albuminemia, thrombocytopenia, and liver function abnormalities. Diagnosis of TSS still depends on a constellation of findings rather than one specific finding and on a lack of evidence of other possible infections (Table 152-2). These other diagnoses include drug toxicities, viral exanthems, Rocky Mountain spotted fever, sepsis, and Kawasaki disease. Illness occurs only in persons who lack antibody to TSST-1. Recurrences are possible if antibody fails to develop after the illness. STAPHYLOCOCCAL SCALDED-SKIN SYNDROME  SSSS primarily affects newborns and children. The illness may vary from a localized blister to exfoliation of much of the skin surface. The skin is usually fragile and often tender, with thin-walled, fluid-filled bullae (Fig. 152-5). Gentle pressure results in rupture of the lesions, leaving denuded underlying skin. The mucous membranes are usually spared. In more generalized infection, there are often constitutional symptoms, including fever, lethargy, and irritability with poor feeding. Significant amounts of fluid can be lost in more extensive cases. Illness usually follows localized infection at one of several possible sites. SSSS is much less common among adults but can follow infections caused by exfoliative toxin– producing strains. NON–S. AUREUS STAPHYLOCOCCAL INFECTIONS Although less virulent than S. aureus, NSaS are among the most com­ mon causes of prosthetic-device infections, including endocarditis. They also are increasingly a cause of native-valve endocarditis and life-threatening bloodstream infections in neonates and in neutropenic patients. Approximately half of the identified NSaS species have been associated with human infections. Of these species, Staphylococcus

FIGURE 152-5  Staphylococcal scalded skin syndrome in a 6-year-old boy. Nikolsky’s sign, with separation of the superficial layer of the outer epidermal layer, is visible. (Adapted from LA Schenfeld: Staphylococcal scalded skin syndrome: N Engl J Med 342:1178, 2000.) epidermidis is the most common human pathogen. It is part of the normal human flora and is found on the skin (where it is the most abundant bacterial species) as well as in the oropharynx and vagina. Staphylococcus saprophyticus, a novobiocin-resistant species, is a com­ mon pathogen in UTIs. ■ ■PATHOGENESIS S. epidermidis is the NSaS species most often associated with pros­ thetic-device infections. Infection is a two-step process, with initial adhesion to the device followed by colonization. S. epidermidis is uniquely adapted to colonize these devices because of its capacity to elaborate the extracellular polysaccharide (glycocalyx or slime) that facilitates formation of a protective biofilm on the device surface. PART 5 Infectious Diseases Implanted prosthetic material is rapidly coated with host matrix molecules such as fibrinogen or fibronectin. These molecules serve as potential bridging ligands, facilitating initial bacterial attachment to the device surface. A number of staphylococcal surface-associated proteins, such as autolysin (AtlE), fibrinogen-binding protein, and accumulation-associated protein (AAP), appear to play a role in attach­ ment to either modified or unmodified prosthetic surfaces. The poly­ saccharide intercellular adhesin facilitates subsequent staphylococcal colonization, aggregation, and accumulation on the device surface. Intercellular adhesin (ica) genes are more commonly found in strains of S. epidermidis that are associated with device infections than in strains associated with colonization of mucosal surfaces. Biofilm acts as a barrier, protecting bacteria from host defense mechanisms as well as from antibiotics while providing a suitable environment for bacterial maturation, survival, and potential spread to other tissue sites. Two additional NSaS species, Staphylococcus lugdunensis and Staph­ ylococcus schleiferi, produce more serious infections (native-valve endocarditis and osteomyelitis) than do other NSaS. The basis for this enhanced virulence is not known, although both species appear to share more virulence determinants with S. aureus (e.g., clumping factor and lipase) than do other NSaS. The capacity of S. saprophyticus to cause UTIs in young women appears related to the presence of adhesins that facilitate adherence to uroepithelial cells. A 160-kDa hemagglutinin/adhesin may contribute to this affinity. ■ ■DIAGNOSIS Although the detection of NSaS at sites of infection or in the blood­ stream by standard microbiologic culture methods is not difficult, interpretation of these results is frequently problematic. Because these organisms are present in large numbers on the skin, they often con­ taminate cultures. It has been estimated that only 10–20% of blood cul­ tures positive for NSaS reflect true bacteremia. Similar problems arise with cultures obtained from other sites. Among the clinical findings

suggestive of true bacteremia are fever, evidence of local infection (e.g., erythema or purulent drainage at the IV catheter site), leukocytosis, and systemic signs of sepsis. Laboratory findings suggestive of true bacteremia include repeated isolation of the same strain (i.e., the same species with the same antibiogram or with a closely related DNA fin­ gerprint) from separate cultures, growth of the strain within 48 h, and bacterial growth in both aerobic and anaerobic bottles. ■ ■CLINICAL SYNDROMES NSaS cause a variety of prosthetic device–related infections, including those that involve prosthetic cardiac valves and joints, vascular grafts, intravascular devices, and CNS shunts. In all of these settings, the clinical presentation is similar. The signs of localized infection are often subtle, the rate of disease progression is slow, and the systemic findings are often limited. Signs of infection, such as purulent drainage, pain at the site, or loosening of prosthetic implants, are sometimes evident. Fever is frequently but not always present, and there may be mild leu­ kocytosis. Acute-phase reactant levels, erythrocyte sedimentation rate, and C-reactive protein concentration may be elevated. Infections that are not associated with prosthetic devices include, as noted, native-valve endocarditis due to NSaS, which accounts for ~5% of cases. Infections in preterm infants and neutropenic patients are often associated with the need for intravascular devices. S. lugdunensis

appears to be a more aggressive pathogen in this setting, causing greater mortality and rapid valvular destruction with abscess formation than other NSaS. TREATMENT Staphylococcal Infections GENERAL PRINCIPLES OF THERAPY Source control (e.g., incision and drainage of suppurative collec­ tions or removal of infected prosthetic devices), coupled with rapid institution of appropriate antimicrobial therapy, is essential for the management of all staphylococcal infections. The emergence of MRSA as a community-based pathogen has increased the impor­ tance of culturing all sites of infection to determine antimicrobial susceptibility and optimize oral treatment regimens. DURATION OF ANTIMICROBIAL THERAPY Therapy for S. aureus bacteremia is generally prolonged (4–6 weeks) because of the high risk of complications (e.g., endocarditis, meta­ static foci of infection). Among the findings associated with com­ plicated bacteremias are (1) persistently positive blood cultures 96 h after institution of therapy, (2) failure to promptly remove or drain an identified focus of infection (i.e., an intravascular cathe­ ter), (3) the presence of deep-seated infections, and (4) acquisition of the infection in the community. Patients with uncomplicated bacteremias are defined by a removable focus of infection, prompt response to antimicrobial therapy (i.e., no fever or positive blood cultures after 3–4 days), no evidence of metastatic foci of infection, and no implanted prostheses. In these latter infections, shortcourse therapy (2 weeks) can be given; however, these findings are not always predictive of an uncomplicated bacteremia. Given these concerns, caution is therefore needed in instituting a short course of therapy. Transesophageal echocardiography to rule out endocarditis is generally necessary because neither clinical nor laboratory findings can reliably detect cardiac involvement. A thorough radiologic investigation to identify potential metastatic collections is also indicated. All symptomatic body sites must be carefully evaluated. Recent studies have demonstrated that parenteral therapy is not always necessary to complete a course of treatment for invasive staphylococcal infections such as endocarditis or osteomyelitis for carefully selected patients. These include patients with uncompli­ cated staphylococcal bacteremia. NSaS treatment is complicated by the possibility that a single iso­ late may be a contaminant. Therapy for 7–14 days is recommended

for documented infections (i.e., blood cultures of the same strain ≥24 h apart) in the absence of endocarditis or additional sites of infection. CHOICE OF ANTIMICROBIAL AGENTS The choice of antimicrobial agents to treat both coagulase-positive and coagulase-negative staphylococcal infections is often difficult because of the prevalence of multidrug-resistant strains and the limited number of clinical trials that have compared the available TABLE 152-3  Antimicrobial Therapy for Staphylococcal Infectionsa SENSITIVITY/ RESISTANCE OF ISOLATE DRUG OF CHOICE ALTERNATIVE(S) COMMENTS Parenteral Therapy for Serious Infections Sensitive to penicillin Penicillin G (4 mU q4h) Nafcillin or oxacillin (2 g q4h), cefazolin (2 g q8h), vancomycin (15–20 mg/kg q8hb) Sensitive to methicillin; resistant to penicillin Nafcillin or oxacillin (2 g q4h), cefazolin (2 g q8h) Daptomycin (6–10 mg/kg IV q24hb,d), vancomycin (15–20 mg/kg q8hb), ceftobiprole (500 mg IV q6hg) Resistant to methicillin Vancomycin (15–20 mg/kg

q8–12hb), daptomycin (6–10 mg/kg IV q24hb,d) for bacteremia, endocarditis, osteomyelitis, and complicated skin infections Linezolid (600 mg q12h PO or IV), ceftaroline (600 mg IV q8–12h), telavancin (7.5–10 mg/kg IV q24h)b, TMP-SMX (5 mg [based on TMP]/kg IV q8–12h)f Additional agents include tedizolid (200 mg once daily IV), oritavancin (single dose of 1200 mg), dalbavancin (single dose of 1500 mg), delafloxacin (300 mg q 12 h IV), omadacycline 100 mg OD). Ceftobiprole (500 mg IV q6hg) Resistant to methicillin with intermediate or complete resistance to vancomycine Daptomycin (6–10 mg/kg q24hb,d) for bacteremia, endocarditis, osteomyelitis, and complicated skin infections Same as for methicillin-resistant strains (check antibiotic susceptibilities) or     Ceftaroline (600 mg IV q8–12h) Newer agents include tedizolid (200 mg once daily IV or PO), oritavancin (single dose of 1200 mg), and dalbavancin (single dose of 1500 mg). These drugs are approved only for the treatment of skin and soft tissue infections. Not yet known (i.e., empirical therapy) Vancomycin (15–20 mg/kg q8–12hb), daptomycin

(6–10 mg/kg q24hb,d) for bacteremia, endocarditis, osteomyelitis, and complicated skin infections — Empirical therapy is given when the susceptibility of the isolate is not known. Vancomycin with or without a b-lactam is recommended for suspected community- or hospital-acquired Staphylococcus aureus infections because of the increased frequency of methicillin-resistant strains in the community. If isolates with an elevated MIC to vancomycin (≥1.5 μg/mL) are common in the community, daptomycin may be preferable. Oral Therapy for Skin and Soft Tissue Infections Sensitive to methicillin Dicloxacillin (500 mg qid), cephalexin (500 mg qid), or cefadroxil (1 g q12h) Minocycline or doxycycline (100 mg q12hb), TMPSMX (1 or 2 DS tablets bid), clindamycin (300–450 mg tid), linezolid (600 mg PO q12h), tedizolid

(200 mg PO q24h) Resistant to methicillin Clindamycin (300–450 mg tid), TMP-SMX (1 or 2 DS tablets bid), minocycline or doxycycline (100 mg q12hb), linezolid (600 mg bid), or tedizolid (200 mg once daily) Delafloxacin 450 mg q12 h, omadacycline 300 mg once a day aRecommended dosages are for adults with normal renal and hepatic function. bThe dosage must be adjusted for patients with reduced creatinine clearance. cFor the treatment of prosthetic-valve endocarditis, the addition of gentamicin (1 mg/kg q8h) and rifampin (300 mg PO q8h) is recommended, with adjustment of the gentamicin dosage if the creatinine clearance rate is reduced. dDaptomycin cannot be used for the treatment of pneumonia. eVancomycin-resistant S. aureus isolates from clinical infections have been reported. fTMP-SMX may be less effective than vancomycin. gAdditional studies are needed. Abbreviations: DS, double-strength; TMP-SMX, trimethoprim-sulfamethoxazole; VISA, vancomycin-intermediate S. aureus; VRSA, vancomycin-resistant S. aureus. Source: Modified from C Liu et al: Clin Infect Dis 52:285, 2011; DL Stevens et al: Clin Infect Dis 59:148, 2014; DL Stevens et al: Med Lett Drugs Ther 56:39, 2014; and LM Baddour et al: Circulation 132:1435, 2015.

agents. Staphylococcal resistance to most antibiotic families, includ­ ing β-lactams, aminoglycosides, fluoroquinolones, and (to a lesser extent) glycopeptides, has increased. This trend is even more appar­ ent with NSaS; >80% of nosocomial isolates are resistant to methi­ cillin, and these methicillin-resistant strains are often resistant to other antibiotics. Because the selection of antimicrobial agents for S. aureus infections is similar to that for NSaS infections, treatment options for these pathogens are discussed together and are sum­ marized in Table 152-3.

Fewer than 5% of isolates are sensitive to penicillin. The clinical microbiology laboratory must verify that the strain is not a b-lactamase producer. Patients with a penicillin allergy can be treated with a cephalosporin if the allergy does not involve an anaphylactic or accelerated reaction; desensitization to b-lactams may be indicated in selected cases of serious infection when maximal bactericidal activity is needed (e.g., prostheticvalve endocarditisc). Vancomycin is a less effective option than a b -lactam. Sensitivity testing is necessary before an alternative drug is selected. The efficacy of adjunctive therapy is not well established in many settings. Linezolid, ceftaroline, and telavancin have in vitro activity against most VISA and VRSA strains. See footnote for treatment of prostheticvalve endocarditis.c CHAPTER 152 Staphylococcal Infections Same as for methicillin-resistant strains; check antibiotic susceptibilities. Ceftaroline is used either alone or in combination with daptomycin.   It is important to know the antibiotic susceptibility of isolates in the specific geographic region. All collections should be drained, and drainage should be cultured. It is important to know the antibiotic susceptibility of isolates in the specific geographic region. All collections should be drained, and drainage should be cultured.

Few strains of staphylococci (≤5%) remain susceptible to peni­ cillin. This is a result of the widespread dissemination of plasmids containing the enzyme penicillinase. Penicillin-resistant isolates are treated with semisynthetic penicillinase-resistant penicillins (SPRPs), such as oxacillin or nafcillin. Methicillin, the first of the SPRPs, is no longer used. Cephalosporins are alternative therapeutic agents for these infections. In patients with a history of serious β-lactam allergies, alternatives to SPRPs for the treatment of invasive infec­ tions should be used only after careful consideration. Desensitiza­ tion to β-lactams remains an option for life-threatening infections. Second- and third-generation cephalosporins offer no therapeutic advantage over first-generation cephalosporins for the treatment of staphylococcal infections, and some third-generation cephalosporins (e.g., ceftazidime, ceftriaxone) have considerably less activity and should be avoided. The carbapenems have excellent activity against methicillin-sensitive S. aureus but not against MRSA.

The isolation of MRSA was reported within 1 year of the intro­ duction of methicillin. Since then, the prevalence of MRSA has steadily increased. In many U.S. hospitals and elsewhere, 40–50% of S. aureus isolates are resistant to methicillin. Resistance to methicil­ lin indicates resistance to all SPRPs as well as to all cephalosporins (except ceftaroline). Production of a novel penicillin-binding pro­ tein (PBP2a) is responsible for methicillin resistance. This protein is synthesized by the mecA gene, which (as stated above) is part of a large mobile genetic element—a pathogenicity or genomic island—called SCCmec. It is hypothesized that mecA was acquired via horizontal transfer from related staphylococcal species. Phe­ notypic expression of methicillin resistance may be constitutive (i.e., expressed in all cells in a population) or heterogeneous (i.e., displayed by only a proportion of the total cell population). Detec­ tion of methicillin resistance is enhanced by growth of cultures at reduced temperatures (≤35°C for 24 h) and with increased concen­ trations of salt in the medium. Culture techniques are increasingly being replaced by PCR-based or other methods (e.g., latex aggluti­ nation) that allow for the rapid detection of methicillin resistance. PART 5 Infectious Diseases Either vancomycin or daptomycin is recommended as the drug of choice for the treatment of invasive MRSA infections. MRSA susceptibility to vancomycin has decreased in many areas of the world. It is important to note that vancomycin is less effective than SPRPs for the treatment of infections due to methicillin-susceptible strains. Three types of staphylococcal resistance to vancomycin have emerged. (1) Minimal inhibitory concentration (MIC; an in vitro measure of susceptibility) “creep” refers to the incremental increase in vancomycin MICs that has been detected in various geographic areas. Studies suggest that morbidity and mortality may be increased in infections due to S. aureus strains with vancomycin MICs of ≥1.5 μg/mL. (2) In 1997, an S. aureus strain with reduced suscepti­ bility to vancomycin (vancomycin-intermediate S. aureus [VISA]) was reported from Japan. Subsequently, additional VISA clinical isolates were reported. These strains were resistant to methicillin and many other antimicrobial agents. The VISA strains appear to evolve (under vancomycin selective pressure) from strains that are susceptible to vancomycin but are heterogeneous, with a small proportion of the bacterial population expressing the resistance phenotype. The mechanism of VISA resistance is in part due to an abnormally thick cell wall. Vancomycin is trapped by the abnormal peptidoglycan cross-linking and is unable to gain access to its target site. Regulatory genes involved in cell wall metabolism appear to play an important role in this type of resistance. (3) In 2002, the first clinical isolate of fully vancomycin-resistant S. aureus (VRSA) was reported. Resistance in this and several additional clinical isolates was due to the presence of vanA, the gene responsible for expression of vancomycin resistance in enterococci. This observa­ tion suggested that resistance was acquired as a result of horizontal conjugal transfer from a vancomycin-resistant strain of Enterococcus faecalis. Several of the patients infected with the VRSA strain had both MRSA and vancomycin-resistant enterococci cultured from infection sites. The vanA gene is responsible for the synthesis of

the dipeptide d-Ala-d-Lac in place of d-Ala-d-Ala. Vancomycin cannot bind to the altered peptide. While isolates with MICs of ≥1.5 μg/mL have been relatively common in some areas, VISA and VRSA isolates are uncommon. Daptomycin, a parenteral bactericidal agent with antistaphylo­ coccal activity, is approved for the treatment of bacteremia (includ­ ing right-sided endocarditis) and complicated skin infections. It is not effective in respiratory infections. This drug has a unique mechanism of action: it disrupts the cytoplasmic membrane. Staph­ ylococcal resistance to daptomycin has been reported. Resistance can emerge during therapy; patients previously treated with van­ comycin may have elevated daptomycin MICs. Patients need to be monitored for rhabdomyolysis with creatine phosphokinase mea­ surement and for eosinophilic pneumonia. Linezolid—the first oxazolidinone—is bacteriostatic against staphylococci; it offers the advantage of comparable bioavailability after oral or parenteral administration. Cross-resistance with other inhibitors of protein synthesis has not been detected. Resistance to linezolid is rare but has been reported. Serious adverse reactions to linezolid include thrombocytopenia, occasional cases of neutro­ penia, and rare instances of lactic acidosis or peripheral and optic neuropathy. These reactions tend to occur after relatively prolonged courses of therapy. Tedizolid, a second oxazolidinone, is available as both oral and parenteral preparations. It exhibits enhanced in vitro activity against antibiotic-resistant gram-positive bacteria, including staph­ ylococci. Tedizolid is administered once a day. Data on its efficacy for the treatment of deep-seated infections are limited. Ceftaroline is a fifth-generation cephalosporin with bactericidal activity against MRSA (including strains with reduced susceptibil­ ity to vancomycin and daptomycin). It is generally well tolerated. Ceftaroline is approved for use in nosocomial pneumonias and for SSTIs. It has increasingly been used to treat invasive MRSA infec­ tions with or without glycopeptides. Telavancin is a parenteral lipoglycopeptide derivative of van­ comycin that is approved for the treatment of complicated SSTIs and for nosocomial pneumonias. The drug has two targets: the cell wall and the cell membrane. It remains active against VISA strains. Because of its potential nephrotoxicity, telavancin should be avoided in patients with renal disease. Dalbavancin and oritavancin are long-acting, parenterally administered lipoglycopeptides that have been used to treat com­ plicated SSTIs. Because of their long half-lives, they can be admin­ istered on a weekly basis. Both have been used as single-dose regimens for the treatment of SSTIs. Emerging data support their use for the treatment of invasive staphylococcal infections. Although the quinolones are active against staphylococci in vitro, the frequency of staphylococcal resistance to these agents has increased, especially among methicillin-resistant isolates. Of particular concern in MRSA is the possibility of quinolone resis­ tance emerging during therapy. Therefore, quinolones are not recommended for the treatment of MRSA infections. Resistance to the quinolones is most commonly chromosomal and results from mutations of the topoisomerase IV or DNA gyrase genes, although multidrug efflux pumps also may contribute. Although the newer quinolones exhibit increased in vitro activity against staphylococci, it is uncertain whether this increase translates into enhanced in vivo activity. Delafloxacin, a fluoroquinolone with broad-spectrum activity, has excellent activity against MRSA, retaining activity against some isolates resistant to other fluoroquinolones. Tigecycline, a broad-spectrum minocycline analogue, has bac­ teriostatic activity against MRSA and is approved for use in SSTIs as well as intraabdominal infections caused by S. aureus. It is not recommended for the treatment of invasive infections. Other older antibiotics, such as minocycline, doxycycline, clindamycin, and trimethoprim-sulfamethoxazole, continue to be successfully used to treat MRSA infections. Ceftobiprole is a new antibiotic with excellent activity against both MRSA and methicillin-susceptible S. aureus (MSSA). It has

been shown in clinical trials to be effective in treating complicated staphylococcal bacteremias, SSTIs, and pneumonia. The benefit of antistaphylococcal combinations to enhance bactericidal activity in the treatment of deep-seated infections remains controversial. Clinical studies have not documented a therapeutic benefit from the addition of gentamicin to single-drug regimens; recent reports have raised concern about the potential nephrotoxicity of gentamicin and adverse reactions from, or drug interactions with, rifampin. As a result, the use of gentamicin in combination with β-lactams or other antimicrobial agents is no longer routinely recommended for the treatment of invasive infec­ tions such as native-valve endocarditis. Rifampin continues to be used for the treatment of prosthetic device–related infections and for osteomyelitis. Omadacycline and eravacycline are broad-spectrum semisyn­ thetic tetracycline derivatives with activity against MRSA. They are currently approved for the treatment of SSTIs. The use of bacteriophages with activity against staphylococci is now being investigated in clinical trials as adjunctive therapy in invasive infections. ANTIMICROBIAL THERAPY FOR SELECTED SETTINGS Empirical Therapy  Empirical coverage for MRSA is indicated when antibiotic susceptibility is not known. Vancomycin or daptomycin is generally recommended. It remains uncertain whether daptomycin is preferable when elevated vancomycin MICs (>1.5 μg/mL) are com­ mon in a specific locale. Salvage Therapy  Salvage therapy for complicated S. aureus infec­ tions is sometimes needed when the bacteremia persists (i.e., for 3 days) despite appropriate treatment. The risk of a poor outcome (i.e., increased mortality, metastatic infections) is increased with the duration of bacteremia. Prolonged bacteremia can occur with both MRSA and MSSA. There is limited high-quality evidence to serve as a guide to salvage therapy. The combination of daptomycin or vancomycin with a β-lactam antibiotic (e.g., ceftaroline) has been successfully used to treat patients with persistent MRSA bactere­ mia, even those patients with isolates displaying reduced suscep­ tibility to these antimicrobial agents. This combination appears to enhance the bactericidal activity of daptomycin by reducing the bacterial cell-surface charge and thus allowing enhanced dapto­ mycin binding. For vancomycin, the combination may allow more strategic binding to the target site with reduced cell-wall thickness. Other combinations have included trimethoprim-sulfamethoxazole or rifampin combined with daptomycin. Linezolid and ceftaroline have also been used as single alternative agents. Endocarditis  S. aureus endocarditis is usually an acute, lifethreatening infection. Thus, prompt collection of blood for cul­ tures should be followed by immediate institution of empirical antimicrobial therapy. For native-valve endocarditis, therapy with a β-lactam is recommended. If a MRSA strain is isolated, vancomycin (15–20 mg/kg every 8–12 h, given in equal doses up to a total of 2 g, with the dose adjusted in the case of renal disease) or daptomycin (6–10 mg/kg every 24 h) is recommended. The vancomycin dose should be adjusted based on area under the curve (AUC)-based dosing, although measurement of trough levels may also be used. Patients are generally treated for 6 weeks. For prosthetic-valve endocarditis, surgery in addition to antibiotic therapy is often nec­ essary. The combination of a β-lactam agent—or, if the isolate is β-lactam-resistant, vancomycin or daptomycin—with an aminogly­ coside (gentamicin, 1 mg/kg IV every 8 h) for 2 weeks and rifampin (300 mg orally or IV every 8 h) for ≥6 weeks is recommended. Infectious diseases and, if necessary, surgical consultation should be considered. Bone and Joint Infections  For hematogenous osteomyelitis or septic arthritis in children, a 4-week course of therapy is usually adequate. In adults, treatment is often more prolonged. For chronic forms of osteomyelitis, surgical debridement is necessary in com­ bination with antimicrobial therapy. For joint infections, a critical

component of therapy is the repeated aspiration or arthroscopy of the affected joint to prevent damage from leukocytes. The combi­ nation of rifampin with ciprofloxacin has been used successfully to treat or suppress prosthetic-joint infections, especially when the device cannot be removed. The efficacy of this combination may reflect enhanced activity against staphylococci in biofilms as well as the attainment of effective intracellular concentrations.

Skin and Soft Tissue Infections  The increase in SSTIs caused by CA-MRSA has drawn attention to the need for initiation of appropriate empirical therapy. Even small abscesses appear to ben­ efit from antibiotic therapy in addition to incision and drainage. Antibiotics are selected depending on local antibiotic susceptibility data; several oral agents have been used to treat these infections, including clindamycin, trimethoprim-sulfamethoxazole, doxycy­ cline, linezolid, and tedizolid. Parenteral therapy is reserved for more complicated infections. Toxic Shock Syndrome  Treatment of shock is the mainstay of therapy for TSS. Both fluids and pressors may be necessary. Tam­ pons or other packing material should be promptly removed. Some investigators recommend therapy with a combination of clindamy­ cin and a semisynthetic penicillin or (if the isolate is resistant to methicillin) vancomycin. Clindamycin is advocated because, as a protein synthesis inhibitor, it reduces toxin production. Linezolid also appears to be effective. A semisynthetic penicillin or a glyco­ peptide is recommended to eliminate any potential focus of infec­ tion as well as to eradicate persistent carriage that might increase the possibility of recurrence. Intravenous immunoglobulin to treat TSS is of uncertain benefit. Glucocorticoids are not recommended for the treatment of this disease. CHAPTER 152 Other Toxin-Mediated Diseases  Therapy for staphylococcal food poisoning is entirely supportive. For SSSS, antistaphylococcal therapy targets the primary site of infection. NONTRADITIONAL APPROACHES TO ANTISTAPHYLOCOCCAL THERAPY In addition to the development of new antibiotics, new and non­ traditional approaches to therapy are currently being investigated. These include the use of phages or phage-derived peptides, as well as probiotics and antivirulence strategies that target selected viru­ lence determinants. Staphylococcal Infections ■ ■PREVENTION Primary prevention of S. aureus infections in the hospital setting involves hand washing and careful attention to appropriate isolation procedures. Through careful screening for MRSA carriage and strict isolation practices, several Scandinavian countries have been remark­ ably successful at preventing the introduction and dissemination of MRSA in hospitals. Decolonization strategies, using both universal and targeted approaches with topical agents (e.g., mupirocin) to eliminate nasal colonization and/or chlorhexidine to eliminate colonization of addi­ tional body sites with S. aureus, have been successful in some clinical settings where the risk of infection is high (e.g., intensive care units). An analysis of clinical trials suggests that decolonization can reduce the incidence of postsurgical infections among people nasally colonized with S. aureus. The risk of recurrent admissions among patients with

S. aureus bacteremia following discharge is high (~22% within 30 days). Decolonization following discharge with mupirocin and chlorhexidine can lower the incidence of recurrent infections. “Bundling” (the application of selected medical interventions in a sequence of prescribed steps) has reduced rates of nosocomial infec­ tions related to procedures such as the insertion of intravenous cath­ eters, in which staphylococci are among the most common pathogens (see Table 147-1). A number of immunization strategies to prevent S. aureus infections—both active (e.g., capsular polysaccharide–protein conjugate vaccine) and passive (e.g., clumping factor antibody)—have been investigated. However, to date, none has been successful for either prophylaxis or therapy in clinical trials.

35 - 153 Streptococcal Infections

153 Streptococcal Infections

Strategies to prevent recurrent S. aureus infections in the community have had limited success. Decolonization with intranasal mupirocin and chlorhexidine washes of the infected individual and the additional decolonization of household members combined with environmental cleaning of surfaces and personal items have all been studied. For indi­ viduals with extensive skin disease and recurrent infections, the use of bleach baths (e.g., one-half cup of household bleach in a half-filled bathtub) for 15 minutes three times weekly may be useful.

■ ■FURTHER READING Becker K et al: Coagulase-negative staphylococci. Clin Microbiol Rev 27:870, 2014. Cheung GYC et al: Pathogenicity and virulence of Staphylococcus aureus. Virulence 12:547, 2021. DeLeo FR et al: Community-associated methicillin-resistant Staphylo­ coccus aureus. Lancet 375:1557, 2010. Holland TL et al: Ceftobiprole for treatment of complicated Staphylo­ coccus aureus bacteremia. N Engl J Med 389:1390, 2023. Lee AS et al: Methicillin-resistant Staphylococcus aureus. Nat Rev Dis Primers 4:18033:1, 2018. Minter DJ et al: Contemporary management of Staphylococcus aureus bacteremia—Controversies in clinical practice. Clin Infect Dis 77:e57, 2023. Rose W et al: Current paradigms of combination therapy in methicillinresistant Staphylococcus aureus (MRSA) bacteremia: Does it work, which combination, and for which patients? Clin Infect Dis 73:2353, 2021. Siciliano V et al: Difficult-to-treat pathogens: A review on the man­ agement of multidrug- resistant Staphylococcus epidermidis. Life 13:1126, 2023. Tong SY et al: Staphylococcus aureus infections: Epidemiology, patho­ PART 5 Infectious Diseases physiology, clinical manifestations, and management. Clin Microbiol Rev 28:603, 2015. Michael R. Wessels

Streptococcal Infections Many varieties of streptococci are found as part of the normal flora colonizing the human respiratory, gastrointestinal, and genitourinary tracts. Several species are important causes of human disease. Group A Streptococcus (GAS; Streptococcus pyogenes) is responsible for streptococcal pharyngitis, one of the most common bacterial infec­ tions of school-age children, and for the postinfectious syndromes of TABLE 153-1  Classification of Streptococci LANCEFIELD GROUP REPRESENTATIVE SPECIES HEMOLYTIC PATTERN TYPICAL INFECTIONS A S. pyogenes β Pharyngitis, impetigo, cellulitis, scarlet fever B S. agalactiae β Neonatal sepsis and meningitis, puerperal infection, urinary tract infection, diabetic ulcer infection, endocarditis C, G S. dysgalactiae subsp. equisimilis β Cellulitis, bacteremia, endocarditis D Enterococcia: E. faecalis, E. faecium Usually nonhemolytic Urinary tract infection, nosocomial bacteremia, endocarditis Nonenterococci: S. gallolyticus (formerly S. bovis) Usually nonhemolytic Bacteremia, endocarditis Variable or nongroupable Viridans streptococci: S. sanguis, S. mitis α Endocarditis, dental abscess, brain abscess Intermedius or milleri group: S. intermedius,

S. anginosus, S. constellatus Variable Brain abscess, visceral abscess Anaerobic streptococcib: Peptostreptococcus magnus Usually nonhemolytic Sinusitis, pneumonia, empyema, brain abscess, liver abscess aSee Chap. 154. bSee Chap. 182.

acute rheumatic fever (ARF) and poststreptococcal glomerulonephri­ tis (PSGN). Group B Streptococcus (GBS; Streptococcus agalactiae) is a leading cause of bacterial sepsis and meningitis in newborns and a major cause of endometritis and fever in parturient women. Viridans streptococci are a common cause of bacterial endocarditis. Enterococci, which are morphologically similar to streptococci, are now con­ sidered a separate genus on the basis of DNA homology studies. Thus, the species previously designated as Streptococcus faecalis and Streptococcus faecium have been renamed Enterococcus faecalis and Enterococcus faecium, respectively. The enterococci are discussed in Chap. 154. Streptococci are gram-positive, spherical to ovoid bacteria that characteristically form chains when grown in liquid media. Most strep­ tococci that cause human infections are facultative anaerobes, although some are strict anaerobes. Streptococci are relatively fastidious organ­ isms, requiring enriched media for growth in the laboratory. Clinicians and clinical microbiologists identify streptococci by several classifica­ tion systems, including hemolytic pattern, Lancefield group, species name, and common or trivial name. Many streptococci associated with human infection produce a zone of complete (β) hemolysis around the bacterial colony when cultured on blood agar. The β-hemolytic streptococci that form large (≥0.5-mm) colonies on blood agar can be classified by the Lancefield system, a serologic grouping based on the reaction of specific antisera with bacterial cell-wall carbohydrate antigens. With rare exceptions, organisms belonging to Lancefield groups A, B, C, and G are all β-hemolytic, and each is associated with characteristic patterns of human infection. Other streptococci produce a zone of partial (α) hemolysis, often imparting a greenish appearance to the agar. These α-hemolytic streptococci are further identified by biochemical testing and include Streptococcus pneumoniae (Chap. 151), an important cause of pneumonia, meningitis, and other infec­ tions, and the several species referred to collectively as the viridans streptococci, which are part of the normal oral flora and are important agents of subacute bacterial endocarditis. Finally, some streptococci are nonhemolytic, a pattern sometimes called γ hemolysis. Among the organisms classified serologically as group D streptococci, the entero­ cocci are assigned to a distinct genus (Chap. 154). The classification of the major streptococcal groups causing human infections is outlined in Table 153-1. GROUP A STREPTOCOCCI Lancefield group A consists of a single species, S. pyogenes. As its spe­ cies name implies, this organism is associated with a variety of sup­ purative infections. In addition, GAS can trigger the postinfectious syndromes of ARF (which is uniquely associated with S. pyogenes infection; Chap. 371) and PSGN (Chap. 326). Worldwide, GAS infections and their postinfectious sequelae (pri­ marily ARF and rheumatic heart disease) account for an estimated 500,000 deaths per year. Although data are incomplete, the incidence of all forms of GAS infection and that of rheumatic heart disease are thought to be tenfold higher in resource-limited countries than in

Presence of rheumatic heart disease (cases per 1000) 0.3 0.8 1.8 FIGURE 153-1  Prevalence of rheumatic heart disease in children 5–14 years old. The circles within Australia and New Zealand represent indigenous populations (and also Pacific Islanders in New Zealand). (Reproduced with permission from JR Carapetis et al: The global burden of group A streptococcal diseases. Lancet Infect Dis 5:685, 2005.) developed countries (Fig. 153-1). As has been observed with infections due to other pathogens associated with the human respiratory tract, the incidence of streptococcal pharyngitis and of invasive GAS infection fell during 2020–2022 in association with COVID-19-related social distancing restrictions. A rebound in GAS infection has been observed in many countries beginning in the last quarter of 2022. Unusually high rates of invasive GAS infections in both children and adults were reported in 2023 and 2024 from centers in Europe and the United States. Prominent among these cases have been severe pneumonia and empy­ ema, sometimes as a co-infection with influenza or another respiratory virus, as well as necrotizing soft tissue infections and streptococcal toxic shock. A GAS strain designated M1UK first emerged in the United Kingdom in association with an upsurge in scarlet fever and has since been implicated as a dominant strain in invasive GAS infec­ tions in the United Kingdom and other European countries. Multiple other lineages of GAS have been identified in invasive infections else­ where in Europe and around the world. ■ ■PATHOGENESIS GAS elaborates a number of cell-surface components and extracel­ lular products important in both the pathogenesis of infection and the human immune response. The cell wall contains a carbohydrate antigen that may be released by acid treatment. The reaction of such acid extracts with group A–specific antiserum is the basis for definitive identification of a streptococcal strain as S. pyogenes. Rarely, the group A antigen may be present on isolates of S. dysgalactiae ssp. equisimilis, which usually express the group C or G antigen (see “Streptococci of Groups C and G,” below). The major surface protein of GAS is M protein, which is the basis for the serotyping of strains with specific antisera. The M protein molecules are fibrillar structures anchored in the cell wall of the organism that extend as hairlike projections away from the cell surface. The amino acid sequence of the distal or aminoterminal portion of the M protein molecule is variable, accounting for the antigenic variation of the different M types, while more proximal regions of the protein are relatively conserved. Traditional M-typing by serologic methods has been largely supplanted by use of the polymerase chain reaction to amplify the variable region of the emm gene, which encodes M protein. DNA sequence analysis of the amplified gene seg­ ment can be compared with an extensive database (developed at the Centers for Disease Control and Prevention [CDC]) for assignment of emm type. Use of emm typing has increased the number of identified emm types to more than 200. This method eliminates the need for typing sera, which are available in only a few reference laboratories.

1.0 1.3 3.5 2.2 5.7 The presence of M protein on a GAS isolate correlates with its capacity to resist phagocytic killing in fresh human blood. This phenomenon appears to be due, at least in part, to the binding of plasma fibrinogen to M protein molecules on the streptococcal surface, which interferes with complement activation and deposition of opsonic complement fragments on the bacterial cell. This resistance to phagocytosis may be overcome by M protein–specific antibodies; thus, individuals with antibodies to a given M type acquired as a result of prior infection are protected against subsequent infection with organisms of the same M type but not against infection with different M types. CHAPTER 153 Streptococcal Infections GAS also elaborates, to varying degrees, a polysaccharide capsule composed of hyaluronic acid. While most clinical isolates of GAS produce a hyaluronic acid capsule, strains of M type 4 or 22 lack a capsule, as do some isolates of M type 89. The fact that acapsular strains have been asso­ ciated with pharyngitis and invasive infection implies that the capsule is not essential for virulence. The production of large amounts of capsule by certain strains imparts a characteristic mucoid appearance to the colonies. The capsular polysaccharide plays an important role in protecting GAS from ingestion and killing by phagocytes. In contrast to M protein, the hyaluronic acid capsule is a weak immunogen, and antibodies to hyal­ uronate have not been shown to be important in protective immunity. The presumed explanation is the apparent structural identity between streptococcal hyaluronic acid and the hyaluronic acid of mammalian con­ nective tissues. The capsular polysaccharide may also play a role in GAS colonization of the pharynx by binding to CD44, a hyaluronic acid–binding protein expressed on human pharyngeal epithelial cells. GAS produces a large number of extracellular products that may be important in local and systemic toxicity and in the spread of infec­ tion through tissues. These products include streptolysins S and O, toxins that damage cell membranes and account for the hemolysis produced by the organisms; streptokinase; DNAses; SpyCEP, a serine protease that cleaves and inactivates the chemoattractant cytokine interleukin 8, thereby inhibiting neutrophil recruitment to the site of infection; and several pyrogenic exotoxins. Previously known as erythrogenic toxins, the pyrogenic exotoxins cause the rash of scarlet fever. Since the mid-1980s, pyrogenic exotoxin–producing strains of GAS have been linked to unusually severe invasive infections, includ­ ing necrotizing fasciitis and the streptococcal toxic shock syndrome (TSS). Several extracellular products stimulate specific antibody responses useful for serodiagnosis of recent streptococcal infection. Tests for antibodies to streptolysin O and DNase B are used most commonly for detection of preceding streptococcal infection in cases of suspected ARF or PSGN.

■ ■CLINICAL MANIFESTATIONS

Pharyngitis  Although seen in patients of all ages, GAS pharyn­ gitis is one of the most common bacterial infections of childhood, accounting for 20–40% of all cases of exudative pharyngitis in chil­ dren; it is rare among those under the age of 3. Younger children may manifest streptococcal infection with a syndrome of fever, malaise, and lymphadenopathy without exudative pharyngitis. Infection is acquired through contact with another individual carrying the organism. Respi­ ratory droplets are the usual mechanism of spread, although other routes, including food-borne outbreaks, have been well described. The incubation period is 1–4 days. Symptoms include sore throat, fever and chills, malaise, and sometimes abdominal complaints and vomiting, particularly in children. Both symptoms and signs are variable, rang­ ing from mild throat discomfort with minimal physical findings to high fever and severe sore throat associated with intense erythema and swelling of the pharyngeal mucosa and the presence of purulent exu­ date over the posterior pharyngeal wall and tonsillar pillars. Enlarged, tender anterior cervical lymph nodes commonly accompany exudative pharyngitis. The differential diagnosis of streptococcal pharyngitis includes the many other bacterial and viral etiologies (Table 153-2). Streptococ­ cal infection is an unlikely cause when symptoms and signs sugges­ tive of viral infection are prominent (conjunctivitis, coryza, cough, hoarseness, or discrete ulcerative lesions of the buccal or pharyngeal mucosa). Because of the range of clinical presentations of streptococcal pharyngitis and the large number of other agents that can produce the same clinical picture, diagnosis of streptococcal pharyngitis on clinical grounds alone is not reliable. The throat culture remains the diagnostic gold standard. Culture of a throat specimen that is properly collected (i.e., by vigorous rubbing of a sterile swab over both tonsillar pillars) and properly processed is the most sensitive and specific means of PART 5 Infectious Diseases TABLE 153-2  Infectious Etiologies of Acute Pharyngitis ORGANISM ASSOCIATED CLINICAL SYNDROME(S) Viruses Rhinovirus Common cold Coronavirus Common cold, COVID-19 Adenovirus Pharyngoconjunctival fever Influenza virus Influenza Parainfluenza virus Cold, croup Coxsackievirus Herpangina, hand-foot-and-mouth disease Herpes simplex virus Gingivostomatitis (primary infection) Epstein-Barr virus Infectious mononucleosis Cytomegalovirus Mononucleosis-like syndrome HIV Acute (primary) infection syndrome Bacteria Group A streptococci Pharyngitis, scarlet fever Group C or G streptococci Pharyngitis Mixed anaerobes Vincent’s angina Arcanobacterium haemolyticum Pharyngitis, scarlatiniform rash Neisseria gonorrhoeae Pharyngitis Treponema pallidum Secondary syphilis Francisella tularensis Pharyngeal tularemia Corynebacterium diphtheriae Diphtheria Yersinia enterocolitica Pharyngitis, enterocolitis Yersinia pestis Plague Chlamydiae Chlamydia pneumoniae Bronchitis, pneumonia Chlamydia psittaci Psittacosis Mycoplasmas Mycoplasma pneumoniae Bronchitis, pneumonia

definitive diagnosis. A rapid diagnostic test for latex agglutination or enzyme immunoassay of swab specimens is a useful adjunct to throat culture. Rapid diagnostic tests typically have a specificity of >95%. Thus, a positive result can be relied upon for definitive diagnosis and eliminates the need for throat culture. In settings in which the incidence of rheumatic fever is low, a confirmatory throat culture is not recom­ mended for routine evaluation of most adults with a negative rapid test. However, because rapid diagnostic tests are less sensitive than throat culture (relative sensitivity in comparative studies, 70–90%), a negative result should be confirmed by throat culture for individuals at higher risk such as those with a history of rheumatic fever or immunocompro­ mise or a family member with such a history; patients living in congre­ gate settings of young adults such as dormitories or military facilities where the incidence of GAS pharyngitis may be elevated; individuals with household exposure to someone with proven GAS infection; and those living in an area in which rheumatic fever is endemic. National or professional organization guidelines in some countries with a very low incidence of ARF recommend symptomatic treatment for patients with mild symptoms and restricting diagnostic testing and/or anti­ biotic treatment to those with suggestive clinical features (e.g., fever, tonsillar swelling or exudate, tender anterior cervical adenopathy, and/ or absence of cough) or special risk factors. TREATMENT GAS Pharyngitis In the usual course of uncomplicated streptococcal pharyngitis, symptoms resolve after 3–5 days. The course is shortened little by treatment, which is given primarily to prevent suppurative compli­ cations and ARF. Prevention of ARF depends on eradication of the organism from the pharynx, not simply on resolution of symptoms, and requires 10 days of penicillin treatment (Table 153-3). A firstgeneration cephalosporin, such as cephalexin or cefadroxil, may be substituted for penicillin in cases of penicillin allergy if the nature of the allergy is not an immediate hypersensitivity reaction (anaphy­ laxis or urticaria) or another potentially life-threatening manifesta­ tion (e.g., severe rash and fever). Alternative agents are erythromycin and azithromycin. Azithro­ mycin offers the advantages of better gastrointestinal tolerability, once-daily dosing, and a 5-day treatment course. Resistance to erythromycin and other macrolides is common among isolates from several countries, including Spain, Italy, Finland, Japan, and Korea. Macrolide resistance may be becoming more prevalent TABLE 153-3  Treatment of Group A Streptococcal Infections INFECTION TREATMENTa Pharyngitis Benzathine penicillin G (1.2 mU IM) or penicillin V

(250 mg PO tid or 500 mg PO bid) × 10 days (Children <27 kg: Benzathine penicillin G [600,000 units IM] or penicillin V [250 mg PO bid or tid] × 10 days) Impetigo Same as pharyngitis Erysipelas/cellulitis Severe: Penicillin G (1–2 mU IV q4h) Mild to moderate: Procaine penicillin (1.2 mU IM bid) Necrotizing fasciitis/ myositis Surgical debridement plus penicillin G (2–4 mU IV q4h) plus clindamycinb (600–900 mg IV q8h) Pneumonia/empyema Penicillin G (2–4 mU IV q4h) plus drainage of empyema Streptococcal toxic shock syndrome Penicillin G (2–4 mU IV q4h) plus clindamycinb

(600–900 mg IV q8h) plus IV immunoglobulin (2 g/kg as a single dose) aPenicillin allergy: A first-generation cephalosporin, such as cephalexin or cefadroxil, may be substituted for penicillin in cases of penicillin allergy if the nature of the allergy is not an immediate hypersensitivity reaction (anaphylaxis or urticaria) or another potentially life-threatening manifestation (e.g., severe rash and fever). Alternative agents for oral therapy are erythromycin (10 mg/kg PO qid, up to a maximum of 250 mg per dose) and azithromycin (a 5-day course at a dose of 12 mg/kg once daily, up to a maximum of 500 mg/d). Vancomycin is an alternative for parenteral therapy. bLinezolid (600 mg IV q12h) may be substituted for isolates resistant to clindamycin. See text for discussion.

elsewhere with the increasing use of this class of antibiotics. Data from the CDC and the Active Bacterial Core surveillance program indicate a rise in macrolide resistance among invasive GAS isolates in the United States to >30% since 2021. In areas with resistance rates exceeding 5–10%, macrolides should be avoided unless results of susceptibility testing are known. Follow-up culture after treatment is not routinely recommended but may be warranted in selected cases, such as those involving patients or families with frequent streptococcal infections or those occurring in situations in which the risk of ARF is thought to be high (e.g., when cases of ARF have recently been reported in the community). Complications  Suppurative complications of streptococcal phar­ yngitis have become uncommon with the widespread use of antibiot­ ics for most symptomatic cases. These complications result from the spread of infection from the pharyngeal mucosa to deeper tissues by direct extension or by the hematogenous or lymphatic route and may include cervical lymphadenitis, peritonsillar or retropharyngeal abscess, sinusitis, otitis media, meningitis, bacteremia, endocarditis, and pneumonia. Local complications, such as peritonsillar or para­ pharyngeal abscess formation, should be considered in a patient with unusually severe or prolonged symptoms or localized pain associated with high fever and a toxic appearance. Nonsuppurative complications include ARF (Chap. 370) and PSGN (Chap. 326), both of which are thought to result from immune responses to streptococcal infection. Penicillin treatment of streptococcal pharyngitis reduces the likelihood of ARF but not that of PSGN. BACTERIOLOGIC TREATMENT FAILURE AND THE ASYMPTOMATIC CARRIER STATE Surveillance cultures have shown that up to 20% of individuals in certain populations may have asymptomatic pharyngeal colonization with GAS. There are no definitive guidelines for management of these asymptomatic carriers or of asymptomatic patients who still have a positive throat culture after a full course of treatment for symptomatic pharyngitis. A reasonable course of action is to give a single 10-day course of penicillin for symptomatic pharyngitis and, if positive cul­ tures persist, not to re-treat unless symptoms recur. Studies of the natural history of streptococcal carriage and infection have shown that the risk both of developing ARF and of transmitting infection to others is substantially lower among asymptomatic carriers than among indi­ viduals with symptomatic pharyngitis. Therefore, aggressive attempts to eradicate carriage probably are not justified under most circum­ stances. An exception is the situation in which an asymptomatic carrier is a potential source of infection to others. Outbreaks of food-borne infection and nosocomial puerperal infection have been traced to asymptomatic carriers who may harbor the organisms in the throat, vagina, or anus or on the skin. TREATMENT Asymptomatic Pharyngeal Colonization with GAS When a carrier is transmitting infection to others, attempts to erad­ icate carriage are warranted. Data are limited on the best regimen to clear GAS after penicillin alone has failed. Regimens reported to have efficacy superior to that of penicillin alone for eradication of carriage include (1) a first-generation cephalosporin such as cephalexin (30 mg/kg; 500 mg maximum) twice daily for 10 days or (2) oral clindamycin (7 mg/kg; 300 mg maximum) three times daily for 10 days. A 10-day course of oral vancomycin (250 mg four times daily) and rifampin (600 mg twice daily) has eradicated rectal colonization. Single-dose azithromycin (20 mg/kg; 1000 mg maxi­ mum) has been used for mass prophylaxis/eradication of coloniza­ tion in outbreak situations. In vitro susceptibility to clindamycin or azithromycin should be confirmed before prescribing either of these antibiotics for eradication of colonization.

FIGURE 153-2  Scarlet fever exanthem. Finely punctate erythema has become confluent (scarlatiniform); petechiae can occur and have a linear configuration within the exanthem in body folds (Pastia’s lines). (From TB Fitzpatrick, RA Johnson, K Wolff: Color Atlas and Synopsis of Clinical Dermatology, 4th ed, New York, McGraw-Hill, 2001, with permission.) CHAPTER 153 Scarlet Fever  Scarlet fever consists of streptococcal infection, usually pharyngitis, accompanied by a characteristic rash (Fig. 153-2). The rash arises from the effects of one of several toxins, currently designated streptococcal pyrogenic exotoxins and previously known as erythrogenic or scarlet fever toxins. In the past, scarlet fever was thought to reflect infection of an individual lacking toxin-specific immunity with a toxin-producing strain of GAS. Susceptibility to scarlet fever was correlated with results of the Dick test, in which a small amount of erythrogenic toxin injected intradermally produced local erythema in susceptible individuals but elicited no reaction in those with specific immunity. Subsequent studies have suggested that development of the scarlet fever rash may reflect a hypersensitivity reaction requiring prior exposure to the toxin. For reasons that are not clear, scarlet fever has become less common in recent years, although large outbreaks have occurred recently in China and the United Kingdom. The symptoms of scarlet fever are the same as those of pharyngitis alone. The rash typi­ cally begins on the first or second day of illness over the upper trunk, spreading to involve the extremities but sparing the palms and soles. The rash is made up of minute papules, giving a characteristic “sand­ paper” feel to the skin. Associated findings include circumoral pallor, “strawberry tongue” (enlarged papillae on a coated tongue, which later may become denuded), and accentuation of the rash in skin folds (Pas­ tia’s lines). Subsidence of the rash in 6–9 days is followed after several days by desquamation of the palms and soles. The differential diagnosis of scarlet fever includes other causes of fever and generalized rash, such as measles and other viral exanthems, Kawasaki disease, toxic shock syndrome, and systemic allergic reactions (e.g., drug eruptions). Streptococcal Infections Skin and Soft Tissue Infections  GAS—and occasionally other streptococcal species—can cause a variety of infections involving the skin, subcutaneous tissues, muscles, and fascia. While several clinical syn­ dromes offer a useful means for classification of these infections, not all cases fit exactly into one category. The classic syndromes are gen­ eral guides to predicting the level of tissue involvement in a particular patient, the probable clinical course, and the likelihood that surgical intervention or aggressive life support will be required. IMPETIGO (PYODERMA) Impetigo, a superficial infection of the skin, is caused primarily by GAS and occasionally by other streptococci or Staphylococcus aureus. Impetigo is seen most often in young children, tends to occur during

FIGURE 153-3  Impetigo is a superficial streptococcal or Staphylococcus aureus infection consisting of honey-colored crusts and erythematous weeping erosions. Occasionally, bullous lesions may be seen. (Courtesy of Mary Spraker, MD; with permission.) warmer months, and is more common in semitropical or tropical climates than in cooler regions. Infection is more common among children living under conditions of poor hygiene. Prospective studies have shown that colonization of unbroken skin with GAS precedes clinical infection. Minor trauma, such as a scratch or an insect bite, may then serve to inoculate organisms into the skin. Impetigo is best prevented, therefore, by attention to adequate hygiene. The usual sites of involvement are the face (particularly around the nose and mouth) and the legs, although lesions may occur at other locations. Individual lesions begin as red papules, which evolve quickly into vesicular and then pustular lesions that break down and coalesce to form characteristic honeycomb-like crusts (Fig. 153-3). Lesions generally are not painful, and patients do not appear ill. Fever is not a feature of impetigo and, if present, suggests either infection extend­ ing to deeper tissues or another diagnosis. The classic presentation of impetigo usually poses little diagnostic difficulty. Cultures of impetiginous lesions often yield S. aureus as well as GAS. In almost all cases, streptococci are isolated initially, and staphylococci appear later, presumably as secondary colonizing flora. In the past, penicil­ lin was nearly always effective against these infections. However, an increasing frequency of penicillin treatment failure suggests that S. aureus may have become more prominent as a cause of impetigo. Bullous impetigo due to S. aureus is distinguished from typical strep­ tococcal infection by more extensive, bullous lesions that break down and leave thin paper-like crusts instead of the thick amber crusts of streptococcal impetigo. Other skin lesions that may be confused with impetigo include herpetic lesions—either those of orolabial herpes simplex or those of chickenpox or zoster. Herpetic lesions can gener­ ally be distinguished by their appearance as more discrete, grouped vesicles and by a positive Tzanck test or by herpes simplex virus- or varicella-zoster virus-specific PCR. In difficult cases, cultures of vesicular fluid should yield GAS (or Staphylococcus aureus) in impe­ tigo and the responsible virus in herpesvirus infections. PART 5 Infectious Diseases TREATMENT Streptococcal Impetigo Treatment of streptococcal impetigo is the same as that for strepto­ coccal pharyngitis. In view of evidence that S. aureus has become a relatively frequent cause of impetigo, empirical regimens should cover both streptococci and S. aureus. For example, either dicloxa­ cillin or cephalexin can be given at a dose of 250 mg four times daily for 10 days. Topical mupirocin ointment also is effective. Culture may be indicated to rule out methicillin-resistant S. aureus,

especially if the response to empirical treatment is unsatisfactory. In most areas of the world, ARF is not a sequela to streptococcal skin infections, although PSGN may follow either skin or throat infec­ tion. The reason for this difference is not known. One hypothesis is that the immune response necessary for development of ARF occurs only after infection of the pharyngeal mucosa. In addition, the strains of GAS that cause pharyngitis are generally of different M protein types than those associated with skin infections; thus, the strains that cause pharyngitis may have rheumatogenic potential, while the skin-infecting strains may not. An exception to this gen­ eral rule may occur among indigenous people in northern Australia and in certain Pacific Island groups. Acute rheumatic fever and rheumatic heart disease are prevalent in these populations as is streptococcal impetigo/pyoderma, but not pharyngitis. This epide­ miologic pattern has led investigators to suggest that skin infection may trigger acute rheumatic fever in this setting. CELLULITIS Inoculation of organisms into the skin may lead to cellulitis: infection involving the skin and subcutaneous tissues. The portal of entry may be a traumatic or surgical wound, an insect bite, or any other break in skin integrity. Often, no entry site is apparent. One form of strep­ tococcal cellulitis, erysipelas, is characterized by a bright red appear­ ance of the involved skin, which forms a plateau sharply demarcated from surrounding normal skin (Fig. 153-4). The lesion is warm to the touch, may be tender, and appears shiny and swollen. The skin often has a peau d’orange texture, which is thought to reflect involvement of superficial lymphatics; superficial blebs or bullae may form, usually 2–3 days after onset. The lesion typically develops over a few hours and is associated with fever and chills. Erysipelas tends to occur on the malar area of the face (often with extension over the bridge of the nose to the contralateral malar region) or on the lower extremi­ ties. After one episode, recurrence at the same site—sometimes years later—is not uncommon. Classic cases of erysipelas, with typical features, are almost always due to β-hemolytic streptococci, usually GAS and occasionally group C or G. Often, however, the appearance of streptococcal cellulitis is not sufficiently distinctive to permit a specific diagnosis on clinical grounds. The anatomic area involved may not be typical for erysipelas, the lesion may be less intensely red than usual and may fade into surrounding skin, and/or the patient may appear only mildly ill. In such cases, it is prudent to broaden the spectrum of empirical antimicrobial therapy to include other patho­ gens, particularly S. aureus, that can produce cellulitis with the same FIGURE 153-4  Erysipelas is a streptococcal infection of the superficial dermis and consists of well-demarcated, erythematous, edematous, warm plaques.

appearance. Staphylococcal infection should be suspected if cellulitis develops around a wound or an ulcer. Streptococcal cellulitis tends to develop at anatomic sites in which normal lymphatic drainage has been disrupted, such as sites of prior cellulitis, the arm ipsilateral to a mastectomy and axillary lymph node dissection, a lower extremity previously involved in deep venous thrombosis or chronic lymphedema, or the leg from which a saphe­ nous vein has been harvested for coronary artery bypass grafting. The organism may enter via a dermal breach some distance from the eventual site of clinical cellulitis. For example, some patients with recurrent leg cellulitis following saphenous vein removal may stop having recurrent episodes only after treatment of tinea pedis on the affected extremity. Fissures in the skin presumably serve as a portal of entry for streptococci, which then produce infection more proximally in the leg at the site of previous injury. Streptococcal cellulitis may also involve recent surgical wounds. GAS is among the few bacterial pathogens that typically produce signs of wound infection and sur­ rounding cellulitis within the first 24 h after surgery. These wound infections are usually associated with a thin exudate and may spread rapidly, either as cellulitis in the skin and subcutaneous tissue or as a deeper tissue infection (see below). Streptococcal wound infection or localized cellulitis may also be associated with lymphangitis, mani­ fested by red streaks extending proximally along superficial lymphat­ ics from the infection site. TREATMENT Streptococcal Cellulitis See Table 153-3 and Chap. 134. DEEP SOFT TISSUE INFECTIONS Necrotizing fasciitis (hemolytic streptococcal gangrene) involves the superficial and/or deep fascia investing the muscles of an extremity or the trunk. The source of the infection is either the skin, with organ­ isms introduced into tissue through trauma (sometimes trivial), or the bowel flora, with organisms released during abdominal surgery or from an occult enteric source, such as a diverticular or appendiceal abscess. The inoculation site may be inapparent and is often some distance from the site of clinical involvement; e.g., the introduction of organisms via minor trauma to the hand may be associated with clinical infection of the tissues overlying the shoulder or chest. Cases associated with the bowel flora are usually polymicrobial, involving a mixture of anaerobic bacteria (such as Bacteroides fragilis or anaero­ bic streptococci) and facultative organisms (usually gram-negative bacilli). Cases unrelated to contamination from bowel organisms are most commonly caused by GAS alone or in combination with other organisms (most often S. aureus). Overall, GAS is implicated in ~60% of cases of necrotizing fasciitis. The onset of symptoms is usually quite acute and is marked by severe pain at the site of involvement, malaise, fever, chills, and a toxic appearance. The physical findings, particularly early on, may not be striking, with only minimal erythema of the overlying skin. Pain and tenderness are usually severe. In contrast, in more superficial cellulitis, the skin appearance is more abnormal, but pain and tenderness are only mild or moderate. As the infection pro­ gresses (often over several hours), the severity and extent of symptoms worsen, and skin changes become more evident, with the appearance of dusky or mottled erythema and edema. The marked tenderness of the involved area may evolve into anesthesia as the spreading inflam­ matory process produces infarction of cutaneous nerves. Although myositis is more commonly due to S. aureus infection, GAS occasionally produces abscesses in skeletal muscles (streptococcal myositis), with little or no involvement of the surrounding fascia or overlying skin. The presentation is usually subacute, but a fulminant form has been described in association with severe systemic toxicity, bacteremia, and a high mortality rate. The fulminant form may reflect the same basic disease process seen in necrotizing fasciitis, but with the necrotizing inflammatory process extending into the muscles them­ selves rather than remaining limited to the fascial layers.

TREATMENT Deep Soft Tissue Streptococcal Infections Once necrotizing fasciitis is suspected, early surgical exploration is both diagnostically and therapeutically indicated. Surgery reveals necrosis and inflammatory fluid tracking along the fascial planes above and between muscle groups, without involvement of the muscles themselves. The process usually extends beyond the area of clinical involvement, and extensive debridement is required. Drain­ age and debridement are central to the management of necrotizing fasciitis; antibiotic treatment is a critical adjunct (Table 153-3), but surgery is life-saving. Treatment for streptococcal myositis consists of surgical drainage—usually by an open procedure that permits evaluation of the extent of infection and ensures adequate debride­ ment of involved tissues—and high-dose penicillin (Table 153-3). Infection models in animals and multiple retrospective clinical studies support the addition of clindamycin to the antibiotic treat­ ment regimen for severe GAS necrotizing soft tissue infections. In vitro and animal models suggest that the growth of GAS slows after the organism reaches a high density in infected tissues and that penicillin and other beta-lactam antibiotics have reduced efficacy under these circumstances as they target cell wall biosynthesis. By contrast, protein synthesis inhibitors retain activity regardless of growth phase of the bacteria. If the infecting strain is not known to be susceptible to clindamycin, linezolid is an alternative. Pneumonia and Empyema  GAS is an occasional cause of pneu­ monia, generally in previously healthy individuals. The onset of symp­ toms may be abrupt or gradual. Pleuritic chest pain, fever, chills, and dyspnea are the characteristic manifestations. Cough is usually present but may not be prominent. Approximately one-half of patients with GAS pneumonia have an accompanying pleural effusion. In contrast to the sterile parapneumonic effusions typical of pneumococcal pneu­ monia, those complicating streptococcal pneumonia are almost always infected. The empyema fluid is usually visible by chest radiography on initial presentation, and its volume may increase rapidly. These pleural collections should be drained early, as they tend to become loculated rapidly, resulting in a chronic fibrotic reaction that may require thora­ cotomy for removal. Bacteremia, Puerperal Sepsis, and Streptococcal Toxic Shock Syndrome  In adults, GAS bacteremia is usually associated with an identifiable local infection, whereas children may have bacteremia without an associated focal infection. Bacteremia occurs rarely with otherwise uncomplicated pharyngitis, occasionally with cellulitis or pneumonia, and relatively frequently with necrotizing fasciitis. Bacte­ remia without an identified source raises the possibility of endocarditis, an occult abscess, or osteomyelitis. A variety of focal infections may arise secondarily from streptococcal bacteremia, including endocardi­ tis, meningitis, septic arthritis, osteomyelitis, peritonitis, and visceral abscesses. GAS is occasionally implicated in infectious complications of childbirth, usually endometritis and associated bacteremia. In the preantibiotic era, puerperal sepsis was commonly caused by GAS; cur­ rently, it is more often caused by GBS. Several nosocomial outbreaks of puerperal GAS infection have been traced to an asymptomatic carrier, usually someone present at delivery. The site of carriage may be the skin, throat, anus, or vagina. CHAPTER 153 Streptococcal Infections Beginning in the late 1980s, several reports described patients with GAS infections associated with shock and multisystem organ failure. This syndrome was called streptococcal toxic shock syndrome (TSS) because it shares certain features with staphylococcal TSS. A case definition for streptococcal TSS was formulated in 1993 and updated in 2010 (Table 153-4). The general features of the illness include fever, hypotension, renal impairment, and respiratory distress syndrome. Various types of rash have been described, but rash usually does not develop. Laboratory abnormalities include a marked shift to the left in the white blood cell differential, with many immature granulocytes; hypocalcemia; hypoalbuminemia; and thrombocytopenia, which usu­ ally becomes more pronounced on the second or third day of illness.

TABLE 153-4  Case Definition for Streptococcal Toxic Shock Syndromea I. Isolation of group A streptococci (Streptococcus pyogenes) A. From a normally sterile site B. From a nonsterile site II. Clinical signs of severity A. Hypotension and B. ≥2 of the following signs

  1. Renal impairment
  2. Coagulopathy
  3. Liver function impairment
  4. Adult respiratory distress syndrome
  5. A generalized erythematous macular rash that may desquamate
  6. Soft tissue necrosis, including necrotizing fasciitis or myositis; or gangrene aAn illness fulfilling criteria IA, IIA, and IIB is defined as a definite case. An illness fulfilling criteria IB, IIA, and IIB is defined as a probable case if no other etiology for the illness is identified. Source: Modified from Streptococcal Toxic Shock Syndrome (STSS) (Streptococcus pyogenes) 2010 Case Definition: Centers for Disease Control and Prevention (https:// ndc.services.cdc.gov/case-definitions/streptococcal-toxic-shock-syndrome-2010/). In contrast to patients with staphylococcal TSS, the majority with streptococcal TSS are bacteremic. The most common associated infection is a soft tissue infection—necrotizing fasciitis, myositis, or cellulitis—although a variety of other associated local infections have been described, including pneumonia, peritonitis, osteomyelitis, and myometritis. Streptococcal TSS is associated with a mortality rate of ≥30%, with most deaths secondary to shock and respiratory failure. Because of its rapidly progressive and lethal course, early recognition of the syndrome is essential. Patients should receive aggressive supportive care (fluid resuscitation, pressors, and mechanical ventilation) in addi­ tion to antimicrobial therapy and, in cases associated with necrotizing fasciitis, should undergo surgical debridement. Exactly why certain patients develop this fulminant syndrome is not known. Early studies of the streptococcal strains isolated from these patients demonstrated a strong association with the production of pyrogenic exotoxin A. This association has been inconsistent in subsequent case series. Pyrogenic exotoxin A and several other streptococcal exotoxins act as superanti­ gens to trigger release of inflammatory cytokines from T lymphocytes. Fever, shock, and organ dysfunction in streptococcal TSS may reflect, in part, the systemic effects of superantigen-mediated cytokine release. PART 5 Infectious Diseases TREATMENT Streptococcal Toxic Shock Syndrome High-dose penicillin is the agent of choice for streptococcal TSS. For empiric treatment of sepsis before the etiology is known, a broad-spectrum antibiotic regimen should include an agent with excellent activity against GAS such as an extended-spectrum peni­ cillin, cephalosporin, or a carbapenem. In light of the possible role of pyrogenic exotoxins or other streptococcal toxins in streptococ­ cal TSS, adjunctive treatment with clindamycin has been advocated by some authorities (Table 153-3), who argue that, through its direct action on protein synthesis, clindamycin is more effective in rapidly terminating toxin production than is penicillin—a cell-wall agent. As discussed previously (see “Treatment of Deep Soft Tis­ sue Streptococcal Infections,” above), support for this view comes from studies of an experimental model of streptococcal myositis, in which mice given clindamycin had a higher rate of survival than those given penicillin. Comparable data on the treatment of human infections are not available, although retrospective analysis has suggested a better outcome when patients with invasive soft tissue infection and streptococcal TSS are treated with clindamycin rather than with cell wall–active antibiotics alone. As clindamycin resistance occurs in 30% or more of invasive GAS isolates in the United States and many other countries, susceptibility to clindamy­ cin should not be assumed in the absence of in vitro testing of the

infecting strain. Linezolid is an appropriate alternative in the setting of clindamycin resistance or unknown susceptibility. IV immuno­ globulin also has been used as adjunctive therapy for streptococcal TSS (Table 153-3). Pooled immunoglobulin preparations contain antibodies capable of neutralizing the effects of streptococcal toxins and may have opsonic activity against GAS. Intravenous immu­ noglobulin (IVIG) also has immunomodulatory properties, which may mitigate the pathophysiologic effects of inflammatory cyto­ kines. Anecdotal reports and case series have suggested favorable clinical responses to IVIG, but no adequately powered, prospective, controlled trials have been reported. A meta-analysis of five studies of streptococcal TSS patients treated with clindamycin found that IVIG use was associated with a reduction in mortality rate from 33.7% to 15.7%. ■ ■PREVENTION No vaccine against GAS is commercially available. A formulation that consists of recombinant peptides containing epitopes of 26 M-protein types has undergone phase 1 and 2 testing in volunteers. Early results indicate that the vaccine is well tolerated and elicits type-specific anti­ body responses. Vaccines based on a conserved region of M protein or on a mixture of other conserved GAS protein antigens are in earlier stages of development. Household contacts of individuals with invasive GAS infection (e.g., bacteremia, necrotizing fasciitis, or streptococcal TSS) are at greater risk of invasive infection than the general population. Asymptomatic pharyngeal colonization with GAS has been detected in up to 25% of persons with >4 h/d of same-room exposure to an index case. However, the CDC does not recommend antibiotic prophylaxis routinely for contacts of patients with invasive disease because such an approach (if effective) would require treatment of hundreds of contacts to prevent a single case. Prophylaxis may be considered for contacts of unusually severe cases or for individuals at increased risk for invasive infection. STREPTOCOCCI OF GROUPS C AND G Group C and group G streptococci are β-hemolytic bacteria that occasionally cause human infections similar to those caused by GAS. Strains that form small colonies on blood agar (<0.5 mm) are generally members of the Streptococcus milleri group (Streptococcus intermedius, Streptococcus anginosus; see “Viridans Streptococci,” below). Large-colony group C and G streptococci of human origin are now considered a single species, Streptococcus dysgalactiae subspecies equisimilis (SDSE). Genomic studies have demonstrated extensive overlap between the genomes of SDSE and GAS and evidence of cross-species recombina­ tion. These organisms have been associated with pharyngitis, cellulitis and soft tissue infections, pneumonia, bacteremia, endocarditis, and septic arthritis. Puerperal sepsis, meningitis, epidural abscess, intraab­ dominal abscess, urinary tract infection, and neonatal sepsis also have been reported. SDSE bacteremia most often affects elderly or chroni­ cally ill patients and, in the absence of obvious local infection, is likely to reflect endocarditis. Septic arthritis, sometimes involving multiple joints, may complicate endocarditis or develop in its absence. Distinct streptococcal species of Lancefield group C cause infections in domes­ ticated animals, especially horses and cattle; some human infections are acquired through contact with animals or consumption of unpasteur­ ized milk. These zoonotic organisms include Streptococcus equi subspe­ cies zooepidemicus and S. equi subspecies equi. TREATMENT Group C or G Streptococcal Infection Penicillin is the drug of choice for treatment of group C or G strep­ tococcal infections. Antibiotic treatment is the same as for similar syndromes due to GAS (Table 153-3). Patients with bacteremia or septic arthritis should receive IV penicillin (2–4 mU every 4 h). All group C and G streptococci are sensitive to penicillin; nearly all are inhibited in vitro by concentrations of ≤0.03 μg/mL. Occasional isolates exhibit tolerance: although inhibited by low concentrations

of penicillin, they are killed only by significantly higher concentra­ tions. The clinical significance of tolerance is unknown. Because of the poor clinical response of some patients to penicillin alone, the addition of gentamicin (1 mg/kg every 8 h for patients with normal renal function) is recommended by some authorities for treatment of endocarditis or septic arthritis due to group C or G streptococci; however, combination therapy has not been shown to be superior to penicillin treatment alone. Patients with joint infections often require repeated aspiration or open drainage and debridement for cure; the response to treatment may be slow, particularly in debili­ tated patients and those with involvement of multiple joints. Infec­ tion of prosthetic joints almost always requires prosthesis removal in addition to antibiotic therapy. GROUP B STREPTOCOCCI Identified first as a cause of mastitis in cows, streptococci belonging to Lancefield group B have since been recognized as a major cause of sepsis and meningitis in human neonates. GBS is also a frequent cause of peripartum fever in women and an occasional cause of serious infec­ tion in nonpregnant adults. Since the widespread institution of prenatal screening for GBS in the 1990s, the incidence of neonatal infection per 1000 live births has fallen from ~2–3 cases to ~0.6 case. During the same period, GBS infection in adults with underlying chronic illnesses has become more common; adults now account for a larger propor­ tion of invasive GBS infections than do newborns. Lancefield group B consists of a single species, S. agalactiae, which is definitively identified with specific antiserum to the group B cell wall–associated carbohy­ drate antigen. A streptococcal isolate can be classified presumptively as GBS on the basis of biochemical tests, including hydrolysis of sodium hippurate (in which 99% of isolates are positive), hydrolysis of bile esculin (in which 99–100% are negative), bacitracin susceptibility (in which 92% are resistant), and production of CAMP factor (in which 98–100% are positive). CAMP factor is a phospholipase produced by GBS that causes synergistic hemolysis with β lysin produced by certain strains of S. aureus. Its presence can be demonstrated by crossstreaking of the test isolate and an appropriate staphylococcal strain on a blood agar plate. GBS organisms causing human infections are encapsulated by 1 of 10 antigenically distinct polysaccharides. The capsular polysaccharide is an important virulence factor. Antibodies to the capsular polysaccharide afford protection against GBS of the same (but not of a different) capsular type. ■ ■INFECTION IN NEONATES Two general types of GBS infection in infants are defined by the age of the patient at presentation. Early-onset infections occur within the first week of life, with a median age of 20 h at onset. Approximately half of these infants have signs of GBS disease at birth. The infection is acquired during or shortly before birth from the colonized maternal genital tract. Surveillance studies have shown that 5–40% of women are vaginal or rectal carriers of GBS. Approximately 50% of infants delivered vaginally by carrier mothers become colonized, although only 1–2% develop clinically evident infection. Prematurity, prolonged labor, obstetric complications, and maternal fever are risk factors for early-onset infection. The presentation of early-onset infection is the same as that of other forms of neonatal sepsis. Typical findings include respiratory distress, lethargy, and hypotension. Essentially all infants with early-onset disease are bacteremic, one-third to one-half have pneumonia and/or respiratory distress syndrome, and one-third have meningitis. Late-onset infections occur in infants 1 week to 3 months old and, in rare instances, in older infants (mean age at onset, 3–4 weeks). The infecting organism may be acquired during delivery (as in early-onset cases) or during later contact with a colonized mother, nursery person­ nel, or another source. Meningitis is the most common manifestation of late-onset infection and in most cases is associated with a strain of capsular type III. Infants present with fever, lethargy or irritability, poor feeding, and seizures. The various other types of late-onset infection include bacteremia without an identified source, osteomyelitis, septic

arthritis, and facial cellulitis associated with submandibular or preau­ ricular adenitis.

TREATMENT Group B Streptococcal Infection in Neonates Penicillin is the agent of choice for all GBS infections. Empirical broad-spectrum therapy for suspected bacterial sepsis, consisting of ampicillin and gentamicin, is generally administered until culture results become available. If cultures yield GBS, many pediatricians continue to administer gentamicin, along with ampicillin or peni­ cillin, for a few days until clinical improvement becomes evident. Infants with bacteremia or soft tissue infection should receive peni­ cillin at a dosage of 200,000 units/kg per day in divided doses. For meningitis, infants ≤7 days of age should receive 250,000–450,000 units/kg per d in three divided doses; infants >7 days of age should receive 450,000–500,000 units/kg per day in four divided doses. Meningitis should be treated for at least 14 days because of the risk of relapse with shorter courses. Prevention  The incidence of GBS infection is unusually high among infants of women with risk factors: preterm delivery, early rupture of membranes (>24 h before delivery), prolonged labor, fever, or chorio­ amnionitis. Because the usual source of the organisms infecting a neo­ nate is the mother’s birth canal, efforts have been made to prevent GBS infections by the identification of high-risk carrier mothers and their treatment with various forms of antibiotic prophylaxis or immunopro­ phylaxis. Prophylactic administration of ampicillin or penicillin to such patients during delivery reduces the risk of infection in the newborn. This approach has been hampered by logistical difficulties in identifying colonized women before delivery; the results of vaginal cultures early in pregnancy are poor predictors of carrier status at delivery. The CDC recommends screening for anogenital colonization at 35–37 weeks

of pregnancy by a swab culture of the lower vagina and anorectum; intrapartum chemoprophylaxis is recommended for culture-positive women and for women who, regardless of culture status, have previ­ ously given birth to an infant with GBS infection or have a history of GBS bacteriuria during pregnancy. Women whose culture status is unknown and who develop premature labor (<37 weeks), prolonged rupture of membranes (>18 h), or intrapartum fever or who have a positive intrapartum nucleic acid amplification test for GBS also should receive intrapartum chemoprophylaxis. The recommended regimen for chemoprophylaxis is a loading dose of 5 million units of penicil­ lin G followed by 2.5 million units every 4 h until delivery. Cefazolin is an alternative for women with a history of penicillin allergy who are thought not to be at high risk for anaphylaxis. For women with a history of immediate hypersensitivity, clindamycin may be substi­ tuted, but only if the colonizing isolate has been demonstrated to be susceptible. If susceptibility testing results are not available or indicate resistance, vancomycin should be used in this situation. CHAPTER 153 Streptococcal Infections Treatment of all pregnant women who are colonized or have risk fac­ tors for neonatal infection will result in exposure of up to one-third of pregnant women and newborns to antibiotics, with the attendant risks of allergic reactions and selection for resistant organisms. Although still in the developmental stages, a GBS vaccine may ultimately offer a better solution to prevention. Because transplacental passage of mater­ nal antibodies produces protective antibody levels in newborns, efforts are underway to develop a vaccine against GBS that can be given to childbearing-age women before or during pregnancy. Results of phase 1 clinical trials of GBS capsular polysaccharide–protein conjugate vac­ cines suggest that a multivalent conjugate vaccine would be safe and highly immunogenic. ■ ■INFECTION IN ADULTS The majority of GBS infections in otherwise healthy adults are related to pregnancy and parturition. Peripartum fever, the most common manifestation, is sometimes accompanied by symptoms and signs of endometritis or chorioamnionitis (abdominal distention and uterine

or adnexal tenderness). Blood and vaginal swab cultures are often posi­ tive. Bacteremia is usually transitory but occasionally results in men­ ingitis or endocarditis. Infections in adults that are not associated with the peripartum period generally involve individuals who are elderly or have an underlying chronic illness, such as diabetes mellitus or a malignancy. Among the infections that develop with some frequency in adults are cellulitis and soft tissue infection (including infected diabetic skin ulcers), urinary tract infection, pneumonia, endocardi­ tis, and septic arthritis. Other reported infections include meningitis, osteomyelitis, and intraabdominal or pelvic abscesses. Relapse or recurrence of invasive infection weeks to months after a first episode is documented in ~4% of cases.

TREATMENT Group B Streptococcal Infection in Adults GBS is less sensitive to penicillin than GAS, requiring somewhat higher doses. Adults with serious localized infections (pneumonia, pyelonephritis, abscess) should receive divided doses of ~12 million units of penicillin G daily; patients with endocarditis or meningitis should receive 18–24 million units per day in divided doses. Van­ comycin is an acceptable alternative for penicillin-allergic patients. NONENTEROCOCCAL GROUP D STREPTOCOCCI The main nonenterococcal group D streptococci that cause human infections were previously considered a single species, Streptococcus bovis. The organisms encompassed by S. bovis have been reclassified into two species, each of which has two subspecies: Streptococcus gal­ lolyticus subspecies gallolyticus, S. gallolyticus subspecies pasteurianus, Streptococcus infantarius subspecies infantarius, and S. infantarius subspecies coli. Endocarditis caused by these organisms is often associ­ ated with neoplasms of the gastrointestinal tract—most frequently, a colon carcinoma or polyp—but is also reported in association with other bowel lesions. When occult gastrointestinal lesions are carefully sought, abnormalities are found in >60% of patients with endocarditis due to S. gallolyticus or S. infantarius. In contrast to the enterococci, nonenterococcal group D streptococci like these organisms are reliably killed by penicillin as a single agent, and penicillin is the agent of choice for the infections they cause. PART 5 Infectious Diseases VIRIDANS AND OTHER STREPTOCOCCI ■ ■VIRIDANS STREPTOCOCCI Consisting of multiple species of α-hemolytic streptococci, the viridans streptococci are a heterogeneous group of organisms that are impor­ tant agents of bacterial endocarditis (Chap. 133). Several species of viridans streptococci, including Streptococcus salivarius, Streptococcus mitis, Streptococcus sanguis, and Streptococcus mutans, are part of the normal flora of the mouth, where they live in close association with the teeth and gingiva. Some species contribute to the development of dental caries. Previously known as Streptococcus morbillorum, Gemella morbillo­ rum has been placed in a separate genus, along with Gemella haemoly­ sans, on the basis of genetic-relatedness studies. These species resemble viridans streptococci with respect to habitat in the human host and associated infections. The transient viridans streptococcal bacteremia induced by eating, toothbrushing, flossing, and other sources of minor trauma, together with adherence to biologic surfaces, is thought to account for the pre­ dilection of these organisms to cause endocarditis (see Fig. 133-1). Viridans streptococci are also isolated, often as part of a mixed flora, from sites of sinusitis, brain abscess, and liver abscess. Viridans streptococcal bacteremia occurs relatively frequently in neutropenic patients, particularly after bone marrow transplantation or high-dose chemotherapy for cancer. Some of these patients develop a sepsis syndrome with high fever and shock. Risk factors for viri­ dans streptococcal bacteremia include chemotherapy with high-dose

cytosine arabinoside, prior treatment with trimethoprim-sulfamethox­ azole or a fluoroquinolone, treatment with antacids or histamine antagonists, mucositis, and profound neutropenia. The S. milleri group (also referred to as the S. intermedius or S. anginosus group) includes three species that cause human disease: S. intermedius, S. anginosus, and Streptococcus constellatus. These organ­ isms are often considered viridans streptococci, although they differ somewhat from other viridans streptococci in their small colony size on solid medium (they typically form colonies <0.5 mm in diameter), their hemolytic pattern (they may be α-, β-, or nonhemolytic), and the disease syndromes they cause. This group commonly produces suppurative infections, particularly abscesses of brain and abdominal viscera, and infections related to the oral cavity or respiratory tract, such as peritonsillar abscess, sinusitis and complications such as orbital cellulitis, subdural and epidural abscess, and cerebral venous sinus thrombosis, lung abscess, and empyema. TREATMENT Infection with Viridans Streptococci Isolates from neutropenic patients with bacteremia are often resis­ tant to penicillin; thus, these patients should be treated presump­ tively with vancomycin until the results of susceptibility testing become available. Viridans streptococci isolated in other clinical settings usually are sensitive to penicillin. Susceptibility testing should be performed to guide treatment of serious infections. ■ ■ABIOTROPHIA AND GRANULICATELLA SPECIES (NUTRITIONALLY VARIANT STREPTOCOCCI) Occasional isolates cultured from the blood of patients with endocar­ ditis fail to grow when subcultured on solid media. These nutrition­ ally variant streptococci require supplemental thiol compounds or active forms of vitamin B6 (pyridoxal or pyridoxamine) for growth in the laboratory. The nutritionally variant streptococci are generally grouped with the viridans streptococci because they cause similar types of infections. However, they have been reclassified on the basis of 16S ribosomal RNA sequence comparisons into two separate genera: Abiotrophia, with a single species (Abiotrophia defectiva), and Granuli­ catella, with three species associated with human infection (Granulica­ tella adiacens, Granulicatella para-adiacens, and Granulicatella elegans). TREATMENT Infection with Nutritionally Variant Streptococci Treatment failure and relapse appear to be more common in cases of endocarditis due to nutritionally variant streptococci than in those due to the usual viridans streptococci. Thus, the addition of gentamicin (1 mg/kg every 8 h for patients with normal renal func­ tion) to the penicillin regimen is recommended for endocarditis due to the nutritionally variant organisms. ■ ■OTHER STREPTOCOCCI Streptococcus suis is an important pathogen in swine and has been reported to cause meningitis in humans, usually in individuals with occupational exposure to pigs. S. suis has been reported to be the most common cause of bacterial meningitis in Vietnam, and it has been responsible for outbreaks in China. Strains of S. suis associated with human infections have generally reacted with Lancefield group R typ­ ing serum and sometimes with group D typing serum as well. Isolates may be α- or β-hemolytic and are sensitive to penicillin. Streptococcus iniae, a pathogen of fish, has been associated with infections in humans who have handled live or freshly killed fish. Cellulitis of the hand is the most common form of human infection, although bacteremia and endocarditis have been reported. Anaerobic streptococci, or pepto­ streptococci, are part of the normal flora of the oral cavity, bowel, and vagina. Infections caused by the anaerobic streptococci are discussed in Chap. 182.

36 - 154 Enterococcal Infections

154 Enterococcal Infections

■ ■FURTHER READING Bruckner L, Gigliotti F: Viridans group streptococcal infections among children with cancer and the importance of emerging antibi­ otic resistance. Semin Pediatr Infect Dis 17:153, 2006. Parks T et al: Polyspecific intravenous immunoglobulin in clindamy­ cin-treated patients with streptococcal toxic shock syndrome: A systematic review and meta-analysis. Clin Infect Dis 67:1434, 2018. Raabe V, Shane A: Group B Streptococcus (Streptococcus agalactiae), in Gram-Positive Pathogens, 3rd ed, Fischetti V et al (eds). Washing­ ton, DC, ASM Press, 2019, pp 228–238. Shulman ST et al: Clinical practice guideline for the diagnosis and management of group A streptococcal pharyngitis: 2012 update by the Infectious Diseases Society of America. Clin Infect Dis 55:1279, 2012. Stevens DL, Bryant AE: Necrotizing soft tissue infections. N Engl J Med 377:2253, 2017. Xie O et al: Inter-species gene flow drives ongoing evolution of Strepto­ coccus pyogenes and Streptococcus dysgalactiae subsp. equisimilis. Nat Commun 15:2286, 2024. William R. Miller, Cesar A. Arias,

Barbara E. Murray*

Enterococcal Infections Enterococci have been recognized as potential human pathogens for well over a century, but only in recent years have these organisms acquired prominence as important causes of nosocomial infections. The ability of enterococci to survive and/or disseminate in the hospital environment and to acquire antibiotic resistance determinants makes the treatment of some enterococcal infections in critically ill patients a difficult challenge. Enterococci were first mentioned in the French literature in 1899; the “entérocoque” was found in the human gastro­ intestinal tract. The first pathologic description of an enterococcal infection dates to the same year. A clinical isolate from a patient who died as a consequence of endocarditis was initially designated Micro­ coccus zymogenes, was later named Streptococcus faecalis subspecies zymogenes, and would now be classified as Enterococcus faecalis. The ability of this isolate to cause severe disease in both rabbits and mice illustrated its potential lethality in the appropriate settings. ■ ■MICROBIOLOGY AND TAXONOMY Enterococci are gram-positive organisms. In clinical specimens, they are usually observed as single cells, diplococci, or short chains (Fig. 154-1), although long chains are noted with some strains. Enterococci were originally classified as streptococci because organ­ isms of the two genera share many morphologic and phenotypic char­ acteristics, including a generally negative catalase reaction. Only DNA hybridization studies and later 16S rRNA sequencing clearly demon­ strated that enterococci should be grouped as a genus distinct from the streptococci. Unlike the majority of streptococci, enterococci hydrolyze esculin in the presence of 40% bile salts and grow at high salt concen­ trations (e.g., 6.5%) and at high temperatures (46°C). Enterococci are usually reported by the clinical laboratory to be nonhemolytic on the basis of their inability to lyse the ovine or bovine red blood cells (RBCs) commonly used in agar plates; however, some strains of E. faecalis do lyse RBCs from humans, horses, and rabbits due to the presence of an acquired hemolysin/cytolysin gene. The majority of clinically relevant enterococcal species hydrolyze pyrrolidonyl-β-naphthylamide (PYR); *Deceased.

FIGURE 154-1  Gram’s stain of cultured blood from a patient with enterococcal bacteremia. Oval gram-positive bacterial cells are arranged as diplococci and short chains. (Courtesy of Audrey Wanger, PhD.) CHAPTER 154 this characteristic is helpful in differentiating enterococci from organ­ isms of the Streptococcus gallolyticus group (formerly known as S. bovis, which includes S. gallolyticus, S. pasteurianus, and S. infantarius) and from Leuconostoc species. Although many species of enterococci have been isolated from human infections, the overwhelming majority of cases are caused by two species, E. faecalis and E. faecium. Less fre­ quently isolated species include Enterococcus gallinarum, E. durans, E. hirae, and E. avium. Enterococcal Infections ■ ■PATHOGENESIS Enterococci are normal inhabitants of the large bowel of human adults, although they usually make up <1% of the culturable intestinal micro­ biota. In the healthy human host, enterococci are typical symbionts that coexist with other gastrointestinal bacteria; in fact, the utility of certain enterococcal strains as probiotics in the treatment of diarrhea suggests their possible role in maintaining the homeostatic equilibrium of the bowel. These commensals play a role in colonization resistance, or the ability of a healthy gastrointestinal microbiota to impede the establish­ ment of a population of drug-resistant bacteria such as vancomycinresistant enterococci (VRE). Colonization resistance arises from a complex set of metabolic and immunologic interactions between the host, pathogen, and intestinal microbiota, many of which are disrupted in hospitalized or chronically ill patients. Several studies have shown that a higher level of gastrointestinal colonization is a critical factor in the pathogenesis of enterococcal infections. Physical factors, such as stomach pH and the mucin layer on the interior of the intestinal lumen, provide a barrier and limit pathogen access to the intestinal epithelium. In the hospital setting, administration of medications that suppress stomach acid secretion, or degradation of the mucin layer by gut commensals during periods of decreased oral intake, can disrupt these protective layers. The avail­ ability of specific carbon sources also has been shown to influence the ability of enterococci to colonize the intestine. Depletion of fructose by members of the genus Olsenella leads to impaired growth and a decrease in the enteric burden of VRE in a mouse-colonization model, while the presence of lactose favors the outgrowth of enterococci. Fur­ ther, enterococci may potentiate the fitness and virulence of pathogens such as Clostridioides difficile by altering the availability of fermentable amino acids, by promoting C. difficile fitness, and by depletion of argi­ nine, which leads to increased C. difficile toxin production.

One of the most important factors that promotes increased gastro­ intestinal colonization by enterococci is the administration of antimi­ crobial agents since enterococci are intrinsically resistant to a variety of commonly used antibacterial drugs. In particular, antibiotics that are excreted in the bile and have broad-spectrum activity (e.g., certain cephalosporins and antimicrobials with anaerobic activity) are usually associated with the recovery of higher numbers of enterococci from feces. However, the increased colonization by hospital-associated strains of E. faecium in the presence of antimicrobial agents appears to be due to more than the simple filling of a “biologic niche.” Studies of coloniza­ tion dynamics in mouse intestines suggest host-derived antimicrobial peptides produced by the innate immune system (such as the lectin RegIIIγ) and compounds such as lantibiotics or bacteriocins produced by members of the microbiota itself are important mediators of coloni­ zation resistance. Activation of Toll-like receptors by lipopolysaccharide (an important component of the gram-negative cell envelope) leads, in mice, to increased production of RegIIIγ, and loss of this stimulation by antibiotic-induced disruptions of commensal gram-negative bacteria impairs clearance of VRE from the intestines. Similarly, antimicrobial lantibiotics produced by commensal bacteria (such as Blautia pro­ ducta) are active against VRE in vitro, but this organism may require a cooperative partnership with other members of the microbiota to effectively provide colonization resistance. Disruption of these partner­ ships by antibiotic administration can lead to an environment where VRE can flourish. Another factor that may contribute to enterococcal survival in the gastrointestinal tract is the production of bacteriocins (molecules that kill competing bacteria). Strains of E. faecalis harboring pheromone-producing plasmids that code for bacteriocins are capable of outcompeting enterococci lacking such plasmids, and conjugative transfer of these elements enhances the survival of recipient enterococci. In the absence of antibiotics, hospital-associated lineages of E. faecium seem to be less adapted for survival in the gastrointestinal tract than are commensal E. faecium strains. Studies examining the rate of carriage of VRE in patients after discharge from the hospital document a median time to clearance between 2 and 4 months in patients without ongoing risk factors, such as continued antibiotic use, residence in a long-term care facility, or need for hemodialysis.

PART 5 Infectious Diseases Colonization by enterococci may also have important effects on the treatment of noninfectious syndromes. In hematopoietic stem cell transplant patients, domination of the gastrointestinal tract by entero­ cocci is associated with increased inflammation and a greater severity of illness and mortality in graft-versus-host disease. Certain enterococ­ cal species, such as E. faecium, E. durans, and E. hirae, modulate the host immune response and can potentiate the activity of anti-PD-L1 immune checkpoint inhibitor therapy. This effect is mediated by the presence of salA, a gene which encodes a peptidoglycan hydrolase with D,L-endopeptidase activity. The muropeptides generated by SalA induce host activation of NF-κB via NOD2, leading to an enhanced antitumor immune response. Several vertebrate, worm, and insect models have been developed to study the role of possible pathogenic determinants in both E. faecalis and E. faecium. Three main groups of virulence factors may increase the ability of enterococci to colonize the gastrointestinal tract and/or cause disease. The first group, enterococcal secreted factors, are mol­ ecules released outside the bacterial cell that contribute to the process of infection. The best studied of these molecules include enterococ­ cal hemolysin/cytolysin and two enterococcal proteases (gelatinase and serine protease). Enterococcal cytolysin is a heterodimeric toxin produced by some strains of E. faecalis that is capable of lysing human (as well as equine but not ovine) RBCs as well as polymorphonuclear leukocytes and macrophages. A newly described pore-forming toxin, Epx, specifically targets human leukocyte antigen type 1, and has been shown to be active against peripheral blood mononuclear cells and intestinal organoids in vitro. E. faecalis gelatinase and serine protease are thought to mediate virulence by several mechanisms, including the degradation of host tissues and the modification of critical components of the immune system. Mutants lacking the genes corresponding to these proteins are highly attenuated in experimental animal models of peritonitis, endocarditis, and endophthalmitis.

A second group of virulence factors, enterococcal surface components, includes adhesins and is thought to contribute to bacterial attachment to extracellular matrix molecules in the human host. Several molecules on the surface of enterococci have been characterized and shown to play a role in the pathogenesis of enterococcal infections. Among the characterized adhesins is aggregation substance of E. faecalis, which mediates the attachment of bacterial cells to each other, thereby facilitating conjugative plasmid exchange. Several lines of evidence indicate that aggregation substance and enterococcal cytolysin act synergistically to increase the virulence potential of E. faecalis strains in experimental endocarditis. The surface protein adhesin of collagen of E. faecalis (Ace) and its E. faecium homologue (Acm) are microbial surface components adhering to matrix molecules (MSCRAMMs); they recognize adhesive matrix molecules involved in bacterial attach­ ment to host proteins such as collagen, fibronectin, and fibrinogen. Both Ace and Acm are collagen adhesins that are important in the pathogenesis of experimental endocarditis. Pili of both E. faecalis and E. faecium are important mediators of attachment to and invasion of host tissues. Mutants of E. faecalis lacking pili are attenuated in biofilm production, experimental endocarditis, and urinary tract infections (UTIs). Other surface proteins that share structural homology with MSCRAMMs and appear to play a role in enterococcal attachment to the host and in virulence include the E. faecalis surface protein Esp and its E. faecium homologue Espfm, the second collagen adhesin of E. faecium (Scm), the surface proteins of E. faecium (Fms), SgrA (which binds to components of the basal lamina), and EcbA (which binds to collagen type V). Additional surface components apparently asso­ ciated with pathogenicity include the Erl protein (a protein from the WxL family) and polysaccharides, which are thought to interfere with phagocytosis of the organism by host immune cells. Teichoic acids on the enterococcal surface appear to be immunogenic, and antibodies to these molecules are protective in some animal models. The third group of virulence factors has not been well character­ ized but includes the E. faecalis stress protein Gls24, which has been associated with enterococcal resistance to bile salts and appears to be important in the pathogenesis of endocarditis, and the hylEfm-containing plasmids of E. faecium, which are transferable between strains and increase gastrointestinal colonization by E. faecium. In mouse peritoni­ tis, acquisition of these plasmids increased the lethality of a commensal strain of E. faecium and enhanced colonization of the uroepithelium. A gene encoding a regulator of oxidative stress (AsrR) has been identified as an important virulence factor of E. faecium. ■ ■EPIDEMIOLOGY According to data collected from 2018 to 2021 by the National Health­ care Safety Network of the Centers for Disease Control and Prevention, enterococci are the third most common isolates (after Escherichia coli and staphylococci) from hospital-associated infections in the United States. Although E. faecalis remains the predominant species recovered from nosocomial infections, the isolation of E. faecium has increased substantially in the past 20 years and accounts for approxi­ mately one-third of all enterococcal infections identified to the species level. This point is important, since E. faecium is by far the most resis­ tant and challenging enterococcal species to treat. More than 90% of E. faecium isolates are resistant to ampicillin (historically the most effec­ tive β-lactam agent against enterococci), while ampicillin resistance in E. faecalis is uncommon. Vancomycin resistance in E. faecium isolates ranges from 50–70% in acute care hospitals in the United States to up to 80% in long-term care facilities. Resistance to vancomycin in E. faecalis isolates is less common, with a higher prevalence in long-term care facilities (10–12%) than in acute care hospitals (2–5%). The dynamics of enterococcal transmission and dissemination in the hospital environment have been extensively studied, with a focus on VRE. These studies have revealed that VRE colonization of the gastrointestinal tract is a critical step in the development of enterococ­ cal disease and that a substantial proportion of patients colonized with VRE remain colonized for prolonged periods (sometimes >1 year) and are more likely than patients without VRE colonization to develop an Enterococcus-related illness (e.g., bacteremia). Important factors

associated with VRE colonization and persistence in the gut include prolonged hospitalization; long courses of antibiotic therapy; hospi­ talization in long-term care facilities, surgical units, and/or intensive care units; organ transplantation; renal failure (particularly in patients undergoing hemodialysis) and/or diabetes; high APACHE (Acute Physiology and Chronic Health Evaluation) scores; and physical prox­ imity to patients infected or colonized with VRE or these patients’ rooms. Once a patient becomes colonized with VRE, several key factors are involved in the organisms’ dissemination in the hospital environ­ ment. VRE can survive exposure to heat and certain disinfectants and have been found on numerous inanimate objects in the hospital, including bed rails, medical equipment, doorknobs, gloves, telephones, and computer keyboards. Thus, health care workers and the environ­ ment play pivotal roles in enterococcal transmission from patient to patient, and infection control measures are crucial in breaking the chain of transmission. Moreover, two meta-analyses have found that, independent of the patient’s clinical status, VRE infection increases the risk of death over that among individuals infected with a glycopeptidesusceptible enterococcal strain. The epidemiology of enterococcal disease and the emergence of VRE have followed slightly different trends in other parts of the world than in the United States. In Europe, the emergence of VRE in the mid-1980s was seen primarily in isolates recovered from animals and healthy humans rather than from hospitalized patients. The presence of VRE was associated with the use of the glycopeptide avoparcin as a growth promoter in animal feeds; this association prompted the European Union to ban the use of this compound in animal husbandry in 1996. However, after an initial decrease in the isolation of VRE from animals and humans, the prevalence of hospital-associated VRE infections has slowly increased, from 8.1% in 2012 to 17.6% in 2022. Furthermore, there are important regional differences in rates of van­ comycin resistance among E. faecium, with rates below 5–10% in most of western Europe and Scandinavia, and 25–50% across much of south­ ern and eastern Europe. Despite regional differences, Europe has seen a general trend of increasing rates of VRE over the past decade, although these rates continue to be much lower than in the United States. The reasons are not totally understood, but it has been postulated that this difference is related to the higher levels of human antibiotic use in the United States. Recent data have also shown increasing rates of entero­ coccal resistance to vancomycin in Latin American countries, with 34% of clinical E. faecium isolates found to be resistant in a multicenter study including hospitals from Colombia, Venezuela, Ecuador, and Peru. In Asia, rates of vancomycin resistance among enterococci appear to be similar to those in U.S. hospitals. The ability to sequence bacterial genomes has increased our understanding of bacterial diversity, evolution, pathogenesis, and mechanisms of antibiotic resistance. Ongoing collection of enterococcal isolates as a part of epidemiologic surveillance has allowed researchers to trace the evolutionary trajectory of enterococci from their origin to the emergence of hospital-adapted clones. Sequence analysis suggests the genus appeared ~400 million years ago with the advent of terrestrial animals. Several key features aided in this transi­ tion, including the ability to recombine large portions of chromosomal DNA from the core genome and a malleable accessory genome consist­ ing of plasmids, phages, and mobile genetic elements. This genomic plasticity contributes to the rising rates of antibiotic resistance seen within the genus and, in particular, in E. faecium. A large proportion of the genomes available for analysis belong to E. faecium, due to its importance as a nosocomial pathogen and the epidemiologic surveillance projects to track the spread of vancomycin-

resistant strains. The population can be divided into two large groups, or clades, of organisms: a hospital-associated clade A and a commu­ nity-associated clade B. The hospital-associated clade appears to be evolving rapidly via a series of historic and ongoing recombination events. Certain regions of the chromosome show a convergence of genomic diversity both within and between clades, suggesting there are several “hot spots” for recombination. Importantly, these regions contain the polysaccharide biosynthesis cluster, which has been impli­ cated in altering the antigenic capsular polysaccharide in streptococci,

and the LiaFSR locus, a major cell envelope stress system that medi­ ates resistance to membrane-active antibiotics and host antimicrobial peptides. Strains belonging to clade A are more frequently identified as isolates causing invasive disease and are more likely to carry drug resistance determinants, whereas clade B isolates largely retain a sus­ ceptible phenotype.

One reason for the propensity of clade A strains to acquire resistance determinants is that they more frequently lack a functional CRISPRCas system (short for clustered regularly interspaced short palindromic repeats). These systems serve as a primitive “immune system” and pro­ vide a genome defense for bacteria to protect them from foreign DNA, such as phages, but they also serve to reduce the frequency of acquisi­ tion of resistance genes borne on mobile genetic elements. Another reason for their survival in the hospital environment is that clade A isolates tend to possess alleles of penicillin-binding protein 5 (PBP5) associated with high-level β-lactam resistance in E. faecium and may express higher levels of this enzyme than commensal strains. A notable feature of the distribution of strains in clade A in some studies is that they share a relatively recent common ancestor with E. faecium of livestock origin. Use of antibiotics in animal husbandry as both therapeutics and growth promoters has been linked to resistance in several important contexts, including glycopeptides, as mentioned above. This phenomenon suggests that continued surveillance, and an expanding understanding of the population structure of enterococci, may help identify potential reservoirs of resistance and inform policy to limit their spread. ■ ■CLINICAL SYNDROMES CHAPTER 154 Urinary Tract Infection and Prostatitis  Enterococci are wellknown causes of nosocomial UTI—the most common infection caused by these organisms (Chap. 140). Enterococcal UTIs are usually associ­ ated with indwelling catheterization, instrumentation, or anatomic abnormalities of the genitourinary tract, and it is often challenging to differentiate between true infection and colonization (particularly in patients with chronic indwelling catheters). Their role as pathogens in otherwise healthy premenopausal woman with acute cystitis is less clear, with data from one study suggesting that enterococci recovered from midstream urine cultures were not predictive of bacteriuria in a subsequent catheterized specimen. The presence of leukocytes in the urine in conjunction with systemic manifestations (e.g., fever) or local signs and symptoms of infection with no other explanation and a positive urine culture (≥105 CFU/mL) suggests the diagnosis. Moreover, enterococcal UTIs often occur in critically or chronically ill patients whose comorbidities may obscure the diagnosis. In many cases, removal of the indwelling catheter may suffice to eradicate the organism without specific antimicrobial therapy. In rare circumstances, UTIs caused by enterococci may run a complicated course, with the development of pyelonephritis and perinephric abscesses that may be a portal of entry for bloodstream infections (see below). Enterococci are also known causes of chronic prostatitis, particularly in men whose urinary tract has been manipulated surgically or endoscopically. These infections can be difficult to treat since the agents most potent against enterococci (i.e., aminopenicillins and glycopeptides) penetrate pros­ tatic tissue poorly. Chronic prostatic infection can be a source of recur­ rent enterococcal bacteremia. Enterococcal Infections Bacteremia and Endocarditis  Bacteremia without endocarditis is another frequently encountered presentation of enterococcal dis­ ease. Intravascular catheters and other devices are commonly associ­ ated with these bacteremic episodes (Chap. 147). Other well-known sources of enterococcal bacteremia include the gastrointestinal and hepatobiliary tracts; pelvic and intraabdominal foci; and, less fre­ quently, wound infections, UTIs, and bone infections. In the United States, enterococci are ranked second (after staphylococci) as etiologic agents of central line–associated bacteremia. Patients with enterococcal bacteremia usually have comorbidities and have been in the hospital for prolonged periods; they commonly have received several courses of antibiotics. Several studies indicate that the isolation of E. faecium from the blood may lead to worse outcomes and higher mortality

rates than when other enterococcal species are isolated; this finding may be related to the higher prevalence of vancomycin and ampicillin resistance in E. faecium than in other enterococcal species, with the consequent reduction of therapeutic options. In some cases (usually when the gastrointestinal tract is the source), enterococcal bacteremia may be polymicrobial, with gram-negative organisms isolated at the same time. In addition, several cases have been documented in which enterococcal bacteremia was associated with Strongyloides stercoralis hyperinfection syndrome in immunocompromised patients.

Enterococci are important causes of community- and health care– associated endocarditis, ranking second after staphylococci in the latter infections, and recent studies have implicated E. faecalis as the leading cause of endocarditis after transcatheter aortic valve implan­ tation (TAVI). The presumed initial source of bacteremia leading to endocarditis is the gastrointestinal or genitourinary tract—e.g., in patients who have malignant and inflammatory conditions of the gut or have undergone procedures in which these tracts are manipulated. The affected patients tend to be male and elderly and to have other debilitating diseases and heart conditions. Both prosthetic and native valves can be involved; mitral and aortic valves are affected most often. Community-associated endocarditis (usually caused by E. faecalis) also occurs in patients with no apparent risk factors or cardiac abnormali­ ties. Endocarditis in women of childbearing age was well described in the past. The typical presentation of enterococcal endocarditis is a subacute course of fever, weight loss, malaise, and cardiac murmur; typical stigmata of endocarditis (e.g., petechiae, Osler’s nodes, Roth’s spots) are found in only a minority of patients. Atypical manifestations include arthralgias and manifestations of metastatic disease (splenic abscesses, hiccups, pain in the left flank, pleural effusion, and spondy­ lodiscitis). Embolic complications are variable and can affect the brain. Heart failure is a common complication of enterococcal endocarditis, and valve replacement may be critical in curing this infection, particu­ larly when multidrug-resistant organisms or major complications are involved. A clinical scoring system (DENOVA) has been proposed to help differentiate enterococcal bacteremia from true endocarditis and determine the need for echocardiography. The duration of therapy is usually 4–6 weeks, with more prolonged courses suggested for multi­ drug-resistant isolates in the absence of valvular replacement. Risk of relapse for endocarditis due to E. faecalis varies across studies, but it is higher in patients who are treated with antibiotics alone (2–11%) than in those who undergo surgery (0–3%); relapse may occur out to 1-year post-treatment. Meningitis  Enterococcal meningitis is an uncommon disease (accounting for only ~4% of meningitis cases) that is usually associ­ ated with neurosurgical interventions and conditions such as shunts, central nervous system (CNS) trauma, and cerebrospinal fluid (CSF) leakage. In some instances—usually in patients with a debilitating condition, such as cardiovascular or congenital heart disease, chronic renal failure, malignancy, receipt of immunosuppressive therapy, or HIV/AIDS—presumed hematogenous seeding of the meninges is seen in infections such as endocarditis or bacteremia. Fever and changes in mental status are common, whereas overt meningeal signs are less so. CSF findings are consistent with bacterial infection—i.e., pleocytosis, with a predominance of polymorphonuclear leukocytes (average, ~500/μL), an elevated serum protein level (usually >100 mg/dL), and a decreased glucose concentration (average, 28 mg/dL). Gram’s staining yields a positive result in about half of cases, with a high rate of organ­ ism recovery from CSF cultures; the most common species isolated are E. faecalis and E. faecium. Complications include hydrocephalus, brain abscesses, and stroke. As mentioned before for bacteremia, an associa­ tion with Strongyloides hyperinfection also has been documented. Intraabdominal, Pelvic, and Soft Tissue Infections  As men­ tioned above, enterococci are part of the commensal microbiota of the gastrointestinal tract and can produce spontaneous peritonitis in cirrhotic individuals and in patients undergoing chronic ambulatory peritoneal dialysis (Chap. 137). These organisms are commonly found (usually along with other bacteria, including enteric gram-negative species and anaerobes) in clinical samples from intraabdominal and PART 5 Infectious Diseases

pelvic collections. The presence of enterococci in intraabdominal infections is sometimes considered to be of little clinical relevance. Several studies have shown that the role of enterococci in intraabdomi­ nal infections originating in the community and involving previously healthy patients is minor since surgery and broad-spectrum antimi­ crobial drugs that do not target enterococci are often sufficient to treat these infections successfully. In the past few decades, however, these organisms have become prominent as a cause of intraabdominal infec­ tions in hospitalized patients because of the emergence and spread of vancomycin resistance among enterococci and the increase in rates of nosocomial infections due to multidrug-resistant E. faecium isolates. In fact, several studies have now documented treatment failures due to enterococci, with consequently increased rates of postoperative com­ plications and death among patients with intraabdominal infections. Thus, anti-enterococcal therapy is recommended for nosocomial peri­ tonitis in immunocompromised and severely ill patients who have had a prolonged hospital stay, have undergone multiple procedures, have persistent abdominal sepsis and collections, or have risk factors for the development of endocarditis (e.g., prosthetic or damaged heart valves). Conversely, specific treatment for enterococci in the first episode of intraabdominal infection originating in the community and affecting previously healthy patients with no important cardiac risk factors for endocarditis does not appear to be beneficial. Enterococci are commonly isolated from soft tissue infections (Chap. 134), particularly those involving surgical wounds (Chap. 147). In fact, these organisms rank third as agents of nosocomial surgical-site infections, with E. faecalis the most frequently isolated species. The clini­ cal relevance of enterococci in some of these infections—as in intraab­ dominal infections—is a matter of debate; differentiating between colonization and true infection is sometimes challenging, although in some cases, enterococci have been recovered from lung, liver, and skin abscesses. Diabetic foot and decubitus ulcers are often colonized with enterococci and may be the portal of entry for bone infections. Other Infections  Enterococci are well-known causes of neonatal infections, including sepsis (mostly late-onset), bacteremia, meningitis, pneumonia, and UTI. Outbreaks of enterococcal sepsis in neonatal units have been well documented. Risk factors for enterococcal disease in newborns include prematurity, low birth weight, indwelling devices, and abdominal surgery. Enterococci have also been described as etio­ logic agents of bone and joint infections, including vertebral osteomy­ elitis, usually in patients with underlying conditions such as diabetes or endocarditis. Similarly, enterococci have been isolated from bone infections in patients who have undergone arthroplasty or reconstruc­ tion of fractures with the placement of hardware. Since enterococci can produce a biofilm that is likely to alter the efficacy of anti-enterococcal agents, treatment of infections that involve foreign material is chal­ lenging, and removal of the hardware may be necessary to eradicate the infection. Rare cases of enterococcal pneumonia, lung abscess, and spontaneous empyema have been described. TREATMENT Enterococcal Infections GENERAL PRINCIPLES Enterococci are intrinsically resistant and/or tolerant to several antimicrobial agents. (Tolerance is defined as lack of killing by drug concentrations 32 times higher than the minimal inhibitory con­ centration [MIC].) Monotherapy for endocarditis with a β-lactam antibiotic (to which many enterococci are tolerant) has produced disappointing results, with high relapse rates after the end of therapy. However, the addition of an aminoglycoside to a cell wall– active agent (a β-lactam or a glycopeptide) increases cure rates and eradicates the organisms; moreover, this combination is synergistic and bactericidal in vitro. Therefore, for many decades, combina­ tion therapy with a cell wall–active agent and an aminoglycoside was the standard of care for endovascular infections caused by enterococci. This synergistic effect can be explained, at least in part,

by the increased penetration of the aminoglycoside into the bacte­ rial cell, presumably as a result of cell-wall alterations produced by the β-lactam (or glycopeptide). Nonetheless, attaining syner­ gistic bactericidal activity in the treatment of severe enterococcal infections—particularly those caused by E. faecium—has become increasingly difficult because of the development of resistance to virtually all antibiotics available for this purpose. The treatment of E. faecalis differs substantially from that of E. faecium (Tables 154-1 and 154-2), mainly because of differences in resistance profiles (see below). For example, resistance to ampicil­ lin and vancomycin is rare in E. faecalis, whereas these antibiotics are only infrequently useful against current isolates of E. faecium. Moreover, as a consequence of the challenges and therapeutic limi­ tations posed by the emergence of drug resistance in enterococci, valve replacement may need to be considered in the treatment of endocarditis caused by multidrug-resistant enterococci. Less severe infections are often related to indwelling intravascular catheters; removal of the catheter increases the likelihood of enterococcal eradication by a subsequent short course of appropriate antimicro­ bial therapy. CHOICE OF ANTIMICROBIAL AGENTS Among the β-lactams, the most active are the aminopenicillins (ampicillin, amoxicillin); next most active are penicillin G, imipe­ nem, and ureidopenicillins (i.e., piperacillin). Cephalosporins are not active as monotherapy. The only two aminoglycosides recom­ mended for synergistic therapy in severe enterococcal infections are gentamicin and streptomycin. This is because the most common acquired enzyme conferring high-level resistance to gentamicin also is active against tobramycin and amikacin but not streptomy­ cin, and the resistance mechanisms causing high-level resistance to streptomycin do not affect gentamicin. The use of amikacin is strongly discouraged because it is infrequently active, and tobramy­ cin should never be used for the treatment of E. faecium infections due to the presence of a chromosomally encoded, species-specific, tobramycin-modifying enzyme. Aminoglycoside monotherapy should not be employed. Vancomycin is an alternative to β-lactam drugs for the treatment of E. faecalis infections but is less useful against E. faecium because resistance is common. As mentioned above, use of the aminoglycoside–ampicillin com­ bination for E. faecalis infections has become increasingly prob­ lematic because of toxicity in critically ill patients and increased rates of high-level resistance to aminoglycosides. An observational, nonrandomized, comparative study encompassing a multicenter cohort was conducted in 17 Spanish hospitals and 1 Italian hospital; the results indicated that a 6-week course of ampicillin plus ceftri­ axone is as effective as ampicillin plus gentamicin in the treatment of E. faecalis endocarditis, with less risk of toxicity. Therefore, this regimen should be considered in patients at risk for aminoglycoside toxicity or those with isolates displaying high-level resistance to aminoglycosides, and it is now recommended as first-line therapy for E. faecalis endocarditis. Use of dual β-lactam regimens for ampicillin-susceptible isolates of E. faecium has not been studied in the clinical setting. Limited in vitro data suggest that synergism between ampicillin and ceftriaxone is not reliably active against these isolates. Linezolid is the only agent approved by the U.S. Food and Drug Administration (FDA) for the treatment of VRE infections (Table 154-2). Linezolid is not bactericidal, and its use in severe endovas­ cular infections has produced mixed results; therefore, it is recom­ mended only as an alternative to other agents for such infections. In addition, linezolid may cause significant toxicities (thrombocytope­ nia, peripheral neuropathy, optic neuritis, and lactic acidosis) when used in regimens given for >2 weeks. Nonetheless, linezolid may play a role in the treatment of enterococcal meningitis and other CNS infections, although clinical data are limited. The lipopeptide daptomycin is a bactericidal antibiotic with in vitro activity against all enterococci. Although daptomycin is not approved by the FDA for the treatment of VRE or E. faecium

TABLE 154-1  Suggested Regimens for the Management of Infections Caused by Enterococcus faecalis CLINICAL SYNDROME SUGGESTED THERAPEUTIC OPTIONSa Endovascular infections (including endocarditis) • Ampicillinb (12 g/d IV in divided doses q4h) plus ceftriaxone (2 g IV q12h)  • Ampicillinb (12 g/d IV in divided doses q4h or by continuous infusion) or penicillin (18–30 mU/d IV in divided doses q4h or by continuous infusion) plus an aminoglycosidec  • Vancomycind (15 mg/kg IV per dose) plus an aminoglycosidec • High-dose daptomycine ± another active agentf • Ampicillinb plus imipenem Nonendovascular bacteremiag • Ampicillinb (12 g/d IV in divided doses q4h) or penicillin (18 mU/d IV in divided doses q4h) ± an aminoglycosidec or ceftriaxone  • Vancomycind (15 mg/kg IV per dose) • High-dose daptomycine ± another active agentf • Linezolid (600 mg IV/PO q12h) Meningitis • Ampicillin (20–24 g/d IV in divided doses q4h) or penicillin (24 mU/d IV in divided doses q4h) plus an aminoglycosidec,h and consider adding ceftriaxone

(2 g IV q12h)   • Vancomycin (500–750 mg IV q6h)d plus an aminoglycosidec or rifampin • Linezolid • High-dose daptomycine (plus intrathecal daptomycin) ± CHAPTER 154 another active agentf Urinary tract infections (uncomplicated) • Amoxicillin (500 mg PO q8h)   • Fosfomycin (3 g PO, one dose)i • Ampicillin (500 mg IV or PO q6h) • Nitrofurantoin (100 mg PO q6h) Enterococcal Infections aAuthors’ preferences are underlined for each category; many of the regimens are off-label. bIn rare cases, β-lactamase-producing isolates may be present. Because these isolates are not detected by conventional determination of the minimal inhibitory concentration, additional tests (e.g., the nitrocefin disk) are recommended for isolates from endocarditis. The use of ampicillin/sulbactam (12–24 g/d) is suggested in these cases. If choosing a β-lactam other than ampicillin for an invasive infection, susceptibility testing for penicillin should be performed to identify penicillin-resistant, ampicillin-susceptible (PRASEF) isolates. Limited in vitro data suggest that synergistic killing by dual β-lactams (i.e., ampicillin and ceftriaxone) may be compromised in PRASEF isolates. Some suggest caution with the combination of ampicillin and ceftriaxone in PRASEF isolates and would consider an alternate therapy (such as daptomycin plus ampicillin) guided by susceptibility testing in cases of persistent bacteremia or signs of clinical failure. cOnly if the organism does not exhibit high-level resistance (HLR) to aminoglycosides. This test is performed by the clinical microbiology laboratory only for gentamicin or streptomycin (growth of enterococci on agar containing gentamicin [500 μg/mL] or streptomycin [2000 μg/mL]). If HLR is documented, the aminoglycoside will not act synergistically with the other agent in the combination. However, HLR to one of these aminoglycosides does not indicate resistance to the other agent (as reported individually). HLR to gentamicin implies lack of synergism with tobramycin and with amikacin. Gentamicin (1–1.5 mg/kg IV q8h) and streptomycin (15 mg/ kg per day IV/IM in two divided doses) are the only two recommended aminoglycosides. dVancomycin is recommended only as an alternative to β-lactam agents in cases of allergy or toxicity plus the inability to desensitize. Specific pharmacologic targets for trough concentrations have not been clinically evaluated in enterococcal bacteremia; trough concentrations of 15–20 mg/L have been associated with increased rates of nephrotoxicity. Vancomycin-resistant strains of E. faecalis have been reported. eConsider doses of 10–12 mg/kg once daily, adjusted for renal function. Monitoring of creatine phosphokinase levels is recommended throughout therapy because of possible rhabdomyolysis. fPotentially active agents may include an aminoglycoside (if HLR is not detected), ampicillin, ceftaroline, tigecycline, eravacycline, or a fluoroquinolone (which, if the isolate is susceptible, may be favored in meningitis). The presence of mutations in LiaFSR seems to increase susceptibility to ampicillin and ceftaroline, and combinations of daptomycin with these compounds are bactericidal in vitro against such strains. gIn selected cases of catheter-associated bacteremia, removal of the catheter and a short course of therapy (~5–7 days) may be sufficient. A single positive blood culture that is likely to be associated with a catheter in a patient who is otherwise doing well may not require therapy after removal of the catheter. Patients at high risk for endovascular infections or with severe disease may benefit from synergistic combination therapy. hThe addition of intrathecal or intraventricular therapy with gentamicin (2–10 mg/d) if the organism does not exhibit HLR or with vancomycin (10–20 mg/d) when the isolate is susceptible has been suggested by some authorities. The addition of systemic rifampin (a good CSF-penetrating agent) may be considered. The combination of ampicillin and ceftriaxone may have clinical benefit (by analogy with endocarditis), but no cases treated with this combination have been reported; the authors would use this combination. iApproved by the U.S. Food and Drug Administration only for uncomplicated urinary tract infections caused by vancomycin-susceptible E. faecalis.

TABLE 154-2  Suggested Regimens for the Management of Infections Caused by Vancomycin- and Ampicillin-Resistant Enterococcus faecium CLINICAL SYNDROME SUGGESTED THERAPEUTIC OPTIONSa Endovascular infections (including endocarditis) • High-dose daptomycinb plus another agentc ± an aminoglycosided  • Linezolid (600 mg IV q12h) • High-dose ampicillin (if MIC is ≤64 μg/mL) ± an aminoglycosided Nonendovascular bacteremiae • High-dose daptomycinb ± another agentc ± an aminoglycosided  • Linezolid (600 mg IV q12h) Meningitis • Linezolid (600 mg IV q12h) ± another

CSF-penetrating active agentf  • High-dose daptomycinb (plus intraventricular daptomycin) ± another CSF-penetrating active agentf,g Urinary tract infections • Fosfomycin (3 g PO, one dose)h  • Nitrofurantoin (100 mg PO q6h) • Ampicillin or amoxicillin (2 g IV/PO q4–6h)i aAuthors’ preferences are underlined for each category; many of these regimens are off-label. bDaptomycin at doses of 10–12 mg/kg once daily is suggested (off-label). Close monitoring of creatine phosphokinase levels is recommended throughout therapy because of possible rhabdomyolysis. cPotentially active agents may include ampicillin or ceftaroline (even if the infecting strain is resistant in vitro), tigecycline, eravacycline, or a fluroquinolone. In vitro synergism of daptomycin with ampicillin or ceftaroline has been observed against some isolates that subsequently become nonsusceptible to daptomycin during therapy. The synergism of daptomycin and β-lactams is associated with mutations in LiaFSR. The authors prefer use of combination therapy for all endovascular infections or for bacteremic patients in whom source control cannot be achieved, as in vitro data suggest combination therapy may prevent or delay the emergence of daptomycin resistance. dOnly if the organism does not exhibit high-level resistance to aminoglycosides (see Table 154-1, footnote c). eIn selected cases of catheter-associated bacteremia, removal of the catheter and a short course of therapy (~5–7 days) may be sufficient. A single positive blood culture that is likely to be associated with a catheter in a patient who is otherwise doing well may not require therapy after removal of the catheter. fFluoroquinolones (e.g., moxifloxacin) and rifampin (if the isolate is susceptible to each agent) reach therapeutic levels in the cerebrospinal fluid. gIntrathecal gentamicin (2–10 mg/d) if high-level resistance is not detected. Intraventricular daptomycin has been used in two cases of meningitis. hApproved by the U.S. Food and Drug Administration (FDA) only for uncomplicated urinary tract infections caused by vancomycin-susceptible E. faecalis. iConcentrations of amoxicillin and ampicillin in urine far exceed those in serum and may be potentially effective even against isolates with high MICs. Doses up to 12 g/d are suggested for isolates with MICs of ≥64 μg/mL. Of note, quinupristin-dalfopristin (Q-D) lost FDA approval for infections due to vancomycin-resistant Enterococcus, and it was discontinued by the manufacturer in 2022. PART 5 Infectious Diseases infections, it has been used alone (at high dosage) or in combina­ tion with other agents (ampicillin, ceftaroline, and tigecycline) with apparent success against multidrug-resistant enterococcal infections (Tables 154-1 and 154-2). The main adverse reactions to daptomycin are elevated creatine phosphokinase levels and eosinophilic pneumonitis (rare). Daptomycin is not useful against pulmonary infections because the pulmonary surfactant inhibits its antibacterial activity. Several meta-analyses have examined the question of which agent should be preferred for VRE bacteremia—linezolid or dap­ tomycin. These studies concluded either no difference between the two drugs or favored linezolid due to lower all-cause and infectionrelated mortality, but they were limited by small patient num­ bers and heterogenous outcomes. A subsequent large retrospective observational study from the Veterans Affairs database reported lower rates of all-cause mortality at 30 days and less microbiologic failure (i.e., positive cultures despite therapy) with daptomycin compared to linezolid. One important observation from these investigations is that the efficacy of daptomycin is dependent on the dose, with improved outcomes seen with high-dose dapto­ mycin therapy (≥10 mg/kg) compared to standard-dose therapy (6 mg/kg). Genome sequencing of clinical isolates has revealed that mutations in genes associated with daptomycin resistance are not uncommon (see “Antimicrobial Resistance,” below) and were associated with the emergence of resistance to daptomycin at lower

simulated dosing regimens (6 mg/kg) in experimental models of infection. These data led the Clinical Laboratory and Standards Institute (CLSI) to change the daptomycin breakpoints in 2019. For E. faecium, all isolates with an MIC of ≤4 mg/L are placed in a “susceptible dose-dependent” category based on a dosing regimen of 8–12 mg/kg, while those with an MIC ≥8 mg/L are considered resistant. For all other enterococci, isolates are considered suscep­ tible with an MIC of ≤2 mg/L, intermediate with an MIC of 4 mg/L, and resistant with an MIC ≥8 mg/L. The glycylcycline drug tigecycline is active in vitro against all enterococci, regardless of the isolates’ vancomycin susceptibility. However, its use as monotherapy for endovascular or severe entero­ coccal infections is not recommended because of low attainable blood levels. Newer-generation tetracyclines, such as eravacycline and omadacycline, also display in vitro activity, but their role in the treatment of enterococcal infections remains to be evaluated. Telavancin, a lipoglycopeptide approved by the FDA for the treatment of skin and soft tissue infections as well as hospitalassociated pneumonia, is active against vancomycin-susceptible enterococci but not VRE. Likewise, dalbavancin, a lipoglycopeptide antibiotic with a long terminal half-life, has FDA approval for skin and soft tissue infections due to susceptible strains of E. faecalis, but no activity against VRE. Oritavancin, a novel glycopeptide with activity against VRE, has been approved for the treatment of acute bacterial skin and soft tissue infections caused by susceptible organisms, including vancomycin-susceptible E. faecalis. The MICs of oritavancin against VRE are low, and this compound may be a promising drug for VRE treatment in the future. Lastly, tedizolid—a new oxazolidinone now available for clinical use—is approved only for the treatment of E. faecalis infections. Tedizolid is more potent than linezolid in vitro against VRE strains; however, its role in severe VRE infections remains to be determined. ANTIMICROBIAL RESISTANCE Resistance to β-lactam agents continues to be observed only infre­ quently in E. faecalis but is characteristic of E. faecium. The mechanism of ampicillin resistance in E. faecium is related to a penicillin-binding protein (PBP) designated PBP5, which is the critical target of β-lactam antibiotics. PBP5 exhibits low affinity for ampicillin and can synthesize cell wall in the presence of this antibiotic, even when other PBPs are inhibited. The version of this protein found in ampicillin-resistant hospital-associated strains has multiple amino-acid differences that even further decrease the affinity of PBP5 for ampicillin; recent structural work has demon­ strated that these changes lead to a more dynamic binding pocket with an increase in the hydrolysis of ampicillin at the active site, thus allowing for increased turnover of the enzyme. Increased pro­ duction of PBP5 is also seen in high-level ampicillin-resistant (e.g., MIC >32 μg/mL) clinical strains. In E. faecalis, ampicillin resistance is extremely rare; however, an emerging phenotype of penicillinresistant but ampicillin-susceptible E. faecalis (PRASEF) has been described. Similar to E. faecium, changes in the promoter of PBP4 (the E. faecalis homologue of PBP5) or the amino-acid sequence of the enzyme itself are associated with increases in the MIC of ampi­ cillin (although still below the susceptible breakpoint), penicillin, piperacillin, and imipenem; in vitro data suggest that these changes may also impair the synergistic activity of ampicillin and ceftriax­ one against some PRASEF isolates. Vancomycin is a glycopeptide antibiotic that inhibits cell-wall peptidoglycan synthesis in susceptible enterococci and has been widely used against enterococcal infections in clinical practice when the utility of β-lactams is limited by resistance, allergy, or adverse reactions. This effect is mediated by binding of the anti­ biotic to peptidoglycan precursors (UDP-MurNAc-pentapeptides) upon their exit from the bacterial cell cytoplasm. The interaction of vancomycin with the peptidoglycan is specific and involves the last two d-alanine residues of the precursor. The first isolates of VRE were documented in 1986, and vancomycin resistance (particularly in E. faecium) has since increased considerably around the world.

37 - 155 Diphtheria and Other Corynebacterial Infections

155 Diphtheria and Other Corynebacterial Infections

The mechanism involves the replacement of the last d-alanine residue of peptidoglycan precursors with d-lactate (e.g., VanA and VanB) or d-serine (e.g., VanC), with consequent high- and low-level resistance, respectively. There is significant heterogeneity among isolates, but either substitution substantially decreases the affinity of vancomycin for the peptidoglycan; with the d-lactate substitution, the affinity for binding to the pentapeptide precur­ sor is decreased by ~1000-fold. Vancomycin-resistant organisms also produce enzymes that destroy the d-alanine-d-alanine ending precursors, ensuring that additional binding sites for vancomycin are not available. The genes encoding the machinery responsible for vancomycin resistance are located in the van operon and likely originated in soil bacteria. Several variants of the operon have been described, but VanA is the most common in clinical isolates in the United States, Latin America, and Europe, whereas VanB isolates are more fre­ quent in Australia. Two enterococcal species, E. gallinarum and E. casseliflavus, have intrinsic low-level resistance to vancomycin due to the presence of the VanC operon in the chromosome. High-level resistance to aminoglycosides (of which gentamicin and streptomycin are the only two tested by clinical laboratories) abolishes the synergism observed between cell wall–active agents and the aminoglycoside. This important phenotype is routinely sought by the clinical laboratory in isolates from serious infec­ tions (Tables 154-1 and 154-2). Genes encoding aminoglycosidemodifying enzymes are usually the cause of high-level resistance to these compounds and are widely disseminated among enterococci, decreasing the options for the treatment of severe enterococcal infections. Additionally, ribosomal methyltransferases, enzymes that methylate rRNA and, as a consequence, disrupt the binding site for aminoglycosides, also can lead to high-level resistance. Resistance to daptomycin has now been well documented in both E. faecalis and E. faecium. Daptomycin exerts its action by complexing with calcium and binding to phosphatidylglycerol in the bacterial membrane. After binding, daptomycin forms oligo­ mers, with recent data suggesting that it displaces enzymes impor­ tant for cell envelope synthesis (MurG and PlsX) and that it can form a complex with lipid II molecules critical for cell-wall synthe­ sis, among other effects on the membrane. Resistance to this antibi­ otic in enterococci arises via two main pathways. The first involves mutations in genes that coordinate the cell-wall and cell-membrane stress response, most commonly a three-component system desig­ nated LiaFSR (for lipid II interfering antibiotics). These mutations lead to activation of the system, with increased expression of an extracellular protein known as LiaX capable of binding daptomycin and enhancing the signaling response. In clinical isolates, mutations in LiaFSR may lead to tolerance (loss of bactericidal activity)—usually in isolates with MICs near the daptomycin breakpoint (i.e., 3–4 mg/L). The second pathway involves changes in genes involved in phospholipid metabolism. It is thought that mutations priming the stress response system occur first, with the subsequent accrual of phospholipid changes leading to a fully resistant phenotype. Prior exposure to daptomycin has been identified as a risk factor for the emergence of daptomycin-resistant E. faecium in cancer patients. Resistance in the absence of exposure to the drug has also been well described, possibly due to the similarity of this antibiotic to antimicrobial peptides of the innate immune system. Thus, careful consideration of patient characteristics, bacterial phenotype, and daptomycin dose is warranted, and it is advisable to obtain infec­ tious diseases consultation in complicated VRE infections. The oxazolidinones (linezolid and tedizolid) act by binding to the ribosome and inhibiting the binding of aminoacyl-tRNAs, thus preventing protein synthesis. Resistance to this class of antibiotics is usually due to alterations of the binding site, either via mutations in the 23S rRNA genes or via the presence of an rRNA methylase. Since enterococci carry multiple copies of the gene encoding the 23S rRNA, prolonged exposure to oxazolidinones can select for increasing levels of resistance by favoring propagation of the resis­ tance allele via recombination. Changes in accessory ribosomal

proteins have also been associated with linezolid resistance and may act to mitigate the fitness defects of mutations in the rRNA. More concerning is the emergence of transferable resistance genes, which can readily move between enterococcal strains. Several of these genes were first recognized in bacterial isolates of animal origin, likely under the selective pressure of antibiotics such as florfenicol. The cfr (chloramphenicol-florfenicol resistance) gene encodes an rRNA methylase that modifies the 23S rRNA, leading to increases in the MIC of linezolid. Tedizolid tends to exhibit lower MICs in the presence of cfr; however, animal models suggest that some variants of the enzyme may compromise the activity of this drug. Two other transmissible resistance genes, optrA and poxtA, encode a ribosomal protection factor that has been implicated in linezolid resistance in enterococcal strains of human and animal origin. While still relatively rare to encounter in clinical practice, these determinants have been identified across the globe and could be an emerging source of resistance.

Newer tetracycline agents, such as tigecycline, omadacycline, and eravacycline, retain activity in the presence of typical tetra­ cycline resistance determinants, including drug efflux pumps and ribosomal protection factors. However, resistance has been docu­ mented and appears to be related to changes in the S10 ribosomal protein, which is situated near the binding site for the drug. Acknowledgment The authors dedicate this chapter to the memory of Dr. Barbara Murray, a trailblazer in infectious diseases and a fearless leader. Dr. Murray was an exceptional scientist and an avid adventurer. She will be remembered as an everlasting mentor who touched many lives with her wisdom. Her indelible legacy will remain with us forever. CHAPTER 155 ■ ■FURTHER READING Berge A et al: The DENOVA score efficiently identifies patients with monomicrobial Enterococcus faecalis bacteremia where echocardiog­ raphy is not necessary. Infection 47:45, 2019. Contreras GA et al: Contemporary clinical and molecular epidemiol­ Diphtheria and Other Corynebacterial Infections ogy of vancomycin-resistant enterococcal bacteremia: A prospective multicenter cohort study (VENOUS I). Open Forum Infect Dis 9: ofab616, 2021. Lebreton F et al: Tracing the enterococci from Paleozoic origins to the hospital. Cell 169:849, 2017. Rogers R, Rice LB: State-of-the-art review: Persistent enterococcal bacteremia. Clin Infect Dis 78:e1, 2024. Satlin MJ et al: Development of daptomycin susceptibility breakpoints for Enterococcus faecium and revision of the breakpoints for other enterococcal species by the Clinical and Laboratory Standards Institute. Clin Infect Dis 70:1240, 2020. William R. Bishai, John R. Murphy

Diphtheria and Other Corynebacterial Infections DIPHTHERIA Diphtheria is a nasopharyngeal and skin infection caused by Coryne­ bacterium diphtheriae. Toxigenic strains of C. diphtheriae produce a protein toxin that causes systemic toxicity, myocarditis, and polyneu­ ropathy. The toxin is associated with the formation of pseudomem­ branes in the pharynx during respiratory diphtheria. While toxigenic strains most frequently cause pharyngeal diphtheria, nontoxigenic strains commonly cause cutaneous disease.

■ ■ETIOLOGY C. diphtheriae is a gram-positive bacillus that is unencapsulated, nonmotile, and nonsporulating. The organism was first identified microscopically in 1883 by Klebs and a year later was isolated in pure culture by Löffler in Robert Koch’s laboratory. The bacteria have a characteristic club-shaped bacillary appearance and typically form clusters of parallel rays, or palisades, that are referred to as “Chinese characters.” The specific laboratory media recommended for the culti­ vation of C. diphtheriae rely upon tellurite, colistin, or nalidixic acid for the organism’s selective isolation from other autochthonous pharyngeal microbes. C. diphtheriae may be isolated from individuals with both nontoxigenic (tox–) and toxigenic (tox+) phenotypes. Uchida and Pap­ penheimer demonstrated that corynebacteriophage beta carries the structural gene tox, which encodes diphtheria toxin, and that a family of closely related corynebacteriophages are responsible for toxigenic conversion of tox– C. diphtheriae to the tox+ phenotype. Moreover, lysogenic conversion from a nontoxigenic to a toxigenic phenotype has been shown to occur in situ. Growth of toxigenic strains of C. diphthe­ riae under iron-limiting conditions leads to the optimal expression of diphtheria toxin and is believed to be a pathogenic mechanism dur­ ing human infection. Less commonly, diphtheria-like disease may be caused by Corynebacterium ulcerans and Corynebacterium pseudotu­ berculosis, which express the same toxin and are considered members of the C. diphtheriae group (discussed below).

■ ■EPIDEMIOLOGY While in many areas diphtheria has been controlled in recent years with effective vaccination, there have been sporadic outbreaks throughout the 1970s in the United States and the 1990s in Europe. Diphtheria is still common in parts of Africa, Asia, Latin America, and the Caribbean where mass immunization programs are not enforced. Large-scale epidemics of diphtheria have occurred in the post–Soviet Union independent states in the late 1990s and, more recently, in Nigeria and Yemen during 2022–2023. In temperate regions, respiratory diphtheria occurs year-round but is most com­ mon during winter months. PART 5 Infectious Diseases C. diphtheriae is transmitted via the aerosol route, usually during close contact with an infected person. Untreated individuals with respi­ ratory diphtheria are thought to be infectious for ~18.5 days and the R0 (basic reproduction number) is 1.7–4.3. There are no significant reser­ voirs other than humans. The mean incubation period for respiratory diphtheria is 1.4 days, but disease onset has occurred as late as 10 days after exposure. Prior to the vaccination era, most individuals over the age of 10 were immune to C. diphtheriae; infants were protected by maternal IgG antibodies but became susceptible after ~6 months of age. Thus, the disease primarily affected children and nonimmune young adults. Cutaneous diphtheria is usually a secondary infection that fol­ lows a primary skin lesion due to trauma, allergy, or autoimmunity. Most often, these isolates lack the tox gene and thus do not express diphtheria toxin. In tropical latitudes, cutaneous diphtheria is more common than respiratory diphtheria. Toxin-producing diphtheria in symptomatic individuals, regardless of site (respiratory or cutaneous), is a reportable disease in the United States, while nontoxigenic disease is not. Nontoxigenic strains of C. diphtheriae have been associated with pharyngitis in Europe, causing outbreaks among men who have sex with men and persons who use illicit IV drugs. The development of diphtheria antitoxin in 1898 by von Behring and of the diphtheria toxoid vaccine in 1924 by Ramon led to the near elimination of diphtheria in Western countries. The annual incidence rate in the United States peaked in 1921, with 206,000 cases (191 cases per 100,000) and 15,520 deaths. In contrast, current U.S. rates are exceeding low, with only 14 cases reported from 1996–2018, with the last case of respiratory diphtheria occurring in 2003 in a returning trav­ eler from Haiti. Nevertheless, pockets of colonization persist in North America, and groups or individuals who resist vaccination remain at risk. Immunity to diphtheria induced by childhood vaccination gradu­ ally decreases in adulthood. An estimated 30% of men 60–69 years old have antitoxin titers below the protective level. In addition to older age

and lack of vaccination, risk factors for diphtheria outbreaks include alcoholism, low socioeconomic status, crowded living conditions, and Native American ethnic background. An outbreak of diphtheria in Seattle, Washington, between 1972 and 1982 comprised 1100 cases, most of which were cutaneous. During the 1990s in the states of the former Soviet Union, a much larger diphtheria epidemic included more than 140,000 cases and more than 4000 deaths; at its peak in 1995, more than 50,412 cases were reported. Clonally related toxigenic C. diphtheriae strains of the ET8 complex were associated with this outbreak. Beginning in 1998, this epidemic was controlled by mass vaccination programs, and between 2000 and 2009, the diphtheria incidence fell by >95%, with high-burden countries such as Latvia reporting fewer than 10 cases. Despite the World Health Organization (WHO) estimate that ~84% of the global population of children have been adequately vaccinated, 8638 diphtheria cases were reported globally in 2021, and many more cases are likely to have gone unreported. The recent coronavirus disease 2019 (COVID-19) pandemic, socioeconomic instability, migration, vaccine hesitancy, and other factors remain as threats to diphtheria control. For example, the WHO reported that the percentage of chil­ dren who received three doses of diphtheria, tetanus, and pertussis immunization fell by 5% (86% to 81%) during 2019–2021 due to health system strains from COVID-19. Additionally, multiple European nations have reported high rates of diphtheria (often cutaneous disease) among unvaccinated asylum seekers, with West Africa and Afghanistan as common source regions. ■ ■PATHOGENESIS AND IMMUNOLOGY Diphtheria toxin produced by tox+ strains of C. diphtheriae is the pri­ mary virulence factor in clinical disease. The toxin is synthesized in precursor form; is released as a 535-amino-acid, single-chain protein; and, in sensitive species (e.g., guinea pigs and humans, but not mice or rats), has a 50% lethal dose of ~100 ng/kg of body weight. The toxin is produced in the pseudomembranous lesion in the pharynx and is taken up in the bloodstream with subsequent distribution to all organs. Once bound to its cell surface receptor (a heparin-binding epidermal growth factor–like precursor), the toxin is internalized by receptor-mediated endocytosis and enters the cytosol from an acidified early endosomal compartment. In vitro, the toxin may be separated into two chains by digestion with serine proteases: the N-terminal A fragment and the C-terminal B fragment. Delivery of the A fragment into the eukaryotic cell cytosol results in irreversible inhibition of protein synthesis by NAD+-dependent ADP-ribosylation of elongation factor 2 and subse­ quent cell death. In 1926, Ramon at the Institut Pasteur found that formalinization of diphtheria toxin resulted in the production of a nontoxic but highly immunogenic diphtheria toxoid. Subsequent studies showed that immunization with diphtheria toxoid elicited antibodies that neutral­ ized the toxin and prevented most disease manifestations. In the 1930s, mass immunization of children and susceptible adults with diphtheria toxoid commenced in the United States and Europe. Individuals with a diphtheria antitoxin titer of >0.01 U/mL are at low risk of disease. In populations where a majority of individuals have protective antitoxin titers, the carrier rate for toxigenic strains of C. diphtheriae decreases, and the overall risk of diphtheria among susceptible individuals is reduced. Nevertheless, individuals with non­ protective titers may contract diphtheria either through travel or expo­ sure to individuals who have recently returned from regions where the disease is endemic. Characteristic pathologic findings of diphtheria include mucosal ulcers with a pseudomembranous coating composed of an inner band of fibrin and a luminal band of neutrophils. Initially white and firmly adherent, in advanced diphtheria the pseudomembranes turn gray or even green or black as necrosis progresses. Mucosal ulcers result from toxin-induced necrosis of the epithelium accompanied by edema, hyperemia, and vascular congestion of the submucosal base. A sig­ nificant fibrinosuppurative exudate from the ulcer develops into the pseudomembrane. Ulcers and pseudomembranes in severe respiratory diphtheria may extend from the pharynx into medium-sized bronchial

airways. Expanding and sloughing membranes may result in fatal air­ way obstruction. APPROACH TO THE PATIENT Diphtheria Diphtheria, although rare in the United States and other developed countries, should be considered when a patient has severe pharyn­ gitis, particularly when there is difficulty swallowing, respiratory compromise, or signs of systemic disease (e.g., myocarditis or gen­ eralized weakness). The leading causes of pharyngitis are respira­ tory viruses (rhinoviruses, influenza viruses, parainfluenza viruses, coronaviruses, adenoviruses; ~25% of cases), group A streptococci (15–30%), group C streptococci (~5%), atypical bacteria such as Mycoplasma pneumoniae and Chlamydia pneumoniae (15–20% in some series), and other viruses such as herpes simplex virus (~4%) and Epstein-Barr virus (<1% in infectious mononucleosis). Less common causes are acute HIV infection, gonorrhea, fusobacte­ rial infection (e.g., Lemierre’s syndrome), thrush due to Candida albicans or other Candida species, and diphtheria. The presence of a pharyngeal pseudomembrane or an extensive exudate should prompt consideration of diphtheria (Fig. 155-1). ■ ■CLINICAL MANIFESTATIONS Respiratory Diphtheria  The clinical diagnosis of diphtheria is based on the constellation of sore throat; adherent tonsillar, pha­ ryngeal, or nasal pseudomembranous lesions; and low-grade fever. In addition, diagnosis requires the isolation of C. diphtheriae or his­ topathologic isolation of compatible gram-positive organisms. The FIGURE 155-1  Respiratory diphtheria due to toxigenic C. diphtheriae producing exudative pharyngitis in a child displaying a pseudomembrane extending from the uvula to the pharyngeal wall. The characteristic white pseudomembrane is caused by diphtheria toxin–mediated necrosis of the respiratory epithelial layer, producing a fibrinous coagulative exudate. Submucosal edema adds to airway narrowing. The pharyngitis is acute in onset, and respiratory obstruction from the pseudomembrane may occur in severe cases. Inoculation of pseudomembrane fragments or submembranous swabs onto Löffler’s or tellurite selective medium reveals C. diphtheriae. (Photograph courtesy of the Centers for Disease Control and Prevention and Immunization Action Coalition, used by permission.)

Centers for Disease Control and Prevention (CDC) recognizes con­ firmed respiratory diphtheria (laboratory proven or epidemiologically linked to a culture-confirmed case) and probable respiratory diphtheria (clinically compatible but not laboratory proven or epidemiologically linked). Carriers are defined as individuals who have positive cultures for C. diphtheriae and who either are asymptomatic or have symptoms but lack pseudomembranes. Most patients seek medical care for sore throat and fever several days into the illness. Occasionally, weakness, dysphagia, headache, and voice change are the initial manifestations. Neck edema and difficulty breathing are evident in more advanced cases and carry a poor prognosis.

The systemic manifestations of diphtheria stem from the effects of diphtheria toxin and include weakness as a result of neurotoxicity and cardiac arrhythmias or congestive heart failure due to myocardi­ tis. Most commonly, the pseudomembranous lesion is located in the tonsillopharyngeal region. Less commonly, the lesions are located in the larynx, nares, and trachea or bronchial passages. Large pseudo­ membranes are associated with severe disease and a poor prognosis. A few patients develop massive swelling of the tonsils and present with “bull-neck” diphtheria, which results from edema of the sub­ mandibular and paratracheal region and is further characterized by foul breath, thick speech, and stridorous breathing. The diphtheritic pseudomembrane is gray or whitish and sharply demarcated. Unlike the exudative lesion associated with streptococcal pharyngitis, the pseudomembrane in diphtheria is tightly adherent to the underly­ ing tissues. Attempts to dislodge the membrane may cause bleeding. Hoarseness suggests laryngeal diphtheria, in which laryngoscopy may be diagnostically helpful. CHAPTER 155 Cutaneous Diphtheria  This dermatosis is characterized by punched-out ulcerative lesions with necrotic sloughing or pseudo­ membrane formation (Fig. 155-2). The diagnosis requires cultivation of C. diphtheriae from lesions, which most commonly occur on the lower and upper extremities, head, and trunk. Diphtheria and Other Corynebacterial Infections Infections Due to Non-diphtheriae Corynebacterium Species and Nontoxigenic C. diphtheriae  Non-diphtheriae species of Corynebacterium and related genera (discussed below) as well as non­ toxigenic strains of C. diphtheriae itself have been found in bloodstream and respiratory infections, often in individuals with immunosuppres­ sion or chronic respiratory disease. These organisms can cause disease manifestations and should not necessarily be dismissed as colonizers. Other Clinical Manifestations  C. diphtheriae causes rare cases of endocarditis and septic arthritis, most often in patients with preexist­ ing risk factors, such as abnormal cardiac valves, injection drug use, or cirrhosis. FIGURE 155-2  Cutaneous diphtheria due to nontoxigenic C. diphtheriae on the lower extremity. (From the Centers for Disease Control and Prevention, Public Health Image Library [PHIL]. #1941.)

■ ■COMPLICATIONS Airway obstruction poses a significant early risk in patients presenting with advanced diphtheria and accounts for 60–65% of deaths typically in the first 1–2 weeks after symptom onset. Pseudomembranes may slough and obstruct the airway or may advance to the larynx or into the tracheobronchial tree. Children are particularly prone to obstruction because of their small airways.

Cardiomyopathy and polyneuropathy are late toxic manifestations of diphtheria arising 1 week or more after respiratory symptoms. Based on systematic reviews, toxic cardiomyopathy accounted for 20–25% of deaths and is typically associated with arrhythmias and dilated cardiomyopathy. Polyneuropathy is seen 3–5 weeks after the onset of diphtheria and has a slow indolent course but may account for 15% of deaths. Patients may develop severe and prolonged neurologic abnormalities. The disorders typically occur in the mouth and neck, with lingual or facial numbness as well as dysphonia, dysphagia, and cranial nerve paresthe­ sias. More ominous signs include weakness of respiratory and abdomi­ nal muscles and paresis of the extremities. Sensory manifestations and sensory ataxia also are observed. Cranial nerve dysfunction typically precedes disturbances of the trunk and extremities because of proxim­ ity to the site of infection. Autonomic dysfunction also is associated with polyneuropathy and can lead to hypotension. Polyneuropathy is typically reversible in patients who survive the acute phase. Other complications of diphtheria include pneumonia, renal failure, encephalitis, cerebral infarction, pulmonary embolism, and serum sickness from antitoxin therapy. ■ ■DIAGNOSIS The diagnosis of diphtheria is based on clinical signs and symptoms plus laboratory confirmation. Respiratory diphtheria should be consid­ ered in patients with sore throat, pharyngeal exudates, and fever. Other symptoms may include hoarseness, stridor, or palatal paralysis. The presence of a pseudomembrane should prompt strong consideration of diphtheria. Once a clinical diagnosis of diphtheria is made, diphtheria antitoxin should be obtained and administered as rapidly as possible. PART 5 Infectious Diseases Laboratory diagnosis of diphtheria is based either on cultivation of C. diphtheriae or toxigenic C. ulcerans from the site of infection or on the demonstration of local lesions with characteristic histopathology. Corynebacterium pseudodiphtheriticum, a nontoxigenic organism, is a common component of the normal throat flora and does not pose a significant risk. Throat samples should be submitted to the laboratory for culture with the notation that diphtheria is being considered. This information should prompt cultivation on special selective medium and subsequent biochemical testing to differentiate C. diphtheriae from other nasopharyngeal commensal corynebacteria. All laboratory iso­ lates of toxigenic C. diphtheriae should be reported to the state health department. A diagnosis of cutaneous diphtheria requires laboratory confirma­ tion since the lesions are not characteristic and are indistinguishable from other dermatoses. Diphtheritic ulcers occasionally—but not consistently—have a punched-out appearance (Fig. 155-2). Patients in whom cutaneous diphtheria is identified should have the nasopharynx cultured for C. diphtheriae. The laboratory medium for cutaneous diphtheria specimens is the same as that used for respiratory diphthe­ ria: Löffler’s or Tinsdale’s selective medium in addition to nonselective medium such as blood agar. As has been mentioned, isolation of toxi­ genic strains of C. diphtheriae in symptomatic individuals is notifiable disease in the United States, regardless of the body site of origin. TREATMENT Diphtheria DIPHTHERIA ANTITOXIN Prompt administration of diphtheria antitoxin is critical in the management of respiratory diphtheria. Diphtheria antitoxin, a horse antiserum, is effective in reducing the extent of local disease as well as the risk of complications of myocarditis and neuropathy.

Rapid institution of antitoxin therapy is also associated with a sig­ nificant reduction in mortality risk. Because diphtheria antitoxin cannot neutralize cell-bound toxin, prompt initiation is impor­ tant. This product, which is no longer commercially available in the United States, can be obtained from the CDC Emergency Operations Center at 770-488-7100 (www.cdc.gov/diphtheria/hcp/ dat/index.html) after first contacting the state health department. The current protocol for the use of diphtheria antitoxin involves a test dose to rule out immediate hypersensitivity. Patients who demonstrate hypersensitivity require desensitization before a full therapeutic dose of antitoxin is administered. Given that the world supply of equine anti–diphtheria toxin is limited, a human monoclonal antibody with the potential to provide a safer alternative to equine antitoxin therapy is being developed. ANTIMICROBIAL THERAPY Antibiotics are used in the management of diphtheria primarily to prevent transmission to susceptible contacts. Antibiotics also prevent further toxin production and may reduce the severity of local infection. Recommended treatment options for patients with respiratory diphtheria are as follows: • Erythromycin, 500 mg IV q6h (for children: 40–50 mg/kg per day IV in two or four divided doses) until the patient can swal­ low comfortably; then 500 mg PO qid to complete a 14-day course • Procaine penicillin G, 600,000 U IM q12h (for children: 12,500– 25,000 U/kg IM q12h) until the patient can swallow comfortably; then oral penicillin V, 125–250 mg qid to complete a 14-day course A clinical study in Vietnam found that penicillin was associated with a more rapid resolution of fever and a lower rate of bacterial resistance than erythromycin; however, relapses were more com­ mon in the penicillin group. Erythromycin therapy targets protein synthesis and thus offers the presumed benefit of stopping toxin synthesis more quickly than a cell wall–active β-lactam agent. Alternative therapeutic agents for patients who are allergic to peni­ cillin or cannot take erythromycin include rifampin and clindamy­ cin. Other reasonable antibiotics are clarithromycin, azithromycin, linezolid, and vancomycin, although they have not been studied in comparison to the agents above. Eradication of C. diphtheriae should be documented after anti­ microbial therapy is complete. A repeat throat culture 2 weeks later is recommended. For patients in whom the organism is not eradi­ cated after a 14-day course of erythromycin or penicillin, an addi­ tional 10-day course followed by repeat culture is recommended. Drug-resistant strains of C. diphtheriae exist and are appearing at higher frequency; several reports have described multidrugresistant strains. Drug resistance should be considered when efforts at pathogen eradication fail. Cutaneous diphtheria should be treated as described above for respiratory disease. Individuals infected with toxigenic strains should receive antitoxin. It is important to treat the underlying cause of the dermatoses in addition to the superinfection with C. diphtheriae. Patients who recover from respiratory or cutaneous diphtheria should have antitoxin levels measured. If diphtheria antitoxin has been administered, this test should be performed 6 months later. Patients who recover from respiratory or cutaneous diphtheria should receive the appropriate vaccine to ensure the development of protective antibody titers. MANAGEMENT STRATEGIES Patients in whom diphtheria is suspected should be hospitalized in respiratory isolation rooms with close monitoring of cardiac and respiratory function. A cardiac workup is recommended to assess the possibility of myocarditis. In patients with extensive pseudomembranes, an anesthesiology or an ear, nose, and throat

consultation is recommended because of the possible need for tra­ cheostomy or intubation. In some settings, pseudomembranes can be removed surgically. Treatment with glucocorticoids has not been shown to reduce the risk of myocarditis or polyneuropathy. ■ ■PROGNOSIS A systematic review of over 20 reported outbreaks found the diph­ theria case fatality ratio among unvaccinated, untreated individuals to be 29%, with children under the age of 5 being at a 1.5-fold higher risk of mortality. Fatal pseudomembranous diphtheria typically occurs in patients with nonprotective antibody titers and in unimmunized patients. The pseudomembrane may actually increase in size from the time it is first noted. Risk factors for death include bullneck diphtheria; myocarditis with ventricular tachycardia; atrial fibrillation; complete heart block; an age of >60 years or <6 months; alcoholism; extensive pseudomembrane elongation; and laryngeal, tracheal, or bronchial involvement. Another important predictor of fatal outcome is the interval between the onset of local disease and the administration of antitoxin. Cutaneous diphtheria has a low mortality rate and is rarely associated with myocarditis or peripheral neuropathy. ■ ■PREVENTION Vaccination  Diphtheria toxoid-based vaccine efficacy is estimated to be 87% in the prevention of symptomatic disease, and sustained campaigns for vaccination of children and adequate boosting vac­ cination of adults are responsible for the exceedingly low incidence of diphtheria in most developed nations. Diphtheria toxoid vaccine has typically been coadministered with tetanus vaccine (with or without acellular pertussis). DTaP (full-level diphtheria toxoid, tetanus toxoid, and acellular pertussis vaccine) is currently recommended for children in five doses up to the age of 6 years; DTaP replaced the earlier wholecell pertussis vaccine DTP in 1997. Tdap is a tetanus toxoid, reduced diphtheria toxoid, and acellular pertussis vaccine formulated for adolescents and adults and is the recommended booster for children 11–12 years old. Tdap is recommended for all adults if they have not received it previously regardless of the interval since the last dose of Td (tetanus and reduced-dose diphtheria toxoids, adsorbed). Tdap vaccination is a priority for health care workers, pregnant women, adults anticipating contact with infants, and adults not previously vaccinated for pertussis. Adults who have received acellular pertussis vaccine should continue to receive decennial Td booster vaccinations. In 2018, a hexavalent vaccine that combined diphtheria-tetanus tox­ oids, acellular pertussis adsorbed (DTaP), inactivated poliovirus (IPV), Haemophilus influenzae type b (Hib) conjugate, and recombinant hepatitis B (HepB) known as DTaP-IPV-Hib-HepB was approved by the U.S. Food and Drug Administration; this product may be used to replace the initial three childhood doses of DTaP (2, 4, and 6 months) or as part of a catch-up schedule in children under 5 years old. The vaccine schedule is detailed in Chap. 129. Prophylaxis Administration to Contacts  Close contacts of diph­ theria patients should undergo throat culture to determine whether they are carriers. After samples for throat culture are obtained, antimicrobial prophylaxis should be considered for all contacts, even those whose cul­ tures are negative. The options are 7–10 days of oral erythromycin or one dose of IM benzathine penicillin G (1.2 million units for persons ≥6 years of age or 600,000 units for children <6 years of age). Contacts of diphtheria patients whose immunization status is uncertain should receive the appropriate diphtheria toxoid–containing vaccine. The Tdap vaccine (rather than Td) is now the booster vaccine of choice for adults who have not recently received an acellular pertus­ sis–containing vaccine. Carriers of C. diphtheriae in the community should be treated and vaccinated when identified. OTHER CORYNEBACTERIAL AND RHODOCOCCUS INFECTIONS Nondiphtherial corynebacteria, referred to as diphtheroids or coryne­ forms, are frequently considered colonizers or contaminants; how­ ever, they have been associated with invasive disease, particularly in

immunocompromised patients. Importantly, even though they are termed nondiphtherial corynebacteria, C. ulcerans and C. pseudotu­ berculosis may produce diphtheria toxin and therefore cause severe human illness. These organisms have been isolated from the blood­ stream, especially in association with catheter infection, endocarditis, prosthetic valve infection, meningitis, brain abscess, osteomyelitis, and peritonitis. Risk factors include indwelling intravenous or perito­ neal catheters and neurosurgical shunts. Patients infected with these organisms are often immunosuppressed or have significant medical comorbidities. The nondiphtherial coryneforms are a collection of bacteria that are taxonomically grouped together in the genus Cory­ nebacterium on the basis of their 16S rDNA signature nucleotides. Despite the shared rDNA signatures, these isolates are quite diverse. For example, their guanine-cytosine content ranges from 45 to 70%. Several nondiphtheroid corynebacteria, including Corynebacterium jeikeium and Corynebacterium urealyticum, are associated with resis­ tance to multiple antibiotics. Rhodococcus equi is associated with necrotizing pneumonia and granulomatous infection, particularly in immunocompromised individuals.

■ ■MICROBIOLOGY AND LABORATORY DIAGNOSIS These organisms are non-acid-fast, catalase-positive, aerobic or fac­ ultatively anaerobic rods. Their colonial morphologies on blood agar vary widely; some species are small and α-hemolytic (similar to lac­ tobacilli), whereas others form large white colonies (similar to yeasts). Many nondiphtherial coryneforms require special media, such as Löf­ fler’s, Tinsdale’s, or tellurite medium. These cultivation idiosyncrasies have led to a complex taxonomic categorization of the organisms. CHAPTER 155 ■ ■EPIDEMIOLOGY Humans are the natural reservoirs for several nondiphtherial coryne­ forms, including C. xerosis, C. pseudodiphtheriticum, C. striatum, C. minutissimum, C. jeikeium, C. urealyticum, and Arcanobacterium haemolyticum. Animal reservoirs including milk are responsible for carriage of C. ulcerans and C. pseudotuberculosis. Soil is the natural reservoir for R. equi. Diphtheria and Other Corynebacterial Infections ■ ■CLINICAL MANIFESTATIONS C. ulcerans  This organism causes a diphtheria-like illness and pro­ duces both diphtheria toxin and a dermonecrotic toxin. The organism is a commensal in horses and cattle and has been isolated from cow’s milk. In contrast to diphtheria, this infection is considered a zoonosis, and cases have been traced to contact with animal carriers, including dogs and pigs. C. ulcerans causes exudative pharyngitis, primarily dur­ ing summer months, in rural areas, and among individuals exposed to animals. Treatment with antitoxin and antibiotics should be initiated when respiratory C. ulcerans is identified, and a contact investigation should be conducted (including throat cultures to determine the need for antimicrobial prophylaxis and, in unimmunized contacts, adminis­ tration of the appropriate diphtheria toxoid–containing vaccine). The organism grows on Löffler’s, Tinsdale’s, and tellurite agars as well as blood agar. In addition to exudative pharyngitis, cutaneous disease due to C. ulcerans has been reported. C. ulcerans is susceptible to a wide panel of antibiotics. Erythromycin and other macrolides appear to be the first-line agents. C. pseudotuberculosis  Infection caused by C. pseudotuberculosis is an important animal pathogen (most notably of sheep) that rarely causes human disease. C. pseudotuberculosis causes suppurative granu­ lomatous lymphadenitis and an eosinophilic pneumonia syndrome among individuals who handle sheep; horses, cattle, goats, deer, and raw milk has also been implicated. Surgical excision of affected lymph nodes should be performed when feasible, and successful treatment with erythromycin or tetracycline has been reported. Some strains express diphtheria toxin and produce a diphtheria-like disease, which should be treated with antitoxin. C. jeikeium (Group JK)  Originally described in American hospi­ tals, C. jeikeium infection was subsequently reported in Europe. After a 1976 survey of diseases caused by nondiphtherial corynebacteria,

CDC group JK emerged as an important opportunistic pathogen among neutropenic and HIV-infected patients. The organism has now been designated a separate species. C. jeikeium forms small, gray to white, glistening, nonhemolytic colonies on blood agar. It lacks urease and nitrate reductase and does not ferment most carbo­ hydrates. The predominant syndrome associated with C. jeikeium is sepsis, sometimes with associated pneumonia, endocarditis, menin­ gitis, osteomyelitis, or epidural abscess. Risk factors for C. jeikeium infection include hematologic malignancy, neutropenia from comor­ bid conditions, prolonged hospitalization, exposure to multiple anti­ biotics, and skin disruption. There is evidence that C. jeikeium is part of the inguinal, axillary, genital, and perirectal flora of hospitalized patients.

Broad-spectrum antimicrobial therapy appears to select for coloni­ zation. The organisms appear as gram-positive coccobacillary forms slightly resembling streptococci. C. jeikeium is resistant to the majority of antibiotic classes except oxazolidinones (e.g., linezolid) and glyco­ peptides (e.g., vancomycin). Effective therapy involves removal of the infectious source, whether a catheter, prosthetic joint, or prosthetic valve. Efforts have been made to prevent C. jeikeium infection with strict institution of infection control protocols for high-risk patients, particularly those in intensive care units. C. urealyticum (Group D2)  Identified as a urease-positive non­ diphtherial Corynebacterium in 1972, C. urealyticum is an opportunis­ tic pathogen causing sepsis and urinary tract infection. C. urealyticum appears to be the etiologic agent of a severe urinary tract syndrome known as alkaline-encrusted cystitis, a chronic inflammatory bladder infection associated with deposition of ammonium magnesium phos­ phate on the surface and walls of ulcerating lesions in the bladder. In addition, C. urealyticum has been associated with pneumonia, perito­ nitis, endocarditis, osteomyelitis, and wound infection. It is similar to C. jeikeium in its resistance to most antibiotics except oxazolidinones and glycopeptides. Vancomycin therapy has been used successfully in severe infections. PART 5 Infectious Diseases C. minutissimum (Erythrasma)  Erythrasma is a cutaneous infection producing reddish-brown, macular, scaly, pruritic inter­ triginous patches. The dermatologic presentation under the Wood’s lamp is of coral red fluorescence. C. minutissimum appears to be a common cause of erythrasma, although there is evidence for a polymicrobial etiology in certain settings. This microbe has also been associated with bacteremia in patients with hematologic malig­ nancy. Erythrasma responds to topical erythromycin, clarithromycin, clindamycin, or fusidic acid, although more severe infections may require oral macrolide therapy. Other Nondiphtherial Corynebacteria  C. xerosis is a human commensal found in the conjunctiva, nasopharynx, and skin. This nontoxigenic organism is occasionally identified as a source of invasive infection in immunocompromised or postoperative patients and pros­ thetic joint recipients. C. amycolatum is a closely related species but tends to demonstrate more antibiotic resistance. C. striatum is found in the anterior nares, skin, face, and upper torso of healthy individu­ als. Also nontoxigenic, this organism has been associated with inva­ sive opportunistic infections in severely ill or immunocompromised patients. C. glucuronolyticum is a nonlipophilic species that causes male genitourinary tract infections such as prostatitis and urethritis. These infections may be successfully treated with a wide variety of antibacterial agents, including β-lactams, rifampin, aminoglycosides, or vancomycin; however, the organism appears to be resistant to fluoroquinolones, macrolides, and tetracyclines. C. imitans has been identified in eastern Europe as a nontoxigenic cause of pharyngitis. C. auris has been identified in children with otitis media; it is sus­ ceptible to fluoroquinolones, rifampin, tetracycline, and vancomycin but resistant to penicillin G and variably susceptible to macrolides. C. pseudodiphtheriticum is a nontoxigenic species that is part of the normal human flora. Human infections—particularly endocarditis of either prosthetic or natural valves and invasive pneumonia—have been reported only rarely. Although C. pseudodiphtheriticum may be

isolated from the nasopharynx of patients with suspected diphtheria, it is part of the normal flora and does not produce diphtheria toxin. C. propinquum, a close relative of C. pseudodiphtheriticum, is part of CDC group D-1 and has been isolated from the human respiratory tract and blood. C. afermentans and subspecies belong to CDC group ANF-1; it is a rare human pathogen that has been isolated from human blood and abscesses. Rhodococcus  Rhodococcus species are phylogenetically related to the corynebacteria. These gram-positive coccobacilli have been asso­ ciated with tuberculosis-like infections in humans with granulo­ matous pathology. While R. equi is best known, other near-relative species have been identified in human infections including R. fascians, R. erythropolis, R. rhodochrous, Gordonia bronchialis, G. sputi, G. terrae, and Tsukamurella paurometabola. R. equi has been recognized as a cause of pneumonia in horses since the 1920s and as a cause of related infections in cattle, sheep, and swine. It is found in soil as an environmental microbe. The organisms vary in length; appear as spherical to long, curved, clubbed rods; and produce large irregular mucoid colonies. R. equi cannot ferment carbohydrates or liquefy gelatin and is often acid fast. An intracellular pathogen of macrophages, R. equi can cause granulomatous necrosis and caseation. This organism has most commonly been identified in pulmonary infection, but infections of brain, bone, and skin also have been reported. Most commonly, R. equi disease manifests as nodular and/or cavitary pneumonia of the upper lobe—a picture similar to that seen in tuberculosis or nocardiosis. Most patients are immunocompromised, often by HIV infection. Subcutaneous nodular lesions also have been identified. The involvement of R. equi should be considered when any patient presents with a tuberculosis-like syndrome. Infection due to R. equi has been treated successfully with antibiot­ ics that penetrate intracellularly, including macrolides, clindamycin, rifampin, and trimethoprim-sulfamethoxazole. β-Lactam antibiotics have not been useful. The organism is routinely susceptible to vanco­ mycin, which is considered the drug of choice. Arcanobacteria  Arcanobacterium haemolyticum was identified as an agent of wound infections in U.S. soldiers in the South Pacific during World War II. It appears to be a human commensal of the nasopharynx and skin, but it is known to cause true pharyngitis as well as chronic skin ulcers. In contrast to the much more common pharyngitis caused by Streptococcus pyogenes, A. haemolyticum pharyngitis is associated with a scarlatiniform rash on the trunk and proximal extremities in about half of cases; this illness is occasionally confused with toxic shock syndrome. Because A. haemolyticum pharyngitis primarily affects teenagers, it has been postulated that the rash–pharyngitis syndrome may represent co-pathogenicity, synergy, or opportunistic second­ ary infection with Epstein-Barr virus. A. haemolyticum has also been reported as a cause of bacteremia, soft tissue infections, osteomyelitis, and cavitary pneumonia, predominantly in the setting of underlying diabetes mellitus. The organism is susceptible to most β-lactams, mac­ rolides, fluoroquinolones, clindamycin, vancomycin, and doxycycline. However, resistance to trimethoprim-sulfamethoxazole as well as tet­ racycline is common. ■ ■FURTHER READING Kim R, Reboli AC: Other coryneform bacteria and Rhodococcus, in Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases, 9th ed. JE Bennett et al (eds). Philadelphia, Elsevier, 2020, pp 2532–2542. Moore LS et al: Corynebacterium ulcerans cutaneous diphtheria. Lancet Infect Dis 15:1100, 2015. Saleeb PG: Corynebacterium diphtheriae (diphtheria), in Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases, 9th ed. JE Bennett et al(eds). Philadelphia, Elsevier, 2020, pp 2526–2531. Sharma NC et al: Diphtheria. Nat Rev Dis Primers 5:81, 2019. Truelove SA et al: Clinical and epidemiologic aspects of diphtheria: A systematic review and pooled analysis. Clin Infect Dis 71:89, 2020.

38 - 156 Listeria monocytogenes Infections

156 Listeria monocytogenes Infections

Karen P. Neil, Jennifer P. Collins,

Patricia M. Griffin

Listeria monocytogenes

Infections Listeria monocytogenes is a ubiquitous environmental saprophyte and an intracellular pathogen in several animals. Humans develop L. monocytogenes infection—listeriosis—primarily through foodborne transmission. The clinical spectrum of listeriosis ranges from febrile gastroenteritis in healthy persons to invasive disease, including bac­ teremia and meningoencephalitis. The major risk groups for invasive disease are pregnant women and their neonates, older adults, and immunocompromised persons. Febrile gastroenteritis is rarely diag­ nosed because Listeria are not detected by routine stool cultures or in rapid diagnostic tests for stool specimens, so the term listeriosis gener­ ally refers to invasive infection. ■ ■MICROBIOLOGY L. monocytogenes lives in soil and decaying vegetable matter. Numer­ ous bird and mammal species are reservoirs. L. monocytogenes is a nonsporulating, facultatively anaerobic, short, gram-positive rod that grows well on blood agar, demonstrating small zones of β-hemolysis. Organisms sometimes appear gram-variable and resemble cocci, dip­ lococci, or diphtheroids; this appearance can obscure the diagnosis. On light microscopy, L. monocytogenes demonstrates characteristic tumbling motility. It grows optimally at 30–37°C but can grow at refrigerator temperatures as low as 4°C. In addition to its ability to grow at cold temperatures, Listeria’s tolerance to low-pH and high-salt environments facilitates its environmental survival. Serotypes are usu­ ally determined on the basis of somatic (O) and flagellar (H) antigens. Nearly all human illness is caused by serotypes 1/2a, 1/2b, and 4b. ■ ■PATHOGENESIS Infection with L. monocytogenes typically occurs through ingestion of contaminated food. The infectious dose has not been well established but is likely to be very low for persons with severely impaired cellular immunity. Increased gastric pH, such as that due to proton pump inhibitors, probably promotes the organism’s survival in the gastroin­ testinal tract. After transcytosis across the intestinal epithelium, the bacteria travel via mesenteric lymph nodes and the bloodstream to the liver and spleen, its target organs; dissemination to other organs can occur. L. monocytogenes can also migrate across the blood-brain bar­ rier and the placenta. Virulence factors include internalins (InlA, InlB), which promote invasion into normally nonphagocytic host cells. A pore-forming cytolysin (listeriolysin O; LLO) and phospholipases facilitate evasion of intracellular killing by mediating escape from the internalization vacuole into the cell cytosol. The surface protein ActA facilitates direct cell-to-cell movement within the cytosol, allowing L. monocytogenes to avoid encountering components of the host immune system, such as antibodies and complement, during dissemination. Iron promotes growth of the organism in vitro, an effect that explains why liste­ riosis has been associated with iron-overload conditions, including hemochromatosis. ■ ■IMMUNE RESPONSE Although L. monocytogenes is ubiquitous in the environment, infection is rare because of both innate and adaptive host immune responses. Studies of mice have contributed to a detailed understanding of the immune response to infection. Activation of innate immunity is important for host survival. Interferon γ and tumor necrosis factor α (TNF-α) are among the key cytokines involved in this response. T cells are the primary drivers of the adaptive immune response, furthering the clearance of infected cells. Cytotoxic (CD8+) T cells are the main contributors to long-term immunity.

These immune mechanisms explain the association between inva­ sive listeriosis and immunocompromising conditions, particularly impaired cellular immunity. In light of numerous reports of invasive listeriosis in patients treated with TNF-α inhibitors, the U.S. Food and Drug Administration added listeriosis to the boxed warning for this drug class. Because L. monocytogenes induces a vigorous cell-mediated immune response, attenuated strains that express foreign antigens are undergoing clinical trials as a cancer immunotherapy.

■ ■EPIDEMIOLOGY L. monocytogenes was first reported as a human pathogen in 1929. Foodborne transmission was not identified until over 50 years later, when a 1983 outbreak investigation implicated coleslaw. It is now known that L. monocytogenes transmission is almost always food­ borne. Listeriosis is a nationally notifiable disease in the United States. According to the Centers for Disease Control and Prevention’s (CDC’s) Foodborne Diseases Active Surveillance Network (FoodNet), the incidence of invasive listeriosis was 2.1–3.3 cases per million persons during 2011–2022 (Fig. 156-1). Although illness is uncommon, liste­ riosis is a substantial contributor to morbidity from foodborne illness because virtually all patients are hospitalized and 18% die. Only about 3% of reported cases of listeriosis are part of a recog­ nized outbreak—i.e., have a source determined; however, outbreak investigations provide data on major foods linked to illness. Strains of L. monocytogenes have survived in production facilities for years. Refrigerated, ready-to-eat foods are of particular concern because the organism’s ability to grow at refrigerator temperatures can result in a small amount of contamination during production growing to much higher levels. PulseNet, the laboratory network for bacterial foodborne disease surveillance, implemented routine subtyping for L. monocyto­ genes in the United States in 1998 and transitioned to whole-genome sequencing in 2013. Improvements in the ability to detect and investi­ gate outbreaks using laboratory-based surveillance has provided more information on known food sources and facilitated recognition of new food sources. CHAPTER 156 Listeria monocytogenes Infections Hot dogs and deli meats were the major sources of U.S. outbreaks until after 2002, when an outbreak linked to turkey deli meat resulted in eight deaths and the recall of >30 million pounds of meat. After that outbreak, the U.S. Department of Agriculture’s Food Safety and Inspection Service (FSIS) issued new regulations requiring vali­ dated L. monocytogenes control programs and intensified testing for L. monocytogenes in ready-to-eat meat and poultry plants. Isolations of L. monocytogenes from ready-to-eat meats continued to decline during 2003–2012 (Fig. 156-1), and outbreaks linked to these products decreased. However, the incidence of listeriosis, which had declined markedly from 1989 through 2001, has not declined significantly since the measures were implemented (Fig. 156-1). Dairy products remain an important source, especially soft cheese made with raw (unpasteur­ ized) milk or produced from pasteurized milk in unsanitary facilities; ice cream and raw milk also have caused outbreaks. Raw produce has been increasingly linked to outbreaks; implicated items include pack­ aged salad, sprouts, enoki mushrooms, cantaloupe and other fruit, caramel apples, and frozen vegetables. Most people with invasive listeriosis are older adults, whose risk increases with each decade over 59 years of age. Most other patients have impaired cellular immunity associated with hematologic malig­ nancy, solid organ or bone marrow transplantation, HIV infection, or receipt of glucocorticoid or other immunosuppressive drugs. The group at highest risk of infection is pregnant women, who almost always have only mild flulike symptoms but who transmit the infec­ tion to the fetus through the placenta. Some neonates may acquire infection in the hospital, as illustrated by an outbreak associated with contaminated mineral oil. Rarely, children and adults with no recog­ nized risk factors develop invasive listeriosis, probably through highly contaminated food. The diagnosis of listeriosis in a hospitalized patient with new symp­ toms should prompt investigation into the food provided during hospi­ talization as a source. In fact, outbreaks have been traced to food served to hospitalized patients, especially those with immunocompromising

Products yielding L. monocytogenes, % ( ) 1989: Case associated with turkey franks; new regulatory policies, industry efforts

1998: PulseNet began subtyping

2000: Listeriosis made nationally notifiable

1988 1990 1992 1994 1996 1998 2000 2002 2004 Year 2006 2008 2010 2012 2014 2016 2018 2020 2022 FIGURE 156-1  Incidence of listeriosis and percentage of ready-to-eat meat and poultry products from Food Safety and Inspection Service (FSIS) testing with cultures that yielded L. monocytogenes, United States, 1989–2022. Incidence data are from the Centers for Disease Control and Prevention’s active sentinel site surveillance and include data from an early surveillance system (1986–1995) and from the Foodborne Diseases Active Surveillance Network (FoodNet) database (1996–2022). The incidence was 7.3 cases per million in 1986. Product data for 1990–2022 are from the U.S. Department of Agriculture Food Safety and Inspection Service Microbiological Testing Program for Ready-to-Eat Meat and Poultry Products and related publications (P Levine et al: J Food Prot 64:1188, 2001; SW Mamber et al: J Food Prot 83:1598, 2020; and the 2010 FSIS Comparative Risk Assessment for L. monocytogenes in RTE Meat and Poultry Deli Meats). conditions; implicated foods include sandwiches, butter, precut celery, Camembert cheese, sausage, tuna salad, and ice cream. Although very rare, transmission of L. monocytogenes has also occurred through transfusion of platelet products, and febrile illness with bacteremia has occurred in persons who received an attenuated strain as a vaccine vector. PART 5 Infectious Diseases Listeriosis is an important problem worldwide. A large 2017–2018 outbreak of listeriosis in South Africa, linked to a ready-to-eat pro­ cessed meat product, disproportionately affected people living with HIV and pregnant women. No outbreak-associated cases were detected in the 15 countries that imported the product; this discrepancy sug­ gests that listeriosis is underrecognized in low- to middle-income countries, particularly those with a high prevalence of HIV infection. ■ ■CLINICAL MANIFESTATIONS L. monocytogenes infection can manifest in several ways. The incu­ bation period differs according to host factors and dose consumed: on average, this interval is <24 h for gastroenteritis and ~11 days for invasive disease, although it can be much longer. Data from outbreak investigations suggest that the incubation period is longer in pregnant women than in nonpregnant adults. Febrile Gastroenteritis  Listeria organisms typically pass through healthy people without causing symptoms, but acute febrile gastroen­ teritis can occur. Outbreak investigations of L. monocytogenes febrile gastroenteritis have identified high organism density in implicated foods, suggesting that a large inoculum must be ingested to cause ill­ ness. Major manifestations are fever, diarrhea, headache, and constitu­ tional symptoms. Illness is usually self-limited, with symptoms lasting an average of 1–3 days. Bacteremia  Bacteremia without a focus is the most common mani­ festation of invasive listeriosis. Major features are fever, chills, myalgias, and arthralgias, sometimes preceded by nausea or diarrhea—markers of the initial gut infection. Bacteremia can cause neurolisteriosis or localized infection at other sites, in which case the diagnosis may be suggested by neurologic or other focal findings. In a large French cohort study, the 3-month mortality rate for L. monocytogenes bacte­ remia was 46%; factors associated with death included multiorgan fail­ ure, neoplasia, worsening of preexisting comorbidities, monocytopenia (<200 cells/μL), older age, and female sex. Neurolisteriosis  L. monocytogenes has an affinity for the cen­ tral nervous system. Neurolisteriosis is the second most common

Incidence per million persons ( )

2003: New regulations for ready-to-eat meat and poultry plants

manifestation of invasive listeriosis. Signs of meningitis along with altered mental status, seizures, or focal neurologic findings sug­ gest meningoencephalitis. A French cohort study found that 84% of patients with neurolisteriosis presented with meningoencephalitis. The onset of neurologic disease can be sudden or subacute and occur over several days. Patients typically present with signs and symp­ toms similar to those in other bacterial meningitides. Compared with other bacterial meningitides, cerebrospinal fluid (CSF) pleocytosis and CSF neutrophil predominance are typically less pronounced; most patients (~60–75%) have CSF white blood cell counts of <1000/μL, but some have high levels. Approximately 30–40% of patients have low CSF glucose levels. Gram’s staining of CSF sediment can show the expected gram-positive rods but commonly shows no organisms and sometimes shows gram-positive cocci, diplococci, or diphtheroids. In a U.S. study, L. monocytogenes caused <5% of cases of community-acquired bacte­ rial meningitis in adults. Uncommon neurolisteriosis manifestations include cerebritis, focal abscess, and rhombencephalitis (encephalitis of the cerebellum and brainstem). Patients with macroscopic abscesses often have positive blood cultures, but CSF findings may be normal in the absence of concurrent meningitis. Abscesses may be misdiagnosed as a primary or metastatic malignancy; they rarely occur in the cerebellum or spinal cord. Rhombencephalitis disproportionately affects otherwise healthy older adults. The classic presentation is biphasic, beginning with fever and headache and continuing after several days with signs of brainstem or cerebellar involvement, such as asymmetric cranial nerve palsies, ataxia, tremor, hemiparesis, or hemisensory deficits. Nearly half of patients with rhombencephalitis experience respiratory failure. The diagnosis may be delayed by the subacute course and by CSF findings, which are often only minimally abnormal. Magnetic resonance imag­ ing (MRI) is superior to computed tomography (CT) for the diagnosis of neurolisteriosis, including rhombencephalitis. Overall, the 3-month mortality rate for neurolisteriosis was 30% in a French cohort study; death was associated with the same risk factors as those documented for bacteremia. Neurolisteriosis-associated mortal­ ity was also higher among patients with a positive blood culture and among those treated with dexamethasone. Nearly half of survivors had long-term neurologic impairment. Focal Infections  Hematogenous dissemination of L. monocyto­ genes infrequently causes endocarditis, pneumonia, localized abscesses in the liver or other internal organs, peritonitis, septic arthritis, osteo­ myelitis, urinary tract infection, or skin lesions. Direct inoculation has

been reported as a rare cause of ocular infection, skin infection, and lymphadenitis. Infection in Pregnant Women and Neonates  The risk of Listeria infection during pregnancy is increased due to impaired maternal cell-mediated immunity. Pregnancy-associated listeriosis is most common in the third trimester, although this may in part reflect underdiagnosis of earlier infections. Typically, pregnant women are either asymptomatic or have a mild, flulike illness with fever, headache, myalgias, or arthralgias. Neurolisteriosis and death are rare in pregnant women without other risk factors. Although nearly all infected women fully recover, only a minority (~5%) have a normal delivery and post­ partum course, and three-quarters of the live-born infants are ill. In a French cohort study with 107 pregnancies in which L. monocytogenes was isolated from the mother, fetus, or neonate, 24% ended with fetal loss, 45% with premature birth, 6% with late-onset listeriosis in the neonate, and 21% with term delivery with fever or signs of fetal distress (i.e., meconium release into amniotic fluid, abnormal fetal heart rate). Fetal loss is uncommon after 29 weeks of gestation. Granulomatosis infantiseptica is a severe in utero infection caused by L. monocytogenes and characterized by disseminated microabscesses and granulomas in the skin, liver, and spleen; most infants with this condition are stillborn or die soon after birth. Neonatal listeriosis usually manifests in one of two ways: early-onset sepsis is hypothesized to result from in utero infection because it is typically diagnosed within 48 h after birth and is often associated with prematurity, whereas late-onset meningitis is thought to result from infection acquired at or soon after birth because it is typically diagnosed at ~2 weeks of age in full-term infants. A recent update from the above study that included additional years of data found that 70% of the 189 live-born infants developed early-onset listeriosis and 6% developed late-onset listeriosis; the other infants appeared uninfected (i.e., had negative cultures) and were well (14%) or had nonspecific signs (10%) that the authors attributed likely to prematurity. Overall, 3% of the 189 live-born infants died. ■ ■DIAGNOSIS Because symptoms of listeriosis overlap with those of other infections, a high index of suspicion can facilitate timely diagnosis. Pregnant women with suspected listeriosis should have blood drawn for cultures, although blood cultures are positive only about half the time. Isolation of L. monocytogenes from a normally sterile site, such as blood, CSF, amniotic fluid, placental tissue, or fetal tissue, is diagnostic. Listeria must be distinguished from other gram-positive rods, especially diph­ theroids. L. monocytogenes can be isolated from sterile specimens on routine medium; selective enrichment medium (such as PALCAM Listeria Selective Agar or Oxford Agar) enhances the capacity for isola­ tion of the organism from nonsterile specimens, such as stool. Stool culture is not indicated in the evaluation of invasive listeriosis; culture on selective medium can be helpful for outbreak investigations of febrile gastroenteritis. Commercially available multiplex polymerase chain reaction panels for CSF specimens include L. monocytogenes as a target and may be a useful adjunct to culture. Matrix-assisted laser desorption/ ionization time-of-flight (MALDI-TOF) mass spectrometry can rapidly identify an isolate as L. monocytogenes. Whole-genome sequencing has been a valuable tool for solving outbreaks, including a nosocomial out­ break associated with ice cream served in hospital milkshakes. TREATMENT Infections Caused by Listeria monocytogenes L. monocytogenes treatment has not been evaluated in clinical tri­ als. Recommendations are based on in vitro animal studies and observational clinical data. High-dose ampicillin (adult dose, 2 g IV every 4 h) or penicillin G (adult dose, 4 million units IV every 4 h) constitutes first-line therapy. Because penicillins are only weakly bactericidal against L. monocytogenes, many experts recom­ mend adding gentamicin for synergy (1.0–1.7 mg/kg every 8 h if renal function is normal), particularly if the infection is severe. Small studies have had varying results with regard to the benefits

of gentamicin. A large study provided evidence favoring amoxi­ cillin-gentamicin as first-line therapy. Patients who are allergic to penicillin should undergo desensitization or be treated with trime­ thoprim-sulfamethoxazole (TMP-SMX; 5 mg/kg per dose of the tri­ methoprim component, given IV every 6–12 h). TMP-SMX should be avoided during the first trimester because it has been associated with neural tube and cardiovascular defects and in the perinatal period because it may increase the risk of kernicterus. Resistance to TMP-SMX has been reported; thus, antibiotic susceptibility testing should be performed if this drug is considered. Treatment failures have been reported with meropenem despite in vitro susceptibility of the organism. L. monocytogenes is susceptible in vitro to several other drugs, including vancomycin, linezolid, tetracycline, macro­ lides, and fourth-generation fluoroquinolones (e.g., moxifloxacin), but relevant clinical reports are limited. Cephalosporins are not effective. A study with 252 patients with neurolisteriosis found significantly reduced survival rates among the 32 patients treated with adjunctive dexamethasone; the authors suggest avoiding this drug in neurolisteriosis. Prepartum antibiotic treatment of preg­ nant women with listeriosis, initiated at least 1 day before delivery, enhances the chance of delivering a healthy infant.

The optimal duration of antibiotic therapy has not been estab­ lished. Treatment duration usually depends on the clinical syn­ drome, disease severity, patient attributes, and response to treatment. The typical minimal treatment duration is 2 weeks for bacteremia, 2 weeks for early-onset neonatal disease, 3 weeks for meningitis, 4–6 weeks for endocarditis, and 6–8 weeks for brain abscess or encephalitis. Longer courses may be needed when patients are immunocompromised or are not improving as expected. Patients with neurolisteriosis do not routinely require a follow-up lumbar puncture if they are improving clinically during antibiotic therapy. CHAPTER 156 ■ ■PREVENTION Implementation of general precautions to prevent foodborne illness can help prevent listeriosis. These measures include fully cooking meats; washing fresh produce; cleaning hands, utensils, and kitchen surfaces after handling uncooked foods; and avoiding unpasteurized dairy products. Persons at increased risk for listeriosis should take addi­ tional precautions, including avoiding unpasteurized milk, soft cheeses (particularly those made with unpasteurized milk), refrigerated pâté or meat spreads, and premade deli salads (e.g., coleslaw; potato, tuna, or chicken salad). Hot dogs, fermented or dry sausages, and refriger­ ated, ready-to-eat delicatessen foods (e.g., smoked fish, deli meat, cheese sliced at a deli) should be avoided or heated until the internal temperature is 165°F or until they are steaming hot. Additional CDC recommendations can be found at www.cdc.gov/listeria/prevention

.html. Testing and treatment are not indicated for an asymptomatic person who has eaten a product recalled because of L. monocytogenes contamination, even if the person has risk factors for invasive listerio­ sis. Hospital dietary services should implement safe food-preparation procedures for immunocompromised patients and should not serve these patients higher-risk foods. Care of patients with listeriosis includes only standard precautions because person-to-person trans­ mission is rare. TMP-SMX given as prophylaxis for Pneumocystis jir­ ovecii infection (e.g., to people infected with HIV or organ transplant recipients) helps prevent listeriosis. Listeria monocytogenes Infections ■ ■FURTHER READING Charlier C et al: Clinical features and prognostic factors of listeriosis: The MONALISA National Prospective Cohort Study. Lancet Infect Dis 17:510, 2017. Charlier C et al: Neonatal listeriosis presentation and outcome:

A prospective study of 189 cases. Clin Infect Dis 74:8, 2022. Conrad AR et al: Listeria monocytogenes illness and deaths associated with ongoing contamination of a multiregional brand of ice cream products, United States, 2010–2015. Clin Infect Dis 76:89, 2023. Farley MM: Listeria monocytogenes, in Principles and Practice of Pediatric Infectious Diseases, 6th ed, Long SS et al (eds). Philadelphia, Elsevier, 2023, pp 797–802.

39 - 157 Tetanus

157 Tetanus

Gottlieb SL et al: Multistate outbreak of listeriosis linked to turkey

deli meat and subsequent changes in US regulatory policy. Clin Infect Dis 42:29, 2006. Hof H: An update on the medical management of listeriosis. Expert Opin Pharmacother 5:1727, 2004. Koopmans MM et al: Human listeriosis. Clin Microbiol Rev 36:e0006019, 2023. Thomas J et al: Outbreak of listeriosis in South Africa associated with processed meat. N Engl J Med 382:632, 2020. C. Louise Thwaites, Hai Duong Ha Thi

Tetanus Tetanus is a preventable disease manifested by skeletal muscle spasms and autonomic nervous system disturbance. It is caused by a powerful neurotoxin produced by the bacterium Clostridium tetani, which is found globally. Tetanus commonly occurs in areas with low vaccination coverage. In developed countries, the disease is seen occasionally in individuals who are incompletely vaccinated. Even though the mortal­ ity rate has decreased significantly over the past two decades, the dis­ ease causes around 50,000 deaths annually worldwide. The mortality rate of tetanus varies depending on staff skills, clinical practices, and equipment capacity. PART 5 Infectious Diseases ■ ■DEFINITION Tetanus diagnosis is based on clinical manifestations with limited sup­ portive laboratory confirmation. Case definitions are often used to facilitate clinical and epidemiologic assessments. The Centers for Dis­ ease Control and Prevention (CDC) defines probable tetanus as “in the absence of a more likely diagnosis, an acute illness with muscle spasms or hypertonia, and diagnosis of tetanus by a health care provider.” Neonatal tetanus is defined by the World Health Organization (WHO) as “an illness occurring in a child who has the normal ability to suck and cry in the first 2 days of life but who loses this ability between days 3 and 28 of life and becomes rigid and has spasms.” Given the unique presentation of neonatal tetanus, the history generally permits accurate classification of the illness with a high degree of probability. Maternal tetanus is defined by the WHO as “tetanus during pregnancy or within 6 weeks after the end of pregnancy (whether with birth, miscarriage, or abortion).” ■ ■ETIOLOGY C. tetani is an anaerobic, gram-positive, spore-forming rod whose spores are highly resilient and can survive readily in the environment throughout the world. The spores resist boiling and many disinfectants. C. tetani spores and bacilli survive in the intestinal systems of many animals, and fecal carriage is common. The spores or bacteria enter the body through abrasions, wounds, or the umbilical stump (in the case of neonates). Once in a suitable anaerobic environment, the organisms grow, multiply, and release tetanus toxin that enters the nervous system and causes disease. Approximately 20–30% tetanus cases have unclear entry wounds. Superficial abrasions to the limbs are the most common entry sites in adults. Deeper injuries or infections (e.g., open fractures, drug injec­ tion) are associated with more severe clinical presentations. Home delivery with unhygienic umbilical cutting and inadequate umbilicalcord care are the main causes of neonatal tetanus. Circumcision or ear-piercing also can result in neonatal tetanus. Tetanus occurs when immunity is lacking. Unvaccinated individuals or those with incomplete vaccination history are at increased risk of developing tetanus.

■ ■EPIDEMIOLOGY The incidence of tetanus has decreased gradually over the past few decades as vaccine coverage has increased. This disease is rare in devel­ oped countries. In 2022, 28 tetanus cases were reported in the United States. Most tetanus cases occur in individuals who have not received recommended tetanus vaccinations and booster shots every 10 years. In the United States between 2009 and 2017, tetanus cases occurred primarily in those between the ages of 20 and 64 (64%), with only 13% of cases occurring in those younger than 20. Three cases of neonatal tetanus were reported in this period. Diabetes and intravenous drug use were associated with increased tetanus risk. The global incidence of neonatal tetanus has reduced significantly following a concerted elimination program by WHO partnering with the United Nations Children’s Fund (UNICEF) and the United Nations Population Fund (UNFPA); however, approximately 25,000 neonates with tetanus died in 2018. The incidence of tetanus among older children and adults (who have a high risk of tetanus due to the deterioration of immunity and lack of booster shots) is unknown. As few countries have good surveillance systems, in 2015, there were esti­ mated to be between 30,000 and 62,000 deaths from tetanus in older children and adults. ■ ■PATHOGENESIS Genome sequencing of C. tetani has allowed identification of several exotoxins and virulence factors. Only those bacteria producing tetanus toxin can cause tetanus. Tetanus toxin undergoes retrograde transport into the central nervous system (CNS) and thus produces clinical effects. Tetanus toxin is intra-axonally transported to motor nuclei of the cranial nerves or ventral horns of the spinal cord. This toxin is pro­ duced as a single 150-kDa protein that is cleaved to produce heavy (100-kDa) and light (50-kDa) chains linked by a disulfide bond and noncovalent forces. The carboxy terminal of the heavy chain binds to specific membrane components in presynaptic α-motor nerve ter­ minals; evidence suggests binding to both polysialogangliosides and membrane proteins. This binding results in toxin internalization and uptake into the nerves. Once inside the neuron, the toxin enters a retrograde transport pathway, whereby it is carried proximally to the motor neuron body. It is known that tetanus toxin exhibits several dif­ ferent pH-dependent conformations and therefore can interact with a variety of different receptors. During its passage from the periphery to the central nervous system, tetanus toxin can access neuronal traffick­ ing systems and evade degradation. Following retrograde transport in the motor neuron, the tetanus toxin undergoes translocation across the synapse to the GABA-ergic presynaptic inhibitory interneuron terminals. Here the light chain, which is a zinc-dependent endopeptidase, cleaves vesicle-associated membrane protein 2 (VAMP2, also known as synaptobrevin). This mol­ ecule is necessary for presynaptic binding and release of neurotrans­ mitter; thus, tetanus toxin prevents transmitter release and effectively blocks inhibitory interneuron discharge. The result is unregulated activity in the motor nervous system. Similar activity in the autonomic system accounts for the characteristic features of skeletal muscle spasm and autonomic system disturbance. The increased circulating cat­ echolamine levels in severe tetanus are associated with cardiovascular complications. Relatively little is known about the processes of recovery from tetanus. Recovery can take several weeks. Peripheral nerve sprouting is involved in recovery from botulism, and similar CNS sprouting may occur in tetanus. Other evidence suggests toxin degradation as a mechanism of recovery. APPROACH TO THE PATIENT Tetanus The clinical manifestations of tetanus occur only after tetanus toxin has reached presynaptic inhibitory nerves. Treatment should not be delayed once the diagnosis of tetanus is confirmed. Management

strategies aim to neutralize remaining unbound toxin, support vital functions, treat symptoms, and control complications until the effects of the toxin have worn off. Patients usually recover after 4–6 weeks (see “Treatment,” below). ■ ■CLINICAL MANIFESTATIONS Clinical presentations of tetanus are diverse and are divided into the following categories: generalized, localized, cephalic, and neonatal teta­ nus. In the mild form of local tetanus, only isolated areas of the body are affected and only small areas of local muscle spasm may be appar­ ent. Localized tetanus can progress to generalized tetanus. If the cranial nerves are involved in localized cephalic tetanus, the pharyngeal or laryngeal muscles may spasm, with consequent aspiration, respiratory failure, or airway obstruction. Generalized tetanus is the most com­ mon form of clinical presentation, characterized by muscle rigidity and generalized spasms. Neonates with tetanus typically present with an inability to suck, poor feeding, and generalized spasms. The most common initial symptoms are trismus (lockjaw), which progresses to neck and body muscle rigidity, difficulty swallowing, and pharyngeal and laryngeal spasms. As the disease progresses, generalized muscle spasms develop and cause pain. Laryngeal spasm can cause respi­ ratory failure or apnea and is a life-threatening event; without immediate respiratory support, this is the most common cause of death in tetanus. Autonomic nervous system disturbance (ANSD) occurs during the second week of severe tetanus, and death due to cardiovascular events becomes the major risk. Clinical symptoms of ANSD include fluctuated heart rate (tachycardia or bradycardia) accompanied by a fluctuation in blood pressure (hypertension or hypotension) resulting from alteration in the activities of the sympathetic or parasympathetic nervous systems. Autonomic involvement is evidenced by gastrointes­ tinal stasis, sweating, and increased tracheal secretions. The Ablett score classifications of the clinical presentation of tetanus are outlined in Table 157-1. ■ ■DIAGNOSIS The diagnosis of tetanus is based on clinical findings. Confirma­ tory laboratory tests are limited. Positive culture of C. tetani from wounds cannot confirm a diagnosis of tetanus. Serum antitetanus immunoglobulin G may also be measured in a sample taken before the administration of antitoxin or immunoglobulin; levels >0.1 IU/mL

(measured by standard enzyme-linked immunosorbent assay) are deemed protective and do not support the diagnosis of tetanus. If levels are below this threshold, a bioassay for serum tetanus toxin may be helpful, but a negative result does not exclude the diagnosis, and these levels are not generally performed. Polymerase chain reaction and recombinase polymerase amplification have been developed to detect the tetanus neurotoxin gene; these techniques have proven promising in recent years. A diagnosis of tetanus requires differentiation from other diseases. The few conditions that mimic generalized tetanus include strychnine poisoning, dystonic reactions to antidopaminergic drugs, somatic symptom disorder, stiff person syndrome, and neuroleptic malignant syndrome. Abdominal muscle rigidity is characteristically continu­ ous in tetanus and should be differentiated from peritonitis or acute abdominal emergency. Cephalic tetanus can be confused with trismus TABLE 157-1  Ablett Classification of Severity of Tetanus GRADE SEVERITY SYMPTOMS I Mild Mild trismus, general spasticity, no respiratory compromise, no spasms, no dysphagia II Moderate Moderate trismus, rigidity, short spasms, mild dysphagia, moderate respiratory involvement, respiratory rate >30 breaths/min III Severe Severe trismus, generalized rigidity, prolonged spasms, severe dysphagia, apneic spells, pulse >120 beats/min, respiratory rate >40 breaths/min IV Very severe Grade 3 with autonomic dysfunction

of other etiologies, such as oropharyngeal infection, cranial nerve diseases, intracranial hemorrhage, submaxillary lymphadenitis, or peritonsillar infections. Hypocalcemia and meningoencephalitis are included in the differential diagnosis of neonatal tetanus.

TREATMENT Tetanus If possible, the entry wound should be identified, cleaned, and debrided of necrotic material in order to remove anaerobic foci of infection and prevent further toxin production. Wound care should be performed several hours after anti-toxin administration. Fail­ ure to remove devitalized tissue and treat infection may result in recurrent or prolonged tetanus. Metronidazole (400 mg rectally or 500 mg IV every 6 h for 7 days) is preferred for antibiotic therapy. Although not a first choice for therapy, an alternative is penicillin (100,000–200,000 IU/kg per day); this drug theoretically may exac­ erbate spasms due to its ability to bind to the GABA receptor and, in one study, was associated with increased mortality. Antitoxin should be given early in an attempt to deactivate any circulating tetanus toxin and prevent its uptake into the nervous system. Two preparations are available: human tetanus immuno­ globulin (HTIG) and equine antitoxin. HTIG is the preparation of choice, as it is less likely to be associated with anaphylactoid reac­ tions. A single IM dose (500–5000 IU) is given. Equine-derived antitoxin is available widely and is used in low- and middle-income countries; after hypersensitivity testing, 10,000–20,000 U is admin­ istered IM. According to a recent randomized controlled trial in Vietnam, there were no significant differences in the outcome and complications between groups using either intramuscular equine antitoxin or HTIG. Additional intrathecal antitoxin (HTIG) showed no benefit. CHAPTER 157 Benzodiazepines are commonly used to control spasms, and patients can tolerate them in high doses. The use of intermittent or continuous sedation depends on the severity of spasms and the availability of appropriate resources, e.g., mechanical ventilators. High-dose diazepam may cause hyperosmolarity and lactic acido­ sis. Midazolam is another option for controlling spasms in tetanus; it has fewer side effects and can be used continuously. Infusions of propofol can be used to control spasms and provide sedation; how­ ever, consideration should be given to the likely long duration of therapy. Sedatives should be reduced in the elderly and patients who have liver diseases due to drug accumulation and slow excretion. Tetanus When sedatives alone cannot control spasms, a combination of neuromuscular blocking agents is recommended. Nondepolarizing neuromuscular blockers are used in clinical practice, depending upon availability. IV magnesium sulfate has been used as a muscle relaxant. Patients who are using these agents require mechani­ cal ventilation support. In those settings with limited availability of mechanical ventilators, controlling spasms while maintaining adequate ventilation is problematic. Patients may require ventilator support for several weeks. It is important to establish a secure airway early in severe teta­ nus. Ideally, patients should be nursed in calm, quiet environments because light and noise can trigger spasms. Dysphagia due to pharyngeal involvement combined with hyperactivity of laryngeal muscles makes endotracheal intubation difficult. Tracheostomy is the better option for securing the airway in severe tetanus. Magnesium sulfate has been used in autonomic nervous system dysfunction to control high blood pressure and tachycardia; main­ taining a plasma concentration of 2–4 mmol/L is recommended. If magnesium alone cannot control tachycardia, short-acting betablockers (labetalol, propranolol, esmolol) or calcium antagonists can be used with strict monitoring. When the parasympathetic nervous system predominates—resulting in prolonged low blood pressure and bradycardia—vasopressors are required. A complication arising from treatment with diazepam injection is thrombophlebitis. Long-term hospitalization with high-dose sedation

creates a high risk of hospital-acquired infections (ventilator-associated pneumonia, bloodstream infection, urinary tract infection, sepsis), deep vein thrombosis, pneumonia emboli, myocardiopathy, myo­ cardial infarction, stress ulcers, muscle weakness, and pressure sores. Many of these patients require long-term rehabilitation.

Patients must be given a full primary course of immunization as tetanus toxin is poorly immunogenic and the immune response following natural infection is inadequate. ■ ■PROGNOSIS Rapid development of tetanus is associated with more severe disease; it is important to note time of onset and length of incubation period. More sophisticated modeling has revealed other important predic­ tors of prognosis. In many adults, particularly in the elderly, surviv­ ing tetanus is associated with reduced long-term functional outcome measures. Studies of children and neonates have suggested a higher incidence of neurologic sequelae. Neonates may be at increased risk of learning disabilities, behavioral problems, cerebral palsy, and deafness. Tetanus has a high survival rate if complications can be controlled and interventions conducted promptly. In assessing prognosis, the incubation period (time from wound to first symptom) and the period of onset (time from first symptom to first generalized spasm or pharyngeal or laryngeal spasm) are of par­ ticular significance. The shorter these periods, the worse is the prog­ nosis. Among the three main scales available to predict the severity and outcome of tetanus, the Tetanus Severity Score (TSS) (Table 157-2) is superior to the Dakar and Philips scores, with a sensitivity of 66% and a specificity of 91%. PART 5 Infectious Diseases ■ ■PREVENTION Tetanus is prevented by good wound management and vaccination (Chap. 129). Safe delivery, hygienic umbilical-cord care, and maternal vaccination are recommended to prevent neonatal tetanus. Individuals sustaining wounds should undergo passive immunization (see “Treat­ ment of Tetanus,” above) if their vaccination status is incomplete or unknown or if their last booster was given >10 years earlier. Vaccina­ tion programs and recommended prevention measures vary somewhat according to individual countries. The rate of primary vaccination coverage in infancy (three doses of DTP [diphtheria, pertussis, and tetanus]) is 86%, but rates for the subsequent boosters necessary for long-term protection are unknown. WHO guidelines for tetanus vaccination consist of a primary course of three doses and should start at 6 weeks of age; the interval between doses is 4 weeks. The first booster is given from 12–23 months of age. The second and third boosters are at 4–7 and 9–15 years of age, respec­ tively. There should be at least 4 years between booster doses. In 2022, the CDC reported the rate of infants who had received a full primary vaccination course of three doses of the diphtheria, tetanus toxoid (DTP3), and pertussis vaccine was 94%. There is a lack of data about boosters in older children, adolescents, and adults in many coun­ tries. The CDC recommends five doses of tetanus vaccine for infants and children at 2 months, 4 months, 6 months, 15 through 18 months, and 4 through 6 years of age, an additional booster dose at 11–12 years of age, and every 10 years thereafter. For those with delayed primary vaccination, catch-up immunization schedules are recommended. There are separate schedules for children ages 4 months through 6 years of age and from ages 7 through 18 years of age. Complete maternal vaccination reduces the incidence of neona­ tal tetanus by an estimated 94%. WHO recommends that pregnant women who have not been vaccinated with the tetanus vaccine should receive at least two doses, with an interval of 4 weeks between doses. The second dose should be given at least 2 weeks before delivery. The third dose should be received at least 6 months later, with the fourth and fifth doses following an interval of 1 year or during subsequent pregnancies. A total of five doses can provide long-term immunity. The CDC provides a special schedule for those with partial vaccination. In high-risk areas, women of childbearing age should receive a primary course of vaccination and education on safe delivery and postnatal practices.

TABLE 157-2  Tetanus Severity Score (TSS) (Sensitivity 66%, Specificity 91%), Cutoff Point to Predict Death ≥8 VARIABLES SCORE Age (year)   ≤70   71–80   >80

Time from first symptom to admission (days)   ≤2   3–5   >5

–5 –6 Difficulty breathing on admission   No   Yes

Coexisting medical condition   Fit and well   Minor illness or injury   Moderately severe illness   Severe illness not immediately life threatening   Immediately life-threatening illness

Entry sites   Internal or injection   Other (including unknown)

Highest systolic blood pressure recorded during first day in

hospital (mmHg)   ≤130   131–140   >140

Highest heart rate recorded during first day in hospital (beats/min)   ≤100   101–110   111–120   >120

Lowest heart rate recorded during first day in hospital (beats/min)   ≤110   >110

–2 Highest temperature recorded during first day in hospital (oC)   ≤38.5   38.6–39   39.1–40   >40

Since March 2022, 47 countries have achieved maternal and neo­ natal tetanus elimination. Despite this relative success, immunization programs need to be continued and promoted to maintain individual long-term protective immunity and to eliminate the incidence of tetanus gradually. Dedicated public health initiatives still need to be improved, and the continuing reports of sizable case series in the medi­ cal literature suggest that tetanus continues to pose a significant global health burden. Acknowledgment The authors wish to thank Dr. Lam Minh Yen for her contributions to this chapter in previous editions. ■ ■FURTHER READING Borrow R et al: The immunological basis for immunization series. Module 3: Tetanus update 2018. Edited by Vaccines and Biologicals Immunization. World Health Organization, 2018. Kyu HH et al: Mortality from tetanus between 1990 and 2015: Findings from the global burden of disease study 2015. BMC Public Health 17:179, 2017. Rodrigo C et al: Pharmacological management of tetanus: An evidencebased review. Crit Care 18:217, 2014.

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158 Botulism

Thwaites CL et al: Magnesium sulfate for treatment of severe tetanus: A randomised controlled trial. Lancet 368:1436, 2006. Thwaites CL et al: Predicting the clinical outcome of tetanus: The tetanus severity score. Trop Med Int Health 11:279, 2006. Van Hao N et al: Human versus equine intramuscular antitoxin, with or without human intrathecal antitoxin, for the treatment of adults with tetanus: A 2 x 2 factorial randomised controlled trial. Lancet Glob Health 10:e862, 2022. Yen LM, Thwaites CL: Tetanus. Lancet 393:1657, 2019. ■ ■WEBSITES Centers for Disease Control and Prevention: Pink Book. Teta­ nus. 2021. www.cdc.gov/vaccines/pubs/pinkbook/downloads/tetanus

.pdf. Centers for Disease Control and Prevention: Recommended catch-up immunization schedule for children and adolescents who start late or who are more than 1 month behind. https://www.cdc.gov/ vaccines/schedules/hcp/imz/catchup.html. Health Protection Agency: Tetanus: Information for health professionals. 2013. www.gov.uk/government/publications/ tetanus-advice-for-health-professionals. Statista: Reported cases of tetanus in the United States from 1980 to 2022, by year. www.statista.com/statistics/1122819/ tetanus-cases-us-by-year/. World Health Organization: Immunization coverage. www.who

.int/news-room/fact-sheets/detail/immunization-coverage. World Health Organization: Maternal and neonatal teta­ nus (MNT) elimination. www.who.int/immunization/diseases/ MNTE_initiative/en/. World Health Organization: Tetanus. www.who.int/news-room/ fact-sheets/detail/tetanus. Carolina Lúquez, Jeremy Sobel

Botulism Botulism is a rare, life-threatening disease characterized by cranial nerve palsies and symmetric descending flaccid paralysis. Four forms of naturally occurring botulism have been described: foodborne botulism, infant botulism, wound botulism, and adult intestinal colonization. Other forms of botulism include iatrogenic botulism and inhalational botulism. Effective treatment depends on early clinical diagnosis. ■ ■ETIOLOGY AND PATHOGENESIS Botulism is caused by botulinum neurotoxins (BoNTs), which are produced by Clostridium botulinum. Rare strains of Clostridium butyricum and Clostridium baratii can also produce BoNTs. Seven distinct serotypes of BoNT (A through G) are well characterized; sero­ types A, B, E, and F reportedly cause disease in humans. Novel serotypes— BoNT/FA (or H or HA), BoNT/En, and BoNT/X—have been proposed, but the scientific community has not yet reached a consensus as to whether each represents a new serotype or a combination of known serotypes, as in the case of BoNT/FA (or H or HA), or whether they represent true toxins or botulinum-like proteins, as in the case of BoNT/En and BoNT/X. BoNTs are encoded by the bont gene, which is also diverse in its DNA sequence. At least 40 unique subtypes of BoNT have been identified within serotypes A, B, E, and F. By definition, a variant of BoNT represents a new subtype when its amino acid sequence differs by at least 2.6% from those of all known subtypes within that particular serotype. Although 2.6% is an arbitrary thresh­ old, this figure has provided the basis for genetic subtype designations

for the past decade, aiding in the classification of BoNTs as new DNA or amino acid sequences become publicly available. In addition, bont genes typically reside within two types of gene clusters. One type includes ha genes encoding hemagglutinin proteins, which facilitate the absorption of toxins across the epithelial barrier. The other type of cluster includes orfX genes that encode proteins with unknown func­ tions. Both cluster types include an ntnh gene, which encodes for a nontoxic nonhemagglutinin protein. It has been proposed that these accessory proteins form a complex with BoNTs and protect them from external proteolytic activity.

Despite their structural variability, BoNTs all have a similar mecha­ nism of action: they target neurons and block neurotransmission by cleaving SNARE-family proteins in the host, with consequent inhibi­ tion of acetylcholine release. BoNTs are metalloproteases composed of a light chain and a heavy chain. The light chain has catalytic activity, and the heavy chain contains a translocation domain and a receptorbinding domain. The receptor-binding domain of the heavy chain mediates the neurospecific binding of BoNTs, which leads to its inter­ nalization within endocytic compartments. Interaction of the trans­ location domain of the heavy chain with the membrane of endocytic vesicles leads to the translocation of the light chain into the cytosol. Once in the cytosol, the light chain cleaves specific SNARE-family proteins. Serotypes A and E cleave SNAP-25; serotypes B, D, F, and G cleave VAMP; and serotype C cleaves SNAP-25 and syntaxin. Cleavage of any of these proteins disrupts the assembly of synaptic fusion com­ plexes, and this disruption inhibits the fusion of the membrane of the synaptic vesicle containing acetylcholine with the neuronal cell mem­ brane. Clinically, the result is flaccid paralysis of voluntary muscles. The irreversible binding of BoNTs to their targets has a clinical conse­ quence: once toxin binding has occurred, the resulting paralysis per­ sists for weeks or months, until nerve endings have been regenerated. CHAPTER 158 BoNTs are produced by C. botulinum and a few strains of C. butyricum and C. baratii, which are all gram-positive, rod-shaped, spore-forming, anaerobic bacteria. Under most environmental conditions, C. botulinum exists as spores that are heat-resistant and ubiquitous in soil. In gen­ eral, C. botulinum spores require temperatures above boiling to ensure destruction; their thermal resistance increases with higher pH and lower salt content. Spores present in foods can survive most preserva­ tion methods and, if the conditions allow it, can germinate and pro­ duce BoNTs in significant amounts to cause disease. Commercial retort canning reliably destroys botulinal spores. Botulism BoNTs are among the most toxic substances known. Extremely small amounts of BoNT can cause severe disease and death. Sever­ ity of disease varies with dose, serotype, and route of exposure. The lethal dose of BoNT in humans is not known but can be estimated by extrapolation of toxicity data from animal studies. The estimated human lethal dose of BoNT acquired via the IV or IM route is 0.1–1 ng/kg of body weight. The human lethal dose of BoNT acquired by inhalation of aerosolized toxin is estimated at 1–75 ng/kg. The degree of toxicity of BoNT acquired by the oral route is estimated to be much lower: 0.1–1 μg/kg. As stated above, four naturally occurring and two non–naturally occurring forms of botulism are known. Foodborne botulism is caused by the ingestion of foods contaminated with BoNT. Wound botulism occurs when spores of BoNT-producing species of Clostridium con­ taminate a wound and then germinate, multiply, and produce toxin. Infant botulism is caused by BoNT-producing species of Clostridium colonizing the intestinal tract of infants ≤1 year of age. Adult intestinal colonization is similar to infant botulism but affects persons >1 year of age. Iatrogenic botulism occurs when a patient given injections of BoNT experiences signs of systemic botulism. BoNTs can also be aerosolized and used as a bioweapon, entering the human body by inhalation. Foodborne Botulism  Foodborne botulism is the most common form reported in many countries. Every case of foodborne botulism represents a public health emergency because of the potential for caus­ ing outbreaks. Foodborne botulism is an intoxication in which food containing preformed toxin is ingested. Spores of BoNT-producing

species of Clostridium are ubiquitous in soil and can be found on veg­ etables and other foodstuffs. C. botulinum type E is commonly found in aquatic environments and in aquatic animals. Because the spores are found in many foods, improper preparation or storage may pro­ duce the confluence of conditions that allow germination and growth of BoNT-producing species of Clostridium, which in turn result in production of BoNT. Both historically and at the present time, canned foods are of concern because they create anaerobic environments. To render these foods safe, proper processing procedures in conditions of enough heat and pressure to inactivate Clostridium spores, along with sufficient acidity, salinity, or other preservative methods to limit the organism’s growth and its production of BoNT, are required. Low-acidity foods, such as corn, peppers, potatoes, and beets, represent a higher risk. A series of botulism outbreaks from commercially canned foods in the early twentieth century resulted in standardization of retort can­ ning methods and promulgation and enforcement of production safety codes. Consumption of fish or other foods of marine origin can cause botulism if prepared or conserved improperly. Most foodborne botu­ lism cases in the United States are caused by home-canned vegetables such as green beans; however, commercially prepared foods, including chicken broth, carrot juice, hot dog chili sauce, and nacho cheese, also have been implicated in recent outbreaks. Marine mammal and fish products traditionally prepared by Alaskan Natives and First Peoples are the main source of botulism in Alaska and Canada.

Wound Botulism  Wound botulism is caused by germination and growth of C. botulinum spores in a wound or necrotic tissue where they produce BoNT, which then enters circulation and produces systemic disease. Few cases of wound botulism were described in the United States until 1981, when the first case associated with injection drug use was reported. Since then, botulism cases due to injection drug use, especially in association with subcutaneous or tissue injection (skin popping) of black tar heroin, have substantially increased in the United States. Black tar heroin was introduced into the United States in the 1970s and, since the late 1980s, has become the predominant form of heroin west of the Mississippi River. Black tar heroin is contaminated with by-products of the manufacturing process, adulterants, and diluents and therefore is considered the most probable source of C. botulinum spores. In recent decades, the few cases of wound botu­ lism not associated with injection drug use have been associated with vehicle crashes, gunshot wounds, open-fracture wounds, and penetrat­ ing wounds caused by contaminated objects. PART 5 Infectious Diseases Infant Botulism  Infant botulism is the most common form of botulism in the United States. It affects infants ≤1 year old, with a mean age at onset of 14 weeks. It has been suggested that the intestinal microbiota in infants may induce susceptibility to botulism; animal models seem to support this claim. Spores of BoNT-producing species of Clostridium can enter the body by ingestion. The highly resistant spores survive passage through the stomach and colonize the intes­ tine, where they germinate, grow, and produce BoNT in situ. Infants can continue excreting C. botulinum for weeks after clinical recovery. Spores of BoNT-producing species of Clostridium have been found in honey. Consumption of honey has been epidemiologically implicated in infant botulism; therefore, honey should not be fed to babies ≤1 year of age. Honey exposure, however, explains only a small proportion of cases. As spores are found in dust and soil, most infant botulism patients probably acquire BoNT-producing species of Clostridium by swallowing dust particles. Why only a few dozen infants are affected each year when presumably most infants regularly ingest clostridial spores remains unknown. Adult Intestinal Botulism  Similar to infant botulism, adult intestinal colonization is caused by spores of BoNT-producing species of Clostridium colonizing the large intestine, growing, and producing BoNT in situ. Although spores are routinely ingested and excreted by humans, the adult intestinal tract does not support spore germination and toxin production under normal circumstances. Adult intestinal colonization is usually associated with inborn anatomic abnormalities, gastrointestinal surgery, or prolonged use of antibiotics, which may

alter the normal intestinal microbiota and facilitate colonization by BoNT-producing species of Clostridium. Although these associated conditions are relatively common, fewer than 30 cases of adult intesti­ nal colonization have been reported worldwide. Iatrogenic Botulism  Iatrogenic botulism occurs in patients injected with large doses of BoNT for treatment of muscle complica­ tions related to such conditions as cerebral palsy and spastic dystonia. The small doses of botulinum toxin used for wrinkle elimination in dermatologic practice are usually insufficient to cause systemic dis­ ease. In 2004, an outbreak of four cases caused by the injection of an unlicensed, highly concentrated BoNT product for cosmetic purposes occurred in the United States. Similarly, in 2017, an outbreak of nine cases occurred in Egypt in association with an unlicensed, highly con­ centrated BoNT preparation. Weaponized Inhalational Botulism  BoNTs were weaponized by the biological weapons programs of several countries in the twentieth century. Aerosolized BoNTs can be used as a bioweapon, exerting their effect by entering the body through inhalation. In the United States, BoNTs are designated as Tier 1 select agents—i.e., agents that present the greatest risk of deliberate misuse with significant potential for mass casualties or devastating effects on the economy, critical infrastructure, or public confidence. Tier 1 agents pose a severe threat to public health and safety. Terrorists have attempted to use BoNT as a bioweapon: Aum Shinrikyo, a Japanese cult, tried unsuccessfully to aerosolize BoNT in terrorism attacks at multiple sites in Japan between 1990 and 1995. ■ ■EPIDEMIOLOGY Foodborne Botulism  In the United States, foodborne botulism is the third most common form of botulism. From 2001 to 2019, 362 foodborne botulism cases were reported, with a mean of 19 cases per year. Most cases (64%) were caused by serotype A BoNT, which was followed in frequency by serotype E (26%). Serotypes B and F caused 7% and 2% of foodborne botulism cases, respectively. Outbreaks caused by serotype E usually had a shorter incubation period, those caused by type A had higher numbers of patients who required mechanical ventilation, and those caused by type B had lower numbers of deaths. Foodborne botulism cases are usually sporadic (i.e., cases occur singly), but small and large outbreaks also can occur. From 2001 to 2019, 5 foodborne botulism outbreaks affecting 10 or more people were reported in the United States (Table 158-1). Every case of food­ borne botulism is considered a public health emergency because it may be the first in an outbreak involving additional patients. Most foodborne botulism cases in the United States are due to a wide variety of home-canned vegetables and pickled vegetables (e.g., beets, green beans, carrots, mushrooms, asparagus, peppers, beans, mustard greens, corn, tomato sauce, olives, pumpkin butter), vegeta­ bles baked in aluminum foil (e.g., potatoes, beets), home-canned meatbased foods (e.g., tuna, pickled pigs’ feet, stew, pasta in meat sauce), TABLE 158-1  Total Foodborne Botulism Outbreaks of 10 or More Cases Reported in the United States Between 2001 and 2019 NO. OF CONFIRMED CASES YEAR STATE FOOD SOURCE

Texas Chili

Multistate Commercially canned hot dog chili sauce

Ohio Home-canned potatoes used to prepare a potato salad, served at a church potluck

Mississippi Pruno, illegal alcoholic beverage consumed by persons who were incarcerated at a federal facility

California Commercially produced nacho cheese, sold at a convenience store

oil-based foods (e.g., pasta and jarred pesto, homemade garlic-infused oil), herbal deer antler tea, home-prepared fermented tofu, commercial clam chowder, or commercial grain and vegetable products. In Alaska, traditional Alaskan Native foods linked to foodborne botulism cases have included seal oil, seal blubber, dried herring in seal oil, fermented seal flipper, stinkheads and other fermented fish heads, stinkfish, salmon eggs, beaver tail, whitefish, fish eggs, fermented beluga, and whale blubber. Commercial food manufacturing processes include retort canning, in which high temperature and pressure destroy the highly resistant clostridial spores, and manipulations that inhibit bacterial growth, such as acidification or addition of growth inhibitors that prevent germination and growth of BoNT-producing species of Clostridium and the production of BoNT. However, commercial foods occasionally still cause botulism if safe manufacturing processes are not followed or fail or if foods are stored or used inappropriately by the retailer or consumer. For instance, an outbreak of 10 cases associated with com­ mercially canned hot dog chili sauce occurred in 2007 as a result of deficiencies in the canning process. Other commercial food–associated outbreaks that occurred in the United States between 2001 and 2019 include a 2001 outbreak of 16 cases linked to chili that was stored at inappropriate temperatures and later served at a church event in Texas and a 2006 outbreak linked to commercial carrot juice, which included four cases in the United States and two cases in Canada. The investiga­ tion of the latter outbreak led to an international product recall. The juice, which had no added sugar, salt, or preservatives, was stored at inappropriate temperatures. Pruno, an illicit prison-brewed alcoholic beverage, first caused a botulism outbreak in a California prison in 2004, affecting 4 prisoners. In 2011, a second outbreak due to pruno was reported and involved 8 patients at a prison in Utah. In 2012, 2 outbreaks associated with pruno occurred in a single prison in Arizona, with 4 and 8 cases, respectively. The largest outbreak from pruno occurred in 2016 in a Mississippi prison; 31 cases were identified, including 19 confirmed and 12 suspected. Wound Botulism  Wound botulism was once rare in the United States, but its frequency has been increasing for decades, and it is now the second most common form of botulism. Between 2001 and 2019, 452 cases of wound botulism were reported, with an average of 24 cases per year. Most cases (92%) were caused by BoNT serotype A and by serotype B (5%). Most cases (96%) were among persons who injected drugs (mainly black tar heroin), and the remaining 4% of cases were due to traumatic injuries. Infant Botulism  Infant botulism is the most common form of botulism in the United States. Between 2001 and 2019, 2172 infant botulism cases were reported. BoNT serotypes A and B caused most cases (40% and 58%, respectively). Only 2 cases were due to serotype E. Of these 2 cases, 1 was due to C. botulinum type E and the other to C. butyricum type E; both cases represented the first report anywhere in this country of infant botulism due to those respective organisms. A small fraction (<1%) of cases were caused by serotype F. Of note, 22 infant botulism cases were due to strains of C. botulinum that can produce two BoNT serotypes (A and B or B and F). Botulism of Other Etiologies  Between 2001 and 2019, 51 cases were reported as being of “unknown or other etiology.” This category includes laboratory-confirmed botulism cases that do not meet the definition of foodborne, infant, or wound botulism. Most of these cases were caused by serotype A (63%) and serotype F (27%). Many were thought to be cases of adult intestinal colonization, although confirma­ tion of this form of botulism is not always possible. ■ ■CLINICAL MANIFESTATIONS Botulism produces a syndrome characterized by bilateral cranial nerve palsies that may be followed by symmetric, descending flaccid paralysis that may cause respiratory arrest. There are no sensory deficits; patients are fully conscious, with normal intellectual function, although cranial nerve palsies may give a mistaken impression of altered consciousness.

The typical incubation period (based on data for foodborne botulism cases, where exposure can be identified) is 1 or 2 days, but a range of 6 h to >7 days has been reported. Several recent systematic reviews substantiate long-known observations that the syndrome is essentially identical for all types of botulism in patients of all ages, although elici­ tation of the typical signs and symptoms may be challenging in infants and young children. A recent systematic review of 16 cases of botulism in pregnant women reported the same clinical syndrome as in non­ pregnant individuals. In all botulism syndromes, the first neurologic manifestation usually is ptosis, which can be striking. Ocular findings of fuzzy vision or frank diplopia are caused by extraocular muscle paralysis due to palsies of cranial nerves III, IV, and VI. Flat, youthfully unlined, expressionless facies are produced by cranial nerve VII (facial nerve) palsy. Dysarthria is also a prominent manifestation. Oral and nasal regurgitation of foods or beverages is caused by cranial nerve IX (glossopharyngeal nerve) palsy. The autonomic system may be affected, producing anhidrosis manifesting as severe pharyngeal pain and erythema that has been mistaken for pharyngitis; paradoxically, other patients experience an inability to manage copious oral secre­ tions. Autonomic dysfunction may produce hemodynamic instability requiring monitoring. Cranial nerve palsy may produce pharyngeal muscle flaccidity, causing airway collapse and respiratory arrest early in the course of illness, while reduction in diaphragmatic and accessory muscle function may cause respiratory compromise hours or days later. Cranial nerve palsies may be followed by descending symmetric flaccid paralysis of the muscles of the neck, shoulders, upper limbs, and lower limbs; proximal muscle groups of each limb are affected before distal muscle groups.

CHAPTER 158 A recent analysis of 332 U.S. botulism cases found the following frequencies for patient-reported symptoms: difficulty swallowing, 86%; fatigue, 85%; blurred vision, 80%; slurred speech, 78%; double vision, 76%; shortness of breath, 65%; and dry mouth, 62%. The analysis also reported the following frequencies of observed signs: afebrile body temperature, 99%; descending paralysis, 93%; alert and oriented status, 93%; ptosis, 81%; limb weakness, 78%; decreased palatal reflex, 54%; facial palsy, 47%; and dilated pupils, 20%. Sixty-six percent of patients were intubated and received mechanical ventilation. These findings are similar to those reported in many smaller series. Rarely, asymmetry of cranial nerve palsies or distal muscle paralysis is reported and, at least in some cases (especially those described in reports based on chart abstractions), may reflect an incomplete or incompletely recorded neurologic examination. Despite intact sensorium, symptoms such as ptosis, dysarthria, and gait instability may be mistaken for diminished consciousness and lack of coordination and may be erroneously attrib­ uted to intoxication from alcohol or other substances. Paresthesias have been reported in some patients; these sensations are not explained by the known activity of botulinum toxin. Paralysis of the diaphragm and accessory muscles of respiration may occur, producing respiratory compromise. Distal tendon reflexes diminish symmetrically. Constipa­ tion due to intestinal paralysis develops in almost all patients. Nausea and vomiting may occur in foodborne botulism, preceding neurologic symptoms. Whether these manifestations are due to BoNT, other prod­ ucts of BoNT-producing species of Clostridium, or other contaminants of spoiled food is unknown. These gastrointestinal symptoms have not been reported in wound botulism. Botulism Death in untreated patients during the first hours to days of illness is caused by airway obstruction resulting from pharyngeal muscle paralysis and inadequate tidal volume resulting from paralysis of dia­ phragmatic and accessory respiratory muscles. The combination of expressionless facies from cranial nerve paralysis and immobility from voluntary muscle paralysis may give patients with botulism a placid appearance that masks the agitation expected with respiratory distress. Respiratory compromise occurs early in the course of disease in a sub­ stantial proportion of patients: the largest systematic literature review to date of foodborne and wound botulism cases (402 patients) reported that the average time from symptom onset to hospitalization was 2 days and that, at hospital admission, 42% of patients had respiratory symp­ toms; of these patients, 42% presented with no extremity weakness. In the same review, 87% of patients who required mechanical ventilation

were intubated during the first 2 days of hospitalization. The severity of disease varies greatly between patients and is probably governed by the dose of toxin to which they have been exposed. Without treatment, some patients do not progress beyond ptosis and mild palsy in one or two cranial nerves; others experience fulminant cranial nerve palsies and rapidly progressive descending flaccid paralysis eventually affect­ ing most or all voluntary muscles as well as respiratory failure requiring intubation and mechanical ventilation within hours.

The different BoNT serotypes are associated with variations in the botulism syndrome. BoNT type A is associated with more rapid disease progression, more frequent respiratory compromise and mechanical ventilation, and longer duration of paralysis. Type B is associated with a milder syndrome, with less severe and shorter-duration paralysis. Intoxication with the rarely occurring type F produces a syndrome of rapidly progressing paralysis that often leads to respiratory failure, with more rapid recovery than occurs with other toxin types. However, all toxin types causing human illness can cause severe disease; the clinical approach is the same for all. The paralysis of botulism can last for weeks or months—the time required for regeneration of affected nerve endings and recovery of voluntary muscle function. For severely affected patients with exten­ sive paralysis, management consists of protracted intensive care, with detection and treatment of attendant risks not specific to botulism, such as ventilator-associated pneumonia, decubitus ulcers, and psy­ chological trauma. More than 95% of noninfant botulism patients in the United States recover; hospital discharge is often followed by protracted rehabilitative care. The survival rate for infant botulism is near 100%. PART 5 Infectious Diseases ■ ■CLINICAL DIAGNOSIS AND LABORATORY CONFIRMATION Rapid clinical diagnosis is essential. A diagnostic aid for botulism, “Clinical Criteria to Trigger Suspicion of Botulism,” has been published by botulism consultants at the Centers for Disease Control and Pre­ vention (CDC; accessible at academic.oup.com/cid/article/66/suppl_1/ S38/4780423). The paralysis of botulism lasts for weeks or months, and administration of equine-source botulinum antitoxin (BAT)—the specific therapy to arrest the progression of paralysis—depends on the correct diagnosis. At this time, laboratory confirmation of botulism, which may require ≥24 h, must take place at a specialized public health laboratory. Therefore, effective, timely treatment relies on rapid clini­ cal diagnosis of botulism in a patient with clinically compatible find­ ings, without awaiting laboratory confirmation. A clinician suspecting noninfant botulism in a patient should immediately contact the state health department’s emergency 24-h line. The state will connect the clinician with a botulism clinical consultant at the CDC (or, in Alaska, California, and Colorado, at the state health department), who will review the case with the clinician, assist in the shipping of appropri­ ate specimens to a public health laboratory for definitive diagnosis, and, when indicated, arrange for immediate shipping of BAT from the federal stockpile at no charge. A clinician suspecting infant botulism in a patient should immediately contact the Infant Botulism Treat­ ment and Prevention Program’s on-call physician at (510)231-7600, who will provide consultation, assist with specimen collection, and, when indicated, assist with the provision of human-derived botulinum antitoxin (BabyBIG), a specific treatment licensed for treatment of infant botulism. The neurologic examination is the key to clinical diagnosis of botu­ lism, as it readily uncovers the cranial nerve deficits that are invariably present in botulism and focuses the differential diagnosis. In principle, the distinct syndrome of bilateral cranial palsies and descending flac­ cid paralysis in a fully conscious patient should render the diagnosis and prompt treatment of botulism straightforward. The presentation of two or more patients with this syndrome is almost pathognomonic, since other illnesses considered during the differential diagnosis of botulism do not produce outbreaks. In practice, however, sporadic (lone) cases of botulism are misdiagnosed, and sometimes the diagno­ sis is missed even in the setting of an outbreak. In part, these failures may be due to the rarity of botulism and the clinician’s unfamiliarity

with its presentation. A possible cause of misdiagnosis is failure to perform a complete neurologic examination; indeed, review of some botulism patients’ charts reveals documentation of the first neurologic examination, which suggested the correct diagnosis, days after hospital admission. As stated earlier, the combination of ptosis, dysarthria, and perceived gait instability from muscle paralysis in some cases may be misinterpreted as intoxication from alcohol or other substances. In other cases, rapidly progressing botulism may result in pharyngeal col­ lapse and respiratory distress relatively early in the course, leading the clinical team to focus on airway management and primary respiratory diagnoses and thus delaying the neurologic evaluation. Standard clinical studies, including bloodwork and radiology, are not useful in diagnosing botulism. In contrast to the findings in Guillain-Barré syndrome (GBS; see below), lumbar-puncture cere­ brospinal fluid (CSF) values—and specifically the protein level—are usually normal in botulism. The CSF protein level may be very slightly elevated in a minority of botulism cases. The fact that botulism pro­ duces no abnormal findings on brain imaging may help rule out rare basilar strokes that produce nonlateralizing symptoms. Serological antibody tests and rapid nerve stimulation tests can rule out myas­ thenia gravis. Electromyography, when performed by an experienced practitioner, can provide support for the diagnosis. Botulism is indi­ cated by findings consistent with neuromuscular junction blockage, normal axonal conduction, and potentiation with rapid repetitive stimulation in affected muscles. Once a neurologic examination reveals the cranial nerve palsies of botulism and any additional bilateral flaccid paralysis, the differential diagnosis may include GBS, myasthenia gravis, Lambert-Eaton syn­ drome, and tick paralysis. Less likely conditions include tetrodotoxin or shellfish poisoning, antimicrobial-associated paralysis, and rarer poisonings. A careful history and physical examination can further narrow the range of diagnoses. GBS is a rare (~1 case per 100,000 population per year in the United States) autoimmune demyelinating polyneuropathy that follows acute infection by Campylobacter jejuni, certain viruses, and other bacteria. In 95% of cases, GBS presents as an ascending paralysis. Recent reports from Peru indicate massive outbreaks of GBS of unknown cause, challenging the previously held notion that conditions causing flaccid paralysis other than botulism occur only as sporadic cases. The 5% of GBS cases presenting as the Miller Fisher variant are characterized by the triad of ophthalmoplegia, ataxia, and areflexia, which may resemble early descending paralysis. The CSF protein level is elevated in GBS, but the increase may take place days after symptom onset; thus, normal CSF levels should be taken into account along with the duration of symptoms, and lumbar puncture may need to be repeated. Electromyography performed by an experienced operator may yield findings indicative of GBS and not botulism. Serological antibody tests or rapid nerve stimulation tests can confirm myasthenia gravis; borderline positive Tensilon tests have been reported in botulism patients. In most stroke patients, the physi­ cal examination should reveal asymmetric paralysis and upper motor neuron signs; brain imaging can help reveal rare basilar strokes that can produce symmetric bulbar palsies. The history and physical examina­ tion should rule out Lambert-Eaton syndrome, which is characterized by proximal limb weakness in patients with advanced cancer. Laboratory testing confirms clinically diagnosed botulism cases and determines the BoNT serotype causing the disease. In addition, labo­ ratory testing can confirm epidemiologic data by demonstrating pres­ ence of BoNT in the suspected food. Botulism cases are confirmed by the laboratory when BoNT is identified in serum or stool specimens or when a BoNT-producing species of Clostridium is isolated from stool specimens or wound cultures. Identification of preformed BoNT in food consumed by patients also confirms foodborne botulism. The gold standard for identification and serotyping of BoNT in clinical or food specimens is the mouse bioassay. The drawback is that this highly sensitive and specific method requires the use of animals. Specimens are injected IP into the mice with and without antitoxin; the mice are then observed for up to 96 h for signs of botulism. If the specimen contains BoNT at levels sufficient to affect the mice quickly, results may be available within 24 h of injection. Low levels of toxin

may produce signs later, so mice should be monitored for 4 days after injection. Many in vitro methods have been developed for detection of BoNT and BoNT-producing species of Clostridium in clinical and food specimens. For instance, public health laboratories in the United States can use a real-time polymerase chain reaction test that detects bont genes encoding serotypes A through G. This test is a useful screening method to determine whether BoNT-producing species of Clostridium are present in cultures of clinical specimens, but positive results must be confirmed. Another in vitro method, the Endopep mass spectrom­ etry (Endopep-MS) assay, is highly sensitive and specific and can detect BoNT in clinical specimens and foods. The advantage of Endopep-MS is that it detects active BoNT and therefore represents an ideal alterna­ tive to the mouse bioassay. Immune-based assays can provide rapid and sensitive results; their main limitation is that they detect antigens, which may not necessarily represent active BoNT. Cell-based in vitro assays are also a possible alternative to the mouse bioassay as they detect biologic activity of BoNT. TREATMENT Botulism Treatment for botulism consists of two components: meticulous monitoring and supportive care, including admittance to the inten­ sive care unit when indicated, and administration of botulinum antitoxin, the only specific therapy for botulism, as quickly as possible. Paralysis from botulism can be rapidly progressive. Vital capacity, and often hemodynamic parameters, should be frequently monitored and mechanical ventilation instituted immediately if needed. Paralysis induced by BoNT lasts weeks or months, and patients with extensive paralysis require painstaking care to avoid complications associated with protracted immobilization, including respirator-dependent pneumonia, decubitus ulcers, and psycho­ logical trauma. Patients who have recovered from severe botulism report that their appearance and immobility often led caregivers to assume they were unconscious; as a consequence, patients were sometimes subjected to painful procedures without warning and to insensitive comments. Signage should remind all caregivers that botulism patients are conscious but “locked in.” Psychological sup­ port should be instituted for intubated botulism patients from the outset. With proper supportive care, >95% of botulism patients in the United States recover, even without antitoxin therapy; however, antitoxin, if promptly administered, can substantially reduce the extent and duration of illness (see below). Botulinum antitoxin is the only specific treatment for botulism. The antitoxin prevents the progression of paralysis but does not reverse existing paralysis. If given early enough in the course of disease, it may avert respiratory compromise, obviate mechanical intubation, and forestall protracted paralysis and hospitalization along with associated complications. Accordingly, it is essential to administer antitoxin as soon as possible. A recent systematic literature review and meta-analysis covering nearly a century of the published literature in noninfant botulism patients confirmed long-known findings from smaller studies by showing significantly reduced mortality rates among patients treated with equine anti­ toxin, especially when treatment was administered within 48 h of symptom onset. Another large systematic literature review of pediatric noninfant botulism recently showed significantly reduced mortality risk among children treated with equine antitoxin. Pub­ lished studies have demonstrated a substantial reduction in the duration and severity of illness among patients with infant botulism who are treated with human-derived botulinum antitoxin. The equine botulinum antitoxin used to treat noninfant botulism consists of antibodies produced in horses immunized with botu­ linum toxoids (inactivated toxins) and toxins. The antibodies are type-specific (anti-A neutralizes BoNT type A and so forth). The currently licensed antitoxin product in the United States, heptava­ lent botulinum antitoxin (BAT), contains antibodies to BoNT types A, B, C, D, E, F, and G. These equine antibodies have undergone

despeciation to reduce antigenicity and the risk of anaphylaxis to foreign protein. A recent systematic literature review, along with studies of BAT use, indicated that <2% of recipients experience serious adverse reactions. Administration of one vial of BAT elicits circulating antitoxin concentrations sufficient to neutralize toxin levels one to two orders of magnitude higher than those found in the serum of most botulism patients. As noted earlier, clinicians suspecting botulism in a patient should immediately call their state health department’s emergency contact to be put in touch with a botulism clinical consultant who will review the case and assist in its management, including shipment of BAT from the federal stock­ pile at no charge. The botulinum antitoxin used to treat infants, BabyBIG, consists of human antibodies obtained from hyperim­ munized volunteers. The product is licensed for treatment of infant botulism due to BoNT types A and B and, as noted earlier, can be obtained through the Infant Botulism Treatment and Prevention Program.

There is no prophylactic treatment for botulism. Persons who may have been exposed to botulinum toxin should be evaluated by a physician and carefully observed for the development of symp­ toms of botulism. If symptoms appear, the patient should be treated immediately with botulinum antitoxin. ■ ■PREVENTION No vaccine is licensed for the prevention of botulism. In the United States, a botulinum toxoid vaccine was available through the CDC until 2011, but it was discontinued because of a decline in immunogenicity of some serotypes and an increase in occurrence of moderate local reactions. Several vaccine candidates are currently in clinical trials. CHAPTER 158 Because most foodborne botulism cases are caused by home-canned or home-preserved foods, the prevention of foodborne botulism depends mainly on proper preparation and preservation that ensures the destruction of spores of BoNT-producing species of Clostridium that may be present in the food or on the creation of an environment that will not allow the germination and growth of these spores, such as low pH or low water activity. Water activity is a measure of how much water is free, unbound, and thus available to microorganisms to use for growth. If foods have low water activity, it means they do not have much free water, and growth of C. botulinum will be limited or inhib­ ited. Using pressure canners and properly cleaning items employed in the canning process can reduce the risk of foodborne botulism. Among other resources, the USDA Complete Guide to Home Canning provides a detailed description of safe home-canning practices. Other ways of preventing foodborne botulism include refrigerating homemade oils infused with garlic or herbs and discarding any of these oils that have not been used after 4 days; maintaining baked potatoes or similar foods wrapped in aluminum foil at temperatures above 140°F until served and then refrigerating leftovers; refrigerating canned or pickled foods after opening; and boiling home-canned foods before eating, especially those foods that are low in acid. Botulism Wound botulism largely affects people who inject drugs, especially black tar heroin. Using safe injection practices may help prevent wound botulism and many other infections, such as HIV and hepatitis C virus infections. Thus, educating injection drug users on the prevention of wound botulism and other infections is vital in protecting their health. As wound botulism can also follow traumatic injuries, keeping wounds clean is key. The risk factors for infant botulism are not fully understood, but pos­ sible sources of spores of BoNT-producing species of Clostridium include foods and dust. In most cases of infant botulism, no source of spores is identified. Honey is the only food that has been identified as an epide­ miologically associated reservoir of spores of BoNT-producing species of Clostridium. Honey should not be fed to infants ≤1 year of age. ■ ■GLOBAL CONSIDERATIONS Botulism has been reported from all parts of the world. The European Centre for Disease Prevention and Control has reported an average of 110 botulism cases each year from 2007 to 2018. During that period,

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159 Gas Gangrene and Other Clostridial Infections

1315 botulism cases were reported from 25 countries, with the most cases in Italy (311 cases), Romania (239 cases), and Poland (202 cases). Foodborne botulism is the most common form of botulism in Europe. Most laboratory-confirmed cases reported from Italy, Romania, and Poland were due to BoNT serotype B. The country of Georgia has a high incidence of botulism (0.9 case per 100,000 persons) relative to rates in the European Union (<0.1/100,000) and the United States (0.01/100,000). From 1980 to 2002, a total of 879 cases of botulism were reported in Georgia; all of them were foodborne, most were associated with home-preserved vegetables, and the majority were due to serotype B. From 1958 to 1983, 986 foodborne botulism outbreaks affecting 4377 individuals were reported from China. Most cases were due to serotype A and were associated with bean products. Botulism in Thailand has been associated with fermented bamboo shoots and fermented soybeans. In 2006, a large foodborne botulism outbreak associated with bamboo shoots occurred in Thailand and affected 209 people who attended a local festival. In South America, Brazil and Argentina have reported several outbreaks of foodborne botulism. For instance, between 2001 and 2008, Brazil reported 18 outbreaks, most of which were associated with meat-based foods such as home-canned meat, homemade pork liver pâté, and commercially canned liver pâté. From 1994 to 2007, Argentina reported 36 outbreaks, most frequently involving home-canned vegetables. Although reports of foodborne botulism in Africa are rare, 5 outbreaks were reported in South Africa between 1959 and 2002, with the majority due to serotype B and associ­ ated with noncommercial foods. In addition, 1 outbreak of 91 cases was reported in Egypt in 1991 and was due to serotype E associated with a traditional salted fish.

Wound botulism cases have been reported most frequently from the United States, next most frequently from the United Kingdom, and occasionally from Italy, France, and Australia. Clusters of wound botulism are rare, but, according to a report from the European Cen­ tre for Disease Prevention and Control, 23 cases of wound botulism among people who had injected heroin were reported in Norway and Scotland between December 2014 and February 2015. Other countries that have reported wound botulism cases include Argentina, China, and Ecuador. PART 5 Infectious Diseases Although rarely reported, infant botulism cases have been noted on all continents except Africa. Outside the United States (where there were 2419 cases), Argentina reported the largest number of cases (366) and Australia the next largest number (32) between 1976 and 2006. Canada, Italy, and Japan also reported a relatively large number of cases (27, 26, and 22, respectively). ■ ■FURTHER READING Centers for Disease Control and Prevention: Botulism in the United States, 1899–1996. Handbook for Epidemiologists, Clinicians, and Laboratory Workers. Atlanta, Centers for Disease Control and Prevention, 1998. Centers for Disease Control and Prevention: National Botulism Surveillance. Available at https://www.cdc.gov/botulism/php/nationalbotulism-surveillance/. Accessed December 19, 2023. Dorner MB et al: A large travel-associated outbreak of iatrogenic botulism in four European countries following intragastric botulinum neurotoxin injections for weight reduction, Türkiye, February to March 2023. Euro Surveill 28:2300203, 2023. European Centre for Disease Prevention and Control: Botulism. Available at www.ecdc.europa.eu/en/botulism.   Accessed September 27, 2020. Fleck-Derderian S et al: The epidemiology of foodborne botulism outbreaks: A systematic review. Clin Infect Dis 66:S73, 2017. Koepke R et al: Global occurrence of infant botulism, 1976–2006. Pediatrics 122:e73, 2008. National Center for Home Food Preservation: USDA Complete Guide to Home Canning, 2015 Revision. Available at nchfp.uga.edu/ papers/guide/INTRO_HomeCanrev0715.pdf.   Accessed March 18, 2024. Peck M et al: Historical perspectives and guidelines for botulinum neurotoxin subtype nomenclature. Toxins (Basel) 9:38, 2017.

Pirazzini M et al: Botulinum neurotoxins: Biology, pharmacology, and toxicology. Pharmacol Rev 69:200, 2017. Rao AK et al: Clinical criteria to trigger suspicion for botulism: An evidence-based tool to facilitate timely recognition of suspected cases during sporadic events and outbreaks. Clin Infect Dis 66:S38, 2018. Rossetto O et al: Botulinum neurotoxins: Mechanism of action. Handb Exp Pharmacol 263:35, 2021. Yu PA et al: Safety and improved clinical outcomes in patients treated with new equine-derived heptavalent botulinum antitoxin. Clin Infect Dis 66:S57, 2017. Amy E. Bryant, Dennis L. Stevens

Gas Gangrene and Other Clostridial Infections The genus Clostridium encompasses >60 species that may be com­ mensals of the gut microflora or may cause a variety of infections in humans and animals through the production of a plethora of pro­ teinaceous exotoxins. C. tetani and C. botulinum, for example, cause specific clinical disease by elaborating single but highly potent toxins. In contrast, C. perfringens and C. septicum cause aggressive necrotizing infections that are attributable to multiple toxins, including bacterial proteases, phospholipases, and cytotoxins. ETIOLOGIC AGENT Vegetative cells of Clostridium species are pleomorphic, rod-shaped, and arranged singly or in short chains (Fig. 159-1); the cells have rounded or sometimes pointed ends. Although clostridia stain gram-positive in the early stages of growth, they may appear to be gram-negative or gram-variable later in the growth cycle or in infected tissue specimens. Most strains are motile by means of peritrichous flagella; C. septicum swarms on solid media. Nonmotile species include C. perfringens, C. ramosum, and C. innocuum. Most species are obligately anaerobic, although clostridial tolerance to oxygen varies widely; some species (e.g., C. septicum, C. tertium) will grow but will not sporulate in air. Clostridia produce more protein toxins than any other bacterial genus, and more than 25 clostridial toxins lethal to mice have been identified. These proteins include neurotoxins, enterotoxins, cytotoxins, FIGURE 159-1  Scanning electron micrograph of C. perfringens.

collagenases, permeases, necrotizing toxins, lipases, lecithinases, hemo­ lysins, proteinases, hyaluronidases, DNases, ADP-ribosyltransferases, and neuraminidases. Botulinum and tetanus neurotoxins are the most potent toxins known, with lethal doses of 0.2–10 ng/kg for humans. Epsilon toxin, a 33-kDa protein produced by C. perfringens types B and D, causes edema and hemorrhage in the brain, heart, spinal cord, and kidneys of animals. It is among the most lethal of the clostridial toxins and is considered a potential agent of bioterrorism (Chap. S4). The genomic sequences of some pathogenic clostridia are now available and are likely to facilitate a comprehensive approach to understanding the virulence factors involved in clostridial pathogenesis. EPIDEMIOLOGY AND TRANSMISSION Clostridium species are widespread in nature, forming endospores that are commonly found in soil, feces, sewage, and marine sediments. The ecology of C. perfringens in soil is greatly influenced by the degree and duration of animal husbandry in a given location and is relevant to the incidence of gas gangrene caused by contamination of wounds with soil. For example, the incidence of clostridial gas gangrene is higher in agricultural regions of Europe than in the Sahara Desert of Africa. Similarly, the incidences of tetanus and food-borne botulism are clearly related to the presence of clostridial spores in soil, water, and many foods. Clostridia are present in large numbers in the indigenous microbiota of the intestinal tract of humans and animals, in the female genital tract, and on the oral mucosa. It should be noted that not all commensal clostridia are toxigenic. Clostridial infections remain a serious public health concern worldwide. In developing nations, food poisoning, necrotizing enterocolitis, and gas gangrene are common because large por­ tions of the population are poor and have little or no immediate access to health care. These infections remain prevalent in developed coun­ tries as well. Gas gangrene commonly follows knife or gunshot wounds or vehicular accidents or develops as a complication of surgery or gas­ trointestinal carcinoma. Severe clostridial infections have emerged as a health threat to injection drug users and to women undergoing child­ birth or abortion. Historically, clostridial gas gangrene has been the scourge of the battlefield. The global political situation portends another possible scenario involving mass casualties of war or terrorism, with extensive injuries conducive to gas gangrene. Therefore, there is an ongoing need to develop novel strategies to prevent or attenuate the course of clostridial infections in both civilians and military personnel. Vaccination against exotoxins important in pathogenesis would be of great benefit in developing nations and could also be used safely in atrisk populations such as the elderly, patients with diabetes who may require lower-limb surgery due to trauma or poor circulation, and those undergoing intestinal surgery. Moreover, a hyperimmune globu­ lin would be a valuable tool for prophylaxis in victims of acute TABLE 159-1  Treatment of Clostridial Infections CONDITION ANTIBIOTIC TREATMENT PENICILLIN ALLERGY ADJUNCTIVE TREATMENT/NOTE Wound contamination None — Treatment should be based on clinical signs and symptoms as listed below and not solely on bacteriologic findings. Polymicrobial anaerobic infections involving clostridia (e.g., abdominal wall, gynecologic) Ampicillin (2 g IV q4h) plus Clindamycin (600–900 mg IV q6–8h) plus Ciprofloxacin (400 mg IV q6–8h) Vancomycin (1 g IV q12h) plus Metronidazole (500 mg IV q6h) plus Ciprofloxacin (400 mg IV q6–8h) Clostridial sepsis Penicillin (3–4 mU IV q4–6h) plus Clindamycin (600–900 mg IV q6–8h) Clindamycin alone or Metronidazole (as above) or Vancomycin (as above) Gas gangrene Penicillin G (4 mU IV q4–6h) plus Clindamycin (600–900 mg IV q6–8h) Cefoxitin (2 g IV q6h) plus Clindamycin (600–900 mg IV q6–8h)

traumatic injury or for attenuation of the spread of infection in patients with established gas gangrene.

CLINICAL SYNDROMES Life-threatening clostridial infections range from intoxications (e.g., food poisoning, tetanus) to necrotizing enteritis/colitis, bacteremia, myonecrosis, and toxic shock syndrome (TSS). Tetanus and botulism are discussed in Chaps. 157 and 158, respectively. Colitis due to C. difficile is discussed in Chap. 139. ■ ■CLOSTRIDIAL WOUND CONTAMINATION Of open traumatic wounds, 30–80% reportedly are contaminated with clostridial species. In the absence of devitalized tissue, the pres­ ence of clostridia does not necessarily lead to infection. In traumatic injuries, clostridia are isolated with equal frequency from both sup­ purative and well-healing wounds. Thus, diagnosis and treatment of clostridial infection should be based on clinical signs and symptoms and not solely on bacteriologic findings. ■ ■POLYMICROBIAL INFECTIONS

INVOLVING CLOSTRIDIA Clostridial species may be found in polymicrobial infections also involv­ ing microbial components of the endogenous flora. In these infections, clostridia often appear in association with non-spore-forming anaerobes and facultative or aerobic organisms. Head and neck infections, conjunc­ tivitis, brain abscess, sinusitis, otitis, aspiration pneumonia, lung abscess, pleural empyema, cholecystitis, septic arthritis, and bone infections all may involve clostridia. These conditions are often associated with severe local inflammation but may lack the characteristic systemic signs of tox­ icity and rapid progression seen in other clostridial infections. In addi­ tion, clostridia are isolated from ~66% of intraabdominal infections in which the mucosal integrity of the bowel or respiratory system has been compromised. In this setting, C. ramosum, C. perfringens, and C. bifer­ mentans are the most commonly isolated species. Their presence does not invariably lead to a poor outcome. Clostridia have been isolated from suppurative infections of the female genital tract (e.g., ovarian or pelvic abscess) and from diseased gallbladders. Although the most frequently isolated species is C. perfringens, gangrene is not typically observed; however, gas formation in the biliary system can lead to emphysematous cholecystitis, especially in diabetic patients. C. perfringens in association with mixed aerobic and anaerobic microbes can cause aggressive lifethreatening type I necrotizing fasciitis or Fournier’s gangrene. CHAPTER 159 Gas Gangrene and Other Clostridial Infections The treatment of mixed aerobic/anaerobic infection of the abdo­ men, perineum, or gynecologic organs should be based on Gram staining, culture, and antibiotic sensitivity information. Reasonable empirical treatment consists of ampicillin or ampicillin/sulbactam combined with either clindamycin or metronidazole (Table 159-1). Empirical therapy should be initiated. Therapy should be based on Gram stain and culture results and on sensitivity data when available. Add gram-negative coverage if indicated (see text). Transient bacteremia without signs of systemic toxicity may be clinically insignificant. Emergent surgical exploration and thorough debridement are extremely important. Hyperbaric oxygen therapy may be considered after surgery and antibiotic initiation.

Broader gram-negative coverage may be necessary if the patient has recently been hospitalized or treated with antibiotics. Such coverage can be obtained by substituting ticarcillin/clavulanic acid, piperacillin/

sulbactam, or a carbapenem antibiotic for ampicillin or by adding a fluoroquinolone or an aminoglycoside to the regimen. Empirical treatment should be given for 10–14 days or until the patient’s clinical condition improves.

■ ■ENTERIC CLOSTRIDIAL INFECTIONS C. perfringens type A is one of the most common bacterial causes of food-borne illness in the United States and Canada. The foods typically implicated include improperly cooked meat and meat products (e.g., gravy) in which residual spores germinate and proliferate during slow cooling or insufficient reheating. Illness results from the ingestion of food containing at least ~108 viable vegetative cells, which sporulate in the alkaline environment of the small intestine, producing C. perfrin­ gens enterotoxin in the process. The diarrhea that develops within 7–30 h of ingestion of contaminated food is generally mild and self-limiting; however, in the very young, the elderly, and the immunocompromised, symptoms are more severe and occasionally fatal. Enterotoxin-producing C. perfringens has been implicated as an etiologic agent of persistent diarrhea in elderly patients in nursing homes and tertiary-care institu­ tions and has been considered to play a role in antibiotic-associated diarrhea without pseudomembranous colitis. C. perfringens strains associated with food poisoning possess the gene (cpe) coding for enterotoxin, which acts by forming pores in host cell membranes. C. perfringens strains isolated from nonfood-borne diseases, such as antibiotic-associated and sporadic diar­ rhea, carry cpe on a plasmid that may be transmitted to other strains. Several methods have been described for the detection of C. perfringens enterotoxin in feces, including cell culture assay (Vero cells), enzymelinked immunosorbent assay, reversed-phase latex agglutination, and polymerase chain reaction (PCR) amplification of cpe. Each method has its advantages and limitations. Interestingly, spores from these strains are particularly resistant to heat, cold, and chemical preserva­ tives. In addition, the extracellular sialidase produced by C. perfringens facilitates pathogenesis. PART 5 Infectious Diseases Enteritis necroticans (gas gangrene of the bowel) is a fulminating clinical illness characterized by extensive necrosis of the intestinal mucosa and wall. Cases can occur sporadically in adults or as epidemics in people of all ages. Enteritis necroticans is caused by

α toxin– and β toxin–producing strains of C. perfringens type C; β toxin is located on a plasmid and is mainly responsible for pathogenesis. This life-threatening infection causes ischemic necrosis of the jejunum. In Papua New Guinea during the 1960s, enteritis necroticans (known in that locale as pigbel) was found to be the most common cause of death in childhood; it was associated with pig feasts and occurred both spo­ radically and in outbreaks. Intramuscular immunization against the

β toxin resulted in a decreased incidence of the disease in Papua New Guinea, although the condition remains common. Enteritis necroti­ cans has also been recognized in the United States, the United Kingdom, Germany (where it is known as darmbrand), and other developed nations; especially affected are adults who are malnourished or who have diabetes, alcoholic liver disease, or neutropenia. Necrotizing enterocolitis, a disease resembling enteritis necroticans but associated with C. perfringens type A, has been found in North America in previously healthy adults. It is also a serious gastrointes­ tinal disease of low-birth-weight (premature) infants hospitalized in neonatal intensive care units. The etiology and pathogenesis of this disease have remained enigmatic for more than four decades. Pathologic similarities between necrotizing enterocolitis and enteritis necroticans include the pattern of small-bowel necrosis involving the submucosa, mucosa, and muscularis; the presence of gas dissecting the tissue planes; and the degree of inflammation. In contrast to enteritis necroticans, which most commonly involves the jejunum, necrotizing enterocolitis affects the ileum and frequently the ileocecal valve. Both diseases may manifest as intestinal gas cysts, although this feature is more common in necrotizing enterocolitis. The sources of the gas, which contains hydrogen, methane, and carbon dioxide, are probably

the fermentative activities of intestinal bacteria, including clostridia. Epidemiologic data support an important role for C. perfringens or other gas-producing microorganisms (e.g., C. neonatale, certain other clostridia, or Klebsiella species) in the pathogenesis of necrotizing enterocolitis. Patients with suspected clostridial enteric infection should undergo nasogastric suction and receive IV fluids. Pyrantel is given by mouth, and the bowel is rested by fasting. Benzylpenicillin (1 mU) is given IV every 4 h, and the patient is observed for complications requiring surgery. Patients with mild cases recover without surgical intervention. However, if surgical indications are present (gas in the peritoneal cav­ ity, absent bowel sounds, rebound tenderness, abdominal rigidity), the mortality rate ranges from 35 to 100%; a fatal outcome is due in part to perforation of the intestine. As pigbel continues to be a common disease in Papua New Guinea, consideration should be given to the use of a C. perfringens type C β toxoid vaccine in local areas. Two doses given 3–4 months apart are preventive. ■ ■CLOSTRIDIAL BACTEREMIA Clostridium species are important causes of bloodstream infections. Molecular epidemiologic studies of anaerobic bacteremia have identi­ fied C. perfringens and C. tertium as the two most frequently isolated species; these organisms cause up to 79 and 5%, respectively, of clos­ tridial bacteremias. Occasionally, C. perfringens bacteremia occurs in the absence of an identifiable infection at another site. When associated with myonecrosis, bacteremia has a grave prognosis. C. septicum is also commonly associated with bacteremia. This spe­ cies is isolated only rarely from the feces of healthy individuals but may be found in the normal appendix. More than 50% of patients whose blood cultures are positive for this organism have some gastrointestinal anomaly (e.g., diverticular disease) or underlying malignancy (e.g., carcinoma of the colon). In addition, a clinically important associa­ tion of C. septicum bacteremia with neutropenia of any origin—and, more specifically, with neutropenic enterocolitis involving the terminal ileum or cecum—has been observed. Patients with diabetes mellitus, severe atherosclerotic cardiovascular disease, or anaerobic myonecrosis (gas gangrene) also may develop C. septicum bacteremia. C. septicum has been recovered from the bloodstream of cirrhotic patients, as have C. perfringens, C. bifermentans, and other clostridia. Infections of the bloodstream by C. sordellii and C. perfringens have been associated with TSS. Of note, Clostridium sordellii has been recently renamed Paeniclostridium sordellii. However, throughout this text, the authors have used the original nomenclature for this pathogen. Bloodstream infection by C. tertium, either alone or in combination with C. septicum or C. perfringens, can be found in patients with seri­ ous underlying disease such as malignancy or acute pancreatitis, with or without neutropenic enterocolitis; the frequency has not been sys­ tematically studied. C. tertium may present special problems in terms of both identification and treatment. This organism may stain gramnegative; is aerotolerant; and is resistant to metronidazole, clindamy­ cin, and cephalosporins. Other clostridia from the C. clostridioforme group (including C. clostridioforme, C. hathewayi, and C. bolteae) can cause bacteremia. The clinical importance of recognizing clostridial bacteremia— especially that due to C. septicum—and starting appropriate treatment immediately (Table 159-1) cannot be overemphasized. Patients with this condition usually are gravely ill, and infection may metastasize to distant anatomic sites, resulting in spontaneous myonecrosis (see next section). Alternative methods to identify bacteremia-causing clostridial species, such as PCR or other rapid diagnostic tests, are not currently available. Anaerobic blood cultures and Gram’s stain inter­ pretation remain the best diagnostic tests at this point. ■ ■CLOSTRIDIAL SKIN AND SOFT TISSUE INFECTIONS Histotoxic clostridial species such as C. perfringens, C. histolyticum, C. septicum, C. novyi, and C. sordellii cause aggressive necrotizing infections of the skin and soft tissues. These infections are attributable

in part to the elaboration of bacterial proteases, phospholipases, and cytotoxins. Necrotizing clostridial soft tissue infections are rapidly progressive and are characterized by marked tissue destruction, gas in the tissues, and shock; they frequently end in death. Severe pain, crepitus, brawny induration with rapid progression to skin sloughing, violaceous bullae, and marked tachycardia are characteristics found in the majority of patients. Clostridial Myonecrosis (Gas Gangrene)  •  TRAUMATIC GAS GANGRENE  C. perfringens myonecrosis (gas gangrene) is one of the most fulminant gram-positive bacterial infections of humans. Even with appropriate antibiotic therapy and management in an intensive care unit, tissue destruction can progress rapidly. Gas gangrene is accompanied by bacteremia, hypotension, and multiorgan failure and is invariably fatal if untreated. Gas gangrene is a true emergency and requires immediate surgical debridement. The development of gas gangrene requires an anaerobic environ­ ment and contamination of a wound with spores or vegetative organ­ isms. Devitalized tissue, foreign bodies, and ischemia reduce locally available oxygen levels and favor outgrowth of vegetative cells and spores. Thus, conditions predisposing to traumatic gas gangrene include crush-type injury, laceration of large or medium-sized arter­ ies, and open fractures of long bones that are contaminated with soil or bits of clothing containing the bacterial spores. Gas gangrene of the abdominal wall and flanks follows penetrating injuries such as knife or gunshot wounds that are sufficient to compromise intestinal integrity, with resultant leakage of the bowel contents into the soft tissues. Prox­ imity to fecal sources of bacteria is a risk factor for cases following hip surgery, adrenaline injections into the buttocks, or amputation of the leg for ischemic vascular disease. In addition, cutaneous gas gangrene caused by C. perfringens, C. novyi, and C. sordellii has been described in the United States and northern Europe among persons injecting blacktar heroin subcutaneously. The incubation period for traumatic gas gangrene can be as short as 6 h and is usually <4 days. The infection is characterized by the sudden onset of excruciating pain at the affected site and the rapid develop­ ment of a foul-smelling wound containing a thin serosanguineous discharge and gas bubbles. Brawny edema and induration develop and give way to cutaneous blisters containing bluish to maroon-colored fluid. Such tissue later may become liquefied and slough. The margin between healthy and necrotic tissue often advances several inches per hour despite appropriate antibiotic therapy, and radical amputation remains the single best life-saving intervention. Shock and organ fail­ ure frequently accompany gas gangrene; when patients become bacte­ remic, the mortality rate exceeds 50%. Diagnosis of traumatic gas gangrene is not difficult because the infection always begins at the site of significant trauma, is associated with gas in the tissue, and is rapidly progressive. Gram staining of drainage or tissue biopsy is usually definitive, demonstrating large gram-positive (or gram-variable) rods, an absence of inflammatory cells, and widespread soft tissue necrosis. SPONTANEOUS (NONTRAUMATIC) GAS GANGRENE  Spontaneous gas gangrene generally occurs via hematogenous seeding of normal muscle with histotoxic clostridia—principally C. perfringens, C. septicum, and C. novyi and occasionally C. tertium—from a gastrointestinal tract portal of entry (as in colonic malignancy, inflammatory bowel disease, diverticulitis, necrotizing enterocolitis, cecitis, or distal ileitis or after gastrointestinal surgery, including colonoscopic polypectomy). These gastrointestinal pathologies permit bacterial access to the bloodstream; consequently, aerotolerant C. septicum can proliferate in normal tis­ sues. Patients surviving bacteremia or spontaneous gangrene due to C. septicum should undergo aggressive diagnostic studies to rule out gastrointestinal pathology. Additional predisposing host factors include leukemia, lymphopro­ liferative disorders, cancer chemotherapy, radiation therapy, and AIDS. Cyclic, congenital, or acquired neutropenia also is strongly associ­ ated with an increased incidence of spontaneous gas gangrene due to C. septicum; in such cases, necrotizing enterocolitis, cecitis, or distal ileitis is common, particularly among children.

CHAPTER 159 FIGURE 159-2  Radiograph of patient with spontaneous gas gangrene due to C. septicum, demonstrating gas in the affected arm and shoulder. The first symptom of spontaneous gas gangrene may be confusion followed by the abrupt onset of excruciating pain in the absence of trauma. These findings, along with fever, should heighten suspicion of spontaneous gas gangrene. However, because of the lack of an obvious portal of entry, the correct diagnosis is frequently delayed or missed. The infection is characterized by rapid progression of tissue destruction with demonstrable gas in the tissue (Fig. 159-2). Swelling increases, and bullae filled with clear, cloudy, hemorrhagic, or purplish fluid appear. The surrounding skin has a purple hue, which may reflect vascular compromise resulting from the diffusion of bacterial toxins into surrounding tissues. Invasion of healthy tissue rapidly ensues, with quick progression to shock and multiple-organ failure. Mortality rates in this setting range from 67 to 100% among adults; among children, the mortality rate is 59%, with the majority of deaths occurring within 24 h of onset. Gas Gangrene and Other Clostridial Infections PATHOGENESIS OF GAS GANGRENE  In traumatic gas gangrene, organ­ isms are introduced into devitalized tissue. It is important to recognize that for C. perfringens and C. novyi, trauma must be sufficient to inter­ rupt the blood supply and thereby to establish an optimal anaerobic environment for growth of these species. These conditions are not strictly required for the more aerotolerant species such as C. septicum and C. tertium, which can seed normal tissues from gastrointestinal lesions. Once introduced into an appropriate niche, the organisms proliferate locally and elaborate exotoxins. The major C. perfringens extracellular toxins implicated in gas gangrene are α toxin and θ toxin. A lethal hemolysin that has both phospholipase C and sphingomyelinase activities, α toxin has been implicated as the major virulence factor of C. perfringens: immuniza­ tion of mice with the C-terminal domain of α toxin provides protection against lethal challenge with C. perfringens, and isogenic α toxin– deficient mutant strains of C. perfringens are not lethal in a murine model of gas gangrene. Recently, a human single chain recombinant antibody against α toxin has been developed having significant preven­ tative and therapeutic efficacy in mice. It has been shown in experimental models that the severe pain, rapid progression, marked tissue destruction, and absence of neutro­ phils in C. perfringens gas gangrene are attributable in large part to

Platelet P-selectin gpIIb/IIIa Fibrinogen PSGL-1 CD11b/CD18 Carbohydrates Leukocyte FIGURE 159-3  Schematic illustration of the molecular mechanisms of C. perfringens α toxin–induced platelet/neutrophil aggregates. Homotypic aggregates of platelets (not shown) and heterotypic aggregates of platelets and leukocytes are due to α toxin–induced activation of the platelet fibrinogen receptor gpIIb/IIIa and upregulation of leukocyte CD11b/CD18. Binding of fibrinogen (red) bridges the connection between these adhesion molecules on adjacent cells. An auxiliary role for α toxin–induced upregulation of platelet P-selectin and its binding to leukocyte P-selectin glycoprotein ligand 1 (PSGL-1) or other leukocyte surface carbohydrates also has been demonstrated. α toxin–induced occlusion of blood vessels by heterotypic aggregates of platelets and neutrophils. The formation of these aggregates, which occurs within minutes, is largely mediated by α toxin’s ability to activate the platelet adhesion molecule gpIIb/IIIa (Fig. 159-3); the implication is that platelet glycoprotein inhibitors (e.g., eptifibatide, abciximab) may be therapeutic for maintaining tissue blood flow. PART 5 Infectious Diseases C. perfringens θ toxin (perfringolysin, PFO) is a member of the thiolactivated cytolysin family known as cholesterol-dependent cytolysins, which includes streptolysin O from group A Streptococcus, pneumoly­ sin from Streptococcus pneumoniae, and several other toxins. Cholesteroldependent cytolysins bind as oligomers to cholesterol in host cell membranes. At high concentrations, these toxins form ring-like pores resulting in cell lysis. At sublytic concentrations, PFO hyperactivates phagocytes and vascular endothelial cells. PFO-mediated activation of the macrophage inflammasome, with production of IL-1β, has also been reported. Cardiovascular collapse and end-organ failure occur late in the course of C. perfringens gas gangrene and are largely attributable to both direct and indirect effects of α and θ toxins. In experimental models, θ toxin causes markedly reduced systemic vascular resistance but increased car­ diac output (i.e., “warm shock”), probably via induction of endogenous mediators (e.g., prostacyclin, platelet-activating factor) that cause vasodi­ lation. This effect is similar to that observed in gram-negative sepsis. In sharp contrast, α toxin directly suppresses myocardial contractility; the consequence is profound hypotension due to a sudden reduction in car­ diac output. The roles of other endogenous mediators, such as cytokines (e.g., tumor necrosis factor, interleukins 1 and 6) and vasodilators (e.g., bradykinin) have not been fully elucidated. C. septicum produces four main toxins—α toxin (lethal, hemolytic, necrotizing activity), β toxin (DNase), γ toxin (hyaluronidase), and Δ toxin (septicolysin, an oxygen-labile hemolysin)—as well as a protease and a neuraminidase. Unlike the α toxin of C. perfringens, that of C. septicum does not possess phospholipase activity. The mechanisms remain to be fully elucidated, but it is likely that each of these toxins contributes uniquely to C. septicum gas gangrene. TREATMENT Gas Gangrene Patients with suspected gas gangrene (either traumatic or sponta­ neous) should undergo prompt surgical inspection of the infected

FIGURE 159-4  Histopathology of experimental gas gangrene due to C. perfringens, demonstrating widespread muscle necrosis, a paucity of leukocytes in infected tissues, and accumulation of leukocytes in adjacent vessels (arrows). These features are due to the effects of α and θ toxins on muscle cells, platelets, leukocytes, and endothelial cells. site. Direct examination of a gram-stained smear of the involved tissues is of major importance. Characteristic histologic findings in clostridial gas gangrene include widespread tissue destruction, a paucity of leukocytes in infected tissues in conjunction with an accumulation of leukocytes in adjacent vessels (Fig. 159-4), and the presence of gram-positive rods (with or without spores). Com­ puted tomography (CT) and magnetic resonance imaging (MRI) are invaluable for determining whether the infection is localized or is spreading along fascial planes, and needle aspiration or punch biopsy may provide an etiologic diagnosis in at least 20% of cases. However, these techniques should not replace surgical exploration, Gram’s staining, and histopathologic examination. When spontane­ ous gas gangrene is suspected, blood should be cultured since bac­ teremia usually precedes cutaneous manifestations by several hours. For patients with evidence of clostridial gas gangrene, thor­ ough emergent surgical debridement is of extreme importance. All devitalized tissue should be widely resected back to healthy via­ ble muscle and skin so as to remove conditions that allow anaerobic organisms to continue proliferating. Closure of traumatic wounds or compound fractures should be delayed for 5–6 days until it is certain that these sites are free of infection. Antibiotic treatment of traumatic or spontaneous gas gangrene (Table 159-1) consists of the administration of penicillin and clindamycin for 10–14 days. Penicillin is recommended on the basis of in vitro sensitivity data; clindamycin is recommended because of its superior efficacy over penicillin in animal models of C. perfrin­ gens gas gangrene and in some clinical reports. Controlled clinical trials comparing the efficacy of these agents in humans have not been performed. In the penicillin-allergic patient, clindamycin may be used alone. The superior efficacy of clindamycin is probably due to its ability to inhibit bacterial protein toxin production, its insensitivity to the size of the bacterial load or the stage of bacterial growth, and its ability to modulate the host immune response. Although C. perfringens remains largely susceptible to first-line antibiotics, antibiotic resistance has been reported. Case reports from the United Kingdom and from Spain found clindamycinresistant C. perfringens in cellulitis and in a spontaneous abscess, respectively. Larger studies from Canada and Taiwan also showed increasing resistance to clindamycin among bloodstream isolates. In 2014, Marchand-Austin et al. published a 2-year prospective Canadian study that examined antimicrobial susceptibility of anaer­ obic bacteria isolated from blood, body fluids, and abscesses. Of 1412 isolates submitted for susceptibility testing, 68 were C. per­ fringens. Of these, all were universally susceptible to penicillin, but 3.8% were clindamycin-resistant. Notably, for Clostridium species other than C. perfringens (n = 289), 14.2% were penicillin-resistant

and 21.6% clindamycin-resistant. A more recent study from Iran found that 21.2% of C. perfringens isolates were resistant to penicil­ lin. Lastly, a 2019 study from Hungary found resistance to penicillin (2.6%) and clindamycin (3.8%) among C. perfringens isolates (n = 313) from tissues with gas gangrene. Among the non-perfringens gas gangrene isolates (n = 59), higher resistance to penicillin and clindamycin was observed (6.8% and 8.5%, respectively). These findings, though not universal, highlight the importance of good anaerobic microbiology susceptibility testing to provide up-to-date information to guide optimal clinical management decisions for clostridial infections. C. tertium is resistant to penicillin, cephalosporins, and clindamy­ cin. Appropriate antibiotic therapy for C. tertium infection is vanco­ mycin (1 g every 12 h IV) or metronidazole (500 mg every 8 h IV). The value of adjunctive treatment with hyperbaric oxygen (HBO) for gas gangrene remains controversial. Basic science studies sug­ gest that HBO can inhibit the growth of C. perfringens but not that of the more aerotolerant C. septicum. In vitro, blood and macerated muscle inhibit the bactericidal potential of HBO. Numerous stud­ ies in animals demonstrate little efficacy of HBO alone, whereas antibiotics alone—especially those that inhibit bacterial protein synthesis—confer marked benefits. Addition of HBO to the thera­ peutic regimen provides some additional benefit, but only if surgery and antibiotic administration precede HBO treatment. In conclusion, gas gangrene is a rapidly progressive infection whose outcome depends on prompt recognition, emergent surgery, and timely administration of antibiotics that inhibit toxin produc­ tion. Gas gangrene associated with bacteremia probably represents a later stage of illness and is associated with the worst outcomes. Emergent surgical debridement is crucial to ensure survival, and ancillary procedures (e.g., CT or MRI) or transport to HBO units should not delay this intervention. Some trauma centers associ­ ated with HBO units may have special expertise in managing these aggressive infections, but proximity and speed of transfer must be carefully weighed against the need for haste. PROGNOSIS OF GAS GANGRENE  The prognosis for patients with gas gangrene is more favorable when the infection involves an extremity rather than the trunk or visceral organs, since debridement of the latter sites is more difficult. Gas gangrene is most likely to progress to shock and death in patients with associated bacteremia and intravascular hemolysis. Mortality rates are highest for patients in shock at the time of diagnosis. Mortality rates are relatively high among patients with spontaneous gas gangrene, especially that due to C. septicum. Survivors of gas gangrene may undergo multiple debridements and face long periods of hospitalization and rehabilitation. PREVENTION OF GAS GANGRENE  Initial aggressive debridement of devitalized tissue can reduce the risk of gas gangrene in contaminated deep wounds. Interventions to be avoided include prolonged applica­ tion of tourniquets and surgical closure of traumatic wounds; patients with compound fractures are at significant risk for gas gangrene if the wound is closed surgically. Vaccination against α toxin is protective in experimental animal models of C. perfringens gas gangrene but has not been investigated in humans. In addition, as mentioned above, a hyper­ immune globulin would represent a significant advance for prophylaxis in victims of acute traumatic injury or for attenuation of the spread of infection in patients with established gas gangrene. Toxic Shock Syndrome  Clostridial infection of the endometrium, particularly that due to C. sordellii, can develop after gynecologic procedures, childbirth, or abortion (spontaneous or elective, surgical, or medical) and, once established, proceeds rapidly to TSS and death. Systemic manifestations, including edema, effusions, profound leu­ kocytosis, and hemoconcentration, are followed by the rapid onset of hypotension and multiple-organ failure. Elevation of the hematocrit to 75–80% and leukocytosis of 50,000–200,000 cells/μL, with a left shift, are characteristic of C. sordellii infection. Pain may not be a prominent feature, and fever is typically absent. In one series, 18% of 45 cases of

C. sordellii infection were associated with normal childbirth, 11% with medically induced abortion, and 0.4% with spontaneous abortion; the case-fatality rate was 100% in these groups. Of the infections in this series that were not related to gynecologic procedures or childbirth, 22% occurred in injection drug users, and 50% of these patients died. Other infections followed trauma or surgery (42%), mostly in healthy persons, and 53% of these patients died. Overall, the mortality rate was 69% (31 of 45 cases). Of patients who succumbed, 85% died within 2–6 days after infection onset or following procedures. Rapidly fatal spontaneous C. bifermentans necrotizing endometritis with toxic shock, leukemoid reaction, and capillary leak has also been described.

Early diagnosis of C. sordellii infections often proves difficult for several reasons. First, the prevalence of these infections is low. Sec­ ond, the initial symptoms are nonspecific and frankly misleading. Early in the course, the illness resembles any number of infectious diseases, including viral syndromes. Given these vague symptoms and an absence of fever, physicians usually do not aggressively pursue additional diagnostic tests. The absence of local evidence of infection and the lack of fever make early diagnosis of C. sordellii infection par­ ticularly problematic in patients who develop deep-seated infection following childbirth, therapeutic abortion, gastrointestinal surgery, or trauma. Such patients are frequently evaluated for pulmonary embo­ lization, gastrointestinal bleeding, pyelonephritis, or cholecystitis. Unfortunately, such delays in diagnosis increase the risk of death, and as in most necrotizing soft tissue infections, patients are hypotensive with evidence of organ dysfunction by the time local signs and symp­ toms become apparent. In contrast, infection is more readily suspected in injection drug users presenting with local swelling, pain, and redness at injection sites; early recognition probably contributes to the lower mortality rates in this group. CHAPTER 159 Physicians should suspect C. sordellii infection in patients who pres­ ent within 2–7 days after injury, surgery, drug injection, childbirth, or abortion and who report pain, nausea, vomiting, and diarrhea but are afebrile. There is little information regarding appropriate treatment for C. sordellii infections. In fact, the interval between onset of symptoms and death is often so short that there is little time to initiate empirical antimicrobial therapy. Indeed, anaerobic cultures of blood and wound aspirates are time-consuming, and many hospital laboratories do not routinely perform antimicrobial sensitivity testing on anaerobes. Anti­ biotic susceptibility data from older studies suggest that C. sordellii, like most clostridia, is susceptible to β-lactam antibiotics, clindamycin, tetracycline, and chloramphenicol but is resistant to aminoglycosides and sulfonamides. Antibiotics that suppress toxin synthesis (e.g., clindamycin) may possibly prove useful as therapeutic adjuncts since they are effective in necrotizing infections due to other toxin-produc­ ing gram-positive organisms. With the adoption of restrictive legisla­ tion that reduces or prohibits access to medically supervised abortions, the incidence of these deadly clostridial infections could increase as patients undergo unsafe pregnancy termination. Gas Gangrene and Other Clostridial Infections Other Clostridial Skin and Soft-Tissue Infections  Crepitant cellulitis (also called anaerobic cellulitis) occurs principally in diabetic patients and characteristically involves subcutaneous tissues or retro­ peritoneal tissues, whereas the muscle and fascia are not involved. This infection can progress to fulminant systemic disease. Cases of C. histolyticum infection with cellulitis, abscess forma­ tion, or endocarditis have also been documented in injection drug users. Endophthalmitis due to C. sordellii or C. perfringens has been described. C. ramosum is also isolated frequently from clinical speci­ mens, including blood and both intraabdominal and soft tissues. This species may be resistant to clindamycin and multiple cephalosporins. ■ ■FURTHER READING Aldape MJ et al: Clostridium sordellii infection: Epidemiology, clinical findings, and current perspectives on diagnosis and treatment. Clin Infect Dis 43:1436, 2006. Aronoff DM et al: Infections caused by Clostridium perfringens and Paeniclostridium sordellii after unsafe abortion. Lancet Infect Dis 23:e48, 2023. Erratum in: Lancet Infect Dis 22:e310, 2022.

42 - SECTION 6 Diseases Caused by Gram-Negative Bacteria

SECTION 6 Diseases Caused by Gram-Negative Bacteria

Bodey GP et al: Clostridial bacteremia in cancer patients. A 12-year

experience. Cancer 67:1928, 1991. Bos J et al: Fatal necrotizing colitis following a foodborne outbreak of enterotoxigenic Clostridium perfringens type A infection. Clin Infect Dis 40:e78, 2005. Bryant AE et al: Clostridial gas gangrene II: Phospholipase C–induced activation of platelet gpIIb/IIIa mediates vascular occlusion and myo­ necrosis in C. perfringens gas gangrene. J Infect Dis 182:808, 2000. Li J et al: Clostridium perfringens sporulation and sporulation-associated toxin production. Microbiol Spectr 4:10.1128/microbiolspec.TBS0022-2015, 2016. Li J et al: NanJ is the major sialidase for Clostridium perfringens Type F food poisoning strain 01E809. Infect Immun 91:e0005323, 2023. Marchand-Austin A et al: Antimicrobial susceptibility of clinical iso­ lates of anaerobic bacteria in Ontario, 2010-2011. Anaerobe 28:120, 2014. Obladen M: Necrotizing enterocolitis—150 years of fruitless search for the cause. Neonatology 96:203, 2009 Sayeed S et al: Beta toxin is essential for the intestinal virulence of Clostridium perfringens type C disease isolate CN3685 in a rabbit ileal loop model. Mol Microbiol 67:15, 2008. Smith LDS, Williams BL: The Pathogenic Anaerobic Bacteria, 3rd ed. Springfield, IL, Charles C Thomas, 1984. Stevens DL, Bryant AE: Necrotizing soft tissue infections. N Engl J Med 377:2253, 2017. Stevens DL et al: Clostridium, in Manual of Clinical Microbiology, 11th ed, J Versalovic (ed). ASM Press, 2014, pp. 940–966. Stevens DL et al: Practice guidelines for the diagnosis and manage­ ment of skin and soft tissue infections: 2014 update by the Infectious Diseases Society of America. Clin Infect Dis 59:e10, 2014. Wang C et al: Hyperbaric oxygen for treating wounds: A systematic PART 5 Infectious Diseases review of the literature. Arch Surg 138:272, 2003. Section 6 Diseases Caused by

Gram-Negative Bacteria Manish Sadarangani, Andrew J. Pollard

Meningococcal

Infections ■ ■DEFINITION Infection with Neisseria meningitidis most commonly manifests as asymptomatic colonization in the nasopharynx of healthy adolescents and adults. Invasive disease occurs rarely, usually presenting as either bacterial meningitis or meningococcal septicemia. Patients may also present with occult bacteremia, pneumonia, septic arthritis, conjuncti­ vitis, and chronic meningococcemia. ■ ■ETIOLOGY AND MICROBIOLOGY N. meningitidis is a gram-negative aerobic diplococcus that colonizes humans only and causes disease after transmission to a susceptible individual. Several related neisserial organisms have been recognized, including the pathogen N. gonorrhoeae and the commensals N. lactam­ ica, N. flavescens, N. mucosa, N. sicca, and N. subflava. N. meningitidis is a catalase- and oxidase-positive organism that utilizes glucose and maltose to produce acid. Meningococci associated with invasive disease are usually encap­ sulated with polysaccharide, and the antigenic nature of the capsule determines an organism’s capsular group (serogroup) (Table 160-1).

TABLE 160-1  Structure of the Polysaccharide Capsule of Common Disease-Causing Meningococci MENINGOCOCCAL CAPSULAR GROUP CHEMICAL STRUCTURE OF OLIGOSACCHARIDE CURRENT DISEASE EPIDEMIOLOGY A 2-Acetamido-2-deoxyD-mannopyranosyl phosphate Epidemic disease mainly in sub-Saharan Africa; sporadic cases worldwide B α-2,8-Nacetylneuraminic acid Sporadic cases worldwide; propensity to cause hyperendemic disease C α-2,9-O-acetylneuraminic acid Small outbreaks and sporadic disease Y 4-O-α-D-glucopyranosylN-acetylneuraminic acid Sporadic disease and occasional small institutional outbreaks W 4-O-α-Dgalactopyranosyl-Nacetylneuraminic acid Sporadic disease; outbreaks of disease associated with mass gatherings; epidemics in subSaharan Africa X (α1→4) N-acetylD-glucosamine-1phosphate Sporadic disease and large outbreaks in the meningitis belt of Africa In total, 12 capsular groups have been identified (A–C, X–Z, E, W, H–J, and L), but just six of these—A, B, C, X, Y, and W (formerly W135)—account for the majority of cases of invasive disease. Group D is often listed as the thirteenth capsular group but has been identified as an unencapsulated variant of group C. Meningococci are commonly isolated from the nasopharynx in studies of carriage; the lack of capsule often is a result of phase variation of capsule expression, but as many as 16% of isolates lack the genes for capsule synthesis and assembly. These “capsule-null” meningococci and those that express capsules other than A, B, C, X, Y, and W are only rarely associated with invasive disease and are most commonly identified in the nasopharynx of asymptomatic carriers. Beneath the capsule, meningococci are surrounded by an outer phospholipid membrane containing lipopolysaccharide (LPS, endo­ toxin) and multiple outer-membrane proteins (Figs. 160-1 and 160-2). Antigenic variability in porins expressed in the outer membrane defines the serotype (PorB) and serosubtype (PorA) of the organ­ ism, and structural differences in LPS determine the immunotype. Serologic methods for typing meningococci are restricted by the limited availability of serologic reagents that can distinguish among the organisms’ highly variable surface proteins. Where available, high-throughput antigen gene sequencing has superseded serology for FIGURE 160-1  Electron micrograph of Neisseria meningitidis. Black dots are gold-labeled polyclonal antibodies binding surface opacity proteins. Blebs of outer membrane can be seen being released from the bacterial surface (arrow). (Photo courtesy of D. Ferguson, Oxford University.)

43 - 160 Meningococcal Infections

160 Meningococcal Infections

Bodey GP et al: Clostridial bacteremia in cancer patients. A 12-year

experience. Cancer 67:1928, 1991. Bos J et al: Fatal necrotizing colitis following a foodborne outbreak of enterotoxigenic Clostridium perfringens type A infection. Clin Infect Dis 40:e78, 2005. Bryant AE et al: Clostridial gas gangrene II: Phospholipase C–induced activation of platelet gpIIb/IIIa mediates vascular occlusion and myo­ necrosis in C. perfringens gas gangrene. J Infect Dis 182:808, 2000. Li J et al: Clostridium perfringens sporulation and sporulation-associated toxin production. Microbiol Spectr 4:10.1128/microbiolspec.TBS0022-2015, 2016. Li J et al: NanJ is the major sialidase for Clostridium perfringens Type F food poisoning strain 01E809. Infect Immun 91:e0005323, 2023. Marchand-Austin A et al: Antimicrobial susceptibility of clinical iso­ lates of anaerobic bacteria in Ontario, 2010-2011. Anaerobe 28:120, 2014. Obladen M: Necrotizing enterocolitis—150 years of fruitless search for the cause. Neonatology 96:203, 2009 Sayeed S et al: Beta toxin is essential for the intestinal virulence of Clostridium perfringens type C disease isolate CN3685 in a rabbit ileal loop model. Mol Microbiol 67:15, 2008. Smith LDS, Williams BL: The Pathogenic Anaerobic Bacteria, 3rd ed. Springfield, IL, Charles C Thomas, 1984. Stevens DL, Bryant AE: Necrotizing soft tissue infections. N Engl J Med 377:2253, 2017. Stevens DL et al: Clostridium, in Manual of Clinical Microbiology, 11th ed, J Versalovic (ed). ASM Press, 2014, pp. 940–966. Stevens DL et al: Practice guidelines for the diagnosis and manage­ ment of skin and soft tissue infections: 2014 update by the Infectious Diseases Society of America. Clin Infect Dis 59:e10, 2014. Wang C et al: Hyperbaric oxygen for treating wounds: A systematic PART 5 Infectious Diseases review of the literature. Arch Surg 138:272, 2003. Section 6 Diseases Caused by

Gram-Negative Bacteria Manish Sadarangani, Andrew J. Pollard

Meningococcal

Infections ■ ■DEFINITION Infection with Neisseria meningitidis most commonly manifests as asymptomatic colonization in the nasopharynx of healthy adolescents and adults. Invasive disease occurs rarely, usually presenting as either bacterial meningitis or meningococcal septicemia. Patients may also present with occult bacteremia, pneumonia, septic arthritis, conjuncti­ vitis, and chronic meningococcemia. ■ ■ETIOLOGY AND MICROBIOLOGY N. meningitidis is a gram-negative aerobic diplococcus that colonizes humans only and causes disease after transmission to a susceptible individual. Several related neisserial organisms have been recognized, including the pathogen N. gonorrhoeae and the commensals N. lactam­ ica, N. flavescens, N. mucosa, N. sicca, and N. subflava. N. meningitidis is a catalase- and oxidase-positive organism that utilizes glucose and maltose to produce acid. Meningococci associated with invasive disease are usually encap­ sulated with polysaccharide, and the antigenic nature of the capsule determines an organism’s capsular group (serogroup) (Table 160-1).

TABLE 160-1  Structure of the Polysaccharide Capsule of Common Disease-Causing Meningococci MENINGOCOCCAL CAPSULAR GROUP CHEMICAL STRUCTURE OF OLIGOSACCHARIDE CURRENT DISEASE EPIDEMIOLOGY A 2-Acetamido-2-deoxyD-mannopyranosyl phosphate Epidemic disease mainly in sub-Saharan Africa; sporadic cases worldwide B α-2,8-Nacetylneuraminic acid Sporadic cases worldwide; propensity to cause hyperendemic disease C α-2,9-O-acetylneuraminic acid Small outbreaks and sporadic disease Y 4-O-α-D-glucopyranosylN-acetylneuraminic acid Sporadic disease and occasional small institutional outbreaks W 4-O-α-Dgalactopyranosyl-Nacetylneuraminic acid Sporadic disease; outbreaks of disease associated with mass gatherings; epidemics in subSaharan Africa X (α1→4) N-acetylD-glucosamine-1phosphate Sporadic disease and large outbreaks in the meningitis belt of Africa In total, 12 capsular groups have been identified (A–C, X–Z, E, W, H–J, and L), but just six of these—A, B, C, X, Y, and W (formerly W135)—account for the majority of cases of invasive disease. Group D is often listed as the thirteenth capsular group but has been identified as an unencapsulated variant of group C. Meningococci are commonly isolated from the nasopharynx in studies of carriage; the lack of capsule often is a result of phase variation of capsule expression, but as many as 16% of isolates lack the genes for capsule synthesis and assembly. These “capsule-null” meningococci and those that express capsules other than A, B, C, X, Y, and W are only rarely associated with invasive disease and are most commonly identified in the nasopharynx of asymptomatic carriers. Beneath the capsule, meningococci are surrounded by an outer phospholipid membrane containing lipopolysaccharide (LPS, endo­ toxin) and multiple outer-membrane proteins (Figs. 160-1 and 160-2). Antigenic variability in porins expressed in the outer membrane defines the serotype (PorB) and serosubtype (PorA) of the organ­ ism, and structural differences in LPS determine the immunotype. Serologic methods for typing meningococci are restricted by the limited availability of serologic reagents that can distinguish among the organisms’ highly variable surface proteins. Where available, high-throughput antigen gene sequencing has superseded serology for FIGURE 160-1  Electron micrograph of Neisseria meningitidis. Black dots are gold-labeled polyclonal antibodies binding surface opacity proteins. Blebs of outer membrane can be seen being released from the bacterial surface (arrow). (Photo courtesy of D. Ferguson, Oxford University.)

Iron-binding proteins e.g., FetA RmpM Pilus Phospholipid bilayer NadA LPS PorA Opa PorB fHbp Transporter protein e.g., FbpA, SodC Pilus assembly apparatus Inner membrane transporter complex e.g., FbpB, FbpC FIGURE 160-2  Cross-section through surface structures of Neisseria meningitidis. LPS, lipopolysaccharide. (Reproduced with permission from M Sadarangani, AJ Pollard: Serogroup B meningococcal vaccines–an unfinished story. Lancet Infect Dis 10:112, 2010.) meningococcal typing. A large database of antigen gene sequences for the outer-membrane proteins PorA, PorB, FetA, Opa, NadA, neisse­ rial heparin binding antigen (NHBA), and factor H–binding protein (fHbp) is available online (pubmlst.org/organisms/neisseria-spp). The number of specialized iron-regulated proteins found in the meningo­ coccal outer membrane (e.g., FetA and transferrin-binding proteins) highlights the organisms’ dependence on iron from human sources. A thin peptidoglycan cell wall separates the outer membrane from the cytoplasmic membrane. The structure of meningococcal populations involved in local and global spread was first studied with multilocus enzyme electro­ phoresis (MLEE), which characterizes isolates according to differ­ ences in the electrophoretic mobility of cytoplasmic enzymes, followed by multilocus sequence typing (MLST), in which meningococci are characterized by sequence types assigned on the basis of sequences of internal fragments of seven housekeeping genes and, more recently, whole genome sequencing (>60,000 genomes are listed in PubMLST [https://pubmlst.org/organisms/neisseria-spp/]). While many distinct genotypes exist, a limited number of hyperinvasive lineages of N. men­ ingitidis have been recognized, persist over decades, and are responsi­ ble for the majority of cases of invasive meningococcal disease worldwide. Hyperinvasive lineages may be associated with more than one capsular group. The apparent genetic stability of these meningo­ coccal clones over decades and during wide geographic spread indi­ cates that they are well adapted to the nasopharyngeal environment of the host and to efficient transmission. The group B meningococcal genome is >2 megabases in length and contains 2158 coding regions. Many genes undergo phase variation that makes it possible to control their expression; this capacity is likely to be important in meningococcal adaptation to the host environment and evasion of the immune response. Meningococci can obtain DNA from their environment and can acquire new genes—including the capsular operon—such that capsule switching from one capsular group to another can occur. ■ ■EPIDEMIOLOGY Patterns of Disease  Up to 500,000 cases of meningococcal disease are thought to occur worldwide each year, although the numbers have been declining recently as a result of both immunization programs and secular trends. About 10% of affected individuals die. There are several patterns of disease: epidemic, outbreak (small clusters of cases), hyper­ endemic, and sporadic or endemic. Epidemics have continued since the original descriptions of menin­ gococcal disease, especially affecting the sub-Saharan meningitis belt of

Africa, where tens to hundreds of thou­ sands of cases (caused mainly by capsular group A but also by capsular groups C, W, and X) may be reported over a season and rates may be as high as 1000 cases per 100,000 population. Capsular group A epidemics took place in Europe and North America after the First and Sec­ ond World Wars, and capsular group A outbreaks have been documented over the past 40 years in New Zealand, China, Nepal, Mongolia, India, Pakistan, Poland, and Russia. However, 65% of outbreaks reported in the meningitis belt between 2010 and 2017 were caused by capsular group C and 35% by capsular group W meningococci, following an immuniza­ tion campaign to control capsular group A outbreaks. New vaccines covering A, C, W, Y, and X are becoming available globally to extend control of outbreaks.

Polysaccharide capsule Outer membrane Periplasmic space Cytoplasmic membrane Clusters of cases occur where there is an opportunity for increased trans­ mission—i.e., in closed or semi-closed communities such as schools, colleges, universities, military training centers, and refugee camps. Over the past 4 decades, such clusters have been especially strongly linked with a particular clone (sequence type 11) that is mainly associated with capsular group C or W but was first described in association with capsular group B. Clusters of capsular group W disease associated with the Hajj pilgrimage in 2000/2001 led to a requirement for vaccination against meningococcal disease for travel to Saudi Arabia. Wider and more prolonged community outbreaks (hyper­ endemic disease) due to single clones of capsular group B meningococci account for ≥10 cases per 100,000. Regions affected in the past 35 years include the U.S. Pacific Northwest, New Zealand (both islands), and the province of Normandy in France. CHAPTER 160 Meningococcal Infections Most countries experience predominantly sporadic cases (0.0–2.8 cases per 100,000 population, but recently rates up to 10 cases per 100,000 have been reported in Africa), with many different disease-causing clones involved and usually no clear epidemiologic link between one case and another. The disease rate and the distribution of meningococcal strains vary in different regions of the world and also in any one location over time. For example, in the United States, the rate of meningococcal dis­ ease fell from 1.2 cases per 100,000 population in 1997 to 0.06 cases per 100,000 in 2021 (Fig. 160-3). Meningococcal disease in the United States was previously dominated by capsular groups B and C; however, in 2011– 2021, group B alone was predominant in children age <5 years, whereas disease in children over 11 years of age, adolescents, and adults was dominated by capsular groups C, W, and Y (Fig. 160-4). There has been a sharp, but small, increase in capsular group Y starting in 2022, with the highest rates in Americans of black ethnicity, individuals over 30 years of age, and those living with HIV. In contrast, rates of disease in England and Wales rose to >5 cases per 100,000 during the 1990s because of an increase in cases caused by the ST11 capsular group C clone. A mass immunization program against capsular group C was undertaken begin­ ning in 1999 and resulted in a large impact against the disease, leaving capsular group B meningococci as the predominant cause of infection in the past quarter century (87% of United Kingdom cases in 2021–2022). Introduction of a group B meningococcal (MenB) vaccine for infants in the United Kingdom in 2015 also led to a significant reduction in group B cases. A hyperinvasive ST11 clone bearing a W capsule emerged in South America and spread to various countries in Europe and in Aus­ tralia with cases peaking in 2016 in the United Kingdom. During the same decade, increases in capsular group Y disease were noted in various countries including Europe, Canada, and South Africa, highlighting the continuing emergence and re-emergence of capsular groups and geno­ types over time. Nevertheless, over the past 15 years, most industrialized nations have observed a decrease in meningococcal disease linked to the introduction of immunization against capsular group C meningococci

1.4 1.2 Cases per 100,000 population

0.8 0.6 0.4 0.2

FIGURE 160-3  Meningococcal disease in the United States, 1997–2021. ABCs, active bacterial cores. (Adapted from ABC Surveillance data, Centers for Disease Control and Prevention; https://www.cdc.gov/abcs/reports-findings/surv-reports.html.) in young children or teenagers and the use of adolescent immunization programs for capsular groups A, C, Y, and W. However, other factors, including changes in natural population immunity (induced by exposure from nasopharyngeal colonization) and prevalent clones of meningo­ cocci (factors that, in combination, probably explain the historical cyclic nature of meningococcal disease rates) as well as a reduction in smoking and passive exposure to tobacco smoke (driven by bans on smoking in buildings and public spaces) across wealthy countries, are likely to have contributed to the fall in cases. There are also data from the United King­ dom indicating that reductions in contacts between individuals during the COVID-19 pandemic led to further substantial declines in meningo­ coccal disease and that contact patterns remain altered and could have a sustained effect on transmission. PART 5 Infectious Diseases Factors Associated with Disease Risk and Susceptibility  The principal determinant of disease susceptibility is age, with the peak incidence in the first year of life (Fig. 160-5). The susceptibility of the Quebec (Canada) January–July 2017

Europe

Israel

United States*

Brazil† African meningitis belt countries‡

A B

C Chile

W Argentina

Y

Other

NG FIGURE 160-4  Global percentage distribution of meningococcal capsular groups causing invasive meningococcal disease, 2017–2019. NG, non-groupable plus capsular groups other than B, C, W, and Y. (Adapted from C Pardo de Santayana et al: Epidemiology of invasive meningococcal disease worldwide from 2010–2019: A literature review. Epidemiol Infect 151:e57, 2023.)

B C Y Other

Year very young presumably results from an absence of specific adaptive immunity in combination with very close contact with colonized indi­ viduals, including parents. Compared with other age groups, infants appear to be particularly susceptible to capsular group B disease: >30% of capsular group B cases in the United States occur during the first year of life. In the early 1990s prior to use of vaccines in North Amer­ ica, the median ages for patients with disease due to capsular groups B, C, Y, and W were 6, 17, 24, and 33 years, respectively. In populations where capsular group A, C, W, and Y vaccines are being used with good coverage, disease from these capsular groups has become very rare in children. After early childhood, a second peak of disease occurs among ado­ lescents and young adults (15–25 years of age) in Europe and North America. It is thought that this peak relates to social behaviors and environmental exposures in this age group, as discussed below. Most cases of infection with N. meningitidis in developed countries today are sporadic, and the rarity of the disease suggests that individual Russia

A–0.23

Australia

New Zealand

South Africa

0.6 Incidence rate per 100,000 persons 0.5 0.4 0.3 0.2 0.1

<1 1–4 5–10 11–14 15–18 19–22 23–26 27–64 65+ Age in years FIGURE 160-5  Age distribution of capsular groups B and ACWY meningococcal disease United States, 2012–2021. (Adapted from https://www.cdc.gov/meningococcal/php/surveillance/.) susceptibility may be important. A number of factors probably contrib­ ute to individual susceptibility, including the host’s genetic constitu­ tion, environment, and contact with a carrier or a case. The best-documented genetic association with meningococcal dis­ ease is complement deficiency, chiefly of the terminal complement components (C5–9), properdin, or factor D or those treated with complement inhibitors such as eculizumab; such a deficiency increases the risk of disease by up to 600-fold and may result in recurrent attacks. Complement components are believed to be important for the bacteri­ cidal activity of serum, which is considered the principal mechanism of immunity against invasive meningococcal disease. However, when inves­ tigated, complement deficiency is found in only a very small proportion of individuals with meningococcal disease (0.3%). Conversely, 7–20% of persons whose disease is caused by the less common capsular groups (W, X, Y, Z, E) have a complement deficiency. Complement deficiency appears to be associated with capsular group B disease only rarely. Indi­ viduals with recurrences of meningococcal disease, particularly those caused by non-B capsular groups, should be assessed for complement deficiency by measurement of total hemolytic complement activity. There is also limited evidence that hyposplenism (through reduction in phagocytic capacity), hypogammaglobulinemia (through absence of specific antibody), and HIV increase the risk of meningococcal disease. Genetic studies have revealed various associations with disease suscep­ tibility, including complement and mannose-binding lectin deficiency, single-nucleotide polymorphisms in Toll-like receptor (TLR) 4 and complement factor H, and variants of Fc gamma receptors. Factors that increase the chance of a susceptible individual’s acquir­ ing N. meningitidis via the respiratory route also increase the risk of meningococcal disease. Acquisition occurs through close contact with carriers as a result of overcrowding (e.g., in poor socioeconomic settings, in refugee camps, during the Hajj pilgrimage to Mecca, dur­ ing freshman-year residence in college dormitories), recruitment into the military, and certain social behaviors (e.g., attendance at bars and nightclubs, kissing). Secondary cases may occur in close contacts of an index case (e.g., household members, persons kissing the infected individual); the risk to these contacts may be as high as 1000 times the background rate in the population. Factors that damage the naso­ pharyngeal epithelium also increase the risk of both colonization with N. meningitidis and invasive disease. The most important of these factors are tobacco smoking (odds ratio, 4.1) and passive exposure to tobacco smoke. In addition, recent viral respiratory tract infection, infection with Mycoplasma species, and winter or the dry season (in sub-Saharan Africa) have been associated with meningococcal disease; all of these factors presumably either increase the expression of adhe­ sion molecules in the nasopharynx, thus enhancing meningococcal adhesion, or facilitate meningococcal invasion of the bloodstream. ■ ■PATHOGENESIS N. meningitidis has evolved as an effective colonizer of the human nasopharynx, with asymptomatic infection rates of >25% described in some series of adolescents and young adults and among residents

of crowded communities. Point-prevalence stud­ ies reveal widely divergent rates of carriage for different types of meningococci. This variation suggests that some types may be adapted to a short duration of carriage with frequent transmission to maintain the population, while others may be less efficiently transmitted but may overcome this dis­ advantage by colonizing for a long period. Despite the high rates of carriage among adolescents and young adults, only ~10% of adults carry menin­ gococci, and colonization is very rare in early childhood. Many of the same factors that increase the risk of meningococcal disease also increase the risk of carriage. Colonization of the nasopharynx involves a series of interactions of meningococcal adhesins (e.g., Opa proteins and pili) with their ligands on the epithelial mucosa. N. meningitidis produces an IgA1 protease that is likely to reduce interruption of colonization by mucosal IgA.

B ACWY Colonization should be considered the normal state of meningococ­ cal infection, with an increased risk of invasion being the unfortunate consequence (for both host and organism) of adaptations of hyperin­ vasive meningococcal lineages to favor their survival. The meningo­ coccal capsule is an important virulence factor: acapsular strains only very rarely cause invasive disease. The capsule provides resistance to phagocytosis and may be important in preventing desiccation during transmission between hosts. Antigenic diversity in surface structures and an ability to vary levels of their expression probably have evolved as important factors in maintaining meningococcal populations within and between individual hosts. CHAPTER 160 Invasion through the mucosa into the bloodstream occurs rarely, usually within a few days of acquisition of an invasive strain by a sus­ ceptible individual. Only occasional cases of prolonged colonization prior to invasion have been documented. Once the organism is in the bloodstream, its growth may be limited if the individual is partially immune, although bacteremia may allow seeding of another site, such as the meninges or the joints. Alternatively, unchecked proliferation may continue, resulting in high bacterial counts in the circulation. During growth, meningococci release blebs of outer membrane (Fig. 160-1) containing outer-membrane proteins and LPS. Endotoxin binds cell-bound CD14 in association with TLR4 to initiate an inflammatory cascade with the release of high levels of various mediators, including tumor necrosis factor (TNF) α, soluble TNF receptor, interleukin (IL) 1, IL-1 receptor antagonist, IL-1β, IL-6, IL-8, IL-10, plasminogenactivator inhibitor 1 (PAI-1), and leukemia inhibitory factor. Soluble CD14-bound endotoxin acts as a mediator of endothelial activation. The severity of meningococcal disease is related both to the levels of endotoxin in the blood and to the magnitude of the inflammatory response. The latter is determined to some extent by polymorphisms in the inflammatory response genes (and their inhibitors), and the release of the inflammatory cascade heralds the development of meningococ­ cal septicemia (meningococcemia). Endothelial injury is central to many clinical features of meningococcemia, including increased vas­ cular permeability, pathologic changes in vascular tone, loss of throm­ boresistance, intravascular coagulation, and myocardial dysfunction. Endothelial injury leads to increased vascular permeability (attributed to loss of glycosaminoglycans and endothelial proteins), with subse­ quent gross proteinuria. Leakage of fluid and electrolytes into the tis­ sues from capillaries (“capillary leak syndrome”) leads to hypovolemia, tissue edema, and pulmonary edema. Initial compensation results in vasoconstriction and tachycardia, although cardiac output eventually falls. While resuscitation fluids may restore circulating volume, tissue edema will continue to increase, and, in the lung, the consequence may be respiratory failure. Meningococcal Infections Intravascular thrombosis (caused by activation of procoagulant pathways in association with upregulation of tissue factor on the endothelium) occurs in some patients with meningococcal disease and results in purpura fulminans and infarction of areas of skin or even of whole limbs. At the same time, multiple anticoagulant pathways

are downregulated through loss of endothelial thrombomodulin and protein C receptors and decreases in levels of antithrombin III, protein C, protein S, and tissue factor pathway inhibitor. Thrombolysis is also profoundly impaired in meningococcal sepsis through the release of high levels of PAI-1.

Shock in meningococcal septicemia appears to be attributable to a combination of factors, including hypovolemia, which results from the capillary leak syndrome secondary to endothelial injury, and myo­ cardial depression, which is driven by hypovolemia, hypoxia, meta­ bolic derangements (e.g., hypocalcemia), and cytokines (e.g., IL-6). Decreased perfusion of tissues as a result of intravascular thrombosis, vasoconstriction, tissue edema, and reduced cardiac output in menin­ gococcal septicemia can cause widespread organ dysfunction, includ­ ing renal impairment and—later in the disease—a decreased level of consciousness due to central nervous system involvement. Bacteria that reach the meninges cause a local inflammatory response—with release of a spectrum of cytokines similar to that seen in septicemia—that presents clinically as meningitis and is thought to determine the severity of neuronal injury. Local endothelial injury may result in cerebral edema and rapid onset of raised intracranial pressure in some cases. ■ ■CLINICAL MANIFESTATIONS As discussed above, the most common form of infection with N. men­ ingitidis is asymptomatic carriage of the organism in the nasopharynx. Despite the location of infection in the upper airway, meningococcal pharyngitis is rarely reported; however, upper respiratory tract symp­ toms are common prior to presentation with invasive disease. It is not clear whether these symptoms relate to preceding viral infection (which may promote meningococcal acquisition and/or invasion) or to meningococcal acquisition itself. After acquiring the organism, suscep­ tible individuals develop disease manifestations in 1–10 days (usually <4 days, although colonization for 11 weeks has been documented). PART 5 Infectious Diseases Along the spectrum of presentations of meningococcal disease, the most common clinical syndromes are meningitis and meningo­ coccal septicemia. In fulminant cases, death may occur within hours of the first symptoms. Occult bacteremia is also recognized and, if untreated, progresses in two-thirds of cases to focal infection, includ­ ing meningitis or septicemia. Meningococcal disease may also present as pneumonia, pyogenic arthritis or osteomyelitis, purulent pericar­ ditis, endophthalmitis, conjunctivitis, primary peritonitis, or (rarely) urethritis. Perhaps because it is difficult to diagnose, meningococcal pneumonia is not commonly reported but is associated with capsu­ lar groups Y, W, and Z and appears most often to affect individuals

10 years of age. Rash  A nonblanching rash (petechial or purpuric) develops in 80% of cases of meningococcal disease; however, the rash is often absent early in the illness. Usually initially blanching in nature (mac­ ules, maculopapules, or urticaria) and indistinguishable from more common viral rashes, the rash of meningococcal infection becomes petechial or frankly purpuric over the hours after onset. In the most severe cases, large purpuric lesions develop (purpura fulminans; Fig. A1-41). Some patients (including those with overwhelming sepsis) may have no rash. While petechial rash and fever are important signs of meningococcal disease, <10% of children (and, in some clinical settings, <1% of patients) with this presentation are found to have meningococcal disease. Most patients presenting with a petechial or purpuric rash have a viral infection (Table 160-2). The skin lesions exhibit widespread endothelial necrosis and occlusion of small vessels in the dermis and subcutaneous tissues, with a neutrophilic infiltrate. Meningitis  Meningococcal meningitis commonly presents as non­ specific manifestations, including fever, vomiting, and (especially in infants and young children) irritability, and is indistinguishable from other forms of bacterial meningitis unless there is an associated pete­ chial or purpuric rash, which occurs in two-thirds of cases. Headache is rarely reported in early childhood but is more common in later child­ hood and adulthood. When headache is present, the following features, in association with fever or a history of fever, are suggestive of bacterial

TABLE 160-2  Common Causes of Petechial or Purpuric Rashes Enteroviruses Influenza and other respiratory viruses Measles virus Epstein-Barr virus Cytomegalovirus Parvovirus Deficiency of protein C or S (including post-varicella protein S deficiency) Platelet disorders (e.g., idiopathic thrombocytopenic purpura, drug effects, bone marrow infiltration) Henoch-Schönlein purpura, connective tissue disorders, trauma (including nonaccidental injuries in children) Pneumococcal, streptococcal, staphylococcal, or gram-negative bacterial sepsis meningitis: neck stiffness, photophobia, decreased level of conscious­ ness, seizures or status epilepticus, and focal neurologic signs. Classic signs of meningitis, such as neck stiffness and photophobia, are often absent in infants and young children with bacterial meningitis, who more usually present with fever and irritability and may have a bulging fontanelle. While 30–50% of patients present with a meningitis syndrome alone, up to 40% of meningitis patients also present with some features of septicemia. Most deaths from meningococcal meningitis alone (i.e., without septicemia) are associated with raised intracranial pressure presenting as a reduced level of consciousness, relative bradycardia and hypertension, focal neurologic signs, abnormal posturing, and signs of brainstem involvement—e.g., unequal, dilated, or poorly reac­ tive pupils; abnormal eye movement; and impaired corneal responses (Chap. 30). Septicemia  Meningococcal septicemia alone accounts for up to 20% of cases of meningococcal disease. The condition may progress from early nonspecific symptoms to death within hours. Mortality rates among children with this syndrome have been high (25–40%), but early aggressive management (as discussed below) may reduce the figure to <10%. Early symptoms are nonspecific and suggest an influ­ enza-like illness with fever, headache, and myalgia accompanied by vomiting and abdominal pain. As discussed above, the rash, if present, may appear to be viral early in the course until petechiae or purpuric lesions develop. Purpura fulminans occurs in severe cases (Fig. A1-41), with multiple large purpuric lesions and signs of peripheral ischemia. Surveys of patients have indicated that limb pain, pallor (including a mottled appearance and cyanosis), and cold hands and feet may be prominent. Shock is manifested by tachycardia, poor peripheral per­ fusion, tachypnea, and oliguria. Decreased cerebral perfusion leads to confusion, agitation, or decreased level of consciousness. With progressive shock, multiorgan failure ensues; hypotension is a late sign in children, who more commonly present with compensated shock (tachycardia, poor peripheral perfusion, and normal blood pressure). Poor outcome is associated with an absence of meningism, hypoten­ sion, young age, coma, relatively low temperature (<38°C), leukopenia, and thrombocytopenia. Spontaneous hemorrhage (pulmonary, gastric, or cerebral) may result from consumption of coagulation factors and thrombocytopenia. Chronic Meningococcemia  Chronic meningococcemia, which is rarely recognized, presents as repeated episodes of petechial rash (Fig. A1-42) associated with fever, joint pain, features of arthritis, and splenomegaly that may progress to acute meningococcal septicemia if untreated. During the relapsing course, bacteremia characteristically clears without treatment and then recurs. The differential diagnosis includes bacterial endocarditis, acute rheumatic fever, Henoch-Schön­ lein purpura, infectious mononucleosis, disseminated gonococcal infection, and immune-mediated vasculitis. This condition has been associated with complement deficiencies in some cases and with inad­ equate sulfonamide therapy in others.

A study from the Netherlands found that half of isolates from patients with chronic meningococcemia had an underacylated lipid A (part of the surface LPS molecule) due to an lpxL1 gene mutation, which markedly reduces the inflammatory response to endotoxin. Postmeningococcal Reactive Disease  In a small proportion of patients, an immune complex disease develops ~4–10 days after the onset of meningococcal disease, with manifestations that include a maculopapular or vasculitic rash (2% of cases), arthritis (up to 8% of cases), iritis (1%), pericarditis, and/or polyserositis associated with fever. The immune complexes involve meningococcal polysaccharide antigen and result in immunoglobulin and complement deposition with an inflammatory infiltrate. These features resolve spontaneously without sequelae. It is important to recognize this condition since a new onset of fever and rash, and/or arthritis, can lead to concerns about relapse of meningococcal disease and unnecessarily prolonged antibiotic treatment. ■ ■DIAGNOSIS Like other invasive bacterial infections, meningococcal disease may pro­ duce elevations of the white blood cell (WBC) count and of values for inflammatory markers (e.g., C-reactive protein and procalcitonin levels or the erythrocyte sedimentation rate). Values may be normal or low in rapidly progressive disease, and a lack of rise in these laboratory test val­ ues does not exclude the diagnosis. However, in the presence of fever and a petechial rash, these elevations are suggestive of meningococcal disease. In patients with severe meningococcal septicemia, common laboratory findings include hypoglycemia, acidosis, hypokalemia, hypocalcemia, hypomagnesemia, hypophosphatemia, anemia, and coagulopathy. Although meningococcal disease is often diagnosed on clinical grounds, in suspected meningococcal meningitis or meningococcemia, blood should routinely be sent for culture to confirm the diagnosis and to facilitate public health investigations; blood cultures are positive in up to 75% of cases. Culture media containing sodium polyanethol sul­ fonate, which may inhibit meningococcal growth, should be avoided. Meningococcal viability is reduced if there is a delay in transport of the specimen to the microbiology laboratory for culture or in plating of cerebrospinal fluid (CSF) samples. In countries where treatment with antibiotics before hospitalization is recommended for meningococcal disease, the majority of clinically suspected cases are culture negative. Real-time polymerase chain reaction (PCR) analysis of whole-blood samples increases the diagnostic yield by >40%, and results obtained with this method may remain positive for several days after adminis­ tration of antibiotics. Unless contraindications exist (raised intracranial pressure, uncor­ rected shock, disordered coagulation, thrombocytopenia, respiratory insufficiency, local infection, ongoing convulsions), lumbar puncture should be undertaken to identify and confirm the etiology of suspected meningococcal meningitis, whose presentation cannot be distinguished from that of meningitis of other bacterial causes. Some authorities have recommended a computed tomography (CT) brain scan prior to lum­ bar puncture because of the risk of cerebral herniation in patients with raised intracranial pressure. However, a normal CT scan is not uncom­ mon in the presence of raised intracranial pressure in meningococcal meningitis, and the decision to perform a lumbar puncture should be made on clinical grounds. CSF features of meningococcal meningitis (elevated protein level and WBC count, decreased glucose level) are indistinguishable from those of other types of bacterial meningitis unless a gram-negative diplococcus is identified. (Gram’s staining is up to 80% sensitive for meningococcal meningitis.) CSF should be submit­ ted for culture (sensitivity, 90%) and (where available) PCR analysis. CSF antigen testing with latex agglutination is insensitive and should be replaced by molecular diagnosis when possible. Lumbar puncture should generally be avoided in meningococcal septicemia, as positioning for the procedure may critically compromise the patient’s circulation in the context of hypovolemic shock. Delayed lumbar puncture may still be useful when the diagnosis is uncertain, particularly if molecular diagnostic technology is available.

In other types of focal infection, culture and PCR analysis of nor­ mally sterile body fluids (e.g., synovial fluid) may aid in the diagnosis. Although some authorities have recommended cultures of scrapings or aspirates from skin lesions, this procedure adds little to the diag­ nostic yield when compared with a combination of blood culture and PCR analysis. Urinary antigen testing also is insensitive, and serologic testing for meningococcal infection has not been adequately studied. Because N. meningitidis is a component of the normal human nasopha­ ryngeal flora, identification of the organism on throat swabs has lim­ ited diagnostic value, but strains identified in the nasopharynx in the context of a probable case are likely to be those responsible for disease.

TREATMENT Meningococcal Infections Death from meningococcal disease is associated most commonly with hypovolemic shock (meningococcemia) and occasionally with raised intracranial pressure (meningococcal meningitis). Therefore, management should focus on the treatment of these urgent clinical issues in addition to the administration of specific antibiotic ther­ apy. Delayed recognition of meningococcal disease or its associated physiologic derangements, together with inadequate emergency management, is associated with poor outcome. Since the disease is rare, protocols for emergency management have been developed (see https://www.meningitis.org/healthcare-professionals/resources). Airway patency may be compromised if the level of conscious­ ness is depressed as a result of shock (impaired cerebral perfusion) or raised intracranial pressure; this situation may require inter­ vention. In meningococcemia, pulmonary edema and pulmonary oligemia (presenting as hypoxia) require oxygen therapy or elec­ tive endotracheal intubation. In cases with shock, aggressive fluid resuscitation (with replacement of the circulating volume several times in severe cases) and inotropic support may be necessary to maintain cardiac output. If shock persists after volume resuscita­ tion at 40 mL/kg, the risk of pulmonary edema is high, and elective intubation is recommended to improve oxygenation and decrease the work of breathing. Metabolic derangements, including hypo­ glycemia, acidosis, hypokalemia, hypocalcemia, hypomagnesemia, hypophosphatemia, anemia, and coagulopathy, should be antici­ pated and corrected. However, aggressive fluid resuscitation with unbuffered electrolyte solutions was found to increase mortality in febrile African children. Studies of the effects of lower volumes of buffered solutions and similar studies in resource-rich settings are required. In the presence of raised intracranial pressure, manage­ ment includes correction of coexistent shock and neurointensive care to maintain cerebral perfusion. CHAPTER 160 Meningococcal Infections Empirical antibiotic therapy for suspected meningococcal disease consists of a third-generation cephalosporin such as ceftriaxone (75–100 mg/kg per d [maximum, 4 g/d] in one or two divided IV doses) or cefotaxime (200 mg/kg per day [maximum, 8 g/d] in four divided IV doses) to cover the various other (potentially penicillinresistant) bacteria that may produce an indistinguishable clinical syndrome. In many settings, vancomycin (usually 4–60 mg/kg

per d in two to four divided IV doses) is also recommended for the empiric management of sepsis and/or suspected bacterial menin­ gitis. Although unusual in most isolates, reduced meningococcal sensitivity to penicillin (a minimal inhibitory concentration of 0.12–1.0 μg/mL) has been reported. Use of penicillin is appropriate once information on antimicrobial resistance patterns is available. Both meningococcal meningitis and meningococcal septicemia are conventionally treated for 7 days, although courses of 3–5 days may be equally effective. Furthermore, a single dose of ceftriaxone or an oily suspension of chloramphenicol has been used success­ fully in resource-poor settings. No data are available to guide the duration of treatment for meningococcal infection at other foci (e.g., pneumonia, arthritis); antimicrobial therapy is usually continued until clinical and laboratory evidence of infection has resolved. Cultures usually become sterile within 24 h of initiation

of appropriate antibiotic chemotherapy. Eye infections (including keratoconjunctivitis and endophthalmitis) should be treated with a combination of topical and systemic IV therapy, with some small studies suggesting an increased risk of bacteremia when treated with topical therapy alone.

The use of glucocorticoids for adjunctive treatment of meningo­ coccal meningitis remains controversial since no relevant studies have had sufficient power to determine true efficacy in this condi­ tion. One large study in adults did indicate a trend toward benefit, and in clinical practice, a decision to use glucocorticoids would best precede a definite microbiologic diagnosis. Therapeutic doses of glucocorticoids are not recommended in meningococcal septice­ mia, but many intensivists recommend replacement glucocorticoid doses for patients who have refractory shock in association with impaired adrenal gland responsiveness, management that is sup­ ported by limited evidence. Various other adjunctive therapies for meningococcal disease have been considered, but few have been subjected to clinical tri­ als and none can currently be recommended. An antibody to LPS (HA1A) failed to confer a demonstrable benefit. Recombinant bactericidal/permeability-increasing protein (which is not currently available) was tested in a study that had inadequate power to show an effect on mortality rates; however, there were trends toward lower mortality rates among patients who received a complete infu­ sion, and this group also had fewer amputations, fewer blood-prod­ uct transfusions, and a significantly improved functional outcome. Given that protein C concentrations are reduced in meningococcal disease, the use of activated protein C has been considered. A survival benefit was demonstrated in adult sepsis trials; however, trials in pediatric sepsis (of particular relevance for meningococcal disease) found no benefit and indicated a potential risk of bleeding complications with use of activated protein C. PART 5 Infectious Diseases The postmeningococcal immune-complex inflammatory syn­ drome has been treated with nonsteroidal anti-inflammatory agents until spontaneous resolution occurs. ■ ■COMPLICATIONS About 10% of patients with meningococcal disease die despite the availability of antimicrobial therapy and other intensive medical inter­ ventions. The most common complication of meningococcal disease (10% of cases) is scarring after necrosis of purpuric skin lesions, for which skin grafting may be necessary. The lower limbs are most often affected; next in frequency are the upper limbs, the trunk, and the face. On average, 13% of the skin surface area is involved. Amputations are necessary in 1–2% of survivors of meningococcal disease because of a loss of tissue viability after peripheral ischemia or compartment syn­ dromes. Unless there is local infection, amputation should usually be delayed to allow the demarcation between viable and nonviable tissue to become apparent. Approximately 5% of patients with meningococcal disease suffer hearing loss, and 7% have neurologic complications. In one study, pain was reported by 21% of survivors, and in an analysis of capsular group B meningococcal disease (the MOSAIC study), as many as one-quarter of survivors had psychological disorders. In some investigations, the rate of complications is higher for capsular group C disease (mostly associated with the ST11 clone) than for capsular group B disease. In patients with severe hypovolemic shock, renal perfusion may be impaired and prerenal failure is common, but permanent renal replacement therapy is rarely needed. Several studies suggest adverse psychosocial outcomes after menin­ gococcal disease, with reduced quality of life, lowered self-esteem, and poorer neurologic development, including increased rates of attention deficit/hyperactivity disorder and special educational needs. Other studies have not found evidence of such outcomes. ■ ■PROGNOSIS Several prognostic scoring systems have been developed to identify patients with meningococcal disease who are least likely to survive. Factors associated with a poorer prognosis are shock; young age

(infancy), old age, and adolescence; coma; purpura fulminans; dis­ seminated intravascular coagulation; thrombocytopenia; leukopenia; absence of meningitis; metabolic acidosis; low plasma concentrations of antithrombin and proteins S and C; high blood levels of PAI-1; and a low erythrocyte sedimentation rate or C-reactive protein level. The Glasgow Meningococcal Septicaemia Prognostic Score (GMSPS) performs well and may be clinically useful for severity assessment in meningococcal disease. However, scoring systems do not direct the cli­ nician to specific interventions, and the priority in management should be recognition of compromised airways, breathing, or circulation and direct, urgent intervention. Most patients improve rapidly with appro­ priate antibiotics and supportive therapy. Fulminant meningococcemia is more likely to result in death or ischemic skin loss than is meningitis; optimal emergency management may reduce mortality rates among the most severely affected patients. ■ ■PREVENTION Since mortality rates in meningococcal disease remain high despite improvements in intensive care management, immunization is the only rational approach to prevention at a population level. Secondary cases are common among household and “kissing” contacts of cases, and secondary prophylaxis with antibiotics is widely recommended for these contacts (see below). Polysaccharide Vaccines  Purified meningococcal capsular poly­ saccharide was first used for immunization in the 1960s. Meningo­ coccal polysaccharide vaccines were formulated as either bivalent (capsular groups A and C) or quadrivalent (capsular groups A, C, Y, and W), with 50 μg of each polysaccharide per dose. Local reactions (erythema, induration, and tenderness) occurred in up to 40% of vac­ cinees, but serious adverse events (including febrile convulsions in young children) are very rarely reported. In adults, the vaccines are immunogenic, but immunity is relatively short-lived (with antibody levels above baseline for only 2–10 years), and booster doses do not induce a further rise in antibody concentration. Indeed, a state of immunologic hyporesponsiveness has been widely reported to follow booster doses of plain polysaccharide vaccines. The repeating sugar units of these vaccines cross-link B-cell receptors to drive specific memory B cells to become plasma cells and produce protective anti­ body. Because meningococcal polysaccharides are T cell–independent antigens, no memory B cells are produced after immunization, and the memory B-cell pool is depleted such that fewer polysaccharidespecific cells are available to respond to a subsequent dose of vaccine (Fig. 160-6). The clinical relevance of hyporesponsiveness is unknown. Plain polysaccharide vaccines generally are not immunogenic in early childhood, possibly because marginal-zone B cells are involved in polysaccharide responses and maturation of the splenic marginal zone is not complete until 18 months to 2 years of age. The efficacy of the meningococcal capsular group C component is >90% in young adults, but there is no protection in infants; no efficacy data are available for the capsular group Y and W polysaccharides in this age group. Group A meningococcal polysaccharides are exceptional in that they are effective in preventing disease at all ages. Two doses admin­ istered 2–3 months apart to children 3–18 months of age or a single dose administered to older children or adults has a protective efficacy rate of >95%. The vaccine was previously used widely in the control of outbreaks of meningococcal disease in the African meningitis belt, pro­ viding protection for 3–5 years. The plain polysaccharide vaccines have been largely superseded by protein–polysaccharide conjugate vaccines. There is no meningococcal capsular group B plain polysaccharide vaccine because α-2,8-N-acetylneuraminic acid is expressed on the surface of neural cells in the fetus such that the B polysaccharide is perceived as “self” and therefore is not immunogenic in humans. Conjugate Vaccines  The poor immunogenicity of plain polysac­ charide vaccines in infancy has been overcome by chemical conjuga­ tion of the polysaccharides to a carrier protein (CRM197, tetanus toxoid, or diphtheria toxoid). Conjugates that contain monovalent capsular group C polysaccharide and quadrivalent vaccines with A, C, Y, and W polysaccharides were developed, and vaccines with other antigen

Polysaccharide IgG2 and IgM BCR Depletion of memory B-cell pool B cell Plasma cell No production of memory B cells A Polysaccharide Carrier protein Polysaccharidespecific plasma cell IgG1 and IgG3 BCR Polysaccharidespecific B cell Internalization and processing of carrier protein MHC Class II CD40 CD80 or CD86 CD28 CD40L TCR Carrier peptide– specific T cell B FIGURE 160-6  A. Polysaccharides from the encapsulated bacteria that cause disease in early childhood stimulate B cells by cross-linking the B-cell receptor (BCR) and driving the production of immunoglobulins. There is no production of memory B cells, and the B-cell pool may be depleted by this process such that subsequent immune responses are decreased. B. The carrier protein from protein–polysaccharide conjugate vaccines is processed by the polysaccharide-specific B cell, and peptides are presented to carrier peptide–specific T cells, with the consequent production of both plasma cells and memory B cells. MHC, major histocompatibility complex; TCR, T-cell receptor. (Reproduced with permission from AJ Pollard: Maintaining protection against invasive bacteria with protein–polysaccharide conjugate vaccines. Nat Rev Immunol 9:213, 2009.) combinations were produced for some markets (e.g., tetanus conjugates with capsular group C and/or Y polysaccharide and Haemophilus influ­ enzae type b polysaccharide). After immunization, peptides from the carrier protein are conventionally thought to be presented by polysac­ charide-specific B cells to peptide-specific T cells in association with major histocompatibility complex (MHC) class II molecules. (Some data suggest that carrier protein peptide may actually be presented in association with an oligosaccharide and MHCII.) The result is a

T cell–dependent immune response that allows production of antibody and generation of an expanded B-cell memory pool. Unlike responses to booster doses of plain polysaccharides, responses to booster doses of conjugate vaccines have the characteristics of memory responses. Indeed, conjugate vaccines overcome the hyporesponsiveness induced by plain polysaccharides by replenishing the memory pool. The reacto­ genicity of conjugate vaccines is similar to that of plain polysaccharide vaccines. The first widespread use of capsular group C meningococcal conju­ gate vaccine (MenC) came in 1999 in the United Kingdom after a rise in capsular group C disease. A mass vaccination campaign involving all individuals <19 years of age was undertaken, and the number of

laboratory-confirmed capsular group C cases fell from 955 in 1998–1999 to just 29 in 2011–2012. The effectiveness of the immunization pro­ gram was attributed both to direct protection of immunized persons and to reduced transmission of the organism in the population as a result of decreased rates of colonization among the immunized (i.e.,

Differentiation Antibody production Antibody production CHAPTER 160 T-cell help Memory response Polysaccharidespecific memory B cell Meningococcal Infections herd immunity). Data on immunogenicity and effectiveness have shown that the duration of protection is short when the vaccine is administered in early childhood; thus, booster doses are needed to maintain population immunity. In contrast, immunity after a dose of vaccine given in adolescence appears to be more prolonged. In 2005, the first quadrivalent conjugate meningococcal vaccine containing A, C, Y, and W polysaccharides conjugated to diphtheria toxoid was initially recommended for all children >11 years of age in the United States and for persons 2–55 years of age in Canada. Such vaccines are now recommended by the Advisory Committee on Immu­ nization Practices (ACIP) for routine administration to individuals 11–12 years of age, with a booster dose at 16 years of age; only a single dose is given to persons >16 years of age. These vaccines are also rec­ ommended for high-risk persons from 2 months to 55 years of age (see https://www.cdc.gov/mmwr/volumes/69/rr/rr6909a1.htm). Uptake was slow initially, but U.S. data show an efficacy rate of 82% in the first year after vaccination and 69% at 8 years (diphtheria conju­ gate vaccine). Early reports of an increase in the risk of Guillain-Barré syndrome after immunization with the diphtheria conjugate vaccine have not been substantiated with further observation. Quadrivalent conjugate vaccines with tetanus or CRM197 as carrier protein are now available in many countries and are used for high-risk groups and in routine programs for toddlers and adolescents. Use of the A, C, W, and Y conjugate vaccine for adolescents since 2015 in the United Kingdom has led to a large reduction in meningococcal disease caused by these

capsular groups. This conjugate vaccine provided direct protection for vaccinated individuals (combined vaccine effectiveness of 94% against C, W, and Y disease) and marked reductions in carriage—a 36% reduc­ tion in carriage of capsular groups C, W, and Y combined at 2 months postvaccination—along with evidence of herd immunity. Indeed, modeling of the decline of cases and carriage in the United Kingdom indicates elimination of these capsular groups over the coming decade.

A monovalent capsular group A vaccine, manufactured in India, was licensed in 2010 and rolled out to countries in the sub-Saharan African meningitis belt in a mass immunization campaign. There is strong evidence that this vaccine has been highly effective in controlling epi­ demic meningococcal disease in the region, with >90% reduction in disease in vaccinated populations. New combination vaccines covering A, C, W, X, and Y have been developed and are set to replace monova­ lent MenA vaccines in Africa as part of the WHO’s global roadmap “Defeating Meningitis by 2030” (https://www.who.int/publications/i/ item/9789240026407). Vaccines Based on Subcapsular Antigens  The lack of immu­ nogenicity of the group B capsule has led to the development of vac­ cines based on subcapsular antigens. Various surface components have been studied in early-phase clinical trials. Outer-membrane vesicles (OMVs) containing outer-membrane proteins, phospholipid, and LPS can be extracted from cultures of N. meningitidis by detergent treat­ ment (Fig. 160-7). OMVs prepared in this way were used in efficacy trials with a Norwegian outbreak strain and reduced the incidence of group B disease among 14- to 16-year-old schoolchildren by 53%. Sim­ ilarly, OMV vaccines constructed from local outbreak strains in Cuba and New Zealand have had reported efficacy rates of >70%. These OMV vaccines appear to produce strain-specific immune responses, with only limited cross-protection, and are therefore best suited to clonal outbreaks (e.g., those in Cuba and New Zealand as well as others in Norway and the province of Normandy in France). PART 5 Infectious Diseases Several purified surface proteins have been evaluated in phase 1 clinical trials but have not yet been developed further because of anti­ genic variability or poor immunogenicity (e.g., transferrin-binding proteins, neisserial surface protein A). Other vaccine candidates have been identified since sequencing of the meningococcal genome. The combination vaccine 4CMenB, which includes the New Zealand OMV vaccine and three recombinant proteins (neisserial adhesin A, factor FIGURE 160-7  Illustration of meningococcal outer-membrane vesicle containing outer-membrane structures.  

H–binding protein, and neisserial heparin-binding antigen), is immu­ nogenic from infancy and has been licensed for use in the United States, Canada, Europe, and Australia. This vaccine has been used with appar­ ent success in the control of several university outbreaks in the United States and in a community outbreak in an area of Quebec, Canada. The 4CMenB vaccine has an acceptable safety profile, with fever prominent among infants and injection-site pain frequently reported among older children and adults. In September 2015, 4CMenB was recommended for routine use in the United Kingdom for all infants born from May 2015 onward; a recent analysis reported a 75% reduction in age groups that were fully eligible for vaccination, with a high coverage rate of 95%. The licensed schedule is three priming doses before 6 months of age and a booster dose at 12 months of age (but is used in the United Kingdom as two priming doses under 6 months of age and a booster at 1 year). A nonsignificant vaccine effectiveness of 53% was seen after two doses, and 59% effectiveness was found after the booster dose at 1 year of age. As of 2022, protein-based MenB vaccines were authorized in 58 countries. Of these countries, 15 have a universal program in at least one age group, 21 have a recommendation for high-risk groups based on medical conditions, and 13 have a recommendation based on increased risk of exposure (e.g., laboratory staff). Because the disease is so rare, the cost-effectiveness of capsular group B vaccine in infant immunization programs, as assessed with conventional thresholds, is borderline in the United Kingdom. Since infants are not commonly colonized with capsular group B meningo­ cocci, any impact on the total population burden of carried organisms will be small. It is therefore unlikely that an infant immunization pro­ gram will provide additional value through induction of herd immu­ nity. Rates of capsular group B carriage are higher among teenagers and young adults than at other ages (apart from infancy). A large clusterrandomized trial in Australia found no effect of 4CMenB on carriage of disease-causing meningococci, highlighting that the benefit of this vaccine is likely to be via direct protection only. An immunogenic vaccine based on two variants of the lipoprotein factor H–binding protein (MenB-fHBP) has been developed for use in adolescents and is licensed in the United States and Europe. The vac­ cine is immunogenic against representative indicator strains, inducing fourfold rises in bactericidal antibody titer in 50–92% of individuals. MenB-fHBP has an acceptable safety profile, with pain at the injection site, fatigue, and headache commonly reported. This vaccine can be used with a range of vaccines routinely administered in adolescence, including Tdap (tetanus–diphtheria–acellular pertussis), human papil­ lomavirus, and MenACWY vaccines. MenB-fHBP has been used to control outbreaks of meningococcal disease in educational institutions in the United States, but no formal studies of its effectiveness have yet been undertaken due to the absence of any public health programs with this vaccine. Studies in the United Kingdom are currently evaluating the impact of both 4CMenB and MenB-fHBP against meningococcal carriage among teenagers. Both of the new capsular group B meningococcal vaccines are licensed for use in the United States for persons 10–25 years of age but are not recommended for routine use. ACIP advises that the vaccines can be used in a two-dose schedule following shared decision-making between the doctor, patient, and/or family. In addition, ACIP recom­ mends their administration to individuals at high risk of capsular group B disease in a two-dose schedule (4CMenB) or a three-dose schedule (MenB-fHBP). ■ ■MANAGEMENT OF CONTACTS Close (household and kissing) contacts of individuals with meningo­ coccal disease are at increased risk of developing secondary disease (up to 1000 times the rate for the general population); a secondary case follows as many as 3% of sporadic cases. About one-fifth of secondary cases are actually co-primary cases—i.e., cases that occur soon after the primary case and in which transmission is presumed to have originated from the same third party. The rate of secondary cases is highest during the week after presentation of the index case. The risk falls rapidly but remains above baseline for up to 1 year after the index case; 30% of sec­ ondary cases occur in the first week, 20% in the second week, and most

44 - 161 Gonococcal Infections

161 Gonococcal Infections

of the remainder over the next 6 weeks. In outbreaks of meningococcal disease, mass prophylaxis has been used; however, limited data support population intervention, and significant concerns have arisen about adverse events and the development of resistance. For these reasons, prophylaxis is usually restricted to (1) persons at greatest risk who are intimate and/or household contacts of the index case and (2) health care workers who have been directly exposed to respiratory secretions. In most cases, members of wider communities (e.g., at schools or col­ leges) are not offered prophylaxis. The aim of prophylaxis is to eradicate colonization of close contacts with the strain that has caused invasive disease in the index case. Prophylaxis should be given to all contacts at the same time to avoid recolonization by meningococci transmitted from untreated contacts and should also be used as soon as possible to treat early disease in secondary cases. If the index patient is treated with an antibiotic that does not reliably clear colonization (e.g., penicillin), the patient should be given a prophylactic agent at the end of treatment to prevent relapse or onward transmission. Although rifampin has been most widely used and studied, it is not the optimal agent because it fails to eradicate carriage in 15–20% of cases, rates of adverse events have been high, compliance is affected by the need for four doses, and emerging resis­ tance has been reported. Ceftriaxone as a single IM or IV injection is highly (97%) effective in carriage eradication and can be used at all ages and in pregnancy. Reduced susceptibility of isolates to ceftriaxone has occasionally been reported. Ciprofloxacin or ofloxacin is preferred in some countries; these agents are highly effective and can be adminis­ tered by mouth but are not recommended in pregnancy. Resistance to fluoroquinolones has been reported in some meningococci in North America, Europe, and Asia. In documented capsular group A, B, C, Y, or W disease, contacts may be offered immunization (with either the MenACWY conjugate vaccine or the MenB vaccine, as appropriate) in addition to chemo­ prophylaxis to provide protection beyond the duration of antibiotic therapy. Mass vaccination has been used successfully to control disease during outbreaks in closed communities (educational and military establishments) as well as during epidemics in open communities. ■ ■FURTHER READING Carr JP et al: Impact of meningococcal ACWY conjugate vaccines on pharyngeal carriage in adolescents: Evidence for herd protection from the UK MenACWY programme. Clin Microbiol Infect 28:1649. e1, 2022. Castilla J et al: Effectiveness of a meningococcal group B vaccine (4CMenB) in children. N Engl J Med 388:427, 2023. Christensen H et al: Meningococcal carriage by age: A systematic review and meta-analysis. Lancet Infect Dis 10:853, 2010. Haidara FC et al: Meningococcal ACWYX conjugate vaccine in 2-to29-year-olds in Mali and Gambia. N Engl J Med 388:1942, 2023. Ladhani SN et al: Vaccination of infants with meningococcal group B vaccine (4CMenB) in England. N Engl J Med 382:309, 2020. Marshall HS et al: Meningococcal B vaccine and meningococcal car­ riage in adolescents in Australia. N Engl J Med 382:318, 2020. Mbaeyi SA et al: Meningococcal vaccination: Recommendations of the Advisory Committee on Immunization Practices, United States, 2020. MMWR Recomm Rep 69:1, 2020. Pardo de Santayana C et al: Epidemiology of invasive meningococ­ cal disease worldwide from 2010–2019: A literature review. Epide­ miol Infect 151:e57, 2023. Pollard AJ et al: Maintaining protection against invasive bacteria with protein–polysaccharide conjugate vaccines. Nat Rev Immunol 9:213, 2009. Read RC et al: Effect of a quadrivalent meningococcal ACWY glyco­ conjugate or a serogroup B meningococcal vaccine on meningococcal carriage: An observer-blind, phase 3 randomised clinical trial. Lancet 384:2123, 2014. Vieusseux M: Memoire sur le maladie qui a regne a Geneva au print­ emps de 1805. J Med Clin Pharm 11:163, 1805. Wang B et al: 4CMenB sustained vaccine effectiveness against invasive meningococcal B disease and gonorrhoea at three years post pro­ gramme implementation. J Infect 87:95, 2023.

Sanjay Ram, Peter A. Rice

Gonococcal Infections ■ ■DEFINITION Gonorrhea is a sexually transmitted infection (STI) of epithelium and commonly manifests as cervicitis, urethritis, proctitis, and conjunctivi­ tis. If untreated, infections at these sites can lead to local complications such as endometritis, salpingitis, tuboovarian abscess, bartholinitis, peritonitis, and perihepatitis in female patients; periurethritis and epi­ didymitis in male patients; and ophthalmia neonatorum in newborns. Disseminated gonococcemia is an uncommon event whose manifesta­ tions include skin lesions, tenosynovitis, septic arthritis, and (in rare cases) endocarditis or meningitis. ■ ■MICROBIOLOGY Neisseria gonorrhoeae is a gram-negative, nonmotile, non-spore-forming organism that grows singly and in pairs (i.e., as monococci and dip­ lococci, respectively). Exclusively a human pathogen, the gonococcus contains, on average, three genome copies per coccal unit; this poly­ ploidy permits a high level of antigenic variation and the survival of the organism in its host. Gonococci, like all other Neisseria species, are oxidase positive. They are distinguished from other neisseriae by their ability to grow on selective media and to use glucose but not maltose, sucrose, or lactose. CHAPTER 161 ■ ■EPIDEMIOLOGY The incidence of gonorrhea had been declining steadily in the United States, but in 2021, there were ~710,000 newly reported cases—up 136% since their historic low in 2009. With 82.4 million cases esti­ mated by the World Health Organization to have occurred globally in 2020 among adults aged 15 to 49 years, gonorrhea remains a major public health problem worldwide, is a significant cause of morbidity in developing countries, and may play a role in enhancing transmission of HIV. Gonococcal Infections Gonorrhea predominantly affects young, nonwhite, unmarried, less educated members of urban populations. The number of reported cases probably represents half of the true number of cases—a dis­ crepancy resulting from underreporting, self-treatment, nonspecific treatment without a laboratory-proven diagnosis, and asymptomatic infection. The number of reported new cases of gonorrhea in the United States rose from ~250,000 in the early 1960s to a high of 1.01 million in 1978. The recorded incidence of gonorrhea in modern times peaked in 1975, with 468 reported new cases per 100,000 population in the United States. This peak was attributable to the interaction of several variables, including improved accuracy of diagnosis, changes in patterns of contraceptive use, and changes in sexual behavior. A decline in the overall incidence of gonorrhea in the United States over the past quarter-century may have reflected increased condom use resulting from public health efforts to curtail HIV transmission. Nevertheless, in 2019, 214 new cases per 100,000 population were reported in this country, representing a 1-year increase of 4.6%; this figure is the highest among industrialized countries. Simultaneously, antibiotic resistance is increasing in the United States and other coun­ tries, prompting the U.S. Centers for Disease Control and Prevention (CDC) to name antibiotic-resistant N. gonorrhoeae as one of the three most urgent threats of its kind. At present, the attack rate in the United States is highest among 15- to 24-year-old women (730.5 per 100,000) and 20- to 29-year-old men (813.6 per 100,000); >70% of all reported cases occur in these two groups. From the standpoint of ethnicity, rates are highest among African Americans (652.9 per 100,000) and lowest among persons of Asian descent (37.8 per 100,000). The incidence of gonorrhea is higher in developing countries than in industrialized nations. The exact incidence of any STI is difficult to ascertain in developing countries because of limited surveillance and variable diagnostic criteria. Extremely high rates of gonorrhea have

been reported among aboriginal populations in Namibia and Australia. Studies in Africa have clearly demonstrated that nonulcerative STIs such as gonorrhea (in addition to ulcerative STIs) are an independent risk factor for the transmission of HIV (Chap. 208).

Gonorrhea is transmitted from males to females more efficiently than in the opposite direction. The rate of transmission to a woman during a single unprotected sexual encounter with an infected man is ~50–80%. Oropharyngeal gonorrhea occurs in ~20% of women who practice fellatio with infected partners. Transmission in either direc­ tion by cunnilingus is rare. In any population, there exists a small minority of individuals who have high rates of new-partner acquisition. These “core-group members” or “high-frequency transmitters” are vital in sustaining STI transmission at the population level. Another instrumental factor in sustaining gonorrhea in the population is the large number of infected individuals who are asymptomatic or have minor symptoms that are ignored. These persons, unlike symptomatic individuals, may not cease sexual activity and therefore may continue to transmit the infec­ tion. This situation underscores the importance of contact tracing and empirical treatment of the sex partners of index cases. ■ ■PATHOGENESIS, IMMUNOLOGY, AND ANTIMICROBIAL RESISTANCE Outer-Membrane Proteins  •  PILI  Fresh clinical isolates of N. gonorrhoeae initially form piliated (fimbriated) colonies distinguish­ able on translucent agar. Pilus expression is rapidly switched off with unselected subculture because of rearrangements in pilus genes. This change is a basis for antigenic variation of gonococci. Piliated strains adhere better to cells derived from human mucosal surfaces and are more virulent in organ culture models and human inoculation experi­ ments than nonpiliated variants. In a fallopian tube explant model, pili mediate gonococcal attachment to nonciliated columnar epithelial cells. This event initiates gonococcal adherence, invasion and transport through these cells to intercellular spaces near the basement membrane or directly into the subepithelial tissue. Pili are also essential for genetic competence and transformation of N. gonorrhoeae, which permit horizontal transfer of genetic material between different gonococcal lineages in vivo. PART 5 Infectious Diseases OPACITY-ASSOCIATED PROTEIN  Another gonococcal surface protein that is important in adherence to epithelial cells is opacity-associated protein (Opa; formerly called protein II). Opa contributes to inter­ gonococcal adhesion, which is responsible for the opaque nature of gonococcal colonies on translucent agar and the organism’s adherence to a variety of eukaryotic cells, including polymorphonuclear leuko­ cytes (PMNs). Certain Opa variants promote invasion of epithelial cells, and this effect has been linked with the ability of Opa to bind vitronectin, heparan sulfate proteoglycans, and several members of the carcinoembryonic antigen–related cell adhesion molecule (CEACAM) receptor family. Epithelial CEACAM-binding gonococci prevent exfo­ liation of epithelium through a mechanism that involves nitric oxide that is produced during anaerobic bacterial metabolism and upregu­ lation of CD105 (a member of the transforming growth factor-beta receptor family), which may interfere with bacterial clearance.

N. gonorrhoeae Opa proteins that bind CEACAM1, which is expressed by primary CD4+ T lymphocytes, suppress the activation and pro­ liferation of these lymphocytes. Select Opa proteins can engage CEACAM3, which is expressed on neutrophils, with consequent non­ opsonic phagocytosis (i.e., phagocytosis independent of antibody and complement) and killing of bacteria. PORIN  Porin (previously designated protein I) is the most abundant gonococcal surface protein. Porin molecules exist as trimers that provide anion-transporting aqueous channels through the otherwise hydrophobic outer membrane. Porin exhibits stable interstrain anti­ genic variation and forms the basis for gonococcal serotyping. Two main serotypes have been identified; PorB.1A strains are often associ­ ated with disseminated gonococcal infection (DGI), whereas PorB.1B strains usually cause local genital infections only. DGI strains are

generally resistant to the killing action of normal human serum and do not incite a significant local inflammatory response; therefore, they may not cause symptoms at genital sites. These characteristics may be related to the ability of PorB.1A strains to bind to complement-inhibitory molecules, resulting in a diminished inflammatory response. Porin can translocate to the cytoplasmic membrane of host cells—a process that could initiate gonococcal endocytosis and invasion. PorB.1B present in outer membrane vesicles shed during bacterial growth inhibits the abil­ ity of dendritic cells to induce T-cell proliferation and may contribute to the ability of gonococci to subvert adaptive immunity. OTHER OUTER-MEMBRANE PROTEINS  Other notable outer-membrane proteins include H.8, a lipoprotein that is present in high concentra­ tion on the surface of all gonococcal strains and is an excellent target for antibody-based diagnostic testing. Transferrin-binding proteins (Tbp1 and Tbp2), lactoferrin-binding proteins (LbpA and LbpB), and hemoglobin/haptoglobin binding proteins (HpuA and HpuB) are required for scavenging iron from transferrin, lactoferrin, and heme in vivo. Transferrin and iron have been shown to enhance the attachment of iron-deprived N. gonorrhoeae to human endometrial cells. TdfH and TdfJ enable gonococci to scavenge host zinc from calprotectin and S100 calcium binding protein A7 (psoriasin). IgA1 protease is produced by N. gonorrhoeae and may protect the organism from the action of mucosal IgA. Lipooligosaccharide  Gonococcal lipooligosaccharide (LOS) con­ sists of a lipid A and a core oligosaccharide that lacks the repeating O-carbohydrate antigenic side chain seen in many other gram-negative bacteria. Gonococcal LOS possesses marked endotoxic activity and contributes to the local cytotoxic effect in a fallopian tube model. LOS core sugars undergo a high degree of phase variation under different conditions of growth; this variation reflects genetic regulation and expression of glycotransferase genes that dictate the carbohydrate structure of LOS. These phenotypic changes may affect interactions of N. gonorrhoeae with elements of the humoral immune system (antibodies and complement) and may also influence direct binding of organisms to both professional phagocytes and nonprofessional phagocytes (epithelial cells). For example, gonococci that are sialylated at their LOS sites inhibit the classic pathway of complement by reduc­ ing binding of IgG and also bind complement factor H to inhibit the alternative pathway of complement. LOS sialylation may also decrease nonopsonic Opa-mediated association with neutrophils and inhibit the oxidative burst in PMNs. The binding of the unsialylated terminal lactosamine residue of LOS to an asialoglycoprotein receptor on male epithelial cells facilitates adherence and subsequent gonococcal inva­ sion of these cells. Moreover, oligosaccharide structures in LOS can modulate host immune responses. For example, the terminal mono­ saccharide expressed by LOS determines the C-type lectin receptor on dendritic cells that is targeted by the bacteria. In turn, the specific C-type lectin receptor engaged influences whether a TH1- or TH2-type response is elicited; the latter response may be less favorable for clear­ ance of gonococcal infection. Host Factors  In addition to gonococcal structures that interact with epithelial cells, host factors seem to be important in mediating entry of gonococci into nonphagocytic cells. Activation of phospha­ tidylcholine-specific phospholipase C and acidic sphingomyelinase by N. gonorrhoeae, which results in the release of diacylglycerol and ceramide, is a requirement for the entry of N. gonorrhoeae into epi­ thelial cells. Ceramide accumulation within cells leads to apoptosis, which may disrupt epithelial integrity and facilitate entry of gonococci into subepithelial tissue. Release of chemotactic factors as a result of complement activation contributes to inflammation, as does the toxic effect of LOS and peptidoglycan in provoking the release of inflamma­ tory cytokines. The importance of humoral immunity in host defenses against neisserial infections is best illustrated by the predisposition of persons deficient in terminal complement components (C5 through C9) to have recurrent bacteremic gonococcal infections and recurrent menin­ gococcal meningitis or meningococcemia. Gonococcal porin induces

T cell–proliferative responses in persons with urogenital gonococcal disease. A significant increase in porin-specific interleukin (IL)

4–producing CD4+ as well as CD8+ T lymphocytes is seen in individu­ als with mucosal gonococcal disease. A portion of these lymphocytes that show a porin-specific TH2-type response could traffic to mucosal surfaces and play a role in immune protection against the disease. Few data clearly indicate that protective immunity is acquired from a previ­ ous gonococcal infection, although bactericidal and opsonophagocytic antibodies to porin and LOS may offer partial protection. On the other hand, women who are infected and acquire high levels of antibody to another outer-membrane protein, Rmp (reduction modifiable protein, formerly called protein III), may be especially likely to become rein­ fected with N. gonorrhoeae because Rmp antibodies block the effect of bactericidal antibodies to porin and LOS. Rmp shows little, if any, interstrain antigenic variation; therefore, Rmp antibodies potentially may block antibody-mediated killing of all gonococci. The mechanism of blocking has not been fully characterized, but Rmp antibodies may noncompetitively inhibit binding of porin and LOS antibodies because of the proximity of these structures in the gonococcal outer membrane. In male volunteers who have no history of gonorrhea, the net effect of these events may influence the outcome of experimental challenge with N. gonorrhoeae. Because Rmp bears extensive homology to enterobac­ terial OmpA and meningococcal class 4 proteins, it is possible that these blocking antibodies result from prior exposure to cross-reacting proteins from these species and also play a role in first-time infection with N. gonorrhoeae. Gonococcal Resistance to Antimicrobial Agents  It is no surprise that N. gonorrhoeae, with its remarkable capacity to alter its antigenic structure and adapt to changes in the microenvironment, has become resistant to numerous antibiotics. The first effective agents against gonorrhea were the sulfonamides, which were introduced in the 1930s and became ineffective within a decade. Penicillin was then used as the drug of choice for the treatment of gonorrhea. By 1965, 42% of gonococcal isolates had developed low-level resistance to penicillin G. Resistance due to the production of penicillinase arose later. Gonococci become fully resistant to antibiotics either by chromo­ somal mutations or by acquisition of R factors (plasmids). Two types of chromosomal mutations have been described. The first type, which is drug specific, is a single-step mutation leading to highlevel resistance. The second type involves mutations at several chromo­ somal loci that combine to determine the level as well as the pattern of resistance. Strains with mutations in chromosomal genes were first observed in the late 1950s. As recently as 2007, chromosomal muta­ tions accounted for resistance to penicillin, tetracycline, or both in ~16% of strains surveyed in the United States. β-Lactamase (penicillinase)–producing strains of N. gonorrhoeae (PPNG) carrying β-lactamase plasmids had rapidly spread worldwide by the early 1980s. N. gonorrhoeae strains with plasmid-borne tetra­ cycline resistance (TRNG) can mobilize some β-lactamase plasmids, and PPNG and TRNG occur together, sometimes along with strains exhibiting chromosomally mediated resistance (CMRNG). Penicillin, ampicillin, and tetracycline are no longer reliable for the treatment of gonorrhea and should not be used. Quinolone-containing regimens also were recommended for treat­ ment of gonococcal infections; the fluoroquinolones offered the advan­ tage of antichlamydial activity when administered for 7 days. However, quinolone-resistant N. gonorrhoeae (QRNG) appeared soon after these agents were first used to treat gonorrhea. QRNG is particularly common in the Pacific Islands (including Hawaii) and Asia, where, in certain areas, all gonococcal strains are now resistant to quinolones. At present, QRNG is also common in parts of Europe and the Middle East. In the United States, QRNG has been identified in all areas but predominantly in states on the Pacific coast, where resistant strains were first seen. Alterations in DNA gyrase and topoisomerase IV have been implicated as mechanisms of fluoroquinolone resistance. Third-generation cephalosporins have remained highly effective as single-dose therapy for gonorrhea, but the recent isolation of strains highly resistant to ceftriaxone (minimal inhibitory concentrations

[MICs], 2 μg/mL) in Asia, some European countries and recently, in the United States, is cause for concern. Even though the MICs of ceftriaxone against certain strains may reach 0.015–0.125 μg/mL (higher than the MICs of 0.0001–0.008 μg/mL for fully susceptible strains), these levels are greatly exceeded in the blood, the urethra, and the cervix when the routinely recommended parenteral dose of ceftriaxone is administered. The rising MICs of oral cefixime (the pre­ viously recommended alternative oral third-generation cephalosporin) against N. gonorrhoeae, combined with this drug’s limited capacity to reach levels sufficiently higher than MICs in the blood, the urethra, the cervix, and especially the pharynx, have resulted in the removal of cefixime from the list of first-line agents for treatment of uncom­ plicated gonorrhea. N. gonorrhoeae strains with reduced susceptibility to ceftriaxone and cefixime (i.e., cephalosporin-intermediate/resistant strains) contain mutations in (1) the penA allele, which is the princi­ pal resistance determinant and encodes a penicillin-binding protein (PBP2) whose sequence can differ in up to 60–70 amino acids from that of wild-type PBP2; (2) the multiple transferable resistance regulator (mtrR) gene that results in increased drug efflux through the MtrCDE efflux pump; and (3) penB, which decreases drug influx through PorB.

Spectinomycin has been used as an alternative agent because it is not associated with resistance to other antibiotics and can be reserved for use against multidrug-resistant strains of N. gonorrhoeae. In China, spectinomycin is recommended as an alternative agent for primary treatment of urogenital gonorrhea and is often used there instead of ceftriaxone. Nevertheless, outbreaks caused by strains resistant to spectinomycin have been documented in Korea and England when the drug had been used for primary treatment of gonorrhea. CHAPTER 161 Resistance to azithromycin can result from alterations of the ribo­ somal binding target by azithromycin and—as with cephalosporins— the over- and underexpression of efflux and influx systems. Combined resistance to cephalosporins and azithromycin has been reported in several instances throughout the world. Gonococcal Infections ■ ■CLINICAL MANIFESTATIONS Gonococcal Infections in Men  Acute urethritis is the most com­ mon clinical manifestation of gonorrhea in male patients. However, there is a reservoir of infected men who remain asymptomatic. The usual incubation period after exposure is 2–7 days before symptoms develop although the interval can be longer. Strains of the PorB.1A serotype tend to cause a greater proportion of cases of mild and asymp­ tomatic urethritis than do PorB.1B strains. When they occur, urethral discharge and dysuria, usually without urinary frequency or urgency, are the major symptoms. The discharge initially is scant and mucoid but becomes profuse and purulent within a day or two. Gram staining of the urethral discharge may reveal PMNs and gram-negative intracel­ lular monococci and diplococci (Fig. 161-1). The clinical manifesta­ tions of gonococcal urethritis are usually more severe and overt than FIGURE 161-1  Gram stain of urethral discharge from a male patient with gonorrhea shows gram-negative intracellular monococci and diplococci. (Source: © All rights reserved. Canadian Guidelines on Sexually Transmitted Infections. Public Health Agency of Canada, modified 2020. Adapted and reproduced with permission from the Minister of Health, 2021.)

those of nongonococcal urethritis, including urethritis caused by Chla­ mydia trachomatis (Chap. 194); however, exceptions are common, and it is often impossible to differentiate the causes of urethritis on clinical grounds alone. The majority of cases of urethritis seen in the United States today are not caused by N. gonorrhoeae and/or C. trachomatis. Although a number of other organisms may be responsible, many cases do not have a specific etiologic agent identified. Certain clones of Neisseria meningitidis, the second member of the pathogenic Neisseria species, have been associated with urethritis in men who have sex with men (MSM) in Europe and in heterosexual men in the southern and midwestern United States.

Most symptomatic men with gonorrhea seek treatment and cease to be infectious. The remaining men, who are largely asymptomatic, accumulate in number over time and constitute the majority of all infected men at any point in time; together with men incubating the organism who shed the organism but have yet to become symptomatic, they serve as the source of spread of infection. Before the antibiotic era, symptoms of urethritis persisted for ~8 weeks. Epididymitis is now an uncommon complication, and gonococcal prostatitis occurs rarely, if at all. Other unusual local complications of gonococcal urethritis include edema of the penis due to dorsal lymphangitis or thrombophlebitis, submucous inflammatory “soft” infiltration of the urethral wall, peri­ urethral abscess or fistula, inflammation or abscess of Cowper’s gland, and seminal vesiculitis. Balanitis may develop in uncircumcised men. Gonococcal Infections in Women  •  GONOCOCCAL CERVICITIS 

Mucopurulent cervicitis is a common STI diagnosis in American women and may be caused by N. gonorrhoeae, C. trachomatis, and other organisms, including Mycoplasma genitalium (Chap. 193). Cervicitis is often associated with more that one bacterial species, for example, coinfection with C. trachomatis, and may also coexist with candidal or trichomonal vaginitis. N. gonorrhoeae primarily infects the columnar epithelium of the cervical os. Bartholin’s glands occasionally become infected. PART 5 Infectious Diseases About one-half of women infected with N. gonorrhoeae develop symptoms. Women who either remain asymptomatic or have only minor symptoms may delay seeking medical attention. These minor symptoms may include scant vaginal discharge issuing from the inflamed cervix (without vaginitis or vaginosis per se) and dysuria (often without urgency or frequency) that may be associated with gonococcal urethritis. Although the incubation period of gonorrhea is less well defined in women than in men, symptoms usually develop within 10 days of infection and are more acute and intense than those of chlamydial cervicitis. The speculum examination reveals a mucopurulent discharge (mucopus) issuing from the cervical os or a reddened (inflamed) cervix even in the absence of reported symptoms. Because Gram stain is not sensitive for the diagnosis of gonorrhea in women, specimens should be submitted for culture or a nonculture assay (see “Laboratory Diagnosis,” below). Edematous and friable cervical ectopy and endo­ cervical bleeding induced by gentle swabbing are more often seen in chlamydial infection. Gonococcal infection may extend deep enough to produce dyspareunia and lower abdominal or back pain. In such cases, it is imperative to consider a diagnosis of pelvic inflammatory disease (PID) and to administer treatment for that disease (Chaps. 141 and 194). N. gonorrhoeae may also be recovered from the urethra and rectum of women with cervicitis, but these are rarely the only infected sites. Urethritis in women may produce symptoms of internal dysuria, which is often attributed to “cystitis.” Pyuria in the absence of bacteriuria visi­ ble on Gram stain of unspun urine, accompanied by urine cultures that fail to yield >102 colonies of bacteria usually associated with urinary tract infection, signifies the possibility of urethritis usually due to C. trachomatis. Urethral infection with N. gonorrhoeae also may occur in this context, but in this instance, urethral cultures are usually positive. GONOCOCCAL VAGINITIS  The vaginal mucosa of healthy women is lined by stratified squamous epithelium and is rarely infected by N. gonorrhoeae. However, gonococcal vaginitis can occur in anestro­ genic women (e.g., prepubertal girls and postmenopausal women),

in whom the vaginal stratified squamous epithelium is often thinned down to the basilar layer, which can be infected by N. gonorrhoeae. The intense inflammation of the vagina makes the physical (speculum and bimanual) examination extremely painful. The vaginal mucosa is red and edematous, and an abundant purulent discharge is often present. Infection in the urethra and in Skene’s and Bartholin’s glands often accompanies gonococcal vaginitis. Inflamed cervical erosion or abscesses in nabothian cysts may also occur. Coexisting cervicitis may result in pus in the cervical os. Anorectal Gonorrhea  Because the female anatomy permits the spread of cervical exudate to the rectum, N. gonorrhoeae is sometimes recovered from the rectum of women with uncomplicated gonococcal cervicitis. The rectum is the sole site of infection in only 5% of women with gonorrhea. Such women are usually asymptomatic but occasion­ ally have acute proctitis manifested by anorectal pain or pruritus, tenesmus, purulent rectal discharge, and rectal bleeding. Among MSM, the frequency of gonococcal infection, including rectal infection, fell by ≥90% throughout the United States in the early 1980s. A resurgence of gonorrhea among MSM has been documented in several cities since the 1990s; the estimated rates of reported cases having more than doubled in a recent 3-year period. Gonococcal isolates from the rec­ tum of MSM tend to be more resistant to antimicrobial agents than are gonococcal isolates from other sites. Gonococcal isolates with a muta­ tion in mtrR or in the promoter region of the gene that encodes for this transcriptional regulator develop increased resistance to antimicrobial hydrophobic agents such as bile acids and fatty acids in feces and thus are found with increased frequency in MSM. This situation may have been responsible for higher rates of failure of treatment for rectal gon­ orrhea with older regimens consisting of penicillin or tetracyclines. Pharyngeal Gonorrhea  Pharyngeal gonorrhea is usually mild or asymptomatic, although symptomatic pharyngitis does occasion­ ally occur with cervical lymphadenitis. The mode of acquisition is oral–genital sexual exposure, with fellatio being a more efficient means of transmission than cunnilingus. In certain female adolescent populations in the United States, pharyngeal gonorrhea has become as common as genital gonorrhea. Most cases resolve spontaneously and transmission from the pharynx to sexual contacts is rare. Pharyngeal infection almost always coexists with genital infection. Swabs from the pharynx should be plated directly onto gonococcal selective media. Pharyngeal colonization with N. meningitidis needs to be differentiated from that with other Neisseria species. Because commensal oropha­ ryngeal neisseriae are often resistant to antimicrobials, horizontal gene transfer between these organisms and N. gonorrhoeae may be impor­ tant in the development of antimicrobial resistance of N. gonorrhoeae. Ocular Gonorrhea in Adults  Ocular gonorrhea in an adult usu­ ally results from autoinoculation of N. gonorrhoeae from an infected genital site. As in genital infection, the manifestations range from severe to occasionally mild or asymptomatic disease. The variability in clinical manifestations may be attributable to differences in the ability of the infecting strain to elicit an inflammatory response. Infection may result in a markedly swollen eyelid, severe hyperemia and chemosis, and a profuse purulent discharge. The massively inflamed conjunctiva may be draped over the cornea and limbus. Lytic enzymes from the infiltrating PMNs occasionally cause corneal ulceration and rarely cause perforation. Prompt recognition and treatment of this condition are of para­ mount importance. Gram stain and culture of the purulent discharge establish the diagnosis. Genital cultures also should be performed. Gonorrhea in Pregnant Women, Neonates, and Children  Gon­ orrhea in pregnancy can have serious consequences for both the mother and the infant. Recognition of gonorrhea early in pregnancy also identifies a population at risk for other STIs, particularly chlamyd­ ial infection, syphilis, and trichomoniasis. The risks of salpingitis and PID—conditions associated with a high rate of fetal loss—are highest during the first trimester. Pharyngeal infection, most often asymptom­ atic, may be more common during pregnancy because of altered sexual

practices. Prolonged rupture of the membranes, premature delivery, chorioamnionitis, funisitis (infection of the umbilical cord stump), and sepsis in the infant (with N. gonorrhoeae detected in the newborn’s gas­ tric aspirate during delivery) are common complications of maternal gonococcal infection at term. Other conditions and microorganisms, including Mycoplasma hominis, Mycoplasma genitalium, Ureaplasma urealyticum, C. trachomatis, and bacterial vaginosis (often accompa­ nied by infection with Trichomonas vaginalis), have been associated with similar complications. The most common form of gonorrhea in neonates is ophthalmia neonatorum, which results from exposure to infected cervical secre­ tions during parturition. Ocular neonatal instillation of 0.5% erythro­ mycin ophthalmic ointment, a prophylactic agent, prevents ophthalmia neonatorum but is not effective for its treatment, which requires systemic antibiotics. One-percent (1%) silver nitrate eye drops and ointments that contain silver nitrate or tetracycline can also be used for prophylaxis but are no longer available in the United States. The clini­ cal manifestations are acute and usually begin 2–5 days after birth. An initial nonspecific conjunctivitis with a serosanguineous discharge is followed by tense edema of the eyelids, chemosis, and a profuse, thick, purulent discharge. Corneal ulcerations that result in nebulae or per­ foration may lead to anterior synechiae, anterior staphyloma, panoph­ thalmitis, and blindness. Infections described at other mucosal sites in infants, including vaginitis, rhinitis, and anorectal infection, are likely to be asymptomatic. Pharyngeal colonization has been demonstrated in 35% of infants with gonococcal ophthalmia, and coughing is the most prominent symptom in these cases. Septic arthritis (see below) is the most common manifestation of systemic infection or disseminated gonococcal infection (DGI) in the newborn. The onset usually comes at 3–21 days of age, and polyarticular involvement is common. Sepsis, meningitis, and pneumonia are seen in rare instances. Any STI in children beyond the neonatal period raises the possibil­ ity of sexual abuse. Gonococcal vulvovaginitis is the most common manifestation of gonococcal infection in children beyond infancy. Anorectal and pharyngeal infections are common in these children and are frequently asymptomatic. The urethra, Bartholin’s and Skene’s glands, and upper genital tract are rarely involved. All children with gonococcal infection should also be evaluated for chlamydial infection, syphilis, and possibly HIV infection. Gonococcal Arthritis  Dissemi­ nated gonococcal infection (DGI; gono­ coccal arthritis) results from gonococcal bacteremia. In the 1970s, DGI occurred in ~0.5–3% of persons with untreated gonococcal mucosal infection. The lower incidence of DGI at present is probably attributable to a decline in the preva­ lence of particular strains that are likely to disseminate. Nonetheless, sporadic outbreaks of DGI still occur in North America. DGI strains resist the bacteri­ cidal action of human serum and gener­ ally do not incite inflammation at genital sites, probably because of limited gen­ eration of chemotactic factors. Strains recovered from DGI cases in the 1970s were often of the PorB.1A serotype, were highly susceptible to penicillin, and had special growth requirements—including arginine, hypoxanthine, and uracil—that made the organism more fastidious and more difficult to isolate. A B C Menstruation is a risk factor for dis­ semination, and approximately twothirds of cases of DGI are in women. In about half of affected women, symptoms of DGI begin within 7 days of onset of menses. Complement deficiencies, D E F FIGURE 161-2  Characteristic skin lesions in patients with proven gonococcal bacteremia. The lesions are in various stages of evolution. A. Very early petechia on finger. B. Early papular lesion, 7 mm in diameter, on lower leg. C. Pustule with central eschar resulting from early petechial lesion. D. Pustular lesion on finger. E. Mature lesion with central necrosis (black) on hemorrhagic base. F. Bullae on anterior tibial surface. (Reprinted with permission from KK Holmes et al: Disseminated gonococcal infection. Ann Intern Med 74:979, 1971.)

especially of the components involved in the assembly of the mem­ brane attack complex (C5 through C9), predispose to neisserial bac­ teremia, and persons with more than one episode of DGI should be screened with an assay for total hemolytic complement activity. DGI is also associated with the use of the complement C5–blocking monoclo­ nal antibody eculizumab.

The clinical manifestations of DGI have sometimes been classified into two stages: a bacteremic stage, which is less common today, and a joint-localized stage with suppurative arthritis. A clear-cut progres­ sion usually is not evident. Patients in the bacteremic stage have higher temperatures, and chills more frequently accompany their fever. Painful joints are common and often occur together with tenosynovitis and skin lesions. Polyarthralgias usually include the knees, elbows, and more distal joints; the axial skeleton is generally spared. Skin lesions are seen in ~75% of patients and include papules and pustules, often with a hem­ orrhagic component (Fig. 161-2; see also Fig. A1-43 in the Atlas of Rashes Associated with Fever). Other manifestations of noninfectious dermatitis, such as nodular lesions, urticaria, and erythema multiforme, have been described. These lesions are usually on the extremities and number between 5 and 40. The differential diagnosis of the bacteremic stage of DGI includes reactive arthritis, acute rheumatoid arthritis, sarcoidosis, erythema nodosum, drug-induced arthritis, and viral infec­ tions (e.g., hepatitis B and acute HIV infection). The distribution of joint symptoms in reactive arthritis differs from that in DGI (Fig. 161-3), as do the skin and genital manifestations (Chap. 374). Suppurative arthritis involves one or two joints, most often the knees, wrists, ankles, and elbows (in decreasing order of frequency); other joints occasionally are involved. Most patients who develop gonococcal septic arthritis do so without prior polyarthralgias or skin lesions; in the absence of symptomatic genital infection, this disease cannot be distinguished from septic arthritis caused by other patho­ gens. The differential diagnosis of acute arthritis in young adults is dis­ cussed in Chap. 135. Rarely, osteomyelitis complicates septic arthritis involving small joints of the hand. CHAPTER 161 Gonococcal Infections Gonococcal endocarditis, although rare today, was a relatively common complication of DGI in the preantibiotic era, accounting for about one-quarter of reported cases of endocarditis. Another unusual complication of DGI is meningitis.

Disseminated gonococcal infection (N = 102) Reactive arthritis (N = 173) Hand and fingers Wrist Elbow Shoulder Sternal* Spine and SI† Hip Knee Ankle Foot and toes

Percent of patients FIGURE 161-3  Distribution of joints with arthritis in 102 patients with disseminated gonococcal infection and 173 patients with reactive arthritis. *Includes the sternoclavicular joints. †SI, sacroiliac joint. Gonococcal Infections in HIV-Infected Persons  The asso­ ciation between gonorrhea and the acquisition of HIV has been demonstrated in several well-controlled studies, mainly in Kenya and Zaire. The nonulcerative STIs enhance the transmission of HIV three- to fivefold; transmission of HIV-infected immune cells and increased viral shedding by persons with urethritis or cervicitis may contribute (Chap. 208). HIV has been detected by polymerase chain reaction (PCR) more commonly in ejaculates from HIV-positive men with gonococcal urethritis than in those from HIV-positive men with nongonococcal urethritis. PCR positivity diminishes twofold after appropriate therapy for urethritis. Not only does gonorrhea enhance the transmission of HIV, but it may also increase the individual’s risk for acquisition of HIV. A proposed mechanism is the significantly greater number of CD4+ T lymphocytes and dendritic cells that can be infected by HIV in endocervical secretions from women with nonul­ cerative STIs than in those from women with ulcerative STIs. PART 5 Infectious Diseases ■ ■LABORATORY DIAGNOSIS A rapid diagnosis of gonococcal infection in men may be obtained by Gram staining of urethral exudates (Fig. 161-1). The detection of gram-negative intracellular monococci and diplococci is usually highly specific and sensitive in diagnosing gonococcal urethritis in symptomatic males but is only ~50% sensitive in diagnosing gono­ coccal cervicitis. Samples should be collected with Dacron or rayon swabs. Part of the sample should be inoculated onto a plate of modi­ fied Thayer-Martin or other gonococcal selective medium for culture. It is important to process all samples immediately because gonococci do not tolerate drying. If plates cannot be incubated immediately, they can be held safely for several hours at room temperature in can­ dle extinction jars prior to incubation. If processing is to occur within 6 h, transport of specimens may be facilitated by the use of nonnu­ tritive swab transport systems such as Stuart or Amies medium. For longer holding periods (e.g., when specimens for culture are to be mailed), culture media with self-contained CO2-generating systems (such as the JEMBEC or Gono-Pak systems) may be used. Specimens should also be obtained for the diagnosis of chlamydial infection (Chap. 194). PMNs are often seen in the endocervix on a Gram stain, and an abnormally increased number (≥30 PMNs per field in five 1000× oil-immersion microscopic fields) establishes the presence of an inflammatory discharge. Unfortunately, the presence or absence of gram-negative intracellular monococci or diplococci in cervical smears does not accurately predict which patients have gonorrhea, and the

diagnosis in this setting should be made by culture or another suitable nonculture diagnostic method. The sensitivity of a single endocervical culture is ~80–90%. If a history of rectal sex is elicited, a rectal wall swab (uncontaminated with feces) should be cultured. A presumptive diagnosis of gonorrhea cannot be made on the basis of gram-negative diplococci in Gram-stained smears from the pharynx, where other Neisseria species are also components of the pharyngeal flora. Several nucleic acid amplification tests (NAATs), including the Roche COBAS AMPLICOR, Gen-Probe Aptima Combo 2, Cepheid Xpert® CT/NG Assay and BD ProbeTec ET, are now widely available on semiautomated or fully automated platforms and are commonly employed diagnostic tests for gonorrhea. These tests also detect C. trachomatis and are more sensitive than culture for identification of either

N. gonorrhoeae or C. trachomatis. These tests offer the advantage that urine samples can be tested with a sensitivity similar to or greater than that obtained when urethral or cervical swab samples are assessed by other non-NAATs or culture, respectively. NAAT tests performed on self-collected vaginal swabs are as sensitive and specific as physiciancollected samples and may be used in women to facilitate sample collection when a pelvic exam is not indicated as part of their clinic evaluation. A point-of-care NAAT-based test (Binx io®) for gonorrhea and chlamydia with a 30-minute turnaround time is now approved by the U.S. Food and Drug Administration (FDA). In MSM, it is impor­ tant to screen the rectum and pharynx because screening urine alone will miss the majority of cases. A disadvantage of non-culture-based assays is that N. gonorrhoeae cannot be grown from the transport systems. Thus, a culture-confirmatory test and formal antimicrobial susceptibility testing, if needed, cannot be performed. Because of the legal implications, the preferred method for the diag­ nosis of gonococcal infection in children is a standardized culture. Two positive NAATs, each targeting a different nucleic acid sequence, may be substituted for culture of the cervix or the urethra as legal evidence of infection in children. Although nonculture tests for gonococcal infection have not been approved by the FDA for use with specimens obtained from the pharynx and rectum of infected children, NAATs from these sites are preferred for diagnostic evaluation in adult victims of suspected sexual abuse, especially if the NAATs have been evalu­ ated by the local laboratory and found to be superior. Cultures should be obtained from the pharynx and anus of both girls and boys, the urethra of boys, and the vagina of girls; cervical specimens are not rec­ ommended for prepubertal girls. For boys with a urethral discharge, a meatal specimen of the discharge is adequate for culture. Presumptive colonies of N. gonorrhoeae should be identified definitively by at least two independent methods. Blood should be cultured in suspected cases of DGI. The use of Isolator blood culture tubes may enhance the yield. The probability of positive blood cultures decreases after 48 h of illness. Synovial fluid should be inoculated into blood culture broth medium and plated onto chocolate agar rather than selective medium because this fluid is not likely to be contaminated with commensal bacteria. Gonococci are infrequently recovered from early joint effusions containing <20,000 leukocytes/μL but may be recovered from effusions containing >80,000 leukocytes/μL. The organisms are seldom recovered from blood and synovial fluid of the same patient. TREATMENT Gonococcal Infections Treatment failure can lead to continued transmission and the emergence of antibiotic resistance. The importance of adequate treatment with a regimen that the patient will adhere to cannot be overemphasized. Single-dose regimens have been developed for uncomplicated gonococcal infections. The Centers for Disease Control and Prevention (CDC) Gonorrhea Treatment Recom­ mendations in 2020 and the 2021 Sexually Transmitted Infections (STI) Treatment Guidelines are summarized in Table 161-1. The third-generation cephalosporin ceftriaxone is now recommended as the first-line regimen for use at twice the previous dose (now,

TABLE 161-1  Recommended Treatment for Gonococcal Infections: Adapted from the 2021 Guidelines for Gonococcal Infection of the Centers for Disease Control and Prevention DIAGNOSIS TREATMENT OF CHOICEa Uncomplicated gonococcal infection of the cervix, urethra, pharynx,b or rectum   First-line regimen Ceftriaxone (500 mg IM, single dose)   plus   Doxycycline (100 mg orally twice a day for 7 days) for treatment of chlamydial infection if chlamydial infection cannot be excluded Alternative regimens if ceftriaxone is not available Gentamicin (240 mg IM, single dose) plus azithromycin (2 g orally as a single dose)c or Cefixime (800 mg PO, single dose) or spectinomycin (2 g IM, single dose)d,e plus Doxycycline (100 mg orally twice a day for 7 days) for treatment of chlamydial infection if chlamydial infection cannot be excluded Epididymitis See Chap. 141 Pelvic inflammatory disease See Chap. 141 Gonococcal conjunctivitis in an adult Ceftriaxone (1 g IM, single dose)f Ophthalmia neonatorumg Ceftriaxone (25–50 mg/kg IV, single dose, not to exceed 125 mg) Disseminated gonococcal infectionh     Initial therapyi       Patient tolerant of β-lactam drugs Ceftriaxone (1 g IM or IV q24h; recommended) or cefotaxime

(1 g IV q8h) or ceftizoxime (1 g IV q8h)     Patients allergic to β-lactam drugs Spectinomycin (2 g IM q12h)d   Continuation therapyj Cefixime (400 mg PO bid) Meningitis or endocarditis See text for specific recommendationsk aTrue failure of treatment with a recommended regimen is rare and should prompt an evaluation for reinfection, infection with a drug-resistant strain, or an alternative diagnosis. bCeftriaxone is the most reliable agent recommended for treatment of pharyngeal infection. cIn vitro synergistic killing of N. gonorrhoeae by gentamicin plus azithromycin is mild to moderate; azithromycin is for treatment chlamydial infection, primarily. dSpectinomycin is unavailable in the United States; in uncomplicated gonococcal infection it should be used at a higher dose (4 g IM, single dose) in areas of the world where increased resistance to spectinomycin exists. eSpectinomycin may be ineffective for the treatment of pharyngeal gonorrhea. fPlus lavage of the infected eye with saline solution (once). gProphylactic regimens are discussed in the text. hHospitalization is indicated if the diagnosis is uncertain, if the patient has the joint-localized stage with suppurative arthritis, or if the patient cannot be relied on to adhere to treatment. iAll initial regimens should also include doxycycline (100 mg orally twice a day for 7 days) for treatment of chlamydial infection if chlamydial infection cannot be excluded; jgonococcal therapy should be continued for 24–48 h after clinical improvement begins, at which time the switch may be made to an oral agent (e.g., cefixime) if antimicrobial susceptibility can be documented by culture of the causative organism. If no organism is isolated and the diagnosis is secure, then treatment with ceftriaxone should be continued for at least 1 week. kHospitalization is indicated to exclude suspected meningitis or endocarditis. 500 mg IM, single dose) based on doubling of mean inhibitory concentrations (MICs) of current strains compared with MICs over 20 years ago. The development of decreased sensitivity to ceftri­ axone throughout the world will require the development of new effective regimens. Azithromycin, which had been recommended to provide additional treatment of gonorrhea (also to include treat­ ment of chlamydial infection) is no longer recommended as part of a first line regimen. Resistance to azithromycin of U.S. isolates of N. gonorrhoeae, which had been less than 0.6% over a number of years, has increased more than sevenfold to 4.7% in 2021. If chla­ mydial infection cannot be excluded, concurrent treatment with doxycycline (100 mg orally twice a day for 7 days) is recommended.

The recommendations for uncomplicated gonorrhea apply to HIVinfected as well as HIV-uninfected patients.

The currently recommended regimen for the treatment of uncom­ plicated gonococcal infection of the urethra, cervix, rectum, or pharynx (a single IM dose of ceftriaxone) almost always results in an effective cure. Quinolone-containing regimens are no longer recommended in the United States as first-line treatment because of widespread resistance. Rising MICs of cefixime worldwide have led the CDC to discontinue its recommendation of this agent as first-line treatment for uncomplicated gonorrhea. Multicenter trials of treat­ ment for uncomplicated gonorrhea in the United States have shown ≥99.5% efficacy of two combination regimens and 96% efficacy in one single-agent regimen: gemifloxacin (320 mg, single oral dose) plus azithromycin (2 g, single oral dose); gentamicin (a single IM dose of 240 mg or, in individuals who weigh ≤45 kg, 5 mg/kg) plus azithromycin (2 g, single oral dose), and zoliflodacin (2 or 3 g, single oral dose). At this time, however, none of these regimens is recom­ mended by CDC as first-line treatment; gentamicin plus azithromy­ cin is recommended as an alternative regimen. Co-infection with C. trachomatis occurs frequently; concurrent treatment with doxycycline (100 mg orally twice daily for 7 days) is effective against chlamydial infection. Spectinomycin has been used as an alternative agent for the treatment of uncomplicated gonococcal infections in penicillin-allergic persons outside the United States but is not currently available in the United States. Of note, the limited effectiveness of spectinomycin and gentamicin for the treatment of pharyngeal infection reduces the utility of this regi­ men in populations among whom gonococcal infection is common, such as MSM. CHAPTER 161 Persons with uncomplicated genital or rectal infections who receive ceftriaxone or an alternative regimen do not need a test of cure; however, cultures for N. gonorrhoeae should be performed if symptoms persist after therapy with an established regimen, and any gonococci isolated should be tested for antimicrobial suscep­ tibility. Persons with pharyngeal infection should undergo a test of cure regardless of the treatment regimen, 7–14 days after treatment to ensure eradication or detection of a possible treatment failure. Symptomatic gonococcal pharyngitis is more difficult to eradicate than genital infection. Persons who cannot tolerate cephalosporins may be treated with an alternative regimen. Treatment with spec­ tinomycin results in a cure rate of ≤52%; persons given spectino­ mycin should have a subsequent pharyngeal sample cultured early (3–5 days) following treatment as a test of cure. A single 2-g dose of azithromycin may be used if the infecting organism is known to be sensitive or in areas where rates of resistance to azithromycin are low. Quinolones may be used if the infecting organism is known to be sensitive. If culture is not readily available and NAAT is positive, every effort should be made to perform a confirmatory culture. All isolates from test-of-cure cultures should undergo antimicrobial susceptibility testing. Because of high rates of reinfection with

N. gonorrhoeae (and C. trachomatis) within 6–12 months, persons previously treated for gonorrhea should be retested 3 months after treatment. Gonococcal Infections Treatments for gonococcal epididymitis and PID are discussed in Chap. 141. Ocular gonococcal infections in older children and adults should be managed with a single dose of ceftriaxone com­ bined with saline irrigation of the conjunctivae (both undertaken expeditiously), and patients should undergo a careful ophthalmo­ logic evaluation that includes a slit-lamp examination. DGI, particularly the joint-localized stage with suppurative arthritis, may require higher dosages and longer durations of therapy (Table 161-1). Hospitalization is indicated if the diagnosis is uncertain, if the patient has localized suppurative arthritis that requires aspiration, or if the patient cannot be relied on to comply with treatment. Open drainage is necessary only occasionally— e.g., for management of hip infections that may be difficult to drain percutaneously. Nonsteroidal anti-inflammatory agents may be indicated to alleviate pain and hasten clinical improvement of affected joints.

45 - 162 Haemophilus and Moraxella Infections

162 Haemophilus and Moraxella Infections

Gonococcal meningitis and endocarditis should be treated in the hospital with high-dose IV ceftriaxone (1–2 g IV every 12–24 h); therapy should continue for 10–14 days for meningitis and for at least 4 weeks for endocarditis. All persons who experience more than one episode of DGI should be evaluated for complement deficiency.

■ ■PREVENTION AND CONTROL Condoms, if properly used, provide effective protection against the transmission and acquisition of gonorrhea as well as other infections that are transmitted to and from genital mucosal surfaces. Spermici­ dal preparations used with a diaphragm or cervical sponges impreg­ nated with nonoxynol-9 offer some protection against gonorrhea and chlamydial infection. However, the frequent use of preparations that contain nonoxynol-9 is associated with mucosal disruption that paradoxically may enhance the risk of HIV infection in the event of exposure. All patients should be instructed to refer sex partners for evaluation and treatment. All sex partners of persons with gonorrhea should be evaluated and treated for N. gonorrhoeae and C. trachomatis infections if their last contact with the patient took place within

60 days before the onset of symptoms or the diagnosis of infection in the patient. If the patient’s last potential sexual exposure to infection was >60 days before onset of symptoms or diagnosis, the patient’s most recent sex partner should be treated. Partner-delivered medications or prescriptions for medications to treat gonorrhea and chlamydial infec­ tion diminish the likelihood of reinfection (or relapse) in the infected patient. This approach is permissible (or potentially allowable) in all 50 states and is an option for partner management. Patients should be instructed to abstain from sexual intercourse until therapy is com­ pleted and until they and their sex partners no longer have symptoms. Greater emphasis must be placed on prevention by public health education, individual patient counseling, and behavior modification, particularly the use of condoms. Sexually active persons, especially adolescents, should be offered screening for STIs. For most male patients, NAAT of urine or a urethral swab may be used for screening. Preventing the spread of gonorrhea may help reduce the transmis­ sion of HIV. No effective vaccine for gonorrhea is yet available, but efforts to test several candidates are underway including a field trial of a licensed group B meningococcal vaccine (4CMenB or Bexsero®), which in retrospective epidemiologic analyses has been associated with reduced rates of gonorrhea. PART 5 Infectious Diseases ■ ■FURTHER READING Bolan GA et al: The emerging threat of untreatable gonococcal infec­ tion. N Engl J Med 366:485, 2012. Bolan GA et al: Morb Mortal Wkly Rep 70 (No. 4), 2021. Gonococcal Infections, in Sexually Transmitted Infections Treatment Guidelines, 2021. U.S. Centers for Disease Control and Prevention (CDC), 2021, pp 71-80. Golden MR et al: Effect of expedited treatment of sex partners on recurrent or persistent gonorrhea or chlamydial infections. N Engl J Med 352:676, 2005. Petousis-Harris EH et al: Effectiveness of a group B outer membrane vesicle meningococcal vaccine against gonorrhoea in New Zealand: A retrospective case-control study. Lancet 390:1603, 2017. Rice PA: Gonococcal arthritis (disseminated gonococcal infection). Infect Dis Clin North Am 19:853, 2005. Su X et al: Neisseria gonorrhoeae infection in women increases with rising gonococcal burdens in male partners: Chlamydia coinfection in women increases their gonococcal burdens. J Infect Dis 226:2192, 2022. Taylor SN et al: Single-dose zoliflodacin (ETX0914) for treatment of urogenital gonorrhea. N Engl J Med 379:1835, 2018. Unemo M et al: Antimicrobial resistance expressed by Neisseria gon­ orrhoeae: A major global public health problem in the 21st century. Microbiol Spectr 4:10.1128/microbiolspec.EI10-0009-2015, 2016. Unemo MM et al: Gonorrhoea. Nat Rev Dis Primers 5:80, 2019.

Kristian Riesbeck, Timothy F. Murphy

Haemophilus and

Moraxella Infections HAEMOPHILUS INFLUENZAE ■ ■MICROBIOLOGY Haemophilus influenzae was first recognized in 1892 by Pfeiffer, who erroneously concluded that the bacterium was the cause of influenza. H. influenzae is a small (1- × 0.3-μm) gram-negative organism of vari­ able shape; thus, it is often described as a pleomorphic coccobacillus. In clinical specimens such as cerebrospinal fluid (CSF) and sputum,

H. influenzae frequently stains only faintly with safranin and therefore can easily be overlooked. H. influenzae grows both aerobically and anaerobically. As the Latin name implies, Haemophilus is the “blood-loving bacterium.” A prereq­ uisite for growth is a requirement for the media to contain blood, but the blood substrate needs to be preheated; hence, H. influenzae grows on chocolate agar plates (slowly heated to 80°C [176°F]), whereas ordinary blood plates (without preheating) will not sustain growth. Its aerobic growth requires two factors: hemin (designated X factor) and nicotinamide adenine dinucleotide (V factor). While MALDI-ToF (Matrix-Assisted Laser Desorption/Ionization - Time of Flight) is now widely used for bacterial identification, these requirements are still sometimes applied in clinical laboratories. However, using phenotypic methods for differentiating among Haemophilus species has limitations, as the growing number of whole-genome sequences of Haemophilus isolates from the human respiratory tract is revealing complex genetic relationships among Haemophilus species (see “Diagnosis,” below). Six major serotypes of H. influenzae have been identified; designated a through f, they are based on antigenically distinct polysaccharide capsules. In addition, some strains lack a polysaccharide capsule and are referred to as nontypeable strains. Type b and nontypeable strains are the most relevant strains clinically (Table 162-1), although encapsulated strains other than type b can cause disease. An H. influ­ enzae type b isolate was the first free-living organism to have its entire genome sequenced in the early 1990s. The antigenically distinct type b capsule is a linear polymer composed of ribosyl-ribitol phosphate. Strains of H. influenzae type b (Hib) cause disease primarily in infants and children <6 years of age, although it is nowadays rare due to effective vaccine campaigns. Nontypeable strains are primarily mucosal pathogens but occasionally cause invasive disease, particularly in immunocompromised hosts and the elderly. ■ ■EPIDEMIOLOGY AND TRANSMISSION H. influenzae, an exclusively human pathogen, is spread by airborne droplets or by direct contact with secretions or fomites. Between TABLE 162-1  Characteristics of Type b and Nontypeable Strains of Haemophilus influenzae FEATURE TYPE b STRAINS NONTYPEABLE STRAINS Capsule Ribosyl-ribitol phosphate Unencapsulated Pathogenesis Invasive infections due to hematogenous spread Mucosal infections due to contiguous spread Clinical manifestations Meningitis and invasive infections in incompletely immunized infants and children Otitis media in infants and children; lower respiratory tract infections in adults with chronic bronchitis Evolutionary history Basically clonal Genetically diverse Vaccine Highly effective conjugate vaccines Protein D used as carrier protein in pneumococcal vaccine approved in Europe: GSK Synflorix. Others under development

30 and 60% of healthy preschool children are intermittent carriers. Colonization with nontypeable H. influenzae is a dynamic process; new strains are acquired and other strains are replaced periodically. This is particularly evident in patients with chronic obstructive pulmonary disease (COPD). The widespread use of Hib conjugate vaccines in many indus­ trialized countries has resulted in striking decreases in the rate of nasopharyngeal colonization by Hib and in the incidence of Hib infection (Fig. 162-1). On a global basis, invasive Hib disease occurs predominantly in unimmunized children and in those who have not completed the primary immunization series. Most World Health Organization member countries have introduced Hib conjugate vac­ cination, but a large number of the world’s children remain unimmu­ nized, principally in countries without national vaccine programs. In addition to immunocompromised hosts, certain groups have a higher incidence of invasive disease with encapsulated H. influenzae (along with capsular serotypes other than Hib) than the general population, including African Americans, Australian Aboriginal children, and Native American groups. Although this increased incidence has not yet been accounted for, several factors may be relevant, including age at exposure to the bacterial species, socioeconomic conditions, and genetic differences. ■ ■PATHOGENESIS Hib strains cause systemic disease by invasion and hematogenous spread from the respiratory tract to distant sites such as the meninges, Incidence of Invasive H. influenzae Disease, United States Hib Non-b serotypes Nontypeable

Incidence per 100,000

A Estimated U.S. Incidence of Invasive H. influenzae Disease by Age Group and Serotype

Incidence per 100,000

<1 ≥65 50–64 35–49 18–34 Age (years) 1–4 5–17 ABCs cases from 2018–2022 and estimated to the U.S. population *2022 data are preliminary B FIGURE 162-1  Estimated incidence rates (per 100,000) of invasive disease due to Haemophilus influenzae caused by Hib, non-b serotypes, and nontypeable bacteria in the United States, 1994–2022. A. Disease caused by Hib remains low. In 2020–2021, disease caused by non-b serotypes and nontypeable bacteria increased. B. Estimated incidence rates (per 100,000) of invasive H. influenzae disease by serotype and age in the United States, 2018–2022. (Source: Active Bacterial Core surveillance, Centers for Disease Control and Prevention, https://www.cdc.gov/hi-disease/surveillance.html.)

bones, and joints. The type b polysaccharide capsule is an important virulence factor affecting the bacterium’s ability to avoid opsonization and cause systemic disease.

Nontypeable strains cause disease by local invasion of mucosal sur­ faces. Otitis media results when bacteria reach the middle ear by way of the eustachian tube. Adults with COPD experience recurrent lower respiratory tract infection due to nontypeable strains. In addition, nontypeable H. influenzae persist in the lower airways of adults with COPD in both extracellular and intracellular locations, contributing to the airway inflammation that is a hallmark of the disease. Nontypeable strains that cause infection in adults with COPD differ in pathogenic potential and genome content from strains that cause otitis media. In the middle ear, nontypeable strains form biofilms. More resistant to host clearance mechanisms and to antibiotics than are planktonic bac­ teria, biofilms are associated with chronic and recurrent otitis media. Nontypeable H. influenzae persist in the human respiratory tract and cause infection by altering expression of genes through slipped-strand mispairing and through phase-variable expression of DNA methylase genes that control the expression of multiple genes that play a role in virulence. The incidence of invasive disease caused by nontypeable strains is low but appears to have increased over the past decade; one possible explanation for this increase might be that unencapsulated and hence nontypeable H. influenzae were overlooked in the past during the pre­ vaccine era due to the high incidence of Hib. Most strains that cause invasive disease are genetically and phenotypically diverse. ■ ■IMMUNE RESPONSE Antibody to the capsule is important in protection from infection by Hib strains. The level of (maternally acquired) serum antibody to the capsular polysaccha­ ride, which is a polymer of polyribitol ribose phosphate (PRP), declines from birth to 6 months of age and, in the absence of vaccination, remains low until ~2 or 3 years of age. The age at the antibody nadir correlates with that of the peak incidence of type b disease. Antibody to PRP then appears partly due to exposure to Hib or cross-reacting antigens. Systemic Hib disease is unusual after the age of 6 years because of the presence of protective antibody. Vaccines in which PRP is conju­ gated to protein carrier molecules have been developed and are now used widely. These vaccines generate an antibody response to PRP in infants and effectively prevent invasive infections in infants and children. CHAPTER 162 Haemophilus and Moraxella Infections Since nontypeable strains lack a capsule, the immune response to infection is directed at noncapsu­ lar antigens. These antigens have generated consider­ able interest as immune targets and potential vaccine components. The human immune response to non­ typeable strains appears to be strain-specific, a charac­ teristic that accounts in part for the propensity of these strains to cause recurrent otitis media and recurrent exacerbations of chronic bronchitis in immunocom­ petent hosts. Hia Hib Hic Hid Hie Hif Non-typeable ■ ■CLINICAL MANIFESTATIONS Hib  The most serious manifestation of infection with Hib is meningitis (Chap. 143), which primarily affects children <2 years of age. The clinical manifesta­ tions of Hib meningitis are similar to those of menin­ gitis caused by other bacterial pathogens. Fever and altered central nervous system function are the most common features at presentation. Nuchal rigidity may or may not be evident. Subdural effusion, the most common complication, is suspected when, despite 2 or 3 days of appropriate antibiotic therapy, the infant has seizures, hemiparesis, or continued obtundation. The overall mortality rate from Hib meningitis is ~5%, and the morbidity rate is high. Of survivors, 6% have

permanent sensorineural hearing loss, and about one-fourth have a significant disability of some type. If more subtle disabilities are sought, up to half of survivors are found to have some neurologic sequelae, such as partial hearing loss and delayed language development.

Epiglottitis (Chap. 37) is a life-threatening Hib infection involving cellulitis of the epiglottis and supraglottic tissues. It can lead to acute upper-airway obstruction. Its unique epidemiologic features are its occurrence in an older age group (2–7 years old) than other Hib infec­ tions and its absence among Navajo Native Americans and Alaskan Eskimos. Sore throat and fever rapidly progress to dysphagia, drooling, and airway obstruction. In countries with Hib vaccine child immuniza­ tion programs, epiglottitis now occurs primarily in adults. Cellulitis (Chap. 134) due to Hib occurs in young children. The most common location is on the head or neck; the involved area some­ times takes on a characteristic bluish-red color. Most patients have bacteremia, and 10% have an additional focus of infection. Hib causes pneumonia in infants. The infection is clinically indistin­ guishable from other types of bacterial pneumonia (e.g., pneumococcal pneumonia) except that Hib is more likely to involve the pleura. Several less common invasive conditions can be important clinical manifesta­ tions of Hib infection in children. These include osteomyelitis, septic arthritis, pericarditis, orbital cellulitis, endophthalmitis, urinary tract infection, abscesses, and bacteremia without an identifiable focus. Non–type b encapsulated strains of H. influenzae (types a, c, d, e, and f) are unusual causes of invasive infection manifested predomi­ nantly by bacteremia and pneumonia. H. influenzae type a infections are seen with increased frequency in indigenous populations of North America, and these strains are predominantly clonal. Most infections due to non–type b encapsulated strains occur in the setting of underly­ ing conditions including immunosuppression. PART 5 Infectious Diseases Nontypeable H. influenzae  Nontypeable H. influenzae is the most common bacterial cause of exacerbations of COPD; these exacerba­ tions are characterized by increased cough, sputum production, and shortness of breath. Fever is low-grade, and no infiltrates are evident on chest x-ray. Nontypeable strains also cause community-acquired bacterial pneumonia in adults, especially among patients with COPD or HIV or other immunocompromised hosts. The clinical features of H. influenzae pneumonia are similar to those of other types of bacte­ rial pneumonia, including pneumococcal pneumonia. In recent years, pneumonia due to nontypeable H. influenzae has been more common than pneumonia caused by Streptococcus pneumoniae (Fig. 162-2). Nontypeable H. influenzae is one of the three most common causes of childhood otitis media (the other two being S. pneumoniae and Moraxella catarrhalis) (Chap. 37). Infants are febrile and irritable, while older children report ear pain. Symptoms of viral upper-respi­ ratory infection often precede otitis media. The diagnosis is made by M. catarrhalis (12%) H. influenzae (33%) S. aureus (12%) Other microbes (15%) S. pneumoniae (5%) S. pneumoniae (28%) A FIGURE 162-2.  Importance of nontypeable Haemophilus influenzae and Moraxella catarrhalis in community-acquired pneumonia (CAP) and in exacerbations of COPD patients. A. Meta-analysis of bacterial detection in lower respiratory tract specimens using molecular panels or quantitative assays from patients with CAP. B. Cumulative results of a prospective study (1994–2004) of bacterial infection in chronic obstructive pulmonary disease (COPD) showing etiology of exacerbations. Numbers of exacerbations shown indicate acquisition of a new strain simultaneous with clinical symptoms of an exacerbation. (Data source for A: NJ Gadsby et al: Clin Infect Dis 62:817, 2016; DN Gilbert et al: Diagn Microbiol Infect Dis 99:115246, 2021; S Serigstad et al: Sci Rep 12:326, 2022; M Fally et al: Infect Dis (Lond) 53:122, 2021.)

pneumatic otoscopy. An etiologic diagnosis, although not routinely sought, can be established by tympanocentesis and culture of middleear fluid. Clinical features associated with H. influenzae otitis media include a history of recurrent episodes, treatment failure, concomitant conjunctivitis, bilateral otitis media, and recent antimicrobial therapy. The increasing use of pneumococcal polysaccharide conjugate vaccines in most countries has resulted in an overall decrease in otitis media and its complications. However, a relative increase in the proportion of otitis media caused by H. influenzae in children failing initial anti­ microbial therapy or with recurrent episodes has occurred. Contin­ ued monitoring of the incidence and etiology of otitis media will be important. Nontypeable H. influenzae also causes puerperal sepsis and is an important cause of neonatal bacteremia. These nontypeable strains, provisionally named Haemophilus quentini, are closely related to but distinct from H. haemolyticus, tend to be of biotype IV, and cause inva­ sive disease after colonizing the female genital tract. Nontypeable H. influenzae causes sinusitis (Chap. 37) in adults and children. In addition, the bacterium is a less common cause of various invasive infections. These infections include bacteremia, empyema, adult epiglottitis, pericarditis, cellulitis, septic arthritis, osteomyelitis, endocarditis, cholecystitis, intraabdominal infections, urinary tract infections, mastoiditis, and aortic graft infection. Most H. influenzae invasive infections in countries where Hib vaccines are used widely are caused by nontypeable strains, and a recent increased incidence of such infections has been observed. Although most strains of nontypeable

H. influenzae that cause invasive infections are genetically diverse, recent localized clusters of infections have been caused by clonally related strains. Continued monitoring will be important. Many patients with H. influenzae bacteremia have an underlying condition, such as HIV infection, cardiopulmonary disease, alcoholism, or cancer. ■ ■DIAGNOSIS The most reliable method for establishing a diagnosis of invasive

H. influenzae infection is recovery of the organism in culture in a normally sterile body site, such as blood, CSF, or joint fluid. Isolation of bacteria is important for antimicrobial susceptibility testing. Rapid polymerase chain reactions (PCR) and other DNA-based methods are commonly used in modern clinical laboratories. H. influenzae isolated from the respiratory tract must be distin­ guished from a complex flora and from other Haemophilus species. Particular caution must be used to distinguish H. influenzae from Haemophilus haemolyticus, a respiratory tract commensal with identi­ cal growth requirements. H. haemolyticus has classically been distin­ guished from H. influenzae by the hemolysis of the former species on horse blood agar. However, a significant proportion of isolates of H. haemolyticus have now been recognized as nonhemolytic. Analysis of M. catarrhalis (39%) H. influenzae (48%) P. aeruginosa (8%) B

various genotypic markers, including 16S ribosomal sequences, super­ oxide dismutase, outer-membrane protein P6, protein D, and fuculose kinase, can be used to distinguish these two species. The availability of whole genome sequences of an increasing number of Haemophilus isolates from the human upper respiratory tract has revealed com­ plex genomic relationships among Haemophilus species, suggesting a genetic continuum between some Haemophilus species. The presence of gram-negative coccobacilli in Gram-stained CSF is strong evidence for Hib meningitis. Recovery of the organism or detec­ tion of H. influenzae DNA from CSF confirms the diagnosis. The same applies for other normally sterile body fluids, such as blood, joint fluid, pleural fluid, pericardial fluid, and subdural effusion, and are thus also confirmatory in other infections. Because nontypeable H. influenzae is primarily a mucosal pathogen, it is a component of a mixed flora; thus, etiologic diagnosis is chal­ lenging. Nontypeable H. influenzae infection is strongly suggested by the predominance of gram-negative coccobacilli among abundant polymorphonuclear leukocytes in a Gram-stained sputum specimen from a patient in whom pneumonia is suspected. Although bacteremia is detectable in a small proportion of patients with pneumonia due to nontypeable H. influenzae, most such patients have negative blood cultures. A diagnosis of otitis media is based on the detection by pneumatic otoscopy of fluid in the middle ear. An etiologic diagnosis requires tympanocentesis but is not routinely sought. An invasive procedure is also required to determine the etiology of sinusitis; thus, treatment is often empirical once the diagnosis is suspected in light of clinical symptoms and sinus radiographs. TREATMENT Haemophilus influenzae Initial therapy for meningitis due to Hib should consist of a cepha­ losporin such as ceftriaxone or cefotaxime. For children, the dosage of ceftriaxone is 75–100 mg/kg daily given in two doses 12 h apart. The pediatric dosage of cefotaxime is 200 mg/kg daily given in four doses 6 h apart. Adult dosages are 2 g every 12 h for ceftriaxone and 2 g every 4–6 h for cefotaxime. An alternative regimen for initial therapy is ampicillin (200–300 mg/kg daily in four divided doses) plus chloramphenicol (75–100 mg/kg daily in four divided doses). Therapy should continue for a total of 1–2 weeks. Administration of glucocorticoids to patients with Hib menin­ gitis reduces the incidence of neurologic sequelae. The presumed mechanism is reduction of the inflammation induced by bacterial cell-wall mediators of inflammation when cells are killed by antimi­ crobial agents. Dexamethasone (0.6 mg/kg per day intravenously in four divided doses for 2 days) is recommended for the treatment of Hib meningitis in children >2 months of age. Invasive infections other than meningitis are treated with the same antimicrobial agents. For epiglottitis, the dosage of ceftriax­ one is 50 mg/kg daily, and the dosage of cefotaxime is 150 mg/kg daily, given in three divided doses 8 h apart. Epiglottitis constitutes a medical emergency, and maintenance of an airway is critical. The duration of therapy is determined by the clinical response. A course of 1–2 weeks is usually appropriate. Many infections caused by nontypeable strains of H. influenzae, such as otitis media, sinusitis, and exacerbations of COPD, can be treated with oral antimicrobial agents. Approximately 30–40% of nontypeable strains produce β-lactamase (with the exact proportion depending on geographic location), and these strains are resistant to ampicillin as well as amoxicillin. Several agents have excellent activity against nontypeable H. influenzae, including amoxicillin/ clavulanic acid, various extended-spectrum cephalosporins, and the macrolides azithromycin and clarithromycin. Fluoroquinolones are highly active against H. influenzae and are useful in adults with exacerbations of COPD. However, fluoroquinolones are not currently recommended for the treatment of children or pregnant women because of possible effects on articular cartilage.

In addition to β-lactamase production, alteration of penicillinbinding proteins—a second mechanism of ampicillin resistance— has been detected in isolates of H. influenzae. Although still rare in the United States, these β-lactamase-negative ampicillin-resistant strains are common in Japan and are increasing in prevalence in Europe. Resistance to macrolides is also being observed with increasing frequency globally. Continued monitoring of the evolving antimicro­ bial susceptibility patterns of H. influenzae will be important.

■ ■PREVENTION Vaccination  (See also Chap. 129) Three monovalent conjugate vaccines that prevent invasive infections with Hib in infants and chil­ dren are licensed in the United States. In addition to eliciting protective antibodies, these vaccines prevent disease by reducing rates of pharyn­ geal colonization with Hib. The widespread use of conjugate vaccines has dramatically reduced the incidence of Hib disease in developed countries. Even though the manufacture of Hib vaccines is costly, vaccination is cost-effective. The Global Alliance for Vaccines and Immunizations has recognized the underuse of Hib conjugate vaccines. The disease burden has been reduced in developing countries that have implemented routine vaccination (e.g., The Gambia, Chile). An important obstacle to more widespread vaccination is the lack of data on the epidemiology and burden of Hib disease in many developing countries. All children should be immunized with a Hib conjugate vaccine, receiving the first dose at ~2 months of age, the rest of the primary series at 2–6 months of age, and a booster dose at 12–15 months of age. Specific recommendations vary for the different conjugate vaccines. The reader is referred to the recommendations of the American Acad­ emy of Pediatrics (Chap. 129 and https://www.aap.org/en/patient-care/ immunizations/vaccination-recommendations-by-the-aap/). CHAPTER 162 Currently, no vaccines are available specifically for the prevention of disease caused by nontypeable H. influenzae. However, a vaccine that contains protein D—a surface protein of H. influenzae—conjugated to pneumococcal polysaccharides is licensed in other countries and is used widely throughout the world. The vaccine has shown partial efficacy in preventing H. influenzae otitis media in clinical trials. Vac­ cine formulations that include surface protein antigens are currently in clinical trials, and additional progress in the development of vaccines against nontypeable H. influenzae is anticipated. Haemophilus and Moraxella Infections Chemoprophylaxis  The risk of secondary disease is greater than normal among household contacts of patients with Hib disease. There­ fore, all children and adults (except pregnant women) in households with an index case and at least one incompletely immunized contact <4 years of age should receive prophylaxis with oral rifampin. When two or more cases of invasive Hib disease have occurred within 60 days at a child-care facility attended by incompletely vaccinated children, administration of rifampin to all attendees and personnel is indicated, as it is for household contacts. Chemoprophylaxis is not indicated in nursery and child-care contacts of a single index case. The reader is referred to the recommen­ dations of the American Academy of Pediatrics. HAEMOPHILUS DUCREYI Haemophilus ducreyi is the etiologic agent of chancroid (Chap. 141), a sexually transmitted disease characterized by genital ulceration and inguinal adenitis. In addition to being a cause of morbidity in itself, chancroid is associated with HIV infection because of the role played by genital ulceration in HIV transmission. Chancroid increases the efficiency of transmission of and the degree of susceptibility to HIV infection. H. ducreyi has also been recognized as an important cause of non–sexually transmitted cutaneous ulcers. ■ ■MICROBIOLOGY H. ducreyi is a highly fastidious coccobacillary gram-negative bacte­ rium whose growth requires X factor (hemin). Although in light of this requirement, the bacterium has been classified in the genus Haemophi­ lus, DNA homology and chemotaxonomic studies have established substantial differences between H. ducreyi and other Haemophilus

species. Taxonomic reclassification of the organism is likely in the future but awaits further study. Ulcers contain predominantly T cells. The fact that patients who have had chancroid may have repeated infections indicates that infection does not confer protection.

■ ■EPIDEMIOLOGY AND PREVALENCE The prevalence of chancroid has steadily declined in the United States and worldwide over the past decade and a half. The infection appears to be more common in developing countries. Transmission is predomi­ nantly heterosexual, and cases in males have outnumbered those in females by ratios of 3:1 to 25:1 during outbreaks. Contact with com­ mercial sex workers and illicit drug use are strongly associated with chancroid. Most cases in developed countries are sporadic. H. ducreyi has emerged as a major cause of cutaneous ulcers in children in developing countries, particularly in the South Pacific and Africa. Strains that cause cutaneous ulcers have genome sequences that are nearly identical to class I strains (of two related classes) of

H. ducreyi that cause genital ulcers. ■ ■CLINICAL MANIFESTATIONS AND DIFFERENTIAL DIAGNOSIS Infection is acquired as the result of a break in the epithelium during sexual contact with an infected individual. After an incubation period of 4–7 days, the initial lesion—a papule with surrounding erythema— appears. In 2 or 3 days, the papule evolves into a pustule, which sponta­ neously ruptures and forms a sharply circumscribed ulcer that generally is not indurated (Fig. 162-3). The ulcers are painful and bleed easily; little or no inflammation of the surrounding skin is evident. Approxi­ mately half of patients develop enlarged, tender inguinal lymph nodes, which frequently become fluctuant and spontaneously rupture. Patients usually seek medical care after 1–3 weeks of painful symptoms. PART 5 Infectious Diseases The presentation of chancroid does not usually include all of the typical clinical features and is sometimes atypical. Multiple ulcers can coalesce to form giant ulcers. Ulcers can appear and then resolve, with inguinal adenitis (Fig. 162-3) and suppuration following 1–3 weeks later; this clinical picture can be confused with that of lympho­ granuloma venereum (Chap. 194). Multiple small ulcers can resemble folliculitis. Other differential diagnostic considerations include the various infections causing genital ulceration, such as primary syphilis, secondary syphilis (condyloma latum), genital herpes, and donovano­ sis. In rare cases, chancroid lesions become secondarily infected with bacteria; the result is extensive inflammation. Non–sexually transmitted cutaneous ulcers caused by H. ducreyi resemble those of yaws caused by Treponema pallidum subspecies FIGURE 162-3  Chancroid with characteristic penile ulcers and associated left inguinal adenitis (bubo).

pertenue, which is endemic in regions where H. ducreyi cutaneous ulcers are seen. Ulcers caused by H. ducreyi are less likely than those of yaws to show central granulating tissue and less likely to have indurated edges, but substantial overlap in clinical characteristics exists. ■ ■DIAGNOSIS Clinical diagnosis of chancroid is often inaccurate, and laboratory confirmation should be attempted in suspected cases. An accurate diagnosis of chancroid relies on culture of H. ducreyi from the lesion or from an aspirate of suppurative lymph nodes. Since the organism can be difficult to grow, the use of selective and supplemented media is nec­ essary. DNA detection by PCR is in use in larger clinical laboratories. A probable diagnosis of sexually transmitted chancroid can be made when the following criteria are met: (1) one or more painful genital ulcers; (2) no evidence of T. pallidum infection by dark-field examination of ulcer exudate or by a negative serologic test for syphilis performed at least 7 days after ulcer onset; (3) a typical clinical presen­ tation for chancroid; and (4) a negative test for herpes simplex virus in the ulcer exudate. A serologic test for syphilis does not distinguish cutaneous ulcers due to H. ducreyi from those due to yaws, but PCR should be performed. TREATMENT Haemophilus ducreyi Treatment regimens for both genital and cutaneous infections include either (1) a single 1-g oral dose of azithromycin; (2) ceftri­ axone (250 mg intramuscularly in a single dose); (3) ciprofloxacin (500 mg by mouth twice a day for 3 days); or (4) erythromycin base (500 mg by mouth three times a day for 7 days). Isolates from patients who do not respond promptly to treatment should be tested for antimicrobial resistance. In patients with HIV infection, healing may be slow and longer courses of treatment may be neces­ sary. Clinical treatment failure in HIV-seropositive patients may reflect co-infection, especially with herpes simplex virus. Contacts of patients with chancroid should be identified and treated, whether or not symptoms are present, if they have had sexual contact with the patient during the 10 days preceding the patient’s onset of symptoms. MORAXELLA CATARRHALIS ■ ■MICROBIOLOGY M. catarrhalis is an unencapsulated gram-negative diplococcus whose ecologic niche is the human respiratory tract. The organism was initially designated Micrococcus catarrhalis. Its name was changed to Neisseria catarrhalis in 1970 because of phenotypic similarities to commensal Neisseria species. On the basis of more rigorous analysis of genetic relatedness, Moraxella catarrhalis is now the widely accepted name for this species. ■ ■EPIDEMIOLOGY Nasopharyngeal colonization by M. catarrhalis is common in infancy, with colonization rates ranging between 33 and 100% and depend­ ing on geographic location. Several factors probably account for this geographic variation, including living conditions, day-care attendance, hygiene, household smoking, and population genetics. The prevalence of colonization decreases steadily with age. The widespread use of pneumococcal conjugate vaccines in some countries has resulted in alterations in patterns of nasopharyngeal colonization in resident populations. A relative increase in colonization by nonvaccine pneumococcal serotypes, nontypeable H. influenzae, and M. catarrhalis has occurred. These changes in colonization pat­ terns may be altering the distribution of pathogens of both otitis media and sinusitis in children. ■ ■PATHOGENESIS M. catarrhalis causes mucosal infections of the respiratory tract by con­ tiguous spread from its colonizing site in the upper airway. A preceding

47 - 164 Legionella Infections

164 Legionella Infections

have been reported. Shewanella species also cause chronic ulcers of the lower extremities, bacteremia, osteomyelitis, biliary tract infections, pneumonia, sepsis, and potentially chronic otitis media. A fulminant course is associated with cirrhosis, hemochromatosis, diabetes mellitus, malignancy, or other severe underlying conditions. These organisms are often susceptible to fluoroquinolones, third- and fourth-generation cephalosporins, β-lactam/β-lactamase inhibitors, carbapenems, and aminoglycosides (Table 163-2), but multidrug resistance is increasing. Chromobacterium violaceum is a facultative anaerobic organism found in soil and water in tropical or subtropical regions. After expo­ sure, it can cause rare but serious—often fatal—skin and soft tissue infections of limbs, although several recent reports suggest a more benign course with lower mortality. Life-threatening infections with severe sepsis and metastatic abscesses occur most often in patients with underlying illness, particularly in children with defective neutrophil function (e.g., those with chronic granulomatous disease). C. violaceum is frequently resistant to multiple drugs; carbapenems are most often used empirically. Fluoroquinolones and trimethoprim-sulfamethoxazole also can be active (Table 163-2). Organisms that can cause nosocomial infections of medical devices in compromised hosts, such as central venous catheters, include Ochrobac­ trum anthropi, Pseudomonas (formerly Flavimonas) oryzihabitans, and Sphingobacterium. Alcaligenes faecalis has been associated with hospitalacquired infections, such as bloodstream infections, due to contaminated hemodialysis and IV fluids or respirators. Sphingomonas koreensis was associated with a small cluster of nosocomial cases at one hospital and was traced to a reservoir in the plumbing system. Ralstonia species also can contaminate water supplies, including hospital water systems. Cases of bacteremia, osteomyelitis, pneumonia, and meningitis have been described. Pandoraea commovens primarily infects patients with cystic fibrosis. However, an outbreak described in Germany resulted in critical illness. Patients were more likely to be receiving mechanical ventilation and have had prior surgery or antimicrobial exposure. Sphingomonas paucimobilis, a rare cause of infection in both healthy and immunocom­ promised patients, can cause bloodstream infections, respiratory distress, and sepsis. It has a predilection for bone and soft tissue infection, osteo­ myelitis, and septic arthritis. Other organisms can cause rare human infections such as Weeksella species; Bergeyella species; various Centers for Disease Control and Prevention (CDC) groups; and Oligella urethra­ lis. The reader is advised to consult subspecialty texts and references for further guidance on these and other organisms. ■ ■FURTHER READING Bläckberg A et al: Infective endocarditis caused by HACEK group bacteria—a registry-based comparative study. Eur J Clin Microbiol 40:1919, 2021. Dernoncourt A et al: Prognostic factors of Pasteurella infections: A single-center retrospective cohort study over a 14-year period (2005–2018). Int J Infect Dis 116:197, 2022. Spencer HK et al: An overview of the treatment of less common non-lactose-fermenting gram-negative bacteria. Pharmacotherapy 40:936, 2020. Steven A. Pergam, Thomas R. Hawn

Legionella Infections Bacteria of Legionella species cause two primary human diseases: Legionella pneumonia (often referred to as Legionnaires’ disease) and Pontiac fever; collectively, these diseases are referred to as legionellosis. Legionnaires’ disease was first described in 1976 in an outbreak among members of the American Legion participating in a conference at a hotel in Philadelphia, Pennsylvania. Since their original description,

■ ■PATHOGEN AND PATHOGENICITY Legionellae are aerobic gram-negative bacteria that are ubiquitous in aquatic environments, damp soil, and compost. Of the more than 60 Legionella species, approximately half have been documented to lead to clinical disease, but most clinical disease is driven by Legionella pneumophila, primarily serotype 1. The primary habitats for growth and replication of Legionella are amoebae and other free-living proto­ zoa, in which these bacterial species can thrive intracellularly; humans are accidental hosts. Legionellae are reliant on host-derived amino acids and nutrients for intracellular replication. The organisms have a biphasic life cycle: a replicative phase in nutrient-rich conditions (e.g., in their protozoal hosts) and a noninfective transmissive phase under scarcity of resources. Therefore, they can persist in complex biofilms in both natural and engineered water systems (e.g., premise plumbing—a building’s hot and cold water piping systems) and are phagocytized by waterborne protozoa. In premise plumbing systems, where tempera­ ture and nutrients support the protozoal hosts of legionellae, the bacte­ ria can replicate to concentrations sufficient to cause human infection. CHAPTER 164 After exposure to Legionella through inhalation or aspiration of small aerosol particles, the organisms attach to immune cells and are phagocytized. After phagocytosis, they can evade intracellular defenses and replicate in human alveolar macrophages and monocytes. Patho­ genic Legionella species have numerous virulence systems that they use to evade the human immune system, including the development of Legionella-containing vacuoles within immune cells, downregula­ tion of cytokine receptors, inhibition of host protein synthesis, and avoidance of lysosomal degradation. Despite their ability to replicate and persist in an intracellular environment, innate immune compo­ nents that target intracellular pathogens—specifically, pattern recogni­ tion receptors, including Toll-like receptors and nucleotide-binding oligomerization domain–like receptors—activate immune responses. Adaptive CD4 and CD8 cytotoxic T-cell involvement and these innate immune responses eventually lead to the production of interferon γ and tumor necrosis factor, the promotion of neutrophil recruitment into the lung, and other proinflammatory responses. This cascade can be beneficial and result in clearance of the pathogen. However, these inflammatory responses can also cause immunopathology and adverse outcomes. L. pneumophila is more cytopathogenic than most nonpneumophila Legionella species, a characteristic that may be partially responsible for its association with severe disease. Legionella Infections ■ ■EPIDEMIOLOGY Legionella species are responsible for >50% of all waterborne outbreaks and >10% of disease related to drinking water in the United States. A National Academies of Sciences, Engineering, and Medicine report estimates that 50,000–70,000 Americans develop Legionnaires’ disease per year. Incidence rates of legionellosis in the United States are report­ edly 2–3 cases per 100,000 persons, but higher rates have been reported in other parts of the world. Numerous global epidemiologic studies assessing legionellosis have shown an increasing prevalence over the past few decades; this increase has been hypothesized to be due to a variety of causes, including an aging population, improved diagnostics, global temperature changes, and an aging water infrastructure. Legio­ nellosis is associated with substantial health care costs.

Legionella species are found throughout the world, but most epi­ demiologic data focus on legionellosis in large metropolitan areas in Australia/New Zealand, Europe, and North America. Rates of infection in other parts of the world are unknown, as surveillance systems and laboratory testing are less readily available in large portions of Africa and Asia. More than 80% of cases of Legionnaires’ disease are linked to L. pneumophila—in particular to serotype 1, which is the most frequently isolated Legionella pathogen. Although L. pneumophila pre­ dominates as a cause of disease, species predilection varies regionally. In Australia and New Zealand, for example, the rate of disease due to Legionella longbeachae approaches or exceeds that for L. pneumophila.

As previously mentioned, most reported cases are due to L. pneumophila serotype 1—a reflection of its pathogenicity. However, this predomi­ nance is also due to the frequency and ease of use of urinary antigen testing that targets this pathogen and allows more effective diagnosis in the community. It is unclear how large a role non-pneumophila spe­ cies and non–serotype 1 L. pneumophila play in disease. However, in studies in Europe, where respiratory cultures are more frequently col­ lected, nearly 10% of Legionnaires’ disease patients were infected with species other than L. pneumophila. In the United States, nearly 10% of culture-confirmed cases are due to non–serogroup 1 L. pneumophila. Immunosuppressed patients, such as cancer patients and transplant recipients, may be more likely to develop pneumonia caused by nonpneumophila species such as Legionella micdadei, Legionella bozemanii, and L. longbeachae. Despite increases in cases in the United States (Fig. 164-1) and throughout the world, incident cases are still thought to be underre­ ported. Many cohort studies of community-acquired pneumonia do not require routine testing for Legionella or assess only for L. pneumophila serotype 1 (by urinary antigen testing) and therefore may underestimate true prevalence. For example, a large administrative database of studies shows that, of patients with clinically proven community-acquired pneu­ monia, only 26% underwent Legionella-specific testing; even patients with documented risk factors for legionellosis are not always tested for Legionella. In studies that routinely assess for legionellosis, the prevalence of Legionella pneumonia ranges between 2 and 10% of all communityacquired pneumonia cases. In addition, extrapulmonary presentations and Pontiac fever are less likely to be identified or to result in presenta­ tion for health care, and this trend leads to further underestimation of the true burden of legionellosis. PART 5 Infectious Diseases Seasonality and Climate  Geoclimatic changes, storms, and sea­ sonality are thought to be important components of Legionella’s epide­ miology. The incidence of Legionella disease increases in the summer and fall—specifically, in warmer weather and with increased rain and humidity. Studies that screen all respiratory samples for Legionella find that legionellosis is indeed diagnosed most frequently in the United States during warmer summer/fall months and periods of greater 3.50 Incidence (cases per 100,000 population) 3.00 2.50 2.00 1.50 1.00 0.50 0.00

Year FIGURE 164-1  Increasing Legionella disease incidence in the United States over the past two decades (2000–2019). (From https://www.cdc.gov/legionella/about/history.html.)

humidity. Furthermore, seasonal storms, which may disrupt water pipes or cause increased flooding, can result in contamination of water systems with soil and lead to Legionella exposures. There is concern that, with ongoing climate shifts and rising global temperatures, cases of legionellosis may continue to increase. Community and Health Care–Associated Outbreaks  Small and large clusters and point-source outbreaks of Legionella cases lead to public health investigations, but these situations account for only ~5–10% of all Legionella cases yearly. Outbreaks occur when two or more people become ill after shared exposures in a community. In health care systems, a single proven case should trigger a Legionella investigation. The Centers for Disease Control and Prevention (CDC) recommends an outbreak investigation if a single patient with Legio­ nella is identified who did not leave the facility/campus for the 10 days prior to illness onset. Additionally, an outbreak investigation within a health care system is warranted if there are at least two possible Legio­ nella patients who spent any time in the hospital/long-term care facility within 12 months of each other (see “Clinical Presentations” below). Most common outbreaks are linked to water sources dispersing aerosol droplets that increase the area of particle spread (e.g., cooling towers or fountains) or to large building structural water systems that cause multiple prolonged exposures (e.g., those in hospitals, hotels, or apartments). The most commonly reported sources include not only cooling towers and fountains but also water misters; centralized heat­ ing, ventilation, and air-conditioning systems; hot tubs/spas; pools; ice machines; and showerheads and sinks in large premise plumbing struc­ tures (Fig. 164-2). When used as primary sources of water, ground­ water and wells have also been associated with Legionella exposures. The majority of exposures are related to engineered hot-water systems, which are often maintained at temperatures that limit scalding but are ideally suited for Legionella’s growth. Legionella can also be found in cold water, particularly in warmer summer months, as a consequence of the warming water temperature; engineering issues (e.g., heating lamps in fountains); or unexpected breaks in plumbing systems (e.g., malfunctioning thermostatic mixing valves), which can lead to hotwater contamination of cold-water systems. Buildings with inconsistent use patterns, such as hotels in seasonal travel destinations, can be linked to outbreaks of legionellosis, as water stagnation leads to low chlorine/disinfectant levels and organism proliferation can reach high enough levels to cause disease. Outbreaks have also been linked to cruise ships and boats. Following stay-athome orders during the SARS-CoV-2 pandemic, when buildings (e.g., hotels) were reopened, limited water movement and stagnation led to increases in cases of legionellosis. Modern buildings with water-saving devices, which aim to limit water and energy use, may increase the risk of legionellosis, as they can decrease water temperatures and limit water flow. Outbreaks in health care and long-term care facilities are identified more frequently than outbreaks in other facilities, as they often bring together at-risk patients, prolonged water expo­ sures, accessible testing, elevated awareness, and regulations that help ensure that cases are more easily linked to common sources. The outbreak examples listed in Table 164-1 demonstrate the wide variety of common sources and the number of cases associated with such factors. As previously mentioned, most large outbreaks involve cooling towers, which can spread aero­ sol droplets over a wide area. The largest out­ break reported to date involved a cooling tower in Spain that was linked to 449 documented cases of Legionnaires’ disease. Outbreaks are increasingly discussed in the media, such as outbreaks linked to cooling towers in the Bronx neighborhood of New York City, a large hotel outbreak in Atlanta, and the outbreaks associ­ ated with the Flint, Michigan, water crisis that

FIGURE 164-2  Sources of waterborne Legionella exposures and spectrum of presentation. The spectrum of sporadic to common-source outbreaks is a continuum. For example, premise plumbing in a large office building can lead to a large outbreak, and travel exposures can be related to large outbreaks. Most sporadic cases have no documented source of exposure, while outbreaks often involve mechanisms that spread water aerosol droplets over long distances (e.g., cooling towers), with a consequent ability to infect more individuals. (Reproduced with permission from Kyoko Kurosawa.) led to numerous deaths. It is not uncommon for lawsuits to be initiated when deaths are linked to outbreaks. Sporadic Cases  The vast majority of cases of Legionnaires’ disease occur sporadically in the community, manifesting as communityacquired pneumonia. Identification of the transmission source is more difficult in community-acquired cases than in nosocomial cases, despite reporting and review by local public health jurisdictions. In nearly 90% of all cases of legionellosis, a source of exposure is never identified. Since the spectrum of water exposures in the community is so broad and incubation periods can be long, identifying indi­ vidual exposures often is not possible. Transient exposures to common sources, travel-related exposures, and exposures to less commonly linked sources (e.g., potting soil and compost) may also be hard to identify. Furthermore, studies of domestic hot water have demon­ strated that 5–30% of households may have Legionella species detected, but the role that households play in clinical legionellosis is hard to determine, as home water testing is infrequently a part of usual contact TABLE 164-1  Examples of Legionella Common Source Outbreaks, Indicating the Wide Variety of Sources and Casesa SITE YEAR ORGANISMa REPORTED SOURCE(S) CASES Hotelb

L. pneumophila serotype 1 Potable water, fountain, spa 85 (29 suspect) Hospitalc

L. pneumophila Potable water 22 cases Communityd

L. pneumophila serotype 1 Cooling tower 334 cases Hospital/communitye 2014-15 L. pneumophila Potable water, household, cooling towers Long-term care facilityf

L. pneumophila Potable water 74 cases Communityg

L. pneumophila Hotel cooling towers 128 cases Hospitalh

L. pneumophila serotype 1 Potable water, showers 13 cases Hoteli

L. pneumophila Fountain 13 LP (66 suspect) Communityj

L. pnuemophilak Hot-tub display 141 cases Communityl

L. pneumophila serotype 1 Cooling tower 17 cases aLarge community outbreaks most commonly linked to cooling towers. Cases not noted to be a specific serotype, were not reported. bSmith SS et al: Open Forum Infect Dis 2:ofv164, 2015. cDepartment of Veteran’s Affairs Inspector General. https://www.va.gov/oig/pubs/VAOIG-13-00994-180.pdf. dShivaji T et al: Eurosuveillance 19:20991, 2014. eSmith AF et al: Environ Health Perspect 127:127001, 2019. fState of Illinois, Auditor General. https://auditor.illinois.gov/Audit-Reports/Performance-Special-Multi/ Performance-Audits/2019_Releases/19-Quincy-Legionnaires-Disease-Perf-Digest.pdf. gNew York City Department of Health and Mental Hygiene. https://www1.nyc.gov/ assets/doh/downloads/pdf/han/alert/legionella-in-bronx-source-identified.pdf. hKessler MA et al: Am J Infect Control. 49:1014, 2021. iBrown E: New York Times. 2019. https://www.nytimes.com/2019/08/16/us/legionnaires-disease-atlanta-hotel-reopen.html. jNorth Carolina Department of Health. https://epi.dph.ncdhhs.gov/cd/legionellosis/ MSFOutbreakReport_FINAL.pdf. kNon–serotype 1. lGrossman NV et al: Morbid Mortal Wkly Rep 72:1315, 2023. https://www.cdc.gov/mmwr/volumes/72/wr/mm7249a1.htm.

investigations. Because of underdiagno­ sis, it is likely that diagnosed sporadic community-acquired cases represent only patients who are ill enough to present to health care for evaluation. Risk Factors  A number of epidemio­ logic and demographic risk factors are associated with legionellosis. Older age is a risk factor; most studies suggest that risk begins to increase at an age of ~40 years. Furthermore, elderly patients are at the highest risk for major complica­ tions. Males are at approximately three times greater risk for Legionella disease than are females in most large epide­ miologic studies. Children are thought to be less likely to develop severe infec­ tions. However, since routine testing is less common among children, cases may be underreported.

Sporadic Premise plumbing Aspiration Travel Cooling towers Water features Common source/outbreak Smoking has been strongly linked to legionellosis. Inhalation of smoke leads to anatomic changes in the airway epithe­ lium, impairs neutrophil and monocyte phagocytosis, and has negative effects on airway ciliary clearance—all of which can increase the risk of pneumonia. Studies have shown that cigarette smoking is a dose-dependent risk factor. Smoking cannabis has also been associated with increased risk. Risk and severity of illness are further associated with smoking-related pulmonary diseases such as chronic obstructive pulmonary disease or emphysema, which in turn lead to increased risk for complications. Patients with other organ dysfunction/failure, such as those with renal disease (including those on dialysis), hepatic disease, nonsmoking pulmonary disease, and cardiac disease, are at increased risk for legionellosis, although it is unclear whether these factors are related to disease severity or to greater awareness and con­ sequent recognition by health care providers. CHAPTER 164 Legionella Infections Immunosuppressed patients are at increased risk for legionellosis and Legionella-related complications. Patients undergoing treatment for cancer (including recipients of hematopoietic cell transplantation) and solid organ transplant recipients are at high risk for legionellosis due to immunosuppression as well as disease- and treatment-related comorbidities. Use of prednisone and other glucocorticoids is strongly associated with legionellosis; however, in light of the heterogeneity of immunosuppressive agents and their use, it remains unclear whether 86 cases

most other single agents are as strongly associated with the disease. Combination immunosuppressive regimens increase risk. Patients treated with these regimens are more likely to develop non-pneumophila legionellosis and non–serotype 1 L. pneumophila infections that may be missed by routine urinary antigen testing. Patients with autoim­ mune diseases receiving tumor necrosis factor inhibitors, either with or without concomitant glucocorticoid use, are also at increased risk for legionellosis. Furthermore, studies suggest a possible association of legionellosis with genetic polymorphisms in components of the innate immune system that are important in recognizing and responding to intracellular pathogens (e.g., Toll-like receptors and interferon genes). There has also been transmission to lung transplant recipients from donor lungs.

Transmission  The Legionella species involved in human disease are usually waterborne pathogens. However, disease development requires sufficient levels of the organism at the exposure site, the formation of small particles that can be inhaled or aspirated into pulmonary alveoli, and an at-risk host. Legionella-containing aerosol particles must be <10 μm in diameter for deposition into the alveoli. The infective dose during exposures is unknown but likely depends on the host: disease development in at-risk individuals may require a more limited expo­ sure. Strain virulence is also thought to be important in disease devel­ opment: L. pneumophila serotype 1 is more apt to lead to outbreaks and disease than, for example, Legionella anisa, which has only rarely been associated with disease in high-risk patients. Because of the necessity for these various factors, estimated attack rates during an exposure are only ~5% for pneumonic presentations. Attack rates for Pontiac fever (see below) are thought to be higher—up to 90% among those exposed. Most exposures occur through the inhalation of contaminated aerosols from mists, sprays, or other mechanisms that produce small water droplets that can be inhaled into the distal alveoli. In homes, the most common sites of exposure are showerheads and sinks, which are especially apt to produce particles small enough for inhalation. The role played by aspiration or microaspiration in exposures is more con­ troversial but is hypothesized to be a secondary route for developing pneumonia. Although human-to-human transmission is not a common pathway, a single presumptive case has been reported. After exposure, L. pneumophila has an incubation period of ~2–10 days; this period has been reported to be longer in immunosuppressed hosts. In contrast, symptoms of Pontiac fever occur within 24–48 h after exposure. PART 5 Infectious Diseases ■ ■CLINICAL PRESENTATIONS Legionella Pneumonia  Legionella pneumonia is the most common manifestation of legionellosis. In clinical practice, Legionella pneumo­ nia is often referred to by clinicians as an “atypical pneumonia” (i.e., pneumonia that lacks the classic signs and symptoms of bronchopneu­ monia). Other bacterial pathogens, such as Chlamydia pneumoniae and Mycoplasma pneumoniae, are also considered as etiologic agents of atypical pneumonia. Initial symptoms of Legionella pneumonia are nonspecific and include fever, myalgias, headache, shortness of breath, and either a dry or a productive cough (Table 164-2). Patients with pneumonia who present with neurologic or gastrointestinal symptoms such as anorexia, nausea, or vomiting may be more likely than others to have legionellosis. Immunosuppressed patients may present without typical symptoms such as fever. Patients who have recently traveled, who present during a known or possible Legionella outbreak, or who develop pneumonia while hospitalized should undergo testing for legionellosis. Patients with severe pneumonia presentations, including acute respiratory failure, and those with pneumonia and sepsis-like presentations should undergo testing for Legionella as per current community-acquired pneumonia guidelines. Patients with Legionella pneumonia classically present with rales, rhonchi, and—when consolidation is present—egophony and dull­ ness to percussion. Not all patients, particularly immunosuppressed patients, present with pulmonary findings on clinical examination. Ini­ tial laboratory findings in patients with Legionella pneumonia include leukocytosis or leukopenia, thrombocytopenia, and elevated liver enzyme levels; hyponatremia and/or renal dysfunction are frequent.

TABLE 164-2  Clinical and Epidemiologic Features of Legionella Pneumonia (Legionnaires’ Disease) and Pontiac Fever LEGIONELLA PNEUMONIA PONTIAC FEVER FEATURE Incubation period 2–10 daysa 24–72 h Pathogenesis Legionella infection Legionella infection or exposure Common symptoms Abdominal or chest pain Anorexia Cough, sputum production Confusionb Cough Diarrhea Fatigue Fever/chills Headache Myalgias Nausea/vomiting Vertigo Diarrheab Fatigue Fever/chills Headache Myalgias Nausea/vomitingb Shortness of breath Risk factors Age >40 years Male Smoker Immunosuppressed host Neurologic disease Chronic lung disease Organ dysfunction/ chronic illness Factors associated with increased exposure Attack rate among exposed individuals ~5%c ~90% Hospitalization rate

90% <1% ICU admission rate 30–50% Extremely low Treatment Antibiotics (macrolide or fluoroquinolone) Supportive care Case-fatality rated 10% Extremely low aIncubation period in immunosuppressed hosts may be longer than 14 days. bThis symptom is strongly associated with Legionella pneumonia. cAttack rates are highly dependent on method of exposure, level of the pathogen in source water, and host’s level of risk. dCase-fatality rates are much higher among immunosuppressed patients and those with severe underlying lung disease, ranging from 30 to 50%. Abbreviation: ICU, intensive care unit. Source: Modified from https://www.cdc.gov/legionella/clinicians/clinical-features. html. Levels of nonspecific laboratory markers of inflammation, such as C-reactive protein, can also be elevated; however, procalcitonin levels may not be as useful as a diagnostic tool. Although clinical symptoms and laboratory findings tend to be nonspecific, a number of clinical prediction tools, such as the Winthrop-University Hospital Criteria and the Legionella Score, have been developed to assist with the diag­ nosis of Legionella pneumonia. These scoring systems may be more useful for their negative than for their positive predictive value. An important subset of Legionella pneumonia cases are those that are linked to health care systems—i.e., nosocomial cases. Although cases of hospital-acquired legionellosis are rare, their identification is necessary as they may be harbingers of contamination of water systems, devices, and/or potable water sources. Because of the rarity of nosocomial cases, outbreaks have sometimes occurred over years before the source is identified within the health care system. In this regard, the CDC offers the following definitions: (1) A presumptive health care–associated case of Legionnaires’ disease is one developing in a patient with Legionella pneumonia after ≥10 days of continuous stay at a health care facility during the 14 days before onset of symp­ toms. (2) A possible case is one that develops in a patient with Legionella pneumonia who has spent a portion of the 14 days before symptom onset in one or more health care facilities but not enough time to meet the criteria for a presumptive case. To ensure that singular cases lead to

more system-wide evaluations, the CDC also recommends an investi­ gation if a health care system detects one or more cases of presumptive health care–associated Legionnaires’ disease at any time or two or more possible cases within 12 months of one another. ■ ■PONTIAC FEVER Pontiac fever is an influenza-like illness whose primary symptoms are fever, headache, myalgias, chills, vertigo, nausea, vomiting, and diar­ rhea (Table 164-2). Compared with Legionella pneumonia, Pontiac fever is a milder, self-limited illness that is defined by the absence of pneumonia. Although studies have shown that Pontiac fever is associ­ ated with exposure to higher counts of colony-forming units in water sources, the role of the pathogen in the disease is not clear. Symptoms usually develop 24–48 h after exposure and can last for 2–5 days. Since many other illnesses resemble Pontiac fever, the diagnosis usually relies on the recognition of typical clinical features during an outbreak situation; therefore, sporadic cases are likely to be missed even when patients present for health care. Studies documenting specific Legionella species as the cause of Pontiac fever clusters find that most are due to L. pneumophila exposure; however, non-pneumophila species such as L. anisa have also been associated with this presentation. Extrapulmonary Disease  A number of rare presentations for legionellosis have been described. Skin and soft tissue infections that resemble cellulitis, including cases due to tap water contamination of postsurgical wounds, have been reported. Endocarditis, primarily culture-negative prosthetic valve endocarditis, and myocarditis and pericarditis have also been reported. Rarely, Legionella species have been associated with septic arthritis and sinusitis. ■ ■DIAGNOSIS The diagnosis of legionellosis on the basis of clinical findings alone is difficult. Additional workup is needed to make a definitive diagnosis, even when cases are potentially linked to a possible outbreak. To make a diagnosis, laboratory confirmation is needed, and invasive proce­ dures may be required—e.g., bronchoscopy, particularly for patients whose results on urinary antigen testing are negative and who cannot produce sufficient sputum for testing or for patients with severe disease requiring intensive care unit (ICU) admission. As current treatment guidelines for community-acquired pneumonia recommend empiri­ cal coverage that includes antibiotics active against Legionella species, diagnostic testing is not routine even among persons who meet the criteria for Legionella-specific testing. Furthermore, not all currently available diagnostic laboratory assays, including urinary antigen test­ ing, are accessible or rapidly available in primary care clinics, urgent care facilities, and emergency rooms where patients may present with their initial symptoms. Radiologic Findings  On chest radiography, Legionella pneumo­ nia presents as focal infiltrates or consolidations, most frequently in the lower lobes, that are indistinguishable from those due to other causes of pneumonia (Fig. 164-3). On computed tomography (CT), air-space disease in one or more lobes is often with associated ground-glass opacities (Fig. 164-4); pleural effusions and lymphadenopathy are less frequently seen. In immunocompromised patients, Legionella can pres­ ent with similar lower-lobe consolidations or atypically as pulmonary nodules—with or without cavitation—that mimic fungal infections (Fig. 164-5) or even as lung abscesses. Progression during early therapy is not uncommon in immunosuppressed patients. Laboratory Diagnostics  •  CULTURE  Cultures—of sputum, bronchoalveolar lavage fluid, lung tissue, or extrapulmonary sites—are the gold standard for diagnosis of Legionella pneumonia because they are critical for epidemiologic investigations. Legionella species require special nutrients, such as cysteine, for growth and therefore require specialized media, such as buffered charcoal yeast extract (BCYE) agar. Legionellae grow slowly, usually over 3–5 days, with non-pneumophila species often requiring longer incubation times. Once growth is seen, Legionella can be stained with standard Gram stain, and colonies often fluoresce blue or white under ultraviolet light. L. micdadei is the only

FIGURE 164-3  Chest x-ray of a patient with Legionella pneumonia and rightlower-lobe consolidation. A 64-year-old woman presented with fever, dry cough, and shortness of breath 7 days after returning from international travel. Legionella urinary antigen testing was positive for L. pneumophila serotype 1. CHAPTER 164 Legionella species that is also modified-acid-fast positive. Sensitivity varies with the sample but is highest among lower respiratory tract samples. At some referral centers, lower-tract samples from high-risk immunosuppressed patient populations are routinely sent for culture. Unfortunately, because of current community-acquired pneumonia guidelines, patients are often treated empirically, and many either never have samples sent for Legionella-specific cultures or have such samples collected only after antibiotic administration, which decreases sensitivity. Respiratory cultures from patients with legionellosis are crucial during outbreak investigations, as clinical and environmental cultures can be compared molecularly to help identify common-source outbreaks; cultures are also used for serotyping of L. pneumophila. Legionella Infections URINARY ANTIGEN TESTING  Legionella urinary antigen tests are widely available at many hospitals and commercial laboratories and are char­ acterized by ease of use, simple specimen collection, rapid turnaround time, high sensitivity, and the ability to detect the most prevalent Legionella species associated with clinical disease—L. pneumophila sero­ type 1. Urinary antigen testing has limitations, however: it detects only L. pneumophila serotype 1 and gives false-negative results in most cases caused by clinically important non–serotype 1 L. pneumophila and nonpneumophila species. Sensitivity for L. pneumophila serotype 1 is ~70% for most assays, but specificity is very high. The urinary antigen test can be negative very early in the disease and can remain positive for months after an infection, particularly in immunosuppressed patient popula­ tions; it cannot be used for patients who are anuric. Urinary antigen testing is not recommended for routine use in screening for exposures among asymptomatic patients in outbreak investigations. SEROLOGY  Acute- and convalescent-phase titers of antibody to Legionella have limited sensitivity in diagnosing acute Legionnaires’ disease but can be useful during outbreak investigations. A case is confirmed by documenting a fourfold or greater rise in titer of specific serum antibody to L. pneumophila serogroup 1. A case is suspected in tests using pooled antigens by (1) a fourfold or greater rise in antibody titer to specific species (e.g., L. longbeachae) or non–serogroup 1 L. pneumophila or (2) a fourfold or greater rise in antibody titer to mul­ tiple species of Legionella. Some experts think that a single antibody level of ≥1:256 may be an adequate basis for diagnosing a presumptive case, but most prefer paired serology for confirmation. Serology is an imperfect tool; data suggest that as many as 20–30% of patients with

PART 5 Infectious Diseases A B FIGURE 164-4  Right-upper-lobe infiltrate in a patient with L. pneumophila pneumonia on chest x-ray and computed tomography (CT). An immunosuppressed patient from a long-term care facility presented with cough, sputum production, fever, and chills. New renal insufficiency and hyponatremia were documented. A chest x-ray (A) was consistent with a small right-upper-lobe infiltrate (white arrow), which was confirmed by CT (B). Urinary antigen testing for L. pneumophila serotype 1 was negative, but polymerase chain reaction on bronchoalveolar lavage fluid was positive for L. pneumophila. A B FIGURE 164-5  Nodular disease presentation on computed tomography (CT) in an immunosuppressed patient infected with L. micdadei. A. CT scan in a hematopoietic cell transplant recipient presenting with fever and cough. A pulmonary nodule was noted in the right upper lobe. Bronchoscopy was performed; cultures were positive on day 5 for small white colonies on buffered charcoal yeast extract plates, and these colonies were eventually identified as L. micdadei. B. Repeat CT scan at day 12 demonstrated an enlarging nodule, diffuse infiltrates, and possible cavitation. The patient required intensive care unit admission and intubation despite appropriate targeted antibiotic therapy. proven legionellosis may not mount an antibody response that is suf­ ficient for diagnosis, and the sensitivity and specificity of seroconver­ sion with regard to non-pneumophila Legionella species are unclear among patients with altered immunity. Serology can provide important information for epidemiologic investigations, helping to identify addi­ tional cases missed by other diagnostic methods. In addition, the use of serologic testing during outbreak studies allows the investigation of patients without severe disease (e.g., those with Pontiac fever). DIRECT FLUORESCENT ANTIBODY TESTING  The sensitivity of direct fluorescent antibody (DFA) testing of sputum is lower than that of other testing modalities, ranging from 20 to 70% depend­ ing on the assay used. Most available assays target specific species (e.g., L. pneumophila) or serotypes. DFA testing may have a higher positive predictive value in patients with severe pneumonia or symptoms consistent with Legionnaires’ disease, but it is not recom­ mended for screening of low-risk patients because of the frequency of false-positive results. MOLECULAR TESTING  Polymerase chain reaction (PCR), loop-mediated isothermal amplification (LAMP), and other nucleic acid amplifica­ tion tests are highly sensitive for lower respiratory tract specimens

(e.g., sputum) and are becoming more widely available. Molecular methods can detect Legionella from multiple sources but are most com­ monly used for respiratory specimens such as sputum and bronchoal­ veolar lavage fluid. PCR is more sensitive than culture; in some studies, up to two to four times as many cases of lower tract disease were detected only by molecular methods. Molecular techniques also are useful in diagnosing infection in patients during antibiotic therapy. However, PCR methods are not used to determine L. pneumophila serotypes—information that is needed for epidemiologic investigations—and most commercially available assays target only L. pneumophila. Multiplex PCR tests for pneumonia and other respiratory pathogens are increasingly available and may include L. pneumophila. TREATMENT Legionella Pneumonia Treatment of Legionella pneumonia involves antibiotics that target intracellular pathogens, whereas patients with Pontiac fever do not require antibiotic therapy. Macrolides and fluoroquinolones are the first-line agents for Legionella pneumonia according to guidelines in the United States and Europe (Table 164-3). Macrolides disrupt protein production critical for survival of the organism. Although erythromycin and clarithromycin are both effective, azithromycin is the preferred agent, as it is easier to tolerate and has fewer drugdrug interactions. Azithromycin and clarithromycin also reach higher intracellular concentrations than erythromycin. Fluoroquinolones are potent agents against Legionella species. Data from both in vitro and in vivo models of infection suggest that fluoroquinolones may be more effective than macrolides, but no randomized clinical trials have yet compared the two drug classes for treatment of legionellosis. In nonrandomized observational studies, fluoroquinolones have been shown to be more effective than macrolides (erythromycin and clarithromycin) in terms of fever resolution and decreased duration of hospitalization; other such studies have shown no difference in outcome. Both macrolides and fluoroquinolones are available as IV and oral formulations. Most experts prefer IV therapy during the first TABLE 164-3  Legionella Treatment Options DISEASE SEVERITY DISEASE MILD MODERATE/SEVEREa,b Pontiac fever None N/A Legionella pneumonia A) Fluoroquinolone: Levofloxacin, 750 mg PO once daily or Ciprofloxacin, 500 mg PO twice daily or Moxifloxacin, 400 mg PO once daily A) Fluoroquinolone:c Levofloxacin, 750 mg IV once daily or Ciprofloxacin, 500 mg IV twice daily or Moxifloxacin, 400 mg PO twice daily or B) Macrolide: Azithromycin, 500 mg PO once daily (day 1), followed by 250 mg PO once daily (for minimum of 4 days) or B) Macrolide:c Azithromycin, 500 mg IV daily or Clarithromycin, 400 mg IV twice daily or Clarithromycin, 400 mg PO daily Note: Agents in bold type are considered first-line treatment. aAll immunosuppressed patients should be considered to have moderate or severe disease and started on IV therapy if possible. bAll patients requiring inpatient care should receive IV therapy until improving, when they can be switched to an oral agent. cConsider dual therapy, either with dual fluoroquinolone and macrolide therapy, or either agent with another secondary agent (e.g., rifampin). Secondary agents include doxycycline, minocycline, rifampin, and trimethoprimsulfamethoxazole, all with varying efficacy for treatment. Abbreviations: IV, intravenous; N/A, not applicable; PO, oral.

few days of treatment for patients with severe Legionella pneumo­ nia. Secondary agents, such as rifampin, doxycycline, minocycline, and, less frequently, trimethoprim-sulfamethoxazole, have also been used, with mixed responses. Tigecycline, a third-generation glycylcycline related to tetracyclines, has been used for treatment of patients with significant antibiotic allergies. The novel aminometh­ ylcycline antibiotic omadacycline appears to be efficacious in vitro, but its clinical efficacy has not been studied to date, and it is not currently recommended for routine use. Although data are limited, combination therapy does not appear to improve outcomes.

The optimal duration of treatment for Legionnaire’s disease is unknown. For community-acquired pneumonia, guidelines rec­ ommend treatment until the patient achieves clinical stability and for a minimum of 5 days. In the absence of data, a similar duration of treatment for Legionella is a reasonable approach. For immunosuppressed patients and patients with severe disease, a more protracted course of therapy is recommended. The duration of therapy for extrapulmonary manifestations of Legionella infec­ tion is unknown and depends on the site involved and clinical improvement. Resistance to macrolides and fluoroquinolones has been reported only rarely. Susceptibility testing is not routinely performed but is available in specialized laboratories and public health departments. ■ ■OUTCOMES Legionella infections are associated with significant morbidity and mortality, leading to hospitalization and ICU admission of most patients who develop pneumonia. Case-fatality rates of Legionella pneumonia are reported to be ~10%, with death more likely among patients who are admitted to the ICU or have major comorbidities. Among patients in whom antibiotic treatment is delayed, mortality rates are approximately three times higher than among those treated earlier. Patients who develop nosocomial pneumonia attributable to health care–associated exposures, particularly those due to L. pneu­ mophila, have case-fatality rates of ~25%. Death is a much more com­ mon outcome among immunocompromised hosts, whose mortality rates can reach ~30–50%. Assessment of long-term follow-up among patients who recover from severe Legionella pneumonia demonstrates that more than one-quarter have ongoing complications after recovery, including recurrent hospitalizations, acute renal failure, respiratory complications, and recurrent pneumonias. In contrast, recovery from Pontiac fever usually takes place within 3–5 days, as the disease is selflimiting; hospitalization, complications, and death related to Pontiac fever are extremely rare. CHAPTER 164 Legionella Infections ■ ■PREVENTION Prevention of legionellosis starts with addressing water systems. Large municipal water systems provide water throughout the globe, but the quality of these systems varies regionally; many areas have limited access to potable water. Only limited regions have the resources to address Legionella water contamination; most water-monitoring agen­ cies focus on control of enteric pathogens, such as Escherichia coli and other coliform bacteria, and do not have an adequate infrastructure to address Legionella. Even in countries and cities with more complex water systems, there is wide variation in how waterborne pathogens are addressed, and rules and regulations are often country dependent. In the Netherlands, for example, chlorination is not routine, whereas the United Kingdom and most countries in the European Union use chlorine routinely as the primary mode of disinfection for public water systems. Although regulated by the Environmental Protection Agency, management and treatment strategies in the United States vary by state and, in some instances, by city. Prevention in the United States focuses on health care organizations and hospitals, where water-based exposures are more often linked to case fatalities. Federal requirements to reduce Legionella risk in the United States were first established in June 2017, when the Centers for Medicare and Medicaid Services required that all health care organi­ zations develop and adhere to water management plans. These plans require the development of multidisciplinary teams, an understanding

48 - 165 Pertussis and Other Bordetella Infections

165 Pertussis and Other Bordetella Infections

of the organization’s water system, identification of high-risk areas (e.g., transplant units, oncology floors), identification of at-risk struc­ tures for Legionella growth, implementation and monitoring of con­ trol measures, methods for intervention if control measures fail, and procedures to assure documentation that policies are followed. All medical centers are required to have an awareness of water quality and to have systems in place to help prevent nosocomial Legionella pneu­ monia. Such policies leave water quality assessment, including testing for Legionella, up to the individual facility. In addition to hospitals, an increasing number of cities, including New York City, require similar water-management plans for cooling towers, with registration, testing, and mitigation options.

Even if detected in regional water systems, Legionella becomes a human pathogen only after replication in premise plumbing systems. In buildings, Legionella finds the ideal environment for logarithmic growth, which leads to exposures and subsequent disease. An impor­ tant first step in prevention within hospitals is a review of plumbing systems to identify areas of concern and a review of impact areas such as dental clinics, ICUs, rehabilitation units, and units that house high-risk patients. Specific water features, such as therapy pools, ice machines, and decorative fountains, need policies for cleaning and disinfection. Targeted approaches to management of cooling towers, such as high-efficiency drift eliminators and routine maintenance, are important considerations. In addition, areas that have undergone recent construction or renovation should be flagged, with prevention policies in place to address the associated risks. New construction or structural updates can lead to water stagnation, while modifications to plumbing can disrupt biofilms. Units with older premise plumbing are thought to be at higher risk, but even brand new facilities can become colonized during construction, with consequent outbreaks. PART 5 Infectious Diseases Testing for Legionella is an important step when presumptive or possible nosocomial pneumonia cases occur and can help address a facility’s potential risks. There are a number of methods for environ­ mental testing for Legionella, but environmental cultures are used in most hospitals because they quantify Legionella levels, allow species identification/serotyping, and can link environmental sources to nosocomial outbreaks. Testing usually focuses on locations where the index patient(s) may have had potential waterborne exposures (e.g., at showers and sinks). Other adjacent areas, along with those noted to be high-risk locations within the hospital, should be considered for additional testing; positive results should widen the testing area. Pro­ active testing is increasingly being used to preclude nosocomial cases; however, if testing is planned, it should be coupled with a management plan that addresses how Legionella will be dealt with if it is found in the water system and where and how frequently testing should be done; we recommend biannual or quarterly testing of select sites within hospital systems. If a common-source outbreak is discovered, a number of approaches can be used to address Legionella. Regardless of source, immediate limitation of ongoing water exposures for patients in the affected room, unit, or floor is a crucial step in avoiding additional cases. Removing or replacing water features associated with exposures, such as decorative fountains and affected equipment or plumbing devices, may be needed. Immediate interventions such as heat shock (increasing water tempera­ tures for a limited period) and hyperchlorination may also be useful as short-term steps in addressing an outbreak. The addition of a disinfectant to the water system is one of the most common ways to address the presence of Legionella. Chemical disinfec­ tion with agents such as chlorine or monochloramine and copper and silver ionization are commonly used for secondary disinfection. Use of disinfectants requires routine maintenance and monitoring of chemical or ion levels to assure that they are sufficient for prevention. Lack of monitoring and system failures have led to breakthrough nosocomial Legionella cases. Another option is water filtration, which either can serve as a primary method for prevention or can be used in combina­ tion with secondary disinfection. Filters—either in-line with plumbing or at point-of-use sites—can be considered for either short- or longterm prevention during an outbreak. However, filters have a limited life span, can weaken water pressure, and are costly to maintain.

■ ■FURTHER READING Cassell K et al: Estimating the true burden of Legionnaires’ disease. Am J Epidemiol 188:1686, 2019. Centers for Disease Control and Prevention: Developing a water management program to reduce Legionella growth and spread in buildings: A practical guide to implementing industry standards. June 24, 2021. Available at https://www.cdc.gov/legionella/wmp/tool­ kit/.  Accessed October 10, 2023. Centers for Disease Control and Prevention: Legionnaires’ dis­ ease surveillance summary report, United States 2018–2019. Available at https://www.cdc.gov/legionella/health-depts/surv-reporting/201819-surv-report-508.pdf.  Accessed October 20, 2023. Kato H et al: Meta-analysis of fluoroquinolones versus macrolides for treatment of legionella pneumonia. J Infect Chemother 27:424, 2021. National Academies of Sciences, Engineering, and Medicine: Management of Legionella in Water Systems. Washington, DC, The National Academies Press, 2020. Pierre DM et al: Diagnostic testing for Legionnaires’ disease. Ann Clin Microbiol Antimicrob 16:1, 2017. May S. ElSherif, Scott A. Halperin

Pertussis and Other

Bordetella Infections Pertussis is an acute infection of the respiratory tract caused by Bordetella pertussis. The word pertussis means “violent cough,” which aptly describes the most consistent and prominent feature of the illness. The inspiratory sound made at the end of an episode of paroxysmal coughing gives rise to the common name for the illness, “whooping cough.” However, this feature is variable: it is uncommon among infants ≤6 months of age and is frequently absent in older children and adults. The Chinese name for pertussis is “the 100-day cough,” which describes the clinical course of the illness accurately. The identification of B. pertussis was first reported by Bordet and Gengou in 1906, and vaccines were produced over the following two decades. ■ ■MICROBIOLOGY Of the 10 identified species in the genus Bordetella, only four are of major medical significance. B. pertussis infects only humans and is the most important Bordetella species causing human disease. B. paraper­ tussis causes an illness in humans that is similar to pertussis but is typi­ cally milder; co-infections with B. parapertussis and B. pertussis have been documented. With improved polymerase chain reaction (PCR) diagnostic methodology, up to 20% of patients with a pertussis-like syndrome have been found to be infected with B. holmesii, formerly thought to be an unusual cause of bacteremia. B. bronchiseptica is an important pathogen of domestic animals that causes kennel cough in dogs, atrophic rhinitis and pneumonia in pigs, and pneumonia in cats. Both respiratory infection and opportunistic infection due to B. bronchiseptica are reported occasionally in humans. B. petrii, B. hinzii, and B. ansorpii have been isolated from patients who are immunocompromised. Bordetella species are gram-negative pleomorphic aerobic bacilli that share common genotypic characteristics. B. pertussis and

B. parapertussis are the most similar of the species, but B. parapertussis does not express the gene coding for pertussis toxin. B. pertussis is a slow-growing fastidious organism that requires selective medium and forms small, glistening, bifurcated colonies. Suspicious colonies are presumptively identified as B. pertussis by direct fluorescent antibody testing or by agglutination with species-specific antiserum. B. pertussis

is further differentiated from other Bordetella species by biochemical and motility characteristics. B. pertussis produces a wide array of toxins and biologically active products that are important in its pathogenesis and in immunity. Most of these virulence factors are under the control of a single genetic locus that regulates their production, resulting in antigenic modula­ tion and phase variation. Although these processes occur both in vitro and in vivo, their importance in the pathobiology of the organism is unknown; they may play a role in intracellular persistence and personto-person spread. The organism’s most important virulence factor is pertussis toxin, which is composed of a B oligomer–binding subunit and an enzymatically active A protomer that ADP-ribosylates a gua­ nine nucleotide–binding regulatory protein (G protein) in target cells, producing a variety of biologic effects. Pertussis toxin has important mitogenic activity, affects the circulation of lymphocytes, and serves as an adhesin for bacterial binding to respiratory ciliated cells. Other important virulence factors and adhesins are filamentous hemaggluti­ nin, a component of the cell wall, and pertactin, an outer-membrane protein. Fimbriae, bacterial appendages that play a role in bacterial attachment, are the major antigens against which agglutinating anti­ bodies are directed. These agglutinating antibodies have historically been the primary means of serotyping B. pertussis strains. Other virulence factors include tracheal cytotoxin, a peptidoglycan fragment, which causes inflammatory respiratory epithelial damage; adenylate cyclase-hemolysin toxin, which impairs host phagocytic cell function; dermonecrotic toxin, which may contribute to respiratory mucosal damage; and lipooligosaccharide, which has properties similar to those of other gram-negative bacterial endotoxins. Since 2010, the emer­ gence of pertactin-negative strains worldwide has been attributed to immune pressure resulting from the use of pertactin-containing acel­ lular pertussis vaccines, reaching >80% dominance in some countries. ■ ■EPIDEMIOLOGY Pertussis is a highly communicable disease, with attack rates of 80–100% among unimmunized household contacts and 20% within households in well-immunized populations. The infection has a world­ wide distribution, with cyclical outbreaks every 3–5 years (a pattern that has persisted despite widespread immunization). Pertussis occurs in all months; however, in North America, its activity peaks in autumn and winter. In developing countries, pertussis remains an important cause of infant morbidity and mortality despite the reported worldwide decrease in inci­ dence following improved vaccine coverage (Fig. 165-1). Monitored as a 100% 80% 60% Coverage 40% 20% 0%

Coverage - Global, DTP-containing vaccine, 1st dose, WHO/UNICEF Estimates of National Immunization Coverage Coverage - Global, DTP-containing vaccine, 3rd dose, WHO/UNICEF Estimates of National Immunization Coverage Number of reported cases - Global, Pertussis FIGURE 165-1  Global annual reported cases of pertussis and rate of coverage with DTP3 (diphtheria toxoid, tetanus toxoid, and pertussis vaccine; one and three doses), 1980–2022. (Reproduced with permission from Diphtheria tetanus toxoid and pertussis (DTP) vaccination coverage (who.int). World Health Organization Immunization Data Portal; WHO 2023.)

surrogate to evaluate immunization programs, 2022 DTP3 (diphtheria, tetanus, and pertussis) coverage rates are still <50% in many developing nations while globally recovering to 84% from 2021’s 81%; the World Health Organization (WHO) estimates that 90% of the burden of pertussis is in developing regions. The overreporting of immunization coverage and underreporting of disease result in substantial underesti­ mation of the global burden of pertussis. WHO estimates are 40 million cases globally each year, with 400,000 deaths, mostly among infants

<3 months of age.

Before the institution of widespread immunization programs in the developed world, pertussis was one of the most common infectious causes of morbidity and death. In the United States before the 1940s, between 115,000 and 270,000 cases of pertussis were reported annu­ ally, with an average yearly rate of 150 cases per 100,000 population. With universal childhood immunization, the number of reported cases fell by >90%, and mortality rates decreased even more dramatically. Only 1010 cases of pertussis were reported in 1976 (Fig. 165-2). After that historic low, rates of pertussis increased slowly. In recent years, pertussis epidemics have been reported with increasing frequency in high-income countries, including Australia, the United Kingdom, and the United States. The United States experienced widespread pertus­ sis outbreaks in 2005, 2010, 2012, 2014, and 2015 at levels not seen in 40–50 years (48,000 reported cases in 2012). Although thought of as a disease of childhood, pertussis can affect people of all ages and is a known cause of prolonged coughing illness in adolescents and adults. In unimmunized populations, pertussis inci­ dence peaks during the preschool years, and well over half of children have the disease before reaching adulthood. In highly immunized popu­ lations such as those in North America, the peak incidence is among infants <1 year of age who have not completed the three-dose primary immunization series. An increase in pertussis incidence among adoles­ cents and adults began in the late 1990s and led to the introduction of an adolescent booster dose across North America by 2006. While the dis­ ease burden among adolescents decreased initially, children 7–10 years of age emerged as a high-risk group during a major outbreak in 2010. Most of the affected children were fully immunized. Subsequent outbreaks in 2012, 2014, and 2015 showed a shift in epidemiology, with pertussis incidence increasing among adolescents while still remaining elevated among 10-year-olds. The most highly affected cohorts were those who received acellular pertussis vaccines in infancy. Although adults contrib­ ute a smaller proportion of reported cases of pertussis than do children and adolescents, this difference may be related to a greater degree of underrecognition and underreporting. Several studies of prolonged CHAPTER 165 Pertussis and Other Bordetella Infections 2,500,000 2,000,000 Number of reported cases 1,500,000 1,000,000 500,000

Year

300,000 DTP 250,000 200,000 Number of cases 150,000 100,000 50,000

FIGURE 165-2  Reported cases of pertussis by year: United States, 1922–2021, marking the introduction of different vaccine types. (From the Centers for Disease Control and Prevention, Pertussis Surveillance Trend Reporting and Case Defintion | CDC. Available at https://www.cdc.gov/pertussis/surv-reporting.html. Accessed December 26, 2023.) coughing illness suggest that B. pertussis may be the etiologic agent in 12–30% of adults with cough that does not improve within 2 weeks. In one study of the efficacy of an acellular pertussis vaccine in adolescents and adults, the incidence of pertussis in the placebo group was 3.7–4.5 cases per 1000 person-years. Although this prospective cohort study yielded a lower estimate than the studies of cough illness, its results still translate to ~1 million cases of pertussis annually among adults in the United States. In addition, asymptomatic pertussis infection, estimated at 56% by a systemic review of tested household contacts, is common and appears to contribute to disease transmission. PART 5 Infectious Diseases Severe morbidity and high mortality rates, however, are restricted almost entirely to infants. In the United States between 2000 and 2017, infants <2 months old accounted for 84% of pertussis deaths with annual mean number of cases, hospitalizations, and deaths at 2957 (range 1803–4994), 1122 (range 544–1938), and 15 (range 3–35), respectively. Although school-age children are the source of infection for most households, adults are often the source for cases in high-risk infants and may serve as the reservoir of infection between epidemic years. ■ ■PATHOGENESIS Infection with B. pertussis is initiated by attachment of the organism to the ciliated epithelial cells of the nasopharynx. Attachment is mediated by surface adhesins (e.g., pertactin and filamentous hemagglutinin), which bind to the integrin family of cell-surface proteins, probably in conjunction with pertussis toxin. The role of fimbriae in adhesion and in maintenance of infection has not been fully delineated. Perhaps the result of redundancy of adhesins, no differences in virulence or clinical mani­ festations have been detected with the emergence of pertactin-negative strains. At the site of attachment, the organism multiplies, producing a variety of other toxins that cause local mucosal damage (tracheal cyto­ toxin, dermonecrotic toxin). Impairment of host defense by B. pertussis is mediated by pertussis toxin and adenylate cyclase-hemolysin toxin. There is local cellular invasion, with intracellular bacterial persistence; however, systemic dissemination does not occur. Systemic manifesta­ tions (lymphocytosis) result from the effects of the toxins. The pathogenesis of the clinical manifestations of pertussis is poorly understood. It is not known what causes the hallmark paroxysmal cough. A pivotal role for pertussis toxin has been proposed but has not been confirmed. It is thought that neurologic events in pertussis, such as seizures and encephalopathy, are due to hypoxia from cough­ ing paroxysms or apnea rather than to the effects of specific bacterial products. B. pertussis pneumonia, which occurs in up to 22% of infants with pertussis, is usually a diffuse bilateral primary infection. In older children and adults with pertussis, pneumonia is often due to second­ ary bacterial infection with streptococci or staphylococci. Deaths from

60,000 50,000 40,000 30,000 20,000 10,000

Tdap DTaP

pertussis among young infants are frequently associated with very high levels of leukocytosis and pulmonary hypertension. ■ ■IMMUNITY Both humoral and cell-mediated immunity are thought to be impor­ tant in pertussis. Although immunity after natural infection was thought to be lifelong, seroepidemiologic evidence demonstrates that it is not and that subsequent episodes of clinical pertussis are prevented by intermittent subclinical infection. Pertussis agglutinins were corre­ lated with protection in early studies of whole-cell pertussis vaccines. Antibodies to pertussis toxin, filamentous hemagglutinin, pertactin, and fimbriae are all protective in animal models. Serologic correlates of protection conferred by acellular pertussis vaccines have not been established, although antibody to pertactin, fimbriae, and (to a lesser degree) pertussis toxin correlated best with protection in two efficacy trials. The duration of immunity after whole-cell pertussis vaccina­ tion is short-lived, with little protection remaining after 10–12 years. Waning of immunity is even more rapid in adolescents and children who have received all their immunizations with acellular vaccines— i.e., within 2–4 years after the fifth or sixth dose. The type of immune response elicited may affect duration of protection; natural infection and whole-cell pertussis vaccine elicit a Th1/Th17-predominant response, whereas acellular pertussis vaccines stimulate a Th2-biased response. Controlled Human Infection Models of pertussis are under development to facilitate better understanding of immunity after infec­ tion and after vaccination. ■ ■CLINICAL MANIFESTATIONS Pertussis is a prolonged coughing illness with clinical manifestations that vary by age (Table 165-1). Although not uncommon among ado­ lescents and adults, classic pertussis is most often seen in preschool and TABLE 165-1  Clinical Features of Pertussis, by Age Group and Diagnostic Status PERCENTAGE OF PATIENTS ADOLESCENTS AND ADULTS INFANTS AND CHILDREN FEATURE Cough     Paroxysmal 70–99 89–93 Worse at night 61–87

Whoop 8–82 69–92 Post-tussive vomiting 17–65 48–60 Source: Reproduced with permission from PE Kilgore et al: Pertussis: Microbiology, disease, treatment, and prevention. Clin Microbiol Rev 29:449, 2016.

school-age children. After an incubation period averaging 7–10 days, an illness develops that is indistinguishable from the common cold and is characterized by coryza, lacrimation, mild cough, low-grade fever, and malaise. After 1–2 weeks, this catarrhal phase evolves into the par­ oxysmal phase: the cough becomes more frequent and spasmodic with repetitive bursts of 5–10 coughs, often within a single expiration. Posttussive vomiting is frequent, with a mucous plug occasionally expelled at the end of an episode. The episode may be terminated by an audible whoop, which occurs upon rapid inspiration against a closed glottis at the end of a paroxysm. During a spasm, there may be impressive neck-vein distension, bulging eyes, tongue protrusion, and cyanosis. Paroxysms may be precipitated by noise, eating, or physical contact. Between attacks, the patient’s appearance is normal, but increasing fatigue is evident. The frequency of paroxysmal episodes varies widely, from several per hour to 5–10 per day. Episodes are often worse at night and interfere with sleep. Most complications occur during the parox­ ysmal stage. Fever is uncommon and suggests bacterial superinfection. After 2–4 weeks, the coughing episodes become less frequent and less severe—changes heralding the onset of the convalescent phase. This phase can last 1–3 months and is characterized by gradual resolution of coughing episodes. For 6–12 months, intercurrent viral infections may be associated with a recrudescence of paroxysmal cough. Not all individuals who develop pertussis have classic disease. The clinical manifestations in adolescents and adults are more often atypical. The cough is severe, prolonged, and often paroxysmal. Though uncom­ mon, a whoop and vomiting with cough are more specific signs of pertussis in adults with prolonged cough. Other suggestive features are a cough at night, sweating episodes between paroxysms of coughing, and exposure to other individuals with a prolonged coughing illness. ■ ■COMPLICATIONS Complications are frequently associated with pertussis and are more common among infants than among older children or adults. Sub­ conjunctival hemorrhages, abdominal and inguinal hernias, pneumo­ thoraces, and facial and truncal petechiae can result from increased intrathoracic pressure generated by severe fits of coughing. Weight loss can follow decreased caloric intake. Urinary incontinence, rib fracture, carotid artery aneurysm, and cough syncope have also been reported in adolescents and adults with pertussis. In a series of >1100 children <2 years of age who were hospitalized with pertussis, 27.1% had apnea, 9.4% had pneumonia, 2.6% had seizures, and 0.4% had encephalopa­ thy; 10 children (0.9%) died. Pneumonia is reported in <5% of ado­ lescents and adults and increases in frequency after 50 years of age. In contrast to the primary B. pertussis pneumonia that develops in infants, pneumonia in older children, adolescents, and adults with pertussis is usually caused by a secondary infection with encapsulated organisms such as Streptococcus pneumoniae or Haemophilus influenzae. ■ ■DIAGNOSIS If the classic symptoms of pertussis are present, clinical diagnosis is not difficult. However, particularly in older children and adults, it is difficult to differentiate infections caused by B. pertussis and B. par­ apertussis from other respiratory tract infections on clinical grounds. Therefore, laboratory confirmation should be attempted in all cases. Lymphocytosis (absolute lymphocyte count >108–109/L) is common among young children, in whom it is unusual with other infections, but not among adolescents and adults. Culture of nasopharyngeal secre­ tions remains the gold standard of diagnosis because of its 100% speci­ ficity, although DNA detection by PCR has replaced culture in most laboratories because of substantially increased sensitivity and quicker results. Multitarget real-time PCR methodology includes primers that differentiate between B. pertussis, B. parapertussis, and B. holmesii. The best specimen is collected by nasopharyngeal aspiration, in which a fine flexible plastic catheter attached to a 10-mL syringe is passed into the nasopharynx and withdrawn while gentle suction is applied. Since B. pertussis is highly sensitive to drying, secretions for culture should be inoculated without delay onto appropriate medium (Bordet-Gengou or Regan-Lowe), or the catheter should be flushed with a phosphatebuffered saline solution for culture and/or PCR. An alternative to the

aspirate is a Dacron or rayon nasopharyngeal swab; again, inoculation of culture plates should be immediate or an appropriate transport medium (e.g., Regan-Lowe charcoal medium) should be used. Results of PCR can be available within hours; cultures become positive by day 5 of incubation.

Nasopharyngeal cultures in untreated pertussis remain positive for a mean of 3 weeks after the onset of illness; these cultures become negative within 5 days of the institution of appropriate antimicrobial therapy. The duration of a positive PCR in untreated pertussis or after therapy is not known but exceeds that of positive cultures. Since much of the period during which the organism can be recovered from the nasopharynx falls into the catarrhal phase, when the etiology of the infection is not suspected, there is only a small window of opportunity for culture- or PCR-proven diagnosis. Cultures and PCR from infants and young children are more frequently positive than those from older children and adults; this difference may reflect earlier presentation of the former age group for medical care. As a result of the difficulties with laboratory diagnosis of pertussis in adolescents, adults, and patients who have been symptomatic for

4 weeks, increasing attention is being given to serologic diagnosis. Enzyme immunoassays detecting IgA and IgG antibodies to pertussis toxin, filamentous hemagglutinin, pertactin, and fimbriae have been developed and assessed for reproducibility. Two- or fourfold increases in antibody titer are suggestive of pertussis, although cross-reactivity of some antigens (such as filamentous hemagglutinin and pertactin) among Bordetella species makes it difficult to depend diagnostically on seroconversion involving a single type of antibody. Criteria for serologic diagnosis based on comparison of results for a single serum specimen with established population values are gaining acceptance, and serologic measurement of antibody to pertussis toxin is becoming more widely standardized and available for diagnostic purposes, par­ ticularly in outbreak settings and for surveillance. CHAPTER 165 ■ ■DIFFERENTIAL DIAGNOSIS A child presenting with paroxysmal cough, post-tussive vomiting, and whoop is likely to have an infection caused by B. pertussis or B. paraper­ tussis; lymphocytosis increases the likelihood of a B. pertussis etiology. Viruses such as respiratory syncytial virus, rhinovirus, and adenovirus have been isolated from patients with clinical pertussis but probably represent co-infection, particularly in children <1 year of age. Pertussis and Other Bordetella Infections In adolescents and adults, who often do not have paroxysmal cough or whoop, the differential diagnosis of a prolonged coughing illness is more extensive. Pertussis should be suspected when any patient has a cough that does not improve within 14 days, a paroxysmal cough of any duration, a cough followed by vomiting (adolescents and adults), or any respiratory symptoms after contact with a laboratory-confirmed case of pertussis. Other etiologies to consider include infections caused by Mycoplasma pneumoniae, Chlamydia pneumoniae, adenovirus, influenza virus, and other respiratory viruses. Use of angiotensinconverting enzyme (ACE) inhibitors, reactive airway disease, and gas­ troesophageal reflux disease are well-described noninfectious causes of prolonged cough in adults. TREATMENT Pertussis ANTIBIOTICS The purpose of antibiotic therapy for pertussis is to eradicate the infecting bacteria from the nasopharynx; therapy does not sub­ stantially alter the clinical course unless given early in the catarrhal phase. Macrolide antibiotics are the drugs of choice for treatment of pertussis (Table 165-2); macrolide-resistant B. pertussis strains have been reported but are rare. Trimethoprim-sulfamethoxazole is rec­ ommended as an alternative for individuals allergic to macrolides. SUPPORTIVE CARE Young infants have the highest rates of complication and death from pertussis; therefore, most infants (and older children with

TABLE 165-2  Antimicrobial Therapy for Pertussis PRIMARY AGENTS ALTERNATE AGENT PATIENT AGE GROUP AZITHROMYCIN (AZ) ERYTHROMYCIN (ER) CLARITHROMYCIN <1 month AZ is the recommended agent for this age group, at 10 mg/kg per day in a single dose for 5 days ER is not preferred, only use if AZ unavailable; 40–50 mg/kg per day in 4 divided doses for 14 days 1-5 months 10 mg/kg per day in a single dose for 5 days 40–50 mg/kg per day in 4 divided doses for 14 days Infants (≥6 months) and children 10 mg/kg on day 1, 5 mg/kg mg per day (maximum 500 mg) on days 2–5 40–50 mg/kg per day (maximum

2 g/d) in 4 divided doses for

14 days Adults 500 mg on day 1, 250 mg/d on days 2–5 2 g/d in 4 divided doses for

14 days Comments Abdominal discomfort, prescribed with caution to cardiac patients Frequent gastrointestinal side effects, hypersensitivity reactions aTrimethoprim-sulfamethoxazole (TMP-SMZ) replaces macrolides only in the event of a macrolide-resistant strain of Bordetella pertussis or when patients older than 2 months are allergic to or cannot tolerate macrolides. Source: T Tiwari et al: Recommended antimicrobial agents for the treatment and postexposure prophylaxis of pertussis: 2005 CDC guidelines. MMWR Recomm Rep

54(RR-14):1, 2005. severe disease) should be hospitalized. A quiet environment may decrease the stimulation that can trigger paroxysmal episodes. Use of β-adrenergic agonists and/or glucocorticoids has been advocated by some authorities but has not been proven to be effective. Cough suppressants are not effective and play no role in the management of pertussis. INFECTION CONTROL MEASURES Hospitalized patients with pertussis should be placed in respiratory isolation, with the use of precautions appropriate for pathogens spread by large respiratory droplets. Isolation should continue for 5 days after initiation of macrolide therapy or, in untreated patients, for 3 weeks (i.e., until nasopharyngeal cultures are consistently negative). PART 5 Infectious Diseases ■ ■PREVENTION Chemoprophylaxis  Because the risk of transmission of B. pertussis within households is high, chemoprophylaxis is widely recommended for household contacts of pertussis cases regardless of their immuniza­ tion status and should be initiated within 21 days of cough onset in the index case. The effectiveness of chemoprophylaxis is supported by sev­ eral epidemiologic studies of institutional and community outbreaks. In the only randomized, placebo-controlled study, erythromycin esto­ late (50 mg/kg per day; maximum dose, 1 g/d) was effective in reducing the incidence of bacteriologically confirmed pertussis by 67%; how­ ever, there was no decrease in the incidence of clinical disease. Despite these results, authorities continue to recommend chemoprophylaxis, particularly in households with members at high risk of severe disease (children <1 year of age, pregnant women). Data on the use of the newer macrolides for chemoprophylaxis are not available, but these drugs are commonly used because of their increased tolerability and their effectiveness. Immunization (See also Chap. 129)  The mainstay of pertussis prevention is active immunization. Pertussis vaccine became widely used in North America after 1940; the reported number of pertussis cases subsequently fell by >90%. Whole-cell pertussis vaccines are pre­ pared through the heating, chemical inactivation, and purification of whole B. pertussis organisms. Despite their efficacy (average estimate, 85%; range for different products, 30–100%), whole-cell pertussis vaccines are associated with adverse events—both common (fever; injection-site pain, erythema, and swelling; irritability) and uncom­ mon (febrile seizures, hypotonic-hyporesponsive episodes). Alleged associations of whole-cell pertussis vaccine with encephalopathy,

TRIMETHOPRIM (TMP)- SULFAMETHOXAZOLE (SMZ)a Not recommended in this age group Contraindicated in infants <2 months 15 mg/kg per day in 2 divided doses for 7 days Only for infants ≥2 months (otherwise contraindicated); TMP 8 mg/kg per day, SMZ 40 mg/kg per day in 2 divided doses for 14 days 15 mg/kg per day (maximum 1 g/d) in 2 divided doses for 7 days TMP 8 mg/kg per day, SMZ 40 mg/kg per day in 2 divided doses for 14 days 1 g/d in 2 divided doses for 7 days TMP 320 mg/d, SMZ 1600 mg/d in 2 divided doses for 14 days Epigastric distress and cramps For patients allergic to macrolides, data on effectiveness are limited sudden infant death syndrome, and autism, although not substantiated, spawned an active anti-immunization lobby. The development of acel­ lular pertussis vaccines, which are effective and less reactogenic, has greatly alleviated concerns about the inclusion of pertussis vaccine in the combined infant immunization series. Although a wide variety of acellular pertussis vaccines were devel­ oped, only a few are still marketed widely; all contain pertussis toxoid and filamentous hemagglutinin. One acellular pertussis vaccine also contains pertactin, and another contains pertactin and two types of fimbriae. Adult formulations of acellular pertussis vaccines have been shown to be safe, immunogenic, and efficacious in clinical trials in adolescents and adults and are now recommended for routine immu­ nization of these groups in several countries. Although whole-cell vaccines are still used extensively in devel­ oping regions of the world, acellular pertussis vaccines are used exclusively for childhood immunization in much of the developed world. In light of evidence of early waning of immunity among chil­ dren who received acellular pertussis vaccine in infancy, the WHO Strategic Advisory Group of Experts (SAGE) recommends that countries using whole-cell pertussis vaccine for the primary infant immunization series continue to do so. In countries using acellular pertussis vaccines in infancy, additional booster immunizations in older children, adolescents, and adults are recommended to prevent pertussis in high-risk infants. Pertussis immunization is also recom­ mended during pregnancy to increase passive transfer of maternal antibodies to the fetus. Studies in high-income countries demonstrate that immunization of women during pregnancy is 90–93% effective at preventing pertussis in infants <2 months of age and is safe. In North America, acellular pertussis vaccines for children are given as a threedose primary series at 2, 4, and 6 months of age, with a reinforcing dose at 15–18 months of age and a booster dose at 4–6 years of age. Adolescents (11–18 years of age) and all unvaccinated adults should receive a dose of the adult-formulation diphtheria–tetanus–acel­ lular pertussis vaccine. Immunization is specifically recommended for health care providers, individuals in close contact with infants, and women at 27–36 weeks of every pregnancy, preferably in the earlier weeks of this range. Pertussis vaccine coverage among U.S. adolescents was 89.9% in 2022; among pregnant women, it was 55.4% during October 2022 to January 2023; and it was low among adults, at 30.1% in 2019. Further improvements in adult vaccine coverage may permit better control of pertussis across the age spectrum, with collateral protection of infants too young to be immunized. However, more effective vaccines with longer-lasting protection will ultimately be needed to control this disease.

49 - 166 Diseases Caused by Gram-Negative Enteric Bacilli

166 Diseases Caused by Gram-Negative Enteric Bacilli

■ ■FURTHER READING Craig R et al: Asymptomatic infection and transmission of pertussis in households: A systematic review. Clin Infect Dis 70:152, 2020. Forsyth KD et al: Recommendations to control pertussis prioritized relative to economies: A Global Pertussis Initiative update. Vaccine 36:7270, 2018. Havers FP et al: Use of tetanus toxoid, reduced diphtheria toxoid, and acellular pertussis vaccines: Updated recommendations of the Advi­ sory Committee on Immunization Practices—United States, 2019. MMWR Morb Mortal Wkly Rep 69:77, 2020. Ma L et al: Pertactin-deficient Bordetella pertussis, vaccine-driven evo­ lution, and reemergence of pertussis. Emerg Infect Dis 27:1561, 2021. Macina D et al: Estimating the pertussis burden in adolescents and adults in the United States between 2007 and 2019. Hum Vaccin Immunother 19:2208514, 2023. Miguelena Chamorro B et al: Bordetella bronchiseptica and Bordetella pertussis: Similarities and differences in infection, immuno-modula­ tion, and vaccine considerations. Clin Microbiol Rev 36:e0016422, 2023. Skoff TH: Sources of infant pertussis infection in the United States. Pediatrics 136:635, 2015. Tatti KM et al: Novel multitarget real-time PCR assay for rapid detec­ tion of Bordetella species in clinical specimens. J Clin Microbiol 49:4059, 2011. Winter K et al: Pertussis in California: A tale of 2 epidemics. Pediatr Infect Dis J 37:324, 2018. Wright J et al: Uptake of pertussis immunization in pregnancy and determinants of vaccination in Toronto, Canada. Vaccine 41: 6895, 2023. Thomas A. Russo, Yohei Doi

Diseases Caused by

Gram-Negative Enteric

Bacilli GENERAL FEATURES AND PRINCIPLES The postantibiotic era has begun. For most patients, this is the first time in their lives when an effective treatment for a bacterial infection may not exist. Species in the order Enterobacterales are at the forefront of this evolving public health crisis. For example, the Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) have designated carbapenem-resistant Enterobacterales (CRE) as representing a threat level of “urgent” and “priority one, critical.” CRE are estimated to have caused more than 100,000 deaths in 2019 globally, and the disease burden is especially high in low- and middleincome countries (e.g., Indian Subcontinent). These pathogens cause a wide variety of infections involving diverse anatomic sites, mostly in compromised hosts but also in healthy individuals. Therefore, a thor­ ough knowledge of clinical presentations and appropriate therapeutic choices is necessary for optimal outcomes. Escherichia coli, Klebsiella, Proteus, Enterobacter, Serratia, Citrobacter, Morganella, Providencia, Cronobacter, and Edwardsiella are enteric gram-negative bacilli (GNB) within the order Enterobacterales that commonly cause extraintestinal infections. Salmonella, Shigella, and Yersinia, which also are in the order Enterobacterales but more commonly cause gastrointestinal infections, are discussed in Chaps. 171, 172, and 176, respectively. ■ ■EPIDEMIOLOGY E. coli, Klebsiella, Proteus, Enterobacter, Serratia, Citrobacter, Morgan­ ella, Providencia, Cronobacter, and Edwardsiella are components of the

normal animal and human colonic microbiota and/or the microbiota in various environmental habitats, including long-term-care facilities (LTCFs) and hospitals. As a result, except for certain pathotypes of intestinal pathogenic E. coli, these genera are global pathogens. The incidence of infection due to these agents is increasing because of the combination of an aging population and increasing antimicrobial resis­ tance. In healthy humans, E. coli is the predominant species of GNB in the colonic microbiota, followed by Klebsiella and Enterobacter. GNB can also colonize the oropharynx and intact skin but, in healthy individuals, tend to do so only transiently. By contrast, in LTCFs and hospital settings, a variety of GNB emerge as the dominant colonizers of both mucosal and skin epithelial surfaces, particularly in associa­ tion with antimicrobial use, severe illness, and extended length of stay. LTCFs are emerging as an important reservoir for resistant GNB. Such colonization with GNB may lead to subsequent extraintestinal infection; for example, oropharyngeal colonization may lead to pneu­ monia, and colonic/perineal colonization may lead to urinary tract infection (UTI). The use of ampicillin or amoxicillin was associated with an increased risk of subsequent infection due to the hypervirulent pathotype of Klebsiella pneumoniae in Taiwan; this association suggests that changes in the quantity or prevalence of colonizing bacteria may significantly influence the risk of infection. Serratia, Enterobacter, and, less commonly, Citrobacter infection may be acquired directly through a variety of contaminated infusates (e.g., medications, blood products, non–U.S. Food and Drug Administration [FDA]-approved stem cell products). A multistate outbreak of highly resistant Serratia due to con­ taminated eyedrops has occurred. Edwardsiella infections are acquired through freshwater and marine environment exposures and are most common in Southeast Asia.

CHAPTER 166 ■ ■STRUCTURE AND FUNCTION Enteric GNB possess an extracytoplasmic outer membrane consisting of a lipid bilayer with associated proteins, lipoproteins, and polysac­ charides (capsule, lipopolysaccharide). The outer membrane interfaces with the external environment, including the human host. A variety of components of the outer membrane are critical determinants in patho­ genesis (e.g., capsule, lipopolysaccharide) and antimicrobial resistance (e.g., permeability barrier, efflux pumps). In addition, secreted prod­ ucts play an important role in both host infection (e.g., iron acquisition molecules) and environmental niche survival and colonization (e.g., type VI secretion systems). Diseases Caused by Gram-Negative Enteric Bacilli
■ ■PATHOGENESIS Multiple bacterial virulence factors are required for the pathogenesis of infections caused by GNB. Possession of specialized virulence genes defines pathogens and enables them to infect the host efficiently. Hosts and their cognate pathogens have been co-adapting throughout evo­ lutionary history. During the host–pathogen “chess match” over time, various and redundant strategies have emerged in both the pathogens and their hosts (Table 166-1). Intestinal pathogenic (diarrheagenic) mechanisms are discussed below. The members of the order Enterobacterales that cause extrain­ testinal infections are primarily extracellular pathogens and therefore share certain pathogenic features. The two principal components of host defense against Enterobacterales, regardless of species, are innate immunity (including intact skin and mucosal barriers; the withholding of nutrients; and the activities of complement, antimicrobial peptides, and professional phagocytes) and humoral immunity. Both suscep­ tibility to and severity of infection are increased with dysfunction or deficiencies of these host components. By contrast, the virulence traits of intestinal pathogenic E. coli—i.e., the distinctive strains that can cause diarrheal disease—are for the most part different from those of extraintestinal pathogenic E. coli (ExPEC) and other GNB that cause extraintestinal infections. This distinction reflects site-specific dif­ ferences in host environments, defense mechanisms, and physiologic derangements that lead to disease. A given enterobacterial strain usually possesses multiple adhesins for binding to a variety of host cells (e.g., in E. coli: type 1, S, and F1C fimbriae; P pili). Nutrient acquisition (e.g., of iron via

TABLE 166-1  Interactions of Extraintestinal Pathogenic Escherichia coli with the Human Host: A Paradigm for Extracellular, Extraintestinal, Gram-Negative Bacterial Pathogens BACTERIAL GOAL HOST OBSTACLE BACTERIAL SOLUTION Extraintestinal attachment Flow of urine, mucociliary escalator Multiple adhesins (e.g., type 1, S, and F1C fimbriae; P pili) Nutrient acquisition for growth Nutrient sequestration (e.g., iron via intracellular storage and extracellular scavenging via lactoferrin and transferrin) Cellular lysis (e.g., hemolysin), multiple mechanisms for competing for iron (e.g., siderophores) and other nutrients Initial avoidance of host bactericidal activity Complement, phagocytic cells, antimicrobial peptides Capsular polysaccharide, lipopolysaccharide Dissemination (within host and between hosts) Intact tissue barriers Irritant tissue damage resulting in increased excretion (e.g., toxins such as hemolysin), invasion of brain endothelium Late avoidance of host bactericidal activity Acquired immunity (e.g., specific antibodies), treatment with antibiotics Cell entry, acquisition of antimicrobial resistance siderophores) requires many genes that are necessary but not sufficient for pathogenesis. The ability to resist the bactericidal activity of com­ plement and phagocytes in the absence of antibody (e.g., as conferred by capsule or the O antigen component of lipopolysaccharide) is one of the defining traits of an extracellular pathogen. Tissue damage (e.g., as mediated by E. coli hemolysin) may facilitate nutrient acquisition and spread within the host. Without doubt, many important virulence genes await identification. PART 5 Infectious Diseases The ability to induce septic shock is another defining feature of these genera. GNB are the most common causes of this potentially lethal syndrome. Pathogen-associated molecular pattern molecules (PAMPs; e.g., the lipid A moiety of lipopolysaccharide) stimulate a proinflam­ matory host response via pattern recognition receptors (e.g., Toll-like or C-type lectin receptors) that activate host defense signaling path­ ways; if overly exuberant, this response results in shock (Chap. 315). Direct bacterial damage of host tissue (e.g., by toxins) or collateral damage from the host response can result in the release of damageassociated molecular pattern molecules (DAMPs; e.g., HMGB1) that can propagate a detrimental proinflammatory host response. Many antigenic variants (serotypes) exist in most genera of GNB. For example, E. coli has >150 O (somatic) antigens, 80 K (capsular) antigens, and 53 H (flagellar) antigens. This antigenic variability, which permits immune evasion and allows recurrent infection by different strains of the same species, has impeded vaccine development (Chap. 129). ■ ■INFECTIOUS SYNDROMES Depending on both the host and the pathogen, GNB can infect nearly every organ or body cavity. E. coli can cause either intestinal or extrain­ testinal infection, depending on the pathotype, and Edwardsiella tarda can cause both intestinal and extraintestinal infection. Klebsiella causes primarily extraintestinal infection, but a toxin-producing variant of Klebsiella oxytoca has been associated with hemorrhagic colitis, and Providencia alcalifaciens and Escherichia albertii have been associated with gastroenteritis. E. coli and—to a lesser degree—Klebsiella account for most extrain­ testinal infections due to GNB. These species (for K. pneumoniae, primarily its hypervirulent pathotype) are the most virulent pathogens within this group, as demonstrated by their ability to cause severe infections in healthy, ambulatory hosts from the community. However, the other genera of GNB are also important extraintestinal pathogens, especially among LTCF residents and hospitalized patients, in large part because of the intrinsic or acquired antimicrobial resistance of these organisms and the increasing number of individuals with

compromised host defenses. The mortality rate is substantial in many GNB infections and correlates with severity of illness, underlying host status, and in some cases the antimicrobial resistance of the infecting pathogen, which can result in suboptimal therapy. Especially problem­ atic are pneumonia, sepsis, and septic shock (arising from any site of infection), for which the associated mortality rates are 20–60%. ■ ■DIAGNOSIS Isolation of GNB from sterile sites almost always implies infection, whereas their isolation from nonsterile sites, particularly open wounds, the respiratory tract, and urine in the presence of an indwelling cath­ eter, requires careful clinical correlation to differentiate colonization from infection. Clinical microbiology laboratories are increasingly replacing identification by biochemical tests with newer diagnostic methodologies such as matrix-assisted laser desorption–ionization– time-of-flight mass spectrometry (MALDI-TOF-MS), nucleic acid amplification tests (NAATs) and sequencing, and immunoassays to enhance the sensitivity, accuracy, and rapidity of reporting on patho­ gen identification and resistance genes. This information can be used to increase the timeliness of initiation and/or the accurate selection of empirical antimicrobial therapy, thereby improving outcomes. These new diagnostic modalities have also resulted in the identification from clinical specimens of unfamiliar species (e.g., K. grimontii, Enterobacter hormaechei). It is best to assume such isolates possess a similar patho­ genic potential as their more familiar counterparts until more data become available. TREATMENT Principles Guiding Treatment in the Era of Increasing Antimicrobial Resistance (See also Chap. 149) Initiation of appropriate empirical anti­ microbial therapy early in the course of infections due to GNB (particularly the more serious ones) leads to improved outcomes. The ever-increasing prevalence of multidrug-resistant (MDR) and extensively drug-resistant (XDR) GNB; the lag between published and current resistance rates; and variations in antimicrobial suscep­ tibility by species, geographic location, regional antimicrobial use, and hospital site (e.g., intensive care units [ICUs] vs wards) neces­ sitate familiarity with evolving patterns of antimicrobial resistance for the selection of appropriate empirical therapy. Patient factors predictive of resistance in a given isolate include recent antimicrobial use, a health care association (e.g., recent or ongoing hospitalization, dialysis, residence in an LTCF, transplant, hematologic malignancy), or international travel (e.g., to Asia, Latin America, Africa, Eastern Europe). Of concern are an increasing number of reports of resistant Enterobacterales causing infections in ambulatory patients without known risk factors. In this era of increasing antimicrobial resistance, it is critical to culture the primary site of infection before initiating antimicrobial therapy and, for systemically ill patients, to obtain blood cultures. In vitro testing may not always detect antimicrobial resistance; therefore, it is important to assess the patient’s clinical response to treatment. Moreover (see discussion of AmpC β-lactamases below), resistance may emerge during therapy. In addition, drainage of abscesses, resection of necrotic tissue, and removal of infected for­ eign bodies, sometimes referred to collectively as “source control,” are often required for cure. For appropriately selected patients, it may be prudent initially, pending antimicrobial susceptibility results, to use two potentially active agents to increase the likelihood that at least one agent will be active against the patient’s organism. If broad-spectrum treatment has been initiated, it is important to switch to the most appropriate narrower-spectrum agent once antimicrobial suscep­ tibility results become available. Such responsible antimicrobial stewardship should help disrupt the ever-escalating cycle of selec­ tion for increasingly resistant bacteria, plus decrease the likelihood of Clostridioides difficile infection, decrease costs, and maximize

the useful longevity of available antimicrobial agents. Likewise, it is important to avoid treatment of patients who are colonized but not infected (e.g., who have a positive sputum culture without evidence of pneumonia, or a positive urine culture without clinical manifes­ tations of UTI). At present, the most reliably and broadly active antimicrobial agents in vitro against Enterobacterales are the carbapenems (except­ ing imipenem, to which Proteeae [Proteus, Morganella, Providencia] are intrinsically resistant); the aminoglycoside amikacin (excepting the Proteeae); the fourth-generation cephalosporin cefepime; the β-lactamase inhibitor combination agents piperacillin-tazobactam, ceftolozane-tazobactam, ceftazidime-avibactam, meropenemvaborbactam, and imipenem/cilastatin-relebactam; and the novel cephalosporin-siderophore cefiderocol. A limitation of imipenem/ cilastatin-relebactam; the tetracycline derivatives tigecycline, oma­ dacycline, and eravacycline; and the polymyxins B and E (colistin) (which are otherwise very active) is their poor activity against

Proteeae and Serratia. Furthermore, the tetracycline derivatives achieve suboptimal concentrations at several anatomic sites (including urine and blood). Clinical data are limited for cefidero­ col outside of UTIs and hospital-acquired ventilator-associated bacterial pneumonias; thus, caution is in order for serious infec­ tions at other sites. The number of antimicrobial agents active against certain strains of Enterobacterales is shrinking, and truly pandrug-resistant GNB exist. Accordingly, the currently available antimicrobial drugs must be used judiciously. Extensive resistance to available agents may leave the clinician with few or no ideal therapeutic options. However, use of a regimen that considers the site of infection, achievable drug levels at that site (e.g., higher concentrations of many agents in urine), and pharmacodynamically guided administration strategies (e.g., pro­ longed infusion of β-lactam agents to maintain drug levels above the minimal inhibitory concentration [MIC]) may increase the chance for a successful outcome. Point-of-care, NAAT-based identification of resistance mechanisms in GNB is becoming available and will enable a strain-specific, patient-specific, precision medicine–based treatment approach that would be predicted to improve outcome. GNB are commonly involved in polymicrobial infections, in which the role of each individual pathogen is uncertain (Chap. 182). Although some GNB are more pathogenic than others, it is usu­ ally prudent, if possible, to design an antimicrobial regimen active against all the GNB identified, because each is typically capable of pathogenicity in its own right. For patients treated initially with a broad-spectrum empirical regimen, the regimen should be deescalated as expeditiously as possible once susceptibility results are known and the patient has responded to therapy. Treatment duration is best individualized based on underlying host status and site of infection. However, for selected non–critically ill patients with source control and a satisfactory clinical response to therapy, 7 days of treatment may suffice for many infections. ANTIMICROBIAL TREATMENT AND RESISTANCE MECHANISMS The most common resistance mechanisms possessed by Enterobac­ terales are summarized in Table 166-2. Enzymatic hydrolysis (e.g., β-lactamases, of which >3000 variants have been described) and modification of antimicrobials are the major mediators of resistance in GNB and will be discussed below. Importantly, it is becoming increasingly recognized that MDR and XDR GNB often possess multiple plasmids and genes that encode for multiple β-lactamases. Broad-spectrum β-lactamases mediate resistance to many peni­ cillins and first-generation cephalosporins and are frequently expressed in enteric GNB. These enzymes are inhibited by all avail­ able β-lactamase inhibitors (e.g., clavulanate, sulbactam, tazobac­ tam, avibactam, relebactam, vaborbactam). In their wild-type form, they do not hydrolyze third- and fourth-generation cephalosporins or cephamycins (e.g., cefoxitin). Extended-spectrum β-lactamases (ESBLs) are modified broadspectrum enzymes that hydrolyze third-generation cephalosporins,

TABLE 166-2  Common Antimicrobial Resistance Mechanisms Possessed by the Enterobacterales ANTIMICROBIALS MOST SIGNIFICANTLY AFFECTED COMMON MEDIATORS

OF RESISTANCE MECHANISM Efflux Tetracyclines, fluoroquinolones (FQ) Efflux pumps Decreased permeability Fosfomycin Alterations in uptake system Target site alteration or overproduction FQ, trimethoprimsulfamethoxazole (TMPSMX), and polymyxins DNA gyrase or topoisomerase IV for FQ; enzymes for folic acid synthesis for TMP-SMX Lipid A for polymyxins Enzymatic hydrolysis of antimicrobials Penicillins, cephalosporins, cephamycins, carbapenems Broad-spectrum β-lactamases (e.g., TEM, SHV) ESBLs (e.g., CTX-M, modified TEM and SHV) AmpC β-lactamases Carbapenemases (e.g., serinebased KPC, OXA; metallobased NDM, VIM, IMP) Enzymatic modification of antimicrobials Aminoglycosides AAC, ANT, APH Abbreviations: AAC, N-acetyltransferases; ANT, O-adenylyltransferases; APH, O-phosphotransferases; CTX, cefotaxime β-lactamase; ESBL, extendedspectrum β-lactamase; IMP, active on imipenem; KPC, Klebsiella pneumoniae carbapenemase; NDM, New Delhi metallo-β-lactamase; OXA, oxacillinase; SHV, sulfhydryl reagent variable β-lactamase; TEM, Temoniera β-lactamase; VIM, Verona integron-mediated metallo-β-lactamase. CHAPTER 166 aztreonam, and (in some instances) fourth-generation cephalo­ sporins, in addition to the drugs hydrolyzed by broad-spectrum β-lactamases. GNB that produce ESBLs may also exhibit porin mutations that result in decreased uptake of relevant β-lactam agents (cephalosporins, β-lactam/β-lactamase inhibitor combina­ tions, and carbapenems), further reducing susceptibility to these agents. The prevalence of acquired ESBL production, particularly of CTX-M-type enzymes, is increasing in GNB worldwide, largely due to the presence of the corresponding genes on transferable plas­ mids, which also variably confer or are associated with resistance to fluoroquinolones, trimethoprim-sulfamethoxazole (TMP-SMX), aminoglycosides, tetracyclines, and (more recently) fosfomycin. To date, ESBLs are most prevalent in E. coli (especially ST131), K. pneumoniae, and K. oxytoca, but these enzymes can occur in all species of Enterobacterales. The approximate regional prevalence of ESBL-producing GNB currently follows a descending gradient as follows: China > Eastern Europe > other parts of Asia (e.g., India)

Latin America and Africa > Western Europe, the United States, Canada, and Australia. Travel to high-prevalence regions increases the likelihood of colonization with these strains. The incidence of community-acquired infections due to ESBL-producing Entero­ bacterales has increased worldwide, including in the United States. Diseases Caused by Gram-Negative Enteric Bacilli
Carbapenems are the most reliably active β-lactam agents against ESBL-producing strains. Piperacillin-tazobactam, when active in vitro, has been used as a carbapenem-sparing alternative, but recent data from the MERINO trial do not support its use for bloodstream infections. Ceftazidime-avibactam, ceftolozane-tazobactam (less active against Klebsiella, Enterobacter, and Citrobacter) are also active against most ESBL-producing strains but have limited clinical data that support potential utility. The roles for tigecycline, eravacy­ cline, and omadacycline are unclear despite these agents’ excellent in vitro activity against most Enterobacterales; however, they are inactive against Proteus, Morganella, Providencia, and Serratia. Oral options for the treatment of ESBL-producing strains are very limited. Fosfomycin, nitrofurantoin (for E. coli, 75–90% sus­ ceptible), pivmecillinam (recently approved in the United States), and omadacycline are the most reliably active agents. Older tetra­ cyclines (e.g., doxycycline and minocycline) also are often active,

although urine levels may be insufficient and clinical experience with gram-negative infections is limited.

AmpC β-lactamases, when induced or stably derepressed to high levels of expression, confer resistance to the same substrates as do ESBLs plus to the cephamycins (e.g., cefoxitin and cefotetan), except to the fourth-generation cephalosporins. The genes encod­ ing these enzymes are primarily chromosomal and therefore may not exhibit the linked resistance to TMP-SMX, aminoglycosides, and tetracyclines that is common with ESBLs. These enzymes are problematic for the clinician: resistance may develop dur­ ing therapy with third-generation cephalosporins and result in clinical failure, particularly in the setting of bacteremia. Although chromosomal AmpC β-lactamases are present in nearly all mem­ bers of the order Enterobacterales (with the notable exceptions of

K. pneumoniae, K. oxytoca, and Proteus mirabilis), the risk of clinically significant induction of high-level expression or selection of stably derepressed mutants with cephalosporin treatment is not uniform across species, being greatest with Enterobacter cloacae, Klebsiella (formerly Enterobacter) aerogenes, Citrobacter freundii, and Hafnia alvei, and less with Serratia marcescens, Providencia, and Morganella morganii. In addition, rare strains of E. coli, K. pneumoniae, and other Enterobacterales have acquired plasmids that contain AmpC β-lactamase genes. For AmpC-producing strains, carbapenems are an appropriate treatment option, especially for severely ill patients. Meta-analyses support piperacillin-tazobactam as a possible option. The fourthgeneration cephalosporin cefepime may be an appropriate option if the concomitant production of an ESBL can be excluded (a task that currently exceeds the capability of most clinical microbiol­ ogy laboratories) and source control is achieved. Ceftazidimeavibactam and cefiderocol are active in vitro, but clinical data are limited. Other carbapenem-sparing alternatives to consider if isolates are susceptible in vitro include fluoroquinolones, TMPSMX, and aminoglycosides. Tigecycline, eravacycline, and oma­ dacycline are active in vitro (except against Proteus, Morganella, Providencia, and Serratia). PART 5 Infectious Diseases Carbapenemases of Ambler class A (serine-β-lactamases; e.g., K. pneumoniae carbapenemase [KPC]) and class B (metallo-βlactamases [MBLs]; e.g., New Delhi metallo-β-lactamase [NDM], Verona integron-mediated metallo-β-lactamase [VIM], imipen­ emase [IMP]) confer resistance to the same drugs as do ESBLs, plus to cephamycins and carbapenems. By contrast, Ambler class D carbapenemases (serine-β-lactamases; e.g., oxacillinase-48 [OXA-48]) hydrolyze carbapenems and penicillins, but they have minimal activity against extended-spectrum cephalosporins. As with ESBLs, carbapenemase-encoding genes may be present on transferable plasmids, which often encode linked resistance to fluoroquinolones, TMP-SMX, tetracyclines, and aminoglycosides. Transposon-mediated spread (e.g., Tn4401 for KPC) also is impor­ tant. Although all major carbapenemases have been described around the globe, KPC is most common in the Americas, NDM in Asia, and OXA in Europe. Asymptomatic intestinal carriage of producing bacteria may facilitate spread. Carbapenemase-producing Enterobacterales (CPE) are most prevalent in K. pneumoniae, followed by Enterobacter spp. and

E. coli, but have been described in nearly all members of the order. M. morganii, Proteus, and Providencia exhibit intrinsic low-level imipenem resistance. A variety of genotypic and phenotypic meth­ ods can detect carbapenemase genes or activity, which could inform epidemiologic surveillance, infection control efforts, antimicrobial stewardship, and treatment decisions, especially if susceptibility data for selected agents are not available. For the treatment of infections due to Enterobacterales that produce class A or D carbapenemases (serine-β-lactamases; KPC, OXA), ceftazidime-avibactam is emerging as a first-line agent particularly for bacteremia, but suboptimal efficacy has been observed with pneumonia and in patients on renal replacement therapy, and resistance has developed in up to 10% of cases. Clini­ cal success against KPC-producing CRE has also been reported

for meropenem-vaborbactam and, to a lesser extent, imipenem/ cilastatin-relebactam; unlike ceftazidime-avibactam, however, neither of these agents is active against OXA-producing CRE. Ceftazidime, cefepime, and aztreonam are active against OXA48-producing CRE, unless other enzymes such as ESBL an AmpC are co-produced. Treatment of infections due to class B MBL–producing CRE is more challenging. The polymyxins B and E currently constitute one of the last lines of defense against strains that produce MBLs (e.g., NDM). However, these agents’ nephrotoxicity and neurotoxicity potential, their limited clinical efficacy, and the recent emergence of the polymyxin resistance threaten their utility. Aztreonam is stable against MBLs but is hydrolyzed by ESBLs and AmpC β-lactamases, which are often coproduced in XDR strains. Ongoing clinical trials are assessing aztreonam plus avibac­ tam, a promising combination with in vitro activity against class A, B, and D enzymes, for the treatment of CRE strains that produce MBLs like NDM. A currently available workaround involving approved drugs is co-administration of ceftazidime-avibactam and aztreonam; avibactam protects aztreonam from hydrolysis from ESBLs and AmpC β-lactamases. Cefiderocol is active in vitro against most strains producing KPC, MBLs, and OXA-48 (i.e., classes A, B, and D enzymes); clinical trials suggest that it may be as efficacious for serious CRE infections as the standard of care, but real-world data are limited. Although tigecycline, eravacycline, and omadacycline are active in vitro, pharmacokinetic-pharmacodynamic limitations exist, and along with the polymyxins, they exhibit poor activity against the tribe Proteeae and Serratia. Aminoglycosides, especially amikacin, may have some utility for combination therapy. Fosfomycin is often active in vitro, but clinical data in the treatment of serious infections due to CPE are limited and resistance may develop with monotherapy. Carbapenem resistance in the absence of carbapenemases can occur in the presence of ESBLs or AmpC β-lactamase production in combination with porin mutations (non-CP-CRE); however, most laboratories will not be able to differentiate CPE from non-CP-CRE. The non-CP-CRE phenotype is most commonly seen in E. coli and Enterobacter spp. In general, resistance to noncarbapenem antimi­ crobial classes is less, but data are limited on the optimal manage­ ment approach for non-CP-CRE. Resistance to classic β-Lactamase inhibitors is an uncommon (4% of E. coli/K. pneumoniae blood isolates) but increasingly recog­ nized phenotype that is characterized by resistance to β-lactamase inhibitors but not to third-generation cephalosporins. This mecha­ nism of resistance is distinct from production of ESBLs, AmpC β-lactamases, and carbapenemases, and it is still being delineated. Limited evidence suggests that ceftriaxone is an appropriate treat­ ment option for such strains. Resistance to newer β-lactamase inhibitors, especially avibactam, is increasingly reported in KPCproducing CPE and is due to mutations leading to structural modi­ fications of the KPC enzyme. These strains remain susceptible to meropenem-vaborbactam. Fluoroquinolone resistance is usually due to alterations in or protection of the target sites in DNA gyrase and topoisomerase IV, with or without decreased permeability and active efflux. Fluo­ roquinolone resistance is increasingly prevalent among GNB and is associated with resistance to other antimicrobial classes; for example, 20–80% of ESBL-producing enteric GNB are also resis­ tant to fluoroquinolones. At present, fluoroquinolones should be considered unreliable as empirical therapy for GNB infections in critically ill patients. Aminoglycoside resistance in Enterobacterales is conferred via enzymatic modification by N-acetyltransferases, O-adenylyltrans­ ferases, or O-phosphotransferases, which in turn affects ribosomal binding. Amikacin is less affected by these transferases than gen­ tamicin and tobramycin and therefore is generally more active. A yet uncommon resistance mechanism involves acquired 16S ribosomal RNA methyltransferases, which prevent all parenterally

administered aminoglycosides from binding to their target ribo­ somes. To date, these methyltransferases are most common in strains that produce MBLs (e.g., NDM). ■ ■PREVENTION (See also Chap. 147) Certain measures are broadly applicable for decreasing infection risk. Antimicrobial stewardship programs should be instituted to facilitate appropriate antimicrobial use, which will minimize the development of resistance. Diligent adherence to hand hygiene protocols by health care personnel and cleaning/disinfection or single-patient use of objects that come into contact with patients (e.g., stethoscopes and blood pressure cuffs) are essential. Indwelling devices (e.g., urinary and intravascular catheters) should be used only when necessary and inserted according to an appropriate protocol; protocols for daily-use evaluation and prompt removal should be implemented. Multiuse medication vials should be avoided if possible. Oral application of chlorhexidine decreases the incidence of pneumo­ nia among patients on ventilators. Increasing data support the imple­ mentation of universal decolonization (e.g., chlorhexidine bathing) to prevent infection in ICU patients or nursing home residents. The public health threat from CRE has resulted in additional recommen­ dations, especially for carbapenemase-producing CRE, which are an even greater concern. These recommendations include contact precau­ tions for patients colonized or infected with CRE, notification to the receiving facility from facilities transferring such a patient, and daily environmental cleaning. Screening of contacts and active surveillance for these bacteria also may be appropriate. ESCHERICHIA COLI INFECTIONS All E. coli strains share a core genome of ~2,000 genes. In con­ trast, an E. coli strain’s ability to cause infection and the nature of such infections are defined largely by accessory (i.e., noncore, nonessential) genes that encode various virulence factors. The compo­ sition of the E. coli accessory genome is continuously in flux, as dem­ onstrated by the recent evolution of Shiga toxin–producing enteroaggregative E. coli. ■ ■COMMENSAL STRAINS Commensal E. coli variants are an important constituent of the normal intestinal microbiota that confer benefits to the host (e.g., resistance to colonization with pathogenic organisms). Such strains generally lack the specialized virulence traits that enable extraintestinal and intestinal pathogenic E. coli strains to cause disease outside and within the gastrointestinal tract, respectively. However, even commensal

E. coli strains can be involved in extraintestinal infections in the pres­ ence of an aggravating factor, such as a foreign body (e.g., a urinary catheter), host compromise (e.g., local anatomic or functional abnor­ malities [including urinary or biliary tract obstruction] or systemic TABLE 166-3  Intestinal Pathogenic Escherichia coli PATHOTYPE EPIDEMIOLOGY CLINICAL SYNDROMEa DEFINING MOLECULAR TRAIT STEC/EHEC/ ST-EAEC Food, water, person-to-person; all ages, industrialized countries Hemorrhagic colitis, hemolyticuremic syndrome ETEC Food, water; young children in and travelers to developing countries Traveler’s diarrhea Heat-stable and labile enterotoxins, colonization factors EPEC Person-to-person; young children and neonates in developing countries Watery diarrhea, persistent diarrhea EIEC Food, water; children in and travelers to developing countries Watery diarrhea, occasionally dysentery EAEC ?Food, water; children in and travelers to developing countries; all ages, industrialized countries Traveler’s diarrhea, acute diarrhea, persistent diarrhea aClassic syndromes; see text for details on disease spectrum. bPathogenesis involves multiple genes, including genes in addition to those listed. Abbreviations: EAEC, enteroaggregative E. coli; EHEC, enterohemorrhagic E. coli; EIEC, enteroinvasive E. coli; EPEC, enteropathogenic E. coli; ETEC, enterotoxigenic E. coli; ST-EAEC, Shiga toxin–producing enteroaggregative E. coli; STEC, Shiga toxin–producing E. coli.

immunocompromise), or an inoculum that is large or contains a mixture of bacterial species (e.g., fecal contamination of the peritoneal cavity).

■ ■EXTRAINTESTINAL PATHOGENIC STRAINS ExPEC strains are the most common enteric GNB to cause communityacquired and health care–associated bacterial infections. The emerging propensity of these strains to acquire new mechanisms of antimicrobial resistance (e.g., FQ resistance mutations, ESBLs, carbapenemases) poses novel challenges in managing ExPEC infection. Several ExPEC clonal groups (e.g., sequence types [STs] ST131, ST95, ST69, ST73) are recognized to have undergone global dissemination. The mecha­ nisms underlying the epidemiologic success of such disseminated lineages remain an area of active investigation. In the case of ST131, efficient human-to-human transmission followed by colonization and long-term persistence within the intestinal microbiota appears to be a critical factor. Although acquisition of ESBL-producing E. coli from the food chain has been described, this appears to occur relatively uncommonly. Like commensal E. coli (but unlike intestinal pathogenic E. coli), ExPEC strains are often found in the intestinal microbiota of healthy individuals and, except for rare chimeric ExPEC/intestinal pathogenic E. coli strains, do not cause gastroenteritis in humans. Entry from their site of colonization (e.g., the colon, vagina, or oropharynx) into a normally sterile extraintestinal site (e.g., the urinary tract, peritoneal cavity, or lungs) is the rate-limiting step for infection. ExPEC strains have acquired accessory genes encoding diverse virulence factors that enable the bacteria to cause infections outside the gastrointestinal tract in both normal and compromised hosts (Table 166-1). These virulence genes define ExPEC and, for the most part, are distinct from the viru­ lence genes that enable intestinal pathogenic strains to cause diarrheal disease (Table 166-3). All age groups, all types of hosts, and nearly all organs and anatomic sites are susceptible to infection by ExPEC. Even previously healthy hosts can become severely ill or die when infected with ExPEC; however, adverse outcomes are more common among hosts with comorbid illnesses and host defense abnormalities. The diversity and the medical and economic impact of ExPEC infections are evident from consideration of the following specific syndromes. CHAPTER 166 Diseases Caused by Gram-Negative Enteric Bacilli
Extraintestinal Infectious Syndromes  •  URINARY TRACT INFECTION  The urinary tract is the site most frequently infected by ExPEC. UTI is an exceedingly common infection among ambulatory patients, accounting for 1% of ambulatory care visits in the United States and second only to lower respiratory tract infection among infections responsible for hospitalization. UTIs are best considered by clinical syndrome (e.g., cystitis, pyelonephritis, catheter-associated UTI) and within the context of specific hosts (e.g., premenopausal women, immunocompromised hosts; Chap. 140). E. coli is the single most common pathogen for all UTI syndrome/host group combinations. RESPONSIBLE GENETIC ELEMENTb Shiga toxin Lambda-like Stx1- or Stx2encoding bacteriophage Virulence plasmid(s) Localized adherence, attaching and effacing lesion on intestinal epithelium EPEC adherence factor plasmid pathogenicity island (locus for enterocyte effacement [LEE]) Invasion of colonic epithelial cells, intracellular multiplication, cell-to-cell spread Multiple genes contained primarily in a large virulence plasmid Aggregative/diffuse adherence, virulence factors regulated by AggR Chromosomal or plasmidassociated adherence and toxin genes

Each year in the United States, E. coli causes 80–90% of the estimated 6–8 million episodes of cystitis that occur in ambulatory, premeno­ pausal women with an anatomically and functionally normal urinary tract (i.e., uncomplicated cystitis). Furthermore, 20% of women with an initial cystitis episode develop frequent recurrences.

Uncomplicated cystitis, the most common acute UTI syndrome, is characterized by dysuria, urinary frequency and urgency, and supra­ pubic pain. Progression to more severe infection is rare; the natural history is slow spontaneous symptom resolution, which antimicrobial therapy hastens. Fever and/or back pain suggest progression to pyelo­ nephritis. Even when pyelonephritis is treated effectively, fever may take 5–7 days to resolve completely. Persistently elevated or increasing fever, flank pain, and neutrophil counts should prompt evaluation for intrarenal or perinephric abscess and/or obstruction. Pyelonephritis uncommonly causes renal parenchymal damage and loss of renal func­ tion, primarily in association with urinary obstruction, which can be preexisting or, rarely, occurs de novo in diabetic patients who develop renal papillary necrosis due to kidney infection. Pregnant women are at unusually high risk for developing pyelonephritis, which can adversely affect the outcome of pregnancy. As a result, prenatal screening for and treatment of asymptomatic bacteriuria during pregnancy are standard. Prostatic infection (prostatitis), a potential complication of UTI in men, can present either acutely (severe), which is rare, or in a chronic manner (recurrent cystitis), which is much more common. Acute pyelonephritis, acute prostatitis, and other systemic illnesses due to UTI can be designated collectively as urosepsis, febrile UTI, or systemic UTI, and may or may not be accompanied by bacteremia. The diagno­ sis and treatment of UTI, as detailed in Chap. 140, should be tailored to the individual host, the nature and site of infection, and local patterns of antimicrobial susceptibility. PART 5 Infectious Diseases ABDOMINAL AND PELVIC INFECTION  The abdomen/pelvis is the second most common site of extraintestinal infection due to E. coli. A wide variety of clinical syndromes occur in this location, includ­ ing acute peritonitis secondary to fecal contamination, spontane­ ous bacterial peritonitis, dialysis-associated peritonitis, diverticulitis, appendicitis, intraperitoneal or visceral abscesses (hepatic, pancreatic, splenic), infected pancreatic pseudocysts, and septic cholangitis and/or cholecystitis. In intraabdominal infections, E. coli can be isolated either alone or, as occurs more often, in combination with other facultative and/or anaerobic members of the intestinal microbiota (Chap. 137). PNEUMONIA  E. coli is not usually considered an important cause of pneumonia (Chap. 131). Indeed, enteric GNB account for only 1–3% of cases of community-acquired pneumonia, in part because these organisms colonize the oropharynx only transiently in a minority of healthy individuals. However, rates of oral colonization with E. coli and other GNB increase with severity of illness and antibiotic use. Consequently, GNB are a more common cause of pneumonia among residents of LTCFs and of hospital-acquired pneumonia (Chap. 147), particularly among postoperative and ICU patients (e.g., ventilatorassociated pneumonia). Pulmonary infection is usually acquired by small-volume aspira­ tion but occasionally occurs via hematogenous spread, in which case multifocal nodular infiltrates can be seen. Tissue necrosis, probably due in part to bacterial cytotoxins, is common. Despite significant institutional variation, E. coli is generally the third or fourth most commonly isolated type of GNB in hospital-acquired pneumonia, accounting for 5–8% of episodes in both U.S.-based and Europe-based studies. Regardless of the host, pneumonia due to ExPEC is a serious disease, with high crude and attributable mortality rates (20–60% and 10–20%, respectively). MENINGITIS  (See also Chap. 143) E. coli is one of the leading causes of neonatal meningitis, together with group B Streptococcus. Most E. coli strains that cause neonatal meningitis possess the K1 capsular antigen and derive from a limited number of meningitis-associated clonal groups (ST95, ST59, ST62). Ventriculomegaly occurs com­ monly. After the first month of life, E. coli meningitis is uncommon and usually accompanies surgical or traumatic disruption of the meninges or hepatic cirrhosis. In patients with cirrhosis who develop meningitis,

the meninges are presumably seeded due to poor hepatic clearance of portal vein bacteremia. CELLULITIS/MUSCULOSKELETAL INFECTION  E. coli contributes fre­ quently to infections of decubitus ulcers and occasionally to infections of lower-extremity ulcers and wounds in diabetic patients and other hosts with neurovascular compromise. Osteomyelitis secondary to contiguous spread can occur in these settings. E. coli also causes cel­ lulitis or infections of burn sites and surgical wounds (accounting for ~10% of surgical site infections), particularly when the infection origi­ nates close to the perineum. E. coli causes hematogenously acquired osteomyelitis, especially of vertebral discs and bodies, accounting for up to 10% of cases in some series (Chap. 136). E. coli occasionally causes orthopedic device–associated infection or septic arthritis and rarely causes hematogenous myositis. Myositis or fasciitis of the thigh due to E. coli should prompt an evaluation for an abdominal source with contiguous spread. ENDOVASCULAR INFECTION  Despite being one of the most common causes of bacteremia, E. coli rarely seeds native heart valves. When the organism does infect native valves, it usually does so in the setting of prior valvular disease. E. coli infections of aneurysms, the portal vein (pylephlebitis), and vascular grafts are uncommon. MISCELLANEOUS INFECTIONS  E. coli can cause infection in nearly every organ and anatomic site. It occasionally causes postoperative mediastinitis or complicated sinusitis and uncommonly causes endo­ phthalmitis, ecthyma gangrenosum, or brain abscess. BACTEREMIA  E. coli bacteremia can arise from infection at any extrain­ testinal site. In addition, E. coli bacteremia can arise from percutaneous intravascular devices, transrectal prostate biopsy, and the increased intes­ tinal mucosal permeability seen in neonates and patients with advanced cirrhosis, neutropenia, chemotherapy-induced mucositis, trauma, and extensive burns. E. coli bacteremia due to an ESBL-producing strain also has been reported after fecal microbiota transplant in patients with increased mucosal permeability. Roughly equal proportions of E. coli bacteremia cases originate in the community and in health care settings. Isolation of E. coli from the blood is almost always clinically significant and may be accompanied by the sepsis syndrome (dysfunction of at least one organ or system) or septic shock (Chap. 315). The urinary tract is the most common source for E. coli bacteremia, accounting for one-half to two-thirds of episodes. Bacteremia from a urinary tract source is particularly common among patients with pyelonephritis, urinary tract obstruction, or urinary instrumentation in the presence of infected urine. The abdomen is the second most common source, accounting for ~25% of episodes. Although many of these episodes result from biliary obstruction (stones, tumor) and overt bowel disruption, which typically are readily apparent, some abdominal sources (e.g., abscesses) are remarkably silent clinically and require identification via imaging studies (e.g., computed tomogra­ phy). Therefore, especially given the high prevalence of asymptomatic bacteriuria among elderly and functionally compromised individuals, the physician should be cautious in attributing E. coli bacteremia to a urinary source in the absence of characteristic signs and symptoms of UTI. Soft tissue, bone, pulmonary infections, and intravascular cath­ eter infections are other sources of E. coli bacteremia. Diagnosis  Strains of E. coli that cause extraintestinal infections usu­ ally grow both aerobically and anaerobically within 24 h on standard diagnostic media and are identified readily by the clinical microbiology laboratory according to routine biochemical criteria. More than 90% of ExPEC strains are rapid lactose fermenters and are indole-positive. However, MALDI-TOF MS is increasingly replacing biochemical methods. TREATMENT Extraintestinal E. coli Infections E. coli does not possess clinically significant intrinsic resistance to antimicrobials; however, increasing acquired resistance is

making treatment problematic. Although geographic differences exist, in general, the prevalence of resistance is >20% for ampi­ cillin, amoxicillin-clavulanate, ampicillin-sulbactam, cefazolin, TMP-SMX, and fluoroquinolones, even in community-acquired infections. This resistance precludes empirical use of these agents for serious infections. Travel outside of the United States, prior exposure to an antimicrobial agent, or exposure to a health care setting further increases the likelihood of resistance. Fortunately,

90% of isolates that cause uncomplicated cystitis remain suscep­ tible to nitrofurantoin and fosfomycin. From 2015 to 2017, the U.S. National Healthcare Safety Network (USNHSN) identified 24% of E. coli clinical isolates as ESBLproducers. Higher prevalences are reported from Asia, Eastern Europe, South America, and Africa; prevalence is also greater in isolates from health care settings, especially LTCFs. Unfortunately, community-acquired UTIs caused by E. coli strains that produce CTX-M ESBLs are increasingly common. Oral treatment options for ESBL-producers are limited. However, in vitro and limited clini­ cal data indicate that fosfomycin, pivmecillinam, and nitrofurantoin are most active and can be used for cystitis (but not pyelonephritis); omadacycline is an option for pulmonary of soft-tissue infection. For parenteral therapy of carbapenem-susceptible strains, the most predictably active agents (>90%) include carbapenems, ami­ kacin, ceftazidime-avibactam, ceftolozane-tazobactam, piperacillintazobactam, polymyxins, cefiderocol, tigecycline, eravacycline, and omadacycline with the caveat that site-specific concentration and potential efficacy are agent dependent. Treatment of carbapenemaseproducing strains is dependent on the class of enzyme produced (see “Carbapenemase” above). Uncertainty exists on the optimal treatment for non-CP-CR E. coli. Empirical treatment decisions for critically ill patients should be dictated by local susceptibility patterns and patient-specific risk fac­ tors (1.2% prevalence from the USNHSN 2015−2017 data). Equally important as prompt institution of effective empirical therapy for seriously ill patients is use of appropriate narrower-spectrum agents for definitive therapy whenever possible and avoidance of treatment for patients who are colonized but not infected. ■ ■INTESTINAL PATHOGENIC STRAINS Pathotypes  Certain strains of E. coli are capable of causing diar­ rheal disease. (Other important intestinal pathogens are discussed in Chaps. 138, 139, and 171–174.) At least in the industrialized world, intestinal pathogenic E. coli strains are rarely encountered in the fecal flora of healthy persons, and instead appear to be essentially obligate pathogens. These strains have evolved a special ability to cause enteritis, enterocolitis, and colitis when ingested in sufficient quantities by a naïve host. At least five distinct pathotypes of intestinal pathogenic E. coli exist: (1) Shiga toxin–producing E. coli (STEC), which includes the subsets enterohemorrhagic E. coli (EHEC) and the recently evolved Shiga toxin–producing enteroaggregative E. coli (STEAEC); (2) enterotoxigenic E. coli (ETEC); (3) enteropathogenic E. coli (EPEC); (4) enteroinvasive E. coli (EIEC); and (5) enteroaggregative

E. coli (EAEC). Diffusely adherent E. coli (DAEC) and cytodetach­ ing E. coli are additional putative pathotypes. Lastly, a variant termed adherent invasive E. coli (AIEC) has been associated with Crohn disease (although a causal role remains unproven) but does not cause acute diarrheal disease. Contaminated food and water are the primary transmission vehicles for ETEC, STEC/EHEC/ST-EAEC, EIEC, and EAEC, whereas personto-person spread (direct or indirect) is the primary transmission route for EPEC and a secondary transmission route for STEC/EHEC/ ST-EAEC. Gastric acidity confers some protection against infection; therefore, persons with decreased stomach acid levels are especially susceptible. Humans are the major reservoir for such strains (except for STEC/EHEC, for which bovines are the main carriers); host range appears to be dictated by species-specific attachment factors. Although some overlap exists, each pathotype possesses a distinctive combina­ tion of virulence traits that results in a pathotype-specific pathogenic

mechanism (Table 166-3). With rare exceptions (e.g. DAEC), these strains are largely incapable of causing disease outside the intestinal tract. Whereas disease due to STEC/EHEC/ST-EAEC occurs primar­ ily in high-income countries, disease due to ETEC, EPEC, and EIEC occurs primarily in low- and middle-income countries in Asia, Africa, and Latin America, and disease due to EAEC occurs globally.

SHIGA TOXIN–PRODUCING E. COLI  STEC/EHEC/ST-EAEC strains are pathogens that can cause hemorrhagic colitis and the hemolyticuremic syndrome (HUS). In contrast to other intestinal pathotypes, STEC/EHEC/ST-EAEC causes infections more frequently in highincome countries than in low and middle-income countries (LMICs). Several large outbreaks resulting from the consumption of fresh produce (e.g., lettuce, spinach, sprouts) and of undercooked ground beef have received significant media attention. In addition, a dramatic 2011 outbreak—mainly in Germany—involved an EAEC strain that acquired a Shiga toxin–encoding phage, resulting in a novel genotype, ST-EAEC (O104:H4). This strain was transmitted to the primary cases by sprouted fenugreek seeds, with subsequent human-to-human trans­ mission, and resulted in >4000 cases and 54 deaths. STEC strains are the fourth most commonly reported cause of bac­ terial diarrhea in the United States (after Campylobacter, Salmonella, and Shigella). O157:H7 is the most prominent serotype among STEC strains, but many other serogroups have been described, including O6, O26, O45, O55, O91, O103, O111, O113, O121, and O145. Domesti­ cated ruminant animals, particularly cattle and young calves, serve as the major reservoir for STEC/EHEC. Ground or mechanically tender­ ized beef—the most common food source of STEC/EHEC strains—is often contaminated with intestinal bacteria from the source ani­ mals during processing. Furthermore, manure from cattle or other animals (including in the form of fertilizer) can contaminate produce (potatoes, lettuce, spinach, sprouts, fallen fruits, nuts, strawberries), and fecal runoff from manure can contaminate water systems. Dairy products and petting zoos are additional sources of infection. CHAPTER 166 Diseases Caused by Gram-Negative Enteric Bacilli
It is estimated that <102 colony-forming units (CFU) of STEC/ EHEC/ST-EAEC can cause disease. Therefore, not only can low levels of food or environmental contamination (e.g., in water swallowed while swimming) result in disease, but person-to-person transmission (e.g., at day-care centers and in institutions) is an important route for secondary spread. Laboratory-associated infections also occur. Illness due to this group of pathogens peaks in the summer months and occurs both as outbreaks and as sporadic cases. For STEC/EHEC/ST-EAEC, production of Shiga toxin (Stx2a–g and/or Stx1a,c,d) is a critical factor for occurrence of clinical dis­ ease, as demonstrated by the 2011 ST-EAEC outbreak. The stx gene is present on chromosomally integrated prophages, and various combinations of stx types and subtypes can occur in a given strain. Shigella dysenteriae strains that produce the closely related Shiga toxin Stx can also cause hemorrhagic colitis and HUS. Stx2 (especially Stx2a,c,d) appears to be more important than Stx1 in the development of HUS. All Shiga toxins studied to date are multimers; they comprise one A subunit that is enzymatically active and five identical B subunits that mediate binding to globosyl ceramides, which are membraneassociated glycolipids expressed on certain host cells. As in ricin, the Stx A subunit cleaves an adenine from the host cell’s 28S rRNA, thereby irreversibly inhibiting ribosomal function (i.e., protein synthesis) and potentially leading to apoptosis. For full pathogenicity, STEC strains require additional properties such as acid tolerance and epithelial cell adherence. Most disease-caus­ ing isolates possess the chromosomal locus for enterocyte effacement (LEE). This pathogenicity island was first described in EPEC strains; it contains genes that mediate adherence to intestinal epithelial cells and a system that subverts host cells by the translocation of bacterial pro­ teins (type III secretion system). EHEC strains make up the subgroup of STEC strains that possess stx1 and/or stx2, as well as LEE. By con­ trast, the 2011 ST-EAEC outbreak strain lacked LEE yet was associated with a higher proportion of patients developing HUS (22%) than the historic average for STEC/EHEC outbreaks (2–8%). Data support the essential role of the 2011 outbreak strain’s EAEC-associated virulence

factors (e.g., AAF/I fimbriae, serine proteases SigA, SepA) in adher­ ence, increased inflammation, and disruption of the intestinal epithe­ lial barrier, which in turn increased the systemic translocation of Stx2a.

After exposure to STEC/EHEC/ST-EAEC and a 3- to 4-day incuba­ tion period, colonization of the colon and perhaps the ileum results in symptoms. Colonic edema and an initial nonbloody secretory diarrhea may progress to the hallmark syndrome of grossly bloody diarrhea (iden­ tified by history or examination). Significant abdominal pain and fecal leukocytes are common (70% of cases), whereas fever is not; absence of fever can incorrectly lead to consideration of noninfectious conditions (e.g., intussusception and inflammatory or ischemic bowel disease). Occasionally, infections caused by C. difficile, K. oxytoca (see “Klebsiella Infections,” below), Campylobacter, and Salmonella present in a similar fashion. STEC/EHEC disease is usually self-limited, lasting 5–10 days. A feared complication of infection with STEC/EHEC strains is HUS, which occurs 2–14 days after diarrhea, most often in young children (estimated to occur in 15% of infected children <10 years of age) or elderly patients. It is estimated that in the United States >50% of all HUS cases—and 90% of HUS cases in children, which is a leading cause of acute renal failure in this latter population—are caused by STEC/EHEC. By contrast, with ST-EAEC infection, HUS occurs more commonly among nonelderly adults, especially young women. HUS is mediated by the systemic translocation of Shiga toxins. Erythrocytes may serve as carriers of Stx to endothelial cells located in the small vessels of the kidney and brain. The subsequent development of throm­ botic microangiopathy (perhaps with direct toxin-mediated effects on various nonendothelial cells) commonly produces some combination of fever, hemolytic anemia (hematocrit <30%), thrombocytopenia (<150,000/mm3), renal failure, and encephalopathy. Stx-mediated complement activation also plays a role in the development of HUS. Although with dialysis support the mortality rate of HUS is <10%, sur­ vivors often have persisting renal and neurologic dysfunction. PART 5 Infectious Diseases ENTEROTOXIGENIC E. COLI  ETEC is a major cause of endemic diar­ rhea in low- and middle-income countries and is responsible for an estimated 800 million cases annually. After weaning, children in these locales commonly experience several episodes of ETEC infection dur­ ing the first 3 years of life. The incidence of disease diminishes with age, a pattern that correlates with the development of mucosal immu­ nity to colonization factors (i.e., adhesins). In industrialized countries, ETEC is the most common agent of traveler’s diarrhea, causing 25–75% of cases. The incidence of infection may be decreased by prudent avoidance of potentially contaminated fluids and foods, particularly items that are raw, insufficiently cooked, peeled, or unrefrigerated (Chap. 130). ETEC infection is uncommon in the United States, but outbreaks secondary to consumption of food products imported from endemic areas have occurred. A large inoculum (106–108 CFU) is needed to produce disease, which usually develops after an incubation period of 12–72 h. After adherence of ETEC to enterocytes via colonization factors (e.g., CFA/I, CS), disease is mediated, primarily by a heat-labile toxin (LT) and/or a heat-stable toxin (ST), leading to diarrheal disease. Disease is less severe with strains that produce only LT. Both LT and ST cause net fluid secretion via activation of adenylate cyclase and/or guanylate cyclase C (ST) in the jejunum and ileum. The result is watery diarrhea accompanied by cramps. LT consists of an A and a pentameric B subunit and is structur­ ally and functionally similar to cholera toxin. Strong binding of the B subunit to the GM1 ganglioside on intestinal epithelial cells leads to the intracellular translocation of the A subunit, which functions as an ADP-ribosyltransferase. Mature ST is an 18- or 19-amino-acid secreted peptide that leads to increased intracellular concentrations of cGMP. Characteristically absent in ETEC-mediated disease are histopatho­ logic changes within the small bowel; mucus, blood, and inflammatory cells in stool; and fever. The disease spectrum of ETEC infection ranges from mild illness to a life-threatening, cholera-like syndrome. Although symptoms are usu­ ally self-limited (typically lasting for 3–5 days), infection may result in significant morbidity and mortality (>250,000 deaths annually, mostly

from profound volume depletion) when access to health care or suit­ able rehydration fluids is limited and when small and/or undernour­ ished children are affected. ENTEROPATHOGENIC E. COLI  EPEC causes disease primarily in young children, including neonates. The first E. coli pathotype recognized as an agent of diarrheal disease, EPEC was responsible for outbreaks of infantile diarrhea (including in hospital nurseries) in industrial­ ized countries in the 1940s and 1950s. At present, EPEC infection is uncommon in high-income countries, but among infants in low- and middle-income countries, it is an important cause of diarrhea (both sporadic and epidemic), often accompanied by vomiting and fever. Breast-feeding diminishes the incidence of EPEC infection. Rapid person-to-person spread may occur. Symptoms develop after colonization of the small bowel and a brief incubation period (1 or 2 days). Initial localized adherence to enterocytes via type IV bundle-forming pili leads to a charac­ teristic effacement of microvilli, with the formation of cuplike, actinrich pedestals mediated by factors in the LEE. Diarrhea production is a complex and regulated process in which host cell modulation by a type III secretion system plays an important role. Strains lacking bundleforming pili have been categorized as atypical EPEC (aEPEC); increas­ ing data support a role for these strains as intestinal pathogens in all age groups and among HIV-infected individuals. Diarrheal stool often contains mucus but not blood. Although EPEC diarrhea is usually selflimited (lasting 5–15 days), it may persist for weeks. ENTEROINVASIVE E. COLI  EIEC, a relatively uncommon (or per­ haps underrecognized) cause of diarrhea, is rarely identified in the United States, although a few food-related outbreaks have been described. In low- and middle-income countries, sporadic disease is recognized infrequently in children and travelers. EIEC shares many genetic and clinical features, as well as a common ancestor, with Shigella. Both are intracellular pathogens for which viru­ lence is mediated by the presence of specific factors and by the loss or inactivation of other factors (antivirulence genes), which presumably occurred during these organisms’ transition from an extracellular to an intracellular lifestyle. Colonization and invasion of the colonic mucosa, followed by replica­ tion therein and cell-to-cell spread (in part via a type III secretion sys­ tem), result in the development of inflammatory colitis. However, unlike Shigella, EIEC produces disease only with a large inoculum (108–1010 CFU) and is less virulent, typically causing only mild, self-limited (7–10 days), watery diarrhea. Onset generally follows an incubation period of 1–3 days. Occasionally, EIEC can cause a shigellosis-like (dysentery) syndrome characterized by fever, abdominal pain, tenesmus, and scant stool containing mucus, blood, and inflammatory cells. ENTEROAGGREGATIVE AND DIFFUSELY ADHERENT E. COLI  EAEC has been described primarily in low- and middle-income countries and in young children. However, recent studies indicate that it may also be a relatively common cause of diarrhea in all age groups in industrialized countries. EAEC has been recognized increasingly as an important cause of traveler’s diarrhea. It is highly adapted to humans—the prob­ able reservoir. A large inoculum is required for infection, which usually manifests as watery and sometimes persistent diarrhea in healthy but also malnourished or HIV-infected hosts. In vitro, EAEC cells exhibit a diffuse or “stacked-brick” pattern of adherence to small-intestine epithelial cells. Virulence factors that probably are necessary for disease are regulated in large part by the transcriptional activator AggR. The pathogenesis of EAEC disease begins with intestinal adherence, which results from stimulation of epithelial mucus production and bacterial biofilm formation, the latter mediated by fimbriae and possibly the mucinase Pic and dispersin. Inflammation ensues, resulting in epithelial cell exfoliation and intestinal secretion, which is mediated by the enterotoxins Pet, EAST-1, ShET1, and HlyE. An additional enteric pathotype, DAEC, is a heterogenous group associated with diarrheal disease, primarily in children 2–6 years of age in some LMICs, and may cause traveler’s diarrhea. DAEC can also cause UTI. Diffuse adherence is observed on epithelial cells. The Afa/ Dr adhesins may contribute to the pathogenesis of such infections.

Diagnosis  Acute infectious diarrhea can be classified as nonin­ flammatory or inflammatory; the latter is suggested by grossly bloody or mucoid stools or a positive test for fecal leukocytes, lactoferrin, or calprotectin (Chap. 138). ETEC, EPEC, DAEC, and EAEC cause non­ inflammatory diarrhea. Identification of these agents can be achieved with commercial multiplex molecular panels (e.g., the BioFire FilmArray Gastrointestinal Panel can detect STEC, ETEC, EPEC, EAEC, and EIEC). However, organism identification is rarely needed because the associated diseases are self-limited. ETEC causes the majority and EAEC a minority of cases of noninflammatory traveler’s diarrhea; here again, however, definitive diagnosis generally is not necessary for man­ agement (as discussed below). If diarrhea persists for >10 days despite treatment, Giardia or Cryptosporidium (or, in immunocompromised hosts, certain opportunistic pathogens) should be sought. Because of the considerable public-health importance of STEC/ EHEC/ST-EAEC infections, including the threat of HUS, the CDC now recommends that all patients with community-acquired diarrhea, whether inflammatory or not, be evaluated for these pathogens by simultaneous culture (to provide an isolate for strain typing and for outbreak detection and control) and detection of Shiga toxin or the corresponding genes. The rationale for testing all cases of communityacquired diarrhea, regardless of clinical features, is that bloody stool and fecal white blood cells (or lactoferrin) are not reliably present with STEC/EHEC/ST-EAEC infection. In addition, the use of both tests increases diagnostic sensitivity over that with either test alone. O157 STEC/EHEC may be identified via culture by screening for E. coli strains that do not ferment sorbitol, with subsequent serotyp­ ing and testing for Shiga toxin. Selective or screening media are not available for culture-based detection of non-O157 STEC/EHEC/ ST-EAEC strains. Detection of Shiga toxins or toxin genes via DNAbased, enzyme-linked immunosorbent, and cytotoxicity assays offers the advantages of rapidity and detection of non-O157 STEC/EHEC/ ST-EAEC strains. Specimens positive for toxin but culture-negative for O157 should be forwarded to the local or state public-health laboratory for specialized testing. TREATMENT Intestinal E. coli Infections The mainstay of treatment for all diarrheal syndromes is replace­ ment of water and electrolytes. This measure is especially important for STEC/EHEC/ST-EAEC infection because appropriate volume expansion may protect against renal injury and improve outcome. The use of prophylactic antibiotics to prevent traveler’s diarrhea generally should be discouraged, especially in light of high rates of antimicrobial resistance. However, in selected patients (e.g., those who cannot afford a brief illness or are predisposed to infection), the use of rifaximin, which is nonabsorbable and is well tolerated, is reasonable. When stools are free of mucus and blood, early patient-initiated treatment of traveler’s diarrhea with a fluoroquinolone or azithro­ mycin decreases the duration of illness, and the use of loperamide may halt symptoms within a few hours. Although dysentery caused by EIEC is self-limited, antimicrobial therapy hastens the resolution of symptoms, particularly in severe cases. By contrast, antimicro­ bial therapy for STEC/EHEC/ST-EAEC infection (the presence of which is suggested by grossly bloody diarrhea without fever) should be avoided because antibiotics may increase the incidence of HUS (possibly via increased production/release of Stx). In the treatment of HUS, plasmapheresis is not recommended and the use of eculi­ zumab (inhibition of C5) should be limited to clinical trials. KLEBSIELLA INFECTIONS K. pneumoniae is the most important Klebsiella species from a medical standpoint, causing community-acquired, LTCF-acquired, and noso­ comial infections. K. oxytoca complex and K. (formerly Enterobacter) aerogenes are primarily pathogens in LTCFs and hospitals. Klebsiella

species are broadly prevalent in the environment and colonize the mucosal surfaces of mammals. In healthy humans, the prevalence of K. pneumoniae colonization is 5–35% in the colon and 1–5% in the oropharynx; skin is usually colonized only transiently.

Most Klebsiella infections in Western countries are caused by “classic”

K. pneumoniae (cKp) and occur in hospitals and LTCFs. The most common clinical syndromes due to cKp are pneumonia, UTI, abdomi­ nal infection, intravascular device infection, surgical site infection, soft tissue infection, and secondary bacteremia. cKp strains have gained notoriety because of their propensity for acquiring treatmentconfounding antimicrobial resistance determinants and causing both localized and widespread outbreaks, such as with the global spread of cKp strains producing NDM-group MBLs. Clonal groups 11, 15, 101, 307, and 258, many members of which produce carbapenemases, are undergoing international dissemination. Transmission within or between institutions is common. K. pneumoniae is nearly fourfold more transmissible than E. coli, and, disconcertingly, carbapenemaseproducing strains are associated with increased spread compared with carbapenem-susceptible strains. In addition, hypervirulent K. pneumoniae (hvKp) strains that are phenotypically and clinically distinct from cKp have emerged recently, after their initial recognition in Taiwan in 1986. Although hvKp infections have occurred globally in all ethnic groups, most cases have been reported in individuals of Asian ethnicity residing in countries from the Asian Pacific Rim, but also in Asians living in other countries. Affected individuals often have diabetes mel­ litus. These demographics raise the possibility of a locale-specific distribution of the organism or an increased susceptibility of Asian hosts, especially those who are diabetic. In contrast to the usual health care–associated context for cKp infections in the West, hvKp can cause serious life- and organ-threatening infections in younger, healthy individuals from the community and can spread metastati­ cally from the primary site of infection or present with multiple sites of infection. Of concern, recent reports from Asian countries have demonstrated that hvKp is responsible for an increasing number of health care–associated or hospital-acquired infections. CHAPTER 166 Diseases Caused by Gram-Negative Enteric Bacilli
hvKp infection initially was characterized and distinguished from traditional infections caused by cKp strains by its (1) presenta­ tion as community-acquired monomicrobial pyogenic liver abscess

(Fig. 166-1, top), (2) occurrence in patients lacking a history of hepatobiliary disease, and (3) propensity for metastatic spread to distant sites. Subsequently, the hvKp pathotype has been recognized as the cause of extrahepatic abscesses and infections with or without liver involvement, including pneumonia; meningitis (in the absence of trauma or neurosurgery); endophthalmitis (Fig. 166-1, middle); splenic, psoas, prostatic, epidural, and brain abscesses; and necrotiz­ ing fasciitis. Survivors often suffer catastrophic morbidity, such as vision loss and major neurologic sequelae. Most recently, clinicians are faced with an even greater challenge—the confluence of antimi­ crobial resistance determinants such as carbapenemase and ESBL genes possessed by cKp and the virulence factors possessed by hvKp on the same or coexisting plasmids. The result is the evolution of MDR and XDR hvKp. K. pneumoniae subspecies rhinoscleromatis is the causative agent of rhinoscleroma, a granulomatous mucosal upper-respiratory infection that progresses slowly (over months or years) and causes necrosis and occasionally obstruction of the nasal passages. K. pneumoniae subspe­ cies ozaenae has been implicated as a cause of chronic atrophic rhinitis and rarely of invasive disease in compromised hosts. K. (Calymma­ tobacterium) granulomatis, a sexually transmitted pathogen, is the causative agent of granuloma inguinale (donovanosis) that results in chronic genital ulcers (Chap. 178). These Klebsiella pathotypes are usually isolated from patients in tropical climates and are genomically distinct from both cKp and hvKp. ■ ■INFECTIOUS SYNDROMES Pneumonia  Although cKp accounts for only a small proportion of cases of community-acquired pneumonia in Western countries

PART 5 Infectious Diseases FIGURE 166-1  Hypervirulent pathotype of K. pneumoniae (hvKp). Top: Abdominal CT scan of a previously healthy 24-year-old Vietnamese man shows a primary liver abscess (red arrow) with metastatic spread to the spleen (black arrow). (Courtesy of Drs. Chiu-Bin Hsaio and Diana Pomakova.) Middle: A previously healthy 33-yearold Chinese man presented with endophthalmitis. (AS Shon, RP Bajwa, TA Russo: Hypervirulent (hypermucoviscous) Klebsiella pneumoniae: A new and dangerous breed. Virulence 4:107, 2013.) Bottom: A hypermucoviscous phenotype (which does not necessarily equate with a mucoid phenotype) has been associated with hvKp strains. A positive string test is shown. However, this test is not optimally sensitive or specific. Identification of all 5 of the biomarkers iucA, iroB, peg-344, rmpA, and rmpA2 is presently the most accurate means to identify hvKp.

(Chap. 131), cKp and K. oxytoca are common causes of pneumonia among LTCF residents and hospitalized patients because of increased rates of oropharyngeal colonization with these organisms in such indi­ viduals. Mechanical ventilation is an important risk factor. In Asia and South Africa, community-acquired pneumonia due to hvKp is becom­ ing increasingly common, rivaling Streptococcus pneumoniae, and may occur in younger patients with no underlying disease. Klebsiella is also a common cause of pneumonia in severely malnourished children in LMICs. As in all pneumonias due to enteric GNB, typical manifestations include production of purulent sputum and evidence of airspace dis­ ease. Presentation with earlier, less extensive infection is now more common than is the classically described lobar infiltrate, bulging fis­ sure, and currant-jelly sputum. Pulmonary infection due to hvKp that has spread metastatically (e.g., from a hepatic abscess) usually includes nodular bilateral densities, more commonly in the lower lobes. Pulmo­ nary necrosis, pleural effusion, and empyema can occur with disease progression. UTI  cKp accounts for only 1–2% of UTI episodes among other­ wise healthy adults but for 5–17% of episodes of UTI in patients with anatomic and functional abnormalities of the urinary tract, including indwelling urinary catheter use (complicated UTI). UTI due to hvKp presents more commonly as renal or prostatic abscess due to bac­ teremic spread than as ascending infection from the urethra and bladder. Abdominal Infection  cKp causes a spectrum of abdominal infec­ tions similar to that caused by E. coli but is less frequently isolated than E. coli. hvKp is a common cause of monomicrobial communityacquired pyogenic liver abscess; in the Asian Pacific Rim, it has been recovered with steadily increasing frequency over the past two decades, replacing E. coli as the most common pathogen causing this syndrome. hvKp also is increasingly described as a cause of spontaneous bacterial peritonitis and splenic abscess. Other Infections  When cKp and K. oxytoca cause cellulitis or soft tissue infection, the process most frequently involves devitalized tissue (e.g., decubitus and diabetic ulcers, burn wounds) and immunocom­ promised hosts. cKp and K. oxytoca cause some cases of surgical site infection and nosocomial sinusitis as well as occasional cases of osteo­ myelitis contiguous to soft tissue infection, nontropical myositis, and meningitis (during the neonatal period and after neurosurgery). By contrast, hvKp has become an important cause of community-acquired monomicrobial necrotizing fasciitis, meningitis, endophthalmitis (Fig. 166-1, middle), and abscesses within the brain, subdural space, and epidural space, particularly in the Asian Pacific Rim but also glob­ ally. Cytotoxin-producing strains of K. oxytoca have been implicated as a cause of non–C. difficile antibiotic-associated hemorrhagic colitis. Bacteremia  Klebsiella infection at any site can produce bactere­ mia. Infections of the urinary tract, respiratory tract, and abdomen (especially hepatic abscess) each account for 15–30% as the source of Klebsiella bacteremia. Intravascular device–related infections account for another 5–15% of episodes, and surgical site and miscellaneous infections account for the rest. Klebsiella is an occasional cause of sepsis in neonates and of bacteremia in neutropenic patients. However, like other enteric GNB, Klebsiella rarely causes endocarditis or other endovascular infections, although endocarditis can involve extensive valvular destruction when it occurs. ■ ■DIAGNOSIS Klebsiellae are readily isolated and identified in the laboratory. These organisms usually ferment lactose, although the subspecies rhinoscle­ romatis and ozaenae are nonfermenters and are indole-negative. hvKp frequently possesses a hypermucoviscous phenotype (Fig. 166-1, bottom), although the sensitivity and specificity of the string test are less than optimal. Identification of all 5 of the biomarkers iucA, iroB, peg-344, rmpA, and rmpA2 is presently the most accurate means to identify hvKp in both antimicrobial sensitive and MDR isolates, although cur­ rently, this test is not routinely available.

TREATMENT Klebsiella Infections K. (formerly Enterobacter) aerogenes has a similar resistance profile to E. cloacae, the treatment of which is discussed below. K. pneu­ moniae and K. oxytoca have similar antibiotic resistance profiles; both are intrinsically resistant to ampicillin. The prevalence of acquired resistance in K. pneumoniae and K. oxytoca is generally

30% for amoxicillin-clavulanate, ampicillin-sulbactam, nitrofu­ rantoin, and TMP-SMX and ~10−20% for fluoroquinolones, piper­ acillin-tazobactam, fosfomycin, and omadacycline. USNHSN data from 2015−2017 identified 25% of K. pneumoniae as ESBL-producing strains; higher rates are reported from Asia, South America, and Africa. Although prevalence of ESBL-producing strains is greatest in LTCF, isolates of cKp that produce CTX-M ESBLs are increasingly described from the community. Oral treat­ ment for infection due to ESBL-producing strains is more challeng­ ing with Klebsiella than with E. coli because of the comparatively poor activity of nitrofurantoin, the lesser activity of fosfomycin (~80%), and limited available data regarding pivmecillinam (>80%) and omadacycline (75–100% susceptible for ESBL-producing iso­ lates, but 60% if resistant to tetracycline). Predictably, the ESBL-driven use of carbapenems has selected for strains of cKp and K. oxytoca that produce carbapenemases (8–18% based on the study and locale, 8.6% prevalence from 2015−2017 USNHSN data). Treatment can be problematic for such organisms, especially those producing MBLs (e.g., NDM), for which the high­ est prevalences are in cKp and K. oxytoca isolates from Eastern Europe and Asia and among health care–associated isolates. Likewise, hvKp strains from Asia are also increasingly reported to produce ESBLs and carbapenemases. Treatment options for carbapenem-resistant Klebsiella are similar to those described for E. coli and depend on the class of carbapen­ emase produced (see “Carbapenemase,” above); consultation with infectious disease experts is advised. For carbapenem-susceptible strains, the most predictably active agents include carbapenems, amikacin, ceftazidime-avibactam, ceftolozane-tazobactam, poly­ myxins, cefiderocol, tigecycline, eravacycline, and omadacycline. Empirical treatment decisions for the critically ill patient should be dictated by local susceptibility patterns, patient-specific risk factors, and the site of infection. PROTEUS INFECTIONS Proteus species are part of the colonic flora of a wide variety of mam­ mals, birds, fish, and reptiles. The ability of these GNB to generate histamine from contaminated fish has implicated them in the patho­ genesis of scombroid (fish) poisoning (Chap. 471). P. mirabilis causes 90% of Proteus infections, which occur in the community, LTCFs, and hospitals. By contrast, Proteus vulgaris and Proteus penneri are associated primarily with infections acquired in LTCFs or hospitals. Correspondingly, P. mirabilis colonizes healthy humans (up to 50%), whereas P. vulgaris and P. penneri are isolated primarily from individuals with underlying disease. By far the most common site of Proteus infection is the urinary tract, where the prin­ cipal known urovirulence factors of Proteus include adhesins, flagella, IgA-IgG protease, iron acquisition systems, and urease. Proteus less commonly causes infection at a variety of other extraintestinal sites. ■ ■INFECTIOUS SYNDROMES UTI  P. mirabilis causes only 1–2% of UTIs in healthy women, and Proteus species collectively cause only 5% of hospital-acquired UTIs. However, Proteus is responsible for 10–15% of cases of complicated UTI, primarily those associated with catheterization; indeed, Proteus accounts for 20–45% of urine isolates from chronically catheterized patients. This high prevalence is due in part to bacterial production of urease, which hydrolyzes urea to ammonia and results in alkaliza­ tion of the urine. In alkaline urine, organic and inorganic compounds

precipitate, contributing to the formation of struvite and carbonate– apatite crystals, biofilms on catheters, and/or frank calculi. Proteus becomes associated with the stones and biofilms; thereafter, it usually cannot be eradicated without removal of the stones or catheter. Over time, staghorn calculi may form within the renal pelvis and lead to obstruction and renal failure. Although biologically plausible, clinical support is lacking for the concept that urine samples exhibiting unex­ plained alkalinity should be cultured, and that isolation of a Proteus species (or other urea-splitting organism) should prompt consideration of an evaluation for urolithiasis.

Other Infections  Proteus occasionally causes pneumonia (primar­ ily in LTCF residents or hospitalized patients), nosocomial sinusitis, intraabdominal abscesses, biliary tract infection, surgical site infec­ tion, soft tissue infection (especially decubitus and diabetic ulcers), and osteomyelitis (primarily contiguous); in rare cases, it causes non­ tropical myositis. In addition, Proteus uncommonly causes neonatal meningitis, with the umbilicus frequently implicated as the source; this disease is often complicated by development of a cerebral abscess. Otogenic brain abscess also occurs. Bacteremia  Most episodes of Proteus bacteremia originate from the urinary tract, although intravascular devices and any of the less com­ mon sites of Proteus infection also are potential sources. Endovascular infection is rare. Proteus species are occasional agents of sepsis in neo­ nates and of bacteremia in neutropenic patients. ■ ■DIAGNOSIS Proteus is readily isolated and identified in the laboratory. Most strains are lactose-negative, produce H2S, and demonstrate characteristic swarming motility and distinct odor on agar plates. P. mirabilis and

P. penneri are indole-negative, whereas P. vulgaris is indole-positive. The inability to produce ornithine decarboxylase differentiates P. penneri from P. mirabilis. CHAPTER 166 Diseases Caused by Gram-Negative Enteric Bacilli
TREATMENT Proteus Infections Intrinsic resistance occurs in all Proteus spp. to nitrofurantoin, polymyxins, imipenem, and the tetracycline derivatives (e.g., tige­ cycline, eravacycline, omadacycline) and, in P. vulgaris and P. penneri, also to ampicillin and the first- and second-generation cephalosporins. Acquired resistance (% of isolates) occurs in P. mirabilis to ampicillin (15–65%), and in Proteus spp. to fluoroqui­ nolones (10–55%), fosfomycin (7–22%), and TMP-SMX (20–50%). In P. mirabilis, ampicillin-sulbactam is more active than ampicillin, with resistance rates of 6–18%, but the prevalence of ESBL produc­ tion (which confers ampicillin-sulbactam resistance) is increasing in the United States (5–10%) and Asia (up to 60%). Isolates of P. mirabilis that produce CTX-M ESBLs have been recovered from ambulatory patients with no recent health care contact (see the sec­ tion on the treatment of extraintestinal E. coli infections for treat­ ment considerations). Acquired carbapenem resistance remains relatively infrequent (<10%). However, production of MBLs (e.g., NDM) limits treatment options due to the inherent resistance of Proteus spp. to polymyxins and tetracycline derivatives (see “Carbapenemase,” above). For critically ill patients, agents with excellent activity overall against Proteus spp. (90–100% of isolates susceptible) include carbapenems (excepting imipenem), amikacin, piperacillin-tazobactam, aztreonam, cefepime, ceftazidime-avibactam, ceftolozane-tazobactam. ENTEROBACTER AND CRONOBACTER INFECTIONS The E. cloacae complex is responsible for most Enterobacter infections, whereas Cronobacter sakazakii (formerly Enterobacter sakazakii), Crono­ bacter malonaticus, E. cancerogenus, E. asburiae, E. hormaechei, E. kobei, E. ludwigii, and E. gergoviae are less commonly isolated (<1% for each). Enterobacter bugandensis has been recently described as an agent of

sepsis in neonates and was isolated from the International Space Station. Enterobacter spp. cause primarily health care–related infections. The organisms are widely prevalent in foods, environmental sources (includ­ ing equipment at health care facilities), and a variety of animals.

Colonization with these organisms is uncommon among healthy humans but increases significantly with LTCF residence or hospitaliza­ tion. Although colonization is an important prelude to infection, direct introduction via IV lines (e.g., contaminated IV fluids or pressure monitors) or contaminated non-FDA-approved stem cell products also occurs. Extensive antibiotic resistance has developed in Enterobacter spp. and probably has contributed to these organisms’ emergence as prominent nosocomial pathogens. Risk factors for Enterobacter infec­ tion include prior antibiotic treatment, comorbid disease, and ICU residency. Enterobacter spp. causes a spectrum of extraintestinal infec­ tions similar to those described for other GNB. ■ ■INFECTIOUS SYNDROMES The most commonly encountered syndromes include pneumonia, UTI (particularly catheter-associated), intravascular device–related infection, surgical site infection, and abdominal infection (primarily postoperative or related to devices such as biliary stents). Nosocomial sinusitis, men­ ingitis related to neurosurgical procedures (including use of intracranial pressure monitors), osteomyelitis, and endophthalmitis after eye surgery are less frequent. Neonates (particularly if low-birth-weight) are at risk for C. sakazakii infection, including neonatal bacteremia, necrotiz­ ing enterocolitis, and meningitis (which is often complicated by brain abscess or ventriculitis). Contaminated powdered infant formula has been implicated as a source for such neonatal infections. The WHO recommends that, to reduce the initial number of bacteria, powdered infant formula should be reconstituted with hot water (>70°C) and, to limit replication of residual bacteria, the reconstituted formula should be stored at <5°C or its storage time minimized. PART 5 Infectious Diseases Enterobacter bacteremia can result from primary infection at any anatomic site. In bacteremia of unclear origin, particularly in an out­ break setting, sources for consideration should include contaminated IV fluids or medications, blood components or plasma derivatives, catheter-flushing fluids, pressure monitors, and dialysis equipment. Enterobacter can also cause bacteremia in neutropenic patients. Entero­ bacter endocarditis is rare, occurring primarily in association with IV drug use or prosthetic valves. ■ ■DIAGNOSIS Enterobacter is readily isolated and identified in the laboratory. Most strains are lactose-positive and indole-negative. TREATMENT Enterobacter Infections E. cloacae is intrinsically resistant to ampicillin, ampicillin-sulbac­ tam, ampicillin-clavulanate, the first-generation cephalosporins, and the cephamycins. The prevalence of acquired resistance has ranged from 15 to 40% for piperacillin-tazobactam, 5 to 23% for polymyxins, 15 to 17% for fosfomycin, 15 to 30% for TMP-SMX, and 5 to 20% for fluoroquinolones and is ~10% for omadacycline (53% if tetracycline resistant). USNHSN data from 2015−2017 identified at least 9% of E. cloacae isolates as presumptively ESBLproducing, based on cefepime resistance. The prevalence of ESBLs in E. cloacae outside of the United States is 20−50%. The use of third-generation cephalosporins can induce or select for stable derepression of AmpC β-lactamase. Because resistance may emerge during therapy (in one study, this phenomenon was documented in 20% of clinical isolates), these agents should be avoided in the treatment of severe Enterobacter infection. Cefepime is stable to hydrolysis by AmpC β-lactamases; thus, it is a suitable option for treatment of Enterobacter infections so long as ESBL is not co-produced. Overall, resistance prevalence generally ranges from 10 to 25% for cefepime and 25 to 50% for aztreonam and the third-generation cephalosporins. Carbapenem resistance remains relatively uncommon (USNHSN data from 2015−2017 identified a

5% prevalence) and is more commonly associated with a combina­ tion of increased AmpC expression and decreased permeability due to porin mutations rather than carbapenemase production, although acquisition of carbapenemase genes is increasing (see “Carbapen­ emase,” above). Uncertainty exists on the optimal treatment for non-CP-CR-Enterobacter spp. Fortunately, overall, the percentage of susceptibility is high (90–99%) for carbapenems, amikacin, ceftazi­ dime-avibactam, cefiderocol, tigecycline, eravacycline, and omada­ cycline (the latter three for tetracycline-susceptible isolates). Once susceptibility data for a patient’s isolate become available, de-escalation of the antimicrobial regimen is advisable whenever possible. SERRATIA INFECTIONS S. marcescens causes >90%, and Serratia liquefaciens complex <10%, of Serratia infections. Serratiae are found primarily in the environment (including in health care institutions), particularly in moist settings. Serratiae have been isolated from a variety of animals, insects, and plants, but only infrequently from healthy humans. In LTCFs and hospitals, reservoirs for the organisms include the hands and finger­ nails of health care personnel, food, milk (on neonatal units), sinks, medical equipment or devices, IV solutions or parenteral medications (particularly those generated by compounding pharmacies), prefilled syringes and multiple-access medication vials (e.g., for heparin, pro­ pofol, saline), blood products (e.g., platelets), hand soaps and lotions, irrigation solutions, and even disinfectants such as chlorhexidine. Infection results from either direct inoculation (e.g., via con­ taminated injected substances [IV fluids, medications, or recreational drugs] or snake bite) or colonization (primarily of the respiratory tract). Sporadic infection is most common, but outbreaks (often involving MDR strains in adult and neonatal ICUs) also occur. Hygiene, medication-compounding standards, sterile technique, and infection control programs are critical measures to prevent infection. The spectrum of extraintestinal infections caused by Serratia is similar to that for other GNB. Serratia species are usually considered to cause mainly health care–associated infections; they account for 1–3% of hospital-acquired infections. However, population-based laboratory surveillance studies in Canada and Australia have demonstrated that community-acquired Serratia infections occur more commonly than was previously appreciated, and case reports have documented serious infection in otherwise healthy hosts. Serratia also is one of the patho­ gens associated with chronic granulomatous disease. ■ ■INFECTIOUS SYNDROMES The most common primary sites of Serratia infection are the respira­ tory and genitourinary tracts, intravascular devices, the eye (contact lens–associated keratitis and other ocular infections), surgical wounds, and the bloodstream (from contaminated infusions), although most episodes of Serratia bacteremia arise from one of the listed focal infec­ tions rather than contaminated infusate. Less common syndromes are soft tissue infections (including myositis, fasciitis, mastitis), osteomy­ elitis, abdominal and biliary tract infections (usually postprocedural), and septic arthritis (primarily from intraarticular injections). Serratiae are uncommon causes of neonatal meningitis; postsurgical meningitis, endophthalmitis, or breast implant infection; and bacteremia in neu­ tropenic patients. Endocarditis is rare, occurring most commonly in IV drug users. ■ ■DIAGNOSIS Serratiae are readily cultured and identified by the laboratory and are usually lactose- and indole-negative. The red pigmentation of some

S. marcescens strains and Serratia rubidaea can produce distinctive clin­ ical findings (e.g., pink breast milk or hypopyon; pseudohemoptysis). TREATMENT Serratia Infections Most Serratia strains (>80%) are intrinsically resistant to ampi­ cillin, amoxicillin-clavulanate, ampicillin-sulbactam, first- and

second-generation cephalosporins, cephamycins, nitrofurantoin, and polymyxins; likewise, tetracycline derivatives are poorly active. By contrast, fluoroquinolones, TMP-SMX, piperacillin-tazobactam, fosfomycin, and omadacycline are active against 85−95% of U.S. and European isolates, including those resistant to tetracycline. Both in the United States and globally, the prevalence of ESBLproducing isolates is generally low (<10%), but rates of 20–30% have been reported in Asia and Latin America. Induction or selec­ tion of variants with stable de-repression of chromosomal AmpC β-lactamases during therapy with third-generation cephalosporins is considered to be uncommon. Resistance prevalence generally ranges from 10 to 20% for aztreonam and the third-generation cephalosporins. Acquisition of carbapenemase-encoding genes is uncommon but increasing. Production of MBL (e.g., NDM) limits treatment options due to Serratia’s predictable resistance to poly­ myxins and tetracycline derivatives (see “Carbapenemase,” above). For critically ill patients, the most active agents overall (>90% susceptible) are carbapenems, piperacillin-tazobactam, cefepime, amikacin, ceftazidime-avibactam, and ceftolozane-tazobactam. CITROBACTER INFECTIONS C. freundii and Citrobacter koseri cause most human Citrobacter infec­ tions, which are epidemiologically and clinically similar to Entero­ bacter infections. Citrobacter species are commonly present in water, food, soil, and certain animals. Colonization with these organisms is uncommon among healthy humans but increases significantly with LTCF residence or hospitalization. Citrobacter species account for 1–2% of nosocomial infections. The affected hosts are usually immu­ nocompromised and/or have comorbid disease or disruption of skin or mucosal barriers. Infection from treatment with contaminated, non-FDA-approved stem cell products has been described. Citrobacter causes extraintestinal infections like those described for other GNB. ■ ■INFECTIOUS SYNDROMES The urinary tract accounts for 40–50% of Citrobacter infections. Less commonly involved sites include the biliary tree (particularly with stones or obstruction), the respiratory tract, surgical sites, soft tissue (e.g., decubitus ulcers), the peritoneum, and intravascular devices. Osteomyelitis (usually from a contiguous focus), central nervous sys­ tem infection in adults (from neurosurgical or other types of meningeal disruption), and myositis occur rarely. Citrobacter (primarily C. koseri) also causes 1–2% of neonatal meningitis cases, of which 50–80% are complicated by brain abscess. Further, case reports in adults suggest that C. koseri infection has a predilection for abscess formation. Citro­ bacter bacteremia is most often due to UTI, biliary/abdominal infec­ tion, or intravascular device infection, and occurs in some neutropenic patients. Endocarditis and other endovascular infections are rare. ■ ■DIAGNOSIS Citrobacter species are readily isolated and identified; 35–50% of iso­ lates are lactose-positive, and 100% are oxidase-negative. C. freundii is indole-negative, whereas C. koseri is indole-positive. TREATMENT Citrobacter Infections C. freundii is more antibiotic-resistant than is C. koseri. Most C. freundii isolates are intrinsically resistant to ampicillin, ampicillinsulbactam, amoxicillin-clavulanate, first-generation cephalospo­ rins, and cephamycins. C. koseri exhibits intrinsic resistance to ampicillin and ampicillin-sulbactam. Overall, the prevalence of acquired resistance generally ranges from 15 to 35% for third-gen­ eration cephalosporins, piperacillin-tazobactam, fluoroquinolones, and TMP-SMX and is ~10% for nitrofurantoin and omadacycline (but 39% for omadacycline if tetracycline-resistant). The prevalence of ESBL production ranges from 5 to 30%. The use of third-generation cephalosporins may result in the induction or selection of variants with stable de-repression of chromosomal AmpC β-lactamases

during therapy. Presently, <10% of isolates have acquired car­ bapenemases (see “Carbapenemase,” above). Carbapenems, ami­ kacin, fosfomycin, polymyxins, cefepime, ceftolozane-tazobactam, ceftazidime-avibactam, cefiderocol, tigecycline, eravacycline, and omadacycline (the latter three if tetracycline-susceptible) are the most active agents against Citrobacter isolates (>90% susceptible).

MORGANELLA AND PROVIDENCIA INFECTIONS M. morganii, Providencia stuartii, and (less frequently) Providencia rettgeri are the members of their respective genera that cause systemic human infections. P. alcalifaciens has been implicated as a cause of food-borne gastroenteritis. These organisms’ epidemiologic asso­ ciations, pathogenic properties, and clinical manifestations resemble those of Proteus species. Morganella and Providencia occur more commonly among LTCF residents than among hospitalized patients, largely resulting from chronic urinary catheter use. Because of these organisms’ intrinsic resistance to polymyxins and tigecycline, they may become more common in settings with extensive use of these agents. ■ ■INFECTIOUS SYNDROMES These species are primarily urinary tract pathogens, causing UTIs that are most often associated with long-term (>30-day) catheterization. Such infections commonly lead to biofilm formation and catheter encrustation (sometimes causing catheter obstruction) or the devel­ opment of struvite bladder or renal stones (sometimes causing renal obstruction, abscess, and extrarenal extension, and serving as foci for relapse). They can cause purple urine (“purple bag syndrome”), as can P. mirabilis, K. pneumoniae, E. coli, and P. aeruginosa. Morganella is also commonly isolated from snakebite infection. CHAPTER 166 Other, less common infectious syndromes due to Morganella and Providencia include surgical site infection, soft tissue infection (pri­ marily involving decubitus and diabetic ulcers), burn site infection, pneumonia (particularly ventilator-associated), intravascular device infection, and intraabdominal infection. Rarely, the other extraintesti­ nal infections described for GNB also occur. Bacteremia is uncommon; when it does occur, any infected site can serve as the source, but the urinary tract accounts for most cases, followed by surgical site, soft tis­ sue, and hepatobiliary infections. Diseases Caused by Gram-Negative Enteric Bacilli
■ ■DIAGNOSIS M. morganii and Providencia are readily isolated and identified. Nearly all isolates are lactose-negative and indole-positive. TREATMENT Morganella and Providencia Infections Morganella and Providencia are intrinsically resistant to ampicil­ lin, ampicillin-clavulanate, ampicillin-sulbactam, first-generation cephalosporins, nitrofurantoin, tetracyclines and derivatives (e.g., tigecycline), imipenem (but not the other carbapenems), and the polymyxins. P. stuartii additionally exhibits intrinsic resistance to gentamicin and tobramycin, as does M. morganii to second-generation cephalosporins. Fosfomycin is poorly active (>50% resistance). The prevalence of resistance generally ranges from 10 to 30% for the third-generation cephalosporins, from 10 to 40% for fluoroquino­ lones, and from 20 to 40% for TMP-SMX; the prevalence is more widely variable for piperacillin-tazobactam. The prevalence of ESBL production is generally <10%. The prevalence of acquired car­ bapenemase production is <10%. Production of MBL (e.g., NDM) limits treatment options due to the inherent resistance of Proteeae to polymyxins and tetracycline derivatives (see “Carbapenemase,” above). Overall, the most active agents (>90% of isolates suscep­ tible) are carbapenems (excepting imipenem), amikacin, cefepime, ceftazidime-avibactam, ceftolozane-tazobactam, and cefiderocol. Removal of a colonized urinary catheter or stone is critical for eradication of UTI.

50 - 167 Acinetobacter Infections

167 Acinetobacter Infections

EDWARDSIELLA INFECTIONS E. tarda is the only member of the genus Edwardsiella that is associated with human disease. This organism is found predominantly in freshwa­ ter and marine environments and in the associated aquatic animal spe­ cies. Human acquisition occurs primarily from interaction with these reservoirs or ingestion of raw or inadequately cooked aquatic animals. E. tarda infection is rare in the United States, where acquisition occurs mainly along the Gulf of Mexico; recently reported cases are mostly from Asia. This pathogen shares clinical features with Salmonella spe­ cies (as an intestinal pathogen; Chap. 171), Vibrio vulnificus (as an extraintestinal pathogen; Chap. 173), and Aeromonas hydrophila (as both an intestinal and an extraintestinal pathogen; Chap. 173).

■ ■INFECTIOUS SYNDROMES Gastroenteritis is the predominant Edwardsiella-associated infectious syndrome (50–80% of reported cases). Self-limiting watery diarrhea is most common, but severe colitis also occurs. The most common extraintestinal infection is wound infection due to direct inoculation, which is often associated with brackish or freshwater injuries, snake­ bites, or fish-related trauma. A case of pneumonia occurred after a near-drowning incident. Cholecystitis, cholangitis, and hepatic abscess may be due to ascending infection via the biliary tree. Other infec­ tious syndromes result from invasion of the gastrointestinal tract and subsequent bacteremia. A primary bacteremic syndrome, sometimes complicated by meningitis, has a 40% case–fatality rate; hematogenous seeding may result in hepatic and intra- and extraperitoneal abscesses, endocarditis, mycotic aneurysm, septic arthritis, osteomyelitis, nec­ rotizing fasciitis, and empyema. Most hosts who develop systemic Edwardsiella infection have significant comorbidities (e.g., hepatobili­ ary disease, iron overload, cancer, or diabetes mellitus). PART 5 Infectious Diseases ■ ■DIAGNOSIS Although E. tarda can readily be isolated and identified, most laborato­ ries do not routinely screen for or identify it in stool samples. Produc­ tion of hydrogen sulfide is a characteristic biochemical property. TREATMENT Edwardsiella Infections E. tarda is susceptible to most antimicrobial agents appropriate for use against GNB. Gastroenteritis is generally self-limiting, but treat­ ment with a fluoroquinolone may hasten resolution. In the setting of severe sepsis, fluoroquinolones, third- and fourth-generation cephalosporins, carbapenems, and amikacin—either alone or in combination—are the safest choices pending susceptibility data. INFECTIONS CAUSED BY MISCELLANEOUS GENERA Other gram-negative organisms such as Hafnia, Kluyvera, Cedecea, Pantoea, Ewingella, Leclercia, Raoultella, and Photorhabdus spp. are occasionally isolated from diverse clinical specimens, including blood, sputum, urine, cerebrospinal fluid, joint fluid, bile, and wounds. Such organisms cause infection predominantly in compromised hosts or in association with an invasive procedure or foreign body. Cephalo­ sporinases from Kluyvera have been implicated as the progenitors of CTX-M ESBLs. Kluyvera and Raoultella may produce carbapenemases. ■ ■FURTHER READING Antimicrobial Resistance Collaborators: Global burden of bac­ terial antimicrobial resistance in 2019: A systematic analysis. Lancet 399:629, 2022. [Erratum in Lancet 400:1102, 2022.] Bonten M et al: Epidemiology of Escherichia coli bacteremia: A sys­ tematic literature review. Clin Infect Dis 72:1211, 2021. Cheng MP et al: Beta-lactam/beta-lactamase inhibitor therapy for potential AmpC-producing organisms: A systematic review and meta-analysis. Open Forum Infect Dis 6:ofz248, 2019. David S et al: Epidemic of carbapenem-resistant Klebsiella pneumoniae in Europe is driven by nosocomial spread. Nat Microbiol 4:1919, 2019.

Harris PNA et al: Effect of piperacillin-tazobactam vs meropenem on 30-day mortality for patients with E coli or Klebsiella pneumoniae bloodstream infection and ceftriaxone resistance: A randomized clinical trial [published correction appears in JAMA 321:2370, 2019]. JAMA 320:984, 2018. Holy O, Forsythe S: Cronobacter spp. as emerging causes of health­ care-associated infection. J Hosp Infect 86:169, 2014. Kamiyama S et al: Edwardsiella tarda bacteremia, Okayama, Japan, 2005–2016. Emerg Infect Dis 25:1817, 2019. Russo TA, Marr CM: Hypervirulent Klebsiella pneumoniae. Clin Microbiol Rev 32:e00001, 2019. Tamma PD et al: Infectious Diseases Society of America 2022 Guid­ ance on the Treatment of Extended-Spectrum β-lactamase Producing Enterobacterales (ESBL-E), Carbapenem-Resistant Enterobacterales (CRE), and Pseudomonas aeruginosa with Difficult-to-Treat Resis­ tance (DTR-P. aeruginosa). Clin Infect Dis 75:187, 2022. van Duin D et al: Molecular and clinical epidemiology of carbapenemresistant Enterobacterales in the USA (CRACKLE-2): A prospective cohort study Lancet Infect Dis 20:731, 2020. [Erratum in Lancet Infect Dis 19:30755, 2020.] Weiner-Lastinger LM et al: Antimicrobial-resistant pathogens asso­ ciated with adult healthcare-associated infections: Summary of data reported to the National Healthcare Safety Network, 2015–2017. Infect Control Hosp Epidemiol 41:1, 2020. Rossana Rosa, Christine A. Vu

Acinetobacter Infections ■ ■DEFINITION Acinetobacter species were first described in 1911 and named Micro­ coccus calcoaceticus. Thereafter, the genus was renamed multiple times; since 1950, it has been known as Acinetobacter. Acinetobacter species are gram-negative, oxidase-negative, nonmotile, nonferment­ ing coccobacilli that are easily recovered on standard culture media. Differentiation among Acinetobacter species on the basis of phenotypic characteristics alone is very difficult. Molecular-based methods such as matrix-assisted laser desorption–ionization–time-of-flight mass spec­ trometry (MALDI-TOF-MS) and quantitative real-time polymerase chain reaction (PCR) are usually necessary to identify Acinetobacter baumannii, the most clinically relevant species of the genus. ■ ■ETIOLOGY AND EPIDEMIOLOGY Acinetobacter species are naturally encountered in water and soil and have also been recovered from fruits and vegetables. In humans, Acinetobacter can be found on the skin and in the respiratory and gastrointestinal tracts. A. baumannii is capable of surviving environ­ mental desiccation for weeks; this characteristic is important from an infection-control perspective as it allows this organism to persist in the hospital environment and on equipment. Acinetobacter was historically considered a pathogen of hot and humid climates. In recent years, however, hospital outbreaks caused by A. baumannii have been reported worldwide, even in temperate climates. In the United States, the Centers for Disease Control and Prevention (CDC) estimates that 12,000 Acinetobacter infections occur every year, 7300 of which are caused by multidrug-resistant strains, with 500 attributable deaths. The increase in the number of infections with A. baumannii is suspected to be due to the rapid spread of certain genetically distinct lineages; of the three international clonal lineages (ICLs), ICL I and ICL II are multidrug resistant. The predominance of these lineages remains unexplained, although it has been proposed that this population structure is the result of two waves of expansion.

The first wave followed a bottleneck (possibly linked to a restricted ecologic niche) that occurred in the distant past. The second wave is ongoing and is being driven by the rapid expansion of a limited num­ ber of multidrug-resistant clones. The COVID-19 pandemic resulted in a setback to the efforts to control the spread of multidrug-resistant organisms, with significant increases in the rates of infections with carbapenem-resistant Acinetobacter reported worldwide. Analysis of the A. baumannii pangenome (the sum of the core and dispensable genomes) has shown that its organization is charac­ terized by a small core genome and a large accessory or disposable genome. This organization reflects A. baumannii’s high plasticity, which enables it to acquire exogenous genetic material. With few exceptions, gene functions associated with virulence are found in the core genome; this observation suggests a limited role for the acquisi­ tion of new virulence traits in the recent nosocomial expansion of A. baumannii clones. Genes associated with resistance to antimicrobial agents are found in both the species core genome and the accessory genome. In the accessory genome, these genes have been found in alien islands, often flanked by integrases, transposases, or insertion sequences. This pattern suggests possible acquisition by horizontal gene transfer from other Acinetobacter strains or even from different bacterial species present in the immediate environment. Acquisition of these antimicrobial resistance genes is hypothesized to have led to the recent rapid expansion of highly homogeneous clonal lineages, whose main difference from nonclonal A. baumannii appears to be their anti­ microbial resistance. Health Care–Associated Infections  Infections caused by A. baumannii occur frequently among patients admitted to intensive care units (ICUs). Risk factors for colonization and infection with this pathogen include nursing home residence, prolonged ICU stay, central venous catheterization, tracheostomy, mechanical ventilation, enteral feedings, and treatment with third-generation cephalosporins, fluoro­ quinolones, and carbapenems. Acquisition of carbapenem-resistant A. baumannii is most common among patients exposed to carbapenems. Spread of A. baumannii across different regions is facilitated by the movement of patients between health care systems and throughout the continuum of health care. Within the hospital, environmental spread of A. baumannii occurs as a result of inappropriate hand hygiene among workers providing health care for infected or colonized patients and the contamination of hospital equipment, such as respiratory therapy and ventilation equipment. The air surrounding the patient may also play a role in environmental colonization with A. baumannii, especially in inpatient areas without physical barriers between patients and with an inadequate number of air exchanges. A. baumannii strains identified during hospital outbreaks are typi­ cally resistant to more antibiotic classes than strains from the commu­ nity. The prevalence of colonization with A. baumannii at the time of admission or during a stay in a long-term acute-care hospital (LTACH) or nursing home is variable and depends on regional flora. Outbreaks of A. baumannii in acute-care hospitals and LTACHs that “share” patients have been described in Ohio, Michigan, Illinois, and Indiana. Community-Acquired Infections  Community-acquired infec­ tions caused by Acinetobacter have been described in Australia and Asia. Few cases have been reported in regions with a temperate climate, and even those few cases have taken place during warm and humid months. Risk factors for community-acquired pneumonia due to this organism include a history of alcohol abuse, diabetes mellitus, smok­ ing, and chronic lung disease. War Zone–Associated Infections  Infections caused by Acineto­ bacter in war zones include skin and soft tissue infections associated with traumatic injuries and bloodstream infections. Outbreak investi­ gations of A. baumannii infections among military personnel returning from Iraq and Afghanistan suggested the acquisition of A. baumannii in field hospitals rather than colonization of the skin before an injury. This view is supported by the recovery of A. baumannii isolates with similar genetic characteristics from inanimate surfaces in field hospi­ tals and from patients.

Disaster Medicine  A. baumannii is linked to infections among victims of trauma during tsunamis, earthquakes, and terrorist attacks. The types of infections most frequently observed in these settings are soft tissue injuries, but bloodstream infections and pneumonia have also been reported. In addition, outbreaks of A. baumannii infection in ICUs caring for disaster victims have been described.

■ ■PATHOGENESIS Mechanisms of pathogenesis and virulence in Acinetobacter spe­ cies have not been fully elucidated. However, A. baumannii seems to have greater virulence potential than other Acinetobacter species, as evidenced by its ability to grow at 37°C and to resist uptake by macrophages. Initial A. baumannii colonization of the host and the environment is facilitated by the organism’s ability to adhere to surfaces and human cells and to create biofilms. The ability to form a biofilm is phenotypi­ cally associated with exopolysaccharide production and pilus forma­ tion. A quorum-sensing molecule encoded by the abaI autoinducer synthase gene has been implicated in A. baumannii biofilm formation on abiotic surfaces. Outer-membrane porins appear to mediate cell apoptosis. A. baumannii can survive in harsh environments within the host and on inanimate surfaces by modifying the structure of its lipid A, with a consequent decrease in susceptibility to antibiotics and antimicrobial peptides and an increase in survival upon desiccation. Acinetobacter species produce an extracellular capsule that protects the bacteria from external threats, including complement-mediated killing. Studies of mouse models showed that Acinetobacter species can increase capsule production in the presence of subinhibitory levels of antibiotic—an ability that leads to increased resistance to complementmediated killing and a hypervirulent phenotype. CHAPTER 167 Phospholipase C and phospholipase D have been identified as viru­ lence factors in A. baumannii. These enzymes exert cytotoxic effects on epithelial cells and facilitate their invasion. Iron-acquisition systems are also important virulence mechanisms in A. baumannii. Through secretion of siderophores (low-molecularmass ferric-binding compounds), A. baumannii is able to grow despite iron deficiencies in the surrounding environment (e.g., in the human host). Acinetobacter Infections Several protein-secretion systems have been identified in A. bau­ mannii. The most recently described is a type II secretion system. The substrate for this system, the LipA lipase, is required for growth on medium containing lipids as a sole carbon source. Mutants lack­ ing the genes for the type II secretion system or its substrate exhibit defective in vivo growth in a neutropenic murine model of bacteremia.

A. baumannii also has a type VI secretion system whose primary function seems to be to secrete antibacterial toxins that kill competing bacteria, including other strains in the same species. The type V autotransporter system has been characterized in

A. baumannii. In a murine systemic model of Acinetobacter infection, the Acinetobacter trimeric autotransporter mediates biofilm formation and maintenance; adherence to extracellular matrix components such as collagen I, II, and IV; and virulence. Outer-membrane vesicles (OMVs) play a special role in protein secretion. Many A. baumannii strains secrete OMVs containing vari­ ous virulence factors, including outer-membrane protein A (OmpA), proteases, and phospholipases. The membrane proteins in OMVs are responsible for eliciting a potent innate immune response. Several studies have shown that A. baumannii OMVs could be used as an acel­ lular vaccine to effectively control A. baumannii infections. Nosocomial strains of Acinetobacter can deploy multiple mecha­ nisms of resistance, including alterations in porins and efflux pumps and expression of β-lactamases. More specifically, Acinetobacter spe­ cies can reduce the expression of porins, thus hindering the passage of β-lactam antibiotics into the periplasmic space. These species can overexpress bacterial efflux pumps and decrease the concentration of β-lactam antibiotics in the periplasmic space. Efflux pumps can also actively remove quinolones, tetracyclines, chloramphenicol, disinfec­ tants, and tigecycline. Acinetobacter species possess chromosomally encoded cephalosporinases and are capable of acquiring β-lactamases,

including serine and metallo-β-lactamases. AmpC β-lactamases are class C β-lactamases intrinsic to all A. baumannii strains. Although these enzymes are expressed at low levels and are not inducible, the addition of the insertion sequence ISAba1 next to the AmpC gene increases β-lactamase production, with resulting resistance to most cephalosporins.

Carbapenem resistance in Acinetobacter species is mostly tied to the emergence of Ambler class D oxacillinases of group 2d, some of which are intrinsic and chromosomal (e.g., OXA-51-like) while others are acquired and are found in plasmids or are chromosomally encoded (e.g., OXA-23-like, 24 [33-like, 40-like], 58-like, 143-like, and 235-like). ■ ■CLINICAL MANIFESTATIONS Pneumonia  A. baumannii is a notorious cause of nosocomial pneu­ monia, most frequently among patients requiring prolonged mechani­ cal ventilation. The onset of disease tends to be later than that caused by other gram-negative bacilli; however, clinical symptoms of hospitalacquired or ventilator-associated pneumonia due to A. baumannii are similar to those of nosocomial or ventilator-associated pneumonia due to other nosocomial pathogens. Thus, the most common indica­ tors of infection include fever and increased sputum production. The positivity of respiratory cultures in most cases may present a challenge for the clinician since airway colonization with A. baumannii may not always indicate a diagnosis of pneumonia, but it is a known risk factor for infection itself. In addition, radiologic findings are nonspecific and can include lobar consolidations and pleural effusions, with cavitations being rarely seen. The crude mortality rates associated with nosoco­ mial pneumonia due to A. baumannii are reported as high as 65%. However, since these infections occur in debilitated patients, their attributable mortality has been difficult to establish. PART 5 Infectious Diseases Community-acquired pneumonia due to A. baumannii is relatively rare. Its clinical presentation is characterized by fever, severe respira­ tory symptoms, and multiple-organ dysfunction. Patients frequently have a cough productive of purulent sputum, shortness of breath, and chest pain. Imaging studies usually show lobar consolidation. Mortality rates associated with this process are >50%. Bloodstream Infections  Bloodstream infections due to A. bau­ mannii are most frequent among ICU patients and usually occur in the presence of a central venous catheter or as a secondary complication of hospital-acquired or ventilator-associated pneumonia. Polymicrobial growth has been reported in 20–36% of bacteremia episodes. Fever is the most common sign of infection (developing in >95% of cases), and presentation with septic shock and disseminated intravascular coagulopathy has been described in as many as 25 to 30% of patients, respectively. A. baumannii bloodstream infections often result in higher hospitalization costs and longer ICU stays. Crude mortality rates from this infection are as high as 40%; however, rates can be as high as 70% from infections caused by carbapenem-resistant isolates. In patients with infections caused by extremely drug-resistant strains, poor outcomes are thought to be driven by delays in the initiation of adequate antimicrobial therapy. Skin and Soft Tissue Infections  Acinetobacter species have been described as part of the skin flora, yet the majority of the organisms from this genus that colonize the skin are not those associated with nosocomial infections. Discerning infection from wound coloniza­ tion is challenging. Gunshot wounds and the presence of orthopedic external-fixation devices are common among patients with combat trauma–associated A. baumannii skin and soft tissue infections. The report on a case series of eight U.S. military patients described the clinical presentation of their infections as evolving from an edematous peau d’orange appearance to a sandpaper appearance with overlying vesicles and then to a necrotizing process with hemorrhagic bullae. Other case series have also included necrotizing fasciitis. A. baumannii is an important pathogen in burn units worldwide. Large burns pro­ vide ideal conditions for A. baumannii and facilitate patient-to-patient transmission. The presence of A. baumannii in wounds contributes to healing delays and graft loss. In addition, wound colonization is a risk

factor for bloodstream infections among patients with extensive burn injuries. A. baumannii infections resulting from trauma to soft tissues in the setting of natural disasters, such as tsunamis and earthquakes, have been reported. The implication is that A. baumannii should be con­ sidered in the differential diagnosis of soft tissue infections following exposure to tropical and subtropical environments. Urinary Tract Infections  A. baumannii is an infrequent cause of urinary tract infections. The majority of cases reported are catheterassociated infections, reflecting the ability of A. baumannii to form biofilms on these devices. A few reports have described communityacquired infections occurring in the setting of nephrolithiasis and after renal transplantation. Meningitis  Central nervous system infections with A. baumannii have been reported in the context of outbreaks, traumatic injuries, neurosurgical procedures, and external ventricular drains. One case series described a petechial rash in up to 30% of patients. Acinetobacter species may look similar to Neisseria meningitidis on a Gram stain of cerebrospinal fluid; both appear as gram-negative paired cocci. Eradi­ cation of A. baumannii from the cerebrospinal fluid can be challenging and requires careful selection of antibiotics that adequately penetrate the site of infection. Other Miscellaneous Infections  A few cases of A. baumannii ker­ atitis associated with the use of contact lenses have been reported. Cases of native- and prosthetic-valve endocarditis have also been described. TREATMENT Acinetobacter Infections Treatment of Acinetobacter infections is challenging due to difficul­ ties in differentiating colonization versus infection and because Acinetobacter can develop resistance to most available antibiotics. Therefore, the choice of empirical therapy should be based on local epidemiology and, if available, the patient’s colonization status with a carbapenem-resistant isolate. Definitive therapy should be determined by antimicrobial susceptibility testing. Antimicrobial options for the management of infections caused by A. baumannii are displayed in Table 167-1. Acinetobacter species possess intrinsic β-lactamases that inacti­ vate first- and second-generation cephalosporins. Through acquisi­ tion of extended-spectrum β-lactamases, these organisms can also become resistant to third- and fourth-generation cephalosporins, along with carbapenems. Nevertheless, when the isolate is suscep­ tible, β-lactam agents should be used. Ampicillin-sulbactam (due to its sulbactam component) is the treatment of choice, with cefepime, meropenem, and imipenem as alternative options based on in vitro susceptibility testing. Currently, there is no antibiotic regimen that has been proven superior for the treatment of carbapenem-resistant A. baumannii. The 2024 Infectious Diseases Society of America (IDSA) “Guidance on the Treatment of Antimicrobial Resistant Gram-Negative Infec­ tions” recommends sulbactam-durlobactam in combination with a carbapenem as their preferred regimen, and high-dose ampicillinsulbactam in combination with either polymyxin B, minocycline, tigecycline, or cefiderocol is an alternative. Pairing of sulbactam with durlobactam makes a novel diazabicyclooctane non-β-lactam β-lactamase inhibitor with activity against the Acinetobacterderived cephalosporinases and class D β-lactamases including car­ bapenemases of the OXA family. High-dose ampicillin-sulbactam is recommended in combination with either polymixn B, mino­ cycline, tigecycline, or cefiderocol. The use of high-dose over standard dose ampicillin-sulbactam increases binding of sulbactam to its penicillin-binding proteins (PBP) targets (PBP2 and PBP3) in order to optimize inhibition of cell wall synthesis. This recom­ mendation is based on two meta-analyses of small clinical trials and observational data. In a randomized clinical trial including 125 patients with carbapenem-resistant A. baumannii, patients treated

TABLE 167-1  Therapeutic Options for the Management of MultidrugResistant Acinetobacter baumannii Infections ANTIBIOTIC DOSINGa COMMENTS Sulbactam 6–9 g/d Unavailable as single drug in many countries (including the United States). Different dosing strategies proposed if administered with ampicillin. Ampicillinsulbactam 3 g q4h 9 g q8h 27 g q24h Infuse over 30 min Infuse over 4 h Infuse as continuous infusion Sulbactamdurlobactam 1 g/1 g q6h Infuse over 3 h Meropenem 2 g q8h Carbapenem-susceptible isolates only; infuse over 3 h Imipenemcilastatin 500 mg q6h Carbapenem-susceptible isolates only; infuse over 3 h Cefiderocol 2g q8h Use in combination therapy; infuse over 3 h Colistin Dosing per the international consensus guidelines on polymixins (Tsuji BT et al, Pharmacotherapy 39:10, 2019) Colistin is preferred for urinary tract infections. Polymyxin B Dosing per the international consensus guidelines on polymixins (Tsuji BT et al, Pharmacotherapy 39:10, 2019) Polymyxin B is preferred over colistin for bloodstream infections. Tigecycline 200-mg loading dose followed by 100 mg q12h Use in combination therapy Minocycline 200 mg q12h IV/PO. Use in combination therapy aAll drugs are given by the IV route unless otherwise stated. with sulbactam-durlobactam had a 28-day all-cause mortality of 19% compared to 32% in patients treated with colistin, and with lower rates of nephrotoxicity in the sulbactam-durlobactam arm. Cefiderocol is a siderophore cephalosporine with in vitro stabil­ ity against the Acinetobacter-derived cephalosporinase and other -Hand hygiene -Contact precautions Health care worker’s hands A. baumannii– positive patient Shared equipment Health care environment -Physical separation from A. baumannii–negative patients -Rectal surveillance -Cohorting nursing personnel -Chlorhexidine baths -Antibiotic stewardship -Daily and terminal disinfection -Limits on shared equipment -Disinfection of equipment between patients FIGURE 167-1  Strategies for the prevention of dissemination of Acinetobacter baumannii in health care facilities.

extended-spectrum β-lactamases. However, A. baumannii isolates with reduced cefiderocol susceptibility have been described, and in a randomized clinical trial that included 54 critically ill patients with carbapenem-resistant A. baumannii, the end-of-study mortal­ ity was 50% in the cefiderocol arm, compared to 18% in the best available therapy arm (mostly consisting of colistin).

Polymyxins are cationic detergents that have become less popu­ lar as a result of nephrotoxicity and neurotoxicity. Additionally, polymyxins are difficult to dose, have a narrow therapeutic win­ dow, and do not reach optimal tissue concentration in the lungs, which is a common site of infection for A. baumannii. Despite its disadvantages, polymyxin B and polymyxin E (colistin) have been reintroduced in clinical practice as they retain in vitro activity against carbapenem-resistant A. baumannii. In a randomized study of patients with pneumonia due to carbapenem-resistant Acineto­ bacter, patients receiving colistin in combination with high-dose ampicillin-sulbactam had a higher rate of clinical improvement by day 5 compared to those receiving colistin monotherapy. The combination of colistin plus meropenem was long favored due to in vitro synergy; however, two randomized controlled trials showed this strategy had comparable outcomes to colistin monotherapy. Several tetracycline derivatives have in vitro activity against A. baumannii. Of them, tigecycline and minocycline could be con­ sidered as part of a combination regimen, used at high doses when minimum inhibitory concentrations are low. Although doxycycline is a widely available agent with established breakpoints against A. baumannii, it is usually less active than minocycline. Eravacycline is a newer tetracycline with promising activity against A. baumannii; however its use has been limited; it is currently being marketed to treat more resistant strains. CHAPTER 167 Bacteriophage therapy against multidrug-resistant A. baumannii has been reported with varied success rates. Furthermore, dosing and duration of therapy vary by syndrome and resistance can also arise during treatment. Acinetobacter Infections ■ ■COMPLICATIONS AND PROGNOSIS Infections caused by A. baumannii can be associated with high mor­ tality rates. Factors contributing to higher mortality are thought to include severity of the patient’s underlying illness and drug resistance in the infecting strain. A. baumannii– negative patient -Physical separation from A. baumannii–positive patients -Cohorting nursing personnel -Chlorhexidine baths -Antibiotic stewardship

51 - 168 Helicobacter pylori Infections

168 Helicobacter pylori Infections

■ ■INFECTION CONTROL AND PREVENTION Acinetobacter species are capable of surviving on hospital surfaces for prolonged periods. In the hospital environment, A. baumannii has been associated with establishment of a fecal patina; this term refers to a coating of enteric organisms that can cover the skin of colonized patients and extend to their surrounding environment. Concentrations of enteric organisms are highest in the colonized patient’s rectum, with spread in a target-like concentric pattern covering the patient’s body and the surrounding environment. High-frequency touch areas in rooms occupied by patients colonized with A. baumannii are more likely to be contaminated. The hands, gloves, and gowns of health care workers can be contaminated after entry into the room of a patient colonized with A. baumannii (Fig. 167-1).

Outbreaks caused by A. baumannii are frequently mono- or oligo­ clonal. A common source of infection has been identified in ~50% of outbreaks. These sources include respiratory therapy equipment, the hands of health care workers, bedside humidifiers, warm bathwater, hospital-prepared distilled water, bedpans, urine jugs, heparinized saline solution, mattresses, reusable pressure transducers in arterial lines, and fluids used for pressure lavage of wounds. Control of multidrug-resistant Acinetobacter outbreaks starts with early recognition, with subsequent halting of the spread of infec­ tion throughout a facility and prevention of the establishment of an endemic strain. It is important to identify the outbreak strain and dif­ ferentiate it from nonoutbreak strains so that infection control activi­ ties can be better targeted. Traditionally, the strain was identified with phenotypic typing systems (biotyping) or by determination of antimi­ crobial susceptibility patterns. Whole genome sequencing (WGS) has been proven an effective tool during outbreak investigations, usually deployed in response to an increase in the number of patients infected or colonized with a multidrug-resistant organism. However, an emerg­ ing body of evidence supports the prospective use of WGS for infec­ tion prevention, as it leads to early detection of transmission pathways and the potential for halting spread sooner and limiting cluster sizes through targeted infection prevention interventions. PART 5 Infectious Diseases During outbreaks, the simultaneous introduction of multiple (“bun­ dled”) measures makes it difficult to assess the impact of each indi­ vidual measure. These interventions include aggressive cleaning of the general environment, active surveillance, contact isolation of colonized or infected patients, cohorting of medical staff, reinforcement of com­ pliance with hand hygiene by health care workers, and use of aseptic care devices. Colonization with A. baumannii is a strong predictor of subsequent clinical infection by this organism. Exposure to carbapenems is a risk factor for initial acquisition of this pathogen; therefore, efforts to cur­ tail unnecessary use of antibiotics are fundamental to the prevention of A. baumannii colonization of patients and the organism’s establishment in health care facilities. ■ ■FURTHER READING Adams-Haduch JM et al: Molecular epidemiology of carbapenemnonsusceptible Acinetobacter baumannii in the United States. J Clin Microbiol 49:3849, 2011. Antunes LC et al: Acinetobacter baumannii: Evolution of a global pathogen. Pathog Dis 71:292, 2014. Chen W: Host innate immune responses to Acinetobacter baumannii infection. Front Cell Infect Microbiol 10:486, 2020. Dexter C et al: Community-acquired Acinetobacter baumannii: Clini­ cal characteristics, epidemiology and pathogenesis. Expert Rev Anti Infect Ther 13:567, 2015. Lee CR et al: Biology of Acinetobacter baumannii: Pathogenesis, antibi­ otic resistance mechanisms, and prospective treatment options. Front Cell Infect Microbiol 7:55, 2017. Munoz-Price LS: Controlling multi-drug resistant gram-negative bacilli in your hospital: A transformational journey. J Hosp Infect 89:254, 2015. Ong SWX et al: Genomic surveillance uncovers ongoing transmission of carbapenem-resistant Acinetobacter baumannii (CRAB) and iden­ tifies actionable routes of transmissions in an endemic setting. Infect Control Hosp Epidemiol 44:460, 2023.

Peleg AY et al: Acinetobacter baumannii: Emergence of a successful pathogen. Clin Microbiol Rev 21:538, 2008. Shields RK et al: Navigating available treatment options for carbape­ nem-resistant Acinetobacter baumannii-calcoaceticus complex infec­ tions. Clin Infect Dis 76:S179, 2023. Tamma PD et al: Infectious Diseases Society of America 2024 guidance on the treatment of antimicrobial-resistant gram-negative infections. Clin Infect Dis 2024:ciae403. Available at https://doi.org/10.1093/cid/ ciae403. Accessed February 14, 2025. Tal-Jasper R et al: Clinical and epidemiological significance of car­ bapenem resistance in Acinetobacter baumannii infections. Antimi­ crob Agents Chemother 60:3127, 2016. Wong D et al: Clinical and pathophysiological overview of Acinetobacter infections: A century of challenges. Clin Microbiol Rev 30:409, 2017. Emad M. El-Omar, Martin J. Blaser

Helicobacter pylori

Infections Helicobacter pylori colonizes the stomach in ~40% of the world’s human population, essentially for life unless eradicated by antibiotic treatment. Colonization with this organism is the main risk factor for peptic ulceration (Chap. 335) as well as for gastric adenocarcinoma and gastric mucosa-associated lymphoid tissue (MALT) lymphoma (Chap. 85). Treatment for H. pylori has revolutionized the manage­ ment of peptic ulcer disease, providing a permanent cure in most cases. Such treatment also represents first-line therapy for patients with low-grade gastric MALT lymphoma. Treatment of H. pylori is of no benefit in the treatment of gastric adenocarcinoma, but preven­ tion of H. pylori colonization or eradicative treatment could poten­ tially prevent gastric malignancy and peptic ulceration. In contrast, increasing evidence indicates that lifelong H. pylori colonization may offer some protection against complications of gastroesopha­ geal reflux disease (GERD), including esophageal adenocarcinoma, although this point is still uncertain. Recent research has focused on whether H. pylori colonization is also a risk factor for some extra­ gastric diseases and whether it is protective against some recently emergent medical problems, such as childhood-onset asthma and other allergic and metabolic conditions. ■ ■ETIOLOGIC AGENT Helicobacter pylori  H. pylori is a gram-negative bacillus that has naturally colonized humans for at least 100,000 years, and probably throughout human evolution. It lives in gastric mucus, with a propor­ tion of the bacteria adherent to the mucosa and possibly a very small number of the organisms entering cells or penetrating the mucosa; the organism’s distribution is mucosal rather than systemic. Its spiral shape and flagella render H. pylori motile in the mucus environment. The organism has several acid-resistance mechanisms, most notably a highly expressed urease that catalyzes urea hydrolysis to produce buff­ ering ammonia. H. pylori is microaerophilic (i.e., grows in low levels of oxygen), is slow-growing, and requires complex growth media in vitro. Other Helicobacter Species  A small proportion of gastric Helicobacter infections are due to species other than H. pylori, possibly acquired as zoonoses. These non-pylori gastric helicobacters are associ­ ated with low-level inflammation and occasionally with disease. In immu­ nocompromised hosts, several nongastric (intestinal) Helicobacter species can cause disease with clinical features resembling those of Campylobacter infections; these species are covered in Chap. 169.

■ ■EPIDEMIOLOGY Prevalence and Risk Factors  The prevalence of H. pylori among adults is <30% in most parts of the United States, Europe, and Ocea­ nia as opposed to >60% in many parts of Africa, South America, and West Asia. In the United States, prevalence varies with age but has been dropping; an estimated 18% of adults and 14% of children and adolescents are positive. H. pylori is usually acquired in childhood. The age association is due mostly to a birth-cohort effect whereby cur­ rent 60-year-olds were more commonly colonized as children than are current children. Spontaneous acquisition or loss of H. pylori in adult­ hood is uncommon. Childhood acquisition explains why the main risk factors for infection are markers of crowding and social deprivation in childhood. Longitudinal studies have shown declining prevalences over the past half-century, concomitant with socioeconomic develop­ ment and widespread antibacterial treatments. Transmission  Humans are the only important reservoir of H. pylori. Children may acquire the organism from their parents (most often the primary caregiver) or from other children. The former is more com­ mon in developed countries and the latter in less developed countries. Whether transmission takes place more often by the fecal–oral or the oral–oral route is unknown, but H. pylori is easily cultured from vomitus and gastroesophageal refluxate and is much less easily cultured from stool. Most acquisition of H. pylori is during the early years of childhood. ■ ■PATHOLOGY AND PATHOGENESIS Long-term H. pylori colonization induces chronic superficial gastritis, a tissue response in the stomach that includes infiltration of the mucosa by both mononuclear and polymorphonuclear cells. (The term gastritis should be used specifically to describe histologic features; it has also been used to describe endoscopic appearances and even symptoms, but only magnification endoscopy correlates with microscopic findings or even with the presence of H. pylori, and even this is insufficient for diagno­ sis.) Although H. pylori is capable of numerous adaptations that prevent excessive stimulation of the immune system, colonization is accompa­ nied by a considerable persistent local and systemic immune response, including the production of antibodies and cell-mediated responses. However, these responses are ineffective in clearing the bacterium. This inefficient clearing appears to be due in part to H. pylori’s downregula­ tion of the immune system, which fosters its own persistence. Most H. pylori–colonized persons do not develop clinical sequelae. That some persons develop overt disease whereas others do not is related to a combination of factors: bacterial strain differences, host susceptibility to disease, and environmental factors. Bacterial Virulence Factors  Several H. pylori virulence fac­ tors are more common among strains that are associated with disease than among those that are not. The cag island is a group of genes that encodes a bacterial type IV secretion system. Through this system, an effector protein, CagA, is translocated into epithelial cells, where it may be activated by phosphorylation and induces host cell signal transduction; proliferative, cytoskeletal, and inflammatory changes in the cell result. The protein at the tip of the secretory appa­ ratus, CagL, binds to integrins on the cell surface, transducing further signaling. Finally, soluble components of the peptidoglycan cell wall enter the cell, mediated by the same secretory system. These compo­ nents are recognized by the intracellular bacterial receptor Nod1, which stimulates a proinflammatory cytokine response resulting in an enhanced tissue response. Carriage of cag-positive strains increases the risk of both peptic ulcer and gastric adenocarcinoma. A second major host-interaction factor is the vacuolating cytotoxin VacA, which forms pores in cell membranes. VacA is polymorphic, and carriage of more active forms also increases the risk of ulcer disease and gastric cancer. Other bacterial factors that are associated with increased disease risk include adhesins, such as BabA (which binds to blood group antigens on epithelial cells). Host Genetic and Environmental Factors  There are currently no host genetic factors that have been reproducibly associated with risk

of acquisition of H. pylori infection. The best-characterized host deter­ minants of disease are genetic polymorphisms leading to enhanced activation of the innate immune response, including polymorphisms in cytokine genes and in genes encoding bacterial recognition pro­ teins such as Toll-like receptors. For example, colonized people with polymorphisms in the interleukin 1 gene that increase the production of this cytokine in response to H. pylori infection are at increased risk of gastric adenocarcinoma. Another host genetic factor that has been linked with increased risk of gastric cancer in multiple populations is a single-nucleotide polymorphism (SNP) in the prostate stem cell antigen (PSCA) rs2294008. In addition to host genetics, environmental cofactors are important in pathogenesis. Smoking increases the risks of duodenal ulcers and gastric cancer in H. pylori–positive individuals. Diets high in salt and preserved foods increase cancer risk, whereas diets high in antioxidants and vitamin C are modestly protective.

Distribution of Gastritis and Differential Disease Risk  The pattern of gastric tissue response is associated with disease risk: antralpredominant gastritis is most closely linked with duodenal ulceration, whereas pan-gastritis and corpus-predominant gastritis are linked with gastric ulceration and adenocarcinoma. This difference probably explains why patients with duodenal ulceration are not at high risk of developing gastric adenocarcinoma later in life, despite being colonized by H. pylori. PATHOGENESIS OF DUODENAL ULCERATION  How gastric coloniza­ tion causes duodenal ulceration is now becoming clearer. H. pylori– induced tissue responses in the gastric antrum diminish the number of somatostatin-producing D cells. Because somatostatin inhibits gastrin release, gastrin levels are higher than in H. pylori–negative persons, and these higher levels lead to increased meal-stimulated acid secretion from the relatively spared gastric corpus. How this situation increases duodenal ulcer risk remains controversial, but the increased acid secretion may contribute to the formation of potentially acidprotective gastric metaplasia in the duodenum. Gastric metaplasia in the duodenum may become colonized by H. pylori and subsequently inflamed and ulcerated. CHAPTER 168 Helicobacter pylori Infections PATHOGENESIS OF GASTRIC ULCERATION AND GASTRIC ADENOCAR­ CINOMA  The pathogenesis of these conditions is less well understood, although both arise in association with pan- or corpus-predominant gastritis. The hormonal changes described above still occur, but the tissue responses in the gastric corpus mean that it produces less acid (hypochlorhydria) despite hypergastrinemia. Gastric ulcers commonly occur at the junction of antral and corpus-type mucosa, an area that is often particularly inflamed. Gastric cancer usually arises in stom­ achs with extensive atrophic gastritis and hypo- or achlorhydria, and probably stems from progressive DNA damage and the survival of abnormal epithelial cell clones. The DNA damage is thought to be due principally to reactive oxygen and nitrogen species arising from inflammatory cells, perhaps in relation to other bacteria that survive in a hypochlorhydric stomach. Longitudinal analyses of gastric biopsy specimens taken years apart from the same patient show that the common intestinal type of gastric adenocarcinoma follows stepwise changes from simple gastritis to gastric atrophy, metaplasia, and dys­ plasia. A second, diffuse type of gastric adenocarcinoma found more commonly in younger adults may arise directly from chronic gastritis without atrophic changes. In recent years, there has been a progressive rise in gastric cancers centered on the gastric corpus and occurring in younger adults (<50 years old) and disproportionately in females; this appears to be in the absence of H. pylori. PATHOGENESIS OF GASTRIC MALT LYMPHOMA  Low-grade B-cell MALT lymphomas are rare malignancies, reported at a rate of ~1 per million population per year prior to the discovery of H. pylori. Since then, reported rates have increased substantially, possibly reflecting overdiagnosis. These tumors arise from the substrate of chronic stimu­ lation of lymphocyte populations by the persistent H. pylori coloniza­ tion. Importantly, there have been numerous reports of these low-grade tumors responding dramatically to H. pylori eradication therapies. However, the boundary between true malignancy and benign lymphoid

hypertrophy is uncertain. Among responders to H. pylori eradication, most do not have the characteristic t(11;18)(q21;q21) translocation of the malignancy and may not have true malignancies but rather benign polyclonal lymphoid proliferation. CagA-positive H. pylori strains have been significantly associated with the t(11;18)(q21;q21)–positive gastric MALT lymphoma compared with translocation-negative cases.

■ ■CLINICAL MANIFESTATIONS Essentially all H. pylori–colonized persons have histologic gastritis, but only ~10–15% develop associated illnesses such as peptic ulceration, gastric adenocarcinoma, or gastric lymphoma (Fig. 168-1). Despite similar rates of H. pylori colonization, rates of these diseases among women are less than half of those among men. ■ ■PEPTIC ULCER DISEASE Worldwide, ~70% of duodenal ulcers and ~50% of gastric ulcers are related to H. pylori colonization (Chap. 335). However, in particular, the proportion of gastric ulcers caused by aspirin and nonsteroidal anti-inflammatory drugs (NSAIDs) is increasing, and in many devel­ oped countries, these drugs have overtaken H. pylori as a cause of gastric ulceration. The main lines of evidence supporting an ulcerpromoting role for H. pylori are that (1) the presence of the organism is a risk factor for the development of ulcers, (2) non-NSAID-induced ulcers rarely develop in the absence of H. pylori, (3) eradication of H. pylori virtually abolishes long-term ulcer relapse, and (4) experimental H. pylori infection of gerbils can cause gastric ulceration. Thus, H. pylori is neither necessary nor sufficient for the development of peptic ulcer disease, but it is a very strong risk factor for its occurrence, and removal of H. pylori changes the natural history of ulcer disease. PART 5 Infectious Diseases Gastric Adenocarcinoma and Lymphoma  Prospective nested case–control studies have shown that H. pylori colonization is a risk factor for adenocarcinomas of the distal (noncardia) stomach (Chap. 85). Long-term experimental infection of gerbils also may result in gastric adenocarcinoma. Moreover, H. pylori may induce primary gastric lymphoma, although this condition is much less com­ mon, and the approaches to histopathologic and cytogenetic evalua­ tions are not standardized. Many of the diagnosed low-grade gastric B-cell lymphomas are dependent on H. pylori for continuing growth Tissue response (inflammation) Primary phenomenon: Hyperacidity Atrophic gastritis Secondary phenomenon: Antigenic stimulation ? B-cell lymphoma Noncardia gastric adenocarcinoma Reflux esophagitis and sequelae Duodenal ulceration Clinical outcome: Association with H. pylori (OR): 3–8 3–6 6–50 0.2–0.6 FIGURE 168-1  Schematic of the relationships between colonization with Helicobacter pylori and diseases of the upper gastrointestinal tract. Essentially all persons colonized with H. pylori develop a host response, which is generally termed chronic gastritis. The nature of the host’s interaction with the particular bacterial population determines the clinical outcome. H. pylori colonization increases the lifetime risk of peptic ulcer disease, noncardia gastric cancer, and B-cell non-Hodgkin’s gastric lymphoma (odds ratios [ORs] for all, >3). In contrast, a growing body of evidence indicates that H. pylori colonization (especially with cagA+ strains) protects against adenocarcinoma of the esophagus (and the sometimes related gastric cardia) and premalignant lesions such as Barrett’s esophagus (ORs, <1). Although the incidences of peptic ulcer disease (cases not due to nonsteroidal anti-inflammatory drugs) and noncardia gastric cancer are declining in developed countries, the incidence of adenocarcinoma of the esophagus is increasing. (Reproduced with permission from MJ Blaser: Hypothesis: The changing relationships of Helicobacter pylori and humans: Implications for health and disease. J Inf Dis 179:1523, 1999.)

and proliferation, and these tumors may regress either fully or partially after H. pylori eradication. However, they require careful short- and long-term monitoring; any that are not confined to the superficial mucosa (and, indeed, some that are) require additional treatment with chemotherapeutic agents or radiotherapy. Functional Dyspepsia  Many patients have upper gastrointestinal symptoms but have normal results on upper gastrointestinal endoscopy (so-called functional or nonulcer dyspepsia; Chap. 335). Because H. pylori is common, some of these patients will be colonized with the organism. H. pylori eradication leads to symptom resolution up to 15% more commonly than does placebo treatment. Whether such patients have peptic ulcers in remission at the time of endoscopy or whether a small subgroup of patients with “true” functional dyspepsia respond to H. pylori treatment is unclear. Either way, because functional dyspepsia is often persistent and difficult to treat, most consensus conference guidelines recommend H. pylori eradication in these patients. If this advice is followed, it is important to realize that only a small subgroup of patients who are treated will benefit. Protection Against Peptic Esophageal Disease, Including Esophageal Adenocarcinoma  Much interest has focused on a protective role for H. pylori against GERD (Chap. 334), Barrett’s esophagus (Chap. 334), and adenocarcinoma of the esophagus and gastric cardia (Chap. 85). The main lines of evidence for this role are (1) that there is a temporal relationship between a falling prevalence of gastric H. pylori colonization and a rising incidence of these condi­ tions; (2) that, in most studies, the prevalence of H. pylori colonization (especially with proinflammatory cagA+ strains) is significantly lower among patients with these esophageal diseases than among control participants; and (3) that, in prospective nested studies (see above), the presence of H. pylori is inversely related to these cancers. The mechanism underlying this protective effect is likely H. pylori–induced hypochlorhydria. Because, at the individual level, GERD severity may decrease, worsen, or remain unchanged after H. pylori treatment, con­ cerns about GERD should not affect decisions about whether to treat H. pylori in an individual patient if a clear-cut indication exists; if there is no clear indication, clinicians should carefully balance consider­ ations of benefit and harm. Other Pathologies  H. pylori has an increasingly recognized role in other gastric pathologies. It may predispose some patients to iron deficiency through occult blood loss and/ or hypochlorhydria and reduced iron absorp­ tion. In addition, several extragastrointestinal pathologies have been linked with H. pylori colonization, although evidence of causality is less strong. Studies of H. pylori treatment in idiopathic thrombocytopenic purpura have consistently described improvement in or even normalization of platelet counts. Potentially important but even more controversial (pro­ tective) associations are with ischemic heart disease and cerebrovascular disease. However, the strength of the latter associations is reduced if confounding factors are taken into account, and our present knowledge is incomplete. Most authorities consider the associations to be non­ causal. An increasing number of studies have shown an inverse association of cagA+ H. pylori with childhood-onset asthma, hay fever, and atopic disorders. These associations have been shown to be causal in animal models, but the effect size in humans has not been established. ■ ■DIAGNOSIS Tests for H. pylori fall into two groups: tests that require upper gastrointestinal endoscopy and simpler tests that can be performed in the clinic (Table 168-1).

TABLE 168-1  Tests Commonly Used to Detect Helicobacter pylori TEST ADVANTAGES DISADVANTAGES Tests Based on Endoscopic Biopsy Biopsy urease test Quick, simple Some commercial tests not fully sensitive before 24 h Histology May give additional histologic information Sensitivity dependent on experience and use of special stains Culture Permits determination of antibiotic susceptibility Sensitivity dependent on experience Noninvasive Tests Serology Inexpensive and convenient; not affected by recent antibiotics or proton pump inhibitors to the same extent as breath and stool tests Cannot be used to monitor treatment success; some commercial kits inaccurate, and most less accurate than urea breath test 13C urea breath test Inexpensive and simpler than endoscopy; useful for follow-up after treatment Requires fasting; not as convenient as blood or stool tests Stool antigen test Inexpensive and convenient; Stool-based tests disliked by people from some cultures useful for follow-up after treatment; may be particularly useful in children Endoscopy-Based Tests  Endoscopy is usually unnecessary in the initial management of young patients with simple dyspepsia but is commonly used to exclude malignancy and make a positive diagnosis in older patients or those with “alarm” symptoms. If endoscopy is per­ formed, the most convenient biopsy-based test is the biopsy urease test, in which one large or two small gastric biopsy specimens are placed into a gel containing urea and an indicator. The H. pylori urease enzyme hydrolyses urea in gastric juice and generates ammonia. This leads to a rise in pH and therefore to a color change, which often occurs within minutes but can require up to 24 h. Histologic examination of biopsy specimens for H. pylori also is accurate, provided that a special stain (e.g., a modified Giemsa, silver, or immuno-stain) permitting optimal visual­ ization of the organism is used. If biopsy specimens are obtained from both antrum and corpus, histologic study yields additional information, including the degree and pattern of inflammation and the presence of any atrophy, metaplasia, or dysplasia. Microbiologic culture is most spe­ cific but may be insensitive because of difficulty with H. pylori isolation. Once the organism is cultured, its identity as H. pylori can be confirmed by its typical appearance on Gram’s stain and its positive reactions in oxidase, catalase, and urease tests. Moreover, the organism’s susceptibility to antibiotics can be determined, and this information can be clinically useful in difficult cases. The occasional biopsy specimens containing the less common non-pylori gastric helicobacters give weakly positive results in the biopsy urease test. Positive identification of these bacteria requires visualization of the characteristic long, tight spirals in histologic sections; they cannot easily be cultured. Noninvasive Tests  Noninvasive H. pylori testing is the norm if gastric cancer does not need to be excluded by endoscopy. The longestestablished test (and a very accurate one) is the urea breath test. In this simple test, the patient drinks a solution of urea labeled with the nonradioactive isotope 13C and then blows into a tube. If H. pylori urease is present, the urea is hydrolyzed, and labeled carbon dioxide is detected in breath samples. The stool antigen test, a simple and accu­ rate test using monoclonal antibodies specific for H. pylori antigens, is more convenient and less expensive than the urea breath test, but some patients dislike sampling stool. The simplest tests for ascertaining

H. pylori status are serologic assays measuring specific IgG levels in serum by enzyme-linked immunosorbent assay or immunoblot. The best of these tests are nearly as accurate as other diagnostic methods, but many commercial tests—especially rapid office tests—do not perform well. Use of Tests to Assess Treatment Success  The urea breath test, the stool antigen test, and biopsy-based tests can all be used to assess

the success of treatment (Fig. 168-2). However, because these tests are dependent on H. pylori load, their use <4 weeks after treatment may yield false-negative results. Early suppression of bacterial numbers may lead to false-negative results since regrowth of the organism can result in its detection weeks later. For the same reason, these tests are unreli­ able if performed within 4 weeks of intercurrent treatment with antibi­ otics or bismuth compounds or within 2 weeks of the discontinuation of proton pump inhibitor (PPI) treatment. In the assessment of treat­ ment success, noninvasive tests are normally preferred. However, after gastric ulceration, endoscopy should be repeated to ensure healing and exclude gastric carcinoma by further histologic sampling; if PPIs have been stopped for at least 2 weeks and no antibiotics or bismuth com­ pounds have been given for at least 6 weeks, there is an opportunity to assess treatment success with biopsy-based tests. Serologic tests are not used to monitor treatment success, as the gradual drop in titer of H. pylori–specific antibodies is too slow (requiring >14 weeks) to be of practical use.

TREATMENT Helicobacter pylori Infection INDICATIONS The most clear-cut indications for treatment are H. pylori–related duodenal or gastric ulceration or low-grade gastric B-cell MALT lymphoma. Whether or not the ulcers are currently active, H. pylori should be eradicated in patients with documented ulcer disease to prevent relapse (Fig. 168-2). Guidelines have recommended H. pylori treatment for colonized patients with functional dyspepsia in case they are among the small percentage who will benefit from such therapy (beyond placebo effects). H. pylori eradication in the treatment of conditions not definitively known to respond has also been recommended but is not universally supported; such condi­ tions include idiopathic thrombocytopenic purpura, vitamin B12 deficiency, and iron-deficiency anemia where other causes have been carefully excluded. For individuals with a strong family his­ tory of gastric cancer, treatment to eradicate H. pylori in the hope of reducing cancer risk is reasonable but of unproven value: it slightly reduces future cancer incidence, but there is no evidence it reduces all-cause mortality. For older dyspeptic patients in the community or those who have “alarm” symptoms (e.g., weight loss) associated with their dyspepsia, upper gastrointestinal endoscopy is indicated to seek a diagnosis and test for H. pylori; the decision regarding whether to eradicate the organism can then be based on indication. Endoscopy is usually considered unnecessary for young dyspeptic patients in the community who have no alarm symptoms (with the precise age cutoff dependent on local guidelines). If the community prevalence of H. pylori is below ~20%, such patients are treated with a short course of acid suppression using a PPI. If these patients do not respond or relapse when treatment is stopped, or if the

H. pylori community prevalence is >20%, many national guidelines recommend a strategy of testing for H. pylori noninvasively and eradicating it if it is found. This strategy will benefit patients who have peptic ulcers and the ~5−10% of patients who have functional dyspepsia responsive to H. pylori eradication, but most patients will be treated unnecessarily. Currently, widespread community screening for and treatment of H. pylori as primary prophylaxis for gastric cancer and peptic ulcers are not recommended in most countries, mainly because the extent of the consequent reduction in cancer risk is not known. Several studies have found a modestly reduced cancer risk after treatment, but the period of follow-up is still fairly short, and the magnitude of the effect in different populations remains unclear. Other reasons not to treat H. pylori in asymptomatic populations at present include (1) the adverse side effects (which are common and can be severe in rare cases) of the multiple-antibiotic regimens used; (2) antibiotic resistance, which may emerge in H. pylori or other incidentally carried bac­ teria; (3) the anxiety that may arise in otherwise healthy people, especially if treatment is unsuccessful; and (4) the existence of a CHAPTER 168 Helicobacter pylori Infections

Indication for H. pylori treatment (e.g., peptic ulcer disease or new-onset dyspepsia) Test for H. pylori Positive First-line treatment (Table 168-2) Wait at least 1 month after treatment finishes (no antibiotics, bismuth compounds, or proton pump inhibitors in the meantime) Positive Urea breath test or stool antigen test* Second-line treatment (Table 168-2) Positive after second-line treatment Third-line treatment; endoscopy with H. pylori culture and sensitivity testing; treat according to known antibiotic sensitivities FIGURE 168-2  Algorithm for the management of Helicobacter pylori infection. *Note that either the urea breath test or the stool antigen test can be used in this algorithm. Occasionally, endoscopy and a biopsy-based test are used instead of either of these tests in follow-up after treatment. The main indication for these invasive tests is in follow-up after gastric ulceration; in this condition, as opposed to duodenal ulceration, it is important to check healing and exclude underlying gastric adenocarcinoma. However, even in this situation, patients undergoing endoscopy may still be receiving proton pump inhibitor therapy, which precludes H. pylori testing. Thus, a urea breath test or a stool antigen test is still required at a suitable interval after the end of therapy to determine whether treatment has been successful (see text). Some authorities use empirical third-line regimens, of which several have been described. PART 5 Infectious Diseases subset of people who will develop GERD symptoms after treat­ ment. Despite the absence of screening strategies, many doctors treat H. pylori if it is known to be present (particularly in children and younger adults), even when the patient is asymptomatic. The rationale is that it reduces patient concern and may reduce future gastric cancer risk and that any reduction in risk is likely to be greater in younger patients. However, such practices do not factor in any potential benefits of H. pylori colonization. Overall, despite widespread clinical activity in this area, most treatment of persons with asymptomatic H. pylori carriage is given with no firm evi­ dence base. Because a proportion of patients (up to 70%) of those diagnosed with gastric low-grade B-cell MALT lymphomas respond to H. pylori eradication, it should be used in all cases, regardless of whether H. pylori can be detected by the diagnostic modalities used since there may be falsely negative results. However, not all of these cases represent true malignancies, so the reported success TABLE 168-2  Commonly Recommended Treatment Regimens for Helicobacter pylori REGIMENa (DURATION) DRUG 1 DRUG 2 DRUG 3 DRUG 4 Regimen 1: OCM (14 days)b Omeprazole (20 mg bidc) Clarithromycin (500 mg bid) Metronidazole (500 mg bid) — Regimen 2: OCA (14 days)b Omeprazole (20 mg bidc) Clarithromycin (500 mg bid) Amoxicillin (1 g bid) — Regimen 3: OBTM (14 days)d Omeprazole (20 mg bidc) Bismuth subsalicylate

(2 tabs qid) Regimen 4: concomitant (14 days)e Omeprazole (20 mg bidc) Amoxicillin (1 g bid) Clarithromycin (500 mg bid) Tinidazole (500 mg bidf) Regimen 5: OAL (10 days)g Omeprazole (20 mg bidc) Amoxicillin (1 g bid) Levofloxacin (500 mg bid or 200 mg bid) — aThe recommended first-line regimens for most of the world are shown in bold type. bThis regimen should be used only for populations in which the prevalence of clarithromycin-resistant strains is known to be <20%. In practice, this restriction limits the regimens’ appropriate range mainly to northern Europe. cMany authorities and some guidelines recommend doubling this dose of omeprazole as trials show resultant increased efficacy with some antibiotic combinations. Omeprazole may be replaced with any proton pump inhibitor (PPI) at an equivalent dosage. Because extensive metabolizers of PPIs are prevalent among Caucasian populations, many authorities recommend esomeprazole (40 mg bid) or rabeprazole (20 mg bid), particularly for regimens 4 and 5. dData supporting this regimen come mainly from Europe and are based on the use of bismuth subcitrate (1 tablet qid) and metronidazole (400 mg tid). This is a recommended first-line regimen in most countries and is the recommended secondline regimen in northern Europe. eThis regimen may be used as an alternative to regimen 3. fMetronidazole (500 mg bid) may be used as an alternative. gThese regimens are used as second-line treatment in many countries (particularly where quadruple or concomitant therapy is used as the first-line regimen) and as third-line treatment in others. They may be less effective where rates of fluoroquinolone use are high and are more likely to be ineffective if there is a personal history of fluoroquinolone use for previous treatment of other infections.

Negative H. pylori not the cause Any remaining symptoms are not due to H. pylori Negative Positive after third-line treatment Refer to specialist Consider whether treatment is still indicated rate may reflect the eradication of benign processes. Examination of tissues for the characteristic chromosomal translocations should be done to help distinguish benign and malignant processes and to guide further therapeutic approaches. These generally are slowly progressive tumors, so the time needed for H. pylori eradication and subsequent evaluation will not interfere with the use of subsequent chemotherapy and/or radiotherapy, if needed. REGIMENS Although H. pylori is susceptible to a wide range of antibiotics in vitro, monotherapy is not usually successful, probably because of inadequate active antibiotic delivery to the colonization niche. Clini­ cal failure of monotherapy prompted the development of multidrug regimens. Current regimens consist of a PPI and two or three antimi­ crobial agents given for 10–14 days (Table 168-2). The optimal regi­ mens vary in different parts of the world, depending on the known Tetracycline HCl (500 mg qid) Metronidazole (500 mg tid)

52 - 169 Infections Due to Campylobacter and Related Organisms

169 Infections Due to Campylobacter and Related Organisms

rates of primary antibiotic resistance in most H. pylori strains in a particular locale. For this reason, guidelines on optimal regimens for H. pylori eradication in individual countries are evolving, and physi­ cians should refer to the most up-to-date local guideline. The two most important factors in successful H. pylori treat­ ment are the patient’s close compliance with the regimen and the use of drugs to which the patient’s strain of H. pylori has not acquired resistance. Treatment failure following minor lapses in compliance is common and often leads to acquired resistance. To stress the importance of compliance, written instructions should be given to the patient, and minor side effects of the regimen should be explained. Increasing levels of primary H. pylori resistance to macrolides (azithromycin and clarithromycin are primarily used), levofloxacin, and—to a lesser extent—metronidazole are of grow­ ing concern. In most parts of the world (the main exception being northwestern Europe), the rate of primary macrolide resistance is sufficiently high that regimens containing clarithromycin plus one other antibiotic often fail; regimens with clarithromycin and two other antibiotics remain an option as the other two antibiotics are likely to eradicate H. pylori even if the strain is macrolide -resistant. When a patient is known to have been exposed—even remotely in time—to clarithromycin or a fluoroquinolone, these antibiotics usually should be avoided. Resistance to amoxicillin or tetracycline is unusual, even if these antibiotics have been given previously, and resistance to metronidazole is only partial; thus, there is no need to avoid using these antibiotics whether or not they have been previously prescribed. Whichever antibiotic regimen is used, meta-analyses show that using high rather than moderate doses of acid-suppressive PPIs with the antibiotics increases the effective­ ness of the regimen. Similarly, use of vonoprazan, a highly effective potassium-competitive acid blocker, originally licensed in Japan and now in several countries, including the United States, was asso­ ciated with higher eradication rates in conjunction with amoxicillin and clarithromycin, than when a PPI was used for acid suppression. Assessment of antibiotic susceptibilities before treatment would be optimal but is not usually undertaken because endoscopy and mucosal biopsy are necessary to obtain H. pylori for culture and because most microbiology laboratories are inexperienced in H. pylori culture. If initial H. pylori treatment fails, the usual approach is empirical re-treatment with another drug regimen (Table 168-2). The third-line approach ideally should be endoscopy, biopsy, and culture plus treatment based on documented antibiotic sensitivities. However, empirical third-line therapies are often used. Non-pylori gastric helicobacters are treated in the same way as H. pylori. However, in the absence of trials, it is unclear whether a positive outcome always represents successful treatment or whether it is sometimes due to natural clearance of the bacteria. ■ ■PREVENTION Carriage of H. pylori has considerable public health significance in economically richer countries, where it is associated with peptic ulcer disease and gastric adenocarcinoma, and in some, but not all, economi­ cally poorer countries, where gastric adenocarcinoma may be an even more common cause of cancer death late in life. If mass prevention were contemplated, vaccination would be the most obvious method: experi­ mental immunization of animals has given promising results, but vac­ cines in humans have thus far not been successful. However, given that H. pylori has co-evolved with its human host over millennia, preventing colonization on a population basis may have biological and clinical costs. For example, lifelong absence of H. pylori may be a risk factor for GERD complications, including esophageal adenocarcinoma. We have specu­ lated that the disappearance of H. pylori may also be associated with an increased risk of other emergent diseases reflecting aspects of the current Western lifestyle, such as childhood-onset asthma and allergy, as sup­ ported by both epidemiologic and animal model studies. Acknowledgment The authors wish to thank John C. Atherton, MD, FRCP, for his prior contributions to this chapter.

■ ■FURTHER READING Amieva M, Peek RM: Pathobiology of Helicobacter pylori–induced

gastric cancer. Gastroenterology 150:64, 2016. Anderson WF et al: The changing face of noncardia gastric cancer inci­ dence among US non-Hispanic whites. J Natl Cancer Inst 110:608, 2018. Arnold IC et al: Helicobacter pylori infection prevents allergic asthma in mouse models through the induction of regulatory T cells. J Clin Invest 121:3088, 2011. Chen MJ et al: Molecular testing-guided therapy versus susceptibility testing-guided therapy in first-line and third-line Helicobacter pylori eradication: Two multicentre, open-label, randomised controlled, non-inferiority trials. Lancet Gastroenterol Hepatol 8:623, 2023. Chen Y, Blaser MJ: Inverse associations of Helicobacter pylori with asthma and allergies. Arch Intern Med 167:821, 2007. Chen Y et al: Association between Helicobacter pylori and mortality in the NHANES II study. Gut 62:1262, 2013. Chen YC et al: Global prevalence of Helicobacter pylori infection and incidence of gastric cancer between 1980 and 2022. Gastroenterology 166:605, 2024. Chow WH et al: An inverse relation between cagA+ strains of Heli­ cobacter pylori infection and risk of esophageal and gastric cardia adenocarcinoma. Cancer Res 58:588, 1998. Ford AC et al: Helicobacter pylori eradication therapy to prevent gastric cancer in healthy asymptomatic infected individuals: System­ atic review and meta-analysis of randomized controlled trials. BMJ 348:g3174, 2014. Graham DY et al: Rifabutin-based triple therapy (RHB-105) for Heli­ CHAPTER 169 cobacter pylori eradication: A double-blind, randomized, controlled trial. Ann Intern Med 172:795, 2020. Hooi JKY et al: Global prevalence of Helicobacter pylori infection: Sys­ tematic review and meta-analysis. Gastroenterology 153:420, 2017. Liou JM et al: Second-line levofloxacin-based quadruple therapy ver­ sus bismuth-based quadruple therapy for Helicobacter pylori eradi­ cation and long-term changes to the gut microbiota and antibiotic resistome: A multicentre, open-label, randomised controlled trial. Lancet Gastroenterol Hepatol 8:228, 2023. Marshall BJ, Warren JR: Unidentified curved bacilli in the stomach Infections Due to Campylobacter and Related Organisms of patients with gastritis and peptic ulceration. Lancet 1:1311, 1984. Plummer M et al: Global burden of gastric cancer attributable to Heli­ cobacter pylori. Int J Cancer 136:487, 2015. Tshibangu-Kabamba E, Yamaoka Y: Helicobacter pylori infection and antibiotic resistance: From biology to clinical implications. Nat Rev Gastroenterol Hepatol 18:613, 2021. Martin J. Blaser

Infections Due to

Campylobacter and Related Organisms ■ ■DEFINITION Bacteria of the genus Campylobacter and of the related genera Arcobacter and Helicobacter (Chap. 168) cause a variety of inflamma­ tory conditions. Although acute diarrheal illnesses are most common, these organisms may cause infections in virtually all parts of the body, especially in compromised hosts, and these infections may have late nonsuppurative sequelae. The designation Campylobacter comes from the Greek for “curved rod” and refers to the organism’s vibrio-like morphology.

■ ■ETIOLOGY Campylobacters are motile, non-spore-forming, curved, gram-negative rods. Originally known as Vibrio fetus, these bacilli were reclassified as a new genus in 1973 after their dissimilarity to other vibrios was recog­ nized. More than 20 species have since been identified. These species are currently divided into three genera: Campylobacter, Arcobacter, and Helicobacter. Not all of the species are pathogens of humans. The human pathogens fall into two major groups: those that primarily cause diarrheal disease and those that cause extraintestinal infection. The principal diarrheal pathogen is Campylobacter jejuni, which accounts for 80–90% of all cases of recognized illness due to campylo­ bacters and related genera. Other organisms that can cause diarrheal disease include Campylobacter coli, Campylobacter upsaliensis, Cam­ pylobacter lari, Campylobacter hyointestinalis, Campylobacter fetus, Arcobacter butzleri, Arcobacter cryaerophilus, Helicobacter cinaedi, and Helicobacter fennelliae. The two Helicobacter species causing diarrheal disease, H. cinaedi and H. fennelliae, are intestinal rather than gastric organisms; in terms of the clinical features of the illnesses they cause, these species most closely resemble Campylobacter rather than Helico­ bacter pylori (Chap. 168) and thus are considered in this chapter. The pathogenic roles of Campylobacter concisus (a member of the normal human oral microbiome), Campylobacter ureolyticus, and Campylo­ bacter troglodytis are uncertain. A new subspecies—C. fetus subspecies testudinum—has been described, chiefly in Asian patients; the very close resemblance of human isolates to strains isolated from reptiles suggests a food source.

The major species causing extraintestinal illnesses is C. fetus. However, any of the diarrheal agents listed above may cause systemic or localized infection as well, especially in compromised hosts. Nei­ ther aerobes nor strict anaerobes, these microaerophilic organisms are adapted for survival in the gastrointestinal mucous layer. This chapter focuses on C. jejuni and C. fetus as the major pathogens and prototypes for their groups. The key features of infection are listed by species (excluding C. jejuni, described in detail in the text below) in Table 169-1. PART 5 Infectious Diseases ■ ■EPIDEMIOLOGY Campylobacters are found in the gastrointestinal tract of many animals used for food (including poultry, cattle, sheep, and swine) and many household pets (including birds, dogs, and cats). These microorgan­ isms often do not cause illness in their animal hosts, but occasionally TABLE 169-1  Clinical Features Associated with Infection Due to “Atypical” Campylobacter and Related Species Implicated as Causes of Human Illness SPECIES COMMON CLINICAL FEATURES LESS COMMON CLINICAL FEATURES ADDITIONAL INFORMATION Campylobacter coli Fever, diarrhea, abdominal pain Bacteremiaa Clinically indistinguishable from C. jejuni Campylobacter fetus Bacteremiaa, sepsis, meningitis, vascular infections Diarrhea, relapsing fevers Not usually isolated from media containing cephalothin or incubated at 42°C Campylobacter upsaliensis Watery diarrhea, low-grade fever, abdominal pain Bacteremiaa, abscesses Difficult to isolate because of cephalothin susceptibility Campylobacter lari Abdominal pain, diarrhea Colitis, appendicitis Seagulls frequently colonized; organism often transmitted to humans via contaminated water Campylobacter hyointestinalis Watery or bloody diarrhea, vomiting, abdominal pain Bacteremiaa Causes proliferative enteritis in swine Helicobacter fennelliae Chronic mild diarrhea, abdominal cramps, proctitis Bacteremiaa Best treated with fluoroquinolones Helicobacter cinaedi Chronic mild diarrhea, abdominal cramps, proctitis Bacteremiaa Best treated with fluoroquinolones; identified in healthy hamsters Campylobacter jejuni subspecies doylei Diarrhea Chronic gastritis, bacteremiab Uncertain role as human pathogen Arcobacter cryaerophilus Diarrhea Bacteremiaa Poultry, seafood sources. Cultured under aerobic conditions Arcobacter butzleri Fever, diarrhea, abdominal pain, nausea; or asymptomatic Bacteremiaa, appendicitis Cultured under aerobic conditions; enzootic in nonhuman primates Campylobacter sputorum Pulmonary, perianal, groin, and axillary abscesses; diarrhea Bacteremia Three clinically relevant biovars: sputorum, faecalis, and paraureolyticus aIn compromised hosts, especially including the elderly, and patients with immunodeficiencies, diabetes, and infection with HIV. bIn children. Source: Adapted from BM Allos, MJ Blaser: Clin Infect Dis 20:1092, 1995.

this can occur (especially in puppies); in such circumstances, puppy-tohuman transmission may be detected. In most cases, campylobacters are transmitted to humans in raw or undercooked food products or through direct contact with infected animals. In the United States and other developed countries, ingestion of contaminated poultry that has not been sufficiently cooked is the most common mode of acquisition (30–70% of cases). Other modes include ingestion of raw (unpasteur­ ized) milk or untreated water, contact with infected household pets, ingestion of contaminated seafood, travel to developing countries (campylobacters being a leading cause of traveler’s diarrhea; Chaps. 130 and 138), oral–anal sexual contact, cross-contamination from any of these sources, and (occasionally) contact with an index case who is incontinent of stool. Campylobacter infections are common. Active surveillance of food­ borne infections in the United States estimates the incidence of diarrheal disease due to campylobacters at ~20 cases per 100,000 persons—similar in incidence to Salmonella and more common than Shigella. Infections occur throughout the year, but the incidence peaks during summer and early autumn. Persons of all ages are affected; however, attack rates for C. jejuni are highest among young children and young adults, whereas those for C. fetus are highest at the extremes of age. Systemic infections due to C. fetus (and to other Campylobacter and related species) are most common among compromised hosts. Persons at increased risk include those with AIDS, immunoglobulin deficiencies, neoplasia, liver disease, diabetes mellitus, and generalized atherosclerosis as well as neonates and pregnant women; proton pump inhibitor use also increases risk. However, apparently healthy nonpreg­ nant persons occasionally develop transient Campylobacter bacteremia as part of a gastrointestinal illness (0.1–1% of cases). In contrast, in many developing countries where sanitation is poor, C. jejuni infections are hyperendemic, with the highest rates among children <2 years old. According to large prospective cohort studies in low- to middle-income countries, C. jejuni infections—even when asymptomatic—are associated with short stature (stunting) and with a particular microbiome signature. Rates of clinically apparent infec­ tion fall with age, as does the illness-to-infection ratio, consistent with progressive development of immunity. ■ ■PATHOLOGY AND PATHOGENESIS C. jejuni infections may be subclinical, especially in hosts in developing countries who have had multiple prior infections and may be partially

immune. Symptomatic infections mostly occur within 2–4 days (range, 1–7 days) of exposure to the organism. The sites of tissue injury include the jejunum, ileum, and colon. Biopsies show an acute nonspecific inflammatory reaction, with neutrophils, monocytes, and eosinophils in the lamina propria, as well as damage to the epithelium, including loss of mucus, glandular degeneration, and crypt abscesses. Biopsy findings may be consistent with Crohn’s disease or ulcerative colitis, but these “idiopathic” chronic inflammatory diseases should not be diagnosed unless infectious colitis, specifically including that due to infection with Campylobacter species and related organisms, has been ruled out. The components of protective immunity to Campylobacter in humans are poorly understood. The high frequency of C. jejuni infections and their severity and recurrence among immunoglobulindeficient patients suggest that antibodies are important in protective immunity. Experience from field studies and human experimental infection models suggests that immune protection may be short-lived or incomplete in the absence of continuous exposure. Knowledge of the pathogenesis of infection is also incomplete. Both the motility of the strain and its capacity to adhere to host tissues appear to favor disease, but classic enterotoxins and cytotoxins (including cytolethal distending toxin) appear not to play substantial roles in tissue injury or disease production. The organisms have been visualized within the epithelium, albeit in low numbers. The documentation of a significant tissue response and occasionally of C. jejuni bacteremia further sug­ gests that tissue invasion is clinically significant, and in vitro studies are consistent with this pathogenic feature. The pathogenesis of C. fetus infections is better defined. Virtually all clinical isolates of C. fetus possess a proteinaceous capsule-like struc­ ture (an S-layer) that renders the organisms resistant to complementmediated killing and opsonization. As a result, C. fetus can cause bacteremia and can seed sites beyond the intestinal tract. The ability of the organism to switch the S-layer proteins expressed—a phenomenon that results in antigenic variability—may contribute to the chronicity and high rate of recurrence of C. fetus infections in compromised hosts. ■ ■CLINICAL MANIFESTATIONS The clinical features of infections due to Campylobacter and the related Arcobacter and intestinal Helicobacter species causing enteric disease appear to be highly similar. C. jejuni can be considered the prototype, in part because it is by far the most common enteric pathogen in the group. A prodrome of fever, headache, myalgia, and/or malaise often occurs 12–48 h before the onset of diarrheal symptoms. The most common signs and symptoms of the intestinal phase are diarrhea, abdominal pain, and fever. The degree of diarrhea varies from several loose watery stools to visibly bloody stools (~10% of cases in adults); most patients presenting for medical attention have ≥10 bowel move­ ments on the worst day of illness. Abdominal pain usually consists of cramping and may be the most prominent symptom. Pain is usually generalized but may become localized; C. jejuni infection may cause pseudoappendicitis. Fever may be the only initial manifestation of C. jejuni infection, a situation mimicking the early stages of typhoid fever. Febrile young children may develop convulsions, and both myocar­ ditis and pericarditis have been observed, especially in young men. Campylobacter enteritis is generally self-limited; however, symptoms persist for >1 week in 10–20% of patients seeking medical attention, and clinical relapses occur in 5–10% of untreated patients. Studies of common-source epidemics indicate that milder illnesses or asymptom­ atic infections may commonly occur. C. fetus may cause a diarrheal illness similar to that due to C. jejuni, especially in immunocompetent hosts. This organism also may cause either intermittent diarrhea or nonspecific abdominal pain without localizing signs. Sequelae are uncommon, and the outcome is benign. C. fetus may also cause a prolonged relapsing systemic illness (with fever, chills, and myalgias) with bacteremia that has no obvi­ ous primary source; this manifestation is especially common among compromised hosts. Secondary seeding of an organ (e.g., meninges, brain, bone, urinary tract, or soft tissue) complicates the course, which may be fulminant. C. fetus infections have a tropism for vascular sites:

endocarditis, mycotic aneurysm, and septic thrombophlebitis may all occur. Infection during pregnancy often leads to fetal death. A variety of Campylobacter species and H. cinaedi can cause recurrent cellulitis with fever and bacteremia in immunocompromised hosts.

■ ■COMPLICATIONS About 90% of bacteremic infections are caused by C. jejuni and C. fetus, in roughly equal proportions, despite the manyfold greater incidence of C. jejuni infections. Overall, mortality has been reported at ~12%. Except in infection with C. fetus, bacteremia is uncommon, developing most often in compromised hosts—including those who are immuno­ compromised and diabetic—and at the extremes of age. Three patterns of extraintestinal infection have been noted: (1) transient bacteremia in a normal host with enteritis (benign course, no specific treatment needed); (2) sustained bacteremia or focal infection in a normal host (bacteremia originating from enteritis, with patients responding well to antimicrobial therapy); and (3) sustained bacteremia or focal infec­ tion in a compromised host. Enteritis may not be clinically apparent. Immediate antimicrobial therapy, possibly prolonged, is necessary for suppression or cure of these infections. Campylobacter, Arcobacter, and intestinal Helicobacter infections in the elderly as well as in patients with AIDS or immunoglobulindeficient patients (most often common variable immunodeficiency) may be severe, persistent, and extraintestinal; relapse after cessation of therapy is common. Immunoglobulin-deficient patients also may develop osteomyelitis and an erysipelas-like rash or cellulitis. Local suppurative complications of infection include cholecystitis, pancreatitis, and cystitis; distant complications include meningitis, endocarditis, and endovasculitis (leading to mycotic aortic aneurysm), osteoarticular infection, peritonitis, cellulitis, and septic abortion. All these complications are rare, except in immunocompromised or elderly hosts. Hepatitis, interstitial nephritis, and the hemolytic-uremic syndrome occasionally complicate acute infection. The two most com­ mon postinfectious sequelae are reactive arthritis and Guillain-Barré syndrome. Reactive arthritis has been reported in up to 2.5% of cases, although nonspecific rheumatologic symptoms are more common (~10%). Reactive arthritis may develop several weeks after infection; population-based analysis shows that there is no association with the HLA-B27 phenotype. The knees are most frequently involved, but involvement of the ankles, wrists, and small joints of the hands is com­ mon, with an average of 3.2 joints affected. Guillain-Barré syndrome or its Miller Fisher (cranial polyneuropathy) variant follow either symp­ tomatic or asymptomatic Campylobacter infections uncommonly—i.e., in 1 of every 1000–2000 cases or, for certain C. jejuni serotypes (such as O19), in 1 of every 100–200 cases. Despite the low frequency of this complication, it is estimated that Campylobacter infections, because of their high incidence, may trigger 20–40% of all cases of Guillain-Barré syndrome. The presence of sialylated lipopolysaccharides on C. jejuni strains prompts a form of molecular mimicry that promotes autoim­ mune recognition of sialylated cell-surface molecules on axons. Immu­ noproliferative small-intestinal disease (alpha chain disease), a form of lymphoma that originates in small-intestinal mucosa-associated lymphoid tissue (MALToma), has been associated with C. jejuni; anti­ microbial therapy has led to marked clinical improvement. CHAPTER 169 Infections Due to Campylobacter and Related Organisms ■ ■DIAGNOSIS In patients with Campylobacter enteritis, peripheral leukocyte counts reflect the severity of the inflammatory process. In addition, stools from nearly all Campylobacter-infected patients presenting for medi­ cal attention in the United States contain leukocytes or erythrocytes. Gram- or Wright-stained fecal smears should be examined in all suspected cases. When the diagnosis of Campylobacter enteritis is suspected on the basis of findings indicating inflammatory diarrhea (fever, fecal leukocytes), clinicians can ask the microbiology laboratory to attempt the visualization of organisms with characteristic vibrioid morphology by direct microscopic examination of stools with Gram’s staining or to use phase-contrast or dark-field microscopy to identify the organisms’ characteristic “darting” motility. Confirmation of the diagnosis of Campylobacter infection is based on identification of an

isolate from cultures of stool, blood, or another site; specific species can be identified by matrix-assisted laser desorption/ionization–time of flight (MALDI-TOF) mass spectrometry. Campylobacter-specific media should be used to culture stools from all patients with inflam­ matory or bloody diarrhea. Because all Campylobacter species are fastidious, they will not be isolated unless selective media or other selective techniques are used. Failure to isolate campylobacters from stool by culture does not entirely rule out their presence. Although culture remains the diagnostic gold standard, species-specific real-time polymerase chain reaction (PCR) techniques appear more sensitive than culture. Although PCR and other culture-independent diagnostic test (CIDTs), including antigen detection tests, may detect nonviable bacteria and may be falsely positive, they are now used frequently to diagnose infection with Campylobacter and other enteric bacteria in clinical microbiology laboratories. The detection of the organisms in stool in the United States by culture almost always implies active or recent infection, but CIDT positivity is more questionable.

In any event, follow-up testing after the clinical resolution of an acute infection is rarely needed. Campylobacter sputorum, C. concisus, and related organisms ubiquitously found in the oral cavity are com­ mensals that only rarely have pathogenic significance. In patients with microscopic colitis, C. concisus can be more frequently detected than in controls, but whether detection of this common oral organism is a marker for colonic disease or is involved in pathogenesis is uncertain. Because of the low levels of metabolic activity of Campylobacter spe­ cies in standard blood culture media, Campylobacter bacteremia may be difficult to detect. ■ ■DIFFERENTIAL DIAGNOSIS The symptoms of Campylobacter enteritis are not sufficiently unusual to distinguish this illness from that due to Salmonella, Shigella, Yersinia, enterohemorrhagic Escherichia coli, and other pathogens. The com­ bination of fever and fecal leukocytes or erythrocytes is indicative of inflammatory diarrhea, and definitive diagnosis is based on culture, CIDTs, or demonstration of the characteristic organisms on stained fecal smears. Extraintestinal Campylobacter illness is diagnosed by culture. Infection due to Campylobacter should be suspected in the setting of septic abortion, and that due to C. fetus should be suspected specifically in the setting of septic thrombophlebitis. It is important to reiterate that (1) the presentation of Campylobacter enteritis may mimic that of ulcerative colitis or Crohn’s disease, (2) Campylobacter enteritis is much more common than either of the latter (especially among young adults), and (3) biopsy may not distinguish among these entities. Thus, a diagnosis of inflammatory bowel disease should not be made until Campylobacter infection has been ruled out, especially in persons with a history of foreign travel, significant animal contact, immunodeficiency, or exposure incurring a high risk of transmission. PART 5 Infectious Diseases TREATMENT Campylobacter Infection Fluid and electrolyte replacement is central to the treatment of diarrheal illnesses (Chap. 138). Even among patients presenting for medical attention with Campylobacter enteritis, not all clearly benefit from specific antimicrobial therapy. Indications for therapy include high fever, bloody diarrhea, severe diarrhea, persistence for

1 week, and worsening of symptoms. A 3-day course of azithromy­ cin (500 mg once daily) is the regimen of choice. A 1-day regimen of azithromycin (1000 mg given as two 500-mg tablets) can also be used. Alternative regimens for adults consist of fluoroquinolones— ciprofloxacin (500 mg by mouth twice daily for 3 days) or levofloxacin (750 mg daily for 3 days)—but resistance to this class of agents as well as to tetracyclines is substantial; ~27% of U.S. human isolates of Campylobacter in 2014 were resistant to ciprofloxacin, and rates are higher in many other countries; thus, travel-related Campylobacter infections should be considered a priori to be fluoroquinoloneresistant. Because macrolide resistance usually is much less com­ mon (<10%), these drugs are the empirical agents of choice. Patients infected with antibiotic-resistant strains are at increased risk of

adverse outcomes. Use of antimotility agents, which may prolong the duration of symptoms and have been associated with toxic megacolon and with death, is not recommended. Of note, C. jejuni and C. coli are resistant to trimethoprim and β-lactam antibiotics, including penicillin and most cephalosporins. For patients with immunocompromising conditions and uncom­ plicated enteritis caused by C. jejuni, therapy duration should be extended to 7–14 days. Bacteremic infections should always be treated with appropriate antibiotics to reduce the high rate of mor­ tality associated with these infections. For bacteremic or other sys­ temic infections, treatment with a carbapenem (imipenem, 500 mg IV every 6 h; or meropenem, 1–2 g IV every 8 h) should be started empirically, and susceptibility testing should always be performed. For life-threatening illness, gentamicin (1.0–1.7 mg/kg IV every 8 h after a loading dose of 1.5–2 mg/kg) can be added. In the absence of endovascular involvement, therapy for systemic infections should be administered for 7–14 days. For immunocompromised patients with systemic infections due to C. fetus and for patients with endovascular infections due to any species, prolonged therapy (up to 4 weeks) is usually necessary. For recurrent infections in immunocompromised hosts, lifelong therapy/prophylaxis is sometimes necessary. ■ ■PROGNOSIS Nearly all patients recover fully from Campylobacter enteritis, either spontaneously or after antimicrobial therapy. Volume depletion prob­ ably contributes to the few deaths that are reported. As stated above, occasional patients develop reactive arthritis or Guillain-Barré syn­ drome or its variants. Systemic infection with C. fetus is much more often fatal than that due to related species; this higher mortality rate reflects in part the population affected. Prognosis depends on the rapidity with which appropriate therapy is begun. Otherwise healthy hosts usually survive C. fetus infections without sequelae. Compro­ mised hosts often have recurrent and/or life-threatening infections due to a variety of Campylobacter species. ■ ■FURTHER READING Amour C et al: Epidemiology and impact of Campylobacter infection in children in 8 low-resource settings: Results from the MAL-ED Study. Clin Infect Dis 63:1171, 2016. Costa D, Iraola G: Pathogenomics of emerging Campylobacter Species. Clin Microbiol Rev 32:e00072, 2019. Dai L et al: New and alternative strategies for the prevention, control, and treatment of antibiotic-resistant Campylobacter. Transl Res 223:76, 2020. Man SM: The clinical importance of emerging Campylobacter species. Nat Rev Gastroenterol Hepatol 8:669, 2011. Marder EP et al: Incidence and trends of infections with pathogens transmitted commonly through food and the effect of increasing use of culture-independent diagnostic tests on surveillance—Foodborne Diseases Active Surveillance Network, 10 U.S. sites, 2013–2016. Morb Mortal Wkly Rep 66:397, 2017. Nielsen HL: High risk of microscopic colitis after Campylobacter concisus infection: Population-based cohort study. Gut 69:1952, 2020. Riddle MS et al: ACG clinical guideline: Diagnosis, treatment, and prevention of acute diarrheal infections in adults. Am J Gastroenterol 111:602, 2016. Rouhani S et al: Gut microbiota features associated with Campylo­ bacter burden and postnatal linear growth deficits in a Peruvian birth cohort. Clin Infect Dis 71:1000, 2020. Same RG, Tamma PD: Campylobacter jejuni infections in children. Pediatr Rev 39:533, 2018. Ternhag A et al: A meta-analysis of the effects of antibiotic treatment on duration of symptoms caused by infection with Campylobacter species. Clin Infect Dis 44:696, 2007. Tinévez C et al: Retrospective multicentric study on Campylobacter spp. bacteremia in France: The campylobacteremia study. Clin Infect Dis 75:702, 2022. Watkins LKF et al: Ongoing outbreak of extensively drug-resistant Campylobacter jejuni infections associated with US pet store puppies, 2016-2020. JAMA Netw Open 4:e2125203, 2021.

53 - 170 Infections Due to Pseudomonas, Burkholderia, and Stenotrophomonas Species

170 Infections Due to Pseudomonas, Burkholderia, and Stenotrophomonas Species

Kartikeya Cherabuddi, Reuben Ramphal

Infections Due to

Pseudomonas, Burkholderia,

and Stenotrophomonas Species The pseudomonads are a heterogeneous group of gram-negative bacteria that have in common an inability to ferment lactose. Formerly classified in the genus Pseudomonas, the members of this group have been assigned to three medically important genera—Pseudomonas, Burkhold­ eria, and Stenotrophomonas—whose biologic behaviors encompass both similarities and marked differences and whose genetic repertoires differ in many respects. The pathogenicity of most pseudomonads is based on opportunism; the exceptions are Burkholderia pseudomallei and Burkholderia mallei, which are primary pathogens. The genus Pseudomonas now contains >140 species. Pseudomonas aeruginosa, the major pathogen of the group, is a significant cause of infections in hospitalized patients and in patients with cystic fibrosis (CF; Chap. 302). Cytotoxic chemotherapy, mechanical ventilation, chronic lung diseases, and broad-spectrum antibiotic therapy set up conditions that predispose to colonization and infection of increasing numbers of hospitalized patients by this pathogen. Other significant members of the genus—Pseudomonas putida, Pseudomonas fluorescens, Pseudomonas oryzihabitans, and Pseudomonas stutzeri—infect humans infrequently and are generally opportunists that are always present in the environment. The genus Burkholderia comprises >20 species, of which Burk­ holderia cepacia is most frequently encountered in Western countries. Similar to P. aeruginosa, B. cepacia (now referred to as the B. cepacia complex species) is both an opportunistic nosocomial pathogen and a cause of infection in CF. The other medically important members of this genus are B. pseudomallei and B. mallei, the etiologic agents of melioidosis and glanders, respectively. The genus Stenotrophomonas contains one species of medical sig­ nificance, Stenotrophomonas maltophilia. This organism is strictly an opportunist that “overgrows” in the setting of broad-spectrum antibi­ otic use. PSEUDOMONAS AERUGINOSA ■ ■EPIDEMIOLOGY P. aeruginosa is found in most moist environments. Soil, plants, veg­ etables, tap water, and countertops are all potential reservoirs for this microbe, as it has simple nutritional needs. Given the ubiquity of P. aeruginosa, it is clear that simple contact with the organism is not sufficient for colonization or infection. Clinical and experimental observations suggest that infection by P. aeruginosa occurs concomi­ tantly with compromised host defenses, mucosal trauma, physiologic derangement, and antibiotic-mediated suppression of normal flora. Thus, it comes as no surprise that the majority of P. aeruginosa infec­ tions occur in intensive care units (ICUs), where these factors fre­ quently converge. Although the organism is initially acquired from environmental sources, patient-to-patient spread occurs in CF clinics and may occur in closed hospital units. In the past, patients with burns appeared to be unusually suscep­ tible to P. aeruginosa. For example, in 1959–1963, Pseudomonas burnwound sepsis was the principal cause of death in 60% of patients with burns dying at the U.S. Army Institute of Surgical Research. For reasons that are unclear, P. aeruginosa infection in burns is no longer the major problem that it was during the 1950s and 1960s. Similarly, in the 1960s, P. aeruginosa appeared as a common pathogen in patients receiving cytotoxic chemotherapy at many institutions in the United States, but it has subsequently diminished in importance. Despite this subsidence,

P. aeruginosa remains one of the most feared pathogens in this population because of its high attributable mortality.

In some parts of Asia and Latin America, P. aeruginosa continues to be the most common cause of gram-negative bacteremia in neutrope­ nic patients.

In contrast to the trends for patients with burns or neutropenia in the United States, the incidence of P. aeruginosa infections among patients with CF has not changed. P. aeruginosa remains the most com­ mon contributing factor to respiratory failure in CF and is responsible for the majority of deaths among CF patients. ■ ■LABORATORY FEATURES P. aeruginosa is a nonfastidious, motile, gram-negative rod that grows on most common laboratory media, including blood and MacConkey agars. It is easily identified in the laboratory on primary-isolation agar plates by pigment production that confers a yellow to dark green or even bluish appearance. Colonies have a shiny “gun-metal” appearance and a characteristic fruity odor. Two of the identifying biochemical characteristics of P. aeruginosa are an inability to ferment lactose on MacConkey agar and a positive reaction in the oxidase test. Most strains are identified on the basis of these readily detectable labora­ tory features even before extensive biochemical testing is done. Some isolates from CF patients are easily identified by their mucoid appear­ ance, which is due to the production of large amounts of the mucoid exopolysaccharide or alginate. Recently, there has been increasing use of molecular testing with multiplex polymerase chain reaction (PCR) platforms, which rapidly identify P. aeruginosa in respiratory and blood samples much earlier than the classical methods. ■ ■PATHOGENESIS Unraveling the mechanisms that underlie disease caused by P. aeruginosa has proved challenging. Of the common gram-negative bacteria, no other species produces such a large number of putative virulence fac­ tors (Table 170-1). Yet P. aeruginosa rarely initiates an infectious pro­ cess in the absence of host injury or compromise, and few of its putative virulence factors have been shown definitively to be involved in disease in humans. Despite its metabolic versatility and possession of multiple colonizing factors, P. aeruginosa exhibits no competitive advantage over enteric bacteria in the human gut; it is not a normal inhabitant of the healthy human gastrointestinal tract, despite the host’s continuous environmental exposure to the organism. CHAPTER 170 Infections Due to Pseudomonas, Burkholderia, and Stenotrophomonas Species Virulence Attributes Involved in Acute P. aeruginosa Infec­ tions  •  MOTILITY AND COLONIZATION  A general tenet of bac­ terial pathogenesis is that most bacteria must adhere to surfaces or colonize a host niche in order to initiate disease. Most gram-negative bacteria examined thus far possess adherence factors called adhesins. P. aeruginosa is no exception. Among its many adhesins are its pili, which demonstrate adhesive properties for a variety of cells and adhere best to injured cell surfaces. In the organism’s flagellum, the flagel­ lin molecule binds to cells, and the flagellar cap attaches to mucins through the recognition of glycan chains. Other P. aeruginosa adhesins TABLE 170-1  Main Putative Virulence Factors of Pseudomonas aeruginosa SUBSTANCE/ ORGANELLE FUNCTION VIRULENCE IN ANIMAL DISEASE Pili Adhesion to cells ? Flagella Adhesion, motility, inflammation Yes Lipopolysaccharide Antiphagocytic activity, inflammation Yes Type III secretion system Toxic activity (ExoU, ExoS) Yes Type II secretion system Toxic activity Yes Proteases Proteolytic activity ? Phospholipases Cytotoxicity ? Exotoxin A Cytotoxicity ? Pyocyanin Cytotoxicity Yes

include the outer core of the lipopolysaccharide (LPS) molecule, which binds to the cystic fibrosis transmembrane conductance regulator (CFTR) and aids in internalization of the organism, and the alginate coat of mucoid strains, which enhances adhesion to cells and mucins. In addition, membrane proteins and lectins have been proposed as colonization factors. The deletion of any given adhesin by itself is not sufficient to abrogate the ability of P. aeruginosa to colonize surfaces probably because of the redundancy of adhesins. Motility is important in host invasion via mucosal surfaces or burned skin in animal models of infection; however, nonmotile strains are not uniformly avirulent. It has been well demonstrated that nonmotile strains of P. aeruginosa are poorly phagocytosed in vitro, possibly leading to enhancement of the virulence of this organism in vivo.

EVASION OF HOST DEFENSES  The transition from bacterial coloniza­ tion to disease requires the evasion of host defenses followed by inva­ sion by the microorganism. P. aeruginosa appears to be well equipped for evasion. Attached bacteria inject four known toxins (ExoS or ExoU, ExoT, and ExoY) via a type III secretion system that allows the bacteria to evade phagocytic cells either by direct cytotoxicity or by inhibi­ tion of phagocytosis. Clinical studies suggest that the mortality rate is higher among patients infected by strains that secrete the ExoU toxin. Another secretion system—the type II system—secretes toxins that can kill animals, and some of its secreted toxins, such as exotoxin A, have the potential to kill phagocytic cells. Multiple proteases secreted by this system may degrade host effector molecules, such as cytokines and chemokines, that are released in response to infection and appear to play a role in corneal infections in mice. TISSUE INJURY  Among gram-negative bacteria, P. aeruginosa prob­ ably produces the largest number of substances that are toxic to cells and thus have the potential to injure tissues. The toxins secreted by the organism’s type III secretion system are capable of injuring tissue. However, their delivery requires the adherence of the organism to cells. Thus, the effects of these toxins are likely to be local or to depend on the presence of large numbers of bacteria at the site of an infection or in the bloodstream. On the other hand, diffusible toxins, secreted by the organism’s type II secretion system, can act freely wherever they come into contact with cells. Possible effectors of this system include exotoxin A, at least four different proteases, and at least two phospholipases. In addition to these secreted toxins, rhamnolipids, pyocyanins—the pigments that confer the characteristic color and odor of P. aeruginosa colonies—and hydrocyanic acid, are produced by P. aeruginosa and are all capable of causing host tissue injury and even neutrophil death. PART 5 Infectious Diseases INFLAMMATORY COMPONENTS  The inflammatory responses to the lipid A component of Pseudomonas LPS and to its flagellin, mediated through the Toll-like receptor (TLR) system (principally TLR4 and TLR5, respectively), are thought to represent important factors in dis­ ease causation. Although these inflammatory responses are required for successful defense against P. aeruginosa (i.e., in their absence, ani­ mals are defenseless against P. aeruginosa infection), florid responses are likely to result in severe disease. Thus, when the sepsis syndrome and septic shock develop in P. aeruginosa infection, they are probably the result of the host response to one or both of these substances, but injury to the lung by Pseudomonas toxins may also result in sepsis syndromes, possibly by causing cell death and the release of cellular components (e.g., heat-shock proteins) that may activate the TLR or another proinflammatory system. Thus, the virulence of this bacterium in acute infections is likely to be multifactorial with a great redundancy of effector molecules being produced. Chronic P. aeruginosa Infections  Chronic infection due to P. aeruginosa occurs mainly in the lungs in the setting of structural pulmonary diseases. The classic example is CF; others include bron­ chiectasis and chronic relapsing panbronchiolitis, a disease seen in Japan and some Pacific Islands. A hallmark of these illnesses is severely defective mucociliary clearance leading to mucus stasis and mucus accumulation in the lungs. There is probably a common factor that selects for P. aeruginosa colonization in these lung diseases—perhaps

the adhesiveness of P. aeruginosa for mucus, a phenomenon that is not noted for most other common gram-negative bacteria, and/or the ability of P. aeruginosa to evade host defenses in mucus. Furthermore,

P. aeruginosa undergoes evolutionary adaptations and diversification in ways that allow its prolonged survival in the lung without an early fatal outcome for the host. The strains found in CF patients exhibit minimal production of virulence factors. Many strains lose the ability to pro­ duce pili and flagella, and most become complement-sensitive because of the loss of the O side chain of their LPS molecules. In addition, most strains found in CF patients overproduce a mucoid exopolysaccharide. These changes probably dampen the host response, allowing the organ­ ism to survive in CF mucus. P. aeruginosa is also believed to lose its ability to secrete many of its injectable toxins during growth in mucus. Although the alginate coat is thought to play a role in the organism’s survival, alginate is not essential as nonmucoid strains may predomi­ nate for long periods. In short, virulence in chronic infections may be mediated by the chronic but attenuated host inflammatory response, which injures the lungs over decades. ■ ■CLINICAL MANIFESTATIONS P. aeruginosa causes infections at almost all sites in the body but shows a rather marked predilection for the lungs. The infections encountered most commonly in hospitalized patients are described below. Bacteremia  Crude mortality rates exceeding 50% have been reported among patients with P. aeruginosa bacteremia. Consequently, this clinical entity has been much feared, and its management has been attempted with the use of multiple antibiotics. Recent publica­ tions report attributable mortality rates of 28–44%, with the precise figure depending on the adequacy and timing of treatment and the seriousness of the underlying disease. In the past, the patient with P. aeruginosa bacteremia classically was neutropenic or had a burn injury. Today, however, a minority of such patients have bacteremic P. aeruginosa infections. Rather, P. aeruginosa bacteremia is seen most often in patients in ICUs, with the lungs, the urinary tract, central venous lines, or wounds being the most important portals for systemic invasion. The clinical presentation of P. aeruginosa bacteremia rarely differs from that of sepsis in general (Chap. 315). Patients are usually febrile, but those who are most severely ill may be in shock or even hypother­ mic. The only point differentiating this entity from gram-negative sep­ sis due to other bacteria may be the distinctive skin lesions (ecthyma gangrenosum) of Pseudomonas infection, which occur almost exclu­ sively in markedly neutropenic patients and patients with AIDS. These small or large, painful, reddish, maculopapular lesions have a geographic margin; they are initially pink, then darken to purple, and finally become black and necrotic (Fig. 170-1). Histopathologic studies indicate that the lesions are due to vascular invasion and are teeming with bacteria. Although similar lesions may occur in aspergillosis, mucormycosis, and occasionally Staphylococcus aureus bacteremia, their presence in a neutropenic patient generally suggests P. aeruginosa bacteremia as the most likely cause. TREATMENT P. aeruginosa Bacteremia (Table 170-2) Antimicrobial treatment of P. aeruginosa bacte­ remia has been controversial. Combination therapy with an FIGURE 170-1  Ecthyma gangrenosum in a neutropenic patient 3 days after onset.

TABLE 170-2  Antibiotic Treatment of Infections Due to Pseudomonas aeruginosa and Related Species INFECTION ANTIBIOTICS AND DOSAGES OTHER CONSIDERATIONS Bacteremia   Nonneutropenic host Ceftazidime (2 g q8h IV) or cefepime (2 g q8h IV) or piperacillin/tazobactam (4.5 g q6h IV) or imipenem (500 mg q6h IV) or meropenem (1–2 g q8h IV) or doripenem (500 mg q8h IV) Optional: Amikacin (7.5 mg/kg q12h or 15 mg/kg q24h IV)   Neutropenic host Cefepime (2 g q8h IV) or any of the other agents above (except doripenem) in the above dosages Endocarditis Antibiotic regimens as for bacteremia for 6–8 weeks Resistance during therapy is common. Surgery is required for relapse. Pneumonia Drugs and dosages as for bacteremia, except that the available carbapenems should not be the sole primary drugs because of high rates of resistance during therapy. Bone infection, malignant otitis externa Cefepime or ceftazidime at the same dosages as for bacteremia; aminoglycosides not a necessary component of therapy; ciprofloxacin (500–750 mg q12h PO) may be used Central nervous system infection Ceftazidime or cefepime (2 g q8h IV) or meropenem

(2 g q8h IV) Eye infection   Keratitis/ulcer Topical therapy with tobramycin/ciprofloxacin/levofloxacin eyedrops   Endophthalmitis Ceftazidime or cefepime as for central nervous system infection plus Topical therapy Urinary tract infection (UTI) Ciprofloxacin (500 mg q12h PO) or levofloxacin (750 mg q24h) or any aminoglycoside (total daily dose given once daily). Cefepime or ceftazidime (1g q8h) or piperacillin/tazobactam (3.375 g q6h) Multidrug- and extreme drugresistant P. aeruginosa infection Ceftazidime/avibactam (2.5 g q8h, infused over 2 h) or ceftolozane/tazobactam (1.5–3 g q8h) or imipenem/ relebactam (500 mg q6h) or cefiderocol (2 g q8h) or colistin (100 mg q12h IV for the shortest possible period to obtain a clinical response) Burkholderia cepacia complex infection Meropenem (2 g q8h IV) or TMP-SMX (1600/320 mg q12h IV) for 14 days Melioidosis (B. pseudomallei), glanders (B. mallei) Ceftazidime (2 g q6h) or meropenem (1 g q8h) or imipenem (500 mg q6h) for 2 weeks followed by TMP-SMX (1600/320 mg q12h PO) for 3 months Stenotrophomonas maltophilia infection TMP-SMX (1600/320 mg q12h IV) plus either levofloxacin (750 mg q24h) or minocycline (100-200 mg q12h) or ticarcillin/clavulanate (3.1 g q4h IV) for 7 to 14 days Abbreviations: MIC, minimum inhibitory concentration; TMP-SMX, trimethoprim-sulfamethoxazole. antipseudomonal β-lactam and an aminoglycoside became the standard of care because of the dismal outcome of single-drug ther­ apy, mainly with aminoglycosides and polymixins, prior to 1971— first for P. aeruginosa bacteremia in febrile neutropenic patients and then extrapolated to all P. aeruginosa bacteremic infections in both neutropenic and nonneutropenic patients. Following the introduction of new antipseudomonal drugs, a number of studies have revisited the choice between combina­ tion treatment and monotherapy for Pseudomonas bacteremia. Although some clinicians still favor combination therapy, most recent observational studies indicate that a single modern antip­ seudomonal β-lactam agent to which the isolate is sensitive is as efficacious as a combination. Even in patients at greatest risk of early death from P. aeruginosa bacteremia (i.e., those with fever and neutropenia), empirical antipseudomonal monotherapy is deemed to be as efficacious as empirical combination therapy by the practice guidelines of the Infectious Diseases Society of America (IDSA).

Add an aminoglycoside empirically for patients in shock and in regions or hospitals where rates of resistance to the primary β-lactam agents are high. Tobramycin may be used instead of amikacin (susceptibility permitting). A duration of 6–10 days of therapy can be used for uncomplicated bacteremia. Febrile neutropenic patients should be treated until no longer neutropenic. Add aminoglycoside or ciprofloxacin, as for bacteremia, until sensitivities available. The duration of therapy is 7 days. Duration of therapy varies with the drug used and type of infection (e.g., 6 weeks for a β-lactam agent; except in puncture-wound osteomyelitis, for which the duration should be 2–4 weeks; oral therapy can be used). Abscesses or other closed-space infections may require drainage. The duration of therapy is ≥2 weeks. Use maximal strengths available or compounded by pharmacy. Therapy should be administered for 2 weeks or until the resolution of eye lesions, whichever is shorter.   CHAPTER 170 Uncomplicated cystitis may be treated for 3 days with oral agents. Relapse may occur if an obstruction or a foreign body is present. The duration of therapy for complicated cystitis and uncomplicated pyelonephritis is 5–7 days. Infections Due to Pseudomonas, Burkholderia, and Stenotrophomonas Species Use 3-g dose of ceftolozane/tazobactam for pneumonia. Meropenem-vaborbactam offers minimal benefit in carbapenem resistant strains. Alternatives to colistin are preferred, if available. Colistin dosing requires renal adjustment and expertise in its use. Inhaled colistin may be added for pneumonia (100 mg q12h). Resistance to both agents is increasing. Do not use them in combination because of possible antagonism.   Broad-spectrum antibiotic therapy leads to respiratory tract colonization and often warrants no treatment. Ceftazidimeavibactam plus aztreonam, cefiderocol, or tigecycline are alternatives for XDR strains. Combination therapy should be used for bacteremia, especially in immunosuppressed patients. One firm conclusion is that monotherapy with an aminoglycoside is not optimal. There are, of course, institutions and countries where rates of susceptibility of P. aeruginosa to first-line antibiotics are <80%. Thus, when a septic patient with a high probability of P. aeruginosa infection is encountered in such settings, empirical combination therapy should be administered until the pathogen is identified and susceptibility data become available. Thereafter, whether one or two agents should be continued remains a matter of individual preference. Recent studies suggest that extended or continuous infusions of β-lactams such as cefepime, piperacillin-tazobactam, or meropenem may result in better outcomes of Pseudomonas bac­ teremia and possibly of Pseudomonas pneumonia. The duration of antibiotic therapy has now become an important consideration due to the increasing isolation of multiple drug-resistant (MDR) and extensively drug-resistant (XDR) P. aeruginosa strains. Recently published studies now strongly support the use of shorter courses of

therapy (7 days) rather than the longer duration (10−14 days) that is commonly recommended for many cases of Pseudomonas bacte­ remia. As in S. aureus bacteremia, catheter removal is important.

Acute Pneumonia  Respiratory infections are the most common of all infections caused by P. aeruginosa. P. aeruginosa is common in both hospital-acquired pneumonia (HAP) and ventilator-associated pneumonia (VAP). This organism appears first or second among the causes of VAP. However, much debate centers on the actual role of

P. aeruginosa in VAP. Many of the relevant data are based on cultures of sputum or endotracheal tube aspirates and may represent nonpatho­ genic colonization of the tracheobronchial tree, biofilms on the endo­ tracheal tube, or simple tracheobronchitis. The increasing use of PCR testing on endotracheal samples has further compounded this issue. PCR coupled with the widespread use of computed tomography with poor specificity for pneumonia, and overlap in appearance with fluid overload in patients with low to intermediate clinical suspicion, may contribute to the overdiagnosis of HAP and VAP. This scenario empha­ sizes the importance of clinical judgment in adjudicating P. aeruginosa colonization versus infection. Older reports of P. aeruginosa pneumonia described patients with an acute clinical syndrome of fever, chills, cough, and necrotizing pneu­ monia indistinguishable from other gram-negative bacterial pneumo­ nias. The traditional accounts described a fulminant infection. Chest radiographs demonstrated bilateral pneumonia, often with nodular densities with or without cavities. This picture is now remarkably rare. Today, the typical patient is on a ventilator, has a slowly progressive infiltrate, and has been colonized with P. aeruginosa for days. While some cases may progress rapidly over 48–72 h, they are the exceptions. Nodular densities are not commonly seen. However, infiltrates may go on to necrosis. Necrotizing pneumonia has also been seen in the community (e.g., after inhalation of hot-tub water contaminated with P. aeruginosa). The typical patient has fever, leukocytosis, tachypnea, hypoxemia, and purulent sputum, and the chest radiograph shows a new infiltrate or the expansion of a preexisting infiltrate. A sputum Gram’s stain showing mainly polymorphonuclear leukocytes (PMNs) in conjunction with a culture positive for P. aeruginosa in this setting suggests a diagnosis of acute P. aeruginosa pneumonia. PART 5 Infectious Diseases There have been increasing reports of the occurrence of communityacquired P. aeruginosa pneumonia among patients with underlying lung diseases. While this undoubtedly occurs, it is difficult to make this diagnosis with a great degree of certainty with the use of sputum cultures in a population prone to airway colonization by multiple strains of bacteria. The patient population in whom the possibility of a community-acquired P. aeruginosa pneumonia should be considered is the neutropenic patient, given the pivotal role that neutrophils play in defense against this bacterium. Such a patient, whether hospitalized or admitted from the community with a pneumonia, should be treated empirically for P. aeruginosa. TREATMENT Acute Pneumonia (Table 170-2) Therapy for P. aeruginosa pneumonia remains unsat­ isfactory. Reports suggest mortality rates of 40–80%, but how many of these deaths are attributable to underlying disease remains unknown. The drugs of choice for P. aeruginosa pneumonia are similar to those given for bacteremia. A potent antipseudomonal β-lactam drug is the mainstay of therapy. Failure rates were high when aminoglycosides were used as single agents, possibly because of their poor penetration into the airways and their binding to airway secretions. Nonetheless, for the treatment of patients at high risk of death, some experts suggest the combination of a β-lactam agent and an antipseudomonal fluoroquinolone or aminoglycoside. As for the duration of therapy, recent IDSA/American Thoracic Society (ATS) guidelines recommend 7 days of treatment for HAP or VAP, even when P. aeruginosa is the offending organism. However, the outcome in neutropenic patients is poor, especially

if accompanied by bacteremia; thus, therapy needs to be extended until neutropenia resolves. In addition, therapy longer than 7 days should be used in patients with P. aeruginosa necrotizing pneumo­ nia as, functionally, this entity is similar to a lung abscess. Chronic Respiratory Tract Infections  P. aeruginosa is respon­ sible for chronic infections of the airways associated with a number of underlying or predisposing conditions—most commonly CF (Chap. 302). A state of chronic colonization beginning early in child­ hood is seen in some Asian populations with chronic or diffuse pan­ bronchiolitis, a disease of unknown etiology. P. aeruginosa is one of the organisms that colonizes damaged bronchi in bronchiectasis, a disease secondary to multiple causes in which profound structural abnormali­ ties of the airways result in mucus stasis. TREATMENT Chronic Respiratory Tract Infections Optimal management of chronic P. aeruginosa lung infection has not been determined. Patients respond clinically to antipseudomonal therapy, but the organism is rarely eradicated. Because eradication is unlikely, the aim of treatment for chronic infection is to quell exac­ erbations of inflammation. The regimens used are similar to those used for pneumonia, but an aminoglycoside is almost always added because resistance is common in chronic disease. However, it may be more appropriate to use an inhaled aminoglycoside preparation in order to maximize airway drug levels. MDR strains are now com­ monly found in such patients given their increased life span and the repeated courses of antibiotics they receive. Endovascular Infections  Infective endocarditis of native valves due to P. aeruginosa is most commonly seen in those who use IV drugs. This organism has also been reported to cause prosthetic-valve endocarditis. Sites of prior native-valve injury due to the injection of foreign material such as talc or fibers probably serve as niduses for bacterial attachment to the heart valve. The manifestations of P. aeru­ ginosa endocarditis resemble those of other forms of endocarditis in those who use IV drugs except that the disease is more indolent than

S. aureus endocarditis. While most disease involves the right side of the heart, left-sided involvement is not rare, and multivalvular disease is common. Fever is a common manifestation, as is pulmonary involve­ ment (due to septic emboli to the lungs). Thus, patients may also experience chest pain and hemoptysis. Involvement of the left side of the heart may lead to signs of cardiac failure, systemic emboli, and local cardiac involvement with sinus of Valsalva abscesses and conduction defects. Skin manifestations other than injection site infections are rare in this disease, and ecthyma gangrenosum is not common. Vertebral osteomyelitis and sternoclavicular joint septic arthritis are uncommon but pathognomic complications of this disease. The diagnosis is based on positive blood cultures along with clinical signs of endocarditis. TREATMENT Endovascular Infections (Table 170-2) It has been customary to use synergistic antibiotic combinations in treating P. aeruginosa endocarditis because of the development of resistance during therapy with a single antipseudo­ monal β-lactam agent. Which combination therapy is preferable is unclear, as all combinations have failed. Treatment is likely to more often be successful in cases of right-sided endocarditis. Cases of P. aeruginosa endocarditis that relapse during or fail to respond to therapy are often caused by resistant organisms and may require surgical therapy. Other considerations for valve replacement are similar to those in other forms of endocarditis (Chap. 133). Bone and Joint Infections  P. aeruginosa is an infrequent cause of bone and joint infections. However, Pseudomonas bacteremia or

infective endocarditis caused by the injection of contaminated illicit drugs has been documented to result in vertebral osteomyelitis and sternoclavicular joint arthritis. The clinical presentation of vertebral

P. aeruginosa osteomyelitis is more indolent than that of staphylococcal osteomyelitis. The duration of symptoms in IV drug users with verte­ bral osteomyelitis due to P. aeruginosa varies from weeks to months. Fever is not uniformly present; when present, it tends to be low grade. There may be mild tenderness at the site of involvement. Blood cul­ tures are usually negative unless there is concomitant endocarditis. The erythrocyte sedimentation rate (ESR) is generally elevated. Vertebral osteomyelitis due to P. aeruginosa has also been reported in the elderly, in whom it originates from urinary tract infections (UTIs). The infec­ tion generally involves the lumbosacral area because of a shared venous drainage (Batson’s plexus) between the lumbosacral spine and the pel­ vis. Sternoclavicular septic arthritis due to P. aeruginosa is seen almost exclusively in persons who use IV drugs. This disease may occur with or without endocarditis, and a primary site of infection often is not found. Plain radiographs show joint or bone involvement. Treatment of these forms of disease is generally successful. Pseudomonas osteomyelitis of the foot most often follows punc­ ture wounds through sneakers and mostly affects children. The main manifestation is pain in the foot, sometimes with superficial cellulitis around the puncture wound and tenderness on deep palpation of the wound. Multiple joints or bones of the foot may be involved. Systemic symptoms are generally absent, and blood cultures are usually negative. Radiographs may or may not be abnormal, but the bone scan is usu­ ally positive, as are magnetic resonance imaging (MRI) studies. Needle aspiration usually yields a diagnosis. Prompt surgery, with exploration of the nail puncture tract and debridement of the involved bones and cartilage, is generally recommended in addition to antibiotic therapy. Osteomyelitis due to P. aeruginosa is also seen following trauma and with decubitus ulcers. In these settings, the cause of osteomyelitis is often polymicrobial, and the role of P. aeruginosa can be questioned. It is therefore critical that deep bone biopsies be requested to ascertain its significance prior to starting treatment that targets P. aeruginosa. TREATMENT Bone and Joint Infections The treatment of bone and joint infections due to P. aeruginosa is often governed by the primary Pseudomonas infection. Since endocarditis is often the primary infection, the agents used for endocarditis will dic­ tate treatment. In other situations, a 6-week course of therapy with an antipseudomonal β-lactam is recommended, and in case of puncturewound osteomyelitis, oral ciprofloxacin may be used. Central Nervous System (CNS) Infections  CNS infections due to P. aeruginosa are relatively rare. Involvement of the CNS is almost always secondary to a surgical procedure, head trauma, implanted devices, temporary external ventricular drains, and rarely bacteremia. The entity seen most often is postoperative or posttraumatic meningi­ tis. Subdural or epidural infection occasionally results from contamina­ tion of these areas. Embolic disease arising from endocarditis in users of IV drugs and leading to brain abscesses has also been described. The cerebrospinal fluid (CSF) profile of P. aeruginosa meningitis is no dif­ ferent from that of pyogenic meningitis of any other etiology. TREATMENT Central Nervous System Infections (Table 170-2) Treatment of Pseudomonas meningitis is difficult; little information has been published. However, the general prin­ ciples involved in the treatment of meningitis apply, including the need for high doses of bactericidal antibiotics to attain high drug levels in the CSF. The agent with which there is the most published experience in P. aeruginosa meningitis is ceftazidime, but other antipseudomonal β-lactam drugs that attain reasonable CSF concentrations, such as cefepime, piperacillin/tazobactam, and

meropenem, have also been used successfully. Other forms of

P. aeruginosa CNS infection, such as brain abscesses and epidural and subdural empyema, generally require surgical drainage and removal of devices, in addition to antibiotic therapy.

Eye Infections  Eye infections due to P. aeruginosa occur mainly as a result of direct inoculation into the tissue during trauma or sur­ face injury by contact lenses. Keratitis and corneal ulcers are the most common types of eye disease and are often associated with contact lenses (especially the extended-wear variety). Keratitis can be slowly or rapidly progressive, but the classic description is disease progress­ ing over 48 h to involve the entire cornea, with opacification and sometimes perforation. P. aeruginosa keratitis should be considered a medical emergency because of the rapidity with which it can progress to loss of sight. P. aeruginosa endophthalmitis secondary to bacteremia is the most devastating of P. aeruginosa eye infections. The disease is fulminant, with severe pain, chemosis, decreased visual acuity, anterior uveitis, vitreous involvement, and panophthalmitis. It is also a rare complication of cataract removal with lens insertion. In the United States, a recent outbreak associated with artificial tears with carbape­ nem-resistant P. aeruginosa led to serious eye infections and, in some cases, vision loss, enucleation, or death. TREATMENT Eye Infections (Table 170-2) The usual therapy for keratitis is the administration of topical antibiotics. Therapy for endophthalmitis includes the use of high-dose local and systemic antibiotics (to achieve higher drug concentrations in the eye) and vitrectomy. CHAPTER 170 Ear Infections  P. aeruginosa infections of the ears vary from mild swimmer’s ear to serious life-threatening infections with neurologic sequelae. Swimmer’s ear is common among children and results from infection of moist macerated skin of the external ear canal. Most cases resolve with treatment, but some patients develop chronic drainage. Swimmer’s ear is managed with topical antibiotic agents (otic solu­ tions). The use of hearing aids may also predispose to this type of infec­ tion. The most serious form of Pseudomonas infection involving the ear has been given various names: two of these designations, malignant otitis externa and necrotizing otitis externa, are now used for the same entity. This disease was originally described in elderly patients with diabetes, in whom the majority of cases still occur. However, it has also been described in patients with AIDS and in elderly patients without underlying diabetes or immunocompromise. The usual presenting symptoms are decreased hearing and ear pain, which may be severe and lancinating. The pinna is usually painful, and the external canal may be tender. The ear canal almost always shows signs of inflamma­ tion, with granulation tissue and exudate. Tenderness anterior to the tragus may extend as far as the temporomandibular joint and mastoid process. A small minority of patients have systemic symptoms. Patients in whom the diagnosis is made late may present with cranial nerve palsies, most commonly cranial nerve VII, or even with cavernous venous sinus thrombosis. The ESR is invariably elevated (≥100 mm/h). The diagnosis is made on clinical grounds in severe cases; however, the “gold standard” is a positive technetium-99 bone scan in a patient with otitis externa due to P. aeruginosa. In diabetic patients, a positive bone scan constitutes presumptive evidence for this diagnosis and should prompt biopsy or empirical therapy. However, it should be kept in mind that S. aureus and Aspergillus spp. can also cause this entity. Infections Due to Pseudomonas, Burkholderia, and Stenotrophomonas Species TREATMENT Ear Infections (Table 170-2) Given the infection of the ear cartilage, sometimes with mastoid or petrous ridge involvement, patients with malignant (necrotizing) otitis externa are treated as for osteomyelitis.

Urinary Tract Infections  UTIs due to P. aeruginosa generally occur as a complication of a catheter in the urinary tract, an obstruc­ tion or stone in the genitourinary system, urinary tract instrumenta­ tion, or surgery. A P. aeruginosa UTI occurring in the community often signals the presence of an abnormality in the urinary tract. It has been reported that the urinary tract is the second most important site of infection leading to Pseudomonas bacteremia.

TREATMENT Urinary Tract Infections (Table 170-2) Most P. aeruginosa UTIs are considered complicated infections that must be treated longer than uncomplicated cystitis. In general, a 7- to 10-day course of treatment suffices. Urinary cath­ eters, stents, or stones should be removed to prevent relapse, which is common and may not be due to antibiotic resistance but rather to factors such as a foreign body that has been left in place or an ongo­ ing obstruction. Removal of a urinary catheter will allow shorter courses of antibiotic therapy if that is the only predisposing factor. Skin and Soft Tissue Infections  Besides pyoderma (ecthyma) gangrenosum in neutropenic patients, folliculitis and other papular or vesicular lesions due to P. aeruginosa have been extensively described and are collectively referred to as dermatitis. Multiple outbreaks have been linked to whirlpools, spas, and swimming pools. To prevent such outbreaks, the growth of P. aeruginosa in the home and in recreational environments must be controlled by proper chlorination of water. Most cases of hot-tub folliculitis are self-limited, requiring only the avoid­ ance of exposure to the contaminated source of water. Approximately one-third of reported outbreaks are associated with hotel facilities. Patients may also have ear pain, skin rashes, and eye irritation. PART 5 Infectious Diseases Toe-web infections occur especially often in the tropics, and the “green-nail syndrome” is caused by P. aeruginosa paronychia, which results from frequent submersion of the hands in water. In the latter entity, the green discoloration results from diffusion of pyocyanin into the nail bed. P. aeruginosa remains a prominent cause of burn wound infections in some parts of the world. The management of these infec­ tions is best left to specialists in burn wound care. Infections in Febrile Neutropenic Patients  In febrile neutro­ penia, P. aeruginosa has historically been the organism against which empirical coverage is always essential. Although in Western countries these infections are now less common, their importance has not diminished because of persistently high mortality rates. In other parts of the world, P. aeruginosa continues to be a significant problem in febrile neutropenia, causing a larger proportion of infections in febrile neutropenic patients than any other single organism. For example,

P. aeruginosa was responsible for 28% of documented infections in 499 febrile neutropenic patients in one study from the Indian subcontinent and for 31% of such infections in another. In a large study of infections in leukemia patients from Japan, P. aeruginosa was the most frequently documented cause of bacterial infection. In studies performed in North America, northern Europe, and Australia, the incidence of

P. aeruginosa bacteremia in febrile neutropenia was quite variable. In a review of 97 reports published between 1987 and 1994, the incidence was reported to be 1–2.5% among febrile neutropenic patients given empirical therapy and 5–12% among patients with microbiologi­ cally documented infections. The most common clinical syndromes encountered were bacteremia, pneumonia, and soft tissue infections manifesting mainly as ecthyma gangrenosum. TREATMENT Infections in Febrile Neutropenic Patients (Table 170-2) Compared with rates three decades ago, improved rates of response to antibiotic therapy have been reported in many studies. A study of 127 patients demonstrated a reduction in the mortality rate from 71 to 25% with the introduction of ceftazidime

and imipenem. Because neutrophils—the normal host defenses against this organism—are absent in febrile neutropenic patients, maximal doses of antipseudomonal β-lactam antibiotics should be used for the management of P. aeruginosa bacteremia in this setting. Infections in Patients with AIDS  P. aeruginosa infections were well documented in patients with AIDS before the advent of antiret­ roviral therapy. Since the introduction of protease inhibitors, P. aeru­ ginosa infections in patients with AIDS have been seen less frequently but still occur, particularly in the form of sinusitis. While this entity is now uncommon in developed nations, there are still large numbers of patients with untreated HIV infection or poorly controlled disease in developing nations who are likely to suffer from P. aeruginosa infec­ tions. The clinical presentation of Pseudomonas infection (especially pneumonia and bacteremia) in patients with AIDS is remarkable in that, although the illness may appear not to be severe, the infection may nonetheless be fatal. Patients with bacteremia may have only a lowgrade fever and may present with ecthyma gangrenosum. Pneumonia, with or without bacteremia, is perhaps the most common type of

P. aeruginosa infection. Patients with P. aeruginosa pneumonia exhibit the classic clinical signs and symptoms of pneumonia, such as fever, productive cough, and chest pain. The infection may be lobar or mul­ tilobar and shows no predisposition for any particular location. The most striking feature is the high frequency of cavitary disease. TREATMENT Infections in Patients with AIDS Therapy for any of these conditions in AIDS patients is no different from that in other patients. However, relapse is the rule unless the patient’s CD4+ T-cell count rises to >50/μL or suppressive antibiotic therapy is given. In attempts to achieve cures and prevent relapses, therapy tends to be more prolonged. Gastrointestinal Infections  A poorly understood syndrome caused by P. aeruginosa has been described in the Far East and has been called Shanghai fever and Pseudomonas enterocolitis. This syndrome occurs in young children; its occurrence in adults appears to be rare. Shanghai fever manifests as severe enteric disease, sepsis with inva­ sive disease, and complications, whereas Pseudomonas enterocolitis is characterized by prolonged fever with bloody or mucoid diarrhea mimicking bacterial enterocolitis. The mortality rate ranges between 23 and 89%, with ecthyma gangrenosum occurring in >50% of cases. Early recognition and treatment have led to a reduction in the mortal­ ity rate. There is an above-average occurrence of the exoU gene among Pseudomonas isolates from patients with this syndrome. Multidrug-Resistant Infections  (Table 170-2) P. aeruginosa has a notorious propensity to develop antibiotic resistance. Over three decades, the impact of resistance was minimized by the rapid develop­ ment of several potent antipseudomonal β-lactams and fluoroquino­ lones. However, rates of resistance to these agents that revolutionized the treatment of P. aeruginosa have risen to the point where some are almost unusable empirically because of the worldwide emergence of strains carrying determinants that mediate resistance. Extremely high rates of MDR strains have been reported from eastern and southern Europe, Latin America, India, and China, especially in ICUs. Physi­ cians have had to resort to drugs such as colistin and polymyxin B, which were discarded decades ago. This surge in resistance is mediated by multiple mechanisms sometimes converging in individual strains. Chief among these are chromosomal or plasmid-borne penicillinases, extended-spectrum β-lactamases, cephalosporinases, and carbapen­ emases. Any of these may be combined with permeability mutations and efflux pump overexpression. The greatest nemesis in this regard is the worldwide presence of carbapenemases in P. aeruginosa leading to resistance to most β-lactams except some of the newest agents recently developed. These new agents are generally combinations of a cephalo­ sporin or a carbapenem most often with a novel β-lactamase inhibitor. Several have been approved for clinical use, and all are active against

MDR P. aeruginosa to varying degrees. Currently approved agents include ceftolozane-tazobactam, ceftazidime-avibactam, meropenemvaborbactam, and imipenem-relebactam. A novel cephalosporin, cefiderocol, which uses the iron uptake pathway of P. aeruginosa, also demonstrates activity against MDR strains. Since MDR and XDR

P. aeruginosa are unpredictable in regard to the underlying mecha­ nisms of resistance, laboratory testing is absolutely required before the use of any of these agents. Most academic institutions restrict the use of these agents as there are increasing reports of resistance even to these agents, as well the cost implications of misuse. BURKHOLDERIA SPECIES ■ ■BURKHOLDERIA CEPACIA COMPLEX The B. cepacia complex (BCC) gained notoriety as the cause of a rap­ idly fatal syndrome of respiratory distress and septicemia (the “cepacia syndrome”) in CF patients. Of the more than 20 species of this com­ plex, the three most frequently seen in CF patients are B. cenocepacia, B. multivorans, and B. stabilis. In addition to their occurrence in CF, members of this complex were not uncommonly encountered in ICU patients (previously designated Pseudomonas cepacia) and patients with chronic granulomatous disease, in whom they caused lung dis­ ease. BCC organisms are environmental organisms that inhabit moist environments and are found in the rhizosphere. They possess multiple virulence factors that may play roles in disease as well as colonizing factors that are capable of binding to lung mucus—an ability that may explain the predilection of B. cepacia for the lungs in CF. B. cenocepacia is motile, secretes elastase, and possesses components of an injectable toxin-secretion system like that of P. aeruginosa; its LPS is among the most potent of all LPSs in stimulating an inflammatory response in the lungs. Inflammation may be the major cause of the lung disease seen in the “cepacia” syndrome. Besides infecting the lungs in CF, the BCC organisms appear as airway colonizers during broad-spectrum antibiotic therapy and are causes of VAP, catheter-associated infections, and wound infections. B. cenocepacia has emerged as a barrier to lung transplantation in CF, with relatively high mortality rates after a year compared to infection with other members of this complex. TREATMENT B. cepacia Complex Infections BCC organisms are intrinsically resistant to many antibiotics, ren­ dering empiric treatment difficult. Therefore, treatment must be tailored according to sensitivities. Trimethoprim-sulfamethoxazole (TMP-SMX), meropenem, and minocycline are the most active agents in vitro and may be started as first-line agents (Table 170-2). However, recent reports indicate that there has been increasing resistance to these agents especially in CF patients. Some strains are susceptible to third-generation ureidopenicillins, advanced cepha­ losporins, and fluoroquinolones, and these agents may be used against isolates known to be susceptible. Newer antibiotics such as ceftolozane-tazobactam and ceftazidime-avibactam show good activity against MDR strains in vitro. However, there is very limited clinical experience with these agents. ■ ■BURKHOLDERIA PSEUDOMALLEI B. pseudomallei is the causative agent of melioidosis, a disease of humans and animals that is geographically restricted to Southeast Asia and northern Australia, with occasional cases in countries such as India and China. This organism may be isolated from individuals returning directly from these endemic regions and from military per­ sonnel who have served in endemic regions. Symptoms of this illness may develop only at a later date because of the organism’s ability to cause latent infections, which has been attributed to its ability to sur­ vive within cells. B. pseudomallei is found in soil and water. Humans and animals are infected by inoculation, inhalation, or ingestion; only rarely is the organism transmitted from person to person. Humans are not colonized without being infected. Among the pseudomonads,

B. pseudomallei is perhaps the most virulent species. Host compromise is not an essential prerequisite for disease, although many patients have common underlying medical diseases (e.g., diabetes, renal failure, or alcohol abuse). B. pseudomallei is a facultative intracellular organism whose replication in PMNs and macrophages may be aided by the possession of a polysaccharide capsule. The organism also possesses elements of a type III secretion system that plays a role in its intracel­ lular survival. During infection, there is a florid inflammatory response whose role in disease is unclear.

B. pseudomallei causes a wide spectrum of conditions, ranging from asymptomatic infection to abscesses, pneumonia, and disseminated disease. It is a significant cause of fatal community-acquired pneu­ monia and septicemia in endemic areas, with mortality rates as high as 44% reported in Thailand. Acute pulmonary infection is the most commonly diagnosed form of melioidosis. Pneumonia may be asymp­ tomatic (with routine chest radiographs showing mainly upper-lobe infiltrates) or may present as severe necrotizing disease. B. pseudom­ allei also causes chronic pulmonary infections with systemic mani­ festations that mimic those of tuberculosis, including chronic cough, fever, hemoptysis, night sweats, and cavitary lung disease. Besides pneumonia, the other principal form of B. pseudomallei disease is skin ulceration with associated lymphangitis and regional lymphadenopa­ thy. Spread from the lungs or skin, which is most often documented in debilitated individuals, gives rise to septicemic forms of melioidosis that carry a high mortality rate. TREATMENT B. pseudomallei Infections CHAPTER 170 B. pseudomallei is susceptible to advanced penicillins, cephalosporins, and carbapenems (Table 170-2). Treatment is divided into two stages: an intensive 2-week phase of therapy with ceftazidime or a carbape­ nem followed by at least 12 weeks of oral TMP-SMX to eradicate the organism and prevent relapse. Australian guidelines for treating this condition recommend longer periods of intensive therapy—4−8 weeks for severe infections, osteomyelitis, and CNS infections. The recogni­ tion of this bacterium as a potential agent of biologic warfare has stimulated interest in the development of a vaccine. Infections Due to Pseudomonas, Burkholderia, and Stenotrophomonas Species ■ ■BURKHOLDERIA MALLEI B. mallei causes the equine disease glanders in Africa, Asia, and South America. The organism was eradicated from Europe and North America decades ago. The last case seen in the United States occurred in 2001 in a laboratory worker; before that, B. mallei had last been seen in this coun­ try in 1949. In contrast to the other organisms discussed in this chapter, B. mallei is not an environmental organism and does not persist outside its equine hosts. Consequently, B. mallei infection is an occupational risk for handlers of horses, equine butchers, and veterinarians in areas of the world where it still exists. Diabetics are thought to be especially susceptible to infection by this organism. The polysaccharide capsule is a critical virulence determinant. The organism is transmitted from animals to humans by inoculation into the skin, where it causes local infection with nodules and lymphadenitis. Regional lymphadenopathy is common. Respiratory secretions from infected horses are extremely infectious. Inhalation results in clinical signs of typical pneumonia but may also cause an acute febrile illness with ulceration of the trachea. The organism may disseminate from the skin or lungs to cause septicemia with signs of sepsis. The septicemic form is frequently associated with shock and a high mortality rate. The infection may also enter a chronic phase and present as disseminated abscesses. B. mallei infection may present as early as 1–2 days after inhalation or (in cutaneous disease) may not become evident for months. TREATMENT B. mallei Infections The antibiotic susceptibility pattern of B. mallei is similar to that of B. pseudomallei; in addition, the organism is susceptible to the

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171 Salmonellosis

macrolides azithromycin and clarithromycin. B. mallei infection should be treated with the same drugs and for the same duration as melioidosis.

STENOTROPHOMONAS MALTOPHILIA S. maltophilia is the only potential human pathogen among a genus of ubiquitous organisms found in the rhizosphere (i.e., the soil that surrounds the roots of plants). The organism is an opportunist that is acquired from the environment but is even more limited than P. aeruginosa in its ability to colonize patients or cause infections. Immunocompromise is not sufficient to permit these events; rather, major perturbations of the human flora are usually necessary for the establishment of S. maltophilia. Accordingly, most cases of human infection occur in the setting of very broad-spectrum antibiotic ther­ apy with agents such as advanced cephalosporins and carbapenems, which eradicate the normal flora and other pathogens. The remark­ able ability of S. maltophilia to resist virtually all classes of antibiotics is attributable to the possession of antibiotic efflux pumps and of two β-lactamases (L1 and L2) that mediate β-lactam resistance, including that to carbapenems. It is fortunate that the virulence of S. maltophilia appears to be limited. Although a serine protease is present in some strains, virulence is probably a result of the host’s inflammatory response to components of the organism such as LPS and flagellin.

S. maltophilia is most commonly found in the respiratory tract of ven­ tilated patients, where the distinction between its roles as a colonizer and as a pathogen is often difficult to make. However, S. maltophilia does cause pneumonia and bacteremia in such patients, and these infections have led to septic shock. A severe form of pneumonia with significant mortality has been reported in neutropenic patients. Also common is central venous line–associated infection (with or without bacteremia), which has been reported most often in patients with cancer. S. maltophilia is a rare cause of ecthyma gangrenosum in neu­ tropenic patients. It has been isolated from ~5% of CF patients but is not believed to be a significant pathogen in this setting. There are also less common sites of infection such as the endocardium, meninges, urinary tract, and skin; however, the sites of major concern remain the blood and lungs. PART 5 Infectious Diseases TREATMENT S. maltophilia Infections The intrinsic resistance of S. maltophilia to most antibiotics ren­ ders infection difficult to treat. The antibiotics to which it is most often (although not uniformly) susceptible are TMP-SMX, ticarcillin-clavulanate, levofloxacin, minocycline, cefiderocol and tigecycline (Table 170-2). Ceftazidime is no longer recommended for treatment. A few retrospective studies show utility of mino­ cycline, but relevant clinical outcomes have not been thoroughly evaluated. Consequently, combination therapy with TMP-SMX plus ticarcillin-clavulanate, levofloxacin, or high-dose minocycline is recommended for initial therapy pending susceptibility testing. Catheters must be removed early in the treatment of bactere­ mia. The treatment of proven, high-inoculum infections due to

S. maltophilia is associated with frequent development of resistance during therapy. The newest β-lactam/β-lactamase inhibitor combi­ nations show mixed results against this organism probably because of the presence of two different β-lactamases, a penicillinase and a cephalosporinase, compounded by the presence of efflux pumps. Resistance to one or the other of the newer agents has already been noted, including to cefiderocol. The most controversial aspect of therapy of S. maltophilia is whether pulmonary cultures in a nonneutropenic patient with lung consolidation require treatment or represent colonization. Many clinicians opt not to treat such patients given the fact that the development of resistance is quite common, compromising a serious subsequent infection such as a bacteremia.

■ ■FURTHER READING Bauer KA et al: Extended-infusion cefepime reduces mortality in patients with Pseudomonas aeruginosa infections. Antimicrob Agents Chemother 57:2907, 2013. Bowers DR et al: Outcomes of appropriate empiric combination versus monotherapy for Pseudomonas aeruginosa bacteremia. Antimicrob Agents Chemother 157:1270, 2013. Brooke JS: Stenotrophomonas maltophilia: An emerging global oppor­ tunistic pathogen. Clin Microbiol Rev 25:2, 2012. Cattaneo C et al: P. aeruginosa bloodstream infections among hemato­ logical patients: An old or new question? Ann Hematol 91:1299, 2012. Fabre V et al: Antibiotic therapy for Pseudomonas aeruginosa bloodstream infections: How long is long enough? Clin Infect Dis 69:2011, 2019. Gupte A et al: High pyocyanin production and non-motility of Pseu­ domonas aeruginosa isolates are correlated with septic shock or death in bacteremic patients. PLoS One 16:e0253259, 2021. Horcajada JP et al: Epidemiology and treatment of multidrug-resistant and extensively drug-resistant Pseudomonas aeruginosa infections. Clin Microbiol Rev 32:e00031, 2019. Kalil AC et al: Executive summary: Management of adults with hospital-acquired and ventilator-associated pneumonia: 2016 clinical practice guidelines by the Infectious Diseases Society of America and the American Thoracic Society. Clin Infect Dis 63:575, 2016. Peña C et al: Influence of virulence genotype and resistance profile in the mortality of Pseudomonas aeruginosa bloodstream infections. Clin Infect Dis 60:539, 2015. Wunderlink RG et al: Cefiderocol versus high-dose, extendedinfusion meropenem for the treatment of Gram-negative nosocomial pneumonia (APEKS-NP): A randomized, double-blind, phase 3, non-inferiority trial. Lancet Infect Dis 21:213, 2021. David A. Pegues, Samuel I. Miller

Salmonellosis Bacteria of the genus Salmonella are highly adapted for growth in both humans and animals and cause a wide spectrum of disease. The growth of serotypes Salmonella Typhi and Salmonella Paratyphi is restricted to human hosts, in whom these organisms cause enteric (typhoid) fever. The remaining serotypes (nontyphoidal Salmonella, or NTS) can colonize the gastrointestinal tracts of a broad range of animals, includ­ ing mammals, reptiles, birds, and insects. More than 200 serotypes of Salmonella are pathogenic to humans, in whom they often cause gastroenteritis and can be associated with localized infections and/or bacteremia. ■ ■ETIOLOGY This large genus of gram-negative bacilli within the family Enterobac­ teriaceae consists of two species: Salmonella enterica, which contains six subspecies, and Salmonella bongori. S. enterica subspecies I includes almost all the serotypes pathogenic for humans. Members of the seven Salmonella subspecies are classified into >2600 serotypes (serovars); for simplicity, Salmonella serotypes (most of which are named for the city where they were identified) are often used as the species designa­ tion. For example, the full taxonomic designation S. enterica subspe­ cies enterica serotype Typhimurium can be shortened to Salmonella serotype Typhimurium or simply S. Typhimurium. Serotyping is based on antigenically diverse surface structures: the somatic O antigen (lipopolysaccharide cell-wall components), the surface Vi antigen (restricted to S. Typhi and S. Paratyphi C), and the flagellar H antigen. Salmonellae are gram-negative, non-spore-forming, facultatively anaerobic bacilli that measure 2–3 μm by 0.4–0.6 μm. The initial

identification of salmonellae in the clinical microbiology laboratory is based on growth characteristics. Salmonellae, like other Enterobacte­ riaceae, produce acid on glucose fermentation, reduce nitrates, and do not produce cytochrome oxidase. In addition, all salmonellae except Salmonella Gallinarum-Pullorum are motile by means of peritrichous flagella, and all but S. Typhi produce gas (H2S) on sugar fermentation. Notably, only 1% of clinical isolates ferment lactose; a high level of suspicion must be maintained to detect these rare clinical lactosefermenting isolates. Although serotyping of all surface antigens can be used for formal identification, most laboratories perform a few simple agglutination reactions that define specific O-antigen serogroups, designated A, B, C1, C2, D, and E. Strains in these six serogroups cause ~99% of Salmonella infections in humans and other warm-blooded animals. Wholegenome sequencing is used in epidemiologic investigations to identify the source of foodborne outbreaks and to explore the international transmission of multidrug-resistant Salmonella strains. ■ ■PATHOGENESIS All Salmonella infections begin with ingestion of organisms, most com­ monly in contaminated food or water. The infectious dose ranges from 200 colony-forming units (CFU) to 106 CFU, and the ingested dose is an important determinant of incubation period and disease severity. Conditions that decrease either stomach acidity (an age of <1 year, acid suppression therapy, or achlorhydric disease) or intestinal integrity (inflammatory bowel disease, cytotoxic chemotherapy, prior gastroin­ testinal surgery, or alteration of the intestinal microbiome by antibiotic administration) increase susceptibility to Salmonella infection. Once S. Typhi and S. Paratyphi reach the small intestine, they penetrate the mucus layer of the gut and traverse the intestinal layer through phagocytic microfold (M) cells that reside within Peyer’s patches. Salmonellae can trigger the formation of membrane ruffles in normally nonphagocytic epithelial cells. These ruffles reach out and enclose adherent bacteria within large vesicles by bacterium-mediated endocytosis. This process is dependent on the direct delivery of

Salmonella proteins into the cytoplasm of epithelial cells by the special­ ized bacterial type III secretion system. These bacterial proteins medi­ ate alterations in the actin cytoskeleton that are required for Salmonella uptake. After crossing the epithelial layer of the small intestine, S. Typhi and S. Paratyphi, which cause enteric (typhoid) fever, are phagocytosed by macrophages. These salmonellae survive the antimicrobial environ­ ment of the macrophage by sensing environmental signals that trigger alterations in regulatory systems of the phagocytosed bacteria. For example, PhoP/PhoQ (the best-characterized regulatory system) trig­ gers the alteration of the outer membrane by increasing the synthesis and transport of different outer-membrane proteins, lipopolysaccha­ rides, and glycerophospholipids, so that the altered bacterial surface can resist microbicidal activities and potentially alter host cell signal­ ing. In addition, salmonellae encode a second type III secretion system that directly delivers bacterial proteins across the phagosome mem­ brane into the macrophage cytoplasm. This secretion system functions to remodel the Salmonella-containing vacuole, promoting bacterial survival and replication. Once phagocytosed, typhoidal salmonellae disseminate throughout the body in macrophages via the lymphatics and colonize reticulo­ endothelial tissues (liver, spleen, lymph nodes, and bone marrow). Patients have relatively few or no signs and symptoms during this ini­ tial incubation stage. Signs and symptoms, including fever and abdom­ inal pain, probably result from secretion of cytokines by macrophages and epithelial cells in response to bacterial products that are recognized by innate immune receptors when a critical number of organisms have replicated. Over time, the development of hepatosplenomegaly is likely to be related to the recruitment of mononuclear cells and the develop­ ment of a specific acquired cell-mediated immune response to S. Typhi colonization. The recruitment of additional mononuclear cells and lymphocytes to Peyer’s patches during the several weeks after initial colonization/infection can result in marked enlargement and necrosis of the Peyer’s patches, which may be mediated by bacterial products

that promote cell death as well as the inflammatory response. In the case of S. Typhi and S. Paratyphi A, many strains produce a toxin when localized within host cells. This toxin is then transported extracel­ lularly and probably contributes to systemic symptoms as well as the unusual neuropsychiatric states that can be seen in severe typhoidal illness.

In contrast to enteric fever, which is characterized by an infiltration of mononuclear cells into the small-bowel mucosa, NTS gastroenteritis is characterized by massive polymorphonuclear leukocyte infiltration into both the large- and small-bowel mucosa. This response appears to depend on the induction of interleukin 8, a strong neutrophil chemotactic factor, which is secreted by intestinal cells because of nontyphoidal Salmonella colonization and translocation of bacterial proteins and LPS into host cell cytoplasm with subsequent activation of inflammasomes. The degranulation and release of toxic substances by neutrophils may result in damage to the intestinal mucosa, causing the inflammatory diarrhea observed with nontyphoidal gastroenteritis. An additional important factor in the persistence of NTS in the intestinal tract and the organism’s capacity to compete with endogenous flora is the ability to utilize the sulfur-containing compound tetrathionate for metabolism in a microaerophilic environment. In the presence of intestinal inflammation, tetrathionate is generated from thiosulfate produced by epithelial cells through inflammatory cell production of reactive oxygen species. In contrast to nontyphoidal Salmonellae, typhoidal Salmonellae do not effectively colonize the intestinal tract but have evolved as systemic pathogens, largely through gene loss and perhaps by the loss of the ability to utilize butyrate within the human intestine. CHAPTER 171 ENTERIC (TYPHOID) FEVER Enteric (typhoid) fever is a systemic disease characterized by fever and abdominal pain and caused by dissemination of S. Typhi or S. Paraty­ phi. The disease was initially called typhoid fever because of its clini­ cal similarity to typhus. In the early 1800s, typhoid fever was clearly defined pathologically as a unique illness based on its association with enlarged Peyer’s patches and mesenteric lymph nodes. In 1869, given the anatomic site of infection, the term enteric fever was proposed as an alternative designation to distinguish typhoid fever from typhus. However, to this day, the two designations are used interchangeably. Salmonellosis ■ ■EPIDEMIOLOGY In contrast to other Salmonella serotypes, the etiologic agents of enteric fever—S. Typhi and S. Paratyphi serotypes A, B, and C—have no known hosts other than humans. Most commonly, food-borne or waterborne transmission results from fecal contamination by ill or asymptomatic chronic carriers. Sexual transmission between male partners has been described. Health care workers occasionally acquire enteric fever after exposure to infected patients or during processing of clinical specimens and cultures. With improvements in food handling and water/sewage treatment, enteric fever has become rare in developed nations. An estimated 9.2– 21 million cases of typhoid fever, 5 million cases of paratyphoid fever, and 110,000–280,000 deaths occur each year. The highest estimated annual incidence rates of typhoid fever are in the Indian subcontinent, including Pakistan, Bangladesh, Nepal, India, and Eastern Mediterra­ nean and African regions, and exceed 1000 cases per 100,000 children in some urban areas (Fig. 171-1). A high incidence of enteric fever cor­ relates with mixing of drinking water with human sewage. In endemic regions, enteric fever is more common in poor neighborhoods in large cities than rural areas and among young children and adolescents than among other age groups. Risk factors include fecally contaminated drinking water or ice, flooding, food and drinks purchased from street vendors, raw fruits and vegetables grown in fields fertilized with sew­ age, ill household contacts, lack of hand washing and toilet access, and evidence of prior Helicobacter pylori infection (an association probably related to chronically reduced gastric acidity). Multidrug-resistant (MDR) strains of S. Typhi emerged in the 1980s in China and Southeast Asia and have since disseminated widely. These strains contain plasmids encoding resistance to chloramphenicol,

FIGURE 171-1  Estimated national typhoid fever incidence and worldwide typhoid conjugate vaccine introduction, 2019–2022. (Reproduced from M Hancuh et al: Typhoid fever surveillance incidence estimates, and progress toward typhoid conjugate vaccine introduction – Worldwide, 2018–2022. MMWR Morb Mortal Wkly Rep 72:171, 2023.) ampicillin, and trimethoprim—antibiotics long used to treat enteric fever. With the increased use of fluoroquinolones to treat MDR enteric fever in the 1990s, MDR strains of S. Typhi and S. Paratyphi with decreased susceptibility to ciprofloxacin (DSC; minimal inhibitory concentration [MIC], ≥0.125 μg/mL) or ciprofloxacin resistance (MIC, ≥1 μg/mL) emerged on the Indian subcontinent and have spread with human migration first to Southern Asia and more recently to Eastern and Southern Africa. These strains represent clone H58, which is increasingly associated with clinical treatment failure of fluoroqui­ nolones. Since emerging in 2016 in urban slums of Sindh province in Southeastern Pakistan, an extensively drug-resistant (XDR) S. Typhi H58 clone with plasmid-mediated extended-spectrum beta-lactamase (ESBL) resistance has now become the dominant cause of typhoid fever in Pakistan. Air travel from Pakistan has facilitated the international spread of this XDR strain. Azithromycin resistance has emerged in multiple countries where azithromycin is used for first-line treatment of enteric fever and for mass administration for trachoma. PART 5 Infectious Diseases In the United States, the Centers for Disease Control and Preven­ tion (CDC) estimates that typhoid fever affects 5700 persons each year, a number far in excess of the ~350 cases of typhoid fever and ~90 cases of paratyphoid fever reported annually. In 2015, the median age of patients with typhoid fever was 23 years, and it was 29 years for paratyphoid fever. Most cases of enteric fever were associated with international travel (78%), predominantly to Indian, Pakistan, and Bangladesh, and visiting friends and family. Only 3% of travelers diagnosed with typhoid fever had received S. Typhi vaccine within the previous 5 years. In 2015, 66% of S. Typhi in the United States were DSC, and ~10% were resistant to ampicillin, chloramphenicol, and trimethoprim-sulfamethoxazole (TMP-SMX). Infection with DSC

S. Typhi was associated with travel to the Indian subcontinent. In the United States, domestically acquired cases of enteric fever are less often DSC or MDR compared with travel-associated cases and are most often sporadic, although outbreaks linked to contaminated food products and previously unrecognized chronic carriers continue to occur. ■ ■CLINICAL COURSE Enteric fever is a misnomer, in that the hallmark features of this disease— fever and abdominal pain—are variable. While fever is documented at presentation in >75% of cases, abdominal pain is reported in only 30–40%. Thus, a high index of suspicion for this potentially fatal systemic illness is necessary when a person presents with fever and a history of recent travel to a developing country.

<10 10−<100 100−<500 ≥500 Not available Not applicable Introduced TCV The mean incubation period for S. Typhi is 10–14 days but ranges from 5 to 21 days, depending on the inoculum size and the host’s health and vaccination status. The most prominent symptom is pro­ longed fever (38.8°–40.5°C [101.8°–104.9°F]), which can continue for up to 4 weeks if untreated. S. Paratyphi A is thought to cause milder disease than S. Typhi, with predominantly gastrointestinal symptoms. However, a prospective study of 669 consecutive cases of enteric fever in Kathmandu, Nepal, found that the infections caused by these organisms were clinically indistinguishable. In this series, symptoms reported on initial medical evaluation included headache (80%), chills (35–45%), cough (30%), sweating (20–25%), myalgias (20%), malaise (10%), and arthralgia (2–4%). Gastrointestinal manifestations included anorexia (55%), abdominal pain (30–40%), nausea (18–24%), vomit­ ing (18%), and diarrhea (22–28%) more commonly than constipation (13–16%). Physical findings included coated tongue (51–56%), spleno­ megaly (5–6%), and abdominal tenderness (4–5%). Early physical findings of enteric fever include rash (“rose spots”; 30%), hepatosplenomegaly (3–6%), epistaxis, and relative bradycardia at the peak of high fever (<50%). Rose spots (Fig. 171-2; see also Fig. A1-9) make up a faint, salmon-colored, blanching, maculopapular rash located FIGURE 171-2  “Rose spots,” the rash of enteric fever due to Salmonella Typhi or Salmonella Paratyphi.

primarily on the trunk and chest. The rash is evident in ~30% of patients at the end of the first week and resolves without a trace after 2–5 days. Patients can have two or three crops of lesions, and Salmonella can be cultured from punch biopsies of these lesions. The faintness of the rash makes it difficult to detect in highly pigmented patients. Complications of typhoid fever are estimated to occur in ~27% of hospitalized patients and correlate with a longer duration of symptoms before hospitalization, host factors (host genetics, immunosuppression, acid suppression therapy, previous exposure, and vaccination status), strain virulence and inoculum, and choice of antibiotic therapy. Gas­ trointestinal bleeding (6%) and intestinal perforation (1%) most com­ monly occur in the third and fourth weeks of illness and result from hyperplasia, ulceration, and necrosis of the ileocecal Peyer’s patches at the initial site of Salmonella infiltration (Fig. 171-3). Both complica­ tions are life-threatening and require immediate fluid resuscitation and surgical intervention, with broadened antibiotic coverage for polymicrobial peritonitis (Chap. 137) and treatment of gastrointestinal hemorrhages, including bowel resection. Neurologic manifestations occur in 2–40% of patients and include meningitis, Guillain-Barré syndrome, neuritis, and neuropsychiatric symptoms (described as “muttering delirium” or “coma vigil”), with picking at bedclothes or imaginary objects. Uncommon complications whose incidences are reduced by prompt antibiotic treatment include disseminated intravascular coagulation, hemophagocytic syndrome, pancreatitis, hepatitis, hepatic and splenic abscesses and granulomas, endocarditis, pericarditis, myocarditis, orchitis, glomerulonephritis, pyelonephritis and hemolytic-uremic syndrome, severe pneumonia, arthritis, osteomyelitis, endophthalmi­ tis, and parotitis. Up to 10% of patients develop mild relapse, usually within 2–3 weeks of fever resolution and associated with the same strain and susceptibility profile. Up to 10% of untreated patients with typhoid fever excrete S. Typhi in the feces for up to 3 months, and 2–5% develop chronic asymptom­ atic carriage, shedding S. Typhi in either urine or stool for >1 year. Chronic carriage is more common among women, infants, and persons who have biliary abnormalities or concurrent bladder infection with Schistosoma haematobium. S. Typhi and other salmonellae are adapted to survive in the gallbladder environment by forming biofilms on gallstones and invading gallbladder epithelial cells. Chronic carriage is associated with an increased risk of gallbladder cancer, which is much more common in locales where S. Typhi is common, such as the Indian subcontinent. ■ ■DIAGNOSIS Because the clinical presentation of enteric fever is relatively non­ specific, the diagnosis needs to be considered in any febrile traveler FIGURE 171-3  Typical ileal perforation associated with Salmonella Typhi infection. (From JM Saxe, R Cropsey: Is operative management effective in treatment of perforated typhoid? Am J Surg 189:342, 2005.)

returning from a developing region, especially the Indian subconti­ nent, and the Southeast Asian or African region. Other diagnoses that should be considered in these travelers include malaria, viral hepatitis, bacterial enteritis, dengue fever, rickettsial infections, leptospirosis, amebic liver abscesses, and acute HIV infection (Chap. 130). Other than a positive culture, no specific laboratory test is diagnostic for enteric fever. In 15–25% of cases, leukopenia and neutropenia are detectable. Leukocytosis is more common among children, during the first 10 days of illness, and in cases complicated by intestinal perfora­ tion or secondary infection. Other nonspecific laboratory findings include moderately elevated values in liver function tests and muscle enzyme levels.

The definitive diagnosis of enteric fever requires the isolation of S. Typhi or S. Paratyphi from blood, bone marrow, other sterile sites, rose spots, stool, or intestinal secretions. The diagnostic sensitivity of blood culture is only ~40–60% and is lower with low blood sample volume and among patients with prior antimicrobial use or in the first week of illness, reflecting the small number of S. Typhi organisms (i.e., <15/mL) typically present in the blood. Because almost all S. Typhi organisms in blood are associated with the mononuclear cell/platelet fraction, centrifugation of blood and culture of the buffy coat can substantially reduce the time to isolation of the organism but do not increase sensitivity. Bone marrow culture is ~80% sensitive for detection of S. Typhi or

S. Paratyphi, and, unlike that of blood culture, its yield is not reduced by up to 5 days of prior antibiotic therapy. Culture of intestinal secretions (best obtained by a noninvasive duodenal string test) can be positive despite a negative bone marrow culture. If blood, bone marrow, and intestinal secretions are all cultured, the yield is >90%. Stool cultures, although negative in 60–70% of cases during the first week, can become positive during the third week of infection in untreated patients. CHAPTER 171 Rapid immunodiagnostic commercial tests, including Tubex and Typhidot, mainly focus on detection of IgM and IgG antibodies to O and H antigens and are widely used at point of care to diagnose typhoid and paratyphoid fever because they are simple and low cost. In a 2017 systematic review, these rapid diagnostic tests had sensitivi­ ties ranging from ~70% to 80% and specificities ranging from ~80% to 90% and thus are not sufficiently accurate to replace blood cultures as the main approach to diagnose enteric fever. PCR detection of

S. Typhi and S. Paratyphi in the blood have sensitivities of ~40–100%, depending on the gene targets. Molecular-based test platforms were scarce in resource-limited settings, but advancements and investments in molecular diagnostics arising from the SARS-CoV-2 pandemic have made feasible the widespread use of molecular tests for diagnosis of enteric fever. Salmonellosis TREATMENT Enteric (Typhoid) Fever Enteric fever is associated with an overall case-fatality rate of 2.5%, but mortality rates rise to 4.5% among hospitalized patients and to 10–30% if untreated. Prompt administration of appropriate anti­ biotic therapy prevents severe complications of enteric fever and reduces mortality to <1%. The initial choice of antibiotics depends on the susceptibility of the S. Typhi and S. Paratyphi strains in the area of residence or travel (Table 171-1). A 2022 systematic review of 27 randomized clinical trials for the treatment of enteric fever found no difference between ceftriaxone, fluoroquinolone, or azithromycin in comparative risk of treatment failure, microbio­ logic failure, relapse, convalescent carriage, or adverse events. Oral cefixime also can be used to treat enteric fever but may increase the risk of clinical failure and time to defervescence compared with fluoroquinolones. Of note, most of the trials included in this review were small and conducted >20 years ago, and data on antimicrobial resistance could not be analyzed. Because of the high prevalence of strains of S. Typhi and S. Paratyphi with decreased susceptibility to ciprofloxacin (MIC >0.125 μg/mL) on the Indian subcontinent and in some locales in Africa, fluoroquinolones should no longer

TABLE 171-1  Antibiotic Therapy for Enteric Fever in Adults INDICATION AGENT DOSAGE (ROUTE) DURATION, DAYS Empirical Treatment   Ceftriaxonea 2 g/d (IV) 10–14   Ciprofloxacinb 500 mg bid (PO) or 400 mg q12h (IV) 5–7   Azithromycinc 1 g/d (PO)

Fully Susceptible Optimal treatment Ceftriaxone 2 g/d (IV) 10–14   Ciprofloxacin 500 mg bid (PO) or 400 mg q12h (IV) 5–7   Azithromycin 1 g/d (PO)

Alternative treatment Amoxicillin 1 g tid (PO) or 2 g q6h (IV)

Chloramphenicol 25 mg/kg tid (PO or IV) 14–21   Trimethoprimsulfamethoxazole 160/800 mg bid (PO) 7–14 Multidrug-Resistant, Depending on the Susceptibility Pattern Optimal treatment Ceftriaxone 2 g/d (IV) 10–14   Ciprofloxacin 500 mg bid (PO) or 400 mg q12h (IV) 5–7   Azithromycin 1 g/d (PO)

Ceftriaxone-Resistant Optimal treatment Meropenemd 1 g q8h (IV) 1 g/d (PO) 10–14

Azithromycin PART 5 Infectious Diseases Eradication of Carriage Optimal treatment Ciprofloxacin 500–750 mg bid (PO)

Alternative treatment Azithromycin 500 mg (PO)

aOr another third-generation cephalosporin (e.g., cefotaxime, 2 g q8h IV; or cefixime, 400 mg bid PO). bOr 1 g on day 1 followed by 500 mg/d PO for 6 days. cOr ofloxacin, 400 mg bid PO for 2–5 days. dOr imipenem 500 mg q6h IV. be used for empirical treatment of enteric fever in these regions. Patients with concern for ceftriaxone-resistant S. Typhi infection based on a history of travel to Pakistan should be treated empiri­ cally with a carbapenem or azithromycin. Ceftriaxone, cefotaxime, and (oral) cefixime are effective for treatment of MDR enteric fever in adults and children, including that caused by DSC and fluoroquinolone-resistant strains. These agents clear fever in ~1 week, with failure rates of ~5–10%, fecal carriage rates of <3%, and relapse rates of 3–6%. Fluoroquinolones are effective against susceptible strains, with cure rates of ~98% and relapse and fecal carriage rates of <2%. Oral azithromycin is recommended for the treatment of uncomplicated enteric fever, including ESBL, DSC, and fluoroquinolone-resistant strains, and results in defervescence in 4–6 days, with rates of relapse and con­ valescent stool carriage of <3%. Against DSC strains, azithromycin is associated with lower rates of treatment failure and shorter dura­ tions of hospitalization than are fluoroquinolones. Carbapenems are increasingly being used to treat complicated XDR S. Typhi infections, but cost and IV route of administration are significant barriers. Most patients with uncomplicated enteric fever can be man­ aged at home with oral antibiotics and antipyretics. Patients with persistent vomiting, diarrhea, and/or abdominal distension should be hospitalized and given supportive therapy as well as a parenteral third-generation cephalosporin, a fluoroquinolone, or carbapenem, depending on the susceptibility profile. Therapy should be adminis­ tered for at least 10 days or for 5 days after fever resolution. In a randomized, prospective, double-blind study of critically ill patients with enteric fever (i.e., those with shock and obtun­ dation) in Indonesia in the early 1980s, the administration of

dexamethasone (an initial dose of 3 mg/kg followed by eight doses of 1 mg/kg every 6 h) with chloramphenicol was associated with a substantially lower mortality rate than was treatment with chlor­ amphenicol alone (10% vs 55%). Although this study has not been repeated in the “post-chloramphenicol era,” severe enteric fever remains one of the few indications for glucocorticoid treatment of an acute bacterial infection. Steroid treatment beyond 48 hours may increase the relapse rate. The 2–5% of patients who develop chronic carriage of fluoro­ quinolone-susceptible S. Typhi can be treated for 4 weeks with oral ciprofloxacin or other fluoroquinolones, with an eradication rate of ~80%. A 4-week course of oral azithromycin can potentially be used to treat carriers with fluoroquinolone-resistant strains, but clinical experience is limited. Oral amoxicillin is associated with lower eradication rates than fluoroquinolones but can be considered in persons with fluoroquinolone-resistant strains that are susceptible to ampicillin. In cases of anatomic abnormality (e.g., biliary, kidney, or bladder stones), eradication often requires both antibiotic therapy and surgical correction. ■ ■PREVENTION AND CONTROL Theoretically, it is possible to eliminate the salmonellae that cause enteric fever because they survive only in human hosts and are spread by contaminated food and water. However, given the high prevalence of the disease in developing countries that lack adequate sewage disposal and water treatment, this goal is currently unrealistic. Thus, travelers to developing countries should be advised to monitor their food and water intake carefully and to strongly consider immunization against S. Typhi. Two unconjugated typhoid vaccines are commercially available in the United States: (1) Ty21a, an oral live attenuated S. Typhi vaccine (given on days 1, 3, 5, and 7, with revaccination with a full four-dose series every 5 years); and (2) Vi CPS, a parenteral vaccine consisting of purified Vi polysaccharide from the bacterial capsule (given in a single dose, with a booster every 2 years). The minimal age for vaccination is 6 years for Ty21a and 2 years for Vi CPS. In a 2018 meta-analysis of 18 randomized clinical trials of vaccines for preventing typhoid fever in populations in endemic areas, the cumulative efficacy was 50% for Ty21a at 2.5 to 3 years and 55% for Vi CPS at 3 years. Although data on typhoid vaccines in travelers are limited, recent evidence suggests that typhoid vaccines are moderately effective (80%) in U.S. travelers. Currently, there is no licensed vaccine for paratyphoid fever. Unconjugated typhoid vaccines are poorly immunogenic in children <5 years of age because of limited ability to elicit T cell–dependent immune responses and immunologic memory. Compared with uncon­ jugated vaccines, typhoid Vi polysaccharide conjugated vaccines are effective in children <2 years of age and elicit substantially longer dura­ tion of protection. The World Health Organization has recommended two typhoid conjugate vaccines (TCV) prioritized to prevent typhoid fever in countries with high incidence rates—Typbar TCV (manufac­ tured by Bharat Biotech) in 2018 and TYPHIBEV (manufactured by Biological E) in 2020. A single intramuscular 0.5-mL dose of either TCV is safe and 79–95% effective, with antibody response persisting up to 7 years. As of 2023, TCV has been routinely introduced in national immunization programs in Pakistan, Nepal, Liberia, Zimbabwe, Malawi, and Samoa (Fig. 171-1). Typhoid vaccine is not required for international travel, but it is rec­ ommended for travelers to areas where there is a moderate to high risk of exposure to S. Typhi, especially those who are traveling to southern Asia and other developing regions of Asia, Africa, the Caribbean, and Central and South America and who will be exposed to potentially contaminated food and drink. Typhoid vaccine should be considered even for persons planning <2 weeks of travel to high-risk areas. In addition, clinical microbiology or research laboratory staff at risk of occupational exposure to S. Typhi and household contacts of known S. Typhi carriers should be vaccinated. Because the protective efficacy of vaccine can be overcome by the high inocula that are commonly encountered in food-borne exposures, immunization is an adjunct and

not a substitute for the avoidance of high-risk foods and beverages. Immunization is not recommended for the management of persons who may have been exposed in a common-source outbreak. Enteric fever is a notifiable disease in the United States. Individual health departments have their own guidelines for allowing ill or colo­ nized food handlers or health care workers to return to their jobs. The reporting system enables public health departments to identify poten­ tial source patients and to treat chronic carriers in order to prevent further outbreaks. In addition, because 1–4% of patients with S. Typhi infection become chronic carriers, it is important to monitor patients (especially child-care providers and food handlers) for chronic carriage and to treat this condition if indicated. NONTYPHOIDAL SALMONELLOSIS ■ ■EPIDEMIOLOGY Worldwide, NTS causes ~93–150 million enteric infections and ~60,000–155,000 deaths annually. In the United States, the CDC esti­ mates that NTS causes ~1.35 million illnesses, 26,500 hospitalizations, and 420 deaths each year. In 2022, the incidence of NTS infection in the United States was 14.5 cases per 100,000 persons—the second highest rate after Campylobacter (17.4 cases per 100,000 persons) among the 8 food-borne enteric pathogens under active surveillance. Although declining modestly since 2017, the incidence rate remains above the U.S. Healthy People 2030 goal of 11.5 cases per 100,000 persons. Glob­ ally, S. Typhimurium and S. Enteritidis are the most common serotypes causing human disease, with large differences in serotype distributions between regions but lesser differences between countries within the same region. The incidence of NTS infection is highest during the rainy season in tropical climates and during the warmer months in temperate climates—a pattern coinciding with the peak in food-borne outbreaks. Rates of morbidity and mortality associated with NTS are highest among the elderly, infants, and immunocompromised individuals, including those with hemoglobinopathies, HIV infection, or infections that cause blockade of the reticuloendothelial system (e.g., bartonel­ losis, malaria, schistosomiasis, histoplasmosis). Over the past three decades, bloodstream infection caused by inva­ sive NTS, predominantly associated with closely related lineages of

S. Typhimurium sequence type (ST) 313, as well as S. Enteritidis ST11, have emerged in sub-Saharan Africa and have spread to South Asia, the United Kingdom, and Brazil. These invasive NTS strains are adapted to person-to-person transmission through stepwise loss-of-function mutations and typically present with nonspecific febrile illness similar to enteric fever and uncommonly cause diarrhea. In 2017, there were ~535,000 invasive NTS cases and ~77,500 deaths, most of which (~80%) occurred in sub-Saharan Africa. Most (75%) S. Typhimurium ST313 isolates are MDR to ampicillin, trimethoprim-sulfamethoxazole, and chloramphenicol, and some are also resistant to ceftriaxone or ciproflox­ acin, especially in South Asia. Recently, a sublineage of S. Typhimurium ST313 combining MDR with both ceftriaxone and azithromycin resis­ tance has emerged in the Democratic Republic of the Congo. The incidence of invasive NTS infection is highest in children <5 years of age, exceeding 100 per 100,000 person-years in several West African countries, and risk is associated with malaria, HIV, malnutrition, and unclean drinking water sources. Transmission from asymptomatic stool carriers in the household and food or water sources has been proposed, but the sources of invasive NTS infection remain uncertain. Unlike S. Typhi and S. Paratyphi, whose only reservoir is humans, NTS can be acquired from multiple animal and plant reservoirs that are part of the typical food supply. Transmission is most commonly associated with food products of animal origin (especially eggs, poul­ try, undercooked ground meat, and dairy products), fresh produce contaminated with animal waste, and contact with animals or their environments. In the United States, NTS are the second most com­ mon cause of food-borne outbreaks after norovirus, causing 30% of outbreaks and 35% of outbreak-associated illnesses. S. Enteritidis infection associated with chicken eggs emerged as a major cause of food-borne disease during the 1980s and 1990s.

S. Enteritidis infection of the ovaries and upper oviduct tissue of hens results in contamination of egg contents before shell deposition. Infec­ tion is spread to egg-laying hens from breeding flocks and through contact with rodents and manure. The number of S. Enteritidis outbreaks and the proportion attributable to egg-containing foods have continued to decline since the mid-1990s; these declines have coincided with interventions in the egg-producing and food service industries. Despite these control efforts, outbreaks of S. Enteritidis infection associated with shell eggs continue to occur. Transmission via contaminated eggs can be prevented by cooking eggs until the yolk is solidified and pasteurizing egg products. Increasingly, outbreaks of S. Enteritidis infection are associated with other foods, including meat, chicken, vegetables, dairy, and baked goods.

Salmonella serotype 4,[5],12:i:–, an antigenic variant of S. Typhimurium that lacks the second-stage flagellar antigen, has emerged since the 1990s as a foodborne pathogen primarily associated with pigs and pork products. This serotype is the second most common NTS in Europe and the fifth most common in the United States. These strains are MDR, with resistance to ampicillin, streptomycin, sulfonamides, and tetracycline. Increasing reports of plasmid-mediated colistin resistance in these strains have been linked to international travel to Thailand and swine farm isolates. Centralization of food processing and widespread food distribution have contributed to the increased incidence of NTS in developed coun­ tries. NTS accounts for a significant majority of illnesses and hospital­ izations associated with multistate foodborne outbreaks and retail food establishment outbreaks in the United States. Manufactured foods to which recent multistate Salmonella outbreaks have been traced include peanut butter; milk products, including powdered infant formula; and various processed foods, including packaged breakfast cereal, salsa, frozen prepared meals, and snack foods. Large outbreaks have also been linked to fresh produce, including alfalfa sprouts, nuts/seeds, can­ taloupe, mangoes, papayas, tomatoes, and the herbal substance kratom consumed for its stimulant effect; these items become contaminated by manure or water at a single site and then are widely distributed. CHAPTER 171 Salmonellosis In the United States, NTS infection associated with exotic pets is an ongoing clinical and public health problem, especially from contact with reptiles or amphibians, including iguanas, snakes, turtles, and lizards. Other pets, including hedgehogs, rodents, birds, baby chicks, ducklings, dogs, and cats, also are potential sources of NTS. Compared to foodborne outbreaks, outbreaks of NTS linked to animal contact more commonly affect young children (<1−4 years of age), result in hospitalization, and are more sustained. Increasing antibiotic resistance in NTS species is a global problem and has been linked to the widespread use of antimicrobial agents in food animals and especially in animal feed. In the United States, clini­ cally important resistant NTS infections, defined as resistance to ampi­ cillin or ceftriaxone or nonsusceptibility to ciprofloxacin, increased an estimated 40% during 2015–2016 (annual incidence ~220,000) com­ pared with 2004–2008 (~159,000 infections). The incidence (51.0 per 100,000 persons per year) and proportion of invasive NTS infections that are multidrug-resistant (75% with co-resistance to ampicillin, trimethoprim-sulfamethoxazole, and chloramphenicol) is dramatically higher across all sub-Saharan African regions. In the United States, infections caused by NTS with any antimicrobial resistance compared to NTS with no resistance are less likely to be associated with an out­ break and more likely to be associated with international travel, an increased risk of hospitalization, hospital length of stay, and death. Outbreaks and sporadic cases of NTS resistant to third-generation cephalosporins have been reported, and international travel and adop­ tion may have contributed to the global spread. Resistance is most commonly mediated by a transferable plasmid containing the ampC (blaCMY) gene and has been linked to the widespread use of the vet­ erinary cephalosporin ceftiofur. In 2021, 3.0% of NTS isolates from humans in the United States were ceftriaxone resistant (MIC, ≥4 µg/ mL). Ceftriaxone resistance is more common among invasive NTS isolates from sub-Saharan Africa (>5%). Carbapenem-producing NTS have been reported sporadically in Europe, North Africa, and South Asia but remain extremely rare in the United States.

Since the early 2000s, strains of DSC NTS (MIC, ≥0.125 μg/mL) have emerged and have been associated with delayed response and treatment failure. In 2021, 10.6% of NTS isolates in the United States were DSC but only 0.5% of isolates were resistant to ciprofloxacin. These strains have diverse resistance mechanisms, including single and multiple mutations in the DNA gyrase genes gyrA and gyrB, mutations in the chromosomally encoded quinolone resistance–determining region, and plasmid-encoded quinolone resistance genes that are not reliably detected by nalidixic acid susceptibility testing or standard ciprofloxacin disk diffusion. In 2012, the U.S. Clinical Laboratory Stan­ dards Institute proposed a lower ciprofloxacin susceptibility breakpoint (≥0.06 μg/mL) for all Salmonella species to address this issue. Because some commercial test systems do not contain ciprofloxacin concentra­ tions as low as the revised ≥0.06 μg/mL susceptibility breakpoint, labo­ ratories can also determine the ciprofloxacin MIC by Etest or another alternative method.

In the United States, azithromycin-resistant NTS strains (MIC, ≥32 μg/mL) remain uncommon (1.1% in 2021), but most such isolates were also MDR. Azithromycin resistance is conferred by plasmid-encoded macrolide resistance genes, and their emergence may be related to international travel, especially to Thailand, and azithromycin overuse. Sporadic cases of carbapenemase-resistant NTS have been reported in Europe, North Africa, and southern Asia. ■ ■CLINICAL MANIFESTATIONS Gastroenteritis  Infection with NTS most often results in gastroen­ teritis indistinguishable from that caused by other enteric pathogens. Nausea, vomiting, and diarrhea occur 6–48 h after the ingestion of contaminated food or water. Patients often experience abdominal cramping and fever (38–39°C [100.5–102.2°F]). Diarrheal stools are usually loose, nonbloody, and of moderate volume. However, largevolume watery stools, bloody stools, or symptoms of dysentery may occur. Rarely, NTS causes pseudoappendicitis or an illness that mimics inflammatory bowel disease. PART 5 Infectious Diseases Gastroenteritis caused by NTS is usually self-limited. Diarrhea resolves within 3–7 days and fever within 72 h. Stool cultures remain positive for 4–5 weeks after infection—and in rare cases of chronic carriage (<1%) for >1 year. Persistent NTS infection and relapsing diarrhea have been described in a small fraction of Israeli patients and were associated with in-host single-nucleotide mutations in key viru­ lence regulators. For acute NTS gastroenteritis, antibiotic treatment usually is not recommended and may prolong fecal carriage. Neonates, the elderly, and immunosuppressed patients (e.g., transplant recipi­ ents, HIV-infected persons) with NTS gastroenteritis are especially susceptible to dehydration and invasive infection and may require hospitalization and antibiotic therapy. Acute NTS gastroenteritis was associated with a threefold increased risk of dyspepsia and irritable bowel syndrome at 1 year in a study from Spain. Bacteremia and Endovascular Infections  Up to 8% of patients with NTS gastroenteritis develop bacteremia; of these, 5–10% develop localized infections. Bacteremia and metastatic infection are most common with Salmonella Choleraesuis and Salmonella Dublin and among infants, the elderly, and immunocompromised patients, espe­ cially those with HIV infection. NTS endovascular infection should be suspected in high-grade or persistent bacteremia, especially with preexisting valvular heart disease, atherosclerotic vascular disease, prosthetic vascular graft, or aortic aneurysm. Arteritis should be suspected in elderly patients with prolonged fever and back, chest, or abdominal pain developing after an episode of gastroenteritis. Endo­ carditis and arteritis are rare (<1% of cases) but are associated with serious and potentially fatal complications, including valve perfora­ tion, endomyocardial abscess, infected mural thrombus, pericarditis, mycotic aneurysms, aneurysm rupture, aortoenteric fistula, and verte­ bral osteomyelitis. Invasive NTS disease is among the most common causes of bacte­ remia in children and in HIV-infected adults in sub-Saharan Africa and Southeast Asia, causing 39% of community-acquired bloodstream infection in one study. NTS bacteremia among these children is not

associated with diarrhea and has been associated with poor nutritional status, malaria, sickle cell disease, and HIV infection. S. Typhimurium ST 131, the most common cause of invasive NTS disease in sub-Saharan Africa, forms a specific clade that is associated with genome reduction and loss of traits required for environmental stress resistance, likely contributing to making this strain more human adapted. Localized Infections  •  INTRAABDOMINAL INFECTIONS 

Intraabdominal infections due to NTS are rare and usually manifest as hepatic or splenic abscesses or as cholecystitis. Risk factors include hepatobiliary anatomic abnormalities (e.g., gallstones), abdominal malignancy, and sickle cell disease (especially with splenic abscesses). Eradication of the infection often requires surgical correction of abnor­ malities and percutaneous drainage of abscesses. CENTRAL NERVOUS SYSTEM INFECTIONS  NTS meningitis most com­ monly develops in infants 1–4 months of age and in adults with HIV infection. It often results in severe sequelae (including seizures, hydro­ cephalus, brain infarction, and mental retardation), with death in up to 60% of cases. Other rare central nervous system infections include ventriculitis, subdural empyema, and brain abscesses. PULMONARY INFECTIONS  NTS pulmonary infections usually present as lobar pneumonia, and complications include lung abscess, empy­ ema, and bronchopleural fistula formation. The majority of cases occur in patients with lung cancer, structural lung disease, sickle cell disease, or glucocorticoid use. URINARY AND GENITAL TRACT INFECTIONS  Urinary tract infections caused by NTS present as either cystitis or pyelonephritis. Risk factors include malignancy, urolithiasis, structural abnormalities, HIV infec­ tion, and renal transplantation. NTS genital infections are rare and include ovarian and testicular abscesses, prostatitis, and epididymitis. Like other focal infections, both genital and urinary tract infections can be complicated by abscess formation. BONE, JOINT, AND SOFT TISSUE INFECTIONS  Salmonella osteomyelitis most commonly affects the femur, tibia, humerus, or lumbar vertebrae and is most often seen in association with sickle cell disease, hemo­ globinopathies, or preexisting bone disease (e.g., fractures). Prolonged antibiotic treatment is recommended to decrease the risk of relapse and chronic osteomyelitis. Septic arthritis occurs in the same patient population as osteomyelitis and usually involves the knee, hip, or shoulder joints. Reactive arthritis can follow NTS gastroenteritis and is seen most frequently in persons with the HLA-B27 histocompatibility antigen. NTS rarely can cause soft tissue infections, usually at sites of local trauma in immunosuppressed patients. ■ ■DIAGNOSIS The diagnosis of NTS infection is based on isolation of the organ­ ism from freshly passed stool or from blood or another ordinarily sterile body fluid. Salmonella is increasingly identified by cultureindependent molecular diagnostic tests due to increased sensitivity, rapid turnaround, and ability to detect multiple enteric pathogens in one test. Culture-independent positive specimens should have primary isolation performed to replicate results and recover NTS isolates. All NTS isolates should be referred to local public health departments for serotyping. Blood cultures should be obtained whenever a patient has prolonged or recurrent fever. Endovascular infection should be suspected if there is high-grade bacteremia (>50% of three or more sets of blood cultures positive). Echocardiography, CT, and indiumlabeled white cell scanning are used to identify localized infection. When a localized infection is suspected, joint fluid aspiration, bone biopsy, abscess drainage, or cerebrospinal fluid should be cultured, as clinically indicated. TREATMENT Nontyphoidal Salmonellosis Antibiotics should not be used routinely to treat uncomplicated NTS gastroenteritis. The symptoms are usually self-limited, and the duration of fever and diarrhea is not significantly decreased by

TABLE 171-2  Antibiotic Therapy for Nontyphoidal Salmonella Infection in Adults INDICATION AGENT DOSAGE (ROUTE) DURATION, DAYS Preemptive Treatmenta   Ciprofloxacinb 500 mg bid (PO) 2–3 Severe Gastroenteritisc   Ciprofloxacin 500 mg bid (PO) or

400 mg q12h (IV) 500 mg once daily

Azithromycin

Trimethoprimsulfamethoxazole 160/800 mg bid (PO)

Amoxicillin 1 g tid (PO)

Ceftriaxone 1–2 g/d (IV)

Bacteremia   Ceftriaxoned 2 g/d (IV) 7–14   Ciprofloxacin 400 mg q12h (IV), then 500 mg bid (PO)   Endocarditis or Arteritis   Ceftriaxone 2 g/d (IV)

Ciprofloxacin 400 mg q8h (IV), then 750 mg bid (PO)     Ampicillin 2 g q4h (IV)   Meningitis   Ceftriaxone 2 g q12h (IV) 14–21   Ampicillin 2 g q4h (IV)   Other Localized Infection   Ceftriaxone 2 g/d (IV) 14–28   Ciprofloxacin 500 mg bid (PO) or

400 mg q12h (IV)     Ampicillin 2 g q6h (IV)   aConsider for neonates; persons >50 years of age with possible atherosclerotic vascular disease; and patients with immunosuppression, endovascular graft, or joint prosthesis. bOr ofloxacin, 400 mg bid (PO). cConsider on an individualized basis for patients with severe diarrhea and high fever who require hospitalization.

dOr cefotaxime, 2 g q8h (IV). antibiotic therapy. In addition, antibiotic treatment has been asso­ ciated with increased rates of relapse, prolonged gastrointestinal carriage, and adverse drug reactions. Dehydration secondary to diarrhea should be treated with fluid and electrolyte replacement. Preemptive antibiotic treatment (Table 171-2) should be con­ sidered for patients at increased risk for invasive NTS infection, including neonates (probably up to 3 months of age); persons

50 years of age with known or suspected atherosclerosis; and patients with immunosuppression, cardiac valvular or endovas­ cular abnormalities, or significant joint disease. Treatment should consist of an oral or IV antibiotic administered for 48–72 h or until the patient becomes afebrile. Immunocompromised persons may require up to 7–14 days of therapy. The <1% of persons who develop chronic carriage of NTS should receive a prolonged anti­ biotic course, as described above for chronic carriage of S. Typhi. Because of the increasing prevalence of antibiotic resistance, empirical therapy for life-threatening NTS bacteremia or focal NTS infection should include a third-generation cephalosporin or a fluo­ roquinolone (Table 171-2). If the bacteremia is low-grade (<50% of blood cultures positive), the patient should be treated for 7–14 days. Patients with HIV/AIDS and NTS bacteremia should receive 1–2 weeks of IV antibiotic therapy followed by 4 weeks of oral therapy with a fluoroquinolone. Patients whose infections relapse after this regimen should receive long-term suppressive therapy with a fluoroquinolone, azithromycin, or TMP-SMX, as indicated by bacterial sensitivities. If the patient has endocarditis or arteritis, treatment for 6 weeks with an IV β-lactam antibiotic (such as ceftriaxone or ampicillin)

is indicated. IV ciprofloxacin followed by prolonged oral therapy is an option. Early surgical resection of infected aneurysms or other infected endovascular sites is recommended. Patients with infected prosthetic vascular grafts that cannot be resected have been main­ tained successfully on chronic suppressive oral therapy. For extrain­ testinal nonvascular infections, a 2- to 4-week course of antibiotic therapy (depending on the infection site) is usually recommended. In chronic osteomyelitis, abscess, or urinary or hepatobiliary infec­ tion associated with anatomic abnormalities, surgical resection or drainage may be required in addition to prolonged antibiotic therapy for eradication of infection.

■ ■PREVENTION AND CONTROL Immunization against Invasive NTS  Despite an urgent need for safe and effective vaccines, there is no available vaccine to protect against invasive NTS disease, and few candidate vaccines have pro­ gressed into early clinical trials. Candidate human NTS vaccines are O-antigen based and contain S. Typhimurium and S. Enteritidis biva­ lent components. NTS vaccines will need to target young children and protect those in at-risk groups, including persons with HIV infection and with malaria and those with sickle cell disease. In addition, rapid, point-of-care diagnostics are critically needed to reduce the morbidity and mortality associated with invasive NTS infection. Despite widespread efforts to prevent or reduce bacterial contami­ nation of animal-derived food products and to improve food-safety education and training, recent declines in the incidence of NTS in the United States have been modest compared with those of other food-borne pathogens. This observation probably reflects the complex epidemiology of NTS. Identifying effective risk-reduction strategies requires monitoring of every step of the food supply chain, including farm sources, slaughter and processing of raw animal or plant products, storage and transport, and preparation of finished foods. Contami­ nated food can be made safe for consumption by pasteurization, irradi­ ation, or proper cooking. All cases of NTS infection should be reported to local public health departments because tracking and monitoring of these cases can identify the source(s) of infection and help authorities anticipate large outbreaks. Prudent use of antimicrobial agents in both humans and animals is needed to limit the emergence of MDR and XDR Salmonella. CHAPTER 171 Salmonellosis ■ ■FURTHER READING Cruz Espinoza LM et al: Occurrence of typhoid fever complications and their relation to duration of illness preceding hospitalization: A systematic literature review and meta-analysis. Clin Infect Dis 69:S435, 2019. Kuehn R et al: Treatment of enteric fever (typhoid and paraty­ phoid fever) with cephalosporins. Cochrane Database Syst Rev 11:CD010452, 2022. Marchello CS et al: Complications and mortality of non-typhoidal salmonella invasive disease: A global systematic review and metaanalysis. Lancet Infect Dis 22:692, 2022. Medalla F et al: Increased Incidence of Antimicrobial-Resistant Non­ typhoidal Salmonella Infections, United States, 2004-2016. Emerg Infect Dis 27:1662-1672, 2021. Milligan R et al: Vaccines for preventing typhoid fever. Cochrane Database Syst Rev 5:CD001261, 2018. Onwuezobe IA et al: Antimicrobials for treating symptomatic non-typhoidal Salmonella infection. Cochrane Database Syst Rev 11:CD001167, 2012. Pulford CV et al: Stepwise evolution of Salmonella Typhimurium ST313 causing bloodstream infection in Africa. Nat Microbiol 6:327, 2021. Watkins LKF et al: Clinical outcomes of patients with nontyphoidal Salmonella infections by isolate resistance—foodborne diseases active surveillance network, 10 US sites, 2004 – 2018. Clin Infect Dis 78:535, 2024.

55 - 172 Shigellosis

172 Shigellosis

Philippe J. Sansonetti, Jean Bergounioux

Shigellosis The discovery of Shigella as the etiologic agent of dysentery—a clinical syndrome of fever, intestinal cramps, and frequent passage of small, bloody, mucopurulent stools—is attributed to the Japanese microbi­ ologist Kiyoshi Shiga, who isolated the Shiga bacillus (now known as Shigella dysenteriae type 1) from patients’ stools in 1897 during a large and devastating dysentery epidemic. Shigella cannot be distinguished from Escherichia coli by genome comparison and remains a separate species only on historical and clinical grounds. ■ ■ETIOLOGIC AGENT Shigella is a non-spore-forming, gram-negative bacterium that, unlike E. coli, is nonmotile and does not produce gas from sugars, decarbox­ ylate lysine, or hydrolyze arginine. Some serovars produce indole, and occasional strains utilize sodium acetate. Shigella dysenteriae, Shigella flexneri, Shigella boydii, and Shigella sonnei (serogroups A, B, C, and D, respectively) can be differentiated on the basis of biochemical and serologic characteristics. Genome sequencing of E. coli K12, S. flexneri 2a, S. sonnei,

S. dysenteriae type 1, and S. boydii has revealed that these species have ~93% of genes in common. The three major genomic “signa­ tures” of Shigella are (1) a 215-kb virulence plasmid that carries most of the genes required for pathogenicity (particularly invasive capacity); (2) the lack or alteration of genetic sequences encoding products (e.g., lysine decarboxylase) that, if expressed, would attenuate pathogenicity; and (3) in S. dysenteriae type 1, the presence of genes encoding Shiga toxin, a potent cytotoxin. PART 5 Infectious Diseases ■ ■EPIDEMIOLOGY The human intestinal tract is the major reservoir of Shigella, which is also found (albeit rarely) in the higher primates. Because excretion of shigel­ lae is greatest in the acute phase of disease, the bacteria are transmitted most efficiently by the fecal–oral route via hand carriage; however, some outbreaks reflect foodborne or waterborne transmission. In impover­ ished areas, Shigella can be transmitted by flies. The high-level infectivity of Shigella is reflected by the very small inoculum required for experi­ mental infection of volunteers (100 colony-forming units [CFU]), by the very high attack rates during outbreaks in day-care centers (33–73%), and by the high rates of secondary cases among family members of sick children (26–33%). Shigellosis can also be transmitted sexually. Throughout history, bacillary dysentery epidemics have often occurred in settings of human crowding under conditions of poor hygiene—e.g., among soldiers in campaigning armies, inhabitants of besieged cities, groups on pilgrimages, and refugees in camps. Epidemics followed a cyclical pattern in areas such as the Indian subcontinent and sub-Saharan Africa. These patterns of devastating epidemics, which were most often caused by S. dysenteriae type 1, were characterized by high attack and mortality rates. In Bangladesh, for instance, an epidemic caused by

S. dysenteriae type 1 was associated with a 42% increase in mortality rate among children 1–4 years of age. This pattern of severe cyclic disease has steeply declined over the last few decades. Along with the increasingly rare isolation of S. dysenteriae 1, the current epidemiology of shigellosis is generally attributed to a significant global reduction of extreme poverty; this coincides with the tremendous decrease in child mortality. Nevertheless, apart from these severe epidemics of the past, shig­ ellosis remains an endemic disease, with 99% of cases occurring in developing countries, with the highest prevalence in the most impov­ erished areas, where personal and general hygiene is below standard. S. flexneri isolates predominate in the least developed areas, whereas S. sonnei is more prevalent in economically emerging countries and in the industrialized world. Prevalence in the Developing World  In a review published under the auspices of the World Health Organization (WHO), the total

annual number of cases in 1966–1997 was estimated at 165 million, and 69% of these cases occurred in children <5 years of age. In this review, the annual number of deaths was calculated to range between 500,000 and 1.1 million. Data (2000–2004) from six Asian countries indicate that, even though the incidence of shigellosis remains stable, mortality rates associated with this disease may have decreased sig­ nificantly, possibly as a result of improved nutritional status. However, extensive and essentially uncontrolled use of antibiotics, which may also account for declining mortality rates, has increased the emergence of multidrug-resistant Shigella strains. A 2013 prospective, matched, case-control study of children <5 years of age emphasized the importance of Shigella in the burden and etiology of diarrheal diseases in developing countries. Shigella was one of the top four pathogens associated with moderate to severe diarrhea and ranked first among children 12–59 months of age. These moderate to severe cases accounted for an 8.5-fold increase in mortality incidence over the average diarrheal disease-related mortality. The study con­ cluded that Shigella remained a major pathogen to be targeted by health care programs. Global reevaluation of the Shigella disease burden is warranted because an often-overlooked complication of shigellosis is the short- and long-term impairment of the nutritional status of infected children in endemic areas. Combined with anorexia, the exudative enteropathy resulting from mucosal abrasions contributes to rapid deterioration of the patient’s nutritional status. Shigellosis thus now appears as a major contributor to stunted growth among children in endemic regions. Peaking in incidence in the pediatric population, endemic shigel­ losis is rare among young and middle-aged adults, probably because of naturally acquired immunity. Incidence then increases again in the elderly population. Prevalence in the Industrialized World  In pediatric popula­ tions, local outbreaks occur when proper and adapted hygiene policies are not implemented in group facilities such as day-care centers and institutions for the developmentally disabled. In adults, as in children, sporadic cases occur among travelers returning from endemic areas, and rare outbreaks of varying size can follow waterborne or foodborne infections. ■ ■PATHOGENESIS AND PATHOLOGY Shigella infection occurs essentially through oral contamination via direct fecal–oral transmission, the organism being poorly adapted to survive in the environment. Resistance to low-pH conditions allows Shigella to survive passage through the gastric barrier. The watery diarrhea that usually precedes dysenteric symptoms is attributable to active secretion and abnormal water reabsorption in the jejunum, as described in experimentally infected rhesus monkeys. This initial purge is possibly due to the combined action of an entero­ toxin (ShET-1) and mucosal inflammation. The dysenteric syndrome, manifested by bloody and mucopurulent stools, reflects invasion of the colonic mucosa. The pathogenesis of Shigella is essentially determined by a large virulence plasmid of 214 kb comprising ~100 genes, of which 25 encode a type III secretion system that inserts into the membrane of the host cell to allow effectors to transit from the bacterial cytoplasm to the host cell cytoplasm (Fig. 172-1). Bacteria are thereby able to invade intestinal epithelial cells by inducing their own uptake either directly at the opening of colonic crypts or following the initial crossing of the epithelial barrier through M cells (the specialized translocating epithelial cells in the follicle-associated epithelium that covers mucosal lymphoid nodules). Shigella induces apoptosis of subepithelial resident macrophages. Once inside the cytoplasm of intestinal epithelial cells, Shigella effectors trigger the cytoskeletal rearrangements necessary to direct uptake of the organism into the epithelial cell. The Shigellacontaining vacuole is then quickly lysed, releasing bacteria into the cytosol. Intracellular shigellae next use cytoskeletal components to propel themselves inside the infected cell; when the moving organism and the host cell membrane come into contact, cellular protrusions form and

Shigella M cell Activation of NF-κB caused by IL-1β and intracellular NLR activation IcsA + IpaA Macrophages IL-8 IL-1β Disruption of epithelial permeability barrier by PMNs Massive invasion of epithelium IL-18 FIGURE 172-1  Invasive strategy of Shigella flexneri. IL, interleukin; NF-κB, nuclear factor κB; NLR,

NOD-like receptor; PMN, polymorphonuclear leukocyte. are engulfed by neighboring cells. This series of events permits bacte­ rial cell-to-cell spread. Cytokines released by a growing number of infected intestinal epithelial cells massively attract immune cells—particularly poly­ morphonuclear leukocytes [PMNs]—to the infected site, thus further destabilizing the epithelial barrier, exacerbating inflammation, and leading to the acute colitis that characterizes shigellosis. Evidence indi­ cates that some type III secretion system–injected effectors can control the extent of inflammation, thus facilitating bacterial survival. Shiga toxin produced by S. dysenteriae type 1 increases disease severity. This toxin belongs to a group of A1-B5 protein toxins whose B subunit binds to the receptor globotriaosylceramide on the target cell surface and whose catalytic A subunit is internalized by receptormediated endocytosis and interacts with the subcellular machinery to inhibit protein synthesis by expressing RNA N-glycosidase activity on 28S ribosomal RNA. This process leads to inhibition of binding of the amino-acyl-tRNA to the 60S ribosomal subunit and thus to a general shutoff of cell protein biosynthesis. Shiga toxins are translocated from the bowel into the circulation. After binding of the toxins to target cells in the kidney, pathophysiologic alterations may result in hemolyticuremic syndrome (HUS; see below). ■ ■CLINICAL MANIFESTATIONS The presentation and severity of shigellosis depend to some extent on the infecting serotype but even more on the age and the immuno­ logic and nutritional status of the host. Poverty and poor standards of hygiene are strongly related to the number and severity of diarrheal episodes, especially in children <5 years following weaning. Shigellosis typically evolves through four phases: incubation, watery diarrhea, dysentery, and the postinfectious phase. The incubation period usually lasts 1–4 days but may be as long as 8 days. Typical initial manifestations are transient fever, limited watery diarrhea, malaise, and anorexia. Signs and symptoms may range from mild abdominal discomfort to severe cramps, diarrhea, fever, vomiting, and tenesmus. The manifestations are usually exacerbated in children, with temperatures up to 40°–41°C (104.0°–105.8°F) and more severe anorexia and watery diarrhea. This initial phase may represent the only clinical manifestation of shigellosis, especially in developed countries. Otherwise, dysentery follows within hours or days and is characterized by uninterrupted excretion of small volumes of bloody mucopurulent stools with increased tenesmus and abdominal cramps. At this stage, Shigella produces acute colitis involving mainly the distal colon and the rectum. Unlike most diarrheal syndromes, dysenteric syndromes rarely present with dehydration as a major feature. Endoscopy, if performed, shows an edematous and hemorrhagic mucosa, with ulcerations and possibly overlying exudates resembling pseudomembranes. The extent of the lesions correlates with the number and frequency of stools and

with the degree of protein loss by exudative mecha­ nisms. Most episodes are self-limited and resolve without treatment in 1 week. With appropriate treatment, recovery takes place within a few days to a week, with no sequelae.

Epithelial cells Acute life-threatening complications are seen most often in children <5 years (particularly those who are malnourished) and in elderly patients. Risk factors for death in a clinically severe case include nonbloody diarrhea, moderate to severe dehydration, bacteremia, absence of fever, abdomi­ nal tenderness, and rectal prolapse. Major com­ plications are predominantly intestinal (e.g., toxic megacolon, intestinal perforations, rectal prolapse) or metabolic (e.g., hypoglycemia, hyponatremia, dehydration). Bacteremia is rare and is reported most frequently in severely malnourished and HIV-infected patients. Alterations of conscious­ ness, including seizures, delirium, and coma, may occur, especially in children <5 years, and are associated with a poor prognosis; fever and severe metabolic alterations are more often the major causes of altered consciousness than is meningitis or the Ekiri syn­ drome (toxic encephalopathy associated with bizarre posturing, cere­ bral edema, and fatty visceral degeneration), which has been reported mostly in Japanese children. Pneumonia, vaginitis, and keratocon­ junctivitis due to Shigella are rarely reported. In the absence of serious malnutrition, severe and very unusual clinical manifestations, such as meningitis, may be linked to genetic defects in innate immune func­ tions (i.e., deficiency in interleukin 1 receptor–associated kinase 4 [IRAK-4]) and may require genetic investigation. Cell-to-cell spread IpaB IpaC type III secretion Macrophage apoptosis Caspase-I activation by IpaB Bacterial survival Initiation of inflammation CHAPTER 172 Two complications of particular importance are toxic megacolon and HUS. Toxic megacolon is a consequence of severe inflammation extending to the colonic smooth-muscle layer and causing paralysis and dilation. The patient presents with abdominal distention and ten­ derness, with or without signs of localized or generalized peritonitis. The abdominal x-ray characteristically shows marked dilation of the transverse colon (with the greatest distention in the ascending and descending segments); thumbprinting caused by mucosal inflamma­ tory edema; and loss of the normal haustral pattern associated with pseudopolyps, often extending into the lumen. Pneumatosis coli is an occasional finding. If perforation occurs, radiographic signs of pneu­ moperitoneum may be apparent. Predisposing factors (e.g., hypokale­ mia and use of opioids, anticholinergics, loperamide, psyllium seeds, and antidepressants) should be investigated. Shigellosis Shiga toxin produced by S. dysenteriae type 1 has been linked to HUS in developing countries but rarely in industrialized countries, where enterohemorrhagic E. coli (EHEC) predominates as the etiologic agent of this syndrome. HUS is an early complication that most often develops after several days of diarrhea. Clinical examination shows pallor, asthenia, and irritability and, in some cases, bleeding of the nose and gums, oliguria, and increasing edema. HUS is a nonimmune (Coombs-negative) hemolytic anemia defined by a diagnostic triad: microangiopathic hemolytic anemia (hemoglobin level typically <80 g/L [<8 g/dL]), thrombocytopenia (mild to moderate in severity; typically <60,000 platelets/μL), and acute renal failure due to thrombosis of the glomerular capillaries (with markedly elevated creatinine levels). Anemia is severe, with fragmented red blood cells (schizocytes) in the peripheral smear, high serum concentrations of lactate dehydrogenase and free circulating hemoglobin, and elevated reticulocyte counts. Acute renal failure occurs in 55–70% of cases; however, renal function recovers in most of these cases (up to 70% in various series). Leuke­ moid reactions, with leukocyte counts of 50,000/μL, are sometimes present in association with HUS. The postinfectious immunologic complication known as reactive arthritis can develop weeks or months after shigellosis, especially in patients expressing the histocompatibility antigen HLA-B27. About 3% of patients infected with S. flexneri later develop this syndrome, with arthritis, ocular inflammation, and urethritis—a condition that can

last for months or years and can progress to difficult-to-treat chronic arthritis. Postinfectious arthritis occurs only after infection with S. flexneri and not after infection with the other Shigella serotypes.

■ ■LABORATORY DIAGNOSIS The differential diagnosis in patients with a dysenteric syndrome depends on the clinical and environmental context. In developing areas, infec­ tious diarrhea caused by other invasive pathogenic bacteria (Salmonella,

Campylobacter jejuni, Clostridium difficile, Yersinia enterocolitica) or par­ asites (Entamoeba histolytica) should be considered. Only bacteriologic and parasitologic examinations of stool can truly differentiate among these pathogens. A first flare of inflammatory bowel disease, such as Crohn’s disease or ulcerative colitis (Chap. 337), should be considered in patients in industrialized countries. Despite the similarity in symptoms, anamnesis discriminates between shigellosis, which usually follows recent travel in an endemic zone, and these other conditions. Microscopic examination of stool smears shows erythrophagocytic trophozoites with very few PMNs in E. histolytica infection, whereas bacterial enteroinvasive infections (particularly shigellosis) are char­ acterized by high PMN counts in each microscopic field. However, because shigellosis often manifests only as watery diarrhea, systematic attempts to isolate Shigella are necessary. The “gold standard” diagnosis of Shigella infection remains the isola­ tion and identification of the pathogen from fecal material. One major difficulty, particularly in endemic areas where laboratory facilities are not immediately available, is the fragility of Shigella and its com­ mon disappearance during transport, especially with rapid changes in temperature and pH. In the absence of a reliable enrichment medium, buffered glycerol saline or Cary-Blair medium can be used as a holding medium, but prompt inoculation onto isolation medium is essential. The probability of isolation is higher if the portion of stools that con­ tains bloody and/or mucopurulent material is directly sampled. Rectal swabs can be used, as they offer the highest rate of successful isolation during the acute phase of disease. Blood cultures are positive in fewer than 5% of cases but should be done when a patient presents with a clinical picture of severe sepsis. PART 5 Infectious Diseases In addition to quick processing, the use of several media increases the likelihood of successful isolation: a nonselective medium such as bromocresol-purple agar lactose; a low-selectivity medium such as Mac­ Conkey or eosin-methylene blue; and a high-selectivity medium such as Hektoen, Salmonella-Shigella, or xylose-lysine-deoxycholate agar. After incubation on these media for 12–18 h at 37°C (98.6°F), shigellae appear as non-lactose-fermenting colonies that measure 0.5–1 mm in diameter and have a convex, translucent, smooth surface. Suspected colonies on nonselective or low-selectivity medium can be subcultured on a highselectivity medium before being specifically identified or can be identi­ fied directly by standard commercial systems on the basis of four major characteristics: glucose positivity (usually without production of gas), lactose negativity, H2S negativity, and lack of motility. The four Shigella serogroups (A–D) can then be differentiated by additional characteris­ tics. This approach adds time and difficulty to the identification process; however, after presumptive diagnosis, the use of serologic methods (e.g., slide agglutination, with group- and then type-specific antisera) should be considered. Group-specific antisera are widely available; in contrast, because of the large number of serotypes and subserotypes, type-specific antisera are rare and more expensive and thus are often restricted to reference laboratories. Molecular methods of diagnostics, including polymerase chain reac­ tion based on Shigella-specific virulence gene sequences or mass spec­ trometry, are not yet standardized for global use. However, serotyping may soon be replaced by genome-based techniques. TREATMENT Shigellosis ANTIBIOTIC SUSCEPTIBILITY OF SHIGELLA As an enteroinvasive disease, shigellosis requires antibiotic treat­ ment. Since the mid-1960s, however, increasing resistance to

multiple drugs has been a dominant factor in treatment deci­ sions. Resistance rates are highly dependent on the geographic area. Clonal spread of particular strains and horizontal transfer of resistance determinants, particularly via plasmids and transposons, contribute to multidrug resistance. The current global status—i.e., high rates of resistance to classic first-line antibiotics such as amoxicillin—has led to a rapid switch to quinolones such as nali­ dixic acid. However, resistance to such early-generation quinolones has also emerged and spread quickly as a result of chromosomal mutations affecting DNA gyrase and topoisomerase IV; this resis­ tance has necessitated the use of later-generation quinolones as first-line antibiotics in many areas. For instance, a review of the antibiotic resistance history of Shigella in India found that, after their introduction in the late 1980s, the second-generation quino­ lones norfloxacin, ciprofloxacin, and ofloxacin were highly effective in the treatment of shigellosis, including cases caused by multidrugresistant strains of S. dysenteriae type 1. However, investigations of subsequent outbreaks in India and Bangladesh detected resistance to norfloxacin, ciprofloxacin, and ofloxacin in 5% of isolates. In the United States, the resistance rate of Shigella to fluoroquinolones reached 87% during 2014−2015. The incidence of multidrug resis­ tance parallels the widespread, uncontrolled use of antibiotics and calls for the rational use of effective drugs. Despite the alarming proportion of resistant Shigella, there is a lack of studies assessing the resistance of community-acquired strains. ANTIBIOTIC TREATMENT OF SHIGELLOSIS (TABLE 172-1) With effective antibiotic therapy, clinical improvement occurs within 48 h, resulting in a decreased risk of complications and death, shorter duration of symptoms, and elimination of Shigella from the stools. Because of the transmissibility of Shigella, current public health recommendations in the United States are that every case be treated with antibiotics. The use of fluoroquinolones (firstline, preferably ciprofloxacin) and cephalosporins and β-lactams (second-line) for 7−10 days is recommended for the treatment of shigellosis. Whereas infections caused by non-dysenteriae Shigella in immunocompetent individuals are routinely treated with a 3-day course of antibiotics, it is recommended that S. dysenteriae type 1 infections be treated for 5 days and that Shigella infections in immunocompromised patients be treated for 7–10 days. TABLE 172-1  Recommended Antimicrobial Therapy for Shigellosis TREATMENT SCHEDULE ANTIMICROBIAL AGENT LIMITATIONS CHILDREN ADULTS First-Line Ciprofloxacin 15 mg/kg 500 mg   2 times per day for 3 days, PO   Second-Line Pivmecillinam 20 mg/kg 100 mg Cost   4 times per day for 5 days PO No pediatric formulation Frequent administration Emerging resistance Ceftriaxone 50–100 mg/kg — Efficacy not validated Must be injected   Once a day IM for 2–5 days   Azithromycin 6–20 mg/kg 1–1.5 g Cost   Once a day for 1–5 days PO Efficacy not validated Minimum inhibitory concentration near serum concentration Rapid emergence of resistance and spread to other bacteria Source: Reproduced with permission from World Health Organization: Guidelines for the control of shigellosis, including epidemics due to Shigella dysenteriae type 1. https://www.who.int/publications/i/item/9241592330.

56 - 173 Cholera and Other Vibrioses

173 Cholera and Other Vibrioses

Treatment for shigellosis must be adapted to the clinical context, with the recognition that the most fragile patients are children <5 years, who represent two-thirds of all cases worldwide and are the group most at risk of severe complications. There are few data on the use of quinolones in children, but Shigella dysentery is an accepted indication. The half-life of ciprofloxacin is longer in infants than in older individuals. The ciprofloxacin dose gener­ ally recommended for children is 30 mg/kg per day in two divided doses. Adults living in areas with high standards of hygiene are likely to develop milder, shorter-duration disease, whereas infants in endemic areas can develop severe, sometimes fatal, dysentery. In the former setting, treatment will remain minimal and bacteriologic proof of infection will often come after symptoms have resolved; in the latter setting, antibiotic treatment and more aggressive mea­ sures, possibly including resuscitation, are often required. REHYDRATION AND NUTRITION Shigella infection rarely causes significant dehydration, particularly in industrialized countries. It is recommended that rehydration should be oral unless the patient is comatose or in shock. Because of the improved effectiveness of reduced-osmolarity oral rehydration solutions, WHO and UNICEF now recommend a standard solu­ tion of 245 mOsm/L (sodium, 75 mmol/L; chloride, 65 mmol/L; glucose [anhydrous], 75 mmol/L; potassium, 20 mmol/L; citrate, 10 mmol/L). In shigellosis, the coupled transport of sodium and glucose may be variably affected, but oral rehydration therapy remains the easiest and most efficient form of rehydration, espe­ cially in severe cases. Nutrition should be started as soon as possible, keeping in mind that in developing countries, malnutrition remains the primary indicator of the risk of diarrhea-related death. Early refeeding is safe, well tolerated, and clinically beneficial. Because breast-feeding reduces diarrheal losses and the need for oral rehydration in infants, it should be maintained in the absence of contraindications (e.g., maternal HIV infection). NONSPECIFIC, SYMPTOM-BASED THERAPY Antimotility agents have been implicated in prolonged fever in volunteers with shigellosis. These agents are suspected of increasing the risk of toxic megacolon and are thought to have been respon­ sible for HUS in children infected by EHEC strains. For safety reasons, it is better to avoid antimotility agents in bloody diarrhea. TREATMENT OF COMPLICATIONS There is no consensus regarding the best treatment for toxic mega­ colon. The patient should be assessed frequently by both medical and surgical teams. Anemia, dehydration, and electrolyte deficits (particularly hypokalemia) may aggravate colonic atony and should be actively treated. Nasogastric aspiration helps to deflate the colon. Parenteral nutrition has not been proven to be beneficial. Fever persisting beyond 48–72 h raises the possibility of local perforation or abscess. Most studies recommend colectomy if, after 48–72 h, colonic distention persists. However, some physicians recommend continuation of medical therapy for up to 7 days if the patient seems to be improving clinically despite persistent megacolon without perforation. Intestinal perforation, either isolated or complicating toxic megacolon, requires surgical treatment and intensive medical support. Rectal prolapse must be treated as soon as possible. With the health care provider using surgical gloves or a soft warm wet cloth and the patient in the knee-chest position, the prolapsed rectum is gently pushed back into place. If edema of the rectal mucosa is evident, making reduction difficult, it can be osmotically reduced by application of gauze impregnated with a warm solution of satu­ rated magnesium sulfate. Rectal prolapse often relapses but usually resolves along with the resolution of dysentery. HUS must be treated by water restriction, including discontinu­ ation of oral rehydration solutions and potassium-rich nutrition. Hemofiltration or peritoneal dialysis is often required.

■ ■PREVENTION Hand washing after defecation or handling of children’s feces and before handling of food is recommended. Stool decontamination (e.g., with sodium hypochlorite), together with a cleaning protocol for medi­ cal staff as well as for patients, has proven useful in limiting the spread of infection during Shigella outbreaks. Ideally, patients should have a negative stool culture before their infection is considered cured. Recur­ rences are rare if therapeutic and preventive measures are correctly implemented. Protection against fly intrusion in commonly infested sites such as kitchens and latrines is strongly advised in endemic areas.

Although several live attenuated oral and subunit parenteral vaccine candidates have been produced and are undergoing clinical trials, no vaccine against shigellosis is currently available. Especially given the rapid progression of antibiotic resistance in Shigella and its increasing recognition as a cause of child stunting, a vaccine is urgently needed. Most recent evidence indicates that synthetic polysaccharide conjugate vaccines (based on chemically synthesized, concatenated lipopolysac­ charide O-side chain mimics) elicit high levels of protective IgG anti­ bodies, hence promising protection. Protection would be particularly important for children <3 years, the population most susceptible to severe forms of shigellosis. ■ ■FURTHER READING Arena ET et al: Bioimage analysis of Shigella infection reveals targeting of colonic crypts. Proc Natl Acad Sci USA 112:E3282, 2015. Bennish ML, Wojtyniak BJ: Mortality due to shigellosis: Community and hospital data. Rev Infect Dis 13(Suppl 4):S245, 1991. Cohen D et al: Safety and immunogenicity of a synthetic carbohydrate CHAPTER 173 conjugate vaccine against Shigella flexneri 2a in healthy adult volun­ teers: A phase 1, dose-escalating, single-blind, randomised, placebocontrolled study. Lancet Infect Dis 21:546, 2021. Cossart P, Sansonetti PJ: Bacterial invasion: The paradigms of enteroinvasive pathogens. Science 304:242, 2004. Kotloff KL et al: The incidence, aetiology, and adverse clinical conse­ Cholera and Other Vibrioses quences of less severe diarrhoeal episodes among infants and children residing in low-income and middle-income countries: A 12-month case-control study as a follow-on to the Global Enteric Multicenter Study (GEMS). Lancet Glob Health 7:E568, 2019. Mani S et al: Status of vaccine research and development for Shigella. Vaccine 34:2887, 2016. Niyogi SK: Shigellosis. J Microbiol 43:133, 2005. Phalipon A, Sansonetti PJ: Shigella’s ways of manipulating the host intestinal innate and adaptive immune system: A tool box for sur­ vival? Immunol Cell Biol 85:119, 2007. Traa BS et al: Antibiotics for the treatment of dysentery in children. Int J Epidemiol 39:i70, 2010. World Health Organization: Guidelines for the control of shigel­ losis, including epidemics due to Shigella dysenteriae type 1. WHO Library Cataloguing-in-Publication Data. www.who.int/cholera/ publications/shigellosis/en/. Matthew K. Waldor, Edward T. Ryan

Cholera and Other

Vibrioses Members of the genus Vibrio cause several important infectious syndromes. Classic among them is cholera, a devastating diarrheal disease caused by Vibrio cholerae that has been responsible for seven global pandemics and much suffering over the past two centuries. Epidemic cholera remains a significant public-health concern in the

developing world today. Other vibrioses caused by other Vibrio spe­ cies include syndromes of diarrhea, soft tissue infection, or primary sepsis. All Vibrio species are highly motile, facultatively anaerobic, curved, gram-negative rods with one or more flagella. In nature, vib­ rios most commonly reside in tidal rivers and bays under conditions of moderate salinity. They proliferate in the summer months when water temperatures exceed 20°C, and the illnesses they cause also increase in frequency during the warm months.

CHOLERA ■ ■DEFINITION Cholera is an acute diarrheal disease that can, in a matter of hours, result in profound, rapidly progressive dehydration and death. Accord­ ingly, cholera gravis (the severe form) is a much-feared disease, par­ ticularly in its epidemic presentation. Fortunately, prompt aggressive fluid repletion and supportive care can obviate the high mortality that is historically associated with cholera. Although the term cholera has occasionally been applied to any severely dehydrating secretory diar­ rheal illness, whether infectious in etiology or not, it now refers to dis­ ease caused by V. cholerae serogroup O1 or O139—i.e., the serogroups with epidemic potential. ■ ■MICROBIOLOGY AND EPIDEMIOLOGY The species V. cholerae is classified into >200 serogroups based on the carbohydrate constituents of their lipopolysaccharide (LPS) O anti­ gens. Although some non-O1 V. cholerae serogroups (strains that do not agglutinate in antisera to the O1 group antigen) have occasionally caused sporadic outbreaks of diarrhea, serogroup O1 was, until the emergence of serogroup O139 in 1992 (see below), the exclusive cause of epidemic cholera. The O1 serogroup is further subdivided into two serotypes, termed Inaba and Ogawa. Two biotypes of V. cholerae O1, classic and El Tor, have been described, but the former is thought to be extinct. PART 5 Infectious Diseases Yearly incidence rate per 100.000 inhabitants, 2020–2022 0–0.1 No reported case 0,1–1 1–10 10–100 100–1000 FIGURE 173-1  World distribution of cholera in 2020–2022. WHO, World Health Organization. (Reproduced with permission from Dr. M. Piarroux, Université de la Méditerranée, France.)

The natural habitat of V. cholerae is coastal salt water and brack­ ish estuaries, where the organism lives in close relation to plankton. V. cholerae can also exist in freshwater in the presence of adequate nutrients and warmth. Humans become infected incidentally but, once infected, can act as vehicles for spread. Ingestion of water contaminated by human feces is thought to be the most common means of acquisi­ tion of V. cholerae. However, consumption of contaminated food and human-to-human transmission also contribute to spread. There is no known animal reservoir. Although the infectious dose is relatively high, it is markedly reduced in hypochlorhydric persons, in those using antacids, and when gastric acidity is buffered by a meal. Cholera is predominantly a pediatric disease in endemic areas, but it affects adults and children equally when newly introduced into a population. In endemic areas, the burden of disease is often greatest during “cholera seasons” associated with high temperatures, heavy rainfall, and flood­ ing, but cholera can occur year-round. Cholera is native to the Ganges delta on the Indian subcontinent. Since 1817, seven global pandemics have occurred. The current (seventh) pandemic—the first due to the El Tor biotype—began in Indonesia in 1961 and spread in serial waves throughout Asia as V. cholerae El Tor displaced the endemic classic biotype, which is thought to have caused the previous six pandemics. In the early 1970s, El Tor cholera erupted in Africa, causing major epidemics before becoming a persistent endemic problem. Currently, >95% of cholera cases reported annually to the World Health Organization (WHO) are from Africa and Asia (Fig. 173-1), but the true burden and distribution of cholera are unknown because the diagnosis is often syndromic and many countries with endemic cholera do not report cholera to the WHO. In 2022 and 2023, large outbreaks involving more than 10,000 cases per country were being reported every few weeks to the World Health Organization. It is possible that >1–4 million cases of cholera occur yearly (of which only ~400,000 are reported to the WHO) and that these cases result in >20,000–140,000 deaths annually (of which <2000 are reported to the WHO). Imported cases only

After a century without cholera in Latin America, the current cholera pandemic reached Central and South America in 1991. Fol­ lowing an initial explosive spread that affected millions, the burden of disease has markedly decreased in Latin America. In 2010, a severe cholera outbreak began in Haiti, a country with no recorded history of this disease. Several lines of evidence indicate that cholera was likely introduced into Haiti by United Nations security forces from Asia, raising the possibility that asymptomatic carriers of V. cholerae play an important role in transmitting cholera over long distances. In 2016, an outbreak of cholera began in Yemen in the setting of a civil war and population displacement and the breakdown of health infrastructure. The recent history of cholera has been punctuated by such severe outbreaks, especially among impoverished or displaced persons. These outbreaks are often precipitated by war or other circumstances that lead to the breakdown of public-health measures. Such was the case in the camps for Rwandan refugees set up in 1994 around Goma, Zaire; in 2008–2009 in Zimbabwe; in 2015 in South Sudan and the Democratic Republic of the Congo; and in 2022 in Lebanon and the Syrian Arab Republic. Sporadic endemic infections due to V. cholerae O1 strains related to the seventh-pandemic strain have been recognized along the U.S. Gulf Coast of Louisiana and Texas. These infections are typically associated with the consumption of contaminated, locally harvested shellfish. Occasionally, cases in U.S. locations remote from the Gulf Coast have been linked to shipped-in Gulf Coast seafood. In October 1992, a large-scale outbreak of clinical cholera caused by a new serogroup, O139, occurred in southeastern India. The organ­ ism appears to be a derivative of El Tor O1 but has a distinct LPS and an immunologically related O-antigen polysaccharide capsule. (O1 organisms are not encapsulated.) After an initial spread across 11 Asian countries, V. cholerae O139 has once again been almost entirely replaced by O1 strains. The clinical manifestations of disease caused by V. cholerae O139 are indistinguishable from those of O1 cholera. Immunity to one, however, is not protective against the other. ■ ■PATHOGENESIS In the final analysis, cholera is a toxin-mediated disease. The watery diarrhea characteristic of cholera is due to the action of cholera toxin, a potent protein enterotoxin elaborated by the organism in the small intestine. The toxin-coregulated pilus (TCP), so named because its synthesis is regulated in parallel with that of cholera toxin, is essential for V. cholerae to survive and multiply in (colonize) the small intestine. Production of cholera toxin, TCP, and several other virulence factors are coordinately regulated by ToxR. This protein modulates the expres­ sion of genes coding for virulence factors in response to environmental signals via a cascade of regulatory proteins. Additional regulatory processes, including bacterial responses to the density of the bacterial population (in a phenomenon known as quorum sensing), modulate the virulence of V. cholerae. Once established in the human small bowel, the organism produces cholera toxin, which consists of a monomeric enzymatic moiety (the A subunit) and a pentameric binding moiety (the B subunit). The B pentamer binds to GM1 ganglioside, a glycolipid on the surface of epithelial cells that serves as the toxin receptor and makes possible the delivery of the A subunit to its cytosolic target. The activated A subunit (A1) irreversibly transfers ADP-ribose from nicotinamide adenine dinucleotide to its specific target protein, the GTP-binding regulatory component of adenylate cyclase. The ADP-ribosylated G protein upregulates the activity of adenylate cyclase; the result is the intracellular accumulation of high levels of cyclic adenosine mono­ phosphate (AMP). In intestinal epithelial cells, cyclic AMP inhibits the absorptive sodium-transport system in villus cells and activates the secretory chloride-transport system in crypt cells, and these events lead to the accumulation of sodium chloride in the intestinal lumen. Because water moves passively to maintain osmolality, isotonic fluid accumulates in the lumen. When the volume of that fluid exceeds the capacity of the rest of the gut to resorb it, watery diarrhea results. Unless the wasted fluid and electrolytes are adequately replaced, shock (due to profound dehydration) and acidosis (due to loss of bicarbonate)

follow. Although perturbation of the adenylate cyclase pathway is the primary mechanism by which cholera toxin causes excess fluid secre­ tion, cholera toxin also enhances intestinal secretion via prostaglandins and/or neural histamine receptors.

The V. cholerae genome is composed of two circular chromo­ somes. Lateral gene transfer has played a key role in the evolution of epidemic V. cholerae. The genes encoding cholera toxin (ctxAB) are part of the genome of a bacteriophage, CTXΦ. The recep­ tor for this phage on the V. cholerae surface is the intestinal coloniza­ tion factor TCP. Because ctxAB is part of a mobile genetic element (CTXΦ), horizontal transfer of this bacteriophage may account for the emergence of new toxigenic V. cholerae strains. Many of the other genes important for V. cholerae pathogenicity, including the genes encoding the biosynthesis of TCP, accessory colonization factors, and virulence regulators, are clustered together in the V. cholerae pathogenicity island. Similar clustering of virulence genes is found in other bacterial pathogens. It is believed that pathogenicity islands are acquired by horizontal gene transfer. V. cholerae O139 is probably derived from an El Tor O1 strain that acquired the genes for O139 O-antigen synthesis by horizontal gene transfer. ■ ■CLINICAL MANIFESTATIONS Individuals infected with V. cholerae O1 or O139 exhibit a range of clinical manifestations. Some individuals are asymptomatic or have only mild diarrhea; others present with the sudden onset of explosive and life-threatening diarrhea (cholera gravis). The reasons for the range in signs and symptoms of disease are incompletely understood but include the level of preexisting immunity, blood type (persons with type O blood are at greatest risk of severe disease if infected, whereas those with type AB are at least risk), and nutritional status. In a nonimmune individual, after a 24- to 48-h incubation period, chol­ era characteristically begins with the sudden onset of painless watery diarrhea that may quickly become voluminous. Patients often vomit. In severe cases, volume loss can exceed 250 mL/kg in the first 24 h. If fluids and electrolytes are not replaced, hypovolemic shock and death may ensue. Fever is usually absent. Muscle cramps due to electrolyte disturbances are common. The stool has a characteristic appearance: a nonbilious, gray, slightly cloudy fluid with flecks of mucus, no blood, and a somewhat fishy, inoffensive odor. It has been called “rice-water” stool because of its resemblance to the water in which rice has been washed (Fig. 173-2). Clinical symptoms parallel volume contraction: CHAPTER 173 Cholera and Other Vibrioses FIGURE 173-2  Rice-water cholera stool. Note floating mucus and gray watery appearance. (Courtesy of Dr. A. S. G. Faruque, International Centre for Diarrhoeal Disease Research, Dhaka; with permission.)

at losses of <5% of normal body weight, thirst develops; at 5–10%, pos­ tural hypotension, weakness, tachycardia, and decreased skin turgor are documented; and at >10%, oliguria, weak or absent pulses, sunken eyes (and, in infants, sunken fontanelles), wrinkled (“washerwoman”) skin, somnolence, and coma are characteristic. Complications derive exclusively from the effects of volume and electrolyte depletion and include renal failure due to acute tubular necrosis. Thus, if the patient is adequately treated with fluid and electrolytes, complications are averted and the process is self-limited, resolving in a few days.

Laboratory data usually reveal an elevated hematocrit (due to hemo­ concentration) in nonanemic patients; mild neutrophilic leukocytosis; elevated levels of blood urea nitrogen and creatinine consistent with prerenal azotemia; normal sodium, potassium, and chloride levels; a markedly reduced bicarbonate level (<15 mmol/L); and an elevated anion gap (due to increases in serum lactate, protein, and phosphate). Arterial pH is usually low (~7.2). ■ ■DIAGNOSIS Cholera should be suspected when a patient ≥5 years of age develops acute watery diarrhea in an area known to have cholera or develops severe dehydration or dies from acute watery diarrhea, even in an area where cholera is not known to be present. The clinical suspicion of cholera can be confirmed by the identification of V. cholerae in stool; however, the organism must be specifically sought. With experience, it can be detected directly by dark-field microscopy on a wet mount of fresh stool, and its serotype can be discerned by immobilization with specific antisera. Laboratory isolation of the organism requires the use of a selective medium such as taurocholate–tellurite–gelatin (TTG) agar or thiosulfate–citrate–bile salts–sucrose (TCBS) agar. If a delay in sample processing is expected, Cary-Blair transport medium and/ or alkaline-peptone water-enrichment medium may be used as well. In endemic areas, there is little need for biochemical confirmation and characterization, although these tasks may be worthwhile in places where V. cholerae is an uncommon isolate. Standard microbiologic bio­ chemical testing for Enterobacteriaceae will suffice for identification of V. cholerae. All vibrios are oxidase-positive. Point-of-care antigendetection cholera dipstick assays are commercially available for use in the field or where laboratory facilities are lacking. PART 5 Infectious Diseases TREATMENT Cholera Death from cholera is due to hypovolemic shock; thus, treat­ ment of individuals with cholera first and foremost requires fluid resuscitation and management. In light of the level of dehydration (Table 173-1) and the patient’s age and weight, euvolemia should first be rapidly restored, and adequate hydration should then be maintained to replace ongoing fluid losses (Table 173-2). Admin­ istration of oral rehydration solution (ORS) takes advantage of the hexose-Na+ co-transport mechanism to move Na+ across the gut mucosa together with an actively transported molecule such as glu­ cose (or galactose); Cl– and water follow. This transport mechanism remains intact even when cholera toxin is active. ORS may be made by adding safe water to prepackaged sachets containing salts and sugar or by adding 0.5 teaspoon (i.e., half a small spoonful) of table TABLE 173-1  Assessing the Degree of Dehydration in Patients with Cholera DEGREE OF DEHYDRATION CLINICAL FINDINGS None or mild, but diarrhea Thirst in some cases; <5% loss of total body weight Moderate Thirst, postural hypotension, weakness, tachycardia, decreased skin turgor, dry mouth/ tongue, no tears; 5–10% loss of total body weight Severe Unconsciousness, lethargy, or “floppiness”; weak or absent pulse; inability to drink; sunken eyes (and, in infants, sunken fontanelles); >10% loss of total body weight

TABLE 173-2  Treatment of Cholera, Based on Degree of Dehydrationa DEGREE OF DEHYDRATION, PATIENT’S AGE (WEIGHT) TREATMENTb None or Mild, but Diarrheac <2 years 1/4–1/2 cup (50–100 mL) of ORS, to a maximum of 0.5 L/d 2–9 years 1/2–1 cup (100–200 mL) of ORS, to a maximum of 1 L/d ≥10 years As much ORS as desired, to a maximum of 2 L/d Moderatec,d <4 months (<5 kg) 200–400 mL of ORS 4–11 months (5–<8 kg) 400–600 mL of ORS 12–23 months (8–<11 kg) 600–800 mL of ORS 2–4 years (11–<16 kg) 800–1200 mL of ORS 5–14 years (16–<30 kg) 1200–2200 mL of ORS ≥15 years (≥30 kg) 2200–4000 mL of ORS Severec All ages and weights Undertake IV fluid replacement with Ringer’s lactate (or, if not available, normal saline). Give 100 mL/kg in the first 3-h period (or the first 6-h period for children <12 months old); start rapidly, then slow down. Give a total of 200 mL/kg in the first 24 h. Continue until the patient is awake, can ingest ORS, and no longer has a weak pulse. aAdapted from World Health Organization: Outbreak Response Field Manual, Global Task on Cholera Control; https://www.gtfcc.org/wp-content/uploads/2020/05/gtfcccholera-outbreak-response-field-manual-2024.pdf. bContinue normal feeding during treatment. cReassess regularly; monitor stool and vomit output. dVolumes of ORS listed should be given within the first 4 h. Abbreviation: ORS, oral rehydration solution. salt and 6 level teaspoons (i.e., 6 small spoonfuls) of table sugar to 1 L of safe water. Potassium intake in bananas or green coconut water should be encouraged. A number of ORS formulations are available, and the WHO now recommends “low-osmolarity” ORS for treatment of individuals with dehydrating diarrhea of any cause (Table 173-3). If available, rice-based ORS is considered superior to standard ORS in the treatment of cholera. ORS can be administered via a nasogastric tube to individuals who cannot ingest fluid; how­ ever, optimal management of individuals with severe dehydration includes the administration of IV fluid and electrolytes. Because profound acidosis (pH <7.2) is common in this group, Ringer’s lac­ tate is the best choice among commercial products (Table 173-4); it must be used with additional potassium supplements, prefer­ ably given by mouth. The total fluid deficit in severely dehydrated patients (>10% of body weight) can be replaced safely within the first 3–4 h of therapy, half within the first hour. Transient muscle cramps and tetany are common. Thereafter, oral therapy can usually TABLE 173-3  Composition of World Health Organization ReducedOsmolarity Oral Rehydration Solution (ORS)a,b CONSTITUENT CONCENTRATION, mmol/L Na+

K+

Cl–

Citratec

Glucose

Total osmolarity

aContains (per package, to be added to 1 L of drinking water): NaCl, 2.6 g; Na3C6H5O7·2H2O, 2.9 g; KCl, 1.5 g; and glucose (anhydrous), 13.5 g. bIf prepackaged ORS is unavailable, a simple homemade alternative can be prepared by combining 3.5 g (~1/2 teaspoon) of NaCl with either 50 g of precooked rice cereal or 6 teaspoons of table sugar (sucrose) in 1 L of drinking water. In that case, potassium must be supplied separately (e.g., in orange juice or coconut water). c10 mmol of citrate per liter, which supplies 30 mmol of HCO3/L.

TABLE 173-4  Electrolyte Composition of Cholera Stool and of Intravenous Rehydration Solution CONCENTRATION, mmol/L SUBSTANCE NA+ K+ CL– BASE Stool         Adult

Child

Ringer’s lactate

4a

aPotassium supplements, preferably administered by mouth, are required to replace the usual potassium losses from stool. be initiated, with the goal of maintaining fluid intake equal to fluid output. However, patients with continued large-volume diarrhea may require prolonged IV treatment to match gastrointestinal fluid losses. Severe hypokalemia can develop but will respond to potas­ sium given either IV or orally. In the absence of adequate staff to monitor the patient’s progress, the oral route of rehydration and potassium replacement is safer than the IV route. Although not necessary for cure, the use of an antibiotic to which the organism is susceptible diminishes the duration and volume of fluid loss and hastens clearance of the organism from the stool. Adjunctive antibiotics should therefore be administered to patients with moderate or severe dehydration due to cholera. In many areas, macrolides such as erythromycin (adults, 250 mg orally four times a day for 3 days; children, 12.5 mg/kg per dose four times a day for 3 days) or azithromycin (adults, a single 1-g dose; children, a single 20-mg/kg dose) are the agents of choice. Increasing resistance to tetracyclines is widespread; however, in areas with confirmed susceptibility, tetracycline (nonpregnant adults, 500 mg orally four times a day for 3 days; children >8 years old, 12.5 mg/kg per dose four times a day for 3 days) or doxycycline (nonpregnant adults, a 300-mg single dose; children >8 years old, a single dose of 4–6 mg/ kg) may be used. Similarly, increasing resistance to fluoroquino­ lones is being reported, but in areas with confirmed susceptibil­ ity, a fluoroquinolone such as ciprofloxacin may be used (adults, 500 mg twice a day for 3 days; children, 15 mg/kg twice a day for 3 days). Oral administration of supplemental zinc is associated with decreased volume and severity of diarrhea in young children, including in those with cholera. Children <6 months of age with cholera should be treated with 10 mg of zinc daily for 10 days; chil­ dren from 6 to <60 months of age should be treated with 20 mg of oral zinc daily for 10 days. ■ ■PREVENTION Provision of safe water and of facilities for sanitary disposal of feces, improved nutrition, and attention to food preparation and storage in the household can significantly reduce the incidence of cholera. In addition, precautions should be taken to prevent the spread of cholera via infected and potentially asymptomatic persons from endemic to nonendemic regions of the world (as was probably the case in the out­ break in Haiti; see “Microbiology and Epidemiology,” above). Much effort has been devoted to the development of an effective cholera vaccine over the past few decades, with a particular focus on oral vaccine strains. In an attempt to maximize mucosal responses, two types of oral cholera vaccine have been developed: oral killed vac­ cines and live attenuated vaccines. Currently, three oral killed cholera vaccines have been prequalified by the WHO. BivWC (ShancholTM; Shantha Biotechnics, Hyderabad, India) contains both biotypes and serotypes of V. cholerae O1 and V. cholerae O139 without supplemen­ tal cholera toxin B subunit, but its manufacture is being terminated. A related vaccine is produced in South Korea (EuvicholTM, EuvicholPlusTM; Eubiologics, Seoul) and currently accounts for the vast majority of cholera vaccine available to global health control programs. WC-rBS (Dukoral®; Valneva, Sweden AB, Stockholm,) contains both biotypes and serotypes of V. cholerae O1 supplemented with 1 mg of recombi­ nant cholera toxin B subunit per dose. The vaccines are administered

as a two- or three-dose regimen, with doses usually separated by 14 days. They provide ~60–85% protection for the first few months. Booster immunizations of WC-rBS are recommended after 2 years for individuals ≥6 years of age and after 6 months for children 2–5 years of age. For BivWC, no formal recommendation regarding booster immu­ nizations exists. However, BivWC was associated with ~60% protection over 5 years among recipients of all ages in a study in Kolkata, India; the rate of protection among children ≤5 years of age approximated 40%. In outbreak situations, even a single dose of BivWC can provide some protection: 40% and 63% adjusted protection for 6 months for all and severely dehydrating cholera, respectively—although there was no evidence of protection in children younger than 5 years of age. Models predict significant herd immunity when vaccination coverage rates exceed 50%. The killed vaccines have been safely administered among populations with high rates of HIV infection.

Oral live attenuated vaccines for V. cholerae O1 also are in develop­ ment. These strains have in common their lack of the genes encod­ ing cholera toxin. One such vaccine, CVD 103-HgR (VaxchoraTM; Emergent BioSolutions, Washington, DC), is derived from a classic strain of V. cholerae and has been approved by the U.S. Food and Drug Administration for use in travelers to cholera-endemic regions but is not prequalified by the WHO. The vaccine was 90% and 80% efficacious against severe cholera after experimental infection of North American volunteers 10 days and 90 days after vaccination, respectively. Vaxchora is approved for use in individuals 2–64 years of age; no recommendations concerning the timing or need for booster vaccinations are currently available. Other live attenuated vaccine can­ didate strains have been prepared from El Tor and O139 V. cholerae and have been tested in studies of volunteers. An advantage of live attenuated cholera vaccines is that they may induce more potent pro­ tection after a single oral dose. Conjugate and subunit cholera vaccines also are being developed. CHAPTER 173 Recognizing that it may be decades before safe water and adequate sanitation become a reality for those most at risk of cholera, the WHO has recommended incorporation of cholera vaccination into compre­ hensive control strategies and has established an international stockpile of oral killed cholera vaccine to assist in outbreak responses. A global strategy on cholera control was launched in 2017 with the WHO hosting the secretariat of the Global Task Force on Cholera Control (GTFCC). This country-by-country approach aims to reduce cholera deaths by 90% and to eliminate cholera in as many as 20 countries by 2030. Integral components of this strategy are advancing water, sanita­ tion, and hygiene (WASH) programs, as well as use of cholera vaccine. From 2016 to 2023, almost 300 million doses of cholera vaccine have been requested from the Global Vaccine Stockpile, and approximately 150 million doses have been shipped to requesting countries for use in control programs. Due to vaccine shortages, cholera vaccines have most recently been almost solely used in reactive and not preventative campaigns, and are often given as single doses. Cholera and Other Vibrioses OTHER VIBRIO SPECIES The genus Vibrio includes several human pathogens that do not cause cholera. Abundant in coastal waters throughout the world, noncholera vibrios can reach high concentrations in the tissues of filter-feeding mollusks. As a result, human infection commonly follows the inges­ tion of seawater or of raw or undercooked shellfish (Table 173-5). Most noncholera vibrios can be cultured on blood or MacConkey agar, which contains enough salt to support the growth of these halophilic species. In the microbiology laboratory, the species of noncholera vibrios are distinguished by standard biochemical tests. The most important of these organisms are Vibrio parahaemolyticus and Vibrio vulnificus. Vibriosis causes an estimated 80,000 illnesses and 100 deaths in the United States every year. The two major types of syndromes for which these noncholera vib­ rios are responsible are gastrointestinal illness (due to V. parahaemo­ lyticus, non-O1/O139 V. cholerae, V. mimicus, V. fluvialis, V. hollisae, and

V. furnissii) and soft tissue infections (due to V. vulnificus, V. alginolyticus, and V. damselae). V. vulnificus is also a cause of primary sepsis in some immunocompromised individuals.

TABLE 173-5  Features of Selected Noncholera Vibrioses ORGANISM VEHICLE OR ACTIVITY HOST AT RISK SYNDROME Vibrio parahaemolyticus Shellfish, seawater Normal Gastroenteritis   Seawater Normal Wound infection Non-O1/O139 Vibrio cholerae Shellfish, travel Normal Gastroenteritis   Seawater Normal Wound infection, otitis media Vibrio vulnificus Shellfish Immunosuppresseda Sepsis, secondary cellulitis   Seawater Normal, immunosuppresseda Wound infection, cellulitis Vibrio alginolyticus Seawater Normal Wound infection, cellulitis, otitis   Seawater Burned, other immunosuppressed Sepsis aEspecially with liver disease or hemochromatosis. Source: Table 161-3 in Harrison’s Principles of Internal Medicine, 14th edition. ■ ■SPECIES ASSOCIATED PRIMARILY WITH GASTROINTESTINAL ILLNESS V. parahaemolyticus  Widespread in marine environments, the halophilic V. parahaemolyticus is the leading seafood-borne bacterial cause of enteritis worldwide. This species was originally implicated in enteritis in Japan in 1953, accounting for 24% of reported cases in one study—a rate that presumably was due to the common practice of eating raw seafood in that country. In the United States, commonsource outbreaks of diarrhea caused by this organism have been linked to the consumption of undercooked or improperly handled seafood or of other foods contaminated by seawater. Since the mid-1990s, the incidence of V. parahaemolyticus infections has increased in several countries, including the United States. Serotypes O3:K6, O4:K68, and O1:K-untypable, which are genetically related to one another, account in part for this increase. Recent reports from China and Thailand suggest that serotype O10:K4 may be an emerging serotype. The enteropathogenicity of V. parahaemolyticus is associated with its abil­ ity to cause hemolysis via a thermostable direct hemolysin (Vp-TDH). Although the mechanisms by which the organism causes diarrhea are not fully defined, most V. parahaemolyticus genomes encode two type III secretion systems, which directly inject toxic bacterial proteins into host cells. The activity of one of these secretion systems is required for intestinal colonization and virulence in animal models. V. parahaemo­ lyticus should be considered a possible etiologic agent in all cases of diarrhea that can be linked epidemiologically to seafood consumption or to the sea itself. The incidence of V. parahaemolyticus infection in the United States has increased from 0.06 per 100,000 persons in 1996 to 0.9 cases per 100,000 in 2019. PART 5 Infectious Diseases Infections with V. parahaemolyticus can result in two distinct gas­ trointestinal presentations. The more common of the two presenta­ tions (including nearly all cases in North America) is characterized by watery diarrhea, usually occurring in conjunction with abdominal cramps, nausea, and vomiting and accompanied in ~25% of cases by fever and chills. After an incubation period of 4 h to 4 days, symptoms develop and persist for a median of 3 days. Dysentery, the less common presentation, is characterized by severe abdominal cramps, nausea, vomiting, and bloody or mucoid stools. V. parahaemolyticus also causes rare cases of wound infection and otitis and very rare cases of sepsis. Most cases of V. parahaemolyticus–associated gastrointestinal ill­ ness, regardless of the presentation, are self-limited. Fluid replacement should be stressed. Antimicrobial agents may be of benefit in moderate or severe disease. Doxycycline, fluoroquinolones, macrolides, or thirdgeneration cephalosporins are usually used. Deaths are extremely rare among immunocompetent individuals. Severe infections are associated with underlying diseases, including diabetes, preexisting liver disease, iron-overload states, or immunosuppression. Non-O1/O139 (Noncholera) V. cholerae  The heterogeneous non-O1/O139 V. cholerae organisms cannot be distinguished from V. cholerae O1 or O139 by routine biochemical tests but do not agglu­ tinate in O1 or O139 antiserum. Non-O1/O139 strains have caused several well-studied food-borne outbreaks of gastroenteritis and have also been responsible for sporadic cases of otitis media, wound

infection, and bacteremia. Generally, non-O1/O139 V. cholerae strains do not produce cholera toxin and do not cause large epidemics of diar­ rheal disease. Like other vibrios, non-O1/O139 V. cholerae organisms are widely distributed in marine environments. In most instances, recognized cases in the United States have been associated with the consumption of raw oysters or with recent travel. The broad clinical spectrum of diarrheal illness caused by these organisms is probably due to the group’s heterogeneous virulence attributes. The typical incubation period for gastroenteritis due to these organ­ isms is <2 days, and the illness lasts for ~2–7 days. Patients’ stools may be copious and watery or may be partly formed, less voluminous, and bloody or mucoid. Diarrhea can result in severe dehydration. Many cases include abdominal cramps, nausea, vomiting, and fever. Like those with cholera, patients who are seriously dehydrated should receive oral or IV fluids; the value of antibiotics is not clear. Extraintestinal infections due to non-O1/O139 V. cholerae com­ monly follow occupational or recreational exposure to seawater. Around 10% of non-O1/O139 V. cholerae isolates come from cases of wound infection, 10% from cases of otitis media, 20% from cases of bacteremia (which is particularly likely to develop in patients with liver disease), and approximately 40% from stool. Extraintestinal infec­ tions should be treated with antibiotics. Information to guide antibiotic selection and dosing is limited, but most strains are sensitive in vitro to tetracycline, ciprofloxacin, and third-generation cephalosporins. ■ ■SPECIES ASSOCIATED PRIMARILY WITH SOFT TISSUE INFECTION OR BACTEREMIA (See also Chap. 134) V. vulnificus  Infection with V. vulnificus is rare, but this organism is the most common cause of severe Vibrio infections in the United States. Like most vibrios, V. vulnificus proliferates in the warm summer months and requires a saline environment for growth. In the United States, infections in humans typically occur in coastal states between May and October and most commonly affect men >40 years of age. V. vulnifi­ cus has been linked to two distinct syndromes: primary sepsis, which usually occurs in patients with underlying liver disease, and primary wound infection, which generally affects people without underlying disease. (Vulnificus is Latin for “wound maker.”) Some authors have suggested that V. vulnificus also causes gastroenteritis independent of other clinical manifestations. V. vulnificus is endowed with a number of virulence attributes, including a capsule that confers resistance to phagocytosis and to the bactericidal activity of human serum as well as a cytolysin. Measured as the 50% lethal dose in mice, the organism’s virulence is considerably increased under conditions of iron overload; this observation is consistent with the propensity of V. vulnificus to infect patients who have hemochromatosis. Primary sepsis most often develops in patients who have cirrho­ sis or hemochromatosis. However, V. vulnificus bacteremia can also affect individuals who have hematopoietic disorders or chronic renal insufficiency, those who are using immunosuppressive medications or alcohol, or (in rare instances) those who have no known underly­ ing disease. After a median incubation period of 16 h, the patient develops malaise, chills, fever, and prostration. One-third of patients

57 - 174 Brucellosis

174 Brucellosis

develop hypotension, which is often apparent at admission. Cutaneous manifestations develop in most cases (usually within 36 h of onset) and characteristically involve the extremities (the lower more often than the upper). In a common sequence, erythematous patches are followed by ecchymoses, vesicles, and bullae. In fact, sepsis and hem­ orrhagic bullous skin lesions suggest the diagnosis in appropriate set­ tings. Necrosis and sloughing also may be evident. Laboratory studies reveal leukopenia more often than leukocytosis, thrombocytopenia, or elevated levels of fibrin-split products. V. vulnificus can be cultured from blood or cutaneous lesions. The mortality rate approaches 50%, with most deaths due to uncontrolled sepsis (Chap. 315). Accord­ ingly, prompt treatment is critical and should include empirical antibiotic administration, aggressive debridement, and general sup­ portive care. V. vulnificus is sensitive in vitro to a number of antibiot­ ics, including tetracycline, fluoroquinolones, and third-generation cephalosporins. Optimal treatment usually involves administration of a third-generation cephalosporin such as ceftazidime with doxycycline or a fluoroquinolone and wound debridement. V. vulnificus–associated soft tissue infection can complicate either a fresh or an old wound that comes into contact with seawater; the patient may or may not have underlying disease. After a short incubation period (4 h to 4 days; mean, 12 h), the disease begins with swelling, ery­ thema, and (in many cases) intense pain around the wound. These signs and symptoms are followed by cellulitis, which spreads rapidly and is sometimes accompanied by vesicular, bullous, or necrotic lesions. Meta­ static events are uncommon. Most patients have fever and leukocytosis. V. vulnificus can be cultured from skin lesions and occasionally from the blood. Prompt antibiotic therapy and debridement are usually curative. V. alginolyticus  First identified as a pathogen of humans in 1973, V. alginolyticus occasionally causes eye, ear, and wound infections. This species is the most salt-tolerant of the vibrios and can grow in salt concentrations of >10%. Most clinical isolates come from superinfected wounds that presumably become contaminated at the beach. Although its severity varies, V. alginolyticus infection tends not to be serious and generally responds well to antibiotic therapy and drainage. Cases of otitis externa, otitis media, and conjunctivitis due to this pathogen have been described. Tetracycline treatment usually results in cure. V. algi­ nolyticus is a rare cause of bacteremia in immunocompromised hosts. ■ ■FURTHER READING Brumfield KD et al: Environmental parameters associated with inci­ dence and transmission of pathogenic Vibrio spp. Environ Microbiol 23:7314; 2021. Collins JP et al: Preliminary incidence and trends of infections with pathogens transmitted commonly through food—Foodborne Dis­ eases Active Surveillance Network, 10 U.S. Sites, 2016–2021. MMWR Morb Mortal Wkly Rep 71:1260, 2022. Domman D et al: Integrated view of Vibrio cholerae in the Americas. Science 358:789, 2017. Global Task Force on Cholera Control: Early detection and response to contain outbreaks. www.gtfcc.org. Accessed December 6, 2023. Huang Y et al: New variant of Vibrio parahaemolyticus, sequence type 3, serotype O10:K4, China, 2020. Emerg Infect Dis 28:1261; 2022. Kayembe HC et al: Drivers of the dynamics of the spread of cholera in the Democratic Republic of the Congo, 2000–2018: An ecoepidemiological study. PLoS Negl Trop Dis 17:e0011597, 2023. Mavian CN et al: Ancestral origin and dissemination dynamics of reemerg­ ing toxigenic Vibrio cholerae, Haiti. Emerg Infect Dis 29:2073, 2023. Weill FX et al: Genomic history of the seventh pandemic of cholera in Africa. Science 358:785, 2017. World Health Organization: Cholera vaccines: WHO position paper. Wkly Epidemiol Rec 92:477, 2017. World Health Organization: Cholera, 2022. Wkly Epidemiol Rec 38:432, 2023. World Health Organization: Outbreak response field manual, Global Task Force on Cholera Control. https://www.gtfcc.org/wp-content/ uploads/2020/05/gtfcc-cholera-outbreak-response-field-manual-2024.pdf. Accessed March 21, 2024.

Nicholas J. Beeching

Brucellosis ■ ■DEFINITION Brucellosis is a bacterial zoonosis transmitted directly or indirectly to humans from infected animals, predominantly domesticated rumi­ nants and swine. The disease is known colloquially as undulant fever because of its remittent character. Although brucellosis commonly presents as an acute febrile illness, its clinical manifestations vary widely, and definitive signs indicative of the diagnosis may be lacking. Thus, the clinical diagnosis usually must be supported by the results of bacteriologic and/or serologic tests. ■ ■ETIOLOGIC AGENTS Human brucellosis is caused by strains of the genus Brucella, with several species groups defined by differences in chromosomal struc­ ture, host preference, and epidemiologic patterns of infection. Brucella melitensis is the most common cause of symptomatic disease in humans, for which the main sources are sheep, goats, and camels; B. abortus is usually acquired from cattle or buffalo; B. suis is generally acquired from swine but has one variant that is enzootic in reindeer and caribou and another in rodents; B. canis is acquired from dogs. B. ovis causes reproductive disease in sheep but has not been clearly implicated in human disease. Rare human infections have been reported with B. neotomae, which is found in desert rodents. Two species, B. ceti and B. pinnipedialis, have been identified in marine mammals, including seals and dolphins. At least one case of laboratory-acquired human disease due to one of these species has been described, and several cases of natural human infection have been reported. As infections in marine mammals appear to be widespread, more cases of zoonotic infection in humans may be identified. Other reported species include B. microti (isolated from field voles), B. papionis (from baboons), B. vulpis (from foxes), and B. inopinata (from a patient with a breast implant). Addi­ tional novel strains have been described in diverse species, including frogs, bats, and various rodents, and the genus likely will expand further in forthcoming years. The genus Brucella is closely related to the genus Ochrobactrum, which includes free-living environmental bacteria that can occasionally cause opportunistic infections. Recent changes in taxonomy now place Ochrobactrum spp. as subspecies of Brucella on genetic grounds, although the ecology, physiology, clinical niche, and antimicrobial sensitivity of these organisms are completely different. CHAPTER 174 Brucellosis All brucellae are small, gram-negative, unencapsulated, nonsporu­ lating rods or coccobacilli. They grow aerobically on peptone-based medium incubated at 37°C; the growth of some types is improved by supplementary CO2. In vivo, brucellae behave as facultative intracel­ lular parasites. The organisms are sensitive to sunlight, ionizing radia­ tion, and moderate heat; they are killed by boiling and pasteurization but are resistant to freezing and drying. Their resistance to drying renders brucellae stable in aerosol form, facilitating airborne transmis­ sion. The organisms can survive for up to 2 months in soft cheeses made from goat’s or sheep’s milk; for at least 6 weeks in dry soil con­ taminated with infected urine, vaginal discharge, or placental or fetal tissues; and for at least 6 months in damp soil or liquid manure kept in cool dark conditions. Brucellae are easily killed by a wide range of common disinfectants used under optimal conditions but are likely to be much more resistant at low temperatures or in the presence of heavy organic contamination. ■ ■EPIDEMIOLOGY Brucellosis is a zoonosis whose occurrence and control are closely related to its prevalence in domesticated animals. Its distribution is worldwide apart from the few countries where it has been eradicated from the animal reservoir. The true global prevalence of human bru­ cellosis is unknown because of the imprecision of diagnosis and the inadequacy of reporting and surveillance systems in many countries.

Recent estimates suggest there may be more than 2 million cases of human infection a year worldwide. There has been increased recog­ nition of brucellosis in India, Pakistan, Sri Lanka, and China, and of importations to countries in Oceania, such as Fiji, and in Asia, such as Thailand and Vietnam. In Europe, the incidence of brucellosis in a country is inversely related to gross domestic product, and in both developed and less well-resourced settings, human brucellosis is related to rural poverty and inadequate access to medical care. Failure of veterinary control programs due to conflicts or for economic reasons contributes further to the emergence and re-emergence of disease, as seen currently in some eastern Mediterranean countries.

Even in well-resourced settings, the true incidence of brucellosis in domesticated animals may be 10–20 times higher than the reported figures. Bovine brucellosis has been the target of control programs in many parts of the world and has been eradicated from the cattle popula­ tions of much of northern Europe, Australia, New Zealand, and Canada, among other nations. Its incidence has been reduced to a low level in the United States and most western European countries, with a varied pic­ ture in other parts of the world. Efforts to eradicate B. melitensis infec­ tion from sheep and goat populations have been much less successful. These efforts have relied heavily on vaccination programs, which have tended to fluctuate with changing economic and political conditions. In some countries (e.g., Israel), B. melitensis has caused serious outbreaks in cattle. Infections with B. melitensis still pose a major public health problem in Mediterranean countries; in western, central, and southern Asia; and in parts of Africa and South and Central America. Infections with B. abortus are common in cattle-rearing communities in African countries such as Kenya and Uganda. Canine infection with B. canis is present on most continents—the incidence appears to be increasing in North America and in several European countries, often associated with importation of dogs from an endemic area. PART 5 Infectious Diseases Human brucellosis is usually associated with occupational or domestic exposure to infected animals or their products. Farmers, shepherds, goatherds, veterinarians, and employees in slaughterhouses and meat-processing plants in endemic areas are occupationally exposed to infection. Feral pig hunters are at risk of infection with B. suis in several countries, including Australia. Family members of individuals involved in animal husbandry may be at risk, although it is often difficult to differentiate food-borne infection from environ­ mental contamination under these circumstances. Laboratory workers who handle cultures or infected samples also are at risk. Travelers and urban residents usually acquire the infection through consumption of contaminated foods. In countries that have eradicated the disease, new cases are most commonly acquired abroad. Dairy products, espe­ cially soft cheeses, unpasteurized milk, and ice cream, are the most frequently implicated sources of infection; raw meat and bone marrow may be sources under exceptional circumstances. Infections acquired through cosmetic treatments using materials of fetal origin have been reported. Person-to-person transmission is extremely rare, as is trans­ fer of infection by blood or tissue donation. Although brucellosis is a chronic intracellular infection, there is no evidence for increased prevalence or severity among individuals with HIV infection or with immunodeficiency or immunosuppression of other etiologies. Brucellosis may be acquired by ingestion, inhalation, or mucosal or percutaneous exposure. Accidental injection or ingestion of the live vaccine strains of B. abortus (S19 and RB51) and B. melitensis (Rev 1) can cause disease. B. melitensis and B. suis have historically been devel­ oped as biological weapons by several countries and could be exploited for bioterrorism (Chap. S4). This possibility should be borne in mind in the event of sudden unexplained outbreaks. ■ ■IMMUNITY AND PATHOGENESIS Exposure to brucellosis elicits both humoral and cell-mediated immune responses. The mechanisms of protective immunity against human brucellosis are presumed to be similar to those documented in laboratory animals, but such generalizations must be interpreted with caution. The response to infection and its outcome are influenced by the virulence, phase, and species of the infecting strain. Differences have been reported between B. abortus and B. suis in modes of cellular

entry and subsequent compartmentalization and processing. Antibod­ ies promote clearance of extracellular brucellae by bactericidal action and by facilitation of phagocytosis by polymorphonuclear and mono­ nuclear phagocytes; however, antibodies alone cannot eradicate infec­ tion. Organisms taken up by macrophages and other cells can establish persistent intracellular infections. The key target cell is the macro­ phage, and bacterial mechanisms for suppressing intracellular killing and apoptosis result in very large intracellular populations. Opsonized bacteria are actively phagocytosed by neutrophilic granulocytes and by monocytes. In these and other cells, initial attachment takes place via specific receptors, including Fc, C3, fibronectin, and mannose-binding proteins. Opsonized—but not unopsonized—bacteria trigger an oxida­ tive burst inside phagocytes. Unopsonized bacteria are internalized via similar receptors but at much lower efficiency. Smooth strains enter host cells via lipid rafts. Smooth lipopolysaccharide (LPS), β-cyclic glu­ can, and possibly an invasion–attachment protein (Alb) are involved in this process. Tumor necrosis factor α (TNF-α) produced early in the course of infection stimulates cytotoxic lymphocytes and activates macrophages, which can kill intracellular brucellae (probably mainly through production of reactive oxygen and nitrogen intermediates) and may clear infection. However, virulent Brucella cells can suppress the TNF-α response, and control of infection in this situation depends on macrophage activation and interferon γ (IFN-γ) responses. Cyto­ kines such as interleukin (IL) 12 promote production of IFN-γ, which drives TH1-type responses and stimulates macrophage activation. Inflammatory cytokines, including IL-4, IL-6, and IL-10, downregulate the protective response. As in other types of intracellular infection, it is assumed that initial replication of brucellae takes place within cells of the lymph nodes draining the point of entry. Subsequent hematog­ enous spread may result in chronic localizing infection at almost any site, although the reticuloendothelial system, musculoskeletal tissues, and genitourinary system are most frequently targeted. Both acute and chronic inflammatory responses develop in brucellosis, and the local tissue response may include granuloma formation with or with­ out necrosis and caseation. Abscesses may also develop, especially in chronic localized infection. The determinants of pathogenicity of Brucella have not been fully characterized, and the mechanisms underlying the manifestations of brucellosis are incompletely understood. The organism is a “stealth” pathogen whose survival strategy is centered on several processes that avoid triggering innate immune responses and that permit survival within monocytic cells. These processes include evasion of intracellular destruction by restricting the fusion of type IV secre­ tion system–dependent Brucella-containing vacuoles with lysosomal compartments, inhibition of apoptosis of infected mononuclear cells, and prevention of dendritic cell maturation, antigen presentation, and activation of naïve T cells. The smooth Brucella LPS, which has an unusual O-chain and core-lipid composition, has relatively low endo­ toxin activity and plays a key role in pyrogenicity and in resistance to phagocytosis and serum killing in the nonimmune host. In addition, LPS is believed to play a role in suppressing phagosome–lysosome fusion and diverting the internalized bacteria into vacuoles located in endoplasmic reticulum, where intracellular replication takes place. Specific exotoxins have not been isolated, but a type IV secretion sys­ tem (Vir) that regulates intracellular survival and trafficking has been identified. In B. abortus, this system can be activated extracellularly, but in B. suis, it is activated (by low pH) only during intracellular growth. Brucellae then produce acid-stable proteins that facilitate the organisms’ survival in phagosomes and may enhance their resistance to reactive oxygen intermediates. A type III secretion system based on modified flagellar structures also has been inferred, although not yet confirmed. Virulent brucellae are resistant to defensins and produce a Cu-Zn superoxide dismutase that increases their resistance to reactive oxygen intermediates. A hemolysin-like protein may trigger the release of brucellae from infected cells. ■ ■CLINICAL FEATURES Brucellosis almost invariably causes fever, which may be associated with profuse sweats, especially at night. In endemic areas, brucellosis may be

difficult to distinguish from other causes of fever. However, two features recognized in the nineteenth century distinguish brucellosis from other tropical fevers, such as typhoid and malaria: (1) Left untreated, the fever of brucellosis shows an undulating pattern that persists for weeks before the commencement of an afebrile period that may be followed by relapse. (2) The fever of brucellosis is associated with musculoskeletal symptoms and signs in about one-half of all patients. The clinical syndromes caused by the different species are similar, although B. melitensis tends to be associated with a more acute and aggressive presentation and B. suis with focal abscess induction. B. abortus infections may have a more insidious onset and are more likely to become chronic. B. canis infections are generally regarded as less severe but, like other species, can cause serious disease such as endocarditis. The incubation period varies from 1 week to several months, and the onset of fever and other symptoms may be abrupt or insidi­ ous. In addition to experiencing fever and sweats, patients become increasingly apathetic and fatigued; lose appetite and weight; and have nonspecific myalgia, headache, and chills. Overall, the presentation of brucellosis often fits one of three patterns: febrile illness that resembles typhoid but is less severe; fever and acute monoarthritis, typically of the hip or knee, in a young child; and long-lasting fever, misery, and lowback or hip pain in an older person (especially men). In an endemic area (e.g., much of the Middle East), a patient with fever and difficulty walking into the clinic would be suspected to have brucellosis until it was proven otherwise. Diagnostic clues in the patient’s history include travel to an endemic area, employment in a diagnostic microbiology laboratory, consump­ tion of unpasteurized milk products (including soft cheeses), contact with animals, accidental inoculation with veterinary Brucella vaccines, and—in an endemic setting—a history of similar illness in the family (documented in almost 50% of cases). Focal features are present in the majority of patients. The most common are musculoskeletal pain and physical findings in the peripheral and axial skeleton (~40% of cases). Osteomyelitis more commonly involves the lumbar and low thoracic vertebrae than the cervical and high thoracic spine. Individual joints that are most commonly affected by septic arthritis are the knee, hip, sacroiliac, shoulder, and sternoclavicular joints; the pattern may be one of monoarthritis or polyarthritis. Osteomyelitis may also accompany septic arthritis. In addition to the usual causes of vertebral osteomyelitis or septic arthritis, the most important disease in the differential diagnosis is tuberculosis. This point influences the therapeutic approach as well as the prognosis, given that several antimicrobial agents used to treat brucellosis are also used to treat tuberculosis. Septic arthritis in brucel­ losis progresses slowly, starting with small pericapsular erosions. In the vertebrae, anterior erosions of the superior end plate are typically the first features to become evident, with eventual involvement and sclero­ sis of the whole vertebra. Anterior osteophytes eventually develop, but vertebral destruction or impingement on the spinal cord is rare and usually suggests tuberculosis (Table 174-1). Other systems may be involved in a manner that resembles typhoid. About one-quarter of patients have a dry cough, usually with few changes visible on the chest x-ray, although pneumonia, empyema, intrathoracic adenopathy, or lung abscess can occur. Sputum or pleural effusion cultures are rarely positive in such cases, which respond well to standard brucellosis treatment. One-quarter of patients have hepa­ tosplenomegaly, and 10–20% have significant lymphadenopathy; the differential diagnosis includes glandular fever–like illness such as that caused by Epstein-Barr virus, Toxoplasma, cytomegalovirus, HIV, or Mycobacterium tuberculosis. Up to 10% of men have acute epididymoorchitis, which must be distinguished from mumps and from surgical problems such as torsion. Prostatitis, inflammation of the seminal vesicles, salpingitis, and pyelonephritis all occur. There is an increased incidence of fetal loss among infected pregnant women, although tera­ togenicity has not been described and the tendency toward abortion is much less pronounced in humans than in animals. Neurologic involvement is common, with depression and lethargy whose severity may not be fully appreciated by either the patient or the physician until after treatment. A small proportion of patients develop

TABLE 174-1  Radiology of the Spine: Differentiation of Brucellosis from Tuberculosis   BRUCELLOSIS TUBERCULOSIS Site Lumbar and others Dorsolumbar Vertebrae Multiple or contiguous Contiguous Diskitis Late Early Body Intact until late Morphology lost early Canal compression Rare Common Epiphysitis Anterosuperior General: upper and lower disk regions, central, subperiosteal Osteophyte Anterolateral (parrot beak) Unusual Deformity Wedging uncommon Anterior wedge, gibbus Recovery Sclerosis, whole-body Variable Paravertebral abscess Small, well-localized Common and discrete loss, transverse process Psoas abscess Rare More likely lymphocytic meningoencephalitis that mimics mild neurotuberculosis, atypical leptospirosis, or noninfectious conditions. Rare manifestations include intracerebral abscess, a variety of cranial nerve deficits, or rup­ tured mycotic aneurysms. Endocarditis occurs in ~1% of cases, most often affecting the aortic valve (natural or prosthetic). Any site in the body may be involved in metastatic abscess formation or inflammation; the female breast and the thyroid gland are affected particularly often. Nonspecific maculo­ papular rashes and other skin manifestations are uncommon and are rarely noticed by the patient even if they develop. CHAPTER 174 ■ ■DIAGNOSIS Because the clinical picture of brucellosis is not distinctive, the diagnosis must be based on a history of potential exposure, a presentation con­ sistent with the disease, and supporting laboratory findings. Results of routine biochemical assays are usually within normal limits, although serum levels of hepatic enzymes and bilirubin may be elevated. Periph­ eral leukocyte counts are usually normal or low, with relative lympho­ cytosis. Mild anemia may be documented. Thrombocytopenia and disseminated intravascular coagulation with raised levels of fibrinogen degradation products can develop. The erythrocyte sedimentation rate and C-reactive protein levels are often normal but may be raised. Brucellosis In body fluids such as cerebrospinal fluid (CSF) or joint fluid, lym­ phocytosis and low glucose levels are the norm. Elevated CSF levels of adenosine deaminase cannot be used to distinguish tubercular meningitis, as they may also be found in brucellosis. Biopsied samples of tissues such as lymph node or liver may show noncaseating granu­ lomas without acid-fast bacilli. The radiologic features of bony disease develop late and are much more subtle than those of tuberculosis or septic arthritis of other etiologies, with less bone and joint destruction. Isotope scanning is more sensitive than plain x-ray and continues to give positive results long after successful treatment. Isolation of brucellae from blood, CSF, bone marrow, or joint fluid or from a tissue aspirate or biopsy sample is definitive, and attempts at isolation are usually successful in 50–70% of cases. Blood culture using modern nonradiometric or similar signaling systems (e.g., Bactec) usu­ ally become positive within 7 days. Clinicians should alert the labora­ tory to the possibility of brucellosis if suspected, as all cultures should be handled under containment conditions appropriate for dangerous pathogens. Brucella species may be misidentified as Agrobacterium, Ochrobactrum, or Psychrobacter (Moraxella) phenylpyruvicus by the gallery identification strips that may still be used in the diagnostic laboratory. In recent years, matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) has emerged as a powerful tool for bacterial identification in well-resourced labora­ tories. Earlier implementations of MALDI-TOF often failed to identify Brucella species correctly as a genus or at species level. Following recent changes in taxonomy, Ochrobactrum isolates may now be reported as a

subspecies of Brucella. This can lead to confusion and have important clinical and public health consequences, as management differs com­ pletely. Clinicians should confirm the meaning of any culture report suggestive of brucellosis with laboratory colleagues before making therapeutic decisions.

The peripheral blood–based polymerase chain reaction (PCR) has enormous potential to detect bacteremia, to predict relapse, and to exclude “chronic brucellosis.” This method is more sensitive and quicker than blood culture, and it does not carry the attendant biohaz­ ard risk posed by culture. However, it is not perfect, and false-negative results are sometimes observed in patients with positive blood cultures. Nucleic acid amplification techniques are now quite widely used, although no single standardized procedure has been adopted. Prim­ ers for the spacer region between the genes encoding the 16S and 23S ribosomal RNAs (rrs-rrl), various outer-membrane protein–encoding genes, the insertion sequence IS711, and the protein BCSP31 are sensi­ tive and specific. Blood and other tissues are the most suitable samples for analysis. The clinical significance of prolonged PCR positivity, com­ monly seen in blood after successful treatment, remains controversial. Serologic examination often provides the only positive laboratory findings in brucellosis. In acute infection, IgM antibodies appear early and are followed by IgG and IgA. All these antibodies are active in agglutination tests, whether performed by tube, plate, or microagglu­ tination methods. The majority of patients have detectable agglutinins at this stage. As the disease progresses, IgM levels decline, and the avidity and subclass distribution of IgG and IgA change. The result is reduced or undetectable agglutinin titers. However, the antibodies are detectable by alternative tests, including the complement fixation test, Coomb’s antiglobulin test, and enzyme-linked immunosorbent assays. There is no clear cutoff value for a diagnostic titer. Rather, serology results must be interpreted in the context of exposure history and clinical presentation. In endemic areas or in settings of potential occupational exposure, agglutinin titers of 1:320–1:640 or higher are considered diagnostic; in nonendemic areas, a titer of ≥1:160 is consid­ ered significant. Repetition of tests after 2–4 weeks may demonstrate a rising titer. PART 5 Infectious Diseases In most centers, the standard agglutination test (or a derivative such as the microagglutination test) is still the mainstay of serologic diag­ nosis. In an endemic setting, >90% of patients with acute bacteremia have standard agglutination titers of at least 1:320 at the time of clinical presentation. Some centers rely on the Rose Bengal test, particularly for screening, but it has been only partially validated for human diagnostic use. A variety of near-patient or point-of-care tests are still in develop­ mental stages. Antibody to the Brucella LPS O chain—the dominant antigen—is detected by all the conventional tests that employ smooth B. abortus cells as antigen. Because B. abortus cross-reacts with B. melitensis and B. suis, there is no advantage in replicating the tests with these antigens. Cross-reactions also occur with the O chains of some other gramnegative bacteria, including Yersinia enterocolitica O:9, Escherichia coli O157, Francisella tularensis, Salmonella enterica group N, Stenotroph­ omonas maltophilia, and Vibrio cholerae. The cell-surface antigens of rough Brucella strains such as B. canis or B. ovis do not react in these serologic tests, which will give false-negative results. Specific antigenbased tests used in veterinary practice to detect B. canis and B. ovis produce inconsistent results in human diagnosis and are not validated for this purpose. Clinicians should consult with the laboratory about test selection if B. canis infection is suspected. Similarly, live B. abortus vaccine strain RB51 does not elicit antibody responses in serologic tests that use smooth antigens, and this fact must be taken into account if serologic tests are employed in attempts to identify or follow the course of infections in persons accidentally exposed to the vaccine. TREATMENT Brucellosis The broad aims of antimicrobial therapy are to treat and relieve the symptoms of current infection and to prevent relapse. Focal disease

presentations may require specific intervention in addition to more prolonged and tailored antibiotic therapy. In addition, tuberculosis must always be excluded, or—to prevent the emergence of resistance— therapy should be tailored to specifically exclude drugs active against tuberculosis (e.g., rifampin used alone) or to include a full antituberculous regimen. Early experience with streptomycin or tetracycline monotherapy showed that relapse was common; thus dual therapy with both agents became the norm. This is still the most effective combina­ tion, but alternatives may be used, with the options depending on local or national policy about the use of rifampin for the treatment of nonmycobacterial infection. For the several antimicrobial agents that are active in vivo, efficacy can usually be predicted by in vitro testing. However, numerous Brucella strains show in vitro sensitiv­ ity to a whole range of antimicrobials that are therapeutically inef­ fective, including assorted β-lactams. The use of fluoroquinolones remains controversial despite the good in vitro activity and whitecell penetration of most agents of this class. Low intravacuolar pH is probably a factor in the poor performance of these drugs. For adults with acute nonfocal brucellosis (duration <1–2 months), a 6-week course of therapy incorporating at least two antimicro­ bial agents is required. Complex or focal disease may necessitate

≥3 months of therapy. Adherence to the therapeutic regimen is very important, and poor adherence underlies almost all cases of appar­ ent treatment failure; such failure is rarely due to the emergence of drug resistance, although increasing resistance to trimethoprimsulfamethoxazole (TMP-SMX) is being reported. The gold standard for the treatment of brucellosis in adults is IM streptomycin (0.75–1 g daily for 14–21 days) together with doxy­ cycline (100 mg twice daily for 6 weeks). In both clinical trials and observational studies, relapse follows such treatment in 5–10% of cases. The usual alternative regimen favored by the World Health Organization and many national guidelines consists of rifampin (600–900 mg/d) plus doxycycline (100 mg twice daily) for 6 weeks. This is easier for the patient and health care provider but requires sustained treatment adherence. The relapse/failure rate of this regimen is ~10% in trial conditions but can rise to >20% in many nontrial situations, possibly because doxycycline levels are reduced and clearance rates increased by concomitant rifampin administra­ tion. Patients who cannot tolerate or receive tetracyclines (children, pregnant women) can be given high-dose TMP-SMX instead (two or three standard-strength tablets twice daily for adults, depending on weight). Good evidence supports the use of an aminoglycoside such as gentamicin (5–6 mg/kg per day for 1–2 weeks) instead of strep­ tomycin, and this is recommended in U.S. and U.K. guidelines. Shorter courses of gentamicin have been associated with high fail­ ure rates in adults. Early experience with fluoroquinolone mono­ therapy was disappointing, but it has been suggested that ofloxacin or ciprofloxacin, given together with rifampin for 6 weeks, might be an acceptable alternative to the other 6-week regimens for adults. A substantial meta-analysis did not support the use of fluoroqui­ nolones in first-line treatment regimens, and these drugs were not recommended by an expert consensus group (the Ioannina Recommendations) except in the context of well-designed clinical trials. However, a more recent meta-analysis is more supportive of the efficacy of these drugs, and adequately powered prospective studies will be needed to resolve their role in standard combination therapy. Although triple-drug regimens are superior to double-drug regimens in meta-analyses, they are not indicated for uncompli­ cated brucellosis. A triple-drug regimen such as doxycycline and rifampin, enhanced by initial aminoglycoside, should be considered for all patients with complicated disease and for those for whom treatment adherence is likely to be a problem. Focal neurologic disease due to Brucella species requires pro­ longed treatment (i.e., for 3–6 months), usually with ceftriaxone supplementation of a standard regimen. Brucella endocarditis is treated with at least three drugs (an aminoglycoside, a tetracy­ cline, and rifampin), and many experts add ceftriaxone and/or a

58 - 175 Tularemia

175 Tularemia

fluoroquinolone to reduce the need for valve replacement. Treat­ ment is usually given for at least 4–6 months, and clinical endpoints for its discontinuation are often difficult to define. Surgery is still required for the majority of cases of infection of prosthetic heart valves and prosthetic joints. There is good evidence for the benefit of antimicrobial pro­ phylaxis to reduce the risk of infection after occupational or other exposure to Brucella organisms, inadvertent immunization with live vaccine intended for use in animals, or exposure to deliberately released brucellae. However, there is little evidence to inform the duration of prophylaxis or preference for dual or single therapy. Based on historical practices, many national guidelines recommend the administration of rifampin plus doxycycline for 3 weeks. How­ ever, such regimens are poorly tolerated, and doxycycline mono­ therapy of the same duration may be used instead. Monotherapy is the standard recommendation in the United Kingdom but not in the United States. Rifampin should be omitted after exposure to vaccine strain RB51, which is resistant to rifampin, and replaced by another agent such as TMP-SMX in combination with doxycy­ cline. After significant brucellosis exposure, expert consultation is advised for women who are (or may be) pregnant. ■ ■PROGNOSIS AND FOLLOW-UP Relapse occurs in up to 30% of poorly compliant patients. Thus, patients should ideally be followed clinically for up to 2 years to detect relapse, which responds to a prolonged course of the same therapy used originally. The general well-being and the body weight of the patient are more useful guides than serology to lack of relapse. IgG antibody levels detected by the standard agglutination test and its variants can remain in the diagnostic range for >2 years after successful treatment. Complement fixation titers usually fall to normal within 1 year of cure. Immunity is not solid; patients can be reinfected after repeated expo­ sures. Fewer than 1% of patients die of brucellosis. When the outcome is fatal, death is usually a consequence of cardiac involvement; more rarely, it results from severe neurologic disease. Despite the low mor­ tality rate, recovery from brucellosis is slow, and the illness can cause prolonged inactivity, with domestic and economic consequences. The existence of a prolonged chronic brucellosis state after success­ ful treatment remains controversial. Evaluation of patients in whom this state is considered (often those with work-related exposure to bru­ cellae) includes careful exclusion of malingering, nonspecific chronic fatigue syndromes, and other causes of excessive sweating, such as alco­ hol abuse and obesity. In the future, the availability of more sensitive assays to detect Brucella antigen or DNA may help to identify patients with ongoing infection. ■ ■PREVENTION Vaccines based on live attenuated Brucella strains, such as B. abortus strain 19BA or 104M, have been used in some countries to protect high-risk populations but have displayed only short-term efficacy and high reactogenicity. Subunit vaccines have been developed but are of uncertain value and cannot be recommended at present. Research in this area has been stimulated by interest in biodefense (Chap. S4) and may eventually yield new products. The mainstay of veterinary pre­ vention is a national commitment to testing and slaughter of infected herds/flocks (with compensation for owners), control of animal movement, and active immunization of animals. These measures are usually sufficient to control human disease as well. In their absence, pasteurization of all milk products before consumption is sufficient to prevent nonoccupational animal-to-human transmission. All cases of brucellosis in animals and humans should be reported to the appropri­ ate public health authorities. ■ ■FURTHER READING Beeching NJ et al: Brucellosis. BMJ Best Practice, 2023. https://

bestpractice.bmj.com/topics/en-us/911. Bosilkovski M et al: The current therapeutical strategies in human brucellosis. Infection 49:823, 2021.

Centers for Disease Control and Prevention: Brucellosis.

https://www.cdc.gov/brucellosis/index.html. Dean AS et al: Clinical manifestations of human brucellosis: A sys­ tematic review and meta-analysis. PLoS Negl Trop Dis 6:e1929, 2012. Norman FF et al: Imported brucellosis: A case series and literature review. Travel Med Infect Dis 14:182, 2016. Zagorsky P et al: Laboratory diagnosis of human brucellosis. Clin Microbial Rev 33:e00073, 2019. Anders F. Johansson

Tularemia ■ ■DEFINITION Tularemia is a zoonotic disease that can naturally be transmitted from vertebrate animals to humans, causing a febrile illness. Synonyms include deer-fly fever, rabbit fever, market men’s disease, water-rat trappers’ disease, wild hare disease (yato-byo), Francis’ disease, and Ohara’s disease. ■ ■ETIOLOGY Tularemia is caused by Francisella tularensis, a small (0.2–0.7 × 0.2 µm), facultatively intracellular, aerobic, gram-negative, pleomorphic bac­ terium. F. tularensis is highly virulent for humans and numerous mammals, including rodents, hares, and rabbits. It is known to cause airborne laboratory-acquired infection; therefore, laboratory work, including culturing of the bacterium, must be performed by trained staff in biologic safety cabinets under biosafety level 3 conditions. The disease tularemia was described by the American researchers McCoy and Chapin as a “plague-like disease of rodents” in California in 1911, 1 year after they cultivated and described the causative bacterium as “Bacterium tularense.” Francisella was later named after Edward Fran­ cis, an American bacteriologist who extensively studied the etiologic agent and the pathogenesis of tularemia over several decades. There are two F. tularensis subspecies of high clinical importance: F. tularensis subspecies tularensis (aka Jellison type A) with human disease docu­ mentation exclusively from the North American continent, and sub­ species holarctica (Jellison type B) with human disease documentation both from North America and from other parts of the world. A third subspecies, F. tularensis subspecies mediasiatica, has been isolated from vertebrate animals in Central Asia and Russia, but there is no docu­ mented disease in humans. The division into type A and type B strains of F. tularensis is based on minor biochemical test differences, with type A strains being significantly more lethal in experimentally infected rabbits. Type A but not type B cultures can ferment glycerol and are positive in a citrulline ureidase assay. Microbial genome sequencing has revealed an average nucleotide identity >99% between type A and type B strains of F. tularensis, but current and historic epidemiologic studies mirror the experimental findings in rabbits—tularemia mortal­ ity among humans is higher with type A in the United States. However, recent research using high-resolution genetic subtyping, combined with analyses of disease outcomes, shows there is heterogeneity in mor­ tality risks among different type A genetic varieties, with some being less virulent for humans than the type B varieties. CHAPTER 175 Tularemia Genetic neighbors of F. tularensis, including Francisella novicida (sometimes referred to as a fourth subspecies of F. tularensis with a genomic average nucleotide identity of ~97% with other subspecies) and the genetically more distant Francisella philomiragia, are oppor­ tunistic pathogens that may cause disease in humans with symptoms varying according to the immune status of the patient. Moreover, mod­ ern microbiology methods and intense research efforts have resulted in the identification of additional Francisella bacteria, including

pathogens of fish, endosymbiont bacteria of ticks, soil bacteria, and bacteria isolated from sea and freshwater, some of which may be opportunistic human pathogens. Taxonomists now recognize at least nine distinct Francisella species, including Francisella tularensis, the causative agent of tularemia.

■ ■EPIDEMIOLOGY F. tularensis infects humans through bites of arthropods functioning as disease vectors (e.g., mosquitoes, ticks, tabanid flies), inhaling infec­ tious aerosols, handling infected animals, or ingesting contaminated water. The reported human cases represent bridging between endemic F. tularensis maintenance in nature, or spillover events from massive amplification in infected rodents, hares, or rabbits during disease outbreaks among animals (epizootics). The disease is endemic to the Northern Hemisphere but has a patchy and geographically uneven dis­ tribution, which is believed to be the result of F. tularensis being main­ tained long-term in smaller geographic areas experiencing repeated outbreaks, often at intervals of several years. In the United States, reported human cases have dropped sharply from several thousand annual cases before 1950 to 200–300 cases annually from 2010–2020, with most of these cases being associated with tick bites in Arkansas, Kansas, Missouri, and Oklahoma according to Centers for Disease Control and Prevention (CDC) statistics. Surveillance data from 2006– 2021 with available F. tularensis culture data showed 66% type A and 34% type B cases in humans. In some parts of Europe and Asia where type B only causes human disease, tularemia was relatively more com­ mon in 2010–2022, with several hundred to >2000 cases in peak years in Finland, Sweden, and Turkey, respectively. In the Northern Boreal Forest regions of Sweden, Finland, and Russia, tularemia is primarily associated with mosquito bites, while tick bites are more common in Western and Eastern Europe. Most cases in the United States, as well PART 5 Infectious Diseases Type A Tularemia in North America Donor and Reservoir Vector Reservoir and Vector Type B Tularemia in North America, Europe, and Asia Environmental reservoir? Donor and Reservoir Reservoir and Vector Vectors in the Boreal Forests FIGURE 175–1  Schematic illustration of the type A and type B disease ecology concepts. (Adapted from P. Keim et al: Molecular epidemiology, evolution, and ecology of Francisella. Ann N Y Acad Sci 1105:30, 2007, Figure 11.)

as in Finland and Sweden, occur during summer, likely reflecting increased human exposure to blood-feeding arthropods, infected wild­ life mammals, or aerosols from carcasses of infected animals during outdoor activities. In contrast to this seasonal pattern, most cases occur in the winter months in Turkey, where the predominant infection route to humans in some rural areas is nonchlorinated drinking water from natural springs that may be contaminated by infected rodents. Colder temperatures enhance the survival of the bacterium in water during winter. In the Southern Hemisphere, rare human and animal infections have been documented from Australia. It is uncertain whether the lack of reporting from other areas of the Southern Hemisphere is due to lack of disease or from underreporting. Based on extensive historic field studies performed in the United States and the former Soviet Union during the 1950s–1970s, it is believed that distinct disease ecologies characterize F. tularensis sub­ species tularensis (type A) and F. tularensis subspecies holarctica (type B), respectively. This concept recognizes the association of type A tulare­ mia with rabbits, ticks, domestic cats, and sheep in comparatively dry environments, in contrast to the association of type B tularemia with streams, ponds, lakes, rivers, and semiaquatic animals such as muskrats and beavers (Fig. 175–1). It remains unknown exactly how the bacterium is maintained in nature. F. tularensis is a generalist eukaryotic host-associated microbe that can infect many animals and cell types, including amphibians, birds, rodents, rabbits, carnivores, and ruminant mammals. Most of these animals are accidental hosts or rapidly die from septic disease. It is unclear if any can maintain the bacterium for a long duration. However, when an animal dies from a tularemia infection with a high level of bacteria in their bodies, this can potentially help temporarily maintain the bacteria in the environment. Over the long term, decay­ ing infectious tissues might also contribute to the bacteria’s persistence Environmental reservoir?

under certain conditions. Another hypothesis for maintenance of F. tularensis is persistence in water with protozoa such as amoeba serving as host cells. Since the 1940s, research has shown that F. tularensis can infect multiple species of soft and hard ticks, supporting the idea of tularemia as an obligate vector-borne disease with ticks as reservoirs. In North America, tularemia in humans is mainly tick-borne, but the role of ticks in maintaining and spreading the disease geographically is more debated compared with other pathogens like Babesia and Borrelia; these latter pathogens are better adapted for transmission between the different life stages of a tick. Contemporary well-designed experiments involving mice and American dog ticks have tested F. tularensis type A strains from three main genetic clades and a type B strain. They dem­ onstrated rapid F. tularensis acquisition by adult ticks from bacteremic mice, an interrupted blood-feeding behavior, and high transmission efficiency to uninfected mice (58–89%), indicating that adult ticks can serve as highly effective disease vectors to humans during outbreaks. However, ticks of the more immature nymph life stage showed low survival after F. tularensis ingestion and low transmission efficiency (0–13.5%), indicating that these are ineffective disease vectors. Overall, both older and newer studies suggest that F. tularensis infection poses a high burden for immature tick stages, and the literature is conflicted on the presence of transovarial transmission in ticks. It has been suggested that ticks, like other blood-feeding arthropods including tabanid flies in North America and mosquitoes in the Boreal Forest regions of Northern Europe, are important for transmitting disease to humans during tularemia epizootics but may not be obligate for main­ taining F. tularensis. To better understand the zoonotic potential and the epidemiology seen in humans, a better understanding of how

F. tularensis is maintained between outbreaks will be required. ■ ■PATHOGENESIS Natural transmission to humans usually occurs through the skin or by inhalation of aerosols containing F. tularensis, but ingesting infected food or water also may cause infection. An extremely low dose of 10–50 bacteria inoculated through microtrauma of the skin or inhaled into the lungs suffices to cause disease in humans. In contrast, a dose in the range of 106–108 bacteria is required to infect humans and other primates by the oral route. Tissue-resident macrophages, including alveolar macrophages in the lung and Langerhans cells in the skin, are believed to be important in the transport of F. tularensis by blood or lymph vessels to lymph nodes, spleen, and bone marrow. In the skin, F. tularensis undergoes phagocytosis by neutrophils at initial stage of infection. After the bacteria have been taken up by tissue-resident macrophages and other local phagocytic cells, F. tularensis escapes from the phagosome into the cell cytosol, replicates, leaves the cell, and disseminates throughout the host. F. tularensis is a “stealth pathogen,” implying that the bacterium is able to manipulate host-cell signaling, thus rendering itself a safe milieu inside the host cell suitable for its rep­ lication. In experimental oral infection of mice, there is minimal local gastrointestinal pathology despite the large infection dose required. It remains unknown what cell types mediate F. tularensis to cross the intestinal epithelium to cause fatal systemic infection. Gross and histopathology findings in nonhuman primates experi­ mentally infected by the inhalation route have shown 0.5- to 1-mm necrotic lesions with live bacteria in the spleen at day 4 after the infection, and subsequent larger foci in liver and lymph nodes at days 5–7, which likely mirror disease events and progression in more severe human tularemia cases. At day 6 after infection of nonhuman primates, randomly scattered subacute abscesses and pyogranulomas 6–20 mm in diameter have been observed on the lung surfaces. From day 7 and onward, granuloma formation and necrotic lesions in tissues surrounded by layers of macrophages, lymphocytes, and giant cells are typical reaction patterns in lymph nodes, lungs, spleen, kidney, and liver, corroborated by historic reports of similar autopsy findings in fatal human tularemia. The immune invasion strategy of F. tularensis—delaying the immune response and permitting rapid systemic distribution—has made stud­ ies of early immune responses challenging. The mechanisms behind

cell-mediated responses against F. tularensis after a primary infection or vaccination by the live vaccine strain of F. tularensis, however, are well described and require both B and T cells, resulting in antigenspecific recall T cell responses persisting 25–30 years after infection or vaccination. Nevertheless, although the bacterium can manipulate and delay them, effective early immune responses occur and involve complement, antibodies, neutrophils, inducible NO synthase, phago­ cyte oxidase, and cytokines such as interferon γ, tumor necrosis factor α, and interleukin 12.

APPROACH TO THE PATIENT Clinical recognition of tularemia may be challenging, especially if the patient does not spontaneously report fever. Sporadic cases of patients seeking health care can easily be misdiagnosed with a diversity of other conditions, including lung cancer, head and neck cancer, lymphoma, tuberculosis, or another disease with lymphoid tissue engagement or a suspected tumor. Tularemia may not be suspected before extensive and costly medical investigations have already been performed. Awareness of the disease and a high index of suspicion is critical. Even in high-endemic areas, patients with tularemia often initially are mistakenly managed and treated for other conditions, including unspecified viral infection, common skin and soft tissue infection, common community-acquired pneu­ monia, or undifferentiated fever or sepsis. A careful disease history of a patient with tularemia typically reveals an acute fever onset episode with influenza-like illness. If the patient seeks care immedi­ ately, an acute fever onset can be easily recollected; if care is sought several weeks after initial symptoms, acute onset of fever may not be mentioned spontaneously. The acute illness may be described as “a severe virus” with high fever, muscle pain, headache, and other constitutional symptoms. Fever may be persistent or intermittent. A tularemia-focused disease history should include risk factors for exposure to F. tularensis and epidemiologic information (e.g., household member or friend having similar symptoms and shar­ ing the same exposure). Exposure risk factors or activities include wild animal contact (e.g., rodents, rabbits, hares), contact with or inhalation of dust contaminated with their urine or feces, recent arthropod bites (tick, fly, or mosquitoes), performing brush cutting or grass trimming, handling of wood, handling of hay, and walking or hunting in the forest or other natural habitats of wildlife species. CHAPTER 175 Tularemia When tularemia is suspected it is important to define patient factors with relevance for coping with infection, to define the infec­ tion syndrome, and to decide on an appropriate level for patient management: • Consider factors that may increase the risk of severe infection (e.g., geriatric frailty, immunosuppression, severe lung and/or heart function impairment, other significant comorbidities). • By physical examination, determine any anatomic location and extent of inflammation (e.g., presence of pneumonitis, lymph node inflammation, skin inflammation including minute ulcers or pustules, pharyngitis). Collect information on the rate of pro­ gression and judge the severity of infection. • Determine an appropriate level of care (e.g., localized symptoms likely suitable for outpatient care, multiorgan involvement/ hemodynamic instability suggesting inpatient care). ■ ■CLINICAL MANIFESTATIONS The most common initial manifestation of tularemia is an influenzalike illness with high fever. The incubation period is usually 3–5 days but may range from 1 to 21 days. Inflamed swollen lymphoid tissue is typical for tularemia. The initial infection route determines what lym­ phoid tissue is engaged (Fig. 175–2). If bacteria are acquired through skin or oral mucous membranes of the pharynx, palpable enlarged tender regional lymph node(s) can result. If bacteria are inhaled, the result will be lymph node enlargement in the mediastinum. Different recognized clinical forms of tularemia are the same for type A and type B tularemia and depend on the initial F. tularensis infection route as outlined in Table 175-1.

Preauricular Cervical, tonsillar, and supraclavicular Infraclavicular Mediastinal Axillary Hilar Spleen Epitrochlear and brachial Inguinal and femoral Popliteal PART 5 Infectious Diseases FIGURE 175–2  Characteristic sites of lymphoid tissue engagement in tularemia. (From https://training.seer.cancer.gov/lymphoma/anatomy/lymph-nodes.html.) The primary tularemia ulcer is often a small break in the skin, with limited signs of inflammation, and may require careful clinical examination to be detected. In some cases, it may not be detectable and the disease form will then be classified as glandular. Red, tender, swollen lymph regional nodes are often the most remarkable clinical examination findings in patients with acute ulceroglandular tularemia (Fig. 175–3). In oropharyngeal and oculoglandular tularemia, there is more often pronounced local inflammation at the primary site of infection with acute pharyngitis or conjunctivitis, respectively. The respiratory or pneumonic form of tularemia acquired by inhalation of F. tularensis may, more often than other clinical forms, be bacteremic and progress to severe disease requiring inpatient care, particularly in patients aged 65 and over and those with comorbidities. In the respiratory/pneumonic clinical form of tularemia, onset of dry cough may occur more than 7 TABLE 175-1  Clinical Forms of Tularemia FORMa F. TULARENSIS INFECTION ROUTE Ulceroglandular or glandular Skin inoculation by blood-feeding arthropods or direct contact (touching infected animal or F. tularensis contaminated material) Oropharyngeal Ingestion of contaminated water or food Oculoglandular Touching the eye with contaminated fingers or exposure to contaminated aerosol Respiratory/pneumonic Inhalation of contaminated aerosol Typhoidal Unknown (likely by inhalation, rarely by ingestion) aAdditional less common but well-described clinical invasive manifestations of tularemia exist, including primary tularemia meningitis, endocarditis, and bone and joint infections. In patients with immunosuppression, tularemia manifestations may be indolent with few focal symptoms and signs. Source: Reproduced with permission from A Tärnvik (ed): WHO Guidelines on Tularaemia: Epidemic and Pandemic Alert and Response. Geneva, World Health Organization Press, 2007.

FIGURE 175–3  Patient presenting with ulceroglandular tularemia 3 weeks after onset of high fever and influenza-like illness. Lymph nodes of the left thigh were red, tender, and swollen. A tularemia ulcer was present on the left ankle (not shown). (Photo from patient care at Region Västerbotten, Sweden.) days after onset of the influenza-like illness. Bronchoscopy findings may include signs of local tracheitis and pneumonitis, and the lymphoid tissue engaged will be in the mediastinum or lung hila. “Typhoidal tularemia” is an older medical classification term referring to a disease syndrome like typhoid fever with prolonged high fever, fatigue, headache, and nausea. It is recommended to avoid using this term, if possible, as it is believed that historically, most cases of typhoidal tularemia were acquired by inhalation and were, in fact, respiratory/pneumonic tularemia. For type A tularemia, historic disease descriptions suggest that respiratory/pneumonic tularemia or meningitis may also occur as secondary manifestations in severe cases of ulceroglandular disease. Data from the United States in 2006–2021 on 1046 cases with primary disease manifestations available showed 47% ulceroglandular, 18% glandular, 17% pneumonic or respiratory, 14% typhoidal, 2% oculoglandular, and 2% oropharyngeal disease according to CDC statistics. Routine biochemical testing of blood generally is not helpful in tularemia because findings are unspecific and may or may not reveal elevated liver enzymes or red blood cells in the urine. In a recent more extensive case series from Sweden of type B tularemia of primarily outpatients, white blood cell counts remained within normal limits for 70% of patients, and the median C-reactive protein level was moderately elevated, peaking at day 7–9 after disease onset at a median of 78 mg/L (range, 30–130). A recent analysis of 33 patients hospitalized with severe respiratory type B tularemia showed peak C-reactive protein levels at 100–400 mg/L among patients with F. tularensis growth in blood cultures and 50–220 mg/L among nonbacteremic patients. ■ ■DIAGNOSIS Laboratory confirmation is recommended and may be based on recovery of an F. tularensis isolate, F. tularensis–specific antigens or nucleic acids, or the detection of F. tularensis–specific antibodies in

blood by serology testing. Appropriate clinical specimens to collect for direct detection of F. tularensis by nucleic acids or culture include ulcer specimens, lower respiratory tract specimens (sputum/tracheal washing/bronchoalveolar lavage), lymph node biopsies or aspirates, and blood cultures. Additional clinical specimens with F. tularensis growth reported include pharyngeal and ventricular washings. Impor­ tantly, sampling from ulcers should be performed from the edge of the wound, preferably selecting a swab with a slightly stiffer stick and some rubbing of the tissue so that it becomes somewhat blood-tinged. If there is a scab, this should be removed before sampling. If culture specimens are sent to a clinical microbiology laboratory, a suspicion of tularemia must be clearly communicated to the laboratory in advance to minimize the risk of laboratory-acquired infections in lab workers. Direct detection of F. tularensis–specific nucleic acid in clinical speci­ mens by polymerase chain reaction or another detection method is a rapid and reliable diagnostic method. For ulcer specimens, the method is comparable with culture in sensitivity and specificity. There is clinical experience of F. tularensis nucleic acid detection from other specimen types, including sputum, bronchoalveolar lavage, and lymph node biop­ sies/aspirates, but formal scientific evaluation of the diagnostic perfor­ mance is lacking. With increased availability of molecular methods in some clinical microbiology laboratories, nucleic acid–based subtyping without a prior culture step can be performed directly from clinical specimens and may help clinicians to identify F. tularensis varieties rapidly. Identification of the subspecies and genetic variety involved in tularemia infections is currently limited in most laboratories but may be of greatest importance in North America, where type A and type B tula­ remia exist side by side. Cultivation of F. tularensis requires Biosafety Level 3 (BSL-3) laboratories, and because F. tularensis is a U.S. Depart­ ment of Health and Human Services (HHS) Tier 1 Select Agent, an accurate culture diagnosis in the clinic is challenging. However, culture identification provides a definitive diagnosis and allows for subsequent genetic methods to identify different F. tularensis varieties with varying risks of mortality. Unexpected findings in blood cultures with matrixassisted laser desorption/ionization–time-of-flight (MALDI-TOF) mass spectrometry identification of F. tularensis is a scenario that has become more common in recent years with the improvement of blood culture systems and MALDI-TOF reference libraries. F. tularensis is a slow-growing bacterium, and an incubation period of 10 days is recommended for optimal sensitivity in blood-culture diagnostics. Serology remains commonly used to confirm tularemia and is the mainstay diagnostic in many clinical microbiology laboratories. Serology is particularly useful in patients when tularemia is suspected as a differential diagnosis at a later stage—the antibody responses against F. tularensis are generally detectable in the blood of patients 10–20 days post-infection using an enzyme-linked immunosorbent assay or a microagglutination test. Some commercial serologic tests may detect F. tularensis–specific anti­ bodies as early as 1–2 weeks after disease onset. Importantly, the persis­ tence of F. tularensis antibodies is prolonged, meaning that up to at least 1 year after acute infection, single titers of antibodies remain above the cutoff set by laboratories to detect acute infections. Therefore, a reliable serologic diagnosis of tularemia in the acute phase should rely on the demonstration of a significant increase of antibody titers between two samples taken 2–4 weeks apart. Chest imaging is useful in respiratory tularemia. Single or multifocal consolidation may be present, accompanied by pleural effusion with hilar and mediastinal lymphadenopathy (Fig. 175–4). Computed tomography (CT) scans in the acute phase may reveal multiple rounded consolidations, often in the subpleural region and sometimes with signs of inclusion necrosis with a diffuse marginal zone. Lymph node enlargements in the mediastinal and hilar regions on a CT scan may also contain necrotic inclusions. A relatively com­ mon scenario for sporadic cases of tularemia is pulmonary nodules or dense consolidations, which can mimic lung cancer, resulting in more extensive clinical investigation paths to exclude cancer, including CT and positron emission tomography (PET) scans (Fig. 175–5). Endo­ bronchial ultrasound bronchoscopy procedures with biopsy may be performed in some cases before tularemia is suspected. The resolution of tularemia pathology on chest imaging may be slow and take months.

FIGURE 175–4  Frontal projection chest x-ray with acute respiratory/pneumonic tularemia (type B) with consolidation at the right side of the lungs with hilar enlargement. Fever and chills started 10 days before. (Photo from patient care at Region Västerbotten, Sweden.) CHAPTER 175 ■ ■PATHOLOGY Analyses of tularemia skin lesions of humans infected by skinning of rabbits with tularemia have suggested a scenario with accumulation of lymphocytes, plasma cells, and neutrophils at the infection site during the first week after F. tularensis infection of the skin. Microabscesses with neutrophils occur in subcutis along with histiocytes and Langerhans cells in dilated lymph vessels. A skin ulcer and granuloma formation occur only after the first week, along with lymph node engagement. Histopathology findings in other tissues of patients with more advanced tularemia and a more extended disease history include irregular micro­ abscesses and granulomas in samples from liver, spleen, kidney, and lymph nodes. Autopsy samples from the lungs of humans have shown necrotizing pneumonia characterized by abundant fibrin, cellular debris, and neutrophils within alveolar walls and alveolar spaces. Tularemia TREATMENT Specific antimicrobial treatment is highly recommended for tula­ remia as outlined in Table 175-2. The antibiotics of choice are cip­ rofloxacin, levofloxacin, gentamicin, or doxycycline. Early specific antimicrobial treatment is essential to avoid complications irrespec­ tive of targeting type A or type B tularemia. Several commonly used antimicrobials, including all the β-lactam drugs and clindamycin, lack treatment efficacy against F. tularensis. Macrolide antibiotics, including erythromycin and azithromycin, show an in vitro effect against F. tularensis type A and type B varieties known to be pres­ ent in the United States, but globally there are very scarce treatment outcome data for these drugs in humans. Generally, macrolide antibiotics are not recommended for tularemia because macrolideresistant type B strains are common in Europe and Asia, resulting in total lack of treatment effect and overall little experience using these drugs for tularemia treatment. Azithromycin, however, may be an alternative treatment choice for patients with verified type A infection according to recent CDC guidelines. Streptomycin, classically used to treat tularemia, has been the drug of choice due to its high cure rate in a U.S. case series reported from 1949–1988, but is now considered a less attractive

A PART 5 Infectious Diseases B C FIGURE 175–5  Dense consolidation in the left lung of patient with a history of weight loss and cough for 3–4 months. A. CT scan showed dense consolidation (2.5 × 2.5 × 2.0 cm) in the basal left upper lobe extending to the apical part of the lower lobe and increased hilar lymphoid tissue. B. Eight days later, PET scan showed increased metabolic activity and progress of the consolidation to 5.0 × 4.5 × 3.5 cm. Tissue biopsy revealed granuloma formation in the adjacent pleura and no malignant cells. C. A tularemia diagnosis was confirmed by serology, the patient was treated with ciprofloxacin, and a CT scan performed 2 months later showed resolution of the consolidation. (Photos from patient care at Region Västerbotten, Sweden.) alternative to gentamicin due to its drawbacks, including the need for intramuscular administration, limited clinical availability, and a higher risk of vestibular and renal toxicity compared with newer aminoglycoside antibiotics. Chloramphenicol is another drug with a higher risk of adverse effects that, for this reason, is seldom used systemically in modern medicine. Chloramphenicol has an in vitro effect against F. tularensis and was reportedly effective in treating tularemia meningitis in the past. ■ ■COMPLICATIONS Inflamed lymph nodes can progress into an abscess with local and/ or systemic reaction and may occur in about one-third of patients, especially if there were delays in F. tularensis–specific antibiotic treatment. An antibiotic treatment delay of >2–3 weeks after disease onset

TABLE 175-2  Treatment and Postexposure Prophylaxis Recommendations for Adult Patients with Tularemia or with Significant Exposure to an F. tularensis Aerosol ADULTS, INCLUDING PREGNANT WOMEN   DRUG DOSEa DURATION (DAYS) Preferred choices Ciprofloxacin 400 mg every 8 hrs IV or 750 mg every 12 hrs PO

Levofloxacin 750 mg every 24 hrs IV or PO

Gentamicin 5–7 mg/kg IV or IM per day, given every 12 hrs or 24 hrs

Alternative choiceb Doxycyclinec 200 mg loading dose, then 100 mg every 12 hrs IV or PO 14–21 Postexposure prophylaxisd Doxycycline 100 mg PO twice daily

Ciprofloxacin 500 mg every 12 hrs PO

aPatients beginning with IV treatment of ciprofloxacin, levofloxacin, or doxycycline can switch to oral administration when clinically indicated. bOther alternative choices having less supporting clinical data are high-dose moxifloxacin, ofloxacin, amikacin, tobramycin, or plazomycin for 10 days. In verified type A-infection, azitromycin may be considered based on in vitro susceptibility analysis. cDoxycycline may safely be included as a preferred choice for patients with nonsevere disease but is associated with a higher frequency of fever relapse and needs a more extended treatment duration. Tetracycline 500 mg every 6 hrs IV or PO for 14-21 days is equivalent to doxycycline. dFor low-risk exposures, preparedness for early treatment may suffice using daily “fever watch”: the patient monitors temperature with instructions to seek immediate treatment if developing a fever. Postexposure prophylaxis is recommended for high-risk exposures, including performing aerosol-generating procedures with F. tularensis cultures outside a biologic safety cabinet or handling infected animals. There are extensive human data showing that doxycycline is effective. Source: Adapted from J Terriquez, C Nelson (eds): Clinical infectious diseases: Tularemia: Update on treatment and clinical findings. Clin Infect Dis 78:s1, 2024. has been statistically associated with lymph node abscess formation. In some cases, the progression to a lymph node abscess may occur despite ongoing appropriate antibiotic treatment initiated late in the disease course. It is unclear if extending the antibiotic treatment duration is of clinical benefit. Culture specimens from an abscess at this disease stage are typically F. tularensis negative, but an additional 10–day course of appropriate antibiotic treatment is often used to avoid further disease progression and may be successful. If the clinician considers that there is a clear and immediate risk of spontaneous rupture of the skin or the mucus membranes in the pharynx with suppuration of pus, open surgical drainage or needle aspiration is recommended for treatment. A more invasive procedure with total surgical excision may be needed in more complicated and long-standing cases. Another complication of uncertain genesis occasionally reported by patients includes postinfectious persisting fatigue and disability. Additional complications or disease manifestations of tularemia have been reported, including adult respiratory distress syndrome, myocarditis, pericarditis, endocarditis, meningitis, osteomyelitis, hepatitis, and renal failure. ■ ■PROGNOSIS Prior to the use of antibiotics to treat tularemia, case fatality rates in the United States were typically 5%–15% and could be as high as 60% for patients with respiratory/pneumonic tularemia. Recent surveillance data of tularemia in the United States between 2006 and 2021 show that of 903 patients with illness outcome available, 27 patients (3.0%) died. Culture of F. tularensis from a clinical specimen was present in approximately 50% of cases and associated with significantly reduced odds of survival (odds ratio [OR], 0.1; 95% confidence interval [CI], 0.04–0.4). Treatment with at least one high-efficacy antimicrobial drug class (aminoglycoside, fluoroquinolone, or tetracycline) was independently associated with increased odds for survival (adjusted OR, 10.4; 95% CI, 4.4–24.5 after controlling for disease severity). Notably, previous surveillance data from 1964–2004 considering the modern genetic distinction of three epidemiologically important type A genetic varieties in the United States found a 24% fatality outcome for the most lethal

59 - 176 Plague and Other Yersinia Infections

176 Plague and Other Yersinia Infections

type A variety. For the type B F. tularensis variety, the same surveillance data from 1964–2004 showed a 7% fatality outcome. Caution in inter­ preting these latter data is warranted, as they likely are overestimates of mortality due to selected reporting and disease severity bias from the inclusion of culture-positive cases only. Recent analyses of patients with respiratory/pneumonic type B tularemia in Sweden revealed a 1.5% fatality. Outcome analyses considering all clinical forms of tulare­ mia suggest an overall type B tularemia fatality rate of <1% in Sweden. The frequency of disease complications, including lymphadenopathy, among 327 patients was <10%, much lower than previously reported for tularemia, likely because appropriate antibiotics were often given early, at a median of 7 days after disease onset. ■ ■PREVENTION Standard precautions are recommended for infection control and pre­ vention in the clinic. Face protection using goggles or a face shield and a fluid-resistant medical mask should be used in procedures with a risk of splashes from potentially infected body fluids. Only procedures with a very high risk of aerosol formation that may contain live F. tularensis, such as the surgical evacuation of pus from an abscess by applying high pressure or autopsy work with rotating tools, warrant respiratory protec­ tion with an N95 mask. Tularemia patients do not need to be isolated, given there is no human-to-human transmission. Because F. tularensis is a known risk for laboratory-acquired infection, suspected isolates should, at a minimum, be manipulated at BSL-2 using BSL-3 precautions in a biosafety cabinet. In the United States, specialized reference labora­ tories should always be consulted and involved in attempts for complete identification and antimicrobial susceptibility testing of F. tularensis. Vaccination is a potentially important measure for preventing tularemia. However, no U.S. Food and Drug Administration (FDA)-approved F. tularensis vaccines exist. The efficacy of the only available vaccine, LVS, is uncertain given concerns about adverse reactions to the vaccine, potential for reversion, unknown correlates of protection, and variable immunogenicity. Prevention strategies for the public include measures to avoid F. tularensis exposure. It is advised to avoid tick-infested areas and mosquito bites in tularemia-endemic areas, wear trousers and longsleeved shirts, use arthropod repellants, and remove attaching ticks promptly. Exposure to and touching dead or sick wild mammals should be avoided. If touching the mammal is necessary, gloves should be worn and hands should be washed thoroughly after removing the gloves. In addition to avoiding direct contact, exposure to potentially F. tularensis– contaminated aerosols should also be avoided, e.g., from handling dry hay or grain where carcasses of rodents are found. ■ ■GLOBAL CONSIDERATIONS Due to its past development as a biologic weapon, there is a risk that

F. tularensis could be used as a biothreat agent. F. tularensis is considered an HHS Tier 1 Select Agent in the United States because it presents the greatest risk of deliberate misuse with the most significant potential for mass casualties or devastating effects to the economy, critical infra­ structure, or public confidence. From 1932 to 1945, Japanese research units examined the utility of F. tularensis as a biologic weapon. After World War II, there were continuous military studies of tularemia. In the United States, there were federally funded biologic warfare pro­ grams from 1943 to 1969, with the development of weapons to dissemi­ nate type A F. tularensis aerosols. By 1973, the entire F. tularensis weapon arsenal of the United States had been destroyed. The former Soviet Union, as part of the civilian component of its offensive Biopreparat program, also incorporated F. tularensis into weapons. In 1970, the World Health Organization published a report estimating that an aero­ sol dispersal of 50 kg of virulent type A F. tularensis over a metropolitan area with 5 million inhabitants would result in 250,000 incapacitating causalities, including 19,000 deaths. Efficacy trials of tularemia vaccines in humans against highly viru­ lent type A tularemia are not feasible, and animal studies are judged to be the best option for screening and evaluating new vaccine candidates. There is ongoing research on the matter, including evaluating vaccines under the FDA “Animal Rule,” aimed at bridging the outcomes in ani­ mals to use in humans.

New tularemia treatment options with promising results, tested in experimentally infected animals, include the use of passive immuniza­ tion by F. tularensis–specific antibodies and a new triazaacenaphthylene antibiotic, gepotidacin. This new antibiotic class blocks bacterial DNA replication via inhibition of DNA gyrase and topoisomerase IV and is claimed to have no cross-resistance with the fluoroquinolones. Impor­ tantly, experience from treatment of humans with tularemia is lacking for both these treatments.

Considerations for tularemia in pregnant women include a recom­ mendation to use antibiotic treatment to avoid a risk of loss of preg­ nancy. Literature is scarce, but the available data suggest that the risk of adverse outcomes, including lymph node complications in the mother, maternal bleeding, spontaneous abortion, intrauterine fetal death, and preterm birth, is lowered with prompt institution of antibiotic treat­ ment. F. tularensis findings in transplacental villi and transplacental transmission to the fetus have been reported without treatment. ■ ■FURTHER READING Baskerville A et al: The pathology of untreated and antibiotic-treated experimental tularemia in monkeys. Br J Exp Pathol 59:615, 1978. Enderlin G et al: Streptomycin and alternative agents for the treatment of tularemia: Review of the literature. Clin Infect Dis 19:42, 1994. Glynn AR et al: Comparison of experimental respiratory tularemia in three nonhuman primate species. Comp Immunol Microbiol Infect Dis 39:13, 2015. Harrell JE et al: Current vaccine strategies and novel approaches to combatting Francisella infection. Vaccine 42:2171, 2024. Lamps LW et al: Histologic and molecular diagnosis of tularemia: A CHAPTER 176 potential bioterrorism agent endemic to North America. Mod Pathol 17:489, 2004. Lindgren H et al: Kinetics of the serological response up to one year after tularemia. Front Cell Infect Microbiol 12:1072703, 2023. Maurin M et al: Tularemia treatment: Experimental and clinical data. Front Microbiol 14:1348323, 2024. Roberts LM et al: Identification of early interactions between Fran­ Plague and Other Yersinia Infections cisella and the host. Infect Immun 82:2504, 2014. Tärnvik A (ed): WHO Guidelines on Tularaemia: Epidemic and Pan­ demic Alert and Response. Geneva, Switzerland; World Health Orga­ nization Press, 2007. Telford SR 3rd, Goethert HK: Ecology of Francisella tularensis. Annu Rev Entomol 65:351, 2020. Widerström M et al: Treatment outcome of severe respiratory Type B tularemia using fluoroquinolones. Clin Infect Dis 78:S38, 2024. Wu HJ et al: Tularemia clinical manifestations, antimicrobial treat­ ment, and outcomes: An analysis of US surveillance data, 2006–2021. Clin Infect Dis 78:S29, 2024. Yanes H et al: Evaluation of in-house and commercial serological tests for diagnosis of human tularemia. J Clin Microbiol 56:e01440, 2017. Michael B. Prentice

Plague and Other

Yersinia Infections PLAGUE Plague is a systemic zoonosis caused by Yersinia pestis. It predominantly affects small rodents in rural areas of Africa, Asia, and the Americas and is usually transmitted to humans by an arthropod vector (the flea). Less often, infection follows contact with animal tissues or respiratory droplets. Plague is an acute febrile illness that is treatable with antimi­ crobial agents, but mortality rates among untreated patients are high.

Ancient DNA studies have confirmed that both the fourteenth-century Black Death and the sixth-century Plague of Justinian in Europe were due to Y. pestis infection. Patients can present with the bubonic, sep­ ticemic, or pneumonic form of the disease. Although there is concern about epidemic spread of plague by the respiratory route, this is not the most common route of plague transmission, and established infectioncontrol measures for respiratory plague exist. However, the fatalities associated with plague and the capacity for infection via the respiratory tract mean that Y. pestis fits the profile of a potential agent of bioter­ rorism (Chap. S4). Consequently, measures have been taken to restrict access to the organism, including legislation affecting diagnostic and research procedures in some countries (e.g., the United States).

■ ■ETIOLOGY The genus Yersinia comprises gram-negative bacteria of the order Enterobacterales (class Gammaproteobacteria). Overwhelming taxo­ nomic and paleogenomic evidence shows Y. pestis recently evolved from Yersinia pseudotuberculosis, an enteric pathogen of mammals spread by the fecal–oral route, and thus has a phenotype distinctly dif­ ferent from that of Y. pestis. When grown in vivo or at 37°C, Y. pestis forms an amorphous capsule made from a plasmid-specified fimbrial protein, Caf or fraction 1 (F1) antigen, which is an immunodiagnostic marker of infection. ■ ■EPIDEMIOLOGY Human plague generally follows an outbreak in a host rodent popula­ tion (epizootic). Mass deaths among the rodent primary hosts lead to a search by fleas for new hosts, with consequent incidental infec­ tion of other mammals. The precipitating cause for an epizootic may ultimately be related to climate or other environmental factors. The reservoir for Y. pestis causing enzootic plague in natural endemic foci between epizootics (i.e., when the organism may be difficult to detect in rodents or fleas) is a topic of ongoing research and may not be the same in all regions. The enzootic/epizootic pattern may be the result of complex dynamic interactions of host rodents that have different plague susceptibilities with different flea vectors; alternatively, an envi­ ronmental reservoir may be important. PART 5 Infectious Diseases ■ ■GLOBAL FEATURES In general, the enzootic areas for plague are lightly populated regions of Africa, Asia, and the Americas (Fig. 176-1). Between January 2013 and December 2018, 2886 cases of plague with a global case-fatality rate of 17% were notified to the World Health Organization (WHO) under the International Health Regulations. More than 97% of these Countries reporting human plague cases, 1970−2005 Probable sylvatic foci FIGURE 176-1  Approximate global distribution of Yersinia pestis. (Compiled from WHO, CDC, and country sources. Reprinted with permission from DT Dennis, GL Campbell: Plague and other Yersinia infections, in Harrison’s Principles of Internal Medicine, 17th ed, AS Fauci et al [eds]. New York, McGraw-Hill, Chap. 152, 2008.)

cases were in Africa. The majority of cases in each year were from the island of Madagascar, which in 2017 experienced an urban outbreak of over 2400 clinically suspected cases, with an unusually high proportion of pneumonic plague (78%). A decline in reports from the Democratic Republic of the Congo (DRC) may reflect ongoing conflict in that country affecting surveillance rather than a true decrease. In the past decade, outbreaks of pneumonic plague have been recorded in the DRC, Uganda, Algeria, Madagascar, China, and Peru. Plague was introduced into North America via the port of San Francisco in 1900 as part of the Third Pandemic, which spread around the world from Hong Kong. The disease is presently enzootic on the western side of the continent from southwestern Canada to Mexico. Most human cases in the United States occur in two regions: “Four Corners” (the junction point of New Mexico, Arizona, Colorado, and Utah), especially northern New Mexico, northern Arizona, and south­ ern Colorado; and further west in California, southern Oregon, and western Nevada (https://www.cdc.gov/plague/maps-statistics/). From 1970 to 2020, 496 cases of plague were reported in the United States; in recent decades, incidence has fallen to an average of seven cases per year. Most cases occur from May to October—the time of year when people are outdoors and rodents and their fleas are most plentiful. Prior animal contact occurs in at least 50% of cases, and about 60% of these include domestic animals (usually dogs or cats) that brought wild animals or plague-infected fleas home. Infected cats or dogs may transmit plague directly to humans by the respiratory route. A slightly lower percentage of prior animal contacts involve direct handling of living or dead wild small mammals (e.g., rabbits, hares, prairie dogs) or wild carnivores (e.g., wildcats, coyotes, or mountain lions). In 2014, an outbreak of nonfatal pneumonic plague in Colorado affected four people exposed to an infected dog, with possible interhuman transmis­ sion in one case. Prior to this report, the most recent case of person-toperson transmission in the United States occurred in the Los Angeles pneumonic plague outbreak of 1924. Plague most often develops in areas with poor sanitary conditions and infestations of rats—in particular, the widely distributed roof rat Rattus rattus and the brown rat Rattus norvegicus (which serves as a laboratory model of plague). Rat control in warehouses and shipping facilities has been recognized as important in preventing the spread of plague since the early twentieth century and features in the current WHO International Health Regulations. Urban rodents acquire infec­ tion from wild rodents, and the proximity of the former to humans increases the risk of transmission. The oriental rat flea Xenopsylla cheopis is the most efficient vector for transmission of plague among rats and onward to humans in Asia, Africa, and South America.

Worldwide, bubonic plague is the predominant form reported. A minority of patients (10–20%) present with primary septicemic plague (i.e., systemic Y. pestis sepsis with no bubo; see “Clinical Manifestations,” below) or pneumonic disease. Primary pneumonic plague is generally the least common and most fatal of the main plague presentations, but, as in the 2017 Madagascar outbreak, it is occasionally predominant. Rare outbreaks of pharyngeal plague following consumption of raw or undercooked camel or goat meat have been reported. A total of 744 (82%) of the 913 plague cases with clinically docu­ mented features (out of 1006 cases reported in total) in the United States from 1900 to 2012 were bubonic disease, 87 (10%) were septice­ mic disease, and 74 (8%) were pneumonic disease; 6 cases (1%) were pharyngeal. Sixteen percent of cases were fatal in the postantibiotic era from 1942 onward compared with 66% in the period 1900–1941. This century (2000–2018), mortality in the United States has been 8% in patients receiving any antimicrobial and 4% in patients receiving one of the high-efficacy antimicrobials currently recommended as therapy. A systematic review and meta-analysis of the worldwide literature estimated a death rate for treated pneumonic plague of 17%, compared with 98% for untreated pneumonic plague. ■ ■PATHOGENESIS As mentioned earlier, genetic evidence shows Y. pestis is a clone derived from the enteric pathogen Y. pseudotuberculosis in the recent evolutionary past (7000–50,000 years ago). The change from infection by the fecal–oral route to a two-stage life cycle, with alternate parasitization of arthropod and a wide range of mammalian hosts, occurred as a result of two plasmid gene acquisitions (pla on pPCP1/pPst and ymt on pFra/pMT1), and the inactivation of a handful of Y. pseudotuberculosis genes, in conjunction with preexisting proper­ ties of the Y. pseudotuberculosis ancestor, including the presence of a virulence plasmid, pYV, and the capacity to cause septicemia. In the arthropod-parasitizing portion of its life cycle, Y. pestis multiplies and forms biofilm-embedded aggregates in the flea midgut after ingestion of a blood meal containing bacteria. In some fleas, biofilm-embedded bacteria eventually fill the proventriculus (a valve connecting the esophagus to the midgut) and block normal blood feeding. Both “blocked” fleas and those containing masses of biofilm-embedded Y. pestis without complete blockage inoculate Y. pestis into each bite site. The ability of Y. pestis to colonize and multiply in fleas fed on infected blood from most hosts requires phospholipase D encoded by the ymt gene on the pFra (pMT1) plasmid, and biofilm synthesis requires the chromosomal hms locus shared with Y. pseudotuberculosis. Recently, it has been shown Y. pestis does not require an intact ymt gene to infect and be transmitted from fleas fed on blood from the brown rat (Rattus norvegicus), although this is required for blood from the black rat (Rattus rattus). Three Y. pseudotuberculosis genes inhibiting biofilm formation or promoting its degradation are inactivated in Y. pestis, together with urease (urease activity otherwise causes acute flea gastrointestinal toxicity). Blockage takes days or weeks to come about after initial infec­ tion of the flea and is followed by the flea’s death. Many flea vectors (including X. cheopis) are also able to transmit plague in an early-phase unblocked state for up to a week after feeding, but 10 fleas in this state are required to infect a mammalian host (mass transmission). Y. pestis disseminates from the site of inoculation in the mammalian host in a process initially dependent on plasminogen activator Pla, which is encoded by the small pPCP1 (pPst) plasmid. This surface protease activates mammalian plasminogen, degrades complement, and adheres to the extracellular matrix component laminin. Pla is essential for the high-level virulence of Y. pestis in mice by subcutane­ ous or intradermal injection (laboratory proxies for fleabites) and for the development of primary pneumonic plague. When actual fleabite inoculation is used in mouse models, the fimbrial capsule-forming protein (Ca1 or fraction 1; F1 antigen) encoded on pFra increases the efficiency of transmission, and plasminogen activator is required for the formation of buboes. Paleogenomics (sequencing of DNA extracts from teeth of ancient human remains) shows that the 14th-century Black Death and the 6thcentury Plague of Justinian were caused by Y. pestis and suggests Black

Death mortality selected for protective immune response gene variants now associated with autoimmune disease. It has also revealed that Y. pestis infection was a common cause of death in Eurasia in the Bronze Age and Neolithic period. Remarkably, the ymt gene is absent from the pFra (pMT1) plasmid in Y. pestis sequences from some remains >4000 years old, whereas pla is present with intact urease and biofilm regulatory genes. This suggests that plague was a common fatal human infection before flea-borne transmission was fully optimized, possibly spread by the pneumonic or gastrointestinal route.

Macrophages, neutrophils, and dendritic cells are all involved in the innate immune response to flea-transmitted Y. pestis. The organ­ ism is taken up by macrophages but avoids being killed by autophagy and can also survive and replicate in neutrophils. Rapid transport of the bacteria to regional lymph nodes occurs. Y. pestis then undergoes extracellular replication with full expression of its antiphagocytic sys­ tems: the type III secretion machines and their effectors encoded by pYV as well as the F1 capsule. These factors prevent neutrophil uptake, and the type III secretion effectors also block extrusion of microbicidal DNA by neutrophils and trigger apoptotic cell death. Immune cell targeting follows binding of the N-formylpeptide receptor (FPR1) on phagocytic cells by LcrV, the needle cap protein of the type III secre­ tion system. Overproduction of LcrV also exerts an anti-inflammatory effect, reducing host immune responses. Likewise, Y. pestis lipopolysac­ charide is modified to minimize stimulation of host Toll-like receptor 4, thereby reducing protective host inflammatory responses during peripheral infection and prolonging host survival with high-grade bacteremia—an effect that probably enhances the pathogen’s subse­ quent transmission by fleabite. CHAPTER 176 Replication of Y. pestis in a regional lymph node results in the local swelling of the lymph node and periglandular region known as a bubo. On histology, the node is found to be hemorrhagic or necrotic, with thrombosed blood vessels, and the lymphoid cells and normal architec­ ture are replaced by large numbers of bacteria and fibrin. Periglandular tissues are inflamed and also contain large numbers of bacteria in a serosanguineous, gelatinous exudate. Plague and Other Yersinia Infections Continued spread through the lymphatic vessels to contiguous lymph nodes produces second-order primary buboes. Infection is initially contained in the infected regional lymph nodes, although transient bacteremia can be detected. As infection progresses, spread via efferent lymphatics to the thoracic duct produces high-grade bacte­ remia. Hematogenous spread to the spleen, liver, and secondary buboes follows, with subsequent uncontrolled septicemia leading to death. In some patients, this septicemic phase occurs without obvious prior bubo development or lung disease (septicemic plague). Hematogenous spread to the lungs results in secondary plague pneumonia, with bac­ teria initially more prominent in the interstitium than in the air spaces (the reverse being the case in primary plague pneumonia). Hematog­ enous spread to other organs, including the meninges, can occur. ■ ■CLINICAL MANIFESTATIONS Bubonic Plague  After an incubation period of 2–6 days, the onset of bubonic plague is sudden and is characterized by fever (>38°C), malaise, myalgia, dizziness, and increasing pain due to progressive lymphadenitis in the regional lymph nodes near the fleabite or other inoculation site. Lymphadenitis manifests as a tense, tender swelling (bubo) that, when palpated, has a boggy consistency with an underly­ ing hard core. Generally, there is one painful and erythematous bubo with surrounding periganglionic edema. The bubo is most commonly inguinal but can also be crural, axillary (Fig. 176-2), cervical, or sub­ maxillary, depending on the site of the bite. Abdominal pain from intraabdominal node involvement can occur without other visible signs. Children are most likely to present with cervical or axillary buboes. The differential diagnosis includes acute focal lymphadenopathy of other etiologies, such as streptococcal or staphylococcal infection, tularemia, cat-scratch disease, tick typhus, infectious mononucleosis, or lymphatic filariasis. These infections do not progress as rapidly, are not as painful, and are associated with visible cellulitis or ascending lymphangitis—both of which are absent in plague.

FIGURE 176-2  Plague patient in the southwestern United States with a left axillary bubo and an unusual plague ulcer and eschar at the site of the infective flea bite. (Reprinted with permission from DT Dennis, GL Campbell: Plague and other Yersinia infections, in Harrison’s Principles of Internal Medicine, 17th ed, AS Fauci et al [eds]. New York, McGraw-Hill, Chap. 152, 2008.) PART 5 Infectious Diseases Without treatment, Y. pestis dissemination occurs and causes seri­ ous illness, including pneumonia (secondary pneumonic plague) and meningitis. Secondary pneumonic plague can be the source of personto-person transmission of respiratory infection by productive cough (droplet infection), with the consequent development of primary plague pneumonia. Appropriate treatment of bubonic plague results in fever resolution within 2–5 days, but buboes may remain enlarged for

1 week after initial treatment and can become fluctuant. Primary Septicemic Plague  A minority (10–25%) of infections with Y. pestis present as gram-negative septicemia (hypotension, shock) without preceding lymphadenopathy. Septicemic plague occurs in all age groups, but persons >40 years of age are at elevated risk. Some chronic conditions may predispose to septicemic plague: in 2009 in the United States, a fatal laboratory-acquired infection with an attenu­ ated Y. pestis strain manifested as septicemic plague in a 60-year-old FIGURE 176-3  Sequential chest radiographs of a patient with fatal primary plague pneumonia. Left: Upright posteroanterior film taken at admission to hospital emergency department on third day of illness, showing segmental consolidation of right upper lobe. Center: Portable anteroposterior film taken 8 h after admission, showing extension of pneumonia to right middle and right lower lobes. Right: Portable anteroposterior film taken 13 h after admission (when patient had clinical acute respiratory distress syndrome), showing diffuse infiltration throughout right lung and patchy infiltration of left lower lung. A cavity later developed at the site of initial right-upper-lobe consolidation. (Reprinted with permission from DT Dennis, GL Campbell: Plague and other Yersinia infections, in Harrison’s Principles of Internal Medicine, 17th ed. AS Fauci et al [eds]. New York, McGraw-Hill, Chap. 152, 2008.)

researcher with diabetes mellitus and undiagnosed hemochromato­ sis. These conditions also carry an increased risk of septicemia with other pathogenic Yersinia species. The term septicemic plague can be confusing since most patients with buboes have detectable bacteremia at some stage, with or without systemic signs of sepsis. In laboratory experiments, however, septicemic disease without histologic changes in lymph nodes is seen in a minority of mice infected via fleabites. Pneumonic Plague  Primary pneumonic plague results from inha­ lation of infectious bacteria in droplets expelled from another person or an animal with primary or secondary plague pneumonia. This syndrome has a short incubation period, averaging from a few hours to 2–3 days (range, 1–7 days), and is characterized by a sudden onset of fever, headache, myalgia, weakness, nausea, vomiting, and diz­ ziness. Respiratory signs—cough, dyspnea, chest pain, and sputum production with hemoptysis—typically arise after 24 h. Progression of initial segmental pneumonitis to lobar pneumonia and then to bilat­ eral lung involvement may occur (Fig. 176-3). The possible release of aerosolized Y. pestis bacteria in a bioterrorist attack, manifesting as an outbreak of primary pneumonic plague in nonendemic regions or in an urban setting where plague is rarely seen, has been a source of public health concern. Secondary pneumonic plague is a consequence of bacteremia occurring in ~10–15% of patients with bubonic plague. Bilateral alveolar infiltrates are seen on chest x-ray, and diffuse intersti­ tial pneumonitis with scanty sputum production is typical. Meningitis  Meningeal plague is uncommon, occurring in ≤6% of plague cases reported in the United States. Presentation with headache and fever typically occurs >1 week after the onset of bubonic or sep­ ticemic plague and may be associated with suboptimal antimicrobial therapy (delayed therapy, penicillin administration, or low-dose tetra­ cycline treatment) and cervical or axillary buboes. Pharyngitis  Symptomatic plague pharyngitis can follow the con­ sumption of contaminated meat from an animal dying of plague or contact with persons or animals with pneumonic plague. This condi­ tion can resemble tonsillitis, with peritonsillar abscess and cervical lymphadenopathy. Asymptomatic pharyngeal carriage of Y. pestis can also occur in close contacts of patients with pneumonic plague. ■ ■LABORATORY DIAGNOSIS Because of the scarcity of laboratory facilities in regions where human Y. pestis infection is most common, and because of the potential sig­ nificance of Y. pestis isolation in a nonendemic area or an area from which human plague has been absent for many years, the WHO recom­ mends an initial presumptive diagnosis followed by reference labora­ tory confirmation (Table 176-1). In the United States, comprehensive national diagnostic facilities for plague have been in place since 1999 (Laboratory Response Network for Biological Threats [LRN-B];

https://emergency.cdc.gov/lrn/index.asp) to detect possible use of bio­ logical terrorism agents, including Y. pestis. Routine diagnostic clinical

TABLE 176-1  World Health Organization Case Definitions of Plague Suspected Case Clinical presentation suggestive of plague And Epidemiological context suggesting possible exposure to plague (exposure to infected humans or animals, or residence in or travel to a known endemic focus within 10 days prior to onset of the disease) Probable Case Meeting the definition of a suspected case plus 1 of the following • F1 antigen detected in bubo aspirate, sputum, blood, or postmortem tissues by rapid antigen test or direct immunofluorescence • A single anti-F1 serology without evidence of previous Yersinia pestis infection or immunization • Direct microscopy in a clinical sample: gram-negative coccobacilli that display bipolar staining with Wayson or Wright-Giemsa stain Confirmed Case Meeting the definition of a suspected case Plus at least 1 of the following • Isolation of Y. pestis from a clinical sample (based on appropriate colonial morphology and at least 2 of the following tests positive: phage lysis of cultures at 20–25°C and 37°C; biochemical profile; F1 antigen detection) • Seroconversion or a fourfold difference in anti-F1 antibody titer in paired serum samples drawn at least 2 weeks apart • Y. pestis DNA positive by species-specific PCR on either clinical sample or culture Not a case (exclusion of diagnosis) Meeting the definition of a suspected case And either of the following • At least two laboratory tests (rapid antigen test or direct immunofluorescence against F1 antigen, direct microscopy, convalescent serology, culture, PCR) are conducted AND they are negative • When no confirmatory tests can be performed, 2 negative rapid antigen tests for F1 antigen on 2 clinical specimens collected at 24-h intervals Abbreviation: PCR, polymerase chain reaction. Source: Reproduced with permission from E Bertherat, S Jullien. Revision of the international definition of plague cases. Wkly Epidemiol Rec 24:238, 2021. microbiology laboratories that are included in this network as sentinellevel laboratories use joint protocols from the Centers for Disease Control and Prevention (CDC) and the American Society for Micro­ biology (https://asm.org/Articles/Policy/Laboratory-Response-NetworkLRN-Sentinel-Level-C) to identify suspected Y. pestis isolates and to refer these specimens to LRN-B reference laboratories for confirmatory tests. Y. pestis is designated a “Tier 1 select agent” under the Public Health Security and Bioterrorism Preparedness and Response Act of 2002 and subsequent executive orders; the provisions of this act, the Patriot Act of 2001, and related executive orders apply to all U.S. labo­ ratories and individuals working with Y. pestis. Details of the applicable regulations are available from the CDC (www.selectagents.gov). Yersinia species are gram-negative coccobacilli (short rods with rounded ends) 1–3 μm in length and 0.5–0.8 μm in diameter. Y. pestis in particular appears bipolar (with a “closed safety pin” appearance) and pleomorphic when stained with a polychromatic stain (Wayson or Wright-Giemsa; Fig. 176-4). Its lack of motility distinguishes Y. pestis from other Yersinia species, which are motile at 25°C and nonmotile at 37°C. Transport medium (e.g., Cary-Blair medium) preserves the viability of Y. pestis if transport is delayed. The appropriate specimens for diagnosis of bubonic, pneumonic, and septicemic plague are bubo aspirate, bronchoalveolar lavage fluid or sputum, and blood, respectively. Culture of postmortem organ biopsy samples also can be diagnostic. A bubo aspirate is obtained by injection of 1 mL of sterile normal saline into a bubo under local anesthetic and aspiration of a small amount of (usually blood-stained) fluid. The WHO has provided guidance on how to aspirate buboes and

FIGURE 176-4  Peripheral-blood smear from a patient with fatal plague septicemia and shock, showing characteristic bipolar-staining Yersinia pestis bacilli (Wright’s stain, oil immersion). (Reprinted with permission from DT Dennis, GL Campbell: Plague and other Yersinia infections, in Harrison’s Principles of Internal Medicine, 17th ed, AS Fauci et al [eds]. New York, McGraw-Hill, Chap. 152, 2008.) collect sputum from patients with suspected pneumonic plague (https:// www.who.int/emergencies/outbreak-toolkit/disease-outbreak-toolboxes/ plague-outbreak-toolbox; https://www.who.int/publications/i/item/ operational-guidelines-on-plague-surveillance-diagnosis-preventionand-control). Gram’s staining of these specimens may reveal gramnegative rods, which are shown by Wayson or Wright-Giemsa staining to be bipolar. These bacteria may even be visible in direct blood smears in septicemic plague (Fig. 176-4); this finding indicates very high num­ bers of circulating bacteria and a poor prognosis. CHAPTER 176 Plague and Other Yersinia Infections Y. pestis grows on nutrient agar and other standard laboratory media but forms smaller colonies than do other Enterobacteriaceae. Speci­ mens should be inoculated onto nutrient-rich media such as sheep blood agar (SBA), into nutrient-rich broth such as brain-heart infusion broth, and onto selective agar such as MacConkey or eosin methylene blue (EMB) agar. Yersinia-specific CIN (cefsulodin, triclosan [Irgasan], novobiocin) agar can be useful for culture of contaminated speci­ mens, such as sputum. Blood should be cultured in a standard blood culture system. The optimal growth temperature is <37°C (25–29°C), with pinpoint colonies only on SBA at 24 h. Slower growth occurs at 37°C. Y. pestis is oxidase-negative, catalase-positive, urease-negative, indole-negative, and lactose-negative. Automated biochemical or mass spectrometry identification systems can misidentify Y. pestis as Y. pseu­ dotuberculosis or other bacterial species. Reference laboratory tests for definitive identification of isolates include direct immunofluorescence for F1 antigen; polymerase chain reaction (PCR) for specific Y. pestis targets (see below); and specific bacteriophage lysis. PCR can also be applied to diagnostic specimens, as can direct immunofluorescence for F1 antigen (produced in large amounts by Y. pestis) by slide microscopy. An immunochromato­ graphic test strip for F1 antigen detection by monoclonal antibodies in clinical specimens (rapid diagnostic test based on the F1 antigen [F1RDT]) has been devised in Madagascar. This method is effective for both laboratory and near-patient use on bubo aspirates and sputum and is now widely used in endemic countries. A similar test strip for Pla antigen has been developed and could be used to detect wild-type or engineered F1-negative virulent strains. Recent clinical experience, including the 2017 Madagascar outbreak, found F1RDT to be at least as sensitive as laboratory culture for diagnosis of bubonic or pneumonic plague. However, there was a low diagnostic yield of culture in this outbreak, possibly due to widespread prehospital use of antimicrobials. The specificity of F1RDT for pneumonic plague diagnosed by culture was 71%; thus, while a positive sputum test is useful in determining whether a symptomatic patient in an endemic area is a probable case requiring treatment, additional laboratory tests are required to confirm

a diagnosis of pneumonic plague. F1RDT had a high negative predic­ tive value in the limited studies available; thus, when other laboratory tests are unavailable, two negative F1RDT tests on clinical specimens 1 day apart can help exclude a diagnosis of plague in a symptomatic patient. This use of F1RDT has been incorporated in the revised WHO international definition of plague introduced following the 2017 Mada­ gascar outbreak.

The WHO requires diagnostic PCR or real-time PCR (RT-PCR) assays to show positivity for at least two different targets specific for Y. pestis from a short list comprising the plasmid-based caf1 (specifying the F1 antigen) and pla (specifying plasminogen activator) genes and three other chromosomal genes. Y. pestis is included in the U.S. Food and Drug Administration (FDA)-authorized Biofire FilmArray Next Generation Diagnostic System (NGDS) Warrior Panel for use with the FilmArray 2.0 system (Biomérieux) as a medical diagnostic device suit­ able for whole blood (ethylenediaminetetraacetic acid [EDTA]), blood cultures, and sputum specimens used by U.S. Department of Defense laboratories and laboratories in the CDC-managed LRN-B network. It is also one of 14 viral, bacterial, and protozoan pathogens diagnosable from blood samples with the BioFire Global Fever Special Pathogens Panel recently authorized by the FDA. Detailed phylogeographic DNA sequence data based on culture collections have been accumulated to trace plague evolution, and this approach could be adapted in the future to real-time clinical plague epidemiology. In the absence of other positive laboratory diagnostic tests, a retro­ spective serologic diagnosis may be made on the basis of rising titers of hemagglutinating antibody to F1 antigen. Enzyme-linked immuno­ sorbent assays (ELISAs) for IgG and IgM antibodies to F1 antigen are also available. PART 5 Infectious Diseases The white blood cell (WBC) count is generally raised (to 10,000– 20,000/μL) in plague, with neutrophilic leukocytosis and a left shift (numerous immature neutrophils); in some cases, however, the WBC count is normal or leukopenia develops. WBC counts are occasionally very high, especially in children (>100,000/μL). Levels of fibrinogen degradation products are elevated in a majority of patients, but plate­ let counts are usually normal or low-normal. However, disseminated intravascular coagulation, with low platelet counts, prolonged pro­ thrombin times, reduced fibrinogen, and elevated fibrinogen degrada­ tion product levels, occurs in a significant minority of patients. TREATMENT Plague Guidelines for the treatment of plague are given in Table 176-2. A 10- to 14-day course of antimicrobial therapy (or a course con­ tinued until 2 days after fever subsides) is recommended. Strepto­ mycin has historically been the parenteral treatment of choice for plague and is approved for this indication by the FDA. Although not yet approved by the FDA for plague, gentamicin has proved safe and effective in clinical trials in Tanzania and Madagascar and in retrospective reviewed cases in the United States. In view of streptomycin’s adverse-reaction profile and limited availability, some experts now recommend gentamicin over streptomycin. The FDA has approved levofloxacin, moxifloxacin, and ciprofloxacin for prophylaxis and treatment of plague (including septicemic and pneumonic plague) under a regulatory approach based on animal studies alone, known as the Animal Rule. Levofloxacin has more efficacy than ciprofloxacin in postexposure prophylaxis of inhalational anthrax in animal models and has also received FDA approval for this indication (Chap. S4); thus, it is a suitable agent for prophylaxis against two diseases in possible bioterrorism expo­ sures. The WHO issued new guidelines in 2021 adding these three fluoroquinolones to the recommended list of first-line medicines for treating bubonic, pneumonic, or septicemic plague. It also rec­ ommended moxifloxacin and ofloxacin for plague meningitis, and ciprofloxacin for postexposure prophylaxis. While systemic chloramphenicol therapy is available in the resource-poor countries primarily affected by plague, it is less

TABLE 176-2  Guidelines for the Treatment of Plague DOSING INTERVAL, h ROUTE DRUG DAILY DOSE Gentamicin Adult 5 mg/kga

IM/IV Child 4.5–7.5 mg/kga

IM/IV Streptomycin Adult 2 g

IM Child 30 mg/kg (maximum 1 g per dose)

IM Levofloxacin Adult (child >50 kg) 750 (500–750) mg

PO/IV Child <50 kg and ≥6 months of age 16 mg/kg (maximum, 250 mg/ dose)

PO/IV Ciprofloxacin Adult 1500 mg

PO 1200 mg

IV Child 30–45 mg/kg (maximum, 500 mg/dose) 8–12 PO 20–30 mg/kg (maximum, 400 mg/dose) 8–12 IV Moxifloxacin Adult 400 mg

PO/IV Doxycycline Adult and child ≥45 kg 200 mg (200 mg loading dose)

PO/IV Child <45 kg 4.4 mg/kg (maximum, 100 mg/ dose, 4.4 mg/kg loading dose)

PO/IV Tetracycline Adult 2 g

PO/IV Child >8 y 40–50 mg/kg

PO/IV Chloramphenicol Adult 50–100 mg/kg

PO/IV Child >2 y 50–100 mg/kg (maximum, 4 g)

PO/IV aAminoglycoside dose is adjusted with impaired renal function. No trial data have been published for once-daily gentamicin therapy for plague in adults or children, but this regimen is efficacious in gram-negative sepsis of other etiologies and has been successful in a recent outbreak of pneumonic plague in the Democratic Republic of the Congo. Neonates (up to 1 week of age) and premature infants should receive gentamicin at 4 mg/kg IV once daily. Source: CA Nelson et al: Antimicrobial treatment and prophylaxis of plague: Recommendations for naturally acquired infections and bioterrorism response. MMWR Recomm Rep 70(No. RR-3):1, 2021. Provides detailed guidelines on recommended regimens for pneumonic versus bubonic plague, plague meningitis, treatment during pregnancy and lactation, and neonatal infection. Recommends dual therapy with two different classes of antimicrobials for initial treatment of patients with severe pneumonic or septicemic plague and patients infected after intentional release of Yersinia pestis. likely to be available or used in high-income countries because of its adverse effect profile. Tetracyclines are also effective and can be given by mouth but are not generally recommended for children age <7 years because of tooth discoloration. Doxycycline is the tet­ racycline of choice; at an oral dosage of 100 mg twice daily, this drug was as effective as intramuscular gentamicin (2.5 mg/kg twice daily) in a trial in Tanzania. There is recent evidence that doxycycline does not cause dental staining in children because it binds calcium less readily than other tetracyclines. Although Y. pestis is sensitive to β-lactam drugs in vitro and these drugs have been efficacious against plague in some animal models, the response to penicillins has been poor in some clinical cases; thus β-lactams and macrolides are not generally recom­ mended as first-line therapy. Chloramphenicol, alone or in com­ bination, is recommended for some focal complications of plague (e.g., meningitis, endophthalmitis, myocarditis) because of its tis­ sue penetration properties. Fluoroquinolones, effective in vitro

and in animal models, are recommended in guidelines for possible bioterrorism-associated pneumonic plague and are increasingly used in plague therapy. ■ ■PREVENTION In endemic areas, the control of plague in humans is based on reduc­ tion of the likelihood of being bitten by infected fleas or exposed to infected droplets from either humans or animals with plague pneumo­ nia. In the United States, residence and outdoor activity or contact with wild or pet animals in rural areas of western states where epizootics occur are the main risk factors for infection. To assess potential risks to humans in specific areas, surveillance for Y. pestis infection among animal plague hosts and vectors is carried out regularly as well as in response to observed animal die-offs. Personal protective measures include avoidance of areas where a plague epizootic has been identified and publicized (e.g., by warning signs or closure of campsites). Sick or dead animals should not be handled by the general public. Hunters, zoologists, and pet owners should wear gloves if handling wild-animal carcasses in endemic areas. General measures to avoid rodent fleabite during outdoor activity are appropriate and include the use of insect repellent, insecticide, and protective clothing. General measures to reduce peridomestic and occupational human contact with rodents are advised and include rodent-proofing of buildings and food-waste stores and removal of potential rodent habitats (e.g., woodpiles and junk heaps). Flea control by insecticide treatment of wild rodents is an effective means of minimizing human contact with plague if an epizo­ otic is identified in an area close to human habitation. Any attempt to reduce rodent numbers must be preceded by flea suppression to reduce the migration of infected fleas to human hosts. An oral F1-V subunit vaccine using raccoon poxvirus (RCN) as a vector (sylvatic plague vac­ cine) is partially protective against plague when administered to wild prairie dogs in field trials and may in the future provide a means of reducing the risk of human exposure to Y. pestis. Patients in whom pneumonic plague is suspected should be managed in isolation (with negative pressure, if available), with droplet precau­ tions observed until pneumonia is excluded or effective antimicrobial therapy has been given for 48 h. Review of the literature published before the advent of antimicrobial agents suggests that the main infec­ tive risk is posed by patients in the final stages of disease who are cough­ ing up sputum with plentiful visible blood and/or pus. Cotton and gauze masks were protective in these circumstances. Current surgical masks capable of barrier protection against droplets, including large respira­ tory particles, are probably protective, but the differential diagnosis of fever and hemoptysis in plague-endemic areas includes small airborne particle-transmitted infections such as tuberculosis. In addition, WHO guidance recommends that personal protective equipment for potential aerosol-generating procedures (e.g., collection of respiratory samples from patients with suspected or confirmed plague) or handling the remains of someone who was infected with plague should include a fit-tested N95 face mask, a gown, gloves, and a face shield or goggles. Antimicrobial Prophylaxis  Postexposure antimicrobial prophy­ laxis lasting 7 days is recommended following household, hospital, or other close contact with persons with untreated pneumonic plague. (Close contact is defined as contact with a patient at <2 m.) In animal aerosol-infection studies, levofloxacin and ciprofloxacin are associated with higher survival rates than doxycycline (Table 176-3). Immunization  Studies with candidate plague vaccines in animal models show that neutralizing antibody provides protection against exposure but that cell-mediated immunity is critical for protection and clearance of Y. pestis from the host. A killed whole-cell vaccine used in humans required multiple doses, caused significant local and systemic reactions, and was not protective against pneumonic plague; this vac­ cine is not currently available. A live attenuated vaccine based on strain EV76 is still used in countries of the former Soviet Union and China but has significant side effects. Different subunit vaccines devised by governmental agencies in the United States, United Kingdom, and China all comprising recombinant F1 (rF1) and various recombinant

TABLE 176-3  Guidelines for Plague Prophylaxis DOSING INTERVAL, h ROUTE DRUG DAILY DOSE Doxycycline Adult 200 mg 12 or 24 PO Child ≥8 y ≥45 kg: adult dose

PO ≤45 kg: 4.4 mg/kg (maximum, 200 mg)

PO Tetracycline Adult 2 g 6 or 12 PO Child ≥8 y 40 mg/kg (maximum 500 mg dose) 6 or 12 PO Levofloxacin Adult and child >50 kg 500–750 mg

PO Child <50 kg and ≥6 months of age 16 mg/kg (maximum, 250 mg/dose)

PO Ciprofloxacin Adult 1–1.5 g

PO Child 30 mg/kg (maximum 750 mg dose)

PO Source: TV Inglesby et al: Plague as a biological weapon: Medical and public health management. Working Group on Civilian Biodefense. JAMA 283:2281, 2000; https:// www.cdc.gov/plague/healthcare/clinicians.html; CA Nelson et al: Antimicrobial treatment and prophylaxis of plague: Recommendations for naturally acquired infections and bioterrorism response. MMWR Recomm Rep 70(No. RR-3):1, 2021. CHAPTER 176 V (rV) proteins produced in Escherichia coli, combined either as a fusion protein or as a mixture, purified, and adsorbed to aluminum hydroxide for injection are close to licensing. This combination pro­ tects mice and various nonhuman primates in laboratory models of bubonic and pneumonic plague and has been evaluated in phase 2 clinical trials. Prelicensing field-efficacy studies (phase 3 trials) are dif­ ficult to devise because of plague epidemiology. In the United States, the FDA will assess plague vaccines for human use under the Animal Rule, using efficacy data from animal studies and antibodies and other correlates of immunity from human vaccinees (https://www.fda.gov/ emergency-preparedness-and-response/mcm-regulatory-science/animal-rule-

information), and the rF1-V subunit vaccine has orphan drug status. The WHO has produced a Target Product Profile (TPP) for phase 3 trial design and prioritization of the vaccine candidates. Candidate vac­ cines include protein subunit, live-attenuated, and bacterial, viral, and bacteriophage vectors, DNA, and mRNA vaccines. Antigens other than F1 and V are being investigated because of the recovery of F1-negative Y. pestis strains from natural sources and the observation that F1 antigen is not required for virulence in primate models of pneumonic plague. Plague and Other Yersinia Infections YERSINIOSIS Yersiniosis is a zoonotic infection with an enteropathogenic Yersinia species, usually Y. enterocolitica or Y. pseudotuberculosis. The usual hosts for these organisms are pigs and other wild and domestic ani­ mals; humans are usually infected by the oral route, and outbreaks from contaminated food occur. Yersiniosis is most common in child­ hood and in colder climates. Patients present with abdominal pain and sometimes with diarrhea (which may not occur in up to 50% of cases). Y. enterocolitica is more closely associated with terminal ileitis and Y. pseudotuberculosis with mesenteric adenitis, but both organisms may cause mesenteric adenitis and symptoms of abdominal pain and tenderness that result in pseudoappendicitis, with the surgical removal of a normal appendix. Diagnosis was historically based on culture of the organism or convalescent serology, but some proprietary multiplex PCR systems for gastrointestinal infection diagnosis now include Y. enterocolitica (but not Y. pseudotuberculosis). Y. pseudotuberculosis and some rarer strains of Y. enterocolitica are especially likely to cause systemic infection, which is also more likely in patients with diabetes or iron overload. Systemic sepsis is treatable with antimicrobial agents, but postinfective arthropathy responds poorly to such therapy. Over

twenty other Yersinia species lacking the virulence plasmid pYV com­ mon to Y. pestis, Y. pseudotuberculosis, and Y. enterocolitica are now recognized, primarily from genome sequencing. These are, at most, opportunistic pathogens of humans (including Y. aldovae, Y. aleksiciae, Y. bercovieri, Y. entomophaga, Y. frederiksenii, Y. hibernica, Y. interme­ dia, Y. kristensenii, Y. massiliensis, Y. mollaretii, Y. nurmii, Y. pekkanenii, Y. rohdei, Y. similis, Y. ruckeri, and Y. wautersii). Molecular phylogeny shows that Y. enterocolitica is more distantly related to Y. pseudotu­ berculosis than these other Yersinia species, and the similar virulence plasmid they share has probably been acquired independently by at least one of the two since the species diverged.

■ ■EPIDEMIOLOGY Y. enterocolitica  Y. enterocolitica is found worldwide and has been isolated from a wide variety of wild and domestic animals and environ­ mental samples, including samples of food and water. In vitro, Y. entero­ colitica is resistant to predation by the protozoon Acanthamoeba castellani and can survive inside it, suggesting a possible mode of environmental persistence. Strains are classically differentiated by biochemical reactions (biovar or biotype) combined with serogroup and, increasingly, by whole genome sequence data used for core genome multilocus sequence typing (cgMLST) (see “Laboratory Diagnosis,” below). Yersiniosis, >99% due to Y. enterocolitica, remains the third most common bacterial food-borne zoonosis reported in Europe, especially prevalent in Germany and Scandinavia. The incidence is highest among children; children <4 years of age are more likely to present with diarrhea than are older children. Abdominal pain with mesenteric ade­ nitis and terminal ileitis is more prominent among older children and adults. Septicemia is more likely in patients with preexisting conditions such as diabetes mellitus, liver disease, any condition involving iron overload (including thalassemia and hemochromatosis), advanced age, malignancy, or HIV/AIDS. As in enteritis of other bacterial etiologies, postinfective complications such as reactive arthritis occur mainly in individuals who are HLA-B27 positive. Erythema nodosum (Fig. A1-39) following Yersinia infection is not associated with HLA-B27 and is more common among women than among men. PART 5 Infectious Diseases Consumption or preparation of raw pork products (such as chitter­ lings) and some processed pork products is strongly linked with infec­ tion because a high percentage of pigs carry pathogenic Y. enterocolitica strains. Outbreaks of Y. enterocolitica infection have been associated with consumption of milk (pasteurized, unpasteurized, and chocolate-flavored) and various ready-to-eat vegetables including fresh spinach that were processed or washed with water. Person-to-person transmission is sus­ pected in a few cases (e.g., in nosocomial and familial outbreaks) but is much less likely with Y. enterocolitica than with other causes of gastro­ intestinal infection, such as Salmonella. A multivariate analysis indicates that contact with companion animals is a risk factor for Y. enterocolitica infection among children in Sweden, and low-level colonization of dogs and cats with Y. enterocolitica has been reported. Transfusion-associated septicemia due to Y. enterocolitica, while recognized as a very rare but frequently fatal event for >30 years, has been difficult to eradicate. Y. pseudotuberculosis  Y. pseudotuberculosis is much less frequently reported as a cause of human disease than Y. enterocolitica, and infec­ tion with Y. pseudotuberculosis is more likely to present as fever and abdominal pain due to mesenteric lymphadenitis and be identified from a blood culture isolate. This organism is associated with wild mammals (rodents, rabbits, and deer), birds, and domestic pigs. Although outbreaks are generally rare, several have recently occurred associated by food culture or dietary history with consumption of lettuce or raw carrots (Finland, New Zealand), tomatoes (France), or unpasteurized milk (Finland). Strains have historically been differ­ entiated by combined biochemical reactions (biovar) and serogroup. cgMLST is now used for investigation of both Y. enterocolitica and Y. pseudotuberculosis outbreaks in several countries. ■ ■PATHOGENESIS The usual route of infection is oral. Studies with both Y. enterocolitica and Y. pseudotuberculosis in animal models suggest that initial replication

in the small intestine is followed by invasion of Peyer’s patches of the distal ileum via M cells, with onward spread to mesenteric lymph nodes. The liver and spleen can also be involved after oral infection. The characteristic histologic appearance of enteropathogenic Yersinia after invasion of host tissues is as extracellular microabscesses sur­ rounded by an epithelioid granulomatous lesion. Experiments involving oral infection of mice with tagged Y. entero­ colitica show that only a very small proportion of bacteria in the gut invade tissues. Individual bacterial clones from an orally inoculated pool give rise to each microabscess in a Peyer’s patch, and the host restricts the invasion of previously infected Peyer’s patches. A prior model positing progressive bacterial spread from Peyer’s patches and mesenteric lymph nodes to the liver and spleen appears to be inaccu­ rate: spread of Y. pseudotuberculosis and Y. enterocolitica to the liver and spleen of mice occurs independently of regional lymph node coloniza­ tion and in mice lacking Peyer’s patches. Invasion requires the expression of several nonfimbrial adhesins, such as invasin (Inv) and—in Y. pseudotuberculosis—Yersinia adhe­ sin A (YadA). Inv interacts directly with β1 integrins, which are expressed on the apical surfaces of M cells but not enterocytes. YadA of Y. pseudotuberculosis interacts with extracellular matrix proteins such as collagen and fibronectin to facilitate host cell integrin association and invasion. YadA of Y. enterocolitica lacks a crucial N-terminal region and binds collagen and laminin but not fibronectin and does not cause inva­ sion. Inv is chromosomally encoded, whereas YadA is encoded on the virulence plasmid pYV. YadA also helps to confer serum resistance in Y. enterocolitica by binding host complement regulators such as factor H and C4-binding protein. Another chromosomal gene, ail (attachment and invasion locus), encodes the extracellular protein Ail, which is the main factor conferring serum resistance in Y. pseudotuberculosis by bind­ ing these complement regulators. By binding to host cell surfaces, YadA allows targeting of immune effector cells by the pYV plasmid–encoded type III secretion system (injectisome). As a consequence, the host’s innate immune response is altered; toxins (Yersinia outer proteins, or Yops) are injected into host macrophages, neutrophils, and dendritic cells, affecting signal transduc­ tion pathways, resulting in reduced phagocytosis and inhibited produc­ tion of reactive oxygen species by neutrophils, and triggering apoptosis of macrophages. Other factors functional in invasive disease include yersiniabactin (Ybt), a siderophore produced by some strains of Y. pseu­ dotuberculosis and Y. enterocolitica as well as other Enterobacterales. Ybt allows bacteria to access iron from saturated lactoferrin during infection and reduces production of reactive oxygen species by innate immune effector cells, thereby decreasing bacterial killing. Y. pseudotuberculosis and Y. pestis make other siderophores apart from Ybt. ■ ■CLINICAL MANIFESTATIONS Self-limiting diarrhea is the most common reported presentation in infection with pathogenic Y. enterocolitica, especially in children <4 years of age, who form the single largest group in most case series. Blood may be detected in diarrheal stool. Older children and adults are more likely than younger children to present with abdominal pain, which can be localized to the right iliac fossa—a situation that often leads to lapa­ rotomy for presumed appendicitis (pseudoappendicitis). Appendec­ tomy is not indicated for Yersinia infection causing pseudoappendicitis. Thickening of the terminal ileum and cecum is seen on endoscopy and ultrasound, with elevated round or oval lesions that may overlie Peyer’s patches. Mesenteric lymph nodes are enlarged. Ulcerations of the mucosa are noted on endoscopy. Gastrointestinal complications include granulomatous appendicitis, a chronic inflammatory condition affecting the appendix that is responsible for ≤2% of cases of appendici­ tis; Yersinia is involved in a minority of cases. Y. enterocolitica infection can present as acute pharyngitis with or without other gastrointesti­ nal symptoms. Fatal Y. enterocolitica pharyngitis has been recorded. Mycotic aneurysm can follow Y. enterocolitica bacteremia, as can focal infection (abscess) in many other sites and body compartments (liver, spleen, kidney, bone, meninges, endocardium). Y. pseudotuberculosis infection is more likely to present as abdom­ inal pain and fever than as diarrhea. A superantigenic toxin—

Y. pseudotuberculosis mitogen (YPM)—is produced by strains seen in eastern Russia in association with Far Eastern scarlet-like fever (FESLF), a childhood illness with desquamating rash, arthralgia, and toxic shock. A similar illness is recognized in Japan (Izumi fever) and Korea. Similarities have been noted with Kawasaki disease, the idio­ pathic acute systematic vasculitis of childhood. There is an epidemio­ logic link between exposure of populations to superantigen-positive Y. pseudotuberculosis and an elevated incidence of Kawasaki disease. Y. enterocolitica or Y. pseudotuberculosis septicemia presents as a severe illness with fever and leukocytosis, often without localizing fea­ tures, and is significantly associated with predisposing conditions such as diabetes mellitus, liver disease, and iron overload. Hemochromatosis combines several of these risk factors. Administration of iron chela­ tors like desferrioxamine, which provide iron accessible to Yersinia (and have an inhibitory effect on neutrophil function), may result in Yersinia septicemia in patients with iron overload who presumably have an otherwise mild gastrointestinal infection. HIV/AIDS has been associated with Y. pseudotuberculosis septicemia. The unusual phe­ nomenon of transfusion-associated septicemia is linked to the ability of Y. enterocolitica to multiply at refrigerator temperature (psychrotro­ phy). Typically, the transfused unit has been stored for >20 days, and it is believed that small numbers of yersiniae from an apparently healthy donor with subclinical bacteremia are amplified to very high numbers by growth inside the bag at ≤4°C, with consequent septic shock after transfusion. Complete prevention of this very rare event (one case in several million transfused units in countries such as the United States and France) without unacceptable restriction in the blood supply has not yet been devised. ■ ■POSTINFECTIVE PHENOMENA As in other invasive intestinal infections (salmonellosis, shigellosis), reactive arthritis (articular arthritis of multiple joints developing within 2–4 weeks of a preceding infection) occurs as a result of autoim­ mune activity initiated by the deposition of bacterial components (not viable bacteria) in joints in combination with the immune response to invading bacteria. The majority of individuals affected by reactive arthritis due to Yersinia are HLA-B27 positive. Myocarditis with elec­ trocardiographic ST-segment abnormalities may occur with Yersiniaassociated reactive arthritis. Most Yersinia-associated cases follow Y. enterocolitica infection (presumably because it is more common than infection with other species), but Y. pseudotuberculosis–associated reactive arthritis is also well documented in Finland, where sporadic and outbreak infections with Y. pseudotuberculosis are more common than in other countries. Of infected individuals identified in a recent Y. pseudotuberculosis serotype O:3 outbreak in Finland, 12% developed reactive arthritis affecting the small joints of the hands and feet, knees, ankles, and shoulders and lasting >6 months in most cases. Erythema nodosum (Fig. A1-39) occurs after Yersinia infection (more commonly in women) with no evidence of HLA-B27 linkage. There is a long-standing association between antithyroid and antiYersinia antibodies. Antibody evidence of prior Y. enterocolitica infec­ tion in Graves’ disease and increased levels of antithyroid antibody in patients with Y. enterocolitica antibodies were first noted in the 1970s. Y. enterocolitica contains a thyroid-stimulating hormone (TSH)–binding site that is recognized by antibodies to TSH from Graves’ disease patients. Raised titers of antibodies to Y. enterocolitica whole cells and Yops have been found in some series of Graves’ disease patients but not in others. It remains unclear whether this cross-reactivity is significant in the etiology of Graves’ disease. ■ ■LABORATORY DIAGNOSIS Standard laboratory culture methods can be used to isolate entero­ pathogenic Yersinia species from sterile samples, including blood and cerebrospinal fluid. Culture on specific selective media (CIN agar), with or without pre-enrichment in broth or phosphate-buffered saline at either 4°C or 16°C, is the basis of most schema for isolation of yersiniae from stool or other nonsterile samples. Outside known highincidence areas, specific culture may only be carried out by laboratories on request, or if a multiplex PCR screen detects Y. enterocolitica–specific

DNA in feces. Several CE-marked, FDA-approved kits for enteric pathogens now offer Y. enterocolitica detection (the precise assay targets are not disclosed), and their use has increased detection of Y. enterocolitica. These kits generally do not detect Y. pseudotuberculosis. A standard for PCR detection of pathogenic Y. enterocolitica and Y. pseudotuberculosis in food samples is available from the International Organization for Standardization.

Matrix-assisted laser desorption ionization time of flight (MALDITOF) mass spectrometry systems can speciate isolates of Y. enteroco­ litica and Y. pseudotuberculosis (but cannot separate Y. pestis from Y. pseudotuberculosis). Most clinical infections from typical presenta­ tions described above are associated with virulence plasmid-containing low-pathogenic strains, which genome sequencing has assigned to four phylogroups that correspond to the classical groups of Biotype 2/3 serogroup O:9 (BT2/3 O:9), BT4 O:3, BT2/3 O:5,27, and BT5. A highly pathogenic (mouse-lethal) virulence plasmid-containing phylogroup (BT1B O:8) used to be commonly reported from North America, where it is now rare, but occasional cases are now reported from Europe and Japan. A current area under active research is the clinical significance of a further phylogroup comprising Biotype 1A Y. enterocolitica strains of various serotypes. These comprise >50% of Y. enterocolitica fecal isolates in recent clinical studies from England, France, and China and >20% in New Zealand. They have generally been regarded as nonpathogenic because they lack the virulence plas­ mid and are not pathogenic in mouse-infection models. However, they do contain other genes associated with Yersinia pathogenesis, invade epithelial cells and macrophages in vitro, and are pathogenic in an insect model. They form the majority of Y. enterocolitica cultures from food sampled in the United Kingdom and France. CHAPTER 176 Because of the frequency with which the virulence plasmid is lost on laboratory subculture, combined biochemical identification (with biotyping according to a standard schema) and serologic identification was usually required to interpret the significance of an isolate of Y. enterocolitica from a nonsterile site. Whole genome DNA sequenc­ ing applying a Yersinia genus wide seven-gene multilocus sequence typing (MLST) scheme can now speciate Y. enterocolitica, Y. pestis, and Y. pseudotuberculosis and differentiate Y. enterocolitica biotypes. A cgMLST scheme provides a more detailed population structure and has been used for outbreak tracing and revealing novel, as yet phenotypi­ cally undefined Yersinia species. Plague and Other Yersinia Infections Agglutinating or ELISA antibody titers to specific O-antigen types are used in the retrospective diagnosis of both Y. enterocolitica and Y. pseudotuberculosis infections. IgA and IgG antibodies persist in patients with reactive arthritis. Serologic cross-reactions between Y. enterocolitica serogroup O:9 and Brucella are due to the similarity of their lipopolysaccharide structures. Multiple assays are required to cover even the predominant serogroups (Y. enterocolitica O:3, O5,27, and O:9; Y. pseudotuberculosis O:1a, O:1b, and O:3), and these assays are generally available only in reference laboratories. ELISA and western blot tests for antibodies to Yops, which are expressed by all pathogenic strains of Y. enterocolitica and Y. pseudotuberculosis, are also available; most of the positivity in these assays probably relates to previous infection with Y. enterocolitica. TREATMENT Yersiniosis Most cases of diarrhea caused by enteropathogenic Yersinia are self-limiting. Data from clinical trials do not support antimicrobial treatment for adults or children with Y. enterocolitica diarrhea. Systemic infections with bacteremia or focal infections outside the gastrointestinal tract generally require antimicrobial therapy. Infants <3 months of age with documented Y. enterocolitica infec­ tion may require antimicrobial treatment because of the increased likelihood of bacteremia in this age group. Y. enterocolitica strains nearly always express β-lactamases. Because of the relative rarity of systemic Y. enterocolitica infection, there are no clinical trial data to guide antimicrobial choice or to suggest the optimal dose and

60 - 177 Bartonella Infections, Including Cat-Scratch Disease

177 Bartonella Infections, Including Cat-Scratch Disease

duration of therapy. On the basis of retrospective case series and in vitro sensitivity data, fluoroquinolone therapy is effective for bacteremia in adults; for example, ciprofloxacin is given at a typical dose of 500 mg twice daily by mouth or 400 mg twice daily IV for at least 2 weeks (longer if positive blood cultures persist). A thirdgeneration cephalosporin is an alternative—e.g., cefotaxime (typical dose, 6–8 g/d in 3 or 4 divided doses) or ceftriaxone. In children, third-generation cephalosporins are effective; for example, cefo­ taxime is given to children ≥1 month of age at a typical dose of 75–100 mg/kg per day in 3 or 4 divided doses, with an increase to 150–200 mg/kg per day in severe cases (maximal daily dose, 8–10 g). Amoxicillin and amoxicillin-clavulanate have shown poor efficacy in case series. Trimethoprim-sulfamethoxazole, gentamicin, and imipenem are all active in vitro. Y. pseudotuberculosis strains do not express β-lactamase but are intrinsically resistant to polymyxin. Because human infection with Y. pseudotuberculosis is less common than that with Y. enterocolitica, less case information is available; however, studies in mice suggest that ampicillin is ineffective. Drugs similar to those used against Y. enterocolitica should be used. The best results have been obtained with a quinolone.

Some trials of treatment for reactive arthritis (with a large proportion of cases due to Yersinia) found that 3 months of oral ciprofloxacin therapy did not affect outcome. One trial in which the same therapy was given specifically for Y. enterocolitica–reactive arthritis found that, while outcome indeed was not affected, there was a trend toward faster remission of symptoms in the treated group. Follow-up 4–7 years after initial antibiotic treatment of reactive arthritis (predominantly following Salmonella and Yersinia infections) demonstrated apparent efficacy in the prevention of chronic arthritis in HLA-B27-positive individuals. A trial showing that azithromycin therapy did not affect outcome in reactive arthri­ tis included cases thought to have followed yersiniosis, although no breakdown of cases was provided. PART 5 Infectious Diseases ■ ■PREVENTION AND CONTROL Current control measures are similar to those used against other enteric pathogens like Salmonella and Campylobacter, which colonize the intestine of food animals. The focus is on safe handling and pro­ cessing of food. No vaccine is effective in preventing intestinal coloni­ zation of food animals by enteropathogenic Yersinia. Consumption of food made from raw pork (which is popular in Germany and Belgium) should be discouraged at present because it is not possible to eliminate contamination with the enteropathogenic Yersinia strains found world­ wide in pigs. Exposure of infants to raw pig intestine during domestic preparation of chitterlings is inadvisable. Modification of abattoir technique in Scandinavian countries from the 1990s onward included the removal of pig intestines in a closed plastic bag; levels of carcass contamination with Y. enterocolitica were reduced, but such contami­ nation was not eliminated. Experimental pig herds free of pathogenic

Y. enterocolitica O:3 (and also of Salmonella, Campylobacter, Toxoplasma, and Trichinella) have been established by selective breeding in Norway but remain rare. In the food industry, vigilance is required because of the potential for large outbreaks if small numbers of enteropathogenic yersiniae contaminate any ready-to-eat food whose safe preservation is based on refrigeration before consumption. The rare phenomenon of contamination of blood for transfusion has proved impossible to eradicate. However, leukodepletion is now practiced in most blood transfusion centers, primarily to prevent non­ hemolytic febrile transfusion reactions and alloimmunization against HLA antigens. This measure reduces but does not eliminate the risk of Yersinia blood contamination. Notification of yersiniosis is now obligatory in some countries. ■ ■FURTHER READING Plague Campbell SB et al: Animal exposure and human plague, United States, 1970–2017. Emerg Infect Dis 25:2270, 2019.

Demeure C et al: Yersinia pestis and plague: An updated view on evo­ lution, virulence determinants, immune subversion, vaccination and diagnostics. Microbes Infect 21:202, 2019. Klunk J et al: Evolution of immune genes is associated with the Black Death. Nature 611:312, 2022. Nelson CA et al: Antimicrobial treatment and prophylaxis of plague: Recommendations for naturally acquired infections and bioterrorism response. MMWR Recomm Rep 70(No. RR-3):1, 2021. Salam AP et al: Deaths associated with pneumonic plague, 1946–2017. Emerg Infect Dis 26: 2432, 2020. World Health Organization: WHO guidelines for plague manage­ ment: Revised recommendations for the use of rapid diagnostic tests, fluoroquinolones for case management and personal protective equip­ ment for prevention of post-mortem transmission, May 27, 2021. Available at: https://www.who.int/publications/i/item/9789240015579. Accessed February 16, 2024. Yang R et al: Yersinia pestis and plague: Some knowns and unknowns. Zoonoses (Burlingt) 3:5, 2023. Yersiniosis Francis MS et al: The pathogenic Yersiniae–advances in the under­ standing of physiology and virulence. Front Cell Infect Microbiol 9:119. 2019. Savin C et al: Genus-wide Yersinia core-genome multilocus sequence typing for species identification and strain characterization. Microb Genom 5:e000301, 2019. Šumilo D et al: Forgotten but not gone: Yersinia infections in England, 1975 to 2020. Eurosurveillance 28:2200516, 2023. Michael Giladi, Moshe Ephros

Bartonella Infections,

Including Cat-Scratch

Disease Bartonella species are fastidious, facultative intracellular, slow-growing, gram-negative bacteria that cause a broad spectrum of diseases in humans. This genus includes >40 distinct species or subspecies, of which at least 20 have been recognized as confirmed or potential human pathogens; Bartonella bacilliformis, Bartonella quintana, and Bartonella henselae are most commonly identified (Table 177-1). Most Bartonella species have successfully adapted to survival in specific domestic or wild mammals. Prolonged intraerythrocytic and endothe­ lial cell infection in these animals creates a niche where the bacteria are protected from both innate and adaptive immunity and which serves as a reservoir for human infections. Chronic bacteremia, which may per­ sist even following prolonged antimicrobial therapy, is a typical finding in Bartonella-hosting mammals. Adhesion and entry to endothelial cells and erythrocytes is facilitated by the Bartonella adhesin A or vari­ ably expressed outer-membrane proteins. Bartonella characteristically evades the host immune system by modification of its virulence factors (e.g., lipopolysaccharides or flagella) and by attenuation of the immune response. Arthropod vectors are often involved. Humans are incidental hosts for most Bartonella species with the exception of B. quintana and B. bacilliformis, which are not considered zoonotic agents. Isolation and characterization of Bartonella species are difficult and require special techniques. Clinical presentation generally depends on both the infecting Bartonella species and the immune status of the infected individual. Bartonella species are susceptible to many antibiotics in vitro; however, clinical responses to therapy and studies in animal

TABLE 177-1  Bartonella Species Known or Suspected to Be Human Pathogens BARTONELLA SPECIESa DISEASE(S)b RESERVOIR HOST(S)c ARTHROPOD VECTOR B. alsatica Endocarditis, lymphadenitis, vascular graft infection B. ancashensis Verruga peruana, bacteremia Unknown Unknown B. bacilliformis Carrión’s disease Humans Sandflies (Lutzomyia verrucarum) B. clarridgeiae Possibly cat-scratch disease Cats, dogs Unknown B. doshiae Bacteremia, arthralgia, blurred vision Voles, rats Unknown B. elizabethae Endocarditis Rats, dogs Fleas B. grahamii Lymphadenopathy, possibly retinitis and retinal vascular occlusion B. henselae Cat-scratch disease, bacillary angiomatosis, bacillary peliosis, bacteremia, endocarditis B. koehlerae Endocarditis Cats Unknown B. kosoyi Lymphadenopathy Rats Fleas “Candidatus Bartonella mayotimonensis”d Endocarditis Bats Unknown “Candidatus Bartonella melophagi”d Various clinical manifestations Sheep Sheep keds B. quintana Trench fever, chronic bacteremia, bacillary angiomatosis, endocarditis B. rochalimae Bacteremia, fever, splenomegaly Foxes, coyotes, dogs, skunks, rats, other rodents B. schoenbuchensis Bacteremia, fever, myalgia, deer ked dermatitis? B. tamiae Bacteremia, fever, myalgia, rash Rats Unknown B. tribocorum Bacteremia, myalgia, headache Rats, mice Unknown B. vinsonii subsp. arupensis Endocarditis, febrile illness Mice, dogs Ticks B. vinsonii subsp. berkhoffii Endocarditis Domestic dogs, coyotes, gray foxes Ticks “Candidatus Bartonella washoensis”d Endocarditis, myocarditis, meningitis Squirrels, rabbits, prairie dogs, possibly other rodents aMany other Bartonella species exist but are not recognized as human pathogens. bAnimal-associated Bartonella species (B. henselae, B. doshiae, B. schoenbuchensis, and B. tribocorum) were isolated from blood of patients who reported tick bites and chronic symptoms such as fatigue and myalgia. DNA of B. henselae, B. vinsonii subsp. berkhoffii, B. koehlerae, or B. melophagi or co-infection with more than one Bartonella species was detected by polymerase chain reaction in blood samples from patients with extensive arthropod and animal exposure who presented with chronic neurologic or neurocognitive syndromes. The causal relationship between bacteremia with these pathogens, tick bites, and clinical manifestations needs to be established. cAnimals are implicated when existing evidence supports their infection with Bartonella species. Data supporting animal-to-human transmission may be lacking. dCandidatus is a taxonomic status for bacteria that cannot be described in sufficient detail to warrant establishment of a novel taxon or cannot be cultured or propagated in culture media. The phylogenetic relatedness of these bacteria has been determined by gene amplification and sequence analysis. models suggest that the minimal inhibitory concentrations of many antimicrobial agents correlate poorly with the drugs’ in vivo efficacies in patients with Bartonella infections. CAT-SCRATCH DISEASE ■ ■DEFINITION AND ETIOLOGY Usually a self-limited illness, cat-scratch disease (CSD) has two general clinical presentations. Typical CSD, the more common, is characterized by subacute regional lymphadenopathy; atypical or extranodal CSD is the collective designation for numerous extranodal manifestations involving various organs. B. henselae is the principal etiologic agent of CSD. Rare cases have been associated with Afipia felis and other Bartonella species. ■ ■EPIDEMIOLOGY CSD occurs worldwide, favoring warm and humid climates. In temper­ ate climates, incidence peaks during fall and winter. Adults are affected nearly as frequently as children. Intrafamilial clustering is rare, and person-to-person transmission does not occur. Apparently healthy bacteremic cats constitute the major reservoir of B. henselae, and cat fleas (Ctenocephalides felis) may be responsible for cat-to-cat transmis­ sion. CSD usually follows contact with cats (especially kittens), but other animals (e.g., dogs) have been implicated as possible reservoirs in rare instances. In the United States, the estimated annual disease incidence is 4–9 outpatient cases per 100,000 population with a higher incidence among those who lived in the southern United States and among chil­ dren 5–9 years of age. About 5–10% of patients in the United States are

Rabbits Fleas Mice, voles, rats, deer Fleas Cats, other felines Cat fleas (Ctenocephalides felis): associated with cat-to-cat, but not with cat-to-human, transmission Humans Human body lice (Pediculus humanus corporis) Fleas Cattle, roe deer, moose Unknown CHAPTER 177 Fleas, ticks Bartonella Infections, Including Cat-Scratch Disease hospitalized. A higher proportion of hospitalization has been reported in Israel. ■ ■PATHOGENESIS Although cat fleas are likely responsible for cat-to-cat transmission, the mode of cat-to-human transmission is undetermined. B. henselae–infected cats’ saliva spreads to claws by self-licking, and B. henselae–contaminated flea feces can be inoculated by a scratch or a bite. Infection of mucous membranes or conjunctivae via droplets or licking may occur as well. With lymphatic drainage to one or more regional lymph nodes in immunocompetent hosts, a TH1 response can result in necrotizing granulomatous lymphadenitis. Dendritic cells, along with their associ­ ated chemokines, play a role in the host inflammatory response and granuloma formation. ■ ■CLINICAL MANIFESTATIONS AND PROGNOSIS Of patients with CSD, 85–90% have typical disease. The primary lesion, a small (0.3- to 1-cm) painless erythematous papule or pustule, devel­ ops at the inoculation site within days to 2 weeks in about one-third to two-thirds of patients (Fig. 177-1A, B). Lymphadenopathy develops 1–3 weeks or longer after cat contact. The affected lymph node(s) are enlarged and usually painful, sometimes have overlying erythema, and suppurate in ~10% of cases (Fig. 177-1C, D, and E). Axillary/epitrochlear nodes are most commonly involved, followed by head/neck nodes and inguinal/femoral nodes. Approximately 50% of patients have fever, malaise, and anorexia. A smaller proportion experience weight loss and night sweats mimicking the presentation of lymphoma. Fever is usually low-grade but infrequently rises to ≥39°C. Resolution is slow, requiring

A PART 5 Infectious Diseases B C

D E F FIGURE 177-1  Manifestations of cat-scratch disease. A. Primary inoculation lesion. Axillary and epitrochlear lymphadenitis appeared 2 weeks later. B. Primary inoculation lesion. Submental lymphadenitis appeared 10 days later. C. Axillary lymphadenopathy of 2 weeks’ duration. The overlying skin appears normal. D. Cervical lymphadenopathy of 6 weeks’ duration. The overlying skin is red. Thick, odorless pus (12 mL) was aspirated. E. Preauricular lymphadenopathy. F. Left-eye neuroretinitis. Note papilledema and stellate macular exudates (“macular star”).

weeks (for fever, pain, and accompanying signs and symptoms) to months (for node shrinkage). Atypical CSD occurs in 10–15% of patients in the absence or presence of lymphadenopathy. Atypical disease includes Parinaud’s oculoglandular syndrome (conjunctivitis with ipsilateral preauricular lymphadenitis; Fig. 177-1E), hepatosplenic disease, neuroretinitis (often presenting as unilateral deterioration of vision; Fig. 177-1F) and other ophthalmologic manifestations, neurologic manifestations (encephalitis, seizures, myelitis, facial and other cranial nerve palsies, peripheral neuritis), fever of unknown origin (FUO), pneumonitis, debilitating myalgia, arthritis or arthralgia (affecting mostly women

20 years old), osteomyelitis (including multifocal disease), and der­ matologic manifestations (including erythema nodosum [see Fig. A1-39], sometimes accompanying arthropathy). CSD-associated FUO is a unique syndrome that may be severe and debilitating, often mimics malignancy, and may present with multiorgan involvement, including hepatosplenic space-occupying lesions, abdominal/mediastinal lymph­ adenopathy, ocular disease, and multifocal osteomyelitis. Fever may be continuous or relapsing. Other manifestations and syndromes (e.g., idiopathic thrombocytopenic purpura, Henoch-Schönlein purpura, erythema multiforme [see Fig. A1-24A], myocarditis) also have been associated with CSD; however, a causal role for B. henselae has yet to be established. In elderly patients (>60 years old), lymphadenopathy is more often absent but encephalitis and FUO are more common than in younger patients. In immunocompetent individuals, CSD—whether typical or atypical—usually resolves without treatment and without sequelae, although some of the ophthalmologic manifestations may occasionally result in moderate to severe vision loss. Lifelong immunity is the rule. ■ ■DIAGNOSIS Routine laboratory tests usually yield normal or nonspecific results. Histopathology initially shows lymphoid hyperplasia and later dem­ onstrates stellate granulomata with necrosis, coalescing microab­ scesses, and occasional multinucleated giant cells—findings that, although nonspecific, may narrow the differential diagnosis. Sero­ logic testing (immunofluorescence or enzyme immunoassay) is the most commonly used laboratory diagnostic approach, with variable sensitivity and specificity. CSD serodiagnosis is often based on the presence of IgG alone (i.e., in the absence of IgM), and seroconver­ sion may take a few weeks; these two factors may pose difficulties in the interpretation of serologic results. Other tests are of low sensitiv­ ity (culture, Warthin-Starry silver staining), of low specificity (cytol­ ogy, histopathology), or of limited availability in routine diagnostic laboratories (polymerase chain reaction [PCR], immunohistochemis­ try). PCR of pus aspirated from lymph nodes or the primary inocula­ tion lesion is highly sensitive and specific and is particularly useful for definitive and rapid diagnosis in seronegative patients. PCR of a lymph node biopsy specimen may be less sensitive, perhaps because of sampling error. APPROACH TO THE PATIENT Cat-Scratch Disease A history of cat contact, a primary inoculation lesion, and regional lymphadenopathy—especially axillary/epitrochlear lymphadenopa­ thy—are highly suggestive of CSD. A characteristic clinical course and corroborative laboratory tests make the diagnosis very likely. Conversely, when acute- and convalescent-phase sera are nega­ tive (as is the case in 10–20% of CSD patients), when spontaneous regression of lymph node size does not occur, and particularly when constitutional symptoms persist, malignancy must be ruled out. Pyogenic lymphadenitis, mycobacterial infection, brucellosis, syphilis, tularemia, plague, toxoplasmosis, sporotrichosis, and his­ toplasmosis also should be considered. In clinically suspected CSD in a seronegative individual, fine-needle aspiration may be adequate and PCR can confirm the diagnosis. When data are less supportive of CSD, lymph node biopsy rather than fine-needle aspiration is

preferred. In seronegative CSD patients with lymphadenopathy and severe complications (e.g., encephalitis or neuroretinitis), early biopsy is important to establish a specific diagnosis. TREATMENT Cat-Scratch Disease (Table 177-2) Treatment regimens are based on only minimal data. Suppurative nodes should be drained by large-bore needle aspira­ tion and not by incision and drainage to avoid chronic draining tracts. Systemic antibiotics are recommended in immunocompro­ mised patients. TABLE 177-2  Antimicrobial Therapy for Disease Caused by Bartonella Species in Adults DISEASE ANTIMICROBIAL THERAPY Typical cat-scratch disease Not routinely indicated; for patients with extensive lymphadenopathy, consider azithromycin (500 mg PO

on day 1, then 250 mg PO once a day for 4 days) Cat-scratch disease neuroretinitis Value of systemic antibiotics is controversial, particularly when visual acuity is not significantly compromised. For more severe cases, doxycycline (100 mg PO bid) plus rifampin (300 mg PO bid) for 4–6 weeks is given. Consider adding systemic glucocorticoids. Other atypical catscratch disease manifestationsa As per neuroretinitis. Treatment duration should be individualized. CHAPTER 177 Trench fever or chronic bacteremia with B. quintana Gentamicin (3 mg/kg IV once a day for 14 days) plus doxycycline (200 mg PO once a day or 100 mg PO bid for 6 weeks) Suspected Bartonella endocarditis Gentamicinb (1 mg/kg IV q8h for ≥14 days) plus doxycycline (100 mg PO/IV bid for 6 weeksc) plus ceftriaxone (2 g IV once a day for 6 weeks) Bartonella Infections, Including Cat-Scratch Disease Confirmed Bartonella endocarditis As for suspected Bartonella endocarditis minus ceftriaxone Bacillary angiomatosis Erythromycind (500 mg PO qid for 3 months) or Doxycycline (100 mg PO bid for 3 months) Bacillary peliosis Erythromycind (500 mg PO qid for 4 months) or Doxycycline (100 mg PO bid for 4 months) Carrión’s disease Oroya fever Chloramphenicol (500 mg PO/IV qid for 14 days) plus another antibiotic (β-lactam preferred) or Ciprofloxacin (500 mg PO bid for 10 days) ± ceftriaxone (1–2 g IV once a day for 10 days) Verruga peruana Azithromycin (500 mg PO once a day for 7 days) or Ciprofloxacin (500 mg PO bid for 7–10 days) or Rifampin (10 mg/kg PO once a day, to a maximum of

600 mg, for 14 days) or Streptomycin (15–20 mg/kg IM once a day for 10 days) aData on treatment efficacy for encephalitis and hepatosplenic cat-scratch disease are lacking. Therapy similar to that given for neuroretinitis is reasonable. bSome experts recommend gentamicin at 3 mg/kg IV once a day. If gentamicin is contraindicated, rifampin (300 mg PO bid) can be added to doxycycline for documented Bartonella endocarditis. cSome experts recommend extending oral doxycycline therapy for 3–6 months and measuring anti-Bartonella IgG titers every 8–10 weeks as a guide to duration of therapy, particularly when the infected valve was not resected. dOther macrolides are probably effective and may be substituted for erythromycin or doxycycline. Source: Recommendations are modified from JM Rolain et al: Antimicrob Agents Chemother 48:1921, 2004.

■ ■PREVENTION Avoiding cats (especially kittens) and instituting flea control are options for immunocompromised patients and for patients with valvular heart disease.

TRENCH FEVER AND CHRONIC BACTEREMIA ■ ■DEFINITION AND ETIOLOGY Trench fever, also known as 5-day fever or quintan fever, is a febrile ill­ ness caused by B. quintana. It was first described as an epidemic in the trenches of World War I; however, recent paleomicrobiological stud­ ies have provided evidence that B. quintana has been associated with human infection for 4000 years. This infection recently reemerged as chronic bacteremia seen most often in homeless people, also referred to as urban or contemporary trench fever. ■ ■EPIDEMIOLOGY In addition to epidemics in World Wars I and II, sporadic outbreaks of trench fever have been reported in many regions of the world. The human body louse has been identified as the vector and humans as the only known reservoir for human infections. Recently, macaque monkeys from China and Japan also were found to be natural hosts of B. quintana, without evidence for macaque-to-human transmission. After a hiatus of several decades during which trench fever was almost forgotten, small clusters of cases of B. quintana chronic bacteremia were reported sporadically, primarily from the United States and France, in homeless people without HIV infection. Alcoholism and louse infesta­ tion were identified as risk factors. PART 5 Infectious Diseases ■ ■CLINICAL MANIFESTATIONS The typical incubation period is 15–25 days (range, 3–38 days). “Classic” trench fever, as described in 1919, ranges from a mild febrile illness to a recurrent or protracted and debilitating disease. Fever is often peri­ odic, lasting 4–5 days with 5-day (range, 3- to 8-day) intervals between episodes. Other symptoms and signs include headache, back and limb pain, profuse sweating, shivering, myalgia, arthralgia, splenomegaly, a maculopapular rash in occasional cases, and nuchal rigidity in some cases. Untreated, the disease usually lasts 4–6 weeks. Death is rare. The clinical spectrum of B. quintana bacteremia in homeless people ranges from asymptomatic infection to a febrile illness with headache, severe leg pain, and thrombocytopenia. Endocarditis sometimes develops. ■ ■DIAGNOSIS Definitive diagnosis requires isolation of B. quintana by blood culture. Some patients have positive blood cultures for several weeks. Patients with acute trench fever typically develop significant titers of antibody to Bartonella, whereas those with chronic B. quintana bacteremia may be seronegative. Patients with high titers of IgG antibodies should be evaluated for endocarditis. In epidemics, trench fever should be differ­ entiated from epidemic louse-borne typhus and relapsing fever, which occur under similar conditions and share many features. TREATMENT B. quintana Bacteremia (Table 177-2) In a small, randomized, placebo-controlled trial involving homeless people with B. quintana bacteremia, therapy with gentamicin and doxycycline was superior to administration of placebo in eradicating bacteremia. Treatment of bacteremia is important, even in clinically mild cases, to prevent endocarditis. Optimal therapy for trench fever without documented bacteremia is uncertain. BARTONELLA ENDOCARDITIS ■ ■DEFINITION AND ETIOLOGY Coxiella burnetii (Chap. 192) and Bartonella species are the most common pathogens of culture-negative endocarditis (Chap. 133). In

France, for example, Bartonella species were identified as the etiologic agents in 28% of 348 cases of culture-negative endocarditis. Preva­ lence, however, varies by geographic location and epidemiologic setting. B. quintana and B. henselae are the most common Bartonella species implicated in endocarditis, and other Bartonella species have report­ edly caused rare cases (Table 177-1). ■ ■EPIDEMIOLOGY Bartonella endocarditis has been reported worldwide. Most patients are adults; more are male than female. Risk factors associated with B. quintana endocarditis include homelessness, alcoholism, and body louse infestation; however, individuals with no risk factors have had Bartonella endocarditis diagnosed as well. B. henselae endocarditis is associated with exposure to cats. Most cases involve native rather than prosthetic valves; the aortic valve accounts for ~60% of cases. Patients with B. henselae endocarditis usually have preexisting valvulopathy, whereas B. quintana often infects normal valves. ■ ■CLINICAL MANIFESTATIONS Clinical manifestations are usually characteristic of subacute endocar­ ditis of any etiology. However, a substantial number of patients have a prolonged, minimally febrile or even afebrile indolent illness, with mild nonspecific symptoms lasting weeks or months before the diagnosis is made. Initial echocardiography may not show vegetations. Acute, aggressive disease is rare. ■ ■DIAGNOSIS Blood cultures, even with use of special techniques (lysis centrifugation or EDTA-containing tubes), are positive in only ~25% of cases—mostly those caused by B. quintana and only rarely those caused by B. henselae. Prolonged incubation of cultures (up to 6 weeks) is required. Serologic tests—either immunofluorescence or enzyme immunoassay—usually demonstrate high-titer (≥1:800) IgG antibodies to Bartonella. Because of cross-antigenicity, routine serology does not distinguish between B. quintana and B. henselae and may also be cross-reactive with other pathogens, such as C. burnetii and Chlamydia species. Approximately 40–50% of patients with Q fever endocarditis have antibodies that significantly react with B. henselae and/or B. quintana; however, in the majority of cases, IgG titers against the true pathogen are several dilutions higher than those for the pathogen causing the false-positive serology, thus allowing the correct diagnosis to be made. Identification of Bartonella to the species level is usually accomplished by application of PCR and DNA sequencing methods to valve tissue or, infrequently, blood specimens. TREATMENT Bartonella Endocarditis (Table 177-2) For patients with culture-negative endocarditis sus­ pected to be due to Bartonella species, empirical treatment consists of gentamicin, doxycycline, and ceftriaxone; the major role of cef­ triaxone in this regimen is to adequately treat other potential causes of culture-negative endocarditis, including members of the HACEK group (Chap. 163). Once a diagnosis of Bartonella endocarditis has been established, ceftriaxone is discontinued. Aminoglycosides, the only antibiotics known to be bactericidal against Bartonella, should be included in the regimen for ≥2 weeks. Indications for valvular surgery are the same as in subacute endocarditis due to other patho­ gens; however, the proportion of patients who undergo surgery (~60%) is high, probably as a consequence of delayed diagnosis. BACILLARY ANGIOMATOSIS AND PELIOSIS ■ ■DEFINITION AND ETIOLOGY Bacillary angiomatosis (sometimes called bacillary epithelioid angio­ matosis or epithelioid angiomatosis) is a disease of severely immuno­ compromised patients, is caused by B. henselae or B. quintana, and is characterized by neovascular proliferative lesions involving various organs. Both species cause cutaneous lesions; hepatosplenic lesions

are caused only by B. henselae, whereas subcutaneous and lytic bone lesions are more frequently associated with B. quintana. Bartonella angiogenic factor A, a proangiogenic autotransporter identified in B. henselae, activates the host vascular endothelial growth factor path­ way to drive angiogenesis. Bacillary peliosis is a closely related angiop­ roliferative disorder caused by B. henselae and involving primarily the liver (peliosis hepatis) but also the spleen and lymph nodes. Bacillary peliosis is characterized by blood-filled cystic structures whose size ranges from microscopic to several millimeters. ■ ■EPIDEMIOLOGY Bacillary angiomatosis and bacillary peliosis occur primarily in HIVinfected persons (Chap. 208) with CD4+ T-cell counts of <100/μL but also affect other immunosuppressed patients and, in rare instances, immunocompetent patients. The incidence has decreased since the introduction of effective antiretroviral therapy and the routine use of rifabutin and macrolides to prevent Mycobacterium avium complex infection in AIDS patients. Contact with cats or cat fleas increases the risk of B. henselae infection. Risk factors for B. quintana infection are low income, homelessness, and body louse infestation. ■ ■CLINICAL MANIFESTATIONS Bacillary angiomatosis presents most commonly as one or more cutaneous lesions that are not painful and may be tan, red, or purple in color. Subcutaneous, often tender nodules, superficial ulcerated plaques (Fig. 177-2), and verrucous growths are also seen. Nodular forms resemble those seen in fungal or mycobacterial infections. Pain­ ful osseous lesions, most often involving long bones, may underlie cutaneous lesions and occasionally develop in their absence. Other organs are rarely involved. Patients usually have constitutional symp­ toms, including fever, chills, malaise, headache, anorexia, weight loss, and night sweats. In patients with advanced immunodeficiency, B. henselae and B. quintana are important causes of FUO. In osseous disease, lytic FIGURE 177-2  Lesions of cutaneous bacillary angiomatosis (BA) in three severely immunocompromised AIDS patients. Left panel shows a 1.5-cm ulcerated, bleeding BA lesion with an erythematous base; middle panel shows numerous small, 2-mm, scattered angiomatous BA lesions; right panel shows a 2.0-cm friable BA lesion on the thigh. (Photos courtesy of Timothy Berger, MD; Jordan Tappero, MD, MPH; and Jane Koehler, MA, MD.)

lesions are generally seen on radiography, and technetium scan shows focal uptake. The differential diagnosis of cutaneous bacillary angio­ matosis includes Kaposi’s sarcoma, pyogenic granuloma, subcutaneous tumors, and verruga peruana. In bacillary peliosis, hypodense hepatic areas are usually evident on imaging.

■ ■PATHOLOGY Bacillary angiomatosis consists of lobular proliferations of small blood vessels lined by enlarged endothelial cells interspersed with mixed infiltrates of neutrophils and lymphocytes, with predominance of the former. Histologic examination of organs with bacillary peliosis reveals small blood-filled cystic lesions partially lined by endothelial cells that can be several millimeters in size. Peliotic lesions are surrounded by fibromyxoid stroma containing inflammatory cells, dilated capillar­ ies, and clumps of granular material. Warthin-Starry silver staining of bacillary angiomatosis and peliosis lesions reveals clusters of bacilli. Cultures are usually negative. ■ ■DIAGNOSIS Bacillary angiomatosis and bacillary peliosis are diagnosed by histo­ logic examination. Blood cultures may be positive. TREATMENT Bacillary Angiomatosis and Peliosis (Table 177-2) Prolonged therapy with a macrolide or doxycycline is recommended for both bacillary angiomatosis and bacillary peliosis. CHAPTER 177 ■ ■PREVENTION Reasonable strategies for HIV-infected persons consist of control of cat-flea infestation and avoidance of cat scratches (for prevention of Bartonella Infections, Including Cat-Scratch Disease

61 - 178 Donovanosis

178 Donovanosis

B. henselae) and avoidance and treatment of body louse infestation (for prevention of B. quintana). Primary prophylaxis is not recommended, but suppressive therapy with a macrolide or doxycycline is indicated in HIV-infected patients with bacillary angiomatosis or bacillary peliosis until CD4+ T-cell counts are >200/μL. Relapse may necessitate lifelong suppressive therapy in individual cases.

CARRIÓN’S DISEASE (OROYA FEVER AND VERRUGA PERUANA) ■ ■DEFINITION AND ETIOLOGY Carrión’s disease is a biphasic disease caused by B. bacilliformis. Oroya fever is the initial, bacteremic, systemic form, and verruga peruana—or “Peruvian warts”—is its late-onset, eruptive manifestation. ■ ■EPIDEMIOLOGY AND PREVENTION Infection is endemic to the geographically restricted Andes valleys of Peru, Ecuador, and Colombia (~500–3200 m above sea level). Sporadic epidemics occur. The disease is transmitted by the phlebotomine sandfly Lutzomyia verrucarum. Maternal-fetal transmission as well as transmission by blood transfusion have been reported. Humans are the only known reservoir of B. bacilliformis. Sandfly control measures (e.g., insecticides) and personal protection measures (e.g., repellents, screen­ ing, bed nets) may decrease the risk of infection. ■ ■PATHOGENESIS After inoculation by the sandfly, bacteria invade the blood ves­ sel endothelium and proliferate; the reticuloendothelial system and various organs also may be involved. Upon reentry into blood vessels,

B. bacilliformis invades, replicates, and ultimately destroys erythro­ cytes, with consequent massive hemolysis and sudden, severe anemia. Microvascular thrombosis results in end-organ ischemia. Survivors sometimes develop cutaneous hemangiomatous lesions characterized by various inflammatory cells, endothelial proliferation, and the pres­ ence of B. bacilliformis (verruga peruana). PART 5 Infectious Diseases ■ ■CLINICAL MANIFESTATIONS The incubation period is 3 weeks (range, 2–14 weeks). Oroya fever may present as a nonspecific bacteremic febrile illness without anemia or as an acute, severe hemolytic anemia with hepatomegaly and jaundice of rapid onset leading to vascular collapse and clouded sensorium. Myalgia, arthralgia, lymphadenopathy, and abdominal pain may develop. Temperature is elevated but not extremely so; high fever may suggest intercurrent infection. Subclinical asymptomatic infection also occurs. Secondary infection with Salmonella (typhi and non-typhi) is common, though other secondary infections have been reported and should be suspected when fever persists or recurs after defervescence. In verruga peruana, red, hemangioma-like, cutaneous vascular lesions of various sizes appear either weeks to months after systemic illness or with no previous suggestive history. These lesions persist for months up to 1 year. Mucosal and internal lesions also may develop. ■ ■DIAGNOSIS AND APPROACH TO THE PATIENT Systemic illness (with or without anemia) or the development of cutaneous lesions in a person who has been to an endemic area raises the possibility of B. bacilliformis infection. Severe anemia with exuberant reticulocytosis—and sometimes thrombocytopenia—can occur. In systemic illness, Giemsa-stained blood films may show typical intraerythrocytic bacilli. Blood and bone marrow cultures may be positive, but growth is slow (1−6 weeks) and requires lower incubation temperature. Serologic assays may be helpful. Diagnosis of verruga peruana is largely clinical, although biopsy may be required to confirm the diagnosis. Several PCR assays have been described; however, their role in diagnosis remains to be clinically validated. Differential diagnosis includes coendemic systemic febrile illnesses (e.g., typhoid fever, malaria, brucellosis) and diseases producing cuta­ neous vascular lesions (e.g., hemangiomata, bacillary angiomatosis, Kaposi’s sarcoma).

TREATMENT Carrión’s Disease (Table 177-2) Antibiotic therapy for systemic B. bacilliformis infec­ tion usually results in rapid defervescence. Additional antibiotic treatment of intercurrent infection is often required. Blood transfu­ sion may be necessary. Treatment of verruga peruana is not always required. Patients with numerous lesions, especially lesions that have been present for only a short period, may respond well to antibiotic therapy. ■ ■COMPLICATIONS AND PROGNOSIS Mortality rates associated with Oroya fever have been reported to be as high as 40% without treatment but are considerably lower (~10%) with treatment. Complications such as bacterial superinfection and neuro­ logic and cardiac manifestations occur frequently. Generalized massive edema (anasarca) and petechiae are associated with poor outcome. Permanent immunity usually develops. ■ ■FURTHER READING Fournier PE et al: Epidemiologic and clinical characteristics of Bartonella quintana and Bartonella henselae endocarditis: A study of 48 patients. Medicine (Baltimore) 80:245, 2001. Gomes C, Ruiz J: Carrion’s disease: The sound of silence. Clin Microbiol Rev 31:e56, 2018. Koehler JE et al: Molecular epidemiology of Bartonella infections in patients with bacillary angiomatosis-peliosis. N Engl J Med 337:1876, 1997. Landes M et al: Cat scratch disease presenting as fever of unknown origin is a unique clinical syndrome. Clin Infect Dis 71:2818, 2020. Rolain JM et al: Recommendations for treatment of human infections caused by Bartonella species. Antimicrob Agents Chemother 48:1921, 2004. Rose SR, Koehler JE: Bartonella including cat scratch disease, in Principles and Practice of Infectious Diseases, 9th ed, GL Mandell et al (eds). Philadelphia, Elsevier, Inc. 2020, pp 2824–2843. Wagner A, Dehio C: Role of distinct type-IV-secretion systems and secreted effector sets in host adaptation by pathogenic Bartonella spe­ cies. Cell Microbiol 21:e13004, 2019. Nigel O’Farrell

Donovanosis Donovanosis is a chronic, progressive bacterial infection that usually involves the genital region. The condition is generally regarded as a sexually transmitted infection of low infectivity. This infection has been known by many other names, the most common being granuloma inguinale. ■ ■ETIOLOGY The causative organism has been reclassified as Klebsiella granulomatis comb nov on the basis of phylogenetic analysis, although there is ongo­ ing debate about this decision. Some authorities consider the original nomenclature (Calymmatobacterium granulomatis) to be more appro­ priate in light of analysis of 16S rRNA gene sequences. Donovanosis was first described in Calcutta in 1882, and the causative organism was recognized by Charles Donovan in Madras in 1905. He identified the characteristic Donovan bodies, measuring

1.5 × 0.7 μm, in macrophages and the stratum malpighii. The organism was not reproducibly cultured until the mid-1990s, when its isolation in peripheral-blood monocytes and human epithelial cell lines was reported.

■ ■EPIDEMIOLOGY Donovanosis has an unusual geographic distribution that has included Papua New Guinea, parts of southern Africa, India, the Caribbean, French Guyana, Brazil, and Aboriginal communities in Australia. In Australia, donovanosis has been almost entirely eliminated through a sustained program backed by strong political commitment and resources at the primary health care level. In South Africa, donova­ nosis is also very close to elimination. Although few cases are now reported in the United States, donovanosis was once prevalent in this country, with 5000–10,000 cases recorded in 1947. The largest epi­ demic recorded was in Dutch South Guinea, where 10,000 cases were identified in a population of 15,000 (the Marind-anim) between 1922 and 1952. Donovanosis is associated with poor hygiene and is more common in lower socioeconomic groups than in those who are better off and in men than in women. Infection in sexual partners of index cases occurs to a limited extent. Donovanosis is a risk factor for HIV infection (Chap. 208). Globally, the incidence of donovanosis has decreased significantly in recent times. This decline probably reflects a greater focus on effective management of genital ulcers because of their role in facilitating HIV transmission. ■ ■CLINICAL FEATURES A lesion starts as a papule or subcutaneous nodule that later ulcerates after trauma. The incubation period is uncertain, but experimental infections in humans indicate a duration of ~50 days. Four types of lesions have been described: (1) the classic ulcerogranulomatous lesion (Fig. 178-1), a beefy red ulcer that bleeds readily when touched; (2) a hypertrophic or verrucous ulcer with a raised irregular edge; (3) a necrotic, offensive-smelling ulcer causing tissue destruction; and (4) a sclerotic or cicatricial lesion with fibrous and scar tissue. The genitals are affected in 90% of patients and the inguinal region in 10%. The most common sites of infection are the prepuce, coronal sulcus, frenum, and glans in men and the labia minora and fourchette in women. Cervical lesions may mimic cervical carcinoma. In men, lesions are associated with lack of circumcision. Lymphadenitis is uncommon. Extragenital lesions occur in 6% of cases and may involve the lip, gums, cheek, palate, pharynx, larynx, and chest. Hematogenous spread with involvement of liver and bone has been reported. During pregnancy, lesions tend to develop more quickly and respond more slowly to treatment. Polyarthritis and osteomyelitis are rare complica­ tions. In newborn infants, donovanosis may present with ear infection. Cases in children have been attributed to sitting on the laps of infected adults. As the incidence of donovanosis has decreased, the number of unusual case reports has appeared to be increasing. Complications include neoplastic changes, pseudoelephantiasis, and stenosis of the urethra, vagina, or anus. FIGURE 178-1  Ulcerogranulomatous penile lesion of donovanosis, with some hypertrophic features.

FIGURE 178-2  Pund cell stained by rapid Giemsa (RapiDiff) technique. Numerous Donovan bodies are visible. CHAPTER 178 ■ ■DIAGNOSIS A clinical diagnosis of donovanosis made by an experienced practitio­ ner on the basis of the lesion’s appearance usually has a high positive predictive value. The diagnosis is confirmed by microscopic identifi­ cation of Donovan bodies (Fig. 178-2) in tissue smears. Preparation of a good-quality smear is important. If donovanosis is suspected on clinical grounds, the smear for Donovan bodies should be taken before swab samples are collected to be tested for other causes of genital ulcer­ ation so that enough material can be collected from the ulcer. A swab should be rolled firmly over an ulcer previously cleaned with a dry swab to remove debris. Smears can be examined in a clinical setting by direct microscopy with a rapid Giemsa or Wright’s stain. Alternatively, a piece of granulation tissue crushed and spread between two slides can be used. Donovan bodies can be seen in large, mononuclear (Pund) cells as gram-negative intracytoplasmic cysts filled with deeply staining bodies that may have a safety-pin appearance. These cysts eventually rupture and release the infective organisms. Histologic changes include chronic inflammation with infiltration of plasma cells and neutrophils. Epithelial changes include ulceration, microabscesses, and elongation of rete ridges. Donovanosis A diagnostic polymerase chain reaction (PCR) test was based on the observation that two unique base changes in the phoE gene eliminate Hae111 restriction sites, enabling differentiation of K. granulomatis comb nov from related Klebsiella species. PCR analysis with a colori­ metric detection system can now be used in routine diagnostic labo­ ratories. A genital ulcer multiplex PCR that includes K. granulomatis has been developed. Serologic tests are only poorly specific and are not currently used. The differential diagnosis of donovanosis includes primary syphilitic chancres, secondary syphilis (condylomata lata), chancroid, lympho­ granuloma venereum, genital herpes, neoplasm, and amebiasis. Mixed infections are common. Histologic appearances should be distinguished from those of rhinoscleroma, leishmaniasis, and histoplasmosis. TREATMENT Donovanosis Many patients with donovanosis present quite late with extensive ulceration. They may be embarrassed and have low self-esteem related to their disease. Reassurance that they have a treatable

62 - SECTION 7 Miscellaneous Bacterial Infections

SECTION 7 Miscellaneous Bacterial Infections

TABLE 178-1  Effective Antibiotics for the Treatment of Donovanosis ANTIBIOTIC ORAL DOSE Azithromycin 1 g on day 1, then 500 mg daily for 7 days

or 1 g weekly for 4 weeks Trimethoprim-sulfamethoxazole 960 mg bid for 14 days Doxycycline 100 mg bid for 14 days Erythromycin 500 mg qid for 14 days (in pregnant women) Tetracycline 500 mg qid for 14 days condition is important, as are the administration of antibiotics and the monitoring of patients for an adequate interval (see below). Epidemiologic treatment of sexual partners and advice about how to improve genital hygiene are recommended. The recommended drug regimens for donovanosis are shown in Table 178-1. Gentamicin can be added if the response is slow. Ceftriaxone, chloramphenicol, and norfloxacin also are effective. Patients treated for 14 days should be monitored until lesions have healed completely. Those treated with azithromycin probably do not need such rigorous follow-up. Surgery may be indicated for very advanced lesions. ■ ■CONTROL AND PREVENTION Donovanosis is probably the cause of genital ulceration that is most readily recognizable clinically. Donovanosis is now limited to a few specific locations, and its global eradication is a distinct possibility. ■ ■FURTHER READING Muller EE, Kularatne R: The changing epidemiology of genital PART 5 Infectious Diseases ulcer disease in South Africa: Has donovanosis been eliminated? Sex Transm Infect 96:596, 2020. O’Farrell N: Donovanosis, in Sexually Transmitted Diseases, 4th ed. KK Holmes et al (eds). McGraw-Hill, 2008, pp 701–708. Rajam RV, Rangiah PN: Donovanosis (granuloma inguinale, granuloma venereum). Monogr Ser World Health Organ 24:1, 1954. Sehgal VN, Prasad AL: Donovanosis. Current concepts. Int J Dermatol 5:8, 1986. Section 7 Miscellaneous Bacterial Infections Michael S. Abers, Gregory A. Filice

Nocardiosis ■ ■INTRODUCTION Bacteria of the genus Nocardia are saprophytic filamentous aerobes ubiquitous in soil and water worldwide. In the past, the majority of isolates associated with pneumonia and systemic disease were iden­ tified biochemically as Nocardia asteroides, but the development of genome sequencing has demonstrated that at least 53 of the more than 100 species of Nocardia are associated with human disease. Most cases of systemic nocardiosis are caused by N. farcinica, N. cyriacigeorgica,

N. nova, N. abscessus, N. otitidiscaviarum, N. transvalensis, N. brasilien­ sis, N. pseudobrasiliensis, N. paucivorans, or N. brevicatena (Fig. 179-1). N. brasiliensis is usually associated with disease limited to the skin. Nocardiosis is usually an opportunistic infection, occurring primarily in individuals with impairments in host defenses. Infections follow­ ing local inoculation and pulmonary or systemic disease have distinct pathogenesis, microbiology, and management.

■ ■MICROBIOLOGY Nocardiae are Gram-positive, weakly acid-fast, and catalase positive. Like other members of the Mycobacteriales order, the cell walls of nocardiae contain mycolic acids (45–65 carbon atoms) and trehalose. Nocardia tend to clump together when grown in liquid media. On solid agar, Nocardia species typically form chalky, wrinkled colonies with a whitish-yellow or orange-brown hue. Growth is optimal at 37°C but can occur at temperatures up to 45°C. ■ ■EPIDEMIOLOGY AND RISK FACTORS Nocardiae are ubiquitous environmental saprophytes found in soil, water, and decaying organic matter worldwide. Humans are frequently exposed via inhalation or direct inoculation of the skin or an eye. Most immunocompetent individuals readily clear nocardiae without devel­ oping clinical disease. Certain impairments in host defenses predispose to invasive infection after inhalation. Primary cutaneous infection usually remains local. Nearly all cases are sporadic, but outbreaks have been reported in nosocomial settings among immunocompromised patients and in immunocompetent individuals related to surgical pro­ cedures or intravenous drug use. Person-to-person spread is not well documented. There is no known seasonality. The incidence of nocardiosis in the general population, estimated on three continents (North America, Europe, and Australia), is approxi­ mately ~0.375 cases per 100,000 persons per year. Nocardia infections are more common among adults than among children and more common among males than females. In contrast to the other forms of nocardiosis that occur worldwide, cases of actinomycetoma have been reported mostly in tropical and subtropical regions, especially in Mexico, Sudan, and India. The most important risk factors are lower socioeconomic status and frequent contact with soil or vegetable matter. Many cases are in laborers. Most cases of systemic nocardiosis occur in patients with host defense defects, including cell-mediated immunity and specific phago­ cyte defects. Those with nocardiosis typically possess one or more of the following risk factors: solid organ transplantation, hematopoietic stem cell transplantation, systemic corticosteroid use or Cushing syndrome, immunosuppressive therapy, or HIV infection. Among transplant recipients, risk factors for nocardiosis include corticoste­ roid dose, recent augmentation of immunosuppression for rejection or graft-versus-host disease, elevated calcineurin inhibitor levels, and patient age. Rare but well-described syndromes associated with nocar­ diosis include pulmonary alveolar proteinosis (PAP), neutralizing autoantibodies to granulocyte-macrophage colony-stimulating factor (GM-CSF), chronic granulomatous disease (CGD), and interleukin 12 (IL-12) or IL-12R deficiency. In the absence of any major risk factor for Nocardia infection, children and adults with nocardiosis should be tested for CGD. ■ ■PATHOGENESIS Pulmonary and systemic nocardiosis both occur following inhalation of fragmented bacterial mycelia. In healthy individuals, a variety of host defense mechanisms, including both innate and adaptive immune responses, help control and eliminate nocardiae from the respiratory tract without causing clinical disease. The importance of neutrophils and macrophages in antinocardial host defense is suggested by the fre­ quency of nocardiosis in patients with CGD. Nocardiae have evolved a number of properties that enable survival within phagocytes, including neutralization of oxidants, prevention of phagosome–lysosome fusion, and prevention of phagosome acidification. Neutrophils phagocytose nocardiae and limit their growth but do not kill them efficiently. Neutralizing autoantibodies against GM-CSF have been found in the majority of patients with autoimmune PAP and appear to be central to the pathogenesis of this disease. Nocardiae stimulate the production of GM-CSF in phagocytes in vitro, and extrapulmonary nocardiosis has been observed in several patients with autoantibodies to GM-CSF, most of whom had not had pulmonary alveolar proteinosis. The rela­ tionships between pulmonary alveolar proteinosis, nocardiosis, and antibodies to GM-CSF remain incompletely defined.

63 - 179 Nocardiosis

179 Nocardiosis

TABLE 178-1  Effective Antibiotics for the Treatment of Donovanosis ANTIBIOTIC ORAL DOSE Azithromycin 1 g on day 1, then 500 mg daily for 7 days

or 1 g weekly for 4 weeks Trimethoprim-sulfamethoxazole 960 mg bid for 14 days Doxycycline 100 mg bid for 14 days Erythromycin 500 mg qid for 14 days (in pregnant women) Tetracycline 500 mg qid for 14 days condition is important, as are the administration of antibiotics and the monitoring of patients for an adequate interval (see below). Epidemiologic treatment of sexual partners and advice about how to improve genital hygiene are recommended. The recommended drug regimens for donovanosis are shown in Table 178-1. Gentamicin can be added if the response is slow. Ceftriaxone, chloramphenicol, and norfloxacin also are effective. Patients treated for 14 days should be monitored until lesions have healed completely. Those treated with azithromycin probably do not need such rigorous follow-up. Surgery may be indicated for very advanced lesions. ■ ■CONTROL AND PREVENTION Donovanosis is probably the cause of genital ulceration that is most readily recognizable clinically. Donovanosis is now limited to a few specific locations, and its global eradication is a distinct possibility. ■ ■FURTHER READING Muller EE, Kularatne R: The changing epidemiology of genital PART 5 Infectious Diseases ulcer disease in South Africa: Has donovanosis been eliminated? Sex Transm Infect 96:596, 2020. O’Farrell N: Donovanosis, in Sexually Transmitted Diseases, 4th ed. KK Holmes et al (eds). McGraw-Hill, 2008, pp 701–708. Rajam RV, Rangiah PN: Donovanosis (granuloma inguinale, granuloma venereum). Monogr Ser World Health Organ 24:1, 1954. Sehgal VN, Prasad AL: Donovanosis. Current concepts. Int J Dermatol 5:8, 1986. Section 7 Miscellaneous Bacterial Infections Michael S. Abers, Gregory A. Filice

Nocardiosis ■ ■INTRODUCTION Bacteria of the genus Nocardia are saprophytic filamentous aerobes ubiquitous in soil and water worldwide. In the past, the majority of isolates associated with pneumonia and systemic disease were iden­ tified biochemically as Nocardia asteroides, but the development of genome sequencing has demonstrated that at least 53 of the more than 100 species of Nocardia are associated with human disease. Most cases of systemic nocardiosis are caused by N. farcinica, N. cyriacigeorgica,

N. nova, N. abscessus, N. otitidiscaviarum, N. transvalensis, N. brasilien­ sis, N. pseudobrasiliensis, N. paucivorans, or N. brevicatena (Fig. 179-1). N. brasiliensis is usually associated with disease limited to the skin. Nocardiosis is usually an opportunistic infection, occurring primarily in individuals with impairments in host defenses. Infections follow­ ing local inoculation and pulmonary or systemic disease have distinct pathogenesis, microbiology, and management.

■ ■MICROBIOLOGY Nocardiae are Gram-positive, weakly acid-fast, and catalase positive. Like other members of the Mycobacteriales order, the cell walls of nocardiae contain mycolic acids (45–65 carbon atoms) and trehalose. Nocardia tend to clump together when grown in liquid media. On solid agar, Nocardia species typically form chalky, wrinkled colonies with a whitish-yellow or orange-brown hue. Growth is optimal at 37°C but can occur at temperatures up to 45°C. ■ ■EPIDEMIOLOGY AND RISK FACTORS Nocardiae are ubiquitous environmental saprophytes found in soil, water, and decaying organic matter worldwide. Humans are frequently exposed via inhalation or direct inoculation of the skin or an eye. Most immunocompetent individuals readily clear nocardiae without devel­ oping clinical disease. Certain impairments in host defenses predispose to invasive infection after inhalation. Primary cutaneous infection usually remains local. Nearly all cases are sporadic, but outbreaks have been reported in nosocomial settings among immunocompromised patients and in immunocompetent individuals related to surgical pro­ cedures or intravenous drug use. Person-to-person spread is not well documented. There is no known seasonality. The incidence of nocardiosis in the general population, estimated on three continents (North America, Europe, and Australia), is approxi­ mately ~0.375 cases per 100,000 persons per year. Nocardia infections are more common among adults than among children and more common among males than females. In contrast to the other forms of nocardiosis that occur worldwide, cases of actinomycetoma have been reported mostly in tropical and subtropical regions, especially in Mexico, Sudan, and India. The most important risk factors are lower socioeconomic status and frequent contact with soil or vegetable matter. Many cases are in laborers. Most cases of systemic nocardiosis occur in patients with host defense defects, including cell-mediated immunity and specific phago­ cyte defects. Those with nocardiosis typically possess one or more of the following risk factors: solid organ transplantation, hematopoietic stem cell transplantation, systemic corticosteroid use or Cushing syndrome, immunosuppressive therapy, or HIV infection. Among transplant recipients, risk factors for nocardiosis include corticoste­ roid dose, recent augmentation of immunosuppression for rejection or graft-versus-host disease, elevated calcineurin inhibitor levels, and patient age. Rare but well-described syndromes associated with nocar­ diosis include pulmonary alveolar proteinosis (PAP), neutralizing autoantibodies to granulocyte-macrophage colony-stimulating factor (GM-CSF), chronic granulomatous disease (CGD), and interleukin 12 (IL-12) or IL-12R deficiency. In the absence of any major risk factor for Nocardia infection, children and adults with nocardiosis should be tested for CGD. ■ ■PATHOGENESIS Pulmonary and systemic nocardiosis both occur following inhalation of fragmented bacterial mycelia. In healthy individuals, a variety of host defense mechanisms, including both innate and adaptive immune responses, help control and eliminate nocardiae from the respiratory tract without causing clinical disease. The importance of neutrophils and macrophages in antinocardial host defense is suggested by the fre­ quency of nocardiosis in patients with CGD. Nocardiae have evolved a number of properties that enable survival within phagocytes, including neutralization of oxidants, prevention of phagosome–lysosome fusion, and prevention of phagosome acidification. Neutrophils phagocytose nocardiae and limit their growth but do not kill them efficiently. Neutralizing autoantibodies against GM-CSF have been found in the majority of patients with autoimmune PAP and appear to be central to the pathogenesis of this disease. Nocardiae stimulate the production of GM-CSF in phagocytes in vitro, and extrapulmonary nocardiosis has been observed in several patients with autoantibodies to GM-CSF, most of whom had not had pulmonary alveolar proteinosis. The rela­ tionships between pulmonary alveolar proteinosis, nocardiosis, and antibodies to GM-CSF remain incompletely defined.

Amox-clav TMP-SMX Linezolid N. farcinica N. cyriacigeorgica N. nova complexa N. abscessus complexb N. beijingensis N. brasiliensis N. brevicatena/paucivorans N. otitidiscaviarum N. transvalensis complexc N. pseudobrasiliensis aIncludes N. nova, N. veterana, N. africana, N. elegans, N. aobensis, N. kruczakiae bIncludes N. abscessus, N. arthitidis, N. asiatica, N. pneumoniae. Note: N. beijingensis is a member of N. abscessus complex, but is listed separately cIncludes N. transvelensis, N. wallacei, N. blacklockiae FIGURE 179-1.  Nocardia species most commonly associated with human disease and their in vitro susceptibility patterns. TMP-SMX: trimethoprim-sulfamethoxazole. (Adapted from multiple sources.) Primary cutaneous nocardiosis typically follows penetrating injury of the skin by an object contaminated with soil or vegetable matter. Thorn-associated injuries, such as those associated with gardening, are a particularly common source of infection. Nocardiae may spread to draining lymph nodes, but dissemination to anatomically distant sites is uncommon. The ocular epithelium serves as a barrier that excludes nocardiae from the deeper tissues. Traumatic injury of the corneal epithelium provides a portal of entry for nocardiae to enter the ocular tissues, resulting in keratitis. The infectious process is limited to the superficial tissues. ■ ■PATHOLOGY The characteristic histologic feature of nocardiosis is an abscess with extensive infiltration of neutrophils and prominent necrosis. Granula­ tion tissue may surround the lesion, but extensive fibrosis and encap­ sulation are uncommon. Pyogranulomatous inflammation occurs in a minority of cases. In contrast, actinomycetoma is characterized by granulomatous inflammation and the formation of sinus tracts. ■ ■CLINICAL MANIFESTATIONS Pleuropulmonary Disease  The onset of pulmonary nocardiosis is typically subacute, with symptoms evolving over days to weeks prior to presentation. Among severely immunocompromised patients, the onset may be more acute. Cough is prominent and produces small amounts of thick, purulent sputum that is not malodorous. Fever, anorexia, and malaise are common; weight loss, dyspnea, pleuritic chest pain, and hemoptysis are uncommon. Without definitive treat­ ment, patients frequently experience remissions and exacerbations over a period of weeks to months. Typical radiographic patterns include infiltrates, nodules, and masses in one or more lobes (Figs. 179-2 and 179-3). Nodules fre­ quently cavitate (Fig. 179-3). Pleural effusion or empyema occurs in about one-third of cases. In some cases, empyema may occur in the absence of lung involvement. Extrapulmonary Disease  Dissemination to anatomically distant sites occurs in up to one-third of patients with pneumonia. While blood cultures are frequently negative, nocardiae likely reach meta­ static sites of infection via hematogenous spread. Virtually any organ can be involved, but the central nervous system (CNS) and skin and subcutaneous tissues are the most common sites.

Clarithromycin Ciprofloxacin Moxifloxacin Doxycycline Minocycline Tobramycin Ceftriaxone Imipenem Amikacin

90% of isolates are susceptible 50–90% of isolates are susceptible 15–50% of isolates are susceptible <15% of isolates are susceptible CHAPTER 179 The CNS is involved in up to two-thirds of cases of disseminated nocardiosis. The vast majority of patients present with one or more brain abscesses. One-third of patients have no symptoms of CNS involvement, and one-fourth of patients do not have apparent pulmo­ nary disease at presentation. Brain imaging demonstrates one or more ring-enhancing lesions that characteristically have a multilobulated appearance (Fig. 179-4). Multiple abscesses are seen in two-thirds of patients. Many abscesses can be detected with contrast-enhanced computed tomography (CT), but magnetic resonance imaging (MRI) with intravenous gadolinium is more sensitive. N. farcinica appears to have a special predilection for CNS involvement. Meningitis is uncommon and is usually due to spread from a nearby brain abscess. Nocardiosis FIGURE 179-2.  Nocardia pneumonia. A dense infiltrate with a possible cavity and several nodules are apparent in the right lung.

FIGURE 179-3.  Pulmonary nocardiosis. A computed tomography scan shows solid nodules (yellow arrows) and a cavitated nodule (yellow arrowhead) in the right lung as well as a right-sided pleural effusion and an abscess of the right chest wall (red asterisk). Bronchiectasis, which was unrelated to nocardiosis, is present in the left lung. A neutrophilic pleocytosis is typically seen in the cerebrospinal fluid, but cultures are rarely positive. Involvement of the skin and subcutaneous tissues occurs in onefifth of patients with disseminated disease. Typical manifestations include subcutaneous abscesses, papules, nodules, and ulcers. Skin abscesses involve adjacent muscle in a minority of cases. Abscesses involving virtually every organ have been described with the most common sites being kidney, adrenal gland, muscle, bone, joint, eye, lymph node, and thyroid gland. Endocarditis has been reported and can affect native or prosthetic valves. Central venous catheter infec­ tions have been reported. Peritonitis has been reported in patients undergoing peritoneal dialysis. PART 5 Infectious Diseases Primary Cutaneous Nocardiosis  Primary cutaneous nocardio­ sis follows transcutaneous nocardial inoculation and takes one of three forms: cellulitis, lymphocutaneous syndrome, or actinomycetoma. Cellulitis begins 1–3 weeks after a recognized breach of the skin, often with soil contamination. Pain, swelling, erythema, and warmth develop over days to weeks. The lesions are usually firm and not fluc­ tuant. Depending on the nature of the inoculation injury, disease may spread to nearby muscles, tendons, bones, and joints. Dissemination is rare. Lymphocutaneous disease usually begins as a pyodermatous nodule at the site of inoculation, followed by central ulceration and purulent or honey-colored drainage. Subcutaneous nodules often appear along lymphatics that drain the primary lesion. Most cases follow inoculation of a limb, but cases involving the face have been reported, especially in children. Most cases of nocardial lymphocutaneous syndrome FIGURE 179-4.  Multiloculated Nocardia brain abscess in the left cerebellum.

are associated with N. brasiliensis. Similar disease occurs with other pathogens, most notably Sporothrix schenckii (Chap. 225) and Myco­ bacterium marinum (Chap. 185). Mycetoma is an indolent, slowly progressive infection of the skin and subcutaneous tissues with nodular swellings and draining sinuses. Actinomycetoma refers to cases of mycetoma caused by filamentous bacteria. Nearly all cases of Nocardia-associated actinomycetoma are caused by N. brasiliensis. Clinical manifestations usually begin with a nodular swelling, sometimes at a site of local trauma. Lesions (Fig. 179-5) typically develop on the feet or lower legs but may involve the hands, the posterior part of the neck, the upper back, the head, and other sites. The nodule eventually breaks down, and a fistula appears, typically followed by others. The fistulae tend to come and go, with new ones forming as old ones disappear. The discharge is serous or purulent, may be bloody, and often contains 0.1­ to 2-mm white granules consist­ ing of masses of mycelia (Fig. 179-5). The lesions spread slowly along fascial planes to involve adjacent areas of skin, subcutaneous tissue, and bone. Over months or years, there may be extensive deformation of the affected part. Lesions involving soft tissues are only mildly pain­ ful; those affecting bones or joints are more so (Fig. 179-5). Systemic symptoms are absent or minimal, but mycetoma cases are often associ­ ated with prolonged, severe disability. Infection rarely disseminates from actinomycetoma, but lesions on the head, neck, and trunk can invade locally to involve deep organs. The World Health Organization has designated mycetoma a neglected tropical disease. Primary Ocular Disease  Primary ocular nocardiosis includes keratitis, scleritis, and exogenous endophthalmitis. These infections typically occur in otherwise healthy individuals. Patients may report recent exposure to soil. Ocular trauma, surgery, and the use of contact lenses are common risk factors. The onset of disease is typically sub­ acute. Signs and symptoms alone cannot distinguish nocardiosis from other ocular diseases. Spread beyond the eye does not occur. ■ ■DIAGNOSIS The first step in diagnosis is examination of sputum or pus for branch­ ing, beaded, Gram-positive filaments 1 μm wide and up to 50 μm long (Fig. 179-6). These filaments tend to fragment; this may produce apparent cocci and coccobacilli forms. Most nocardiae are acid-fast in direct smears if a weak acid is used for decolorization (e.g., in the mod­ ified Kinyoun, Ziehl-Neelsen, and Fite-Faraco methods). The organ­ isms often take up silver stains. Recovery from specimens containing mixed flora can be improved with selective media (colistin–nalidixic acid agar, modified Thayer-Martin agar, or buffered charcoal–yeast extract agar). Nocardiae grow well on most fungal and mycobacterial media, but procedures used for decontamination of specimens for mycobacterial culture can kill nocardiae and should not be used when nocardiosis is on the differential diagnosis. Nocardiae grow relatively slowly; colonies may take up to 2 weeks to appear and may not develop their characteristic appearance—white, yellow, or orange, with aerial hyphae—for up to 4 weeks. When the diagnosis of nocardiosis is being considered, clinicians should inform the microbiology laboratory so that cultures can be incubated for pro­ longed periods of time. Sputum smears are frequently negative in patients with pulmonary nocardiosis. In such cases, bronchoalveolar lavage fluid or a lung biopsy specimen should be obtained. Isolation of Nocardia from a respiratory specimen does not always indicate pulmonary infection. Nocardia may colonize the respiratory tract of patients with underly­ ing bronchiectasis or other structural lung diseases. In such cases, isolation of Nocardia from a respiratory sample must be considered in the greater clinical context. When nocardiae are present in respira­ tory specimens from patients with intact host defenses, pulmonary nocardiosis should be diagnosed only when clinical and radiographic features are supportive. Nocardiosis is less likely if Gram-stained specimens are negative and cultures are not consistently positive. In contrast, a positive culture in an immunosuppressed patient usually reflects clinical disease, and empirical antimicrobial therapy should be started.

A B C D FIGURE 179-5.  Nocardia brasiliensis mycetoma. A. Draining sinuses and giant white grains with a seropurulent discharge. B. Radiography of the foot showing marked soft tissue enlargement and bony lytic lesions. C. Direct microscopy of grains stained with Lugol’s iodine (×40). D. Periodic acid–Schiff stain of skin biopsy (×40). (Images provided by Roberto Arenas and Mahreen Ameen, St. John’s Institute of Dermatology, Guy’s & St Thomas’ NHS Trust, London, UK. Reprinted from R Arenas, M Ameen: Lancet Infect Dis 10:66, 2010, with permission from Elsevier.) Occasionally, Nocardia isolated from blood cultures may represent contamination in individuals without risk factors for nocardiosis, par­ ticularly those who lack clinical manifestations suggestive of nocardial disease. Actinomycetoma, eumycetoma (cases involving fungi; Chap. 225), and botryomycosis (cases involving cocci or bacilli, often Staphylo­ coccus aureus) are difficult to distinguish clinically but are readily distinguished with microbiologic testing or biopsy. Granules should be sought in any discharge. Suspect particles should be washed in saline, examined microscopically, and cultured. Granules in actinomycetoma are usually white, pale yellow, pink, or red. They consist of tight masses of fine filaments (0.5–1 μm wide) radiating outward from a central core (Fig. 179-5). Granules in eumycetoma cases are white, yellow, FIGURE 179-6.  Gram-stained brain biopsy specimen from a patient with CNS nocardiosis. (Reproduced with permission from N Hauser et al: An immunocompromised woman with a brain lesion. Am J Med 133:e516-e517.)

CHAPTER 179 Nocardiosis brown, black, or green; under the microscope, they appear as masses of broader filaments (2–5 μm wide) encased in a matrix. Granules of botryomycosis consist of loose masses of cocci or bacilli. Organisms can also be seen in wound discharge or histologic specimens. Culture is the most reliable method for determining the causative organism in cases of mycetoma. When possible, isolates should be speciated at least to a complex level, by either matrix-assisted laser desorption/ionization–time of flight mass spectrometry (commonly referred to as MALDI-TOF) or genomic sequencing (typically 16S rRNA, secA1, hsp65, or gyrB). When possible, antimicrobial susceptibility testing should be performed, ideally with a Clinical Laboratory Standards Institute–approved broth dilution test. E-tests are less definitive. Nocardiae grow more slowly than most clinically important bacteria, and they tend to clump in sus­ pension so that susceptibility test endpoints are difficult to interpret. Thus, experience is required for interpretation of susceptibility testing results. ■ ■EVALUATION OF PATIENTS WITH ESTABLISHED NOCARDIOSIS Timely identification of metastatic sites of infection is a critical com­ ponent of managing patients with nocardiosis. Disseminated nocar­ diosis is exceedingly uncommon in patients with primary cutaneous or ocular disease that develops following traumatic injury. Patients with other forms of nocardiosis, regardless of symptoms, should undergo contrast-enhanced imaging of the CNS to identify undiagnosed brain abscesses. MRI is the preferred imaging modality, with CT reserved for scenarios when MRI cannot be performed. The presence of a previously undiagnosed immunologic defect should be considered in patients with disseminated nocardiosis who lack typical risk factors for infection. Such patients should undergo testing for HIV infection. Further testing for underlying immuno­ logic disorders should be guided by a careful history and physical examination.

TABLE 179-1  Treatment Duration for Nocardiosis DISEASE DURATION Pulmonary or systemic     Intact host defenses 6–12 months   Deficient host defenses 12 monthsa   CNS disease 12 monthsb Primary cutaneous disease (cellulitis, lymphocutaneous disease) 2 months Osteomyelitis, arthritis, laryngitis, sinusitis 4 months Actinomycetoma 6–12 months after clinical cure Keratitis Topical: until clinical cure   Systemic: until 2–4 months after clinical cure aIn some patients with AIDS and CD4+ T lymphocyte counts of <200/μL or with chronic granulomatous disease, therapy for pulmonary or systemic disease may be continued indefinitely. bIf all apparent central nervous system (CNS) disease has been excised, the duration of therapy may be reduced to 6 months. TREATMENT Nocardiosis Trimethoprim-sulfamethoxazole (TMP-SMX) is the drug of choice for most cases of nocardiosis (Fig. 179-1 and Table 179-1). At the outset, 10–20 mg/kg of TMP and 50–100 mg/kg of SMX are given each day in two divided doses. Later, daily doses can be reduced to as little as 5 mg/kg and 25 mg/kg, respectively. In persons with sulfonamide allergies, desensitization usually allows continuation of therapy with these effective and inexpensive drugs. PART 5 Infectious Diseases Linezolid is highly bioavailable and uniformly active against all Nocardia species. Treatment-limiting toxicity frequently develops after 2–3 weeks of standard-dose therapy. Tedizolid is associated with less toxicity, but clinical experience is limited. Amikacin is active against nearly all Nocardia species with the notable exceptions of N. transvalensis and some N. pseudobrasiliensis isolates. Doses of 5–7.5 mg/kg every 12 h or 15 mg/kg every 24 h are typically used. Serum drug levels should be monitored during prolonged therapy, especially in patients with diminished renal function and in the elderly. Among the β-lactams, ceftriaxone and imipenem have activ­ ity against many Nocardia species and are frequently used when combination therapy is indicated. Ceftriaxone is not active against N. farcinica, which is among the most common species isolated in nocardiosis. In patients with CNS involvement, meropenem is preferred to imipenem as the latter is associated with a higher risk of seizures. Amoxicillin (875 mg) combined with clavulanate (125 mg), given twice a day, has been effective in treating N. brasiliensis and some cases of N. farcinica. Among the quinolones, moxifloxacin appears to be most active. Minocycline (100–200 mg twice daily) is often effective; other tetra­ cyclines are usually less effective. Tigecycline appears to be active in vitro against some species, but clinical experience is limited. Empirical treatment of patients with severe disease should con­ sist of combination therapy with two or three of the following agents: TMP-SMX, linezolid, amikacin, and imipenem (or merope­ nem if the CNS is involved). Clinical improvement is usually noticeable after 1–2 weeks of therapy but may take longer. After definite clinical improvement, therapy can be continued with a single oral drug, usually TMP-SMX. Some experts use two or more drugs for the entire course of therapy, but whether multiple drugs are better than a single agent is not known, and additional drugs increase the risk of toxicity. Surgical management of nocardial disease is similar to that of other bacterial diseases. Brain abscesses should be aspirated, drained, or excised if the diagnosis is unclear, if an abscess is large,

or if an abscess fails to respond to antimicrobial therapy. Surgi­ cal options include needle aspiration and open drainage/excision. Needle aspiration is less invasive, but patients frequently require multiple aspirations. Small or inaccessible brain abscesses can be treated medically. Brain imaging should be repeated to document the resolution of lesions, although radiographic improvement typi­ cally lags behind clinical improvement. With appropriate treatment, the mortality rate for localized pulmonary nocardiosis is <10%. Disseminated nocardiosis car­ ries a higher mortality rate, especially among patients with CNS involvement. Primary cutaneous and ocular nocardiosis often respond to anti­ microbial therapy. Occasionally, surgical management is required for patients with actinomycetoma or scleritis that fails to response to antimicrobial therapy. The mortality rate for actinomycetoma is exceedingly low, but patients suffer often from substantial mor­ bidity, disfigurement, disability, and/or stigmatization. Patients with nocardial keratitis should not receive adjunctive topical corticosteroids. ■ ■PREVENTION Use of TMP-SMX in high-risk populations to prevent Pneumocystis disease may reduce but does not eliminate the risk of nocardiosis. The incidence of nocardiosis is low enough that prophylaxis solely to prevent a first episode of Nocardia infection (i.e., primary pro­ phylaxis) is not recommended. The role of secondary prophylaxis to prevent recurrent nocardiosis has not been adequately studied. Some experts recommend secondary prophylaxis for patients who remain at high risk for Nocardia infection after the completion of antimicro­ bial therapy. ■ ■FURTHER READING Averbuch D et al: Nocardia infections in hematopoietic cell transplant recipients: A multicenter international retrospective study of the Infectious Diseases Working Party of the European Society for Blood and Marrow Transplantation. Clin Infect Dis 75:88, 2022. Corsini Campioli C et al: Clinical presentation, management, and outcomes of patients with brain abscess due to Nocardia species. Open Forum Infect Dis 8:ofab067, 2021. Hamdi AM et al: Retrospective analysis of antimicrobial susceptibility profiles of Nocardia species from a tertiary hospital and reference lab­ oratory, 2011 to 2017. Antimicrob Agents Chemother 64(3):e01868, 2020. Lebeaux D et al: Outcome and treatment of nocardiosis after solid organ transplantation: New insights from a European study. Clin Infect Dis 64:1396, 2017. Margalit I et al: How do I manage nocardiosis? Clin Microbiol Infect 27:550, 2021. Passerini M et al: Trimethoprim-sulfamethoxazole significantly reduces the risk of nocardiosis in solid organ transplant recipients: Systematic review and individual patient data meta-analysis. Clin Microbiol Infect 30:170, 2024. Restrepo A et al: Nocardia infections in solid organ transplantation: Guidelines from the Infectious Diseases Community of Practice of the American Society of Transplantation. Clin Transplant 33:e13509, 2019. Rosen LB et al: Nocardia-induced granulocyte macrophage colonystimulating factor is neutralized by autoantibodies in disseminated/ extrapulmonary nocardiosis. Clin Infect Dis 60:1017, 2015. Traxler RM et al: Updated review on Nocardia species: 2006–2021. Clin Microbiol Rev 35:e0002721, 2022. Wang H et al: Epidemiology and antimicrobial resistance profiles of the Nocardia species in China, 2009 to 2021. Microbiol Spectr 10:2,

Yetmar ZA et al: Mortality after nocardiosis: Risk factors and evalu­ ation of disseminated infection. Open Forum Infect Dis 10:ofad409, 2023.

64 - 180 Actinomycosis

180 Actinomycosis

Thomas A. Russo, John C. Hu

Actinomycosis Actinomycosis is uncommon, and most physicians’ personal experience with its clinical presentations is limited. Laboratory identification of the etiologic agents from the order Actinomycetales is not routine. Thus, actinomycosis remains a diagnostic challenge, even for a skilled clinician. However, this infection is usually curable with medical therapy alone. Therefore, an awareness of the full spectrum of clinical syndromes can expedite diagnosis and treatment and minimize unnecessary surgical interventions, morbidity, and mortality. Classical actinomycosis is an indolent, slowly progressive infection caused by anaerobic or microaerophilic bacteria, primarily of the genus Actinomyces, that colonize the mouth, colon, and vagina. Mucosal disruption may lead to infection at virtually any site in the body. In vivo growth of actinomycetes usually results in the formation of characteristic clumps called grains or sulfur granules. The clinical presentations of actinomycosis are myriad. Common in the preantibiotic era, actinomycosis has diminished in incidence, as has its timely recognition. Actinomycosis has been called the most misdiagnosed disease, and it has been said that no disease is so often missed by experienced diagnosticians. Three “classic” clinical presentations that should prompt consideration of this unique infection are (1) the combination of chronicity, progression across tissue boundaries, and mass-like features (mimicking malignancy, with which it is often confused); (2) the development of a sinus tract, which may spontaneously resolve and recur; and (3) a refractory or relapsing infection after a short course of therapy, since cure of established actinomycosis requires prolonged treatment. ■ ■ETIOLOGIC AGENTS Actinomycosis is most commonly caused by A. israelii, A. naeslundii, Schaalia (Actinomyces) odontolyticus, A. viscosus, Schaalia (Actinomyces) meyeri, A. graevenitzii, and A. gerencseriae. Infections due to Winkia (Actinomyces) neuii have been increasingly recognized. Most if not all actinomycotic infections are polymicrobial. Aggregatibacter (Actinobacillus) actinomycetemcomitans, Eikenella corrodens, Enterobacteriaceae, and species of Fusobacterium, Bacteroides, Capnocytophaga, Staphylococcus, and Streptococcus are commonly isolated with actinomycetes in various combinations, depending on the site of infection. Their contribution to the pathogenesis of actinomycosis is uncertain. Genome-based analysis and comparative 16S rRNA gene sequencing have led to the identification of an ever-expanding list of Actinomyces species and a reclassification of some species to other genera. In recent years, many prior Actinomyces species have been placed into new genera including Schaalia, Winkia, Gleimia, and Pauljensenia, though most publications have yet to adapt these new taxonomic changes. At present, 33 species remain in the Actinomyces genus with at least 26 species implicated as causes of human disease. Gleimia (Actinomyces) europaeus, A. radingae, Schaalia (Actinomyces) turicensis, Schaalia (Actinomyces) cardiffensis, A. urogenitalis, Pauljensenia (Actinomyces) hongkongensis, Schaalia (Actinomyces) georgiae, Schaalia (Actinomyces) massiliensis, A. timonensis, Schaalia (Actinomyces) funkei, Trueperella (Arcanobacterium) pyogenes, Trueperella (Arcanobacterium) bernardiae, and Propionibacterium propionicum are additional causes of human actinomycosis, albeit not always with a “classic” presentation. ■ ■EPIDEMIOLOGY Actinomycosis has no geographic boundaries and occurs throughout life, with a peak incidence in the middle decades. Males have a threefold higher incidence than females, possibly because of poorer dental hygiene and/or more frequent trauma. Improved dental hygiene and the initiation of antimicrobial treatment before actinomycosis fully develops have probably contributed to a decrease in incidence since the advent of antibiotics. Individuals who do not seek or have access to health care, those who have an intrauterine contraceptive device (IUD) in place for a prolonged period (see “Pelvic Disease,” below), and those

who receive bisphosphonate treatment (see “Oral–Cervicofacial Disease,” below) are probably at higher risk.

■ ■PATHOGENESIS AND PATHOLOGY The etiologic agents of actinomycosis are members of the normal oral flora and are often cultured from the bronchi, the gastrointestinal tract, and the female genital tract. The critical step in the development of actinomycosis is disruption of the mucosal barrier. Local infection may ensue. Once established, actinomycosis spreads contiguously in a slow, progressive manner, ignoring tissue planes. Although acute inflammation may initially develop at the infection site, the hallmark of actinomycosis is the characteristic chronic, indolent phase manifested by lesions that usually appear as single or multiple indurations. Central necrosis consisting of neutrophils and sulfur granules develops and is virtually diagnostic. The fibrotic walls of the mass are typically described as “wooden.” The responsible bacterial and/or host factors have not been identified. Over time, sinus tracts to the skin, adjacent organs, or bone may develop. In rare instances, distant hematogenous seeding may occur; lymphatic spread and associated lymphadenopathy are uncommon. As mentioned above, these unique features of actinomycosis mimic malignancy, with which it is often confused. Foreign bodies appear to facilitate infection. This association most frequently involves IUDs. Reports have described an association of actinomycosis with HIV infection; transplantation; common variable immunodeficiency; chronic granulomatous disease; treatment with anti–tumor necrosis factor α agents, glucocorticoids, or bisphosphonates; and radio- or chemotherapy. Actinomycosis after SARS-CoV-2 infection is reported but the association is not well-established. Ulcerative mucosal infections (e.g., by herpes simplex virus or cytomegalovirus) may facilitate disease development. CHAPTER 180 ■ ■CLINICAL MANIFESTATIONS Oral–Cervicofacial Disease  Actinomycosis occurs most frequently at an oral, cervical, or facial site, usually as a soft tissue swelling, abscess, mass, or ulcerative lesion that is often mistaken for a neoplasm. Dental diseases or procedures are common precipitating factors. The angle of the jaw is generally involved, but a diagnosis of actinomycosis should be considered with any mass lesion or relapsing infection in the head and neck. Radiation therapy and medicationrelated osteonecrosis of the jaw (MRONJ) due to antiresorptive therapy with bisphosphonates and anti–receptor activator of nuclear factor-κβ ligand (RANKL) such as denosumab, angiogenesis inhibitors, and tyrosine kinase inhibitors have all been recognized as contributing to an increasing incidence of actinomycotic infection of the mandible and maxilla (Fig. 180-1). Canaliculitis (commonly due to P. propionicum), Actinomycosis FIGURE 180-1  Bisphosphonate-associated maxillary osteomyelitis due to Actinomyces viscosus. A sulfur granule is seen within the bone. (Reprinted with permission from NH Naik, TA Russo: Bisphosphonate related osteonecrosis of the jaw: The role of Actinomyces. Clin Infect Dis 49:1729, 2009. © 2009 Oxford University Press.)

A FIGURE 180-2  Thoracic actinomycosis. A. A chest wall mass from extension of pulmonary infection. B. Pulmonary infection is complicated by empyema (open arrow) and extension to the chest wall (closed arrow). (Courtesy of Dr. C. B. Hsiao, Division of Infectious Diseases, Department of Medicine, State University of New York at Buffalo.) otitis, sinusitis, and laryngeal disease also can develop. Pain, fever, and leukocytosis are variably reported. Contiguous extension to the cra­ nium, cervical spine, or thorax is a potential sequela. Thoracic Disease  Thoracic actinomycosis, which may be facili­ tated by aspirated foreign material such as animal bones or teeth, usually follows an indolent progressive course, with involvement of the pulmonary parenchyma and/or the pleural space. Chest pain, fever, and weight loss are common. A cough, when present, is vari­ ably productive. The usual radiographic finding is either a mass lesion or pneumonia. On computed tomography (CT), central areas of low attenuation and ring-like rim enhancement may be seen; cavitary disease may develop. More than 50% of cases include pleural thicken­ ing, effusion, or empyema (Fig. 180-2). Rarely, pulmonary nodules or endobronchial lesions occur. Lesions suggestive of actinomycosis include those that cross fissures or pleura; extend into the mediasti­ num, contiguous bone, or chest wall (empyema necessitatis or empyema necessitans); or are associated with a sinus tract. In the absence of these findings, thoracic actinomycosis is usually mistaken for a neoplasm or pneumonia due to more usual causes. PART 5 Infectious Diseases Mediastinal infection is uncommon, usually arising from thoracic extension but rarely from perforation of the esophagus, trauma, or extension of head and neck or abdominal disease. The structures within the mediastinum and the heart can be involved in various com­ binations; consequently, the possible presentations are diverse. Primary endocarditis (in which W. neuii has been increasingly described), esophageal infection, and isolated disease of the breast occur. A B FIGURE 180-3  Hepatic–splenic actinomycosis. A. Computed tomogram showing multiple hepatic abscesses and a small splenic lesion due to Actinomyces israelii. Arrow indicates extension outside the liver. Inset: Gram stain of abscess fluid demonstrating beaded filamentous gram-positive rods. B. Subsequent formation of a sinus tract. (Reprinted with permission from M Saad: Actinomyces hepatic abscess with cutaneous fistula. N Engl J Med 353:e16, 2005. © 2005 Massachusetts Medical Society. All rights reserved.)

B Abdominal Disease  Abdominal actinomycosis poses a great diag­ nostic challenge. Months or years usually pass from the inciting event (e.g., appendicitis, diverticulitis, peptic ulcer disease, spillage of gall­ stones or bile during cholecystectomy, foreign-body perforation, bowel surgery, or ascension from IUD-associated pelvic disease) to clinical recognition. Because of the flow of peritoneal fluid and/or the direct extension of primary disease, virtually any abdominal organ, region, or space can be involved. The disease usually presents as an abscess, a mass, or a mixed lesion that is often fixed to underlying tissue and mistaken for a tumor. On CT, enhancement is most often heterogeneous and adjacent bowel is thickened. Sinus tracts to the abdominal wall, to the perianal region, or between the bowel and other organs may develop and mimic inflammatory bowel disease (Chap. 337). Recurrent disease or a wound or fistula that fails to heal suggests actinomycosis. Hepatic infection usually presents as one or more abscesses or masses (Fig. 180-3). Isolated disease presumably develops via hema­ togenous seeding from cryptic foci. Imaging and percutaneous tech­ niques have resulted in improved diagnosis and treatment. All levels of the urogenital tract can be infected. Renal disease usu­ ally presents as pyelonephritis and/or renal and perinephric abscess. Bladder involvement, usually due to extension of pelvic disease, may result in ureteral obstruction or fistulas to bowel, skin, or uterus. Acti­ nomyces can be detected in urine with appropriate stains and cultures. Pelvic Disease  Actinomycotic involvement of the pelvis occurs most commonly in association with an IUD but can also be associated with other foreign bodies, such as surgical mesh. When an IUD is in place or

FIGURE 180-4  Computed tomogram showing pelvic actinomycosis associated with an intrauterine contraceptive device. The device is encased by endometrial fibrosis (solid arrow); also visible are paraendometrial fibrosis (open triangular arrowhead) and an area of suppuration (open arrow). has been used but removed, pelvic symptoms should prompt consider­ ation of actinomycosis. The risk, although not quantified, appears small. The disease rarely develops when the IUD has been in place for <1 year, but the risk increases with time. Symptoms are typically indolent; fever, weight loss, abdominal pain, and abnormal vaginal bleeding or discharge are the most common. The earliest stage of disease—often endometritis— commonly progresses to pelvic masses or a tuboovarian abscess (Fig. 180-4). Unfortunately, because the diagnosis is often delayed, a “frozen pelvis” mimicking malignancy or endometriosis can develop by the time of recognition, which may lead to unnecessary surgery. Cancer antigen 125 levels may be elevated, further contributing to misdiagnosis. In contrast to malignancy and tuberculosis, pelvic actinomycosis only uncommonly includes ascites and lymphadenopathy. An endometrial biopsy may enable diagnosis in a minimally invasive fashion. Actinomyces-like organisms (ALOs), which are identified in Papa­ nicolaou-stained specimens in (on average) 7% of women using an IUD, have a low positive predictive value for diagnosis. The detection of ALOs in an asymptomatic patient warrants education and close follow-up but not removal of the IUD unless a suitable contraceptive alternative is agreed on. In the presence of symptoms that cannot be accounted for, it seems prudent to remove the IUD and—if advanced disease is excluded—to initiate a 14-day course of empirical treatment for possible early endometritis. Central Nervous System Disease  Actinomycosis of the central nervous system (CNS) is rare. Single or multiple brain abscesses are most common. Individuals with hereditary hemorrhagic telangiectasia are at increased risk for brain abscess with Actinomyces as the potential etiologic agent. An abscess usually appears on CT as a ring-enhancing lesion with a thick wall that may be irregular or nodular. Magnetic res­ onance perfusion and spectroscopy findings have also been described, as have primary meningitis, epidural or subdural space infection, and cavernous sinus syndrome. Musculoskeletal and Soft Tissue Infection  Actinomycotic infection of bones and joints is usually due to adjacent soft tissue infec­ tion but may be associated with trauma, injections, surgery (e.g., pros­ theses), osteoradionecrosis and bisphosphonate osteonecrosis (limited to mandibular and maxillary bones), or hematogenous spread. Because of slow disease progression, new bone formation and bone destruction can be seen concomitantly. Infection of soft tissue is uncommon and is usually a result of trauma. Actinomycetoma is a slowly progressive infection of the skin and subcutaneous tissue that is usually seen in warm climates. Despite the name being suggestive of Actinomyces as a causative agent, it is most commonly caused by Nocardia or Actino­ madura species (Chap. 179).

Disseminated Disease  Hematogenous dissemination of disease from any location rarely results in multiple-organ involvement. S. meyeri is most commonly involved. The lungs and liver are most often affected, with the presentation of multiple nodules mimicking disseminated malignancy. The clinical presentation may be surprisingly indolent given the extent of disease.

■ ■DIAGNOSIS The diagnosis of actinomycosis is rarely considered. All too often, actinomycosis is first mentioned by the pathologist after extensive surgery. Since medical therapy alone is frequently sufficient for cure, the challenge for the clinician is to consider the possibility of acti­ nomycosis, to diagnose it in the least invasive fashion, and to avoid unnecessary surgery. The clinical and radiographic presentations that suggest actinomycosis are discussed above. Of note, hypermetabolism has been demonstrated by 18F-fluorodeoxyglucose positron emission tomography (FDG-PET) in actinomycotic disease. Aspirations and biopsies (with or without CT or ultrasound guidance) are being used successfully to obtain clinical material for diagnosis, although surgery may be required. The microscopic identification of sulfur granules (an in vivo matrix of bacteria, calcium phosphate, and host material) in pus or tissues, which increases with the examination of additional histo­ pathologic sections and the use of positively charged slides to optimize adhesion, is the most common means of diagnosis. Occasionally, these granules are identified grossly from draining sinus tracts or pus. Peri­ odic acid–Schiff (PAS), Grocott methenamine silver (GMS), and Gram stains may be helpful to identify actinomycotic aggregates in surgical specimens. On hematoxylin-eosin stain, the granules may be eosino­ philic or variably surrounded by a radiating fringe of eosinophilic clubs called the Splendore-Hoeppli phenomenon (Fig. 180-5). Although sulfur granules are a defining characteristic of actinomycosis, granules also are found in mycetoma (Chaps. 179 and 225) and botryomycosis (a chronic suppurative bacterial infection of soft tissue or, in rare cases, visceral tissue that produces clumps of bacteria resembling granules). These entities can easily be differentiated from actinomycosis with appropriate histopathologic and microbiologic studies. Microbiologic identification of actinomycetes is often precluded by prior antimicro­ bial therapy or failure to perform appropriate microbiologic cultures. For optimal yield, the avoidance of even a single dose of antibiotics is mandatory. Although some species can grow aerobically, isolation is maximized under anaerobic conditions, usually requiring 5–7 days but potentially up to 2–4 weeks. The use of 16S rRNA gene amplification and sequencing by clinical microbiology laboratories is increasing and is enhancing diagnostic sensitivity and specificity. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) holds similar promise, but databases are still being optimized. Because actinomycetes are components of the normal oral and genitaltract flora, their identification in the absence of sulfur granules in sputum, bronchial washings, and cervicovaginal secretions may reflect colonization rather than infection. CHAPTER 180 Actinomycosis TREATMENT Actinomycosis Decisions about treatment are based on the collective clinical experience of the past 75 years. Actinomycosis requires prolonged treatment with high doses of antimicrobial agents; suitable antimi­ crobial agents and those deemed unreliable are listed in Table 180-1. The need for intensive treatment is presumably due to the drugs’ poor penetration of the thick-walled masses common in this infec­ tion and/or the sulfur granules themselves, which may represent a biofilm. Although therapy must be individualized, the IV adminis­ tration of 18–24 million units of penicillin daily for 2–6 weeks, fol­ lowed by oral therapy with penicillin or amoxicillin (total duration, 6–12 months), is a reasonable guideline for serious infections and bulky disease. For penicillin-allergic patients, tetracyclines, ceftri­ axone, or carbapenems are reasonable alternatives. Less extensive disease, particularly that involving the oral–cervicofacial region or

PART 5 Infectious Diseases TABLE 180-1  Appropriate and Inappropriate Antibiotic Therapy for Actinomycosisa CATEGORY AGENT Extensive successful clinical experienceb Penicillin: 3–4 million units IV q4hc,d Amoxicillin: 500 mg PO q6h Erythromycin: 500–1000 mg IV q6h or 500 mg PO q6hc Tetracycline: 500 mg PO q6h Doxycycline: 100 mg IV or PO q12h Minocycline: 100 mg IV or PO q12h Clindamycin: 900 mg IV q8h or 300–450 mg PO q6hc Anecdotal successful clinical experience Ceftriaxoned Imipenem-cilastatin Piperacillin-tazobactam Agents predicted to be efficacious on the basis of in vitro activity Vancomycin Dalbavancin Linezolid Rifampin Ertapenemd Meropenem Tigecyclined Eravacycline Azithromycind Agents that should be avoided Metronidazole Aminoglycosides Oxacillin, dicloxacillin Cephalexin Ceftazidime Daptomycin Fluoroquinolones aAdditional coverage for concomitant “companion” bacteria may be required. bControlled evaluations have not been performed. Dose and duration require individualization depending on the host, site, and extent of infection. As a general rule, a maximal parenteral antimicrobial dose for 2–6 weeks followed by oral therapy, for a total duration of 6–12 months, is required for serious infections and bulky disease, whereas a shorter course may suffice for less extensive disease, particularly in the oral–cervicofacial region. Monitoring the impact of therapy with computed tomography or magnetic resonance imaging is advisable when appropriate. cRecent in vitro data have demonstrated resistance in up to 33% of isolates. dThis agent can be considered for at-home parenteral therapy; penicillin requires a continuous infusion pump. A B FIGURE 180-5  Microscopic evaluation of actinomycotic sulfur granules. A. Actinomycotic sulfur granule with gram-positive Actinomyces organisms surrounded by eosinophilic, proteinaceous coating called the Splendore-Hoeppli phenomenon. B. Actinomycotic granule appearance with Grocott methenamine silver stain. (Courtesy of Ayesha Arshad, MD, VA Western New York Healthcare System.) the isolation of Actinomyces in the absence of tissue changes associ­ ated with actinomycosis, may be cured with a shorter course. For home IV therapy, the ease of once-a-day dosing makes ceftriaxone appealing in certain circumstances; however, a greater body of lit­ erature supporting its efficacy would be desirable. The availability of portable infusion pumps for home therapy allows for both the appropriate dosing and practical administration of IV penicillin. For infections in critical sites (e.g., CNS), this approach remains the safest until more information is available on other agents. The phar­ macokinetic properties, availability of oral and parenteral formula­ tions, and potential efficacy of azithromycin also make this agent appealing. Unfortunately, few in vitro and no clinical data exist on its use to treat actinomycosis. If therapy is extended beyond the resolution of measurable disease, the risk of relapse—a clinical hall­ mark of this infection—will be minimized; CT and magnetic reso­ nance imaging (MRI) are generally the most sensitive and objective techniques by which to accomplish this goal. A similar approach is reasonable for immunocompromised patients, although refractory disease has been described in HIV-infected individuals. While the role played by “companion” microbes in actinomycosis is unclear, many isolates are pathogens in their own right, and a regimen cov­ ering these organisms during the initial treatment course is reason­ able. Isolation of Actinomyces from blood cultures in the absence of defined infection may represent contamination or transient bacte­ remia from a mucosal site of colonization, in which case treatment may not be necessary. Combined medical–surgical therapy is still advocated in some reports. However, an increasing body of literature now supports an initial attempt at cure with medical therapy alone, even in extensive disease. CT and MRI should be used to monitor the response to therapy. In most cases, either surgery can be avoided or a less exten­ sive procedure can be used. This approach is particularly valuable in sparing critical organs, such as the bladder or the reproductive organs in women of childbearing age. For a well-defined abscess, percutaneous drainage in combination with medical therapy is a reasonable approach. When a critical location is involved (e.g., the epidural space, the CNS), when there is significant hemoptysis, or when suitable medical therapy fails, surgical intervention may be appropriate. In the absence of optimal data, the combination of a prolonged course of antimicrobial therapy and resection—at least of necrotic bone for radiation- and medication-related osteonecrosis of the jaw—is a reasonable approach.

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181 Whipple Disease

■ ■FURTHER READING Barberis C et al: Antimicrobial susceptibility of clinical isolates of Actinomyces and related genera reveals an unusual clindamycin resistance among Actinomyces urogenitalis strains. J Glob Antimicrob Resist 8:115, 2017. Bonnefond S et al: Clinical features of actinomycosis: A retrospective, multicenter study of 28 cases of miscellaneous presentations. Medi­ cine 95:e3923, 2016. Brody A et al: Targeted histological evaluation shows high incidence of Actinomyces infection in medication-related osteonecrosis of the jaws. Sci Rep 12:3406, 2022. Fong P et al: Identification and diversity of Actinomyces species in a clinical microbiology laboratory in the MALDI-TOF MS era. Anaer­ obe 54:151, 2018. Heo SH et al: Imaging of actinomycosis in various organs: A compre­ hensive review. Radiographics 34:19, 2014. Jeffery-Smith A et al: Is the presence of Actinomyces spp. in blood culture always significant? J Clin Microbiol 54:1137, 2016. Karanfilian KM et al: Cervicofacial actinomycosis. Int J Dermatol 59:1185, 2020. Kononen E, Wade WG: Actinomyces and related organisms in human infections. Clin Microbiol Rev 28:419, 2015. Lo Muzio L et al: The contribution of histopathological examination to the diagnosis of cervico-facial actinomycosis: A retrospective analysis of 68 cases. Eur J Clin Microbiol Infect Dis 33:1915, 2014. Lynch T et al: Species-level identification of Actinomyces isolates caus­ ing invasive infections: Multiyear comparison of Vitek MS (matrixassisted laser desorption ionization-time of flight mass spectrometry) to partial sequencing of the 16S rRNA gene. J Clin Microbiol 54:712, 2016. Qiu L et al: Pulmonary actinomycosis imitating lung cancer on (18) F-FDG PET/CT: A case report and literature review. Korean J Radiol 16:1262, 2015. Yang WT, Grant M: Actinomyces neuii: A case report of a rare cause of acute infective endocarditis and literature review. BMC Infect Dis 19:511, 2019. Thomas A. Russo, Seth R. Glassman

Whipple Disease Whipple disease (WD), described by George Whipple in 1907, is a chronic infection caused by Tropheryma whipplei. Most commonly, years pass from the onset of symptoms to the recognition of the dis­ ease because of its rarity, its various manifestations mimicking other conditions, and the need to perform nonroutine diagnostic tests. The long-held belief that WD is an infection was supported by observations on its responsiveness to antimicrobial therapy in the 1950s and the identification of bacilli via electron microscopy in small-bowel biopsy specimens in the 1960s. This hypothesis was finally confirmed by amplification and sequencing of a partial 16S rRNA polymerase chain reaction (PCR)–generated amplicon from duodenal tissue in 1991. The subsequent successful cultivation of T. whipplei enabled whole genome sequencing and the development of additional diagnostic tests. The development of PCR-based diagnostics has broadened our understanding of both the epidemiology of and the clinical syndromes attributable to T. whipplei. Exposure to T. whipplei, which appears to be much more common than has been appreciated, can be followed by asymptomatic carriage, acute disease, or chronic infection. Chronic infection—WD—is a rare development after exposure. “Classic” WD is manifested by some combination of arthralgias/arthritis, weight loss, chronic diarrhea, abdominal pain, and fever. Variable involvement at

other sites also occurs; neurologic and cardiac disease are most com­ mon. Acute infection and chronic organ disease in the absence of intes­ tinal involvement (see “Isolated Infection,” below) are described with increasing frequency. Since untreated WD is often fatal and delayed diagnosis may lead to irreparable organ damage (e.g., in the central nervous system [CNS]), knowledge of the clinical scenarios in which WD should be considered and of an appropriate diagnostic strategy is mandatory.

■ ■ETIOLOGIC AGENT T. whipplei is a weakly staining gram-positive bacillus. Genomic sequence data have revealed that the organism has a small (<1-megabase) chromosome, with many biosynthetic pathways absent or incomplete. This finding is consistent with a host-dependent intracellular pathogen or a pathogen that requires a nutritionally rich extracellular environment. It is one of the slowest growing human pathogens, with a doubling time of 18 days. A genotyping scheme based on a variable region has disclosed >100 genotypes to date. All genotypes appear to be capable of causing similar clinical syndromes. ■ ■EPIDEMIOLOGY WD is rare but has been increasingly recognized since the advent of PCR-based diagnostic tools. Prevalence had been previously esti­ mated at 1−3 cases per 1 million population, although a recent U.S. epidemiologic survey places the number closer to 10 cases per million. Seroprevalence studies indicate that ~50% of Western Europeans and ~75% of Africans from rural Senegal have been exposed to T. whipplei. Higher prevalence may be attributable to differences in sanitation. Humans are the only known host. In most studies, males more com­ monly develop WD; WD is more common in Caucasians and increases with age. To date, no clear animal or environmental reservoir has been demonstrated. However, the organism has been identified by PCR in sewage water and human feces. Workers with direct exposure to sewage are more likely to be asymptomatically colonized than controls, a pat­ tern suggesting fecal–oral spread. Fecal PCR detection rates of 38% among family members of carriers or patients with infection support oral–oral or fecal–oral spread, although a common environmental exposure cannot be excluded. Further, the development of acute T. whipplei pneumonia in children raises the possibility of droplet or airborne transmission. CHAPTER 181 Whipple Disease ■ ■PATHOGENESIS AND PATHOLOGY Rates of asymptomatic carriage of T. whipplei are far higher than rates of chronic infection (<0.01% of those exposed). Both decreased host pathogen-specific inflammatory response and pathogen-driven modulation of host inflammatory response likely play a role in estab­ lishing chronic infection. The human leukocyte antigen (HLA) alleles DRB1()13 and DQB1()06, which stimulate humoral rather than cellmediated immune responses, are associated with an increased risk of infection. However, only a minority of infected patients possess these haplotypes, suggesting a role for other host factors. IRF4, a transcrip­ tion factor involved with the immune response, could be such a factor as evidenced by four related family members with WD who possessed IFR4 haploinsufficiency due to a loss-of-function mutation; the distri­ bution of WD in this extended family was consistent with an autosomal dominant trait with incomplete penetrance. Flow cytometry performed in WD patients demonstrates B-cell sub­ set abnormalities when compared to matched controls. Chronic infec­ tion is associated with an impaired TH1 response, enhanced production of anti-inflammatory cytokines, increased activity of regulatory T cells, M2 polarization of macrophages with diminished antimicrobial activity and impaired phagosome–lysosome fusion and ensuing apoptosis, and blunted development of T. whipplei–specific T cells. Therapies that blunt cell-mediated host immune responses (e.g. systemic glucocorticoids or anti–tumor necrosis factor α [TNF-α] agents) may accelerate progression of chronic disease. Impaired cell-mediated immunity may play a role in establishing chronic carriage of T. whipplei, as is evidenced by higher rates of detection in the secretions of HIV-infected persons. T. whipplei has a tropism for myeloid cells, which it invades and in which it can avoid being killed. Infiltration of infected tissue by

large numbers of foamy macrophages containing periodic acid–Schiff (PAS)–staining inclusions (representing ingested bacteria) is a char­ acteristic and most common finding. With gastrointestinal disease progression, villus atrophy, lymphangiectasia, crypt hyperplasia, and apoptosis of surface epithelial cells are observed in the small intestine, with resultant diarrhea due to decreased absorption and increased leak of water and solutes. Occasionally, involvement of lymphatic or hepatic tissue may manifest as noncaseating granulomas that can mimic sar­ coid or granulomatous vasculitis.

■ ■CLINICAL MANIFESTATIONS Asymptomatic Colonization/Carriage  Studies using primar­ ily PCR have detected T. whipplei sequence in stool, saliva, duodenal tissue, and (rarely) blood in the absence of symptoms. Although prevalence rates are still being defined, in Western European countries, detection in saliva (0.2%) is less common than that in stool (1–11%) and appears to occur only with concomitant fecal carriage. The preva­ lence of fecal carriage is elevated among individuals with exposure to waste water or sewage (12–26%) and among children living in tropical Africa and Asia (20–48%). A duration of carriage of 7 years for the same strain has been described in a sewer worker. Evolution of the car­ rier state into chronic disease is uncommon. Bacterial loads are lighter in asymptomatic carriage than in active disease. Acute Infection  T. whipplei has been implicated as a cause of acute gastroenteritis in children. It was also detected via PCR in the blood of 4.6% of febrile patients (75% of whom were <15 years of age) from two rural villages in Senegal as opposed to 0.25% of healthy controls. Further, T. whipplei has been implicated as a cause of acute pneumonia. These data suggest that primary acquisition may result in symptomatic pulmonary or intestinal infection or a febrile syndrome, which perhaps are more common than is generally appreciated. PART 5 Infectious Diseases Chronic Infection  •  “CLASSIC” WD  So-called classic WD was the initial clinical syndrome recognized, with consequent identifica­ tion of T. whipplei. This chronic infection is defined by involvement of the duodenum and/or jejunum that develops over years. In most individuals, the initial phase of disease manifests primarily as intermit­ tent, often symmetrical, occasionally chronic, and rarely destructive migratory oligo- or polyarthralgias/seronegative arthritis involving the knees, wrists, ankles, and metacarpal-interphalangeal joints most commonly. Less frequently, spondylitis, sacroiliitis, discitis, tenosyno­ vitis, bursitis, and prosthetic hip infection also have been described. Intermittent fever, myalgias, and skin nodules may accompany joint symptoms. Tests for rheumatoid factor and antinuclear antibody are usually negative. This initial stage is often confused with a variety of rheumatologic disorders and, on average, lasts 6–8 years before gastrointestinal symptoms commence. Treatment of presumed inflam­ matory arthritis with immunosuppressive agents (e.g., glucocorticoids, anti-TNF-α, anakinra) can accelerate progression of the disease pro­ cess; thus, screening for WD prior to initiation of immunosuppres­ sant therapy may be appropriate, depending on the clinical scenario. Alternatively, antimicrobial therapy for another indication may reduce symptoms, and this situation should also prompt consideration of WD. The intestinal symptoms that develop in the majority of cases are characterized by diarrhea with accompanying weight loss and may be associated with fever and abdominal pain. Occult gastrointestinal blood loss, vitamin deficiencies, hepatosplenomegaly (10–15%), and ascites (10%) are less common. Anemia and hypereosinophilia may be detected. The most common finding on abdominal computed tomography is mesenteric and/or retroperitoneal lymphadenopathy (usually raising concern about lymphoma). The endoscopic or videocapsule observation of pale, yellow, or shaggy mucosa with erythema or ulceration past the first portion of the duodenum suggests WD (Fig. 181-1). When endoscopy with duodenal biopsy is nondiagnostic, a video-capsule study may assist in identifying more distal lesions for subsequent biopsy. 18F-Fluorodeoxyglucose positron emission tomog­ raphy (FDG-PET) studies in patients with WD suggest the entire small bowel can be involved. Diagnostic misdirection can be caused by

FIGURE 181-1  Endoscopic view of the jejunal mucosa demonstrating a thickened, granular mucosa and “white spots” due to dilated lacteals. (Reprinted with permission from J Bureš et al: Whipple’s disease: Our own experience and review of the literature. Gastroenterol Res Pract, 2013.) co-infection with Giardia lamblia, which is occasionally identified. The intestinal phase can also be confused with Crohn or celiac disease. In addition to rheumatologic and intestinal disease, neurologic (6–63%), cardiac (17–55%), pulmonary (10–50%), lymphatic (10–55%), ocular (5–10%), dermal (5–30%), and less commonly other sites are variably involved in classic WD. Neurologic Disease  CNS disease, defined by PCR-based detec­ tion of T. whipplei in cerebrospinal fluid (CSF), develops in ~50% of patients, many of whom are asymptomatic. A variety of neurologic manifestations have been reported and portend a poor prognosis. The most common are cognitive changes including memory impairment progressing to dementia, personality and mood alterations, hypotha­ lamic involvement (e.g., polyuria/polydipsia, sleep-cycle disorders), and supranuclear ophthalmoplegia. In addition, neuro-ophthalmologic manifestations of WD include supranuclear gaze palsy (usually verti­ cal), oculomasticatory and oculofacial myorhythmia (highly suggestive of WD), nystagmus, and retrobulbar neuritis. Focal neurologic pre­ sentations (dependent on lesion location), seizures, ataxia, meningitis, encephalitis, rhomobo- or limbic encephalitis, hydrocephalus, myelop­ athy, myoclonus, choreiform movements, and distal polyneuropathy also have been described. Neurologic sequelae occur with CNS disease, and the mortality risk is significant. Magnetic resonance imaging (MRI) results may be normal. Identified lesions (solitary or multifocal) are usually T2 and fluid-attenuated inver­ sion recovery (FLAIR) hyperintense and may enhance with gadolinium. All sites can be involved, and the nature of lesions is variable (e.g., nodu­ lar, infiltrative, tumor-like). Although imaging findings are myriad and are not diagnostic, the median temporal lobe, midbrain, hypothalamus, and thalamus are commonly affected. FDG-PET may reveal increased uptake. CSF analysis may be normal; when abnormal, leukocytosis (gen­ erally lymphocyte-predominant) and an elevated protein concentration are common. A low CSF glucose level has been reported. Cardiac Disease  Endocarditis is increasingly recognized in WD (85% of cases in males), causes 2.6−6.3% of culture-negative endocarditis cases, and may be complicated by congestive heart failure (40% of cases), embolic events, arrhythmias, mycotic aneurysm, or rarely hypoten­ sion. Fever is often absent, and the Duke clinical criteria are rarely met.

Vegetations are identified by echocardiography in 50–75% of cases. All valves, alone or in combination, can be affected; most commonly involved are the aortic and mitral valves. Preexisting valvular disease is found in only a minority of cases, although infection of bioprosthetic valves has been described. Mural, myocardial, aortic (aortitis), or peri­ cardial disease also occurs alone or in combination with valvular involve­ ment. Constrictive pericarditis develops infrequently. The diagnosis of cardiac disease is rarely made prior to surgical intervention. Pulmonary Disease  Some combination of interstitial disease, nodules, parenchymal infiltrate, and pleural effusion is observed. An association with pulmonary hypertension has also been reported. The clinical significance of T. whipplei sequence identified in bronchoalveo­ lar lavage fluid (BALF) from asymptomatic HIV-infected individuals or in a case of interstitial lung disease is unresolved but suggests caution in diagnosing “isolated” pneumonia based on sequence alone. Notably, while the bacterium seems to exist in the airways of HIV-infected per­ sons at higher rates, its presence is not clearly associated with increased inflammation or a discernible decrease in lung function. Lymphatic Disease  Mesenteric and retroperitoneal lymphadenopa­ thy are common with intestinal disease, and mediastinal adenopathy may be associated with pulmonary infection. Peripheral adenopathy is less common. Ocular Disease (Non–Neuro-Ophthalmologic)  Uveitis is the most common form of ocular disease, usually presenting as a change in vision or “floaters.” Anterior (anterior chamber), intermediate (vitreous), and posterior (retina/choroid) uveitis can occur alone or in combination. Treatment with glucocorticoids alone can worsen uveitis and unmask extraocular disease. Likewise, use of local or systemic glucocorticoids after ocular surgery can precipitate ocular infection, likely as a result of asymptomatic or subclinical disease. Keratitis, crystalline keratopathy, and optic neuritis also have been reported. Patients may be misdiag­ nosed with sarcoid or Behçet’s disease prior to the recognition of WD. Dermatologic Disease  Skin hyperpigmentation (melanoderma), particularly in light-exposed areas in the absence of adrenal dysfunction, is suggestive of WD. A variety of other cutaneous manifestations have been described, including erythematous macular lesions, nonthrombo­ cytopenic purpura, subcutaneous nodules, and hyperkeratosis. Miscellaneous Sites  Thyroid, renal, testicular, epididymal, gall­ bladder, skeletal muscle, and bone marrow involvement and membra­ nous nephropathy have all been described. In fact, almost any organ can be involved in classic WD, with varying frequency, variable com­ binations, and myriad signs and symptoms. As a result, WD should be considered in the setting of a chronic multisystemic process. Despite its rarity, the combination of rheumatologic and intestinal disease with weight loss, with or without neurologic and cardiac involvement, war­ rants heightened suspicion. ISOLATED INFECTION  This entity has been defined as infection in the absence of intestinal symptoms, although an occasional small-bowel biopsy may be PAS-positive or more commonly PCR-positive in this setting. “Isolated infection” is something of a misnomer since multiple nonintestinal sites of T. whipplei infection are not uncommon. Infec­ tion at the same nonintestinal sites (single or multiple) that are variably involved in classic WD may also present as “isolated infection.” Further, intestinal disease can subsequently develop. Endocarditis, neurologic disease, uveitis, rheumatologic manifestations, and pulmonary involve­ ment are most commonly described. Signs and symptoms are similar to those described for T. whipplei infection of these sites in classic WD. With enhanced PCR-based diagnostic capabilities, T. whipplei infection without concomitant intestinal involvement (of which endocarditis is the best example) will probably be diagnosed increasingly often. REINFECTION/RELAPSING DISEASE/IMMUNE RECONSTITUTION INFLAMMATORY SYNDROME (IRIS)  It has been suggested that, if an underlying host immune defect places an individual at risk for chronic infection, then that person may be at risk for reinfection due to occupational exposure or contact with family members who are

asymptomatically colonized. One case of apparent reinfection that was due to a different genotype supports this contention.

Optimal treatment regimens and durations are still being defined. However, it is clear, especially in the setting of occult or overt CNS disease, that treatment with oral tetracycline or trimethoprimsulfamethoxazole (TMP-SMX) alone may result in disease relapse. Relapses or perhaps reinfections occurring years to decades after initial therapy have been described. As in patients treated for HIV or mycobacterial disease, IRIS has been described in up to 17% of patients treated for T. whipplei infec­ tion. Prior immunosuppressive therapy increases the likelihood of IRIS, in which inflammation recurs after an initial clinical response to treatment and loss of PCR detection of T. whipplei. In patients with chronic WD, IRIS may be related to sustained disruption of the epithe­ lial barrier, leading to increased translocation of gut-derived microbial products and dysbalanced T-cell restoration. Manifestations include the development of fever, arthritis, skin lesions, subcutaneous nodules, pleuritis, uveitis, and orbital and periorbital inflammation; some cases have been fatal. ■ ■DIAGNOSIS Considering T. whipplei infection and ensuring that the appropriate tests are performed are the critical steps in making the diagnosis, which otherwise will likely be missed. Serology is of little value since patients with active infection usually mount a poor IgM/IgG response to

T. whipplei and a positive result most likely reflects prior exposure and clearance. The clinical presentation will in part dictate which clinical specimens are most likely to enable the diagnosis. In the presence (and perhaps the absence) of gastrointestinal symptoms, postbulbar duo­ denal biopsies should be performed, although a normal macroscopic appearance is common. As a general rule, the diagnostic yield is greater for tissue specimens than for body fluids. Biopsy of normal-appearing skin may detect T. whipplei in the setting of classic WD and serve as a minimally invasive means to establish the diagnosis. It is prudent to collect CSF even in the absence of CNS symptoms; asymptomatic disease is common, the CNS is the most common site for relapse, and thus the information gained by CSF examination could influence the design and duration of the treatment regimen. CHAPTER 181 Whipple Disease The diagnosis of classic WD was originally based on histologic find­ ings in intestinal biopsy specimens. Although this diagnostic procedure remains important, it is not optimally sensitive. Infiltration of the lamina propria with macrophages containing PAS-positive inclusions that are resistant to diastase is observed. However, PAS is nonspecific, also yield­ ing positive results with mycobacteria as well as other microorganisms. Staining of other tissues or fluids (e.g., ocular aspirations) for PASpositive inclusions in macrophages can be performed to support the diagnosis. The sensitivity of identification of PAS-positive inclusions in WD may be decreased by anti-TNF-α therapy. Electron microscopy can be used to identify the trilaminar cell wall of T. whipplei. When available, immunohistochemistry has greater specificity and sensitivity than PAS staining and can be performed on archived fixed tissue. Alternatively, the use of fluorescence in situ hybridization (FISH) has been reported as a complementary diagnostic tool with various tissue samples. The development and implementation of specific PCR-based diag­ nostics have significantly increased the sensitivity and specificity of

T. whipplei identification. PCR can be applied to affected tissues (with greater sensitivity for non-formalin-fixed than for formalin-fixed tis­ sue) in support of histologic findings and to various body fluids. It is important to note that the interpretation of a PCR-based diagnostic approach must take into account limitations such as false-positive results due to sample contamination, false-negative results due to low organism load, poor sample quality, inadequate DNA extraction, and variability in performance of various PCR assays. Quantitative com­ parisons from different sites can add specificity to PCR-based diagnos­ tics of WD and distinguish between WD patients and asymptomatic carriers. In patients suspected of having WD, PCR testing of duo­ denal biopsy specimens with a cycle threshold value of ≤30 can help confirm the diagnosis, even in cases with negative PAS staining. As with all diagnostic tests, consideration of pretest probability is critical

for interpretation, and a negative result does not exclude WD. Urine PCR for T. whipplei infection may hold promise for the noninvasive diagnosis of classic and isolated WD. In one study of 12 cases, urine PCR was positive in nine cases (75%) prior to treatment compared to zero (0%) of 110 controls, including 11 controls that were presumed carriers in whom feces PCR was positive, although there was no evi­ dence of disease. In addition, urine PCR is a potential tool to evaluate the success of WD therapy. Saliva and fecal PCR are inappropriate as the sole diagnostic tools for WD due to low positive predictive values, which more commonly identify colonization, not disease; a positive result requires confirmation from an appropriate end-organ tissue or body fluid.

Next-generation sequencing techniques to evaluate for cell-free DNA (cfDNA) in plasma may lead to increased recognition of T. whipplei as a cause of endocarditis. T. whipplei has been successfully cultured from blood, CSF, synovial fluid, BALF, valve tissue, duodenal tissue, skeletal muscle, and lymph nodes, but culture is not practical since it takes months to obtain a positive result. Affected anatomic sites in WD patients may demonstrate uptake on FDG-PET, which in turn could guide tissue sampling for use in specific tests. TREATMENT Whipple Disease Data on treatment are emerging, but the optimal regimen and dura­ tion for chronic infection, which may depend on the sites involved (e.g., CNS and heart valve), are unclear. Appropriate treatment usu­ ally results in a rapid—and at times remarkable—clinical response (e.g., in CNS disease), but eradication requires prolonged treat­ ment. Maintenance of a durable response has been more challeng­ ing because of both relapse and host predisposition to reinfection. PART 5 Infectious Diseases Rates of relapse, particularly of CNS disease, were unacceptable with oral tetracycline or TMP-SMX monotherapy. Sequence data now indicate that TMP is not active against T. whipplei (given the absence of dihydrofolate reductase in T. whipplei) and that resistance to SMX and sulfadiazine can occur. However, a randomized controlled trial in 40 patients, who received either ceftriaxone (2 g IV q24h) or merope­ nem (1 g IV q8h) for 2 weeks followed by oral TMP-SMX (160/800 mg)

twice a day for 1 year, demonstrated outstanding efficacy. The only case in which therapy failed—an asymptomatic CNS infection that was not eradicated by either regimen—was subsequently cured with oral minocycline and chloroquine (250 mg/d after a loading dose). A follow-up trial reported similar efficacy with a regimen of cef­ triaxone (2 g IV q24h) for 2 weeks followed by oral TMP-SMX for

3 months. One issue in these trials was that the doses—and perhaps the duration of ceftriaxone and meropenem treatment as well—were not optimal for CNS infection. By contrast, in a small retrospective series, outcome was better in patients treated with oral doxycycline (100 mg twice a day) plus hydroxychloroquine (200 mg three times a day to raise phagosome pH and increase drug activity in vitro) than in patients initially treated with TMP-SMX. Until more data become available, it seems prudent—at least in asymptomatic/symptomatic CNS disease (which is present in many cases of WD)—first to administer CNS-optimized doses of IV ceftriaxone (2 g q12h) or meropenem (2 g q8h) for 2–4 weeks and then to treat with oral doxycycline, or minocycline plus hydroxy­ chloroquine for at least 1 year, if tolerated. Although TMP-SMX has been frequently used as the oral alternative with reported success, a number of relapses or reinfections with TMP-SMX treatment have been reported, thereby suggesting caution for its use in patients with infection in critical locations such as the CNS and the heart. Although data on the use of PCR to guide therapy do not exist, it seems reasonable that continued T. whipplei detection by PCR, especially in the CSF and perhaps urine, should dictate at least continuation of therapy or perhaps consideration of an alternative regimen when in conjunction with a poor clinical response.

Timely recognition may result in cure with medical manage­ ment alone. Surgery may be needed in the setting of endocarditis with significant valve dysfunction or myocardial abscess. Current European guidelines for the treatment of endocarditis caused by

T. whipplei recommend oral doxycycline plus hydroxychloroquine for ≥18 months or, alternatively, ceftriaxone (2 g q24h IV) or peni­ cillin (2 million units q4h IV) plus streptomycin (1 g q24h IV) for 2–4 weeks followed by oral TMP-SMX (800 mg q12h); a small study from Spain reported that treatment durations of 12–13 months with these regimens or variations were efficacious. Data on isolated infection and certain site-specific treatment issues are even more limited. Anecdotal reports describe suc­ cessful treatment of uveitis with oral TMP-SMX with or without rifampin, whereas treatment with tetracycline alone has resulted in relapse. Although a role for adjunctive intraocular therapy has been reported, the data are unclear on this point. There is a single case report of clearance of infection in a chronically relapsing patient by the addition of interferon gamma to antimicrobials; supplementa­ tion to antimicrobials may be a consideration to address refractory disease or potential issues with antibiotic resistance. Although data on the treatment of foreign body–associated infection are virtually nonexistent, medical treatment for a pros­ thetic hip infection was apparently successful; however, follow-up was limited. The occurrence of a Jarisch-Herxheimer reaction within 24 h of treatment initiation has been described, with rapid resolution. The addition of glucocorticoids may be beneficial in the management of IRIS, and thalidomide has been used in steroid-refractory cases. Importantly, although data are lacking, due to the inherent risk of relapse or reinfection, lifelong suppressive therapy with doxy­ cycline after completion of the initial treatment regimen has been advocated. Regardless of the therapeutic approach chosen, an effort to ensure compliance and close follow-up for potential relapse or reinfection, which can occur many years after an apparent cure, will maximize the chances for a good outcome. PREVENTION Attempts are underway to develop a peptide-based vaccine against T. whipplei. However, the role for this may be limited given the rela­ tive rarity of infection. ■ ■FURTHER READING Bally JF et al: Systematic review of movement disorders and oculomo­ tor abnormalities in Whipple’s disease. Mov Disord 33:1700, 2018. Boumaza A et al: Whipple’s disease and Tropheryma whipplei infec­ tions: From bench to bedside. Lancet Infect Dis 22:e280, 2022. Crews NR et al: Diagnostic approach for classic compared with local­ ized Whipple disease. Open Forum Infect Dis 5:ofy136, 2018. Damaraju D et al: Clinical problem-solving: A surprising cause of chronic cough. N Engl J Med 373:561, 2015. de Vries CR et al: 710. Non-invasive diagnosis of Whipple endocardi­ tis using next-generation sequencing for microbial cell-free DNA in plasma. Open Forum Infect Dis 7:S407, 2020. Guérin A et al: IRF4 haploinsufficiency in a family with Whipple’s disease. Elife 7:e32340, 2018. Gunther U et al: Gastrointestinal diagnosis of classical Whipple disease: Clinical, endoscopic, and histopathologic features in 191 patients. Medicine 94:e714, 2015. Ioannou P et al: Whipple’s disease-associated infective endocarditis: A systematic review. Infect Dis (Lond) 55:447, 2023. Lagier JC, Raoult D: Whipple’s disease and Tropheryma whipplei infections: When to suspect them and how to diagnose and treat them. Curr Opin Infect Dis 31:463, 2018. Meyer S et al: Contribution of PCR to differential diagnosis between patients with Whipple disease and Tropheryma whipplei carriers. J Clin Microbiol 61:e0145722, 2023. Moter A et al: Potential role for urine polymerase chain reaction in the diagnosis of Whipple’s Disease. Clin Infect Dis 68:1089, 2019.

66 - 182 Infections Due to Mixed Anaerobic Organisms

182 Infections Due to Mixed Anaerobic Organisms

Neeraj K. Surana, Dennis L. Kasper

Infections Due to Mixed Anaerobic Organisms Anaerobes constitute the predominant class of bacteria of the normal human microbiota that reside on mucous membranes and predomi­ nate in many infectious processes, particularly those arising from mucosal surfaces. These organisms generally cause disease subsequent to the breakdown of mucosal barriers and the leakage of the microbiota into normally sterile sites. Infections resulting from contamination by the microbiota are usually polymicrobial and involve both aerobic and anaerobic bacteria. However, the difficulties encountered in handling specimens in which anaerobes may be important and the technical challenges entailed in cultivating and identifying these organisms in clinical microbiology laboratories continue to leave the anaerobic etiology of an infectious process unproven in many cases. Therefore, an understanding of the types of infections in which anaerobes can play a role is crucial in selecting appropriate microbiologic tools to identify the organisms in clinical specimens and in choosing the most appropriate treatment, including antibiotics and surgical drainage or debridement of the infected site. This chapter focuses on infections caused by anaerobic bacteria other than Clostridium species, which are covered elsewhere (Chaps. 139 and 159). ■ ■HISTORIC PERSPECTIVE Anaerobic organisms were first identified by Antonie van Leeuwen­ hoek in 1680—nearly a century before oxygen itself was discovered. Leeuwenhoek set up culture medium (crushed pepper powder and clean rainwater) in two glass tubes—one open to ambient air and the other sealed closed—that he incubated for several days. Although he did not expect to observe anything in the sealed tube, he was surprised to find “animalcules” in both tubes. He noted that these bacteria in the sealed tube were “bigger than the biggest sort” in the tube left open to air. It was not until the mid- to late nineteenth century that Leeuwen­ hoek’s findings were confirmed by Pasteur and others. However, these principles described by Leeuwenhoek underlie the basic pathogenesis of anaerobic infections: development of an anaerobic environment in a closed space is due to consumption of oxygen by aerobic organisms and results in the outgrowth of anaerobic organisms. ■ ■DIFFERENCES BETWEEN ANAEROBIC AND AEROBIC ORGANISMS Anaerobic bacteria can be categorized as obligate anaerobes (killed in the presence of ≥0.5% oxygen), aerotolerant organisms (can tolerate the presence of oxygen but cannot use it for growth), and facultative anaer­ obes (can grow in the presence or absence of oxygen). Most clinically relevant anaerobes, such as Bacteroides fragilis, Prevotella melaninogenica, and Fusobacterium nucleatum, are relatively aerotolerant. These organ­ isms contrast with obligate aerobes, which require high concentrations of oxygen for growth, and microaerophilic organisms, which are dam­ aged by atmospheric concentrations of oxygen (~21%) but require low concentrations of oxygen (typically 2–10%) for growth. Given that molecular oxygen can reduce to superoxide (O2 −) and hydrogen peroxide (H2O2), which are damaging to cells, the ability to tolerate the presence of oxygen is due, in part, to the expression of superoxide dis­ mutase and catalase. The variation in anaerobic organisms tolerating anywhere from <0.5 to 8% O2 may reflect the amount of these enzymes that is produced. Furthermore, aerobic and anaerobic organisms differ in their energy metabolism. Cellular respiration requires establishment of an electrochemical gradient across the membrane, resulting in an electric potential (often related to a proton gradient) across the membrane. In aerobic respiration, electrons are shuttled through an electron transport chain, with oxygen as the final electron acceptor. Anaerobic organisms can metabolize energy by either anaerobic respiration or fermentation.

Given that the final electron acceptor in anaerobic respiration (e.g., sulfate, nitrate, carbon dioxide, or fumarate) is not as highly oxidizing as oxygen, this pathway is less efficient than aerobic respiration and produces less ATP per glucose molecule. In contrast, fermentation does not use an electrochemical gradient. Rather, it releases energy from an organic molecule (e.g., pyruvate and its derivatives) via substrate-level phosphorylation and is therefore a less efficient process than either aerobic or anaerobic respiration; for comparison, fermentation results in ~5% of the energy released by aerobic respiration. For these reasons, facultative anaerobes will preferentially utilize oxygen if it is available; in oxygen-limiting situations, organisms will use anaerobic respiration rather than fermentative processes, if possible.

■ ■ANAEROBES OF THE HUMAN MICROBIOTA Most human mucocutaneous surfaces harbor a rich indigenous nor­ mal microbiota composed of aerobic and anaerobic bacteria. These surfaces are dominated by anaerobic bacteria, which often account for 99.0–99.9% of the cultivable microbiota and range in concentration from 103/mL in the nose to 1012/mL in gingival scrapings and the colon (Table 182-1). It is interesting that anaerobes inhabit many areas of the body that are exposed to air: skin, nose, mouth, and throat. Anaer­ obes are thought to reside in the portions of these sites that either are relatively well protected from oxygen (e.g., gingival crevices) or have a local anaerobic environment conferred by neighboring aerobic organ­ isms (e.g., tooth surfaces). The ability to cultivate these organisms is improving, and—with strict attention to anaerobic conditions—more than 80% of the microscopic counts in fecal samples can be cultured. However, culture-independent approaches (e.g., sequencing of the 16S rDNA gene) show that the overwhelmingly diverse low-abundance bacterial species present in the microbiota remain uncultivated. Several projects, including the Human Microbiome Project (funded by the U.S. National Institutes of Health) and MetaHIT (financed by the European Commission), have characterized the normal microbiota of healthy individuals and have demonstrated the presence of >10,000 different bacterial species in the collective human microbiota. The human gut CHAPTER 182 Infections Due to Mixed Anaerobic Organisms TABLE 182-1  The Anaerobic Human Microbiota: An Overview ANAEROBIC/ AEROBIC RATIO POTENTIAL PATHOGEN(S) TOTAL BACTERIAa ANATOMIC SITE Nose 103–104 2:1 Peptostreptococcus spp., Prevotella spp. Oral cavity Saliva 108–109 10:1 Fusobacterium nucleatum, Prevotella melaninogenica, Prevotella oralis group, Bacteroides ureolyticus group, Peptostreptococcus spp. Tooth surface 1010–1011 1:1 Gingival crevices 1011–1012 103:1 Gastrointestinal tract Stomach 100–103 1:10 Lactobacillus spp. Duodenum 101–105 1:1 Lactobacillus spp., Streptococcus spp. Jejunum 103–106 1:1 Streptococcus spp., Lactobacillus spp., Peptostreptococcus spp. Ileum 104–109 10:1 Bacteroides spp., Streptococcus spp., Enterococcus spp. Cecum and colon 1011–1012 103:1 Bacteroides spp. (principally members of the B. fragilis group), Prevotella spp., Clostridium spp. Female genital tract 107–109 10:1 Peptostreptococcus spp., Bacteroides spp., Prevotella bivia Skin 104–106 100:1 Cutibacterium acnes aPer gram or milliliter.

alone harbors >1000 bacterial species, with 100–200 species present in any given individual.

The major reservoir of anaerobic bacteria is the lower gastrointesti­ nal tract, but these organisms are also present in considerable numbers in the oral cavity, skin, and female genital tract (Table 182-1). In the oral cavity, the ratio of anaerobic to aerobic bacteria ranges from 1:1 on the surface of a tooth to 1000:1 in the gingival crevices. Prevotella and Porphyromonas species make up much of the indigenous oral anaerobic microbiota. Fusobacterium and Bacteroides (non–B. fragilis group) species are present in lower numbers. Anaerobic bacteria are not found in appreciable numbers in the normal stomach and proximal small intestine. In the distal ileum, the microbiota begins to resemble that of the colon, where the ratio of anaerobes to aerobic species is high (~1000:1). The predominant anaerobes in the human intestine belong to the phyla Bacteroidetes and Firmicutes and include a number of Prevotella and Bacteroides species (e.g., members of the B. fragilis group, such as B. fragilis, B. thetaiotaomicron, B. ovatus, B. vulgatus, B. uniformis, and Parabacteroides distasonis) as well as various Clos­ tridium, Peptostreptococcus, Blautia, and Fusobacterium species. In the female genital tract, there are ~109 organisms/mL of secretions, with an anaerobe-to-aerobe ratio of ~10:1. The predominant anaerobes in the female genital tract are Prevotella, Bacteroides, Fusobacterium, Clos­ tridium, and the anaerobic Lactobacillus species. The skin microbiota contains anaerobes as well; Cutibacterium acnes (which was previously Propionibacterium acnes and will be considered as one of the Propioni­ bacterium species for the remainder of this chapter) is the predominant species, and other species of propionibacteria and peptostreptococci are present in lower numbers. PART 5 Infectious Diseases ■ ■ANAEROBES AND HUMAN HEALTH Commensal anaerobes have been implicated as crucial mediators of physiologic, metabolic, and immunologic functions in the mammalian host. The intestinal microbiota is essential for fermenting dietary car­ bohydrates into forms that are more usable by the host, among which polysaccharides are the most abundant biologic source of energy. Of the organisms found within the intestines, Bacteroides species express the widest array of polysaccharide-degrading enzymes, providing important nutrients for both the host and other commensal organisms. For example, B. thetaiotaomicron expresses 172 glycosyl hydrolases. The anaerobic intestinal microbiota is also responsible for the produc­ tion of secreted products that promote human health (e.g., vitamin K and bile acids useful in fat absorption and cholesterol regulation). One of the most important roles that anaerobes serve as components of the normal colonic microbiota is the promotion of resistance to colonization. The presence of commensal bacteria effectively interferes with colonization by potentially pathogenic bacterial species through the depletion of oxygen and nutrients, the production of enzymes and toxic end products, and the modulation of the host’s intestinal innate immune response. For example, the normal intestinal microbiota plays an important role in protection against enteric infections, including those due to Salmonella enterica serotype Typhimurium and Clostridium difficile. The anaerobic intestinal microbiota also has immunomodulatory properties that help regulate the immune system. The first example of this role was demonstrated with B. fragilis, which can balance the effector functions of T cells in the peripheral immune system and induce colonic regulatory T cells via expression of polysaccharide A (PSA). Moreover, B. fragilis expresses a glycosphingolipid that regu­ lates the number of colonic invariant natural killer T cells. There are now numerous examples of commensal anaerobes that can modulate different aspects of the intestinal and extraintestinal immune system— everything from specific effector T cells to dendritic cells to antimicro­ bial peptides. Clearly, the gut microbiota confers many benefits, and its dysregula­ tion may play a role in the pathogenesis of diseases characterized by inflammation and aberrant immune responses, such as inflammatory bowel disease, rheumatoid arthritis, multiple sclerosis, asthma, and type 1 diabetes. Furthermore, the gut microbiota has been associated with obesity and metabolic syndrome. A more complete discussion of

the intersection between the microbiota and human health is covered elsewhere (Chap. 484). ■ ■ETIOLOGY There are >10,000 species of bacteria—the overwhelming majority of which are anaerobes—in the human microbiota, with each individual colonized by hundreds of species. Anaerobic infections occur when the harmonious relationship between the host and the host’s microbiota is disrupted. Any site in the body is susceptible to infection with these indigenous organisms if they are introduced into otherwise sterile tis­ sue, either through disruption of mucosal surfaces (e.g., intestinal per­ foration, ischemia, surgery) or via direct inoculation of organisms into tissue (e.g., bite wounds, trauma). Because the sites that are colonized by anaerobes contain many species of bacteria, the resulting infections are often polymicrobial, involving multiple species of anaerobes in combination with synergistically acting facultative and/or microaero­ philic organisms. Despite the complex array of bacteria in the normal microbiota, relatively few genera are isolated commonly from human infections (Fig. 182-1). While the specific organisms identified vary with the site and source of infection, the etiologic agents typically reflect the neighboring microbiota. For example, organisms normally found in the oro- and nasopharyngeal microbiota (e.g., P. melaninogenica, Fusobacterium necrophorum, F. nucleatum, Peptostreptococcus species, Porphyromonas gingivalis, Porphyromonas asaccharolytica, and Actino­ myces species) can cause disease in contiguous areas, including odon­ togenic infections, peripharyngeal space infections, chronic sinusitis, and pleuropulmonary infections. In female genital tract infections, organisms normally colonizing the vagina (e.g., Prevotella bivia and Prevotella disiens) are the most common isolates. Escherichia coli and B. fragilis, both of which are components of the intestinal microbiota, are the most commonly identified isolates from intraabdominal abscesses. Indeed, the B. fragilis group, which encompasses 25 species and includes B. thetaiotaomicron, B. vulgatus, B. uniformis, and B. ovatus, contains the anaerobic organisms among the most frequently isolated from clinical infections. It is useful to think about anaerobic infectious etiologies with regard not only to their anatomic location but also to their microbiologic features. While many anaerobic gram-negative bacilli cause disease (e.g., Prevotella, Bacteroides, Fusobacterium, and Porphyromonas species), Veillonella species, which are part of the oral and intestinal microbiota, are among the few anaerobic gram-negative cocci that have been impli­ cated in human disease. Similarly, the peptostreptococci (e.g., P. micros, P. asaccharolyticus, and P. anaerobius) and Finegoldia magnus (which was previously Peptostreptococcus magnus and will be considered as part of the peptostreptococci for the remainder of this chapter) are the chief anaerobic gram-positive cocci that have pathogenic potential. Clostridium species are the primary anaerobic spore-forming grampositive rods that produce human disease (Chap. 159). Uncommonly, anaerobic gram-positive non-spore-forming bacilli cause infection;

Gram-positive cocci Clostridium spp. Other Gram-positive rods Bacteroides fragilis Other Bacteroides spp. Prevotella spp. Fusobacterium spp. Porphyromonas spp. Other Gram-negative rods Veillonella spp. FIGURE 182-1  Distribution of anaerobic organisms isolated from clinical materials. (Data combined from Y Park et al: Clinical features and prognostic factors of anaerobic infections: A 7-year retrospective study. Korean J Intern Med 24:13, 2009; and Japanese Association for Anaerobic Infections Research: Anaerobic infections (general): Epidemiology of anaerobic infections. J Infect Chemother 17:4, 2011.)

C. acnes, a component of the skin microbiota and a cause of foreignbody infections, and Actinomyces species are relevant examples. ■ ■PATHOGENESIS First and foremost, anaerobic infections require an anaerobic environ­ ment with a lowered oxidation-reduction potential. In some circum­ stances, this environment can occur directly—e.g., in tissue ischemia, trauma, surgery, or a perforated viscus. In many other situations, the infection is polymicrobial, and the facultative organisms maintain a lowered oxidation-reduction potential in the local microenvironment that allows for the propagation of obligate anaerobes. Once the proper anaerobic environment is established, the organisms must still contend with the host’s immune defenses. Similar to aerobic organisms, anaer­ obes express an array of virulence factors that help evade host defenses, they can form abscesses as a protective measure, and they can act syn­ ergistically with other bacteria to better persist in the host. Virulence factors associated with anaerobes typically confer the abil­ ity to evade host defenses, adhere to cell surfaces, produce toxins and/ or enzymes, or display surface structures such as capsular polysaccha­ rides and lipopolysaccharides that contribute to pathogenic potential. The ability of an organism to adhere to host tissues is often critical to the establishment of infection. Some oral species adhere to the epithe­ lium in the oral cavity. P. gingivalis, a common isolate in periodontal disease, has fimbriae that facilitate attachment. In supragingival plaque, many oral anaerobes are able to attach directly to aerobic bacteria (e.g., Streptococcus species) that are adherent to the tooth’s surface. F. nuclea­ tum is a notable example of these secondary colonizers: it expresses receptors to which almost all oral bacteria can bind and serves as an important bridge between the primary colonizers and subsequent lay­ ers of bacteria. B. fragilis synthesizes pili, fimbriae, and hemagglutinins that aid in attachment to host cell surfaces in the intestine. Anaerobic bacteria produce several exoproteins that can enhance the organisms’ virulence. P. gingivalis produces a collagenase that enhances tissue destruction. Exotoxins produced by clostridial species, including botulinum toxins, tetanus toxin, C. difficile toxins A and B, and five toxins produced by Clostridium perfringens, are among the most virulent bacterial toxins in mouse lethality assays. Anaerobic gram-negative bacteria, such as B. fragilis, P. gingivalis, and Prevotella intermedia, possess lipid A molecules (endotoxins) that are 100–1000 times less biologically potent than endotoxins associated with aero­ bic gram-negative bacteria; these differences relate to variations in acylation status, length of fatty acids, and number of phosphate groups. This relative biologic inactivity may account for the lower frequency of disseminated intravascular coagulation and purpura in anaerobic gram-negative bacteremia than in facultative and aerobic gramnegative bacillary bacteremia. An exception is the lipopolysaccharide from Fusobacterium, which may account for the severity of Lemierre’s syndrome (see “Complications of Anaerobic Head and Neck Infec­ tions,” below). The most extensively studied virulence factor of the nonsporulating anaerobes is the capsular polysaccharide complex of B. fragilis. This organism is unique among anaerobes in its potential for virulence dur­ ing growth at normally sterile sites. Although it constitutes only 0.5–1% of the normal colonic microbiota, B. fragilis is the anaerobe most com­ monly isolated from intraabdominal infections and bacteremia. In an animal model of intraabdominal sepsis, the capsular polysaccharide was identified as the major virulence factor of B. fragilis; this polymer plays a specific, central role in the induction of abscesses. A series of detailed biologic and molecular studies of this virulence factor showed that B. fragilis produces at least eight distinct capsular polysaccharides, far more than the number reported for any other encapsulated bac­ terium. B. fragilis can exhibit distinct surface polysaccharides either alone or in combination by regulating the expression of these differ­ ent capsules in an on–off manner through a reversible inversion of DNA segments within the promoters for operons containing the genes required for polysaccharide synthesis. Structural analysis of two of these polysaccharides, PSA and polysaccharide B (PSB), revealed that each polymer consists of repeating units with positively charged free amino groups and negatively charged groups. This structural feature is

rare among bacterial polysaccharides, and the ability of PSA—and, to a lesser extent, PSB—to induce abscesses in animals depends on this zwitterionic charge motif. Intraabdominal abscess induction is related to the capacity of PSA to stimulate macrophages to release cytokines and chemokines—in particular, interleukin (IL) 8, IL-17, and tumor necrosis factor α (TNF-α)—from resident peritoneal cells through a Toll-like receptor 2–dependent mechanism. The release of cytokines and chemokines results in the chemotaxis of polymorphonuclear neu­ trophils (PMNs) into the peritoneum, where they adhere to mesothelial cells induced by TNF-α to upregulate their expression of intercellular adhesion molecule 1 (ICAM-1). PMNs adherent to ICAM-1-expressing cells probably represent the nidus for an abscess. PSA also activates T cells to produce certain cytokines, including IL-17 and interferon γ, that are necessary for abscess formation.

These virulence factors not only promote persistence of the anaer­ obe that produces them but also aid in the survival of bystander organ­ isms and result in bacterial synergies. Clinically, these synergies are evidenced by the fact that anaerobic infections typically involve three to six different organisms. Examples of this synergistic pathogenesis include creation of a favorable environment for growth (e.g., establish­ ment and maintenance of an anaerobic environment by facultative organisms), inhibition of host defenses (e.g., production of short-chain fatty acids and succinic acid that inhibit the ability of phagocytes to clear facultative organisms), provision of necessary growth factors for other organisms (e.g., oral diphtheroids that produce vitamin K, which is needed by P. melaninogenica), and creation of tissue damage that promotes spread of the infection. In these ways, facultative and obligate anaerobes synergistically potentiate abscess formation. CHAPTER 182 APPROACH TO THE PATIENT Infections Due to Anaerobic Bacteria The physician must consider several points when approaching the patient with a possible infection due to anaerobic bacteria. Infections Due to Mixed Anaerobic Organisms

  1. The organisms colonizing mucosal sites are commensals, very few of which typically cause disease. When these organisms do cause disease, it often occurs in proximity to the mucosal site they colonize.
  2. For anaerobes to cause tissue infection, they must spread beyond the normal mucosal barriers.
  3. Conditions favoring the propagation of anaerobic bacteria, par­ ticularly a lowered oxidation-reduction potential, are necessary. These conditions exist at sites of trauma, tissue destruction, compromised vascular supply, and necrosis.
  4. Frequently, a complex array of infecting microbes can be found, occasionally with >10 different species isolated from a suppura­ tive site.
  5. Anaerobic organisms tend to be found in abscess cavities or in necrotic tissue. The failure of an abscess to yield organisms on routine culture is a clue that the abscess is likely to contain anaerobic bacteria. Often smears of this “sterile pus” are found to be teeming with bacteria when Gram’s stain is applied. Although some facultative organisms (e.g., Staphylococcus aureus) are also capable of causing abscesses, abscesses in organs or deeper body tissues should call anaerobic infection to mind.
  6. Gas is found in many anaerobic infections of deep tissues but is not diagnostic because it can be produced by aerobic bacteria as well.
  7. Although a putrid-smelling infection site or discharge is con­ sidered diagnostic for anaerobic infection, this manifestation usually develops late in the course and is present in only 30–50% of cases.
  8. Some species (the best example being the B. fragilis group) require specific therapy. However, many synergistic infections can be cured with antibiotics directed at some but not all of the organisms involved. Antibiotic therapy, combined with debride­ ment and drainage, disrupts the interdependent relationship

among the bacteria, and some species that are resistant to the antibiotic do not survive without the co-infecting organisms. 9. Manifestations of severe sepsis and disseminated intravascu­ lar coagulation are unusual in patients with purely anaerobic infection. ■ ■EPIDEMIOLOGY Difficulties in the performance of appropriate cultures, contamination of cultures by components of the normal microbiota, and the lack of readily available, reliable culture techniques have made it challenging to obtain accurate data on the incidence or prevalence of anaerobic infections. However, anaerobic infections are encountered frequently, with anaerobes constituting 7–8% and 13–15% of bacteria isolated from inpatients and outpatients, respectively. Bacteremia and soft tissue infections are the most common types of anaerobic infection (Fig. 182-2). Typically, anaerobic bacteria account for <1% of all cases of bacteremia. ■ ■CLINICAL MANIFESTATIONS Although anaerobes can cause infection anywhere in the body, cer­ tain clinical findings and characteristics are commonly found. These include abscess formation, putrid purulence (due to volatile fatty acid by-products), septic thrombophlebitis, tissue necrosis, and failure to respond clinically to broad-spectrum antibiotics that lack activity against anaerobes. Anaerobic Infections of the Mouth, Head, and Neck  Anaero­ bic bacteria are commonly involved in infections of the mouth, head, and neck (Chap. 37). The predominant isolates are components of the normal microbiota of the upper airways—mainly Prevotella species,

P. asaccharolytica, Fusobacterium species, peptostreptococci, and microaerophilic streptococci. PART 5 Infectious Diseases OROFACIAL INFECTIONS  The most common oral infections are odontogenic and include dental caries and periodontal disease (gingi­ vitis and periodontitis). While dental caries usually manifest with pain, sensitivity, and discoloration of the tooth, periodontal disease involves inflammation of the gums and underlying tissue. In its more severe forms, periodontitis can result in difficulty chewing, loose teeth, and occasionally tooth loss. Severe orofacial infections typically develop as a consequence of dental infection, and the infection can spread from the tooth to different anatomic areas that provide the least resistance, resulting in periapical, periodontal, or pericoronal infections. If the dental surface is completely breached, an endodontic infection (pul­ pitis) can occur. In late stages of pulpitis, the tooth is generally very sensitive to heat, but cold stimuli may provide relief. Left untreated, pulpitis can progress to invade the alveolar bone and develop into a periapical abscess. The abscesses, particularly those involving the sec­ ond and third molars, can occasionally extend into the submandibular, Head and neck Lung Abdomen Soft tissue and joints Bacteremia Catheter-related Surgical site infection FIGURE 182-2  Distribution of types of infection from which anaerobic organisms were cultured at a single hospital over a 7-year period. Head and neck infections included sinusitis, otitis media, and retropharyngeal abscess; abdominal infections included liver abscess, biliary tract infection, bowel obstruction, and intraabdominal abscess; catheter-related infections included those related to peritoneal dialysis catheters and ventriculoperitoneal shunts. (Data from Y Park et al: Clinical features and prognostic factors of anaerobic infections: A 7-year retrospective study. Korean J Intern Med 24:13, 2009.)

sublingual, and submental spaces (Ludwig’s angina). This infection results in marked local swelling of tissues, with pain, trismus, and supe­ rior and posterior displacement of the tongue. Submandibular swelling of the neck and obstruction by the tongue can impair swallowing and cause respiratory obstruction. In some cases, tracheotomy is lifesaving. Microbiologically, dental caries begin with the binding of Strep­ tococcus mutans and Streptococcus sanguis to the tooth surface, with subsequent further colonization by anaerobes. In contrast, periodon­ titis is typically associated with P. gingivalis, Tannerella forsythia, Aggregatibacter actinomycetemcomitans, and Treponema denticola. Fusobacterium, Actinomyces, Peptostreptococcus, and Bacteroides spe­ cies (other than B. fragilis) are the organisms most commonly isolated from periapical abscesses. ACUTE NECROTIZING ULCERATIVE GINGIVITIS  Gingivitis may become a necrotizing infection (trench mouth, Vincent’s stomatitis). The onset of disease is usually sudden and is associated with painful bleeding gums, foul breath, and a bad taste. The gingival mucosa, especially the papillae between the teeth, becomes ulcerated and may be covered by a yellowish-white or gray “pseudomembrane,” which is removable with gentle pressure. Patients may become systemically ill, developing fever, malaise, cervical lymphadenopathy, and leukocytosis. The infection can sometimes extend into the pharynx, resulting in an extremely sore throat, foul breath, and tonsillar pillars that are swollen, red, ulcerated, and covered by a pseudomembrane. Prevotella, Treponema, and Fusobacterium species have been implicated. In some cases, acute necrotizing gingivitis can rapidly progress to noma (cancrum oris), a gangrenous infection that destroys the soft and hard tissues related to the oral cavity. Noma occurs most frequently in young children (1–4 years of age) who have immune dysfunction related to malnutrition and endemic infections (particularly measles). This infection occurs worldwide but is most common in sub-Saharan Africa, where the incidence is 1–7 cases per 1000 children. Although the pathogenesis is not fully understood, infections with F. necrophorum and P. intermedia are thought to be key drivers of this disease. Without treatment, the mortality rate is 70–90%. PERIPHARYNGEAL SPACE INFECTIONS  These infections arise from the spread of organisms from the upper airways to potential spaces formed by the fascial planes of the head and neck. The etiology is typically polymicrobial and represents the normal microbiota of the mucosa of the originating site. Peritonsillar abscess (quinsy) is the most common peripharyngeal infection and occurs as a complication of acute tonsillitis. Consistent with its association with tonsillitis, adolescents are most commonly affected. Patients present with a sore throat, dysphagia, peritonsillar swelling, muffled voice, and uvular deviation to the contralateral side. The abscess material typically grows group A Streptococcus in conjunc­ tion with obligate anaerobes (e.g., Bacteroides, Prevotella, and Pepto­ streptococcus species) (Chap. 37). Retropharyngeal abscesses typically occur in children 2–4 years of age, although they can occur at any age. Although a suppurative infection of the retropharyngeal lymph nodes is the usual precursor to these abscesses in children, foreign-body ingestion and/or local trauma is more commonly the inciting factor in adults. The clinical presentation shares many features with peritonsil­ lar abscesses, but difficulty extending the neck and torticollis are more common with retropharyngeal abscesses. The etiologic agents are the same as in peritonsillar abscesses, with additional aerobic organisms (e.g., S. aureus, viridans streptococci) also playing a role. SINUSITIS AND OTITIS  Anaerobic bacteria have been implicated in chronic sinusitis but play little role in acute sinusitis. Numerous studies related to the microbiology of chronic sinusitis have been con­ ducted; on average, anaerobic bacteria have been found in two-thirds of patients, with many studies demonstrating their presence in >90% of patients. Anaerobic bacteria represent ~40% of all bacteria cultured, with Peptostreptococcus, Prevotella, and Porphyromonas species the most commonly isolated anaerobes. S. aureus and Enterobacteriaceae are the aerobes most commonly recovered in chronic sinusitis. Anaero­ bic bacteria have been isolated in ~60% of cases of chronic suppurative otitis media in children, but they are not involved in acute otitis media.

COMPLICATIONS OF ANAEROBIC HEAD AND NECK INFECTIONS  Direct extension of these infections into contiguous areas can result in addi­ tional disease manifestations. Cranial spread of these infections can result in osteomyelitis of the skull or mandible or in intracranial infec­ tions, such as brain abscess and subdural empyema. Caudal spread can produce mediastinitis or pleuropulmonary infection. Hematogenous complications can also result from anaerobic infections of the head and neck. Bacteremia, which occasionally is polymicrobial, can lead to endocarditis or other distant infections. Lemierre’s syndrome (Chap. 37), which is usually due to F. necrophorum, is an acute oro­ pharyngeal infection with secondary septic thrombophlebitis of the internal jugular vein and frequent septic emboli, most commonly to the lung. This infection typically begins with pharyngitis, which is fol­ lowed by local invasion in the lateral pharyngeal space, with resultant internal jugular vein thrombophlebitis. Central Nervous System (CNS) Infections  CNS infections associated with anaerobic bacteria are brain abscess, epidural abscess, and subdural empyema, in which anaerobes are recovered in up to 30, 20, and 10% of cases, respectively. The frequency with which anaerobes are recovered depends in large part on the underlying reason for the infection. For example, brain abscesses are typically due to hematog­ enous seeding, contiguous spread, penetrating head trauma, or recent surgical intervention. Anaerobic bacteria are most commonly associ­ ated with brain abscesses resulting from contiguous spread (related to otogenic, odontogenic, and sinus infections), and the pathogens recovered are the same as in these antecedent infections. Facultative or microaerophilic streptococci and coliforms are often part of a mixed infecting flora in brain abscesses. The location of the abscess may also provide insight into the pathogens. Abscesses in the frontal lobe (often associated with sinusitis) are due to anaerobes, streptococci, and staphylococci; temporal lobe and cerebellar abscesses are often related to the oral microbiota and middle-ear pathogens. Obligate anaerobes rarely cause meningitis. Only one obligate anaerobe was identified in a seminal study of 188 bacterial meningitis isolates, and a U.S. national surveillance study of 18,642 such isolates collected between 1977 and 1981 found only five obligate anaerobes. This low incidence may be due, in part, to the fact that many clinical microbiology laboratories do not routinely culture cerebrospinal fluid (CSF) for anaerobes. Pleuropulmonary Infections  The lungs are constantly seeded with organisms from the oral microbiota via subclinical microaspira­ tion that normally occurs in all people. Even though the lung is the site of oxygen exchange and is therefore an overwhelmingly aerobic envi­ ronment, the organisms most abundant in the lower respiratory tract (as assessed by culture-independent methods) include anaerobes such as Prevotella and Veillonella species, with oral microaerophilic strep­ tococcal species (e.g., the Streptococcus milleri group) also present in significant abundances. In patients who have impaired bacterial clear­ ance (due to decreased cough, dysfunctional mucociliary transport, or alcohol intoxication) and/or increased rates of aspiration (due to neu­ rologic disorders, impaired consciousness, or swallowing dysfunction), these anaerobic bacteria can establish an infection and result in aspira­ tion pneumonia, lung abscess, or empyema. These anaerobic infections have an indolent course that may serve as a clinical clue differentiating them from conditions with other etiologies (e.g., chemical pneumoni­ tis, pneumococcal pneumonia) that often present more acutely. ASPIRATION PNEUMONIA  Bacterial aspiration pneumonia must be distinguished from two other clinical syndromes associated with aspiration that are not of bacterial etiology. One syndrome results from aspiration of food or, rarely, other foreign bodies. Obstruction of major airways typically results in difficulty breathing, atelectasis, and moderate nonspecific inflammation. Therapy consists of removal of the foreign body. The second aspiration syndrome relates to chemical pneumonitis caused by inhalation or aspiration of alveolar irritants. Perhaps the most common cause of chemical pneumonitis is Mendelson syndrome, which results from regurgitation and aspira­ tion of acidic gastric juices. Pulmonary inflammation—including the

destruction of the alveolar lining, with transudation of fluid into the alveolar space—occurs with remarkable rapidity. This syndrome typi­ cally develops within 4–6 h, often following anesthesia when the gag reflex is depressed. The patient becomes tachypneic, tachycardic, and hypoxic, often in the absence of fever. The leukocyte count may rise, and the chest x-ray may evolve from normal to a complete bilateral “whiteout” within 8–24 h. Sputum production is minimal. The pulmo­ nary signs and symptoms often resolve quickly with symptom-based therapy, but this condition can culminate in respiratory failure due, in part, to pulmonary edema. Antibiotic therapy is not indicated unless bacterial superinfection occurs.

In contrast to these syndromes, bacterial aspiration pneumonia develops over a period of several days or weeks rather than hours. The pathogenesis includes some combination of an increased bacterial burden, increased virulence of the organisms aspirated, and potential airway damage related to aspiration of gastric fluid. Patients gener­ ally report fever, malaise, and sputum production. In some patients, weight loss and anemia reflect a more chronic process. Usually the history reveals factors predisposing to aspiration, such as significant alcohol consumption or neurologic impairment due to a previ­ ous stroke. Severe dental disease is often associated with aspiration pneumonia, but it is not clear whether this association relates to an increased number of oral microbes and/or the presence of organisms with increased virulence. Sputum characteristically is not malodorous unless the process has been ongoing for at least a week. Chest x-rays show consolidation in dependent pulmonary segments: in the basilar segments of the lower lobes if the patient has aspirated while upright and in either the posterior segment of the upper lobe (usually on the right side, given that the right mainstem bronchus has a more vertical orientation) or the superior segment of the lower lobe if the patient has aspirated while supine. CHAPTER 182 A mixed bacterial population with many PMNs is evident on Gram’s staining of sputum. Expectorated sputum is unreliable for anaerobic cultures because of inevitable contamination by the normal oral micro­ biota. Reliable specimens for culture can be obtained by transtracheal or transthoracic aspiration—techniques that are rarely used at present. Although the culture of protected-brush specimens or bronchoalveolar lavage fluid obtained by bronchoscopy is controversial, more recent data suggest that these approaches can be used without oropharyngeal contamination and can recover anaerobic organisms from the lower respiratory tract in a site-directed manner. Further research is needed to determine how these approaches compare with the previous gold standards. Infections Due to Mixed Anaerobic Organisms ANAEROBIC LUNG ABSCESSES  (See also Chap. 132) These abscesses result from subacute anaerobic pulmonary infection. The clinical presentation typically involves a history of constitutional signs and symptoms (including malaise, weight loss, fever, night sweats, and foul-smelling sputum) that have typically persisted for 1–3 weeks prior to hospitalization. Patients who develop lung abscesses often, but not always, have an antecedent dental infection. Abscess cavities may be single or multiple and generally occur in dependent pulmonary segments (Fig. 182-3). The differential diagnosis for lung abscesses includes pneumonia (including necrotizing pneumonia), a purulent pleural effusion with a bronchopleural fistula, and a pneumatocele. Of note, infection with some aerobic organisms, particularly S. aureus, can develop into a lung abscess without an anaerobic component. Approximately 90% of cases have an anaerobe identified—usually three to six isolates per sample—if careful attention is paid to handling and processing of the abscess sample. The most common isolates include peptostreptococci, Prevotella and Porphyromonas species, and F. nucleatum. An important finding is that ~90% of cultures also demonstrate the presence of aerobic organisms, such as S. aureus, Streptococcus pneumoniae, and Klebsiella pneumoniae. Consistent with the notion that anaerobes are contributing to disease, patients often do not improve clinically until they receive an antibiotic regimen that includes anaerobic coverage. EMPYEMA  Empyema is a manifestation of long-standing anaerobic pulmonary infection and manifests with thick, purulent material in

FIGURE 182-3  Chest radiograph (left) and CT image (right) of a lung abscess. The patient aspirated while supine and developed an abscess in the posterior segment of the right upper lobe. Cultures were pretreated and grew only Klebsiella pneumoniae. (Images provided by Gita N. Mody, MD, MPH, Division of Cardiothoracic Surgery, Department of Surgery, The University of North Carolina at Chapel Hill.) PART 5 Infectious Diseases the pleural space, often in association with a bronchopleural fistula. Alternatively, a subdiaphragmatic infection may extend into the pleural space and similarly result in an empyema. The clinical presentation resembles that of other anaerobic pulmonary infections and may include foul-smelling sputum, pleuritic chest pain, and marked chestwall tenderness. This disease process must be differentiated from a parapneumonic effusion resulting from more routine causes of pneu­ monia (e.g., S. pneumoniae). In the latter instance, the fluid is a thin exu­ date that has a mean white blood cell (WBC) count of ~5000 cells/mL,

a lactate dehydrogenase level of >200 IU/L, and a pH of ~7.4. In contrast, empyema is characterized by foul-smelling thick pus with a mean WBC count of ~55,000 cells/mL, a lactate dehydrogenase level of >1000 IU/L, and a pH of <7.2 as well as loculations and a thick pleural peel on imaging. Drainage and occasionally decortication of the visceral and parietal pleura are required. Defervescence, a return to a feeling of well-being, and resolution of the process may require several months, particularly in the absence of surgical intervention. Intraabdominal Infections  Breach of the gut mucosal surface (e.g., due to trauma, intestinal perforation, or malignancy) allows members of the microbiota to enter the normally sterile perito­ neum. Accordingly, the offending organisms reflect the microbiota in the affected intestinal region. For example, recovery of Candida species from intraabdominal infections should prompt evaluation of the stomach and proximal small bowel for potential perforation. Furthermore, a study of patients with perforated and gangrenous appendicitis demonstrated that virtually all samples yielded E. coli

and members of the B. fragilis group; peptostreptococci and Bilophila wadsworthia—additional components of the appendiceal and colonic microbiota—also were recovered from >50% of samples. Notably, some studies have identified an average of 10 different bacterial species, with an anaerobe-to-aerobe ratio of ~3:1. Given that >1000 bacterial species are present in the colonic microbiota, the dominance of such a limited repertoire of bacterial genera and species recovered in intraabdominal infections reflects a combina­ tion of two factors: the increased propensity of these organisms to result in intraabdominal abscesses and the difficulty faced by clinical microbiology laboratories in culturing the diverse organisms pres­ ent in these samples. See Chap. 137 for a complete discussion of intraabdominal infections.

Neutropenic enterocolitis (typhlitis) involves marked thickening of the bowel wall (typically >4 mm) in the setting of neutropenia, abdominal pain, and fever. This condition most commonly affects the cecum and may extend to the neighboring terminal ileum and/or proximal colon, but any intestinal region may be involved. Typhlitis generally occurs after 1–2 weeks of chemotherapy-induced neutrope­ nia associated with treatment of hematologic or, less commonly, solid tumor malignancies, but it can occur regardless of the cause of neutro­ penia. At least 5% of adults hospitalized for malignancy are thought to develop typhlitis, but this is likely an underestimate. Although the right lower quadrant is the most common location of abdominal pain and tenderness, these symptoms are absent in nearly half of cases; more­ over, some patients, particularly those taking glucocorticoids, may not experience abdominal pain at all. Given the weakened integrity of the bowel wall and the associated neutropenia, patients often develop bacteremia due to one or more organisms related to the microbiota of the affected intestinal segment. Patients who develop bacteremia due to Clostridium septicum often have relatively severe disease, and identi­ fication of this organism is highly associated with the presence of malignancy—notably, colon cancer. Medical management including bowel rest, intestinal decompression, and broad-spectrum antibiotic administration is generally successful, although surgical intervention may be required in cases of persistent intestinal bleeding, necrotic bowel, or clinical deterioration suggestive of an ongoing intestinal process. Pelvic Infections  Anaerobes are frequently encountered in pel­ vic inflammatory disease, pelvic abscess, endometritis, tubo-ovarian abscess, septic abortion, and postoperative or postpartum infections. These infections are often of mixed etiology, involving both anaerobes and coliforms; pure anaerobic infections without coliform or other facultative bacterial species occur more often in pelvic than in intraab­ dominal sites. The major anaerobic pathogens in pelvic abscesses are P. bivia, P. disiens, and the B. fragilis group, but many other anaerobes also have been implicated. See Chap. 141 for a complete discussion of pelvic inflammatory disease. Anaerobic bacteria have been thought to be contributing factors in bacterial vaginosis. This syndrome of unknown etiology is character­ ized by a profuse malodorous discharge and a change in bacterial ecology that results in replacement of the Lactobacillus-dominated normal microbiota with an overgrowth of anaerobic bacterial species.

Culture-based and culture-independent approaches have identified numerous organisms, including Gardnerella vaginalis, peptostrepto­ cocci, genital mycoplasmas, and species within the genera Prevotella, Mobiluncus, Atopobium, Leptotrichia, Megasphaera, and Eggerthella. This wide array of implicated bacteria may reflect differences in the overall disease spectrum of bacterial vaginosis and/or a shared physi­ ologic response to these different organisms. Skin and Soft Tissue Infections  Similar to other anatomic sites, skin or soft tissue injured by trauma, ischemia, or surgery creates a suitable environment for anaerobic infections. The infecting bacteria either are introduced directly (e.g., wounds associated with intestinal surgery, decubitus ulcers, or human bites) or originate in contiguous areas (e.g., cutaneous abscesses, rectal abscesses, and axillary sweat gland infections [hidradenitis suppurativa]). Anaerobes also are often cultured from foot ulcers of diabetic patients. The most common loca­ tions for anaerobic cellulitis include the neck, trunk, groin (including the genitalia), and legs. The deep soft tissue infections associated with anaerobic bacteria are gas gangrene, synergistic cellulitis (both progres­ sive and necrotizing), necrotizing fasciitis, and myositis (Chaps. 134 and 159). Gas gangrene (crepitus cellulitis) is most often due to C. perfringens, although other clostridial species have been implicated as well. This infection involves extensive gas formation in the tissue leading to crepitus and a thin, dark, occasionally malodorous discharge. True gas gangrene typically presents with fever and tenderness around the lesion and can rapidly spread; in contrast, there are somewhat more indolent forms of anaerobic cellulitis that may involve some gas formation but often present without fever or extensive local pain and can spread over the course of days rather than minutes. Progressive bacterial synergistic gangrene (Meleney gangrene) is characterized by an area of exquisite pain, redness, and swelling fol­ lowed by ulceration. As the ulcer enlarges, it is surrounded by a viola­ ceous ring that fades into a pink edematous border. If it is not promptly treated, the ulcer continues to enlarge, and new, distant ulcers may emerge. Symptoms are limited to pain; fever is not typical. Peptostrep­ tococci and microaerophilic streptococci are commonly found in the leading edge of the lesions, and S. aureus and Proteus species can be isolated from the ulcerated lesion. Treatment includes surgical removal of necrotic tissue and antimicrobial administration. In contrast, syn­ ergistic necrotizing cellulitis involves the deep fascia and occurs near the point of bacterial entry. Pain, fever, and systemic symptoms are common. If this form of cellulitis involves the scrotum, perineum, and anterior abdominal wall, it is referred to as Fournier gangrene. S. aureus, the B. fragilis group, Peptostreptococcus species, Clostridium species, Fusobacterium species, and members of the family Enterobacteriaceae are the predominant organisms identified. Necrotizing fasciitis, a rapidly spreading destructive disease of the fascia, is usually attributed to group A streptococci (Chap. 153) but can also be a mixed infection involving anaerobes and aerobes. Polymicro­ bial necrotizing fasciitis differs from stereotypical group A streptococ­ cal necrotizing fasciitis in that the initial erythematous, swollen, tender lesions progress over 3–5 days (as opposed to 1–3 days), with conse­ quent skin breakdown and cutaneous gangrene. Fever, subcutaneous gas, development of anesthesia (often before skin necrosis), and a foulsmelling discharge are common. The particular clinical findings some­ times suggest the causative agent: regional lymphadenopathy suggests the B. fragilis group; necrosis and gangrene suggest Clostridium species, peptostreptococci, the B. fragilis group, and Enterobacteriaceae; bul­ lous lesions suggest Enterobacteriaceae; a foul-smelling odor suggests Bacteroides and Clostridium species; and subcutaneous gas suggests peptostreptococci, Clostridium species, and the B. fragilis group. More­ over, diabetic infections are often associated with Bacteroides species, Enterobacteriaceae, and S. aureus, and infections related to trauma are associated with Clostridium species. Although S. aureus is the typical cause of myositis, anaerobes— particularly C. perfringens—are often recovered from patients with pyogenic myositis. In anaerobic streptococcal myonecrosis, pepto­ streptococci are often identified along with group A streptococci or

S. aureus. Patients typically present with fever, muscle pain, fatigue, and an elevated creatinine kinase level suggestive of muscle inflammation.

Bone and Joint Infections  A comprehensive review of the world literature on anaerobic bone infections through 1975 included >650 cases. Of these, ~400 cases were caused by Actinomyces species; anaero­ bic cocci and Bacteroides, Fusobacterium, and Clostridium species were most commonly identified in the remaining cases. Actinomycotic involvement of the jaw was the most common bone infection, with the mandible involved four times as frequently as the maxilla. Patients with cervicofacial actinomycosis (Chap. 180) are often described as having a “lumpy jaw” because of the prominent soft tissue swelling that is sometimes mistaken for malignancy or granulomatous disease. These infections can be chronic in nature, can include the development of sinus tracts, can progress across normal tissue boundaries, and can require prolonged antibiotic treatment to prevent relapse. The verte­ brae are the second most common location for Actinomyces infection; involvement of the thorax, abdomen, or pelvis is much less frequent. Osteomyelitis involving anaerobes other than Actinomyces species most commonly develops by extension of an adjacent infection (e.g., soft tissue, paranasal sinus, or middle-ear infection). For example, diabetic foot ulcers and decubitus ulcers may be complicated by mixed aerobic–anaerobic osteomyelitis (Chap. 136). Hematogenous seeding of bone by anaerobes is uncommon and is thought to occur in fewer than 10% of cases. The most common sites of anaerobic osteomyelitis are the head (skull and jaw) and the extremities. Fusobacteria have been isolated in pure culture from infections of the mastoid, mandible, and maxilla. Clostridium species have been reported as anaerobic pathogens in cases of osteomyelitis of the long bones following trauma. Anaerobic and microaerophilic cocci are most frequently isolated from infections involving the skull or mastoid; usually, these organisms are present in mixed cultures. CHAPTER 182 In contrast to anaerobic osteomyelitis, anaerobic arthritis (Chap. 135) is uncommon, typically involving a single isolate, and most cases are secondary to hematogenous spread. Although Fusobacterium spe­ cies accounted for nearly one-third of cases in the preantibiotic era, C. acnes, peptostreptococci, and B. fragilis are now among the more frequent causes of anaerobic septic arthritis. Peptostreptococci and

C. acnes are often found in association with prosthetic joints, Fusobac­ terium species have a predilection for the sternoclavicular and sacro­ iliac joints, and clostridial arthritis is especially common after trauma. As a frequent cause of bacteremia, B. fragilis is a common cause of anaerobic septic arthritis; however, arthritis occurs in fewer than 5% of patients with B. fragilis bacteremia. Infections Due to Mixed Anaerobic Organisms Bacteremia  B. fragilis is the anaerobe most commonly isolated from blood cultures. Although the frequency of positive cultures appeared to be decreasing in the 1980s, more recent evidence suggests that the rate is now increasing and that the increase may be related to changing demographics, with more patients who are elderly, immunocompro­ mised, and/or receiving medications that may disrupt the mucosal barrier (e.g., chemotherapy). The source of bacteremia is most often an abscess in the abdomen, female genital tract, or soft tissue. At a large tertiary-care U.S. hospital, 0.8% of all positive blood cultures yielded an anaerobic gram-negative bacillus, with 0.5% yielding B. fragilis. A similar study in France revealed that 0.6% of all positive blood cultures yielded an anaerobic organism; 60% of these isolates were Bacteroides species, and 22% were Clostridium species. Peptostreptococcus and Fusobacterium species are also recovered with significant frequency. Once the organism in the blood has been identified, both the portal of bloodstream entry and the underlying problem that probably led to seeding of the bloodstream can often be deduced from an understand­ ing of the organism’s normal site of residence. For example, mixed anaerobic bacteremia including B. fragilis usually implies a colonic pathology, with mucosal disruption from neoplasia, diverticulitis, or some other inflammatory lesion. The initial manifestations are determined by the portal of entry and reflect the localized condition. Although the clinical manifestations of B. fragilis bacteremia (e.g., rigors, hectic fevers) are similar to those of aerobic gram-negative

bacillary bacteremia, the incidence of septic shock is lower with B. fra­ gilis. This difference may be due to differences in the immunostimula­ tory effects of the different endotoxin structures.

In virtually all cases, isolation of a member of the B. fragilis group from blood indicates underlying infection that is associated with a mortality rate of 60% if untreated. It has been suggested that the mor­ tality rate depends in part on the species recovered (B. thetaiotaomicron

P. distasonis > B. fragilis), but it is unclear whether differences in mortality rates relate to intrinsic differences in the virulence of these organisms, in their antimicrobial susceptibility profiles, and/or in the host’s immune response. Case–fatality rates appear to increase with the increasing age of the patient (with reported rates of >66% among patients >60 years old), with the isolation of multiple species from the bloodstream, and with the failure to surgically remove a focus of infection. Endocarditis  (See also Chap. 133) Although gram-negative anaer­ obic bacteria only rarely cause endocarditis, their involvement is asso­ ciated with significant mortality rates (21–43%). Members of the B. fragilis group are the most commonly identified gram-negative anaer­ obes in endocarditis. Anaerobic streptococci, which are often classified incorrectly, are likely responsible for this disease more frequently than is generally appreciated. Compared with aerobic bacterial endocarditis, endocarditis due to Bacteroides species is less likely to be associated with a history of cardiovascular disease and more likely to involve thromboembolic complications. ■ ■DIAGNOSIS There are three critical steps in the diagnosis of anaerobic infection: (1) proper collection of specimens; (2) rapid transport of the specimens to the microbiology laboratory, preferably in anaerobic transport media; and (3) proper handling of the specimens by the laboratory. Specimens must be collected by meticulous sampling of infected sites, with avoid­ ance of contamination by the normal microbiota. Samples from sites known to harbor numerous anaerobes (e.g., the mouth, nose, vagina, feces) are not acceptable for anaerobic culture as the presence of the normal microbiota will complicate interpretation of the results in a clinically meaningful manner. In contrast, samples from normally sterile locations (e.g., blood, pleural fluid, peritoneal fluid, CSF, and aspirates or biopsy samples from normally sterile sites) are appropriate for anaerobic culture in clinical microbiology laboratories. As a general rule, liquid or tissue specimens are preferred; if swab specimens must be used, special anaerobic swab systems should be used to help main­ tain persistence of anaerobes. Liquid samples should be collected in an air-free syringe that is then capped, injected into anaerobic transport bottles, or quickly transported to the clinical microbiology laboratory for immediate culture. PART 5 Infectious Diseases Because of the time and difficulty involved in the isolation of anaer­ obic bacteria, the diagnosis of anaerobic infections must frequently be based on presumptive evidence. As mentioned previously, anaerobic infections are sometimes suggested by specific clinical factors, such as origins from a site with an anaerobic-rich microbiota (e.g., the intesti­ nal tract, oropharynx), the presence of an abscess, involvement of sites with lowered oxidation-reduction potential (e.g., avascular necrotic tissues), a foul odor, and the presence of gas in tissues. None of these features is necessarily pathognomonic or required for the diagnosis of an anaerobic infection, but these are helpful clues to keep in mind when constructing a differential diagnosis. When cultures of obviously infected sites or purulent material yield no growth, streptococci only, or a single aerobic species (such as E. coli) and Gram’s staining reveals a mixed bacterial population, the involvement of anaerobes should be suspected; the implication is that the anaerobic microorganisms have failed to grow because of inadequate transport and/or culture techniques. It is also important to remember that prior antibiotic therapy reduces the cultivability of these bacteria. Failure of an infection to respond to antibiotics that are not active against anaerobes (e.g., aminoglycosides and—in some circumstances—penicillin, cephalosporins, or tetracyclines) suggests an anaerobic etiology.

TREATMENT Anaerobic Infections Similar to successful therapy for other types of infection, treatment for anaerobic infections requires the administration of appropri­ ate antibiotics, surgical debridement of devitalized tissues, and drainage of any large abscess. Any mucosal breach must be closed promptly to prevent ongoing infection. ANTIBIOTIC THERAPY AND RESISTANCE The antibiotics used to treat anaerobic infections should be active against both aerobic and anaerobic organisms because many of these infections are of mixed etiology. Antibiotic regimens can usu­ ally be selected empirically on the basis of the location of infection (which provides insight into the likely species involved), the sever­ ity of infection, and knowledge of local antimicrobial resistance pat­ terns. Other factors influencing the selection of antibiotics include need for penetration into certain organs (such as the brain) and associated toxicity (Chap. 149). As with all infections, the general maxim is to use the the narrowest-spectrum agent(s) possible so as to minimize the impact on the normal microbiota and the develop­ ment of resistance. Because of the slow growth rate of many anaerobes, the lack of standardized testing methods and of clinically relevant standards for resistance, and the generally good results obtained with empirical therapy, the role of antibiotic susceptibility testing of these organ­ isms has been limited in most clinical microbiology laboratories. Instead, isolates are sent to reference laboratories for susceptibility testing when an infection is serious (e.g., brain abscess, meningitis, joint infection), is refractory, or requires prolonged therapy (e.g., osteomyelitis, prosthetic joint infection, endocarditis). Such testing should also be considered when a patient is not responding to anti­ microbial therapy as expected; multidrug-resistant anaerobes have been reported. Antimicrobial susceptibility testing is also helpful in monitoring the activity of new drugs and recording current resis­ tance patterns among anaerobic pathogens. The need for susceptibility testing of anaerobic organisms is highlighted by increasing rates of antimicrobial resistance, geo­ graphic and institutional differences in susceptibility profiles, spe­ cies-specific antibiograms, and the potential for worse clinical outcomes when ineffective antibiotics are used. These differences preclude making any sweeping generalizations regarding antibiotic therapy for anaerobic infections. For example, rates of resistance to piperacillin-tazobactam have remained low (≤1%) for all Bacteroi­ des species in the United States, but B. thetaiotaomicron isolates in Korea have a notably higher resistance rate (17%). Clindamycin was historically effective against members of the B. fragilis group, but rates of resistance have increased to 30–43% in the United States and are >80% in some parts of the world. Furthermore, metronida­ zole is effective against many different anaerobic organisms and is considered a first-line agent for many anaerobic infections world­ wide, but, in a population of Colombian patients with refractory periodontitis, 45% of Fusobacterium isolates and 25% of Prevotella and Porphyromonas strains were resistant to metronidazole; this finding underscores the importance of understanding the local antibiogram and of assessing susceptibility profiles in refractory disease. Empirical Therapy  Not every anaerobe isolated must be specifically targeted by the antibiotic regimen. Given that infections involving anaerobes are typically polymicrobial, that the cultivation and iden­ tification of anaerobes are challenging (i.e., not all organisms may be recovered), and that organisms often depend on one another for persistence, clinical resolution of the infection is often achieved with empirical antibiotics targeting the bulk of the organisms recovered. Antibiotics that demonstrate no useful activity against anaerobes include aminoglycosides, monobactams, and trimethoprim-sulfa­ methoxazole. With the caveat that susceptibility profiles may change with time and geography, the antibiotics that are commonly used

TABLE 182-2  Antimicrobial Therapy That Is Typically Active against Commonly Encountered Anaerobes ANTIBIOTIC(S) CAVEATS Metronidazole This drug is clinically unreliable against

gram-positive non-spore-forming anaerobes (e.g., Actinomyces spp., Propionibacterium spp., Peptostreptococcus spp.). Rates of resistance are increasing in some

gram-negative anaerobes. The newer cephalosporin/β-lactamase combinations have limited anaerobic activity. β-Lactam/β-lactamase inhibitor combinations (ampicillin-sulbactam, ticarcillin–clavulanic acid, piperacillin-tazobactam) Clindamycin Rates of resistance are increasing in Bacteroides spp. Carbapenems (meropenem, imipenem, ertapenem, doripenem) Rates of resistance are currently very low (<5%), although some carbapenemase-producing strains have been identified. Chloramphenicol Some clinical failures have been noted, even

when the isolate is found to be susceptible by in vitro testing. as empirical therapy against anaerobic bacteria include metroni­ dazole, β-lactam/β-lactamase inhibitor combinations, clindamycin, carbapenems, and chloramphenicol (Table 182-2). Metronidazole is active against gram-negative anaerobes, includ­ ing nearly all isolates of Bacteroides species, and gram-positive spore-forming organisms, such as C. difficile (Chap. 139) and other Clostridium species. Given intrinsically reduced susceptibility, metronidazole is clinically unreliable against gram-positive non- spore-forming organisms, such as Actinomyces, Propionibacterium, Lactobacillus, Bifidobacterium, Eubacterium, and Peptostreptococ­ cus. Of note, a few metronidazole-resistant Bacteroides isolates have been identified in the United States, and rates of such resistance have been increasing in Europe. Moreover, the rate of resistance to metronidazole has probably been greatly underestimated in some countries (e.g., the United Kingdom) that use metronida­ zole susceptibility to discriminate between obligate and facultative anaerobes (with obligate anaerobes defined by their susceptibility). Although the majority of metronidazole-resistant isolates have been identified in patients who have been exposed to the drug, resistant organisms have also been found in metronidazole-naïve patients. More than 90% of clinical isolates from the B. fragilis group produce β-lactamases that are predominantly active against cepha­ losporins and that are highly active, cell associated, and produced constitutively. Thus, members of the B. fragilis group are presumed to be resistant to penicillin and ampicillin but may remain suscep­ tible to extended-spectrum penicillins, particularly in combination with a β-lactamase inhibitor (e.g., ampicillin-sulbactam, piperacillin-

tazobactam). Rates of resistance to ampicillin-sulbactam are increas­ ing, particularly in P. distasonis, which has a reported resistance rate of 21% in the United States. Because β-lactamase production is not common in Clostridium species, these combination agents are usually effective. Of note, the newer cephalosporin/β-lactamase inhibitors (e.g., ceftolozane-tazobactam, ceftazidime-avibactam) have limited anaerobic activity. Clindamycin is active against many anaerobes. However, rates of resistance to clindamycin among Bacteroides species increased in the United States from 7% in 1981 to 33% in 2010–2012. Resistance to clindamycin among non-Bacteroides gram-negative anaerobes is much less common (<10%). Some Clostridium species are resistant to clindamycin, although C. perfringens typically is not. Carbapenems (ertapenem, doripenem, meropenem, and imipe­ nem) are active against anaerobes, with fewer than 3% of Bacteroides isolates resistant. There is little difference among resistance rates for specific species, and, of the carbapenems, imipenem typically has the lowest resistance rate. Although the β-lactamase produced by most Bacteroides species is unable to inactivate carbapenems, rare B. fragilis strains have been reported to produce a carbapenemase.

Resistance to chloramphenicol is rare in Bacteroides species. Nationwide surveys in the United States have identified no resistant organisms, but some isolates with elevated minimal inhibitory con­ centrations (MICs)—i.e., 16 μg/mL—have been noted. Although chloramphenicol has excellent in vitro activity against all clinically relevant anaerobes, some clinical failures have been documented. Therefore, this drug may be less preferable if other active agents are available.

Other antibiotics with more variable activity against anaerobes include the fluoroquinolones and tigecycline. Although many fluoroquinolones (e.g., ciprofloxacin, levofloxacin, ofloxacin) display reasonable activity against anaerobic organisms other than Bacteroides species, these agents exhibit poor activity against the B. fragilis group. Rates of resistance to moxifloxacin are relatively high (39–83%) among Bacteroides isolates obtained in the United States but are much lower among B. fragilis and B. thetaiotaomicron isolates collected in Korea (8 and 2%, respectively) or Taiwan (8 and 15%, respectively). Tigecycline is active against most anaerobic bacteria, although MICs are somewhat higher for Clostridium species. Tigecycline’s efficacy for treatment of complicated intraabdominal infections is comparable to that of imipenem, and it is therefore recommended as single-agent therapy for these infections. Infections at Specific Sites  In clinical situations, specific antibi­ otic regimens and durations must be tailored to the initial site of infection; the reader is referred to specific chapters on infections at specific sites for recommendations. In general, anaerobic infections are often broadly categorized as originating above or below the dia­ phragm. This distinction is clinically useful in that the predominant pathogens—and therefore the empirical antibiotic regimens—differ between these two categories of infection. CHAPTER 182 Infections above the diaphragm usually reflect the orodental microbiota, which includes Prevotella, Porphyromonas, Fusobacte­ rium, and Bacteroides species other than the B. fragilis group along with streptococci (both aerobic and microaerophilic). Accordingly, antibiotic regimens should cover both aerobic and anaerobic bacteria. Given that >70% of these infections include a β-lactamase-producing organism, β-lactam drugs (penicillins and cephalosporins) are poor options as monotherapy. The recommended regimens include clindamycin, a β-lactam/β-lactamase inhibitor combination, or met­ ronidazole in combination with a drug active against microaerophilic and aerobic streptococci (e.g., penicillin). Infections Due to Mixed Anaerobic Organisms Anaerobic infections arising below the diaphragm (e.g., colonic and intraabdominal infections) must be treated specifically with agents active against Bacteroides species, including B. fragilis. Single agents suitable for this purpose include cefoxitin, moxi­ floxacin, a β-lactam/β-lactamase inhibitor combination, or a carbapenem. A two-drug regimen is an alternative, with one drug active against anaerobes and the other against coliforms (e.g., metronidazole with either a cephalosporin or a fluoroquinolone). In addition, if the clinician suspects that gram-positive facultative organisms such as enterococci are involved, therapeutic regimens should include ampicillin or vancomycin. Although clindamycin and cefotetan were previously considered acceptable options for intraabdominal infections involving anaerobes, these drugs are no longer recommended because of escalating rates of resistance in the B. fragilis group. Ampicillin-sulbactam is not recommended because of high rates of resistance among community-acquired strains of E. coli rather than because of resistance in anaerobic bacteria. CNS infections involving anaerobic organisms may be treated with metronidazole, a carbapenem, chloramphenicol, or—if only gram-positive anaerobes are involved—penicillin. Clindamycin and cefoxitin have poor penetration into the CSF and should not be used. Cases of osteomyelitis in which a polymicrobial infection is identified from a bone biopsy specimen should be treated with a regimen that covers both aerobes and anaerobes, as some organisms that are often regarded as a contaminant (e.g., C. acnes) may have a pathogenic role. When an anaerobic organism is recognized as a

67 - SECTION 8 Mycobacterial Diseases

SECTION 8 Mycobacterial Diseases

major or sole pathogen infecting a joint, the duration of treatment should be similar to that used for arthritis caused by aerobic bacteria (Chap. 135).

Although not every anaerobe needs to be covered with pathogendirected therapy in most polymicrobial infections, several studies of Bacteroides bacteremia have clearly demonstrated that patients receiving effective therapy have lower mortality rates and more rapid sterilization of blood cultures than patients receiving ineffec­ tive therapy. FAILURE OF THERAPY Anaerobic infections that fail to respond to treatment or that relapse should be reassessed. Potential causes include an uncontrolled source of infection (e.g., ongoing intestinal leak into the perito­ neum), superinfection with a new organism, and/or antibiotic fail­ ure. Additional imaging may be useful to discern whether surgical drainage or debridement is warranted. Obtaining additional culture specimens will help identify whether an organism resistant to the antibiotics being used is present. Strong consideration should be given to obtaining susceptibility profiles for the isolates. ■ ■FURTHER READING Cooley L, Teng J: Anaerobic resistance: Should we be worried? Curr Opin Infect Dis 32:523, 2019. Debreuil LJ: Fifty years devoted to anaerobes: Historical, lessons, and highlights. Eur J Clin Microbiol Infect Dis 43:1, 2024. Finegold SM: Anaerobes: Problems and controversies in bacteriology, infections, and susceptibility testing. Rev Infect Dis 12:S223, 1990. Reissier S et al: Recent trends in antimicrobial resistance among PART 5 Infectious Diseases anaerobic clinical isolates. Microorganisms 11:1474, 2023. Styrt B, Gorbach SL: Recent developments in the understanding of the pathogenesis and treatment of anaerobic infections (2). N Engl J Med 321:240, 1989. Wexler HM: Bacteroides: The good, the bad, and the nitty-gritty. Clin Microbiol Rev 20:593, 2007. Section 8 Mycobacterial Diseases Mario C. B. Raviglione, Andrea Gori

Tuberculosis Tuberculosis (TB), which is caused by bacteria of the Mycobacterium tuberculosis complex, is one of the oldest diseases known to affect humans. In 2023, after being replaced by COVID-19 during the three previous years, TB probably returned to be the top cause of infectious death from a single infectious agent. Population genomic studies suggest that M. tuberculosis may have emerged ~70,000 years ago in Africa and subsequently disseminated along with anatomi­ cally modern humans, expanding globally during the Neolithic Age as human density started to increase. This disease most often affects the lungs, although other organs are involved in up to one-third of cases. If properly treated, TB caused by drug-susceptible strains is curable in the vast majority of cases. If untreated, the disease may be fatal in more than 70% of people. Transmission usually takes place through the airborne spread of droplet nuclei produced by patients with infectious pulmonary TB. Through pharmacologic prophylaxis, the development of the disease can be prevented in those who have contracted TB infection.

ETIOLOGIC AGENT Mycobacteria belong to the family Mycobacteriaceae and the order Actinomycetales. Of the pathogenic species belonging to the M. tuber­ culosis complex, which comprises eight distinct subgroups, the most common and important agent of human disease by far is M. tuberculosis (sensu stricto). A closely related organism isolated from cases in West, Central, and East Africa is M. africanum. The complex includes some zoonotic members, such as M. bovis (the bovine tubercle bacillus— characteristically resistant to pyrazinamide, once an important cause of TB transmitted by unpasteurized milk, and currently responsible for 140,000 human cases worldwide in 2020, half of them in Africa) and M. caprae (related to M. bovis). In addition, other organisms that have been reported rarely as causing TB include M. pinnipedii (a bacillus infecting seals and sea lions in the southern hemisphere and recently isolated from humans), M. mungi (isolated from banded mongooses in southern Africa), M. orygis (described in oryxes and other Bovidae in Africa and Asia and a potential cause of infection in humans), and

M. microti (the “vole” bacillus, a less virulent organism). Finally, M. canetti is a rare isolate from East African cases that produces unusual smooth colonies on solid media and is considered closely related to a supposed progenitor type. There is no known environmen­ tal reservoir for any of these organisms. M. tuberculosis is a rod-shaped, non-spore-forming, thin aero­ bic bacterium measuring 0.5 μm by 3 μm. Mycobacteria, including M. tuberculosis, are often neutral on Gram staining. However, once stained, the bacilli cannot be decolorized by acid alcohol; this charac­ teristic justifies their classification as acid-fast bacilli (AFB; Fig. 183-1). Acid fastness is due mainly to the organisms’ high content of mycolic acids, long-chain cross-linked fatty acids, and other cell-wall lipids. Microorganisms other than mycobacteria that display some acid fast­ ness include species of Nocardia and Rhodococcus, Legionella micdadei, and the protozoa Isospora and Cryptosporidium. In the mycobacterial cell wall, lipids (e.g., mycolic acids) are linked to underlying ara­ binogalactan and peptidoglycan. This structure results in very low FIGURE 183-1  Acid-fast bacillus smear showing M. tuberculosis bacilli. (Courtesy of the Centers for Disease Control and Prevention, Atlanta.)

68 - 183 Tuberculosis

183 Tuberculosis

major or sole pathogen infecting a joint, the duration of treatment should be similar to that used for arthritis caused by aerobic bacteria (Chap. 135).

Although not every anaerobe needs to be covered with pathogendirected therapy in most polymicrobial infections, several studies of Bacteroides bacteremia have clearly demonstrated that patients receiving effective therapy have lower mortality rates and more rapid sterilization of blood cultures than patients receiving ineffec­ tive therapy. FAILURE OF THERAPY Anaerobic infections that fail to respond to treatment or that relapse should be reassessed. Potential causes include an uncontrolled source of infection (e.g., ongoing intestinal leak into the perito­ neum), superinfection with a new organism, and/or antibiotic fail­ ure. Additional imaging may be useful to discern whether surgical drainage or debridement is warranted. Obtaining additional culture specimens will help identify whether an organism resistant to the antibiotics being used is present. Strong consideration should be given to obtaining susceptibility profiles for the isolates. ■ ■FURTHER READING Cooley L, Teng J: Anaerobic resistance: Should we be worried? Curr Opin Infect Dis 32:523, 2019. Debreuil LJ: Fifty years devoted to anaerobes: Historical, lessons, and highlights. Eur J Clin Microbiol Infect Dis 43:1, 2024. Finegold SM: Anaerobes: Problems and controversies in bacteriology, infections, and susceptibility testing. Rev Infect Dis 12:S223, 1990. Reissier S et al: Recent trends in antimicrobial resistance among PART 5 Infectious Diseases anaerobic clinical isolates. Microorganisms 11:1474, 2023. Styrt B, Gorbach SL: Recent developments in the understanding of the pathogenesis and treatment of anaerobic infections (2). N Engl J Med 321:240, 1989. Wexler HM: Bacteroides: The good, the bad, and the nitty-gritty. Clin Microbiol Rev 20:593, 2007. Section 8 Mycobacterial Diseases Mario C. B. Raviglione, Andrea Gori

Tuberculosis Tuberculosis (TB), which is caused by bacteria of the Mycobacterium tuberculosis complex, is one of the oldest diseases known to affect humans. In 2023, after being replaced by COVID-19 during the three previous years, TB probably returned to be the top cause of infectious death from a single infectious agent. Population genomic studies suggest that M. tuberculosis may have emerged ~70,000 years ago in Africa and subsequently disseminated along with anatomi­ cally modern humans, expanding globally during the Neolithic Age as human density started to increase. This disease most often affects the lungs, although other organs are involved in up to one-third of cases. If properly treated, TB caused by drug-susceptible strains is curable in the vast majority of cases. If untreated, the disease may be fatal in more than 70% of people. Transmission usually takes place through the airborne spread of droplet nuclei produced by patients with infectious pulmonary TB. Through pharmacologic prophylaxis, the development of the disease can be prevented in those who have contracted TB infection.

ETIOLOGIC AGENT Mycobacteria belong to the family Mycobacteriaceae and the order Actinomycetales. Of the pathogenic species belonging to the M. tuber­ culosis complex, which comprises eight distinct subgroups, the most common and important agent of human disease by far is M. tuberculosis (sensu stricto). A closely related organism isolated from cases in West, Central, and East Africa is M. africanum. The complex includes some zoonotic members, such as M. bovis (the bovine tubercle bacillus— characteristically resistant to pyrazinamide, once an important cause of TB transmitted by unpasteurized milk, and currently responsible for 140,000 human cases worldwide in 2020, half of them in Africa) and M. caprae (related to M. bovis). In addition, other organisms that have been reported rarely as causing TB include M. pinnipedii (a bacillus infecting seals and sea lions in the southern hemisphere and recently isolated from humans), M. mungi (isolated from banded mongooses in southern Africa), M. orygis (described in oryxes and other Bovidae in Africa and Asia and a potential cause of infection in humans), and

M. microti (the “vole” bacillus, a less virulent organism). Finally, M. canetti is a rare isolate from East African cases that produces unusual smooth colonies on solid media and is considered closely related to a supposed progenitor type. There is no known environmen­ tal reservoir for any of these organisms. M. tuberculosis is a rod-shaped, non-spore-forming, thin aero­ bic bacterium measuring 0.5 μm by 3 μm. Mycobacteria, including M. tuberculosis, are often neutral on Gram staining. However, once stained, the bacilli cannot be decolorized by acid alcohol; this charac­ teristic justifies their classification as acid-fast bacilli (AFB; Fig. 183-1). Acid fastness is due mainly to the organisms’ high content of mycolic acids, long-chain cross-linked fatty acids, and other cell-wall lipids. Microorganisms other than mycobacteria that display some acid fast­ ness include species of Nocardia and Rhodococcus, Legionella micdadei, and the protozoa Isospora and Cryptosporidium. In the mycobacterial cell wall, lipids (e.g., mycolic acids) are linked to underlying ara­ binogalactan and peptidoglycan. This structure results in very low FIGURE 183-1  Acid-fast bacillus smear showing M. tuberculosis bacilli. (Courtesy of the Centers for Disease Control and Prevention, Atlanta.)

permeability of the cell wall, thus reducing the effectiveness of most antibiotics. The complete genome sequence of M. tuberculosis comprises 4.4 million base pairs, 4043 genes encoding 3993 proteins, and 50 genes encoding stable RNAs; its high guanine-plus-cytosine content (65.6%) is indicative of an aerobic “lifestyle.” A large propor­ tion of genes are devoted to the production of enzymes involved in cell wall metabolism. Substantial genetic variability exists among the innu­ merable M. tuberculosis strains from different parts of the world. Based on such genetic variability it is possible to distinguish and compare different strains. Their distinction is important to study transmission dynamics and identify outbreaks. Starting in the 1990s, reproducible genotyping methods were developed to type the bacterium. Initially, they included insertion sequence 6110 (IS6110), restriction fragment length polymorphism (RFLP) typing, and spoligotyping. Lately, most studies utilize mycobacterial interspersed repetitive unit variable num­ ber tandem repeats (MIRU-VNTRs) and whole genome sequencing analysis. EPIDEMIOLOGY In 2023, 8.2 million new cases of TB (all forms, both pulmonary and extrapulmonary) were reported to the World Health Organization (WHO), around 97% of which were reported from low- and middleincome countries. However, because of insufficient case detection and incomplete notification, reported cases represent only about two-thirds of the total estimated cases. The WHO estimated that 10.8 million (range, 10.1–11.7 million, rate 134 per 100,000 persons) new (incident) cases of TB occurred worldwide in 2023. Eight countries accounted for two thirds of all cases: India (26%), Indonesia (10%), China (6.8%), the Philippines (6.8%), Pakistan (6.3%), Nigeria (4.6%), Bangladesh (3.5%) and the Democratic Republic of the Congo (3.1%). Of all cases, 55% occurred in male patients, 33% in female patients, and 12% in children. It is further estimated that 1.25 million (range 1.13–1.37 million) deaths from TB, including 160,000 among persons with HIV co-infection, occurred in 2023; 98% of these deaths were in low- and middle-income countries. Estimates of TB incidence and mortality FIGURE 183-2  Estimated tuberculosis (TB) incidence rates (per 100,000 population) in 2022. The designations used and the presentation of material on this map do not imply the expression of any opinion whatsoever on the part of the World Health Organization (WHO) concerning the legal status of any country, territory, city, or area or of its authorities or concerning the delimitation of its frontiers or boundaries. Dotted, dashed, and white lines represent approximate border lines for which there may not yet be full agreement. (Reproduced with permission from Global Tuberculosis Report 2023. Geneva, World Health Organization; 2023.)

rates (per 100,000 population) in 2022 are depicted in Figs. 183-2 and 183-3, respectively).

During the past few years, numbers of reported cases have stabilized or are slowly declining in most high-income countries. In the United States, TB cases and incidence rates steadily decreased from 1992 to 2021. In 2022, 8300 cases of TB (2.5 cases per 100,000 population) were reported to the U.S. Centers for Disease Control and Prevention (CDC), a slight increase from the 7882 cases reported in 2021. In the United States, TB is uncommon among young white adults of European descent, who have only rarely been exposed to M. tuberculosis infection during recent decades. In contrast, because of a high risk of transmission in the past, the prevalence of M. tuberculosis infection is relatively high among elderly whites; overall, 13 million persons are estimated to be “latently” infected. In general, adults ≥65 years of age have the highest incidence rate per capita and children <14 years of age the lowest. Of the total 7882 cases in 2021, 28% were among U.S.-born persons and 71% among non-U.S.-born persons. Non-Hispanic Black or African American persons accounted for the highest proportion of cases (34%), followed by non-Hispanic White persons (29%), and Hispanic/Latinos (24%). Among non-U.S.-born persons in the United States in 2021, 48% occurred in persons born in Asia, 33% among Hispanic/Latino persons, and 12% among African American persons. Overall, the high­ est rates per capita were among non-U.S.-born persons of more than one ethnicity (30 cases per 100,000 population). In 2020 in the United States 600 deaths were caused by TB, the highest number since 2006. In 2021 in Canada, TB cases and rates per 100,000 population were 1829 and 4.8, respectively. Of these cases, 76% occurred in foreignborn persons. However, the highest rates per 100,000 population were among Inuit (135) and First Nation persons (16). Similarly, in Europe, TB has reemerged as an important public health problem, mainly as a result of cases among immigrants from high-incidence countries and among marginalized populations, often in large urban settings. In most Western European countries, there are more cases annually among foreign-born than native populations. CHAPTER 183 Tuberculosis In 2023 TB incidence was decreasing in Africa, resuming a slow decline in the European, Eastern Mediterranean, and Southeast Asia Incidence per 100,000 population per year 0–9.9 10–99 100–199 200–299 300–499 ≥500 No data Not applicable

FIGURE 183-3  Estimated tuberculosis (TB) mortality rates in HIV-negative persons in 2022. (See disclaimer in Fig. 183-2. Reproduced with permission from Global Tuberculosis Report 2023. Geneva, World Health Organization; 2023.) PART 5 Infectious Diseases regions following the COVID-19 pandemic, and still increasing in the Americas and the Western Pacific. Globally, TB incidence has increased by nearly 5% since 2020 as a result of essential service disrup­ tions during the worst period of the pandemic. Of the estimated 10.8 million new cases of TB in 2023, 6.1% were in people living with HIV, with the majority of them occurring in Africa. Furthermore, an estimated 400,000 (range, 360,000-440,000) cases of rifampin-resistant (also called rifampicin-resistant) TB (RR-TB) and multidrug-resistant TB (MDR-TB)—a form of the disease caused by bacilli resistant at least to isoniazid and rifampin—occurred in 2023, representing 3.2% and 16%, respectively, of all new and previously treated cases. Only 189,000 MDR/RR-TB cases were enrolled on treatment in 2023 because of a lack of culture and drug susceptibility testing (DST) capacity in many settings worldwide. The countries of the former Soviet Union remain those with the highest proportions of MDR/RR disease among new TB cases, reaching up to one-third of the total. Overall, more than 50% of all MDR/RR-TB cases occur in India (27%), the Russian Federation (7.4%), Indonesia (7.4%), China (7.3%) and the Philippines (7.2%). Cases of extensively drug-resistant TB (XDR-TB), in which MDR-TB is compounded by additional resis­ tance to any fluoroquinolones and at least one additional group A drug (e.g., bedaquiline and linezolid), and cases of pre-XDR-TB, a form in which MDR/RR-TB strains are also resistant to any fluoroquinolone, occur worldwide. However, the vast majority of XDR-TB cases remain undiagnosed because reliable methods for DST are still lacking and laboratory capacity in low-income countries is limited. ■ ■FROM EXPOSURE TO INFECTION M. tuberculosis is most commonly transmitted from a person with infectious pulmonary TB by droplet nuclei containing M. tuberculosis bacteria, which are aerosolized by coughing, sneezing, or speaking. The tiny droplets dry rapidly; the smallest (<5–10 μm in diameter) may remain suspended in the air for several hours and may reach the terminal air passages when inhaled. Other routes of transmission of tubercle bacilli (e.g., through the skin or the placenta) are uncommon and of no epidemiologic significance. The risk of transmission and of subsequent acquisition of M. tuberculosis infection is determined mainly by exogenous factors, although endogenous factors also may

Mortality per 100,000 population per year 0–0.9 1–4.9 5–19 20–39 ≥40 No data Not applicable play a role. The probability of contact with a person who has an infec­ tious form of TB, the intimacy and duration of that contact, the degree of infectiousness of the case, and the shared environment in which the contact takes place are all important determinants of the likelihood of transmission. The most infectious patients whose sputum contains AFB visible by microscopy have cavitary pulmonary disease or, much less commonly, laryngeal TB and produce sputum containing as many as 105–107 AFB/mL. Patients with sputum smear–negative/culture-pos­ itive TB are less infectious, although they have been responsible for up to 20% of transmission in some studies in the United States. Those with culture-negative pulmonary TB and extrapulmonary TB are essentially noninfectious. Because persons with both HIV infection and TB are less likely to have cavitations, they may be less infectious than those without HIV co-infection. Crowding in poorly ventilated rooms is one of the most important factors in the transmission of tubercle bacilli because it increases the intensity of contact with a case. The virulence of the transmitted organism is also an important factor in establishing infection. Endogenous factors such as the degree of immune compe­ tence also are important. In particular, HIV-infected patients, persons undergoing cancer treatment, or those administered immunosuppres­ sive drugs may be at higher risk of TB infection acquisition. Attempts to estimate the basic reproductive number R0 for TB have resulted in a wide range of values depending on environmental conditions and social behaviors of populations: from 0.24 in the Netherlands during the period 1933−2007 to 4.3 in China in 2012, reflecting the status of disease control. ■ ■FROM INFECTION TO DISEASE Unlike the risk of acquiring infection with M. tuberculosis, the risk of developing disease after being infected depends largely on endogenous factors, such as the individual’s innate immunologic and nonimmu­ nologic defenses and the level at which the individual’s cell-mediated immunity is functioning. Clinical illness directly following infection is classified as primary TB and is common among children in the first few years of life and among immunocompromised persons. Although primary TB may be severe and disseminated, it generally is not asso­ ciated with high-level transmissibility. When infection is acquired later in life, the chance is greater that the mature immune system will

TABLE 183-1  Risk Factors for Active Tuberculosis in Persons Who Have Been Infected with Tubercle Bacilli FACTOR RELATIVE RISK/ODDSa Recent infection (<1 year) 12.9 Fibrotic lesions (spontaneously healed) 2–20 Comorbidities and iatrogenic causes   HIV infection 21–>30   Silicosis

  Chronic renal failure/hemodialysis 10–25   Diabetes 2–4   IV drug use 10–30   Excessive alcohol use

  Immunosuppressive treatment

  Tumor necrosis factor α inhibitors 4–5   Gastrectomy 2–5   Jejunoileal bypass 30–60   Posttransplantation period (renal, cardiac) 20–70 Tobacco smoking 2–3 Malnutrition and severe underweight

aOld infection = 1. contain it at least temporarily. Bacilli, however, may persist for years before reactivating to produce secondary (or postprimary) TB, which, because of frequent cavitation, is more often infectious than is primary disease. Overall, it is estimated that up to 10% of infected persons will eventually develop active TB in their lifetime—half of them during the first 18 months after infection. The risk is much higher among immu­ nocompromised individuals and, particularly, HIV-infected persons. Reinfection of a previously infected individual, which is common in areas with high rates of TB transmission, may also favor the develop­ ment of disease. At the height of the TB resurgence in the United States in the early 1990s, molecular typing and comparison of strains of

M. tuberculosis suggested that up to one-third of cases of active TB in some inner-city communities were due to recent transmission rather than to reactivation of old infection. Age is an important determinant of the risk of disease after infection. Among infected persons, the incidence of TB is highest during late adolescence and early adulthood; the reasons are unclear. The incidence among women peaks at 25–34 years of age. In this age group, rates among women may be higher than those among men, whereas at older ages the opposite is true. The risk increases in the elderly, possibly because of waning immunity and comorbidity. A variety of diseases and conditions favor the development of active TB (Table 183-1). In absolute terms, the most potent risk factor for TB among infected individuals is HIV co-infection, which suppresses cellular immunity. The risk that infection will proceed to active disease is directly related to the patient’s degree of immunosuppression. In a study of people living with HIV (PLWH), those who were tuberculin skin test (TST)–positive had risks varying from 2.6 to 13.3 cases per 100 person-years, which increased as the CD4+ T cell count decreased. ■ ■NATURAL HISTORY OF DISEASE Studies conducted in various countries before the advent of antimi­ crobial TB therapy showed that untreated TB is often fatal. About one-third of patients died within 1 year after diagnosis. Historic data also show that 55% of sputum smear-positive cases were dead within 5 years and up to 86% (weighted mean, 70%) within 10 years. A lower case fatality rate, around 20%, was estimated for untreated paucibacil­ lary smear-negative cases at 5 years. Of the survivors at 5 years, ~60% had undergone spontaneous remission, while the remainder were still excreting tubercle bacilli. With effective, timely, and proper antimi­ crobial TB treatment, patients have a very high chance of being cured. However, improper use of anti-TB drugs, while reducing mortality rates, may also result in large numbers of chronic infectious cases, often with drug-resistant bacilli.

PATHOGENESIS AND IMMUNITY

■ ■INFECTION AND MACROPHAGE INVASION The interaction of M. tuberculosis with the human host begins when droplet nuclei containing viable microorganisms, propelled into the air by infectious patients, are inhaled by a close bystander. Although the majority of inhaled bacilli are trapped in the upper airways and expelled by ciliated mucosal cells, a fraction (usually <10%) reach the alveoli, a unique immunoregulatory environment. There, in the very early phases of infection, the predominant cells infected by M. tuber­ culosis are myeloid dendritic cells. Subsequently, alveolar macrophages that have not yet been activated (prototypic alternatively activated macrophages) phagocytose the bacilli. Adhesion of mycobacteria to macrophages results largely from binding of the bacterial cell wall to a variety of macrophage cell-surface receptor molecules, including complement receptors, the mannose receptor, the immunoglobulin G Fcγ receptor, and type A scavenger receptors. Surfactants may also play a role in the early phase of interaction between the host and the pathogen, and surfactant protein D can prevent phagocytosis. Phago­ cytosis is enhanced by complement activation, leading to opsonization of bacilli with C3 activation products such as C3b and C3bi. Con­ comitantly, binding of certain receptors, such as the mannose receptor, regulates postphagocytic events like phagosome–lysosome fusion and inflammatory cytokine production. After a phagosome forms, the sur­ vival of M. tuberculosis in the cell seems to depend in part on reduced acidification due to lack of assembly of a complete vesicular protonadenosine triphosphatase. A complex series of events is generated by the bacterial cell-wall lipoglycan lipoarabinomannan, which inhibits the intracellular increase of Ca2+. Thus, the Ca2+/calmodulin pathway (leading to phagosome–lysosome fusion) is impaired, and the bacilli survive within the phagosomes by blocking fusion. The M. tuberculosis phagosome inhibits the production of phosphatidylinositol 3-phosphate, which normally earmarks phagosomes for membrane sorting and mat­ uration (including phagolysosome formation), which would destroy the bacteria. Bacterial factors block the host defense of autophagy, in which the cell sequesters the phagosome in a double-membrane vesicle (autophagosome) that is destined to fuse with lysosomes. If the bacilli are successful in arresting phagosome maturation, then bacterial rep­ lication begins and the macrophage eventually ruptures and releases its bacillary contents. This process is mediated by the ESX-1 secretion system that is encoded by genes contained in the region of difference 1 (RD1). Other uninfected phagocytic cells are then recruited to con­ tinue the infection cycle by ingesting dying macrophages and their bacillary content, thus, in turn, becoming infected themselves and expanding the infection. CHAPTER 183 Tuberculosis ■ ■VIRULENCE OF TUBERCLE BACILLI M. tuberculosis must be viewed as a complex formed by a multi­ tude of strains that differ in virulence and are capable of produc­ ing a variety of manifestations of disease. Since the elucidation of the M. tuberculosis genome in 1998, large mutant collections have been generated, and many bacterial genes that contribute to M. tuberculosis virulence have been found. Moreover, different patterns of virulence defects have been defined in various animal models—predominantly mice but also guinea pigs, rabbits, and nonhuman primates. The katG gene encodes for a catalase/peroxidase enzyme that protects against oxidative stress and is required for isoniazid activation and subsequent bactericidal activity. RD1 is a 9.5-kb locus that encodes two key small protein antigens—the 6-kDa early secretory antigen (ESAT-6) and culture filtrate protein 10 (CFP-10)—as well as a putative secretion apparatus that may facilitate their egress; the absence of this locus in the vaccine strain M. bovis bacille Calmette-Guérin (BCG) is a key attenuating mutation. Mutants lacking key enzymes of bacterial bio­ synthesis become auxotrophic for the missing substrate and often are totally unable to proliferate in animals; these include the leuCD and panCD mutants, which require leucine and pantothenic acid, respec­ tively. The isocitrate lyase gene (icl1) encodes a key step in the glyoxyl­ ate shunt that facilitates bacterial growth on fatty acid substrates; this gene is required for long-term persistence of M. tuberculosis infection

in mice with chronic TB. M. tuberculosis mutants in regulatory genes such as sigma factor C and sigma factor H (sigC and sigH) are associ­ ated with normal bacterial growth in mice, but they fail to elicit full tissue pathology. Finally, the mycobacterial protein CarD (expressed by the carD gene) seems essential for the control of rRNA transcription that is required for mycobacterial replication and persistence in the host cell. Its loss exposes mycobacteria to oxidative stress, starvation, DNA damage, and ultimately sensitivity to killing by a variety of host mutagens and defense mechanisms.

■ ■INNATE RESISTANCE TO INFECTION Several observations suggest that genetic factors play a key role in innate resistance to infection with M. tuberculosis and the devel­ opment of disease. The existence of this resistance, which is poly­ genic in nature, is suggested by the differing degrees of susceptibility to TB in different populations. This mechanism of elimination of the pathogen may be accompanied by negative results in the TST and interferon γ (IFN-γ) release assays (IGRAs). In mice, a gene called Nramp1 (natural resistance–associated macrophage protein 1) plays a regulatory role in resistance/susceptibility to mycobacteria. The human homologue NRAMP1, which maps to chromosome 2q, may play a role in determining susceptibility to TB, as is suggested by a study among West Africans. Studies of mice identified a novel host resistance gene, ipr1, that is encoded within the sst1 locus; ipr1 encodes an IFN-induc­ ible nuclear protein that interacts with other nuclear proteins in mac­ rophages primed with IFNs or infected by M. tuberculosis. In addition, polymorphisms in multiple genes, such as those encoding for various major histocompatibility complex alleles, IFN-γ, T cell growth factor β, interleukin (IL) 10, mannose-binding protein, IFN-γ receptor, Toll-like receptor 2, vitamin D receptor, and IL-1, have been associated with susceptibility to TB. PART 5 Infectious Diseases ■ ■THE HOST RESPONSE, GRANULOMA FORMATION, AND “LATENCY” In the initial stage of host–bacterium interaction, prior to the onset of an acquired cell-mediated immune (CMI) response, M. tuberculosis disseminates widely through the lymph vessels, spreading to other sites in the lungs and other organs, and undergoes a period of extensive growth within naïve inactivated macrophages; additional naïve macro­ phages are recruited to the early granuloma. How the bacillus accesses the parenchymal tissue still needs to be elucidated: it may directly infect epithelial cells or transmigrate through infected macrophages across the epithelium. Infected dendritic cells or monocytes then begin to transport bacilli to the lymphatic system. Studies suggest that

M. tuberculosis uses specific virulence mechanisms to subvert host cellular signaling and to elicit an early regulated proinflammatory response that promotes granuloma expansion and bacterial growth during this key early phase. A study of M. marinum infection in zebraf­ ish has delineated one molecular mechanism by which mycobacteria induce granuloma formation. The mycobacterial protein ESAT-6 induces secretion of matrix metalloproteinase 9 (MMP9) by nearby epithelial cells that are in contact with infected macrophages. MMP9 in turn stimulates recruitment of naïve macrophages, thus inducing gran­ uloma maturation and bacterial growth. Disruption of MMP9 function results in reduced bacterial growth. Another study has shown that M. tuberculosis–derived cyclic AMP is secreted from the phagosome into host macrophages, subverting the cell’s signal transduction path­ ways and stimulating an elevation in the secretion of tumor necrosis factor α (TNF-α) as well as further proinflammatory cell recruitment. Ultimately, the chemoattractants and bacterial products released dur­ ing the repeated rounds of cell lysis and infection of newly arriving macrophages enable dendritic cells to access bacilli; these cells migrate to the draining lymph nodes and present mycobacterial antigens to T lymphocytes. At this point, the development of cell-mediated and humoral immunity begins. These initial stages of infection are usually asymptomatic. About 2–4 weeks after infection, two host responses to M. tuber­ culosis develop: a macrophage-activating CMI response and a tis­ sue-damaging response. The macrophage-activating response is a

T cell–mediated phenomenon resulting in the activation of macrophages that are capable of killing and digesting tubercle bacilli. The tissuedamaging response is the result of a delayed-type hypersensitivity reac­ tion to various bacillary antigens; it destroys inactivated macrophages that contain multiplying bacilli but also causes caseous necrosis of the involved tissues (see below). Although both of these responses can inhibit mycobacterial growth, it is the balance between the two that determines the forms of TB that will develop subsequently. With the development of specific immunity and the accumulation of large numbers of activated macrophages at the site of the primary lesion, granulomatous lesions (tubercles) are formed. These lesions consist of accumulations of lymphocytes and activated macrophages that evolve toward epithelioid and giant cell morphologies. Initially, the tissue-damaging response can limit mycobacterial growth within mac­ rophages. As stated above, this response, mediated by various bacterial products, not only destroys macrophages but also produces early solid necrosis in the center of the tubercle. Although M. tuberculosis can survive, its growth is inhibited within this necrotic environment by low oxygen tension and low pH. At this point, some lesions may heal by fibrosis, with subsequent calcification, whereas inflammation and necrosis occur in other lesions. Some observations have challenged the traditional view that any encounter between mycobacteria and macrophages results in chronic infection. It is possible that an immune response capable of eradicating early infection may sometimes develop as a consequence, for instance, of disabling mutations in mycobacterial genomes rendering their replication ineffective. Individual granulomas that are formed during this phase of infection can vary in size and cell composition; some can contain the spread of mycobacteria, while others cannot. TB infection ensues as a result of this dynamic balance between the microorganism and the host. For many years, TB infection has been called “latent TB infection (LTBI).” This terminology was used to define a state of persistent immune response to stimulation by M. tuberculosis antigens with no evidence of clinically manifest, active TB. The qualification “latent” may offer some convenience of distin­ guishing infection from disease, albeit an inaccurate description of a process that encompasses bacterial generations that are not dormant. Latency may be an inaccurate term because bacilli may remain active during this “latent” stage, forming biofilms in necrotic areas within which they temporarily hide. Therefore, some have proposed the term persister as a more accurate descriptor of the behavior of the bacilli in this phase. It is important to recognize that infection and disease do not represent a binary state but rather a continuum along which infection will eventually move in the direction of full containment or disease. The ability to predict, through systemic biomarkers, which infected individuals will progress toward disease would be of immense value in devising prophylactic interventions at scale. ■ ■MACROPHAGE-ACTIVATING RESPONSE Cell-mediated immunity is critical at this early stage. In the majority of infected individuals, local macrophages are activated when bacillary antigens processed by macrophages stimulate T lymphocytes to release a variety of lymphokines. These activated macrophages aggregate around the lesion’s center and effectively neutralize tubercle bacilli without causing further tissue destruction. In the central part of the lesion, the necrotic material resembles soft cheese (caseous necrosis)— a phenomenon that may also be observed in other conditions, such as neoplasms. Even when healing takes place, viable bacilli may remain within macrophages or in the necrotic material for many years. These “healed” lesions in the lung parenchyma and hilar lymph nodes may later undergo calcification. ■ ■DELAYED-TYPE HYPERSENSITIVITY In a minority of cases, the macrophage-activating response is weak, and mycobacterial growth can be inhibited only by intensified delayed hypersensitivity reactions, which lead to lung tissue destruction. The lesion tends to enlarge further, and the surrounding tissue is progres­ sively damaged. At the center of the lesion, the caseous material lique­ fies. Bronchial walls and blood vessels are invaded and destroyed, and cavities are formed. The liquefied caseous material, containing large

amounts of bacilli, is drained through bronchi. Within the cavity, tubercle bacilli multiply, spill into the airways, and are discharged into the environment through expiratory maneuvers such as coughing and talking. In the early stages of infection, bacilli are usually transported by macrophages to regional lymph nodes, from which they gain access to the central venous return; from there they reseed the lungs and may also disseminate beyond the pulmonary vasculature throughout the body via the systemic circulation. In young children with poor natural immunity, hematogenous dissemination may result in fatal miliary TB or tuberculous meningitis. ■ ■ROLE OF MACROPHAGES AND MONOCYTES While cell-mediated immunity confers partial protection against M. tuberculosis, humoral immunity plays a less well-defined role in protection (although evidence is accumulating on the existence of antibodies to lipoarabinomannan, which may prevent dissemination of infection in children). In cell-mediated immunity, two types of cells are essential: macrophages, which directly phagocytose tubercle bacilli, and T cells (mainly CD4+ T lymphocytes, although the role of CD8+ T cells has recently been the subject of much research), which induce protection through the production of cytokines, especially IFN-γ. After infection with M. tuberculosis, alveolar macrophages secrete various cytokines responsible for several events (e.g., the formation of granulo­ mas) as well as systemic effects (e.g., fever and weight loss). However, alternatively activated alveolar macrophages may be particularly sus­ ceptible to M. tuberculosis growth early on, given their more limited proinflammatory and bactericidal activity, which is related in part to being bathed in surfactant. New monocytes and macrophages attracted to the site are key components of the immune response. Their primary mechanism is probably related to production of oxidants (such as reac­ tive oxygen intermediates or nitric oxide) that have antimycobacterial activity and increase the synthesis of cytokines such as TNF-α and IL-1, which in turn regulate the release of reactive oxygen intermediates and reactive nitrogen intermediates. In addition, macrophages can undergo apoptosis—a defensive mechanism to prevent the release of cytokines and bacilli via their sequestration in the apoptotic cell. Recent work also describes the involvement of neutrophils in the host response, although the timing of their appearance and their effectiveness remain uncertain. ■ ■ROLE OF T LYMPHOCYTES Alveolar macrophages, monocytes, and dendritic cells are also critical in processing and presenting antigens to T lymphocytes, primarily CD4+ and CD8+ T cells; the result is the activation and proliferation of CD4+ T lymphocytes, which are crucial to the host’s defense against M. tuberculosis. Qualitative and quantitative defects of CD4+ T cells explain the inability of HIV-infected individuals to contain mycobacte­ rial proliferation. Activated CD4+ T lymphocytes can differentiate into cytokine-producing TH1 or TH2 cells. TH1 cells produce IFN-γ—an activator of macrophages and monocytes—and IL-2. TH2 cells produce IL-4, IL-5, IL-10, and IL-13 and may also promote humoral immunity. The interplay of these various cytokines and their cross-regulation determine the host’s response. The role of cytokines in promoting intracellular killing of mycobacteria, however, has not been entirely elucidated. IFN-γ may induce the generation of reactive oxygen and nitrogen intermediates and regulate genes involved in bactericidal effects. TNF-α also is important. Although its precise mechanisms are complex and not yet fully clarified, there is a suggested model that foresees an ideal setting for TNF-α between excessive activation—with consequent worsening of immunopathologic reactions—and insuf­ ficient activation—with resulting lack of containment—in the control of TB infection. Observations made originally in transgenic knockout mice and more recently in humans suggest that other T cell subsets, especially CD8+ T cells, may play an important role. CD8+ T cells have been associated with protective activities via cytotoxic responses and lysis of infected cells as well as with production of IFN-γ and TNF-α. Finally, natural killer cells act as co-regulators of CD8+ T-cell lytic activities, and γδ T cells are increasingly thought to be involved in protective responses in humans.

■ ■MYCOBACTERIAL LIPIDS AND PROTEINS Lipids are involved in mycobacterial recognition by the innate immune system, and lipoproteins (such as 19-kDa lipoprotein) trigger potent signals through Toll-like receptors present in blood dendritic cells. M. tuberculosis possesses various protein antigens. Some are present in the cytoplasm and cell wall; others are secreted. That the latter are more important in eliciting a T lymphocyte response is suggested by experiments documenting the appearance of protective immunity in animals after immunization with live, protein-secreting mycobacteria. Among the antigens that may play a protective role are some proteins of the antigen 85 complex are the 30-kDa major secretory protein (85B) and ESAT-6 antigens. Protective immunity is probably the result of reactivity to many different mycobacterial antigens. These antigens are being incorporated into newly designed vaccines on various platforms.

■ ■SKIN-TEST REACTIVITY Coincident with the appearance of immunity, delayed-type hypersensi­ tivity to M. tuberculosis develops. This reactivity is the basis of the TST, which is used primarily for the diagnosis of M. tuberculosis infection in persons without symptoms. The cellular mechanisms responsible for TST reactivity are related mainly to previously sensitized CD4+ T lymphocytes, which are attracted to the skin-test site. There, they proliferate and produce cytokines. Although delayed hypersensitivity is associated with protective immunity (TST-positive persons are less susceptible to a new M. tuberculosis infection than are TST-negative persons), it by no means guarantees protection against reactivation. In fact, cases of active TB are often accompanied by strongly positive skintest reactions. There is also evidence of reinfection with new strains of M. tuberculosis in patients previously treated for active disease. This evidence underscores the fact that previous infection or active TB does not necessarily confer fully protective immunity. CHAPTER 183 CLINICAL MANIFESTATIONS TB is classified as pulmonary, extrapulmonary, or both. Depending on several factors linked to host immunologic status and bacterial strains, extrapulmonary TB may occur in 10–40% of patients. Furthermore, up to two-thirds of HIV-infected patients with TB may have both pulmo­ nary and extrapulmonary TB or extrapulmonary TB alone. Tuberculosis ■ ■PULMONARY TB Pulmonary TB is traditionally categorized as primary or postprimary (adult-type, secondary). This distinction has been challenged by molecular evidence from TB-endemic areas indicating that a large percentage of cases of adult pulmonary TB result from recent infection (either primary infection or reinfection) and not from reactivation. Primary Disease  Primary pulmonary TB occurs soon after the initial infection. It may be asymptomatic or may present with fever and occasionally pleuritic chest pain. In areas of high TB transmission, this form of disease is often seen in children. Because most inspired air is distributed to the middle and lower lung zones, these areas are most commonly involved in primary TB. The lesion forming after initial infection (Ghon focus) is usually peripheral and accompanied by transient hilar or paratracheal lymphadenopathy, which may or may not be visible on standard chest radiography (CXR) (Fig. 183-4). Some patients develop erythema nodosum on the legs (see Fig. A1-39) or phlyctenular conjunctivitis. In the majority of cases, the lesion heals spontaneously and becomes evident only as a small calcified nodule. Pleural reaction overlying a subpleural focus also is common. The Ghon focus, with or without overlying pleural reaction, thickening, and regional lymphadenopathy, is referred to as the Ghon complex. In young children with immature cell-mediated immunity and in persons with impaired immunity (e.g., those with malnutrition or HIV infection), primary pulmonary TB may progress rapidly to clinical illness. The initial lesion increases in size and can evolve in different ways. Pleural effusion, which is found in up to two-thirds of cases, results from the penetration of bacilli into the pleural space from an adjacent subpleural focus. In severe cases, the primary site rapidly enlarges, its central portion undergoes necrosis, and cavitation develops (progressive primary TB). TB in young children is almost

FIGURE 183-4  Chest radiograph showing right hilar lymph node enlargement with infiltration into the surrounding lung tissue in a child with primary tuberculosis. (Courtesy of Prof. Robert Gie, Department of Paediatrics and Child Health, Stellenbosch University, South Africa; with permission.) invariably accompanied by hilar or paratracheal lymphadenopathy due to the spread of bacilli from the lung parenchyma through lymphatic vessels. Enlarged lymph nodes may compress bronchi, causing total obstruction with distal collapse, partial obstruction with large-airway wheezing, or a ball-valve effect with segmental/lobar hyperinflation. Lymph nodes may also rupture into the airway with development of pneumonia, often including areas of necrosis and cavitation, distal to the obstruction. Bronchiectasis (Chap. 301) may develop in any segment/lobe damaged by progressive caseating pneumonia. Occult hematogenous dissemination commonly follows primary infection. However, in the absence of a sufficient acquired immune response, which usually contains the infection, disseminated or miliary disease may result (Fig. 183-5). Small granulomatous lesions develop in multiple organs and may cause locally progressive disease or result in tuberculous meningitis; this is a particular concern in very young children and immunocompromised persons (e.g., PLWH). PART 5 Infectious Diseases FIGURE 183-5  Chest radiograph showing bilateral miliary (millet-sized) infiltrates in a child. (Courtesy of Prof. Robert Gie, Department of Paediatrics and Child Health, Stellenbosch University, South Africa; with permission.)

FIGURE 183-6  Chest radiograph showing a right-upper-lobe infiltrate and a cavity with an air-fluid level in a patient with active tuberculosis. (Courtesy of Dr. Andrea Gori, Infectious Diseases Unit, Fondazione IRCCS Ca’ Granda Ospediale Maggiore Policlinico, University of Milan, Milan, Italy; with permission.) Postprimary (Adult-Type) Disease  Also referred to as reacti­ vation or secondary TB, postprimary TB is probably most accurately termed adult-type TB because it may result from endogenous reac­ tivation of distant or recent infection (primary infection or reinfec­ tion). It is usually localized to the apical and posterior segments of the upper lobes, where the substantially higher mean oxygen tension (compared with that in the lower zones) favors mycobacterial growth. The superior segments of the lower lobes also are frequently involved. The extent of lung parenchymal involvement varies greatly, from small infiltrates to extensive cavitary disease. With cavity formation, lique­ fied necrotic contents are ultimately discharged into the airways and may undergo bronchogenic spread, resulting in satellite lesions within the lungs that may in turn undergo cavitation (Figs. 183-6 and 183-7). Massive involvement of pulmonary segments or lobes, with coales­ cence of lesions, produces caseating pneumonia. While up to one-third of untreated patients reportedly succumb to severe pulmonary TB FIGURE 183-7  CT scan showing a large cavity in the right lung of a patient with active tuberculosis. (Courtesy of Dr. Elisa Busi Rizzi, National Institute for Infectious Diseases, Spallanzani Hospital, Rome, Italy; with permission.)

within a few months after onset (the classic “galloping consumption” of the past), others may undergo a process of spontaneous remission or proceed along a chronic, progressively debilitating course (“consump­ tion” or phthisis). Under these circumstances, some pulmonary lesions become fibrotic and may later calcify, but cavities persist in other parts of the lungs. Individuals with such chronic disease continue to discharge tubercle bacilli into the environment. Most patients respond to treatment, with defervescence, decreasing cough, weight gain, and a general improvement in well-being within several weeks. Early in the course of disease, symptoms and signs are often non­ specific and insidious, consisting mainly of fever, often diurnal, and night sweats due to defervescence, weight loss, anorexia, general mal­ aise, and weakness. However, in up to 90% of cases, cough eventually develops—often initially nonproductive and limited to the morning and subsequently accompanied by the production of purulent sputum, sometimes with blood streaking. Hemoptysis develops in 20–30% of cases, and massive hemoptysis may ensue as a consequence of the erosion of a blood vessel in the wall of a cavity. Hemoptysis, however, may also result from rupture of a dilated vessel in a cavity (Rasmussen’s aneurysm) or from aspergilloma formation in an old cavity. Pleuritic chest pain sometimes develops in patients with subpleural parenchy­ mal lesions or pleural disease. Extensive disease may produce dyspnea and, in rare instances, adult respiratory distress syndrome. Physical findings are of limited use in pulmonary TB. Many patients have no abnormalities detectable by chest examination, whereas others have detectable rales in the involved areas during inspiration, especially after coughing. Occasionally, rhonchi due to partial bronchial obstruction and classic amphoric breath sounds in areas with large cavities may be heard. Systemic features include fever (often low-grade and intermit­ tent) in up to 80% of cases and wasting. Absence of fever, however, does not exclude TB. In some recurrent cases and among patients with low Karnofsky score, finger clubbing has been reported. The most common hematologic findings are mild anemia, leukocytosis, and thrombocytosis with a slightly elevated erythrocyte sedimentation rate and/or C-reactive protein level. None of these findings is consistent or sufficiently accurate for diagnostic purposes. Hyponatremia due to the syndrome of inappropriate secretion of antidiuretic hormone also has been reported. ■ ■EXTRAPULMONARY TB In descending order of frequency, the extrapulmonary sites most involved in TB are the lymph nodes, pleura, genitourinary tract, bones and joints, meninges, peritoneum, and pericardium. However, virtu­ ally any organ system may be affected. As a result of hematogenous dissemination in PLWH, extrapulmonary TB is seen more commonly today than in the past in settings of high HIV prevalence. Lymph Node TB (Tuberculous Lymphadenitis)  The most common presentation of extrapulmonary TB in both HIV-seronegative individuals and PLWH (35% of cases worldwide and >40% of cases in the United States in recent series), lymph node disease is particularly frequent among PLWH and among children (Fig. 183-8). In the United States, besides children, women (particularly non-Caucasians) seem to be especially susceptible. Once caused mainly by M. bovis, tuberculous lymphadenitis today is due largely to M. tuberculosis. Lymph node TB presents as painless swelling of the lymph nodes, most commonly at posterior cervical and supraclavicular sites (a condition historically referred to as scrofula). Lymph nodes are usually discrete in early dis­ ease but develop into a matted nontender mass over time; a fistulous tract draining caseous material may result. Associated pulmonary disease is present in fewer than 50% of cases, and systemic symptoms are uncommon except in HIV-infected patients. The diagnosis is established by fine-needle aspiration biopsy (with a yield of up to 80%) or surgical excision biopsy. Bacteriologic confirmation is achieved in most cases, granulomatous lesions with or without visible AFBs are typically seen, and cultures are positive in 70–80% of cases. Among PLWH, granulomas are less well organized and are frequently absent entirely, but bacterial loads are heavier than in HIV-seronegative patients, with higher yields from microscopy and culture. Differential

FIGURE 183-8  Tuberculous lymphadenitis affecting the cervical lymph nodes in a 2-year-old child from Malawi. (Courtesy of Prof. S. Graham, Centre for International Child Health, University of Melbourne, Australia; with permission.) diagnosis includes a variety of infectious conditions, neoplastic dis­ eases such as lymphomas or metastatic carcinomas, and rare disorders like Kikuchi disease (necrotizing histiocytic lymphadenitis), Kimura disease, and Castleman disease. CHAPTER 183 Pleural TB  Involvement of the pleura accounts for ~20% of extra­ pulmonary cases in the United States and elsewhere. Isolated pleural effusion usually reflects recent primary infection, and the collection of fluid in the pleural space represents a hypersensitivity response to mycobacterial antigens. Pleural disease may also result from con­ tiguous parenchymal spread, as in many cases pleurisy accompanies postprimary disease. Depending on the extent of reactivity, the effusion may be small, remain unnoticed, and resolve spontaneously or may be sufficiently large to cause symptoms such as fever, pleuritic chest pain, and dyspnea. Physical findings are those of pleural effusion: dullness to percussion and absence of breath sounds. CXR reveals the effusion and, in up to one-third of cases, also shows a parenchymal lesion. Thoracentesis is required to ascertain the nature of the effusion and to differentiate it from manifestations of other etiologies. The fluid is straw-colored and at times hemorrhagic; it is an exudate with a protein concentration >50% of that in serum (usually ~4–6 g/dL), a normal to low glucose concentration, a pH of ~7.3 (occasionally <7.2), and detectable white blood cells (usually 500–6000/μL). Neutrophils may predominate in the early stage, but lymphocyte predominance is typical later. Mesothelial cells are generally rare or absent. AFBs are rarely seen on direct smear, and cultures often may be falsely negative for M. tuber­ culosis; positive cultures are more common among postprimary cases. An elevated pleural concentration of adenosine deaminase is consistent with TB, while TB may be excluded if the value is very low. Lysozyme also is present in the pleural effusion. Measurement of IFN-γ, either directly or through stimulation of sensitized T cells with mycobacterial antigens, can be diagnostically helpful. Needle biopsy of the pleura is often required for diagnosis and is recommended over pleural fluid analysis; it reveals granulomas and/or yields a positive culture in up to 80% of cases. Pleural biopsy can yield a positive result in ~75% of cases when real-time automated nucleic acid amplification is used (the Xpert MTB/RIF assay [Cepheid; Sunnyvale, CA]; see “Nucleic Acid Ampli­ fication Technology,” below); testing of pleural fluid with this assay is not recommended because of low sensitivity. This form of pleural TB responds rapidly to chemotherapy and may resolve spontaneously. Concurrent glucocorticoid administration may reduce the duration of fever and/or chest pain but is not of proven benefit. Tuberculosis Tuberculous empyema is a less common complication of pulmonary TB. It is usually the result of the rupture of a cavity, with spillage of a large number of organisms into the pleural space. This process may create a bronchopleural fistula with evident air in the pleural space.

CXR shows hydropneumothorax with an air-fluid level. The pleural fluid is purulent and thick and contains large numbers of lymphocytes. Acid-fast smears and mycobacterial cultures are often positive. Surgical drainage is usually required as an adjunct to chemotherapy. Tubercu­ lous empyema may result in severe pleural fibrosis and restrictive lung disease. Removal of the thickened visceral pleura (decortication) is occasionally necessary to improve lung function.

TB of the Upper Airways  Nearly always a complication of advanced cavitary pulmonary TB, TB of the upper airways may involve the larynx, pharynx, and epiglottis. Symptoms include hoarseness, dys­ phonia, and dysphagia in addition to chronic productive cough. Find­ ings depend on the site of involvement, and ulcerations may be seen on laryngoscopy. Acid-fast smear of the sputum is often positive, but biopsy may be necessary in some cases to establish the diagnosis. Car­ cinoma of the larynx may have similar features but is usually painless. Genitourinary TB  Genitourinary TB, which accounts for ~10– 15% of all extrapulmonary cases in the United States and elsewhere, may involve any portion of the genitourinary tract. Clinical manifesta­ tions are cryptic and protean. Patients may be asymptomatic and their disease discovered only after destructive lesions of the kidneys have developed. Symptoms are often nonspecific and include those of uri­ nary tract infection with frequency, dysuria, nocturia and hematuria, and abdominal or flank pain. Without a high index of suspicion, this form of TB may result in delayed diagnosis with irreversible organ damage. Up to 75% of patients have abnormalities on CXR suggesting previous or concomitant pulmonary disease. Urinalysis gives abnormal results in 90% of cases, revealing pyuria and hematuria. The docu­ mentation of culture-negative pyuria in acidic urine should raise the suspicion of TB. IV pyelography, abdominal CT, or MRI (Fig. 183-9) may show deformities and obstructions; calcifications and ureteral strictures are suggestive findings. Culture of three morning urine specimens yields a definitive diagnosis in nearly 90% of cases. Severe ureteral strictures may lead to hydronephrosis, serious renal damage, and, ultimately, renal failure. Genital TB is diagnosed more commonly in female than in male patients. In female patients, it affects the fallo­ pian tubes and the endometrium and may cause infertility, pelvic pain, and menstrual abnormalities. Diagnosis requires biopsy or culture of specimens obtained by dilation and curettage. In male patients, genital TB preferentially affects the epididymis, producing a slightly tender mass that may drain externally through a fistulous tract; orchitis and PART 5 Infectious Diseases FIGURE 183-9  MRI of culture-confirmed renal tuberculosis. T2-weighted coronary plane: coronal sections showing several renal lesions in both the cortical and the medullary tissues of the right kidney. (Courtesy of Dr. Alberto Matteelli, Department of Infectious Diseases, University of Brescia, Italy; with permission.)

FIGURE 183-10  CT scan demonstrating destruction of the right pedicle of T10 due to Pott’s disease. The patient, a 70-year-old Asian woman, presented with back pain and weight loss and had biopsy-proven tuberculosis. (Courtesy of Charles L. Daley, MD, University of California, San Francisco; with permission.) prostatitis also may develop. In almost half of cases of genitourinary TB, urinary tract disease is also present. Genitourinary TB responds well to chemotherapy. Skeletal TB  In the United States, TB of the bones and joints is responsible for ~10% of extrapulmonary cases. In bone and joint dis­ ease, pathogenesis is related to reactivation of hematogenous foci or to spread from adjacent paravertebral lymph nodes. Weight-bearing joints (the spine in 40% of cases, the hips in 13%, and the knees in 10%) are most commonly affected. Spinal TB (Pott’s disease or tuberculous spondylitis; Fig. 183-10) often involves two or more adjacent vertebral bodies. Whereas the upper thoracic spine is the most common site of spinal TB in children, the lower thoracic and upper lumbar vertebrae are usually affected in adults. From the anterior superior or inferior angle of the vertebral body, the lesion slowly reaches the adjacent body, later affecting the intervertebral disk. With advanced disease, collapse of vertebral bodies results in kyphosis (gibbus). A paravertebral “cold” abscess also may form. In the upper spine, this abscess may track to and penetrate the chest wall, presenting as a soft tissue mass; in the lower spine, it may reach the inguinal ligaments or present as a psoas abscess. CT or MRI reveals the characteristic lesion and suggests its etiology. The differential diagnosis includes tumors and other infec­ tions. Pyogenic bacterial osteomyelitis involves the disk very early and produces rapid sclerosis. Aspiration of the abscess or bone biopsy confirms the tuberculous etiology, as cultures are usually positive and histologic findings highly typical. A catastrophic complication of Pott’s disease is paraplegia, which is usually due to an abscess or a lesion compressing the spinal cord. Paraparesis due to a large abscess is a medical emergency and requires rapid drainage. TB of the hip joints, usually involving the head of the femur, causes pain; TB of the knee produces pain and swelling. If the disease goes unrecognized, the joints may be destroyed. Diagnosis requires examination of the synovial fluid, which is thick in appearance, with a high protein concentration and a variable cell count. Although synovial fluid culture is positive in a high percentage of cases, synovial biopsy and tissue culture may be neces­ sary to establish the diagnosis. Skeletal TB responds to chemotherapy, but severe cases may require surgery. Tuberculous Meningitis and Tuberculoma  TB of the central nervous system (CNS) accounts for ~5% of extrapulmonary cases in the United States. It is seen most often in young children but also devel­ ops in adults, especially those infected with HIV. Tuberculous menin­ gitis results from the hematogenous spread of primary or postprimary pulmonary TB or from the rupture of a subependymal tubercle into the subarachnoid space. In more than half of cases, evidence of old pulmonary lesions or a miliary pattern is found on CXR. The disease

often presents subtly as headache and slight mental changes after a prodrome of weeks of low-grade fever, malaise, anorexia, and irritabil­ ity. If not recognized, tuberculous meningitis may evolve acutely with severe headache, confusion, lethargy, altered sensorium, and neck rigidity. Typically, the disease evolves over 1–2 weeks, a course longer than that of typical bacterial meningitis. Because meningeal involve­ ment is pronounced at the base of the brain, paresis of cranial nerves (ocular nerves in particular) is a frequent finding, and the involvement of cerebral arteries may produce focal ischemia. The ultimate evolution is toward coma, with hydrocephalus and intracranial hypertension. Lumbar puncture is the cornerstone of diagnosis. In general, exami­ nation of cerebrospinal fluid (CSF) reveals a high leukocyte count (up to 1000/μL), usually with a predominance of lymphocytes but some­ times with a predominance of neutrophils in the early stage; a protein content of 1–8 g/L (100–800 mg/dL); and a low glucose concentration. However, any of these three parameters can be within the normal range. AFBs are infrequently seen on direct smear of CSF sediment, but repeated lumbar punctures increase the yield. Culture of CSF is diag­ nostic in up to 80% of cases and remains the gold standard. Real-time automated nucleic acid amplification (the Xpert MTB/RIF assay) has a sensitivity of up to 80% and is the preferred initial diagnostic option. Treatment should be initiated immediately upon a positive Xpert MTB/ RIF result. A negative result does not exclude a diagnosis of TB and requires further diagnostic workup. Imaging studies (CT and MRI) may show hydrocephalus and abnormal enhancement of basal cisterns or ependyma. If unrecognized, tuberculous meningitis is uniformly fatal. This disease responds to chemotherapy; however, neurologic sequelae are documented in 25% of treated cases, in most of which the diagnosis has been delayed. Clinical trials have demonstrated that patients given adjunctive glucocorticoids may experience faster resolu­ tion of CSF abnormalities and elevated CSF pressure, resulting in lower rates of death or severe disability and relapse. In one study, adjunctive dexamethasone significantly enhanced the chances of survival among persons >14 years but did not reduce the frequency of neurologic sequelae. The dexamethasone schedule was (1) 0.4 mg/kg per day given intravenously with tapering by 0.1 mg/kg per week until the fourth week, when 0.1 mg/kg per day was administered, followed by (2) 4 mg/d given by mouth with tapering by 1 mg per week until the fourth week, when 1 mg/d was administered. The WHO now recommends that adjuvant glucocorticoid therapy with either dexamethasone or pred­ nisolone, tapered over 6–8 weeks, should be used in CNS TB. However, among PLWH, a recent placebo-controlled study demonstrated no benefit with respect to survival or secondary endpoints from a 6- to 8-week tapering course of adjunctive dexamethasone. Tuberculoma, an uncommon manifestation of TB of the CNS, presents as one or more space-occupying lesions and usually causes seizures and focal signs. CT or MRI reveals contrast-enhanced ring lesions, but biopsy is necessary to establish the diagnosis. Gastrointestinal TB  Gastrointestinal TB is uncommon, making up only 3.5% of extrapulmonary cases in the United States. Various pathogenetic mechanisms are involved: swallowing of sputum with direct seeding, hematogenous spread, or (largely in developing areas) ingestion of milk from cows affected by bovine TB. Although any por­ tion of the gastrointestinal tract may be affected, the terminal ileum and the cecum are the sites most commonly involved. Abdominal pain (at times similar to that associated with appendicitis) and swell­ ing, obstruction, hematochezia, and a palpable mass in the abdomen are common findings at presentation. Fever, weight loss, anorexia, and night sweats also are common. With intestinal wall involvement, ulcerations and fistulae may simulate Crohn disease; the differential diagnosis of this entity is always difficult. Anal fistulae should prompt an evaluation for rectal TB. Because surgery is required in most cases, the diagnosis can be established by histologic examination and culture of specimens obtained intraoperatively. Tuberculous peritonitis follows either the direct spread of tubercle bacilli from ruptured lymph nodes and intraabdominal organs (e.g., genital TB in women) or hematogenous seeding. Nonspecific abdomi­ nal pain, fever, and ascites should raise the suspicion of tuberculous

peritonitis. The coexistence of cirrhosis (Chap. 355) in patients with tuberculous peritonitis complicates the diagnosis. In tuberculous peritonitis, paracentesis reveals an exudative fluid with a high protein content and leukocytosis that is usually lymphocytic (although neutro­ phils occasionally predominate). The yield of direct smear and culture is relatively low; culture of a large volume of ascitic fluid can increase the yield, but peritoneal biopsy (with a specimen best obtained by lapa­ roscopy) is often needed to establish the diagnosis.

Pericardial TB (Tuberculous Pericarditis)  Due either to direct extension from adjacent mediastinal or hilar lymph nodes or to hematogenous spread, pericardial TB has often been a disease of the elderly in countries with low TB prevalence. However, it also develops frequently in PLWH. Case fatality rates are as high as 40% in some series. The onset may be subacute, although an acute presentation, with dyspnea, fever, dull retrosternal pain, and a pericardial friction rub, is possible. An effusion eventually develops in many cases; cardiovascu­ lar symptoms and signs of cardiac tamponade may ultimately appear (Chap. 281). In the presence of effusion, TB must be suspected if the patient belongs to a high-risk population (HIV-infected, originating in a high-prevalence country); if there is evidence of previous TB in other organs; or if echocardiography, CT, or MRI shows effusion and thickness across the pericardial space. A definitive diagnosis can be obtained by pericardiocentesis under echocardiographic guidance. The pericardial fluid must be submitted for biochemical, cytologic, and microbiologic evaluation. The effusion is exudative in nature, with a high count of lymphocytes and monocytes. Hemorrhagic effusion is common. Direct smear examination is very rarely positive. Culture of pericardial fluid reveals M. tuberculosis in up to two-thirds of cases, whereas pericardial biopsy has a higher yield. High levels of adenosine deaminase, lysozyme, and IFN-γ may suggest a tuberculous etiology. CHAPTER 183 Without treatment, pericardial TB is usually fatal. Even with treat­ ment, complications may develop, including chronic constrictive peri­ carditis with thickening of the pericardium, fibrosis, and sometimes calcification, which may be visible on a chest radiograph. Systematic reviews and meta-analyses show a trend toward benefit from gluco­ corticoid treatment with regard to death and constrictive pericarditis. However, the largest and most recent study—the IMPI study—failed to show such a benefit. Of the patients enrolled in this trial, 67% were PLWH, and only a fraction was receiving antiretroviral treatment (ART). A supplemental analysis among HIV-negative patients showed a small mortality benefit, as did another small study among PLWH. The WHO currently recommends that, in patients with tuberculous pericarditis, initial adjuvant glucocorticoid therapy may be used. The 2016 guidelines of the American Thoracic Society (ATS), the CDC, and the Infectious Diseases Society of America (IDSA), on the other hand, suggest that glucocorticoid therapy should not be routinely administered. Tuberculosis Caused by direct extension from the pericardium or by retrograde lymphatic extension from affected mediastinal lymph nodes, tubercu­ lous myocarditis is extremely rare. Usually, it is fatal and is diagnosed postmortem. Miliary or Disseminated TB  Miliary TB is due to hematogenous spread of tubercle bacilli. Although in children it is often the conse­ quence of primary infection, in adults it may be due to either recent infection or reactivation of old disseminated foci. The lesions are usu­ ally yellowish granulomas 1–2 mm in diameter that resemble millet seeds (thus the term miliary, coined by nineteenth-century patholo­ gists). Clinical manifestations are nonspecific and protean, depending on the predominant site of involvement. Fever, night sweats, anorexia, weakness, and weight loss are presenting symptoms in the majority of cases. At times, patients have a cough and other respiratory symp­ toms due to pulmonary involvement as well as abdominal symptoms. Physical findings include hepatomegaly, splenomegaly, and lymphade­ nopathy. Eye examination may reveal choroidal tubercles, which are pathognomonic of miliary TB, in up to 30% of cases. Meningismus occurs in fewer than 10% of cases. A high index of suspicion is required for the diagnosis of miliary TB. Frequently, CXR (Fig. 183-5) reveals a miliary reticulonodular pattern

(more easily seen on underpenetrated film), although no radiographic abnormality may be evident early in the course and among PLWH. Other radiologic findings include large infiltrates, interstitial infiltrates (especially in PLWH), and pleural effusion. Sputum-smear microscopy is negative in most cases. Various hematologic abnormalities may be seen, including anemia with leukopenia, lymphopenia, neutrophilic leukocytosis and leukemoid reactions, and polycythemia. Dissemi­ nated intravascular coagulation has been reported. Elevation of alka­ line phosphatase levels and other abnormal liver function tests are detected in patients with severe hepatic involvement. TST results may be negative in up to half of cases, but reactivity may be restored during chemotherapy. Bronchoalveolar lavage and transbronchial biopsy are more likely to provide bacteriologic confirmation, and granulomas are evident in liver or bone-marrow biopsy specimens from many patients. If it goes unrecognized, miliary TB is lethal; with proper early treat­ ment, however, it is amenable to cure. Glucocorticoid therapy has not proved beneficial.

A rare presentation seen in the elderly, cryptic miliary TB has a chronic course characterized by mild intermittent fever, anemia, and—ultimately—meningeal involvement preceding death. An acute septicemic form, nonreactive miliary TB, occurs very rarely and is due to massive hematogenous dissemination of tubercle bacilli. Pancy­ topenia is common in this form of disease, which is rapidly fatal. At postmortem examination, multiple necrotic but nongranulomatous (“nonreactive”) lesions are detected. Less Common Extrapulmonary Forms  TB may cause chorio­ retinitis, uveitis, panophthalmitis, and painful hypersensitivity-related phlyctenular conjunctivitis. Tuberculous otitis is rare and presents as hearing loss, otorrhea, and tympanic membrane perforation. In the nasopharynx, TB may simulate granulomatosis with polyangiitis. Cutaneous manifestations of TB include primary infection due to direct inoculation, abscesses and chronic ulcers, scrofuloderma, lupus vulgaris (a smoldering disease with cutaneous nodules, plaques, and fissures), miliary lesions, and erythema nodosum. Tuberculous mas­ titis results from retrograde lymphatic spread, often from the axillary lymph nodes. Adrenal TB is a manifestation of disseminated disease presenting rarely as adrenal insufficiency. Finally, congenital TB results from transplacental spread of tubercle bacilli to the fetus or from PART 5 Infectious Diseases FIGURE 183-11  Estimated HIV prevalence in new and relapse tuberculosis (TB) cases in 2022. (See disclaimer in Fig. 183-2. Reproduced with permission from Global Tuberculosis Report 2022. Geneva, World Health Organization; 2023.)

ingestion of contaminated amniotic fluid. This rare disease affects the liver, spleen, lymph nodes, and various other organs. Post-TB Lung Disease and Complications  A substantial proportion of TB survivors considered cured experience chronic impairment of lung function, dyspnea, residual cough, bronchiectasis, and pulmonary hypertension. In addition, aspergillomas and chronic pulmonary aspergillosis (Chap. 223) have been seen post-TB. Chronic pulmonary aspergillosis may manifest as simple aspergilloma (fungal ball) or chronic cavitary aspergillosis. In the presence of large residual cavities, Aspergillus fumigatus may colonize the lesion and produce symptoms such as respiratory impairment, hemoptysis, persistent fatigue, and weight loss, often resulting in the erroneous diagnosis of TB recurrence. The detection of Aspergillus precipitins (IgG) in the blood suggests chronic pulmonary aspergillosis, as do radiographic abnormalities such as thickening of the pleura and cavitary walls or the presence of a fungal ball inside the cavity. Treatment is difficult. Recent preliminary studies on the use of triazoles for ≥6 months indicate improvement or stabilization of 60–75% of the radiologic and clinical manifestations. Surgical removal of lesions is risky except in simple aspergilloma. HIV-Associated TB (See also Chap. 208)  TB is one of the most common diseases among PLWH worldwide. Responsible for up to a quarter of all HIV-related mortality (167,000 deaths per year), TB is still one of the main causes of death in this population, especially in Africa (Fig. 183-11). Classic studies showed that a person with a positive TST who acquires HIV infection has a 3–13% annual risk of developing active TB, with the exact risk depending on the degree of immunosuppression when observation begins. Furthermore, a new TB infection acquired by a PLWH may evolve into active disease in a matter of weeks rather than months or years. TB can appear at any stage of HIV infection, and its presentation varies with the stage. When cell-mediated immunity is only partially compromised, pulmonary TB presents in a typical man­ ner (Figs. 183-6 and 183-7), with upper-lobe infiltrates and cavitation and without significant lymphadenopathy or pleural effusion. In later stages of HIV infection, when the CD4+ T-cell count is <200/μL, a primary TB–like pattern, with diffuse interstitial and subtle infiltrates, HIV prevalence in new and relapse TB cases, all ages (%) 0–4.9 5–9.9 10–19 20–49 ≥50 No data Not applicable

little or no cavitation, pleural effusion, and intrathoracic lymphade­ nopathy, is more common. However, these forms are becoming less common because of the expanded use of ART. Extrapulmonary TB is common among PLWH. In various series, extrapulmonary TB—alone or in association with pulmonary disease—has been documented in 40–60% of all cases in PLWH. The most common forms are lymphatic, disseminated, pleural, and pericardial. Mycobacteremia and meningitis also are common, particularly in advanced HIV disease. The diagnosis of TB in PLWH can be complicated. The Xpert MTB/RIF assay is the preferred initial diagnostic option for pulmonary TB, ensuring a sen­ sitivity of more than 80% and a specificity of 98%; therapy should be started on the basis of a positive result because treatment delays may be fatal. A negative Xpert MTB/RIF result, however, does not exclude a diagnosis of TB. Culture remains the gold standard. Detection of myco­ bacterial lipoarabinomannan antigen in urine has shown favorable results in assisting with the diagnosis of TB in PLWH (see “Additional Diagnostic Procedures,” below). The immune reconstitution inflammatory syndrome (IRIS) or TB immune reconstitution disease consists of exacerbations in systemic manifestations (lymphadenopathy, fever) or respiratory signs (worsen­ ing of pulmonary infiltrations, pleural effusion) as well as laboratory or radiographic manifestations of TB. This syndrome is rarely seen among non-HIV-infected persons, is associated with the administration of ART, and occurs in ~10% of HIV-infected TB patients. Usually devel­ oping 1–3 months after initiation of ART, IRIS is more common among patients with advanced immunosuppression and extrapulmonary TB. “Unmasking IRIS” may develop after the initiation of ART in patients with undiagnosed subclinical TB. The earlier ART is started after TB treatment is initiated and the lower the baseline CD4+ T-cell count, the greater the risk of IRIS. Death due to IRIS is relatively infrequent and occurs mainly among patients who have a high preexisting mor­ tality risk. The presumed pathogenesis of IRIS consists of an immune response that is rapidly improved by HIV suppression and is stimulated by antigens released as bacilli are killed during effective chemotherapy. There is no diagnostic test for IRIS, and its confirmation relies heav­ ily upon case definitions incorporating clinical and laboratory data; a variety of case definitions have been suggested. The first priority in the management of a possible case of IRIS is to ensure that the clinical syndrome does not represent a failure of TB treatment or the develop­ ment of another infection. Mild paradoxical reactions can be managed with symptom-based treatment and do not worsen outcomes of treat­ ment for TB. However, IRIS can result in serious neurologic complica­ tions or death in patients with CNS TB. Therefore, ART should not be initiated during the first 8 weeks of TB treatment in patients with TB meningitis. Glucocorticoids have been used for severe paradoxical reactions; prednisolone given for 4 weeks at a low dosage (1.5 mg/kg

per day for 2 weeks and half that dose for the remaining 2 weeks) has reduced the need for hospitalization and therapeutic procedures and has hastened alleviation of symptoms, as reflected by Karnofsky per­ formance scores, quality-of-life assessments, radiographic response, and C-reactive protein levels. The effectiveness of glucocorticoids in alleviating the symptoms of IRIS is probably linked to suppression of proinflammatory cytokine concentrations, as these medications reduce serum concentrations of IL-6, IL-10, IL-12p40, TNF-α, IFN-γ, and IFN-γ-inducible protein 10. Recommendations for the prevention and treatment of TB in PLWH are provided below. DIAGNOSIS The key to the early diagnosis of TB is a high index of suspicion. Diag­ nosis is not difficult in persons belonging to high-risk populations who present with typical symptoms and a classic chest radiograph showing upper-lobe infiltrates with cavities (Fig. 183-6). On the other hand, the diagnosis can easily be missed in an elderly nursing-home resident or a teenager with a focal infiltrate. Often, the diagnosis is first entertained when the chest radiograph of a patient being evaluated for respiratory symptoms is abnormal. If the patient has no complicating medical con­ ditions that cause immunosuppression, the chest radiograph may show typical upper-lobe infiltrates with cavitation (Fig. 183-6). The longer the delay between the onset of symptoms and the diagnosis, the more

likely is the finding of cavitary disease. In contrast, immunosuppressed patients, including PLWH, may have “atypical” findings on CXR—e.g., lower-zone infiltrates without cavity formation—or interstitial disease only.

The several approaches to the diagnosis of TB require, above all, a well-organized microbiology laboratory network with an appropriate distribution of tasks at different levels of the health care system. Besides clinical assessment and radiography, screening and referral are the principal tasks at the peripheral and community levels. Diagnosis at a secondary level (e.g., a traditional district hospital in a high-incidence setting) can be accomplished nowadays through real-time automated nucleic acid amplification technology (e.g., the Xpert MTB/RIF assay, which also allows detection of drug resistance) or through traditional AFB microscopy where new tools have not yet been introduced. At a tertiary level, molecular tests, rapid culture, and DST should be applied. ■ ■NUCLEIC ACID AMPLIFICATION TECHNOLOGY Several test systems based on amplification of mycobacterial nucleic acid have become available in the past few years and are now the pre­ ferred first-line diagnostic tests. These tests are progressively replacing smear microscopy, as they ensure rapid confirmation of all types of TB. One system that permits rapid diagnosis of TB with high speci­ ficity and sensitivity (approaching that of liquid culture) is the fully automated, real-time nucleic acid amplification technology known as the Xpert MTB/RIF assay. Xpert MTB/RIF can simultaneously detect TB and rifampin resistance in <2 h and has minimal biosafety and training requirements. Therefore, it can be housed in nonconventional laboratory settings as long as a stable and uninterrupted power supply can be assured. The WHO recommends its use worldwide as the firstline diagnostic test in all adults and children with signs or symptoms of active TB. Given the test’s high sensitivity, the WHO also recom­ mends its use as the initial diagnostic test for PLWH in whom TB is suspected. In the diagnosis of pulmonary TB, this test has an overall sensitivity of 85% reaching 98% among AFB-positive cases and ~70% among AFB-negative specimens; its specificity is 98%. When compared with phenotypic drug susceptibility testing for simultaneous detection of rifampin resistance, Xpert MTB/RIF has an overall sensitivity of 96% and a specificity of 98%. The newer Xpert MTB/RIF Ultra assay (Ultra), which uses the same GeneXpert diagnostic platform, has an overall sensitivity of 90% including “trace calls” (i.e., the “noise” produced by detection of DNA from nonviable bacilli) as positive with the greatest increases among smear-negative, culture-positive cases (+17%) and among PLWH (+12%). If “trace calls” are excluded, sensitivity decreases to 86%. Because of this greater sensitivity and the capacity to also detect nonviable bacilli, the new Ultra cartridge has 2% lower specificity than the original test. However, excluding “trace calls,” specificity increases to 98%. Among PLWH Ultra sensitivity is 88% and specificity 95%. Sensitivity and specificity for detection of rifampin resistance by Ultra are 94% and 99%, respectively, similar to those by the Xpert MTB/RIF assay. CHAPTER 183 Tuberculosis In the diagnosis of extrapulmonary TB, Xpert MTB/RIF or Ultra should be the initial tests applied to CSF from patients in whom TB meningitis is suspected as well as a replacement test (preferable to con­ ventional microscopy, culture, and histopathology) for selected non­ respiratory specimens—those obtained by gastric lavage, fine-needle aspiration, or pleural or other biopsies. Sensitivity varies according to specimen type with the lowest in pleural fluid (50% with Xpert MTB/ RIF and 71% with Ultra) and the highest in synovial fluid (97%) and lymph node biopsy (100% with Ultra). “Trace calls” in specimens from persons with extrapulmonary TB, as well as for PLWH and children, should be considered true positives, given the high risk of severe mor­ bidity and premature death, while among other cases they warrant additional tests to confirm the diagnosis of TB and prevent overtreat­ ment. Among patients with a recent history of TB, “trace calls” may represent DNA from dead bacilli under degradation. Truenat MTB and MTB Plus are two other newly introduced rapid adopted rapid molecular tests with sensitivities of 73% and 80%, respectively, if compared with bacteriological culture, and specificities of 98% and 96%, respectively. Truenat MTB-Rif Dx detects

rifampin resistance with a sensitivity of 84% and a specificity of 97%. These rapid tests, being portable and battery-operated, can be used in peripheral care settings rather than smear microscopy, culture, and phenotypic DST. New high-throughput automated platforms for TB diagnosis and drug-resistant variants are now available (Abbott Real­ Time MTB and RIF/INH, FluoroType MTBDR and MTB, BD Max MDR-TB, cobas MTB and MTB-RIF/INH). These platforms are suit­ able for centralized laboratories and have the advantage of processing a large number of samples in a reasonable time. Sensitivity is higher than 91% and specificity ranges from 97 to 100%. Head-to-head studies with Xpert MTB/RIF have shown comparable performance. Another available molecular test for detection of M. tuberculosis is based on loop-mediated isothermal amplification (LAMP), a temperature-inde­ pendent technology that amplifies DNA, is relatively simple to use, and is interpreted through a visual display. It may be used as a replacement for sputum-smear microscopy for the diagnosis of adult pulmonary TB and as a follow-up test to smear microscopy for the further inves­ tigation of smear-negative specimens from adults with suspected pul­ monary TB. The TB-LAMP assay should not replace rapid molecular tests that detect both TB and rifampin resistance, and its usefulness in PLWH in whom TB is suspected remains unclear.

■ ■AFB MICROSCOPY In some low- and middle-income settings, a presumptive diagnosis is still often based on the finding of AFB on microscopic examination, such as a smear of expectorated sputum or of tissue (e.g., a lymph node biopsy). Although inexpensive, AFB microscopy has relatively low sensitivity (40–60%) in culture-confirmed cases of pulmonary TB and does not distinguish TB from nontuberculous mycobacteria. The traditional method—light microscopy of specimens stained with ZiehlNeelsen basic fuchsin dyes—is satisfactory, although time consuming and operator dependent. Most modern laboratories processing large numbers of diagnostic specimens use auramine–rhodamine staining and fluorescence microscopy; this approach is more sensitive than the Ziehl-Neelsen method. However, it is expensive because it requires high-cost mercury vapor light sources and a darkroom. Less expensive light-emitting diode (LED) fluorescence microscopes are now recom­ mended by the WHO as the microscopy tool of choice. They are as sensitive as—or more sensitive than—traditional fluorescence micro­ scopes. As a result, conventional light and fluorescence microscopes are being replaced with this more recent technology, especially in developing countries. For patients with signs or symptoms of pulmo­ nary TB, it has been recommended that one or two sputum specimens, preferably collected early in the morning, should be submitted to the laboratory for AFB smear and mycobacterial culture. If tissue is obtained, it is critical that the portion of the specimen intended for culture not be put in preservation fluid such as formaldehyde. The use of AFB microscopy in examining urine or gastric lavage fluid is lim­ ited by the low numbers of organisms, which can cause false-negative results, or the presence of commensal mycobacteria, which can cause false-positive results. PART 5 Infectious Diseases ■ ■MYCOBACTERIAL CULTURE Definitive diagnosis depends on the isolation and identification of M. tuberculosis from a clinical specimen. Commercial liquid-culture systems are recommended by the WHO as the reference standard for culture. The MGIT (mycobacteria growth indicator tube) system uses a fluorescent compound sensitive to the presence of oxygen dissolved in the liquid medium. The appearance of fluorescence, detected by fluo­ rometric technology, indicates active growth of mycobacteria. MGIT cultures usually become positive after a period ranging from 10 days to 2–3 weeks; the tubes are read weekly until the eighth week of incuba­ tion before the result is declared to be negative. Specimens may also be inoculated onto egg- or agar-based medium (e.g., Löwenstein-Jensen or Middlebrook 7H10 or 7H11) and incubated at 37°C (under 5% CO2 for Middlebrook medium). Because most species of mycobacteria, includ­ ing M. tuberculosis, grow slowly, 4–8 weeks may be required before growth is detected on these conventional culture media. Although M. tuberculosis may be identified presumptively on the basis of growth

time and colony pigmentation and morphology, a variety of biochemi­ cal tests have traditionally been used to speciate mycobacterial isolates. In modern, well-equipped laboratories, commercial liquid culture for isolation and species identification by molecular methods or highpressure liquid chromatography of mycolic acids has replaced isolation on solid media and identification by biochemical tests. A low-cost, rapid immunochromatographic lateral-flow assay based on detection of MTP64 antigen may also be used for species identification of the M. tuberculosis complex in culture isolates. These new methods, which are increasingly used in limited-resource settings, have decreased the time required for bacteriologic confirmation of TB to 2–3 weeks. ■ ■DRUG SUSCEPTIBILITY TESTING Universal DST is considered by the WHO as the current standard of care for all TB patients and should consist of DST to at least rifampin for all initial isolates of M. tuberculosis, as rifampin resistance is an excellent proxy for MDR-TB diagnosis. Expanded and rapid suscep­ tibility testing for isoniazid and key second-line anti-TB drugs (espe­ cially the fluoroquinolones and the injectable drugs) is mandatory when RR-TB is found in order to guide selection of the appropriate treatment regimens. Susceptibility testing may be conducted directly by molecular techniques (with the clinical specimen) or indirectly (with mycobacterial cultures) on solid or liquid medium. Results are obtained rapidly by direct susceptibility testing on liquid medium, with an average reporting time of 3 weeks. With indirect testing on solid medium, results may not be available for ≥8 weeks. Highly reliable genotypic methods for the rapid identification of genetic mutations in gene regions known to be associated with resistance to rifampin (such as those in rpoB) and isoniazid (such as those in katG and inhA) have been developed and are being widely implemented for screening of patients at increased risk of drug-resistant TB. Apart from the Xpert MTB/RIF, Xpert MTB/RIF Ultra, and Truenat MTB-Rif Dx assays, which effectively detect rifampin resistance, the most widely used tests are molecular line probe assays (LPAs). LPAs are a family of DNA strip-based tests capable of detecting bacterial DNA and identifying drug resistance–associated mutations. After extraction of DNA from M. tuberculosis isolates or from clinical specimens, the resistance gene regions are amplified by polymerase chain reaction (PCR) and labeled and probe-hybridized PCR products are detected by colorimetric development. This assay confirms the presence of M. tuberculosis as well as mutations in target resistance-gene regions. Given the rapidity and accuracy of commercially available LPAs, the WHO recommends that they are used to detect resistance to isoniazid and rifampin when patients have sputum smear–positive specimens or a cultured isolate of M. tuberculosis. These recommendations do not eliminate the need for conventional phenotypic culture-based DST to identify resistance to other drugs and to monitor emergence of additional drug resistance. A similar approach has been developed for second-line anti-TB drugs, such as the fluoroquinolones. Therefore, second-line LPAs (instead of phenotypic culture-based DST) are now recommended by the WHO as the initial test for rapid detection of resistance to the fluoroquino­ lones or the second-line injectable drugs in isolates from patients with confirmed RR-TB or MDR-TB. As with first-line LPAs, these recom­ mendations do not eliminate the need for conventional phenotypic, culture-based DST to identify resistance to other drugs and to monitor for the emergence of additional resistance. Detection of pyrazinamide resistance is important among persons with MDR/RR-TB. The WHO has recently recommended the use of a LPA with reverse hybridizationbased technology in culture isolates rather than phenotypic culturebased DST. Whole genome sequencing (WGS) of M. tuberculosis can provide comprehensive information on mutations conferring resistance, but it has been hampered by the requirement for a culture sample before DNA processing. Amplification and sequencing of relevant genomic targets directly from sputum samples have been successfully tested, and targeted new-generation sequencing (tNGS) is now recommended by the WHO to detect drug resistance after TB diagnosis in order to guide decisions on treatment. This class of diagnostics is particularly useful for patients requiring comprehensive DST with faster results

than phenotypic DST. However, its suboptimal sensitivity for some new and repurposed drugs still requires confirmation by phenotypic DST. ■ ■RADIOGRAPHIC PROCEDURES CXR is a rapid imaging technique that has historically been used as a primary tool to detect pulmonary TB. CXR has high sensitivity but poor specificity. Although TB may often present with typical patterns strongly suggesting the disease, some abnormalities seen in TB are also present in several other lung conditions. The initial suspicion of pulmonary TB is often based on abnormal CXR findings in a patient with respiratory symptoms. The presence of lesions suggestive of TB should prompt bacteriologic investigations in all cases, without excep­ tion. Although the “classic” picture is that of upper-lobe disease with infiltrates and cavities (Fig. 183-6), virtually any radiographic pat­ tern—from a normal film or a solitary pulmonary nodule to diffuse alveolar infiltrates in a patient with adult respiratory distress syndrome— may be seen. In the era of HIV/AIDS, no radiographic pattern can be considered pathognomonic, but CXR can assist in diagnosing TB or ruling it out before initiation of any preventive treatment. CXR is also helpful as a screening test preceding rapid molecular assays to improve their predictive value. Digital CXR technology, which allows display of images in a digital format on a computer screen instead of on x-ray film, offers several advantages: the procedure time is reduced, the run­ ning costs are lower, the imaging is of superior quality, and telemedi­ cine assistance is available, including computer-aided detection (CAD) and interpretation of findings using software programs that analyze digital imaging for abnormalities compatible with TB. However, lim­ ited evidence suggests that while sensitivity may be high, specificity is variable. A recent systematic review of CAD studies concluded that the diagnostic accuracy of this technology is still limited and that general­ izability to low-prevalence settings is still uncertain. CT (Fig. 183-7) may be useful in interpreting questionable findings on plain CXR and in diagnosing some forms of extrapulmonary TB (e.g., intrabdominal disease, Pott’s disease; Fig. 183-10). MRI is useful in the diagnosis of bone lesions and intracranial TB. ■ ■ADDITIONAL DIAGNOSTIC PROCEDURES Other diagnostic tests may be used when pulmonary TB is suspected. Sputum induction by ultrasonic nebulization of hypertonic saline may be useful for patients who cannot produce a sputum specimen spontaneously. Frequently, patients with radiographic abnormalities that are consistent with other diagnoses (e.g., bronchogenic carci­ noma) undergo fiberoptic bronchoscopy with bronchial brushings and endobronchial or transbronchial biopsy of the lesion. Bronchoalveo­ lar lavage of a lung segment containing an abnormality also may be performed. In all cases, it is essential that specimens be submitted for molecular testing with the Xpert MTB/RIF assay, mycobacterial culture, and AFB smear. For the diagnosis of primary pulmonary TB in children, who often do not expectorate sputum, induced sputum specimens and specimens from early-morning gastric lavage may yield positive results in the Xpert MTB/RIF assay or on culture. Recently, a promising artificial intelligence (AI)-powered monitoring of cough counts has been tested for prediction of TB disease and treatment monitoring, and AI-powered classification of cough sounds for TB screening. Once fully developed, these non-invasive tools may be use­ ful complements to the diagnostic armamentarium. Invasive diagnostic procedures are indicated for patients with sus­ pected extrapulmonary TB. In addition to testing of specimens from involved sites (e.g., CSF for tuberculous meningitis, pleural fluid and biopsy samples for pleural disease), biopsy and culture of bone marrow and liver tissue have good diagnostic yields in disseminated (miliary) TB, particularly in PLWH, who also have a high frequency of positive blood cultures. Xpert MTB/RIF should always be the initial diagnostic test in patients in whom TB meningitis is suspected; any positive results should prompt immediate treatment initiation, while negative results should be followed up by additional testing. In some cases, the results of culture or Xpert MTB/RIF are negative but a clinical diagnosis of TB is supported by consistent epidemiologic evidence (e.g., a history of close contact with an infectious patient) and a compatible clinical and

radiographic response to treatment. In the United States and other industrialized countries with low rates of TB, some patients with lim­ ited abnormalities on CXR and sputum positive for AFB are infected with nontuberculous mycobacteria, most commonly organisms of the M. avium complex or M. kansasii (Chap. 185). Factors favoring the diagnosis of nontuberculous mycobacterial disease over TB include an absence of risk factors for TB and the presence of underlying chronic pulmonary disease.

Patients with HIV-associated TB pose several diagnostic problems (see “HIV-Associated TB,” above). PLWH with sputum culture–posi­ tive, AFB-positive TB may present with a normal chest radiograph. The Xpert MTB/RIF assay is the preferred rapid diagnostic test in this population of patients because of its simplicity and increased sensitiv­ ity (~60–70% among AFB-negative, culture-positive cases and 97–98% among AFB-positive cases). With the advent of ART, the occurrence of disseminated M. avium complex disease that can be confused with TB has become much less common. A test based on the detection of mycobacterial lipoarabinomannan antigen in urine has emerged as a potentially useful point-of-care test for TB in PLWH with low CD4+ T-cell counts. The lateral-flow urine lipoarabinomannan (LF-LAM) assay can be performed manually and read by eye. The WHO recom­ mends that this assay be used to assist in the diagnosis of TB only (1) in HIV-positive adults and children in inpatient settings who have signs and symptoms of TB, or who have advanced HIV disease or are seriously ill, or, irrespective of signs and symptoms of TB, who have a CD4+ T-cell count of ≤200 cells/μL or (2) in outpatient settings in HIV-positive adults and children who have signs and symptoms of TB, or irrespective of signs and symptoms, who have a CD4+ T cell count of <100 cells/μL. CHAPTER 183 ■ ■BIOMARKERS In view of the limitations of current diagnostics, research on TB biomarkers and multiple marker biosignatures that could be used as a point-of-care test for disease or triage is a high priority and has been crystallized in well-defined target product profiles by the WHO. Recent systematic reviews revealed that promising host biomarkers under study, such as antibodies, cytokines, chemokines, and RNA signatures, by far exceed pathogen biomarkers that can be obtained from urine or blood. However, currently, candidate biomarkers require additional studies to fully assess their performance. Tuberculosis ■ ■DIAGNOSIS OF M. TUBERCULOSIS INFECTION Two modalities currently exist for identification of individuals with TB infection: the skin tests and IGRA, both of which measure host immu­ nologic response to TB antigens. These tests have limitations, especially in settings or populations with high TB and/or HIV prevalence. Skin Testing  In 1891, Robert Koch discovered that components of M. tuberculosis in a concentrated liquid-culture medium, subsequently named “old tuberculin,” were capable of eliciting a skin reaction when injected subcutaneously into patients with TB. In 1932, Seibert and Munday purified this product by ammonium sulfate precipitation to produce an active protein fraction known as tuberculin purified protein derivative (PPD). In 1941, PPD-S, developed by Seibert and Glenn, was chosen as the international standard. Later, the WHO and UNICEF sponsored large-scale production of a master batch of PPD (RT23) and made it available for general use. The greatest limitation of PPD is its lack of mycobacterial species specificity, a property due to the large number of proteins in this product that are highly conserved in the various species. In addition, subjectivity of the skin-reaction interpre­ tation that is dependent on the operator, deterioration of the product, and batch-to-batch variations limit the usefulness of PPD. The skin test with tuberculin PPD (TST) is most widely used in screening for TB infection. It probably measures the response to antigenic stimulation by T cells that reside in the skin rather than the response of recirculating memory T cells. The test is of limited value in the diagnosis of active TB because of its relatively low sensitivity and specificity and its inability to discriminate between TB infection and active disease. False-negative reactions are common in immunosup­ pressed patients and in those with overwhelming TB. False-positive

reactions may be caused by infections with nontuberculous myco­ bacteria (Chap. 185) and by BCG vaccination. A repeated TST can produce larger reaction sizes due to either boosting or true conversion. The “boosting phenomenon” is a spurious TST conversion resulting from boosting of reactivity on a subsequent TST 1–5 weeks after the initial test. Distinguishing boosting from true conversion is difficult yet important and can be based on clinical and epidemiologic consider­ ations. For instance, true conversions are likely after BCG vaccination in a previously TST-negative person or in a close contact of an infec­ tious patient.

Recently, new Mycobacterium tuberculosis antigen-based skin tests (TBSTs) using ESAT-6 and CFP-10 antigens have been introduced. Combining the simplicity of the TST approach with the specificity of IGRAs, three different TBSTs were assessed by the WHO and consid­ ered to have an accuracy similar to that of IGRAs and greater than that of TST, therefore being useful also for PLWH, children, and BCGvaccinated persons. IFN-γ Release Assays  In-vitro assays that measure T cell release of IFN-γ in response to stimulation with the highly TB-specific RD1encoded antigens ESAT-6 and CFP-10 were introduced in the early 2000s and are commercially available. The T-SPOT.TB test (Oxford Immunotec; Oxford, UK) is an enzyme-linked immunospot assay, and the QuantiFERON-TB Gold test (Qiagen GmbH; Hilden, Ger­ many) is a whole-blood enzyme-linked immunosorbent assay for measurement of IFN-γ. The QuantiFERON-TB Gold In-Tube (QFTGIT) assay, which facilitates blood collection and initial incubation, also contains another specific antigen, TB7.7. These tests mainly measure the response of recirculating memory CD4+ T cells—normally part of a reservoir in the spleen, bone marrow, and lymph nodes—to persisting bacilli-producing antigenic signals. However, CD8+ cells can also release IFN-γ in vitro in response to stimulation with TB antigens, and they seem to do so especially in the early phase of infection and in the phase of reactivation. Therefore, a new version of the QFT-GIT assay, called QuantiFERON-TB Gold Plus (QFT-Plus), has been developed and operates through two antigen tubes: TB1, containing long peptides from ESAT-6 and CFP-10 and inducing a CD4+ T cell response, and TB2, which also contains shorter peptides stimulating CD8+ cells. PART 5 Infectious Diseases Potential advantages of IGRAs include logistical convenience, the need for only one patient visit to complete testing, and the avoidance of somewhat subjective measurements (e.g., skin induration). However, IGRAs require that blood be drawn and then delivered to the labora­ tory in a timely fashion. IGRAs also require that testing be performed by specially trained technicians in a laboratory setting. These require­ ments pose challenges similar to those faced with the TST, including cold-chain requirements and batch-to-batch variations. Because of higher specificity and greater availability of resources, IGRAs have usually replaced the TST for TB infection diagnosis in low-incidence, high-income settings. However, in high-incidence TB and HIV set­ tings and population groups, evidence regarding the performance and usefulness of IGRAs is still limited, and cost considerations may limit wider use. A number of national guidelines on the use of IGRAs for TB infec­ tion testing have been issued. In the United States, an IGRA is preferred to the TST for most persons over the age of 5 years who are being screened for TB infection. However, for individuals at high risk of progression to active TB (e.g., PLWH), either test—or, to optimize sen­ sitivity, both tests—may be used. Because of the paucity of data on the use of IGRAs in children, the TST is preferred for TB infection testing of children aged <5 years. In Canada and some European countries, a two-step approach for those with positive TSTs—i.e., an initial TST fol­ lowed by an IGRA—is often recommended. However, a TST may boost an IGRA response if the interval between the two tests exceeds 3 days. Because the IGRA uses elaboration of IFN-γ for its readout, patients with anti-IFN-γ autoantibodies may have an indeterminate test result. In conclusion, both the TST and IGRA, although useful as diagnos­ tic aids, are imperfect tests for TB infection: while they can identify TB infected persons, they have low predictive value in identifying

TABLE 183-2  Recommended Dosagea for Initial Treatment of Tuberculosis in Adults and Children   DAILY DOSE DRUG ADULT PEDIATRIC Isoniazid 5 mg/kg, max 300 mg 10 (7–15) mg/kg, max 300 mg Rifampin 10 mg/kg, max 600 mg 15 (10–20) mg/kg, max 600 mg Pyrazinamide 25 mg/kg, max 2 g 35 (30–40) mg/kg Ethambutol 15 mg/kg 20 (15–25) mg/kg aThe duration of treatment with individual drugs varies by regimen, as detailed in Table 183-3. Source: Based on recommendations of the American Thoracic Society/Infectious Diseases Society of America/Centers for Disease Control and Prevention and the World Health Organization. individuals with the highest risk of progression toward disease, cannot differentiate between active TB and TB infection, cannot distinguish new infections from reinfections, and display reduced sensitivity in immunocompromised patients. TREATMENT Tuberculosis The two main aims of TB treatment are (1) to prevent morbidity and death by curing TB while preventing recurrences and emer­ gence of drug resistance, and (2) to interrupt transmission by rendering patients noninfectious. Four major drugs are considered first-line agents for the treatment of TB: isoniazid, rifampin, pyra­ zinamide, and ethambutol. Table 183-2 presents currently recom­ mended dosages in adults and children. An additional rifamycin, rifapentine, also is available and is recommended in some treatment and prevention regimens. For a detailed discussion of the drugs used for the treatment of TB, see Chap. 186. Because of a lower degree of effectiveness and tolerability, several classes of second-line drugs are generally used mainly for the treat­ ment of patients with drug-resistant TB. These agents have previ­ ously been classified in various manners to facilitate a standardized approach to their use. In the latest WHO guidance on the treatment of MDR-TB, they are now grouped in three ranked categories for the purpose of designing more individualized regimens of 18–20 months’ duration (Table 183-3). Group A drugs include three TABLE 183-3  Groups of Drugs Recommended for Use in Longer MDR-TB Regimens and Approach to the Design of a Longer Regimen for Adults and Children GROUP DRUG Group A: All three drugs should be included to ensure that at least four likely effective agents (including one from group B below) are started and at least three are included for the rest of treatment if bedaquiline is stopped Levofloxacin or moxifloxacin Bedaquiline Linezolid Group B: At least one of these drugs should always be included Clofazimine Cycloserine or terizidone Group C: Drugs to be used to complete the regimen and when drugs from groups A and B cannot be useda Ethambutol Delamanid Pyrazinamide Imipenem-cilastatin or meropenem Amikacin (or streptomycin if amikacin is not available) Ethionamide or prothionamideb p-Aminosalicylic acidb aKanamycin and capreomycin are not to be included in the longer regimen. bTo be included only if bedaquiline, linezolid, clofazimine, or delamanid are not used, or if better options are not possible. Source: Adapted from the World Health Organization, 2022.

classes of oral agents: the fluoroquinolones levofloxacin and moxi­ floxacin; the oxazolidinone linezolid; and the diarylquinoline beda­ quiline. Group B drugs include two other oral agents: clofazimine and cycloserine (or its analogue terizidone). Group C drugs include the nitroimidazole delamanid; imipenem-cilastatin or meropenem; the injectable aminoglycosides amikacin and streptomycin (the latter formerly a first-line agent, now rarely used for drug-resistant TB because of their toxicity and high resistance levels worldwide); ethionamide or prothionamide; and P-aminosalicylic acid (PAS). In addition, the first-line anti-TB drugs ethambutol and pyr­ azinamide as well as high-dose isoniazid are used for MDRTB treatment. Information about drugs used in the treatment of drug-resistant TB (including dosages) can be found in the following WHO Handbook: https://iris.who.int/bitstream/han dle/10665/365308/9789240063129-eng.pdf?sequence=1. This classi­ fication scheme excludes the second-line injectable aminoglycoside kanamycin and the polypeptide capreomycin. Amithiozone, which has been associated with severe and at times fatal skin reac­ tions—including Stevens-Johnson syndrome—among PLWH, is no longer recommended. REGIMENS Standard regimens are traditionally divided into an intensive (bac­ tericidal) phase and a continuation (sterilizing) phase. During the intensive phase, the majority of tubercle bacilli are killed, symp­ toms resolve, and usually the patient becomes noninfectious. The continuation phase is required to eliminate persisting mycobacteria and prevent relapse. TABLE 183-4  Recommended Antituberculosis Treatment Regimens   INDICATION DURATION, MONTHS DRUGS DURATION, MONTHS DRUGS New drug-susceptible pulmonary or extrapulmonarya TB cases

HRZEb

HRb,c New drug-susceptible pulmonary TB (12 years or older)

HPMZb,d

HPMb,d New non-severee TB in children and adolescents aged 3 months to

16 years

HRZ(E)b

HRb Pregnancy

HREf

HR Intolerance to Z

HRE

HR Relapses, treatment default, failures Tailored according to rapid drug susceptibility testing Resistance (or intolerance) to H Throughout (6) RZELfx MDR/RR-TB (see text for further details) Throughout (6) BPaLMg for patients aged ≥14 years without previous exposure to B, L, and Pa. Throughout (6) BDLLfxCfz for patients with no previous exposure to B, D, and L, including children, adolescents, and pregnant and breastfeeding women. Either Lfx and Cfz may be omitted depending on fluoroquinolone drug susceptibility testing. This regimen may be used in place of 9-month or longer regimens described below. Throughout (9) BLMZ or BLLfxCfzZ or BDLLfxZ for patients with no previous exposure to B, D, and L, and in whom resistance to fluoroquinolones has been excluded. These regimens may be used in place of currently recommended longer (18-month) regimens described below. Alternatively: all-oral, B-containing, 9-month regimen: 4 months of B (used for up to 6 months), Lfx or M, Eto or Pto, E, Hh, Z, Cfz followed by 5 months of Lfx or M, E, Z, Cfz for adults and children with no previous exposure to second-line treatment including B, in whom resistance to fluoroquinolones has been excluded, and in the absence of extensive pulmonary TB or severe forms of extrapulmonary TB. In more complex forms of MDR/RR-TB (e.g., XDR-TB) longer (≥18 months) individualized regimens need to be formulated as per Table 183-3. aExcept for TB of central nervous system, bone, or joint, for which longer therapy should be used. bAll drugs should be given daily. cThe American Thoracic Society, the Centers for Disease Control and Prevention, and the Infectious Diseases Society of America suggest that a 2-month continuation phase could be used in HIV-seronegative patients with sputum smear–negative and culture-negative TB. dRifapentine is given at the daily dose of 1200 mg. eIt is considered non-severe: TB of peripheral lymph nodes, intrathoracic lymph node without air obstruction, uncomplicated pleural effusion, or paucibacillary, non-cavitary disease confined to one lobe of the lungs and without a miliary pattern. fThe 6-month regimen with pyrazinamide can probably be used safely during pregnancy and is recommended by the WHO and the International Union Against Tuberculosis and Lung Disease. If pyrazinamide is not included in the initial treatment regimen, the minimal duration of therapy is 9 months. gLinezolid is given at the dosage of 600 mg daily. This regimen may be used without moxifloxacin in case of documented resistance to fluoroquinolones. Abbreviations: B, bedaquiline; Cfz, clofazimine; D, delamanid; E, ethambutol; Eto, ethionamide; H, isoniazid; Hh, high-dose isoniazid (900 mg); L, linezolid; Lfx, levofloxacin; M, moxifloxacin; MDR/RR-TB, multidrug-resistant and rifampin-resistant tuberculosis; Pa, pretomanid; Pto, prothionamide; R, rifampin; WHO, World Health Organization; XDR-TB, extensively drug-resistant tuberculosis; Z, pyrazinamide.

Six-month regimen  The treatment regimen of choice for virtually all forms of drug-susceptible TB in adults consists of a 2-month initial (intensive) phase of isoniazid, rifampin, pyrazinamide, and ethambutol followed by a 4-month continuation phase of isonia­ zid and rifampin (2HRZE/4HR) (Table 183-4). This regimen can cure TB in >90% of patients. In children, most forms of TB in the absence of HIV infection or suspected isoniazid resistance can be safely treated without ethambutol in the intensive phase. Treatment should be given daily throughout the course, as systematic reviews have demonstrated that the use of intermittent (twice-weekly or thrice-weekly) regimens are associated with increased risk of treatment failure, relapse, and acquisition of drug resistance. The latest guidelines by the ATS, the CDC, and the IDSA, while recom­ mending daily administration of drugs, include a provision for use of intermittent thrice-weekly supervised regimens mainly among patients who are not infected with HIV, do not have cavitary dis­ ease, and are at low risk of relapse.

Patients with cavitary pulmonary TB and delayed sputum-culture conversion (i.e., those who remain culture-positive at 2 months) should be retested immediately for drug-resistant TB, and a change of regimen should be considered. A full course of 6 months with four-drug therapy should be performed not including interruptions of >4 weeks. Four-month regimens  In children and adolescents between 3 months and 16 years of age with nonsevere TB, the WHO also recommends the use of a 4-month regimen consisting of a 2-month initial (intensive) phase of isoniazid, rifampin, pyrazinamide, and CHAPTER 183 INITIAL PHASE CONTINUATION PHASE Tuberculosis

ethambutol followed by a 2-month continuation phase of isoniazid and rifampin.

The recent Tuberculosis Trials Consortium Study 31/AIDS Clin­ ical Trials Group A5349 (Study 31/A5349) showed that a 4-month daily regimen that included isoniazid, pyrazinamide (for the first 2 months only), rifapentine at a daily dose of 1200 mg, and moxi­ floxacin at a daily dose of 400 mg was noninferior to the standard 6-month regimen and had a similar adverse-event profile. This option (2HPMZ/2HPM) is now recommended by the WHO and CDC as an alternative for patients aged 12 years or older, including PLWH, provided that rigorous antibacterial stewardship is ensured especially to prevent fluoroquinolone resistance. This regimen has not been evaluated, and therefore should not be used, in patients with body weight <40 kg; children aged <12 years; patients with severe extrapulmonary or disseminated forms of TB; PLWH with a CD4+ count of <100/μL, pregnant, breastfeeding or postpar­ tum women; or those with history of prolonged QT syndrome or receiving medications with known clinically relevant drug–drug interactions or infected with an isolate known or suspected to be resistant to the regimen drugs. Alternative regimens for patients who exhibit drug intolerance or adverse reactions are listed in Table 183-4. However, severe side effects prompting discontinuation of any of the first-line drugs and use of these alternative regimens are uncommon. To prevent isoniazid-related neuropathy, pyridoxine (10–25 mg/d) should be added to the regimen given to persons at high risk of vitamin B6 deficiency (e.g., those with alcohol use dis­ order; malnourished persons; pregnant and lactating women; and patients with conditions such as chronic renal failure, diabetes, and HIV infection, which are also associated with neuropathy). Finally, to facilitate absorption of rifampin, the drug should be taken on an empty stomach and without meals. PART 5 Infectious Diseases PATIENT CARE AND SUPPORT Poor adherence to treatment is one of the most important impedi­ ments to cure. Moreover, the tubercle bacilli harbored by patients who do not fully adhere to the prescribed regimen are likely to become resistant to the drugs to which they are irregularly exposed. Both patient- and provider-related factors may affect adherence. Patient-related factors include a lack of belief that the illness is worth the cost of adherence; the existence of concomitant medical conditions (notably alcohol or substance abuse); lack of social sup­ port; fear of the stigma and discrimination associated with TB; and poverty, with attendant joblessness and homelessness. Providerrelated factors that may prevent adherence include lack of support, education, and encouragement of patients and inconvenient clinical services. A variety of interventions to increase the chances of completion of the months-long treatment course are available. First, a package of social support interventions that are complementary and not mutually exclusive, consisting of educational, psychological, and material goods and services, may enable patients with TB to address hurdles to treatment adherence. Health education and counseling on the disease’s seriousness and solutions and on the importance of treatment adherence until cure should be provided to all patients at the start of and throughout the course of TB therapy. Psychological support (i.e., counseling sessions or peer-group support) can be particularly relevant in the context of the stigma and discrimination often affecting persons with TB and their families. Material support (e.g., food or financial support in forms such as meals, food baskets, food supplements, food vouchers, transport subsidies, living allow­ ances, housing incentives, or financial bonuses) reduces indirect costs incurred by patients or their attendants in accessing health services and mitigates the consequences of income loss related to the disease. The recent RATIONS clinical trial in India showed that nutritional support provided to a cohort of undernourished TB patients resulted in rapid weight gain and a substantially decreased mortality. In the same trial, nutritional support of household con­ tacts was associated with a 39–48% reduction in TB incidence in the household during 2 years of follow-up.

Second, it is paramount that health services be arranged to meet the needs and reasonable expectations of patients. Components of optimal health services include a suitable geographic location, a schedule responsive to patients’ needs, functional channels of com­ munication between patients and their health care providers (e.g., a telephone short-messaging system, audio/video call capability, home or workplace visits), and a staff willing and competent to care for patients with TB, to address their concerns, and to base the care they provide on sound ethical standards. Third, it is crucial to offer the patient a suitable option for treat­ ment administration that minimizes the chance of nonadherence. Such options traditionally include unsupervised, self-administered therapy; in-person directly observed therapy (DOT); and nondaily DOT (e.g., supervision not for every dose but weekly or a few times per week) at a location mutually agreed on by patient and health care provider, with supervisory responsibility delegated to a qualified person. Direct supervision of adherence is crucial in view of the lack of tools to accurately predict adherence to self-administered treatment and of the public health importance of TB. The WHO, along with the ATS, the CDC, and the IDSA, states that ideally all patients should have their therapy directly supervised, especially during the initial phase, with proper social support based on a patient-centered approach as described above. In several countries, personnel to supervise therapy are usually available through TB control programs of local public health departments, often involving members of the community who are accepted by the patient and who have been properly trained and educated by health workers to undertake the supervisory role. Direct supervision with social support has been shown to significantly increase the proportion of patients completing treat­ ment in all settings and to lessen the chances of treatment failure, relapse, and default. In general, community- or home-based DOT is recommended over health facility–based DOT or unsupervised treatment; DOT administered by trained lay providers or health care workers is recommended over DOT administered by family members. Recently, comparison of video-observed therapy (VOT) with in-person DOT has shown similar outcomes. In a multicenter, analyst-blinded, randomized, controlled superiority trial of VOT through daily remote observation using a smartphone app versus DOT done 3−5 times weekly at home, community, or clinic set­ tings, VOT was superior to DOT in ensuring scheduled obser­ vation of drug intake. Therefore, VOT can replace DOT when Internet access is good and video communication technology (e.g., smartphones, tablets, computers) is available. The system can be appropriately organized and operated by health care providers and patients. Other digital health tools can facilitate the monitoring of adherence, including digital medication monitors; these monitors can register when the pillbox is opened, with options to emit audio signals or a short message to remind patients to take medicines. These tools are customized to the needs and preferences of the individual patient and the provider. In addition to the above measures promoting adherence, provi­ sion of fixed-dose combination products that reduce the number of tablets the patient needs to swallow is recommended by the WHO over separate drug formulations. Various fixed-dose combi­ nation products are available (e.g., isoniazid/rifampin, isoniazid/

rifampin/pyrazinamide, and isoniazid/rifampin/pyrazinamide/ ethambutol). Fixed-dose combinations increase patient satisfac­ tion and minimize the likelihood of prescription error or of development of drug resistance resulting from monotherapy if a drug is out of stock or the patient prefers one drug over others. In addition, these combinations facilitate programmatic manage­ ment of procurement and supply. In the past, the bioavailability of rifampin was found to be substandard in some formulations of fixed-dose combinations. Medical regulatory authorities should ensure that combination products are of good quality; however, top standards for drug quality assurance are not always operative, especially in limited-resource countries. Prescribers should be aware of this potential problem.

MONITORING TREATMENT RESPONSE

AND DRUG TOXICITY Bacteriologic evaluation through commercial liquid-culture systems (or—when liquid-culture capacity is not yet available—through smear microscopy) is essential in monitoring the response to TB treatment. In addition, the patient’s weight should be monitored regularly and the drug dosage adjusted with any significant weight change. Patients with pulmonary disease should have their sputum examined monthly until cultures become negative to allow early detection of treatment failure. With the recommended 6-month standard first-line regimen, >80% of drug-susceptible TB patients will have negative sputum cultures at the end of the second month of treatment. By the end of the third month, the sputum of virtually all patients should be culture negative. In some patients, especially those with extensive cavitary disease and large numbers of organ­ isms, AFB smear conversion may lag behind culture conversion as a result of the expectoration and microscopic visualization of dead bacilli. Therefore, as capacity is built, smear microscopy should be progressively abandoned as a monitoring tool in favor of liquid culture. As noted above, patients with cavitary disease in whom sputum culture conversion does not occur by 2 months require immediate testing or retesting for drug resistance. When a patient’s sputum cultures or smears remain positive at ≥3 months despite good adherence, treatment failure caused by drug resistance is likely. Nontuberculous mycobacteria may confound and confuse AFB microscopy. The pattern of drug resistance should guide the choice of the best treatment option (see below). A sputum specimen should be collected at the end of treatment to document cure. In settings where mycobacterial cultures are not yet available, moni­ toring by AFB smear examination should be undertaken at 2, 5, and 6 months. Bacteriologic monitoring of patients with extrapul­ monary TB is more difficult and often is not feasible. In these cases, the response to treatment must be assessed clinically with the help of medical imaging. Monitoring of the response to chemotherapy by nucleic acid amplification technology, such as the Xpert MTB/RIF assay, is not suitable because these tests can produce positive results due to nonviable bacilli. Likewise, serial chest radiographs are not recom­ mended because radiographic changes may lag behind bacteriologic response and are not highly sensitive. After the completion of treat­ ment, neither sputum examination nor CXR is recommended for routine follow-up purposes. However, a chest radiograph obtained at the end of treatment may be useful for comparative purposes should the patient develop symptoms of recurrent TB months or years later. Patients should be instructed to report promptly for medical assessment if they develop any such symptoms. During treatment, patients should be monitored for drug tox­ icity. The most common adverse reaction of significance among those treated for drug-susceptible TB is hepatitis. Patients should be carefully educated about the signs and symptoms of drug-induced hepatitis (e.g., dark urine, loss of appetite, nausea) and should be instructed to discontinue treatment promptly and see their health care provider if these manifestations occur. Although biochemical monitoring is not routinely recommended, all adult patients should undergo baseline assessment of liver function (e.g., measurement of serum levels of hepatic aminotransferases and bilirubin). Older patients, those with concomitant diseases, those with a history of hepatic disease (especially hepatitis C), and those using alcohol daily should be monitored especially closely (i.e., monthly), with repeated measurements of aminotransferases, during the initial phase of treatment. Up to 20% of patients have small increases (up to three times the upper limit of normal) in serum levels of aspartate aminotransferase that are not accompanied by symptoms and are of no consequence. Suspension of treatment should be considered for patients with symptomatic hepatitis, especially when accompanied by at least a three-fold increase in serum levels of AST and/or ALT, and for patients without symptoms of hepatic injury who have marked (at least fivefold) elevations in serum levels of AST and/or ALT. Drugs should be reintroduced one at a time after

liver functions have returned to normal. Hypersensitivity reactions usually require the discontinuation of all drugs and rechallenge to determine which agent is the culprit. Because of the variety of regi­ mens available, it usually is not necessary—although it is possible—to desensitize patients. Hyperuricemia and arthralgia caused by pyra­ zinamide can usually be managed by the administration of acetyl­ salicylic acid; however, pyrazinamide treatment should be stopped if the patient develops gouty arthritis. Individuals who develop autoimmune thrombocytopenia secondary to rifampin therapy should not receive the drug thereafter. Similarly, the occurrence of optic neuritis with ethambutol is an indication for permanent discontinuation of this drug. Other common manifestations of drug intolerance, such as pruritus and gastrointestinal upset, can gener­ ally be managed without the interruption of therapy. Treatment with second-line agents for drug-resistant TB is associated with a variety of adverse drug reactions that are more frequent and severe than in patients receiving first-line TB regimens (see below). The likelihood of drug–drug interactions also is higher when secondline regimens are used.

TREATMENT FAILURE AND RELAPSE Treatment failure should be suspected when a patient’s cultures (or sputum smears, when cultures are not available) remain positive after 3 months of treatment. In the management of such patients, it is imperative that the current isolate be urgently retested (or tested for the first time if, for some reason, rapid molecular susceptibility testing was not performed at the start of treatment) for susceptibil­ ity to first-line agents. If resistance to rifampin is detected, test­ ing should be done to second-line agents as well. The treatment approach should start with molecular testing for—at the least— resistance to rifampin and isoniazid. Because results are expected to become available within a few days, changes in the regimen can be postponed until that time. However, if the patient’s clinical condition is deteriorating rapidly, an earlier change in regimen may be indicated. A cardinal rule in the latter situation is always to add more than one drug, preferably two or three, at a time to a failing regimen; in practice, starting an empirical regimen for MDR-TB (see “Drug-Resistant TB,” below) is warranted. The patient may continue to take isoniazid and rifampin along with these new agents pending the results of susceptibility tests. CHAPTER 183 Tuberculosis Patients who experience a recurrence after apparently success­ ful treatment (i.e., a relapse) are less likely to harbor drug-resistant strains than are patients in whom treatment has failed. Acquired resistance is uncommon among strains from patients in whom relapse follows the proper completion of a standard 6-month regi­ men. The treatment decision depends on a general assessment of the risk of drug resistance, the severity of the case, and the results of rapid susceptibility testing. Patients whose treatment has been interrupted and who have a high likelihood of MDR-TB should receive an MDR-TB regimen that includes second-line agents as soon as possible and according to DST results (Table 183-4). DRUG-RESISTANT TB Strains of M. tuberculosis resistant to individual drugs arise by spontaneous point mutations in the mycobacterial genome that occur at low but predictable rates (10−7–10−10 for the key drugs). Resistance to rifampin is associated with mutations in the rpoB gene in 95% of cases, that to isoniazid with mutations mainly in the katG gene (50–95% of cases) and the inhA gene promoter region (up to 45%), that to pyrazinamide in the pncA gene (up to 98%), that to ethambutol in the embB gene (50–65%), that to the fluoroquinolones in the gyrA–gyrB genes (75–95%), and that to the aminoglycosides mainly in the rrs gene (up to 80%); the C-12T mutation is the most common mutation in the eis promoter region associated with aminoglycoside resistance, especially in Eastern European countries. Because there is no cross-resistance among the commonly used classes of drugs, the probability that a strain will be resistant to two drug classes is the product of the probabilities of resistance to each drug class and thus is low. The development of

drug-resistant TB almost invariably follows monotherapy—i.e., the failure of the health care provider to prescribe at least two drugs to which tubercle bacilli are susceptible; of the patient to absorb or take properly prescribed therapy; or of the bioavailability of poorquality drugs or preparations (e.g., due to crushing of tablets). Drug-resistant TB may be either primary or acquired. In primary drug resistance, the patient is infected from the start by a drugresistant strain. Acquired resistance develops in the infecting strain during treatment. In North America, Western Europe, most of Latin America, and the Persian Gulf states, rates of primary resis­ tance are generally low and isoniazid resistance is most common. In the United States, although rates of primary isoniazid resistance have been stable at ~7–8%, the rate of primary MDR-TB has declined from 2.5% in 1993 to <1% since 2000. MDR-TB is a serious problem in some regions, especially in the countries of the former Soviet Union and some countries of Asia (Fig. 183-12). Even more serious is the occurrence of MDR strains that are also resistant to additional second-line agents used in treatment, such as the fluoro­ quinolones. Creation of drug-resistant TB can be prevented by adherence to the principles of sound treatment: inclusion of at least two quality-assured, bactericidal drugs to which the organism is susceptible; use of effective combination regimens; supervision of treatment with patient support; and verification that patients com­ plete the prescribed course. The use of fixed-dose combination products may prevent selective drug intake and therefore possibly protect against the creation of drug resistance. Transmission of drug-resistant strains can be prevented by the implementation of respiratory infection-control measures (see below) and by early detection of persons with active TB followed by immediate initia­ tion of treatment with an effective regimen.

PART 5 Infectious Diseases Isoniazid-Resistant TB  For the treatment of patients with isoni­ azid-resistant but rifampin-susceptible disease, a combination of rifampin, ethambutol, pyrazinamide, and levofloxacin for 6 months is recommended. This fluoroquinolone-containing regimen should not be used until rifampin resistance has been excluded by a reliable FIGURE 183-12  Percentage of new cases of multidrug-resistant/rifampin-resistant tuberculosis (TB) in all countries surveyed by the World Health Organization (WHO) Global Drug Resistance Surveillance Project during 1994–2022. Figures are based on the most recent year for which data have been reported, which varies among countries. Data reported before the year 2002 are not shown. (See disclaimer in Fig. 183-2. Reproduced with permission from Global Tuberculosis Report 2023. Geneva, World Health Organization; 2023.)

diagnostic test to avoid inadvertent treatment of MDR-TB with an inadequate regimen. Susceptibility should also be tested for the fluoroquinolones and pyrazinamide. If the fluoroquinolone is contraindicated because of intolerance or resistance, the patient can be given a 6-month regimen of rifampin, ethambutol, and pyra­ zinamide. Isoniazid probably does not contribute to a successful outcome in these regimens but may be retained (also to facilitate treatment with the four-drug fixed-dose formulation). Other drugs, such as the injectable aminoglycosides, are unlikely to play a role in the treatment of most isoniazid-resistant TB cases. However, they may be considered only in the presence of additional resistance or of drug intolerance. RR-, MDR-TB  MDR-TB, in which bacilli are resistant to (at least) isoniazid and rifampin, is more difficult to manage than is disease caused by drug-susceptible organisms because these two bactericidal drugs are potent first-line agents and because associated resistance to other first-line drugs as well (e.g., eth­ ambutol) is not uncommon. Treatment for RR-TB and MDR-TB has traditionally been a topic of much debate, given its complex­ ity, long duration, toxicity, and limited efficacy; the cost of most second-line drugs; and the lack of randomized controlled clinical trials to support combinations. Recent developments include the accrual of individual datasets for patients treated worldwide; the release of findings from randomized, controlled, phase 3 clinical trials (the STREAM stage 1 trial comparing a 9-month, shorter MDR-TB regimen with the previous optimized WHO background regimen, and Otsuka’s phase 3 trial 213 comparing the addition of the new drug delamanid to the previous optimized WHO background regimen with the addition of placebo); results of the Nix-TB and ZeNix trials enrolling highly drug-resistant cases on a regimen composed of three oral drugs (bedaquiline, pretomanid, and linezolid [BPaL]) at different dosages; the TB-PRACTECAL trial testing BPaL-based regimens against multiple comparators; the NExT trial testing several drug combinations for 6–9 months; the assessment of programmatic data from South Africa on Percentage of cases 0–2.9 3–5.9 6–11 12–17 18–24 ≥25 No data Not applicable

Tuberculosis

CHAPTER 183 the large-scale use of shorter all-oral bedaquiline-containing regimens; the BEAT-Tuberculosis trial testing a new 6-month fully oral regimen consisting of BDLLfxCfz; and the endTB trial testing five different 9-month regimens consisting of different combinations of BLfx (or M) LCfzD. The assessment of this information resulted in a recent 2024 update of WHO guidance for the treatment of MDR-TB and all other RR-TB cases in which isoniazid resistance is absent or unknown (https://iris.who.int/ bitstream/handle/10665/378472/B09123-eng.pdf?sequence=1). As a result, the WHO is now recommending four approaches to treat MDR/RR-TB (see Table 183-4): (1) a 6-month, fully oral regimen composed of bedaquiline, pretomanid, linezolid (600 mg) and moxifloxacin; (2) a 6-month oral regimen consisting of beda­ quiline, delamanid, and linezolid (600 mg), plus levofloxacine and clofazimine; (3) different 9-months oral bedaquiline-containing regimens; and (4) an individualized longer regimen of at least 18 months’ duration consisting of an optimal combination of oral drugs chosen according to a rational approach and using the WHO priority grouping of medicines (Table 183-4). All-oral regimens are now the preferred options, and the use of either a shorter or a longer regimen depends on the assessment of the severity of dis­ ease, knowledge of drug resistance pattern, and history of previous treatment. Six-month, All-Oral, Bedaquiline, Pretomanid, Linezolid, and Moxi­ floxacin (BPaLM) Regimen  New evidence generated through three recent clinical trials (Nix-TB, Ze-Nix, and TB-PRACTECAL) prompted WHO in 2022 to recommend that a regimen composed of the four drugs bedaquiline, the new nitroimidazole compound pretomanid, linezolid 600 mg daily, and moxifloxacin (BPaLM) should be preferred to previously recommended regimens for MDR/RR-TB. This regimen has shown treatment success rates of nearly 90% and, with a linezolid dosage of 600 mg daily, fewer adverse events than in the previous Nix-TB trial when the drug was administered at the dosage of 1200 mg daily. DST for fluoro­ quinolones should be obtained at the start of treatment and guide the decision to retain moxifloxacin in the regimen or administer only the other three drugs. This regimen is recommended for (1) patients with MDR/RR-TB or pre-XDR-TB aged 14 years or older regardless of HIV status, (2) all forms of disease except for dis­ seminated, central nervous system, and osteoarticular TB, and (3) patients with no previous or <1 month exposure to bedaquiline, pretomanid and linezolid (with the possibility of usage also among those previously treated as long as susceptibility is confirmed). Pregnant and breast-feeding women should not be administered this regimen given the incomplete evidence on pretomanid safety in those conditions. Six-Month Regimen, All-Oral (BDLLfxCfz)  A 6-month regimen consisting of bedaquiline, delamanid, and linezolid (600 mg), plus levofloxacin and clofazimine has been recently shown to be noninferior to the 9-month or longer regimens. Either levofloxacin or clofazimine can be omitted depending on fluoroquinolone drug susceptibility testing: in case of proven resistance to fluoroquino­ lone, clofazimine can be administered while in case of fluoroqui­ nolone susceptibility, clofazimine can be avoided. This regimen can be used in all patients who have not been exposed (or who have been exposed for less than 1 month) to bedaquiline, delamanid, and linezolid. It can also be used in children, adolescent, and pregnant or breastfeeding women. 9-Month, All-Oral MDR-TB Regimens  Although the 6-month regimens described above should be the first and second choice, in MDR/RR-TB patients without resistance to fluoroquinolones and who have not been exposed (or who have been exposed for less than 1 month) to bedaquiline, delamanid and linezolid, the use of 9-month, all-oral, bedaquiline-containing regimens has been proven to be non-inferior to longer, 18-month regimens. These regimens consist of, in order of preference: (i) bedaquiline, line­ zolid, moxifloxacin and pyrazinamide (BLMZ); (ii) bedaquiline, linezolid, levofloxacin, clofazimine, and pyrazinamide (BLLfxCfzZ); and (iii) bedaquiline, delamanid, linezolid, levofloxacin, and pyra­ zinamide (BDLLfxZ). In addition, in patients with no extensive pulmonary disease or severe extrapulmonary disease, with no or <1 month exposure to second-line drugs such as bedaquiline, fluoroquinolones, ethionamide, linezolid, and clofazimine (with the possibility of usage also among those previously treated as long as susceptibility is confirmed), an all-oral, bedaquiline-containing regimen may be used rather than the longer (18-month) regimens. Observational programmatic data from South Africa showed that a fully oral regimen starting with 6 months of bedaquiline accompa­ nied by 4−6 months of levofloxacin or moxifloxacin, ethionamide, ethambutol, pyrazinamide, high-dose isoniazid (10–15 mg/kg per day), and clofazimine, and followed by 5 months of levofloxacin (or moxifloxacin), clofazimine, pyrazinamide, and ethambutol, was associated with low toxicity and better outcomes than the older, standardized, injectable-containing regimen. This regimen may also be used in children as well as in pregnant women for whom ethionamide is replaced with linezolid 600 mg daily. As in all TB cases, DST is essential to detect resistance, especially to rifampin and fluoroquinolones, before starting the 9-month regimen. Longer MDR-TB Regimens  In some MDR/RR-TB patients a lon­ ger regimen could be administered. Table 183-3 shows the priority grouping of drugs recommended by the WHO and the approach to the design of a longer regimen for both adults and children. As much as possible, the regimen is composed of all three group A agents and at least one group B agent to ensure that at least four active drugs likely to be effective are administered and at least three of them can be continued throughout the treatment if bedaquiline is discontinued. A second group B agent should be added if one or two group A agents cannot be used. Group C agents can be added to complete a regimen when group A or B agents alone cannot be used as described. Levofloxacin or moxifloxacin, bedaquiline, and linezolid should always be included in longer regimens. Clofazi­ mine and cycloserine (or terizidone) are the two group B options to be added to group A drugs. Group C drugs can replace group A and B agents that cannot be used, and the choice should be based on drug susceptibility testing, drug resistance levels in the population, the patient’s history of previous use of these drugs, and potential intolerance or toxicity. The injectable agents kanamycin and capreo­ mycin should not be included in any regimen, since they have been associated with higher risks of failure and relapse when compared with longer regimens in which other agents were used instead. Ami­ kacin (or streptomycin if the other aminoglycoside is not available) may be used among those aged ≥18 years as long as susceptibility is demonstrated and adverse reactions are strictly monitored. The use of ethionamide (or prothionamide) and PAS is restricted to situa­ tions in which bedaquiline, linezolid, clofazimine, and delamanid are not used, while clavulanic acid should not be included. A treat­ ment course of at least 18–20 months is recommended, but dura­ tion may depend on patient response. In principle, a duration of 15–17 months after culture conversion is suggested. Most patients should receive an intensive course of 6–7 months. Important considerations when treating MDR-TB patients include the safety and effectiveness of several agents especially when used for long periods of time. As in past recommendations, informed consent should be sought from patients treated with all MDR-TB regimens, and active TB drug safety monitoring is recommended. Patients taking QT interval– prolonging drugs (bedaquiline, delamanid, clofazimine, and fluoro­ quinolones) should be closely monitored, with electrocardiography performed at the start of treatment and repeated during treatment; patients with a QTc interval >500 ms or a history of ventricular arrhythmias should not be given these drugs. Patients taking amikacin should undergo serial audiometry to detect any hearing loss early on. Incentives and other forms of support can encourage patients not to interrupt treatment. The design of regimens for complex patterns of MDR-TB, includ­ ing XDR-TB, follows the same principles outlined in Table 183-3

through the selection of agents likely to be effective and tolerated. Observational studies have shown that aggressive management in such patients, with early drug susceptibility testing, use of a rational combination of effective drugs, strict adherence to directly observed therapy, monthly bacteriologic monitoring, and intensive patient support, may—besides interrupting transmission—increase the chances of cure and avert death. For patients with localized dis­ ease and sufficient pulmonary reserve, lobectomy or wedge resec­ tion may be considered as part of treatment.

Because the management of MDR-TB is complicated by both social and medical factors, care of seriously ill patients is ideally provided in specialized centers or, in their absence, in the context of programs with adequate resources and capacity, including com­ munity support. When patients are in stable condition, treatment and care on an ambulatory basis at a decentralized health care facil­ ity should be prioritized, as this approach may increase treatment success and reduce loss to follow-up. This approach should not, however, preclude hospitalization when it is necessary. Respiratory infection-control measures should be observed throughout. As part of a patient-centered approach, palliative and end-of-life care should be provided as a priority when all recommended treatment options have been exhausted. HIV-ASSOCIATED TB Several observational studies and randomized controlled trials have shown that treatment of HIV-associated TB with anti-TB drugs and simultaneous use of ART is associated with significant reductions in mortality risk and AIDS-related events. Evidence from randomized controlled trials shows that early initiation of ART during antiTB treatment is associated with a 34–68% reduction in mortality rates, with especially good results in patients with CD4+ T cell counts of <50/μL. Therefore, the main aim in the management of HIV-associated TB is to initiate anti-TB treatment and to immedi­ ately consider initiating or continuing ART. All HIV-infected TB patients, regardless of CD4+ T cell count, are candidates for ART, which optimally is initiated as soon as possible after the diagnosis of TB and with the strong recommendation to start within the first 2 weeks of anti-TB therapy, especially for profoundly immuno­ suppressed patients with CD4+ T cell counts of <50/μL. Notably, patients affected by TB involving the CNS require special consid­ eration. In these instances, it is recommended to delay the initia­ tion of ART for the initial 8 weeks, irrespective of the CD4+ T cell count. In general, the standard 6-month daily regimen is equally effective in HIV-negative and HIV-positive patients with drugsusceptible TB. However, in the uncommon situation in which a PLWH cannot receive ART, prolongation of the continuation phase of TB treatment by 3 months can be considered. The new 4-month daily regimen including isoniazid, pyrazinamide, rifapentine at a daily dose of 1200 mg, and moxifloxacin at a daily dose of 400 mg (2HPMZ/2HPM) can also be used among PLWH and a CD4+ T cell counts of >100/μL. As in any other TB patient, intermittent regimens should not be used in PLWH. As for any other adult PLWH (Chap. 208), first-line ART for TB patients consists of two nucleoside reverse transcriptase inhibitors plus a nonnucleoside reverse transcriptase inhibitor or an integrase or protease inhibi­ tor. Recent guidelines have also considered a two-drug treatment consisting of one nucleoside reverse transcriptase inhibitor plus an integrase inhibitor. Although TB treatment modalities are similar to those in HIV-negative patients, adverse drug reactions may be more pronounced in PLWH. In this regard, three important consider­ ations are relevant: an increased frequency of paradoxical reactions, interactions between ART components and rifamycins, and devel­ opment of rifampin monoresistance with intermittent treatment. IRIS—i.e., the exacerbation of symptoms and signs of TB—has been described above. Rifampin, a potent inducer of enzymes of the cytochrome P450 system, lowers serum levels of many HIV prote­ ase inhibitors, some nonnucleoside reverse transcriptase inhibi­ tors, and some integrase inhibitors—essential drugs used in ART. Typically, the ART regimens used alongside a rifampicin-based TB PART 5 Infectious Diseases

regimen include the integrase inhibitors dolutegravir or raltegravir (but not bictegravir, cabotegravir, or elvitegravir) at double the standard dose, combined with two nucleoside analogues. Other­ wise, the nonnucleoside reverse transcriptase inhibitor efavirenz is a valid alternative to the integrase inhibitor. Alternatively, rifabutin, which has much less enzyme-inducing activity, has been used in place of rifampin. However, dosage adjustments for rifabutin and protease or integrase inhibitors are still being assessed. Several clinical trials have found that PLWH whose degree of immuno­ suppression is advanced (e.g., CD4+ T cell counts of <100/μL) are prone to treatment failure and relapse with rifampin-resistant organisms when treated with “highly intermittent” (i.e., once- or twice-weekly) rifamycin-containing regimens. Consequently, it is now recommended that all TB patients who are infected with HIV, like all other TB patients with rifampin-susceptible disease, receive a rifampin-containing regimen on a daily basis. Because recom­ mendations are frequently updated, consultation of the following websites is advised: www.who.int/health-topics/hiv-aids, www.who

.int/health-topics/tuberculosis, www.cdc.gov/hiv, and www.cdc.gov/tb. SPECIAL CLINICAL SITUATIONS Although comparative clinical trials of treatment for extrapulmo­ nary TB are limited, the available evidence indicates that most forms of disease should be treated with a 6-month regimen recom­ mended for patients with pulmonary disease. For TB meningitis, the ATS, the CDC, and the IDSA recommend extension of the continuation phase for 7–10 months. The WHO and the Ameri­ can Academy of Pediatrics recommend that children with bone and joint TB, tuberculous meningitis, or miliary TB receive up to 12 months of treatment (2-month induction treatment followed by 10-month consolidation treatment). Treatment for TB may be complicated by underlying medical problems that require special consideration. As a rule, patients with chronic renal failure should never receive aminoglycosides and should receive ethambutol only if serum drug levels can be monitored. Isoniazid, rifampin, and pyrazinamide may be given in the usual doses in cases of mild to moderate renal failure, but the dosage of pyrazinamide should be modified for patients with renal failure. Patients with hepatic disease pose a special problem because of the hepatotoxicity of isoniazid, rifampin, and pyrazinamide. Patients with severe hepatic disease may be treated with ethambutol, streptomycin, and possibly another drug (e.g., a fluoroquinolone); if required, isoniazid and rifampin may be administered under close supervision. The use of pyrazinamide in patients with liver failure should be avoided. Silicotuberculosis necessitates the extension of therapy by at least 2 months. The regimen of choice for pregnant women (Table 183-4) is 9 months of treatment with isoniazid and rifampin supplemented by ethambutol for the first 2 months. Although the WHO has rec­ ommended routine use of pyrazinamide for pregnant women in combination with isoniazid and rifampin, this drug has not been recommended for pregnant women in the United States because of insufficient data documenting its safety in pregnancy. Streptomycin is contraindicated because it is known to cause eighth-cranial-nerve damage in the fetus. The thioamides, bedaquiline, and delamanid also should be avoided in the treatment of pregnant women with MDR-TB. Treatment for TB is not a contraindication to breastfeeding; most of the drugs administered will be present in small quantities in breast milk, albeit at concentrations far too low to provide any therapeutic or prophylactic benefit to the child. Medical consultation on difficult-to-manage cases is provided by the US CDC Regional Training and Medical Consultation Centers (https://www.cdc.gov/tb-programs/php/about/tb-coe.html). PREVENTION The primary way to prevent TB is to diagnose and isolate infectious cases rapidly and to administer appropriate treatment until patients are rendered noninfectious (usually 2–4 weeks after the start of proper

treatment) and the disease is cured. Additional strategies include BCG vaccination and preventive treatment of persons with TB infection who are at high risk of developing active disease. ■ ■BCG VACCINATION One of the most used vaccines in the history of medicine, BCG was derived from an attenuated strain of M. bovis and was first admin­ istered to humans in 1921. Many BCG vaccines are available world­ wide; all are derived from the original strain, but the vaccines vary in efficacy, ranging from 80% to nil in randomized, placebo-controlled trials. A similar range of efficacy was found in observational studies (case-control, historic cohort, and cross-sectional) in areas where infants are vaccinated at birth. These studies and a meta-analysis also found higher rates of efficacy in the protection of infants and young children from serious disseminated forms of childhood TB, such as tuberculous meningitis and miliary TB. BCG vaccine is safe and rarely causes serious complications except in those with underly­ ing immunodeficiencies. The local tissue response begins 2–3 weeks after vaccination, with scar formation and healing within 3 months. Side effects—most commonly, ulceration at the vaccination site and regional lymphadenitis—occur in 1–10% of vaccinated persons. Some vaccine strains have caused osteomyelitis in ~1 case per million doses administered. Disseminated BCG infection (“BCGitis”) and death have occurred in 1–10 cases per 10 million doses administered, although this problem is restricted almost exclusively to persons with impaired immunity, such as children with severe combined immuno­ deficiency syndrome or adults with HIV infection. BCG vaccination induces TST reactivity, which tends to wane with time. The presence or size of TST reactions after vaccination does not predict the degree of protection afforded. BCG vaccine is recommended for routine use at birth in countries or among populations with high TB prevalence. However, because of the low risk of transmission of TB in the United States and other high-income countries, the variability in protection afforded by BCG, and its impact on the TST, the vaccine is not recommended for gen­ eral use. HIV-infected adults and children should not receive BCG vaccine. Moreover, infants whose HIV status is unknown but who have signs and symptoms consistent with HIV infection or who are born to HIV-infected mothers should not receive BCG, nor should siblings of those with immunodeficiencies unless they are known to be unaffected. Over the past decade, renewed research and development efforts have been made toward a new TB vaccine, and several candidates have been developed and tested. A promising candidate vaccine, M72/AS01E, a subunit vaccine pairing two M. tuberculosis antigens (Mtb32A and Mtb39A) with the adjuvant system AS01E, was tested in a randomized phase 2b trial showing an estimated efficacy of about 50% at 36 months. Adverse events were not different in the vaccine and placebo groups. This vaccine is now under development. The investigators aim to enroll up to 20,000 participants, including people living with HIV, at 60 trial sites in South Africa and six other countries including Zambia, Malawi, Mozambique, Kenya, Indonesia, and Vietnam. Participants in the double-blind trial will receive either M72/AS01E or a placebo. Completion of this trial is expected to take at least 5 years. As of the end of 2024, 15 candidate vaccines were in the 4 stages of clinical trials. They included mycobacterial live attenuated or inac­ tivated vaccines, viral vector vaccines, adjuvant recombinant protein vaccines and RNA vaccines. Several challenges must be faced in the development of a TB vaccine. For instance, the lack of predictive ani­ mal models and protection correlates renders trials long and expensive. Furthermore, the decision about whether a candidate vaccine should be developed for prevention of infection (preexposure) or prevention of reactivation (postexposure) without an exact understanding of its precise mechanism of action is complex. ■ ■TB PREVENTIVE TREATMENT (TPT) It is estimated that nearly 2 billion individuals—a quarter of the human population—have been infected with M. tuberculosis in their lifetime.

Although only a small fraction of these infections will progress toward active disease, new active cases will continue to emerge from this pool of infected individuals. Therefore, TPT (also called chemoprophylaxis or preventive chemotherapy, and previously referred to as treatment of latent TB infection) is a fundamental intervention in TB control and elimination strategies.

Infection can be tested using TST, the new Mycobacterium tubercu­ losis antigen-based skin tests (TBSTs), or IGRA, although these tests just measure host immune response to TB antigens. Unfortunately, at present, there is no gold-standard diagnostic test that can confirm true infection (as opposed to immunologic memory of previous exposure) or predict which infected individuals will develop active TB. As a result, decisions to treat infection should include consideration of the risk of progression in an individual. For skin testing, five tuberculin units of polysorbate-stabilized PPD should be injected intradermally into the volar surface of the forearm (i.e., the Mantoux method). Mul­ tipuncture tests are not recommended. Reactions are read at 48–72 h as the transverse diameter (in millimeters) of induration; the diameter of erythema is not considered. In some persons, TST reactivity wanes with time but can be recalled by a second skin test administered ≥1 week after the first (i.e., two-step testing). For persons periodically undergoing the TST, such as health care workers and individuals admitted to long-term-care institutions, initial two-step testing may preclude subsequent misclassification of those who have boosted reac­ tions as TST converters. The cutoff for a positive TST (and thus for TPT) is related both to the probability that the reaction represents true infection and to the likelihood that the individual, if truly infected, will develop TB. Table 183-5 suggests possible conventional cutoff by risk group. Thus, positive reactions for PLWH, recent close contacts of infectious cases, organ transplant recipients, previously untreated per­ sons whose chest radiograph shows fibrotic lesions consistent with old CHAPTER 183 Tuberculosis TABLE 183-5  Tuberculin Reaction Size and Cutoff for Tuberculosis (TB) Preventive Treatment TUBERCULIN REACTION SIZE, mm RISK GROUP HIV-infected persons ≥5 Recent contacts of a patient with TB ≥5a Organ transplant recipients ≥5 Persons with fibrotic lesions consistent with old TB on chest radiography ≥5 Persons who are immunosuppressed—e.g., due to the use of glucocorticoids or tumor necrosis factor α inhibitors ≥5 Persons with high-risk medical conditionsb ≥5 Recent immigrants (≤5 years) from high-prevalence countries ≥10 Injection drug users ≥10 Mycobacteriology laboratory personnel; residents and employees of high-risk congregate settingsc ≥10 Children <5 years of age; children and adolescents exposed to adults in high-risk categories ≥10 Low-risk personsd ≥15 aTuberculin-negative contacts, especially children, should receive prophylaxis for 2–3 months after contact ends and should then undergo repeat tuberculin skin testing (TST). Those whose results remain negative should discontinue prophylaxis. HIV-infected contacts should receive a full course of treatment regardless of TST results. bThese conditions include silicosis and end-stage renal disease managed by hemodialysis. cThese settings include correctional facilities, nursing homes, homeless shelters, and hospitals and other health care facilities. dExcept for employment purposes where longitudinal TST screening is anticipated, TST is not indicated for these low-risk persons. A decision to treat should be based on individual risk/benefit considerations. Source: Adapted from Centers for Disease Control and Prevention: https://www.cdc. gov/tb/publications/factsheets/testing/Tuberculin_Skin_Testing_Information_for_ Health_Care_Providers.pdf.

TB, and persons receiving drugs that suppress the immune system are defined as an area of induration ≥5 mm in diameter. A 10-mm cutoff is used to define positive reactions in most other at-risk persons. For persons with a very low risk of developing TB if infected, a cutoff of 15 mm is used. (Except for employment purposes where longitudinal screening is anticipated, the TST is not indicated for these low-risk persons.) Criteria for positive TBSTs are based on manufacturer rec­ ommendations, and some require induration with a diameter cutoff of 5 mm. A positive IGRA also is based on manufacturer recom­ mendations. Good clinical practice requires that, in addition to test results, epidemiologic and clinical factors also guide the decision to implement TPT and that active TB be definitively excluded before the initiation of a prophylactic regimen. The WHO recommends system­ atic testing for infection and TPT for the following groups at high risk of progression from infection to disease or of exposure and infection: adults, adolescents and children older than 12 months living with HIV; infants with HIV aged <12 months who are contacts of persons with TB; all household contacts of patients with infectious pulmonary TB including children <5 years of age; patients with silicosis, patients starting anti-TNF treatment, patients on dialysis, and patients pre­ paring for organ or hematologic transplantation. In addition, testing and TPT may be considered for persons living or working in at-risk institutional or crowded settings, such as prisoners, health care work­ ers, recent immigrants from high-TB-burden countries, and homeless persons who use drugs.

Some skin test- and IGRA-negative individuals are also candidates for TPT. Once an appropriate clinical evaluation has excluded active TB, infants and children <5 years of age who were in contact with infectious cases should be offered TPT even in the absence of a positive test for TB infection. PLWH >1 year of age who have been exposed to an infectious TB patient should receive TPT regardless of the results of a TB infection test. Any HIV-infected candidate for TPT must be screened carefully to exclude active TB, which would necessitate full disease treatment. The use of a clinical algorithm based on four signs/ symptoms (current cough, fever, weight loss, and night sweats) helps to decide which PLWH can start TPT. The absence of all four symptoms tends to exclude active TB in PLWH. The presence of one of these four manifestations, on the other hand, warrants further investigation for active TB before TPT is started. Although a test for TB infection is pru­ dent before starting TPT, this test is not an absolute requirement—given the logistical challenges—among contacts aged <5 years and PLWH in high-TB-incidence and low-resource settings. PART 5 Infectious Diseases Among PLWH receiving ART, conversion of the TST from negative to positive can occur during the first few months of TPT. Conversions (from negative to positive) and reversions (from positive to negative) are more common with IGRAs than with TSTs among serially tested health care workers in the United States. TPT in selected persons at risk aims to prevent active disease and, in the absence of an immunizing vaccine, is a critical component of TB elimination strategies. This intervention is based on the results of a large number of randomized, placebo-controlled clinical trials dem­ onstrating that a 6- to 9-month course of isoniazid reduces the risk of active TB in infected patients by up to 90%. Analysis of available data indicated that the optimal duration of treatment with this drug was ~9 months. In the absence of reinfection, the protective effect is believed to be lifelong. Clinical trials have shown that isoniazid reduces rates of TB among TST-positive PLWH. Studies in HIVinfected patients have also demonstrated the effectiveness of shorter TPT regimens containing a rifamycin. Several TPT regimens (Table 183-6) can be used. The most widely used has been that based on isoniazid alone at a daily dose of 5 mg/kg (up to 300 mg/d) for 9 months. On the basis of cost–benefit analyses and concerns about feasibility, a 6-month period of treatment at the same dose is considered adequate by the WHO. A clinical trial showed that a regimen of isoniazid (900 mg) and rifapentine (900 mg), given once weekly for 12 weeks, is as effec­ tive as the standard 9-month isoniazid regimen. This regimen was associated with higher rates of treatment completion (82% vs 69%) and less hepatotoxicity (0.4% vs 2.7%) than isoniazid alone, although the rate of permanent discontinuation due to an adverse event was

TABLE 183-6  Recommended Regimens and Drug Dosages for Tuberculosis Preventive Treatmenta REGIMEN DOSE ADVERSE EVENTS Isoniazid alone for

6 or 9 months Adults: 5 mg/kg (max, 300 mg) per day Children <10 years of age: 10 mg/kg per day (range, 7–15 mg) Drug-induced liver injury, nausea, vomiting, abdominal pain, skin rash, peripheral neuropathy, dizziness, drowsiness, seizure Rifapentine plus isoniazid for

3 months Adults and children: Isoniazid: 15 mg/kg (900 mg) weekly Rifapentine: 15–30 mg/kg (900 mg) weekly Hypersensitivity reactions, petechial skin rash, druginduced liver injury Anorexia, nausea, abdominal pain Hypotensive reactions Isoniazid plus rifampin for

3 months As below As above Rifampin alone for

4 months Adults: 10 mg/kg per day Children <10 years of age: 15 mg/kg (range, 10–20 mg) per day Flulike syndrome, skin rash, drug-induced liver injury, anorexia, nausea, abdominal pain, neutropenia, thrombocytopenia, renal reactions (e.g., acute tubular necrosis and interstitial nephritis) Rifapentine plus isoniazid for

1 month Age >13 years only: isoniazid 300 mg and rifapentine 600 mg daily (28 doses) Essentially similar to those of rifapentine plus isoniazid for

3 months with neutropenia more common and elevation in liver enzyme levels and neuropathy less common Levofloxacin for

6 months Daily: adults 10–15 mg/kg;

children 15–20 mg/kg (maximum 750 mg) No grade 3, 4 or serious adverse events reported aSee text for full description of evidence on and limitations of these regimens. Source: Reproduced with permission from World Health Organization. higher (4.9% vs 3.7%). Currently, the isoniazid–rifapentine regimen is not recommended for children <2 years of age or pregnant women. A 3-month regimen of daily isoniazid and rifampin is used in some countries (e.g., the United Kingdom) for both adults and children who are known not to have HIV infection. An alternative regimen for adults is 4 months of daily rifampin, which should also be effective against isoniazid-resistant strains. Recently, an open-label, randomized, phase 3 noninferiority trial has shown that among PLWH, a 1-month regimen of daily rifapen­ tine plus isoniazid was noninferior to the 9-month daily isoniazid regimen and ensured a higher treatment completion. As a result, the WHO has included a 1-month regimen composed of daily isoniazid (300 mg) and rifapentine (600 mg) among the available options for patients aged 13 years or more. Rifampin and rifapentine are contra­ indicated in PLWH receiving protease inhibitors or most nonnucleo­ side reverse transcriptase inhibitors (e.g., nevirapine), as well as in those with chronic hepatitis B receiving tenofovir alafenamide. Efavi­ renz and tenofovir disoproxil can be used for simultaneous adminis­ tration with a rifamycin without dose adjustment. However, the dose of the integrase inhibitor dolutegravir needs to be increased to 50 mg twice daily when given together with rifampin, a dose that is usually well tolerated and gives equivalent efficacy in viral suppression and recovery of CD4+ cell count compared with efavirenz. Administra­ tion of rifapentine with raltegravir was found to be safe and well tolerated. A recent phase 1/2 trial of a 3-month regimen of isoniazid plus rifapentine and dolutegravir in adults with HIV reported good tolerance and viral load suppression, reported no adverse events of grade >3, and did not indicate that rifapentine reduced dolutegravir levels sufficiently to require dose adjustment. Clinical trials to assess the efficacy of long-term isoniazid administration (i.e., for at least

3 years) among PLWH in high-TB-transmission settings have shown that this regimen can be more effective than 9 months of isoniazid

and is therefore recommended under those circumstances. Studies looking at whether briefer treatment with rifapentine-based regimens could achieve similar efficacies have been undertaken. Isoniazid should not be given to persons with active liver disease. All isoniazid recipients at increased risk of hepatotoxicity (e.g., those abusing alco­ hol daily and those with a history of liver disease) should undergo baseline and then monthly assessment of liver function; they should be carefully educated about hepatitis and instructed to discontinue use of the drug immediately should any symptoms develop. More­ over, these patients should be seen and questioned monthly during therapy about adverse reactions and should be given no more than a 1-month supply of drug at each visit. Persons receiving high-dose isoniazid and who are at risk of vitamin B6 (pyridoxine) deficiency should receive pyridoxine to prevent peripheral neuropathy. TPT among persons likely to have been infected by a multidrugresistant strain is a challenge because no clinical trial results are avail­ able to guide treatment. Close observation for early signs of disease is one option. However, in selected high-risk household contacts of patients with MDR-TB (e.g., children, recipients of immunosuppres­ sive therapy), TPT may be considered on the basis of individualized risk assessment and clinical criteria. In the absence of evidence of effi­ cacy of any regimen, important factors in the decision to treat include intensity of exposure, certainty about a source case, information on the drug resistance pattern of the index case, and potential adverse events. Confirmation of infection with available testing is generally required. Drug selection should be based on the drug susceptibility profile of the index case. WHO recommends daily levofloxacin for 6 months in people exposed to MDR-TB. Some studies are currently investigating the role of delamanid and bedaquiline as TPT. It may be more difficult to ensure adherence to TPT than when treating those with active TB. If family members of patients with active TB are being treated, adherence and monitoring may be easier. When feasible, supervised therapy may increase the likelihood of completion. As in active cases, the provision of incentives also may be helpful. Cur­ rently, no evidence shows that large-scale use of TPT leads to signifi­ cant development of drug resistance. However, before TPT begins, it is mandatory to carefully exclude active TB in order to prevent under­ treatment and development of drug resistance. ■ ■PRINCIPLES OF TB CONTROL The highest priority in any TB control program is the early detection of all infectious cases and the provision of treatment under optimal case-management conditions with social support until cure. In addi­ tion, regular screening of high-risk groups, including immigrants from high-prevalence countries, migrant workers, prisoners, home­ less individuals, substance abusers, and HIV-seropositive persons, is recommended to detect either infection or disease and treat promptly. Contact investigation is an important component of efficient TB control. A great deal of attention should been given to interrupt and prevent transmission of TB in institutional settings such as hospitals, homeless shelters, and prisons. Measures to limit such transmission include respiratory isolation of persons with suspected TB until they are proven to be noninfectious, proper ventilation in rooms of patients with infectious TB, use of ultraviolet irradiation in areas of increased risk of TB transmission, correct use of personal protective equipment, and periodic screening of personnel who may come into contact with known or unsuspected cases of TB. In high-prevalence countries, the essential elements of good TB care and control consist of well-defined interventions including early detection of cases and bacteriologic confirmation of the diagnosis and drug resistance pattern; administration of the proper regimen ensur­ ing adherence to treatment and social support to patients; availability of drugs of proven quality, with an effective supply and management system; and a monitoring and evaluation system, including assessment of treatment outcomes and measurement of the impact of control measures on indicators such as mortality, incidence, and prevalence of drug resistance. In the era of the United Nations (UN) Sustainable Development Goals (2016–2030) and of high political visibility of TB

with international targets established by the UN General Assembly, the TB response is multisectoral and holistic. Engagement beyond dedicated programs and even the health sector is essential, as is multi­ sectoral accountability. The “End TB” strategy promoted by the WHO since 2016 builds on three pillars and relies on increased investments and efforts by all governments, their national programs, and a multi­ tude of partners within and beyond the health sector: (1) integrated, patient-centered care and prevention; (2) bold policies and supportive systems; and (3) intensified research and innovation. A “fourth” multisectoral pillar is however necessary to pursue TB elimination in the distant future. In fact, besides specific clinical care and control inter­ ventions as described in this chapter, elimination of TB in a society ultimately will require control and mitigation of the multitude of direct risk factors (e.g., HIV infection, smoking, alcohol abuse, diabetes) and socioeconomic determinants (e.g., extreme poverty, inadequate living conditions and poor housing, undernutrition, indoor air pollution) with clearly implemented policies within the health sector and other sectors linked to human development and welfare.

■ ■FURTHER READING Bhargava A et al: Nutritional support for adult patients with micro­ biologically confirmed pulmonary tuberculosis: Outcomes in a pro­ grammatic cohort nested within the RATIONS trial in Jharkhand, India. Lancet Glob Health 11:1402, 2023. Conradie F et al: Bedaquiline–pretomanid–linezolid regimens for drug-resistant tuberculosis. N Engl J Med 387:810, 2022. Dorman SE et al: Four-month rifapentine regimens with or without moxifloxacin for tuberculosis. N Engl J Med 384:1705, 2021. Migliori GB et al: Clinical standards for the assessment, management CHAPTER 183 and rehabilitation of post-TB lung disease. Int J Tuberc Lung Dis 25:797, 2021. Nahid P et al: An Official American Thoracic Society/Centers for Disease Control and Prevention/European Respiratory Society/ Infectious Diseases Society of America clinical practice guideline: Treatment of drug-resistant tuberculosis. Am J Respir Crit Care Med 200:e93, 2019. Nyang’wa BT et al: A 24-week, all-oral regimen for rifampin resistant Tuberculosis tuberculosis. N Engl J Med 387:2331, 2022. Reid M et al: Scientific advances and the end of tuberculosis: A report from the Lancet Commission on Tuberculosis. Lancet 402:1473, 2023. Swindells S et al: One month of rifapentine plus isoniazid to prevent HIV-related tuberculosis. N Engl J Med 380:1001, 2019. Uplekar M et al: WHO’s new End TB strategy. Lancet 385:1799, 2015. Walker TM et al: The 2021 WHO catalogue of Mycobacterium tuber­ culosis complex mutations associated with drug resistance: A geno­ typic analysis. Lancet Microbe 3:e265, 2022. ■ ■WEBSITES United Nations General Assembly: Political Declaration of the High-Level Meeting on the Fight Against Tuberculosis. https://docu­ ments.un.org/doc/undoc/gen/n23/306/91/pdf/n2330691.pdf World Health Organization: Global tuberculosis report 2024. Geneva: World Health Organization; 2024. https:// www.who.int/teams/global-tuberculosis-programme/tb-reports/ global-tuberculosis-report-2024 World Health Organization: WHO consolidated guidelines on tuberculosis: Module 3: diagnosis: rapid diagnostics for tuberculosis detection, 3rd ed. Geneva: World Health Organization; 2024. https:// www.who.int/publications/i/item/9789240089488 World Health Organization: WHO consolidated guidelines on tuberculosis. Module 4: treatment - drug-resistant tuberculosis treatment, 2022 update. Geneva: World Health Organization; 2022

.https://www.who.int/publications/i/item/9789240063129 World Health Organization: WHO consolidated guidelines on tuberculosis. Module 4: treatment: drug-susceptible tuberculosis treatment. Geneva: World Health Organization; 2022. https://www

.who.int/publications/i/item/9789240048126

69 - 184 Leprosy

184 Leprosy

Jan H. Richardus, Hemanta K. Kar,

Zoica Bakirtzief, Wim H. van Brakel

Leprosy Leprosy, also referred to as Hansen’s disease, is a chronic infectious disease caused by Mycobacterium leprae. The clinical manifestations are largely confined to the skin, peripheral nervous system, eyes, and upper respiratory tract. The differing immune responses to M. leprae result in a spectrum of disease ranging from tuberculoid to leproma­ tous leprosy. M. leprae has a predilection for peripheral nerves, and immunologically mediated reactional states can cause nerve damage to the face, arms, and legs; this damage often results in disability, which in turn can lead to stigma and social exclusion. The physical disfigurement that accompanies leprosy has left marks on society that have endured long after the disease’s disappearance in many countries. In everyday language, leprosy has become a metaphor for a horrible condition that warrants social exclusion. Leprosy is a neglected disease and is often thought no longer to exist. However, 174,087 new cases from 182 countries were reported in 2022. A general lack of awareness among both the public and medical practitioners often delays diagnosis and treatment and thus results in irreversible impairments. Early diagnosis and treatment of leprosy and leprosy reactions can cure the disease and prevent most chronic complications. ■ ■ETIOLOGY M. leprae is an obligate, intracellular, acid-fast staining, rod-shaped bacterium, measuring 1–8 μm in length and 0.3 μm in diameter.

M. leprae mostly appears irregularly stained and fragmented or granu­ lar, in which case the organism is usually considered to be dead. The few bacteria that are brightly and uniformly stained are thought to be solid, viable bacilli. The morphologic index is a measure of uniformly stained solid bacilli on slit-skin smear examination and is calculated as the percentage of viable bacilli among the total number of bacilli counted under oil-immersion microscopy. On slit-skin smear exami­ nation at the lepromatous end of the disease spectrum, M. leprae is pre­ dominantly found in clumps or globi within macrophages (lepra cells). Inside these cells, M. leprae multiplies in unrestricted fashion, and hun­ dreds of bacilli may be present; the organisms are arranged in parallel arrays placed side by side as a result of the presence of surface lipids (glial substances). The bacteriologic index is a logarithmic-scaled mea­ sure of the density of bacilli of all forms found in the dermis upon slitskin smear examination, varying from 0 to 6+ (with or without globi) from the tuberculoid to the lepromatous end of the disease spectrum. The bacteriological index falls an average of 1 log unit per year with multidrug therapy. M. leprae infects mainly macrophages and Schwann cells. It has never been grown in artificial media. Reproduction occurs by binary fission, and the organism grows slowly (over 12–14 days) in the footpads of mice. The temperature required for survival and

proliferation—between 27°C and 30°C—explains the greater impact of the disease on surface areas such as the skin, peripheral nerves, testicles, and upper airways, with less inner visceral involvement. M. leprae remains viable for 9 days in the environment. PART 5 Infectious Diseases Ultrastructural Characteristics of M. leprae  Electron micros­ copy reveals that M. leprae has a cytoplasm, plasma membrane, cell wall, and capsule. The cytoplasm contains structures common in grampositive microorganisms. The plasma membrane has a permeable lipid bilayer containing interacting proteins—the protein surface antigens. Similar to that of other mycobacteria, M. leprae’s cell wall, which is attached to the plasma membrane, is composed of peptidoglycans bound to branched-chain polysaccharides; these peptidoglycans are arabinogalactans, which support mycolic acids, and lipoarabinoman­ nan (LAM). The capsule—the outermost structure—contains lipids, particularly phthiocerol dimycocerosate and phenolic glycolipid (PGL-1), which has a trisaccharide bound to lipid by a molecule of phenol.

Because this trisaccharide is antigenically specific for M. leprae, its detection is helpful in serologic diagnosis of leprosy. Genome of M. leprae  Comparative analysis of the genomics of single-nucleotide polymorphisms indicates that four distinct strains of M. leprae originated in East Africa or Central Asia. A mutation spread to Europe and subsequently underwent two separate mutations that were then followed by spread to West Africa and the Americas. The genome of M. leprae is circular. Its estimated molecular mass is 2.2 × 109 Da, with 3,268,203 base pairs and a guanine-plus-cytosine content of 57.8%. Culture Difficulties  Compared to the genome of Mycobacterium tuberculosis, that of M. leprae underwent reductive evolution, result­ ing in a smaller genome rich in inactive or entirely deleted genes. This reductive evolution, gene decay, and genome downsizing all may explain the unusually long generation time and may account for the inability to culture the leprosy bacillus in artificial media. As a result, propagation of M. leprae has been restricted to animal models, includ­ ing the armadillo and normal, athymic, and gene-knockout mice. These systems have provided the basic resources for genetic, metabolic, and antigenic studies of the bacillus. Growth of M. leprae in mouse footpads also provides a tool for assessing the viability of the bacteria and testing the drug susceptibility of clinical isolates. Immunologic Properties of M. leprae  M. leprae induces both humoral and cell-mediated immune responses. The immunogenic components of M. leprae include polysaccharides and proteins. Poly­ saccharide components induce mainly a humoral immune response, whereas protein components induce both humoral and cell-mediated immune responses. The immunogens in M. leprae form two distinct groups: cytoplasmic antigens and antigens from the mycobacterial cell. As mentioned above, a species-specific phenolic glycolipid, PGL-1, has been identified in M. leprae. Other varieties of M. leprae antigens identified with monoclonal antibodies include antigens of 18, 28, 7, 14, 36, 65, and 70 kDa that may possibly induce an immune response. Mycobacterium lepromatosis  In 2008, a new mycobacterial species, M. lepromatosis, was isolated from patients with a special type of dif­ fuse lepromatous leprosy known as diffuse leprosy of Lucio and Latapí. This clinical variety of leprosy is found mainly in Mexico and Central America. M. lepromatosis is very similar to M. leprae microbiologically and clinically. Microbiologically, both species are acid-fast and noncul­ tivable and preferentially infect skin and peripheral nerves. Clinically, differentiation of M. lepromatosis from M. leprae in individual patients is not diagnostically necessary since both organisms respond well to the same antimycobacterial regimens. ■ ■EPIDEMIOLOGY Incidence, Prevalence, and Disability  The true incidence of leprosy is difficult to establish because the figure is very low and because the initial signs and symptoms are often insidious, and thus not all cases are detected as they occur. In 2022, as stated earlier, 174,087 new cases were reported to the World Health Organization (WHO) from 182 countries. New case detection per year is commonly used as a proxy for incidence, but operational factors, such as the inten­ sity of case detection, the use of surveys, the use of contact tracing, the level of community awareness, and the quality and availability of health care, have a profound effect on case detection rates. In nonendemic countries around the world, leprosy is often misdiagnosed simply because it is not considered. The registered prevalence of leprosy is defined as the number of patients receiving treatment at a point in time (usually at the end of a calendar year). The registered prevalence is a proxy measure for true prevalence, which would include existing cases that have not yet been detected. The two factors that determine the registered prevalence are the new case detection rate and the duration of treatment; changes in either factor will affect the registered prevalence. The WHO leprosy disability grading system scores patients accord­ ing to the presence of disabilities of the eyes, hands, and feet. For the

900,000 800,000 700,000 600,000 500,000 400,000 300,000 200,000 100,000

FIGURE 184-1  Global trend in leprosy new-case detection, 1990–2022. hands and feet, grade 0 means no anesthesia and no visible impair­ ment; grade 1 signifies anesthesia but no visible impairment; and grade 2 indicates visible impairment. For the eyes, grade 0 signifies no eye problems due to leprosy and no evidence of visual loss; grade 1 signifies eye problems due to leprosy without severe effects on vision; and grade 2 indicates severe visual impairment (vision score worse than 6/60; inability to count fingers at 6 meters) and also includes lagophthalmos, iridocyclitis, and corneal opacities. The sum score for these six body sites is called the Eye-Hand-Foot (EHF) score and is used as an overall indicator of the impairment status of a person with leprosy. Leprosyrelated grade 2 disability is usually reported as the proportion of people with such disability at any site among patients newly diagnosed with leprosy in a specific year. The global trend in new case detection since 1990 is presented in Fig. 184-1. The trend was remarkably static up to the year 2001, with a peak around the year 2000; fell dramatically between 2001 and 2005; and has leveled off from 2006 until 2016. Since 2017, a continuous decline has been observed, but the figures for 2020 and 2021 are unreli­ able due to underreporting during the COVID-19 pandemic. The most important factor contributing to the fast downward trend after the year 2000 was the decline in leprosy control activities following the decla­ ration by the WHO in 2000 that leprosy was eliminated as a “public health problem.” Elimination was defined as a prevalence of <1 case per 10,000 population at the global level. The decline in new case detection since 2016 to below 200,000 cases per year indicates that transmission of M. leprae is becoming less and that elimination of leprosy could become possible in the next 25 years or so. Sex, Age, and Geographic Distribution  Approximately 40% of all reported leprosy patients are women, but the low proportion in some countries raises concerns about underdiagnosis in women due to poor access to health services, illiteracy, low status, and other cultural factors. The age-specific incidence often shows a bimodal pattern, with peaks in the teenage years and in adulthood. Around 8% of all newly detected cases are found in children (<15 years of age), a measure that is often taken as an indicator of continued (recent) transmission. Leprosy is rare among children <5 years of age. Around 5% of all patients have a grade 2 disability. There are large variations among world regions and countries in new case detection rates. Approximately 80% of global new case detec­ tion is reported from India, Brazil, and Indonesia. There are also dis­ tinct geographic variations within countries, with differences between urban and rural communities and clustering of cases at the village or neighborhood level. Geographic variations can be due to differences in health service provision, socioeconomic development, isolation,

and poverty. Figure 184-2 depicts the geographic distribution of new leprosy cases in 2022. Transmission  Understanding of the transmission of M. leprae is limited. The existing evidence is largely circumstantial because of the long incubation period from exposure to disease, the inability to culture M. leprae, and the difficulty of diagnosing both infection and early disease. M. leprae organisms can be shed in large numbers from the mouth and nose of patients with untreated multibacillary leprosy (droplet infection) and sometimes from damaged skin, but it is unclear whether patients with paucibacillary leprosy can spread the bacillus. There is evidence for transmission between humans and—in southern U.S. states—for zoonotic transmission through wild armadillos. The main route of entry into the body is assumed to be the respiratory tract, but in patients with wounds or tattoos, transmission through the skin also is possible. CHAPTER 184 Leprosy Reservoirs of Infection  It is assumed that humans are the main reservoir of infection for M. leprae. The armadillo is also a reservoir for human infection. Certain species of monkeys and red squirrels are infected with M. leprae in the wild, but there is no evidence of transmission to humans through contact with these animals. Evidence is weak for the potential of water and soil as environmental sources of M. leprae. The higher incidence rate of leprosy among household con­ tacts of multibacillary cases than among those of paucibacillary cases suggests that multibacillary cases represent an important reservoir for undetected and untreated cases in the community; that is, a prolonged period between the onset of signs of leprosy and treatment due to a delay in diagnosis and initiation of multidrug therapy increases expo­ sure in the community. Persons with subclinical leprosy are likely to be a main source of infection, given that multidrug therapy for clinical leprosy apparently has not made an impact on transmission. Incubation Period, the Role of Contacts, and Genetic Sus­ ceptibility  The incubation period of leprosy is estimated to range from 2 to ≥10 years. The incubation period for multibacillary leprosy appears to be longer (5 to ≥15 years) than that for paucibacillary lep­ rosy (~2–5 years). Poverty-associated factors such as low level of edu­ cation, poor hygiene, and food shortages have been identified as risk factors for leprosy, but the most important risk factors are associated with intimacy and duration of contact with a leprosy patient, in par­ ticular with an index case with multibacillary leprosy, and the intensity of contact with and physical distance from the index patient. Increasing evidence from studies in twins and from observational studies supports host genetic susceptibility to leprosy. Ongoing studies are exploring the

New leprosy cases, 2022 – Nouveaux cas de lèpre en 2022

1–10 11–100 101–1000 1001–10,000

10,000 No data – Aucune donnée FIGURE 184-2  Geographic distribution of new leprosy cases, 2022. (Reproduced with permission from Global leprosy (Hansen disease) update, 2022: new paradigm-control to elimination. Wkly Epidemiol Rec 98:409, 2023.) PART 5 Infectious Diseases mechanism underlying genetic susceptibility to leprosy and its clinical manifestations. ■ ■PATHOGENESIS Whatever the route of M. leprae’s entry into the human body, the pathogenic process usually starts in the peripheral nerves. Once bacilli are engulfed by Schwann cells, the histopathologic changes in nerve and skin—and thus the type of leprosy that develops—depend on the immunologic resistance of the person infected, in particular on the cell-mediated immune (CMI) response to the bacillus and its antigens. Ridley-Jopling Classification of Leprosy  In 1962, Ridley and Jopling described five overlapping categories of leprosy: tuberculoid (TT), borderline tuberculoid (BT), mid-borderline (BB), borderline lepromatous (BL), and lepromatous (LL). An early clinical manifes­ tation is recognized and referred to as indeterminate leprosy (IL). Immunologic resistance is strong at the tuberculoid end of the spec­ trum, gradually diminishes through the borderline spectrum, and is weakest in lepromatous leprosy. The LL and TT types of leprosy are relatively stable, with little or no change in clinical disease expression over time, while the BL, BB, and BT types are unstable both clinically and immunologically. Further distinction indicates that subpolar types of TT and LL leprosy (TTs and LLs) are less stable than polar types (TTp and LLp). The immune reaction depends on predisposing genetic factors and the extent of exposure to M. leprae. The host tissue’s reac­ tion and related damage are largely due to delayed hypersensitivity. In response to the presence of M. leprae, a granuloma is formed either by macrophage–lymphocyte interaction when there is immunity or otherwise by macrophages only. The formation of a granuloma is pre­ ceded by a stage of infiltration by lymphocytes alone, as is seen in IL. Because of the strong immune response toward the tuberculoid end of the spectrum, macrophages, along with many lymphocytes, become fixed epithelioid cells, and groups of these cells become giant cells. The tuberculoid granuloma leads to nerve destruction resulting in anes­ thesia and muscle weakness. The cellular response is less focal and less destructive in the borderline portion of the spectrum; consequently, there is less damage to nerves and few bacilli are present. In BL leprosy, there are macrophage granulomas along with lymphocytes, but little nerve damage and more bacilli. In LL leprosy, bacilli multiply within

Schwann cells and perineural cells. Liberated bacilli from these cells are engulfed by histiocytes, becoming wandering macrophages and travel­ ing throughout the body to other nerves and tissues via blood, lymph, and tissue fluids. In addition, there are diffuse lepromas in LL leprosy that consist of histiocytes and/or macrophages, with very few lym­ phocytes and plasma cells. The bacilli are packed within macrophages called globi and outside macrophages either singly or in small groups. WHO Simplified Clinical Classification of Leprosy  RidleyJopling classification requires clinical and pathologic expertise that does not exist in many settings. The WHO has therefore introduced a simplified classification system based on slit-skin smear: patients with negative slit-skin smear results at all body sites are classified as having paucibacillary leprosy, whereas patients with positive smears at any body site are classified as having multibacillary leprosy. However, because slit-skin smear facilities are not available or dependable in many countries, most leprosy control programs use clinical criteria only for classifying leprosy and deciding on the appropriate treatment regimen for individual patients. In this circumstance, paucibacil­ lary leprosy is defined as one to five skin lesions and no or only one involved peripheral nerve, while multibacillary leprosy is defined as six or more skin lesions and/or more than one involved peripheral nerve. ■ ■CLINICAL MANIFESTATIONS Leprosy is a disease affecting mainly the skin, cutaneous and peripheral nerves, mucous membranes, and, less commonly, other sites such as joints, lymph nodes, eyes, and testes. Other systemic manifestations may occur, particularly in BL and LL disease, with or without leprosy reactions. Most dermal and cutaneous nerves feeding skin lesions are affected—e.g., the supraorbital, great auricular, radial cutaneous, infrapatellar, superficial fibular, and sural nerves and the cutaneous nerves of the thigh. The peripheral nerves involved include the ulnar, median, radial (in upper limbs), lateral popliteal, and posterior tibial (in lower limbs). The cranial nerves commonly involved are the tri­ geminal and facial. Indeterminate Leprosy (IL)  This early clinical type manifests as one or a few hypopigmented or faintly erythematous, ill-defined to well-defined macular lesions measuring 1–5 cm in diameter. These lesions invariably occur on the external aspects of the limbs, buttocks,

FIGURE 184-3  Tuberculoid (TT) leprosy. Hypopigmented macular lesion with a welldefined edge and loss of fine-touch sensation. (From Dr. H. K. Kar, with permission.) and face, with mild to moderate impairment of touch and/or thermal sensations. There is no thickening of the corresponding cutaneous and peripheral nerves. IL is often, but not always, the first clinical sign of leprosy. This type either heals spontaneously or progresses to a determinate form of the disease (TT, BT, BB, BL, or LL), depending on CMI status. Tuberculoid (TT) Leprosy  TT leprosy (Fig. 184-3) presents either as a well-defined, hypopigmented macule or as a raised,

erythematous/brown/copper-colored plaque with a well-defined edge. The lesions may be found on any part of the skin and are characterized by complete loss of fine touch and temperature sensations over their surface. Skin lesions are single or few (up to three) in number and can be of any size, but they seldom measure >10 cm in diameter. In plaquetype lesions, the raised clear-cut edge often slopes inward to a flattened and sometimes hypopigmented central area, acquiring an annular con­ figuration. The skin surface of both macular and plaque lesions is dry, hairless, and anesthetic because of destruction of underlying superficial cutaneous nerves. Larger corresponding cutaneous nerves are thick­ ened in a limited number of cases. On the face, sensory impairment may be difficult to demonstrate because of the generous and bilateral supply of sensory nerve endings. Autonomic nerve damage within the lesion is responsible for surface dryness and loss of sweating over the lesion. A solitary peripheral-nerve trunk in the vicinity of a lesion may be thickened, with sensory loss of the area supplied and with or with­ out motor disfigurement. On slit-skin smear examination, no acid-fast bacilli (AFB) are normally found. The lepromin skin test is strongly positive, signifying good host CMI status. Borderline Tuberculoid (BT) Leprosy  BT leprosy (Fig. 184-4) is characterized by either macular or plaque-type lesions numbering three to nine or more and asymmetrically located on any part of the body, with variable sizes and contours. The margins of the lesions range from poorly defined to well defined; sometimes both forms of margin are seen in one lesion. There may be smaller satellite lesions around a larger one, especially on sides where the margin is less defined; this characteristic indicates downgrading of the lesion from TT to BT leprosy. The edges of plaque lesions may slope outward in contrast to TT lesions, which slope inward; plaques may gradually fade outward and eventually blend into normal-looking skin. Loss of sensation is less intense than it is in TT lesions and dryness on the surface less conspicuous. Several peripheral nerves are likely to be enlarged in an asymmetrical pattern, with sensory and motor deficits. One of the most striking features of BT leprosy is susceptibility to a type 1 leprosy reaction (T1R; see below) that exacerbates skin lesions and/or periph­ eral nerves. If not diagnosed and treated early, disease in these patients tends to downgrade across the spectrum to BB, BL, or LLs leprosy, with an increasing bacteriologic index and a regressed CMI response caus­ ing nerve damage along the way. Slit-skin smears show bacteriologic indices varying from negative to 1+.

FIGURE 184-4  Borderline tuberculoid (BT) leprosy. Macular lesion with irregular, moderately defined edge and satellite lesion, with loss of sensation. (From Dr. W. H. van Brakel, with permission from NLR.) Mid-Borderline (BB) Leprosy  This form of leprosy is unstable. Many cases downgrade toward BL and LL disease, especially if not treated. There are multiple plaque lesions and, not infrequently, macu­ lar lesions; the lesions are of various shapes and sizes, are bilateral, and usually occur in a more or less symmetrical distribution. In annular lesions, the inner edge is well demarcated and “punched out,” and the outer edge is ill defined and merges with normal-looking skin. The surface of the lesions is moderately shiny, and the central area looks pale. There is minimal loss of sensation over the lesions. Nerve dam­ age is variable in BB leprosy. Many nerves may be thickened, and this effect may be asymmetrical. BB leprosy is not commonly observed and rapidly changes its spectrum—rarely to BT leprosy but more often to BL disease. The lepromin test is negative. Slit-skin smears of lesions show a moderate number of AFB (2+ to 3+). CHAPTER 184 Leprosy Borderline Lepromatous (BL) Leprosy  In BL leprosy

(Fig. 184-5), there are numerous bilateral, round or oval, macular, dif­ fusely infiltrated, erythematous or hypopigmented lesions with moder­ ately defined borders. The lesions are usually 2–3 cm in diameter, may have a coppery hue, and tend to become symmetrical. Some loss of FIGURE 184-5  Borderline lepromatous (BL) leprosy. Numerous diffusely infiltrated erythematous and hypopigmented macules, downgrading from borderline tuberculoid to lepromatous leprosy. (From Dr. C. L. M. van Hees, Department of Dermatology, Erasmus MC, University Medical Center, Rotterdam, the Netherlands, with permission.)

FIGURE 184-6  Lepromatous (LL) leprosy. Multiple nodules on ears and face and loss of eyebrows. (From Dr. K. Mponda, Department of Dermatology, Queen Elisabeth Central Hospital, Blantyre, Malawi, with permission.) PART 5 Infectious Diseases sensation may be detected, particularly over older lesions; however, no loss of sensation is observed over fresh lesions. With disease progres­ sion, papules, nodules, and plaques develop over the macular lesions. In untreated patients, new ill-defined skin lesions continue to develop. Widespread but asymmetrical thickening of peripheral nerves, with or without tenderness, leads to sensory and motor deficits. The lepromin test gives negative results, as it does in all degrees of lepromatous lep­ rosy. Slit-skin smear examination of lesions shows a bacteriologic index varying from 3+ to 4+. Lepromatous (LL) Leprosy  LL leprosy (Fig. 184-6) pres­ ents with innumerable bilateral, symmetrically distributed, diffusely indurated, erythematous, copper-colored or skin-colored patches or plaques. There is no loss of sensation over these lesions, which have a smooth, shiny surface. The lesions spread over the face, earlobes, ears, extensor aspects of the upper and lower extremities, back, and but­ tocks. Induration can readily be recognized when lesions are viewed tangentially under natural sunlight. The induration initially is of a finer type but gradually becomes coarse, and lesions then progress to papules, plaques, and nodules. Bilateral earlobe thickening and eyebrow loss occur. Coarse induration on the face sometimes results in gross skin folds that lead to an appearance referred to as “lion face,” particularly when associated with loss of eyebrows and thickening of earlobes. Of all cases of LL leprosy, 10–15% are of the polar type (LLp) from the time of lesion onset; the remaining cases downgrade from the untreated borderline spectrum to subpolar LLs leprosy. Patients with LLs disease develop nerve damage during the borderline stages. In LLp disease, involvement of peripheral nerves occurs late and is bilateral and symmetrical, with sensory loss in a “glove-and-stocking” distribu­ tion. Slit-skin smear examination shows a bacteriologic index of 4+ to 6+ with globi. SYSTEMIC INVOLVEMENT  In LL leprosy, AFB are found in the lymph nodes, spleen, liver, bone marrow, adrenal glands, smooth and striated muscles, tooth pulp, testes, oral cavity, nose, larynx, and eyes. Involve­ ment of the testes leads first to sterility and then to gynecomastia and impotence. Eye involvement includes corneal anesthesia; early on, this manifestation is due to bacillary infiltration of corneal nerves, while

later it arises from damage to the ophthalmic division of the trigeminal nerve. In addition, eye involvement includes episcleritis, iridocyclitis, iris atrophy, cataract and glaucoma, lagophthalmos, corneal ulcer­ ation and perforation, and blindness. The nose is a portal of entry for

M. leprae and is the earliest site of involvement in LL leprosy. Edema and mucosal thickening occur in the inferior turbinate and nasal septum, with crusting and epistaxis. Later, patients develop chronic rhinitis with loss of smell sensation. Septal perforation due to bony destruction, with typical saddle-nose disfigurement, is common in advanced LL disease. In late-stage LL leprosy, ulceration of the tongue, pharynx, hard and soft palates (leading to palate perforation), tonsillar pillars, and uvula occurs. In the hands, slow resorption sets in, starting from the distal end of the terminal phalanx and proceeding proximally to involve the middle and proximal phalanges. HISTOID LEPROSY  Histoid leprosy is a rare form of LL leprosy in which waxy, shiny, firm, symmetrical or asymmetrical nodules and plaques are observed over normal-looking skin. Histologic examina­ tion of these lesions shows specific spindle-cell granulomas. Slit-skin smear examination reveals high bacteriologic and microbiologic indi­ ces without globi in most cases. DIFFUSE LEPROSY OF LUCIO AND LATAPÍ  This rare form of nonnodu­ lar LL leprosy occurring in Mexico and Central America is character­ ized by diffuse shiny infiltration of the skin and widespread sensory loss. The skin looks waxy and has a shiny appearance (“lepra bonita,” or beautiful leprosy), with obvious diffuse induration of the earlobes and forehead as well as loss of eyebrows, sometimes eyelashes, and not infrequently all body hair. This form of leprosy can be complicated by an unusual reaction known as Lucio’s phenomenon (see below). Primary Neuritic Leprosy  In some countries, such as India and Nepal, primary neuritic disease is observed in 2–10% of all leprosy cases, with only peripheral nerve involvement and no skin lesions. Nerve thickening and sensory loss occur in the affected area, with or without a motor deficit. Primary neuritic leprosy, even though not described by Ridley and Jopling, can manifest at different points along the disease spectrum. For practical purposes, primary neuritic leprosy is classified as paucibacillary or multibacillary on the basis of the absence or presence of AFB in nerve biopsy sections or the number of thickened nerves (single or multiple). ■ ■LEPROSY REACTIONS Leprosy reactions are immunologic phenomena that occur before, dur­ ing, or after treatment. They are severe complications that need to be diagnosed and treated early to prevent nerve function impairment and subsequent disfigurement as well as blindness. Type 1 Leprosy Reaction (T1R)  T1R is a delayed hypersen­ sitivity reaction associated with sudden alteration of CMI status and leading to a shift in the patient’s position on the leprosy spectrum. This reaction is marked by infiltration of lesions by activated CD4+ T lymphocytes, especially T helper cells. T1R is also called a reversal reaction because of the upgrading of CMI status. T1R is usually observed in the borderline portion of the spectrum. Skin lesions are characterized by acute swelling and redness (Fig. 184-7). Nerves may be painful and tender because of neuritis, with consequent nerve damage and disfig­ urement. In the severe form of T1R, nerve abscesses may be formed. Loss of nerve function can be much less obvious than usual when it occurs without other signs of inflammation. This “silent neuritis” may lead to sensory and motor impairment in the hands, feet, and face. Arthralgia or arthritis sometimes occurs. Rarely, the patient may develop fever and malaise, tenosynovitis, and edema of the feet and hands. Type 2 Leprosy Reaction (T2R)  T2R, also known as ENL (erythema nodosum leprosum), is an immune complex–mediated syndrome (i.e., an antigen–antibody reaction involving complement) that causes inflammation of the skin, nerves, and other organs as well as general malaise. ENL is an example of a type III hypersensitivity reaction (Coombs and Gell classification) or Arthus phenomenon.

FIGURE 184-7  Type 1 leprosy reaction. Increased inflammation of existing lesions. (From Dr. W. H. van Brakel, with permission from NLR.) This reaction occurs mostly during multidrug therapy but can also develop in untreated patients. Evanescent, pink-to-red, maculopapular, papular, nodular, or plaque lesions suddenly appear and are usually accompanied by constitutional symptoms like malaise and fever, with or without painful swelling in the joints (Fig. 184-8). These crops of skin lesions present on the outer aspects of the thighs, legs, and face. They are painful or tender and warm, blanch with light finger pres­ sure, and last for a few days. The lesions change in color from pink/ red to bluish and brownish after 24–48 h and turn dark in a week. Rarely, ENL lesions become vesicular, pustular, bullous, and necrotic and break down to produce ulceration (erythema nodosum necroti­ cans). The patient may have other associated signs such as lymph node enlargement, myositis, arthritis, synovitis, rhinitis, epistaxis, laryngitis, iridocyclitis, glaucoma, painful dactylitis, acute epididymoorchitis, nephritis and renal failure, hepatosplenomegaly, anemia, and—at a later stage—amyloidosis. Severe T2R may include swollen, painful, and tender nerve trunks with sensory and motor deficits. Lucio’s Phenomenon  Lucio’s phenomenon is observed in diffuse leprosy of Lucio and Latapí and may be a variant of erythema nodosum necroticans. Marked vasculitis and thrombosis of the superficial and deep vessels result in hemorrhage and infarction of the skin. Clinically, the skin reaction begins as slightly indurated, bluish-red, ill-defined, painful, and rarely palpable plaques with an erythematous halo, usu­ ally developing on one limb but sometimes on other areas of the body. The lesions are irregular or triangular. After a few days, they become purplish at the center; a central hemorrhagic infarct may develop with or without blister formation, and a necrotic eschar that detaches easily FIGURE 184-8  Type 2 leprosy reaction. Erythema nodosum leprosum, with pustular lesions. (From Dr. H. K. Kar, with permission.)

and leaves an ulcer of irregular shape may follow later. The ulcer heals, leaving a superficial scar. Patients remain afebrile throughout.

Nerve Function Impairment, Neuritis, and Disfigurement 

The terms nerve function impairment, nerve damage, neuropathy, and neuritis are often used interchangeably for the sensory, motor, and/ or autonomic nerve deficits that occur because of the pathologic pro­ cesses resulting from M. leprae infection of the nerve. Neuritis (nerve inflammation) in leprosy is usually a subacute, demyelinating, and unremitting event involving cutaneous nerves and larger peripheral nerves. “Silent neuritis” or “quiet nerve paralysis” is defined as progres­ sive sensory or motor impairment in the absence of symptoms such as pain, paresthesia, or tenderness of the nerve and with no obvious signs of leprosy reactions. Neuritis can occur at any time during leprosy but is more common and severe during leprosy reactions, mainly in T1R. Sensory and motor neuropathy can lead to secondary impairments in the upper and lower extremities, such as muscle atrophy, mobile- and fixed-joint contractures, bone absorption of digits, and cracks and wounds. ■ ■DIAGNOSIS Clinical Diagnosis  Three cardinal signs indicate a diagnosis of leprosy. The diagnosis can be established when at least one of these three signs are present:

  1. Hypopigmented or erythematous skin lesion(s) with definite loss or impairment of sensation: The clinical presentation of skin patches or plaques is diagnostic when it is associated with a definite loss or impairment of sensation (light touch, pain, and/or temperature). Diagnostic dilemmas arise in the indeterminate stage of leprosy because of variable loss of sensation and the presence of facial lesions (i.e., because the density of innervation in the face can com­ pensate for damage to certain nerve branches).
  2. Involvement of the peripheral nerves, as demonstrated by definite CHAPTER 184 Leprosy thickening with sensory impairment: Thickening of a peripheral nerve should be assessed by palpation of the affected nerve and comparison with the corresponding contralateral nerve. In mul­ tibacillary leprosy, thickening of nerves is often bilateral. Nerve tenderness is established by the application of mild pressure on the nerve during palpation with the fingertips. The peripheral nerves commonly palpated in a leprosy patient are the greater auricular, ulnar, radial, radial cutaneous, median, lateral popliteal, posterior tibial, sural, and superficial peroneal nerves.
  3. A positive result for AFB in slit-skin smears, establishment of the presence of AFB in a skin smear or biopsy sample, or a positive result in a biopsy polymerase chain reaction (PCR). Diagnostic Tools  •  TESTING OF SKIN SENSATION  Light-touch sensation is tested with cotton, wool, or a feather. Pain is assessed as the patient’s ability to distinguish between the sharp and blunt ends of a wooden or bamboo toothpick. Thermal sensation thresholds are assessed with computer-assisted sensory testing equipment. SLIT-SKIN SMEAR  Normally a slit-skin smear is taken from four sites: the right earlobe, the forehead above the eyebrows, the chin, and the left buttock in men or the left upper thigh in women. The mate­ rial is stained with Ziehl-Neelsen reagent and examined with a light microscope. The bacteriologic index is determined with a standard logarithmic scale and graded from 0 to 6. The microbiologic index is determined as the percentage of solid, stained AFB. SKIN BIOPSY  A skin biopsy is done to confirm the diagnosis of leprosy, to classify the disease, to support the diagnosis of reactions, and to deter­ mine cure after the completion of multidrug therapy. When macular lesions are suspected of reflecting IL, a biopsy sample should be taken from the middle of a lesion; with plaques, a sample should be obtained from the active indurated edge. When there are numerous skin lesions with different morphologies, more than one biopsy sample is required for proper evaluation of the disease spectrum. Identification of early lesions of leprosy by histopathologic techniques is enhanced by immu­ nochemical staining, which reveals the presence of M. leprae antigens.

PGL-1 ANTIBODY TEST  PGL-1 is a specific lipid on the M. leprae cell wall. A PGL-1 enzyme-linked immunosorbent assay (ELISA) has been used for serologic diagnosis of leprosy, yielding positive results in 90–95% of multibacillary cases and in 25–60% of paucibacillary cases. Using PGL-1 antigen and adopting an immunochromatographic technique, a rapid lateral-flow assay—the ML flow test—has been developed for detection of antibody to PGL-1. This assay gives posi­ tive results in 92–97% of patients with multibacillary leprosy and in 32–40% of patients with paucibacillary disease. Recently, a quantitative UPC-LFA (upconverting phosphor lateral flow assay) test has become available, which has a higher specificity in paucibacillary disease.

LEPROMIN TEST  The lepromin (or Mitsuda) skin test measures cel­ lular immunity against lepromin. A bacillary suspension standardized by the number of inactivated M. leprae it contains is injected just under the skin. The reaction to lepromin is measured as induration in

millimeters 3–4 weeks after intradermal inoculation. The result pro­ vides information about the ability of an individual’s T cells to respond to M. leprae and the likelihood of granuloma formation in that indi­ vidual. A negative lepromin test is generally seen in patients with LL or BL leprosy, indicating the lack of a protective cellular response. GENE AMPLIFICATION (PCR) TECHNIQUE  Gene amplification signifi­ cantly enhances the detection of M. leprae, especially in bacteriologic index–negative leprosy and cases that do not fulfill the criteria for the cardinal signs of leprosy. The several PCR methods developed to amplify different gene stretches in M. leprae include conventional DNA-based PCR, reverse-transcription PCR, and multiplex PCR. As major genes for detection of disease targets, PCR uses M. leprae–specific genes encoding 36-kDa antigen, 18-kDa antigen, 65-kDa antigen com­ plex 85, 16S ribosomal RNA (rRNA), and repetitive sequences. These assays are sensitive to as few as 1–10 bacilli and yield positive results in 60–75% of smear-negative cases. Multiplex PCR employing the genes encoding the repetitive element RLEP, SodA, and 16S rRNA can be used for early diagnosis and for the diagnosis of subclinical infection among household contacts. PART 5 Infectious Diseases Differential Diagnosis  Leprosy is often diagnosed late, with a consequent increase in the risk of nerve damage and its ensuing dis­ abilities. The hypopigmented macules of leprosy must be differentiated from a variety of conditions, including pityriasis alba, vitiligo, pro­ gressive macular hypomelanosis, pityriasis versicolor, pityriasis rosea, postinflammatory hypopigmentation, sarcoidosis, post–kala-azar dermal leishmaniasis, and morphea. In the analysis of plaques and nodular lesions, conditions such as granuloma annulare, cutaneous sarcoidosis, cutaneous leishmaniasis, lupus miliaris disseminatus faciei, nodu­ lar histiocytosis, lupus erythematosus, cutaneous T-cell lymphomas (especially mycosis fungoides), and secondary syphilis should be kept in mind. ENL lesions must be differentiated from erythema nodosum of other etiologies, nodular vasculitis, and cutaneous polyarteritis nodosa. In the case of mononeuropathy lesions, diabetes, amyloidosis, and myxedema must be considered. With polyneuropathy lesions of acute onset, Guillain-Barré syndrome and toxic polyneuropathy must be given consideration. Diagnostic Tools for Nerve Function Impairment  All sen­ sory modalities, autonomic function, and motor function of motor nerves may be affected in leprosy to varying degrees. The modalities mediated by small unmyelinated fibers, such as pain and warm tem­ perature sensation and autonomic function, are often affected first. Clinically detectable impairment of touch sensation and motor func­ tion frequently follows after several months. Unfortunately, tools that allow reliable and safe testing of pain and temperature sensation and autonomic function often are not available at peripheral health facili­ ties, but simple and reliable tests of touch sensation and motor function do provide a reflection of the underlying neuropathy. TOUCH SENSATION TESTING  The ulnar and median nerves and the posterior tibial nerve are usually tested for touch sensation. The most reliable test is the Semmes-Weinstein monofilament (SWM) test. If the impairment is of <6 months’ duration and/or new nerve function

impairment is diagnosed, glucocorticoid treatment should be given. Because filaments are not available in most peripheral health centers, the WHO recommends that a ballpoint pen be used instead. The test­ ing protocol is the same as in the SWM test: the stimulus is delivered by touching the test sites with the tip of a ballpoint pen held at an angle of ~45° relative to the skin. VOLUNTARY MUSCLE TESTING  Motor function of the hands and feet should be evaluated by voluntary muscle testing. The muscle functions most affected in leprosy are eye closure (facial nerve), finger abduc­ tion (ulnar nerve), thumb opposition (median nerve), wrist exten­ sion (radial nerve), and ankle extension (common peroneal nerve). Strength is assessed with a WHO-recommended system as strong, weak, or paralyzed. NERVE CONDUCTION TESTS  Testing of nerve conduction parameters is sensitive in detecting early signs of peripheral neuropathy in lep­ rosy. Sensory nerve conduction parameters are often affected several months ahead of clinical tests (e.g., the SWM test). However, a trial of glucocorticoid treatment of such early changes did not show improved long-term outcomes, perhaps suggesting that the glucocorticoids are unable to switch off or reverse the pathologic process. ULTRASOUND TESTING OF NERVES  Palpable enlargement of certain peripheral nerves is one of the cardinal signs of leprosy. Definite enlargement is easy to establish, but milder degrees are much harder to diagnose by palpation. Ultrasound imaging and measurement of nerve diameters—even with portable equipment—can detect nerve enlarge­ ment accurately. This technique may be used to support the diagnosis of leprosy and may indicate the onset of neuropathy that warrants antiinflammatory treatment. OTHER TESTS OF PERIPHERAL NERVE FUNCTION  Pain and tempera­ ture sensation are commonly affected in leprosy neuropathy. However, these sensations are difficult to test safely and reliably under field conditions. Studies have shown that heat detection thresholds are often affected several months before touch sensation is impaired. Laser Doppler measurement of autonomic vasomotor reflexes is a sensitive method for detection of peripheral autonomic nerve damage in leprosy patients. TREATMENT Leprosy, Leprosy Reactions, and Other Major Manifestations TREATMENT OF LEPROSY Multidrug Therapy  Only one multidrug regimen is recommended by the WHO for the treatment of leprosy. This regimen consists of a combination of two or three of the following drugs: rifampin, dapsone, and clofazimine (Table 184-1). The keystone of WHOrecommended multidrug therapy for multibacillary leprosy is a monthly dose of rifampin together with daily doses of dapsone and daily and monthly doses of clofazimine. Patients with paucibacil­ lary leprosy are treated with two drugs, receiving monthly doses of rifampin and daily doses of dapsone. The treatment duration is 12 months for multibacillary disease and 6 months for paucibacil­ lary disease. Provided that patients complete therapy, treatment failure rates are very low. Some studies have investigated a uniform regimen of three drugs for 6 months. In a recent systematic review of evidence on the potential benefits and risks of this shorter regimen, the WHO concluded that relevant evidence is limited and inconclusive, with a potential increase in the risk of relapse. Therefore, the WHO does not recommend a shortened treatment duration for multibacillary leprosy. The WHO further recommends supervised intake, but actual practice varies among countries. Through the WHO, multidrug therapy is provided free of charge as blister packs for adults to all countries reporting leprosy. Blister packs are also provided for

TABLE 184-1  WHO-Recommended Multidrug Treatment for Leprosy PAUCIBACILLARY LEPROSYa MULTIBACILLARY LEPROSYb DRUG, AGE GROUP Dapsone Adult 100 mg/d 100 mg/d Child age 10–14 years 50 mg/d 50 mg/d Child <10 years Dose adjusted to body weight Dose adjusted to body weight Rifampin Adult 600 mg monthly 600 mg monthly Child 10–14 years 450 mg monthly 450 mg monthly Child <10 years Dose adjusted to body weight Dose adjusted to body weight Clofaziminec Adult — 50 mg/d plus 300 mg monthly Child 10–14 years — 50 mg/d plus 150 mg monthly Child <10 years — Dose adjusted to body weight aDuration: 6 doses (6 blister packs). bDuration: 12 doses (12 blister packs). cIn 2018, the World Health Organization (WHO) suggested including clofazimine in the multidrug therapy regimen for paucibacillary leprosy as well, but it is questionable whether this suggestion will be implemented because of the possibility that skin discoloration might compromise compliance. In addition, this alteration would involve a major change in the production of blister packs, which currently do not include clofazimine for paucibacillary leprosy patients (in line with the original WHO recommendation). 10- to 14-year-olds, while younger children are given doses adjusted according to body weight (Table 184-1). Adverse Events  •  Rifampin  Rifampin acts by inhibiting DNA-dependent RNA polymerase, thereby interfering with bacte­ rial RNA synthesis. Rifampin is well absorbed orally. Hepatotoxicity may occur with a mild transient elevation of hepatic aminotrans­ ferases, but this reaction is rare at the dosages and intervals recom­ mended for leprosy and is not an indication for discontinuation of treatment. Because rifampin is given only monthly in WHOrecommended multidrug therapy regimens, the adverse effects recognized from its use in tuberculosis probably do not occur. A monthly dose of rifampin does not cause induction of hepatic cyto­ chrome p450. Urine discoloration occurs but is harmless. Dapsone  Dapsone (4,4-diaminodiphenyl sulfone [DDS]) acts by blocking folic acid synthesis and is only weakly bactericidal. Oral absorption is good, and the drug has a long half-life averaging 28 h. Dapsone has a poor safety profile, and its use should be monitored carefully. In the doses recommended for leprosy, it can cause mild hemolysis and may cause anemia or, rarely, psychosis. Glucose6-phosphate dehydrogenase deficiency seldom causes a problem, and enzyme levels are not routinely tested before the start of mul­ tidrug treatment. On the other hand, the “DDS syndrome” (also called the dapsone hypersensitivity syndrome) is a severe adverse event that is not uncommon in some countries. It usually devel­ ops 6 weeks after the commencement of dapsone administration and manifests as fever, skin rash, eosinophilia, lymphadenopathy, hepatitis, and encephalopathy. Other rare but severe cutaneous adverse reactions are erythema multiforme, Stevens-Johnson syn­ drome, toxic epidermal necrolysis, and exfoliative dermatitis. The fatality rate for DDS syndrome is 10%, with death occurring from liver failure, sepsis, and bone marrow failure. Most patients require treatment with systemic glucocorticoids. In all cases, dapsone treat­ ment must be stopped. Agranulocytosis, hepatitis, and cholestatic jaundice occur rarely with dapsone therapy. Clofazimine  Clofazimine is a brick-red, fat-soluble crystalline dye. The mechanism of its weakly bactericidal action against M. leprae is not known. High drug concentrations are found in the intestinal

mucosa, mesenteric lymph nodes, and body fat. The most notice­ able adverse event is skin discoloration ranging from red to purple or black, with the degree of discoloration depending on the dosage. Clofazimine can accumulate in active leprosy skin lesions, thus making them more prominent. The abnormal pigmentation usually fades within 6–12 months of clofazimine discontinuation, although traces of discoloration may remain for up to 4 years. The skin dis­ coloration associated with clofazimine is psychologically distressing for many people. Patients often stop taking the drug because the discoloration is socially disabling for them, alerting their social environment to the fact that they are taking anti-leprosy medication and thus breaking confidentiality about treatment. Urine, sputum, and sweat may become pink during clofazimine administration. Clofazimine also produces a characteristic ichthyosis on the shins and forearms. Adverse gastrointestinal events ranging from mild cramps to diarrhea and weight loss may result from clofazimine crystal deposition in the wall of the small bowel.

Relapse  The cure rate for leprosy with multidrug therapy is 99%, but relapse is possible. In multibacillary leprosy, relapse is defined as the multiplication of M. leprae, with an increase of at least 2+ over the previous value in the bacteriologic index at any single site; this change usually occurs in conjunction with evidence of clinical deterioration (e.g., new skin patches or nodules and/or new nerve damage). Relapse rates are well below 1% except among a small pro­ portion of patients who have a very high bacillary load at the start of treatment (bacteriologic index ≥4). In different studies, four to seven relapses were recorded per 100 person-years. These relapses usually occurred <5 years after the end of multidrug therapy. Since antimicrobial resistance to the combination of drugs used in multi­ drug treatment is rare, patients with relapse can be re-treated with the same multibacillary regimen. CHAPTER 184 Recognizing a relapse in paucibacillary leprosy can be difficult, as symptoms may resemble T1R. However, relapse of paucibacil­ lary disease is very rare. Administration of a therapeutic trial with glucocorticoids to patients with new lesions may help distinguish between these two phenomena: a definite improvement within 4 weeks of initiation of glucocorticoid therapy indicates T1R, whereas a lack of response favors the diagnosis of a clinical relapse. Patients with multibacillary disease who present with a relapse are re-treated with the multidrug regimen regardless of any change in classification. Patients with paucibacillary disease require 2 years of monitoring after treatment and patients with multibacillary disease at least 5 years. Reinfection by different strains of M. leprae is possible and can be confused with relapse. Leprosy Rifampin Resistance and Second-Line Drugs  Resistance to rifampin has been reported from several countries, although the number of patients involved is small. Evidence on the potential benefits and risks of using alternative regimens for drug-resistant leprosy is not available. Therefore, recommendations provided by the WHO for second-line regimens are based on expert opinion and the known activity of alternative drugs, including the likeli­ hood of cross-resistance. For rifampin-resistant leprosy, the WHO guidelines recommend daily treatment with at least two second-line drugs—clarithromycin, minocycline, or a quinolone (ofloxacin, levofloxacin, or moxifloxacin)—plus clofazimine for 6 months, followed by clofazimine plus one of the second-line drugs daily for an additional 18 months. Leprosy patients infected with M. leprae resistant to both rifampin and ofloxacin may be treated daily with the following regimen: clarithromycin, minocycline, and clofazi­ mine for 6 months, followed by clarithromycin or minocycline plus clofazimine for an additional 18 months. TREATMENT OF LEPROSY REACTIONS Type 1 Reactions  Oral, short-acting glucocorticoids are the treat­ ment of choice for T1R. Prednisolone is used most often in an initial dose of 1 mg/kg of body weight once a day, usually with a maximum of 60–80 mg. If standard treatment protocols are followed, as they are in most leprosy programs in endemic countries, an initial dose

PART 5 Infectious Diseases of 40 mg of prednisolone is recommended by the WHO. The dose is tapered slowly, usually by 5 mg every 2 weeks over a period of 20 weeks—a schedule that results in better outcomes and lower reaction relapse rates than the previously recommended 12-week glucocorticoid regimen. However, the clinical response should guide treatment. Patients should be examined every 2 weeks, and the examination should include a quick nerve function assessment. Not infrequently, the reaction flares up again once the daily gluco­ corticoid dose is tapered to <10–20 mg. The potential benefits of longer treatment should be balanced against the risks of prolonged glucocorticoid use, especially at higher doses. Type 2 Reactions  Mild first-time T2R (or ENL) reactions with localized skin nodules may be treated with aspirin and pentoxifyl­ line. If a rapid effect is needed, the most effective drug to date is tha­ lidomide, which rapidly suppresses clinical signs, including nerve impairment and iritis. However, the drug is blacklisted in many countries because of its teratogenicity. If available, it should be given with great caution to women of childbearing age—only after careful counseling and a negative pregnancy test and with strict adherence to contraception. A dose of 100–200 mg is given either once or twice daily. In acute first episodes, thalidomide treatment should be tapered down and stopped after 1–2 weeks. If tissues other than the skin are affected—e.g., the eyes (iritis/uveitis), testes (orchitis), kidneys (nephritis), or joints (arthritis)—longer treatment may be needed until signs and symptoms have resolved. In patients with severe recurrent ENL, a daily thalidomide maintenance dose of 50 mg may be effective in suppressing new episodes. Because of the restricted availability and use of thalidomide, patients with acute ENL are usually treated with glucocorticoids. T2R tends to be transient, often resolving in ~2 weeks. The treatment strategy is therefore to suppress the acute signs and symptoms with high-dose oral prednisolone, quickly tapering treatment in 2–3 weeks either to zero or to a low maintenance dose if the patient has had previous attacks. High-dose clofazimine also is effective in preventing recur­ rent ENL, but attainment of a maximal effect takes several weeks. The usual regimen is 300 mg daily for 1 month, followed by 200 mg daily for 1 month and, subsequently, 100 mg daily as a mainte­ nance dose for as long as necessary. Prolonged use of high-dose clofazimine may cause significant adverse gastrointestinal effects. An important side effect of clofazimine is a dark discoloration of the skin. While discoloration resolves gradually after the drug is discontinued, it is one main reason that patients dislike or even refuse to take clofazimine. TREATMENT AND PROGNOSIS OF NERVE FUNCTION IMPAIRMENT Episodes of sensory or motor nerve function impairment without skin signs are common. Neuropathy may occur without obvious neuritis. Still, the treatment of such “silent neuropathy” is the same as that for T1R. High-dose prednisolone is the drug of choice. Some experts think that patients will benefit from nerve decompression surgery, but evidence from randomized controlled trials is lacking. If glucocorticoid treatment is started shortly after the develop­ ment of nerve function impairment, the prognosis for full recov­ ery is good. Generally, some recovery can still be expected up to 6 months after onset, but the likelihood of recovery diminishes with every new episode. Generally, nerve function impairment that has persisted for >6 months does not benefit from glucocorticoid treatment. TREATMENT OF (NEUROPATHIC) PAIN Pain is common in people affected by leprosy and is often of neuro­ pathic origin. Little evidence-based information is currently avail­ able on the origin and treatment of pain in leprosy. Generally, for the treatment of neuropathic pain, three classes of medication are available: tricyclic antidepressants, phenothiazines, and anticonvul­ sants (carbamazepine, oxcarbazepine, gabapentin, and pregabalin). These agents can be combined with analgesics and anti-inflamma­ tory drugs according to the patient’s needs. DISEASE MANAGEMENT DURING TREATMENT Leprosy can be cured effectively, but the long duration of multidrug therapy means that careful management is needed to help the patient complete treatment. Regular visits to a health center may invoke questions from community members that may threaten the patient’s privacy, thus causing the patient mental distress and jeopardizing treatment adherence. Counseling is essential, as are patient-friendly arrangements for collecting treatment drugs. The disease, its treat­ ment, and its possible complications should be discussed, including a consideration of disease prognosis, the resolution of skin patches, skin discoloration by clofazimine, the lack of contagiousness during multidrug therapy, and the capacity for unrestricted family relations, including marital life and sexual activity. Possible stigmatization, including self-stigmatization, also should be discussed. Because of the diverse complications that are possible, especially in patients with multibacillary leprosy, a multidisciplinary approach to patient management is required. In low-income countries, the responsibility for treatment usually lies with a leprosy control offi­ cer or a general medical practitioner. In middle- and high-income countries, the main treatment responsibility usually falls to a der­ matologist. Additional support should come from a neurologist or neurophysiologist for the diagnosis of nerve function impairment, and a rehabilitation physician, physiotherapist, infectious disease specialist, and/or psychologist may be needed. Occasionally, spe­ cialist support with regard to orthotics as well as in ophthalmology, occupational therapy, reconstructive surgery, and/or communitybased rehabilitation is indicated. Supervised Multidrug Therapy  Regular treatment is important, especially the supervised 4-weekly dose of rifampin and clofazi­ mine. However, treatment adherence can be facilitated by flexible arrangements; for example, patients can be allowed to take home more than one 4-week blister pack if they will be away for travel or seasonal labor. In such cases, a family member or another respon­ sible person can be asked to supervise the monthly dose. Monthly Nerve Function Assessment  Since nerve damage can be insidious and silent, it is important to conduct a brief nerve function assessment at each clinic visit during multidrug therapy. This regular assessment is especially important in patients with known risk fac­ tors for nerve function impairment. At highest risk are patients with multibacillary disease, who already have nerve damage at the start of treatment. Their risk of additional nerve damage is as high as 65%. Multibacillary leprosy patients without nerve function impairment at diagnosis and paucibacillary leprosy patients with such impair­ ment at diagnosis have a 16% chance of developing damage and additional damage, respectively. Patients with paucibacillary disease who do not have nerve function impairment at diagnosis are at low­ est risk (3%); for them, an assessment at the start and completion of multidrug therapy can be sufficient. Leprosy reactions and new nerve damage may also occur after completion of multidrug treat­ ment. While the risk diminishes with time, these manifestations can occur up to 3 years after the conclusion of therapy. Health Education  During treatment, patients will have questions that need to be addressed in order to ensure their treatment adher­ ence. Sensitive questions may arise regarding everyday life within the family and at work that, if not addressed properly, could lead to social withdrawal and mental health issues. Crucial points for health education are at diagnosis and at completion of treatment. When communicating the diagnosis, the physician must explain that the disease is caused by a curable microbial infection and must cover the possible discomforts of drug intake, the interruption of disease transmission through drug intake, and the importance of adhering to treatment to achieve a cure. At the completion of multidrug therapy, the emphasis should be on separating the con­ cept of cure (bacterial activity) from the sequelae of the disease (nerve function impairment, leprosy reactions, and disabilities) and explaining that the patient may need to continue receiving health care, including reconstructive surgery, for the sequelae. Patients often associate cure with the absence of symptoms, which

is not accurate in leprosy. Some patients will experience discomfort during bacterial activity but will have no sequelae after treatment. In others, nerve function impairment or leprosy reactions may cause disfigurement with physical discomfort after cure. These sequelae will need further management and patients at risk should be warned that such posttreatment complications may occur and to report without delay should this happen. Disabilities such as claw hand or neuropathic foot require chronic care. Guidelines After the Completion of Multidrug Therapy  Patients should receive counseling at release from treatment. The topics covered should include reassurance that the person is no longer contagious, that in some patients hypopigmentation in skin lesions may not resolve for a long time, and that skin discoloration due to clofazimine will gradually disappear in the following months. Nerve impairment may continue to improve after release from treatment, but this is by no means certain. Most important, patients should be instructed to return to the clinic if any new skin signs or fresh nerve damage occurs. This situation is not uncommon, is usually due to a leprosy reaction, and should be managed carefully from both a medical and a social perspective, since patients and persons in their environment will interpret this development as “leprosy coming back.” Patients at risk of further episodes of reaction and/or addi­ tional nerve function impairment (e.g., patients with preexisting nerve function impairment and multibacillary infection or patients who have experienced a reactional episode during therapy) should be asked to return for a check-up every 6 months for at least 3 years after being released from treatment. ■ ■REHABILITATION AND SOCIAL ASPECTS Physical Rehabilitation  Peripheral neuropathy and its second­ ary disabling consequences often require physical rehabilitation. This effort may include reconstructive surgery in the case of facial, ulnar, median, or posterior tibial paralysis. In this case, pre- and postopera­ tive physical therapy is of crucial importance. Physical therapy is also indicated when muscles are not completely paralyzed or when con­ tractures are too stiff to allow surgery. Since paralysis is usually accom­ panied by sensory and autonomic neuropathy, occupational therapy also is helpful; therapists teach patients how to minimize the risk of further injury and other techniques for prevention of disabilities. The key principle is teaching patients and former patients to self-manage their disabilities. In many programs, this teaching occurs in the setting of self-care groups. A well-tested and evidence-based self-care routine for hands and feet consists of inspection, soaking, scraping, and oiling (ISSO). Specifically, in ISSO, the person inspects the affected limbs for hotspots (evidence of too much stress on an area of skin): wounds, cracks, and calluses. Next, the affected limb(s) are soaked in plain water for 15 minutes. While the skin is wet, areas with excess calluses are scraped with a rough stone or another rough object. The skin is then rubbed with petroleum jelly or another nonfragrant oil in order to trap moisture in the skin. If this routine is performed daily, the skin can be kept supple and in good condition, despite sensory and autonomic damage. If sensation on the soles of the feet is impaired, the person must wear protective footwear. Simple footwear (e.g., sandals or sneak­ ers) available at the local market is adequate as long as it has a strong sole and a soft insole of ethylene-vinyl acetate or microcellular rubber that distributes pressure—an especially important feature when foot muscles are weak or paralyzed or the architecture of the foot is dam­ aged, as is often the case with neuropathy. In high-resource settings, tailor-made orthopedic shoes can be provided. Mental and Social Support  Like other chronic health conditions, leprosy requires patients to cope with the burden of new routines in everyday life. In addition to coping with stigma, they must organize themselves for prolonged treatment, prevention of disabilities, and rehabilitation activities. Moreover, like other neglected tropical dis­ eases (see “Neglected Tropical Diseases,” below), leprosy may lead to poor mental health. Such diseases are accompanied by social exclusion in the form of poor access to services such as health care, education,

employment, and housing. This exclusion accounts for common mental health comorbidities in leprosy patients and their family mem­ bers, including depression, anxiety, and suicidal thoughts. Leprosy is probably the most notorious of all stigmatized health conditions, and social stigmatization is the most common issue that triggers mental suffering. Other infectious diseases that raise this issue include HIV infection, tuberculosis, and neglected tropical diseases like lymphatic filariasis, Buruli ulcer, and dermal leishmaniasis. Although the rea­ sons for stigmatization vary, the manifestations and interventions that effectively reduce stigma are similar across conditions and countries. Therefore, joint interventions addressing health-related stigmas for multiple conditions would be strategically and financially attractive. The need to introduce mental health care in leprosy services is press­ ing. Therapeutic group meetings among institutionalized patients and self-care groups at the community level, with a focus on prevention of disabilities and mental well-being, are known to ameliorate depression, encourage self-acceptance, and promote confidence.

NEGLECTED TROPICAL DISEASES  Leprosy is one of a medically diverse group of 20 neglected tropical diseases (NTDs). This group includes infectious diseases caused by bacteria, viruses, fungi, and parasites as well as some noninfectious conditions, such as podoco­ niosis and snakebite. NTDs have been grouped together because they affect 1.5 billion of the poorest people on Earth and have been widely neglected in domains such as public policy, funding, and the develop­ ment of diagnostics and treatments. Leprosy is the archetypical NTD, featuring all of the common characteristics: a treatable infectious dis­ ease, a known population at risk, available preventive chemotherapy, disease complications that may lead to severe disabilities, and a per­ vasive social stigma that leads to discrimination, social exclusion, and severe mental health consequences. Nevertheless, the priority accorded to leprosy on the public health agenda of most endemic countries is very low. By joining hands in advocacy, fundraising, and development of joint control strategies, health care organizations can substantially raise the priority profile of NTDs, benefiting each of the individual disease control programs. Such a joint approach serves the goal of universal health coverage and helps to strengthen health services more effectively than vertical programs are ever able to do on their own. CHAPTER 184 Leprosy ■ ■PREVENTION AND CONTROL Interruption of Transmission and Novel Preventive

Strategies  Leprosy control was traditionally based on early case detection and multidrug treatment. Apart from health education and leprosy awareness campaigns, no preventive measures were available. In the 1990s, authorities hoped that the transmission of M. leprae in the community could be interrupted through timely detection of cases and provision of multidrug therapy, leading to a decline in leprosy incidence. Unfortunately, this has not been the case (Fig. 184-1). The inability to reduce leprosy incidence in many countries and the heightened interest in NTDs have invigorated research into new techniques for the diagno­ sis of disease and infection, leprosy vaccines, enhanced postexposure chemoprophylaxis regimens, epidemiologic tools (e.g., geographic infor­ mation systems for identifying leprosy hotspots), surveillance of antimi­ crobial resistance, and alternative drugs and drug treatment regimens. Vaccines Against Leprosy  The bacille Calmette-Guérin (BCG) vaccine used against tuberculosis provides varying degrees of protec­ tion against leprosy and is used routinely as postexposure immuno­ prophylaxis for contacts of leprosy patients in Brazil. Two promising vaccine candidates are in the pipeline: the MIP vaccine from India, which is based on killed Mycobacterium indicus pranii, and the syn­ thetic LepVax vaccine developed by the University of Washington’s Infectious Disease Research Institute in the United States. If proven effective, these vaccines, like the BCG vaccine, will be used as postex­ posure prophylaxis for contacts of leprosy patients. Trials are in early stages, and sufficient proof of efficacy will take years. Postexposure Chemoprophylaxis  The introduction of post­ exposure chemoprophylaxis (PEP) for household and other close contacts of leprosy patients is an important innovation. A large

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185 Nontuberculous Mycobacterial Infections

randomized controlled trial has shown that single-dose rifampin, given once to household contacts, neighbors, and social contacts, reduces the recipients’ risk of leprosy by ~60%. Implementation studies have shown that PEP with single-dose rifampin is feasible and well accepted by patients, contacts, and health workers in a variety of health care settings. Furthermore, modeling studies have indicated the potential impact of PEP on transmission of M. leprae in endemic populations. This intervention was included in the 2018 WHO Guidelines for the Diagnosis, Treatment, and Prevention of Leprosy and is currently being introduced in many countries. Research is ongoing into enhanced PEP regimens for those close contacts who are at increased risk of leprosy (e.g., blood-related household contacts and close contacts of multi­ bacillary leprosy patients).

“Zero Leprosy”  The WHO has formulated its new Global Lep­ rosy Strategy 2021–2030. As in the organization’s previous strategy, a holistic approach to leprosy control is advocated, focusing on zero infection and disease, zero disability, and zero stigma and discrimina­ tion. For 2030, the WHO is setting ambitious targets of achieving 120 countries with zero new autochthonous leprosy cases, reducing the annual number of new cases detected by 70%, reducing the rate of new cases with grade 2 disability per million population (as a proxy for detection delay) by 90%, and reducing the rate of new child cases with leprosy per million children (as a proxy for recent transmission) by 90%. Widespread implementation of PEP with single-dose rifampin is one of the key strategies to achieve these goals. The “Triple Zero Strat­ egy” (zeroleprosy.org) has also been embraced by the partners united in the Global Partnership for Zero Leprosy, the International Federation of Anti-Leprosy Associations, the Novartis Foundation, the Sasakawa Health Foundation, and the International Association for Integration, Dignity, and Economic Advancement. PART 5 Infectious Diseases In July 2023, the WHO launched new “Technical Guidance on Interruption of Transmission and Elimination of Leprosy Disease” (www.who.int/publications/i/item/9789290210467). This contains clear milestones, definitions, and cutoffs for interruption of transmission and elimination of leprosy disease. Using the Leprosy Elimination Framework and the accompanying tools—the Leprosy Elimination Monitoring Tool and Leprosy Programme and Transmission Assessment— countries can track their progress toward these milestones in detail. The outlook for achieving “zero leprosy” is better than ever before, but this goal is admittedly very ambitious. It can be reached only when all leprosy-endemic countries enhance their leprosy control activities to include (1) active case-finding strategies, including improved diag­ nosis; (2) contact screening; (3) implementation of PEP; (4) improved prevention of disability services; and (5) activities to reduce stigma and discrimination and to promote the social inclusion and mental well-being of affected patients and their families. Coincident with these efforts, an important threat must be confronted. With the waning of interest in leprosy and the integration of management of the disease into nonspecialized health systems, the number of medical doctors and health workers at the primary care level who have experience in diagnosing and treating leprosy has decreased substantially all over the world. Once lost, expertise is difficult to regain. Therefore, new energy and resources need to be invested in bolstering technical capacity for all aspects of leprosy services, with a view to strengthening the health system in an integrated way and leaving no one behind. Acknowledgment We thank Dr. Colette L.M. van Hees, dermatologist at Erasmus MC, University Medical Center Rotterdam, for critical review of this chapter. ■ ■FURTHER READING Bratschi MW et al: Current knowledge on Mycobacterium leprae transmission: A systematic literature review. Lepr Rev 86:142, 2015. Collin SM et al: Systematic review of Hansen disease attributed to Mycobacterium lepromatosis. Emerg Infect Dis 29:1376, 2023. Fróes LAR Jr et al: Bacterial, fungal and parasitic co-infections in leprosy: A scoping review. PLoS Negl Trop Dis 17:e0011334, 2023. Kumar B, Kar HK (eds): IAL Textbook of Leprosy, 2nd ed. New Delhi, Jaypee Brothers Medical Publishers (P) Ltd, 2017.

Scollard DM, Gillis TP (eds): International Textbook of Leprosy. Available at https://internationaltextbookofleprosy.org. Accessed February 17, 2024. Smith WC et al: The missing millions: A threat to the elimination of leprosy. PLoS Negl Trop Dis 9:e0003658, 2015. World Health Organization: Guidelines for the diagnosis, treat­ ment and prevention of leprosy. New Delhi, WHO Regional Office for South-East Asia, 2018. Available at https://apps.who.int/iris/ handle/10665/274127.  Accessed February 17, 2024. Steven M. Holland

Nontuberculous

Mycobacterial Infections Several terms—nontuberculous mycobacteria (NTM), atypical myco­ bacteria, mycobacteria other than tuberculosis, and environmental mycobacteria—all refer to mycobacteria other than Mycobacterium tuberculosis, its close relatives (M. bovis, M. caprae, M. africanum,

M. pinnipedii, M. canetti), and M. leprae. The number of identified species of NTM is growing and will continue to do so because of the use of DNA sequence typing for speciation. The number of known spe­ cies currently exceeds 199. NTM are highly adaptable and can inhabit hostile environments, including industrial solvents. ■ ■EPIDEMIOLOGY NTM are ubiquitous in soil and water. Specific organisms have recurring niches, such as M. simiae in certain aquifers, M. fortuitum in pedicure baths, and M. immunogenum in metalworking fluids. Most NTM cause disease in humans only rarely unless some aspect of host defense is impaired, as in bronchiectasis, or breached, as by inoculation (e.g., liposuction, trauma, cardiac surgery). There are few instances of human-to-human transmission of NTM, which occurs almost exclusively in cystic fibrosis. Because infections due to NTM are rarely reported to health agencies and because their identification is sometimes problematic, reliable data on incidence and prevalence are lacking. Disseminated disease denotes significant immune dysfunction (e.g., advanced HIV infection), whereas pulmonary disease, which is much more common, is highly associated with pulmonary epithelial defects but not with systemic immunodeficiency. In the United States, the incidence and prevalence of pulmonary infection with NTM, mostly in association with bronchiectasis (Chap. 301), have for many years been severalfold higher than the corresponding figures for tuberculosis, and rates of the former are increasing among the elderly as rates of tuberculosis continue to fall. Among patients with cystic fibrosis, who often have bronchiectasis, rates of clinical infection with NTM range from 3 to 15%, with even higher rates among older patients. Although NTM may be recovered from the sputa of many individuals, it is critical to differentiate active disease from commensal harboring of the organisms. A scheme to help with the proper diagnosis of pulmonary infection caused by NTM has been developed by the American Thoracic Society and is widely used (https:// doi.org/10.1093/cid/ciaa241). The bulk of nontuberculous mycobacte­ rial disease in North America is due to M. kansasii, organisms of the M. avium complex (MAC), and organisms in the M. abscessus complex. In Europe, Asia, and Australia, the distribution of NTM in clinical specimens is roughly similar to that in North America, with MAC spe­ cies and rapidly growing organisms such as M. abscessus encountered frequently. M. xenopi and M. malmoense are especially prominent in northern Europe. M. ulcerans causes the distinct clinical entity Buruli ulcer, which occurs throughout tropical zones, especially in western Africa.

M. marinum is a common cause of cutaneous and tendon infections in coastal regions and among individuals exposed to fish tanks or swim­ ming pools. The true international epidemiology of infections due to NTM is hard to determine because the isolation of these organisms often is not reported and speciation often is not performed for M. tuberculosis or NTM. The latter issue poses an especially important problem during therapy for tuberculosis when smears positive for acid-fast bacilli are considered evidence of treatment failure. The increasing ease of iden­ tification and speciation of these organisms is already having a major impact on the description of the dynamic international epidemiology of tuberculosis and NTM infections. ■ ■PATHOBIOLOGY Because exposure to NTM is essentially universal and disease is rare, it can be assumed that normal host defenses against these organisms must be strong and that otherwise healthy individuals in whom sig­ nificant disease develops are highly likely to have specific susceptibility factors that permit NTM to become established, multiply, and cause disease. At the advent of HIV infection, CD4+ T lymphocytes were recognized as key effector cells against NTM; the development of dis­ seminated MAC disease was highly correlated with a decline in CD4+ T lymphocyte numbers. Such a decrease has also been implicated in disseminated MAC infection in patients with idiopathic CD4+ T lymphocytopenia. Potent inhibitors of tumor necrosis factor α (TNF-α), such as infliximab, adalimumab, certolizumab, golimumab, and etan­ ercept, neutralize this critical cytokine, with consequent inhibition of granuloma formation. The occasional result is severe mycobacterial or fungal infection; these associations indicate that TNF-α is a crucial element in mycobacterial control. However, in cases without the above risk factors, much of the basis of susceptibility to disseminated infec­ tion with NTM is accounted for by specific mutations in the interferon γ (IFN-γ)/interleukin 12 (IL-12) synthesis and response pathways or autoantibodies to IFN-γ itself. Mycobacteria are typically phagocytosed by macrophages, which respond with the production of IL-12, a heterodimer composed of IL-12p35 and IL-12p40 moieties that together make up IL12p70. IL-12 activates T lymphocytes and natural killer cells through binding to its receptor (composed of IL-12Rβ1 and IL-12Rβ2/IL-23R), with consequent phosphorylation of STAT4. IL-12 stimulation of STAT4 leads to secretion of IFN-γ, which activates neutrophils and macrophages to produce reactive oxidants, to increase expression of the major histocompatibility complex and Fc receptors, and to concen­ trate certain antibiotics intracellularly. Signaling by IFN-γ through its receptor (composed of IFN-γR1 and IFN-γR2) leads to phosphoryla­ tion of STAT1, which in turn regulates IFN-γ-responsive genes, such as those coding for IL-12 and TNF-α. TNF-α signals through its own receptor via a downstream complex containing the nuclear factor-κB (NF-κB) essential modulator (NEMO). Therefore, the positive feed­ back loop between IFN-γ and IL-12/IL-23 drives the immune response to mycobacteria and other intracellular infections. These genes are known to be the critical ones in the pathway of mycobacterial control: specific Mendelian mutations have been identified in IFNG, IFNGR1, IFNGR2, STAT1, GATA2, ISG15, IRF8, IL-12A, IL-12RB1, IL-12RB2, CYBB (which encodes the gp91phox protein of the NADPH oxidase), SPP2A, MCTS1, and IKBKG (which encodes NEMO) (Fig. 185-1). Despite the identification of genes associated with disseminated dis­ ease, only ~70% of cases of disseminated nontuberculous mycobacte­ rial infections that are not associated with HIV infection have a genetic diagnosis; the implication is that more mycobacterial susceptibility genes and pathways remain to be identified. In contrast to the recognized genes and mechanisms associated with disseminated nontuberculous mycobacterial infection, the bestrecognized underlying condition for pulmonary infection with NTM is bronchiectasis (Chap. 301). Most of the well-characterized forms of bronchiectasis, including cystic fibrosis, primary ciliary dyskinesia, STAT3-dominant negative hyper-IgE syndrome (Job’s syndrome), and idiopathic bronchiectasis, have high rates of association with nontu­ berculous mycobacterial infection. The precise mechanism by which

IL-2 IL-2R T/NK IFNf a1 IL-12R a2 IL-18 ? IL-15 IFNfR STAT1 GATA2

IRF8 ISG15 IL-12 NEMO AFB Salm. TNF NRAMP1 MΦ TLR TNFR CD14 LPS FIGURE 185-1  Cytokine interactions of infected macrophages (MΦ) with T and natural killer (NK) lymphocytes. Infection of macrophages by mycobacteria (AFB) leads to the release of heterodimeric interleukin 12 (IL-12). IL-12 acts on its receptor complex (IL-12R), with consequent STAT4 activation and production of homodimeric interferon γ (IFNγ). Through its receptor (IFNγR), IFNγ activates STAT1, stimulating the production of tumor necrosis factor α (TNFα) and leading to the killing of intracellular organisms such as mycobacteria, salmonellae (Salm.), and some fungi. Homotrimeric TNFα acts through its receptor (TNFαR) and requires nuclear factor-κB essential modulator (NEMO) to activate nuclear factor-κB, which also contributes to the killing of intracellular bacteria. Both IFNγ and TNFα lead to upregulation of IL-12. TNFα-blocking antibodies work either by blocking the ligand (infliximab, adalimumab, certolizumab, golimumab) or by providing soluble receptor (etanercept). Mutations in IFNG, IFNGR1, IFNGR2, IL12B, IL12RB1, IL12RB2, STAT1, GATA2, ISG15, IRF8, CYBB, MCTS1, and IKBKG (NEMO) have been associated with predisposition to mycobacterial infections. Other cytokines, such as IL-15 and IL-18, also contribute to IFNγ production. Signaling through the Toll-like receptor (TLR) complex and CD14 also upregulates TNFα production. IRF8, interferon regulatory factor 8; ISG15, interferon-stimulated gene 15; LPS, lipopolysaccharide; NRAMP1, natural resistance-associated macrophage protein 1. CHAPTER 185 Nontuberculous Mycobacterial Infections bronchiectasis predisposes to locally destructive but not systemic involvement is unknown. Unlike disseminated or pulmonary infection, “hot-tub lung” rep­ resents pulmonary hypersensitivity to NTM—most commonly MAC organisms—growing in underchlorinated water, often in indoor hot tubs. ■ ■CLINICAL MANIFESTATIONS Disseminated Disease  Disseminated MAC or M. kansasii infec­ tions in people with advanced HIV infection are now uncommon in North America because of effective antimycobacterial prophylaxis and improved treatment of HIV infection. When such mycobacterial disease was common, the portal of entry was the bowel, with spread to bone marrow and the bloodstream. Surprisingly, disseminated infec­ tions with rapidly growing NTM (e.g., M. abscessus, M. fortuitum) are very rare in people with advanced HIV infection. Because these organ­ isms are of low intrinsic virulence and disseminate only in conjunction with impaired immunity, disseminated disease can be indolent and progressive over weeks to months. Typical manifestations of malaise, fever, and weight loss are often accompanied by organomegaly, lymph­ adenopathy, and anemia. Because special cultures or stains are required to identify the organisms, the most critical step in diagnosis is to sus­ pect infection with NTM. Blood cultures may be negative, but involved organs typically have significant organism burdens, sometimes with a grossly impaired granulomatous response. Disseminated involvement (i.e., involvement of two or more organs) without an underlying iatrogenic cause should prompt a genetic investigation of the IFN-γ/IL-12 pathway. Recessive muta­ tions in IFNGR1 and IFNGR2 typically ablate IFN-γ signaling and lead to severe infection with NTM. In contrast, dominant negative muta­ tions in IFNGR1, which lead to overaccumulation of a defective inter­ fering mutant receptor on the cell surface, inhibit but do not abolish

normal IFN-γ signaling and cause nontuberculous mycobacterial osteomyelitis. Dominant negative mutations in STAT1 and recessive mutations in IL-12RB1 can produce variable phenotypes consistent with their residual capacities for IFN-γ synthesis and response. Male patients who have disseminated nontuberculous mycobacterial infec­ tions along with bacterial or viral infections; conical, peg, or missing teeth; or an abnormal hair pattern should be evaluated for defects in the pathway that activates NF-κB through NEMO (IKBKG). These patients may have associated immune globulin defects as well. Patients with myelodysplasia and mycobacterial disease should be investigated for GATA2 deficiency. A recently recognized group of patients who often develop disseminated infections with both MAC and rapidly growing NTM (predominantly M. abscessus) as well as other opportu­ nistic infections such as Talaromyces have high-titer neutralizing auto­ antibodies to IFN-γ. This syndrome has been reported most frequently in East Asian female patients.

IV catheters can become infected with NTM, usually as a conse­ quence of contaminated water. M. abscessus and M. fortuitum some­ times infect deep indwelling lines as well as fluids used in eye surgery, subcutaneous injections, and local anesthetics. Infected catheters should be removed. Pulmonary Disease  Lung disease is by far the most common form of nontuberculous mycobacterial infection in North America and the rest of the industrialized world. In North America, rates of NTM lung disease far exceed rates of tuberculosis. The clinical presentation typi­ cally consists of months or years of throat clearing, nagging cough, and slowly progressive fatigue. Patients will often have seen physicians mul­ tiple times and received symptom-based or transient therapy before the diagnosis is entertained and samples are sent for mycobacterial stains and cultures. Because not all patients can produce sputum, bronchos­ copy may be required for diagnosis. The typical lag between onset of symptoms and diagnosis is ~5 years in older women. Predisposing fac­ tors include underlying lung diseases such as bronchiectasis (Chap. 301), pneumoconiosis (Chap. 300), chronic obstructive pulmonary disease (Chap. 303), primary ciliary dyskinesia (Chap. 301), α1 antitrypsin deficiency (Chap. 303), and cystic fibrosis (Chap. 302). Bronchiectasis and nontuberculous mycobacterial infection often coexist and progress in tandem. This situation makes causality difficult to determine in a given index case, but bronchiectasis is certainly among the most critical predisposing factors that are exacerbated by infection. PART 5 Infectious Diseases MAC are the most common cause of pulmonary nontuberculous mycobacterial infection in North America, but rates vary somewhat by region. MAC infection most commonly develops during the sixth or seventh decade of life in women who have had months or years of nagging intermittent cough and fatigue, with or without sputum pro­ duction or chest pain. The constellation of pulmonary disease due to NTM in a tall and thin woman who may have chest wall abnormalities is often referred to as “Lady Windermere syndrome,” after an Oscar Wilde character of the same name. In fact, pulmonary MAC infec­ tion does afflict older nonsmoking white women more than men, with onset at ~60 years. Patients tend to be taller and thinner than the general population, with high rates of scoliosis, mitral valve prolapse, and pectus anomalies. Whereas male smokers with upper-lobe cavi­ tary disease tend to carry the same single strain of MAC indefinitely, nonsmoking females with nodular bronchiectasis tend to carry several strains of MAC simultaneously, with changes over the course of their disease. M. kansasii can cause a clinical syndrome that strongly resembles tuberculosis, consisting of hemoptysis, chest pain, and cavitary lung disease. The rapidly growing NTM, such as M. abscessus, have been associated with esophageal motility disorders such as achalasia. Patients with pulmonary alveolar proteinosis are prone to pulmonary nontuberculous mycobacterial and Nocardia infections; the underlying mechanism may be inhibition of alveolar macrophage function due to the autoantibodies to granulocyte-macrophage colony-stimulating fac­ tor found in many of these patients. Cervical Lymph Nodes  The most common form of nontubercu­ lous mycobacterial infection among young children in North America

is isolated cervical lymphadenopathy, caused most frequently by MAC organisms but also by other NTM. The cervical swelling is typically firm and relatively painless, with a paucity of systemic signs. Because the differential diagnosis of painless adenopathy includes malignancy, many children have infection with NTM diagnosed inadvertently at biopsy; cultures and special stains may not have been requested because mycobacterial disease was not ranked high in the differential. Local fistulae usually resolve completely with resection and/or anti­ biotic therapy. Likewise, the entity of isolated pediatric intrathoracic nontuberculous mycobacterial infection, which is probably related to cervical lymph node infection, is usually mistaken for cancer. In neither isolated cervical nor isolated intrathoracic infections with NTM have children with underlying immune defects been commonly identified, nor do the affected children usually go on to develop other opportunistic infections. Skin and Soft Tissue Disease  Cutaneous involvement with NTM usually requires a break in the skin for introduction of the bacteria. Pedicure bath–associated infection with M. fortuitum is more likely if skin abrasion (e.g., during leg shaving) has occurred just before the pedicure. Outbreaks of skin infection are often caused by rapidly growing NTM (especially M. abscessus, M. fortuitum, and M. chelonae) acquired via skin contamination from surgical instruments (especially in cosmetic surgery), injections, and other procedures. These infec­ tions are typically accompanied by painful, erythematous, draining subcutaneous nodules, usually without associated fever or systemic symptoms. M. marinum lives in many water sources and can be acquired from fish tanks, swimming pools, barnacles, and fish scales. This organ­ ism typically causes papules or ulcers (“fish-tank granuloma”), but the infection can progress to tendinitis with significant impairment of manual dexterity. Lesions appear days to weeks after inoculation of organisms by a typically minor trauma (e.g., incurred during the cleaning of boats or the handling of fish). Tender nodules due to

M. marinum can advance up the arm in a pattern also seen with Sporothrix schenckii (sporotricoid spread). The typical carpal tendon involvement may be the first presenting manifestation and may lead to surgical exploration or steroid injection. The index of suspicion for M. marinum infections must be high to ensure that proper specimens obtained during procedures are sent for culture. M. ulcerans, another waterborne skin pathogen, is found mainly in the tropics, especially in tropical areas of Africa. Infection follows skin trauma or insect bites that allow admission to contaminated water. The skin lesions are typically painless, clean ulcers that slough and can cause osteomyelitis. The toxin mycolactone accounts for the dimin­ ished host inflammatory response and the painless ulcerations. ■ ■DIAGNOSIS NTM can be detected on acid-fast or fluorochrome smears of sputum or other body fluids. When the organism burden is high, the organ­ isms may appear as gram-positive beaded rods, but this finding is unreliable. (In contrast, nocardiae may appear as gram-positive and beaded but filamentous bacteria.) Again, the requisite and most sensi­ tive step in the diagnosis of any mycobacterial disease is to think of including it in the differential. In almost all laboratories, mycobacte­ rial sample processing, staining, and culture are conducted separately from routine bacteriologic tests; thus, many infections go undiagnosed because of physician failure to request the appropriate test. In addition, mycobacteria usually require separate blood culture media. NTM are broadly differentiated into rapidly growing (<7 days) and slowly growing (≥7 days) forms. Because M. tuberculosis typically takes ≥2 weeks to grow, many laboratories refuse to consider culture results final until 6 weeks have elapsed. Newer techniques using liquid culture media permit more rapid isolation of mycobacteria from specimens than is possible with traditional media. Species more readily detected with incubation at 30°C include M. marinum, M. haemophilum, and

M. ulcerans. M. haemophilum prefers iron supplementation or blood, whereas M. genavense requires supplemented medium with the addi­ tive mycobactin J. Bacterial formation of pigment in light conditions

(photochromogenicity) or dark conditions (scotochromogenicity) or a lack of bacterial pigment formation (nonchromogenicity) was his­ torically used to help categorize NTM. In contrast to NTM colonies,

M. tuberculosis colonies are beige, rough, dry, and flat. Current iden­ tification schemes reliably use biochemical, nucleic acid, or cell wall composition, as assessed by high-performance liquid chromatography or mass spectrometry, for speciation. With the remarkable decline in U.S. cases of tuberculosis over recent decades, NTM have become the mycobacteria most commonly isolated from humans in North America. However, not all isolations of NTM, especially from the lung, reflect pathology and require treatment. Whereas identification of an organism in a blood or organ biopsy specimen in a compatible clinical setting is considered diagnostic, the American Thoracic Society rec­ ommends that pulmonary infection due to NTM be diagnosed only when disease is clearly demonstrable—i.e., in an appropriate clinical and radiographic setting (nodules, bronchiectasis, cavities) and with repeated isolation of NTM from expectorated sputum or recovery of NTM from bronchoscopy or biopsy specimens. Given the large num­ ber of species of NTM and the importance of accurate diagnosis for the implementation of proper therapy, identification of these organisms is ideally taken to the species level. The purified protein derivative (PPD) of tuberculin is delivered intradermally to evoke a memory T-cell response to mycobacterial antigens. This test is variously referred to as the PPD test, the tubercu­ lin skin test, and the Mantoux test, among other designations. Unfor­ tunately, the cutaneous immune response to these tuberculosis-derived filtrate proteins does not differentiate well between infection with some NTM and that with M. tuberculosis. Because intermediate reactions (~10 mm) to PPD in latent tuberculosis and nontuberculous myco­ bacterial infections can overlap significantly, the progressive decline in active tuberculosis in the United States means that NTM probably account for increasing proportions of PPD reactivity. In addition, bac­ ille Calmette-Guérin (BCG) can cause some degree of cross-reactivity in PPD testing, posing problems of interpretation for patients who have received BCG vaccine. Assays to measure the elaboration of IFN-γ in response to the relatively tuberculosis-specific proteins ESAT6 and CFP10 form the basis for IFN-γ-release assays (IGRAs). These assays can be performed with whole blood or on membranes. It is important to note that M. marinum, M. kansasii, and M. szulgai also have ESAT6 and CFP10 and may cause false-positive reactions in IGRAs. Despite cross-reactivity with NTM, large PPD reactions (>15 mm) most com­ monly signify tuberculosis. Conversely, in the setting of anti-IFNγ autoantibodies, the IGRA test is indeterminate (failure of IFNγ detec­ tion in response to specific antigens and mitogens, due to neutralizing anti-IFNγ autoantibodies). Isolation of NTM from blood specimens is clear evidence of disease. Whereas rapidly growing mycobacteria may proliferate in routine blood culture media, slow-growing NTM typically do not; therefore, it is imperative to suspect the diagnosis and to use the correct bottles for cultures. Isolation of NTM from a biopsy specimen constitutes strong evidence for infection, but cases of laboratory contamination do occur. Identification of organisms on stained sections of biopsy material confirms the authenticity of the culture. Certain NTM require lower incubation temperatures (M. genavense) or special additives (M. haemophilum) for growth. Some NTM (e.g., M. tilburgii) remain noncultivable but can be identified molecularly in clinical samples. The radiographic appearance of nontuberculous mycobacterial dis­ ease in the lung depends on the underlying disease, the severity of the infection, and the imaging modality used. The advent and increase in the use of computed tomography (CT) scanning has allowed the iden­ tification of characteristic changes that are highly consistent with non­ tuberculous mycobacterial infection, such as the “tree-in-bud” pattern of bronchiolar inflammation (Fig. 185-2). Involvement of the lingual and right-middle lobes is commonly seen on chest CT but is difficult to appreciate on plain film. Severe bronchiectasis and cavity formation are common in more advanced disease. Isolation of NTM from respiratory samples can be confusing.

M. gordonae is often recovered from respiratory samples but is not usually seen on smear and is almost never a pathogen. Patients with

FIGURE 185-2  Chest computed tomography of a patient with pulmonary Mycobacterium avium complex infection. Arrows indicate the “tree-in-bud” pattern of bronchiolar inflammation (peripheral right lung) and bronchiectasis (central right and left lungs). bronchiectasis occasionally have NTM recovered from sputum culture with a negative smear. The American Thoracic Society has developed guidelines for the diagnosis of infection with MAC, M. abscessus, and M. kansasii. A positive diagnosis requires the growth of NTM from two of three sputum samples, regardless of smear findings; a positive bronchoscopic alveolar sample, regardless of smear findings; or a pul­ monary parenchyma biopsy sample with granulomatous inflammation or mycobacteria found on section and NTM found on culture. These guidelines probably apply to other NTM as well. CHAPTER 185 Although many laboratories use DNA probes to identify M. tuber­ culosis, MAC, M. gordonae, and M. kansasii, speciation of NTM helps determine the antimycobacterial therapy to be used. Only testing of MAC organisms for susceptibility to clarithromycin and of M. kansasii for susceptibility to rifampin is indicated; few data support other in vitro susceptibility tests, attractive though they appear. MAC isolates that have not been exposed to macrolides are almost always suscepti­ ble. NTM that have persisted beyond a course of antimicrobial therapy are often tested for antibiotic susceptibility, but the value and meaning of these tests are undetermined. Nontuberculous Mycobacterial Infections ■ ■PREVENTION Prophylaxis of MAC disease in patients infected with HIV is started when the CD4+ T lymphocyte count falls to <50/μL. Azithromycin (1200 mg weekly), clarithromycin (1000 mg daily), and rifabutin (300 mg daily) are effective. Macrolide prophylaxis in immunodefi­ cient patients who are susceptible to NTM (e.g., those with defects in the IFN-γ/IL-12 axis) has not been prospectively validated but seems prudent. TREATMENT Nontuberculous Mycobacteria NTM cause chronic infections that evolve relatively slowly over a period of weeks to years. Therefore, it is rarely necessary to initi­ ate treatment on an emergent basis before the diagnosis is clear and the infecting species is known. Treatment of NTM is complex, often poorly tolerated, and potentially toxic. Just as in tuberculosis, inadequate single-drug therapy is almost always associated with the emergence of antimicrobial resistance and relapse. MAC infection often requires multidrug therapy, the founda­ tion of which is a macrolide (clarithromycin or azithromycin), ethambutol, and a rifamycin (rifampin or rifabutin). For dis­ seminated nontuberculous mycobacterial disease in HIV-infected patients, the use of rifamycins poses special problems—i.e., rifa­ mycin interactions with protease inhibitors. For pulmonary MAC disease, thrice-weekly administration of a macrolide, a rifamycin,

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186 Antimycobacterial Agents

and ethambutol has been successful. Therapy is prolonged, gen­ erally continuing for 12 months after culture conversion; typically, a course lasts for at least 18 months. Other drugs with activity against MAC organisms include IV and aerosolized aminogly­ cosides, fluoroquinolones, and clofazimine. In elderly patients, rifabutin can exert significant toxicity. However, with only mod­ est efforts, most antimycobacterial regimens are well tolerated by most patients. Resection of cavitary lesions or severely bronchi­ ectatic segments has been advocated for some patients, especially those with macrolide-resistant infections. The success of therapy for pulmonary MAC infections depends on whether disease is nodular or cavitary and on whether it is early or advanced, rang­ ing from 20 to 80%.

M. kansasii lung disease is similar to tuberculosis in many ways and is also effectively treated with isoniazid (300 mg/d), rifampin (600 mg/d), and ethambutol (15 mg/kg per day). Other drugs with very high-level activity against M. kansasii include macrolides, fluoroquinolones, and aminoglycosides. Treatment should con­ tinue until cultures have been negative for at least 1 year. In most instances, M. kansasii infection is easily cured. Bulky, severe, necro­ tizing M. kansasii lymphadenopathy, especially in the mediastinum, is strongly associated with GATA2 deficiency. Rapidly growing mycobacteria pose special therapeutic prob­ lems. Extrapulmonary disease in an immunocompetent host is usually due to inoculation (e.g., via surgery, injections, or trauma) or to line infection and is often treated successfully with a macrolide and another drug (with the choice based on in vitro susceptibility), along with removal of the offending focus. In contrast, pulmonary disease, especially that caused by M. abscessus, is extremely dif­ ficult to cure. Repeated courses of treatment are usually effective in reducing the infectious burden and symptoms. Therapy generally includes a macrolide along with an IV-administered agent such as amikacin, a carbapenem, cefoxitin, or tigecycline. Other oral agents (used according to in vitro susceptibility testing and tolerance) include fluoroquinolones, doxycycline, linezolid, and the newer tetracycline family drugs, omadacycline and eravacycline. Because nontuberculous mycobacterial infections are chronic, care must be taken in the long-term use of drugs with neurotoxicities, such as linezolid and ethambutol. Prophylactic pyridoxine has been sug­ gested in these cases. Durations of therapy for M. abscessus lung disease are difficult to predict because so many cases are chronic and require intermittent therapy. Expert consultation and manage­ ment are strongly recommended. PART 5 Infectious Diseases Once recognized, M. marinum infection is highly responsive to antimicrobial therapy and is cured relatively easily with any combination of a macrolide, ethambutol, and a rifamycin. Therapy should be continued for 1–2 months after clinical resolution of iso­ lated soft-tissue disease; tendon and bone involvement may require longer courses in light of clinical evolution. Other drugs with activity against M. marinum include sulfonamides, trimethoprimsulfamethoxazole, doxycycline, and minocycline. Treatment of the other NTM is less well defined, but macrolides and aminoglycosides are usually effective, with other agents added as indicated. Expert consultation is strongly encouraged for difficult or unusual infections due to NTM. ■ ■PROGNOSIS The outcomes of nontuberculous mycobacterial infections are closely tied to the underlying condition (e.g., IFN-γ/IL-12 pathway defect, cystic fibrosis) and can range from recovery to death. With no or inad­ equate treatment, symptoms and signs can be debilitating, including persistent cough, fever, anorexia, and severe lung destruction. With treatment, patients typically regain strength and energy. The optimal duration of therapy when NTM persist in sputum is unknown, but treatment in this situation can be prolonged. In general, for severe underlying immunodeficiencies, hematopoietic stem cell transplanta­ tion is recommended and may be helpful in the resolution of severe mycobacterial disease.

■ ■GLOBAL CONSIDERATIONS In many countries, pulmonary tuberculosis is diagnosed by smear alone, which is also the method used for monitoring of response and relapse. However, examination of mycobacteria from the affected “relapsed” patients shows that a significant proportion of isolates are actually NTM. Overall, as rates of tuberculosis decline, the proportion of positive smears caused by NTM will increase. Advances in specia­ tion will distinguish tuberculosis from nontuberculous mycobacterial infections and thereby affect rates of assumed relapse and resistance, leading to more targeted and appropriate therapy. ■ ■FURTHER READING Blakney RA et al: Incidence of nontuberculous mycobacterial pul­ monary infection, by ethnic group, Hawaii, USA, 2005-2019. Emerg Infect Dis 28:1543, 2022. Daley CL et al: Treatment of nontuberculous mycobacterial pulmo­ nary disease: An official ATS/ERS/ESCMID/IDSA Clinical Practice Guideline. Clin Infect Dis 71:905, 2020. Holland SM et al: Case 28-2017. A 13-month-old girl with pneumo­ nia and a 33-year-old woman with hip pain. N Engl J Med 377:1077, 2017. Hong GH et al: Natural history and evolution of anti-interferon-γ autoantibody-associated immunodeficiency syndrome in Thailand and the United States. Clin Infect 71:53, 2020. Lange C et al: Consensus management recommendations for less common non-tuberculous mycobacterial pulmonary diseases. Lancet Infect Dis 22:e178, 2022. Marras TK et al: Relative risk of all-cause mortality in patients with nontuberculous mycobacterial lung disease in a US managed care population. Respir Med 145:80, 2018. Marshall JE et al: Nontuberculous mycobacteria testing and culture positivity in the United States. BMC Infect Dis 24:288, 2024. Prevots DR et al: Global epidemiology of nontuberculous mycobacte­ rial pulmonary disease: A review. Clin Chest Med 44:675, 2023. Spinner MA et al: GATA2 deficiency: A protean disorder of hemato­ poiesis, lymphatics, and immunity. Blood 123:809, 2014. Szymanski EP et al: Pulmonary nontuberculous mycobacterial infec­ tion. A multisystem, multigenic disease. Am J Respir Crit Care Med 192:618, 2015. Divya Reddy, Sebastian G. Kurz,

Max R. O’Donnell

Antimycobacterial Agents Agents used for the treatment of mycobacterial infections, includ­ ing tuberculosis (TB), leprosy, and infections due to nontuberculous mycobacteria (NTM), are administered in multiple-drug regimens for prolonged courses. Currently, >180 species of mycobacteria have been identified, the majority of which do not cause disease in humans. While the overall incidence of disease caused by Mycobacterium tuberculosis has been declining, there has been a recent increase in incidence in the context of the COVID-19 pandemic, and TB remains a leading cause of morbidity and mortality in low- and middle-income countries—especially in sub-Saharan Africa where TB/HIV co-infection is common. Wellorganized infrastructure for early diagnosis, treatment of TB infection and disease, and development of effective drug regimens and vaccines remain vital to the global strategies for TB control (Chaps. 183 and 485). Infections with NTM have gained in clinical prominence in the United States and other developed countries. These largely environ­ mental organisms often establish infection in immunocompromised patients or in persons with structural lung disease.

TABLE 186-1  Regimens for the Treatment of Latent Tuberculosis Infection in Adults REGIMEN SCHEDULE DURATION COMMENTS Isoniazid plus rifapentine 900 mg (15 mg/kg) weekly plus 900 mg (for weight >50 kg) weekly 3 months Directly observed therapy is recommended for once-weekly treatment in HIV-positive and -negative individuals. This regimen may be supplemented with pyridoxine (25–50 mg/d). Rifampin 600 mg/d (10 mg/kg) 4 months Recommended in HIV-negative individuals and in children. Data on effectiveness in HIV-positive patients are unavailable. Isoniazid plus rifampin 300 mg/d (5 mg/kg) plus 600 mg/d (10 mg/kg) 3 months Risk of hepatotoxicity may be higher with the combination regimen compared to that of the individual drugs. Isoniazid 300 mg/d (5 mg/kg) Alternative: 900 mg twice weekly (15 mg/kg) 6–9 months (6 months acceptable) Supplement with pyridoxine (25–50 mg daily) 6 months’ duration strongly recommended for HIV-negative patients and conditional for

HIV-positive patients. 9 months may be more effective but with higher risk of hepatic toxicity.

Twice-weekly regimens require directly observed therapy. Source: TR Sterling et al: Guidelines for the treatment of latent tuberculosis infection: Recommendations from the National Tuberculosis Controllers Association and CDC, 2020. MMWR Recomm Rep 69:1, 2020. TUBERCULOSIS ■ ■GENERAL PRINCIPLES The earliest recorded human case of TB dates back 9000 years. Early treatment modalities, such as bloodletting, were replaced by the sanatorium movement in the late nineteenth century, which focused on fresh air, nutrition, and bedrest to treat consumptive patients and came with the benefit of isolating infected individuals. The isolation of streptomycin from Streptomyces griseus in 1943 launched the era of antibiotic treatment for TB. Over subsequent decades, the discovery of additional agents and the use of multiple-drug regimens allowed progressive shortening of the treatment course from years to as little as 6 months for drug-susceptible TB. Latent TB infection (LTBI) and active TB disease are diagnosed by history, physical examination, radiographic imaging, tuberculin skin test, interferon-γ release assays, acid-fast staining, mycobacterial cultures, and/or new molecular diagnostics. LTBI is treated with isoniazid plus rifapentine (weekly for 3 months), rifampin (daily for 4 months), isoniazid plus rifampin (daily for 3 months), or isoniazid (optimally daily or twice weekly for 6−9 months) (Table 186-1). The 3-month, weekly regimen of iso­ niazid with rifapentine is currently the regimen of choice in children

2 years of age and in all adults including HIV-positive individuals. The regimen is not recommended for pregnant women and for persons with hypersensitivity reactions to isoniazid or rifampin. Shorter duration rifamycin-based regimens (rifampin alone for 4 months or for 3 months in combination with isoniazid) are currently preferred for the treatment of LTBI over isoniazid for 6−9 months in adults and children due to their effectiveness, safety, and tolerability. Caution is advised in HIV-positive individuals due to potential for drug interactions, lack of definitive data on effectiveness, and the possibility of subclinical TB disease that could facilitate the development of rifampin resistance. TABLE 186-2  Simplified Approach to Treatment of Active Tuberculosis (TB) in Adults CULTURE RESULTS INTENSIVE PHASE CONTINUATION PHASE EXTENSION OF TOTAL TREATMENT Culture-positive, drug-susceptible HRZE for 2 months, dailya or 3 times per week (with dose adjustment) HR for 4 months, daily or 5 days per week or HR for 4 months, 3 times per weekb

(with dose adjustment) Culture-negative HRZE for 2 months HR for 2 months, daily or 2 or 3 times per weekd Continuation phase extended to 4 months if the patient is infected with HIV Extrapulmonary, drug-susceptible HRZE for 2 months HR for 4–7 months, daily or 5 days per weeke Continuation phase extended to 10 months in TB meningitis; 7 months recommended by some authorities for bone/joint TB aDaily treatment is preferred; however, thrice-weekly therapy in the intensive phase (with or without an initial 2 weeks of daily therapy) may be considered in patients who are not infected with HIV and are at low risk of relapse (i.e., in pulmonary tuberculosis caused by drug-susceptible organisms that, at the start of treatment, is noncavitary and/or smear negative). bUse regimen with caution in HIV patients and/or those with cavitary disease, as missed doses can lead to treatment failure, relapse, and acquired drug resistance. cCulture conversion is prolonged if it occurs beyond 2 months. dTwice-weekly treatment regimens are not recommended in patients infected with HIV and those with cavitary pulmonary disease suspected to be TB. eStandard daily 6-month TB treatment regimen is considered to be adequate for most forms of extrapulmonary TB, including miliary TB. For TB meningitis, the addition of glucocorticoids is recommended. Abbreviations: E, ethambutol; H, isoniazid; R, rifampin; Z, pyrazinamide. Sources: Official American Thoracic Society/Centers for Disease Control and Prevention/Infectious Diseases Society of America: Clinical practice guidelines: Treatment of drug-susceptible tuberculosis. Clin Infect Dis 63:e147, 2016.

Completions rates of a self-administered, once-weekly regimen of isoniazid plus rifapentine for 3 months with monthly monitoring were found to be noninferior to those seen with directly observed therapy (DOT) in the United States, and thus, this regimen is considered an acceptable strategy for treating LTBI in countries with a focus on sec­ ondary prevention of TB disease. Recently, a 1-month daily regimen of rifapentine and isoniazid in HIV-positive individuals was found to be noninferior to 9 months of isoniazid; this regimen is included in the 2020 World Health Organization (WHO) LTBI treatment guidelines. For active or suspected TB disease, clinical factors, including HIV co-infection, symptom duration, radiographic appearance, and public health concerns about TB transmission, drive diagnostic testing and treatment initiation. Confirmation of active TB relies on detection of M. tuberculosis via culture or molecular testing. A combination of drugs is used for the treatment of TB disease (Table 186-2). For drugsusceptible disease, a standardized regimen is used with an intensive phase consisting of four drugs—isoniazid (H), rifampin (R), pyrazin­ amide (Z), and ethambutol (E)—given for 2 months, which is followed by a continuation phase of isoniazid and rifampin for 4 months, for a total treatment duration of 6 months. U.S. guidelines recommend extension of the continuation phase to 7 months (for a total treatment duration of 9 months) for patients with cavitary disease; if the 2-month course of pyrazinamide is not completed; or if sputum cultures remain positive beyond 2 months of treatment (delayed culture conversion), which also warrants evaluation for development of drug resistance. CHAPTER 186 Antimycobacterial Agents In 2020, a large multinational randomized trial showed that a 4-month regimen composed of daily rifapentine, isoniazid, pyrazin­ amide, and moxifloxacin for 8 weeks, followed by rifapentine, isonia­ zid, and moxifloxacin for 9 weeks, was noninferior to the traditional 6-month HRZE regimen. Patients with HIV and a CD4 count >100 were included. This regimen now has conditional recommendation by Continuation phase extended to 7 months if 2 months of Z is not completed, if the patient is infected with HIV and is not receiving antiretroviral therapy, or if culture conversion is prolonged and/or cavitation is evident on chest radiography (U.S. guidelines)c

the WHO and U.S. guidelines. Remaining concerns are higher daily pill burden, potential side effects from prolonged use of moxifloxacin, and need for fluoroquinolone resistance testing in areas where resistance is prevalent.

Treatment of TB in patients co-infected with HIV poses signifi­ cant challenges, but some progress is being made. To improve sur­ vival, current recommendations include initiation of antiretroviral therapy (ART) in HIV patients co-infected with M. tuberculosis within

2 weeks of the initiation of treatment for TB (except TB meningitis) if the CD4+ T-cell count is ≤50/μL and by 8–12 weeks of TB treatment initiation if the CD4+ T-cell count is ≥50/μL. Interactions of rifampin with protease inhibitors or nonnucleoside reverse transcriptase inhibi­ tors can be significant and require close monitoring and dose adjust­ ments. Reassuringly, a recent study comparing the safety and efficacy of rifampin for 4 months in patients with LTBI showed that it was as effective as isoniazid for 9 months and was also well tolerated and safe for treatment in persons living with HIV. Rifabutin is an alternative drug of choice in HIV patients co-infected with M. tuberculosis, as it is a less potent cytochrome P3A inhibitor than rifampin. The TB immune reconstitution inflammatory syndrome (IRIS) may appear as early as 1 week after initiation of ART and manifests as paradoxical worsen­ ing or unmasking of existing TB infection. Conservative management consists of continued administration of ART and TB medications. However, severe or debilitating IRIS has been treated in reported case series with varying doses of glucocorticoids. A randomized, doubleblind, placebo-controlled trial showed that a 4-week course of pred­ nisone significantly reduced the need for hospitalization and hastened symptom improvement and quality of life in TB IRIS. Intermittent antimycobacterial therapy in patients infected with HIV and M. tuber­ culosis has been associated with low plasma levels of several key TB PART 5 Infectious Diseases TABLE 186-3  Monitoring and Clinical Management of Tuberculosis (TB) Treatment in Adultsa DRUG ASSESSMENT MANAGEMENT LTBI Treatment With hepatic risk factorsb, check ALT and bilirubin at baseline. If ALT is ≥3× ULN or total bilirubin is >2× ULN, defer treatment and reevaluate. Isoniazid Determine whether hepatic risk factors are present. If so, obtain baseline and periodic ALT and bilirubin values If ALT is 5× ULN (or 3× ULN with symptoms)c or if bilirubin reaches jaundice levels (usually >2× ULN), interrupt treatment. With normalization, consider an alternative agent. Rifampin Same as above Same as above TB Treatment Check ALT, bilirubin, platelets, creatinine, and hepatitis panel for all patients at baseline. If hepatic risk factors are present, check ALT and bilirubin monthly. Isoniazid If ALT is >5× ULN (or >3× ULN with hepatitis symptoms)c Obtain history of alcohol consumption and concomitant drug use. In most instances, discontinue H, Z, R, and other hepatotoxic drugs. Consider alternative agents. Obtain viral hepatitis serologies. Rechallenge: With normalization of liver enzymes, R and H may be sequentially reintroduced. With no recurrence of hepatotoxicity, Z is not resumed in many cases. Alternative rechallenge protocols have been used. Rifampin If primary elevation is in bilirubin and alkaline phosphatase, most likely due to rifampin Discontinue R if total bilirubin reaches jaundice levels (usually >2× ULN). May try to reintroduce; if not tolerated, may substitute Q. Ethambutol Decrease in visual acuity or color vision on monthly screening Discontinue ethambutol and repeat ocular examination. Peripheral neuropathy may be a precursor of ocular toxicity; if it occurs, consider repeat ocular examination. Pyrazinamide If ALT is >5× ULN (or >3× ULN with symptoms)c Same as for H. Fluoroquinolone, bedaquiline, delamanid QTc prolongation is a concern and should be monitored, especially if drugs are used in combination Asymptomatic QTc prolongation should prompt consideration of stopping known QT-prolonging drugs and/or close monitoring, depending on the clinical situation and degree of prolongation. Symptomatic QTc prolongation (e.g., palpitations or arrhythmias) should prompt discontinuation of drugs. Linezolid Visual impairment; monitor for peripheral neuropathy and bone marrow suppression including anemia, thrombocytopenia, and leukopenia Discontinue linezolid if visual toxicity develops. Rechallenge after complete resolution, especially with a lower dose, is an option. Stop if peripheral neuropathy or bone marrow suppression develops. aAll regimens require monthly clinical monitoring. bHepatic risk factors: chronic alcohol use, viral hepatitis, preexisting liver disease, pregnancy or ≤3 months postpartum, hepatotoxic medications. cRelevant manifestations include nausea, vomiting, abdominal pain, jaundice, or unexplained fatigue. Abbreviations: ALT, alanine aminotransferase; H, isoniazid; LTBI, latent tuberculosis infection; Q, fluoroquinolone; QTc, corrected QT interval; R, rifampin; ULN, upper limit of normal; Z, pyrazinamide. Sources: JJ Saukkonen et al: An official ATS statement: Hepatotoxicity of antituberculosis therapy. Am J Respir Crit Care Med 174:935, 2006; American Thoracic Society/ Centers for Disease Control and Prevention/Infectious Diseases Society of America: Treatment of tuberculosis. Am J Respir Crit Care Med 167:603, 2003; WHO consolidated guidelines on drug-resistant tuberculosis treatment. Geneva: World Health Organization; 2019. License: CC BY-NC-SA 3.0 IGO.

drugs and with higher rates of treatment failure or relapse; therefore, intermittent twice-weekly therapy for TB in HIV-co-infected individu­ als is not recommended. Adherence to medications is critical in achieving a cure with anti­ mycobacterial therapy. In addition to DOT by trained staff, either in the clinic or at home, case management interventions such as patient education/counseling, field/home visits, and patient reminders are also recommended to improve treatment adherence. Use of mobile health technologies, including video DOT, text messaging, and next-generation electronic pillboxes, shows promise in promoting TB adherence. In drug-susceptible TB, monthly dispensing of TB medications is also advised for all patients to allow essential clinical monitoring for hepa­ totoxicity due to these medications. Clinical monitoring includes at least monthly assessment for symptoms (nausea, vomiting, abdominal discomfort, and unexplained fatigue) and signs (jaundice, dark urine, light stools, diffuse pruritus) of hepatotoxicity, although the latter represent comparatively late manifestations (Table 186-3). The pres­ ence of such symptoms and signs mandates provisional discontinua­ tion of potentially hepatotoxic agents; discontinuation at the onset of hepatitis symptoms reduces the risk of progression to fatal hepatitis. Although biochemical monitoring is not routinely recommended, baseline assessment of liver function is recommended in adults includ­ ing testing of at least serum alanine aminotransferase (ALT) and total bilirubin levels (Table 186-3). (See Chap. 183 for further details.) For patients with active TB, monthly mycobacterial cultures of sputum are recommended until it is certain that the organisms have been cleared and the patient has responded to therapy or until no sputum is avail­ able for culture. If significant clinical improvement does not occur or the patient’s condition deteriorates over the course of therapy, possibilities include

treatment failure due to incomplete adherence, poor medication absorption, or the development of resistance. For patients co-infected with HIV and M. tuberculosis, IRIS, which is a diagnosis of exclusion, should also be a consideration. Drug susceptibility testing should be repeated at this point. If resistance is documented or strongly sus­ pected, at least two efficacious drugs to which the isolate is susceptible or which the patient has not already taken should be added to the therapeutic regimen. Multidrug-resistant tuberculosis (MDR-TB) is defined as disease caused by a strain of M. tuberculosis that is resistant to both isoniazid and rifampin—the most efficacious of the first-line TB drugs. The risk of MDR-TB is elevated in patients presenting from geographic areas in which ≥5% of incident cases are MDR-TB and in patients previously treated for TB. Treatment regimens for MDR-TB are rapidly evolving, and in 2019, the WHO issued a new classification of second-line agents to treat drug-resistant disease (Table 183-4). New 2022 WHO recom­ mendations emphasize an all-oral bedaquiline-containing regimen with the goal to limit treatment duration to 6 months compared to conventional durations of 9 months or longer (Table 186-4). Results from several recent large clinical trials have formed the basis of these recommendations. The “Bangladesh regimen” was the first shortcourse MDR-TB regimen systematically studied in the STREAM-1 trial and was able to reduce treatment duration to 9−12 months with favorable outcomes in up to 90% of patients. It consists of a seven-drug intensive phase (kanamycin, prothionamide, isoniazid, fluoroquino­ lone, ethambutol, pyrazinamide, and clofazimine) and a four-drug continuation phase (fluoroquinolone, ethambutol, pyrazinamide, and clofazimine). In 2018, a large meta-analysis, which pooled individual data from >12,000 patients enrolled in 50 trials, assessed the role of individual drugs to treat MDR-TB. This analysis showed an association of significantly better treatment outcomes with the use of linezolid, bedaquiline, clofazimine, carbapenems, and later generation fluoro­ quinolones and worse outcomes with kanamycin and capreomycin in these patients. As a result of this analysis, oral drug combinations are now prioritized, while several traditional second-line drugs, including kanamycin and capreomycin, are no longer recommended. The shift toward all-oral regimens of shortened duration has been made possible by the introduction of novel drugs, most prominently bedaquiline and pretomanid, as well as the repurposing of existing agents for MDR-TB treatment (e.g., linezolid, clofazimine). A further TABLE 186-4  Simplified Approach to Treatment of Drug-Resistant Tuberculosis (TB) in Adultsa CULTURE RESULTS INTENSIVE PHASE CONTINUATION PHASE EXTENSION OF TOTAL TREATMENT Resistant to H Lfx RZEb for 6 months … Prolonged culture conversion and/or evidence of cavitation on chest radiography. Resistant to HR (MDR)c   Bdq, Pa, Lz, Mfx for 6 months (may drop Mfx if documented Q resistance) BPaLM Regimend WHO short-course regimen (9-month all-oral regimen)e Bdq plus Lfx or Mfx, Eto, E, Z, Hh, Cfz for 4–6 months At least four effective second-line agents, including all three group A and at least one group B; add group C if intolerant to A or B drugs for 5–7 months WHO extended regimenf aDrug-resistant TB treatment regimens should be constructed and care provided in close consultation with experienced drug-resistant TB clinicians. Surgical management should also be considered in appropriate cases. bProlonged pyrazinamide duration may be associated with increased risk of liver toxicity. cMonoresistance to R is rare and should be treated as MDR. The BPaLM regimen is now the preferred MDR treatment regimen for patients without significant prior exposure to Bdq, Dlm, Pa, or Lz. eThe WHO short-course regimen is recommended for patients not qualifying for BPaLM regimen (availability, medical comorbidities, drug resistance, prior exposure to Pa or Lz) with no prior exposure to second-line drugs and documented fluoroquinolone susceptibility only. Patients with treatment intolerance to antimycobacterial agents, disseminated TB, or pregnancy should be excluded from short-course regimens. fPatients who do not qualify for WHO short-course regimens should be treated using extended MDR-TB treatment regimens. The construction of extended regimens is guided by the requirement for selection of effective antimycobacterial agents, the need to combine sufficient medicines to maximize relapse-free survival, and the need to minimize toxicity. Abbreviations: Bdq, bedaquiline; Cfz, clofazimine; E, ethambutol; Eto, ethionamide; H, isoniazid; Hh, high-dose isoniazid; Lfx, levofloxacin; Pa, Pretomanid; Dlm, Delamanid;

Lz, Linezolid; MDR, multidrug resistant; Mfx, moxifloxacin; Pa, pretomanid; Q, fluoroquinolone; R, rifampin; WHO, World Health Organization; Z, pyrazinamide. Sources: Official American Thoracic Society/Centers for Disease Control and Prevention/Infectious Diseases Society of America: Clinical practice guidelines: Treatment of drug-resistant tuberculosis. Am J Respir Crit Care Med 200:e93, 2019; World Health Organization consolidated guidelines on drug-resistant tuberculosis treatment. WHO 2022.

step toward a shortened all-oral regimen was the Nix-TB study, which showed that a 6-month regimen of bedaquiline, pretomanid, and linezolid (BPaL regimen) for treatment of highly drug-resistant TB was associated with favorable outcomes (absence of clinical or bac­ teriologic treatment failure or relapse within 6 months of treatment completion) in 89% of patients. While a major breakthrough, caution has been raised regarding the higher rate of side effects, mostly due to linezolid, and lack of a control arm. The TB PRACTECAL trial was an open-label, multicenter, randomized, controlled, noninferiority trial that evaluated the safety and efficacy of a 24-week regimen of bedaqui­ line, pretomanid, linezolid, and moxifloxacin (BPaLM) compared to a 9- to 20-month standard-care regimen for the treatment of rifampicinresistant TB. In this study, the 6-month BPaLM regimen was found to be noninferior in both the intention-to-treat and per-protocol analyses, with a lower percentage of serious adverse events compared to the standard-care regimen. High cost, limited access to these new drugs, and the threat of baseline and emergent resistance, especially to bedaquiline, are barriers that need to be addressed to facilitate global adaptation of these new regimens.

■ ■FIRST-LINE ANTITUBERCULOSIS DRUGS The following discussion of individual anti-TB agents focuses on treat­ ment of TB in adults, unless otherwise noted. Several agents are being actively investigated during the current remarkable period of drug development for TB treatment. Isoniazid  Isoniazid is a critical drug for treatment of both TB dis­ ease and LTBI. Isoniazid has excellent bactericidal activity against both intracellular and extracellular, actively dividing M. tuberculosis. This drug is bacteriostatic against slowly dividing organisms. In treatment of LTBI, isoniazid is generally well tolerated, has well-established effi­ cacy, and is inexpensive. In this setting, the drug is taken daily, which is the preferred dosing schedule, or intermittently (i.e., twice weekly) using DOT for 6 months, which has been found to be equivalent to the traditional 9 months in most settings. A weekly isoniazid and rifapen­ tine regimen, administered over 3 months under DOT, has been shown to be noninferior to daily isoniazid given for 9 months and had a higher treatment completion rate than the single-drug regimen. More recent evidence also suggests that completion rates of a self-administered 3-month regimen of weekly isoniazid and rifapentine are noninferior CHAPTER 186 Antimycobacterial Agents     Failed culture conversion at month 4–6, delayed clinical response. Treatment interruption of >7 days is made up by adding on to the total treatment duration. Lfx or Mfx, Cfz, Z, E for 5 months At least 4 drugs for a total of

18–20 months or for 15–17 months after culture conversion

to those seen with DOT in the United States. It is expected that a 1-month daily regimen in combination with rifapentine will be added to new WHO guidelines.

For treatment of TB disease, isoniazid is used in combination with other agents to ensure killing of both actively dividing M. tuberculosis and slowly growing “persister” mycobacteria. Unless the organism is resistant, the standard regimen includes isoniazid, rifampin, ethambu­ tol, and pyrazinamide (Table 186-2). Isoniazid is often given together with 25–50 mg of pyridoxine daily to prevent drug-related peripheral neuropathy. MECHANISM OF ACTION  Isoniazid is a prodrug activated by the mycobacterial KatG catalase-peroxidase; isoniazid is coupled with reduced nicotinamide adenine dinucleotide (NADH). The resulting isonicotinic acyl–NADH complex blocks the mycobacterial ketoenoyl­ reductase known as InhA through binding to its substrate and inhibit­ ing fatty acid synthase and ultimately mycolic acid synthesis. Mycolic acids are essential components of the mycobacterial cell wall. KatG activation of isoniazid also results in the release of free radicals that have antimycobacterial activity, including nitric oxide. The minimal inhibitory concentrations (MICs) of isoniazid for wild-type (untreated) susceptible strains are <0.1 μg/mL for M. tuber­ culosis and 0.5–2 μg/mL for M. kansasii. PHARMACOLOGY  Isoniazid is the hydrazide of isonicotinic acid, a small, water-soluble molecule. The usual adult oral daily dose of

300 mg results in peak serum levels of 3–5 μg/mL within 30 min to 2 h after ingestion—well in excess of the MICs for most susceptible strains of M. tuberculosis. Both oral and IM preparations of isoniazid reach effective levels in the body, although antacids and high-carbohydrate meals may interfere with oral absorption. Isoniazid diffuses well throughout the body, reaching therapeutic concentrations in body cavities and fluids, with concentrations in cerebrospinal fluid (CSF) comparable to those in serum. PART 5 Infectious Diseases Isoniazid is metabolized in the liver via acetylation by N-

acetyltransferase 2 (NAT2) and hydrolysis. Both fast- and slow-acetylation phenotypes occur; patients who are “fast acetylators” may have lower serum levels of isoniazid, whereas “slow acetylators” may have higher levels and experience more toxicity. Satisfactory isoniazid levels are attained in the majority of homozygous fast NAT2 acetylators given a dose of 6 mg/kg and in the majority of homozygous slow acetylators given only 3 mg/kg. Genotyping is increasingly being used to charac­ terize isoniazid-related pharmacogenomic responses. Isoniazid’s interactions with other drugs are due primarily to its inhibition of the cytochrome P450 system. Among the drugs with significant isoniazid interactions are warfarin, carbamazepine, ben­ zodiazepines, acetaminophen, clopidogrel, maraviroc, dronedarone, salmeterol, tamoxifen, eplerenone, and phenytoin. DOSING  The recommended daily dose of isoniazid for the treatment of TB is 5 mg/kg for adults and 10 mg/kg for children (U.S. guidelines recommend 10−15 mg), with a maximal daily dose of 300 mg for both. For intermittent therapy in adults (usually twice per week), the dose is 15 mg/kg, with a maximal daily dose of 900 mg. Isoniazid does not require dosage adjustment in patients with renal disease. When the 12-dose, 3-month weekly LTBI regimen is used, the dose of isoniazid is 15 mg/kg, with a maximal dose of 900 mg, and the drug is co-administered with rifapentine. The novel 1-month regimen uses isoniazid 300 mg in conjunction with rifapentine for people aged >13 years without weight adjustment. RESISTANCE  Although isoniazid, along with rifampin, is the mainstay of TB treatment regimens, ~7% of clinical M. tuberculosis isolates in the United States are resistant. Rates of primary isoniazid resistance among untreated patients are significantly higher in many populations born outside the United States. Five separate pathways for isoniazid resistance have been elucidated. Most strains have amino acid changes in either the catalase-peroxidase gene (katG) or the mycobacterial ketoenoylreductase gene (inhA). Less frequently, alterations in kasA, the gene for an enzyme involved in mycolic acid elongation, and loss of NADH dehydrogenase 2 activity confer isoniazid resistance.

In 20–30% of isoniazid-resistant M. tuberculosis isolates, increased expression of efflux pump genes, such as efpA, mmpL7, mmr, p55, and the Tap-like gene Rv1258c, has been implicated as the underlying mechanism of resistance. ADVERSE EFFECTS  Although isoniazid is generally well tolerated, drug-induced liver injury and peripheral neuropathy are significant adverse effects associated with this agent. Isoniazid may cause asymp­ tomatic transient elevation of aminotransferase levels (often termed hepatic adaptation) in up to 20% of recipients. Other adverse reactions include rash (2%), fever (1.2%), anemia, acne, arthritic symptoms, a systemic lupus erythematosus–like syndrome, optic atrophy, seizures, and psychiatric symptoms. Symptomatic hepatitis occurs in <0.1% of persons treated with isoniazid alone for LTBI, and fulminant hepatitis with hepatic failure occurs in <0.01%. Isoniazid-associated hepatitis is idiosyncratic, but its incidence increases with age, with daily alcohol consumption, and in women who are within 3 months postpartum. In patients who have liver disorders or HIV infection, who are preg­ nant or in the 3-month postpartum period, who have a history of liver disease (e.g., hepatitis B or C, alcoholic hepatitis, or cirrhosis), who use alcohol regularly, who have multiple medical problems, or who have other risk factors for chronic liver disease, the risks and benefits of isoniazid treatment for LTBI should be weighed. If treatment is undertaken, these patients should have serum concentrations of ALT determined at baseline. Routine baseline hepatic ALT testing based solely on an age of >35 years is optional and depends on individual concerns. Monthly biochemical monitoring during isoniazid treat­ ment is indicated for patients whose baseline liver function tests yield abnormal results and for persons at risk for hepatic disease, including the groups just mentioned. Guidelines recommend that isoniazid be discontinued in the presence of hepatitis symptoms or jaundice and an ALT or AST level three times the upper limit of normal or in the absence of symptoms with an ALT or AST level five times the upper limit of normal (Table 186-3). Peripheral neuropathy associated with isoniazid occurs in up to 2% of patients given 5 mg/kg. Isoniazid appears to interfere with pyridox­ ine (vitamin B6) metabolism. The risk of isoniazid-related neurotoxic­ ity is greatest for patients with preexisting disorders that also pose a risk of neuropathy, such as HIV infection; for those with diabetes mellitus, alcohol abuse, or malnutrition; and for those simultaneously receiving other potentially neuropathic medications, such as stavudine. These patients should be given prophylactic pyridoxine (25–50 mg/d). Rifampin  Rifampin is a semisynthetic derivative of Amycolatopsis rifamycinica (formerly known as Streptomyces mediterranei). The most active antimycobacterial agent available, rifampin is the keystone of first-line treatment for TB. Introduced in 1968, this drug eventually permitted dramatic shortening of the TB treatment course. Rifampin has both sterilizing and bactericidal activity against dividing and nondividing M. tuberculosis. The drug is also active against an array of other organisms, including some gram-positive and gram-negative bacteria, Legionella, M. kansasii, and Mycobacterium marinum. MECHANISM OF ACTION  Rifampin exerts both intracellular and extracellular bactericidal activities. Like other rifamycins, rifampin specifically binds to and inhibits mycobacterial DNA-dependent RNA polymerase, blocking RNA synthesis. Susceptible strains of M. tubercu­ losis as well as M. kansasii and M. marinum are inhibited by rifampin concentrations of 1 μg/mL. PHARMACOLOGY  Rifampin is a fat-soluble, complex macrocyclic molecule readily absorbed after oral administration. Serum levels of 10–20 μg/mL are achieved 2.5 h after the usual adult oral dose of 10 mg/kg (given without food). Rifampin has a half-life of 1.5–5 h. The drug distributes well throughout most body tissues, including CSF. Rifampin turns body fluids such as urine, saliva, sputum, and tears a reddish-orange color—an effect that offers a simple means of assessing patients’ adherence to this medication. Rifampin is excreted primarily through the bile and enters the enterohepatic circulation; <30% of a dose is renally excreted. As a potent inducer of the hepatic cytochrome P450 system, rifampin can decrease the half-life of some drugs, such as digoxin,

warfarin, phenytoin, prednisone, cyclosporine, methadone, oral con­ traceptives, clarithromycin, azole antifungal agents, quinidine, anti­ retroviral protease inhibitors, and nonnucleoside reverse transcriptase inhibitors. The Centers for Disease Control and Prevention (CDC) has issued guidelines for the management of drug interactions during treatment of HIV and M. tuberculosis co-infection (www.cdc.gov/tb/). DOSING  The daily dosage of rifampin is 10 mg/kg for adults and 10–20 mg/kg for children, with a maximum of 600 mg/d for both. The drug is given once daily, twice weekly, or three times weekly. No adjustments of dose or frequency are necessary in patients with renal insufficiency. RESISTANCE  Resistance to rifampin in M. tuberculosis, M. leprae, and other organisms is the consequence of spontaneous, mostly missense point mutations in a core region of the bacterial gene coding for the β subunit of RNA polymerase (rpoB). RNA polymerase altered in this manner is no longer subject to inhibition by rifampin. Most rapidly and slowly growing NTM harbor intrinsic resistance to rifampin, for which the mechanism has yet to be determined. ADVERSE EFFECTS  Adverse events associated with rifampin are infrequent and generally mild. Hepatotoxicity due to rifampin alone is uncommon in the absence of preexisting liver disease and often con­ sists of isolated hyperbilirubinemia rather than aminotransferase eleva­ tion. Other adverse reactions include rash, pruritus, gastrointestinal symptoms, and pancytopenia. Rarely, a hypersensitivity reaction may occur with intermittent therapy, manifesting as fever, chills, malaise, rash, and—in some instances—renal and hepatic failure. Pyrazinamide  A nicotinamide analogue, pyrazinamide is an important bactericidal drug used in the initial phase of TB treatment. Its administration for the first 2 months of therapy with rifampin and isoniazid allows treatment duration to be shortened from 9 to 6 months and decreases rates of relapse. MECHANISM OF ACTION  Pyrazinamide’s antimycobacterial activity is essentially limited to M. tuberculosis. The drug is more active against slowly replicating organisms than against actively replicating organ­ isms. Pyrazinamide is a prodrug that is converted by the mycobacte­ rial pyrimidase to the active form, pyrazinoic acid (POA). This agent is active only in acidic environments (pH <6.0), as are found within phagocytes or granulomas. The exact mechanism of action of POA is unclear, but fatty acid synthetase I may be the primary target in M. tuberculosis. Susceptible strains of M. tuberculosis are inhibited by pyra­ zinamide concentrations of 16–50 μg/mL at pH 5.5. PHARMACOLOGY AND DOSING  Pyrazinamide is well absorbed after oral administration, with peak serum concentrations of 20–60 μg/mL at 1–2 h after ingestion of the recommended adult daily dose of 15–30 mg/kg (maximum, 2 g/d). It distributes well to various body compart­ ments, including CSF, and is an important component of treatment for tuberculous meningitis. The serum half-life of the drug is 9–11 h with normal renal and hepatic function. Pyrazinamide is metabolized in the liver to POA, 5-hydroxypyrazinamide, and 5-hydroxy-POA. A high proportion of pyrazinamide and its metabolites (~70%) is excreted in the urine. The dosage must be adjusted according to the level of renal function in patients with reduced creatinine clearance. ADVERSE EFFECTS  At the higher dosages used previously, hepatotox­ icity was seen in as many as 15% of patients treated with pyrazinamide. However, at the currently recommended dosages, hepatotoxicity now occurs less commonly when this drug is administered with isoniazid and rifampin during the treatment of TB. Older age, active liver dis­ ease, HIV infection, and low albumin levels may increase the risk of hepatotoxicity. The use of pyrazinamide with rifampin for the treat­ ment of LTBI is no longer recommended because of unacceptable rates of hepatotoxicity and death in this setting. Hyperuricemia is a common adverse effect of pyrazinamide therapy that usually can be managed conservatively. Clinical gout is rare. Although pyrazinamide is recommended by international TB orga­ nizations for routine use in pregnancy, it is not recommended in the United States because of inadequate teratogenicity data.

RESISTANCE  The basis of pyrazinamide resistance in M. tubercu­ losis is a mutation in the pncA gene coding for pyrazinamidase, the enzyme that converts the prodrug to active POA. Resistance to pyra­ zinamide is associated with loss of pyrazinamidase activity, which pre­ vents conversion of pyrazinamide to POA. Of pyrazinamide-resistant

M. tuberculosis isolates, 72–98% have mutations in pncA. Conventional methods of testing for susceptibility to pyrazinamide may produce both false-negative and false-positive results because the high-acidity environment required for the drug’s activation also inhibits the growth of M. tuberculosis. There is some controversy as to the clinical signifi­ cance of in vitro pyrazinamide resistance.

Ethambutol  Ethambutol is a bacteriostatic antimycobacterial agent first synthesized in 1961. A component of the standard first-line regi­ men, ethambutol provides synergy with the other drugs in the regimen and is generally well tolerated. Susceptible species include M. tubercu­ losis, M. marinum, M. kansasii, and organisms of the Mycobacterium avium complex (MAC); however, among first-line drugs, ethambutol is the least potent against M. tuberculosis. This agent is also used in combination with other agents in the continuation phase of treatment when patients cannot tolerate isoniazid or rifampin or are infected with organisms resistant to either of the latter drugs. MECHANISM OF ACTION  Ethambutol is bacteriostatic against

M. tuberculosis. Its primary mechanism of action is the inhibition of the arabinosyltransferases involved in cell wall synthesis, which prob­ ably inhibits the formation of arabinogalactan and lipoarabinomannan. The MIC of ethambutol for susceptible strains of M. tuberculosis is 0.5–2 μg/mL. CHAPTER 186 PHARMACOLOGY AND DOSING  From a single dose of ethambutol, 75–80% is absorbed within 2–4 h of administration. Serum levels peak at 2–4 μg/mL after the standard adult daily dose of 15 mg/kg. Etham­ butol is well distributed throughout the body except in the CSF; a dos­ age of 25 mg/kg is necessary for attainment of a CSF level half of that in serum. For intermittent therapy, the dosage is 25–35 mg/kg thrice weekly. To prevent toxicity, the dosage must be lowered and the fre­ quency of administration reduced for patients with renal insufficiency. Antimycobacterial Agents ADVERSE EFFECTS  Ethambutol is usually well tolerated and has no significant interactions with other drugs. Optic neuritis, the most serious adverse effect reported, typically presents as reduced visual acuity, central scotoma, and loss of the ability to see green (or, less commonly, red). The cause of this neuritis is unknown, but it may be due to an effect of ethambutol on the amacrine and bipolar cells of the retina. Symptoms typically develop several months after initiation of therapy, but ocular toxicity soon after initiation of ethambutol has been described. The risk of ocular toxicity is dose dependent, with occurrence in 1–5% of patients, and can be increased by renal insuf­ ficiency. The routine use of ethambutol in younger children is not recommended because monitoring for visual complications can be difficult. If drug-resistant TB is suspected, ethambutol can be used for treatment of children. All patients starting therapy with ethambutol should have a baseline test for visual acuity, visual fields, and color vision and should undergo an examination of the optic fundus. Visual acuity and color vision should be monitored monthly or less often as needed. Cessation of ethambutol in response to early symptoms of ocular toxicity usually results in reversal of the deficit within several months. Recovery of all visual function may take up to 1 year. In the elderly and in patients whose symptoms are not recognized early, deficits may be permanent. Some experts think that supplementation with hydroxycobalamin (vitamin B12) is beneficial for patients with ethambutol-related ocular toxicity. Other adverse effects of ethambutol are rare. Peripheral sen­ sory neuropathy occurs in rare instances. RESISTANCE  Ethambutol resistance in M. tuberculosis and NTM is associated primarily with missense mutations in the embB gene that encodes for arabinosyltransferase. Mutations have been found in resis­ tant strains at codon 306 in 50–70% of cases. Mutations at embB306 can cause significantly increased MICs of ethambutol, resulting in clinical resistance.

■ ■OTHER RIFAMYCIN DRUGS

Rifabutin  Rifabutin, a semisynthetic derivative of rifamycin S, inhibits mycobacterial DNA-dependent RNA polymerase. Rifabutin is recommended in place of rifampin for the treatment of TB in HIV-co-infected individuals who are taking protease inhibitors or nonnucleoside reverse transcriptase inhibitors, particularly nevirap­ ine. A study in India showed better TB treatment outcomes in HIVco-infected patients given daily rifabutin plus atazanavir/ritonavir than in those given thrice-weekly rifabutin plus atazanavir/ritonavir. Rifabutin’s effect on hepatic enzyme induction is less pronounced than that of rifampin. Protease inhibitors may cause significant increases in rifabutin levels through inhibition of hepatic metabolism. Rifabutin is more active in vitro than rifampin against MAC organisms and other NTM, but its clinical superiority has not been established. PHARMACOLOGY  Like rifampin, rifabutin is lipophilic and is absorbed rapidly after oral administration, reaching peak serum levels 2–4 h after ingestion. Rifabutin distributes best to tissues, reaching levels 5–10 times higher than those in plasma. Unlike rifampin, rifabutin and its metabolites are partially cleared by the hepatic microsomal system. Rifabutin’s slow clearance results in a mean serum half-life of 45 h— much longer than the 3- to 5-h half-life of rifampin. Clarithromycin (but not azithromycin) and fluconazole appear to increase rifabutin levels by inhibiting hepatic metabolism. ADVERSE EFFECTS  The most common adverse effects of rifabutin treatment are gastrointestinal; other reactions include rash, headache, asthenia, chest pain, myalgia, and insomnia. Less common adverse reactions include fever, chills, a flulike syndrome, anterior uveitis, hepatitis, Clostridium difficile–associated diarrhea, a diffuse polymy­ algia syndrome, and yellow skin discoloration (“pseudo-jaundice”). Laboratory abnormalities include neutropenia, leukopenia, thrombo­ cytopenia, and increased levels of liver enzymes. Rifabutin appears to be better tolerated by the majority (72%) of adult TB patients who have developed rifampin-related adverse effects. Female patients, those coinfected with hepatitis B or hepatitis C, and those with rifampin-related arthralgias, dermatologic reactions, and cholestasis are more likely to develop mild to severe rifabutin-related adverse effects. PART 5 Infectious Diseases RESISTANCE  Similar to rifampin resistance, rifabutin resistance is mediated by mutations in rpoB. Rifapentine  Rifapentine is a semisynthetic cyclopentyl rifamycin, sharing a mechanism of action with rifampin. Rifapentine is lipophilic and has a prolonged half-life that permits weekly or twice-weekly dosing. Therefore, rifapentine is the subject of intensive clinical inves­ tigation aimed at determining optimal dosing and frequency of admin­ istration. Currently, it is an alternative to rifampin in the continuation phase of treatment for noncavitary drug-susceptible pulmonary TB in HIV-seronegative patients who have negative sputum smears at completion of the initial phase of treatment. When administered in these specific circumstances, rifapentine (10 mg/kg, up to 600 mg) is given once weekly with isoniazid. Because of higher rates of relapse, this regimen is not recommended for patients with TB disease and HIV co-infection; moreover, it has not been approved for children <12 years of age. In a phase 2 study, substituting daily rifapentine for rifampin yielded higher rates of sputum sterilization after 2 months of intensive treatment. Higher doses of rifapentine (20 mg/kg vs 10 mg/kg) had better results and were safe and well tolerated. Regimens containing high doses of rifapentine are being evaluated to see whether they can shorten the TB treatment course to <6 months. PHARMACOLOGY  Rifapentine’s absorption is improved when the drug is taken with food. After oral administration, rifapentine reaches peak serum concentrations in 5–6 h and achieves a steady state in

10 days. The half-life of rifapentine and its active metabolite, 25-desacetyl rifapentine, is ~13 h. The administered dose is excreted via the liver (70%). ADVERSE EFFECTS  The adverse effects profile of rifapentine is similar to that of other rifamycins. Rifapentine is teratogenic in animal models and is relatively contraindicated in pregnancy.

RESISTANCE  Rifapentine resistance is mediated by mutations in rpoB. Mutations that cause resistance to rifampin also cause resistance to rifapentine. ■ ■SECOND-LINE ANTITUBERCULOSIS DRUGS Second-line anti-TB agents are indicated for treatment of drug-resistant TB, for patients who are intolerant or allergic to first-line agents, and when first-line supplemental agents are unavailable. According to their usability, they are divided into three WHO groups. Group A  •  FLUOROQUINOLONES  Fluoroquinolones inhibit mycobacterial DNA gyrase and topoisomerase IV, preventing cell repli­ cation and protein synthesis, and are bactericidal. Given their excellent activity, they have been investigated for their potential to shorten the course of treatment for drug-susceptible TB from 6 to 4 months. In contrast to prior trials, a recent large, open-label, randomized controlled trial (TBTC Study 31) yielded promising results for shortening of TB treatment. Patients with drug-susceptible TB disease were randomized to receive either a standard 6-month TB regimen or a 4-month regimen containing rifapentine (8 weeks of once-daily rifapentine, isoniazid, pyrazinamide, and ethambutol followed by 9 weeks of once-daily rifa­ pentine and isoniazid) or a 4-month regimen containing rifapentine and moxifloxacin (8 weeks of once-daily rifapentine, isoniazid, pyrazin­ amide, and moxifloxacin followed by 9 weeks of once-daily rifapentine, isoniazid, and moxifloxacin). The trial demonstrated that a 4-month regimen using daily rifapentine and moxifloxacin (but not the rifapen­ tine-only regimen) was noninferior to the standard 6-month TB treat­ ment regimen using an end point of TB-free survival 12 months after randomization. Combining once-daily rifapentine with moxifloxacin allows for synergistic action on sputum conversion in a compliancefriendly once-daily option. Current recommendations continue to be for a standard 6-month regimen, although it is anticipated that these results will inform future guidelines. Gatifloxacin has fallen out of favor because of significant dysglycemia. Ciprofloxacin and ofloxacin are no longer recommended for the treatment of TB because of poor efficacy. Despite documented resistance to early-generation fluoroquinolones (e.g., ofloxacin and ciprofloxacin), use of a later-generation fluoroquino­ lone in patients with drug-resistant TB has been associated with favor­ able outcomes. Fluoroquinolones are also considered safe alternatives for patients who develop treatment-limiting adverse effects from first-line agents. Levofloxacin and moxifloxacin have both been used effectively in the treatment of MDR-TB. The optimal dose of levofloxacin for this indication is being actively studied, but doses of at least 750 mg are commonly used. High-dose moxifloxacin (800 mg) is recommended for standardized shorter MDR-TB regimens. The fluoroquinolones are well absorbed orally, reach high serum levels, and distribute well into body tissues and fluids. Their absorp­ tion is decreased by co-ingestion with products containing multivalent cations, such as antacids. Adverse effects are relatively infrequent (0.5–10% of patients) and include gastrointestinal intolerance, rashes, dizziness, and headache. Most studies of fluoroquinolone side effects have been based on relatively short-term administration for bacterial infections, but trials have now shown the relative safety and tolerability of fluoroquinolones administered for months during TB treatment in adults. Although the potential to prolong the QTc interval, leading to cardiac arrhythmias, has been a source of concern with fluoroquino­ lones, cessation of treatment due to this adverse effect is rare. Because the benefits may outweigh the risks in treatment of drug-resistant TB, there is increasing interest in the use of fluoroquinolones in children, which has traditionally been avoided because of the risks of tendon rupture and cartilage damage. Multiple courses of empirical fluoroquinolone therapy for presumed community-acquired pneumonia are associated with delayed diagnosis of active pulmonary TB and increased fluoroquinolone resistance in

M. tuberculosis. Mutations in the genes encoding for DNA gyrase (gyrA and gyrB) are implicated in the majority of cases—but not all cases—of clinical resistance to fluoroquinolones. DIARYLQUINOLINES  Bedaquiline (TMC207 or R207910) is a dia­ rylquinoline with a novel mechanism of action: inhibition of the

mycobacterial ATP synthetase proton pump. Bedaquiline is bacteri­ cidal for M. tuberculosis. Resistance has been reported due to point mutations in the atpE gene encoding for subunit c of ATP synthetase. Clinical bedaquiline resistance has also been reported due to non­ target mutations in mmpR or Rv0678 (a negative repressor of the MmpS5–MmpL5 efflux pump) and PepQ (a cytoplasmic peptidase), both of which may cause cross-resistance to clofazimine. Bedaquiline is metabolized by the hepatic cytochrome CYP3A4. Rifampin lowers bedaquiline levels by 50%, and protease inhibitors also interact sig­ nificantly with this drug. Because efavirenz induces CYP3A4, there is concern about lower bedaquiline levels with co-administration. In a study of co-treatment with bedaquiline and efavirenz in healthy volun­ teers, bedaquiline levels were reduced by only 20%; however, in a study modeling chronic co-administration of these two drugs, the reduction in bedaquiline levels was estimated to be 50%, leading many national TB programs to avoid efavirenz co-administration with bedaquiline. The oral bioavailability of bedaquiline appears to be excellent. The dosage is 400 mg/d for the first 2 weeks and then 200 mg thrice weekly typically for 6 months total. The elimination half-life is long (>14 days). A single dose of this drug can inhibit the growth of M. tuberculosis for up to 1 week through a combination of long plasma half-life, high-level tissue penetration, and long tissue half-life. Bedaquiline added to a background regimen improved the 2-month sputum culture–conversion rate in multicenter, randomized, placebo-controlled trials, and these results led to approval by the U.S. Food and Drug Administration (FDA). However, the death rate in one trial was higher in the bedaqui­ line arm than in the control arm (11.4 vs 2.5%); the result was a “black box” warning from the FDA, which also included QT prolongation. Subsequent studies have not found an association with significant mortality. The CDC has made a provisional recommendation for the use of bedaquiline for 24 weeks in adults with laboratory-confirmed pulmonary MDR-TB when no other effective treatment regimen can be provided. Bedaquiline is an integral part of all shorter course, oral MDR treatment regimens endorsed by the WHO. OXAZOLIDINONES  Linezolid is an oxazolidinone used primarily for the treatment of drug-resistant gram-positive bacterial infections. However, this drug is active in vitro against M. tuberculosis and NTM. Several case series have suggested that linezolid may help clear mycobacteria rela­ tively rapidly when included in a regimen for the treatment of complex cases of drug-resistant TB. Linezolid’s mechanism of action is disruption of protein synthesis by binding to the 50S bacterial ribosome. Linezolid has nearly 100% oral bioavailability, with good penetration into tis­ sues and fluids, including CSF. Clinical resistance to linezolid has been reported and is typically associated with mutations in the 23S rRNA and in two ribosomal proteins, L3 (rplC) and L4 (rplD). Adverse effects may include optic and peripheral neuropathy, pancytopenia, and lactic acidosis and are usually associated with higher doses. Linezolid is a weak monoamine oxidase inhibitor and can be associated with the serotonin syndrome when given concomitantly with serotonergic drugs (primarily antidepressants such as selective serotonin reuptake inhibitors). It has been shown that ~80% of patients with MDR-TB can be successfully treated with linezolid-containing, individualized anti-TB regimens based on drug sensitivity testing. Replacement of ethambutol with linezolid for 2–4 weeks during the intensive phase of treatment of drug-susceptible TB is currently being evaluated for possible faster sputum conversion and a shorter treatment regimen. For MDR-TB treatment, linezolid is usually administered at a dose of 600 mg (or less in some cases) once daily, which appears to be effective. A single daily dose is associated with fewer adverse events than twice-a-day dosing. Sutezolid, a modified version of oxazolidinones and protein syn­ thesis inhibitor, is found to have higher early bactericidal activity compared to linezolid and is currently undergoing phase 2A trials. It is currently FDA approved for complex skin infections and appears to have less frequent side effects compared to linezolid; the adverse effects profile of long-term exposure compared with that of linezolid needs further investigation. Group B  •  CLOFAZIMINE  Clofazimine is a fat-soluble rimino­ phenazine dye used primarily in the treatment of leprosy worldwide.

It is currently gaining popularity in the management of drug-resistant TB because of its low cost and its intracellular and extracellular activ­ ity. By increasing reactive oxidant species and causing membrane destabilization, clofazimine may promote killing of antibiotic-tolerant

M. tuberculosis “persister” organisms. In addition to antimicrobial activity, the drug has other pharmacologic activities, such as antiinflammatory, pro-oxidative, and immunopharmacologic properties. Clofazimine has a half-life of ~70 days in humans, and average steadystate concentrations are achieved at ~1 month. Intake with fatty meals can improve its low and variable rates of absorption (45–62%). Com­ mon side effects include gastrointestinal intolerance and reversible orange to brownish discoloration of skin, bodily fluids, and secretions. Dose adjustment may be necessary in patients with severe hepatic impairment. Clofazimine was studied as part of a regimen developed in Bangladesh for potential shortening of the MDR-TB treatment course. A meta-analysis suggested that inclusion of clofazimine in a multidrug regimen for treatment of MDR-TB was associated with a favorable outcome. Newer analogues with improved pharmacokinetics and alternative formulations of clofazimine (liposomal, nanosuspension, inhalational) are being studied.

CYCLOSERINE  Cycloserine is an analogue of the amino acid d-alanine and prevents bacterial cell-wall synthesis. It inhibits the action of enzymes, including alanine racemase, that are involved in the produc­ tion of peptidoglycans. Cycloserine is active against a range of bacteria, including M. tuberculosis. Mechanisms of mycobacterial resistance are not well understood, but overexpression of alanine racemase can confer resistance in Mycobacterium smegmatis. Cycloserine is well absorbed after oral administration and is widely distributed through­ out body fluids, including CSF. The usual adult dosage is 250 mg two or three times per day. Serious potential side effects include seizures and psychosis (with suicide in some cases), peripheral neuropathy, head­ ache, somnolence, and allergic reactions. Drug levels are monitored to achieve optimal dosing and to reduce the risk of adverse effects, especially in patients with renal failure. Cycloserine should be adminis­ tered as DOT only with caution and with support from experienced TB physicians to patients with epilepsy, active alcohol abuse, severe renal insufficiency, or a history of depression or psychosis. CHAPTER 186 Antimycobacterial Agents Group C  •  NITROIMIDAZOLES  The prodrugs delamanid (OPC67683) and pretomanid (PA 824) are novel nitro-dihydro-imidaz­ ooxazole derivatives that are activated by M. tuberculosis–specific flavin-dependent nitroreductases and whose antimycobacterial activity is attributable to inhibition of mycolic acid biosynthesis. Delamanid was shown in a randomized, placebo-controlled, multinational clinical trial to significantly improve the culture conversion rate at 2 months. QT prolongation occurred significantly more often in delamanidtreated patients, but no clinically relevant events were reported. In a subsequent randomized phase 3 trial, there was no significant differ­ ence in 6-month sputum conversion between delamanid and placebo among patients with an optimized background regimen. Currently, it is part of several ongoing clinical trials including combination with beda­ quiline. It is recommended for use in children younger than 6 years with rifampicin-resistant TB. Usual adult dose is 100 mg twice daily. Pretomanid, the second novel agent from this class, has shown promising results in the treatment of drug-resistant TB in combination with bedaquiline. A combination of pretomanid with moxifloxacin and pyrazinamide for treatment of drug-susceptible TB was found to have higher culture conversation rates at 8 weeks compared to HRZE; how­ ever, a subsequent phase 3 study raised concern for higher frequency of potentially fatal hepatotoxicity. It is currently being evaluated in several phase 3 clinical trials in various combinations, including with fluo­ roquinolones and pyrazinamide. Based on the previously mentioned results with the BPaL regimen (Nix-TB study), the FDA has granted approval for specific highly resistant TB cases. Adult treatment dose is 200 mg administered daily. AMOXICILLIN-CLAVULANATE AND CARBAPENEMS  β-Lactam agents are largely ineffective for the treatment of M. tuberculosis because of resistance conferred by a hydrolyzing class A β-lactamase, BlaC.

Carbapenems are poor substrates of BlaC, and clavulanic acid leads to irreversible inhibition. While the use of either amoxicillin-clavulanic acid or carbapenems alone for highly resistant forms of TB has been anecdotally reported with unclear results, the combination of meropenem and clavulanic acid turned out to be highly active in vitro. Recently, the combination was found to have effective early bacteri­ cidal activity, and in a large individual patient data meta-analysis, the combination was associated with positive outcomes. Nevertheless, the need to administer these carbapenems intravenously and the lack of information on the drugs’ long-term side effects have restricted their use to certain severe cases only. Recommended daily doses are either imipenem-cilastatin 1 g (each component) IV twice daily or merope­ nem 1 g IV three times daily, each in combination with clavulanic acid 125 mg oral twice daily, which is only available in combination with amoxicillin.

AMINOGLYCOSIDES  Aminoglycosides have played a time-honed role in the treatment of mycobacterial infections. Amikacin and streptomy­ cin are aminoglycosides that exert mycobactericidal activity by bind­ ing to the 16S ribosomal subunit. The spectrum of antibiotic activity for amikacin and streptomycin includes M. tuberculosis, several NTM species, and aerobic gram-negative and gram-positive bacteria. Due to the need of intravenous or painful intramuscular injections and their serious side effect profile, the WHO recommends limiting their use with the increased availability of novel oral agents. Kanamycin and capreomycin, a cyclic polypeptide similar to aminoglycosides, are no longer recommended due to worse treatment outcomes and increased mortality. This recommendation is based on a large individual patientlevel meta-analysis of observational cohort studies and is likely due to increased toxicity seen with these agents. Streptomycin was the first antimycobacterial agent used for the treatment of TB. Derived from Streptomyces griseus, streptomycin is bactericidal against dividing M. tuberculosis organisms but has only low-level early bactericidal activity. In developing countries, it continues to be widely used due to its low cost. The usual daily dose of streptomycin (given IM either daily or 5 days per week) is 15 mg/kg for adults and 20–40 mg/kg for children, with a maximum of 1 g/d for both with dose reduction recommended for patients ≥60 years of age or with renal impairment. Central nervous system penetration is poor. PART 5 Infectious Diseases Amikacin resistance is less widespread, and streptomycin-resistant strains may still be susceptible. The usual daily adult dosage is 15–30 mg/kg given IM or IV (maximal daily dose, 1 g). It is frequently used to treat severe NTM infections. Mycobacterial resistance to aminoglycosides is due to mutations in the genes encoding the 16S ribosomal RNA gene (rrs). Adverse effects of both amikacin and streptomycin include ototoxicity (in up to 10% of recipients, with auditory dysfunction occurring more commonly than vestibulotoxicity), nephrotoxicity, and neurotoxicity. ETHIONAMIDE  Ethionamide is a derivative of isonicotinic acid. Its mechanism of action is through inhibition of the inhA gene product enoyl–acyl carrier protein (acp) reductase, which is involved in mycolic acid synthesis. Ethionamide is bacteriostatic against metabolically active M. tuberculosis and some NTM. It is used in the treatment of drug-resistant TB, but its use is limited by severe gastrointestinal reactions (including abdominal pain, nausea, and vomiting) as well as significant central and peripheral neurologic side effects, revers­ ible hepatitis (in ~5% of recipients), hypersensitivity reactions, and hypothyroidism. Ethionamide should be taken with food to reduce gastrointestinal effects and with pyridoxine (50–100 mg/d) to limit neuropathic side effects. PARA-AMINOSALICYLIC ACID  Para-aminosalicylic acid (PAS; 4-ami­ nosalicylic acid) is an oral agent used in the treatment of drug-resistant TB. Its bacteriostatic activity is due to inhibition of folate synthesis and of iron uptake. PAS has relatively little activity as an anti-TB agent. Adverse effects may include high-level nausea, vomiting, and diarrhea. PAS may cause hemolysis in patients with glucose-6-phosphate dehy­ drogenase deficiency. The drug should be taken with acidic foods to improve absorption. Enteric-coated PAS granules (4 g orally every

8 h) appear to be better tolerated than other formulations and produce higher therapeutic blood levels. PAS has a short half-life (1 h), and 80% of the dose is excreted in the urine. ■ ■DRUGS IN DEVELOPMENT The pipeline of novel TB drugs is rapidly changing. We direct the reader to the Working Group on New TB Drugs for the most up-todate information (https://www.newtbdrugs.org/pipeline/clinical). NONTUBERCULOUS MYCOBACTERIA More than 180 species of NTM have been identified. Only a minor­ ity of these environmental organisms, which are extensively found in soil and water, are important human pathogens. NTM cause extensive disease primarily in persons with preexisting pulmonary disease or immunocompromise but can also cause nodular/bronchiectatic dis­ ease in otherwise seemingly healthy hosts. Disseminated infections with NTM are common in immunocompromised individuals. NTM are also important causes of skin and soft tissue infections in surgi­ cal settings. The two major classes of NTM are the slow-growing and rapidly growing species; subcultures of the latter grow within 1 week. The growth characteristics of NTM have diagnostic, therapeutic, and prognostic implications. The rate of growth can provide useful pre­ liminary information within a specific clinical context, in that growth within 2–3 weeks is much more likely to indicate an NTM than M. tuberculosis. When NTM do grow from cultures, colonization should be distinguished from active disease to optimize the risk and benefit of prolonged treatment with multiple medications. According to the recommendations of the American Thoracic Society and the Infectious Diseases Society of America, significant clinical manifestations and/ or radiographic evidence of progressive disease consistent with NTM infection as well as either reproducible sputum culture results or a single positive culture from bronchoscopy are required for the diagno­ sis of NTM pulmonary disease. Isolation of NTM from blood or from an infected extrapulmonary site, such as soft tissue or bone, is usually indicative of disseminated or local NTM infection (Chap. 185). Treat­ ment of NTM disease is prolonged and requires multiple medications. Side effects of the regimens employed are common, and intermittent therapy is often used to mitigate these adverse events. Treatment regimens depend on the NTM species, the extent or type of disease, and—to some degree—drug susceptibility test results. ■ ■THERAPEUTIC CONSIDERATIONS FOR

SPECIFIC NTM Slowly Growing Mycobacteria  Slowly growing mycobacteria can be divided into three categories based on their pigment-producing capabilities and—if they do produce pigment—their requirement for light to do so. Photochromogens, including M. marinum and M. kansasii, can produce yellowish-orange pigment only when exposed to light. Scotochromogens, including Mycobacterium gordonae and Mycobac­ terium scrofulaceum, can make pigment regardless of light exposure. MAC organisms and Mycobacterium ulcerans are nonchromogens—i.e., are incapable of making pigment irrespective of light exposure. MYCOBACTERIUM AVIUM COMPLEX  Among the NTM, MAC organisms most commonly cause human disease. In immunocompetent hosts, MAC species are most often found in conjunction with underlying significant lung disease, such as chronic obstructive pulmonary dis­ ease or bronchiectasis. For patients with nodular or bronchiectatic MAC lung disease, an initial regimen consisting of clarithromycin or azithromycin, rifampin or rifabutin (the latter is preferred for HIV patients receiving ART), and ethambutol is given three times per week for at least 12 months after culture conversion. A daily regimen of these three drugs, with consideration of amikacin or streptomycin in the initial treatment phase, is recommended for patients with fibrocavitary MAC lung disease or severe nodular/bronchiectatic disease. Routine initial testing for macrolide resistance is recommended, as is testing at 6 months with a failing regimen (i.e., with cultures persistently positive for NTM). Interpretation of susceptibility tests to drugs other than macrolides and aminoglycosides is hampered by poor correlation with

clinical outcomes. Amikacin has been reformulated as a liposomal sus­ pension for inhalation with increased penetration into airway biofilms. The CONVERT trial showed that addition of inhaled liposomal amika­ cin to standard three-drug regimen of azithromycin or clarithromycin, rifampin, and ethambutol in treatment-refractory (persistent sputum positivity after at least 6 months) MAC lung disease significantly increased culture conversion rates from 9 to 26% at 6 months. Respira­ tory adverse events (primarily dysphonia, cough, and dyspnea) were reported in 87.4% of patients receiving inhaled liposomal amikacin compared to 50% in the standard therapy group; however, rates of seri­ ous adverse events were not different between the regimens. Inhaled liposomal amikacin is now approved for use in refractory pulmonary MAC infections (persistent positive cultures after at least 6 months of treatment). It is currently being evaluated as a first-line agent and as a replacement for rifampin in the treatment of MAC lung disease. Surgical resection should be considered for individuals whose infec­ tion is localized to one lung, who have adequate lung function to toler­ ate lung resection, who have had a poor response to medical therapy, and/or who have developed macrolide-resistant MAC disease. Treatment of MAC in persons living with HIV should be initiated in consultation with an infectious diseases specialist. For HIV-infected patients with well-controlled HIV disease and CD4 T-cell counts in the normal range, MAC treatment is identical to patients without HIV disease except that drug-drug interactions between antimycobacterial agents and ART should be carefully considered. HIV-infected patients with low CD4 count (CD4+ T-cell count <100/μL) are at risk for dis­ seminated MAC infection. MAC disease in these patients is generally treated with clarithromycin, ethambutol, and rifabutin. Azithromycin may be preferred to clarithromycin depending on adverse effects and patient tolerance. Amikacin and fluoroquinolones are often used in salvage regimens. Treatment for disseminated MAC infection in AIDS patients may be lifelong in the absence of immune reconstitution. Therapy is recommended for at least 12 months after culture conver­ sion and at least 6 months of effective immune reconstitution with ART (CD4+ T-cell count >100/μL). MYCOBACTERIUM KANSASII  M. kansasii is the second most common NTM causing human disease in the United States. It is also the second most common cause of NTM pulmonary disease in the United States, where it is most commonly reported in the southeastern region. M. kansasii infection can be treated with rifampin, ethambutol, and either isoniazid or macrolide; therapy continues for at least 18 months or for 12 months after culture conversion. The American Thoracic Society and the Infectious Diseases Society of America recommend routine susceptibility testing to rifampin only. Resistance to isoniazid and eth­ ambutol can be acquired during therapy but is usually associated with rifampin resistance as well. Rifampin-resistant M. kansasii is treated with a three-drug regimen including agents such as ciprofloxacin, azithromy­ cin, ethambutol, rifabutin, amikacin, trimethoprim-sulfamethoxazole, and streptomycin after drug susceptibility testing. MYCOBACTERIUM MARINUM  M. marinum is an NTM found in salt water and freshwater, including swimming pools and fish tanks. It is a cause of localized soft tissue infections, which may require surgical management. Combination regimens include clarithromycin and either ethambutol or rifampin. Other agents with activity against M. marinum include doxycycline, minocycline, and trimethoprim-sulfamethoxazole. Drug susceptibility testing is recommended only if the swab remains culture positive after 3 months of appropriate therapy. Rapidly Growing Mycobacteria  Rapidly growing mycobacteria causing human disease include Mycobacterium abscessus, Mycobac­ terium fortuitum, and Mycobacterium chelonae. Treatment of these mycobacteria is complex and should be undertaken with input from experienced clinicians. It is important to note that testing rapidly growing mycobacteria for macrolide resistance is tricky, as an induc­ ible erm gene may confer in vivo macrolide resistance to isolates that are susceptible in vitro. M. abscessus is the third most common NTM pathogen in the United States. It is endemic in the southeastern states between Texas

and Florida. Skin, soft tissue, and bone infections occur, usually after accidental trauma or surgery. This organism appears to have a pre­ dilection to cause lung infections in white nonsmoking women aged

60 who have no preexisting lung disease. M. abscessus isolates are usually resistant to standard anti-TB regimens. Skin and soft tissue infections are usually treated for a minimum of 4 months with a mac­ rolide (clarithromycin or azithromycin) and a parenteral agent such as amikacin, cefoxitin, or imipenem. Bone infections are treated for at least 6 months. This regimen can be used for the treatment of lung infections but is often unsuccessful because of drug adverse effects and toxicities. A regimen comprising a combination of at least three active drugs (amikacin, linezolid, tigecycline, imipenem, azithromycin, pro­ vided the organism is macrolide susceptible) is recommended based on in vitro drug susceptibility testing. A recent meta-analysis has shown that overall therapeutic efficiency rates in M. abscessus lung infection are low at ~35%; however, incorporation of amikacin, imipenem, line­ zolid, and/or tigecycline was associated with improved outcomes. Con­ versely, macrolide resistance has been associated with worse outcomes. Surgical resection should be considered in all patients with good lung reserve and a localized infection.

■ ■DRUGS FOR THE TREATMENT OF NTM Clarithromycin  Clarithromycin is a macrolide antibiotic with broad activity against many gram-positive and gram-negative bacteria as well as NTM. This drug is active against MAC organisms and many other NTM species, inhibiting protein synthesis by binding to the 50S mycobacterial ribosomal subunit. NTM resistance to macrolides is probably caused by overexpression of the gene ermB, with consequent methylation of the binding site. Strains of M. abscessus subsp. abscessus harbor an inducible macrolide resistance mechanism coded by erm41, which leads to ribosomal methylation and becomes apparent after macrolide incubation of 3−5 days, significantly hampering treatment success. Twenty percent of strains have a nonfunctional erm41 gene. Clarithromycin is well absorbed orally and distributes well to tissues. It is cleared both hepatically and renally; the dosage should be reduced in renal insufficiency. Clarithromycin is a substrate for and inhibits cytochrome 3A4 and should not be administered with cisapride, pimo­ zide, or terfenadine because cardiac arrhythmias may occur. Numerous drugs interact with clarithromycin through the CYP3A4 metabolic pathway. Rifampin lowers clarithromycin levels; conversely, rifampin levels are increased by clarithromycin. However, the clinical relevance of this interaction does not appear to be great. CHAPTER 186 Antimycobacterial Agents For patients with nodular/bronchiectatic MAC infection, the dosage of clarithromycin is 500 mg, given morning and evening three times a week. For the treatment of fibrocavitary or severe nodular/bronchiectatic MAC infection, a dose of 500–1000 mg is given daily. Disseminated MAC infection is treated with 1000 mg daily. Clarithromycin is used in combination regimens that typically include ethambutol and a rifamy­ cin in order to avoid the development of macrolide resistance. Adverse effects include frequent gastrointestinal intolerance, hepatotoxicity, headache, rash, and rare instances of hypoglycemia. Clarithromycin is contraindicated during pregnancy because of its teratogenicity in animal models. Azithromycin  Azithromycin is a derivative of erythromycin. Although technically an azalide and not a macrolide, it works similarly to macrolides, inhibiting protein synthesis through binding to the 50S ribosomal subunit. Azithromycin is preferred over clarithromycin due to once-daily dosing, better tolerability, fewer drug interactions, and equal efficacy. Resistance to azithromycin is almost always associated with com­ plete cross-resistance to clarithromycin. Azithromycin is well absorbed orally, with good tissue penetration and a prolonged half-life (~48 h). The usual dosage for treatment of MAC infection is 250 mg daily or 500 mg three times per week. Azithromycin is used in combination with other agents to avoid the development of resistance. For prophylaxis against dis­ seminated MAC infection in immunocompromised individuals, a dose of 1200 mg once per week is given. Because azithromycin is not metabolized by cytochrome P450, it interacts with few drugs. Adjustment of the dos­ age on the basis of renal function is not necessary.

72 - SECTION 9 Spirochetal Diseases

SECTION 9 Spirochetal Diseases

Amikacin Liposome Inhalation Suspension (ALIS)  ALIS is a new formulation of the aminoglycoside amikacin, which allows for improved penetration in the lung with reduced toxicity. In the CONVERT study, treatment with amikacin liposome inhalation sus­ pension in addition to standard background regimen was associated with significantly increased culture conversion (29 vs 8.9%; p < .0001) by month 6 in patients with treatment-refractory MAC lung disease compared to standard background regimen alone. It is now approved for treatment of refractory MAC lung infection with persistent sputum positivity at 6 months while on appropriate background regimen. The typical dose is 590 mg (one vial once a day) for 6 months along with the standard three-drug regimen of macrolide, rifampin, and ethambutol. Dosage adjustments in patients with hepatic and renal dysfunction are not required. Half-life elimination typically occurs in ~5.9–9.5 h. Respiratory side effects such as bronchospasm, cough, dysphonia, and dyspnea are common. Monitoring for systemic aminoglycoside toxicity should be considered.

Imipenem  Imipenem primarily inhibits cell-wall biosynthesis by binding to the penicillin-binding proteins. It is rapidly gaining impor­ tance for the treatment of M. abscessus, with a meta-analysis showing improved outcomes with its inclusion in a multidrug regimen. It is dosed at 500 mg to 1 g twice to three times a day as part of a combina­ tion regimen for the treatment of M. abscessus. Half-life of imipenem is ~1 h, and because it is metabolized in the kidneys, dosing adjustment is needed with renal dysfunction. Adverse effects include anemia, throm­ bocythemia, and liver dysfunction. Cefoxitin  Cefoxitin is a second-generation parenteral cephalosporin with activity against rapidly growing NTM, particularly M. abscessus and M. chelonae. Its mechanism of action against NTM is unknown but may involve inactivation of cell-wall synthesis enzymes. High doses are used for treatment of NTM: 200 mg/kg IV three or four times per day, with a maximal daily dose of 12 g. The half-life of cefoxitin is ~1 h, with primary renal clearance that requires adjustment in renal insufficiency. Adverse effects are uncommon but include gastrointestinal manifesta­ tions, rash, eosinophilia, fever, and neutropenia. PART 5 Infectious Diseases Newer Drugs  Three newer classes of drugs—the oxazolidinones, the glycylcyclines, and the ketolides—are currently being evaluated for possible use in the treatment of NTM infections, especially those caused by M. abscessus. Approximately 50% of M. abscessus isolates have shown some degree of susceptibility in vitro to linezolid, an oxazolidinone. Tigecycline, which is a glycylcycline and a tetracycline derivative, and telithromycin, a ketolide, also appear to have in vitro activity against M. abscessus. These drugs, however, have not yet been prospectively tested for NTM in patients. In addition, some anti-TB drugs, including clofazimine and beda­ quiline, are being evaluated as alternative agents for the treatment of refractory NTM infections. In particular, clofazimine appears to act synergistically in combination with amikacin, bedaquiline, or tigecy­ cline. The exact role of these agents in the treatment of refractory NTM infections remains unclear. Suppressive therapy with periodic parenteral/ oral drugs to limit disease progression and control symptoms may be an appropriate alternative to curative treatment. CONCLUSION Treatment of mycobacterial infections requires multiple-drug regi­ mens that often exert significant side effects with the potential to limit tolerability. The prolonged duration of treatment has vastly improved results over those obtained in past decades, but drugs and regimens that will shorten treatment duration and limit adverse drug effects and interactions are needed. ■ ■FURTHER READING Collaborative Group for the Meta-Analysis of Individual Patient Data in Mdr-Tb Treatment–2017: Treatment correlates of successful outcomes in pulmonary multidrug-resistant tuberculo­ sis: An individual patient data meta-analysis. Lancet 392:821, 2018.

Daley CL et al: Treatment of nontuberculous mycobacterial pulmo­ nary disease: An official ATS/ERS/ESCMID/IDSA clinical practice guideline. Clin Infect Dis 71:e1, 2020. Nahid P et al: Official American Thoracic Society/Centers for Disease Control and Prevention/Infectious Diseases Society of America clini­ cal practice guidelines: Treatment of drug-susceptible tuberculosis. Clin Infect Dis 63:e147, 2016. Sterling TR et al: Guidelines for the treatment of latent tuberculosis infection: Recommendations from the National Tuberculosis Con­ trollers Association and CDC, 2020. MMWR Recomm Rep 69:1, 2020. World Health Organization: WHO consolidated guidelines on tuberculosis Module 4: Treatment, Drug-resistant tuberculosis treat­ ment Geneva: World Health Organization, 2022. Section 9 Spirochetal Diseases Sheila A. Lukehart

Syphilis DEFINITION Syphilis, a chronic systemic infection caused by Treponema pallidum subspecies pallidum, is usually sexually transmitted and is character­ ized by episodes of active disease interrupted by asymptomatic periods (latency). After an incubation period averaging 2–6 weeks, a primary lesion appears—often associated with regional lymphadenopathy—and then resolves without treatment. The secondary stage, with generalized mucosal and cutaneous lesions and generalized lymphadenopathy, also resolves spontaneously and is followed by a latent period of subclini­ cal infection lasting years or decades. Central nervous system (CNS) invasion may occur early in infection, and CNS involvement may be symptomatic or asymptomatic. In the preantibiotic era, one-third of untreated patients developed tertiary syphilis, characterized by destructive mucocutaneous, skeletal, or parenchymal lesions; aortitis; or late CNS manifestations. ETIOLOGY The Spirochaetales include five genera that are pathogenic for humans and for a variety of other animals: Leptospira species (leptospirosis, Chap. 189); Borrelia and Borreliella species (relapsing fever and Lyme disease, respectively; Chaps. 190 and 191); Brachyspira species (gastro­ intestinal infections); and Treponema species (syphilis and the endemic treponematoses; see also Chap. 188). The Treponema pallidum subspe­ cies include T. pallidum subsp. pallidum (venereal syphilis), T. pallidum subsp. pertenue (yaws), and T. pallidum subsp. endemicum (bejel).

T. carateum (pinta), for which no extant strains are available for molec­ ular studies, is still classified as a separate species. Historically, the pathogenic Treponema were distinguished by the clinical syndromes they produce, but phylogenetic analyses of whole genome sequences from several strains (excluding T. carateum) yield the three named subspecies groupings. Whether these groupings represent geographi­ cal variation or actual biological differences is unclear. The crossing of subspecies boundaries by some “molecular signatures” and the recent recognition of treponemes of the endemicum genotype in sexually acquired genital ulcers (chancres) and secondary rashes (Chap. 188) support the concept of a genetic and clinical “continuum” among strains and subspecies of the pathogenic treponemes. T. pallidum subspecies are thin spiral organisms, with a cell body surrounded by a trilaminar cytoplasmic membrane, a delicate

73 - 187 Syphilis

187 Syphilis

Amikacin Liposome Inhalation Suspension (ALIS)  ALIS is a new formulation of the aminoglycoside amikacin, which allows for improved penetration in the lung with reduced toxicity. In the CONVERT study, treatment with amikacin liposome inhalation sus­ pension in addition to standard background regimen was associated with significantly increased culture conversion (29 vs 8.9%; p < .0001) by month 6 in patients with treatment-refractory MAC lung disease compared to standard background regimen alone. It is now approved for treatment of refractory MAC lung infection with persistent sputum positivity at 6 months while on appropriate background regimen. The typical dose is 590 mg (one vial once a day) for 6 months along with the standard three-drug regimen of macrolide, rifampin, and ethambutol. Dosage adjustments in patients with hepatic and renal dysfunction are not required. Half-life elimination typically occurs in ~5.9–9.5 h. Respiratory side effects such as bronchospasm, cough, dysphonia, and dyspnea are common. Monitoring for systemic aminoglycoside toxicity should be considered.

Imipenem  Imipenem primarily inhibits cell-wall biosynthesis by binding to the penicillin-binding proteins. It is rapidly gaining impor­ tance for the treatment of M. abscessus, with a meta-analysis showing improved outcomes with its inclusion in a multidrug regimen. It is dosed at 500 mg to 1 g twice to three times a day as part of a combina­ tion regimen for the treatment of M. abscessus. Half-life of imipenem is ~1 h, and because it is metabolized in the kidneys, dosing adjustment is needed with renal dysfunction. Adverse effects include anemia, throm­ bocythemia, and liver dysfunction. Cefoxitin  Cefoxitin is a second-generation parenteral cephalosporin with activity against rapidly growing NTM, particularly M. abscessus and M. chelonae. Its mechanism of action against NTM is unknown but may involve inactivation of cell-wall synthesis enzymes. High doses are used for treatment of NTM: 200 mg/kg IV three or four times per day, with a maximal daily dose of 12 g. The half-life of cefoxitin is ~1 h, with primary renal clearance that requires adjustment in renal insufficiency. Adverse effects are uncommon but include gastrointestinal manifesta­ tions, rash, eosinophilia, fever, and neutropenia. PART 5 Infectious Diseases Newer Drugs  Three newer classes of drugs—the oxazolidinones, the glycylcyclines, and the ketolides—are currently being evaluated for possible use in the treatment of NTM infections, especially those caused by M. abscessus. Approximately 50% of M. abscessus isolates have shown some degree of susceptibility in vitro to linezolid, an oxazolidinone. Tigecycline, which is a glycylcycline and a tetracycline derivative, and telithromycin, a ketolide, also appear to have in vitro activity against M. abscessus. These drugs, however, have not yet been prospectively tested for NTM in patients. In addition, some anti-TB drugs, including clofazimine and beda­ quiline, are being evaluated as alternative agents for the treatment of refractory NTM infections. In particular, clofazimine appears to act synergistically in combination with amikacin, bedaquiline, or tigecy­ cline. The exact role of these agents in the treatment of refractory NTM infections remains unclear. Suppressive therapy with periodic parenteral/ oral drugs to limit disease progression and control symptoms may be an appropriate alternative to curative treatment. CONCLUSION Treatment of mycobacterial infections requires multiple-drug regi­ mens that often exert significant side effects with the potential to limit tolerability. The prolonged duration of treatment has vastly improved results over those obtained in past decades, but drugs and regimens that will shorten treatment duration and limit adverse drug effects and interactions are needed. ■ ■FURTHER READING Collaborative Group for the Meta-Analysis of Individual Patient Data in Mdr-Tb Treatment–2017: Treatment correlates of successful outcomes in pulmonary multidrug-resistant tuberculo­ sis: An individual patient data meta-analysis. Lancet 392:821, 2018.

Daley CL et al: Treatment of nontuberculous mycobacterial pulmo­ nary disease: An official ATS/ERS/ESCMID/IDSA clinical practice guideline. Clin Infect Dis 71:e1, 2020. Nahid P et al: Official American Thoracic Society/Centers for Disease Control and Prevention/Infectious Diseases Society of America clini­ cal practice guidelines: Treatment of drug-susceptible tuberculosis. Clin Infect Dis 63:e147, 2016. Sterling TR et al: Guidelines for the treatment of latent tuberculosis infection: Recommendations from the National Tuberculosis Con­ trollers Association and CDC, 2020. MMWR Recomm Rep 69:1, 2020. World Health Organization: WHO consolidated guidelines on tuberculosis Module 4: Treatment, Drug-resistant tuberculosis treat­ ment Geneva: World Health Organization, 2022. Section 9 Spirochetal Diseases Sheila A. Lukehart

Syphilis DEFINITION Syphilis, a chronic systemic infection caused by Treponema pallidum subspecies pallidum, is usually sexually transmitted and is character­ ized by episodes of active disease interrupted by asymptomatic periods (latency). After an incubation period averaging 2–6 weeks, a primary lesion appears—often associated with regional lymphadenopathy—and then resolves without treatment. The secondary stage, with generalized mucosal and cutaneous lesions and generalized lymphadenopathy, also resolves spontaneously and is followed by a latent period of subclini­ cal infection lasting years or decades. Central nervous system (CNS) invasion may occur early in infection, and CNS involvement may be symptomatic or asymptomatic. In the preantibiotic era, one-third of untreated patients developed tertiary syphilis, characterized by destructive mucocutaneous, skeletal, or parenchymal lesions; aortitis; or late CNS manifestations. ETIOLOGY The Spirochaetales include five genera that are pathogenic for humans and for a variety of other animals: Leptospira species (leptospirosis, Chap. 189); Borrelia and Borreliella species (relapsing fever and Lyme disease, respectively; Chaps. 190 and 191); Brachyspira species (gastro­ intestinal infections); and Treponema species (syphilis and the endemic treponematoses; see also Chap. 188). The Treponema pallidum subspe­ cies include T. pallidum subsp. pallidum (venereal syphilis), T. pallidum subsp. pertenue (yaws), and T. pallidum subsp. endemicum (bejel).

T. carateum (pinta), for which no extant strains are available for molec­ ular studies, is still classified as a separate species. Historically, the pathogenic Treponema were distinguished by the clinical syndromes they produce, but phylogenetic analyses of whole genome sequences from several strains (excluding T. carateum) yield the three named subspecies groupings. Whether these groupings represent geographi­ cal variation or actual biological differences is unclear. The crossing of subspecies boundaries by some “molecular signatures” and the recent recognition of treponemes of the endemicum genotype in sexually acquired genital ulcers (chancres) and secondary rashes (Chap. 188) support the concept of a genetic and clinical “continuum” among strains and subspecies of the pathogenic treponemes. T. pallidum subspecies are thin spiral organisms, with a cell body surrounded by a trilaminar cytoplasmic membrane, a delicate

peptidoglycan layer, and a lipid-rich outer membrane. Endoflagella wind around the cell body in the periplasmic space and are responsible for motility. Historically, T. pallidum subspecies could not be cultured in vitro, but long-term propagation of multiple strains of T. pallidum subsp. pallidum and one strain of subsp. endemicum in complex medium with eukaryotic cells is now possible. To date, the pertenue subspecies has not been cultured. All T. pallidum subspecies have severely limited metabolic capabilities and are highly dependent on host-derived amino acids, carbohydrates, and lipids. Genetic analyses have revealed the existence of a 12-member gene family (tpr) encoding outer-membrane antigens. One member, TprK, has discrete variable regions that undergo antigenic variation during infection, providing a mechanism for immune evasion and persistence. The only known natural host for T. pallidum subsp. pallidum (referred to hereafter as T. pallidum) is the human. T. pallidum can infect many mammals, but only humans, higher apes, and a few labora­ tory animals develop syphilitic lesions. Rabbits are used to propagate T. pallidum and serve as the animal model that best reflects human disease and immunopathology. TRANSMISSION AND EPIDEMIOLOGY Nearly all cases of syphilis are acquired by sexual contact with infec­ tious lesions (i.e., the chancre, mucous patch, skin rash, or condylo­ mata lata; see Fig. A1-20). T. pallidum DNA has also been detected in swabs of normal-appearing oral mucosa, in saliva, in urine, and in semen, raising the possibility of transmission by these routes, but the infectivity of these organisms has not been assessed. Less common modes of transmission include nonsexual skin contact, infection in utero, blood transfusion, and organ transplantation. ■ ■SYPHILIS IN THE UNITED STATES Following the introduction of penicillin therapy in the 1940s, the number of reported cases of syphilis of all stages in the United States declined 95% to a low of 31,575 cases in 2000, with 5979 reported cases of primary and secondary (P&S) syphilis. (P&S cases are infectious and are a better indicator of disease activity than total syphilis cases.) Since 2000, total cases have increased 6.6-fold to 207,255, and the number of P&S cases has increased tenfold, with 59,061 cases reported in 2022 (Fig. 187-1). Nationally, ~45% of these cases were in men who have sex with men (MSM), ~40% of whom are co-infected with HIV. In two years, from 2020 to 2022, P&S cases rose 31.7% among all men and 85.4% among women, with increases in all racial and ethnic groups and in all geographic regions of the United States. Because the incidence of congenital syphilis parallels that of infectious syphilis in women, the striking increase in early syphilis in women has resulted in a dramatic increase in congenital syphilis. In 2022, 3755 cases of congenital syphi­ lis were reported, resulting in 282 congenital syphilis-related stillbirths and deaths. In the last decade, the number of reported cases in infants

Men Women

Number of cases

FIGURE 187-1  Primary and secondary syphilis in the United States, 1990–2022, by sex. (Data from the Centers for Disease Control and Prevention.)

<1 year of age has increased from 334 to 3755, or elevenfold. The vast majority of these cases resulted from late or no prenatal care, but some were in women who had tested positive for syphilis but were not treated prior to delivery. A recent study of substance use in pregnant women with syphilis found that illicit use of opioids and other illicit nonprescription substances was six and four times higher, respectively, in persons with a congenital syphilis outcome than in those without a congenital syphilis outcome. Other risk factors include homelessness and unstable housing, transactional sex, and incarceration.

The populations at highest risk for acquiring syphilis have changed over time, with outbreaks among MSM in the pre-HIV era of the late 1970s and early 1980s, as well as at present. The current dramatic decade-long increase in syphilis and other sexually transmitted infec­ tions in MSM may be due to unprotected sex between persons who are HIV concordant and to disinhibition facilitated by highly effective antiretroviral therapy (ART) or pre- and postexposure prophylaxis (PrEP and PEP). Many MSM diagnosed with syphilis have had syphilis previously, and reinfections may be asymptomatic in persons with multiple past episodes. Thus, more frequent (every 3 months) screen­ ing for syphilis and other sexually transmitted infections is warranted in high-risk populations. Cases of P&S syphilis among African Americans increased 5.5-fold between 2002 and 2022, and the rate (44.4 per 100,000 population) remains higher than rates for other racial/ethnic groups except American Indians/Alaska Natives (67 per 100,000). Of individuals named as sexual contacts of persons with early syphi­ lis, some will have developed manifestations of syphilis when they are first seen, and others will develop infectious syphilis if not treated; overall, ~30–60% of persons exposed to P&S syphilis will develop syphilis if not treated. Thus, identification and treatment of all recently exposed sexual contacts continue to be important aspects of syphilis control. Recent data suggest that PEP with 200 mg of doxycycline (Doxy-PEP), taken within 72 h of exposure, will significantly reduce the likelihood of syphilis (and other) infections in MSM. Data from a single study of Doxy-PEP in women have been disappointing, but these results may be due to lack of compliance in the test population. Concerns have been raised that widespread use of Doxy-PEP (perhaps misused as PrEP) may result in selection for resistance in sexually transmitted infection pathogens. CHAPTER 187 Syphilis ■ ■GLOBAL SYPHILIS Syphilis remains a significant health problem globally; the number of new infections is estimated at ~8 million per year, with 22.3 million prevalent cases. The regions most affected include sub-Saharan Africa, South America, and central and south Asia. Rates of P&S syphilis have increased dramatically among MSM in many European, Asian, and South American countries. Globally, although efforts by the World Health Organization (WHO) reduced the incidence of congenital syphilis in the 2010s, that progress is being lost as recent increases are reported in nearly all regions of the world. In 2022, there were an estimated 700,000 cases of congenital syphilis and 390,000 adverse birth outcomes: 150,000 early fetal deaths and stillbirths, 70,000 neo­ natal deaths, 55,000 preterm or low-birth-weight infants, and 115,000 infants with clinical evidence of infection. NATURAL COURSE AND PATHOGENESIS OF UNTREATED SYPHILIS T. pallidum rapidly penetrates intact mucous membranes or microscopic abrasions in skin and, within a few hours, enters the lymphatics and blood to produce systemic infection and metastatic foci long before the appearance of a primary lesion. Blood from a patient with incubating or early syphilis is infectious. The generation time of T. pallidum during early disease is estimated to be ~33 h, and the incubation period of syphi­ lis is inversely proportional to the number of organisms transmitted. The 50% infectious dose for intradermal inoculation in humans has been calculated to be 57 organisms, and the treponeme concentration gener­ ally reaches 107/g of tissue before a clinical lesion appears. The median incubation period in humans (~21 days) suggests an average inoculum of 500–1000 infectious organisms for naturally acquired disease; the incubation period rarely exceeds 6 weeks.

The primary lesion appears at the site of inoculation, usually persists for 4–6 weeks, and then heals spontaneously. Histopathologic exami­ nation shows perivascular infiltration, chiefly by CD4+ and CD8+

T lymphocytes, plasma cells, and macrophages, with capillary endo­ thelial proliferation and subsequent obliteration of small blood vessels. The cellular infiltration produces a TH1-type cytokine profile, consis­ tent with the activation of macrophages. Phagocytosis of opsonized organisms by activated macrophages ultimately causes their destruc­ tion, resulting in spontaneous resolution of the chancre and later the secondary rash.

The generalized parenchymal, constitutional, mucosal, and cutane­ ous manifestations of secondary syphilis usually appear ~6–12 weeks after infection, although primary and secondary manifestations may occasionally overlap. In contrast, some patients may enter the latent stage without ever recognizing secondary lesions. The histopathologic features of secondary maculopapular skin lesions include hyperkera­ tosis of the epidermis, capillary proliferation with endothelial swelling in the superficial dermis, and—in the deeper dermis—perivascular infiltration by CD8+ and CD4+ T lymphocytes, macrophages, and variable numbers of plasma cells. T. pallidum disseminates during the first days to weeks of infection, invading many tissues, including the CNS; cerebrospinal fluid (CSF) abnormalities can be detected in as many as 40% of patients during the secondary stage. Clinical hepatitis and immune complex–induced glomerulonephritis are rare, but recognized, manifestations of secondary syphilis. Generalized nontender lymphadenopathy is noted in 85% of patients with second­ ary syphilis. The paradoxical appearance of secondary manifestations, even after the development of an immune response that clears primary lesions, likely results from immune evasion due to antigenic variation of exposed portions of the TprK surface protein. Secondary lesions generally subside within 2–6 weeks, and the infection enters the latent stage, which is detectable only by serologic testing. In the preantibiotic era, up to 25% of untreated patients experienced at least one cutaneous relapse of secondary lesions, usually during the first year. Therefore, identification and examination of sexual contacts are most important for patients with syphilis of <1 year in duration. PART 5 Infectious Diseases In the preantibiotic era, about one-third of patients with untreated latent syphilis developed clinically apparent tertiary disease, the most common types being the gumma (a usually benign granulomatous lesion); cardiovascular syphilis (usually involving the vasa vasorum of the ascending aorta and resulting in aneurysm); and late symptomatic neurosyphilis (tabes dorsalis and paresis). In Western countries today, specific treatment for early and latent syphilis and coincidental therapy (i.e., therapy with antibiotics active against treponemes, but given for other conditions) have nearly eliminated tertiary syphilis. Asymptom­ atic CNS involvement, however, is still demonstrable in up to 40% of persons with early syphilis and 25% of patients with late latent syphi­ lis, and modern cases of general paresis and tabes dorsalis are being reported from China. The factors that contribute to the development and progression of tertiary disease are unknown. The course of untreated syphilis was studied retrospectively in a group of nearly 2000 patients with primary or secondary disease diagnosed clinically (the Oslo Study, 1891–1951) and was assessed prospectively in 431 African-American men with seropositive latent syphilis of ≥3 years in duration (the notorious Tuskegee Study, 1932–1972). In the Oslo Study, serious late complications were nearly twice as common among men as among women. In the Tuskegee Study, untreated syphilis increased the death rate 17% compared to uninfected subjects, largely due to cardiovascular syphilis. The ethical issues eventually raised by the Tuskegee Study, begun in the pre-penicillin era but continuing into the early 1970s, had a major influence on the development of current guide­ lines for protection of human subjects and still contribute to a reluctance of some African Americans to participate in clinical research. CLINICAL MANIFESTATIONS ■ ■PRIMARY SYPHILIS The typical primary chancre usually begins as a single painless pap­ ule that rapidly erodes and becomes indurated, with a characteristic

FIGURE 187-2  Primary syphilis with a firm, nontender chancre. cartilaginous consistency on palpation of the edge and base of the ulcer. Multiple primary lesions are seen in a minority of patients. In heterosexual men, the chancre is usually located on the penis, where it is readily seen (Fig. 187-2; see also Fig. A1-17), but in MSM, it may also be found in the anal canal, rectum, or mouth. Oral sex has been identified as the source of infection in some MSM. In women, com­ mon primary sites are the cervix, vaginal wall, and labia, as well as anal canal and mouth. Consequently, primary syphilis goes unrecognized in women and MSM more often than in heterosexual men. Atypical primary lesions are common, and may be multiple, small, or partially resolved. Therefore, syphilis should be considered in the evaluation of trivial or atypical dark-field-negative genital lesions. The lesions that most commonly must be differentiated from those of pri­ mary syphilis include those caused by herpes simplex virus infection (Chap. 197), chancroid (Chap. 162), traumatic injury, and donovano­ sis (Chap. 178). Regional (usually inguinal) lymphadenopathy accom­ panies the primary syphilitic lesion, appearing within 1 week of lesion onset. The nodes are firm, nonsuppurative, and painless. Inguinal lymphadenopathy is bilateral and may occur with anal as well as with genital chancres. The chancre generally heals within 4–6 weeks (range, 2–12 weeks), but lymphadenopathy may persist for months. ■ ■SECONDARY SYPHILIS The classical manifestations of the secondary stage include mucocu­ taneous or cutaneous lesions and generalized nontender lymphade­ nopathy. The healing primary chancre may still be present in ~15% of cases—more frequently in persons with concurrent HIV infection. The skin rash consists of macular, papular, papulosquamous, and occasion­ ally pustular syphilides; often more than one form is present simulta­ neously. The eruption may be very subtle, and 25% of patients with a discernible rash may be unaware that they have dermatologic manifes­ tations. Initial lesions are pale red or pink, nonpruritic, discrete mac­ ules distributed on the trunk and extremities; these macules progress to papular lesions that are distributed widely and that frequently involve the palms and soles (Fig. 187-3; see also Figs. A1-18 and and A1-19). Rarely, severe necrotic lesions (lues maligna) may appear and are more commonly reported in HIV-infected individuals with low CD4+ T-cell counts. Involvement of the hair follicles may result in patchy alopecia of the scalp hair, eyebrows, or beard in up to 5% of cases.

FIGURE 187-3  Secondary syphilis. Left: Maculopapular truncal eruption. Middle: Papules on the palms. Right: Papules on the soles. (Photos courtesy of Jill McKenzie and Christina Marra.) In warm, moist, intertriginous areas (commonly the perianal region, vulva, and scrotum), papules can enlarge to produce broad, moist, pink or gray-white, highly infectious lesions (condylomata lata; see Fig. A1-20) in 10% of patients with secondary syphilis. Superficial mucosal erosions (mucous patches) occur in 10–15% of patients and commonly involve the oral or genital mucosa (see Fig. A1-21). The typical mucous patch is a painless silver-gray erosion surrounded by a red periphery. T. pallidum DNA has been detected in oral mucosal swabs from persons with early and latent syphilis, but who have no vis­ ible oral lesions. The implications of this finding for transmission are unclear but warrant further research. Constitutional signs and symptoms that may accompany or precede secondary syphilis include sore throat (15–30%), fever (5–8%), weight loss (2–20%), malaise (25%), anorexia (2–10%), headache (10%), and meningismus (5%). Acute meningitis occurs in only 1–2% of cases, but CSF cell and protein concentrations are increased in up to 40% of early syphilis cases, and viable T. pallidum organisms have been recovered from CSF during primary and secondary syphilis in 30% of cases, sometimes without other CSF abnormalities. Persons with current or recent secondary syphilis may present with ocular or otic manifestations. Ocular findings include pupillary abnormalities and optic neuritis as well as the classic iritis or uveitis. The diagnosis of ocular syphilis is often considered in affected patients only after they fail to respond to topical steroid therapy. Anterior uve­ itis has been reported in 5–10% of patients with secondary syphilis, and T. pallidum has been demonstrated in aqueous humor from such patients. Permanent blindness may result without prompt diagnosis and treatment. Otic syphilis may present as sensorineural hearing loss, vertigo, or tinnitus, and deafness may result if untreated. The publica­ tion of several reports of ocular and otic syphilis reminds clinicians to inquire about neurologic manifestations in all stages of syphilis infection. In a recent study, 7.9% of patients with syphilis, when asked, reported recent vision or hearing changes, and more than half of those had abnormal CSF or ophthalmologic findings consistent with syphilis. Less often recognized complications of secondary syphilis include hepatitis, nephropathy, gastrointestinal involvement (hypertrophic gastritis, patchy proctitis, or a rectosigmoid mass—sometimes mis­ takenly assumed to be malignant), arthritis, and periostitis. Hepatic involvement is common in syphilis; although it is usually asymptom­ atic, up to 25% of patients may have abnormal liver function tests. Frank syphilitic hepatitis is rare. Renal involvement usually results from immune complex deposition and produces proteinuria associ­ ated with an acute nephrotic syndrome. Like those of primary syphilis, most manifestations of the secondary stage resolve spontaneously, usually within 1–6 months. ■ ■LATENT SYPHILIS Positive serologic tests for syphilis, together with a normal CSF exami­ nation and the absence of clinical manifestations of syphilis, indicate a diagnosis of latent syphilis in an untreated person. The diagnosis may be made following routine serologic screening or may be suspected due to a history of primary or secondary lesions, a history of exposure to

syphilis, or the delivery of an infant with congenital syphilis. A previous nonre­ active serologic test or clear history of lesions or exposure may help to estab­ lish the duration of infection, which is an important factor in the selection of appropriate therapy. Early latent syphilis is limited to the first year after infection, whereas late latent syphilis is defined as that of ≥1 year in duration or unknown duration. The classical definition of early latent syphilis could include an asymp­ tomatic person whose secondary rash has resolved, as well as a person whose chancre has healed but who has not yet developed secondary manifestations. Accordingly, the Centers for Disease Control and Prevention (CDC) have revised the case definitions for surveillance and reporting purposes to better reflect the recognition that some clinical presentations may be seen at several stages of infec­ tion. These definitions include the traditional primary and secondary stages, as well as “syphilis, early nonprimary nonsecondary,” describing infections of <12 months in duration, and “syphilis, unknown duration or late,” encompassing the previous late latent and late (tertiary) clas­ sifications. In this new scheme, neurologic, ocular, otic, and late clinical manifestations are reported separately in the context of their separate primary, secondary, early nonprimary nonsecondary, and unknown duration or late categories.

CHAPTER 187 It was previously thought that untreated late latent syphilis had three possible outcomes: (1) persistent lifelong infection; (2) development of tertiary syphilis; or (3) spontaneous cure, with reversion of serologic tests to negative. Although progression to clinically evident late syphilis is very rare today, the occurrence of spontaneous microbiologic cure is in doubt. Syphilis Because T. pallidum continues to be present throughout untreated infection, it may seed the bloodstream intermittently during the latent stage, and a pregnant woman with latent syphilis may infect the fetus in utero. Moreover, syphilis has been transmitted through blood transfu­ sion or organ donation from patients with latent syphilis. ■ ■REINFECTION SYPHILIS A growing number of individuals, particularly MSM, acquire multiple episodes of syphilis, with important implications for clinical presenta­ tion and serologic testing. Although no national data are available, 32% of enrollees (mostly MSM) in an 18-year longitudinal study of CNS involvement were known to have had multiple episodes of syphilis. It is well recognized that, after treatment, persons with past syphilis are less likely to revert to nonreactive in the Venereal Disease Research Labora­ tory (VDRL)/rapid plasma reagin (RPR) tests than persons with first episode syphilis, and treponemal tests will remain reactive. However, several recent studies also indicate that subsequent episodes of syphilis are more likely to be asymptomatic than initial episodes, less likely to have T. pallidum identified in blood or CSF, and less likely to have laboratory-defined neurosyphilis. These cases would be detectable only by serologic screening, reinforcing the utility of frequent screening in high-risk populations to identify reinfection. ■ ■INVOLVEMENT OF THE CNS Traditionally, neurosyphilis has been considered a late manifestation of syphilis, but this view is inaccurate. CNS syphilis represents a con­ tinuum encompassing early invasion (usually within the first weeks of infection), months to years of asymptomatic involvement, and, in some cases, development of early or late neurologic manifestations. Early neurosyphilis includes asymptomatic or symptomatic meningitis and meningovascular syphilis; late neurosyphilis includes tabes dorsalis and general paresis. Asymptomatic Neurosyphilis  The diagnosis of asymptomatic neurosyphilis is made in patients who lack neurologic symptoms and signs but who have CSF abnormalities, including mononuclear

pleocytosis, increased protein concentration, or reactivity in the CSF VDRL test. CSF abnormalities are demonstrated in up to 40% of cases of untreated primary or secondary syphilis and in 25% of cases of untreated latent syphilis. T. pallidum has been recovered by inocula­ tion into rabbits of CSF from up to 30% of patients with primary or secondary syphilis but less frequently from patients with syphilis of

1 year in duration. The presence of T. pallidum in CSF is often asso­ ciated with other CSF abnormalities, but organisms can be recovered from patients with otherwise normal CSF. Although the prognostic implications of these findings in early syphilis are uncertain, it may be appropriate to conclude that even patients with early syphilis who have CSF abnormalities do indeed have asymptomatic neurosyphilis and should be treated for neurosyphilis; such treatment is particularly important in patients with concurrent untreated HIV infection. Before the advent of penicillin, the risk of development of clinical neurosyphi­ lis in untreated asymptomatic persons was roughly proportional to the intensity of CSF changes. In several large studies, neurosyphilis was associated with a serum RPR titer of ≥1:32, regardless of clinical stage or HIV infection status. Most experts agree that clinical neurosyphilis is more common among persons with untreated HIV infection, and that the immune reconstitution seen with effective ART may have a protective effect against development of clinical neurosyphilis in some HIV-infected persons with syphilis. Nonetheless, RPR titer ≥1:32 is still associated with reactive CSF VDRL, even in persons taking effec­ tive ART. HIV-uninfected persons with untreated latent syphilis and normal CSF probably run a very low risk of subsequent neurosyphilis. Symptomatic Neurosyphilis  The major clinical categories of symptomatic neurosyphilis include early meningeal and meningovas­ cular and late parenchymatous syphilis. The last category includes gen­ eral paresis and tabes dorsalis. The onset of symptoms usually occurs <1 year after infection for meningeal syphilis, up to 10 years after infection for meningovascular syphilis, at ~20 years for general paresis, and at 25–30 years for tabes dorsalis. Neurosyphilis is more frequently symptomatic in patients co-infected with untreated HIV, particularly those with low CD4+ T lymphocyte counts.

PART 5 Infectious Diseases Meningeal syphilis may present as headache, nausea, vomiting, neck stiffness, cranial nerve involvement, seizures, and changes in mental status. This condition may be concurrent with or may follow the sec­ ondary stage. Patients presenting with uveitis, iritis, or hearing loss often have meningeal syphilis, but these clinical findings can also be seen in patients with normal CSF. Meningovascular syphilis reflects meningitis together with inflam­ matory vasculitis of small, medium, or large vessels. The most common presentation is a stroke syndrome involving the middle cerebral artery of a relatively young adult. However, unlike the usual thrombotic or embolic stroke syndrome of sudden onset, meningovascular syphilis often becomes manifest after a subacute encephalitic prodrome (with headaches, vertigo, insomnia, and psychological abnormalities), which is followed by a gradually progressive vascular syndrome. The manifestations of general paresis reflect widespread late paren­ chymal damage and include abnormalities corresponding to the mnemonic paresis: personality, affect, reflexes (hyperactive), eye (e.g., Argyll Robertson pupils), sensorium (illusions, delusions, hallucina­ tions), intellect (a decrease in recent memory and in the capacity for orientation, calculations, judgment, and insight), and speech. Tabes dorsalis is a late manifestation of syphilis that presents with symptoms and signs of demyelination of the posterior columns, dorsal roots, and dorsal root ganglia, including ataxia, foot drop, paresthesia, bladder disturbances, impotence, areflexia, and loss of positional, deep-pain, and temperature sensations. The small, irregular Argyll Robertson pupil, a feature of both tabes dorsalis and paresis, reacts to accommo­ dation but not to light. Optic atrophy also occurs frequently in associa­ tion with tabes. ■ ■OTHER MANIFESTATIONS OF LATE SYPHILIS The slowly progressive inflammatory process leading to tertiary dis­ ease begins early during infection, although these manifestations may not become clinically apparent for years or decades. Early syphilitic aortitis first becomes evident soon after secondary lesions subside, and

treponemes that trigger the development of gummas may have seeded the tissue years earlier. Cardiovascular Syphilis  Cardiovascular manifestations, usually appearing 10–40 years after infection, are attributable to endarteritis obliterans of the vasa vasorum, which provide the blood supply to large vessels; T. pallidum DNA has been detected by polymerase chain reaction (PCR) in aortic tissue. Cardiovascular involvement results in uncomplicated aortitis, aortic regurgitation, saccular aneurysm (usu­ ally of the ascending aorta), or coronary ostial stenosis. In the prean­ tibiotic era, symptomatic cardiovascular complications developed in ~10% of persons with untreated late syphilis. Today, cardiovascular syphilis is rarely seen in the developed world. Late Benign Syphilis (Gumma)  Gummas are usually solitary lesions ranging from microscopic to several centimeters in diameter. Histologic examination shows a granulomatous inflammation, with a central area of necrosis due to endarteritis obliterans. T. pallidum in low numbers have been detected by PCR in these lesions, and penicillin treatment results in rapid resolution, confirming the treponemal stim­ ulus for the inflammation. Common sites include the skin and skeletal system; however, any organ (including the brain) may be involved. Gummas of the skin produce indolent, painless, indurated nodular or ulcerative lesions that may resemble other chronic granulomatous con­ ditions. Skeletal gummas may affect any bone or cartilage. Upper respi­ ratory gummas can lead to perforation of the nasal septum or palate. ■ ■CONGENITAL SYPHILIS Transmission of T. pallidum across the placenta from a pregnant per­ son with syphilis to the fetus may occur at any stage of pregnancy, but fetal damage generally does not occur until after the fourth month of gestation when fetal immunologic competence begins to develop. This timing suggests that the pathogenesis of congenital syphilis, like that of adult syphilis, depends on the host immune response rather than on a direct toxic effect of T. pallidum. The risk of fetal infection during untreated early maternal syphilis is ~75–95%, decreasing to ~35% for maternal syphilis of >2 years in duration. Adequate treatment of the woman before the 16th week of pregnancy should prevent fetal dam­ age, and treatment before the third trimester should adequately treat the infected fetus. Untreated maternal infection may result in a rate of fetal loss of up to 40% with second-trimester spontaneous abortion, stillbirth, prematurity, and neonatal death. Among infants born alive, only fulminant congenital syphilis is clinically apparent at birth, and these babies have a very poor prognosis. The most common clinical problem is the healthy-appearing baby born to a mother with a posi­ tive serologic test. Routine serologic testing for syphilis in early pregnancy is costeffective in virtually all populations, even in areas with a low pre­ natal prevalence of syphilis. Low-tech point-of-care tests have been developed and widely implemented to facilitate antenatal testing in resource-poor settings. Globally, congenital syphilis incidence has increased dramatically, particularly in Africa, South America, and the United States. Periodic lack of benzathine penicillin (BPG) availability in low- and middle-income countries and the current critical shortage in the United States and Europe complicate treatment of seropositive women. Globally, integration of programs to prevent congenital syphi­ lis with programs to prevent maternal transmission of HIV would be highly cost-effective but is often hampered by the restrictions placed on HIV-focused funds. All pregnant women should be serologically screened at their first antenatal visit. Where the prevalence of syphilis in women is high or when the patient is at high risk of reinfection, testing should be repeated at 28 weeks and at delivery. Those testing positive should be treated immediately, even before receiving results of confirmatory tests. During the current BPG shortage, many clinics are treating nonpreg­ nant patients with doxycycline, thus reserving BPG for seropositive pregnant women. Neonatal congenital syphilis must be differentiated from other generalized congenital infections, including rubella, cyto­ megalovirus or herpes simplex virus infection, and toxoplasmosis, as well as from erythroblastosis fetalis.

Manifestations of congenital syphilis may appear early (within the first 2 years of life, often at 2–10 weeks of age) or late (after 2 years). The earliest manifestations of congenital syphilis include rhinitis, or “snuffles” (23%); mucocutaneous lesions (35–41%); bone changes (61%), including periostitis detectable by x-ray examination of long bones; hepatosplenomegaly (50%); lymphadenopathy (32%); anemia (34%); jaundice (30%); thrombocytopenia; and leukocytosis. CNS invasion by T. pallidum is detectable in 22% of infected neonates. Neonatal death is usually due to pulmonary hemorrhage, second­ ary bacterial infection, or severe hepatitis. Late congenital syphilis (untreated after 2 years of age) is subclinical in 60% of cases; the clini­ cal spectrum in the remainder of cases may include interstitial keratitis (which occurs at 5–25 years of age), eighth-nerve deafness, and recur­ rent arthropathy. Neurosyphilis was documented in about one-quarter of untreated patients with late congenital syphilis in the preantibiotic era. Gummatous periostitis occurs at 5–20 years of age and, as in bejel, tends to cause destructive lesions of the palate and nasal septum. Classic stigmata include Hutchinson’s teeth (centrally notched, widely spaced, peg-shaped upper central incisors), “mulberry” molars (sixthyear molars with multiple, poorly developed cusps), saddle nose, and saber shins. LABORATORY EXAMINATIONS ■ ■DEMONSTRATION OF THE ORGANISM Historically, dark-field microscopy and immunofluorescence antibody staining have been used to identify T. pallidum in moist lesions such as chancres or condylomata lata, but these tests are rarely available outside of research laboratories. Sensitive and specific PCR tests have been developed but are not commercially available, although a number of laboratories perform in-house validated PCR testing. The recent advances in cultivation of T. pallidum in a tissue culture system have not yet been implemented in clinical laboratories. T. pallidum can be found in tissue by immunofluorescence or immu­ nohistochemical methods using specific monoclonal antibodies to

T. pallidum; some commercial polyclonal antibodies are cross-reactive with other spirochetes and should be avoided. Silver stains should be interpreted with caution because artifacts resembling T. pallidum are often seen. T. pallidum DNA has been detected by PCR in lesion swabs, tissue samples, blood, CSF, ocular fluid, urine, and oropharyngeal and rectal swabs. ■ ■SEROLOGIC TESTS FOR SYPHILIS Treponemal and Lipoidal Tests  There are two types of serologic tests for syphilis: lipoidal (formerly called “nontreponemal”) and trepo­ nemal. Both are reactive in persons with any treponemal infection, including syphilis, yaws, pinta, and bejel. The most widely used lipoidal antibody tests are the RPR and VDRL tests, which measure IgG and IgM directed against a cardiolipinlecithin-cholesterol antigen complex. The RPR test is easier to perform and uses unheated serum or plasma; it is the test of choice for rapid serologic diagnosis in a clinical setting. The VDRL test remains the standard for examining CSF and is superior to the RPR for this pur­ pose. Either test is recommended for screening and for quantitation of serum antibody. The titer generally reflects disease activity, rising during early syphilis, often exceeding 1:32 in secondary syphilis, and declining slowly thereafter without therapy. After treatment for early syphilis, a persistent fall by fourfold or more (e.g., a decline from 1:32 to 1:8) is considered an adequate response. VDRL titers do not cor­ respond directly to RPR titers, and sequential quantitative testing (as for response to therapy) must employ a single test. A reactive VDRL/ RPR screening test must be confirmed by a treponemal test to rule out a biological false-positive reaction. Treponemal tests measure antibodies to native or recombinant

T. pallidum antigens and include the fluorescent treponemal antibody– absorbed (FTA-ABS) test and the T. pallidum particle agglutination (TPPA) test, both of which are more sensitive for primary syphilis than the lipoidal tests. When used to confirm reactive lipoidal test results,

treponemal tests have a very high positive predictive value for diagno­ sis of syphilis.

Treponemal enzyme or chemiluminescence immunoassays (EIAs/ CIAs), based largely on reactivity to recombinant antigens, are auto­ mated and now used as screening tests by large laboratories. When these tests are used for screening, a high proportion of sera reactive by EIA/CIA are nonreactive by subsequent lipoidal tests. Such sera should be examined in the TPPA test, which includes different antigens and a different platform. If the TPPA test is nonreactive, the patient is unlikely to have syphilis; if it is reactive, the patient is likely to have current or past syphilis. Both lipoidal and treponemal tests may be nonreactive in early primary syphilis, although treponemal tests are slightly more sensitive (85–90%) during this stage than lipoidal tests (~80%). All tests are reactive during secondary syphilis. (Fewer than 1% of patients with high titers have a lipoidal test that is nonreactive or weakly reactive with undiluted serum but is reactive with diluted serum—the prozone phenomenon.) VDRL and RPR sensitivity and titers may decline in untreated persons with late latent syphilis, but treponemal tests remain reactive in late syphilis. After treatment for early syphilis, lipoidal test titers will generally decline or the tests will become nonreactive, whereas treponemal tests often remain reactive after therapy and are not helpful in determining the infection status of persons with past syphilis. There is some concern in the literature about persons in whom the lipoidal test titer fails to become nonreactive or remains reactive in low titer after treatment; this is more commonly seen in persons with repeated episodes of syphilis. The implications in such cases are unclear, but re-treatment rarely achieves the desired goal and is not recommended in the absence of clinical findings. CHAPTER 187 False-Positive Serologic Tests for Syphilis  The lipid antigens of lipoidal tests are similar to those found in human tissues, and these tests may be reactive (usually with titers ≤1:8) in persons without treponemal infection, largely limited to persons with autoimmune conditions or injection drug use. Among patients being screened for syphilis because of risk factors, clinical suspicion, or history of expo­ sure, ~1% of reactive lipoidal tests are falsely positive. In a patient with a false-positive lipoidal test, syphilis is excluded by a nonreactive treponemal test. Syphilis False-positive reactions may also occur with treponemal tests, par­ ticularly the EIA/CIA tests. Screening a low-prevalence population for syphilis with a treponemal test may result in true-positive reactions being outnumbered by false-positive reactions, leading to unnecessary treatment. Thus, screening with lipoidal tests is highly recommended. ■ ■EVALUATION FOR NEURO-, OCULAR, AND

OTIC SYPHILIS Involvement of the CNS is detected by examination of CSF for mono­ nuclear pleocytosis (>5 white blood cells/μL), increased protein con­ centration (>45 mg/dL), or CSF VDRL reactivity. Elevated CSF cell counts and protein concentrations are not specific for neurosyphilis and may be confounded by HIV co-infection. Because CSF pleocytosis may also be due to HIV, some studies have suggested using a CSF white cell cutoff of 20 cells/μL as diagnostic of neurosyphilis in HIV-infected patients with syphilis. The CSF VDRL test is highly specific and, when reactive, is considered diagnostic of neurosyphilis; however, this test is insensitive and may be nonreactive even in cases of symptomatic neu­ rosyphilis. The RPR test should not be substituted for the VDRL test for CSF examination. The FTA-ABS test on CSF is reactive far more often than the CSF VDRL test in all stages of syphilis, but reactivity may reflect passive transfer of serum antibody into the CSF. A nonreactive FTA-ABS test on CSF, however, may be used to rule out asymptomatic neurosyphilis. All T. pallidum–infected patients with signs or symptoms consistent with neurologic disease (e.g., meningitis, hearing loss) should have a CSF examination, regardless of disease stage. Persons with suspected ophthalmic disease (e.g., uveitis, iritis) should have a thorough ocular examination, with cranial nerve evaluation. Hearing loss, which can occur at any stage of syphilis, may be an isolated finding or may be due

to neurosyphilis (involvement of the eighth cranial nerve). If there is no cranial nerve dysfunction in persons with ocular or otic manifesta­ tions, no CSF exam is required; conversely, a finding of cranial nerve involvement indicates the need for CSF examination. All persons with ocular or otic syphilis should nonetheless be treated as for neurosyphi­ lis regardless of CSF findings.

The appropriate management of asymptomatic persons is less clear. Lumbar puncture on all asymptomatic patients with untreated syphilis is impractical and unnecessary. Even at high doses, penicillin G ben­ zathine fails to result in treponemicidal drug levels in CSF, and viable

T. pallidum have been isolated from the CSF of patients (with and with­ out HIV infection) after penicillin G benzathine treatment for early syphilis. Therefore, it is important to identify persons at higher risk for having or developing neurosyphilis so that appropriate treatment may be given. Large-scale prospective studies have shown that patients with RPR titers of ≥1:32 are at higher risk of having neurosyphilis (11fold and 6-fold higher in HIV-infected and HIV-uninfected persons, respectively), as are HIV-infected patients with CD4+ T-cell counts of ≤350/μL. Persons with active tertiary syphilis and those in whom treatment failure is suspected should also have their CSF examined to determine appropriate therapy. ■ ■EVALUATION OF HIV-INFECTED PATIENTS FOR SYPHILIS Because persons at highest risk for syphilis are also at increased risk for HIV infection, these two infections frequently coexist. There is evidence that syphilis and other genital ulcer diseases are important risk factors for acquisition and transmission of HIV infection. Some manifestations of syphilis may be altered in patients with concurrent untreated HIV infection, and multiple cases of neurologic relapse after standard therapy have been reported in these patients. PART 5 Infectious Diseases Persons with newly diagnosed HIV infection should be tested for syphilis; conversely, all patients with newly diagnosed syphilis should be tested for HIV infection. Some authorities, persuaded by reports of persistent T. pallidum in CSF of HIV-infected persons after standard therapy for early syphilis, have recommended CSF examination for evidence of neurosyphilis for all co-infected patients, regardless of the stage of syphilis, with treatment for neurosyphilis if CSF abnormalities are found. Others, on the basis of their own clinical experience, think that standard therapy—without CSF examination—is sufficient for all cases of early syphilis in HIV-infected patients without neurologic signs or symptoms. All persons with HIV infection and early syphilis should receive careful neurologic, ophthalmic, and otologic examina­ tions, including cranial nerve assessment; if cranial nerve dysfunction or neurologic signs or symptoms are found, a CSF examination is war­ ranted to inform treatment. As described above, RPR titer and CD4+ T-cell count can be used to identify patients at higher risk of neurosyphilis who might benefit from lumbar puncture, although some cases of neurosyphilis will be missed even when these criteria are used. Serologic testing after treat­ ment is important for all patients with syphilis, particularly for those also infected with HIV. TREATMENT Syphilis TREATMENT OF ACQUIRED SYPHILIS The CDC’s 2021 guidelines for the treatment of syphilis are sum­ marized in Table 187-1 and are discussed below. Penicillin G is the drug of choice for all stages of syphilis. T. pallidum is killed by very low concentrations of penicillin G, although a long period of exposure to penicillin is required because of the unusually slow rate of multiplication of the organism. Penicillin G benzathine is the preferred treatment for uncomplicated syphilis, with aqueous peni­ cillin being used for ocular, otic, and neurosyphilis. The efficacy of penicillin against syphilis remains undiminished after 75 years of use, and there is no evidence of penicillin resistance in T. pallidum. The current (2023) extreme shortage of penicillin G benzathine

TABLE 187-1  Recommendations for the Treatment of Syphilisa PATIENTS WITHOUT PENICILLIN ALLERGY PATIENTS WITH CONFIRMED PENICILLIN ALLERGY STAGE OF SYPHILIS Primary, secondary, or early latent CSF normal or not examined: Penicillin G benzathine (single dose of 2.4 mU IM) CSF abnormal: Treat as neurosyphilis CSF normal or not examined: Doxycycline (100 mg PO bid) or tetracycline HCl (500 mg PO qid) for 2 weeks CSF abnormal: Treat as neurosyphilis Late latent (or latent of unknown duration), cardiovascular, or benign tertiary CSF normal or not examined: Penicillin G benzathine (2.4 mU IM weekly for 3 weeks) CSF abnormal: Treat as neurosyphilis CSF normal and patient not infected with HIV: Doxycycline (100 mg PO bid) or tetracycline HCl (500 mg PO qid) for 4 weeks CSF normal and patient infected with HIV: Desensitize and treat with penicillin if compliance cannot be assured CSF abnormal: Treat as neurosyphilis Neurosyphilis, ocular syphilis, or otic syphilis Aqueous crystalline penicillin G (18–24 mU/d IV, given as 3–4 mU q4h or continuous infusion) for 10–14 days or Aqueous procaine penicillin G (2.4 mU/d IM) plus oral probenecid (500 mg qid), both for 10–14 days Desensitize and treat with penicillin Syphilis in pregnancy According to stage Desensitize and treat with penicillin aSee text for indications for CSF examination. Abbreviations: CSF, cerebrospinal fluid; mU, million units. Source: Adapted from the 2021 Sexually Transmitted Diseases Treatment Guidelines from the Centers for Disease Control and Prevention. Available from https://www. cdc.gov/std/treatment-guidelines/default.htm. has significantly complicated the management and control of the syphilis outbreak in the United States and elsewhere, increasing the need to use alternative antibiotics. Doxycycline, normally the second-line treatment, is now being used widely for men and non­ pregnant women with uncomplicated syphilis, reserving penicillin G benzathine for pregnant women. Other antibiotics effective in syphilis include the tetracyclines and the cephalosporins. Amino­ glycosides and spectinomycin inhibit T. pallidum only in very large doses, and the sulfonamides and most quinolones are inactive. Azithromycin showed significant promise as an effective oral agent against T. pallidum; however, strains harboring 23S rDNA muta­ tions that confer macrolide resistance are widespread. Such strains represent >99% of recent isolates from large U.S., European, and Chinese cities, although the prevalence of resistant strains varies by geographic location. Routine treatment of syphilis with azithromy­ cin is not recommended. Careful follow-up of any patient treated for syphilis with azithromycin must be assured. Several additional antibiotics are actively being tested for efficacy against human syphilis infection. Early Syphilis Patients and Their Contacts  Penicillin G benza­ thine is the most widely used agent for the treatment of early syphi­ lis (2.4 million units; Table 187-1) and for preventive treatment of individuals exposed to infectious syphilis within the previous 3 months. The regimens recommended for prevention are the same as those recommended for early syphilis. Penicillin G benzathine cures >95% of cases of early syphilis, although clinical relapse can follow treatment, particularly in patients with untreated HIV infection. Because the risk of neurologic relapse may be higher in HIV-infected patients, CSF examination may be recommended for

HIV-seropositive individuals with syphilis of any stage, particularly those with a serum RPR titer of ≥1:32 or a CD4+ T-cell count of ≤350/μL. Therapy appropriate for neurosyphilis should be given if there is any evidence of CNS infection. Late Latent Syphilis or Syphilis of Unknown Duration  If the CSF is normal or is not examined, the recommended treatment is penicillin G benzathine (7.2 million units total; Table 187-1). If CSF abnormalities are found, the patient should be treated for neurosyphilis. Tertiary Syphilis  This category includes persons with gummas (“benign”), cardiovascular syphilis, or signs and symptoms of late neurosyphilis. CSF examination should be performed. If the CSF is normal, the recommended treatment is penicillin G benzathine (7.2 million units total; Table 187-1). If CSF is abnormal, the patient should be treated for neurosyphilis. The clinical response to treat­ ment for benign tertiary syphilis is usually impressive, but responses in cardiovascular syphilis are not dramatic because aortic aneurysm and aortic regurgitation cannot be reversed by antibiotics. Syphilis in Penicillin-Allergic Patients  For penicillin-allergic patients with syphilis, a 2-week (early syphilis) or 4-week (late or late latent syphilis) course of therapy with doxycycline or tetracy­ cline is recommended (Table 187-1). These regimens appear to be quite effective in early syphilis but have not been tested for late or late latent syphilis, and compliance may be problematic. Limited studies suggest that ceftriaxone (1 g/d, given IM or IV for 8–10 days) is effective for early syphilis, and there are reports of the suc­ cess of 2 g/d for ocular syphilis. These nonpenicillin regimens have not been carefully evaluated in HIV-infected individuals and should be used with caution. If compliance and follow-up are not assured, penicillin-allergic HIV-infected persons with late latent or late syphilis should be desensitized and treated with penicillin. Neurosyphilis  Penicillin G benzathine, even at high doses, does not produce treponemicidal concentrations of penicillin G in CSF and should not be used for treatment of neurosyphilis. Asymp­ tomatic neurosyphilis may relapse as symptomatic disease after treatment with benzathine penicillin, and the risk of relapse may be higher in immunosuppressed HIV-infected patients. Both symp­ tomatic and asymptomatic neurosyphilis should be treated with aqueous penicillin (Table 187-1). Administration of either IV aque­ ous crystalline penicillin G or of IM aqueous procaine penicillin G plus oral probenecid in recommended doses is thought to ensure treponemicidal concentrations of penicillin G in CSF. The clinical response to penicillin therapy for meningeal syphilis is dramatic, but treatment of neurosyphilis with existing parenchymal damage may only arrest disease progression. No data suggest that additional therapy (e.g., penicillin G benzathine for 3 weeks) would be benefi­ cial after treatment for neurosyphilis. The use of antibiotics other than penicillin G for the treat­ ment of neurosyphilis has not been studied, although limited data suggest that 1−2 g/d of IV ceftriaxone for 10−14 days and oral doxycycline, 200 mg twice daily for 21 days, may be used. Until further studies confirm these regimens, for patients with confirmed penicillin allergy, desensitization and treatment with penicillin are recommended. Management of Syphilis in Pregnancy  Every pregnant woman should undergo a lipoidal screening serologic test at the first pre­ natal visit and, if at high risk of re-exposure, again in the third trimester and at delivery. In the untreated pregnant patient with presumed syphilis, expeditious treatment appropriate to the stage of the disease is essential. Patients should be warned of the risk of a Jarisch-Herxheimer reaction, which may be associated with mild premature contractions but rarely results in premature delivery. Penicillin is the only recommended agent for the treatment of syphilis in pregnancy. If the patient has a documented penicillin allergy, desensitization and penicillin therapy should be undertaken according to the CDC’s 2021 guidelines. After treatment during early pregnancy, a quantitative lipoidal test should be repeated

8 weeks following treatment unless reinfection is suspected and again at delivery. Treated women may not achieve a fourfold decline in titer before delivery, but for those in whom antibody titers rise by fourfold, re-treatment is warranted, with careful evaluation of the neonate at birth. EVALUATION AND MANAGEMENT OF CONGENITAL SYPHILIS All infants born to women who are untreated or who were treated after a reactive lipoidal serologic test result should be carefully examined at birth for evidence of congenital syphilis. Samples from suspicious lesions and the placenta or umbilical cord should be examined (immunohistochemistry or Clinical Laboratory Improvement Amendments [CLIA]-validated PCR) for presence of T. pallidum. Long bone radiographs may provide evidence of the periostitis of congenital syphilis.

Whether or not they are infected, newborn infants of women with reactive serologic tests may themselves have reactive tests because of transplacental transfer of maternal IgG antibodies. If the neonatal titer is fourfold higher than the mother’s, infec­ tion is indicated and treatment is warranted; CSF examination is recommended. For asymptomatic infants born to women treated adequately with penicillin during the first or second trimester of pregnancy, regular quantitative lipoidal tests may be performed to monitor for appropriate reduction in neonatal antibody titers. Ris­ ing or persistent titers indicate infection, and the infant should be treated. Detection of neonatal IgM antibody is insensitive, and no commercially available test is currently recommended. CHAPTER 187 An infant born to a seropositive mother should be treated at birth if (1) the treatment status of the seropositive mother is unknown; (2) the mother received inadequate or nonpenicillin therapy; (3) the mother received penicillin therapy in the third trimester; or (4) the infant may be difficult to follow. The CSF should be examined to obtain baseline values before treatment. Penicillin is the only recom­ mended drug for the treatment of syphilis in infants. More detailed recommendations for the treatment of infants and older children are included in the CDC’s 2021 treatment guidelines. JARISCH-HERXHEIMER REACTION A dramatic although self-limited reaction consisting of fever, chills, myalgia, headache, tachycardia, and increased respiratory rate may follow the initiation of treatment for syphilis. This reaction is thought to be a response to lipoproteins released by dying T. pal­ lidum organisms. The Jarisch-Herxheimer reaction occurs in ~50% of patients with primary syphilis, 90% of those with secondary syphilis, and a lower proportion of persons with later-stage disease. Defervescence takes place within 12–24 h. In secondary syphilis, erythema and edema of cutaneous lesions may increase. Patients should be warned to expect such developments, which can be managed with symptom-based treatment; steroid therapy is not required for this mild transient reaction. Syphilis FOLLOW-UP EVALUATION OF RESPONSES TO THERAPY Efficacy of treatment should be assessed by clinical evaluation and monitoring of the quantitative VDRL or RPR titer for a fourfold decline (e.g., from 1:32 to 1:8). Patients with primary or secondary syphilis should be examined 6 and 12 months after treatment, and persons with latent or late syphilis at 6, 12, and 24 months. More fre­ quent clinical and serologic examination (3, 6, 9, 12, and 24 months) is recommended for patients concurrently infected with HIV, regard­ less of the stage of syphilis. After successful treatment of seropositive first-episode primary or secondary syphilis, the VDRL or RPR titer progressively declines; the test becomes nonreactive by 12 months in 40–75% of seroposi­ tive primary cases and in 20–40% of secondary cases. A minority of patients treated for early syphilis may experience a one-dilution titer increase within 14 days after treatment; however, this early ele­ vation does not significantly affect the serologic outcome at 6 months after treatment. In patients with HIV infection or a history of prior syphilis, VDRL and RPR tests are less likely to become nonreactive.

74 - 188 Endemic Treponematoses

188 Endemic Treponematoses

Rates of decline of serologic titers appear to be slower, and serologi­ cally defined treatment failures more common among HIV-infected patients than among those without HIV co-infection; however, effective ART may reduce these differences. Re-treatment should be considered if serologic responses are not adequate or if clinical signs persist or recur. The CSF may be examined, with treatment for neurosyphilis if CSF is abnormal and treatment for late latent syphilis if CSF is normal. Patients treated for late latent syphilis frequently have low initial VDRL or RPR titers and may not have a fourfold decline after therapy with penicillin. In such patients, re-treatment is not warranted unless the titer rises or signs and symptoms of syphilis appear. Because treponemal tests may remain reactive despite treatment for seropositive syphilis, these tests are not useful in following the response to therapy.

The activity of neurosyphilis (symptomatic or asymptomatic) correlates best with CSF pleocytosis, and this measure provides the most sensitive index of response to treatment. Repeat CSF exami­ nations may be performed every 6 months until the cell count is normal. An elevated CSF cell count falls to normal in 3–12 months in adequately treated HIV-uninfected patients. The persistence of mild pleocytosis in HIV-infected patients may be due to the pres­ ence of HIV in CSF; this scenario may be difficult to distinguish from treatment failure. Elevated levels of CSF protein fall more slowly, and the CSF VDRL titer declines gradually over several years. In patients treated for neurosyphilis, a fourfold reduction in serum RPR titer has been positively correlated with normalization of CSF abnormalities; this correlation is stronger in HIV-uninfected patients and in HIV-infected patients receiving effective ART. Thus, multiple follow-up CSF examinations may not be necessary in patients treated for neurosyphilis in whom serologic titers are falling appropriately. PART 5 Infectious Diseases IMMUNITY TO SYPHILIS The rate of development of acquired resistance to T. pallidum after nat­ ural or experimental infection depends on both the size of the infecting inoculum and the duration of infection before treatment. Both humoral and cellular responses are important in the healing of early lesions. Cel­ lular infiltration, predominantly by T lymphocytes and macrophages, produces an interferon γ–dominated cytokine milieu and results in the clearance of organisms by activated macrophages. Specific antibod­ ies to surface antigens enhance phagocytosis. Antigenic variation of the TprK protein contributes to development of subsequent stages of syphilis, persistence of infection, and susceptibility to reinfection with another strain. Comparative genomic studies have revealed genes with sequence variations among T. pallidum strains, leading to development of molecular typing methods used to examine syphilis outbreaks. Sev­ eral laboratories are actively working to develop effective vaccines that stimulate the relevant protective immunologic functions to attenuate or prevent lesion development and reduce dissemination of T. pallidum to distant anatomic sites. ■ ■FURTHER READING Hamill M et al: State of the art review: Neurosyphilis. Clin Infect Dis 78:e57, 2024. Luetkemeyer A et al: Postexposure doxycycline to prevent bacterial sexually transmitted infections. N Engl J Med 388:1296, 2023. Lukehart S et al: Immunization with a tri-antigen syphilis vaccine significantly attenuates chancre development, reduces bacterial load, and inhibits dissemination of T. pallidum. Vaccine 40:7676, 2022. Tantalo L et al: Antimicrobial susceptibility of Treponema pallidum subspecies pallidum: An in-vitro study. Lancet Microbe 4:e994,

Workowski K et al: Sexually transmitted infections treatment guide­ lines, 2021. MMWR Recomm Rep 70:39, 2021.

Sheila A. Lukehart, Lorenzo Giacani

Endemic Treponematoses The endemic treponematoses are chronic diseases that are generally trans­ mitted by direct contact, usually during childhood and, like syphilis, can cause severe late manifestations years after initial infection. These diseases are caused by spirochete bacteria closely related to Treponema pallidum subspecies pallidum, the etiologic agent of syphilis (Chap. 187). Yaws, pinta, and bejel (endemic syphilis) have traditionally been distinguished from venereal syphilis by mode of transmission, age of acquisition, geo­ graphic distribution, and clinical features; however, there is overlap for each of these factors. Most of our “knowledge” about these infections is based on observations by health care workers who have visited endemic areas during the past 70 years. Except for the ongoing programs of mass drug administration (MDA) for yaws eradication promoted by the World Health Organization (WHO), virtually no well-designed studies of the natural history, diagnosis, or treatment of these infections have been conducted. The four classically defined treponemal infections, modified by current knowledge, are compared in Table 188-1. ■ ■EPIDEMIOLOGY Generally, yaws flourishes in moist tropical areas (Fig. 188-1); bejel has been found primarily in arid climates of West Africa and the Middle East; and pinta has been found in temperate foci in the Americas. Because of the ongoing yaws eradication programs, some survey data are available for that disease, but no epidemiologic data are available for bejel and pinta, aside from sparse case reports. Thus, the current extent of these infections is unknown. The endemic treponematoses have traditionally been limited to rural areas of developing nations and have been seen in developed countries primarily among recent immigrants from endemic regions. In a WHO-sponsored mass eradication campaign from 1952 to 1969, >160 million people in Africa, Asia, and South America were examined for treponemal infections, and >50 million cases, contacts, and persons with latent infections were treated. This campaign reduced the prevalence of active yaws from >20% to <1% in many areas. In sub­ sequent decades, lack of focused surveillance and diversion of resources resulted in the documented resurgence of these infections in some regions. Of nearly 100 countries previously endemic for yaws, there are 16 countries with reported yaws cases during 2017–2022, and 3 others with suspected cases; there are no data for the remaining countries. In 2022, a total of 168,239 suspected cases were reported, primarily from Papua New Guinea, Côte d’Ivoire, Ghana, and Solomon Islands, all countries in which focused or integrated yaws detection programs and treatment trials are ongoing. Other areas of resurgent yaws morbidity in Africa include Cameroon, Togo, Benin, Central African Republic, Nigeria, Congo, Liberia, and Democratic Republic of the Congo. In Asia and the Pacific Islands, reports document active cases of yaws in Papua New Guinea, the Solomon Islands, Timor Leste, Vanuatu, the Philippines, and Indonesia. After years of focused programs, India was declared yaws-free in 2016. In the Americas, suspected yaws cases have been reported in Haiti, Colombia, and Ecuador, with no recent data for Peru, Brazil, Guyana, Suriname, and many Caribbean islands. The prevalence of bejel is estimated to be >10% in some regions of northern Ghana, Mali, Niger, Burkina Faso, and Senegal, although data are scarce. No data are available from formerly endemic regions of the Middle East. Recent molecular studies, however, have reported genital lesions caused by T. pallidum subspecies endemicum in persons from Cuba and Japan, and in a person with sexual contact in Pakistan. Pinta is thought to be limited to Central America and northern South America, where it is found rarely and only in very remote vil­ lages. The WHO lists 15 countries in Latin America where pinta was previously endemic; however, due to the lack of surveillance, the cur­ rent prevalence of pinta is unknown. In the early 1980s, clinical evi­ dence of pinta was discovered in 20% of the examined inhabitants of a

TABLE 188-1  Classic Comparison of the Agents of the Human Treponematoses and Their Associated Diseases FEATURE SYPHILIS YAWS BEJEL (ENDEMIC SYPHILIS) PINTA Organism T. pallidum subsp. pallidum T. pallidum subsp. pertenue T. pallidum subsp. endemicum T. carateum Common modes of transmission Sexual, transplacental, skin-to-skin Skin-to-skin Mouth-to-mouth or via shared drinking/eating utensils, skin to skin, sexuala Usual age of acquisition Sexual maturity or in utero Childhood Early childhood, adulthooda Late childhood Primary lesion Mucocutaneous ulcer (chancre) Papilloma, often ulcerative Mucosal papule, rarely seen Nonulcerating papule with satellites, pruritic Common location Genital, oral, anal Extremities Oral, occasionally sexuala Extremities, face Secondary lesions Cutaneous rash and mucosal lesions; condylomata lata, ocular and otic syphilis Cutaneous papillomatous or ulcerative lesions; condylomata lata, osteoperiostitis Infectious relapses ~25% Common Unknown Unknown Late complications Gummas, cardiovascular and central nervous system involvement Destructive gummas of skin, bone, cartilageb Destructive gummas of skin, bone, cartilageb Nondestructive, dyschromic, achromic macules aSexual transmission has been recently postulated for bejel (see text). bCentral nervous system involvement and congenital infection in the endemic treponematoses have been postulated by some investigators (see text). remote village in Panama. In 1987 and 1993, pinta cases were reported in indigenous populations living in the Amazon border region of Bra­ zil, Colombia, and Peru. More recently, in 1999, an active pinta lesion was identified in a Cuban tourist visiting Austria, and in 2021, a case of pinta was confirmed in southern Brazil. It is likely, therefore, that pinta is still endemic in some remote areas of Latin America. It is important to note the vast majority of the data described for the endemic treponematoses are reported as “suspected cases” that are based on clinical diagnosis of ulcers and are not confirmed. Serologic confirmation is only occasionally available, and even reactive trepone­ mal serologies may reflect past or latent treponemal infection that is unrelated to the etiology of the ulcer. Molecular studies of lesion swabs from Papua New Guinea indicate that ~30% of suspected yaws lesions contain T. pallidum DNA; most of these had T. pallidum DNA detected alone, while some contained both T. pallidum and Haemophilus ducreyi DNA (dual infection). Approximately 50% of lesions contained DNA Number of reported cases, 2022 ≥ 10,000 1,000 – 9,999 < 1,000 Interrupted transmission FIGURE 188-1  Geographic distribution of yaws in 2022. (Reproduced with permission from World Health Organization.)

Skin-to-skin Mucocutaneous lesions (mucous patch, split papule, condylomata lata); osteoperiostitis Pintides, pigmented, pruritic from H. ducreyi, but not T. pallidum. Data from other sites confirm these approximate proportions. Of the 20% of lesions that had neither of these pathogens identified, Streptococcus pyogenes was the most abundant organism detected in a metagenomic analysis. Both H. ducreyi and S. pyogenes have been shown to be common skin and environmen­ tal contaminants in yaws-endemic areas but may also serve as primary pathogens or secondary infections. Thus, the accuracy of the diagnosis in “yaws cases” reported to WHO is unclear. CHAPTER 188 Evidence of yaws-like and genital lesions, with treponemal serore­ activity, has been found in several species of wild nonhuman primates (NHP) in sub-Saharan Africa, providing evidence that there is an ani­ mal reservoir for yaws treponemes. At the genomic level, these organ­ isms are virtually identical to known human T. pallidum subspecies pertenue isolates. Although direct NHP-human transmission has not yet been confirmed, this finding likely has important implications for yaws eradication efforts. Endemic Treponematoses

Countries with suspected cases Previously endemic countries (current status unknown) Non-endemic countries Not applicable

■ ■MICROBIOLOGY

The etiologic agents of the endemic treponematoses are listed in Table 188-1. These little-studied organisms are morphologically identical to T. pallidum subspecies pallidum (the agent of syphilis), and no definitive antigenic differences among them have been identified to date. A controversy has existed for decades about whether the patho­ genic treponemes are truly separate organisms or represent a genetic continuum. Current genome sequencing indicates that, although yaws, bejel, and syphilis isolates clearly cluster in separate branches of phyloge­ netic trees, these organisms are 99.8% identical at the genomic level, and several studies support the ability of these pathogens to exchange DNA between subspecies. Genomic studies of many more isolates from differ­ ing geographic regions are needed to further understand the genomic relationship among the pathogenic treponemes to better inform nomen­ clature decisions. Currently, three of the four etiologic agents are classi­ fied as subspecies of T. pallidum; the fourth (T. carateum) remains a separate species simply because no pinta organisms have been available for genetic studies. Based on analysis of a limited number of strains and clinical samples available for genetic studies, molecular signatures that can differentiate the known strains of T. pallidum subspecies have been identified using approaches ranging from restriction fragment length polymorphism to whole genome sequencing. No obvious genetic poly­ morphisms have been identified that might be related to any distinct clinical characteristic of these diseases. ■ ■CLINICAL FEATURES All of the treponemal infections, including syphilis, are chronic and are characterized by defined disease stages, with a localized primary lesion, disseminated secondary lesions, periods of latency, and possible late lesions. Primary and secondary stages are more frequently over­ lapping in yaws and bejel than in syphilis, and the late manifestations of pinta are very mild relative to the destructive lesions of the other treponematoses. The current preference is to divide the clinical course of the endemic treponematoses into “early” and “late” stages, and this terminology is increasingly used for syphilis as well. PART 5 Infectious Diseases Historically, the major clinical distinctions made between syphilis and the other human treponematoses are the apparent lack of congenital transmission and of central nervous system (CNS) involvement in the “nonvenereal” infections. It is not known whether these distinctions are entirely accurate. Because of the high degree of genetic relatedness among the organisms, there is little biologic reason to think that T. pal­ lidum subspecies endemicum and T. pallidum subspecies pertenue would be unable to cross the blood–brain barrier or to invade the placenta. These organisms are like T. pallidum subspecies pallidum in that they obviously disseminate from the site of initial infection and can persist for decades. The lack of recognized congenital infection may be because childhood infections often reach the latent stage (characterized by a low to undetectable bacterial load) before girls reach sexual maturity, thus reducing the likelihood of fetal infection. Neurologic involvement may go unrecognized because of the lack of trained medical personnel in endemic regions, the delay of many years between infection and possible CNS manifestations, or a low rate of symptomatic CNS disease. Some D C B A FIGURE 188-2  Clinical manifestations of early yaws. A. Primary ulcer. B. Secondary papillomata. C. Periostitis, D. Polydactylitis. (Photos were taken during a yaws elimination trial in Papua New Guinea and are published with permission from Dr. Oriol Mitjà.)

published evidence supports congenital transmission as well as cardio­ vascular, ophthalmologic, and CNS involvement in yaws and endemic syphilis. Although the reported studies have been small, have failed to control for other causes of CNS abnormalities, and in some instances have not included serologic confirmation, it may be erroneous to accept unquestioningly the frequently repeated belief that these organisms fail to cause such manifestations. Yaws  Also known as pian, framboesia, or bouba, yaws is characterized by the development of one or several primary lesions (“mother yaw”) followed by multiple disseminated skin lesions. All early skin lesions are infectious and may persist for many months; cutaneous relapses are common during the first 5 years. Late manifestations, affecting ~10% of untreated persons, are destructive lesions of skin, bone, and joints. The infection is transmitted by direct contact with infectious lesions, often during play or group sleeping, and may be enhanced by disruption of the skin by insect bites or abrasions. While T. pallidum subspecies pertenue DNA has been detected on flies and fomites from endemic regions, there is not yet convincing evidence of insect or fomite transmis­ sion of infection. After an average of 3–4 weeks, the first lesion begins as a papule—usually on an extremity—and then enlarges (particularly during moist warm weather) to become ulcerated (Fig 188-2A) or pap­ illomatous (“raspberry-like”—thus the name “framboesia”). Regional lymphadenopathy develops, and the lesion usually heals within 6 months; dissemination is thought to occur during the early weeks of infection. A generalized secondary eruption, accompanied by general­ ized lymphadenopathy, appears either concurrent with or after the primary lesion, and may take several forms—macular, papular, or papil­ lomatous (Fig. 188-2B). Painful papillomatous lesions on the soles of the feet result in a crablike gait (“crab yaws”), and periostitis (Fig. 188-2C) may result in nocturnal bone pain and polydactylitis (Fig. 188-2D). Late yaws is manifested by gummas of the skin and long bones, hyperkerato­ ses of the palms and soles, osteitis and periostitis, and hydrarthrosis. The late gummatous lesions are characteristically extensive. Destruction of the nose, maxilla, palate, and pharynx is termed gangosa and is similar to the destructive lesions seen in leprosy and leishmaniasis. Bejel  The early lesions of bejel (endemic syphilis, siti, dichuchwa, njovera, skerljevo) are usually localized to mucocutaneous and mucosal surfaces. The infection is reportedly transmitted by direct contact, by kissing, by premastication of food, or by sharing of drinking and eating utensils. Recently, however, T. pallidum subspecies endemicum has been identified in genital lesions (clinically diagnosed as primary syphilitic chancres) and in secondary lesions in several settings, sug­ gesting sexual transmission. The initial lesion, usually an intraoral papule, may go unrecognized and is followed by mucous patches on the oral mucosa (Fig. 188-3A) and mucocutaneous lesions resembling the condylomata lata of secondary syphilis. This eruption may last for months or even years, and treponemes can readily be demonstrated in early lesions. Periostitis and regional lymphadenopathy are common. After a variable period of latency, late manifestations may appear, including osseous and cutaneous gummas. Destructive gummas, oste­ itis, and gangosa are more common in bejel than in yaws.

75 - 189 Leptospirosis

189 Leptospirosis

A B FIGURE 188-3  Clinical manifestations of bejel and pinta. A. Mucous patches of early bejel. B. Pigmented macules of early pinta. (Photos reprinted with permission from the Handbook of Endemic Treponematoses, PL Perine et al, Geneva, World Health Organization, Color Plates 54, 60; 1984.) Pinta  Pinta (mal del pinto, carate, azul, purupuru) is the most benign of the treponemal infections. This disease has three stages that are characterized by marked changes in skin color (Fig. 188-3B), but pinta does not appear to cause destructive lesions or to involve tissues other than the skin. The initial papule is most often located on the extremities or face and is pruritic. After 1 to many months of infection, numerous disseminated secondary lesions (pintides) appear. These lesions are initially red but become deeply pigmented, ultimately turning a dark slate blue. The secondary lesions are infectious and highly pruritic and may persist for years. Late pigmented lesions are called dyschromic macules and contain treponemes. Over time, most pigmented lesions show varying degrees of depigmentation, becoming brown and eventually white and giving the skin a mottled appearance. White achromic lesions are characteristic of the late stage. ■ ■DIAGNOSIS Diagnosis of the endemic treponematoses is based on clinical manifes­ tations and, when available, dark-field microscopy and serologic test­ ing. The same serologic tests—detecting antibodies to either lipoidal or treponemal antigens—that are used for syphilis (Chap. 187) become reactive during all treponemal infections. To date there is no antibody test that can discriminate among the treponemal infections. The nonsyphilis treponemal infections should also be considered in the evalu­ ation of a reactive syphilis serology in any person who has emigrated from an endemic area. Sensitive nucleic acid amplification–based assays can be used to confirm treponemal infection and to identify the etiologic agent in research and selected clinical laboratories. TREATMENT Endemic Treponematoses The current WHO-recommended therapy for patients and their con­ tacts includes either azithromycin (30 mg/kg, up to a maximum of

2 g) or benzathine penicillin G (1.2 million units IM for adults; 600,000 units for children <10 years old); these two drugs have been shown to be equivalent for early yaws. The recommended dose of benzathine penicillin G is half of that recommended for early syphi­ lis, yet no controlled efficacy studies have been conducted. Evidence of genetic resistance to penicillin is lacking, although relapsing lesions have been reported after penicillin treatment in Papua New Guinea. The efficacy of single-dose azithromycin provided the WHO’s revitalized yaws eradication program with a much easier regimen for use in mass treatment. Macrolide resistance has become common in circulating strains of T. pallidum subspecies pallidum in many parts of the world (Chap. 187), and analyses of yaws samples from Papua New Guinea and elsewhere have yielded evidence of mutations con­ ferring resistance to macrolide antibiotics, including azithromycin,

in a small number of treated patients. Careful molecular surveil­ lance is essential to monitor developing resistance in yaws-endemic areas. Limited data suggest the efficacy of tetracycline for treatment of yaws, but no data exist for other endemic treponematoses. Based solely on experience with syphilis, it is likely that doxycycline, tetra­ cycline (at doses appropriate for syphilis; Chap. 187), or ceftriaxone are alternatives, in addition to azithromycin, for patients allergic to penicillin. A Jarisch-Herxheimer reaction (Chap. 187) may follow treatment of endemic treponematoses. Lipoidal serologic titers (in the Venereal Disease Research Laboratory [VDRL] slide test or the

rapid plasma reagin [RPR] test) usually decline after effective therapy, but patients may not become seronegative.

■ ■CONTROL Buoyed by the successful elimination of yaws in India and the avail­ ability of an inexpensive, single-dose oral drug for treatment, in 2012, the WHO renewed its efforts to eradicate yaws globally by 2020. Based on the results of several pilot programs of MDA, however, the target year for eradication was extended to 2030. Initial enthusiasm has been dampened by several factors: (1) Pilot studies have indicated that a very high level of MDA coverage must be achieved and that multiple rounds of MDA are needed in the affected areas. Treatment must be followed by careful case detection and targeted treatment of cases and contacts. (2) Azithromycin resistance has emerged during MDA studies in Papua New Guinea. Although subsequent treatment with benzathine penicillin G was able to contain the spread of resistant organisms, such evidence suggests that there may be only a short window of time during which countries can successfully use azithromycin for yaws eradica­ tion. Antibiotic resistance is of particular concern because multiple rounds of MDA are likely to be required. Further, given the ongoing campaigns against trachoma using low-dose azithromycin MDA, often in populations also at high risk for yaws, more widespread macrolide resistance seems inevitable. (3) Lastly, the possible animal reservoir needs to be evaluated, particularly in Africa. Yaws elimination will require rapid implementation and scale-up of high-level drug coverage in endemic areas, and continued careful surveillance by local health centers will be essential for success of this timely and important effort. CHAPTER 189 Leptospirosis ■ ■FURTHER READING Giacani L, Lukehart SA: The endemic treponematoses. Clin Micro­ biol Rev 27:89, 2014. Janecková K et al: The genomes of the yaws bacterium, Treponema pallidum subsp. pertenue, of nonhuman primate and human origin are not genomically distinct. PLoS Negl Trop Dis 17:e0011602, 2023. John LN et al: Trial of three rounds of mass azithromycin administra­ tion for yaws eradication. N Engl J Med 386:47, 2022. Marco Goeijenbier, Jiři F. P. Wagenaar

Leptospirosis Leptospirosis is a globally important zoonotic disease whose appar­ ent reemergence is illustrated by recent outbreaks on virtually all continents. The disease is caused by pathogenic Leptospira species and is characterized by a broad spectrum of clinical manifestations, varying from asymptomatic infection to fulminant, fatal disease. In its mild form, leptospirosis may present as nonspecific symptoms such as fever, headache, and myalgia. Severe leptospirosis, characterized by the triad of jaundice, renal dysfunction, and hemorrhagic diathesis, is often referred to as Weil’s syndrome. With or without jaundice, severe pulmonary hemorrhage is increasingly recognized as an important presentation of severe disease.

PART 5 Infectious Diseases workers in the fishing industry. Risk factors include direct or indirect contact with animals, including exposure to water and soil contami­ nated with animal urine. Leptospirosis has also been recognized in deteriorating inner cities and suburban areas where rat and mouse populations are expanding. FIGURE 189-1  Differentiation of pathogenic, intermediate, and nonpathogenic (saprophytic) Leptospira species by molecular phylogenetic analysis using core genomes comparison (CgMLST). (Reproduced with permission from Dr. A Ahmed, Leptospirosis Reference Center, Academic Medical Center, Medical Microbiology, Amsterdam, The Netherlands.) ■ ■ETIOLOGIC AGENT Leptospira species are spirochetes belonging to the order Spirochaetales and the family Leptospiraceae. Traditionally, the genus Leptospira com­ prised two species: the pathogenic L. interrogans and the free-living L. biflexa, now designated L. interrogans sensu lato and L. biflexa sensu lato, respectively. Sixty-four Leptospira species with pathogenic (17 spe­ cies), intermediate (21 species), and nonpathogenic (26 species) status have now been described based on phylogenetic analyses (Fig. 189-1). Genome sequences of all Leptospira species have been published, and this will undoubtedly lead to a better understanding of the pathogen­ esis of leptospirosis. However, classification based on serologic differ­ ences better serves clinical, diagnostic, and epidemiologic purposes. Pathogenic Leptospira species are divided into serovars according to their antigenic composition. There are more than 260 known patho­ genic serovars, which are arranged in 26 serogroups. Leptospires are coiled, thin, highly motile organisms that have hooked ends and two periplasmic flagella, with polar extrusions from the cytoplasmic membrane that are responsible for motility (Fig. 189-2). These organisms are 6–20 μm long and ~0.1 μm in diameter; they stain poorly but can be seen microscopically by dark-field examination and after silver impregnation staining of tissues. Leptospires require special media and conditions for growth; it may take weeks to months for cultures to become positive. ■ ■EPIDEMIOLOGY Leptospirosis has a worldwide distribution. Infection occurs most commonly in the tropics and subtropics because the climate and occa­ sionally poor hygienic conditions favor the pathogen’s survival and distribution. In most countries, leptospirosis is an underappreciated problem. Most cases occur in men, with a peak incidence during the summer and fall in both the Northern and Southern Hemispheres and during the rainy season in the tropics. Reliable data on morbidity and mortality from leptospirosis have gradually started to appear. Current information on global human leptospirosis varies but indicates that ~1 million severe cases occur per year, with a mean case–fatality rate of nearly 10%. As a zoonosis, leptospirosis affects almost all mammalian species and represents a significant veterinary burden. Rodents, especially rats, are the most important reservoir, although other wild mammals as well as domestic and farm animals may also harbor these microor­ ganisms. Leptospires establish a symbiotic relationship with their host and can persist in the urogenital tract for years. Some serovars are generally associated with specific animals—e.g., Icterohaemorrhagiae and Copenhageni with rats, Grippotyphosa with voles, Hardjo with cattle, Canicola with dogs, and Pomona with pigs—but may occur in other animals as well. Leptospirosis presents as both an endemic and an epidemic disease. Transmission of leptospires may follow direct contact with urine, blood, or tissue from an infected animal or, more commonly, exposure to environmental contamination. The dogma that human-to-human transmission is very rare is challenged by recent findings on household clustering, asymptomatic renal colonization, and prolonged excretion of leptospires. Both of the latter features could imply human infection sources that are not recognized. Because leptospires can survive in a humid environment for many months, water is an important vehicle in their transmission. Epidemics of leptospirosis are not well understood. Outbreaks may result from exposure to floodwaters contaminated by urine from infected animals, as has been reported from several coun­ tries. However, it is also true that outbreaks may occur without floods, and floods often occur without outbreaks. The vast majority of infections with Leptospira cause no or only mild disease in humans. A small percentage of infections (~1%) lead to severe, potentially fatal complications. The proportion of leptospirosis cases that are mild is unknown because patients either do not seek or do not have access to medical care or because the nonspecific symp­ toms are interpreted as an influenza-like illness. Reported cases surely represent a significant underestimation of the total number. Certain occupational groups are at especially high risk, including veterinarians, agricultural workers, sewage workers, slaughterhouse employees, and

FIGURE 189-2  Transmission electron microscopic image of Leptospira interrogans invading equine conjunctival tissue. (Image kindly provided by Dr. JE Nally, National Animal Disease Center, U.S. Department of Agriculture, Ames, IA.) Recreational exposure and domestic-animal contact are prominent sources of leptospirosis. Recreational freshwater activities, such as canoeing, windsurfing, swimming, and waterskiing, place persons at risk for infection. Also, several out­ breaks have followed sporting events. For example, an outbreak took place in 1998 among athletes after a triath­ lon in Springfield, Illinois. Ingestion of one or more swallows of lake water during the swimming leg of the tri­ athlon was a prominent risk factor for illness. Heavy rains that preceded the triathlon, with consequent agri­ cultural runoff, are likely to have increased the level of leptospiral contamination in the lake water. In another outbreak, 42% of partici­ pants contracted leptospirosis during the 2000 Eco-Challenge-Sabah mul­ tisport endurance race in Malaysian Borneo. Swimming in the Segama River was shown to be an indepen­ dent risk factor. Furthermore, out­ breaks among athletes participating in the recently popular mud-runs are increasingly reported. Approximate time scale Incubation period Incubation Leptospires present in Blood CSF Urine Antibody titers High Low “Negative” Laboratory investigations Culture/PCR PCR In addition, leptospirosis is a trav­ eler’s disease. Large proportions of patients acquire the infection while traveling in tropical countries, usually during adventurous activities such as whitewater rafting, jungle trekking, and caving or spelunking. Recent data from the GeoSentinel Global Surveil­ lance Network described in detail 180 returned travelers (mostly male; 74%) with leptospirosis from January 1997 Serology Phases FIGURE 189-3  Biphasic nature of leptospirosis and relevant investigations at different stages of disease. Specimens 1

and 2 for serology are acute-phase serum samples; specimen 3 is a convalescent-phase serum sample that may facilitate detection of a delayed immune response; and specimens 4 and 5 are follow-up serum samples that can provide epidemiologic information, such as the presumptive infecting serogroup. CSF, cerebrospinal fluid. (Used with permission of [ASM], from Leptospirosis, PN Levett, 14:296, 2001; permission conveyed through Copyright Clearance Center, Inc. and Turner, Leptospirosis I, Transactions of the Royal Society of Tropical Medicine & Hygiene, 61:842, 1967. Permission is granted as per the terms of the STM Permissions Guidelines. Reproduced by permission of Oxford University Press on behalf of the Royal Society of Tropical Medicine & Hygiene.)

through December 2016. Infection was predominantly acquired in Southeast Asia (52% [n = 93]; mainly [n = 52] from Thailand); overall, 110 patients (59%) were hospitalized, and one patient died. Transmis­ sion via laboratory accidents has been reported but is rare. New data indicate that leptospirosis may develop after unanticipated immersion in contaminated water (e.g., in an automobile accident) more frequently than has generally been thought and can also result from an animal bite.

■ ■PATHOGENESIS Transmission occurs through cuts, abraded skin, or mucous mem­ branes, especially the conjunctival and oral mucosa. After entry, the highly motile organisms proliferate, cross tissue barriers, and dis­ seminate hematogenously to all organs (leptospiremic phase). During this initial incubation period, leptospires can be isolated from the bloodstream (Fig. 189-3). Clearly, Leptospira can survive in the non­ immune host by evading parts of the innate immune response such as complement-mediated killing and phagocytosis; however, earlier stud­ ies have highlighted the relation between an exaggerated proinflamma­ tory immune response and mortality. During the immune phase, the appearance of antibodies coincides with the disappearance of lepto­ spires from the blood. However, the bacteria persist in various organs, including liver, lung, kidney, heart, and brain. Autopsy findings illus­ trate the involvement of multiple organ systems in severe disease. Renal pathology shows both acute tubular damage and interstitial nephritis. Acute tubular lesions progress in time to interstitial edema and acute tubular necrosis. Severe nephritis is observed in patients who survive long enough to develop it and seems to be a secondary response to acute epithelial damage. The reported deregulation of the expression of several transporters along the nephron contributes to impaired sodium absorption, tubular potassium wasting, and polyuria. Histopathology of the liver shows focal necrosis (widespread hepatocellular necrosis is usually not found), foci of inflammation, and plugging of bile cana­ liculi. Hepatocyte apoptosis has also been documented. Experimental work showed infiltration of Leptospira in Disse space (perisinusoidal space) and migration between hepatocytes with detachment of the CHAPTER 189 Leptospirosis Months-years

Years Week 1

Acute stage Convalescent stage Uveitis ? Interstitial nephritis 2-30 days Fever Reservoir host Convalescent shedder Normal response Titers decline at varying rates Delayed Early treatment Anamnestic Blood CSF Urine Urine

Leptospiremia Leptospiruria and immunity

FIGURE 189-4  Severe pulmonary hemorrhage in leptospirosis. Left panel: Chest x-ray. Right panel: Gross appearance of right lower lobes of lung at autopsy. This patient, a 15-year-old from the Peruvian Amazonian city of Iquitos, died several days after presentation with acute illness, jaundice, and hemoptysis. Blood culture yielded Leptospira interrogans serovar Copenhageni/Icterohaemorrhagiae. (Adapted with permission from E Segura et al: Clin Infect Dis 40:343, 2005. © 2005 by the Infectious Diseases Society of America.) intercellular junctions and disruption of bile canaliculi leading to bile leakage. Petechiae and hemorrhages are observed in the heart, lungs (Fig. 189-4), kidneys (and adrenals), pancreas, liver, gastrointesti­ nal tract (including retroperitoneal fat, mesentery, and omentum), muscles, prostate, testes, and brain (subarachnoid bleeding). Several studies show an association between hemorrhage and thrombocytope­ nia. Although the underlying mechanisms of thrombocytopenia have not been elucidated, it seems likely that platelet consumption plays an important role. A consumptive coagulopathy may occur, with elevated markers of coagulation activation (thrombin–antithrombin complexes, prothrombin fragments 1 and 2, D-dimer), diminished anticoagulant markers (antithrombin, protein C), and deregulated fibrinolytic activ­ ity. Overt disseminated intravascular coagulation (DIC) has been doc­ umented in several clinical studies. Elevated plasma levels of soluble E-selectin and von Willebrand factor in patients with leptospirosis reflect endothelial cell activation. More specifically, markers of endo­ thelial cell activation correlate to disease severity in patients with severe leptospirosis. Experimental models show that pathogenic leptospires or leptospiral proteins are able to activate endothelial cells in vitro and to disrupt endothelial-cell barrier function, thus increasing permeability and promoting dissemination. Platelets have been shown to aggre­ gate on activated endothelium in the human lung, whereas histology reveals swelling of activated endothelial cells but no evident vasculitis or necrosis. Immunoglobulin and complement deposition have been demonstrated in lung tissue involved in pulmonary hemorrhage. PART 5 Infectious Diseases Leptospira species have a typical double-membrane cell wall struc­ ture harboring a variety of membrane-associated proteins, including an unusually high number of lipoproteins. The peptidoglycan layer is located close to the cytoplasmic membrane. The lipopolysaccharide (LPS) in the outer membrane has an unusual structure with relatively low endotoxic potency. However, host immunity depends on the pro­ duction of circulating antibodies to serovar-specific LPS. It is unclear whether other antigens play a significant role in protective humoral immunity. Pathogenic Leptospira contain a variety of genes coding for proteins involved in motility and in cell and tissue adhesion and invasion that represent (potential) virulence factors. Many of these are surfaceexposed outer-membrane proteins (OMPs). It is likely that several surface-exposed proteins mediate pathogen–host cell interactions, and these proteins may represent candidate vaccine components. Although animal-model studies have shown various degrees of vaccine efficacy for various putative virulence-associated OMPs, it is not yet clear whether such proteins elicit acceptable levels of sterilizing immunity. Ongoing breakthroughs in genetic manipulation of Leptospira and whole genome sequencing will undoubtedly provide more insight into the biology and virulence of this pathogen. ■ ■CLINICAL MANIFESTATIONS Although leptospirosis is a potentially fatal disease with bleeding and multiorgan failure as its clinical hallmarks, the majority of cases are

thought to be relatively mild, presenting as the sudden onset of a febrile illness. The incubation period is usually 1–2 weeks but ranges from 2 to 30 days. Leptospirosis is classically described as biphasic. The acute leptospiremic phase is char­ acterized by fever of 3–10 days’ duration, during which time the organism can be cultured from blood and detected by polymerase chain reaction (PCR). During the immune phase, resolution of symptoms may coincide with the appearance of antibodies, and leptospires can be cultured from the urine. The distinction between the first and second phases is not always clear: milder cases do not always include the second phase, and severe disease may be monophasic and fulminant. The idea that distinct clinical syndromes are associ­ ated with specific serogroups has been refuted, although some serovars tend to cause more severe disease than others. Mild Leptospirosis  Most patients are asymptomatic or only mildly ill and do not seek medical attention. Serologic evidence of past inapparent infection is frequently found in persons who have been exposed but have not become ill. Mild symptomatic leptospirosis usually presents as a flulike illness of sudden onset, with fever, chills, headache, nausea, vomiting, abdominal pain, conjunctival suffusion (redness without exudate), and myalgia. Muscle pain is intense and especially affects the calves, back, and abdomen. The headache is intense, localized to the frontal or retroorbital region (resembling that occurring in dengue), and sometimes accompanied by photophobia. Aseptic meningitis may be present and is more common among children than among adults. Although Leptospira can be cultured from the cere­ brospinal fluid (CSF) in the early phase, the majority of cases follow a benign course with regard to the central nervous system; symptoms disappear within a few days but may persist for weeks. Physical examination may include any of the following findings, none of which is pathognomonic for leptospirosis: fever, conjunctival suffusion, pharyngeal injection, muscle tenderness, lymphadenopathy, rash, meningismus, hepatomegaly, and splenomegaly. If present, the rash is often transient; may be macular, maculopapular, erythematous, or hemorrhagic (petechial or ecchymotic); and may be misdiagnosed as due to scrub typhus or viral infection. Lung auscultation may reveal crackles. Mild jaundice may be present. The natural course of mild leptospirosis usually involves spontane­ ous resolution within 7–10 days, but persistent symptoms have been documented. In the absence of a clinical diagnosis and antimicrobial therapy, the mortality rate in mild leptospirosis is low. Severe Leptospirosis  Although the onset of severe leptospirosis may be no different from that of mild leptospirosis, severe disease is often rapidly progressive and is associated with a case–fatality rate ranging from 1 to 50%. Higher mortality rates are associated with an age >40 years, altered mental status, acute renal failure, respiratory insufficiency, hypotension, and arrhythmias. The classic presentation, often referred to as Weil’s syndrome, encompasses the triad of hemor­ rhage, jaundice, and acute kidney injury. Patients die of multiorgan failure after septic shock and/or severe bleeding complications that most commonly involve the lungs (pul­ monary hemorrhage), gastrointestinal tract (melena, hematemesis), urogenital tract (hematuria), and skin (petechiae, ecchymosis, and bleeding from venipuncture sites). Pulmonary hemorrhage (with or without jaundice) is now recognized as a widespread public health problem, presenting with cough, chest pain, respiratory distress, and hemoptysis that may not be apparent until patients are intubated. Jaundice occurs in 5–10% of all patients with leptospirosis; it can be profound and give an orange cast to the skin but usually is not associ­ ated with fulminant hepatic necrosis. Physical examination may reveal an enlarged and tender liver. Acute kidney injury is common in severe disease, presenting after several days of illness, and can be either nonoliguric or oliguric. Typical

electrolyte abnormalities include hypokalemia and hyponatremia. Loss of magnesium in the urine is uniquely associated with leptospiral nephropathy. Hypotension is associated with acute tubular necrosis, oliguria, or anuria, requiring fluid resuscitation and sometimes vaso­ pressor therapy. Hemodialysis can be lifesaving, with renal function typically returning to normal in survivors. In severe leptospirosis, an altered mental status may reflect lepto­ spiral meningitis. The diagnosis of leptospirosis meningitis may be challenging since patients may be anicteric or lack other diagnostic hallmarks of severe leptospirosis. Without proper antibiotic treatment, a mortality rate of 13% has been reported; in contrast, among patients treated with antibiotics, the mortality rate is 2%. Neurologic sequelae are described until months after acute illness. Other syndromes include (necrotizing) pancreatitis, cholecystitis, skeletal muscle involvement, and rhabdomyolysis with moderately ele­ vated serum creatine kinase levels. Cardiac involvement is commonly reflected on the electrocardiogram as nonspecific ST- and T-wave changes. Repolarization abnormalities and arrhythmias are considered poor prognostic factors. Myocarditis has been described. Rare hemato­ logic complications include hemolysis, thrombotic thrombocytopenic purpura, and hemolytic-uremic syndrome. Long-term symptoms following severe leptospirosis include fatigue, myalgia, malaise, and headache and may persist for years. Autoim­ mune-associated uveitis, a potentially chronic condition, is a recog­ nized sequela of leptospirosis. ■ ■DIAGNOSIS The clinical diagnosis of leptospirosis should be based on an appropri­ ate exposure history combined with any of the protean manifestations of the disease. Returning travelers from endemic areas usually have a history of recreational freshwater activities or other mucosal or per­ cutaneous contact with contaminated surface waters or soil. For non­ travelers, recreational or accidental water/soil contact and especially occupational hazards that involve direct or indirect animal contact should be explored (see “Epidemiology,” above). Although biochemical, hematologic, and urinalysis findings in acute leptospirosis are nonspecific, certain patterns may suggest the diagnosis. Laboratory results usually show signs of a bacterial infec­ tion, including leukocytosis with a left shift and elevated markers of inflammation (C-reactive protein level, procalcitonin, and erythrocyte sedimentation rate). Thrombocytopenia (platelet count ≤100 × 109/L) is common and is associated with bleeding and renal failure. In severe disease, signs of coagulation activation may be present, varying from borderline abnormalities to a serious derangement compatible with DIC as defined by international criteria. The kidneys are invariably involved in leptospirosis. The absence of renal involvement does not rule out leptospirosis. However, when the kidneys are involved, related findings range from urinary sediment changes (leukocytes, erythro­ cytes, and hyaline or granular casts) and mild proteinuria in mild dis­ ease to renal failure and azotemia in severe leptospirosis. Nonoliguric hypokalemic renal insufficiency (see “Clinical Manifestations,” above) is characteristic of early leptospirosis. Serum bilirubin levels may be high, whereas rises in aminotransferase and alkaline phosphatase levels are usually moderate. Although clinical symptoms of pancreatitis are not a common finding, amylase levels are often elevated. When symp­ toms of meningitis develop, examination of the CSF shows pleocytosis that can range from a few cells to >1000 cells/μL, with a predominance of lymphocytes. Predominant polymorphonuclear pleocytosis has been reported. This phenomenon may be related to the timing of the lumbar puncture: polymorphonuclear cells are thought to be found in early disease and are later replaced by lymphocytes. Although protein and glucose levels in the CSF are usually normal, protein levels may be slightly elevated. In severe leptospirosis, pulmonary radiographic abnormalities are more common than would be expected based on physical examination (Fig. 189-4). The most common radiographic finding is a patchy bilat­ eral alveolar pattern that corresponds to scattered alveolar hemorrhage. These abnormalities predominantly affect the lower lobes. Other find­ ings include pleura-based densities (representing areas of hemorrhage)

and diffuse ground-glass attenuation typical of acute respiratory dis­ tress syndrome (ARDS).

A definitive diagnosis of leptospirosis is based on isolation of the organism from the patient, on a positive result in the PCR, or on seroconversion or a rise in antibody titer. In cases with strong clinical evidence of infection, a single antibody titer of 1:200–1:800 (depend­ ing on whether the case occurs in a low- or high-endemic area) in the microscopic agglutination test (MAT) is required. Preferably, a fourfold or greater rise in titer is detected between acute- and convalescentphase serum specimens. Antibodies generally do not reach detectable levels until the second week of illness. The antibody response can be affected by early treatment with antibiotics. The MAT, which uses a battery of live leptospiral strains, and the enzyme-linked immunosorbent assay (ELISA), which uses a broadly reacting antigen, are the standard serologic procedures. The MAT usu­ ally is available only in specialized laboratories and is used for deter­ mination of the antibody titer and for tentative identification of the involved leptospiral serogroup—and, when epidemiologic background information is available, the putative serovar. This point underscores the importance of testing antigens representative of the serovars preva­ lent in the particular geographic area. However, cross-reactions occur frequently, and thus definitive identification of the infecting serovar or serogroup is not possible without isolation of the causative organism. Because serologic testing lacks sensitivity in the early acute phase of the disease (up to day 5), it cannot be used as the basis for a timely decision about whether to start treatment. In addition to the MAT and the ELISA, various rapid tests with diagnostic value have been developed, and some of these are com­ mercially available. These rapid tests mainly apply lateral flow, (latex) agglutination, or ELISA methodology and are reasonably sensitive and specific, although results reported in the literature vary, probably as a consequence of differences in test interpretation, (re)exposure risks, serovar distribution, and the use of biased serum panels. These methods do not require culture or MAT facilities and are useful in settings that lack a strong medical infrastructure. PCR methodologies, notably real-time PCR, have become increasingly widely implemented. Compared with serology, PCR offers a great advantage: the capacity to confirm the diagnosis of leptospirosis with a high degree of accuracy during the first 5 days of illness. CHAPTER 189 Leptospirosis ■ ■DIFFERENTIAL DIAGNOSIS The differential diagnosis of leptospirosis is broad, reflecting the diverse clinical presentations of the disease. Although leptospirosis transmission is more common in tropical and subtropical regions, the absence of a travel history does not exclude the diagnosis. When fever, headache, and myalgia predominate, influenza, SARS-CoV-2, and other common and less common (e.g., dengue and chikungunya) viral infections should be considered. Malaria, typhoid fever, ehrlichiosis, viral hepatitis, and acute HIV infection may mimic the early stages of leptospirosis and are important to recognize. Rickettsial diseases, den­ gue, and hantavirus infections (hemorrhagic fever with renal syndrome or hantavirus cardiopulmonary syndrome) share epidemiologic and clinical features with leptospirosis. Dual infections have been reported. In this light, it is advisable to conduct serologic testing for rickettsiae, dengue virus, and hantavirus when leptospirosis is suspected. When bleeding is detected, dengue hemorrhagic fever and other viral hemor­ rhagic fevers, including hantavirus infection, yellow fever, Rift Valley fever, filovirus infections, and Lassa fever, should be considered. TREATMENT Leptospirosis Severe leptospirosis should be treated with IV penicillin (Table 189-1) as soon as the diagnosis is considered. Leptospira are highly susceptible to a broad range of antibiotics, including the β-lactam antibiotics, cephalosporins, aminoglycosides, and macrolides, but are not susceptible to vancomycin, rifampicin, metronidazole, and chloramphenicol. Early intervention may prevent the development

76 - 190 Relapsing Fever

190 Relapsing Fever

TABLE 189-1  Treatment and Chemoprophylaxis of Leptospirosis in Adultsa INDICATION REGIMEN Treatment Mild leptospirosis Doxycyclineb (100 mg PO bid) or Amoxicillin (500 mg PO tid) or Ampicillin (500 mg PO tid) Moderate/severe leptospirosis Penicillin (1.5 million units IV or IM q6h) or Ceftriaxone (2 g/d IV) or Cefotaxime (1 g IV q6h) or Doxycyclineb (loading dose of 200 mg IV, then 100 mg IV q12h) Chemoprophylaxis   Doxycyclineb (200 mg PO once a week) or Azithromycin (250 mg PO once or twice a week) aAll regimens are given for 7 days. bDoxycycline should not be given to pregnant women or children. cThe efficacy of doxycycline prophylaxis in endemic or epidemic settings remains unclear. Experiments in animal models and a cost-effectiveness model indicate that azithromycin has a number of characteristics that may make it efficacious in treatment and prophylaxis. of major organ-system failure or lessen its severity. Although stud­ ies supporting antibiotic therapy have produced conflicting results, clinical trials are difficult to perform in settings patients frequently present for medical care in the later stages of disease. Antibiotics are less likely to benefit patients in whom organ damage has already occurred. Two open-label randomized studies comparing penicillin with parenteral cefotaxime, parenteral ceftriaxone, and doxycycline showed no significant differences among the antibiotics with regard to complications and mortality risk. Thus ceftriaxone, cefotaxime, or doxycycline is a satisfactory alternative to penicillin for the treat­ ment of severe leptospirosis. Antimicrobial susceptibility testing is not routine practice in individual cases of leptospirosis; to date, however, antibiotic resistance has not been reported in isolates from patients or the environment. PART 5 Infectious Diseases In mild cases, oral treatment with doxycycline, azithromycin, ampicillin, or amoxicillin is recommended. In regions where rick­ ettsial diseases are coendemic, doxycycline or azithromycin is the drug of choice. In rare instances, a Jarisch-Herxheimer reaction develops within hours after the initiation of antimicrobial therapy. Aggressive supportive care for leptospirosis is essential and can be life-saving. Patients with nonoliguric renal dysfunction require aggressive fluid and electrolyte resuscitation to prevent dehydra­ tion and precipitation of oliguric renal failure. Peritoneal dialysis or hemodialysis should be provided to patients with oliguric renal failure. Rapid initiation of hemodialysis has been shown to reduce mortality risk and typically is necessary only for short periods. Patients with pulmonary hemorrhage may have reduced pulmonary compliance (as seen in ARDS) and may benefit from mechanical ventilation with low tidal volumes to avoid high ventilation pres­ sures. Evidence is contradictory for the use of glucocorticoids and desmopressin as adjunct therapy for pulmonary involvement asso­ ciated with severe leptospirosis. ■ ■PROGNOSIS Most patients with leptospirosis recover. However, post-leptospirosis symptoms, mainly of a depression-like nature, may occur and persist for years after the acute disease. Mortality rates are highest among patients who are elderly and those who have severe disease (pulmo­ nary hemorrhage, Weil’s syndrome). Leptospirosis during pregnancy is associated with high fetal mortality rates. Long-term follow-up of patients with renal failure and hepatic dysfunction has documented good recovery of both renal and hepatic function. ■ ■PREVENTION Individuals who may be exposed to Leptospira through their occupa­ tions or their involvement in recreational freshwater activities should

be informed about the risks. Measures for controlling leptospirosis include avoidance of exposure to urine and tissues from infected animals through proper eyewear, footwear, and other protective equip­ ment. Targeted rodent control strategies have a potential benefit. Vaccines for agricultural and companion animals are generally available, and their use should be encouraged. The veterinary vaccine used in each area should contain the serovars known to be present in that area in order to prevent vaccine serovar mismatch. Unfortunately, some vaccinated animals potentially still excrete leptospires in their urine. Commercial vaccines for human leptospirosis are available in Japan, China, Cuba, and France. These vaccines, made with bacterins— inactivated Leptospira—provide short-term, serovar-specific immunity but cause strong adverse side effects; both usability and availability remain problematic. One of the largest-scale trials of vaccine efficacy for leptospirosis in humans has been reported from Cuba. However, no conclusions can be drawn about efficacy and adverse reactions because of insufficient study design and outcome details. The effi­ cacy of chemoprophylaxis with doxycycline (200 mg once a week) or azithromycin (in pregnant women and children) is being disputed, but focused pre- and postexposure administration is indicated in instances of well-defined short-term exposure (Table 189-1). Acknowledgment The authors gratefully acknowledge Dr. Marga G. A. Goris for her sub­ stantial contributions to this chapter in previous editions. ■ ■FURTHER READING Adler A: Leptospira and Leptospirosis. Berlin Heidelberg, SpringerVerlag, 2015. Azevedo IR et al: Human leptospirosis: In search for a better vaccine. Scand J Immunol 98:1, 2023. de Vries SG et al: Leptospirosis among returned travelers: A GeoSentinel Site Survey and Multicenter Analysis−1997−2016. Am J Trop Med Hyg 99:127, 2018. Haake DA, Levett PN: Leptospirosis in humans. Curr Top Microbiol Immunol 387:65, 2015. Levett PN: Leptospirosis. Clin Microbiol Rev 14:296, 2001. Petakh P et al: Current treatment options for leptospirosis: A minireview. Front Microbiol 15:1403765, 2024. Sykes JE et al: A global one health perspective on leptospirosis in humans and animals. J Am Vet Med Assoc 260:1589,2022. van Samkar A et al: Suspected leptospiral meningitis in adults: Report of four cases and review of the literature. Neth J Med 73:464, 2015. Vincent AT et al: Revisiting the taxonomy and evolution of pathoge­ nicity of the genus Leptospira through the prism of genomics. PLoS Negl Trop Dis 13:e0007270, 2019. Alan G. Barbour

Relapsing Fever Relapsing fever is caused by infection with any of several species of Borrelia spirochetes. It occurs in three different clinical and epide­ miologic forms: louse-borne relapsing fever (LBRF), soft tick relaps­ ing fever (STRF), and hard tick relapsing fever (HTRF). Physicians in ancient Greece distinguished LBRF from other febrile disorders by its characteristic clinical presentation: two or more fever episodes sepa­ rated by varying periods of well-being. In the nineteenth century, LBRF was one of the first diseases to be associated with a specific microbe by virtue of its characteristic laboratory finding: the presence of large numbers of spirochetes of the genus Borrelia in the blood. The host responds with systemic inflammation that results in an illness ranging from a flulike syndrome to sepsis. Other manifestations

are the consequences of central nervous system (CNS) involvement and disordered hemostasis. Antigenic variation of the spirochetes’ surface proteins accounts for the infection’s relapsing course. Acquired immunity follows the serial development of antibodies to each of the several variants appearing during an infection. Treatment with antibi­ otics results in rapid cure but at the risk of a moderate to severe JarischHerxheimer reaction. LBRF caused large epidemics well into the twentieth century and currently occurs in northeastern Africa and among migrants from that area. At present, however, most cases of relapsing fever are tick-borne in origin, with transmission from either soft-bodied ticks or hardbodied ticks. Sporadic cases and small outbreaks of STRF are focally distributed on most continents, with Africa and Central Asia most affected. In North America, the majority of reports of relapsing fever have been from the western United States and Canada and northern Mexico. Two other members of the genus, Borrelia miyamotoi and B. lonestari, are causes of HTRF, an acute febrile illness with nonspecific constitu­ tional symptoms and occasionally meningoencephalitis. ■ ■ETIOLOGIC AGENT Coiled, thin microscopic filaments that swim in one direction and then coil up before heading in another were first observed in the blood of patients with relapsing fever in the 1880s. These microbes were categorized as spirochetes and assigned to the genus Borrelia. The breakthrough cultivation medium was rich in ingredients, ranging from simple (e.g., N-acetylglucosamine) to more complex (e.g., serum). The limited biosynthetic capacity of Borrelia cells is accounted for by a genome content one-quarter that of Escherichia coli. Like other spirochetes, the helix-shaped Borrelia cells have two membranes, the outer of which is more loosely secured than in other double-membrane bacteria, such as E. coli. As a consequence, fixed organisms with damaged membranes can assume a variety of morphologies in smears and histologic preparations. The flagella of spirochetes run between the two membranes and are not on the cell surface. Although technically gram-negative, the 10- to 20-μm-long Borrelia cells, with a diameter of 0.2–0.3 μm, are too narrow to be seen by microscopy of Gram-stained slides. ■ ■EPIDEMIOLOGY Borrelia recurrentis, unlike the other relapsing fever Borrelia species, is acquired from an insect, the body louse (Pediculus humanus corporis), with humans serving as the sole reservoir in its life cycle (Table 190-1). Acquisition occurs not from the bite itself but from either rubbing the crushed insect’s hemolymph into the bite site or auto-inoculation into the conjunctivae or a wound. Although LBRF transmission is currently limited to Ethiopia, Eritrea, and Somalia, the disease has had a global TABLE 190-1  Relapsing Fever Borrelia Species, by RF Type, Geographic Region, and Vector SPECIES RELAPSING FEVER TYPE REGION(S) ARTHROPOD VECTOR(S) B. crocidurae Soft tick RF (STRF) West Africa Ornithodoros sonrai B. duttonii STRF East Africa, southern and central Africa O. moubata B. hermsii STRF Western North America O. hermsi B. hispanica STRF North Africa, southern Europe O. erraticus B. kalaharica STRF West Africa, southern Africa O. savignyi B. lonestari Hard tick RF (HTRF) Southern and eastern United States Amblyomma americanum B. mazzotti STRF Mexico, Central America O. talaje B. miyamotoi HTRF North America, Asia, Europe Ixodes pacificus, I. persulcatus, I. ricinus, I. scapularis B. nietonii STRF Western North America O. hermsii B. persica STRF Central Asia, Middle East O. tholozani B. puertoricensis STRF Central America O. puertoricensis B. recurrentis Louse-borne RF Africa, globala Pediculus humanus corporis (human body louse) B. turicatae STRF Southwestern United States, northern Mexico O. turicata B. venezuelensis STRF Central America, South America O. rudis aAlthough transmission is currently limited to Ethiopia and adjacent countries, B. recurrentis infection has had a global distribution in the past, and that potential remains.

FIGURE 190-1  Ornithodoros turicata soft ticks of different ages. distribution in the past, and that potential remains. Outbreaks of LBRF, often in association with typhus, can occur under circumstances of famine, refugee migration, war, homelessness, and incarceration. LBRF can occur in camps of migrants at a distance from their home countries. The several species of Borrelia that cause STRF have geographic distributions that correspond with those of their vectors: soft ticks of the genus Ornithodoros (Fig. 190-1). STRF is found on most continents but is absent in arctic environments. For most species, the reservoirs of infection are small to medium-sized mammals, usually rodents, but also pigs and other domestic animals living around human habitats. However, one species, Borrelia duttonii in sub-Saharan Africa, is largely maintained by tick transmission between human hosts. In North America, STRF occurs as single cases or small case clusters through transient exposure of persons to tick-infested buildings or caves where mammals have nests or sleep. The two main Borrelia species involved in North America are Borrelia hermsii and Borrelia nietonii in the mountainous west and Borrelia turicatae in arid southwestern and south-central regions. The soft tick vectors typically feed for no more than 30 min, usually while the victim is sleeping, and then leave undetected. Transovarial transmission from one generation of ticks to the next means that infection risk may persist in a dwelling long after incriminated mammalian reservoirs have been removed. CHAPTER 190 Borrelia miyamotoi is transmitted to humans from other mammals by different species of Ixodes hard ticks (e.g., I. scapularis in the eastern United States and I. ricinus in Europe) that also transmit Lyme dis­ ease, babesiosis, anaplasmosis, and a viral encephalitis. B. miyamotoi is acquired through outdoor activities and through contact with ticks in forested and shrubby areas during recreation, work, or activities around the home, similarly to Lyme disease (Chap. 191). Among residents of most areas where B. miyamotoi and Borreliella (also called Borrelia) burgdorferi coexist, the prevalence of antibodies to the former is about one-fourth of that to the latter. In contrast to B. burgdorferi, B. miyamotoi is transmitted to the host soon after the tick begins to feed, and it may be acquired from tick larvae as well as nymphs and adults. A less common cause of HTRF is B. lonestari, which is trans­ mitted by Amblyomma americanum ticks of the southern and eastern United States. Relapsing Fever

■ ■PATHOGENESIS AND IMMUNITY STRF and HTRF spirochetes enter the body in the tick’s saliva with the onset of feeding. From an inoculum of a few cells, STRF spirochetes proliferate in the blood, doubling every 6 h to numbers of 106–107/mL or more. HTRF spirochetes grow more slowly in a mammalian host and attain lower peak concentrations in the blood. Borrelia species are extra­ cellular pathogens; their presence inside cells connotes dead bacteria after phagocytosis. Binding of the spirochetes to erythrocytes leads to aggregation of red blood cells, their sequestration in the spleen and liver, and hepatosplenomegaly and anemia. A bleeding disorder is probably the consequence of thrombocytopenia, impaired hepatic production of clotting factors, and/or blockage of small vessels by aggregates of spiro­ chetes, erythrocytes, and platelets. Some species (e.g., B. turicatae) are neurotropic and enter the brain, where they are comparatively sheltered from host immunity. Relapsing fever spirochetes can cross the maternalfetal barrier and cause placental damage and inflammation, leading to intrauterine growth retardation and congenital infection.

Although Borrelia species do not have potent exotoxins or a lipo­ polysaccharide endotoxin, they have abundant lipoproteins that acti­ vate Toll-like receptors on host cells, which leads to a proinflammatory process similar to that in endotoxemia, with elevations of tumor necro­ sis factor α, interleukin 6, and interleukin 8 concentrations. IgM antibodies specific for the serotype-defining surface lipoprotein appear after a few days of infection and soon reach a concentration that causes lysis of bacteria in the blood through either direct bactericidal action or opsonization. The release of lipoproteins and other bacterial products from dying bacteria provokes a “crisis,” during which there can be an increase in temperature, hypotension, and other signs of shock. A similar phenomenon occurring in some patients soon after the initiation of antibiotic treatment is characterized by an abrupt worsening of the patient’s condition, which is called a Jarisch-Herxheimer reaction (JHR). PART 5 Infectious Diseases ■ ■CLINICAL MANIFESTATIONS STRF and LBRF present with the sudden onset of fever. Febrile periods are punctuated by intervening afebrile periods of a few days; this pat­ tern occurs at least twice in STRF. The patient’s temperature is ≥39°C and may be as high as 43°C. The first fever episode often ends in a crisis lasting ~15–30 min and consisting of rigors, a further elevation in temperature, and increases in pulse and blood pressure. The crisis phase is followed by profuse diaphoresis, falling temperature, and hypotension, which usually persist for several hours. In LBRF, the first episode of fever is unremitting for 3–6 days; it is usually followed by a single milder episode. In STRF, multiple febrile periods last 1–3 days each. In both forms, the interval between fevers ranges from 4 to 14 days, sometimes with symptoms of malaise and fatigue. The symptoms that accompany the fevers are usually nonspecific. Headache, neck stiffness, arthralgia, myalgia, and vomiting may accompany the first and subsequent febrile episodes. An enlarging spleen and liver cause abdominal pain. A nonproductive cough is com­ mon during LBRF and—in combination with fever and myalgias—may suggest influenza. Acute respiratory distress syndrome may occur dur­ ing LBRF or STRF. On physical examination, the patient with LBRF or STRF may be delirious or apathetic. There may be body lice in the patient’s clothes or signs of insect bites. In regions with B. miyamotoi or B. lonestari infection, a hard tick may be embedded in the skin. Jaundice, epistaxis, and sub­ conjunctival hemorrhages are common during LBRF but not in STRF or HTRF. Splenomegaly or spleen tenderness is common in LBRF and STRF. Localizing neurologic findings are more common in STRF than in LBRF or HTRF. In North America, B. turicatae infection has neuro­ logic manifestations, including aseptic meningitis and cranial neuri­ tis, more often than B. hermsii or B. nietonii infection. Unilateral or bilateral Bell’s palsy is the most common form of cranial neuritis and typically presents in the second or third febrile episode, not the first. Visual impairment from unilateral or bilateral iridocyclitis or panoph­ thalmitis may be permanent. In LBRF, neurologic manifestations such as altered mental state are thought to be secondary to systemic inflam­ mation or small hemorrhages in the brain rather than to direct invasion of the nervous system.

Myocarditis in STRF or LBRF is evidenced by gallops on cardiac auscultation, a prolonged QTc interval, and cardiomegaly and pulmo­ nary edema on chest radiography. General laboratory studies in all forms of relapsing fever are not specific. Mild to moderate normocytic anemia is common, but frank hemolysis and hemoglobinuria do not develop. Leukocyte counts range from slightly elevated to leukopenic. Mild to moderate throm­ bocytopenia is common; platelet counts may fall below 50,000/μL in some cases. C-reactive protein and procalcitonin levels are elevated. Laboratory evidence of hepatitis can be found, with elevated serum concentrations of aminotransferases; the prothrombin and partial thromboplastin times may be moderately prolonged. Analysis of the cerebrospinal fluid (CSF) is indicated in all cases of suspected relapsing fever with signs of meningitis or meningoencepha­ litis. The presence of mononuclear pleocytosis and mildly to moder­ ately elevated protein levels justifies intravenous antibiotic therapy in relapsing fever. The manifestations and course of B. miyamotoi disease are not as distinctive as those of relapsing fever. The most common presentation is acute onset of fever with chills, headache, and myalgias starting 1–2 weeks after a tick bite. Patients have been hospitalized with a pre­ sumptive diagnosis of undifferentiated sepsis. There may be a second fever episode in untreated patients. Meningoencephalitis or meningitis was documented in adults with deficiencies in humoral immunity, including the effects of anti–B cell antibodies such as rituximab. If the patient has coexisting early Lyme disease, there may be erythema migrans, the localized skin rash. ■ ■DIAGNOSIS STRF or LBRF should be considered in a patient with the characteristic fever pattern and a history of recent exposure—i.e., within 1–2 weeks before illness onset—to body lice or soft ticks in geographic areas with documented current or past transmission. Because of the longevity of the ticks and the transovarial transmission of the pathogen in the ticks, a case of relapsing fever may be diagnosed many years after the last case reported in that locale. The lice may be on the clothes of a migrant, refugee, or unhoused person. While the epidemiologic risks for B. miyamotoi HTRF are similar to those for Lyme disease, prompt removal of an embedded tick upon discovery may not reduce the risk of infection of this pathogen. The bedrock for laboratory diagnosis of LBRF and STRF remains direct detection of the spirochetes by microscopy of the blood. Manual differential counts of white blood cells by Wright, Giemsa, or GiemsaWright stain usually reveal spirochetes in thin blood smears if their concentration is ≥105/mL and several oil-immersion fields are exam­ ined (Fig. 190-2). The peak density of B. miyamotoi or B. lonestari in the blood may not be high enough for use of a blood smear alone for diagnosis. For LBRF and STRF, the preferred time to obtain a blood specimen is at or just before fever’s peak. Lower concentrations of spirochetes may be revealed by a thick blood smear that is treated with 0.5% acetic acid before staining. An alternative is a wet mount of citrated blood mixed with saline and examined by phase-contrast or dark-field microscopy for motile spirochetes. Polymerase chain reaction (PCR) and similar nucleic acid amplifica­ tion test (NAAT) procedures are increasingly used for examination of blood or CSF in cases of suspected relapsing fever. Overall, NAAT is more sensitive than thin blood smear, particularly for samples obtained between febrile episodes. PCR is the preferred procedure for direct detection of B. miyamotoi or B. lonestari in blood or CSF. Culture of blood or CSF in Barbour-Stoenner-Kelly broth medium or equivalent is an option for isolation of Borrelia species. However, visible growth takes several days, and few laboratories offer this service. Options for serologic confirmation of infection are limited, and results may be misleading. Whole cell–based assays, such as enzymelinked immunosorbent assay (ELISA) and the C6 peptide ELISA for Lyme disease, may be positive in relapsing fever through antigenic cross-reactivities among these spirochetes. A commercially available assay based on GlpQ—a protein antigen of STRF, HTRF, and LBRF Borrelia species but not of any Lyme disease species—has better

FIGURE 190-2  Photomicrograph of tick-borne relapsing fever spirochete (Borrelia turicatae) in a Giemsa-Wright–stained thin blood smear. Included in the figure are a polymorphonuclear leukocyte and two platelets. specificity but commonly is negative at a time when a blood smear or PCR assay would be positive. The results of a GlpQ-based assay or an indirect immunofluorescence assay of whole cells cannot be used to differentiate between different Borrelia species as to etiology. A positive IgG immunoassay of a single specimen may be the consequence of a past infection and not the present illness. ■ ■DIFFERENTIAL DIAGNOSIS Depending on the patient’s history of residential, occupational, travel, and recreational exposures, the differential diagnosis of STRF includes one or more of the following infections that feature either periodicity in Tick-borne relapsing fever Louse-borne relapsing fever Meningitis/encephalitis Oral therapy Sequential therapy Intravenous ceftriaxone 2 g qd or Na penicillin G, 5 million U q6h for 14 days First choice Age ≥9 years, not pregnant: doxcycline, 100 mg bid Age <9 years: erythromycin, 12.5 mg/kg per day Second choice Age ≥9 years, not pregnant: tetracycline 500 mg qid Third choice Age ≥9 years: erythromycin, 500 mg qid Duration: 10 days FIGURE 190-3  Algorithm for treatment of relapsing fever. If it is not known whether the patient has tick-borne or louse-borne relapsing fever, the patient should be treated for the tick-borne form. The dashed line indicates that central nervous system invasion in louse-borne relapsing fever is uncommon.

the fever pattern or an extended single febrile period with nonspecific constitutional symptoms: Rocky Mountain spotted fever and other rickettsioses, ehrlichiosis, anaplasmosis, tick-borne viral infection, and babesiosis in North America, Europe, Russia, and northeastern Asia. STRF with Bell’s palsy may be misdiagnosed as Lyme disease in areas endemic for both diseases. Elsewhere in the Americas and Asia and in most of Africa, malaria, typhoid fever, typhus and other rickettsio­ ses, dengue, and leptospirosis also may be considered. Other agents transmitted by the body louse are Rickettsia prowazekii, the cause of typhus, and Bartonella quintana, the cause of trench fever. There may be co-infections of malaria, typhus, or typhoid with STRF or LBRF.

B. miyamotoi infection may coexist with Lyme disease.

TREATMENT Relapsing Fever Penicillin and tetracyclines have been the antibiotics of choice for LBRF and STRF for several decades. Erythromycin has been a long-standing alternative choice. There is no evidence of acquired resistance to these antibiotics. Borrelia species are also susceptible to second- and third-generation cephalosporins. These spirochetes are relatively resistant to rifampin, sulfonamides, and aminoglyco­ sides. Spirochetes are no longer detectable in the blood within a few hours after the first dose of an effective antibiotic. Under conditions of limited resources or in the midst of an epidemic, a single dose of antibiotic usually suffices for successful treatment of LBRF (Fig. 190-3). For adults, a single dose of oral doxycycline (200 mg), oral tetracycline (500 mg), or intramuscular ceftriaxone 250 mg for adults and 125 mg for children is effective. The corresponding doses for children are oral tetracycline at 12.5 mg/kg, oral doxycycline at 5 mg/kg, and intramuscular penicillin G pro­ caine at 200,000–400,000 units. When an adult patient is stuporous or nauseated, the intravenous dose of tetracycline is 250–500 mg. Tetracyclines are contraindicated in pregnant and nursing women; for individuals in these groups who are allergic to penicillin, oral erythromycin (500 mg for adults and 12.5 mg/kg for children) is an alternative. While there is little reported experience with other macrolides, such as azithromycin, these are likely to be as effective as erythromycin. CHAPTER 190 Relapsing Fever First choice Single dose of penicillin G ceftriaxone IM 250 mg adults and 125 mg children 800,000 U adults 200,000–400,000 U children then (4–12 h later) Age ≥9 years, not pregnant: doxycycline, 100 mg bid or tetracycline 500 mg qid Age <9 years or pregnant: erythromycin, 12.5 mg/kg per day or 500 mg qid Duration: 7 days Second choice (penicillin allergy): erythromycin, 500 mg qid ≥9 years or 12.5 mg/kg per day <9 years Duration: 7 days

77 - 191 Lyme Borreliosis

191 Lyme Borreliosis

There are shortcomings of single-dose therapies for LBRF. With penicillin alone, recurrence may occur in up to 20% of patients, and the frequency of JHR was higher after tetracycline than penicillin. For treatment of LBRF in adults in Ethiopia, a regimen that reduces rates of both recurrence and JHR was a single dose of 400,000 units of intramuscular penicillin G procaine followed several hours later or the next day by doxycycline (100 mg orally twice daily) or tetra­ cycline (500 mg or 12.5 mg/kg orally every 6 h) for 7 days.

The accumulated anecdotal reports on STRF therapy indicate a recurrence rate of ≥20% after single-dose treatment, plausibly due to the propensity of some tick-borne species to invade the CNS. Accordingly, multiple antibiotic doses are recommended. The pre­ ferred treatment for adults is a 10-day course of doxycycline (100 mg twice daily) or tetracycline (500 mg or 12.5 mg/kg orally every 6 h). When tetracyclines are contraindicated, the alternatives are oral penicillin V potassium (500 mg or 12.5 mg/kg every 6–8 h) or erythromycin (500 mg or 12.5 mg/kg orally every 6 h) for 10 days. If a β-lactam antibiotic is given and CNS involvement is confirmed or suspected, it is preferably administered intravenously rather than orally. For adults, the regimen is penicillin G (5 million units IV every 6 h) or ceftriaxone (2 g IV daily) for 10–14 days. The JHR during treatment of LBRF or STRF can be severe and may end in death if precautions are not in place for close monitor­ ing for at least 24 h and with provision of parenteral cardiovascular and volume support as needed. Apprehension, rigors, fever, and hypotension occur within 1–3 h of initiation of antibiotic treat­ ment and may be accompanied by a further decrease in the platelet count. The incidence of the JHR is 20–60% in LBRF after the first antibiotic dose. JHR may also be encountered when a patient with unsuspected relapsing fever is treated with other types of antibiot­ ics, such ciprofloxacin, that have suboptimal effects. PART 5 Infectious Diseases Experience with the treatment of B. miyamotoi or B. lonestari HTRF is limited, but these organisms likely have the same antibiotic susceptibilities as other Borrelia species. Therapy for B. miyamotoi disease follows the guidelines for Lyme disease. This would include parenteral therapy for CNS involvement. In absence of contraindi­ cations, doxycycline (100 mg twice daily) is the preferred choice for uncomplicated B. miyamotoi infection because of the antibiotic’s efficacy for anaplasmosis and Lyme disease. If JHR occurs, it is generally milder than is observed in relapsing fever. ■ ■PROGNOSIS The mortality rates for untreated LBRF and STRF are in the ranges of 10–70% and 4–10%, respectively, and are largely determined by coexisting conditions, such as malnutrition or another infection, and by the availability of medical support. With prompt antibiotic treat­ ment, the mortality rate is 2–5% for LBRF and <2% for STRF. There are no reported deaths from HTRF. Features associated with a poor prognosis of LBRF or STRF include concurrence with malaria, typhus, or typhoid; pregnancy; stupor or coma on admission; diffuse bleeding; poor liver function; myocarditis; and bronchopneumonia. The mortal­ ity rate from the JHR in LBRF, in the absence of adequate monitoring and resuscitation measures, is ~5%. LBRF or STRF during pregnancy frequently leads to abortion, stillbirth, or perinatal death, but con­ genital malformations have not been reported. Although spirochetes or their remnants may persist in the CNS or other sequestered sites after bacteremia has resolved, posttreatment sequelae and prolonged disability have not been documented for any form of relapsing fever. Partial immunity against reinfection seems to develop in residents of areas with perennial elevated risk. ■ ■PREVENTION There is no vaccine for LBRF, STRF, or HTRF. Reduction of expo­ sure to lice and ticks is the key strategy for prevention. LBRF can be prevented through improved personal hygiene, reduction of crowd­ ing, better access to hot water (≥60° C) for clothes washing, and selected use of pesticides. Clothing is an important factor in main­ taining the human body louse. The risk of STRF can be reduced by construction of houses with concrete or sealed plank floors and

without thatched roofs or mud walls. Dwellings in forested areas pose a risk in western North America when rodents nest in the roof, attic, or wall spaces or under the structure. Buildings infested with Ornithodoros ticks can be treated with pesticides and then rodent-proofed. If residing in a high-risk environment, individuals should not sleep on the floor, and beds should be moved away from the wall. Individuals with recreational or occupational exposure to caves, where mammals may reside, merit advice about the risk of STRF. Following exposure at a site of STRF risk, treatment with doxycycline (a single dose of 100 mg or 200 mg on day 1 followed by 100 mg/d for 4 days) was efficacious in preventing infection in a placebo-controlled trial. Recommendations for preventing B. miya­ motoi infection follow those for reducing risk of Lyme disease from exposure to the vector, hard ticks (Chap. 191). ■ ■FURTHER READING Barbour AG, Schwan TG: Borrelia, in Bergey’s Manual of Systematics of Archaea and Bacteria, WB Whitman et al (eds). Hoboken, Wiley, 2015. Beeson AM et al: Soft tick relapsing fever — United States, 2012–2021. MMWR Morb Mortal Wkly Rep 72:777, 2023. Butler T: The Jarisch-Herxheimer reaction after antibiotic treatment of spirochetal infections: A review of recent cases and our under­ standing of pathogenesis. Am J Trop Med Hyg 96:46, 2017. Isenring E et al: Infectious disease profiles of Syrian and Eritrean migrants presenting in Europe: A systematic review. Travel Med Infect Dis 25:65, 2018. Kahlig E et al: Louse-borne relapsing fever-A systematic review and analysis of the literature: Part 1-Epidemiology and diagnostic aspects. PLoS Negl Trop Dis 15:e0008564, 2021. Kahlig E et al: Louse-borne relapsing fever-A systematic review and analysis of the literature: Part 2-Mortality, Jarisch-Herxheimer reac­ tion, impact on pregnancy. PLoS Negl Trop Dis 15:e0008656, 2021. McCormick DW et al: Characteristics of hard tick relapsing fever caused by Borrelia miyamotoi, United States, 2013–2019. Emerg Infect Dis 29:1719, 2023. Vazquez LJ et al: Relapsing fever caused by Borrelia lonestari after tick bite in Alabama. Emerg Infect Dis 29:441, 2023. Warrell DA: Louse-borne relapsing fever (Borrelia recurrentis infec­ tion). Epidemiol Infect 147:e106, 2019. Wormser GP et al: Aggregation of data from 4 clinical studies dem­ onstrating efficacy of single-dose doxycycline postexposure for pre­ vention of the spirochetal infections: Lyme disease, syphilis, and tick-borne relapsing fever. Diagn Microbiol Infect Dis 99:115293 2021. Allen C. Steere, Jacob E. Lemieux

Lyme Borreliosis ■ ■DEFINITION Lyme borreliosis is caused by a closely related group of spirochetes, Borrelia burgdorferi sensu lato (also called Borreliella spp.), transmitted by ticks of the Ixodes ricinus complex. The infection usually begins with a characteristic expanding skin lesion, erythema migrans (EM; stage 1, localized infection). After several days or weeks, the spirochete may spread to many different sites (stage 2, disseminated infection). Possi­ ble manifestations of disseminated infection include additional annular skin lesions, meningitis, cranial neuritis, radiculoneuritis, peripheral neuritis, carditis, atrioventricular nodal block, or migratory muscu­ loskeletal pain. Months or years later (usually after periods of latent infection), intermittent or persistent arthritis, chronic encephalopathy

or polyneuropathy, or acrodermatitis may develop (stage 3, persistent infection). Most patients experience early symptoms of the illness dur­ ing the summer, but the infection may not become symptomatic until it progresses to stage 2 or 3. Lyme disease was recognized as a separate entity in 1976 because of a geographic cluster of children in Lyme, Connecticut, who were thought to have juvenile rheumatoid arthritis. It became apparent that Lyme disease was a multisystem illness that affected primarily the skin, nervous system, heart, and joints. Epidemiologic studies of patients with EM implicated certain Ixodes ticks as vectors of the disease. Early in the twentieth century, EM had been described in Europe and attributed to I. ricinus tick bites. In 1982, a previously unrecognized spirochete, now called Borrelia burgdorferi, was recovered from Ixodes scapularis ticks and then from patients with Lyme disease. The entity is now called Lyme disease or Lyme borreliosis. ■ ■ETIOLOGIC AGENT B. burgdorferi, the causative agent of Lyme disease, is a fastidious microaerophilic bacterium. The spirochete’s genome is quite small (~1.5 Mb) and consists of a highly unusual genome organi­ zation with a linear chromosome of 950 kb and 17–21 linear and circu­ lar plasmids. The most remarkable aspect of the B. burgdorferi genome is that there are sequences for more than 100 known or predicted lipoproteins—a larger number than in almost any other organism. Most of these lipoproteins are encoded on plasmids and exported to the outer leaflet of the outer membrane, where they interact with the infected host or tick vector. The spirochete has few proteins with bio­ synthetic activity and depends on its host for most of its nutritional requirements. It has no sequences for recognizable toxins. Currently, 20 closely related borrelial species are collectively referred to as B. burgdorferi sensu lato (i.e., “B. burgdorferi in the general sense”) or Borreliella spp. The human infection Lyme borreliosis is caused primarily by four pathogenic genospecies: B. burgdorferi sensu stricto (“B. burgdorferi in the strict sense,” hereafter referred to simply as B. burgdorferi), Borrelia garinii, Borrelia bavariensis, and Borrelia afzelii. B. burgdorferi is the major cause of the infection in the United States; all four genospecies are found in Europe, and B. garinii, B. Afzelii, and B. bavariencis are the major causes in Asia. Strains of B. burgdorferi have been subdivided according to several typing schemes: one based on sequence variation of outer-surface protein C (OspC), a second based on differences in the 16S–23S rRNA intergenic spacer region (RST or IGS), a third called multilocus sequence typing, and a fourth based on whole genome sequencing (WGS). From these typing systems, it is apparent that strains of B. burgdorferi differ in pathogenicity. WGS type A, which includes OspC type A (RST1) strains, has the largest pangenome and the largest number of plasmidencoded lipoproteins, which are often immunogenic, and is especially likely to disseminate. Thus, this strain is particularly virulent and may have played a role in the emergence of Lyme disease in epidemic form in the northeastern United States in the late twentieth century. ■ ■EPIDEMIOLOGY The >20 known genospecies of B. burgdorferi sensu lato live in nature in enzootic cycles involving 14 species of ticks that are part of the I. ricinus complex. I. scapularis (Fig. 472-1) is the principal vector in the eastern United States from Maine to Georgia and in the midwestern states of Wisconsin, Minnesota, Indiana, and Michigan. I. pacificus is the vector in the western states of California and Oregon. The disease is acquired throughout Eurasia, from Ireland and Great Britain to Scan­ dinavia to western Russia, where I. ricinus is the vector, and in eastern Russia, China, and Japan, where I. persulcatus is the vector. These ticks may transmit other agents as well. In the United States, I. scapularis also transmits Babesia microti, Anaplasma phagocytophilum, Ehrlichia muris–like agent, Borrelia miyamotoi, Borrelia mayonii, and Powassan virus (the deer tick virus) (see “Differential Diagnosis,” below). In Europe and Asia, I. ricinus and I. persulcatus also transmit tick-borne encephalitis virus. Ticks of the I. ricinus complex have larval, nymphal, and adult stages. They require a blood meal at each stage. The risk of infection

FIGURE 191-1  A classic erythema migrans lesion (9 cm in diameter) is shown near the right axilla. The lesion has partial central clearing, a bright red outer border, and a target center. (Courtesy of Vijay K. Sikand, MD; with permission.) in a given area depends largely on the density of these ticks as well as their feeding habits and animal hosts, which have evolved differently in different locations. For I. scapularis in the northeastern United States, the white-footed mouse and certain other rodents are the preferred hosts of the immature larvae and nymphs. It is critical that both of the tick’s immature stages feed on the same host because the life cycle of the spirochete depends on horizontal transmission: in early summer from infected nymphs to mice and in late summer from infected mice to lar­ vae, which then molt to become the infected nymphs that will begin the cycle again the following year. It is the tiny nymphal tick that is primar­ ily responsible for transmission of the disease to humans, which peaks during the early summer months. White-tailed deer, which are not involved in the life cycle of the spirochete, are the preferred host for the adult stage of I. scapularis and seem to be critical to the tick’s survival. CHAPTER 191 Lyme Borreliosis Lyme disease is now the most common vector-borne infection in the United States and Europe. Since surveillance was begun by the Centers for Disease Control and Prevention (CDC) in 1982, the number of cases in the United States has increased dramatically. More than 30,000 new cases are now reported each summer, but the number of true cases is estimated at 476,000 annually. In Europe, reported frequencies of the disease are highest in the middle of the continent and in Scandinavia. ■ ■PATHOGENESIS AND IMMUNITY To maintain its complex enzootic cycle, B. burgdorferi must adapt to two markedly different environments: the tick and the mammalian host. The spirochete expresses outer-surface protein A (OspA) in the midgut of the tick, whereas OspC is upregulated as the organism travels to the tick’s salivary gland. There, OspC binds a tick salivary-gland pro­ tein (Salp15), which is required for infection of the mammalian host. The tick usually must be attached for at least 24 h for transmission of B. burgdorferi. After injection into the human skin, the spirochete downregulates OspC and upregulates the VlsE lipoprotein. This protein undergoes extensive antigenic variation, which is necessary for spirochetal survival. After several days to weeks, B. burgdorferi may migrate out­ ward in the skin, producing EM, and may spread hematogenously or in the lymph to other organs. The only known virulence factors of

B. burgdorferi are surface proteins that allow the spirochete to attach to mammalian proteins, integrins, glycosaminoglycans, or glycoproteins. For example, spread through the skin and other tissue matrices may be facilitated by the binding of human plasminogen and its activators to the surface of the spirochete. Several Borrelia strains bind components of complement, such as Factor H and other complement regulators, which help to protect spirochetes from complement-mediated lysis. Dissemination of the organism in the blood is facilitated by binding to the fibrinogen receptor (αIIbβ3) on activated platelets and the vit­ ronectin receptor (αvβ3) on endothelial cells. As the name indicates, spirochetal decorin-binding proteins A and B bind decorin, a glycos­ aminoglycan on collagen fibrils, and B. burgdorferi also binds directly

to native type 1 collagen lattices. This binding may explain why the organism is commonly aligned with collagen fibrils in the extracellular matrix in the heart, nervous system, or joints.

To control and eradicate B. burgdorferi, the host mounts both innate and adaptive immune responses, resulting in macrophage- and antibody-mediated killing of the spirochete. As part of the innate immune response, complement may lyse the spirochete in the skin. Cells at affected sites release potent proinflammatory cytokines, including interleukin 6, tumor necrosis factor α, interleukin 1β, and interferon γ (IFN-γ). Patients who are homozygous for a Toll-like receptor 1 polymorphism (1805GG), particularly when infected with highly inflammatory B. burgdorferi RST1 strains, have exceptionally high levels of proinflammatory cytokines. The purpose of the adaptive immune response appears to be the production of specific antibodies, which opsonize the organism—a step necessary for optimal spirochetal killing. Studies with protein arrays expressing ~1200 B. burgdorferi proteins detected antibody responses to a total of 120 spirochetal proteins (particularly outer-surface lipoproteins) in a population of patients with Lyme arthritis. Histologic examination of all affected tis­ sues reveals an infiltration of lymphocytes, macrophages, and plasma cells with some degree of vascular damage, sometimes including oblit­ erative microvascular lesions. In enzootic infection, B. burgdorferi spirochetes must survive this immune assault only during the summer months before returning to larval ticks to begin the cycle again the following year. In contrast, infection of humans is a dead-end event for the spirochete. Within sev­ eral weeks or months, innate and adaptive immune mechanisms—even without antibiotic treatment—control widely disseminated infection, and generalized systemic symptoms wane. Thus, immune mechanisms seem to succeed eventually in the near or total eradication of B. burg­ dorferi from selected niches, including the joints or nervous system, and symptoms resolve in most patients. However, without antibiotic therapy, spirochetes may survive in localized niches for several more years. For example, B. burgdorferi infection in the United States may cause persistent arthritis or, in rare cases, subtle encephalopathy or polyneuropathy, and B. afzelii may cause acrodermatitis. PART 5 Infectious Diseases ■ ■CLINICAL MANIFESTATIONS Early Infection: Stage 1 (Localized Infection)  Because of the small size of nymphal ixodid ticks, most patients do not remember the preceding tick bite. After an incubation period of 3–32 days, EM usually begins as a red macule or papule at the site of the tick bite that expands slowly to form a large annular lesion (Fig. 191-1). As the lesion increases in size, it often develops a bright red outer border and partial central clearing. The center of the lesion sometimes becomes intensely erythematous and indurated, vesicular, or necrotic. In other instances, the expanding lesion remains an even, intense red; several red rings are found within an outside ring; or the central area turns blue before the lesion clears. Although EM can be located anywhere, the thigh, groin, and axilla are particularly common sites. The lesion is warm but not often painful. Approximately 20% of patients do not exhibit this characteristic skin manifestation. Early Infection: Stage 2 (Disseminated Infection)  In cases in the United States, B. burgdorferi often spreads hematogenously to many sites within days or weeks after the onset of EM. In these cases, patients may develop secondary annular skin lesions similar in appearance to the initial lesion. Skin involvement is commonly accompanied by severe headache, mild stiffness of the neck, fever, chills, migratory musculoskel­ etal pain, arthralgias, and profound malaise and fatigue. Less common manifestations include generalized lymphadenopathy or splenomegaly, hepatitis, sore throat, nonproductive cough, conjunctivitis, iritis, or orchitis. Except for fatigue and lethargy, which are often constant, the early signs and symptoms of Lyme disease are typically intermittent and changing. Even in untreated patients, the early symptoms usually become less severe or disappear within several weeks. In ~15% of patients, the infection presents with these nonspecific systemic symptoms. Symptoms suggestive of meningeal irritation may develop early in Lyme disease when EM is present but usually are not associated with

cerebrospinal fluid (CSF) pleocytosis or an objective neurologic deficit. After several weeks or months, ~15% of untreated patients develop frank neurologic abnormalities, including meningitis, subtle encepha­ litic signs, cranial neuritis (most commonly involving the facial nerve, resulting in unilateral or bilateral facial palsy), motor or sensory radiculoneuropathy, peripheral neuropathy, mononeuritis multiplex, cerebellar ataxia, or myelitis—alone or in various combinations. In children, the optic nerve may be affected because of inflammation or increased intracranial pressure, and these effects may lead to blindness. In the United States, the usual pattern consists of fluctuating symptoms of meningitis accompanied by facial palsy and peripheral radiculo­ neuropathy. Lymphocytic pleocytosis (~100 cells/μL) is found in CSF, often along with elevated protein levels and normal or slightly low glucose concentrations. In Europe and Asia, the first neurologic sign is characteristically radicular pain, which is followed by the development of CSF pleocytosis (meningopolyneuritis or Bannwarth’s syndrome); meningeal or encephalitic signs are frequently absent. These early neu­ rologic abnormalities usually resolve completely within months, but in rare cases, chronic neurologic disease may occur later. Within several weeks after the onset of illness, ~8% of patients develop cardiac involvement. The most common abnormality is a fluctuating degree of atrioventricular block (first-degree, Wenckebach, or complete heart block). Some patients have more diffuse cardiac involvement, including electrocardiographic changes indicative of acute myopericarditis, left ventricular dysfunction evident on radionu­ clide scans, or (in rare cases) cardiomegaly or fatal pancarditis. Cardiac involvement lasts for only a few weeks in most patients but may recur in untreated patients. A few cases of mitral or aortic valve endocarditis have been reported, in one case occurring years after acute cardiac involvement of Lyme disease. Chronic cardiomyopathy caused by

B. burgdorferi has been reported in Europe. During this stage, musculoskeletal pain is common. The typical pattern consists of migratory pain in joints, tendons, bursae, muscles, or bones (usually without joint swelling) lasting for hours or days and affecting one or two locations at a time. Late Infection: Stage 3 (Persistent Infection)  Months after the onset of infection, ~60% of patients in the United States who have received no antibiotic treatment develop frank arthritis. The typical pattern comprises intermittent attacks of oligoarticular arthritis in large joints (especially the knees), lasting for weeks or months in a given joint. A few small joints or periarticular sites also may be affected, primar­ ily during early attacks. The number of patients who continue to have recurrent attacks decreases each year. However, in a small percentage of cases, involvement of large joints—usually one or both knees—is persis­ tent and may lead to erosion of cartilage and bone. White cell counts in joint fluid range from 500 to 110,000/μL (aver­ age, 25,000/μL); most of these cells are polymorphonuclear leukocytes. Tests for rheumatoid factor or antinuclear antibodies usually give negative results, but a low-titer antinuclear antibody value may occur. Examination of synovial biopsy samples reveals fibrin deposits, villous hypertrophy, vascular proliferation, microangiopathic lesions, and a heavy infiltration of lymphocytes and plasma cells. Although most patients with Lyme arthritis respond well to antibiotic therapy, a small percentage in the northeastern United States have persis­ tent postinfectious (also called postantibiotic or antibiotic-refractory) Lyme arthritis for months or even for several years after receiving oral and IV antibiotic therapy for 2 or 3 months. Although more often these patients are initially infected with OspA type A (RST1) strains of

B. burgdorferi, this complication is not thought to result from persistent infection. Results of culture and polymerase chain reaction (PCR) for B. burgdorferi in synovial tissue obtained in the postantibiotic period have been uniformly negative. Rather, the basic pathogenetic feature of postinfectious Lyme arthritis is the development of an excessive, dysregulated proinflammatory immune response during the infection, characterized by exceptionally high IFN-γ levels, which persist in the postinfectious period. Risk factors for excessively high IFN-γ responses include presentation of an epitope of B. burgdorferi OspA (OspA164-175)

by certain class II major histocompatibility complex molecules

(particularly HLA-DRBI∗0401); a Toll-like receptor 1 polymorphism 1805GG in patients who were infected with OspC type A (RST1)

B. burgdorferi strains; and an imbalance of the CD4+ T effector/

regulatory cell ratio in which the majority of CD4+CD25+ T cells, which are ordinarily regulatory T cells, become IFN-γ-secreting T effector cells. The consequences of this excessive proinflammatory response in Lyme synovia include vascular damage, autoimmune and cytotoxic processes, and tumor-like fibroblast proliferation and fibrosis. An important driver of innate immune responses may be persistence of

B. burgdorferi peptidoglycan in synovial fluid, which may be especially difficult to clear. In addition, seven autoantigens that are targets of T- and B-cell responses in patients with Lyme disease, particularly those with postinfectious arthritis, have now been identified. These include three autoantigens associated with the vasculature (i.e., endothelial cell growth factor, apolipoprotein B-100, and annexin A2) and four extracellular matrix (ECM) proteins (i.e., matrix metalloproteinase 10, fibronectin-1, laminin B2, and collagen Vα1). Autoantibodies against vascular antigens are associated with obliterative microvascular lesions, and T-cell responses to epitopes of ECM proteins are associated with significantly longer durations of postinfectious arthritis. Although rare, chronic neurologic involvement also may become apparent months to several years after the onset of infection, some­ times after long periods of latent infection. The most common form of chronic central nervous system involvement is subtle encepha­ lopathy affecting memory, mood, or sleep, and the most common form of peripheral neuropathy is an axonal polyneuropathy manifested as either distal paresthesia or spinal radicular pain. Patients with encephalopathy frequently have evidence of memory impairment in neuropsychological tests and abnormal results in CSF analyses. In cases of polyneuropathy, electromyography generally shows extensive abnor­ malities of proximal and distal nerve segments. Encephalomyelitis or leukoencephalitis, a rare manifestation of Lyme borreliosis associated primarily with B. garinii infection in Europe, is a severe neurologic dis­ order that may include spastic paraparesis, upper motor neuron blad­ der dysfunction, and, rarely, lesions in the periventricular white matter. Acrodermatitis chronica atrophicans, the late skin manifestation of Lyme borreliosis, has been associated primarily with B. afzelii infection in Europe and Asia. It has been observed especially often in elderly women. The skin lesions, which are usually found on the acral surface of an arm or leg, begin insidiously with reddish-violaceous discolor­ ation; they become sclerotic or atrophic over a period of years. The basic patterns of Lyme borreliosis are similar worldwide, but there are regional variations, primarily between the illness found in North America, which is caused exclusively by B. burgdorferi, and that found in Europe, which is caused primarily by B. afzelii, B. garinii, and B. bavariensis. With each of the Borrelia species, the infection usually begins with EM. However, B. burgdorferi strains in the eastern United States often disseminate widely; they are particularly arthritogenic, and especially OspC type A (RST1) strains may lead to postinfectious arthritis. B. garinii and B. bavariensis typically disseminate less widely, but are especially neurotropic, are more likely to cause typical neu­ roborreliosis (Bannwarth’s syndrome) and rarely may cause borrelial encephalomyelitis. B. afzelii often infects only the skin but may persist in that site, where it may cause several different dermatoborrelioses, including acrodermatitis chronica atrophicans. Posttreatment Lyme Disease Syndrome (PTLDS)  Despite resolution of the objective manifestations of the infection with antibi­ otic therapy, ~10% of patients (although the reported percentages vary widely) continue to have subjective pain, neurocognitive manifesta­ tions, or fatigue symptoms. This disabling problem has been known for years following certain other infections but has been brought to fore recently with postacute sequelae of COVID-19. In Lyme disease, these symptoms usually improve and resolve within months but may last for years. At the far end of the spectrum, the symptoms may be similar to or indistinguishable from chronic fatigue syndrome (Chap. 461) and fibromyalgia (Chap. 385). Compared with symptoms of active Lyme disease, post-Lyme symptoms tend to be more generalized or disabling. They include marked fatigue, severe headache, diffuse musculoskeletal

pain, multiple symmetric tender points in characteristic locations, pain and stiffness in many joints, diffuse paresthesias, difficulty with concentration, and sleep disturbances. Patients with this condition lack evidence of joint inflammation, have normal neurologic test results, and may exhibit anxiety and depression. In contrast, late mani­ festations of Lyme disease, including arthritis, encephalopathy, and neuropathy, are usually associated with minimal systemic symptoms. Currently, no evidence indicates that persistent subjective symptoms after recommended courses of antibiotic therapy are caused by active infection. Randomized controlled trials have shown than repeated courses of antibiotics do not improve the symptoms of PTLDS and are not recommended.

■ ■DIAGNOSIS The culture of B. burgdorferi in Barbour-Stoenner-Kelly (BSK) medium permits definitive diagnosis, but this method has been used primarily in research studies. Moreover, with a few exceptions, positive cultures have been obtained only early in the illness—particularly from biopsy samples of EM skin lesions, less often from plasma samples, and occa­ sionally from CSF samples. Later in the infection, PCR is greatly supe­ rior to culture for the detection of B. burgdorferi DNA in joint fluid; this is the major use for PCR testing in Lyme disease. However, because B. burgdorferi DNA may persist for at least weeks after spirochetal kill­ ing with antibiotics, detection of spirochetal DNA in joint fluid is not an accurate test of active joint infection in Lyme disease and cannot be used reliably to determine the adequacy of antibiotic therapy. The sensitivity of PCR determinations in CSF from patients with neurobor­ reliosis has been much lower than that in joint fluid. With current methods, there seems to be little if any role for PCR in the detection of B. burgdorferi DNA in blood or urine samples. CHAPTER 191 Because of the problems associated with direct detection of

B. burgdorferi, Lyme disease is usually diagnosed by the recognition of a characteristic clinical picture accompanied by serologic confirmation. Although serologic testing may yield negative results during the first several weeks of infection, almost all patients have a positive antibody response to B. burgdorferi after that time when a two-test approach of enzyme-linked immunosorbent assay (ELISA) and Western blot or a protocol of two enzyme immunoassays (EIAs) is used. The limita­ tion of serologic tests is that they do not clearly distinguish between active and inactive infection. After antibiotic therapy, the amount of antibody declines but the results of Western blot, a nonquantitative test, do not change much (or very slowly). Thus, patients with previous Lyme disease—particularly in cases progressing to late stages—often remain seropositive for years, even after adequate antibiotic therapy. In addition, ~10% of patients are seropositive because of asymptom­ atic infection. If individuals with past or asymptomatic B. burgdorferi infection subsequently develop another illness, the positive serologic test for Lyme disease may cause diagnostic confusion. According to an algorithm published by the American College of Physicians (Table 191-1), serologic testing for Lyme disease is recommended only for patients with at least an intermediate pretest probability of Lyme disease, such as those with oligoarticular arthritis. It should not be used as a screen­ ing procedure in patients with pain or fatigue syndromes. In such patients, the probability of a false-positive serologic result is higher than that of a true-positive result. Lyme Borreliosis TABLE 191-1  Algorithm for Testing for and Treating Lyme Disease PRETEST PROBABILITY EXAMPLE RECOMMENDATION High Patients with erythema migrans Empirical antibiotic treatment without serologic testing Intermediate Patients with oligoarticular arthritis Serologic testing and antibiotic treatment if test results are positive Low Patients with nonspecific symptoms (myalgias, arthralgias, fatigue) Neither serologic testing nor antibiotic treatment Source: Adapted from the recommendations of the American College of Physicians (G Nichol et al: Ann Intern Med 128:37, 1998).

For serologic analysis of Lyme disease in the United States, the CDC recommends a two-step approach in which samples are first tested by ELISA, and equivocal or positive results are then tested by Western blot. This is called the conventional two-test approach. During the first weeks of infection, both IgM and IgG responses to the spirochete should be determined, preferably in both acute- and convalescentphase serum samples. Approximately 20–30% of patients have a positive response detectable in acute-phase samples (usually only a positive IgM response), whereas ~70–80% have a positive response during convales­ cence (2–4 weeks later). After 4–8 weeks of infection (by which time most patients with active Lyme disease have disseminated infection), the sensitivity and specificity of the IgG response to the spirochete are both very high—in the range of 99%—as determined by the two-test approach of ELISA and Western blot. At this point and thereafter, a single test (that for IgG) is usually sufficient. In persons with illness of >2 months’ duration, a positive IgM test result alone is likely to be false-positive and therefore should not be used to support the diagnosis.

According to current criteria adopted by the CDC, an IgM Western blot is considered positive if two of the following three bands are pres­ ent: 23, 39, and 41 kDa. However, the combination of two such bands may still represent a false-positive result. Misuse or misinterpretation of IgM blots has been a factor in the incorrect diagnosis of Lyme dis­ ease in patients with other illnesses. An IgG blot is considered positive if 5 of the following 10 bands are present: 18, 23, 28, 30, 39, 41, 45, 58, 66, and 93 kDa. In European cases, no single set of criteria for the interpretation of immunoblots results in high levels of sensitivity and specificity in all countries. A new methodology called the modified two-test approach, which is now approved by the U.S. Food and Drug Administration, is a two-test approach using two EIAs, thereby dispensing with the Western blot. One such method employs a whole–B. burgdorferi sonicate ELISA followed by a VlsE C6 peptide IgG ELISA. This approach, which gives simply a positive or a negative result, increases sensitivity during the first several weeks of infection without compromising specificity. For more complex cases or in those with late infection, it is still valuable to determine antibody specificities to multiple spirochetal proteins, as is done with Western blots. More recently, line immunoblots or other multiplexed antibody platforms have been developed as substitutes for Western blots. These assays allow more objective interpretation, and some platforms can provide quantitative data about antibody responses to many spirochetal proteins. After successful antibiotic treatment, antibody titers decline slowly, but responses (including that to the VlsE C6 peptide) may persist for years. Moreover, not only the IgG but also the IgM response may persist for years after therapy. Therefore, even a positive IgM response cannot be interpreted as confirmation of recent infection or reinfection unless the clinical picture is appropriate. PART 5 Infectious Diseases ■ ■DIFFERENTIAL DIAGNOSIS Classic EM is a slowly expanding erythema, often with partial central clearing. If the lesion expands little, it may represent the red papule of an uninfected tick bite. If the lesion expands rapidly, it may represent cellulitis (e.g., streptococcal cellulitis) or an allergic reaction, perhaps to tick saliva. Patients with secondary annular lesions may be thought to have erythema multiforme, but neither the development of blistering mucosal lesions nor the involvement of the palms or soles is a feature of B. burgdorferi infection. In the eastern United States, an EM-like skin lesion, sometimes with mild systemic symptoms, may be associated with Amblyomma americanum tick bites. However, the cause of this southern tick-associated rash illness (STARI) has not yet been identified. This tick may also transmit Ehrlichia chaffeensis, a rickettsial agent (Chap. 192). As stated above, I. scapularis ticks in the United States may transmit not only B. burgdorferi but also B. microti, the red blood cell parasite causing babesiosis (Chap. 232); A. phagocytophilum, the agent of human granulocytotropic anaplasmosis (Chap. 192); B. miyamotoi, a relapsing fever spirochete (Chap. 190); B. mayonii and E. muris–like agent, newly recognized species that occur in the upper midwestern United States; or less commonly, Powassan virus (the deer tick virus, which is closely related to European tick-borne encephalitis virus), which may cause fatal infection (Chap. 215). Babesiosis, anaplasmosis,

B. miyamotoi, and B. mayonii typically cause an influenza-like syn­ drome with fever, myalgias, and cytopenia, but symptoms may range from asymptomatic infection to severe or even fatal disease, particu­ larly in the young or the elderly. Co-infected patients may have more severe or persistent symptoms than patients infected with a single agent. Standard blood counts may yield clues regarding the presence of co-infection. Anaplasmosis and B. miyamotoi may cause leukopenia or thrombocytopenia, and babesiosis may cause thrombocytopenia and hemolytic anemia. IgM serologic responses may confuse the diagnosis. For example, A. phagocytophilum may elicit a positive IgM response to B. burgdorferi. PCR of blood is the diagnostic test of choice for co-infection with A. phagocytophilum, B. microti, or B. miyamotoi. Alternatively, examination of a peripheral blood smear can be used to detect B. microti, but blood smear analysis is insensitive for A. phago­ cytophilum and B. miyamotoi. The frequency of co-infection in differ­ ent studies has been variable. In one prospective study, 4% of patients with EM had evidence of co-infection, although this appears to be an increasing problem in early infection. Facial palsy caused by B. burgdorferi, which occurs in the early dis­ seminated phase of the infection (often in July, August, or September), is usually recognized by its association with EM. However, facial palsy without EM may be the presenting manifestation of Lyme disease. In such cases, both the IgM and the IgG responses to the spirochete are usually positive. The most common infectious agents that cause facial palsy are herpes simplex virus type 1 (Bell’s palsy; Chap. 197) and varicella-zoster virus (Ramsay Hunt syndrome; Chap. 198). Later in the infection, oligoarticular Lyme arthritis most resembles peripheral spondyloarthropathy in an adult or the pauciarticular form of juvenile idiopathic arthritis in a child. Patients with Lyme arthritis usually have the strongest IgG antibody responses seen in Lyme bor­ reliosis, with reactivity to many spirochetal proteins. The most common problem in the diagnosis of early Lyme disease is to miss the diagnosis, either because of an atypical morphology of EM or the erroneous assumption that a negative serologic test for B. burgdorferi excludes acute disease. Clinicians should be aware of the typical and atypical manifestations of EM, recognize the limited value of serologic testing in evaluating patients with early Lyme disease, and understand that the diagnosis of early Lyme disease is a clinical one. The most common problem in diagnosis of late-stage Lyme disease is to mistake chronic fatigue syndrome (Chap. 461) or fibromyalgia (Chap. 385) for Lyme disease. This difficulty is compounded by the fact that a small percentage of patients with Lyme disease do in fact develop these chronic pain or fatigue syndromes in association with or soon after Lyme disease. Moreover, a counterculture has emerged that ascribes pain and fatigue syndromes to chronic Lyme disease when there is little or no evidence of B. burgdorferi infection. In such cases, the term chronic Lyme disease, which is equated with chronic

B. burgdorferi infection, is a misnomer, and the use of repeated courses of antibiotic treatment is not warranted. Well-controlled randomized trials have found no benefit to antibiotic therapy for PTLDS, whereas there is well-documented risk of harm. TREATMENT Lyme Borreliosis ANTIBIOTIC TREATMENT As outlined in the algorithm in Fig. 191-2, the various manifes­ tations of Lyme disease can usually be treated successfully with orally administered antibiotics; the exceptions are severe objective neurologic abnormalities and third-degree atrioventricular heart block, which are generally treated with IV antibiotics, and arthri­ tis that does not respond to oral therapy. For early Lyme disease, doxycycline is effective and can be administered to men, non­ pregnant women, and children older than age 8. An advantage of this regimen is that it is also effective against A. phagocytophilum,

B. miyamotoi, and B. mayonii, which are transmitted by the same tick that transmits the Lyme disease agent. Amoxicillin, cefuroxime axetil, and erythromycin or its congeners are second-, third-, and

Skin Erythema migrans Acrodermatitis Joint Arthritis* Heart AV block Nervous system Facial palsy alone Meningitis Radiculoneuritis Encephalopathy Polyneuropathy 1˚, 2˚ 3˚ Oral therapy First choice Age ≥9 years, not pregnant: doxycycline, 100 mg bid Age <9 years: amoxicillin, 50 mg/kg per day Second choice for adults: amoxicillin, 500 mg tid Third choice for all ages: cefuroxime axetil, 500 mg bid Fourth choice for all ages: erythromycin, 250 mg qid Intravenous therapy First choice: ceftriaxone, 2 g qd Second choice: cefotaxime, 2 g q8h Third choice: Na penicillin G, 5 million U q6h Guidelines for duration of therapy Localized skin infection: 14 days Early disseminated infection: 21 days Acrodermatitis: 30 days Arthritis: 30−60 days** Neurologic involvement: 14–28 days Cardiac involvement: 28 days; complete course with oral therapy when patient is no longer in high-degree AV block FIGURE 191-2  Algorithm for the treatment of the various early or late manifestations of Lyme borreliosis. AV, atrioventricular. *For arthritis, oral therapy should be tried first; if arthritis is unresponsive, IV therapy should be administered. **For Lyme arthritis, IV ceftriaxone (2 g given once a day for 14–28 days) also is effective and is necessary for patients who do not respond to oral therapy. However, compared with oral treatment, this regimen is less convenient to administer, has more side effects, and is more expensive. fourth-choice alternatives, respectively, for the treatment of Lyme disease. In children, amoxicillin is effective (not >2 g/d); in cases of penicillin allergy, cefuroxime axetil or erythromycin may be used. In contrast to second- or third-generation cephalosporin antibiot­ ics, first-generation cephalosporins, such as cephalexin, are not effective. For patients with infection localized to the skin, a 10-day course of doxycycline or a 14-day course of amoxicillin is generally sufficient; in contrast, for patients with early disseminated infec­ tion, a 14- to 21-day course is recommended. Approximately 15% of patients experience a Jarisch-Herxheimer-like reaction during the first 24 h of therapy. In multicenter studies, >90% of patients whose early Lyme disease was treated with these regimens had satisfactory outcomes. Although some patients reported symptoms after treat­ ment, objective evidence of persistent infection or relapse was rare, and re-treatment was usually unnecessary. Oral administration of doxycycline or amoxicillin for 30 days is recommended for the initial treatment of Lyme arthritis in patients who do not have concomitant neurologic involvement. Among patients with arthritis who have an incomplete response to oral antibiotics, a second, 30-day course of oral antibiotics may be suc­ cessful. However, among patients with arthritis who have minimal or no response to oral antibiotics, re-treatment with IV ceftriaxone for 28 days is appropriate. In patients with arthritis in whom joint inflammation persists for months or even several years after both oral and IV antibiotics, treatment with nonsteroidal anti-inflammatory agents, therapy with disease-modifying antirheumatic drugs, or synovectomy may be successful. In the United States, parenteral antibiotic therapy is usually used for severe objective neurologic abnormalities. Patients with such abnormalities are most commonly treated with IV ceftriaxone for 14–28 days, but IV cefotaxime or IV penicillin G for the same

duration also may be effective. In Europe, similar results have been obtained with oral doxycycline and IV antibiotics in the treatment of acute neuroborreliosis. Although systematic trials have not been conducted in the United States, oral doxycycline is now used by many clinicians in this country for the treatment of patients with less severe neurologic abnormalities, such as facial palsy alone or uncomplicated Lyme meningitis. In patients with high-degree atrio­ ventricular block or a PR interval of >0.3 s, IV therapy for at least part of the course and cardiac monitoring are recommended, but the insertion of a permanent pacemaker is not necessary.

It is unclear how and whether asymptomatic infection should be treated, but patients with such infection are often given a course of oral antibiotics. Because maternal–fetal transmission of B. burgdorferi seems to occur rarely (if at all), standard therapy for the manifesta­ tions of the illness is recommended for pregnant women. Long-term persistence of B. burgdorferi has not been documented in any large series of patients after treatment with currently recommended regi­ mens, but there are a few case reports of persistent infection after such regimens. Although an occasional patient requires a second course of antibiotics, there is no indication for multiple, repeated antibiotic courses in the treatment of Lyme disease. CHRONIC LYME DISEASE After appropriately treated Lyme disease, a small percentage of patients continue to have subjective symptoms, primarily musculo­ skeletal pain, neurocognitive difficulties, or fatigue. This syndrome, termed posttreatment Lyme disease syndrome (PTLDS), is sometimes a disabling condition that is similar to chronic fatigue syndrome or fibromyalgia. Five double-blind, placebo-controlled trials conducted in the United States and Europe have failed to show benefit of further antibiotic therapy in these patients. For example, in a large study, one group of patients with PTLDS received IV ceftriaxone for 30 days

followed by oral doxycycline for 60 days, while another group received IV and oral placebo preparations for the same durations. No significant differences were found between groups in the numbers of patients reporting that their symptoms had improved, become worse, or stayed the same. Such patients are best treated for the relief of symptoms rather than with prolonged courses of antibiotics. PROPHYLAXIS AFTER A TICK BITE The risk of infection with B. burgdorferi after a recognized tick bite is so low that antibiotic prophylaxis is not routinely indicated. However, if an attached, engorged I. scapularis nymph is found or if follow-up is anticipated to be difficult, a single 200-mg dose of doxycycline, which usually prevents Lyme disease when given within 72 h after the tick bite, may be administered. CHAPTER 191 Lyme Borreliosis ■ ■PROGNOSIS The response to treatment is best early in the disease. Later treatment of Lyme borreliosis is still effective, but the period of convalescence may be longer. Eventually, most patients recover with minimal or no residual deficits. ■ ■REINFECTION Reinfection may occur after EM when patients are treated with antimi­ crobial agents. In such cases, the immune response is not adequate to provide protection from subsequent infection. However, patients who develop an expanded immune response to the spirochete over a period of months (e.g., those with Lyme arthritis) have protective immunity for a period of years and rarely, if ever, acquire the infection again. ■ ■PREVENTION Protective measures for the prevention of Lyme disease may include the avoidance of tick-infested areas, the use of repellents and acari­ cides, tick checks, and modification of landscapes in or near residential areas. Although a vaccine for Lyme disease used to be available, the manufacturer has discontinued its production. Another company is currently testing a similar vaccine in both the United States and Europe. However, no vaccine is currently available commercially for the prevention of this infection.

78 - SECTION 10 Diseases Caused by Rickettsiae, Mycoplasmas, and Chlamydiae

SECTION 10 Diseases Caused by Rickettsiae, Mycoplasmas, and Chlamydiae

■ ■FURTHER READING Arvikar SL, Steere AC: Diagnosis and treatment of Lyme arthritis.

Infect Dis Clin North Am 36:563, 2022. Aucott JN: Posttreatment Lyme disease syndrome. Infect Dis Clin North Am 29:309, 2015. Branda JA, Steere AC: Laboratory diagnosis of Lyme borreliosis. Clin Micro Rev 34:e00018, 2021. Branda JA et al: Two-tiered antibody testing for Lyme disease with use of 2 enzyme immunoassays, a whole-cell sonicate enzyme immu­ noassay followed by a VlsE C6 peptide enzyme immunoassay. Clin Infect Dis 53:541, 2011. Klempner MS et al: Two controlled trials of antibiotic treatment in patients with persistent symptoms and a history of Lyme disease. N Engl J Med 345:85, 2001. Lantos PM et al: Clinical practice guidelines by the Infectious Diseases Society of America (IDSA), American Academy of Neurology, and the American College of Rheumatology (ACR): 2020 guidelines for the prevention, diagnosis, and treatment of Lyme disease. Clin Infect Dis 72:1, 2021. Lemieux JE et al: Whole genome sequencing of human Borrelia burg­ dorferi isolates reveals linked blocks of accessory genome elements located on plasmids and associated with human dissemination. PLoS Pathog 17:e1011243, 2023. Li X et al: Burden and viability of Borrelia burgdorferi in skin or joints of patients with erythema migrans or Lyme arthritis. Arthritis Rheum 63:2238, 2011. Lochhead RB et al: Lyme arthritis, linking infection, inflammation, and autoimmunity. Nat Rev Rheumatol 17:449, 2021. Oschmann P et al: Stages and syndromes of neuroborreliosis. J Neurol PART 5 Infectious Diseases 245:262, 1998. Steere AC: Lyme disease. N Engl J Med 345:115, 2001. Steere AC: Posttreatment Lyme disease syndromes: Distinct pathogenesis caused by maladaptive host responses. J Clin Invest 130:2148, 2020. Steere AC et al: Prospective study of serologic tests for Lyme disease. Clin Infect Dis 47:188, 2008. Steere AC et al: Lyme borreliosis. Nat Rev Dis Primers 2:16090, 2016. Ursinus J et al: Prevalence of persistent symptoms after treatment for Lyme borreliosis: A prospective observational cohort study. Lancet Reg Health Eur 6:100142, 2021. Erratum in: Lancet Reg Health Eur 27:100622, 2023. Section 10 Diseases Caused by Rickettsiae, Mycoplasmas, and Chlamydiae David H. Walker, J. Stephen Dumler,

Lucas S. Blanton, Chantal P. Rovers

Rickettsial Diseases Rickettsiae are a heterogeneous group of small, obligately intracel­ lular, gram-negative coccobacilli and short bacilli, most of which are transmitted by a tick, mite, flea, or louse vector. Except in the case of louse-borne typhus, humans are incidental hosts. Among rickettsiae, Coxiella burnetii, Rickettsia prowazekii, and Rickettsia typhi have the well-documented ability to survive for an extended period outside the reservoir or vector and to be extremely infectious: inhalation of a single Coxiella microorganism can cause pneumonia. High-level infectivity and severe illness after inhalation make R. prowazekii, R. rickettsii, R. typhi, R. conorii, and C. burnetii bioterrorism threats (Chap. S4). Clinical infections with rickettsiae can be classified according to (1) the taxonomy and diverse microbial characteristics of the agents, which

belong to seven genera (Rickettsia, Orientia, Ehrlichia, Anaplasma, Neor­ ickettsia, “Candidatus Neoehrlichia,” and Coxiella); (2) epidemiology; or (3) clinical manifestations. The clinical manifestations of all the acute presentations are similar during the first 5 days: fever, headache, and myalgias with or without nausea, vomiting, and cough. As the course progresses, clinical manifestations—including a macular, maculopapu­ lar, or vesicular rash; eschar; pneumonitis; and meningoencephalitis— vary from one disease to another. Given the many etiologic agents with varied mechanisms of transmission, geographic distributions, and asso­ ciated disease manifestations, the consideration of rickettsial diseases as a single entity poses complex challenges (Table 192-1). Establishing the etiologic diagnosis of rickettsioses is very diffi­ cult during the acute stage of illness, and definitive diagnosis usually requires the examination of serum samples during the acute and con­ valescent phases of illness. Heightened clinical suspicion is based on epidemiologic data, history of exposure to vectors or reservoir animals, travel to endemic locations, clinical manifestations (sometimes includ­ ing rash or eschar), and characteristic laboratory findings (including thrombocytopenia, normal or low white blood cell [WBC] counts, elevated hepatic enzyme levels, and hyponatremia). Such suspicion should prompt empirical treatment. Doxycycline is the empirical drug of choice for most of these infections. Only one agent, C. burnetii, has been documented to cause chronic illness. One other species, R. prowazekii, causes recrudescent illness (Brill-Zinsser disease) when latent infection is reactivated years after resolution of the acute illness. Rickettsial infections dominated by fever may resolve without fur­ ther clinical evolution. However, after nonspecific early manifestations, the illnesses can also evolve along one or more of several principal clinical lines: (1) development of a macular or maculopapular rash; (2) development of an eschar at the site of tick or mite feeding, which can occur during the incubation period; (3) development of a vesicular rash (often in rickettsialpox, R. parkeri infection, and African tick-bite fever); (4) development of pneumonitis with chest radiographic opaci­ ties and/or rales (Q fever and severe cases of Rocky Mountain spotted fever [RMSF], Mediterranean spotted fever [MSF], louse-borne typhus, human monocytotropic ehrlichiosis [HME], human granulocytotropic anaplasmosis [HGA], scrub typhus, and murine typhus); (5) develop­ ment of meningoencephalitis (louse-borne typhus and severe cases of RMSF, scrub typhus, HME, murine typhus, MSF, and [rarely] Q fever); and (6) progressive hypotension and multiorgan failure as seen with sepsis or toxic shock syndromes (RMSF, MSF, louse-borne typhus, murine typhus, scrub typhus, HME, HGA, and neoehrlichiosis). Epidemiologic clues to the transmission of a particular pathogen include (1) environmental exposure to ticks, fleas, or mites during the season of activity of the vector species for the disease in the appropri­ ate geographic region (spotted fever and typhus rickettsioses, scrub typhus, ehrlichiosis, anaplasmosis); (2) travel to or residence in an endemic geographic region during the incubation period (Table 192-1); (3) exposure to parturient ruminants, cats, and dogs (Q fever); (4) exposure to flying squirrels (R. prowazekii infection); and (5) history of previous louse-borne typhus (recrudescent typhus). Clinical laboratory findings such as thrombocytopenia (particularly in spotted fever and typhus rickettsioses, ehrlichiosis, anaplasmosis, and scrub typhus), normal or low WBC counts, mild to moderate serum elevations of hepatic aminotransferases, and hyponatremia sug­ gest some common pathophysiologic mechanisms. Application of these clinical, epidemiologic, and laboratory prin­ ciples requires consideration of a rickettsial diagnosis and knowledge of the individual diseases. TICK-, MITE-, LOUSE-, AND FLEA-BORNE RICKETTSIOSES These diseases, caused by organisms of the genera Rickettsia and Ori­ entia in the family Rickettsiaceae, result from endothelial cell infection and increased vascular permeability. Pathogenic rickettsial species are very closely related, have small genomes (as a result of reductive evolu­ tion, which eliminated many genes for biosynthesis of intracellularly available molecules), and are traditionally separated into typhus and spotted fever groups on the basis of lipopolysaccharide antigens. Some

79 - 192 Rickettsial Diseases

192 Rickettsial Diseases

■ ■FURTHER READING Arvikar SL, Steere AC: Diagnosis and treatment of Lyme arthritis.

Infect Dis Clin North Am 36:563, 2022. Aucott JN: Posttreatment Lyme disease syndrome. Infect Dis Clin North Am 29:309, 2015. Branda JA, Steere AC: Laboratory diagnosis of Lyme borreliosis. Clin Micro Rev 34:e00018, 2021. Branda JA et al: Two-tiered antibody testing for Lyme disease with use of 2 enzyme immunoassays, a whole-cell sonicate enzyme immu­ noassay followed by a VlsE C6 peptide enzyme immunoassay. Clin Infect Dis 53:541, 2011. Klempner MS et al: Two controlled trials of antibiotic treatment in patients with persistent symptoms and a history of Lyme disease. N Engl J Med 345:85, 2001. Lantos PM et al: Clinical practice guidelines by the Infectious Diseases Society of America (IDSA), American Academy of Neurology, and the American College of Rheumatology (ACR): 2020 guidelines for the prevention, diagnosis, and treatment of Lyme disease. Clin Infect Dis 72:1, 2021. Lemieux JE et al: Whole genome sequencing of human Borrelia burg­ dorferi isolates reveals linked blocks of accessory genome elements located on plasmids and associated with human dissemination. PLoS Pathog 17:e1011243, 2023. Li X et al: Burden and viability of Borrelia burgdorferi in skin or joints of patients with erythema migrans or Lyme arthritis. Arthritis Rheum 63:2238, 2011. Lochhead RB et al: Lyme arthritis, linking infection, inflammation, and autoimmunity. Nat Rev Rheumatol 17:449, 2021. Oschmann P et al: Stages and syndromes of neuroborreliosis. J Neurol PART 5 Infectious Diseases 245:262, 1998. Steere AC: Lyme disease. N Engl J Med 345:115, 2001. Steere AC: Posttreatment Lyme disease syndromes: Distinct pathogenesis caused by maladaptive host responses. J Clin Invest 130:2148, 2020. Steere AC et al: Prospective study of serologic tests for Lyme disease. Clin Infect Dis 47:188, 2008. Steere AC et al: Lyme borreliosis. Nat Rev Dis Primers 2:16090, 2016. Ursinus J et al: Prevalence of persistent symptoms after treatment for Lyme borreliosis: A prospective observational cohort study. Lancet Reg Health Eur 6:100142, 2021. Erratum in: Lancet Reg Health Eur 27:100622, 2023. Section 10 Diseases Caused by Rickettsiae, Mycoplasmas, and Chlamydiae David H. Walker, J. Stephen Dumler,

Lucas S. Blanton, Chantal P. Rovers

Rickettsial Diseases Rickettsiae are a heterogeneous group of small, obligately intracel­ lular, gram-negative coccobacilli and short bacilli, most of which are transmitted by a tick, mite, flea, or louse vector. Except in the case of louse-borne typhus, humans are incidental hosts. Among rickettsiae, Coxiella burnetii, Rickettsia prowazekii, and Rickettsia typhi have the well-documented ability to survive for an extended period outside the reservoir or vector and to be extremely infectious: inhalation of a single Coxiella microorganism can cause pneumonia. High-level infectivity and severe illness after inhalation make R. prowazekii, R. rickettsii, R. typhi, R. conorii, and C. burnetii bioterrorism threats (Chap. S4). Clinical infections with rickettsiae can be classified according to (1) the taxonomy and diverse microbial characteristics of the agents, which

belong to seven genera (Rickettsia, Orientia, Ehrlichia, Anaplasma, Neor­ ickettsia, “Candidatus Neoehrlichia,” and Coxiella); (2) epidemiology; or (3) clinical manifestations. The clinical manifestations of all the acute presentations are similar during the first 5 days: fever, headache, and myalgias with or without nausea, vomiting, and cough. As the course progresses, clinical manifestations—including a macular, maculopapu­ lar, or vesicular rash; eschar; pneumonitis; and meningoencephalitis— vary from one disease to another. Given the many etiologic agents with varied mechanisms of transmission, geographic distributions, and asso­ ciated disease manifestations, the consideration of rickettsial diseases as a single entity poses complex challenges (Table 192-1). Establishing the etiologic diagnosis of rickettsioses is very diffi­ cult during the acute stage of illness, and definitive diagnosis usually requires the examination of serum samples during the acute and con­ valescent phases of illness. Heightened clinical suspicion is based on epidemiologic data, history of exposure to vectors or reservoir animals, travel to endemic locations, clinical manifestations (sometimes includ­ ing rash or eschar), and characteristic laboratory findings (including thrombocytopenia, normal or low white blood cell [WBC] counts, elevated hepatic enzyme levels, and hyponatremia). Such suspicion should prompt empirical treatment. Doxycycline is the empirical drug of choice for most of these infections. Only one agent, C. burnetii, has been documented to cause chronic illness. One other species, R. prowazekii, causes recrudescent illness (Brill-Zinsser disease) when latent infection is reactivated years after resolution of the acute illness. Rickettsial infections dominated by fever may resolve without fur­ ther clinical evolution. However, after nonspecific early manifestations, the illnesses can also evolve along one or more of several principal clinical lines: (1) development of a macular or maculopapular rash; (2) development of an eschar at the site of tick or mite feeding, which can occur during the incubation period; (3) development of a vesicular rash (often in rickettsialpox, R. parkeri infection, and African tick-bite fever); (4) development of pneumonitis with chest radiographic opaci­ ties and/or rales (Q fever and severe cases of Rocky Mountain spotted fever [RMSF], Mediterranean spotted fever [MSF], louse-borne typhus, human monocytotropic ehrlichiosis [HME], human granulocytotropic anaplasmosis [HGA], scrub typhus, and murine typhus); (5) develop­ ment of meningoencephalitis (louse-borne typhus and severe cases of RMSF, scrub typhus, HME, murine typhus, MSF, and [rarely] Q fever); and (6) progressive hypotension and multiorgan failure as seen with sepsis or toxic shock syndromes (RMSF, MSF, louse-borne typhus, murine typhus, scrub typhus, HME, HGA, and neoehrlichiosis). Epidemiologic clues to the transmission of a particular pathogen include (1) environmental exposure to ticks, fleas, or mites during the season of activity of the vector species for the disease in the appropri­ ate geographic region (spotted fever and typhus rickettsioses, scrub typhus, ehrlichiosis, anaplasmosis); (2) travel to or residence in an endemic geographic region during the incubation period (Table 192-1); (3) exposure to parturient ruminants, cats, and dogs (Q fever); (4) exposure to flying squirrels (R. prowazekii infection); and (5) history of previous louse-borne typhus (recrudescent typhus). Clinical laboratory findings such as thrombocytopenia (particularly in spotted fever and typhus rickettsioses, ehrlichiosis, anaplasmosis, and scrub typhus), normal or low WBC counts, mild to moderate serum elevations of hepatic aminotransferases, and hyponatremia sug­ gest some common pathophysiologic mechanisms. Application of these clinical, epidemiologic, and laboratory prin­ ciples requires consideration of a rickettsial diagnosis and knowledge of the individual diseases. TICK-, MITE-, LOUSE-, AND FLEA-BORNE RICKETTSIOSES These diseases, caused by organisms of the genera Rickettsia and Ori­ entia in the family Rickettsiaceae, result from endothelial cell infection and increased vascular permeability. Pathogenic rickettsial species are very closely related, have small genomes (as a result of reductive evolu­ tion, which eliminated many genes for biosynthesis of intracellularly available molecules), and are traditionally separated into typhus and spotted fever groups on the basis of lipopolysaccharide antigens. Some

TABLE 192-1  Features of Selected Rickettsial Infections DISEASE ORGANISM TRANSMISSION Rocky Mountain spotted fever (RMSF) Rickettsia rickettsii Tick bite: Dermacentor andersoni, D. variabilis,

D. similis Amblyomma cajennense,

A sculptum, A. patinoi,

A. mixtum, A. aureolatum Rhipicephalus sanguineus Mexico, South America, United States Mediterranean spotted fever (MSF) R. conorii Tick bite: R. sanguineus,

R. pumilio African tick-bite fever R. africae Tick bite: A. hebraeum,

A. variegatum Maculatum disease R. parkeri Tick bite: A. maculatum,

A. triste, A. tigrinum, A. ovale Pacific Coast tick fever Rickettsia ssp californica Tick bite: D. occidentalis United States 3–9 5–14

+++ Rickettsialpox R. akari Mite bite: Liponyssoides sanguineus Tick-borne lymphadenopathy R. slovaca,

R. raoultii Tick bite: D. marginatus,

D. reticularis Flea-borne spotted fever R. felis Flea (mechanism undetermined): Ctenocephalides felis Epidemic typhus R. prowazekii Louse feces: Pediculus humanus humanus, fleas and lice of flying squirrels, or recrudescence Murine typhus R. typhi Flea feces: Xenopsylla cheopis, C. felis, others Human monocytotropic ehrlichiosis (HME) Ehrlichia chaffeensis Tick bite: A. americanum, D. variabilis Ewingii ehrlichiosis E. ewingii Tick bite: A. americanum United States 1–21 4–21

None   Unnamed ehrlichiosis E. muris ssp. eauclairensis Tick bite: Ixodes scapularis United States Unknown 3–14

None   Human granulocytotropic anaplasmosis (HGA) Anaplasma phagocytophilum Tick bite: I. scapularis, I. ricinus, I. pacificus, I. persulcatus, Haemaphysalis concinna Unnamed disease A. capra I. persulcatus Northeastern China, France Neoehrlichiosis “Candidatus Neoehrlichia mikurensis” Tick bite: I. ricinus, I. persulcatus, Haemaphysalis concinna Scrub typhus Orientia tsutsugamushi Mite bite: Leptotrombidium deliense, others O. chuto Mite bite Africa and Western Asia Candidatus O. chiloensis Mite bite Chile Q fever Coxiella burnetii Inhalation of aerosols of infected parturition material (goats, sheep, cattle, cats, others), ingestion of infected milk or milk products a++++, severe; +++, marked; ++, moderate; +, present in a small proportion of cases; —, not a noted feature.

GEOGRAPHIC RANGE INCUBATION PERIOD, DAYS DURATION, DAYS RASH, % ESCHAR, % LYMPHADENOPATHYa United States 2–14 10–20

<1 + Central/

South America Southern Europe, Africa, Middle East, central Asia 5–7 7–14

Sub-Saharan Africa, West Indies 4–10 4–19

+++ United States, South America 2–10 6–16

++ United States, Ukraine, Turkey, Mexico, Croatia 10–17 3–11

+++ Europe 7–9 17–180

++++ CHAPTER 192 Worldwide 8–16 8–16

— Worldwide 7–14 10–18

None — Rickettsial Diseases Worldwide 8–16 9–18

None — United States 1–21 3–21

None ++ United States, Europe, Asia 4–8 3–14 Rare None — Unknown 11–21

Europe, China ≥8 11–75

None   Asia, Australia, Pacific and Indian Ocean islands 9–18 6–21

+++ Worldwide except New Zealand, Antarctica 3–30 5–57 4–18% None —

diseases and their agents (e.g., R. africae, R. parkeri, and R. sibirica) are too similar to require separate descriptions. Indeed, the similarities among MSF (R. conorii [all strains] and R. massiliae), North Asian tick typhus (R. sibirica), Japanese spotted fever (R. japonica), and Flinders Island spotted fever (R. honei) far outweigh their minor variations. The Rickettsiaceae that cause life-threatening infections are, in order of decreasing case–fatality rate, R. rickettsii (RMSF); R. prowazekii (louse-borne typhus); Orientia tsutsugamushi (scrub typhus); R. cono­ rii (MSF); R. typhi (murine typhus); and, in rare cases, other spotted fever–group (SFG) organisms. Some agents (e.g., R. parkeri, R. africae, Rickettsia 364D, R. akari, R. slovaca, R. honei, R. felis, R. massiliae,

R. helvetica, R. heilongjiangensis, R. aeschlimannii, and R. monacensis) have never been documented to cause a fatal illness. The most preva­ lent SFG rickettsia in the United States, R. amblyommatis, has been circumstantially associated with asymptomatic seroconversion in most persons and with self-limited illness in others.

■ ■ROCKY MOUNTAIN SPOTTED FEVER Epidemiology  RMSF occurs in 47 states (with the highest preva­ lence in the south-central and southeastern states) as well as in Canada, Mexico, and Central and South America. The infection is transmit­ ted by Dermacentor variabilis, the American dog tick, in the eastern two-thirds of the United States; by D. andersoni, the Rocky Mountain wood tick, in the western United States; by D. similis in California; by Rhipicephalus sanguineus, the brown dog tick, in Mexico, Arizona, and probably Colombia; and by Amblyomma sculptum, A. mixtum, A. patinoi, A. cajennense, A. tonelliae, and A. aureolatum in Central and/ or South America. Maintained partially by transovarian transmission from one generation of ticks to the next, R. rickettsii can be acquired by uninfected ticks through the ingestion of a blood meal from rick­ ettsemic small mammals or by co-feeding adjacent to an infected tick. PART 5 Infectious Diseases Humans become infected during tick season (in the Northern Hemisphere, from April to September), although some cases occur in winter. The mortality rate was 20–25% in the preantibiotic era and has been reported at ~3–5% in the postantibiotic era, principally because of delayed diagnosis and treatment. Recent reporting of a relatively low mortality rate (0.4%) for spotted fever rickettsiosis is likely an artifact related to the abundance of less pathogenic SFG rickettsial species likely causing subclinical or undiagnosed infection with cross-reactive anti-SFG antibodies and to a relatively low proportion of diagnostically confirmed cases. Indeed, the reported case–fatality rates in confirmed cases in the United States and in parts of Arizona, where R. rickettsii is the sole infecting SFG species, are 9% and 10%, respectively. The case– fatality rate is highest among children (<10 years of age) and in the later decades of life (>70 years). For unknown reasons, the case–fatality rate of RMSF in Mexico and Brazil approaches 50%. Pathogenesis  R. rickettsii organisms are inoculated into the dermis along with secretions of the tick’s salivary glands after ≥6 h of feeding. The rickettsiae spread lymphohematogenously throughout the body and infect numerous foci of contiguous endothelial cells. The dosedependent incubation period is ~1 week (range, 2–14 days). Occlusive thrombosis and ischemic necrosis are not the fundamental pathologic bases for tissue and organ injury. Instead, increased vascular perme­ ability, with resulting edema, hypovolemia, and ischemia, is responsible. Consumption of platelets results in thrombocytopenia in 32–52% of patients, but disseminated intravascular coagulation (DIC) with hypofi­ brinogenemia is rare. Activation of platelets, generation of thrombin, and activation of the fibrinolytic system all appear to be homeostatic physi­ ologic responses to endothelial injury by nonocclusive hemostatic plugs. Clinical Manifestations  Early in the illness, when medical atten­ tion usually is first sought, RMSF is difficult to distinguish from many self-limiting viral illnesses. Fever, headache, malaise, myalgia, nausea, vomiting, and anorexia are the most common symptoms during the first 3 days. The patient becomes progressively more ill as vascular infec­ tion and injury advance. In one large series, only one-third of patients were diagnosed with presumptive RMSF early in the clinical course and treated appropriately as outpatients. In the tertiary-care setting, RMSF is

all too often recognized only when late severe manifestations, develop­ ing at the end of the first week or during the second week of illness in patients without appropriate treatment, prompt return to a physician or hospital and admission to an intensive care unit. The progressive nature of the infection is clearly manifested in the skin. Rash is evident in only 14% of patients on the first day of illness and in only 49% during the first 3 days. Erythematous macules (1–5 mm) appear first on the wrists and ankles and then on the remainder of the extremities and the trunk. Later, more severe vascular damage results in frank hemor­ rhage at the center of the maculopapule, producing a petechia that does not disappear upon compression (Fig. 192-1). This sequence of events is sometimes delayed or aborted by effective treatment. However, the rash is a variable manifestation, appearing on day 6 or later in 20% of cases and not appearing at all in ~10% of cases. Petechiae occur in 41–59% of cases, appearing on or after day 6 in 74% of cases that manifest a rash. Involve­ ment of the palms and soles, often considered diagnostically important, usually develops relatively late in the course (after day 5 in 43% of cases) and does not develop at all in 18–64% of cases. Hypovolemia leads to prerenal azotemia and (in 17% of cases) hypo­ tension. Infection of the pulmonary microcirculation leads to noncar­ diogenic pulmonary edema; 12% of patients have acute respiratory distress syndrome, and 8% require mechanical ventilation. Cardiac involvement manifests as dysrhythmia in 7–16% of cases. Besides respiratory failure, central nervous system (CNS) involve­ ment is the other important determinant of the outcome of RMSF. Encephalitis, presenting as confusion or lethargy, is apparent in 26–28% of cases. Progressively severe encephalitis manifests as stupor or delirium in 21–26% of cases, ataxia in 18%, coma in 10%, and sei­ zures in 8%. Numerous focal neurologic deficits have been reported. A B FIGURE 192-1  A. Petechial lesions of Rocky Mountain spotted fever on the lower legs and soles of a young, previously healthy patient. B. Close-up of lesions from the same patient. (Photos courtesy of Dr. Lindsey Baden; with permission.)

Meningoencephalitis results in cerebrospinal fluid (CSF) pleocytosis in 34–38% of cases; usually there are 10–100 cells/μL and a mono­ nuclear predominance, but occasionally there are >100 cells/μL and a polymorphonuclear predominance. The CSF protein concentration is increased in 30–35% of cases, but the CSF glucose concentration is usually normal. Acute kidney injury, often reversible with rehydration, is caused by acute tubular necrosis in severe cases with shock. Hepatic injury with increased serum aminotransferase concentrations (38% of cases) is due to multifocal death of individual hepatocytes without hepatic failure. Jaundice is recognized in 9% of cases and an elevated serum bilirubin concentration in 18–30%. Life-threatening bleeding is rare. Anemia develops in 30% of cases and is severe enough to require transfusions in 11%. Blood is detected in the stool or vomitus of 10% of patients, and death has followed mas­ sive upper-gastrointestinal hemorrhage. Other characteristic clinical laboratory findings include increased plasma levels of proteins of the acute-phase response (C-reactive pro­ tein, procalcitonin, fibrinogen, ferritin, and others), hypoalbuminemia, and hyponatremia (in 56% of cases) due to the appropriate secretion of antidiuretic hormone in response to the hypovolemic state. Myositis occurs occasionally, with marked elevations in serum creatine kinase levels and multifocal rhabdomyonecrosis. Ocular involvement includes conjunctivitis in 30% of cases and retinal vein engorgement, flame hemorrhages, arterial occlusion, and papilledema with normal CSF pressure in some instances. Severe RMSF can present as sepsis or septic shock. In untreated fatal cases, death occurs 8–15 days after onset. A rare presentation, fulmi­ nant RMSF, is fatal within 5 days after onset. This fulminant presenta­ tion is seen most often in male black patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency and may be related to an undefined effect of hemolysis on the rickettsial infection. Although survivors of RMSF usually return to their previous state of health, permanent sequelae, including neurologic deficits and gangrene necessitating amputation of extremities, may follow severe illness. Diagnosis  The diagnosis of RMSF during the acute stage is more difficult than is generally appreciated. The most important epidemio­ logic factor is a history of exposure to a potentially tick-infested envi­ ronment within the 14 days preceding disease onset during a season of possible tick activity. However, only 60% of patients actually recall being bitten by a tick during the incubation period. The differential diagnosis for early clinical manifestations of RMSF (fever, headache, and myalgia without a rash) includes influenza, enteroviral infection, infectious mononucleosis, viral hepatitis, lep­ tospirosis, typhoid fever, gram-negative or gram-positive bacterial sepsis, HME, HGA, murine typhus, sylvatic flying-squirrel typhus, and rickettsialpox. Enterocolitis may be suggested by nausea, vomit­ ing, and abdominal pain; prominence of abdominal tenderness has resulted in exploratory laparotomy. CNS involvement can masquerade as bacterial or viral meningoencephalitis. Cough, pulmonary signs, and chest radiographic opacities can lead to a diagnostic consideration of bronchitis or pneumonia. At presentation during the first 3 days of illness, only 3% of patients exhibit the classic triad of fever, rash, and history of tick exposure. When a rash appears, a diagnosis of RMSF should be considered. However, many illnesses considered in the differential diagnosis also can be associ­ ated with a rash, including rubeola, rubella, meningococcemia, dissemi­ nated gonococcal infection, secondary syphilis, toxic shock syndrome, drug hypersensitivity, immune thrombocytopenic purpura, thrombotic thrombocytopenic purpura, Kawasaki syndrome, and immune complex vasculitis. Conversely, any person in an endemic area with a provisional diagnosis of one of the above illnesses could have RMSF. Thus, if a viral infection is suspected during RMSF season in an endemic area, it should always be kept in mind that RMSF can mimic viral infection early in the course; if the illness worsens over the next couple of days after initial presentation, the patient should return for reevaluation. The most common serologic test for confirmation of the diagnosis is the indirect immunofluorescence assay. Not until 7–10 days after onset

is a reactive titer of ≥64 first detectable. The sensitivity and specific­ ity of the indirect immunofluorescence IgG assay are 89–100% and 99–100%, respectively. Detection of IgM is no more sensitive in early illness and is subject to nonspecific cross-reactivity. It is important to understand that serologic tests for RMSF are usually negative at the time of presentation for medical care and that treatment should not be delayed while a positive serologic result is awaited. Because of significant seropositivity in the expanding geographic distribution of Amblyomma americanum ticks, a positive serologic titer in the first 5–6 days of illness represents preexisting antibody unrelated to the current illness.

The only diagnostic test that has proven useful during the acute ill­ ness is immunohistologic examination of a cutaneous biopsy sample from a rash lesion for R. rickettsii. Examination of a 3-mm punch biopsy from such a lesion is 70% sensitive and 100% specific, and poly­ merase chain reaction (PCR) on a rash biopsy would likely yield even higher sensitivity. PCR amplification for detection of R. rickettsii DNA in peripheral blood is not adequately sensitive. Although rickettsiae are present in large quantities in heavily infected foci of endothelial cells, there are relatively low quantities in the circulation. Cultiva­ tion of rickettsiae in cell culture is feasible but is seldom undertaken because of technical difficulty and biohazard concerns. The recent dramatic increase in the reported incidence of RMSF correlates with the use of single-titer SFG cross-reactive enzyme immunoassay serol­ ogy. Few cases are specifically determined to be caused by R. rickettsii. Currently, many febrile persons who do not have RMSF present with cross-reactive antibodies, possibly because of previous exposure to the highly prevalent SFG rickettsia R. amblyommatis. CHAPTER 192 TREATMENT Rocky Mountain Spotted Fever The drug of choice for the treatment of both children and adults with RMSF is doxycycline. Because of the severity of RMSF, imme­ diate empirical administration of doxycycline should be strongly considered for any patient with a consistent clinical presentation in the appropriate epidemiologic setting. Doxycycline is adminis­ tered orally (or, with coma or vomiting, intravenously) at 100 mg twice daily. For children with suspected RMSF, up to five courses of doxycycline may be administered with minimal risk of den­ tal staining. In patients with allergy to doxycycline, desensitiza­ tion should be considered. Once considered an alternative during pregnancy, chloramphenicol is not readily available in the United States. Although available in much of the world, it is less effective than doxycycline. Fortunately, there is little evidence to support the occurrence of tetracycline-associated adverse events in moth­ ers (hepatotoxicity) and fetuses (staining of deciduous teeth and teratogenicity) who receive doxycycline. The antirickettsial drug should be administered until the patient is afebrile and improving clinically—usually 3–5 days after defervescence. β-Lactam antibiot­ ics, erythromycin, and aminoglycosides have no role in the treat­ ment of RMSF, and sulfa-containing drugs are associated with more adverse outcomes than no treatment at all. There is little clinical experience with fluoroquinolones, clarithromycin, and azithromy­ cin, which are not recommended. The most seriously ill patients are managed in intensive care units, with careful administration of fluids to achieve optimal tissue perfusion without precipitating noncardiogenic pulmonary edema. In some severely ill patients, hypoxemia requires intubation and mechanical ventilation; oliguric or anuric acute renal failure requires renal replacement therapy; seizures necessitate the use of antiseizure medication; anemia or severe hemorrhage necessitates transfusions of packed red blood cells; or bleeding with severe thrombocytopenia requires platelet transfusions. Rickettsial Diseases Prevention  Avoidance of tick bites is the only available preventive approach. Use of protective clothing and tick repellents, inspection of the body once or twice a day, and removal of ticks before they inoculate

rickettsiae reduce the risk of infection. Prophylactic doxycycline treat­ ment of tick bites has no proven role in preventing RMSF.

■ ■MEDITERRANEAN SPOTTED FEVER (BOUTONNEUSE FEVER), AFRICAN TICK-BITE FEVER, AND OTHER TICK-BORNE SPOTTED FEVERS Epidemiology and Clinical Manifestations  R. conorii is preva­ lent in southern Europe, Africa, and southwestern and south-central Asia. The disease is characterized by high fever, rash, and—in most geographic locales—an inoculation eschar (tâche noire) that appears before the onset of fever at the site of the tick bite. A severe form of the disease (mortality rate, 50%) occurs in patients with diabetes, alcohol­ ism, or heart failure. African tick-bite fever, caused by R. africae, occurs in rural areas of sub-Saharan Africa and in the Caribbean islands and is transmitted by Amblyomma hebraeum and A. variegatum ticks. The average incuba­ tion period is 4–10 days. The mild illness consists of headache, fever, eschar, and regional lymphadenopathy. Amblyomma ticks, a high por­ tion of which are infected with R. africae, often feed in groups, with the consequent development of multiple eschars. Rash may be vesicular, sparse, or absent altogether. Because of tourism in sub-Saharan Africa, African tick-bite fever is the rickettsiosis most frequently imported into Europe and North America. Maculatum disease, a similar disease caused by the closely related species R. parkeri, is transmitted by A. maculatum and found in a low percentage of A. americanum ticks in the United States. It is also transmitted by A. triste in South America and Arizona as well as A. tigrinum and A. ovale in South America. R. japonica causes Japanese spotted fever, which also occurs in Korea, Thailand, Laos, and China. Similar diseases in northern Asia are caused by R. sibirica and R. heilongjiangensis. Queensland tick typhus due to R. australis is transmitted by Ixodes holocyclus ticks. Flinders Island spotted fever, found on the island for which it is named as well as in Tasmania, mainland Australia, and Asia, is caused by R. honei. In Europe, patients infected with R. slovaca by a wintertime Dermacentor tick bite usually manifest an afebrile illness with an eschar (usually on the scalp) and painful regional lymphadenopathy; R. raoultii causes a similar syndrome in Europe and Asia. PART 5 Infectious Diseases Diagnosis  Diagnosis of these tick-borne spotted fevers is based on clinical and epidemiologic findings and is confirmed by serology, immunohistochemical demonstration of rickettsiae in skin biopsy specimens, cell-culture isolation of rickettsiae, or PCR of skin biopsy, eschar biopsy or swab, or blood samples. Serologic diagnosis detects antibodies to antigens shared among SFG rickettsiae, hindering iden­ tification of the etiologic species. In an endemic area, a possible diag­ nosis of rickettsial spotted fevers should be considered when patients present with fever, rash, and/or a skin lesion consisting of a black necrotic lesion or a crust surrounded by erythema. TREATMENT Tick-Borne Spotted Fevers As with RMSF, severe cases should be treated with doxycycline (100 mg bid orally) continued for 3–5 days after defervescence. Alternative agents for milder disease include doxycycline (100 mg bid orally for 1–5 days), chloramphenicol (500 mg qid orally for 7–10 days), ciprofloxacin (750 mg bid orally for 7 days), and josamycin (3 g/d orally for 5 days) where available. Data on the efficacy of treatment of mildly ill children with clarithromycin or azithromycin should not be extrapolated to adults or to patients with moderate or severe illness. ■ ■RICKETTSIALPOX R. akari infects mice and their mites (Liponyssoides sanguineus), which maintain the organisms by transovarial transmission. Epidemiology  Rickettsialpox is recognized principally in New York City, but cases have also been reported in other urban and rural

FIGURE 192-2  Eschar at the site of the mite bite in a patient with rickettsialpox. (Reprinted from A Krusell et al: Emerg Infect Dis 8:727, 2002. Photo obtained by Dr. Kenneth Kaye.) locations in the United States and in Ukraine, Croatia, Mexico, and Turkey. Investigation of eschars suspected of representing bioterror­ ism-associated cutaneous anthrax revealed that rickettsialpox occurs more frequently than previously realized. Clinical Manifestations  A papule forms at the site of the mite’s feeding, develops a central vesicle, and becomes a 1- to 2.5-cm painless black crusted eschar surrounded by an erythematous halo (Fig. 192-2). Enlargement of the regional lymph nodes draining the eschar suggests initial lymphogenous spread. After an incubation period of 10–17 days, during which the eschar and regional lymphadenopathy frequently go unnoticed, disease onset is marked by malaise, chills, fever, headache, and myalgia. A macular rash appears 2–6 days after onset and usually evolves sequentially into papules, vesicles, and crusts that heal without scarring (Fig. 192-3); in some cases, the rash remains macular or maculopapular. Some patients develop nausea, vomiting, abdominal pain, cough, con­ junctivitis, or photophobia. Without treatment, fever lasts 6–10 days. Diagnosis and Treatment  Clinical, epidemiologic, and conva­ lescent serologic data establish the diagnosis of a SFG rickettsiosis that is seldom pursued further. Doxycycline is the drug of choice for treatment. ■ ■FLEA-BORNE SPOTTED FEVER Rickettsia felis is suspected to cause an emerging rickettsiosis world­ wide. Maintained transovarially in the geographically widespread cat flea, Ctenocephalides felis, the infection has been described as moder­ ately severe, with fever, rash, and headache as well as CNS, gastroin­ testinal, and pulmonary symptoms on the basis of PCR, which often detects organisms in healthy persons. Patient isolates are lacking. ■ ■EPIDEMIC (LOUSE-BORNE) TYPHUS Epidemiology  The human body louse (Pediculus humanus humanus) lives in clothing under poor hygienic conditions and usually in impov­ erished cold areas. Lice acquire R. prowazekii when they ingest blood from a rickettsemic patient. The rickettsiae multiply in the louse’s mid­ gut epithelial cells and are shed in its feces. The infected louse leaves a febrile person and deposits infected feces on its subsequent host during its blood meal; the patient autoinoculates the organisms by scratching. The louse is killed by the rickettsiae and does not pass R. prowazekii to its offspring. Epidemic typhus haunts regions afflicted by wars and disasters. An outbreak involved 100,000 people in refugee camps in Burundi in 1997. A small focus was documented in Russia in 1998, sporadic cases were reported from Algeria, and frequent outbreaks occurred in Peru, Rwanda and Burundi. Eastern flying squirrels (Glaucomys volans) and their lice and fleas maintain R. prowazekii in a zoonotic cycle and transmit infection to humans.

A B FIGURE 192-3  A. Papulovesicular lesions on the trunk of the patient with rickettsialpox shown in Fig. 192-2. B. Close-up of lesions from the same patient. (Reprinted from A Krusell et al: Emerg Infect Dis 8:727, 2002. Photos obtained by Dr. Kenneth Kaye.) Brill-Zinsser disease is a recrudescent illness occurring years after acute epidemic typhus, probably as a result of waning immunity.

R. prowazekii remains latent for years; its reactivation results in spo­ radic cases of disease in louse-free populations or in epidemics in louse-infested populations. Recrudescence has been documented after flying squirrel–associated typhus. Rickettsiae are potential agents of bioterrorism (Chap. S4). Infec­ tions with R. prowazekii and R. rickettsii have high case–fatality ratios. These organisms cause difficult-to-diagnose diseases and are highly infectious when inhaled as aerosols. Organisms resistant to tetracycline or chloramphenicol have been developed in the laboratory. Clinical Manifestations  After an incubation period of ~1–2 weeks, the onset of illness is abrupt, with prostration, severe headache, and fever rising rapidly to 38.8°–40.0°C (102°–104°F). Cough is promi­ nent, developing in 70% of patients. Myalgias are usually severe. A rash begins on the upper trunk, usually on the fifth day, and then becomes generalized, involving the entire body except the face, palms, and soles. Initially, this rash is macular; without treatment, it becomes maculo­ papular, petechial, and confluent. The rash often goes undetected on black skin; 60% of African patients have spotless epidemic typhus. Pho­ tophobia, with considerable conjunctival injection, is common. The tongue may be dry, brown, and furred. Confusion and coma associated with meningoencephalitis are common. Skin necrosis and gangrene of the digits as well as interstitial pneumonia may occur in severe cases. Untreated disease is fatal in 7–40% of cases, with outcome depending primarily on the condition of the host. Patients with untreated infec­ tions develop renal insufficiency and multiorgan involvement in which

neurologic manifestations are frequently prominent. Overall, 12% of patients with epidemic typhus have neurologic involvement. Infection associated with North American flying squirrels is a milder illness; whether this milder disease is due to host factors (e.g., better health status) or attenuated virulence is unknown.

Diagnosis and Treatment  Epidemic typhus is sometimes misdiag­ nosed as typhoid fever in tropical countries (Chap. 171). This means even serologic studies are often unavailable in settings of louse-borne typhus. Epidemics can be recognized by the serologic or immunohisto­ chemical diagnosis of a single case or by detection of R. prowazekii in a louse found on a patient. Doxycycline (100 mg bid) is administered orally or—if the patient is comatose or vomiting—intravenously and continued until 3–5 days after defervescence. Under epidemic conditions, a single 200-mg oral dose can be tried but fails in some cases. Pregnant patients should be evaluated individually and treated with chloramphenicol early in pregnancy or with doxycycline late in pregnancy. Prevention  Prevention of epidemic typhus involves control of body lice. Clothes should regularly be changed and laundered in hot water, and insecticides can be used every 6 weeks to control the louse population. ■ ■ENDEMIC MURINE TYPHUS Epidemiology  R. typhi is maintained in mammalian host–flea cycles, with rats (Rattus rattus and R. norvegicus) and the Oriental rat flea (Xenopsylla cheopis) as the classic zoonotic niche. Fleas acquire R. typhi from rickettsemic rats and carry the organism throughout their life span. Nonimmune rats and humans are infected when rickettsialaden flea feces contaminate pruritic bite lesions; less frequently, the flea bite transmits the organisms. Transmission can also occur via inha­ lation of aerosolized rickettsiae from flea feces. Infected rats appear healthy, although they are rickettsemic for ~2 weeks. CHAPTER 192 In the United States, murine typhus occurs mainly in Texas, where the disease is expanding northward, and in southern California, where it appears to affect people experiencing homelessness disproportion­ ally. The incidence is steadily increasing in both of these states, where the classic rat–flea cycle is absent and an opossum–cat flea (C. felis) cycle is highly suspected. Globally, endemic typhus occurs mainly in warm (often coastal) areas throughout the tropics and subtropics, where it is highly prevalent though often unrecognized. The incidence peaks from April through July in Texas and during the warm months of summer and early fall in other geographic locations. Patients seldom recall exposure to fleas, although exposure to animals such as cats, opossums, and rats is reported in nearly 40% of cases. Rickettsial Diseases Clinical Manifestations  The incubation period of experimental murine typhus averages 11 days (range, 8–16 days). Headache, myalgia, arthralgia, nausea, and malaise develop 1–3 days before onset of chills and fever. Patients often experience nausea and vomiting. The duration of untreated illness averages 12 days (range, 9–18 days). Rash occurs in approximately half of all patients. It is present in only 13% of patients at presentation for medical care (usually ~4 days after onset of fever), appearing an average of 2 days later in half of the remaining patients. The initial macular rash is often faint and detected by careful inspection of the axilla or the inner surface of the arm. Sub­ sequently, the rash becomes maculopapular, involving the trunk more often than the extremities; it is seldom petechial and rarely involves the face, palms, or soles. A rash is detected in only 20% of patients with darkly pigmented skin. Pulmonary involvement is frequently prominent; 35% of patients have a hacking, nonproductive cough, and 23% of patients who undergo chest radiography have pulmonary densities due to intersti­ tial pneumonia, pulmonary edema, and pleural effusions. Bibasilar rales are the most common pulmonary sign. Less common clinical manifestations include abdominal pain, confusion, stupor, seizures, ataxia, coma, and jaundice. Clinical laboratory studies frequently reveal anemia and leukopenia early in the course, leukocytosis late in the course, thrombocytopenia, hyponatremia, hypoalbuminemia, increased serum levels of hepatic aminotransferases, and prerenal

azotemia. Complications can include respiratory failure, hematemesis, cerebral hemorrhage, and hemolysis. Severe illness necessitates the admission of 10% of hospitalized patients to an intensive care unit. Greater severity is generally associated with old age, underlying dis­ ease, and treatment with a sulfonamide; the case–fatality rate is 0.4%.

Diagnosis and Treatment  Serologic studies of acute- and con­ valescent-phase serum samples can provide a diagnosis, and an immunohistochemical method for identification of typhus groupspecific antigens in biopsy samples has been developed. Cultivation is used infrequently and is not widely available. PCR of the blood is not adequately sensitive. When endemic typhus is suspected, patients should be treated empirically with doxycycline (100 mg twice daily by mouth for 7 days). Where available, chloramphenicol is an alternative. Ciprofloxacin and azithromycin are active in vitro, but clinically, they are less effective than doxycycline. ■ ■SCRUB TYPHUS Epidemiology  O. tsutsugamushi differs substantially from Rickett­ sia species both genetically and in cell-wall composition (i.e., it lacks lipopolysaccharide). O. tsutsugamushi is maintained by transovarial transmission in trombiculid mites. After hatching, infected larval mites (chiggers, the only stage that feeds on a host) inoculate organisms into the skin. Infected chiggers are particularly likely to be found in areas of heavy scrub vegetation during the wet season, when mites lay eggs. Scrub typhus is endemic and reemerging in eastern and southern Asia, northern Australia, and islands of the western Pacific and Indian Oceans. Infections are prevalent in these regions; in some areas, >3% of the population is infected or reinfected each month. Immunity to the homologous strain wanes over 1–3 years, and the organisms exhibit remarkable antigenic diversity with loss of cross-protective immunity in as short a period as 1 month. Emerging cases in Chile, serologic evidence of infection in other parts of South and Central America and Africa, and the discovery of Orientia chuto infection in southwestern Asia challenge the classic epidemiology of scrub typhus. PART 5 Infectious Diseases Clinical Manifestations  Illness varies from mild and self-limiting to fatal. After an incubation period of 6–21 days, onset is characterized by fever, headache, myalgia, cough, and gastrointestinal symptoms. Some patients recover spontaneously after a few days. The classic case description includes an eschar where the chigger has fed, regional lymphadenopathy, and a maculopapular rash—signs that are seldom seen in indigenous patients. In fact, <50% of returning travelers develop an eschar, and <40% develop a rash (on day 4–6 of illness). Severe cases typically manifest with encephalitis and interstitial pneumonia due to vascular injury. The case–fatality rate for untreated classic cases is 6% but would probably be lower if all mild cases were diagnosed. Diagnosis and Treatment  Serologic assays (indirect fluorescent antibody, indirect immunoperoxidase, enzyme immunoassays, and rapid diagnostic assays using lateral flow chromatographic tests) are the mainstays of laboratory diagnosis. PCR amplification of Orientia genes from eschars is effective, but less so for blood. Patients are treated with oral doxycycline (100 mg twice daily for 7–15 days), azithromycin (500 mg for 3 days), or chloramphenicol (500 mg four times daily for 7–15 days). In severe scrub typhus, the combination of doxycycline and azithromycin is beneficial compared to monotherapy of either agent. Some cases of scrub typhus in Thailand are poorly responsive to doxy­ cycline or chloramphenicol but respond to azithromycin and rifampin. EHRLICHIOSES AND ANAPLASMOSES Ehrlichioses are acute febrile infections caused by members of the family Anaplasmataceae, which is made up of obligately intracellular organ­ isms of more than five genera, including those with established human pathogens: Ehrlichia, Anaplasma, Wolbachia, “Candidatus Neoehrlichia,” and Neorickettsia. The bacteria reside in vertebrate reservoirs and target vacuoles of hematopoietic and, for some species, endothelial cells (Fig. 192-4). Four Ehrlichia species, two Anaplasma species, and one Neoehrlichia species are transmitted by ticks to humans and cause infec­ tion that can be severe and prevalent. E. chaffeensis, the agent of HME,

FIGURE 192-4  Peripheral-blood smear from a patient with human granulocytotropic anaplasmosis. A neutrophil contains two morulae (vacuoles filled with Anaplasma phagocytophilum). (Photo courtesy of Dr. J. Stephen Dumler.) and E. muris subsp. eauclairensis infect predominantly mononuclear phagocytes; E. ewingii and A. phagocytophilum infect neutrophils. Infec­ tions with “Candidatus Neoehrlichia mikurensis,” A. capra, and A. bovis are less well characterized but have been reported to grow in endothe­ lium, erythrocytes, and monocytes, respectively. Ehrlichia, “Candidatus Neoehrlichia,” and Anaplasma are main­ tained by horizontal tick–mammal–tick transmission, and humans are only inadvertently infected. Wolbachiae are associated with human filariasis, since they are important for filarial viability and pathogenic­ ity; antibiotic treatment targeting wolbachiae is a strategy for filariasis control. Neorickettsiae parasitize flukes (trematodes) that in turn parasitize aquatic snails, fish, and insects. Only a single pathogen of human neorickettsiosis has been described: N. sennetsu causes sen­ netsu fever, an infectious mononucleosis–like illness first identified in 1953 in association with the ingestion of raw fish containing N. sen­ netsu–infected flukes. ■ ■HUMAN MONOCYTOTROPIC EHRLICHIOSIS Epidemiology  More than 25,480 cases of E. chaffeensis infec­ tion had been reported to the U.S. Centers for Disease Control and Prevention (CDC) as of November 2023. However, active prospective surveillance documented an incidence as high as 414 cases per 100,000 population in some U.S. regions. Most E. chaffeensis infections are identified in the south-central, southeastern, and mid-Atlantic states, but cases have also been recognized in California, New York, New Eng­ land, and midwestern states. All stages of the Lone Star tick (A. ameri­ canum), which is expanding its geographic range, feed on white-tailed deer—a major reservoir. Dogs and coyotes also serve as reservoirs and often lack clinical signs. Tick bites and exposures are frequently reported by patients in rural areas, and 64% of infections occur in May through July. Transfusion-related infections are documented, as are infections in patients with organ transplants. The incidence of HME is highest in patients 60–84 years of age; however, 9% of infections occur in children ≤19 years of age, and these include severe and fatal infec­ tions. Of patients with HME, 59% are male. E. chaffeensis has been detected in South and Central America, Africa, and Asia. Clinical Manifestations  E. chaffeensis disseminates hematog­ enously from the dermal blood pool created by the feeding tick. After a median incubation period of 8 days, illness develops. Clinical manifes­ tations are undifferentiated and include fever (97% of cases), headache (70%), myalgia (68%), and malaise (77%). Less frequently observed are nausea, vomiting, and diarrhea (28–57%); cough (30%); rash (29% overall, 6% at presentation); and confusion (20%). HME can be severe:

77% of patients with confirmed cases are hospitalized, and 2% die. Lifethreatening complications include renal failure, meningoencephalitis, acute respiratory distress syndrome, a DIC-like syndrome, pneumonia, septic shock, cardiac failure, hepatitis, hemorrhage, hemophagocytic lymphohistiocytosis and—in immunocompromised patients— overwhelming ehrlichial infection; patients with diabetes, cancer, organ transplantation, asplenia, hepatitis C, or HIV infection have a 2.3 relative risk for death. Laboratory findings are valuable in the differen­ tial diagnosis of HME; 66% of patients have leukopenia (initially lym­ phopenia, later neutropenia), 86% have thrombocytopenia, and 89% have elevated serum levels of hepatic aminotransferases. Despite low blood cell counts, the bone marrow is hypercellular, and noncaseating granulomas can be present. Vasculitis is not a component of HME. Diagnosis  HME can be fatal. If not given empirical doxycycline treatment, 39% and 40% of patients with HME require admission to an intensive care unit and mechanical ventilation, respectively; these measures are not necessary in patients receiving prompt empirical treatment. In addition, hospital stay and illness duration are length­ ened in untreated patients by 8 and 12 days, respectively. The diagnosis is suggested by fever, known tick exposure in the preceding 3 weeks, thrombocytopenia and/or leukopenia, and increased serum amino­ transferase activities. Morulae are demonstrated in <10% of peripheralblood smears. HME can be confirmed during active infection by PCR amplification of E. chaffeensis nucleic acids in blood obtained before the start of doxycycline therapy. Retrospective serodiagnosis requires a consistent clinical picture and a fourfold increase in E. chaffeensis antibody titer to ≥128 in paired serum samples obtained ~3 weeks apart. Separate specific diagnostic tests are necessary for HME and HGA (see below). ■ ■EWINGII EHRLICHIOSIS AND EHRLICHIA MURIS EAUCLAIRENSIS INFECTIONS Ehrlichia ewingii resembles E. chaffeensis in its tick vector (A. americanum) and vertebrate reservoirs (white-tailed deer and dogs). E. muris eauclai­ rensis causes human infections after Ixodes scapularis tick exposure in Wisconsin and Minnesota. E. ewingii and E. muris illnesses are similar to but less severe than HME. Many cases occur in immunocompromised patients. Human infections with E. canis have been documented as subclinical ehrlichemia. No specific serologic diagnostic tests for these other ehrlichiae are readily available, and E. chaffeensis serologic tests can be positive when the infecting agent is actually a different species of Ehrlichia. ■ ■“CANDIDATUS NEOEHRLICHIA MIKURENSIS” INFECTION “Candidatus Neoehrlichia mikurensis,” a bacterium in a phylogenetic clade between Ehrlichia and Anaplasma, was originally identified in Ixodes ricinus ticks from the Netherlands and in mice and Ixodes ovatus ticks from Japan. By means of broad-range 16S rRNA gene amplifica­ tion and sequence analysis, this organism was identified as the cause of severe and sometimes prolonged febrile illnesses in European immu­ nocompromised patients with tick bites or exposures and in Chinese patients developing a mild febrile illness after being bitten by Ixodes persulcatus and Haemaphysalis concinna ticks. The clinical presenta­ tion is similar to those of HME and HGA except for an often prolonged course and a predilection for venous thrombosis. Specific diagnostic methods have been developed but are not widely available. TREATMENT Ehrlichioses Doxycycline is effective for HME as well as other ehrlichioses; the use of this drug in “Candidatus N. mikurensis” infection is associated with disease resolution. Therapy with doxycycline (100 mg given PO or IV twice daily) or tetracycline (250–500 mg given PO every 6 h) lowers hospitalization rates and shortens fever duration. E. chaffeen­ sis is not susceptible to chloramphenicol in vitro, and the use of this drug is controversial. While a few reports document E. chaffeensis

persistence in humans, this finding is rare; most infections are cured by short courses of doxycycline continuing for 3–5 days after defer­ vescence. Although poorly studied for this indication, rifampin may be suitable when doxycycline is contraindicated.

Prevention  HME, E. ewingii ehrlichiosis, E. muris ehrlichiosis, and “Candidatus N. mikurensis” infection can be prevented by the avoid­ ance of ticks in endemic areas. The use of protective clothing and tick repellents, careful postexposure tick searches, and prompt removal of attached ticks probably diminish infection risk. ■ ■HUMAN GRANULOCYTOTROPIC ANAPLASMOSIS Epidemiology  As of November 2023, 64,225 cases of HGA had been reported to the CDC, most in the upper Midwest and north­ eastern United States. The global geographic distribution is similar to that of Lyme disease because of the shared Ixodes tick vectors. Natural reservoirs for A. phagocytophilum are small mammals, such as whitefooted mice and squirrels, and large animals such as white-tailed deer in the United States and red deer in Europe. HGA incidence peaks in May through July, but the disease can occur throughout the year with exposure to Ixodes ticks. HGA often affects males (59%), and the inci­ dence of infection in persons older than 60 years is 7 times higher than in those younger than 15 years. Clinical Manifestations  Seroprevalence rates are high in endemic regions; thus, it seems likely that most individuals develop subclinical infections. The incubation period for HGA is 4–8 days, after which the disease manifests as fever (75–100% of cases), myalgia (73%), headache (82%), and malaise (97%). A minority of patients develop nausea, vom­ iting, or diarrhea (22–40%); cough (27%); or confusion (17%). A rash in HGA (5%) most often reflects co-infection with Borrelia, resulting in erythema migrans. Most patients develop thrombocytopenia (80%) and/or leukopenia (63%) with increased serum hepatic aminotransfer­ ase levels (80%). CHAPTER 192 Rickettsial Diseases Life-threatening complications occur most often in the elderly and include renal failure, acute respiratory distress syndrome, a toxic shock–like syndrome, pneumonia, a DIC- or sepsis-like syndrome and hemophagocytic lymphohistiocytosis. Meningoencephalitis is rare in documented cases of HGA. Other documented neurologic sequelae include brachial plexopathy, cranial nerve involvement, and demyelinat­ ing polyneuropathy. Infection of patients with a preexisting immuno­ compromising condition (diabetes, immunosuppressive medications, asplenia, arthritis) is associated with a 3.0 relative risk for life-threatening complications. Of patients with HGA, 31% are hospitalized, and 7% require intensive care. The case–fatality rate is 0.3%, but the relative risk for death is 16 if infection occurs with an immunosuppressive condi­ tion. Neither vasculitis nor granulomas are components of HGA. While patients can be co-infected with Borrelia burgdorferi and Babesia microti (transmitted by the same tick vector[s]), there is little evidence that these infections increase the severity or persistence of HGA. HGA transmit­ ted by transfusion (including the transfusion of leukoreduced blood or platelets) has now been reported in at least nine cases, including a fatality. Diagnosis  HGA should be included in the differential diagnosis of influenza-like illnesses during seasons with Ixodes tick activity (May through December), especially in the context of a known tick bite or exposure. Concurrent thrombocytopenia, leukopenia, or elevated serum levels of alanine or aspartate aminotransferase further increase the likelihood of HGA. Many HGA patients develop Lyme disease antibodies in the absence of clinical findings consistent with that diag­ nosis. Thus, HGA should be considered in the differential diagnosis of atypical severe Lyme disease presentations. Peripheral-blood film examination for neutrophil morulae can yield a diagnosis in 20–75% of infections. PCR testing of blood from patients with active disease before doxycycline therapy is sensitive and specific. Serodiagnosis is retrospective, requiring a fourfold increase in A. phagocytophilum anti­ body titer (to ≥128) in paired serum samples obtained 1 month apart. Since seroprevalence is high in some regions, a single acute-phase titer should not be used for diagnosis.

Anaplasma capra and A. bovis Infection  Human infection by A. capra, first isolated from goat blood, was identified in 28 patients from northeastern China. Patients presented with fever, headache, malaise, dizziness, myalgias, and chills, but these manifestations were less severe than in HGA. Hospitalization was recorded for 18% of patients, and 14% had underlying disorders, including hyperglycemia, hypertension, coronary heart disease, diabetes, and cancer. Five patients had severe manifestations, including one with encephalitic signs and A. capra DNA present in CSF. A. capra is found most often in Haemaphysalis longicornis ticks, and at least seven additional species can carry it over a distribution across Asia and eastern Europe. All patients responded to doxycycline treatment and survived.

Anaplasma bovis was recently identified in four human patients in a surveillance study of suspected Lyme borreliosis, HGA, HME, and babesiosis in the United States and in two patients with fever, myalgias, and rash in China. All six patients were between the ages of 35 and 71 years; four were male, and two were female. The apparent tick vec­ tors are Dermacentor variabilis in the United States and Haemaphysalis longicornis in China. Both Chinese patients recovered after antimicro­ bial treatment. TREATMENT Human Granulocytotropic Anaplasmosis No prospective studies of therapy for HGA have been conducted. However, doxycycline (100 mg PO twice daily) is effective. Rifampin therapy is associated with improvement of HGA in pregnant women and children. Most treated patients defervesce within 24–48 h. PART 5 Infectious Diseases Prevention  HGA prevention requires tick avoidance. Transmission can be documented as few as 4 h after a tick bite. Q FEVER The agent of Q fever is C. burnetii, a small pleomorphic coccobacillus with a gram-negative cell wall, that was first isolated in 1935 and called a rickettsia due to its presence in ticks, intracellular replication, small size, and staining characteristics, but it is now known to be genetically quite distinct from Rickettsiaceae and to have a number of unique features. It survives in harsh environments, escapes intracellular killing in macrophages by inhibiting the final step in phagosome maturation, and has adapted to the acidic phagolysosome. Epidemiology  Q fever is a zoonosis: transmission of C. burnetii to humans typically occurs by inhalation after it has been shed by animals. The primary sources of human infection are infected sheep, goats, and cattle. At parturition, when large amounts of C. burnetii are present in the fetus, placenta, membranes, and fluids, the bacterium readily contaminates the environment. Smaller amounts can be shed in milk for weeks to months after parturition, but also in urine and feces. Once shed, C. burnetii can remain viable in manure, hay, soil, etc., for many years, after which it can be aerosolized and inhaled, even after traveling miles from the source by wind. A variety of other vertebrate animals can be hosts of C. burnetii, including birds, cats, dogs, rabbits, skunks, raccoons, deer, bears, sloths, kangaroos, and marine animals. C. burnetii has also been found in several tick species, which could be important for maintenance of the agent in veterinary populations, but the majority of human Q fever cases are associated with aerosol transmission from infected livestock. Infections in animals are usu­ ally asymptomatic, but abortions and stillbirth have been observed in pregnant goats and sheep. Because it is easily dispersed as an aerosol and because of the extremely low infectious dose required for human infection (probably between 1 and 10 viable bacteria), C. burnetii is a potential agent of bioterrorism (Chap. S4), with a high infectivity rate and pneumonia as the major manifestation. Persons most at risk for Q fever include abattoir workers, veterinar­ ians, farmers, and other individuals who have contact with infected animals (particularly newborn animals), but outbreaks often affect people without any livestock contact. In Canada and the Netherlands, 65% and 72%, respectively, of persons living and/or working on dairy

cattle farms were seropositive, and in the United States, 22% of veteri­ narians were seropositive, compared to ~3% of the population overall. Seroprevalence data confirm that Q fever is underreported in many countries. An outbreak of Q fever associated with ingestion of raw milk confirmed the oral route of transmission, although this route is uncommon. In rare instances, person-to-person transmission follows childbirth in an infected woman, surgery, autopsy, blood transfusion, or organ transplantation of an infected individual. Multiple outbreaks involving laboratory staff have been reported in the past. Some evi­ dence suggests that C. burnetii can be sexually transmitted among humans. Some unusual modes of C. burnetii transmission to humans include treatment with live fetal sheep cells and percutaneous infection after crushing an infected tick between the fingers. Infections due to C. burnetii occur in most geographic locations except New Zealand and Antarctica. Several factors influence the epi­ demiology: environmental conditions such as high concentrations of animals, high animal pregnancy rates, dry weather, and the strength and direction of winds. In addition to differences between strains of C. burnetii, the variability in human susceptibility to C. burnetii can influ­ ence transmission and development of disease. Some people become sick after exposure, whereas others have only mild symptoms that are not sufficient to lead them to seek medical assistance, and ~60% have asymptomatic seroconversion. Q fever continues to be endemic in Australia and France. In Cayenne, French Guiana, Q fever is hyper­ endemic: 40% of all community-acquired pneumonias are caused by C. burnetii. The largest known outbreak occurred between 2007 and 2010 in the Netherlands. Over 4000 cases were reported, and over 40,000 people were infected. The outbreak was due to a combination of high-density goat farming in areas with large urban populations and environmental factors. Farms where spread did not occur had high vegetation density and lower groundwater concentrations. Young age seems to be protective against disease caused by C. burnetii. In a large outbreak in Switzerland, symptomatic infection occurred five times more often among persons >15 years of age than among younger individuals. In many outbreaks, men are affected more commonly than women. Clinical Manifestations  •  ACUTE Q FEVER  The incubation period is 3–30 days. The primary manifestations of acute Q fever dif­ fer geographically. During the Dutch outbreak, but also in Canada and Croatia, pneumonia is the more common presentation. In some countries where Q fever is endemic, such as France and Israel, hepatitis is more common. These differences could reflect the route of infec­ tion (i.e., ingestion of contaminated milk for hepatitis and inhalation of contaminated aerosols for pneumonia) or strain differences. In the Dutch outbreak, sequelae of infection in pregnant women were rare; this was not the case among pregnant women elsewhere. Pericarditis, myocarditis, acalculous cholecystitis, pancreatitis, lymphadenitis, spon­ taneous rupture of the spleen, transient hypoplastic anemia, hemolytic anemia, hemophagocytic lymphohistiocytosis, meningitis, encephalitis, optic neuritis, and erythema nodosum are less common manifestations. The symptoms of acute Q fever are nonspecific; common among them are fever, extreme fatigue, photophobia, and severe headache that is frequently retro-orbital. Other symptoms include chills, sweats, nau­ sea, vomiting, and diarrhea. Cough develops in about half of patients with Q fever pneumonia. A nonspecific rash may be evident in 4–18% of patients. The WBC count is usually normal. Thrombocytopenia occurs in ~25% of patients, and reactive thrombocytosis frequently develops during recovery. Biochemical markers of autoimmunity, such as anticytoplasmic antibodies (ANCA), antinuclear antibodies (ANA), anti–smooth muscle antibodies, or antiphospholipid antibodies, are often present in acute Q fever. Chest radiography can show opacities similar to those seen in pneumonia caused by other pathogens. Case– fatality rate of acute Q fever is ~1–2%. Acute Q fever occasionally complicates pregnancy. In one series, it resulted in premature birth in 35% of cases and in abortion or neo­ natal death in 43%. Neonatal death and lower infant birth weight are reported up to three times more often among women seropositive for C. burnetii in some areas but not others.

Rickettsial Diseases

CHAPTER 192 TABLE 192-2  Diagnostic Criteria for Chronic Q Fever as Defined by the Dutch Q Fever Consensus Group PROVEN CHRONIC Q FEVER PROBABLE CHRONIC Q FEVER POSSIBLE CHRONIC Q FEVER 1.  Positive Coxiella burnetii PCR in blood or tissuea or 2.  IFA ≥1:800 or 1:1024 for C. burnetii phase I IgG and Definite endocarditis according to the modified Duke criteria or Proven large vessel or prosthetic infection by imaging studies (18FDG-PET, CT, MRI, or AUS) IFA ≥1:800 or 1:1024 for C. burnetii phase I IgG AND AT LEAST ONE OF THE FOLLOWING: Valvulopathy not meeting the major criteria of the modified Duke criteria Known aneurysm and/or vascular or cardiac valve prosthesis without signs of infection by means of TEE/TTE, 18FDG-PET, CT, MRI, or AUS Suspected osteomyelitis or hepatitis as manifestation of chronic Q fever Pregnancy Symptoms and signs of chronic infection, such as fever, weight loss, night sweats, hepatosplenomegaly, and persistently raised ESR and CRP Granulomatous tissue inflammation, proven by histologic examination Immunocompromised state IFA ≥1:800 or 1:1024 for C. burnetii phase I IgG without manifestations meeting the criteria for proven or probable chronic Q fever aIn absence of acute infection. Abbreviations: AUS, abdominal ultrasound; CRP, C-reactive protein; CT, computed tomography; 18FDG-PET, fluorodeoxyglucose positron emission tomography; ESR, erythrocyte sedimentation rate; IFA, indirect fluorescent antibody assay; MRI, magnetic resonance imaging; PCR, polymerase chain reaction; TEE, transesophageal echocardiography; TTE, transthoracic echocardiography. Q FEVER FATIGUE SYNDROME  Prolonged fatigue follows acute Q fever in up to 20% of cases and can be accompanied by a constellation of symptoms, including headaches, sweats, arthralgia, and myalgias. Sev­ eral hypotheses regarding the etiology exist, including host and genetic factors, cytokine dysregulation, and a partly biopsychological etiology with C. burnetii acting as a trigger for fatigue development. A random­ ized controlled trial including 155 patients with strictly diagnosed Q fever fatigue syndrome showed that long-term treatment with doxy­ cycline did not reduce fatigue severity compared to placebo, so antibi­ otics should not be prescribed for these patients. Cognitive behavioral therapy aimed at fatigue-related cognitions and behaviors thought to perpetuate symptoms was effective in reducing fatigue severity in the short term. The beneficial effect of this treatment, however, was not maintained after 1 year. CHRONIC Q FEVER  Following symptomatic or asymptomatic primary infection, 1−5% of all patients develop chronic Q fever. Chronic Q fever endocarditis, infected aneurysms, or infected vascular prosthe­ ses are most frequently observed. The primary infection often has not been recognized or was asymptomatic, and the median duration between primary infection and manifestation of chronic infection is 13 months, with the largest observed interval between acute infection and diagnosis of chronic Q fever being >9 years. Risk factors for the development of chronic Q fever include male sex, older age, valvu­ lopathy or prior valve surgery, aneurysms, vascular prostheses, renal insufficiency, and immunocompromised state. Diagnosing chronic Q fever is difficult, as patients are sometimes asymptomatic until com­ plications occur and often present with nonspecific symptoms, such as fever, night sweats, weight loss, fatigue, and malaise. Fever can be absent and is frequently low grade. C-reactive protein is often low or even normal. Chronic Q fever endocarditis differs from endocarditis caused by other bacteria, manifesting as endothelium-covered nodules on the valves, aortic root abscess, or new or rapidly increasing valve insufficiency. A high index of suspicion is necessary for timely diag­ nosis. Patients with chronic Q fever are often ill for >1 year before the diagnosis is made. The disease should be suspected in all patients with culture-negative endocarditis. In addition, all patients with valvular heart disease, an aneurysm or vascular prosthesis and unexplained weight loss, fever, elevated inflammatory markers, stroke, unexpected aneurysm growth, and/or progressive heart failure should be tested for C. burnetii infection. Other manifestations of chronic Q fever include lymphadenitis and bone infection including vertebral osteomyelitis and prosthetic joint infection. Of 313 patients with proven chronic Q fever in the Netherlands, 65% developed complications. The most frequently observed complications were acute aneurysms, heart failure, and noncardiac abscesses. One in six patients with vascular chronic Q fever develops arterial fistula, including aortoenteric fistula, aortocaval fistula, aortobronchial fistula, and arteriocutaneous fistula. PCR posi­ tivity at any time during the disease, presence of prosthetic material, and older age were associated with complications. Q fever–related mortality was 25% in patients diagnosed with proven chronic Q fever after the Dutch outbreak. Chronic Q fever–related mortality was high­ est in patients with both endocarditis and vascular infection (33%), followed by patients with vascular infection only (25%), and was lowest in endocarditis patients (12%). Diagnosis  Culture of C. burnetii from buffy-coat blood samples or tissue specimens is possible but requires a biosafety level 3 laboratory and is not used in clinical practice. PCR detects C. burnetii DNA in blood and tissue specimens, including paraffin-embedded samples. The detection of antibodies to C. burnetii is the most commonly used method for the diagnosis of Q fever. Available serologic assays are indirect fluorescent antibody (IFA) assay, enzyme-linked immuno­ sorbent assay (ELISA), and complement fixation test (CFT), with IFA being the gold standard. IFA tests are useful for the detection of and discrimination between acute and chronic infection and have excellent sensitivity and specificity. The diagnosis of acute Q fever is dependent on seroconversion, defined as a fourfold increase in IgG titer for phase II antigens between acute- and convalescent-phase samples. In the first 1−2 weeks of illness, PCR on blood or serum can also be positive. A high phase I IgG titer (e.g., >512) is suggestive of chronic Q fever, but alone, it is not enough for a definite diagnosis. A positive PCR for C. burnetii in blood or tissue in the absence of an acute infection confirms the diagnosis, but PCR on blood is negative in the majority of patients and tissue samples are often very difficult to obtain. The diagnosis of chronic Q fever should be based on a combination of clinical, labora­ tory, and imaging criteria. There has been debate on the optimal set of criteria, but the Dutch literature-based consensus guideline (Table 192-2) appears to be the most sensitive and is easy to use. Valvular vegetations are detected in only 12% of patients with Q fever endocarditis by transthoracic echocardiography, but the rate of detection is higher (21–50%) with transesophageal echocardiography. Fluorodeoxyglucose positron emission tomography combined with computed tomography (FDG-PET/CT) can detect not only prosthetic valvular infection but also intravascular infection elsewhere, osteo­ myelitis, and lymphadenitis. In native valve endocarditis, specificity is very high but sensitivity is low, so a normal FDG-PET/CT scan cannot exclude native valve endocarditis. A study including 273 FDG-PET/ CT scans performed at diagnosis in patients suspected of chronic Q fever showed that, even after serology, PCR, and often ultrasound or CT had been performed, FDG-PET/CT led to a change in diagnosis or treatment in 20% of patients. Adding FDG uptake in a heart valve as a major criterion to the Duke criteria led to a 1.9-fold increase of definite endocarditis diagnoses. Of 218 scans performed during follow-up, 57% resulted in treatment adjustment. In case of suspected chronic Q fever, FDG-PET/CT should be considered. TREATMENT Q Fever ANTIBIOTICS In vitro, C. burnetii is susceptible to doxycycline, quinolones, trimethoprim-sulfamethoxazole (TMP-SMX), macrolides, and

80 - 193 Infections Due to Mycoplasmas

193 Infections Due to Mycoplasmas

rifampin. Although antimicrobial susceptibility testing is not rou­ tinely performed and resistance to doxycycline does not appear to be a common problem in clinical practice, doxycycline-resistant isolates do exist.

Treatment of acute Q fever with doxycycline (100 mg twice daily for 14 days) is usually successful. Quinolones also are effective. When Q fever is diagnosed during pregnancy, treatment with TMPSMX is recommended for the duration of the pregnancy. Treatment with doxycycline and hydroxychloroquine for 6−12 months following acute infection should be considered in patients with valve abnormalities, a prosthetic heart valve, an aneurysm, or vascular prosthesis. This appeared to be effective in preventing progression to chronic Q fever in patients with valvulopathy. The exact indications and duration of prophylaxis should be based on a careful consideration of possible benefits and side effects. Decisions on treatment of chronic Q fever are challenging, so consultation with an infectious diseases expert is recommended. There is no indication for antibiotic therapy in those with possible chronic Q fever (only elevated phase I IgG without symptoms or an infectious focus). Addition of hydroxychloroquine (to alkalinize the phagolysosome) renders doxycycline bactericidal against C. burnetii in vitro, and the combination of doxycycline 100 mg twice daily with 200 mg hydroxychloroquine three times daily is the favored regi­ men. It is advised to determine serum levels of doxycycline aiming for concentrations between 5 and 10 mg/L, often requiring higher doses than 200 mg per day. Patients treated with this regimen must be advised about photosensitivity, but side effects should not lead to cessation of doxycycline too easily since it appears to be the most effective approach for this serious infection that has a high mortality despite treatment. Patients treated with hydroxychloroquine are at risk for developing retinopathy, so they should be evaluated by an ophthalmologist before starting treatment and every 6−12 months during the course of therapy. If doxycycline-hydroxychloroquine cannot be used, the regimen chosen should include at least two anti­ biotics active against C. burnetii. In a study including 322 patients with chronic Q fever, treatment with doxycycline combined with a quinolone appeared to be a safe alternative. PART 5 Infectious Diseases Minimum treatment duration is 18 months after PCR on blood had become negative (if positive before) and adequate doxycycline levels have been reached for native valve endocarditis and other manifestations without prosthetic material and 24 months for patients with prosthetic valve endocarditis or infected vascular prostheses. Many patients with vascular infection need prolonged treatment before the infection resolves, and surgical intervention is often necessary to remove an infected graft if the patient does not respond to antibiotic therapy. Abscesses need drainage for antibiotic therapy to be successful. Defining cure of chronic Q fever after the minimum treatment duration should be based on a combination of imaging (if abnormal at diagnosis), decline of serologic titers, nega­ tivity of PCR on blood or serum, and improvement of symptoms. FOLLOW-UP After acute Q fever, patients without risk factors for developing chronic Q fever should be evaluated clinically and serologically after 6 months. When IgG phase I is <1024 and clinical symptoms do not suggest chronic infection, follow-up can be stopped. For patients with a very high risk of developing chronic Q fever who have received antibiotics for 6−12 months, patients with immu­ nosuppression or other risk factors not treated with antibiotics for a prolonged period of time, or patients with possible chronic Q fever (only phase I IgG ≥1024), follow-up with serology and PCR every 3−6 months for 2 years is recommended. During treatment of chronic Q fever, patients should be followed every 3 months to evaluate symptoms, side effects, serology, and PCR. When new complications are suspected, imaging should be repeated. After the end of treatment, relapse has been described up to 5 years later. It is therefore recommended to continue monitoring with serology and PCR until a minimum of 5 years after end of treatment.

Prevention  A whole-cell vaccine (Q-Vax) licensed in Australia effectively prevents Q fever in abattoir workers. Vaccine is given only to people without a history of Q fever and negative results in both serologic and skin testing that is performed with intradermal diluted

C. burnetii vaccine to prevent side effects. Cases among abattoir work­ ers in Australia declined dramatically as a result of a vaccination pro­ gram, but the vaccine has not been approved outside Australia. Good animal-husbandry practices are important in preventing widespread contamination of the environment by C. burnetii. These practices include isolating aborting animals for up to 14 days, raising feed bunks to prevent contamination of feed by excreta, destroying aborted materials (by burning and burying fetal membranes and still­ born animals), and wearing masks and gloves when handling aborted materials. Vaccination of sheep and goats and a culling program were effective in the Dutch outbreak. During an outbreak of Q fever and for 4 weeks after it ceases, blood donations should not be accepted from individuals who live in the affected area. Acknowledgment The authors thank Thomas Marrie, MD, for his significant contributions to this chapter in the previous editions. ■ ■FURTHER READING Biggs HM et al: Diagnosis and management of tickborne rickettsial diseases: Rocky Mountain spotted fever and other spotted fever group rickettsioses, ehrlichioses, and anaplasmosis—United States. MMWR 65:1, 2016. Buijs SB et al: Still new chronic Q fever cases diagnosed 8 years after a large Q fever outbreak. Clin Infect Dis 73:1476, 2021. Gillespie J, Salje J: Orientia and Rickettsia: Different flowers from the same garden. Curr Opin Microbiol 74:102318, 2023. Ismail N, Mcbride JW: Tick-borne emerging infections: Ehrlichiosis and anaplasmosis. Clin Lab Med 37:317, 2017. Varghese GM et al: Intravenous doxycycline, azithromycin, or both for severe scrub typhus. N Engl J Med 388:792, 2023. R. Doug Hardy

Infections Due to

Mycoplasmas Mycoplasmas are prokaryotes of the class Mollicutes. Their size (150–350 nm) is closer to that of viruses than to that of typical bacteria. Unlike viruses, however, mycoplasmas grow in cell-free culture media; in fact, they are the smallest organisms capable of independent replication. The entire genomes of many Mycoplasma species have been sequenced and have been found to be among the smallest of all prokaryotic genomes. Sequencing information for these genomes has helped define the minimal set of genes necessary for cellular life. The absence of genes related to the synthesis of amino acids, fatty acid metabolism, and cholesterol dictates the mycoplasmas’ parasitic or saprophytic dependence on a host for exogenous nutrients and neces­ sitates the use of complex fastidious media to culture these organisms. Mycoplasmas lack a cell wall and are bound only by a cell membrane. The absence of a cell wall explains the inactivity of β-lactam antibiotics (penicillins and cephalosporins) against infections caused by these organisms. At least 13 Mycoplasma species, 2 Acholeplasma species, and 2 Urea­ plasma species have been isolated from humans. Most of these species are thought to be normal inhabitants of oral and urogenital mucous

membranes. M. pneumoniae, M. hominis, M. genitalium, U. urealyti­ cum, and U. parvum have been shown conclusively to be pathogenic in immunocompetent humans. M. pneumoniae primarily infects the respiratory tract, while M. hominis, M. genitalium, U. urealyticum, and U. parvum are associated with a variety of genitourinary tract disorders and neonatal infections. Other mycoplasmas may cause disease in immunocompromised persons. MYCOPLASMA PNEUMONIAE ■ ■PATHOGENESIS M. pneumoniae is generally thought to act as an extracellular pathogen. Although the organism has been shown to exist and replicate within human cells, it is not known whether these intracellular events contrib­ ute to the pathogenesis of disease. M. pneumoniae attaches to ciliated respiratory epithelial cells by means of a complex terminal organelle at the tip of one end of the organism. Cytoadherence is mediated by interactive adhesins and accessory proteins clustered on this organelle. After extracellular attachment, M. pneumoniae causes injury to host respiratory tissue. The mechanism of injury is thought to be mediated by the production of hydrogen peroxide and of an ADP-ribosylating and vacuolating cytotoxin of M. pneumoniae that has many similarities to pertussis toxin. Because mycoplasmas lack a cell wall, they also lack cell wall–derived stimulators of the innate immune system, such as lipopolysaccharide, lipoteichoic acid, and murein (peptidoglycan) frag­ ments. However, lipoproteins from the mycoplasmal cell membrane appear to have inflammatory properties, probably acting through Toll-like receptors (primarily TLR2) on macrophages and other cells. Lung biopsy specimens from patients with M. pneumoniae respiratory tract infection reveal an inflammatory process involving the trachea, bronchioles, and peribronchial tissue, with a monocytic infiltrate that coincides with a luminal exudate of polymorphonuclear leukocytes. Experimental evidence indicates that innate immunity provides most of the host’s defense against mycoplasmal infection in the lungs, whereas cellular immunity may actually play an immunopathogenic role, exacer­ bating mycoplasmal lung disease. Humoral immunity appears to provide protection against dissemination of M. pneumoniae infection; patients with humoral immunodeficiencies do not have more severe lung disease than do immunocompetent patients in the early stages of infection but more often develop disseminated infection resulting in syndromes such as arthritis, meningitis, and osteomyelitis. The immunity that follows severe M. pneumoniae infections is more protective and longer-lasting than that following mild infections. Genuine second attacks of M. pneu­ moniae pneumonia have been reported infrequently. ■ ■EPIDEMIOLOGY M. pneumoniae infection occurs worldwide. It is likely that the inci­ dence of upper respiratory illness due to M. pneumoniae is up to 20 times that of pneumonia caused by this organism. Infection is spread from one person to another by respiratory droplets expectorated dur­ ing coughing and results in clinically apparent disease in an estimated 80% of cases. The incubation period for M. pneumoniae is 2–4 weeks; therefore, the time-course of infection in a specific population may be several weeks long. Intrafamilial attack rates are as high as 84% among children and 41% among adults. Outbreaks of M. pneumoniae illness often occur in institutional settings such as military bases, boarding schools, and summer camps. Infections tend to be endemic, with spo­ radic epidemics every 4–7 years. Most significantly, M. pneumoniae is a major cause of communityacquired respiratory illness in both children and adults and is often grouped with Chlamydia pneumoniae and Legionella species as one of the most important bacterial causes of “atypical” community-acquired pneumonia. For community-acquired pneumonia in adults, M. pneu­ moniae is the most frequently detected “atypical” organism. Analysis of 13 studies of community-acquired pneumonia published between 1996 and 2001 (which included 6207 ambulatory and hospitalized adults) showed that the overall prevalence of M. pneumoniae was 22.7%; by comparison, the prevalence of C. pneumoniae was 11.7%, and that of Legionella species was 4.6%. The summation of 26 more recent

investigations of “atypical” organisms in community-acquired pneu­ monia in adults published between 2002 and 2015 found the overall prevalence of M. pneumoniae was 7.2%; by comparison, the prevalence of C. pneumoniae was 4.3%, and that of Legionella species was 2.8%.

M. pneumoniae pneumonia is also referred to as Eaton agent pneumo­ nia (the organism having first been isolated in the early 1940s by Mon­ roe Eaton), primary atypical pneumonia, and “walking” pneumonia.

■ ■CLINICAL MANIFESTATIONS Upper Respiratory Tract Infections and Pneumonia  Acute

M. pneumoniae infections generally manifest as pharyngitis, tracheo­ bronchitis, reactive airway disease/wheezing, or a nonspecific upper respiratory syndrome. Little evidence supports the commonly held belief that this organism is an important cause of otitis media, with or without bullous myringitis. Pneumonia develops in 3–13% of infected individu­ als; its onset is usually gradual, occurring over several days, but may be more abrupt. Although Mycoplasma pneumonia may begin with a sore throat, the most common presenting symptom is cough. The cough is typically nonproductive, but some patients produce sputum. Headache, malaise, chills, and fever are noted in the majority of patients. On physical examination, wheezes or rales are detected in ~80% of patients with M. pneumoniae pneumonia. In many patients, however, pneumonia can be diagnosed only by chest radiography. The most common radiographic pattern is that of peribronchial pneumonia with thickened bronchial markings, streaks of interstitial infiltration, and areas of subsegmental atelectasis. Segmental or lobar consolidation is not uncommon. While clinically evident pleural effusions are infre­ quent, lateral decubitus views reveal that up to 20% of patients have pleural effusions. CHAPTER 193 Overall, the clinical presentation of pneumonia in an individual patient is not useful for differentiating M. pneumoniae pneumonia from other types of community-acquired pneumonia. The possibility of M. pneumoniae infection deserves particular consideration when community-acquired pneumonia fails to respond to treatment with a penicillin or a cephalosporin—antibiotics that are ineffective against mycoplasmas. Symptoms usually resolve within 2–3 weeks after the onset of illness. Although M. pneumoniae pneumonia is generally selflimited, appropriate antimicrobial therapy significantly shortens the duration of clinical illness. Infection uncommonly results in critical illness and only rarely in death. In some patients, long-term recur­ rent wheezing or reactive airway disease may follow the resolution of acute pneumonia. The significance of chronic infection, especially as it relates to asthma, is an area of active investigation. Infections Due to Mycoplasmas Extrapulmonary Manifestations  An array of extrapulmonary manifestations may develop during M. pneumoniae infection. The most significant are neurologic, dermatologic, cardiac, rheumatologic, and hematologic in nature. Extrapulmonary manifestations can be a result of disseminated infection, especially in patients with humoral immunode­ ficiencies (e.g., septic arthritis); postinfectious autoimmune phenomena (e.g., Guillain-Barré syndrome); or possibly ADP-ribosylating toxin. Overall, these manifestations are uncommon, given the frequency of

M. pneumoniae infection. Notably, many patients with extrapulmonary M. pneumoniae disease do not have respiratory disease. Skin eruptions described with M. pneumoniae infection include erythematous (macular or maculopapular), vesicular, bullous, petechial, and urticarial rashes. In some reports, 17% of patients with M. pneu­ moniae pneumonia have had an exanthem. Erythema multiforme major (Stevens-Johnson syndrome) is the most clinically significant skin erup­ tion associated with M. pneumoniae infection; it appears to occur more commonly with M. pneumoniae than with other infectious agents. A wide spectrum of neurologic manifestations has been reported with M. pneumoniae infection. The most common are meningoen­ cephalitis, encephalitis, Guillain-Barré syndrome, and aseptic menin­ gitis. M. pneumoniae has been implicated as a likely etiologic agent in 5–7% of cases of encephalitis. Other neurologic manifestations may include cranial neuropathy, acute psychosis, cerebellar ataxia, acute demyelinating encephalomyelitis, cerebrovascular thromboembolic events, and transverse myelitis.

TABLE 193-1  Diagnostic Tests for Respiratory Mycoplasma pneumoniae Infectiona TEST SENSITIVITY, % SPECIFICITY, % Respiratory culture ≤60

Respiratory PCR 65–90 90–100 Serologic studiesb 55–100 55–100 aA combination of PCR and serology is suggested for routine diagnosis. If macrolide resistance is suspected, resistance testing by culture and/or PCR is available. bAcute- and convalescent-phase serum samples are recommended. Abbreviation: PCR, polymerase chain reaction. aAntimicrobial resistance has been reported in mycoplasmas, as described in the text. Hematologic manifestations of M. pneumoniae infection include hemolytic anemia, aplastic anemia, cold agglutinins, disseminated intravascular coagulation, and hypercoagulopathy. When anemia does occur, it generally develops in the second or third week of illness. In addition, hepatitis, glomerulonephritis, pancreatitis, myocarditis, pericarditis, rhabdomyolysis, and arthritis (septic and reactive) have been convincingly ascribed to M. pneumoniae infection. Septic arthritis has been described most commonly in hypogammaglobulinemic patients. ■ ■DIAGNOSIS Clinical findings, nonmicrobiologic laboratory tests, and chest radi­ ography are not useful for differentiating M. pneumoniae pneumonia from other types of community-acquired pneumonia. In addition, since M. pneumoniae lacks a cell wall, it is not visible on Gram stain. Although of historical interest, the measurement of cold agglutinin titers is no longer recommended for the diagnosis of M. pneumoniae infection because the findings are nonspecific and assays specific for M. pneumoniae are now available. PART 5 Infectious Diseases Acute M. pneumoniae infection can be diagnosed by polymerase chain reaction (PCR) detection of the organism in respiratory tract secretions or by isolation of the organism in culture (Table 193-1). Oropharyngeal, nasopharyngeal, and pulmonary specimens are all acceptable for diag­ nosing M. pneumoniae pneumonia. Other bodily fluids, such as cerebro­ spinal fluid, are acceptable for extrapulmonary infection. M. pneumoniae culture (which requires special media) is not recommended for routine diagnosis because the organism may take weeks to grow and is often dif­ ficult to isolate from clinical specimens. In contrast, PCR allows rapid, specific diagnosis earlier in the course of clinical illness. The diagnosis can also be established by serologic tests for IgM and IgG antibodies to M. pneumoniae in paired (acute- and conva­ lescent-phase) serum samples; enzyme-linked immunoassay is the recommended serologic method. An acute-phase sample alone is not adequate for diagnosis, as antibodies to M. pneumoniae may not develop until 2 weeks into the illness; therefore, it is important to test paired samples. In addition, IgM antibody to M. pneumoniae can persist for up to 1 year after acute infection. Thus, its presence may indicate recent rather than acute infection. The combination of PCR of respiratory tract secretions and sero­ logic testing constitutes the most sensitive and rapid approach to the diagnosis of M. pneumoniae infection. TREATMENT Mycoplasma pneumoniae Infections Although in the majority of untreated cases symptoms resolve within 2–3 weeks without significant associated morbidity,

M. pneumoniae pneumonia can be a serious illness that responds to appropriate antimicrobial therapy (Table 193-2). Randomized, double-blind, placebo-controlled trials in adults have demonstrated that antimicrobial treatment significantly decreases the duration of fever, cough, malaise, hospitalization, and radiologic abnor­ malities in M. pneumoniae pneumonia. Treatment options for acute

M. pneumoniae infection include macrolides (e.g., oral azithromy­ cin, 500 mg on day 1, then 250 mg/d on days 2–5), tetracyclines (e.g., oral doxycycline, 100 mg twice daily for 7–14 days), and respiratory fluoroquinolones. However, ciprofloxacin and ofloxacin are not rec­ ommended because of their high minimal inhibitory concentrations

TABLE 193-2  Antimicrobial Agents of Choice for Mycoplasma Infectionsa ORGANISM(S) DRUGS Mycoplasma pneumoniae Azithromycin, clarithromycin, erythromycin, doxycycline, levofloxacin, moxifloxacin, gemifloxacin (not ciprofloxacin or ofloxacin) Ureaplasma urealyticum, Ureaplasma parvum Azithromycin, clarithromycin, erythromycin, doxycycline Mycoplasma hominis Doxycycline, clindamycin Mycoplasma genitalium Azithromycin, moxifloxacin, doxycycline against M. pneumoniae isolates and their poor performance in experimental studies. A 7- to 14-day course of quinolone therapy appears adequate. Even though appropriate antibiotic therapy signif­ icantly reduces the duration of respiratory illness, it does not appear to shorten the duration of detection of M. pneumoniae by culture or PCR; therefore, a test of cure for eradication is not suggested. In Asian countries, a high prevalence (range 34–76%) of

M. pneumoniae resistant to macrolides has been reported. In Europe and in the United States, macrolide-resistant M. pneu­ moniae is less common. In the United States, national surveillance from 2018 found that 10.2% of isolates demonstrated macrolide resistance. Furthermore, national surveillance from 2015–2018 found macrolide resistance of 15.2–21.7% in the eastern United States and 1.9–2.8% in the western United States. Clinical studies have demonstrated that, when treated with macrolides, patients with community-acquired pneumonia due to macrolide-resistant

M. pneumoniae experience a significantly longer duration of symp­ toms than do patients infected with macrolide-sensitive organisms; thus, macrolide resistance in M. pneumoniae does appear to have clinical significance. In addition, clinical investigations have indi­ cated that for macrolide-refractory M. pneumoniae pneumonia, tetracycline class therapy results in shorter duration of fever and hospital length of stay compared with macrolide therapy. If macro­ lide resistance is prominent in a particular geographic locale or is suspected, then a nonmacrolide antibiotic should be considered for treatment; in addition, in these instances, a respiratory sample may be sent to a Mycoplasma reference laboratory for the detection of macrolide resistance by culture or PCR. While the 2019 Infectious Diseases Society of America and American Thoracic Society guidelines do not recommend routinely using corticosteroids in community-acquired pneumonia, growing clinical literature suggests that the addition of glucocorticoids to an antibiotic regimen may be of value for the treatment of severe or refractory M. pneumoniae pneumonia. A 2019 meta-analysis of 24 randomized controlled trials in children found that use of corticosteroids in macrolide-refractory M. pneumoniae pneumonia significantly reduced hospital days and duration of fever. Clinical literature in adults also shows benefit, but these data are limited and more observational in nature. The roles of antimicrobial drugs, glucocorticoids, and IV immunoglobulin in the treatment of neurologic disease due to

M. pneumoniae remain unknown. UROGENITAL MYCOPLASMAS ■ ■EPIDEMIOLOGY M. hominis, M. genitalium, U. urealyticum, and U. parvum can cause urogenital tract disease. The significance of isolation of these organ­ isms in a variety of other syndromes is unknown and in some cases is being investigated. M. fermentans has not been shown convincingly to cause human disease. While urogenital mycoplasmas may be transmitted to a fetus during passage through a colonized birth canal, sexual contact is the major mode of transmission, and the risk of colonization increases dra­ matically with increasing numbers of sexual partners. In asymptom­ atic women, these mycoplasmas may be found throughout the lower

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194 Chlamydial Infections

urogenital tract. The vagina yields the largest number of organisms; next most densely colonized are the periurethral area and the cervix. Ureaplasmas are isolated less often from urine than from the cervix, but M. hominis is found with approximately the same frequency at these two sites. Ureaplasmas are isolated from the vagina of 40–80% of sexually active, asymptomatic women and M. hominis from 21–70%. The two microorganisms are found concurrently in 31–60% of women. In men, colonization with each organism is less prevalent. A large U.S. prospective multicenter study of a nucleic acid amplification test for M. genitalium including male and female patients seeking care in diverse geographic regions found overall prevalence to be 10.3%. Mycoplasmas have been isolated from urine, semen, and the distal urethra of asymptomatic men. ■ ■CLINICAL MANIFESTATIONS Urethritis, Pyelonephritis, and Urinary Calculi  In many episodes of Chlamydia-negative nongonococcal urethritis, ureaplasmas may be the causative agent. These organisms may also cause chronic voiding symptoms in women. The common presence of ureaplasmas in the urethra of asymptomatic men may suggest either that only certain serovars are pathogenic or that predisposing factors, such as lack of immunity, must exist in persons who develop symptomatic infection. Alternatively, disease may develop only upon initial exposure to urea­ plasmas. Ureaplasmas have been implicated in epididymitis. M. geni­ talium also appears to cause urethritis. M. genitalium and ureaplasmas do not have a known role in prostatitis. M. hominis does not appear to play a primary etiologic role in urethritis, epididymitis, or prostatitis. Evidence suggests that M. hominis causes up to 5% of cases of acute pyelonephritis. Ureaplasmas have not been associated with this disease. Ureaplasmas play a limited role in the production of urinary calculi. The frequency with which ureaplasmas reach the kidney, the predis­ posing factors that allow them to do so, and the relative frequency of urinary tract calculi induced by this organism (compared with other organisms) are not known. Pelvic Inflammatory Disease  M. hominis can cause pelvic inflammatory disease. In most episodes, M. hominis occurs as part of a polymicrobial infection, but the organism may play an independent role in a limited number of cases. Data also support an association of M. genitalium with pelvic inflammatory disease. Ureaplasmas are not thought to cause pelvic inflammatory disease. Postpartum and Postabortal Infection  Studies implicate

M. hominis as the primary pathogen in ~5–10% of women who have post­ partum or postabortal fever; ureaplasmas have been implicated to a lesser degree. These infections are generally self-limited; however, if symptoms persist, specific antimicrobial therapy should be given. Ureaplasmas also appear to play a role in occasional postcesarean wound infections. Nonurogenital Infection  In rare instances, M. hominis causes nonurogenital infections, such as brain abscess, wound infection, poststernotomy mediastinitis, endocarditis, and neonatal meningitis. These infections are most common among immunocompromised and hypogammaglobulinemic patients. Ureaplasmas and M. hominis can cause septic arthritis in immunodeficient patients. Ureaplasmas probably cause neonatal pneumonitis; their possible causal role in the development of bronchopulmonary dysplasia—the chronic lung disease of premature infants—has been extensively investigated, with most studies indicating at least a significant association. It is unclear whether ureaplasmas and M. hominis cause infertility, spontaneous abortion, premature labor, low birth weight, or chorioamnionitis. ■ ■DIAGNOSIS Culture and PCR are both appropriate methods for the isolation of urogenital mycoplasmas. Culture of these organisms, however, requires special techniques and media that generally are available only at larger medical centers and reference laboratories. Serologic testing is not recommended for the clinical diagnosis of urogenital Mycoplasma infections. Antibiotic resistance testing to guide appropriate therapy is becoming more available to clinical practice.

TREATMENT Urogenital Mycoplasma Infections Because colonization with urogenital mycoplasmas is common, it appears at present that their isolation from the urogenital tract in the absence of disease generally does not warrant treatment. Patients with recurrent urethritis or cervicitis after treatment failure of appropriate antibiotic therapy for sexually transmitted infections should receive M. genitalium testing. It is not recom­ mended to screen for M. genitalium in those who are asymptomatic. However, sex partners of patients with symptomatic M. genitalium infection should receive testing. Those with positive test results should receive treatment with antibiotics to possibly reduce the risk for reinfection. Macrolides and doxycycline are considered the antimicrobial agents of choice for Ureaplasma infections (Table 193-2). Ure­ aplasma resistance to macrolides, doxycycline, quinolones, and chloramphenicol has been reported. M. hominis is resistant to mac­ rolides. Doxycycline is generally the drug of choice for M. hominis infections, although resistance has been reported. Clindamycin is generally active against M. hominis. Quinolones are active in vitro against M. hominis. For M. genitalium, treatment failure and/or antibiotic resistance has increased for both azithromycin and moxifloxacin. The Centers for Disease Control and Prevention’s 2021 Sexually Transmitted Infections Treatment Guidelines recommend two-stage therapy based on antibiotic resistance testing, if available: doxycycline 100 mg orally 2 times per day for 7 days, followed by either azithromycin 1 g orally initial dose, followed by 500 mg orally once daily for 3 additional days (if macrolide sensitive), or moxifloxacin 400 mg orally once daily for 7 days (if macrolide resistant). If M. genitalium antibiotic resistance testing is not available, then doxycycline fol­ lowed by moxifloxacin is recommended. Test of cure for M. genita­ lium infection is not recommended for asymptomatic persons who received treatment with a recommended regimen. CHAPTER 194 Chlamydial Infections ■ ■FURTHER READING Waites KB et al: Mycoplasma pneumoniae from the respiratory tract and beyond. Clin Microbiol Rev 30:747, 2017. Waites KB et al: Macrolide-resistant Mycoplasma pneumoniae in the United States as determined from a national surveillance program. J Clin Microbiol 57:e00968, 2019. Wang G et al: Global prevalence of resistance to macrolides in Myco­ plasma pneumoniae: A systematic review and meta-analysis. J Anti­ microb Chemother 77:2353, 2022. Workowski KA et al: Sexually transmitted diseases treatment guide­ lines, 2021. MMWR Recomm Rep 70:1, 2021. Matthew M. Hamill, Thomas C. Quinn

Chlamydial Infections Chlamydiae are obligate intracellular bacteria that cause a wide variety of diseases in humans and nonhuman animals. ETIOLOGIC AGENTS The chlamydiae were originally classified as four species in the genus Chlamydia: C. trachomatis, C. pneumoniae, C. psittaci, and C. pecorum (the last species being found in ruminants). The C. psittaci group has been separated into three species: C. psittaci, C. felis, and C. abortus. The mouse pneumonitis strain (MoPn) is now classified as

C. muridarum, and the guinea pig inclusion conjunctivitis strain (GPIC) is now designated C. caviae.

C. trachomatis is divided into two biovars: trachoma and LGV (lym­ phogranuloma venereum). The trachoma biovar causes two major types of disease in humans: ocular trachoma, the leading infectious cause of preventable blindness in the developing world; and urogenital infections, which are predominantly sexually transmitted but can be neonatally transmitted. The 18 serovars of C. trachomatis fall into three groups: the trachoma serovars A, B, Ba, and C; the oculogenital serovars D–K; and the LGV serovars L1–L3. Serovars can be distinguished by serologic typing with monoclonal antibodies or by molecular gene typing. How­ ever, serovar identification usually is not important clinically, since the antibiotic susceptibility pattern is the same for all three groups. The one exception applies when LGV is suspected on clinical grounds; in this situation, serovar determination is important because a longer treatment duration is typically required for LGV strains. BIOLOGY, GROWTH CYCLE, AND PATHOGENESIS ■ ■BIOLOGY During their intracellular growth, chlamydiae produce characteristic intracytoplasmic inclusions that can be visualized by direct fluorescent antibody or Giemsa staining of infected clinical material, such as con­ junctival scrapings or cervical or urethral epithelial cells. Chlamydiae are nonmotile, gram-negative, obligate intracellular bacteria that replicate within the cytoplasm of host cells, forming the characteristic membrane-bound inclusions that are the basis for some diagnostic tests. Originally considered to be large viruses, chlamydiae differ from viruses in possessing RNA and DNA as well as a cell wall that is quite similar in structure to the cell wall of typical gram-negative bacteria. However, chlamydiae lack peptidoglycan; their structural integrity depends on disulfide binding of outer-membrane proteins. PART 5 Infectious Diseases ■ ■GROWTH CYCLE Among the defining characteristics of chlamydiae is a unique growth cycle that involves alternation between two highly specialized morpho­ logic forms (Figs. 194-1 and 194-2): the elementary body, which is the infectious form and is specifically adapted for extracellular survival, and the metabolically active and replicating reticulate body, which is not infectious, is adapted for an intracellular environment, and does not survive well outside the host cell. The biphasic growth cycle begins with attachment of the elementary body (diameter, 0.25–0.35 μm) at specific sites on the surface of the host cell. The elementary body enters the cell through a process similar to receptor-mediated endocytosis and resides in an inclusion, where the entire growth cycle is completed. The chlamydiae prevent phagosome–lysosome fusion. The inclusion membrane is modified by insertion of chlamydial antigens. Once the elementary body has entered the cell, it reorganizes into a reticulate body, which is larger (0.5–1 μm) and contains more RNA. After ~8 h, the reticulate body starts to divide by binary fission. The intracytoplasmic, membrane-bound inclusion body containing the reticulate bodies increases in size as the reticulate bodies multiply. Approximately 18–24 h after infection of the cell, these reticulate bodies begin to become elementary bodies by a reorganization or condensation pro­ cess that is poorly understood. After rupture of the inclusion body, the elementary bodies are released to initiate another cycle of infection. Chlamydiae are susceptible to many broad-spectrum antibiotics and possess a number of enzymes, but they have a very restricted metabolic capacity. None of these metabolic reactions result in the production of energy. Chlamydiae have thus been considered to be energy parasites that use the ATP produced by the host cell for their own metabolic functions. Many aspects of chlamydial molecular biology are not well understood, but the sequencing of several chlamydial genomes and new proteomics research have provided researchers with many relevant tools for elucidating the biology of the life cycle. ■ ■PATHOGENESIS Genital infections are primarily caused by C. trachomatis serovars D–K, with serovars D, E, and F involved most frequently. Molecular

FIGURE 194-1  Chlamydial intracellular inclusions filled with smaller dense elementary bodies and larger reticulate bodies. (Reprinted with permission from WE Stamm: Chlamydial infections, in Harrison’s Principles of Internal Medicine, 17th ed. AS Fauci et al [eds]. New York, McGraw-Hill, 2008, p 1070.) 2. Initial inclusions 3. Fusion of inclusions; appearance of RBs

  1. Uptake of chlamydial EBs Cell membrane Cell cytoplasm Cell nucleus
  2. Multiplication of RBs; enlargement of inclusion
  3. Release of EBs
  4. Conversion of RBs to EBs
  5. Return to normal cycle with IFN-γ removal
  6. Persistence associated with IFN-γ exposure; large aberrant RBs FIGURE 194-2  Chlamydial life cycle. EBs, elementary bodies; IFN-γ, interferon γ; RBs, reticulate bodies. (Reproduced with permission from WE Stamm: Chlamydial infections, in AS Fauci et al [eds]: Harrison’s Principles of Internal Medicine, 17th ed. New York, McGraw-Hill, 2008.)

typing of the major outer-membrane protein gene (omp1) from which serovar differences arise has been used to demonstrate that polymor­ phisms can occur in isolates from patients who are exposed frequently to multiple infections, while less variation is observed in isolates from less sexually active populations. Polymorphisms in the major outermembrane protein may provide antigenic variation, and the different forms allow persistence in the community because immunity to one is not protective against the others. The trachoma biovar is essentially a parasite of squamocolumnar epithelial cells; the LGV biovar is more invasive and involves lymphoid cells. As is typical of chlamydiae, C. trachomatis strains are capable of causing chronic, clinically inapparent, asymptomatic infections. Because the duration of the chlamydial growth cycle is ~48–72 h, the incubation period of sexually transmitted chlamydial infections is relatively long—generally 1–3 weeks. C. trachomatis causes cell death as a result of its replicative cycle and can induce cell damage whenever it persists. However, few toxic effects are demonstrated, and cell death because of chlamydial replication is not sufficient to account for dis­ ease manifestations, the majority of which are due to immunopatho­ logic mechanisms or nonspecific host responses to the organism or its by-products. In recent years, the entire genomes of various chlamydial species have been sequenced, the field of proteomics has become estab­ lished, host innate immunity has been more precisely delineated, and innovative host cell–chlamydial interaction studies have been conducted. As a result, many insights have been gained into how chla­ mydiae adapt and replicate in their intracellular environment and produce disease. These insights into pathogenesis include information on the regulation of gene expression, protein localization, the type III secretion system, the roles of CD4+ and CD8+ T lymphocytes in the host response, and T lymphocyte trafficking. The chlamydial heat-shock protein, which shares antigenic epitopes with similar proteins of other bacteria and with human heat-shock protein, may sensitize the host, and repeated infections may cause host cell damage. Persistent or recurrent chlamydial infections are associ­ ated with fibrosis, scarring, and complications following simple squa­ mocolumnar epithelial infections. A common endpoint of these late consequences is scarring of mucous membranes. Genital complications can lead to pelvic inflammatory disease (PID) and its late consequences of tubal factor infertility, ectopic pregnancy, and chronic pelvic pain, while ocular infections may lead to blinding trachoma. High levels of antibody to human heat-shock protein have been associated with tubal factor infertility and ectopic pregnancy. Without adequate therapy, chlamydial infections may persist for several years, although symptoms— if present—usually abate. Pathogenic mechanisms of C. pneumoniae have yet to be completely elucidated. The same is true for C. psittaci, except that this agent infects cells very efficiently and causes disease that may reflect direct cytopathic effects. C. TRACHOMATIS INFECTIONS ■ ■GENITAL INFECTIONS (SEE ALSO CHAP. 141) Spectrum  Although chlamydiae cause a number of human dis­ eases, localized lower genital tract infections caused by C. trachomatis and the sequelae of such infections are the most important in terms of medical and economic impact. Oculogenital infections due to C. trachomatis serovars D–K are transmitted during sexual contact or from mother to baby during childbirth and are associated with many syndromes, including cervicitis, salpingitis, acute urethral syndrome, endometritis, ectopic pregnancy, infertility, and PID in female patients; urethritis, proctitis, and epididymitis in male patients; and reactive arthritis, conjunctivitis, and pneumonia in infants. Women bear the greatest burden of morbidity because of the serious sequelae of these infections. Untreated infections may lead to PID, and multiple episodes of PID can lead to tubal factor infertility and chronic pelvic pain. Studies estimate that up to 80–90% of women and >50% of men with

C. trachomatis genital infections lack symptoms; other patients have

very mild symptoms. Additionally, extragenital infection in the phar­ ynx and rectum is typically asymptomatic and may persist for years without treatment. Thus, a large reservoir of infected persons contin­ ues to transmit infection to sexual partners.

As their designations reflect, the LGV serovars (L1, L2, and L3) cause LGV, an invasive sexually transmitted infection (STI) characterized by a transient and typically painless genital ulcer followed by acute lymph­ adenitis with bubo formation and/or acute hemorrhagic proctitis (see “Lymphogranuloma Venereum,” below). Epidemiology  •  GLOBAL EPIDEMIOLOGY  C. trachomatis geni­ tal infections are global in distribution. The World Health Organiza­ tion (WHO) estimated that in 2020, 50 million (95% uncertainty interval [UI] 36-67) prevalent cases in men and 77 million [UI 67-90] prevalent cases in women and 59 million [95% UI 34-90] new cases of chlamydia in men and 70[95% UI 44-98] new cases in women. These figures make chlamydial infection the most prevalent bacterial STI in the world. There are large differences by WHO region; in 2020 in the Americas, the male incidence (per 1000) was 48 (95% UI, 23–86), compared with 12 (95% UI, 5–25) in Southeast Asia. In women in the Americas, there were 68 (95% UI, 39–104) incident cases (per 1000) compared with 16 (95% UI, 7–28) in Southeast Asia. The associated morbidity is substantial, and the economic cost is high. U.S. EPIDEMIOLOGY  In the United States, these infections are the most commonly reported of all infectious diseases. Chlamydia is noti­ fiable nationally and is reported to the U.S. Centers for Disease Control and Prevention (CDC). In 2022, 1,649,716 cases were reported to the CDC, representing a 5-year decline of 6.2%. As chlamydial infections are typically asymptomatic, case reports are profoundly influenced by screening coverage. The disruptions in STI-related screening, preven­ tion, and care activities during the COVID-19 pandemic necessitate that the trend for chlamydia surveillance data collected during the pandemic should be interpreted with caution. Higher rates among women than among men reflect the focus on expansion of screening programs for women during the past 25 years. Use of increasingly sen­ sitive diagnostic nucleic acid amplification tests, an increased emphasis on case reporting, and improvements in the information systems used have elevated the number of cases reported every year. The CDC and other professional organizations recommend annual screening of all sexually active women <25 years of age and women aged >25 years who are at increased risk of infection, as well as rescreening of previ­ ously infected individuals at 3 months. Additionally, the CDC recom­ mends that all pregnant women aged <25 years and those >25 years who are at increased risk be routinely screened for chlamydia during the first prenatal visit. Women who remain at risk for infection should be retested in the third trimester to prevent maternal complications and neonatal infection. In pregnancy, a test-of-cure is recommended 4 weeks after treatment. The 2022 U.S. total case count corresponds to 495 cases per 100,000 population. Women have the highest infec­ tion rates (621.2 cases per 100,000) compared to the rate among men (363.7 cases per 100,000). With the increased availability of urine test­ ing and extragenital testing, men—including gay, bisexual, and other men who have sex with men (MSM)—are increasingly being tested for chlamydial infection. From 2021 to 2022, rates of chlamydial infection in men increased by 1.8%, whereas rates in women fell by 1.2% during this period. Chlamydia rates remain highest among adolescents and young adults; in 2022, 57.7% of all cases were reported among persons aged 15–24 years. Chlamydial infection rates vary considerably among different racial and ethnic populations as well as between states. For example, the rate in Louisiana per 100,000 population in 2022 was 788.6, compared with 198.0 in Vermont; the rate among blacks/African Americans was 1113.3 per 100,000 population compared with 100.6 in Asians. CHAPTER 194 Chlamydial Infections The aforementioned statistics are based on case reporting; in 2018, the CDC estimated there were 4 million cases of chlamydia in the United States. Although screening programs have demonstrated reduc­ tions in PID rates, evidence is lacking to support general screening of young sexually active men. In contrast, sexually active MSM should undergo at least annual screening at anatomically exposed sites. Studies

based on screening surveys estimate that the U.S. prevalence of C. tra­ chomatis cervical infection was 5.9% and 8.8% among asymptomatic male and female college students, respectively; between 3.3 and 8.3% among prenatal patients; 7% for women seen in family planning clin­ ics; and >10% for women seen in STI clinics. In a cohort of female college students, incident chlamydial infection was also associated with bacterial vaginosis and high-risk human papillomavirus infection. The prevalence of C. trachomatis in the cervix of pregnant women is approximately six times higher than that of Neisseria gonorrhoeae. The prevalence of genital infection with either agent is highest among women who are between the ages of 20 and 24. Recurrent infections are common in groups with greater vulnerability to STIs and are often acquired from untreated sexual partners. The use of hormonal contraception and the presence of cervical ectopy are also associated with an increased risk. The proportion of infections that are asymp­ tomatic appears to be higher for C. trachomatis (83.9% in men, 87.1% in women) than for N. gonorrhoeae, (41.3% in men, 68.4% in women), and symptomatic C. trachomatis infections are clinically less severe. Mild or asymptomatic C. trachomatis infections of the fallopian tubes nonetheless cause ongoing tubal damage and infertility. The costs of

C. trachomatis infections and their complications to the U.S. health care system have recently been estimated to be >$516.7 million annually.

Clinical Manifestations  •  NONGONOCOCCAL AND POSTGONO­ COCCAL URETHRITIS  C. trachomatis is the most common cause of nongonococcal urethritis (NGU) and postgonococcal urethritis (PGU). The designation PGU refers to NGU developing in men 2–3 weeks after treatment of gonococcal urethritis with single doses of agents such as penicillin or cephalosporins, which lack antimicrobial activity against chlamydiae. Current treatment regimens for gonorrhea have evolved and now include combination therapy with ceftriaxone and doxycycline unless chlamydia has been excluded; if chlamydia has been excluded, then gonorrhea is treated with ceftriaxone mono­ therapy. Thus, both the incidence of PGU and the causative role of C. trachomatis in this syndrome have declined. PART 5 Infectious Diseases In the United States, most of the estimated 2 million cases of acute urethritis are NGU, and C. trachomatis is implicated in 30–50% of these cases. The cause of most of the remaining cases of NGU is due to Myco­ plasma genitalium; Trichomonas vaginalis and herpes simplex virus (HSV) cause some cases. Other etiologies include Haemophilus species, N. meningitidis, and adenovirus. The rate of involvement of C. tracho­ matis in urethral infection ranges from 3–7% among asymptomatic men to 15–20% among symptomatic men attending STI clinics. One multisite study of men in Baltimore, Seattle, Denver, and San Francisco reported an overall chlamydial prevalence of 7% in urine samples assessed by nucleic acid amplification tests (NAATs)—molecular tests that amplify the nucleic acids in clinical specimens. As in women, infection in men is age related, with young age most strongly associ­ ated with chlamydial urethritis. The prevalence among men is highest at 20–24 years of age. In STI clinics, urethritis is usually less prevalent among MSM than among heterosexual men. NGU is diagnosed by documentation of leukocytes on urethral exudate and by exclusion of gonorrhea by Gram’s staining, NAAT, or culture. C. trachomatis urethritis is generally less severe than gonococ­ cal urethritis, although in any individual patient, these two forms of urethritis cannot reliably be differentiated solely on clinical grounds. Symptoms include urethral discharge (often whitish and mucoid rather than frankly purulent), dysuria, and urethral itching. Physical examination may reveal meatal erythema and tenderness as well as a urethral exudate that is often demonstrable only by stripping or milk­ ing of the urethra. At least one-third of male patients with C. trachomatis urethral infection have no evident signs or symptoms of urethritis. The avail­ ability of NAATs for first-void urine specimens has facilitated broaderbased testing for asymptomatic infection in male patients. As a result, asymptomatic chlamydial infection has been demonstrated in 5–10% of sexually active male adolescents screened at school-based clinics or community centers. Such patients generally have pyuria (≥15 leuko­ cytes per 400× microscopic field in the sediment of first-void urine), a

positive leukocyte esterase test, or an increased number of leukocytes on a Gram-stained smear prepared from a urogenital swab inserted 1–2 cm into the anterior urethra. When specific diagnostic tests for chlamydiae are not available, the examination of an endourethral speci­ men for increased leukocytes is useful in differentiating between true urethritis and functional symptoms in symptomatic patients. Alterna­ tively, urethritis can be assayed noninvasively by examination of a firstvoid urine sample for pyuria, either by microscopy or by the leukocyte esterase test. Urine (or a urethral swab) can also be tested directly for chlamydiae by DNA amplification methods (NAATs), as described below (see “Detection Methods”). Urine testing for urethral STIs in men is much more acceptable than endourethral swab collection. EPIDIDYMITIS  Chlamydial urethritis may be followed by acute epi­ didymitis (<6 weeks’ duration), but this condition is rare, generally occurring in sexually active patients <35 years of age; in older men, epididymitis is usually associated with gram-negative bacterial infec­ tion and/or instrumentation procedures. An estimated 50–70% of cases of acute epididymitis are caused by C. trachomatis. The condition usu­ ally presents as unilateral scrotal pain with tenderness, swelling, and fever in a young man, often occurring in association with chlamydial urethritis. The illness may be mild enough to treat with oral antibiot­ ics on an outpatient basis or severe enough to require hospitalization and parenteral therapy. Testicular torsion should be excluded promptly by imaging or surgical exploration in a teenager or young adult who presents with acute unilateral testicular pain without urethritis. The possibility of testicular tumor or chronic infection (>6 weeks’ duration) (e.g., tuberculosis) should be excluded when a patient with unilateral intrascrotal pain and swelling does not respond to appropriate antimi­ crobial therapy. REACTIVE ARTHRITIS  Reactive arthritis consists of conjunctivitis, urethritis (or, in female patients, cervicitis), arthritis, and characteristic mucocutaneous lesions. It may develop in 1–2% of cases of NGU; the rate of chlamydia-induced arthritis has been estimated to be 4–15% and is thought to be the most common type of peripheral inflamma­ tory arthritis in young men. When reactive arthritis is triggered by an STI, it may also be referred to as sexually acquired reactive arthri­ tis (SARA). C. trachomatis has been recovered from the urethra of 16–44% of patients with reactive arthritis and 69% of men with signs of urogenital inflammation at the time of examination. Antibodies to C. trachomatis have also been detected in 46–67% of patients with reactive arthritis, and Chlamydia-specific cell-mediated immunity has been documented in 72%. In addition, C. trachomatis has been isolated from synovial biopsy samples from 15 of 29 patients in a number of small series and from a smaller proportion of synovial fluid specimens. Chlamydial nucleic acids have been identified in synovial membranes and chlamydial elementary bodies in joint fluid. The pathogenesis of reactive arthritis is unclear, but this condition probably represents an abnormal host response to a number of infectious agents, including those associated with bacterial gastroenteritis (e.g., Salmonella, Shigella, Yersinia, or Campylobacter), or to infection with C. trachomatis or N. gonorrhoeae. An association between reactive arthritis and the HLA-B27 phenotype in white individuals has been described, but not in patients from sub-Saharan Africa, where the prevalence of HLAB27 is much lower. Since other mucosal infections produce an identi­ cal syndrome, chlamydial infection is thought to initiate an aberrant hyperreactive immune response that produces inflammation of the involved target organs in these genetically predisposed individuals. Evidence of exaggerated cell-mediated and humoral immune responses to chlamydial antigens in reactive arthritis supports this hypothesis. The finding of chlamydial elementary bodies and DNA in joint fluid and synovial tissue from patients with reactive arthritis suggests that chlamydiae may actually spread from genital to joint tissues in these patients—perhaps in macrophages. NGU is the initial manifestation of reactive arthritis in 80% of patients, typically occurring within 14 days after sexual exposure. The urethritis may be mild and may even go unnoticed by the patient. Similarly, gonococcal urethritis may precede reactive arthritis, but coinfection with an agent of NGU is difficult to rule out. The urethral

discharge may be purulent or mucopurulent, and patients may or may not report dysuria. Accompanying prostatitis, usually asymptomatic, has been described. Arthritis usually begins ~4 weeks after the onset of urethritis but may develop sooner or, in a small percentage of cases, may actually precede urethritis. The knees are most frequently involved; next most commonly affected are the ankles and small joints of the feet. Sacroiliitis, either symmetrical or asymmetrical, is docu­ mented in two-thirds of patients. Mild bilateral conjunctivitis, iritis, keratitis, or uveitis is sometimes present but lasts for only a few days. Finally, dermatologic manifestations occur in up to 50% of patients. The initial lesions—usually papules with a central yellow spot—most often involve the soles and palms and, in ~25% of patients, eventually epithelialize and thicken to produce keratoderma blenorrhagicum. Circinate balanitis is usually painless and occurs in fewer than half of patients. The initial episode of reactive arthritis usually lasts 2–6 months. PROCTITIS  Primary anal or rectal infections with C. trachomatis have been described in women and MSM who practice receptive anal intercourse. Symptoms are characterized by anorectal pain, a bloody mucopurulent discharge, and tenesmus. Oculogenital serovars D–K and LGV serovars L1, L2, and L3 have been found to cause proctitis. The LGV serovars are far more invasive and typically cause more severely symptomatic disease, including severe ulcerative proctocolitis that can be clinically confused with HSV proctitis. However, asymptomatic or paucisymptomatic LGV infection is increasingly recognized in MSM. Histologically, LGV proctitis may resemble Crohn’s disease in that giant cell formation and granulomas are detected. In the United States and Europe, cases of LGV proctitis occur almost exclusively in MSM, many of whom have HIV infection and other STI coinfections. The less invasive non-LGV serovars of C. trachomatis cause mild proctitis. Many infected individuals are asymptomatic, and in these cases, infection is diagnosed only by routine NAAT of rectal swabs. The number of fecal leukocytes is usually abnormal in both asymptomatic and symptomatic cases. Sigmoidoscopy may yield normal findings or may reveal mild inflammatory changes or small erosions or follicles in the lower 10 cm of the rectum. Histologic examination of rectal biopsies generally shows anal crypts and prominent follicles as well as neutrophilic infiltration of the lamina propria. Chlamydial proctitis is best diagnosed by isolation of C. trachomatis from the rectum and documentation of a response to appropriate therapy. NAATs are the diagnostic test of choice. MUCOPURULENT CERVICITIS  Although most women with chla­ mydial infections of the cervix have no symptoms, almost half gener­ ally have local signs of infection on examination. Cervicitis is usually characterized by the presence of a mucopurulent discharge, with >20 neutrophils per microscopic field visible in strands of cervical mucus in a thinly smeared, Gram-stained preparation of endocervical exudate. Hypertrophic ectopy of the cervix may also be evident as an edematous area near the cervical os that is congested and bleeds easily on minor trauma (e.g., when a specimen is collected with a swab). A Papanico­ laou smear shows increased numbers of neutrophils as well as a char­ acteristic pattern of mononuclear inflammatory cells, including plasma cells, transformed lymphocytes, and histiocytes. Cervical biopsy shows a predominantly mononuclear cell infiltrate of the subepithelial stroma. Clinical experience and collaborative studies indicate that a cutoff of >30 polymorphonuclear leukocytes (PMNs)/1000× field in a Gram-stained smear of cervical mucus correlates best with chlamydial or gonococcal cervicitis. However, the wide availability of NAATs and limited access to microscopy outside of specialist centers has decreased the role of cervical mucus examination. Clinical recognition of chlamydial cervicitis depends on a high index of suspicion and careful cervical examination. No genital symp­ toms are specifically correlated with chlamydial cervical infection. The differential diagnosis of a mucopurulent discharge from the endocer­ vical canal in a young, sexually active woman includes gonococcal endocervicitis, salpingitis, endometritis, and intrauterine contraceptive device–induced inflammation. Diagnosis of cervicitis is based on the presence of PMNs on a cervical swab as noted above; the presence of chlamydiae is confirmed by NAAT.

PELVIC INFLAMMATORY DISEASE  Inflammation of sections of the fal­ lopian tube is often referred to as salpingitis or PID. The proportion of acute salpingitis cases caused by C. trachomatis varies geographically and with the population studied. It has been estimated that C. tracho­ matis causes up to 50% of PID cases in the United States. Recent studies report that the proportions of PID cases attributable to N. gonorrhoeae or C. trachomatis is decreasing; approximately 50% of women diag­ nosed with acute PID tested positive for either organism. PID occurs via ascending intraluminal spread of C. trachomatis or N. gonorrhoeae from the lower genital tract. Mucopurulent cervicitis is often followed by endometritis, endosalpingitis, and finally pelvic peritonitis. Evidence of mucopurulent cervicitis is often found in women with laparoscopi­ cally verified salpingitis. Similarly, endometritis, demonstrated by an endometrial biopsy showing plasma cell infiltration of the endometrial epithelium, is documented in most women with laparoscopy-verified chlamydial (or gonococcal) salpingitis. Chlamydial endometritis can also occur in the absence of clinical evidence of salpingitis. Histologic evidence of endometritis has been correlated with a syndrome consist­ ing of vaginal bleeding, lower abdominal pain, and uterine tenderness in the absence of adnexal tenderness. Chlamydial salpingitis produces milder symptoms than gonococcal salpingitis and may be associated with less marked adnexal tenderness. Thus, mild adnexal or uterine tenderness in a sexually active woman with cervicitis may suggest chlamydial PID.

Screening and treating sexually active women for chlamydia and gonorrhea reduce their risk for PID. Chronic untreated endometrial and tubal inflammation can result in tubal scarring, impaired tubal function, tubal occlusion, and infertility, even among women who report no prior treatment for chlamydial infection. C. trachomatis has been particularly implicated in “subclinical” PID on the basis of a lack of history of PID among Chlamydia-seropositive women with tubal damage and detection of chlamydial DNA or antigen among asymp­ tomatic women with tubal infertility. These data suggest that the best method to prevent PID and its sequelae is surveillance and control of lower genital tract infections along with diagnosis and treatment of sex partners and prevention of reinfections. Promotion of early symptom recognition and health care presentation may reduce the frequency and severity of sequelae of PID. CHAPTER 194 Chlamydial Infections PERIHEPATITIS  Fitz-Hugh–Curtis syndrome was originally described as a complication of gonococcal PID. However, studies over the past several decades have suggested that chlamydial infection is more com­ monly associated with perihepatitis than is N. gonorrhoeae. Perihepati­ tis should be suspected in young, sexually active women who develop right-upper-quadrant pain, fever, or nausea. Evidence of salpingitis may or may not be found on examination. Frequently, perihepatitis is strongly associated with extensive tubal scarring, adhesions, and inflammation observed at laparoscopy, and high titers of antibody to the 57-kDa chlamydial heat-shock protein have been documented. Culture and/or serologic evidence of C. trachomatis has been found in three-fourths of women with this syndrome. URETHRAL SYNDROME IN WOMEN  In the absence of infection with uropathogens such as coliforms or Staphylococcus saprophyticus, C. tra­ chomatis is the pathogen most commonly isolated from college women with dysuria, frequency, and pyuria. Screening studies can recover C. trachomatis at both the cervix and the urethra; in up to 25% of infected women, the organism is isolated only from the urethra. The urethral syndrome in women consists of dysuria and frequency in conjunction with chlamydial urethritis, pyuria, and no bacteriuria or urinary patho­ gens. Although symptoms of the urethral syndrome may develop in some women with chlamydial infection, the majority of women attend­ ing STI clinics for urethral chlamydial infection do not have dysuria or frequency. Even in women with chlamydial urethritis causing the acute urethral syndrome, signs of urethritis such as urethral discharge, meatal redness, and swelling are uncommon. However, mucopurulent cervicitis in a woman presenting with dysuria and frequency strongly suggests C. trachomatis urethritis. Other correlates of chlamydial urethral syndrome include a duration of dysuria of >7–10 days, lack of hematuria, and lack of suprapubic tenderness. Abnormal urethral

Gram’s stains showing >10 PMNs/1000× field in women with dysuria but without coliform bacteriuria support the diagnosis of chlamydial urethritis. Other possible diagnoses include gonococcal or trichomonal infection of the urethra.

INFECTION IN PREGNANCY AND THE NEONATAL PERIOD  Infections during pregnancy can be transmitted to infants during delivery. Approximately 20–30% of infants exposed to C. trachomatis in the birth canal develop conjunctivitis, and 10–15% subsequently develop pneumonia. All newborn infants receive ocular prophylaxis at birth to prevent ophthalmia neonatorum. The agents used to prevent gonococ­ cal eye infection are not effective against chlamydial conjunctivitis. The most effective measure to prevent neonatal chlamydial conjunc­ tivitis is through screening and treatment of chlamydial infections in pregnant women. Without treatment, conjunctivitis usually develops at 5–19 days of life and often results in a profuse mucopurulent dis­ charge. Roughly half of infected infants develop clinical evidence of inclusion conjunctivitis. However, it is impossible to differentiate chlamydial conjunctivitis from other forms of neonatal conjunctivitis (e.g., that due to N. gonorrhoeae, Haemophilus influenzae, Streptococcus pneumoniae, or HSV) on clinical grounds; thus, laboratory diagnosis is required. Inclusions within epithelial cells are often detected in Giemsa-stained conjunctival smears, but these smears are considerably less sensitive than cultures or NAATs for chlamydiae. Gram-stained smears may show gonococci or occasional small gram-negative coc­ cobacilli in Haemophilus conjunctivitis, but smears should be accom­ panied by cultures or NAATs for these agents. C. trachomatis has also been isolated frequently and persistently from the nasopharynx, rectum, and vagina of infected infants—occa­ sionally for >1 year in the absence of treatment. In some cases, otitis media results from perinatally acquired chlamydial infection. Pneumo­ nia may develop in infants from 2 weeks to 4 months of age. C. tracho­ matis is estimated to cause 20–30% of pneumonia cases in infants <6 months of age. Epidemiologic studies have linked chlamydial pul­ monary infection in infants with increased occurrence of subacute lung disease (bronchitis, asthma, wheezing) in later childhood. PART 5 Infectious Diseases LYMPHOGRANULOMA VENEREUM  C. trachomatis serovars L1, L2, and L3 cause LGV, an invasive systemic STI. The peak incidence of LGV corresponds with the age of greatest sexual activity: the second and third decades of life. The worldwide incidence of LGV is falling, but the disease is still endemic and a major cause of morbidity in parts of Asia, Africa, South America, and the Caribbean. LGV is rare in indus­ trialized countries; for more than a decade, the reported incidence in the United States has been only 0.1 case per 100,000 population. In the Bahamas, an apparent outbreak of LGV was described in association with a concurrent increase in heterosexual infection with HIV. Reports of outbreaks with the newly identified variant L2b in Europe, Australia, and the United States indicate that LGV is becoming more prevalent among MSM. These cases have usually presented as hemorrhagic proc­ tocolitis in HIV-positive men. However, LGV is increasingly described as an asymptomatic rectal infection in 30–50% of cases and is fre­ quently seen in MSM without HIV. More widespread use of NAATs for identification of rectal infections may have enhanced case recognition. LGV begins as a small painless papule that tends to ulcerate at the site of inoculation, often escaping attention. This primary lesion heals in a few days without scarring and is usually recognized as LGV only in retrospect. LGV strains of C. trachomatis have occasionally been recov­ ered from genital ulcers and from the urethra of men and the endo­ cervix of women who present with inguinal adenopathy; these areas may be the primary sites of infection in some cases. Proctitis is more common among people who practice receptive anal intercourse, and an elevated white blood cell count in anorectal smears may predict LGV in these patients. Ulcer formation may facilitate transmission of HIV infection and other sexually transmitted and blood-borne infections. As NAATs for C. trachomatis are standard of care in high-income settings, increasing numbers of cases of LGV proctitis are being rec­ ognized in MSM. Such patients present with anorectal pain and muco­ purulent, bloody rectal discharge. Sigmoidoscopy reveals ulcerative proctitis or proctocolitis, with purulent exudate and mucosal bleeding.

Histopathologic findings in the rectal mucosa include granulomas with giant cells, crypt abscesses, and extensive inflammation. These clinical, sigmoidoscopic, and histopathologic findings may closely resemble those of Crohn’s disease of the rectum. The most common presenting picture in heterosexual men and women is the inguinal syndrome, which is characterized by pain­ ful inguinal lymphadenopathy beginning 2–6 weeks after presumed exposure; in rare instances, the onset comes after a few months. The inguinal adenopathy is unilateral in two-thirds of cases, and palpable enlargement of the iliac and femoral nodes is often evident on the same side as the enlarged inguinal nodes. The nodes are initially dis­ crete, but progressive periadenitis results in a matted mass of nodes that becomes fluctuant and suppurative. The overlying skin becomes fixed, inflamed, and thin, and multiple draining fistulas finally develop. Extensive enlargement of chains of inguinal nodes above and below the inguinal ligament (“the groove sign”) is not specific and, although not uncommon, is documented in only a minority of cases. Spontane­ ous healing usually takes place after several months; inguinal scars or granulomatous masses of various sizes persist for life. Massive pelvic lymphadenopathy may lead to exploratory laparotomy. Constitutional symptoms are common during the stage of regional lymphadenopathy and, in cases of proctitis, may include fever, chills, headache, meningismus, anorexia, myalgias, and arthralgias. Other systemic complications are infrequent but include arthritis with ster­ ile effusion, aseptic meningitis, meningoencephalitis, conjunctivitis, hepatitis, and erythema nodosum (Fig. A1-39). Complications of untreated anorectal infection include perirectal abscess; anal fistulas; and rectovaginal, rectovesical, and ischiorectal fistulas. Secondary bacterial infection probably contributes to these complications. Rectal stricture is a late complication of anorectal infection and usually devel­ ops 2–6 cm from the anal orifice—i.e., at a site within reach on digital rectal examination. A small percentage of cases of LGV in men present as chronic progressive infiltrative, ulcerative, or fistular lesions of the penis, urethra, or scrotum. Associated lymphatic obstruction may pro­ duce elephantiasis. When urethral stricture occurs, it usually involves the posterior urethra and causes incontinence or difficulty with urina­ tion. In women, esthiomene of the genitalia is a late, rare complication of untreated LGV infection resulting in massive labial elephantiasis and ulceration followed by scarring. Diagnosis  •  DETECTION METHODS  Historically, chlamydiae were cultivated in the yolk sac of embryonated eggs. The organisms can be grown more easily in tissue culture, but cell culture—once con­ sidered the diagnostic gold standard—has been replaced by nonculture assays (Table 194-1). In general, culture for chlamydiae in clinical specimens is now performed only in specialized laboratories. The first nonculture assays, such as direct fluorescent antibody staining of clinical material and enzyme immunoassay (EIA), have been replaced by NAATs, which are currently recommended by the CDC as the diagnostic assays of choice. At present, numerous NAAT assays cleared by the U.S. Food and Drug Administration (FDA) are commercially available, some of which are available as high-throughput robotic plat­ forms. Three point-of-care rapid diagnostic assays are available; they are of increasing interest since patients can potentially be treated before leaving the clinic, thus preventing forward transmission while patients wait for results from tests with longer turnaround times. However, turnaround time and cost of goods currently limit their widespread adoption. Over-the-counter tests are also in development. CHOICE OF SPECIMEN  Cervical and urethral swabs have traditionally been used for the diagnosis of STIs in female and male patients, respec­ tively. However, given the greatly increased sensitivity and specificity of NAATs, less invasive samples (e.g., urine for both sexes and vaginal swabs for women) can be used. For screening of women, the CDC now recommends that self-collected or clinician-collected vaginal swabs, which are slightly more sensitive than urine, be used; a recent analysis demonstrated the superiority of vaginal swabs over urine in women. Urine screening tests are often used in outreach screening programs, however. For symptomatic women undergoing a pelvic examination, cervical swab samples may be used because they have slightly higher

TABLE 194-1  Diagnostic Tests for Sexually Transmitted and Perinatal Chlamydia trachomatis Infection INFECTION SUGGESTIVE SIGNS/SYMPTOMS PRESUMPTIVE DIAGNOSISa CONFIRMATORY TEST OF CHOICE Men NGU, PGU Discharge, dysuria Gram’s stain with ≥2 WBCs/high power field (HPF) in highprevalence settings (e.g., STI clinics) or ≥5 WBCs/HPF in lower-prevalence settings. No gram-negative intracellular diplococci (GNID). In the absence of microscopy: positive leukocyte esterase test on first catch urine Epididymitis Unilateral intrascrotal swelling, pain, tenderness; fever; NGU Gram’s stain with ≥2 (or ≥5) WBCs/HPF; no GNID; urinalysis with pyuria Women Cervicitis Mucopurulent cervical discharge, sustained endocervical bleeding easily induced by nontraumatic passage of a swab through the cervical os Leukorrhea, defined as >10 WBCs/HPF on microscopic examination of vaginal fluid, might be a sensitive indicator of cervical inflammation with a high negative predictive value Salpingitis Lower abdominal pain, cervical motion tenderness, adnexal tenderness or masses C. trachomatis always potentially present in salpingitis Vaginal (or cervical) NAAT for C. trachomatis Urethritis Dysuria and frequency without hematuria MPC; sterile pyuria; negative routine urine culture Urine NAAT for C. trachomatis Adults of Either Sex Proctitis Rectal pain, discharge, tenesmus, bleeding; history of receptive anorectal intercourse Negative gonococcal NAAT and Gram’s stain; at least 1 WBC/HPF in rectal Gram’s stain Reactive arthritis NGU, arthritis, conjunctivitis, typical skin lesions Gram’s stain with ≥5 WBC/HPF; lack of GNID Urine or vaginal NAAT for C. trachomatis LGV Regional adenopathy, primary lesion, proctitis, systemic symptoms None NAAT for C. trachomatis. Molecular testing for LGV is not widely available or not FDA cleared. If available, molecular PCR testing for C. trachomatis serovars L1, L2, or L3 can confirm diagnosis LGV Neonates Conjunctivitis Purulent conjunctival discharge 5–12 days after birth Negative culture and Gram’s stain for gonococci, Haemophilus spp., pneumococci, staphylococci Infant pneumonia Subacute, afebrile pneumonia in infants aged 1–3 months None Chlamydial tissue culture, DFA, or NAAT (not FDA cleared) of nasopharyngeal specimen aA presumptive diagnosis of chlamydial infection is often made in the syndromes listed when gonococci are not found. A positive test for Neisseria gonorrhoeae does not exclude the involvement of C. trachomatis, which often is present in patients with gonorrhea. Abbreviations: DFA, direct fluorescent antibody; FDA, U.S. Food and Drug Administration; HPF, high-power field; LGV, lymphogranuloma venereum; MPC, mucopurulent cervicitis; NAAT, nucleic acid amplification test; NGU, nongonococcal urethritis; PGU, postgonococcal urethritis; WBC, white blood cell. Source: Based on Centers for Disease Control and Prevention Sexually Transmitted Infections Treatment Guidelines, 2021. https://www.cdc.gov/std/treatment-guidelines/ STI-Guidelines-2021.pdf. chlamydial counts. For male patients, a first-catch urine specimen is the sample of choice, but self-collected penile-meatal swabs have been shown to be very effective. ALTERNATIVE SPECIMEN TYPES  Ocular samples from babies and adults can be assessed by NAATs. However, NAATs are not cleared by the FDA for detecting chlamydia from conjunctival swabs, and clinical laboratories should verify the procedure according to Clini­ cal Laboratory Improvement Amendments regulations. Samples from extragenital rectal and pharyngeal sites are used to detect chlamydiae by NAATs; in 2019, the CDC cleared the first NAATS for use on extra­ genital samples. OTHER DIAGNOSTIC ISSUES  Because NAATs detect nucleic acids instead of live organisms, they should be used with caution as testof-cure assays. Residual nucleic acid from cells rendered noninfective by antibiotics may continue to yield a positive result in NAATs for as long as 3 weeks after therapy when viable organisms have actually been eradicated. Therefore, clinicians should not use NAATs for test of cure until after 3 weeks. The CDC currently does not recommend a test of cure after treatment for infection with C. trachomatis except in pregnancy. However, because incidence studies have demonstrated that previous chlamydial infection increases the probability of becoming

Urine NAAT for C. trachomatis Urine NAAT for C. trachomatis Vaginal (or cervical) NAAT for C. trachomatis Rectal NAAT for C. trachomatis CHAPTER 194 Chlamydial Infections Tissue culture, conjunctival NAAT for C. trachomatis (not FDA cleared); DFA-stained scraping of conjunctival material reinfected, the CDC does recommend that previously infected indi­ viduals be rescreened 3 months after treatment. SEROLOGY  Serologic testing may be helpful in the diagnosis of LGV and neonatal pneumonia caused by C. trachomatis. The serologic test of choice is the microimmunofluorescence (MIF) test, in which high-titer purified elementary bodies mixed with embryonated chicken yolk sac material are affixed to a glass microscope slide to which dilutions of sera are applied. After incubation and washing, fluorescein-conjugated IgG or IgM antibody is applied. The test is read with an epifluorescence microscope, with the highest dilution of serum producing visible fluorescence designated as the titer. The MIF test is not widely available except in research laboratories and is highly labor intensive. Although the complement fixation (CF) test can also be used, it employs lipopolysaccharide (LPS) as the antigen and therefore identifies the pathogen only to the genus level. Single-point titers of >1:64 support a diagnosis of LGV, for which it is difficult to demonstrate rising antibody titers—i.e., paired serum samples are difficult to obtain since the disease often results in the patient’s being seen by the physician after the acute stage. Any anti­ body titer of >1:16 is considered significant evidence of exposure to chlamydiae. However, serologic testing is never recommended for

diagnosis of uncomplicated genital infections of the cervix, urethra, and lower genital tract or for C. trachomatis screening of asymptom­ atic individuals.

TREATMENT C. trachomatis Genital Infections A 7-day course of oral doxycycline (100 mg twice daily) is the recommended regimen of treatment for uncomplicated chlamydial infections. A single 1-g oral dose of azithromycin or oral levofloxa­ cin 500 mg once daily for 7 days are alternatives. Doxycycline has slightly better efficacy than azithromycin in treatment of genital chlamydia infection and has demonstrated significantly higher microbiological cure rates in the treatment of rectal infection. The single-dose regimen of azithromycin has great appeal for the treatment of patients with uncomplicated chlamydial infection (especially those without symptoms and those with a likelihood of adherence challenges) and of the sexual partners of infected patients. These advantages must be weighed against the lower efficacy and greater cost of azithromycin. Whenever possible, the single 1-g dose should be given as directly observed therapy. Although not approved by the FDA for use in pregnancy, this regi­ men appears to be safe and effective for this purpose. Amoxicillin (500 mg three times daily for 7 days) can also be given as an alterna­ tive to pregnant women. The fluoroquinolones are contraindicated in pregnancy. A 2-week course of treatment is recommended for complicated chlamydial infections (e.g., PID, epididymitis). Tradi­ tionally, a 3-week course of doxycycline (100 mg orally twice daily), azithromycin 1 g orally once weekly for 3 weeks, or erythromycin base (500 mg orally four times daily) is recommended treatment for both bubonic and anogenital LGV. However, several studies have demonstrated the efficacy of shorter-course treatment for LGV. Failure of treatment with a tetracycline in genital infections usually indicates limited adherence or reinfection rather than involvement of a drug-resistant strain. To date, clinically significant drug resis­ tance has not been observed in C. trachomatis. PART 5 Infectious Diseases Treatment or testing for chlamydiae should be considered among patients with N. gonorrhoeae because of the frequency of coinfection. Systemic treatment with erythromycin base or ethylsuc­ cinate has been recommended for ophthalmia neonatorum and for C. trachomatis pneumonia in infants. Data on azithromycin efficacy are limited, but it may be effective for use in neonates. For the treatment of adult inclusion conjunctivitis, a single 1-g dose of azithromycin was as effective as standard 10-day treatment with doxycycline. SEX PARTNERS The continued high prevalence of chlamydial infections in most parts of the United States is due primarily to the failure to diag­ nose—and therefore treat—patients with symptomatic or asymp­ tomatic infection and their sex partners. Urethral or cervical infection with C. trachomatis has been well documented in a high proportion of the sex partners of patients with NGU, epididymitis, reactive arthritis, salpingitis, and endocervicitis. If possible, con­ firmatory laboratory tests for chlamydiae should be undertaken in these individuals, but even persons without positive tests or evi­ dence of clinical disease who have recently been exposed to proven or possible chlamydial infection (e.g., NGU) should be offered therapy. A novel approach is partner-delivered therapy, in which index patients receive treatment and are also provided with treat­ ment to give to their sex partner(s). NEONATES AND INFANTS In neonates with conjunctivitis or infants with pneumonia, eryth­ romycin ethylsuccinate or base can be given orally at a dosage of

50 mg/kg per day, preferably in four divided doses, for 2 weeks. Careful attention must be given to adherence to therapy—a fre­ quent problem. Relapses of eye infection may follow oral erythro­ mycin therapy. Thus, careful follow-up is required after treatment;

the efficacy of erythromycin treatment for ophthalmia neonatorum is ~80%; therefore, a second course of therapy might be required. Both parents should be examined for C. trachomatis infection and, if diagnostic testing is not readily available, should be treated with doxycycline or azithromycin. Prevention  Since many chlamydial infections are asymptomatic, effective control and prevention must involve periodic screening of individuals at risk. Selective cost-effective screening criteria have been developed. Among women, young age (generally <25 years) is a critical risk factor for chlamydial infections in nearly all studies. Other risk factors include mucopurulent cervicitis; multiple, new, or symptomatic male sex partners; and lack of barrier contraceptive use. In some settings, screening based on young age may be as sensitive as criteria that incorporate behavioral and clinical measures. Another strategy is universal testing of all patients in high-prevalence clinic populations (e.g., STI clinics, juvenile detention facilities, and family planning clinics). The effectiveness of selective screening in reducing the prevalence of chlamydial infection among women has been demonstrated in several studies. In the Pacific Northwest, where extensive screening began in family planning clinics in 1998 and in STI clinics in 1993, the preva­ lence declined from 10% in the 1980s to <5% in 2000. Similar trends have occurred in association with screening programs elsewhere. In addition, screening can reduce upper genital tract disease. In Seattle, women at a large health maintenance organization who were screened for chlamydial infection on a routine basis had a lower incidence of symptomatic PID than did women who received standard care and underwent more selective screening. In settings with low to moderate prevalence, the prevalence at which selective screening becomes more cost-effective than universal screening must be defined. Most studies have concluded that univer­ sal screening is preferable in settings with a chlamydial prevalence of

3–7%. Depending on the criteria used, selective screening is likely to be more cost-effective when prevalence falls below 3%. The avail­ ability of highly sensitive and specific diagnostic NAATs using urine specimens and self-obtained vaginal swabs makes it feasible to mount an effective nationwide Chlamydia control program, with screening of individuals with increased vulnerability for STI acquisition in tradi­ tional health care settings and in novel outreach and community-based settings. The U.S. Preventive Services Task Force has named Chlamydia screening as a Grade B recommendation, which means that private insurance and Medicare will cover the cost of screening under the Affordable Care Act. More recently, home-based, self-collected, mailin testing platforms have expanded the testing for chlamydia and other STIs outside of clinical settings. Since 2018, data have emerged on the efficacy of doxycycline postexposure prophylaxis. Efficacy data exist for MSM and transgender women, where a 200-mg single oral dose of doxycycline taken within 24–72 h of condomless sex reduced the risk of chlamydia acquisition by ~70–80%. ■ ■TRACHOMA Epidemiology  Trachoma—a sequela of ocular disease predomi­ nantly in resource-limited areas—continues to be a leading cause of preventable infectious blindness worldwide. The WHO estimates that ~6 million people have been blinded by trachoma and that ~1.3 million people in developing countries still suffer from preventable blindness due to trachoma; certainly, hundreds of millions live in trachomaendemic areas. Foci of trachoma persist in Africa, Asia, Latin America, the Middle East, and the Pacific Rim. C. trachomatis serovars A, B, Ba, and C are isolated from patients with clinical trachoma in areas of endemicity in countries in Africa, the Middle East, Asia, and South America. The trachoma-hyperendemic areas of the world are in northern and sub-Saharan Africa, the Middle East, drier regions of the Indian sub­ continent, and Southeast Asia. In hyperendemic areas, the prevalence of trachoma is essentially 100% by the second or third year of life. Active disease is most common among young children, who are the

reservoir for trachoma. By adulthood, active infection is infrequent, but sequelae result in blindness. In such areas, trachoma constitutes the major cause of blindness. Trachoma is transmitted through contact with discharges from the eyes of infected patients. Transmission is most common under poor hygienic conditions and most often takes place between fam­ ily members or between families with shared facilities. Eye-seeking flies (e.g., Musca sorbens) can also transfer the mucopurulent ocular discharges, carrying the organisms on their legs from one person to another. The International Trachoma Initiative founded by the WHO in 1998 aimed to eliminate blinding trachoma globally by 2020. The Neglected Tropical Disease road map 2021–2030 has set 2030 as the new target year for global elimination of trachoma as a public health problem. Clinical Manifestations  The clinical manifestations of trachoma include two phases: active trachoma (conjunctivitis) and cicatricial disease (conjunctival scarring). Both endemic trachoma and adult inclusion conjunctivitis present initially as conjunctivitis characterized by small lymphoid follicles in the conjunctiva. In regions with hyperen­ demic classic blinding trachoma, the disease usually starts insidiously before the age of 2 years. Reinfection is common and probably contrib­ utes to the pathogenesis of trachoma. Studies using polymerase chain reaction (PCR) or other NAATs indicate that chlamydial DNA is often present in the ocular secretions of patients with trachoma, even in the absence of positive cultures. Thus, persistent infection may be more common than was previously thought. The cornea becomes involved, with inflammatory leukocytic infil­ trations and superficial vascularization (pannus formation). As the inflammation continues, conjunctival scarring eventually distorts the eyelids, causing them to turn inward so that the lashes constantly abrade the eyeball (trichiasis and entropion); eventually the corneal epithelium is abraded and may ulcerate, with subsequent corneal scarring, opacification, and blindness. Destruction of the conjunctival goblet cells, lacrimal ducts, and lacrimal gland may produce a “dry-eye” syndrome, with resultant corneal opacity due to drying (xerosis) or secondary bacterial corneal ulcers. Communities with blinding trachoma often experience seasonal epidemics of conjunctivitis due to H. influenzae that contribute to the intensity of the inflammatory process. In such areas, the active infec­ tious process usually resolves spontaneously in affected persons at 10–15 years of age, but conjunctival scars continue to shrink, produc­ ing trichiasis and entropion with subsequent corneal scarring in adults. In areas with milder and less prevalent disease, the process may be much slower, with active disease continuing into adulthood; blindness is rare in these cases. Eye infection with oculogenital C. trachomatis strains in sexually active young adults presents as an acute onset of unilateral follicular conjunctivitis and preauricular lymphadenopathy similar to that seen in acute conjunctivitis caused by adenovirus or HSV. If untreated, the disease may persist for 6 weeks to 2 years. It is frequently associated with corneal inflammation in the form of discrete opacities (“infil­ trates”), punctate epithelial erosions, and minor degrees of superficial corneal vascularization. Very rarely, conjunctival scarring and eyelid distortion occur, particularly in patients treated for many months with topical glucocorticoids. Recurrent eye infections develop most often in patients whose sexual partners are not treated with antimicrobial agents. Diagnosis  The clinical diagnosis of classic trachoma can be made if two of the following signs are present: (1) lymphoid follicles on the upper tarsal conjunctiva; (2) typical conjunctival scarring; (3) vascular pannus; or (4) limbal follicles or their sequelae, Herbert pits. The clinical diagnosis of endemic trachoma should be confirmed by laboratory tests in children with relatively marked degrees of inflam­ mation. Intracytoplasmic chlamydial inclusions are found in 10–60% of Giemsa-stained conjunctival smears in such populations, but chlamydial NAATs are more sensitive and are often positive when smears or cultures are negative. While NAATS are the most sensitive

and specific diagnostic assays they are prohibitively expensive and require laboratory infrastructure that are not available in most endemic regions. Follicular conjunctivitis in European or American adults living in trachomatous regions is rarely due to trachoma.

TREATMENT Trachoma Adult inclusion conjunctivitis responds well to treatment with the same regimens used in uncomplicated genital infections—namely, azithromycin (a 1-g single oral dose) or doxycycline (100 mg twice daily for 7 days). Chlamydial resistance to azithromycin has not been documented. Topical tetracycline (1% eye ointment twice daily for 6 weeks) can be used as an alternative, but adherence rates are likely to be suboptimal. In the setting of eye infection with oculogenital strains, simultaneous treatment of all sexual partners is necessary to prevent ocular reinfection and chlamydial genital disease. Topical antibiotic treatment is not required for patients who receive systemic antibiotics. PSITTACOSIS Psittacine birds and many other avian species act as natural reser­ voirs for C. psittaci–type organisms, common pathogens in domestic mammals and birds. The species C. psittaci, which now includes only avian strains, affects humans only as a zoonosis. (The other strains previously included in this species have been placed into different species that reflect the animals they infect: C. abortus,

C. muridarum, C. suis, C. felis, and C. caviae.) Although all birds are susceptible, pet birds (parrots, parakeets, macaws, and cockatiels) and poultry (turkeys and ducks) are most frequently involved in transmission of C. psittaci to humans. Exposure is greatest in poul­ try-processing workers and in owners of pet birds. Infectious forms of the organisms are shed from both symptomatic and apparently healthy birds and may remain viable for several months. C. psittaci can be transmitted to humans by direct contact with infected birds or by inhalation of aerosols from avian nasal discharges and from infectious avian fecal or feather dust. Transmission from person to person has never been demonstrated. CHAPTER 194 Chlamydial Infections The diagnosis is usually established serologically. Psittacosis in humans may present as acute primary atypical pneumonia (which can be fatal in up to 10% of untreated cases); as severe chronic pneumonia; or as a mild illness or asymptomatic infection in persons exposed to infected birds. ■ ■EPIDEMIOLOGY True incidence and prevalence of psittacosis is difficult to ascertain, in part due to lack of routine testing and the varying performance of commonly used diagnostic tests. Since 2010, the CDC has typically received 10 reports annually of confirmed cases of psittacosis, although many more cases probably occur than are reported. Outbreaks have been observed in, for example, poultry plant workers. Control of psit­ tacosis depends on control of avian sources of infection. A pandemic of psittacosis was once stopped by banning shipment or importation of psittacine birds. Birds can receive prophylaxis in the form of a tetracycline-containing feed. Imported birds are currently quarantined for 30 days of treatment. ■ ■CLINICAL MANIFESTATIONS Typical symptoms include fever, chills, muscular aches and pains, severe headache, hepato- and/or splenomegaly, and gastrointestinal symptoms. Hepatitis and neurologic complications can occur. Cardiac complications may involve endocarditis and myocarditis. Fatal cases were common in the preantibiotic era. As a result of quarantine of imported birds and improved veterinary-hygienic measures, outbreaks and sporadic cases of psittacosis are now rare. Severe pneumonia requiring management in an intensive care unit may develop. The incubation period is usually 5–19 days but can last as long as 28 days.

■ ■DIAGNOSIS Previously, the most widely used serologic test for diagnosing chlamyd­ ial infections was the genus-specific CF test, in which assay of paired serum specimens often shows fourfold or greater increases in antibody titer. The CF test remains useful, but the gold standard of serologic tests is now the MIF test, which is not widely available (see section on diagnosis of C. trachomatis genital infection, above). Any antibody titer above 1:16 is considered significant evidence of exposure to chla­ mydiae, and a fourfold titer rise in paired sera in combination with a clinically compatible syndrome can be used to diagnose psittacosis. Some commercially available serologic tests based on measurement of antibodies to LPS can be useful when the clinical diagnosis is consis­ tent with bird exposure; however, since these tests are reactive for all chlamydiae (i.e., all chlamydiae contain LPS), caution must be used in their interpretation. NAATs from respiratory samples are highly sensi­ tive and specific but are not widely available in laboratories. C. psittaci is now considered a biohazard category B biothreat agent and has been associated with laboratory-acquired infections.

TREATMENT Psittacosis The antibiotic of choice is doxycycline; the dosage for adults is 100 mg twice a day, continued for 10–21 days; it is unclear if lon­ ger courses prevent relapse. Severely ill patients may need intra­ venous doxycycline and cardiovascular and respiratory support. Tetracycline (500 mg four times a day by mouth) and azithromycin 250–500 mg by mouth daily for 7 days are alternative therapies. Erythromycin (500 mg four times a day by mouth) appears to be inferior to other agents in animal models. PART 5 Infectious Diseases C. PNEUMONIAE INFECTIONS C. pneumoniae is a common cause of human respiratory diseases, such as pneumonia and bronchitis. This organism reportedly accounts for as many as 10% of cases of community-acquired pneumonia, but more typically 1–2%. PCR is the preferred method for diagnosis of acute infections. Serologic studies have linked C. pneumoniae to atheroscle­ rosis; isolation and PCR detection in cardiovascular tissues have also been reported. These findings suggest an expanded range of diseases and syndromes for C. pneumoniae. Large-scale case–cohort studies have demonstrated some association of C. pneumoniae with lung can­ cer, as evaluated by serology. ■ ■EPIDEMIOLOGY Primary infection occurs mainly in school-aged children and reinfec­ tion in adults. Seroprevalence rates of 40–70% show that C. pneu­ moniae is widespread in both industrialized and developing countries. Seropositivity usually is first detected at school age, and rates generally increase by ~10% per decade. About 50% of individuals have detectable antibody at 30 years of age, and most have detectable antibody by the eighth decade of life. Although, as mentioned, serologic evidence sug­ gests that C. pneumoniae may be associated with up to 10% of cases of community-acquired pneumonia, most of this evidence is based not on paired serum samples but rather on a single high IgG titer. Some doubt exists about the true prevalence and etiologic role of C. pneumoniae in atypical pneumonia, especially since reports of cross-reactivity have raised questions about the specificity of serology when only a single serum sample is used for diagnosis. ■ ■PATHOGENESIS Little is known about the pathogenesis of C. pneumoniae infection. It begins in the upper respiratory tract and may persist as a prolonged asymptomatic condition of the upper respiratory mucosal surfaces. However, evidence of replication within vascular endothelium and synovial membranes of joints shows that, in at least some individuals, the organism is transported to distant sites, perhaps within macro­ phages. A C. pneumoniae outer-membrane protein may induce host

immune responses whose cross-reactivity with human proteins results in an autoimmune reaction. The role of C. pneumoniae in the etiology of atherosclerosis has been discussed since 1988, when Finnish researchers presented serologic evidence of an association of this organism with coronary heart disease and acute myocardial infarction. Subsequently, the organism was identified in atherosclerotic lesions by culture, PCR, immunohistochemistry, and transmission electron microscopy; how­ ever, discrepant study results (including those of animal studies) and failure of large-scale treatment studies have raised doubts as to the etiologic role of C. pneumoniae in atherosclerosis. Epidemiologic studies have demonstrated an association between serologic evi­ dence of C. pneumoniae infection and atherosclerotic disease of the coronary and other arteries. In addition, C. pneumoniae has been identified in atherosclerotic plaques by electron microscopy, DNA hybridization, and immunocytochemistry. The organism has been recovered in culture from atheromatous plaques—a result indicating the presence of viable replicating bacteria in vessels. Evidence from animal models supports the hypothesis that C. pneumoniae infection of the upper respiratory tract is followed by recovery of the organism from atheromatous lesions in the aorta and that the infection acceler­ ates the process of atherosclerosis, especially in hypercholesterolemic animals. Antimicrobial treatment of the infected animals reverses the increased risk of atherosclerosis. In humans, two small trials in patients with unstable angina or recent myocardial infarction sug­ gested that antibiotics reduce the likelihood of subsequent cardiac events. However, larger-scale trials have not documented an effect of various antichlamydial regimens on the risk of these events. ■ ■CLINICAL MANIFESTATIONS C. pneumoniae was first reported as the etiologic agent of mild atypi­ cal pneumonia in military recruits and college students. The clinical spectrum of C. pneumoniae infection includes acute pharyngitis, sinusitis, bronchitis, and pneumonitis, primarily in young adults. The clinical manifestations of primary infection appear to be more severe and prolonged than those of reinfection. The pneumonitis of C. pneu­ moniae pneumonia resembles that of Mycoplasma pneumonia in that leukocytosis is frequently lacking and patients often have prominent antecedent upper respiratory tract symptoms, fever, nonproductive cough, mild to moderate illness, minimal findings on chest ausculta­ tion, and small segmental infiltrates on chest x-ray. In elderly patients, pneumonia due to C. pneumoniae can be especially severe and may necessitate hospitalization and respiratory support. Chronic infection with C. pneumoniae has been reported among patients with chronic obstructive pulmonary disease and may play a role in the natural history of asthma, including exacerbations. The clinical symptoms of respiratory infections caused by C. pneumoniae are nonspecific and do not differ from those caused by other agents of atypical pneumonia, such as Mycoplasma pneumoniae. ■ ■DIAGNOSIS PCR amplification of respiratory secretions is the preferred method of diagnosis; several commercial assays are available. Serology and culture can be used to diagnose C. pneumoniae infection. Serology was the traditional diagnostic method. Serology is not FDA approved because of its poor predictive value and is not routinely used clinically. Cell cul­ ture, which can perform similarly to PCR and allows for antimicrobial susceptibility testing, is seldom used as most clinical laboratories are not equipped to culture Chlamydia spp. The organism is very difficult to grow in tissue cultures but has been cultivated in HeLa cells, HEp-2 cells, and HL cells. Previously, the gold standard serologic test was the MIF test (see section on diagnosis of C. trachomatis genital infection, above). Any antibody titer >1:16 is considered significant evidence of exposure to chlamydiae. According to a CDC-sponsored expert work­ ing group, the diagnosis of acute C. pneumoniae infection requires demonstration of a fourfold rise in titer in paired serum samples. There are no official recommendations for diagnosis of chronic infections, although many research studies have used high titers of IgA as an indicator. The older CF tests and EIAs for LPS are not recommended,

82 - SECTION 11 Viral Diseases- General Considerations

SECTION 11 Viral Diseases: General Considerations

as they are not specific for C. pneumoniae but identify the chlamydiae only to the genus level. TREATMENT C. pneumoniae Infections Although few controlled trials of treatment have been reported, C. pneumoniae is inhibited in vitro by erythromycin, tetracy­ cline, azithromycin, clarithromycin, gatifloxacin, and gemifloxacin. Directed therapies include azithromycin 500 mg orally once fol­ lowed by 250 mg on days 2–5; doxycycline 100 mg orally twice daily; or clarithromycin 500 mg twice daily. The fluoroquinolones levofloxacin (750 mg orally once daily) and moxifloxacin (400 mg orally once daily) are alternatives, but their role is limited by increasing safety concerns due to serious side effects. For most patients, a 5-day course of therapy is sufficient. Beta-lactams and trimethoprim/sulfamethoxazole are not active. Acknowledgment The authors wish to thank Dr. Charlotte A. Gaydos for her contributions to this chapter in previous editions. ■ ■FURTHER READING Centers for Disease Control and Prevention: Sexually Trans­ mitted Infections Surveillance, 2022. Atlanta, GA: U.S. Department of Health and Human Services, 2024. https://www.cdc.gov/std/statistics/2022/ default.htm. Centers for Disease Control and Prevention: Sexually trans­ mitted infections treatment guidelines, 2021. MMWR Recomm Rep 70:1, 2021. Elwell C et al: Chlamydia cell biology and pathogenesis. Nat Rev Microbiol 14:385, 2016. Gaydos CA, Essiq A: Chlamydiaceae, in Manual of Clinical Microbiol­ ogy, 11th ed. JH Jorgensen et al (eds). Washington, DC, ASM Press, 2015, pp 1106–1121. Goller JL et al: Population attributable fraction of pelvic inflamma­ tory disease associated with chlamydia and gonorrhoea: A crosssectional analysis of Australian sexual health clinic data. Sex Transm Infect 92:525, 2016. Gregory ECW, Ely DM: Trends and characteristics of sexually transmitted infections during pregnancy: United States, 2016–2018. National Vital Statistics Reports 69:1, 2020. Hammerschlag MR et al: Chlamydia pneumoniae, in Mandell, Doug­ las, and Bennett’s Principles and Practice of Infectious Diseases, 9th ed. JE Bennett, R Dolin, MJ Blaser (eds). Philadelphia, Elsevier, 2020, Chapter 182. Hughes Y et al: Universal lymphogranuloma venereum (LGV) testing of rectal chlamydia in men who have sex with men and detection of asymptomatic LGV. Sex Transm Infect 98:582, 2022. Kuypers J et al: Principles of laboratory diagnosis of STIs, in Sexu­ ally Transmitted Diseases, 4th ed. KK Holmes et al (eds). New York, McGraw-Hill, 2008, pp 937–948. Luetkemeyer AF et al: Postexposure doxycycline to prevent bacterial sexually transmitted infections. N Engl J Med 388:1296, 2023. Papp JR et al: Recommendations for the laboratory-based detection of Chlamydia trachomatis and Neisseria gonorrhoeae, 2014. MMWR 63:1, 2014. Rowley J et al: Chlamydia, gonorrhea, trichomoniasis and syphilis: Global prevalence and incidence estimates, 2016. Bull World Health Organ 97:548, 2019. Schachter J, Stephens RS: Biology of Chlamydia trachomatis, in Sexually Transmitted Diseases, 4th ed. KK Holmes et al (eds). New York, McGraw-Hill, 2008, pp 555–574. Taylor HR: Trachoma: A Blinding Scourge from the Bronze Age to the Twenty-First Century. East Melbourne, Victoria, Australia, Centre for Eye Research Australia/Haddington Press, 2008.

Section 11 Viral Diseases: General Considerations David M. Knipe, Max L. Nibert

Principles of Medical

Virology Viruses are obligate intracellular parasites that must enter cells to replicate and propagate themselves to spread to other cells. Infection often injures the host cell—hence the name “virus,” derived from the Latin word virus for poison or toxin. Viruses are one of the simplest life forms and, at the minimum, have a nucleic acid genome with a protein coat. They do not divide by division, as do cells; instead, viruses are programmed to disassemble inside cells, to use their nucleic acid genome to encode viral proteins that replicate their genomic nucleic acid, and then to assemble the progeny genomes into viral particles. The progeny viruses are secreted or released from the host cell as extra­ cellular virions that infect surrounding cells. Viruses depend on the host cell for many of the enzymes and organelles that synthesize carbo­ hydrates, lipids, nucleic precursors and nucleic acids, and high-energy molecules, including the host cell’s ribosomes, which are used to make viral proteins. In the process of taking over the host cell, viruses inhibit normal cell metabolic pathways and cause damage to the cell in a pro­ cess that results in the cytopathic effect (CPE). Injury to cells and cell death can cause tissue damage and contribute to virus-induced disease. CHAPTER 195 Viruses are distinct from other intracellular parasites such as viroids, virusoids, prions, and intracellular bacteria. Viroids are small, circular, single-stranded RNA infectious pathogens of plants that do not have a protein coat, while virusoids are small, circular-RNA, infectious patho­ gens that depend on viruses to provide the proteins for their replica­ tion and protein coat. Prions are misfolded proteins that spread from one cell to another, causing the same protein molecules to misfold in the new cell. The misfolded proteins in prions cause cellular damage (Chap. 449). Principles of Medical Virology VIRUS STRUCTURE There are many different virus structures, but nearly all are formed from a few fundamental structural elements. The minimal virion particle is composed of a complex of nucleic acids (the genome) and a protein shell (the capsid) (Fig. 195-1). The combination of the genome and the capsid is called the nucleocapsid. The genome is protected within the capsid. The external surface of virions can consist of either the protein capsid or a lipid envelope around the capsid (Fig. 195-1). Viral genomes can consist of single- or double-stranded RNA or DNA and can comprise one or more genome segments. Singlestranded (ss) genomes are designated as positive strand (+) if they con­ tain the sequences encoding the open reading frames for viral proteins, while they are designated as negative strand (–) if they contain only complementary sequences. Thus, a positive-strand RNA viral genome can be translated into a viral protein upon entry into the host cell, while a negative-strand genome must be copied into complementary RNA molecules for translation. This dilemma is solved in negative-strand viruses by the loading of transcriptases onto the viral genome prior to encapsidation; these enzymes transcribe the genome into viral mRNA upon entry into and uncoating within the cell. Viral capsids are made of repeating protein subunits because their genomes have limited coding capacity. The capsids are constructed with a few structural units or capsomers packed into a symmetrical arrangement. Capsids are usually organized in one of two ways: (1) an icosahedral or spherical symmetry based on an icosahedron with two-, three-, and fivefold axes of symmetry formed from 20 triangular faces or (2) a helical symmetry. However, viruses occasionally have more complex structures (e.g., the poxviruses) (Fig. 195-2).

83 - 195 Principles of Medical Virology

195 Principles of Medical Virology

as they are not specific for C. pneumoniae but identify the chlamydiae only to the genus level. TREATMENT C. pneumoniae Infections Although few controlled trials of treatment have been reported, C. pneumoniae is inhibited in vitro by erythromycin, tetracy­ cline, azithromycin, clarithromycin, gatifloxacin, and gemifloxacin. Directed therapies include azithromycin 500 mg orally once fol­ lowed by 250 mg on days 2–5; doxycycline 100 mg orally twice daily; or clarithromycin 500 mg twice daily. The fluoroquinolones levofloxacin (750 mg orally once daily) and moxifloxacin (400 mg orally once daily) are alternatives, but their role is limited by increasing safety concerns due to serious side effects. For most patients, a 5-day course of therapy is sufficient. Beta-lactams and trimethoprim/sulfamethoxazole are not active. Acknowledgment The authors wish to thank Dr. Charlotte A. Gaydos for her contributions to this chapter in previous editions. ■ ■FURTHER READING Centers for Disease Control and Prevention: Sexually Trans­ mitted Infections Surveillance, 2022. Atlanta, GA: U.S. Department of Health and Human Services, 2024. https://www.cdc.gov/std/statistics/2022/ default.htm. Centers for Disease Control and Prevention: Sexually trans­ mitted infections treatment guidelines, 2021. MMWR Recomm Rep 70:1, 2021. Elwell C et al: Chlamydia cell biology and pathogenesis. Nat Rev Microbiol 14:385, 2016. Gaydos CA, Essiq A: Chlamydiaceae, in Manual of Clinical Microbiol­ ogy, 11th ed. JH Jorgensen et al (eds). Washington, DC, ASM Press, 2015, pp 1106–1121. Goller JL et al: Population attributable fraction of pelvic inflamma­ tory disease associated with chlamydia and gonorrhoea: A crosssectional analysis of Australian sexual health clinic data. Sex Transm Infect 92:525, 2016. Gregory ECW, Ely DM: Trends and characteristics of sexually transmitted infections during pregnancy: United States, 2016–2018. National Vital Statistics Reports 69:1, 2020. Hammerschlag MR et al: Chlamydia pneumoniae, in Mandell, Doug­ las, and Bennett’s Principles and Practice of Infectious Diseases, 9th ed. JE Bennett, R Dolin, MJ Blaser (eds). Philadelphia, Elsevier, 2020, Chapter 182. Hughes Y et al: Universal lymphogranuloma venereum (LGV) testing of rectal chlamydia in men who have sex with men and detection of asymptomatic LGV. Sex Transm Infect 98:582, 2022. Kuypers J et al: Principles of laboratory diagnosis of STIs, in Sexu­ ally Transmitted Diseases, 4th ed. KK Holmes et al (eds). New York, McGraw-Hill, 2008, pp 937–948. Luetkemeyer AF et al: Postexposure doxycycline to prevent bacterial sexually transmitted infections. N Engl J Med 388:1296, 2023. Papp JR et al: Recommendations for the laboratory-based detection of Chlamydia trachomatis and Neisseria gonorrhoeae, 2014. MMWR 63:1, 2014. Rowley J et al: Chlamydia, gonorrhea, trichomoniasis and syphilis: Global prevalence and incidence estimates, 2016. Bull World Health Organ 97:548, 2019. Schachter J, Stephens RS: Biology of Chlamydia trachomatis, in Sexually Transmitted Diseases, 4th ed. KK Holmes et al (eds). New York, McGraw-Hill, 2008, pp 555–574. Taylor HR: Trachoma: A Blinding Scourge from the Bronze Age to the Twenty-First Century. East Melbourne, Victoria, Australia, Centre for Eye Research Australia/Haddington Press, 2008.

Section 11 Viral Diseases: General Considerations David M. Knipe, Max L. Nibert

Principles of Medical

Virology Viruses are obligate intracellular parasites that must enter cells to replicate and propagate themselves to spread to other cells. Infection often injures the host cell—hence the name “virus,” derived from the Latin word virus for poison or toxin. Viruses are one of the simplest life forms and, at the minimum, have a nucleic acid genome with a protein coat. They do not divide by division, as do cells; instead, viruses are programmed to disassemble inside cells, to use their nucleic acid genome to encode viral proteins that replicate their genomic nucleic acid, and then to assemble the progeny genomes into viral particles. The progeny viruses are secreted or released from the host cell as extra­ cellular virions that infect surrounding cells. Viruses depend on the host cell for many of the enzymes and organelles that synthesize carbo­ hydrates, lipids, nucleic precursors and nucleic acids, and high-energy molecules, including the host cell’s ribosomes, which are used to make viral proteins. In the process of taking over the host cell, viruses inhibit normal cell metabolic pathways and cause damage to the cell in a pro­ cess that results in the cytopathic effect (CPE). Injury to cells and cell death can cause tissue damage and contribute to virus-induced disease. CHAPTER 195 Viruses are distinct from other intracellular parasites such as viroids, virusoids, prions, and intracellular bacteria. Viroids are small, circular, single-stranded RNA infectious pathogens of plants that do not have a protein coat, while virusoids are small, circular-RNA, infectious patho­ gens that depend on viruses to provide the proteins for their replica­ tion and protein coat. Prions are misfolded proteins that spread from one cell to another, causing the same protein molecules to misfold in the new cell. The misfolded proteins in prions cause cellular damage (Chap. 449). Principles of Medical Virology VIRUS STRUCTURE There are many different virus structures, but nearly all are formed from a few fundamental structural elements. The minimal virion particle is composed of a complex of nucleic acids (the genome) and a protein shell (the capsid) (Fig. 195-1). The combination of the genome and the capsid is called the nucleocapsid. The genome is protected within the capsid. The external surface of virions can consist of either the protein capsid or a lipid envelope around the capsid (Fig. 195-1). Viral genomes can consist of single- or double-stranded RNA or DNA and can comprise one or more genome segments. Singlestranded (ss) genomes are designated as positive strand (+) if they con­ tain the sequences encoding the open reading frames for viral proteins, while they are designated as negative strand (–) if they contain only complementary sequences. Thus, a positive-strand RNA viral genome can be translated into a viral protein upon entry into the host cell, while a negative-strand genome must be copied into complementary RNA molecules for translation. This dilemma is solved in negative-strand viruses by the loading of transcriptases onto the viral genome prior to encapsidation; these enzymes transcribe the genome into viral mRNA upon entry into and uncoating within the cell. Viral capsids are made of repeating protein subunits because their genomes have limited coding capacity. The capsids are constructed with a few structural units or capsomers packed into a symmetrical arrangement. Capsids are usually organized in one of two ways: (1) an icosahedral or spherical symmetry based on an icosahedron with two-, three-, and fivefold axes of symmetry formed from 20 triangular faces or (2) a helical symmetry. However, viruses occasionally have more complex structures (e.g., the poxviruses) (Fig. 195-2).

Glycoprotein Genome Genome Capsid Envelope Enveloped virion with icosahedral capsid Nonenveloped icosahedral virion A B FIGURE 195-1  Schematic diagrams of the major forms of human viruses. A. Icosahedral capsid without an envelope. B. Icosahedral capsid with a lipid envelope. C. Helical capsid with a lipid envelope. D. Complex virion. Positive-strand RNA viruses Name Picornaviridae Caliciviridae Hepeviridae Matonaviridae Togaviridae Genome size (kb) 7.5

6.7–10 No Yes Envelope No Caspsid symmetry Icosahedral Icosahedral Icosahedral Negative-strand RNA viruses PART 5 Infectious Diseases Name Rhabdoviridae Filoviridae Genome size (kb) 11–12 15–19 Envelope Yes Yes Caspsid symmetry Helical Helical Segmented negative-strand RNA viruses Segmented double-strand RNA viruses Retroviruses Name Orthomyxoviridae Peribunyaviridae Hantaviridae Nairoviridae Arenaviridae Genome size (kb)

Envelope Yes Yes Yes Caspsid symmetry Helical Helical Helical DNA viruses 100 nm Papillomaviridae Polyomaviridae Parvoviridae Name Hepadnaviridae 5 Kb Genome size 5–9 kbp 3 kbp No No Envelope Caspsid symmetry Icosahedral Icosahedral Icosahedral FIGURE 195-2  Schematic diagrams of viruses of the major families that infect humans. The viruses are grouped by genotype, and the virions are drawn approximately to scale. Prototype viruses of each family are listed in Table 195-1. (Source: Modified from Fig. 185-2 in Harrison’s Principles of Internal Medicine, 20th ed.)

Genome Genome Complex virion Enveloped virion with helical nucleocapsid D C Flaviviridae Coronaviridae 9–13 25–32 Yes Yes Icosahedral Helical Pneumoviridae Paramyxoviridae 14–22 Yes Helical Retroviridae Sedoreoviridae Spinareoviridae 7–13

Yes No Icosahedral Icosahedral Adenoviridae Orthoherpesviridae Poxviridae 36–38 kbp 125–240 kbp 190 kbp Yes No Yes Yes Icosahedral Icosahedral Complex

Enveloped viruses (e.g., measles virus) are efficient in infecting cells because the viral lipid membrane fuses easily with the plasma membrane of the host cell or with internal membranes to deliver the nucleocapsid to the cytoplasm of the host cell. Thus, these viruses are highly transmissible. The lipid envelope is susceptible to disruption by detergents or organic solvents; thus, enveloped viruses such as measles virus, coronaviruses, and influenza viruses can be inactivated by soap and water or alcohol-based hand sanitizers. In contrast, unenveloped viruses (e.g., norovirus or poliovirus) have a tough protein shell whose resistance to small-intestine bile salts—a surfactant that emulsifies lipids—allows them to infect the intestine. Unenveloped viruses, espe­ cially those that infect the gastrointestinal tract, are not inactivated by detergents or organic solvents and must be inactivated by peroxide or hypochlorite or removed by washing with soap and water. CLASSIFICATION OF VIRUSES Viruses are classified as a free-standing groups because they are not formally related to organisms within any of the major kingdoms. The highest level of viral classification was originally the family, but there have been efforts to classify viruses into higher ranks, culminating in kingdoms and realms. This higher classification is largely not that relevant to medical virology because the major viruses of clinical interest can be conveniently classified into a number of families (Table 195-1), each of which has characteristic virion and genome structures (Fig. 195-2). Classification of viruses into families, genera, and species was previously based on multiple criteria, including type of genomic nucleic acid (i.e., RNA or DNA; ss positive or negative strand or double strand), capsid symmetry (helical, icosahedral, or complex), presence or absence of an envelope, mode of replication, and tropism (preferred cell type for replication) or type of disease it causes. Recent sequence analysis of viral genomes has refined and revised some of the original virus classifications. The International Committee on Taxonomy of Viruses specifies both formal and common names for viruses. For example, herpes simplex virus (HSV) is the common name for species simplex virus human alpha 1. VIRAL REPLICATION IN CELLS Viral replication takes place in the host cell by the following steps: binding, entry, uncoating, transport to the site of replication, transcrip­ tion of mRNA, translation of viral proteins, replication of the input genome, assembly of progeny viral particles, and egress from the cell. All viruses must enter cells by mechanisms that allow virus binding to the cell surface and subsequent crossing of the plasma membrane and/ or other membranes to gain entry into the cytoplasm. After entry, the mechanisms of replication diverge for the different viruses, depending on the nature of the viral genome. ■ ■VIRAL ENTRY Viruses bind to specific receptors on the cell surface and generally enter cells by one of three pathways: (1) fusion of the envelope with the surface plasma membrane; (2) endocytosis followed by fusion with the endo­ some membrane; or (3) lysis of the endosome or formation of pores in the endosome. Viruses often bind to a charged molecule on the surface of cells to concentrate themselves thereon. They then bind more specifically to a protein or carbohydrate molecule, and this binding triggers endocy­ tosis or fusion of the viral envelope with the cellular plasma membrane. Endocytosis can occur by any of several mechanisms, including clathrinmediated endocytosis, macropinocytosis, micropinocytosis, and caveo­ lar endocytosis. After viral entry into endocytic vesicles, acidification of the vesicles leads to conformational changes in the viral glycoproteins, fusion of the viral envelope with the endocytic membrane, and release of the nucleocapsid into the cytoplasm. At the entry stage or later, the genome must be uncoated or the capsid opened sufficiently to allow transcription, translation, and/or replication. ■ ■VIRAL REPLICATION STRATEGIES Positive-Strand RNA Viruses  The RNA genomes of the picorna­ viruses, caliciviruses, hepeviruses, togaviruses, flaviviruses, and corona­ viruses can be translated in the cytoplasm directly after removal of the

capsid coat or uncoating. The picornaviral and flaviviral genomic RNA is translated into a polyprotein that is cleaved by viral and cellular proteases to generate (1) nonstructural proteins that replicate the genomic RNA to complementary negative-strand molecules and then back to positivestrand RNA molecules and (2) structural proteins that assemble capsids for progeny virions. Replication of positive-strand viral RNA takes place in replication complexes associated with cytoplasmic membranes, often in membrane sacs that concentrate the components, protect them from host responses, and provide the redox environment needed for optimal replication. Progeny virions are released when the host cell lyses. The positive-strand genome RNA of the caliciviruses, hepatitis E virus (a hepevirus), and the togaviruses is translated to generate a polyprotein, which, when cleaved by viral and cellular proteases, yields the nonstruc­ tural proteins that replicate the viral genome to a negative-strand copy and then synthesize new full-length positive strands and a subgenomic mRNA that encodes the structural proteins. Progeny virions are released by budding or cell lysis, depending on whether the virus is enveloped or not. Replication of the genome to the negative strand is followed by a transition back to the positive-strand genome for translation and encap­ sidation. Progeny virions are released by budding.

Negative-Strand RNA Viruses  The rhabdoviruses, filoviruses, and paramyxoviruses have a single negative strand of genome RNA that is transcribed by a virion-associated RNA-dependent RNA poly­ merase (transcriptase) to yield subgenomic mRNAs that encode the replicase and structural proteins. The replicase copies the full-length negative-strand RNA to a full-length positive-strand RNA and then back to a full-length negative strand, which is assembled into nucleo­ capsids that bud out of the cell to form progeny virions. CHAPTER 195 The influenza viruses, peri bunyaviruses, and arenaviruses have seg­ mented negative RNA genomes that are transcribed by virion-associated transcriptases to yield mRNAs that encode nonstructural and struc­ tural proteins. The replicase enzyme complex copies the negativestrand RNA genomes to full-length positive-strand copies and back to full-length negative-strand RNA molecules. The peri bunyaviruses and arenaviruses replicate entirely in the cytoplasm. In contrast, influenza viral transcription takes place in the nucleus, with nascent cellular transcripts serving as primers to yield mRNAs that are transported to the cytoplasm for translation. Viral proteins are transported into the nucleus to promote genome replication, and progeny negative-strand RNAs are transported to the cytoplasm to bud into progeny virions. Some of the bunyaviruses and the arenaviruses have open reading frames on the “negative strand.” Thus, these viruses use both negative- and positive-sense or ambisense coding of their RNA genomes. The full-length negative strands are assembled in the correct assortment in capsid proteins and then bud to yield infectious progeny virions. Principles of Medical Virology Double-Stranded RNA Viruses  The reovirus and rotavirus genomes consist of multiple double-stranded (ds) RNA molecules that are transcribed by virion-associated, RNA-dependent RNA poly­ merases (transcriptases) to yield mRNAs encoding nonstructural and structural proteins. Following viral protein synthesis, replication of positive-strand RNAs to form dsRNA molecules and assembly into viral capsids occur in cytoplasmic viral factories. Progeny viruses are released when infected cells lyse. Double-Stranded DNA Viruses  Most dsDNA viral genomes are transported to the infected cell’s nucleus for transcription and replication. The host cell recognizes foreign DNA that is not fully loaded with histone nucleosomes with a normal pattern and tries to epigenetically silence these molecules; DNA viruses have evolved mechanisms to overcome these epigenetic silencing mechanisms. The dsDNA genomes of the papovaviruses and papillomaviruses are coated with nucleosomal chromatin in the virion and therefore are delivered to the nucleus in a form that is not recognized as foreign. Viral early gene expression is promoted by an enhancer adjacent to the early gene promoter, which is transcribed by host cell RNA polymerase II to yield the early mRNAs. The early proteins promote viral DNA replication by host enzymes, and late genes are then transcribed. The late proteins encode the capsid proteins to assemble progeny virions.

TABLE 195-1  Major Families of Human Pathogenic Viruses FAMILY REPRESENTATIVE VIRUSES TYPE OF RNA/DNA LIPID ENVELOPE Picornaviridae Coxsackievirus Echovirus Enteroviruses, including poliovirus Rhinoviruses Hepatitis A virus Caliciviridae Norovirus (+) RNA No Hepeviridae Hepatitis E virus (+) RNA No Matonaviridae Rubella virus (+) RNA Yes Togaviridae Eastern equine encephalitis virus Western equine encephalitis virus Flaviviridae Yellow fever virus Dengue virus St. Louis encephalitis virus West Nile virus Zika virus Hepatitis C virus Hepatitis G virus Coronaviridae SARS-CoV-1 SARS-CoV-2 Middle East respiratory syndrome virus Rhabdoviridae Rabies virus Vesicular stomatitis virus Filoviridae Marburg virus Ebola virus PART 5 Infectious Diseases Pneumoviridae Respiratory syncytial virus (–) RNA Yes Paramyxoviridae Parainfluenza virus Newcastle disease virus Mumps virus Rubeola (measles) virus Orthomyxoviridae Influenza A, B, and C viruses (–) RNA, 8 segments Yes Peribunyaviridae California encephalitis virus (–) RNA, 3 segments Yes Hantaviridae Hantavirus (–) RNA, 3 segments Yes Nairoviridae Crimean–Congo hemorrhagic fever virus (–) RNA, 3 segments Yes Arenaviridae Lymphocytic choriomeningitis virus Lassa fever virus South American hemorrhagic fever virus Sedoreoviridae Rotavirus dsRNA, 11 segments No Spinareoviridae Reovirus Colorado tick fever virus Retroviridae Human T lymphotropic virus 1 and 2 Human immunodeficiency virus 1 and 2 Hepadnaviridae Hepatitis B virus dsDNA with ss portions Yes Parvoviridae Parvovirus B19 ssDNA No Papillomaviridae Human papillomaviruses dsDNA No Polyomaviridae JC virus BK virus Merkel cell polyoma virus Adenoviridae Human adenoviruses dsDNA No Orthoherpesviridae Herpes simplex virus 1 and 2 Varicella-zoster virus Epstein-Barr virus Cytomegalovirus Human herpesvirus 6 Human herpesvirus 7 Kaposi’s sarcoma–associated herpesvirus Poxviridae Variola (smallpox) virus Orf virus Molluscum contagiosum virus Abbreviations: ds, double-stranded; ss, single-stranded.

(+) RNA No (+) RNA Yes (+) RNA Yes (+) RNA Yes (–) RNA Yes (–) RNA Yes (–) RNA Yes (–) RNA, 2 segments Yes dsRNA, 10–12 segments No (+) RNA, 2 identical segments Yes … … dsDNA Yes dsDNA Yes

The dsDNA genomes of adenoviruses are delivered to the infected cell’s nucleus coated with a viral protein that hides the viral genomes from the host’s epigenetic silencing mechanisms. Viral DNA genomes are transported to and released through the nuclear pores and are tran­ scribed by host cell RNA polymerase II to yield pre-early mRNAs. The pre-early proteins promote the transcription of early mRNAs, whose proteins promote viral DNA replication. The late proteins encode structural proteins of the virion. The dsDNA genomes of the herpesviruses, which are not coated with histones in the virion, are transported to the infected cell’s nuclear pores and released into the nucleus. The naked DNA is rapidly loaded with histones bearing silencing modifications by host cell mechanisms; however, a viral enhancer and a virion protein that uses host enzymes to drive chromatin reorganization allow immediate-early gene tran­ scription and expression. Immediate-early proteins promote early gene transcription. Among the E proteins, eight or nine viral proteins including the viral DNA polymerase are essential for viral DNA syn­ thesis. Late genes then encode proteins for virion assembly. In contrast, the poxviruses replicate entirely in the cytoplasm—an unusual site for replication of a dsDNA virus. As a result, they encode many of the enzymes and factors needed for viral transcription and genome replication. A virus-encoded, virion-associated, DNA-dependent RNA polymerase transcribes the viral genome in the infected cell’s cytoplasm to yield early mRNAs. The early mRNAs encode additional transcription factors and DNA replication factors, including a viral DNA polymerase. After DNA replication, the full set of viral proteins needed for viral progeny assembly is generated by intermediate and late transcription. Single-Stranded DNA Viruses  The ssDNA genomes of the parvoviruses are delivered to the infected cell’s nucleus, and host cell enzymes copy the ssDNA into dsDNA. The dsDNA is then transcribed by the cell’s RNA polymerase II to yield mRNAs encoding proteins that promote viral DNA replication and assemble progeny capsids. How the parvoviruses deal with host epigenetic silencing mechanisms is not known. Retroviruses  The retrovirus genome consists of two identical positive-strand ssRNA molecules, which are not translated but instead copied into dsDNA by the virion RNA-dependent DNA polymerase or reverse transcriptase upon entry into the host cell’s cytoplasm. The dsDNA is transported with the reverse transcriptase–integrase com­ plex into the nucleus, where the viral integrase catalyzes the integration of the viral DNA molecule into the host cell’s chromosomes to yield the provirus. Transcription of the provirus by host RNA polymerase II yields mRNA for translation of viral proteins and for viral full-length transcripts for assembly of progeny virions. VIRAL EFFECTS ON THE HOST CELL Many viruses inhibit cellular macromolecular processes, such as host cell transcription and protein synthesis, in an attempt to optimize their own replication by usurping the host cell’s machinery and bio­ chemical precursors. These inhibitory events can lead to cell injury and ultimately to cell death, or necrosis. The effects are often manifest by progressive changes in cell structure, detachment from the substrate and rounding up, and eventual lysis. Collectively, these changes are referred to as the CPE. Cells may detect infection as described below and initiate a pathway called programmed cell death, or apoptosis, in an attempt to limit viral infection. Some viruses induce host cell growth to optimize their own replica­ tion or to amplify the host cells. Papovaviruses, papillomaviruses, and adenoviruses induce the cellular S phase to activate functions needed for viral DNA replication. These viruses also target cellular proteins that control cell growth, inactivating or degrading them to allow the cell cycle to progress to the S phase. Studies of the mechanisms of these viral effects on host cells have identified cellular tumor-suppressor genes such as the p53 and retinoblastoma pRB genes. Epstein-Barr virus induces proliferation to amplify its latent-infection host cell, a B cell. However, the viral mechanisms sometimes induce immor­ talization of a cell that has already undergone or later undergoes the

oncogenic transformation leading to a cancer cell. Some retroviruses encode altered versions of host genes that can induce transformation. Collectively, these DNA viruses and retroviruses are called tumor viruses.

HOST ANTIVIRAL RESPONSES AND VIRAL ANTAGONISTIC MECHANISMS Host cells have evolved numerous mechanisms for resisting viral infec­ tion. They encode constitutively expressed proteins that inhibit viral replication in a process called intrinsic resistance. One well-known host resistance factor is the rhesus macaque Trim5α protein, which inhibits human immunodeficiency virus (HIV) type 1 infection soon after the viral core enters the cytoplasm. Viruses have in turn evolved mechanisms by which to evade or neutralize resistance factors in cells of their host species. The promy­ elocytic leukemia (PML) protein and its associated proteins in nuclear domain 10 (ND-10) structures in the nucleus of human cells restrict HSV replication, but HSV has evolved a gene product—infected cell protein 0 (ICP0), an E3 ubiquitin ligase—that promotes the degra­ dation of the PML protein and thwarts this antiviral mechanism. Similarly, IFI16 can restrict HSV infection, but the viral ICP0 protein promotes its degradation. Nevertheless, PML and IFI16 protein expres­ sion are increased by interferon (IFN) signaling, and the elevated levels of these interferon-stimulated genes (ISGs) are sufficient to reduce wild-type HSV infection. Thus, during HSV infection, there is a race between cellular IFN and viral ICP0 expression. ■ ■TYPES OF CELLULAR INFECTIONS The balance of proviral and antiviral factors in a cell defines whether it is permissive or nonpermissive for viral replication. An infection in which progeny virus is produced is a productive infection. If a cell becomes infected but does not die, a virus may establish a persistent infection. A chronic infection can result if infectious virus is continu­ ally produced. An abortive infection occurs when infection begins but is not completed. In abortive infections, the cell may (1) die, if enough CPEs are exerted, as described above; (2) undergo oncogenic transfor­ mation; or (3) harbor a latent infection in which no infectious virus is found but the virus can reactivate at a later time. Examples of these outcomes are the abortive oncogenic infection of cells by Merkel cell polyomavirus, chronic infection of liver cells by hepatitis B virus, and latent infection of neurons by HSV. CHAPTER 195 Principles of Medical Virology ■ ■STAGES OF INFECTION OF A HOST The stages of viral infection are (1) entry into the host, (2) primary replication and disease at the site of entry, (3) spread through the host, (4) secondary replication and disease at new sites, (5) persistence or clearance by the host immune response, and (6) transmission or release from the host. Infection of a host can be acute, chronic, or latent. Entry  Keratinized skin cells are not viable and therefore are not good host cells for viral replication. Thus, viruses must enter the host at a mucosal surface (e.g., at oral, respiratory, and nasal sites), through a body opening (e.g., by inhalation or ingestion), or through a break in the skin (e.g., the sites of mosquito or other insect bites). For example, papillomaviruses and HSV enter at breaks in the skin, while Zika and dengue viruses can be introduced via insect bites. Primary Replication and Disease  Viruses replicate at the site of entry into the body (i.e., the primary site of infection), are shed back into the environment, and may cause entry-site disease and/or spread to cause systemic illness. For example, influenza viruses can infect the respiratory mucosa. Noroviruses and rotaviruses can infect epithelial cells in the gastrointestinal tract. Dengue and Zika viruses can infect dendritic cells in the tissues after a mosquito bite. If viral infection injures cells and tissues and causes disease at the entry site, the incuba­ tion period between exposure and disease can be as short as 1 or 2 days. Viral Spread  Although some viral infections remain localized at the primary site, others spread from the primary site to secondary sites where the viruses infect new cells and cause disease. This spread may take place through the lymph and the bloodstream (viremia). Measles

virus, for example, replicates initially in the respiratory epithelium, and infected dendritic cells spread through the lymph to lymph nodes where T cells and monocytes are infected and transmit virus through the bloodstream to organs and lymph nodes throughout the body. Systemic disease can result from the disseminated infection, and viral spread into the skin causes the classic measles rash. The incubation period of 10–14 days from exposure to clinical symptoms reflects the time involved for multiple rounds of viral replication and spread within the body before the classic rash symptoms appear. Similarly, dendritic cells and macrophages infected with dengue virus can travel through the circulatory system and transmit virus to secondary sites where infection and disease can follow.

Alternatively, viral spread may occur via neuronal pathways by transsynaptic spread of virions. Rabies virus spreads transsynaptically from the periphery to the central nervous system to cause encephalitis. HSV-1 causes a primary infection at mucosal surfaces and then enters the axon of a sensory neuron and establishes latent infection in the neuron’s cell body. Reactivation usually leads to a recurrent infection at the site of primary infection, but occasionally, the virus can move along nerve tracts to the central nervous system and cause encephalitis. Host Immune Responses  Acute viral infection is blunted by the rapid host innate immune response and then controlled by the later adaptive immune response. INNATE IMMUNITY  The first arm of the host’s immune response—the innate immune response—is rapid, with recognition of general pat­ terns of viral molecules but not of specific antigens, whose recognition occurs during the later adaptive response. Using pattern recogni­ tion receptors, host cells recognize foreign molecules with patterns contained in microbes—i.e., pathogen-associated molecular patterns (PAMPs). Recognition of the foreign molecules leads to activation of innate signaling pathways that induce the expression of IFNs, cyto­ kines, and other host gene products, including those attributable to IFN-stimulated genes, which serve as antiviral effector molecules. Viral ssRNA is recognized by Toll-like receptor 7 (TLR7) and TLR8, which induce transcription of type I IFN genes and IFN-stimulated genes. IFNs act on the producing cell in an autocrine manner and on sur­ rounding cells in a paracrine manner to induce expression of antiviral genes and to activate antiviral mechanisms. dsRNA is recognized by TLR3, which activates expression of type I IFNs. ssRNA and dsRNA are recognized by retinoic acid–inducible gene I (RIG-I) and melanoma differentiation-associated antigen 5 (MDA5), which induce type I IFN expression. Viral glycoproteins are recognized by TLR2 and TLR4. Viral DNA is recognized by the cytoplasmic cGAS receptor, which PART 5 Infectious Diseases Entry Uncoating Synthesis of viral proteins Assembly of progeny virus Copying of viral nucleic acids Release B A FIGURE 195-3  Steps in viral infection of a host cell and effects of immune effector mechanisms. A. Steps in viral infection of a host cell. The steps include entry into the cell, uncoating of the viral genomic nucleic acid, synthesis of viral proteins, copying of viral nucleic acids, assembly of progeny virus, egress, and release from the host cell. B. Mechanisms of immune effector mechanisms. Antibodies can bind to the extracellular virion and neutralize infectivity by preventing binding to the cellular receptor, preventing entry at other steps, preventing uncoating, or preventing other steps of infection. T cells recognize antigenic peptides presented on the surface of infected cells and produce antiviral cytokines and/or activate cell killing.

activates type I IFN expression, and by the nuclear IFN-inducible pro­ tein 16 (IFI16) receptor, which activates IFN expression in some cell types and epigenetic silencing of the viral DNA genome in many cell types. IFI16 can therefore act as a constitutively expressed resistance factor or as an IFN-stimulated gene. Innate responses also direct the induction of the later, more specific adaptive immune responses. ADAPTIVE IMMUNITY  Viral antigens are presented as peptides to both CD4+ and CD8+ T cells by antigen-presenting cells to induce these T cells to develop into antigen-specific T cells. Viral antigens are also presented to B cells, which induce differentiation of antibodyproducing B cells. Antibodies can bind to virions and neutralize their infectivity by preventing their binding to receptors, their entry, their uncoating, or other steps in infection (Fig. 195-3). Antibodies can also bind to viral antigens on the surface of virions and infected cells and promote phagocytosis, antibody-dependent cytotoxicity, and complement-mediated lysis. T cells recognize viral peptides bound to major histocompatibility complex molecules on the surface of infected cells and produce cytokines that exert an antiviral effect or activate cellkilling mechanisms. Thus, the host’s adaptive immune responses can target either virions or infected cells and can clear infection. Long-Term Effects of Infection  Persistent infections can lead to continued pathology due to the ongoing immune response, but even acute infection can lead to long-term effects on the host. The chronic sequelae following SARS-CoV-2 infection or long COVID brought attention to this puzzling aspect of viral infection, but this type of viral pathogenesis had been long recognized in various forms of post-acute infection syndromes (PAIS). These included the chronic symptoms following infectious mononucleosis, or acute Epstein-Barr infection, post-dengue fatigue syndrome, and post-polio syndrome, among oth­ ers. The persisting symptoms may be the result of (1) persisting viral replication or antigens; (2) activation of autoimmune responses; (3) alteration of endogenous bacteria or viruses; and/or (4) irreparable tis­ sue damage. The large disease burden of long COVID and other forms of PAIS make this a priority for future studies of viral pathogenesis. VIRAL EVOLUTION Because viral RNA-dependent RNA polymerases are error-prone and most do not have editing functions, sequence changes are frequently introduced into their genomes. These alterations can lead to popula­ tions or swarms of viruses with divergent sequences among a viral population in an individual. Upon drug selection, immune pressure, or host restriction, preexisting variants can emerge as the new major form of a virus. Differences in replicative ability can lead to enrichment Antibody Entry Uncoating Synthesis of viral proteins Assembly of progeny virus Copying of viral nucleic acids T-cell Release Antibody

of more fit viruses and loss of less fit variants. This trend was observed in the COVID-19 pandemic as more fit variants became the dominant forms of SARS-CoV-2 in the population. Viruses with segmented genomes can undergo genome reassort­ ment in cells co-infected with two viral strains, the result being a new genetic composition for a given virus. For example, new segments can arise in influenza virus isolates thought to be reassortants between the extant human strains and animal or avian strains, such as those from porcine or avian species. This type of event is the cause of the major shifts in influenza viruses that occur periodically over a decade. These major changes due to reassortment and acquisition of a new genome segment are referred to as antigenic shift, as opposed to the small changes due to sequence variation, which are designated antigenic drift. Especially in DNA viruses but—under special circumstances—also in RNA viruses such as coronaviruses, viral genomes can undergo recombination between two strains of virus and generate recombinant genomes with new combinations of genes that may be more or less fit. Viral variants can acquire the ability to infect cells of new host spe­ cies or to jump species barriers. Zoonotic infection occurs when a virus spreads from animals to humans, as is thought to have occurred with both SARS-CoV-1 and SARS-CoV-2. The original viral ancestor of these viruses—probably endemic in bats—is thought to have spread to other animals sold in the markets of China, and viral variants then arose that could efficiently infect humans. Evolution of variants that could efficiently infect and be transmitted by humans as agents of respiratory infection led to the COVID-19 pandemic. MOLECULAR EPIDEMIOLOGY OF VIRUSES Several molecular techniques allow easy genotyping of virus isolates. Direct sequencing, analysis of polymorphisms in restriction endo­ nuclease cleavage sites, and polymerase chain reaction (PCR) analysis allow a search for genotypic markers in isolates, with sequencing being the most precise definition of a viral strain. When these types of tests are applied, some viruses (e.g., influenza virus and measles virus) are found to have mainly one strain prevalent in the population at a given time. Thus, only one virus strain spreads through the population. For other viruses, such as HIV or HSV, nearly every unrelated isolate can be differentiated by these tests, and many strains are latent and spread­ ing within the population and are evolving in parallel. With these molecular techniques, genotypic markers can be used to determine whether a virus has been transmitted from one individual to another. Genomic sequencing studies of SARS-CoV-2 have identified a num­ ber of major strains circulating at any given time. As new variants have arisen, each has become the dominant circulating strain. DETECTION AND QUANTIFICATION

OF VIRUSES Viruses and viral infections need to be detected and quantified for both clinical and scientific purposes. Diagnostic virology employs the scientific principles described above to detect viruses and evidence of infection in clinical samples, to define the type of virus present in a sample, and in some cases to quantify the amount of virus or the viral load in a patient. Scientific studies use these principles for detection and quantification of viruses in laboratory stocks and for measurement of viral replication. ■ ■DETECTION OF INFECTIOUS VIRUS Biologic assays must be used to detect and measure infectious virus. Infectivity can be measured as either the ability to infect animals and cause disease or the ability to infect cultured cells and cause CPE. For example, SARS-CoV-1 virus was first isolated by the introduction of an oropharyngeal swab sample into Vero cell cultures and detection of CPE. ■ ■DETECTION OF VIRAL PARTICLES, THEIR COMPONENTS, AND VIRAL GENE PRODUCTS Viral Particles  Electron microscopy (EM) must be used to visual­ ize virions directly, because viruses (other than the poxviruses) are smaller than the resolution of the light microscope. Virions can be visualized by EM with negative staining of the virions themselves or by transmission EM of infected cells. As stated above, SARS viral particles

were first visualized in sections of Vero cells infected with samples from patients. The cell culture supernatant showed coronavirus par­ ticles by negative-staining EM. The latter method has also been used to detect viral particles in stool during outbreaks of gastroenteritis. Antibodies specific for viral capsid proteins are often used in this assay to concentrate the virus and enhance its detection.

Viral Nucleic Acids  Viral nucleic acids are detected by amplifica­ tion methods involving PCR with specific primers, which amplifies very small numbers of viral nucleic acid molecules. These methods can use direct amplification of DNA in clinical samples to detect and quantify viral DNA genomes; alternatively, they can use reverse transcription of RNA followed by PCR to detect a DNA product in clinical samples as a means to detect viral RNA sequences. Multiple primers can be used in a multiplex reaction to detect multiple pathogens. The process of nucleic acid isolation, reverse transcription, and PCR has been automated, and high-throughput instruments measure the HIV load in serum samples. HSV-1 DNA can be measured in cerebrospinal fluid as a rapid assay for HSV encephalitis. These methods have also been transferred to rapid assays for point-of-care detection of viral genomes. Viral Antigens  Viral antigens can be detected by immunologic methods such as immunofluorescence and enzyme immunosorbent assay (EIA). Immunofluorescence involves fixation and permeabili­ zation of cells or tissues from clinical specimens and reaction with either (1) an antiviral antibody conjugated to a fluorophore (direct immunofluorescence) or (2) an antiviral antibody followed by an anti-immunoglobulin antibody conjugated to a fluorophore (indirect immunofluorescence), with detection of the fluorophore by fluores­ cence microscopy in either case. CHAPTER 195 The EIA entails the immobilization of an antiviral antibody on a substrate such as a microtiter well, incubation of the patient’s sample in the well, and further incubation with an antibody linked to an enzyme. The bound enzyme is then measured by production of a colored sub­ strate that can be read spectrophotometrically or detected in a rapid antigen test kit. Hemagglutination  Some viruses have the ability to cross-link and agglutinate red blood cells of specific species, a process called hemag­ glutination. Viral titer is measured by the inverse of the last dilution of the sample that causes hemagglutination. Quantitative Assays of Viruses  Viruses can be quantified in terms of virion particle numbers and/or infectivity. The number of virion particles in a sample can be determined by negative staining and observation by EM. The numbers of viral DNA genomes can be deter­ mined by PCR, and RNA genomes can be determined by reverse tran­ scriptase PCR (RT-PCR), as described above. Alternatively, purified viral particles can be quantified biochemically by spectrophotometric assays that measure viral protein. Principles of Medical Virology The number of infectious particles can be quantified by an endpoint dilution assay in which the virus is diluted until only one-half of cul­ tures are infected; this concentration is designated the tissue culture infectious dose for 50% of cultures, or TCID50. An alternative assay can determine at what dose one-half of experimental animals die of viral disease (lethal dose for 50% of test animals, or LD50). A more quantita­ tive assay of infectivity is the plaque assay. A plaque is an area of visual­ ized localized CPE. In the plaque assay, dilutions of the virus sample are placed on cells attached to a culture dish, and after adsorption of the virus to cells, the cells are overlaid with semisolid medium or medium containing antibody, which prevents virus diffusion through the medium. Virus then spreads only cell to cell, causing a restricted area of CPE—a plaque—on the cellular monolayer. The number of plaques formed by each dilution of virus defines the titer in plaque-forming units (PFUs) per volume of virus stock. For viruses that infect humans, the ratio of viral particles to infec­ tious units, or the particle-to-PFU ratio, is always much greater than 1—usually 10–1000. This result signifies a large excess of particles that are defective and/or that do not score as infectious in laboratory assays. Thus, for experimental purposes, following input virus particles, either visually or biochemically, does not guarantee that the observer is

84 - 196 Antiviral Chemotherapy, Excluding Antiretroviral Drugs

196 Antiviral Chemotherapy, Excluding Antiretroviral Drugs

following the real infection pathway. In addition, clinical preparations of viruses used for vaccines, vaccine vectors, gene therapy vectors, and oncolytic viruses need to be defined precisely and specifically in terms of particles versus infectious units for accurate and safe dosing. As an example, a recent adenovirus-based COVID vaccine was quantified on the basis of spectrophotometric measurement of purified virions. After the trial was initiated, lower than expected immune responses led to a reexamination of the vaccine dose. An excipient discovered in the vaccine was found to cause errors in spectrophotometric measure­ ment that led to an overestimate of the virus concentration. Parallel measurements of viral genomes with RT-PCR allowed a more accurate measurement of the vaccine vector batches, and the dose was revised to one-half of the original level. This example illustrates the importance of precise measurements of viral particles and infectious particles in preparations of viruses for clinical use. DETECTION OF VIRUS-SPECIFIC ANTIBODIES The presence of virus-specific antibodies provides evidence of prior infection with a virus or prior exposure to viral antigens through immunization; thus, antibody tests are extremely important clinically. The most common tests for antibodies are the enzyme-linked immuno­ sorbent assay (ELISA) and the Western blot or immunoblot assay. An ELISA involves the immobilization of viral antigen on a substrate such as a microtiter well, its incubation with the patient’s serum, and further incubation with an antibody to human IgG coupled to an enzyme. The amount of bound antibody is measured by detection of a colored product made by the bound enzyme. The Western blot assay involves the resolution of viral proteins in a polyacrylamide gel, their transfer to a membrane, incubation with the patient’s serum, and further incuba­ tion with antibody to human IgG coupled to an enzyme. Proteins with bound antibodies are detected as a colored product made by the bound antibody. The Western blot detects antigen of a specific size and there­ fore is more specific than ELISA. For example, HIV serologic testing involves high-throughput ELISA screening followed by a Western blot assay to confirm the specificity of any positive ELISA result.

PART 5 Infectious Diseases In a hemagglutination inhibition assay, antibodies specific for viral surface proteins are detected by their ability to block hemagglutination. IMMUNIZATION AGAINST VIRAL DISEASES Viral vaccines are among the most effective biomedical and public health measures that have been implemented: millions of deaths have been pre­ vented by their use. These vaccines are safe because extensive protocols have been developed for monitoring vaccine safety both before and after licensure. Historically, viral vaccines were based on either inactivated virus or live attenuated viruses, as exemplified by the Salk polio vaccine and the Sabin live attenuated polio vaccine, respectively. Both of these vaccines were quite successful, offering individual advantages. Further vaccine types have been developed, including those based on recombi­ nant proteins, viral vectors, and, most recently, mRNA. For each virus, the optimal antigen and immunization strategy must be developed on the basis of the virus-specific immune correlates, antibodies, or T cells needed for immunologic protection against infection and disease. These concepts are discussed in greater detail in Chap. 129. ANTIVIRAL THERAPEUTICS ■ ■ANTIVIRAL DRUGS Viruses replicate in human cells and use much of the host cell’s machin­ ery. Therefore, antiviral drugs must target virus-specific events to optimize safety. Viral targets for drugs have been identified in studies of the mechanisms of viral infection and replication (Chap. 196). Many of the most successful antiviral drugs target viral enzymes; examples include the anti-HSV drugs that target the virus DNA polymerases and thymidine kinase (Chap. 196) and the HIV drugs that target the virus reverse transcriptase, protease, and integrase (Chap. 208). ■ ■VIRUSES AS THERAPEUTICS Viruses have been engineered for a number of medical purposes, including gene delivery and tumor cell killing. As described above,

viruses have been developed as vaccines and vaccine vectors. For exam­ ple, vesicular stomatitis virus–based vectors have been employed as Ebola vaccines. Adenovirus-based vectors have been used as AIDS vac­ cine vectors and have been used as COVID-19 vaccine vectors. Viral recombinants, including those of retroviruses and adeno-associated viruses, have been approved as vectors for delivery of genes to cells for treatment of single-gene defects. Retroviruses integrate into the cell’s chromosomes and are maintained with stable expression of the trans­ gene, although some concerns have arisen about possible activation of neighboring promoters and adverse effects due to that activation. Adeno-associated viruses are not integrated but are stably maintained and capable of durable expression of the transgene. Adenoviruses and herpesviruses are also being tested as gene therapy vectors. Finally, an attenuated strain of HSV expressing granulocyte-macrophage colonystimulating factor has been approved for treatment of melanoma because of its oncolytic and immunotherapeutic properties. Many additional studies are assessing viruses for use as vectors and for immu­ notherapeutic and oncolytic applications. SUMMARY As obligate intracellular parasites, viruses enter host cells, replicate, and spread in the form of progeny viruses. Injury to the host cell resulting from viral entry may lead to tissue and organ damage. Basic knowledge of the mechanisms underlying infection by and replication of viruses that infect humans is the foundation for medical studies of viral pathogenesis, viral vaccines, antiviral drugs, and the use of viruses as therapeutics. There are many interactions between different viruses; thus, a broad knowledge of all viruses is essential to our preparedness for the next viral epidemic or pandemic. ■ ■FURTHER READING Choutka J et al: Unexplained post-acute infection syndromes. Nat Med 28:911, 2022. Howley PM et al (eds): Fields Virology: Vol. 4: Fundamentals, 7th ed. Philadelphia, Wolters Kluwer/Lippincott Williams & Wilkins Health, 2023. Knipe DM et al: Ensuring vaccine safety. Science 370:1274, 2020. Ksiazek TG et al: A novel coronavirus associated with severe acute respiratory syndrome. N Engl J Med 348:1953, 2003. Sodroski CN, Knipe DM: Nuclear interferon-stimulated gene product maintains heterochromatin on the herpes simplex viral genome to limit lytic infection. Proc Natl Acad Sci USA 120:e2310996120, 2023. Voysey M et al: Safety and efficacy of the ChAdOx1 nCoV-19 vaccine (AZD1222) against SARS-CoV-2: An interim analysis of four ran­ domised controlled trials in Brazil, South Africa, and the UK. Lancet 397:99, 2021. Zhou P et al: A pneumonia outbreak associated with a new coronavi­ rus of probable bat origin. Nature 579:270, 2020. Jeffrey I. Cohen, Marc G. Ghany

Antiviral Chemotherapy, Excluding Antiretroviral

Drugs Most antiviral drugs inhibit viral DNA or RNA replication, but other activities, such as virus entry, viral RNA transcription, cleavage of pro­ teins by the viral protease, virus uncoating after infection, and virus release from cells, are all targeted by different licensed antiviral agents. Inhibition of viral replication does not eliminate the virus in the cell; host cell immune responses are important for viral clearance. Antiviral

drugs usually do not eradicate latent viral infections but instead inhibit viral replication; thus, when treatment is stopped, the virus can reac­ tivate and replicate again. Resistance to antiviral agents due to muta­ tions in viral proteins is not uncommon and is more common for RNA viruses with a higher mutation rate than for DNA viruses. This differ­ ence may explain the observation that drug-resistant DNA viruses are a greater problem in immunocompromised patients, whereas drugresistant RNA viruses can be found in healthy persons as well. Patients may harbor a mixture of drug-resistant and drug-sensitive viruses that is dynamic and changes under pressure from the drug. Combina­ tion therapy with more than one antiviral agent, each with a differ­ ent mechanism of action, may be more effective than monotherapy, particularly against RNA viruses, which may be present as mixtures with different resistance patterns. Antiviral testing can be performed in patients who do not respond to antiviral drugs or whose response diminishes. For some viruses, such testing involves the sequencing of selected viral genes; however, in many cases, it involves the growth of virus in the presence of different concentrations of the drug, which is a laborious, time-consuming process. Response to antiviral therapy has traditionally been assessed clinically, but quantitative PCR has been useful in monitoring the response to therapy for viruses that circulate in the blood (e.g., cytomegalovirus [CMV], hepatitis B and C viruses [HBV and HCV, respectively]). Systemic therapy with antivirals is usu­ ally more effective than topical therapy but is more commonly associ­ ated with side effects. ANTIVIRAL DRUGS FOR HERPESVIRUS INFECTIONS ■ ■ACYCLOVIR, VALACYCLOVIR, FAMCICLOVIR, AND PENCICLOVIR Acyclovir is an analogue of deoxyguanosine and is phosphorylated to the monophosphate form by viral thymidine kinase in cells infected with herpes simplex virus (HSV) or varicella-zoster virus (VZV). Cellular protein kinases further phosphorylate the drug to the active triphosphate form, which inhibits viral DNA polymerase; the drug is incorporated into viral DNA to terminate its replication. Valacyclovir, a valine ester of acyclovir, is absorbed much better than acyclovir; its rapid conversion to acyclovir in the liver and intestine results in plasma acyclovir levels approximately four times higher than are attained with oral acyclovir. Acyclovir and valacyclovir are approved by the U.S. Food and Drug Administration (FDA) for treatment of initial episodes of genital herpes, recurrent genital herpes, varicella, and zoster (Table 196-1). Valacyclovir is also approved for treatment of herpes labialis (cold sores), for suppres­ sion of recurrences of genital herpes, and for reduction of transmission of genital HSV. The doses of acyclovir and valacyclovir used for treating VZV infections are higher than those used for HSV infections since VZV is less susceptible to inhibition by these drugs. Both drugs exhibit poor activity against CMV. Intravenous acyclovir is used for severe dis­ ease requiring hospitalization; oral acyclovir or valacyclovir is used for outpatient therapy; and topical acyclovir, penciclovir, and docosanol are approved for treatment of orolabial herpes but are much less effective than the oral drugs. Acyclovir is excreted by the kidneys. Thus the dose of acyclovir or valacyclovir needs to be reduced with renal insufficiency. Central nervous system (CNS) side effects that occur with IV acyclovir or oral valacyclovir are more common with the higher drug levels seen in persons with renal insufficiency. Reversible renal insufficiency due to crystallization of the drug in renal tubules can occur with IV acyclovir, especially in persons who are dehydrated. Headache, nausea, rash, and diarrhea have been reported with acyclovir. Mutations in the HSV or VZV thymidine kinase or, less commonly, in viral DNA polymerase can result in resistance to acyclovir or valacyclovir. Viruses lacking thymidine kinase activity are also resistant to famciclovir and ganci­ clovir. Acyclovir- and valacyclovir-resistant HSV and VZV are rare in immunocompetent persons. Resistant virus is treated with foscarnet or, less commonly, cidofovir. Mucosal disease due to resistant virus in immunocompromised persons is sometimes treated with topical foscarnet, trifluridine, or cidofovir.

Famciclovir is a diacetyl ester of penciclovir that is converted to penciclovir in the intestine and liver. Penciclovir is a guanosine analogue that is less potent than acyclovir, but, because of its longer intracellular half-life, its activity is similar to that of acyclovir. Pen­ ciclovir is phosphorylated by HSV and VZV thymidine and cellular kinases and has activity similar to that of acyclovir for HSV and VZV infections. Famciclovir is approved for treatment of zoster, suppres­ sion of genital herpes, and treatment of recurrent mucocutaneous herpes in patients with HIV infection. Famciclovir is excreted by the kidneys, and the dose is adjusted for renal insufficiency. Side effects are uncommon and can include headache, nausea, and diar­ rhea. Resistance due to mutations in viral thymidine kinase or DNA polymerase can occur.

Oral acyclovir reduces the duration of pain and other symptoms, time to healing, and shedding in patients with their first episode of genital herpes when treatment is begun within 6 days of infection. Acyclovir, valacyclovir, and famciclovir are all effective for treatment of primary and recurrent genital and orolabial herpes as well as for sup­ pressive therapy for these conditions. Topical acyclovir cream reduces shedding and time to healing by 1–2 days if given within 1 day of symp­ tom onset in persons with recurrent genital or orolabial herpes. Oral acyclovir or valacyclovir reduces the severity of varicella when given within 1 day of onset of the rash. Oral acyclovir, famciclovir, or vala­ cyclovir shortens the duration of pain and rash associated with zoster if given within 3 days of onset. Oral valacyclovir is more effective than oral acyclovir and is generally preferred since it has better oral bioavail­ ability and does not need to be given as frequently. Suppressive vala­ cyclovir therapy for genital herpes reduces transmission to uninfected partners by 50%. Intravenous acyclovir is used for herpes encephalitis and disseminated HSV or VZV disease. CHAPTER 196 ■ ■GANCICLOVIR AND VALGANCICLOVIR Ganciclovir is a deoxyguanosine analog that is phosphorylated by UL97 protein kinase in cells infected with CMV and converted to its active form, ganciclovir triphosphate, by cellular protein kinases. Ganciclovir triphosphate inhibits both viral DNA polymerase and incorporation of guanosine triphosphate into viral DNA. Valganciclovir is a valine ester of ganciclovir and is converted to ganciclovir in the liver and intestine. Valganciclovir has much better oral bioavailability than ganciclovir; plasma levels of oral valganciclovir and IV ganciclovir are similar. Ganciclovir and valganciclovir are used for treatment and prevention of CMV disease in immunocompromised patients and are approved for prevention of CMV infection in transplant recipients and for treatment of CMV retinitis. Ganciclovir is effective against HSV, VZV, human herpesvirus type 6 (HHV-6), and herpes B virus. This drug is excreted by the kidneys, and dose adjustment is required in renal insufficiency. Ganciclovir therapy often results in neutropenia and thrombocytope­ nia after 1 week. Less commonly, ganciclovir has been associated with CNS symptoms, particularly at high plasma drug levels. Mutations in CMV UL97 protein kinase or, less commonly, UL54 viral DNA poly­ merase can result in resistance to ganciclovir or valganciclovir. CMV with mutations in protein kinase is usually sensitive to foscarnet and cidofovir, while CMV with mutations in both protein kinase and DNA polymerase is usually sensitive only to foscarnet. Mutations are more common among persons who are highly immunocompromised and who have been taking the drug for a long time. Resistant virus is treated with foscarnet or cidofovir. Antiviral Chemotherapy, Excluding Antiretroviral Drugs Ganciclovir and valganciclovir are used for treating severe CMV infections in immunocompromised patients, including colitis, pneu­ monitis, retinitis, and encephalitis. Induction therapy, given two or three times daily, is usually followed by less frequently administered maintenance therapy. Oral valganciclovir has activity similar to that of intravenous ganciclovir. Ganciclovir and valganciclovir are used for prevention of CMV infection in transplant recipients when given either preemptively (on the basis of viremia) or prophylactically. Ganciclovir reduces developmental delay in infants with congenital CMV disease involving the CNS and reduces hearing loss in infants with asymptom­ atic congenital CMV infection. Ganciclovir and valganciclovir are used for treatment of HHV-6 encephalitis, HHV-8–associated Castleman

TABLE 196-1  Antiviral Drugs for Herpesvirus Treatment and Prophylaxis in Adults DISEASE DRUG ROUTE ADULT DOSE COMMENTS Orolabial herpes, primary episode Acyclovir Valacyclovir Famciclovir Oral Oral Oral 400 mg tid × 7–10 d 1 g bid × 7–10 d 500 mg bid or 250 mg tid × 7–10 d Orolabial herpes, recurrence Acyclovir Valacyclovir Famciclovir Oral Oral Oral 400 mg 5 times daily × 5 d 2 g bid × 1 d 1500 mg × 1 d Orolabial herpes, suppression Acyclovir Valacyclovir Famciclovir Oral Oral Oral 400 mg bid 500 mg or 1 g once daily 500 mg bid Genital herpes, primary episode Acyclovir Valacyclovir Famciclovir Oral Oral Oral 400 mg tid or 200 mg 5 times daily × 7–10 d 1 g bid × 7–10 d 250 mg tid × 7–10 d Genital herpes, recurrence Acyclovir Valacyclovir Famciclovir Oral Oral Oral 800 mg tid × 2 d or 400 mg tid × 5 d 500 mg bid × 3 d or 1 g daily × 5 d 500 mg once, then 250 mg bid × 2 d Genital herpes suppression Acyclovir Valacyclovir Famciclovir Oral Oral Oral 400 mg bid 250 mg bid 500 mg to 1 g daily HSV encephalitis Acyclovir IV 10–15 mg/kg q8h × 14–21 d Reduces mortality and sequelae HSV keratitis Acyclovir Trifluridine Vidarabine Topical Topical Topical 3% ophthalmic ointment, 5 times daily 1% ophthalmic solution, 1 drop q2h when awake

(9 drops daily max) 3% ointment, 0.5-inch ribbon 5 times daily Mucocutaneous herpes in immunocompromised patient Acyclovir Valacyclovir Famciclovir IV Oral Oral 5 mg/kg q8h × 7–14 d 500 mg to 1 g bid × 7–10 d 500 mg bid × 7–10 d PART 5 Infectious Diseases Varicella Acyclovir Valacyclovir Oral Oral 20 mg/kg (800 mg max) 5 times daily × 5 d 20 mg/kg (1 g max) tid × 5 d Zoster Acyclovir Valacyclovir Famciclovir Oral Oral Oral 800 mg 5 times daily × 7 d 1 g tid × 7 d 500 mg tid × 7 d Varicella or zoster, disseminated Acyclovir IV 10 mg/kg q8h × 7 d Reduces time for last new lesion formation and virus shedding; reduces cutaneous dissemination Cytomegalovirus disease Ganciclovir IV 5 mg/kg q12h × 14–21 d, then 5 mg/kg daily (maintenance dose) 900 mg bid × 14–21 d, then 90 mg daily (maintenance dose) 60 mg/kg q8h × 14–21 d, then 90–120 mg daily (maintenance dose) 5 mg/kg once weekly twice, then every other week 400 mg bid  Valganciclovir Oral Foscarnet IV Cidofovir Maribavir IV   Oral Cytomegalovirus prophylaxis Letermovir Oral IV 480 mg qd 480 mg qd disease in patients with poorly controlled HIV infection, and severe HSV or VZV disease when acyclovir is unavailable. ■ ■FOSCARNET Foscarnet is a pyrophosphate analogue that directly inhibits herpesvi­ rus DNA polymerases by blocking the pyrophosphate binding site in the enzyme. Foscarnet does not require additional phosphorylation (unlike acyclovir, cidofovir, or ganciclovir) in virus-infected cells for its activity. This drug is approved for treatment of CMV retinitis and mucocutaneous acyclovir-resistant HSV disease. It is also used to treat ganciclovir-resistant CMV and acyclovir-resistant VZV. Foscarnet is given intravenously and is excreted by the kidneys; dose adjustment is required in renal insufficiency. Up to one-third of patients receiving foscarnet develop nephrotoxicity with elevated levels of creatinine and

Reduces duration of fever, lesions, and virus shedding Reduces duration of lesions by 1–2 d if given during prodrome In patients with >6 recurrences per year, reduces number of recurrences by ~50% and increases time to first recurrence Reduces duration of symptoms, genital lesions, and virus shedding by 2, 4, and 7 d, respectively Reduces duration of symptoms, genital lesions, and virus shedding by 1–2 d In patients with >6 recurrences per year, reduces recurrence rates from 80–85% to 25–30%, reduces virus shedding and transmission Shortens duration of disease; acyclovir better tolerated, especially with prolonged treatment IV acyclovir reduces time to healing, duration of pain, and duration of virus shedding Has modest effect on symptoms, reduces fever duration by 1 day Reduces time for last new lesion formation, virus shedding, and pain duration Neutropenia and thrombocytopenia common after 1 week Levels and side effects similar to ganciclovir Nephrotoxicity, electrolyte abnormalities; give with additional saline Nephrotoxicity; give with probenecid and saline Used for disease refractory to ganciclovir, foscarnet, or cidofovir Antagonizes activity of ganciclovir Numerous drug interactions  Numerous drug interactions Given 100 days after stem cell transplant, 200 days after kidney transplant blood urea nitrogen, and proteinuria. Renal tubular acidosis and inter­ stitial nephritis also have been reported. Renal insufficiency is more common among persons who are dehydrated, given other nephrotoxic drugs, or given high doses or rapid infusions of foscarnet. Administer­ ing IV saline before and after each foscarnet dose and giving the drug over an adequate period can reduce nephrotoxicity. Renal insufficiency is often reversible after treatment when the drug is stopped. Other side effects include hypomagnesemia and hypocalcemia, which can be asso­ ciated with arrhythmias, paresthesias, and seizures. Other metabolic abnormalities include hypokalemia, hypophosphatemia, or hyper­ phosphatemia. Foscarnet can also cause headache, fever, rash, diar­ rhea, acute dystonia, tremors, hemorrhagic cystitis, genital ulcerations, anemia, and abnormal liver function values. Mutations in CMV DNA polymerase (UL54) or in HSV or VZV DNA polymerase can result

in resistance to foscarnet. CMV, HSV, and VZV can become resistant to foscarnet; some strains of CMV are resistant to foscarnet, ganci­ clovir, and cidofovir; and HSV can become resistant to acyclovir and foscarnet. Foscarnet is typically used to treat CMV retinitis, HHV-6 encephalitis, or drug-resistant severe CMV, HSV, or VZV infections in immunocompromised patients. Topical foscarnet has been used to treat acyclovir-resistant mucosal infections due to HSV. ■ ■CIDOFOVIR Cidofovir is an analogue of deoxycytidine monophosphate and is phos­ phorylated in cells to its active diphosphate form. The diphosphate form of cidofovir competes with deoxycytidine triphosphate for incorpora­ tion into herpesvirus DNA. The drug inhibits replication of all human herpesviruses as well as poxviruses, papillomaviruses, polyomaviruses, and adenoviruses. Cidofovir is approved for treatment of CMV retinitis in patients with AIDS; it is also used for treatment of infections caused by CMV exhibiting ganciclovir resistance due to mutations in UL97 protein kinase and those caused by HSV or VZV displaying mutations in thymidine kinase. Because cidofovir is excreted by the kidneys, dose adjustment is required in renal insufficiency. About one-fifth of patients receiving cidofovir develop nephrotoxicity, and the drug is associated with metabolic acidosis and glucosuria. Cidofovir therapy is preceded by at least 1 L of saline, and probenecid is given 3 h before, 2 h after, and 8 h after each dose to reduce nephrotoxicity. An addi­ tional 1 L of saline is recommended during treatment or immediately thereafter. About one-fourth of patients receiving cidofovir develop neutropenia; additional side effects include ocular hypotony, uveitis, iritis, headache, nausea, vomiting, diarrhea, and rash. Mutations in CMV DNA polymerase (UL54) or HSV DNA polymerase can result in resistance to cidofovir. Some strains of CMV exhibiting ganciclovir resistance due to mutations in viral DNA polymerase are resistant to cidofovir, whereas many CMV and HSV strains exhibiting foscarnet resistance due to mutations in DNA polymerase may retain sensitiv­ ity to cidofovir. Cidofovir is typically used to treat ganciclovir- and/or foscarnet-resistant severe CMV disease or acyclovir- and/or foscarnetresistant HSV disease in immunocompromised patients. Cidofovir has been used as preemptive therapy against CMV infection in transplant recipients. It has also been used to treat severe adenovirus infections, adenovirus or BK virus hemorrhagic cystitis, BK nephropathy, and severe molluscum contagiosum, although controlled studies have not been performed. Topical cidofovir has been used to treat acyclovirresistant HSV mucosal infections and anogenital warts. ■ ■LETERMOVIR Letermovir is a dihydroquinazolin that inhibits the CMV DNA ter­ minase complex (UL51, UL59), which is required for cleavage and packaging of CMV into nucleocapsids. The drug has no activity against other human herpesviruses. Letermovir is approved for prophylaxis of CMV infection and disease in adult CMV-seropositive recipients of an allogeneic hematopoietic stem cell transplant or donor CMV-positive, recipient CMV-negative kidney transplant recipients. Letermovir is metabolized by the liver and excreted in the feces; dose adjustment is not required if the creatinine clearance rate (CrCl) is >10 mL/min. The dose of letermovir must be decreased in persons taking cyclospo­ rine. Letermovir therapy results in reduced levels of voriconazole and increased levels of sirolimus, tacrolimus, cyclosporine, and other drugs metabolized by CYP2C8 or transported by OAT1B1/3. Side effects of letermovir include headache, nausea, diarrhea, and peripheral edema. Letermovir does not cause nephrotoxicity and is not myelosuppressive. Resistance to letermovir occurs more frequently in vitro than resistance to ganciclovir or foscarnet, and clinically significant letermovir resis­ tance due to mutations in UL56 in patients with CMV disease has been reported; resistance may be less common when the drug is used for pro­ phylaxis in patients with low or undetectable CMV levels. When given to CMV-seropositive patients, starting a median of 8 days after hemato­ poietic stem cell transplantation and continuing for 14 weeks, letermo­ vir reduced the incidence of clinically significant CMV infection by 38% compared with placebo. While anecdotes describe the use of letermovir for treatment of CMV disease, resistance may develop quickly.

■ ■MARIBAVIR Maribavir is a benzimidazole that inhibits the CMV UL97 protein kinase and CMV replication, and reduces the egress of viral particles from the nucleus. Maribavir is approved for treating adults and chil­ dren with posttransplant CMV infection/disease that is refractory to treatment (with or without proven genotype resistance) with ganciclo­ vir, cidofovir, or foscarnet. Since maribavir inhibits the UL97 protein kinase, it can antagonize ganciclovir and should not be given with the latter drug. Resistance to maribavir has been reported, and such CMV strains are also resistant to ganciclovir. Maribavir can increase concen­ trations of several drugs including tacrolimus, sirolimus, everolimus, and cyclosporine. The most common side effect of maribavir is taste disturbance.

■ ■TRIFLURIDINE AND VIDARABINE Trifluridine is a thymidine analogue that is incorporated into viral DNA and inhibits its synthesis. Vidarabine is approved for topical therapy of herpes keratitis and has also been used topically to treat acyclovir-resistant mucosal HSV infections. Trifluridine is active against acyclovir-resistant HSV, CMV, and vaccinia virus. Vidarabine is an adenosine analogue that is incorporated into viral DNA and inhibits viral DNA polymerase. Both trifluridine and vidarabine are used for topical therapy only. ■ ■INVESTIGATIONAL AND OTHER AGENTS Brincidofovir is a phospholipid conjugate of cidofovir that is rapidly taken up by cells and converted into cidofovir. It is active against herpesviruses (including most strains of ganciclovir-resistant CMV), poxviruses, adenovirus, and polyomaviruses. It does not cause neph­ rotoxicity and is not myelosuppressive. Diarrhea is the most common side effect. The drug has been associated with intestinal toxicity and acute graft-versus-host disease of the gastrointestinal tract. The drug did not meet its primary endpoints in trials for adenovirus disease or CMV prophylaxis. At the time of this writing, it is not available as part of an expanded access program. An intravenous formulation that, it is hoped, will cause less gastrointestinal toxicity is being tested for adenovirus viremia. CHAPTER 196 Antiviral Chemotherapy, Excluding Antiretroviral Drugs Pritelivir inhibits the helicase–primase complex required for repli­ cation of HSV. This drug has reduced viral shedding in patients with recurrent genital herpes and is being tested for use against acyclovirresistant HSV mucocutaneous infection. Pritelivir is available as an expanded access drug for acyclovir-resistant HSV infection. Amenamevir is a helicase–primase inhibitor under development for HSV and VZV infections. ANTIVIRAL DRUGS FOR RESPIRATORY VIRUS INFECTIONS ■ ■INFLUENZA Neuraminidase Inhibitors  Oseltamivir, zanamivir, and perami­ vir are neuraminidase inhibitors that inhibit cleavage of sialic acid, which is required for the release of influenza virus from infected cells and its spread to other cells. Oseltamivir phosphate is an oral prodrug that is cleaved by esterases in the liver, gastrointestinal tract, and blood to oseltamivir carboxyl­ ate, the more active form. It is approved for treatment of uncompli­ cated influenza A or B disease when given ≤48 h after symptom onset and for prophylaxis of influenza A and B in persons ≥1 year of age (Table 196-2). Oseltamivir is much less active against influenza B

than against influenza A. The drug is excreted by the kidneys, and the dose is adjusted in renal insufficiency. The most common side effects are nausea, abdominal pain, and vomiting. Although CNS side effects have been reported, particularly in children, it is unclear whether they are due to the drug or to influenza virus infection itself. Resis­ tance to oseltamivir can develop as a result of mutations in the viral neuraminidase or in the hemagglutinin. Oseltamivir-resistant virus has been transmitted from person to person. Resistance has been reported in ~15% of healthy children and ~1% of adults; resistance is more common among immunocompromised persons.

TABLE 196-2  Antiviral Drugs for Respiratory Virus Treatment and Prophylaxis in Adults DISEASE DRUG ROUTE ADULT DOSE COMMENTS Influenza A, B Oseltamivir Oral Treatment: 75 mg bid × 5 d Prophylaxis: 75 mg/d Influenza A, B Zanamivir Inhaled Treatment: 10 mg bid × 5 d Prophylaxis: 10 mg/d Influenza A, B Peramivir IV 600 mg once Shortens duration of symptoms by 1–2 d when given within 2 d of onset Influenza A, B Baloxavir Oral Treatment or postexposure prophylaxis: 40 mg once; if

80 kg, 80 mg once Influenza A Amantadine Oral Treatment: 100 mg bid × 5 d Prophylaxis: 200 mg/d Influenza A Rimantadine Oral Treatment: 100 mg bid × 5 d Prophylaxis: 200 mg/d Respiratory syncytial virus Ribavirin Inhaled Aerosol from reservoir containing 20 mg/mL for

12–18 h/d × 3–7 d SARS-CoV-2 Nirmatrelvir/ ritonavir Oral 300 mg/100 mg bid × 5 days Reduces rate of hospitalization by 50% within 30 days after diagnosis SARS-CoV-2 Remdesivir IV 200 mg on day 1, then 100 mg qd × 2 d for outpatients, × 4 days for inpatients SARS-CoV-2 Molnupiravir Oral 800 mg q12h × 5 days Recommended for patients when nirmatrelvir or remdesvir are not available, unable to be used, or not appropriate. Approved under EUA. Abbreviation: EUA, emergency use authorization by the U.S. Food and Drug Administration (FDA). PART 5 Infectious Diseases Zanamivir is approved for treatment of uncomplicated influenza A and B in adults and children ≥7 years of age who have had symptoms for ≤2 days and for prophylaxis in persons ≥5 years of age. Because zanamivir has poor oral bioavailability, it is given as a powder through an inhaler. Thus, use of the drug can be difficult for young children and some elderly patients. Inhalation of zanamivir may cause bron­ chospasm, particularly in persons with underlying lung disease; it is not recommended for persons with asthma, chronic obstructive pul­ monary disease, or other airway disease. Zanamivir is more active than oseltamivir against influenza B. It is also active against some isolates of influenza virus that are resistant to oseltamivir; resistance to zanamivir is less common than that to oseltamivir. Peramivir is approved for treating uncomplicated influenza in patients ≥2 years of age who have had symptoms for ≤2 days. Because of its long half-life, it is given as a single IV dose. Peramivir is highly active against both influenza A and B. The drug is excreted by the kid­ neys, and the dose is adjusted in renal insufficiency. The most common side effect is diarrhea. While peramivir-resistant virus is rare in healthy persons, peramivir-resistant virus has been isolated from immuno­ compromised persons. Oseltamivir, zanamivir, and peramivir are effective for treatment of uncomplicated influenza A and B, including disease caused by avian influenza viruses (e.g., H5N1, H7N9, and H9N2). None of the neuraminidase inhibitors is approved by the FDA for complicated influenza or for persons requiring hospitalization for the disease. While not licensed for the treatment of persons with complicated disease, inpatients, and pregnant women, oseltamivir is considered the drug of choice in these settings. The efficacy of zanamivir is similar to that of oseltamivir in hospitalized patients. Treatment is most effective when begun within 2 days of symptom onset and should be started as early as possible; such early treatment reduces symptoms by ~1 day in per­ sons with uncomplicated disease. For persons with influenza requiring hospitalization and with pneumonia, treatment with oseltamivir or zanamivir is recommended even later. Treatment may reduce the risk of complications and death in hospitalized patients with influenza. Oseltamivir and zanamivir (but not peramivir) are approved for prophylaxis of influenza, especially in institutions where outbreaks can be severe, and for prophylaxis in persons who have been exposed to the virus, are at high risk for disease complications, and have not recently been vaccinated. The efficacy of oseltamivir and zanamivir for

Shortens duration of symptoms by 1 d when given within 2 d of onset; reduces complications; considered drug of choice for patients with complications of influenza Shortens duration of symptoms by 1–2 d when given within 2 d of onset; requires patient training for use; can cause bronchospasm; not recommended for persons with asthma or chronic obstructive pulmonary disease Shortens duration of symptoms by 1 d when given within 2 d of onset; active against virus resistant to neuraminidase inhibitors Most influenza virus strains are resistant; use only if virus is known to be sensitive. Most influenza virus strains are resistant; use only if virus is known to be sensitive. Reduces severity of symptoms in hospitalized infants with lower respiratory tract disease; anecdotal reports of reduced progression to lower respiratory tract disease and mortality in stem cell transplant patients Reduces duration of hospitalization in some studies. Duration of treatment extended up to 10 days if no improvement. prophylaxis is estimated to be ~70–90%. For persons at institutions, prophylaxis is given for at least 2 weeks and for up to 1 week after out­ breaks resolve. For other high-risk persons, prophylaxis is given within 2 days of exposure and continued for 1 week after exposure. Since neuraminidase inhibitors reduce virus release from cells, they should not be given 2 days before or within 2 weeks after receipt of live, attenu­ ated influenza vaccine. Resistance has been reported during treatment with oseltamivir or peramivir, especially in immunocompromised persons; oseltamivir-resistant viruses are usually sensitive to zanamivir. Baloxavir  Baloxavir inhibits the cap-dependent endonuclease that is important in initiating synthesis of influenza virus mRNA. This drug is approved by the FDA as a single oral dose for postexposure pro­ phylaxis of influenza and for treatment of uncomplicated influenza in persons ≥12 years of age who have had symptoms for ≤48 h. Baloxavir inhibits influenza A and B viruses, including avian strains and strains that are resistant to neuraminidase inhibitors. The drug’s efficacy is similar to that of the neuraminidase inhibitors in persons with uncom­ plicated influenza and reduces symptoms by ~1 day. In addition, baloxa­ vir exhibits efficacy similar to that of oseltamivir for reducing symptoms in high-risk patients. However, its effectiveness in patients hospitalized with complications of influenza is unknown. Reduced sensitivity of influenza virus to baloxavir has been associated with mutations in the viral polymerase acidic protein after one dose. The incidences of nausea and vomiting are lower with baloxavir than with oseltamivir. Levels of the drug are lower if it is taken with dairy products, polyvalent cation–containing laxatives or antacids, or oral supplements containing calcium, iron, magnesium, selenium, or zinc. Since baloxavir reduces virus replication, it should not be given 2 days before or within 2 weeks after receipt of live, attenuated influenza vaccine. Adamantanes  Amantadine and rimantadine inhibit the influenza virus’s M2 protein and its uncoating and membrane fusion. While these drugs are active against influenza A, resistance is widespread and can develop rapidly; thus, the adamantanes are not recommended as treatment or prophylaxis for influenza unless the virus is known to be sensitive. ■ ■RESPIRATORY SYNCYTIAL VIRUS Ribavirin  Ribavirin is an analogue of guanosine and inhibits replication of numerous RNA and DNA viruses. The drug inhibits

viral RNA synthesis and capping of viral mRNA and in some cases increases the viral RNA mutation rate to lethal levels for some viruses. Ribavirin inhibits replication of respiratory syncytial virus (RSV), influenza virus, parainfluenza virus, and many other RNA viruses in vitro. While the drug has been used to treat numerous viral infections, including Lassa fever and hepatitis E, it is approved by the FDA only for use against RSV and as a component of combination therapy for hepatitis C. Aerosolized ribavirin is approved for treatment of hospi­ talized infants and young children with severe lower respiratory tract infections due to RSV; it is given for 12–18 h per day and is most effec­ tive when used early in the course of these severe infections. Ribavirin is given in a generator that yields an aerosol of particles small enough to reach the lower respiratory tract; the level of systemic absorption is low. The aerosolized form of the drug can induce bronchospasm, sud­ den deterioration of respiratory function (especially in infants), and rash and can precipitate in ventilators, interfering with their function. Ribavirin is mutagenic and teratogenic in animals; accordingly, it is not recommended for use in pregnant women, and the exposure of health care workers should be minimized with personal protective equipment. In early studies, ribavirin reduced the shedding of RSV and the severity of symptoms in hospitalized infants with lower respiratory tract disease who were not on mechanical ventilation, the duration of oxygen sup­ plementation, and the duration of time on mechanical ventilation in infants. More recent analyses of the literature suggest that the efficacy of the drug in these settings is much less certain, and the drug is not recommended for routine use by the American Academy of Pediatrics. In retrospective studies, ribavirin has been reported to reduce the risk of progression of RSV from upper to lower respiratory tract disease in stem cell transplant recipients and to reduce mortality rates in these patients. In a retrospective study, the outcome of treatment with oral ribavirin was similar to that obtained with the aerosolized drug in hematopoietic stem cell transplant recipients with RSV disease. Riba­ virin has not been shown to affect the clinical course of patients with parainfluenza and is not recommended for their treatment. Aerosol­ ized ribavirin costs more than $25,000 per day. Nirsevimab-alip  Nirsevimab-alip, a human monoclonal antibody that targets the prefusion form of the RSV F protein, is approved for prevention of RSV lower respiratory tract disease in neonates and infants born during or entering their first RSV season, and for children up to 24 months who are vulnerable to severe RSV disease through their second RSV season. Palivizumab  Palivizumab, a humanized monoclonal antibody to RSV F protein, is approved for prevention of lower respiratory tract dis­ ease due to RSV in pediatric patients at high risk of RSV disease, includ­ ing premature infants and children with bronchopulmonary dysplasia. ■ ■SARS-COV-2 (SEE CHAP. 204) Remdesivir is converted in cells to an adenosine triphosphate analogue that inhibits the RNA-dependent RNA polymerase of several viruses. The drug is approved by the FDA for treatment of persons ≥12 years of age with SARS-CoV-2 requiring hospitalization; it shortens the dura­ tion of hospitalization in persons with lower respiratory tract disease. While the results of studies with the drug vary, it is recommended by the National Institutes of Health (NIH) for patients with SARS-CoV-2 who require supplemental oxygen while hospitalized. The drug is given intravenously and is not recommended in persons with a glomerular filtration rate (GFR) <30 mL/min. Serum transaminase elevations have been reported in healthy persons receiving remdesivir, and liver enzymes should be monitored before and during treatment. Chloro­ quine inhibits the activity of remdesivir in vitro; hydroxychloroquine or chloroquine phosphate should not be given with remdesivir. Nirmatrelvir, a SARS-CoV-2 main protease inhibitor, boosted with ritonavir, a CYP3 and HIV protease inhibitor, is approved for treatment of mild to moderate COVID-19 in adults at high risk for progression to severe COVID-19. The drug should be given as soon as possible after infection and within 5 days of onset of symptoms. The dose should not be given with drugs highly dependent on CYP3A in which elevated lev­ els can be associated with severe reactions, such as statins, sirolomus,

tacrolimus, colchicine, and many other drugs. Use of the drug with medications that induce CYP3A can reduce the levels of nirmatrelivir or ritonavir with loss of effectiveness. The dose should be reduced for renal impairment.

Molnupiravir is an oral ribonucleoside analogue that inhibits rep­ lication of SARS-CoV-2. The drug reduced the risk of hospitalization or death in patients with mild to moderate COVID-19 by ~50% in a phase 3 clinical trial. AT-527 is an oral nucleotide prodrug that reduced SARS-CoV-2 viral loads in patients hospitalized with COVID-19 in a phase 2 clinical trial. PF-07321332 is an oral SARS-CoV-2 protease inhibitor that is being tested in combination with low-dose ritonavir in a phase 2/3 clinical trial for prevention of COVID-19 infection. Remdesivir and nirmatrelvir boosted with ritonavir are approved by the FDA and recommended as first-line therapy for COVID-19 by the NIH guidelines; molnupiravir is approved under an emergency use authorization by the FDA and is considered second-line therapy. At the time of this writing, Pemgarda, a monoclonal antibody to SARSCoV-2, can be given under emergency use authorization to prevent COVID-19 in immunocompromised persons age 12 and older. ■ ■INVESTIGATIONAL AGENTS FOR RESPIRATORY VIRUS INFECTIONS Favipiravir (T705) inhibits viral RNA polymerases and is active against influenza and other RNA viruses. It is approved for treatment of emerg­ ing influenza viruses in Japan. Presatovir is an RSV fusion inhibitor that was ineffective in two trials of RSV disease. DAS181 (Fludase) is a siali­ dase that cleaves sialic acid, a receptor for influenza A and B and parain­ fluenza viruses; it did not improve the clinical outcomes of patients with influenza, but in case reports transplant recipients with parainfluenza have improved clinically with the drug. Laninamivir octanoate inhibits the neuraminidase of influenza A and B viruses and is approved for treat­ ing influenza in Japan. RSV604 interacts with the RSV nucleocapsid and is undergoing phase 2 studies in transplant recipients. CHAPTER 196 Antiviral Chemotherapy, Excluding Antiretroviral Drugs ANTIVIRAL DRUGS FOR HUMAN PAPILLOMAVIRUS AND POXVIRUS INFECTIONS Interferon α (IFN-α) inhibits replication of many RNA and DNA viruses in vitro. IFN-α is approved by the FDA for intralesional treat­ ment of external anogenital warts caused by human papillomavirus (HPV). It is effective in resolving lesions in ~50% of cases, with a recur­ rence rate of ~25%. Imiquimod is a toll-like receptor 7 agonist that induces production of IFN-α and other cytokines. It is approved as a topical cream for treatment of external genital and perianal warts caused by HPV in persons ≥12 years of age. This drug is effective in resolving lesions in ~40% of cases. Tecovirimat is approved by the FDA for treatment of smallpox and inhibits replication of mpox and vaccinia viruses. Resistance to teco­ virimat developed in a person treated with the drug for progressive vaccinia and has been reported in persons with mpox. INVESTIGATIONAL ANTIVIRAL DRUGS

FOR PICORNAVIRUS Pocapavir inhibits picornaviruses by inhibiting virus uncoating and is being developed to reduce poliovirus shedding; resistance to the drug develops rapidly. ANTIVIRAL DRUGS FOR HEPATITIS B

VIRUS INFECTION Eight drugs representing two classes are approved for the treatment of chronic HBV infection in the United States. One class, the nucleos(t)ide

analogues, act as chain terminators of nascently replicating DNA thereby competitively inhibiting HBV reverse transcriptase; the other class, exogenous IFNs, mimic and augment the role of endogenous interferons (Table 196-3). The goal of therapy for chronic hepatitis B is to prevent progression to cirrhosis, liver failure, and hepatocellular carcinoma. This can be achieved through long-term inhibition of viral replication with reduction in hepatic inflammation, the driver of liver

TABLE 196-3  Antiviral Drugs for Chronic Hepatitis B Treatment in Adults DEVELOPMENT OF RESISTANCE COMMON SIDE EFFECTSa TREATMENT MONITORING COMMENTS DRUG ROUTE AND DOSE Interferons SC injection; 180 μg/week for 48 weeks SC injection;

1.5 μg/kg per week for 48 weeks Not described in longterm studies. Side effects are common and include fevers, chills, myalgia, fatigue, neurotoxicity, and leukopenia. Autoantibodies can develop, particularly antithyroid antibodies. Pegylated a2a   Pegylated a2b Nucelos(t)ide Analogues Lamivudine Oral; 100 mg daily 30% after 1 year; 70% after 5 years Malaise or fatigue, GI symptoms (nausea/ vomiting, abdominal pain, diarrhea), headache, upper respiratory tract infection Adefovir Oral; 10 mg daily 20–29% after 5 years Adefovir is usually active against lamivudine-resistant HBV strains. Headache, asthenia, GI symptoms (abdominal pain, nausea) Telbivudine Oral; 600 mg daily 11–25% after 2 years Cross-resistance is common between lamivudine- and telbivudine-resistant HBV strains. Headache, fatigue, GI symptoms (abdominal pain) PART 5 Infectious Diseases Entecavir Oral; 0.5–1 mg daily 1–2% after 5 years in nucleos(t)ide-naïve patients; 50% after 5 years in lamivudine-resistant patients Headache, fatigue, elevated alanine aminotransferase level Tenofovir disoproxil Oral; 300 mg daily No resistance after up to 10 years of treatment Headache, fatigue, nasopharyngitis, upper respiratory tract infection, nausea Tenofovir alafenamide Oral; 25 mg daily No resistance after up to 3 years of treatment Headache, fatigue, nasopharyngitis, upper respiratory tract infection Emtricitabine Oral; 200 mg daily Not defined Headache, GI symptoms (nausea, diarrhea, abdominal pain), fatigue, depression, insomnia, abnormal dreams, rash, asthenia, increased cough, rhinitis aFor emtricitabine, side effects were assessed only in combination with antiretroviral therapy. Abbreviation: GI, gastrointestinal. fibrosis. Virologic responses (defined by suppression of HBV replica­ tion), biochemical responses (improvement or normalization of liver function values), and histologic responses (reduction in inflammation and fibrosis on liver biopsy) are often achievable with current treat­ ments. However, loss of hepatitis B e antigen (HBeAg) (an intermedi­ ate treatment endpoint), viral clearance with loss of hepatitis B surface antigen (HBsAg), and immune control (defined by a hepatitis B surface

Complete blood counts should be performed biweekly for the first month and then monthly, renal and liver function testing monthly, thyroid function testing every 3 months. Recommended as first-line therapy. Best treatment response seen among patients with HBV genotype A and B infections. Contraindicated in clinically significant portal hypertension and pregnancy.  Renal and liver function testing every 3–6 months Assessment of lactic acid level HBV DNA and serologic testing every 3–6 months Monotherapy recommended if duration of therapy is to be <1 year, as in prophylaxis against HBV reactivation with immunosuppression or chemotherapy. Renal and liver function testing every 6 months Assessment of lactic acid level HBV DNA and serologic testing every 3–6 months — Measurement of creatine kinase level if there is concern about myopathy Renal and liver function testing every 3–6 months Assessment of lactic acid level HBV DNA and serologic testing every 3–6 months — Renal and liver function testing every 3–6 months Assessment of lactic acid level HBV DNA and serologic testing every 3–6 months Recommended as first-line therapy. Dose of 0.5 mg daily in treatment-naïve patients, 1 mg daily in treatment-experienced patients. Dose adjusted in renal dysfunction. Renal and liver function testing every 3–6 months Phosphorus assessment in patients with chronic kidney disease Assessment of lactic acid level HBV DNA and serologic testing every 3–6 months Recommended as first-line therapy. Dosing frequency— but not dose—reduced in chronic kidney disease. May be used during pregnancy; possible risk of low birth weight. Renal and liver function testing every 3–6 months Phosphorus assessment in patients with chronic kidney disease Assessment of lactic acid level HBV DNA and serologic testing every 3–6 months Recommended as first-line therapy. May be used during pregnancy; possible risk of low birth weight. Renal and liver function testing every 3–6 months Assessment of lactic acid level HBV DNA and serologic testing every 3–6 months While not approved for treatment of chronic HBV infection, used interchangeably with lamivudine. Dosing frequency adjusted in chronic kidney disease. antibody [HBsAb] level of >10 IU/mL) are uncommon with current therapies. Treatment with a nucleos(t)ide analogue is considered first-line ther­ apy for chronic HBV infection because of its antiviral potency, favor­ able side-effect profile, and ease of administration. All drugs in this class are given by mouth once daily. While all nucleos(t)ide analogues carry a black box warning for lactic acidosis and severe hepatomegaly,

these adverse events were observed in patients taking older nucleoside analogues (such as stavudine and didanosine for the treatment of HIV) and have not occurred in clinical trials of newer nucleos(t)ides for chronic HBV infection. Once initiated, nucleos(t)ide therapy must be continued for a long duration because of the risk of virus rebound and subsequent hepatitis flare if treatment is stopped. This can occur in up to 40–50% of patients and rarely may lead to hepatic decompensation. Viral rebound occurs because nucleos(t)ides analogues do not target the covalently closed circular DNA—an episomal form of viral DNA. Comparative studies of nucleos(t)ide analogues have demonstrated that newer drugs (entecavir, tenofovir, disoproxil, and tenofovir alaf­ enamide) are associated with lower rates of viral resistance than older agents (lamivudine, telbivudine, and adefovir), but, if viral replication is effectively suppressed, histologic and biochemical improvement will occur in ~60–75% of patients without significant differences between antiviral agents or combinations. However, rates of HBsAg clearance remain extremely low (<1–5%). Pegylated IFN-α2a also is considered a first-line therapy for chronic hepatitis B infection. Pegylated IFN-α2a has certain advantages over nucleos(t)ide analogues—including a finite dosing period of 48 weeks, absence of viral resistance, and higher rates of serologic response—but it has lower rates of biochemical and virologic responses (<40% for both). Downsides to pegylated IFN-α2a include its poor tolerability because of numerous side effects; it is contraindicated in patients with clinically significant portal hypertension and pregnancy. Response rates are higher when pegylated IFN-α2a is combined with nucleos(t)ide therapy in treatment-naïve patients: overall rates of HBsAg loss after 48 weeks of combination therapy with pegylated IFN-α2a and tenofovir disoproxil fumarate (TDF) were low but signifi­ cantly higher than when either was given alone: 9.1% versus 0% with TDF alone (p<.001) and 2.8% with IFN alone (p<.005). Combination therapy is not recommended because long-term nucleos(t)ide analogue monotherapy can achieve similar rates of HBsAg loss as 1 year of combination therapy and greater viral suppression. There is minimum benefit to adding pegylated IFN-α2a to ongoing nucleos(t)ide therapy. The choice of a class of agent is dependent on the presence of comorbid conditions that prevent the use of one agent over another and patient preference. ■ ■LAMIVUDINE Lamivudine is an oral cytidine analogue that competitively inhibits the viral reverse transcriptase activity of both HIV and HBV, prevent­ ing viral replication. Lamivudine was the first oral agent approved for therapy of chronic hepatitis B. Its long-term use was limited by high rates of viral resistance, approaching 30% among patients treated for 1 year and ~70% after 5 years of therapy. Its use in chronic hepatitis B has been superseded by agents with better resistance profiles. While not approved for the treatment of chronic HBV infection, emtricitabine is a cytosine analogue similar in structure, activity, and resistance to lamivudine. Used alone, it offers no advantage over lami­ vudine, but in combination with tenofovir (both TDF and tenofovir alafenamide fumarate [TAF]) it is used as part of an antiretroviral regi­ men to treat patients with HIV/HBV co-infection requiring lifelong antiviral therapy and off-label in selected cases of established nucleo­ side resistance. ■ ■ADEFOVIR Adefovir dipivoxil is the oral prodrug of adefovir—a monophosphate nucleotide analogue of adenosine. This drug is active against HBV, HIV, some herpesviruses (HSV and CMV), and poxviruses. Adefovir is effective for the management of HBV in treatment-naïve patients and those infected with lamivudine-resistant HBV. Viral resistance to adefovir is slower to emerge than resistance to lamivudine but still develops in 20–30% of patients after 5 years of treatment. Adefovir has been replaced with nucleos(t)ide analogues with higher barriers to resistance as first-line treatment for chronic hepatitis B. ■ ■TELBIVUDINE Telbivudine, a β-L enantiomer of thymidine, was approved by the FDA in 2006 for the treatment of chronic HBV infection. It has little or no

activity against HIV replication. Telbivudine is generally well tolerated and effective against HBV replication, but the risk of viral resistance (25% in HBeAg-positive and 11% in HBeAg-negative patients after 2 years of use), myopathy, peripheral neuropathy, and fatigue limited its use. Telbivudine was withdrawn from the U.S. market primarily for economic reasons.

■ ■ENTECAVIR Entecavir is a cyclopentyl guanosine analogue that, once triphosphory­ lated, blocks HBV polymerase in multiple ways, inhibiting priming and reverse transcription of the HBV negative strand and positive-strand synthesis. Entecavir effectively inhibits HBV replication, with resulting biochemical and histologic improvement. This drug is active against some lamivudine-resistant HBV strains, but only at concentrations 20- to 30-fold higher than those obtained with the standard 0.5-mg dose; thus, a higher dose (1 mg daily) of entecavir is recommended for patients with previous lamivudine exposure. Entecavir resistance leading to viral rebound and clinical hepatitis is uncommon among previously untreated patients but may occur in up to 50% of patients with prior lamivudine resistance after 5 years of entecavir treatment. Entecavir-resistant strains retain susceptibility to tenofovir and occa­ sionally adefovir. Entecavir is generally well tolerated and highly bioavailable but should be taken on an empty stomach because food interferes with its absorption. The drug is renally cleared, and dosing should be adjusted for a CrCl of <50 mL/min. ■ ■TENOFOVIR Tenofovir is a nucleotide analogue of adenosine monophosphate with activity against both retroviruses and hepadnaviruses. Two prodrug forms, TDF and TAF, are approved by the FDA for the treatment of both HIV infection and HBV infection. Tenofovir potently inhib­ its HBV replication. Rates of viral suppression are similar between TDF and TAF at 3 years. After 10 years of continuous use, 96–98% of patients receiving TDF achieve complete viral suppression. TAF is associated with higher biochemical response compared with TDF. Clinical resistance to tenofovir has not been observed with up to 8 years of therapy. Both formulations are renally eliminated, and renal toxicity—including acute renal failure, Fanconi syndrome, and diabetes insipidus—has been reported. The risk is higher with TDF compared to TAF. In clinical trials of TAF, there have been no reported cases of Fanconi syndrome or proximal renal tubulopathy. Small declines in bone mineral density (~2.3% at 5 years with TDF and <1% at 3 years with TAF) have been observed. Routine monitoring of renal func­ tion during therapy with both agents is indicated, and dose frequency should be reduced in patients with GFR <50 mL/min. CHAPTER 196 Antiviral Chemotherapy, Excluding Antiretroviral Drugs ■ ■INTERFERONS IFNs have a broad spectrum of antiviral activity in addition to modu­ lating the immune system. Recombinant α, β, γ, and λ IFNs have been evaluated in a variety of viral infections. Standard IFN- α2b was the first drug approved for treatment of chronic hepatitis B, but it has been largely replaced by pegylated IFN-α2a. Pegylation of IFN, through linkage of IFN to polyethylene glycol, results in slower absorption, decreased clearance, and more sustained serum IFN concentrations, thereby permitting a more convenient once-weekly dosing schedule. Consequently, pegylated IFN has supplanted standard IFN. IFNs are associated with numerous adverse effects including fever, myalgia, fatigue, somnolence, depression, confusion, leukopenia, and develop­ ment of autoantibodies, including antithyroid antibodies, that limit its tolerability and patient acceptance. Pegylated IFN-α2a is approved by the FDA for therapy in patients with chronic hepatitis B and C. Pegylated IFN-α2b is no longer avail­ able in the United States. The administration of pegylated IFN-α2a for 48 weeks in patients with HBeAg-positive infection resulted in the loss of markers for HBV replication (e.g., HBeAg and HBV DNA in 29–36% and 8–14% of cases, respectively; 2–7% of patients also cleared HBsAg). In most patients who lose HBeAg and HBV DNA, serum aminotransferases return to normal levels, and the viral and biochemical responses are maintained

in the long term. Predictors of a favorable response to pegylated IFN-α2a therapy include low pretherapy levels of HBV DNA, high pretherapy serum levels of alanine aminotransferase (ALT), a short duration of chronic HBV infection, HBV genotypes A and B, and active liver inflammation on biopsy. Poor responses are seen in patients with HBeAg-negative and immunosuppressed patients, including those infected with HIV.

ANTIVIRAL DRUGS FOR HEPATITIS C INFECTION The goal of HCV treatment is long-term suppression of viral replica­ tion or a sustained virologic response (SVR). SVR is achieved when levels of HCV RNA in the serum remain undetectable 12 weeks after the end of treatment. SVR is considered synonymous with cure, as it is associated with durable suppression of HCV replication, lower allcause and liver-related mortality, and a reduced risk of hepatocellular carcinoma. These benefits have been confirmed in patients with and without advanced liver disease and cirrhosis who received IFN-based and IFN-sparing, combination direct-acting antiviral drugs (DAAs). Several targeted therapies with DAAs are effective against HCV (Table 196-4). Three classes of DAAs that target the NS5B RNAdependent RNA polymerase, the NS3/4 protease, and NS5A, a zincbinding phosphoprotein that is integral for HCV RNA replication, form the basis of curative regimens for chronic HCV infection. A combination of two or three DAAs is now the standard of care for the treatment of chronic HCV infection, regardless of genotype or fibro­ sis stage. Two pangenotypic regimens, glecaprevir/pibrentasvir and sofosbuvir/velpatasvir administered for 8 to 12 weeks, respectively, are PART 5 Infectious Diseases TABLE 196-4  Antiviral Drugs for Hepatitis C Treatment in Adultsa MECHANISM(S) OF ACTION DRUG FORMULATION ROUTE, DOSE, DURATION Sofosbuvir Oral; 400 mg daily; duration varies (12–24 weeks) Nucleoside analogue Genotypes 1–6 Headache, fatigue Should be combined with at least one other DAA from a different class. Sofosbuvir/ledipasvir Oral; 400 mg/90 mg daily; 8, 12, or 24 weeks Nucleoside analogue/ NS5A inhibitor Sofosbuvir/velpatasvir Oral; 400 mg/100 mg daily; 12 weeks Nucleoside analogue/ NS5A inhibitor Sofosbuvir/velpatasvir/ voxilaprevir Oral; 400 mg/100 mg/100 mg once daily; 12 weeks Nucleoside analogue/ NS5A inhibitor/protease inhibitor Elbasvir/grazoprevir Oral; 50 mg/100 mg once daily; 12 or 16 weeks NS5A inhibitor/protease inhibitor Glecaprevir/ pibrentasvir Oral; 3 100-mg tablets/40 mg once daily; 8, 12, or 16 weeks NS5A inhibitor/protease inhibitor Daclatasvir Oral; 60-mg tablet once daily; 12 weeks Dose reduced to 30 mg once daily when taken with a strong CYP3A inhibitor Dose increased to 90 mg once daily when taken with moderate CYP3A inducers NS5A inhibitor Genotypes 1 and 3 Ribavirin Oral; 3–6 200-mg capsules once daily or in divided doses, based on weight, history of cardiovascular disease, and renal function Nucleoside analogue, also unknown mechanisms aWhile these drugs are approved by the FDA for chronic but not acute HCV, they have been recommended for acute HCV by both the Infectious Diseases Society of America and the American Association for the Study of Liver Diseases. Abbreviation: DAA, directly acting antiviral agent.

the most widely used regimens with SVR rates that exceed 95% for all HCV genotypes. Two pangenotypic regimens with high SVR rates are approved specifically for re-treatment of chronic HCV infection after initial treatment failure: glecaprevir/pibrentasvir and sofosbuvir/velpatasvir/ voxilaprevir. In the setting of unfavorable resistance-associated variants (RAVs) or cirrhosis, re-treatment efficacy can frequently be improved by extension of the treatment course or the addition of ribavirin. Review of the online joint American Association for the Study of Liver Diseases/Infectious Diseases Society of America’s HCV Guidelines is useful for selecting the appropriate DAA regimen. In addition, for all DAA-based treatments, checking for drug–drug interactions before the initiation of therapy is recommended. Most regimens are well tolerated, but all DAAs carry a black-box warning about reactivation of HBV—mostly among HBsAg-positive persons and to a lesser extent patients with isolated anti-HBc following HCV suppression. In some cases, fulminant hepatitis, hepatic flare, and death have occurred in patients with untreated HBV infection who underwent treatment for chronic HCV infection. These risks are rare and can be safely managed with routine monitoring; treatment of HCV should not be deferred because of HBV co-infection. ■ ■NS5B POLYMERASE AND NS5A-CONTAINING REGIMENS Sofosbuvir  Sofosbuvir is the prodrug of a uridine inhibitor of the HCV NS5B RNA-dependent RNA polymerase. The active uridine nucleoside triphosphate results in termination of viral RNA replica­ tion. Sofosbuvir is approved by the FDA for the treatment of HCV SPECTRUM OF ACTIVITY COMMON SIDE EFFECTS COMMENTS Genotypes 1, 4, 5, and 6 Headache, fatigue Avoid coadministration with antacid medications. Genotypes 1–6 Headache, fatigue Avoid coadministration with antacid medications. Genotypes 1–6 Headache, fatigue, diarrhea, nausea Approved for re-treatment of patients with previous DAA experience. Avoid coadministration with antacid medications. Genotypes 1 and 4 Fatigue, anemia, headache, nausea Pretreatment testing for resistanceassociated variants recommended in patients infected with genotype 1a. Monitor hepatic function panel at 8 weeks and again at 12 weeks if patient is receiving 16 weeks of treatment. Genotypes 1–6 Headache, fatigue — Headache, fatigue Use recommended only along with sofosbuvir—with or without ribavirin—for genotype 1 or 3 infection; no longer considered a first- or second-line regimen. Unknown, used for all genotypes Anemia, nausea, teratogenic in pregnancy Used only as combined therapy with DAAs or interferon. Complete blood counts should be monitored after 2 weeks of treatment and as clinically indicated thereafter. Dose may be adjusted based on anemia and renal function.

genotypes 1–4 and is active against genotypes 1–6. Resistance to sofos­ buvir is conferred by an S282T substitution in the NS5B protein, but clinically significant resistance to sofosbuvir treatment has rarely been encountered and virologic breakthrough during sofosbuvir treatment is exceedingly rare. Sofosbuvir is approved for use with other DAAs as part of three fixed-dose combination regimens: as two-drug regimens with the NS5A protein inhibitors ledipasvir and velpatasvir, and as a three-drug regimen with velpatasvir and the protease inhibitor voxi­ laprevir. Both sofosbuvir and its active metabolite are renally cleared, and while the FDA has approved this drug only for patients with an estimated GFR of ≥30 mL/min, several studies have demonstrated its safety and efficacy in end-stage renal disease and for patients undergo­ ing dialysis. Sofosbuvir has not been associated with significant toxicity or drug interactions with one notable exception: sofosbuvir potentiates amiodarone and may cause severe bradycardia, especially if coadmin­ istered with amiodarone and a beta blocker. Sofosbuvir/Ledipasvir  Ledipasvir is an NS5A protein inhibitor that is available only in combination with sofosbuvir. The fixed-dose combination of ledipasvir and sofosbuvir is effective against genotypes 1, 4, 5, and 6 with SVR rates of 95–100%. The standard duration of treatment is 12 weeks for genotypes 1 (all subgenotypes), 4, 5, and 6; however, treatment duration may be reduced to 8 weeks in treatmentnaïve, genotype 1–infected noncirrhotic patients with baseline HCV RNA levels below 6 million copies/mL. Treatment should be extended to 24 weeks or ribavirin should be added in patients who have decom­ pensated cirrhosis or previous DAA exposure. Ledipasvir is excreted via the biliary route, and no adjustment is needed for mild or moder­ ate renal impairment. Several studies have shown that sofosbuvir/

ledipasvir is safe in end-stage renal disease, but it remains FDA approved only for patients with a CrCl of >30 mL/min. No dose reduc­ tion is required for decompensated cirrhosis (Child–Turcotte–Pugh class B or C). Ledipasvir absorption is improved with food intake and is inhibited by antacids or proton pump inhibitors. Ledipasvir is an inhibitor of P-glycoprotein and may increase levels of tenofovir; renal function should be monitored in patients receiving both medications, although clinically significant interactions are unlikely during the relatively short period of treatment. Ledipasvir is generally well toler­ ated, and clinical trials have shown only a small increase in side effects, including headache and fatigue, over those occurring with placebo. Sofosbuvir/Velpatasvir  While chemically similar to ledipas­ vir, velpatasvir has an expanded spectrum of activity and exhibits improved efficacy over ledipasvir against HCV genotypes 2 and 3. Velpatasvir is available only in combination with sofosbuvir for the treatment of naïve patients with genotype 1–6 infection and all stages of fibrosis, including decompensated cirrhosis. SVR rates for patients without cirrhosis were 96–100% for all HCV genotypes. In contrast to sofosbuvir/ledipasvir treatment, shortening of the duration of sofosbuvir/ velpatasvir therapy in these patients is not required. Similar to ledipas­ vir, velpatasvir should be taken with food, and coadministration with antacids or proton pump inhibitors should be avoided. Velpatasvir is in general well tolerated, and reported side effects are minimal. Sofosbuvir/Velpatasvir/Voxilaprevir  Available in a triple-drug combination with sofosbuvir and velpatasvir, voxilaprevir is a NS3/ NS4A protease inhibitor that is active against HCV genotypes 1–6. The fixed-dose combination for 12 weeks is recommended for the re-treatment of patients with genotype 1–6 infection in whom SVR has not been attained after previous combination DAA treatment and for treatment-naïve genotype 3–infected patients with cirrhosis and the NS5A resistance-associated variant Y93H. Patients with genotype 3 infection who have failed an NS5A protein inhibitor–regimen have lower SVR rates to re-treatment with sofosbuvir/velpatasvir/voxilaprevir for 12 weeks; thus, it is recommended either to add ribavirin or, if riba­ virin cannot be tolerated, to extend the duration of therapy to 24 weeks. Voxilaprevir is not recommended for patients with decompensated cirrhosis (see “Protease Inhibitors and Protease Inhibitor–Containing Regimens,” below) or those with significant renal impairment and a CrCl of <30 mL/min. Voxilaprevir, like other protease inhibitors, is

metabolized by the CYP3A system, and the effect of voxilaprevir may be reduced in the presence of other CYP inducers.

Sofosbuvir/Daclatasvir  The combination of sofosbuvir with daclatasvir—the only NS5A protein inhibitor available individually rather than coformulated with other DAAs—is approved for the treat­ ment of HCV genotypes 1 and 3. Daclatasvir binds the N terminus of the NS5A protein, both inhibiting viral RNA replication and blocking virion assembly. It is given in combination with sofosbuvir for 12 weeks and is safe for the treatment of patients with decompensated cirrhosis. Daclatasvir is a substrate of CYP3A, and the dose should be reduced if daclatasvir is given with a strong CYP3A inhibitor and increased if it is given with moderate CYP3A4 inducers. Daclatasvir absorption is not affected by food, and daclatasvir is highly protein bound. The dose does not need to be adjusted for renal impairment, and side effects are uncommon. ■ ■PROTEASE INHIBITOR–CONTAINING REGIMENS Protease inhibitors are specifically designed to inhibit the HCV NS3/4A serine protease by mimicking the HCV polypeptide and, when bound by the viral protease, form a covalent bond with the catalytic NS3 serine residues, blocking further activity and preventing proteolytic cleavage of the HCV polyprotein into NS4A, NS4B, NS5A, and NS5B proteins. As a class, the protease inhibitors are hepatically metabolized and therefore should not be administered to patients with decompensated (Child–Turcotte–Pugh class B or C) cirrhosis. For patients receiving protease inhibitors, the current recommendation is that liver function tests should be monitored monthly. CHAPTER 196 Glecaprevir/Pibrentasvir  Glecaprevir is a pangenotypic NS3/ NS4A protease inhibitor that is coformulated with pibrentasvir, a pangenotypic NS5A protein inhibitor. Each medication individually has a high genetic barrier to resistance and is active against HCV geno­ types 1–6. In patients infected with genotypes other than genotype 3, baseline resistance has no influence on glecaprevir treatment efficacy, and NS3/NS4A baseline polymorphisms have not been noted to cor­ relate with virologic failure. Treatment duration varies with fibrosis and treatment experience: an 8-week course of therapy is recommended for treatment-naïve patients who are infected with any genotype and have any degree of fibrosis up to compensated cirrhosis, including patients with genotype 3 infection. This results in SVR rates of 95–100% across all HCV genotypes. Treatment-experienced cirrhotic patients should receive 12 weeks of treatment, and patients with prior NS5A protein inhibitor exposure with or without compensated cirrhosis should receive 16 weeks of therapy. The combination of glecaprevir/pibrentasvir should be taken with food. Clearance is via biliary excretion; therefore, no dose adjustment is required in end-stage renal disease. Because of the protease component, the combination of glecaprevir/pibrentasvir is not appropriate for patients with decompensated cirrhosis. Glecaprevir and pibrentasvir are only weak CYP3A inducers, but they inhibit the P glycoprotein, breast cancer resistance protein (BCRP), and organic anion transporter P1 (OATP1) drug transporters. When taken with other drugs that are substrates for these transporters, concentrations of both drugs may be increased. The combination regimen is generally well tolerated; mild headache, fatigue, diarrhea, and nausea have been reported. Antiviral Chemotherapy, Excluding Antiretroviral Drugs Elbasvir/Grazoprevir  The coformulation of elbasvir, an NS5A replication complex inhibitor, and grazoprevir, an NS3/NS4A protease inhibitor, is active against HCV genotypes 1 and 4. However, its effi­ cacy in the treatment of HCV genotype 1a is reduced in the presence of baseline RAVs in the NS5A protein at positions M28, Q30, L31, and Y93; thus, in patients infected with genotype 1a, baseline resistance testing should be performed and, if the result is positive, ribavirin should be added and therapy should be extended to improve response rates. Susceptibility to grazoprevir is reduced with NS5A protein D168 substitutions, but few resistant isolates have been noted in cases of virologic failure; thus, testing for these substitutions before therapy is not recommended. Treatment duration is 12 weeks (genotype 1b or genotype 1a without baseline RAVs) or 16 weeks (in combination with

ribavirin in patients with baseline NS5A protein polymorphisms and in genotype 4–infected patients with previous IFN exposure). Absorp­ tion of grazoprevir and elbasvir is unaffected by food, and the dose does not need to be adjusted in patients with chronic kidney disease or those who are undergoing dialysis. Elbasvir, like grazoprevir, is a sub­ strate of the CYP3A system; coadministration with moderate or strong CYP3A inducers or with strong inhibitors is not recommended. Both components are well tolerated, and few side effects have been reported. The use of this drug combination, as with all those containing protease inhibitors, is contraindicated in decompensated cirrhosis.

■ ■INTERFERONS Several IFN preparations have been studied and approved as therapeu­ tic options for chronic HCV infection. Approved regimens combined IFN/pegylated IFN with ribavirin, a nonspecific nucleoside analogue with the antiviral effects discussed below. The approval of direct acting antiviral agents in 2014 led to revised guidance, and IFN therapy is no longer recommended for the treatment of hepatitis C. ■ ■RIBAVIRIN Ribavirin, a synthetic oral triazole guanosine analogue, weakly inhibits both DNA and RNA polymerases, but its primary mechanism in HCV treatment is not well understood. It may promote infidelity of RNA viral replication, giving rise to unfit or less fit viral mutations, and also appears to stimulate IFN-response genes and modulate adaptive immune responses. The role of ribavirin in HCV therapy has changed over time. Ribavirin played an integral role in HCV treatment during the IFN era to prevent virologic relapse and, combined with sofosbuvir, was required as part of IFN-sparing regimens before other DAAs were available. However, adverse drug effects associated with higher doses (in heavier patients)—including hemolytic anemia, which is increased with renal failure—were frequently treatment-limiting. Other side effects include rash, myalgia, and fatigue. Ribavirin is teratogenic, and its use in women with child-bearing potential is therefore limited. PART 5 Infectious Diseases With the advent of several combination DAA-only, IFN-sparing regimens, there are often multiple ribavirin-free options for treatment. However, there are still several indications for ribavirin augmenta­ tion of combination DAA-based therapy. Most importantly, ribavirin improves the SVR rate by an average of 5% in treatment-naïve and treatment-experienced patients with genotype 1 infection, particularly that due to subgenotype 1a. The addition of ribavirin to treatment with paritaprevir/ritonavir/ombitasvir plus dasabuvir is recommended for patients with genotype 1a or 4 infection as well as for patients infected with genotype 1a who are receiving elbasvir/grazoprevir with baseline NS5A protein RAVs to overcome reduced susceptibility to elbasvir. Ribavirin is frequently included in regimens for re-treatment of geno­ type 1–infected, therapy-experienced patients with cirrhosis in order to preserve SVR rates while shortening re-treatment duration. SVR rates at 12 weeks were comparable in treatment-experienced cirrhotic patients receiving 24 weeks of ledipasvir/sofosbuvir and those receiv­ ing 12 weeks of ledipasvir/sofosbuvir plus ribavirin. Ribavirin also improves outcomes in treatment-experienced patients with genotype 3 infection—an ongoing therapeutic challenge even in the setting of cur­ rent pangenotypic regimens. Ribavirin improves treatment response in other clinical settings as well, specifically in patients with decompen­ sated cirrhosis for whom treatment protease inhibitors cannot be used and in patients with genotype 2 infection in resource-limited settings where ribavirin is more affordable than fixed-dose combination DAA regimens. Because of its broad antiviral effects, ribavirin is not known to select for any particular RAVs. Absorption of ribavirin is improved by administration with food, and the drug is excreted renally. Lowering the dose of the drug may reduce toxicity. While determining red blood cell counts and hemo­ globin levels after 2 weeks of therapy is recommended to monitor for hemolytic anemia, ribavirin can be administered safely to most patients for the relatively short period of DAA-based therapy. In patients with renal insufficiency and those with end-stage renal disease who are undergoing dialysis, the dose must be adjusted and the patient closely monitored for anemia.

In a recent large-scale study, ribavirin was effective in the treatment of chronic infection with hepatitis E virus, which can cause chronic inflammatory hepatitis in immunosuppressed patients, particularly solid-organ transplant recipients. ANTIVIRAL DRUGS FOR HEPATITIS D INFECTION ■ ■INTERFERONS At high doses, IFN-α and pegylated IFN-α are active against hepatitis D virus infection. In off-label use for hepatitis D, SVR was achieved in 25–35% of patients treated with IFN-α and 17–43% of patients treated with pegylated IFN-α for 48 weeks. Virologic and biochemical relapse occur frequently after stopping IFN. Extending the duration is associated with maintenance of clinical response and HBsAg loss in a few cases. ■ ■ENTRY INHIBITORS Bulevirtide is a synthetic lipopeptide that mimics a region within pre-S1 of the large HBsAg and irreversibly binds to the sodium tauro­ cholate cotransporting polypeptide, the hepatocyte entry receptor for both HDV and HBV. Bulevirtide indirectly reduces HDV replication by blocking viral entry and new rounds of infection. Bulevirtide is approved by the European Medicines Agency (EMA) at a dose of 2 mg subcutaneously once daily for use in patients with compensated chronic HDV infection. In off-label use, bulevirtide was shown to be effective in patients with decompensated liver disease due to chronic hepatitis D infection. In a clinical trial, bulevirtide given 2 or 10 mg subcutaneously once daily was able to suppress HDV viremia by at least 100-fold and normalize alanine aminotransferase (ALT) levels in 45% and 48% of patients, respectively; undetectable viremia was achieved in 12.2% and 20% of patients, respectively. Combined bulevirtide and pegylated interferon was superior to bulevirtide alone to reduce HDV RNA to an undetectable level. No emergence of viral resistance has been observed to date. Asymptomatic elevation in bile acids and injectionsite reactions are the most common adverse reactions. Bulevirtide is not yet approved in the United States. Acknowledgment The authors gratefully acknowledge the contributions of Dr. Eleanor Wilson to this chapter in the previous edition. ■ ■FURTHER READING Acosta E et al: Advances in the development of therapeutics for cyto­ megalovirus infections. J Infect Dis 221:S32, 2020. American Association for the Study of Liver Diseases/Infectious Diseases Society of America: Recommendations for testing, man­ aging, and treating hepatitis C. Available at http://www.hcvguidelines

.org.  Accessed October 28, 2023. Asselah T, Rizzetto M: Hepatitis D virus infection. N Engl J Med 389:58, 2023. Asselah T et al: Bulevirtide combined with pegylated interferon for chronic hepatitis D. N Engl J Med 391:133,2024. Avery R et al: Maribavir for refractory cytomegalovirus infections with or without resistance post-transplant: Results from a phase 3 randomized clinical trial. Clin Infect Dis 75:690, 2022. Chou R et al: Screening for hepatitis C virus infection in adolescents and adults: Updated evidence report and systematic review for the US Preventive Services Task Force. JAMA 323:976, 2020. Drysdale SB et al: Nirsevimab for prevention of hospitalizations due to RSV in infants. N Engl J Med 389:2425, 2023. Gnann JW Jr, Whitley RJ: Genital herpes. N Engl J Med 375:666, 2016. Hanula R et al: Evaluation of oseltamivir used to prevent hospitaliza­ tion in outpatients with influenza: A systematic review and metaanalysis. JAMA Intern Med 184:18, 2024. Ikematsu H et al: Baloxavir marboxil for prophylaxis against influenza in household contacts. N Engl J Med 383:309, 2020. Ison MG et al: Early treatment with baloxavir marboxil in high-risk adolescent and adult outpatients with uncomplicated influenza

85 - SECTION 12 Infections Due to DNA Viruses

SECTION 12 Infections Due to DNA Viruses

(CAPSTONE-2): A randomised, placebo-controlled, phase 3 trial. Lancet Infect Dis 20:1204, 2020. Saullo JL, Miller RA: Cytomegalovirus therapy: Role of letermovir in prophylaxis and treatment in transplant recipients. Annu Rev Med 74:89, 2023. Tang LE et al: Chronic hepatitis B infection: A review. JAMA 319:1802, 2018. Uyeki T et al: Clinical practice guidelines by the Infectious Diseases Society of America: 2018 update on diagnosis, treatment, chemopro­ phylaxis, and institutional outbreak management of seasonal influenza. Clin Infect Dis 68:e1, 2019. Venkatesan S et al: Neuraminidase inhibitors and hospital length of stay: A meta-analysis of individual participant data to determine treatment effectiveness among patients hospitalized with nonfatal 2009 pandemic influenza A (H1N1) virus infection. J Infect Dis 221:356, 2020. Vyas A et al: Real-world outcomes associated with letermovir use for cytomegalovirus primary prophylaxis in allogeneic hematopoietic cell transplant recipients: A systematic review and meta-analysis of observational studies. Open Forum Infect Dis 10:ofac687, 2022. Yardeni D et al: Current best practice in hepatitis B management and understanding long-term prospects for cure. Gastroenterology 164:42, 2023. Section 12 Infections Due to DNA Viruses Lawrence Corey

Herpes Simplex

Virus Infections ■ ■DEFINITION Herpes simplex viruses (HSV-1, HSV-2; Herpesvirus hominis) produce a variety of infections involving mucocutaneous surfaces, the periph­ eral nervous system (PNS), the central nervous system (CNS), and—on occasion—visceral organs. Prompt recognition and treatment reduce the morbidity and mortality rates associated with HSV infections. ■ ■ETIOLOGIC AGENT The genome of HSV is a 152-kb linear, double-stranded DNA molecule (molecular weight, ~100 × 106) that encodes >90 tran­ scription units with 84 identified proteins. The genomic structures of the two HSV subtypes are similar. The overall genomic sequence homology between HSV-1 and HSV-2 is ~50%, whereas the proteome homology is >80%. The homologous sequences are distributed over the entire genome map, and most of the polypeptides specified by one viral type are antigenically related to polypeptides of the other viral type. Many type-specific regions unique to HSV-1 and HSV-2 proteins do exist, and a number of them appear to be important in host immunity. These type-specific regions have been used to develop serologic assays that distinguish between the two viral subtypes. The most commonly used protein is glycoprotein G (US-4), which differs markedly in size and antigenic sites between HSV-1 and HSV-2. Either restriction endo­ nuclease analysis or sequencing of viral DNA can be used to distinguish between the two subtypes and among strains of each subtype. Recombi­ nant viruses (HSV-1/HSV-2) do circulate in nature. The variability of nucleotide sequences from clinical strains of HSV-1 and HSV-2 is such that HSV isolates obtained from two individuals can be differentiated by restriction enzyme patterns or genomic sequences. Epidemiologically related sources, such as sexual partners, mother–infant pairs, or persons involved in a common-source outbreak, can be inferred from such

patterns. Deep sequencing of sequential isolates suggests that more than one variant of HSV-1 or HSV-2 can be found in a single individual and minor mutational changes do occur within anatomic sites and over time.

The viral genome is packaged in a regular icosahedral protein shell (capsid) composed of 162 capsomeres (Chap. 195). The outer covering of the virus is a lipid-containing membrane (envelope) acquired as the DNA-containing capsid buds through the inner nuclear membrane of the host cell. Between the capsid and lipid bilayer of the envelope is the tegument. Viral replication has both nuclear and cytoplasmic phases. Only four of the 12 glycosylated envelope proteins appear to be essential for cell entry: glycoprotein D (gD), gH, gL, and gB. gD binds to cellular co-receptors that belong to the heparin sulfate or tumor necrosis factor receptor family of proteins, the immunoglobulin superfamily (nectin family), triggering a conformational change that alters activation of the gH-gL heterodimer complex that then activates gB and the fusogen glycoprotein gC. The ubiquity of these receptors contributes to the wide host range of herpesviruses. HSV replication is highly regulated. After fusion and entry, the nucleocapsid enters the cytoplasm and several viral proteins are released from the virion. Some of these viral proteins shut off host protein synthesis (by increasing cellular RNA degradation), whereas others “turn on” the transcrip­ tion of immediate early genes of HSV replication. These immediate early gene products, designated α genes, are required for synthesis of the subsequent polypeptide group: the β polypeptides, many of which are regulatory proteins and enzymes required for DNA replication. Most current antiviral drugs interfere with β proteins, such as viral thymidine kinase (TK) and DNA polymerase. The third (γ) class of HSV genes encodes viral structural and tegument proteins and mostly requires viral DNA replication for expression. New antiviral drugs directed at viral assembly and release are under development. CHAPTER 197 After viral genome replication and structural protein synthesis, nucleocapsids are assembled in the cell’s nucleus. Specific viral pro­ teases clip the end of the DNA into procapsid. In the nucleus, the nucleocapsid binds through the inner nuclear membrane to genetic vessels that fuse with the outer membrane and moves the capsid into the cytoplasm. In some cells, viral replication in the nucleus forms two types of inclusion bodies: type A basophilic Feulgen-positive bodies that contain viral DNA and eosinophilic inclusion bodies that are devoid of viral nucleic acid or protein and represent a “scar” of viral infection. The cytoplasmic capsids move along microtubules to the Golgi network where a second round of envelopment occurs. The capsids acquire their lipid envelope and most of the tegument. Cellular machinery transports the infectious virus out of the cell. Herpes Simplex Virus Infections Viral genomes are maintained by some neuronal cells in a repressed state called latency. Latency, which is associated with transcription of only a limited number of virus-encoded RNAs, accounts for the pres­ ence of viral DNA and RNA in neural tissue at times when infectious virus cannot be isolated. Maintenance and growth of neural cells from latently infected ganglia in tissue culture result in production of infec­ tious virions (explantation) and in subsequent permissive infection of susceptible cells (co-cultivation). Activation of the viral genome may then occur, resulting in reactivation—the normal pattern of regulated viral gene expression and replication and HSV release. The release of virions from the neuron follows a complex process of anterograde transport down the length of neuronal axons. In experimental animals, ultraviolet light, systemic and local immunosuppression, and trauma to the skin or ganglia are associated with reactivation. A noncoding region of the viral genome initially felt to be three noncoding regions and now felt to be a more diverse set of noncod­ ing RNAs and micro-RNAs (miRNAs) collectively referred to as the latency-associated transcripts (LATs) are found in the nuclei of latently infected neurons, and deletion mutants of the LAT region exhibit reduced efficiency in their later reactivation. HSV DNA copy number is highly variable between neurons, with no direct correlation between HSV DNA copy numbers and LAT positivity. About 10% of ganglionic neurons contain viral DNA and only about 1% of these neurons express LATs. Substitution of HSV-1 LATs for HSV-2 LATs induces an HSV-1 reactivation pattern, suggesting this region of the genome apparently

86 - 197 Herpes Simplex Virus Infections

197 Herpes Simplex Virus Infections

(CAPSTONE-2): A randomised, placebo-controlled, phase 3 trial. Lancet Infect Dis 20:1204, 2020. Saullo JL, Miller RA: Cytomegalovirus therapy: Role of letermovir in prophylaxis and treatment in transplant recipients. Annu Rev Med 74:89, 2023. Tang LE et al: Chronic hepatitis B infection: A review. JAMA 319:1802, 2018. Uyeki T et al: Clinical practice guidelines by the Infectious Diseases Society of America: 2018 update on diagnosis, treatment, chemopro­ phylaxis, and institutional outbreak management of seasonal influenza. Clin Infect Dis 68:e1, 2019. Venkatesan S et al: Neuraminidase inhibitors and hospital length of stay: A meta-analysis of individual participant data to determine treatment effectiveness among patients hospitalized with nonfatal 2009 pandemic influenza A (H1N1) virus infection. J Infect Dis 221:356, 2020. Vyas A et al: Real-world outcomes associated with letermovir use for cytomegalovirus primary prophylaxis in allogeneic hematopoietic cell transplant recipients: A systematic review and meta-analysis of observational studies. Open Forum Infect Dis 10:ofac687, 2022. Yardeni D et al: Current best practice in hepatitis B management and understanding long-term prospects for cure. Gastroenterology 164:42, 2023. Section 12 Infections Due to DNA Viruses Lawrence Corey

Herpes Simplex

Virus Infections ■ ■DEFINITION Herpes simplex viruses (HSV-1, HSV-2; Herpesvirus hominis) produce a variety of infections involving mucocutaneous surfaces, the periph­ eral nervous system (PNS), the central nervous system (CNS), and—on occasion—visceral organs. Prompt recognition and treatment reduce the morbidity and mortality rates associated with HSV infections. ■ ■ETIOLOGIC AGENT The genome of HSV is a 152-kb linear, double-stranded DNA molecule (molecular weight, ~100 × 106) that encodes >90 tran­ scription units with 84 identified proteins. The genomic structures of the two HSV subtypes are similar. The overall genomic sequence homology between HSV-1 and HSV-2 is ~50%, whereas the proteome homology is >80%. The homologous sequences are distributed over the entire genome map, and most of the polypeptides specified by one viral type are antigenically related to polypeptides of the other viral type. Many type-specific regions unique to HSV-1 and HSV-2 proteins do exist, and a number of them appear to be important in host immunity. These type-specific regions have been used to develop serologic assays that distinguish between the two viral subtypes. The most commonly used protein is glycoprotein G (US-4), which differs markedly in size and antigenic sites between HSV-1 and HSV-2. Either restriction endo­ nuclease analysis or sequencing of viral DNA can be used to distinguish between the two subtypes and among strains of each subtype. Recombi­ nant viruses (HSV-1/HSV-2) do circulate in nature. The variability of nucleotide sequences from clinical strains of HSV-1 and HSV-2 is such that HSV isolates obtained from two individuals can be differentiated by restriction enzyme patterns or genomic sequences. Epidemiologically related sources, such as sexual partners, mother–infant pairs, or persons involved in a common-source outbreak, can be inferred from such

patterns. Deep sequencing of sequential isolates suggests that more than one variant of HSV-1 or HSV-2 can be found in a single individual and minor mutational changes do occur within anatomic sites and over time.

The viral genome is packaged in a regular icosahedral protein shell (capsid) composed of 162 capsomeres (Chap. 195). The outer covering of the virus is a lipid-containing membrane (envelope) acquired as the DNA-containing capsid buds through the inner nuclear membrane of the host cell. Between the capsid and lipid bilayer of the envelope is the tegument. Viral replication has both nuclear and cytoplasmic phases. Only four of the 12 glycosylated envelope proteins appear to be essential for cell entry: glycoprotein D (gD), gH, gL, and gB. gD binds to cellular co-receptors that belong to the heparin sulfate or tumor necrosis factor receptor family of proteins, the immunoglobulin superfamily (nectin family), triggering a conformational change that alters activation of the gH-gL heterodimer complex that then activates gB and the fusogen glycoprotein gC. The ubiquity of these receptors contributes to the wide host range of herpesviruses. HSV replication is highly regulated. After fusion and entry, the nucleocapsid enters the cytoplasm and several viral proteins are released from the virion. Some of these viral proteins shut off host protein synthesis (by increasing cellular RNA degradation), whereas others “turn on” the transcrip­ tion of immediate early genes of HSV replication. These immediate early gene products, designated α genes, are required for synthesis of the subsequent polypeptide group: the β polypeptides, many of which are regulatory proteins and enzymes required for DNA replication. Most current antiviral drugs interfere with β proteins, such as viral thymidine kinase (TK) and DNA polymerase. The third (γ) class of HSV genes encodes viral structural and tegument proteins and mostly requires viral DNA replication for expression. New antiviral drugs directed at viral assembly and release are under development. CHAPTER 197 After viral genome replication and structural protein synthesis, nucleocapsids are assembled in the cell’s nucleus. Specific viral pro­ teases clip the end of the DNA into procapsid. In the nucleus, the nucleocapsid binds through the inner nuclear membrane to genetic vessels that fuse with the outer membrane and moves the capsid into the cytoplasm. In some cells, viral replication in the nucleus forms two types of inclusion bodies: type A basophilic Feulgen-positive bodies that contain viral DNA and eosinophilic inclusion bodies that are devoid of viral nucleic acid or protein and represent a “scar” of viral infection. The cytoplasmic capsids move along microtubules to the Golgi network where a second round of envelopment occurs. The capsids acquire their lipid envelope and most of the tegument. Cellular machinery transports the infectious virus out of the cell. Herpes Simplex Virus Infections Viral genomes are maintained by some neuronal cells in a repressed state called latency. Latency, which is associated with transcription of only a limited number of virus-encoded RNAs, accounts for the pres­ ence of viral DNA and RNA in neural tissue at times when infectious virus cannot be isolated. Maintenance and growth of neural cells from latently infected ganglia in tissue culture result in production of infec­ tious virions (explantation) and in subsequent permissive infection of susceptible cells (co-cultivation). Activation of the viral genome may then occur, resulting in reactivation—the normal pattern of regulated viral gene expression and replication and HSV release. The release of virions from the neuron follows a complex process of anterograde transport down the length of neuronal axons. In experimental animals, ultraviolet light, systemic and local immunosuppression, and trauma to the skin or ganglia are associated with reactivation. A noncoding region of the viral genome initially felt to be three noncoding regions and now felt to be a more diverse set of noncod­ ing RNAs and micro-RNAs (miRNAs) collectively referred to as the latency-associated transcripts (LATs) are found in the nuclei of latently infected neurons, and deletion mutants of the LAT region exhibit reduced efficiency in their later reactivation. HSV DNA copy number is highly variable between neurons, with no direct correlation between HSV DNA copy numbers and LAT positivity. About 10% of ganglionic neurons contain viral DNA and only about 1% of these neurons express LATs. Substitution of HSV-1 LATs for HSV-2 LATs induces an HSV-1 reactivation pattern, suggesting this region of the genome apparently

maintains—rather than establishes—latency. Viral miRNA appears to silence expression of the key neurovirulence factor infected-cell protein 34.5 (ICP34.5) and to bind in an antisense configuration to the immediate-early protein ICP0 messenger RNA to prevent expression, which is vital to HSV reactivation. While certain viral transcripts are known to be necessary for reactivation from latency, the molecular mechanisms of HSV latency are not fully understood and strategies to interrupt or maintain latency in neurons are incompletely understood.

While latency is the predominant state of virus on a per-neuron basis, the high frequency of oral and genital tract reactivation for HSV-1 and HSV-2 suggests that the viruses are rarely quiescent within the entire biomass of ganglionic tissue. The virus appears to be in a dynamic state—“mostly suppressed”—but with continual individual cells showing various degrees of viral transcriptional activity, and only a few of these infected neurons giving rise to actual reactivation. There is increasing recognition that HSV infection of the autonomic ganglia plays an important role in both initial and reactivation infections. In fact, deaths of animals from HSV-2 infection appear to be related to autonomic dysfunction of the bowel. Both HSV-1 and HSV-2 are shed subclinically. Most persons infected with HSV-2 and HSV-1 have frequent subclinical bursts of reactivation lasting 2–6 h, and the host tissue-based immune system can contain viral reactivation in the tissue before the development of clinical reactivation. ■ ■PATHOGENESIS Exposure to HSV at mucosal surfaces or abraded skin sites permits entry of the virus into cells of the epidermis and dermis and initiation of viral replication therein. HSV infections are usually acquired sub­ clinically. Whether clinical or subclinical, HSV acquisition is associated with sufficient viral replication to permit infection of sensory and/or autonomic nerve endings. On entry into the neuronal cell, the virus— or, more likely, the nucleocapsid—is transported intra-axonally to the nerve cell bodies in ganglia. Viral particles tether onto cellular proteins that motor along microtubules from axon tips (neurite endings) to neuronal cell bodies. In humans, the transit interval of spread to the ganglia after virus inoculation into peripheral tissue is unknown. Dur­ ing the initial phase of infection, viral replication occurs in ganglia and contiguous neural tissue. Virus then spreads to other mucocutaneous surfaces through centrifugal migration of infectious virions via periph­ eral nerves. This mode of spread helps explain the large surface area involved, the high frequency of new lesions distant from the initial crop of vesicles that is characteristic in patients with primary genital or oral– labial HSV infection, and the ability to recover virus from neural tissue distant from neurons innervating the inoculation site. Contiguous spread of locally inoculated virus also may take place and allow further mucosal extension of disease. Recent studies have demonstrated HSV viremia—another mechanism for extension of infection throughout the body—in ~30–40% of persons with primary HSV-2 infection; latent infection with both viral subtypes in both sensory and auto­ nomic ganglia has been demonstrated. For HSV-1 infection, trigeminal ganglia are most commonly infected, although extension to the inferior and superior cervical ganglia also occurs. With genital infection, sacral nerve root ganglia (S2–S5) are most commonly affected. Autonomic ganglia, pelvic nerves, and vaginal nerve roots are commonly infected. PART 5 Infectious Diseases After resolution of primary disease, infectious HSV can no longer be cultured from the ganglia; however, neuronal infection, as defined by the presence of viral DNA, persists in ganglionic cells in the anatomic regions of the initial infection. The mechanism of reactivation from latency is unknown, although increasingly evidence of limited viral genes or miRNAs is identified in latently infected neurons. Evidence exists for viral antigen and activated host T cells at the ganglia and periphery, and immune responses in ganglia as well as peripheral tissue appear to influence the frequency and severity of HSV reactiva­ tion. HSV-specific T cells have been recovered from peripheral nerve root ganglia. Many of these resident CD8+ T cells are juxtaposed with latently HSV-1-infected neurons in the trigeminal ganglia and can block reactivation with both interferon (IFN) γ release and gran­ zyme B–mediated degradation of the immediate-early protein ICP4. In addition, there appears to be a latent viral load in the ganglia that

correlates positively with the number of neurons infected and the rate of reactivation but inversely with the number of T cells present. It is not known whether reactivating stimuli transiently suppress these immune cells, independently upregulate transcription of lytic genes, or both. Moreover, host containment in the mucosa has been demonstrated. Once virus reaches the dermal–epidermal junction, there are three possible outcomes: (1) rapid host containment of infection near the site of reactivation; (2) spread of small amounts of virus into the epidermis, with a micro-ulceration associated with low-titer subclinical shedding; and (3) widespread replication and necrosis of epithelial cells and sub­ sequent clinical recurrence (the latter defined clinically by a skin blister and ulceration). Histologically, herpetic lesions involve a thin-walled vesicle or ulceration in the basal region, multinucleated cells that may include intranuclear inclusions, necrosis, and an acute inflammatory response. Re-epithelialization occurs once viral replication is restricted, almost always in the absence of a scar. Analysis of the DNA from sequential isolates of HSV or from iso­ lates from multiple infected ganglia in any one individual has revealed similar, if not identical, restriction endonuclease or DNA sequence patterns in most persons. As more sensitive genomic technologies are developed, evidence of multiple strains of the same subtype is increas­ ingly being reported. For example, infection of individual neurons with multiple strains of drug-susceptible and drug-resistant virus in severely immunosuppressed patients indicates that ganglia can be reseeded during chronic infection. Because exposure to mucosal shedding is relatively common during a person’s lifetime, current data suggest that exogenous infection with different strains of the same subtype does occur. The role strain variation plays in the varied reactivation pattern of disease is unknown. ■ ■IMMUNITY Host responses influence the acquisition of HSV disease, the severity of infection, resistance to the development of latency, the maintenance of latency, and the frequency of recurrences. Both antibody-mediated and cell-mediated reactions are clinically important. Immunocom­ promised patients with defects in cell-mediated immunity experience more severe and more extensive HSV infections than those with deficits in humoral immunity, such as agammaglobulinemia. Experi­ mental ablation of lymphocytes indicates that T cells play a major role in preventing lethal disseminated disease, although antibodies help reduce titers of virus in neural tissue. Some clinical manifestations of HSV appear to be related to the host immune response (e.g., stromal opacities associated with recurrent herpetic keratitis). The surface viral glycoproteins have been shown to be targets of antibodies that mediate neutralization and immune-mediated cytolysis (antibody-dependent cell-mediated cytotoxicity [ADCC]). Monoclonal antibodies to HSV viral glycoproteins have, in experimental infections, conferred pro­ tection against subsequent neurologic disease or ganglionic latency, and reduced subsequent reactivation in animals model experiments. Human studies of monoclonal antibodies are underway. Multiple cell populations, including neutrophils, macrophages, and a variety of T lymphocytes, play a role in host defenses against HSV infections, as do lymphokines generated by T lymphocytes. In animals, passive transfer of primed lymphocytes confers protection from subsequent HSV challenge. Maximal protection usually requires the activation of multiple T-cell subpopulations, including cytotoxic T cells and T cells responsible for delayed hypersensitivity. The latter may confer protection by the antigen-stimulated release of lymphokines (e.g., IFNs), which in turn have a direct antiviral effect and both activate and enhance a variety of specific and nonspecific effector cells. Cellular and humoral immune responses to HSV have been detected both in human ganglia and in mucosal tissue at the site of reactivation. The HSV virion contains a variety of genes that are directed at the inhibi­ tion of host responses. These include gene ICP47, which can bind to the cellular transporter-activating protein TAP-1 and reduce the ability of this protein to bind HSV peptides to human leukocyte antigen class I, thereby reducing recognition of viral proteins by cytotoxic T cells of the host. This effect can be overcome by the addition of IFN-γ, but this reversal requires 24–48 h; thus, the virus has time to replicate and

invade other host cells. Entry of infectious HSV-1 and HSV-2 inhibits several signaling pathways of both CD4+ and CD8+ T cells, leading to their functional impairment in killing and influencing the spectrum of their cytokine secretion. HSV-specific CD8+ T-cell responses appear to be an important component in viral clearance from lesions. Immunosuppressed patients with frequent and prolonged HSV lesions have fewer functional CD8+ T cells directed at HSV. HSV-specific CD8+ T cells have been shown to persist in the genital skin at the dermal–epidermal junction contiguous to nerve endings for months after lesion resolution. Even during clini­ cal quiescence, these CD8+ T cells make both antiviral and cytotoxic proteins indicative of immune surveillance. These resident memory CD8+ T cells appear to be “first responders” capable of controlling viral reactivation at the site of viral release into the dermis. The com­ munication with surrounding keratinocytes using cytokine release of interferon gamma initiates antiviral resistance mechanisms to HSV epithelial cell infection. Innate immunity also influences HSV infec­ tion. Severe HSV-1 infections occur in individuals with natural killer (NK) cell defects. Severe familial HSV-1 infections are associated with TLR3 polymorphisms, and the TLR signaling protein UNC93B and TLR2 polymorphisms influence disease reactivation. This rapid “on and off” interplay between the virus and the host helps explain the variability in clinical disease severity between episodes in any single individual. Differences of 30–60 min in host responses can result in 100- to 1000-fold differences in viral levels and can determine whether an episode of disease is subclinical or clinical. There is a strong association between the magnitude of the CD8+ T-lymphocyte response and the clearance of virus from genital lesions. The location, effectiveness, and longevity of the CD8+ T lymphocytes (and other influencers of immune effector functions such as natural killer or CD4+ T-cell responses) may be important in the expression of disease and the likelihood of transmission over time. ■ ■EPIDEMIOLOGY Seroepidemiologic studies have documented HSV infections world­ wide. The global prevalence of HSV-1 is estimated at 66% of the population (3.7 billion people), while 13.2% or 492 million people aged 15–49 were estimated to live with HSV-2. The past 15 years have shown that the prevalence of HSV-2 is even higher in the developing than in the developed world. In sub-Saharan Africa, HSV-2 seroprevalence among pregnant women may approach 60%, and annual acquisition rates among teenage girls may verge on 20%. The global incidence has been estimated at ~23.9 million infections per year, with 491.5 million infected persons worldwide. As in the developed world, the rate of HSV-2 coital acquisition as well as the serologic prevalence are higher among women than among men. Most of this HSV-2 acquisition is preceded by acquisition of HSV-1; the frequency of genital HSV-1 in middle- and low-income countries is low at present. Infection with HSV-1 is acquired more frequently and earlier in life than infection with HSV-2. From 70 to 90% of adults have antibodies to HSV-1 by the fifth decade of life. In populations of low socioeconomic status, most persons acquire HSV-1 infection before the third decade of life. Antibodies to HSV-2 are not detected routinely until puberty. Antibody prevalence rates correlate with past sexual activity and vary greatly among different population groups. There is evidence that the prevalence of HSV-2 has decreased slightly over the past decade or so in the United States. Serosurveys indicate that 15–20% of the U.S. pop­ ulation has antibodies to HSV-2. In most routine obstetric and family planning clinics, 15–30% of women have HSV-2 antibodies, although only 10% of those who are seropositive for HSV-2 report a history of genital lesions. As many as 50% of heterosexual adults attending sexu­ ally transmitted disease clinics have antibodies to HSV-2. A wide variety of serologic surveys has catalogued the widespread epidemic of HSV-2 in Central America, South America, and Africa. In Africa, HSV-2 sero­ prevalence has ranged from 40 to 70% in obstetric and other sexually experienced populations. Antibody prevalence rates average ~5–10% higher among women than among men. Many studies continue to show that both incident and—more importantly—prevalent HSV-2 infection enhances the acquisition rate

of HIV-1. More specifically, HSV-2 infection is associated on a popula­ tion basis with a two- to fourfold increase in HIV-1 acquisition. This association has been amply demonstrated in heterosexual men and women in both the developed and developing worlds. Mathematical models suggest that ~33–50% of HIV-1 infections may be attributable to HSV-2 both in men who have sex with men (MSM) and in hetero­ sexual women in sub-Saharan Africa. Epidemiologically, regions of the world with high HSV-2 prevalence and selected populations within such regions have a higher population-based incidence of HIV-1.

HSV-2 facilitates the spread of HIV into low-risk populations; prev­ alent HSV-2 appears to increase the risk of HIV infection by seven- to ninefold on a per-coital basis. In addition, HSV-2 is more frequently reactivated in and transmitted by persons co-infected with HIV-1 than in persons not co-infected. Thus, most areas of the world with a high HIV-1 prevalence also have a high HSV-2 prevalence. The shedding of HIV-1 virions from herpetic lesions in the genital region facilitates the spread of HIV through sexual contact. HSV-2 reactivation is associated with a localized persistent inflammatory response consisting of high concentrations of CCR5-enriched CD4+ T cells as well as inflamma­ tory dendritic cells in the submucosa of the genital skin. These cells can support HIV infection and replication and thus are likely to account for the increased risk of HIV acquisition among persons with genital herpes. Unfortunately, antiviral therapy does not reduce this subclini­ cal postreactivation inflammation, probably because of the inability of current antiviral agents to prevent the release of small amounts of HSV antigen into the genital mucosa. Several studies suggest that many cases of “asymptomatic” genital HSV-2 infection are, in fact, simply unrecognized or confined to ana­ tomic regions of the genital tract that are not easily visualized. Asymp­ tomatic seropositive persons shed virus on mucosal surfaces almost as frequently as do those with symptomatic disease. This large reservoir of unidentified carriers of HSV-2 and the frequent asymptomatic reac­ tivation of the virus from the genital tract have fostered the continued spread of genital herpes throughout the world. CHAPTER 197 Herpes Simplex Virus Infections HSV infections occur throughout the year. Transmission can result from contact with persons who have active ulcerative lesions or with persons who have no clinical manifestations of infection but who are shedding HSV from mucocutaneous surfaces. HSV reactivation on genital skin and mucosal surfaces is common. Most HSV-1 and HSV-2 episodes last 2–6 h; thus, replication of the virus and clearance by the host are rapid. Even with once-daily sampling, HSV DNA can be detected on 20% of days by polymerase chain reac­ tion (PCR). Corresponding figures for HSV-1 in oral secretions are similar. Rates of shedding are highest during the initial years after acquisition, with viral shedding occurring on as many as 30–50% of days during this period. Immunosuppressed patients shed HSV from mucosal sites at an even higher frequency (20–80% of days). These high rates of mucocutaneous reactivation suggest that exposure to HSV from sexual or other close contact (kissing, sharing of glasses or silverware) is common and help explain the continuing spread and high seroprevalence of HSV infections worldwide. Reactivation rates vary widely among individuals. Among people living with HIV, a low CD4+ T-cell count and a high HIV-1 load are associated with increased rates of HSV reactivation. Daily antiviral chemotherapy for HSV-2 infection can reduce shedding rates but does not eliminate shedding, as measured by PCR or culture. ■ ■CLINICAL SPECTRUM HSV has been isolated from nearly all visceral and mucocutaneous sites. The clinical manifestations and course of HSV infection depend on the anatomic site involved, the age and immune status of the host, and the antigenic type of the virus. Primary HSV infections (i.e., first infections with either HSV-1 or HSV-2 in which the host lacks HSV antibodies in acute-phase serum) are frequently accompanied by systemic signs and symptoms. Compared with recurrent episodes, pri­ mary infections, which involve both mucosal and extramucosal sites, are characterized by a longer duration of symptoms and virus isolation from lesions. The incubation period ranges from 1 to 26 days (median, 6–8 days). Both viral subtypes can cause genital and oral–facial

infections, and the infections caused by the two subtypes are clinically indistinguishable. However, the frequency of reactivation of infection is influenced by anatomic site and virus type. Genital HSV-2 infection is twice as likely to reactivate and recurs 8–10 times more frequently than genital HSV-1 infection. Conversely, oral–labial HSV-1 infection recurs more frequently than oral–labial HSV-2 infection. Asymptom­ atic shedding rates follow the same pattern.

Oral–Facial Infections  Gingivostomatitis and pharyngitis are the most common clinical manifestations of first-episode HSV-1 infec­ tion, whereas recurrent herpes labialis is the most common clinical manifestation of reactivation HSV-1 infection. HSV pharyngitis and gingivostomatitis usually result from primary infection and are most common among children and young adults. Clinical symptoms and signs, which include fever, malaise, myalgias, inability to eat, irritabil­ ity, and cervical adenopathy, may last 3–14 days. Lesions may involve the hard and soft palate, gingiva, tongue, lip, and facial area. HSV-1 or HSV-2 infection of the pharynx usually results in exudative or ulcer­ ative lesions of the posterior pharynx and/or tonsillar pillars. Lesions of the tongue, buccal mucosa, or gingiva may occur later in the course in one-third of cases. Fever lasting 2–7 days and cervical adenopathy are common. It can be difficult to differentiate HSV pharyngitis clini­ cally from bacterial pharyngitis, Mycoplasma pneumoniae infections, and pharyngeal ulcerations of noninfectious etiologies (e.g., StevensJohnson syndrome). No substantial evidence suggests that reactivation of oral–labial HSV infection is associated with symptomatic recurrent pharyngitis. Reactivation of HSV from the trigeminal ganglia may be associ­ ated with asymptomatic virus excretion in the saliva, development of intraoral mucosal ulcerations, or herpetic ulcerations on the vermilion border of the lip or external facial skin. About 50–70% of seropositive patients undergoing trigeminal nerve-root decompression and 10–15% of those undergoing dental extraction develop oral–labial HSV infec­ tion a median of 3 days after these procedures. Clinical differentiation of intraoral mucosal ulcerations due to HSV from aphthous, traumatic, or drug-induced ulcerations is difficult. PART 5 Infectious Diseases In immunosuppressed patients, HSV infection may extend into mucosal and deep cutaneous layers. Friability, necrosis, bleeding, severe pain, and inability to eat or drink may result. The lesions of HSV mucositis are clinically similar to mucosal lesions caused by cytotoxic drug therapy, trauma, or fungal or bacterial infections, and co-infections are common. Persistent ulcerative HSV infections are among the most common infections in patients with AIDS. HSV and Candida infections often occur concurrently. Systemic antiviral therapy speeds the rate of healing and relieves the pain of mucosal HSV infections in immunosuppressed patients. The frequency of HSV reactivation during the early phases of transplantation or induction chemotherapy is high (50–90%), and prophylactic systemic antiviral agents such as intravenous (IV) acyclovir and penciclovir or the oral congeners of these drugs are used to reduce reactivation rates. Patients with atopic eczema may also develop severe oral–facial HSV infections (eczema herpeticum), which may rapidly involve extensive areas of skin and occasionally disseminate to visceral organs. Extensive eczema her­ peticum has resolved promptly with the administration of IV acyclovir. Erythema multiforme may also be associated with HSV infections (see Figs. 59-9 and A1-24); some evidence suggests that HSV infec­ tion is the precipitating event in ~75% of cases of cutaneous erythema multiforme. HSV antigen has been demonstrated both in circulatory immune complexes and in skin lesion biopsy samples from these cases. Patients with severe HSV-associated erythema multiforme are candi­ dates for chronic suppressive oral antiviral therapy. HSV-1 and varicella-zoster virus (VZV) have been implicated in the etiology of Bell’s palsy (flaccid paralysis of the mandibular portion of the facial nerve). Some but not all trials have documented quicker reso­ lution of facial paralysis with the prompt initiation of antiviral therapy, with or without glucocorticoids. However, other trials have shown little benefit. There are advantages to the use of both antiviral drugs and glucocorticoids for moderate to severe Bell’s palsy. Some experts feel glucocorticoids alone are preferred for mild disease.

FIGURE 197-1  Genital herpes: primary vulvar infection, with multiple, extremely painful, punched-out, confluent, shallow ulcers on the edematous vulva and perineum. Micturition is often very painful. Associated inguinal lymphadenopathy is common. (Reprinted with permission from K Wolff et al: Fitzpatrick’s Color Atlas & Synopsis of Clinical Dermatology, 5th ed. New York, McGraw-Hill, 2005.) Genital Infections  First-episode primary genital herpes is char­ acterized by fever, headache, malaise, and myalgias. Pain, itching, dysuria, vaginal and urethral discharge, and tender inguinal lymph­ adenopathy are the predominant local symptoms. Widely spaced bilateral lesions of the external genitalia are characteristic (Fig. 197-1). Lesions may be present in varying stages, including vesicles, pustules, or painful erythematous ulcers. The cervix and urethra are involved in

80% of women with first-episode infections. First episodes of genital herpes in patients who have had prior HSV-1 infection are occasionally associated with systemic symptoms: prior HSV-1 infection is associ­ ated with faster healing than true primary genital herpes. Detection of HSV DNA in serum has been found in ~30% of cases of true primary genital herpes. The clinical courses of acute first-episode genital herpes are similar for HSV-1 and HSV-2 infection. However, the recurrence rates of genital disease differ with the viral subtype: the 12-month recurrence rates among patients with first-episode HSV-2 and HSV-1 infections are ~90% and ~55%, respectively (median number of recur­ rences, 4 and <1, respectively). Recurrence rates for genital HSV-2 infections vary greatly among individuals and over time within the same individual. HSV has been isolated from the urethra and urine of men and women without external genital lesions. A clear mucoid dis­ charge and dysuria are characteristics of symptomatic HSV urethritis. HSV has been isolated from the urethra of 5% of women with the dysuria–frequency syndrome. Occasionally, HSV genital tract disease is manifested by endometritis and salpingitis in women and by pros­ tatitis in men. About 15% of cases of HSV-2 acquisition are associated with nonlesional clinical syndromes, such as aseptic meningitis, cer­ vicitis, or urethritis. A more complete discussion of the differential diagnosis of genital herpes is presented in Chap. 141. Both HSV-1 and HSV-2 can cause symptomatic or asymptomatic rectal and perianal infections. HSV proctitis is usually associated with rectal intercourse. However, subclinical perianal shedding of HSV is detected in women and men who report no rectal intercourse. This phenomenon is due to the establishment of latency in the sacral der­ matome or sacral autonomic ganglia from prior genital tract infection, with subsequent reactivation in epithelial cells in the perianal region. Such reactivations are often subclinical. Symptoms of HSV proctitis include anorectal pain, anorectal discharge, tenesmus, and constipa­ tion. Sigmoidoscopy reveals ulcerative lesions of the distal 10 cm of the rectal mucosa. Rectal biopsies show mucosal ulceration, necrosis,

polymorphonuclear and lymphocytic infiltration of the lamina propria, and (in occasional cases) multinucleated intranuclear inclusion-bearing cells. Perianal herpetic lesions are also found in immunosuppressed patients receiving cytotoxic therapy. Extensive perianal herpetic lesions and/or HSV proctitis is common among patients with HIV infection. The recent outbreak of Mpox infections globally has made the dif­ ferentiation of HSV anal rectal infection from Mpox infection of clini­ cal and therapeutic importance. PCR-based assays clearly distinguish between the two entities. Acyclovir should be used for HSV-2 infection and tecovirimat for monkeypox infection. Herpetic Whitlow  Herpetic whitlow—HSV infection of the finger— may occur as a complication of primary oral or genital herpes by inoculation of virus through a break in the epidermal surface or by direct introduction of virus into the hand through occupational or some other type of exposure; either viral type may be isolated from the lesion. Clinical signs and symptoms include abrupt-onset edema, erythema, and localized tenderness of the infected finger. Vesicular or pustular lesions of the fingertip that are indistinguishable from lesions of pyogenic bacterial infection are seen. Fever, lymphadenitis, and epi­ trochlear and axillary lymphadenopathy are common. The infection may recur. Prompt diagnosis (to avoid unnecessary and potentially exacerbating surgical therapy and/or transmission) is essential. Antivi­ ral therapy is usually recommended (see below). Herpes Gladiatorum  HSV may infect almost any area of skin. Mucocutaneous HSV infections of the thorax, ears, face, and hands have been described among wrestlers. Transmission of these infections is facilitated by trauma to the skin sustained during wrestling. Outbreaks of HSV among competitive wrestlers have illustrated the importance of prompt diagnosis and therapy to contain the spread of this infection. Eye Infections  HSV infection of the eye is the most common cause of corneal blindness in the United States. HSV keratitis presents as an acute onset of pain, blurred vision, chemosis, conjunctivitis, and char­ acteristic dendritic lesions of the cornea. Use of topical glucocorticoids may exacerbate symptoms and lead to involvement of deep structures of the eye. Debridement, topical antiviral treatment, and/or IFN ther­ apy hasten healing. However, recurrences are common, and the deeper structures of the eye may sustain immunopathologic injury. Stromal keratitis due to HSV appears to be related to T-cell–dependent destruc­ tion of deep corneal tissue. An HSV-1 epitope that is autoreactive with T cell–targeting corneal antigens has been postulated to be a factor in this infection. Chorioretinitis, usually a manifestation of disseminated HSV infection, may occur in neonates or in patients with HIV infection. HSV and VZV can cause acute necrotizing retinitis as an uncommon but severe manifestation. While VZV infection is the most common cause of acute retinal necropsy, both HSV-1 and HSV-2 may also be associated with this syn­ drome. Emergent ophthalmology consulta­ tion is recommended as residual blindness may occur; both systemic and intravitical antiviral therapy is recommended. Central and Peripheral Nervous System Infections  HSV accounts for 10–20% of all cases of sporadic viral encephalitis in the United States. The estimated incidence is ~2.3 cases per 1 million persons per year. Cases are distributed throughout the year, and the age distribution appears to be biphasic, with peaks at 5–30 and >50 years of age. HSV-1 causes

95% of cases. The pathogenesis of HSV encephalitis varies. In children and young adults, primary HSV infection may result in encephalitis; presumably, exogenously acquired virus enters the CNS by neuro­ tropic spread from the periphery via the FIGURE 197-2  Computed tomography and diffusion-weighted magnetic resonance imaging scans of the brain of a patient with left-temporal-lobe herpes simplex virus encephalitis.

olfactory bulb. However, most adults with HSV encephalitis have clini­ cal or serologic evidence of mucocutaneous HSV-1 infection before the onset of CNS symptoms. In ~25% of the cases examined, the HSV-1 strains from the oropharynx and brain tissue of the same patient differ; thus, some cases may result from reinfection with another strain of HSV-1 that reaches the CNS. Two theories have been proposed to explain the development of actively replicating HSV in localized areas of the CNS in persons whose ganglionic and CNS isolates are similar. Reactivation of latent HSV-1 infection in trigeminal or autonomic nerve roots may be associated with extension of virus into the CNS via nerves innervating the middle cranial fossa. HSV DNA has been dem­ onstrated by DNA hybridization in brain tissue obtained at autopsy— even from healthy adults. Thus, reactivation of long-standing latent CNS infection may be another mechanism for the development of HSV encephalitis. Recent studies have identified genetic polymorphisms among families with a high frequency of HSV encephalitis. Peripheralblood mononuclear cells, fibroblasts, and neurons from these patients (predominantly children) appear to secrete reduced levels of IFN in response to HSV. Genetic mutations in TLR3 documented in patients with HSV encephalitis suggest that some cases of sporadic HSV encephalitis may be related to host genetic determinants. The clinical hallmark of HSV encephalitis has been the acute onset of fever and focal neurologic symptoms and signs, especially in the temporal lobe (Fig. 197-2); gadolinium enhancing lesions are seen in the temporal lobe by magnetic resonance imaging (MRI). Clinical differentiation of HSV encephalitis from other viral encephalitides, focal infections, or noninfectious processes is difficult. Elevated cere­ brospinal fluid (CSF) protein levels, leukocytosis (predominantly lym­ phocytes), and red blood cell counts due to hemorrhagic necrosis are common in HSV encephalitis. While brain biopsy has been the gold standard for defining HSV encephalitis, a highly sensitive and specific PCR for detection of HSV DNA in CSF has largely replaced biopsy for defining HSV CNS infection. Although titers of antibody to HSV in CSF and serum increase in most cases of HSV encephalitis, they rarely do so earlier than 10 days into the illness and, therefore, although useful in retrospect, generally are not helpful in establishing an early clinical diagnosis. In rare cases, demonstration of HSV antigen, HSV DNA, or HSV replication in brain tissue obtained by biopsy is highly sensitive; examination of such tissue also provides the opportunity to identify alternative, potentially treatable causes of encephalitis. Antiviral therapy with acyclovir reduces the rate of death from HSV encephalitis. Most authorities recommend the administration of IV CHAPTER 197 Herpes Simplex Virus Infections

acyclovir to patients with presumed HSV encephalitis until the diag­ nosis is confirmed or an alternative diagnosis is made. All confirmed cases should be treated with IV acyclovir (30 mg/kg per day in three divided doses for 14–21 days). After the completion of therapy, the clinical recurrence of encephalitis requiring more treatment has been reported. For this reason, some authorities prefer to treat initially for 21 days, and many continue therapy until HSV DNA has been eliminated from the CSF. Even with therapy, neurologic sequelae are common, especially among persons >50 years of age.

HSV DNA has been detected in CSF from 3 to 15% of persons pre­ senting to the hospital with aseptic meningitis. HSV meningitis, which is usually seen in association with primary genital HSV infection, is an acute, self-limited disease manifested by headache, fever, and mild photophobia and lasting 2–7 days. Lymphocytic pleocytosis in the CSF is characteristic. Neurologic sequelae of HSV meningitis are rare. HSV is the most commonly identified cause of recurrent lymphocytic meningitis (Mollaret’s meningitis). Demonstration of HSV antibodies in CSF or persistence of HSV DNA in CSF can establish the diagno­ sis. For persons with frequent recurrences of HSV meningitis, daily antiviral therapy has reduced the frequency of recurrent episodes of symptomatic meningitis. Autonomic nervous system dysfunction, especially of the sacral region, has been reported in association with both HSV and VZV infections. Numbness, tingling of the buttocks or perineal areas, urinary retention, constipation, CSF pleocytosis, and (in males) impotence may occur. Symptoms appear to resolve slowly over days or weeks. Occasionally, hypoesthesia and/or weakness of the lower extremities persists for many months. Transitory hypoesthesia of the area of skin innervated by the trigeminal nerve and vestibular system dysfunction (as measured by electronystagmography) are the predominant signs of disease. Rarely, transverse myelitis, manifested by a rapidly progressive symmetric paralysis of the lower extremities or Guillain-Barré syndrome, follows HSV infection. Similarly, PNS involvement (Bell’s palsy) or cranial polyneuritis may be related to reactivation of HSV-1 infection. PART 5 Infectious Diseases There is increasing experimental evidence suggesting an association between herpesvirus pathogens, specifically HSV-1, and the develop­ ment of sporadic Alzheimer’s disease (AD). HSV-1 DNA is detected in brain tissue of patients with AD, and epidemiologically, HSV-1 anti­ bodies are a significant risk factor for later AD onset. A wide variety of models of AD indicate that HSV-1 infection can induce neuronal death, tau phosphorylation, and intracellular expression of isoforms of amyloid precursor protein cleavage products that produce multicellu­ lar-like plaque structures associated with AD. There are no cogent data to indicate antiviral therapy would be of benefit to anyone with AD. An adequately powered prospective study of prolonged antiviral therapy has not been conducted. HSV Lymphadenitis  HSV-1 or HSV-2 lymphadenitis has been reported, with increasing frequency especially among patients with chronic lymphocytic leukemia. This condition represents a diagnos­ tic dilemma often confused with lymphomatous transformation and occurs absent of oral or genital lesions. One histologic hint between HSV and lymphoblastic transformation is the presence of necrosis in biopsy tissue in HSV lymphadenitis. Isolation of HSV or demonstra­ tion of HSV DNA in tissue confers the diagnosis. Concomitant HSV viremia may also be present. Intravenous acyclovir has been associated with resolution of infection and clinical improvement. Visceral Infections  HSV infection of visceral organs usually results from viremia, and multiple-organ involvement is common. Occasionally, however, the clinical manifestations of HSV infection involve only the esophagus, lung, or liver. HSV esophagitis may result from direct extension of oral–pharyngeal HSV infection into the esophagus or may occur de novo by reactivation and spread of HSV to the esophageal mucosa via the vagus nerve. The predominant symp­ toms of HSV esophagitis are odynophagia, dysphagia, substernal pain, and weight loss. Multiple oval ulcerations appear on an erythematous base with or without a patchy white pseudomembrane. The distal esophagus is most commonly involved. With extensive disease, diffuse

friability may spread to the entire esophagus. Neither endoscopic nor barium examination can reliably differentiate HSV esophagitis from Candida esophagitis or from esophageal ulcerations due to thermal injury, radiation, or corrosives. Endoscopically obtained secretions— for cytologic examination and culture or DNA detection by PCR—provide the most useful material for diagnosis. Systemic antiviral therapy usu­ ally reduces the severity and duration of symptoms and heals esopha­ geal ulcerations. HSV pneumonitis is uncommon except in severely immunosup­ pressed patients and may result from extension of herpetic tracheo­ bronchitis into lung parenchyma. Focal necrotizing pneumonitis usually ensues. Hematogenous dissemination of virus from sites of oral or genital mucocutaneous disease may also occur, producing bilateral interstitial pneumonitis. Bacterial, fungal, and parasitic pathogens are commonly present in HSV pneumonitis. The mortality rate from untreated HSV pneumonia in immunosuppressed patients is high (>80%). HSV has also been isolated from the lower respiratory tract of persons with acute respiratory distress syndrome and prolonged intu­ bation. Most authorities believe that the presence of HSV in tracheal aspirates in such settings is due to reactivation of HSV in the tracheal region and localized tracheitis in persons with long-term intubation. Such patients should be evaluated for extension of HSV infection into the lung parenchyma. While retrospective reviews of HSV tracheitis in intensive care unit patients suggest benefit from antiviral therapy, well-powered controlled trials assessing the role of antiviral agents used against HSV in ventilation-associated morbidity and mortality have not been conducted. The role of lower respiratory tract HSV infection in overall rates of morbidity and mortality associated with these condi­ tions is unclear. HSV is an uncommon cause of hepatitis in immuno­ competent patients. HSV infection of the liver is associated with fever, abrupt elevations of bilirubin and serum aminotransferase levels, and leukopenia (<4000 white blood cells/μL). Disseminated intravascular coagulation may also develop. Other reported complications of HSV infection include monar­ ticular arthritis, adrenal necrosis, idiopathic thrombocytopenia, and glomerulonephritis. Disseminated HSV infection in immunocompe­ tent patients is rare. In immunocompromised patients, burn patients, or malnourished individuals, HSV occasionally disseminates to other visceral organs, such as the adrenal glands, pancreas, small and large intestines, and bone marrow. Dissemination, even recurrent dis­ semination, is being increasingly recognized in persons with chronic lymphocytic leukemia. Rarely, primary HSV infection in pregnancy disseminates and may be associated with the death of both mother and fetus. This uncommon event is usually related to the acquisition of primary infection in the third trimester. Disseminated HSV infection is best detected by the presence of HSV DNA in plasma or blood. Neonatal HSV Infections  Of all HSV-infected populations, neo­ nates (infants <6 weeks) have the highest frequency of visceral and/or CNS infection. Without therapy, the overall rate of death from neonatal herpes is 65%; <10% of neonates with CNS infection develop normally. Although skin lesions are the most commonly recognized features of disease, many infants do not develop lesions at all or do so only well into the course of disease. Neonatal infection is usually acquired perinatally from contact with infected genital secretions at delivery. Congenitally infected infants have been reported. Of neonatal HSV infections, 30–50% are due to HSV-1 and 50–70% to HSV-2. A recent review of U.S. Medicaid databases indicates a frequency of ~1000 cases of neonatal HSV yearly, a rate of between 1 per 2000 and 1 per 3000 live births. The risk of developing neonatal HSV infection is 10 times higher for an infant born to a mother who has recently acquired HSV than for other infants. Neonatal HSV-1 infections may also be acquired through postnatal contact with immediate family members who have symptomatic or asymptomatic oral–labial HSV-1 infection or through nosocomial transmission within the hospital. All neonates with presumed herpes should be treated with IV acyclovir and then placed on maintenance oral antiviral therapy for the first 6–12 months of life. Antiviral chemotherapy with high-dose IV acyclovir (60 mg/kg per day) has reduced the mortality rate from neonatal herpes to ~15%.

However, rates of morbidity, especially among infants with HSV-2 infection involving the CNS, are still very high. HSV in Pregnancy  In the United States, 21% of all pregnant women and 51% of non-Hispanic black pregnant women are seroposi­ tive for HSV-2. However, the risk of mother-to-child transmission of HSV in the perinatal period is highest when the infection is acquired near the time of labor—that is, in previously HSV-seronegative women. The clinical manifestations of recurrent genital herpes—including the frequency of subclinical versus clinical infection, the duration of lesions, pain, and constitutional symptoms—are similar in pregnant and nonpregnant women. Recurrences increase in frequency over the course of pregnancy. However, when women are seropositive for HSV-2 at the outset of pregnancy, no effect on neonatal outcomes (including birth weight and gestational age) is seen. First-episode infections in pregnancy have more severe consequences for mother and infant. Maternal visceral dissemination during the third trimester occasionally occurs, as does premature birth or intrauterine growth retardation. The acquisition of primary disease in pregnancy, whether related to HSV-1 or HSV-2, carries the risk of transplacental transmis­ sion of virus to the neonate and can result in spontaneous abortion, although this outcome is relatively uncommon. For newly acquired genital HSV infection during pregnancy, most authorities recommend treatment with acyclovir (400 mg three times daily) or valacyclovir (500–1000 mg twice daily) for 7–10 days. However, the impact of this intervention on transmission is unknown. The high HSV-2 prevalence rate in pregnancy and the low incidence of neonatal disease in these women (1 case per 6000–20,000 live births) indicate that only a few infants are at risk of acquiring HSV. Therefore, cesarean section is not warranted for all women with recurrent genital disease. Because intrapartum transmission of infection accounts for the majority of cases, abdominal delivery need be considered only for women who are shedding HSV at delivery. Several studies have shown no correlation between recurrence of viral shedding before delivery and viral shed­ ding at term. Hence, weekly virologic monitoring and amniocentesis are not recommended. The frequency of transmission from mother to infant is markedly higher among women who acquire HSV near term (30–50%) than among those in whom HSV-2 infection is reactivated at delivery (<1%). Although maternal antibody to HSV-2 is protective, antibody to HSV-1 offers little or no protection against neonatal HSV-2 infection. Primary genital infection with HSV-1 leads to a particularly high risk of trans­ mission during pregnancy and accounts for an increasing proportion of neonatal HSV cases. Moreover, during reactivation, HSV-1 appears more transmissible to the neonate than HSV-2. Only 2% of women who are seropositive for HSV-2 have HSV-2 isolated from cervical secretions at delivery, and only 1% of infants exposed in this man­ ner develop infection, presumably because of the protective effects of maternally transferred antibodies and perhaps lower viral titers during reactivation. Despite the low frequency of transmission of HSV in this setting, 30–50% of infants with neonatal HSV are born to mothers with established genital herpes. Isolation of HSV by cervicovaginal swab at the time of delivery is the greatest risk factor for intrapartum HSV transmission (relative risk = 346); however, culture-negative, PCR-positive cases of intrapartum trans­ mission are well described. New acquisition of HSV (odds ratio [OR] = 49), isolation of HSV-1 versus HSV-2 (OR = 35), cervical versus vulvar HSV detection (OR = 15), use of fetal scalp electrodes (OR = 3.5), and young maternal age confer further risk of transmission, whereas cesar­ ean delivery is protective (OR = 0.14). Physical examination poorly predicts the absence of shedding, and PCR far exceeds culture in terms of sensitivity and speed. Therefore, PCR detection at the onset of labor should be used to aid clinical decision-making for women with HSV-2 antibody. Because cesarean section appears to be an effective means of reducing maternal–fetal transmission, patients with recurrent genital herpes should be encouraged to come to the hospital early at the time of delivery for careful examination of the external genitalia and cervix as well as collection of a swab sample for viral isolation. Women who have no evidence of lesions can have a vaginal delivery. The presence

of active lesions on the cervix or external genitalia is an indication for cesarean delivery.

If first-episode exposure has occurred (e.g., if HSV serologies show that the mother is seronegative or if the mother is HSV-1-seropositive and the isolate at delivery is found to be HSV-2), many authorities would initiate antiviral therapy for the infant with IV acyclovir. At a minimum, samples for viral cultures and PCR should be obtained from the throat, nasopharynx, eyes, and rectum of these infants immediately and at 5- to 10-day intervals. Lethargy, skin lesions, or fever should be evaluated promptly. All infants from whom HSV is isolated 24 h after delivery should be treated with IV acyclovir at recommended doses. ■ ■DIAGNOSIS Both clinical and laboratory criteria are useful for diagnosing HSV infections. A clinical diagnosis can be made accurately when charac­ teristic multiple vesicular lesions on an erythematous base are present. However, herpetic ulcerations may resemble skin ulcerations of other etiologies and considerable overlap including coinfection between HSV and Mpox infections exist. Mucosal HSV infections may also present as urethritis or pharyngitis without cutaneous lesions. Thus, laboratory studies to confirm the diagnosis and to guide therapy are recommended. While staining of scrapings from the base of the lesions with Wright’s, Giemsa’s (Tzanck preparation), or Papanicolaou’s stain to detect giant cells or intranuclear inclusions of Herpesvirus infection is a well-described procedure, few clinicians are skilled in this tech­ nique, the sensitivity of staining is low (<30% for mucosal swabs), and these cytologic methods do not differentiate between HSV and VZV infections. CHAPTER 197 HSV infection is best confirmed in the laboratory by detection of virus, viral antigen, or viral DNA in scrapings from lesions. HSV DNA detection by PCR is the most sensitive laboratory technique for detect­ ing mucosal or visceral HSV infections and is the recommended test for laboratory confirmation of a diagnosis. HSV causes a discernible cytopathic effect in a variety of cell culture systems, and this effect can be identified within 48–96 h after inoculation. Spin-amplified culture with subsequent staining for HSV antigen has shortened the time needed to identify HSV to <24 h. Culture is indicated when antiviral sensitivity testing is required. The sensitivity of all detection methods depends on the stage of the lesions (with higher sensitivity for vesicu­ lar than for ulcerative lesions), on whether the patient has a first or a recurrent episode of the disease (with higher sensitivity in first than in recurrent episodes), and on whether the sample is from an immu­ nosuppressed or an immunocompetent patient (with more antigen or DNA in immunosuppressed patients). Laboratory confirmation permits subtyping of the virus; information on subtype may be useful epidemiologically and may help to predict the frequency of reactivation after first-episode oral–labial or genital HSV infection. Herpes Simplex Virus Infections Both type-specific and type-common antibodies to HSV develop during the first several weeks after infection and persist indefinitely. Serologic assays with whole-virus antigen preparations, such as com­ plement fixation, neutralization, indirect immunofluorescence, passive hemagglutination, radioimmunoassay, and enzyme-linked immuno­ sorbent assay, can differentiate uninfected (seronegative) persons from those with past HSV-1 or HSV-2 infection, but they do not reliably distinguish between the two viral subtypes or help establish an active clinical diagnosis of HSV-2. Serologic assays that identify antibodies to the type-specific glycoprotein G of the two viral subtypes (G1 and G2) are available commercially and can be used to distinguish between the human antibody responses to HSV-1 and HSV-2. Point-of-care assays that provide results from capillary blood or serum during a clinic visit are available. False-positive and false-negative assays do occur, and there is a need for commercially available assays to distinguish prevalent HSV-2 infection, especially on the backgroiund of earlier acquired HSV-1. A western blot assay that can detect several HSV type-specific proteins can also be used and is, at present, the best con­ firmatory assay. The presence of type-specific HSV-2 antibody implies past HSV-2 infection—i.e., latent infection, likely subclinical reactiva­ tion, increased risk of HSV-1 acquisition, and potential transmission to others.

PART 5 Infectious Diseases Acute- and convalescent-phase serum samples can be useful in demonstrating seroconversion during primary HSV-1 or HSV-2 infec­ tion. However, few available tests report titers, and increases in index values do not reflect first episodes in all patients. Serologic assays based on type-specific proteins should be used to identify asymptomatic car­ riers of HSV-1 or HSV-2. No reliable IgM method for defining acute HSV infection is available. Several studies have shown that persons with previously unrecog­ nized HSV-2 infection can be taught to identify symptomatic reactiva­ tions. Individuals seropositive for HSV-2 should be told about the high frequency of subclinical reactivation on mucosal surfaces that are not visible to the eye (e.g., cervix, urethra, perianal skin) or in microscopic ulcerations that may not be clinically symptomatic. Transmission of infection during such episodes is well established. HSV-2-seropositive persons should be educated about the high likelihood of subclinical shedding and the role that condoms (male or female) may play in reducing transmission. Antiviral therapy with valacyclovir (500 mg once daily) has been shown to reduce but not eliminate the transmis­ sion of HSV-2 between sexual partners. TREATMENT Herpes Simplex Virus Infections Many aspects of mucocutaneous and visceral HSV infections are amenable to antiviral chemotherapy. For mucocutaneous infec­ tions, acyclovir and its congeners famciclovir and valacyclovir have been the mainstays of therapy. Several antiviral agents are available for topical use in HSV eye infections: idoxuridine, trifluorothymi­ dine, topical vidarabine, and cidofovir and, more recently, topical pritelivir. For HSV encephalitis and neonatal herpes, IV acyclovir is the treatment of choice. Most licensed antiviral agents for use against HSV inhibit the viral DNA polymerase. One class of drugs, typified by the drug acyclovir, is made up of substrates for the HSV enzyme TK. Acyclovir, ganciclovir, famciclovir, and valacyclovir are all selectively phos­ phorylated to the monophosphate form in virus-infected cells. Cellular enzymes convert the monophosphate form of the drug to the triphosphate, which is then incorporated into the viral DNA chain. Acyclovir is the agent most frequently used for the treat­ ment of HSV infections and is available in IV, oral, and topical formulations. Valacyclovir, the valyl ester of acyclovir, offers greater bioavailability than acyclovir and thus can be administered less frequently. Famciclovir, the oral formulation of penciclovir, is clini­ cally effective in the treatment of a variety of HSV-1 and HSV-2 infections. Ganciclovir is active against both HSV-1 and HSV-2; however, it is more toxic than acyclovir, valacyclovir, and famci­ clovir and generally is not recommended for the treatment of HSV infections. Anecdotal case reports suggest that ganciclovir may also be less effective than acyclovir for the treatment of HSV infections. All three recommended compounds—acyclovir, valacyclovir, and famciclovir—have proved effective in shortening the duration of symptoms and lesions of mucocutaneous HSV infections in both immunocompromised and immunocompetent patients (Table 197-1). IV and oral formulations prevent reactivation of HSV in sero­ positive immunocompromised patients during induction chemo­ therapy or in the period immediately after bone marrow or solid organ transplantation. Chronic daily suppressive therapy reduces the frequency of reactivation disease among patients with frequent genital or oral–labial herpes. Only valacyclovir has been subjected to clinical trials that demonstrated reduced transmission of HSV-2 infection between sexual partners. IV acyclovir (30 mg/kg per day, given as a 10-mg/kg infusion over 1 h at 8-h intervals) is effective in reducing rates of death and morbidity from HSV encephalitis. Early initiation of therapy is a critical factor in outcome. The major side effect associated with IV acyclovir is transient renal insufficiency, usually due to crystallization of the compound in the renal paren­ chyma. This adverse reaction can be avoided if the medication is given slowly over 1 h and the patient is well hydrated. Because CSF levels of acyclovir average only 30–50% of plasma levels, the dos­ age of acyclovir used for treatment of CNS infection (30 mg/kg per day) is double that used for treatment of mucocutaneous or visceral disease (15 mg/kg per day). Even higher doses of IV acyclovir are used for neonatal HSV infection (60 mg/kg per day in three divided doses). Antiviral drugs neither eradicate latent infection nor affect the risk, frequency, or severity of subclinical or clinical recurrence after the drug is discontinued. Increasingly, shorter courses of therapy are being used for recur­ rent mucocutaneous infection with HSV-1 or HSV-2 in immuno­ competent patients. One-day courses of famciclovir and valacyclovir are clinically effective, more convenient, and generally less costly than longer courses of therapy (Table 197-1). These short-course regimens should be reserved for immunocompetent hosts. SUPPRESSION OF MUCOCUTANEOUS HERPES Recognition of the high frequency of subclinical reactivation pro­ vides a well-accepted rationale for the use of daily antiviral therapy to suppress reactivations of HSV, especially in persons with frequent clinical reactivations (e.g., those with recently acquired genital HSV infection). Immunosuppressed persons, including those with HIV infection, may also benefit from daily antiviral therapy. Daily acy­ clovir and valacyclovir reduce the frequency of HSV reactivations among HIV-positive persons. Regimens used include acyclovir (400–800 mg twice daily), famciclovir (500 mg twice daily), and valacyclovir (500 mg twice daily); valacyclovir at a dose of 4 g/d was associated with thrombotic thrombocytopenic purpura in one study of HIV-infected persons. Daily acyclovir therapy is associated with a modest reduction in the titer of HIV RNA in plasma (0.5-log10 reduction) and in the genital mucosa (0.33-log10 reduction). Epi­ sodes of subclinical reactivation still occur with chronic acyclovir or valaciclovir therapy. REDUCED HSV TRANSMISSION TO SEXUAL PARTNERS Once-daily valacyclovir (500 mg) has been shown to reduce trans­ mission of HSV-2 between sexual partners. Transmission rates are higher from males to females and among persons with frequent HSV-2 reactivation. Serologic screening can be used to identify at-risk couples. Daily valacyclovir appears to be more effective at reducing subclinical shedding than daily famciclovir. ACYCLOVIR RESISTANCE Clinically relevant acyclovir-resistant strains of HSV do occur. Most of these strains have an altered substrate specificity for phos­ phorylating acyclovir. Thus, cross-resistance to famciclovir and valacyclovir is usually found. Occasionally, an isolate with altered TK specificity arises and is sensitive to famciclovir but not to acy­ clovir. In some patients infected with TK-deficient virus, higher doses of acyclovir are associated with clearing of lesions. In others, clinical disease progresses despite high-dose therapy. The major­ ity of clinically significant acyclovir resistance has been seen in immunocompromised patients. HSV-2 isolates appear to be more resistant than HSV-1 strains. The frequency of acyclovir resistance is not systematically monitored; the lack of appreciable change in the frequency of acyclovir resistance in the general population past 45 years probably reflects the reduced transmission of TK-deficient mutants. Isolation of HSV from lesions persisting despite adequate dosages and blood levels of acyclovir should raise the suspicion of acyclovir resistance. Clinical management of acyclovir resistance can be challenging. Therapy with the antiviral drug foscarnet (40–80 mg/kg IV every 8 h until clinical resolution) is the only cur­ rently well demonstrated approach (Chap. 196). Because of its toxic­ ity and cost, this drug is usually reserved for patients with extensive mucocutaneous infections. Cidofovir is a nucleotide analogue and exists as a phosphonate or monophosphate form. Most TK-deficient strains of HSV are sensitive to cidofovir. Cidofovir ointment speeds healing of acyclovir-resistant lesions, but the topical drug itself can cause mucocutaneous ulcerations. No well-controlled trials of systemic cidofovir have been reported. Occasional cases may respond to topical imiquimod. Anecdotal case reports of successful

TABLE 197-1  Antiviral Chemotherapy for Herpes Simplex Virus (HSV) Infection I.  Mucocutaneous HSV infections A. Infections in immunosuppressed patients

  1. Acute symptomatic first or recurrent episodes: IV acyclovir (5 mg/kg q8h) or oral acyclovir (400 mg qid), famciclovir (500 mg bid or tid), or valacyclovir

(500 mg bid) is effective. Treatment duration may vary from 7 to 21 days. IV therapy may be given for 2–10 days until clinical improvement and followed by oral therapy. 2. Suppression of reactivation disease (genital or oral–labial): IV acyclovir (5 mg/kg q8h) or oral valacyclovir (500 mg bid) or acyclovir (400–800 mg 3–5 times per day) prevents recurrences during the 30-day period immediately after transplantation. Longer-term HSV suppression is often used for persons with continued immunosuppression. In bone marrow and renal transplant recipients, oral valacyclovir (2 g/d) is also effective in reducing cytomegalovirus infection. Oral valacyclovir at a dose of 4 g/d has been associated with thrombotic thrombocytopenic purpura after extended use in HIV-positive persons. In HIV-infected persons, oral acyclovir (400–800 mg bid), valacyclovir (500 mg bid), or famciclovir (500 mg bid) is effective in reducing clinical and subclinical reactivations of HSV-1 and HSV-2. B. Infections in immunocompetent patients

  1. Genital herpes a. First episodes: Oral acyclovir (200 mg 5 times per day or 400 mg tid), valacyclovir (1 g bid), or famciclovir (250 mg tid) for 7–14 days is effective. IV acyclovir (5 mg/kg q8h for 5 days) is given for severe disease or neurologic complications such as aseptic meningitis. b. Symptomatic recurrent genital herpes: Short-course (1- to 3-day) regimens are preferred because of low cost, likelihood of adherence, and convenience. Oral acyclovir (800 mg tid for 2 days), valacyclovir (500 mg bid for 3 days), valacyclovir (1 g orally once a day for 3 days), or famciclovir (750 or 1000 mg bid for 1 day, a 1500-mg single dose, or 500 mg stat followed by 250 mg q12h for 2 days) effectively shortens lesion duration. Other options include oral acyclovir (200 mg 5 times per day), valacyclovir (500 mg bid), and famciclovir (125 mg bid for 5 days). c. Suppression of recurrent genital herpes: Oral acyclovir (400–800 mg bid) or valacyclovir (500 mg daily) is given. Patients with >9 episodes per year should take oral valacyclovir (1 g daily or 500 mg bid) or famciclovir (250 mg bid or 500 mg bid).
  2. Oral–labial HSV infections a. First episode: Oral acyclovir is given (200 mg 5 times per day or 400 mg tid); an oral acyclovir suspension can be used (600 mg/m2 qid). Oral famciclovir

(250 mg bid) or valacyclovir (1 g bid) has been used clinically. The duration of therapy is 5–10 days. b. Recurrent episodes: If initiated at the onset of the prodrome, single-dose or 1-day therapy effectively reduces pain and speeds healing. Regimens include oral famciclovir (a 1500-mg single dose or 750 mg bid for 1 day) or valacyclovir (a 2-g single dose or 2 g bid for 1 day). Self-initiated therapy with 6-timesdaily topical penciclovir cream effectively speeds healing of oral–labial HSV infection. Topical acyclovir cream has also been shown to speed healing or single dose oral amenamevir 1200 mg. c. Suppression of reactivation of oral–labial HSV: If started before exposure and continued for the duration of exposure (usually 5–10 days), oral acyclovir (400 mg bid) prevents reactivation of recurrent oral–labial HSV infection associated with severe sun exposure. 3. Surgical prophylaxis of oral or genital HSV infection: Several surgical procedures, such as laser skin resurfacing, trigeminal nerve-root decompression, and lumbar disk surgery, have been associated with HSV reactivation. IV acyclovir (3–5 mg/kg q8h) or oral acyclovir (800 mg bid), valacyclovir (500 mg bid), or famciclovir (250 mg bid) effectively reduces reactivation. Therapy should be initiated 48 h before surgery and continued for 3–7 days. 4. Herpetic whitlow: Oral acyclovir (200 mg) is given 5 times daily (alternative: 400 mg tid) for 7–10 days. 5. HSV proctitis: Oral acyclovir (400 mg 5 times per day) is useful in shortening the course of infection. In immunosuppressed patients or in patients with severe infection, IV acyclovir (5 mg/kg q8h) may be useful. 6. Herpetic eye infections: In acute keratitis, topical trifluorothymidine, vidarabine, idoxuridine, acyclovir, penciclovir, and interferon are all beneficial. Debridement may be required. Topical steroids may worsen disease. II.  Central nervous system HSV infections A. HSV encephalitis: IV acyclovir (10 mg/kg q8h; 30 mg/kg per day) is given for 10 days or until HSV DNA is no longer detected in cerebrospinal fluid. B. HSV aseptic meningitis: No studies of systemic antiviral chemotherapy exist. If therapy is to be given, IV acyclovir (15–30 mg/kg per day) should be used. C. Autonomic radiculopathy: No studies are available. Most authorities recommend initial therapy with IV acyclovir (5–10 mg/kg q8h) followed by oral therapy for 21 days. III.  Neonatal HSV infections: IV acyclovir (60 mg/kg per day, divided into 3 doses) is given. The recommended duration of IV treatment is 21 days. Monitoring for relapse should be undertaken. Continued suppression with oral acyclovir suspension should be given for 3–4 months. IV.  Visceral HSV infections A. HSV esophagitis: IV acyclovir (15 mg/kg per day) is given. In some patients with milder forms of immunosuppression, oral therapy with valacyclovir or famciclovir is effective. B. HSV pneumonitis and lymphadenitis: No controlled studies exist. IV acyclovir (15 mg/kg per day) should be considered. V.  Disseminated HSV infections: No controlled studies exist. IV acyclovir (5 mg/kg q8h) should be tried. Adjustments for renal insufficiency may be needed. No definite evidence indicates that therapy will decrease the risk of death. VI.  Erythema multiforme associated with HSV: Anecdotal observations suggest that oral acyclovir (400 mg bid or tid) or valacyclovir (500 mg bid) will suppress erythema multiforme. VII.  Infections due to acyclovir-resistant HSV: IV foscarnet (40 mg/kg IV q8h) should be given until lesions heal. The optimal duration of therapy and the usefulness of its continuation to suppress lesions are unclear. The helicase-primase inhibitor pritelivir, 400 mg oral loading dose on day 1 followed by 100 mg once daily for 21–28 days, has reported effectiveness. Some patients may benefit from cutaneous application of trifluorothymidine or 1% cidofovir gel, both of which must be compounded at a pharmacy. These preparations should be applied once daily for 5–7 days. Topical imiquimod can be considered. IV cidofovir (5 mg/kg weekly) may be considered. VIII.  Acyclovir and pregnancy: No adverse effects to the fetus or newborn have been attributable to acyclovir. Acyclovir can be used in all stages of pregnancy and among women who are breastfeeding (the drug can be found in breast milk). Suppressive acyclovir treatment in late pregnancy (acyclovir 400 mg orally tid or valacyclovir 500 mg orally bid from ~34 weeks until delivery) reduces the frequency of cesarean delivery among women with recurrent genital herpes. Such treatment may not protect against transmission to neonates. treatment of acyclovir resistant HSV with oral pritelivir have been reported in immunocompetent persons. Drugs from a new class that inhibit HSV-specific helicase/primase activity, amenamevir and pritelivir, are under clinical investigation and appear to offer a better toxicity profile for the treatment of acyclovir-resistant strains of HSV. Amenamevir is licensed for varicella-zoster infections in

CHAPTER 197 Herpes Simplex Virus Infections Japan, and a recent study has shown promise in recurrent oral– labial HSV. ACYCLOVIR EFFICACY IN THE DEVELOPING WORLD Initial studies of acyclovir-like drugs were performed solely in the developed world. While acyclovir, valacyclovir, and famciclovir are

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198 Varicella-Zoster Virus Infections

effective in the developing world, their clinical and virologic ben­ efits, especially in reducing the frequency of genital lesions among patients in Africa, seem reduced from those in European and U.S. populations. The mechanism of this phenomenon is uncertain. Acyclovir therapy does not reduce the rate of HIV acquisition; however, HIV load among MSM in the United States decreased by 1.3 log10 in contrast to 0.9 log10 among Peruvian MSM and 0.5 log10 among African women. Curiously, the anti-HIV drug tenofovir reduces HSV-2 acquisition among women in Africa, although it has no demonstrable clinical benefit or antiviral effects among persons with established HSV-2 infection in studies in the United States. The reasons for these disparate results are unclear.

■ ■PREVENTION Efforts to control HSV disease on a population basis through suppres­ sive antiviral therapy and/or educational programs have been limited. Barrier forms of contraception (especially condoms) decrease the like­ lihood of transmission of HSV infection, particularly during periods of asymptomatic viral excretion. When lesions are present, HSV infection may be transmitted by skin-to-skin contact despite the use of a condom. Nevertheless, the available data suggest that consistent condom use is an effective means of reducing the risk of genital HSV-2 transmission. Chronic daily antiviral therapy with valacyclovir can also be partially effective in reducing acquisition of HSV-2, especially among suscep­ tible women. There are no comparative efficacy studies of valacyclovir versus condom use. Most authorities suggest both approaches. The need for a vaccine to prevent acquisition of HSV infection is great, especially in light of the role HSV-2 plays in enhancing the acquisition and transmission of HIV-1. PART 5 Infectious Diseases A substantial portion of neonatal HSV cases could be prevented by reducing the acquisition of HSV by women in the third trimester of pregnancy. Neonatal HSV infection can result from either the acquisi­ tion of maternal infection near term or the reactivation of infection at delivery in the already-infected mother. Women without known genital herpes should be counseled to abstain from vaginal intercourse during the third trimester with partners known to have or suspected of hav­ ing genital herpes. Some authorities have recommended that antiviral therapy with acyclovir or valacyclovir be given to HSV-2-infected women in late pregnancy as a means of reducing reactivation of HSV-2 at term. Data are not available to support the efficacy of this approach, and the high treatment-to-prevention ratio makes this a difficult if not dubious public health strategy, even though it can reduce the frequency of HSV-associated cesarean delivery. ■ ■FURTHER READING Centers for Disease Control and Prevention: 2021 Sexually transmitted diseases treatment guidelines. Available at https://www

.cdc.gov/std/treatment-guidelines/herpes.htm. James C et al: Herpes simplex virus: Global infection prevalence and incidence estimates, 2016. Bull World Health Organ 98:315, 2020. Kawashima M et al: A phase 3, randomized, double-blind, placebocontrolled study evaluating a single, patient-initiated dose of ame­ namevir for recurrent herpes labialis. J Dermatol 50:311, 2022. Looker KJ et al: Global and regional estimates of the contribution of herpes simplex virus type 2 infection to HIV incidence: A population attributable fraction analysis using published epidemiological data. Lancet Infect Dis 20:240, 2020. Mahant S et al: Neonatal herpes simplex virus infection among Medicaid-enrolled children: 2009–2015. Pediatrics 143:e20183233, 2019. Serris A et al: Pritelivir for recurrent acyclovir-resistant herpes simplex virus 2 infections in immunocompromised patients. J Antimicrob Chemother 77:2303, 2022.

Richard J. Whitley

Varicella-Zoster Virus

Infections ■ ■DEFINITION Varicella-zoster virus (VZV) causes two distinct clinical syndromes: varicella (chickenpox) and herpes zoster (shingles). Chickenpox, a ubiquitous and extremely contagious infection, is usually a benign ill­ ness of childhood characterized by an exanthematous vesicular rash. With reactivation of latent VZV (which is most common after the sixth decade of life), herpes zoster presents as a dermatomal vesicular rash and is usually associated with severe pain. ■ ■HISTORY AND ETIOLOGY Early in the twentieth century, similarities in the histopathologic fea­ tures of skin lesions resulting from varicella and herpes zoster were described. Viral isolates from patients with each of these diseases produced similar pathology in tissue culture—specifically, the appear­ ance of eosinophilic intranuclear inclusions and multinucleated giant cells. These results suggested that the viruses were biologically similar. Restriction endonuclease analyses of viral DNA from a patient with chickenpox who subsequently developed herpes zoster verified the molecular identity of the two viruses responsible for these different clinical presentations. VZV is a member of the family Herpesviridae, sharing with other members such structural characteristics as a lipid envelope surround­ ing a nucleocapsid with icosahedral symmetry, a total diameter of ~180–200 nm, and centrally located double-stranded DNA that is ~125,000 bp in length. ■ ■PATHOGENESIS AND PATHOLOGY Primary Infection  Transmission occurs readily by the respiratory route; the subsequent localized replication of the virus at an undefined site (presumably the nasopharynx) leads to seeding of the lymphatic/ reticuloendothelial system and ultimately to the development of viremia. Viremia in patients with chickenpox manifests as a diffuse and scattered skin lesions that can be confirmed by the recovery of VZV from the blood (rarely) or routinely by the detection of viral DNA in either blood or lesions by polymerase chain reaction (PCR). Vesicles involve the corium or dermis, with degenerative changes characterized by balloon­ ing, the presence of multinucleated giant cells, and eosinophilic intranu­ clear inclusions. Infection may involve localized blood vessels of the skin, resulting in necrosis and epidermal hemorrhage. With the evolution of disease, the vesicular fluid becomes cloudy because of the recruitment of polymorphonuclear leukocytes and the presence of degenerated cells and fibrin. Ultimately, the vesicles either rupture and release their fluid (which includes infectious virus) or are gradually reabsorbed. Recurrent Infection  The mechanism of reactivation of VZV that results in herpes zoster is unknown. The virus infects dorsal root ganglia during chickenpox, where it remains latent until reactivated. Histopathologic examination of representative dorsal root ganglia dur­ ing active herpes zoster demonstrates hemorrhage, edema, and lym­ phocytic infiltration. Latent virus has been detected in sensory (dorsal, cranial, and enteric) ganglia. Active replication of VZV in other organs, such as the lung or the brain, can occur during either chickenpox or herpes zoster but is uncommon in the immunocompetent host. Pulmonary involvement is characterized by interstitial pneumonitis, multinucleated giant cell formation, intranuclear inclusions, and pulmonary hemorrhage. Central nervous system (CNS) infection leads to histopathologic evidence of perivascular cuffing similar to that encountered in measles and other viral encephalitides. Focal hemorrhagic necrosis of the brain, characteristic of herpes simplex virus (HSV) encephalitis, develops infrequently in VZV infection.

■ ■EPIDEMIOLOGY AND CLINICAL MANIFESTATIONS Chickenpox  Humans are the only known reservoir for VZV. Chickenpox is highly contagious, with an attack rate of at least 90% among susceptible (seronegative) individuals. Persons of both sexes and all races are infected equally. The virus is endemic in the popula­ tion at large; however, it becomes epidemic among susceptible indi­ viduals during seasonal peaks—namely, late winter and early spring in the temperate zone. For the most part, our knowledge of the disease’s natural history and incidence predates the licensure of the chicken­ pox vaccine in 1995. Historically, children 5–9 years old were most commonly affected, accounting for 50% of all cases. Most other cases involved children 1–4 and 10–14 years old. Approximately 10% of the population of the United States over the age of 15 was susceptible to infection. VZV vaccination during the second year of life and followed by a preschool booster has dramatically changed the epidemiology of infection, causing a significant decrease in the annualized incidence of chickenpox, as noted below. The incubation period of chickenpox ranges from 10 to 21 days but is usually 14–17 days. Secondary attack rates in susceptible siblings within a household are 70–90%. Patients are infectious ~48 h before the onset of the vesicular rash, during the period of vesicle formation (which generally lasts 4–5 days), and until all vesicles are crusted. Clinically, chickenpox presents as a rash, low-grade fever, and malaise, although a few patients develop a prodrome 1–2 days before onset of the exanthem. In the immunocompetent patient, chickenpox is usually a benign illness associated with lassitude and with body temperatures of 37.8°–39.4°C (100°–103°F) of 3–5 days’ duration. The skin lesions—the hallmark of the infection—include maculopapules, vesicles, and scabs in various stages of evolution (Fig. 198-1; see also Fig. A1-30). These lesions, which evolve from maculopapules to vesi­ cles over hours to days, appear on the trunk and face and rapidly spread to involve other areas of the body. Most are small and have an ery­ thematous base with a diameter of 5–10 mm. Successive crops appear over a 2- to 4-day period. Lesions can also be found on the mucosa of the pharynx and/or the vagina. Their severity varies from one person to another. Some individuals have very few lesions, while others have as many as 2000. Younger children tend to have fewer vesicles than older individuals. Within families, secondary and tertiary cases are associ­ ated with a larger number of vesicles than the first family case. Immu­ nocompromised patients—both children and adults, particularly those FIGURE 198-1  Varicella lesions at various stages of evolution: vesicles on an erythematous base, umbilical vesicles, and crusts.

with leukemia—have lesions (often with a hemorrhagic base) that are more numerous and take longer to heal than those of immunocompe­ tent individuals. Immunocompromised individuals are also at greater risk for visceral complications, which occur in 30–50% of cases and are fatal 15% of the time in the absence of antiviral therapy.

The most common infectious complication of varicella is secondary bacterial superinfection of the skin, which is usually caused by Strep­ tococcus pyogenes or Staphylococcus aureus, including strains that are methicillin-resistant. Skin infection results from excoriation of lesions after scratching. Gram stain of skin lesions may help clarify the etiology of unusually erythematous and pustulated lesions. The most common extracutaneous site of involvement in children is the CNS. The syndrome of acute cerebellar ataxia and meningeal inflammation generally appears ~21 days after onset of the rash and rarely develops in the pre-eruptive phase. The cerebrospinal fluid (CSF) contains lymphocytes and elevated levels of protein. CNS involvement is a benign complication of VZV infection in immunocompetent children and generally does not require hospitalization. Aseptic meningitis, encephalitis, transverse myelitis, and Guillain-Barré syn­ drome can also occur. Encephalitis is reported in 0.1–0.2% of children with chickenpox. Reye syndrome, an historical complication for the most part, can occur in children concomitantly treated with aspirin; therefore, aspirin is no longer utilized. Other than supportive care, no specific therapy (e.g., although acyclovir is administered by some physicians) has proved efficacious for patients with CNS involvement. Varicella pneumonia, the most serious complication following chickenpox, develops more often in adults (up to 20% of cases) than in children and is particularly severe in pregnant women. Pneumonia due to VZV usually has its onset 3–5 days into the illness and is associ­ ated with tachypnea, cough, dyspnea, and fever. Cyanosis, hypoxemia, pleuritic chest pain, and hemoptysis are frequently noted. Roentgeno­ graphic evidence of disease consists of nodular infiltrates and intersti­ tial pneumonitis. Resolution of pneumonitis parallels improvement of the skin rash; however, patients may have persistent fever and compro­ mised pulmonary function for weeks. CHAPTER 198 Varicella-Zoster Virus Infections Other complications of chickenpox include myocarditis, corneal lesions, nephritis, arthritis, bleeding diatheses, acute glomerulonephri­ tis, and hepatitis. Hepatic involvement, distinct from Reye syndrome and usually asymptomatic, is common in chickenpox and is generally characterized by elevated levels of liver enzymes, particularly aspartate and alanine aminotransferases. Perinatal varicella is associated with mortality rates as high as 30% when maternal disease develops within 5 days before delivery or within 48 h thereafter. Illness in this setting is unusually severe because the newborn does not receive protective transplacental antibodies and has an immature immune system. Congenital varicella, with clinical mani­ festations of limb hypoplasia, cicatricial skin lesions, and microcephaly at birth, is extremely uncommon. Herpes Zoster  Herpes zoster (shingles) is a sporadic disease that results from reactivation of latent VZV from dorsal root ganglia. Most patients with shingles have no history of recent exposure to other individuals with VZV infection. Herpes zoster occurs at all ages, but its incidence is highest (5–10 cases per 1000 persons) among individu­ als in the sixth decade of life and beyond. Data suggest that at least 1.2 million cases occur annually in the United States. Vaccination has, likely, decreased the incidence of shingles. Recurrent herpes zoster is exceedingly rare except in immunocompromised hosts, especially those with AIDS. Herpes zoster is characterized by a unilateral vesicular dermatomal eruption, often associated with severe pain. The dermatomes from T3 to L3 are most frequently involved. If the ophthalmic branch of the trigeminal nerve is involved, zoster ophthalmicus results. The factors responsible for the reactivation of VZV are not known. In children, reactivation is usually benign; in adults, it can be debilitating because of pain. The onset of disease is heralded by pain within the dermatome, which may precede lesions by 48–72 h; an erythematous maculo­ papular rash evolves rapidly into vesicular lesions (Fig. 198-2). In the normal host, these lesions may remain few in number and continue to

FIGURE 198-2  Close-up of lesions of disseminated zoster. Note lesions at different stages of evolution, including pustules and crusting. (Photo courtesy of Lindsey Baden; with permission.) FIGURE 198-3  Herpes zoster in an HIV-infected patient is seen as hemorrhagic vesicles and pustules on an erythematous base grouped in a dermatomal distribution. form for only 3–5 days. The total duration of disease is generally 7–10 days; however, it may take as long as 2–4 weeks for the skin to return to normal. Patients with herpes zoster can transmit infection to sero­ negative individuals, resulting in chickenpox. In a few patients, char­ acteristic localization of pain to a dermatome with serologic evidence of herpes zoster has been reported in the absence of skin lesions, an entity known as zoster sine herpetica. When branches of the trigeminal nerve are involved, lesions may appear on the face, particularly the tip of the nose (Hutchinson sign), in the mouth, in the eye, or on the tongue. Zoster ophthalmicus is usually a debilitating condition that can result in blindness in the absence of antiviral therapy. In Ramsay Hunt syndrome, pain and vesicles appear in the external auditory canal, and patients lose their sense of taste in the anterior two-thirds of the tongue while developing ipsilateral facial palsy. The geniculate ganglion of the sensory branch of the facial nerve is involved. PART 5 Infectious Diseases In both normal and immunocompromised hosts, the most debilitat­ ing complication of herpes zoster is pain associated with acute neuritis and postherpetic neuralgia. Postherpetic neuralgia is uncommon in young individuals; however, at least 50% of patients over age 50 report some degree of pain in the involved dermatome for months after the resolution of cutaneous disease. Changes in sensation in the derma­ tome, resulting in either hypo- or hyperesthesia, are common. CNS involvement may follow localized herpes zoster. Many patients without signs of meningeal irritation have CSF pleocytosis and mod­ erately elevated levels of CSF protein. Symptomatic meningoencepha­ litis is characterized by headache, fever, photophobia, meningitis, and vomiting. A rare manifestation of CNS involvement is granulomatous angiitis with contralateral hemiplegia, which can be diagnosed by cere­ bral arteriography. Other neurologic manifestations include transverse myelitis with or without motor paralysis. Like chickenpox, herpes zoster is more severe in immunocompro­ mised than immunocompetent individuals. Lesions continue to form for >1 week, and scabbing is not complete in most cases until 3 weeks into the illness. Patients with Hodgkin disease and non-Hodgkin lymphoma are at greatest risk for progressive herpes zoster. Cutaneous dissemination (Fig. 198-3) develops in ~40% of these patients in the absence of therapy. Among patients with cutaneous dissemination, the risk of pneumonitis, meningoencephalitis, hepatitis, and other serious complications is increased by 5–10%. However, even in immunocom­ promised patients, disseminated zoster is rarely fatal. Recipients of hematopoietic stem cell transplants are at particularly high risk of VZV infection. Of all cases of posttransplantation VZV infection, 30% occur within 1 year (50% of these within 9 months);

45% of the patients involved have cutaneous or visceral dissemination. The mortality rate in this situation is 10%. Postherpetic neuralgia, scarring, and bacterial superinfection are especially common in VZV infections occurring within 9 months of transplantation. Among infected patients, concomitant graft-versus-host disease increases the chance of dissemination and/or death. ■ ■DIFFERENTIAL DIAGNOSIS The diagnosis of chickenpox is usually based on clinical presentation and is not difficult. The characteristic rash and a history of recent exposure should lead to a prompt diagnosis. Other viral infections that can mimic chickenpox include disseminated HSV infection in patients with atopic dermatitis and the disseminated vesiculopapular lesions sometimes associated with coxsackievirus infection, echovirus infec­ tion, or atypical measles. However, these rashes are more commonly morbilliform with a hemorrhagic component rather than vesicular or vesiculopustular. Rickettsialpox (Chap. 192) is sometimes confused with chickenpox; however, rickettsialpox can be distinguished eas­ ily by detection of the “herald spot” at the site of the mite bite and the development of a more pronounced headache. Serologic testing is also useful in differentiating rickettsialpox from varicella and can confirm susceptibility in adults unsure of their chickenpox history. Mpox should be considered in travelers returning from endemic areas (Chap. 201) and has been a recent concern in HIV-infected individu­ als, leading to vaccination campaigns. Smallpox concern has increased because of the threat of bioterrorism (Chap. S4). The lesions of small­ pox are larger than those of chickenpox and are all at the same stage of evolution at any given time. Unilateral vesicular lesions in a dermatomal pattern should lead rapidly to the diagnosis of herpes zoster, although the occurrence of shingles without a rash has been reported. Both HSV and coxsacki­ evirus infections can cause dermatomal vesicular lesions. Supportive diagnostic virology and fluorescent staining of skin scrapings with monoclonal antibodies are helpful in ensuring the proper diagnosis. In the prodromal stage of herpes zoster, the diagnosis can be exceed­ ingly difficult and may be made only after lesions have appeared or by retrospective serologic assessment. ■ ■LABORATORY FINDINGS Unequivocal confirmation of the diagnosis is possible only through the isolation of VZV in susceptible tissue-culture cell lines, the

demonstration of either seroconversion or a fourfold or greater rise in antibody titer between acute-phase and convalescent-phase serum specimens, or the detection of VZV DNA by PCR. Specimens for detection of VZV DNA by PCR include lesions, blood, and saliva. A rapid impression can be obtained by a Tzanck smear, with scraping of the base of the lesions in an attempt to demonstrate multinucleated giant cells; however, the sensitivity of this method is low (~60%). PCR detection of viral DNA in vesicular fluid is available in many diagnostic laboratories and is the diagnostic method of choice. Direct immuno­ fluorescent staining of cells from the lesion base or detection of viral antigens by other assays (such as the immunoperoxidase assay) is also useful. The most frequently employed serologic tools for assessing host response are the immunofluorescent detection of antibodies to VZV membrane antigens, the fluorescent antibody to membrane antigen (FAMA) test, immune adherence hemagglutination, and enzymelinked immunosorbent assay (ELISA). The FAMA test and the ELISA appear to be most sensitive. TREATMENT Varicella-Zoster Virus Infections Medical management of chickenpox in the immunologically nor­ mal host is directed toward the prevention of avoidable com­ plications. Obviously, good hygiene includes daily bathing and soaks. Secondary bacterial infection of the skin can be avoided by meticulous skin care, particularly with close cropping of fin­ gernails. Pruritus can be decreased with topical dressings or the administration of antipruritic drugs. Tepid water baths and wet compresses are better than drying lotions for the relief of itching. Administration of aspirin to children with chickenpox should be avoided because of its association with the development of Reye syndrome. Acyclovir (800 mg by mouth five times daily), valacy­ clovir (1 g three times daily), or famciclovir (250 mg three times daily) for 5–7 days is recommended for adolescents and adults with chickenpox of ≤24 h in duration. (Valacyclovir is licensed for use in children and adolescents. Famciclovir is recommended but not licensed for varicella.) Likewise, acyclovir therapy may be of benefit to children <12 years of age if initiated early in the disease (<24 h) at a dose of 20 mg/kg every 6 h. The advantages (i.e., pharmacokinetics) of the second-generation agents valacyclovir and famciclovir are described in Chap. 196. Aluminum acetate soaks for the management of herpes zoster can be both soothing and cleansing. Patients with herpes zoster benefit from oral antiviral therapy, as evidenced by accelerated healing of lesions and resolution of zoster-associated pain with acyclovir, valacyclovir, or famciclovir. Acyclovir is administered at a dosage of 800 mg five times daily for 7–10 days. However, valacyclovir and famciclovir have superior pharmacokinetics and pharmacodynamics and should be used preferentially. Famciclovir, the prodrug of penciclovir, is at least as effective as acyclovir and perhaps more so; the dose is 500 mg by mouth three times daily for 7 days. Valacyclovir, the prodrug of acyclovir, accelerates heal­ ing and resolution of zoster-associated pain more promptly than acyclovir. The dose is 1 g by mouth three times daily for 5–7 days. Compared with acyclovir, both famciclovir and valacyclovir offer the advantage of less frequent administration. All three of these drugs are now available as generic products. In severely immunocompromised hosts (e.g., transplant recipients, patients with cancer, particularly lymphoproliferative malignancies), both chickenpox and herpes zoster (including dis­ seminated disease) should be treated, at least at the outset, with IV acyclovir, which reduces the occurrence of visceral complications but has little effect on healing of skin lesions or pain. The dose is 10 mg/kg every 8 h for 7 days. For low-risk immunocompromised hosts, oral therapy with valacyclovir or famciclovir appears ben­ eficial. If medically feasible, it is desirable to decrease immuno­ suppressive treatment concomitant with the administration of IV acyclovir.

Patients with varicella pneumonia typically require ventilatory support. Persons with zoster ophthalmicus should be referred immediately to an ophthalmologist. Therapy for this condition consists of the administration of analgesics for severe pain and the use of atropine. Acyclovir, valacyclovir, and famciclovir all acceler­ ate healing. Decisions regarding the use of corticosteroids should be made by the ophthalmologist.

The management of acute neuritis and/or postherpetic neuralgia can be particularly difficult. In addition to the judicious use of anal­ gesics ranging from nonnarcotics to narcotic derivatives, drugs such as gabapentin, pregabalin, amitriptyline hydrochloride, lidocaine (patches), and fluphenazine hydrochloride are reportedly beneficial for pain relief. In one study, glucocorticoid therapy administered early in the course of localized herpes zoster significantly acceler­ ated such quality-of-life improvements as a return to usual activity and termination of analgesic medications. The dose of prednisone administered orally was 60 mg/d on days 1–7, 30 mg/d on days 8–14, and 15 mg/d on days 15–21. This regimen is appropriate only for relatively healthy elderly persons with moderate or severe pain at presentation. Patients with osteoporosis, diabetes mellitus, glycosuria, or hypertension may not be appropriate candidates. Glucocorticoids should not be used without concomitant antiviral therapy. ■ ■PREVENTION Three methods are used for the prevention of VZV infections. First, a live attenuated varicella vaccine (Oka) is recommended for all children

1 year of age (up to 12 years of age) who have not had chickenpox and for adults known to be seronegative for VZV. Two doses are recommended for all children: the first at 12–15 months of age and the second at ~4–6 years of age. VZV-seronegative persons >13 years of age should receive two doses of vaccine at least 1 month apart. The vaccine is both safe and efficacious. Breakthrough cases are mild and may result in spread of the vaccine virus to susceptible con­ tacts. The universal vaccination of children has resulted in a decreased incidence of chickenpox in sentinel communities. Furthermore, the inactivated Oka vaccine, when administered to hematopoietic stem cell recipients, decreases the incidence of herpes zoster. CHAPTER 198 Varicella-Zoster Virus Infections In individuals >50 years of age, the shingles vaccine of choice is Shingrix. It is a subunit vaccine (HZ/su) that consists of VZV glyco­ protein E and the AS01B adjuvant. A randomized, placebo-controlled study administered two doses of vaccine or placebo 1 month apart to 15,411 participants aged 50 years or older. Overall vaccine efficacy for the prevention of herpes zoster virus was 97.2% (95% confidence interval, 93.7–99.0%; p <.001). Injection-site and systemic reactions were more frequent in vaccine recipients, but the proportions of participants who had serious adverse events were similar in the vaccine and placebo groups. The Advisory Committee on Immunization Practices recom­ mends that persons in this age group be offered this vaccine in order to reduce the frequency of shingles and the severity of postherpetic neu­ ralgia. Of note, vaccine immunity wanes over time, and reassessment of current recommendations or the use of a promising inactivated vaccine in development will be required. A second approach is to administer varicella-zoster immune globulin (VZIG) to individuals who are susceptible, are at high risk for developing complications of varicella, and have had a significant exposure. This product should be given within 96 h (preferably within 72 h) of the exposure but may be administered up to 10 days with some efficacy. Indications for administration of VZIG appear in Table 198-1, which has been adapted from the American Academy of Pediatrics Red Book. Lastly, antiviral therapy can be given as prophylaxis to individuals at high risk who are ineligible for vaccination or who are beyond the 96-h window after direct contact. While the initial studies have used acyclovir, similar benefit can be anticipated with either valacyclovir or famciclovir. Therapy is instituted 7 days after intense exposure. At this time, the host is midway into the incubation period. This approach significantly decreases disease severity, if not totally preventing disease.

88 - 199 Epstein-Barr Virus Infections, Including Infectious Mononucleosis

199 Epstein-Barr Virus Infections, Including Infectious Mononucleosis

TABLE 198-1  Recommendations for VZIG Administration Exposure Criteria

  1. Significant exposure to a person with chickenpox or zoster a. Household: residence in the same household b. Playmate: face-to-face indoor play c. Hospital Varicella: same 2- to 4-bed room or adjacent beds in a large ward, face-toface contact with an infectious staff member or patient, visit by a person deemed contagious Zoster: intimate contact (e.g., touching or hugging) with a person deemed contagious d. Newborn infant: onset of varicella in the mother ≤5 days before delivery or ≤48 h after delivery; VZIG not indicated if the mother has zoster

  2. Patient should receive VZIG as soon as possible but not >96 h after exposure. Candidates (Provided They Have Significant Exposure) Include

  3. Immunocompromised susceptible children without a history of varicella or varicella immunization

  4. Susceptible pregnant women

  5. Newborn infants whose mother had onset of chickenpox within 5 days before or within 48 h after delivery

  6. Hospitalized premature infant (≥28 weeks of gestation) whose mother lacks a reliable history of chickenpox or serologic evidence of protection against varicella

  7. Hospitalized premature infant (<28 weeks of gestation or ≤1000-g birth weight), regardless of maternal history of varicella or VZV serologic status Abbreviation: VZIG, varicella-zoster immune globulin. Source: Table is adapted from the American Academy of Pediatrics Red Book. PART 5 Infectious Diseases ■ ■FURTHER READING Arvin A: Aging, immunity, and the varicella-zoster virus. N Engl J Med 352:2266, 2005. Arvin A, Abendroth A: Varicella-zoster virus, in Fields Virology: DNA Viruses. Vol 2. 7th ed, Howley P et al (eds). Philadelphia, PA, Wolters Kluwer, 2021, pp 445-488. Cohen JI: A new vaccine to prevent herpes zoster. N Engl J Med 372: 2149, 2015. Gershon AA et al: Varicella zoster virus infection. Nat Rev Dis Primers 1:15016, 2015. Gnann JW, Whitley RJ: Herpes zoster. N Engl J Med 347:340,

Kimberlin DW et al (eds): Redbook: 2021-2024 Report of the Committee on Infectious Diseases, 32nd ed. Itasca, IL, American Academy of Pediatrics, 2021. Lai H et al: Efficacy of an adjuvanted herpes zoster subunit vaccine in older adults. N Engl J Med 372:2087, 2015. Marin M et al: Monitoring varicella vaccine impact on varicella inci­ dence in the United States: Surveillance challenges and changing epidemiology, 1995-2019. J Infect Dis 226:S392, 2022. Morrison VA et al: Long-term persistence of zoster vaccine efficacy. Clin Infect Dis 60:900, 2015. Shaw J, Gershon AA: Varicella virus vaccination in the United States. Viral Immunol 31:96, 2018. Whitley RJ, Arvin A: Chickenpox and herpes zoster (varicella-zoster virus), in Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases, 10th ed. Blaser MJ, Cohen JI, Holland SM (eds). Philadelphia, PA, Elsevier Press. In press. Wutzler P et al: Varicella vaccination: The global experience. Expert Rev Vaccines 16:833, 2017.

Jeffrey I. Cohen

Epstein-Barr Virus

Infections, Including

Infectious Mononucleosis ■ ■DEFINITION Epstein-Barr virus (EBV) is the cause of heterophile-positive infec­ tious mononucleosis (IM), which is characterized by fever, sore throat, lymphadenopathy, and atypical lymphocytosis. EBV is also associated with several tumors, including nasopharyngeal and gastric carcinoma, Burkitt’s lymphoma, Hodgkin’s lymphoma, T-cell lymphoma, and (in patients with immunodeficiencies) B-cell lymphoma and smooth muscle tumors. Large epidemiology studies show a strong association of EBV with multiple sclerosis. The virus is a member of the family Herpesviridae. The two types of EBV that are widely prevalent in nature are not distinguishable by conventional serologic tests. ■ ■EPIDEMIOLOGY EBV infections occur worldwide. These infections are most common in early childhood, with a second peak during late adolescence. By adulthood, >90% of individuals have been infected and have antibodies to the virus. IM is usually a disease of young adults. In lower socio­ economic groups and in areas of the world with deficient standards of hygiene (e.g., developing regions), EBV tends to infect children at an early age, and IM is uncommon. In areas with higher standards of hygiene, infection with EBV is often delayed until adulthood, and IM is more prevalent. EBV is spread by contact with oral secretions. The virus is frequently transmitted from asymptomatic adults to infants and among young adults by transfer of saliva during kissing. Transmission by less inti­ mate contact is rare. EBV has been transmitted by blood transfusion and by bone marrow transplantation. More than 90% of asymptomatic seropositive individuals shed the virus in oropharyngeal secretions. Shedding is increased in immunocompromised patients and those with IM. ■ ■PATHOGENESIS EBV is transmitted by salivary secretions. The virus infects the epithe­ lium of the oropharynx and the salivary glands and is shed from these cells. While B cells may become infected after contact with epithelial cells, studies suggest that lymphocytes in the tonsillar crypts can be infected directly. The virus then spreads through the bloodstream. The proliferation and expansion of EBV-infected B cells along with reactive T cells during IM result in enlargement of lymphoid tissue. Polyclonal activation of B cells leads to the production of antibodies to host-cell and viral proteins. During the acute phase of IM, up to 1 in every 100 B cells in the peripheral blood is infected by EBV; after recovery, 1–50 in every 1 million B cells is infected. During IM, there is an inverted CD4+/CD8+ T-cell ratio. The percentage of CD4+ T cells decreases, while there are large clonal expansions of CD8+ T cells; up to 40% of CD8+ T cells are directed against EBV antigens during acute infection. Memory B cells, not epithelial cells, are the reservoir for EBV in the body. When patients are treated with acyclovir, shedding of EBV from the oropharynx stops but the virus persists in B cells. The EBV receptor (CD21) on the surface of B cells is also the recep­ tor for the C3d component of complement. Another EBV receptor (CD35) on B cells binds to CD21. Human leukocyte antigen class II serves as a co-receptor for EBV entry into B cells. EBV infection of epithelial cells occurs by virus binding to ephrin A2 and results in viral replication and production of virions. When B cells are infected by EBV in vitro, they become transformed and can proliferate indefinitely. During latent infection of B cells, the EBV nuclear antigens (EBNAs), latent membrane proteins (LMPs), multiple microRNAs, and small EBV RNAs (EBERs) are expressed in vitro. EBV-transformed B cells

secrete immunoglobulin; only a small fraction of these cells produce virus. Cellular immunity is more important than humoral immunity in controlling EBV infection. In the initial phase of infection, suppressor T cells, natural killer (NK) cells, and nonspecific cytotoxic T cells are important in controlling the proliferation of EBV-infected B cells. Lev­ els of markers of T-cell activation and serum interferon γ are elevated. Later in infection, human leukocyte antigen–restricted cytotoxic T cells that recognize EBNAs and LMPs and destroy EBV-infected cells are generated. If T-cell immunity is compromised, EBV-infected B cells may begin to proliferate. When EBV is associated with lymphoma in immuno­ competent persons, virus-induced proliferation is but one step in a multistep process of neoplastic transformation. In many EBV-containing tumors, LMP-1 mimics members of the tumor necrosis factor receptor family (e.g., CD40), transmitting growth-proliferating signals. ■ ■CLINICAL MANIFESTATIONS Signs and Symptoms  Most EBV infections in infants and young children either are asymptomatic or present as mild pharyngitis with or without tonsillitis. In contrast, ~75% of infections in adolescents pres­ ent as IM. IM in the elderly often presents with nonspecific symptoms, including prolonged fever, fatigue, myalgia, and malaise. In contrast, pharyngitis, lymphadenopathy, splenomegaly, and atypical lympho­ cytes are relatively rare in elderly patients. The incubation period for IM in young adults is ~4–6 weeks. A pro­ drome of fatigue, malaise, and myalgia may last for 1–2 weeks before the onset of fever, sore throat, and lymphadenopathy. Fever is usually low-grade and is most common in the first 2 weeks of the illness; how­ ever, it may persist for >1 month. Common signs and symptoms are listed along with their frequencies in Table 199-1. Lymphadenopathy and pharyngitis are most prominent during the first 2 weeks of the illness, while splenomegaly is more prominent during the second and third weeks. Lymphadenopathy most often affects the posterior cervi­ cal nodes but may be generalized. Enlarged lymph nodes are frequently tender and symmetric but are not fixed in place. Pharyngitis, often the most prominent sign, can be accompanied by enlargement of the tonsils with an exudate resembling that of streptococcal pharyngitis. A morbilliform or papular rash, usually on the arms or trunk, devel­ ops in ~5% of cases (Fig. 199-1). Earlier studies reported that many patients treated with penicillin derivatives develop a macular rash; penicillin-associated rashes are not predictive of future adverse reac­ tions to penicillins. More recent studies suggest that EBV-associated rashes may occur with similar frequency in those exposed to penicillin When CNS complications develop, they usually do so during the first 2 weeks of EBV infection; in some patients, especially children, TABLE 199-1  Signs and Symptoms of Infectious Mononucleosis MEDIAN PERCENTAGE OF

PATIENTS (RANGE) MANIFESTATION Symptoms Sore throat 75 (50–87) Malaise 47 (42–76) Headache 38 (22–67) Abdominal pain, nausea, or vomiting 17 (5–25) Chills 10 (9–11) Signs Lymphadenopathy 95 (83–100) Fever 93 (60–100) Pharyngitis or tonsillitis 82 (68–90) Splenomegaly 51 (43–64) Hepatomegaly 11 (6–15) Rash 10 (0–25) Periorbital edema 13 (2–34) Palatal enanthem   7 (3–13) Jaundice   5 (2–10)

FIGURE 199-1  Rash in a patient with infectious mononucleosis due to Epstein-Barr virus. (Courtesy of Maria Turner, MD; with permission.) derivatives and those not taking these drugs. Erythema nodosum (Fig. A1-39) and erythema multiforme (Fig. A1-24) also have been described (Chap. 61). The severity of the disease correlates with the levels of CD8+ T cells and EBV DNA in the blood. Most patients have symptoms for 2–4 weeks, but nearly 10% have fatigue that persists for ≥6 months. CHAPTER 199 Laboratory Findings  The white blood cell count is usually ele­ vated and peaks at 10,000–20,000/μL during the second or third week of illness. Lymphocytosis is usually demonstrable, with >10% atypical lymphocytes. The latter cells are enlarged lymphocytes that have abun­ dant cytoplasm, vacuoles, and indentations of the cell membrane (Fig. 199-2). CD8+ T cells predominate among the atypical lympho­ cytes. Low-grade neutropenia and thrombocytopenia are common during the first month of illness. Liver function is abnormal in >90% of cases. Serum levels of aminotransferases and alkaline phosphatase are usually mildly elevated. The serum concentration of bilirubin is elevated in ~40% of cases. Epstein-Barr Virus Infections, Including Infectious Mononucleosis
Complications  Most cases of IM are self-limited. Deaths are very rare and are most often due to central nervous system (CNS) complications, splenic rupture, upper-airway obstruction, or bacterial superinfection. FIGURE 199-2  Atypical lymphocytes from a patient with infectious mononucleosis due to Epstein-Barr virus.

they are the only clinical manifestations of acute infection. Hetero­ phile antibodies and atypical lymphocytes may be absent. Meningitis and encephalitis are the most common neurologic abnormalities, and patients may present with headache, meningismus, or cerebellar ataxia. Acute hemiplegia and psychosis also have been described. The cerebro­ spinal fluid contains mainly lymphocytes, with occasional atypical lym­ phocytes. Most cases resolve without neurologic sequelae. Acute EBV infection has also been associated with cranial nerve palsies (especially those involving cranial nerve VII), Guillain-Barré syndrome, acute transverse myelitis, and peripheral neuritis.

Autoimmune hemolytic anemia occurs in ~2% of cases during the first 2 weeks. In most cases, the anemia is Coombs-positive, with cold agglutinins directed against the red blood cell i antigen. Most patients with hemolysis have mild anemia that lasts for 1–2 months, but some patients have severe disease with hemoglobinuria and jaundice. Nonspecific antibody responses may also include rheumatoid factor, antinuclear antibodies, anti–smooth muscle antibodies, antiplatelet antibodies, and cryoglobulins. IM has been associated with red-cell aplasia, severe granulocytopenia, thrombocytopenia, pancytopenia, and hemophagocytic lymphohistiocytosis. The spleen ruptures in <0.5% of cases. Splenic rupture is more common among male than female patients and may manifest as abdominal pain, referred shoulder pain, or hemodynamic compromise. Hypertrophy of lymphoid tissue in the tonsils or adenoids can result in upper-airway obstruction, as can inflammation and edema of the epiglottis, pharynx, or uvula. About 10% of patients with IM develop streptococcal pharyngitis after their initial sore throat resolves. Other rare complications associated with acute EBV infection include hepatitis (which can be fulminant), myocarditis or pericarditis, pneumonia with pleural effusion, interstitial nephritis, genital ulcer­ ations, and vasculitis. PART 5 Infectious Diseases EBV-Associated Diseases Other Than IM  EBV-associated lymphoproliferative disease has been described in patients with con­ genital or acquired immunodeficiency, including those with severe combined immunodeficiency, patients with AIDS, and recipients of bone marrow or organ transplants who are receiving immunosuppres­ sive drugs (especially cyclosporine). Proliferating EBV-infected B cells infiltrate lymph nodes and multiple organs, and patients present with fever and lymphadenopathy or gastrointestinal symptoms. Pathologic studies show B-cell hyperplasia or poly- or monoclonal lymphoma. X-linked lymphoproliferative disease is a recessive disorder of young boys who have a normal response to childhood infections but develop fatal lymphoproliferative disorders after infection with EBV. The protein associated with most cases of this syndrome (SAP, encoded by SH2D1A) binds to a protein that mediates interac­ tions of B and T cells. Most patients with this syndrome die of acute IM. Others develop hypogammaglobulinemia, malignant B-cell lym­ phomas, aplastic anemia, or agranulocytosis. Disease resembling X-linked lymphoproliferative disease, but with more prominent hemo­ phagocytosis, has also been associated with mutations in BIRC4. Muta­ tions in ITK, MAGT1, CORO1A, TNFRSF9, IL27RA, CD70, or CD27 are associated with inability to control EBV and lymphoma. Mutations in other genes, such as GATA2, PIK3CD, CTPS1, RLTPR, RSGRP1, TNFRSF9, and several genes associated with severe combined immu­ nodeficiency, also can predispose to severe or fatal EBV disease as well as other infections. Moreover, IM has proved fatal to some patients with no obvious preexisting immune abnormality. Oral hairy leukoplakia (Fig. 199-3) is an early manifestation of infection with HIV in adults (Chap. 208). Most patients present with raised, white corrugated lesions on the tongue (and occasionally on the buccal mucosa) that contain EBV DNA. Children infected with HIV can develop lymphoid interstitial pneumonitis; EBV DNA is often found in lung tissue from these patients. Patients with chronic fatigue syndrome may have titers of antibody to EBV that are elevated but are not significantly different from those in healthy EBV-seropositive adults. These patients do not have elevated levels of EBV DNA in the blood. While some patients have malaise and fatigue that persist for weeks or months after IM, persistent EBV

FIGURE 199-3  Oral hairy leukoplakia often presents as white plaques on the lateral surface of the tongue and is associated with Epstein-Barr virus infection. infection is not a cause of chronic fatigue syndrome. EBV reactivation (largely based on viral serology) has been associated with post-acute COVID-19 syndrome (PACS), but it is unclear that EBV is a cause of the symptoms of PACS. Chronic active EBV infection is very rare and is distinct from chronic fatigue syndrome. The affected patients have an illness lasting >3 months, with elevated levels of EBV DNA in the blood (in T or NK cells); high titers of antibody to EBV; and evidence of organ involvement, including hepatosplenomegaly, lymphadenopathy, and hepatitis, pneumonitis, uveitis, or neurologic disease. Some have somatic mutations in DD3X and other tumor driver genes. EBV is associated with several malignancies. About 15% of cases of Burkitt’s lymphoma in the United States and ~90% of those in Africa are associated with EBV (Chap. 113). African patients with Burkitt’s lymphoma have high levels of antibody to EBV, and their tumor tissue usually contains viral DNA. Malaria in African patients may impair cellular immunity to EBV and induce polyclonal B-cell activation with an expansion of EBV-infected B cells. In addition, malaria may target B cells and result in expansion of germinal centers, with consequently increased activity of activation-induced cytidine deaminase, which can mutate DNA. These changes may enhance the proliferation of B cells with elevated EBV DNA in the bloodstream, thereby increasing the likelihood of a c-myc translocation—the hallmark of Burkitt’s lym­ phoma. EBV-containing Burkitt’s lymphoma also occurs in patients with AIDS. Anaplastic nasopharyngeal carcinoma is common in southern China and is uniformly associated with EBV; the affected tissues con­ tain viral DNA and antigens. Patients with nasopharyngeal carcinoma often have elevated titers of antibody to EBV (Chap. 82). Antibody to an EBV protein, BNLF1, in serum is a useful screening marker. Measurement of EBV DNA in plasma is useful for early detection of nasopharyngeal carcinoma. High levels of EBV plasma DNA before treatment or detectable levels of EBV DNA after radiation therapy correlate with lower rates of overall survival and relapse-free survival among patients with nasopharyngeal carcinoma. Worldwide, the most common EBV-associated malignancy is gastric carcinoma. About 9% of these tumors are EBV-positive including >90% of gastric lymphoepithelioma-like carcinomas (Chap. 85). EBV has been associated with Hodgkin’s lymphoma, especially the mixed-cellularity type (Chap. 114). Patients with Hodgkin’s lymphoma often have elevated titers of antibody to EBV. In about half of cases in the United States, viral DNA and antigens are found in Reed-Sternberg cells. The risk of EBV-positive Hodgkin’s lymphoma is significantly increased in young adults for several years after EBV-seropositive IM. About 50% of non-Hodgkin’s lymphomas in patients with AIDS are EBV-positive. EBV is present in B cells of lesions from patients with lymphomatoid granulomatosis. In some cases, EBV DNA has been detected in tumors from immunocompetent patients with angiocentric nasal NK/T-cell lym­ phoma, aggressive NK leukemia/lymphoma, T-cell lymphoma, and CNS lymphoma. Studies have demonstrated viral DNA in leiomyosarcomas from AIDS patients and in smooth-muscle tumors from organ transplant recipients. Virtually all CNS lymphomas in AIDS patients are associated

Antibody titer

Anti-VCA IgM Anti-VCA IgG

Anti-EBNA 1 week

1 month 2 months 3 months Time of symptoms FIGURE 199-4  Pattern of Epstein-Barr virus (EBV) serology during acute infection. EBNA, Epstein-Barr nuclear antigen; VCA, viral capsid antigen. (Reproduced with permission from JI Cohen, in NS Young et al [eds]: Clinical Hematology. Philadelphia, Mosby, 2006.) with EBV. EBV has been associated with multiple sclerosis; an epidemi­ ology study of over a million military personnel found that the risk of multiple sclerosis was 32-fold higher after primary EBV infection, but not after other virus infections. In addition, a history of IM and higher levels of antibodies to EBNA before the onset of disease is more common in persons with multiple sclerosis than in the general population; additional research on the role of EBV in multiple sclerosis is needed. ■ ■DIAGNOSIS Serologic Testing (Fig. 199-4)  The heterophile test is used for the diagnosis of IM in children and adults. In the test for this antibody, human serum is absorbed with guinea pig kidney, and the heterophile titer is defined as the greatest serum dilution that agglutinates sheep, horse, or cow erythrocytes. The heterophile antibody does not interact with EBV proteins. A titer of ≥40 is diagnostic of acute EBV infec­ tion in a patient who has symptoms compatible with IM and atypical lymphocytes. Tests for heterophile antibodies are positive in 40% of patients with IM during the first week of illness and in 80–90% during the third week. Therefore, repeated testing may be necessary, espe­ cially if the initial test is performed early. Tests usually remain positive for 3 months after the onset of illness, but heterophile antibodies can persist for up to 1 year. These antibodies usually are not detectable in children <5 years of age, in the elderly, or in patients presenting with symptoms not typical of IM. The commercially available monospot test for heterophile antibodies is somewhat more sensitive than the classic heterophile test. The monospot test is ~75% sensitive and ~90% spe­ cific compared with EBV-specific serologies (see below). False-positive monospot results are more common among persons with connective tissue disease, lymphoma, viral hepatitis, and malaria. EBV-specific antibody testing is used for patients with suspected acute EBV infection who lack heterophile antibodies and for patients TABLE 199-2  Differential Diagnosis of Infectious Mononucleosis   SIGN OR SYMPTOM   ETIOLOGY FEVER ADENOPATHY SORE THROAT ATYPICAL LYMPHOCYTES DIFFERENCES FROM EBV MONONUCLEOSIS EBV infection + + + + — CMV infection + ± ± + Older age at presentation, longer duration of fever HIV infection + + + ± Diffuse rash, oral/genital ulcers, aseptic meningitis Toxoplasmosis + + ± ± Less splenomegaly; exposure to cats or raw meat HHV-6 infection + + + + Older age at presentation Streptococcal pharyngitis + + + – No splenomegaly, less fatigue Viral hepatitis + ± – ± Higher aminotransferase levels Rubella + + ± ± Maculopapular rash, no splenomegaly Lymphoma + + + + Fixed, nontender lymph nodes Drugsa + + – ± Occurs at any age aMost commonly phenytoin, carbamazepine, sulfonamides, or minocycline. Abbreviations: CMV, cytomegalovirus; EBV, Epstein-Barr virus; HHV, human herpesvirus.

with atypical infections. Titers of IgM and IgG antibodies to viral capsid antigen (VCA) are elevated in the serum of >90% of patients at the onset of disease. IgM antibody to VCA is most useful for the diagnosis of acute IM because it is present at elevated titers only during the first 2–3 months of the disease; in contrast, IgG anti­ body to VCA usually is not useful for diagnosis of IM but often is used to assess past exposure to EBV because it persists for life. Seroconversion to EBNA positivity also is useful for the diagnosis of acute infection with EBV. Antibodies to EBNA become detectable relatively late (3–6 weeks after the onset of symptoms) in nearly all cases of acute EBV infection and persist for the lifetime of the patient. These antibodies may be lacking in immunodeficient patients and in those with chronic active EBV disease. Titers of other antibodies also may be elevated in IM; however, these elevations are less useful for diagnosis. Antibodies to early antigens are detectable 3–4 weeks after the onset of symptoms in patients with IM. About 70% of individuals with IM have antibodies to early antigen diffuse (EA-D) during the illness; the presence of EA-D antibodies is especially likely in patients with relatively severe disease. These anti­ bodies usually persist for only 3–6 months. Levels of EA-D antibodies are elevated in patients with nasopharyngeal carcinoma or chronic active EBV infection. Antibodies to early antigen restricted (EA-R) are often found at elevated titers in patients with African Burkitt’s lym­ phoma or chronic active EBV infection; however, they are not useful for diagnosis. IgA antibodies to EBV antigens have proved useful for the identification of patients with nasopharyngeal carcinoma and of persons at high risk for the disease.

Heterophile CHAPTER 199 Other Studies  Detection of EBV DNA, RNA, or proteins has been valuable in demonstrating the association of the virus with various malignancies. The polymerase chain reaction has been used to detect EBV DNA in the cerebrospinal fluid of some AIDS patients with CNS lymphomas and to monitor the amount of EBV DNA in the blood of patients with lymphoproliferative disease. Detection of high levels of EBV DNA in blood for a few days to several weeks after the onset of IM may be useful if serologic studies yield equivocal results. Culture of EBV from throat washings or blood is not helpful in the diagnosis of acute infection, since EBV persists in the oropharynx and in B cells for the lifetime of the infected individual. Epstein-Barr Virus Infections, Including Infectious Mononucleosis
Differential Diagnosis  Whereas ~90% of cases of IM are due to EBV, 5–10% of cases are due to cytomegalovirus (CMV) (Chap. 200). CMV is the most common cause of heterophile-negative mononucleo­ sis; less common causes of IM and differences from IM due to EBV are shown in Table 199-2.

89 - 200 Cytomegalovirus and Human Herpesvirus Types 6, 7, and 8

200 Cytomegalovirus and Human Herpesvirus Types 6, 7, and 8

TREATMENT EBV-Associated Disease Therapy for IM consists of supportive measures, with rest and analgesia. Excessive physical activity during the first month should be avoided to reduce the possibility of splenic rupture, which often necessitates splenectomy. Glucocorticoid therapy is not indicated for uncomplicated IM and in fact may predispose to bacterial superinfection. Prednisone (40–60 mg/d for 2–3 days, with subse­ quent tapering of the dose over 1–2 weeks) has been used for the prevention of airway obstruction in patients with severe tonsillar hypertrophy, for autoimmune hemolytic anemia, for hemophago­ cytic lymphohistiocytosis, and for severe thrombocytopenia. Glu­ cocorticoids have also been administered to rare patients with severe malaise and fever and to patients with severe CNS or cardiac disease. Acyclovir has had no significant clinical impact on IM in con­ trolled trials. In one study, the combination of acyclovir and pred­ nisolone had no significant effect on the duration of symptoms of IM. Acyclovir, at a dosage of 400–800 mg five times daily, has been effective for the treatment of oral hairy leukoplakia (despite common relapses). Posttransplantation EBV lymphoproliferative disease (Chap. 148) generally does not respond to antiviral ther­ apy. When possible, therapy should be directed toward reduction of immunosuppression. Antibody to CD20 (rituximab) has been effective in some cases. Infusions of donor lymphocytes are often effective for stem cell transplant recipients, although graft-versushost disease can occur. Infusions of HLA-matched EBV-specific cytotoxic T cells have been used to prevent EBV lymphoprolif­ erative disease in high-risk settings as well as to treat the disease. Interferon α administration, cytotoxic chemotherapy, and radiation therapy (especially for CNS lesions) also have been used. Infusion of autologous EBV-specific cytotoxic T lymphocytes has shown promise in small studies of patients with nasopharyngeal carcinoma and Hodgkin’s lymphoma. Treatment of several cases of X-linked lymphoproliferative disease with antibody to CD20 resulted in a successful outcome of what otherwise would probably have been fatal acute EBV infection. PART 5 Infectious Diseases ■ ■PREVENTION The isolation of patients with IM is unnecessary. A vaccine directed against the major EBV glycoprotein reduced the frequency of IM but did not affect the rate of asymptomatic infection in a phase 2 trial. Additional vaccines are in clinical trials. ■ ■FURTHER READING Bjornevik K et al: Longitudinal analysis reveals high prevalence of Epstein-Barr virus associated with multiple sclerosis. Science 375:296, 2022. Chan KCA et al: Analysis of plasma Epstein-Barr virus DNA to screen for nasopharyngeal cancer. N Engl J Med 377:513, 2017. Cohen JI et al: Epstein-Barr virus NK and T cell lymphoproliferative disease: Report of a 2018 international meeting. Leuk Lymphoma 61:808, 2020. Dierickx D, Habermann TM: Post-transplantation lymphoprolifera­ tive disorders in adults. N Engl J Med 378:549, 2018. Li T et al: Anti-Epstein-Barr virus BNLF2b for mass screening for nasopharyngeal cancer. N Engl J Med 389:808, 2023. Mahaden JM et al: Tabelecleucel for allogeneic haematopoietic stemcell or solid organ transplant recipients with Epstein-Barr viruspositive post-transplant lymphoproliferative disease after failure of rituximab or rituximab and chemotherapy (ALLELE): A phase 3, multicentre, open-label trial. Lancet Oncol 25:376, 2024. Murray PG, Young LS: An etiological role for the Epstein-Barr virus in the pathogenesis of classical Hodgkin lymphoma. Blood 134:591, 2019. Tangye SG, Latour S: Primary immunodeficiencies reveal the molec­ ular requirements for effective host defense against EBV infection. Blood 135:644, 2020.

Camille Nelson Kotton, Martin S. Hirsch

Cytomegalovirus and

Human Herpesvirus

Types 6, 7, and 8 CYTOMEGALOVIRUS ■ ■DEFINITION Cytomegalovirus (CMV), which was initially isolated from patients with congenital cytomegalic inclusion disease, is now recognized as an important pathogen in all age groups. In addition to inducing severe birth defects, CMV causes a wide spectrum of disorders in older chil­ dren and adults, ranging from an asymptomatic subclinical infection to a mononucleosis syndrome in healthy individuals to disseminated disease in immunocompromised patients. Human CMV is one of sev­ eral related species-specific viruses that cause similar diseases in vari­ ous animals. All are associated with the production of characteristic enlarged cells—hence the name cytomegalovirus. CMV, a β-herpesvirus, has double-stranded DNA, four species of mRNA, a protein capsid, and a lipoprotein envelope. Like other her­ pesviruses, CMV demonstrates icosahedral symmetry, replicates in the cell nucleus, and can cause either a lytic and productive or a latent infection. CMV can be distinguished from other herpesviruses by cer­ tain biologic properties, such as host range and type of cytopathology. Viral replication is associated with the production of large intranuclear inclusions and smaller cytoplasmic inclusions. CMV appears to repli­ cate in a variety of cell types in vivo; in tissue culture it grows preferen­ tially in fibroblasts. Although there is little clinical evidence that CMV is oncogenic in vivo, it does transform fibroblasts in rare instances, and genomic transforming fragments have been identified. ■ ■EPIDEMIOLOGY CMV has a worldwide distribution. In many low- and middle-income regions, nearly all adults are seropositive for CMV, whereas only half of adults in the United States and Canada are seropositive. Immunocom­ promised CMV seropositive adults are more likely to undergo reactiva­ tion disease rather than primary infection. Data generated in specific regions should be considered in the context of local seropositivity rates, when appropriate. Based on a recent systematic review, the overall prevalence of con­ genital CMV is estimated to be 0.67%, with a threefold higher preva­ lence in low- and middle-income countries (1.42%) compared with high-income countries (0.48%). Communal living and poor personal hygiene facilitate spread. Perinatal and early childhood infections are common. CMV may be present in breast milk, saliva, feces, and urine. Transmission can occur among young children in day-care centers and has been traced from infected toddler to pregnant mother to develop­ ing fetus. When an infected child introduces CMV into a household, 50% of nonimmune family members seroconvert within 6 months. CMV is not readily spread by casual contact but rather requires repeated or prolonged intimate exposure for transmission. In late adolescence and young adulthood, CMV is often transmitted sexually, and asymptomatic carriage in semen or cervical secretions is common. Transfusion of CMV-seropositive blood products containing viable leukocytes may transmit CMV, with a frequency of 0.14–10% per unit transfused, although use of leukocyte-reduced or CMV-seronegative blood significantly decreases the risk of CMV transmission. Once infected, an individual generally carries CMV for life, similar to other herpes viruses. The infection usually remains silent. CMV reactivation syndromes develop more frequently, however, when

T lymphocyte–mediated immunity is compromised—for example, after organ transplantation, with lymphoid neoplasms and certain acquired immunodeficiencies (in particular, HIV infection; Chap. 208), or during critical illness in intensive care units. Most primary CMV

infections in organ transplant recipients (Chap. 148) result from trans­ mission via the graft or blood products. In CMV-seropositive trans­ plant recipients, infection results from reactivation of latent virus in the recipients or from infection by a new strain from the donor. CMV infection may be associated with diseases as diverse as coronary artery stenosis and malignant gliomas, although these associations require further validation. ■ ■PATHOGENESIS Congenital CMV infection can result from either primary or reactiva­ tion infection of the mother. However, clinical disease in the fetus or newborn is related largely to primary maternal infection (Table 200-1). The major factors determining the severity of congenital infection are unclear, although a deficient capacity to produce precipitating antibod­ ies and to mount T cell responses to CMV is associated with relatively severe disease. Primary infection with CMV in late childhood or adulthood is often associated with a vigorous T lymphocyte response that may contrib­ ute to the development of a mononucleosis syndrome similar to that observed with Epstein-Barr virus (Chap. 199). The hallmark of such infection is the appearance of atypical lymphocytes in the peripheral blood; these cells are predominantly activated CD8+ T lymphocytes. Polyclonal activation of B cells by CMV contributes to the development of rheumatoid factors and other autoantibodies during mononucleosis. Once acquired, CMV persists indefinitely in host tissues. The sites of persistent infection may include multiple cell types and various organs. Transmission via blood transfusion or organ transplantation is due primarily to silent infection in these tissues. If the host’s T cell responses become compromised by disease or by iatrogenic immuno­ suppression, latent virus can reactivate to cause a variety of syndromes. Chronic antigenic stimulation in the presence of immunosuppression (for example, after organ transplantation) appears to be an ideal set­ ting for CMV activation and CMV disease. Certain particularly potent suppressants of T cell immunity (e.g., antithymocyte globulin, alem­ tuzumab) are associated with a high rate of clinical CMV syndromes. CMV may itself contribute to further T lymphocyte hyporesponsive­ ness, which often precedes superinfection with other opportunistic pathogens such as bacteria, molds, other viruses, and Pneumocystis. ■ ■PATHOLOGY Cytomegalic cells in vivo (presumed to be infected epithelial cells) are two to four times larger than surrounding cells and often contain an 8- to 10-μm intranuclear inclusion that is eccentrically placed and is surrounded by a clear halo, producing an “owl’s eye” appearance. Smaller granular cytoplasmic inclusions are demonstrated occasion­ ally. Cytomegalic cells are found in a wide variety of organs, includ­ ing the salivary gland, lung, liver, kidney, intestine, pancreas, adrenal gland, and central nervous system. The cellular inflammatory response to infection consists of plasma cells, lymphocytes, and monocyte-macrophages. Granulomatous reac­ tions occasionally develop, particularly in the liver. Immunopatho­ logic reactions may contribute to CMV disease. Immune complexes TABLE 200-1  Cytomegalovirus (CMV) Disease in the Immunocompromised Host POPULATION RISK FACTORS PRINCIPAL SYNDROME(S) TREATMENT PREVENTION Fetus/neonate Primary maternal infection/ early pregnancy, reactivation infection Cytomegalic inclusion disease Ganciclovir followed by valganciclovir for symptomatic neonates Organ transplant recipient Seropositivity of donor and/or recipient; potent immunosuppressive regimen; treatment of rejection Febrile leukopenia (CMV syndrome); gastrointestinal disease; pneumonia; early infection may be asymptomatic Hematopoietic stem cell transplant recipient Graft-vs-host disease; older age of recipient; seropositive recipient; viremia Pneumonia; gastrointestinal disease Person with HIV <50 CD4+ T cells/μL; CMV seropositivity Retinitis; gastrointestinal disease; neurologic disease

have been detected in infected infants, sometimes in association with CMV-related glomerulopathies. Immune-complex glomerulopathy has also been observed in some CMV-infected patients after renal transplantation.

■ ■CLINICAL MANIFESTATIONS Congenital CMV Infection  Fetal infections range from subclini­ cal to severe and disseminated. CMV seroconversion rates during preg­ nancy range from 1% to 7%. Of infants born to mothers with primary CMV infections during pregnancy, 5–20% will develop clinical mani­ festations, with a mortality rate of ~5%. Petechiae, hepatosplenomeg­ aly, and jaundice are the most common presenting features (60–80% of cases). They can have “blueberry muffin”–like hemorrhagic purpuric eruptions, which when biopsied show histopathology with dermal erythropoiesis. Infections during the first trimester are associated with up to 40−50% of infected neonates developing sensorineural complica­ tions. Microcephaly with or without cerebral calcifications, intrauter­ ine growth retardation, and prematurity are reported in 30–50% of cases. Inguinal hernias and chorioretinitis are less common. Labora­ tory abnormalities include elevated alanine aminotransferase levels in serum, thrombocytopenia, conjugated hyperbilirubinemia, hemolysis, and elevated protein levels in cerebrospinal fluid. The prognosis for severely infected infants is poor, and few survivors escape intellectual or hearing difficulties later in childhood. The differential diagnosis of cytomegalic inclusion disease in infants includes syphilis, toxoplasmo­ sis, bacterial sepsis, and infection with a variety of viruses, including rubella, Zika, or herpes simplex virus. Most congenital CMV infections are clinically inapparent at birth. Of asymptomatically infected infants, 7−11% develop sensorineural hearing loss over a 5-year period. CHAPTER 200 Perinatal CMV Infection  The newborn may acquire CMV at delivery by passage through an infected birth canal or by postnatal con­ tact with infected breast milk or other maternal secretions. Of infants who are breast-fed for >1 month by seropositive mothers, 40–60% become infected. Iatrogenic transmission can result from blood trans­ fusion; use of leukocyte-reduced or CMV-seronegative blood products for transfusion into low-birth-weight seronegative infants or seronega­ tive pregnant women decreases risk. Cytomegalovirus and Human Herpesvirus Types 6, 7, and 8
The great majority of infants infected at or after delivery remain asymptomatic. However, protracted interstitial pneumonitis has been associated with perinatally acquired CMV infection, particularly in premature infants, and occasionally has been accompanied by infection with Chlamydia trachomatis, Pneumocystis, or Ureaplasma urealyticum. Poor weight gain, adenopathy, rash, hepatitis, anemia, and atypical lymphocytosis may also be found, and CMV excretion often persists for months or years. CMV Mononucleosis  The most common clinical manifesta­ tion of CMV infection in immunocompetent hosts beyond the neonatal period is a heterophile antibody–negative mononucleosis syndrome, which may develop spontaneously or follow transfusion of Avoidance of exposure; education of pregnant women about risks Ganciclovir or valganciclovir, ± CMV immunoglobulin for severe or resistant/refractory disease Prophylaxis with ganciclovir, valganciclovir, or letermovir, or preemptive therapy Maribavir, foscarnet, or cidoforvir for resistant/ refractory infection Prophylaxis with letermovir, ganciclovir, or valganciclovir, or preemptive therapy Ganciclovir, valganciclovir, foscarnet, or cidofovir Highly active antiretroviral therapy; prophylaxis with valganciclovir

leukocyte-containing blood products. Although the syndrome occurs at all ages, it most often involves sexually active young adults. With incubation periods of 20–60 days, the illness generally lasts for 2–6 weeks. Prolonged high fevers, sometimes with chills, profound fatigue, and malaise, characterize this disorder. Myalgias, headache, and splenomegaly are common, but in CMV mononucleosis (as opposed to Epstein-Barr virus mononucleosis), exudative pharyngitis and cervical lymphadenopathy are rare. Occasional patients develop rubelliform rashes, often after exposure to ampicillin or certain other antibiotics. Less common are interstitial or segmental pneumonia, myocarditis, pleuritis, arthritis, splanchnic vein thrombosis, and encephalitis. In rare cases, Guillain-Barré syndrome complicates CMV mononucleosis. The characteristic laboratory abnormality of CMV mononucleosis is relative lymphocytosis in peripheral blood, with >10% atypical lymphocytes. Total leukocyte counts may be low, normal, or markedly elevated. Although significant jaundice is uncom­ mon, serum aminotransferase and alkaline phosphatase levels are often moderately elevated. Heterophile antibodies are absent; however, transient immunologic abnormalities are common and may include the presence of cryoglobulins, rheumatoid factors, cold agglutinins, and antinuclear antibodies. Hemolytic anemia, thrombocytopenia, and granulocytopenia complicate recovery in rare instances.

Most patients recover without sequelae, although postviral asthe­ nia may persist for months. The excretion of CMV in urine, genital secretions, and/or saliva often continues for months or years. Rarely, CMV infection is fatal in immunocompetent hosts; survivors can have recurrent episodes of fever and malaise, sometimes associated with autonomic nervous system dysfunction (e.g., attacks of sweating or flushing). PART 5 Infectious Diseases CMV Infection in the Immunocompromised Host  (Table 200-1) CMV is the most common viral pathogen complicating organ trans­ plantation (Chap. 148). In recipients of kidney, heart, lung, liver, pan­ creas, and vascularized composite (hand, face, other) transplants, CMV infection may result in a variety of clinical manifestations, including fever and leukopenia, hepatitis, colitis, pneumonitis, esophagitis, gas­ tritis, and retinitis. CMV disease is an independent risk factor for both graft loss and death. Without prophylaxis, the period of maximal risk is between 1 and 4 months after transplantation. Disease likelihood and viral replication levels generally are greater after primary infection than after reactivation. Molecular studies indicate that seropositive organ transplant recipients are susceptible to infection with donorderived, genotypically variant CMV strains. Reactivation infection, although common, is less likely than primary infection to be clinically significant. The overall risk of clinical disease is related to various factors, such as serologic mismatch (donor seropositive, recipient seronegative), degree of immunosuppression, use of antilymphocyte antibodies, lack of anti-CMV prophylaxis, and co-infection with other pathogens. The transplanted organ is particularly vulnerable as a tar­ get for CMV infection; thus, there is a tendency for CMV hepatitis to follow liver transplantation and for CMV pneumonitis to follow lung transplantation. CMV viremia occurs in roughly one-third of hematopoietic stem cell transplant (HSCT) recipients; the risk of severe disease may be reduced by prophylaxis or preemptive therapy with antiviral drugs. The risk is greatest in the first 100 days after transplantation, and identified risk factors include certain types of immunosuppressive therapy, an allogeneic (rather than an autologous) graft, acute graft-versus-host disease, older age, and recipient seropositivity prior to transplant. CMV is an important pathogen in persons with HIV (PWH) (Chap. 208), in whom it may cause retinitis or disseminated disease, particularly when peripheral-blood CD4+ T cell counts fall below 50/μL. In PWH and CD4+ T cells under 200/uL, high blood CMV viral loads are a predictor of severe disease and death. As treatment for underlying HIV infection has improved, the incidence of serious CMV infections (e.g., retinitis) has decreased. During the first few weeks after starting highly active antiretroviral therapy, however, acute flare-ups of CMV retinitis may occur secondary to an immune reconstitution inflamma­ tory syndrome (IRIS).

Syndromes produced by CMV in immunocompromised hosts (“CMV syndrome”) often begin with fatigue, fever, malaise, anorexia, night sweats, and arthralgias or myalgias. Liver function abnormali­ ties, leukopenia, thrombocytopenia, and atypical lymphocytosis may be observed during these episodes. Without treatment, CMV infection may progress to more severe end-organ disease. The development of tachypnea, hypoxemia, and nonproductive cough signals respiratory involvement. Radiologic examination of the lung often shows bilateral interstitial or reticulonodular infiltrates that begin in the periphery of the lower lobes and spread centrally and superiorly; localized seg­ mental, nodular, or alveolar patterns are less common. The differential diagnosis includes Pneumocystis infection; other viral, bacterial, or fungal infections; pulmonary hemorrhage; and injury secondary to irradiation or to treatment with cytotoxic drugs. Gastrointestinal CMV involvement may be localized or extensive and almost exclusively affects immunocompromised hosts. Colitis is the most common clinical manifestation in organ transplant recipients. Ulcers of the esophagus, stomach, small intestine, or colon may result in bleeding or perforation. Clinicians should be aware that blood tests such as CMV antigenemia and viral load testing may yield negative results in the setting of intestinal disease. CMV infection may lead to exacerbations of underlying ulcerative colitis. Hepatitis occurs fre­ quently, particularly after liver transplantation. Acalculous cholecysti­ tis and adrenalitis also have been described. CMV rarely causes meningoencephalitis in otherwise healthy individuals. Two forms of CMV encephalitis are seen in PWH. One resembles HIV encephalitis and often presents as progressive demen­ tia; the other is a ventriculoencephalitis characterized by cranial-nerve deficits, nystagmus, disorientation, lethargy, and ventriculomegaly. In immunocompromised patients, CMV can also cause subacute progres­ sive polyradiculopathy, which is often reversible if recognized and treated promptly. CMV retinitis is an important cause of blindness in immunocom­ promised patients, particularly patients with advanced AIDS (Chap. 208). Early lesions consist of small, opaque, white areas of granular retinal necrosis that spread in a centrifugal manner and are later accompanied by hemorrhages, vessel sheathing, and retinal edema (Fig. 200-1). CMV retinopathy must be distinguished from that due to other condi­ tions, including toxoplasmosis, candidiasis, and herpes simplex virus infection. Fatal CMV infections are often associated with persistent viremia and the involvement of multiple organ systems. Progressive pulmo­ nary infiltrates, pancytopenia, hyperamylasemia, and hypotension are characteristic features that are frequently found in conjunction with a terminal bacterial, fungal, or protozoan superinfection. Extensive adrenal necrosis with CMV inclusions is often documented at autopsy, as is CMV involvement of many other organs. FIGURE 200-1  Cytomegalovirus infection in a patient with AIDS may appear as an arcuate zone of retinitis with hemorrhages and optic disk swelling. Often CMV is confined to the retinal periphery, beyond view of the direct ophthalmoscope.

■ ■DIAGNOSIS CMV infection usually cannot be diagnosed reliably on clinical grounds alone. Isolation of CMV or detection of its antigens or DNA in appropriate clinical specimens is the preferred approach. The most common method of detection is quantitative nucleic acid testing (QNAT) for CMV by polymerase chain reaction (PCR) technology, for which blood or other specimens can be used; some centers use a CMV antigenemia test, an immunofluorescence assay that detects CMV antigens (pp65) in peripheral-blood leukocytes. CMV DNA in cerebrospinal fluid is useful in the diagnosis of CMV encephalitis or polyradiculopathy. Virus excretion and/or viremia is readily detected by culture of appropriate specimens on human fibroblast monolayers. If CMV titers are high, as is common in congenital disseminated infection and in AIDS, characteristic cytopathic effects may be detected within a few days. However, in some situations (e.g., CMV mononucleosis), viral titers are low, and cytopathic effects may take several weeks to appear. Many laboratories expedite diagnosis with an overnight tissue-culture method (shell vial assay) involving centrifugation and an immunocy­ tochemical detection technique employing monoclonal antibodies to an immediate-early CMV antigen. Isolation of CMV from urine, stool, or saliva does not, by itself, constitute proof of acute infection, since excretion from these sites may continue for months or years after ill­ ness. Detection of viremia by QNAT or antigenemia testing is a better predictor of acute infection. A variety of serologic assays detect antibody to CMV. An increased level of IgG antibody to CMV may not be detectable for up to 4 weeks after primary infection. Detection of CMV-specific IgM is sometimes useful in the diagnosis of recent or active infection; however, circulat­ ing rheumatoid factors may result in occasional false-positive IgM tests. Serology is more helpful when used to predict risk of CMV infec­ tion and disease in transplant recipients and is not recommended to diagnose acute disease in immunocompromised patients. ■ ■PREVENTION Prevention of CMV infection and disease in organ transplant and HSCT recipients is usually based on one of two methods: universal prophylaxis or preemptive therapy. With universal prophylaxis, antivi­ ral drugs are used for a defined period, often 3 or 6 months. One clini­ cal trial demonstrated that, in CMV-seronegative kidney transplant recipients with seropositive donors, prophylaxis with valganciclovir was more effective at prevention when given for 200 days rather than 100 days. With preemptive therapy, patients are monitored weekly for CMV viremia, and antiviral treatment is initiated once viremia is detected. Because of the bone marrow–suppressive effects of universal prophylaxis with ganciclovir/valganciclovir, preemptive therapy has been more commonly employed in HSCT recipients; letermovir pro­ vides excellent prophylaxis in higher-risk patients. For PWH, CMV end-organ disease is best prevented by using antiretroviral therapy sufficient to maintain CD4+ T-cell counts above 100/μL; primary pro­ phylaxis with ganciclovir or valganciclovir is not recommended. Several additional measures are useful for the prevention of CMV transmission to CMV-naïve, high-risk patients. The use of CMVseronegative or leukocyte-depleted blood significantly decreases the rate of transfusion-associated transmission. In a placebo-controlled trial, a CMV glycoprotein B vaccine reduced infection rates among 464 CMV-seronegative women; this outcome raises the possibility that a vaccine will reduce rates of congenital infection, but further stud­ ies must validate this approach. A conditionally replication-defective CMV, termed V160, was recently shown in a phase 2 clinical trial to be well tolerated and immunogenic although three doses of vaccine did not reduce the incidence of primary CMV infection in CMVseronegative women compared with placebo. The Triplex vaccine is in clinical trials in stem cell and liver transplant recipients; this vaccine has a recombinant viral vector with genes expressing three immuno­ dominant proteins: UL83 (pp65), UL123 (IE1), and UL122 (IE2). An mRNA CMV vaccine, mRNA-1647, is in clinical trials; it contains six mRNAs that encode the membrane-bound pentamer complex and the full-length membrane-bound glycoprotein B. CMV immune

globulin (intravenous immunoglobulin enriched from CMV-positive donors) has been studied in a variety of clinical situations (primary CMV infection in pregnancy, HSCT, solid organ transplantation), with conflicting results, and is used much less often in the era of multiple effective antiviral agents. A recent trial in pregnant women using CMV immunoglobulin before 24 weeks’ gestation did not result in a lower incidence of congenital CMV infection or perinatal death compared with placebo.

Prophylactic acyclovir or valacyclovir at high doses may reduce rates of CMV infection and disease in renal transplant recipients; neither drug is effective in the treatment of active CMV disease. Valacyclovir may also reduce the risk of maternal-fetal CMV transmission in preg­ nant women with primary CMV infection. TREATMENT Cytomegalovirus Infection Ganciclovir is a guanosine derivative that has considerably more activity against CMV than its congener acyclovir. After intracel­ lular conversion by a viral phosphotransferase encoded by CMV gene region UL97, ganciclovir triphosphate is a selective inhibitor of CMV DNA polymerase. Several clinical studies have indicated response rates of 70–90% among people with HIV who are given ganciclovir for the treatment of CMV retinitis or colitis. In severe infections (e.g., CMV pneumonia in HSCT recipients), ganciclovir is sometimes combined with CMV immune globulin. Prophylactic or suppressive ganciclovir may be useful in high-risk HSCT or organ transplant recipients (e.g., those who are CMV-seropositive before transplantation). In many PWH, those with persistently low CD4+ T cell counts, and those with CMV disease, clinical and virologic relapses occur promptly if treatment with ganciclovir is discontinued. Therefore, prolonged prevention regimens may be recommended for such patients. Resistance to ganciclovir is more common among patients exposed to antivirals for >3 months and is usually related to mutations in the CMV UL97 gene (or, less commonly, the UL54 gene, or UL56 after letermovir exposure, or UL27 after maribavir exposure). The advent of CMV genotyping for resistance mutations has made it possible to rapidly obtain information regarding optimal treatment approaches against clini­ cally resistant CMV. CHAPTER 200 Cytomegalovirus and Human Herpesvirus Types 6, 7, and 8
Valganciclovir is an orally bioavailable prodrug that is rap­ idly metabolized to ganciclovir in intestinal tissues and the liver. Approximately 60–70% of an oral dose of valganciclovir is absorbed. An oral valganciclovir dose of 900 mg results in ganciclovir blood levels similar to those obtained with an IV ganciclovir dose of 5 mg/kg. Valganciclovir appears to be as effective as IV ganciclovir for both CMV induction (treatment) and prevention regimens, also offering the advantage of oral dosing. Ganciclovir or valganciclovir therapy for CMV disease consists of a 14- to 21-day induction course (5 mg/kg IV twice daily for ganciclovir or 900 mg PO twice daily for valganciclovir), sometimes followed by suppressive therapy (e.g., valganciclovir, 900 mg/d). Peripheral-blood neutropenia develops in roughly one-quarter of treated patients but may be ameliorated by granulocyte colonystimulating factor. Guidelines recommend checking weekly blood­ work for CMV NAT or antigenemia to ascertain virologic response. Whether to use suppressive therapy should depend on the overall level of immunocompromise and the risk of recurrent disease. Discontinuation of preventative regimens should be considered in PWH who, while receiving antiretroviral therapy, have a sustained (3- to 6-month) increase in CD4+ T cell counts to >100/μL. Com­ pared with shorter (6-week) courses, prolonged (6-month) courses of valganciclovir had beneficial effects on hearing and developmen­ tal outcomes in infants with congenital CMV infection. For treat­ ment of CMV retinitis, some clinicians use intravitreal injections of ganciclovir or foscarnet (see below) plus oral valganciclovir or intravenous ganciclovir, although no clinical trials have compared these approaches.

Foscarnet (sodium phosphonoformate) inhibits CMV DNA polymerase. Because this agent does not require phosphorylation to be active, it is also effective against most ganciclovir-resistant isolates. Foscarnet is less well tolerated than ganciclovir and causes considerable toxicity, including renal dysfunction, hypomagnese­ mia, hypokalemia, hypocalcemia, genital ulcers, dysuria, nausea, and paresthesia. Moreover, foscarnet administration requires the use of an infusion pump and close clinical monitoring. With aggres­ sive hydration and dose adjustments for renal dysfunction, the toxicity of foscarnet can be reduced. The use of foscarnet should be avoided when a saline load cannot be tolerated (e.g., in cardiac insufficiency). The approved induction regimen is 60 mg/kg every 8 h for 2 weeks, although 90 mg/kg every 12 h is equally effective and no more toxic. Maintenance infusions should deliver 90–120 mg/kg

once daily. No oral preparation is available. Foscarnet-resistant virus may emerge during extended therapy. This drug is used more frequently after HSCT than in other situations to avoid the myelo­ suppressive effects of ganciclovir; in general, foscarnet has been the first choice for infections with ganciclovir-resistant CMV. Maribavir is now being used for resistant/refractory CMV infections, as it was shown in a phase 3 trial to be more effective and have less toxicity compared with alternative therapies. Letermovir has efficacy for prophylaxis after HSCT and organ transplantation but induces rapid development of resistance when used during active infection; some experts recommend letermovir for prevention after treatment of resistant/refractory CMV.

Cidofovir is a nucleotide analogue with a long intracellular halflife that allows intermittent IV administration. Induction regimens of 5 mg/kg weekly for 2 weeks are followed by secondary preven­ tion regimens of 3–5 mg/kg every 2 weeks. Cidofovir can cause severe nephrotoxicity through dose-dependent proximal tubular cell injury; however, this adverse effect can be tempered somewhat by saline hydration and probenecid. Cidofovir is used primarily for ganciclovir-resistant virus but is being replaced by maribavir; it may be useful for prevention after treatment of resistant/refractory CMV, as it could be given every 2 weeks. PART 5 Infectious Diseases HUMAN HERPESVIRUS (HHV)

TYPES 6, 7, AND 8 ■ ■HHV-6 AND HHV-7 HHV-6 and -7 seropositivity rates are generally high throughout the world. HHV-6 was first isolated in 1986 from peripheral-blood leuko­ cytes of six persons with various lymphoproliferative disorders. There are two genetically distinct variants (HHV-6A and HHV-6B). HHV-6 appears to be transmitted by saliva and possibly by genital secretions. HHV 6 DNA integrates into chromosomal telomeres in several cell types, including germ cells, in about 1% of people. As a result, offspring may carry a copy of the viral genome in every nucleated cell (known as inherited chromosomally integrated or iciHHV-6). The full clinical implications of iciHIV-6 remain unclear; clinicians should be aware that they will always be HHV-6 NAT positive. Pregnant women with iciHHV-6 may be at higher risk for pre-eclampsia or spontaneous abortion. Infection with HHV-6 frequently occurs during infancy as maternal antibody wanes. The peak age of acquisition is 9–21 months; by 24 months, seropositivity rates approach 80%. Older siblings appear to serve as a common source of transmission. In addition, congenital infection may occur, and ~1% of newborns are infected with HHV-6; transplacental infection with HHV-6 accounts for about 14%, with the others related to infection from inherited chromosomally integrated HHV-6. Congenital infection is generally asymptomatic, although sub­ tle neurologic defects have been described. Most postnatally infected children develop symptoms (fever, fussiness, and diarrhea). A minority develop exanthem subitum (roseola infantum; see Fig. A1-5), a com­ mon illness characterized by fever with subsequent rash. In addition, ~10–20% of febrile seizures without rash during infancy are caused by HHV-6. After initial infection, HHV-6 persists in peripheral-blood

mononuclear cells as well as in the central nervous system, salivary glands, and female genital tract. In older age groups, HHV-6 has been associated with mononucleo­ sis syndromes; in immunocompromised hosts, encephalitis, pneu­ monitis, syncytial giant-cell hepatitis, and disseminated disease are seen. In transplant recipients, HHV-6 infection may also be associated with graft dysfunction. Acute HHV-6-associated limbic encephalitis has been reported in hematopoietic stem cell transplant recipients and is characterized by memory loss, confusion, seizures, hyponatre­ mia, and abnormal electroencephalographic and MRI results. High plasma loads of HHV-6 DNA in HSCT recipients are associated with allelic-mismatched donors, use of glucocorticoids, delayed monocyte and platelet engraftment, development of limbic encephalitis, and increased all-cause mortality rates. Mesial temporal lobe epilepsy has been associated with HHV-6 infections, and, like many other viruses, HHV-6 (as well as EBV, another herpes virus) has been implicated in the pathogenesis of multiple sclerosis, although further study is needed to distinguish between association and etiology. HHV-7 was isolated in 1990 from T lymphocytes from the periph­ eral blood of a healthy 26-year-old man. The virus is frequently acquired during childhood, albeit at a later age than HHV-6. HHV-7 is commonly present in saliva, which is presumed to be the principal source of infection; breast milk and cervical secretions also may carry the virus. Viremia can be associated with either primary or reactiva­ tion infection. The most common clinical manifestations of childhood HHV-7 infections are fever and seizures. Some children present with respiratory or gastrointestinal signs and symptoms. An association has been made between HHV-7 and pityriasis rosea, but evidence is insuf­ ficient to indicate a causal relationship. Clustering of HHV-6, HHV-7, and CMV infections in transplant recipients can make it difficult to sort out the roles of the various agents in individual clinical syndromes. HHV-6 and HHV-7 appear to be susceptible to ganciclovir and foscarnet, although definitive evidence of clinical response is lacking. Use of the narrower-spectrum, CMVspecific agent letermovir for CMV prevention after HSCT was shown not to result in higher rates of HHV-6. ■ ■HHV-8 Unique herpesvirus-like DNA sequences were reported during 1994 and 1995 in tissues derived from Kaposi’s sarcoma (KS) and body cavity–based lymphoma occurring in people with HIV. The virus from which these sequences were derived is designated HHV-8 or Kaposi’s sarcoma–associated herpesvirus (KSHV). HHV-8, which infects B lymphocytes, macrophages, and both endothelial and epithelial cells, appears to be causally related not only to KS and a subgroup of AIDS-related B cell body cavity–based lymphomas (primary effusion lymphomas) but also to multicentric Castleman disease, a lymphop­ roliferative disorder of B cells. The association of HHV-8 with several other diseases has been reported but not confirmed. HHV-8 seropositivity occurs worldwide, with areas of high ende­ micity influencing rates of disease. Unlike other herpesvirus infec­ tions, HHV-8 infection is much more common in some geographic areas (e.g., central and southern Africa) than in others (North America, Asia, northern Europe). Pockets of increased HHV-8 infections have been observed among men who have sex with men (MSM) with HIV in the southern United States; risk factors for dual infections with HIV and HHV-8 in this population include oral/anal or oral/penile sex and methamphetamine use. In high-prevalence areas, infection occurs in childhood, and seropositivity is associated with families having numerous children who share eating and drink­ ing utensils; HHV-8 may be transmitted in saliva. In low-prevalence areas, infections typically occur in adults, probably via sexual trans­ mission. Concurrent epidemics of HIV-1 and HHV-8 infections among certain populations (e.g., MSM) in the late 1970s and early 1980s appear to have resulted in the frequent association of AIDS and KS. Transmission of HHV-8 may also be associated with organ transplantation, injection drug use, and blood transfusion; however, transmission via organ transplantation or blood transfusion in the United States appears to be quite rare.

90 - 201 Monkeypox, Molluscum Contagiosum, and Other Poxvirus Infections

201 Monkeypox, Molluscum Contagiosum, and Other Poxvirus Infections

Primary HHV-8 infection in immunocompetent children may manifest as fever and maculopapular rash. Among individuals with intact immunity, chronic asymptomatic infection is the rule, and neo­ plastic disorders generally develop only after subsequent immunocom­ promise. Immunocompromised persons with primary infection may present with fever, splenomegaly, lymphoid hyperplasia, pancytopenia, or rapid-onset KS. Quantitative analysis of HHV-8 DNA suggests a predominance of latently infected cells in KS lesions and frequent lytic replication in multicentric Castleman disease. The KS-associated herpesvirus inflammatory cytokine syndrome (KICS)—consisting of fever, lymphadenopathy, hepatosplenomegaly, cytopenias, and high levels of HHV-8, human and viral interleukin 6, and human interleukin 10—has been described in some HIV-infected patients and is associ­ ated with a high mortality rate. Effective antiretroviral therapy for HIV-infected individuals has led to a marked reduction in rates of KS among persons dually infected with HHV-8 and HIV in resource-rich areas. HHV-8 itself is suscep­ tible in vitro to ganciclovir, foscarnet, and cidofovir. A small, random­ ized, double-blind, placebo-controlled, crossover trial suggested that oral valganciclovir administered once daily reduced HHV-8 replica­ tion. However, clinical benefits of valganciclovir or other drugs in HHV-8 infection have not yet been demonstrated. Sirolimus inhibits the progression of dermal KS in kidney transplant recipients while providing effective immunosuppression. Rituximab alone or in combi­ nation with chemotherapy can lead to a survival of >90% at 5 years in HHV-8–associated multicentric Castleman disease. ■ ■FURTHER READING Cytomegalovirus Avery RK et al: Maribavir for refractory cytomegalovirus infections with or without resistance post-transplant: Results from a Phase 3 randomized clinical trial. Clin Infect Dis 75:690, 2022. Chatzakis C et al: The effect of valacyclovir on secondary prevention of congenital cytomegalovirus infection, following primary maternal infection acquired periconceptionally or in the first trimester of pregnancy. An individual data meta-analysis. Am J Obstet Gynecol 230:109, 2024. Das R et al: Safety, efficacy, and immunogenicity of a replicationdeficient human cytomegalovirus vaccine, V160, in cytomegalovirusseronegative women: a double-blind, randomized, placebo-controlled, phase 2b trial. Lancet Infect Dis 23:1383, 2023. Drutman SB et al: Fatal cytomegalovirus infection in an adult with inherited NOS2 deficiency. N Engl J Med 382:437, 2020. Fang M et al: High cytomegalovirus viral load is associated with 182day all-cause mortality in hospitalized people with human immuno­ deficiency virus. Clin Infect Dis 76:1266, 2023. Hughes BL et al: A trial of hyperimmune globulin to prevent congeni­ tal cytomegalovirus infection. N Engl J Med 385:436, 2021. Kotton CN et al: The Third International Consensus Guidelines on the management of cytomegalovirus in solid-organ transplantation. Transplantation 102:900, 2018. Limaye AP et al: Letermovir vs valganciclovir for prophylaxis of cyto­ megalovirus in high-risk kidney transplant recipients: A randomized clinical trial. JAMA 330:33, 2023. Ssentongo P et al: Congenital cytomegalovirus infection burden and epidemiologic risk factors in countries with universal screening: A systematic review and meta-analysis. JAMA Netw Open 4:e2120736, 2021. Human Herpesvirus (HHV) Types 6, 7, and 8 Gabrielli L et al: Inherited chromosomally integrated human herpes­ virus 6: Laboratory and clinical features. Microrganisms 11:548, 2023. Gaccioli F et al: Fetal inheritance of chromosomally integrated human herpesvirus 6 predisposes the mother to pre-eclampsia. Nat Microbiol 5:901, 2020. Kampouri E et al: Human herpesvirus-6 reactivation and disease after allogeneic haematopoietic cell transplantation in the era of letermovir for cytomegalovirus prophylaxis. Clin Microbiol Infect 29:1450.e1, 2023.

Knights SM et al: High seroprevalence of Kaposi sarcoma–associated

herpesvirus in men with HIV in the southern United States. Open Forum Infect Dis 10:ofad160, 2023. Miura H et al: Inherited chromosomally integrated human herpesvi­ rus 6 is a risk factor for spontaneous abortion. J Infect Dis 223:1717, 2021. Pellett Madan R et al: Human herpesvirus 6, 7, and 8 in solid organ transplantation: Guidelines from the American Society of Transplan­ tation Infectious Diseases Community of Practice. Clin Transplant 33:e13518, 2019. Faisal Syed Minhaj, Christina L. Hutson

Monkeypox, Molluscum

Contagiosum, and Other Poxvirus Infections POXVIRUSES CHAPTER 201 ■ ■DEFINITION AND ETIOLOGY Poxviruses (Poxviridae) are a family of double-stranded DNA viruses whose genomic structure is generally conserved across subfamilies, genera, and species. The central portion of the genome, which can range up to 200 kb, encodes the open reading frames (ORFs) required for replication or packaging of virions. The left and right ends of the genome encode genes with predicted functions in immune eva­ sion, host interaction, or unknown roles. The complement of ORFs across different genera is largely responsible for differences in disease manifestations and/or virus host range. Four genera of poxviruses (Orthopoxvirus, Parapoxvirus, Yatapoxvirus, and Molluscipoxvirus) include species that can infect humans. Additionally, a currently unclassified poxvirus has been reported to cause human illness. Table 201-1 identifies these viruses, the majority of which are zoonotic, and lists some of their epidemiologic characteristics. Monkeypox, Molluscum Contagiosum, and Other Poxvirus Infections ■ ■EPIDEMIOLOGY Most poxviruses that infect humans are spread through direct contact; notable exceptions are some species of Orthopoxvirus (i.e., variola and monkeypox viruses [MPXV]). Variola virus (the virus that causes smallpox) is transmitted primarily by close contact and via respira­ tory secretions. In what seems to have been a rare circumstance near the end of global efforts to eradicate smallpox, it was reported that variola virus appeared to transmit via aerosol in a German hospital in Meschede. The degree to which potential aerosols had a role in small­ pox transmission remains debated. There are two geographically, genetically, and clinically distinct clades of MPXV. Clade I (formally Congo Basin clade) is endemic to Central Africa (Democratic Republic of the Congo, Republic of the Congo, Central African Republic, Cameroon, Gabon). Clade II (for­ mally West African clade) is endemic to West Africa (Nigeria, Liberia, Sierra Leone, Cameroon, Cote d’Ivoire). Clade I MPXV causes a higher proportion of individuals with severe disease than Clade II, which is further subdivided into clade IIa, found endemically in West Africa, and clade IIb, which spread worldwide in 2022, with low levels of virus circulation continuing to date. In endemic areas, MPXV infections historically clustered around rural villages and within households with significant wildlife exposure. Spread to humans is primarily through direct contact (e.g., handling during food preparation) with infected animals leading to percutaneous or permucosal exposure. Human-tohuman spread historically was primarily via close skin-to-skin contact;

TABLE 201-1  Poxviruses Causing Infection in Humans GENUS, SPECIES GEOGRAPHY ZOONOTIC CHARACTERISTICS Orthopoxvirus Variola (smallpox) Eradicated, formerly worldwide Solely a human pathogen Monkeypox Historically endemic to West and Central Africa, but spread worldwide in 2022 Squirrel species, Gambian rats, and dormice implicated as potential reservoir species; other species effective in transmitting disease to humans (pet North American prairie dogs, non-human primates); can be acquired during hunting/preparation of African wildlife for nutritional protein source Cowpox Europe Rodents as reservoir; outbreaks associated with rodent pet trade; cats also effective transmitters of illness; previously, dairy cow teat lesions linked to human cutaneous lesions Vaccinia and vaccinia-like viruses (e.g., buffalopox, Cantagalo, Araçatuba) Europe, India, and South America Rodents suspected as a potential reservoir; localized lesions on cattle or other ruminants (e.g., water buffalo for buffalopox) responsible for most human infections Borealpox (formally known as Alaskapox) United States (Alaska) Northern red-backed voles, squirrels, and shrews suspected as potential reservoir species; potential transmission to humans suspected from contact of above species or pets (e.g., cats, dogs) who were infected PART 5 Infectious Diseases Akhmeta Georgia (country) Woodmice (Apodemus spp.); cows can be infected and possibly transmit to humans. Molluscipoxvirus Molluscum contagiosum Worldwide Thought to be solely a human pathogen; closely related viruses described in other mammals Parapoxvirus Orf Worldwide Handling of infected sheep and goats primarily responsible for transmission to humans Pseudocowpox Worldwide Handling of infected dairy cattle Bovine papular stomatitis Worldwide Handling of infected beef cattle Deerpox U.S. deer herds Handling of infected deer Sealpox Seal/pinniped colonies worldwide Handling of infected pinnipeds Yatapoxvirus Tanapox Africa Possible nonhuman primate reservoir; potential arthropod mediator Unclassified poxvirus NY-014a United States (New York State) Unknown aPossibly an orthopoxvirus. however, spread via respiratory secretions also was believed to occur. After decades without detection of MPXV infections, Nigeria expe­ rienced a rapid increase in mpox cases in 2017. As routine smallpox vaccination (which provides cross-protection for mpox) ended in 1980, it is unclear what role waning orthopoxvirus immunity played in its reemergence. These cases in Nigeria were unusual in that they occurred in urban centers in persons without known animal con­ tact. In 2021, eight travel-associated mpox cases in persons who flew from Nigeria were identified in four countries (United States, United Kingdom, Israel, and Singapore), with limited subsequent human-tohuman transmission. In 2022, clade IIb mpox spread globally, with

transmission occurring primarily via close, intimate (e.g., sexual) contact and predominantly affecting gay, bisexual, and other men who have sex with men (MSM). MSM, particularly Black and Hispanic MSM in the United States, remain disproportionately affected by mpox to date. Additionally, up to 50% of those diagnosed with mpox in this outbreak are also living with HIV. Other orthopoxviruses (apart from variola virus) (Table 201-1) are thought to spread only via direct contact or percutaneous/permucosal exposures to infected animals (or humans). The orthopoxvirus infec­ tions caused by cowpox and the vaccinia-like viruses are typically acquired initially through contact with an infected animal. Humanto-human transmission can also occur via contact with the lesion(s) of the infected human. In Europe, human cowpox infections have been associated with the pet rat trade, and vaccinia-like viruses (e.g., Belo Horizonte, Cantagalo, Araçatuba) are reported in handlers of dairy cattle in South America. Similarly, buffalopox has been reported in inhabitants of the Indian subcontinent exposed to infectious lesions on water buffalo. In the United States, vaccinia, the virus historically known as the substrate for smallpox vaccine, has caused infections in laboratory workers studying the virus. With all orthopoxvirus infec­ tions, the illness is considered infectious from symptom onset until all lesions have crusted, the crust(s) have separated from the skin, and a fresh layer of healthy skin has formed underneath. The most common poxvirus encountered in practice is molluscum contagiosum virus (MCV), which is a molluscipoxvirus. MCV likely spreads through direct contact with and percutaneous exposure to another infected human. Like variola virus (an orthopoxvirus), MCV is considered to be a pathogen of humans only. Infections are com­ monly seen in pediatrics where transmission occurs through play activities. In adults, the disease can manifest similarly, but genital involvement also is noted as transmission often occurs through sexual exposure. The epidemiology of tanapox (a yatapoxvirus) is poorly under­ stood. Simian reservoirs and the potential for an arthropod vector are hypothesized. Rare cases of tanapox have only been seen in the United States from travelers returning from West or Central Africa. Human infections with parapoxviruses occur through direct contact with and percutaneous exposure to lesions developing at the site of contact with an infected animal. ■ ■PATHOGENESIS The pathogenesis of orthopoxvirus infections is thought to involve sys­ temic spread of disease from the site of virus inoculation to local lymph nodes, lymphoreticular tissue seeding, and finally the development of symptomatic (febrile) viremia and viral skin tropism. Disease severity is affected by the degree to which the innate immune and interferon responses control the initial stages of infection. During illness, patients may experience lymphadenopathy, fever, pain, and malaise. In immu­ nocompromised persons more severe systemic manifestations are seen. Cases exemplifying this were seen throughout the global mpox outbreak in 2022. Individuals with intact immune systems develop lesions (often at the exposure site) about 1 week after exposure; these lesions progress through specified stages over the next 7–14 days, fol­ lowed by scabbing and complete resolution by 14–21 days after rash onset. Lesion scabs contain viable virus, and it is only once all lesions scab over, the scabs separate from the skin, and newly formed intact skin forms that the infectious period ends. In contrast, persons with severe immunocompromised states (e.g., advanced HIV [i.e., CD4 T cell count <200 cells/mm3], organ transplantation) can develop severe mpox. In these instances, the spread or growth of MPXV goes unchecked, and systemic spread of disease results in wide dissemina­ tion of the rash, with large confluent lesions persisting for months and additional organ involvement. Other poxvirus infections—with the possible exception of yatapox­ virus infection, in which disease pathogenesis is poorly understood— likely involve only local growth of the virus at the site of inoculation or reinoculation. In some immunocompromised hosts, the lesions caused by Parapoxvirus infections can become quite large; such lesions are referred to as “giant orf.”

■ ■CLINICAL MANIFESTATIONS Systemic poxvirus infections (i.e., Variola Virus, Monkeypox Virus [Orthopoxvirus genome] and Tanapox [Yatapoxvirus genome]) Following exposure, both smallpox and mpox have a similar incuba­ tion period of up to 17 days. Classically, smallpox and mpox present as follows: the first clinical sign is fever, which is followed by rash onset days later. Other prodromal symptoms include malaise, sore throat, and headache. When lymphadenopathy was present during this stage and throughout illness, it was a key differentiating factor between smallpox (absent) and mpox (present) in endemic countries. The rash evolves through classic macular, papular, vesicular, and pustular phases (the last with central umbilication), with each stage lasting 2–3 days and lesions in the same anatomic location typically in the same stage of development. Diffuse, well-circumscribed, centrifugally distributed (i.e., lesions more prominent on the face and extremities, including palms and soles, than on the trunk) lesions are classic. Lesions are typi­ cally painful as they emerge and subsequently become pruritic during later stages. However, during the emergence of clade IIb mpox in 2022, several clinical manifestations differed from this classic presentation. During the clade IIb outbreak, patients often presented with rash as their first sign or symptom of disease or concurrently or in the absence of prodromal symptoms; lesions were frequently localized, primarily in the anogenital region (Fig. 201-1). The lesions themselves were smaller, fewer in number (often <10), and often in different stages at the same anatomic site. There does not appear to be an increase in disease severity in those without severe immunocompromise (i.e., HIV with CD4 T cell count >200 cells/mL). However, a minority of patients have developed severe manifestations of mpox, many of whom who are severely immunocompromised (e.g., advanced HIV, organ transplanta­ tion, comparable severe immunocompromise). Severe Mpox  Rare severe manifestations of mpox include ocular infection, neurologic complications, myopericarditis, and uncontrolled viral replication in severely immunocompromised patients. Mpoxrelated ocular disease can occur via autoinoculation (i.e., touching a lesion then the eye) or local spread from a nearby lesion. Symptoms include eye pain, redness, vision changes or loss, or periorbital swell­ ing. Ocular disease can manifest as blepharitis, conjunctivitis, con­ junctival lesions, keratitis, and vision loss. Neurologic complications include rare reports of encephalitis and myelitis. Patients may have severe headache, neck pain, altered mentation, or focal deficits. Myo­ pericarditis has been reported in some patients with mpox, including complaints of shortness of breath or palpitations with elevations in car­ diac biomarkers and electrocardiographic changes. Mucosal complica­ tions can affect alimentation, urination, or defecation due to painful or obstructive lesions. These can lead to strictures, edema, and severe lymphadenopathy. Complications from uncontrolled viral replication commonly occur in severely immunocompromised persons such as those with advanced HIV or organ transplantation. These patients often develop numerous large, coalescing, or necrotic lesions and can have other organ involvement including the gastrointestinal tract, liver, lungs, and brain, which can manifest as organ dysfunction. These patients may have disease lasting many months in which immune system optimization is crucial to recovery. Most deaths have occurred in the United States, predominantly among people with advanced HIV with CD4 <50 cells/mm3. Tanapox  Patients infected with tanapox virus initially present with a very high fever, are often thought to have malaria (given the endemic location), and later develop 1–10 nodular lesions. These nodules are often in anatomic areas not typically covered by clothing. Lesions are seldom filled with fluid and more often contain necrotic tissue. The lesion size peaks around 2 weeks after initial formation, and lesions typically disappear spontaneously within 6 weeks. Other Orthopoxvirus Infections  Other orthopoxvirus infec­ tions are more localized in their presentation, with lesions likely devel­ oping directly at the site of contact with the virus. Akhmeta, Borealpox (formally known as Alaskapox), vaccinia, vaccinia-like, and cowpox virus infections are typically associated with a localized rash or lesion

evolving through classical papular, vesicular, and pustular phases. In immunocompromised patients, presentation of these orthopoxvirus infections can be protracted or disseminated and, rarely, lead to death.

Other Poxvirus Infections  Individuals infected with other pox­ viruses that cause localized disease (parapoxviruses and MCV) seldom report a febrile phase and instead experience slow and gradual devel­ opment of a lesion or lesions. The lesion of molluscum contagiosum has a classic pearly appearance that sometimes umbilicates as it matures (Fig. 201-2). There is little inflammation that surrounds the painless lesions, which persist for months but gradually regress after 6–12 months. Patients with immunocompromised status can have severe and prolonged disease; in patients with uncontrolled HIV and advanced HIV, immune reconstitution is usually sufficient to clear the virus. The rash lesions of parapoxvirus infections begin as ery­ thematous papules, develop into a “target” lesion, and then become nodular and papilloma-like. “Giant” parapoxvirus infections have been reported in immunocompromised individuals. ■ ■DIFFERENTIAL DIAGNOSIS A patient usually presents to the clinician with nodular or vesicu­ lopustular lesions. Important elements of the history include travel, occupation (with risk varying dependent on the poxvirus; greater risk is in laboratory workers working with poxviruses, farmers, hunters, and sex workers), animal exposures, lesion evolution, sexual history, and symptom timing with respect to rash onset. Additionally, given the appearance and location, mpox lesions in the anogenital area may resemble common sexually transmitted infections (STIs) such as gon­ orrhea, chlamydia, or syphilis; up to 10% of patients with mpox may be coinfected with an STI. Other differential diagnoses in poxvirus infec­ tions include varicella, yaws, papillomavirus infection, herpes simplex virus, and (particularly in parapoxvirus infections) cutaneous anthrax. While the characteristic lesions of poxvirus infection coupled with an indicative exposure history are helpful in narrowing the differential, laboratory testing is needed to confirm the diagnosis. CHAPTER 201 Monkeypox, Molluscum Contagiosum, and Other Poxvirus Infections Currently, the most common laboratory tool for diagnosis of pox­ virus infection involves nucleic acid (i.e., molecular) testing. Nucleic acid–based diagnostics include polymerase chain reaction (PCR) and sequencing to fully characterize the isolate in some cases. This technol­ ogy has led to the identification of a number of new poxviruses that can cause human infection, including Akhmeta, Borealpox, and NY-014. Orthopoxvirus molecular testing options drastically expanded during the global 2022 mpox outbreak that was caused by clade IIb MPXV. Prior to the outbreak, only select PCR assays (developed by the Centers for Disease Control and Prevention [CDC] for smallpox prepared­ ness) were available within a subset of the Public Health Laboratories (PHL) within the Laboratory Response Network. During the 2022 outbreak, CDC and the U.S. Food and Drug Administration (FDA) collaborated to increase testing availability in commercial laboratories within the United States. PCR assays specific to non-variola ortho­ poxvirus (NVO), orthopoxvirus generic, generic mpox, and mpox clade II are now readily available for testing of most orthopoxviruses. Other countries experienced similar growth in orthopoxvirus testing options. Many laboratories have also introduced multiplex testing options that include both NVO- and MPXV-specific targets. Some laboratories (U.S. Government and PHL) offer mpox clade-specific testing. The orthopoxviruses also grow well in most standard clinical laboratory tissue cultures. The parapoxviruses are difficult to isolate via culture (primary cells are best), and MCV cannot be cultured. Electron microscopy identifies the characteristic large, brick-shaped virus par­ ticles on negative stain if orthopoxvirus, yatapoxvirus, or MCV is pres­ ent. Parapoxviruses have an ovoid structure with crisscross spicules on negative-stain electron microscopy. MCV has a classic appearance, with Henderson-Patterson bodies, on pathologic analysis of a biopsy sample. Serologic assays can demonstrate orthopoxvirus reactivity, but most are unable to distinguish between orthopoxvirus species because of their broad antigenic similarity. Efforts in serology tests during the mpox outbreak are ongoing in order to differentiate infection from vaccination.

PART 5 Infectious Diseases A   D B   E C   F FIGURE 201-1  Mpox lesions. A–D. Standard lesions seen in mpox. Notice the well-circumscribed nature of the lesions even on mucosal surfaces; many of the lesions display the central umbilication unique to poxviruses, which often develop in later stages of illness. E and F. Severe mpox manifestations in a patient with advanced HIV. Notice the large coalescing lesion on the back, where the large lesion border is composed of individual lesions; additionally, large healing lesions are seen on the neck and hands. (Source: CDC.)

A   B FIGURE 201-2  Molluscum contagiosum lesions. Notice the classic pearly appearance of molluscum contagiosum in A and B. B also displays central umbilication. (Source: CDC.) TREATMENT Poxvirus Treatment of poxvirus infection is largely supportive. Typical sup­ portive care goals include prevention of autoinoculation to sec­ ondary sites and bacterial superinfection, pain control, and scar minimization. Disease-specific therapies are generally reserved for severe illness. Recently, as part of smallpox preparedness efforts, two antiviral agents active against the orthopoxviruses have been approved by FDA for the treatment of smallpox. As these agents did not require comparative human trials, their role in human orthopoxvirus infections continues to be investigated. Both anti­ viral agents are virustatic; therefore an immune response is crucial to recovery for any orthopoxvirus infection. For mpox specifically, given the high proportion of individuals coinfected with HIV, a key aspect of management is initiation, continuation, or reinitiation of antiretroviral therapy. The degree of HIV control is directly corre­ lated with hospitalization, severe mpox manifestations, and mortal­ ity. As in other etiologies of immunosuppression, immune system optimization is critical for recovery. Immunosuppressive agents should be avoided or held if possible during illness. Tecovirimat is an inhibitor of a viral egress protein that prevents cell-to-cell dissemination of mature virions; it is virustatic, so concomitant optimized immune function is essential to favorable clinical outcomes. It was used as an investigational drug in isolated cases of vaccinia, cowpox, and borealpox, and used extensively dur­ ing the global mpox outbreak. It has a favorable safety profile and is the drug of choice for severe orthopoxviral infections; however, its effectiveness for treatment of human orthopoxvirus infections has not been systematically evaluated. Tecovirimat is dosed 600 mg orally or 200 mg intravenously every 12 h (with higher or more frequent dosing depending on patient weight) typically for 2 weeks. A fatty meal is necessary to optimize enteral bioavailability. For patients with severe immunocompromise and severe manifesta­ tions of mpox, the benefits of a prolonged treatment course (i.e., beyond the standard 14-day duration) may outweigh the harms (tecovirimat resistance). Importantly, a single-point amino acid change in the viral target of the drug can confer resistance to teco­ virimat; therefore it should be reserved for severe orthopoxvirus infections given its primary purpose as therapy during a smallpox incident. Additionally, genotypic tecovirimat resistance does not

consistently correlate with phenotypic resistance. To complicate tecovirimat resistance further, there have been different ranges of resistant virus observed, and it is unclear how these results should be interpreted for clinical treatment purposes. It is also unknown if resistance observed within one lesion specimen correlates with viral populations throughout the infected individual, and it has been observed that resistance develops differently (with different viral mutations detected) in swabs from different parts of the body. Therefore, it is reasonable to continue therapy even when resistance is suspected or detected in a specimen.

The other FDA-approved therapy for smallpox is brincidofovir, a prodrug of cidofovir. Either cidofovir or brincidofovir can be given along with tecovirimat for severe poxviral infections. Brincidofovir is only available orally and dosed at 200 mg weekly for two doses. Cidofovir can be used if intravenous therapy is required at 5 mg/kg weekly with concurrent probenecid. Limited animal data suggest that brincidofovir may be synergistic with tecovirimat in orthopox­ virus disease. Therefore, combination therapy can be considered in severe infections. Cidofovir and brincidofovir have higher barriers to resistance, therefore, it is less likely to occur. Topical cidofovir has been used in orthopoxvirus and MCV infections with mixed results. Vaccinia immune globulin (VIG) is licensed for the treatment of adverse reactions to live, replicating smallpox (vaccinia virus) vac­ cine. The standard dose is 6000 U/kg intravenously; dosing can be repeated, and doses of up to 9000 U/kg can be used. Given antigenic similarities across the orthopoxvirus genus, VIG was used exten­ sively for severe mpox with unclear efficacy. It is used primarily for people who may not be able to mount a sufficiently robust immune response to clear virus in patients with severe immunocompromise. Monoclonal antibodies similar to VIG are currently being studied for variola virus. CHAPTER 201 Trifluridine is active against ocular orthopoxviral infections and can be administered for treatment or ocular prophylaxis for peri­ orbital lesions. Treatment dosing is instillation of one drop into the affected eye(s) every 2 h while awake for the first 2 weeks and then four times daily for an additional 2 weeks. Monkeypox, Molluscum Contagiosum, and Other Poxvirus Infections Treatment for MCV infection is on a case-by-case basis. Immu­ nomodulatory therapies such as imiquimod have been used. If quicker resolution is desired, curettage and topical liquid nitrogen is available along with the FDA-approved agent cantharidin, which is applied by a clinician to lesions every 21 days for up to four doses. ■ ■PROGNOSIS In immunocompetent hosts most poxvirus infections are self-limited, resolving in weeks or, in the case of molluscum contagiosum, months. The exceptions are the generalized orthopoxvirus infections caused by MPXV and variola virus, whose case–fatality rates are greater. In unvaccinated individuals smallpox carries a mortality of up to 30%, and MPXV clade I, IIa, and IIb mortality rates are 1.4–10%, 1%, and <1%, respectively. Immunocompromised hosts may have more severe orthopoxvirus and parapoxvirus infections (e.g., severe mpox, pro­ gressive vaccinia, eczema vaccinatum, severe borealpox) leading to higher mortality, or they may have atypical presentations (e.g., giant orf). In patients with advanced HIV, effective antiretroviral therapy is essential to favorable clinical outcomes. MCV infections can be diffuse in immunocompromised persons. Immune reconstitution inflamma­ tory syndrome (IRIS) has been associated with recrudescence of MCV infections. Poxvirus reinfections are seldom reported. ■ ■PREVENTION ACAM2000 is a live, replicating vaccinia virus vaccine administered as a single dose via a percutaneous needle. JYNNEOS is a live, nonrepli­ cating, modified vaccinia Ankara-Bavarian Nordic (MVA-BN) vaccine administered via two subcutaneous doses 28 days apart. Awareness of occupational risks and adherence to appropriate barrier precautions effectively prevent most poxvirus infections. Pre-exposure vaccination is recommended for specific persons at risk of occupational exposure

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202 Parvovirus Infections

(e.g., research laboratorians) to orthopoxviruses via either ACAM2000 or JYNNEOS. Booster doses for these persons at sustained risk for occupational exposure to orthopoxviruses should be administered either 2 years, 3 years, or 10 years after primary vaccination depending on the vaccine administered and reason for vaccination. JYNNEOS is also recommended for persons at risk of mpox during mpox outbreaks and on the routine immunization schedule for persons with specific risk factors defined by CDC’s Advisory Committee on Immunization Practices (ACIP). There is currently no recommendation for booster doses for these persons at risk or for persons to whom JYNNEOS was recommended during the mpox outbreak that started in 2022. Effec­ tiveness of ACAM2000 is inferred from use of similar live, replicating vaccines during the smallpox eradication era when administration of a qualified vaccine 3–5 years earlier was viewed as 100% protective against variola virus. During mpox surveillance efforts in Democratic Republic of the Congo in the 1980s, smallpox vaccination 3–19 years earlier was 85% protective against disease among household contacts of people with mpox. The duration of efficacy is unclear. JYNNEOS was widely used during the global mpox outbreak, and effectiveness against mpox ranged from 36% to 75% for one-dose vaccination and 66% to 89% for two-dose vaccination. The duration of immunity following JYNNEOS vaccination compared with live, replicating vaccinia virus vaccines is unclear.

Acknowledgment Inger K. Damon contributed to this chapter in the last edition and some material from that chapter has been retained here. ■ ■FURTHER READING Chen X et al: Molluscum contagiosum virus infection. Lancet Infect PART 5 Infectious Diseases Dis 13:877, 2013. Rao AK et al: Interim clinical treatment considerations for severe manifestations of mpox — United States, February. MMWR Morb Mortal Wkly Rep 72:232, 2023. Thornhill JP et al: Monkeypox virus infection in humans across 16 countries — April–June 2022. N Engl J Med 387:679, 2022. Maria Söderlund-Venermo

Parvovirus Infections Parvoviruses, members of the large family Parvoviridae, are small (diameter, ~22 nm), nonenveloped, icosahedral viruses with a lin­ ear single-stranded DNA genome of ~5000 nucleotides. The family includes viruses infecting many different animal hosts, from mammals to insects. Five main groups of parvoviruses infect humans: parvovirus B19 (B19V), adeno-associated viruses (AAVs), parvovirus 4 (parv4), human bocaviruses (HBoVs), and human protoparvoviruses (bufavirus and cutavirus). PARVOVIRUS B19 ■ ■DEFINITION B19V belongs to the genus Erythroparvovirus, so named due to its narrow tropism of erythrocyte precursors in the bone marrow. B19V is divided into three genotypes (1, 2, and 3), with similar antigenic, pathogenic, and biological properties. ■ ■EPIDEMIOLOGY B19V exclusively infects humans, and infection is common in virtually all parts of the world. Genotype 1 is currently predominant world­ wide, whereas genotype 2 nowadays rarely causes active infections but

remains persistent in tissues of older individuals. Genotype 3 is the most diverse and appears to be more common in the western parts of Africa. Outbreaks of B19V infection, causing childhood rash (erythema infectiosum), are most common in schools and day-care centers and occur as epidemics a few years apart, in temperate climates, mostly in winter and spring. Within households, schools, and day-care centers, the infection rates approach 50%. The risk of infection increases in pro­ portion to the number of children. Transmission occurs primarily via the respiratory route and occurs before the onset of rash or arthralgia. By the age of 15 years, ~50% of children have detectable IgG antibody to B19V; this seroprevalence may rise to 80% among the elderly. Especially in patients with a hemolytic disorder or compromised immune system, the viral load of B19V in blood can be extremely high (up to 1014 particles/mL), which increases the risk of transmission to hospital staff and family. Transmission can also occur via transfusion, particularly of pooled blood products. However, plasma pools are nowadays screened for B19V DNA, and high-titer pools are discarded. B19V is quite resistant to both heat and solvent-detergent inactivation. ■ ■PATHOGENESIS B19V replicates in erythroid progenitors. This specificity may be due in part to a limited tissue distribution of the yet unknown primary B19V receptor that is recognized by the N-terminal unique region of B19-virus protein 1 (VP1u). Another important receptor, the blood group P antigen (globoside), recognized by the common VP region, VP2, is needed at a later intracellular step. Individuals who lack this P antigen are naturally resistant to B19V infection. Infection leads to high-titer viremia, with 104-12 virus particles/mL detectable in the blood at the apex (Fig. 202-1), and virus-induced cytotoxicity results in cessation of red cell production. The viral load will, however, quickly drop, leaving very low-level B19V DNA in the blood for months and even years after the acute infection. B19V DNAemia in nonacute infections has, however, also been shown to be due to nonencapsidated naked DNA being released from injured tissues. In immunocompetent individuals with normal hemopoiesis, the arrest of erythropoiesis is transient, with only a minimal drop in hemoglobin levels, which resolves as the immune response is mounted. However, in individuals with increased erythropoiesis (especially with hemolytic anemia), the cessation of red cell production can induce a transient crisis with severe anemia (Fig. 202-1). Similarly, if an individual (or a fetus) does not induce neutralizing antibodies to halt the lytic infection, erythroid production is compromised, and chronic anemia develops (Fig. 202-1). In immunocompetent individuals, the immune-mediated phase of illness, which begins 2–3 weeks after acute infection as the IgM response peaks, manifests as the rash of erythema infectiosum or fifth disease alone or together with arthralgia and/or frank arthritis (see “Clinical Manifestations”). If immunocompromised patients with chronic B19V-induced anemia are given immunoglobulins, they may also present with a rash, which is due to antigen-antibody complexes in skin. Even if B19V requires erythroid precursor cells for its replication, it can also enter nonpermissive cells, such as B cells, monocytes, and endothelial cells, by antibody-dependent enhancement (ADE), and remain presumably dormant for life in multiple tissues, such as the heart, liver, kidneys, synovia, brain, and even bones. This persistent presence of B19V DNA in our tissues does not generally seem to com­ plicate normal health but may nevertheless be responsible for some disease presentations in predisposed individuals, as has been suggested in myocarditis, for example. ■ ■CLINICAL MANIFESTATIONS Erythema Infectiosum  Most B19V infections are asymptomatic or exhibit only a mild nonspecific illness. The main manifestation of symptomatic B19V infection is erythema infectiosum, also known as fifth disease or slapped-cheek disease (Figs. 202-2 and A1-1A). Infection may begin with a minor febrile prodrome ~7–10 days after exposure, but it is often absent, and the classic facial rash develops

B19 Virus B19 Antibodies Hemoglobin (g%) Clinical manifestations B19 Virus B19 Antibodies Hemoglobin (g%) Clinical manifestations

IgM

IgG

1.0

Reticulocytes (g%) 0.2

Rash, arthralgia Fever, chills, headache, myalgia 2 6 10

Days Inoculation or infection 2 6 10

Days Infection Normals A B FIGURE 202-1  Schematic of the time course of parvovirus B19 infection in (A) normals (erythema infectiosum), (B) transient aplastic crisis (TAC), and (C) chronic anemia/ pure red cell aplasia (PRCA). (From The New England Journal of Medicine, Parvovirus B19, NS Young, KE Brown: 350:586. Copyright @2004 Massachusetts Medical Society. Reprinted with permission from Massachusetts Medical Society.) FIGURE 202-2  Young child with erythema infectiosum, or fifth disease, showing typical “slapped-cheek” appearance.

B19 Virus B19 Antibodies Hemoglobin (g%) Clinical manifestations

IgM

IgG IgM and IgG

Reticulocytes (g%) Reticulocytes (g%)

Symptoms of anemia CHAPTER 202 Symptoms of anemia 2 6 10

Days Infection PRCA TAC Parvovirus Infections C suddenly several days later. After 2–3 days, the erythematous maculo­ papular rash may spread to the trunk and extremities in a lacy reticular pattern. However, its pattern, intensity, and distribution vary, and B19V-induced rash is difficult to clinically distinguish from other viral exanthems, so a laboratory test should be used when a definite diagnosis is necessary, such as in pregnant women. Typically, the rash may recur for weeks when exercising or sunbathing, but the child is no longer infectious and can go to school. Adults typically do not exhibit the “slapped-cheek” appearance but present with arthralgia, with or without a macular rash. In children, arthritis and encephalitis are rare complications. Polyarthropathy Syndrome  Although uncommon among children, arthropathy occurs in ~50% of adults and is more common among women than among men. The distribution of the affected joints is often symmetrical, with arthralgia affecting the small joints of the hands and occasionally the ankles, knees, and wrists. Resolution usually occurs within a few weeks, but recurring symptoms can continue for months. The illness may mimic rheumatoid arthritis, and rheumatoid factor can often be detected in serum. However, the mere presence of B19V DNA in synovia is not enough to prove a causative relation, since healthy individuals also may exhibit viral DNA in their synovia. Transient Aplastic Crisis  Asymptomatic transient reticulocy­ topenia occurs in most individuals with B19V infection. However, in patients who depend on continual rapid production of red cells, infection can cause a transient aplastic crisis (TAC). B19V is the pre­ dominant cause of TAC in individuals with hemolytic disorders, hemo­ globinopathies, red cell enzymopathies, and autoimmune hemolytic anemias. Patients present with severe to life-threatening anemia and a

low reticulocyte count, and bone marrow examination reveals charac­ teristic giant pronormoblasts and an absence of erythroid precursors. However, reticulocytopenia in sickle-cell patients with acute worsening of anemia is diagnostic without bone marrow examination. Patients are often febrile and very ill, often including other complications. As its name indicates, the illness is transient, and anemia resolves with the cessation of cytopathic infection in the erythroid progenitors, and lifelong immunity follows.

Pure Red Cell Aplasia (PRCA)/Chronic Anemia  Chronic B19V infection has been reported in a wide range of immunocom­ promised patients who are unable to mount a neutralizing immune response, including those with certain congenital immunodeficien­ cies, AIDS (Chap. 208), lymphoproliferative disorders (especially acute lymphocytic leukemia), and transplantation (Chap. 148). PRCA patients have persistent anemia with reticulocytopenia, absent or low levels of B19V IgG, extremely high titers of B19V DNA in serum, and typically scattered giant pronormoblasts on bone marrow examina­ tion. Nonerythroid hematologic lineages are rarely affected, but tran­ sient neutropenia, lymphopenia, and thrombocytopenia (including idiopathic thrombocytopenic purpura) have been observed. B19V occasionally causes a hemophagocytic syndrome. The suspicion of B19V infection in such cases is often difficult due to the lack of normal B19V-related symptoms like rash or arthralgia, which are immune mediated. Diagnosis is, however, important due to the existence of effective therapy in form of repeated immunoglobulin administrations. Co-infection with Plasmodium and B19V has been suggested to play a role in the development of severe anemia in young children with malaria. B19V-infected immunocompetent individuals seldom show PRCA or chronic anemia. PART 5 Infectious Diseases Hydrops Fetalis  B19V infection during pregnancy can lead to hydrops fetalis and/or fetal loss, due to either miscarriage (before 22 weeks of gestation) or fetal death (after 22 weeks of gestation). B19V probably causes 10–20% of all cases of nonimmune hydrops, which is character­ ized by gross edema and severe anemia. The risk of transplacental fetal infection is ~30%, and the excess risk of fetal loss (when the mother is infected before gestational week 20) is ~9%, but very low thereafter. Although B19V does not appear to be teratogenic, rare cases of eye damage, central nervous system (CNS) abnormalities, and congenital anemia have been reported. B19V infection may not cause any symp­ toms in the pregnant mother, so exposed seronegative mothers should undergo tests for B19V infection, and if found positive, they should be monitored regularly throughout pregnancy. Most fetal infections resolve themselves, but sometimes intrauterine red cell transfusions are needed. Unusual Manifestations  B19V infection may rarely cause hepati­ tis, vasculitis, myocarditis, glomerulosclerosis, or meningoencephalitis. A variety of other cardiac manifestations, CNS diseases, and autoim­ mune diseases have also been reported in conjunction with B19V infection. However, B19V DNA can be detected by polymerase chain reaction (PCR) for life in many tissues; therefore, this finding is of no known clinical significance, but its interpretation may cause confusion regarding B19V disease association. TABLE 202-1  Diseases Associated with Human Parvovirus B19 Infection and Methods of Diagnosis DISEASE HOSTS IgM IgG PCR QUANTITATIVE PCR Fifth disease Healthy children Positive Positive Positive

104 IU/mL Polyarthropathy syndrome Healthy adults (more often women) Positive within 3 months of onset Transient aplastic crisis Patients with increased Negative/positive Negative/positive Positive Often >1012 IU/mL, but rapidly decreases erythropoiesis Persistent anemia/pure red cell aplasia Immunodeficient or immunosuppressed patients Negative/weakly positive Hydrops fetalis/ congenital anemia Fetuses (of mothers infected <20 weeks) Negative/positive Positive Positive amniotic fluid or tissue Abbreviations: IU, international units (1 IU equals ~1 genome); n/a, not applicable; PCR, polymerase chain reaction.

■ ■DIAGNOSIS Diagnosis of B19V infection in immunocompetent individuals is gen­ erally based on detection of B19V antibodies (Table 202-1). IgM can be detected by indirect enzyme immunoassay (EIA) at the time of the rash in erythema infectiosum and by the third day of TAC in patients with hematologic disorders, and may remain detectable for ~3 months or longer. B19V IgG is detectable by the seventh day of illness and persists throughout life, whereby IgG positivity marks immunity. However, serum samples taken 2 weeks apart that show seroconversion or a four­ fold or greater increase in IgG titer are considered diagnostic for acute infection. Modern serology can further measure the quality of IgG; as the immune response matures with time, the initially low avidity of IgG gradually increases within 6 months and can be measured with a dena­ turing EIA. Another way of timing the B19V infection is by comparing the IgG responses toward linear versus conformational B19V VP2 epitopes using epitope-type-specific (ETS) EIA. Both avidity and ETS EIAs differentiate between acute and past infection and thus increase the specificity of the diagnosis. Detection of B19V DNA in serum (or amniotic fluid) by PCR provides further help, especially in pregnancy, TAC, or chronic anemia. In acute infection at the height of viremia,

1012 B19V DNA IU/mL of serum can be detected; nevertheless, the viral load falls rapidly within a few days but can remain detectable by PCR for months or even years after acute infection, even in healthy individuals, necessitating a quantitative (q)PCR. Of note, in tissue material, PCR alone should not be used to establish a B19V etiology because viral DNA remains in healthy bodies for decades. TREATMENT Parvovirus B19 Infection No antiviral drugs against B19V are available for patient use, and treatment of B19V infection often targets symptoms only. However, cidofovir, and its lipid conjugate brincidofovir, as well as hydroxy­ urea, seem to inhibit B19V replication in vitro. TAC caused by B19V infection frequently necessitates treatment with repeated blood transfusions. In patients receiving chemotherapy, temporary ces­ sation of treatment may result in an immune response and resolu­ tion. If this approach is unsuccessful or not applicable, commercial immunoglobulin can cure or ameliorate chronic B19V infection in immunosuppressed or otherwise immunocompromised patients. Generally, the intravenous IgG (IVIG) dose is 400 mg/kg daily for 5–10 days and the patient should be monitored for relapses. Administration of IVIG is not beneficial for the immune-mediated erythema infectiosum or B19V-associated arthropathies, which generally are self-limited. Intrauterine blood transfusion can pre­ vent fetal loss in some cases of fetal hydrops; however, the risks need to be evaluated. ■ ■PREVENTION No vaccine has been approved for the prevention of B19V infection, although vaccines based on B19V virus–like particles expressed in insect cells are known to be highly immunogenic. Phase 1 trials of a putative vaccine were discontinued because of adverse side effects, but others are under development. Positive Positive 104 IU/mL Negative/weakly positive Positive Often >1012 IU/mL, but should be >106 in the absence of treatment n/a

92 - 203 Human Papillomavirus Infections

203 Human Papillomavirus Infections

HUMAN BOCAPARVOVIRUSES ■ ■DEFINITION Human bocavirus 1 (HBoV1) of the genus Bocaparvovirus, discovered in 2005, causes both upper and lower respiratory tract infections in young children and affects almost all children before the age of 5 years. Three related bocaviruses, HBoV2, HBoV3, and HBoV4, are mainly found in fecal samples and are thus considered enteric, with disputed roles in gastroenteritis. ■ ■EPIDEMIOLOGY HBoVs are very common worldwide; the seroprevalences in Europe, already at 6 years of age, are 80% for HBoV1, 50% for HBoV2, 10% for HBoV3, and 0–1% for HBoV4, the latter being slightly more prevalent in China. ■ ■CLINICAL MANIFESTATIONS AND DIAGNOSIS HBoVs cause systemic infections with short viremia and induction of antibodies, which can be cross-reacting between the four HBoVs. HBoV1 causes mild to severe, rarely even life-threatening, respiratory tract infections (RTIs) in 0.5- to 5-year-old children (mean 2 years); it is the second to fourth most common finding (2–20%) in nasopharyn­ geal secretions of pediatric RTI patients. HBoV1 DNA, in decreasing levels, often remains detectable by sensitive PCRs in the airways for weeks or months after acute RTI, which is problematic, since RTI is commonly diagnosed by qualitative PCRs in respiratory secretions. This may lead to clinically incorrect interpretations of etiology, codetections with other respiratory pathogens, and doubts of the role of HBoV1 in pathogenesis. However, clinical disease due to HBoV1 is associated with evidence of acute primary infection based on other diagnostic methods: sole infection, serology (IgG seroconversion or fourfold or greater increase in titer, IgM positivity, and/or low IgG avidity), viremia, and presence in respiratory secretions of spliced viral mRNA, high-load HBoV1 DNA (>104 genome copies/mL), or antigens. A combination of these methods should be applied in cases of severe or uncommon disease manifestations or for epidemiologic studies. HBoV1 causes both upper and lower RTIs, such as common cold, bronchiolitis, pneumonia, and exacerbations of asthma, with symptoms similar to those of other viral RTIs, with fever, cough, and wheezing commonly reported. In addition, diarrhea and acute otitis media often complicate HBoV1 infection. The role of the enteric HBoVs in childhood gastroenteritis remains to be established, but they have, like HBoV1, also been shown to cause encephalitis. There are no specific treatments or vaccines for HBoV infections. OTHER HUMAN PARVOVIRUSES ■ ■BUFA- AND CUTAVIRUSES Bufavirus and cutavirus of the genus Protoparvovirus were recently discovered in diarrheal stools by metagenomics. The global serop­ revalence of bufavirus varies extensively from 3 to 85%, whereas that of cutavirus remains <6%. Although bufavirus DNA is found in 0.2–4% of diarrhea stools, the viral loads are low, and its pathogenic role is disputed. Interestingly, skin-persistent cutavirus has been shown to be associated with cutaneous T-cell lymphoma and its precursor, para­ psoriasis en plaques, the latter with up to 67% genoprevalence in skin biopsies. Whether this association is causal or consequential remains to be confirmed. ■ ■PARVOVIRUS 4 Parvovirus 4 (parv4), in the genus Tetraparvovirus, was initially discov­ ered in a patient with an acute viral syndrome and has since been found in pooled plasma donations and blood or tissues of injection drug users and hemophiliacs, suggesting parenteral spread. Even though parv4 in general is considered apathogenic, occasional studies have detected its DNA in patients with rash and respiratory, gastrointestinal, or central nervous system infections. ■ ■ADENO-ASSOCIATED VIRUSES Adeno-associated viruses (AAV, with many serotypes), of the genus Dependoparvovirus, depend on helper functions from other viruses

for their replication. Because of their ability to remain latent and their apathogenic nature, they have been developed as successful vectors in gene therapy. Nevertheless, in 2022, AAV2 was surprisingly associated with acute childhood hepatitis, but a causative role was not confirmed.

Acknowledgment I thank the previous author, Kevin E. Brown, who wrote the prior edition’s chapter. Some material from that chapter has been retained here. ■ ■FURTHER READING Christensen A et al: Human bocaviruses and paediatric infections. Lancet Child Adolesc Health 3:418, 2019. Crabol Y et al: Intravenous immunoglobulin therapy for pure red cell aplasia related to human parvovirus B19 infection: A retrospec­ tive study of 10 patients and review of the literature. Clin Infect Dis 56:968, 2013. Maple PA et al: Identification of past and recent parvovirus B19 infection in immunocompetent individuals by quantitative PCR and enzyme immunoassays: A dual-laboratory study. J Clin Microbiol 52:947, 2014. Matthews PC et al: Human parvovirus 4 ‘PARV4’ remains elusive despite a decade of study F1000 Res 6:82, 2017. Phan T et al: Cutavirus: A newly discovered parvovirus on the rise. Infect Gen Evol 80:104175, 2020. Qiu J et al: Human parvoviruses. Clin Microbiol Rev 30:43, 2017. Söderlund-Venermo M: Emerging human parvoviruses: The rocky road to fame. Ann Rev Virol 6:71, 2019. Xiuong Y et al: The risk of maternal parvovirus B19 infection during CHAPTER 203 pregnancy on fetal loss and fetal hydrops: A systematic review and meta-analysis. J Clin Virol 114:12, 2019. Human Papillomavirus Infections Darron R. Brown, Aaron C. Ermel

Human Papillomavirus

Infections Interest in human papillomavirus (HPV) infection began in earnest in the 1980s after Harold zur Hausen postulated that infection with these viruses was associated with cervical cancer. It is now recognized that HPV infection of the human genital tract is extremely common and causes clinical conditions ranging from asymptomatic infection to genital warts (condylomata acuminata); dysplastic lesions and invasive cancers of the anus, penis, vulva, vagina, and cervix; and a subset of oropharyngeal cancers. This chapter describes the epidemi­ ology of HPV as a virus and a pathogen, the natural history of HPV infections and associated cancers, strategies to prevent infection and HPV-associated disease, and treatment modalities for some conditions caused by HPV. ■ ■PATHOGENESIS Overview  HPV is an icosahedral, nonenveloped, 8000-basepair, double-stranded DNA virus with a diameter of 55 nm. Like the genomes of other papillomaviruses, HPV’s genome consists of an early (E) gene region, a late (L) gene region, and a noncoding region, which contains regulatory elements. The E1, E2, E5, E6, and E7 pro­ teins are expressed early in the growth cycle and are necessary for viral replication and cellular transformation. The E6 and E7 proteins are responsible for malignant transformation, targeting the human cellcycle regulatory molecules p53 and Rb (retinoblastoma protein) for degradation, respectively. Translation of the L1 and L2 transcripts and splicing of an E1^E4 transcript occur later. The L1 gene encodes the

54-kDa major capsid protein that makes up the majority of the virus shell; the 77-kDa L2 minor protein contributes a smaller percentage of the capsid mass.

More than 200 HPV types have been identified and are numerically designated on the basis of a unique L1 gene sequence. Approximately 40 HPV types are regularly identified in the anogenital tract; these types are subdivided into high-risk and low-risk categories depending on the associated risk of cervical cancer. For example, HPV types 6 and 11 cause genital warts and ~10% of low-grade cervical lesions and are thus designated low risk. HPV types 16 and 18 cause dysplastic lesions and a high percentage of invasive cancers of the cervix and are there­ fore considered high risk. HPV is a tissue-tropic virus and targets basal keratinocytes of specific anatomic tissues after microtrauma allows exposure of these cells to the virus. HPV 1, for example, causes plantar warts but does not infect genital epithelium. The HPV replication cycle is completed as keratinocytes undergo differentiation. Virions are assembled in the nuclei of differentiated keratinocytes and can be visualized by electron microscopy. Infection is transmitted by contact with virus contained in these desquamated keratinocytes (or with free virus) from an infected individual. The Immune Response to HPV Infection  Unlike many viral infections, HPV infection has no viremic phase. This lack of viremia may account for the incomplete antibody response to HPV infec­ tion. Natural HPV infection of the genital tract gives rise to a detect­ able serum antibody response in 60–70% of individuals. Significant, although incomplete, protection against type-specific reinfection is associated with the presence of neutralizing antibodies. Serum anti­ bodies likely reach the cervical epithelium and secretions by transuda­ tion and exudation. Therefore, protection against infection relates to the amount of neutralizing antibody at the site of infection and lasts as long as sufficient levels of neutralizing antibodies are present. PART 5 Infectious Diseases A cell-mediated immune response plays an important role in controlling progression of HPV infection. Histologic examination of lesions in individuals who experience regression of genital warts demonstrates infiltration by T cells and macrophages. CD4+ T cell regulation is particularly important in controlling HPV infections, as evidenced by the higher rates of infection and disease in immunosup­ pressed individuals, particularly those who are infected with HIV. Spe­ cific T cell responses may be measured against HPV proteins, the most important of which appear to be the E2 and E6 proteins. In women with HPV16 cervical infection, a strong T cell response to HPV16– derived E2 protein is associated with a lack of progression of cervical disease. However, measurable changes occur in the innate and adaptive immune systems of patients with HPV-associated cancers. There is suppression of the antigen-presentation process as well as suppression of antitumor activity. The end result is a reduction of HPV-specific antitumor immune responses and an increase in immunosuppressive cellular responses. ■ ■THE NATURAL HISTORY OF HPV-ASSOCIATED MALIGNANCY HPV is transmitted by vaginal or anal intercourse, by oral sex, and probably by touching a partner’s genitalia. In cross-sectional and longitudinal studies, ~50% of young women demonstrate evidence of HPV infection, with peaks during the teens and early twenties, within a few months after first coital experience. The number of lifetime sexual partners correlates with the likelihood of HPV infection and the subse­ quent risk of HPV-associated malignancy. HPV infection may occur in a monogamous person if that person’s partner is infected. Most HPV infections become undetectable after 6 to 9 months, a phenomenon known as “clearance.” However, with prolonged followup and frequent sampling, the same HPV types may again be detected months or even years later. It is still debated whether such episodic detection indicates viral latency followed by reactivation or represents reinfection with an identical HPV type. Most evidence indicates that reactivation of latent virus is the cause of episodic detection of a spe­ cific HPV type.

While HPV is the causative agent of several cancers, most attention has focused on cervical cancer, which is the second most common cancer in women worldwide. More than 600,000 women are diagnosed and 300,000 die from invasive cervical cancer annually. More than 85% of all cervical cancer cases, as well as deaths, occur in women living in low-income countries, especially countries in sub-Saharan Africa, Asia, and South and Central America. Evidence collected over 25 years shows that HPV causes nearly 100% of cervical cancers. Persistent HPV infection is the most significant risk factor for cervical cancer; relative risks range from 10 to 20 and exceed 100 in case–control studies and prospective studies, respectively. The time from HPV infection to cervical cancer may exceed 20 years. Cer­ vical cancer peaks in the fifth and sixth decades of life for women living in developed countries and a decade or more earlier for women living in resource-poor countries. Persistent carriers of oncogenic HPV types are at greatest risk for high-grade cervical dysplasia and cancer. Why HPV infections in some women but not others eventually lead to malignancy is not clear. Although oncogenic HPV infection is necessary for the development of cervical malignancy, only ~3–5% of infected women will ever develop this cancer, even in the absence of cytologic screening. Biomarkers that can predict which women will develop cervical cancer are not available, or incompletely char­ acterized. Immunosuppression in general plays a significant role in redetection/reactivation of HPV infections, while other factors, such as smoking, hormonal changes, chlamydial infection, and nutritional deficits, have an impact on viral persistence and cancer. The International Agency for Research on Cancer (IARC) has con­ cluded that HPV types 16, 18, 31, 33, 35, 39, 45, 51, 52, 56, 58, and 59 are carcinogenic in the uterine cervix. HPV type 16 is particularly virulent and causes at least 50% of cervical cancers. Worldwide, HPV types 16 and 18 cause at least 70% of cervical squamous cell carcinomas and 85% of cervical adenocarcinomas. Oncogenic types other than 16 or 18 cause the remaining 30% of cervical cancers. HPV types 16 and 18 also cause nearly 90% of anal cancers worldwide. In addition to cervical and anal cancer, other HPV-associated can­ cers include vulvar and vaginal cancer (caused by HPV in 50–70% of cases), penile cancer (caused by HPV in 50% of cases), and at least 65% of oropharyngeal squamous cell carcinomas (OPSCCs). Over the past two decades, an epidemic of OPSCC related to oncogenic HPV infection, primarily HPV type 16, has developed. Rates of OPSCC in the United States have been increasing in men from a low of 0.27 case per 100,000 in 1973 to 0.57 case per 100,000 per year in 2004; rates in women have remained relatively stable at ~0.17 per 100,000 per year. The greatest increase in the incidence of OPSCC is among white men 40–50 years of age. Nearly 14,000 new cases were diagnosed in the United States in 2013. OPSCCs of the base of the tongue and tonsil cancer have increased annually by rates of 1.3 and 0.6%, respectively. Few data are available from developing countries about OPSCC. ■ ■THE EFFECTS OF HIV ON HPV-ASSOCIATED DISEASE HIV infection accelerates the natural history of HPV infections. HIVinfected individuals are more likely than other individuals to develop genital warts, and their lesions are more recalcitrant to treatment. HIV infection has been consistently associated with precancerous cervi­ cal lesions, including low-grade cervical intraepithelial lesions (CIN) and CIN 3, the immediate precursor to cervical cancer. Women with HIV/AIDS have significantly higher rates of cervical cancer as well as subsets of some vulvar, vaginal, and oropharyngeal tumors (Chap. 75) than women in the general population. Studies indicate a direct rela­ tionship between low CD4+ T lymphocyte count and the risk of cervi­ cal cancer. Some studies show a reduced likelihood of HPV infection and precancerous lesions of the cervix in HIV-infected women given antiretroviral therapy (ART). However, the incidence of cervical cancer in HIV-infected women has not changed significantly since ART was introduced, possibly because of preexisting oncogenic HPV infections that occurred before ART was initiated. The burden of HPV-associated cancers is expected to increase in HIV-infected patients, given the prolonged life expectancies provided

with ART. For women living in developing countries where cervical cancer screening is not widely available, this trend will have significant consequences. Thus, elucidating the interactions of HIV infection and cervical cancer with cofactors such as diet, other sexually transmit­ ted infections, and environmental exposures is an important focus of research that impacts women living in low- and middle-income countries. Similar to that of cervical cancer, the incidence of anal cancer is strongly influenced by HIV infection. HIV-infected men who have sex with men (MSM) and HIV-infected women have much higher rates of anal cancer than HIV-uninfected populations. Specifically, the incidence among HIV-infected MSM has been found to be as high as 130 cases per 100,000 as opposed to 5 cases per 100,000 among HIVnegative MSM. The advent of ART has not impacted the incidence of anal cancer and high-grade anal intraepithelial neoplasia in the HIVinfected patient population. More information regarding screening, prevention, and treatment in the HIV-infected population can be found at the Department of Health and Human Services website (https://clinicalinfo.hiv.gov/en/ guidelines). ■ ■CLINICAL MANIFESTATIONS OF HPV INFECTION HPV infects the male urethra, penis, and scrotum and the female vulva, vagina, and cervix. Perianal, anal, and oropharyngeal infections occur in both genders. Genital warts are caused primarily by HPV type 6 or 11 and appear as soft sessile growths with a surface that is either smooth or rough with multiple finger-like projections. Penile genital warts are usually 2–5 mm in diameter and often occur in groups. A second type of penile lesion, the keratotic plaque, is slightly raised above normal epi­ thelium and has a rough, often pigmented surface. Figs. 203-1 to 203-3 show vulvar and vaginal, penile, and perianal warts, respectively. Vulvar warts are soft, whitish papules that are either sessile or have multiple fine, finger-like projections. These lesions are most often located in the introitus and labia. In nonmucosal areas, vulvar lesions are similar in appearance to those in men: dry and keratotic. Vulvar lesions can appear as smooth, sometimes pigmented papules that may coalesce. Vaginal lesions appear as multiple areas of elongated papillae. Biopsy of vulvar or vaginal lesions may reveal malignancy; differentia­ tion based on clinical exam is not always reliable. Subclinical cervical HPV infections are common, and the cervix may appear normal on examination. Cervical lesions often appear FIGURE 203-1  Warts of the vulva and vagina caused by human papillomavirus. (Reproduced with permission from K Wolff et al: Fitzpatrick’s Color Atlas and Synopsis of Clinical Dermatology, 8th ed. New York: McGraw-Hill, 2013.)

FIGURE 203-2  Penile genital warts caused by human papillomavirus. (Reproduced with permission from K Wolff et al: Fitzpatrick’s Color Atlas and Synopsis of Clinical Dermatology, 8th ed. New York: McGraw-Hill, 2013.) as papillary proliferations near the transformation zone. Irregular vascular loops are present beneath the surface epithelium. Patients who develop cervical cancer from HPV infection may present with a variety of symptoms. Early carcinomas appear eroded and bleed eas­ ily. More advanced carcinomas present as ulcerated lesions or as an exophytic cervical mass. Some cervical carcinomas are in the cervical canal and may be difficult to see. Bleeding, symptoms of a mass lesion in late stages, and metastatic disease that may manifest as bowel or bladder obstruction due to direct extension of the tumor also have been described. CHAPTER 203 Patients with squamous cell cancer of the anus (Chap. 86) have more variable presentations. The most common presentations include rectal bleeding and pain or a mass sensation. Twenty percent of patients who are diagnosed with anal cancer may not present with any specific symptoms at the time of diagnosis, and the lesion is found fortuitously. Human Papillomavirus Infections ■ ■PREVENTION OF HPV INFECTION AND DISEASE Behaviors That Can Reduce Exposure to HPV  HPV infec­ tions are transmitted through direct contact with infected genital skin FIGURE 203-3  Perianal warts caused by human papillomavirus. (Reproduced with permission from K Wolff et al: Fitzpatrick’s Color Atlas and Synopsis of Clinical Dermatology, 8th ed. New York: McGraw-Hill, 2013.)

or mucosal surfaces and secretions. Abstinence may possibly reduce HPV infections: for both men and women, numerous studies indicate that HPV infection and HPV-associated diseases correlate with the number of lifetime sexual partners, and people with no history of sexual intercourse have a lower detection rate of HPV. Fewer studies look at nonpenetrative sex on the risk of HPV infection and disease, but several studies indicate that HPV can be spread by any sexual intimacy, including touching, oral sex, or use of sex toys. It is therefore possible that individuals who have not partaken in sexual intercourse can become infected.

Use of latex condoms reduces the risk of HPV infection and HPVassociated disease, such as genital warts and cervical precancers. Correct and consistent condom use has also been associated with regression of CIN in women and regression of HPV-associated penile lesions in men. As a preventive measure, condom use should be con­ sidered partially effective at best and not a substitute for cervical cancer screening or vaccination against HPV. HPV Vaccines  The development of HPV vaccines effective in preventing infection and HPV-associated disease represents a major development in the past decade. The vaccines use virus-like particles (VLPs) that consist of the HPV L1 major capsid protein. The L1 protein self-assembles into VLPs when expressed in eukaryotic cells (i.e., yeast or insect cells). These VLPs contain the same epitopes as actual HPV virions. However, they do not contain genetic material and therefore cannot transmit infection. The immunogenicity of the HPV vaccines relies on development of conformational neutralizing antibodies directed toward epitopes displayed on viral capsids. Several large vaccine trials have been completed and demonstrate the high degree of safety and efficacy of HPV vaccines. There have been three HPV vaccines developed, tested, and U.S. Food and Drug Administration (FDA) approved, as described below. PART 5 Infectious Diseases BIVALENT VACCINE (CERVARIX)  The bivalent HPV vaccine con­ tains L1 VLPs of HPV types 16 and 18 and is marketed under the name of Cervarix (GlaxoSmithKline). This vaccine was tested in 18,644 women 15−25 years of age residing in the United States, South America, Europe, and Asia. It is administered by intramuscular injec­ tion three times (months 0, 1, and 6). The primary endpoints of the study included vaccine efficacy against persistent infections with HPV types 16 and 18. Investigators also assessed vaccine efficacy against CIN grade 2 or higher due to HPV 16 and 18 in women who had no evidence of HPV 16 or 18 infection at baseline. Vaccine efficacy related to HPV 16 or HPV 18 was 94.9% (95% confidence interval [CI], 87.7–98.4%) against CIN 2 or worse; 91.7% (95% CI, 66.6–99.1%) against CIN 3 or worse; and 100% (95% CI, –8.6–100%) against adeno­ carcinoma in situ (AIS). Adverse events associated with the bivalent vaccine were evaluated in phase 3 trials in a subset of 3077 women who received vaccine and 3080 women who received hepatitis A vaccine. Injection-site adverse events (pain, redness, and swelling) and systemic adverse events (fatigue, headache, and myalgia) were reported more frequently in the HPV vaccine group than in the control group. Seri­ ous adverse events, new-onset chronic disease, or medically significant conditions occurred in the same proportion (3.5%) of HPV vaccine recipients and control vaccine recipients. The bivalent HPV vaccine is approved in the United States for prevention of cervical cancer, CIN2 or worse, AIS, and CIN 1 caused by HPV types 16 and 18. This vaccine is approved for females 9−25 years of age. Cervarix is not currently marketed in the United States. QUADRIVALENT VACCINE (GARDASIL)  The quadrivalent L1 VLP vaccine (HPV types 6, 11, 16, and 18) is marketed under the name Gardasil (Merck). It is administered intramuscularly three times (months 0, 2, and 6). A combined efficacy analysis based on data from four randomized double-blind clinical studies including >20,000 participants was performed; results demonstrated that vaccine effi­ cacy against external genital warts was 98.9% (95% CI, 93.7−100%). Vaccine efficacy was 95.2% (95% CI, 87.2−98.7%) against CIN; 100% (95% CI, 92.9−100%) against type 16- or 18-related CIN 2/3 or AIS; and 100% (95% CI, 55.5−100.0%) against type 16- or 18-related vulvar

intraepithelial neoplasia grades 2 and 3 (VIN 2/3) and against vaginal intraepithelial neoplasia grades 2 and 3 (VaIN 2/3). Safety data on the quadrivalent HPV vaccine are available from at least seven clinical trials, including nearly 12,000 women 9–26 years of age who received the vaccine and ~10,000 women who received aluminum-containing or saline placebo. A larger proportion of young women reported injection-site adverse events in the vaccine groups than in the placebo groups. Systemic adverse events were reported by similar proportions of vaccine and placebo recipients and were described as mild or moderate for most participants. The types of serious adverse events reported were similar for the two groups. Ten persons who received the quadrivalent vaccine and seven persons who received placebo died during the course of the trials; no deaths were considered to be vaccine related. During the course of studies on the quadrivalent HPV vaccine, surveillance data for development of new medical conditions were collected for up to 4 years after vaccination. No statistically significant differences in the incidence of any medical conditions between vaccine and placebo recipients were demonstrated; this result indicated a very high safety profile for the vaccine. A recent safety review by the FDA and the Centers for Disease Control and Prevention (CDC) exam­ ined events related to Gardasil that had been reported to the Vaccine Adverse Events Reporting System (VAERS). The adverse events were consistent with what was seen in previous safety studies of the vaccine. Of note, rates of syncope and venous thrombotic events were higher with Gardasil than those usually observed with other vaccines. The quadrivalent HPV vaccine is approved for (1) vaccination of females ages 9−26 years of age to prevent genital warts and cervical cancer caused by HPV types 6, 11, 16, and 18; (2) vaccination of the same population to prevent precancerous or dysplastic lesions, including cervical AIS, CIN 2/3, VIN 2/3, VaIN 2/3, and CIN 1; (3) vaccination of males 9−26 years of age to prevent genital warts caused by HPV types 6 and 11; and (4) vaccination of patients ages 9−26 years to prevent anal cancer and associated precancerous lesions due to HPV types 6, 11, 16, and 18. While the duration of protection has not been established, no evidence of waning protection has been found after a three-dose series of the quadrivalent HPV vaccine, even after 10 years of follow-up from clinical trials. The quadrivalent HPV vaccine is no longer available in the United States but is still available in many other countries, although production is not likely to continue in the future. NINE-VALENT VACCINE (GARDASIL-9)  In 2014, the FDA approved a new nine-valent L1 VLP vaccine. The nine-valent vaccine is marketed under the name Gardasil-9 (Merck). It is administered intramuscularly two times (months 0 and 6) for males and females from age 9 to the fifteenth birthday, then three times (months 0, 2, and 6) for males and females from age 15 through age 45. Three doses of HPV vaccine are recommended for persons starting the vaccination series on or after the fifteenth birthday and for persons with certain immunocompromising conditions, including HIV/AIDS. The nine-valent vaccine targets HPV types 6, 11, 16, and 18 (the types also targeted by the quadrivalent HPV vaccine) as well as five additional oncogenic HPV types (31, 33, 45, 52, and 58). HPV types 16 and 18 together cause up to 80% of all cervical cancers worldwide, and worldwide data show that HPV types 31, 33, 35, 45, 52, and 58 are the next most frequently detected types in invasive cervical cancers. Mathematical models estimate that the level of protection conferred by the nine-valent HPV vaccine against all HPV-associated squamous cell cancers worldwide could be raised to at least 90%. In clinical studies of females 16−26 years of age, the nine-valent HPV vaccine generated a noninferior antibody response to HPV types 6, 11, 16, and 18 compared with the quadrivalent HPV vaccine. Bridging immunologic studies in male and female vaccine recipients 9−15 years of age and in males 16−26 years of age indicated that the lower bound of the 95% CIs of the geometric mean titer ratio and seroconversion rates met criteria for noninferiority for all HPV types represented in the vaccine. In female recipients 16−26 years of age, vaccine efficacy against the combined endpoint of high-grade cervi­ cal, vulvar, or vaginal disease caused by any of the five additional

oncogenic HPV types was 96.7% (95% CI, 80.9–99.8%). Like the other available HPV vaccines, the nine-valent HPV vaccine is safe and extremely well tolerated. The nine-valent HPV vaccine is approved for 9- to 45-year-old males and females and has an FDA indication for prevention of cervical, vaginal, vulvar, and anal cancer and genital warts due to vaccine types. CROSS-PROTECTION OF HPV VACCINES  Women who receive any of the available HPV vaccines produce neutralizing antibodies to virus types that are closely related to type 16 or 18. Analyses of data from clinical trials suggest that the HPV vaccines may offer limited crossprotection against nonvaccine virus types. Over short periods, the bivalent vaccine appears more efficacious against HPV types 31, 33, and 45 than the quadrivalent vaccine, but differences in study design make direct comparisons difficult if not impossible. In addition, in the bivalent vaccine trials, vaccine efficacy against persistent infections with HPV types 31 and 45 waned over time, whereas efficacy against persistent infection with HPV type 16 or 18 remained stable. These results suggest that cross-protection is likely to be shorter lived than efficacy against infection and disease caused by vaccine types. SINGLE-DOSE VACCINATION  As a cost saving measure, the World Health Organization has recommended that 9- to 15-year-olds may be vaccinated with a single dose of an HPV vaccine. This strategy appears to yield similar antibody titers to a two-dose regimen, but the level and duration of protection against infection and disease are not fully understood. A single-dose strategy has not been recommended by the FDA or other regulatory agencies in the United States. RECOMMENDATIONS FOR HPV VACCINATION  The most recent guide­ lines for HPV vaccination from the Advisory Committee on Immuni­ zation Practices (ACIP) are summarized below and provided in detail at https://www.cdc.gov/vaccines/hcp/acip-recs/vacc-specific/hpv.html. No prevaccination testing of any kind is recommended to establish whether the HPV vaccine should be administered to an individual. The HPV vaccine should be administered, if possible, before exposure to HPV through sexual activity because the vaccines are preventa­ tive against specific HPV types and have no effect on preexisting, type-specific HPV infections. Either the bivalent (where available) or nine-valent HPV vaccines may be used. An individual can begin a vaccine series with one HPV vaccine and then complete the series with another. For those who have completed a vaccination series with the bivalent or quadrivalent vaccine, an additional full series (two or three doses, depending on age as indicated above) of vaccination with the nine-valent vaccine may be given, but there are no data to determine the effectiveness of this approach. For children, adolescents, and adults (male and female) 9−26 years of age, the ACIP recommends HPV vaccination at age 11 or 12 years, although vaccination can be initiated at 9 years of age per FDA approval as above. “Catch-up” HPV vaccination is recommended for men and women through 26 years of age who are not adequately vaccinated. For adults (male and female) 27−45 years of age, catch-up HPV vaccination is not routinely recommended. Instead, the ACIP now recommends “shared clinical decision-making” (see below) regarding HPV vaccination for adults in this age range who are not adequately vaccinated. HPV vaccines are not licensed for use in adults older than 45 years of age. For women, cervical cancer screening should continue according to age-specific guidelines regardless of having received an HPV vaccine (see cervical cancer screening section below). SHARED CLINICAL DECISION-MAKING FOR ADULTS (MALES AND FEMALES) 27−45 YEARS OF AGE  A discussion with adults 27−45 years of age should occur prior to routine recommendation of the HPV vaccine. HPV infection occurs soon after first sexual activity in most people, and vaccine effectiveness is therefore lower in older individuals due to prior infections. HPV exposure usually decreases among older age groups. Although HPV vaccination is safe for adults 27−45 years of age, the benefit to the population is likely to be minimal. However, some men and women who are not vaccinated may be at risk for acquisition of new HPV infections and could therefore benefit from HPV vaccination.

In considering HPV vaccination of adults 27−45 years of age, some key points emphasized by the ACIP that should be discussed include the following:

• HPV is a common sexually transmitted infection, and most HPV infections are asymptomatic and do not lead to clinical disease. • Most sexually active adults have been exposed to HPV, although not necessarily all of the HPV types targeted by vaccines. • Some adults are at risk for acquiring new HPV infections through sexual activity. For example, having a new sex partner is a risk factor for acquiring a new HPV infection. • Persons in a long-term, mutually monogamous sexual partnership are unlikely to acquire a new HPV infection. • Antibody testing cannot determine whether a person is immune or susceptible to a specific HPV type. • HPV vaccines are very effective in persons who have not been exposed to vaccine-type HPV before vaccination. • Vaccine effectiveness is likely to be lower among persons with mul­ tiple lifetime sex partners because these individuals have probably had previous infections with vaccine-type HPV. • HPV vaccines are prophylactic (i.e., they prevent new HPV infec­ tions). They have no utility in preventing established HPV infection from progressing to clinical disease, and they do not have a role in treatment of HPV-associated disease. RECOMMENDATIONS FOR HPV VACCINATION IN PEOPLE LIVING WITH HIV (PLWH)  Guidelines for HPV vaccination of PLWH are summarized below and can be found in detail at https://hivinfo.nih.gov. HPV vaccines are safe in PLWH. Administration of HPV vaccines generates high levels of antibody against HPV types represented in vaccine, although antibody levels are generally lower than in those who are HIV-uninfected. In addition, immune responses appear stron­ ger among PLWH who have the highest CD4 counts and the lowest HIV viral loads. Studies also indicate that HPV vaccination induces an anamnestic response in PLWH. Regarding efficacy in protect­ ing against HPV-associated disease, one randomized, double-blind, clinical trial evaluated the efficacy of the quadrivalent HPV vaccine in adults with HIV infection older than 27 years in prevention of new anal HPV infections or improvement in high-grade dysplastic anal lesions. The trial did not show efficacy, but many study participants had HPV infection detected at baseline, prior to vaccination. CHAPTER 203 Human Papillomavirus Infections HPV vaccination is recommended (three doses) for girls and boys with HIV infection 11−26 years of age. Because some individuals with HIV infection (similar to HIV-uninfected individuals) have had many sex partners prior to vaccination, HPV vaccination may be less benefi­ cial in these patients than in those with few or no lifetime sex partners. Current data do not strongly support vaccination for those PLWH older than 26 years. The public health benefit for HPV vaccination of PLWH in this age range is likely to be minimal. However, although many PLWH ages 27–45 years will not fully benefit from the vaccine, there may be situations that suggest the possibility of vaccine ben­ efit, and the same shared clinical decision-making (described above) between the provider and patient is recommended. ■ ■SCREENING FOR HPV-ASSOCIATED CANCER Once HPV infection occurs, prevention of HPV-associated disease relies on screening. At present, screening for cervical cancer is widely accepted as cost-effective in preventing cervical cancer. Anal screen­ ing is accepted for screening in high-risk groups, though no national guidelines exist for screening intervals or ages for initiation and ces­ sation of screening. In resource-rich countries, the primary method of cervical cancer screening is cytology via Pap smear. The American Society of Colposcopy and Cervical Pathology (ASCCP) guidelines recommend initiation of cervical cancer screening at age 21, no mat­ ter the age of sexual debut. Women 21−29 years old should have a Pap smear every 3 years if their initial and subsequent Pap smears are normal. Although adolescent and young women often test HPV DNA positive, they are at very low risk of cervical cancer. Because the pres­ ence of HPV DNA does not correlate with the presence of high-grade

squamous intraepithelial neoplasia, co-testing (testing for HPV DNA at the time of Pap smear) is not recommended for women in this age group.

As a method of determining the need for colposcopy, HPV DNA co-testing is recommended for women 25−29 years of age in whom cytology detects abnormal squamous cells of undetermined signifi­ cance (ASCUS). Women 30−65 should have a Pap smear every 3 years if testing for HPV DNA is not performed. The screening interval for women in this age group can be extended to every 5 years if HPV DNA co-testing is performed and results are negative. HPV testing is not recommended for partners of women with HPV or for screening of conditions other than cervical cancer. The role of HPV DNA testing as a primary screen for cervical cancer is changing. In the United States, there are three commercially avail­ able assays (cobas HPV Test [Roche Diagnostics], the BD Onclarity HPV Assay [Becton, Dickinson and Company], and Alinity m HR HPV Assay (Abbott Molecular Diagnostics)) that are FDA approved for primary screening using HPV DNA testing. However, more assays may gain approval for usage as the feasibility and evidence for their use in various populations globally come to light. These tests can be used to detect HPV DNA in specimens obtained from the cervix without cervical cytology for women ≥25 years of age. A positive result for HPV type 16 or 18 has a high enough positive predictive value in the general population that these women should have colposcopy performed. If high-risk HPV types other than HPV 16 or HPV 18 are detected, then cytology can be obtained. The complete set of algorithms for appropri­ ate age-specific screening guidelines, HPV DNA testing, and the man­ agement of abnormal Pap smears are available through the ASCCP at http://asccp.org/guidelines. PART 5 Infectious Diseases For women ≤30 years of age who are infected with HIV, cervical cytology is the preferred method of cervical cancer screening and HPV DNA co-testing is not recommended. Cervical cancer screening should begin within 1 year of diagnosis of HIV infection, regardless of the mode of HIV transmission. If the first Pap smear is normal, then subsequent Pap smears should be performed annually until three negative tests are obtained. Cytology can then be obtained every 3 years. For women ≥30 years old, Pap testing is performed in the same manner as for younger women. However, HPV DNA co-testing can be used in women of this age group. If cytology and HPV DNA co-testing are negative, the next exam can be performed in 3 years. Positive HPV DNA co-test results are treated in the same manner as in HIV-uninfected women. Women residing in developing countries with a lack of access to cervical screening programs have a higher rate of cervical cancer and a poorer cancer-specific survival. Approximately 75% of women liv­ ing in developed countries have been screened in the past 5 years, as opposed to ~5% of women living in developing countries. Economic and logistic obstacles likely impede routine cervical cancer screening for these populations. Many poor countries rely on an alternative method—visual inspection with acetic acid (VIA)—for cervical can­ cer screening. While some studies show a reduction in cervical cancer mortality in communities where VIA is widely utilized, other stud­ ies do not. In addition, the low specificity of VIA is problematic. As newer methods that use detection of oncogenic HPV DNA become available, even resource-limited countries may be able to replace VIA with such methods and achieve a reduction in cervical cancers as a result. There are no anal cancer screening guidelines endorsed by organiza­ tions such as the United States Preventative Services Task Force. The International Anal Neoplasia Society has published consensus guide­ lines regarding screening for anal cancer in patients at higher risk such as those with HIV. Current HIV treatment guidelines suggest that there may be a benefit to screening, but an effect on the associated morbidity and mortality of anal squamous cell cancer has not been consistently demonstrated. Although the most effective method of screening for anal cancer has not been determined, data published from a random­ ized controlled treatment trial noted a significant reduction in progres­ sion of high-grade precancerous lesions of the anus to anal cancer in those who received treatment. Further studies on optimal screening strategies for anal cancer are ongoing.

The incidence of HPV-associated head and neck cancers in the United States has overtaken the incidence of cervical cancer as of 2020, but there are no established guidelines for screening for HPV-associated head and neck cancers. However, HPV vaccination is likely to be effec­ tive for both anal and head and neck cancers associated with HPV. TREATMENT HPV-Associated Disease A variety of treatment modalities are available for various HPV infections, but none has been proven to eliminate HPV from tissue adjacent to the destroyed and infected tissue. Treatment efficacies are limited by frequent recurrences, presumably due to reinfection from an infected partner, reactivation of latent virus, or autoinoculation from nearby infected cells. The goals of treatment include prevention of viral transmission, eradication of premalig­ nant lesions, and reduction of symptoms. Therapies are generally successful in eliminating visible lesions and grossly diseased tissue. Different therapies are indicated for geni­ tal warts, vaginal and cervical disease, and perianal and anal disease. THERAPEUTIC OPTIONS Imiquimod  Imiquimod (5 or 3.75% cream) is a patient-applied topical immunomodulatory agent thought to activate immune cells by binding to a Toll-like receptor that leads to an inflamma­ tory response. Imiquimod 5% cream is applied to genital warts at bedtime three times per week for up to 16 weeks. Warts are cleared in ~56% of patients, more often in women than in men; recurrence rates approach 13%. Local inflammatory side effects are com­ mon. Rates of clearance of genital warts are not as high with the 3.75% formulation as with the 5% preparation, but the duration of treatment is shorter (daily application required for a maximum of 8 weeks) and fewer local and systemic adverse reactions occur. Imiquimod should not be used to treat vaginal, cervical, or anal lesions. The safety of imiquimod during pregnancy has not been established. Interferon  Recombinant interferon α is used for intralesional treatment of genital warts, including perianal lesions. The recom­ mended dosage is 1.0 × 106 IU of interferon into each lesion three times weekly for 3 weeks. Interferon therapy causes clearance of infected cells by immune-boosting effects. Adverse events include headache, nausea, vomiting, fatigue, and myalgia. Interferon ther­ apy is costly and should be reserved for severe cases that do not respond to less expensive treatments. Interferon should not be used to treat vaginal, cervical, or anal lesions. Cryotherapy  Cryotherapy (liquid nitrogen treatment) for HPVassociated lesions causes cellular death. Genital warts usually disap­ pear after two or three weekly sessions but often recur. Cryotherapy, which is nontoxic and is not associated with significant adverse reactions, can also be used for diseased cervical tissue. Local pain occurs frequently. Surgical Methods  Exophytic lesions can be surgically removed after intradermal injection of 1% lidocaine. This treatment is well tolerated but can cause scarring and requires hemostasis. Genital warts can also be destroyed by electrocautery, in which no addi­ tional hemostasis is required. Laser Therapy  Laser treatment affords destruction of exophytic lesions and other HPV-infected tissue while preserving normal tissue. Local anesthetics are generally adequate. Efficacy for genital lesions is at least equal to that of other therapies (60–90%), with low recurrence rates (5–10%). Complications include local pain, vagi­ nal discharge, periurethral swelling, and penile or vulvar swelling. Laser therapy has also been used successfully for cervical dysplasia and anal disease caused by HPV. Therapeutic Vaccines  The innate and adaptive immune systems are altered in patients with HPV-associated cancers. Antitumor

93 - SECTION 13 Infections Due to DNA and RNA Respiratory Viruses

SECTION 13 Infections Due to DNA and RNA Respiratory Viruses

TABLE 203-1  Recommended Treatments for Genital Warts Caused by Human Papillomavirusa TREATMENT IMIQUIMOD CRYOTHERAPY INTERFERON SURGICAL REMOVAL LASER Effectiveness Good Good Good Excellent Excellent Recurrence Frequent Frequent Frequent Frequent Frequent Adverse effects Frequent, mild to moderate Mild, well tolerated Frequent, moderately severe Mild, well tolerated Mild to moderate, well tolerated Availability Fair Good Fair Good Fair Cost Expensive Inexpensive Very expensive Moderately expensive Very expensive aImiquimod can be self-administered. All other treatments must be administered by a clinician. immune responses are blunted by specific viral mechanisms. Numerous therapeutic vaccines that are being developed are designed to enhance the cell-mediated response to the HPV E6 and E7 oncoproteins, which are expressed in HPV-associated cancers. Such vaccines would enhance the ability to treat HPV-associated cancers, conditions that are very difficult to treat with current modalities. However, while progress has been made, no HPV vac­ cine is currently available for treatment of HPV infection or HPVassociated disease. Other Therapies  Both trichloroacetic acid and bichloroacetic acid are caustic agents that destroy warts by coagulation of proteins. Nei­ ther of these agents is recommended for treatment. Sinecatechins (15% ointment) and podophyllotoxin (0.05% solution or gel and 0.15% cream) are occasionally used for external genital warts, but other modalities listed above are as or more effective and are better tolerated. RECOMMENDATIONS FOR TREATMENT Table 203-1 lists available treatments for genital warts. An optimal therapy for HPV-related genital tract disease that combines high efficacy, low toxicity, low cost, and low recurrence is not available. For genital warts of the penis or vulva, cryotherapy is the safest, least expensive, and most effective modality. However, all available modalities for treatment of genital warts carry high rates of recur­ rence. Guidelines for the treatment of anogenital warts can be found on the CDC website (https://www.cdc.gov/std/treatment-guidelines/ anogenital-warts.htm). Women with vaginal lesions should be referred to a gynecologist experienced in colposcopy and treatment of these lesions. Treat­ ment of cervical disease involves careful inspection, biopsy, and histopathologic grading to determine the severity and extent of disease. Women with evidence of HPV-associated cervical disease should be referred to a gynecologist familiar with HPV and experi­ enced in colposcopy. Optimal follow-up of these patients includes colposcopic examination of the cervix and vagina on a yearly basis. Guidelines from the American College of Obstetricians and Gyne­ cologists are available for the treatment of cervical dysplasia and cancer. For anal or perianal lesions, cryotherapy or surgical removal is safest and most effective. Anoscopy and/or sigmoidoscopy should be performed in patients with perianal lesions, and suspicious lesions should be biopsied to rule out malignancy. ■ ■COUNSELING PATIENTS REGARDING

HPV DISEASE Most sexually active adults will be infected with HPV during their lives. The only way to avoid acquiring an HPV infection is to abstain from sexual activity, including intimate touching and oral sex. Practicing safe sex (partner reduction, use of condoms) may help reduce HPV transmission. Most HPV infections will be controlled by the immune system and cause no symptoms or disease. Some infections lead to genital warts and cervical precancers. Genital warts can be treated for cosmetic reasons and to prevent spread of infection to others. Even after resolution of genital warts, latent HPV may persist in normalappearing skin or mucosa and thus theoretically may be transmitted to uninfected partners. Precancerous cervical lesions should be treated to prevent progression to cancer.

■ ■FURTHER READING Akhatova A et al: Prophylactic human papillomavirus vaccination: From the origin to the current state. Vaccines (Basel) 10:1912, 2022. Clifford GM et al: Carcinogenicity of human papillomavirus (HPV) types in HIV-positive women: A meta-analysis from HPV infection to cervical cancer. Clin Infect Dis 64:1228, 2017. Garland SM et al: Impact and effectiveness of the quadrivalent human papillomavirus vaccine: A systematic review of 10 years of real-world experience. Clin Infect Dis 63:519, 2016. Gavinski K, DiNardo D: Cervical cancer screening. Med Clin North Am 107:259, 2023. Gelbard MK, Munger K: Human papillomaviruses: Knowns, mysteries, and unchartered territories. J Med Virol 95:e29191, 2023. Giuliano AR et al: Efficacy of quadrivalent HPV vaccine against HPV infection and disease in males. N Engl J Med 364:401, 2011. Gravitt PE, Winer RL: Natural history of HPV infection across the CHAPTER 204 lifespan: Role of viral latency. Viruses 9:265, 2017. Palefsky J et al: Treatment of anal high-grade squamous intraepithe­ lial lesions to prevent anal cancer. N Engl J Med 386:2273, 2022. Rosenblum HG et al: Declines in prevalence of human papilloma­ virus vaccine-type infection among females after introduction of vaccine — United States, 2003–2018. MMWR Morb Mortal Wkly Rep 70:415, 2021. Schiffman M et al: Carcinogenic human papillomavirus infection. Common Viral Respiratory Infections, Other Than COVID-19
Nat Rev Dis Primers 2:16086, 2016. Section 13 Infections Due to DNA and RNA Respiratory Viruses James E. Crowe, Jr.

Common Viral

Respiratory Infections,

Other Than COVID-19 The most common and frequent infections in humans are respiratory virus infections. Influenza viruses and coronaviruses have been the agents responsible for the largest infectious disease pandemics. These viruses are easily transmitted by contact, droplets, and fomites. Fur­ thermore, transmission can occur before the appearance of symptoms. These viruses are also associated with a large reproductive number (the number of secondary infections generated from one infected individual to others). Some classical respiratory viruses (e.g., rhinoviruses) enter the body through the respiratory tract, replicating and causing disease only in cells of the respiratory epithelium. Other, more systemic viruses (e.g., measles virus and severe acute respiratory syndrome coronavirus [SARS-CoV]) spread via the bloodstream and cause systemic disease;

94 - 204 Common Viral Respiratory Infections, Other Than COVID-19

204 Common Viral Respiratory Infections, Other Than COVID-19

TABLE 203-1  Recommended Treatments for Genital Warts Caused by Human Papillomavirusa TREATMENT IMIQUIMOD CRYOTHERAPY INTERFERON SURGICAL REMOVAL LASER Effectiveness Good Good Good Excellent Excellent Recurrence Frequent Frequent Frequent Frequent Frequent Adverse effects Frequent, mild to moderate Mild, well tolerated Frequent, moderately severe Mild, well tolerated Mild to moderate, well tolerated Availability Fair Good Fair Good Fair Cost Expensive Inexpensive Very expensive Moderately expensive Very expensive aImiquimod can be self-administered. All other treatments must be administered by a clinician. immune responses are blunted by specific viral mechanisms. Numerous therapeutic vaccines that are being developed are designed to enhance the cell-mediated response to the HPV E6 and E7 oncoproteins, which are expressed in HPV-associated cancers. Such vaccines would enhance the ability to treat HPV-associated cancers, conditions that are very difficult to treat with current modalities. However, while progress has been made, no HPV vac­ cine is currently available for treatment of HPV infection or HPVassociated disease. Other Therapies  Both trichloroacetic acid and bichloroacetic acid are caustic agents that destroy warts by coagulation of proteins. Nei­ ther of these agents is recommended for treatment. Sinecatechins (15% ointment) and podophyllotoxin (0.05% solution or gel and 0.15% cream) are occasionally used for external genital warts, but other modalities listed above are as or more effective and are better tolerated. RECOMMENDATIONS FOR TREATMENT Table 203-1 lists available treatments for genital warts. An optimal therapy for HPV-related genital tract disease that combines high efficacy, low toxicity, low cost, and low recurrence is not available. For genital warts of the penis or vulva, cryotherapy is the safest, least expensive, and most effective modality. However, all available modalities for treatment of genital warts carry high rates of recur­ rence. Guidelines for the treatment of anogenital warts can be found on the CDC website (https://www.cdc.gov/std/treatment-guidelines/ anogenital-warts.htm). Women with vaginal lesions should be referred to a gynecologist experienced in colposcopy and treatment of these lesions. Treat­ ment of cervical disease involves careful inspection, biopsy, and histopathologic grading to determine the severity and extent of disease. Women with evidence of HPV-associated cervical disease should be referred to a gynecologist familiar with HPV and experi­ enced in colposcopy. Optimal follow-up of these patients includes colposcopic examination of the cervix and vagina on a yearly basis. Guidelines from the American College of Obstetricians and Gyne­ cologists are available for the treatment of cervical dysplasia and cancer. For anal or perianal lesions, cryotherapy or surgical removal is safest and most effective. Anoscopy and/or sigmoidoscopy should be performed in patients with perianal lesions, and suspicious lesions should be biopsied to rule out malignancy. ■ ■COUNSELING PATIENTS REGARDING

HPV DISEASE Most sexually active adults will be infected with HPV during their lives. The only way to avoid acquiring an HPV infection is to abstain from sexual activity, including intimate touching and oral sex. Practicing safe sex (partner reduction, use of condoms) may help reduce HPV transmission. Most HPV infections will be controlled by the immune system and cause no symptoms or disease. Some infections lead to genital warts and cervical precancers. Genital warts can be treated for cosmetic reasons and to prevent spread of infection to others. Even after resolution of genital warts, latent HPV may persist in normalappearing skin or mucosa and thus theoretically may be transmitted to uninfected partners. Precancerous cervical lesions should be treated to prevent progression to cancer.

■ ■FURTHER READING Akhatova A et al: Prophylactic human papillomavirus vaccination: From the origin to the current state. Vaccines (Basel) 10:1912, 2022. Clifford GM et al: Carcinogenicity of human papillomavirus (HPV) types in HIV-positive women: A meta-analysis from HPV infection to cervical cancer. Clin Infect Dis 64:1228, 2017. Garland SM et al: Impact and effectiveness of the quadrivalent human papillomavirus vaccine: A systematic review of 10 years of real-world experience. Clin Infect Dis 63:519, 2016. Gavinski K, DiNardo D: Cervical cancer screening. Med Clin North Am 107:259, 2023. Gelbard MK, Munger K: Human papillomaviruses: Knowns, mysteries, and unchartered territories. J Med Virol 95:e29191, 2023. Giuliano AR et al: Efficacy of quadrivalent HPV vaccine against HPV infection and disease in males. N Engl J Med 364:401, 2011. Gravitt PE, Winer RL: Natural history of HPV infection across the CHAPTER 204 lifespan: Role of viral latency. Viruses 9:265, 2017. Palefsky J et al: Treatment of anal high-grade squamous intraepithe­ lial lesions to prevent anal cancer. N Engl J Med 386:2273, 2022. Rosenblum HG et al: Declines in prevalence of human papilloma­ virus vaccine-type infection among females after introduction of vaccine — United States, 2003–2018. MMWR Morb Mortal Wkly Rep 70:415, 2021. Schiffman M et al: Carcinogenic human papillomavirus infection. Common Viral Respiratory Infections, Other Than COVID-19
Nat Rev Dis Primers 2:16086, 2016. Section 13 Infections Due to DNA and RNA Respiratory Viruses James E. Crowe, Jr.

Common Viral

Respiratory Infections,

Other Than COVID-19 The most common and frequent infections in humans are respiratory virus infections. Influenza viruses and coronaviruses have been the agents responsible for the largest infectious disease pandemics. These viruses are easily transmitted by contact, droplets, and fomites. Fur­ thermore, transmission can occur before the appearance of symptoms. These viruses are also associated with a large reproductive number (the number of secondary infections generated from one infected individual to others). Some classical respiratory viruses (e.g., rhinoviruses) enter the body through the respiratory tract, replicating and causing disease only in cells of the respiratory epithelium. Other, more systemic viruses (e.g., measles virus and severe acute respiratory syndrome coronavirus [SARS-CoV]) spread via the bloodstream and cause systemic disease;

however, they also may enter through and cause disease in the respi­ ratory tract. Although infections with systemic viruses often induce lifelong immunity against disease, respiratory viruses that do not cause high-grade viremia usually can reinfect the same host many times throughout life. Reinfection with the same virus is common because of incomplete or waning immunity after natural infection. Hundreds of different viruses cause infection of the respiratory tract, and within each virus type, there can be a nearly unlimited diversity of field strains that vary antigenically, geographically, and over time (e.g., antigenically drifting influenza viruses or coronaviruses). Specific antiviral treat­ ment options are limited, and only a few licensed vaccines are available. For further discussion of common respiratory virus infections, see Chap. 37 and syndrome-specific chapters. For further discussion of SARS-CoV-2 virus infections, see Chap. 205.

Common viral respiratory infections can be categorized in several ways, including by site of anatomic involvement, disease syndrome, or etiologic agent. ANATOMIC SITES IN THE HUMAN RESPIRATORY TRACT The type of respiratory disease that develops during virus infection is dictated to a large degree by the cell types and tissue organization in the respiratory tract. The vocal cords mark the transition between the upper and lower respiratory tracts. The upper respiratory tract is a complex anatomic system with interconnected structures, including the sinuses, middle-ear spaces, Eustachian tubes, conjunctiva, naso­ pharynx, oropharynx, and larynx. The tonsils and the adenoids are large collections of lymphoid tissue in the pharynx that participate in immunity but also are susceptible to infections. The lower respiratory tract structures include the trachea, bronchi, bronchioles, alveolar spaces, and lung tissue, including epithelial cells and blood vessels. The epithelial cell types that line the respiratory tract are varied in morphol­ ogy and function, and their susceptibility to different virus infections varies. The principal types of cells in the major airways are ciliated or nonciliated epithelial cells, goblet cells, and Clara cells. Smooth-muscle cells form major tissue structures around the epithelial structures of the large airways of the lower respiratory tract down to the level of the bronchioles, and these cells are reactive to intrinsic and extrinsic signals, including viral infection or exposure to allergens or pollut­ ants. The pathologic process of wheezing is driven by smooth-muscle contraction and obstruction of airways caused by mucus accumulation and epithelial sloughing in the lumen. Reactive airways causing wheez­ ing are most often due to constriction of lumen size at the level of the bronchioles (which have the narrowest lumen diameter of the airways). The lung does not have smooth-muscle or ciliated cells but instead possesses pneumocytes of types I and II. Pneumonia (Chap. 131) is an infection of the pneumocytes in the lung tissue and the alveolar spaces. The alveolar spaces also contain cells of the monocyte lineage, such as macrophages, which patrol the air spaces. PART 5 Infectious Diseases DISEASE SYNDROMES Since different respiratory viruses tend to have a predilection for repli­ cation in differing cells or regions of the respiratory tract, it is possible for the well-trained clinician with epidemiologic information to under­ stand the most likely associations of viruses with clinical syndromes. The clinical diagnoses for virus infections in the upper respiratory tract are rhinitis or the common cold, sinusitis, otitis media, conjunctivitis, pharyngitis, tonsillitis, and laryngitis. Some upper respiratory tract infections affect more than one upper respiratory tract anatomic site during a single infection, such as the classical pattern of pharyngo­ conjunctival fever during adenovirus infection. Lower respiratory tract syndromes also can be associated easily with anatomic region, including tracheitis, bronchitis, bronchiolitis, pneumonia, and exacer­ bations of reactive airway disease or asthma. Bronchiolitis is a disease condition characterized by trapping of air in the lungs with difficulty in expiration (i.e., wheezing); it is caused by inflammation or infec­ tion of the bronchioles, the smallest and most highly resistant airways. Again, mixed syndromes occur, such as laryngotracheitis, usually termed croup. Croup, a disease condition characterized by difficulty

in inspiration associated with a barky cough, is caused by inflamma­ tion or infection of the larynx, trachea, and bronchi. When respiratory symptoms occur in the context of a respiratory viral illness with signifi­ cant systemic signs, infection with particular agents can be suspected (e.g., influenza, measles, SARS-CoV, SARS-CoV-2, or hantavirus pul­ monary syndrome [HPS]), with exposure history taken into account. ETIOLOGIC AGENTS ■ ■RESPIRATORY VIRUSES CAUSING DISEASE IN IMMUNOCOMPETENT HOSTS Children have more frequent respiratory virus infections than adults; thus, it was natural that many early discoveries about the viral causes of respiratory infections came from pediatric studies. The principal causes of acute viral respiratory infections were determined in large epidemio­ logic studies in the 1960s and 1970s, when cell culture of infectious agents became available. More recently, studies of viral epidemiology have been conducted in adults, especially in special populations such as the elderly, nursing home residents, and immunocompromised individuals. Rapid antigen detection tests (based on immunoassays for detection of viral proteins) became available for respiratory syncytial virus (RSV) and influenza virus in the 1980s. With the availability of sensitive and specific molecular tests, such as reverse transcription combined with the polymerase chain reaction (RT-PCR), studies in the past several decades have greatly increased the extent to which we understand the causes of viral respiratory infections. Multiplex panels of RT-PCR tests capable of detecting a dozen or more viruses are com­ monly available for clinical testing of respiratory secretions. Nested multiplex PCR assays performed in two stages provide sensitive tests that have been especially helpful in studies of infection in adults, who often shed much lower concentrations of virus in secretions than do children. Typically, influenza viruses, RSV, and human metapneumo­ virus (hMPV) are the most common causes of serious lower respira­ tory tract disease in otherwise healthy subjects; parainfluenza viruses (PIVs) and adenoviruses also cause substantial disease. Rhinoviruses (the most common cause of the common cold syndrome) have been increasingly associated with lower respiratory tract syndromes. Rhino­ virus infection is so common, even in asymptomatic individuals, that it has been hard to establish clear figures for the role of rhinovirus in lower respiratory disease. COVID-19 and the associated public health measures deployed in 2020−2021 altered the epidemiology of respira­ tory viruses such that conventional viruses were greatly reduced in incidence, exhibited altered seasonality, or even disappeared (influenza type B Yamagata lineage viruses) in the years immediately following. Generally, about two-thirds of cases of respiratory illness in a research setting can be associated with a specific viral agent. Besides the viruses mentioned above (and discussed below), several additional viruses identified with molecular tools have been associated with respiratory illness. Still, our diagnostic tools remain suboptimal since a specific infectious agent is not identified in approximately one-third of clinical respiratory illnesses in large surveillance studies. It is likely that in most of these cases pathogens are not detected because of the very low titers of virus in patient samples at the time of clinical presentation, which may occur after the period of peak virus shedding. It is also possible that novel agents are yet to be identified. As emerging tools for metage­ nomic and “virome” studies (with sequencing of all nucleic acids in a sample) are applied in these settings in coming years, new agents and new associations with disease will probably be discovered. ■ ■RESPIRATORY VIRUSES CAUSING DISEASE IN IMMUNOCOMPROMISED HOSTS Special populations of patients are susceptible not only to the conven­ tional respiratory viruses discussed above but also to agents causing symptoms during reactivation of latent viruses or new infections with opportunistic agents. Most prominently, reactivating latent viruses, such as herpes simplex virus (HSV) and cytomegalovirus (CMV) and adenoviruses, cause disease in immunocompromised humans. Patients at most risk are those with hematopoietic stem cell or solid organ transplantation, leukopenia caused by chemotherapy, or advanced

HIV-AIDS. In immunosuppressed patients with pneumonia, CMV is the virus recovered most frequently during deep respiratory tract diag­ nostic procedures such as bronchoalveolar lavage. These patients also are highly susceptible to more frequent and more severe disease caused by common respiratory viruses, including RSV, hMPV, PIVs, influenza viruses, rhinoviruses, and adenoviruses. Conventional acute respira­ tory viruses can cause chronic and sometimes fatal infections in these populations. Nosocomial transmission of respiratory viruses occurs in hematopoietic stem cell transplantation units, and the frequency of transmission can be high, with entire units affected. ■ ■SPECIFIC VIRAL CAUSES OF RESPIRATORY DISEASE Orthomyxoviridae: Influenza Viruses  (See also Chap. 206) Influenza virus infection and influenza syndrome usually are associ­ ated with fever, myalgias, fatigue, sore throat, headache, and cough. Influenza causes severe and even fatal pneumonia, particularly in elderly patients, nursing home residents, immunocompromised per­ sons, and very young children. Influenza pneumonia has an unusually high rate of complication by bacterial superinfection, with staphylo­ coccal and streptococcal bacterial pneumonia occurring in as many as 10% of cases in some clinical series. Influenza is a single-stranded, segmented, negative-sense, RNA genome virus of the family Orthomyxoviridae. There are four (sero) types of influenza viruses: A, B, C and D. Influenza A and C viruses infect multiple species, whereas influenza B virus infects humans almost exclusively. Type D viruses primarily infect cattle and are not known to infect humans. Type A viruses appear to be the most virulent for humans and most commonly cause severe disease manifestations, although type B viruses cause substantial morbidity. Based on antibody response, influenza A viruses can be subdivided into 18 different hem­ agglutinin (H) surface protein subtypes and 11 neuraminidase (N) sur­ face protein subtypes. The subtypes that have caused major pandemics in humans are H1N1, which caused the 1918 pandemic; H2N2, which caused the 1957 pandemic; H3N2, which caused the 1968 pandemic; and H1N1pdm2009, which caused the 2009 pandemic. Currently, two type A subtypes (H1N1 and H3N2) and one type B lineage (Victoria) cause annual seasonal epidemics. Major pandemics caused by new influenza viruses are always pos­ sible. Many highly pathogenic influenza viruses circulate in aquatic birds. Occasionally, avian viruses infect humans directly after close contact with infected wild birds or poultry. Co-housing of pigs (which have both avian and human influenza virus receptors) with poultry may increase the risk of reassortment of human and animal or bird viruses; reassortment can make the zoonotic viruses more fit for repli­ cation in humans. Several outbreaks of avian influenza have occurred in limited numbers of humans to date, and there is the risk of a world­ wide pandemic with avian influenza viruses if a strain acquires the potential to spread efficiently from human to human. H5N1 influenza virus infection of humans, predominantly by direct chicken-to-human transmission, occurred during an epizootic in Hong Kong’s poultry population in 1997. The disease affected many types of wild and domestic birds and caused a high rate of systemic disease and death in infected humans. This virus, carried in the gastrointestinal tract of wild birds, has spread throughout Asia and beyond and continues to evolve antigenically. An H5N1 virus was detected widely in dairy cattle in the United States in 2024 and inferred to be a highly pathogenic avian influenza (HPAI) virus. Avian H7N7 and H7N9 viruses also have caused zoonotic outbreaks. A significant outbreak of H7N9 virus infec­ tion began in China in March 2013, with high mortality, and there have been six outbreaks to date, the largest in 2016−2017 with 766 human infections. H7N9 is considered to have high potential to cause a future pandemic. H1N2 virus is endemic in pigs and affects humans with close contact. An H3N2 variant virus that differs antigenically from seasonal human viruses is endemic in pigs and occasionally infects children who have close contact with pigs in the United States. H3N8 is a subtype of equine influenza viruses that can infect humans who are in close contact with pigs. Rare human cases caused by H6, H9, and H10

subtype viruses have been reported. Type B influenza viruses co-circulate in humans during seasonal epidemics. Type B viruses mutate less frequently than type A viruses. The slower evolution of type B viruses is probably linked to the fact that they are almost exclusively human pathogens. There is only one B type of influenza, but these viruses began to diverge into two antigenically distinguishable lineages in the 1970s. The two virus lineages were named after the initial designated representative strains—B/Victoria/2/87 and B/Yamagata/16/88—and can be distinguished by serologic or genotyping laboratory tests. The evolution of B viruses over time spurred the inclusion of two type B virus antigens in seasonal influenza vaccines, expanding some multiva­ lent vaccines from trivalent (H1N1, H3N2, B) to a quadrivalent format. During the COVID-19 pandemic, the diversity of influenza in humans has been reduced, as strains in lineage B/Yamagata and one clade of H3N2 known as 3c3.A were not detected. In 2023, the World Health Organization recommended future vaccines to be trivalent with only B/Victoria for the type B component.

Pneumoviridae/Paramyxoviridae (the formal species names of family Pneumoviridae were updated in 2023; Table 204-1) 

•  RESPIRATORY SYNCYTIAL VIRUS  Human RSV (hRSV) (species name Human orthopneumovirus hominis) is a single-stranded, negativesense, nonsegmented, RNA genome virus of the genus Respirovirus (formerly Pneumovirus) in the family Paramyxoviridae. Infection is ubiquitous, affecting most humans in the first several years of life and causing reinfections throughout life. RSV is among the most trans­ missible viruses of humans. Disease epidemics occur yearly, typically between October or November and March in temperate regions. RSV is one of the most common viral causes of severe lower respiratory tract illness in the elderly and in children; it is among the most important causes of hospitalization of elderly and infant patients throughout the world. There is only one serotype of RSV, but antigenic variability does occur in circulating field strains. In immune serum reciprocal crossneutralization studies, the two antigenic subgroups, A and B, appear to be ~25% antigenically related; this relatedness may partially explain the susceptibility of humans to reinfection, which is very common and can be caused by viruses of the same subgroup or even the same strain. However, reinfection in otherwise healthy adults usually is associated with mild disease confined to the upper respiratory tract. Severe lower respiratory tract disease is common in the elderly, especially in frail institutionalized elderly populations. Immunocompromised patients of any age also are at risk of severe or prolonged disease, especially recipi­ ents of hematopoietic stem cell transplants. Wheezing is common with primary infection in children (bronchiolitis), and there is a strong asso­ ciation of RSV infection early in life and subsequent asthma, although it is unclear whether severe childhood RSV causes asthma or is the first manifestation of reactive airway disease. RSV causes exacerbations of asthma and is associated with acute exacerbations of chronic obstruc­ tive pulmonary disease (COPD), also referred to as acute exacerbations of chronic bronchitis (AECB). CHAPTER 204 Common Viral Respiratory Infections, Other Than COVID-19
HUMAN METAPNEUMOVIRUS  hMPV (species name Metapneumo­ virus hominis) was discovered only in 2001 but probably has always been present in human populations. Infection occurs first in early childhood, and reinfections are common throughout life. This virus is similar in many respects to RSV, but it now is classified in the fam­ ily Pneumoviridae and is a member of the genus Metapneumovirus. It causes both upper and lower respiratory disease. It appears to be TABLE 204-1  Family Pneumoviridae, Human Pathogens with Current Species Names, the International Committee on Taxonomy of Viruses: 2023 Release CURRENT SPECIES NAME FORMER SPECIES NAME(S) GENUS Metapneumovirus Metapneumovirus hominis Human metapneumovirus (hMPV) Orthopneumovirus Human orthopneumovirus hominis Human respiratory syncytial virus (hRSV) or Human orthopneumovirus

somewhat less virulent than RSV, causing about half as much severe lower respiratory tract disease, probably because it does not possess the nonstructural genes that RSV expresses in infected cells to abrogate the effect of host innate immune effectors like interferons. The clinical features of lower respiratory tract infections caused by hMPV are like those of such infections caused by other paramyxoviruses, most often including cough, coryza, and wheezing. Like RSV, hMPV plays an important role in exacerbations of asthma or COPD and causes pneu­ monia or wheezing in frail and institutionalized elderly individuals and immunocompromised patients.

Paramyxoviridae (the formal species names of family Para­ myxoviridae were updated in 2023; Table 204-2)  •  HUMAN PARAINFLUENZA VIRUSES  The human PIVs (hPIV) are a group of four distinct serotypes (designated 1–4) of single-stranded, negativesense RNA viruses belonging to the family Paramyxoviridae. hPIV3 (species name Respirovirus pneumoniae) most commonly causes severe disease, and repeated infection is common throughout life, although secondary infections often are mild or asymptomatic. Primary infec­ tions in children manifest as laryngotracheitis (croup), while subse­ quent infections typically are limited to the upper respiratory tract. hPIVs are detected with sensitive RT-PCR tests or, more classically, by cell culture with immunofluorescent microscopy or hemadsorption in reference laboratories. MEASLES VIRUS  (See also Chap. 211) Measles virus (species name Morbillivirus hominis) is also a paramyxovirus but of the genus Morbil­ livirus. This virus causes a systemic infection known as rubeola but also can manifest with respiratory symptoms. Measles virus probably is the most contagious respiratory virus infection of humans: it is transmitted efficiently not only by direct contact with infected persons or fomites (like other respiratory viruses) but also by small-particle aerosols. Mea­ sles virus infection is preventable by vaccination, but the pathogen is so transmissible that cases are inevitable—even in the United States— whenever vaccination rates fall below 90–95% in a population. The virus causes systemic illness, sometimes including severe pneumonia, when primary infection occurs in an unvaccinated adult or an immu­ nocompromised person of any age. Therefore, vigilance in maintaining high vaccination rates is critical. With primary infection, the illness in children is typically milder; however, mortality rates in lower-resource countries are high, especially among persons with underlying risk fac­ tors, including malnutrition. PART 5 Infectious Diseases Symptoms of measles include ≥3 days of high fever and a classical set of upper and lower respiratory tract symptoms sometimes termed “the 3 Cs”: cough, coryza, and conjunctivitis. Unlike most respiratory viruses, measles virus circulates in the bloodstream and thus causes TABLE 204-2  Family Paramyxoviridae Human Pathogens with Current Species Names, the International Committee on Taxonomy of Viruses: 2023 Release CURRENT SPECIES NAME FORMER SPECIES NAME(S) GENUS Respirovirus Respirovirus laryngotracheitidis Human respirovirus 1 or human parainfluenza virus type 1 (hPIV1)   Respirovirus pneumoniae Human parainfluenza virus type 3 (hPIV3) or Human respirovirus 3 Orthorubulavirus Orthorubulavirus parotitidis Mumps virus or Mumps orthorubulavirus Orthorubulavirus laryngotracheitidis Human parainfluenza type 2 (hPIV2) or Human orthorubulavirus 2 Orthorubulavirus hominis Human parainfluenza type 4a (hPIV4a) or Human orthorubulavirus 4 Human parainfluenza type 4b (hPIV4b) or Human orthorubulavirus 4 Orthorubulavirus mammalis Parainfluenza type 5 (PIV5) or Mammalian orthorubulavirus 5

disseminated infection with systemic manifestations. Usually, a charac­ teristic diffuse maculopapular rash appears within days of fever onset. Koplik’s spots (see Fig. A1-2)—typical mucosal lesions in the mouth that appear briefly—are considered diagnostic of measles infection in the setting of the typical rash and fever. Picornaviridae  A wide variety of picornaviruses cause respiratory disease, including nonpolio enteroviruses, rhinoviruses, and parecho­ viruses (Chap. 210). The designations of these viruses can be confus­ ing: the Enterovirus, rhinovirus, and Parechovirus species names were changed (with the approval of the International Committee on Tax­ onomy of Viruses) to remove references to host species names (such as the formerly used terms human, simian, etc.), and they are frequently updated. These changes are summarized in Table 204-3. The genus Enterovirus consists of 15 species, including the enteroviruses and rhinoviruses affecting humans. The genus Parechovirus contains six species, one of which—Parechovirus A—encompasses 19 types: human parechoviruses (HPeVs) 1 and 2 are common human pathogens. These viruses exhibit seasonal patterns that differ from those of most other acute respiratory viruses. Rhinovirus infections occur year-round. Enterovirus infections occur most commonly in the summer months in temperate areas. RHINOVIRUSES  Rhinoviruses have single-stranded, positive-sense RNA genomes. Rhinoviruses A through C represent species in the Enterovirus genus of the family Picornaviridae. Rhinoviruses are the most common viral infective agents in humans and the most frequent cause of the common cold. Field isolates of rhinovirus are exception­ ally diverse; they can be classified by serotyping into >100 serotypes or, alternatively, by genotyping into many genotypes that cause cold symptoms. The viral particles are icosahedral in structure and are non­ enveloped. Rhinoviruses are responsible for at least half of all cases of the common cold. Rhinovirus-induced common colds may be compli­ cated in children by otitis media and in adults by sinusitis. Most adults, in fact, have radiographic evidence of sinusitis during the common cold, which resolves without therapy. Therefore, the primary disease is probably best termed rhinosinusitis. Rhinovirus infection is associated with exacerbations of reactive airway disease in children and asthma TABLE 204-3  Enterovirus, Rhinovirus, and Parechovirus Species Names, the International Committee on Taxonomy of Viruses: 2023 Release GENUS CURRENT SPECIES NAME FORMER SPECIES NAME(S) Enterovirus

(15 species) Enterovirus alphacoxsackie: consists of 25 serotypes, including coxsackieviruses and some nonpolio enteroviruses that cause respiratory disease Human enterovirus A; Enterovirus A Enterovirus betacoxsackie: consists of 63 serotypes, including some coxsackieviruses, echoviruses, and nonpolio enteroviruses Human enterovirus B; Enterovirus B Enterovirus coxsackiepol: consists of 23 serotypes, including the polioviruses Human enterovirus C; Enterovirus C Enterovirus deconjuncti: consists of multiple serotypes and includes enterovirus D68 Human enterovirus D; Enterovirus D Enterovirus alpharhino Human rhinovirus A; Rhinovirus A Enterovirus betarhino Human rhinovirus B; Rhinovirus B Enterovirus cerhino Human rhinovirus C; Rhinovirus C Parechovirus

(6 species) Parechovirus ahumpari: consists of 19 types (1–19); human parechoviruses (HPeVs) 1 and 2 are common human pathogens Parechovirus A; HPeV-1 and HPeV-2 were formerly classified in the genus Enterovirus as echoviruses 22 and 23, respectively

in adults. It is not clear whether rhinovirus is restricted to the upper respiratory tract and only indirectly induces inflammatory responses that affect the lower respiratory tract or whether the viruses spread to the lower respiratory tract. In the past, it was thought that these viruses did not often replicate or cause disease in the lower respiratory tract. However, recent studies have discerned strong epidemiologic associa­ tions of rhinoviruses with wheezing and asthma exacerbations, includ­ ing episodes severe enough to require hospitalization. Rhinovirus C (species Enterovirus cerhino) has been associated with more severe disease syndromes, such as pneumonia or exacerbation of COPD. Rhi­ noviruses likely can infect the lower airways to some degree, inducing a local inflammatory response. Another possibility is that significant local infection of the upper respiratory tract may induce regional elaboration of mediators that causes lower airway disease. The associa­ tion of rhinovirus infection with lower respiratory tract illness is dif­ ficult to study because diagnosis by cell culture is not sensitive. RT-PCR diagnostic tests are difficult to interpret because they are often positive for prolonged periods and even asymptomatic individuals may have a positive test. Comprehensive serologic studies to confirm infection are difficult because of the large number of serotypes. Nevertheless, most experts believe rhinoviruses are a common cause of serious lower respiratory tract illness. ENTEROVIRUSES  Nonpolio enteroviruses are common and distrib­ uted worldwide. Although infection often is asymptomatic, these viruses cause outbreaks of clinical respiratory disease, sometimes with fatal consequences. The species Enterovirus alphacoxsackie (formerly Enterovirus A) consists of 25 serotypes, including coxsackieviruses and some nonpolio enteroviruses that cause respiratory disease. Cox­ sackieviruses cause oral lesions and often are associated in children with hand-foot-and-mouth disease. The pharyngitis associated with this infection characteristically manifests with herpangina, a clinical syndrome of ulcers or small vesicles on the palate that often involves the tonsillar fossa and is associated with fever, difficulty swallowing, and throat pain. Outbreaks commonly occur in young children during the summer. Enterovirus A71 also causes large outbreaks of hand-footand-mouth disease, especially in Asia, sometimes leading to neurologic complications and even death. The species Enterovirus betacoxsackie (formerly Enterovirus B) consists of >90 serotypes, including the echo­ viruses (echo being an acronym for enteric cytopathic human orphan, which may be an archaic notion since most echoviruses are associated with human diseases, most commonly in children). Echoviruses can be isolated from many children with upper respiratory tract infections during the summer months. Echovirus 11 has been associated with laryngotracheitis or croup. Epidemiologic studies also have associated echoviruses with epidemic pleurodynia, an acute illness characterized by sharp chest pain and fever. The species Enterovirus coxsackiepol (formerly Enterovirus C) consists of 23 serotypes, including the polio­ viruses. The species Enterovirus deconjuncti (formerly Enterovirus D)

consists of five serotypes, including enterovirus D68, which has been associated with wheezing and a polio-like syndrome in children marked by acute flaccid myelitis. PARECHOVIRUSES  The genus Parechovirus comprises six species, one of which is Parechovirus ahumpari (formerly Parechovirus A), which can affect humans. The most common member of the genus Parecho­ virus, human parechovirus 1 (HPeV-1), is a frequent human pathogen. The genus also includes the closely related human parechovirus 2 (HPeV-2). HPeVs usually cause mild respiratory or gastrointestinal illness. Most infections occur in young children. The seroprevalence of HPeV-1 and HPeV-2 is high among adults. Adenoviridae  Viruses of the family Adenoviridae infect both humans and animals. As their designation indicates, adenoviruses were first isolated in human lymphoid tissues from surgically removed adenoids. In fact, some serotypes establish persistent asymptomatic infections in tonsil and adenoid tissues, and virus shedding can occur for months or years. These double-stranded DNA viruses are <100 nm in diameter and have nonenveloped icosahedral morphology. The large double-stranded DNA genome is linear and nonsegmented. The seven

major human adenovirus species (designated A through G) fall into over 50 immunologically distinct serotypes. Human respiratory tract infections are caused mainly by the B and C species. Adenovirus infec­ tions can occur throughout the year. Many serotypes cause sporadic outbreaks, while others appear to be endemic in particular locations. Respiratory illnesses include mild disease such as the common cold and lower respiratory tract illnesses including croup, bronchiolitis, and pneumonia. Conjunctivitis is associated with infection by the B and D species. A particular constellation of symptoms referred to as pharyngoconjunctival fever is frequently associated with acute adenovirus infection. In contrast, gastroenteritis has been associated most frequently with virus serotypes 40 and 41 of species F. Immu­ nocompromised patients are highly susceptible to severe disease dur­ ing infection with respiratory adenoviruses. The syndrome of acute respiratory disease (ARD), especially common in stressful or crowded living conditions, was first recognized among military recruits during World War II and has continued to be a problem when vaccination has been suspended temporarily because of lapses in vaccine supply. ARD is most often associated with adenovirus types 4 and 7. Adenovirus vaccine containing live adenovirus types 4 and 7 taken orally as two tablets, which prevents most illness caused by these two virus types, is only available for U.S. military personnel 17−50 years of age. It is recommended by the Department of Defense for military recruits entering basic training or other military personnel at high risk for adenovirus infection.

Coronaviridae  SARS-CoV-2 emerged in an outbreak in Wuhan, China, that spread worldwide, causing the severe pandemic of COVID-19. SARS-CoV-2 is discussed separately in Chap. 205. CHAPTER 204 Other members of the genus Coronavirus also contribute to respira­ tory illness, including severe disease. Dozens of coronaviruses affect animals. In the twentieth century, only two representative strains of human coronaviruses were known to cause disease: 229E (HCoV229E) and OC43 (HCoV-OC43). An outbreak of infection with SARS-associated coronavirus (SARS-CoV) first showed that animal coronaviruses have the potential to cross from other species to humans, with devastating effects. The one major SARS-CoV epidemic to date (2002−2003) encompassed >8000 cases, with mortality rates approach­ ing 10%. SARS-CoV causes a systemic illness with a respiratory route of entry. In contrast to most other viral pneumonias, SARS lacks upper respiratory symptoms, although cough and dyspnea occur in most patients. Typically, patients present with a nonspecific illness manifest­ ing as fever, myalgia, malaise, and chills or rigors; watery diarrhea may occur as well. Investigators have reported the identification of a fourth human coronavirus, HCoV-NL63. Evidence is emerging that this new group 1 coronavirus is a common respiratory pathogen of humans, causing both upper and lower respiratory tract illness. HCoV-HKU1 was first described in January 2005 after its detection in a patient with pneumonia. Several cases of respiratory illness have been associated with this virus, but its infrequent identification suggests that this group 2 coronavirus has caused a low incidence of illness to date. The Middle East respiratory syndrome coronavirus (MERS-CoV), first isolated in 2012, causes severe disease in humans, with ~35% mortality and >2500 cases reported to date. MERS-CoV is a zoonotic virus (transmitted between animals and people). The virus likely emerged from bats in the Middle East, although studies have shown that humans are infected through direct or indirect contact with an intermediate host—infected dromedary camels. Common Viral Respiratory Infections, Other Than COVID-19
Herpesviridae  Several herpesviruses cause upper respiratory infec­ tions, especially infection of the oral cavity. Herpes simplex pharyngitis is associated with characteristic clinical findings, such as acute ulcer­ ative stomatitis and ulcerative pharyngitis. HSV types 1 and 2—human herpesvirus (HHV) 1 (species Simplexvirus humanalpha1) and HHV2 (species Simplexvirus humanalpha2), respectively—both cause oral lesions (Chap. 197), although >90% of oral infections are caused by HSV-1. Primary oral disease can be severe, especially in young children, who sometimes are admitted for rehydration therapy because of poor oral intake. A significant proportion of individuals suffer recurrences

of symptomatic disease consisting of vesicles on the lips. Epstein-Barr virus (EBV) mononucleosis syndrome (Chap. 199) is often marked by acute or subacute exudative pharyngitis; in some cases, tonsillar swelling in EBV pharyngitis is so severe that airway occlusion appears immi­ nent. Most of the viruses in the family Herpesviridae—including CMV

(Chap. 200); EBV; varicella-zoster virus (VZV; Chap. 198); and HHV-6, -7, and -8 (Chap. 200)—can cause severe disease in immunocompro­ mised patients, especially hematopoietic stem cell transplant recipients.

Parvoviridae: Human Bocavirus  Human bocavirus (HBoV) was identified in 2005 in respiratory samples from children with lower respiratory tract disease. Sequence analysis of the genome revealed that the virus is a member of the genus Bocaparvovirus (subfamily Parvo­ virinae, family Parvoviridae). This virus has been identified as the sole agent in a limited number of respiratory samples from individuals with respiratory tract disease, especially hospitalized young children, but the virus is also commonly found by RT-PCR tests in respiratory samples from healthy subjects. Retroviridae: HIV  Pharyngitis occurs with primary HIV infection and may be associated with mucosal erosions and lymphadenopathy (Chap. 208). Papovaviridae: Polyomaviruses  Polyomaviruses are small, dou­ ble-stranded, DNA-genome, nonenveloped icosahedral viruses that may be oncogenic. Two major polyomaviruses, JC and BK viruses, are known to infect humans. Of adults in the United States, ≥80% are sero­ positive for these viruses. JC virus can infect the respiratory system, kidneys, or brain. BK virus infection causes a mild respiratory infec­ tion or pneumonia and can involve the kidneys of immunosuppressed transplant recipients. PART 5 Infectious Diseases EPIDEMIOLOGY ■ ■AGE Age (along with the associated factor of prior exposure history) is a major determinant of risk for symptomatic disease during respiratory virus infection. Primary infection with most of the acute respiratory viruses often is more severe than secondary infection. Indeed, reinfec­ tion with most of these viruses occurs throughout life, but primary infection is much more likely to be associated with severe lower respi­ ratory tract disease, while secondary infection typically is asymptom­ atic or associated with upper respiratory tract symptoms only. As these infections are ubiquitous, most primary infections (and thus many of the severe cases) occur during the first few years of life. Later, exposure to young children (in populations such as parents of young children and daycare workers) is a risk factor for frequent reinfection. Despite a lifetime of previous exposures, the risk of severe disease increases with age in the elderly, probably because of immune senescence and general medical decline. ■ ■SEASON Infections with most of the conventional respiratory viruses (e.g., influ­ enza virus, RSV, and hMPV) occur in winter. Typically, there is one dominant virus sweeping through a local community at any one time, a pattern that suggests some population-level interference with trans­ mission. However, outbreaks can be closely spaced, and co-circulation of different viruses or antigenically diverse strains of one virus does occur. In the United States, some regional differences in seasonality have been noted; for example, RSV often appears in Florida and other southeastern states first. Seasons are, of course, reversed in the North­ ern and Southern hemispheres, so that winter epidemics occur roughly from November to March in the United States but from April to August in Australia; therefore, “winter” epidemics are almost always occurring somewhere in the world. Seasonal variances differ in the tropics, where acute respiratory viral infections are more common in the rainy season. The pandemic starting in 2020 caused by SARS-CoV-2 disrupted the seasonality of the common respiratory viruses for several years. SARS-CoV-2 outbreak patterns do not follow traditional seasonal

patterns like other respiratory viruses, with peaks caused by antigenic variants at any time of the year thus far. ■ ■RISK FACTORS FOR DISEASE Infection with these viruses is nearly universal, but disease expression varies among individuals infected with identical viruses. Therefore, investigators have sought to identify risk factors for severe disease. Most single risk factors identified have a moderate effect on the inci­ dence of severe disease, but an accumulation of factors is associated with high risk. Underlying lung disease is a major factor, especially diseases associated with the need for chronic oxygen supplementation. COPD is one of the most profound risk factors. Other severe underly­ ing medical conditions, especially cardiovascular disease, also enhance risk. Smoking (or exposure to wood smoke), low socioeconomic status, and male gender all contribute to minor increases in the risk of lower respiratory tract illness. Obesity causes a chronic state with features of inflammation that are associated with impaired immunity, reduced response to vaccination, and higher susceptibility to severe disease. Close exposure to infected people is a major factor. For instance, living in close quarters (e.g., housing for military trainees, college dormi­ tories, or nursing homes) puts groups of individuals at risk for rapid outbreaks. A breakdown in isolation and hand-washing compliance procedures can lead to cycles of nosocomial transmission of infection in hospital inpatient wards and intensive care units. In assessments of severe lower respiratory tract illness, a history of travel to an area with unusual agents should be considered carefully (e.g., exposure to avian influenza outbreaks in Asia, exposure to MERS-CoV in the Middle East). In 2020−2021, the dominance of the SARS-CoV-2 outbreak and the associated health measures deployed reduced the incidence of con­ ventional respiratory viruses. ■ ■TRANSMISSION Most respiratory viruses are transmitted by two principal modes: fomites or large-particle aerosols of respiratory droplets spread directly from person to person by coughing or sneezing. Fomite transmission occurs indirectly when infected respiratory droplets are deposited on the hands or on inanimate objects and surfaces, with subsequent transfer of secretions to a susceptible person’s nose or conjunctiva. Most respiratory viruses do not spread by small-particle aerosols across rooms or down halls, although measles virus and VZV do spread in this manner. Therefore, contact and droplet precautions are sufficient to prevent transmission in most settings; hand washing is especially critical in health care settings during the winter. Intensive studies of the SARS-CoV-2 pandemic are ongoing (see previous sections on COVID-19), but many experts agree that exposure to large-particle droplets likely is one of the major ways that SARS-CoV-2 spreads. APPROACH TO THE PATIENT Common Viral Respiratory Infections The principal interventions that make a difference in the care of patients with acute respiratory virus infections are supportive, and these factors should be managed meticulously. Hypoxia is managed with supplemental oxygen and respiratory failure with mechanical ventilation. Because the tachypnea and fever that often accompany pneumonia and wheezing frequently result in dehydration, fluid management is important. The astute clinician can narrow the etio­ logic possibilities based on epidemiologic knowledge; information about viruses circulating in the community (widely available from local reference laboratories, county and state health departments, and the U.S. Centers for Disease Control and Prevention [CDC]); and the patient’s exposure history, age, and immunologic status, including vaccination status. Proper use of rapid diagnostic tests is important. When diagnostic tests are applied only to samples from individuals at high risk of exposure to an infectious agent in the appropriate season, the positive predictive value of the test is increased. A central medical decision is whether to use a specific antibacterial or antiviral agent to treat a respiratory infection.

Antibiotics do not improve the outcome of uncomplicated respi­ ratory virus infections in otherwise healthy subjects. Some viral infections, especially influenza, can be complicated by secondary bacterial infection. There are only a limited number of licensed antiviral drugs, which should be used when a specific viral etiology is determined. Antiviral treatment generally is effective only when administered early in the course of illness. CLINICAL MANIFESTATIONS The common cold is characterized by nasal congestion, sneezing, rhinorrhea, cough, and sore throat. Laryngitis is accompanied by hoarseness or dysphonia. Acute bronchitis is characterized by a dry or productive cough of <3 weeks’ duration (most prevalent in winter) in the absence of signs and symptoms of pneumonia and of evidence of pneumonia on chest radiography and is primarily caused by viruses. Bacteria play a more prominent role in chronic bronchitis. Bronchiol­ itis is an acute illness with wheezing and evidence of upper respiratory infection, primarily seen in the winter in infants and young children. The typical clinical manifestations of acute pneumonia include cough, sputum production, dyspnea, and chest pain. More systemic signs and symptoms also occur in pneumonia, including fever, fatigue, sweats, headache, myalgia, and occasionally nausea, abdominal pain, and diarrhea. DIAGNOSIS The clinical diagnosis of a respiratory syndrome and the anatomic location of infection are based on history, physical examination, and radiography. A specific viral etiology can be determined by specific diagnostic tests. The gold standard for diagnosing a respiratory viral infection is virus isolation, performed by inoculation of cell cultures with fresh secretions and use of multiple cell types in a reference labo­ ratory staffed by experienced technologists. Direct or indirect fluores­ cent antibody detection can be used to visualize virus-infected cells in nasal secretions. Rapid antigen-based diagnostic tests are used to detect influenza virus or RSV proteins in nasopharyngeal secretions. The most sensitive tests typically are RT-PCR molecular diagnostic tests that amplify and detect the presence of viral genomic RNA or DNA in respiratory secretions. Multiplex panels assaying a sample for a dozen or more common respiratory viruses are available. These tests must be used and interpreted carefully because of their extreme sensitivity. If care is not taken, it is relatively easy to contaminate a PCR test in the laboratory with small amounts of DNA from a previous reaction. In addition, because a viral genome can sometimes persist in nasal secre­ tions for weeks after an infection resolves, a positive test may indicate a recently resolved rather than a currently acute infection. Despite these limitations, PCR tests generally are considered the most sensitive and specific tests available. Chest radiographs should be obtained for all patients with suspected pneumonia. TREATMENT Common Viral Respiratory Infections INFLUENZA (SEE ALSO CHAP. 206) Several drugs are licensed in the United States for the treatment or prophylaxis of influenza. Neuraminidase inhibitors act on both influenza A and B viruses by serving as transition-state analogues of the viral neuraminidase that is needed to release newly budded virion progeny from the surface of infected cells. The cell surface normally is coated heavily with the viral receptor sialic acid. Osel­ tamivir is administered orally twice daily and is effective for the prevention or treatment of uncomplicated influenza in otherwise healthy adults. Observational studies indicate that oseltamivir also may be beneficial during serious illness. The drug is generally well tolerated, with primarily gastrointestinal toxicity. Zanamivir, a pow­ der that is administered through oral inhalation, exhibits effective­ ness like that of oseltamivir. Moreover, zanamivir is active against

some influenza virus strains that are resistant to oseltamivir. Inhala­ tion of zanamivir powder may cause bronchospasm in patients with COPD or asthma. Peramivir is a neuraminidase inhibitor that acts as a transition-state analogue inhibitor of the influenza neuramini­ dase enzyme that is administered intravenously as a single 600-mg dose. It is efficacious in acute, uncomplicated influenza and was approved by the U.S. Food and Drug Administration (FDA) in 2014 for treatment of individuals who cannot take oral or inhaled medica­ tions. Laninamivir was approved in Japan for prophylaxis (2013) or treatment (2010) of influenza; it is under investigation in the United States. It is a polymeric zanamivir conjugate that is delivered by oral inhalation, and it exhibits greater potency and longer retention times than conventional zanamivir. Baloxavir marboxil is a relatively new class of drug for influenza. It is a prodrug whose metabolism releases the active agent baloxavir acid that inhibits influenza virus cap-dependent endonuclease activity in infected cells. This activity is used by the virus for a process in which the first 10–20 residues of a host cell RNA are removed and used as the 5′ cap and primer to initiate the synthesis of the influenza mRNA (a process sometimes termed “cap snatching”). Baloxavir marboxil was approved by the FDA in 2018 for treatment of acute uncomplicated flu within 2 days of illness onset in otherwise healthy people 12 years and older or those at high risk of developing flu-related complications. In 2020, the FDA approved an updated indication to include postexposure prevention of influenza for people ≥12 years old after contact with an infected person. The adamantanes amantadine and rimantadine were used in the past for the treatment of influenza A infection. These drugs interfere with the ion channel activity caused by the M2 protein of influenza A viruses, which is needed for viral particle uncoating after endocytosis. Widespread resistance occurred in many currently circulating influenza A viruses. RSV INFECTION Ribavirin is a nucleoside antimetabolite prodrug whose activation by kinases in the cell results in a 5′-triphosphate nucleotide form that inhibits RNA replication. The drug was licensed in an aerosol formula in the United States in 1986 for treatment of children with severe RSV-induced lower respiratory tract infection. The efficacy of aerosolized ribavirin therapy remains uncertain despite several clinical trials. Most centers use it infrequently, if ever, in otherwise healthy infants with severe RSV disease. Intravenous ribavirin has been used for adenovirus, hantavirus, measles virus, PIV, and influ­ enza virus infections, although a good risk/benefit profile has not been clearly established for any of these uses. OTHER VIRAL TARGETS Pleconaril, an oral drug with good bioavailability for treatment of infections caused by picornaviruses, has been tested for treatment of rhinovirus infection. This drug acts by binding to a hydrophobic pocket in the VP1 protein and stabilizing the protein capsid, pre­ venting release of viral RNA into the cell. Pleconaril reduces mucous secretions and other symptoms and is being further examined for this indication. Acyclovir and related compounds are guanineanalogue antiviral drugs used in the treatment of herpesvirus infec­ tions. HSV stomatitis in immunocompromised patients is treated with famciclovir or valacyclovir, and immunocompetent patients with severe oral disease compromising oral intake are sometimes treated with these agents. These compounds have also been used prophylactically to prevent the recurrence of outbreaks, with mixed results. Intravenous acyclovir is effective against HSV or VZV pneu­ monia in immunocompromised patients. Systemic therapy of CMV infection in immunocompromised patients has been studied with numerous small-molecule inhibitor drugs, including ganciclovir, valganciclovir, foscarnet, and cidofovir. The clinical utility of these drugs in the immunocompetent host is uncertain. The nucleotide analogue cidofovir also has activity against many other viruses, including adenoviruses. Intravenous cidofovir has been effective in the management of severe adenoviral infection in immunocompro­ mised patients but may cause serious nephrotoxicity.

CHAPTER 204 Common Viral Respiratory Infections, Other Than COVID-19

COMPLICATIONS: CO-INFECTIONS Co-infections with two or more viruses can occur because of the over­ lap in the winter season of these viruses in temperate areas. In general, in careful studies using cell culture techniques for virus isolation, two or more viruses were isolated from respiratory secretions of otherwise healthy adults with acute respiratory illness in ~5–10% of cases. There is little evidence that more severe disease occurs during co-infections. The incidence of positive results in two molecular diagnostic tests (generally RT-PCR for these RNA viruses) is expected to be higher than that of culture because, as discussed above, molecular tests can remain positive for an extended period after shedding of infectious virus has ended.

PREVENTION ■ ■VACCINES Numerous vaccines against influenza viruses have been licensed. In the United States, trivalent and quadrivalent inactivated intramuscular vaccines (covering H3N2, H1N1, and one or two B antigens) and a live attenuated trivalent vaccine for intranasal administration are available (although components of the live attenuated vaccine were only ~3% effective during the 2013−2016 seasons and that vaccine was not avail­ able during the 2016–2018 seasons). Vaccines are effective when the vaccine strains chosen for inclusion are highly related antigenically to the epidemic strain, but occasional antigenic mismatches cause negligible efficacy of a vaccine component. Antigenic drift caused by point mutations in the hemagglutinin (HA) and neuraminidase (NA) molecules leads to antigenic divergence, requiring the production of new vaccines each year. The segmented influenza genome allows reas­ sortment of two viruses during co-infection of one individual or ani­ mal; sometimes the consequence is a major antigenic shift resulting in a pandemic. On average, pandemics occur every 20–30 years. There is current concern about the potential for an H5N1 or H7N9 pandemic, and experimental vaccines are being tested for these and other avian influenza viruses. PART 5 Infectious Diseases Vaccines were developed for adenovirus serotypes 4 and 7 and were approved for prevention of epidemic respiratory illness among military recruits. Essentially, these vaccines consisted of unmodified viruses given by the enteric route in capsules instead of by the respiratory route—the natural route of infection leading to disease. Inoculation by the altered route resulted in an immunizing asymptomatic infection. Most U.S. military recruits are vaccinated against adenovirus, and epi­ demic disease recurs in the absence of vaccination. Two vaccines for RSV are now available (Arexy [GlaxoSmithKline] and Abrysvo [Pfizer]) based on recombinant subunit protein formula­ tions of the virus surface fusion (F) protein. Both can be used to protect older adults. The CDC recommends a single dose of either vaccine for adults aged 60 and older who decide with their health care provider that RSV vaccination is right for them. Arexy also can be used in indi­ viduals 50 through 59 years of age who are at increased risk for lower respiratory tract disease caused by RSV. Abrysvo can be used to pro­ tect young infants through maternal immunization, leading to passive transfer of enhanced levels of serum antibodies from mother to fetus; a single dose is recommended for pregnant women between 32 and 36 weeks of pregnancy. There are no licensed vaccines against rhinoviruses; as there is little or no cross-protection between serotypes, it will be challenging to develop a vaccine covering >100 serotypes. Efforts to develop seasonal coronavirus vaccines are in the preclinical stage. SARS-CoV-2 vaccines are discussed in Chap. 205. ■ ■PASSIVE PROTECTION WITH IMMUNOTHERAPY Palivizumab, a humanized mouse monoclonal antibody to the F protein of RSV, was licensed for prevention of RSV hospitalization in high-risk infants, in half or more of whom it was effective. Experimen­ tal treatment of both immunocompetent and immunocompromised RSV-infected individuals with antibody was reported, but the efficacy of this approach has not been established. A next-generation RSV neu­ tralizing antibody with higher potency and an extended half-life of

~3 months (nirsevimab) was approved in 2023 for all infants younger than 8 months of age born during RSV season or entering their first RSV season and some young children who are at increased risk for severe RSV disease and entering their second RSV season. ■ ■ISOLATION PROCEDURES, PERSONAL PROTECTIVE EQUIPMENT, AND HAND WASHING Most respiratory viruses are spread by direct contact—i.e., bodysurface to body-surface contact and physical transfer of microorgan­ isms between a susceptible person and an infected person. Poor hand hygiene is probably the most common cause of contact transmission of viruses, which occurs often in family, school, and workplace settings. Transmission between health care workers and patients also takes place when hand-washing compliance is low. Fomites (objects or substances capable of carrying infectious organisms), including instruments, stethoscopes, and other objects in medical environments, can contrib­ ute to transmission. Small-particle-mediated airborne transmission can occur but is probably not the dominant mode of transmission for most respiratory viruses. Particle size affects the epidemiology of airborne pathogens. The composition and size distribution of the generated particles affect the duration of suspension of the infectious agents in the air, the distance across which they can be transported, the interval during which the virus remains infectious, and the site of deposition in the airway of a susceptible host. Direct exposure to large-particle aerosols (e.g., exposure at close range—up to 3 ft—to a cough or sneeze) causes some transmission. Particles of small size can remain suspended in the air for long periods; for instance, particles of ~1 μm can remain suspended for hours. However, in general, only a few respiratory viruses are thought to be transmitted by small-particle aerosols. Protection from transmission in health care environments can be achieved by proper implementation of and adherence to estab­ lished procedures for the appropriate level of precaution. Standard and Contact Precautions  Standard precautions, the basic level of infection control that is always used in the care of all patients, reduces the risk of transmission of viruses from respiratory tract secretions and mucous membranes. Contact precautions, the second level, require a single room for the patient when possible and the use of additional personal protective equipment, including the wearing of clean, nonsterile gloves when touching a patient or coming into contact with secretions. Fluid-resistant nonsterile gowns are used to protect skin and clothing during activities in which contact with secretions is anticipated, and providers should wear each gown for the care of only one patient. A face mask is used when there is potential for direct contact with respiratory secretions. Eye protection (goggles or face shields) is worn in anticipation of potential splashing of respira­ tory secretions. Good hand hygiene should always follow any patient contact, including washing for 20 s with soap and warm water or clean­ ing with an alcohol-based hand rub. Providers should attempt to avoid the contamination of clothing and the transfer of microorganisms to other patients, surfaces, or environments. Droplet Precautions  Large-particle droplets are generated during sneezing and coughing and during the performance of some medical procedures, such as airway suctioning in critical care units or bron­ choscopy. Such droplets may contain viruses, but their range is usually limited to about 3 ft. Transmission of large-particle droplets occurs when they are deposited on the nasal mucosa or conjunctivae. To pre­ vent transmission in these settings, providers should implement drop­ let precautions. They should wear a face mask, such as a surgical mask, for close contact (within 3–6 ft of the patient). Patients also should wear a face mask when exiting the examination room and should avoid coming into close contact with other patients. Airborne Precautions  Airborne transmission occurs through the dissemination of airborne droplet nuclei (particles of ≤5 μm) or evapo­ rated droplets containing viruses that can remain suspended in the air for long periods. Certain viruses that are carried by the airborne route can be inhaled by a susceptible host in the same room or over a long distance from the source patient, depending on environmental factors

95 - 205 SARS-CoV-2 and COVID-19

205 SARS-CoV-2 and COVID-19

such as temperature and ventilation. Viruses transmitted by this route are SARS-CoV, measles virus, and VZV. Patients with these infections should be managed with personal respiratory protection and special ventilation and air handling. Providers should wear an N95 respirator selected with fit-testing, which must be repeated annually. Powered air-purifying respirators (PAPRs) are used in some cases. The patient should be housed in an airborne-infection isolation room—a negativepressure room that has a minimum of six air exchanges per hour and exhausts through high-efficiency particulate air (HEPA) filtration or directly to the outside. GLOBAL CONSIDERATIONS ■ ■HENDRA AND NIPAH VIRUSES These emerging paramyxoviruses, which are grouped in their own new genus (Henipavirus), may not be respiratory pathogens in a con­ ventional sense, but they probably infect humans by the respiratory route. Nipah virus is a newly recognized zoonotic virus, named after the location in Malaysia where it was first identified in 1999, and has caused significant outbreaks in Bangladesh and India. It has caused disease in humans who have had contact with infectious animals. Hendra virus (formerly called equine morbillivirus) is another closely related zoonotic paramyxovirus and was first isolated in Australia in 1994. The viruses have caused only a few localized outbreaks, but their wide host range and ability to cause high mortality raise concerns for the future. The natural host of these viruses is thought to be a certain species of fruit bat present in Australia and the Pacific. Pigs may be an intermediate host for transmission to humans in Nipah infection and horses in Hendra infection. Although the mode of transmission from animals to humans is not defined, inoculation of infected materials onto the respiratory tract probably plays a role. The clinical presenta­ tion usually appears to be an influenza-like syndrome that progresses to encephalitis, includes respiratory illness, and causes death in about half of identified cases. ■ ■BUNYAVIRIDAE: HANTAVIRUS Intermittent outbreaks of hantavirus infection occur in South America and cause a severe lung infection: HPS. In addition, >800 cases of HPS have been reported in the United States, caused by the Sin Nombre hantavirus. The disease was first recognized during an outbreak in 1993. About one-third of recognized cases end in death. The Four Cor­ ners outbreak (at the intersection of the northwestern corner of New Mexico, the northeastern corner of Arizona, the southeastern corner of Utah, and the southwestern corner of Colorado) is well known; how­ ever, cases now have been reported in at least 32 states. Outbreaks of HPS also have occurred in South America, especially in Chile, caused by the related Andes hantavirus. Patients with HPS usually present with an influenza-like illness, including fever. Findings on physical examination are nonspecific, often consisting only of fever and elevated respiratory and heart rates. In addition to respiratory symptoms, abdominal pain is common. Diagnosis is often delayed until illness becomes severe, at which point intubation and mechanical ventilation may be required for respiratory support. SUMMARY Viruses are the leading causes of acute lower respiratory tract infection in most populations. Influenza virus and RSV are the most common pathogens; hMPV, PIV3, and rhinoviruses account for most other acute viral respiratory infections. Infection in otherwise healthy adults gener­ ally leads to partial immunity to these pathogens, with protection against severe lower respiratory disease. However, reinfection, with upper respi­ ratory tract illness, is common throughout life. Special populations such as immunocompromised patients, institutionalized frail elderly patients, and patients with COPD are at highest risk for severe disease. ■ ■FURTHER READING Beard KR et al: Treatment of influenza with neuraminidase inhibitors. Curr Opin Infect Dis 31:51, 2018. Falsey AR et al: Bacterial complications of respiratory tract viral ill­ ness: A comprehensive evaluation. J Infect Dis 208:432, 2013.

Fry AM et al: Seasonal trends of human parainfluenza viral infections:

United States, 1990–2004. Clin Infect Dis 43:1016, 2006. Hammitt LL et al: Nirsevimab for prevention of RSV in healthy latepreterm and term infants. N Engl J Med 386:837, 2022. Liu J-W et al: Comparison of antiviral agents for seasonal influenza outcomes in healthy adults and children: A systematic review and network meta-analysis. JAMA Netw Open 4:e2119151, 2021. Monto AS, Cavallaro JJ: The Tecumseh study of respiratory illness. II. Patterns of occurrence of infection with respiratory pathogens, 1965–1969. Am J Epidemiol 94:280, 1971. Papi A et al: Respiratory syncytial virus prefusion F protein vaccine in older adults. N Engl J Med 388:595, 2023. Walsh EE et al: Efficacy and safety of a bivalent RSV prefusion F vac­ cine in older adults. N Engl J Med 388:1465, 2023. Williams JV et al: Human metapneumovirus infection plays an etio­ logic role in acute asthma exacerbations requiring hospitalization in adults. J Infect Dis 192:1149, 2005. James E. Crowe, Jr.

SARS-CoV-2 and

COVID-19 CHAPTER 205 CORONAVIRUS DISEASE 2019 (COVID-19) Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2; ICTV name Betacoronavirus pandemicum) emerged in 2019 in an outbreak in Wuhan, China, that spread worldwide and caused a severe pandemic. SARS-CoV-2 is the cause of a respiratory disease called COVID-19. The virus is a member of the Betacoronavirus genus that not only includes the highly pathogenic viruses severe acute respiratory syndrome

coronavirus 1 (SARS-CoV-1; ICTV name Betacoronavirus pandemi­ cum) (which caused a smaller epidemic in 2002−2003) and Middle East respiratory syndrome coronavirus (MERS-CoV or HCoV-EMC; ICTV name Betacoronavirus cameli) (which caused small epidemics in 2012, 2015, and 2018 with continuing sporadic cases), but also contains the common cold viruses human coronavirus OC43 (HCoV-OC43 or betacoronavirus 1; ICTV name Betacoronavirus gravedinis) and human coronavirus HKU1 (HCoV-HKU1; ICTV name Betacoronavirus hong­ konense). These are enveloped, positive-sense RNA viruses encoded by a viral RNA genome that is quite large, a single linear RNA segment of nearly 30,000 nucleotides that encodes four structural proteins, desig­ nated the S (spike), E (envelope), M (membrane), and N (nucleocapsid) proteins, and a large polyprotein that is cleaved into 16 nonstructural proteins in infected cells. The trimeric S protein is primed by the transmembrane protease serine 2 (TMPRSS2) to facilitate entry of SARS-CoV-2. SARS-CoV-2 S protein is a type 1 fusion machine that also mediates attachment using a receptor-binding domain (RBD) that binds to the human angiotensin-converting enzyme 2 (hACE2) protein receptor. SARS-CoV-2 and COVID-19 ■ ■EPIDEMIOLOGY The origins of the virus and mode of exposure to the first human cases remain unresolved, but the virus likely existed prior to the outbreak in some form in a bat reservoir. Infection was first detected in humans in 2019 in Wuhan, China; it rapidly spread by human-to-human transmission through all provinces of China and then worldwide. The World Health Organization (WHO) designated SARS-CoV-2 a Public Health Emergency of International Concern on January 30, 2020, and declared the outbreak a pandemic on March 11, 2020, leading to sev­ eral years of global disruption. On May 5, 2023, the WHO declared an end to the global COVID-19 emergency. As of September 2024, the

virus had caused >700 million cases and >7 million deaths worldwide, although most countries have now stopped accurate reporting of cases. The basic reproduction number (R0) (the expected number of cases generated directly by one case in a population in which all individuals are susceptible to infection) of SARS-CoV-2 has been estimated to peak around 6, which is substantially higher than that of seasonal influenza (typically 1–2). Densely populated settings such as prisons, cruise ships, nursing homes, airplanes, and large indoor gatherings facilitate high transmission efficiency. Transmission in outdoor settings is less common. Healthcare workers and those working in dentistry have a high potential for exposure. Certain individuals may have contributed to extraordinarily high transmission events (so-called “superspreaders”). Transmission does occur in school settings, although schools have not been considered a primary driver of population transmission and children have lower rates of severe disease than adults. Spread of SARS-CoV-2 is primarily via respiratory droplets transmitted between persons in proximity when the droplets make direct contact with mucous membranes. Airborne transmission by small particle from person-to-person may occur, but airborne transmission over long dis­ tances is unlikely. Fomite transmission by contact with contaminated surfaces occurs; therefore, hand washing in environments of exposure has been advised. Large-scale frequent surface decontamination efforts have been deployed in public spaces, but the effect of these cleanings on reducing transmission is uncertain.

For replication, coronaviruses use RNA-dependent RNA polymer­ ases that are error prone, allowing viral variants to occur frequently. Most variant residues have no effect or are deleterious to the virus. However, some variations affect transmission, disease severity, anti­ genicity (and thus efficacy and effectiveness of immunity induced by prior vaccination or infection), the effectiveness of therapies, the sensitivity and specificity of diagnostic tools, and the effectiveness of social and public health measures. Rapid evolution of variants has been a distinguishing feature of the COVID-19 pandemic. The WHO has established a Technical Advisory Group on SARS-CoV-2 Virus Evolu­ tion to follow and report variant evolution. The group has developed terminology concerning variants under monitoring (VUMs), variants of interest (VOIs), and variants of concern (VOCs) to draw attention to virus evolution. In 2024, the WHO launched a WHO Coronavirus Network (CoViNet) to facilitate early and accurate detection of coro­ naviruses and variant tracking. The WHO Technical Advisory Group on COVID-19 Vaccine Composition assesses the analyses of the likely effects of emerging VOCs on the performance of COVID-19 vaccines. PART 5 Infectious Diseases Advanced age is the principal risk factor for severe illness from COVID-19 (marked by need for hospitalization, intensive care, and mechanical ventilation). More than 95% of COVID-19 deaths occur in persons older than 45 years, and >80% of deaths occur in those older than 65 years. Preexisting social and health disparities put some groups at increased risk of illness or death from COVID-19, including persons with disabilities and many racial/ethnic minority groups. Male sex is associated with higher risk of severe disease (odds ratio, ~1.8). Most individuals who die have preexisting comorbidities. The risk of severe COVID-19 illness increases markedly with elevated body mass index (BMI). Overweight condition (BMI >25 but <30), obesity (BMI ≥30 but <40), and severe obesity (BMI ≥40) are risk factors for progressively increased severe COVID-19. Substance use, such as alcohol, opioid, or cocaine use disorder, and current or former smoking both increase risk. Pregnant women are more likely to suffer more severe illness. Most other medical conditions increase the risk of severe illness, but conditions that especially increase risk are as follows: (1) chronic lung diseases, including COPD, moderate to severe asthma, cystic fibrosis, and pulmonary hypertension interstitial lung disease; (2) cancer or cancer treatments, including hematologic malignancies, solid organ transplant, and stem cell transplant; (3) immunodeficiency, including primary immunodeficiency caused by inherited genetic defects or sec­ ondary or acquired immunodeficiency caused by prolonged use of cor­ ticosteroids, other immunosuppressive drugs, or HIV type 1 (HIV-1)

infection; (4) hemoglobin blood disorders, including thalassemia or sickle cell disease; (5) cerebrovascular disease, such as stroke; (6) cogni­ tive impairment or other neurologic conditions; (7) heart conditions,

including arterial hypertension, heart failure, coronary artery disease, and cardiomyopathies; (8) obstructive sleep apnea; (9) chronic inflam­ matory, autoimmune, and rheumatic diseases; (10) type 1 or type 2 diabetes mellitus; (11) chronic liver disease, especially cirrhosis; and (12) genetic conditions, especially Down syndrome. A multisystem inflammatory syndrome in children (MIS-C) or pediatric multisystem inflammatory syndrome (PMIS) has been associated with COVID-19, comprising a persistent fever, involvement of multiple organ systems (including gastrointestinal, dermatologic, cardiac, renal, hematologic, and neurologic), and elevated circulating inflammatory markers. The highest-risk individuals for MIS-C in the United States are Black and Latino children age 3–12 years. A similar syndrome in adults (MIS-A) may occur rarely. ■ ■PREVENTATIVE MEASURES Early in the epidemic, public health methods for prevention were limited mostly to nonpharmaceutical interventions (NPIs), including social distancing (staying at least 6 ft from other persons in public to avoid infection), social isolation (staying away from other persons when infected), quarantine (staying at home for 14 days after potential exposure), limiting travel, and working from home. When a local epi­ demic persists, prior to entering health care settings, patients often are screened for clinical signs or symptoms common in COVID-19, espe­ cially fever, respiratory symptoms (cough, dyspnea, sore throat), myal­ gias, and anosmia/hyposmia. Universal masking has been required in many health care settings during epidemic conditions, although effec­ tiveness of masking in community or health settings has been difficult to assess. Most mask studies have been observational and confounded by lack of consistency in concurrent interventions, study of exposure to variants with differing R0 values, differences in mask type (N95, KN95, FFP2, paper medical or surgical, or cloth masks), poor compliance, and other factors. Most experts conclude that masking, use of personal pro­ tective equipment, and hand washing are highly appropriate in health care settings with infected patients and may have some benefit in com­ munity settings. Most organizations and governments responsible for public health decisions have discontinued mandatory policies in favor of softer recommendations. ■ ■CLINICAL MANIFESTATIONS The disease course varies widely, including asymptomatic infection, mild disease, moderate disease, or severe disease requiring hospital­ ization, oxygen therapy, intensive care, and mechanical ventilation. A substantial proportion of patients (possibly a third of those infected) are asymptomatic, but those individuals can transmit the virus to oth­ ers. Most individuals with symptomatic infection have mild disease (no pneumonia). Severe disease, typically requiring hospitalization and involving pneumonia and associated manifestations (dyspnea, radio­ graphic involvement of more than half of the lung, and/or hypoxia with oxygen saturation ≤94%), is common. Critical disease with manifesta­ tions of respiratory failure requiring mechanical ventilation, multior­ gan failure, or shock occurs and requires intensive care. ■ ■DIAGNOSIS OF COVID-19 Testing for COVID-19 is recommended during periods of respiratory symptoms, or 5 days or later after exposure without active symptoms. COVID-19 tests usually use a sample taken from the nasopharynx or sometimes a saliva sample. There are two major types of approved diagnostic tests: antigen tests (for proteins of SARS-CoV-2) and molec­ ular tests (for genetic material of SARS-CoV-2). Antigen tests are often used as screening tests and sometimes called rapid or at-home tests. Approved protein-detection antigen test devices are reliable and accu­ rate, but they are less accurate than molecular tests, especially when testing exposed persons without symptoms. COVID-19 diagnostic tests are widely available from health care professionals, hospitals and clinics, and some pharmacies or other testing sites in the community. In the United States, at-home COVID-19 tests approved by the U.S. Food and Drug Administration (FDA) are available for free or for purchase. Molecular tests are nucleic acid amplification tests (NAATs), most often performed using reverse transcription polymerase chain

reaction (RT-PCR) to convert the viral RNA genome to DNA copies (cDNA) and amplify the copy number for enhanced sensitivity. PCR tests are not rapid and cannot be performed at home, so they are typi­ cally performed in professional health care settings and processed in a reference laboratory using specialized equipment. Nasopharyngeal swabs are used mostly commonly, while saliva testing also has been implemented, especially in large-scale population screening efforts. Other more general laboratory tests have been used during medi­ cal management of severe or critical illness. Most immune profiling tests reveal widespread abnormalities consistent with systemic disease including lymphopenia and thrombocytopenia; elevated inflammatory markers, such as interleukin 6 (IL-6), tumor necrosis factor α, ferritin, and C-reactive protein; elevated liver enzymes and lactate dehydroge­ nase; elevated markers of acute kidney injury; elevated D-dimer and prothrombin time; and elevated troponin and creatine phosphokinase. Research-grade tests show that beneficial components of the adaptive immune response, including antibodies and T cells, also arise during the first 1−2 weeks after exposure. Chest radiographs may exhibit abnormal findings such as consolidation and ground-glass opacities that are distributed bilaterally, especially in the lower lung regions, but may also be normal despite respiratory compromise. Chest computed tomography (CT) has features (ground-glass opacifications with or without mixed consolidation, pleural thickening, interlobular septal thickening, and air bronchograms) that can be systematically inter­ preted as typical, indeterminate, or atypical for COVID-19. Chest CT may be more sensitive than radiographs, but CT should be used prin­ cipally for medical management of respiratory disease, not as a primary diagnostic tool for COVID-19. Lung ultrasound also has been used to image the lungs to detect some COVID-19 abnormalities. ■ ■CLINICAL COURSE The onset of disease manifests typically within 4–5 days after exposure and nearly always within 14 days. Symptoms include cough, fever, myalgia, headache, dyspnea, sore throat, and gastrointestinal symp­ toms of nausea, vomiting, or diarrhea. Sudden onset of dysgeusia and anosmia (loss of taste and smell) occurs in a substantial number of cases, which often resolves in weeks to months. Diverse dermatologic findings occur in patients with COVID-19. General decline of health status, including onset or worsening of dementia, can occur in older individuals, especially those with cognitive impairment. Mental health consequences of the acute disease, isolation measures, and medical management regimens are common, including fatigue, depression, general and social anxiety, sleep disturbances, cognitive deficits, post­ traumatic symptoms, and substance abuse disorder. Long COVID is a chronic condition that occurs after SARS-CoV-2 infection and is present for at least 3 months with a wide range of symptoms or conditions that may improve, worsen, or be ongoing. The symptoms are varied and can include neuropsychiatric disorders and pain syn­ dromes in addition to respiratory and metabolic changes. People with long COVID may experience many symptoms related to alterations in mental health and decreased brain function and experience increased suicidal ideation, which may increase suicide risk. ■ ■COMPLICATIONS Severe complications of infection can occur. The major complication in patients with severe disease is acute respiratory distress syndrome requiring oxygen therapy and mechanical ventilation. Disseminated intravascular coagulation is a complication in severe disease. Throm­ boembolic complications are common in severe disease, mostly occur­ ring as venous thromboembolism, including pulmonary embolism or deep vein thrombosis. Events stemming from arterial thrombosis, including acute stroke or ischemia of the limbs, are reported. Cardiac complications manifest as heart failure, myocardial injury, or arrhyth­ mias and cardiovascular syndromes, especially shock. Acute kidney injury requiring dialysis can occur. Encephalopathy occurs in critically ill patients, and delirium in the intensive care unit setting reduces overall survival. Other neurologic complications including seizures, ataxia, or motor or sensory deficits have been reported. Those with COVID-19 disease and laboratory markers of excessive inflammatory

response can exhibit a pattern of persistent fever and multiorgan dis­ ease with high risk of fatal outcome. An excessive proinflammatory host response to SARS-CoV-2 infection likely contributes directly to pulmonary pathology and severity of COVID-19. Manifestations typi­ cally mediated by autoantibodies have been reported. Disease is usually caused by direct viral pathogenesis in tissues or the associated immune response, but secondary bacterial or fungal infections do occur, usually as bacteremia or respiratory infections.

■ ■GENERAL MEDICAL MANAGEMENT Medical management of COVID-19 is focused on severe respiratory illness and systemic disease manifestations. As bacterial infection is an uncommon complication of COVID-19, antibiotics are not generally indicated, but when the diagnosis is uncertain, empiric antibiotic regi­ mens for community-acquired or health care–associated pneumonia should be considered. Since there is such a substantial risk of throm­ boembolic complications, many experts recommend pharmacologic prophylaxis of venous thromboembolism for all hospitalized patients with COVID-19. Nonsteroidal anti-inflammatory drugs (NSAIDs) are often used as antipyretic agents, but questions have been raised about a possible association between NSAID use and worse outcomes with COVID-19; when possible, the preferred antipyretic agent is acetaminophen. Immunosuppressed individuals are at higher risk of severe illness or death; therefore, on a case-by-case basis, providers should decide whether to continue immunomodulatory agents such as steroids or other immunosuppressive drugs that were indicated for preexisting conditions prior to onset of COVID-19. Generally, experts agree that the best course usually is to continue common preexisting medications of aspirin, statins, and angiotensin-converting enzyme inhibitors or angiotensin receptor blockers. CHAPTER 205 The time to recovery from COVID-19 is affected by the severity of disease, the individual’s preexisting comorbidities, and age. Generally, symptomatic infection is an acute syndrome that resolves in 2 weeks in ~80% of persons, especially following mild or moderate disease. Individuals with severe disease often require longer for recovery, on the order of several months. However, a subset of individuals with infec­ tion progress to a recurring or persisting pattern of symptoms, most commonly including fatigue, cognitive deficits, cough, dyspnea, or chest pain. Those with severe acute pulmonary or cardiac injury may have persisting respiratory or cardiac impairment. Diverse long-term adverse mental health consequences of infection are common, and the public health measures used to manage the pandemic also have led to social isolation with adverse mental health consequences. SARS-CoV-2 and COVID-19 ■ ■SPECIFIC TREATMENTS The approach to specific treatment of COVID-19 of varying levels of severity continues to evolve, especially because emerging SARS-CoV-2 variants exhibit differing properties of transmission efficiency and capacity to cause disease. Most options fall into the categories of smallmolecule viral inhibitors, polyclonal or monoclonal antibodies, and immunomodulators. The recommended use of these medical counter­ measures can be grouped under categories of outpatient management of mild disease or hospital management of severe disease. We recom­ mend consulting up-to-date recommendations from groups authorized to provide expert or governmental guidelines, including the National Institutes of Health (NIH) COVID-19 Treatment Guidelines Panel in the United States or the WHO for international settings (see “Further Reading,” below). Many but not all of the guidelines from such groups are harmonized. During the early years of the pandemic, numerous countermeasures were authorized (i.e., not necessarily fully approved) under the Emergency Use Authorization (EUA) authority that allows the FDA to facilitate the availability and use of medical countermeasures prior to full licensure when the secretary of the Department of Health and Human Services declares that an EUA is appropriate for a public health emergency. However, the WHO ended Public Health Emergency of International Concern status for COVID on May 5, 2023, and now proper drug use once again typically requires FDA approval. Antiviral drugs (including small-molecule inhibitors and polyclonal or monoclonal antibodies) have the most potential to alter the clinical

course early in infection, since they may reduce the peak titer of virus, a parameter that is likely correlated with severity of disease. Later in the clinical course, anti-inflammatory medications may be of more benefit since the pathogenesis of disease is driven increasingly more over time by tissue inflammation and systemic inflammatory responses than by direct viral cytopathic effect.

■ ■OUTPATIENT MANAGEMENT Most patients with COVID-19 can be managed as outpatients. Individ­ uals who are infected but have mild disease can be treated with support­ ive care only. The decision to use antiviral drugs is driven principally by medical risk factors, but it should also consider available resources, the clinical situation, and the patient’s social circumstances. Shared clinical decision-making should be used regarding COVID-19–specific therapy to accommodate the individual’s values and preferences in addition to medical risk factors. Risk-benefit ratios are difficult to assess. Reducing viral load has the potential to reduce acute disease and complications, but COVID medications also have the potential to cause adverse effects or drug–drug interactions, and they carry the risk of causing a rebound COVID-19 phenomenon that then may require prolongation of isola­ tion. COVID-19–specific therapy is not recommended for individuals who have asymptomatic SARS-CoV-2 infection. Symptomatic patients who are most likely to benefit from specific antiviral treatment are individuals who are older, unvaccinated, with multiple medical comor­ bidities, and/or with immunocompromised conditions. Others who lack these risks might benefit to some degree, but since their risk of hospitalization or death is low the impetus to risk side effects is much less compelling. For symptomatic adults who are at increased risk, many experts recommend treating immunocompetent unvaccinated adults

50 years of age, all adults ≥65 years of age regardless of vaccination status, immunocompetent adults of any age who have multiple medical risk factors for progression to severe disease, and immunocompromised adults of any age. As of late 2024, the principal antiviral drug for use in outpatients is nirmatrelvir-ritonavir, which should be administered as soon as possible and within 5 days after symptom onset. The typical dose is nirmatrelvir 300 mg and ritonavir 100 mg orally twice daily for 5 days, but the dose should be adjusted for reduced renal function and should be avoided in patients with severe renal dysfunction. Drug inter­ actions are possible, so prior to prescribing, providers should review all medications and consider potential drug interactions in consultation with a pharmacist or using online tools. In a minority of cases, viral shedding occurs again after an initial improvement, with or without mild symptoms, requiring an extension of the period of isolation. Patients on treatment should be counseled to return for reevaluation if the patients experience worsening or new-onset mental status changes such as confusion or worsening respiratory symptoms such as dyspnea. Patients should be counseled on an appropriate period of self-isolation to achieve infection control, per current recommendations of the U.S. Centers for Disease Control and Prevention (CDC). PART 5 Infectious Diseases If nirmatrelvir-ritonavir is not available or suitable, alternatives that are more difficult to administer are available. Remdesivir is approved, but administration requires three IV doses over 3 days. The processing in vivo of this broad-spectrum antiviral prodrug causes intracellular delivery of an active chemical that serves as a ribonucleotide analogue inhibitor of SARS-CoV-2 RNA polymerase. Molnupiravir is an anti­ viral small-molecule drug that is metabolized into a ribonucleoside analogue that inhibits viral reproduction by promoting widespread mutations in the replication of viral RNA by RNA-directed RNA poly­ merase. In December 2021, the FDA granted an EUA to molnupiravir for use in certain populations where other treatments are not feasible, but with a black box warning about its mutagenic properties that limit its use in patients with pregnancy potential. Virus-neutralizing antibodies delivered as monoclonal human antibodies or antibody combinations or as constituents of high-titer convalescent plasma also have been authorized for postexposure prophylaxis or treatment of outpatient COVID-19, but new variants arise and reduce their effec­ tiveness and often lead to withdrawal of authorization. Many other therapies have been used without specific autho­ rization or approval and are of uncertain benefit. Antibiotics are

not recommended for routine treatment of COVID-19, as bacterial complications are unusual. Dexamethasone, prednisone, and other corticosteroids have been used in the setting of COVID-19 infection accompanied by acute exacerbation of chronic obstructive pulmonary disease or asthma. ■ ■INPATIENT MANAGEMENT Indications for Hospitalization  COVID-19 patients with symp­ toms suggestive of more severe disease—especially moderate to severe dyspnea, cyanosis, change in mental status or chest pain, reduced urine output or anuria, with or without oxygen saturation ≤94%—should be evaluated in an emergency department. The NIH COVID-19 Treat­ ment Guidelines Panel recommends hospitalization for patients with oxygen saturation of <94% on room air, respiratory rate of >30 breaths/ min, PaO2/FiO2 <300 mmHg, or lung infiltrates on chest radiograph occupying >50% of lung fields. The decision to hospitalize varies regionally and due to differences in available resources and social risk factors. Management in Hospital  Upon hospitalization, the initial assess­ ment should seek to define medical comorbidities or immunocompro­ mising factors and to identify dysfunction in critical organ systems. Fever should be controlled, preferably with acetaminophen. The use of NSAIDs has not been aligned with poor COVID-19 outcomes, but most experts suggest avoiding their use or limiting them to lower effective doses. If the patient uses common chronic medications, those generally should be continued, including aspirin (unless there is a clear bleeding risk), statins, and angiotensin-converting enzyme inhibitors or angiotensin receptor blockers. Hospitalized patients should receive therapeutic-dose anticoagulation to prevent venous thromboembo­ lism. For patients without any risk factors, additional care is primarily supportive. Patients with risk factors for severe disease typically are treated with remdesivir. Oxygen should be used to support oxygen saturation. Patients receiving low-flow supplemental oxygen are usu­ ally treated with remdesivir (for antiviral effect) and low-dose dexa­ methasone (for anti-inflammatory treatment). Patients treated with high-dose glucocorticoids should be monitored for common adverse effects, especially hyperglycemia and increased risk of co-infection. If the oxygen requirement escalates during the first 4 days of hospi­ talization despite those treatments, with severe disease and elevated inflammatory markers on laboratory testing, the Janus kinase inhibitor baricitinib or the monoclonal IL-6–blocking antibody tocilizumab can be used. Janus kinase inhibitors typically are used for treatment of rheumatoid arthritis because of their known immunomodulatory effects, which probably also improve inflammation during COVID-19, but baricitinib may also mediate some direct antiviral effects by inter­ fering with viral entry into cells. Alternatively, tocilizumab can be used for immunomodulatory effect. This biologic drug is a humanized monoclonal antibody against the IL-6 receptor that blocks the effect of the cytokine IL-6, which plays an important role in pathogenesis. These immunomodulatory therapies are also usually used for patients admitted to an intensive care unit to receive high-flow supplemental oxygen or noninvasive ventilation, mechanical ventilation, or extracor­ poreal membrane oxygenation. Abatacept or infliximab are alternate immunomodulatory agents that can be used if the first-line immuno­ modulators are not available. ■ ■ANTIBODY-BASED THERAPIES Passive immunization with SARS-CoV-2 antibodies to achieve antivi­ ral immunity or therapeutic effect has been tested using convalescent plasma (blood plasma taken from persons who have recovered from COVID-19) or human monoclonal antibodies (mAbs). The typical overall composite titer of SARS-CoV-2–neutralizing antibodies in convalescent plasma following a single primary infection is moder­ ately low, limiting its effectiveness and reproducibility. Human mAbs are recombinant proteins made in the laboratory based on the genes encoding an antibody obtained typically from a single SARS-CoV-2– specific B cell isolated from the peripheral blood of a convalescent indi­ vidual. Numerous mAb products obtained EUA for use in outpatients

with laboratory-confirmed mild to moderate SARS-CoV-2 infection who were at high risk for progressing to severe disease and/or hospi­ talization, although these EUAs for therapy are now revoked because new variants have escaped their activity. ■ ■TREATMENT OF COMPLICATIONS Severe and complicated COVID-19 is a complex disease that can affect the respiratory and cardiovascular systems and may involve all other major organs. Disease severity and mortality are often determined by complications due to the systemic effects of a hyperinflammatory and hypercoagulable state. Acute respiratory distress syndrome with severe hypoxia and viral sepsis are common, with acute cardiac injury, arrhythmias, thromboembolic events, acute kidney injury, cytokine storm, oxidative stress, and thrombosis and shock. Each of these organ system diseases must be managed in the context of ongoing viral infection. Bacterial superinfection of COVID-19 probably occurs, but the incidence is uncertain. There are insufficient data to recom­ mend empiric broad-spectrum antimicrobial therapy in the absence of another indication, although some experts routinely administer broadspectrum antibiotics as empiric therapy for bacterial pneumonia to all patients with COVID-19 and moderate or severe hypoxemia. Ideally, providers initiating empiric therapy should attempt to deescalate or stop antibiotics if there is no ongoing evidence of bacterial infection. Several EUAs have been issued for medical management of complica­ tions during COVID-19, including replacement solutions for continu­ ous renal replacement therapy and drugs for sedation via continuous infusion in intensive care. Anticoagulation in the face of COVID-19– associated thromboembolic events is an especially complex situation and requires expert consultation. ■ ■ANTIBODY-BASED PREVENTION Antibodies of the IgG isotype typically have an average serum half-life of about 21 days, and thus passive transfer of immune serum or anti­ bodies confers protection of modest duration. During COVID-19, how­ ever, proof of principle for the use of a long-acting antibody (LAAB) combination for longer-term prevention of disease was established with the emergency use authorization of the two-antibody combina­ tion drug tixagevimab co-packaged with cilgavimab. These antibodies were engineered with variant residues in the Fc portion of the antibody that altered the kinetics of binding and retention to the Fc receptor neonatal (FcRn) that regulates antibody half-life in vivo. This first-inhuman authorization of an LAAB enabled protection of individuals who could not be effectively vaccinated, such as the immunocom­ promised. The FDA terminated the EUA in January 2023 due to the widespread emergence of escape variants in circulation. However, this program established a paradigm for vaccine surrogate use of LAABs in high-risk individuals. In 2024, the FDA granted an EUA for a second LAAB, pemivibart, which is currently limited to use when the combined national fre­ quency of variants with substantially reduced susceptibility to pemi­ vibart is ≤90%. Pemivibart requires IV administration because of the large (4.5 g) dose for pre-exposure prophylaxis of COVID-19 in adults and adolescents (≥12 years of age weighing ≥40 kg) who have moderate to severe immune compromise due to certain medical conditions or receipt of certain immunosuppressive medications or treatments and are unlikely to mount an adequate immune response to COVID-19 vaccination. ■ ■PREVENTION BY IMMUNOPROPHYLAXIS Four COVID vaccines—two based on mRNA (Pfizer-BioNTech and Moderna), one on a subunit protein approach (Novavax), and one on viral vectored technology (Janssen)—have been authorized or approved in the United States. The EUA for the viral vector vaccine based on a human adenovirus that was modified to contain the gene for making the SARS-CoV-2 spike protein was withdrawn in June 2023 due to association with thrombosis with thrombocytopenia syndrome (TTS). Currently, the mRNA vaccines are available to everyone 6 months and older, and the subunit protein vaccine is for anyone 12 years and older. The principal goal of vaccination is preventing severe COVID-19 by

reducing critical illness or death, hospitalization, or medically attended illness in emergency departments and urgent care visits for COVID-19. Prevention of all symptomatic SARS-CoV-2 infections is beneficial and often measured as a secondary endpoint in clinical trials. Preliminary data also suggest that vaccinated persons with COVID are less likely than unvaccinated persons to suffer long COVID syndrome.

COVID vaccine designs were developed in record time in 2020 because of years of prior research on candidate vaccines for related coronaviruses, including SARS-CoV-1 and MERS-CoV. The viral sur­ face spike (S) protein that is the target of protective neutralizing anti­ bodies was engineered from the native “metastable” membrane-bound form to a stabilized “prefusion” conformation of soluble protein. This protein then was encoded by recombinant mRNA formulated in lipid nanoparticle delivery components. Phase 3 clinical data for the original mRNA vaccines delivered in a two-dose primary series showed 95% efficacy for preventing symptomatic COVID, a stunning outcome for this rapid development program. The durability of immunity proved relatively short in subsequent real-world effectiveness studies in adults, which showed that the protection from the mRNA waned over time. In addition, antigenic variant viruses continue to emerge on a regu­ lar basis, further eroding the level of vaccine-induced immunity. In response, manufacturers and regulators have supported a program of progressively updating mRNA vaccines to boost levels of immunity and broaden coverage to recognize newer variants. The original mRNA vaccines from both Pfizer and Moderna protected against the original SARS-CoV-2 virus. The mRNAs encoding spike protein antigens have been replaced three times since, with vaccines targeting different vari­ ants of the Omicron virus strain. In 2022, vaccines encoding the S protein from both the original strain and the Omicron variants designated BA.4 and BA.5 (“bivalent” vaccines) were deployed. In 2022, the XBB subvariant of Omicron emerged by recombination of two BA.2 sub­ lineage viruses, leading to the replacement of vaccines in 2023 with a monovalent vaccine based on XBB. In 2024, the mRNA vaccines were updated to protect against KP.2, a further 2024 sublineage known as a FLiRT variant because it contains a phenylalanine (F) to leucine (L) mutation and an arginine (R) to threonine (T) mutation in the S pro­ tein. Even as these updated vaccines were rapidly released, new variants continued to emerge. CHAPTER 205 SARS-CoV-2 and COVID-19 The protein vaccine contains a recombinant form of S protein instead of mRNA encoding S. The gene encoding the S protein is introduced into an insect virus vector (baculovirus) and expressed in insect cells, and then purified and formulated as a nonreplicating nanoparticle that displays multiple copies of S. The protein nanopar­ ticle construct is then formulated with a saponin-based adjuvant named Matrix-M to increase immunogenicity. The protein vaccine demonstrated 100% effectiveness against moderate and severe disease in phase 3 trial results published in 2021. The vaccine was updated for 2024–2025 with antigen based on a JN.1 variant (that emerged prior to the KP.2 variants on which 2024 mRNA vaccines are based), although the manufacturer reported preclinical studies in which the JN.1 nanoparticle vaccine induced cross-reactive antibodies to the later KP.2 and KP.3 variants. ■ ■FURTHER READING Centers for Disease Control and Prevention: COVID-19. Avail­ able at https://www.cdc.gov/covid/. Accessed September 10, 2024. Centers for Disease Control and Prevention: Infection control guidance: SARS-CoV-2. Available at https://www.cdc.gov/covid/hcp/ infection-control/. Accessed September 10, 2024. Infectious Diseases Society of America: IDSA guidelines on the treatment and management of patients with COVID-19. Available at https://www.idsociety.org/practice-guideline/covid-19-guideline-

treatment-and-management/. Accessed September 10, 2024. National Institutes of Health: Coronavirus Disease 2019 (COVID-19) Treatment Guidelines. Available at https://www.ncbi. nlm.nih.gov/books/NBK570371/. Accessed September 10, 2024. World Health Organization: Therapeutics and COVID-19: Liv­ ing guideline. Available at https://www.who.int/publications/i/item/ WHO-2019-nCoV-therapeutics-2023.2. Accessed December 21, 2024.

96 - 206 Influenza

206 Influenza

Kathleen M. Neuzil

Influenza ■ ■DEFINITION The term influenza represents both a clinically defined respiratory illness accompanied by systemic symptoms of fever, malaise, and myalgia and the name of the orthomyxoviruses that cause this syndrome. Although this term is sometimes used more generally to denote any viral respi­ ratory illness, many features distinguish influenza from these other illnesses, most particularly its systemic symptoms, its propensity to cause sharply peaked winter epidemics in temperate climates, and its capacity to spread rapidly among close contacts. The morbidity and mortality associated with influenza epidemics are documented closely in the United States by the Centers for Disease Control and Prevention (CDC), which records clinical cases of influenza-like illness, cases of virologically documented influenza, and excess deaths due to pneumo­ nia and influenza combined. ■ ■ETIOLOGIC AGENTS Three influenza viruses occur in humans: A, B, and C. These viruses are irregularly circular in shape, measure 80–120 nm in diameter, and have a lipid envelope and prominent spikes that are formed by the two surface glycoproteins, hemagglutinin (H) and neuraminidase (N) (Fig. 206-1). The hemagglutinin functions as the viral attachment protein, binding to sialic acid receptors on the cells that line the super­ ficial epithelium of the respiratory tract. The neuraminidase cleaves the virus from the cell membrane to facilitate its release from the cell and prevents self-aggregation of viruses. Influenza A viruses have eight single-strand negative-sense RNA segments in their genomes that encode hemagglutinin and neuraminidase as well as internal genes, including polymerase, matrix, nucleoprotein, and nonstructural genes. The segmented nature of the genome allows gene reassortment; an analogy for reassortment is the shuffling of a deck of cards. Reassort­ ment takes place when a single cell is infected with two different strains of influenza. PART 5 Infectious Diseases Among the influenza viruses, the A viruses are of paramount importance for several reasons: (1) the plasticity of their genomes, which enables them to react to the prevailing immunity in the com­ munity by modifying their immunogenic epitopes, particularly on the hemagglutinin surface protein (antigenic drift); (2) the segmentation of their genomes, which allows genes coding both surface and internal proteins to be reassorted between influenza A variants (antigenic shift); and (3) their extensive mammalian and avian reservoirs, in which multiple variants with distinct hemagglutinin and neuraminidase genes lie in wait. As a result of all of these factors, influenza A virus has the abil­ ity, particularly after an antigenic shift, to cause a worldwide epidemic (pandemic). The most severe influenza A pandemic in modern history FIGURE 206-1  An electron micrograph of influenza A virus (×40,000). (From YZ Cohen, R Dolin: Influenza, in Harrison’s Principles of Internal Medicine, 19th ed. DL Kasper et al [eds]. New York: McGraw-Hill, 2015, p 1209.)

took place in 1918; ~50 million deaths were attributed to the culpable influenza A H1N1 virus in the years surrounding 1918. The influenza A viruses are further classified by their surface glycoproteins (H and N), the geographic location of their isolation, their sequential number among isolated viruses, and their year of isolation. Thus, for the 2024–2025 season, U.S.-licensed egg-based influenza vaccines will contain hemagglutinin derived from an influ­ enza A/Victoria/4897/2022 (H1N1) pdm09-like virus, an influenza A/Thailand/8/2022 (H3N2)-like virus, and an influenza B/Austria/

1359417/2021 (B/Victoria lineage)-like virus. U.S. cell culture–based inactivated and recombinant influenza vaccines will contain HA derived from an influenza A/Wisconsin/67/2022 (H1N1) pdm09-like virus, an influenza A/Massachusetts/18/2022 (H3N2)-like virus, and an influenza B/Austria/1359417/2021 (B/Victoria lineage)-like virus. ■ ■EPIDEMIOLOGY Influenza virus causes outbreaks during the cooler months of the year and thus has a mirror-image season in the antipodes compared with that in the Northern Hemisphere. The circulation of strains in the Southern Hemisphere has some predictive value for vaccine composi­ tion in the Northern Hemisphere, and vice versa. This information is important as the degree of antigenic drift is one determinate of vaccine efficacy. Vaccine composition typically must change in at least one component yearly in anticipation of the predicted circulating strains. A typical outbreak begins in early winter and lasts 4–5 weeks in a given community, although its impact on the country as a whole will be of considerably longer duration. When influenza activity exceeds a predetermined baseline, an influenza outbreak is classified as an epidemic. Influenza’s impact is reflected in increased school and work absenteeism, increased visits to emergency departments and primary care physicians, and increased hospitalizations, particularly of older persons and individuals with underlying cardiopulmonary disease. The impact often is most easily recognized in the pediatric population, whose school absenteeism quickly peaks. Influenza’s global spread and causative strain(s) in a given year are well documented by the surveillance networks of the World Health Organization (WHO) and the CDC. The severity of an epidemic depends on the transmissibility and virulence of the viral strain, the susceptibility of the population, the adaptation of the virus to its human host, and the degree of antigenic match to the recommended vaccine. None of these parameters is totally predictable for influenza A. Influenza is largely spread by small- and large-particle droplets; however, emerging data support a role for aerosol transmission. Transmission is likely modulated by temperature and humidity. Spread is facilitated by the coughing and sneezing that accompany the ill­ ness. Within families, the illness is often introduced by a preschool or school-aged child. In the United States, influenza virus circulation in the first quarter of 2020 declined sharply within 2 weeks of the COVID-19 emergency declaration and widespread implementation of community mitigation measures and travel restrictions. The decline occurred in other Northern Hemisphere countries and the tropics, and in 2020, Southern Hemisphere temperate climates had virtually no influenza circulation. Influenza activity remained at low levels during the 2020–2021 Northern Hemisphere season, and increased in seasons thereafter. While changes in health care–seeking behavior and testing priorities during the pandemic may have contributed, such declines in influenza detection were noted even in areas with continued or increased testing, implicating community mitigation measures as the most likely reason. Influenza A Viruses  When a major shift in the hemagglutinin and/or the neuraminidase occurs, with introduction of a new sero­ type from an animal or avian reservoir, an influenza A strain has the potential to cause a pandemic. In modern influenza history, such shifts occurred in 1918 (H1N1), 1957 (H2N2), 1968 (H3N2), 1977 (H1N1), and 2009 (H1N1pdm) (Table 206-1). On the basis of analysis of serum antibody profiles in the elderly, epidemics that took place in the 1890s have been attributed to H3N2 and H2N2 viruses. Epidemics typical of influenza have been documented throughout recorded history.

TABLE 206-1  Emergence of Antigenic Subtypes of Influenza A Virus Associated with Pandemic or Epidemic Disease YEARS SUBTYPE EXTENT OF OUTBREAK 1889–1890 H2N8a Severe pandemic 1900–1903 H3N8a Moderate epidemic 1918–1919 H1N1b (formerly HswN1) Severe pandemic 1933–1935 H1N1b (formerly H0N1) Mild epidemic 1946–1947 H1N1 Mild epidemic 1957–1958 H2N2 Severe pandemic 1968–1969 H3N2 Moderate pandemic 1977–1978c H1N1 Mild pandemic 2009–2010d H1N1 Pandemic aAs determined by retrospective serologic survey of individuals alive during those years (“seroarchaeology”). bHemagglutinins formerly designated as Hsw and H0 are now classified as variants of H1. cFrom this time until 2016–2017, viruses of the H1N1 and H3N2 subtypes circulated in alternating years or concurrently. dA novel influenza A/H1N1 virus emerged to cause this pandemic. Source: Adapted from YZ Cohen, R Dolin. Influenza. In: Kasper DL, et al, eds. Harrison’s Principles of Internal Medicine, 19th ed. New York, McGraw-Hill, 2015,

p. 1209. In some epidemics, a younger age group proves especially suscep­ tible. This is the case with current H1N1 epidemics, where individuals born before 1968 had likely been exposed to related viral strains and thus were relatively protected against the current strain. The 1918 epi­ demic was striking in this regard: the most severely infected individu­ als were infants and previously healthy young adults—the latter being a group not typically found to have high influenza mortality (Fig. 206-2). The 1918 epidemic increased all-cause mortality and led to more deaths than all military losses in World War I. Despite the attention paid to the risk and impact of pandemic disease, it is generally appre­ ciated that—with the exception of 1918—cumulatively more illness occurs during yearly epidemics combined than in pandemics. All of

Pneumonia/influenza mortality rate per 100,000 <1 Year

65–74 Years

25–34 Years

Influenza epidemics with excess pneumonia/influenza mortality >20/100,000 FIGURE 206-2  Excess pneumonia/influenza deaths in 1900–1953, demonstrating the dramatic peaks of deaths among young infants and young adults (25–34 years of age) in 1918. (Data are from public health records collated by PF Wright.)

the annual influenza A epidemics in the past 50 years have been caused by H1N1 and/or H3N2 strains. H2N2 strains circulated between 1957 and 1968, and H1N1 strains circulated prior to that, including in 1918.

Avian Influenza Viruses  Wild birds are considered the natural hosts for influenza viruses, and potential pandemic viruses continue to emerge with higher-numbered hemagglutinins (e.g., H5, H6, H7, H8, H9) reflecting some of the 18 distinct H subtypes in avian reservoirs. Bird migration contributes to rapid global spread. Highly pathogenic influenza H5 and H7, in particular, have been associated with wide­ spread outbreaks in poultry, human infections through contact with infected birds, and limited human-to-human transmission. Most human infections have occurred in individuals who have had direct contact with domesticated birds or who have visited live-bird markets. Some avian strains—notably H5 strains—are highly pathogenic in humans, as was the 1918 strain. The reasons for the high pathogenic­ ity of certain strains are not entirely clear. After the sequencing of the 1918 virus recovered from the lungs of bodies buried in the Arctic permafrost, the virus was genetically reconstructed under carefully controlled isolation conditions. In animal studies of this viable 1918 virus, both the hemagglutinin and the ribonucleoprotein contributed to high levels of replication accompanied by an abnormally enhanced innate immune response characterized by proinflammatory cytokines. Perhaps this “cytokine storm” is the best explanation for the enhanced illness in young, immunologically vigorous individuals during the 1918 pandemic. Sequencing demonstrated that the 1918 virus was of avian origin. Although the 1918 virus was first identified in military camps in the United States, its impact cannot be attributed to the disruption of war—the illness was well documented in countries such as Iceland that were not directly involved in World War I. CHAPTER 206 The same concerns about a “cytokine storm” have been raised about the H5N1 viruses that first emerged in Hong Kong in 1996. These viruses exhibited high pathogenicity in individuals who had direct con­ tact with domestic fowl, with mortality rates close to 50%, but also dis­ played poor human-to-human transmissibility. Pathogenicity appears to be a function not just of the viruses’ surface proteins, but also of an optimal gene constellation including all eight segmented influenza genes. However, unlike the 1918 strain, the H5N1 viruses have, to date, caused only sporadic disease, as have other limited clusters of a highly pathogenic H7N9 virus. Influenza Sporadic H5 avian infections have been reported in many mammals, including sea lions, sea elephants, foxes, goats, and zoo animals. In 2024, a multistate outbreak of highly pathogenic H5 avian influenza in dairy cows was first reported, with rare human infections from animal exposures. It is unclear why higher-numbered avian hemagglutinin strains have not acquired the degree of transmissibility necessary to cause pandemic disease. Swine Influenza Viruses  Swine play an important role in inter­ species transmission of influenza. It is postulated that epithelial cells in the swine respiratory tract may play a specific role as a “mixing vessel,” allowing the reassortment of genes from avian and human sources and thereby permitting the transmission of avian viruses to humans. The nature of the sialic acid receptors for influenza virus hemagglutinin partially accounts for host preference. Humans have largely α-2,6galactose receptors, while birds have α-2,3-galactose receptors. Swine have both types of receptors on their respiratory epithelial cells—hence their postulated role in facilitating reassortment and host adaptation of avian strains to growth in humans. The swine origin 2009 H1N1pdm strain was a reassorted virus with gene segment origins from avian, human, and swine hosts. Influenza B and C Viruses  The influenza B viruses are more genetically stable than the influenza A viruses and are mainly associ­ ated with human infection. Two lineages of influenza B have circulated for the past 40 years (B/Yamagata-like and B/Victoria-like viruses), and it has proven difficult to predict which strain will be dominant in a given year. Co-circulation of both B lineages—Victoria and Yamagata—began in 2011. This led to the incorporation of representa­ tives of both influenza A lineages plus influenza A/H1N1 and H3N2

Deaths 4,900–51,000 Hospitalizations 100,000–710,000 Illnesses 9,300,000–41,000,000 FIGURE 206-3  Pyramid of impact of influenza illness. Estimated range of annual burden of influenza in the United States from 2010–2023. (From https://www.cdc .gov/flu-burden/php/about/index.html?CDC_AAref_Val=https://www.cdc.gov/flu/ about/burden/index.) viruses into quadrivalent vaccines, first marketed in the United States in 2013. However, since March 2020, B/Yamagata influenza viruses have not circulated in the population. Thus, influenza vaccines in the United States will revert to a trivalent vaccine for the 2024–2025 season, containing only a B/Victoria-like virus. Influenza C viruses cause intermittent mild disease. The clinical information about this virus is limited because of the small number of isolated viruses compared to influenza A or B viruses. Influenza-Associated Morbidity and Mortality  Influenza virus infects people of all ages and causes mild to severe illness, and even death in some cases. The impact of influenza is highly variable from year to year and can be depicted as a pyramid of illnesses, medical visits, hospitalizations, and deaths (Fig. 206-3). Infection rates are highest among children, with complications and hospitalizations from seasonal influenza being greatest among certain high-risk groups during most epidemics. These groups are assigned the highest priority for vaccination and other preventive and therapeutic measures. Their caregivers and close contacts are also prioritized targets of interventions (Table 206-2). PART 5 Infectious Diseases Mortality attributable to influenza, reported as excess over the anticipated sine-wave curve of pneumonia and influenza deaths during the year, varied between 4900 and 51,000 deaths annually from 2010 to 2023. The dramatic effect of the COVID-19 pandemic on excess pneumonia and influenza mortality data is evident from the comparison of 2020 data with data from the prior three seasons (Fig. 206-4). Due to this outsized effect of COVID-19, the method for calculating mortality due to influenza changed starting with the 2023–2024 season and is now restricted to percentage of deaths with influenza listed on the death certificate. In contrast to mortality surveillance in adults, influenzaassociated pediatric mortality is based on laboratory confirmation. Upon normalization of influenza circulation after the COVID-19 pandemic, 184 children died in 2022–2023 and 138 children died in 2023–2024 from laboratory-confirmed influenza. These numbers are undoubtedly underestimates, since all children are not tested for influenza, and even among those who are tested, tests are less sensitive later in the illness. ■ ■PATHOGENESIS AND IMMUNITY At a cellular level, influenza virus binds to sialic acid receptors and enters the epithelial cell through receptor-mediated endocytosis. The virus then enters an endosome, where acidification promotes proteolytic cleavage of the hemagglutinin, exposing a fusion domain. The influenza hemagglutinin undergoes a marked structural reorganization in this cleavage step. Hemagglutinin cleavage may be one of the factors that restrict viral growth to epithelial cells, as a unique protease in the respiratory milieu is required for this cleavage to occur. The fusion domain allows the viral RNA to enter the cytoplasm. The nucleoprotein is transported into the nucleus of the cell, where transcription to a positive-sense RNA and replication take place. Viral proteins then assemble on the apical surface of the infected cell and, after incorporation of cellular membrane, bud from the membrane back into the mucosal milieu.

TABLE 206-2  High-Risk Groups Who Should Be Assigned a High Priority for Influenza Immunization and Treatmenta High-Risk Group Children 6–59 months of age Adults ≥50 years of age Persons with chronic pulmonary (including asthma), cardiovascular (except isolated hypertension), renal, hepatic, neurologic, hematologic, or metabolic disorders (including diabetes mellitus) Persons who are immunocompromised (any cause, including medications or HIV infection) Women who are or plan to be pregnant during the influenza season Children and adolescents (6 months through 18 years of age) who are receiving aspirin- or salicylate-containing medications and who might be at risk for Reye syndrome Residents of nursing homes and other long-term-care facilities American Indians/Alaska Natives Persons who are extremely obese (body mass index ≥40) Contacts and Caregivers Caregivers and contacts of those at risk: health care personnel in inpatient and outpatient care settings who have the potential for exposure to patients or to infectious materials, medical emergency-response workers, autopsy personnel, employees of nursing home and long-term-care facilities who have contact with patients or residents, and students and trainees in these professions who have contact with patients Household contacts and caregivers of children ≤59 months (i.e., <5 years) of age (particularly contacts of infants <6 months old) and adults ≥50 years of age Household contacts (including children) and caregivers of persons who are in a high-risk group aNo hierarchy is implied by order of listing. Source: Centers for Disease Control and Prevention 2023–2024 summary of recommendations for influenza vaccine (https://www.cdc.gov/mmwr/volumes/72/rr/ rr7202a1.htm). Influenza infection is initiated in the upper respiratory tract via aerosolized virus. The cells infected with influenza virus are primarily the ciliated cells of the respiratory tract. Denudation of the superficial epithelium probably accounts for much of the symptomatology and may predispose to secondary bacterial infections. The onset of symptoms follows an incubation period that, for a viral illness, is very short: 48–72 h. The infection spreads to the lungs but, even there, remains confined to the epithelial layer. Influenza virus is associated with systemic symptoms of fever, malaise, and myalgia. These manifestations are presumed to be mediated by cytokines, and excess cytokine production has been implicated in the acute toxicity of H5N1 and other highly pathogenic influenza viruses. The immune response to influenza virus occurs at the systemic and mucosal levels and involves both T and B cells. The B-cell responses are directed primarily toward antigenic epitopes on the two surface glycoproteins—i.e., hemagglutinin and neuraminidase. At a structural level, the four recognized epitopes on the hemagglutinin are largely confined to the globular head of the protein, which collectively constitute the targets for hemagglutination inhibition (HAI) antibodies. HAI and neutralizing antibodies are highly correlated; HAI antibody levels are used as a measure of susceptibility to clinical infection and thus as a measure of vaccine-induced protection. In a child or an adult without prior vaccination or with the emergence of a distinctly new strain, serum HAI antibody is a surrogate for protection. However, in individuals with both vaccine-induced and natural immunity, the protective efficacy of a vaccine based on serum HAI antibody is more difficult to predict. There is considerable research interest in the induction and protective role of broadly neutralizing antibodies that recognize less antigenically variable regions on the stalk of the hemagglutinin. The results of these studies have led to investments toward research and development of a universal influenza vaccine, although no such vaccines are yet available in clinical practice. The role of T-cell immunity, which primarily recognizes internal protein epitopes, remains unclear in humans. However, T-cell

Pneumonia, Influenza, and COVID-19 Mortality from the National Center for Health Statistics Mortality Surveillance System

Number of influenza coded deaths Number of COVID-19 coded deaths % of deaths due to PIC Baseline Threshold

% of all deaths due to PIC

Epidemic threshold

Seasonal baseline

MMWR week

FIGURE 206-4  Pneumonia, influenza, and COVID-19 mortality. MMWR, Morbidity and Mortality Weekly Report; PIC, pneumonia, influenza, COVID-19. Data through the week ending January 23, 2021, as of January 28, 2021. (From https://www.cdc.gov/fluview/?CDC_AAref_Val=https://www.cdc.gov/flu/weekly/index) immunity is thought to play a role in clearance of an influenza infection that quite reproducibly develops 8–10 days after exposure. A role for T cells in protection against acquisition of infection has also been proposed. CLINICAL MANIFESTATIONS Attack rates of clinical influenza vary considerably from year to year. With the advent of molecular diagnostic tests, prospective studies with regular sampling demonstrate that asymptomatic or minimally symptomatic cases of influenza are more common than previously recognized. When symptomatic, influenza is primarily a respiratory illness causing cough, sore throat, and rhinorrhea or nasal congestion. The illness has a sudden onset and is epidemiologically linked to close contact with persons who have similar symptoms and often to community-wide respiratory illness. What distinguishes influenza from most other respiratory viral illnesses is the degree of accompanying fever, chills, fatigue, myalgia, and malaise. SARS-CoV-2 is the exceptional respiratory virus that also has a remarkable systemic component (Chaps. 204 and 205). Symptoms of influenza typically begin within 48–72 h of exposure. Respiratory symptoms, particularly recurrent cough, persist well beyond the 2–5 days of systemic symptoms. There is a postinfectious delay in return to normal levels of activity. Pulmonary function is persistently decreased after acute influenza. Persons with a regular exercise routine (e.g., runners) note a decrease from their prior level of performance that typically lasts for a month or more. In the elderly, the respiratory presentation may be less prominent, but there is often a decline in baseline activity and a loss of appetite. On physical examination, the patient with influenza appears ill, with sweating, coughing, nonpurulent conjunctivitis, and diffuse pharyngeal erythema. With lower respiratory involvement, pulmonary examination typically reveals nonlocalizing scattered rales, rhonchi, and wheezes. When present, localized pulmonary findings suggest relatively complicated pneumonia with a bacterial component. Muscle

Number of deaths

CHAPTER 206 Influenza pain may be elicited by pressure, particularly in the calves and thighs. There are rare gastrointestinal findings. No rash is associated with influenza. ■ ■COMPLICATIONS Most persons who become ill with influenza virus infection recover without serious complications or sequelae. Complications of influenza occur most commonly in persons ≥65 years of age, young children, persons of all ages with underlying cardiopulmonary disease and immunosuppression, and women who are in the second or third trimester of pregnancy. Respiratory Complications  Pneumonia characterized by progressive air hunger, localized pulmonary findings on physical examination, and radiographic findings of diffuse infiltrates or consolidation is the most common complication of influenza. Pneumonia in influenza can be primary influenza viral pneumonia, secondary bacterial pneumonia, or mixed viral and bacterial pneumonia. Primary viral pneumonia is characterized by increasing dyspnea, persistent fever, and—in more severe cases—cyanosis. Primary influenza pneumonia was typical in the 1918 pandemic and occurs with H5N1 virus, as initially described in Hong Kong in 1997. Pathologically, a marked inflammatory reaction in the alveolar septa is characterized by infiltration of monocytes, lymphocytes, and macrophages, with variable numbers of neutrophils. Destruction and hemorrhage are seen in the respiratory epithelium. Large amounts of virus can be recovered from the lungs. In secondary bacterial pneumonia or mixed viral and bacterial pneumonia, illness may be biphasic, with evidence of recovery from the primary influenza illness followed by recrudescence of fever and pulmonary symptoms. Localizing findings may be detected on pulmonary examination and/or x-ray. The development of secondary bacterial infection is not surprising, as influenza de-epithelializes the airways and destroys ciliary function, allowing bacterial contamination. Another proposed mechanism for bacterial/viral enhancement is

the production by Staphylococcus and Pseudomonas of proteases that enhance cleavage of the influenza hemagglutinin and thereby facilitate viral replication. The risk of secondary bacterial disease is greatest in elderly patients and those with chronic obstructive pulmonary disease (COPD).

Some influenza strains cause laryngotracheobronchitis, bronchiol­ itis, or croup in children. Otitis media—a common accompaniment to influenza in children—may also be due to a combination of influenza virus and bacteria. Extrapulmonary Complications  Although influenza is believed to spread only rarely beyond the respiratory epithelial cells, where unique endogenous proteases facilitate hemagglutinin cleavage and productive infection, this disease causes not only prominent systemic complaints but also a variety of extrapulmonary manifestations. The most common extrapulmonary manifestation of influenza is myositis, which is seen more often in influenza B and is characterized by severe muscle pain, elevated creatinine phosphokinase levels, and myoglobin­ uria that can lead to renal failure. The muscles are extremely tender to touch. Myo-/pericarditis is seen less frequently. However, a consistent epidemiologic link exists between influenza epidemics and excess car­ diovascular hospitalizations. Neurologic involvement, while rare, does occur following influenza infection. Influenza-associated encephalopathy or encephalitis is char­ acterized by rapid progression within a few days of influenza infection. Transverse myelitis and parkinsonian symptoms have been reported. Postinfectious acute demyelinating encephalomyelitis can follow influ­ enza as well as other viral infections. Neurologic manifestations are more frequent in children as compared to adults. Children most com­ monly present with febrile seizures, increased seizure frequency among those with seizure disorders, or self-limited encephalopathy. More serious manifestations of meningitis, encephalitis, and focal brain lesions may occur, particularly in children with preexisting neurologic conditions. PART 5 Infectious Diseases Guillain-Barré syndrome can develop after influenza and was reported after a widespread influenza vaccination effort in the fall of 1976 that was undertaken in anticipation of a swine influenza epidemic (which never materialized). Until aspirin was recognized as a cofactor in its precipitation, Reye syndrome, an acute hepatic decompensation, was seen commonly in children and adolescents with influenza, par­ ticularly those infected with influenza B virus. Subsequently, the use of aspirin for fever control and symptom relief in children with viral infections was strongly discouraged, and Reye syndrome has virtually disappeared from clinical practice. ■ ■LABORATORY FINDINGS AND DIAGNOSIS There is a strong argument for establishing a microbiologic diagnosis from both an individual-patient and a public-health perspective. This information is particularly valuable early in the season, when the extent of influenza and the precise circulating strain(s) are uncertain; in the management of high-risk or hospitalized patients; in settings such as long-term-care facilities and hospitals, where the institution of specific infection-control measures is appropriate; and in any patient with influenza-like illness if the test results will influence clinical management. Influenza virus is most easily recovered from nasal or pharyngeal specimens. A number of rapid influenza diagnostic tests (RIDTs) are TABLE 206-3  Categories of Vaccines Licensed for Prevention of Seasonal Influenza, United States   LIVE ATTENUATED STANDARD INACTIVATED HIGH-DOSE INACTIVATED RECOMBINANT ADJUVANTED INACTIVATED Route Intranasal Intramuscular Intramuscular Intramuscular Intramuscular Approved ages 2–49 years ≥6 months ≥65 years ≥18 years ≥65 years HAa

Substrate Eggs Eggs/cell culture Eggs Cell culture Eggs Number of strains

aHemagglutinin content in micrograms per strain.

available. They work by detecting viral antigens and can provide results within 10–15 min. Rapid molecular assays (i.e., nucleic acid amplifica­ tion tests [NAATs]) detect viral genetic material. Several NAATs are authorized for home use, including tests that detect and differentiate between SARS-CoV-2, influenza A, and influenza B in self-collected anterior nasal swab samples. In addition to RIDTs and rapid molecular assays, several influenza diagnostic tests are available in specialized hospital and public health laboratories, including reverse transcription polymerase chain reaction (RT-PCR) and viral culture. Many nucleic acid–based tests are multiplex and target a panel of common respira­ tory pathogens— influenza, respiratory syncytial virus, parainfluen­ zavirus, and coronaviruses including SARS-CoV-2—an advantage in the ill hospitalized patient and during outbreaks of other respiratory pathogens. Clinicians should not use viral culture for initial or primary diagnosis of influenza because results will not be available in a timely manner to inform clinical management, but viral culture can confirm the strain and allow for antiviral sensitivity testing. Serologic confirmation of infection is also possible but requires paired serum samples, with the convalescent-phase sample obtained 2 weeks after infection. Other laboratory tests are of limited value. Mild leu­ kopenia is seen in influenza, and a white blood cell count >15,000/μL suggests a secondary bacterial component in influenza pneumonia. ■ ■DIFFERENTIAL DIAGNOSIS Influenza may be diagnosed clinically based on an acute presentation of a febrile respiratory illness during high periods of influenza circu­ lation. However, less common presentations of influenza and cases occurring outside of peak influenza season are frequently misdiag­ nosed on the basis of symptoms alone. Influenza symptoms and signs may overlap with symptoms of other respiratory viruses. Respiratory syncytial virus often co-circulates with influenza virus; it particularly affects the youngest children, causing bronchiolitis, but it can also infect the elderly, leading to an influenza-like nonspecific respiratory illness and a decline in mobility, nutrition, and pulmonary function, with resultant hospitalization. Persons with COVID-19 have a wide range of symptoms reported, ranging from mild to severe illness. Many of these symptoms—fever, chills, cough, shortness of breath, fatigue, muscle aches, headaches, congestion or runny nose—overlap with the symptoms of influenza. While new loss of taste (ageusia) or smell (anosmia) may distinguish COVID-19 from influenza, they are reported in the minority of infected persons. When SARS-CoV-2 and influenza viruses are cocirculating, clinicians should consider both viruses, as well as co-infection, in patients with acute respiratory illness symptoms. The similar clini­ cal presentations reiterate the importance of testing in order to inform treatment decisions. ■ ■IMMUNIZATION Vaccination is the best approach to prevent influenza. The vaccines currently available in the United States are increasing in number and diversity (Table 206-3). These vaccines fall into two broad cat­ egories: parenterally administered inactivated influenza vaccines and intranasally administered live-attenuated influenza vaccines. Current vaccines are further classified based on production substrate (eggs, cell), antigen dose and valence (trivalent or quadrivalent), and the presence or absence of adjuvants. Current inactivated influenza vac­ cines are designed with the common goal to induce immunity to the NONREPLICATING VACCINES

hemagglutinin surface glycoprotein of the influenza virus. No effort is made to standardize the neuraminidase content. As the viral surface hemagglutinin undergoes frequent antigenic drift, the seasonal influenza vaccine is reformulated as often as twice annually to match the strains projected to circulate in the following influenza season. The decision about vaccine composition must be made ~10 months before the seasonal peak in influenza virus circulation; this decision is made by committees at the WHO. Subsequently, the U.S. Food and Drug Administration (FDA), which has regulatory authority over vaccines in the United States, convenes an advisory committee that considers the recommendations of WHO, reviews and discusses similar data, and makes a final decision regarding vaccine virus composition of influenza vaccines licensed and marketed in the United States. This timing can result in a mismatch of vaccine composition with the viral strains that are actually prevalent in the upcoming season. Influenza vaccine is unique in being given seasonally in the months immediately preceding an outbreak in temperate climates. In the United States, vaccine is typically available starting in August or September. The performance of current influenza vaccines varies by year, vaccine formulation, and the underlying age, health condition, and prior virus and vaccine exposure of the recipient. Unfortunately, the relative contribution of each of these factors has not been well-elucidated, given the many variables involved and the complex interplay of infection and host response. Depending on the degree to which vaccine strains match circulating strains, seasonal influenza vaccines will confer more or less protection, as antibody against influenza is for the most part strain specific. A meta-analysis of randomized controlled trials of influenza vaccine efficacy over 12 influenza seasons showed inactivated influenza vaccines had a pooled efficacy of 59% (95% confidence interval, 51–67%) among those aged 18–65 years. Since 2004−2005, the CDC has estimated the effectiveness of seasonal influenza vaccine to prevent laboratory-confirmed influenza associated with medically attended respiratory illness. During that period, effectiveness ranged from ~40 to 60% across all age groups during seasons when most circulating influenza vaccines are antigenically similar to the recommended influenza vaccine components; effectiveness was lower in years with strain mismatch. Importantly, studies support that influenza vaccine mitigates disease severity. For example, observational studies in children support that influenza vaccination reduces intensive care unit hospitalizations and deaths by an estimated 74 and 65%, respectively. Newer technologies have been developed to overcome some of the limitations of current vaccines. The first fully recombinant vaccine was approved by the FDA in 2017. Both recombinant and cell-based vaccines may overcome the egg adaptation of vaccine strains that may contribute to diminished vaccine effectiveness. Oil-in-water adjuvanted vaccines and high-dose vaccines elicit greater immune responses than traditional inactivated influenza vaccines and are approved in the United States for persons ≥65 years of age. In most head-to-head comparisons, high-dose vaccines have shown superior effectiveness to standard dose. While evidence is more limited, select comparisons of recombinant and adjuvanted vaccines with standard vaccines likewise show improved effectiveness. In head-to-head comparisons in pediatric populations in the 1990s, a live, attenuated, intranasally administered vaccine (LAIV) exhibited an efficacy exceeding that of injected inactivated vaccines. LAIV is a desirable option in children given the ease of intranasal administration and theoretical advantage of stimulating mucosal immunity by the topical route. However, in the 2014−2016 influenza seasons, LAIV had lower replicative fitness and no demonstrable efficacy assignable to the vaccine’s H1N1 component. Consequently, advisory committees in the United States and elsewhere suspended the recommendations for use of LAIV until manufacturing improvements allowed reinstatement of recommendations for its use in 2018. Since that time LAIV has performed comparably to inactivated influenza vaccines in annual effectiveness assessments. Inactivated influenza vaccines have been licensed for >60 years and have a robust safety and tolerability profile. While local reactions are most common following inactivated influenza vaccines, rare adverse

events may occur. These include Guillain-Barré syndrome, identified in 1976 and less frequently during other years; oculorespiratory syndrome, first recognized in 2000; and febrile seizures first reported in young children in Australia in 2010. Adjuvanted vaccines in general cause more local pain and erythema than unadjuvanted vaccines. LAIVs have been associated with excess wheezing and hospitalizations in children younger than 2 years and thus are not licensed for use in this age group.

The recommendations for use, the approved age range of each product, the route of administration, and the anticipated side effects are updated annually by the CDC (https://www.cdc.gov/acip-recs/hcp/ vaccine-specific/flu.html.) In the United States, routine annual influenza vaccination is recommended for all persons 6 months of age and older. For persons 65 years of age and over, higher-dose or adjuvanted vaccines are preferred. For other age groups, no preferential recommendation is made for one influenza vaccine product over another. Two doses of vaccine should be given to children <9 years of age who have not received at least two lifetime doses of influenza vaccine prior to the start of the season. All other children should receive one dose. Groups at special risk of experiencing or transmitting influenza and for whom influenza immunization is a particularly high priority are listed in Table 206-2. History of severe allergic reaction (e.g., anaphylaxis) to any vaccine component is a contraindication to influenza vaccines. A history of Guillain-Barré syndrome within 6 weeks of a previous dose of influenza vaccine is considered a precaution for the use of all influenza vaccines. Egg allergy alone necessitates no additional safety measures for influenza vaccination beyond those recommended for any vaccine recipient, regardless of severity of previous reaction to egg. CHAPTER 206 TREATMENT Influenza Influenza Antiviral therapy for influenza has been limited by the paucity of available drugs, the short duration of symptoms in uncomplicated influenza, and the changing patterns of drug resistance in influenza viral strains. In the past, influenza A infection could be treated with the M-2 channel blockers amantadine and rimantadine. Widespread resistance has currently relegated these compounds to historical interest only. Neuraminidase inhibitors have been the mainstay for treatment of influenza A and B viruses for many years. As their name implies, these drugs inhibit the influenza neuraminidase and thus limit the egress of influenza virus from an infected cell. They are most effective in patients whose influenza illness is recognized early and confirmed by rapid diagnostic testing or on the basis of clinical and epidemiologic evidence. In experimental trials, these drugs hasten the resolution of symptoms if given within 48 h of infection. There are indications for their use both prophylactically—either throughout the season or, when a case is recognized in a close contact, in the short term—and therapeutically. The anticipated effect of early administration is the resolution of symptoms 1–2 days sooner than without treatment. The use of neuraminidase inhibitors is recommended for complicated influenza infections in hospitalized patients in the absence of formal proof of efficacy and when diagnosis may have been delayed. All the available neuraminidase inhibitors carry a risk of development of resistance, particularly with prolonged administration (e.g., to an immunodeficient individual with persistent recovery of influenza virus). Resistance to neuraminidase inhibitors is not widespread among currently circulating influenza A or B strains, but its development has been demonstrated in the laboratory, and clinical resistance could influence the utility of these drugs. The defined risk groups who can benefit from neuraminidase inhibitors include children <2 years of age, adults >65 years of age, patients with chronic conditions, immunosuppressed individuals, pregnant women, women who have delivered infants ≤2 weeks previously, patients <19 years old who are receiving long-term aspirin

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207 The Human Retroviruses

treatment, Native Americans (including Alaska Natives), morbidly obese individuals, and residents of nursing homes or chronic-care facilities. This list resembles that of candidates whose vaccination is a high priority (Table 206-2). Use of neuraminidase inhibitors should be considered in selected high-risk cases despite a history of vaccination.

The available neuraminidase inhibitors are oral oseltamivir, nasal-spray zanamivir, and intravenous peramivir. Oseltamivir, which is most widely used, is an orally absorbed drug that is con­ verted to its active component, oseltamivir carboxylate, in the liver. Gastrointestinal symptoms, especially nausea, may accompany the administration of oseltamivir. Because zanamivir is not orally bio­ available, it is given as an inhaled dry powder dispersed through a Diskhaler device. The usual duration of therapy with either oral oseltamivir or intranasal zanamivir is 5 days, with twice-a-day dosing. Oseltami­ vir is preferred for treatment of pregnant women and is approved for treatment at any age, beginning at 14 days of life in infants. Poor oral intake or absorption is a contraindication to the use of oseltamivir, although this drug can also be given by oro/nasal tube. Asthma and COPD are relative contraindications to the use of intranasal zanamivir; this agent is approved for treatment in persons 7 years and older. For hospitalized patients with suspected or confirmed influenza, initiation of antiviral treatment with oral or enterically administered oseltamivir is recommended as soon as possible. For patients who cannot tolerate or absorb oral or enterically administered oseltamivir, the use of a single infusion of intravenous peramivir should be considered. Peramivir is licensed for individuals ≥2 years of age. The most current recommendations and details on influenza antiviral drug use and release are available through the CDC (https://www.cdc.gov/flu/professionals/antivirals/ summary-clinicians.htm). PART 5 Infectious Diseases In 2018, a first-in-class compound, baloxavir marboxil (XOFLUZA), was approved by the FDA for persons 12 years and older for prophylaxis or treatment of uncomplicated influenza within 2 days of onset of illness. Baloxavir inhibits cap-dependent endonuclease, has activity against influenza A and B, and is a singledose formulation. In clinical studies, if given within 48 h of symp­ toms, baloxavir decreased symptom duration, viral shedding, and antibiotic use in healthy individuals with uncomplicated influenza. However, development of resistance is a concern, with 2–10% of the trial participants who received baloxavir showing viral escape with reduced drug susceptibility. CDC does not recommend use of bal­ oxavir in pregnant women, breastfeeding mothers, outpatients with complicated or progressive illness, severely immunosuppressed people, or hospitalized patients because of lack of information on use of baloxavir for these groups. Other critical aspects of treatment include maintenance of fluid and electrolyte balance, oxygen supplementation, fever control with nonsteroidal anti-inflammatory drugs, and treatment of suspected secondary bacterial complications with antibiotics. Appropriate respiratory isolation of patients should be practiced in accordance with local hospital guidelines. ■ ■FURTHER READING Barry JM: The Great Influenza: The Story of the Deadliest Pandemic in History. New York, Penguin Books, 2005. Chung JR: Effects of influenza vaccination in the United States during the 2018–2019 influenza season. Clin Infect Dis 71:e368, 2020. Erbelding EJ: A universal influenza vaccine: The strategic plan for the National Institute of Allergy and Infectious Diseases. J Infect Dis 218:347, 2018. Fineberg HV: Pandemic preparedness and response—lessons from the H1N1 influenza of 2009. N Engl J Med 370:1335, 2014. Kash JC, Taubenberger JK: The role of viral, host, and secondary bacterial factors in influenza pathogenesis. Am J Pathol 185:1528, 2015. Osterholm MT et al: Efficacy and effectiveness of influenza vaccines: A systemic review and meta-analysis. Lancet Infect Dis 12:36, 2012.

Treanor JJ: Influenza vaccination. N Engl J Med 375:1261, 2016. Uyeki TM et al: Influenza. Lancet 400:693, 2022. Watanabe T et al: 1918 influenza virus hemagglutinin (HA) and the viral RNA polymerase complex enhance viral pathogenicity, but only HA induces aberrant host responses in mice. J Virol 87:5239, 2013. Wright PF et al: Correlates of immunity to influenza as determined by challenge of children with live, attenuated influenza vaccine. Open Forum Infect Dis 3:108, 2016. Section 14 Infections Due to Human Immunodeficiency Virus and Other Human Retroviruses Dan L. Longo, Anthony S. Fauci

The Human

Retroviruses The retroviruses, which make up a large family (Retroviridae), infect mainly vertebrates. These viruses have a unique replication cycle whereby their genetic information is encoded by RNA rather than DNA. Retroviruses contain an RNA-dependent DNA polymerase (a reverse transcriptase) that directs the synthesis of a DNA form of the viral genome after infection of a host cell. The designation retro­ virus denotes that information in the form of RNA is transcribed into DNA in the host cell—a sequence that overturned a central dogma of molecular biology: that information passes unidirectionally from DNA to RNA to protein. The observation that RNA was the source of genetic information in the causative agents of certain animal tumors led to a number of paradigm-shifting biologic insights regarding not only the direction of genetic information passage but also the viral etiology of certain cancers and the concept of oncogenes as normal host genes scavenged and altered by a viral vector. The family Retroviridae includes seven subfamilies (Table 207-1). Members of two of the families infect humans with pathologic con­ sequences: the deltaretroviruses, of which human T-cell lymphotropic virus (HTLV) type 1 is the most important in humans; and the lentivi­ ruses, of which HIV is the most important in humans. The wide variety of interactions of a retrovirus with its host range from completely benign events (e.g., silent carriage of endogenous retroviral sequences in the germline genome of many animal species) to rapidly fatal infections (e.g., exogenous infection with an onco­ genic virus such as Rous sarcoma virus in chickens). The ability of TABLE 207-1  Classification of Retroviruses: The Family Retroviridae GENUS EXAMPLE(S) FEATURE Alpharetrovirus Rous sarcoma virus Contains src oncogene Betaretrovirus Mouse mammary tumor virus Exogenous or endogenous Gammaretrovirus Abelson murine leukemia virus Contains abl oncogene Deltaretrovirus HTLV-1 Causes T-cell lymphoma and neurologic disease Epsilonretrovirus Walleye dermal sarcoma virus Not known to be pathogenic in humans Lentivirus HIV-1, HIV-2 Causes AIDS Spumavirus Simian foamy virus Not known to be pathogenic in humans

retroviruses to acquire and alter the structure and function of host cell genetic sequences has revolutionized our understanding of molecular carcinogenesis. The viruses can insert into the germline genome of the host cell and behave as a transposable or movable genetic element. They can activate or inactivate genes near the site of integration into the genome. They can rapidly alter their own genome by recombina­ tion and mutation under selective environmental stimuli. Most human viral diseases occur as a consequence of tissue destruction either directly by the virus itself or indirectly by the host’s response to the virus. Although these mechanisms are opera­ tive in retroviral infections, retroviruses have additional mechanisms of inducing disease, including the malignant transformation of an infected cell and the induction of an immunodeficiency state through selective destruction or dysfunction of immune-competent cells that renders the host susceptible to opportunistic diseases (infections and neoplasms; Chap. 208). In addition to their role as acquired causes of infections, retroviruses and lentiviruses are employed as tools to alter gene expression in target cells. Gene therapy applications are expanding, and in addition to the benefits they bestow, a new set of viral-mediated adverse events have emerged. STRUCTURE AND LIFE CYCLE All retroviruses are similar in structure, genome organization, and mode of replication. Retroviruses are 70–130 nm in diameter and have a lipid-containing envelope surrounding an icosahedral capsid with a dense inner core. The core contains two identical copies of the singlestrand RNA genome. The RNA molecules are 8–10 kb long and are complexed with reverse transcriptase and tRNA. Other viral proteins, such as integrase, are also components of the virion particle. The RNA has features usually found in mRNA: a cap site at the 5′ end of the mol­ ecule, which is important in the initiation of mRNA translation, and a polyadenylation site at the 3′ end, which influences mRNA turnover (i.e., messages with shorter polyA tails turn over faster than messages with longer polyA tails). However, the retroviral RNA is not translated; instead, it is transcribed into DNA. The DNA form of the retroviral genome is called a provirus. The replication cycle of retroviruses proceeds in two phases (Fig. 207-1). In the first phase, the virus enters the cytoplasm after binding to one or more specific cell-surface receptors; the viral RNA and reverse transcriptase synthesize a double-strand DNA version of the RNA template; and the provirus moves into the nucleus and integrates into the host cell genome. This proviral integration is per­ manent. Although some animal retroviruses integrate into a single specific site of the host genome in every infected cell, the human ret­ roviruses integrate randomly. This first phase of replication depends entirely on gene products in the virus. The second phase includes the synthesis and processing of viral genomes, mRNAs, and proteins using host cell machinery, often under the influence of viral gene products. Virions are assembled and released from the cell by budding from the membrane; host cell membrane proteins are frequently incorporated into the envelope of the virus. Proviral integration occurs during the S-phase of the cell cycle; thus, in general, nondividing cells are resistant to retroviral infection. Only the lentiviruses are able to infect nondivid­ ing cells. Once a host cell is infected, it is infected for the life of the cell. Retroviral genomes include both coding and noncoding sequences (Fig. 207-2). In general, noncoding sequences are important rec­ ognition signals for DNA or RNA synthesis or processing events and are located in the 5′ and 3′ terminal regions of the genome. All retroviral genomes are terminally redundant, containing identical sequences called long terminal repeats (LTRs). The ends of the retrovi­ ral RNA genome differ slightly in sequence from the integrated retro­ viral DNA. In the latter, the LTR sequences are repeated in both the 5′ and the 3′ terminus of the virus. The LTRs contain sequences involved in initiating the expression of the viral proteins, the integration of the provirus, and the polyadenylation of viral RNAs. The primer binding site, which is critical for the initiation of reverse transcription, and the viral packaging sequences are located outside the LTR sequences. The coding regions include the gag (group-specific antigen, core protein),

Adsorption to specific receptor Penetration Reverse transcription Integration Translation Transcription Capsid assembly Budding Provirus A gag pol env Transcription m7G Polyadenylation m7G m7G gag pol env Splicing env mRNA m7G gag pol env env proteins gag pol mRNA CHAPTER 207 m7G gag pol env Genomes m7G gag gag pol env m7G pol gag pol env Proteins The Human Retroviruses B FIGURE 207-1  The life cycle of retroviruses. A. Overview of virus replication. The retrovirus enters a target cell by binding to a specific cell-surface receptor; once the virus is internalized, its RNA is released from the nucleocapsid and is reversetranscribed into proviral DNA. The provirus is inserted into the genome and then transcribed into RNA; the RNA is translated; and virions assemble and are extruded from the cell membrane by budding. B. Overview of retroviral gene expression. The provirus is transcribed, capped, and polyadenylated. Viral RNA molecules then have one of three fates: they are exported to the cytoplasm, where they are packaged as the viral RNA in infectious viral particles; they are spliced to form the message for the envelope polyprotein; or they are translated into Gag and Pol proteins. Most of the messages for the Pol protein fail to initiate Pol translation because of a stop codon before its initiation; however, in a fraction of the messages, the stop codon is missed, and the Pol proteins are translated. (Reproduced with permission from JM Coffin, in BN Fields, DM Knipe [eds]: Fields Virology. New York, Raven, 1990.) pol (RNA-dependent DNA polymerase), and env (envelope) genes. The gag gene encodes a precursor polyprotein that is cleaved to form three to five capsid proteins; a fraction of the Gag precursor proteins also contain a protease responsible for cleaving the Gag and Pol polyprot­ eins. A Gag-Pol polyprotein gives rise to the protease that is responsible for cleaving the Gag-Pol polyprotein. The pol gene encodes three pro­ teins: the reverse transcriptase, the integrase, and the protease. The reverse transcriptase copies the viral RNA into the double-strand DNA provirus, which inserts itself into the host cell DNA via the action of integrase. The protease cleaves the Gag-Pol polyprotein into smaller protein products. The env gene encodes the envelope glycoproteins: one protein that binds to specific surface receptors and determines what cell types can be infected and a smaller transmembrane protein that anchors the complex to the envelope. Fig. 207-3 shows how the retroviral gene products make up the virus structure. HTLVs have a region between env and the 3′ LTR that encodes sev­ eral proteins and transcripts in overlapping reading frames (Fig. 207-2). Tax is a 40-kDa protein that does not bind to DNA but induces the expres­ sion of host cell transcription factors that alter host cell gene expression

LTR MuLV GAG POL LTR MA CA NC RT PR p14 p95 HTLV-I,II p19 p24 p15 GAG LTR MA CA NC p17 p24 p7 HIV-1 RT IN p10 POL p66 p32 GAG PR LTR HIV-2 GAG POL FIGURE 207-2  Genomic structure of retroviruses. The murine leukemia virus MuLV has the typical three structural genes: gag, pol, and env. The gag region gives rise to three proteins: matrix (MA), capsid (CA), and nucleic acid–binding (NC) proteins. The pol region encodes both a protease (PR) responsible for cleaving the viral polyproteins and a reverse transcriptase (RT). In addition, HIV pol encodes an integrase (IN). The env region encodes a surface protein (SU) and a small transmembrane protein (TM). The human retroviruses have additional gene products translated in each of the three possible reading frames. HTLV-1 and HTLV-2 have tax and rex genes with exons on either side of the env gene. HIV-1 and HIV-2 have six accessory gene products: tat, rev, vif, nef, vpr, and either vpu (in HIV-1) or vpx (in HIV-2). The genes for these proteins are located mainly between the pol and env genes. GP, glycoprotein; HBZ, HTLV-1 basic leucine zipper domain–containing protein; LTR, long terminal repeat. PART 5 Infectious Diseases and is capable of inducing cell transformation under certain circum­ stances. Rex is a 27-kDa protein that regulates the expression of viral mRNAs. Other transcripts from this region (p12, p13, and p30) tend to restrict expression of viral genes and diminish the immunogenicity of infected cells. The protein of HBZ, a product of the complementary proviral DNA strand, interacts with many cellular transcription factors and signaling proteins. It stimulates proliferation of infected cells and is the only viral product universally expressed in HTLV-1-infected tumor cells. These proteins are produced from messages that are similar but that are spliced differently from overlapping but distinct exons. HTLV-I HIV-1 SU gp46 gp120 TM p21 gp41 NC p15 p7 PR p14 p10 RT p95 p66 IN –– p32 MA p19 p17 CA p24 p24 RNA 9kb 10kb FIGURE 207-3  Schematic structure of human retroviruses. The surface glycoprotein (SU) is responsible for binding to receptors of host cells. The transmembrane protein (TM) anchors SU to the virus. NC is a nucleic acid–binding protein found in association with the viral RNA. A protease (PR) cleaves the polyproteins encoded by the gag, pol, and env genes into their functional components. RT is reverse transcriptase, and IN is an integrase present in some retroviruses (e.g., HIV-1) that facilitates insertion of the provirus into the host genome. The matrix protein (MA) is a Gag protein closely associated with the lipid of the envelope. The capsid protein (CA) forms the major internal structure of the virus, the core shell.

ENV LTR LTR GP46 p21 SU TM TAX, p40 REX, p27 POL p30 p12 p13 HBZ p23 VIF VPR VPU NEF p27 LTR GP120 GP41 SU ENV TM p15 p16 TAT, p14 REV, p19 ENV NEF LTR VIF VPX VPR TAT REV The lentiviruses in general, and HIV-1 and -2 in particular, contain a larger genome than other pathogenic retroviruses. They contain an untranslated region between pol and env that encodes portions of several proteins, varying with the reading frame into which the mRNA is spliced. Tat is a 14-kDa protein that augments the expression of virus from the LTR. The Rev protein of HIV-1, similar to the Rex protein of HTLV, regulates RNA splicing and/or RNA transport. The Nef protein downregulates CD4, the cellular receptor for HIV; alters host T-cell–activation pathways; and enhances viral infectivity. The Vif protein is necessary for the proper assembly of the HIV nucleoprotein core in many types of cells; without Vif, proviral DNA is not efficiently produced in these infected cells. In addition, the Vif protein targets APOBEC (apolipoprotein B mRNA-editing enzyme catalytic polypep­ tide, a cytidine deaminase that mutates the viral sequence) for pro­ teasomal degradation, thus blocking its virus-suppressing effect. Vpr, Vpu (HIV-1 only), and Vpx (HIV-2 only) are viral proteins encoded by translation of the same message in different reading frames. As noted above, oncogenic retroviruses depend on cell proliferation for their replication; lentiviruses can infect nondividing cells, largely through effects mediated by Vpr. Vpr facilitates transport of the provirus into the nucleus and can induce other cellular changes, such as G2 growth arrest and differentiation of some target cells. Vpx is structurally related to Vpr, but its functions are not fully defined. Vpu promotes the degra­ dation of CD4 in the endoplasmic reticulum and stimulates the release of virions from infected cells. Retroviruses can be either exogenously acquired (by infection with an infected cell or a free virion capable of replication) or transmitted in the germline as endogenous virus. Endogenous retroviruses are often replication defective. The human genome contains endogenous retroviral sequences, but there are no known replication-competent endogenous retroviruses in humans. In general, viruses that contain only the gag, pol, and env genes either are not pathogenic or take a long time to induce disease; these observations indicate the importance of the other regulatory genes in viral disease pathogenesis. The pathogenesis of neoplastic transforma­ tion by retroviruses relies on the chance integration of the provirus at a spot in the genome resulting in the expression of a cellular gene

(protooncogene) that becomes transforming by virtue of its unregu­ lated expression. For example, avian leukosis virus causes B-cell leu­ kemia by inducing the expression of myc. Some retroviruses possess captured and altered cellular genes near their integration site, and these viral oncogenes can transform the infected host cell. Viruses that have oncogenes often have lost a portion of their genome that is required for replication. Such viruses need helper viruses to reproduce, a feature that may explain why these acute transforming retroviruses are rare in nature. All human retroviruses identified to date are exogenous and are not acutely transforming (i.e., they lack a transforming oncogene). These remarkable properties of retroviruses have led to experimen­ tal efforts to use them as vectors to insert specific genes into particular cell types, a process known as gene therapy or gene transfer. The process could be used to repair a genetic defect or to introduce a new property that could be used therapeutically; for example, a gene (e.g., thymidine kinase) that would make a tumor cell susceptible to killing by a drug (e.g., ganciclovir) could be inserted. One source of concern about the use of retroviral vectors in humans is that replication-competent viruses might rescue endogenous retroviral replication, with unpre­ dictable results. This concern is not merely hypothetical: the detection of proteins encoded by endogenous retroviral sequences on the surface of cancer cells implies that the genetic events leading to the cancer were able to activate the synthesis of these usually silent genes. Lentiviruses are being widely examined as gene therapy tools to cor­ rect genetic defects (e.g., thalassemia, sickle cell disease, hemophilia) and to generate effective cancer therapies (e.g., chimeric antigen recep­ tor T cells, natural killer [NK] cells, and macrophages). The success of these therapies has been variable. Gene therapy interventions are often not permanent as expression of the introduced gene wanes over time. The random insertion of genetic material into multiple sites of the genome in some cells has resulted in insertional mutagenesis and the generation of novel malignancies. While gene therapies are overall only rarely associated with second malignancies, the expectation is that the use of directed insertion of genetic material into a specific site in the genome (e.g., with CRISPR technology) will reduce the risk. HUMAN T-CELL LYMPHOTROPIC VIRUS HTLV-1, a delta retrovirus, was isolated in 1980 from a T-cell lym­ phoma cell line from a patient originally thought to have cutaneous T-cell lymphoma. Later it became clear that the patient had a dis­ tinct form of lymphoma (originally reported in Japan) called adult T-cell leukemia/lymphoma (ATL). Serologic data have determined that HTLV-1 is the cause of at least two important diseases: ATL and tropical spastic paraparesis, also called HTLV-1-associated myelopathy (HAM). HTLV-1 may also play a role in infective dermatitis, arthritis, uveitis, and Sjögren’s syndrome. Two years after the isolation of HTLV-1, HTLV-2 was isolated from a patient with an unusual form of hairy cell leukemia that affected T cells. Epidemiologic studies of HTLV-2 failed to reveal a consistent disease association. Similarly, HTLV-3 and HTLV-4 have been identified but have no known disease association. ■ ■BIOLOGY AND MOLECULAR BIOLOGY Because the biology of HTLV-1 and that of HTLV-2 are similar, the following discussion will focus on HTLV-1. Human glucose transporter protein 1 (GLUT-1) functions as a receptor for HTLV-1, probably acting together with neuropilin-1 (NRP1) and heparan sulfate proteoglycans. The heparan sulfate proteoglycans do not appear to be involved with HTLV-2 cell entry. Generally, only T cells are productively infected, but infection of B cells and other cell types is occasionally detected. The most common outcome of HTLV-1 infection is latent carriage of randomly integrated provirus in CD4+ T cells. HTLV-1 does not contain an oncogene and does not insert into a unique site in the genome. Indeed, most infected cells express no viral gene products. The only viral gene product that is routinely expressed in tumor cells transformed by HTLV-1 in vivo is hbz. The tax gene is thought to be critical to the transformation process but is not expressed in the tumor cells of many ATL patients, possibly because of the immunogenicity of tax-expressing cells. Cells

transformed in vitro, by contrast, actively transcribe HTLV-1 RNA and produce infectious virions. Most HTLV-1-transformed cell lines are the result of the infection of a normal host T cell in vitro. It is difficult to establish cell lines derived from authentic ATL cells.

Although tax does not itself bind to DNA, it is located in the nucleus and induces the expression of a wide range of host cell gene products, including transcription factors (especially c-rel/nuclear factor κB [NF-κB], ets-1 and -2, and members of the fos/jun family), cytokines (e.g., interleukin [IL] 2, granulocyte-macrophage colony-stimulating factor, and tumor necrosis factor), membrane proteins and receptors (major histocompatibility [MHC] molecules and IL-2 receptor α), and chromatin remodeling complexes. The genes activated by tax are gen­ erally controlled by transcription factors of the c-rel/NF-κB and cyclic AMP response element binding (CREB) protein families. It is unclear how this induction of host gene expression leads to neoplastic trans­ formation; tax can interfere with G1 and mitotic cell-cycle checkpoints, block apoptosis, inhibit DNA repair, and promote antigen-independent T-cell proliferation. Induction of a cytokine-autocrine loop has been proposed; however, IL-2 is not the crucial cytokine. The involvement of IL-4, IL-7, and IL-15 has been proposed. In light of the irregular expression of tax in ATL cells, it has been suggested that tax is important in the early phases of transformation but is not essential for the maintenance of the transformed state. The maintenance role is thought to be due to hbz expression. As is clear from the epidemiology of HTLV-1 infection, transformation of an infected cell is a rare event and may depend on heterogeneous second, third, or fourth genetic hits. No consistent chromosomal abnormali­ ties have been described in ATL; however, aneuploidy is common, and individual cases with p53 mutations and translocations involving the T-cell receptor genes on chromosome 14 have been reported. Tax may repress certain DNA repair enzymes, permitting the accumula­ tion of genetic damage that would normally be repaired. However, the molecular pathogenesis of HTLV-1-induced neoplasia is not fully understood. CHAPTER 207 The Human Retroviruses ■ ■FEATURES OF HTLV-1 INFECTION Epidemiology  HTLV-1 infection is transmitted in at least three ways: from mother to child, especially via breast milk; through sexual activity, more commonly from men to women; and through the blood—via contaminated transfusions or contaminated needles. The virus is most commonly transmitted perinatally. Compared with HIV, which can be transmitted in cell-free form, HTLV-1 is less infectious, and its transmission usually requires cell-to-cell contact. HTLV-1 is endemic in southwestern Japan and Okinawa, where

1 million persons are infected. Antibodies to HTLV-1 are present in the serum of up to 35% of Okinawans, 10% of residents of the Japanese island of Kyushu, and <1% of persons in nonendemic regions of Japan. Despite this high prevalence of infection, only ~500 cases of ATL are diagnosed in this area each year. Clusters of infection have been noted in other areas of eastern Asia, such as Taiwan; in the Caribbean basin, including northeastern South America; in northwestern South America; in central and southern Africa; in Italy, Israel, Iran, and Papua New Guinea; in the Arctic; and in the southeastern part of the United States (Fig. 207-4). An estimated 10–20 million persons have HTLV-1 infection worldwide. Progressive spastic or ataxic myelopathy developing in an indi­ vidual who is HTLV-1 positive (i.e., who has serum antibodies to HTLV-1) may be due to direct infection of the nervous system with the virus, but destruction of the pyramidal tracts appears to involve HTLV-1-infected CD4+ T cells; a similar disorder may result from infection with HIV or HTLV-2. In rare instances, patients with HAM are seronegative but have detectable antibody to HTLV-1 in cerebro­ spinal fluid (CSF). The cumulative lifetime risk of developing ATL is 3% among HTLV1-infected patients, with a threefold greater risk among men than among women; a similar cumulative risk is projected for HAM (4%), but with women more commonly affected than men. The distribution of these two diseases overlaps the distribution of HTLV-1, with >95%

FIGURE 207-4  Global distribution of HTLV-1 infection. Countries with a prevalence of HTLV-1 infection of 1–5% are shaded darkly. Note that the distribution of infected patients is not uniform in endemic countries. For example, the people of southwestern Japan and northeastern Brazil are more commonly affected than those in other regions of those countries. of affected patients showing serologic evidence of HTLV-1 infection. The latency period between infection and the emergence of disease is 20–30 years for ATL. For HAM, the median latency period is ~3.3 years (range, 4 months to 30 years). The development of ATL is rare among persons infected by blood products; however, ~20% of patients with HAM acquire HTLV-1 from contaminated blood. ATL is more com­ mon among perinatally infected individuals, whereas HAM is more common among persons infected via sexual transmission. PART 5 Infectious Diseases Associated Diseases  •  ATL  Four clinical types of HTLV-1-induced neoplasia have been described: acute, lymphomatous, chronic, and smoldering. All of these tumors are monoclonal proliferations of CD4+ postthymic T cells with clonal proviral integrations and clonal T-cell receptor gene rearrangements. ACUTE ATL  About 60% of patients who develop malignancy have classic acute ATL, which is characterized by a short clinical pro­ drome (~2 weeks between the first symptoms and the diagnosis) and an aggressive natural history (median survival period, 6 months). The clinical picture is dominated by rapidly progressive skin lesions, pulmonary involvement, hypercalcemia, and lymphocytosis with cells containing lobulated or “flower-shaped” nuclei (see Fig. 113-7). The malignant cells have monoclonal proviral integrations and express CD4, CD3, and CD25 (low-affinity IL-2 receptors) on their surface. Serum levels of CD25 can be used as a tumor marker. Anemia and thrombocytopenia are rare. The skin lesions may be difficult to distin­ guish from those in mycosis fungoides. Lytic bone lesions, which are common, do not contain tumor cells but rather are composed of osteo­ lytic cells, usually without osteoblastic activity. Despite the leukemic picture, bone marrow involvement is patchy in most cases. The hypercalcemia of ATL is multifactorial; the tumor cells produce osteoclast-activating factors (tumor necrosis factor α, IL-1, lympho­ toxin) and can also produce a parathyroid hormone–like molecule. Affected patients have an underlying immunodeficiency that makes them susceptible to opportunistic infections similar to those seen in patients with AIDS (Chap. 208). The pathogenesis of the immunodefi­ ciency is unclear. Pulmonary infiltrates in ATL patients reflect leukemic infiltration half the time and opportunistic infections with organisms such as Pneumocystis and other fungi the other half. Gastrointestinal symptoms are nearly always related to opportunistic infection. Stron­ gyloides stercoralis is a gastrointestinal parasite that has a pattern of endemic distribution similar to that of HTLV-1. HTLV-1-infected persons also infected with this parasite may develop ATL more often or more rapidly than those without Strongyloides infections. Serum concentrations of lactate dehydrogenase and alkaline phosphatase are often elevated in ATL. About 10% of patients have leptomeningeal involvement leading to weakness, altered mental status, paresthesia, and/or headache. Unlike other forms of central nervous system (CNS) lymphoma, ATL may be accompanied by normal CSF protein levels.

The diagnosis depends on finding ATL cells in the CSF (Chap. 113). LYMPHOMATOUS ATL  The lymphomatous type of ATL occurs in ~20% of patients and is similar to the acute form in its natural history and clinical course, except that cir­ culating abnormal cells are rare and lymph­ adenopathy is evident. The histology of the lymphoma is variable but does not influence the natural history. In general, the diagnosis is suspected on the basis of the patient’s birthplace (see “Epidemiology,” above) and the presence of skin lesions and hypercal­ cemia. The diagnosis is confirmed by the detection of antibodies to HTLV-1 in serum. CHRONIC ATL  Patients with the chronic form of ATL generally have normal serum levels of calcium and lactate dehydrogenase and no involvement of the CNS, bone, or gastrointestinal tract. The median duration of survival for these patients is 2 years. In some cases, chronic ATL progresses to the acute form of the disease. SMOLDERING ATL  Fewer than 5% of patients have the smolder­ ing form of ATL. In this form, the malignant cells have monoclonal proviral integration; <5% of peripheral-blood cells exhibit typical morphologic abnormalities; hypercalcemia, adenopathy, and hepato­ splenomegaly do not develop; the CNS, the bones, and the gastroin­ testinal tract are not involved; and skin lesions and pulmonary lesions may be present. The median survival period for this small subset of patients appears to be ≥5 years. HAM (TROPICAL SPASTIC PARAPARESIS)  In contrast to ATL, in which there is a slight predominance of male patients, HAM affects female patients disproportionately. HAM resembles multiple sclerosis in certain ways (Chap. 455). The onset is insidious. Symptoms include weakness or stiffness in one or both legs, back pain, and urinary incontinence. Sen­ sory changes are usually mild, but peripheral neuropathy may develop. The disease generally takes the form of slowly progressive and unremit­ ting thoracic myelopathy; one-third of patients are bedridden within 10 years of diagnosis, and one-half are unable to walk unassisted by this point. Patients display spastic paraparesis or paraplegia with hyperre­ flexia, ankle clonus, and extensor plantar responses. Cognitive function is usually spared; cranial nerve abnormalities are unusual. Magnetic resonance imaging (MRI) reveals lesions in both the white matter and the paraventricular regions of the brain as well as in the spi­ nal cord. Pathologic examination of the spinal cord shows symmetric degeneration of the lateral columns, including the corticospinal tracts; some cases involve the posterior columns as well. The spinal meninges and cord parenchyma contain an inflammatory infiltrate that includes CD8+ T cells with myelin destruction. HTLV-1 is not usually found in cells of the CNS but may be detected in a small population of lymphocytes present in the CSF. In general, HTLV-1 replication is greater in HAM than in ATL, and patients with HAM have a stronger immune response to the virus. Antibodies to HTLV-1 are present in the serum and appear to be produced in the CSF of HAM patients, where titers are often higher than in the serum. The pathophysiology of HAM may involve the induction of autoim­ mune destruction of neural cells by T cells with specificity for viral components such as Tax or Env proteins. One theory is that suscep­ tibility to HAM may be related to the presence of human leukocyte antigen (HLA) alleles capable of presenting viral antigens in a fashion that leads to autoimmunity. Insufficient data are available to confirm an HLA association. However, antibodies in the sera of HAM patients have been shown to bind a neuron-specific antigen (heteronuclear ribonuclear protein A1 [hnRNP A1]) and to interfere with neurotrans­ mission in vitro. It is unclear what factors influence whether HTLV-1 infection will cause disease and, if it does, whether it will induce a neoplasm (ATL)

or an autoimmune disorder (HAM). Differences in viral strains, the susceptibility of particular MHC haplotypes, the route of HTLV-1 infection, the viral load, and the nature of the HTLV-1-related immune response are putative factors, but few definitive data are available. OTHER PUTATIVE HTLV-1-RELATED DISEASES  Even in the absence of the full clinical picture of HAM, bladder dysfunction is common in HTLV-1-infected women. In areas where HTLV-1 is endemic, diverse inflammatory and autoimmune diseases have been attributed to the virus, including uveitis, dermatitis, pneumonitis, rheumatoid arthritis, and polymyositis. However, a causal relationship between HTLV-1 and these illnesses has not been established. Prevention  Women in endemic areas should not breast-feed their children, and blood donors should be screened for serum antibodies to HTLV-1. As in the prevention of HIV infection, the practice of safe sex and the avoidance of needle sharing are important. A B Smouldering (5–10% of all cases) Chronic (10–20% of all cases) Features • ≥5% abnormal T cells • ≤4 × 109 lymphocytes per L • Calcium <2·74 mmol per L • LDH ≤1·5 × upper limit of normal • Skin and lung involvement only Features • ≥5% abnormal T cells • ≥4 × 109 lymphocytes per L • Calcium <2·74 mmol per L • LDH ≤2 × upper limit of normal • Lymphadenopathy and involvement of spleen, skin, liver, and lung only Prognosis • 4-year overall survival 52% • Median overall survival 55 months
Prognosis • 4-year overall survival 36% • Median overall survival 31·5 months Symptomatic IFN-α and AZTbased therapy, with or without ATO and topical therapies Asymptomatic Active monitoring Unfavourable chronic Favourable chronic If IFN- and AZT-based therapy is unavailable and tumorous lesions are present: Chemotherapy with or without topical therapies, followed by allogeneic HSCT IFN-α and AZT-based therapy with or without ATO, continued indefinitely If IFN- and AZT-based therapy is unavailable and non-tumorous lesions are present: Skin-directed therapies and active monitoring If IFN- and AZT-based therapy is unavailable, or PD on therapy: Chemotherapy followed by allogeneic HSCT If PD with tumorous lesions or progression during active monitoring or IFN- and AZT-based therapy: Switch strategy for aggressive disease Patients with ATLL who are older or unsuitable for transplantation: Following first-line therapy (reduced dose of chemotherapy or IFN-α and AZT-based therapy, if available), consider maintenance strategies such as sobuzoxane or etoposide, or IFN-α and AZT-based therapy with or without ATO (if available) FIGURE 207-5  An approach to HTLV-1-associated adult T-cell leukemia/lymphoma. ATLL, adult T-cell leukemia or lymphoma; ATO, arsenic trioxide; AZT, azidothymidine; CHOEP, cyclophosphamide, vincristine, doxorubicin, etoposide, and prednisolone; CHOP, cyclophosphamide, vincristine, doxorubicin, and prednisolone; DA-EPOCH, dose-adjusted etoposide, prednisolone, vincristine, and cyclophosphamide; HSCT, hematopoietic stem cell transplantation; Hyper-CVAD, cyclophosphamide, vincristine, doxorubicin, and dexamethasone with alternating high-dose methotrexate and cytarabine; LDH, lactate dehydrogenase; PD, progressive disease; VCAP-AMP-VECP, vincristine, cyclophosphamide, doxorubicin, and prednisolone; doxorubicin, ranimustine, and prednisolone; vindesine, etoposide, carboplatin, and prednisolone. (Reproduced with permission from JS O’Donnell et al: Integrated molecular and immunological features of human T-lymphotropic virus type 1 infection and disease progression to adult T-cell leukaemia or lymphoma. Lancet Haematology 10:e539, 2023.)

TREATMENT HTLV-1 Infection For the small number of patients who develop HTLV-1–related disease, therapies are not curative. In patients with the acute and lymphomatous types of ATL, the disease progresses rapidly. Hyper­ calcemia is generally controlled by glucocorticoid administration and cytotoxic therapy directed against the neoplasm. The tumor is highly responsive to combination chemotherapy that is used against other forms of lymphoma; however, patients are susceptible to over­ whelming bacterial and opportunistic infections, and ATL relapses within 4–10 months after remission in most cases (Fig. 207-5). The combination of interferon α and zidovudine may extend sur­ vival. Because viral replication is not clearly associated with ATL progression, zidovudine is probably effective through its cytotoxic C D Lymphoma (20–25% of all cases) Acute (55–60% of all cases) Features • ≤4 × 109 lymphocytes per L • ≤1% abnormal T cells • Lymphadenopathy diagnosed by histology with or without extranodal lesions Features • All remaining patients with leukaemic manifestations and tumour lesions not classified under any other subtype Prognosis • 4-year overall survival 11% • Median overall survival 8·3 months Prognosis • 4-year overall survival 16% • Median overall survival 10·6 months CHAPTER 207 If non-bulky lymph nodes or tumours: Either IFN-α and AZT-based therapy or intensive chemotherapy Extranodal primary cutaneous variant Intensive chemotherapy The Human Retroviruses If available: Concurrent or sequential lowdose IFN-α and AZT-based therapy If IFN-` and AZT-based therapy is unavailable, or PD on therapy, or if patient has bulky disease: Intensive chemotherapy If available: Early up-front allogeneic HSCT If available: Early up-front allogeneic HSCT Intensive chemotherapy with or without skin-directed therapy, followed by allogeneic HSCT or IFN-α and AZT-based therapy with or without ATO Chemotherapy: Japan: VCAP-AMP-VECP (modified LSG15), or EPOCH Elsewhere: CHOP, CHOEP, DA-EPOCH, or hyper-CVAD Relapsed and refractory disease: • Single-agent or alternative combination chemotherapy containing platinum, etoposide, and high-dose cytarabine • Localised radiotherapy (palliation) In Japan: Mogamulizumab and lenalidomide

99 - 208 Human Immunodeficiency Virus Disease- AIDS and Related Disorders

208 Human Immunodeficiency Virus Disease: AIDS and Related Disorders

effects (as a chain-terminating thymidine analogue) rather than its antiviral effects. Selected series have reported high rates of response and a 40% rate of 5-year survival; however, this level of response has not been universal. LSG15, a multidrug chemotherapy program developed in Japan, induces complete responses in about one-third of patients, about half of whom survive for >2 years; however, the median survival time is about 13 months. High-dose therapy with bone marrow transplantation has been widely tested in Japan. Median survival has not been influenced by this treat­ ment; however, up to 25% of patients survive free of disease for

4 years. Lenalidomide has been reported to have a 42% response rate in patients with relapsed ATL, extending median survival to

20 months despite a short 4-month progression-free survival period. Mogamulizumab, an antibody to CCR4 (a receptor for a number of chemokines, including RANTES and TARC), improved response rates when added to chemotherapy. An experimental approach using an yttrium-90-labeled or toxin-conjugated antibody to the IL-2 receptor appears promising but is not widely available. Patients with the chronic or smoldering form of ATL may be managed with an expectant approach: treat any infections, and watch and wait for signs of progression to acute disease.

Patients with HAM may obtain some benefit from the use of glucocorticoids to reduce inflammation during acute episodes of myelopathy. Antiretroviral regimens have not been effective. In one study, danazol (200 mg three times daily) produced significant neu­ rologic improvement in five of six treated patients, with resolution of urinary incontinence in two cases, decreased spasticity in three, and restoration of the ability to walk after confinement to a wheel­ chair in two. Antibody to IL-15 receptor β chain has been tested with some promising clinical effects in small numbers of patients. Physical therapy and rehabilitation are important components of management. PART 5 Infectious Diseases ■ ■FEATURES OF HTLV-2 INFECTION Epidemiology  HTLV-2 is endemic in certain Native American tribes and in Africa. It is generally considered to be a New World virus that was brought from Asia to the Americas 10,000–40,000 years ago during the migration of infected populations across the Bering land bridge. The mode of transmission of HTLV-2 is probably the same as that of HTLV-1 (see above). HTLV-2 may be less readily transmitted sexually than HTLV-1. Studies of large cohorts of injection drug users with serologic assays that reliably distinguish HTLV-1 from HTLV-2 indicated that the vast majority of HTLV-positive cohort members were infected with HTLV-2. The seroprevalence of HTLV in a cohort of 7841 injection drug users from drug treatment centers in Baltimore, Chicago, Los Angeles, New Jersey (Asbury Park and Trenton), New York City (Brooklyn and Harlem), Philadelphia, and San Antonio was 20.9%, with >97% of cases due to HTLV-2. The seroprevalence of HTLV-2 was higher in the Southwest and the Midwest than in the Northeast. In contrast, the seroprevalence of HIV-1 was higher in the Northeast than in the Southwest or the Midwest. Approximately 3% of the cohort mem­ bers were infected with both HTLV-2 and HIV-1. The seroprevalence of HTLV-2 increased linearly with age. Women were significantly more likely than men to be infected with HTLV-2; the virus is thought to be more effi­ ciently transmitted from male to female than from female to male. Associated Diseases  Although HTLV-2 was isolated from a patient with a T-cell variant of hairy cell leukemia, this virus has not been consistently associated with a particular disease and in fact has been thought of as “a virus searching for a disease.” However, evidence is accumulating that HTLV-2 may play a role in certain neurologic, hematologic, and dermatologic diseases. These data require confirma­ tion, particularly in light of the previous confusion regarding the rela­ tive prevalences of HTLV-1 and HTLV-2 among injection drug users. Prevention  Avoidance of needle sharing, adherence to safe-sex practices, screening of blood (by assays for HTLV-1, which also detect

HTLV-2), and avoidance of breast-feeding by infected women are important principles in the prevention of spread of HTLV-2. HUMAN IMMUNODEFICIENCY VIRUS HIV-1 and HIV-2 are members of the lentivirus subfamily of Retro­ viridae and are the only lentiviruses known to infect humans. The lentiviruses are slower-acting than viruses that cause acute infection (e.g., influenza virus) but not than other retroviruses. The features of acute primary infection with HIV resemble those of more classic acute infections. The characteristic chronicity of HIV disease is consistent with the designation lentivirus. For a detailed discussion of HIV, see Chap. 208. ■ ■FURTHER READING Forlani G et al: HTLV-1 infection and pathogenesis: New insights from cellular and animal models. Int J Mol Sci 22:8001, 2021. Katsuya H et al: Treatment and survival among 1594 patients with ATL. Blood 126:2570, 2015. Letafati A et al: Therapeutic approaches for HTLV-1-associated adult T-cell leukemia/lymphoma: A comprehensive review. Med Oncol 40:295, 2023. Ma G et al: Multifaceted functions and roles of HBZ in HTLV-1 patho­ genesis. Retrovirology 13:16, 2016. Moir S et al: Pathogenic mechanisms of HIV disease. Annu Rev Pathol 6:223, 2011. Ohmoto A, Fuji S: Non-cancerous complications in HTLV-1 carriers. Expert Rev Anti Infect Ther 22:307, 2024. Yamauchi J et al: An update on human T-cell leukemia virus type I (HTLV-1)-associated myelopathy/tropical spastic paraparesis (HAM/ TSP) focusing on clinical and laboratory biomarkers. Pharmacol Ther 218:107669, 2021.

Human Immunodeficiency

Virus Disease: AIDS and

H. Clifford Lane The Acquired Immune Deficiency Syndrome (AIDS) was first rec­ ognized in the United States in the summer of 1981, when the U.S. Centers for Disease Control and Prevention (CDC) reported the unex­ plained occurrence of Pneumocystis jirovecii (formerly P. carinii) pneu­ monia in five previously healthy homosexual men in Los Angeles and of Kaposi’s sarcoma (KS) with or without P. jirovecii pneumonia and other opportunistic infections in 26 previously healthy homosexual men in New York, San Francisco, and Los Angeles. The disease was soon recognized in male and female injection drug users; in hemophil­ iacs and blood transfusion recipients; among female sexual partners of men with AIDS; and among infants born to mothers with AIDS. In 1983, what became known as human immunodeficiency virus (HIV) was isolated from a patient with lymphadenopathy, and by 1984 HIV was demonstrated clearly to be the causative agent of AIDS. In 1985, a sensitive enzyme-linked immunosorbent assay (ELISA) was developed to detect antibodies to HIV. This led to an appreciation of the scope and evolution of the HIV epidemic at first in the United States and other developed nations and ultimately among developing nations throughout the world (see “HIV Infection and AIDS Worldwide,” below). The staggering worldwide evolution of the HIV pandemic has been matched by an explosion of information in the areas of HIV virology, pathogenesis (both immunologic and virologic), treatment of

HIV infection, treatment and prophylaxis of the opportunistic diseases associated with HIV infection, and prevention of HIV infection. The information flow related to HIV disease is enormous and continues to expand, and it has become almost impossible for the health care generalist to stay abreast of the literature. The purpose of this chapter is to present the most current information available on the scope of the pandemic, as well as the pathogenesis, treatment, and prevention of HIV disease. Above all, the aim is to provide a solid scientific basis and practical clinical guidelines for a state-of-the-art approach to the care of persons with HIV. ■ ■DEFINITION The current CDC classification system for HIV infection and AIDS categorizes patients based on clinical conditions associated with HIV infection together with the level of the CD4+ T lymphocyte count. A confirmed HIV case can be classified in one of three clinical stages (A, B, or C) and one of three CD4+ T lymphocyte categories (1, 2, or 3). Advanced HIV disease (AIDS) is classified as stage C, requiring the diagnosis of one or more specific opportunistic illnesses (Table 208-1), and as stage 3 in anyone over 6 years of age if the CD4+ T lymphocyte count is below 200 cells/µL (Table 208-2). The definition and staging criteria of AIDS are complex and com­ prehensive and were established for surveillance purposes rather than for the practical care of patients. Thus, the clinician should not focus on whether the patient fulfills the strict definition of AIDS but should view HIV disease as a spectrum ranging from primary infection, with or without the acute syndrome, to the relatively asymptomatic stage, to advanced stages associated with opportunistic diseases (see “Patho­ physiology and Pathogenesis,” below). TABLE 208-1  CDC Stage 3 (AIDS)-Defining Opportunistic Illnesses in HIV Infection Bacterial infections, multiple or recurrenta Candidiasis of bronchi, trachea, or lungs Candidiasis of esophagus Cervical cancer, invasiveb Coccidioidomycosis, disseminated or extrapulmonary Cryptococcosis, extrapulmonary Cryptosporidiosis, chronic intestinal (>1 month’s duration) Cytomegalovirus disease (other than liver, spleen, or nodes), onset at age >1 month Cytomegalovirus retinitis (with loss of vision) Encephalopathy attributed to HIV Herpes simplex: chronic ulcers (>1 month’s duration) or bronchitis, pneumonitis, or esophagitis (onset at age >1 month) Histoplasmosis, disseminated or extrapulmonary Isosporiasis, chronic intestinal (>1 month’s duration) Kaposi’s sarcoma Lymphoma, Burkitt’s (or equivalent term) Lymphoma, immunoblastic (or equivalent term) Lymphoma, primary, of brain Mycobacterium avium complex or Mycobacterium kansasii, disseminated or extrapulmonary Mycobacterium tuberculosis of any site, pulmonary,b disseminated, or extrapulmonary Mycobacterium, other species or unidentified species, disseminated or extrapulmonary Pneumocystis jirovecii (previously known as Pneumocystis carinii) pneumonia Pneumonia, recurrentb Progressive multifocal leukoencephalopathy Salmonella septicemia, recurrent Toxoplasmosis of brain, onset at age >1 month Wasting syndrome attributed to HIV aOnly among children age <6 years. bOnly among adults, adolescents, and children age ≥6 years. Source: MMWR 63(RR-03), April 11, 2014.

TABLE 208-2  CDC HIV Infection Stages 1–3 Based on Age-Specific CD4+ T Lymphocyte Count or CD4+ T Lymphocyte Percentage

of Total Lymphocytesa AGE ON DATE OF CD4 T+ LYMPHOCYTE TEST 6 YEARS

THROUGH ADULT <1 YEAR 1–5 YEARS STAGEa CELLS/lL % CELLS/lL % CELLS/lL %

≥1500 ≥34 ≥1000 ≥30 ≥500 ≥26

750–1499 26–33 500–999 22–29 200–499 14–25

<750 <26 <500 <22 <200 <14 aThe stage is based primarily on the CD4+ T lymphocyte count; the CD4+

T lymphocyte count takes precedence over the CD4+ T lymphocyte percentage,

and the percentage is considered only if the count is missing. Source: MMWR 63(RR-03), April 11, 2014. ETIOLOGIC AGENT HIV is the etiologic agent of AIDS; it belongs to the family of human retroviruses (Retroviridae) and the subfamily of lentiviruses (Chap. 207). Nononcogenic lentiviruses cause disease in other animal species, including sheep, horses, goats, cattle, cats, and monkeys. The four retroviruses known to cause human disease belong to two distinct groups: the human T lymphotropic viruses (HTLV)-1 and HTLV-2, which are transforming retroviruses; and the human immunodefi­ ciency viruses, HIV-1 and HIV-2, which cause cytopathic effects either directly or indirectly (Chap. 207). The most common cause of HIV disease throughout the world, and certainly in the United States, is HIV-1, which comprises several subtypes with different geographic distributions (see “Molecular Heterogeneity of HIV-1,” below). HIV-2 was first identified in 1986 in West African patients and was originally confined to West Africa. However, cases traced to West Africa or to sexual contacts with West Africans have been identified throughout the world. HIV-1 strains are broken down into one major group (M) and three minor groups (N, O, P), and HIV-2 viruses are divided into groups A through H. Each group likely derives from a separate transfer to humans from a nonhuman primate reservoir. HIV-1 viruses likely came from chimpanzees and/or gorillas, and HIV-2 from sooty mang­ abeys. The AIDS pandemic is primarily caused by the HIV-1 M group viruses. Although HIV-1 group O and HIV-2 viruses have been found in numerous countries, including those in the developed world, they have caused much more localized epidemics. Reported infections with group N and group P viruses are rare and confined almost entirely to residents of Cameroon or travelers from Cameroon. The taxonomic relationship between primate lentiviruses is shown in Fig. 208-1. CHAPTER 208 Human Immunodeficiency Virus Disease: AIDS and Related Disorders
■ ■MORPHOLOGY OF HIV Electron microscopy shows that the HIV virion is an icosahedral struc­ ture (Fig. 208-2) containing numerous external spikes formed by the two major envelope proteins that exist as a trimeric heterodimer, the external gp120 and the transmembrane gp41. The virion buds from the surface of the infected cell (Fig. 208-2A) and incorporates a variety of host cellular proteins into its lipid bilayer. The structure of HIV-1 is schematically diagrammed in Fig. 208-2B. ■ ■REPLICATION CYCLE OF HIV HIV is an RNA virus whose hallmark is the reverse transcription of its genomic RNA to proviral DNA by the enzyme reverse transcriptase. The replication cycle of HIV begins with the high-affinity binding via surface-exposed residues within the gp120 protein to its receptor on the host cell surface, the CD4 molecule (Fig. 208-3). The CD4 molecule is a 55-kDa protein found predominantly on a subset of T lymphocytes that are responsible for helper function in the immune system (Chap. 360). Once it binds to CD4, the gp120 protein under­ goes a conformational change that facilitates binding to one of two major co-receptors. The two major co-receptors for HIV-1 are CCR5 and CXCR4. Both receptors belong to the family of seven-transmem­ brane-domain G protein–coupled cellular receptors, and the use of one or the other or both receptors by the virus for entry into the cell is an important determinant of the cellular tropism of the virus. Cell-to-cell

SIV-SYK SIV-TAL SIV-MUS SIV-GSN HIV-2 and SIV-SMM/MAC A B SIV-VER SIV-GRI SIV-TAN SIV-DRL SIV-RCM SIV-MND2 SIV-SAB SIV-CPZ Pan troglodytes schweinfurthii HIV-1 P and O groups and SIV_Gorilla HIV-1 M and N groups and SIV-CPZ Pan troglodytes troglodytes 0.25 PART 5 Infectious Diseases FIGURE 208-1  A phylogenetic tree based on the nearly complete genomes (gag through nef genes) of primate immunodeficiency viruses. The scale (0.25) indicates a 25% phylogenetically corrected genetic distance at the nucleotide level. Clades in color represent viruses (HIV-1, HIV-2) identified in humans after relatively recent transfers from chimpanzee, gorilla, and sooty mangabey reservoirs. (Prepared by Brian Foley, PhD, of the HIV Sequence Database, Theoretical Biology and Biophysics Group, Los Alamos National Laboratory; additional information at www.hiv.lanl.gov/content/sequence/HelpDocs/subtypes.html.) spread is also facilitated by accessory molecules such as the C-type lectin receptor DC-SIGN expressed on certain dendritic cells (DCs) that bind to the HIV gp120 envelope protein, allowing virus captured on DCs to spread to CD4+ T cells. Following binding of the envelope protein to the CD4 molecule associated with the above-mentioned conformational change in the viral envelope gp120, which facilitates binding to a co-receptor, fusion with the host cell membrane occurs as the newly exposed gp41 molecule penetrates the plasma membrane of the target cell and then folds upon itself to bring the virion and target cell together (Fig. 208-4). Following fusion, uncoating of the capsid protein shell is initiated—a step that facilitates reverse transcription and leads to formation of the preintegration complex, composed of viral RNA, enzymes, and accessory proteins and surrounded by capsid and matrix proteins (Fig. 208-3). All these post-fusion viral components constitute the HIV replication complex, including the outer capsid shell, which plays an integral role in supporting reverse transcription of viral RNA. As the preintegration complex traverses the cytoplasm to reach the nucleus, the viral reverse transcriptase enzyme catalyzes the reverse transcription of the genomic RNA into DNA, resulting in the formation of double-stranded HIV proviral DNA. At several steps of the replication cycle, the virus is vulnerable to various cellular factors that can block the progression of infection. The cytoplasmic tripartite motif-containing protein 5α (TRIM5α) is a host restriction factor that interacts with retroviral capsids, causing their premature disassembly and induction of innate immune responses. The apolipoprotein B mRNA editing enzyme (catalytic polypeptidelike 3 [APOBEC3]) family of cellular proteins also inhibits progression of virus infection after virus has entered the cell and prior to entering the nucleus. APOBEC3 proteins, which are incorporated into virions and released into the cytoplasm of a newly infected cell, bind to the single minus-strand DNA intermediate and deaminate viral cytidine, causing hypermutation of retroviral genomes. HIV has evolved a

powerful strategy to protect itself from APO­ BEC. The viral protein Vif targets APOBEC3 for proteasomal degradation. SAMHD1 is another post-entry host factor that prevents reverse transcription by depleting pools of deoxynucleotides (dNTPs). The type I inter­ feron (IFN)-induced myxovirus resistance protein 2 (MX2) is another restriction factor associated with innate immunity that inhib­ its HIV-1 nuclear entry. SIV-MON SIV-ASC SIV-DEN SIV-DEB SIV-COL SIV_LST SIV-SUN SIV-MND1 Following reverse transcription, the pro­ viral DNA accesses the nuclear pore and is transferred from the cytoplasm to the nucleus, where it is integrated into a host cell chromo­ some through the action of another virally encoded enzyme, integrase (Fig. 208-3). HIV proviral DNA preferentially integrates into the host genomic DNA in regions of active transcription. This provirus may remain tran­ scriptionally inactive (latent) or may manifest varying levels of gene expression, up to active transcription and production of virus. SIV-OLC SIV-WRC Cellular activation plays an important role in the replication cycle of HIV and is critical to the pathogenesis of HIV disease (see “Pathophysiology and Pathogenesis,” below). Following initial binding, fusion, and internalization of the nucleic acid contents of virions into the target cell, incompletely reverse-transcribed DNA intermediates are labile in quiescent cells and do not integrate efficiently into the host cell genome unless cellular activation occurs shortly after infec­ tion. Furthermore, some degree of activation of the host cell is required for the initiation of transcription of the integrated proviral DNA into either genomic RNA or mRNA. This latter process may not necessarily be associated with the detect­ able expression of the classic cell-surface markers of activation. This is reflected in the fact that cell-associated HIV RNA transcribed from competent or defective proviruses can be detected in infected resting CD4+ T cells. In this regard, activation of HIV expression from the latent state depends on the interaction of various cellular and viral fac­ tors. Following transcription, HIV mRNA is translated into proteins that undergo modification through glycosylation, myristoylation, phosphorylation, and cleavage. The viral particle is formed by the assembly of HIV proteins, enzymes, and genomic RNA at the plasma membrane of the cells. Budding of the progeny virion through the lipid bilayer of the host cell membrane is the point at which the core acquires its external envelope and where the host restriction factor tetherin can inhibit the release of budding particles. Tetherin is an IFN-induced type II transmembrane protein that interferes with virion detachment. The HIV accessory protein vpu counteracts this effect through direct interactions with tetherin. During or soon after budding, the virally encoded protease catalyzes the cleavage of the gag-pol precursor to yield the mature virion. Progression through the virus replication cycle is profoundly influenced by a variety of viral regulatory gene products. Likewise, each point in the replication cycle of HIV is a real or potential target for therapeutic intervention. Thus far, the reverse transcriptase, protease, and integrase enzymes as well as the process of virus–target cell binding and fusion have proved to be susceptible to pharmacologic disruption. ■ ■HIV GENOME Figure 208-5 illustrates schematically the arrangement of the HIV genome. Like other retroviruses, HIV-1 has genes that encode the structural proteins of the virus: gag encodes the proteins that form the core of the virion (including p24 antigen); pol encodes the enzymes responsible for protease processing of viral proteins, reverse

A B C FIGURE 208-2  A. Transmission electron micrograph of HIV-1 virus particles (colorized yellow/gold) replicating from an HIV-infected H9 T cell (purple). Budding virus particles that have not yet separated from the cell appear as semicircles. A separated, spherical immature particle is seen at the center of the image. (Image captured at the NIAID Integrated Research Facility [IRF] in Fort Detrick, Maryland. Courtesy of NIAID.) B. Structure of HIV-1, including the gp120 envelope, gp41 transmembrane components of the envelope, genomic RNA, enzyme reverse transcriptase, p18(17) inner membrane (matrix), and p24 core protein (capsid). (Courtesy of George V. Kelvin. Adapted from RC Gallo: Sci Am 256:46, 1987.) C. Scanning electron micrograph of a human H9 T cell (blue/green) infected with HIV-1 virus particles (yellow). (Image captured at NIAID Rocky Mountain Laboratories in Hamilton, Montana. Courtesy of NIAID.) transcription, and integration; and env encodes the envelope glycopro­ teins. However, HIV-1 is more complex than other retroviruses, par­ ticularly those of the nonprimate group, in that it also contains at least six other regulatory genes (tat, rev, nef, vif, vpr, and vpu), which code for proteins involved in the modification of the host cell to enhance virus growth and the regulation of viral gene expression. Several of these proteins are thought to play a role in the pathogenesis of HIV disease; their various functions are listed in Fig. 208-5. Flanking these genes are long terminal repeats (LTRs), which contain regulatory ele­ ments involved in gene expression (Fig. 208-5). The major difference between the genomes of HIV-1 and HIV-2 is the fact that HIV-2 lacks the vpu gene and has a vpx gene not contained in HIV-1. ■ ■MOLECULAR HETEROGENEITY OF HIV-1 Molecular analyses of HIV isolates reveal varying levels of sequence diversity over all regions of the viral genome. For example, the degree of difference in the coding sequences of the viral envelope protein ranges from a few percent (very close, among isolates from the same infected individual) to more than 50% (extreme diversity, between isolates from the different groups of HIV-1: M, N, O, and P). The changes tend to cluster in hypervariable regions. HIV can evolve by several means, including simple base substitution, insertions and

gp41 Matrix Lipid membrane Capsid RNA gp120 Reverse transcriptase CHAPTER 208 Human Immunodeficiency Virus Disease: AIDS and Related Disorders
deletions, recombination, and gain and loss of glycosylation sites. HIV sequence diversity arises directly from the limited fidelity of the reverse transcriptase, i.e., a tendency toward copying errors. The balance of immune pressure and functional constraints on proteins influences the regional level of variation within proteins. For example, envelope, which is exposed on the surface of the virion and is under immune selective pressure from both antibodies and cytolytic T lymphocytes, is extremely variable, with clusters of mutations in hypervariable domains. In contrast, reverse transcriptase, with important enzymatic functions, is relatively conserved in the absence of drug-induced selec­ tive pressure, particularly around the active site. The extraordinary variability of HIV-1 contrasts markedly with the relative stability of HTLV-1 and 2. The four groups (M, N, O, and P) of HIV-1 are the result of four separate chimpanzee-to-human (or possibly gorilla-to-human for groups O and P) transfers. Group M (major), which is responsible for most infections in the world, has diversified into subtypes and inter­ subtype recombinant forms, due to “sub-epidemics” within humans after one of those transfers. Among primate lentiviruses, HIV-1 is most closely related to viruses isolated from chimpanzees and gorillas (Fig. 208-1). The chimpanzee subspecies Pan troglodytes troglodytes has been established to be a

1 Binding and fusion to the host cell surface. HIV gp120 CD4 Co-receptor (CCR5 or CXCR4) Host Cell

Viral DNA is

transported across the nucleus and integrates into the host DNA. PART 5 Infectious Diseases Mature Virion

The virus matures

after protease cleaves long precursor proteins

New viral RNA

and proteins move to the cell surface and an immature virion begins to form. Protease FIGURE 208-3  The replication cycle of HIV. See text for description. (From the National Institute of Allergy and Infectious Diseases.) HIV virion gp41 gp120 CD4 CCR5/ CXCR4 Membrane fusion CD4+ T cell Receptor binding

HIV RNA, reverse transcriptase, integrase, and other viral proteins enter the host cell. Preintegration complex

Viral DNA is

formed by reverse transcription. Viral RNA Reverse transcriptase Integrase Viral DNA Host DNA New viral RNA

New viral RNA is

used as genomic RNA and to make viral proteins. FIGURE 208-4  Binding and fusion of HIV-1 with its target cell. HIV-1 binds to its target cell via the CD4 molecule, leading to a conformational change in the gp120 molecule that allows it to bind to the co-receptor CCR5 (for R5-using viruses). The virus then firmly attaches to the host cell membrane in a coiled-spring fashion via the newly exposed gp41 molecule. Virus–cell fusion occurs as the transitional intermediate of gp41 undergoes further changes to form a hairpin structure that draws the two membranes into close proximity (see text for details). (Adapted from D Montefiori, JP Moore: HIV vaccines. Magic of the occult? Science 283:336, 1999.)

LTR Long terminal repeat vif Viral infectivity factor (p23) Overcomes inhibitory effects of APOBEC3, preventing hypermutation and viral DNA degradation Contains control regions that bind host transcription factors (NF-κβ, NFAT, Sp. 1, TBP) Required for the initiation of transcription Contains RNS trans-acting response element (TAR) that binds Tat R U5 U3

R U5 U3

pol Polymerase Encodes a variety of viral enzymes, including PR (p10), RT and RNAase H (p66/51), and IN (p32) all processed by PR gag Pr55gag Polyprotein processed by PR MA, matrix (p17) Undergoes myristoylation that helps target gag polyprotein to lipid rafts CA capsid (p24) Binds cyclophilin A and CPSF6 Target of TRIM5α NC, nucleocapsid (p7) Zn finger, RNA-binding protein p6 Regulates the terminal steps in virion budding through interactions with TSG101 and ALIX 1 Incorporates Vpr into viral particles FIGURE 208-5  Organization of the genome of the HIV provirus together with a summary description of its 9 genes encoding 15 proteins. (Reproduced with permission from WC Greene et al: Charting HIV’s remarkable voyage through the cell: Basic science as a passport to future therapy. Nat Med 8:673, 2002.) natural reservoir of the HIV-1 M and N groups. The rare viruses of the HIV-1 O and P groups are most closely related to viruses found in Cameroonian gorillas. The M group comprises ten subtypes, or clades, designated A, B, C, D, F, G, H, J, K, and L, as well as more than 150 known circulating recombinant forms (CRFs) and numerous unique recombinant forms. Inter-subtype recombinants are generated by infection of an individual with two subtypes that then recombine and create a virus with a selective advantage. These CRFs range from highly prevalent forms—such as CRF01_AE, common in southeast Asia, and CRF02_AG in west and central Africa—to numerous CRFs that are relatively rare, either because they are of a more recent origin (newly recombined) or because they have not broken out into a major population. The subtypes and CRFs constitute the major lineages of the M group of HIV-1. HIV-1 M group subtype C dominates the global pandemic, and although there is speculation that it is more transmis­ sible than other subtypes, solid data on variations in transmissibility between subtypes are lacking. Human population densities, access to prevention and treatment, prevalence of genital ulcers, iatrogenic transmissions, and other confounding host factors are all possible rea­ sons why one subtype has spread more than another. Figure 208-6 schematically diagrams the worldwide distribution of HIV-1 subtypes by region. Nine strains account for most new HIV infections globally: HIV-1 subtypes A, B, C, D, F, and G and three of the CRFs, CRF01_AE, CRF02_AG, and CRF07_BC. Subtype C viruses (of the M group) are by far the most common form, likely accounting for ~50% of infections worldwide. In sub-Saharan Africa, home to approx­ imately two-thirds of all people living with HIV/AIDS, most infections

vpu Viral protein U Promotes CD4 degradation and influences virion release Overcomes inhibitory effects of tetherin env gp160 envelope protein Cleaved in endoplasmic reticulum to gp120 (SU) and gp 41 (TM) gp120 mediates CD4 and chemokine receptor binding, while gp41 mediates fusion Contains RNA response element (RRE) that binds Rev nef Negative effector (p27) Promotes downregulation of surface CD4 and MHC 1 expression Blocks apoptosis Enhances viron activity Alters state of cellular activation Progression to disease slowed significantly in absence of Nef vpr Viral protein R (p15) Promotes G2 cell-cycle arrest Facilitates HIV infection of macrophages tat Transcriptional activator (p14) Binds TAR In presence of host cyclin T1 and CDK9 enhances RNA Pol II elongation on the viral DNA template rev Regulator of viral gene expression (p19) Binds RRE Inhibits viral RNA splicing and promotes nuclear export of incompletely spliced viral RNAs CHAPTER 208 Human Immunodeficiency Virus Disease: AIDS and Related Disorders
are caused by subtype C, with smaller proportions of infections caused by subtype A, subtype D, CRF02_AG, and other subtypes and recombinants. In South Africa, the country with the largest number of prevalent infections (7.7 million in 2023), 98% of all HIV-1 isolates sequenced are of subtype C. In Asia, HIV-1 isolates of the CRF01_AE lineage and subtypes B and C predominate. CRF01_AE accounts for most infections in south and southeast Asia, while ~97% of infections in India, home to an estimated 2.5 million people with HIV, are of sub­ type C (see “HIV Infection and AIDS Worldwide,” below). Subtype B viruses are overwhelmingly predominant in the United States, Canada, certain countries in South America, western Europe, and Australia. It is thought that, purely by chance, subtype B was seeded into the United States and Europe in the late 1970s, thereby establishing an overwhelming founder effect. Many countries have co-circulating viral subtypes that are giving rise to new CRFs. Sequence analyses of HIV-1 isolates from infected individuals indicate that recombination among viruses of different clades likely occurs when an individual is infected with viruses of more than one subtype, particularly in geographic areas where subtypes overlap, and more often in sub-epidemics driven by injection drug use than in those driven by sexual transmission. The extraordinary diversity of HIV, reflected by the presence of multiple subtypes, circulating recombinant forms, and continuous viral evolution, has implications for possible differential rates of transmission, rates of disease progression, and the development of resistance to antiret­ roviral drugs. This diversity may also prove to be a formidable obstacle to HIV vaccine development, as a broadly useful vaccine would need to induce protective responses against a wide range of viral strains.

76° 76° 38° 38° 0° 0° –38° –38° 5000 km –76° –76° PART 5 Infectious Diseases World FIGURE 208-6  Global geographic distribution of HIV-1 subtypes and recombinant forms. Distributions derived from relative frequency of subtypes among >1.16 million HIV genomic sequences in the Los Alamos National Laboratory HIV Sequence Database. (Additional information available at www.hiv.lanl.gov/components/sequence/HIV/geo/ geo.comp.) TRANSMISSION HIV is transmitted primarily by sexual contact (both heterosexual and male to male); by blood and blood products; and by infected mothers to infants intrapartum, perinatally, or via breast milk. After four decades of experience and observations, there is no evidence that HIV is trans­ mitted by any other modality. Table 208-3 lists the estimated risk of HIV transmission for various types of exposures. ■ ■SEXUAL TRANSMISSION HIV infection is predominantly a sexually transmitted infection (STI) worldwide. By far the most common mode of infection, particularly in developing countries, is heterosexual transmission, although in many western countries male-to-male sexual transmission dominates. Although a wide variety of factors including viral load and the presence of ulcerative genital diseases influence the efficiency of heterosexual transmission of HIV, such transmission is generally inefficient. A systemic review found a low per-act risk of heterosexual transmission in the absence of antiretrovi­ rals: 0.04% for female-to-male transmission and 0.08% for male-to-female transmission during vaginal intercourse in the absence of antiretroviral therapy or condom use (Table 208-3). As discussed below, this risk approaches zero in settings where the infected partner is on effective antiretroviral therapy or when the uninfected partner is on a program of pre-exposure or postexposure prophylaxis (PrEP or PEP). HIV has been demonstrated in seminal fluid both within infected mononuclear cells and in cell-free material. The virus appears to

01_AE

7.9% 02_AG

2.6% 07_BC

2.3% A

5.1% A6

1.8% B 629459 54.0% BF1

1.0% C 169330 14.5% D

2.8% F

1.2% Other

6.7% Total 1166040 100.0% TABLE 208-3  Estimated Per-Act Probability of Acquiring HIV from an Infected Source, By Exposure Act TYPE OF EXPOSURE RISK PER 10,000 EXPOSURES Parenteral Blood transfusion

Needle-sharing during injection drug use

Percutaneous (needle-stick)

Sexual Receptive anal intercourse

Insertive anal intercourse

Receptive penile-vaginal intercourse

Insertive penile-vaginal intercourse

Receptive oral intercourse Low Insertive oral intercourse Low Othera Biting Negligible Spitting Negligible Throwing body fluids (including semen or saliva) Negligible Sharing sex toys Negligible aHIV transmission through these exposure routes is technically possible but unlikely and not well documented. Source: CDC, www.cdc.gov/hivpartners/php/riskandprevention/.

concentrate in the seminal fluid, particularly in situations where there are increased numbers of lymphocytes and monocytes in the fluid, as seen in genital inflammatory states such as urethritis and epididymitis, conditions closely associated with other STIs. The virus has also been demonstrated in cervical smears and vaginal fluid. There is an elevated risk of HIV transmission associated with unprotected receptive anal intercourse (URAI) among both men and women compared to the risk associated with unprotected receptive vaginal intercourse. Although data are limited, the per-act risk for HIV transmission via URAI has been estimated to be ~1.4% (Table 208-3). The risk of HIV acquisition associated with URAI is higher than that seen in penile–vaginal inter­ course probably because only a thin, fragile rectal mucosal membrane separates the deposited semen from potentially susceptible cells in and beneath the mucosa, and microtrauma of the mucosal membrane has been associated with anal intercourse. Anal douching and sexual prac­ tices that traumatize the rectal mucosa also increase the likelihood of infection. It is likely that anal intercourse provides at least two modali­ ties of infection: (1) direct inoculation into blood in cases of traumatic tears in the mucosa; and (2) infection of susceptible target cells, such as Langerhans cells, in the mucosal layer in the absence of trauma. Insertive anal intercourse also confers an increased risk of HIV acqui­ sition compared with insertive vaginal intercourse in the receptive partner since the vaginal mucosa is several layers thicker than the rectal mucosa and less likely to be traumatized during intercourse. Nonethe­ less, the virus can be transmitted to either partner through vaginal intercourse. As noted in Table 208-3, male-to-female HIV transmission is more efficient than female-to-male transmission. The differences in reported transmission rates between men and women may be due in part to the prolonged exposure of the vaginal and cervical mucosa to infected seminal fluid; the endometrium also can be exposed to virus when semen enters through the cervical os. By comparison, the penis and urethral orifice of the uninfected male partner are only exposed relatively briefly to infected vaginal fluid. Among various cofactors examined in studies of heterosexual HIV transmission, the presence of other STIs has been strongly associated with HIV transmission. In this regard, there is a close association between genital ulcerations and transmission, owing to both susceptibility to infection and infectivity. Infections with micro­ organisms such as Treponema pallidum (Chap. 187), Haemophilus ducreyi (Chap. 162), and herpes simplex virus (HSV; Chap. 197) are important causes of genital ulcerations linked to transmission of HIV. In addition, pathogens responsible for non-ulcerative inflam­ matory STIs such as those caused by Chlamydia trachomatis (Chap. 194), Neisseria gonorrhoeae (Chap. 161), and Trichomonas vaginalis (Chap. 236) also are associated with an increased risk of transmission of HIV infection. Bacterial vaginosis, an infection related to sexual behavior, but not strictly an STI, also may be linked to an increased risk of transmission of HIV infection. Multiple studies have suggested that treating STIs and genital tract syndromes may help decrease transmission of HIV. This effect is most prominent in populations in which the prevalence of HIV infection is relatively low. It is notewor­ thy that this principle may not apply to the treatment of HSV infec­ tions since it has been shown that even following anti-HSV therapy with resulting healing of HSV-related genital ulcers, HIV acquisition is not reduced. Biopsy studies revealed that the likely explanation is that HIV receptor–positive inflammatory cells persisted in the genital tissue despite the healing of ulcers, and so HIV-susceptible targets remained at the site. The quantity of HIV-1 in plasma (viral load) is a primary deter­ minant of the risk of HIV-1 transmission. In a cohort of heterosexual couples in Uganda discordant for HIV infection and not receiving anti­ retroviral therapy, the mean serum HIV RNA level was significantly higher among individuals with HIV whose partners seroconverted than among those whose partners did not seroconvert. In fact, trans­ mission was rare when the infected partner had a plasma level of <1700 copies of HIV RNA per milliliter, even when genital ulcer disease was present (Fig. 208-7). The rate of HIV transmission per coital act was highest during the early stage of HIV infection when plasma HIV RNA levels were high and in advanced disease with high viral set points.

No genital ulcer disease Genital ulcer disease Probability of transmission

per 10,000 coital acts

<1700 <1700– 12,499 12,500– 38,499 ≥38,500 HIV load of infected partner, RNA copies/mL FIGURE 208-7  Probability of HIV transmission per coital act among monogamous, heterosexual, HIV-serodiscordant couples in Uganda. (From RH Gray et al: Lancet 357:1149, 2001.) Antiretroviral therapy dramatically reduces plasma viremia in most people with HIV (see “Antiretroviral Therapy” and “HIV Prevention,” below) and is associated with a dramatic reduction in risk of transmis­ sion, an approach widely referred to as treatment as prevention or TasP. Multiple studies have demonstrated that if the viral load of a person with HIV is reduced by antiretroviral therapy to <20 copies/mL as measured by conventional commercial assays, there is essentially no chance of sexual transmission to the person’s sexual partner. This is true for heterosexuals as well as men who have sex with men, leading to the commonly used description of this phenomenon as “undetectable equals untransmittable” or “U = U.” CHAPTER 208 Multiple studies including large, randomized, controlled trials clearly have indicated that male circumcision is associated with a lower risk of acquisition of HIV infection for heterosexual men. Studies also suggest that circumcision is protective against HIV acquisition for men who have sex with men reporting mainly or only insertive sex. The benefit of circumcision may be due to increased susceptibility of uncircumcised men to ulcerative STIs, as well as to other factors such as microtrauma to the foreskin and glans penis. In addition, the highly vascularized inner layer of foreskin tissue contains a high density of Langerhans cells as well as increased numbers of CD4+ T cells, macro­ phages, and other cellular targets for HIV. Finally, the moist environ­ ment under the foreskin may promote the presence or persistence of microbial flora that, via inflammatory changes, may lead to even higher concentrations of target cells for HIV in the foreskin. In addition, ran­ domized clinical trials have demonstrated that male circumcision also reduces HSV type 2, human papillomavirus virus (HPV), and genital ulcer disease in men as well as HPV, genital ulcer disease, bacterial vaginosis, and Trichomonas vaginalis infections among female partners of circumcised men. Thus, there may be an added indirect benefit of diminution of risk for HIV acquisition to the female sexual partners of circumcised men. Human Immunodeficiency Virus Disease: AIDS and Related Disorders
In some studies, the use of oral contraceptives was associated with an increase in incidence of HIV infection over and above that which might be expected by not using a condom for birth control. This phe­ nomenon may be due to drug-induced changes in the cervical mucosa, rendering it more vulnerable to penetration by the virus. Adolescent girls might also be more susceptible to infection upon exposure due to the properties of an immature genital tract with increased cervical ectopy or exposed columnar epithelium. Oral sex is a much less efficient mode of transmission of HIV than is anal intercourse or vaginal intercourse (Table 208-3). Multiple studies have reported that the incidence of transmission of infection by oral sex among couples discordant for HIV is extremely low. However, there have been well-documented reports of HIV transmission that likely resulted from fellatio or cunnilingus. Therefore, the assumption that oral sex is completely safe is not warranted. The association of alcohol consumption and illicit drug use with unsafe sexual behavior, both homosexual and heterosexual, leads to an increased

risk of sexual transmission of HIV. Methamphetamine and other so-called “club drugs” such as 3,4-methylenedioxymethamphetamine (MDMA; also known as “ecstasy”), ketamine, gamma-hydroxybutyrate (GHB), and inhaled nitrites (known as “poppers”), sometimes taken in conjunction with PDE-5 inhibitors such as sildenafil (Viagra), tadalafil (Cialis), or vardenafil (Levitra), have been associated with risky sexual practices and increased risk of HIV infection, particularly among men who have sex with men.

■ ■TRANSMISSION THROUGH INJECTION

DRUG USE HIV can be transmitted to injection drug users (IDUs) who are exposed to HIV while sharing injection paraphernalia such as needles, syringes, the water in which drugs are mixed, or the cotton through which drugs are filtered. Parenteral transmission of HIV during injec­ tion drug use does not require IV puncture; subcutaneous (“skin pop­ ping”) or intramuscular (“muscling”) injections can transmit HIV as well, even though these behaviors are sometimes erroneously perceived as low risk. Among IDUs, the risk of HIV infection increases with the duration of injection drug use; the frequency of needle sharing; the number of partners with whom paraphernalia are shared; comorbid psychiatric conditions such as antisocial personality disorder; the use of cocaine in injectable form or smoked as “crack”; and the use of injection drugs in a geographic location with a high prevalence of HIV infection. As noted in Table 208-3, the per-act risk of transmission from injection drug use with a contaminated needle has been estimated to be approximately 0.6%. ■ ■TRANSMISSION BY TRANSFUSED BLOOD

AND BLOOD PRODUCTS HIV can be transmitted to individuals who receive HIV-contaminated blood transfusions, blood products, or transplanted tissue. The vast majority of HIV infections acquired via contaminated blood trans­ fusions, blood components, or transplanted tissue in resource-rich countries occurred prior to the spring of 1985, when mandatory test­ ing of donated blood for HIV-1 was initiated. It is estimated that >90% of individuals exposed to HIV-contaminated blood products become infected (Table 208-3). Transfusions of whole blood, packed red blood cells, platelets, leukocytes, and plasma are all capable of transmitting HIV infection. In contrast, hyperimmune gamma globulin, hepatitis B

immune globulin, plasma-derived hepatitis B vaccine, and Rho immune globulin have not been associated with transmission of HIV infection. The procedures involved in processing these products either inactivate or remove the virus. PART 5 Infectious Diseases Currently, in the United States and in most developed countries, the following measures have made the risk of transmission of HIV infection by transfused blood or blood products extremely small: the screening of blood donations for antibodies to HIV-1 and HIV-2 and determination of the presence of HIV nucleic acid usually in minipools of several specimens; the careful selection of potential blood donors with health history questionnaires to exclude individuals with risk behaviors; and opportunities for self-deferral and the screening out of HIV-negative individuals with serologic testing for infections that have shared risk factors with HIV, such as hepatitis B and C and syphilis. The chance of infection of a hemophiliac via clotting factor concen­ trates has essentially been eliminated because of standard screening of blood together with the added layer of safety resulting from heat treatment of the concentrates. It is currently estimated that the risk of infection with HIV in the United States via transfused screened blood is approximately 1 in 2 million units. Since nearly 21 million blood components are transfused in the United States each year, eliminat­ ing the risk of transfusion-related HIV transmission likely will not be possible. Transmission of HIV (both HIV-1 and HIV-2) by blood or blood products is still an ongoing threat in certain developing coun­ tries where routine screening of blood is not universally practiced. Furthermore, there have been reports in certain countries of sporadic breakdowns in routinely available screening procedures in which con­ taminated blood or plasma was transfused, resulting in small clusters of patients becoming infected.

■ ■OCCUPATIONAL TRANSMISSION OF HIV:

HEALTH CARE WORKERS, LABORATORY

WORKERS, AND THE HEALTH CARE SETTING There is a small but definite occupational risk of HIV transmission to health care workers and laboratory personnel and potentially others who work with HIV-containing materials, particularly when sharp objects are used. More than 300,000 health care workers are stuck with needles or other sharp medical instruments in the United States each year. The global number of HIV infections among health care workers attributable to sharps injuries has been estimated to be 1000 cases (range, 200–5000) per year. In the United States, a total of 58 documented cases of occupational HIV transmission to health care workers, and 150 possible transmissions, have been reported by the CDC. Since 1999, only one confirmed case (a laboratory technician sustaining a needle puncture while working with a live HIV culture in 2008) has been reported. Exposures that place a health care worker at potential risk of HIV infection are percutaneous injuries (e.g., a needle stick or cut with a sharp object) or contact of mucous membrane or nonintact skin (e.g., exposed skin that is chapped, abraded, or afflicted with dermatitis) with blood, tissue, or other potentially infectious body fluids. Large, multi-institutional studies have indicated that the risk of HIV trans­ mission following skin puncture from a needle or a sharp object that was contaminated with blood from a person with documented HIV infection is ~0.23%, and after a mucous membrane exposure it is ~0.09% (see “HIV and the Health Care Worker,” below) if the injured and/or exposed person is not treated within 24 hours with antiretro­ viral drugs. The risk of hepatitis B virus (HBV) infection following a similar type of exposure is ~6–30% in nonimmune individuals. If a susceptible worker is exposed to HBV, postexposure prophylaxis with hepatitis B immune globulin and initiation of HBV vaccine is >90% effective in preventing HBV infection. The risk of HCV infection fol­ lowing percutaneous injury is ~1.8% (Chap. 350). Rare HIV transmission after nonintact skin exposure has been documented. The average risk for transmission by this route has not been precisely determined; however, it is estimated to be less than the risk for mucous membrane exposure. Transmission of HIV through intact skin has not been documented. All health care workers experi­ encing a puncture wound or mucous membrane exposures involving blood from a patient with documented HIV infection should be treated prophylactically with combination antiretroviral therapy (ART). This practice, referred to as postexposure prophylaxis or PEP, has dramati­ cally reduced the occurrence of puncture-related transmissions of HIV to health care workers. In addition to blood and visibly bloody body fluids, semen and vaginal secretions also are considered potentially infectious; however, they have not been implicated in occupational transmission from patients to health care workers. The following fluids also are consid­ ered potentially infectious: cerebrospinal fluid, synovial fluid, pleural fluid, peritoneal fluid, pericardial fluid, and amniotic fluid. The risk for transmission after exposure to fluids or tissues other than HIV-infected blood has not been quantified, but it is probably considerably lower than the risk after blood exposures. Feces, nasal secretions, saliva, sputum, sweat, tears, urine, and vomitus are not considered potentially infectious for HIV unless they are visibly bloody. Rare cases of HIV transmission via human bites have been reported, but not in the setting of occupational exposure. An increased risk for HIV infection following percutaneous expo­ sures to HIV-infected blood is associated with exposures involving a relatively large quantity of blood, as in the case of a device visibly contaminated with the patient’s blood, a procedure that involves a hollow-bore needle placed directly in a vein or artery, or a deep injury. Factors that might be associated with mucocutaneous transmission of HIV include exposure to an unusually large volume of blood and pro­ longed contact. In addition, the risk increases for exposures to blood from untreated patients with high levels of HIV in the blood. Since the beginning of the HIV epidemic, there have been rare instances where transmission of infection from a health care worker to patients seemed highly probable. Despite this small number of documented cases, the

risk of HIV transmission involving infected health care workers to patients is extremely low in developed countries—in fact, too low to be measured accurately. In this regard, several retrospective epide­ miologic studies have been performed tracing thousands of patients of dentists, physicians, surgeons, obstetricians, and gynecologists with HIV, and no cases of HIV transmission that could be linked to the health care providers were identified other than the already identified documented cases. Breaches in infection control and the reuse of contaminated syringes, failure to properly sterilize surgical instruments, and/or hemodialysis equipment also have resulted rarely in the transmission of HIV from patient to patient in hospitals, nursing homes, and out­ patient settings. Finally, these very rare occurrences of transmission of HIV as well as HBV and HCV to and from health care workers in the workplace underscore the importance of the use of universal precau­ tions when caring for all patients (see below and Chap. 147). ■ ■MOTHER-TO-CHILD TRANSMISSION OF HIV HIV infection can be transmitted from an infected mother to her fetus during pregnancy, during delivery, or by breast-feeding. This remains a persistent form of transmission of HIV infection in certain developing countries. Virologic analyses of aborted fetuses indicate that HIV can be transmitted to the fetus during the first or second trimesters of preg­ nancy. However, maternal transmission occurs most commonly during birth. Two studies performed in Rwanda and the Democratic Republic of Congo (then called Zaire) indicated that among all mother-to-child transmissions of HIV, the relative proportions were 23–30% before birth, 50–65% during birth, and 12–20% via breast-feeding. In the absence of antiretroviral therapy for the mother during preg­ nancy, labor, and delivery, and for the infant prophylactically following birth, the overall probability of transmission of HIV from mother to infant/fetus ranges from 15% to 25% in industrialized countries and from 25% to 35% in developing countries. These differences may relate to the adequacy of prenatal care as well as to the stage of HIV disease and the general health of the mother during pregnancy. Higher rates of transmission have been reported to be associated with many

factors—the best documented of which is the presence of high mater­ nal levels of plasma viremia, with the risk increasing linearly with the level of maternal plasma viremia. It is very unlikely that mother-tochild transmission will occur if the mother’s level of plasma viremia is <1000 copies of HIV RNA/mL of blood and extremely unlikely if the level is <50 copies/mL. Increased mother-to-child transmission is also correlated with closer human leukocyte antigen (HLA) match between mother and child. A prolonged interval between membrane rupture and delivery is another well-documented risk factor for transmission of HIV. Other conditions that are potential risk factors, but that have not been consistently demonstrated, are the presence of chorioamnionitis at delivery; STIs during pregnancy; illicit drug use during pregnancy; cigarette smoking; preterm delivery; and obstetric procedures such as amniocentesis, amnioscopy, fetal scalp electrodes, and episiotomy. Today, the rate of mother-to-child transmission has fallen to <1% in pregnant women who are receiving ART for their HIV infection. Such treatment, combined with cesarean section delivery, has rendered mother-to-child transmission of HIV an extremely unusual event in the United States and other developed nations. In this regard, both the United States Public Health Service and the World Health Organiza­ tion guidelines recommend that all pregnant women with HIV receive lifelong ART for the health of the mother (regardless of plasma HIV RNA copy number or CD4+ T cell count) as well as to prevent perinatal transmission. Breast-feeding is an important modality of transmission of HIV infection in certain developing countries, particularly where untreated mothers continue to breast-feed for prolonged periods. The risk factors for mother-to-child transmission of HIV via breast-feeding by an untreated mother include detectable levels of HIV in breast milk, the presence of mastitis, low maternal CD4+ T cell counts, and maternal vitamin A deficiency. The risk of HIV infection via breastfeeding is highest in the early months of breast-feeding. In addition, exclusive breast-feeding has been reported to carry a lower risk of

HIV transmission than mixed feeding. In developed countries, breastfeeding of babies by a mother with HIV is contraindicated since alter­ native forms of adequate nutrition, i.e., formulas, are readily available. In developing countries, where breast-feeding may be essential for the overall health of the infant, the continuation of ART in the infected mother during the period of breastfeeding markedly diminishes the risk of transmission of HIV to the infant. In fact, treatment of a pregnant woman with ART should be provided for the benefit of the woman as much as for the prevention of mother-to-child transmission and should be continued beyond the pregnancy, for life.

■ ■TRANSMISSION OF HIV BY OTHER BODY FLUIDS Although HIV can be isolated typically in low titers from saliva of a small proportion of infected individuals, there is no convincing evidence that saliva can transmit HIV infection, either through kiss­ ing or through other exposures, such as occupationally to health care workers. Saliva contains endogenous antiviral factors; among these factors, HIV-specific immunoglobulins of IgA, IgG, and IgM isotypes are detected readily in salivary secretions of infected individuals. It has been suggested that large glycoproteins such as mucins and throm­ bospondin 1 sequester HIV into aggregates for clearance by the host. In addition, multiple soluble salivary factors inhibit HIV to various degrees in vitro, probably by targeting host cell receptors rather than the virus itself. Perhaps the best studied of these, secretory leukocyte protease inhibitor (SLPI), blocks HIV infection in several cell culture systems, and it is found in saliva at levels that approximate those required for inhibition of HIV in vitro. In this regard, higher salivary levels of SLPI in breast-fed infants were associated with a decreased risk of HIV transmission through breast milk. It has also been suggested that submandibular saliva reduces HIV infectivity by stripping gp120 from the surface of virions, and that saliva-mediated disruption and lysis of HIV-infected cells occurs because of the hypotonicity of oral secretions. Transmission of HIV by a human bite can occur but is a rare event. Although virus can be detected, if not isolated, from virtually any body fluid, there is no evidence that HIV transmission can occur as a result of exposure to tears, sweat, or urine. However, there have been isolated cases of transmission of HIV infection by body fluids that may or may not have been contaminated with blood. Most of these situations occurred in the setting of a close relative providing intensive nursing care for a person with HIV without observing universal pre­ cautions, underscoring the importance of adhering to such precautions in the handling of body fluids and wastes from people with HIV. CHAPTER 208 Human Immunodeficiency Virus Disease: AIDS and Related Disorders
EPIDEMIOLOGY ■ ■HIV INFECTION AND AIDS WORLDWIDE HIV infection/AIDS is a global pandemic, with cases reported from virtually every country. In 2023, an estimated 39.9 million individuals were living with HIV infection, according to the Joint United Nations Programme on HIV/AIDS (UNAIDS). An estimated 95% of people liv­ ing with HIV/AIDS reside in low- and middle-income countries; ~53% are female, and 1.4 million are children <15 years. The regional distri­ bution of these cases is illustrated in Fig. 208-8. The estimated number North America and Western and North America and Western and Central Europe Eastern Europe and Central Asia Eastern Europe and Central Asia Central Europe 2.3 million [2.0 million–2.7 million] 2.3 million [2.0 million–2.7 million] 2.1 million [1.9 million–2.3 million] Middle East and 2.1 million [1.9 million–2.3 million] Middle East and North Africa North Africa Caribbean Caribbean 210,000 [170,000–280,000] Western and Central 210,000 [170,000–280,000] Western and Central Africa 5.1 million [4.5 million–5.9 million] 340,000 [280,000–390,000] 340,000 [280,000–390,000] Asia and the Pacific Asia and the Pacific 6.7 million [6.0 million–7.5 million] 6.7 million [6.0 million–7.5 million] Africa 5.1 million [4.5 million–5.9 million] Latin America Latin America 2.3 million [2.0 million–2.5 million] 2.3 million [2.0 million–2.5 million] Eastern and Southern Africa Eastern and Southern Africa 20.8 million [19.2 million–23.0 million] 20.8 million [19.2 million–23.0 million] FIGURE 208-8  Estimated number of adults and children living with HIV infection as of December 2023. Total: 39.9 million (36.1 million–44.6 million). (From Joint United Nations Programme on HIV/AIDS [UNAIDS].)

3,500,000 45,000,000 New HIV infections and AIDS-related deaths 3,000,000 2,500,000 2,000,000 1,500,000 1,000,000 15,000,000 500,000 – –

FIGURE 208-9  Global estimates of new HIV cases, AIDS-related deaths, and HIV prevalence, 1990–2023. (From UNAIDS.) of people living with HIV—i.e., the global prevalence—has increased more than fourfold since 1990, reflecting the combined effects of con­ tinued high rates of new HIV infections and the life-prolonging impact of antiretroviral therapy (Fig. 208-9). The overall global prevalence of HIV infection among persons 15–49 years of age is ~0.7%, with rates varying widely by country and region as illustrated in Fig. 208-10. The WHO Eastern and Southern Africa region remains most severely affected, with more than 1 in 20 adults (5.7%) living with HIV and accounting for more than half of the people with HIV worldwide. An estimated 88.4 million people have become infected with HIV since the start of the pandemic. PART 5 Infectious Diseases In 2023, an estimated 1.3 million new cases of HIV infection occurred worldwide, including 120,000 among children <15 years; about one-third of new infections were among people age 15–24 years. The regional distribution of new HIV cases in 2023 is shown No data 0% 0.1% 0.2% 0.5% 1% 2% 5% 10% 20% 50% FIGURE 208-10  Adult HIV prevalence rates by country, 2021. Data are estimates for adults age 15–49 years. (Reproduced from IHME, Global Burden of Disease with major processing by Our World in Data; 2024.)

People living with HIV 40,000,000 35,000,000 People living with HIV 30,000,000 25,000,000 New HIV infections 20,000,000 AIDS-related deaths 10,000,000 5,000,000

in Fig. 208-11. Globally, members of certain high-risk populations are disproportionately affected by HIV infection, often because of factors such as marginalization, discrimination, and in some cases criminalization. Sex workers, people who inject drugs, transgender people, prisoners, gay men and other men who have sex with men, the clients of sex workers, and the sexual partners of these key populations accounted for 55% of all new HIV infections in 2022 (Fig. 208-12). New HIV infections globally have fallen by 60% since their peak in 1995 (Fig. 208-9). In 2023, 1.3 million people were newly infected with HIV, compared with 3.3 million people in 1995. Reductions in global HIV incidence likely reflect progress with HIV prevention efforts and the increased provision to HIV-infected people of antiretroviral ther­ apy, which makes them much less likely to transmit the virus to sexual partners. Between 2010 and 2023, new HIV infections declined by 39%, from 2.1 million to 1.3 million. During the same period a ~62%

FIGURE 208-11  Distribution of new HIV infections, by region, 2023. (Reproduced with permission from UNAIDS.) reduction in HIV infections among children <15 years was observed, from 300,000 in 2010 to 120,000 in 2023. This progress is due largely to the increasing availability of antiretroviral medications to prevent the transmission of HIV from mother to infant. An estimated 30.7 million people with HIV globally, 77% of all people living with HIV, were accessing antiretroviral therapy as of December 2023, up from 7.7 million people in 2010. Among pregnant women with HIV, 84% had access to antiretroviral medicines to prevent transmission of HIV to their child in 2023. In 2023, global AIDS deaths totaled 630,000 (including 76,000 chil­ dren <15 years), a 65% decrease since 2000 that coincides with a rapid expansion of access to antiretroviral therapy (Fig. 208-13). Since the beginning of the HIV pandemic, an estimated 42.3 million persons globally have died of an AIDS-related illness. The HIV epidemic has occurred in “waves” in different regions of the world, each wave having somewhat different characteristics depending on the demographics of the country and region in question and the tim­ ing of the introduction of HIV into the population. Although the AIDS epidemic was first recognized in the United States and shortly thereaf­ ter in Western Europe, it very likely began in sub-Saharan Africa (see above), a region particularly devastated by the epidemic. The 21 countries of the Eastern and Southern Africa region are home to about 7% of the world’s population but had 20.8 million peo­ ple living with HIV in 2023, >50% of the global total (Fig. 208-8). HIV prevalence among adults age 15–49 years across the region is 5.7%. Of 21 countries in the region, 17 have generalized epidemics, that is, their national prevalence is >1%. In 6 countries in the region, >10% of Sex workers (8%) Gay men and men who have sex with other men (20%) Remaining population (45%) Transgender women (1%) People who inject drugs (8%) Clients of sex workers (10%) Sex partners of key populations (non-clients) (8%) FIGURE 208-12  Global distribution of new HIV infections by population. Data for 2022. (From UNAIDS.)

Asia and the Pacific 300,000 Caribbean 15,000 Eastern Europe and central Asia 140,000 Latin America 120,000 Middle East and North Africa 23,000 Western and central Europe and North America 56,000 Eastern and southern Africa 450,000 Western and central Africa 190,000 the adult population age 15–49 has HIV infection (Fig. 208-10). South Africa has the highest number of people living with HIV in the world (7.7 million); Eswatini (formerly known as Swaziland) has the highest adult HIV prevalence globally (25.1%). Heterosexual exposure is the primary mode of HIV transmission in most countries in this region, as is the case throughout sub-Saharan Africa. Women and girls age 15 years and older account for ~63% of all HIV infections in the region. Key populations, notably sex workers and clients of sex workers, accounted for 23% of new infections in the region in 2022. CHAPTER 208 Major progress in the HIV response has been made in the Eastern and Southern Africa region in recent years. The annual number of people acquiring HIV in the region fell by 59% between 2010 and 2023, from 1.1 million to 450,000. AIDS-related deaths decreased by 57% in that period, from 600,000 in 2010 to 260,000 in 2023. As of 2023, 93% of people living with HIV in the region knew their infections status; 83% of those living with HIV (17.4 million people) were on antiretro­ viral therapy, and 78% had suppressed viral loads. Human Immunodeficiency Virus Disease: AIDS and Related Disorders
The 25 countries of the Western and Central Africa region are home to 5.1 million people living with HIV, of whom 3.1 million are women age ≥15 years and 380,000 are children. HIV prevalence in most countries is relatively low compared with Eastern and Southern Africa. HIV prevalence among adults across the region overall stands at 1.2%. About 40% of new infections in the region in 2023 occurred in Nigeria, a large country with an HIV seroprevalence rate of 1.3%. As in Eastern and Southern Africa, heterosexual transmission accounts for most HIV transmission in West and Central Africa, with sex workers and their clients accounting for about one-fifth of new infections. The Middle East and North Africa region has one of the lowest HIV prevalence rates in the world (<0.1%), although annual new infections more than doubled between 2010 and 2023, from 11,000 to 23,000. In 2023, an estimated 230,000 people were living with HIV in the 19 countries in the region. Cases are largely concentrated among IDUs, men who have sex with men, and sex workers and their clients. In Asia and the Pacific, an estimated 6.7 million people were living with HIV at the end of 2023, with an overall adult prevalence rate of 0.2%. With 300,000 new infections in 2023, the region accounted for one-quarter of all new infections globally that year. The region saw a 13% decrease in new infections between 2010 and 2023, with reduc­ tions in Thailand and Vietnam offset by increases in Afghanistan, Bangladesh, Fiji, the Lao People’s Democratic Republic, Papua New Guinea, and the Philippines. In the same time frame, AIDS-related deaths in the region fell by 51% to 150,000. Among countries in this region, only Thailand has an adult seroprevalence rate that reaches 1%. However, the populations of many countries are so large that even low infection and seroprevalence rates result in large numbers of people living with HIV. In this regard, three populous countries—China, India, and Indonesia—account for around three-quarters of all people living with HIV in the region. Key populations (Fig. 208-11) and their

3,000,000 2,500,000 Number of AIDS–related deaths 2,000,000 1,500,000 1,000,000 500,000 0

FIGURE 208-13  Global antiretroviral therapy coverage and number of AIDS-related deaths, 1990-2023. (Data from The Urgency of Now: AIDS at a Crossroads. Geneva: Joint United Nations Programme on HIV/AIDS; 2024.) partners accounted for ~80% of new HIV infections in the region in 2022. Rising numbers of new infections among gay men and other men who have sex with men in this region are a major concern. The HIV epidemic continues to expand in Eastern Europe and Central Asia, with a 20% increase in annual new HIV infections and 34% increase in AIDS deaths between 2010 and 2023. There are about 2.1 million people living with HIV in the region, where the epidemic has been driven by injection drug use but has an increasing proportion of new HIV infections transmitted sexually. Members of key popula­ tions and their sexual partners account for ~94% of new infections in the region. The Russian Federation, Ukraine, Kazakhstan, and Uzbekistan reported 92% of all registered cases in the region in 2023. PART 5 Infectious Diseases Approximately 2.3 million people were living with HIV/AIDS in Latin America at the end of 2023. The annual number of new infec­ tions in the region increased by 9% between 2010 and 2023 to 120,000. About two-thirds of new infections were among key populations and their sex partners, with increasing rates among men who have sex with men, sex workers, and transgender women. AIDS-related deaths declined by 28% to 30,000 from 2010 to 2023. Brazil is home to the largest number of HIV-infected persons (1,00,000) in the region. In the Caribbean, an estimated 340,000 people were living with HIV in 2023, with 15,000 new infections that year. New infections have declined by 22% in the region since 2010. About 90% of new infections in the region in 2023 were in Cuba, the Dominican Republic, Haiti, and Jamaica. People from key populations and their sex partners accounted for almost half of new infections in the region in 2023. Approximately 2.3 million people were living with HIV/AIDS in North America and Western and Central Europe in 2023. While modes of transmission vary greatly by country, HIV in the region dis­ proportionately affects men who have sex with men. New infections in the region fell by 24% between 2010 and 2023 to 56,000, while AIDSrelated deaths fell 34%, to 13,000. Key populations accounted for ~83% of new infections in the region in 2022. ■ ■HIV INFECTION IN THE UNITED STATES As of 2022, an estimated 1.2 million individuals in the United States were living with HIV infection, ~13% of whom were unaware of their infection. Among people with diagnosed HIV in 2022, about 76% have received some HIV care, and about 65% have achieved viral suppres­ sion (see below). Nearly 40% of people living with HIV in the United States are Black/ African American and 26% are Hispanic/Latino; ~60% are men who have sex with men, according to CDC estimates. The HIV prevalence

Percentage of people living with HIV receiving treatment Percentage of people living with HIV receiving treatment

Number of AIDS–related deaths

target

rate among all individuals age 13 years or older in the United States is 0.4%. Approximately 1.4% of Black/African-American adults are living with HIV in the United States, more than any other racial/ethnic group. The estimated annual number of new HIV infections in the United States has fallen by more than two-thirds since its height in the late 1980s of about 130,000 per year. CDC data indicate further progress in recent years, as overall incidence fell 12% from 2018 to 2022, from 36,200 new infections to 31,800. The estimated distribution of new infections in 2022, broken down by transmission category, is shown in Fig. 208-14. In the United States, the burden of HIV infection is not evenly dis­ tributed across states and regions. In most areas of the country, HIV is concentrated in urban areas. In the southern United States, larger per­ centages of diagnoses are in smaller metropolitan and nonmetropolitan areas. HIV has disproportionately affected minority populations in the United States in both urban and rural areas. Among new HIV infec­ tions from 2018 to 2022 in the United States, 37% were among Blacks/ African Americans, a group that constitutes only 12% of the U.S. popu­ lation. Hispanics/Latinos, 18% of the U.S. population, accounted for 33% of new HIV infections. The rate of new HIV infections in 2022 by race/ethnicity in the United States is shown in Fig. 208-15. Perinatal HIV transmission, from an HIV-infected mother to her baby, has declined significantly in the United States, largely due to the implementation of guidelines for the universal counseling and voluntary HIV testing of pregnant women and the use of antiretroviral Heterosexual contact 7000 infections (22%) Male-to-male sexual contact and injection drug use 1100 infections (3%) Injection drug use 2300 infections (7%) Male-to-male sexual contact 21,400 infections (67%) FIGURE 208-14  Estimated distribution of new HIV infections in the United States among people age ≥13 years, by transmission category, 2022. Total: 31,800. (From CDC: HIV Surveillance Supplemental Report 29 [No. 1], 2024.)

Black/African American Multiple races 21.6 Hispanic/Latino 20.7 American Indian/ Alaska Native 9.8 White 4.4 Asian 2.2

Rate/100,000 population FIGURE 208-15  Estimated rate of new HIV infections in the United States among people age ≥13 years, by race/ethnicity (per 100,000 population), 2022. (From CDC: HIV Surveillance Supplemental Report 29 [No. 1], 2024.) therapy for pregnant women and newborn infants to prevent infection. In 2022, 62 children were newly diagnosed with HIV infection in the United States, down from a peak of ~1750 in 1991. The rate of HIV-related deaths in the United States rose steadily through the 1980s, peaked around 1994, and fell rapidly through 1997 (Fig. 208-16). Since then, the rate of death due to HIV disease con­ tinues to decline, decreasing from 2.6 deaths per 100,000 in 2010 to 1.3 deaths per 100,000 in 2022. This trend is likely due to several fac­ tors, including improved prophylaxis and treatment of opportunistic infections, growing experience among the health professions in caring for HIV-infected individuals, improved access to health care, and a decrease in new infections. However, the most influential factor clearly has been the increased use—and continued refinement—of antiretro­ viral therapy (ART), generally administered in a combination of two, three, or four agents. Despite much progress, HIV disease continues to remain among the leading causes of death among all persons age 25–44 years in the United States, ranking 13th in 2022. HIV disease as an underlying cause of death is particularly high among Black/AfricanAmerican persons age 25–44 years, ranking as the 8th leading cause among Black males and the 12th leading cause among Black females in 2022. PATHOPHYSIOLOGY AND PATHOGENESIS The hallmark of HIV disease is a profound immunodeficiency result­ ing primarily from a progressive quantitative and qualitative deficiency of the subset of T lymphocytes referred to as helper T cells occurring

Deaths, No. (in thousands)

Year of death FIGURE 208-16  Trends in annual age-adjusted rates of death in the general population with HIV infection as the underlying cause, United States, 1987–2022. (From CDC.)

in a setting of aberrant immune activation creating a state of immuno­ suppression. The helper subset of T cells is defined phenotypically by the presence on its surface of the CD4 molecule (Chap. 360), which serves as the primary cellular receptor for HIV. A co-receptor also must be present together with CD4 for efficient binding, fusion, and entry of HIV-1 into its target cells (Figs. 208-3 and 208-4). HIV-1 uses two major co-receptors, CCR5 and CXCR4, for fusion and entry; these co-receptors are also the primary receptors for certain chemoattractant cytokines termed chemokines and belong to the seven-transmembranedomain G protein–coupled family of receptors. Multiple mechanisms responsible for cellular depletion and/or immune dysfunction of CD4+ T cells have been demonstrated in vitro. These include direct infection and destruction of these cells by HIV, as well as indirect effects such as immune clearance of infected cells; cell death associated with aberrant immune activation and inflammation, including caspase 1–mediated pyroptosis prompted by tissue CD4+ T cells undergoing abortive/

nonproductive HIV infection; and immune exhaustion due to per­ sistent cellular activation with resulting cellular dysfunction. Patients with CD4+ T cell levels below certain thresholds are at high risk of developing a variety of opportunistic diseases, particularly the infec­ tions and neoplasms that are AIDS-defining illnesses. Some features of AIDS, such as Kaposi’s sarcoma and certain neurologic abnormalities, cannot be explained completely by the immunodeficiency caused by HIV infection, since these complications may occur prior to the devel­ opment of severe immunologic impairment.

34.1 The combination of viral pathogenic and immunopathogenic events that occur during the course of HIV disease from the moment of initial (primary) infection through the development of advanced-stage dis­ ease is complex and varied. It is important to appreciate that the patho­ genic mechanisms of HIV disease are multifactorial and multiphasic and are different at different stages of the disease. Therefore, it is essen­ tial to consider the typical clinical course of an untreated individual with HIV to better appreciate these pathogenic events (Fig. 208-17). CHAPTER 208 Human Immunodeficiency Virus Disease: AIDS and Related Disorders
■ ■EARLY EVENTS IN HIV INFECTION:

PRIMARY INFECTION AND INITIAL DISSEMINATION OF VIRUS Using rectal or vaginal mucosal transmission in nonhuman primates as a model, the earliest events (within hours) that occur following expo­ sure of HIV to the mucosal surface determine whether an infection will be established or aborted as well as the subsequent course of events following infection. Although the mucosal barrier is relatively effective in limiting access of HIV to susceptible targets in the submucosal tis­ sue, the virus can cross the barrier by transport on Langerhans cells, an epidermal type of DC, just beneath the surface or through microscopic rents in the mucosa. Significant disruptions in the mucosal barrier as seen in ulcerative genital disease facilitate viral entry and increase the

Death ±Acute HIV syndrome Wide dissemination of virus Seeding of lymphoid organs Primary infection Constitutional symptoms

CD4+ T lymphocyte count (cells/µL)

Clinical latency

Weeks Years FIGURE 208-17  Typical course of an untreated HIV-infected individual. See text for detailed description. (From G Pantaleo, C Graziosi, AS Fauci: The Immunopathogenesis of Human Immunodeficiency Virus Infection. N Engl J Med 328:327, 1993. Copyright © 1993 Massachusetts Medical Society. Reprinted with permission from Massachusetts Medical Society.) PART 5 Infectious Diseases efficiency of infection. Viruses then seek susceptible targets, which are primarily CD4+ T cells that are spatially dispersed in the mucosa. This spatial dispersion of targets provides a significant obstacle to the estab­ lishment of infection. Such obstacles account for the low efficiency of sexual transmission of HIV (see “Sexual Transmission,” above). Both “partially” resting CD4+ T cells and activated CD4+ T cells serve as early amplifiers of infection. Resting CD4+ T cells are more abundant; however, activated CD4+ T cells support productive infection and thus generate larger amounts of virus. For infection to become established, the basic reproductive rate (R0) must become equal to or greater than 1,

i.e., each infected cell would infect at least one other cell. Once infec­ tion is established, the virus replicates in lymphoid cells in the mucosa, the submucosa, and to some extent the lymphoreticular tissues that drain the gut or genital tissues. For a variable period up to several days, the virus is typically not detected in the plasma. This period is Lamina propria Lymphoid tissue Infected “resting” CD4+ T cells PD-1+CD8+ T cells “Resting” CD4+ T cells HIV virions Crossing the barrier Activated CD4+ T cell DC Infected activated CD4+ T cell Infected cell Macrophage Hours Days Weeks Years FIGURE 208-18  Summary of early events in HIV infection. See text for detailed description. CTLs, cytolytic T lymphocytes; HIV, human immunodeficiency virus. (Adapted from AT Haase: Nat Rev Immunol 5:783, 2005.)

called the “eclipse” phase of infection. As more virus is produced within several days to weeks, it is disseminated, first to the draining lymph nodes and then to other lymphoid compartments where it has easy access to dense concentrations of CD4+ T cell targets, allowing for a burst of high-level plasma viremia that is readily detectable by currently available assays measuring viral RNA (Fig. 20818). The gut-associated lymphoid tissue (GALT) is one target of HIV infection and a location where CD4+ T cells (pri­ marily memory cells) are infected and depleted, both by direct viral effects and by activation-associated apoptosis. Once virus replication reaches this threshold and virus is widely disseminated, infec­ tion is firmly established throughout the lymphoid tissues of the body and persists for the life of the individual. It is important to point out that the effi­ ciency of initial infection of susceptible cells may vary somewhat with the route of infection. Virus that enters directly into the bloodstream via infected blood or blood products (i.e., transfusions, use of contaminated needles for injection drugs, sharp-object injuries, maternalto-fetal transmission either intrapartum or perinatally, or sexual inter­ course where there is enough trauma to cause bleeding) is likely first cleared from the circulation to the spleen and other lymphoid organs, where primary focal infections begin, followed by wider dissemination throughout other lymphoid tissues as described above. Opportunistic diseases HIV RNA copies per mL plasma

It has been demonstrated that sexual transmission of HIV is the result of a single infectious event and that a viral genetic bottleneck exists for transmission with selective transmission of certain viruses. In this regard, certain characteristics of the HIV envelope glycoprotein have a major influence on transmission, at least in subtype A and C viruses. Transmitting viruses, often referred to as “founder viruses,” are typically a small fraction of the circulating viremia of the transmit­ ting partner and are less-diverged viruses with signature sequences including shorter V1–V2 loop sequences and fewer predicted N-linked glycosylation sites relative to the major circulating variants. These Late-responding CTLs Establishment of lymphoidtissue viral reservoir Partial control Immune activation Dissemination of virus Sustained HIV production Regulatory T cells

Founder Replicating virus FIGURE 208-19  As HIV diverges from founder to chronically replicating virus, it accumulates N-linked glycosylation sites. See text for detailed description. (Adapted from CA Derdeyn et al: Science 303:2019, 2004; B Chohan et al: J Virol 79:6528, 2005; and BF Keele et al: Proc Natl Acad Sci USA 105:7552, 2008.) viruses are almost exclusively R5 strains and are usually sensitive to neutralizing antibody. Once replication proceeds in the newly infected partner, the founder virus diverges and accumulates glycosylation sites, becoming progressively more resistant to neutralization (Fig. 208-19). The acute burst of viremia and wide dissemination of virus in pri­ mary HIV infection may be associated with an acute HIV syndrome, which occurs to varying degrees in ~50% of individuals within 2 to

4 weeks of initial infection (see below). This syndrome is usually associated with high levels of plasma viremia reflected in millions of copies of HIV RNA per milliliter of plasma that can last for several weeks. Acute mononucleosis-like symptoms are well correlated with the presence of high levels of plasma viremia. Virtually all patients develop some degree of plasma viremia during primary infection, which contributes to virus dissemination throughout the lymphoid tissue, even though they may remain asymptomatic or not recall experiencing symptoms. The peak level of plasma viremia in primary HIV infection does not necessarily determine the rate of disease progression; however, the set point of the level of steady-state plasma viremia after ~1 year correlates with the rate of disease progression in the untreated patient and with immunologic and virologic aberrancies that may persist in the treated patient. The strikingly high levels of viremia observed in many patients during acute HIV infection is felt to be associated with a higher likelihood of transmission of the virus to others by a variety of routes including sexual transmission, shared needles and syringes, and mother-to-child transmission intrapartum, perinatally, or via breast milk. ■ ■ESTABLISHMENT OF CHRONIC INFECTION Persistence of Virus Replication  HIV infection is unique among human viral infections. Despite the robust cellular and humoral immune responses that are mounted following primary infection (see “Immune Response to HIV,” below), once infection has been estab­ lished the virus succeeds in escaping complete immune-mediated clearance, paradoxically seems to thrive on immune activation, and is never eliminated completely from the body. Rather, a chronic infection develops and persists with varying degrees of continual virus replica­ tion in the untreated patient for a median of ~10 years before the patient becomes clinically ill (see “Advanced HIV Disease,” below). It is this establishment of a chronic, persistent infection that is the hallmark of HIV disease. Throughout the often-protracted course of chronic infection, virus replication can invariably be detected in untreated patients by widely available molecular assays that measure copies of virion-associated HIV RNA in plasma (copies per milliliter). Levels of virus vary greatly in most untreated patients, usually ranging from fewer than 50 to greater than a million copies of HIV RNA per milliliter of plasma. Studies using highly sensitive molecular techniques have demonstrated that even in treated patients in whom plasma viremia is suppressed to below detection limits of commercial assays (20–50 copies of HIV RNA per milliliter depending on assay kit manufacturer)

by ART, there is a continual low level of virion production in most infected patients. In other human viral infections (with some excep­ tions) if the host survives, the virus is completely cleared from the body and a state of immunity against subsequent infection develops. HIV infection very rarely kills the host during primary infection. Certain viruses, such as HSV (Chap. 197), are not completely cleared from the body after infection, but instead enter a latent state; in these cases, clinical latency is accompanied by microbiologic latency. This is not the case with HIV infection as described above. Chronicity associated with persistent virus replication can also be seen in certain cases of HBV and HCV infections (Chap. 352); however, in these infections the immune system is not a target of the virus.

Escape of HIV from Effective Immune System Control  Inher­ ent to the establishment of chronicity of HIV infection is the ability of the virus to evade adequate control and elimination by the cellular and humoral immune responses. There are several mechanisms whereby the virus accomplishes this evasion. Paramount among these is the establishment of a sustained level of replication associated with the generation of viral diversity via mutation and recombination. The selec­ tion of mutants that escape control by CD8+ cytolytic T lymphocytes (CTLs) is critical to the propagation and progression of HIV infection. The high rate of virus replication associated with inevitable mutations also contributes to the inability of antibody to neutralize and/or clear the autologous virus. Furthermore, for reasons that remain unclear, the humoral immune system does not readily produce classic neutralizing antibodies against the HIV envelope and does so only after years of persistent virus replication and after the infection is firmly established (see below). Extensive analyses of sequential HIV isolates and host responses have demonstrated that viral escape from B cell and CD8+ T cell responses occurs early after infection and allows the virus to con­ tinually evade effective immune responses. Virus-specific CD8+ CTLs expand greatly during primary HIV infection, and they likely represent the high-affinity responses that would be expected to be most efficient in eliminating virus-infected cells; however, with very rare exceptions, viral control is incomplete as viral replication persists at relatively high levels in the majority of individuals. In addition to viral escape from CTLs through high rates of mutation, it is thought that the initially strong immune response becomes qualitatively dysfunctional owing to the overwhelming immune activation associated with persistent viral replication, leading to immune “exhaustion” that affects both arms of adaptive immunity. Several studies have indicated that exhaustion of HIV-specific CD8+ T cells during prolonged immune activation is associated with upregulation of several inhibitory receptors, such as the programmed death (PD) 1 molecule (of the B7-CD28 family of mol­ ecules), T cell immunoreceptor with Ig and ITIM domains (TIGIT),

T cell immunoglobulin and mucin domain–containing molecule 3 (Tim-3), and lymphocyte activating gene 3 (Lag-3), collectively referred to as immune-checkpoint receptors. Upregulation of these surface proteins restricts polyreactivity and proliferative capacity, func­ tional attributes of CD8+ T cells that are essential for effective killing of pathogens. Another mechanism contributing to the evasion by HIV of immune system control is the downregulation of HLA class I molecules on the surface of HIV-infected cells by the viral proteins Nef, Tat, and Vpu, resulting in the lack of ability of CD8+ CTLs to recognize and kill infected target cells. Although this downregulation of HLA class I mol­ ecules would seem to favor elimination of HIV-infected cells by natural killer (NK) cells, this latter mechanism does not remove HIV-infected cells effectively (see below). Another potential means of escape of HIVinfected cells from elimination by CD8+ CTLs is the sequestration of infected cells in immunologically privileged sites such as the central nervous system (CNS), as well as the low frequency of virus-specific CD8+ CTLs in areas of lymphoid tissues, namely germinal centers, where HIV actively replicates. CHAPTER 208 Human Immunodeficiency Virus Disease: AIDS and Related Disorders
The principal targets of neutralizing antibodies against HIV are the envelope proteins gp120 and gp41. HIV employs at least three mechanisms to evade neutralizing antibody responses: hypervariability in the primary sequence of the envelope, extensive glycosylation of the envelope, and conformational masking of neutralizing epitopes.

Several studies that have followed the evolution of the humoral immune response to HIV from the earliest points after primary infec­ tion indicate that the virus continually mutates to escape the emerging antibody response such that the sequential antibodies that are induced do not neutralize the currently autologous virus. Broadly neutralizing antibodies capable of neutralizing a wide range of primary HIV isolates in vitro occur in only about 20% of people with HIV, and, when they do occur, 2–3 years of infection with continual virus replication are generally required to drive the affinity maturation of the antibodies. Unfortunately, by the time these broadly neutralizing antibodies are formed, they are ineffective in containing the virus currently replicat­ ing in the patient. Persistent viremia also results in exhaustion of B cells like the exhaustion reported for CD8+ T cells, adding to the defects in the humoral response to HIV.

CD4+ T cell help is essential for the integrity of both humoral and cell-mediated antigen-specific immune responses. HIV preferentially infects activated CD4+ T cells including HIV-specific CD4+ T cells, and so this loss of viral-specific helper T cell responses has profoundly negative consequences for the immunologic control of HIV replica­ tion. Furthermore, this loss occurs early in the course of infection, and animal studies indicate that 40–70% of all memory CD4+ T cells in the GALT are eliminated during acute infection. During chronic HIV viremia, CD4+ T cells also exhibit evidence of exhaustion, reflected in upregulation of the cytotoxic T lymphocyte–associated antigen 4 (CTLA-4), also a member of the B7-CD28 family. Finally, the escape of HIV from immune-mediated elimination dur­ ing primary infection allows the formation of a pool of latently infected CD4+ T cells, referred to as the viral reservoir, which may not be rec­ ognized or completely eliminated by virus-specific CTLs or by ART (see below). Thus, despite a potent immune response and the marked downregulation of virus replication following primary HIV infection, HIV succeeds in establishing a state of chronic infection with a variable degree of persistent virus replication. During this period most patients make the clinical transition from acute primary infection to variable periods of clinical latency or smoldering disease activity (see below). PART 5 Infectious Diseases The HIV Reservoir: Obstacles to the Eradication of Virus 

A pool of latently infected, resting CD4+ T cells that serves as at least one component of the persistent reservoir of virus exists in virtually all people with HIV, including those who are receiving ART. Such cells carry an integrated form of HIV DNA in the genome of the host and can remain in this state until an activation signal drives the expres­ sion of HIV transcripts. Only a small fraction of the latently infected cells in the viral reservoir contains replication-competent virus, with the overwhelming majority of cells containing defective proviruses incapable of a full replication cycle. However, upon activation of the reservoir variable degrees of sustained virus replication invariably occur. This form of latency is to be distinguished from preintegration latency, in which HIV enters a resting CD4+ T cell and, in the absence of an activation signal, reverse transcription of the HIV genome occurs to a certain extent but the resulting proviral DNA fails to integrate into the host genome. This period of preintegration latency may last hours to days, and if no activation signal is delivered to the cell, the proviral DNA loses its capacity to initiate a productive infection. If these cells do become activated prior to decay of the preintegration complex, reverse transcription proceeds to completion and the virus continues along its replication cycle (see above and Fig. 208-20). The pool of cells that are in the postintegration state of latency is established early dur­ ing primary HIV infection. Despite the suppression of plasma viremia to <20–50 copies per milliliter by potent regimens of ART adminis­ tered over several years, this pool of latently infected cells persists and can give rise to replication-competent virus upon cellular activation ex vivo. Modeling studies built on projections of decay curves have estimated that in such a setting of prolonged viral suppression, it would require many years to the entire life of the host for the pool of latently infected cells to be eliminated. This has not been documented to occur spontaneously in any patients very likely because the latent viral reservoir is long-lived and is continually replenished by the low levels of persistent virus replication that may remain below the limits

Resting CD4+ T cell Preintegration latency (unstable) T-cell activation (Ag, cytokines) Degradation of unintegrated HIV DNA T-cell activation (Ag, cytokines) CTLs Resting CD4+ T cell Active Virus Replication Postintegration latency (stable) Cytopathic effect of virus Resting latently infected CD4+ memory T cells T-cell activation (Ag, cytokines) Virus spread FIGURE 208-20  Generation of latently infected, resting CD4+ T cells in people with HIV. See text for details. Ag, antigen; CTLs, cytolytic T lymphocytes. (Courtesy of TW Chun.) of detection of current assays (see below) as well as by the expansion by proliferation of the pool of latently infected cells (Fig. 208-20), even in patients who for the most part are treated successfully. Reservoirs of HIV-infected cells, latent or otherwise, can exist in multiple com­ partments including the lymphoid tissue, peripheral blood, and CNS (likely in cells of the monocyte/macrophage lineage) as well as in other unidentified locations. Over the past several years attempts have been made to eliminate HIV in the latent viral reservoir using agents that activate resting CD4+ T cells and/or reinitiate viral expression without systemic activation during the course of ART; however, such attempts, referred to as “shock and kill,” have thus far been unsuccessful. This persistent reservoir of infected cells remains a major obstacle to the goal of eradication of virus from infected individuals and hence a clas­ sic “cure,” despite the favorable clinical outcomes that have resulted from ART. Consequently, intense efforts are being directed toward investigating the feasibility of achieving ART-free HIV remission through passive transfer of long-acting broadly neutralizing antibodies and therapeutic agents that could enhance the host immune responses against the virus. Viral Dynamics  The dynamics of viral production and turnover have been quantified using mathematical modeling in the setting of the administration of reverse transcriptase and protease inhibitors to people with HIV in clinical studies. Treatment with these drugs resulted in a precipitous decline in the level of plasma viremia, which typically fell by well over 90% within 2 weeks. It was determined on the basis of modeling the kinetics of viral decline and the emergence of resistant mutants during therapy that 93–99% of the circulating virus originated from recently infected, rapidly turning over CD4+ T cells and that ~1–7% of circulating virus originated from longer-lived cells, likely monocytes/macrophages. A negligible amount of circulating virus originated from the pool of latently infected cells (Fig. 208-21).

Replication cycle ~2 days Latently infected CD4+ T cells Rapidly turning over infected CD4+ T cells Half life 1.0 day Uninfected, activated CD4+ T cells ≤1% 93–99% Uninfected CD4+ T cells Circulating HIV virions Half life ~30–60 min 1–7% CD4+ T cells infected with defective viruses Longer-lived cells FIGURE 208-21  Dynamics of HIV infection in vivo. See text for detailed description. (Adapted from Perelson AS et al: HIV-1 dynamics in vivo: Virion clearance rate, infected cell life-span, and viral generation time. Science 271:1582, 1996.) It was also determined that the half-life of a circulating virion was ~30–60 min and that of productively infected cells was 1 day. Given the relatively steady level of plasma viremia and of infected cells, it appears that extremely large amounts of virus (~1010–1011 virions) are produced and cleared from the circulation each day. In addition, data suggest that the minimal duration of the HIV-1 replication cycle in vivo is ~2 days. Other studies have demonstrated that the decrease in plasma viremia that results from treatment with ART correlates closely with a decrease in virus replication in lymph nodes, further confirming that lymphoid tissue is the main site of HIV replication and the main source of plasma viremia. The level of steady-state viremia, called the viral set point, at ~1 year following acquisition of HIV infection has important prognostic impli­ cations for the progression of HIV disease in the untreated patient. It has been demonstrated that, as a group, untreated people with HIV who have a low set point at 6 months to 1 year following infection prog­ ress to AIDS much more slowly than do individuals whose set point is very high at that time (Fig. 208-22). Clinical Latency versus Microbiologic Latency  With the exception of certain long-term nonprogressors and “elite controllers” of HIV replication, the level of CD4+ T cells in the blood inevitably decreases progressively in viremic people with HIV in the absence of ART. The decline in CD4+ T cells may be gradual or abrupt, the latter usually reflecting a significant spike in the level of plasma vire­ mia. Most patients are relatively asymptomatic while this progressive 1.0 Proportion remaining AIDS-free 0.8 0.6 0.4 0.2 0.0 0

Time, years FIGURE 208-22  Relationship between levels of virus and rates of disease progression. Kaplan-Meier curves showing proportion of 1604 untreated patients remaining AIDS-free over 10 years, stratified by baseline HIV-1 RNA categories (copies per milliliter). (From Multicenter AIDS Cohort Study; JW Mellors, A Muñoz, JV Giorgi, JB Margolick, CJ Tassoni, P Gupta, LA Kingsley, JA Todd, AJ Saah, R Detels, JP Phair, CR Rinaldo, Jr.)

decline is taking place (see below) and are often described as being in a state of clinical latency. However, this term is misleading; it does not mean disease latency, since progression, although slow in many cases and often without symptoms, is generally relentless as evidenced by readily detectable plasma viremia, during this period. Furthermore, clinical latency should not be confused with microbiologic latency since varying levels of virus replica­ tion inevitably occur during this period of clinical latency. Even in those rare patients, such as elite controllers, who have <50 copies of HIV RNA per milliliter in the absence of therapy, there is virtually always some degree of lowlevel ongoing virus replication.

■ ■ADVANCED HIV DISEASE In untreated patients or in patients in whom therapy has not adequately controlled virus replication, after a vari­ able period, usually measured in years, the CD4+ T cell count falls below a critical level (<200/μL) and the patient becomes highly susceptible to opportunistic disease (Fig. 208-17). For this reason, the CDC case definition of stage 3 (AIDS) includes all people with HIV >5 years of age with CD4+ T cell counts below this level (Table 208-2). Patients may experience constitutional signs and symptoms or may develop an opportunistic disease abruptly without any prior symptoms. The depletion of CD4+ T cells continues to be progressive and unrelenting in this phase. It is not uncommon for CD4+ T cell counts in the untreated patient to drop to as low as 10/μL or even to zero. In countries where ART as well as prophylaxis and treatment for opportunistic infections are readily accessible, survival is increased dramatically even in those patients with advanced HIV disease. In contrast, untreated patients who prog­ ress to this severest form of immunodeficiency usually succumb to opportunistic infections or neoplasms (see below). CHAPTER 208 Human Immunodeficiency Virus Disease: AIDS and Related Disorders
■ ■LONG-TERM SURVIVORS, LONG-TERM NONPROGRESSORS, AND ELITE CONTROLLERS It is important to distinguish between the terms long-term survivor and long-term nonprogressor. Long-term nonprogressors are by definition long-term survivors; however, the reverse is not always true. Predic­ tions from one study that antedated the availability of effective ART estimated that ~13% of homosexual/bisexual men who were infected at an early age may remain free of clinical AIDS for >20 years. Many of these individuals may have gradually progressed in their degree of immune deficiency; however, they certainly survived for a considerable period. With the advent of effective ART, the survival of people with HIV has dramatically increased. Early in the AIDS pandemic, prior to the availability of antiretroviral therapy, if a patient presented with a life-threatening opportunistic infection, the median survival was 26 weeks from the time of presentation. Currently, a 20-yearold individual with HIV who is appropri­ ately treated with ART can expect to live at least 50 years according to mathematical model projections. In the face of ART, longterm survival is now commonplace. Defi­ nitions of long-term nonprogressors have varied considerably over the years, and so such individuals constitute a heterogeneous group. Long-term nonprogressors were first described in the 1990s. Originally, individu­ als were considered to be long-term nonpro­ gressors if they had been infected with HIV for a long period (≥10 years), their CD4+ T cell counts were in the normal range, their plasma viremia remained relatively low (undetectable to several thousand copies of HIV RNA/mL plasma), and they remained clinically stable over years without receiv­ ing ART. Approximately 5–15% of people <500 500 to 3000 3001 to 10,000 10,001 to 30,000

30,000

with HIV fell into this broader nonprogressor category. However, this group was rather heterogeneous and over time a significant proportion of these individuals progressed and ultimately required antiretroviral therapy. From this broader group, a much smaller subgroup of “elite” controllers was identified, and they constituted a fraction of 1% of people with HIV. These elite controllers, by definition, have extremely low levels of plasma viremia that is often undetectable by standard assays and normal CD4+ T cell counts. It is noteworthy that their HIV-specific immune response, especially HIV-specific CD8+ CTLs that can clear infected CD4+ T cells, is robust and distinctly superior to those of progressors with HIV infection. In this group of elite control­ lers, certain HLA class I haplotypes are overrepresented, particularly HLA-B57-01 and HLA-B27-05. Outside of the subgroup of elite con­ trollers, multiple other genetic factors have been shown to be involved to a greater or lesser degree in the control of virus replication and thus in the rate of HIV disease progression (see “Genetic Factors in HIV-1 and AIDS Pathogenesis,” below).

■ ■LYMPHOID ORGANS AND HIV PATHOGENESIS Regardless of the portal of entry of HIV, lymphoid tissues are the major anatomic sites for the establishment and propagation of HIV infection. Despite the use of measurements of plasma viremia to determine the level of disease activity, virus replication occurs mainly in lymphoid tissue and not in blood; indeed, the level of plasma viremia directly reflects virus production in lymphoid tissue. Some patients experience progressive generalized lymphadenopathy early in the course of the infection; others experience varying degrees of transient lymphadenopathy. Lymphadenopathy reflects the cellular activation and immune response to the virus in the lymphoid tis­ sue, which is generally characterized by follicular or germinal center hyperplasia. Lymphoid tissue involvement is a common denominator of virtually all patients with HIV infection, even those without easily detectable lymphadenopathy. PART 5 Infectious Diseases Examinations of lymph tissue and peripheral blood in patients and monkeys during various stages of HIV and simian immunodeficiency virus (SIV) infection, respectively, have led to substantial insight into the pathogenesis of HIV disease. In most of the original human stud­ ies, peripheral lymph nodes were the predominant sources for analyses into changes in lymphoid tissues associated with HIV and SIV infec­ tion, whereas more recent studies have expanded to include the GALT, where the earliest burst of virus replication occurs associated with marked depletion of CD4+ T cells. A variety of techniques, including sensitive molecular and imaging approaches to visualize virus and cells in location or suspension, have been employed to describe events associated with HIV disease. During acute HIV infection resulting from mucosal transmission, virus replication progressively amplifies from scattered lymphoid cells in the lamina propria of the gut to drain­ ing lymph nodes, leading to high levels of plasma viremia. The GALT plays a major role in the amplification of virus replication, and virus is disseminated from replication in the GALT to peripheral lymphoid tis­ sues. A profound degree of cellular activation occurs within lymphoid tissues (see below) and is reflected in follicular or germinal center hyperplasia. At this time copious amounts of extracellular virions (both infectious and defective) are trapped on the processes of the follicular dendritic cells (FDCs) that form the stromal cell network in the light zones of lymph node germinal centers. Virions that have bound com­ plement components on their surfaces attach to the surface of FDCs via interactions with complement receptors and likely via Fc receptors that bind to antibodies that are attached to the virions. The use of in situ hybridization techniques, including those that allow detection of viral RNA in the context of tissue architecture, has revealed that HIV is primarily expressed in CD4+ T cells of the paracortical area and, to a lesser extent, in specialized CD4+ T cells (see below) in light zones of germinal centers (Fig. 208-23). The persistence of trapped virus on the surface of FDC likely reflects both a long-lived viral reservoir and virus that is replaced by continual expression in nearby CD4+ T cells. The trapped virus, either as whole virion or shed envelope, also serves as a continual activator of CD4+ T cells, thus driving further virus replication.

FIGURE 208-23  HIV in the lymph node of an HIV-infected individual. An individual cell infected with HIV shown expressing HIV RNA by in situ hybridization using a radiolabeled molecular probe. Original ×500. (Reproduced with permission from G Pantaleo et al: HIV infection is active and progressive in lymphoid tissue during the clinically latent stage of disease. Nature 362:355, 1993.) During the early stages of HIV disease, the architecture of lymphoid tissues is generally preserved and may even be hyperplastic owing to an increased presence of B cells and specialized CD4+ T cells called follicular helper CD4+ T cells (TFH) in prominent germinal centers. Extracellular virions can be seen by electron microscopy attached to FDC processes. The trapping of antigen is a physiologically normal function for the FDCs, which present antigen to B cells and secrete factors such as CXCL13 that retain B and TFH cells in the light zones of germinal centers. These FDC functions, along with stimulatory factors produced by TFH cells, contribute to the generation of B cell memory. However, in the case of HIV, persistent cellular activation, resulting in a shift to secretion of proinflammatory cytokines such as interleukin (IL) 1β, tumor necrosis factor (TNF) α, IFN-γ, and IL-6, can induce viral replication (see below) and diminish the effectiveness of the immune response against the virus. In addition, the CD4+ TFH cells that are recruited into the germinal center to provide help to B cells in the generation of an HIV-specific immune response are highly susceptible to infection and may be an important component of the HIV reservoir. Thus, in HIV infection, a normal physiologic function of the immune system, i.e., the generation of an HIV-specific immune response that contributes to the clearance of virus, can also have del­ eterious consequences. As HIV disease progresses, the architecture of lymphoid tissues begins to disrupt. Confocal microscopy reveals destruction of the fibroblastic reticular cell (FRC) and FDC networks in the T cell zone and B cell follicles/germinal centers, respectively. The mechanisms of destruction are not completely understood, but they are thought to be associated with collagen deposition causing fibrosis and a shift in the expression of certain cytokines, namely decreases in IL-7 and lym­ photoxin α, which are critical to the maintenance of lymphoid tissues and their lymphocyte constituents, and increased levels of transform­ ing growth factor (TGF) β. As the disease progresses to an advanced stage, there is complete disruption of the architecture of the lymphoid tissues, accompanied by dissolution of the FRC and FDC networks. At this point, the lymph nodes are “burnt out.” This destruction of lymphoid tissue compounds the immunodeficiency of HIV disease and contributes both to the inability to control HIV replication and to the inability to mount adequate immune responses against opportu­ nistic pathogens and vaccination. The events from primary infection to the ultimate destruction of the immune system are illustrated in

Fig. 208-24. In nonhuman primate studies and some human studies that have examined GALT following SIV or HIV infection, the basal level of cellular activation combined with virus-mediated activation leads to the rapid infection and elimination of an estimated 50–90% of CD4+ T cells in the gut.

Massive viremia Wide dissemination to lymphoid organs Establishment of infection in GALT Primary infection Partial immunologic control of virus replication Destruction of Immune System Accelerated virus replication Rapid CD4+ T cell turnover FIGURE 208-24  Events that transpire from primary HIV infection through the establishment of chronic persistent infection to the ultimate destruction of the immune system. See text for details. GALT, gut-associated lymphoid tissue. ■ ■THE ROLE OF IMMUNE ACTIVATION AND INFLAMMATION IN HIV PATHOGENESIS Activation of the immune system and variable degrees of inflamma­ tion are essential components of any appropriate immune response to a foreign antigen. However, immune activation and inflammation, which are aberrant in certain individuals with HIV, play a critical role in the pathogenesis of HIV disease as well as other chronic conditions associated with HIV infection. Immune activation and inflammation in individuals with HIV contribute substantially to (1) the replication of HIV, (2) the induction of immune dysfunction, and (3) the increased incidence of chronic conditions such as premature cardiovascular disease (Table 208-4). INDUCTION OF HIV REPLICATION BY ABERRANT IMMUNE ACTIVATION 

The immune system is normally in a state of homeostasis, awaiting per­ turbation by foreign antigenic stimuli. Once the immune response deals with and clears the antigen, the system returns to relative quiescence (Chap. 360). This is generally not the case in HIV infection where, in the untreated patient, virus replication is invariably persistent with very few exceptions and as a result immune activation is persistent. HIV replicates most efficiently in activated CD4+ T cells; in HIV infection, chronic activation provides the cell substrates necessary for persistent virus replication throughout the course of HIV disease, particularly in the untreated patient. Even in certain patients receiving ART whose levels of plasma viremia are suppressed to <20 copies per milliliter, there are low but detectable degrees of virus replication that drives lowlevel persistent immune activation. In addition, immune activation may result from RNA transcription of the integrated DNA of defective proviruses. From a virologic standpoint, although quiescent CD4+ T cells can be infected with HIV, albeit inefficiently, reverse transcription, TABLE 208-4  Conditions Associated with Persistent Immune Activation and Inflammation in Patients with HIV Infection Accelerated aging syndrome Bone fragility Cancers Cardiovascular disease Diabetes Kidney disease Liver disease Neurocognitive dysfunction

integration, and virus spread are much more efficient in activated cells. Further­ more, cellular activation induces expres­ sion of virus in cells latently infected with HIV. In essence, immune activation and inflammation provide the engine that drives HIV replication. In addition to endogenous factors such as cytokines, multiple exogenous factors such as other microbes that induce cellular activation can enhance HIV replication and thus may play a role in HIV pathogenesis.

HIV-specific immune response Trapping of virus and establishment of chronic, persistent infection Co-infection with a range of viruses, such as HSV types 1 and 2, cytomegalo­ virus (CMV), human herpesvirus (HHV) 6, Epstein-Barr virus (EBV), HBV, HCV, adenovirus, and HTLV-1 have been shown to upregulate HIV expression. In addition, infestation with nematodes has been shown to be associated with a heightened state of immune activation that facilitates HIV replication; in certain studies, deworming of the infected host has resulted in a decrease in plasma vire­ mia. Two diseases of great global health significance, malaria and tuberculosis (TB), have been shown to increase HIV viral load in dually infected individuals. Globally, Mycobacterium tuberculosis is the most common opportunistic infection in people with HIV (Chap. 183). In addition to the fact that individuals with HIV are more likely to develop active TB after exposure and to reactivate latent TB, it has been demonstrated that active TB can accelerate the course of HIV infection. It has also been shown that levels of plasma viremia are greatly elevated in indi­ viduals with HIV who have active TB and who are not receiving ART, compared with pre-TB levels and levels of viremia after successful treatment of the active TB. The situation is similar in the interaction between HIV and malaria parasites (Chap. 231). Acute infection with Plasmodium falciparum of individuals with HIV increases viral load, and the increased viral load is reversed by effective treatment of malaria. Immune activation mediated by cytokines and HIV envelope-mediated aberrant cell signaling CHAPTER 208 Human Immunodeficiency Virus Disease: AIDS and Related Disorders
MICROBIAL TRANSLOCATION AND PERSISTENT IMMUNE ACTIVATION 

One proposed mechanism of persistent immune activation involves the disruption of the mucosal barrier in the gut due to HIV replication in submucosal lymphoid tissue. As a result of this disruption, there is an increase in the products of bacteria, particularly lipopolysaccharide (LPS), that translocate from the bowel lumen through the damaged mucosa to the circulation, leading to persistent systemic immune activation and inflammation. This effect can persist even after the HIV viral load is brought to <20 copies/mL by ART. Other related factors that are thought to contribute to the pathogenesis of HIV include depletion in the GALT of IL-17–producing T cells, which are responsible for defense against extracellular bacteria and fungi, as well as alterations in gut microbiota and the metabolic pathways involved. PERSISTENT IMMUNE ACTIVATION AND INFLAMMATION INDUCE IMMUNE DYSFUNCTION  The immune activated state in HIV infection is reflected by hyperactivation of B cells leading to hypergammaglobu­ linemia; increased lymphocyte turnover; activation of monocytes; expression of activation markers and immune checkpoint receptors on CD4+ and CD8+ T cells; increased activation-associated cellular apoptosis and pyroptosis; lymph node hyperplasia, particularly during the chronic phase prior to disease progression; increased secretion of proinflammatory cytokines, particularly IL-6 and type I interferons; elevated levels of high-sensitivity C-reactive protein (CRP), CXC che­ mokine ligand 10 (CXCL10), d-dimer, neopterin, β2-microglobulin, soluble (s) CD14, sTNFR, sCD27, sCD163, and sCD40L; and autoim­ mune phenomena (see “Autoimmune Phenomena,” below). Even in the absence of direct infection of a target cell, HIV envelope proteins can interact with cellular receptors (CD4 molecules and chemokine

receptors) to deliver potent activation signals resulting in calcium flux, the phosphorylation of certain proteins involved in signal transduc­ tion, co-localization of cytoplasmic proteins including those involved in cell trafficking, immune dysfunction, and, under certain circum­ stances, apoptosis and pyroptosis. From an immunologic standpoint, chronic exposure of the immune system to a particular antigen over an extended period may ultimately lead to an inability to sustain an adequate immune response to the antigen in question. In many chronic viral infections, including HIV infection, persistent viremia is associ­ ated with “functional exhaustion” of virus-specific T cells, decreasing their capacity to proliferate and perform effector functions. It has been demonstrated that this phenomenon of immune exhaustion may be mediated, at least in part, by the upregulation of inhibitory receptors on HIV-specific T cells, such as PD-1, LAG-3, and Tim-3 that are shared by both CD4+ and CD8+ T cells, as well as CTLA-4 on CD4+ and 2B4 and CD160 on CD8+ T cells. Furthermore, the ability of the immune system to respond to a broad spectrum of non-HIV antigens may be compromised if immunocompetent bystander cells are maintained in a state of chronic activation.

The deleterious effects of chronic immune activation on the pro­ gression of HIV disease are well established. As in most conditions of persistent antigen exposure, the host maintains activation of antigen (HIV)-specific responses but must also prevent excessive activation and potential immune-mediated damage to tissues. Certain studies suggest that normal immunoregulatory mechanisms that act to keep hyperimmune activation in check, particularly CD4+, FoxP3+, and CD25+ regulatory T cells (T-regs), may be dysfunctional or depleted in the context of advanced HIV disease. One possibility is a role for the inhibitory receptor LAG-3 (see below), which is overexpressed on exhausted T cells and shown to inhibit the proliferation of T-regs. PART 5 Infectious Diseases Apoptosis  Apoptosis is a form of programmed cell death that is a normal mechanism for the elimination of effete cells in organogenesis as well as in the cellular proliferation that occurs during a normal immune response (Chap. 360). Apoptosis can occur by intrinsic or extrinsic pathways, the latter of which is largely dependent on cellular activation, and in this regard the aberrant cellular activation associated with HIV disease is correlated with a heightened state of apoptosis. HIV can trigger activation-induced cell death through the upregula­ tion of the death receptors, such as Fas/CD95, TNFR1, or TNF-related apoptosis-inducing ligand (TRAIL) receptors 1 and 2. Their corre­ sponding ligands FasL, TNF, and TRAIL also are upregulated in HIV disease. HIV-induced stress and alterations in homeostasis also can trigger intrinsic apoptosis due to the downregulation of antiapoptotic proteins such as Bcl-2. Other mechanisms of HIV-induced cell death have been described, including autophagy, necrosis, necroptosis, and pyroptosis. The phenomenon of pyroptosis, an inflammatory form of cell death involving the upregulation of the proinflammatory enzyme caspase 1 and release of the proinflammatory cytokines IL-1β and IL-18, has been linked to a bystander effect of HIV replication on depletion of CD4+ T cells (see “Pathophysiology and Pathogenesis,” above). The process of pyroptosis generates multimeric complexes called inflammasomes, which can also be activated by LPS. Certain viral gene products have been associated with enhanced susceptibility to apoptosis; these include Env, Tat, and Vpr. In contrast, Nef has been shown to possess antiapoptotic properties. The intensity of apoptosis correlates with the general state of activation of the immune system and not with the stage of disease or with viral burden. Multiple studies, including those examining lymphoid tissue, have demonstrated that the rate of apoptosis is elevated in HIV infection and that apoptosis is seen in “bystander” cells such as CD8+ T cells and B cells as well as in uninfected CD4+ T cells. It is likely that this bystander apoptosis of immunocompetent cells related to immune activation contributes to the general immunologic abnormalities in HIV disease. MEDICAL CONDITIONS ASSOCIATED WITH PERSISTENT IMMUNE ACTIVATION AND INFLAMMATION IN HIV DISEASE  It has become clear, as the survival of people with HIV has increased, that multiple, previously unrecognized medical complications are associated with HIV disease—and that these complications relate to chronic immune

activation and inflammation (Table 208-4). These complications can appear even after patients have experienced years of ART-induced adequate control of viral replication (plasma viremia <50 copies per milliliter of plasma) for several years. Other chronic conditions that have been reported include bone fragility, certain cancers, diabetes, kidney and liver disease, and neurocognitive dysfunction, thus present­ ing an overall picture of accelerated aging. Autoimmune Phenomena  Autoimmune phenomena are com­ monly observed in people with HIV, and they reflect, at least in part, chronic immune activation and the dysregulation of B and T cells. Although these phenomena usually occur in the absence of autoim­ mune disease, a wide spectrum of clinical manifestations that may be associated with autoimmunity have been described (see “Immu­ nologic and Rheumatologic Diseases,” below). Autoimmune phe­ nomena include antibodies against autoantigens expressed on intact lymphocytes and other cells, or against proteins released from dying cells. Antiplatelet and antierythrocyte antibodies have some clinical relevance in that they may contribute to thrombocytopenia and auto­ immune hemolytic anemia, respectively, in HIV disease (see below). Antibodies to nuclear and cytoplasmic components of cells have been reported, as have antibodies to cardiolipin and phospholipids, as well as surface receptors, including CD4, and serum proteins. However, these manifestations are relatively low in the era of ART. Molecular mimicry, either from opportunistic pathogens or from HIV itself, also is a trigger or cofactor in autoimmunity. Antibodies against the HIV envelope proteins, especially gp41, often cross-react with host proteins; the best-known examples are antibodies directed against the membrane-proximal external region (MPER) of gp41 that also react with phospholipids and cardiolipin. The phenomenon of polyreactive HIV-specific antibodies may be beneficial to the host (see “Immune Response to HIV,” below). The increased occurrence and/or exacerbation of certain autoim­ mune diseases have been reported in HIV infection; these diseases include psoriasis, idiopathic thrombocytopenic purpura, autoimmune hemolytic anemia, Graves’ disease, antiphospholipid syndrome, and primary biliary cirrhosis. Most of these manifestations were described prior to the advent of ART and have decreased in frequency since its widespread use. However, with increasing availability of ART, an immune reconstitution inflammatory syndrome (IRIS) has been increas­ ingly observed in infected individuals, particularly those with low CD4+ T cell counts (see below). IRIS is an autoimmune-like phenom­ enon characterized by a paradoxical deterioration of clinical condition, which is usually compartmentalized to a particular organ system in individuals in whom ART has recently been initiated. It is associated with a decrease in viral load and at least partial recovery of immune competence, which is usually associated with increases in CD4+ T cell counts. The immunopathogenesis of this syndrome is felt to be related to an increase in immune response against the presence of residual antigens that are usually microbial and is most commonly seen with underlying mycobacterial (Mycobacterium tuberculosis [TB] or avium complex [MAC]), fungal (cryptococcal), and viral (CMV, HHV) infec­ tions. This syndrome is discussed in more detail below. ■ ■CYTOKINES AND OTHER SOLUBLE FACTORS

IN HIV PATHOGENESIS The immune system is homeostatically regulated by a complex network of immunoregulatory cytokines, which are pleiotropic and redun­ dant and operate in an autocrine and paracrine manner. They are expressed continuously, even during periods of apparent quiescence of the immune system. On perturbation of the immune system by antigenic challenge, the expression of cytokines increases to varying degrees (Chap. 360). Cytokines that are important components of this immunoregulatory network are thought to play major roles in HIV disease, during both the early and chronic phases of infection. A potent proinflammatory “cytokine storm” is induced during the acute phase of HIV infection, likely a response of inflammatory cells to virus replicating at very high levels. Cytokines and chemokines that are induced during this early phase include the type I interferon IFN-α,

IL-15, and CXCL10, followed by IL-6, IL-12, and TNF-α, and a delayed peak of the anti-inflammatory cytokine IL-10. Soluble factors of innate immunity also are induced shortly after infection, including neopterin and β-microglobulin. Several of these early-expressed cytokines and factors are not downregulated following the early phase of HIV infec­ tion, as seen in other self-resolving viral infections, and persist during the chronic phase of infection and contribute to maintaining high levels of immune activation. Among the cytokines and factors associ­ ated with early innate immune responses, they are intended to contain viral replication, although paradoxically most are potent inducers of HIV expression/replication because of their ability to induce immune activation that leads to enhanced viral production and an increase in readily available target cells for HIV (activated CD4+ T cells). The induction of IFN-α, one of the first cytokines induced during primary HIV infection and an important element of innate immune sensing, is thought to play a particularly important role in HIV pathogenesis by inducing a large number of IFN-associated genes that activate the immune system, alter the homeostasis of CD4+ T cells, and influ­ ence the virus variants that are selected during the HIV transmission bottleneck. Other cytokines that are elevated during the chronic phase of HIV infection and linked to immune activation include IFN-γ, the CC-chemokine RANTES (CCL5), macrophage inflammatory protein (MIP) 1β (CCL4), and IL-18. Multiple cytokines and soluble factors have been associated with HIV pathogenesis at various stages of disease, in various tissues or organs, and in the regulation of HIV replication. Plasma levels of IP-10 are predic­ tive of disease progression, whereas the proinflammatory cytokine IL-6, marker of monocyte/macrophage activation soluble CD14 (sCD14), and coagulation marker d-dimer are associated with increased risk of all-cause mortality in people with HIV. In particular, IL-6, sCD14, and d-dimer are associated with increased risk of cardiovascular disease and other causes of death, even in individuals receiving ART. IL-18 has also been shown to play a role in the development of the HIV-associated lipodystrophy syndrome. Elevated levels of TNF-α and IL-6 have been demonstrated in plasma and cerebrospinal fluid (CSF), and increased expression of TNF-α, IL-1β, IFN-γ, and IL-6 has been demonstrated in the lymph nodes of people with HIV prior to disease progression and a shift to TGF-β in advanced disease (see “Lymphoid Organs and HIV Pathogenesis,” above). RANTES (CCL5), MIP-1α (CCL3), and MIP-1β (CCL4) (Chap. 360) inhibit infection by and spread of R5 HIV-1 strains, while stromal cell–derived factor (SDF) 1 inhibits infection by and spread of X4 strains. The mechanisms whereby the CC-chemokines RANTES (CCL5), MIP-1α (CCL3), and MIP-1β (CCL4) inhibit infection of R5 strains of HIV, or SDF-1 blocks X4 strains of HIV, involve blocking of the binding of the virus to its co-receptors, the CC-chemokine receptor CCR5 and the CXC-chemokine receptor CXCR4, respectively. Other soluble factors that have not yet been fully characterized, such as soluble CD8 antiviral factor (CAF), also have been shown to suppress HIV rep­ lication, independent of co-receptor usage. ■ ■LYMPHOCYTE TURNOVER IN HIV INFECTION The immune systems of patients with HIV infection are characterized by a profound increase in lymphocyte turnover that is immediately reduced with effective ART. Studies utilizing in vivo or in vitro labeling of lymphocytes in the S-phase of the cell cycle have demonstrated a tight correlation between the degree of lymphocyte turnover and plasma vire­ mia. This increase in turnover is seen in CD4+ and CD8+ T lymphocytes as well as B lymphocytes and can be observed in peripheral blood and lymphoid tissue. Mathematical models derived from these data suggest that one can view the lymphoid pool as consisting of dynamically distinct subpopulations of cells that are differentially affected by HIV infection. A major consequence of HIV infection appears to be a shift in cells from a more quiescent pool to a pool with a higher turnover rate. It is likely that a consequence of a higher rate of turnover is a higher rate of cell death. It has been suggested that the more rapid decline in CD4+ compared with CD8+ T cells may be linked to alterations in inflammatory and homeostatic cytokines that cause increased activation-induced death without replenishment of CD4+ T cells. (See Table 208-5 for additional mechanisms of depletion.)

TABLE 208-5  Proposed Mechanisms of CD4+ T Cell Dysfunction and Depletion DIRECT MECHANISMS INDIRECT MECHANISMS Loss of plasma membrane integrity due to viral budding Aberrant intracellular signaling events Accumulation of unintegrated viral DNA Activation of DNA-dependent protein kinase during viral integration into host genome Autoimmunity Interference with cellular RNA processing Innocent bystander killing of viral

antigen–coated cells Intracellular gp120–CD4 autofusion events Apoptosis, pyroptosis (caspase

1–associated inflammation), autophagy Syncytia formation Inhibition of lymphopoiesis from reduced survival cytokines and lymphoid tissue integrity Activation-induced cell death Elimination of HIV-infected cells by virusspecific immune responses ■ ■THE ROLE OF VIRAL RECEPTORS AND CO-RECEPTORS IN HIV PATHOGENESIS CCR5 AND CXCR4  As mentioned above, HIV-1 utilizes two major coreceptors along with CD4 to bind to, fuse with, and enter target cells; these co-receptors are CCR5 and CXCR4, which are also receptors for certain endogenous chemokines. Strains of HIV that utilize CCR5 as a co-receptor are referred to as R5 viruses. Strains of HIV that utilize CXCR4 are referred to as X4 viruses. Many virus strains are dual tropic in that they utilize both CCR5 and CXCR4; these are referred to as R5X4 viruses. CHAPTER 208 Human Immunodeficiency Virus Disease: AIDS and Related Disorders
The natural chemokine ligands for the major HIV co-receptors can readily block entry of HIV. For example, the CC-chemokines RANTES (CCL5), MIP-1α (CCL3), and MIP-1β (CCL4), which are the natu­ ral ligands for CCR5, block entry of R5 viruses, whereas SDF-1, the natural ligand for CXCR4, blocks entry of X4 viruses. The mechanism of inhibition of viral entry is a steric inhibition of binding that is not dependent on signal transduction (Fig. 208-25). The transmitting virus is almost invariably an R5 virus that pre­ dominates during the early stages of HIV disease, although in the era of deep sequencing, more X4 variants have been detected in early disease than were previously reported. In the absence of ART or in therapeutic failures, there is a transition to a predominantly X4 virus in approxi­ mately half of individuals infected with subtype B virus. The transition is often preceded by dual R5X4 strains, and detection of X4 variants is associated with a relatively rapid decline in CD4+ T cell counts, increased HIV plasma viremia, and progression of disease. However, the other half of infected individuals progress in their disease while maintaining predominance of an R5 virus, and individuals infected with non–subtype B clades more rarely switch from CCR5 tropism to CXCR4 tropism than do those infected with subtype B. The reason for this difference is unclear. The basis for the tropism of different envelope glycoproteins for either CCR5 or CXCR4 relates to the ability of the HIV envelope, including the third variable region (V3 loop) of gp120, to interact with these co-receptors. In this regard, binding of gp120 to CD4 induces a conformational change in gp120 that increases its affinity for the relevant co-receptor. Finally, R5 viruses are more efficient in infecting monocytes/macrophages and microglial cells of the brain (see “Neuro­ pathogenesis in HIV Disease,” below). THE INTEGRIN `4a7  The integrin a4β7 is an accessory receptor for HIV. It is not essential for the binding and infection of a CD4+ T cell by HIV; however, it likely plays an important role in the transmission of HIV at mucosal surfaces such as the genital tract and gut and con­ tributes somewhat to the pathogenesis of HIV disease. The integrin a4β7, which is the gut homing receptor for peripheral T cells, binds

ENV HIV HIV CD4 CD4+ Target Cell SDF-1 CXCR4 A ENV HIV HIV CD4 CD4+ Target Cell PART 5 Infectious Diseases CC-Chemokine (RANTES, MIP-1α, MIP-1β) CCR5 B FIGURE 208-25  Model for the role of co-receptors CXCR4 and CCR5 in the efficient binding and entry of X4 (A) and R5 (B) strains of HIV-1, respectively, into CD4+ target cells. Blocking of this initial event in the virus life cycle can be accomplished by inhibition of binding to the co-receptor by the normal ligand for the receptor in question. The ligand for CXCR4 is stromal cell–derived factor (SDF-1); the ligands for CCR5 are RANTES, MIP-1a, and MIP-1b. in its activated form to a specific tripeptide in the V2 loop of gp120, resulting in rapid activation of leukocyte function–associated antigen 1 (LFA-1), the central integrin in the establishment of virologic synapses, which facilitate efficient cell-to-cell spread of HIV. It has been demon­ strated that α4β7high CD4+ T cells are more susceptible to productive infection than are α4β7low–neg CD4+ T cells because this cellular subset is enriched with metabolically active CD4+ T cells that are CCR5high. These cells are present in the mucosal surfaces of the gut and genital tract. Importantly, it has been demonstrated that the virus that is trans­ mitted during sexual exposure binds much more efficiently to α4β7 than does the virus that diversifies from the transmitting virus over time by mutation, particularly involving the accumulation of glycogens on the surface of the HIV envelope (see “Early Events in HIV Infection: Primary Infection and Initial Dissemination of Virus,” above). ■ ■CELLULAR TARGETS OF HIV CD4+ T lymphocytes and to a lesser extent CD4+ cells of the myeloid lineage are the principal targets of HIV and are the only cells that can be productively infected with HIV. Circulating DCs have been reported to express low levels of CD4, although high expression of the restric­ tion factor SAMHD1 in myeloid (mDC) and plasmacytoid (pDC) DCs limits HIV replication in these cells by depleting intracellular pools of dNTPs and directly degrading viral RNA. Epidermal Langerhans cells express CD4 and have been infected by HIV in vivo, although they too restrict replication by high expression of the host restriction fac­ tor langerin. As has been shown in vivo for DCs, FDCs, and B cells,

Langerhans cells are more likely to bind and transfer virus to activated CD4+ T cells than to be productively infected themselves. Of potential clinical relevance is the demonstration that thymic precursor cells, which were assumed to be negative for CD3, CD4, and CD8 molecules, express low levels of CD4 and can be infected with HIV in vitro. In addition, human thymic epithelial cells trans­ planted into an immunodeficient mouse can be infected with HIV by direct inoculation of virus into the thymus. Since these cells may play a role in the normal regeneration of CD4+ T cells, it is possible that their infection and depletion contribute, at least in part, to the impaired ability of the CD4+ T cell pool to completely reconstitute itself in certain infected individuals in whom ART has suppressed plasma viremia to below the level of detection (see below). In addition, CD34+ monocyte precursor cells have been shown to be infected in vivo in patients with advanced HIV disease. It is likely that these cells express low levels of CD4, and therefore it is not essential to invoke CD4-independent mechanisms to explain the infection. The clinical relevance of this finding is unclear. ■ ■QUALITATIVE AND QUANTITATIVE ABNORMALITIES OF MONONUCLEAR CELLS CD4+ T Cells  The primary immunopathogenic lesion in HIV infection involves CD4+ T cells, and the range of CD4+ T cell abnor­ malities in advanced HIV infection is broad. The defects are both quantitative and qualitative and ultimately impact virtually every limb of the immune system, indicating the critical dependence of the integ­ rity of the immune system on the inducer/helper function of CD4+

T cells. In advanced HIV disease, most of the observed immune defects can ultimately be explained by the quantitative depletion of CD4+ T cells. However, T cell dysfunction can be demonstrated in patients early in the course of infection, even when the CD4+ T cell count is in the low–normal range. The degree and spectrum of dysfunctions increase as the disease progresses, reflecting the range of CD4+ T cell functional heterogeneity, especially in lymphoid tissues. One of the first sites of intense HIV replication is in the GALT where CD4+ TH17 cells reside; they are important for host defense against extracellular pathogens in the intestinal mucosa and help maintain the integrity of the gut epithe­ lium. In HIV infection, they are depleted by direct and indirect effects of viral replication and cause loss of gut homeostasis and integrity, as well as a shift toward a TH1 phenotype. Studies have shown that even after many years of ART, normalization of the CD4+ T cells in the GALT remains incomplete. In lymph nodes, HIV perturbs another important subset of the CD4+ helper T lineage, namely TFH cells (see “Lymphoid Organs and HIV Pathogenesis,” above). TFH cells, which are derived either directly from naïve CD4+ T cells or from other TH precursors, migrate into B cell follicles during germinal center reactions and pro­ vide help to antigen-specific B cells through cell–cell interactions and secretion of cytokines to which B cells respond, the most important of which is IL-21. In addition, it has been shown that people with HIV who have broadly neutralizing antibodies have higher frequencies of memory TFH CD4+ T cells. As with TH17 cells, TFH cells are highly susceptible to HIV infection. However, in contrast to TH17 and most other CD4+ T cell subsets, the number of TFH cells is increased in lymph nodes of people with HIV, especially those who are viremic. It is unclear whether this increase is helpful to responding B cells, although the likely outcome is that the increase in numbers is detrimental to the quality of the humoral immune response against HIV (see “Immune Response to HIV,” below). In addition, defects of central memory cells are a critical component of HIV immunopathogenesis. The progressive loss of antigen-specific CD4+ T cells has important implications for the control of HIV infection. In this regard, there is a correlation between the maintenance of HIV-specific CD4+ T cell proliferative responses and improved control of infection. Essentially every T cell function has been reported to be abnormal at some stage of HIV infection. Loss of polyfunctional HIV-specific CD4+ T cells, especially those that produce IL-2, occurs early in disease, whereas IFN-producing CD4+ T cells are maintained longer and do not correlate with control of HIV viremia. Other abnormalities include impaired expression of IL-2 receptors,

defective IL-2 production, reduced expression of the IL-7 receptor (CD127), and a decreased proportion of CD4+ T cells that express CD28, a major co-stimulatory molecule necessary for the normal activation of T cells, which is also depleted due to aging. Cells lacking expression of CD28 do not respond normally to activation signals and may express markers of terminal activation including HLA-DR, CD38, and CD45RO. As mentioned above (“The Role of Immune Activation and Inflammation in HIV Pathogenesis”), a subset of CD4+ T cells referred to as T regulatory cells, or T-regs, may be involved in damp­ ing aberrant immune activation that propagates HIV replication. The presence of these T-reg cells correlates with lower viral loads and higher CD4+/CD8+ T cell ratios. A loss of this T-reg capability with advanced disease may be detrimental to the control of virus replication. It is difficult to explain completely the profound immunodeficiency noted in people with HIV solely based on direct infection and quantita­ tive depletion of CD4+ T cells. This is particularly apparent during the early stages of HIV disease, when CD4+ T cell numbers may be only marginally decreased. In this regard, it is likely that CD4+ T cell dysfunc­ tion results from a combination of depletion of cells due to direct infec­ tion of the cell and multiple virus-related but indirect effects on the cell (Table 208-5). Several of these effects have been demonstrated ex vivo and/or by the analysis of cells isolated from the peripheral blood. Soluble viral proteins, particularly gp120, can bind with high affinity to the CD4 molecules on uninfected T cells and monocytes; in addi­ tion, virus and/or viral proteins can bind to DCs or FDCs. HIV-specific antibodies can recognize these bound molecules and potentially collaborate in the elimination of the cells by ADCC. HIV envelope glycoproteins gp120 and gp160 manifest high-affinity binding to the CD4 molecule as well as to various chemokine receptors. Intracellular signals transduced by gp120 through both CD4 and CCR5/CXCR4 have been associated with a number of immunopathogenic processes including anergy, apoptosis, and abnormalities of cell trafficking. The molecular mechanisms responsible for these abnormalities include dysregulation of the T cell receptor–phosphoinositide pathway, p56lck activation, phosphorylation of focal adhesion kinase, activation of the MAP kinase and ras signaling pathways, and downregulation of the co-stimulatory molecules CD40 ligand and CD80. The inexorable decline in CD4+ T cell counts that occurs in most untreated people with HIV may result in part from the inability of the immune system to regenerate over an extended period of time the rap­ idly turning over CD4+ T cell pool efficiently enough to compensate for both HIV-mediated and naturally occurring attrition of cells. In this regard, the degree and duration of decline of CD4+ T cells at the time of initiation of therapy is an important predictor of the restora­ tion of these cells. A person who maintains a very low CD4+ T cell count for a considerable period before the initiation of ART almost invariably has an incomplete reconstitution of such cells. At least two major mechanisms may contribute to the failure of the CD4+ T cell pool to reconstitute itself adequately over the course of HIV infection. The first is the destruction of lymphoid precursor cells, including thymic and bone marrow progenitor cells; the other is the gradual disruption of the lymphoid tissue architecture and microenvironment, which is essential for efficient regeneration of immunocompetent cells. Finally, during the advanced stages of CD4+ T lymphopenia, there are increased serum levels of the homeostatic cytokine IL-7. It was initially felt that this elevation was a homeostatic response to the lymphopenia; however, recent findings suggest that the increase in serum IL-7 was a result of reduced utilization of the cytokine related to the loss of cells expressing the IL-7 receptor, CD127, which serves as a normal physi­ ologic regulator of IL-7 production. CD8+ T Cells  A relative CD8+ T lymphocytosis is generally asso­ ciated with high levels of HIV plasma viremia and likely reflects an immune response to the virus as well as dysregulated homeostasis asso­ ciated with generalized immune activation. During the late stages of HIV infection, there may be a significant reduction in the numbers of CD8+ T cells despite the presence of high levels of viremia. HIV-specific CD8+ CTLs have been demonstrated in people with HIV early in the course of disease, and their emergence often coincides with a decrease

in plasma viremia—an observation that is a factor in the proposal that virus-specific CTLs can control HIV disease for a finite period of time in a certain percentage of infected individuals. However, emergence of HIV escape mutants that ultimately evade these HIV-specific CD8+ T cells has been described in most people with HIV who are not receiving ART. In addition, as the disease progresses, the functional capability of these cells gradually decreases, at least in part due to the persistent nature of HIV infection that causes functional exhaustion via the upregulation of inhibitory receptors such as PD-1, TIGIT, LAG-3, and TIM-3 on HIV-specific CD8+ T cells (see “The Role of Immune Activation and Inflammation in HIV Pathogenesis,” above). As chronic immune activation persists, there are also systemic effects on CD8+ T cells, such that as a population they assume an abnormal phenotype characterized by expression of activation markers such as co-expression of HLA-DR and CD38 with an absence of expression of the IL-2 receptor (CD25) and a reduced expression of the IL-7 receptor (CD127). In addition, CD8+ T cells lacking CD28 expression are increased in HIV disease, reflecting a skewed expansion of a less differentiated CD8+ T cell subset. This skewing of subsets is also asso­ ciated with diminished polyfunctionality, a qualitative difference that distinguishes elite controllers from progressors. Elite controllers can also be distinguished from progressors by the maintenance in the for­ mer of a high proliferative capacity of their HIV-specific CD8+ T cells coupled to increases in perforin expression and elimination of infected targets, characteristics that are markedly diminished in advanced HIV disease. It has been reported that the phenotype of CD8+ T cells in people with HIV may be of prognostic significance. Those individuals whose CD8+ T cells developed a phenotype of HLA-DR+/CD38–

following seroconversion had stabilization of their CD4+ T cell counts, whereas those whose CD8+ T cells developed a phenotype of HLA-DR+/ CD38+ had a more aggressive course and a poorer prognosis. In addi­ tion to the defects in HIV-specific CD8+ CTLs, functional defects in other MHC-restricted CTLs, such as those directed against influenza and CMV, have been demonstrated. CD8+ T cells secrete a variety of soluble factors that inhibit HIV replication, including the CC-chemokines RANTES (CCL5), MIP-1α (CCL3), and MIP-1β (CCL4) and poten­ tially several yet-unidentified factors. The presence of high levels of HIV viremia in vivo as well as exposure of CD8+ T cells in vitro to HIV envelope, both of which are associated with aberrant immune activation, have been shown to be associated with a variety of cellular functional abnormalities. Furthermore, since the integrity of CD8+ T cell function depends in part on adequate inductive signals from CD4+ T cells, the defect in CD8+ CTLs is likely compounded by the quantita­ tive loss and qualitative dysfunction of CD4+ T cells.

CHAPTER 208 Human Immunodeficiency Virus Disease: AIDS and Related Disorders
B Cells  The predominant defect in B cells from people with HIV is one of aberrant cellular activation, which is reflected by increased propensity to terminal differentiation and immunoglobulin secretion, as well as increased expression of markers of activation and exhaustion. As a result of activation and differentiation in vivo and induction of inhibitory pathways of regulation, B cells from HIV viremic patients manifest a decreased capacity to undergo cell signaling and mount a proliferative response ex vivo. B cells from people with HIV manifest enhanced spontaneous secretion of immunoglobulins in vitro, a pro­ cess that reflects their highly differentiated state in vivo. There is also an increased incidence of EBV-related B cell lymphomas in people with HIV that are likely due to combined effects of defective T cell immune surveillance and increased B cell turnover that increases the risk of oncogenesis. Untransformed B cells cannot be infected with HIV, although HIV or its products can activate B cells directly. B cells from patients with high levels of viremia bind virions to their surface via the CD21 complement receptor. It is likely that in vivo activation of B cells by replication-competent or defective virus as well as viral products during the viremic state accounts at least in part for their activated phe­ notype. B cell subpopulations from people with HIV undergo a number of changes over the course of HIV disease, including the attrition of resting memory B cells and replacement with several aberrant mem­ ory and differentiated B cell subpopulations that collectively express reduced levels of CD21 and either increased expression of activation

markers or inhibitory receptors associated with functional exhaustion. The more activated and differentiated B cells are also responsible for increased secretion of immunoglobulins and increased susceptibility to Fas-mediated apoptosis. In more advanced disease, there is also the appearance of immature B cells associated with CD4+ T cell lymphope­ nia. Despite increased frequencies of germinal center B cells and CD4+ TFH cells, both of which are required for effective humoral immunity, cognate B cell–CD4+ T cell interactions in lymphoid tissues are per­ turbed in people with HIV, especially those with persistent viremia. In vivo, the aberrant activated state of B cells manifests itself by hypergam­ maglobulinemia and by the presence of circulating immune complexes that bind the B cells and restrict their capacity to respond to further stimulation. People with HIV respond poorly to primary and secondary immunizations with protein and polysaccharide antigens. Using immu­ nization with influenza vaccine, it has been demonstrated that there is a memory B cell defect in people with HIV, particularly those with high levels of HIV viremia. There is also evidence that responses to HIV and non-HIV antigens in infected individuals, especially those who remain viremic, are enriched in abnormal subsets of B cells that either are highly prone to apoptosis or show signs of functional exhaustion. Taken together, these B cell defects are likely responsible at least in part for the inadequate humoral response to HIV as well as to decreased response to vaccinations and the increase in certain bacterial infections seen in advanced HIV disease in adults. In addition, they likely contribute to the inadequacy of host defenses against bacterial infections that play a role in the increased morbidity and mortality of children with HIV. The absolute number of circulating B cells also may be depressed in HIV infection; this phenomenon likely reflects increased activation-induced apoptosis as well as a redistribution of cells out of the circulation and into the lymphoid tissue—phenomena that are associated with ongoing viral replication. Monocytes/Macrophages  Circulating monocytes are generally normal in number in people with HIV; however, there is evidence of increased activation within this lineage. The increased level of sCD14 and other biomarkers (see above) reported in people with HIV is an indirect marker of monocyte activation in vivo. Levels of sCD14 can remain elevated in individuals whose plasma viremia has been sup­ pressed by ART for several years, an indicator of the residual immune activation and inflammation observed in HIV infection and effects on the monocyte/macrophage lineage. Multiple other abnormalities of circulating monocytes have been reported in people with HIV, many of which may be related directly or indirectly to aberrant in vivo immune activation. In this regard, increased levels of lipopolysac­ charide (LPS) are found in the sera of people with HIV due, at least in part, to translocation across the gut mucosal barrier (see above). LPS is a highly inflammatory bacterial product that preferentially binds to macrophages through CD14 and Toll-like receptors, resulting in cel­ lular activation. In the peripheral blood, expansion of monocytes that express the intermediate and nonclassical marker CD16 and markers of activation (HLA-DR) and stimulation (CD40 and CD86) has been described, especially in viremic individuals. Activated monocytes are also responsible for secretion of inflammatory cytokines and chemo­ kines observed in HIV infection, including CXCL10, IL-1β, and IL-6. Monocytes express the CD4 molecule and several co-receptors for HIV on their surface, and thus are potential targets of HIV infection. How­ ever, in vivo infection of circulating monocytes is difficult to demonstrate, although infection of tissue macrophages and macrophage-lineage cells in the brain (infiltrating macrophages or resident microglial cells) and lung (pulmonary alveolar macrophages) can be demonstrated easily. Tissue macrophages are an important source of HIV during the inflam­ matory response associated with opportunistic infections and can serve as persistent reservoirs of HIV infection, thus representing an obstacle to the eradication of HIV by antiretroviral drugs. Dendritic and Langerhans Cells  DCs and Langerhans cells are not productively infected with HIV, likely in part due to their expres­ sion of host restriction factors, including APOBEC3G and SAMHD1 (see above). However, they are thought to play an important role in the initiation of HIV infection by virtue of the ability of HIV to bind

PART 5 Infectious Diseases

to cell-surface C-type lectin receptors, particularly DC-SIGN (see above) and langerin. However, while langerin provides a host barrier for replication by trafficking HIV to acidic compartments for degrada­ tion, DC-SIGN retains HIV in early endosomal compartments. This allows efficient presentation of intact virus to CD4+ T cell targets that become infected; complexes of infected CD4+ T cells and DCs provide an optimal microenvironment for virus replication. Furthermore, pDCs secrete large amounts of IFN-α in response to viral infections and as such play an important role in innate sensing of HIV during early phase of infection. The numbers of circulating pDCs and mDCs are decreased in HIV infection through mechanisms that remain unclear, although several studies have shown increased lymphoid tissue recruitment of DCs associated with lymphoid hyperplasia and inflammation. The mDCs are also involved in the initiation of adaptive immunity in drain­ ing lymph nodes by presenting antigen to T cells and B cells, as well as by secreting cytokines such as IL-12, IL-15, and IL-18 that activate other immune cells, although these functions are perturbed in HIV infection. Natural Killer Cells and Innate Lymphoid Cells  NK cells represent the prototypical member of innate lymphoid cells (ILCs) that collectively provide tissue homeostasis and immunosurveillance against virus-infected cells, certain tumor cells, and allogeneic cells (Chap. 360). There are no convincing data that HIV productively infects NK cells in vivo; however, functional abnormalities in NK cells have been observed throughout the course of HIV disease, and the severity of these abnormalities increases as disease progresses. NK cells are part of the innate immune system and act by direct killing of infected cells and secretion of antiviral cytokines and chemokines. In early HIV infection there is an increase in the activation of NK cells, and the capacity to secrete IFN-γ is maintained, although they manifest reduced cytotoxic function as a result of altered maturation. During chronic HIV infection, both NK cell cytotoxicity and cytokine secre­ tion become impaired. Given that HIV infection of target cells down­ regulates HLA-A and B, but not HLA-C and D molecules, this may explain in part the relative inability of NK cells to kill HIV-infected tar­ get cells. However, the NK cell impairments, especially in patients with high levels of virus replication, are associated with an expansion of an “anergic” CD56–/CD16+ NK cell subset. This abnormal subset of NK cells manifests an increased expression of inhibitory NK cell receptors (iNKRs) and a substantial decrease in expression of natural cytotoxic­ ity receptors (NCRs) and shows a markedly impaired lytic activity. The overrepresentation of this abnormal subset of NK cells may explain in part the observed defects in NK cell function in people with HIV and likely begins to occur during primary infection. The relative expression of iNKRs and NCRs—as well as their ligands, which include HLA class I molecules—has an impact on the antiviral functions associated with NK cells, including direct killing and ADCC. Polymorphisms in iNKR and NCR alleles have been linked to HIV-1 disease outcomes, and there are indications that the early control of HIV may be mediated by cytotoxic NK cell-mediated responses. NK cells may also serve as sources of HIV-inhibitory soluble factors, including CC-chemokines such as MIP-1α (CCL3), MIP-1β (CCL4), and RANTES (CCL5). Finally, both inflammatory cytokines and alterations in the GALT of HIV infected individuals disrupt NK cells and other ILCs. ■ ■GENETIC FACTORS IN HIV-1 AND

AIDS PATHOGENESIS Candidate gene approaches and genome-wide association studies (GWAS) have identified polymorphisms in host genes that contribute to inter-individual variation in (1) the risk of acquiring HIV, (2) the steady-state levels of HIV that are established soon after infection (virologic set point), (3) the rate at which untreated HIV infection progresses to AIDS defined by a CD4+ T cell count that is lower than 200 cells/mm3 and/or development of AIDS-defining illnesses, (4) the level of immune reconstitution (e.g., CD4+ cell counts) achieved and risk of non-AIDS-associated diseases after initiation of virally suppressive antiretroviral therapy (ART), and (5) adverse reactions to antiretroviral agents. The key polymorphisms that influence these five outcomes are summarized in Table 208-6, and their identification has

TABLE 208-6  Host Genetic Factors Influencing HIV/AIDS Pathogenesis and Therapy Responses GENEa GENETIC VARIATION MECHANISMSb GENETIC ASSOCIATIONSc Genes in MHC Locus HLA-B B27 and B57 Altered presentation of specific HIV antigens Slower progression to AIDS; lower viral load B35 Restriction of specific HIV peptide presentation Faster progression to AIDS; higher viral load HLA-Bw4 Providing ligands for activating KIR Slower progression to AIDS B57:01 Altered presentation of specific HIV antigens (as above); possible abacavir-specific activation of cytokine-producing CD8+ T cells in carriers of this allele HLA-B? 21M allele Enhanced HLA-E expression levels correlated with higher HLA-A expression and inhibition of NKG2Aexpressing cells B57:03 bearing the rs2523608-A allele Altered presentation of specific HIV antigens Variant overexpressed in HIV-1 controllers of African descent HLA class I allele Homozygosity of HLA-class I alleles Reduced repertoire for epitope recognition Faster progression to AIDS; increased risk of mother-to-child transmission Shared donor–recipient HLA alleles Preadaptation of HIV strains Faster progression to AIDS Rare HLA alleles Limited adaptation of HIV strains; less frequent escape mutants HLA class II allele HLA-DRB1 alleles Influence on protein specificity of CD4+ T cell responses to HIV Gag and Nef proteins HLA extended haplotype A1-B8-DR3-DQ2 (AH 8.1) Increased proinflammatory responses; higher TNF-α production HLA-C rs9264942-C allele (35 kb upstream of HLA-C) in linkage with rs67384697-Del Increased expression of HLA-C by reducing binding of miRNA-148a rs5010528-G (1 kb upstream of HLA-C) Unknown Higher risk of developing nevirapine-associated hypersensitivity HCP5 rs2395029-G Linkage disequilibrium with HLA-B57:01 Lower viral load and slower progression to AIDS MICA Noncoding SNV near MICA, rs4418214-T May affect HLA class I peptide presentation— linkage with protective HLA-B alleles PSORS1C3 rs3131018-A May affect HLA class I peptide presentation Enriched in HIV-1 controllers ZNRD1 rs9261174-C Possible interference in processing of HIV transcripts; influence ZNRD1 expression Chemokine Receptors CCR5 rs333: 32-bp deletion in the ORF (Δ32) found in persons of European descent Truncated CCR5 protein; reduced co-receptor activity of R5 HIV strain Promoter SNVs, haplotypes (HHA to HHG*2) Altered CCR5 expression, e.g., HHE haplotype correlates with high CCR5 expression Increased expression of the lncRNA RP11-24-11.2, which corresponds to an antisense transcript that overlaps CCR5 (CCR5AS); results in increased CCR5 expression rs1015164 G→A (34 kb downstream from CCR5 and close to CCRL2) CCR2 rs1799864: SNV in ORF (64 V→I) Linkage with polymorphisms in CCR5 promoter 64I-bearing haplotype associated with delayed progression to AIDS CCRL2 rs3204849: SNV in ORF (167 Y→F) SNV in linkage with CCR5 haplotype 167F associated with accelerated progression to AIDS and PCP CXCR6 rs2234358: G→T in the 3’ UTR Trafficking of effector T cells and activation of NK T cells; minor HIV co-receptor CX3CR1 SNVs in ORF: rs3732379 (249 V→I) and rs3732378 (280 T→M) Alleles bearing 249I and 280M reduce receptor expression and binding of fractalkine, the CX3CR1 ligand DARC rs2814778: Promoter SNV (–46T→C) found in persons of African descent –46C/C associated with absent DARC expression (Duffy null), low neutrophil counts, and altered circulating chemokine levels as well as HIV binding to RBCs and trans-infection of HIV-1 Chemokines CCL3L, CCL4L Gene copy number of CCL3L and CCL4L High numbers of CCL3L and CCL4L genecontaining segmental duplications correlated with high CCL3L and CCL4L levels CCL5 Promoter SNVs Altered gene expression Influenced HIV-AIDS susceptibility CCL2 rs1024611: Promoter SNV (–2578 T→G) –2578G allele: increased CCL2 expression and monocyte recruitment

Slower progression to AIDS; higher risk of abacavir-associated hypersensitivity Higher viral load, reduced CD4+ counts, and accelerated disease progression Protection against HIV infection HLA-DRB115:02—lower viral load HLA-DRB103:01—higher viral load Faster progression to AIDS Decreased viral load set point CHAPTER 208 Enriched in HIV-1 controllers Human Immunodeficiency Virus Disease: AIDS and Related Disorders
Slower disease progression to AIDS Δ32/Δ32: CCR5-null state associated with resistance to acquiring HIV infection Δ32/wild type: slower progression to AIDS; better CD4+ T cell recovery during ART HHE/HHE: increased HIV susceptibility and faster progression to AIDS rs1015164A allele associated with higher viral load Prevalence of rs2234358-T lower in long-term nonprogressors and viremic controllers of African descent 249I and 280M associated with faster AIDS progression in persons of European descent –46C/C: increased risk of acquiring HIV but slower HIV disease progression; Duffy null–associated low neutrophil trait associated with increased HIV risk Gene copy number lower than population median associated with increased HIV-AIDS susceptibility and lower CD4+ T cell recovery during ART –2578G/G associated with increased risk of developing HIV-1–associated dementia and faster AIDS onset (Continued)

TABLE 208-6  Host Genetic Factors Influencing HIV/AIDS Pathogenesis and Therapy Responses GENEa GENETIC VARIATION MECHANISMSb GENETIC ASSOCIATIONSc CXCL12 rs7919208: Promoter SNV (G→A) rs7919208A creates a new transcription factor binding site associated with increased CXCL12 expression Cytokines IL-6 rs1800795: Promoter SNV (–174 G→C) –174G/G associated with increased IL-6 and CRP levels IL-7RA rs6897932: Coding SNV (244 T→I) 244 I/I associated with increased signal transduction and proliferation in response to IL-7 IL-10 rs1800872: Promoter SNV (–592 C→A) –592A associated with decreased IL-10 levels –592A associated with increased HIV infection risk and AIDS progression rate Drug-Metabolizing Enzyme Gene CYP2B6 Multiple variants (e.g., rs3745274

[516 G→ T], i.e., CYP2B6*6) CYP2B6 variants influence enzyme activity 516T/T associated with higher risk of adverse reactions to efavirenz Innate Immunity Genes MBL Alleles defined by 3 coding SNVs Low plasma concentration and structural variation of MBL protein X allele (promoter SNV –221) Decreased levels of MBL protein Faster progression to AIDS with X/X genotype APOBEC3G rs8177832: ORF SNV (186 H→R) Reduced anti–HIV-1 activity 186R associated with rapid AIDS progression in persons of African descent APOBEC3F Haplotype tagged by rs2076101 in ORF (231 I→V) 231V variant may influence Vif-mediated APOBEC3F degradation TLR7 rs179008: ORF SNV (32A→T) on Chr. X Lower TLR7 mRNA translation efficiency and impaired TLR7-dependent IFN-α production PARD3B rs11884476 near exon 20: C→G Direct interaction with HIV signaling through SMAD family of proteins PART 5 Infectious Diseases IFNL4 rs368234815: Frameshift mutation (TT→ΔG) Polymorphism in IFNL4 gene in linkage with a IFNL3 variant; this haplotype influences IFNL3 levels rs8099917: T→G Unknown rs8099917-G associated with higher susceptibility to KS Others ApoE E4 allele defined by two coding SNVs ApoE is an HIV-1–inducible inhibitor of

HIV-1 replication and infectivity in macrophages ApoL1/MYH9 Several risk haplotypes, including G1 Overexpression of the ApoL1 kidney risk variants may increase kidney cell death RYR3 rs2229116: ORF SNV (A →G) Unknown; potential impact on calcium signaling and homeostasis PROX1 rs17762192-G 36 kb upstream of PROX1 Unknown; presumably due to its impact on PROX1 expression, which is a negative regulator of IFN-γ Gene–Gene Interaction KIR+HLA KIR3DS1 interaction with HLA-Bw4-80Ile Altered NK cell activity required to eliminate

HIV-infected cells KIR2DL3 interaction with HLA-C1 Reduction of inhibitory KIR likely resulting in increased immune activation, impaired killing of latently infected cells, and a higher proviral burden KIR3DL1 I47V interaction with HLA-B57:01 Variation in an immune NK cell receptor that binds B57:01, modifying the protective effect of B*57:01 LILRB2+HLA LILRB2 interaction with HLA class I Regulation of dendritic cells by LILRB2-HLA engagement CCL3L1+ CCR5 Low CCL3L1 gene copies + detrimental CCR5 genotypes Low CCL3L1 and high CCR5 expression Increased HIV/AIDS susceptibility and reduced immune reconstitution during ART aRepresentative genes and polymorphisms and bpossible mechanisms are listed. cSome of the associations are population specific and may display cohort-specific effects. Most of the associations were derived from persons of European descent. Abbreviations: APOBEC, apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like; ApoE, apolipoprotein E; ApoL1, apolipoprotein L1; ART, antiretroviral therapy; CCL, CC ligand; CCL3L, CCL3-like; CCR5, CC chemokine receptor 5; CCR5AS, CCR5 antisense RNA; CCRL2, CC chemokine receptor–like 2; CRP, C-reactive protein; CYP2B6, cytochrome P450 family 2 subfamily B member 6; CXCL12, chemokine (C-X-C motif) ligand 12; CXCR6, chemokine (C-X-C motif) receptor 6; CX3CR1, chemokine (C-X3-C motif) receptor 1; DARC, Duffy antigen receptor for chemokines; Del, deletion; HCP5, HLA class I histocompatibility antigen protein P5; HHE, human haplogroup E; HLA, human leukocyte antigen; IFN, interferon; IFNL3, interferon λ3 gene; IFNL4, interferon λ4 gene; IL, interleukin; IL-7RA, interleukin 7 receptor α; KIR, killer cell immunoglobulin-like receptors; KS, Kaposi’s sarcoma; LILRB2, leukocyte immunoglobulin-like receptor B2; MBL, mannose-binding lectin; MHC, major histocompatibility complex; MICA, MHC class I polypeptide-related sequence A; MYH9, myosin heavy chain 9; NK, natural killer; ORF, open reading frame; PARD3B, par-3 family cell polarity regulator beta; PCP, Pneumocystis jirovecii pneumonia; PROX1, prospero homeobox 1; PSORS1C3, psoriasis susceptibility 1 candidate 3; RYR3, ryanodine receptor 3; SMAD, mothers against decapentaplegic homolog; SNV, single nucleotide variant; rs#, SNV identification number; TLR7, Toll-like receptor 7; TNF-α, tumor necrosis factor α; UTR, untranslated region; VL, viral load; ZNRD1, zinc ribbon domain containing 1; +, present; –, absent. Sources: SK Ahuja, W He. Reviews for additional information; P An et al: Trends Genet 26:119, 2010; J Fellay: Antivir Ther 14:731, 2009; RA Kaslow et al: J Infect Dis 191:S68, 2005; D van Manen et al: Retrovirology 9:70, 2012; MP Martin et al: Immunol Rev 254:245, 2013; S Limou et al: Front Immunol 4:118, 2013; PJ McLaren et al: Curr Opin HIV AIDS 10:110, 2015; PJ McLaren et al: Proc Natl Acad Sci USA 112:14658, 2015; PJ McLaren, M Carrington: Nat Immunol 16:577, 2015; P An et al: PLoS Genet 12:e1005921, 2016; F Pereyra et al: Science 330:1551, 2010; I Bartha et al: PLoS Comput Biol 13:e1005339, 2017; S Kulkarni et al: Nat Immunol 20:824, 2019; S Le Clerc et al: Front Genet 10:799 2019; V Kalidasan et al: Front Microbiol 11:46 2020; SN Gingras et al: Hum Genet 139:865 2020.

(Continued) rs7919208A associated with higher susceptibility to HIV-related non-Hodgkin’s lymphoma –174G/G associated with high risk of KS development and variable recovery of CD4+ T cells during ART 244 I/I associated with faster CD4+ T cell recovery during ART Slow progression to AIDS with heterozygosity for coding SNVs 231V associated with lower viral load, slower progression to AIDS and PCP rs179008-T associated with lower viral load and cell-associated HIV-1 DNA in women rs11884476-G associated with slower progression to AIDS rs368234815-ΔG associated with higher prevalence of AIDS-defining illnesses and potentially increased HIV-1 infection risk E4/E4 associated with rapid AIDS progression and HIV-associated dementia Increased risk for HIV-associated nephropathy rs2229116-G associated with subclinical atherosclerosis during ART rs17762192-G associated with reduced rate of HIV disease progression KIR3DS1/HLA-Bw4-80Ile associated with delayed AIDS onset HLA-C1+ KIR2DL+ associated with better immune recovery during ART Increasing copy numbers of 47V associated with lower viral load in persons carrying HLA-B*57 Control of HIV-1

CCR5 promoter rs2856758 rs2734648 rs1799987 rs1799988 rs41469351 rs1800023 rs1800024 rs1015164 rs1799864 rs333 Haplotype A A HHC HHB HHA V V A A G T G G T T A A C C C C wt wt A A V V A A T T G G T T A G C C T C wt wt HHD HHE HHF1 HHF2 HHG1 HHG2 A G A G A V V I V V A A A G G G G G G G A A A A A C C C C C A A A A A C T T C C C C C C C wt wt wt wt ∆32 HLA-C PSORS1C3 HLA-B MICA HCP5 MICB CCR3 CCR2 CCR5 CCRL2

– Log10 (P value)

Chr. 3p21 CCR5 locus

Chr.

FIGURE 208-26  Schema depicting haplotypes within two regions that contribute significantly to HIV-AIDS susceptibility. Top: Haplotypes (left, CCR5; right, HLA alleles). Bottom: GWAS Manhattan plots schematized. Chr, chromosome. Horizontal dotted line: genome level significance threshold. greatly advanced our understanding of the genes that influence HIVAIDS pathogenesis and ART-associated immune reconstitution. Of particular interest are polymorphisms in two chromosomal regions, as they are associated with consistent effects on HIV acquisition, viro­ logic set point, and/or rates of HIV disease progression: the region in chromosome 3 that includes the gene that encodes the HIV co-receptor CC-chemokine receptor 5 (CCR5) and the major histocompatibility locus (MHC) in chromosome 6 (Fig. 208-26). GENETICS OF CCR5: FROM BENCH TO BEDSIDE  While the discovery of CCR5 as a major co-receptor for cell entry of HIV-1 was established by in vitro studies, genetic association studies established its seminal role in HIV pathogenesis. Initial in vitro studies revealed that a 32-bp deletion (Δ32) in the coding region of CCR5 contributes to resistance to CCR5 using R5 strains of HIV. The CCR5 Δ32 allele encodes a trun­ cated protein that is not expressed on the cell surface. Congruently, genotype-phenotype association studies in large cohorts demonstrated that individuals homozygous for the CCR5 Δ32 allele (Δ32/Δ32) lack CCR5 surface expression and are highly resistant to acquiring HIV infection; heterozygosity for the CCR5 Δ32 allele is associated with a lower risk of acquiring HIV. The distribution of the CCR5 Δ32 allele is population specific. Approximately 1% of individuals of European ancestry are homozy­ gous for the CCR5 Δ32 allele. Depending on the geographic region in Europe, up to 18% of individuals are heterozygous for the CCR5 Δ32 allele. The CCR5 Δ32 allele is rare in other populations. The evolution­ ary pressure that resulted in the emergence of the CCR5 Δ32 allele in the European population remains unknown and has been speculated to be secondary to an ancestral pandemic, such as the plague. Subsequent studies identified single nucleotide variants (SNVs) in the promoter (regulatory) region of CCR5 that influence gene expres­ sion levels. Alleles bearing specific cassettes of linked polymorphisms (haplotypes) were identified and designated as human haplogroups A to G2 (HHA to HHG2) (Fig. 208-26). The CCR5 Δ32 polymorphism is found on the HHG2 haplotype. CCR5 haplotypes A–D versus E–G2 differ by bearing GT versus AC at polymorphic sites rs1799987 and rs1799988 (Fig. 208-26). CCR5-HHA haplotype represents the ances­ tral haplotype (found in chimpanzees) and is associated with lower CCR5 gene expression, whereas the CCR5-HHE haplotype is associ­ ated with higher CCR5 expression. Methylation of DNA is a common epigenetic signaling mechanism that cells use to lock genes in the “off”

rs2395029 rs9264942 rs4418214 rs3131018 HLA-C HLA-B

allele allele 63 67 70 97 M M C C C C C B57:01 B52:01 B27:05 G R E E M S V T C T S N G G M V C C R R E E S C T N T C N K G G C C C C A A M M V V V Cw08:02 C C T T T B14:02 R R R R R N N E N N C S S Y F W R R S R T T T T T N N N Q N G G G G T Cw07:02 B07:02 B35:01 Chr. 6p21 MHC region

CHAPTER 208 position, and polymorphisms in CCR5 haplotypes may mediate their effects by influencing DNA methylation levels in the CCR5 locus. The CCR5-HHE and CCR5-HHA haplotypes are more sensitive and resistant, respectively, to T cell activation–induced demethylation of the CCR5 locus. In worldwide populations, HHE and HHC are prevalent haplotypes, whereas the ancestral HHA haplotype is more common in persons of African ancestry. The associations of CCR5 haplotypes with HIV acquisition and/or HIV disease course are largely consistent with their effects on CCR5 gene expression. For example, homozygosity for the CCR5-HHE haplotype is associated with an increased risk of acquiring HIV, progressing rapidly to AIDS, and reduced immune recovery dur­ ing ART. The HHA haplotype is associated with slower disease progres­ sion in African populations and has been speculated to be a basis for why chimpanzees (who all carry the ancestral CCR5-HHA haplotype) naturally infected with SIV resist disease progression. The pairing of the HHC and CCR5 Δ32-bearing HHG2 haplotypes (HHC/HHG2 genotype) is associated with a lower risk of acquiring HIV infection and slower rate of HIV disease progression, whereas the pairing of the HHE haplotype with the HHG2 haplotype is associated with the opposite effects. The CCR2-64I–bearing HHF2 haplotype is associated with a slower HIV disease course. Human Immunodeficiency Virus Disease: AIDS and Related Disorders
Consistent with these genetic associations, polymorphisms in genes encoding ligands for CCR5 have also been associated with variable HIV susceptibility and disease progression rates. Examples include copy number variations of CCL3L1 and SNVs in CCL5. The sum of these studies established a pivotal role of CCR5 and its ligands in HIVAIDS pathogenesis and, potentially, immune recovery. The discovery that the CCR5 Δ32/Δ32 genotype is associated with strong resistance to HIV infection, and that uninfected persons bear­ ing this genotype did not appear to have impaired immunity, led to the development of two kinds of novel therapies. First, it spurred the development of a new class of therapies approved by the U.S. Food and Drug Administration: entry inhibitors (e.g., maraviroc) that block the interaction of CCR5 with the HIV envelope. Second, it led to the evaluation of novel experimental cellular therapies. A person with HIV and acute myelogenous leukemia was given an allogeneic stem cell transplantation from an HLA-compatible person whose cells lacked expression of CCR5 due to the Δ32/Δ32 genotype. There was no evi­ dence of HIV-1 infection for 13 years in the patient who underwent the transplant; the patient eventually died due to recurrence of leukemia.

This observation provided a proof of concept for an HIV cure and led to the development of additional novel cellular therapies involving autologous transplantation of CD4+ T cells in which the CCR5 gene is inactivated ex vivo using new gene editing procedures. Similar cel­ lular strategies have had mixed success, mainly due to the latent viral reservoir in various tissues.

DISCOVERY OF HLA CLASS I ALLELES THAT ASSOCIATE WITH VIROLOGIC CONTROL OF HIV INFECTION  There is a strong association between variations within the HLA-B gene with protective (e.g., HLA-B57 and HLA-B27 alleles) or detrimental (e.g., HLA-B35 allele) outcomes during HIV infection. Carriage of the HLA-B57 and/or HLA-B27 alleles is associated with slower disease progression. The beneficial effects of these alleles may relate in part to their associations with a lower virologic set point as well as to higher cell-mediated immunity in people with HIV. The protective effect of the HLA-B57 and HLAB27 alleles on the HIV disease course is underscored by the finding that the prevalence of these alleles is higher among persons with long-term nonprogression and persons who control HIV replication spontaneously (elite controllers). In contrast, the HLA-B35 allele has been associated with faster progression to AIDS and higher viral load. The prevalence of the HLA-B alleles differs between populations. HLAB57:01 in Europeans and HLA-B57:03 in persons of African descent are the protective alleles. In some populations (e.g., Japanese) where the HLA-B57/HLA-B27 alleles are absent, HLA-B*51 is associated with a protective phenotype. Possession of the protective HLA-B alleles is associated with broader and stronger CD8+ T cell responses to HIV epitopes. The mechanisms underlying the differential effects of the HLA-B alleles on the course of HIV disease may relate to differences in the ability of antigenpresenting cells to present immunodominant HIV epitopes to T helper or cytotoxic T lymphocytes in the context of MHC-encoded molecules. This may result in differential immune responses that influence viral replication. In this regard, the HLA-B alleles that impact the course of HIV disease differ in their amino acid residues in the HLA-B peptidebinding groove; this difference may play a critical role in virologic control. PART 5 Infectious Diseases The HLA-B−21M allele does not influence HLA-B gene expression; however, it is in linkage with HLA-B haplotypes that are associated with higher HLA-A and HLA-E expression. Higher HLA-A levels associate with poorer control of HIV as well as higher viral load, reduced CD4+ T cell counts, and accelerated progression to AIDS. HLA-E is the ligand for natural killer (NK) cell NKG2A, an inhibitory receptor. Engage­ ment of NKG2A with HLA-E inhibits NK cells that would normally be potent eliminators of virally infected cells. Thus, targeting NKG2A might provide a therapeutic avenue for HIV treatment. Investigators have also examined the influence of extended HLA haplotypes (linked alleles) on the course of HIV disease. The extended HLA ancestral haplotype (AH) 8.1 is defined by the presence of HLAA1, HLA-B8, and HLA-DR3 alleles. AH 8.1 is the most common ances­ tral haplotype in persons of European descent (present in 10%) and is associated with multiple autoimmune diseases in HIV-seronegative persons. These associations of AH 8.1 are thought to be due to a geneti­ cally determined hyperresponsiveness characterized by high TNF-α production and lack of complement C4A. Strong epidemiologic data indicate that carriage of AH 8.1 in HIV-seropositive persons is associ­ ated with a rapid decline in the number of CD4+ T cells and faster pro­ gression to AIDS development. Gene–gene interactions between HLA alleles and other genes (e.g., killer cell immunoglobulin-like receptors) also may influence HIV disease progression rates. POLYMORPHISMS IDENTIFIED BY GWAS THAT ASSOCIATE WITH VIROLOGIC CONTROL AND DISEASE PROGRESSION  GWAS have not identified additional genetic variations that associate with the risk of HIV-1 acquisition, presumably due to a paucity of well-characterized risk cohorts in which level of exposure has been quantified. By con­ trast, large-scale GWAS have identified SNVs, especially in the MHC, that influence HIV viral load, including in a large group of individuals termed HIV controllers (including elite controllers) who spontaneously (without ART) control viral replication. GWAS in people with HIV

-35 kb Indel263 -35 kb Indel263 T C T C – – G G gDNA gDNA HLA-C HLA-C Less silencing Silencing C – T G mRNA mRNA miR-148a binding intact HLA-C expression Viral control HIV outcomes Lower Reduced Worse Higher Increased Better FIGURE 208-27  Linkage disequilibrium between two variants in the HLA-C locus and their influence on binding of miR-148a to the 3′-untranslated region (UTR). Altered binding of miR-148a associates with HLA-C protein expression levels and, in turn, viral control and HIV disease outcomes. Effects associated with T-G (left) and C-del (right) haplotypes are depicted. The C-deletion haplotype prevents binding of miR-148a to 3′-UTR of HLA-C (less silencing). Kb, kilobase. of European ancestry identified four SNVs in genes in the HLA class I loci that associated with virologic control. These SNVs are within or in the vicinity of PSORS1C3, HLA-C, MICA, and HCP5 genes (Fig. 208-26).

As noted in this figure, the individual effects of these alleles are dif­ ficult to discern because of linkage disequilibrium. The protective effects of the SNVs in HCP5 and MICA may relate to their linkage with known protective HLA-B alleles. The protective HCP5 allele is in linkage disequilibrium with the HLA-B57:01 allele, and the protective MICA allele tags with the HLA-B57:01 and HLA-B*27:05 alleles. The protective HLA-C SNV is associated with higher HLA-C expression, which has been associated with viral control and better HIV outcomes. This protective SNV (rs9264942; T→C) resides 35 kb upstream of the HLA-C gene and is in strong linkage disequilibrium with a 3′-UTR indel263 SNV (rs67384697; G→deletion), generating the T-G or C-deletion haplotypes (Fig. 208-27). miR-148a binds to the 3′-UTR region encompassing the rs67384697 SNV and silences HLA-C expres­ sion. Binding of miR-148a to the 3′-UTR is disrupted on the mRNA transcribed from the C-deletion haplotype; this disruption associates with less silencing of the mRNA and therefore higher HLA-C cell surface expression, which associates with better HIV disease outcomes (Fig. 208-27). Conversely, binding of miR-148a to the 3′-UTR is intact on the mRNA transcribed from the T-G haplotype; this binding associates with silencing of the mRNA and therefore lower HLA-C cell surface expression associates with worse HIV disease outcomes

(Fig. 208-27). GWAS in persons of African descent have identified an SNV (rs2523608) that tags the HLA-B*57:03 allele that is known to associate with HIV-1 control and a slower disease course. Together, these GWAS data underscore the importance of variations in HLA class I loci in control of viral replication. A recent GWAS suggested that an SNV (rs1015164G→A) approxi­ mately 34 kb downstream of the CCR5 loci is associated with a higher viral load set point (Fig. 208-26) and lower CD4+ T cell counts in therapy-naïve HIV-seropositive persons. rs1015164 maps to a lncRNA gene in proximity to the CCRL2 gene (Fig. 208-26). The lncRNA is transcribed from the antisense strand of CCR5 and was therefore named CCR5AS. The rs1015164A allele is associated with higher expression of CCR5AS in CD4+ T cells, which in turn was associated with increased levels of CCR5 mRNA. Although the detrimental effect of the rs1015164A allele was suggested to be independent of the det­ rimental effects of the abovementioned CCR5-HHE haplotype, further investigation is warranted as the rs1015164A allele and CCR5-HHE haplotype are in a high degree of linkage disequilibrium. Most GWAS studies have been performed in European populations, limiting generalizability to other populations. Additionally, GWAS are generally not suitable for identifying rare variants (<1% prevalence). Therefore, next-generation sequencing (NGS) approaches were sug­ gested to identify these rare variants. However, a recent NGS study suggests that exonic variants with large effect sizes are unlikely to have

a major contribution to host control of HIV infection. Mathematical modeling revealed that variations in host genes may explain about 10% of the observed variability in HIV viral load, whereas viral genetic diversity may explain 29% of the variability. GENETIC ASSOCIATIONS WITH SPECIFIC AIDS AND NON-AIDS CONDITIONS  Carotid artery disease  Many of the non-AIDS events in HIV-seropositive individuals resemble those attributable to immune senescence and those found in the HIV-seronegative aging population. A functional SNV in the ryanodine receptor 3 (RYR3) gene was found to be associ­ ated with an increased risk of common carotid intima–media thickness (cIMT), which is a surrogate for subclinical atherosclerosis. Functional studies on RYR3 and its isoforms demonstrate a major role of these receptors in modulating endothelial function and atherogenesis via calcium-signaling pathways, providing a biologically plausible mecha­ nism by which the SNV in RYR3 may associate with increased cIMT risk. Kidney disease  HIV-1–associated nephropathy (HIVAN) is a form of focal sclerosing glomerulonephritis caused by direct infection of kid­ ney epithelial cells with HIV. HIVAN is more common in persons of African descent. There is evidence that polymorphisms in the MYH9 gene and in the neighboring APOL1 gene are a strong determinant of susceptibility to HIVAN in persons of African descent. The effect of carrying two APOL1 risk alleles explains nearly 35% of HIVAN. Overexpression of the APOL1 kidney risk variants may associate with increased kidney cell death. HIV-associated neurocognitive disorder  HIV-associated neurocognitive disorder (HAND) comprises a spectrum of neurocognitive deficits due to HIV infection. Variations in the apolipoprotein E (ApoE) gene have strong associations with Alzheimer’s disease in the HIV-seronegative population. In HIV-seropositive persons, possession of the E4/E4 genotype has been associated with dementia, peripheral neuropathy, and impairment in cognition as well as immediate and delayed verbal memory. Macrophage recruitment and activation play a central role in the development of many of the HAND syndromes. Variations in chemokines that play an influential role in macrophage activation and recruitment, namely CCL2 (MCP-1) and CCL3 (MIP-1α), have been shown to influence the risk of developing HAND. Variations in mitochondrial genes also have been associated with a risk of AIDS and HAND. A GWAS identified a polymorphism in chromosome 14 in the T cell receptor α locus that may influence neurocognitive outcomes. HIV-1–associated pneumocystis pneumonia  Human Apobec3 cytidine deaminases are intrinsic resistance factors to HIV-1. However, HIV-1 encodes a viral infectivity factor (Vif) that degrades APOBEC3 pro­ teins. Association studies suggest a role of genetic variations in the APOBEC3 family in HIV disease. A common haplotype derived from 6 SNVs in the APOBEC3F gene and tagged by a codon-changing vari­ ant is associated with a significantly lower viral load set point, slower rate of progression to AIDS, and delayed development of Pneumocystis jirovecii pneumonia (PCP). In addition, a coding SNV in the CCRL2 gene is associated with accelerated progression to AIDS and rapid development of PCP. HIV-related non-hodgkin lymphoma (NHL)  The relative risk of developing NHL in HIV-seropositive persons is highly elevated compared with the general population. NHL represents approximately 34% of all identi­ fied cancers in HIV-seropositive persons. A recent GWAS identified a promoter SNV in the CXCL12 gene that was associated with higher susceptibility to develop HIV-related NHL. The effect of this SNV is likely causal as it creates new transcription factor binding sites, impact­ ing CXCL12 expression. ASSOCIATIONS WITH ART-RELATED ADVERSE EVENTS  Abacavir, an effective antiretroviral agent, is associated with significant risk of hypersensitivity reactions (2–9% of cases). Interestingly, while the HLA-B57:01 allele is associated with a slower HIV disease course, possession of this allele is associated with a higher risk of abacavirassociated hypersensitivity, possibly due to the abacavir-specific acti­ vation of cytokine-producing CD8+ T cells only in HLA-B57:01

carriers. Pharmacogenetic screening for the HLA-B*57:01 allele is recommended before initiation of abacavir treatment.

The antiretroviral agent nevirapine is associated with hypersen­ sitivity reactions in 6–10% of patients, including Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN). rs5010528G, a strong proxy for HLA-C*04:01 carriage, was associated with high risk of SJS and TEN during nevirapine treatment. In addition, efavirenz was among the first antiretroviral agents to be co-formulated into singlepill regimens for mass rollout globally. Several genetic variants in the drug-metabolizing enzyme CYP2B6 have been associated with high efavirenz plasma concentrations and increased risk of adverse neuro­ psychiatric effects. For example, homozygosity for one such variant, rs3745274 T/T, increases the risk of adverse reactions to efavirenz up to fivefold, and this risk genotype is much more common in Africans (13.7%) than Europeans (5.6%). ■ ■NEUROPATHOGENESIS IN HIV DISEASE While there has been a remarkable decrease in the incidence of the severe forms of HIV encephalopathy among those with access to treat­ ment in the era of effective ART, people with HIV can still experience milder forms of neurocognitive impairment despite adequate ART. Factors that contribute to the neurocognitive decline include lack of complete control of HIV replication in the brain; production of HIV proteins that may be neurotoxic; low CD4+ T cell nadir; chronic immune activation; comorbidities such as drug abuse, microvascular disease, older age, and diabetes; and the potential for neurotoxicity of certain antiretroviral drugs. HIV has been demonstrated in the brain and CSF of infected individuals with and without neuropsychiatric abnormalities. As opposed to lymphoid tissues, there are no resident lymphocytes in the brain. The main cell types that are infected in the brain in vivo are the perivascular macrophages and the microglial cells, which can sometimes form syncytia resulting in multinucleated giant cells; low-level viral replication is also seen in perivascular astrocytes. It has been proposed that monocytes that have already been infected in the blood can migrate into the brain, where they then reside as macrophages, or macrophages can be directly infected while residing within the brain. The precise mechanisms whereby HIV enters the brain are unclear; however, they are thought to relate, at least in part, to the ability of virus-infected and immune-activated macrophages to induce adhesion molecules such as E-selectin and vascular cell adhe­ sion molecule 1 (VCAM-1) on brain endothelium. Other studies have demonstrated that HIV gp120 enhances the expression of intercellular adhesion molecule 1 (ICAM-1) in glial cells and HIV Tat protein can disrupt the tight junctions of the brain endothelial cells to facilitate entry of HIV-infected cells into the CNS. Virus isolates from the brain are preferentially R5 strains as opposed to X4 strains; in this regard, people with HIV who are heterozygous for CCR5-Δ32 appear to be relatively protected against the development of HIV encephalopathy. Once HIV enters the brain due to pressures of the local environment, it evolves to develop distinct sequences in the env, tat, and LTR genes. These unique sequences have been associated with neurocognitive dys­ function; however, it is unclear if they are causal (see below). CHAPTER 208 Human Immunodeficiency Virus Disease: AIDS and Related Disorders
People with HIV may manifest white matter lesions as well as neuro­ nal loss. The white matter lesions are due to axonal injury and a disrup­ tion of the blood-brain barrier and not due to demyelination. Given the absence of evidence of HIV infection of neurons, HIV-mediated effects on neurons are thought to involve indirect pathways whereby viral proteins, particularly gp120 and Tat, trigger the release of endogenous neurotoxins from macrophages and to a lesser extent from astrocytes. In addition, it has been demonstrated that both HIV-1 Nef and Tat can induce chemotaxis of leukocytes, including monocytes, into the CNS. Neurotoxins can be released from monocytes as a consequence of infection and/or immune activation. Monocyte-derived neurotoxic factors have been reported to kill neurons via a variety of mechanisms including activation of the N-methyl-d-aspartate (NMDA) receptors and induction of oxidative stress. In addition, HIV gp120 shed by virus-infected monocytes could cause neurotoxicity by antagonizing the function of vasoactive intestinal peptide (VIP), by elevating intracel­ lular calcium levels, and by decreasing neurotrophic factor levels in the

cerebral cortex. A variety of monocyte-derived cytokines can contribute directly or indirectly to the neurotoxic effects in HIV infection; these include TNF-α, IL-1, IL-6, TGF-β, IFN-γ, platelet-activating factor, and endothelin. Furthermore, among the CC-chemokines, elevated levels of monocyte chemotactic protein 1 (MCP-1 or CCL-2) in the brain and CSF have been shown to correlate best with the presence and degree of HIV encephalopathy in ART-naïve patients. In addition, infection and/ or activation of monocyte-lineage cells can result in increased produc­ tion of eicosanoids, quinolinic acid, nitric oxide, excitatory amino acids such as l-cysteine and glutamate, arachidonic acid, platelet-activating factor, free radicals, TNF-α, and TGF-β, which may contribute to neu­ rotoxicity. Astrocytes may play diverse roles in HIV neuropathogenesis. Reactive gliosis or astrocytosis has been demonstrated in the brains of people with HIV, and TNF-α and IL-6 have been shown to induce astrocyte proliferation. In addition, astrocyte-derived IL-6 can induce HIV expression in infected cells in vitro. Furthermore, it has been sug­ gested that astrocytes may downregulate macrophage-produced neuro­ toxins. Evidence of neuronal injury can be demonstrated by measuring neurofilament levels in CSF. Treatment with ART leads to improvement in neuropsychiatric manifestations and a decrease in these cytokine levels in CSF, suggesting that they are driven by the virus or by its products. However, even in patients on long-term ART, there may be evidence of persistently activated lymphocytes in the CSF. It is unclear if these lymphocytes may contribute to neuronal injury in the brain or are critical for controlling the CNS viral reservoir. However, some individuals may develop a subacute encephalitis due to an IRIS reaction (see below). This often occurs weeks or a few months after initiation of ART in individuals with low CD4+ T cell counts. It is thought that the recovery of CD4+ T cells causes a lymphocyte response to the CNS HIV reservoir. The contribution of host genetic factors to development of neuropsychiatric manifestations of HIV infection has not been well studied. However, evidence supports the role of several genetic factors including the E4 allele for apoE in an increased risk of HIV-associated neurocognitive disorders and peripheral neuropathy.

PART 5 Infectious Diseases It has also been suggested that the CNS may serve as a relatively sequestered site for a reservoir of latently infected cells that might be a barrier for the eradication of virus by ART (see “The HIV Reservoir: Obstacles to the Eradication of Virus,” above). ■ ■PATHOGENESIS OF KAPOSI’S SARCOMA There are at least four distinct epidemiologic forms of KS: (1) the clas­ sic form that occurs in older men of predominantly Mediterranean or eastern European Jewish backgrounds with no recognized contribut­ ing factors; (2) the equatorial African form that occurs in all ages, also without any recognized precipitating factors; (3) the form associated with organ transplantation and its attendant iatrogenic immunosup­ pressed state; and (4) the form associated with HIV-1 infection. In the latter two forms, KS is an opportunistic disease; in people with HIV, unlike typical opportunistic infections, its occurrence is not strictly related to the level of depression of CD4+ T cell counts. The pathogen­ esis of KS is complex; fundamentally, it is an angioproliferative disease that is not a true neoplastic sarcoma, at least not in its early stages. It is a manifestation of excessive proliferation of spindle cells that are believed to be of vascular origin and have features in common with endothelial and smooth-muscle cells. In HIV disease the development of KS is dependent on the interplay of a variety of factors including HIV-1 itself, human herpes virus 8 (HHV-8), immune activation, and cytokine secretion. Numerous epidemiologic and virologic stud­ ies have clearly linked HHV-8, which is also referred to as Kaposi’s sarcoma–associated herpesvirus (KSHV), to KS not only in people with HIV but also in individuals with the other forms of KS. HHV-8 is a γ-herpesvirus related to EBV and herpesvirus saimiri. It encodes a homologue to human IL-6 and, in addition to KS, has been implicated in the pathogenesis of body cavity lymphoma, multiple myeloma, and monoclonal gammopathy of undetermined significance. Sequences of HHV-8 are found universally in the lesions of KS, and patients with KS are virtually all seropositive for HHV-8. HHV-8 DNA sequences can be found in the B cells of 30–50% of patients with KS and 7% of patients with AIDS without clinically apparent KS.

Between 1% and 2% of eligible blood donors are positive for anti­ bodies to HHV-8, while the prevalence of HHV-8 seropositivity in men with HIV is 30–35%. The prevalence of HHV-8 seropositivity in women with HIV is ~4%. This finding is reflective of the lower incidence of KS in women. It has been debated whether HHV-8 is the transforming agent in KS; the bulk of the cells in the tumor lesions of KS are not neoplastic cells. However, it has been demonstrated that endothelial cells can be transformed in vitro by HHV-8. In this regard, HHV-8 possesses genes, including homologues of the IL-8 receptor, Bcl-2, and cyclin D, which can potentially transform the host cell. Despite the complexity of the pathogenic events associated with the development of KS in people with HIV, HHV-8 is the etiologic agent of this disease. The initiation and/or propagation of KS requires an activated state and is mediated, at least in part, by cytokines. Mul­ tiple factors, including TNF-α, IL-1β, IL-6, granulocyte-macrophage colony-stimulating factor (GM-CSF), basic fibroblast growth factor, and oncostatin M, function in an autocrine and paracrine manner to sustain the growth and chemotaxis of the KS spindle cells. In this regard, KSHV-derived IL-6 has been demonstrated to induce prolifera­ tion of lymphoma cells and to inhibit the cytostatic effects of IFN-α on KSHV-infected lymphoma cells. IMMUNE RESPONSE TO HIV As detailed above and below, following the initial burst of viremia during primary infection, people with HIV mount robust immune responses that in most cases substantially curtail the levels of plasma viremia and likely contribute to delaying the ultimate development of clinically apparent disease for a median of 10 years in untreated indi­ viduals. This immune response contains elements of both humoral and cell-mediated immunity involving both adaptive and innate immune responses (Table 208-7; Fig. 208-28). It is directed against multiple antigenic determinants of the HIV virion as well as against viral pro­ teins expressed on the surface of infected cells. Ironically, those CD4+ T cells with T cell receptors specific for HIV are theoretically those CD4+ T cells most likely to be activated—and thus to serve as early targets for productive HIV infection and the cell death or dysfunction associated with infection. Thus, an early consequence of HIV infection is interference with and decrease of the helper T cell population needed to generate an effective immune response. Although a great deal of investigation has been directed toward delineating and better understanding the components of this immune response, it remains unclear which immunologic effector mechanisms are most important in delaying progression of infection and which, if any, play a role in the pathogenesis of HIV disease. This lack of knowl­ edge has also hampered the ability to develop an effective vaccine for HIV disease. TABLE 208-7  Elements of the Immune Response to HIV Humoral immunity Neutralizing antibodies   Type specific   Group specific   Broadly neutralizing Antibodies participating in antibody-dependent cellular cytotoxicity (ADCC) Protective Pathogenic (bystander killing) Enhancing antibodies Complement Cell-mediated immunity Helper CD4+ T lymphocytes Class I MHC–restricted cytotoxic CD8+ T lymphocytes CD8+ T cell–mediated inhibition (noncytolytic) ADCC Natural killer cells Abbreviation: MHC, major histocompatibility complex.

Neutralizing antibody Cytotoxic CD8+ T lymphocyte Lysis Class I MHC Activation, proliferation, cytokine and chemokine release TCR Viral antigens Fc receptor ADCC Free gp120 CD4 HIV-infected CD4+ T lymphocyte Bystander killing Uninfected CD4+ T lymphocyte Helper CD4+ T lymphocytes Cytokine release Activation Lysis Class II MHC HIV-infected CD4+ T lymphocyte Natural killer cells FIGURE 208-28  Schematic representation of the different immunologic effector mechanisms thought to be active in the setting of HIV infection. Detailed descriptions are given in the text. ADCC, antibody-dependent cellular cytotoxicity; MHC, major histocompatibility complex; TCR, T cell receptor. ■ ■HUMORAL IMMUNE RESPONSE Antibodies to HIV usually appear within 3–6 weeks and almost invariably within 12 weeks of primary infection (Fig. 208-29); rare exceptions are in individuals who have defects in the ability to produce HIV-specific antibodies. Along with plasma levels of HIV RNA and p24 antigen, detection of these antibodies forms the basis of most diagnostic screening tests for HIV infection. The appearance of HIVbinding antibodies detected by ELISA and Western blot assays occurs prior to the appearance of neutralizing antibodies; the latter generally appear following the initial decreases in plasma viremia and are more closely related to the appearance of HIV-specific CD8+ T lymphocytes. The first antibodies detected are those directed against the immuno­ dominant region of the envelope gp41, followed by the appearance of antibodies to the structural or gag protein p24 and the gag precursor p55. Antibodies to p24 gag are followed by the appearance of antibod­ ies to the outer envelope glycoprotein (gp120), the gag protein p17, and the products of the pol gene (p31 and p66). In addition, one may see antibodies to the low-molecular-weight regulatory proteins encoded by the HIV genes vpr, vpu, vif, rev, tat, and nef. On rare occasions, levels of HIV-specific antibodies may decline during treatment of acute HIV infection; otherwise they remain persistently positive. While antibodies to multiple antigens of HIV are produced, the pre­ cise functional significance of these different antibodies is unclear. The only viral proteins that elicit neutralizing antibodies are the envelope

Seroconversion, Initial viremia ADCC, CTL Autologous NAbs Broadly reactive NAbs

3–10 Years infected FIGURE 208-29  Relationship between initial HIV viremia and the development of antibodies to HIV. Within 3 to 6 weeks of initial HIV infection, nonneutralizing antibodies to HIV appear. These antibodies are capable of mediating antibodydependent cellular cytotoxicity (ADCC). The decline in plasma viremia generally correlates with the appearance of cytotoxic T lymphocytes (CTL). After approximately 3 months, autologous neutralizing antibodies (NAbs) capable of neutralizing prior circulating strains of HIV appear. After 2 or more years, broadly reactive NAbs appear. (Reproduced with permission from of Annual Reviews, Inc. from The Role of Antibodies in HIV Vaccines, JR Mascola and DC Monteori. 28:413, 2010.) proteins gp120 and gp41. Antibodies directed toward the envelope proteins of HIV have been characterized both as being protective and as possibly contributing to the pathogenesis of HIV disease. Among the protective antibodies are those that function to neutralize HIV directly and prevent the spread of infection to additional cells, as well as those that participate in ADCC. The first neutralizing antibodies are directed against the autologous infecting virus and appear after approximately 12 to 24 weeks of infection. Due to its high rate of mutation the virus is usually able to quickly escape these (and subsequent) neutralizing anti­ bodies. One important mechanism of immune escape is the addition of N-linked glycosylation sites, forming a glycan shield that interferes with envelope recognition by these initial antibodies. CHAPTER 208 Human Immunodeficiency Virus Disease: AIDS and Related Disorders
A number of broad and potent HIV-neutralizing envelope-specific antibodies have been isolated from people with HIV in studies designed to better understand the host response to HIV infection. Approximately 20% of patients develop antibodies capable of neutral­ izing highly diverse strains. These usually appear 2 or more years fol­ lowing infection in the face of continual viremia. These studies have revealed at least five major sites within the HIV envelope trimer that are able to elicit broadly neutralizing antibodies. These sites include antibodies directed toward the CD4 binding site (CD4bs) of gp120, those binding glycan-dependent epitopes in the V1/V2 region of gp120, those near the base of the V3 region of gp120, those binding to the gp120/gp41 bridge, and those binding to the membrane-proximal region of gp41 (Fig. 208-30). Several of these antibodies contain unique features including high levels of somatic hypermutation, selec­ tive germ-line gene usage (especially for CD4bs antibodies), and long CD4 binding site V1V2 glycan V3 glycan Silent face center Subunit interface Membrane proximal external region Fusion peptide FIGURE 208-30  Known targets of broadly neutralizing antibodies against HIV-1. (Courtesy of J Stuckey, GY Chuang.)

heavy chain complementary determining regions (especially CDRH3). Of note, while these antibodies are broadly neutralizing in vitro, their precise in vivo significance is unclear and the patients from whom they were derived demonstrate evidence of ongoing viral replication unless treated with ART.

The other major class of protective antibodies are those that partici­ pate in ADCC, a form of cell-mediated immunity (Chap. 360) in which NK cells that bear Fc receptors bind specific anti-HIV antibodies that then juxtapose the NK cells to and destroy cells expressing HIV antigens. The levels of anti-envelope antibodies capable of mediating ADCC are highest in the earlier stages of HIV infection. Antibodies to both gp120 and gp41 have been shown to participate in ADCC-mediated killing of HIV-infected cells. In vitro, IL-2 can augment ADCC-mediated killing. In addition to playing a role in host defense, HIV-specific antibodies have also been implicated in disease pathogenesis. Antibodies directed to gp41, when present in low titer, have been shown in vitro to be capable of facilitating infection of cells through an Fc receptor–mediated mecha­ nism known as antibody enhancement. Thus, the same regions of the envelope protein of HIV that give rise to antibodies capable of mediat­ ing ADCC can also elicit the production of antibodies that can facilitate infection of cells in vitro. In addition, it has been postulated that antigp120 antibodies that participate in the ADCC killing of HIV-infected cells might also kill uninfected CD4+ T cells if the uninfected cells had bound free gp120, a phenomenon referred to as bystander killing. One of the most primitive components of the humoral immune system is the complement system (Chap. 360). This element of innate immunity consists of ~30 proteins that are found circulating in blood or associated with cell membranes. While HIV alone is capable of directly activating the complement cascade, the resulting lysis is weak due to the presence of host cell regulatory proteins captured in the virion envelope during budding. It is possible that complement-opsonized HIV virions have increased infectivity in a manner analogous to antibody-mediated enhancement. PART 5 Infectious Diseases ■ ■CELLULAR IMMUNE RESPONSE T cell–mediated immunity plays a major role in host defense against most viral infections (Chap. 360) and is thought to be an important component of the host immune response to HIV. T cell immunity can be divided into two major categories: that mediated by helper/inducer CD4+ T cells and that mediated by cytotoxic/immunoregulatory CD8+ T cells. HIV-specific CD4+ T cells can be detected in the majority of people with HIV via flow cytometry to measure intracellular cytokine produc­ tion in response to MHC class II tetramers pulsed with HIV peptides or through lymphocyte proliferation assays utilizing HIV antigens such as p24. These cells likely play a critical role in the orchestration of the immune response to HIV by providing help to HIV-specific B cells and CD8+ T cells. They may also be capable of directly killing HIV-infected cells. HIV-specific CD4+ T cells may be preferential targets of HIV infection by HIV-infected antigen-presenting cells during the genera­ tion of an immune response to HIV (Fig. 208-28). However, they also are likely to undergo clonal expansions in response to HIV antigens and thus survive as a population of cells despite the virus. No clear correlations exist between levels of HIV-specific CD4+ T lymphocytes and plasma HIV RNA levels; however, in the setting of high viral loads, CD4+ T cell responses to HIV antigens appear to shift from one of proliferation and IL-2 production to one of IFN-γ production. Thus, while a reverse correlation exists between the level of p24-specific proliferation and levels of plasma HIV viremia, the nature of the causal relationship between these parameters is unclear. MHC class I–restricted, HIV-specific CD8+ T cells have been identified in the peripheral blood of patients with HIV-1 infection. These cells include CTLs that produce perforins and granzyme, and T cells that can be induced by HIV antigens to express an array of cytokines such as IFN-γ, IL-2, MIP-1β, and TNF-α. Multiple HIV antigens, including Gag, Env, Pol, Tat, Rev, and Nef, can elicit CD8+ T cell responses. CTLs have been identified in the peripheral blood of patients within weeks of HIV infection and prior to the appearance of plasma virus. The selective pressure they exert on the evolution of the population of circulating viruses reflects their potential role in

control of HIV infection. These CD8+ T lymphocytes, through their HIV-specific antigen receptors, bind to and cause the lytic destruction of target cells bearing autologous MHC class I molecules presenting HIV antigens. Two types of CTL activity can be demonstrated in the peripheral blood or lymph node mononuclear cells of people with HIV. The first type directly lyses appropriate target cells in culture without prior in vitro stimulation (spontaneous CTL activity). The other type of CTL activity reflects the precursor frequency of CTLs (CTLp); this type of CTL activity can be demonstrated by stimulation of CD8+ T cells in vitro with a mitogen such as phytohemagglutinin or anti-CD3 antibody to expand the population prior to detection. In addition to CTLs, CD8+ T cells capable of being induced by HIV antigens to express cytokines such as IFN-γ also appear in the setting of HIV-1 infection. It is not clear whether these are the same or differ­ ent effector pools compared with those cells mediating cytotoxicity; in addition, the relative roles of each in host defense against HIV are not fully understood. It does appear that these CD8+ T cells are driven to in vivo expansion by HIV antigen. There is a direct correlation between levels of CD8+ T cells capable of producing IFN-γ in response to HIV antigens and plasma levels of HIV-1 RNA. Thus, while these cells are clearly induced by HIV-1 infection, in most instances they are not able to effectively control infection. One exception may be a subset of patients who control viral replication in the absence of antiretroviral drugs and are referred to as elite nonprogressors (see “Long-Term Sur­ vivors, Long-Term Nonprogressors, and Elite Controllers,” above). The peripheral blood of these patients contains a population of CD8+ T cells that undergo substantial in vitro proliferation in response to HIV antigens and express perforin and granzyme. At least three other forms of cell-mediated immunity to HIV have been described: non-cytolytic CD8+ T cell–mediated suppression of HIV replication, ADCC, and NK cell activity. Non-cytolytic CD8+ T cell–mediated suppression of HIV replication refers to the ability of CD8+ T cells from a patient with HIV to inhibit the replication of HIV in tissue culture without killing infected targets. There is no requirement for HLA compatibility between the CD8+ T cells and the HIV-infected cells. This effector mechanism is thus nonspecific and appears to be mediated by soluble factor(s) including the CCchemokines RANTES (CCL5), MIP-1α (CCL3), and MIP-1β (CCL4). These CC-chemokines are potent suppressors of HIV replication and operate at least in part via blockade of the HIV co-receptor (CCR5) for R5 (macrophage-tropic) strains of HIV-1 (see above). ADCC, as described above in relation to humoral immunity, involves the killing of HIV-expressing cells by NK cells armed with specific antibodies directed against HIV antigens. Finally, NK cells alone have been shown to be capable of killing HIV-infected target cells in tissue culture. This primitive cytotoxic mechanism of host defense is directed toward non­ specific surveillance for neoplastic transformation and viral infection through recognition of altered class I MHC molecules. DIAGNOSIS AND LABORATORY MONITORING OF HIV INFECTION The establishment of HIV as the causative agent of AIDS and related syndromes early in 1984 was followed by the rapid development of sen­ sitive screening tests for HIV infection. By March 1985, blood donors in the United States were routinely screened for antibodies to HIV. In 1996, blood banks in the United States added the p24 antigen capture assay to the screening process to help identify the rare, infected individuals who were donating blood in the time (up to 3 months) between infection and the development of antibodies. In 2002, the ability to detect early infection with HIV was further enhanced by the licensure of nucleic acid testing (NAT) as a routine part of blood donor screening. These refinements decreased the interval between infection and detection (window period) from 22 days for antibody testing to 16 days with p24 antigen testing and subsequently to 12 days with NAT. The development of sensitive assays for monitoring levels of plasma viremia ushered in a new era of being able to monitor the progression of HIV disease more closely. Utilization of these tests, coupled with the measurement of levels of CD4+ T lymphocytes in peripheral blood, is an important compo­ nent of the management of persons with HIV infection.

■ ■DIAGNOSIS OF HIV INFECTION The CDC has recommended that screening for HIV infection be performed as a matter of routine health care. The diagnosis of HIV infection depends on the demonstration of antibodies to HIV and/or the direct detection of HIV or one of its components. As noted above, antibodies to HIV generally appear in the circulation 3–12 weeks following infection. In addition to laboratory-based screening tests, several home tests are available. The standard blood screening tests for HIV infection are based on the detection of antibodies to HIV and/or the p24 antigen (see below) of HIV. A common laboratory-based platform is the ELISA, also referred to as an enzyme immunoassay (EIA). This solid-phase assay is an extremely good screening test with a sensitivity of >99.5%. Most diagnostic laboratories use commercial kits that contain antigens from both HIV-1 and HIV-2 and thus can detect antibodies to either. These kits use both natural and recombinant antigens and are continu­ ously updated to increase their sensitivity to newly discovered species, such as group O viruses (Fig. 208-1). The fourth-generation EIA tests combine detection of antibodies to HIV-1 or HIV-2 with detection of the p24 antigen of HIV. EIA tests are generally scored as positive (highly reactive), negative (nonreactive), or indeterminate (partially reactive). While the EIA is an extremely sensitive test, it is not optimal with regard to specificity. This is particularly true in studies of low-risk individuals, such as volunteer blood donors. In this latter population, as few as 10% of EIA-positive individuals are subsequently confirmed to have HIV infection. Among the factors associated with false-positive EIA tests are antibodies to class II antigens (such as may be seen follow­ ing pregnancy, blood transfusion, or transplantation), autoantibodies, hepatic disease, recent influenza vaccination, acute viral infections, and administration of an HIV vaccine. For these reasons, anyone suspected of having HIV infection based on a positive or inconclusive fourth-generation EIA result should have the result confirmed with a more specific assay such as an HIV-1– or HIV-2–specific antibody immunoassay or a plasma HIV RNA level. One can estimate whether an individual has a recent infection with HIV-1 by comparing the results on a standard EIA test that will score positive for all infected individuals with the results on an assay modified to be less sensitive (“detuned assay”) that will score positive for individuals with estab­ lished HIV infection and negative for individuals with recent infection. In rare instances, an individual with HIV treated early in the course of infection may revert to a negative EIA. This does not indicate clearing of infection; rather, it signifies levels of ongoing exposure to virus or viral proteins insufficient to maintain a measurable antibody response. When these individuals have discontinued therapy, viruses and anti­ bodies have reappeared. CDC recommendations indicate that a positive fourth-generation assay confirmed by a second HIV-1– or HIV-2–specific immunoassay or a plasma HIV RNA level is adequate for diagnosis. The Western blot, which had previously been used for a confirmatory test, is no longer used for this purpose. A guideline for the use of these serologic tests in attempting to make a diagnosis of HIV infection is depicted in Fig. 208-31.

In patients in whom HIV infection is suspected, the appropriate initial test is a fourth-generation HIV-1/2 antigen anti­ body immunoassay. If the result is negative, unless there is strong reason to suspect early HIV infection (as in a patient exposed within the previous 3 months), the diag­ nosis is ruled out and retesting should be performed only as clinically indicated. If the immunoassay is indeterminate or positive, the test should be repeated. If the repeat is negative on two occasions, one can assume that the initial positive reading was due to a + – HIV-1 HIV-2 HIV-1 antibodies detected + – Reactive test result Nonreactive test result FIGURE 208-31  CDC algorithm for making a diagnosis of HIV infection using tests for antibody, antigen, and RNA. NAT, nucleic acid test. (Adapted from stacks.cdc.gov/view/cdc/23446.)

technical error in the performance of the assay and that the patient is negative. If the repeat is indeterminate or positive, one should proceed to an HIV-1/HIV-2 antibody differentiation immunoassay such as the Bio-Rad Geenius. If testing is positive for HIV-1 and/or HIV-2 one may make a diagnosis of HIV-1 and/or HIV-2 infection. If the HIV-1/ HIV-2 antibody testing is negative or indeterminate one should pro­ ceed to HIV-1 RNA testing with a nucleic acid test (NAT; see below). If the NAT is positive, in the presence of a negative antibody test, one can make a diagnosis of acute HIV-1 infection. If the NAT test is negative for HIV-1 one should consider additional NAT testing for HIV-2. One can conclude a false-positive fourth-generation test in the setting of repeated negative or indeterminate HIV-2/HIV-2 antibody tests in the setting of negative NAT tests.

In addition to these standard laboratory-based assays for detecting antibodies to HIV, a series of point-of-care tests can provide results in 1 to 60 minutes. While the sensitivity and specificity of these tests are generally quite high, it is generally recommended that any positive results be confirmed with standard laboratory testing. Currently in the United States there is one FDA-approved rapid test kit available for use at home (OraQuick). It provides results in approximately 20 minutes. There are several other tests for which the sample is obtained at home and mailed to the lab. A positive result with any of these tests should be followed with confirmatory laboratory testing by a health care professional. A variety of laboratory tests are available for the direct detection of HIV or its components (Table 208-8). These tests may be of consider­ able help in making a diagnosis of HIV infection when the antibody determination assays are indeterminate. In addition, the tests detecting levels of HIV RNA can be used to determine prognosis and to assess the response to antiretroviral therapies. The simplest, least expensive, and most rarely used of the direct detection tests is the p24 antigen cap­ ture assay. This is an EIA-type assay in which the solid phase consists of antibodies to the p24 antigen of HIV. It detects the viral protein p24 in the blood of people with HIV where it exists either as free antigen or complexed to anti-p24 antibodies. It is currently part of the fourthgeneration HIV-1/2 antigen antibody immunoassay test recommended for initial screening. Overall, ~30% of individuals with untreated HIV infection have detectable levels of free p24 antigen. This increases to ~50% when samples are treated with a weak acid to dissociate antigen– antibody complexes. Throughout the course of HIV infection, an equi­ librium exists between p24 antigen and anti-p24 antibodies. During the first few weeks of infection, before an immune response develops, there is a brisk rise in p24 antigen levels. After the development of anti-p24 antibodies, these levels decline. Late in the course of infection, when CHAPTER 208 Human Immunodeficiency Virus Disease: AIDS and Related Disorders
HIV-1/2 ANTIGEN/ANTIBODY COMBINATION IMMUNOASSAY + – Negative for HIV-1 and HIV-2 antibodies and p24Ag HIV-1/HIV-2 antibody differentiation immunoassay + + HIV-1 HIV-2 – + HIV-1 HIV-2 – – HIV-1 or indeterminate HIV-2 HIV-2 antibodies detected HIV antibodies detected HIV-1 NAT + – HIV-1 NAT Negative for HIV-1 HIV-1 NAT Acute HIV-1 infection

TABLE 208-8  Characteristics of Tests for Direct Detection of HIV TEST TECHNIQUE SENSITIVITYa COST/TESTb Immune complex–dissociated p24 antigen capture assay Measurement of levels of HIV-1 core (p24) protein in an EIA-based format following dissociation of antigen–antibody complexes by weak acid treatment HIV RNA by PCR Target amplification of HIV-1 RNA via reverse transcription followed by PCR Reliable to 20 copies/mL of HIV RNA $75–150 HIV RNA by bDNA Measurement of levels of particle-associated HIV RNA in a nucleic acid capture assay employing signal amplification HIV RNA by TMA Target amplification of HIV-1 RNA via reverse transcription followed by T7 RNA polymerase HIV RNA by NASBA Isothermal nucleic acid amplification with internal controls Reliable to 80 copies/mL of HIV RNA $75–150 aSensitivity figures refer to those approved by the U.S. Food and Drug Administration. bPrices may be lower in large-volume settings. Abbreviations: bDNA, branched DNA; cDNA, complementary DNA; EIA, enzyme immunoassay; NASBA, nucleic acid sequence–based amplification; PCR, polymerase chain reaction; TMA, transcription-mediated amplification. circulating levels of virus are high, p24 antigen levels also increase, particularly when detected by techniques involving dissociation of antigen-antibody complexes. The p24 antigen capture assay has its greatest use as a screening test for HIV infection in patients suspected of having the acute HIV syndrome (see below), as high levels of p24 antigen are present prior to the development of antibodies. Its use as a stand-alone test for routine blood donor screening for HIV infection has been replaced by the use of “fourth-generation” assays that com­ bine antigen and antibody testing or NAT. The ability to measure and monitor levels of HIV RNA in the plasma of patients with HIV infection has been of extraordinary value in fur­ thering our understanding of the pathogenesis of HIV infection, in monitoring the response to ART, and in providing a diagnostic tool in settings where measurements of anti-HIV antibodies may be mislead­ ing, such as in acute infection and neonatal infection. In addition to the commercially available tests for measuring HIV RNA, DNA PCR assays also are employed by research laboratories for making a diagnosis of HIV infection by amplifying HIV proviral DNA from peripheral blood mononuclear cells. The commercially available RNA detection tests have a sensitivity of 20–50 copies of HIV RNA per milliliter of plasma. Research laboratory–based RNA assays can detect as few as one HIV RNA copy per milliliter, while the DNA PCR tests can detect proviral DNA at a frequency of one copy per 10,000–100,000 cells. Thus, these tests are extremely sensitive. One frequent consequence of a high degree of sensitivity is some loss of specificity, and false-positive results have been reported with each of these techniques. For this reason, a positive EIA with a positive HIV RNA assay can be considered the “gold standard” for a diagnosis of HIV infection, and the interpretation of other test results must be done with this in mind. PART 5 Infectious Diseases In the RT-PCR technique, following DNAse treatment, a cDNA copy is made of the HIV RNA species present in plasma typically using tRNALys3 as a primary primer. This is the same host cell primer used during the reverse transcription stage of the viral life cycle (Fig. 208-32). Because HIV is an RNA virus, this will result in the production of DNA copies of the HIV genome in amounts proportional to the amount of HIV RNA present in plasma. This cDNA is then amplified and charac­ terized using standard PCR techniques. In addition to being diagnostic and prognostic tools, RT-PCR and DNA-PCR also are useful for ampli­ fying defined areas of the HIV genome for sequence analysis and have become an important technique for studies of sequence diversity and microbial resistance to antiretroviral agents. In patients with a positive or indeterminate EIA test and an indeterminate Western blot, and in patients in whom serologic testing may be unreliable (such as patients with hypogammaglobulinemia or advanced HIV disease), these tests for quantitating HIV RNA in plasma or detecting proviral DNA in periph­ eral blood mononuclear cells are valuable tools for making a diagnosis of HIV infection; however, they should be used for diagnosis only when standard serologic testing has failed to provide a definitive result. ■ ■LABORATORY MONITORING OF PATIENTS

WITH HIV INFECTION The integration of clinical and laboratory data is essential to optimal management of patients with HIV infection. The close relationship

Positive in 50% of patients; detects down to 15 pg/mL of p24 protein $1–2 Reliable to 50 copies/mL of HIV RNA $75–150 Reliable to 100 copies/mL of HIV RNA $225 between clinical manifestations of HIV infection and CD4+ T cell count has made measurement of CD4+ T cell numbers a routine part of the initial evaluation of people with HIV. The discovery of HIV as the cause of AIDS led to the development of sensitive tests that allow one to monitor the levels of HIV in the blood. Determinations of peripheral blood CD4+ T cell counts and measurements of the plasma levels of HIV RNA provide a powerful set of tools for determining prognosis and monitoring response to therapy. CD4+ T Cell Counts  The CD4+ T cell count is the laboratory test generally accepted as the best indicator of the immediate state of immunologic competence of the patient with HIV infection. This mea­ surement has been shown to correlate very well with the level of immu­ nologic competence. Patients with CD4+ T cell counts <200/μL are at high risk of disease from P. jirovecii, while patients with CD4+ T cell counts <50/μL are also at high risk of disease from CMV, mycobacteria of the M. avium complex (MAC), and/or T. gondii (Fig. 208-32). Once the CD4+ T cell count is <200/μL, patients should be placed on a regimen for P. jirovecii prophylaxis. Once the count is <50/μL, primary prophylaxis for MAC infection is indicated unless the patient is imme­ diately started on ART. As with any laboratory measurement, one may wish to obtain two determinations prior to any significant changes in patient management based on CD4+ T cell count alone. Patients with HIV infection should have CD4+ T cell measurements performed at the time of diagnosis and every 3–6 months for the first 2 years there­ after. More frequent measurements should be made if a declining trend is noted. For patients who have been on ART for at least 2 years with HIV RNA levels persistently <50 copies/mL and CD4 counts 300– 500/μL, monitoring may be decreased to every year. For those with CD4 counts >500/μL, the monitoring of the CD4 count is felt by many to be optional. There are a handful of clinical situations in which the CD4+

T cell count may be misleading. Patients with HTLV-1/HIV co-infection may have elevated CD4+ T cell counts that do not accurately reflect their degree of immune competence. In patients with hypersplenism or those who have undergone splenectomy, and in patients receiving medi­ cations that suppress the bone marrow such as IFN-α, the CD4+ T cell percentage may be a more reliable indication of immune function than the CD4+ T cell count. A CD4+ T cell percentage of 15 is comparable to a CD4+ T cell count of 200/μL. HIV RNA Determinations  Facilitated by highly sensitive tech­ niques for the precise quantitation of small amounts of nucleic acids, the measurement of serum or plasma levels of HIV RNA has become an essential component in the monitoring of patients with HIV infec­ tion. As discussed in “Diagnosis of HIV Infection,” above, the most used technique is the RT-PCR assay. This assay generates data in the form of number of copies of HIV RNA per milliliter of serum or plasma, and commercial assays can reliably detect as few as 20 cop­ ies of HIV RNA per milliliter of plasma. Research-based assays can detect down to one copy per milliliter. While it is common practice to describe levels of HIV RNA below these cut-offs as “undetectable,” this is a term that should be avoided as it is imprecise and leaves the false impression that the level of virus is 0. By utilizing more sensitive,

CD4 (cells/µL3) * *

HSV HZos Crp KS Cry Can PCP NHL DEM PML WS Tox CMV PCP2 MAC Opportunistic illness FIGURE 208-32  Relationship between CD4+ T cell counts and the development of opportunistic diseases. Boxplot of the median (line inside the box), first quartile (bottom of the box), third quartile (top of the box), and mean (asterisk) CD4+ lymphocyte count at the time of the development of opportunistic disease. Can, candidal esophagitis; CMV, cytomegalovirus infection; Crp, cryptosporidiosis; Cry, cryptococcal meningitis; DEM, AIDS dementia complex; HSV, herpes simplex virus infection; HZos, herpes zoster; KS, Kaposi’s sarcoma; MAC, Mycobacterium avium complex bacteremia; NHL, nonHodgkin’s lymphoma; PCP, primary Pneumocystis jirovecii pneumonia; PCP2, secondary P. jirovecii pneumonia; PML, progressive multifocal leukoencephalopathy; Tox, Toxoplasma gondii encephalitis; WS, wasting syndrome. (From Annals of Internal Medicine, RD Moore, RE Chaisson: Natural History of Opportunistic Disease in an HIV-Infected Urban Clinical Cohort. 124(7):633-642, 1996. Copyright © 1996 American College of Physicians. All Rights Reserved. Reprinted with the permission of American College of Physicians, Inc.) nested PCR techniques and by studying tissue levels of virus as well as plasma levels, HIV RNA can be detected in virtually every patient with HIV infection. There are notable exceptions to this that involve patients who underwent cytoreductive therapy followed by transplant from CCR5Δ32 donors. Measurements of changes in HIV RNA levels over time have been of great value in delineating the relationship between levels of virus and rates of disease progression (Fig. 208-22), the rates of viral turnover, the relationship between immune system activation and viral replication, and the time to development of drug resistance. HIV RNA measure­ ments are greatly influenced by the state of activation of the immune system and may fluctuate greatly in the setting of secondary infections or immunization. For these reasons, decisions based on HIV RNA levels should never be made on a single determination. Measurements of plasma HIV RNA levels should be made at the time of HIV diagno­ sis. At the time of diagnosis, ART should be initiated, and HIV RNA levels monitored approximately every 4 weeks until the effectiveness of the therapeutic regimen is determined by the development of a new steady-state level of HIV RNA. In most instances of effective antiretro­ viral therapy, the plasma level of HIV RNA will drop to <50 copies/mL within 6 months of the initiation of treatment. During therapy, levels of HIV RNA should be monitored every 3–6 months to evaluate the continuing effectiveness of therapy. HIV Resistance Testing  The availability of multiple antiretroviral drugs as treatment options has generated a great deal of interest in the potential for measuring the sensitivity of an individual’s HIV viral quasispecies to different antiretroviral agents. HIV resistance testing can be done through either genotypic or phenotypic measurements. In the genotypic assays, sequence analyses of the HIV genomes obtained from patients are compared with sequences of viruses with known antiretroviral resistance profiles. In the phenotypic assays, the in vivo growth of patient-derived viral isolates or genetically constructed pseu­ doviruses is compared with the growth of reference strains of the virus in the presence or absence of different antiretroviral drugs. These tests are quite good at identifying those antiretroviral agents that have been utilized in the past and suggesting agents that may be of future value in a given patient. Resistance testing is recommended at the time of ini­ tial diagnosis and, if therapy is not initiated at that time, at the time of initiation of ART. Drug resistance testing is also indicated in the setting of virologic failure and should be performed while the patient is still

on the failing regimen because of the propensity for the pool of HIV quasi­ species to rapidly revert to wild-type in the absence of the selective pressures of ART. In the hands of experts, resis­ tance testing enhances the short-term ability to decrease viral load by ~0.5 log compared with changing drugs merely based on drug history. In addi­ tion to the use of resistance testing to help in the selection of new drugs in patients with virologic failure, it may also be of value in selecting an ini­ tial regimen for treatment of therapynaïve individuals. This is particularly true in geographic areas with a high level of background resistance. The patient needs to have an HIV-1 RNA level above 500–1000 copies/mL for an accurate resistance determination. Resistance assays lose their consis­ tency at lower levels of plasma viremia.

Co-Receptor Tropism Assays 

Following the licensure of maravi­ roc as the first CCR5 antagonist for the treatment of HIV infection (see below), it became necessary to be able to determine whether a patient’s virus was likely to respond to this treatment. Patients tend to have CCR5-tropic virus early in the course of infection, with a trend toward CXCR4 viruses later in disease. The antiretroviral agent maraviroc is effective only against CCR5-tropic viruses. Because the genotypic determinants of cellular tropism are poorly defined, a phenotypic assay is necessary to determine this prop­ erty of HIV. The Trofile assay (Monogram Biosciences) is available to make this determination. This assay clones the envelope regions of the patient’s virus into an indicator virus that is then used to infect target cells expressing either CCR5 or CXCR4 as their co-receptor. The assay takes weeks to perform and is expensive. Another, less costly option is to obtain a genotypic assay of the V3 region of HIV-1 and then employ a computer algorithm to predict viral tropism from the sequence. While this approach is less expensive than the classic phenotypic assay, there are fewer data to validate its predictive value. CHAPTER 208 Human Immunodeficiency Virus Disease: AIDS and Related Disorders
Other Tests  A variety of other laboratory tests have been studied as potential markers of HIV disease activity. Among these are quantitative culture of replication-competent HIV from plasma, peripheral blood mononuclear cells, or resting memory CD4+ T cells; circulating levels of β2-microglobulin, soluble IL-2 receptor, IgA, acid-labile endogenous IFN, or TNF-α; and the presence or absence of activation markers such as CD38, HLA-DR, and PD-1 on CD4+ or CD8+ T cells. Nonspecific serologic markers of inflammation and/or coagulation such as IL-6, d-dimer, and sCD14 have been shown to have a high correlation with all-cause mortality (Table 208-9). While these measurements have value as markers of disease activity and help to increase our under­ standing of the pathogenesis of HIV disease, they do not currently play a major role in the monitoring of patients with HIV infection. TABLE 208-9  Association between High-Sensitivity CRP, Il-6, and d-Dimer with All-Cause Mortality in Patients with HIV Infection UNADJUSTED ADJUSTED ODDS RATIO (FOURTH/FIRST) P ODDS RATIO (FOURTH/FIRST) P MARKER Hs-CRP 2.0 .05 2.8 .03 IL-6 8.3 <.0001 11.8 <.0001 d-dimer 12.4 <.0001 26.5 <.0001 Abbreviations: Hs-CRP, high-sensitivity C-reactive protein; IL-6, interleukin 6. Source: From LH Kuller et al: PLoS Med 5:e203, 2008.

CLINICAL MANIFESTATIONS The clinical consequences of HIV infection encompass a spectrum ranging from an acute syndrome associated with primary infection to a prolonged asymptomatic state to advanced disease. It is best to regard HIV disease as beginning at the time of primary infection and progressing through vari­ ous stages. As mentioned above, active virus replication and progressive immunologic impairment occur throughout the course of HIV infec­ tion in most patients. Except for the rare, true, “elite” virus controllers or long-term nonprogressors (see “Long-Term Survivors, Long-Term Nonprogressors, and Elite Controllers,” above), HIV disease in untreated patients inexorably progresses even during the clinically latent stage. Since the mid-1990s, ART has had a major impact on preventing and reversing the progression of disease over extended periods of time in a substantial proportion of adequately treated patients. Today, a person diagnosed with HIV infection and treated with ART has a close to normal life expectancy.

■ ■ACUTE HIV INFECTION It is estimated that 50–70% of individuals with HIV infection experi­ ence an acute clinical syndrome ~3–6 weeks after primary infection (Fig. 208-33). Varying degrees of clinical severity have been reported, and although it has been suggested that symptomatic seroconversion leading to the seeking of medical attention indicates an increased risk for an accelerated course of disease, there does not appear to be a cor­ relation between the level of the initial burst of viremia in acute HIV infection and the subsequent course of disease. The typical clinical findings in the acute HIV syndrome are listed in Table 208-10; they occur along with a burst of plasma viremia. It has been reported that several symptoms of the acute HIV syndrome (fever, skin rash, pharyngitis, and myalgia) occur less frequently in those infected by injection drug use compared with those infected by sexual contact. The syndrome is typical of an acute viral syndrome and has been lik­ ened to acute infectious mononucleosis. Symptoms usually persist for one to several weeks and gradually subside as an immune response to HIV develops and the levels of plasma viremia decrease. Opportunistic infections have been reported during this stage of infection, reflecting the immunodeficiency that results from reduced numbers of CD4+ T cells and likely also from the dysfunction of CD4+ T cells owing to viral protein and endogenous cytokine-induced perturbations of cells (Table 208-5) associated with the extremely high levels of plasma viremia. The Fiebig staging system has been used to describe the different stages of acute HIV infection, ranging from stage 1 (HIV RNA positive alone) to stage VI (HIV RNA and full Western blot positive). A number of immunologic abnormalities accompany the acute HIV syndrome, including multiphasic perturbations of the numbers of circulating lymphocyte subsets. The numbers of total lymphocytes and T cell subsets (CD4+ and CD8+) are initially reduced. An inversion of the CD4+/CD8+ T cell ratio occurs later because of a rise in the number PART 5 Infectious Diseases Primary Infection 3–6 weeks Plasma viremia (wide dissemination of virus) Acute syndrome Retrafficking of lymphocytes 1 week–3 months Immune response to HIV Curtailment of plasma viremia Establishment of chronic, persistent infection in lymphoid tissue 1–2 weeks Clinical latency FIGURE 208-33  The acute HIV syndrome. See text for detailed description. (From

G Pantaleo, C Graziosi, AS Fauci: The Immunopathogenesis of Human Immunodeficiency Virus Infection. N Engl J Med 328:327, 1993. Copyright © 1993 Massachusetts Medical Society. Reprinted with permission from Massachusetts Medical Society.)

TABLE 208-10  Clinical Findings in the Acute HIV Syndrome General Neurologic   Fever   Meningitis   Pharyngitis   Encephalitis   Lymphadenopathy   Peripheral neuropathy   Headache/retroorbital pain   Myelopathy   Arthralgias/myalgias Dermatologic   Lethargy/malaise   Erythematous maculopapular rash   Anorexia/weight loss   Mucocutaneous ulceration   Nausea/vomiting/diarrhea Source: Reproduced with permission from B Tindall, DA Cooper: Primary HIV infection: Host responses and intervention strategies. AIDS 5:1, 1991. of CD8+ T cells. In fact, there may be a selective and transient expan­ sion of CD8+ T cell subsets, as determined by T cell receptor analysis (see above). The total circulating CD8+ T cell count may remain elevated or return to normal; however, CD4+ T cell levels usually remain somewhat depressed, although there may be a slight rebound toward normal. Lymphadenopathy occurs in ~70% of individuals with primary HIV infection. Most patients recover spontaneously from this syndrome, and many are left with only a mildly depressed CD4+ T cell count that remains stable for a variable period before beginning its pro­ gressive decline; in some individuals, the CD4+ T cell count returns to the normal range. Approximately 10% of patients manifest a fulminant course of immunologic and clinical deterioration after primary infec­ tion, even after the disappearance of initial symptoms. In most patients, primary infection with or without the acute syndrome is followed by a prolonged period of clinical latency or smoldering low disease activity. ■ ■THE ASYMPTOMATIC STAGE—CLINICAL LATENCY Although the length of time from initial infection to the development of clinical disease varies greatly, the median time for untreated patients is ~10 years. As emphasized above, HIV disease with active virus replica­ tion is ongoing and progressive during this asymptomatic period. The rate of disease progression is directly correlated with HIV RNA levels. Patients with high levels of HIV RNA in plasma progress to symptomatic disease faster than patients with low levels of HIV RNA (Fig. 208-22). Some patients referred to as long-term nonprogressors show little if any decline in CD4+ T cell counts over extended periods of time. These patients generally have extremely low levels of HIV RNA; a subset, referred to as elite nonprogressors, exhibits HIV RNA levels <50 copies/ mL. Certain other patients remain entirely asymptomatic even though their CD4+ T cell counts show a steady progressive decline to extremely low levels. In these patients, the appearance of an opportunistic disease may be the first manifestation of HIV infection. During the asymptom­ atic period of HIV infection, the average rate of CD4+ T cell decline is ~50/μL per year in an untreated patient. When the CD4+ T cell count falls to <200/μL, the resulting state of immunodeficiency is severe enough to place the patient at high risk for opportunistic infections and neoplasms and, hence, for clinically apparent disease. ■ ■SYMPTOMATIC DISEASE Symptoms of HIV disease can appear at any time during the course of HIV infection. Generally, the spectrum of illnesses that one observes changes as the CD4+ T cell count declines. The more severe and lifethreatening complications of HIV infection occur in patients with CD4+ T cell counts <200/μL. A diagnosis of AIDS is made in any individual age 6 years and older with HIV infection and a CD4+ T cell count <200/μL (stage 3, Table 208-2) and in anyone with HIV infec­ tion who develops one of the HIV-associated diseases considered to be indicative of a severe defect in cell-mediated immunity (Table 208-1). While the causative agents of the secondary infections are characteristically opportunistic organisms such as P. jirovecii, atypical mycobacteria, CMV, and other organisms that do not ordinarily cause disease in the absence of a compromised immune system, they also include several common bacterial and mycobacterial pathogens. Following the wide­ spread use of ART and implementation of guidelines for the preven­ tion of opportunistic infections (Table 208-11), the incidence of these

TABLE 208-11  NIH/CDC/IDSA 2024 Guidelines for the Prevention of Opportunistic Infections in Persons Infected with HIV PATHOGEN INDICATIONS FIRST CHOICE(S) ALTERNATIVES Pneumocystis jirovecii CD4+ T cell count <100–200/μL if HIV RNA level above detection limits or CD4+ T cell count <100 /μL regardless of HIV RNA level or Oropharyngeal candidiasis or Prior bout of PCP May stop prophylaxis if CD4+ T cell count

200/μL for ≥3 months Mycobacterium tuberculosis Important to double check drug–drug interactions with ART regimen   Isoniazid sensitive Skin test >5 mm or Positive IFN-γ release assay or Prior positive test without treatment or Close contact with case of active pulmonary TB Same with high probability of exposure to drugresistant TB   Drug resistant Consult local public health authorities Mycobacterium avium complex CD4+ T cell count <50/μL unless ART immediately initiated—or if not able to achieve viral suppression or Prior documented disseminated disease May stop prophylaxis once fully suppressive ART in place Toxoplasma gondii T. gondii IgG antibody positive and CD4+ T cell count <100/μL or Prior toxoplasmic encephalitis and CD4+ T cell count <200/μL May stop prophylaxis if CD4+ T cell count

200/μL and HIV RNA suppressed for ≥3 months Varicella zoster virus Significant exposure to chickenpox or shingles in a patient with no history of immunization or prior exposure to either

Trimethoprim-sulfamethoxazole (TMP-SMX), 1 DS tablet qd PO or TMP-SMX, 1 SS tablet qd PO Dapsone 50 mg bid PO or 100 mg/d PO or Dapsone 50 mg/d PO + Pyrimethamine 50 mg/week PO + Leucovorin 25 mg/week PO or (Dapsone 200 mg PO + Pyrimethamine 75 mg PO + Leucovorin 25 mg weekly PO) or Aerosolized pentamidine, 300 mg via Respirgard II nebulizer every month or Atovaquone 1500 mg/d PO or TMP-SMX 1 DS tablet 3?/week PO Isoniazid 15 mg/kg PO once weekly

(900 mg maximum) plus rifapentine

(600 mg for 25.1-32 kg; 750 mg for

32.1-49.9 kg; 900 mg ≥ 50.0 kg Pyridoxine 50 mg PO weekly for

12 weeks or Isoniazid 300 mg PO daily + rifampin

600 mg PO daily + pyridoxine 25–50 mg PO daily for 3 months Isoniazid 300 mg PO daily + pyridoxine 25–50 mg PO daily for 6–9 months Rifampin 600 mg PO daily for 4 months Isoniazid 300 mg PO daily + rifapentine (<35 kg: 300 mg; 35–45 kg: 450 mg; >45 kg: 600 mg) PO daily + pyridoxine 25–50 mg PO daily for 4 weeks (only for persons on an efavirenz-based ART) CHAPTER 208 Azithromycin 1200 mg weekly PO or

600 mg twice weekly PO or Clarithromycin 500 mg twice daily PO Rifabutin (dose adjusted based on cART regimen) Human Immunodeficiency Virus Disease: AIDS and Related Disorders
TMP-SMX 1 DS tablet PO daily or Sulfadiazine 2000–4000 mg in 2–4 divided doses daily PO + Pyrimethamine 25–50 mg/d PO + Leucovorin 10–25 mg/d PO TMP-SMX 1 DS 3 times weekly PO or TMP-SMX, 1 SS PO daily or Dapsone 50 mg/d PO + Pyrimethamine 50 mg weekly PO + Leucovorin 25 mg weekly PO or (Dapsone 200 mg PO + Pyrimethamine 75 mg PO + Leucovorin 25 mg PO) weekly or Atovaquone 1500 mg PO daily ± (Pyrimethamine 75 mg PO + Leucovorin 10 mg PO) daily or (Clindamycin 600 mg q8h PO + Pyrimethamine 25 mg/d PO + Leucovorin 10mg/d PO) daily Varicella zoster immune globulin, IM, within 10 d of exposure (800-843-7477) Acyclovir 800 mg PO 5 times a day for 5–7 days or Valacyclovir 1 g PO tid for 5–7 days (Continued)

TABLE 208-11  NIH/CDC/IDSA 2024 Guidelines for the Prevention of Opportunistic Infections in Persons Infected with HIV PATHOGEN INDICATIONS FIRST CHOICE(S) ALTERNATIVES Cryptococcus neoformans Prior documented disease—primary prophylaxis not recommended in United States May stop secondary prophylaxis 1 year after initiation of therapy if CD4+ T cell count >100/μL, no evidence of active fungal infection, and HIV RNA levels suppressed for >3 months Histoplasma capsulatum Prior documented disease or CD4+ T cell count <150 μL and high risk (endemic area or occupational exposure) May stop prophylaxis after 1 year if CD4+ T cell count >150/μL and patient on effective ART for ≥6 months Coccidioides immitis Prior documented disease or positive serology and CD4+ T cell count <250/μL if from a disease endemic area. (For this indication prophylaxis can be stopped if CD4+ T cell count ≥250 for 6 months.) Talaromyces (formerly Penicillium) marneffei Prior documented disease Patients with CD4+ T cell counts <100 who live or stay in northern Thailand, Southern China, or Vietnam May stop secondary prophylaxis in patients on ART with CD4+ T cell count >100/μL for ≥6 months Salmonella species Prior recurrent bacteremia—primary prophylaxis not generally recommended Bartonella Prior infection Doxycycline 200 mg/d PO May stop after 3–4 months of therapy and with a CD4+ T cell count >200/μL for >3 months and effective ART for at least 6 months PART 5 Infectious Diseases Cytomegalovirus Prior end-organ disease—primary prophylaxis not recommended May stop secondary prophylaxis if CD4+ T cell count >100/μL for 6 months and no evidence of active CMV disease Restart if CD4+ T cells <100/μL Immunizations Generally Recommended Hepatitis B virus All susceptible (anti-HBc– and anti-HBs– negative) patients Hepatitis A virus All susceptible (anti-HAV–negative) patients Hepatitis A vaccine: 2 doses Influenza virus All patients annually Inactivated trivalent influenza virus vaccine 1 dose yearly COVID-19 Annually or per updated CDC recommendations (www.cdc.gov/covid/vaccines/ immunocompromised-people.html) Streptococcus pneumoniae All patients, preferably before CD4+ T cell count ≤200/μL Reimmunize persons initially immunized at a CD4+ T cell count <100/μL whose CD4+ T cell count then increases to >200/μL Human papillomavirus All patients 13–26 years of age HPV vaccine; 3 doses Mpox All patients with HIV who have potential for mpox exposure Meningococcal disease All patients with HIV infection >18 years Two doses of quadrivalent meningococcal conjugate vaccine Zoster Age <18 years Two-dose series of recombinant zoster vaccine (Shingrix) IM 2–6 months apart

(Continued) Fluconazole 200 mg/d PO Itraconazole 200 mg/d PO Itraconazole 200 mg bid PO Fluconazole 400 mg/d PO Fluconazole 400 mg PO daily Itraconazole 200 mg/d PO Fluconazole 400 mg PO once weekly Based on sensitivity of initial infection or Azithromycin 1200 mg weekly PO or Clarithromycin 500 mg bid PO Valganciclovir 900 mg once daily PO Hepatitis B vaccine: 3 doses Pneumococcal conjugated vaccine (15 or 20) 0.5 mL IM; if 15-valent is used, follow in 8 weeks or more by pneumococcal polysaccharide vaccine (23) Mpox vaccine (MVA-BN); 2 doses (Continued)

TABLE 208-11  NIH/CDC/IDSA 2024 Guidelines for the Prevention of Opportunistic Infections in Persons Infected with HIV PATHOGEN INDICATIONS FIRST CHOICE(S) ALTERNATIVES Recommended for Prevention of Severe or Frequent Recurrences Herpes simplex Frequent/severe recurrences Valacyclovir 500 mg twice daily PO Candida (mucocutaneous) Frequent/severe recurrences—primary prophylaxis not recommended Abbreviations: ART, antiretroviral therapy; bid, twice daily; cART, combination antiretroviral therapy; DS, double-strength; IFN-γ, interferon γ; IM, intramuscular; PCP, Pneumocystis jirovecii pneumonia; PO, by mouth; qd, daily; SS, single-strength; TB, tuberculosis; tid, three times a day. secondary infections has decreased dramatically (Fig. 208-34). Overall, the clinical spectrum of HIV disease is constantly changing as patients live longer and new and better approaches to treatment and prophylaxis are developed. In addition to the classic, original AIDSdefining illnesses, patients with HIV infection also have an increase in several serious non-AIDS illnesses, including non-AIDS-related cancers and cardiovascular, renal, and hepatic disease. Non-AIDS events (Table 208-4) now dominate the disease burden for patients

Incidence/100 person-years No. of opportunistic infections

Pneumocystis pneumonia

Disseminated Mycobacterium avium complex Esophageal candidiasis

Cytomegalovirus retinitis Kaposi’s sarcoma

Cytomegalovirus disease

Cryptococcosis Toxoplasmosis

A Year of observation

per 100 person-years

B

FIGURE 208-34  A. Decrease in the incidence of opportunistic infections and Kaposi’s sarcoma in people with HIV with CD4+ T cell counts <100/μL from 1992 through 1998. (Reproduced with permission from JE Kaplan et al: Epidemiology of human immunodeficiency virus-associated opportunistic infections in the United States in the era of highly active antiretroviral therapy. Clin Infect Dis 30:S5-14, 2000.) B. Quarterly incidence rates of cytomegalovirus (CMV), Pneumocystis jirovecii pneumonia (PCP), and Mycobacterium avium complex (MAC) from 1995 to 2001. (Reproduced with permission from Palella FJ Jr et al; HIV Outpatient Study Investigators. Durability and predictors of success of highly active antiretroviral therapy for ambulatory patients with HIV. AIDS 16:1617, 2002.)

(Continued) or Acyclovir 400 mg twice daily PO or Famciclovir 500 mg twice daily PO Fluconazole 100–200 mg daily PO Posaconazole oral suspension 400 mg daily PO or Itraconazole 200 mg oral suspension daily or Posaconazole tablet 300 mg PO daily with HIV infection successfully treated with ART. In developed coun­ tries, AIDS-related illnesses are responsible for only ~25% of deaths in patients with HIV infection. A similar percentage of deaths are due to non-AIDS-defining malignancies. Cardiovascular disease and liver disease each account for approximately 15% of deaths with 3% of deaths due to suicide The physician providing care to a patient with HIV infection must be well versed in general internal medicine as well as HIV-related opportunistic diseases. In general, it should be stressed that a key element of treatment of symptomatic complications of HIV disease, whether they are primary or secondary, is achieving good control of HIV replication with the use of ART and instituting primary and secondary prophylaxis for opportunistic infections as indicated. CHAPTER 208 Diseases of the Respiratory System  Acute bronchitis and sinusitis are prevalent during all stages of HIV infection. The most severe cases tend to occur in patients with lower CD4+ T cell counts. Sinusitis presents as fever, nasal congestion, and headache. The diag­ nosis is made by CT or MRI. The maxillary sinuses are most commonly involved; however, disease is also frequently seen in the ethmoid, sphenoid, and frontal sinuses. While some patients may improve without antibiotic therapy, radiographic improvement is quicker and more pronounced in patients who have received antimicrobial ther­ apy. It is postulated that this high incidence of sinusitis results from an increased frequency of infection with encapsulated organisms such as

H. influenzae and S. pneumoniae. In patients with low CD4+ T cell counts one may see mucormycosis infections of the sinuses. In contrast to the course of this infection in other patient popula­ tions, mucormycosis of the sinuses in patients with HIV infection may progress more slowly. In this setting aggressive, frequent local debridement in addition to local and systemic amphotericin B may result in effective treatment. Human Immunodeficiency Virus Disease: AIDS and Related Disorders
CMV PCP MAC Pulmonary disease is one of the most frequent complications of HIV infection. The most common manifestation of pulmonary disease is pneumonia. Three of the 10 most common AIDS-defining illnesses are recurrent bacterial pneumonia, tuberculosis, and pneumonia due to the unicellular fungus P. jirovecii. Other major causes of pulmonary infiltrates include other mycobacterial infections, other fungal infec­ tions, nonspecific interstitial pneumonitis, KS, and lymphoma. Bacterial pneumonia is seen with an increased frequency in patients with HIV infection, with 0.8–2.0 cases per 100 person-years. Patients with HIV infection are particularly prone to infections with encapsulated organisms. S. pneumoniae (Chap. 153) and H. influenzae (Chap. 162) are responsible for most cases of bacterial pneumonia in patients with AIDS. This may be a consequence of altered B cell function and/or defects in neutrophil function that may be secondary to HIV disease (see above). Pneumonias due to S. aureus (Chap. 152) and P. aeru­ ginosa (Chap. 170) also are reported to occur with an increased fre­ quency in patients with HIV infection. S. pneumoniae (pneumococcal) infection may be the earliest serious infection to occur in patients with

HIV disease. This can present as pneumonia, sinusitis, and/or bacte­ remia. Patients with untreated HIV infection have a 6-fold increase in the incidence of pneumococcal pneumonia and a 100-fold increase in the incidence of pneumococcal bacteremia. Pneumococcal disease may be seen in patients with relatively intact immune systems. In one study, the baseline CD4+ T cell count at the time of a first episode of pneumo­ coccal pneumonia was ~300/μL. Of interest is the fact that the inflam­ matory response to pneumococcal infection appears proportional to the CD4+ T cell count. Due to this high risk of pneumococcal disease, immunization with the conjugated pneumococcal vaccine followed by booster immunization with the 23-valent pneumococcal polysac­ charide vaccine is one of the generally recommended prophylactic measures for patients with HIV infection. This is likely most effective if given while the CD4+ T cell count is >200/μL and, if given to patients with lower CD4+ T cell counts, should be repeated once the count has been above 200 for 6 months. Although clear guidelines do not exist, it also makes sense to repeat immunization every 5 years. The incidence of bacterial pneumonia is cut in half when patients quit smoking.

Pneumocystis pneumonia (PCP; Chap. 227) is caused by the fungus P. jirovecii and was once the hallmark of AIDS. It has dramatically declined in incidence following the development of effective prophy­ lactic regimens and the widespread use of ART. It is, however, still the single most common cause of pneumonia in patients with HIV infec­ tion in the United States and can be identified as a likely etiologic agent in 25% of cases of pneumonia in patients with HIV infection, with an incidence of about 1 case per 100 person-years. Approximately 30% of cases of HIV-associated PCP occur in patients who are unaware of their HIV status. The risk of PCP is greatest among those who have experienced a previous bout of PCP and those who have CD4+ T cell counts of <200/μL. Overall, 79% of patients with PCP have CD4+ T cell counts <100/μL and 95% of patients have CD4+ T cell counts <200/μL. Recurrent fever, night sweats, thrush, and unexplained weight loss also are associated with an increased incidence of PCP. For these reasons, it is strongly recommended that all patients with CD4+ T cell counts <200/μL (or a CD4 percentage <15) receive some form of PCP prophylaxis. The incidence of PCP is approaching zero in patients with known HIV infection receiving appropriate ART and prophylaxis. In the United States, primary PCP is now occurring at a median CD4+ T cell count of 36/μL, while secondary PCP is occurring at a median CD4+ T cell count of 10/μL. PART 5 Infectious Diseases Patients with PCP generally present with fever and a cough that is usually nonproductive or productive of only scant amounts of white sputum. They may complain of a characteristic retrosternal chest pain that is worse on inspiration and is described as sharp or burning. HIV-associated PCP may have an indolent course characterized by weeks of vague symptoms and should be included in the differential diagnosis of fever, pulmonary complaints, or unexplained weight loss in any patient with HIV infection and <200 CD4+ T cells/μL. The most common finding on chest x-ray is either a normal film, if the disease is suspected early, or a faint bilateral interstitial infiltrate. The classic finding of a dense perihilar infiltrate is unusual in patients with AIDS. In patients with PCP who have been receiving aerosolized pentamidine for prophylaxis, one may see an x-ray picture of upper lobe cavi­ tary disease, reminiscent of TB. Other less common findings on chest x-ray include lobar infiltrates and pleural effusions. Thin-section CT may demonstrate a patchy ground-glass appearance. Routine laboratory evaluation is usually of little help in the differential diagnosis of PCP. A mild leukocytosis is common, although this may not be obvious in patients with prior neutropenia. Elevation of lactate dehydrogenase is common. Arterial blood-gases may indicate hypoxemia with a decline in Pao2 and an increase in the arterial-alveolar (a–a) gradient. Arterial blood-gas measurements not only aid in making the diagnosis of PCP but also provide important information for staging the severity of the disease and directing treatment (see below). A definitive diagnosis of PCP requires demonstration of the organism in samples obtained from induced sputum, bronchoalveolar lavage, transbronchial biopsy, or open-lung biopsy. PCR has been used to detect specific DNA sequences for P. jirovecii in clinical specimens where histologic exami­ nations have failed to make a diagnosis.

In addition to pneumonia, other clinical problems have been reported in people with HIV as a result of infection with P. jirovecii. Otic involvement may be seen as a primary infection, presenting as a polypoid mass involving the external auditory canal. In patients receiv­ ing aerosolized pentamidine for prophylaxis against PCP, one may see a variety of extrapulmonary manifestations of P. jirovecii. These include ophthalmic lesions of the choroid, a necrotizing vasculitis that resembles Buerger disease, bone marrow hypoplasia, and intestinal obstruction. Other organs that have been involved include lymph nodes, spleen, liver, kidney, pancreas, pericardium, heart, thyroid, and adrenals. Organ infection may be associated with cystic lesions that may appear calcified on CT or ultrasound. The standard treatment for PCP or disseminated pneumocystosis is trimethoprim-sulfamethoxazole (TMP-SMX). A high (20–85%) inci­ dence of side effects, particularly skin rash and bone marrow suppres­ sion, is seen with TMP-SMX in patients with HIV infection. Alternative treatments for mild to moderate PCP include dapsone/trimethoprim, clindamycin/primaquine, and atovaquone. IV pentamidine is the treatment of choice for severe disease in the patient unable to tolerate TMP-SMX. For patients with a Pao2 <70 mmHg or with an a–a gradient

35 mmHg, adjunct glucocorticoid therapy should be used in addition to specific antimicrobials. Overall, treatment should be continued for 21 days and followed by secondary prophylaxis. Prophylaxis for PCP is indicated for any individual with HIV who has experienced a prior bout of PCP, any patient with a CD4+ T cell count of <200/μL or a CD4 percentage <15, any patient with unexplained fever for >2 weeks, and any patient with a recent history of oropharyngeal candidiasis. The preferred regimen for prophylaxis is TMP-SMX, one double-strength tablet daily. This regimen also provides protection against toxoplasmo­ sis and some bacterial respiratory pathogens. For patients who cannot tolerate TMP-SMX, alternatives for prophylaxis include dapsone plus pyrimethamine plus leucovorin, aerosolized pentamidine administered by the Respirgard II nebulizer, and atovaquone. Primary or secondary prophylaxis for PCP can be discontinued in those patients treated with ART who maintain good suppression of HIV (<50 copies/mL) and CD4+ T cell counts >200/μL for at least 3 months. M. tuberculosis, once thought to be on its way to extinction in the United States, experienced a resurgence associated with the HIV epi­ demic (Chap. 183). Worldwide, approximately one-third of all AIDSrelated deaths are associated with TB, and TB is the primary cause of death for 10–15% of patients with HIV infection. In the United States ~5% of untreated AIDS patients have active TB. Patients with HIV infection are more likely to have active TB by a factor of 100 when compared with an HIV-negative population. For an asymptomatic HIV-negative person with a positive purified protein derivative (PPD) skin test, the risk of reactivation TB is around 1% per year. For the patient with untreated HIV infection, a positive PPD skin test, and no signs or symptoms of TB, the rate of reactivation TB is 7–10% per year. Untreated TB can accelerate the course of HIV infection. Levels of plasma HIV RNA increase in the setting of active TB and decline in the setting of successful TB treatment. Active TB is most common in patients 25–44 years of age, in African Americans and Hispanics, in patients in New York City and Miami, and in patients in developing countries. In these demographic groups, 20–70% of the new cases of active TB are in patients with HIV infection. The epidemic of TB embedded in the epidemic of HIV infection probably represents the greatest health risk to the general public and the health care profession associated with the HIV epidemic. In contrast to infection with atypical mycobacteria such as MAC, active TB often develops relatively early in the course of HIV infection and may be an early clinical sign of HIV disease. In one study, the median CD4+ T cell count at presentation of TB was 326/μL. The clinical manifestations of TB in people with HIV are quite varied and generally show different patterns as a function of the CD4+ T cell count. In patients with relatively high CD4+ T cell counts, the typical pattern of pulmonary reactivation occurs: patients present with fever, cough, dyspnea on exertion, weight loss, night sweats, and a chest x-ray revealing cavitary apical disease of the upper lobes. In patients with lower CD4+ T cell counts, disseminated disease is more common.

Human Immunodeficiency Virus Disease: AIDS and Related Disorders

CHAPTER 208 In these patients the chest x-ray may reveal diffuse or lower-lobe bilat­ eral reticulonodular infiltrates consistent with miliary spread, pleural effusions, and hilar and/or mediastinal adenopathy. Infection may be present in bone, brain, meninges, GI tract, lymph nodes (particularly cervical lymph nodes), and viscera. Some patients with advanced HIV infection and active TB may have no symptoms of illness, and thus screening for TB should be part of the initial evaluation of every patient with HIV infection. Approximately 60–80% of people with HIV and TB have pulmonary disease, and 30–40% have extrapulmonary disease. Respiratory isolation and a negative-pressure room should be used for patients in whom a diagnosis of pulmonary TB is being considered. This approach is critical to limit nosocomial and community spread of infection. Culture of the organism from an involved site provides a definitive diagnosis. Blood cultures are positive in 15% of patients. This figure is higher in patients with lower CD4+ T cell counts. In the setting of fulminant disease, one cannot rely on the accuracy of a negative PPD skin test to rule out a diagnosis of TB. In addition, IFN-γ release assays may be difficult to interpret due to high backgrounds as a consequence of HIV-associated immune activation. TB is one of the conditions associated with HIV infection for which cure is possible with appropriate therapy. Therapy for TB is generally the same in the patient with HIV as in the HIV-negative patient (Chap. 183). Due to the possibility of multidrug-resistant or extensively drug-resistant TB, drug susceptibility testing should be performed to guide therapy. Due to pharmacokinetic interactions, adjusted doses of rifabutin and/or changes in ART are required when treating TB in the setting of HIV infection. Treatment is most effective in programs that involve directly observed therapy. Initiation of ART and/or anti-TB therapy may be associated with clinical deterioration due to immune reconstitution inflammatory syndrome (IRIS) reactions. These are most common in patients initiating both treatments at the same time, may occur as early as 1 week after initiation of ART therapy, and are seen more fre­ quently in patients with advanced HIV disease. For these reasons it is recommended that initiation of ART be delayed in antiretroviral-naïve patients with CD4 counts ≥50 cells/μL until 2–8 weeks following the initiation of treatment for TB. For patients with CD4 counts <50 cells/μL the benefits of more immediate ART outweigh the risks of IRIS, and ART should be started as soon as possible in those patients. Effective prevention of active TB can be a reality if the health care professional is aggressive in looking for evidence of latent or active TB by making sure that all patients with HIV infection receive a PPD skin test or evaluation with an IFN-γ release assay. Anergy testing is not of value in this setting. Since these tests rely on the host mounting an immune response to M. tuberculosis, patients with CD4+ T cell counts <200 cells/μL should be retested if their CD4+ T cell counts rise to persistently above 200. Patients at risk of continued exposure to TB should be tested annually. People with HIV with a skin-test reaction of

5 mm, those with a positive IFN-γ release assay, or those who are close household contacts of persons with active TB should receive treatment with 12 weeks of once-weekly isoniazid and rifapentin. Atypical mycobacterial infections also are seen with an increased frequency in patients with HIV infection. Infections with at least 12 different mycobacteria have been reported, including M. bovis and representatives of all four Runyon groups. The most common atypical mycobacterial infection is with M. avium or M. intracellulare species— the Mycobacterium avium complex (MAC). Infections with MAC are seen mainly in patients in the United States and are rare in Africa. It has been suggested that prior infection with M. tuberculosis decreases the risk of MAC infection. MAC infections probably arise from organ­ isms that are ubiquitous in the environment, including both soil and water. There is little evidence for person-to-person transmission of MAC infection. The presumed portals of entry are the respiratory and GI tracts. MAC infection is a late complication of HIV infection, occurring predominantly in patients with CD4+ T cell counts of <50/μL. The average CD4+ T cell count at the time of diagnosis is 10/μL. The most common presentation is disseminated disease with fever, weight loss, and night sweats. At least 85% of patients with MAC infection are mycobacteremic, and large numbers of organisms can often be demonstrated on bone marrow biopsy. The chest x-ray is abnormal in ~25% of patients, with the most common pattern being that of a bilateral, lower-lobe infiltrate suggestive of miliary spread. Alveolar or nodular infiltrates and hilar and/or mediastinal adenopathy also can occur. Other clinical findings include endobronchial lesions, abdomi­ nal pain, diarrhea, and lymphadenopathy. Anemia and elevated liver alkaline phosphatase are common. The diagnosis is made by the cul­ ture of blood or involved tissue. The finding of two consecutive sputum samples positive for MAC is highly suggestive of pulmonary infection. Cultures may take 2 weeks to turn positive. Therapy consists of a mac­ rolide, usually clarithromycin, with ethambutol. Some physicians elect to add a third drug from among rifabutin, ciprofloxacin, or amikacin in patients with extensive disease. Therapy is continued until resolution of clinical signs and symptoms, negative cultures, and CD4+ T cell counts 100/μL for 3–6 months in the setting of ART. Primary prophylaxis for MAC is indicated in patients with HIV infection and CD4+ T cell counts <50/μL not immediately starting ART (Table 208-11). This may be discontinued in patients in whom ART induces a sustained suppres­ sion of viral replication regardless of the change in CD4+ T cell count. Rhodococcus equi is a gram-positive, pleomorphic, acid-fast, nonspore-forming bacillus that can cause pulmonary and/or disseminated infection in patients with advanced HIV infection. Fever and cough are the most common presenting signs. Radiographically one may see cavitary lesions and consolidation. Blood cultures are often positive. Treatment is based on antimicrobial sensitivity testing. Fungal infections of the lung, in addition to PCP, can be seen in patients with AIDS. Patients with pulmonary cryptococcal disease present with fever, cough, dyspnea, and, in some cases, hemoptysis. A focal or diffuse interstitial infiltrate is seen on chest x-ray in >90% of patients. In addition, one may see lobar disease, cavitary disease, pleural effusions, and hilar or mediastinal adenopathy. More than half of patients are fungemic, and 90% of patients have concomitant CNS infection. Coccidioides immitis is a mold that is endemic in the south­ west United States. It can cause a reactivation pulmonary syndrome in patients with HIV infection. Most patients with this condition will have CD4+ T cell counts <250/μL. Patients present with fever, weight loss, cough, and extensive, diffuse reticulonodular infiltrates on chest x-ray. One may also see nodules, cavities, pleural effusions, and hilar adenop­ athy. While serologic testing is of value in the immunocompetent host, serologies are negative in 25% of patients with HIV and coccidioidal infection. Invasive aspergillosis is not an AIDS-defining illness and is generally not seen in patients with AIDS in the absence of neutropenia or administration of glucocorticoids. When it does occur, Aspergillus infection may have an unusual presentation in the respiratory tract of patients with AIDS, where it gives the appearance of a pseudomem­ branous tracheobronchitis. Primary pulmonary infection of the lung may be seen with histoplasmosis. The most common pulmonary mani­ festation of histoplasmosis, however, is in the setting of disseminated disease, presumably due to reactivation. In this setting respiratory symptoms are usually minimal, with cough and dyspnea occurring in 10–30% of patients. The chest x-ray is abnormal in ~50% of patients, showing either a diffuse interstitial infiltrate or diffuse small nodules, and the urine will often be positive for Histoplasma antigen. Two forms of idiopathic interstitial pneumonia have been identified in patients with HIV infection: lymphoid interstitial pneumonitis (LIP) and nonspecific interstitial pneumonitis (NIP). LIP, a common finding in children, is seen in about 1% of adult patients with untreated HIV infection. This disorder is characterized by a benign infiltrate of the lung and is thought to be part of the polyclonal activation of lympho­ cytes seen in the context of HIV and EBV infections. Transbronchial biopsy is diagnostic in 50% of the cases, with an open-lung biopsy required for diagnosis in the remainder of cases. This condition is gen­ erally self-limited, and no specific treatment is necessary. Severe cases have been managed with brief courses of glucocorticoids. Although rarely a clinical problem since the use of ART, evidence of NIP may be seen in up to half of all patients with untreated HIV infection. Histologically, interstitial infiltrates of lymphocytes and plasma cells in a perivascular and peribronchial distribution are present. When symptomatic, patients present with fever and nonproductive cough occasionally accompanied by mild chest discomfort. Chest x-ray is

usually normal or may reveal a faint interstitial pattern. Like LIP, NIP is a self-limited process for which no therapy is indicated other than appropriate management of the underlying HIV infection. HIVrelated pulmonary arterial hypertension (HIV-PAH) is seen in ~0.5% of people with HIV. Patients may present with an array of symptoms including shortness of breath, fatigue, syncope, chest pain, and signs of right-sided heart failure. Chest x-ray reveals dilated pulmonary vessels and right-sided cardiomegaly with right ventricular hypertrophy seen on electrocardiogram. ART does not appear to be of clear benefit, and the prognosis is quite poor with a median survival in the range of 2 years.

Neoplastic diseases of the lung including KS and lymphoma are dis­ cussed below in the section on neoplastic diseases. Diseases of the Cardiovascular System  Heart disease is a rela­ tively common postmortem finding in people with HIV (25–75% in autopsy series). The most common form of heart disease is coronary heart disease. In one large series the overall rate of myocardial infarc­ tion (MI) was 3.5/1000 patient-years, 28% of these events were fatal, and MI was responsible for 7% of all deaths in the cohort. In patients with HIV infection, cardiovascular disease may be associated with classic risk factors such as smoking, a direct consequence of HIV infection and the accompanying immune activation, or a complication of ART. In general, patients with HIV infection have higher levels of triglycerides, lower levels of high-density lipoprotein cholesterol, and a higher prevalence of smoking than cohorts of individuals without HIV infection. The finding that the rate of cardiovascular disease events was lower in patients on antiretroviral therapy than in those randomized to undergo a treatment interruption identified a clear association between HIV replication and risk of cardiovascular disease. In one study, a baseline CD4+ T cell count of <500/μL was found to be an independent risk factor for cardiovascular disease comparable in magnitude to that attributable to smoking. While the precise pathogenesis of this associa­ tion remains unclear, it is likely related to the immune activation and increased propensity for coagulation seen because of HIV replication. Exposure to HIV protease inhibitors and certain reverse transcriptase inhibitors has been associated with increases in total cholesterol and/ or risk of MI. Any increases in the risk of death from MI resulting from the use of certain antiretrovirals must be balanced against the marked increases in overall survival brought about by these drugs. PART 5 Infectious Diseases Another form of heart disease associated with HIV infection is a dilated cardiomyopathy associated with congestive heart failure (CHF) referred to as HIV-associated cardiomyopathy. This generally occurs as a late complication of HIV infection and, histologically, displays elements of myocarditis. For this reason, some have advocated it be treated with IV immunoglobulin (IVIg). HIV can be directly demon­ strated in cardiac tissue in this setting, and there is debate over whether HIV plays a direct role in this condition. Patients present with typical findings of CHF including edema and shortness of breath. Patients with HIV infection may also develop cardiomyopathy as side effects of IFN-α or nucleoside analogue therapy. These are reversible once therapy is stopped. KS, cryptococcosis, Chagas’ disease, and toxoplas­ mosis can involve the myocardium, leading to cardiomyopathy. In one series, most patients with HIV infection and a treatable myocarditis were found to have myocarditis associated with toxoplasmosis. Most of these patients also had evidence of CNS toxoplasmosis. Thus, MRI or double-dose contrast CT scan of the brain should be included in the workup of any patient with advanced HIV infection and cardiomyopathy. A variety of other cardiovascular problems are found in patients with HIV infection. Pericardial effusions may be seen in the setting of advanced HIV infection. Predisposing factors include TB, CHF, mycobacterial infection, cryptococcal infection, pulmonary infection, lymphoma, and KS. While pericarditis is quite rare, in one series 5% of patients with HIV disease had pericardial effusions considered to be moderate or severe. Tamponade and death have occurred in asso­ ciation with pericardial KS, presumably owing to acute hemorrhage. Nonbacterial thrombotic endocarditis has been reported and should be considered in patients with unexplained embolic phenomena. IV

pentamidine, when given rapidly, can result in hypotension as a conse­ quence of cardiovascular collapse. Diseases of the Oropharynx and Gastrointestinal System 

Oropharyngeal and GI diseases are common features of HIV infection. They are most frequently due to secondary infections. In addition, oral and GI lesions may occur with KS and lymphoma. Oral lesions, including thrush, hairy leukoplakia, and aphthous ulcers (Fig. 208-35), are particularly common in patients with untreated HIV infection. Thrush, due to Candida infection, and oral hairy leuko­ plakia, associated with EBV, are usually indicative of fairly advanced immunologic decline; they generally occur in patients with CD4+ T cell counts of <300/μL. In one study, 59% of patients with oral can­ didiasis went on to develop AIDS in the next year. Thrush appears as a white, cheesy exudate, often on an erythematous mucosa in the poste­ rior oropharynx. While most commonly seen on the soft palate, early lesions are often found along the gingival vestibule. The diagnosis is made by direct examination of a scraping for pseudohyphal elements. Culturing is of no diagnostic value, as patients with HIV infection may have a positive throat culture for Candida in the absence of thrush. Oral hairy leukoplakia presents as white, frondlike lesions, generally along the lateral borders of the tongue and sometimes on the adjacent buccal mucosa (Fig. 208-35). Despite its name, oral hairy leukoplakia is not considered a premalignant condition. Lesions are associated with florid replication of EBV. While usually more disconcerting as a sign of HIV-associated immunodeficiency than a clinical problem in need of treatment, severe cases of oral hairy leukoplakia have been reported to respond to topical podophyllin or systemic therapy with anti-herpesvirus agents. Aphthous ulcers of the posterior oropharynx also are seen with regularity in patients with untreated HIV infection (Fig. 208-35). These lesions are of unknown etiology and can be quite painful and interfere with swallowing. Topical anesthetics provide immediate symptomatic relief of short duration. The fact that tha­ lidomide is an effective treatment for this condition suggests that the pathogenesis may involve the action of tissue-destructive cytokines. Palatal, glossal, or gingival ulcers may also result from cryptococcal disease or histoplasmosis. Esophagitis (Fig. 208-36) may present with odynophagia and retrosternal pain. Upper endoscopy is generally required to make an accurate diagnosis. Esophagitis may be due to Candida, CMV, or HSV. While CMV tends to be associated with a single large ulcer, HSV infec­ tion is more often associated with multiple small ulcers. The esophagus may also be the site of KS and lymphoma. Like the oral mucosa, the esophageal mucosa may have large, painful ulcers of unclear etiology that may respond to thalidomide. While achlorhydria is a common problem in patients with HIV infection, other gastric problems are generally rare. Among the neoplastic conditions involving the stomach are KS and lymphoma. Infections of the small and large intestine leading to diarrhea, abdominal pain, and occasionally fever are among the most significant GI problems in people with HIV. They include infections with bacteria, protozoa, and viruses. Bacteria may be responsible for infections of the GI tract in patients with HIV infection. Infections with enteric pathogens such as Salmo­ nella, Shigella, and Campylobacter are more common in men who have sex with men and are often more severe and more apt to relapse in patients with HIV infection. Patients with untreated HIV have approxi­ mately a 20-fold increased risk of infection with S. typhimurium. They may present with a variety of nonspecific symptoms including fever, anorexia, fatigue, and malaise of several weeks’ duration. Diarrhea is common but may be absent. Diagnosis is made by culture of blood and stool. Long-term therapy with ciprofloxacin is the recommended treatment. HIV-infected patients also have an increased incidence of S. typhi infection in areas of the world where typhoid is a problem. Shigella spp., particularly S. flexneri, can cause severe intestinal dis­ ease in people with HIV. Up to 50% of patients with GI disease will develop bacteremia. Campylobacter infections occur with an increased frequency in patients with HIV infection. While C. jejuni is the strain most frequently isolated, infections with many other strains have been

A C FIGURE 208-35  Various oral lesions in people with HIV. A. Thrush. B. Hairy leukoplakia. C. Aphthous ulcer. D. Kaposi’s sarcoma. FIGURE 208-36  Barium swallow of a patient with Candida esophagitis. The flow of barium along the mucosal surface is grossly irregular.

B CHAPTER 208 Human Immunodeficiency Virus Disease: AIDS and Related Disorders
D reported. Patients usually present with crampy abdominal pain, fever, and bloody diarrhea. Infection may also present as proctitis. Stool examination reveals the presence of fecal leukocytes. Systemic infec­ tion can occur, with up to 10% of infected patients exhibiting bacte­ remia. Most strains are sensitive to erythromycin. Abdominal pain and diarrhea may be seen with MAC infection, and patients with HIV infection may have persistent diarrhea due to enteroaggregative E. coli. Fungal infections including histoplasmosis, coccidioidomycosis, and penicilliosis have all been identified as a cause of fever and diar­ rhea in patients with HIV infection. Peritonitis has been seen with C. immitis. Cryptosporidia, microsporidia, and Isospora belli (Chap. 236) are the most common opportunistic protozoa that infect the GI tract and cause diarrhea in patients with HIV. Cryptosporidial infection may present in a variety of ways, ranging from a self-limited or intermittent diarrheal illness in patients in the early stages of HIV infection to a severe, life-threatening diarrhea in severely immunodeficient individu­ als. In patients with untreated HIV infection and CD4+ T cell counts of <300/μL, the incidence of cryptosporidiosis is ~1% per year. In 75% of cases the diarrhea is accompanied by crampy abdominal pain, and 25% of patients have nausea and/or vomiting. Cryptosporidia may also cause biliary tract disease in the patient with HIV, leading to cho­ lecystitis with or without accompanying cholangitis and pancreatitis secondary to papillary stenosis. The diagnosis of cryptosporidial diar­ rhea is made by stool examination or biopsy of the small intestine. The

diarrhea is noninflammatory, and the characteristic finding is the pres­ ence of oocysts that stain with acid-fast dyes. Therapy is predominantly supportive and marked improvements have been reported in the set­ ting of effective ART. Treatment with up to 2000 mg/d of nitazoxanide (NTZ) is associated with improvement in symptoms or a decrease in shedding of organisms in about half of patients. Its overall role in the management of this condition remains unclear. Patients can minimize their risk of developing cryptosporidiosis by avoiding contact with human and animal feces, by not drinking untreated water from lakes or rivers, and by not eating raw shellfish.

Microsporidia are small, unicellular, obligate intracellular parasites that reside in the cytoplasm of enteric cells (Chap. 236). The main spe­ cies causing disease in humans is Enterocytozoon bieneusi. The clinical manifestations are similar to those described for cryptosporidia and include abdominal pain, malabsorption, diarrhea, and cholangitis. The small size of the organism may make it difficult to detect; however, with the use of chromotrope-based stains, organisms can be identified in stool samples by light microscopy. Definitive diagnosis generally depends on electron-microscopic examination of a stool specimen, intestinal aspirate, or intestinal biopsy specimen. In contrast to crypto­ sporidia, microsporidia have been noted in a variety of extraintestinal locations, including the eye, brain, sinuses, muscle, and liver, and they have been associated with conjunctivitis and hepatitis. The most effec­ tive way to deal with microsporidia in a patient with HIV infection is to restore the immune system by treating the HIV infection with ART. Albendazole, 400 mg bid, has been reported to be of benefit in some patients. I. belli is a coccidian parasite (Chap. 236) most commonly found as a cause of diarrhea in patients from tropical and subtropical regions. Its cysts appear in the stool as large, acid-fast structures that can be differentiated from those of cryptosporidia based on size, shape, and number of sporocysts. The clinical syndromes of Isospora infection are identical to those caused by cryptosporidia. The important distinction is that infection with Isospora is generally easy to treat with TMP-SMX. While relapses are common, a thrice-weekly regimen of TMP-SMX appears adequate to prevent recurrence. PART 5 Infectious Diseases CMV colitis (Chap. 200) was once seen as a consequence of advanced immunodeficiency in 5–10% of patients with AIDS. It is much less common with the advent of ART. CMV colitis presents as diarrhea, abdominal pain, weight loss, and anorexia. The diarrhea is usually nonbloody, and the diagnosis is achieved through endoscopy and biopsy. Multiple mucosal ulcerations are seen at endoscopy, and biopsies reveal characteristic intranuclear and cytoplasmic inclusion bodies. Secondary bacteremias may result as a consequence of thinning of the bowel wall. Treatment is with either valganciclovir/ganciclovir or foscarnet for 3–6 weeks. Relapses are common, and maintenance therapy is typically necessary in patients whose HIV infection is poorly controlled. Patients with CMV disease of the GI tract should be care­ fully monitored for evidence of CMV retinitis. In addition to disease caused by specific secondary infections, patients with HIV infection may also experience a chronic diarrheal syndrome for which no etiologic agent other than HIV can be identi­ fied. This entity is referred to as AIDS enteropathy or HIV enteropathy. It is most likely a direct result of HIV infection in the GI tract and improves with ART. Histologic examination of the small bowel in these patients reveals low-grade mucosal atrophy with a decrease in mitotic figures, suggesting a hyporegenerative state. Patients often have decreased or absent small-bowel lactase and malabsorption with accompanying weight loss. The initial evaluation of a patient with HIV infection and diarrhea should include a set of stool examinations, including culture, examina­ tion for ova and parasites, and examination for Clostridium difficile toxin. Approximately 50% of the time this workup will demonstrate infection with pathogenic bacteria, mycobacteria, or protozoa. If the initial stool examinations are negative, additional evaluation, including upper and/or lower endoscopy with biopsy, will yield a diagnosis of microsporidial or mycobacterial infection of the small intestine ~30% of the time. In patients for whom this diagnostic evaluation is nonre­ vealing, a presumptive diagnosis of HIV enteropathy can be made if the

History and physical Stool culture for enteric pathogens Stool for ova and parasites × 3 Stool for Clostridium difficile toxin Diagnosis No diagnosis Treat No suspicion of colitis Suspicion of colitis Upper endoscopy and biopsy Diagnosis Colonoscopy and biopsy Treat No diagnosis HIV-Associated Enteropathy FIGURE 208-37  Algorithm for the evaluation of diarrhea in a patient with HIV infection. HIV-associated enteropathy is a diagnosis of exclusion and can be made only after other, generally treatable, forms of diarrheal illness have been ruled out. diarrhea has persisted for >1 month. An algorithm for the evaluation of diarrhea in patients with HIV infection is given in Fig. 208-37. Rectal lesions are common in patients with HIV, particularly the perirectal ulcers and erosions due to the reactivation of HSV (Fig. 208-38). These lesions may appear quite atypical, as denuded skin without vesicles. They typically respond well to treatment with valacyclovir, famciclovir, or foscarnet. Other rectal lesions encountered in patients with HIV infection include condylomata acuminata, KS, and intraepi­ thelial neoplasia (see below). Hepatobiliary Diseases  Diseases of the hepatobiliary system are a major problem in patients with HIV infection. It has been estimated that approximately 15% of the deaths of patients with HIV infection FIGURE 208-38  Severe, erosive perirectal herpes simplex in a patient with AIDS.

T cell count, and use of ART are taken into account. In the setting of HIV and HCV co-infection, levels of HCV are approximately 10-fold higher than in the HIV-negative patient with HCV infection. There is a 50% higher overall mortality rate with a five-fold increased risk of death due to liver disease in patients chronically infected with both HCV and HIV. Use of directly acting agents for the treatment of HCV leads to cure rates approaching 100%, even in patients with HIV coinfection. Successful treatment of HCV in patients with HIV decreases mortality. Hepatitis A virus infection is not seen with an increased frequency in patients with HIV infection. It is recommended that all patients with HIV infection who have not experienced natural infec­ tion be immunized with hepatitis A and/or hepatitis B vaccines. Infec­ tion with hepatitis G virus, also known as GB virus C, is seen in ~50% of patients with HIV infection. For reasons that are currently unclear, there are data to suggest that patients with HIV infection co-infected with this virus have a decreased rate of progression to AIDS. A variety of other infections also may involve the liver. Granuloma­ tous hepatitis may be seen as a consequence of mycobacterial or fungal infections, particularly MAC infection. Hepatic masses may be seen in the context of TB, peliosis hepatis, or fungal infection. Among the fungal opportunistic infections, C. immitis and Histoplasma capsula­ tum (Chap. 218) are those most likely to involve the liver. Biliary tract disease in the form of papillary stenosis or sclerosing cholangitis has been reported in the context of cryptosporidiosis, CMV infection, and KS. When no diagnosis can be made, the term AIDS cholangiopathy is used. Hemophagocytic lymphohistiocytosis of the liver has been seen in the setting of Hodgkin’s disease and may occur prior to diagnosis of the underlying neoplasm. Many of the drugs used to treat HIV infection are metabolized by the liver and can cause liver injury. Fatal hepatic reactions have been reported with a wide array of antiretrovirals including nucleoside ana­ logues, nonnucleoside analogues, and protease inhibitors. Nucleoside

analogues work by inhibiting DNA synthesis. This can result in toxicity to mitochondria, which can lead to disturbances in oxidative metabo­ lism. This may manifest as hepatic steatosis and, in severe cases, lactic acidosis and fulminant liver failure. It is important to be aware of this condition and to watch for it in patients with HIV infection receiving nucleoside analogues. It is reversible if diagnosed early and the offend­ ing agent(s) discontinued. Nevirapine has been associated with at times fatal fulminant and cholestatic hepatitis, hepatic necrosis, and hepatic failure. Indinavir may cause mild to moderate elevations in serum bilirubin in 10–15% of patients in a syndrome similar to Gilbert’s syn­ drome. A similar pattern of hepatic injury may be seen with atazanavir. In the patient receiving ART with an unexplained increase in hepatic transaminases, strong consideration should be given to drug toxicity.

Pancreatic injury is most commonly a consequence of drug toxicity, notably that secondary to pentamidine or dideoxynucleosides. While up to half of patients in some series have biochemical evidence of pancreatic injury, <5% of patients show any clinical evidence of pan­ creatitis that is not linked to a drug toxicity. Diseases of the Kidney and Genitourinary Tract  Diseases of the kidney or genitourinary tract may be a direct consequence of HIV infection, due to an opportunistic infection or neoplasm, or related to drug toxicity. Overall, microalbuminuria is seen in ~20% of untreated patients with HIV; significant proteinuria is seen in closer to 2%. The presence of microalbuminuria has been associated with an increase in all-cause mortality. HIV-associated nephropathy (HIVAN) was first described in IDUs and was initially thought to be IDU nephropathy in patients with HIV infection; it is now recognized as a true direct com­ plication of HIV infection. Although most patients with this condition have CD4+ T cell counts <200/μL, HIV-associated nephropathy can be an early manifestation of HIV infection and is also seen in children. More than 90% of reported cases have been in African-American or Hispanic individuals; the disease is not only more prevalent in these populations but also more severe and at one point was the third leading cause of end-stage renal failure among African Americans age 20–64 years in the United States. Proteinuria is the hallmark of this disorder. Edema and hypertension are rare. Ultrasound examination reveals enlarged, hyperechogenic kidneys. A definitive diagnosis is obtained through renal biopsy. Histologically, focal segmental glomerulosclero­ sis is present in 80%, and mesangial proliferation in 10–15% of cases. Prior to effective antiretroviral therapy, this disease was characterized by relatively rapid progression to end-stage renal disease. Patients with HIV-associated nephropathy should be treated for their HIV infection. Treatment with angiotensin-converting enzyme (ACE) inhibitors and/ or prednisone, 60 mg/d, have been reported to be of benefit in some cases. The incidence of this disease in patients receiving adequate ART has not been well defined; however, the impression is that it has decreased in frequency and severity. It is the leading cause of end-stage renal disease in patients with HIV infection. CHAPTER 208 Human Immunodeficiency Virus Disease: AIDS and Related Disorders
Among the drugs commonly associated with renal damage in patients with HIV disease are pentamidine, amphotericin, adefovir, cidofovir, tenofovir, and foscarnet. Switching from tenofovir disoproxil fumarate (TDF) to tenofovir alafenamide (TAF) may lead to a decrease in renal injury from TDF. TMP-SMX may compete for tubular secre­ tion with creatinine and cause an increase in the serum creatinine level. The pharmacokinetic booster cobicistat, a component of several fixeddrug ART formulations, inhibits renal tubular secretion of creatinine leading to increased serum creatinine levels without a true decline in glomerular filtration rate. Sulfadiazine may crystallize in the kidney and result in an easily reversible form of renal shutdown, while indi­ navir or atazanavir may form renal calculi. Adequate hydration is the mainstay of treatment and prevention for these latter two conditions. Genitourinary tract infections are seen with a high frequency in patients with HIV infection; they present with skin lesions, dysuria, hematuria, and/or pyuria and are managed in the same fashion as in patients without HIV infection. Infections with HSV are covered below (“Dermatologic Diseases”). Infections with T. pallidum, the etio­ logic agent of syphilis, play an important role in the HIV epidemic. In HIV-negative individuals, genital syphilitic ulcers as well as the ulcers

PART 5 Infectious Diseases A B C FIGURE 208-39  Characteristics of lipodystrophy. A. Truncal obesity and buffalo hump. B. Facial wasting. C. Accumulation of intraabdominal fat on CT scan. of chancroid are major predisposing factors for heterosexual transmission of HIV infection. While most people with HIV and syphilis have a typical presentation of the latter, a variety of formerly rare clinical problems may be encountered in the setting of dual infection. Among them are lues maligna, an ulcerating lesion of the skin due to a necrotizing vasculitis and associated with unexplained fever; nephrotic syndrome; and neurosyphilis. The most common presentation of syphilis in the patient with HIV is that of condylomata lata, a form of second­ ary syphilis. Neurosyphilis may be asymptomatic or may present as acute meningitis, neuroretinitis, deafness, or stroke. The rate of neurosyphilis may be as high as 1% in patients with HIV infection, and one should consider a lumbar puncture to look for neurosyphilis in all patients with HIV infec­ tion and secondary syphilis. As a consequence of the immunologic abnormalities seen in the setting of HIV infection, diagnosis of syphilis through standard serologic testing may be challenging. On the one hand, a significant number of patients have false-positive Venereal Disease Research Laboratory (VDRL) tests due to polyclonal B cell activation. On the other hand, the development of a new positive VDRL may be delayed in patients with new infec­ tions, and the anti–fluorescent treponemal antibody (anti-FTA) test may be negative in the setting of immunodeficiency. Thus, dark-field examination of appropriate specimens should be performed in any patient in whom syphilis is suspected, even if the patient has a negative VDRL. Similarly, any patient with a positive serum VDRL test, neurologic findings, and an abnormal spinal fluid examination should be considered to have neurosyphilis and treated accordingly, regardless of the CSF VDRL result. In any setting, patients treated for syphilis need to be carefully monitored to ensure adequate therapy. Approximately one-third of patients with HIV infection will experience a Jarisch-Herxheimer reaction upon initiation of therapy for syphilis. Vulvovaginal candidiasis is a common problem in women with HIV infection. Symptoms include pruritus, discomfort, dyspareunia, and dysuria. Vulvar infection may present as a morbilliform rash that may extend to the thighs. Vaginal infection is usually associated with a white discharge, and plaques may be seen along an erythematous vaginal wall. Diagnosis is made by microscopic examination of the discharge for pseudohyphal ele­ ments in a 10% potassium hydroxide solution. Mild disease can be treated with topical therapy. More serious disease can be treated with fluconazole. Other causes of vaginitis include Trichomonas and mixed bacteria. Diseases of the Endocrine System and Metabolic Disorders  A variety of endocrine and metabolic disorders are seen in the context of HIV infection. These may be a direct consequence of HIV infec­ tion, secondary to opportunistic infections or neoplasms, or related to medication side effects. Between 33% and 75% of patients with HIV infection receiving thymidine analogues or protease inhibitors as a component of ART develop a syndrome often referred to as lipodystro­ phy, consisting of elevations in plasma triglycerides, total cholesterol, and apolipoprotein B, as well as hyperinsulinemia and hyperglycemia. Many of the patients have been noted to have a characteristic set of body habitus changes associated with fat redistribution, consisting of truncal obesity coupled with peripheral wasting (Fig. 208-39). Trun­ cal obesity is apparent as an increase in abdominal girth related to increases in mesenteric fat, a dorsocervical fat pad (“buffalo hump”) reminiscent of patients with Cushing’s syndrome, and enlargement of the breasts. The peripheral wasting, or lipoatrophy, is particularly noticeable in the face and buttocks and by the prominence of the veins in the legs. These changes may develop at any time ranging from ~6 weeks to several years following the initiation of ART. Approximately 20% of the patients with HIV-associated lipodystrophy meet the criteria for the metabolic syndrome as defined by the International Diabetes Federation or the U.S. National Cholesterol Education Program (NCEP) Adult Treatment Panel III. The lipodystrophy syndrome has been reported in association with regimens containing a variety of different drugs, and while initially reported in the setting of protease inhibitor therapy, it appears that similar changes can also be induced by protease-sparing regimens. It has been suggested that the lipoatrophy changes are par­ ticularly severe in patients receiving the thymidine analogues stavudine and zidovudine. Current treatment guidelines avoid these drugs and recommend drugs with fewer of these side effects. NCEP guidelines should be followed in the management of these lipid abnormalities (Chap. 419), with a further recommendation that moderate-intensity statin therapy be given to all individuals with HIV infection between the ages of 40 and 75 years unless their 10-year risk of cardiovascular disease is <5%. In all patients, consideration should be given to chang­ ing the components of ART with avoidance of thymidine analogues (azidothymidine and stavudine) and offending protease inhibitors. Due to concerns regarding drug interactions, the most utilized lipidlowering agents in this setting are gemfibrozil and atorvastatin. Lactic acidosis is associated with certain ART medications. This is most often seen with the nucleoside analogue reverse transcriptase inhibitors and can be fatal.

Patients with advanced HIV disease may develop hyponatremia due to the syndrome of inappropriate antidiuretic hormone (vasopressin) secre­ tion (SIADH) because of increased free-water intake and decreased free-water excretion. SIADH is usually seen in conjunction with pulmonary or CNS disease. Low serum sodium may also be due to adrenal insufficiency; a concomitant high serum potassium should alert one to this possibility. Hyperkalemia may be secondary to adrenal insufficiency, HIV nephropathy; or medications, particularly trim­ ethoprim and pentamidine. Hypokalemia may be seen in the setting of tenofovir or amphotericin therapy. Adrenal gland disease may be due to mycobacterial infections, CMV disease, cryptococcal disease, histoplasmosis, or ketoconazole toxicity. Iatrogenic Cushing’s syn­ drome with suppression of the hypothalamic-pituitary-adrenal axis may be seen with the use of local glucocorticoids (injected or inhaled) in patients receiving ritonavir or cobicistat. This is due to inhibition of the hepatic enzyme CYP3A4 by ritonavir leading to prolongation of the glucocorticoid half-life. Thyroid function may be altered in 10–15% of patients with HIV infection. Both hypo- and hyperthyroidism may be seen. The pre­ dominant abnormality is subclinical hypothyroidism. In the setting of ART, up to 10% of patients have been noted to have elevated thyroidstimulating hormone levels, suggesting that this may be a manifesta­ tion of immune reconstitution. Immune-reconstitution Graves’ disease may occur as a late (9–48 months) complication of ART. In advanced HIV disease, infection of the thyroid gland may occur with opportunis­ tic pathogens, including P. jirovecii, CMV, mycobacteria, Toxoplasma gondii, and Cryptococcus neoformans. These infections are generally associated with a nontender, diffuse enlargement of the thyroid gland. Thyroid function is usually normal. Diagnosis is made by fine-needle aspirate or open biopsy. Depending on the severity of disease, HIV infection is associated with hypogonadism in 20–50% of men and is lowest in the setting of ART. While this is generally a complication of underlying illness, tes­ ticular dysfunction may also be a side effect of ganciclovir therapy. In some surveys, up to two-thirds of patients report decreased libido and one-third complain of erectile dysfunction. Androgen-replacement therapy should be considered in patients with symptomatic hypogo­ nadism. HIV infection does not seem to have a significant effect on the menstrual cycle outside the setting of advanced disease. Immunologic and Rheumatologic Diseases  Immunologic and rheumatologic disorders are common in patients with HIV infection and range from excessive immediate-type hypersensitivity reactions (Chap. 363) to an increase in the incidence of reactive arthritis (Chap. 386) to conditions characterized by a diffuse infiltrative lymphocytosis. The occurrence of these phenomena is an appar­ ent paradox in the setting of the profound immunodeficiency and immunosuppression that characterizes HIV infection and reflects the complex nature of the immune system and its regulatory mechanisms. Drug allergies are the most significant allergic reactions occurring in patients with HIV and appear to become more common as the disease progresses. They occur in up to 65% of patients who receive therapy with TMP-SMX for PCP. In general, these drug reactions are characterized by erythematous, morbilliform eruptions that are pru­ ritic, tend to coalesce, and are often associated with fever. Nonetheless, ~33% of patients can be maintained on the offending therapy, and thus these reactions are not an immediate indication to stop the drug. Ana­ phylaxis is extremely rare in patients with HIV infection, and patients who have a cutaneous reaction during a single course of therapy can still be considered candidates for future treatment or prophylaxis with the same agent. The one exception to this is the nucleoside analogue abacavir, where fatal hypersensitivity reactions have been reported with rechallenge. This hypersensitivity is strongly associated with the HLA-B5701 haplotype, and a hypersensitivity reaction to abacavir is an absolute contraindication to future therapy. For other agents, including TMP-SMX, desensitization regimens are moderately suc­ cessful. While the mechanisms underlying these allergic-type reactions remain unknown, patients with HIV infection have been noted to have elevated IgE levels that increase as the CD4+ T cell count declines. The

numerous examples of patients with multiple drug reactions suggest that a common pathway is involved.

HIV infection shares many similarities with a variety of autoim­ mune diseases, including a substantial polyclonal B cell activation that is associated with a high incidence of antiphospholipid antibodies, such as anticardiolipin antibodies, VDRL antibodies, and lupus-like antico­ agulants. In addition, people with HIV have an increased incidence of antinuclear antibodies. Despite these serologic findings, there is no evidence that people with HIV have an increase in two of the more common autoimmune diseases, i.e., systemic lupus erythematosus and rheumatoid arthritis. In fact, it has been observed that these diseases may be somewhat ameliorated by the concomitant presence of HIV infection, suggesting that an intact CD4+ T cell limb of the immune response plays an integral role in the pathogenesis of these conditions. Similarly, there are anecdotal reports of patients with common vari­ able immunodeficiency (Chap. 362), characterized by hypogamma­ globulinemia, who have had a normalization of Ig levels following the development of HIV infection, suggesting a possible role for overactive CD4+ T cell immunity in certain forms of that syndrome. The one autoimmune disease that may occur with an increased frequency in patients with HIV infection is a variant of primary Sjögren’s syndrome (Chap. 373) in which patients with HIV infection develop a syndrome consisting of parotid gland enlargement, dry eyes, and dry mouth. This condition is associated with lymphocytic infiltrates of the salivary gland and lung. One also can see peripheral neuropathy, polymyositis, renal tubular acidosis, and hepatitis. In contrast to Sjögren’s syndrome, in which the lymphocytic infiltrates are composed predominantly of CD4+ T cells, in patients with HIV infection the infiltrates are com­ posed predominantly of CD8+ T cells. In addition, while patients with Sjögren’s syndrome are mainly women who have autoantibodies to Ro and La and who frequently have HLA-DR3 or B8 MHC haplotypes, people with HIV who have this syndrome are usually African-American men who do not have anti-Ro or anti-La and who most often have the HLA-DR5 haplotype. This syndrome appears to be less common with the increased use of effective ART. The term diffuse infiltrative lymphocytosis syndrome (DILS) is used to describe this entity and to distinguish it from Sjögren’s syndrome. CHAPTER 208 Human Immunodeficiency Virus Disease: AIDS and Related Disorders
Approximately one-third of people with HIV experience arthralgias; furthermore, 5–10% are diagnosed as having some form of reactive arthritis, such as Reiter’s syndrome or psoriatic arthritis as well as undifferentiated spondyloarthropathy (Chap. 374). These syndromes occur with increasing frequency as the competency of the immune system declines. This association may be related to an increase in the number of infections with organisms that may trigger a reactive arthri­ tis with progressive immunodeficiency or to a loss of important regula­ tory T cells. Reactive arthritides in people with HIV generally respond well to standard treatment; however, therapy with methotrexate has been associated with an increase in the incidence of opportunistic infections and should be used with caution and only in severe cases. People with HIV also experience a variety of joint problems without obvious cause that are referred to generically as HIV- or AIDS-associated arthropathy. This syndrome is characterized by subacute oligoarticular arthritis developing over a period of 1–6 weeks and lasting 6 weeks to 6 months. It generally involves the large joints, predominantly the knees and ankles, and is nonerosive with only a mild inflammatory response. X-rays are nonrevealing. Nonsteroidal anti-inflammatory drugs are only marginally helpful; however, relief has been noted with the use of intraarticular glucocorticoids. A second form of arthritis also thought to be secondary to HIV infection is called painful articular syndrome. This condition, reported as occurring in up to 10% of AIDS patients, presents as an acute, severe, sharp pain in the affected joint. It affects primarily the knees, elbows, and shoulders; lasts 2–24 h; and may be severe enough to require narcotic analgesics. The cause of this arthropathy is unclear; however, it is thought to result from a direct effect of HIV on the joint. This condition is reminiscent of the fact that other lentiviruses, in particular the caprine arthritis-encephalitis virus, are capable of directly causing arthritis. A variety of other immunologic or rheumatologic diseases have been reported in people with HIV, either de novo or in association

with opportunistic infections or drugs. Using the criteria of widespread musculoskeletal pain of at least 3 months’ duration and the presence of at least 11 of 18 possible tender points by digital palpation, 11% of a cohort of people with HIV containing 55% IDUs were diagnosed as having fibromyalgia (Chap. 385). While the incidence of frank arthri­ tis was less in this population than in other studied populations that consisted predominantly of men who have sex with men, these data support the concept that there are musculoskeletal problems that occur as a direct result of HIV infection. CNS angiitis and polymyositis also have been reported in people with HIV. Septic arthritis is surprisingly rare, especially given the increased incidence of staphylococcal bactere­ mias seen in this population. When septic arthritis has been reported, it has usually been due to Staphylococcus aureus, systemic fungal infec­ tion with C. neoformans, Sporothrix schenckii, or H. capsulatum or to systemic mycobacterial infection with M. tuberculosis, M. haemophilum, M. avium, or M. kansasii.

Patients with HIV infection treated with ART have been found to have an increased incidence of osteonecrosis or avascular necrosis of the hip and shoulders. In a study of asymptomatic patients, 4.4% were found to have evidence of osteonecrosis on MRI. While precise causeand-effect relationships have been difficult to establish, this complica­ tion has been associated with the use of lipid-lowering agents, systemic glucocorticoids, and testosterone; bodybuilding exercise; alcohol con­ sumption; and the presence of anticardiolipin antibodies. Osteoporosis has been reported in 7% of women with HIV infection, with 41% of women demonstrating some degree of osteopenia. Several studies have documented decreases in bone mineral density of 2–6% in the first 2 years following the initiation of ART. This may be particularly apparent with tenofovir-containing regimens. PART 5 Infectious Diseases Immune Reconstitution Inflammatory Syndrome (IRIS) 

Following the initiation of effective ART, a paradoxical worsening of pre-existing, untreated, or partially treated opportunistic infections may be noted. One may also see exacerbations of pre-existing autoim­ mune conditions or the development of new autoimmune conditions following the initiation of antiretrovirals (Table 208-12). IRIS related to a known pre-existing infection or neoplasm is referred to as para­ doxical IRIS, while IRIS associated with a previously undiagnosed con­ dition is referred to as unmasking IRIS. The term immune reconstitution disease (IRD) is sometimes used to distinguish IRIS manifestations related to opportunistic diseases from IRIS manifestations related to autoimmune diseases. IRD is particularly common in patients with underlying untreated mycobacterial or fungal infections. Some form of IRIS is seen in 10–30% of patients following the initiation of ART, depending on the clinical setting, and is most common in patients starting therapy with CD4+ T cell counts <50 cells/μL who have a precipitous drop in HIV RNA levels following the initiation of ART. Signs and symptoms may appear anywhere from 2 weeks to 2 years after the initiation of ART and can include localized lymphadenitis, prolonged fever, pulmonary infiltrates, hepatitis, increased intracra­ nial pressure, uveitis, sarcoidosis, and Graves’ disease. The clinical course can be protracted, and severe cases can be fatal. The underlying mechanism appears to be related to a phenomenon similar to type IV hypersensitivity reactions and reflects the immediate improvements in immune function that occur as levels of HIV RNA drop and the TABLE 208-12  Characteristics of Immune Reconstitution Inflammatory Syndrome (IRIS) Paradoxical worsening of an existing clinical condition or abrupt appearance of a new clinical finding (unmasking) is seen following the initiation of antiretroviral therapy Occurs weeks to months following the initiation of antiretroviral therapy Is most common in patients starting therapy with a CD4+ T cell count <50/μL who experience a precipitous drop in viral load Is frequently seen in the setting of tuberculosis, particularly when ART is started soon after initiation of anti-TB therapy Can be fatal Abbreviations: ART, antiretroviral therapy; TB, tuberculosis.

TABLE 208-13  Causes of Bone Marrow Suppression in Patients with HIV Infection DISEASES MEDICATIONS HIV infection Mycobacterial infections Fungal infections B19 parvovirus infection Lymphoma Zidovudine Dapsone Trimethoprim-sulfamethoxazole Pyrimethamine 5-flucytosine Ganciclovir Interferon α Trimetrexate Foscarnet immunosuppressive effects of HIV infection are controlled. In severe cases, the use of immunosuppressive drugs such as glucocorticoids may be required to blunt the inflammatory component of these reac­ tions while specific antimicrobial therapy takes effect. Diseases of the Hematopoietic System  Disorders of the hema­ topoietic system including lymphadenopathy, anemia, leukopenia, and/or thrombocytopenia are common throughout the course of HIV infection and may be the direct result of HIV, manifestations of second­ ary infections and neoplasms, or side effects of therapy (Table 208-13). Direct histologic examination and culture of lymph node or bone marrow tissue are often diagnostic. A significant percentage of bone marrow aspirates from patients with HIV infection have been reported to contain lymphoid aggregates, the precise significance of which is unknown. Initiation of ART will lead to reversal of most hematologic complications that are the direct result of HIV infection. Some patients, otherwise asymptomatic, may develop persistent generalized lymphadenopathy as an early clinical manifestation of HIV infection. This condition is defined as the presence of enlarged lymph nodes (>1 cm) in two or more extrainguinal sites for >3 months without an obvious cause. The lymphadenopathy in this setting is due to marked follicular hyperplasia in the node in response to HIV infec­ tion. The nodes are generally discrete and freely movable. This feature of HIV disease may be seen at any point in the spectrum of immune dysfunction and is not associated with an increased likelihood of devel­ oping AIDS. Paradoxically, a loss in lymphadenopathy or a decrease in lymph node size outside the setting of ART may be a prognostic marker of disease progression. In patients with CD4+ T cell counts >200/μL, the differential diagnosis of lymphadenopathy includes TB, KS, Castle­ man’s disease, and lymphoma. In patients with more advanced disease, lymphadenopathy may also be due to atypical mycobacterial infection, toxoplasmosis, systemic fungal infection, or bacillary angiomatosis. While indicated in patients with CD4+ T cell counts <200/μL, lymph node biopsy is not indicated in patients with early-stage disease unless there are signs and symptoms of systemic illness, such as fever and weight loss, or unless the nodes begin to enlarge, become fixed, or coalesce. Monoclonal gammopathy of unknown significance (MGUS) (Chap. 116), defined as the presence of a serum monoclonal IgG, IgA, or IgM in the absence of a clear cause, has been reported in 3% of patients with HIV infection. The overall clinical significance of this finding in patients with HIV infection is unclear, although it has been associated with other viral infections, non-Hodgkin’s lymphoma, and plasma cell malignancy. Anemia is the most common hematologic abnormality in patients with HIV and, in the absence of a specific treatable cause, is indepen­ dently associated with a poor prognosis. While generally mild, anemia can be quite severe and require chronic blood transfusions. Among the specific reversible causes of anemia in the setting of HIV infection are drug toxicity, systemic fungal and mycobacterial infections, nutritional deficiencies, and parvovirus B19 infections. The antiretroviral zid­ ovudine may block erythroid maturation prior to its effects on other marrow elements. A characteristic feature of zidovudine therapy is an elevated mean corpuscular volume (MCV). Another drug used in

patients with HIV infection that has a selective effect on the erythroid series is dapsone. This drug can cause a serious hemolytic anemia in patients who are deficient in glucose-6-phosphate dehydrogenase and can create a functional anemia in others through induction of methemoglobinemia. Folate levels are usually normal in people with HIV; however, vitamin B12 levels may be depressed as a consequence of achlorhydria or malabsorption. True autoimmune hemolytic anemia is rare, although ~20% of patients with HIV infection may have a positive direct antiglobulin test as a consequence of polyclonal B cell activation. Infection with parvovirus B19 also may cause anemia. It is important to recognize this possibility given the fact that it responds well to treat­ ment with IVIg. Erythropoietin levels in patients with HIV infection and anemia are generally lower than expected given the degree of anemia. Treatment with erythropoietin may result in an increase in hemoglobin levels. An exception to this is a subset of patients with zidovudine-associated anemia in whom erythropoietin levels may be quite high. During the course of HIV infection, neutropenia may be seen in approximately half of patients. In most instances it is mild; however, it can be severe and can put patients at risk of spontaneous bacterial infec­ tions. This is most frequently seen in patients with severely advanced HIV disease and in patients receiving potentially myelosuppressive therapies. In the setting of neutropenia, diseases not commonly seen in patients with HIV, such as aspergillosis or mucormycosis, may occur. Both granulocyte colony-stimulating factor (G-CSF) and GM-CSF increase neutrophil counts in patients with HIV infection regardless of the cause of the neutropenia. Earlier concerns about the potential of these agents to also increase levels of HIV were not confirmed in controlled clinical trials. Thrombocytopenia may be an early consequence of HIV infection. Approximately 3% of patients with untreated HIV infection and CD4+ T cell counts ≥400/μL have platelet counts <150,000/μL. For untreated patients with CD4+ T cell counts <400/μL, this incidence increases to 10%. Thrombocytopenia is more common in patients with hepatitis C co-infection, cirrhosis, and/or ongoing high-level HIV replication. Thrombocytopenia is rarely a serious clinical problem in patients with HIV infection and generally responds well to successful ART. Clini­ cally, it resembles the thrombocytopenia seen in patients with idio­ pathic thrombocytopenic purpura (Chap. 120). Immune complexes containing anti-gp120 antibodies and anti-anti-gp120 antibodies have been noted in the circulation and on the surface of platelets in patients with HIV infection. Patients with HIV infection have also been noted to have a platelet-specific antibody directed toward a 25-kDa compo­ nent of the surface of the platelet. Other data suggest that the thrombo­ cytopenia in patients with HIV infection may be due to a direct effect of HIV on megakaryocytes. Whatever the cause, it is very clear that the most effective medical approach to this problem has been the use of ART. For patients with platelet counts <20,000/μL, a more aggressive approach combining IVIg or anti-Rh Ig for an immediate response and ART for a more lasting response is appropriate. Rituximab has been used with some success in otherwise refractory cases. Splenectomy is a rarely needed option and is reserved for patients refractory to medical management. Because of the risk of serious infection with encapsulated organisms, all patients with HIV infection about to undergo splenec­ tomy should be immunized with vaccines to prevent disease from

S. pneumoniae, N. meningitidis, and H. influenzae type b. It should be noted that, in addition to causing an increase in the platelet count, removal of the spleen will result in an increase in the peripheral blood lymphocyte count, making CD4+ T cell counts unreliable markers of immunocompetence. In this setting, the clinician should rely on the CD4+ T cell percentage for making diagnostic decisions with respect to the likelihood of opportunistic infections. A CD4+ T cell percentage of 15 is approximately equivalent to a CD4+ T cell count of 200/μL.

In patients with early HIV infection, thrombocytopenia has also been reported as a consequence of classic thrombotic thrombocytopenic purpura (Chap. 120). This clinical syndrome, consisting of fever, thrombocytopenia, hemolytic anemia, and neurologic and renal dys­ function, is a rare complication of early HIV infection. As in other set­ tings, the appropriate management is the use of salicylates and plasma

exchange. Other causes of thrombocytopenia include lymphoma, mycobacterial infections, and fungal infections.

The incidence of venous thromboembolic disease such as deepvein thrombosis or pulmonary embolus is approximately 1% per year in patients with HIV infection. This is approximately 10 times higher than that seen in an age-matched population. Factors associ­ ated with an increased risk of clinical thrombosis include age >45, history of an opportunistic infection, lower CD4 count, and estrogen use. Abnormalities of the coagulation cascade, including decreased protein S activity, increases in factor VIII, anticardiolipin antibodies, PAR-1 expression on T cells, or lupus-like anticoagulant, have been reported in more than 50% of patients with HIV infection. The clini­ cal significance of this increased propensity toward thromboembolic disease is likely reflected in the observation that elevations in d-dimer are strongly associated with all-cause mortality in patients with HIV infection (Table 208-9). Dermatologic Diseases  Dermatologic problems occur in >90% of patients with HIV infection. From the macular, roseola-like rash seen with the acute seroconversion syndrome to extensive end-stage KS, cutaneous manifestations of HIV disease can be seen throughout the course of HIV infection. Among the more common nonneoplas­ tic problems are seborrheic dermatitis, folliculitis, and opportunistic infections. Extrapulmonary pneumocystosis may cause a necrotizing vasculitis. Neoplastic conditions are covered in a separate section below. Seborrheic dermatitis occurs in 3% of the general population and in up to 50% of patients with HIV infection. Seborrheic dermatitis increases in prevalence and severity as the CD4+ T cell count declines. In patients with HIV, seborrheic dermatitis may be aggravated by concomitant infection with Pityrosporum, a yeastlike fungus; use of topical antifungal agents has been recommended in cases refractory to standard topical treatment. CHAPTER 208 Folliculitis is among the most prevalent dermatologic disorders in patients with HIV infection and is seen in ~20% of patients. It is more common in patients with CD4+ T cell counts <200 cells/μL. Pruritic papular eruption is one of the most common pruritic rashes in patients with HIV infection. It appears as multiple papules on the face, trunk, and extensor surfaces and may improve with ART. Eosinophilic pustular folliculitis is a rare form of folliculitis that is seen with increased fre­ quency in patients with HIV infection. It presents as multiple, urticarial perifollicular papules that may coalesce into plaque-like lesions. Skin biopsy reveals an eosinophilic infiltrate of the hair follicle, which in certain cases has been associated with the presence of a mite. Patients typically have an elevated serum IgE level and may respond to treat­ ment with topical anthelmintics. Pruritus is a common symptom in patients with HIV infection and can lead to prurigo nodularis. Patients with HIV infection have also been reported to develop a severe form of Norwegian scabies with hyperkeratotic psoriasiform lesions. Human Immunodeficiency Virus Disease: AIDS and Related Disorders
Both psoriasis and ichthyosis, although they are not reported to be increased in frequency, may be particularly severe when they occur in patients with HIV infection. Pre-existing psoriasis may become guttate in appearance and more refractory to treatment in the setting of HIV infection. Reactivation herpes zoster (shingles) is seen in 10–20% of patients with HIV infection. This reactivation syndrome of varicella-zoster virus indicates a modest decline in immune function and may be the first indication of clinical immunodeficiency. In one series, patients who developed shingles did so an average of 5 years after HIV infec­ tion. In a cohort of patients with HIV infection and localized zoster, the subsequent rate of the development of AIDS was 1% per month. In that study, AIDS was more likely to develop if the outbreak of zoster was associated with severe pain, extensive skin involvement, or involve­ ment of cranial or cervical dermatomes. The clinical manifestations of reactivation zoster in patients with HIV, although indicative of immu­ nologic compromise, are not as severe as those seen in other immu­ nodeficient conditions. Thus, while lesions may extend over several dermatomes, involve the spinal cord, and/or be associated with frank cutaneous dissemination, visceral involvement has not been reported. In contrast to patients without a known underlying immunodeficiency

state, patients with HIV infection tend to have recurrences of shingles with a relapse rate of ~20%. Valacyclovir, famciclovir, or acyclovir is the treatment of choice. IV acyclovir can be used in severe cases, and foscarnet may be of value in patients with acyclovir-resistant virus.

Infection with herpes simplex virus in people with HIV is associated with recurrent orolabial, genital, and perianal lesions as part of recur­ rent reactivation syndromes (Chap. 197). As HIV disease progresses and the CD4+ T cell count declines, these infections become more frequent and severe. Lesions often appear as beefy red, are exquisitely painful, and tend to occur high in the gluteal cleft (Fig. 208-38). Peri­ rectal HSV may be associated with proctitis and anal fissures. HSV should be high in the differential diagnosis of any patient with HIV who has a poorly healing, painful perirectal lesion. In addition to recurrent mucosal ulcers, recurrent HSV infection in the form of her­ petic whitlow can be a problem in patients with HIV infection, present­ ing with painful vesicles or extensive cutaneous erosion. Valacyclovir, famciclovir, or acyclovir is the treatment of choice in these settings. It is noteworthy that even subclinical reactivation of herpes simplex may be associated with increases in plasma HIV RNA levels. Diffuse skin eruptions due to Molluscum contagiosum may be seen in patients with advanced HIV infection. These flesh-colored, umbilicated lesions resemble those of Talaromyces (formerly Penicil­ lium) marnefei or Cryptococcosis. They tend to regress with effective ART and can also be treated with local therapy. Similarly, condyloma acuminatum lesions may be more severe and more widely distributed in patients with low CD4+ T cell counts. Imiquimod cream may be helpful in some cases. Atypical mycobacterial infections may present as erythematous cutaneous nodules, as may fungal infections, Bartonella, Acanthamoeba, and KS. Cutaneous infections with Aspergillus have been noted at the site of IV catheter placement. PART 5 Infectious Diseases The skin of patients with HIV infection is often a target organ for drug reactions (Chap. 63). Although most skin reactions are mild and not necessarily an indication to discontinue therapy, some patients may have particularly severe cutaneous reactions to drugs, including eryth­ roderma, Stevens-Johnson syndrome, and toxic epidermal necrolysis. This is particularly true for sulfa drugs, nonnucleoside reverse tran­ scriptase inhibitors, abacavir, amprenavir, darunavir, fosamprenavir, and tipranavir. Similarly, patients with HIV infection are often quite photosensitive and burn easily following exposure to sunlight or as a side effect of radiation therapy (Chap. 64). HIV infection and its treatment may be accompanied by cosmetic changes of the skin that are not of great clinical importance but may be troubling to patients. Yellowing of the nails and straightening of the hair, particularly in African-American patients, have been reported as a consequence of HIV infection. Zidovudine therapy has been associ­ ated with elongation of the eyelashes and the development of a bluish discoloration to the nails, again more common in African-American patients. Therapy with clofazimine may cause a yellow-orange discol­ oration of the skin and urine. Neurologic Diseases  Clinical disease of the nervous system accounts for a significant degree of morbidity in a high percentage of patients with HIV infection (Table 208-14). The neurologic problems that occur in people with HIV may be either primary to the patho­ genic processes of HIV infection or secondary to opportunistic infec­ tions or neoplasms. Among the more frequent opportunistic diseases that involve the CNS are toxoplasmosis, cryptococcosis, progressive multifocal leukoencephalopathy, and primary CNS lymphoma. Other less common problems include mycobacterial infections; syphilis; and infection with CMV, herpes zoster, HTLV-1, Trypanosoma cruzi, or Acanthamoeba. Overall, secondary diseases of the CNS have been reported to occur in approximately one-third of patients with AIDS. These data antedate the widespread use of ART, and this frequency is considerably lower in patients with suppressed viral replication. Primary processes related to HIV infection of the nervous system are reminiscent of those seen with other lentiviruses, such as the maedivisna virus of sheep. Neurologic problems directly attributable to HIV occur through­ out the course of infection and may be inflammatory, demyelinating,

TABLE 208-14  Neurologic Diseases in Patients with HIV Infection HIV-1 infection   Aseptic meningitis   HIV-associated neurocognitive disorders Opportunistic infections   Toxoplasmosis   Cryptococcosis   Progressive multifocal (HAND), including HIV encephalopathy/AIDS dementia complex   Myelopathy     Vacuolar myelopathy     Pure sensory ataxia     Paresthesia/dysesthesia   Peripheral neuropathy     Acute inflammatory demyelinating leukoencephalopathy   Cytomegalovirus   Syphilis   Mycobacterium tuberculosis   HTLV-1 infection   Amebiasis Neoplasms   Primary CNS lymphoma   Kaposi’s sarcoma polyneuropathy (Guillain-Barré syndrome)     Chronic inflammatory demyelinating polyneuropathy (CIDP)     Mononeuritis multiplex     Distal symmetric polyneuropathy   Myopathy or degenerative in nature. The term HIV-associated neurocognitive disorders (HAND) is used to describe a spectrum of disorders that range from asymptomatic neurocognitive impairment (ANI) to minor neurocognitive disorder (MND) to clinically severe dementia. The most severe form, HIV-associated dementia (HAD), also referred to as the AIDS dementia complex, or HIV encephalopathy, is considered an AIDS-defining illness. Many HIV-infected patients have some neurologic problem during the course of their disease. Even in the setting of suppressive ART, approximately 50% of people with HIV can be shown to have mild to moderate neurocognitive impairment using sensitive neuropsychiatric testing. As noted in the section on pathogenesis, damage to the CNS may be a direct result of viral infec­ tion of the CNS macrophages or glial cells or may be secondary to the release of neurotoxins and potentially toxic cytokines such as IL-1β, TNF-α, IL-6, and TGF-β. It has been reported that people with HIV who have the E4 allele for apoE are at increased risk for AIDS encephalopathy and peripheral neuropathy. Virtually all patients with HIV infection have some degree of nervous system involvement with the virus. This is evidenced by the fact that CSF findings are abnormal in ~90% of untreated patients, even during the asymptomatic phase of HIV infection. CSF abnormalities include pleocytosis (50–65% of patients), detection of viral RNA (~75%), elevated CSF protein (35%), and evidence of intrathecal synthesis of anti-HIV antibodies (90%). It is important to point out that evidence of infection of the CNS with HIV does not imply impairment of cognitive function. The neurologic function of a person with HIV should be considered normal unless clinical signs and symptoms suggest otherwise. Aseptic meningitis may occur at any time in the course of HIV infection; however, it is rare following the development of AIDS. This suggests that clinical aseptic meningitis in the context of HIV infection is an immune-mediated disease. In the setting of acute primary infec­ tion, patients may experience a syndrome of headache, photophobia, and meningismus. Rarely, an acute encephalopathy due to encephalitis may occur. Cranial nerve involvement may be seen, predominantly cranial nerve VII but occasionally V and/or VIII. CSF findings include a lymphocytic pleocytosis, elevated protein level, and normal glucose level. This syndrome, which cannot be clinically differentiated from other viral meningitides (Chap. 143), usually resolves spontaneously within 2–4 weeks; however, in some patients, signs and symptoms may become chronic. Fungal meningitis is the leading infectious cause of meningitis in patients with AIDS (Chaps. 143 and 144). While the vast majority of these are due to C. neoformans, up to 12% may be due to C. gattii. Cryptococcal meningitis is the initial AIDS-defining illness in ~2% of patients and generally occurs in patients with CD4+ T cell counts <100/μL. Cryptococcal meningitis is particularly common in untreated patients with AIDS in Africa, occurring in ~5% of patients. Most patients present with a picture of subacute meningoencephalitis with

fever, nausea, vomiting, altered mental status, headache, and meningeal signs. The incidence of seizures and focal neurologic deficits is low. The CSF profile may be normal or may show only modest elevations in WBC or protein levels and decreases in glucose. The opening pres­ sure in the CSF is usually elevated. In addition to meningitis, patients may develop cryptococcomas and cranial nerve involvement. Approxi­ mately one-third of patients also have pulmonary disease. Uncommon manifestations of cryptococcal infection include skin lesions that resemble molluscum contagiosum, lymphadenopathy, palatal and glos­ sal ulcers, arthritis, gastroenteritis, myocarditis, and prostatitis. The prostate gland may serve as a reservoir for smoldering cryptococcal infection. The diagnosis of cryptococcal meningitis is made by iden­ tification of organisms in spinal fluid with India ink examination or by the detection of cryptococcal antigen. Blood cultures for fungus are often positive. A biopsy may be needed to make a diagnosis of CNS cryptococcoma and to distinguish inadequately treated infection from immune reconstitution syndrome. Initial treatment is with IV amphotericin B 0.7 mg/kg daily, or liposomal amphotericin 3–4 mg/kg

daily, with flucytosine 25 mg/kg qid for at least 2 weeks if possible. Decreases in renal function in association with amphotericin can lead to increases in flucytosine levels and subsequent bone marrow suppres­ sion. Therapy continues with amphotericin alone until the CSF culture turns negative followed by fluconazole 800 mg/d PO for 8 weeks, and then fluconazole 200 mg/d until the CD4+ T cell count has increased to >200 cells/μL for 6 months in response to ART. Repeated lumbar puncture may be required to manage increased intracranial pressure. Symptoms may recur with initiation of ART as an immune reconstitu­ tion syndrome (see above). For this reason, it is recommended that patients receive 4–6 weeks of antifungal therapy prior to initiation of ART. Other fungi that may cause meningitis in patients with HIV infection are C. immitis and H. capsulatum. Meningoencephalitis has also been reported due to Acanthamoeba or Naegleria. HIV-associated dementia consists of a constellation of signs and symptoms of CNS disease. While this is generally a late complication of HIV infection that progresses slowly over months, it can be seen in patients with CD4+ T cell counts >350 cells/μL. A major feature of this entity is the development of dementia, defined as a decline in cogni­ tive ability from a previous level. It may present as impaired ability to concentrate, increased forgetfulness, difficulty reading, or increased difficulty performing complex tasks. Initially these symptoms may be indistinguishable from findings of situational depression or fatigue. In contrast to “cortical” dementia (such as Alzheimer’s disease), aphasia, apraxia, and agnosia are uncommon, leading some investigators to classify HIV encephalopathy as a “subcortical dementia” character­ ized by defects in short-term memory and executive function (see below). In addition to dementia, patients with HIV encephalopathy may also have motor and behavioral abnormalities. Among the motor problems are unsteady gait, poor balance, tremor, and difficulty with rapid alternating movements. Increased tone and deep tendon reflexes may be found in patients with spinal cord involvement. Late stages may be complicated by bowel and/or bladder incontinence. Behavioral problems include apathy, irritability, and lack of initiative, with pro­ gression to a vegetative state in some instances. Some patients develop a state of agitation or mild mania. These changes usually occur without significant changes in level of alertness. This contrasts with the find­ ing of somnolence in patients with dementia due to toxic/metabolic encephalopathies. HIV-associated dementia is the initial AIDS-defining illness in ~3% of patients with HIV infection and thus only rarely precedes clinical evidence of immunodeficiency. Clinically significant encephalopathy eventually develops in ~25% of untreated patients with AIDS. As immunologic function declines, the risk and severity of HIV-associated dementia increases. Autopsy series suggest that 80–90% of patients with HIV infection have histologic evidence of CNS involvement. Several classification schemes have been developed for grading HIV encephalopathy; a commonly used clinical staging system is outlined in Table 208-15. The precise cause of HIV-associated dementia remains unclear, although the condition is thought to be a result of a combination of

TABLE 208-15  Clinical Staging of HAND According to Frascati Criteria NEUROCOGNITIVE STATUSa FUNCTIONAL STATUSb STAGE Asymptomatic 1 SD below mean in 2 cognitive domains No impairments in activities of daily living Mild neurocognitive disorder 1 SD below mean in 2 cognitive domains Impairments in activities of daily living HIV-associated dementia 2 SD below mean in 2 cognitive domains Notable impairments in activities of daily living aNeurocognitive testing should include assessment of at least 5 domains, including attention/information processing, language, abstraction/executive function, complex perceptual motor skills, memory (including learning and recall), simple motor skills, or sensory perceptual skills. Appropriate norms must be available to establish the number of domains in which performance is below 1 SD. bFunctional status is typically assessed by self-reporting but might be corroborated by a collateral source. No agreed measures exist for HIV-associated neurocognitive disorder criteria. Note that, for diagnosis of HIV-associated neurocognitive disorder, other causes of dementia must be ruled out and potential confounding effects of substance use or psychiatric illness should be considered. Source: Adapted from A Antinori et al: Neurology 69:1789, 2007. direct effects of HIV on the CNS and associated immune activation. HIV has been found in the brains of patients with HIV encephalopathy by Southern blot, in situ hybridization, PCR, and electron microscopy. Multinucleated giant cells, macrophages, and microglial cells appear to be the main cell types harboring virus in the CNS. Histologically, the major changes are seen in the subcortical areas of the brain and include pallor and gliosis, multinucleated giant cell encephalitis, and vacuolar myelopathy. Less commonly, diffuse or focal spongiform changes occur in the white matter. Areas of the brain involved in motor function, language, and judgment are most severely affected. CHAPTER 208 There are no specific criteria for a diagnosis of HIV-associated dementia, and this syndrome must be differentiated from other dis­ eases that affect the CNS of patients with HIV (Table 208-14). The diagnosis of dementia depends on demonstrating a decline in cogni­ tive function. This can be accomplished objectively with the use of a Mini-Mental State Examination (MMSE) in patients for whom prior scores are available. For this reason, it is advisable for all patients with a diagnosis of HIV infection to have a baseline MMSE. However, changes in MMSE scores may be absent in patients with mild HIV encepha­ lopathy. Imaging studies of the CNS, by either MRI or CT, often demonstrate evidence of cerebral atrophy (Fig. 208-40). MRI may also reveal small areas of increased density on T2-weighted images. Lumbar puncture is an important element of the evaluation of patients with Human Immunodeficiency Virus Disease: AIDS and Related Disorders
FIGURE 208-40  AIDS dementia complex. Postcontrast CT scan through the lateral ventricles of a 47-year-old man with AIDS, altered mental status, and dementia. The lateral and third ventricles and the cerebral sulci are abnormally prominent. Mild white matter hypodensity is seen adjacent to the frontal horns of the lateral ventricles.

PART 5 Infectious Diseases HIV infection and neurologic abnormalities. It is generally most help­ ful in ruling out or making a diagnosis of opportunistic infections. In HIV encephalopathy, patients may have the nonspecific findings of an increase in CSF cells and protein level. While HIV RNA can often be detected in the spinal fluid and HIV can be cultured from the CSF, this finding is not specific for HIV encephalopathy. There appears to be no correlation between the presence of HIV in the CSF and the presence of HIV encephalopathy. Elevated levels of monocyte chemoattractant protein-1 (MCP-1), β2-microglobulin, neopterin, and quinolinic acid (a metabolite of tryptophan reported to cause CNS injury) have been noted in the CSF of patients with HIV encephalopathy. These findings suggest that these factors as well as inflammatory cytokines may be involved in the pathogenesis of this syndrome. Combination antiretroviral therapy is of benefit in patients with HIV-associated dementia. Improvement in neuropsychiatric test scores has been noted for both adult and pediatric patients treated with anti­ retrovirals. The rapid improvement in cognitive function noted with the initiation of ART suggests that at least some component of this problem is quickly reversible, again supporting at least a partial role of soluble mediators in the pathogenesis. It should also be noted that these patients have an increased sensitivity to the side effects of neuroleptic drugs. The use of these drugs for symptomatic treatment is associated with an increased risk of extrapyramidal side effects; therefore, patients with HIV encephalopathy who receive these agents must be monitored carefully. It is felt by many physicians that the decrease in the preva­ lence of severe cases of HAND brought about by ART has resulted in an increase in the prevalence of milder forms of this disorder. Seizures may be a consequence of opportunistic infections, neo­ plasms, or HIV encephalopathy (Table 208-16). The seizure threshold is often lower than normal in patients with advanced HIV infection due in part to the frequent presence of electrolyte abnormalities. Seizures are seen in 15–40% of patients with cerebral toxoplasmosis, 15–35% of patients with primary CNS lymphoma, 8% of patients with cryptococcal meningitis, and 7–50% of patients with HIV encepha­ lopathy. Seizures may also be seen in patients with CNS tuberculosis, aseptic meningitis, and progressive multifocal leukoencephalopathy. Seizures may be the presenting clinical symptom of HIV disease. In one study of 100 patients with HIV infection presenting with a first seizure, cerebral mass lesions were the most common cause, responsible for 32 of the 100 new-onset seizures. Of these 32 cases, 28 were due to toxoplasmosis and 4 to lymphoma. HIV encephalopathy accounted for an additional 24 new-onset seizures. Cryptococcal meningitis was the third most common diagnosis, responsible for 13 of the 100 seizures. In 23 cases, no cause could be found, and it is possible that these cases represent a subcategory of HIV encephalopathy. Of these 23 cases, 16 (70%) had 2 or more seizures, suggesting that anticonvulsant therapy is indicated in all patients with HIV infection and seizures unless a rap­ idly correctable cause is found. Due to a variety of drug–drug interac­ tions between antiseizure medications and antiretrovirals, drug levels need to be monitored carefully. Patients with HIV infection may present with focal neurologic defi­ cits from a variety of causes. The most common causes are toxoplasmo­ sis, progressive multifocal leukoencephalopathy, and CNS lymphoma. Other causes include cryptococcal infections (discussed above; also Chap. 221), stroke, and reactivation of Chagas’ disease. FIGURE 208-41  Central nervous system toxoplasmosis. A coronal postcontrast T1-weighted MRI scan demonstrates a peripheral enhancing lesion in the left frontal lobe, associated with an eccentric nodular area of enhancement (arrow); this so-called eccentric target sign is typical of toxoplasmosis. Toxoplasmosis (Chap. 235) has been one of the most common causes of secondary CNS infections in patients with AIDS, but its inci­ dence is decreasing in the era of ART. It is most common in patients from the Caribbean and from France, where the seroprevalence of T. gondii is around 50%. This figure is closer to 15% in the United States. Toxoplasmosis is generally a late complication of HIV infection and usually occurs in patients with CD4+ T cell counts <200/μL. Cerebral toxoplasmosis is thought to represent a reactivation of latent tissue cysts. It is 10 times more common in patients with antibodies to the organism than in patients who are seronegative. Patients diagnosed with HIV infection should be screened for IgG antibodies to T. gondii during the time of their initial workup. Those who are seronegative should be counseled about ways to minimize the risk of primary infec­ tion including avoiding the consumption of undercooked meat and careful hand washing after contact with soil or changing the cat litter box. The most common clinical presentation of cerebral toxoplasmosis in patients with HIV infection is fever, headache, and focal neurologic deficits. Patients may present with seizure, hemiparesis, or aphasia as a manifestation of these focal deficits or with a picture more influenced by the accompanying cerebral edema and characterized by confusion, dementia, and lethargy, which can progress to coma. The diagnosis is usually suspected on the basis of MRI findings of multiple lesions in multiple locations, although in some cases only a single lesion is seen. Pathologically, these lesions generally exhibit inflammation and central necrosis and, as a result, demonstrate ring enhancement on contrast MRI (Fig. 208-41) or, if MRI is unavailable or contraindicated, on double-dose contrast CT. There is usually evidence of surrounding edema. In addition to toxoplasmosis, the differential diagnosis of single or multiple enhancing mass lesions in the patient with HIV includes primary CNS lymphoma and, less commonly, TB or fungal or bacterial abscesses. The definitive diagnostic procedure is brain biopsy. How­ ever, given the morbidity rate that can accompany this procedure, it is usually reserved for the patient who has failed 2–4 weeks of empiric therapy for toxoplasmosis. If the patient is seronegative for T. gondii, the likelihood that a mass lesion is due to toxoplasmosis is <10%. In that setting, one may choose to be more aggressive and perform a brain biopsy sooner. Standard treatment is sulfadiazine and pyrimethamine with leucovorin as needed for a minimum of 4–6 weeks. Alternative therapeutic regimens include clindamycin in combination with pyri­ methamine; atovaquone plus pyrimethamine; and azithromycin plus pyrimethamine plus rifabutin. Relapses are common, and it is recom­ mended that patients with a history of prior toxoplasmic encephalitis receive maintenance therapy with sulfadiazine, pyrimethamine, and leucovorin as long as their CD4+ T cell counts remain <200 cells/μL. Patients with CD4+ T cell counts <100/μL and IgG antibody to Toxoplasma should receive primary prophylaxis for toxoplasmosis. TABLE 208-16  Causes of Seizures in Patients with HIV Infection DISEASE OVERALL CONTRIBUTION TO FIRST SEIZURE, % FRACTION OF PATIENTS WHO HAVE SEIZURES, % HIV encephalopathy 24–47 7–50 Cerebral toxoplasmosis

15–40 Cryptococcal meningitis

Primary central nervous system lymphoma

15–30 Progressive multifocal leukoencephalopathy

Source: From DM Holtzman et al: Am J Med 87:173, 1989.

Fortunately, the same daily regimen of a single double-strength tab­ let of TMP-SMX used for P. jirovecii prophylaxis provides adequate primary protection against toxoplasmosis. Secondary prophylaxis/ maintenance therapy for toxoplasmosis may be discontinued in the setting of effective ART and increases in CD4+ T cell counts to >200/μL for 6 months. JC virus, a human polyomavirus that is the etiologic agent of pro­ gressive multifocal leukoencephalopathy (PML), is an important oppor­ tunistic pathogen in patients with AIDS (Chap. 142). While ~80% of the general adult population has antibodies to JC virus, indicative of prior infection, <10% of healthy adults show any evidence of ongo­ ing viral replication. PML is the only known clinical manifestation of JC virus infection. It is a late manifestation of AIDS and is seen in ~1–4% of patients with AIDS. The lesions of PML begin as small foci of demyelination in subcortical white matter that eventually coalesce. The cerebral hemispheres, cerebellum, and brainstem may all be involved. Patients typically have a protracted course with multifocal neurologic deficits, with or without changes in mental status. Approximately 20% of patients experience seizures. Ataxia, hemiparesis, visual field defects, aphasia, and sensory defects may occur. Headache, fever, nausea, and vomiting are rarely seen. Their presence should suggest another diagnosis. MRI typically reveals multiple, nonenhancing white matter lesions that may coalesce and have a predilection for the occipital and parietal lobes. The lesions show signal hyperintensity on T2-weighted images and diminished signal on T1-weighted images. The measure­ ment of JC virus DNA levels in CSF has a diagnostic sensitivity of 76% and a specificity of close to 100%. Prior to the availability of ART, most patients with PML died within 3–6 months of the onset of symptoms. Paradoxical worsening of PML has been seen with initiation of ART as an immune reconstitution syndrome. There is no specific treat­ ment for PML; however, a median survival of 2 years and survival of

15 years have been reported in patients with PML treated with ART for their HIV disease. Despite having a significant impact on survival, only ~50% of patients with HIV infection and PML show neurologic improvement with ART. Studies with other antiviral agents such as cidofovir have failed to show clear benefit. Factors influencing a favor­ able prognosis for PML in the setting of HIV infection include a CD4+ T cell count >100/μL at baseline and the ability to maintain an HIV viral load of <500 copies/mL. Baseline HIV-1 viral load does not have independent predictive value of survival. PML is one of the few oppor­ tunistic infections that continues to occur with some frequency despite the widespread use of ART. Reactivation American trypanosomiasis may present as acute menin­ goencephalitis with focal neurologic signs, fever, headache, vomiting, and seizures. Accompanying cardiac disease in the form of arrhythmias or heart failure should increase the index of suspicion. The presence of antibodies to T. cruzi supports the diagnosis. In South America, reactivation of Chagas’ disease is considered to be an AIDS-defining condition and may be the initial AIDS-defining condition. Most cases occur in patients with CD4+ T cell counts <200 cells/μL. Lesions appear radiographically as single or multiple hypodense areas, typically with ring enhancement and edema. They are found predominantly in the subcortical areas, a feature that differentiates them from the deeper lesions of toxoplasmosis. T. cruzi amastigotes, or trypanosomes, can be identified from biopsy specimens or CSF. Other CSF findings include elevated protein and a mild (<100 cells/μL) lymphocytic pleocytosis. Organisms can also be identified by direct examination of the blood. Treatment consists of benzimidazole (2.5 mg/kg bid) or nifurtimox (2 mg/kg qid) for at least 60 days, followed by maintenance therapy for the duration of immunodeficiency with either drug at a dose of 5 mg/kg three times a week. As is the case with cerebral toxoplasmosis, successful therapy with antiretrovirals may allow discontinuation of maintenance therapy for Chagas’ disease. Stroke may occur in patients with HIV infection. In contrast to the other causes of focal neurologic deficits in patients with HIV infec­ tion, the symptoms of a stroke are sudden in onset. Patients with HIV infection have an increased prevalence of many classic risk factors associated with stroke, including smoking and diabetes. It has been reported that HIV infection itself can lead to an increase in carotid

artery stiffness. The relative increase in risk for stroke as a consequence of HIV infection is more pronounced in women and in individuals between the ages of 18 and 29. Among the secondary infectious dis­ eases in patients with HIV infection that may be associated with stroke are vasculitis due to cerebral varicella zoster or neurosyphilis and septic embolism in association with fungal infection. Other elements of the differential diagnosis of stroke in the patient with HIV infection include atherosclerotic cerebral vascular disease, thrombotic thrombo­ cytopenic purpura, and cocaine or amphetamine use.

Primary CNS lymphoma is discussed below in the section on neo­ plastic diseases. Spinal cord disease, or myelopathy, is present in ~20% of patients with AIDS, often as part of HIV-associated neurocognitive disorder. In fact, 90% of the patients with HIV-associated myelopathy have some evidence of dementia, suggesting that similar pathologic processes may be responsible for both conditions. Three main types of spinal cord disease are seen in patients with AIDS. The first of these is a vacuolar myelopathy, as mentioned above. This condition is pathologically simi­ lar to subacute combined degeneration of the cord, such as that occur­ ring with pernicious anemia. Although vitamin B12 deficiency can be seen in patients with AIDS as a primary complication of HIV infection, it does not appear to be responsible for most cases of myelopathy seen in patients with HIV infection. However, it should be included in the differential diagnosis. Vacuolar myelopathy is characterized by a sub­ acute onset and often presents with gait disturbances, predominantly ataxia and spasticity; it may progress to include bladder and bowel dysfunction. Physical findings include evidence of increased deep ten­ don reflexes and extensor plantar responses. The second form of spinal cord disease involves the dorsal columns and presents as a pure sensory ataxia. The third form is also sensory in nature and presents with par­ esthesias and dysesthesias of the lower extremities. In contrast to the cognitive problems seen in patients with HIV encephalopathy, these spinal cord syndromes do not respond well to antiretroviral drugs, and therapy is mainly supportive. CHAPTER 208 Human Immunodeficiency Virus Disease: AIDS and Related Disorders
One important disease of the spinal cord that also involves the peripheral nerves is a myelopathy and polyradiculopathy seen in asso­ ciation with CMV infection. This entity is generally seen late in the course of HIV infection and is fulminant in onset, with lower extremity and sacral paresthesias, difficulty in walking, areflexia, ascending sen­ sory loss, and urinary retention. The clinical course is rapidly progres­ sive over a period of weeks. CSF examination reveals a predominantly neutrophilic pleocytosis, and CMV DNA can be detected by CSF PCR. Intravenous therapy with ganciclovir or foscarnet can lead to rapid improvement, and prompt initiation of therapy is important in minimizing the degree of permanent neurologic damage. Combination therapy with both drugs should be considered in patients who have been previously treated for CMV disease. Other diseases involving the spinal cord in patients with HIV infection include HTLV-1-associated myelopathy (HAM) (Chap. 207), neurosyphilis (Chap. 187), infection with herpes simplex (Chap. 197) or varicella-zoster (Chap. 198), TB (Chap. 183), and lymphoma (Chap. 113). Peripheral neuropathies are common in patients with HIV infection. They occur at all stages of illness and take a variety of forms. Early in the course of HIV infection, an acute inflammatory demyelinat­ ing polyneuropathy resembling Guillain-Barré syndrome may occur (Chap. 458). In other patients, a progressive or relapsing-remitting inflammatory neuropathy resembling chronic inflammatory demy­ elinating polyneuropathy (CIDP) has been noted. Patients commonly present with progressive weakness, areflexia, and minimal sensory changes. CSF examination often reveals a mononuclear pleocytosis, and peripheral nerve biopsy demonstrates a perivascular infiltrate sug­ gesting an autoimmune etiology. Plasma exchange or IVIg has been tried with variable success. Because of the immunosuppressive effects of glucocorticoids, they should be reserved for severe cases of CIDP refractory to other measures. Another autoimmune peripheral neurop­ athy seen in patients with AIDS is mononeuritis multiplex (Chap. 457) due to a necrotizing arteritis of peripheral nerves. The most common peripheral neuropathy in patients with HIV infection is a distal sensory polyneuropathy (DSPN) also referred to as painful sensory neuropathy

(HIV-SN), predominantly sensory neuropathy, or distal symmetric peripheral neuropathy. This condition may be a direct consequence of HIV infection or a side effect of ART with dideoxynucleosides. It is more common in taller individuals, older individuals, and those with lower CD4 counts. Two-thirds of patients with AIDS may be shown by electrophysiologic studies to have some evidence of peripheral nerve disease. Presenting symptoms are usually painful burning sensations in the feet and lower extremities. Findings on examination include a stocking-type sensory loss to pinprick, temperature, and touch sensa­ tion and a loss of ankle reflexes. Motor changes are mild and are usu­ ally limited to weakness of the intrinsic foot muscles. Response of this condition to antiretrovirals has been variable, perhaps because antiret­ rovirals are responsible for the problem in some instances. When due to dideoxynucleoside therapy, patients with lower extremity peripheral neuropathy may complain of a sensation that they are walking on ice. Other entities in the differential diagnosis of peripheral neuropathy include diabetes mellitus, vitamin B12 deficiency, and side effects from metronidazole or dapsone. For distal symmetric polyneuropathy that fails to resolve following the discontinuation of dideoxynucleosides, therapy is symptomatic; gabapentin, carbamazepine, tricyclics, or analgesics may be effective for dysesthesias. Treatment-naïve patients may respond to ART.

Myopathy may complicate the course of HIV infection; causes include HIV infection itself, zidovudine, and the generalized wasting syndrome (discussed below). HIV-associated myopathy may range in severity from an asymptomatic elevation in creatine kinase levels to a subacute syndrome characterized by proximal muscle weakness and myalgias. Quite pronounced elevations in creatine kinase may occur in asymptomatic patients, particularly after exercise. The clinical sig­ nificance of this as an isolated laboratory finding is unclear. A variety of both inflammatory and noninflammatory pathologic processes have been noted in patients with more severe myopathy, including myofiber necrosis with inflammatory cells, nemaline rod bodies, cytoplasmic bodies, and mitochondrial abnormalities. Profound muscle wasting, often with muscle pain, may be seen after prolonged zidovudine ther­ apy. This toxic side effect of the drug is dose-dependent and is related to its ability to interfere with the function of mitochondrial polymer­ ases. It is reversible following discontinuation of the drug. Red ragged fibers are a histologic hallmark of zidovudine-induced myopathy. PART 5 Infectious Diseases Ophthalmologic Diseases  Ophthalmologic problems occur in ~50% of patients with advanced HIV infection. The most common abnormal findings on funduscopic examination are cotton-wool spots. These are hard white spots that appear on the surface of the retina and often have an irregular edge. They represent areas of retinal ischemia secondary to microvascular disease. At times they are associated with small areas of hemorrhage and thus can be difficult to distinguish from CMV retinitis. In contrast to CMV retinitis, however, these lesions are not associated with visual loss and tend to remain stable or improve over time. One of the most devastating consequences of HIV infection is CMV retinitis. Patients at high risk of CMV retinitis (CD4+ T cell count <100/μL) should undergo an ophthalmologic examination every 3–6 months. The majority of cases of CMV retinitis occur in patients with a CD4+ T cell count <50/μL. Prior to the availability of ART, this CMV reactivation syndrome was seen in 25–30% of patients with AIDS. In the ART era this has dropped to close to 2%. CMV retinitis usually presents as a painless, progressive loss of vision. Patients may also complain of blurred vision, “floaters,” and scintillations. The dis­ ease is usually bilateral, although typically it affects one eye more than the other. The diagnosis is made on clinical grounds by an experienced ophthalmologist. The characteristic retinal appearance is that of peri­ vascular hemorrhage and exudate. In situations where the diagnosis is in doubt due to an atypical presentation or an unexpected lack of response to therapy, vitreous or aqueous humor sampling with molecu­ lar diagnostic techniques may be of value. CMV infection of the retina results in a necrotic inflammatory process, and the visual loss that develops is irreversible. CMV retinitis may be complicated by rheg­ matogenous retinal detachment as a consequence of retinal atrophy

in areas of prior inflammation. Therapy for CMV retinitis consists of oral valganciclovir, IV ganciclovir, or IV foscarnet, with cidofovir as an alternative. Combination therapy with ganciclovir and foscarnet has been shown to be slightly more effective than either ganciclovir or foscarnet alone in the patient with relapsed CMV retinitis. A 3-week induction course is followed by maintenance therapy with oral valgan­ ciclovir. If CMV disease is limited to the eye, intravitreal injections of ganciclovir or foscarnet may be considered. Intravitreal injections of cidofovir are generally avoided due to the increased risk of uveitis and hypotony. Maintenance therapy is continued until the CD4+ T cell count remains >100 μL for >6 months. The majority of patients with HIV infection and CMV disease develop some degree of uveitis with the initiation of ART. The etiology of this is unknown; however, it has been suggested that this may be due to the generation of an enhanced immune response to CMV as an IRIS (see above). In some instances, this has required the use of topical glucocorticoids. Both HSV and varicella zoster virus can cause a rapidly progressing, bilateral, necrotizing retinitis referred to as the acute retinal necrosis syndrome, or progressive outer retinal necrosis (PORN). This syndrome, in contrast to CMV retinitis, is associated with pain, keratitis, and iritis. It is often associated with orolabial HSV or trigeminal zoster. Ophthalmologic examination reveals widespread pale gray peripheral lesions. This condition is often complicated by retinal detachment. It is important to recognize and treat this condition with IV ganciclovir or IV acyclovir (if definitely due to HSV) as quickly as possible to mini­ mize the loss of vision. Several other secondary infections may cause ocular problems in patients with HIV. P. jirovecii can cause a lesion of the choroid that may be detected as an incidental finding on ophthalmologic examination. These lesions are typically bilateral, are from half to twice the disc diameter in size, and appear as slightly elevated yellow-white plaques. They are usually asymptomatic and may be confused with cotton-wool spots. Chorioretinitis due to toxoplasmosis can be seen alone or, more commonly, in association with CNS toxoplasmosis. KS may involve the eyelid or conjunctiva, while lymphoma may involve the retina. Syphi­ lis may lead to a uveitis that is highly associated with the presence of neurosyphilis. Additional Disseminated Infections and Wasting Syndrome 

Infections with species of the small, gram-negative, Rickettsia-like organism Bartonella (Chap. 177) are seen with increased frequency in patients with HIV infection. While it is not considered an AIDSdefining illness by the CDC, many experts view infection with Bar­ tonella as indicative of a severe defect in cell-mediated immunity. It is usually seen in patients with CD4+ T cell counts <100/μL and is a significant cause of unexplained fever in patients with advanced HIV infection. Among the clinical manifestations of Bartonella infection are bacillary angiomatosis, cat-scratch disease, and trench fever. Bacillary angiomatosis is usually due to infection with B. henselae and is linked to exposure to flea-infested cats. It is characterized by a vascular pro­ liferation that leads to a variety of skin lesions that have been confused with the skin lesions of KS. In contrast to the lesions of KS, the lesions of bacillary angiomatosis generally blanch, are painful, and typically occur in the setting of systemic symptoms. Infection can extend to the lymph nodes, liver (peliosis hepatis), spleen, bone, heart, CNS, respira­ tory tract, and GI tract. Cat-scratch disease is also due to infection with B. henselae and generally begins with a papule at the site of inoculation. This is followed several weeks later by the development of regional adenopathy and malaise. Infection with B. quintana is transmitted by lice and has been associated with case reports of trench fever, endocar­ ditis, adenopathy, and bacillary angiomatosis. The organism is quite difficult to culture, and diagnosis often relies on identifying the organ­ ism in biopsy specimens using the Warthin-Starry or similar stains, PCR, and/or seroconversion. Treatment is with either doxycycline or erythromycin for at least 3 months. Histoplasmosis is an opportunistic infection that is seen most fre­ quently in patients in the Mississippi and Ohio River valleys, Puerto Rico, the Dominican Republic, and South America. These are all areas in which infection with H. capsulatum is endemic (Chap. 218).

Because of this limited geographic distribution, histoplasmosis is only seen in approximately 0.5% of AIDS cases in the United States. Histo­ plasmosis is generally a late manifestation of HIV infection; however, it may be the initial AIDS-defining condition. In one study, the median CD4+ T cell count for patients with histoplasmosis and AIDS was 33/μL. While disease due to H. capsulatum may present as a primary infection of the lung, disseminated disease, presumably due to reactiva­ tion, is the most common presentation in patients with HIV. Patients usually present with a 4- to 8-week history of fever and weight loss. Hepatosplenomegaly and lymphadenopathy are each seen in about 25% of patients. CNS disease, either meningitis or a mass lesion, is seen in 15% of patients. Bone marrow involvement is common, with throm­ bocytopenia, neutropenia, and anemia occurring in 33% of patients. Approximately 7% of patients have mucocutaneous lesions consisting of a maculopapular rash and skin or oral ulcers. Respiratory symptoms are usually mild, with chest x-ray showing a diffuse infiltrate or dif­ fuse small nodules in ~50% of cases. The gastrointestinal tract may be involved. Diagnosis is made by silver staining of tissue, by culturing the organisms from blood, bone marrow, or tissue, or by detecting antigen in blood or urine. Treatment is typically with liposomal amphotericin B followed by maintenance therapy with oral itraconazole until the serum Histoplasma antigen is <2 units, the patient has been on antiret­ rovirals for at least 6 months, and the CD4 count is >150 cells/μL. In the setting of mild infection, it may be appropriate to initiate therapy with itraconazole alone. Following the spread of HIV infection to southeast Asia, dissemi­ nated infection with the fungus Talaromyces (formerly Penicillium) marneffei was recognized as a complication of HIV infection and is considered an AIDS-defining condition in those parts of the world where it occurs. T. marneffei is the third most common AIDS-defining illness in Thailand, following TB and cryptococcosis. It is more fre­ quently diagnosed in the rainy than the dry season. Clinical features include fever, generalized lymphadenopathy, hepatosplenomegaly, anemia, thrombocytopenia, and papular skin lesions with central umbilication resembling the lesions of Molluscum contagiosum. Treat­ ment is with amphotericin B followed by itraconazole until the CD4+ T cell count is >100 cells/μL for at least 6 months. Visceral leishmaniasis (Chap. 233) is recognized with increasing frequency in patients with HIV infection who live in or travel to areas endemic for this protozoal infection transmitted by sandflies. The clin­ ical presentation is one of hepatosplenomegaly, fever, and hematologic abnormalities. Lymphadenopathy and other constitutional symptoms may be present. A chronic, relapsing course is seen in two-thirds of coinfected patients. Organisms can be detected by PCR and, with special techniques, isolated from cultures of bone marrow aspirates. Histologic stains are often diagnostic but may be negative. Antibody titers are of little help. Patients with HIV infection usually respond well initially to standard therapy with amphotericin B or pentavalent antimony com­ pounds. Eradication of the organism is difficult, however, and relapses are common. Patients with HIV infection are at a slightly increased risk of infec­ tion with malaria and of clinical malaria. This is particularly true for patients from nonendemic areas who are at risk for primary infec­ tion and in patients with lower CD4+ T cell counts. HIV-positive A B C FIGURE 208-42  Kaposi’s sarcoma in three patients with AIDS demonstrating (A) periorbital edema and bruising; (B) classic truncal distribution of lesions; and (C) upper extremity lesions.

individuals with CD4+ T cell counts <300 cells/μL have a poorer response to malaria treatment than others. Co-infection with malaria is associated with a modest increase in HIV viral load. The risk of malaria may be decreased with TMP-SMX prophylaxis.

Generalized wasting is an AIDS-defining condition; it is defined as involuntary weight loss of >10% associated with intermittent or constant fever and chronic diarrhea or fatigue lasting >30 days in the absence of a defined cause other than HIV infection. Prior to the widespread use of ART it was the initial AIDS-defining condition in ~10% of patients with AIDS in the United States. Generalized wast­ ing is rarely seen today with the earlier initiation of antiretrovirals. A constant feature of this syndrome is severe muscle wasting with scattered myofiber degeneration and occasional evidence of myositis. Glucocorticoids may be of some benefit; however, this approach must be carefully weighed against the risk of compounding the immunode­ ficiency of HIV infection. Androgenic steroids, growth hormone, and total parenteral nutrition have been used as therapeutic interventions with variable success. Neoplastic Diseases  The neoplastic diseases considered to be AIDS-defining conditions are Kaposi’s sarcoma, non-Hodgkin’s lym­ phoma, and invasive cervical carcinoma. In addition, there is also an increase in the incidence of a variety of non-AIDS-defining malig­ nancies including Hodgkin’s disease; multiple myeloma; leukemia; melanoma; and cervical, brain, testicular, oral, lung, gastric, liver, renal, and anal cancers. Since the introduction of potent ART, there has been a marked reduction in the incidence of KS (Fig. 208-34). The non-AIDS-defining malignancies now account for more morbidity and mortality in patients with HIV infection than the AIDS-defining malignancies and are responsible for approximately 10% of the deaths in patients with HIV infection. Rates of non-Hodgkin’s lymphoma have declined; however, this decline has not been as dramatic as the decline in rates of KS. In contrast, ART has had little effect on human papillomavirus (HPV)-associated malignancies. As patients with HIV infection live longer, a wider array of cancers is seen in this population. While some may only reflect known risk factors (e.g., smoking, alcohol consumption, co-infection with other viruses such as hepatitis B) that are increased in patients with HIV infection, some may be a direct consequence of HIV and are clearly increased in patients with lower CD4+ T cell counts. CHAPTER 208 Human Immunodeficiency Virus Disease: AIDS and Related Disorders
Kaposi’s sarcoma is a multicentric neoplasm presenting as multiple vascular nodules in the skin, mucous membranes, and viscera. The clinical course of KS ranges from indolent, with only minor skin or lymph node involvement, to fulminant, with extensive cutaneous and visceral involvement. In the initial period of the AIDS epidemic, KS was a prominent clinical feature of the first cases of AIDS, occurring in 79% of the patients diagnosed in 1981. By 1989 it was seen in only 25% of cases, by 1992 the number had decreased to 9%, and by 1997 the number was <1%. HHV-8 (KSHV) has been strongly implicated as a viral cofactor in the pathogenesis of KS. Clinically, KS has varied presentations and may be seen at any stage of HIV infection, even in the presence of a normal CD4+ T cell count. The initial lesion may be a small, raised, reddish-purple nodule on the skin (Fig. 208-42), a discoloration on the oral mucosa (Fig. 208-35D),

or a swollen lymph node. Lesions often appear in sun-exposed areas, particularly the tip of the nose, and have a propensity to occur in areas of trauma (Koebner phenomenon). Because of the vascular nature of the tumors and the presence of extravasated red blood cells in the lesions, their colors range from reddish to purple to brown and often take the appearance of a bruise, with yellowish discoloration and tat­ tooing. Lesions range in size from a few millimeters to several centi­ meters in diameter and may be either discrete or confluent. KS lesions most commonly appear as raised macules; however, they can also be papular, particularly in patients with higher CD4+ T cell counts. Con­ fluent lesions may give rise to surrounding lymphedema and may be disfiguring when they involve the face and disabling when they involve the lower extremities or the surfaces of joints. Apart from skin, the lymph nodes, GI tract, and lung are the organ systems most commonly affected by KS. Lesions have been reported in virtually every organ, including the heart and the CNS. In contrast to most malignancies, in which lymph node involvement implies metastatic spread and a poor prognosis, lymph node involvement may be seen very early in KS and is of no special clinical significance. In fact, some patients may present with disease limited to the lymph nodes. These are generally patients with relatively intact immune function and thus the patients with the best prognosis. Pulmonary involvement with KS generally presents with shortness of breath. Some 80% of patients with pulmonary KS also have cutaneous lesions. The chest x-ray characteristically shows bilat­ eral lower lobe infiltrates that obscure the margins of the mediastinum and diaphragm (Fig. 208-43). Pleural effusions are seen in 70% of cases of pulmonary KS, a fact that is often helpful in the differential diagno­ sis. GI involvement is seen in 50% of patients with KS and usually takes one of two forms: (1) mucosal involvement, which may lead to bleeding that can be severe; these patients sometimes also develop symptoms of GI obstruction if lesions become large; and (2) biliary tract involve­ ment. KS lesions may infiltrate the gallbladder and biliary tree, leading to a clinical picture of obstructive jaundice similar to that seen with sclerosing cholangitis. Several staging systems have been proposed for KS. One in common use was developed by the National Institute of Allergy and Infectious Diseases AIDS Clinical Trials Group; it distin­ guishes patients on the basis of tumor extent, immunologic function, and presence or absence of systemic symptoms (Table 208-17).

PART 5 Infectious Diseases A diagnosis of KS is based on biopsy of a suspicious lesion. Histo­ logically one sees a proliferation of spindle cells and endothelial cells, extravasation of red blood cells, hemosiderin-laden macrophages, and, in early cases, an inflammatory cell infiltrate. Included in the differ­ ential diagnosis are lymphoma (particularly for oral lesions), bacillary angiomatosis, and cutaneous mycobacterial infections. FIGURE 208-43  Chest x-ray of a patient with AIDS and pulmonary Kaposi’s sarcoma. The characteristic findings include dense bilateral lower lobe infiltrates obscuring the heart borders and pleural effusions.

TABLE 208-17  National Institute of Allergy and Infectious Diseases AIDS Clinical Trials Group TIS Staging System for Kaposi’s Sarcoma GOOD RISK (STAGE 0): ALL OF THE FOLLOWING POOR RISK (STAGE 1): ANY OF THE FOLLOWING PARAMETER Tumor (T) Confined to skin and/ or lymph nodes and/or minimal oral disease Tumor-associated edema or ulceration Extensive oral lesions GI lesions Nonnodal visceral lesions Immune system (I) CD4+ T cell count ≥200/μL CD4+ T cell count <200/μL Systemic illness (S) No B symptomsa B symptomsa present Karnofsky performance status ≥70 Karnofsky performance

status <70 No history of opportunistic infection, neurologic disease, lymphoma, or thrush History of opportunistic infection, neurologic disease, lymphoma, or thrush aDefined as unexplained fever, night sweats, >10% involuntary weight loss, or diarrhea persisting for more than 2 weeks. Management of KS (Table 208-18) should be conducted in con­ sultation with an expert since definitive treatment guidelines do not exist. In the majority of cases, effective ART will go a long way in achieving control. ART has been associated with the spontaneous regression of KS lesions. Paradoxically, it has also been associated with the initial appearance of KS as a form of IRIS. For patients in whom tumor persists or is compromising vital functions or in whom control of HIV replication is not possible, a variety of options exist. In some cases, lesions remain quite indolent, and many of these patients can be managed with no specific treatment. Fewer than 10% of AIDS patients with KS die as a consequence of their malignancy, and death from secondary infections is considerably more common. Thus, whenever possible one should avoid treatment regimens that may further sup­ press the immune system and increase susceptibility to opportunistic infections. Treatment is indicated under two main circumstances. The first is when a single lesion or a limited number of easily accessible lesions are causing significant discomfort or cosmetic problems, such as with prominent facial lesions, lesions overlying a joint, or lesions in the oropharynx that interfere with swallowing or breathing. Under these circumstances, treatment with localized radiation, intralesional vinblastine, topical 9-cis-retinoic acid, or cryotherapy may be helpful. It should be noted that patients with HIV infection are particularly sensitive to the side effects of radiation therapy. This is especially true with respect to the development of radiation-induced mucositis; doses of radiation directed at mucosal surfaces, particularly in the head and TABLE 208-18  Management of AIDS-Associated Kaposi’s Sarcoma Observation and optimization of antiretroviral therapy Single or limited number of lesions   Radiation   Intralesional vinblastine   Cryotherapy Extensive disease; inadequate response to ART   Initial therapy     Interferon α (if CD4+ T cells >150/μL)     Liposomal daunorubicin   Subsequent therapy     Liposomal doxorubicin     Paclitaxel     Pomalidomide     Combination chemotherapy with low-dose doxorubicin, bleomycin, and vinblastine (ABV)     Targeted radiation

neck region, should be adjusted accordingly. The second indication for KS-directed treatment is for patients with a large number of lesions or in patients with visceral involvement. In these patients, systemic ther­ apy, either IFN-α or chemotherapy, should be considered. The single most important determinant of response appears to be the CD4+ T cell count. This relationship between response rate and baseline CD4+

T cell count is particularly true for IFN-α. The response rate to IFN-α for patients with CD4+ T cell counts >600/μL is ~80%, while the response rate for patients with counts <150/μL is <10%. In contrast to the other systemic therapies, IFN-α provides an added advantage of having antiretroviral activity; thus, it may be the appropriate first choice for single-agent systemic therapy for early patients with dissemi­ nated disease. A variety of chemotherapeutic agents also have been shown to have activity against KS. Five of them—liposomal daunorubi­ cin, liposomal doxorubicin, vinblastine, paclitaxel, and the thalidomide analogue pomalidomide—have been approved by the FDA for this indication. Liposomal daunorubicin and pomalidomide are approved as first-line therapy for patients with advanced KS despite ART. They have fewer side effects than conventional chemotherapy. In contrast, liposomal doxorubicin and paclitaxel are approved only for KS patients who have failed standard chemotherapy. Response rates vary from 23% to 88%, appear to be comparable to what had been achieved earlier with combination chemotherapy regimens, and are greatly influenced by CD4+ T cell count. Vinblastine is most commonly used as an intra­ lesional injection or as part of a combination regimen. Lymphomas (Chaps. 113 and 114) occur with an increased fre­ quency in patients with congenital or acquired T cell immunodeficien­ cies. AIDS is no exception; at least 6% of all patients with AIDS develop lymphoma at some time during the course of their illness. This is a 10- to 20-fold increase in incidence compared with the general popu­ lation. In contrast to the situation with KS, primary CNS lymphoma, and most opportunistic infections, the incidence of AIDS-associated systemic lymphomas has not experienced a dramatic decrease as a con­ sequence of the widespread use of effective ART. Lymphoma occurs in all risk groups, with the highest incidence in patients with hemophilia and the lowest incidence in patients from the Caribbean or Africa with heterosexually acquired infection. Lymphoma is a late manifestation of HIV infection, generally occurring in patients with CD4+ T cell counts <200/μL. As HIV disease progresses, the risk of lymphoma increases. The attack rate for lymphoma increases exponentially with increas­ ing duration of HIV infection and decreasing level of immunologic function. At 3 years following a diagnosis of HIV infection, the risk of lymphoma is 0.8% per year; by 8 years after infection, it is 2.6% per year. As individuals with HIV infection live longer as a consequence of improved ART and better treatment and prophylaxis of opportu­ nistic infections, it is anticipated that the incidence of lymphomas may increase. Three main categories of lymphoma are seen in patients with HIV infection: grade III or IV immunoblastic lymphoma, Burkitt’s lym­ phoma, and primary CNS lymphoma. Approximately 90% of these lymphomas are B cell in phenotype; more than half contain EBV DNA. Some are associated with KSHV. These tumors may be either mono­ clonal or oligoclonal in nature and are probably in some way related to the pronounced polyclonal B cell activation seen in patients with HIV infection. Immunoblastic lymphomas account for ~60% of the cases of lym­ phoma in patients with AIDS. The majority of these are diffuse large B cell lymphomas (DLBCL). They are generally high grade and would have been classified as diffuse histiocytic lymphomas in earlier clas­ sification schemes. This tumor is more common in older patients, increasing in incidence from 0% in people with HIV <1 year old to >3% in those >50 years of age. Two variants of immunoblastic lymphoma that are seen primarily in patients with HIV are primary effusion lym­ phoma (PEL) and its solid variant, plasmacytic lymphoma of the oral cavity. PEL, also referred to as body cavity lymphoma, presents with lymphomatous pleural, pericardial, and/or peritoneal effusions in the absence of discrete nodal or extranodal masses. The tumor cells do not express surface markers for B cells or T cells and are felt to represent a preplasmacytic stage of differentiation. While both KSHV and EBV

DNA sequences have been found in the genomes of the malignant cells from patients with body cavity lymphoma, KSHV is felt to be the driv­ ing force behind the oncogenesis (see above).

Small noncleaved cell lymphoma (Burkitt’s lymphoma) accounts for ~20% of the cases of lymphoma in patients with AIDS. It is most frequent in patients 10–19 years old and usually demonstrates char­ acteristic c-myc translocations from chromosome 8 to chromosome 14 or 22. Burkitt’s lymphoma is not commonly seen in the setting of immunodeficiency other than HIV-associated immunodeficiency, and the incidence of this particular tumor is more than 1000-fold higher in the setting of HIV infection than in the general population. In con­ trast to African Burkitt’s lymphoma, where 97% of the cases contain EBV genome, only 50% of HIV-associated Burkitt’s lymphomas are EBV-positive. Primary CNS lymphoma accounts for ~20% of the cases of lym­ phoma in patients with HIV infection. In contrast to HIV-associated Burkitt’s lymphoma, primary CNS lymphomas are usually positive for EBV. In one study, the incidence of Epstein-Barr positivity was 100%. This malignancy does not have a predilection for any particular age group. The median CD4+ T cell count at the time of diagnosis is ~50/μL. Thus, CNS lymphoma generally presents at a later stage of HIV infec­ tion than does systemic lymphoma. This may explain, at least in part, the poorer prognosis for this subset of patients. The clinical presentation of lymphoma in patients with HIV infec­ tion is quite varied, ranging from focal seizures to rapidly growing mass lesions in the oral mucosa (Fig. 208-44) to persistent unexplained fever. At least 80% of patients present with extranodal disease, and a similar percentage have B-type symptoms of fever, night sweats, and/or weight loss. Virtually any site in the body may be involved. The most com­ mon extranodal site is the CNS, involved in approximately one-third of all patients with lymphoma. Approximately 60% of these cases are primary CNS lymphoma. Primary CNS lymphoma generally presents with focal neurologic deficits, including cranial nerve findings, head­ aches, and/or seizures. MRI or CT generally reveals a limited number (one to three) of 3- to 5-cm lesions (Fig. 208-45). The lesions often show ring enhancement on contrast administration and may occur in any location. Contrast enhancement is usually less pronounced than that seen with toxoplasmosis. Lesions of CNS lymphoma are most commonly seen deep in the white matter. The main diseases in the differential diagnosis are cerebral toxoplasmosis and cerebral Chagas’ disease. In addition to the 20% of lymphomas in people with HIV that are primary CNS lymphomas, CNS disease is also seen in patients with HIV and systemic lymphoma. Approximately 20% of patients with systemic lymphoma have CNS disease in the form of leptomeningeal involvement. This fact underscores the importance of lumbar puncture in the staging evaluation of patients with systemic lymphoma. CHAPTER 208 Human Immunodeficiency Virus Disease: AIDS and Related Disorders
Systemic lymphoma is seen at earlier stages of HIV infection than primary CNS lymphoma. In one series the mean CD4+ T cell count was 226/μL. In addition to lymph node involvement, systemic FIGURE 208-44  Immunoblastic lymphoma involving the hard palate of a patient with AIDS.

FIGURE 208-45  Central nervous system lymphoma. Postcontrast T1-weighted MRI scan in a patient with AIDS, altered mental status, and hemiparesis. Multiple enhancing lesions, some ring-enhancing, are present. The left sylvian lesion shows gyral and subcortical enhancement, and the lesions in the caudate and splenium (arrowheads) show enhancement of adjacent ependymal surfaces. lymphoma may commonly involve the GI tract, bone marrow, liver, and lung. GI tract involvement is seen in ~25% of patients. Any site in the GI tract may be involved, and patients may complain of difficulty swallowing or abdominal pain. The diagnosis is usually suspected on the basis of CT or MRI of the abdomen. Bone marrow involvement is seen in ~20% of patients and may lead to pancytopenia. Liver and lung involvement are each seen in ~10% of patients. Pulmonary disease may present as a mass lesion, multiple nodules, or an interstitial infiltrate. PART 5 Infectious Diseases Both conventional and unconventional approaches have been employed in an attempt to treat HIV-related lymphomas. Systemic lymphoma is generally treated by the oncologist with combination chemotherapy. Earlier disappointing figures are being replaced with more optimistic results for the treatment of systemic lymphoma follow­ ing the availability of more effective ART and the use of rituximab in CD20+ tumors. While there is some controversy regarding the use of antiretrovirals during chemotherapy, there is no question that their use overall in patients with HIV lymphoma has improved survival. Con­ cerns regarding synergistic bone marrow toxicities with chemotherapy and ART are mitigated with the use of ART regimens that avoid myelosuppressive antiretrovirals. As in most situations in patients with HIV disease, those with higher CD4+ T cell counts tend to fare better although not as well as patients with lymphoma without HIV. Response rates as high as 72% with a median survival of 33 months and disease-free intervals up to 9 years have been reported. Treatment of primary CNS lymphoma remains a significant challenge. Treatment is complicated by the fact that this illness usually occurs in patients with advanced HIV disease. Palliative measures such as radiation therapy provide some relief. The prognosis remains poor in this group, with a 2-year survival of 20–30%. Multicentric Castleman’s disease (MCD) is a KSHV-associated lym­ phoproliferative disorder that is seen with an increased frequency in patients with HIV infection. While the incidence of Kaposi’s sarcoma has decreased, the incidence of MCD has increased in the setting of ART. While not a true malignancy, MCD shares many features with lymphoma including generalized lymphadenopathy, hepatospleno­ megaly, and systemic symptoms of fever, fatigue, and weight loss. Pul­ monary symptoms may be seen in ~50% of patients. KS is present in 75–82% of cases. Lymph node biopsies reveal a predominance of inter­ follicular plasma cells and/or germinal centers with vascularization and an “onion skin” (hyaline vascular) appearance. Prior to the availability of ART, patients with HIV and multicentric Castleman’s disease had a 15-fold increased risk of developing non-Hodgkin’s lymphoma com­ pared with patients with HIV in general. Treatment typically involves chemotherapy. Rituximab may be of benefit, but it has been associated

with worsening of coexisting KS. Similarly, the use of corticosteroids may increase the risk of developing KS. The median survival of patients with treated multicentric Castleman’s disease pre-ART was initially reported as 14 months. This has increased to a 2-year survival of more than 90% in the era of ART. Evidence of infection with human papillomavirus (HPV), associated with intraepithelial dysplasia of the cervix or anus, is approximately twice as common in people with HIV as in the general population and can lead to intraepithelial neoplasia and eventually invasive cancer. In a series of studies, men with HIV were examined for evidence of anal dysplasia, and Papanicolaou (Pap) smears were found to be abnormal in 20–80%. These changes tend to persist and are generally not affected by ART, raising the possibility of a subsequent transition to a more malignant condition. While the incidence of an abnormal Pap smear of the cervix is ~5% in otherwise healthy women, the incidence of abnormal cervical smears in women with HIV infection is 30–60%, and invasive cervical cancer is included as an AIDS-defining condition. While only small increases in the absolute numbers of cervical or anal cancers have been seen as a consequence of HIV infection, the relative risk of these conditions when one compares men and women with HIV to men and women without HIV infection is on the order of 10- to 100-fold and decreased with ART. Given the high rates of dysplasia and relative risks for cervical and anal cancer, a comprehensive gyne­ cologic and rectal examination, including Pap smear, is indicated at the initial evaluation and 6 months later for all patients with HIV infec­ tion. If these examinations are negative at both time points, the patient should be followed with yearly evaluations. If an initial or repeat Pap smear shows evidence of severe inflammation with reactive squamous changes, the next Pap smear should be performed at 3 months. If, at any time, a Pap smear shows evidence of squamous intraepithelial lesions, colposcopic examination with biopsies as indicated should be performed. The 2-year survival rate for women with HIV and invasive cervical cancer is 64% compared with 79% for women without HIV. In addition to rectal and cervical lesions, HPV can also lead to head and neck cancers. In one study of men who have sex with men, 25% were found to have oral HPV; high-risk HPV genotypes were three times more common in men with HIV. The most common HPV genotypes in the general population and the genotypes upon which current HPV vaccines are based are 6, 11, 16, and 18. In the population of people with HIV, other genotypes such as 58 and 53 also are prominent. This raises a concern about the level of effectiveness of the current HPV vaccines for patients with HIV. Despite this, it is recommended that patients with HIV infection be vaccinated against HPV. IDIOPATHIC CD4+ T LYMPHOCYTOPENIA A syndrome was recognized in 1992 characterized by an absolute CD4+ T cell count of <300/μL or <20% of total T cells on a minimum of two occasions at least 6 weeks apart; no evidence of HIV-1, HIV-2, HTLV-1, or HTLV-2 on testing; and the absence of any defined immu­ nodeficiency or therapy associated with decreased levels of CD4+

T cells. By mid-1993, ~100 patients had been described. After extensive multicenter investigations, a series of reports were published in early 1993, which together allowed a number of conclusions. Idiopathic CD4+ lymphocytopenia (ICL) is a very rare syndrome, as determined by studies of blood donors and cohorts of HIV-seronegative men who have sex with men. Cases were clearly identified as early as 1983. The definition of ICL based on CD4+ T cell counts coincided with the ready availability of testing for CD4+ T cells in patients suspected of being immunodeficient. However, as a result of immune deficiency, certain patients with ICL develop some of the opportunistic diseases (par­ ticularly cryptococcosis, nontuberculous mycobacterial infections, and HPV disease) seen in patients with HIV. In one study, HPV-associated disease was seen in 29% of patients, cryptococcosis in 24%, molluscum contagiosum in 95%, and nontuberculous mycobacterial disease in 5%. Approximately 10–30% of patients may exhibit an autoimmune disease; this is more common in those with higher CD4 counts. The syndrome is demographically, clinically, and immunologically unlike HIV infection and AIDS. Fewer than half of the reported ICL patients had risk factors for HIV infection, and there were wide geographic

and age distributions. The fact that a significant proportion of initially diagnosed patients did have risk factors probably reflects a selection bias, in that physicians who take care of patients with HIV were more likely to monitor CD4+ T cells. Approximately half of the patients are women, compared with approximately one-fifth among people with HIV in the United States. Many patients with ICL remained clinically stable, and their condition may not deteriorate progressively as is common with seriously immunodeficient patients with HIV. Approxi­ mately 15% of patients with ICL experience spontaneous reversal of the CD4+ T lymphocytopenia. Immunologic abnormalities in ICL are somewhat different from those of HIV infection. ICL patients often have increases in CD4+ T cell activation with decreases in CD8+ T cells and B cells. Furthermore, immunoglobulin levels are either normal or, more commonly, decreased in patients with ICL, compared with the usual hypergammaglobulinemia of people with HIV. Virologic studies of these patients have revealed no evidence of HIV-1, HIV-2, HTLV-1, or HTLV-2 or of any other mononuclear cell–tropic virus. Further­ more, there has been no epidemiologic evidence to suggest that a transmissible microbe was involved. The cases of ICL have been widely dispersed, with no clustering. Close contacts and sexual partners who were studied were clinically well and were serologically, immunologi­ cally, and virologically negative for HIV. ICL is a heterogeneous syn­ drome, and it is highly likely that there is no common cause; however, there may be common causes among subgroups of patients that are currently unrecognized. Patients who present with laboratory data consistent with ICL should be worked up for underlying diseases that could be responsible for the immune deficiency. If no underlying cause is detected, no spe­ cific therapy should be initiated. However, if opportunistic diseases occur, they should be treated appropriately (see above). Depending on the level of the CD4+ T cell count, patients should receive prophylaxis for the commonly encountered opportunistic infections. TREATMENT AIDS and Related Disorders GENERAL PRINCIPLES OF PATIENT MANAGEMENT The CDC guidelines call for the testing for HIV infection to be a part of routine medical care. It is recommended that the patient be informed of the intention to test, as is the case with other routine laboratory determinations, and be given the opportunity to “opt out.” Such an approach is critical to the goal of identifying as many infected individuals as possible since 13% of the 1.2 million indi­ viduals in the United States with HIV are not aware of their status. In the setting of routine testing, although it is difficult, pretest counseling is an important part of the process. No matter how well prepared a patient is for adversity, the discovery of a diagnosis of HIV infection is a devastating event. Thus, physicians should be sensitive to this fact and, where possible, utilize pretest counseling to at least partially prepare the patient should the results demon­ strate the presence of HIV infection. Following a diagnosis of HIV infection, the health care provider should be prepared to immedi­ ately activate support systems for the newly diagnosed patient and initiate ART. These supports should include individuals who can spend time talking to the newly diagnosed person and ensuring that he or she is emotionally stable and ready to begin therapy. Most communities have HIV support centers that can be of great help in these difficult situations. The treatment of patients with HIV infection requires not only a comprehensive knowledge of the possible disease processes that may occur and up-to-date knowledge of and experience with ART, but also the ability to deal with the problems of a chronic, potentially life-threatening illness. A comprehensive knowledge of internal medicine is required to deal with the changing spectrum of illnesses associated with HIV infection, many of which are similar to a state of accelerated aging. The appropriate use of potent ART and other treatment and prophylactic interventions is of critical importance in providing each patient with the best opportunity to

live a long and healthy life with HIV infection. In contrast to the earlier days of this epidemic, a diagnosis of HIV infection needs no longer be equated with having an inevitably fatal disease. In addition to medical interventions, the health care provider has a responsibility to provide each patient with appropriate counseling and education concerning their disease as part of a comprehensive care plan. Patients must be educated about the potential transmis­ sibility of their infection and about the fact that while health care providers may refer to levels of the virus as “undetectable,” this is only a reflection of the sensitivity of the assay being used to measure the virus, rather than a comment on the presence or absence of the virus. It is important for patients to be aware that the virus is still present in virtually all patients who have ever been diagnosed with HIV infection and capable of being transmitted in the absence of effective ART. Thus, there must be frank discussions concerning sexual practices and the sharing of syringes and other parapher­ nalia used in illicit drug use. The treating physician not only must be aware of the latest medications available for patients with HIV infection but also must educate patients concerning the natural history of their illness, listen to their concerns, and be sensitive to their fears. As with other diseases, therapeutic decisions should be made in consultation with the patient, when possible, and with the patient’s proxy if the patient is incapable of making decisions. In this regard, it is recommended that all patients with HIV infection, and in particular those with CD4+ T cell counts <200/μL, designate a trusted individual with durable power of attorney to make medical decisions on their behalf, if necessary.

Following a diagnosis of HIV infection, several examinations and laboratory studies should be performed to help determine the extent of disease and provide baseline standards for future refer­ ence (Table 208-19). In addition to routine chemistry, fasting lipid profile, aspartate aminotransferase, alanine aminotransferase, total and direct bilirubin, fasting glucose and hematology screening pan­ els, Pap smear, urinalysis, and chest x-ray, one should also obtain a CD4+ T cell count, a plasma HIV RNA level, an HIV resistance test, a rapid plasma reagin (RPR) or Venereal Disease Research Laboratory (VDRL) test, an anti-Toxoplasma antibody titer, and serologies for hepatitis A, B, and C. A PPD test or IFN-γ release assay should be done and an MMSE performed and recorded. A pregnancy test should be done for any women for whom the drug efavirenz is being considered, and HLA-B5701 testing should be done in all patients in whom the drug abacavir is being considered. CHAPTER 208 Human Immunodeficiency Virus Disease: AIDS and Related Disorders
TABLE 208-19  Initial Evaluation of the Patient with HIV Infection History and physical examination Routine chemistry and hematology AST, ALT, alkaline phosphatase, direct and indirect bilirubin Lipid profile and fasting glucose CD4+ T lymphocyte count Plasma HIV RNA level HIV resistance testing HLA-B5701 screening RPR or VDRL test Anti-Toxoplasma antibody titer Urinalysis PPD skin test or IFN-γ release assay Mini-Mental Status Examination Serologies for hepatitis A, hepatitis B, and hepatitis C Immunization with pneumococcal polysaccharide; influenza; HPV as indicated Immunization with hepatitis A and hepatitis B if seronegative Counseling regarding natural history and transmission Help contacting others who might be infected Abbreviations: ALT, alanine aminotransferase; AST, aspartate aminotransferase; PPD, purified protein derivative; RPR, rapid plasma reagin; VDRL, Venereal Disease Research Laboratory.

TABLE 208-20  HIV Resources Available on the World Wide Web CLINICALINFO.HIV.GOV US-approved HIV treatment guidelines CLINICALTRIALS.GOV Information on clinical trials HIV.GOV Epidemiological data and prevention information Patients should be immunized with pneumococcal polysaccharide, with annual influenza and COVID-19 shots, and, if seronegative for these viruses, with HPV, hepatitis A, and hepatitis B vaccines. The status of hepatitis C infection should be determined. In addi­ tion, patients should be counseled with regard to sexual practices and needle sharing, and counseling should be offered to people whom the patient knows, or suspects, also may be infected. Once these baseline activities are performed, short- and long-term medi­ cal management strategies should be developed based on the most recent information available and modified as new information becomes available. The field of HIV medicine is changing rapidly, and it is difficult to remain fully up to date. Fortunately, there are a series of excellent sites on the Internet that are frequently updated, and they provide the most recent information on a variety of topics, including consensus panel reports on treatment (Table 208-20). ANTIRETROVIRAL THERAPY Combination antiretroviral therapy (ART), also referred to as highly active antiretroviral therapy (HAART), is the cornerstone of man­ agement of patients with HIV infection and should be initiated as soon as possible following a diagnosis of HIV infection. One excep­ tion to immediate initiation of ART is in the setting of cryptococ­ cal meningitis or TB where several weeks of specific antimicrobial therapy prior to initiation of ART may decrease the risk of severe IRIS. Following the initiation of widespread use of ART in the United States in 1995–1996, marked declines were noted in the inci­ dence of most AIDS-defining conditions (Fig. 208-34). Suppression of HIV replication is an important component in prolonging and improving the quality of life for the patient as well as minimizing the risk of transmission of HIV to others. Adequate suppression of HIV replication requires strict adherence to prescribed regimens of antiretroviral drugs. This has been facilitated by the coformula­ tions of antiretrovirals and the development of once-daily, monthly regimens, and every-six-month regimens with the expectation that even longer-lasting drugs are on the horizon. Among the decisions that need to be made in the context of prescribing ART are selec­ tion of the best initial regimen, determining when a given regimen should be changed, and deciding what regimen should be selected when a change is made. The care provider and patient must come to a mutually agreeable plan based on the best available data. In an effort to facilitate this process, the U.S. Department of Health and Human Services makes available on the Internet (clinicalinfo.hiv.gov/ en/guidelines) a series of periodically updated guidelines, including “Guidelines for the Use of Antiretroviral Agents in HIV-Infected Adults and Adolescents” and “Guidelines for the Prevention and Treatment of Opportunistic Infections in Persons Infected with Human Immu­ nodeficiency Virus.” At present, an extensive clinical trials network, involving both clinical investigators and patient advocates, is in place attempting to develop improved approaches to therapy. Consor­ tia comprising representatives of academia, industry, independent foundations, and the federal government engage in drug develop­ ment, including a wide-ranging series of clinical trials. As a result, new therapies and new therapeutic strategies are continually emerg­ ing. New drugs are often available through expanded-access pro­ grams prior to official licensure. Given the complexity of this field, decisions regarding ART are best made in consultation with experts. PART 5 Infectious Diseases Currently available drugs for the treatment of HIV infection as part of a combination regimen fall into five categories: those that inhibit the viral reverse transcriptase enzyme (nucleoside and nucleo­ tide reverse transcriptase inhibitors; nonnucleoside reverse tran­ scriptase inhibitors), those that inhibit the viral protease enzyme (protease inhibitors), those that inhibit the viral integrase enzyme (integrase strand transfer inhibitors), those that interfere with viral

entry (fusion inhibitors; CCR5 antagonists; CD4 antagonists) and those that interfere with the viral capsid (Table 208-21; Fig. 208-46). A typical regimen will include two nucleoside/nucleotide reverse tran­ scriptase inhibitors (usually a tenofovir-based drug or abacavir +

3TC or FTC) plus a nonnucleoside reverse transcriptase inhibitor, an integrase inhibitor, or a protease inhibitor boosted with a phar­ macokinetic enhancer (ritonavir or cobicistat). More recent studies have also supported the two-drug regimen of dolutegravir plus 3TC for initial therapy in hepatitis B–negative patients with baseline HIV RNA levels <500,000 copies/mL. Numerous fixed-drug formula­ tions combining two or more of these antiretroviral drugs have been licensed (Table 208-22). Prior to initiation of therapy and at any time a change in therapy due to treatment failure is being consid­ ered, drug resistance testing should be performed to help guide the selection of drugs to be used in combination. A summary of known resistance mutations for antiretroviral drugs is shown in Fig. 208-47. While most patients with HIV infection will be infected with HIV-1, some patients, especially those with an epidemiologic link to West Africa, may be infected with HIV-2. While the principles of treatment are the same as those for persons infected with HIV-1, it is important to note that the nonnucleoside reverse transcriptase inhibitors, enfuvirtide, and fostemsavir are not active against HIV-2 and should not be used as part of ART regimens in HIV-2–infected individuals. The FDA-approved reverse transcriptase inhibitors include the nucleoside analogues zidovudine, didanosine, zalcitabine, stavudine, lamivudine, abacavir, and emtricitabine; the nucleotide analogues tenofovir disoproxil and tenofovir alafenamide; and the nonnucleo­ side reverse transcriptase inhibitors nevirapine, delavirdine, efavi­ renz, etravirine, rilpivirine, long-acting rilpivirine, and doravirine (Table 208-21). These represent the first class of drugs licensed for the treatment of HIV infection. They are indicated for this use as part of combination regimens. It should be stressed that none of these drugs should be used as monotherapy for HIV infection due to the relative ease with which drug resistance may develop under such circumstances. Thus, when lamivudine, emtricitabine, or tenofovir is used to treat hepatitis B infection in the setting of HIV infection, one should ensure that the patient is also on additional antiretroviral medication. Similarly, when any of these three medications are discontinued, one needs to be vigilant for a flare of hepatitis B in coinfected patients. The reverse transcriptase inhibitors block the HIV replication cycle at the point of RNAdependent DNA synthesis, the reverse transcription step. While the nonnucleoside reverse transcriptase inhibitors are quite selective for the HIV-1 reverse transcriptase, the nucleoside and nucleotide ana­ logues inhibit a variety of DNA polymerases in addition to those of the HIV-1 reverse transcriptase. For this reason, serious side effects are more varied with the nucleoside analogues and include mitochondrial damage that can lead to hepatic steatosis and lactic acidosis as well as peripheral neuropathy and pancreatitis. The use of either of the thymidine analogues zidovudine and stavudine has been associated with a syndrome of hyperlipidemia, glucose intol­ erance/insulin resistance, and fat redistribution often referred to as lipodystrophy syndrome (discussed in “Diseases of the Endocrine System and Metabolic Disorders,” above). For these reasons, the older drugs in this class, zidovudine, didanosine, zalcitabine, and stavudine, are no longer recommend for use in the United States due to their side-effect profiles. The nucleoside and nucleotide transcriptase inhibitors preferred for use in combination regimens according to the DHHS Panel on the use of antiretroviral drugs are lamivudine, emtricitabine, abacavir, tenofovir disoproxil, and tenofovir alafenamide. Given its renal toxicity, tenofovir disoproxil should be limited to use in patients with creatinine clearance (CrCl)

70 while tenofovir alafenamide should generally be limited to use in patients with CrCl >30. The preferred nonnucleoside reverse transcriptase inhibitors are efavirenz, rilpivirine, and doravirine. Of note, rilpivirine is approved for treatment only in ART-naïve patients with HIV RNA levels <100,000 copies/mL and is contrain­ dicated in patients taking proton pump inhibitors.

TABLE 208-21  Antiretroviral Drugs Licensed in the United States for the Treatment of HIV Infection DOSE IN COMBINATION SUPPORTING DATA TOXICITY DRUGa INDICATION Nucleoside or Nucleotide Reverse Transcriptase Inhibitors Lamivudine (Epivir, 2′3′-dideoxy-3′- thiacytidine, 3TC) In combination with other antiretroviral agents for the treatment of HIV infection 150 mg bid 300 mg qd Emtricitabine

(FTC, Emtriva) In combination with other antiretroviral agents for the treatment of HIV infection 200 mg qd Comparable to lamivudine in combination with stavudine and nevirapine/efavirenz Abacavir (Ziagen) For treatment of HIV infection in combination with other antiretroviral agents 300 mg bid Abacavir + zidovudine + 3TC equivalent to indinavir + zidovudine + 3TC with regard to viral load suppression (~60% in each group with <400 HIV RNA copies/mL plasma) and CD4+ T cell increase (~100/μL in each group) at 24 weeks Tenofovir disoproxil fumarate (Viread) For use in combination with other antiretroviral agents when treatment is indicated 300 mg qd Reduction of ~0.6 log in HIV-1 RNA levels when added to background regimen in treatmentexperienced patients Tenofovir alafenamide (Vemlidy) In combination with emtricitabine and other antiretroviral agents for treatment of HIV-1 infection 25 mg qd 92% of patients treated in combination with emtricitabine, elvitegravir, and cobicistat had HIV-1 RNA levels <50 copies/mL Nonnucleoside Reverse Transcriptase Inhibitors Nevirapine (Viramune) In combination with other antiretroviral agents for treatment of progressive HIV infection 200 mg/d × 14 days then 200 mg bid or 400 mg extended release qd Efavirenz (Sustiva) For treatment of HIV infection in combination with other antiretroviral agents 600 mg qhs Efavirenz + zidovudine + 3TC comparable to indinavir + zidovudine + 3TC with regard to viral load suppression (a higher percentage of the efavirenz group achieved viral load <50 copies/ mL, but the discontinuation rate in the indinavir group was unexpectedly high, accounting for most treatment “failures”); CD4 cell increase (~140/μL in each group) at 24 weeks Etravirine (Intelence) In combination with other antiretroviral agents in treatment-experienced patients whose HIV is resistant to nonnucleoside reverse transcriptase inhibitors and other antiretroviral medications 200 mg bid Higher rates of HIV RNA suppression to <50 copies/mL (56% vs 39%); greater increases in CD4+ T cell count (89 vs 64 cells) compared to placebo when given in combination with an optimized background regimen Rilpivirine (Edurant) In combination with other drugs in previously untreated patients when treatment is indicated 25 mg qd Noninferior to efavirenz with respect to suppression at week 48 in 1368 treatmentnaïve individuals, except in patients with pretherapy HIV RNA levels >100,000 where it was inferior Protease Inhibitors Ritonavir (Norvir) In combination with other antiretroviral agents for treatment of HIV infection when treatment is warranted 600 mg bid (also used in lower doses as pharmacokinetic booster) Atazanavir (Reyataz) For treatment of HIV infection in combination with other antiretroviral agents 400 mg qd or 300 mg qd + ritonavir 100 mg qd when given with efavirenz

In combination with zidovudine superior to zidovudine alone with respect to changes in CD4+ T cell counts in 495 patients who were zidovudine-naïve and 477 patients who were zidovudine-experienced; overall CD4+ T cell counts for the zidovudine group were at baseline by 24 weeks, while in the group treated with zidovudine plus lamivudine they were 10–50 cells/μL above baseline; 54% decrease in progression to AIDS/death compared with zidovudine alone Flare of hepatitis in HBV-coinfected patients who discontinue drug Hepatotoxicity in HBV-co-infected patients who discontinue drug, skin discoloration Hypersensitivity reaction In HLAB5701+ individuals (can be fatal); fever, rash, nausea, vomiting, malaise or fatigue, and loss of appetite Renal, osteomalacia, flare of hepatitis in HBV-co-infected patients who discontinue drug Nausea, less renal toxicity than tenofovir disoproxil fumarate CHAPTER 208 Increase in CD4+ T cell count, decrease in HIV RNA when used in combination with nucleosides Skin rash, hepatotoxicity Human Immunodeficiency Virus Disease: AIDS and Related Disorders
Rash, dysphoria, elevated liver function tests, drowsiness, abnormal dreams, depression, lipid abnormalities, potentially teratogenic Rash, nausea, hypersensitivity reactions Nausea, dizziness, somnolence, vertigo, less CNS toxicity and rash than efavirenz Reduction in the cumulative incidence of clinical progression or death from 34% to 17% in patients with CD4+ T cell count

<100/μL treated for a median of 6 months Nausea, abdominal pain, hyperglycemia, fat redistribution, lipid abnormalities, may alter levels of many other drugs, paresthesias, hepatitis Comparable to efavirenz when given in combination with zidovudine + 3TC in a study of 810 treatment-naïve patients; comparable to nelfinavir when given in combination with stavudine + 3TC in a study of 467 treatmentnaïve patients Hyperbilirubinemia, PR prolongation, nausea, vomiting, hyperglycemia, fat maldistribution, rash transaminase elevations, renal stones (Continued)

TABLE 208-21  Antiretroviral Drugs Licensed in the United States for the Treatment of HIV Infection DOSE IN COMBINATION SUPPORTING DATA TOXICITY DRUGa INDICATION Darunavir (Prezista) In combination with 100 mg ritonavir for combination therapy in treatmentexperienced adults 600 mg + 100 mg ritonavir bid with food Entry Inhibitors Enfuvirtide (Fuzeon) In combination with other agents in treatmentexperienced patients with evidence of HIV-1 replication despite ongoing antiretroviral therapy 90 mg SC bid In treatment of experienced patients, superior to placebo when added to new optimized background (37% vs 16% with <400 HIV RNA copies/mL at 24 weeks; + 71 vs + 35 CD4+ T cells at 24 weeks) Maraviroc (Selzentry) In combination with other antiretroviral agents in adults infected with only CCR5tropic HIV-1 150–600 mg bid depending on concomitant medications (see text) Ibalizumab (Trogarzo) In combination with other antiretroviral agents in patients with multidrugresistant HIV-1 Single loading dose of 2000 mg followed by a maintenance dose of 800 mg every 2 weeks Fostemsavir (Rukobia) In combination with other antiretroviral agents in patients with multi-drug resistant HIV-1 600 mg bid At 96 weeks, 64% of patients with multi-drug resistant HIV-1 treated with an optimized background and fostemsavir achieved an HIV-1 RNA level <200 copies/mL PART 5 Infectious Diseases Integrase Inhibitors Raltegravir (Isentress) In combination with other antiretroviral agents 400 mg bid At 24 weeks, among 436 patients with 3-class drug resistance, 76% of patients randomized to receive raltegravir achieved HIV RNA levels <400 copies/mL compared with 41% of patients randomized to receive placebo Elvitegravir (Available only in combination with cobicistat, tenofovir, and emtricitabine [Stribild]) Fixed-dose combination 1 tablet daily Noninferior to raltegravir or atazanavir/ ritonavir in treatment-experienced patients Dolutegravir (Tivicay) In combination with other antiretroviral agents 50 mg daily for treatment-naïve patients 50 mg twice daily for treatment-experienced patients or those also receiving efavirenz or rifampin Bictegravir (Available only in combination with tenofovir alafenamide and emtricitabine [Biktarvy]) For treatment of HIV infection in adults 50 mg bictegravir/

25 mg tenofovir alafenamide/200 mg emtricitabine qd Cabotegravir (Vocabria) In combination with rilpivirine for treatment of HIV infection in adults Oral lead-in of 30 mg + 25 mg rilpivirine for 1 month; followed by an initial injection of 600 mg (3 mL) IM + 900 mg (3 mL) rilpivirine IM; followed by monthly injections of 400 mg

(2 mL) IM + 600 mg

(2 mL) rilpivirine IM

(Continued) At 24 weeks, patients with prior extensive exposure to antiretrovirals treated with a new combination including darunavir showed a –1.89-log change in HIV RNA levels and a 92-cell increase in CD4+ T cells compared with –0.48 log and 17 cells in the control arm Diarrhea, nausea, headache, skin rash, hepatotoxicity, hyperlipidemia, hyperglycemia Local injection reactions, hypersensitivity reactions, increased rate of bacterial pneumonia At 24 weeks, among 635 patients with CCR5tropic virus and HIV-1 RNA >5000 copies/mL despite at least 6 months of prior therapy with at least 1 agent from 3 of the 4 antiretroviral drug classes, 61% of patients randomized to maraviroc achieved HIV RNA levels <400 copies/mL compared with 28% of patients randomized to placebo Hepatotoxicity, nasopharyngitis, fever, cough, rash, abdominal pain, dizziness, musculoskeletal symptoms At 25 weeks, 50% of patients with multidrugresistant HIV-1 with HIV-1 RNA >1000 copies/ mL treated with an optimized background of 1 active drug and ibalizumab achieved HIV RNA levels <200 copies/mL Rash, diarrhea, nausea QTc prolongation at higher doses, elevation in liver enzymes in those with hepatitis B or C Nausea, headache, diarrhea, CPK elevation, muscle weakness, rhabdomyolysis Diarrhea, nausea, upper respiratory infections, headache Noninferior to raltegravir, superior to efavirenz or darunavir/ritonavir Insomnia, headache, hypersensitivity reactions, hepatotoxicity Noninferior to dolutegravir/tenofovir/ emtricitabine and noninferior to dolutegravir/ abacavir/lamivudine Nausea, diarrhea, headache Noninferior to abacavir/dolutegravir/ lamivudine or dolutegravir + 2 nucleoside/tide reverse transcriptase inhibitors Noninferior to nonnucleoside reverse transcriptase inhibitor + 2 nucleoside/ tide reverse transcriptase inhibitors or a protease inhibitor + 2 nucleoside/tide reverse transcriptase inhibitors or an integrase inhibitor and 2 nucleoside/tide reverse transcriptase inhibitors Injection-site reactions (Continued)

TABLE 208-21  Antiretroviral Drugs Licensed in the United States for the Treatment of HIV Infection DOSE IN COMBINATION SUPPORTING DATA TOXICITY DRUGa INDICATION Lenacapavir (Sunlenca) In combination with other agents for treatment of multidrug-resistant HIV infection in adults Induction with either (927 mg SC + 600 mg orally day 1; 600 mg orally day 2) or (600 mg orally days 1 and 2; 300 mg orally day 8; 927 mg SC day 15) followed by 927 mg SC every 6 months aInitial trade names are provided. Generic forms may be available. Abbreviations: ARC, AIDS-related complex; bid, twice daily; CPK, creatine phosphokinase; HBV, hepatitis B virus; IM, intramuscular; qd, once daily; qhs, once daily at bedtime; SC, subcutaneous. Nucleoside or Nucleotide Reverse Transcriptase Inhibitors O CH3 HN NH2 O O N O O P HN N NH O HOCH2 HO– N N N HO O O H N3 H Zidovudine Zalcitabine Didanosine NH2 O H CH3 N N N H2N O O N O O OH N N O HO F S H H Lamivudine Tenofovir disoproxil fumarate Emtricitabine Stavudine Nonnucleoside Reverse Transcriptase Inhibitors CH3 CH–CH3 CH3 SO2 H CH3 NH N N N N N H N O N H C O N OH CH3 SO2 N C Delavirdine C N NH HN N Rilpivirine Protease Inhibitors O O O O N H N H N N H O N N S S HO O CH3 H3C OH OH CH3 Nelfinavir mesylate Ritonavir NH2 OH OH H N N N H N O N H NH NH N O O NHC(CH3)3 O OH O O NHC(CH3)3 ×CH3SO3H Saquinavir mesylate Indinavir sulfate O H OH H N O O H H N S H SO2 N H OH O O O N H CF3 Tipranavir Darunavir FIGURE 208-46  Molecular structures of antiretroviral agents.

(Continued) At 52 weeks, 83% of patients with multi-drug resistant HIV-1 treated with an optimized background and lenacapavir achieved an HIV-1 level <50 copies/mL Injection site reactions, nausea NH2 HN HO N N N O N N H2N H2SO4 N O OPh N N N OH NH CH3 H3C 1/2 O O CH2OH 2 O Tenofovir alafenamide Abacavir NH2 N N O O H C C N CO2H N O P CHAPTER 208 O O O H CH3 HO2C O O O O OH S O N Human Immunodeficiency Virus Disease: AIDS and Related Disorders
O N H F3C H3C CH3 O H N N O Cl N N N N Br NH2 Nevirapine Efavirenz Etravirine CH3 CH3 H3C S NHtBu O H N N H N O O H N HN N H O OH H3C H Lopinavir NH2 N O HN O O S O OH O O H3CO OCH3 • H2S H N N H H N N H • H2SO4 N O N OH O O Amprenavir Atazanavir O O • C2H5OH H3C NH2 CH3

Entry Inhibitors NH2 NH2 O OH O O O O O O H N N H H2N H N N H H N N H O O O NH H2N HO O O O O OH HN OH O O NH O H2N HN O O H2N NH HO HO O O O O HN N H N H O O H N H N H N N H H2N O O O O HO F N N Me F H N N N O Me Me Maraviroc PART 5 Infectious Diseases O O N N N N N Integrase Inhibitors FO O H N N H N N N H N H N O N F O O OH Me Raltegravir Elvitegravir Me ONa O O F F O

N H N N 11a O H O Cabotegravir Capsid Inhibitor F F F F H N N N O N N F3C Me S O O Lenacapavir FIGURE 208-46  (Continued)

HO HO HO O O O O H N N H H N N H H N N H O O H N N H N H O O O HO N NH Ibalizumab NH2 H N O O N H NH O H N H N O N H N H O O OH O OH O HN O NH O NH2 HN O N H N H Enfuvirtide O NH2 O N N OH OH O H2N O OH P O OH HO Fostemsavir O F Cl N F O O HO O N O F H N N H OH N O Dolutegravir F F O OH Bictegravir Cl N– Na+ S Me N O O CF3

TABLE 208-22  Combination Formulations of Antiretroviral Drugs NAME COMBINATION ABC/3TC (generic) Abacavir 600 mg/lamivudine 300 mg EFV/TDF/FTC (generic)a Efavirenz 600 mg/tenofovir disoproxil fumarate 300 mg/ emtricitabine 200 mg Biktarvya Bictegravir 50 mg/tenofovir alafenamide 25 mg/emtricitabine 200 mg Cabenuvaa Cabotegravir + rilpivirine (long-acting injection) Cimduo Tenofovir disoproxil fumarate 300 mg/lamivudine 300 mg Compleraa Rilpivirine 25 mg/tenofovir disoproxil fumarate 300 mg/ emtricitabine 200 mg Delstrigoa Doravirine 100 mg/tenofovir disoproxil fumarate 300 mg/ lamivudine 300 mg Descovy Tenofovir alafenamide 25 mg/emtricitabine 200 mg Dovatoa Dolutegravir 50 mg/lamivudine 300 mg Genvoyaa Elvitegravir 150 mg/cobicistat 150 mg/tenofovir alafenamide

10 mg/emtricitabine 200 mg Julucaa Dolutegravir 50 mg/rilpivirine 25 mg Odefseya Rilpivirine 25 mg/tenofovir alafenamide 25 mg/emtricitabine 200 mg Stribilda Elvitegravir 150 mg/cobicistat 150 mg/tenofovir disoproxil fumarate 300 mg/emtricitabine 200 mg Symfia Efavirenz 600 mg/tenofovir disoproxil fumarate 300 mg/lamivudine 300 mg Symfi Loa Efavirenz 400 mg/tenofovir disoproxil fumarate 300 mg/lamivudine 300 mg Symtuzaa Darunavir 800 mg/cobicistat 150 mg/tenofovir alafenamide 10 mg/ emtricitabine 200 mg Triumeqa Dolutegravir 50 mg/abacavir 600 mg/lamivudine 300 mg Truvada Tenofovir disoproxil fumarate 300 mg/emtricitabine 200 mg aComplete, once-daily, single-tablet regimens. Source: Guidelines for the Use of Antiretroviral Agents in HIV-Infected Adults and Adolescents, USPHS. The HIV-1 protease inhibitors (saquinavir, indinavir, ritona­ vir, nelfinavir, amprenavir, fosamprenavir, lopinavir/ritonavir, ata­ zanavir, atazanavir/cobicistat, tipranavir, darunavir, and darunavir/

cobicistat) are an important part of the therapeutic armamentarium of antiretrovirals. While possessing antiviral properties of its own, ritonavir is typically used as a pharmacokinetic enhancer due to its high affinity for several isoforms of cytochrome P450 (3A4, 2D6) leading to large increases in the plasma concentrations of coadministered drugs metabolized by these pathways. As in the case of reverse transcriptase inhibitors, resistance to protease inhibitors can develop rapidly in the setting of monotherapy, and thus these agents should be used only as part of combination therapeutic regi­ mens. Based on superior efficacy and side-effect profile, ritonavirboosted darunavir in combination with emtricitabine and tenofovir (disoproxil or alafenamide) is the preferred protease inhibitor strat­ egy according to the DHHS Panel on the use of antiretroviral drugs. Integrase strand transfer inhibitors act by blocking the action of the HIV integrase enzyme and thus preventing integration of the HIV provirus into the host cell genome. They are among the most potent and safest of the antiretroviral drugs and frequently part of initial combination regimens. The five licensed integrase inhibitors are raltegravir, cabotegravir, elvitegravir, dolutegravir, and bictegravir. Cabotegravir is an integrase inhibitor that is given in combination with rilpivirine as a monthly injection. Prior to initiation of the monthly injections, patients should initially be treated with oral preparations of the two drugs to be sure they are well tolerated. Elvitegravir is always given in combination with cobicistat, which acts to boost the concentrations of elvitegravir. Cobicistat also inhibits tubular secretion of creatinine, resulting in increases in serum creatinine, and is not recommended for patients with estimated creatinine clearances <70 mL/min. Bictegravir is available only in combination with tenofovir alafenamide and emtricitabine. When used as part of initial ART, integrase inhibitor– containing regimens have been associated with greater weight gain

than nonnucleoside reverse transcriptase inhibitor– or protease inhibi­ tor–containing regimens.

Entry inhibitors act by interfering with the binding of HIV to its receptor or co-receptor or by interfering with the process of fusion (see above). The first drug in this class to be licensed was the fusion inhibitor enfuvirtide, or T-20, followed by the CCR5 antagonist maraviroc. The anti-CD4 monoclonal antibody ibalizumab was licensed in 2018, and the small molecule fostemsavir in 2020. Given that maraviroc is effective only against CCR5-tropic viruses, a coreceptor tropism assay should be performed when use of this agent is being considered. The capsid inhibitor lenacapavir acts by interfering with viral rep­ lication by binding to the interface between p24 capsid subunits in hexamers. This then interferes with multiple steps in the viral life­ cyle including nuclear uptake of proviral DNA, virus assembly and release, and capsid core formation. It is licensed for treatment of individuals who are heavily treatment-experienced with multidrugresistant virus that is not suppressed on the current regimen. It is given by subcutaneous injection every 6 months after an oral or oral plus subcutaneous loading regimen. PRINCIPLES OF THERAPY The principles of therapy for HIV infection have been articulated by a panel sponsored by the U.S. Department of Health and Human Services as a working group of the NIH Office of AIDS Research Advisory Council. These principles are summarized in Table 208-23. As noted in these guidelines, ART of HIV infection does not lead to eradication or cure of HIV. The possible exceptions are a limited num­ ber of individuals with HIV infection and cancer who received alloge­ neic stem cell transplants from donors who were homozygous for the CCR5Δ32 mutation (see above) and thus resistant to HIV infection. CHAPTER 208 Treatment decisions must consider the fact that one is dealing with a chronic infection that requires life-long therapy. Patients initiating antiretroviral therapy must be willing to commit to ongoing treatment and understand the importance of adherence to their prescribed regi­ men. The importance of adherence is illustrated by the observation that treatment interruption is associated with rapid increases in HIV RNA levels, rapid declines in CD4+ T cell counts, and an increased risk of clinical progression. While it seems reasonable to assume that the complications associated with ART could be minimized by intermittent treatment regimens designed to minimize exposure to the drugs in question, all efforts to do so have paradoxically been associated with an increase in serious adverse events in the patients randomized to intermittent therapy, demonstrating that some “nonAIDS-defining” serious adverse events such as heart attack and stroke are linked to HIV replication. Thus, unless contraindicated for reasons of toxicity, patients started on ART should remain on ART. Human Immunodeficiency Virus Disease: AIDS and Related Disorders
At present, the U.S. Department of Health and Human Services Guidelines panel recommends that everyone with HIV infection be treated with ART and that therapy be initiated a soon as pos­ sible after diagnosis with some exceptions noted below. ART has been associated with a decrease in disease progression in patients at all stages of HIV infection and leads to a decrease in the risk of transmission of infection. In addition, one may wish to administer a 6-week course of therapy to uninfected individuals immediately fol­ lowing a high-risk exposure to HIV. The combination of tenofovir and emtricitabine is also licensed for pre-exposure prophylaxis in individuals at high risk of HIV infection, as is an injectable, longacting formulation of cabotegravir that may be even more effective. For patients diagnosed with an opportunistic infection and HIV infection at the same time and a CD4+ count ≥50 cells/μL, one may consider a 2- to 4-week delay in the initiation of antiretroviral therapy during which time treatment is focused on the opportunis­ tic infection. This delay may decrease the severity of any subsequent immune reconstitution inflammatory syndrome by lowering the antigenic burden of the opportunistic infection. This is particularly true for patients with TB or cryptococcal meningitis. For patients with advanced HIV infection (CD4+ <50 cells/μL), however, ART should be initiated as soon as possible.

PART 5 Infectious Diseases FIGURE 208-47  Amino acid substitutions conferring resistance to antiretroviral drugs. For each amino acid residue, the letter above the bar indicates the amino acid associated with wild-type virus and the letter(s) below indicate the substitution(s) that confer viral resistance. The number shows the position of the mutation in the protein. Mutations selected by protease inhibitors in Gag cleavage sites are not listed. HR1, first heptad repeat; NAMs, nRTI-associated mutations; NNRTI, nonnucleoside reverse transcriptase inhibitor; nRTI, nucleoside reverse transcriptase inhibitor; PI, protease inhibitor. Amino acid abbreviations: A, alanine; C, cysteine; D, aspartate; E, glutamic acid; F, phenylalanine; G, glycine; H, histidine; I, isoleucine; K, lysine; L, leucine; M, methionine; N, asparagine; P, proline; Q, glutamine; R, arginine; S, serine; T, threonine; V, valine; W, tryptophan; Y, tyrosine. (Reproduced with permission from AM Wensing et al: 2022 Update of the drug resistance mutations in HIV-1. 13, 2022. Reproduced with permission from IAS–USA.)

Human Immunodeficiency Virus Disease: AIDS and Related Disorders

CHAPTER 208 FIGURE 208-47  (Continued)

TABLE 208-23  Principles of Therapy of HIV Infection

  1. Ongoing HIV replication leads to immune system damage, progression to AIDS, and systemic immune activation.
  2. Plasma HIV RNA levels indicate the magnitude of HIV replication and the rate of CD4+ T cell destruction. CD4+ T cell counts indicate the current level of competence of the immune system.
  3. Maximal suppression of viral replication is a goal of therapy; the greater the suppression the less likely the appearance of drug-resistant quasispecies.
  4. The most effective therapeutic strategies involve the simultaneous initiation of combinations of effective anti-HIV drugs with which the patient has not been previously treated and that are not cross-resistant with antiretroviral agents that the patient has already received.
  5. The antiretroviral drugs used in combination regimens should be used according to optimum schedules and dosages.
  6. The number of available drugs is limited. Any decisions on antiretroviral therapy have a long-term impact on future options for the patient.
  7. Women should receive optimal antiretroviral therapy regardless of pregnancy status.
  8. The same principles apply to children and adults, but the treatment of HIVinfected children involves unique pharmacologic, virologic, and immunologic considerations.
  9. Compliance is an important part of ensuring maximal effect from a given regimen. The simpler the regimen, the easier it is for the patient to be compliant. Source: Modified from Principles of Therapy of HIV Infection, USPHS, and the Henry J. Kaiser Family Foundation. Once the decision has been made to initiate therapy, the health care provider must decide which drugs to use as the first regi­ men. The decision regarding choice of drugs not only will affect the immediate response to therapy but also will have implications regarding options for future therapeutic regimens. The initial regi­ men is usually the most effective insofar as the virus has yet to be under any selective pressure to develop significant drug resistance. HIV is capable of rapidly developing resistance to any single agent, and therapy must be given as a multidrug combination. Given that patients can be infected with viruses that harbor drug resistance mutations, it is recommended that a viral genotype be done at the time of initiation of therapy to ensure that appropriate antiretroviral agents have been selected. The combination regimens currently rec­ ommended for initial therapy in most treatment-naïve patients are listed in Table 208-24. It is currently debated whether treatmentnaïve individuals with <50 copies/mL of HIV RNA benefit from ART. While these individuals are at low risk of disease progression in the short term, they do have evidence of persistent immune acti­ vation that may have long-term consequences. PART 5 Infectious Diseases Following the initiation of therapy, one should expect a rapid, at least 1-log (10-fold) reduction in plasma HIV RNA levels within 1–2 months and then a slower decline in plasma HIV RNA levels to <50 copies/mL within 6 months. During this same time there should be a rise in the CD4+ T cell count of 100–150/cells μL that is also TABLE 208-24  Initial Combination Regimens Recommended for Most Treatment-Naïve Patients Regardless of HIV RNA Level or CD4 Count For people who do not have a history of using Carbotegravir-LA as PrEP:   Bictegravir + tenofovir alafenamide + emtricitabine (Biktarvy)   Dolutegravir + tenofovira + emtricitabineb   Dolutegravir + lamivudine (Dovato) only for those with HIV

RNA ≤ 500,000 copies/ml For people who have a history of Carbotegravir-LA as PrEP, INSTI genotype resistance testing should be performed; if starting prior to resistance testing results:   Darunavir/cobicistat or darunavir/ritonavir + tenofovira + emtricitabineb aTenofovir alafenamide and tenofovir disoproxil fumarate are two forms of tenofovir approved by FDA. Tenofovir alafenamide has fewer bone and renal toxicities, while tenofovir disoproxil fumarate is associated with lower lipid levels. bLamivudine may substitute for emtricitabine and vice versa. Source: Guidelines for the Use of Antiretroviral Agents in HIV-Infected Adults and Adolescents, USPHS.

TABLE 208-25  Indications for Changing Antiretroviral Therapy in Patients with HIV Infectiona Less than a 1-log drop in plasma HIV RNA by 4 weeks following the initiation of therapy A reproducible significant increase (defined as threefold or greater) from the nadir of plasma HIV RNA level not attributable to intercurrent infection, vaccination, or test methodology Persistently declining CD4+ T cell numbers Clinical deterioration Side effects aGenerally speaking, a change should involve the initiation of at least two drugs felt to be effective in the given patient. The exception to this is when a change is being made to manage toxicity, in which case a single substitution is reasonable. Source: Guidelines for the Use of Antiretroviral Agents in HIV-Infected Adults and Adolescents, USPHS. particularly brisk during the first month of therapy. Subsequently, one should anticipate a CD4+ T cell count increase of 50–100 cells/ year until numbers approach normal. Many clinicians feel that failure to achieve these endpoints is an indication for a change in therapy. Other reasons for a change in therapy include a persistently declining CD4+ T cell count, a consistent increase in HIV RNA levels to >200 copies/mL, clinical deterioration, or drug toxicity (Table 208-25). As in the case of initiating therapy, changing therapy may have a lasting impact on future therapeutic options. When changing therapy because of treatment failure (clinical progression or worsening laboratory parameters), it is important to attempt to provide a regimen with at least two new active drugs. This deci­ sion can be guided by resistance testing (see below). In the patient in whom a change is made for reasons of drug toxicity, a simple replacement of one drug is reasonable. It should be stressed that in attempting to sort out a drug toxicity it may be advisable to hold all therapy for a period of time to distinguish between drug toxicity and disease progression. Drug toxicity will usually begin to show signs of reversal within 1–2 weeks. Prior to changing a treatment regimen because of drug failure, it is important to ensure that the patient has been adherent to the prescribed regimen. As in the case of initial therapy, the simpler the new therapeutic regimen, the easier it is for the patient to be compliant. Plasma HIV RNA levels should be mon­ itored within 2–4 weeks after initiation of ART or following a change in regimen, every 4–8 weeks until HIV RNA levels are suppressed to <200 copies/mL, and then every 3–6 months during therapy. In order to determine an optimal therapeutic regimen for ini­ tial therapy or for a patient on a failing regimen, one may attempt to measure antiretroviral drug susceptibility through genotyping or phenotyping of HIV quasispecies and to determine adequacy of dosing through measurement of drug levels. Genotyping may be done through cDNA sequencing. Phenotypic assays typically measure the enzymatic activity of viral enzymes in the presence or absence of different concentrations of different drugs and have also been used to determine co-receptor tropism. These assays will generally detect quasispecies present at a frequency of ≥10%. Nextgeneration sequencing may allow detection of quasispecies at fre­ quencies down to 1%. It is generally recommended that resistance testing be used in confirming initial therapy choices in settings where the risk of transmission of resistant virus is high (such as the United States and Europe) and in determining new regimens for patients experiencing virologic failure while on therapy. Resistance testing may be of particular value in distinguishing drug-resistant virus from poor patient compliance. Due to the rapid rate at which drug-resistant viruses revert to wild-type, it is recommended that resistance testing performed in the setting of drug failure be con­ ducted while the patient is still on the failing regimen. Measure­ ment of plasma drug levels can also be used to tailor individual treatment. The inhibitory quotient, defined as the trough blood level/IC50 of the patient’s virus, is used by some to determine the adequacy of dosing of a given treatment regimen. Despite the best of efforts there will still be patients with ongoing high levels of HIV

replication while receiving the best available therapy. These patients will receive benefit from remaining on antiretroviral therapy even though it is not fully suppressive. In addition to the licensed medications discussed above, a large number of experimental agents are being evaluated as possible therapies for HIV infection. Therapeutic strategies are being devel­ oped to interfere with virtually every step of the replication cycle of the virus (Fig. 208-3) and in an attempt to eliminate the reservoir of infected cells to “cure” HIV infection. In addition to directly acting antiviral drugs, other strategies, generically referred to as “immune-based therapies,” are being developed as a complement to antiviral therapy. Among the antiviral agents in early clinical trials are additional nucleoside and nucleotide analogues, protease inhibitors, fusion inhibitors, receptor and co-receptor antagonists, and integrase inhibitors—as well as new antiviral strategies includ­ ing long-acting injectables, antisense nucleic acids, and maturation inhibitors. Among the immune-based therapies being evaluated are monoclonal antibodies, bone marrow transplantation, adoptive transfer of lymphocytes genetically modified to resist infection or enhance HIV-specific immunity, active immunotherapy with inac­ tivated HIV or its components, IL-7, and IL-15. HIV AND THE HEALTH CARE WORKER Health care workers, especially those who deal with large numbers of patients with HIV, have a small but definite risk of becoming infected with HIV as a result of professional activities (see “Occupational Trans­ mission of HIV: Health Care Workers, Laboratory Workers, and the Health Care Setting,” above). In the United States, 58 health care workers for whom case investiga­ tions have been completed have had documented seroconversions to HIV following occupational exposures. Only one of these has occurred since 1999. Approximately 85% of the exposures resulting in infection have been due to percutaneous (puncture/cut injury) exposures to HIV-infected blood. In addition, at least 150 possible cases of occu­ pationally acquired HIV infection have been reported among health care personnel in the United States. The number of these workers who actually acquired their infection through occupational exposures is not known. Taken together, data from several large studies suggest that the risk of HIV infection following a percutaneous exposure to HIV-

contaminated blood in an individual who does not receive postexposure prophylaxis (PEP) is ~0.23%, and after a mucous membrane exposure, ~0.09%. Although episodes of HIV transmission after nonintact skin exposure have been documented, the average risk for transmission by this route has not been precisely quantified but is estimated to be less than the risk for mucous membrane exposures. The risk for transmis­ sion after exposure to body fluids or tissues other than HIV-infected blood also has not been quantified but is probably considerably lower than for blood exposures. A seroprevalence survey of 3420 orthopedic surgeons, 75% of whom practiced in an area with a relatively high prevalence of HIV infection and 39% of whom reported percutaneous exposure to patient blood, usually through an accident involving a suture needle, failed to reveal any cases of possible occupational infec­ tion, suggesting that the risk of infection with a suture needle may be considerably less than that with a blood-drawing (hollow-bore) needle. Most cases of health care worker seroconversion have occurred as a result of needle-stick injuries. When one considers the circum­ stances that result in needle-stick injuries, it is immediately obvious that adhering to the standard guidelines for dealing with sharp objects would result in a significant decrease in this type of accident. In one study, 27% of needle-stick injuries resulted from improper disposal of the needle (more than half of these were due to recapping the needle), 23% occurred during attempts to start an IV line, 22% occurred during blood drawing, 16% were associated with an IM or SC injection, and 12% were associated with giving an IV infusion. Occupational exposures to HIV should be considered as a medical emergency to ensure timely postexposure management and admin­ istration of PEP. A delay of even sending 72 h in the initiation of PEP may be the difference preventing and not preventing infection.

Recommendations regarding PEP must take into account that a variety of circumstances determine the risk of transmission of HIV following occu­ pational exposure. In this regard, several factors have been associated with an increased risk for occupational transmission of HIV infection, including deep injury, the presence of visible blood on the instrument causing the exposure, injury with a device that had been placed in the vein or artery of the source patient, and advanced HIV disease in the source patient. Other important considerations when considering PEP in the health care worker include known or suspected pregnancy or breast-feeding, the possibility of exposure to drug-resistant virus, and the toxicities of different PEP regimens. Regardless of the decision to use PEP, the wound should be cleansed immediately and antiseptic applied. If a decision is made to offer PEP, U.S. Public Health Service guidelines recommend that PEP regimens contain three (or more) antiretroviral drugs administered for a 4-week duration for all occupational exposures to HIV. Detailed guidelines are available from the Updated U.S. Public Health Service Guidelines for the Management of Occupational Exposures to HIV and Recommendations for Postexposure Prophylaxis (CDC, 2018). The report emphasizes the importance of adherence to PEP when it is indicated, and close follow-up of exposed workers should be provided including counseling, baseline and follow-up HIV testing, and monitor­ ing for drug toxicity. Follow-up appointments should begin within 72 h of an HIV exposure and may be concluded 4 months after exposure. For consultation on the treatment of occupational exposures to HIV and other bloodborne pathogens, the clinician managing the exposed patient can call the National Clinicians’ Post-Exposure Prophylaxis Hotline (PEPline) at 888-448-4911. This service is available 24 hours a day at no charge. (Additional information is available at www.nccc.ucsf.edu.) PEPline support may be especially useful in challenging situations, such as when drug-resistant HIV strains are suspected or if the health care worker is pregnant.

CHAPTER 208 Health care workers can minimize their risk of occupational HIV infection by following the CDC guidelines of June 2015, which include adherence to universal precautions and assuming that blood and other body fluids from all patients are potentially infectious. Therefore, the following infection control precautions should be adhered to at all times: (1) routinely use barriers (such as gloves and/or goggles) when anticipating contact with blood or body fluids; (2) immediately wash hands and other skin surfaces after contact with blood or body fluids; and (3) carefully handle and dispose of sharp instruments during and after use. For further information contact the CDC at 800-CDC-INFO (232-4636) or see www.cdc.gov/cdc-info/. The risk of HBV infection fol­ lowing a needle-stick injury from a hepatitis antigen–positive patient is much higher than the risk of HIV infection (see “Transmission,” above). There are multiple examples of needle-stick injuries where the patient was positive for both HBV and HIV and the health care worker became infected only with HBV. For these reasons, it is advisable, given the high prevalence of HBV infection in people with HIV, that all health care workers dealing with patients with HIV be immunized with the HBV vaccine. Human Immunodeficiency Virus Disease: AIDS and Related Disorders
TB is another infection common to patients with HIV that can be transmitted to the health care worker. For this reason, all health care workers should know their PPD status, have it checked yearly, and, where appropriate, receive 6 months of isoniazid treatment if their skin test converts to positive. In addition, all patients in whom a diagnosis of active pulmonary TB is being entertained should be placed immedi­ ately in respiratory isolation, pending results of the diagnostic evalua­ tion. The emergence of drug-resistant organisms, including extensively drug-resistant TB strains, has made TB an increasingly important problem for health care workers. This is particularly true for the health care worker with pre-existing HIV infection. HIV PREVENTION Many proven interventions, usually applied in combination, have a role in preventing the transmission of HIV (Fig. 208-48). Education, counseling, and behavior modification are the cornerstones of any HIV prevention strategy. A major problem in the United States and elsewhere is that many infections are passed on by those who do not know that they are infected. Of the ~1.2 million persons in the United States

HIV Testing PrEP PEP Treatment as Prevention Microbicides Clean Syringes and SSPs STI Testing and Treatment Medical Male Circumcision Treatment/ Prevention of Drug/ Alcohol Addiction Blood Supply Screening PMTCT Condoms Education/ Behavior Modification FIGURE 208-48  The HIV prevention “toolkit.” See text for detailed description. PMTCT, prevention of mother-to-child transmission of HIV; PEP, postexposure prophylaxis with antiretroviral drugs; PrEP, pre-exposure prophylaxis with antiretroviral drugs; SSPs, syringe services programs. (From: The White House. 2021. National HIV/AIDS Strategy for the United States 2022–2025. Washington, DC.) with HIV, it is estimated that ~13% do not know their HIV status and that a substantial proportion of all new infections are transmitted by those people. In this regard, the CDC has recommended HIV testing as part of routine medical care and that all individuals between the ages of 13 and 64 years be tested at least once. These individuals should be informed of the testing and be tested without the need for written informed consent. Each individual can “opt out” of testing; however, testing should otherwise be routinely administered. Individuals who are practicing high-risk behavior should be tested more often and should use pre-exposure prophylaxis (PrEP) (see below). Partners engaged in monogamous sexual relationships who wish to be assured of safety should both be tested for HIV antibody. If both are negative, it must be understood that any divergence from monogamy puts both partners at risk; open discussion of the importance of honesty in such relationships should be encouraged. PART 5 Infectious Diseases When the HIV status of either partner is not known, or when one partner is positive, there are a number of options. Use of condoms can markedly decrease the chance of HIV transmission. It should be remembered that condoms are not 100% effective in preventing trans­ mission of HIV infection, and there is a ~10% failure rate of condoms used for contraceptive purposes. Most condom failures result from breakage or improper usage, such as not wearing the condom for the entire period of intercourse. Latex condoms are preferable since viruses have been shown to leak through natural skin condoms. Petroleumbased gels should never be used for lubrication of the condom, since they increase the likelihood of condom rupture. Microbicides composed of gels or rings containing antiretroviral drugs have been shown to be variably efficacious in preventing acquisi­ tion of HIV infection in women engaging in vaginal intercourse. The considerable degree of variability in efficacy relates to the generally poor adherence of participants to the use of the intervention. One product, a vaginal ring that releases the antiretroviral drug dapivirine from the ring into the vagina slowly over 28 days, has been recom­ mended by WHO as an additional prevention choice for women at substantial risk of HIV infection as part of combination prevention approaches. Large, prospective clinical trials have clearly demonstrated that ART for people with HIV has an important role in HIV prevention. The initial results of the HPTN 052 clinical trial published in 2011 demonstrated a 96% reduction in HIV transmission risk among het­ erosexual HIV-discordant couples where the partner with HIV started ART immediately versus delayed ART initiation. The final results of HPTN 052, published in 2016, reported no HIV transmissions within these couples when the partner with HIV had a suppressed viral load (defined as having a viral load of <400 copies of HIV RNA per mil­ liliter). Three subsequent studies reported similar results, with no

genetically linked infections while the partner with HIV was virally suppressed even though couples were engaging in sex without a condom and not using PrEP. These three studies included >500 HIVdiscordant heterosexual couples and >1100 HIV-discordant couples of men who have sex with men. Combined, these couples engaged in >125,000 sex acts without a condom or PrEP over more than 2600 couple-years of observation. Collectively, the studies demonstrated that if the viral load of the infected partner is decreased to below detectable levels by antiretroviral therapy, sexual transmission to the uninfected partner does not occur. This is true for heterosexuals and men who have sex with men, leading, as noted above, to the commonly used phrase “undetectable equals untransmittable” or U = U. Pre-exposure prophylaxis (PrEP) with antiretroviral medication also is highly effective in preventing HIV acquisition by at-risk uninfected men who have sex with men and heterosexual men and women. Accumulated data indicate that high adherence to a PrEP regimen of emtricitabine + tenofovir disoproxil fumarate, taken as 1 pill per day or on demand (immediately before and following a sexual encoun­ ter), is 99% effective in preventing HIV acquisition if subjects adhere strictly to the regimen. Subsequent studies indicated similar, if not better, efficacy with cabotegravir injections given every 2 months as a maintenance regimen. A recent phase 3 clinical trial conducted in South Africa and Uganda indicated that the twice-yearly injectable HIV capsid inhibitor lenacapavir demonstrated even greater efficacy in preventing HIV infection in cisgender women. Additional phase 3 studies of lenacapavir for PrEP are being conducted in cisgender men, transgender women, transgender men, and gender-nonbinary individ­ uals. More limited data demonstrate the utility of PrEP for people who inject drugs. CDC estimates that approximately 1.2 million people in the United States are at “substantial” risk for HIV infection and should be counseled about PrEP. Adult male circumcision, which has been shown to result in a 50–65% reduction in HIV acquisition in the circumcised subject, is currently being pursued, particularly in developing nations, as a component of HIV prevention (see above). The most effective way to prevent trans­ mission of HIV infection among IDUs is to stop the use of injectable drugs. Unfortunately, that is extremely difficult to accomplish unless the individual enters a treatment program. For those who will not or cannot participate in a drug treatment program and who will continue to inject drugs, the avoidance of sharing of needles and other parapher­ nalia (“works”) is the next best way to avoid transmission of infection. However, the cultural and social factors that contribute to the sharing of paraphernalia are complex and difficult to overcome. Under these circumstances, paraphernalia should be cleaned after each usage with a virucidal solution, such as sodium hypochlorite (undiluted house­ hold bleach). Needle exchange programs have been highly successful in decreasing HIV transmission among injection drug users without increasing the use of injection drugs. As noted, above, oral PrEP also is effective in preventing acquisition of HIV infections among IDUs. It is important for IDUs to be tested for HIV infection and counseled to avoid transmission to their sexual partners. Prevention of transmission through blood or blood products and prevention of mother-to-child transmission are discussed in “Transmission,” above. ■ ■HIV VACCINES There is currently no safe and effective vaccine approved for the pre­ vention of HIV infection. Successful vaccines for other diseases are predicated on the assumptions that the body can mount an adequate immune response to the microbe or virus in question during natural infection and that the vaccine will mimic the natural response to infection. Even with serious diseases, such as smallpox, poliomyelitis, measles, and influenza among others, the body usually clears the infec­ tious agent and provides protection, which is usually lifelong against future exposure to the same pathogen. Unfortunately, this is not the case with HIV infection since the natural immune response to HIV infection is unable to clear the virus from the body and cases of super­ infection are not rare. Some of the factors that contribute to the problematic nature of developing a preventive HIV vaccine are (1) the high mutability of the